Deformable backstop as seaward end of coseismic slip in the Nankai Trough seismogenic zone

Wide-angle seismic surveys performed in the last decade have clarified the 3-D crustal structure along the Nankai Trough. The geometry and velocity structure of the southwestern Japan subduction zone are now well constrained. Comparing these observations with the rupture distribution of historic great thrust earthquakes, it appears that the coseismic rupture occurred along plate boundaries deeper than the wedge/backstop boundary (the boundary between the Neogene-Quaternary accretionary wedge and the crust forming the backstop). From the view of spatial relationship, both rupture distributions of the last two large events and the crust forming the backstop are considerably retreated from the trough axis in the west and east off the Kii Peninsula. In both areas, seamount or ridge subduction is apparent in seismic results, geomorphological data and geomagnetic data. The landward indentation of the deformable backstop, which corresponds to the crustal block of old accreted sediments, may be formed by seamount subduction according to published results of sandbox modeling. In particular, the subducted seamount may be a structural factor affecting the recession of the crustal block forming the backstop.     

Geomorphological study on a clastic accretionary prism: The Nankai Trough

Abstract The Nankai Trough, off southwest Japan, is one of the best sites for the study of geomorphic characteristics of a clastic accretionary prism. A recent multibeam survey over the central and eastern parts of the Nankai accretionary prism has revealed a large variation of the topography along the trough axis. Analysis of the bathymetric data suggests the existence of prism deformational features of different scales, such as depressions, embayment structures and cusps. These structures are the results of slope instability caused by basement relief of subducted oceanic plate. Unstable slopes recover by new accretion and development of a low angle thrust. Small-scale deformation due to the subduction of a small isolated seamount is then adjusted to the regional trend. By contrast, a 30 km indentation of the wedge observed in the eastern part of the Nankai Trough, the Tenryu Cusp, has seemed to retain its geometry. The subducted Philippine Sea plate has deformed greatly near the eastern end of the Nankai Trough, because of the collision between the Izu-Ogasawara (Bonin) arc and central Japan. Therefore, the indentation may be the result of the continuous subduction of a basement high, such as the Zenisu Ridge, which has been formed under north-south compression due to the arc-arc collision.     

The formation mechanism of accretionary wedge at Karamay in West Junggar,NW China

Accretionary wedge is the typical product of subduction-zone processes at shallow depths.Determining the location,composition and mechanism of accretionary wedge has important implications for understanding the tectonism of plate subduction.The Central Asian Orogenic Belt(CAOB) is one of the world's largest accretionary orogenic belts,and records the bulk evolution of Paleo-Asian Ocean from opening to closure,with multi-stages and multi-types of crust-mantle interaction in the Paleozoic.West Junggar(western part of Junggar Basin),located in the core area of CAOB,is characterized by a multiple intra-oceanic subduction system during the Paleozoic.In its eastern part crop out Devonian-Carboniferous marine sedimentary rocks,Darbut and Karamay ophiolitic melanges,alkali oceanic island basalts,island arc volcanic rocks and thrusted nappe structure.Such lithotectonic associations indicate the occurrence of accretionary wedge at Karamay.In order to decipher its formation mechanism,this paper presents a synthesis of petrography,structural geology and geochemistry of volcanic rocks.In combination with oceanic subduction channel processes,itis suggested that the accretionary wedge is acomposite melange with multiple stages of formation.The application of oceanic subduction channel model to the Karamay accretionary wedge provides new insights into the accretion and orogenesis of CAOB.     

