intraplate shortening and underthrusting of a large basement ridge in the eastern nankai subduction zone abstract due to the collision of the izu bonin arc with central japan the subducting philippine sea oceanic plate is affected by lithospheric shortening seaward of the eastern nankai trough in the zenisu ridge area seismic reflection and refraction profiles across the ridge show a series of low angle thrusts cutting through the entire oceanic crust they also indicate that the formation of this ridge is related to a single lithospheric thrust with 10 km throw that inserts a corner of upper mantle within the lower crust we propose that the serpentinized mantle crust transition acts as a decoupling level above which distributed shortening results in the formation of a crustal prism although the amount of deformation is much smaller than in a mountain belt such as the pyrenees a parallelism can be proposed between both systems that correspond to intraplate shortening and have in common a single mantle shear zone and overlying distributed crustal deformation the underthrusting below the eastern nankai accretionary prism of a similar and now inactive tectonic structure the paleo zenisu ridge is evidenced by the deformation pattern of the margin this ridge is probably detached on top of the subduction decollement at least where volcanoes are impinging the continental margin part of the ridge may thus be accreted to the prism backstop we show that both zenisu and paleo zenisu systems accommodate the same n s compression related to convergence partitioning process this subduction kinematics can be understood in terms of a steady state model partially constrained by the trails of volcanoes that have impinged the margin n s shortening is accommodated by the thrusts seaward of the trough and right lateral motion is prevalent along the subduction system based on these kinematics we propose that the formation of the paleo zenisu and zenisu ridges are related to the successive collisions of volcanic massifs of the izu bonin arc with the japanese arc the zenisu system is a result of the collision of the izu block since 2 ma while the paleo zenisu system was the result of the earlier collision of the tanzawa massif between 5 and 3 ma

ridge offsets normal faulting and gravity anomalies of slow spreading ridges we develop a model relating mid ocean ridge normal faulting and crustal structure by examining recently available high resolution gravity and multibeam bathymetric data of the mid atlantic ridge and other spreading centers results of the analysis reveal a consistent pattern of positive residual gravity anomalies along the crust paralleling all major atlantic offsets studied especially along the inside corner side of the offsets individual residual gravity anomaly spikes local peaks which have amplitudes of up to 20 mgal and typical across axis spacing of 10 30 km often coincide with individual major fault scarps suggesting crust thinned by normal faulting theoretical calculations indicate that the amplitude and spacing of the observed residual gravity spikes are consistent with the presence of successive ridge parallel low angle faults that originate episodically at inside corners of ridge offset intersections fault scarp heights and the amounts of crustal thinning as inferred from gravity anomalies are consistently larger at inside corners than at outside corners supporting a model in which tectonic extension near ridge offsets is asymmetric with low angle faults occurring preferentially at inside corners these results on spatial variations in seafloor morphology and gravity anomalies further support a three dimensional tectonic faulting model at oceanic spreading centers with three major characteristics 1 low angle faults form preferentially at inside corners where the mantle lithosphere is the strongest and the lithospheric plates are sufficiently decoupled across ridge axis offsets 2 low angle faults decrease in throw toward midpoints of long ridge segments where large low angle faults may not be sustained by a weak lithosphere and 3 the residual gravity peaks are statistically larger at transform than near nontransform offsets indicating that the length of a ridge offset and its tectonic style control the development of low angle faults

deep crustal structure of the eastern nankai trough and zenisu ridge by dense airgun obs seismic profiling abstract an unprecedentedly extensive seismic refraction and wide angle reflection survey using 65 ocean bottom seismographs revealed detailed crustal structure around the eastern nankai trough a previously published crustal model shows an abrupt offset of the moho at the south of the zenisu ridge a prominent topographic high along the oceanward slope of the nankai trough our crustal model indicates that this offset of the moho extends southwestward continuously to 138 e decreasing its gap the survey area experienced the last two great earthquakes in 1854 and 1944 however the northeastern part of the survey area seems to have remained unruptured since the 1854 event factors controlling the size of the rupture area for great earthquakes are still a matter of debate there are several candidates for these factors in the survey area hypothetical tectonic boundaries that may or may not be oceanward prolongation of major on land tectonic lines estimated locations of slab disruption and the extent of moho offset along the strike of the zenisu ridge the main purpose of this survey is to clarify the relation between the crustal structure and these geophysical and geological features bounding the rupture area our crustal model from the trough axis to the continental slope is characterized by a well developed sedimentary wedge bounded by island arc crustal blocks consisting of upper and lower crust to the northwest furthermore the subducting oceanic crust which can be traced down to 25 km depth shows that the down dip angle steepens at 55 km landward from the trough axis on the basis of compilation of our crustal model with previously published models around the eastern nankai trough we derived an image of the entire subducting plate geometry for depths shallower than 20 km which is still poorly constrained by the land observation of microearthquakes significant lateral variations of the crustal structure and the slab geometry are recognized along one prominent canyon and the offset of the moho at the south of the zenisu ridge disappears to the southwest of the canyon moreover it seems that the slab disruption recognized at a depth greater than 20 km is connected to this canyon therefore the lateral variation of the crustal structure along the canyon may be one of the causes to stop rupture propagation of great earthquakes furthermore the crustal variation may also form a tectonic boundary that distinguishes the subduction pattern of the philippine sea plate including the influence of the izu ogasawara collision in the eastern nankai trough from the simple subduction pattern of the western nankai trough

