the philippine sea new survey results reveal the structure and the history of the marginal basins the hydrographic department of japan initiated a long term geological and geophysical survey of the philippine sea in 1983 we here summarize the bathymetry and magnetic anomalies of this area focusing on the structure and the evolutionary process of the marginal basins the shikoku and parece vela basins developed as one backarc system in the later phase of their formation the significant difference between the two basins however is clearest near the extinct spreading centers the steep oki daito escarpment located in the northern west philippine basin was mapped for the first time during this survey the n s trending spreading fabric north of the escarpment is in striking contrast to the nw se to e w lineament in the south our data reveal a more complicated history of the western philippine sea than has been proposed in previous studies

reconstructing subducted oceanic lithosphere by reverse engineering slab geometries the northern philippine sea plate subducting slabs commonly acquire complex geometries from the migration of subduction trenches slab mantle interaction slab tearing and collision of slabs at depth although it is possible to construct three dimensional models of subducted slabs using earthquake hypocenter locations and tomographic models it is often not possible to rigorously test their accuracy here we present a methodology for performing such a test by reverse engineering the presubduction configuration of a slab of oceanic lithosphere from interpretations of its present day morphology we illustrate our approach for the ryukyu and shikoku slabs northwest philippine sea having simulated them as viscoelastic sheets that we unfolded and floated to the surface the net strain distribution of the floated mesh indicated which parts of the original slab model were geometrically viable minimal net strain and which parts of the mesh required additional tears and or zones of localized ductile extension to have enabled the slab to deform during subduction in the instance of the ryukyu and shikoku slabs the palau kyushu and gagua ridges are shown to have both acted as planes of weakness that broke into major vertical slab tears these subducted ridges are connected by a trench parallel tear that represented the former contact between the huatung and west philippine basins the fossil spreading center of the shikoku basin formed a separate slab window upon subduction along the nankai trough the methodology presented herein is a powerful tool to evaluate complex slab morphologies infer the locations of slab tears and therefore reconstruct intricate configurations of subducted oceanic lithosphere

continuing evolution of the pacific juan de fuca north america slab window system a trench ridge transform example from the pacific rim abstract many subduction margins that rim the pacific ocean contain complex records of cenozoic slab window volcanism combined with tectonic disruption of the continental margin the series of slab windows that opened beneath california and mexico starting about 28 5 ma resulted from the death of a series of spreading ridge segments and led to piecewise destruction of a subduction regime the timing and areal extent of the resultant slab window volcanism provide constraints on models that depict the subsequent fragmentation and dispersal of the overlying continental margin the initial pioneer slab window thermally weakened the overlying western transverse ranges and california borderlands region starting about 28 5 ma a second thermal pulse occurred in the same region starting about 19 ma during growth of the monterey slab window this additional heating combined with the capture of a partially subducted monterey plate fragment by the cocos plate initiated the pulling apart and rotation of the adjacent continental margin similarly the capture of guadalupe and magdalena plate fragments by the pacific plate and initiation of the guadalupe magdalena slab window about 12 5 ma are coeval with baja california pulling away from the mexico continental margin with the break along the comondu arc in crust already thermally weakened by about 10 my of volcanism in coastal california distributed crustal extension and subsidence accompanied the new transform plate boundary and continued until the slab windows cooled and plate motion coalesced along a through going system of strike slip faults the transform boundary continues to evolve and forward modeling predicts an instability with the current configuration as a result of convergence between the sierra nevada and peninsular ranges batholiths starting about 2 my in the future the instability may be resolved by a shift in the locus of transform motion from the san andreas fault to the eastern california shear zone or by breaking off another fragment of the mojave or southern sierra nevada crustal blocks and translating it northward

late cenozoic evolution of epithermal gold metallogenic provinces in kyushu japan kyushu island japan is located at the junction of the southwest japan arc and the ryukyu arc there are two major late cenozoic epithermal gold silver provinces in kyushu which are termed the northern and southern provinces the provinces are characterized by 1 pliocene volcanism dominated by calc alkaline andesite followed by quaternary volcanism including extrusion of both calc alkaline and tholeiitic magmas 2 formation of extensional grabens 3 pliocene to pleistocene mineralization which was dominated by abundant low sulfidation ls epithermal deposits with a few high sulfidation hs examples the two epithermal gold silver provinces have evolved differently since about 5 ma the northern province has exhibited diminished hydrothermal activity from the pliocene to pleistocene whereas the southern province has witnessed increased hydrothermal activity mainly in easterly and northerly directions changes of tectonic setting from the pliocene to pleistocene account for the variable trends in epithermal gold deposit formation westward oblique subduction of the philippine sea plate beneath the southwest japan arc caused development of the hohi graben and arc related volcanism at about 6 ma this was associated with widespread ls mineralization in and surrounding the hohi graben as is represented by the bajo and taio deposits the subduction of the relatively buoyant kyushu palau ridge during the early pliocene strengthened the coupling between the slab and overriding ryukyu arc leading to polygenetic andesite volcanism with associated hs kasuga iwato and akeshi and ls kushikino mineral deposits forming in the southern province a change of the subduction direction of the philippine sea plate from west to north northwest in the early pliocene increased the orthogonal convergence rate between the southwest japan arc and the philippine sea plate resulting in a decrease of volcanic and hydrothermal activity in the hohi graben of the northern province the more northerly subduction of the philippine sea plate shifted the locus of the kyushu palau ridge subduction northward resulting in underplating of the older 85 60 ma negatively buoyant amami basin oceanic slab in the southern province rather than continued subduction of the young 27 15 ma buoyant shikoku basin slab this replacement caused steepening of the slab angle and slab rollback in the southern province which was associated with regional extension an eastward shift of the ryukyu volcanic front and development of the kagoshima and shimabara grabens as well as the okinawa trough rhyolite and basalt volcanism in addition to andesite volcanism have occurred since 2 ma in the area of the ryukyu back arc coincident ls mineralization at hishikari and ohkuchi was affiliated with the rhyolite volcanism another change of the subduction direction of the philippine sea plate to the northwest occurred at 2 1 ma the forearc sliver of the southwest japan arc shifted westward in association with right lateral strike slip faulting along the median tectonic line due to the increase of the westward convergence rate this shift resulted in shortening and cessation of graben development in the hohi area restricting the subsequent volcanism and related hydrothermal activity to the central part of the graben

