seismic stratigraphic framework of the forearc basin off central sumatra sunda arc abstract new multichannel seismic reflection data provide information on the stratigraphic framework and geologic history of the forearc basin west of central sumatra we recognize six seismic stratigraphic sequences that reflect the cenozoic history and development of the outer continental shelf and forearc basin southeast of nias island these sequences indicate several episodes of uplift of the subduction complex and filling of the forearc basin early in the development of this margin paleogene slope deposits prograded onto the adjacent basin floor onlapping this assemblage are two units interpreted as younger paleogene trough deposits uplift associated with rejuvenation of subduction in the late oligocene led to erosion of the sumatra shelf and formation of a regional unconformity the early miocene was a period of significant progradation a miocene limestone unit partly downlaps and partly onlaps the older paleogene deposits it is characterized by shallow shelf and oblique progradational facies passing into basin floor facies a buried reef zone occurs near the shelf edge the cutting of an erosional unconformity on the shelf and slope in late miocene early pliocene time culminated this episode of deposition in the late pliocene a large flexure developed at the western boundary of the basin displacing the outer arc ridge upward relative to the basin over 1 km of pliocene to recent sediment was deposited as a wedge in the deep western portion of the basin landward of the outer arc ridge these deposits are characterized by flat lying high amplitude continuous reflections that overstep the late miocene unconformity up to 800 m of shallow water limestone have been deposited on the shelf since mid pliocene time

crustal structure of the java margin from seismic wide angle and multichannel reflection data seismic investigations across the convergent sunda margin off indonesia provide a detailed image of the crustal architecture of the sunda plate boundary the combined analysis and interpretation of wide angle and reflection seismic data along two coincident profiles across the subduction zone are complemented by additional lines within the forearc domain which yield some three dimensional 3 d constraints on the velocity depth structure across the margin a detailed cross section of the subduction zone is presented which is confirmed by supplementary gravity modeling the sunda convergence zone is a prime example of an accretionary margin where sediment accretion has led to the formation of a massive accretionary prism with a total width of 110 km between the trench and the forearc basin it is composed of a frontal wedge which documents ongoing accretion and a fossil part behind the present backstop structure which constitutes the outer high moderate seismic velocities derived from wide angle modeling indicate a sedimentary composition of the outer high the subducting oceanic slab is traced to a depth of almost 30 km underneath the accretionary prism the adjacent forearc domain is characterized by a pronounced morphological basin which is underlain by a layer of increased seismic velocities and a shallow upper plate moho at 16 km depth we speculate that remnant fragments of oceanic crust might be involved in the formation of this oceanic type crust found at the leading edge of the upper plate beneath the forearc basin

subduction of the daiichi kashima seamount in the japan trench lallemand s culotta r and von huene r 1989 subduction of the daiichi kashimi seamount in the japan trench in j p cadet and s uyeda editors subduction zones the kaiko project tectonophysics 160 231 247 in 1984 1985 the kaiko consortium collected seabeam single channel seismic and submersible sampling data in the vicinity of the daiichi kashima seamount and the southern japan trench we performed a prestack migration of a shell multichannel seismic profile that crosses this area and examined it in the light of this unusually diverse kaiko dataset unlike the frontal structure of the northern japan trench where mass wasting appears to be the dominant tectonic process the margin in front of the daiiclr kashima shows indentation imbrication uplift and erosion emplacement of the front one third of the seamount beneath the margin front occurs without accretion we conclude that the daiichi kashima seamount exemplifies an intermediate stage between the initial collision and subduction of a seamount at a continental margin

neotectonic deformation of the central chile margin deepwater forearc basin formation in response to hot spot ridge and seamount subduction 1 underlying the midslope valparaiso terrace the valparaiso basin constitutes a unique and prominent deepwater forearc basin on the central chile margin the basin contains a 3 3 5 km thick sediment fill of late cenozoic age the seismic stratigraphic architecture of the valparaiso basin fill shows that margin wide subsidence above the underthrusting juan fernandez ridge an aseismic volcanic swell crested with seamounts on the nazca plate characterized the basin evolution contemporaneously with margin subsidence and seaward rotation of the upper slope and shelf the outer part of the forearc was deformed by thrust faulting above the leading flank of underthrusting seamounts that rotated the middle slope to a more horizontal attitude and formed seaward vergent outer forearc highs subsidence of the margin above the underthrusting juan fernandez ridge is ascribed to processes of subduction erosion enhanced by factors related to subduction of the ridge that caused dismemberment of upper plate material along the shallow aseismic part of the plate interface and transfer of material to the downgoing nazca plate accelerated midslope subsidence 55 75 km landward of the trench coincides with the seaward limit of interplate seismicity suggesting that enhanced basal erosion is related to the transition from aseismic to stick slip subduction behavior since subduction of the juan fernandez ridge began near 33 s 10 ma ago the chile trench north of 33 s is inferred to have retreated 3 km myr and the midslope valparaiso basin subsided 0 3 0 5 km myr approximating analogous neogene rates for the peruvian margin

mentawai fault zone off sumatra a new key to the geodynamics of western indonesia the geodynamic evolution of the western part of the sunda arc is controlled by the change from frontal subduction of the indo australian plate along java to oblique subduction along sumatra this obliquity gives rise to the sumatra fault zone that links the accretionary zone of the andaman sea to the sunda strait previous studies have shown a decrease of displacement rate of the movement along the fault zone from north to south consequently it has been proposed that the area between the subduction zone and the fault zone i e the sumatra sliver platelet was deformed an oceanographic cruise on the indonesian ship r v baruna jaya iii was designed to study this area seismic reflection data reveal the existence of a major strike slip fault parallel to the sumatra fault zone that we called the mentawai fault zone located in the fore arc area just east of the mentawai islands it is at least 600 km long thus the sumatra sliver plate appears to be composed of several strips that move toward the northwest to accommodate the oblique subduction

