Oceanic Plateau Accretion Research Articles

Late Jurassic oceanic plateau subduction in the Bangong–Nujiang Tethyan Ocean of northern Tibet

Oceanic plateau accretion and subsequent flat-slab subduction in modern convergent settings have profoundly influenced the nature of subduction and mantle dynamics. However, evaluating similar impacts in ancient convergent settings, where oceanic plateaus have been subducted but geological records are limited, remains challenging. In this study, we present geochronological and geochemical data for a suite of ore-associated plutonic rocks from the Gaobaoyue area of northern Tibet. These rocks have zircon U–Pb ages of 152–146 Ma, with high Sr contents and Sr/Y and La/Yb ratios, low MgO, Yb, and Y contents, and depleted Sr–Nd–Hf isotopic compositions, consistent with an adakitic affinity that was generated by the partial melting of subducting oceanic crust. We compare the Late Jurassic adakitic magmatism with the spatiotemporal evolution of magmatism in northern Tibet to infer oceanic plateau subduction and subsequent flat-slab subduction in the Bangong–Nujiang Tethyan Ocean. This tectonic model explains (i) slab-derived adakitic magmatism, (ii) the observed lull in magmatic activity, (iii) intraplate compression and uplift, and (iv) subduction jump and initiation. We also propose that the subduction of heterogeneous oceanic crust (i.e., buoyant oceanic plateau subduction) provided favorable conditions for tectonic exhumation, vertical slab tearing, and the formation of Cu–Au deposits. Our findings not only have implications for establishing the fundamental process of oceanic plateau accretion in ancient subduction zones but also provide an alternative explanation for Late Jurassic complex tectonomagmatic activity in northern Tibet.

Numerical modeling of induced subduction initiation: Insights from the oceanic plateau accretion

Subduction initiation is a crucial process for plate tectonics. Two scenarios of induced subduction initiation–subduction polarity reversal and subduction transference–are primarily attributed to the accretion of oceanic plateaus. However, the factors governing these two distinct types of subduction initiation remain elusive. Here, we employ 2D thermomechanical numerical models to investigate the dynamics of induced subduction initiation during oceanic plateau accretion. Specifically, we explore the impact of the thickness, density, and rheological strength of the oceanic plateau on subduction initiation. Our results reveal three geodynamic regimes: continuous subduction, subduction transference, and subduction polarity reversal. Thin oceanic plateaus tend to subduct along with slabs into the deep mantle, while thick oceanic plateaus often accrete to the margin, triggering subduction initiation. The rheological strength and density of an oceanic plateau play a significant role in determining whether subduction transference or polarity reversal occurs. Increasing the viscosity and decreasing the density of the oceanic plateau promotes strain concentration in front of the oceanic plateau, resulting in subduction polarity reversal. Our model results provide insights into the dynamics of subduction transference in the Caroline Sea and polarity reversal in Solomon in the western Pacific Ocean.

Upper-plate response to ridge subduction and oceanic plateau accretion, Washington Cascades and surrounding region: Implications for plate tectonic evolution of the Pacific Northwest (USA and southwestern Canada) in the Paleogene