an expression of philippine sea plate rotation the parece vela and shikoku basins abstract the philippine sea plate located between the pacific eurasian and australian plates is the world s largest marginal basin plate the motion of the philippine sea plate through time is poorly understood as it is almost entirely surrounded by subduction zones and hence previous studies have relied on palaeomagnetic analysis to constrain its rotation we present a comprehensive analysis of geophysical data within the parece vela and shikoku basins two oligocene to miocene back arc basins which provide independent constraints on the rotational history of the philippine sea plate by means of their seafloor spreading record we have created a detailed plate model for the opening of the parece vela and shikoku basins based on an analysis of all available magnetic gravity and bathymetric data in the region subduction along the izu bonin mariana trench led to trench roll back arc rupture and back arc rifting in the parece vela and shikoku basins at 30 ma seafloor spreading in both basins developed by chron 9o 28 ma and possibly by chron 10o 29 ma as a northward and southward propagating rift respectively the spreading orientation in the parece vela basin was e w as opposed to ene wsw in the shikoku basin the spreading ridges joined by chron 6by 23 ma and formed a r r r triple junction to accommodate the difference in spreading orientations in both basins at chron 6no 20 ma the spreading direction in the parece vela basin changed from e w to ne sw at chron 5ey 19 ma the spreading direction in the shikoku basin changed from ene wsw to ne sw this change was accompanied by a marked decrease in spreading rate cessation of back arc opening occurred at 15 ma a time of regional plate reorganisation in se asia we interpret the dramatic change in spreading rate and direction from e w to ne sw at 20 1 3 ma as an expression of philippine sea plate rotation and is constrained by the spacing between our magnetic anomaly identifications and the curvature of the fracture zones this rotation was previously thought to have begun at 25 ma as a result of a global change in plate motions our results suggest that the philippine sea plate rotated clockwise by about 4 between 20 and 15 ma about a pole located 35 n 84 e this implies that the majority of the 34 clockwise rotation inferred to have occurred between 25 and 5 ma from paleomagnetic data may have in fact been confined to the period between 15 and 5 ma

field measurements of wave overtopping at the rubble mound breakwater of rome ostia yacht harbour abstract this paper describes a new station for full scale measurement of wave overtopping at the rome yacht harbour rubble mound breakwater in ostia italy and the results of the successful first measurement campaign carried out during the winter season 2003 2004 the equipment and the research activities were supported by the eu project clash focusing on scale effects for wave overtopping at coastal structures the site is characterized by a very small tidal range a long shallow foreshore and depth limited breaking waves which interact with a shallow sloping porous rock structure overtopping water is collected by a steel tank installed on the crown slab behind the parapet wall the measurement of water level variation inside the tank by means of two pressure transducers allows the calculation of individual overtopping volumes incident waves sea levels and wind are also measured during seven independent storms more than 400 individual overtopping events were recorded and about 86 h of valid data are available this extensive dataset is presented discussed and then used for comparison with two commonly used overtopping prediction formulae based on small scale model tests showing their tendency to underestimate the prototype results a strong correlation between the hourly mean overtopping discharge and corresponding maximum volume is also presented the paper generally confirms the validity of the approach used in troch et al 2004 troch p geeraets j van de walle b de rouck j van damme l allsop w franco l 2004 full scale wave overtopping measurements on the zeebrugge rubble mound breakwater coastal engineering 51 609 628 for field measurement of wave overtopping

crustal strains in the japanese islands as deduced from dense gps array data from the nationwide continuous gps network operated by the geographical survey institute of japan were used to estimate crustal strains in the japanese islands we used horizontal velocity vectors at 610 sites for the period april 1996 to march 1997 and employed the least squares prediction method to interpolate the velocities to a uniform 7km 7km grid over japan the gridded velocities were then differentiated in space to calculate principal components of strains results clearly portray tectonic strainings of the japanese islands in one year 1 dilatational strains show that the japanese islands are under a compressive strain regime induced by the subducting oceanic plates 2 distribution of high maximum shear strains coincides with the regions of crustal activities and 3 principal axes of the strains indicate influences of e w and nw se compression due to the converging oceanic plates north south extension in kyushu is also visible

tectonic evolution of the japanese island arc system abstract growth of the japanese arc system which has mainly taken place along the continental margin of asia since the permian is the result of subduction of the ancient pacific ocean floor backarc basin formation in the tertiary shaped the present day arc configuration the neotectonic regime which is characterized by strong east west compression has been triggered by the eastward motion of the amur plate in the quaternary the tectonic evolution of the japanese arc system includes formation of rock assemblages common in most orogenic belts because the origin and present day tectonics of these assemblages are better defined in the case of the japanese arc system study of the system provides useful insight into orogenesis and continental crust evolution

a new model of slab tear of the subducting philippine sea plate associated with kyushu palau ridge subduction we suggest that the kyushu palau ridge kpr plays a key role in the subduction process of the philippine sea plate psp and the origin of the abu volcano in the southwestern japan the 3 d p wave velocity structure was imaged to approximately 300 km beneath the abu volcano using a large number of p wave arrivals from local earthquakes and teleseismic events our results indicate that a high velocity anomaly beneath the abu volcano is associated with the subducting psp however the anomaly is not continuous being interrupted apparently by a low velocity anomaly zone extending northwestwards from 80 km to great depth the psp appears to be tearing and then forms a slab window corresponding to kpr subduction at ca 2 ma the low velocity anomaly may indicate hot upper mantle material rising through the slab window and causing partial melting both of the lower crust of the overriding plate and the oceanic crust of the subducted kpr a new model is presented for slab tearing of the psp associated with the subduction of the buoyant wide and thick kpr and directional change in the motion of the plate contributing to better understanding of the abu volcanism c 2014 elsevier b v all rights reserved