Cordilleran-type orogeny and episodic growth of continents: Insights from the circum-Pacific continental margins

A new material migration hypothesis for the plate subduction zone orogeny, so-called ‘the cordilleran-type orogeny’, is proposed on the basis of geological constraints as well as mechanics of accretionary wedges. The major tectonic processes of the hypothesis comprise: (i) episodic, extensive magmatism along the margin of an overriding plate; (ii) supply of voluminous igneous and eroded materials through forearc to trench, with an increase in the net sediment influx into trench; (iii) accelerated accretion of sediments beneath an overriding older accretionary wedge; and (iv) upward material migration within the wedge and resultant exhumation of high-P/T metamorphic rocks near the inland margin of the wedge. This hypothesis was validated by the test using available geo- and thermo-chronological data from two classical types of subduction-related orogens in Southwest Japan and California. The hypothesis, coupled with the thermochronologic point of view, requires the reconsideration of coevality of paired metamorphism. The temporal pairing is to be observed between the beginning of the regional high-T/P metamorphism and that of the uplift and exhumation of high-P/T metamorphism, with some time lag needed for material migration. Where the temporal pairing is examined therefore, the formation age of igneous rocks and related high-T/P metamorphic rocks should be compared to the exhumation age of high-P/T metamorphic rocks. The episodic, extensive magmatism that triggers the cordilleran-type orogeny shows a temporal correlation in the mid-Cretaceous for most circum-Pacific continental margins. The resultant widespread formation of accretionary complexes is also observed in the western part of the circum-Pacific margins. The deduced mid-Cretaceous circum-Pacific orogeny accompanied a gross increase in the continental crust production rate, and was approximately coeval with the Pangea breakup and the Central Pacific superplume episode, implying the orogeny as a part of the mid-Cretaceous pulsation of the Earth.     

Evolution of an accretionary complex along the north arm of the Island of Sulawesi, Indonesia

Abstract Seismic reflections across the accretionary prism of the North Sulawesi provide excellent images of the various structural domains landward of the frontal thrust. The structural domain in the accretionary prism area of the North Sulawesi Trench can be divided into four zones: (i) trench area; (ii) Zone A; (iii) Zone B; and (iv) Zone C. Zone A is an active imbrication zone where a decollement is well imaged. Zone B is dominated by out‐of‐sequence thrusts and small slope basins. Zone C is structurally high in the forearc basin, overlain by a thick sedimentary sequence. The subducted and accreted sedimentary packages are separated by the decollement. Topography of the oceanic basement is rough, both in the basin and beneath the wedge. The accretionary prism along the North Sulawesi Trench grew because of the collision between eastern Sulawesi and the Bangai–Sula microcontinent along the Sorong Fault in the middle Miocene. This collision produced a large rotation of the north arm of Sulawesi Island. Rotation and northward movement of the north arm of Sulawesi may have resulted in southward subduction and development of the accretionary wedge along North Sulawesi. Lateral variations are wider in the western areas relative to the eastern areas. This is due to greater convergence rates in the western area: 5 km/My for the west and 1.5 km/My for the east. An accretionary prism model indicates that the initiation of growth of the accretionary prism in the North Sulawesi Trench occurred approximately 5 Ma. A comparison between the North Sulawesi accretionary prism and the Nankai accretionary prism of Japan reveals similar internal structures, suggesting similar mechanical processes and structural evolution.     

Material transportation and fluid-melt activity in the subduction channel: Numerical modeling

The subduction channel is defined as a planar to wedge-like area of variable size,internal structure and composition,which forms between the upper and lower plates during slab subduction into the mantle.The materials in the channel may experience complex pressure,temperature,stress and strain evolution,as well as strong fluid and melt activity.A certain amount of these materials may subduct to and later exhume from100 km depth,forming high to ultra-high pressure rocks on the surface as widely discovered in nature.Rock deformation in the channel is strongly assisted by metamorphic fluids activities,which change composition and mechanical properties of rocks and thus affect their subduction and exhumation histories.In this study,we investigate the detailed structure and dynamics of both oceanic and continental subduction channels,by conducting highresolution petrological-thermomechanical numerical simulations taking into account fluid and melt activities.The numerical results demonstrate that subduction channels are composed of a tectonic rock melange formed by crustal rocks detached from the subducting slab and the hydrated mantle rocks scratched from the overriding plate.These rocks may either extrude sub-vertically upward through the mantle wedge to the crust of the upper plate,or exhume along the subduction channel to the surface near the suture zone.Based on our numerical results,we first analyze similarities and differences between oceanic and continental subduction channels.We further compare numerical models with and without fluid and melt activity and demonstrate that this activity results in strong weakening and deformation of overriding lithosphere.Finally,we show that fast convergence of orogens subjected to fluid and melt activity leads to strong deformation of the overriding lithosphere and the topography builds up mainly on the overriding plate.In contrast,slow convergence of such orogens leads to very limited deformation of the overriding lithosphere and the mountain building mainly occurs on the subducting plate.     

