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<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE article PUBLIC "-//NLM//DTD Journal Publishing with OASIS Tables v3.0 20080202//EN" "journalpub-oasis3.dtd"> <article xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:oasis="http://docs.oasis-open.org/ns/oasis-exchange/table" dtd-version="3.0"> <front> <journal-meta> <journal-id journal-id-type="publisher">SE</journal-id> <journal-title-group> <journal-title>Solid Earth</journal-title> <abbrev-journal-title abbrev-type="publisher">SE</abbrev-journal-title> <abbrev-journal-title abbrev-type="nlm-ta">Solid Earth</abbrev-journal-title> </journal-title-group> <issn pub-type="epub">1869-9529</issn> <publisher><publisher-name>Copernicus GmbH</publisher-name> <publisher-loc>Göttingen, Germany</publisher-loc> </publisher> </journal-meta> <article-meta> <article-id pub-id-type="doi">10.5194/se-5-1243-2014</article-id><title-group><article-title>Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments</article-title> </title-group><?xmltex \runningtitle{Future accreted terranes}?><?xmltex \runningauthor{J.~L.~Tetreault and S.~J.~H.~Buiter}?> <contrib-group> <contrib contrib-type="author" corresp="yes" rid="aff1"> <name><surname>Tetreault</surname><given-names>J. L.</given-names></name> <email>joya.tetreault@ngu.no</email> </contrib> <contrib contrib-type="author" corresp="no" rid="aff1 aff2"> <name><surname>Buiter</surname><given-names>S. J. H.</given-names></name> <ext-link>https://orcid.org/0000-0002-2493-2377</ext-link></contrib> <aff id="aff1"><label>1</label><institution>Geodynamics Team, Geological Survey of Norway (NGU), Trondheim, Norway</institution> </aff> <aff id="aff2"><label>2</label><institution>Centre for Earth Evolution and Dynamics, University of Oslo, Oslo, Norway</institution> </aff> </contrib-group> <author-notes><corresp id="corr1">J. L. Tetreault (joya.tetreault@ngu.no)</corresp></author-notes><pub-date><day>4</day><month>December</month><year>2014</year></pub-date> <volume>5</volume> <issue>2</issue> <fpage>1243</fpage><lpage>1275</lpage> <history> <date date-type="received"><day>28</day><month>May</month><year>2014</year></date> <date date-type="rev-request"><day>1</day><month>July</month><year>2014</year></date> <date date-type="rev-recd"><day>30</day><month>October</month><year>2014</year></date> <date date-type="accepted"><day>4</day><month>November</month><year>2014</year></date> </history> <permissions> <license license-type="open-access"> <license-p>This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit <ext-link ext-link-type="uri" xlink:href="http://creativecommons.org/licenses/by/3.0/">http://creativecommons.org/licenses/by/3.0/</ext-link></license-p> </license> </permissions> <self-uri xlink:href="https://se.copernicus.org/articles/.html">This article is available from https://se.copernicus.org/articles/.html</self-uri> <self-uri xlink:href="https://se.copernicus.org/articles/.pdf">The full text article is available as a PDF file from https://se.copernicus.org/articles/.pdf</self-uri> <abstract> Allochthonous accreted terranes are exotic geologic units that originated from anomalous crustal regions on a subducting oceanic plate and were transferred to the overriding plate by accretionary processes during subduction. The geographical regions that eventually become accreted allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. These future allochthonous terranes (FATs) contribute to continental crustal growth, subduction dynamics, and crustal recycling in the mantle. We present a review of modern FATs and their accreted counterparts based on available geological, seismic, and gravity studies and discuss their crustal structure, geological origin, and bulk crustal density. Island arcs have an average crustal thickness of 26 km, average bulk crustal density of 2.79 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>, and three distinct crustal units overlying a crust–mantle transition zone. Oceanic plateaus and submarine ridges have an average crustal thickness of 21 km and average bulk crustal density of 2.84 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>. Continental fragments presently on the ocean floor have an average crustal thickness of 25 km and bulk crustal density of 2.81 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>. Accreted allochthonous terranes can be compared to these crustal compilations to better understand which units of crust are accreted or subducted. In general, most accreted terranes are thin crustal units sheared off of FATs and added onto the accretionary prism, with thicknesses on the order of hundreds of meters to a few kilometers. However, many island arcs, oceanic plateaus, and submarine ridges were sheared off in the subduction interface and underplated onto the overlying continent. Other times we find evidence of terrane–continent collision leaving behind accreted terranes 25–40 km thick. We posit that rheologically weak crustal layers or shear zones that were formed when the FATs were produced can be activated as detachments during subduction, allowing parts of the FAT crust to accrete and others to subduct. In many modern FATs on the ocean floor, a sub-crustal layer of high seismic velocities, interpreted as ultramafic material, could serve as a detachment or delaminate during subduction. </abstract> </article-meta> </front> <body> <sec id="Ch1.S1" sec-type="intro"> <title>Introduction</title> Terrane accretion is considered to be one of the main contributors to the growth of continental crust <xref ref-type="bibr" rid="bib1.bibx260 bib1.bibx47 bib1.bibx35" id="paren.1"/>. Although continental crust is lost by erosion and/or recycled into the mantle at subduction zones, crust is also added to continents at subduction zones by accretion and magmatic events. Accreted terranes can be made of tectonically added crustal units of volcanic arcs, oceanic plateaus, continental fragments, seamounts, accretionary prisms, melanges, ophiolites, and flysch. The tectonic accretion of volcanic arcs, oceanic plateaus, and seamounts to continents adds mafic juvenile crust that eventually will mature into felsic compositional continental crust by progressive magmatism and lower crustal foundering <xref ref-type="bibr" rid="bib1.bibx260" id="paren.2"/>. The concept of accreted terranes was first born in the 1970s and has evolved greatly since <xref ref-type="bibr" rid="bib1.bibx196 bib1.bibx136 bib1.bibx57 bib1.bibx257" id="paren.3"/>. <xref ref-type="bibr" rid="bib1.bibx136" id="normal.4"/> was the first to introduce terranes into the geologic lexicon as “an association of geologic features, such as stratigraphic formations, intrusive rocks, mineral deposits, and tectonic history, some or all of which lend a distinguishing character to a particular tract of rocks and which differ from those of an adjacent terrane.” It was in the sutured rock belts of different affinities (oceanic crust and island arc) in the Klamath Mountains that <xref ref-type="bibr" rid="bib1.bibx136" id="normal.5"/> first coined the term after recognizing that these tectonically juxtaposed rocks must have been scraped off in a subduction zone. In following years the attributes of “suspect” or “accreted” were added to specify terranes of allochthonous affinity which were juxtaposed tectonically to autochthonous deposits on continents, such as by accretionary processes at a subduction zone <xref ref-type="bibr" rid="bib1.bibx56 bib1.bibx11 bib1.bibx143" id="paren.6"/>. The quest to identify and map accreted terranes led to the patchwork tapestry of terrane belts of western North America <xref ref-type="bibr" rid="bib1.bibx57" id="paren.7"/> and the idea that continents grew from accretionary processes at subduction zones. In addition to identifying suspect terranes on the continents, researchers sought to map out regions of the oceanic floor that could possibly become future accreted terranes. The advancement of oceanic seismology in the 1980s led to the cataloguing of anomalous crustal regions on oceanic plates that could eventually become accreted terranes <xref ref-type="bibr" rid="bib1.bibx32 bib1.bibx11 bib1.bibx205" id="paren.8"/>. These anomalous crustal regions were initially called “oceanic plateaus”, a term which encompassed every region of anomalously thick crust on the ocean plate. In this context, oceanic plateaus included large igneous provinces (LIPs), island arcs, hot spots, extinct mid-ocean ridges, seamounts, and submarine plateaus with continental crust <xref ref-type="bibr" rid="bib1.bibx11" id="paren.9"/>. Later compilations of anomalous crustal structures on the oceanic floor separated oceanic plateaus, thermal swells, and continental submarine plateaus <xref ref-type="bibr" rid="bib1.bibx245 bib1.bibx185" id="paren.10"/>. <xref ref-type="bibr" rid="bib1.bibx48" id="normal.11"/> designated basaltic oceanic plateaus, active spreading ridges, continental and island arc crust, continental passive margins, and seamounts as “future colliders”. These compilations have focused on constraining the crustal thicknesses and volumes of oceanic plateaus, thermal swells, leaky transforms, and continental submarine plateaus <xref ref-type="bibr" rid="bib1.bibx11 bib1.bibx238 bib1.bibx245 bib1.bibx185" id="paren.12"/>. In the past decade, numerous and advanced marine geophysical and geochemical studies have been undertaken to characterize the crustal composition of oceanic LIPs, submarine ridges, island arcs, continental submarine plateaus, and seamounts. Naturally the following question was posed: can we quantify the likelihood of accretion or subduction of these crustal features? Researchers used analytical studies of the buoyancy forces of oceanic plateaus, continental fragments, and island arcs that prevented or allowed them to subduct or collide in a subduction zone <xref ref-type="bibr" rid="bib1.bibx195 bib1.bibx48 bib1.bibx198" id="paren.13"/>. <xref ref-type="bibr" rid="bib1.bibx195" id="normal.14"/> and <xref ref-type="bibr" rid="bib1.bibx198" id="normal.15"/> suggest the contrast between the external force of slab pull and the internal force produced by buoyant terrane crust will control the amount of terrane crust subducted or accreted. <xref ref-type="bibr" rid="bib1.bibx195" id="normal.16"/> estimate that only 10 km of continental crust is subductable. Based on isostatic analyses of the subductability of oceanic plateaus, island arcs, and continental crust, <xref ref-type="bibr" rid="bib1.bibx48" id="normal.17"/> calculated that collision would occur for oceanic plateaus with a crust <inline-formula><mml:math display="inline"><mml:mrow><mml:mo>></mml:mo><mml:mn>17</mml:mn></mml:mrow></mml:math></inline-formula> km thick, a continental crust <inline-formula><mml:math display="inline"><mml:mrow><mml:mo>></mml:mo><mml:mn>20</mml:mn></mml:mrow></mml:math></inline-formula> km thick, and young, hot island arcs. <xref ref-type="bibr" rid="bib1.bibx250" id="normal.18"/> calculated the forces in the subduction zone necessary for a crustal block of continental affinity to shear off of a subducting plate and concluded that accretion can only occur in a relatively dry (low pore pressure) subduction interface. More recently, analog and numerical geodynamic experiments have examined the subductability of oceanic LIPs, submarine ridges, island arcs, continental submarine plateaus, and microcontinents and the effects on the subduction zone dynamics after subduction <xref ref-type="bibr" rid="bib1.bibx80 bib1.bibx271 bib1.bibx16 bib1.bibx186 bib1.bibx84 bib1.bibx68 bib1.bibx187 bib1.bibx1 bib1.bibx269" id="paren.19"/>. Of course, observations of thick oceanic LIPs subducting <xref ref-type="bibr" rid="bib1.bibx183 bib1.bibx242 bib1.bibx7" id="paren.20"><named-content content-type="pre">such as the Ontong Java and Hikurangi plateaus;</named-content></xref> and the relative absence of entire island arc crusts in the geologic record <xref ref-type="bibr" rid="bib1.bibx55" id="paren.21"/> indicate that the accretion, subduction, and collision of thick crustal regions might not always follow the analytical and geodynamic estimates. The tectonic addition of crustal material to continents at accretionary zones usually occurs by adding slivers of thrusted crustal units to the accretionary prism region <xref ref-type="bibr" rid="bib1.bibx57 bib1.bibx258 bib1.bibx35" id="paren.22"/>, rather than collision and addition of the entire crustal thickness to the continent. In the vein of earlier studies <xref ref-type="bibr" rid="bib1.bibx11 bib1.bibx238 bib1.bibx245 bib1.bibx185" id="paren.23"/>, we catalog the regions of anomalous crust on the ocean floor and compare them to accreted terranes using new geophysical and geological studies from the last couple of decades. We group island arcs, oceanic LIPs, submarine ridges, seamounts, hot spots, submarine continental fragments, and microcontinents all as future allochthonous terranes (FATs). Although accreted terranes can also be units from accretionary prisms and melanges, these pre-accretion units are actually part of the subduction zone and are autochthonous to the convergent margin, and therefore are not covered in this study. In this paper we review the crustal compositions of modern and accreted examples of FATs and discuss the processes that lead to accretion, subduction, or collision for each of these anomalous crustal features on the ocean floor. Geophysical, geological, and geochemical studies provide us with new insight on the crustal layers and constraints on densities of FATs, and we will show in our summary that there are no significant differences between seismic velocity profiles from continental crust and mafic oceanic plateau crust. This compilation will summarize average crustal thicknesses, bulk crustal densities, and crustal structures of FATs. A better understanding of modern analogues of accreted allochthonous terranes will improve our understanding of the volume of crust accreted and subducted, the processes and kinematics affecting accretion and subduction, and collision. We hope therefore that this compilation will constrain future modeling studies of terrane accretion. <?xmltex \floatpos{t}?><fig id="Ch1.F1"><caption>Cartoon schematics of FAT crust in subduction zones for four accretionary processes: <bold>(a)</bold> accretion in the accretionary prism, <bold>(b)</bold> subcretion, <bold>(c)</bold> flake tectonics, and <bold>(d)</bold> collision. In <bold>(a)</bold>, sediments and crustal units from the subducting oceanic plate and FAT are scraped off and accumulated in the accretionary prism in front of the fore-arc. The majority of the FAT crust is subducted. Subcretion <bold>(b)</bold> occurs below the accretionary prism, as crustal slices of the FAT are sheared and thrust onto the overriding continent. <bold>(c)</bold> Flake tectonics is the accretionary process where FAT crust is obducted onto the overriding continent, likely over a thick, strong prism of metasedimentary rocks in the overriding plate. <bold>(d)</bold> Collision will occur for large FATs, after some of the crust has subducted and accreted. The subducting slab may eventually detach. </caption> <?xmltex \igopts{width=165.025984pt}?><graphic xlink:href="https://www.solid-earth.net/5/1243/2014/se-5-1243-2014-f01.pdf"/> </fig> </sec> <sec id="Ch1.S2"> <title>Accretionary orogenesis processes</title> Accreted terranes are typically composed of units scraped off of FATs and mixed in with other subducting sediments or crust in melange or accretionary prism formations. The FAT also undergoes severe internal deformation while accreting/subducting. We observe four types of accretion processes in the Phanerozoic geologic record: incorporation into the accretionary complex, underplating to the overriding crust (sometimes termed subcretion), obduction over the overriding plate (or flake tectonics), and collision (Fig. <xref ref-type="fig" rid="Ch1.F1"/>). Incorporation of FAT crust into the accretionary prism occurs through offscraping or underplating onto the prism <xref ref-type="bibr" rid="bib1.bibx49" id="paren.24"/>. Offscraping of FAT crust into the accretionary wedge or imbricate thrusting onto the front of the accretionary wedge (Fig. <xref ref-type="fig" rid="Ch1.F1"/>a) are observed often in the geologic record <xref ref-type="bibr" rid="bib1.bibx276" id="paren.25"/>. In this type of accretion, the FAT crust does not subduct completely, but instead builds out the accretionary wedge seaward, as in an accretionary plate margin <xref ref-type="bibr" rid="bib1.bibx45" id="paren.26"/>. Landward-verging imbricate thrust faults typically shear off blocks of tens to hundreds of meters of FAT or oceanic crust <xref ref-type="bibr" rid="bib1.bibx159" id="paren.27"/>. For example, the Oso Melange and Oso Igneous Complex in Costa Rica records the history of accreted oceanic plateaus, island arcs, and seamounts which were mixed in with accretionary prism sediments <xref ref-type="bibr" rid="bib1.bibx23" id="paren.28"/>. Underplating of FAT crustal material onto the overriding plate during or after subduction, also called subcretion, is perhaps the most common type of terrane accretion. Crustal units can be offscraped and underplated onto the overriding plate by stacked thrust faults, or they can be sheared and incorporated into the subduction channel (Fig. <xref ref-type="fig" rid="Ch1.F1"/>b) and later exhumed as part of the melange units. <xref ref-type="bibr" rid="bib1.bibx197" id="normal.29"/> suggests that temperature and strain rate control whether mass transfer of material by underplating or diffusive subcretion in the subduction channel is the primary accretion method. Active underplating in a modern subduction zone is clearly observed in seismic refraction studies of the Sagami trough in Japan <xref ref-type="bibr" rid="bib1.bibx160" id="paren.30"/>. In the Borneo wedge, crustal units of a subducted continental fragment underplated the accretionary prism in thrust slices <xref ref-type="bibr" rid="bib1.bibx239" id="paren.31"/>. Thrust slices of underplated FAT crust are often interlaid with thrusted melange units, as seen in the imbricated intraoceanic arc and melange slices of the Klamath Mountains <xref ref-type="bibr" rid="bib1.bibx289" id="paren.32"/>. In addition, weak crustal layers can be activated as detachments that allow for shearing of crustal units <xref ref-type="bibr" rid="bib1.bibx292 bib1.bibx269" id="paren.33"/>. <?xmltex \hack{\newpage}?