Abstract The interaction between subduction zones and oceanic spreading centers is a common tectonic process, and yet our understanding of how it is manifested in the geologic record is limited to a few well-constrained modern and ancient examples. In the Paleogene, at least one oceanic spreading center interacted with the northwestern margin of North America. Several lines of evidence place this triple junction near Washington (USA) and southern British Columbia (Canada) in the early to middle Eocene, and we summarize a variety of new data sets that permit us to track the plate tectonic setting and geologic evolution of this region from 65 to 40 Ma. The North Cascades segment of the voluminous Coast Mountains continental magmatic arc experienced a magmatic lull between ca. 60 and 50 Ma interpreted to reflect low-angle subduction. During this period of time, the Swauk Basin began to subside inboard of the paleo-trench in Washington, and the Siletzia oceanic plateau began to develop along the Farallon plate–Kula plate or Farallon plate–Resurrection plate spreading center. Farther east, peraluminous magmatism occurred in the Omineca belt and Idaho batholith. Accretion of Siletzia and ridge-trench interaction occurred between ca. 53 and 49 Ma, as indicated by: (1) near-trench magmatism from central Vancouver Island to northwestern Washington, (2) disruption and inversion of the Swauk Basin during a short-lived contractional event, (3) voluminous magmatism in the Kamloops-Challis belt accompanied by major E-W extension east of the North Cascades in metamorphic core complexes and supra-detachment basins and grabens, and (4) southwestward migration of magmatism across northeastern Washington. These events suggest that flat-slab subduction from ca. 60 to 52 Ma was followed by slab rollback and breakoff during accretion of Siletzia. A dramatic magmatic flare-up was associated with rollback and breakoff between ca. 49.4 and 45 Ma and included bimodal volcanism near the eastern edge of Siletzia, intrusion of granodioritic to granitic plutons in the crystalline core of the North Cascades, and extensive dike swarms in the North Cascades. Transtension during and shortly before the flare-up led to >300 km of total offset on dextral strike-slip faults, formation of the Chumstick strike-slip basin, and subhorizontal ductile stretching and rapid exhumation of rocks metamorphosed to 8–10 kbar in the North Cascades crystalline core. By ca. 45 Ma, the Farallon–Kula (or Resurrection)–North American triple junction was likely located in Oregon (USA), subduction of the Kula or Resurrection plate was established outboard of Siletzia, and strike-slip faulting was localized on the north-striking Straight Creek–Fraser River fault. Motion of this structure terminated by 35 Ma. These events culminated in the establishment of the modern Cascadia convergent margin.

Subduction Polarity Reversal Triggered by Oceanic Plateau Accretion: Implications for Induced Subduction Initiation

AbstractSubduction polarity reversal, an induced subduction initiation (SI), may occur frequently in geological history as exemplified by the Solomon subduction zone. However, the mechanism and dynamics of polarity reversal remain poorly understood. Here, we use 2D thermomechanical numerical models to investigate the dynamics of subduction polarity reversal during oceanic plateau accretion. Model results show that larger and lighter oceanic plateaus favor subduction polarity reversal, which is manifested by the formation of the Solomon subduction zone after the large Ontong Java Plateau collided with the former Melanesian (Manus‐North Solomon‐Vitiaz) trench. We analyze the stress state during the transient SI process and find that rapid extension of the oceanic plateau is concomitant with subduction polarity reversal, thus questioning the use of extensional structures as geological criteria for spontaneous SI.

Accretion of oceanic plateaus at continental margins: Numerical modeling

The accretion of oceanic plateaus has played a significant role in continental growth during Earth's history, which is evidenced by the presence of oceanic island basalts (OIB) and plume-type ophiolites in many modern orogens. However, oceanic plateaus can also be subducted into the deeper mantle, as revealed by seismic tomography. The controlling factors of accretion versus subduction of oceanic plateaus remain unclear. Here, we investigate the dynamics of oceanic plateau accretion at active continental margins using a thermo-mechanical numerical model. Three major factors for the accretion of oceanic plateaus are studied: (1) a thinned continental margin of the overriding plate, (2) “weak” layers in the oceanic lithosphere, and (3) a young oceanic plateau. For a large oceanic plateau, the modes of oceanic plateau accretion can be classified into one-sided and two-sided subduction–collisional regimes, which mainly depend on the geometry of the continental margin (normal or thinned). For smaller-sized seamounts, accretion occurs only if all three factors are satisfied, of which a thinned continental margin is the most critical. Possible geological analogues for the two-sided subduction–collisional mode include the Taiwan orogenic belt and subduction of the Ontong Java Plateau. The accretion model for small oceanic plateaus applies to the Nadanhada Terrane in Northeast China.