crustal structure of the central sunda margin at the onset of oblique subduction summary the convergent margin of the central sunda arc in indonesia was the target of a reflection and refraction seismic survey conducted in 1998 and 1999 along two seismic lines across the subduction complex off southern sumatra and off sunda strait coincident multichannel and wide angle data were collected complemented by two refraction strike lines in the forearc basin off sumatra the combined analysis of the acquired data allows us to present a detailed model of the subduction zone where initiation of strain partitioning occurs due to the onset of oblique subduction the dip of the subducted plate is well defined along both dip lines and a lateral increase from 5 to 7 from beneath the outer high off sumatra to sunda strait is supported by complementary gravity modelling the downgoing slab is traced to a depth of more than 30 km on both reflection dip lines a clearly developed backstop structure underlying a trench slope break defines the landward termination of the active accretionary prism and separates it from the outer high active subduction accretion is supported by laterally increasing velocities between the deformation front and the active backstop structure seismic velocities of the outer high are moderate along both lines 5 8 km s 1 at 20 km depth suggesting a sedimentary composition reduced reflectivity beneath a rugged top basement traced along the outer high of both dip lines supports a high degree of deformation and material compaction several kilometres of sediment has accumulated in the forearc domain although a distinct morphological basin is only recognized off southern sumatra and is not developed off sunda strait the bathymetric elevation of the java shelf that is encountered in the southern sunda strait corresponds to increased velocities of a basement high there and is connected to extensional structures of the sunda strait transtensional basin differences observed in the morphology of the forearc domain are also reflected in the lower crustal structure off southern sumatra the velocity depth model clearly indicates a continental type crust underlying the forearc basin whereas lower velocities are found beneath the sunda strait forearc domain off sumatra some 3 d constraint on the upper plate structure is gained from the refraction strike lines which in addition is supported by synthetic data modelling

three dimensional numerical simulation of the subduction dynamics in the sunda arc region southeast asia abstract the sunda arc has been modelled as a test subduction zone with geometrical complexities and varying dynamics we simulated spatially the structural heterogeneities of the subduction zone and carried out our computations by a three dimensional finite element approach the aim is to estimate the lateral variation in the effective horizontal compressive stress in the interplate region which can produce the present day observed seismogenic stress field in our simplified approach the stress field has been modelled by varying the traction ridge push while keeping the age dependent gravitational body force slab pull constant our result suggests that there is a considerable amount of stress dissipation in the decoupled segment of java at sumatra the required traction is much greater than that expected from the age dependency relations in our modelling the transition from the dominance of the horizontal compressive stress to that of the gravitational pull in the regional stress field occurs around a depth range of 150 200 km lateral variation in the subducted slab volume emerges crucial to the seismogenic stress field and the derived pattern of displacement subduction of the lighter oceanic rise material should enhance the stress level the effect of abrupt lateral variation in structure e g sudden changes in the slab dip or slab depth may also be significant on the regional stress field viscoelastic modelling attempted for estimating the temporal variation in subduction dynamics suggests that the thrust level will increase southward of the present day thrust zone in java while strike slip fault type stress field will be developed south of sumatra

fore arc deformation and underplating at the northern hikurangi margin new zealand geophysical investigations of the northern hikurangi subduction zone northeast of new zealand image fore arc and surrounding upper lithospheric structures a seismic velocity vp field is determined from seismic wide angle data and our structural interpretation is supported by multichannel seismic reflection stratigraphy and gravity and magnetic modeling we found that the subducting hikurangi plateau carries about 2 km of sediments above a 2 km mixed layer of volcaniclastics limestone and chert the upper plateau crust is characterized by vp 4 9 6 7 km s overlying the lower crust with vp 7 1 km s gravity modeling yields a plateau thickness around 10 km the reactivated raukumara fore arc basin is 10 km deep deposited on 5 10 km thick australian crust the fore arc mantle of vp 8 km s appears unaffected by subduction hydration processes the east cape ridge fore arc high is underlain by a 3 5 km deep strongly magnetic 3 3 a m high velocity zone interpreted as part of the onshore matakaoa volcanic allochthon and or uplifted raukumara basin basement of probable oceanic crustal origin beneath the trench slope we interpret low seismic velocity high attenuation low density fore arc material as accreted and recycled suggesting that underplating and uplift destabilizes east cape ridge triggering two sided mass wasting mass balance calculations indicate that the proposed accreted and recycled material represents 25 100 of all incoming sediment and any remainder could be accounted for through erosion of older accreted material into surrounding basins we suggest that continental mass flux into the mantle at subduction zones may be significantly overestimated because crustal underplating beneath fore arc highs have not properly been accounted for

subduction of seamounts at the java trench a view with long range sidescan sonar abstract we describe here a 1300 km by 45 km gloria long range sidescan sonar swath along the eastern java trench the swath images the trench axis a narrow strip of oceanic crust to the south and the toe of the accretionary wedge to the north between 108 and 120 e sonar images of the ocean crust show a pattern of normal faults typical of the outer wall of trenches these result from tension related to the bending of the oceanic lithosphere into the subduction zone a number of sub circular seamounts are also seen some of which are currently being subducted isolated ponds of flat lying sediments occur in the trench axis although along much of its length the trench is devoid of such sediment accumulations on the inner trench wall the accretionary wedge is recognised by its distinctive grainy texture with the grain aligned sub parallel to the deformation front where subducting seamounts are colliding with the wedge large crescentic areas of very high backscattering correlate with re entrants in the deformation front and large indentations in the wedge the high backscattering within these collision scars is interpreted to arise from outcropping strata and talus covered slopes resulting from erosion of an inner trench wall shortened and oversteepened by collision with a seamount all of the sediment ponds in the trench axis occur in close association with collision scars strongly suggesting localised erosion of the accretionary wedge in the scar areas however the volume of the indentations appears to be an order of magnitude greater than the volume of eroded sediments deposited in the trench indicating that erosion cannot be the primary mechanism by which indentations are created most of the missing material must be displaced landward by thrusting and folding ahead of the seamount our observations at the java trench are broadly comparable with those made at the japan trench by french and japanese workers however some differences in the detail of the collision process are evident in particular oversteepening and erosion occur from the earliest stage of collision at the java trench while uplift and understeepening initially dominate at the japan trench differences in the effects of subducting seamounts in the two areas are attributed to contrasts between the relatively unlithified strata of the java trench inner wall and the more lithified strata of the non accreting japan trench inner wall