volcano tectonic interactions during rapid plate boundary evolution in the kyushu region sw japan evolution of the local plate tectonic and volcanic system relationship at kyushu island is defined by major changes in tectonics and volcanic style at ca 15 10 6 and 2 ma plate reconstructions presented here suggest that prior to 15 ma the pacific plate subduction dominated kyushu tectonics from 15 to 6 ma the evolving relative plate motions shifted the triple junction between the pacific plate philippine sea plate and southwest japan northwards so that the philippine sea plate was subducted beneath kyushu we suggest that a lack of subduction related volcanism from 10 to 6 ma is due to shallow subduction of the young shikoku basin lithosphere by 6 5 ma changes in the philippine sea plate motion led to more rapid nearly trench normal subduction of the eocene west philippine basin crust beneath kyushu this model is supported by an increase in arc like geochemistry of lavas since ca 6 5 ma subduction of fluid rich features such as the kyushu palau ridge introduced large volumes of fluids into the kyushu arc system leading to voluminous volcanism across kyushu focused particularly in areas where the ridge subduction occurs in tandem with local extensional tectonics key issues such as the timing of izu arc collision with central japan and the history of motion of the philippine sea plate are reassessed here resulting in a model that favors izu arc central japan collision at ca 8 6 ma rather than the more widely accepted date of ca 15 ma

a performance test of nearshore wave height prediction with clash datasets abstract the clash database was analyzed for extraction of a set of data having the measured wave heights at both the deep station and the toe of the structure for wave overtopping tests yielding 1214 data from 29 datasets wave heights in front of the toe of the structure were estimated with the goda formulas and compared with the measured ones comparison yielded the overall mean of 1 106 with the standard deviation of 0 155 for the ratio of the estimated to the measured heights which support the use of the goda formulas for prediction of nearshore wave heights another set of 1215 data having the measured wave heights at the deep station and the wave heights calculated with the swan model was also extracted from the clash database a comparative test of the swan model using the wave height estimated with the goda formulas in lieu of the measured height indicated the performance of the swan model being similar to that of the goda formulas but a tendency of underestimation was noticed in shallow water on a beach of very gentle slope

imaging of the subducted kyushu palau ridge in the hyuga nada region western nankai trough subduction zone abstract we performed 3d seismic tomography of the hyuga nada region western nankai subduction zone to investigate the relationship of the subducted part of kyushu palau ridge kpr to coseismic rupture propagation seismicity and shallow very low frequency earthquakes combining active source and passive source data recorded both onshore and offshore we imaged the deep slab from near the trough axis to the coastal area our results show the subducted kpr as a low velocity belt oriented nw se extending down the plate boundary to around 30 km depth at this depth we suggest that the subducted kpr detaches from the slab and becomes underplated on the overriding continental plate as the coseismic slip areas of past large earthquakes do not extend into the subducted kpr we suggest that it may inhibit rupture propagation the interior of the subducted kpr shows active intraslab seismicity with a wide depth distribution shallow very low frequency earthquakes are continuously active above the location of the subducted kpr whereas they are intermittent to the northeast of the subducted kpr thus the subducted kpr appears to be an important factor in coseismic rupture propagation and seismic phenomena in this region

tearing of stagnant slab subducted slabs of oceanic lithosphere below the western pacific tend to be stagnant in the transition zone with poorly known mechanical properties typical examples are the izu bonin and japan slabs that meet each other to form a cusplike junction beneath southwest japan here we show that these two slabs are torn apart at their junction when they bend to flatten over the 660 kilometer discontinuity as is expected from a simple geometric argument we present three lines of evidence for this ongoing slab tear

tomographic evidence for the mantle upwelling beneath southwestern japan and its implications for arc magmatism abstract the seismotectonics and volcanotectonics of southwestern sw japan have been affected by the subduction of the old pacific and the young philippine sea slabs we have attempted to determine the three dimensional p and s wave velocity structures of the upper mantle in sw japan by using a large amount of high quality arrival time data recorded by the dense seismograph network on the japanese islands to understand the tectonic framework of sw japan our results show the existence of a large low velocity anomaly below the philippine sea slab in the chugoku and shikoku districts at least down to depths of 300 km we infer that the low velocity zone is an upwelling flow from depth two shallower low velocity zones derived from this low velocity zone are detected one is toward the quaternary volcanoes in the chugoku district and the other is toward the uppermost mantle beneath the kii peninsula the former suggests that the magma for the quaternary volcanoes in the chugoku district is attributable to the upwelling flow in the upper mantle this idea supports the geochemical and petrological observations that the erupted alkalic basalt in the chugoku district should have originated from a more primitive or enriched mantle the latter is responsible for the absence of a high velocity anomaly corresponding to the subducted philippine sea slab beneath the kii peninsula the philippine sea slab could have been more hydrated locally beneath the kii peninsula than that in the surrounding regions by the interaction with the mantle upwelling consequently dehydration reactions could be involved during the progressive metamorphism accompanied by intensive intraslab seismicity lowering the seismic velocity in the slab aqueous fluids thus released to the overriding plate might be a possible source of mantle type helium and trigger a long term swarm activity in the crust of the overriding plate our results indicate the importance of the heterogeneous structure below the philippine sea slab with regard to the tectonics of sw japan

origin and evolution of the west philippine basin a new interpretation abstract combined analyses of the magnetic lineations and seafloor structures of the west philippine basin show that it was formed by spreading from the central basin spreading center in two distinctly different spreading phases before and after subduction was initiated along the palau kyushu trend at 45 ma b p these episodes are recognized by differences in the strike of the magnetic lineations spreading rates density and strike of fracture zones and basement relief from 60 45 ma b p spreading was ne sw relative to present orientations at a half rate of 44 mm a at about 45 ma b p the spreading direction changed to a more n s direction with a reconfiguration of the central basin spreading center into numerous short e w segments offset by closely spaced n s transform faults spreading slowed to a half rate of 18 mm a and ceased at 35 ma b p thus the west philippine basin originated at 45 ma b p by the trapping of normal ocean crust west of the initial subduction along the palau kyushu trend the 45 35 ma b p period represents the dying phase of spreading on the central basin spreading center following isolation of the west philippine basin from the plate driving forces of the pacific