strain partitioning in the zone of transition from nankai subduction to izu bonin collision central japan implications for an extensional tear within the subducting slab abstract the izu bonin ridge collision with central japan is a type example of collision of two island arcs in a subduction context based on the analysis of active faults paleomagnetic data and recent seismicity in the northern tip of the izu bonin arc and the adjacent oceanic plate we propose a tectonic schema of the izu collision system and its transition to the nankai subduction to the west the dense gps network deployed in central japan allows us to constrain the kinematics of the different blocks identified this new kinematic model demonstrates the essential role of strain partitioning there as the northward component of the izu bonin arc motion is locked in the collision zone 30 of the convergence between the philippine sea plate and central japan is accommodated within the philippine sea slab along the zenisu east izu deformation system we show that this pattern of deformation is related to an extensional tear of the philippine sea slab propagating southward along the volcanic arc we discuss the implications of this kinematic model on the seismogenic potential of the area and the dynamics of the central japan convergence zone

observation and tentative interpretation of a double bsr on the nankai slope seismic data collected during the french japanese kaiko tokai cruise of r v l atalante on the upper slope of the eastern nankai margin reveal the simultaneous presence at two distinct depths below the seafloor of two bottom simulating reflector bsr type reflectors the upper bsr is traced as a continuous reflector over about 10 km as water depth decreases from 850 m to 550 m its depth below seafloor decreases from 200 m to 40 m the lower bsr is traced at 50 100 m below the upper one the two bsrs end abruptly near the summit of the daichii tenryu knoll into an area where the 3 5 khz record suggests active gas expulsion through the seabed the observed depth of the upper bsr fits the predicted one for the base of the methane gas hydrate stability zone as estimated from present temperature and pressure conditions at the seafloor and in the slope sediments thus we interpret the upper bsr as an active methane hydrate bsr we further suggest that the lower bsr is a residual hydrate related bsr this could have followed a recent migration of the base of the methane hydrate stability zone from the lower bsr to the upper one as possible causes for this migration we discuss sea bottom warming and tectonic uplift the bsr migration could have occurred as a response to a 1 2 c sea bottom warming or with an equivalent effect an event of fast uplift of the seafloor by about 90 m we do not discard other interpretations of the lower bsr such as an active hydrate related bsr formed from a mixture of gases

faults configuration around the eastern nankai trough deduced by multichannel seismic profiling abstract the japan marine science and technology center jamstec the ocean research institute university of tokyo and the laboratoire de geologie ecole normale superieure at paris carried out in collaboration a 120 channel reflection seismic survey using an airgun array with total 4200 cubic inch capacity around the eastern nankai trough in 1996 data obtained by r v hakuho university of tokyo were primarily processed by jamstec and were shared by the above three main bodies of this program to complete geoscientific interpretation the fault configuration around the eastern nankai trough is very complicated due to the collision of the izu peninsula against central japan the major remarkable faults around the eastern nankai area are the zenisu the kodaiba and the tokai thrusts the main objective of this paper is to clarify these fault configurations the zenisu thrust dips gently northward from the southern slope of the zenisu ridge and reaches the oceanic moho the location of the tokai thrust may jump at the tenryu canyon and appears to reach the top of the subducting oceanic plate the kodaiba fault penetrates a 5 5 5 6 km s layer which is the crust of the central japan forearc a large fault in the trough axis cuts the entire oceanic crust and has a flower structure which indicates that it has strike slip component its strike is nw se parallel to the plate motion and perpendicular to the strike of the zenisu ridge the top of the subducting oceanic crust of the philippine sea plate was defined beneath the continental slope of the central japan forearc the angle of subduction becomes abruptly steeper 50 km north of the deformation front we also observed at the southern roof of the zenis thrust a drape fold structure which is proof of subduction we suggest a subduction on the southern side of the zenisu ridge strongly deformed oceanic crust at the trough axis and some major faults in the forearc region reaching subducting oceanic crust

deformation of the nankai trough inner accretionary prism the role of inherited structures funding for this research was provided by the national science foundation grant oce 0451790 and a u s science support program post expedition award