>Flake tectonics is the process of obduction of terranes during subduction/collision on top of an overriding strong wedge <xref ref-type="bibr" rid="bib1.bibx208" id="paren.34"/>. Accretion of FATs via flake tectonic mechanics is most notably evident in southwestern Canada where the Paleozoic Quesnellia, Stikinia, and Cache Creek terranes were thrust and subsequently transported hundreds of kilometers inland over a Proterozoic metasedimentary wedge <xref ref-type="bibr" rid="bib1.bibx259 bib1.bibx270 bib1.bibx60" id="paren.35"/>. Other notable examples of accretion via flake tectonics include the Alps <xref ref-type="bibr" rid="bib1.bibx208" id="paren.36"/> and the Archean greenstone belts <xref ref-type="bibr" rid="bib1.bibx128" id="paren.37"/>. The paucity of flake tectonic mechanics in Phanerozoic terrane accretion is explained by the absence of a strong overriding wedge in most subduction zones <xref ref-type="bibr" rid="bib1.bibx79" id="paren.38"/>. Intact accretion of FAT crusts by “docking” is often a collisional process (when subduction ceases) rather than an accretionary process (subduction continues after accretion) and is a method of continental growth via large volume addition of exotic crustal material (Fig. <xref ref-type="fig" rid="Ch1.F1"/>d). Continental fragments and composite terranes typically lead to collision. In terrane docking, it is possible to preserve the whole crustal section of terranes. Many of the larger FATs such as oceanic plateaus and continental fragments are accreted by collision. A notable example of docking of major crustal units is in Canada, where lithospheric suture zones bounding major terranes are identified with seismic refraction lines <xref ref-type="bibr" rid="bib1.bibx50" id="paren.39"/>. Intraoceanic island arcs are often on the overriding plate, on the receiving end of accretion processes. Arc–continent collision in this configuration allows for the overriding island arc to be added as an intact unit to the subducting continent. An additional method of adding crustal material to continents to form suspect terranes is by back-arc basin closure and tectonic switching <xref ref-type="bibr" rid="bib1.bibx54 bib1.bibx34" id="paren.40"/>. During subduction, changing convergence velocities can lead to cycles of slab retreat and advance that can form and close a back-arc basin on the overriding plate <xref ref-type="bibr" rid="bib1.bibx54" id="paren.41"/>. Triassic and Jurassic extension and formation of a back-arc basin in the North American Cordilleran was followed by basin inversion and thrusting, leading to island arc accretion through Nevada and California <xref ref-type="bibr" rid="bib1.bibx74" id="paren.42"/>. Tectonic switching and back-arc basin closure has been used to explain the accretion of terranes in the Lachlan orogen in Australia <xref ref-type="bibr" rid="bib1.bibx54" id="paren.43"/> and the Svecofennian orogen in Sweden <xref ref-type="bibr" rid="bib1.bibx124" id="paren.44"/>. </sec> <sec id="Ch1.S3"> <title>Island arcs</title> <sec id="Ch1.S3.SS1"> <title>Island arcs: general setting</title> Island arcs are volcanic island chains that form on the overriding oceanic plate at subduction zones (Fig. <xref ref-type="fig" rid="Ch1.F2"/>). Extinct intra-oceanic island arcs, also called remnant arcs, back arcs, or ridges, can also become accreted allochthonous terranes of island arc affinity. Continental volcanic arcs are defined as volcanic arcs built on the continental upper plate of a subduction zone and therefore excluded from this compilation. However, some oceanic island arcs are built on fragments of continental crust, most notably Japan, and can eventually become accreted terranes, and those special cases are included. Island arc chains are geographically curvilinear, spanning hundreds of kilometers along strike and about 100 km in width <xref ref-type="bibr" rid="bib1.bibx27" id="paren.45"/>. The topography of island arcs is quite striking, with the elevation rising from sea floor to sometimes a couple of kilometers above sea level over just 10 or 20 km distance. The locations of island arcs <xref ref-type="bibr" rid="bib1.bibx81" id="paren.46"><named-content content-type="pre"><inline-formula><mml:math display="inline"><mml:mrow><mml:mo>∼</mml:mo><mml:mn>120</mml:mn></mml:mrow></mml:math></inline-formula> km from the trench in subduction zones;</named-content></xref> are believed to be dictated by slab dip and melting in the mantle wedge <xref ref-type="bibr" rid="bib1.bibx82" id="paren.47"/> and/or fluid release from the downgoing slab <xref ref-type="bibr" rid="bib1.bibx111" id="paren.48"/>. Remnant arcs are created by either back-arc rifting of the fore-arc or abandonment due to changes in plate motion <xref ref-type="bibr" rid="bib1.bibx148" id="paren.49"/>. The Izu–Bonin–Mariana arc system is one such example: it is composed of several active island arc chains with more than one remnant back-arc produced by changing plate motions <xref ref-type="bibr" rid="bib1.bibx261" id="paren.50"/>. Back-arc basins separate active island arcs from remnant arcs and form by extension in the upper plate due to slab rollback or mantle wedge convection <xref ref-type="bibr" rid="bib1.bibx247" id="paren.51"/>. Back-arc basins are composed of extended arc crust and even oceanic crust. Island arcs are the most widely intuited contributor of continental crustal growth <xref ref-type="bibr" rid="bib1.bibx260" id="paren.52"/>, primarily because the crustal composition is believed to be most similar to the felsic continental crust. Using volume estimates from <xref ref-type="bibr" rid="bib1.bibx55" id="normal.53"/>, we project about 13 % of post-Archean accreted terranes are oceanic island arcs and 55 % are continental arcs. <xref ref-type="bibr" rid="bib1.bibx48" id="normal.54"/> estimated that island arcs greater than 15 km in thickness are buoyant enough to collide with continental crust; however, the paucity of whole crustal sections of island arcs in the geologic record does not agree with this hypothesis <xref ref-type="bibr" rid="bib1.bibx55" id="paren.55"/>. <?xmltex \floatpos{t}?><table-wrap id="Ch1.T1" specific-use="star"><caption>Island arc crustal thicknesses including the crust mantle transition layer (CMTL). All thicknesses are taken from seismic interpretations except for the Tonga Arc, which was derived by gravity modeling. </caption><oasis:table frame="topbot"><oasis:tgroup cols="3"> <oasis:colspec colnum="1" colname="col1" align="left"/> <oasis:colspec colnum="2" colname="col2" align="left"/> <oasis:colspec colnum="3" colname="col3" align="left"/> <oasis:thead> <oasis:row rowsep="1"> <oasis:entry colname="col1">Island Arc</oasis:entry> <oasis:entry colname="col2">Thickness (km)</oasis:entry> <oasis:entry colname="col3">Reference</oasis:entry> </oasis:row> </oasis:thead> <oasis:tbody> <oasis:row> <oasis:entry colname="col1">Aleutian Arc</oasis:entry> <oasis:entry colname="col2">35–37</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx252" id="normal.56"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Aves Ridge</oasis:entry> <oasis:entry colname="col2">26</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx43" id="normal.57"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Bonin Arc (S. Izu Active Arc)</oasis:entry> <oasis:entry colname="col2">25</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx265 bib1.bibx165" id="normal.58"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Chugoku Arc (SW. Japan)</oasis:entry> <oasis:entry colname="col2">30</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx138" id="normal.59"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Daito Ridge</oasis:entry> <oasis:entry colname="col2">20–25</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx203" id="normal.60"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">N. Izu Arc</oasis:entry> <oasis:entry colname="col2">26–32</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx164" id="normal.61"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">S. Izu Rear Arc</oasis:entry> <oasis:entry colname="col2">18</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx265" id="normal.62"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Japan (Honshu Arc)</oasis:entry> <oasis:entry colname="col2">26</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx4" id="normal.63"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Japan (Chikogu segment)</oasis:entry> <oasis:entry colname="col2">30</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx138" id="normal.64"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Kuril Arc</oasis:entry> <oasis:entry colname="col2">33</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx202" id="normal.65"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Kyushu–Palau Ridge</oasis:entry> <oasis:entry colname="col2">20</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx204" id="normal.66"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Lau–Colville Ridge</oasis:entry> <oasis:entry colname="col2">15</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx147" id="normal.67"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Leeward Antilles Arc</oasis:entry> <oasis:entry colname="col2">27</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx181" id="normal.68"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Lesser Antilles Arc</oasis:entry> <oasis:entry colname="col2">24</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx43" id="normal.69"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Lesser Antilles at Montserrat</oasis:entry> <oasis:entry colname="col2">26–34</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx251" id="normal.70"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Luzon Arc</oasis:entry> <oasis:entry colname="col2">25–30</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx290 bib1.bibx75" id="normal.71"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Mariana Arc</oasis:entry> <oasis:entry colname="col2">18</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx28" id="normal.72"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Mariana Arc</oasis:entry> <oasis:entry colname="col2">20</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx263" id="normal.73"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">W. Mariana Ridge</oasis:entry> <oasis:entry colname="col2">17</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx263" id="normal.74"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">New Hebrides Arc (Vanuatu)</oasis:entry> <oasis:entry colname="col2">27–28</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx63 bib1.bibx134" id="normal.75"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Ogasawara Ridge (Bonin Ridge)</oasis:entry> <oasis:entry colname="col2">21</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx265" id="normal.76"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">N. Ryukyu Arc</oasis:entry> <oasis:entry colname="col2">23–27</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx201" id="normal.77"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">S. Ryukyu Arc</oasis:entry> <oasis:entry colname="col2">29–44</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx201" id="normal.78"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Solomon Islands</oasis:entry> <oasis:entry colname="col2">27</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx193" id="normal.79"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">South Sandwich Arc</oasis:entry> <oasis:entry colname="col2">20</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx173" id="normal.80"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Sunda Arc</oasis:entry> <oasis:entry colname="col2">20</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx168" id="normal.81"/> </oasis:entry> </oasis:row> <oasis:row rowsep="1"> <oasis:entry colname="col1">Tonga Arc</oasis:entry> <oasis:entry colname="col2">22.2</oasis:entry> <oasis:entry colname="col3">gravity modeling: <xref ref-type="bibr" rid="bib1.bibx22" id="normal.82"/></oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Average</oasis:entry> <oasis:entry colname="col2">26 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 6</oasis:entry> <oasis:entry colname="col3"/> </oasis:row> </oasis:tbody> </oasis:tgroup></oasis:table></table-wrap> </sec> <sec id="Ch1.S3.SS2"> <title>Island arcs: modern examples</title> There is a noticeable variation in crustal thickness and structure of modern island arcs between arc systems and even along strike within arc systems <xref ref-type="bibr" rid="bib1.bibx27" id="paren.83"/> (Fig. <xref ref-type="fig" rid="Ch1.F3"/>), which can be attributed to the level of maturity in arc crustal evolution <xref ref-type="bibr" rid="bib1.bibx267" id="paren.84"/>, the amount of back arc extension <xref ref-type="bibr" rid="bib1.bibx204" id="paren.85"/>, and the magmatic production rate <xref ref-type="bibr" rid="bib1.bibx43" id="paren.86"/>. Mature island arc systems, such as the Izu–Bonin–Mariana system, have three crustal layers which were developed by partial melting of the initial immature basaltic arc crust <xref ref-type="bibr" rid="bib1.bibx267" id="paren.87"/>. The upper crustal layer often has a sharp velocity gradient and P-wave velocities ranging from 3 to 6 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> (Fig. <xref ref-type="fig" rid="Ch1.F3"/>), which are interpreted to be layers of volcaniclastics, volcanic flows, and sediments. The mid-crustal layer is characterized by seismic velocities of around 6–6.5 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>. This low velocity layer is often interpreted to be a layer of felsic to intermediate igneous rocks in many modern oceanic island arcs (South Sandwich; <xref ref-type="bibr" rid="bib1.bibx175" id="altparen.88"/>; the Izu–Bonin-Mariana system; <xref ref-type="bibr" rid="bib1.bibx164 bib1.bibx263 bib1.bibx265" id="altparen.89"/>; Tonga Arc; <xref ref-type="bibr" rid="bib1.bibx64" id="altparen.90"/>). The felsic mid-crustal unit is produced by repetitive anatexis of the mafic lower crust <xref ref-type="bibr" rid="bib1.bibx267 bib1.bibx233" id="paren.91"/>. Juvenile island arcs are believed to lack this felsic middle layer, as in the cases of the Lesser Antilles and Leeward Antilles <xref ref-type="bibr" rid="bib1.bibx181 bib1.bibx43" id="paren.92"/> and parts of the Kyushu–Palau Ridge <xref ref-type="bibr" rid="bib1.bibx204" id="paren.93"/>. The mid-crustal layer of the mature Aleutian arc, on the other hand, is inferred to be of a more mafic than intermediate composition, based on the higher seismic velocities at depths of 11–20 km <xref ref-type="bibr" rid="bib1.bibx252" id="paren.94"/>. The lower crustal unit of island arcs is typically characterized by seismic velocities ranging from 6.7 to 7.3 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> (Fig. <xref ref-type="fig" rid="Ch1.F3"/>) and is interpreted to be gabbroic in composition, underlain by mafic to ultramafic cumulates. The mafic and ultramafic cumulates are sometimes classified as a separate unit from the lower crust, called the crust–mantle transition layer (CMTL) <xref ref-type="bibr" rid="bib1.bibx263 bib1.bibx265" id="paren.95"/>. The CMTL has typical seismic velocities around 7.0–7.6 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> (Fig. <xref ref-type="fig" rid="Ch1.F3"/>). We include the CMTL as part of the crust because it is above the seismic Moho in modern arcs and also found above mantle rocks in the accreted Talkeetna arc in Alaska <xref ref-type="bibr" rid="bib1.bibx232 bib1.bibx104" id="paren.96"/>, Kohistan arc in Pakistan <xref ref-type="bibr" rid="bib1.bibx167" id="paren.97"/>, and Guanajuato arc in Mexico <xref ref-type="bibr" rid="bib1.bibx172" id="paren.98"/>. Seismic velocities ranging from 7.6 to 8.0 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> are found below the lower crust of the Mariana arc and West Mariana rear arc in a thick layer, but the authors interpret the reflections between this layer and the lower crust as the Moho discontinuity and not the CMTL <xref ref-type="bibr" rid="bib1.bibx263 bib1.bibx264" id="paren.99"/>. Seismic reflections are also observed below this layer <xref ref-type="bibr" rid="bib1.bibx263 bib1.bibx264" id="paren.100"/> and they are attributed to transformation of mafic materials during arc crustal generation rather than melt in the mantle <xref ref-type="bibr" rid="bib1.bibx264 bib1.bibx267" id="paren.101"/>. <?xmltex \floatpos{t}?><table-wrap id="Ch1.T2" specific-use="star"><caption>Bulk crustal densities (in g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>) of modern island arcs determined from seismic velocities using different seismic velocity–density relationships. Crustal densities include the density of the CMTL. Bulk densities are also reported from studies where the authors combined gravity and seismic data to determine crustal density. </caption><oasis:table frame="topbot"><oasis:tgroup cols="5"> <oasis:colspec colnum="1" colname="col1" align="left"/> <oasis:colspec colnum="2" colname="col2" align="left"/> <oasis:colspec colnum="3" colname="col3" align="left"/> <oasis:colspec colnum="4" colname="col4" align="left"/> <oasis:colspec colnum="5" colname="col5" align="left"/> <oasis:thead> <oasis:row> <oasis:entry colname="col1">Island Arcs</oasis:entry> <oasis:entry colname="col2">Nafe–</oasis:entry> <oasis:entry colname="col3">Christensen–</oasis:entry> <oasis:entry colname="col4">Christensen–</oasis:entry> <oasis:entry colname="col5">Reported in</oasis:entry> </oasis:row> <oasis:row rowsep="1"> <oasis:entry colname="col1"/> <oasis:entry colname="col2">Drake</oasis:entry> <oasis:entry colname="col3">Mooney</oasis:entry> <oasis:entry colname="col4">Shaw</oasis:entry> <oasis:entry colname="col5">the study</oasis:entry> </oasis:row> </oasis:thead> <oasis:tbody> <oasis:row> <oasis:entry colname="col1">Aleutians<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">1</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.70</oasis:entry> <oasis:entry colname="col3">2.73</oasis:entry> <oasis:entry colname="col4">2.73</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Aleutians<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.81</oasis:entry> <oasis:entry colname="col3">2.81</oasis:entry> <oasis:entry colname="col4">2.83</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Aleutians<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">3</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.97</oasis:entry> <oasis:entry colname="col3">3.02</oasis:entry> <oasis:entry colname="col4">3.05</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Aves Ridge<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">4</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.77</oasis:entry> <oasis:entry colname="col3">2.71</oasis:entry> <oasis:entry colname="col4">2.70</oasis:entry> <oasis:entry colname="col5">2.70</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Bonin Arc (S. Izu Arc)<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">5</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.86</oasis:entry> <oasis:entry colname="col3">2.86</oasis:entry> <oasis:entry colname="col4">2.85</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Izu–Bonin Arc<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">6</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.81</oasis:entry> <oasis:entry colname="col3">2.82</oasis:entry> <oasis:entry colname="col4">2.79</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">S. Izu Rear Arc<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">5</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.83</oasis:entry> <oasis:entry colname="col3">2.82</oasis:entry> <oasis:entry colname="col4">2.80</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">SW. Japan<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">7</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.77</oasis:entry> <oasis:entry colname="col3">2.80</oasis:entry> <oasis:entry colname="col4">2.76</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Kuril Arc<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">8</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.65</oasis:entry> <oasis:entry colname="col3">2.62</oasis:entry> <oasis:entry