马尼拉俯冲带北段增生楔前缘构造变形和精细结构

马尼拉俯冲带是南海的东部边界,记录了南海形成演化的关键信息,同时也是地震和海啸多发区域.本文利用过马尼拉俯冲带北段的高分辨率多道地震剖面,分析了研究区内海盆和海沟的沉积特征,精细刻画了区内增生楔前缘的构造变形、结构以及岩浆活动特征.研究区内增生楔下陆坡部分由盲冲断层、构造楔和叠瓦逆冲断层构成,逆冲断层归并于一条位于下中新统的滑脱面上,滑脱面向海方向的展布明显受到增生楔之下埋藏海山和基底隆起的影响;上陆坡的反射特征则因变形强烈和岩浆作用而难以识别;岩浆活动开始于晚中新世末期并持续至第四纪.马尼拉俯冲带北段增生楔的形成时间早于16.5 Ma,并通过前展式逆冲向南海方向扩展;马尼拉俯冲带的初始形成时间可能在晚渐新世,而此时南海海盆扩张仍在持续.南海东北缘19°N-21°N区域为南海北部陆坡向海盆的延伸,高度减薄的陆壳的俯冲造成马尼拉海沟北段几何形态明显地向东凹进.     

Subduction of continental margins and the uplift of high-pressure metamorphic rocks

The mechanism by which high-pressure metamorphosed continental material is emplaced at high structural levels is a major unsolved problem of collisional orogenesis. We suggest that the emplacement results from partial subduction of the continental margin which, because of its high flexural rigidity, produces a rapid change in the trajectory of the descending slab. We assume a two-fold increase in effective elastic thickness of the lithosphere as the continental margin approaches the subduction zone, and calculate the flexural profile of a thin plate for progressive downward migration of the zone of increased rigidity. We assess the effect of changes in the flexural profile on the overlying accretionary prism and mantle wedge as the continent approaches by estimating the extra stresses that are imposed on the wedge due to the bending moment exerted by the continental part of the plate. The wedges overlying the subduction zones, and the subducting slab itself, experience substantial extra compressional stress at depths of around 100 km, and extensional stress at shallower depths, as the continental margin passes through the zone of maximum curvature. The magnitudes of such extra stresses are probably adequate to effect significant deformation of the wedge and/or the descending plate, and are experienced in a time interval of less than 5 m.y. for typical subduction rates. The spatial variation of yield stresses in the region of the wedge and descending slab indicates that much of this deformation may be taken up in the crustal part of the descending slab, which is the weakest region in the deeper parts of the subduction zone. This may result in rapid upward migration of the crust of the partially subducted continental margin, against the flow of subduction. High-pressure metamorphosed terranes emplaced by the mechanism envisaged in this paper would be bounded by thrust faults below and normal faults above. Movement on the faults would have been coeval, and would have resulted in rapid unroofing of the high-pressure terranes, synchronous with arrival of the continental margin at the subduction zone and, therefore, relatively early in the history of a collisional orogen.     