Infant intra-oceanic arc magmatism due to initial subduction induced by oceanic plateau accretion: A case study of the Bangong Meso-Tethys, central Tibet, western China

The Meso-Tethyan oceanic plateaus are becoming conspicuous as giant units on the oceanic floor and have played important roles in both continental marginal orogenesis and Tethys oceanic evolution. In this study, we present mineralogical, geochronological, geochemical and Sm–Nd isotopic data for basaltic lavas from the Namco ophiolite and a high-Mg pillow lava–dyke–gabbro association from the Pengco ophiolite in central Tibet. Zircon U–Pb and Ar–Ar dating reveals that the Namco lavas erupted at ∼181 Ma while the Pengco boninitic association formed at ∼164 Ma. The Namco lavas display nearly flat rare-earth element (REE) patterns with no Nb–Ta depletions as well as high εNd values, characteristic of oceanic plateau lava. In contrast, the Pengco high-Mg rocks exhibit low REE concentrations below the normal mid-ocean ridge basalt (N-MORB), ubiquitous Nb–Ta depletions and low εNd values, and the dykes and gabbros are characterized by U-shape REE patterns, indicating that they could have derived from a depleted mantle source that was contaminated by sedimentary flux and marking a mid-Jurassic initial intra-oceanic arc magmatism erupted on the Early Jurassic Meso-Tethyan oceanic plateau represented by the Namco ophiolite. Our Pengco boninitic rocks, along with the literature data, indicate a 167–160 Ma boninitic-like initial intra-oceanic arc within the Bangong Meso-Tethys, running from the Shiquanhe area to the Naqu area with a length of ∼1000 km, which was uniformly built on the Early Jurassic Meso-Tethyan oceanic plateau. Our literature investigation also indicates a ∼175 Ma accretionary orogeny with distinct signature of the oceanic plateau involvements along the southern Qiangtang continental margin, which is manifested by regional metamorphic, magmatic and depositional records. We thus suggest that the accretion of the Early Jurassic Meso-Tethyan oceanic plateau onto the southern Qiangtang continental margin resulted in the extensive orogeny along the continental margin, jammed the subduction zone at ∼175 Ma and induced intra-oceanic subduction initiation as well as the intra-oceanic infant arc magmatism in the Meso-Tethys at ∼164 Ma.

Tectonic setting required for the preservation of sedimentary mélanges in Palaeozoic and Mesozoic accretionary complexes of southwest Japan

The Palaeozoic to Mesozoic accretionary complexes of southwest Japan include various types of mélange. Most mélanges are polygenetic in origin, being sedimentary or diapiric mélanges that were overprinted by tectonic deformation during subduction. Sedimentary mélanges, without a tectonic overprint, are present in the Permian accretionary complexes of the Akiyoshi and Kurosegawa belts and in the Early Cretaceous accretionary complex of the Chichibu Belt. These mélanges are characterized by dominant basalt and limestone clasts, within a mudstone matrix. The basalt and limestone clasts within the sedimentary mélanges were derived from ancient seamounts. Subduction of a seamount results in deformation of the pre-existing accretionary wedge, and it is difficult to incorporate a seamount into an accretionary wedge; therefore, preservation of seamount fragments requires a special tectonic setting. Oceanic plateau accretion might play an important role in interrupting the processes of subduction and accretion during the formation of accretionary complexes. Especially the Mikabu oceanic plateau might have caused the cessation of accretion during the Early Cretaceous. The subduction and accretion of volcanic arcs and oceanic plateaux helps to preserve sedimentary mélanges from tectonic overprinting by preventing further subduction.

Is the Ordos Basin floored by a trapped oceanic plateau?

The Ordos Basin in China has about 10 km of Neoarchean to Quaternary sediments covering an enigmatic basement of uncertain origin. The basement is tectonically stable, has a thick mantle root, low heat flow, few earthquakes, and has been slowly subsiding for billions of years. The basement has geophysical signatures that indicate it is dominantly intermediate to mafic in composition, and is similar to some other cratons world-wide, and also to several major oceanic plateaus. It was accreted to the amalgamated Eastern Block and Central Orogenic belt of the North China Craton (NCC) in the Paleoproterozoic, then involved in several Proterozoic tectonic events including being over-thrust by an accretionary orogen, and intruded by Andean arc-related magmas, and then involved in a continent-continent collision during amalgamation with the Columbia Supercontinent. Thus, the basement rocks are deformed, metamorphosed to granulite facies, and determining their initial origin is difficult. We suggest that the data is consistent with an origin as an oceanic plateau that accreted to the NCC and, later experienced different episodes of differentiation associated with later subduction and collisions. Formation of cratonic lithosphere by accretion of oceanic plateaus may be one mechanism to create stable cratons. Other cratons that apparently formed by partial melting of underplated and imbricated oceanic slabs are stable in some cases, but also re-activated and “de-cratonized” in some cases in Asia, where they have been affected by younger subduction, hydration, slab roll-back, and melt-peridotite reactions. This suggests that the initial mode of craton formation may be a factor in the preservation of stable cratons, and de-cratonization is not only influenced by younger tectonic activity.