slab interactions in 3 d subduction settings the philippine sea plate region abstract the importance of slab slab interactions is manifested in the kinematics and geometry of the philippine sea plate and western pacific subduction zones and such interactions offer a dynamic basis for the first order observations in this complex subduction setting the westward subduction of the pacific sea plate changes along strike from single slab subduction beneath japan to a double subduction setting where pacific subduction beneath the philippine sea plate occurs in tandem with westward subduction of the philippine sea plate beneath eurasia our 3 d numerical models show that there are fundamental differences between single slab systems and double slab systems where both subduction systems have the same vergence we find that the observed kinematics and slab geometry of the pacific philippine subduction can be understood by considering an along strike transition from single to double subduction and is largely independent from the detailed geometry of the philippine sea plate important first order features include the relatively shallow slab dip retreating stationary trenches and rapid subduction for single slab systems pacific plate subducting under japan and front slabs within a double slab system philippine sea plate subducting at ryukyu in contrast steep to overturned slab dips advancing trench motion and slower subduction occurs for rear slabs in a double slab setting pacific subducting at the izu bonin mariana this happens because of a relative build up of pressure in the asthenosphere beneath the philippine sea plate where the asthenosphere is constrained between the converging ryukyu and izu bonin mariana slabs when weak back arc regions are included slab slab convergence rates slow and the middle philippine plate extends which leads to reduced pressure build up and reduced slab slab coupling models without back arcs or with back arc viscosities that are reduced by a factor of five produce kinematics compatible with present day observations

aseismic deep subduction of the philippine sea plate and slab window abstract we have made great efforts to collect and combine a large number of high quality data from local earthquakes and teleseismic events recorded by the dense seismic networks in both south korea and west japan this is the first time that a large number of korean and japanese seismic data sets are analyzed jointly as a result a high resolution 3 d p wave velocity model down to 700 km depth is determined which clearly shows that the philippine sea phs plate has subducted aseismically down to 460 km depth under the japan sea tsushima strait and east china sea the aseismic phs slab is visible in two areas one is under the japan sea off western honshu and the other is under east china sea off western kyushu however the aseismic phs slab is not visible between the two areas where a slab window has formed the slab window is located beneath the center of the present study region where many teleseismic rays crisscross detailed synthetic tests were conducted which indicate that both the aseismic phs slab and the slab window are robust features using the teleseismic data recorded by the japanese stations alone the aseismic phs slab and the slab window were also revealed zhao et al 2012 though the ray paths in the japanese data set crisscross less well offshore the slab window may be caused by the subducted kyushu palau ridge and kinan seamount chain where the phs slab may be segmented hot mantle upwelling is revealed in the big mantle wedge above the pacific slab under the present study region which may have facilitated the formation of the phs slab window these novel findings may shed new light on the subduction history of the phs plate and the dynamic evolution of the japan subduction zone

philippine sea and east asian plate tectonics since 52 ma constrained by new subducted slab reconstruction methods we reconstructed philippine sea and east asian plate tectonics since 52 ma from 28 slabs mapped in 3 d from global tomography with a subducted area of 25 of present day global oceanic lithosphere slab constraints include subducted parts of existing pacific indian and philippine sea oceans plus wholly subducted proto south china sea and newly discovered east asian sea mapped slabs were unfolded and restored to the earth surface using three methodologies and input to globally consistent plate reconstructions important constraints include the following 1 the ryukyu slab is 1000 km n s too short to account for 20 philippine sea northward motion from paleolatitudes 2 the marianas pacific subduction zone was at its present location 200 km since 48 10 ma based on a 1000 km deep slab wall 3 the 8000 2500 km east asian sea existed between the pacific and indian oceans at 52 ma based on lower mantle flat slabs 4 the caroline back arc basin moved with the pacific based on the overlapping coeval caroline hot spot track these new constraints allow two classes of philippine sea plate models which we compared to paleomagnetic and geologic data our preferred model involves philippine sea nucleation above the manus plume 0 150 e near the pacific east asian sea plate boundary large philippine sea westward motion and post 40 ma maximum 80 clockwise rotation accompanied late eocene oligocene collision with the caroline pacific plate the philippine sea moved northward post 25 ma over the northern east asian sea forming a northern philippine sea arc that collided with the sw japan ryukyu margin in the miocene 20 14 ma

plate tearing in the northwestern corner of the subducting philippine sea plate the philippine sea plate phs simultaneously subducts northwestward and collides eastward with the eurasian plate eu in northeast taiwan these two tectonic events induce high seismic activity which makes northeastern taiwan one of the most seismically active zones in the world to understand the mechanical processes at work we used existing geophysical data and the aftershocks recorded following a recent large strike slip event occurring within the phs oceanic crust during this event a nw se trending left lateral sub parallel to the phs eu convergence vector was active as a consequence of the collision subduction plate geometry we show that the lithosphere of the northwestern corner of the phs has been torn in a nw se orientation this tectonic feature is associated with an abrupt tectonic stress boundary and could generate large intra plate earthquakes

enigmatic highly active left lateral shear zone in southwest japan explained by aseismic ridge collision global positioning system gps site velocities and earthquake focal mechanisms reveal an active left lateral shear zone cutting across kyushu in southwest japan surprisingly no active faults have been identified in association with this zone of rapid contemporary deformation to explain the existence of this shear zone we propose a model comprising subduction of an aseismic ridge kyushu palau ridge at the southwest end of the nankai trough because of rapid 40 mm yr along strike migration of the ridge we suggest that the ridge subduction point and resulting left lateral shear zone is never in one place long enough to enable the development of a through going fault zone that can be identified at the ground surface reconciling the mismatch between the gps seismological and geological data in this region our conceptual model is supported by numerical modeling results we also suggest that the along strike change in subducting plate buoyancy explains the recent counterclockwise rotation of the kyushu forearc documented in paleomagnetic studies as is found in many other western pacific subduction margins