full interseismic locking of the nankai and japan west kurile subduction zones an analysis of uniform elastic strain accumulation in japan constrained by permanent gps we analyze permanent global positioning system gps data obtained over japan between 1995 and 1997 to estimate the instantaneous interseismic coupling ratio of the seismogenic zones due to the subduction of the pacific and philippine sea plates below the japanese islands we first derive the gps strain rate fields that characterize the crustal deformation of southern and northern japan and invert them to determine the effective subduction velocity along the central nankai trough on one side and the japan west kurile trench on the other these reference free velocities are close to those predicted by plate motion models with respect to eurasia we conclude that the eurasian reference frame gives a good approximation to the subduction motion and that to first order both subduction zones were fully locked during the period of measurements we then test whether the coupling ratio shows local variations within the seismogenic zones to do this we divide the subduction interface into 35 km 30 km elements that we model by point source groups and we invert the gps velocity field referenced to eurasia to derive the coupling ratio between 0 and 1 on each fault element the results are coherent over the 3 years and confirm that both the central nankai and the japan west kurile seismogenic zones are homogeneously fully locked most of the coupling ratios are close to 1 and a few are close to 0 intermediate values are rare the zones of decoupling correspond either to strong postseismic afterslip associated with the 1994 sanriku oki interplate earthquake japan trench or to a small overestimation of the actual lower limit of the locked zone we conclude that within the resolution of the gps data and the model 1 partial coupling did not exist during these 3 years along the nankai and japan west kurile trenches 2 the small seismic coupling ratio previously derived from earthquakes analysis for the japan and kurile trenches may indicate that a significant part of the elastic energy is dissipated silently through slow earthquakes and postseismic afterslip and 3 the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrast to the homogeneous loading finally we discuss the nonelastic residual deformation within the frame of the long term deformation of the japanese islands

crustal seismic imaging from multifold ocean bottom seismometer data by frequency domain full waveform tomography application to the eastern nankai trough 1 in offshore surveys the deep crust is generally investigated by traveltime tomography applied to sparse ocean bottom seismometer data the inferred velocity models are of limited resolution precluding a quantitative analysis of deep tectonic discontinuities if dense arrays of ocean bottom seismometers can be deployed the resulting data sets become amenable to full waveform processing which should lead to a significant improvement in the resolution of structures such acquisition and processing were achieved on the eastern nankai subduction zone full waveform inversion is entirely implemented in the frequency domain enabling efficient finite difference wave modeling and allowing to limit the inversion to a few discrete frequencies of increasing value such a hierarchical procedure defines a multiresolution approach to seismic imaging the data set was recorded by 93 instruments deployed along a 105 km long profile thirteen frequencies ranging from 3 to 15 hz were inverted sequentially wavelengths imaged by the full waveform inversion typically range between 0 5 and 8 km full waveform tomography reveals an intense compression in the upper prism and underlying backstop as evidenced by several thrusts the frontal ones still active corresponding to negative velocity anomalies the subducting paleo zenisu ridge is also reconstructed above which stands the decollement velocities in the upper mantle beneath the ridge are rather low 7 5 km s 1 suggesting partially serpentinized mantle beneath the ridge this paper demonstrates the feasibility of full waveform tomography based on dense ocean bottom seismometer data sets and its ability to quantitatively image the entire crust with a significantly improved resolution compared to what is usually achieved through traveltime tomography

zenisu ridge mechanical model of formation abstract seabeam seismic and submersible surveys took place during the kaiko project and revealed significant compressive deformation at the northeastern end of the philippine sea plate related to the recent collision of the izu ogasawara arc against central japan intraoceanic thrusting at the base of the zenisu ridge a linear topographic high running a few tens of kilometers south of the nankai trough is supported by tectonic magnetic and gravimetric data we investigate the formation of the zenisu ridge in terms of compressive mechanical failure of a thin elastic perfectly plastic plate subducting at a trench and subject to a regional compressive axial force the rheological envelope concept is used throughout the numerical calculations based on a detailed study of flexure of the present day bending far from the deformation zone we evaluate the bending forces involved the bulge is 120 to 150 m high and the compressive stress all along the nankai trough is about 100 mpa in the zenisu ridge area an additional compressive stress is superimposed due to the nearby collision at izu peninsula we compute the vertical distribution of the deviatoric stress before failure and find that the deviatoric stress is maximum at a depth of 20 25 km in the trench area and again at the surface 60 to 100 km seaward in the vicinity of the bulge the development of a thrust joining these two maxima through the entire thickness of the lithosphere is discussed the model predicts that the formation of the zenisu ridge did not occur before 4 ma and is caused by progressive tectonic uplift due to the redistribution of bending stresses as the ridge approaches the nankai trough

magnetic anomalies in the shikoku basin a new interpretation kaiko surveys over the nankai trough made available new magnetic and structural data for the northern shikoku basin a survey of the oceanic lithosphere subducting below southwest japan along the central nankai trough revealed the existence of several north south basement troughs they are probably transform faults related to a north south spreading system we exanune the possibility of a late phase of north south spreading limited to the axial northernmost shikoku basin active between 14 and 12 ma if this system was already active before that time i e during the n55 opening of the southeastern basin then a triple junction should be found between both areas based on these data and previous studies we present a new interpretation of magnetic anomalies over the whole basin from early east west rifting to late north south spreading opening of the shikoku basin proceeded through multiple episodes of spreading the analysis of magnetic anomalies constrains the kinematic evolution of the basin through time and space two successive counter clockwise rotations of the spreading direction are postulated at anomaly 6 19 ma and at anomaly 5b 14 ma involving segmentation and rotation of the spreading ridge