Petrogenesis of greenstones from the Mino–Tamba belt, SW Japan: Evidence for an accreted Permian oceanic plateau

Permian greenstones in the Jurassic Mino–Tamba accretionary complex, southwest Japan, are divided into three distinct series on the basis of their geological occurrence, mineralogy, and geochemistry. A low-Ti series (LTS) is associated with Lower Permian chert and limestone, and is the most voluminous of the three series. The LTS shows slightly more enriched geochemical and isotopic characteristics than MORB. A transition series (TS) is mainly associated with Lower Permian chert, and has more enriched geochemical signatures than MORB. Its isotopic characteristics are divided into enriched and depleted types. A high-Ti series (HTS) occurs as sills and hyaloclastites within Middle Permian chert and as dikes intruding the TS. Some HTS rocks have high MgO contents. The HTS is characterized by enrichment in incompatible trace elements and an isotopic composition comparable to HIMU-type basalt. The geochemistry of the voluminous LTS is similar to that of the oceanic basalt series of the Kerguelen plateau, suggesting production by partial melting of a shallow mantle plume head below thick oceanic lithosphere in Early Permian time. We infer that the TS formed simultaneously at the margins of the mantle plume head. In contrast, the HTS may have resulted from partial melting of a deep mantle plume tail in Middle Permian time. Permian greenstones in the Mino–Tamba belt may have thus formed by superplume activity in an intra-oceanic setting. Given the presence of two known contemporary continental flood basalt provinces (Siberia and Emeishan) and some accreted oceanic plateau basalts, the vast magmatism of the Mino–Tamba oceanic plateau suggests a large-scale superplume pulse in Permian time. Accretion of oceanic plateaux may have played an important role in the growth of continental margins and island arcs in Japan and elsewhere in the circum-Pacific region.

Oceanic plateau accretion inferred from Late Paleozoic greenstones in the Jurassic Tamba accretionary complex, southwest Japan

Abstract The Jurassic Tamba accretionary complex is divided into two tectono‐stratigraphic suites (Type I and II nappe groups), which are further divided into six complexes (nappes) each of which is characterized by a rock sequence of Late Paleozoic greenstone/limestone, Permian to Jurassic chert and Jurassic terrigenous clastic rocks. The mode of occurrence of the greenstone is divided into two types. The major basal type occurs as a large coherent slab associated with Permian chert and limestone, constituting the basal part of each complex, and the minor mixed type occurs as fragmented allochthonous greenstone blocks and lenses mixed with chert, limestone and sandstone in the Jurassic mudstone matrix. Most of the basal greenstones have uniform geochemical characteristics, which indicate enriched‐mid‐oceanic ridge basalt (MORB) affinity. Their geochemical compositions are akin to the reported Permo‐Carboniferous and Triassic oceanic plateau basalts. Mixed greenstones are divided into two petrochemical types: (i) tholeiitic basalt with normal‐MORB affinity, which is predominant in the uppermost complex of the Type II suite (upper nappe group); and (ii) tholeiitic and alkalic basalts of oceanic island or seamount origin, which are common in all complexes of the Tamba Belt. Geochemical characteristics of the greenstones thus vary in accordance with their occurrences and the structural units to which they belong. This relationship reflects the difference in topographic relief and crustal thickness of the accreted oceanic edifices – the remnants of thick oceanic plateau crust tended to accrete to the continental margin as a large basal greenstone body, whereas thin normal oceanic crust with small seamounts or oceanic islands accreted as mixed greenstones because of their mechanical weakness. The Type II suite (upper nappe group) contains the basal and mixed greenstones, whereas the Type I suite (lower nappe group) includes only mixed greenstones. This distinction may reflect the temporal change of subducting edifices from a thick oceanic plateau to a thin normal oceanic crust, and suggests that the accretion of a large oceanic plateau may be responsible for building accretionary complexes with thick basal greenstones slabs.

