Citations per year. Duplicate citations. The following articles are merged in Scholar. Their combined citations are counted only for the first article. Merged citations. This "Cited by" count includes citations to the following articles in Scholar. Add co-authors Co-authors. Upload PDF. Follow this author. New articles by this author. New citations to this author. Accreted oceanic plateaus and submarine ridges are typically identified in the geologic record as mafic to ultra-mafic basalts unit in accreted terranes.
Kerr 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. Figure 6.
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Crustal structures of modern oceanic plateaus and submarine ridges from seismic imaging studies. Depending on their origin, submarine ridge basalts can also have. It is quite likely that many greenstones and mafic accreted units, identified as accreted ophiolites or oceanic crust, may actually be oceanic plateaus see Table 4 in Kerr et al. For example, the hotspot-related greenstones of the Chugoku. 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 Gorg-ona and Columbia oceanic plateaus of South America. 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 Greene et al.
Wrangellia's exposed units are composed of limestone and pelagic sediments, pillow lavas, massive flood basalts, sub-aerial and submarine flows, and olivine-rich basalts Greene et al. In other accreted oceanic plateaus, the preserved crustal thicknesses can be as low as km thick.
The total reconstructed thickness of the accreted Columbia oceanic plateau is only km, but units from all of the original crustal layers are found Kerr et al. The accreted Colombian oceanic plateau also has preserved units of the ultramafic layer below the lower crust, which include olivine gabbronorites and pyroxenites Kerr et al. 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 Kerr and Tarney, ; Kerretal. Accreted submarine ridges and seamounts are typically only truncated units of crustal layers.
In Central America, various "ophiolitic" units are found with OIB geo-chemical signatures, which are interpreted as hotspot-related seamounts or submarine ridges Hoernle et al. 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 Schmandt and Humphreys, ; McCrory and Wilson, Examples of accreted seamounts, identified primarily by their OIB signature, are the alkali basaltic units found in Japan Isozaki et al.
Typical seamount-derived terranes include thin-skinned units of radiolarian cherts, limestones, serpentinized peridotites, layered gab-bros, and alkali basalts that are on the order of hundreds of meters thick Geldmacher et al. Accreted ocean-island basalts, interpreted to be remnants of seamounts, are often found within accretionary complexes e.
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 Buchs et al. Watts et al. 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. Kerr et al. The basal cumulate layer may be a ductile layer that serves as a detachment to allow for underplating, an idea originally speculated by Schubert and Sandwell to develop in plateaus that exceed 15 km in thickness based on the rheological relationship of strength with depth.
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 Kerr et al. More commonly, detachments at shallower depths will allow for ob-duction 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 Kerr et al. 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 Davy et al.
Modern tectonic accretion of submarine ridges with continental fragments is observed in the accre-tionary system of Southeast Asia, where future collision of the Benham plateau with the Philippine arc is predicted Yu-mul et al.
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Figure 7. Location map of continental fragments shown in black compiled for this study. Ogasawara Plateau are initially subducting and underplating their respective fore-arc regions Shulgin et al. Continental fragments, microcontinents, and continental ribbons are submarine regions of continental crust on the oceanic plate Fig. Continental fragments are bound by oceanic crust on one side and thick.
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 Kimbell et al. Microcontinents, such as Jan Mayen and the Seychelles, are surrounded by oceanic crust.
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 Peron-Pinvidic and Manatschal, ; Reston, 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. 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 Morewood et al. 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. 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 Richardson et al.
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The upper crust has seismic velocities around 5. The seismic velocities of the mid-crustal layer range from 6. The lower crust typically has velocities of 6. In only a few continental fragments, a basal layer with high seismic velocities 7. 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 Harland et al. Under the Rockall Bank, this layer is believed to be serpentinized upper mantle O'Reilly et al.
For the continental fragments off the Australian margin, the high velocity lower layer is interpreted as mafic un-derplating Grobys et al. 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 serpen-tinized mantle or mafic underplating O'Reilly et al. As we expect, the average crustal density of continental fragments and microcontinents is similar to that of the typical continental crust.
Interestingly, the average crustal density determined from the eight seismic studies that constrained their models with gravity measurements is a lower value of 2. The lower densities derived by gravity modeling are mainly from studies on continental fragments with no seismically identified mafic basal layer.
Because the classification and the identification of how such features form offshore of passive margins is relatively new Peron-Pinvidic and Manatschal, , see references therein , 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 Manatschal, , such as the Briangonnais terrane Handy et al.
In Newfoundland, the Dashwoods ter-rane is interpreted to be a rifted microcontinent block on the passive margin of Laurentia that was later reunited with Lau-rentia during the Taconic orogeny Waldron and van Staal, 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 Briangonnais terrane and other accreted continental fragments Reston, In western Norway, mantle peridotite melange units, reinterpreted as hyperextended crust, underlie accreted microcontinent slivers of Gula, Jotunn, and Lindas nappes Andersen et al.
Precambrian terranes with continental affinities gneisses of the Central Asian Oro-genic belt are bound by ophiolitic sutures and interpreted as microcontinents rifted off of the East Gondwana margin Windley et al. It is possible that the ophiolites characterized by sedimentary units, volcanics, and deep marine formations; Windley et al.