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

slope instabilities and gravity processes in fluid migration and tectonically active environment in the eastern nankai accretionary wedge kaiko tokai 96 cruise abstract one main feature imaged on the nankai slope by using the sar pasisar system deep tow single channel seismic streamer during the franco japanese kaiko tokai cruise in april 1996 is the gravitational displacement of large volumes of the sediment this slope failure is assumed to be mainly related to an increase of the slope angle beyond its threshold value tentatively linked to the uplift effect of the subduction of a paleo zenisu ridge and to the existence of a basal shear plane possibly controlled by altered properties of the sediments at the base of the gas hydrate stability field the role of pore fluid or free gas is discussed but remains unknown without any in situ data this role may be major if considering the gassy environment in the sedimentary cover where earthquake loading can cause excess pore pressure and act as a major triggering mechanism for sliding

japan france kaiko tokai project tectonics of subduction in the nankai trough the nankai trough has attracted the interest of earth scientists for a long time as it constitutes a natural laboratory for studying the subduction processes the discovery of the role of uids in the development of accretionary complexes then of their relationship to the way the seismogenic zone works has further boosted scienti c research on subduction tectonics a rst cooperative program was established between japan and france in the early 1980s to survey the subduction trenches around japan followed by more focused programs on the nankai trough kaiko nankai and kaiko tokai all these programs have made use of state of the art technology such as submersibles swath mapping and deep towed systems in this special issue 11 papers are published based on contributions to the international symposium on japan france kaiko tokai project tectonics of subduction in the nankai trough held in tokyo japan in march 1998 they cover a broad range of topics related to subduction tectonics and associated phenomena surface and deep tectonic structure cold seeps gas hydrates gravity and erosional processes all intimately linked in the subduction context of the nankai trough a rst set of three papers present the structural and kinematic background of the eastern nankai trough which is probably now one of the best imaged convergent margins in the world the deep crustal structure of the eastern nankai trench is presented by nakanishi et al 2002 based on an extensive seismic refraction and wide angle re ection survey performed in 1996 it provides a clear image of the subducting plate geometry revealing signi cant lateral variations in crustal structure that may partly control the size of the rupture area of large earthquakes more details of the con guration of the main thrusts are provided by a 120 channel re ection seismic survey also performed in 1996 takahashi et al 2002 the data make it possible to follow the subducting philippine sea plate beneath the continental slope of japan and to image the main thrusts the survey also covered the zenisu ridge an uplifted oceanic crustal block located south of the eastern nankai trough which is interpreted as resulting from intra oceanic thrusting similar intra oceanic thrusting a paleo zenisu ridge is also demonstrated to occur beneath the accretionary wedge mazzotti et al 2002 this rather unusual occurrence of successive intra oceanic shortening is interpreted as successive micro collisions of volcanic massifs of the nearby izu bonin arc during an otherwise steady state subduction process insights into how the collision of the izu bonin arc has proceeded since the pliocene are provided by the reconstructions of the paleobathymetry based on foraminifers akimoto et al 2002 the next set of papers are more closely related

toward a robust workflow for deep crustal imaging by fwi of obs data the eastern nankai trough revisited crustal scale imaging by the full waveform inversion fwi of long offset seismic data is inherently difficult because the large number of wavelengths propagating through the crust makes the inversion prone to cycle skipping therefore efficient crustal scale fwi requires an accurate starting model and a stable workflow minimizing the nonlinearity of the inversion here we attempt to reprocess a challenging 2d ocean bottom seismometer obs dataset from the eastern nankai trough the starting model is built by first arrival traveltime tomography fat which is fwi assisted for tracking cycle skipping we iteratively refine the picked traveltimes and then reiterate the fat until the traveltime residuals remain below the cycle skipping limit subsequently we apply laplace fourier fwi in which progressive relaxation of time damping is nested within frequency continuation to hierarchically inject more data into the inversion these two multiscale levels are complemented by a layer stripping approach implemented through offset continuation the reliability of the fwi velocity model is assessed by means of source wavelet estimation synthetic seismogram modeling ray tracing modeling dynamic warping and checkerboard tests although the visco acoustic approximation is used for wave modeling the synthetic seismograms reproduce most of the complexity of the data with a high traveltime accuracy the revised fwi scheme produces a high resolution velocity model of the entire crust that can be jointly interpreted with migrated images derived from multichannel seismic data this study opens a new perspective on the design of obs crustal scale experiments amenable to fwi however a further assessment of the optimal obs spacing is required for reliable fwi

multiscale seismic imaging of the eastern nankai trough by full waveform inversion 1 classical active seismic methods fail to sharply image the earth s deep crust we present the first crustal scale application of 2 d full waveform inversion based on dense ocean bottom seismic data to investigate the eastern nankai subduction system japan this approach allows to quantify seismic velocities up to an unprecedented degree of resolution results reveal compressive tectonic features within both the subducting oceanic crust and the backstop at depth velocity anomalies along major faults and structural discontinuities bring evidence for the presence of fluids and weakened material and also for a possible coseismic slip partitioning structure index terms 0902 exploration geophysics computational methods seismic 3025 marine geology and geophysics marine seismics 0935 8010 structural geology fractures and faults 8105 tectonophysics continental margins and sedimentary basins 1212 citation dessa j x s operto s kodaira a nakanishi g pascal j virieux and y kaneda 2004 multiscale seismic imaging of the eastern nankai trough by full waveform inversion geophys res lett 31 l18606 doi 10 1029 2004gl020453

