a subducting seamount beneath the nankai accretionary prism off shikoku southwestern japan a multi channel seismic mcs reflection survey was conducted to study the structure of the nankai convergent margin off shikoku island in july 1997 based on reflection characteristics we could identify three major seismic reflection units i e units a b and c the mcs data as well as swath bathymetric data reveal a buried circular seamount subducting beneath the nankai accretionary prism the subducting seamount is responsible for deformation of the accretionary wedge resulting in a compressed uplifted sediment knoll and many steep escarpments with ne sw strike a possible thrust fault was identified on the seaward flank of the seamount indicating compressive deformation a tectonic model for subduction of the seamount is presented based on the interpretation of the mcs data

three dimensional splay fault geometry and implications for tsunami generation megasplay faults very long thrust faults that rise from the subduction plate boundary megathrust and intersect the sea floor at the landward edge of the accretionary prism are thought to play a role in tsunami genesis we imaged a megasplay thrust system along the nankai trough in three dimensions which allowed us to map the splay fault geometry and its lateral continuity the megasplay is continuous from the main plate interface fault upwards to the sea floor where it cuts older thrust slices of the frontal accretionary prism the thrust geometry and evidence of large scale slumping of surficial sediments show that the fault is active and that the activity has evolved toward the landward direction with time contrary to the usual seaward progression of accretionary thrusts the megasplay fault has progressively steepened substantially increasing the potential for vertical uplift of the sea floor with slip we conclude that slip on the megasplay fault most likely contributed to generating devastating historic tsunamis such as the 1944 moment magnitude 8 1 tonankai event and it is this geometry that makes this margin and others like it particularly prone to tsunami genesis

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

basin centered asperities in great subduction zone earthquakes a link between slip subsidence and subduction erosion 1 published areas of high coseismic slip or asperities for 29 of the largest circum pacific megathrust earthquakes are compared to forearc structure revealed by satellite free air gravity bathymetry and seismic profiling on average 71 of an earthquake s seismic moment and 79 of its asperity area occur beneath the prominent gravity low outlining the deep sea terrace 57 of an earthquake s asperity area on average occurs beneath the forearc basins that lie within the deep sea terrace in sw japan slip in the 1923 1944 1946 and 1968 earthquakes was largely centered beneath five forearc basins whose landward edge overlies the 350 c isotherm on the plate boundary the inferred downdip limit of the locked zone basin centered coseismic slip also occurred along the aleutian mexico peru and chile subduction zones but was ambiguous for the great 1964 alaska earthquake beneath intrabasin structural highs seismic slip tends to be lower possibly due to higher temperatures and fluid pressures kilometers of late cenozoic subsidence and crustal thinning above some of the source zones are indicated by seismic profiling and drilling and are thought to be caused by basal subduction erosion the deep sea terraces and basins may evolve not just by growth of the outer arc high but also by interseismic subsidence not recovered during earthquakes basin centered asperities could indicate a link between subsidence subduction erosion and seismogenesis whatever the cause forearc basins may be useful indicators of long term seismic moment release the source zone for cascadia s 1700 a d earthquake contains five large basin centered gravity lows that may indicate potential asperities at depth the gravity gradient marking the inferred downdip limit to large coseismic slip lies offshore except in northwestern washington where the low extends landward beneath the coast transverse gravity highs between the basins suggest that the margin is seismically segmented and could produce a variety of large earthquakes

when seamounts subduct data from an array of seismometers on the sea floor show the complex pattern of earthquakes around subducted seamounts

cross section restoration of salt related deformation best practices and potential pitfalls abstract cross section restoration typically assumes plane strain deformation and area conservation constraints that are usually invalid for salt because of its characteristic three dimensional flow and possible dissolution thus restoration of salt related deformation provides added challenges and uncertainty in this review paper we summarize the historical development of ideas methods and applications of restoration in salt basins while most published restorations do not maintain salt area constraints on its variation range from arbitrary assumptions to quantitatively incorporating isostatic calculations we illustrate several scenarios in which the presence of salt adds ambiguity to restoration primarily because it can hide deformation diapirs can widen during extension and narrow during shortening translating overburden can move into salt and drive allochthonous advance secondary minibasin subsidence can be accommodated at both shallow and deep salt levels and allochthonous salt can record evacuation of deeper salt although we caution against using restoration to test and validate small scale details of interpretations we emphasize that sequential restoration remains an essential tool in structural and basin analyses however because of the uncertainties a regional three dimensional approach and sound geological reasoning are critical for deriving meaningful and useful results from cross section restoration of salt structures