Subduction-channel model of prism accretion,melange formation,sediment subduction,and subduction erosion at convergent plate margins: 1. Background and description

Many geological and geophysical investigations, particularly the Deep Sea Drilling Project, have shown that convergent plate margins are highly diverse features. For example, at some sites of subduction, such as the Lesser Antilles, the bedded sediment atop the incoming oceanic plate is extensively offscraped, whereas at others, such as Mariana, not only is the incoming sediment completely subducted beneath crystalline rock but portions of the overriding plate are undergoing subduction erosion. Earthquakes indicate wide variations in stress distribution within and between sites of plate convergence. Many ancient accretionary complexes include tracts of intensely-deformed subduction melange that contain blocks of mafic greenstones. Some contain bodies of thoroughly recrystallized blueschist that were uplifted from depths of 20 to 30 km. A comprehensive model for convergent plate margins must explain these and numerous other observations. Although the still widely cited imbricatethrust model for prism accretion qualitatively explains some observations at subduction zones, it does not account for many others, such as deep sediment subduction and subduction erosion. The subduction-channel model postulates essentially the same basic mechanics for all convergent plate margins that have attained a quasi-steady state (typically reached after about 20 Ma of subduction at speeds of 10 to 20 km Ma^?1). It assumes that the subducting sediment deforms approximately as a viscous material once it is dragged into a relatively thin shear zone, or subduction channel, between the downgoing plate and the overriding one. It predicts the overall movement patterns of the sediment deforming within the channel and near its inlet, accounts for most of the observed features at convergent plate margins, and quantifies the processes of sediment subduction, offscraping, and underplating, and the formation of subduction melange. The predicted variations in tectonic behavior depend upon such site-specific variables as the speed of subduction, the supply of sediment, the geometry of the descending plate, and the topography and structure of the overriding block.     

马尼拉俯冲带中段增生楔精细构造特征及微型圈闭盆地发育模式探讨

对马尼拉俯冲带中段高精度地形数据编辑后绘制成图,联合之前所获取的地震剖面数据,对俯冲带增生楔的精细构造地貌特征及发育模式进行了深入探讨.在增生楔下构造区发现了大量狭长的微型圈闭盆地,经过对其平面断裂特点与深部应力状态的分析,发现增生楔从俯冲前缘到脊顶区其构造特点的转变对应着微型圈闭盆地4个完整的发育阶段,即初期的加积断裂阶段、中期的圈闭成盆阶段、后期的挤压消亡阶段乃至最终的隆升推覆阶段,揭示出一种新型的俯冲带增生楔发育模式,并推断其本质是深部板片俯冲活动所产生的压应力在海底面的表现.     

Rejuvenated extension of the Philippine Sea plate and its effect on subduction dynamics in the Nankai Trough

The Nankai Trough, Japan, is a subduction zone characterized by the recurrence of disastrous earthquakes and tsunamis. Slow earthquakes and associated tremor also occur intermittently and locally in the Nankai Trough and the causal relationship between slow earthquakes and large earthquakes is important to understanding subduction zone dynamics. The Nankai Trough off Muroto, Shikoku Island, near the southeast margin of the rupture segment of the 1946 Nankai earthquake, is one of three regions where slow earthquakes and tremor cluster in the Nankai Trough. On the Philippine Sea plate, the rifting of the central domain of the Shikoku Basin was aborted at ~15 Ma and underthrust the Nankai forearc off Muroto. Here, the Tosa-Bae seamount and other high-relief features, which are northern extension of the Kinan Seamount chain, have collided with and indented the forearc wedge. In this study, we analyzed seismic reflection profiles around the deformation front of accretionary wedge and stratigraphically correlated them to drilling sites off Muroto. Our results show that the previously aborted horst-and-graben structures, which were formed around the spreading center of the Shikoku Basin at ~15 Ma, were rejuvenated locally at ~6 Ma and more regionally at ~3.3 Ma and have remained active since. The reactivated normal faulting has enhanced seafloor roughness and appears to affect the locations of slow earthquakes and tremors. Rejuvenated normal faulting is not limited to areas near the Nankai Trough, and extends more than 200 km into the Shikoku Basin to the south. This extension might be due to extensional forces applied to the Philippine Sea plate, which appear to be driven by slab-pull in the Ryukyu and Philippine trenches along the western margin of the Philippine Sea plate.     