the five deeps the location and depth of the deepest place in each of the world s oceans abstract the exact location and depth of the deepest places in each of the world s oceans is surprisingly unresolved or at best ambiguous out of date erroneous misleading or non existent data on these locations have propagated uncorrected through online sources and the scientific literature for clarification this study reviews and assesses the best resolution bathymetric datasets currently available from public repositories the deepest place in each ocean are the molloy hole in the fram strait arctic ocean 5669 m 79 137 n 2 817 e the trench axis of the puerto rico trench atlantic ocean 8408 m 19 613 n 67 847 w an unnamed deep in the java trench indian ocean 7290 m 11 20 s 118 47 e challenger deep in the mariana trench pacific ocean 10 925 m 11 332 n 142 202 e and an unnamed deep in the south sandwich trench southern ocean 7385 m 60 33 s 25 28 w however discussed are caveats to these locations that range from the published coordinates for a number of named deeps that require correction some deeps that should fall into abeyance deeps that are currently unnamed and the problems surrounding variable and low resolution bathymetric data recommendations on the above and the nomenclature and definition of deeps as undersea features are provided

sunda strait and central sumatra fault sunda strait marks the limit between java trench frontal subduction and sumatra trench oblique subduction the right lateral central sumatra fault accommodates the oblique subduction it does not pass sunda strait but ends between sumatra and java in a complex pattern of dominantly normal faults associated with subsidence seismicity and volcanism we examine the implications of this phenomenon in sunda strait and in the adjacent fore arc basin and accretionary prism which underwent north south compression due to subduction as well as east west extension we propose that the strait is a consequence of the northwestward motion of the southwestern sumatra block along the central sumatra fault we also propose that the extensional zone in the sunda strait widens considerably to the southwest toward the trench and changes into a composite zone of deformation affected by strike slip as well as normal faulting

seismic stratigraphy and subsidence evolution of the northwest sumatra fore arc basin abstract new seismic reflection data collected with the indonesian rv baruna jaya iii under a french indonesian cooperative program allowed to study the geodynamic evolution and the seismic stratigraphy off northern sumatra within the sumatra fore arc domain the direction of subduction of the indian oceanic crust below the sunda arc is normal in front of java and oblique along sumatra as a result parts of the sumatra margin are moving northwards along two large strike slip faults parallel to the subduction trench the sumatra fault in sumatra and the mentawai fault at sea which separates the accretionary prism from the fore arc basin we describe the relationships between the seismic stratigraphy and the sedimentary history and we propose a tectonic evolution of the fore arc domain after widespread uplift and erosion during the paleogene the fore arc basin becomes subsident during the miocene as evidenced by shelf deposits organized in two transgressive regressive sequences 2a 2b produced by tectonics and eustatism this is interpreted as the result of a limited e w extension linked to the incipient play of the sumatra and mentawai faults in pliocene quaternary times the fore arc basin is segmented into several sub basins aceh simeulue and nias basins by compressional zones or by strike slip faults the subsidence rate increased strongly producing sequence 3 with two or four subunits for the pliocene and sequence 4 for the quaternary during this period local variations of the sediments thickness and of the number of sequences indicate that tectonics prevails over eustatism in the fore arc basin the sumatra margin is one of the best examples in the world to study the relationships between sedimentary and tectonic processes in an oblique subduction zone

structural expression of forearc crust uplift due to subducting asperity abstract new structural observations and mapping of reefal terraces carried out both on the field and on satellite remote sensing data indicate that sumba island is presently undergoing a large amount of extension associated with a significant regional uplift this crustal uplift may have been created by a major thrust emerging in the south of the island the uplift which partly accommodated the australian plate south west banda arc convergence is associated with the general northeastward tilt of the island the consequent anomalous positive topography along the southern coast of the island is being compensated by significant tectonic erosion along large scale curvilinear normal faults in the southeastern half of the island the most important expression of this gravitational collapse is located at the receding side of an advancing circular dome showing striking similarities with accretionary wedges being affected by seamount subduction the part of the forearc basin known as the savu basin is moderately deformed mostly in its central part and appears to act as a rigid buttress in the convergence between the banda arc and the australian plate as a result the convergence appears to be transferred northward within the actively shortening back arc domain which goes from the north of the flores island to the southwest of the timor block the convergent plate boundary shows a transition between a stable domain west of sumba and a tectonized domain east of sumba the latter coinciding with the subduction of the outer australian passive margin the subduction of the ocean continent boundary of the austral indian plate below the banda arc since the lower pliocene may have incorporated some crustal fragments in the plunging benioff zone most likely the integration of the stretched continental lithosphere in the subduction zone caused the uplift the entire forearc domain exhibiting inherited structures of the upper plate

plate tectonics and island arcs the plate tectonic concepts that developed rapidly in the late l960s made possible the understanding of island arcs before that time mobilistic concepts evolved slowly hindered particularly in the united states by an obstructionist geoscience establishment the volcanic belts of island arcs form about 100 km above subducting plates convergent plate boundaries evolve complexly with time and at any one time vary greatly along their lengths seismicity defines positions but not trajectories of descending slabs which sink more steeply than they dip and are overridden by advancing upper plates subduction occurs beneath only one side at a time of an internally rigid plate and the common regime in an overriding plate behind a surficial accretionary wedge is extensional except where a collision is underway back arc basin lithosphere is built behind or by migrating island arcs which lengthen and increase their curvatures a collision can involve two active arcs in which case intervening lithosphere sinks beneath both of them or an active margin and a passive one either type of collision generally is followed by the breaking through of new subduction beneath the composite mass of light crust from a new trench on the outside of the aggregate conversely a new subduction system commonly is a by product of collision a strip of back arc basin crust is in many cases left attached to the aggregate in front of the new trench and becomes the basement for a fore arc basin the leading edge of which is raised as melange is stuffed under it sedimentation in trenches is predominantly longitudinal and can be from distant sources accretionary wedges are dynamic being thickened at both toes and bottoms by tectonic accretion and thinned by gravitational forward flow melange is largely a product of tectonic imbrication and flowage driven by these conflicting processes not of submarine sliding high pressure metamorphic rocks form beneath overriding plates not within wedges in front of them arc magmas incorporate much material from the lithosphere through which they rise and vary correspondingly with the evolving composition of that lithosphere arc crust is inflated into geanticlines by intrusive rocks and thermal expansion submarine island arc volcanic rocks are widely spilitized with na enrichment and ca depletion by hydrothermal reaction with sea water the lower crust of mature island arcs consists of granulite facies rocks of mafic intermediate and felsic intermediate compositions the mohorovicic discontinuity may be primarily a constructional boundary representing the shallow limit off crystallization of voluminous rocks of ultramafic composition or plagioclase free mineralogy

