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

three dimensional numerical modeling of temperature and mantle flow fields associated with subduction of the philippine sea plate southwest japan we investigated temperature and mantle flow distributions associated with subduction of the philippine sea phs plate beneath southwest japan by constructing a three dimensional parallelepiped model incorporating a past clockwise rotation the bathymetry of the philippine sea plate and distribution of the subducting velocity within its slab the geometry of the subducting plate was inferred from contemporary seismic studies and was used as a slab guide integrated with historical plate rotation into the 3 d simulation using the model we estimated a realistic and high resolution temperature field on the subduction plate interface which was constrained by a large number of heat flow data and attempted to clarify its relationship with occurrences of megathrust earthquakes long term slow slip events l sses and nonvolcanic low frequency earthquakes lfes results showed that the oblique subduction coupled with the 3 d geometry of subducting phs plate was a key factor affecting the interplate and intraplate temperature distributions leading to a cold anomaly in the plate interface beneath western shikoku the bungo channel and the kii peninsula temperatures in the slab core in these regions at a depth near the continental moho were nearly 200 c lower than that in eastern shikoku indicating a high thermal lateral heterogeneity within the subducting plate the geothermal control of the lfes beneath western shikoku was estimated to be within a range from 400 to 700 c and the interplate temperature for the l sses with a slip larger than 15 cm beneath the bungo channel was estimated to be approximately 350 500 c a large horizontal temperature gradient of 2 5 c km was present where the lfes occurred repeatedly the steep temperature change was likely to be related to the metamorphic phase transformation from lawsonite or blueschist to amphibolite of hydrous minerals of the mid ocean ridge basalt of the subducting phs plate

formation of distinct granitic magma batches by partial melting of hybrid lower crust in the izu arc collision zone central japan the miocene kofu granitic complex kgc occurs in the izu collision zone where the izu bonin mariana ibm arc has been colliding with the honshu arc since the middle miocene the kgc includes rocks ranging in compositions from biotite bearing granite the shosenkyo and mizugaki plutons and hornblende biotitebearing granodiorite tonalite quartz diorite and granite the shiodaira sanpo hirose and sasago plutons to hornblende bearing tonalite and trondhjemite the ashigawa tonogi pluton indicating that it was constructed from multiple intrusions of magma with different bulk chemistry the sr isotopic compositions corrected to sensitive high resolution ion microprobe shrimp zircon ages sri suggest that the primary magmas of each pluton were formed by anatexis of mixed lower crustal sources involving both juvenile basalt of the ibm arc and shimanto sedimentary rocks of the honshu arc after the primary magmas had formed the individual plutons evolved by crystal fractionation processes without significant crustal assimilation or additional mantle contribution shrimp zircon u pb ages in the kgc range from 16 8 to 10 6ma and overlap the resumption of magmatic activity in the ibmand honshu arcs at c 17ma and the onset of ibm arc honshu arc collision at c 15ma the age of the granite plutons is closely related to the episodic activity of arc magmatism and distinct granitic magma batches could be formed by lower crustal anatexis induced by intrusion of underplated mantle derived arc magmas based on pressures determined with the al in hornblende geobarometer the kgc magmas intruded into the middle crust thus the kgc could represent an example of the middle crust layer indicated throughout the ibm arc by 6 0 6 5 km s seismic velocities this granitic middle crust layer acted buoyantly during the ibm arc honshu arc collision leading to accretion of buoyant ibmarc middle crust to the honshu arc

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

anelastic structure of the upper mantle beneath the northern philippine sea we investigated the upper mantle anelastic structure beneath the northern philippine sea region including the izu bonin subduction zone and the shikoku basin we used regional waveform data from 69 events in the pacific and the philippine sea slabs recorded on f net and j array network broadband stations in western japan using the s p phase pair method we obtained differential attenuation factors t which represent the relative whole path q we conducted a tomographic inversion using 978 t values to invert for a fine scale 50 100 km three dimensional anelastic structure the results shows two high q regions qp 1000 which are consistent with the locations of the pacific and the philippine sea slabs also there is a low q qp 110 area extending to the deeper parts 350 400 km of the model just beneath the old spreading center and the kinan seamount chain in the shikoku basin a small depth dependence of the laterally averaged qp was found with values of 266 0 250 km 301 250 400 km and 413 400 500 km 2003 elsevier b v all rights reserved

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

origin of felsic volcanism in the izu arc intra arc rift an intra arc rift iar is developed behind the volcanic front in the izu arc japan bimodal volcanism represented by basalt and rhyolite lavas and hydrothermal activity is active in the iar the constituent minerals in the rhyolite lavas are mainly plagioclase and quartz whereas mafic minerals are rare and are mainly orthopyroxene without any hydrous minerals such as amphibole and biotite both the phenocryst and groundmass minerals have felsic affinities with a narrow compositional range the petrological and bulk chemical characteristics are similar to those of melts from some partial melting experiments that also yield dry rhyolite melts the hydrous mineral free narrow mineral compositions and low al2o3 affinities of the iar rhyolites are produced from basaltic middle crust under anhydrous low temperature melting conditions the iar basalt lavas display prominent across arc variation with depleted elemental compositions in the volcanic front side and enriched compositions in the rear arc side the across arc variation reflects gradual change in the slab derived components as demonstrated by decreasing ba zr and th zr values to the rear arc side rhyolite lavas exhibit different across arc variations in either the fluid mobile elements or the immobile elements such as nb zr la yb and chondrite normalized rare earth element patterns reflecting that the felsic magmas had different source the preexisting arc crust formed during an earlier stage of arc evolution most probably during the oligocene prior to spreading of the shikoku back arc basin the lack of systematic across arc variation in the iar rhyolites and their dry shallow crustal melting origin combines to suggest re melting of preexisting oligocene middle crust by heat from the young basaltic magmatism

complexities in the tectonic evolution of the west philippine basin abstract the oceanic west philippine basin can be subdivided into a main basin a northwest sub basin and a southern sub basin these basins have undergone different tectonic histories magnetic anomalies 21 17 are present in the main basin and indicate that it formed by two limbed seafloor spreading the now extinct spreading center is located within a zone of rough topography associated with the northwest trending central basin fault cbf the morphology of the cbf is primarily the result of post spreading tectonism the magnetic anomaly pattern of the main basin does not extend into the southern sub basin or the northwest sub basin geometrical constraints indicate that the southern sub basin did not form as part of the southern limb of the main basin the southern sub basin may have formed immediately before a ridge jump to the main basin spreading axis alternatively possible shallower crustal depth in the southern sub basin may indicate that the sub basin is younger than the main basin and formed separately from it in the northwest sub basin which originated as part of the main basin the dominant topographic trends are perpendicular to those in the main basin this difference as well as higher heat flow and shallower depth in the northwest sub basin indicate that it has undergone a deformational event which did not affect the main basin this deformation may be related to subduction along the east luzon margin in mid tertiary time the gagua ridge an uplifted sliver of oceanic crust along the western edge of the northwest sub basin may have originated during this event