the eastern and western ends of nankai trough results of box 5 and box 7 kaiko survey abstract seabeam mapping and detailed geophysical surveying have been conducted over the eastern and western ends of the nankai trough the eastern survey covers the transition between the large izu bonin izu ogasawara ridge collision with honshu and the nankai trough subduction it includes a northeast trending basement ridge the zenisu ridge to the southeast of nankai trough as well as two large channel systems one following the trench the nankai channel and the other coming from the north the tenryu canyon the zenisu ridge is a zone of recent intra plate shortening consisting of three distinct segments with an increasing deformation from southwest to northeast we interpret this gradient of deformation as a way to absorb the kinematic discontinuity between the diffuse shortening prevailing over the izu bonin ridge and the concentrated shortening along the nankai trough the shortening axis appears to change from north south along the zenisu ridge to northwest southeast along the nankai trough and finally to east west within honshu the western survey covers the junction between the nankai and ryukyu trenches and the kyushu palau ridge which is subducted without major internal deformation the ridge appears to act as an indenter upon the margin the tip of the indenter is presently situated under the upper accretionary prism

nankai trough and zenisu ridge a deep sea submersible survey abstract eight submersible dives between 3000 and 4200 m water depth were made off southern japan in the eastern nankai subduction zone benthic communities associated with chemosynthetic processes were discovered along the 800 m wide active tectonic zone at the toe of the accretionary prism a benthic community was also discovered along a zone of active compression at the foot of zenisu ridge 30 km south of nankai trough temperature measurements within the sediments below the benthic communities confirm that upward motion of interstitial water occurs there studies of water samples indicate advection of methane and light hydrocarbons specimens of the benthic community have been shown to have included in their shells carbonate resulting from methane consumption thus the benthic communities are related to overpressure driven fluid advection along tectonic zones with active surface deformation a 300 m high active scarp at the toe of the accretionary prism is related to relative motion in a 280 direction which is close to the 305 average direction of subduction in this area the dives establish further that compressive deformation is presently occurring at the foot of zenisu ridge the previous interpretation of the zenisu ridge as a zone of recent north south intraplate shortening 40 km south of the nankai trench is confirmed we conclude that tectonic evolution might well lead to future detachment of the zenisu ridge and overthrusting of this large piece of oceanic crust over the continental margin such a process might be an efficient one to emplace ophiolites over continents

oblique and near collision subduction sagami and suruga troughs preliminary results of the french japanese 1984 kaiko cruise leg 2 abstract leg 2 of the french japanese 1984 kaiko cruise has surveyed the suruga and the sagami troughs which lie on both sides of the northwestward moving and colliding izu bonin ridge the northernmost part of the philippine sea plate the transition from the nankai trough to the suruga trough is characterized by northward decrease in width of the accretionary prism in good agreement with the increasing obliquity between the through axis and the direction of the convergence as the strike of the convergent boundary changes from ene nne to south north south of the area the southern margin of the zenisu ridge shows contractional deformations this supports the interpretation made by the team of leg 1 who studied the western extension of the area we studied that it is an intra oceanic thrusting of the ridge over the shikoku basin in the sagami trough where the relative motion is highly oblique to the plate boundary active subduction is mostly confined in the east west trending portions of the trough located south of the boso peninsula and along the lower boso canyon near the ttt triple junction in between the present motion is mainly right lateral along the northwest trending boso escarpment however an inactive but recent pliocene to lower pleistocene accretionary prism exists south of the boso escarpment which suggests that the relative motion was more northerly than at present before about 1 ma ago

high pore fluid pressure may cause silent slip in the nankai trough silent slip events have been detected at several subduction zones but the cause of these events is unknown using seismic imaging we detected a cause of the tokai silent slip which occurred at a presumed fault zone of a great earthquake the seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down dip of a subducted ridge we propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip

effects of oceanic ridge subduction on accretionary wedges experimental modeling and marine observations sandbox modeling is used to study the deformation of accretionary wedges caused by the subduction of oceanic ridges the first experiment incorporates a massive ridge within a sand wedge the wedge thickens and shortens when the forward propagation of the basal decollement ceases the wedge thickening results in taper change reactivation of preexisting thrusts and retreat of the frontal part of the sand wedge similar mechanisms may have affected some margins that have undergone ridge subduction such as the tonga margin after the subduction of the oblique louisville oceanic ridge the second experiment shows the effects of an active basement thrust slice as it enters a subduction zone this process may have happened in the eastern nankai accretionary wedge initially the wedge in this experiment behaved similarly to that of the first experiment rapidly the topographic slope changed the wedge thickened above the basement slice generating a slope break in the topography a deeper propagating accretionary wedge again characterized by a small taper developed these results when compared with observations made in the eastern nankai trench are in agreement with the past subduction of a basement thrust slice in this area