coulomb theory applied to accretionary and nonaccretionary wedges possible causes for tectonic erosion and or frontal accretion based on observations from both modem convergent margins and sandbox modeling we examine the possible conditions favoring frontal accretion and or frontal and basal tectonic erosion mean characteristic parameters t t and are used to discuss the mechanical stability of 28 transects across the frontal part of convergent margins where the coulomb theory is applicable natural observations reveal that typical accretionary wedges are characterized by low tapers with smooth surface slope and subducting plate low convergence rates and thick trench sediment while nonaccretionary wedges display large tapers with irregular surface slopes and rough subducting plate high convergence rates and almost no trench fill sandbox experiments were performed to illustrate the effects of seamounts ridges in the subduction zone on the deformation of an accretionary wedge these experiments show that a wedge of sand is first trapped and pushed in front of the seamount which acts as a moving bulldozer this is followed by a tunnelling effect of the subducting seamount through the frontal wedge material which results in considerable sand reworking at an advanced subduction stage the d6collement jumps back from a high level in the wedge to its former basal position we conclude that a high trench sedimentation rate relative to the convergence rate leads to frontal accretion in contrast several conditions may favor tectonic erosion of the upper plate first oceanic features such as grabens seamounts or ridges may trap upper plate material during their subduction process second destabilization of the upper plate material by internal fluid overpressuring causing hydrofracturing is probably another important mechanism

coulomb wedge model applied to the subduction of seamounts in the japan trench results from the french japanese kaiko exploration program of the japan trench and complementary data are used to propose a model of subduction of seamounts examples in two seamount chains one at the junction between the japan and kuril trenches and the other in the southern part of the japan trench off inubo cape illustrate the stages of subduction the japan trench margin evolved as a homogeneous wedge of deformable noncohesive coulomb material during seamount subduction this is illustrated by the subduction of the daiichi kashima seamount in the southern japan trench our model predicts that the first consequence of seamount subduction is a compressive thickening of the toe of the wedge above the advancing flank of the seamount because of the oversteepening of the rigid base of the wedge this agrees with our observations in the daiichi kashima area this thickening shifts landward across the wedge with the advancing flank of the seamount erosion dominates the wedge above the trailing flank of the seamount because of the corresponding understeepening of the rigid base of the wedge a reentrant of the toe of the wedge is thus created it is largest when the oceanic flank of the seamount coincides with the base of the landward slope at this time the trench axis will probably have an abnormally large amount of sediments as illustrated at the junction between the japan and kuril trenches after the subduction of the seamount the edge of the wedge should progressively move back to its initial location frontal accretion is absent or very limited

deformation of accretionary wedges in response to seamount subduction insights from sandbox experiments sandbox experiments using a two dimensional and a three dimensional approach are used to study the deformation of margins in response to seamount subduction successive mechanisms of deformation are activated during the subduction of conical seamounts first reactivation of the frontal thrusts and compaction of the accretionary wedge is observed then back thrusting and conjugate strike slip faulting develops above the leading slope of the subducted seamount the basal d6collement is deflected upward in the wake of the subducting high and a large shadow zone develops behind the seamount trailing slope consequently frontal accretion is inhibited and part of the frontal margin is dragged into the subduction zone when the main d6collement returns to its basal level in the wake of the seamount the margin records a rapid subsidence and a new accretionary wedge develops closing the margin reentrant the sediments underthrusted in the wake of the seamount into the shadow zone are underplated beneath the rear part of the accretionary wedge substantial shortening and thickening of the deformable seaward termination of the upper plate basement associated with basal erosion is observed seamount subduction induces significant material transfer within the accretionary wedge favors large tectonic erosion of the frontal margin and thickening of the rear part of the margin the subduction and underplating of relatively undeformed water ladden sediments associated with fluid expulsion along the fractures affecting the margin could modify the fluid pressure along the basal ddcollement consequently significant variations of the effective basal friction and local mechanical coupling between the two plates could be expected around the subducting seamount

crustal dynamics and active fault mechanics during subduction erosion application of frictional wedge analysis on to the north chilean forearc abstract the forearc region of the non accreting south american plate margin in northern chile is characterised by subduction erosion and regional uplift neotectonic deformation structures reflect simultaneous extensional and contractional fault kinematics in the outer forearc where the brittle crust directly overlies the subducting nazca plate the stress regime changes from extension in the upper part to compression at the base of the forearc wedge as seen in neotectonic surface structures and seismic data in the inner forearc surface structures indicate a compressional stress regime also affecting the western rim of the magmatic arc this stress regime is limited to a brittle crustal wedge segment which overlies the ductile part of the inner forearc lithosphere the shape of the two brittle forearc wedges at the leading edge of the south american plate is controlled by exogenetic surface processes internal deformation processes contemporaneous basal tectonic erosion and underplating mechanical parameter sets controlling and reflecting the recent tectonic processes and geometrical wedge segmentation within the forearc system are evaluated and applied to general frictional wedge models the states of stress within the crustal wedges are controlled by spatial variations of the basal mechanical parameters in the down dip direction of the forearc wedge base the new models illustrate the fundamental kinematics and dynamic processes of coulomb type basal tectonic erosion and mass transfer modes along active non accretive convergent margins the frictional wedge models explain the dynamics of the simultaneous and contrary deformation processes affecting the forearc crust at the north chilean convergent margin shaped by active basal tectonic erosion

comparison of two u37k sea surface temperature records for the last climatic cycle at odp site 658 from the sub tropical northeast atlantic abstract u37k is a proxy indicator of sea surface temperature of the euphotic zone two records of u37k are compared to assess the validity of this indicator as a proxy of paleo sst the site studied odp leg 108 site 658 is located below the upwelling cell off northwest africa the 658a b record has already been reported and the main finding was the detection of rapid and large sst oscillations in terminations magnetic susceptibility data are used to cross correlate the new hole 658c record with that of 658a and b the oxygen isotope stratigraphic record is used to produce a preliminary time scale the two sst records of the last 120 ka reveal similar long term changes for the last climatic cycle which are in agreement with other paleoclimatic indicators such as 18o both the new 658c u37k record and the magnetic susceptibility data suggest that the missing record of stage 4 reported for 658a and b has been recovered in hole 658c detailed comparison of the high resolution records of the first 5 m 25 ka demonstrates that the u37k index is a reproducible measure for sst reconstruction for this segment of the record all features of sst changes reported in the 658a and b curve are confirmed in the new 658c curve furthermore sst oscillations of roughly the same magnitude and duration as the ones reported to occur during terminations have been identified in stages 3 and 4 these new findings indicate that rapid and large sst oscillations can occur not only during the deglaciation transition but also throughout the interglacial and glacial stages other complimentary proxy studies are needed to understand the mechanisms of these sst changes