Incipient subduction of the Ontong Java Plateau along the North Solomon trench

The Ontong Java Plateau (OJP) in the western central Pacific is the largest and thickest oceanic plateau and one of a few oceanic plateaus converging on an island arc (Solomon island arc—SIA). To better understand the evolution of the North Solomon trench (NST), active oblique convergence between the OJP and SIA, and late Neogene development of Malaita accretionary prism (MAP), we present 850 km of multichannel seismic reflection data integrated with 7832 km 2 of IZANAGI side-scan sonar coverage. We have focussed the study at the transition area between the well-defined northwestern end of the North Solomon trench and a diffusely deformed area where the trench is actively propagating in a northwestward direction. The deeper structure beneath the survey area is discussed by Phinney et al. [Oceanic plateau accretion in the Malaita accretionary prism inferred from multi-channel seismic reflection data, this issue] using deeper penetration, multichannel seismic reflection lines. The serial cross sections provided by multichannel seismic profiling combined with the IZANAGI backscattering imagery provides a time series evolution for the development of the North Solomon trench. The main evolutionary stages include (1) the incipient trench in the northern area marked by a diffuse zone of deformation above a broad arch in the crust. Deeper penetration profiles by Phinney et al. show the bulge is related to a deeper decollement fault that is propagating upward and seaward through the crust. (2) The formation of a continuous thrust front in the central area. Deeper penetration profiles by Phinney et al. show this thrust front is surface expression of the same decollement present at depth to the north. The boundary between the surface trace of the thrust and the diffuse area of deformation in the northern area is inferred as a vertical, high-angle tear fault with left-lateral offset. (3) The formation of a deep, elongate trench which controls gravitationally related slumping and sedimentation around the steep edges of the trench fill basin. The areas to the southeast are those that have undergone convergence for the longest period of time and therefore show better developed trench structures and a reduced width of the MAP. Areas to the northwest have undergone convergence for a shorter period of time and show less developed trench structures and a wide area of the MAP.

Oceanic plateau accretion onto the northwestern margin of the Yilgarn Craton, Western Australia: implications for a mantle plume event at ca. 2.0 Ga

The 2.0 Ga Bryah Basin (Group) is one of a series of Palaeoproterozoic basins in the eastern Capricorn Orogen, a major tectonic unit between the Archaean Pilbara and Yilgarn Cratons of Western Australia. The Narracoota Formation forms the bulk of the Bryah Group and consists of ultramafic and mafic schist and metabasaltic hyaloclastites, sheeted dykes and a layered mafic-ultramafic complex, collectively referred to as metabasites, because they are generally metamorphosed to greenschist facies. Chemically these rocks correspond to komatiites, high-Mg basalts and tholeiites. Previous interpretations have purported that these rocks represent rift-related volcanics, ophiolites and mid-ocean ridge settings. In this contribution, I show that the Narracoota Formation metabasites were likely part of an ancient oceanic plateau, created by a 2.0 Ga mantle plume. Geochemical data (eg REE patterns) indicate that the metabasites are identical to modern oceanic plateaux. The presence of orogenic gold lodes further supports the oceanic plateau model. During the 1.96 Ga Glenburgh Orogeny, which brought together an Andean-style magmatic arc (southern Gascoyne Complex) and the northwestern margin of the Yilgarn Craton, the Narracoota oceanic plateau was accreted onto the Yilgarn Craton. The 1.8 Ga Capricorn Orogeny further deformed the Narracoota metabasites, resulting in the present day structural configuration.

Genesis of Archean tonalite-trondhjemite suites: Plume or subduction related?