long term evolution of an accretionary prism the case study of the shimanto belt kyushu japan the shimanto belt in sw japan is commonly described as a paleo accretionary prism whose structure is explained by continuous accretion like in modern accretionary prisms such as nankai we carried out a structural study of the cretaceous to miocene part of the shimanto belt on kyushu to test this hypothesis of continuous accretion most deformation structures observed on the field are top to the se thrusts fitting well the scheme of accretionary wedge growth by frontal accretion or underplating in particular the tectonic melange at the top of the hyuga group records a penetrative deformation reflecting burial within the subduction channel in contrast we documented two stages of extension that require modifying the traditional model of the belt as a simple giant accretionary wedge the first one in the early middle eocene is mostly ductile and localized in the foliated bases of the morotsuka and kitagawa groups the second one postdating the middle miocene is a brittle deformation spread over the whole belt on kyushu integrating these new tectonic features to existing data we propose 2 d reconstructions of the belt evolution leading to the following conclusions 1 erosion and extension of the margin in the early middle eocene resulted from the subduction of a trench parallel ridge 2 the late eocene to early miocene evolution is characterized by rapid growth of the prism followed by a middle miocene stage where large displacements occurred along low angle out of sequence thrusts such as the nobeoka tectonic line 3 from middle miocene the strain regime was extensional

neogene subsidence and stratigraphy of the simeulue forearc basin northwest sumatra abstract interpretation of new multi channel seismic data reveals the neogene subsidence history of the simeulue forearc basin located off northwestern sumatra the trench parallel elongated basin extends over 260 km in nw se direction and 100 km in sw ne direction maximum water depth is about 1300 m and the neogene sedimentary fill shows a thickness of up to 5 s two way traveltime we identified three stages of subsidence evolution after the formation of a regional basal neogene unconformity which are characterized by changing accumulation space sedimentation rates source areas and tectonic phases an early mid miocene stage is marked by occurrence of subsidence in several half grabens forming along the western border of the basin during the late miocene pliocene this small scale subsidence changed to a steadily subsiding trench parallel trough the present setup of the forearc region under the influence of strike slip faults due to oblique subduction is active at least since this time as evidenced by wrench faulting at the end of this stage subsidence expanded significantly eastwards drowning a large carbonate platform that evolved in the then shallow southern and eastern parts of the basin presently the central part of the simeulue basin is subject to inversion probably related to reactivation of early mid miocene half grabens

subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake the nankai trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century with a recurrence time of 100 to 200 years the 1946 nankaido earthquake was unusual with a rupture zone estimated from long period geodetic data that was more than twice as large as that derived from shorter period seismic data in the center of this earthquake rupture zone we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers we propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture

controls on active forearc basin stratigraphy and sediment fluxes the pleistocene of hawke bay new zealand detailed high resolution documentation of forearc basin fi ll is scarce in the literature in this geological and geophysical study we investigated the pleistocene sedimentary record of the tectonically active hawke bay forearc domain of the hikurangi subduction margin of new zealand interpretation of an extensive seismic refl ection data set that is correlated with marine cores and onshore geological maps identifi es the detailed stratigraphic architecture of the last 1 1 m y this analysis reveals the infl uences and interactions of tectonic deformation climate eustasy and isostasy on forearc basin sedimentation eleven 100 k y depositional sequences are recognized in the basin fi ll thus highlighting the dominance of pleistocene climate eustasy on sequence development the stacking pattern and isopach maps of sequences exhibit an overall retrogradational trend and an arcward migration of depocenters these trends progressively develop a basinwide diachronous and composite erosion unconformity formed by the lateral succession and landward encroachment of the 12 sequence bounding unconformities s12 to s1 among these the s5 surface ca 430 ka is an angular unconformity that separates major megasequences of the sedimentary record the forearc domain evolved from a series of ridge parallel basins to a succession of connected basins that have progressively developed around major growing thrust faulted ridges since ca 430 ka this change in basin confi guration and associated signifi cant increase of the preserved sediment fluxes occurred synchronous with the reactivation of major out of sequence thrusts and the completion of the mid pleistocene transition

evolution of tectono sedimentary systems in the kumano basin nankai trough forearc abstract sedimentary deposits in the distal kumano forearc basin of the nankai accretionary margin off kii peninsula japan have been imaged using three dimensional 3d seismic data the seismic data along with logging and core data from the integrated ocean drilling program iodp show that the unconformity between the accretionary prism and overlying forearc sediments is time transgressive the unconformity at site c0002 separates 5 ma prism rocks from 3 65 ma basin deposits at site c0009 it separates 5 6 ma prism from 3 8 ma basin sediments acoustic reflections in the basal deposits are sub parallel to the underlying accretionary prism the acoustic facies varies in thickness from 50 to 750 m the mudstone deposits and laterally equivalent turbidites are interpreted as lower trench slope deposits the condensed slope sediment ss section decreases in age from 3 5 to 1 5 ma at site c0002 to 1 5 0 9 ma at c0009 acoustic sequences within the lower forearc basin lfb contain higher proportions of silt and sand turbidites and progressively onlap the ss unit along a low angle discontinuity kl in a landward direction because of the landward onlap of the lfb unit the oldest lfb strata at c0002 are older than 1 67 ma whereas those at c0009 are younger than 0 9 ma thus the amount of time missing or characterized by condensed sedimentation across the kl unconformity decreases in duration in the landward direction the landward onlapping sequences tilt progressively landward in response to regional uplift along an out of sequence thrust oost mega splay fault regional tilting shifted the basin s depocenter progressively landward expanding that part of the basin from 10 km in width to 30 km the onset of sand silt turbidite deposition in the distal basin began after more accommodation space was created by the uplift of the outer ridge along the splay fault at 1 9 ma conversely turbidites of the upper forearc basin ufb progressively onlap lfb in a seaward direction furthermore the respective thicknesses of the lfb and ufb units switch from the seaward side of the basin c0002 farther landward c0009 the lfb unit is 800 m thick in the seaward region whereas it is only 200 300 m thick in the landward region the ufb unit is