Super-scale Failure of the Southern Oregon Cascadia Margin

—Using SeaBeam bathymetry and multichannel seismic reflection records we have identified three large submarine landslides on the southern Oregon Cascadia margin. The area enclosed by the three arcuate slide scarps is approximately 8000 km², and involves an estimated 12,000–16,000 km³ of the accretionary wedge. The three arcuate slump escarpments are nearly coincident with the continental shelf edge on their landward margins, spanning the full width of the accretionary wedge. Debris from the slides is buried or partially buried beneath the abyssal plain, covering a subsurface area of at least 8000 km². The three major slides, called the Heceta, Coos Basin and Blanco slides, display morphologic and structural features typical of submarine landslides. Bathymetry, sidescan sonar, and seismic reflection profiles reveal that regions of the continental slope enclosed by the scarps are chaotic, with poor penetration of seismic energy and numerous diffractions. These regions show little structural coherence, in strong contrast to the fold thrust belt tectonics of the adjacent northern Oregon margin. The bathymetric scarps correlate with listric detachment faults identified on reflection profiles that show large vertical separation and bathymetric relief. Reflection profiles on the adjacent abyssal plain image buried debris packages extending 20–35 km seaward of the base of the continental slope. In the case of the youngest slide, an intersection of slide debris and abyssal plain sediments, rather than a thrust fault, mark the base of slope. The age of the three major slides decreases from south to north, indicated by the progressive northward shallowing of buried debris packages, increasing sharpness of morphologic expression, and southward increase in post-slide reformation of the accretionary wedge. The ages of the events, derived from calculated sedimentation rates in overlying Pleistocene sediments, are approximately 110 ka, 450 ka, and 1210 ka. This series of slides traveled 25–70 km onto the abyssal plain in at least three probably catastrophic events, which may have been triggered by subduction earthquakes. The lack of internal structure in the slide packages, and the considerable distance traveled suggest catastrophic rather than incremental slip, although there could have been multiple events. The slides would have generated large tsunami in the Pacific basin, possibly larger than that generated by an earthquake alone. We have identified a potential future slide off southern Oregon that may be released in a subduction earthquake. The occurrence of the slides and subsequent subduction of the slide debris, along with evidence for margin subsidence implies that basal subduction erosion has occurred over at least the last 1 Ma. The massive failure of the southern Oregon slope may have been the result of the collision of a seamount province or aseismic ridge with the margin, suggested by the age progression of the slides and evidence for subducted basement highs. The lack of latitudinal offset between the oldest slide debris and the corresponding scarp on the continental slope implies that the forearc is translating northward at a substantial fraction of the margin-parallel convergence rate.     

The Variation of Stresses due to Aseismic Sliding and its Effect on Seismic Activity

— Numerical simulation of recurring large interplate earthquakes in a subduction zone is conducted to explore the effects of aseismic sliding on the variation of stresses and the activity of small earthquakes. The frictional force obeying a rate- and state-dependent friction law is assumed to act on the plate interface in a 2-D model of uniform elastic half-space. The simulation results show that large earthquakes repeatedly occur at a constant time interval on a shallow part of the plate interface and that aseismic sliding migrates from the upper aseismic zone as well as from the lower aseismic zone into the central part of the seismogenic zone before the occurrence of a large interplate earthquake. This spatiotemporal variation of aseismic sliding significantly perturbs the stresses in the overriding plate and in the subducting oceanic plate, leading to the precursory seismic quiescence in the overriding plate and the activation of the intermediate-depth earthquakes of down-dip tension type. After the occurrence of a large interplate earthquake, the activity of the intermediate-depth earthquakes of down-dip compression type in the subducting slab is expected to increase and migrate downward. This is because the downward propagation of postseismic sliding causes the downward migration of compressional-stress increase in the down-dip direction of the plate interface. The simulation result further indicates that episodic events of aseismic sliding may occur when the spatial distributions of friction parameters are significantly nonuniform. The variation of stresses due to episodic sliding is expected to cause seismicity changes.     