surface expression of fluid venting at the toe of the nankai wedge and implications for flow paths abstract cold seeps have been observed with manned submersibles at many sites on the tokai margin most sites are found on the hanging wall of major thrust faults within 1 2 km of their outcrop few sites relate to purely erosional features this relationship between thrusting and venting may be explained either by fluid flow along fault zones by fluid flow along permeable layers outcropping at the fault scarp or by diffuse upward flow of fluids released during footwall compaction we use deep towed side scan sonar and sediment sounder profiles acquired at the toe of the nankai accretionary wedge to identify the context of seepage at a smaller scale modes of fluid expulsion inferred from observations in this zone include flow channelling along strata along thrusts along gravity faults and along joint fractures as well as diffuse flow we then combine visual observation and monitoring of electrochemical potential eh to assess the relative importance of seepage sites we show that bacterial mats are better visual indicators of active seepage than bivalve beds and that the most active seepage sites correlate with outcrops of coarse turbidites cemented by authigenic carbonates these were sampled and have permeability high enough to channel fluid flow 1 4 10 14 m2 we suggest they are the main conduits near the seafloor however seepage sites are also observed directly on the outcrop of major thrusts indicating that they do play a role as fluid conduits we observe negative polarity seismic reflections on these faults below 1 km depth and wonder about their significance

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

structural features of the bonin arc implications for its tectonic history abstract the bonin arc from the shikoku basin to the trench axis is broken into four crustal blocks each separated by major right lateral strike slip faults from north to south they are termed the sumisu jima the sofu gan and the central bonin dislocations these dislocations are delineated by 1 1 changes in trends of en echelon ridge and trough structures of the main arc 2 2 right lateral offsets of the bathymetry and structure of the arc s outer ridge 3 3 right lateral offsets in the free air gravity contours the most likely time of formation of the en echelon structures and fracturing of the arc into the four crustal blocks is between 27 and 17 m y b p miocene during this time japan was moving southeastward relative to asia and the philippine sea plate including the bonin arc was moving northward subducting beneath southwest japan also convergence and subduction of the pacific plate was taking place along the east side of the bonin arc which involved the collision of thick buoyant crustal blocks of the marcus necker ridge near the south end of the arc these movements resulted in compressive forces acting on the bonin arc from the northwest and produced a clockwise torque within the arc the response to these forces included development of ne nne structural trends within the arc and fracturing of the arc into four crustal blocks offset from each other by ne nne right lateral displacements

deformable backstop as seaward end of coseismic slip in the nankai trough seismogenic zone abstract wide angle seismic surveys performed in the last decade have clarified the 3 d crustal structure along the nankai trough the geometry and velocity structure of the southwestern japan subduction zone are now well constrained comparing these observations with the rupture distribution of historic great thrust earthquakes it appears that the coseismic rupture occurred along plate boundaries deeper than the wedge backstop boundary the boundary between the neogene quaternary accretionary wedge and the crust forming the backstop from the view of spatial relationship both rupture distributions of the last two large events and the crust forming the backstop are considerably retreated from the trough axis in the west and east off the kii peninsula in both areas seamount or ridge subduction is apparent in seismic results geomorphological data and geomagnetic data the landward indentation of the deformable backstop which corresponds to the crustal block of old accreted sediments may be formed by seamount subduction according to published results of sandbox modeling in particular the subducted seamount may be a structural factor affecting the recession of the crustal block forming the backstop

a subducting seamount beneath the mediterranean ridge abstract two transverse morphological tracks across the front of the mediterranean ridge were swath mapped with a hydrosweep instrument system along the tracks ridges and troughs at the front of the accretionary prism are disrupted and bent in the direction of relative plate convergence the longer of the two tracks extends to bannock basin a brine filled depression with a closed outer moat around a central mound seismic reflection data and a gravity anomaly show a buried circular seamout beneath bannock basin whose summit is the mound this indicates an origin of the larger transverse track and bannock basin from a subducting seamount beneath bannock basin the seamount has displaced about 340 km 3 of the messinian section and yet the seafloor has not been uplifted therefore considerable evaporitic material is inferred to have been displaced by the seamount and bannock basin is a likely vent for large quantities of dissolved materials from the disrupted messinian evaporitic sequence

intraoceanic thrusts in the nankai trough off the kii peninsula implications for intraplate earthquakes 1 we identified intraoceanic thrusts developed as imbricate structures within the subducting philippine sea plate off the kii peninsula in central japan manifesting as strong amplitude reflections observed in an industry standard three dimensional 3d seismic reflection data set these imbricate intraoceanic thrusts cut through the oceanic crust as a discontinuous thrust plane striking approximately parallel to the trench in our survey area large intraplate earthquakes with moment magnitudes mw over 7 occurred on 5 september 2004 causing strong ground motions on the islands of japan and tsunami waves the locations of the intraoceanic thrusts recognized in the seismic data are distributed around the estimated hypocenters of the mainshocks and aftershocks of the 2004 earthquakes furthermore their geometry extracted from the 3d seismic data could explain the kind of complex rupture pattern observed during the 2004 events therefore we propose that the intraoceanic thrusts are seismogenically active