structural architecture and active deformation of the nankai accretionary prism japan submersible survey results from the tenryu submarine canyon two research cruises that deployed submersible surveys were undertaken along the tenryu submarine canyon to directly observe the structural architecture of the eastern nankai accretionary prism off the coast of southern japan the surveys have demonstrated that the accreted sediments are strongly deformed turbidite sequences that occur in repeated thrust anticline structures from the leading edge of the prism near the trench toward the arc the following deformation zones have been identified within the prism frontal thrust zone prism toe zone imbricate thrust zone and tokai thrust zone or out of sequence thrust or oost zone the frontal thrust zone is characterized by debris deposits within the hanging wall that have an age of 0 0 43 ma as determined from radiolarian biostratigraphy the prism toe zone is characterized by unconsolidated turbidite sequences that are 1 98 3 4 ma these sequences are cut by normal and thrust faults the imbricate thrust zone includes consolidated muddy layers and unconsolidated sandy layers that contain numerous fracture cleavages the oost zone consists of highly deformed consolidated sediments ranging in age from 0 18 to 1 03 ma from the prism toe zone to the imbricate thrust zone the uniaxial compressive strength increases gradually from 0 5 3 0 to 1 0 6 0 mpa while the anisotropy of magnetic susceptibility changes from oblate to prolate shapes and porosity decreases from 40 50 to 30 50 these data indicate that the eastern nankai accretionary prism appears to have been deformed toward the imbricate thrust zone just south of the oost stable isotope analyses of calcite veins and calcite cement of the sandstone samples from the tokai thrust zone have shown that fluid temperatures for calcite precipitation were 24 63 c in the oost zone the occurrence of highly deformed and consolidated rocks within the nankai accretionary prism likely resulted from tectonic transportation of deeply buried rocks along major out of sequence thrust faults such as the tokai oost we infer therefore that out of sequence thrust faults play a major role in transporting deeply buried deformed rocks in accretionary prisms to the shallower depths and even to the seafloor during ongoing subduction

hydrocarbon seepage and its sources at mud volcanoes of the kumano forearc basin nankai trough subduction zone twelve submarine mud volcanoes mv in the kumano forearc basin within the nankai trough subduction zone were investigated for hydrocarbon origins and fluid dynamics gas hydrates diagnostic for methane concentrations exceeding solubilities were recovered from mvs 2 4 5 and 10 molecular ratios c1 c2 40 v pdb indicate that hydrate bound hydrocarbons hcs at mvs 2 4 and 10 are derived from thermal cracking of organic matter considering thermal gradients at the nearby iodp sites c0009 and c0002 the likely formation depth of such hcs ranges between 2300 and 4300 m below seafloor mbsf with respect to basin sediment thickness and the minimum distance to the top of the plate boundary thrust we propose that the majority of hcs fueling the mvs is derived from sediments of the cretaceous to tertiary shimanto belt below pliocene pleistocene to recent basin sediments considering their sizes and appearances hydrates are suggested to be relicts of higher mv activity in the past although the sporadic presence of vesicomyid clams at mv 2 showed that fluid migration is sufficient to nourish chemosynthesis based organisms in places distributions of dissolved methane at mvs 3 4 5 and 8 pointed at fluid supply through one or few mv conduits and effective methane oxidation in the immediate subsurface the aged nature of the hydrates suggests that the major portion of methane immediately below the top of the methane containing sediment interval is fueled by current hydrate dissolution rather than active migration from greater depth