accretion and recent deformation of sediments along the northern cascadia subduction zone new multichannel seismic reflection profiles acoustic imagery and swath bathymetric data from the vancouver island continental margin reveal the surface morphology cross sectional geometry and deformation style of this part of the cascadia accretionary prism the incoming 3 to 5 km thick sediment section fails most commonly along landward dipping thrusts that penetrate to or very close to the top of the underlying oceanic crust the faults are listric with approximately circular cross sections and hanging wall sediments are back tilted in a planar manner high near lithostatic pore pressures in the deeper part of the section are inferred from the flattening of the faults near the decollement and from the long distances over which the decollement has propagated seaward the faults dip more steeply at 40 45 near the surface where pleistocene turbidites are displaced similar folds have developed where the turbidites are particularly thick 4 km and near hydrostatic pore pressures are inferred to occur in the turbidite part of the section an increase in pore pressure with depth is expected as a consequence of sedimentation and tectonic loading particularly because the deeper hemipelagic sediments are probably far less permeable than are the overlying turbidites the top of the downgoing juan de fuca plate which defines the base of the accreted sedimentary wedge beneath the margin is well imaged in the new seismic reflection profiles and is further defined by land reflection data seismic refraction and earthquake hypocenters the landward dipping continental backstop to the prism is also well located if these constraints are used for the geometry of the accretionary prism a general balance is found between the present prism volume and the estimated total sediment supply over the approximately 42 m y accretion history this result along with the observation that deformation front faults penetrate most or all of the sediment section suggests that little or no sediment has been subducted since initiation of the present phase of subduction in the eocene application of the critically tapered wedge model of davis and others to the observed prism geometry suggests that high average pore pressures occur in the prism particularly the seawardmost part beneath the lower slope beneath the continental shelf the dip of the subducting slab increases to greater than 11 a dip that according to the critically tapered wedge model is too steep to allow upward growth of an accretionary prism this provides a simple explanation for ongoing subsidence of the tofino sedimentary basin over the older part of the accretionary complex to the southeast the juan de fuca plate dips much more gently beneath the olympic peninsula of northern washington allowing the accretionary prism to grow upward well above sea level to form the olympic mountains

neotectonics of the sumatran fault indonesia the 1900 km long trench parallel sumatran fault accommodates a significant amount of the right lateral component of oblique convergence between the eurasian and indian australian plates from 10 n to 7 s our detailed map of the fault compiled from topographic maps and stereographic aerial photographs shows that unlike many other great strike slip faults the sumatran fault is highly segmented cross strike width of step overs between the 19 major subaerial segments is commonly many kilometers the influence of these step overs on historical seismic source dimensions suggests that the dimensions of future events will also be influenced by fault geometry geomorphic offsets along the fault range as high as 20 km and may represent the total offset across the fault if this is so other structures must have accommodated much of the dextral component of oblique convergence during the past few million years our analysis of stretching of the forearc region near the southern tip of sumatra constrains the combined dextral slip on the sumatran and mentawai faults to be no more than 100 km in the past few million years the shape and location of the sumatran fault and the active volcanic arc are highly correlated with the shape and character of the underlying subducting oceanic lithosphere nonetheless active volcanic centers of the sumatran volcanic arc have not influenced noticeably the geometry of the active sumatran fault on the basis of its geologic history and pattern of deformation we divide the sumatran plate margin into northern central and southern domains we support previous proposals that the geometry and character of the subducting investigator fracture zone are affecting the shape and evolution of the sumatran fault system within the central domain the southern domain is the most regular the sumatran fault there comprises six right stepping segments this pattern indicates that the overall trend of the fault deviates 4 clockwise from the slip vector between the two blocks it separates the regularity of this section and its association with the portion of the subduction zone that generated the giant m w 9 earthquake of 1833 suggest that a geometrically simple subducting slab results in both simple strike slip faulting and unusually large subduction earthquakes

outer slope faulting associated with the western kuril and japan trenches summary elongated fault escarpments on the outer slopes of the western kuril and japan trenches have been investigated through detailed swath bathymetric mapping numerous horsts and grabens formed by these escarpments were identified distinct n70 e linear alignment of the escarpments parallel to the magnetic anomaly lineations was revealed on the outer slope of the western kuril trench in the japan trench north of 39 00aen most of the escarpments are parallel to the trench axis and oblique to the magnetic lineations a zig zag pattern of faulting exists south of 39 00aen each topographic profile was decomposed by computer analysis into two curves representing 1 the smoothed long wavelength slope of the subducting ocean crust surface and 2 the short wavelength 10 km roughness of plateaus and valleys edged by outwardand inward facing fault escarpments throughout the surveyed areas escarpment heights increase from the crest of the trench outer swell down to a depth of about 6000 m on the slope of the outer trench wall but with no distinct increase below that depth no significant diverence is recognized in fault throws towards and away from the trench it can be concluded that these elongated escarpments originate from normal faults on the upper layer of the oceanic crust under extensional stress in a direction perpendicular to the trench axis which is caused by downward bending of the subducting lithosphere the relationship of escarpment height to escarpment length is similar to that obtained from normal fault escarpments in the east pacific rise crest the maximum length and height of escarpments are small in the kuril trench compared with those in the japan trench implying a diverence in mechanical strength depending on the fault orientation the crust is weakest along the inherited spreading fabric second weakest probably along the non transform ovset direction and strongest in directions very oblique to these orientations seamounts appear to be more rigid than normal ocean crust with no particular weak orientations resulting in fewer but larger faults along the axis of plate bending as most clearly represented in the subducting daiichi kashima seamount