colname="col4">2.51</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Kyushu–Palau Ridge<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">9</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.83</oasis:entry> <oasis:entry colname="col3">2.83</oasis:entry> <oasis:entry colname="col4">2.84</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Leeward Antilles Arc<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>10</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.73</oasis:entry> <oasis:entry colname="col3">2.71</oasis:entry> <oasis:entry colname="col4">2.63</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Lesser Antilles Arc<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">4</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.76</oasis:entry> <oasis:entry colname="col3">2.76</oasis:entry> <oasis:entry colname="col4">2.70</oasis:entry> <oasis:entry colname="col5">2.66</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Mariana Arc<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>10</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.78</oasis:entry> <oasis:entry colname="col3">2.77</oasis:entry> <oasis:entry colname="col4">2.73</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">W. Mariana Ridge<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>11</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.65</oasis:entry> <oasis:entry colname="col3">2.57</oasis:entry> <oasis:entry colname="col4">2.46</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Ogasawara Ridge<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">5</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.89</oasis:entry> <oasis:entry colname="col3">2.91</oasis:entry> <oasis:entry colname="col4">2.91</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">South Sandwich Arc<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>12</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.76</oasis:entry> <oasis:entry colname="col3">2.73</oasis:entry> <oasis:entry colname="col4">2.68</oasis:entry> <oasis:entry colname="col5">2.89</oasis:entry> </oasis:row> <oasis:row rowsep="1"> <oasis:entry colname="col1">Tonga Arc<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>13</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.80</oasis:entry> <oasis:entry colname="col3">2.79</oasis:entry> <oasis:entry colname="col4">2.75</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">average</oasis:entry> <oasis:entry colname="col2">2.79 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 0.08</oasis:entry> <oasis:entry colname="col3">2.79 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 0.10</oasis:entry> <oasis:entry colname="col4">2.75 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 0.14</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> </oasis:tbody> </oasis:tgroup></oasis:table><table-wrap-foot>References are (1) <xref ref-type="bibr" rid="bib1.bibx129" id="normal.102"/>, (2) <xref ref-type="bibr" rid="bib1.bibx178" id="normal.103"/>,(3) <xref ref-type="bibr" rid="bib1.bibx252" id="normal.104"/>, (4) <xref ref-type="bibr" rid="bib1.bibx43" id="normal.105"/>, (5) <xref ref-type="bibr" rid="bib1.bibx265" id="normal.106"/>, (6) <xref ref-type="bibr" rid="bib1.bibx164" id="normal.107"/>, (7) <xref ref-type="bibr" rid="bib1.bibx138" id="normal.108"/>, (8) <xref ref-type="bibr" rid="bib1.bibx202" id="normal.109"/>, (9) <xref ref-type="bibr" rid="bib1.bibx204" id="normal.110"/>, (10) <xref ref-type="bibr" rid="bib1.bibx181" id="normal.111"/>, (11) <xref ref-type="bibr" rid="bib1.bibx263" id="normal.112"/>, (12) <xref ref-type="bibr" rid="bib1.bibx175" id="normal.113"/>, and (13) <xref ref-type="bibr" rid="bib1.bibx64" id="normal.114"/>.</table-wrap-foot></table-wrap> <?xmltex \floatpos{t}?><fig id="Ch1.F2" specific-use="star"><caption>Global location map of island arcs (shown in black) on the present day ocean floor. Arc systems labeled on the map are: <?xmltex \hack{\mbox\bgroup}?>A – Aleutians<?xmltex \hack{\egroup}?>, H – New Hebrides, IBM – Izu–Bonin (Ogasawara)–Mariana arc system, J – Japan Arc, K – Kuril Arc, L – Loyalty Arc, <?xmltex \hack{\mbox\bgroup}?>LA – Lesser<?xmltex \hack{\egroup}?> and Leeward Antilles, Lu – Luzon Arc, OD – Oki–Daito system, PKR – Palau–Kyushu Ridge, NB – New Britain Arc, R – Ryukyu Arc, S – Solomon Arc, SH – Sangihe–Halmahera arc system, SS – South Sandwich Arc, and TKL – Tonga–Lau–Kermadec arc system. Below is a zoom-in of the numerous oceanic island arc systems (in white) in Southeast Asia, with bathymetry from ETOPO 1 <xref ref-type="bibr" rid="bib1.bibx2" id="paren.115"/>. </caption> <?xmltex \igopts{width=341.433071pt}?><graphic xlink:href="https://www.solid-earth.net/5/1243/2014/se-5-1243-2014-f02.pdf"/> </fig> The average crustal thickness of island arcs (including remnant arcs), determined from the thickest regions in 26 seismic and gravity studies of island arcs, is <inline-formula><mml:math display="inline"><mml:mrow><mml:mo>∼</mml:mo><mml:mn>26</mml:mn></mml:mrow></mml:math></inline-formula> <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 6 km (Table <xref ref-type="table" rid="Ch1.T1"/>). Bulk crustal densities were calculated from the P-wave velocities of 17 seismic refraction studies using the Nafe–Drake curve <xref ref-type="bibr" rid="bib1.bibx179" id="paren.116"/>, the <xref ref-type="bibr" rid="bib1.bibx41" id="normal.117"/> relationships for all rocks at 10 km depth intervals, and the <xref ref-type="bibr" rid="bib1.bibx42" id="normal.118"/> curve based on mafic rocks from the mid-Atlantic ridge (Table <xref ref-type="table" rid="Ch1.T2"/>). The densities calculated for the CMTL layer in this compilation range from 3.02 to 3.32 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> using the <xref ref-type="bibr" rid="bib1.bibx41" id="normal.119"/> relationships. These values are within the range of, if not slightly lower than, the densities calculated based on mineral assemblages and sub-Moho conditions (<inline-formula><mml:math display="inline"><mml:mrow><mml:mo>></mml:mo><mml:mn>0.8</mml:mn></mml:mrow></mml:math></inline-formula> MPa and 800–1000 <inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mo>∘</mml:mo></mml:msup></mml:math></inline-formula>C) for the ultramafic pyroxenites from accreted island arcs (<inline-formula><mml:math display="inline"><mml:mrow><mml:mo>∼</mml:mo><mml:mn>3.25</mml:mn></mml:mrow></mml:math></inline-formula>–3.40 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>) <xref ref-type="bibr" rid="bib1.bibx144 bib1.bibx8" id="paren.120"/>. Three seismic refraction studies constrained their crustal structure models with gravity modeling and inferred a whole crustal density for the arcs which we compare to our calculated densities (Table <xref ref-type="table" rid="Ch1.T2"/>). Coincidentally, the average island arc crustal density calculated with the <xref ref-type="bibr" rid="bib1.bibx41" id="normal.121"/> relationship is identical to the average density calculated with the Nafe–Drake curve (2.79 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>). The average crustal densities calculated from the three relationships are lower than the bulk density of average continental crust <xref ref-type="bibr" rid="bib1.bibx41" id="paren.122"><named-content content-type="pre">2.83 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>;</named-content></xref> and the average density for oceanic crust <xref ref-type="bibr" rid="bib1.bibx33" id="paren.123"><named-content content-type="pre">2.86 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>;</named-content></xref>. <?xmltex \floatpos{t}?><fig id="Ch1.F3" specific-use="star"><caption>Seismic crustal structure of modern island arcs and calculated structure of accreted island arcs from previous studies. The thicknesses of units in the accreted arcs are calculated by geobarometric methods <xref ref-type="bibr" rid="bib1.bibx30 bib1.bibx104 bib1.bibx191" id="paren.124"/> and the seismic velocities for the Kohistan units were measured in the lab <xref ref-type="bibr" rid="bib1.bibx191" id="paren.125"/>. For the accreted island arcs, orange represents upper crust, light blue represents middle crust, green is lower crust, and red represents the CMTL. References are (1) <xref ref-type="bibr" rid="bib1.bibx129" id="normal.126"/>, (2) <xref ref-type="bibr" rid="bib1.bibx178" id="normal.127"/>, (3) <xref ref-type="bibr" rid="bib1.bibx252" id="normal.128"/>, (4) <xref ref-type="bibr" rid="bib1.bibx265" id="normal.129"/>, (5) <xref ref-type="bibr" rid="bib1.bibx164" id="normal.130"/>, (6) <xref ref-type="bibr" rid="bib1.bibx138" id="normal.131"/>, (7) <xref ref-type="bibr" rid="bib1.bibx202" id="normal.132"/>, (8) <xref ref-type="bibr" rid="bib1.bibx43" id="normal.133"/>, (9) <xref ref-type="bibr" rid="bib1.bibx263" id="normal.134"/>, (10) <xref ref-type="bibr" rid="bib1.bibx175" id="normal.135"/>, (11) <xref ref-type="bibr" rid="bib1.bibx64" id="normal.136"/>, (12) <xref ref-type="bibr" rid="bib1.bibx204" id="normal.137"/>, (13) <xref ref-type="bibr" rid="bib1.bibx181" id="normal.138"/>, (14) <xref ref-type="bibr" rid="bib1.bibx30" id="normal.139"/>, (15) <xref ref-type="bibr" rid="bib1.bibx104" id="normal.140"/>, and (16) <xref ref-type="bibr" rid="bib1.bibx191" id="normal.141"/>. </caption> <?xmltex \igopts{width=341.433071pt}?><graphic xlink:href="https://www.solid-earth.net/5/1243/2014/se-5-1243-2014-f03.pdf"/> </fig> </sec> <sec id="Ch1.S3.SS3"> <title>Island arcs: accreted examples</title> Accreted island arcs are mostly identified in the geologic record as calc-alkaline volcanic units. The amount of crustal thickness that is actually accreted varies significantly throughout the geologic record. It is not common to find the entire crustal section preserved in terranes of accreted island arcs. Only a few accreted island arc terranes (i.e., Talkeetna, Bonanza, Kohistan, Canyon Mountain, and El Paxtle arcs) contain parts of all of the original crustal layers, but these accreted layers are severely thinned. Geobarometric and geologic studies suggest original crustal thicknesses of 30–35 km for the Talkeetna arc <xref ref-type="bibr" rid="bib1.bibx104 bib1.bibx114" id="paren.142"/>, 24 km for the Bonanza arc <xref ref-type="bibr" rid="bib1.bibx30" id="paren.143"/>, 45 km for the Kohistan arc <xref ref-type="bibr" rid="bib1.bibx191" id="paren.144"/>, and about 30 km for the Canyon Mountain complex <xref ref-type="bibr" rid="bib1.bibx211" id="paren.145"/>. The remaining preserved crustal thicknesses are 18 km thickness for the Talkeetna arc <xref ref-type="bibr" rid="bib1.bibx104" id="paren.146"/>, 15 km for the Bonanza arc <xref ref-type="bibr" rid="bib1.bibx30" id="paren.147"/>, and about 8.3 km for the Canyon Mountain complex <xref ref-type="bibr" rid="bib1.bibx211" id="paren.148"/>. The Kohistan arc is believed to be entirely preserved in crustal thickness <xref ref-type="bibr" rid="bib1.bibx191 bib1.bibx216" id="paren.149"/>. Interestingly, the estimated original crustal thicknesses of these accreted terranes are significantly larger than the average thickness of modern island arcs, most likely because of the large uncertainty and often lack of constraints in estimating the depth of crystallization. Truncated units from all crustal layers are also found in the accreted Alisitos–Teloloapan arc in Mexico <xref ref-type="bibr" rid="bib1.bibx172" id="paren.150"/> and the Alisitos Arc in Baja <xref ref-type="bibr" rid="bib1.bibx25 bib1.bibx26" id="paren.151"/>, but no estimates of original thickness have been made. Based on the few terranes that contain units from the entire arc crust and even the upper mantle, accreted island arcs are composed of three crustal layers. The upper crust in accreted island arcs is mostly composed of volcaniclastics, basalt flows, tuffs, and sediments <xref ref-type="bibr" rid="bib1.bibx172 bib1.bibx211" id="paren.152"/>. The middle layers identified in accreted island arc suites are felsic to intermediate composition plutons such as tonalities, diorites, and trondhjemites (Fig. <xref ref-type="fig" rid="Ch1.F3"/>) <xref ref-type="bibr" rid="bib1.bibx233 bib1.bibx104" id="paren.153"/>. In the accreted Talkeetna arc, the middle crustal layer is composed of intermediate to felsic plutons that produce seismic velocities of 6–6.5 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> <xref ref-type="bibr" rid="bib1.bibx233" id="paren.154"/>. The lower crust is typically mafic in composition, including garnet gabbros, layered gabbros, and pyroxene granulites <xref ref-type="bibr" rid="bib1.bibx69 bib1.bibx104 bib1.bibx172" id="paren.155"/>. Ultramafic cumulates such as pyroxenite gabbros and dolerites are best preserved in the accreted Kohistan arc <xref ref-type="bibr" rid="bib1.bibx167 bib1.bibx191" id="paren.156"/>, but smaller units are also found in the El Paxtle arc in the Guerrero terrane <xref ref-type="bibr" rid="bib1.bibx172" id="paren.157"/>, Talkeetna arc <xref ref-type="bibr" rid="bib1.bibx104" id="paren.158"/>, Canyon Mountain complex <xref ref-type="bibr" rid="bib1.bibx211" id="paren.159"/>, and Bonanza arc <xref ref-type="bibr" rid="bib1.bibx30" id="paren.160"/>. Seismic velocities from the Tonsina pyroxenite unit of the accreted Talkeetna arc are 7.3–7.6 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> <xref ref-type="bibr" rid="bib1.bibx8" id="paren.161"/>, and those from the Jijal garnet pyroxenites of the accreted Kohistan arc are 7.8–8.4 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> <xref ref-type="bibr" rid="bib1.bibx167" id="paren.162"/>, correlative to the CMTL in modern island arcs. The preserved thicknesses of crustal units of island arcs in accreted terranes varies depending on the style of accretion and collision and the subduction polarity in an arc–continent convergence zone <xref ref-type="bibr" rid="bib1.bibx78" id="paren.163"/>. Because island arcs form on the overriding plate at subduction zones, whole-arc accretion is most likely due to a continent entering the subduction zone on the downgoing plate before the arc is obducted or collided onto the continent. This type of tectonic accretion is currently observed at the Luzon Arc in Taiwan <xref ref-type="bibr" rid="bib1.bibx46" id="paren.164"/> and in the early stages in the collision of the Banda arc with Australia <xref ref-type="bibr" rid="bib1.bibx254" id="paren.165"/>. The mostly intact, accreted Kohistan arc in Pakistan is a notable example of arc–continent collision <xref ref-type="bibr" rid="bib1.bibx248" id="paren.166"/>. But in this case the Kohistan arc is believed to have been on the subducting plate in a “backward-facing” arc–continent collision polarity <xref ref-type="bibr" rid="bib1.bibx78" id="paren.167"/>. Besides arc–continent collision, island arcs collide/accrete to another FAT (such as an oceanic plateau) and create a large composite terrane that will collide, suture to continents, and preserve remnants of the island arc crust. A modern example of arc collision including a continental fragment rather than a continent is the Palawan microcontinent–Philippine arc collision <xref ref-type="bibr" rid="bib1.bibx291" id="paren.168"/>. Accreted examples are the Talkeetna arc in Wrangellia composite terrane in Canada <xref ref-type="bibr" rid="bib1.bibx104" id="paren.169"/> and the Stikine arc in Canada <xref ref-type="bibr" rid="bib1.bibx83 bib1.bibx140" id="paren.170"/>. Quite possibly the modern-day Ontong Java Plateau–Solomon islands in the southwest Pacific <xref ref-type="bibr" rid="bib1.bibx215" id="paren.171"/> will be a future accreted composite terrane. And in the case of an island-arc–back-arc system accreting onto a continent, it is likely that the back-arc basin will be accreted along with the active and extinct island arcs. Tethyan ophiolites composed of MORBs and boninites are remnants of back-arc basin closure and obduction during arc–continent collision <xref ref-type="bibr" rid="bib1.bibx85" id="paren.172"/>. However, in most cases only the upper 2–5 km of arc crust are accreted onto continents through thin-skinned thrusting and preserved. This most likely occurs when island arcs are on the subducting plate and arc material is underplated and accreted onto the overriding plate. For example, in the eastern Klamath Mountains of North America, Devonian island arc units are 2.5–3.5 km in thickness and include mafic pillow basalts and a felsic upper unit, indicative of upper to middle crustal layers <xref ref-type="bibr" rid="bib1.bibx73" id="paren.173"/>. Cambrian to Ordovician island arc fragments in the Central Asia orogenic belt are bound by imbricate thrust faults <xref ref-type="bibr" rid="bib1.bibx287 bib1.bibx277" id="paren.174"/>, mirroring the thrust fault-sutured arcs and back-arc basins of Southeast Asia <xref ref-type="bibr" rid="bib1.bibx218" id="paren.175"/>. Detachment faults produced by thinning during back-arc extension or rheologically weak crustal layers can enable accretion of island arc crustal units. <xref ref-type="bibr" rid="bib1.bibx292" id="normal.176"/> suggest that Ordovician terranes of arc and back-arc origins in the Central Newfoundland Annieopsquotch accretionary tract were accreted onto Laurentia because of low angle detachments within the arcs that were produced during back-arc extension. Also, the felsic middle crustal layer could be weakened by metasomatism from fluids released during subduction and act as a décollement layer to underplate arc crustal units onto the continent <xref ref-type="bibr" rid="bib1.bibx272" id="paren.177"/>. Another possible mechanism for accretion is the delamination of the CMTL and increased buoyancy of the remaining island arc crust. The CMTL, composed of ultramafic cumulates and peridotites, is often cited as a layer that delaminates either pre- or syn-accretion <xref ref-type="bibr" rid="bib1.bibx8 bib1.bibx96" id="paren.178"/>. The delamination of the ultramafic CMTL will result in a more felsic overall composition for island arcs, allowing the remaining arc crust to match better with the composition of continental crust <xref ref-type="bibr" rid="bib1.bibx263 bib1.bibx265" id="paren.179"/>. Densities calculated from mineral assemblages and in situ conditions from gabbronites and pyroxenites of the CMTL in accreted island arcs are 0.05–0.25 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> greater than those from mantle material for the same conditions, therefore leading to a negative buoyancy instability <xref ref-type="bibr" rid="bib1.bibx144 bib1.bibx8" id="paren.180"/>. Evidence for CMTL delamination is cited in trench-parallel upper mantle anisotropy observed below modern island arcs <xref ref-type="bibr" rid="bib1.bibx9" id="paren.181"/>. <xref ref-type="bibr" rid="bib1.bibx104" id="normal.182"/> also find that the volume of pyroxenites in the Talkeetna arc is much less than needed to produce the arc's crustal composition, and infer that this discrepancy is due to either foundering of much of the CMTL or the missing pyroxenites were not accreted. On the other hand, the Tonsina pyroxenites of the Talkeetna arc are conformably underlain by upper mantle harzburgites <xref ref-type="bibr" rid="bib1.bibx232" id="paren.183"/>, suggesting the unlikelihood that volumes of the pyroxenite are removed. Furthermore, the depleted rare earth element (REE) signature of the ultramafic section of the Kohistan arc indicates that it did not form from crustal fractionation but as a result of mantle and crust mixing <xref ref-type="bibr" rid="bib1.bibx96" id="paren.184"/>. The thickness of CMTLs cannot be clearly determined through crustal fractionation modeling, and the apparent missing thickness due to delamination may not be valid, at least for the Talkeetna arc. <?xmltex \floatpos{t}?