application of an improved algorithm to high precision relocation of isc test events abstract new location features for possible implementation by the international seismological centre in its standard location procedures are tested using a set of 156 well located and geographically well distributed earthquakes and explosions the tests are performed using the engdahl et al engdahl e r van der hilst r d buland r p 1998 global teleseismic earthquake relocation with improved travel times and procedures for depth determination bull seism soc am 88 3295 3314 ehb location algorithm with the 1 d reference earth model ak135 kennett b l n engdahl e r buland r 1995 constraints on seismic velocities in the earth from travel times geophys j int 122 108 124 weighting by phase variance as a function of distance improves location accuracy by 7 use of later phase arrival times does not result in a significant improvement in location or depth for events with observing stations well distributed in azimuth however with application of an improved phase identification technique depth phases provide significantly better estimates of focal depth

tectonic implications of early miocene oib magmatism in a near trench setting the outer zone of sw japan and the northernmost ryukyu islands abstract the outer zone of the sw japan and northernmost ryukyu arcs was affected by intense igneous activity during the miocene characterized by morb like basalts alkaline basalts and s type with subordinate i type felsic to intermediate volcano plutonic complexes these igneous rocks are inferred to be the products of near trench magmatism early miocene 18 ma alkaline basalt dikes from the shingu otoyo area in central northern shikoku and an alkaline lamprophyre dike from tanegashima one of the northernmost ryukyu islands pre date the middle miocene felsic to intermediate igneous rocks the basalts and lamprophyre have compositions of basanite basalt trachybasalt and phonotephrite they are characterized by elevated large ion lithophile elements liles e g sr ba and th and high concentrations of high field strength elements hfses e g tio 2 nb and zr the geochemical signatures of the basalts and lamprophyre suggest an ocean island basalt type oib type mantle source the occurrence of alkaline basalts and lamprophyre with oib type intraplate geochemical signatures in a near trench setting is unusual with regard to plate tectonic processes we propose that trench ward motion of the overriding plate during the period around the early miocene resulted in a shallowly dipping slab and interplate coupling between the subducting philippine sea plate psp and the overlying crust beneath most of the outer zone in the western part of sw japan and the northernmost ryukyu islands the oib type magmatism in the near trench environment is most plausibly explained by the upwelling of asthenospheric material from beneath the subducting slab which migrated through fractures and or tears in the slab we envisage two possible scenarios for the formation of these fractures or tears 1 the shallowing dip angle of the subducted psp resulted in concave upwards flexure of the slab generating fractures in the flexed region and 2 differential motion within the subducting slab induced slab tearing and segmentation

microbial foraminiferal episodes in the early aptian of the southern tethyan margin ecological significance and possible relation to oceanic anoxic event 1a two regionally significant microbial foraminiferal episodes 150 kyr each occur within the early aptian shallow marine platform in oman and throughout eastern arabia the stratigraphically lower of these two intervals is characterized by isolated or coalescent domes that share similarities with modern open marine stromatolites from the exuma cays bahamas the upper interval is predominantly built by a problematic lithocodium bacinella consortium in buildup and massive boundstone facies based on highresolution chemostratigraphy these shoalwater intervals are coeval with oceanic anoxic event 1a oae1a livello selli field evidence demonstrates that the buildup episodes alternate with stratigraphic intervals dominated by rudist bivalves this biotic pattern is also recognized in other coeval tethyan sections and is perhaps a characteristic shoalwater expression of the oae1a the short lived regional expansions of this microbial foraminiferal out ofbalance facies cannot be explained by local environmental factors salinity and oxygen level alone and the buildup consortia do not occupy stressed refugia in the absence of grazing metazoans judging from recent analogues the main fossil groups i e microbial assemblages macroalgae larger sessile foraminifera and rudist bivalves all favoured elevated trophic levels but with different tolerance limits the implication of this is that the influence of palaeofertility events possibly related to oae1a on carbonate platform community structures must be investigated the observations made in these coastal sections are a significant first step for the improved understanding of the early aptian period of biotic oceanic and climatic change

a note on wave energy dissipation over steep beaches abstract this paper revisits the derivation of the parametric surf zone model proposed by baldock et al baldock t e holmes p bunker s van weert p 1998 cross shore hydrodynamics within an unsaturated surf zone coast eng 34 173 196 we show that a consistent use of the proposed rayleigh distribution for surf zone wave heights results in modification of the expressions for the bulk dissipation rate and enhanced dissipation levels on steep beaches and over saturated surf zone conditions as a consequence the modification proposed herein renders the model robust even on steep beaches where it could otherwise develop a shoreline singularity

reinitiation of subduction and magmatic responses in sw japan during neogene time southwestern japan during paleogene time was affected by subduction of the kula and pacifi c plates beneath the eastern margin of eurasia followed by a brief episode of transform faulting to allow the shikoku basin to open at 27 15 ma subsequent opening of the japan sea back arc basin in middle miocene time broke japan away from eurasia and sw japan rotated clockwise 45o as it drifted south 17 15 ma southward drift required subduction of the philippine sea plate beneath sw japan leading to the formation of a magmatic arc subduction of the hot shikoku basin lithosphere and spreading ridge resulted in distinctive volcanism in the sw japan forearc between 17 and 12 ma this volcanism included midoceanic ridge basalt morb like intrusions and oceanic island basalt oib type alkali basalts felsic plutons in the accretionary prism and high magnesium andesites along the setouchi forearc basin mafi c magmas in the outermost forearc originated from the subducted shikoku basin spreading ridge whereas setouchi high magnesium andesites hma may have resulted from the interaction of melts of the subducted philippine sea plate with the overlying mantle felsic magmas in the forearc resulted from melting of shimanto belt sediments caused by intrusion of hma magmas persistent rear arc volcanism was caused by upwelling asthenosphere associated with opening of the sea of japan rear arc volcanism began ca 25 ma as rift fi ll low alkali tholeiite volcanism and was replaced gradually after 12 ma by alkali basalt volcanism upwelling related alkali volcanism continues up to the present whereas forearc volcanism ceased at 12 ma the volcanic arc narrowed with time as the philippine sea slab descended and slowly cooled adakitic dacites erupted after 1 7 ma above the 100 km depth contour of the subducted philippine sea plate suggesting that melting resulted from interaction of the slab with upwelling asthenosphere interactions between upwelled rear arc asthenosphere and subduction of the hot philippine sea slab appear to have been the main controls of magmatism for the neogene sw japan arc