backstop geometry and accretionary mechanics of the sunda margin the convergent sunda margin off indonesia displays all geological features characteristic of an accretion dominated subduction zone a combined interpretation of prestack depth migrated seismic reflection data and velocity information gained from refraction studies is supplemented by high resolution bathymetric data and for the first time allows the exact mapping of backstop regimes initially the outer high evolved as material was pushed against a static rigid arc framework backstop underlying a forearc basin increasing material strength of the outer high due to lithification formed a dynamic backstop which controls accretion today an out of sequence thrust marks the transition from the recent active frontal accretionary prism to the outer high and may be traced in the seismic and bathymetric data over the whole extent of the study area the existence of a static as well as a dynamic backstop controls the forearc geometry and is associated with the segmentation of the forearc which is observed in regimes of frontal as well as of oblique subduction mass balance calculations which account for porosity changes and metamorphism indicate a subduction history dominated by accretionary processes since the late eocene accretion is associated with the low values of basal friction inferred for the sunda margin structural investigations of conjugate fault planes indicate a very weak basal detachment effective stress analyses reveal that intrinsically weak material causes the high strength ratio of the detachment to the overlying sediments whereas overpressuring within the frontal accretionary prism is negligible

sunda java trench kinematics slab window formation and overriding plate deformation since the cretaceous the kinematics and time dependence of back arc extension or compression is one of the most poorly understood aspects of plate tectonics and has nearly exclusively been studied from snapshots of present day observations here we combine absolute and relative plate motions with reconstructions of now subducted ocean floor to analyse subduction kinematics and upper plate strain from geological observations since 80 ma along the 3200 km long sunda java trench one of the largest subduction systems on earth combining plate motions and slab geometries enables us to reconstruct a time dependent slab window beneath sundaland formed through wharton spreading ridge subduction we find that upper plate advance and retreat is the main influence on upper plate strain but subduction of large bathymetric ridges and slab window effects also play a significant and at times dominant role compression in the sundaland back arc region can be linked to advance of the upper plate extension of the sundaland backarc region correlates with two patterns of upper plate motion a retreat of the upper plate and b advance of the upper plate combined with more rapid advance of the sundaland margin due to hinge rollback subduction of large bathymetric ridges causes compression in the upper plate especially wharton ridge subduction underneath sumatra over the period 15 0 ma our reconstructions unravel the evolving geometry of a slab window underlying the java south sumatra region and we propose that decreased mantle wedge viscosities associated with this slab window exacerbated palaeogene extension in the java sea region via active rifting and enabled sumatran continental extension to continue at 50 35 ma when upper plate advance would otherwise have led to compression

slow rates of subduction erosion estimated from subsidence and tilting of the tonga forearc the convergent margins of the western pacific are marked by widespread tectonic erosion of the overriding plate by the subducting oceanic lithosphere at ocean drilling program site 841 on the mid trench slope of the tonga forearc rates of tectonic trenchward tilting and subsidence of the basement appear to be linked supporting models of basal erosion focused near the trench rates of subsidence significantly exceed those predicted from plate cooling models and require crustal thinning to account for the difference in this extensional environment estimated extension at site 841 is 280 tilting and subsidence rates peaked ca 32 34 ma just after spreading in the south fiji basin at 35 ma however the trenchward direction of tilt and higher extension on the trench slope than on the tonga platform suggest a link to tectonic erosion near the trench rather than extension focused in the arc edifice since this time ca 135 km has been eroded from the edge of the plate of which 80 km is related to the ongoing collision of the louisville ridge a probable paleo collision event at 16 ma caused the forearc at site 841 to be uplifted and tilted arcward forming a major unconformity due to mass wasting steady state erosion averages 1 5 km m y there is no compelling evidence to suggest loss of several hundred kilometers of arc crust since the eocene and thus no reason to suspect that subduction initiation and related magmatism may have predated the ca 45 ma boninitic magmatic activity

the importance of atmospheric input of terrestrial organic material to deep sea sediments the concentrations of lipids were determined in atmospheric particle gas and rain samples collected from the tropical north pacific to assess lipid sources transport mechanisms and fluxes to the ocean surface four lipid compound classes aliphatic hydrocarbons fatty alcohols fatty acid esters and salts all unequivocally show a terrestrial vascular plant source these aerosol lipids originate from wind erosion of asian and american soils and direct emission from vegetation the major fluxes result from rain rather than dry deposition these fluxes are large enough to have a major potential impact on the inventory of terrestrially derived lipid material found in deep sea sediments this has been showm for n alkanes fatty alcohols fatty acids total lipids and for organic carbon by comparing atmospheric and sediment trap fluxes with sediment accumulation rates it is suggested that some biogenic terrestrial material is more protected from degradation than marine derived material

how lime sand islands in the south china sea have responded to global warming over the last 30 years evidence from satellite remote sensing images abstract as global warming threatens their existence there are widespread concerns regarding the persistence of lime sand islands and the future of mid ocean atoll nations to investigate how climate change has affected lime sand islands changes in vegetation and other characteristics of such islands in the south china sea scs were investigated from 1989 to 2019 using 67 satellite images first boundaries of the lime sand islands and their vegetation were extracted using an active contour extraction procedure called the gradient vector flow snake model afterwards the spatial extents were estimated by enclosing the extracted boundaries and the digital shoreline analysis system was used to calculate beach widths finally area growth rates and speeds were used to evaluate areal changes in the lime sand islands and their respective vegetation based on the estimated area growth rates area growth speeds and beach widths the lime sand islands in scs eroded over the past three decades whereas their vegetation expanded further analysis suggested that direct inundation caused by sea level rise might not be clearly identified from the satellite images however other climate change related factors were most likely responsible for the observed island erosions these factors included higher wave energy stronger typhoon intensity and destructiveness and accelerated coral reef degradation in addition the observed expansion of vegetation on the lime sand islands was likely due to the increase in precipitation in a warming world the results show that 1 the lime sand islands will continue to erode whereas vegetation will continue to expand 2 as vegetation growth is significantly inhibited by salt water when it is adjacent to the ocean vegetation areas on lime sand islands may start to continuously decline overall this study is the first to quantitatively examine changes in scs lime sand islands due to global warming