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

subduction of the philippine sea slab in view of p wave anisotropy abstract three dimensional 3d structure of isotropic and anisotropic velocities in the chugoku and shikoku districts southwest japan is retrieved from the first p arrival times from local earthquakes by assuming p wave azimuthal anisotropy with hexagonal symmetry axis the main result of isotropic velocity structure is that the philippine sea phs slab is represented by a high velocity body with the thickness of 20 30 km lying at the around depth of 40 km beneath the shikoku and southern chugoku districts the phs slab subduction appears to be interrupted by a small scale low velocity body under middle chugoku and is not clear under north chugoku these features of 3d isotropic velocity structure are basically similar to those obtained in previous tomographic studies using recent seismic data this result suggests that seismic anisotropy has no significant effect on estimation of isotropic velocity structure from the p arrival time data on the other hand fast propagation directions of p wave azimuthal anisotropy in the crust are basically in close agreement with fast polarization directions estimated from polarization anomaly of shear waves azimuthal anisotropy in the crust of shikoku district is characterized by the fast propagation direction with the e w orientation trend which is nearly parallel to trajectory of maximum principal stress acting on the southwest japan arc and strike directions of zonal geological structure crustal anisotropy in the chugoku district is consistent with the orientation of the geological structure and not with the maximum principal stress direction these results indicate that the crustal anisotropy is attributable not only to alignment of the stress induced cracks but also to lineament of geological structures we could also point out the difference in the upper mantle anisotropy between the shikoku and chugoku districts i e p wave velocity tends to be fast in the n s directions beneath shikoku and southern chugoku and in e w directions under northern chugoku since the n s axis trend of p wave anisotropy is highly correlated with high velocity anomaly of phs slab it is regarded as slab anisotropy the reason why the p wave anisotropy shows the high propagation velocity in the e w direction beneath northern chugoku is discussed on the example of two cases in which the phs slab is absent and present in the upper mantle beneath the region although we could not locate exactly the north leading edge of the phs slab in view of anisotropic velocity structure if the phs slab is defined as a high velocity body with fast propagation axes in the directions of n s system it turns out to exist at least beneath shikoku to south chugoku

two types of antigorite serpentinite controlling heterogeneous slow slip behaviours of slab mantle interface abstract it is known that plate boundaries in subduction zones show heterogeneous slip nature with strongly coupled seismogenic zones and various types of episodic tremor and slip ets zones in order to examine the petrological controls on the large scale structure we compared recent geophysical observations in the shikoku area southwest japan with petrological models of the hanging wall mantle wedge as a result we found a close relationship between mineral assemblages in the mantle wedge and the characteristics of slow slip behaviour recorded in the shikoku area short term etss take place in the antigorite olivine stability field and silent long term slow slip events sses take place in the antigorite brucite stability field based on observations of natural antigorite serpentinites we propose a model that the dominant serpentinization reaction in the mantle wedge changes with increasing depth resulting in variable extents of pore fluid pressures along slip planes the serpentinization reaction in the antigorite brucite stability field olivine h 2 o antigorite brucite proceeds at the expense of water this is consistent with moderately elevated pore pressures inferred for long term sses the existence of weak brucite enhances the development of shear zones oblique to the main foliation the resultant anastomosing network provides fluid pathways that may help reduce pore pressures on slip planes in contrast progress of the serpentinization reaction in the antigorite olivine stability field olivine h 2 o sio 2 antigorite results in a large amount of residual water that contributes to further increase pore fluid pressures on slip planes of short term sses our results imply that understanding of serpentinization reactions and their contributions to fluid networks in mantle wedge is important in constructing quantitative 3 d models for strain evolutions along plate boundaries

three dimensional crustal structure of the mariana island arc from seismic tomography 1 a three dimensional 3 d seismic refraction survey was acquired over the mariana volcanic arc at 14 5 18 5n and 145 147e first arrival traveltimes from this survey and from a separate 2 d survey acquired approximately perpendicular to the arc have been simultaneously inverted for a 3 d p wave velocity model using seismic tomography subject to smoothness constraints the active arc which initiated only 3 4 ma ago has an average crustal thickness of 18 km approximately 40 km to the east the inactive remnant of the rifted eocene arc has an average crustal thickness of 21 km due primarily to a thicker lower crustal layer with velocities of 6 5 7 0 km s 1 crustal production clearly varies both temporally and spatially with some crustal layers including the igneous forearc crust varying in thickness by a factor of up to 2 along strike average p wave velocities within the upper crust of the modern arc are 240 360 m s 1 lower than in the eocene arc but are 280 m s 1 higher within the lower crust middle crust with velocities of 6 0 6 5 km s 1 is best developed beneath the eocene arc these results suggest an evolution of arc structure with increasing age we infer closure of fractures and porosity in the upper crust through hydrothermal circulation and a reduction in the mafic character of the middle to lower crust as a result of intracrustal differentiation although tonalitic rocks may predominate in the transition from upper to middle crust the bulk of the crust is essentially basaltic

the early miocene 25 ma volcanism in the northern kyushu palau ridge enriched mantle source injection during rifting prior to the shikoku backarc basin opening the northern kyushu palau ridge kpr remnant conjugate arc of the izu ogasawara bonin mariana ibm active arc is dominated by basalt andesite except for the komahashi daini seamount where acidic plutonic rocks of 38 ma were recovered these mafic to intermediate volcanics are produced by the rifting volcanism in the proto ibm arc associated with spreading of the shikoku basin the hfse and hree contents and ratios of these volcanics indicate enriched source mantle composition compared to recent volcanic front the lile ratios exhibit similar characteristics to reararc volcanism of the recent izu arc and some enriched volcanics exhibit high abundance of sediment melt inputs based on these observations and compilations of the published data set the replacement event of the wedge mantle under the ibm arc occurred two times the first event occurred between 45 and 38 ma with pacific type mantle being replaced by depleted indian type mantle the second event occurred between 36 and 25 ma enriched mantle flowed from reararc side the slab component during the proto ibm arc rifting was a similar characteristic to recent reararc volcanism of the izu arc and sediment melt added in a local area

seismological evidence for variable growth of crust along the izu intraoceanic arc 1 the processes that create continental crust in an intraoceanic arc setting are a matter of debate to address this issue we conducted an active source wide angle seismic study to examine along arc structural variations of the izu intraoceanic arc the data used were acquired over a 550 km long profile along the volcanic front from sagami bay to tori shima the obtained structural model showed the existence of felsic to intermediate composition middle crust with a p wave velocity vp of 6 0 6 5 km s 1 in its upper part and 6 5 6 8 km s 1 in its lower part the thickness of the middle crust varied markedly from 3 to 13 km the underlying lower crust also consisted of two layers vp of 6 8 7 2 km s 1 in the upper part and vp of 7 2 7 6 km s 1 in the lower part the upper of these layers was interpreted to consist of plutonic gabbro and the lower layer was interpreted to be mafic to ultramafic cumulates average crustal velocities calculated from our model showed remarkable lateral variation which correlated well with arc volcanism low average crustal seismic velocities 6 7 km s 1 due to thick middle crust were obtained beneath basaltic volcanoes e g o shima miyake jima hachijo jima aoga shima while higher average velocities 7 1 km s 1 were obtained beneath rhyolitic volcanoes e g nii jima kurose south hachijo caldera myoji knoll and south sumisu caldera we concluded from these observations that continental crust grows predominantly beneath the basaltic volcanoes of the izu arc and that rhyolitic volcanism may be indicative of a more juvenile stage of crustal evolution or remelting of preexisting continental crust or both

a cause of rupture segmentation and synchronization in the nankai trough revealed by seismic imaging and numerical simulation 1 a giant earthquake occasionally occurs in a subduction zone owing to a simultaneous rupture in adjacent segments which have been previously ruptured by large earthquakes however it is still unknown if a giant earthquake coincidentally occurs or if there is a causal factor to control its generation in this study we show a cause and a growth process of a giant earthquake which may occur along southwestern japan on the basis of seismic images obtained from wide angle seismic data and a numerical simulation incorporating the structural images the wide angle seismic data were acquired along three trough parallel profiles crossing the rupture segmentation boundary between the 1944 tonankai moment magnitude mw 8 1 and the 1946 nankai mw 8 4 earthquakes the seismic imaging detected a high seismic velocity body forming a strongly coupled patch at the segmentation boundary the numerical simulation explained the historic rupture patterns and shows the occurrence of a giant earthquake along the entire nankai trough a distance of over 600 km long mw 8 7 the growth process revealed from the simulated slip history in and around the strongly coupled patch is 1 prior to the giant earthquake a small slow event or earthquake occurs near the segmentation boundary 2 this accelerates a very slow slip slower than the plate convergent rate at the strong patch which reduces a degree of coupling and 3 then a rupture easily propagates through the strong patch when the next earthquake is nucleated near the segmentation boundary consequently growing into a giant earthquake

a geologic test of the kula pacific ridge capture mechanism for the formation of the west philippine basin in spite of nearly 30 yr of investigation the origin of the west philippine basin remains elusive this basin occupies nearly half of the philippine sea plate which remains one of the most poorly understood of the major lithospheric plates on earth establishing the origin of the west philippine basin is fundamental to deciphering the evolution of the western pacific basin including the india asia collision and the opening of numerous marginal basins in the region in this paper we combine new on land kinematic data from southwest japan with existing marine magnetic data from the ancient kula pacific ridge to test an early tectonic model that suggests that the west philippine basin arose by trapping of a part of the kula pacific ridge during the middle eocene the results suggest that this mechanism is viable but that the ridge capture event may have occurred during the middle eocene 10 m y earlier than originally proposed