Discussed are two probable models for lateral growth of continental crust in the Early Precambrian: accretion of island arcs and accretion of oceanic plateaus. Important constraints on geodynamic settings in which Archean crust was generated can be obtained from the study of magmatic products found in granite-greenstone provinces— tholeiitic basalts and rocks of the tonalite-trondhjemite-granodiorite series (TTG). The komatiite-tholeiite and tholeiitic-basalt series are dierentiated into island-arc and plume (oceanic plateau) assemblages based on their geochemical signatures and the character of associated lithologies. Because of the geochemical similarity between Archean TTG suites and adakites (which are generated through oceanic slab melting in subduction zone), the tonalite-trondhjemite-granodiorite series is widely considered to be subduction related. Our case study in the Early Archean TTG suite of the Onot block (SW margin of the Siberian craton) shows that tonalite-trondhjemite suites, unlike adakites, bear no major element evidence that their initial melts interacted with mantle wedge peridotites. The TTG-associated amphibolites, just like model metabasite sources of tonalite melts, have trace element signatures identical to modern and Archean basalts of plume related oceanic plateaus, and not to MORB-like abyssal tholeiites. Archean TTG suites were not restricted solely to subduction related settings, but could also have formed through melting at the bottom a of thick crust generated by accretion of oceanic plateaus. The most promising criteria to reconstruct geodynamic settings can be provided by (i) compositional characteristics of metabasite enclaves in TTG suites and the model source for tonalite- trondhjemite melts, (ii) Mg#'s of TTG rocks and their similarity to experimental melts, and (iii) isotope parameters of TTG rocks and their associated amphibolites.

A Paleoproterozoic ultramafic-mafic assemblage and associated volcanic rocks of the Boromo greenstone belt: fractionates originating from island-arc volcanic activity in the West African craton

The Loraboué Birimian ultramafic-mafic assemblage, located in the Boromo greenstone belt (Burkina Faso), is interpreted as the remains of a magma chamber that crystallized at the base of an island arc. The ultramafic rocks exhibit an heteradcumulate texture, being generally made up of wehrlites and more rarely dunites. The crystallization sequence inferred from the cumulates is olivine+chromite followed by clinopyroxene+amphibole±orthopyroxene±biotite. The gabbroic rocks are mainly layered and grade into more differentiated facies with sub-pegmatitic texture, containing up to 70% modal plagioclase including zircon and, more commonly, apatite crystals. Textural relationships and mineral phase chemistry are indicative of crystallization at elevated pressures (>8 kbar), the parental magma being generated by a moderate to high degree of partial melting of a mantle source affected by previous metasomatic events above a subducted oceanic slab. The Loraboué volcanic formations exhibit a range of geochemical features. They consist dominantly of calc-alkaline basalts, pyroclastites and rhyolite and, more rarely, of basalt, dolerite and gabbro of tholeiitic affinity. These different types of basalt, as well as the dolerite and the isolated massive gabbro, show the classic features of arc magmatic suites, namely LILE and Pb enrichment, depleted HFSE patterns and high Ce/Nb and Th/Nb ratios. Thus, the calc-alkaline plutonic and volcanic assemblages of Loraboué could represent the roots of an island arc and the associated coeval volcanic rocks. The Paleoproterozoic crust of the West African craton was heterogeneous and was not the consequence of a single process of genesis. As some modern igneous province, the Birimian crust was generated by both volcanic arc accretion and oceanic plateau accretion.

Growth of granite–greenstone terranes at convergent margins, and stabilization of Archean cratons

Archean granite–greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events. Most Archean granite greenstone terranes appear to have acquired their first-order structural and metamorphic characteristics at convergent plate margins, where large accretionary wedges similar in aspect to the Chugach, Makran, and Altaids grew through offscraping and accretion of oceanic plateaux, oceanic crustal fragments, juvenile island arcs, rifted continental margins, and pelagic and terrigenous sediments. Buoyant slabs of parts of Archean oceanic lithosphere may have been underplated beneath these orogens, forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere. Back-stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges. Dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas (sanukitoid suite) in the mantle wedge, whereas other magmas (tonalite, trondhjemite, granodiorite or TTG suite) appear to have been generated by melting of hot young subducted slabs. Eventual collision of these juvenile orogens with other continental blocks formed anatectic granites, then thickened the crust beyond its ability to support its own mass, which initiated gravitational collapse and decompressional release of syn- to late-tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs, and aided in the cratonization of the granite–greenstone terranes. Deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid- and upper-crust, forming ore deposits and plutons. Most preserved granite–greenstone terranes have been tectonically stable since the Archean, and form the cratonic interiors of many continents.