Peripheral bulge—a causal mechanism for the Lower/Middle Ordovician unconformity along the western margin of the Northern Appalachians

This report proposes a plate tectonic model that can explain the Early/Middle Ordovician erosional unconformity observed along much of the western margin of the Appalachian orogen. In order for the model to apply, the Taconic allochthons must represent an outer arc (accretionary wedge) and the related subduction zone and Benioff zone must have dipped east (this report reviews the evidence for these assumptions). If these suppositions are correct, then the observed unconformity may have resulted from upwarp along a peripheral bulge (which occurs seaward of present-day oceanic trenches) as the Ordovician continental margin drifted east into the trench. Theoretical calculations show that the amount of uplift experienced by a continental plate over a peripheral bulge is on the order of the amount of uplift observed on the unconformity in Newfoundland. Furthermore, the sequence of events in Taconic times along the western margin of the Appalachian orogen supports the hypothesis that the paleocontinental margin drifted east over a peripheral bulge and on into the trench. The Ordovician shallow-water carbonate bank on the continental margin of the North American plate was uplifted (peripheral bulge) and then rapidly down-dropped to abyssal depths (continental margin entering trench) where it was first covered by flysch and then structurally overlain by the Taconic allochthons (continental margin underthrusting the outer arc). The present western boundary of the maximum relief on the unconformity would delineate the trend and approximate position of the bulge when the craton jammed the subduction zone and ceased convergence with the island arc (in Caradocian times).     

Three-dimensional mantle lithosphere deformation at collisional plate boundaries: A subduction scissor across the South Island of New Zealand

The continental plate collision across the South Island of New Zealand is highly oblique (dextral) and bounded by oppositely verging ocean plate subduction zones. As such, the region can be considered as a type of ‘subduction scissor’. Within this tectonic context, we use three-dimensional computational geodynamic models to consider how convergent mantle lithosphere can be modified by scissor and strike–slip effects. Bounding subduction at both ends of the continental collision causes flow of the descending mantle lithosphere in the direction along strike of the model plate boundary, with thinning in the centre and thickening towards the subduction zones that bifurcates the continental mantle lithosphere root. With dipping bounding subduction, the mantle lithosphere root takes on a more complex morphology that folds over from one subduction polarity to the other, but remains as a continuous feature as it folds under the collision zone. In the absence of bounding subduction, the plate convergence causes a linear (along strike) mantle lithosphere root to develop. A rapid strike–slip motion between the converging plates transfers material in the plate boundary-parallel direction and tends to blur out features that develop in this direction—such as descending viscous instabilities. The along-strike variations in the morphology of the mantle lithosphere root that develop in the models—viz., thickening of the root towards the subduction edges, thinning in the centre—are consistent with recent, albeit poorly constrained, geophysical interpretations of the large-scale lithospheric structure of the South Island. We speculate that this reflects the nature of the evolution of the South Island collision as a limited continental segment of the plate boundary that it is dominated and guided by adjacent well-developed/developing ocean plate subduction.     

Thermodynamic constraints on phase changes as earthquake source mechanisms in subduction zones

Application of thermodynamic equations for phase transitions with a shock wave output from metastable to stable phases that are believed to occur in subduction zones indicates that while many show a volume reduction, and thus may contribute to slab pull mechanisms of plate tectonics, they do not satisfy criteria for the generation of shock waves. Of the reactions considered only one (antigorite + clinoenstatite → forsterite + talc) satisfies the criteria, and for this case the shock wave pressure is, in fact, less than the pressure calculated for the leading edge of the subduction zone. This would imply that if the transition does occur under shock conditions in the subduction zone, dilatational rather than compressional first motions for the associated earthquake are expected.     