a seismic stratigraphic analysis of mariana forearc basin evolution new seismic data collected in the 14 5 18 5 n mariana segment of the izu bonin mariana island arc system image six seismic stratigraphic sequences that can be mapped throughout the inner forearc these sediments were most likely deposited from 35 ma to the present the oldest stratigraphic units 1 2 and 3 are syn rift volcaniclastic deposits unit 4 deposits accumulated during a period of mild structural inversion which resulted in several isolated reverse faulted anticlines within the forearc sedimentary basin a late period of extensional deformation began near the end of unit 5 deposition and continued through unit 6 sedimentation to the present seismic lines show that the basement of the forearc is composed of large rotated fault blocks and half grabens with ne nw and nne trends fault offset calculations show that basement faults with dips between 45 and 50 account for only 4 total extension in the forearc south of 16 3 n normal growth faults initiated during basement extension offset the frontal arc high from a deep forearc basin from correlations with the known geologic history we hypothesize that extension during deposition of units 1 through 3 corresponds to rifting of the eo oligocene arc between 35 ma and 29 ma older deposits being too thin to be seismically resolvable localized compression during unit 4 accumulation occurred some time after eo oligocene rifting in the early miocene late stage normal faulting near the end of deposition of unit 5 and throughout unit 6 accumulation may be associated with the opening of the mariana trough backarc basin from 8 ma years to the present there is a higher density of these later faults in the inner forearc between 15 5 and 17 n and in the outer forearc between 14 n and 18 n recent extension is at least partially accommodated by reactivation of older basement faults with the same ne nw and nne trends stratigraphic relationships indicate that the inner forearc south of 16 3 n has differentially subsided and tilted trenchward possibly as a result of a recent change in subducting slab geometry or subducted relief under the forearc

analysis of normal fault populations in the kumano forearc basin nankai trough japan 1 multiple orientations and generations of faults from 3 d coherency mapping 1 analyses of normal faults in the kumano forearc basin of the nankai trough reveal multiple normal fault populations in a region generally thought to be under compression most faults have offsets of less than 20 m and dips of 60 70 and show no growth structures indicating that the faults were active for short periods of time the oldest generation of faults is older than 0 9 ma and strikes 50 60 the next oldest faults strike 160 170 are older than 0 44 ma and are related to local uplift along the western edge of the region the youngest faults cut the seafloor shallow faults near the se margin of the basin curve from 100 in the middle of the survey area to 145 at the se corner of the area the pattern of the two youngest fault populations is consistent with the regional stress pattern maximum horizontal stress subparallel to the trench orientations of older fault populations are caused by uplift of the underlying accretionary prism implying that the forearc basin region is not as stable as previously thought reconstruction of displacements on the youngest faults shows that the overall horizontal extension is less than 2 concentrated near the seaward edge of the basin the active normal faults distributed throughout the basin support the idea that the horizontal stress parallel to the plate convergence direction does not reach the critical stress to activate or form thrust faults and produce horizontal shortening within the shallow portion of the inner wedge

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

analysis of normal fault populations in the kumano forearc basin nankai trough japan 2 principal axes of stress and strain from inversion of fault orientations 1 we use a high resolution 3 d seismic survey to map a population of recent normal faults within the kumano basin of the nankai subduction zone in order to quantify patterns of strain and stress state over the last 0 44 myr we identify distinct fault populations that define three phases of extension phases 1 and 2 comprise nw se striking faults located along the western basin edge and in the northwestern portion of the study area respectively the ne sw striking faults of phase 3 comprise the largest population and extend 20 km landward from the basin s seaward edge phase 2 faults typically terminate within a few reflectors of the seafloor whereas most phase 3 faults form seafloor scarps inversion of the fault populations documents ne sw extension during phases 1 and 2 and nw se extension during phase 3 consistent with both core scale structures and horizontal stress orientations observed at integrated ocean drilling program iodp boreholes slip on phase 3 faults accommodates strain of up to 1 2 concentrated near the basin s seaward edge inversion for a best fit stress tensor yields a subvertical 1 and subhorizontal 2 and 3 for all faulting phases we find that during phase 3 in most portions of the basin 2 3 shmax shmin reflecting widely varying fault strikes this contrasts with distinct shmax and shmin magnitudes inferred from iodp borehole data these observations may be reconciled if the orientation of maximum horizontal stress fluctuates due to variation of subduction parallel compression through the seismic cycle

mid quaternary decoupling of sediment routing in the nankai forearc revealed by provenance analysis of turbiditic sands coring during integrated ocean drilling program expeditions 315 316 and 333 recovered turbiditic sands from the forearc kumano basin site c0002 a quaternary slope basin site c0018 and uplifted trench wedge site c0006 along the kumano transect of the nankai trough accretionary wedge offshore of southwest japan the compositions of the submarine turbiditic sands here are investigated in terms of bulk and heavy mineral modal compositions to identify their provenance and dispersal mechanisms as they may reflect changes in regional tectonics during the past ca 1 5 myrs the results show a marked change in the detrital signature and heavy mineral composition in the forearc and slope basin facies around 1 ma this sudden change is interpreted to reflect a major change in the sand provenance rather than heavy mineral dissolution and or diagenetic effects in response to changing tectonics and sedimentation patterns in the trench slope basin the sands older than 1 ma were probably eroded from the exposed cretaceous tertiary accretionary complex of the shimanto belt and transported via the former course of the tenryu submarine canyon system which today enters the nankai trough northeast of the study area in contrast the high abundance of volcanic lithics and volcanic heavy mineral suites of the sands younger than 1 ma points to a strong volcanic component of sediment derived from the izu honshu collision zones and probably funnelled to this site through the suruga canyon however sands in the forearc basin show persistent presence of blue sodic amphiboles across the 1 ma boundary indicating continuous flux of sediments from the kumano kinokawa river this implies that the sands in the older turbidites were transported by transverse flow down the slope the slope basin facies then switched to reflect longitudinal flow around 1 ma when the turbiditic sand tapped a volcanic provenance in the izu honshu collision zone while the sediments transported transversely became confined in the kumano basin therefore the change in the depositional systems around 1 ma is a manifestation of the decoupling of the sediment routing pattern from transverse to long distance axial flow in response to forearc high uplift along the megasplay fault