the java margin revisited evidence for subduction erosion off java the eastern sunda margin off indonesia from central java to sumba island remains a little investigated subduction zone contrary to its well studied northwestern segment whereas large portions of the sunda margin are considered a classical accretionary zone subduction characteristics along the central java sector indicate erosive processes as the dominant mode of mass transfer the tectonic framework of the central java margin with a convergence rate of 6 7 cm yr insignificant sediment input and a pronounced seafloor roughness where the oceanic roo rise is subducting underneath java facilitates subduction erosion evidence for erosion comes from newly acquired geophysical data off central java local erosive processes in the wake of seamount subduction are documented by a high resolution bathymetric survey and result in an irregular trend of the deformation front sculpted by seamount collision scars subduction of oceanic basement relief leads to large scale uplift of the forearc as recorded on a reflection seismic profile and to a dismemberment of the previous outer forearc high giving way to isolated topographic elevations the broad retreat of the java trench and deformation front above the leading edge of the roo rise has exposed an area of approximately 25 000 km2 of deeper seafloor formerly covered by the previous frontal prism frontal erosion coincides with a steepening of the lower slope angle in the central java sector compared to the neighbouring segments in global compilations the key geological parameters of the central java margin lie in the erosive regime reflecting the interplay of basement relief subduction negligible sediment supply and a high convergence rate on the evolution of the margin

molecular record of secular sea surface temperature changes on 100 year timescales for glacial terminations i ii and iv high resolution palaeoclimate records based on oxygen isotope data have provided important insights into climate variability and rates of natural climate change on about a thousand year timescale1 3 little is known4 however about the variation of climate and of palaeoceanographic conditions throughout the quaternary on timescales of less than 1 000 years at frequencies far greater than those of the milankovitch orbital cycles here we show that such high time resolution is possible from molecular stratigraphic studies based on biomarker organic molecules alkenones we have sampled alkenone stratigraphic records at 70 to 200 yr intervals across glacial terminations i ii and iv in sediment cores from oop site 658 off northwest africa5 sea surface temperatures ssts derived from the alkenone uk37 index6 8vary rapidly beyond the range of analytical noise by up to 2 5 c in 300 yr showing hitherto unknown cycles with about 600 yr periodicities some of the changes parallel similar events in the oxygen isotope stratigraphy sst oscillations may be linked in part to abrupt breakdowns in atlantic deep water ventilation resulting from meltwater events of quaternary glacial terminations3 9 10

the 1994 java tsunami earthquake slip over a subducting seamount on june 2 1994 a large subduction thrust earthquake ms 7 2 produced a devastating tsunami on the island of java this earthquake had a number of unusual characteristics it was the first recorded large thrust earthquake on the java subduction zone all of the aftershock mechanisms exhibit normal faulting no mechanisms are similar to the main shock also the large tsunami and the relatively low energy radiated by the main shock have led to suggestions that this earthquake might have involved slow shallow rupture near the trench similar to the 1992 nicaragua earthquake we first relocate the main shock and the aftershocks we then invert long period surface waves and broadband body waves to determine the depth and spatial distribution of the main shock slip a dip of 12 hypocenter depth of 16 km and moment of 3 5 l020 n m mw 7 6 give the best fit to the combined seismic data and are consistent with the plate interface geometry the source spectrum obtained from both body and surface waves has a single corner frequency between 10 and 20 mhz implying a stress drop of 0 3 mpa the main energy release was preceded by a small subevent lasting 12 s the main slip occurred at 20 km depth downdip and to the nw of the hypocenter this area of slip is collocated with a prominent high in the bathymetry that has been identified as a subducting seamount we interpret the java earthquake as slip over this subducting seamount which is a locked patch in an otherwise decoupled subduction zone we find no evidence for slow shallow rupture no thrust aftershocks are expected if the entire locked zone slipped during the main shock but extension of the subducting plate behind the seamount would promote normal faulting as observed it seems probable that such a source model could also explain the size and timing of the observed tsunami

molecular stratigraphy a new tool for climatic assessment variations in sea surface temperatures over the past 500 000 years are inferred from the relative abundance behaviour of two organic compounds c37 alkenones over the upper 8 metres of a sediment core from the eastern equatorial atlantic this molecular record ascribed to contributions from prymnesiophyte algae correlates well with the variations in the 18 signal for the calcareous skeletons of certain planktonic foraminifera thus providing the first demonstration of a new stratigraphical technique which may be especially valuable where methods based on carbonate 18 fail

seismic history and seismotectonics of the sunda arc historic records of the last 300 years reveal two great interplate earthquakes 1833 mw 8 1861 mw 8 8 which ruptured major segments of the sumatra fore arc in western indonesia a significant percentage of interplate slip along this portion of the plate boundary appears to occur seismically the ends of these rupture zones are coincident with clusters of large ms 7 and moderate 6 ms 7 shocks and with heterogeneities in the plate interface as inferred from geologic and geophysical data the northern extent of the 1833 rupture zone is coincident with a group of moderate and large earthquakes a structural arch in the fore arc and a fracture zone on the subducted plate the southern portion is coincident with a very prominent cluster and the along strike extension of a sediment filled trough on the subducted plate prior to this study sumatra was characterized as relatively aseismic as inferred from the lack of great earthquakes in the instrumental record of this century java and the lesser sunda islands had major ms 6 earthquakes in the historic record but none are of the same magnitude as the great events near sumatra there is no clear evidence that shocks in the java fore arc were interplate events yet geophysical data indicate recent uplift of the trench slope break and a change in strike of the trench axis where the northernmost flank of the roo rise a prominent bathymetrie high appears to be interacting with the fore arc this feature was formed at a ridge crest and is more buoyant and thus more difficult to subduct than the surrounding seafloor there is no increase in seismicity associated with this tectonism however that may indicate a lack of contact between the crystalline portions of the plates at this time we infer that a majority of slip on the plate interface near java occurs aseismically the variation in occurrence of great interplate earthquakes along the sunda arc may be interpreted in terms of a model that attributes variation in interplate coupling to the age of the subducted lithosphere great interplate earthquakes occur near sumatra where the age of the youngest crust is 46 ma the characteristics of the sunda arc and analogies with pacific arcs imply that the entire length of sumatra has the potential to produce great thrust earthquakes its seismic potential should be considered high as there has been no recurrence of the great events of 1833 and 1861 in contrast no such events have been reported off java and the lesser sunda islands where the oldest crust is 152 ma the plate interface near java and the lesser sunda islands should be considered to have low seismic potential