Episodic Crustal Growth and Mantle Evolution

We propose a new model for global crust-mantle evolution, which connects evidence for variable rates of crustal growth, non-steady or alternating modes of mantle convection, and geochemical evidence for an 'open-system' upper mantle. During periods of penetrative two-layer convection, the upper, depleted mantle is partially replenished by exchange with the lower mantle, and the continental crust grows rapidly, probably through oceanic plateau accretion and underplating. During periods of more restricted twolayer convection the continental crust grows more slowly and only by arc accretion, while the upper mantle is progressively depleted.

Crustal volumes of the continents and of oceanic and continental submarine plateaus

Global topographic data and the assumption of Airy isostasy have been used to estimate the crustal volumes of the continents and the oceanic and continental submarine plateaus. The calculated crustal volumes are 7182 × 10 6 km 3 for the continents, 242 × 10 6 km 3 for continental submarine plateaus, and 369 × 10 6 km 3 for oceanic plateaus. The Falkland Plateau and the Lord Howe Rise are the two largest continental submarine plateaus with volumes of 48 × 10 6 km 3 and 47 × 10 6 km 3, respectively. Total continental crustal volume is 7581 × 10 6 km 3 (including the volume of continental sediments on the ocean floor 160 × 10 6 km 3), in good agreement with previous estimates. Continental submarine plateaus on the seafloor comprise 3.2% of the total continental crustal volume. The largest oceanic plateaus in order of decreasing size are the Ontong-Java Plateau, the Kerguelen Plateau, the Caribbean, the Chagos Laccadive Ridge, the Ninetyeast Ridge, and the Mid-Pacific Mountains. Together they comprise 54% of the total anomalous crustal volume in oceanic plateaus. An upper bound to the continental crust addition rate by the accretion of oceanic plateaus is 3.7 km 3/yr, a value that assumes accretion of all oceanic plateaus, with a total volume of 4.9% of the continental crustal volume, on a 100 Myr time scale. Even if a substantial fraction of the crustal volume in oceanic plateaus is subducted, accretion of oceanic plateaus could make a contribution to continental growth since the upper bound to the addition rate exceeds recent estimates of the island arc addition rate. Subduction of continental submarine plateaus with the oceanic lithosphere on a 100 Myr time scale gives an upper bound to the continental crustal subtraction rate of 2.4 km 3/yr, much larger than recent estimates of crustal subtraction by subduction of seafloor sediments. Effective subduction of all oceanic plateaus implies equally effective subduction of continental submarine plateaus. A potentially important way to recycle continental crust back into the mantle may be the break off of small fragments from the continents, entrapment of the continental fragments in the seafloor, and subduction of the fragments with the oceanic lithosphere. This process may be occurring in the Mediterranean for Corsica and Sardinia.

Accretionary processes at subduction zones in the eastern mediterranean

The tectonic history of the eastern Mediterranean as well as the present tectonic style are affected by the collision and accretion of oceanic plateaus at subduction zones. Small but distinct oceanic plateaus exist today in the eastern Mediterranean away from the plate boundary. Many other oceanic plateaus crowd the plate boundary, particularly in the Hellenic arc area. Their presence results in the unusual complexity of this subduction zone, as is evidenced by the seismicity and bathymetry patterns. The present two arcs system in this region, the Hellenic and Cyprean arcs, is probably the consequence of the collision of a large continental plateau with an older arc. This plateau has been thrusted over the continental mass of Anatolia and pieces of it are to be found in the region between the two arcs. Active subduction in the Hellenic arc takes place in the outer trenches, particularly in the Ionian and Strabo trenches. Subduction of Mediterranean lithosphere in the Pliny and Ptolemy trenches is limited to their western parts. Inner segments of these trenches served as the main subduction trenches before the subduction zones migrated outward following the collision of oceanic plateaus.