double saloon door tectonics in the japan sea fossa magna and the japanese island arc abstract a number of criteria considered diagnostic of double saloon door rifting and seafloor spreading are matched by data from the japanese arc these include i a pair of terranes sw and ne honshu which rotated in opposite directions from 22 21 ma to 14 11 ma ii rotated terranes which comprise a retro arc fold thrust belt attached to an accretionary wedge intruded by a magmatic arc iii contemporaneous backarc extension from 24 to 21 ma which is brought to a halt by progressive collision of the izu bonin and japan arcs from 15 to 5 ma iv isolation of blocks of thicker continental crust by areas of thin continental or oceanic crust during backarc rifting v such isolation may be due to simultaneous rifting or to progressively seaward rifts associated with ridge jumps towards the subduction zone vi opposite rotations are accommodated by subduction rollback demonstrated by seaward migration of the volcanic front from 30 26 ma to 16 15 ma vii concurrent development of a major arc orthogonal rift the fossa magna from 23 18 ma to 14 ma which was thereafter inverted from 15 ma to the recent viii a northeast propagating rift in the northern japan basin demonstrated by the relationship of linear magnetic anomalies to the mapped continent ocean boundary driving mechanisms for double saloon door tectonics are discussed in relation to various reconstructions of the northwest pacific opposite rotational torques leading to opposite terrane rotations may be caused by rollback of a curved trench hingeline or by the divergent slab sinking forces of the pacific and philippine sea plates

slab derived fluids fore arc hydration and sub arc magmatism beneath kyushu japan we estimate the three dimensional 3 d p wave attenuation structure beneath kyushu japan using a large number of high quality waveform data our results show that the mantle wedge is characterized by high attenuation regions in the fore arc corner and in the back arc beneath volcanoes with the two regions separated by a low attenuation area the volcanic gap in central kyushu is underlain by low attenuation below the moho high attenuation in the fore arc is probably associated with serpentinized peridotite while that in the back arc is interpreted as an upwelling flow that is the source of arc magmas the presence of low attenuation mantle that separates the high attenuation hydrated fore arc and back arc mantle regions suggests that fluids are supplied from two depth levels of the slab by different mechanisms low attenuation beneath the volcanic gap probably results from intricate 3 d mantle flow that is caused by tectonic processes such as back arc extension and ridge collision

recent geodynamic evolution of the north fiji basin southwest pacific a cruise of the r v jean charcot seapso iii december 1985 in the north fiji basin between 16 and 22 s obtained new structural and magnetic data concerning the central part of the basin these have been used for the reinterpretation of the national oceanic and atmospheric administration and u s naval research project aeromagnetic survey 1979 the structural surface ship magnetic and aeromagnetic data allow us to present a sketch of the evolution in four stages of the north fiji basin for the past 10 m y

surface wave tomography and azimuthal anisotropy of the philippine sea plate abstract long period and broadband waveform data from 1984 to 2007 extracted mainly from the iris data center with additional data from 1997 to 2007 extracted from the ocean hemisphere project is used to perform a detailed mapping of rayleigh wave group velocity and anisotropic structures of the philippine sea plate psp our results show prominent high rayleigh wave group velocity anomalies and heterogeneity in three marginal basins of the psp at period up to 30 s the convergent boundaries around the psp are associated with low velocity anomalies heterogeneity of velocity distribution at deep depths along the ryukyu trench is also observed our results indicate that the azimuthal anisotropy in the psp is depth dependent the fast direction in the west philippine basin and the shikoku basin is in ne sw at shorter periods which is correlated with fossilized fabric produced by final phase of deformation whereas the fast direction in the parece vela basin is complex at shorter periods and becomes uniformly ne sw oriented at longer periods at the depths near the lithosphere asthenosphere boundary the fast directions become parallel to the absolute plate motion for various parts of the psp the fast directions along subduction zones around the psp are predominately trench normal the relatively simple patterns of azimuthal anisotropy may imply that the observed anisotropy originates from the a type lpo of olivine fabric at shallow depths in the back arc region and with slab entrained mantle flow at deep depths

split philippine sea plate beneath japan 1 the shape of the philippine sea plate subducting beneath western japan is a crucial factor in understanding earthquakes and volcanic activity we propose that the subducted plate was split along an extinct ridge due to an abrupt change of subduction direction followed by elastic deformation of the plate and an accumulation of stress near the ridge this shape is consistent with receiver function images the distribution of deep tremor sources the seismicity and focal mechanisms of intraplate earthquakes and the distribution of anomalous ratios of helium isotopes in ground water the movement history of the plate can explain active tectonics in western japan in the last 2 4 ma the location history of the volcanic front and the tear control where fluids ascend in the crust and these fluids are responsible for the generation of large earthquakes and volcanoes

crustal structure of the southern okinawa trough symmetrical rifting submarine volcano and potential mantle accretion in the continental back arc basin back arc basins are a primary target to understand lithospheric evolution in extension associated with plate subduction most of the currently active back arc basins formed in intraoceanic settings and host well developed spreading centers where seafloor spreading has occurred however rift structure at its initial stage a key to understand how the continental lithosphere starts to break in a magma rich back arc setting is poorly documented here we present seismological evidence for structure of the southern okinawa trough an active rift zone behind the ryukyu subduction zone we find that the southern okinawa trough exhibits an almost symmetric rift system across the rift axis yaeyama rift and that the sedimentary layers are highly cut by inward dipping normal faults the rift structure also accompanies a narrow 2 7 km wide on axis intrusion resulted from passive upwelling of magma on the other hand an active submarine volcano is located 10 km away from the rift axis the p wave velocity vp model derived from seismic refraction data suggests that the crust has been significantly thinned from the original 25 km thick arc crust and the thinnest part with 12 km thickness occurs directly beneath the rift axis the velocity model also reveals that there exists a thick layer with vp of 6 5 7 2 km s at lower crustal levels and may indicate that mantle materials accreted at the bottom of the crust during the crustal stretching the abrupt crustal thinning and the velocity depth profile suggest that the southern okinawa trough is at a transitional stage from continental rifting to seafloor spreading

high resolution magnetic signature of active hydrothermal systems in the back arc spreading region of the southern mariana trough high resolution vector magnetic measurements were performed on five hydrothermal vent fields of the back arc spreading region of the southern mariana trough using shinkai 6500 a deep sea manned submersible a new 3 d forward scheme was applied that exploits the surrounding bathymetry and varying altitudes of the submersible to estimate absolute crustal magnetization the results revealed that magnetic anomaly derived absolute magnetizations show a reasonable correlation with natural remanent magnetizations of rock samples collected from the seafloor of the same region the distribution of magnetic anomaly derived absolute magnetization suggests that all five andesite hosted hydrothermal fields are associated with a lack of magnetization as is generally observed at basalt hosted hydrothermal sites furthermore both the pika and urashima sites were found to have their own distinct low magnetization zones which could not be distinguished in magnetic anomaly data collected at higher altitudes by autonomous underwater vehicle due to their limited extension the spatial extent of the resulting low magnetization is approximately 10 times wider at off axis sites than at on axis sites possibly reflecting larger accumulations of nonmagnetic sulfides stockwork zones and or alteration zones at the off axis sites

p wave anisotropy tomography of central japan insight into subduction dynamics abstract a p wave anisotropic tomography under central japan was determined using a large number of arrival time data from 22 244 earthquakes recorded by 770 seismic stations on the japan islands the results show that the anisotropy exists widely beneath the study area except for the central portion of the mantle wedge the fast velocity directions fvds of the mantle wedge are generally trench parallel under the fore arc area and trench normal under the back arc area which may be attributed to the lattice preferred orientation lpo of olivine changing from b type under the fore arc area to a type under the back arc area with the variation of water content and the occurrence of mantle flow the subducting philippine sea plate phs beneath tokai is revealed as a high velocity anomaly dipping northward with nee to ne sw fvds which are consistent with the spreading direction of the phs during its formation and are inferred as the fossil anisotropy of the phs the anisotropy of the upper portion of the pacific slab under central japan is also revealed in this study the fvds of the upper portion of the pacific slab are sub parallel to the japan trench and izu bonin trench at 60 120 km depths and become almost perpendicular to the trenches below 100 120 km depth after comparisons with the mesozoic magnetic anomaly lineations and the principle stresses in the pacific slab we propose that the fossil fvds of the pacific slab beneath central japan were already rebuilt by the principle stresses in the slab