苏门答腊俯冲带地区壳内密度结构研究

俯冲带上覆板片密度特征是认识俯冲及其引发深部岩浆过程的一个窗口.本文以苏门答腊俯冲带空间重力异常数据为基础,在2.5D密度结构剖面约束下,通过3D密度反演,获得了研究区3D密度结构分布.反演结果表明,俯冲板片角度和下倾极限深度均从研究区西北向东南方向逐渐增加;研究区上覆板片下地壳存在低密度异常,主要集中在东南部,分布范...     

Tectonics of the Easter plate

We present a new model for the Easter plate in which rift propagation has resulted in the formation of a rigid plate between the propagating and dying ridges. We use the distribution of earthquakes, eleven new focal mechanisms, and existing bathymetric and magnetic data to describe the tectonics of this area. Both the Easter-Nazca and Easter-Pacific Euler poles are sufficiently close to the Easter plate to cause rapid changes in rates and directions of motion along the boundaries. The east and west boundaries are propagating and dying ridges; the southwest boundary is a slow-spreading ridge and the northern boundary is a complex zone of convergent and transform motion.The Easter plate may reflect the tectonics of rift propagation on a large scale, where rigid plate tectonics requires boundary reorientation. We use simple schematic models to illustrate the general features and processes which occur at plates resulting from large-scale rift propagation.     

An Evaluation of Proposed Mechanisms of Slab Flattening in Central Mexico

Central Mexico is the site of an enigmatic zone of flat subduction. The general geometry of the subducting slab has been known for some time and is characterized by a horizontal zone bounded on either side by two moderately dipping sections. We systematically evaluate proposed hypotheses for shallow subduction in Mexico based on the spatial and temporal evidence, and we find no simple or obvious explanation for the shallow subduction in Mexico. We are unable to locate an oceanic lithosphere impactor, or the conjugate of an impactor, that is most often called upon to explain shallow subduction zones as in South America, Japan, and Laramide deformation in the US. The only bathymetric feature that is of the right age and in the correct position on the conjugate plate is a set of unnamed seamounts that are too small to have a significant effect on the buoyancy of the slab. The only candidate that we cannot dismiss is a change in the dynamics of subduction through a change in wedge viscosity, possibly caused by water brought in by the slab.     

Cretaceous episodic growth of the Japanese Islands

Abstract The Japanese Islands formed rapidly in situ along the eastern Asian continental margin in the Cretaceous due to both tectonic and magmatic processes. In the Early Cretaceous, huge oceanic plateaus created by the mid-Panthalassa super plume accreted with the continental margin. This tectonic interaction of oceanic plateau with continental crust is one of the significant tectonic processes responsible for continental growth in subduction zones. In the Japanese Islands, Late Cretaceous-Early Paleogene continental growth is much more episodic and drastic. At this time the continental margin uplifted regionally, and intra-continent collision tectonics took place in the northern part of the Asian continent. The uplifting event appears to have been caused by the subduction of very young oceanic crust (i.e. the Izanagi-Kula Plate) along the continental margin. Magmatism was also very active, and melting of the young oceanic slab appears to have resulted in ubiquitous plutons in the continental margin. Regional uplift of the continental margin and intra-continent collision tectonics promoted erosion of the uplifted area, and a large amount of terrigenous sediment was abruptly supplied to the trench. As a result of the rapid supply of terrigenous detritus, the accretionary complexes (the Hidaka Belt in Hokkaido and the Shimanto Belt in Southwest Japan) grew rapidly in the subduction zone. The rapid growth of the accretionary complexes and the subduction of very young, buoyant oceanic crust caused the extrusion of a high-P/T metamorphic wedge from the deep levels of the subduction zone. Episodic growth of the Late Cretaceous Japanese Islands suggests that subduction of very young oceanic crust and/or ridge subduction are very significant for the formation of new continental crust in subduction zones.