><fig id="Ch1.F4" specific-use="star"><caption>Location map of oceanic plateaus (shown in red) and submarine ridges (shown in black). Updated from LIP list of <xref ref-type="bibr" rid="bib1.bibx52" id="normal.185"/> based on the definition of <xref ref-type="bibr" rid="bib1.bibx21" id="normal.186"/>. Oceanic plateaus and submarine ridges labeled in the figure are: <?xmltex \hack{\mbox\bgroup}?>A – Agulhas<?xmltex \hack{\egroup}?> Plateau, B – Benham Rise, BR – Broken Ridge, C – Caribbean Plateau, Ca – Carnegie Ridge, Ch – Chagos–Laccadive Ridge, <?xmltex \hack{\mbox\bgroup}?>Co – Cocos<?xmltex \hack{\egroup}?> Ridge, CR – Conrad Rise, Cro – Crozet Bank, DC – Del Cano Rise, F – Falkland Ridge, FIR – Faroe–Iceland Ridge, G – Galapagos Ridge, H – Hikurangi Plateau, He – Hess Rise, K – Kerguelen Plateau, M – Manihiki Plateau, Ma – Madagascar Ridge, Ml – Malpelo Ridge, Mo – Mozambique Ridge, Mq – Marquesas Ridge, MR – Maud Rise, MT – Madeira–Tore Rise, Na – Nazca Ridge, Ni – Ninetyeast Ridge, NG – Northeast Georgia Rise, Og – Ogasawara Plateau, OJP – Ontong Java Plateau, R – Roo Rise, RG – Rio Grande Rise, Sh – Shatsky Rise, SL – Sierra Leone Rise, U – Urdaneta Rise, T – Tuamotu Plateau, W – Walvis Ridge, and W-C – Wallaby Plateau and Cuvier Plateau.</caption> <?xmltex \igopts{width=398.338583pt}?><graphic xlink:href="https://www.solid-earth.net/5/1243/2014/se-5-1243-2014-f04.pdf"/> </fig> </sec> </sec> <sec id="Ch1.S4"> <title>Oceanic plateaus, submarine ridges, and seamounts</title> <sec id="Ch1.S4.SS1"> <?xmltex \opttitle{Oceanic plateaus, submarine ridges, and\hack{\\} seamounts: general setting}?><title>Oceanic plateaus, submarine ridges, and<?xmltex \hack{\newline}?> seamounts: general setting</title> Oceanic plateaus, submarine ridges, and seamounts (Figs. <xref ref-type="fig" rid="Ch1.F4"/>, <xref ref-type="fig" rid="Ch1.F5"/>) are mafic igneous regions with crust that is thicker than the surrounding oceanic crust; they are often difficult to differentiate from one another as accreted terranes <xref ref-type="bibr" rid="bib1.bibx150" id="paren.187"/>. Oceanic plateaus, submarine ridges, and seamounts all form due to excess magmatism breaching the oceanic plate. Historically, the term “oceanic plateau” has included a large range of geographic features from extinct mid-ocean ridges, continental plateaus, remnant island arcs, oceanic flood basalts, submarine ridges, and seamount chains to hot spot tracks in the global compilations of <xref ref-type="bibr" rid="bib1.bibx11" id="normal.188"/>, <xref ref-type="bibr" rid="bib1.bibx245" id="normal.189"/> and <xref ref-type="bibr" rid="bib1.bibx185" id="normal.190"/>. Now, oceanic plateaus are defined as a type of large igneous province (LIP) formed on oceanic crust. They are vast, wide regions of anomalously thick igneous crust and are submarine analogues to continental flood basalts <xref ref-type="bibr" rid="bib1.bibx150 bib1.bibx151" id="paren.191"/>. LIPS are large igneous regions on continental or oceanic crust that were rapidly emplaced (within short pulses of 1–5 Myr) over areas of more than 100 000 km<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msup></mml:math></inline-formula> <xref ref-type="bibr" rid="bib1.bibx51 bib1.bibx52 bib1.bibx21" id="paren.192"/>. The origin of oceanic plateaus has been a point of vigorous discussion in the literature in terms of whether the feeder magmas originate from deep plumes or in the upper mantle based on geochemical signatures and geodynamic models <xref ref-type="bibr" rid="bib1.bibx229 bib1.bibx87 bib1.bibx29 bib1.bibx122 bib1.bibx127" id="paren.193"/>. Several modern oceanic plateaus were emplaced during the Cretaceous and were later rifted apart at triple junctions, such as the Kerguelen–Broken Ridge <xref ref-type="bibr" rid="bib1.bibx91" id="paren.194"/>, Manihiki–Hikurangi–Ontong Java <xref ref-type="bibr" rid="bib1.bibx268 bib1.bibx67" id="paren.195"/>, and Agulhas–Maud Rise–northeast Georgia Rise plateaus <xref ref-type="bibr" rid="bib1.bibx209" id="paren.196"/>. The accreted Sorachi plateau is related to the Shatsky Rise oceanic plateau <xref ref-type="bibr" rid="bib1.bibx135" id="paren.197"/> and thus could be another possible triple junction-related oceanic plateau <xref ref-type="bibr" rid="bib1.bibx234" id="paren.198"/>. <?xmltex \floatpos{t}?><fig id="Ch1.F5" specific-use="star"><caption>Location map of seamounts (shown in black). Revised from LIP list of <xref ref-type="bibr" rid="bib1.bibx52" id="normal.199"/>. Seamounts labeled are: <?xmltex \hack{\mbox\bgroup}?>Au – Austral<?xmltex \hack{\egroup}?> Seamounts, B – Balleny Islands, C – Corner Seamounts, Ca – Canary Islands, Em – Emperor Seamounts, G – Gilbert Seamounts, <?xmltex \hack{\mbox\bgroup}?>H – Hawaii<?xmltex \hack{\egroup}?>, JFR – Juan Fernandez Ridge, Li – Line Islands, Lo – Louisville Ridge, Ma – Mathematician Seamounts, Mg – Magellan Seamounts, Mr – Marshall Seamounts, Mu – Musician Seamounts, MP – mid-Pacific Mountains, NE – New England Seamounts, SyG – Sala y Gomez chain, S-M-A – Shona–Meteor Rise–Agulhas Ridges, T – Tasmantid Seamounts.</caption> <?xmltex \igopts{width=398.338583pt}?><graphic xlink:href="https://www.solid-earth.net/5/1243/2014/se-5-1243-2014-f05.pdf"/> </fig> Even though the seismic crustal structures of oceanic plateaus and submarine ridges appear similar, their origins are different and submarine ridges are volumetrically smaller <xref ref-type="bibr" rid="bib1.bibx21" id="paren.200"/>. In this review, we follow the definition of oceanic plateaus as outlined by <xref ref-type="bibr" rid="bib1.bibx150" id="normal.201"/>, <xref ref-type="bibr" rid="bib1.bibx151" id="normal.202"/>, and <xref ref-type="bibr" rid="bib1.bibx21" id="normal.203"/> for differentiating between oceanic plateaus and submarine ridges. Some submarine ridges, such as the Nazca Ridge, Cocos Ridge, and the Tuamotu Plateau, have been previously classified as oceanic plateaus; however, based on the definition of <xref ref-type="bibr" rid="bib1.bibx21" id="normal.204"/>, these mafic regions are neither voluminous enough nor formed due to rapid magmatism and therefore must be classified as submarine ridges. Submarine ridges are the result of significant magmatism produced at hot spot tracks, leaky transforms, or now-extinct mid-ocean ridges. In addition to oceanic plateaus and submarine ridges, we include large seamounts and seamount chains in this grouping (Fig. <xref ref-type="fig" rid="Ch1.F5"/>). In general, seamounts are submarine volcanoes, smaller in areal extent than oceanic plateaus and submarine ridges, with geochemical signatures that suggest different sources for different seamount chains. The number of seamounts <inline-formula><mml:math display="inline"><mml:mrow><mml:mo>></mml:mo><mml:mn>1.5</mml:mn></mml:mrow></mml:math></inline-formula> km in height currently on the ocean floor is estimated to be more than 13 000 based on satellite <?xmltex \hack{\mbox\bgroup}?>altimetry<?xmltex \hack{\egroup}?> <xref ref-type="bibr" rid="bib1.bibx283" id="paren.205"/>, and these numerous features often alter subduction zone by blocking the subducting interface or causing uplift in the accretionary prism <xref ref-type="bibr" rid="bib1.bibx280" id="paren.206"/>. Seamounts can be formed by various processes: they can be the result of upper mantle mini-convection cells under mid-ocean ridges or transforms <xref ref-type="bibr" rid="bib1.bibx24" id="paren.207"/>, deep mantle upwellings, short-lived hotspot volcanism, upper asthenospheric upwelling, and lithospheric cracking <xref ref-type="bibr" rid="bib1.bibx86 bib1.bibx19 bib1.bibx237" id="paren.208"/>. The geochemical signature of mafic accreted terranes is important in helping to determine if the accreted terrane was originally a plume-derived oceanic plateau, hot spot track submarine ridge, or the product of excess upper mantle magmatism. <?xmltex \floatpos{t}?><table-wrap id="Ch1.T3" specific-use="star"><caption>Crustal thicknesses of oceanic plateaus and submarine ridges. Thicknesses are derived from seismic studies unless otherwise noted. <inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">1</mml:mn></mml:msup></mml:math></inline-formula>The crustal thickness was extrapolated in the original study because the Moho was not imaged.</caption><oasis:table frame="topbot"><oasis:tgroup cols="3"> <oasis:colspec colnum="1" colname="col1" align="left"/> <oasis:colspec colnum="2" colname="col2" align="left"/> <oasis:colspec colnum="3" colname="col3" align="left"/> <oasis:thead> <oasis:row> <oasis:entry colname="col1">Oceanic plateaus</oasis:entry> <oasis:entry colname="col2">Thickness (km)</oasis:entry> <oasis:entry colname="col3">Reference</oasis:entry> </oasis:row> <oasis:row rowsep="1"> <oasis:entry colname="col1">and submarine ridges</oasis:entry> <oasis:entry colname="col2"/> <oasis:entry colname="col3"/> </oasis:row> </oasis:thead> <oasis:tbody> <oasis:row> <oasis:entry colname="col1">Agulhas Plateau</oasis:entry> <oasis:entry colname="col2">20</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx209" id="normal.209"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">S. Agulhas Plateau</oasis:entry> <oasis:entry colname="col2">25</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx101" id="normal.210"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Alpha Ridge</oasis:entry> <oasis:entry colname="col2">38</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx77" id="normal.211"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Broken Ridge</oasis:entry> <oasis:entry colname="col2">20.5</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx90" id="normal.212"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Caribbean Plateau</oasis:entry> <oasis:entry colname="col2">10–20</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx183 bib1.bibx284 bib1.bibx188" id="normal.213"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Carnegie Ridge</oasis:entry> <oasis:entry colname="col2">13–19</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx236 bib1.bibx235" id="normal.214"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Cocos Ridge</oasis:entry> <oasis:entry colname="col2">21</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx279" id="normal.215"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Crozet Plateau</oasis:entry> <oasis:entry colname="col2">17</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx225" id="normal.216"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Del Cano Rise</oasis:entry> <oasis:entry colname="col2">17.5</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx103" id="normal.217"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Eauripik Ridge</oasis:entry> <oasis:entry colname="col2">16<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">1</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx71" id="normal.218"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Faroe–Iceland Ridge</oasis:entry> <oasis:entry colname="col2">23</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx14" id="normal.219"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Hikurangi Plateau</oasis:entry> <oasis:entry colname="col2">16–23</oasis:entry> <oasis:entry colname="col3">gravity modeling: <xref ref-type="bibr" rid="bib1.bibx67" id="normal.220"/></oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">N. Kerguelen Plateau</oasis:entry> <oasis:entry colname="col2">17</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx37" id="normal.221"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">S. Kerguelen Plateau</oasis:entry> <oasis:entry colname="col2">21–25</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx206" id="normal.222"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Laccadive Ridge</oasis:entry> <oasis:entry colname="col2">24</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx113" id="normal.223"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Madagascar Ridge</oasis:entry> <oasis:entry colname="col2">25</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx256" id="normal.224"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Madeira–Tore Rise</oasis:entry> <oasis:entry colname="col2">17–18</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx212" id="normal.225"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Maldive Ridge (Chagos Laccadive)</oasis:entry> <oasis:entry colname="col2">15</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx89" id="normal.226"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Malpelo Ridge</oasis:entry> <oasis:entry colname="col2">21</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx184" id="normal.227"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Manihiki Plateau</oasis:entry> <oasis:entry colname="col2">21.4<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">1</mml:mn></mml:msup></mml:math></inline-formula>, 25</oasis:entry> <oasis:entry colname="col3"><xref ref-type="bibr" rid="bib1.bibx132" id="normal.228"/>; gravity modeling: <xref ref-type="bibr" rid="bib1.bibx274" id="normal.229"/></oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Marquesas Island</oasis:entry> <oasis:entry colname="col2">15–17</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx31" id="normal.230"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Maud Rise</oasis:entry> <oasis:entry colname="col2">11–14</oasis:entry> <oasis:entry colname="col3">Ørsted Satellite data: <xref ref-type="bibr" rid="bib1.bibx156" id="normal.231"/></oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Mozambique Ridge</oasis:entry> <oasis:entry colname="col2">22–24</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx166 bib1.bibx116" id="normal.232"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Nazca Ridge</oasis:entry> <oasis:entry colname="col2">18–21</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx115 bib1.bibx288 bib1.bibx119" id="normal.233"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Ninetyeast Ridge</oasis:entry> <oasis:entry colname="col2">24</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx108" id="normal.234"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Ogasawara Plateau</oasis:entry> <oasis:entry colname="col2"><inline-formula><mml:math display="inline"><mml:mrow><mml:mo>></mml:mo><mml:mn>20</mml:mn></mml:mrow></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx145" id="normal.235"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Ontong Java Plateau</oasis:entry> <oasis:entry colname="col2">33</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx193" id="normal.236"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Rio Grande Rise</oasis:entry> <oasis:entry colname="col2">11–12</oasis:entry> <oasis:entry colname="col3">gravity modeling: <xref ref-type="bibr" rid="bib1.bibx194" id="normal.237"/></oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Roo Rise</oasis:entry> <oasis:entry colname="col2">12–18</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx255" id="normal.238"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Shatsky Rise</oasis:entry> <oasis:entry colname="col2">26</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx99" id="normal.239"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Tuamotu Plateau</oasis:entry> <oasis:entry colname="col2">21</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx210" id="normal.240"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Wallaby Plateau</oasis:entry> <oasis:entry colname="col2">18</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx190" id="normal.241"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Walvis Ridge</oasis:entry> <oasis:entry colname="col2">12.5</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx39" id="normal.242"/> </oasis:entry> </oasis:row> <oasis:row rowsep="1"> <oasis:entry colname="col1">Zenith Plateau</oasis:entry> <oasis:entry colname="col2">18</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx190" id="normal.243"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Average</oasis:entry> <oasis:entry colname="col2">21 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 4</oasis:entry> <oasis:entry colname="col3"/> </oasis:row> </oasis:tbody> </oasis:tgroup></oasis:table></table-wrap> <?xmltex \floatpos{t}?><table-wrap id="Ch1.T4" specific-use="star"><caption>Bulk crustal densities of oceanic plateaus and submarine ridges. Bulk crustal densities (in g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>) are determined from seismic velocities using different seismic velocity–density relationships. Bulk densities are also reported from studies where the authors combined gravity and seismic data to determine crustal density. References are (1) <xref ref-type="bibr" rid="bib1.bibx209" id="normal.244"/>, (2) <xref ref-type="bibr" rid="bib1.bibx101" id="normal.245"/>, (3) <xref ref-type="bibr" rid="bib1.bibx90" id="normal.246"/>, (4) <xref ref-type="bibr" rid="bib1.bibx235" id="normal.247"/>, (5) <xref ref-type="bibr" rid="bib1.bibx225" id="normal.248"/>, (6) <xref ref-type="bibr" rid="bib1.bibx279" id="normal.249"/>, (7) <xref ref-type="bibr" rid="bib1.bibx14" id="normal.250"/>, (8) <xref ref-type="bibr" rid="bib1.bibx36" id="normal.251"/>, (9) <xref ref-type="bibr" rid="bib1.bibx206" id="normal.252"/>, (10) <xref ref-type="bibr" rid="bib1.bibx113" id="normal.253"/>, (11) <xref ref-type="bibr" rid="bib1.bibx256" id="normal.254"/>, (12) <xref ref-type="bibr" rid="bib1.bibx212" id="normal.255"/>, (13) <xref ref-type="bibr" rid="bib1.bibx132" id="normal.256"/>, (14) <xref ref-type="bibr" rid="bib1.bibx31" id="normal.257"/>, (15) <xref ref-type="bibr" rid="bib1.bibx116" id="normal.258"/>, (16) <xref ref-type="bibr" rid="bib1.bibx119" id="normal.259"/>, (17) <xref ref-type="bibr" rid="bib1.bibx108" id="normal.260"/>, (18) <xref ref-type="bibr" rid="bib1.bibx193" id="normal.261"/>, (19) <xref ref-type="bibr" rid="bib1.bibx255" id="normal.262"/>, (20) <xref ref-type="bibr" rid="bib1.bibx70" id="normal.263"/>, and (21) <xref ref-type="bibr" rid="bib1.bibx210" id="normal.264"/>.