the lau havre taupo back arc basin a southward propagating multi stage evolution from rifting to spreading abstract the lau havre taupo lht system is an active 2000 km long back arc basin associated with pacific australian plate convergence in the sw pacific data from the lht back arc system include odp drillhole swath bathymetric and side scan imagery seismic reflection rock geochemistry magnetic anomaly heat flow shallow seismicity gravity and seismic refraction data the lht forms a longitudinal transect from southward propagating true oceanic spreading in the lau basin near tonga similarly southward propagating arc rifting in the havre trough and southward propagating continental rifting in the taupo volcanic zone tvz in northern new zealand four transitional tectonic settings are identified along the lht north to south between the continuum of spreading to rifting and comprise 1 oceanic spreading of the central lau spreading centre clsc propagating into oceanic crust formed by the eastern lau spreading centre elsc 2 oceanic spreading of the elsc propagating into rifted lau havre arc crust 3 rifting within havre arc crust propagating into rifting new zealand continental crust and 4 rifting of continental crust propagating into as yet non rifted but flexurally subsiding continental crust the enhanced spatial and temporal resolution of observed tectonic and magmatic processes along the strike of the lht is used to constrain a model for the evolution of back arc rifting to back arc spreading a sequence of five stages is identified and comprise 1 a combination of incipient simple and pure shear 2 early distributed and spatially heterogeneous half graben rifting with low angle synthetic and antithetic structures associated with rapid graben sediment infilling ephemeral early morb like rift magmatism and the initial trenchward migration of the arc front 3 a latter phase of half graben rifting mostly confined to a contiguous axial zone with continued ephemeral morb like rift magmatism and further trenchward migration of the arc front 4 initial oceanic spreading with evolving steady state morb magmatism along the axial rift graben system and 5 the reconfiguration of ridge geometry in response to changes in opening orientation leading to the development of ridge propagation into newly created oceanic crust rifting stages 2 and 3 can produce at least 120 km wide back arc crust by a balance of crustal extension with an estimated factor of 3 with rift and constructional arc magmatism in the lht system depending on latitude rates of widening during stages 2 and 3 vary between 15 and 45 mm yr 1 whilst stages 4 and 5 have full spreading rates of 90 mm yr 1

seismic structure of the upper mantle beneath the philippine sea from seafloor and land observation implications for mantle convection and magma genesis in the izu bonin mariana subduction zone abstract we have determined the three dimensional shear wave speed structure of the upper mantle in and around the philippine sea region using seismograms recorded by dense land based and long term broadband ocean bottom seismographic stations we used a surface wave tomography technique in which multimode phase speeds are measured and inverted for a 3 d shear wave speed structure by incorporating the effects of a finite frequency and ray bending the new ocean bottom data provided us with improved spatial resolution 300 km in the philippine sea region in the upper 120 km the shear wave speed structure is well correlated with seafloor age at depths greater than 160 km fast anomalies of the subducting pacific plate are clearly defined we also found slow speed anomalies beneath the kyushu palau ridge at depths greater than 120 km along the izu bonin ogasawara mariana arc we have detected three separate slow anomalies in the mantle wedge at depths shallower than 100 km beneath the back arc each anomaly has a width of 500 km moreover these three anomalies have a close relationship with the three groups of frontal and rear arc volcanoes having distinct sr nd and pb isotope ratios we suggest that each of the anomalies is a site of large scale flow of upper mantle into the mantle wedge and that each already contains a component from the adjacent subducting slab

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

slab segmentation revealed by anisotropic p wave tomography 1 seismic anisotropy is a useful indicator for identifying the physical and chemical condition of the earth s interior such as stress and flow fields and in situ constituent minerals using traveltime tomography we examined three dimensional anisotropic p wave velocity structure of the kii peninsula southwest japan where source regions of megathrust earthquakes along the nankai trough are presumed to exist the tomography revealed that the philippine sea slab beneath the peninsula is segmented i e a high velocity slab with e w anisotropy obtained during seafloor spreading is broken by an anomalous low velocity region with n s anisotropy the anomaly lies along the segmentation boundary of two previous rupture zones the 1944 tonankai and 1946 nankai earthquakes and can be explained by the occurrence of a fracture zone with n s oriented fractures and fault planes revealed by wide angle seismic data it is likely that this anomalous region will form a clear segmentation boundary during future earthquakes

dual subduction tectonics and plate dynamics of central japan shown by three dimensional p wave anisotropic structure abstract the central japanese subduction zone is characterized by a complex tectonic setting affected by the dual subduction of oceanic plates and collisions between the island arcs to better understand of the subduction system we performed an anisotropic tomography analysis using p wave arrival times from local earthquakes to determine the three dimensional structure of p wave azimuthal anisotropy in the overriding plate and the pacific and philippine sea phs slabs the principal characteristics of anisotropy in the subducted and subducting plates are 1 in the overriding plate the distribution pattern of fast direction of crustal anisotropy coincides with that of the strike of geological structure 2 in the two oceanic plates fast propagation directions of p wave were sub parallel to the directions of seafloor spreading additionally our tomographic images demonstrate that 1 the bottom of the median tectonic line the longest fault zone in japan reaches to the lower crust and seems to link to the source region of an inter plate earthquake along the phs slab 2 the segmentation of the phs slab the izu islands arc the nishi shichito ridge and the shikoku basin due to the formation history is reflected in the regional variation of anisotropy the tomographic study further implies that there might be a fragment of the pacific slab suggested by a previous study beneath the tokyo metropolitan area the overall findings strongly indicate that seismic anisotropy analysis provide potentially useful information to understand a subduction zone

structural development of sumisu rift izu bonin arc geophysical swath mapping multichannel seismic profiling and ocean drilling data are used to document the structural evolution of sumisu rift and to analyze the pattern of strain resulting from extension of an intraoceanic island arc the 120 km long 30 50 km wide sumisu rift is bounded to the north and south by structural and volcanic highs west of the sumisu and torishima calderas and longitudinally by curvilinear border fault zones with both convex and concave dip slopes the zig zag pattern of normal faults average strikes 337 and 355 indicates extension oriented 076 10 orthogonal to the volcanic arc three oblique transfer zones divide the rift along strike into four segments with different fault trends and uplift subsidence patterns differential strain across the transfer zones is accommodated by interdigitating rift parallel faults and sometimes by cross rift volcanism rather than by strike or oblique slip faults from estimates of extension 2 5 km the age of the rift 2 ma and the accelerating subsidence we infer that sumisu rift is in the early synrift stage of back arc basin formation following an early sag phase half graben formed with synthetically faulted structural rollovers facing large offset 2 2 5 km throw border fault zones in the three northern rift segments the largest faults are on the arc side and dip 60 75 w whereas in the southern segment they are on the west side and dip 25 50 e the present full graben stage is dominated by hanging wall antithetic faulting basin widening by footwall collapse and a concentration of subsidence in an inner rift the hanging wall collapses but not necessarily as a result of border fault propagation from adjacent rift segments whereas the border faults may penetrate the theologically weak lithosphere te 3 km many of the hanging wall and footwall collapse structures are detached only a few kilometers below the seafloor back arc volcanism usually erupted along faults occurs in the rift and along the protoremnant arc during both stages where drilled the arc margin has been uplifted 1 1 0 5 km concurrently with 1 1 km of rift basin subsidence extremely high sedimentation rates up to 6 m kyr in the inner rift have kept pace with synrift faulting created a smooth basin floor and resulted in sediment thicknesses that mimic the differential basin subsidence a linear zone of weakness caused by the greater temperatures and crustal thickness along the arc volcanic line controls the initial locus of rifting rifts are better developed between the arc edifices intrusions may be accommodating extensional strain adjacent to the arc volcanoes no obvious correlations are observed between the rift structures and preexisting cross arc trends

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

1 d electrical conductivity structure beneath the philippine sea results from an ocean bottom magnetotelluric survey abstract eight months of observation using ocean bottom electro magnetometers obems have allowed us to estimate the regional electrical conductivity structure beneath the philippine sea six obems were deployed along a line crossing the philippine sea from nw to se and five of them recorded useful data the raw time series data were cleaned up before we estimated the magnetotelluric mt impedance tensor conductivity structure at five sites is estimated using 1 d occam s inversion to fit the determinant average of each mt impedance tensor after a correction for the effect of topography we examined effect from two dimensionalities on the 1 d conductivity structure and the robustness of solutions the results of the 1 d conductivity structural model are strongly related to tectonic setting and the crustal age beneath each site the structure beneath the spreading axis of the mariana trough shows a distinct low conductivity structure at depths of 50 150 km and it probably reflects the upwelling dynamics operating beneath the spreading axis these low values are comparable with that of olivine with low hydrogen content implying that 1 the melting process extracts water from minerals such as olivine and 2 the melt beginning depth in the mariana trough is deeper than that of the typical morb source region the off axis conductivity profiles infer the existence of a high conductivity peak or a conductivity gradient change at mid depth the depth level of the peak increases with crustal age suggesting that the conductivity structure is related to a geothermal structure and that these conductivity profiles are explained by the temperature gradient change possibly combined with the presence of partial melt our results suggest that further ocean bottom em study has high potential to investigate the temperature gradient change and amount of hydrogen water and melt in the upper mantle

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

sedimentary volcanic and tectonic processes of the central mariana arc mariana trough back arc basin formation and the west mariana ridge 1 we present new multichannel seismic profiles and bathymetric data from the central marianas that image the west mariana ridge wmr remnant arc both margins of the mariana trough back arc basin the modern arc and eocene frontal arc high these data reveal structure and stratigraphy related to three periods of arc volcanism and two periods of arc rifting we interpret the boundary between accreted back arc basin and rifted arc crust along the mariana trough and support these findings with drilling results and recent seismic refraction and gravity studies we show that with the exception of a few volcanoes behind the volcanic front that straddle the boundary between crustal types the modern mariana arc is built entirely on rifted arc crust between 14 and 19 n our data indicate that there is more accreted back arc seafloor to the west of the mariana trough spreading axis than to the east confirming previous evidence for an asymmetric basin the rifted margin of the wmr remnant arc forms a stepped pattern along the western boundary of the mariana trough between 15 30 and 19 n in this region linear volcanic cross chains behind the wmr are aligned with the trend of mariana trough spreading segments and the wmr ridges extend into the back arc basin along the same strike these ridges are magmatic accommodation zones which to the north along the izu bonin arc punctuate tectonic extension for the wmr we hypothesize that rift basins are more commonly the sites where spreading segment offsets nucleate whereas magmatic centers of spreading segments are sites where magmatism continues from arc volcanism through rifting to back arc spreading the mariana trough is opening nonrigidly and is characterized by two predominant abyssal hill trends nnw sse in the north and n s in the south between the only two basin crossing fracture zones at 15 5 and 17 5 n s axes propagated north at the expense of nnw axes

geodynamic setting of izu bonin mariana boninites the izu bonin mariana ibm forearc is characterized by the occurrence of boninite like lavas the study of the cenozoic setting of the genesis of these boninitic lavas in light of modern geodynamic contexts in the tonga and fiji regions lead us to define three tectonic settings that favour the formation of boninites in back arc basins in addition to previous settings that involve the presence of a mantle plume 1 propagation at low angle between a spreading centre and the associated volcanic arc 2 intersection at a high angle of an active spreading centre and a transform fault at the termination of an active volcanic arc and 3 intersection at a right angle between an active spreading centre and a newly created subduction zone a geodynamic model of the philippine sea plate shows that boninites in the bonin islands are related to the second mechanism mentioned above whereas mariana forearc boninites are relevant to the third mechanism in the early eocene the transform plate boundary bounding the eastern margin of the philippine sea plate at the location of the present day mariana arc evolved into a subduction zone that trends perpendicular to the active spreading centre of the west philippine basin some where around 43 47 ma the presence of a mantle plume in the vicinity of the subduction zone bounding the northern ibm arc explains boninites that erupted in its northern part but only in early eocene time

asthenospheric outflow from the shrinking philippine sea plate evidence from hf nd isotopes of southern mariana lavas abstract at subduction zones sinking of the downgoing lithosphere is thought to enable a return flow of asthenospheric mantle around the slab edges so that the asthenosphere from underneath the slab invades the ambient mantle flowing underneath the volcanic arc and the backarc basin for instance at the northern end of the lau basin trench retreat and slab rollback enable toroidal return flow of samoan mantle beneath a transform margin to provide a supply of fresh undepleted indian mantle that feeds the backarc spreading center questions however arise about the sense of mantle flow when plate kinematics predict that the trench is advancing as seen in the mariana convergent margin does the mantle flow in or does it escape outward through slab tears or gaps here we address the origin and sense of asthenospheric mantle flow supplying the southern mariana convergent margin a region of strong extension occurring above the subducting pacific plate does the asthenosphere flow northward from underneath the pacific plate and caroline hotspot through a slab tear or gap or does it flow outward from the mariana trough which possesses a characteristic indian ocean isotopic signature to address these questions we integrate geodetic data along with new hf nd isotopic data for fresh basaltic lavas from three tectonic provinces in the southernmost marianas the fina nagu volcanic complex the malaguana gadao backarc spreading ridge and the se mariana forearc rift our results indicate that indian mantle flows outward and likely escapes through slab tears or gaps to accommodate shrinking of the philippine sea plate we thus predict that asthenospheric flow around the pacific slab at the southern mariana trench is opposite to that predicted by most subduction driven mantle flow models

tectonics of the southern tip of the parece vela basin philippine sea plate we report new geophysical and petrological data collected at the southern tip of the parece vela basin in the philippine sea the parece vela basin which was formed as a backarc basin behind proto mariana arc trench system from late oligocene to middle miocene provides us a good opportunity to study the nature of successive backarc basin formations in the philippine sea and the relationship between are and backarc magmatisms regional bathymetric map derived from satellite altimetry shows that the southern tip of the basin now located just west of the yap arc trench system has unique morphological and tectonic features which include 1 the absence of spreading center or its trace 2 shallow average depth and 3 enigmatic curved structures our newly collected high resolution bathymetric data reveal that the spreading fabric similar to the central parece vela basin exists to the north of 9 degrees 20 n thus it appears that the present day yap arc and backarc region represent the western half of the seafloor that was produced by the early e w and the following ne sw spreading in the northern and central parece vela basin and that the eastern counterpart now lies west of the west mariana ridge unlike the northern parece vela basin there appears to be no evidence for a systematic propagation of spreading center in the southern part instead two rift segments one which extends from the central parece vela basin and the other which lies within the western remnant arc kyushu palau ridge overlap at the southern tip of the basin producing a complex seafloor that includes curvilinear deeps and deformed topographic highs c 2007 elsevier b v all rights reserved

a synthesis of heat flow determinations and thermal modeling along the nankai trough japan 1 we review marine heat flow data along the nankai trough and show that observations 30 km seaward of the deformation front are 20 below conductive predictions 129 94 mw m 2 but consistent with the global heat flow average for oceanic crust of the same age 16 28 ma heat flow values 30 km seaward of the deformation front are generally 20 higher than conductive predictions this heat flow pattern is consistent with the advection of heat by fluid flow in the subducting oceanic crust and explains both the high heat flux in the vicinity of the trench 200 and 140 mw m 2 and steep landward declines to values of approximately 60 mw m 2 over distances of 65 and 50 km along the muroto and kumano transects respectively along the ashizuri transect the lack of heat flow data precludes a definitive interpretation we conclude that fluid flow in the subducting oceanic crust leads to temperatures that are generally 25 c higher near the toe of the margin wedge and 50 100 c lower near the downdip limit of the seismogenic zone than estimated by purely conductive models

cenozoic tectonic evolution of asia a preliminary synthesis abstract asia has been a major testing ground for various competing models of continental deformation due to its relatively well understood plate boundary conditions in the cenozoic exceptional exposure of active structures and strain distribution and widespread syn collisional igneous activity as a proxy for the thermal state of the mantle and crust two cenozoic orogens dominate the continent the himalayan tibetan orogen in the east induced by the india asia collision and the turkish iranian caucasus orogen in the west induced by the arabia asia collision the development of the two orogens was accomplished by shortening in the early stage followed by strike slip faulting and extension in the late stage in the himalayan tibetan orogen shortening across two discrete thrust belts at 55 30 ma in southern and northern tibet created a large intracontinental basin the paleo qaidam basin in between subsequent crustal thickening and a possible thermal event in the mantle e g convective removal of central tibetan mantle lithosphere may have raised the elevation of this early intra plateau basin up to 2 3 km to its current height collision between india and asia also caused lateral extrusion of southeast asia between 32 ma and 17 ma the latest stage of the india asia collision was expressed by north trending rifting and the development of trench facing v shaped conjugate strike slip faults in central mongolia central tibet eastern afghanistan and southeast asia in the turkish iranian caucasus orogen early crustal thickening in the orogenic interior began at or prior to 30 20 ma this style of deformation was replaced by strike slip faulting at 15 5 ma associated with further northward penetration of arabia into asia westward extrusion of the anatolia turkey block and rapid extension across the sea of crete and sea of aegean the late stage extension in both orogens was locally related to extensional core complex development the continental margin extension of east asia was developed in two stages initially in a widely distributed zone that has an east west width of 500 800 km during 65 35 ma which was followed by localized extension and opening of back arc basins associated with the development of spreading centers at 32 17 ma e g japan sea or east korea sea bohai bay and south china sea opening of the back arc basins could be induced by 1 rapid eastward migration of the western pacific trench system or 2 oblique subduction of pacific plate beneath asia that had produced a series of en echelon right slip primary shear zones linking with back arc spreading centers oriented obliquely to the strike of the nearby trench since 15 ma the eastern margin of asia became contractional in the east west direction as indicated by the collapse of back arc basins in the western pacific and the development of fold thrust belts along the eastern continental margin coeval with the contraction is widespread east west extension in siberia north china and the tibetan plateau the above observations can be explained by a change in boundary condition along the eastern margin of asia that allowed the thickened asian continent to spread eastward causing east west extension in its trailing edge and east west compression in its leading edge in west asia continental margin extension started at about 25 20 ma in the aegean and cretan regions which was associated with a rapid southward retreat of the hellenic arc the complex evolution of cenozoic deformation in asia may be explained by a combined effect of temporal changes in plate boundary conditions thermal evolution of the upper mantle perturbed by collisional tectonics and the built up of gravitational energy through crustal thickening and thermal heating although the past research in asia has treated the india asia and arabia asia convergence as separate collisional processes their interaction may have controlled the far field cenozoic deformation in asia the most pronounced result of this interaction is the creation of a northeast trending 300 400 km wide and 1500 km long zone of northwest striking right slip faults which extends from the zagros thrust belt in the south to western mongolia in the north and links with the active tian shan and altai shan intracontinental orogens cenozoic deformation and coeval igneous activity spatially overlap with one another in the himalayan tibetan and turkish iranian caucasus orogens a large cenozoic magmatic gap exists between tibet in the south and mongolia in the north where cenozoic deformation has not been associated with any coeval igneous activity finally cenozoic igneous activity is always associated with jurassic cretaceous magmatic arcs suggesting a causal relationship between the early arc magmatism and later syn collisional magmatism

peridotites and volcanics from the yap arc system implications for tectonics of the southern philippine sea plate satellite altimetry reveals a continuous n20 e trending bathymetric feature extending from the yap trench through the north yap escarpment nye to the parece vela rift suggesting a genetic relationship we report the results of submersible observations and geochemical study of peridotites and volcanics from the yap trench and the north yap escarpment and use these to understand the genesis and evolution of the yap arc system metamorphosed rocks and gabbros of the parece vela basin origin predominate on the yap islands and for the upper part of the forearc although minor remnant arc volcanics of 25 ma age exist there are also arc volcanics of 11 7 ma age in the forearc whereas depleted arc type mantle peridotites are exposed along faults in the lower part of the forearc landward slope the results show that the yap arc north yap escarpment system might form as an incipient arc system at the propagating tip of the parece vela rift at 25 ma and that the overall yap arc forearc region is composed of several different units

origin of the palau and yap trench arc systems summary this paper attempts to account for several unique morphological and geological characteristics of the palau and yap trench arc systems in the northwestern pacific ocean using a new hydrodynamic model postulated herein the model is a simple mathematical treatment of a stationary viscous flow pattern in the asthenosphere that causes an appreciable amount of mechanical resistance against a vertically subducted rigid slab moving horizontally together with the flat plate a couple of thrusting forces comprising uplifting pressure generated in the frontal edge of the overriding lid and a new type of tectonic erosion along the surface of the vertical slab seem to reasonably explain apparently contradictory features of the palau and yap trench arcs long term gps measurements show that the palau and yap trench arcs are moving over the viscous asthenosphere toward the nww with velocities as great as 10 cm yr 1 stresses along the trench arcs predicted by the model would be sufficiently large to maintain their present morphology 1 water depths greater than 8000 m in spite of the extremely small plate convergence rates and relatively young ages of subducted plate 2 v shaped cross section of the axial bottom of these trenches implying little cover of slumped deposits in spite of very steep landward slopes 3 serpentinized peridotites gabbros basalts and limestones are exposed on the landward slope with neither sediment cover nor ferromanganese hydro oxide coatings 4 abnormally close proximity of the palau and yap islands to the trench axis 5 predominant distribution of barrier reefs in their western periphery apparently opposite to the easterly to northeasterly prevailing wind

motion of the ontong java plateau in the hot spot frame of reference 122 ma present abstract a new model of pacific absolute plate motion between 140 and 0 ma generated in the fixed hot spot frame of reference has been used to track palaeogeographic positions of the ontong java plateau ojp from the time c 122 ma and location c 43 s of its formation to its present location north of the solomon islands the resulting ojp seafloor flow line suggests that changes in pacific plate motion passage over hot spots and pacific rim tectonism all have influenced the continuing structural development and deformation of the plateau satellite derived gravity bathymetry and rayleigh wave tomography potentially reveal the structural fabric of the ojp and adjoining nauru basin including the orientation of probable fracture zones location of possible relict spreading centres and the presence of a thick lithospheric root as well as possible later hot spot related modification of the fabric the most recent phase of ojp deformation which began about 6 ma accelerated at 2 6 ma and continues today has resulted in the uplift of the islands of malaita and santa isabel and the formation of the malaita anticlinorium with slip along the old fracture zones possibly triggering submarine canyon formation on the ne side of the ojp this collision related deformation also is probably responsible for the ongoing uplift and tilting of the islands of nauru and banaba ne of the ojp high plateau

two station measurement of rayleigh wave phase velocities for the huatung basin the westernmost philippine sea with obs implications for regional tectonics summary a broad band ocean bottom seismometer obs deployed 180 km east of taiwan provides a first glimpse into the upper mantle beneath the westernmost section of the philippine sea or the huatung basin hb we measured interstation phase velocities of rayleigh waves between the obs and stations on the eastern coast of taiwan the phase velocities show smooth variations from 3 8 to 3 9 km s 1 for periods of 25 40 s in this short period range phase velocities are comparable to those characterizing the 15 30 ma parece vela basin of the philippine sea modelling of the finite frequency effect proves the validity of the measurement for the average hb the shear wave velocity models inverted from the 25 to 40 s dispersion show a velocity at lithospheric depths about 0 1 km s 1 lower than that of the west philippine sea which agrees with the age effect derived from the pacific pure path model inversions incorporating the less reliable data above 40 s yield a shear velocity 4 0 km s 1 below 150 km an unrealistic value even for a hotspot plume environment the seismological evidence together with the correlation in seafloor depth suggests that the hb and the parece vela basin may have a similar age this is at odds with the previous geochronological study suggesting an early cretaceous age for the hb thermal rejuvenation of the lithosphere was examined as a potential solution to reconciling the two age models

three dimensional shear wave structure beneath the philippine sea from land and ocean bottom broadband seismograms 1 we obtained three dimensional 3 d shear wave speed structure beneath the philippine sea and the surrounding region from seismograms recorded by land based and long term broadband ocean bottom seismographic stations the ocean bottom data gave us a better station coverage to obtain a higher spatial resolution about 300 400 km in the philippine sea than in previous studies we employed a new technique of surface wave tomography in which multimode phase speeds are measured and inverted for a 3 d shear wave speed structure by incorporating the effects of finite frequency and ray bending there is a sharp speed contrast along the izu bonin mariana trench across which the philippine sea side has a significantly slower upper mantle than the pacific ocean side in the upper 120 km the shear wave speed structure is well correlated with the age of the provinces at depths greater than 160 km the pattern is dominated by fast anomalies of the subducted slabs of the pacific plate and two slow anomalies to the south of the daito ridge and in the southernmost part of the philippine sea

late amagmatic extension along the central and eastern segments of the west philippine basin fossil spreading axis the spreading processes within the west philippine basin wpb remain partly unknown this study presents an analysis of the tectono magmatic processes that happened along its spreading axis during the conclusion of the last spreading phase at 33 30 ma we demonstrate that the late episode of n s opening from an e w trending spreading system has been followed by a late tectonic event occurring in the central and eastern parts of the basin this event was responsible for transtensional strain accommodated along the nw se faults cutting through the former e w rift valley in the central part of the basin in its eastern part the same event occurred at a larger extent and led to the creation of a new nw se axis obliquely cutting the older e w spreading segments and their associated spreading fabrics at this location several tens of kilometers of slightly oblique amagmatic extension occurred following a v60 direction we propose that this late event is associated with the onset of e w opening of the parece vela basin located along the eastern border of the wpb at 30 ma extensive stresses within this basin were probably transmitted to the hot and easily deformable rift zone of the wpb the newly created nw se axis most likely propagated from east to west being responsible for scissors opening within the wpb ne sw extension ceased when well organized spreading started at 26 ma in parece vela basin accommodating entirely the global extensive stress pattern 2002 elsevier science b v all rights reserved

ontong java plateau and late neogene changes in pacific plate motion aided by a new geometrical technique called hot spotting we recently proposed a new model for the absolute motion of the pacific plate this model called wk97 assumed the hawaiian hotspot is under loihi and that there was a change in plate motion at 3 6 ma the model located the louisville hotspot close to the hollister ridge a shallow volcanic ridge associated with a geoid high seismicity and recent volcanism most of the tectonic implications of wk97 however were not addressed furthermore although several recent studies suggest that the geochemistry of hollister lavas may indicate a mixture between louisville melt material and pacific antarctic ridge mid ocean ridge basalt these studies strongly disagree with wk97 and consider it seriously flawed here we discuss numerous tectonic predictions of wk97 and present a large amount of observational evidence in support of it we find that a late neogene collision between the ontong java plateau and the northern margin of the australia plate appears to have altered the motion of the pacific plate as inferred from hotspot volcanism by forcing it to rotate counter clockwise this rotation about a low latitude pole appears to have induced right lateral shear stress along the entire pacific plate divergent boundary which resulted in the formation of extensional transform faults microplates and propagating ridges this change in motion also appears to have triggered concomitant circum pacific tectonism including trench migration and back arc rifting the wk97 suggests that long lived plume fed volcanism on the pacific plate appears very limited perhaps only to five to six hotspots hawaii louisville cobb caroline and perhaps marquesas and bowie the geometry of these chains are consistent with our model despite some evidence for volcanic age progressions most other pacific neogene hotspots are most likely crack spots i e sites of extensional volcanism at preexisting zones of weakness reactivated by plate stresses indeed several volcanic ridges such as hollister ridge near the louisville hotspot appear to have formed in response to stresses following the change in absolute plate motion wk97 also obviates the need for simultaneous different directions of mantle flow in the south central pacific transpression at the san andreas and alpine strike slip faults and aleutian arc explosivity since 6 ma also appear to be consequences of the changes in pacific plate motion furthermore both the wk97 model and observed relative plate motions suggest that adjoining plates have changed their absolute motions as well and that large changes in absolute plate motion do not necessarily imply large changes in relative plate motion our findings support slab pull as the dominant plate tectonic driving force highlight the rheological difference between strong oceanic lithosphere and weak continental crust and suggest that wk97 presently provides a unifying albeit preliminary framework for relating both intraplate and circum pacific tectonism and volcanism in the late neogene

constraints of magnetostratigraphic and mineralogical data on the provenance of sediments in the parece vela basin of the western pacific abstract the sediment provenance and sedimentary processes of the parece vela basin in the western pacific during the last ca 1 myr were studied based on the analysis of magnetic properties mineral features and paleomagnetic ages of core c p19 the results show that the sedimentary sequence has a lower limit age younger than 1 072 ma and the sedimentation process can be divided into two stages with sedimentation rates of 4 0 mm kyr during ca 1 0 78 ma and 3 6 mm kyr since 0 78 ma these sedimentation rates are higher than those in the surrounding area where the sedimentation rate is generally less than 2 mm kyr taken together with the mineral features and magnetic parameters these findings show that volcanic sediments eroded from the nearby ridge were transported by currents to the sampling site these volcanic sediments represent the main source of sediment in the area with eolian inputs contributing little and nonbiogenic authigenic magnetite accounting for part of the magnetic fraction the sedimentary environment and sediment provenance have varied during the last approximately 1 million years during 997 880 ka volcanic activity increased and the input of volcanic clasts was high whereas during 880 443 ka the sedimentary environment tended to be stable and the sediment supply decreased with the increase in volcanic activity in the western pacific the sediment input increased again during ca 443 ka present

effect of margin curvature on plate deformation in a 3 d numerical model of subduction zones summary the large amount of data acquired along most subduction zones underlines the importance of three dimensions in the subduction mechanism in this work we use a fully 3 d mechanical numerical model to analyse the impact of the plate boundary geometry on the deformation of the upper plate the model consists of two initially horizontal independent plates overlying an inviscid fluid and continuously pushed towards each other the initial geometry of the contact zone between the two plates controls the stress regime and the strain pattern in the upper plate the case with a linear plate boundary and orthogonal convergence is first considered and used as a reference model with this simple model the effects of some important parameters i e the interplate friction and the lithosphere asthenosphere density contrast are investigated finally cases with curved plate boundaries are considered and the results are used to shed light on the importance of the dip direction of the interplate plane for the upper plate strain pattern unnecessary oceanward convexity causes an accumulation of the subducted material beneath the upper plate and induces an important uplift of the convex area on the contrary the material escapes from an oceanward concavity and provokes subsidence in the forearc zone such a behaviour induces preferential zones of weakeness in the overriding plate and may allow for explaining some local stress regime variations along convergent margins

regional variation of magnetization of oceanic crust subducting beneath the nankai trough 1 we present the characteristics of marine magnetic anomalies in the nankai trough and the northern shikoku basin the marine magnetic anomalies are clearly visible over the shikoku basin but gradually disappear across the nankai trough the expected reason for the decrease in the marine magnetic anomaly amplitudes across the trough is the increase in depth of the magnetic source the subducting oceanic crust forward and inverse magnetic modeling results which take into account the slab depth distribution show variations in magnetization that cannot be explained solely by increasing depth of the slab we identify five zones along the trough axis on the basis of the pattern of the magnetic amplitude profile two ridges a seamount chain and two basement zones the magnetic inversion analysis indicates cyclic intensity variations along the ridge zones the largest contrast in magnetization along the n s lineation before the deformation front occurs off ashizuri as low temperature chemical oxidation effects are most likely the main cause of the reduction in magnetization there are considerable differences in the original carrier of magnetic intensity in the as and kii peninsula kp zones and the original carrier in as is characterized by much higher original magnetic intensities this indicates several possibilities the oceanic crust originated from asymmetric spreading there was magma injection during the late stage volcanism or the as zone was affected by an anomalous heat source it would be concluded from the current data that the basement in as originally had a much stronger magnetic carrier and suffered stronger low temperature oxidation effects than the eastern part of the shikoku basin the regional differences in intensity strongly suggest a variety of magnetic minerals and phase changes associated with temperature and mechanical dislocation

low frequency tremors intraslab and interplate earthquakes in southwest japan from a viewpoint of slab dehydration 1 low frequency tremors lft recently found in the southwest japan fore arc likely occur due to hydro fracturing from fluids leaving the dehydrating philippine sea slab however there are some places without such lft they are n izu kanto e shikoku and s kyushu these are the places where island arc type crust is subducting we propose that dehydration of the subducted crust does not occur beneath these regions because it is composed mainly of tonalite lacking the quantity of hydrous minerals seen in normal subducting oceanic crust we show that almost no earthquake occurs within the subducted crust in such regions consistent with dehydration embrittlement hypothesis for intraslab seismicity the lack of dehydration from the crust would also affect the mode of occurrence of interplate earthquakes because pore fluid pressure is more difficult to rise at the thrust zone this might be reflected in the irregular occurrence of great earthquakes in these regions

slab dehydration and earthquake distribution beneath southwestern and central japan based on three dimensional thermal modeling we developed a 3 d thermal convection model to estimate the thermal regime water content distribution and slab dehydration beneath southwestern and central japan where deep tectonic tremors and short term slow slip events s sses are frequently observed on the plate interface extending from the western shikoku to the tokai district the results showed that the pressure temperature p t conditions for the s sses deep tectonic tremors and regular earthquakes indicate that they probably originated from slab dehydration in association with subduction resulting in a phase transformation from prehnite actinolite lawsonite blueschist to amphibolite with a large thermal gradient slab dehydration and the thermal gradient in the dip direction are considered key factors for controlling the seismogenesis of slow and regular earthquakes in the philippine sea plate beneath southwestern and central japan although the deep tectonic tremors are more complicated because they are likely favored by a comparatively low temperature gradient in southwestern japan

size and shape of saturn s moon titan cassini observations show that saturn s moon titan is slightly oblate a fourth order spherical harmonic expansion yields north polar south polar and mean equatorial radii of 2574 32 0 05 kilometers km 2574 36 0 03 km and 2574 91 0 11 km respectively its mean radius is 2574 73 0 09 km titan s shape approximates a hydrostatic synchronously rotating triaxial ellipsoid but is best fit by such a body orbiting closer to saturn than titan presently does titan s lack of high relief implies that most but not all of the surface features observed with the cassini imaging subsystem and synthetic aperture radar are uncorrelated with topography and elevation titan s depressed polar radii suggest that a constant geopotential hydrocarbon table could explain the confinement of the hydrocarbon lakes to high latitudes

fluid upwelling beneath arc volcanoes above the subducting philippine sea plate evidence from regional electrical resistivity structure 1 at subduction zones aqueous fluids released from the subducting slab play an important role in island arc volcanism triggering partial melting and accretion magmatism we used the network magnetotelluric mt method to obtain the deep 100 km large scale electrical resistivity structure beneath kyushu japan with the aim of identifying regions of deep fluids and magma that feeds subduction zone volcanoes in the region network mt observations in which dipoles of about 10 30 km in length were used to measure the electric potential difference were performed over all of kyushu in the southwest japan arc where the philippine sea plate psp is subducting beneath the eurasian plate at a high angle using the network mt data we obtained the large scale electrical resistivity structure along four profiles across each of four quaternary active volcanoes at the volcanic front of the kyushu subduction zone the resistivity models have two features in common a conductor beneath each volcano whose base extends to the backarc side and a resistor along the hinge line of the subducting psp in the forearc the conductor indicates regions of fluid released from the slab or partial melting of the mantle representing a magma source for subduction zone volcanoes in this region

the west philippine basin and the initiation of subduction revisited 1 new swath bathymetry plus existing geophysical data reveal that the direction of west philippine basin wpb seafloor spreading rotated 100 counter clockwise between 49 ma and 33 ma the curvilinear and multi stranded mindanao fracture zone separates the wpb from the palau basin to the south wpb opening was contemporaneous with and behind early izu bonin mariana ibm subduction whose arc volcanism began by 50 ma this produced over 1000 km of arc parallel spreading or stretching plus magmatism in the mariana segment of the eocene ibm arc forearc the initial ibm subduction cut across rather than followed pre existing structures remnant arcs fracture zones and spreading fabric new models of subduction initiation are required to reproduce such characteristics

contrasting subduction structures within the philippine sea plate hydrous oceanic crust and anhydrous volcanic arc crust we show contrasting subduction structures within the philippine sea plate inferred from active source wide angle reflection data previous studies showed that large amplitude reflections from the slab are observed in southwest japan and indicated that a thin low velocity layer with a high fluid content is formed along the top of the subducting oceanic crust on the contrary we found that the slab reflections have smaller amplitudes in the izu collision zone central japan where the izu bonin volcanic arc has been colliding subducting suggesting that such a low velocity layer does not exist beneath the collision zone this structural difference is also supported by p wave and s wave velocity anomalies by passive source tomography and electrical conductivity and correlates with the regional distribution of deep tremors and intraslab earthquakes both of which are induced by dehydration processes within the downgoing slab based on these comparisons we suggest that the original structure of the incoming plate controls the contrasting subducting systems typical oceanic plate absorbs water by hydrothermal circulation at spreading centers and or seawater infiltration at outer rises whereas volcanic arc crust consumes a large amount of hydrous minerals for melt production and metamorphoses to more stable anhydrous forms before subduction

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

the action of water films at a scales in the earth implications for the nankai subduction system abstract water properties change with confinement within nanofilms trapped between natural charged clay particles we investigated nanofilm characteristics through high stress laboratory compression tests in combination with analyses of expelled pore fluids we utilized sediments obtained from deep drilling of the nankai subduction zone at site c0002 of the integrated ocean drilling program iodp we show that below 1 2 km there should be widespread ultrafiltration of migrating fluids experiments to 100 mpa normal compression collapse pores below a few ion monofilm thicknesses a reduction towards a single condensing dehydrating ion monofilm occurs as stresses rise 100 200 mpa and clay separations are reduced to

paleogeographic reconstruction and origin of the philippine sea abstract our reconstruction of the philippine sea suggests that it formed by two distinct episodes of back arc spreading each of which resulted from seaward retreat of the trench in the first episode the protoizu bonin trench retreated northward and the west philippine basin formed behind the northern half of the palau kyushu ridge in the second episode the izu mariana trench retreated eastward and the shikoku and parece vela basins formed behind it during the last 17 ma the philippine sea basin has been moving northwestward with respect to eurasia shifting the ttt triple junction off central japan westward by about 50 km the motion of the philippine sea with respect to eurasia at the triple junction changed from north northwestward to west northwestward 10 5 ma ago for the period before 17 ma ago we construct two models retreating trench model and anchored slab model the izu bonin trench migrated from south to northeast rotating in a clock wise sense since 48 ma ago in the retreating trench model in the anchored slab model the trench has been fixed with respect to eurasian margin since 43 ma ago we prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of eurasia during 30 17 ma ago is much simpler for this model than for the anchored slab model furthermore rotations of the bonin mariana islands are consistent with those predicted from the retreating trench model the 48 ma ages of the northern part of the palau kyushu ridge and of chichi jima of the bonin islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 ma ago from this and the subparallelism in trend between the northern part of the palau kyushu ridge and the central basin ridge we propose that the major part of the west philippine basin formed by back arc spreading in a n s direction behind the northern part of the palau kyushu ridge the pacific plate was moving northward with respect to hot spots from 48 to 43 ma ago which implies that the pacific plate is not likely to have been subducting beneath the west philippine basin during this time we speculate that another plate existed south of the pacific plate and thai it was subducting beneath both tile west philippine basin and australia the annihilation of this plate might he a cause for the sudden change of the pacific plate motion at 43 ma ago

oblique subduction and collision forearc tectonics of the kuril arc the tectonics of the southwestern kuril arc are a result of the oblique subduction of the pacific plate at the kuril trench in association with forearc sliver migration caused by the oblique subduction collision tectonics occur at the leading margin of the sliver and tensional tectonics take place at the tapering margin as a result of the collision a deep crustal section of island arc is observed at the leading margin of the forearc sliver tectonics of the kuril arc related to oblique subduction are different from those of the western sunda arc where backarc spreading occurs at the leading margin this difference is due to margin morphology of the oblique subduction zone