heterogeneity within the subducting pacific slab beneath the izu bonin mariana arc evidence from tomography using 3d ray tracing inversion techniques distinct zones of seismic heterogeneity along the izu bonin mariana arc have been investigated using complimenting regional bulk sound shear wave speed and p wave tomographic images the distribution of seismic anomalies and the inferred geometry of the subducting pacific plate have been modelled in unprecedented detail using joint tomography and new p wave model using a 3d inversion algorithm the use of 3d ray tracing techniques and smaller cell parameterizations have greatly enhanced the resolution of gradients therefore the models show much more detail about the structure and physical properties of the subduction zone the well defined features from the multiple wave speed images are used to elucidate the distinct morphology change between slab beneath the izu bonin horizontal slab and mariana vertical arcs and the distribution of physical properties in the mantle and the subducting oceanic lithosphere changes in physical properties within the slab tear at the southern end of the izu bonin arc identified as a bgap or thinningq in the tomographic images could be the result of the distortion of the pacific plate as its shape transforms between near horizontal to near vertical a decrease in the rigidity and strength of the lithosphere the subduction of the marcus necker ridge change in subduction velocity or a combination of all these factors strong slow anomalies exist both in the mantle wedge above and below the slab beneath the izu bonin arc but are not present beneath the mariana arc at the junction between the izu bonin and mariana arcs it appears that the there are two separate pieces of the pacific plate the torn slab north of 268 n and a buckled near vertically dipping slab south of the tear the transition between the two distinct slab morphologies coincides with the location of the ogasawara plateau and the trench d 2005 elsevier b v all rights reserved

examination of the influence of several factors on longshore current computation with random waves abstract influence of various factors affecting the longshore currents induced by obliquely incident random waves is examined through numerical calculation seven numerical models for random wave breaking process are found to yield large differences in the wave heights in the surf zone and longshore current velocities the turbulent eddy viscosity formulation by larson and kraus larson m and kraus n c 1991 numerical model of longshore current for bar and trough beaches j waterway port coastal and ocean eng asce 117 4 pp 326 347 functions almost equal to that by battjes battjes j a 1975 modeling of turbulence in the surf zone proc symp modeling techniques pp 1050 1061 but the formulation by longuet higgins longuet higgins m s 1970 longshore current generated by obliquely incident sea waves 1 and 2 j geophys res 75 33 pp 6779 6801 produces excessive diffusion of longshore currents into the offshore zone the generation and decay process of the surface roller is indispensable in the longshore current analysis the random wave transformation model called pegbis parabolic equation with gradational breaker index for spectral waves by goda goda y 2004 a 2 d random wave transformation model with gradational breaker index coastal engineering journal jsce and world scientific 46 1 pp 1 38 produced good agreement with several laboratory and field data of longshore currents

volcanic and structural history of the hohi volcanic zone central kyushu japan more than 5000 km3 of magmatic material was erupted in pliocene pleistocene times in a volcano tectonic depression i e the hohi volcanic zone hvz in central kyushu japan the eruptive deposits consist mainly of andesite lava flows and large scale pyroclastic flow deposits their eruptions were accompanied by the formation of an ew oriented graben 70 km 45 km under regional ns extensional stress pre tertiary basement rocks are absent on the surface of the graben but occur at depth having subsided up to 3 km radiometric ages of volcanic rocks on the surface show zoned isochrons from 5 ma at the margin to 0 3 ma in the center of the hvz the youngest center of age zonation coincides with a 30 mgal negative bouguer gravity anomaly radiometric ages of rocks from drill cores are older toward the bottom of the graben reaching a maximum of at least 4 ma volcanic activity concentrated over time toward the center of the graben and buried successively erupted material areas of active volcanism in the hvz became smaller and changed in style during the 5 ma history of activity volcanism of the early stage 5 2 ma was characterized by voluminous eruptions of andesitic lava flows that formed lava plateaus and were intruded by ew oriented feeder dikes perhaps related to fissure eruptions in contrast late stage volcanism 2 0 ma resulted primarily in andesitic to dacitic lava domes with features of monogenetic volcanoes produced at low eruption rates the hvz shows unimodal volcanism dominated by andesitic and dacitic lavas with a small amount of rhyolite and only traces of basalt these characteristics differ from those that typify volcanism in most other extensional areas erupted material in the hvz is of the calc alkali and high alkali tholeiite series and shows no significant chemical changes over 5 ma except for an increase in k2o after 1 6 ma the net horizontal displacement along normal faults indicates that the hvz widened by about 10 20 across the graben at an average rate of 0 1 cm yr i interpret the hvz to be neither a pull apart structure of the pre tertiary basement nor the result of propagation of the okinawa trough but rather the earliest stage of rifting when vertical subsidence caused by normal faulting is compensated by filling with volcanic material

tears or thinning subduction structures in the pacific plate beneath the japanese islands abstract the nature of a subduction zone at depth is affected by the evolution of its tectonic system and the geometry of the trench line can change over time due to slab roll back or the arrival of a distinctive feature with the incoming oceanic lithosphere the configuration of the plate has to accommodate such changes with buckling thinning or the formation of tears depending on the rate of influx to the trench tomographic imaging is commonly used to recognise the presence of such tears through marked reductions in wavespeed anomalies in localised zones a good example is provided by pacific plate subduction beneath the japanese islands a horizontal tear in the plate below 300 km depth can be recognised at the southern end of the izu bonin arc associated with the change in slab morphology to the much steeper mariana arc beneath southern honshu a break in the fast wavespeeds associated with the pacific plate has been described as a tear based on the evidence of converted phases from the edge of the zone and tensional focal mechanisms for seismic events in the tear zone in the north close to the hokkaido bend in the subduction zone the reduction in the shear wavespeed anomaly is just as dramatic but here the characteristics of high frequency guided waves from deep earthquakes indicate continuity of slab material with thinning of the slab the thinned slab has less wavespeed contrast within the affected cells and so appears in the tomographic images as a weakened anomaly the various modes of slab deformation represent different ways in which the subducted material accommodates the strains imposed by the evolution of the geometry of the subduction scenario not all significant reductions in wavespeed anomalies represent tears and thus it is important that such interpretations be checked against the characteristics of wave propagation through the zone