</caption><oasis:table frame="topbot"><oasis:tgroup cols="5"> <oasis:colspec colnum="1" colname="col1" align="left"/> <oasis:colspec colnum="2" colname="col2" align="left"/> <oasis:colspec colnum="3" colname="col3" align="left"/> <oasis:colspec colnum="4" colname="col4" align="left"/> <oasis:colspec colnum="5" colname="col5" align="left"/> <oasis:thead> <oasis:row> <oasis:entry colname="col1">Oceanic plateaus</oasis:entry> <oasis:entry colname="col2">Nafe–</oasis:entry> <oasis:entry colname="col3">Christensen–</oasis:entry> <oasis:entry colname="col4">Christensen–</oasis:entry> <oasis:entry colname="col5">Reported in</oasis:entry> </oasis:row> <oasis:row rowsep="1"> <oasis:entry colname="col1">and submarine ridges</oasis:entry> <oasis:entry colname="col2">Drake</oasis:entry> <oasis:entry colname="col3">Mooney</oasis:entry> <oasis:entry colname="col4">Shaw</oasis:entry> <oasis:entry colname="col5">the study</oasis:entry> </oasis:row> </oasis:thead> <oasis:tbody> <oasis:row> <oasis:entry colname="col1">Agulhas Plateau<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">1</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.85</oasis:entry> <oasis:entry colname="col3">2.85</oasis:entry> <oasis:entry colname="col4">2.84</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">S. Agulhas Plateau<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.82</oasis:entry> <oasis:entry colname="col3">2.80</oasis:entry> <oasis:entry colname="col4">2.75</oasis:entry> <oasis:entry colname="col5">3.03</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Broken Ridge<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">3</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.82</oasis:entry> <oasis:entry colname="col3">2.82</oasis:entry> <oasis:entry colname="col4">2.80</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Carnegie Ridge<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">4</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.85</oasis:entry> <oasis:entry colname="col3">2.85</oasis:entry> <oasis:entry colname="col4">2.83</oasis:entry> <oasis:entry colname="col5">2.89</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Cocos Ridge<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">5</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.91</oasis:entry> <oasis:entry colname="col3">2.93</oasis:entry> <oasis:entry colname="col4">2.94</oasis:entry> <oasis:entry colname="col5">2.93</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Crozet Plateau<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">6</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.70</oasis:entry> <oasis:entry colname="col3">2.63</oasis:entry> <oasis:entry colname="col4">2.53</oasis:entry> <oasis:entry colname="col5">2.62</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Faroe–Iceland Ridge <inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">7</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.82</oasis:entry> <oasis:entry colname="col3">2.83</oasis:entry> <oasis:entry colname="col4">2.80</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">N. Kerguelen<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">8</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.90</oasis:entry> <oasis:entry colname="col3">2.92</oasis:entry> <oasis:entry colname="col4">2.92</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">S. Kerguelen Plateau<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">9</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.76</oasis:entry> <oasis:entry colname="col3">2.76</oasis:entry> <oasis:entry colname="col4">2.71</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Laccadive Island<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>10</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.87</oasis:entry> <oasis:entry colname="col3">2.89</oasis:entry> <oasis:entry colname="col4">2.88</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Madagascar Ridge<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>11</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.89</oasis:entry> <oasis:entry colname="col3">2.89</oasis:entry> <oasis:entry colname="col4">2.89</oasis:entry> <oasis:entry colname="col5">2.89</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Madeira–Tore Rise<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>12</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.77</oasis:entry> <oasis:entry colname="col3">2.74</oasis:entry> <oasis:entry colname="col4">2.68</oasis:entry> <oasis:entry colname="col5">2.90</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Malpelo Ridge<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">4</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.91</oasis:entry> <oasis:entry colname="col3">2.90</oasis:entry> <oasis:entry colname="col4">2.91</oasis:entry> <oasis:entry colname="col5">2.86</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Manihiki Plateau<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>13</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.79</oasis:entry> <oasis:entry colname="col3">2.80</oasis:entry> <oasis:entry colname="col4">2.77</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Marquesas Island<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>14</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.91</oasis:entry> <oasis:entry colname="col3">2.87</oasis:entry> <oasis:entry colname="col4">2.87</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Mozambique Ridge<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>15</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.70</oasis:entry> <oasis:entry colname="col3">2.70</oasis:entry> <oasis:entry colname="col4">2.62</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Nazca Ridge<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>16</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.88</oasis:entry> <oasis:entry colname="col3">2.89</oasis:entry> <oasis:entry colname="col4">2.89</oasis:entry> <oasis:entry colname="col5">2.88</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Ninetyeast Ridge<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>17</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">3.01</oasis:entry> <oasis:entry colname="col3">3.04</oasis:entry> <oasis:entry colname="col4">3.08</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Ontong Java Plateau<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>13</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.85</oasis:entry> <oasis:entry colname="col3">2.87</oasis:entry> <oasis:entry colname="col4">2.85</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Ontong Java Plateau<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>18</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.88</oasis:entry> <oasis:entry colname="col3">2.91</oasis:entry> <oasis:entry colname="col4">2.90</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Roo Rise<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>19</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.75</oasis:entry> <oasis:entry colname="col3">2.74</oasis:entry> <oasis:entry colname="col4">2.68</oasis:entry> <oasis:entry colname="col5">2.75</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Shatsky Rise<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>20</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.96</oasis:entry> <oasis:entry colname="col3">2.97</oasis:entry> <oasis:entry colname="col4">3.00</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row rowsep="1"> <oasis:entry colname="col1">Tuamotu Plateau<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>21</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.80</oasis:entry> <oasis:entry colname="col3">2.79</oasis:entry> <oasis:entry colname="col4">2.74</oasis:entry> <oasis:entry colname="col5">2.74</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Average</oasis:entry> <oasis:entry colname="col2">2.84 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 0.08</oasis:entry> <oasis:entry colname="col3">2.84 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 0.09</oasis:entry> <oasis:entry colname="col4">2.82 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 0.13</oasis:entry> <oasis:entry colname="col5">2.85 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 0.12</oasis:entry> </oasis:row> </oasis:tbody> </oasis:tgroup></oasis:table></table-wrap> <?xmltex \floatpos{t}?><fig id="Ch1.F6" specific-use="star"><caption>Crustal structures of modern oceanic plateaus and submarine ridges from seismic imaging studies. References are (1) <xref ref-type="bibr" rid="bib1.bibx209" id="normal.265"/>, (2) <xref ref-type="bibr" rid="bib1.bibx101" id="normal.266"/>, (3) <xref ref-type="bibr" rid="bib1.bibx90" id="normal.267"/>, (4) <xref ref-type="bibr" rid="bib1.bibx225" id="normal.268"/>, (5) <xref ref-type="bibr" rid="bib1.bibx36" id="normal.269"/>, (6) <xref ref-type="bibr" rid="bib1.bibx206" id="normal.270"/>, (7) <xref ref-type="bibr" rid="bib1.bibx256" id="normal.271"/>, (8) <xref ref-type="bibr" rid="bib1.bibx132" id="normal.272"/>, (9) <xref ref-type="bibr" rid="bib1.bibx193" id="normal.273"/>, (10) <xref ref-type="bibr" rid="bib1.bibx255" id="normal.274"/>, (11) <xref ref-type="bibr" rid="bib1.bibx70" id="normal.275"/>, (12) <xref ref-type="bibr" rid="bib1.bibx235" id="normal.276"/>, (13) <xref ref-type="bibr" rid="bib1.bibx279" id="normal.277"/>, (14) <xref ref-type="bibr" rid="bib1.bibx14" id="normal.278"/>, (15) <xref ref-type="bibr" rid="bib1.bibx113" id="normal.279"/>, (16) <xref ref-type="bibr" rid="bib1.bibx212" id="normal.280"/>, (17) <xref ref-type="bibr" rid="bib1.bibx31" id="normal.281"/>, (18) <xref ref-type="bibr" rid="bib1.bibx116" id="normal.282"/>, (19) <xref ref-type="bibr" rid="bib1.bibx119" id="normal.283"/>, (20) <xref ref-type="bibr" rid="bib1.bibx108" id="normal.284"/>, (21) <xref ref-type="bibr" rid="bib1.bibx210" id="normal.285"/>, (22) <xref ref-type="bibr" rid="bib1.bibx281" id="normal.286"/>, (23) <xref ref-type="bibr" rid="bib1.bibx59" id="normal.287"/>, (24) <xref ref-type="bibr" rid="bib1.bibx169" id="normal.288"/>, and (25) <xref ref-type="bibr" rid="bib1.bibx170" id="normal.289"/>. </caption> <?xmltex \igopts{width=369.885827pt}?><graphic xlink:href="https://www.solid-earth.net/5/1243/2014/se-5-1243-2014-f06.pdf"/> </fig> </sec> <sec id="Ch1.S4.SS2"> <?xmltex \opttitle{Oceanic plateaus, submarine ridges, and\hack{\\} seamounts: modern examples}?><title>Oceanic plateaus, submarine ridges, and<?xmltex \hack{\newline}?> seamounts: modern examples</title> Oceanic plateau and submarine ridge bathymetry is generally 2–3 km above the surrounding ocean crust. Oceanic plateaus and submarine ridges have similar crustal thicknesses, and, from 32 seismic and geophysical studies, their combined average crustal thickness is approximately 21 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 4 km (Table <xref ref-type="table" rid="Ch1.T3"/>). Even though the 33 km-thick Ontong Java Plateau is commonly used to exemplify the typical crustal thickness of an oceanic plateau, it is anomalously thick for oceanic plateaus (Fig. <xref ref-type="fig" rid="Ch1.F6"/>). Oceanic plateaus and submarine ridges typically have a sedimentary layer, upper crust, lower crust, and mafic underplating identified in seismic interpretations, although several oceanic plateaus and submarine ridges have an additional middle crustal layer (Fig. <xref ref-type="fig" rid="Ch1.F6"/>). Seismic refraction studies indicate an upper layer of 1–4 km thickness of low seismic velocities, correlated to limestones, pelagic sediments, and volcaniclastic sediments. Underlying that is the upper crust with P-wave velocities of 4.5–6.0 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>, commonly interpreted as mixed basaltic flows and pelagic material, altered basalts, and other submarine flows. The upper crust is sometimes correlated to oceanic layer 2 because of the similar seismic velocities. In oceanic plateaus and submarine ridges where three crustal layers are identified, the upper crust has very low seismic velocities (3.5–4.5 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>) and the middle crust has velocities typical of basalts (5.0–6.0 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>). The lower crust typically has seismic velocities of 6.5–7.0 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> in all oceanic plateaus and submarine ridges. Over-thickened lower crusts are common in this group of FATs, especially in submarine ridges. The lower crust is often interpreted to be gabbroic or correlative to oceanic crust layer 3. We caution against relating crustal units of this FAT to oceanic crust because oceanic plateaus and submarine ridges are formed differently from typical oceanic crust. Many oceanic plateaus and submarine ridges have a basal unit of high seismic velocities (7.0–7.9 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>), which is highlighted in a compilation by <xref ref-type="bibr" rid="bib1.bibx231" id="normal.290"/>. <xref ref-type="bibr" rid="bib1.bibx107" id="normal.291"/> and <xref ref-type="bibr" rid="bib1.bibx113" id="normal.292"/> suggest that this mafic basal unit is underplated material due to plume magmatism. Early studies have suggested that the high seismic velocity lower crustal layer was representative of a ductile layer that occurs in crust greater than 15 km thick <xref ref-type="bibr" rid="bib1.bibx245" id="paren.293"/>. However, the theory that all large oceanic igneous provinces will have a ultramafic layer was debunked by the compilation of <xref ref-type="bibr" rid="bib1.bibx231" id="normal.294"/>. We calculated an average crustal density from the P-wave velocities from 23 seismic refraction studies of oceanic plateaus and submarine ridges (Table <xref ref-type="table" rid="Ch1.T4"/>). The average crustal density is estimated to be 2.84 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> from the <xref ref-type="bibr" rid="bib1.bibx41" id="normal.295"/> depth-dependent relationship, 2.84 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> using the Nafe–Drake curve <xref ref-type="bibr" rid="bib1.bibx179" id="paren.296"/>, and 2.82 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> with the <xref ref-type="bibr" rid="bib1.bibx42" id="normal.297"/> depth-dependent relationship (Table <xref ref-type="table" rid="Ch1.T4"/>). Interestingly, these values are close to the densities of average continental crust <xref ref-type="bibr" rid="bib1.bibx41" id="paren.298"><named-content content-type="pre">2.83 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>;</named-content></xref> and average oceanic crust <xref ref-type="bibr" rid="bib1.bibx33" id="paren.299"><named-content content-type="pre">2.86 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>;</named-content></xref>. Generally, the densities of oceanic plateaus and submarine plateaus calculated from the Nafe–Drake and Christensen–Mooney relationships are similar to the densities determined in combined seismic–gravity studies (Table <xref ref-type="table" rid="Ch1.T4"/>). For our review on crustal structure we focus only on large submarine volcanoes (<inline-formula><mml:math display="inline"><mml:mrow><mml:mo>></mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:math></inline-formula> km high) which are included in the list of LIPs by <xref ref-type="bibr" rid="bib1.bibx52" id="normal.300"/>. Many of these large seamounts have heights of 3–5 km above the surrounding ocean floor. The seismic crustal structure of seamounts consists of one or two layers and may contain a thick intrusive volcanic core. Seamounts are volcanoes build up on top of oceanic crust <xref ref-type="bibr" rid="bib1.bibx171" id="paren.301"/>. The <?xmltex \hack{\mbox\bgroup}?>upper<?xmltex \hack{\egroup}?><?xmltex \hack{\mbox\bgroup}?>crustal<?xmltex \hack{\egroup}?> layers of seamounts and oceanic crust correlate with the seismic velocities of basalts. The lower crustal units are interpreted to be gabbros and sheeted dikes. Many seamounts, such as the those in the O'Higgins and Musician seamount chains, have two crustal layers similar to oceanic crust and no seismically discernable intrusive core <xref ref-type="bibr" rid="bib1.bibx169 bib1.bibx170" id="paren.302"/>. Other submarine volcanics, such as Great Meteor seamount and Marcus–Wake seamount chain, have a thick layer that is seismically different from the surrounding oceanic crust and is interpreted as the volcanic core <xref ref-type="bibr" rid="bib1.bibx281 bib1.bibx146" id="paren.303"/>. In some seamounts, such as the Hawaiian chain <xref ref-type="bibr" rid="bib1.bibx174" id="paren.304"/> and La Reunion <xref ref-type="bibr" rid="bib1.bibx38" id="paren.305"/>, the oceanic crust is underplated by a seismically fast layer. Yet other submarine volcanics, including the Louisville hot spot track <xref ref-type="bibr" rid="bib1.bibx59" id="paren.306"/>, Musician seamounts <xref ref-type="bibr" rid="bib1.bibx169" id="paren.307"/>, O'Higgins Seamount <xref ref-type="bibr" rid="bib1.bibx170" id="paren.308"/>, and Marcus–Wake seamount chain <xref ref-type="bibr" rid="bib1.bibx146" id="paren.309"/>, do not have any seismic high-velocity layer below the crust. The high seismic velocities found in the Louisville and Marcus–Wake seamount chains are attributed to mafic intrusions in the lower crust <xref ref-type="bibr" rid="bib1.bibx59 bib1.bibx146" id="paren.310"/>. The subcrustal high-velocity layer in other seamounts is theorized to be from mafic dikes formed as a lithostatic response to loading <xref ref-type="bibr" rid="bib1.bibx174" id="paren.311"><named-content content-type="pre">Hawaii:</named-content></xref>, hot spot material <xref ref-type="bibr" rid="bib1.bibx38" id="paren.312"><named-content content-type="pre">La Reunion:</named-content></xref>, or hydrated lithosphere <xref ref-type="bibr" rid="bib1.bibx170" id="paren.313"><named-content content-type="pre">O'Higgins seamount:</named-content></xref>. </sec> <sec id="Ch1.S4.SS3"> <?xmltex \opttitle{Oceanic plateaus, submarine ridges, and\hack{\\} seamounts: accreted examples}?><title>Oceanic plateaus, submarine ridges, and<?xmltex \hack{\newline}?> seamounts: accreted examples</title> Accreted oceanic plateaus and submarine ridges are typically identified in the geologic record as mafic to ultramafic basalts unit in accreted terranes. <xref ref-type="bibr" rid="bib1.bibx150" id="normal.314"/> presents a diagnostic criteria for identifying ancient oceanic plateaus in the geological record based on geology, petrology, and geochemistry. Oceanic plateaus are composed mainly of <?xmltex \hack{\mbox\bgroup}?>tholeiitic<?xmltex \hack{\egroup}?><?xmltex \hack{\mbox\bgroup}?>basalts<?xmltex \hack{\egroup}?