transient and permanent deformation of central japan estimated by gps 2 strain partitioning and arc arc collision abstract deformation of central japan can be divided into two main components elastic deformation due to the interseismic locking of subduction thrusts and deformation associated with intra arc tectonics in a companion paper henry et al earth planet sci lett 2000 this issue we estimated an elastic deformation field based on the modeling of global positioning system gps strain rates in this study we show that the corrected field gps minus predicted elastic deformation is consistent with geological and seismological observations and should represent long term plastic deformation we use this corrected gps deformation field to describe the kinematics of partitioning of the philippine sea plate amurian plate amu convergence in relation with the izu bonin arc collision about one third of the relative philippine sea amur motion is transferred to the central japan crust north of the collision front this transfer induces high strain rates in southern kanto and in the northern japanese alps the latter region corresponds to the eastern boundary of the amu we propose that the independent motion of central japan results from the delamination and the underthrusting of the izu bonin arc lower crust and the upward bent oceanic slab

intraplate shortening in the central indian ocean determined from a 2100 km long north south deep seismic reflection profile although intraplate deformation is commonly observed on the continents there are only a few places in the oceans where deformation is distributed over a broad area rather than being localized at a single plate boundary the central indian ocean has long been recognized as a site of active widespread compressive deformation of the oceanic lithosphere reflecting an exceptionally high stress level however the strain pattern derived from indirect and incomplete observations remains poorly known a more complete data set was obtained during the phedre cruise 1991 by means of deep seismic reflection profiling one 2100 km long profile runs along the 81 e meridian from lat 14 s to the coast of sri lanka and provides us with the first long cross section through the region of observed crustal level faults fault analysis offers new estimates of the amount of shortening which was obtained across individual faults by measuring the maximum vertical uplift of sedimentary reflectors at the base of the sedimentary cover and converting it to horizontal throw under various assumptions of the fault geometry the contribution of the seismically resolvable faults amounts to 22 37 km of shortening or 2 5 4 3 this is two to three times greater than previous estimates extrapolated from local measurements on shorter seismic reflection profiles

seafloor manifestations of fluid seepage at the top of a 2000 metre deep ridge in the eastern nankai accretionary wedge long lived venting and tectonic implications abstract the results of the 1989 kaiko nankai diving cruise to the eastern nankai accretionary wedge show that a narrow corridor on the top of a 2000 m deep ridge is characterized by concentrated surface manifestations of fluid seepage and chaotic outcrop orientations suggesting active faulting along a right lateral shear zone modern biological indicators of fluid venting include bivalve vesicomyid species and vestimentiferan tube worms ancient manifestations of fluid venting consist of similar vesicomyid species found in 20 and 150 ka concretions this suggests relatively constant biotypes carbonate deposits including fossil cemented shells were observed near active sites either as paving stones diffuse seepage and cementation cut by channels or as pinnacles local seepage and cementation despite its proximity less than 15 km a nearby 3800 m deep site where substantial fluid discharge occurs seems to be completely separate from the 2000 m deep ridge site the 3800 m deep site differs from its shallower counterpart in terms of its seafloor manifestations and the composition of its fluids the sediment at the top of the ridge ranges in age around 500 300 ka which attests that the transition from a depositional to an erosional stage was younger than 800 ka but older than 200 ka this transition can be due to either the incorporation of this unit from the trench fill into the accretionary wedge front or the uplift of a slope basin the first hypothesis leads to a 2 5 cm yr minimum progradation rate associated with a 2 5 mm yr minimum uplift rate from the paleotrench to the present ridge the second hypothesis implies a recent uplift of a piggy back basin at a minimum rate of 1 mm yr

shear partitioning in the eastern nankai trough evidence from submersible dives abstract submersible dives over the wedge of the easternmost nankai trough demonstrate the existence of a left lateral strike slip system parallel to the subduction zone this system develops as the trench turning northward toward land becomes highly oblique to the motion although shear partitioning is well known in other subduction zones the process here has the peculiarity of continuing northward into land

strain modes within the forearc arc and back arc domains in the izu japan and taiwan arc continent collisional settings in this study i examine the strain modes of the forearc arc and back arc domains in arc continent collisional settings leading to arc material subduction delamination and or accretion the study focusses on two well documented colliding island arcs the izu bonin mariana ibm arc in japan and the luzon arc in taiwan both carried by the philippine sea plate firstly there is a body of evidence that both the ibm and the luzon arcs were built on the same late jurassic to early cretaceous proto philippine sea plate crust their internal structure is thus more heterogeneous than expected from paleogene or neogene supposedly intra oceanic island arcs secondly those arc systems and proximal back arcs have similar seismic characteristics attesting either for the presence of a middle crust with continental velocities and or serpentinized uppermost mantle that facilitate crustal shortening slivering and subsequent decoupling from the rest of the subducting plate it is shown that the proximal back arc domain called rear arc in case of paleoarc activity overlying the mantle wedge and the subducting slab may lose its strength if slab derived hydration occur decoupling then occurs below the moho arc delamination likely occurs in mid crustal levels because middle crust heated by nearby magmatism becomes weak accretion of arc material onto the upper plate depends on the characteristics of the arc itself and the geodynamic configuration most of the accreted material is probably underplated rather than frontally accreted