splay fault branching along the nankai subduction zone seismic reflection profiles reveal steeply landward dipping splay faults in the rupture area of the magnitude m 8 1 tonankai earthquake in the nankai subduction zone these splay faults branch upward from the plate boundary interface that is the subduction zone at a depth of approximately 10 kilometers approximately 50 to 55 kilometers landward of the trough axis breaking through the upper crustal plate slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion

deep sea sedimentation controlled by sea level rise during the last deglaciation an example from the kumano trough japan abstract an approach combining interpretation of seismic reflection data with analysis of core lithology especially analysis of the depositional ages of turbidites showed that depositional patterns in the deep sea basin of the eastern kumano trough which is a forearc basin along an active convergent plate margin were different during sea level lowstand slow transgressive rapid transgressive and highstand stages during the lowstand and early transgressive stages turbidites were deposited on the deep sea basin floor where they formed a submarine fan composed of terrigenous sediments supplied by rivers via a submarine canyon subsequent coastline retreat and bay development during the rapid transgression stage caused a decrease in the amount of coarse grained terrigenous sediments reaching the basin floor however turbidites were still deposited within the submarine canyon during the rapid transgressive and highstand stages these deposits contain shelf sediments that have been reworked by tidal currents and storms and sediments from slope failures caused by submarine earthquakes but they do not contain the terrigenous sediments that are characteristic of the turbidites of the lowstand and slow transgression stages

depositional age and triggering event of turbidites in the western kumano trough central japan during the last ca 100 years abstract for correlating a turbidite record with a detailed land record of natural disasters during the last ca 100 years the depositional age of the surface hemipelagic sediments of the kumano trough located in central japan were estimated we attempted to extract mass accumulation rate of hemipelagic fallout from hemipelagic intervals on the basis of 137 cs geochronology x radiograph dry bulk density and grain size distribution based on assumptions of the biogenic mixing effect on the 137 cs appearance horizon and hemipelagite erosion by turbidity current the reconstructed turbidite record from hemipelagic fallout mass accumulation rate is well correlated with the record of major natural disasters in the kumano area based on the correlation most of turbidite layers are flood storm origin except for a well sorted seismo turbidite induced by the tonankai earthquake that occurred the mid 20th century although the wide distribution of a flood induced turbidite layer due to the 19th century tostukawa flood which was a huge disaster in the area caused by heavy rain and dams breaching it does not show a hyperpycnite succession the flood by dams breaching may cause surge like flow angular to subangular sand sized lithic fragments in the turbidite show that sand grains were transported from the kumano river directly plant debris and microfossils are common in both seismo turbidite and flood storm induced turbidites

possible strain partitioning structure between the kumano fore arc basin and the slope of the nankai trough accretionary prism 1 a 12 km wide 56 km long three dimensional 3 d seismic volume acquired over the nankai trough offshore the kii peninsula japan images the accretionary prism fore arc basin and subducting philippine sea plate we have analyzed an unusual trench parallel depression a notch along the seaward edge of the fore arc kumano basin just landward of the megasplay fault system this bathymetric feature varies along strike from a single steep walled 3 5 km wide notch in the northeast to a broader 5 km wide zone with several shallower linear depressions in the southwest below the notch we found both vertical faults and faults which dip toward the central axis of the depression dipping faults appear to have normal offset consistent with the extension required to form a bathymetric low some of these dipping faults may join the central vertical fault s at depth creating apparent flower structures offset on the vertical faults is difficult to determine but the along strike geometry of these faults makes predominantly normal or thrust motion unlikely we conclude therefore that the notch feature is the bathymetric expression of a transtensional fault system by considering only the along strike variability of the megasplay fault we could not explain a transform feature at the scale of the notch strike slip faulting at the seaward edge of fore arc basins is also observed in sumatra and is there attributed to strain partitioning due to oblique convergence the wedge and decollement strength variations which control the location of the fore arc basins may therefore play a role in the position where an along strike component of strain is localized while the obliquity of convergence in the nankai trough is comparatively small 15 we believe it generated the kumano basin edge fault zone which has implications for interpreting local measured stress orientations and suggests potential locations for strain partitioning related deformation in other subduction zones