> with minor amounts of picrites and komatiites and are geochemically distinct from mid-ocean ridge basalt (MORB)-type and ocean-island basalt (OIB)-type mantle sources <xref ref-type="bibr" rid="bib1.bibx150 bib1.bibx122" id="paren.315"/>. Depending on their origin, submarine ridge basalts can also have MORB or ocean-island basalt OIB signatures. It is quite likely that many greenstones and mafic accreted units, identified as accreted ophiolites or oceanic crust, may actually be oceanic plateaus <xref ref-type="bibr" rid="bib1.bibx155" id="paren.316"><named-content content-type="pre">see Table 4 in</named-content></xref>. For example, the hotspot-related greenstones of the Chugoku and Chichibu belts in Japan were reinterpreted as accreted oceanic plateau/submarine ridges rather than the earlier inference of mid-ocean ridge basalts, based on high Zr <inline-formula><mml:math display="inline"><mml:mo>/</mml:mo></mml:math></inline-formula> Y ratios that are more similar to OIB geochemical signatures <xref ref-type="bibr" rid="bib1.bibx266" id="paren.317"/>. The total amount of preserved crustal structure and thickness of oceanic plateaus varies in the observed geological record of accreted terranes. Sometimes the entire crustal thickness is preserved in accreted terranes, as in the Triassic Wrangellia terrane of North America, or only truncated units from all crustal layers are found, as in the accreted Gorgona and Columbia oceanic plateaus of South America. Seismic refraction studies indicate that the total thickness of the Wrangellia composite terrane crust is about 25<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mo>+</mml:mo></mml:msup></mml:math></inline-formula> km in Vancouver <xref ref-type="bibr" rid="bib1.bibx224 bib1.bibx50" id="paren.318"/> and 30 km in Alaska <xref ref-type="bibr" rid="bib1.bibx18" id="paren.319"/>. Approximately 6 km of exposed stratigraphic thickness, correlated to the sedimentary and upper crustal layers of the Wrangellia oceanic plateau, is found in Vancouver Island <xref ref-type="bibr" rid="bib1.bibx106" id="paren.320"/>. Wrangellia's exposed units are composed of limestone and pelagic sediments, pillow lavas, massive flood basalts, subaerial and submarine flows, and olivine-rich basalts <xref ref-type="bibr" rid="bib1.bibx105 bib1.bibx106" id="paren.321"/>. In other accreted oceanic plateaus, the preserved crustal thicknesses can be as low as 2–7 km thick. The total reconstructed thickness of the accreted Columbia oceanic plateau is only 8–15 km, but units from all of the original crustal layers are found <xref ref-type="bibr" rid="bib1.bibx154" id="paren.322"/>. The accreted Colombian oceanic plateau also has preserved units of the ultramafic layer below the lower crust, which include olivine gabbronorites and pyroxenites <xref ref-type="bibr" rid="bib1.bibx154" id="paren.323"/>. In Ecuador, fragments of the Gorgona oceanic plateau include pillow basalts, dolerite sheets, and gabbros of the upper and mid crust, overlying the plume-derived magmas of the lower crust in thin-skinned thrust sheets <xref ref-type="bibr" rid="bib1.bibx152 bib1.bibx154" id="paren.324"/>. Accreted submarine ridges and seamounts are typically only truncated units of crustal layers. In Central America, various “ophiolitic” units are found with OIB geochemical signatures, which are interpreted as hotspot-related seamounts or submarine ridges <xref ref-type="bibr" rid="bib1.bibx126 bib1.bibx97 bib1.bibx23" id="paren.325"/>. The enigmatic Siletz terrane of northern California and Oregon is composed of volcanics with OIB signatures that have been variously interpreted as a hot spot track, slab window, and mid-ocean ridge <xref ref-type="bibr" rid="bib1.bibx243 bib1.bibx189" id="paren.326"/>. Examples of accreted seamounts, identified primarily by their OIB signature, are the alkali basaltic units found in Japan <xref ref-type="bibr" rid="bib1.bibx137" id="paren.327"/>. Typical seamount-derived terranes include thin-skinned units of radiolarian cherts, limestones, serpentinized peridotites, layered gabbros, and alkali basalts that are on the order of hundreds of meters thick <xref ref-type="bibr" rid="bib1.bibx97 bib1.bibx23" id="paren.328"/>. Accreted ocean-island basalts, interpreted to be remnants of seamounts, are often found within accretionary complexes <xref ref-type="bibr" rid="bib1.bibx140" id="paren.329"><named-content content-type="pre">e.g., Cache Creek terrane:</named-content></xref>. Accreted seamounts are often “decapitated” in the accretionary prism instead of underplated to the overriding plate. The seamount terranes of the Oso Igneous Complex in Costa Rica are within an accretionary prism complex, suggesting that the seamounts were decapitated within the prism and subsequently accreted to the Central American active margin <xref ref-type="bibr" rid="bib1.bibx23" id="paren.330"/>. <xref ref-type="bibr" rid="bib1.bibx280" id="normal.331"/> suggest that even small seamounts can be accreted if the subduction channel is narrow, highly coupled, or if the seamount is regionally compensated by a thick, strong lithosphere. <?xmltex \floatpos{t}?><fig id="Ch1.F7" specific-use="star"><caption>Location map of continental fragments (shown in black) compiled for this study. Continental fragments labeled are: AM – Alpha Mendelev Ridge, B – Bower's Ridge, Bo – Bounty Plateau, BL – Bill Bailey and Lousy banks, C – Campbell Plateau, Ch – Challenger Plateau, Ck – Chukchi Plateau, Ct – Chatham Rise, El – Elan Bank, Ex – Exmouth Plateau, ET – East Tasman Plateau, F – Faroe Bank, FC – Flemish Cap, FP – Falkland Plateau, HR – Hatton and Rockall Banks, JM – Jan Mayen, LH – Lord Howe Rise, LM – Lomonosov Ridge, Na – Naturaliste Plateau, N – Norfolk and Fairway Ridges, NR – Northwind Ridge, P – Porcupine Bank, Q – Queensland Plateau, <?xmltex \hack{\mbox\bgroup}?>S – Seychelles<?xmltex \hack{\egroup}?>, and ST – South Tasman Plateau.</caption> <?xmltex \igopts{width=398.338583pt}?><graphic xlink:href="https://www.solid-earth.net/5/1243/2014/se-5-1243-2014-f07.pdf"/> </fig> Accretion of oceanic plateaus and large submarine ridges can occur as collision and whole crustal addition to a continent, or by underplating and accretion of sheared crustal units. <xref ref-type="bibr" rid="bib1.bibx155" id="normal.332"/> suggest that after mafic oceanic plateaus are accreted or collided, causing the subduction zone to jump, silicic magmas intrude and “mature” the accreted plateau lithology towards a more continental crust lithology. The basal cumulate layer may be a ductile layer that serves as a detachment to allow for underplating, an idea originally speculated by <xref ref-type="bibr" rid="bib1.bibx245" id="normal.333"/> to develop in plateaus that exceed 15 km in thickness based on the rheological relationship of strength with depth. Even though this layer is not found in all LIPs and seamounts of great thicknesses <xref ref-type="bibr" rid="bib1.bibx231" id="paren.334"/> (Fig. <xref ref-type="fig" rid="Ch1.F6"/>), the cumulate or underplated magma layer could definitely serve as a ductile layer to initiate detachment within the subduction zone. The Colombian (Gorgona) oceanic plateau is the only documented accreted plateau that has accreted units of the basal ultramafic cumulate layer, most likely due to the onset of collision early after plateau formation <xref ref-type="bibr" rid="bib1.bibx154" id="paren.335"/>, leading us to hypothesize that this ultramafic basal layer commonly serves as a detachment layer; therefore it is not observed in other accreted oceanic plateaus. More commonly, detachments at shallower depths will allow for obduction and imbrication of the upper units, as observed in the upper basaltic units of the Caribbean oceanic plateau that were obducted in the Caribbean islands and Ecuador <xref ref-type="bibr" rid="bib1.bibx153 bib1.bibx284" id="paren.336"/> and in the basaltic units of the Ontong Java Plateau onto Malaita island <xref ref-type="bibr" rid="bib1.bibx214" id="paren.337"/>. In the case of the active oceanic plateau–continent collision of the Hikurangi Plateau with the North Island of New Zealand, obduction of upper volcanics, limestones, and basalt units are observed in the accretionary prism <xref ref-type="bibr" rid="bib1.bibx67" id="paren.338"/>, while the majority of the plateau crust is subducting and underplating New Zealand <xref ref-type="bibr" rid="bib1.bibx242" id="paren.339"/>. Modern tectonic accretion of submarine ridges with continental fragments is observed in the accretionary system of Southeast Asia, where future collision of the Benham plateau with the Philippine arc is predicted <xref ref-type="bibr" rid="bib1.bibx291" id="paren.340"/> or has already initiated in the thrust faults of the East Luzon Trough <xref ref-type="bibr" rid="bib1.bibx223" id="paren.341"/>. Similarly, the Roo Rise and Ogasawara Plateau are converging on the Sunda continental arc and Izu–Bonin oceanic arc, respectively. Much like the subduction of the Hikurangi Plateau under New Zealand <xref ref-type="bibr" rid="bib1.bibx242" id="paren.342"/>, the Roo Rise and Ogasawara Plateau are initially subducting and underplating their respective fore-arc regions <xref ref-type="bibr" rid="bib1.bibx255 bib1.bibx192" id="paren.343"/>. </sec> </sec> <sec id="Ch1.S5"> <title>Continental fragments and microcontinents</title> <sec id="Ch1.S5.SS1"> <?xmltex \opttitle{Continental fragments and microcontinents:\hack{\\} general setting}?><title>Continental fragments and microcontinents:<?xmltex \hack{\newline}?> general setting</title> Continental fragments, microcontinents, and continental ribbons are submarine regions of continental crust on the oceanic plate (Fig. <xref ref-type="fig" rid="Ch1.F7"/>) that are the result of rifting events on passive margins and retreating active margins. Continental fragments are bound by oceanic crust on one side and thick sedimentary basins overlying extremely thinned continental crust on the other. In some cases, extension proceeded far enough in the failed rifts separating continental fragments from the interior that exhumed and serpentinized mantle directly underlies the basin sediments. Exhumed mantle is inferred from seismic and potential field studies for the Porcupine Basin <xref ref-type="bibr" rid="bib1.bibx157" id="paren.344"/>, Phu Khanh Basin <xref ref-type="bibr" rid="bib1.bibx240" id="paren.345"/>, and the Santos Basin <xref ref-type="bibr" rid="bib1.bibx293" id="paren.346"/>. Microcontinents, such as Jan Mayen and the Seychelles, are surrounded by oceanic crust. Modern continental fragments on the ocean floor include the Rockall Bank, Hatton Bank, Campbell Plateau, Lord Howe Rise, and the Norfolk Rise (Fig. <xref ref-type="fig" rid="Ch1.F7"/>). Continental fragments and microcontinents are theorized to form as a result of plume interaction with passive margins <xref ref-type="bibr" rid="bib1.bibx200 bib1.bibx95" id="paren.347"/>, localized thinning on the basins surrounding continental fragments <xref ref-type="bibr" rid="bib1.bibx213" id="paren.348"/>, differential thinning due to inherited structural grains from ancient sutures zones <xref ref-type="bibr" rid="bib1.bibx125" id="paren.349"/>, or back-arc extension over a retreating slab <xref ref-type="bibr" rid="bib1.bibx241 bib1.bibx262" id="paren.350"/>. Because continental fragments and microcontinents are formed during extensional processes, it is likely they are bound by deep crustal detachment faults and are thinned from normal faulting <xref ref-type="bibr" rid="bib1.bibx213 bib1.bibx226" id="paren.351"/>. The continental fragments of the southwest Pacific ocean are formed in a back-arc extensional regime and are thus bounded by back-arc basins similar to those of island arcs in the Pacific. <?xmltex \floatpos{t}?><fig id="Ch1.F8" specific-use="star"><caption>Seismic velocity profiles of modern continental fragments. References are (1) <xref ref-type="bibr" rid="bib1.bibx94" id="normal.352"/>, (2) <xref ref-type="bibr" rid="bib1.bibx110" id="normal.353"/>, (3) <xref ref-type="bibr" rid="bib1.bibx61" id="normal.354"/>, (4) <xref ref-type="bibr" rid="bib1.bibx109" id="normal.355"/>, (5) <xref ref-type="bibr" rid="bib1.bibx15" id="normal.356"/>, (6) <xref ref-type="bibr" rid="bib1.bibx162" id="normal.357"/>, (7) <xref ref-type="bibr" rid="bib1.bibx93" id="normal.358"/>, (8) <xref ref-type="bibr" rid="bib1.bibx98" id="normal.359"/>, (9) <xref ref-type="bibr" rid="bib1.bibx88" id="normal.360"/>, (10) <xref ref-type="bibr" rid="bib1.bibx17" id="normal.361"/>, (11) <xref ref-type="bibr" rid="bib1.bibx176" id="normal.362"/>, (12) <xref ref-type="bibr" rid="bib1.bibx199" id="normal.363"/>, (13) <xref ref-type="bibr" rid="bib1.bibx275" id="normal.364"/>, and (14) <xref ref-type="bibr" rid="bib1.bibx53" id="normal.365"/>.</caption> <?xmltex \igopts{width=312.980315pt}?><graphic xlink:href="https://www.solid-earth.net/5/1243/2014/se-5-1243-2014-f08.pdf"/> </fig> <?xmltex \floatpos{t}?><table-wrap id="Ch1.T5" specific-use="star"><caption>Crustal thicknesses of continental fragments from seismic studies unless otherwise noted.</caption><oasis:table frame="topbot"><oasis:tgroup cols="3"> <oasis:colspec colnum="1" colname="col1" align="left"/> <oasis:colspec colnum="2" colname="col2" align="left"/> <oasis:colspec colnum="3" colname="col3" align="left"/> <oasis:thead> <oasis:row> <oasis:entry colname="col1">Continental fragments</oasis:entry> <oasis:entry colname="col2">Thickness (km)</oasis:entry> <oasis:entry colname="col3">Reference</oasis:entry> </oasis:row> <oasis:row rowsep="1"> <oasis:entry colname="col1">and microcontinents</oasis:entry> <oasis:entry colname="col2"/> <oasis:entry colname="col3"/> </oasis:row> </oasis:thead> <oasis:tbody> <oasis:row> <oasis:entry colname="col1">Alpha–Mendeleev</oasis:entry> <oasis:entry colname="col2">26</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx177" id="normal.366"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Bill Bailey Bank</oasis:entry> <oasis:entry colname="col2">26</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx94" id="normal.367"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Bounty Platform</oasis:entry> <oasis:entry colname="col2">23</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx110" id="normal.368"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Bower's Ridge</oasis:entry> <oasis:entry colname="col2">25</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx61" id="normal.369"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Campbell plateau</oasis:entry> <oasis:entry colname="col2">24</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx109" id="normal.370"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Chatham Rise</oasis:entry> <oasis:entry colname="col2">22</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx110" id="normal.371"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Chatham Rise</oasis:entry> <oasis:entry colname="col2">20</oasis:entry> <oasis:entry colname="col3">gravity modeling: <xref ref-type="bibr" rid="bib1.bibx67" id="normal.372"/></oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">East Greenland Ridge</oasis:entry> <oasis:entry colname="col2">9–11</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx76" id="normal.373"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Elan Bank</oasis:entry> <oasis:entry colname="col2">16</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx15" id="normal.374"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Exmouth Plateau</oasis:entry> <oasis:entry colname="col2">20</oasis:entry> <oasis:entry colname="col3">magnetotellurics: <xref ref-type="bibr" rid="bib1.bibx123" id="normal.375"/></oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Fairway Ridge</oasis:entry> <oasis:entry colname="col2">23</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx162" id="normal.376"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Falkland plateau</oasis:entry> <oasis:entry colname="col2">25–30</oasis:entry> <oasis:entry colname="col3">gravity modeling: <xref ref-type="bibr" rid="bib1.bibx158" id="normal.377"/></oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Faroe Bank</oasis:entry> <oasis:entry colname="col2">27.5</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx94" id="normal.378"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Faroe Islands</oasis:entry> <oasis:entry colname="col2">35–40</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx230" id="normal.379"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Flemish Cap</oasis:entry> <oasis:entry colname="col2">33</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx98" id="normal.380"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Flemish Cap</oasis:entry> <oasis:entry colname="col2">30</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx93" id="normal.381"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Galicia Bank</oasis:entry> <oasis:entry colname="col2">22</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx102" id="normal.382"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Hatton Bank</oasis:entry> <oasis:entry colname="col2">26.5</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx88" id="normal.383"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Hatton Bank</oasis:entry> <oasis:entry colname="col2">23</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx285" id="normal.384"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Jan Mayen</oasis:entry> <oasis:entry colname="col2">16</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx17" id="normal.385"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Jan Mayen</oasis:entry> <oasis:entry colname="col2">19</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx163" id="normal.386"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Lomonosov Ridge</oasis:entry> <oasis:entry colname="col2">26</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx139" id="normal.387"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Lomonosov Ridge</oasis:entry> <oasis:entry colname="col2">26</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx217" id="normal.388"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Lord Howe Rise</oasis:entry> <oasis:entry colname="col2">25</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx162" id="normal.389"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Lord Howe Rise</oasis:entry> <oasis:entry colname="col2">29</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx253" id="normal.390"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Lousy Bank</oasis:entry> <oasis:entry colname="col2">24</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx94" id="normal.391"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Lousy Bank</oasis:entry> <oasis:entry colname="col2">25</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx161" id="normal.392"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Mendeleev Ridge</oasis:entry> <oasis:entry colname="col2">32</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx176" id="normal.393"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Norfolk rise</oasis:entry> <oasis:entry colname="col2">205</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx162" id="normal.394"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Norfolk rise</oasis:entry> <oasis:entry colname="col2">21.6</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx253" id="normal.395"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Porcupine Bank</oasis:entry> <oasis:entry colname="col2">28</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx286" id="normal.396"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Porcupine Bank</oasis:entry> <oasis:entry colname="col2">25</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx199" id="normal.397"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Rockall Bank</oasis:entry> <oasis:entry colname="col2">30</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx275" id="normal.398"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Rockall Bank</oasis:entry> <oasis:entry colname="col2">28.5</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx199" id="normal.399"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Sao Paolo Plateau</oasis:entry> <oasis:entry colname="col2">12–16</oasis:entry> <oasis:entry colname="col3">gravity modeling: <xref ref-type="bibr" rid="bib1.bibx246" id="normal.400"/></oasis:entry> </oasis:row> <oasis:row rowsep="1"> <oasis:entry colname="col1">Seychelles</oasis:entry> <oasis:entry colname="col2">39</oasis:entry> <oasis:entry colname="col3"> <xref ref-type="bibr" rid="bib1.bibx53" id="normal.401"/> </oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Average</oasis:entry> <oasis:entry colname="col2">24.8 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 5.7</oasis:entry> <oasis:entry colname="col3"/> </oasis:row> </oasis:tbody> </oasis:tgroup></oasis:table></table-wrap> <?xmltex \floatpos{t}?