coseismic slip resolution along a plate boundary megathrust the nankai trough southwest japan geodetic survey measurements are used to estimate the coseismic slip distribution in the 1944 tonankai mw 8 1 and 1946 nankaido mw 8 3 earthquakes and to assess quantitatively the degree to which this slip is resolved on the plate boundary megathrust data used include 798 angle changes from triangulation surveys 328 leveling section differences and 5 coseismic tidal gage offsets many of the nominally coseismic triangulation data span 50 years nearly half the earthquake cycle and correction for interseismic deformation using post 1950 observations is applied microseismicity is used to define the configuration of the plate boundary interface and approximate it with a continuous multisegment fault model because the onshore geodetic data have very limited resolving power for offshore fault segments offshore coseismic slip was constrained by satake s 1993 estimation based on tsunami data the majority of the coseismic slip occurs between 15 and 25 km depth although resolution declines toward the trench axis it is sufficiently good to define two distinct high slip regions one off southeastern shikoku island 11 m maximum and the other offshore of kii peninsula 3 m maximum the slip magnitude off southeastern shikoku coupled with the plate convergence rate would imply an recurrence interval of about 270 years much longer than the average repeat time of 120 years for historical great earthquakes on the nankai trough however the maximum coseismic slip is sensitive to the assumed fault geometry and slippage on trough parallel splay faults could significantly decrease the maximum slip to about 6 m

deep structure of japan subduction zone as derived from local regional and teleseismic events we have determined a detailed three dimensional p wave velocity structure of the japan subduction zone to 500 km depth by inverting local regional and teleseismic data simultaneously we used 45 318 p wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the japan islands the arrival times are recorded by the japan university seismic network which covers the entire japan islands densely and uniformly we also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern japan in comparison with the previous results obtained from only local and regional events the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes in the crust and uppermost mantle low velocity zones are clearly visible beneath active volcanoes in the mantle wedge the low velocity zones generally parallel with the slab and exist continuously to a depth of about 200 km which is consistent with the petrological geochemical and geodynamic studies we consider that the existence of volcanism related low velocity anomalies in the mantle wedge is a general seismological characteristic of subduction zones in light of all the available tomographic results for many subduction zones in the world the pacific slab beneath japan is imaged more clearly than in previous studies as a high velocity zone with a thickness of 80 90 km and a p wave velocity 4 6 higher than the normal mantle lower velocity anomalies are found in the mantle below the slab

vp and vs tomography of kyushu japan new insight into arc magmatism and forearc seismotectonics high resolution 3d tomographic images of the crust and upper mantle are determined under the entire arc region of kyushu by inverting a large number of arrival time data of p wave s wave and sp depth phases the hypocenters of the offshore earthquakes are accurately located by applying a combined method that makes use of sp depth phases and double difference of the p wave and s wave arrival times our tomographic results confirmed the major features delineated by previous studies and revealed some new features of structural heterogeneity related to arc magmatism beneath the kyushu arc high velocity anomalies correspond to the subducting philippine sea slab whilst low velocity zones in the mantle wedge are clearly imaged coinciding well with the velocity heterogeneities determined by previous studies strong low velocity anomalies are distributed extensively along the volcanic front and extend to the back arc side in the crust and upper mantle our present seismic images together with the previous results suggest that the fluids supplied by the dehydration processes of the descending philippine sea slab and the hot upwelling materials related to the opening of the okinawa trough might have contributed to the unzen volcanism under the forearc region of the kyushu subduction zone prominent low velocity anomalies are visible at depths of 10 40 km along the upper boundary of the subducting philippine sea slab and large interplate earthquakes are located around the low velocity zones which is similar to the features revealed in the northeast japan subduction zone the spatial distribution of large thrust earthquakes below the offshore region reflects the existence of weak coupled or decoupled portions on the interface of the subducting philippine sea slab

geochemical evidence for the melting of subducting oceanic lithosphere at plate edges most island arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust1 volcanic rocks interpreted to contain a component of melt not just a fluid from the subducting slab itself are uncommon but possible examples have been recognized in the aleutian islands baja california patagonia and elsewhere2 3 4 the geochemically distinctive rocks from these areas termed adakites are often associated with subducting plates that are young and warm and therefore thought to be more prone to melting5 but the subducting lithosphere in some adakite locations such as the aleutian islands appears to be too old and hence too cold to melt6 7 this implies either that our interpretation of adakite geochemistry is incorrect or that our understanding of the tectonic context of adakites is incomplete here we present geochemical data from the kamchatka peninsula and the aleutian islands that reaffirms the slab melt interpretation of adakites2 but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate we conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow the use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction zone environments