the java convergent margin structure seismogenesis and subduction processes the java margin is characterized by a distinct variation in lower to upper plate material transfer and recurring catastrophic tsunamogenic earthquakes both processes are closely linked to the subduction of oceanic basement relief resulting in varying degrees of fore arc deformation tomographic models of refraction seismic profiles and reflection seismic lines in combination with high resolution multibeam bathymetric data reveal the variability in the deep structure and deformation of the java fore arc shallow subduction processes are governed by the sediment supply in the trench as well as by the nature and fabric of the oceanic lithosphere the deep structure of the fore arc reveals a shallow upper plate crust mantle transition present along the entire java margin section the serpentinized fore arc mantle wedge governs the depth extent of the seismogenic zone here which is narrower compared to its sumatran analogue in addition offshore central java high relief oceanic basement features potentially act as asperities as well as barriers to seismic rupture limiting the possible magnitude of subduction thrust earthquakes however the potential for geohazards in particular tsunamis is high along the entire margin this results from tsunamogenic earthquakes ubiquitous splay faults and potentially tsunamogenic landslides which further increase the risk of future tsunamis

detailed structural image around splay fault branching in the nankai subduction seismogenic zone results from a high density ocean bottom seismic survey 1 to investigate megathrust earthquake and tsunami generation in a subduction seismogenic zone it is important to know the detailed structure around the plate boundary and active splay fault system the nankai trough southwestern japan is among the best studied subduction zones with splay faults in the world this paper presents a detailed structural image around the splay fault in the coseismic rupture zone of the 1944 tonankai earthquake in the central nankai trough based on results from a wide angle high density ocean bottom seismograph obs survey our seismic image clearly shows for the first time the subduction structure along with the splay fault reflections imaged on a previously obtained seismic reflection profile the correspondence between the splay fault reflections imaged on the seismic reflection profile and our reflectivity image was confirmed by converting the time migrated section to a depth section by using a velocity model estimated from the wide angle obs data in this study the high resolution seismic image obtained by the high density obs survey demonstrates a zone of low velocity gradient just above and along the splay fault we focused specifically on the detailed structure around the splay fault a simple amplitude analysis of the wide angle reflections showed that it is likely that a thin low velocity layer with velocities 0 5 1 5 km s slower than those in the surrounding rock exists above the splay fault the presence of this layer suggests elevated fluid pressure in the fault zone which cut through the relatively homogeneous rock of the accretionary prism and generated past megathrust earthquakes

early sedimentation and deformation in the kumano forearc basin linked with nankai accretionary prism evolution southwest japan early forearc basin sedimentation and subsidence appear to be linked with convergent margin evolution yet they do not follow predictable spatiotemporal patterns in the kumano basin of southwest japan 3 d seismic mapping of major unconformities combined with biostratigraphic age constraints from two integrated ocean drilling program iodp drill sites illustrates the development of several tectonostratigraphic packages during early evolution and initial creation of accommodation space in the forearc basin between 3 8 ma and 2 06 ma a series of slope basins developed between thrust anticlines oriented along a plate convergence perpendicular axis a thin and sheet like interval likely slope cover unconformably overlies the slope basin sediments deposition of this interval at iodp site c0009 lasted from 2 06 ma until between 1 24 ma and 1 34 ma and was partly coeval with the early upper kumano basin sedimentary packages landward postdating the slope cover sediments a third sedimentary package was deposited before 0 9 ma this section correlates with an interval rich in terrigenous material e g woody debris indicating a possible distinct source or sediment routing system our work supports the idea that early forearc basin sediments may be deposited in an actively deforming outer wedge environment and that the outer to inner wedge transition of the lower forearc basin is likely a multistage process the unsteady spatiotemporal nature of inner outer wedge coupling may lead to erratic stratigraphic patterns such as have been observed in forearc basins worldwide