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. Figure 8. Seismic velocity profiles of modern continental fragments. References are 1 Funck et al. The North Palawan block is the best example of a passive margin fragment currently impinging on an island arc the Philippine Mobile Belt.
Other continental fragments, such as the Sulawesi block and the Bird's Head block, were created during. Often it is the case that FATs will combine before accreting onto a continent - such as oceanic plateau-island arc. Table 5. Crustal thicknesses of continental fragments from seismic studies unless otherwise noted. 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 Bruhn et al. 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 Hall, ; Pubellier and Mer-.
On the Philippine Sea Plate, the Halmahera and Sangihe arcs are colliding with doubly verging subduction zones and closing the Molucca sea Pubellier et al. Another example of arc-arc collision is in central Japan, where the Izu arc collides and underplates the Honshu arc Arai et al. Arc-submarine ridge collision is observed with the subduction of the Ogasawara plateau under the Izu-Bonin arc Miura et al. And the active collision of the Ontong Java oceanic plateau with the Solomon arc Petterson et al. Table 6. Bulk densities gcm 3 of continental fragments and microcontinents determined from seismic velocities using various velocity-density curves.
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 Christensen and Mooney Fig.
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 Tatsumi et al. Foundering of this subcrustal ultramafic layer on mature island arcs would leave a crustal composition that is intermediatecomposition. In the geological record, large volumes of crustal accretion are carried out by the collision of composite terranes or continental fragments onto continents Vink et al. 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 Hammer et al.
Another notable accreted ribbon composite terranes is the Cimmerian superterrane which closed the Tethyan sea Sengor, Figure 9. Average values for FATs and continental crust are plotted as stars. All densities are converted from seismic velocities using the relationships in Christensen and Mooney 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 postcollision magmatism and melting of the continental lower crust Chung et al.
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. 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. Indeed, Archeaen greenstone belts have led some researchers to suggest that accreted oceanic plateaus were the major crustal contributor in the Precam-brian e. However, more recent Paleozoic tectonic growth of continents is believed to be from felsic island arcs or modified post-accretion oceanic plateaus Clift et al. 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 Reyn-ers et al.
In these instances, units from the sedimentary and upper crustal layers are being actively scraped off at the accretionary prism Mann and Taira, or underplated at the plate interface Contreras-Reyes and Carrizo, , 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 mag-matic 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. But not all continental crust will accrete; the sub-ductability of continental crust has been proven by coesite found in exhumed ultrahigh pressure terranes Chopin, and geodynamic modeling Afonso and Zlotnik, 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. 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 Corfield, We would argue that combining gravity measurements with seismic models can narrow the origin of an undetermined FAT crust, as also suggested by Barton 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 Dove et al. 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. 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 Afonso and Zlotnik, ; Tetreault and Buiter, In island arcs, possible delamination units are the felsic middle crust and.
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 Cloos, 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 un-derplated Tetreault and Buiter, Continental fragments also may contain pre-existing faults from their earlier rifting stage that could serve as detachment faults during subduction. 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 Pubellier and Mer-esse, 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 Stern and Scholl, Whether the ul-tramafic 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 Behn and Kelemen, Results from seismic anisotropy studies and crystal fraction-ation 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 P wave velocities Brennanetal. 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 Larter et al. In addition, the ultramafic units below the lower crust could be a rheologically weak layer that leads to decollement-related underplating during subduction. Post-collision magmatism can alter the composition of accreted terranes by introducing melt from the lower crust and mantle.
OR/14/040 Geological Structure
Transitional I-S-type granites in the Sibumasa ter-rane of Malaysia were emplaced post-collision and indicate that melting of the lower crust occurred with additional mantle heat Ghani et al. Post-collision, slab detachment led to asthenospheric upwelling and partial melting of the thickened crust to produce granitoids in the Meguma Ter-rane of Nova Scotia Keppie and Dallmeyer, 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 Beranek et al.
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 accre-tionary or erosive, and slab pull forces.
Deep Structure and Past Kinematics of Accreted Terranes | Geophysical Monograph Series
Numerical experiments have shown that a thin subduction interface will promote shearing of the FAT crust and accretion of the upper crustal layers De Franco et al. The nature of the ac-cretionary prism region can be either erosive or accretionary depending on the convergence rates and sedimentary and erosive fluxes Clift and Vannucchi, ; Scholl and von Huene, , 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 Molnar and Gray, ; Cloos, In addition, eclogitization of the oceanic lithosphere will increase the negative buoyancy of the slab and even allow continental crust to subduct Afonso and Zlotnik, 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 Hyndman et al.
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 Currie and Hyndman, and oceanic back-arcs Honda and Saito, 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 Honda et al. 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 al-lochthonous 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 accre-tionary prism, accreted terranes are interpreted to be km thick. The average crustal densities for island arcs is 2. The different crustal structures of these FATs and their rheological differences can lead to various processes of accretion, including accretionary prism thrusting, underplat-ing, 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 accre-tionary 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.
This paper was improved by the insightful comments and reviews from William Collins and Manuel Pubellier. We also thank the reviews of an anonymous reviewer and Andrew Kerr on a previous iteration of this manuscript. Afonso, J.
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