><table-wrap id="Ch1.T6" specific-use="star"><caption>Bulk densities (g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>) of continental fragments and microcontinents determined from seismic velocities using various velocity–density curves. Bulk densities are also reported from studies where the authors combined gravity and seismic data to determine crustal density. References are (1) <xref ref-type="bibr" rid="bib1.bibx94" id="normal.402"/>, (2) <xref ref-type="bibr" rid="bib1.bibx110" id="normal.403"/>, (3) <xref ref-type="bibr" rid="bib1.bibx61" id="normal.404"/>, (4) <xref ref-type="bibr" rid="bib1.bibx109" id="normal.405"/>, (5) <xref ref-type="bibr" rid="bib1.bibx15" id="normal.406"/>, (6) <xref ref-type="bibr" rid="bib1.bibx162" id="normal.407"/>, (7) <xref ref-type="bibr" rid="bib1.bibx93" id="normal.408"/>, (8) <xref ref-type="bibr" rid="bib1.bibx98" id="normal.409"/>, (9) <xref ref-type="bibr" rid="bib1.bibx88" id="normal.410"/>, (10) <xref ref-type="bibr" rid="bib1.bibx17" id="normal.411"/>, (11) <xref ref-type="bibr" rid="bib1.bibx176" id="normal.412"/>, (12) <xref ref-type="bibr" rid="bib1.bibx199" id="normal.413"/>, (13) <xref ref-type="bibr" rid="bib1.bibx275" id="normal.414"/>, and (14) <xref ref-type="bibr" rid="bib1.bibx53" id="normal.415"/>.</caption><oasis:table frame="topbot"><oasis:tgroup cols="5"> <oasis:colspec colnum="1" colname="col1" align="left"/> <oasis:colspec colnum="2" colname="col2" align="left"/> <oasis:colspec colnum="3" colname="col3" align="left"/> <oasis:colspec colnum="4" colname="col4" align="left"/> <oasis:colspec colnum="5" colname="col5" align="left"/> <oasis:thead> <oasis:row> <oasis:entry colname="col1">Continental fragments</oasis:entry> <oasis:entry colname="col2">Nafe–</oasis:entry> <oasis:entry colname="col3">Christensen–</oasis:entry> <oasis:entry colname="col4">Christensen–</oasis:entry> <oasis:entry colname="col5">Reported in</oasis:entry> </oasis:row> <oasis:row rowsep="1"> <oasis:entry colname="col1">and microcontinents</oasis:entry> <oasis:entry colname="col2">Drake</oasis:entry> <oasis:entry colname="col3">Mooney</oasis:entry> <oasis:entry colname="col4">Shaw</oasis:entry> <oasis:entry colname="col5">the study</oasis:entry> </oasis:row> </oasis:thead> <oasis:tbody> <oasis:row> <oasis:entry colname="col1">Bill Bailey Bank<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">1</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.80</oasis:entry> <oasis:entry colname="col3">2.81</oasis:entry> <oasis:entry colname="col4">2.78</oasis:entry> <oasis:entry colname="col5">2.79</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Bounty Platform<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.83</oasis:entry> <oasis:entry colname="col3">2.86</oasis:entry> <oasis:entry colname="col4">2.87</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Bower's Ridge<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">3</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.90</oasis:entry> <oasis:entry colname="col3">2.92</oasis:entry> <oasis:entry colname="col4">2.93</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Campbell Plateau<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">4</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.78</oasis:entry> <oasis:entry colname="col3">2.79</oasis:entry> <oasis:entry colname="col4">2.75</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Chatham Rise<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.82</oasis:entry> <oasis:entry colname="col3">2.83</oasis:entry> <oasis:entry colname="col4">2.85</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Elan Bank<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">5</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.82</oasis:entry> <oasis:entry colname="col3">2.85</oasis:entry> <oasis:entry colname="col4">2.84</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Fairway Rise<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">6</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.78</oasis:entry> <oasis:entry colname="col3">2.77</oasis:entry> <oasis:entry colname="col4">2.72</oasis:entry> <oasis:entry colname="col5">2.74</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Faroe Bank<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">1</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.79</oasis:entry> <oasis:entry colname="col3">2.81</oasis:entry> <oasis:entry colname="col4">2.77</oasis:entry> <oasis:entry colname="col5">2.77</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Flemish Cap<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">7</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.82</oasis:entry> <oasis:entry colname="col3">2.85</oasis:entry> <oasis:entry colname="col4">2.83</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Flemish Cap<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">8</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.81</oasis:entry> <oasis:entry colname="col3">2.83</oasis:entry> <oasis:entry colname="col4">2.85</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Hatton Bank<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">9</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.92</oasis:entry> <oasis:entry colname="col3">2.96</oasis:entry> <oasis:entry colname="col4">2.98</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Jan Mayen<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>10</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.75</oasis:entry> <oasis:entry colname="col3">2.69</oasis:entry> <oasis:entry colname="col4">2.74</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Lord Howe Rise<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">6</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.81</oasis:entry> <oasis:entry colname="col3">2.82</oasis:entry> <oasis:entry colname="col4">2.79</oasis:entry> <oasis:entry colname="col5">2.77</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Lousy Bank<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">1</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.79</oasis:entry> <oasis:entry colname="col3">2.79</oasis:entry> <oasis:entry colname="col4">2.76</oasis:entry> <oasis:entry colname="col5">2.79</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Mendeleev Ridge<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>11</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.84</oasis:entry> <oasis:entry colname="col3">2.85</oasis:entry> <oasis:entry colname="col4">2.82</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Norfolk Rise<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">6</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.74</oasis:entry> <oasis:entry colname="col3">2.71</oasis:entry> <oasis:entry colname="col4">2.64</oasis:entry> <oasis:entry colname="col5">2.77</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Porcupine Bank<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>12</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.76</oasis:entry> <oasis:entry colname="col3">2.75</oasis:entry> <oasis:entry colname="col4">2.79</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row> <oasis:entry colname="col1">Rockall Bank<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>13</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.85</oasis:entry> <oasis:entry colname="col3">2.88</oasis:entry> <oasis:entry colname="col4">2.89</oasis:entry> <oasis:entry colname="col5">2.83</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Rockall Bank<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>12</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.79</oasis:entry> <oasis:entry colname="col3">2.80</oasis:entry> <oasis:entry colname="col4">2.82</oasis:entry> <oasis:entry colname="col5"/> </oasis:row> <oasis:row rowsep="1"> <oasis:entry colname="col1">Seychelles<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mn>14</mml:mn></mml:msup></mml:math></inline-formula></oasis:entry> <oasis:entry colname="col2">2.89</oasis:entry> <oasis:entry colname="col3">2.94</oasis:entry> <oasis:entry colname="col4">2.92</oasis:entry> <oasis:entry colname="col5">2.86</oasis:entry> </oasis:row> <oasis:row> <oasis:entry colname="col1">Average</oasis:entry> <oasis:entry colname="col2">2.82 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 0.05</oasis:entry> <oasis:entry colname="col3">2.81 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 0.08</oasis:entry> <oasis:entry colname="col4">2.83 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 0.06</oasis:entry> <oasis:entry colname="col5">2.79 <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 0.04</oasis:entry> </oasis:row> </oasis:tbody> </oasis:tgroup></oasis:table></table-wrap> <?xmltex \floatpos{t}?><fig id="Ch1.F9" specific-use="star"><caption><bold>(a)</bold> Velocity profiles for island arcs (red), <bold>(b)</bold> oceanic LIPs (blue), <bold>(c)</bold> continental fragments (green) compared to the average velocity profiles of continental crust (black) from <xref ref-type="bibr" rid="bib1.bibx41" id="normal.416"/>. <bold>(d)</bold> Bulk crustal density versus crustal thickness for oceanic plateaus (blue circles), island arcs (red triangles), continental fragments (green squares) and continental crust (black squares). Average values for FATs and continental crust are plotted as stars. All densities are converted from seismic velocities using the relationships in <xref ref-type="bibr" rid="bib1.bibx41" id="normal.417"/>. <bold>(e)</bold> Velocity profiles for all FATs plotted together.</caption> <?xmltex \igopts{width=398.338583pt}?><graphic xlink:href="https://www.solid-earth.net/5/1243/2014/se-5-1243-2014-f09.pdf"/> </fig> </sec> <sec id="Ch1.S5.SS2"> <?xmltex \opttitle{Continental fragments and microcontinents:\hack{\\} crustal structure}?><title>Continental fragments and microcontinents:<?xmltex \hack{\newline}?> crustal structure</title> Naturally, continental fragments and microcontinents have crustal compositions similar to those of typical continental crust. In general, seismic studies have identified two crustal layers with low seismic velocity values representative of their continental affinity. However, the rifting processes that led to the formation of continental fragments and microcontinents most likely affect their layers and entire thicknesses <xref ref-type="bibr" rid="bib1.bibx199" id="paren.418"/>, as well as adding mafic intrusions to the crust. From 36 geophysical studies of continental fragments we determine an average crustal thickness of <inline-formula><mml:math display="inline"><mml:mrow><mml:mo>∼</mml:mo><mml:mn>24.8</mml:mn></mml:mrow></mml:math></inline-formula> <inline-formula><mml:math display="inline"><mml:mo>±</mml:mo></mml:math></inline-formula> 5.7 km (Table <xref ref-type="table" rid="Ch1.T5"/>). Continental fragments have a sediment layer that can be up to 5 km thick and overlying two to three crustal layers, some of which are underplated with a mafic layer (Fig. <xref ref-type="fig" rid="Ch1.F8"/>). The thick sedimentary layer is generally devoid of volcanics, but some rift-related sills may intrude the sedimentary sequences of continental fragments in regions of high magmatism <xref ref-type="bibr" rid="bib1.bibx230 bib1.bibx66" id="paren.419"/>. The upper crust has seismic velocities around 5.5 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>, most likely from rocks of granitic and gneissic composition. The seismic velocities of the mid-crustal layer range from 6.0 to 6.5 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>. The lower crust typically has velocities of 6.5–7.0 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> and is inferred to be gabbroic. In only a few continental fragments, a basal layer with high seismic velocities (7.4–7.8 km s<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">1</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>) is found above the seismic Moho (Fig. <xref ref-type="fig" rid="Ch1.F8"/>). The high velocity layer under the Faroe Bank is interpreted to be a layer of mafic sill intrusions in the crust related to the Iceland plume or convective upwellings <xref ref-type="bibr" rid="bib1.bibx121" id="paren.420"/>. Under the Rockall Bank, this layer is believed to be serpentinized upper mantle <xref ref-type="bibr" rid="bib1.bibx207" id="paren.421"/>. For the continental fragments off the Australian margin, the high velocity lower layer is interpreted as mafic underplating <xref ref-type="bibr" rid="bib1.bibx109" id="paren.422"/>. Mostly, the high velocity seismic layer is found below the surrounding basins with oceanic or thinned continental crust. In these regions, the high velocity layer is also hypothesized to be either serpentinized mantle or mafic underplating <xref ref-type="bibr" rid="bib1.bibx207 bib1.bibx227 bib1.bibx180" id="paren.423"/>. The average crustal density of continental fragments and microcontinents, determined with the <xref ref-type="bibr" rid="bib1.bibx41" id="normal.424"/> depth-dependent relationship from seismic velocities from 20 studies, is <inline-formula><mml:math display="inline"><mml:mrow><mml:mo>∼</mml:mo><mml:mn>2.81</mml:mn></mml:mrow></mml:math></inline-formula> g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> (Table <xref ref-type="table" rid="Ch1.T6"/>). As we expect, the average crustal density of continental fragments and microcontinents is similar to that of the typical continental crust. Despite having thicknesses much lower than the average continental crust (25 km compared to 41 km) the lower densities calculated because of the smaller depths (<inline-formula><mml:math display="inline"><mml:mrow><mml:mo><</mml:mo><mml:mn>25</mml:mn></mml:mrow></mml:math></inline-formula> km) are balanced by the mafic underplating contribution to several of the continental fragments. Interestingly, the average crustal density determined from the eight seismic studies that constrained their models with gravity measurements is a lower value of 2.79 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>. The lower densities derived by gravity modeling are mainly from studies on continental fragments with no seismically identified mafic basal layer. </sec> <sec id="Ch1.S5.SS3"> <?xmltex \opttitle{Continental fragments and microcontinents:\hack{\\} accreted examples}?><title>Continental fragments and microcontinents:<?xmltex \hack{\newline}?> accreted examples</title> Because the classification and the identification of how such features form offshore of passive margins is relatively new <xref ref-type="bibr" rid="bib1.bibx213" id="paren.425"><named-content content-type="post">see references therein</named-content></xref>, there has been little recognition of such features in the accretionary record. The most recognized accreted continental crustal units are found in the Alps. Many of the crustal units accreted in the Alps are believed to be rifted continent fragments <xref ref-type="bibr" rid="bib1.bibx182" id="paren.426"/>, such as the Briançonnais terrane <xref ref-type="bibr" rid="bib1.bibx120" id="paren.427"/>, gneiss units of the Piemonte units <xref ref-type="bibr" rid="bib1.bibx10" id="paren.428"/>, and the Monte Rosa nappe <xref ref-type="bibr" rid="bib1.bibx92" id="paren.429"/>. In Newfoundland, the Dashwoods terrane is interpreted to be a rifted microcontinent block on the passive margin of Laurentia that was later reunited with Laurentia during the Taconic orogeny <xref ref-type="bibr" rid="bib1.bibx278" id="paren.430"/>. Accretionary and collisional processes could utilize the underlying detachment faults or surrounding exhumed and serpentinized mantle lithosphere. There is evidence for detachment faults that are inherited from initial rifting on the Briançonnais terrane and other accreted continental fragments <xref ref-type="bibr" rid="bib1.bibx226" id="paren.431"/>. In western Norway, mantle peridotite melange units, reinterpreted as hyperextended crust, underlie accreted microcontinent slivers of Gula, Jotunn, and Lindas nappes <xref ref-type="bibr" rid="bib1.bibx3" id="paren.432"/>. Precambrian terranes with continental affinities (gneisses) of the Central Asian Orogenic belt are bound by ophiolitic sutures and interpreted as microcontinents rifted off of the East Gondwana margin <xref ref-type="bibr" rid="bib1.bibx287" id="paren.433"/>. It is possible that the ophiolites <xref ref-type="bibr" rid="bib1.bibx287" id="paren.434"><named-content content-type="pre">characterized by sedimentary units, volcanics, and deep marine formations;</named-content></xref> bounding these continental terranes are hyperextended crust. Modern analogues of continental fragment accretion exist in Southeast Asia, where many continental fragments were created during back-arc basin rifting. In this region, continental fragments are accreting and colliding with arcs and other continental fragments. The North Palawan block is the best example of a passive margin fragment currently impinging on an island arc (the Philippine Mobile Belt). The North Palawan block rifted off of the China margin during the extensional opening of the South China Sea <xref ref-type="bibr" rid="bib1.bibx13" id="paren.435"/> and is colliding with the Philippine continental arc <xref ref-type="bibr" rid="bib1.bibx291" id="paren.436"/>. Other continental fragments, such as the Sulawesi block and the Bird's Head block, were created during back-arc rifting events and are now sutured to basin blocks in the present Sunda continental arc <xref ref-type="bibr" rid="bib1.bibx220 bib1.bibx221" id="paren.437"/>. </sec> </sec> <sec id="Ch1.S6"> <title>Composite terranes</title> Often it is the case that FATs will combine before accreting onto a continent – such as oceanic plateau–island arc <?xmltex \hack{\mbox\bgroup}?>composite<?xmltex \hack{\egroup}?><?xmltex \hack{\mbox\bgroup}?>terranes<?xmltex \hack{\egroup}?