the lithium boron and strontium isotopic systematics of groundwaters from an arid aquifer system implications for recharge and weathering processes abstract saline groundwaters are common to inland australia but their hydrochemical evolution and origin remain largely unknown the saline groundwaters in the alluvial aquifers of the darling river have previously been found to exhibit broad similarity in traditional hydrochemical and isotopic tracers by contrast in this study the trace element isotopes 7li 11b and 87sr 86sr have illuminated more complex hydrogeochemical processes in the same aquifer system this paper reports the first ever set of 7li values in any groundwater system in australia they varied from 5 8 to 16 2 with an average value of 9 7 n 19 in the alluvial aquifers of the darling river catchment the 11b values were all higher than seawater and close to some of the highest 11b values ever reported in the literature for a groundwater system 44 4 to 53 9 average 48 8 n 17 the 87sr 86sr ratios ranged from 0 708 to 0 713 with an average value of 0 709 n 19 the differing signatures in these trace element isotope values highlighted by discovery of the deeper older groundwater system with heavier li isotope values and higher 87sr 86sr is an important finding of this research simple mixing models between river water and saline groundwater cannot explain the observed variation in trace element isotopes hydrochemical evolution was found to be dependent on proximity to the darling river and depth varying degrees of li and b isotopic fractionation during water sediment interaction were interpreted to account for the evolution of the saline groundwaters the measurement of these trace element isotopes has permitted delineation of groundwater end members that would have otherwise not been identified in their absence an inaccurate interpretation of the hydrochemical evolution of these saline groundwaters would have been made this study highlights the importance of a multi tracer approach which includes trace element isotopes in resolving complex geochemical processes in groundwater in semi arid to arid zone environments

crustal velocity field of southwest japan subduction and arc arc collision we investigate crustal deformation in southwest japan over a 3 year period revealed by a permanent dense global positioning system gps array southwest japan is a part of the amurian plate a microplate moving about 10 mm yr toward the east with respect to the eurasian plate it overrides the philippine sea plate at the nankai trough and collides with the northeast japan arc in the central part of japan in this paper we first derive gps site velocities relative to the stable part of the amurian plate in order to isolate signals of crustal deformation caused by the subduction and or the collision the velocity field has a conspicuous feature indicating the interseismic elastic loading by the philippine sea plate slab at the nankai trough characterized by the northwestward movements of points throughout the studied area their amplitudes are the largest at the pacific coast and decay toward the japan sea coast with a subtle systematic shift of azimuths a model assuming an elastic half space the convergence rate at the nankai trough based on a refined euler vector and the strength of the coupling inferred from a thermal model could explain the velocity field in the western part of the studied area to a large extent those in the eastern part systematically deviate from them and the residual components there show east west shortening and north south extension this may represent crustal thickening and trenchward extrusion of crustal blocks caused by the collision between southwest and northeast japan this suggests that the collision between southwest and northeast japan gives rise to not only crustal thickening but also trenchward extrusion of crustal block a velocity contrast was found across the median tectonic line the largest inland active fault in japan but the current permanent gps network is not dense enough for us to discuss its coupling depth

stratigraphy of the kazusa group boso peninsula an expanded and highly resolved marine sedimentary record from the lower and middle pleistocene of central japan major climatic reorganizations including changes in the nature of glacial interglacial cycles through the pleistocene are a key issue for improving the understanding of earth s climate system highly resolved marine sedimentary records are essential to reconstruct the details of these past climatic changes and investigate the mechanisms responsible for them the kazusa group located in the central part of the japanese island chain is well exposed and contains a remarkably continuous and thick deep and shallow water marine sedimentary succession this group also contains well preserved marine microfossils pollen paleomagnetic reversal events geochemical signatures and a large number of tephra beds these features allow us to establish a robust chronological and stratigraphic framework for the kazusa group and provide a rare opportunity to study oceanic and terrestrial climatic and environmental changes at high resolution especially through the lower and lower middle pleistocene in the boso peninsula chiba prefecture the kazusa group is deeply incised yielding spectacular river cut exposures the matuyama brunhes paleomagnetic reversal is observed immediately above the widespread byakubi e byk e tephra in the chiba composite section within the kokumoto formation kazusa group because the matuyama brunhes boundary customarily serves as the primary guide for the lower middle pleistocene subseries boundary the chiba composite section is considered an excellent candidate for its global boundary stratotype section and point gssp this study reviews the published data for the kazusa group hitherto almost exclusively in japanese in order to place the chiba composite section within its broader depositional and chronostratigraphic context

depositional architecture provenance and tectonic eustatic modulation of miocene submarine fans in the shikoku basin results from nankai trough seismogenic zone experiment seismostratigraphy coring and logging while drilling during integrated ocean drilling program expeditions 319 322 and 333 sites c0011 c0012 show three miocene submarine fans in the ne shikoku basin with broadly coeval deposits at ocean drilling program site 1177 and deep sea drilling project site 297 nw shikoku basin the sediment dispersal patterns have major implications for paleogeographies at that time the oldest finer grained kyushu fan has sheet like geometry quartz rich flows were fed mostly from an ancestral landmass in the east china sea during prolonged hemipelagic mud deposition at c0011 c0012 similar to 12 2 to 9 1 ma sand supply continued at sites 1177 and 297 sand delivery to much of the shikoku basin halted during a phase of sinistral strike slip to oblique plate motion after which the daiichi zenisu fan similar to 9 1 to 8 0 ma was fed by submarine channels the youngest fan daini zenisu similar to 8 0 to 7 6 ma has sheet like geometry with thick bedded coarse grained pumiceous sandstones the pumice fragments were fed from a mixed provenance that included the collision zone of the izu bonin and honshu arcs the shift from channelized to sheet like flows was favored by renewal of relatively rapid northward subduction which accentuated the trench as a bathymetric depression increased sand supply appears to correlate with long term eustatic lowstands of sea level the stratigraphic position and 3 d geometry of the sandbodies have important implications for subduction related processes including the potential for focused fluid flow and fluid overpressures above and below the plate boundary fault in sheet like sands pathways for fluid flow have greater horizontal permeability compared with those in channel sands