obduction why how and where clues from analog models obduction is an odd geodynamic process characterized by the emplacement of dense oceanic ophiolites atop light continental plates in convergent settings we herein present analog models specifically designed to explore the conditions i e sharp increase of plate velocities herein coined as acceleration slab interaction with the 660 km discontinuity ridge subduction under which obduction may develop as a result of subduction initiation the experimental setup comprises an upper mantle modeled as a low viscosity transparent newtonian glucose syrup filling a rigid plexiglas tank and high viscosity silicone plates convergence is simulated by pushing a piston with plate tectonics like velocities 1 10 cm yr onto a model comprising a continental margin a weakness zone with variable resistance and dip w an oceanic plate with or without a spreading ridge a preexisting subduction zone s dipping away from the piston and an upper active continental margin below which the oceanic plate is being subducted at the start of the model as for the neotethyan natural example several configurations were tested over thirty five parametric models with special emphasis on comparing different types of weakness zone and the degree of mechanical coupling across them measurements of displacements and internal deformation allow for a precise and reproducible tracking of deformation models consistently demonstrate that once conditions to initiate subduction are reached obduction may develop further depending on the effective strength of w results 1 constrainthe range of physical conditions required for obduction to develop nucleate and 2 underline the key role of such perturbations for triggering obduction particularly plate acceleration they provide an explanation to the short lived peri arabic obduction which took place along thousands of km almost synchronously within 50 10 myr from turkey to oman while the subduction zone beneath eurasia became temporarily jammed they also demonstrate that the emplacement of dense oceanic material on continental lithosphere is not a mysterious process requiring extraordinary boundary conditions but results from large scale normal oceanic then continental subduction processes

zenisu ridge a deep intraoceanic thrust related to subduction off southwest japan abstract during the french japanese kaiko project seabeam seismic and submersible observations were made in the eastern part of the nankai subduction zone close to the area of collision between the izu bonin island arc and the japan margin the most prominent feature is the zenisu ridge an elongated relief of the philippine sea plate running parallel to the trench magnetic anomalies indicate that the crust of the zenisu ridge is a part of the shikoku oceanic basin formed in the early miocene 23 ma ago and presumably uplifted at a later stage structural analysis of seismic data and diving observations lead us to interpret the superficial structure as being due to compressive tectonics mapping the acoustic basement reveals that the southeastern flank of the ridge is bounded by a double thrust both segments being of equal magnitude vertical offset about 1 to 1 5 km geophysical data support the hypothesis of a main thrust cutting through most of the lithosphere and flattening at depth the overall structure of the surrounding area reveals a compressive deformation zone widening toward the east the magnitude of the compressive deformation decreasing westward as well as southward of the zenisu ridge

detailed subduction structure across the eastern nankai trough obtained from ocean bottom seismographic profiles to investigate the deep crustal structure of the philippine sea plate at its northern margin we performed a seismic refraction and wide angle reflection survey around the eastern nankai trough by using an ocean bottom seismograph array with an air gun source we derived a crustal structure model across the trough from the shikoku basin to the continental slope that explains not only our seismic data but also previously published gravity data the origin of the zenisu ridge a conspicuous topographic high along the oceanward slope of the nankai trough is still a matter of argument there has been some controversy as to whether the igneous activity of the izu ogasawara arc or the seafloor spreading of the shikoku basin is responsible for the formation of the ridge although the crustal thickness beneath the ridge is between 8 and 11 km slightly thicker than typical oceanic crust our structure model clearly indicates that the zenisu ridge has an oceanic crust and its structure is very similar to that of the shikoku basin beneath the south flank of the zenisu ridge the moho shows an offset of 5 km in depth this offset may correlate with the recently proposed nascent subduction boundary and subduction collision tectonics of this area the velocity structure beneath the continental slope appears characteristic for a well developed accretionary wedge bounded by the continental upper crust of the japan island arc to the northwest and subducting oceanic crust which can be traced beneath the accretionary wedge and the continental upper crust

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

deep seismic imaging of the eastern nankai trough japan from multifold ocean bottom seismometer data by combined travel time tomography and prestack depth migration 1 the easternmost segment of the nankai trough remained unruptured during the 1944 and 1946 earthquakes that affected the rest of the trench it is therefore a zone of seismic gap that also undergoes a peculiar tectonic regime due to its proximity with the izu collision zone we investigate this area with densely sampled active seismic data one hundred ocean bottom seismometers spaced only 1 km apart were deployed along a line perpendicular to the trough a two step tomographic approach designed for this type of acquisition is applied first a long wavelength velocity model is built by a linearized inversion of first arrival travel times its resolution is assessed and its reliability is confirmed through a semiglobal approach in which a large range of starting models is tested second this tomographic model is used to exploit late reflected arrivals with a prestack depth migration the result of which is validated by classically modeling refraction and reflection travel times in the derived structural model thus both velocities and discontinuities are mapped in our model with a minimum of data interpretation and a priori knowledge our results bring new insights on the deep crustal zones of the margin and its evolution the existence of presently inactivated major thrusts is very clearly evidenced in the backstop a general timing for its evolution and deformation is proposed farther seaward the presence of a subducted ridge beneath the accretionary wedge is confirmed its compressive structure and origin are demonstrated it exhibits a thickened lower crust which nature is discussed besides strong analogies between this paleo zenisu subducting ridge and the zenisu ridge observed farther south are also emphasized bringing evidences for a steady state crustal deformation pattern