the crustal structure of the cocos ridge off costa rica 1 the submarine cocos ridge in the northwestern panama basin a bathymetric feature more than 1000 km long and 250 500 km broad is about 2 km shallower than the adjacent basin it is generally interpreted as the trace of the galapagos hot spot two 127 and 260 km long seismic wide angle sections were recorded along and across this ridge offshore the osa peninsula costa rica crustal thickening is seen everywhere along the sections on the northwestern outer ridge flank increased thickness is exclusively attributed to the upper crust and expressed by 2 km thick flow basalts the quepos plateau caps the upper crust in this area toward the center of the cocos ridge the moho deepens from 11 12 to 21 km depth and crustal thickening is almost entirely attributed to the lower crust which makes up 80 of the crust and is three times the thickness of normal oceanic lower crust it is homogeneously structured and the velocities which range from 6 5 km s at the top to 7 35 km s at the base are comparable to normal lower crust under these depth conditions and suggest no differences to a gabbroic rock composition similarities to the crustal velocity structure of iceland central kerguelen plateau and broken ridge are consistent with a formation of this 13 15 ma old cocos ridge segment by excessive magmatism in a near plate boundary setting

upper plate deformation associated with seamount subduction in many active margins severe deformation is observed at the front of the overriding plate where seamounts or aseismic ridges subduct such deformation appears to be a main tectonic feature of these areas which influences the morphology and the seismicity of the margin to better understand the different stages of seamount subduction we have performed sandbox experiments to study in detail the evolution of deformation both in space and time and thus complement seismic images and bathymetry interpretation we focus in this paper on the surface deformation directly comparable with seafloor morphology two types of subducting seamounts were modelled relatively small conical seamounts and larger flat topped seamounts the indentation of the margin by the seamount inhibits frontal accretion and produces a re entrant the margin uplift includes displacement along backthrusts which propagate from the base of the seamount and out of sequence forethrusts which define a shadow zone located on the landward flank of the seamount when the seamount is totally buried beneath the margin this landward shielded zone disappears and a larger one is created in the wake of the asperity due to

postdepositional aerobic and anaerobic particulate organic matter degradation succession reflected by dinoflagellate cysts the madeira abyssal plain revisited abstract we report on the succession of selective degradation of dinoflagellate cyst species that can be considered representative for discrete particulate organic matter pom classes of different degradability the effects of anaerobic and aerobic degradation as well as bioturbation in a natural setting are documented in high resolution by means of palynological and geochemical analyses on madeira abyssal plain a and f turbidites these turbidites are unique as their initial ungraded sediments are affected by a downward penetrating oxydation front geochemical analyses document the presence of an active downward penetrating oxidation front in the a turbidite and a palaeo oxidation front in the f turbidite in this latter turbidite several zones can be distinguished from top to bottom an oxidised bioturbated zone an oxidised but not bioturbated zone a visible paleo oxidation front and a narrow nitrogenous zone overlying unoxidised sediments we are the first to report that anaerobic degradation within the nitrogenous zones in both turbidites affects cysts of some heterotrophic dinoflagellates the cyst species affected echinidinium aculeatum echinidinium spp cysts of protoperidinium monospinum and brigantedinium spp exponentially decrease in this zone that is further characterised by a strong decrease in sulphur content due to sulphide oxidation degradation rates are different for each species these cysts of heterotrophic dinoflagellates consist of a nitrogen rich glycan in contrast to the cellulosic cysts walls of phototrophic dinoflagellates therefore our observation supports the hypothesis that the quality of organic matter plays an important role in om degradation in oxygen deficient environments with n rich om being more labile than other components in these environments all heterotrophic species are strongly affected by aerobic degradation with their cyst concentrations exponentially decreasing with increasing oxygen exposure degradation rates vary between species and range von highly degradable to slightly degradable most strongly affected are cysts of protoperidinium monospinum brigantedinium spp and echinidinium spp increasingly less affected are selenopemphix nephroides selenopemphix quanta other peridinioids and echinidinium aculeatum an effect of aerobic degradation on photosynthetic species could only be observed for pentapharsodinium dalei all impagidinium species appeared to be resistant to aerobic degradation despite having analysed only a limited number of samples in the bioturbated zone of the f turbidite we have strong indications that the bioturbation can lead to the degradation of pom that is not affected by aerobic degradation alone the cyst species spiniferites ramosus impagidinium paradoxum lingulodinium machaerophorum nematosphaeropsis labyrinthus spiniferites spp impagidinium sphaericum spiniferites elongates and spiniferites mirabilis do not show concentration changes in the non bioturbated aerobic sediments but their concentrations are considerably decreased in the bioturbated part of the f turbidite this supports the hypothesis that bioturbation can increase the degradation of pom by e g increasing oxygen exposure time and or by an alteration of aerobic and anaerobic conditions we furthermore show that the degradation rates of pom components represented by individual cyst species differ between zones with different redox and biological conditions this implies that pom degradation reaction rate coefficients are environment dependent our observation that dinoflagellate cyst species have different degradation rates ranging from extremely labile to extremely recalcitrant within the individual redox bioturbation zones supports the hypothesis that cyst walls have a species specific molecular structure our results support continuum models of organic matter degradation that assume a continuous distribution of organic matter reactivity

subduction of a late cretaceous seamount of the louisville ridge at the tonga trench a model of normal and accelerated tectonic erosion the hotspot generated louisville ridge is a 4000 km chain of seamounts typically 2 2 5 km high and 10 40 km in diameter and an underlying crustal swell 1 5 km high and 100 km wide trending nnw across the southwestern pacific the northwest end of the ridge collides with the north trending tonga trench 26 s which just north of that point is exceptionally deep 10 8 km and lacks both a turbidite wedge and a bordering accretionary complex the collision zone is moving rapidly southward multichannel seismic reflection data in the collision zone show a west dipping subsurface platform 2 3 km beneath the lower western trench slope which is interpreted as the flat summit of a subducted guyot motuku of the louisville chain projected eastward the summit plain passes 1 2 km above the trench floor dredging of the nearby inner trench wall recovered uppermost cretaceous maestrichtian oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted louisville seamount the principal effects of hotspot ridge collision with a sediment starved trench are 1 the impacting seamounts are subducted rather than accreted and 2 although some seamount rocks are temporarily accreted the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km m y accelerated tectonic erosion is related to 1 fracturing shearing and general weakening of arc substrate rocks as they are lifted by the swell penetrated by impacting seamounts and left to collapse as the ridge moves away 2 a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell 3 a possible elevation of the subduction decollement to account for the removal of as much as 30 000 km of material from a 400 km sector of the trench in 1 million years and 4 a reduction in supply of arc derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge normal tectonic erosion in the tonga trench is apparently minor and we conclude that the bulk of the 37 000 km of material which fills subducting grabens each million years is arc derived volcanic and pelagic sediment