rapid forearc basin uplift and megasplay fault development from 3d seismic images of nankai margin off kii peninsula japan abstract offshore kii peninsula japan a large thrust within the overriding forearc the megasplay fault appears to move coseismically during great earthquakes 3d seismic images of the kumano forearc basin that overlies the megasplay correlated with iodp drilling data are a potential record of the history of large scale motion along this structure in the early quaternary uplift occurred in the southwest portion of the basin that may be a preliminary phase of motion along the megasplay more extensive landward tilting of the outer basin sediments across the seismic volume occurred over 300 kyr in the middle to late quaternary 1 3 1 ma this tilting event may represent the major period of motion along the megasplay that formed the modern fault geometry extensive normal faulting that cuts the forearc basin sediments clearly formed subsequent to the late quaternary tilting and in many cases offset the modern seafloor these faults may form either due to gravitational response to the uplift or as a by product of sediment underthusting these results suggest that the megasplay is a recently formed and transient structure and support the idea that out of sequence thrusts serving as the dominant structure for convergence driven shortening in a subduction zone may be short lived geologically but dominate a margin during these intervals

massive methane release triggered by seafloor erosion offshore southwestern japan vast amounts of methane hydrate exist beneath continental margins but whether this methane releases from sediment on a large scale and affects the oceans and atmosphere remains unclear analysis of newly acquired three dimensional seismic images and drilling data from a large gas hydrate province reveal a recently eroded v shaped depression the depression sharply cuts through a relic bottom simulating reflection bsr and hydrate laden sediments the shape of the relic bsr indicates that the seafloor depression was once a large anticline that has recently been eroded and released an estimated 1 51 10 11 m 3 of methane we hypothesize that erosion of the seafloor via bottom water currents unroofed buoyant hydrate laden sediments and subhydrate overpressured free gas zones beneath the anticline once triggered gas driven erosion created a positive feedback mechanism releasing gas and eroding hydrate bearing sediment we suggest that erosive currents in deep water methane hydrate provinces act as hair triggers destabilizing kilometer scale swaths of the seafloor where large concentrations of underlying overpressured methane exist our analysis suggests that kilometer scale degassing events are widespread and that deep water hydrate reservoirs can rapidly release methane in massive quantities

sediment flow routing during formation of forearc basins constraints from integrated analysis of detrital pyroxenes and stratigraphy in the kumano basin japan the evolution of sediment flow routing during complete evolution of the kumano forearc basin is determined through integration of stratigraphic and sediment provenance analyses in the upper nankai forearc a new approach uses the compositional variability of detrital clinopyroxenes and orthopyroxenes collected at eight major rivers in japan and three drill sites in the basin and nearby slope environment including the first drill cuttings retrieved by the integrated ocean drilling program iodp joint interpretation of these datasets reveals that the sedimentation history of the basin is characterised by three main phases separated by newly recognised time transgressive boundaries we show that the kumano basin initiated as a trench slope basin in the early quaternary 1 93 ma and that it progressively evolved towards an upper slope environment with increased turbidite confinement and influence from climatic forcing basin initiation was broadly synchronous with development of the nankai megasplay fault suggesting a causal relationship with construction of the nankai accretionary prism unlike preceding studies documenting long distance longitudinal transport of clastic material along the lower nankai forearc only limited longitudinal transport is documented by detrital pyroxenes in the upper forearc these results suggest that transverse canyons are a major control on the sediment flow routing during maturation of forearc basins and that long distance longitudinal flows along convergent margins are principally restricted to near trench environments even in the presence of large forearc basins

coupled ocean atmosphere responses to recent freshwater flux changes over the kuroshio oyashio extension region abstract observations have indicated a trend of freshwater loss in the global western boundary current extension regions over several recent decades in this paper the coupled ocean atmosphere response to the observed freshwater flux trend defined as evaporation minus precipitation emp over the kuroshio oyashio extension koe region is studied in a series of coupled model experiments the model explicitly demonstrates that the positive emp forcing in the koe region can set up a cyclonic gyre straddling the subtropical and the subpolar gyre which induces anomalous southward cold advection in the west and northward warm advection in the interior this leads to the formation of a temperature dipole in the midlatitudes with a cooling in the west and a warming in the east with the positive emp forcing in the koe the response of the extratropical atmospheric circulation in the north pacific sector is characterized by an equivalent barotropic low originating primarily from the western tropical pacific c