>. In general, the larger mass of these FATs makes accretion by collision inevitable. The currently accreting Yakutat terrane in Alaska has been speculated to be a continental–oceanic composite terrane. Parts of the Yakutat subducting under Alaska involve oceanic basement or oceanic plateau crust, while the accreting eastern region of the crust is of continental composition <xref ref-type="bibr" rid="bib1.bibx20" id="paren.438"/>. Modern examples of composite terranes include arc–arc collisions, arc–oceanic plateau collisions, and arc–continental fragment collisions. The formation of composite terranes is widely observed in Southeast Asia where numerous island arcs and continental fragments are actively subducting and accreting <xref ref-type="bibr" rid="bib1.bibx117 bib1.bibx218" id="paren.439"/>. On the Philippine Sea Plate, the Halmahera and Sangihe arcs are colliding with doubly verging subduction zones and closing the Molucca sea <xref ref-type="bibr" rid="bib1.bibx219" id="paren.440"/>. Another example of arc–arc collision is in central Japan, where the Izu arc collides and underplates the Honshu arc <xref ref-type="bibr" rid="bib1.bibx4" id="paren.441"/>. Arc–submarine ridge collision is observed with the subduction of the Ogasawara plateau under the Izu–Bonin arc <xref ref-type="bibr" rid="bib1.bibx192" id="paren.442"/>. And the active collision of the Ontong Java oceanic plateau with the Solomon arc <xref ref-type="bibr" rid="bib1.bibx214 bib1.bibx183" id="paren.443"/> represents a modern analog to the accreted Yakutat–Wrangellia terrane in North America. In the geological record, large volumes of crustal accretion are carried out by the collision of composite terranes or continental fragments onto continents <xref ref-type="bibr" rid="bib1.bibx273" id="paren.444"/>. In North America, the amalgamation of the Wrangellia and Stikinia terranes resulted in a ribbon continent (SABIYA) that was <inline-formula><mml:math display="inline"><mml:mrow><mml:mo>∼</mml:mo><mml:mn>8000</mml:mn></mml:mrow></mml:math></inline-formula> km long and <inline-formula><mml:math display="inline"><mml:mrow><mml:mo>∼</mml:mo><mml:mn>500</mml:mn></mml:mrow></mml:math></inline-formula> km wide <xref ref-type="bibr" rid="bib1.bibx141" id="paren.445"/>. During the collision of the superterrane with North America, the mantle lithosphere belonging to the microcontinent was also sutured to the continent, as evidenced by seismic reflection lines <xref ref-type="bibr" rid="bib1.bibx118" id="paren.446"/> and mantle xenoliths from both regions <xref ref-type="bibr" rid="bib1.bibx142" id="paren.447"/>. Another notable accreted ribbon composite terranes is the Cimmerian superterrane which closed the Tethyan sea <xref ref-type="bibr" rid="bib1.bibx249" id="paren.448"/>. </sec> <sec id="Ch1.S7"> <title>Discussion</title> <sec id="Ch1.S7.SS1"> <title>FAT similarities and differences</title> This review of the crustal composition of future accreted terranes highlights the variability in crustal thickness and structure between FAT groups as well as within each group. A comparison of modern FATs to their accreted versions can help us understand crustal composition of accreted units, the amount of crust lost during subduction, and the processes that allow for accretion and collision. Based on average crustal thickness and density, there appears to be no significant difference between FAT groups that would indicate that one particular group would be more susceptible to subduction or accretion. The seismic velocity profiles from each of the three FAT groups show considerable overlap with the average continental crust given by <xref ref-type="bibr" rid="bib1.bibx41" id="normal.449"/> (Fig. <xref ref-type="fig" rid="Ch1.F9"/>). However, all three groups show considerable variability in their crustal structure, depending on their formation and tectonic history, and this will play a part in terrane accretion. The crustal structure of island arcs is composed of two to three layers which are commonly underlain by ultramafic cumulates (the CMTL). The main differences in arc crustal composition and thickness are products of maturation: juvenile arcs are more mafic, thinner, and smaller, while mature island arcs have undergone repetitive anatexis to produce a felsic middle layer. The ultramafic cumulate layer found in most arcs could be formed during early anatexis of the initial basaltic island arc crust <xref ref-type="bibr" rid="bib1.bibx267" id="paren.450"/>. Foundering of this subcrustal ultramafic layer on mature island arcs would leave a crustal composition that is <?xmltex \hack{\mbox\bgroup}?>intermediate<?xmltex \hack{\egroup}?><?xmltex \hack{\mbox\bgroup}?>composition<?xmltex \hack{\egroup}?> and a better contributor to the continental crust. However, many accreted terranes from island arcs do contain units from the ultramafic CMTL, so further modification needs to occur to produce a more compositionally similar crust to continents, such as by the addition of adakites from post-collision magmatism and melting of the continental lower crust <xref ref-type="bibr" rid="bib1.bibx44" id="paren.451"/>. Oceanic plateaus and submarine ridges are quite varied in their crustal structure, and some are also underlain by a high seismic velocity layer. Moreover, recognized oceanic plateaus do not have unique seismic crustal structures or thicknesses which can be differentiated from submarine ridges (Fig. <xref ref-type="fig" rid="Ch1.F6"/>). To determine whether a large mafic igneous feature on the ocean floor is an oceanic plateau or submarine ridge, the geochemical and geodynamic history is obviously needed. Accreted mafic terranes, typically greenstone belts, represent oceanic plateaus, submarine ridges, and seamounts that have been added to continents by accretion or collision. The large terranes (<inline-formula><mml:math display="inline"><mml:mrow><mml:mo>></mml:mo><mml:mn>30</mml:mn></mml:mrow></mml:math></inline-formula> km thick) of Wrangellia and Siletz in North America indicate that these mafic bodies are significant contributors to continental crust despite their mafic composition. Indeed, Archeaen greenstone belts have led some researchers to suggest that accreted oceanic plateaus were the major crustal contributor in the Precambrian <xref ref-type="bibr" rid="bib1.bibx222 bib1.bibx72" id="paren.452"><named-content content-type="pre">e.g.,</named-content></xref>. However, more recent (Paleozoic) tectonic growth of continents is believed to be from felsic island arcs or modified post-accretion oceanic plateaus <xref ref-type="bibr" rid="bib1.bibx47 bib1.bibx46 bib1.bibx260" id="paren.453"/>.) There is observational evidence for modern day subduction of oceanic plateaus and submarine ridges: the Hikurangi oceanic plateau subducting seemingly intact to approximately 65 km depth under New Zealand <xref ref-type="bibr" rid="bib1.bibx228" id="paren.454"/>, the Ontong Java Plateau subducting under the Solomon Islands <xref ref-type="bibr" rid="bib1.bibx183" id="paren.455"/>, and the Nazca Ridge under Peru <xref ref-type="bibr" rid="bib1.bibx119" id="paren.456"/>. In these instances, units from the sedimentary and upper crustal layers are being actively scraped off at the accretionary prism <xref ref-type="bibr" rid="bib1.bibx183" id="paren.457"/> or underplated at the plate interface <xref ref-type="bibr" rid="bib1.bibx58" id="paren.458"/>, leaving behind evidence of the oceanic plateau's existence after subduction. Being rifted off fragments of continental crust, continental fragments have crustal compositions similar to continental crust. The accretion of continental fragments or microcontinents does not require post-accretion modification to achieve the average composition of continental crust. The main difference between the crustal structure of continental fragments and that of typical continental crust is the magmatic addition from extension and rifting that leads to the formation of continental fragments. Because of their geographic relation to continents (as part of the passive margin architecture), continental fragments will most likely precede continents into the subduction zone and continent–continent collision. But not all continental crust will accrete; the subductability of continental crust has been proven by coesite found in exhumed ultrahigh pressure terranes <xref ref-type="bibr" rid="bib1.bibx40" id="paren.459"/> and geodynamic modeling <xref ref-type="bibr" rid="bib1.bibx1" id="paren.460"/>. The collision of continental fragments with continents can lead to slab detachment and then exhumation of these continentally derived terranes. In terms of seismic crustal structure, there is too much variation within and between groups to determine whether a crustal profile belongs to an island arc, oceanic plateau and submarine ridge, or continental fragment (Fig. <xref ref-type="fig" rid="Ch1.F9"/>e). While the seismic velocity profiles of continental fragments do appear to best match the average continental crust profile, there is significant overlap between the velocity profiles of continental fragments and oceanic plateaus/submarine ridges (Fig. <xref ref-type="fig" rid="Ch1.F9"/>). Clearly, seismic velocity profiles should not be the sole basis for determining the nature of crustal composition of an unclassified region of anomalous crust on the ocean floor. One example is the recent finding of granite in deep sea drilling of Rio Grande Rise that would reclassify that feature as a continental fragment rather than a submarine ridge <xref ref-type="bibr" rid="bib1.bibx62" id="paren.461"/>. We would argue that combining gravity measurements with seismic models can narrow the origin of an undetermined FAT crust, as also suggested by <xref ref-type="bibr" rid="bib1.bibx6" id="normal.462"/> for calculating densities directly from seismic values. Many regions of anomalous crust on the Arctic ocean floor have been identified as both continental fragments and oceanic plateaus because of the low constraints provided by only using seismic velocities to determine the crustal composition <xref ref-type="bibr" rid="bib1.bibx77 bib1.bibx176 bib1.bibx5" id="paren.463"/>. When determining the true crustal nature, seismic, gravity, and geochemical studies should also be reinforced with tectonic reconstructions to gain insight on the geological history of an unknown FAT. </sec> <sec id="Ch1.S7.SS2"> <title>From FAT to accreted terrane</title> Accretionary orogens are built of accreted terranes that are hundreds of meters thick, characterized by thin-skinned deformation, and suture bound. In terranes where units from the entire crust of island arcs and oceanic LIPs are preserved, the remaining crustal thickness has been severely sheared and thinned. Although buoyancy is an enabling factor in crustal accretion at subduction zones, it is likely that accretion can occur because weak layers in the FAT crust enable detachments and shear zones to develop within the subduction zone as the crust is subducting. Recent geodynamic experiments show that if a weak zone or detachment fault is present within the crust of the subducting crustal region, whether it is an island arc, oceanic plateau, or continental fragment, accretion will occur and leave a severely thinned terrane <xref ref-type="bibr" rid="bib1.bibx1 bib1.bibx269" id="paren.464"/>. In island arcs, possible delamination units are the felsic middle crust and the CMTL. Pre-existing weaknesses in island arcs produced by back-arc rifting can also serve as detachment faults during subduction. Another important factor in tectonic accretion of island arcs to continents is the elevated geotherm resulting in more buoyant crust and mantle <xref ref-type="bibr" rid="bib1.bibx48" id="paren.465"/>. For example, the Moho temperature for the accreted Talkeetna arc is estimated to be around 900 <inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mo>∘</mml:mo></mml:msup></mml:math></inline-formula>C <xref ref-type="bibr" rid="bib1.bibx114" id="paren.466"/>, which is comparable to cold island arc systems but much higher than typical continental Moho temperatures. Active island arcs will have hot and thin lithospheres, and the high geotherms could activate detachments between crustal layers. The depth of the weak layer or detachment determines the amount of crust and the layers of crust that can be underplated <xref ref-type="bibr" rid="bib1.bibx269" id="paren.467"/>. Continental fragments also may contain pre-existing faults from their earlier rifting stage that could serve as detachment faults during subduction. And while there is no observed evidence for delamination of the ultramafic layer underplating oceanic plateaus, we infer that this layer could also act similar to the ultramafic layer found in island arcs and serve as a décollement during accretion. Collision and docking of large FATs can lead to a small jump in the location of the subduction interface as the slab tears from the accreted terrane, creating asthenospheric upwelling and post-collision magmatism <xref ref-type="bibr" rid="bib1.bibx218" id="paren.468"/>. The crustal deficit of most accreted island arcs, oceanic plateaus, submarine ridges, continental fragments, and even seamounts suggests that a significant amount of crustal material is recycled back into the mantle. Perhaps the foundering of the lower crust and CMTL of oceanic plateaus and island arcs, which is considered to be a major mechanism of terrane accretion, can account for the volumetric loss of crustal material <xref ref-type="bibr" rid="bib1.bibx260" id="paren.469"/>. Whether the ultramafic unit below the lower crust in many FATs is dense enough to create instability and delamination can be determined from laboratory studies of accreted ultramafic units. The ultramafic cumulates of the CMTL in island arcs are inferred to have higher densities than upper mantle dunites when calculated with the expected temperatures and pressures at lower crustal depths <xref ref-type="bibr" rid="bib1.bibx8" id="paren.470"/>. Results from seismic anisotropy studies and crystal fractionation modeling of arc crustal magma development support the theory that the ultramafic high velocity layer under island arcs is often delaminated before or during accretion. In the accreted Wrangellia oceanic plateau, seismic refraction studies of the crust do not show any high <inline-formula><mml:math display="inline"><mml:mi>P</mml:mi></mml:math></inline-formula> wave velocities <xref ref-type="bibr" rid="bib1.bibx18 bib1.bibx224" id="paren.471"/>, which can be interpreted as loss of the ultramafic subcrustal layer. However, interestingly enough, combined gravity and seismic studies of modern island arcs, oceanic plateaus, and submarine ridges do not involve a high density unit between the crust and mantle <xref ref-type="bibr" rid="bib1.bibx173 bib1.bibx112 bib1.bibx181 bib1.bibx43 bib1.bibx101 bib1.bibx235 bib1.bibx225 bib1.bibx279 bib1.bibx256 bib1.bibx212 bib1.bibx119 bib1.bibx255 bib1.bibx210" id="paren.472"/>, contrary to the laboratory-derived densities of the arc CMTL rocks. In addition, the ultramafic units below the lower crust could be a rheologically weak layer that leads to décollement-related underplating during subduction. Post-collision magmatism can alter the composition of accreted terranes by introducing melt from the lower crust and mantle. Transitional I-S-type granites in the Sibumasa terrane of Malaysia were emplaced post-collision and indicate that melting of the lower crust occurred with additional mantle heat <xref ref-type="bibr" rid="bib1.bibx100" id="paren.473"/>. Post-collision, slab detachment led to asthenospheric upwelling and partial melting of the thickened crust to produce granitoids in the Meguma Terrane of Nova Scotia <xref ref-type="bibr" rid="bib1.bibx149" id="paren.474"/>. Similarly, recent geochemical work on the plutons of the Barnard Glacier suite predicts it was formed due to asthenospheric upwelling from slab detachment after Wrangellia collided with the Alexander composite terrane <xref ref-type="bibr" rid="bib1.bibx12" id="paren.475"/>. These magmatic sutures help to modify the accreted terrane crust. Besides the crustal features of FATs, other factors that may influence terrane accretion are the thickness of the subduction zone interface, whether the subduction zone is accretionary or erosive, and slab pull forces. Numerical experiments have shown that a thin subduction interface will promote shearing of the FAT crust and accretion of the upper crustal layers <xref ref-type="bibr" rid="bib1.bibx68" id="paren.476"/>. The nature of the accretionary prism region can be either erosive or accretionary depending on the convergence rates and sedimentary and erosive fluxes <xref ref-type="bibr" rid="bib1.bibx45 bib1.bibx244" id="paren.477"/>, and this will factor into whether crust is recycled back into the mantle or not. Finally, the force of the subducting slab drives subduction and can most likely overcome the buoyancy of small crustal units <xref ref-type="bibr" rid="bib1.bibx195 bib1.bibx48" id="paren.478"/>. In addition, eclogitization of the oceanic lithosphere will increase the negative buoyancy of the slab and even allow continental crust to subduct <xref ref-type="bibr" rid="bib1.bibx1" id="paren.479"/>. Another option for loss of ultramafic lower crustal material could be removal by back-arc mantle convection. Small-scale mantle convection in the back-arc region could contribute to lower crustal flow and crustal and lithospheric thinning in a continental back-arc mobile belt <xref ref-type="bibr" rid="bib1.bibx133" id="paren.480"/>. Back-arc extension on an oceanic plate leads to remnant island arcs, and the elevated mantle temperatures will lead to more vigorous small convection that can easily aid in the removal of the CMTL layer in remnant arcs and active arcs. Numerical experiments have shown that small scale convection under continental back-arcs <xref ref-type="bibr" rid="bib1.bibx65" id="paren.481"/> and oceanic back-arcs <xref ref-type="bibr" rid="bib1.bibx130" id="paren.482"/> is necessary to fit heat flow measurements, low viscosity layers under back-arcs, and seismic anisotropy observations. Indeed, small scale convection under the Izu–Bonin Arc, as inferred by the spatial and temporal patterns of volcanic activity <xref ref-type="bibr" rid="bib1.bibx131" id="paren.483"/>, would aid in removal of the CMTL layers under the Izu, Bonin, and Mariana, active island arcs and their remnant arcs. </sec> </sec> <sec id="Ch1.S8" sec-type="conclusions"> <title>Conclusions</title> Regions of high topography and anomalous crust on the oceanic floor that encounter an active subduction zone are likely to become accreted terranes. These future allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. By comparing modern FATs to examples of accreted terranes, we can better constrain the quantities of crust that are subducted and the material parameters that contribute to accretion. We find that modern island arcs have an average crustal thickness of 26 km, oceanic plateaus and submarine ridges have an average thickness of 21 km, and continental fragments and microcontinents have an average crustal thickness of 25 km. Yet most accreted terranes of island arc, oceanic plateau, submarine ridge, seamount, and continental fragment affinity are on the order of meters to kilometers thick. In the cases where collision occurred rather than accretion by underplating or scraping into the accretionary prism, accreted terranes are interpreted to be 25–40 km thick. The average crustal densities for island arcs is 2.79 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula>, 2.84 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> for oceanic plateaus and submarine ridges, and 2.81 g cm<inline-formula><mml:math display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mo>-</mml:mo><mml:mn mathvariant="normal">3</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> for continental fragments and microcontinents. The different crustal structures of these FATs and their rheological differences can lead to various processes of accretion, including accretionary prism thrusting, underplating, and collision. Crustal slivers of island arcs typically underplate and accrete to the overriding continent. Subduction of oceanic plateaus and submarine ridges often leads to accretion by collision. Seamounts and submarine volcanics subduct easily if they are not incorporated into the accretionary prism. Continental fragments likely lead to collision rather than accretion via underplating as they are connected to passive margins. In addition to the buoyancy of FAT crust, weak crustal layers and delamination of the lower crust and subcrustal layers lead to accretion and formation of accreted terranes. </sec> </body> <back><ack><title>Acknowledgements</title>This study was supported by the Norwegian Research Council through NFR project 180449. Figures 1–8 were constructed with GMT software <xref ref-type="bibr" rid="bib1.bibx282" id="paren.484"/>. This paper was improved by the insightful comments and reviews from William Collins and Manuel Pubellier. 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