origin and motion history of the philippine sea plate abstract the philippine sea plate is the one major plate whose tertiary motion is poorly constrained and whose origin is problematical its southern boundary is the sorong fault system which is part of a major left lateral fault system at the northern margin of the australian plate the southern part of the plate in eastern indonesia has been neglected in most syntheses but includes some of the oldest rocks within the plate which are separated from remnant arcs of the daito ridge province of the northern philippine sea by the west philippine central basin the east indonesian islands of the halmahera waigeo region contain a good mesozoic and tertiary stratigraphic record indicating a long arc history for the southern part of the plate new palaeomagnetic data from these islands define two sub areas an area forming part of the philippine sea plate north of the sorong fault and an area within the sorong fault system the area north of the fault records a long term clockwise rotation history whereas that within the fault zone records local rotations interpreted as due to deformation at the plate edge rocks of philippine sea plate origin within both areas record similar latitudinal shifts the rotation of the area north of the sorong fault is considered to represent the motion of the southern part of the philippine sea plate the new data indicate large tertiary clockwise rotations similar to earlier suggestions for other parts of the plate but record a discontinuous and more complex motion history than previously suggested for the southern part of the plate there was 40 rotation with northward translation between 0 and 25 ma no significant rotation between 25 and 40 ma and there was 50 rotation with southward translation between 40 and 50 ma we show that the new palaeomagnetic data form part of a single set with earlier palaeomagnetic data from elsewhere in the plate the translation history of the southern part of the plate in eastern indonesia can be reconciled with northward motions recorded elsewhere and can be used to determine rotation poles for the plate 15 n 160 e for the interval 5 25 ma and 10 n 150 e for the interval 40 50 ma reconstructions based on these poles predict that at 45 ma the palau kyushu ridge had a wnw ese orientation which is very different from that postulated by many models used to explain the widespread boninite volcanism in the izu bonin marians forearc at this time the long arc history of the southern part of the plate and the reconstructions based on the rotation poles calculated from the palaeomagnetic data favour an origin for the west philippine basin by spreading in a backarc basin

imaging the subducting slabs and mantle upwelling under the japan islands summary a high resolution p wave tomography of the crust and mantle down to 700 km depth beneath the japan islands is determined using a large number of high quality arrival time data from local earthquakes and teleseismic events simultaneously the tomography shows that the philippine sea slab is subducting aseismically down to 430 km depth under southwest japan though the seismicity within the slab ends at 180 km depth a low velocity low v zone in the mantle wedge under tohoku and kyushu is found to extend westward from the volcanic front to the backarc under the japan sea and east china sea significant low v anomalies are revealed in the deep portion of the mantle wedge 400 500 km depth above the pacific slab under southwest japan which may reflect hot mantle upwelling associated with fluids from the deep dehydration of the pacific slab low v anomalies appear at 420 700 km depths beneath the pacific slab under eastern japan which may reflect hot mantle upwelling associated with the deep subduction of the pacific slab and its collapsing down to the lower mantle

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

dynamics of the ryukyu izu bonin marianas double subduction system trench motions represent the surface expression of the interaction between subducting plates and the underlying mantle but the inherent dynamics are not fully understood one interesting case is the migration of the izu bonin marianas trench ibm that accommodates the subduction of the pacific beneath the philippine sea plate psp which is in turn subducting beneath the eurasian plate along the ryukyu trench the history of the ibm trench is dominated by fast episodic retreat from 40 to 15 ma however around 10 5 ma the ibm trench reversed its motion from retreat to advance the switch in trench motion occurred soon after the breakoff of the psp slab along the ryukyu trench and the onset of new subduction and represents a fundamental change in the dynamics of the western pacific subduction zones here following the modelling study of ci kova and bina 2015 which suggested a link between ibm trench advance and ryukyu subduction we run 2 d numerical experiments to test the influence of a newly formed ryukyu slab on the established ibm subduction zone we run two dimensional numerical experiments to test the influence of this newly formed ryukyu slab on the ibm subduction zone the results from our geodynamic model compare favourably with the reconstructed trend indicating that the switch in trench motion along the ibm trench may indeed be related to the onset of a new subduction zone along the ryukyu trench our analysis substantiates the idea that advancing trench motions in the western pacific are due to the establishment of a double subduction system further analysis of such dynamics provides insights for the mechanisms controlling subducting plate and trench motions and mantle force transmission

testing global synchronism in peri pacific arc volcanism abstract episodicity of arc volcanism along the pacific rim is quantitatively analysed based on the record preserved by ash layers in deep sea sediments drilled by the deep sea drilling project and the ocean drilling program two major pulses of arc volcanism have been found one during middle miocene times 18 13 ma the other during pliocene quaternary times 5 0 ma these pulses are shown to relate to the tectonic stress regime of the arc lithosphere rather than to subduction rate these episodes of arc volcanism activity coincide with major tectonic events documented on circum pacific subduction zones as a consequence a global control of plate kinematics could influence both volcanic and tectonic evolution of pacific convergent margins

slab rollback suggested by latest miocene to pliocene forearc stress and migration of volcanic front in southern kyushu northern ryukyu arc abstract the northern ryukyu arc has active backarc rift neutral stress forearc and active accretionary prism the okinawa trough has been shaped by the episodic rifting in the backarc paleostresses were inferred in this study from mesoscale faults in neogene forearc sediments called the miyazaki group southeast kyushu in the northern ryukyu arc the forearc stress changed from compressional to extensional from the latest miocene through early pliocene time the stress history is concordant with the transition in tectonic regime from folding to rifting in the backarc the transition in the stress state occurred simultaneously also with trenchward movement of the volcanic front these phenomena suggest that the subducting slab under southern kyushu became steeper in the early pliocene extensional tectonics ceased sometime in the late pliocene or early to mid pleistocene concordant with the counterclockwise change of subducting direction of the philippine sea plate

three dimensional numerical models of the influence of a buoyant oceanic plateau on subduction zones abstract we have investigated potential effects on a subduction zone during oceanic plateau trench collision using three dimensional numerical models an oceanic plateau of varied density was embedded in the slab which was pinned at one end the plateaus strongly influenced the shape of the trench for a plateau with a higher density the trench retreat rate was reduced in the region surrounding the plateau and the plateau subducted along with the slab for lower density plateaus the trench in the region of the plateau advanced and the plateau compressed and resisted subduction spreading laterally along the trench with a weaker slab rheology the arcuate shape of the trench towards the free end of the trench was enhanced beneath the most buoyant plateaus a tear formed in the subducted portion of the slab soon after the slab tip reached the top of the lower mantle we compare the model results with a region in the northwest pacific where the ogasawara plateau meets the trench of the izu bonin mariana subduction zone

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting