radial upper mantle attenuation structure of inactive back arc basins from differential shear wave measurements we measure differential attenuation between ss s and sscs scs phase pairs to characterize the variation of attenuation with depth in the upper mantle of five inactive back arc basins the kuril basin sea of japan banda sea the celebes and sulu seas and the shikoku basin a spectral ratio technique is used to measure the differential attenuation operator of the transversely polarized waveforms over a frequency band of 10 to 83 mhz two algorithms are employed to compute the vertically averaged attenuation structure a spectral stacking procedure and a least squares inversion in the spectral stacking method the individual spectra are corrected for the elastic structure at the ss or sscs bounce point and the differential attenuation operator is computed by spectral division the attenuation operators are then normalized and stacked by source depth to obtain more stable spectra and an average t for sources within a restricted depth range is obtained from the slope of the log amplitude spectrum a model for the depth dependent q structure is then calculated from the t measurements assuming q is frequency independent alternatively t measurements for individual phase pairs are made using a similar technique and analyzed by ray tracing and a least squares inversion to obtain the q 1 estimates the q results obtained from the stacking and inversion methods are generally in good agreement the q 1 structures for the various back arc regions are similar to each other within the uncertainties of the derived q models in addition to computing q models for each individual region we partitioned the data into three tectonic provinces based upon bounce point locations the resulting average radial q structure for an inactive basin shows a q of 54 in the uppermost mantle 115 at intermediate depths and 173 in the transition zone we also find a low q zone of 36 beneath active island arcs the q values for inactive back arc basins are lower than the global averages derived from normal modes but are generally consistent with previous body and surface wave studies of other young oceanic regions the most striking feature of this study is the observation of very strong attenuation concentrated at shallow depths 160 km in the upper mantle beneath these basins

dawsonite formation in the beier sag hailar basin ne china tuff a natural analog for mineral carbon storage abstract we determined the rock types the authigenic minerals the paragenetic sequence and the origin of dawsonite in pyroclastic rocks from the yimin formation of beier sag in the hailar basin china dawsonite a diagenetic mineral is thought to result from a large influx of co 2 and therefore this system represents a natural analogue for in situ mineral carbon storage the studied host rocks are mainly tuffs tuffaceous sandstones which now contain up to 70 vol authigenic carbonates including dawsonite ankerite and siderite the initial alteration of the tuffs yielded minor siderite kaolinite illite and mixed illite smectite then formed as product phases dawsonite and quartz subsequently precipitated in response to co 2 influx apparently coupled to feldspar and perhaps kaolinte dissolution dawsonite reaches a maximum 25 vol of the bulk rock mass balance suggests that this co 2 influx was coupled to the external import of sodium and export of sio 2 ankerite and additional siderite precipitated during the late stage alkaline diagenesis the carbon isotope values of the dawsonite are in the range 4 1 to 2 2 indicating the magmatic origin of the co 2 vitrinite reflectance and thermal gradient constraints suggest that the dawsonite at this location formed at a temperature of 75 c

reconstruction of northeast asian deformation integrated with western pacific plate subduction since 200 ma abstract the configuration and kinematics of continental deformation and its marginal plate tectonics on the earth s surface are intrinsic manifestations of plate mantle coupling the complex interactions of plate boundary forces result in plate motions that are dominated by slab pull and ridge push forces and the effects of mantle drag these interactions also result in continental deformation with a complex basin mountain architecture and evolution the kinematics and evolution of the western pacific subduction and northeast asian continental margin deformation represent a first order tectonic process whose nature and chronology remains controversial this paper implements a deep time reconstruction of the western pacific subduction continental accretion or collision and basin mountain deformation in northeast asia since 200 ma based on a newly revised global plate model we use gplates software to examine strain recovery geological and seismic tomography constraints for the western pacific plate subduction and sequentially backward rotations of deforming features the results indicate a nw se oriented shortening from 200 to 137 ma a nww see oriented extension from 136 to 101 ma a nearly n s oriented extension and uplift with a short term nww see oriented compressional inversion in northeast china from 100 to 67 ma and a nw se and nearly n s oriented extension from 66 ma to the present day the western pacific oceanic plate subducted forward under east asia along mudanjiang honshu island during the jurassic and the trenches retreated to the sikhote alin north shimanto and south shimanto zones from ca 137 128 ma ca 130 90 ma and in ca 60 ma respectively our time dependent analysis of plate motion and continental deformation coupling suggests that the multi plate convergent motion and ocean continent convergent orogeny were induced by advance subduction during the jurassic and earliest cretaceous our analysis also indicates that intra continent rifting and back arc extension were triggered by trench retreat during the cretaceous and that the subduction of the oceanic ridge and arc were triggered by trench retreat during the cenozoic therefore reconstructing the history of plate motion and subduction and tracing the geological and deformation records in continents play a significant role in revealing the effects of complex plate motions and the interactions of plate boundary forces on plate mantle coupling and plate motion intracontinental deformation coupling

low vp vs ratios in the crust and upper mantle beneath the sea of okhotsk inferred from teleseismic pmp smp and sms underside reflections from the moho 1 teleseismic ground motion recordings of deep earthquakes often contain clear seismic wave reflections from the free surface above the source pp sp ss with isolated precursors pmp smp and sms produced by underside reflections from the moho the crust mantle boundary below the free surface reflection points the waveforms of the depth phases and their precursors are readily modeled providing constraints for steep angles of incidence on integrated two way crustal travel times and moho shear and compressional impedance contrasts above deep earthquake source regions we model surface reflection waveforms in stacks of three component long period wwssn and digital broadband seismograms for signals traversing the crust and upper mantle beneath the sea of okhotsk which overlies deep earthquakes in the subducted kurile slab the crust beneath most of the sea of okhotsk is 19 25 km thick indicating a submerged continental margin with minor crustal thinning toward the kuril back arc basin where thin oceanic crust is found modeling pmp and sms phases that sample the same subregion provides tight constraints on the vp vs ratio in the crust and uppermost mantle assuming common density structure and consistent crustal thickness for p and s waves the absolute velocities are not constrained by the data we find unusually low vp vs ratios in the range 1 6 1 7 for the uppermost mantle under the sea of okhotsk and comparably low average vp vs ratios for the crust in several regions the presence of fluids and extensive silica enrichment possibly involving low temperature veining are viable explanations for the anomalous vp vs ratios this may represent an important process of continentalization taking place landward from the volcanic arc in subduction zones

evolution of collision related basins in the eastern end of the kurile basin okhotsk sea northwestern pacific abstract a densely meshed 9400 km long high resolution seismic survey revealed the evolution of sedimentary basins in the southeastern rim of the kurile basin offshore of eastern hokkaido nw pacific hokkaido is situated in a collision zone not only of okhotsk and eurasian plates and also of the kurile and ne japan arcs offshore eastern hokkaido is overlain by n s trending basins hamatonbetu oki monbetu oki and kitami yamato sedimentary basins hosb mosb and kysb respectively and abasiri oki trough aot separated by zones of uplift depositional patterns of the hosb and deformation of the adjacent uplift zone suggest the pull apart origin of the basin related to dextral motions along the central sakhalin fault which extends to hokkaido and binds the okhotsk and eurasian plates the uplift zone adjacent to the mosb suggests the initiation of subsidence in the middle miocene within a splay fault basin with subsequent transformation into a 40 km wide graben in the late miocene and into a half graben in the pliocene the pre late miocene unconformity is widespread the kysb and the aot began as a concatenated basin in the south of mosb in the middle miocene in the pliocene the kitami yamato bank kyb was uplifted and divided the kysb aot basin into kysb and aot the kyb uplift was probably induced by the kurile forearc sliver movement the interpretation of structures suggests that the kurile basin may have opened in a n s direction from dextral shearing along a collision zone in the middle miocene in the pliocene the kinematics changed to a ne se spreading direction because of southwestward migration of the kurile arc with respect to the ne japan arc compressional tectonics after the pliocene may suggest the destruction of the kurile basin

geodynamic mechanism and classification of basins in the earth system abstract the sedimentary basin is a basic geological unit in the earth system its formation and evolution are controlled mainly by plate tectonics and geodynamics which have produced a variety of basin types the development and evolution of various basins depend not only on the tectonic effect of lithospheric plate but also on sublithospheric and earth surface actions basin classification schemes based on the theories of platform geosyncline or plate tectonics could neither include all basin types nor reflect the complex processes of basin formation and evolution from the viewpoint of earth system we proposed a new basin classification scheme based on geodynamic mechanism that is lithospheric plate sublithospheric and earth surface actions in our geodynamic classification we employed a series of geodynamic settings of basin formation and evolution including the lithospheric plate sublithospheric and earth surface environments the dynamic regimes of basin formation and evolution such as tectonic setting extension compression and shearing the thermal processes and gravity effects the various natures of basin basement and crustal types continental oceanic or transitional crust and the history of basin sedimentary filling and tectonic paleogeography successively as the controlling factors for basin classification this new geodynamic classification scheme highlights both the role of the lithosphere and the effects of the sublithospheric and the earth surface actions on controlling the basin formation and evolution the basic dynamic types of basin classification include the prototype basin forming under a single tectonic thermal or gravity regime and the superimposed basin as well as residual basin forming under multistage thermal and or tectonic regimes thus this classification scheme could provide an evolving understanding of lithosphere crust mantle interaction and geodynamic processes as well as of hydrocarbon accumulation and metallogenic mechanisms in sedimentary basins and would help in the synthetic exploration of various resources

mechanism of crystal redistribution in a sheet like magma body constraints from the nosappumisaki and other shoshonite intrusions in the nemuro peninsula northern japan processes of crystal separation in a magma heavily laden with crystals without phase change are investigated from observations on frozen magma systems nosappumisaki and other shoshonite intrusions in the nemuro peninsula japan for which the origin of the crystals and the initial conditions are well constrained the nosappumisaki intrusion is 120 m in thickness and extends for more than 1 5 km it exhibits a wide range of lithological variation principally as a result of crystal redistribution after intrusion crystals in each lithology can be clearly divided into two kinds according to their composition and texture those present before the intrusion of the magma phenocrysts and those that crystallized in situ after intrusion from the vertical change in mode and size of phenocrysts it is shown that 1 augite phenocrysts were rapidly deposited with little overgrowth after intrusion by significant coagulation or clustering on a time scale of more than a few years and 2 plagioclase phenocrysts definitely denser than the melt but concentrated in the upper level floated by counter flow of massive deposition of augite phenocrysts these results indicate that in a magma heavily laden with crystals of a few millimeters in size 20 vol crystal crystal and crystal melt interaction play an important role in the separation of crystals from the host melt

thermal and exhumation history of sakhalin island russia constrained by apatite u pb and fission track thermochronology abstract sakhalin island represents a key locality to study the tectonic evolution of the western pacific the island is located at the amur okhotsk plate margin and records a complex thermotectonic history apatite double dating u pb and fission track and thermal history modelling were applied to three late eocene granitoid massifs within central and southern sakhalin the aniva okhotsk and langeri complexes apatite u pb results yield consistent late eocene 40 37 ma ages suggesting rapid post magmatic cooling apatite fission track results reveal bimodal age distributions with late eocene early oligocene 38 33 ma and early miocene 20 17 ma age populations that can be correlated with variations in uranium and chlorine concentrations thermal history modelling translates the aft age bimodality into two phase cooling histories the timing of the early cooling phase 38 33 ma corresponds with the apatite u pb ages indicating rapid cooling to at least 100 c during the late oligocene the second cooling phase at 20 17 ma cooled the samples to near surface temperatures both cooling phases correspond with regional unconformities and subsequent accelerations in sedimentation rate suggesting that cooling was a response to rapid exhumation in addition our data suggests that the studied terranes record differential exhumation with respect to the structural architecture the miocene exhumation pulse is coeval with the timing of transpressional fault displacement and the subsequent opening of the kuril basin

mode of mixture of oceanic fragments and terrigenous trench fill in an accretionary complex example from southern sakhalin abstract four modes of mixture of oceanic materials such as cherts limestones and basalts and terrigenous trench fill sediments are recognized in the cretaceous accretionary complexes in southern sakhalin far eastern pacific margin of the u s s r the first mode is a thin skinned stacking of pelagic bedded cherts with black shale due to duplex formation in association with shallow underplating or off scraping the second mode is a mixture of the uppermost part of the oceanic crust with terrigenous trench fill sediments due to olistostrome formation reconstruction of the original sequence in ascending order suggests that the basement is pillow basalt with overlying pelagic sediments of chert and limestone olistostrome caused by collapse of oceanic crust due to its bending before encountering a trench and finally trenchfill sediments of turbidite this sequence is imbricated by thrusting which appears to have occurred at the time of underplating the third mode is a tectonic mixing of oceanic fragments and terrigenous sediments due to shear which is related to underthrusting of the oceanic plate a reduction in the supply of trench fill sediments causes copious accretion of the upper part of oceanic crust the fourth mode is in situ basalt volcanism at the trench where thick baked margins of terrigenous shale are formed around the injected basalts this mode appears to represent a ridge trench encounter these four different modes of mixing seem to be usual in ancient accretionary complexes in orogenic belts

changes in depositional systems and sequences in response to basin evolution in a rifted and inverted basin an example from the neogene niigata shin etsu basin northern fossa magna central japan abstract the niigata shin etsu basin which occurs along the eastern margin of the sea of japan was generated in middle miocene time as a failed rift basin in relation to the backarc opening of the japan arc and was converted to a compressional basin in late miocene time the basin filling succession can be divided into four tectonostratigraphic units stages based on characteristics of depositional systems depositional sequences tectonic subsidence rate and sediment supply these units coincide in time with major tectonic events during backarc rifting and subsequent basin inversion unit i 16 13 5 ma represents a syn rift stage characterized by rapid basin subsidence the rate of accommodation exceeded the rate of accumulation resulting in a retrogradational stacking pattern of muddy slope to basin floor systems unit ii 13 5 6 5 ma represents a post rift thermal subsidence stage the rate of accommodation nearly equaled the rate of accumulation resulting in an aggradational stacking pattern of five third order depositional sequences mainly comprising submarine fan turbidite systems affected by supply of coarse clastics unit iii 6 5 1 ma represents the basin inversion stage showing no subsidence or uplift the accumulation rate exceeded the rate of accommodation resulting in rapid progradation of depositional sequences composed of slope shelf shallow marine delta and fluvial systems unit iv 1 ma present represents an intense compressional stress field sedimentation has been restricted to a synclinal area due to syndepositional folding and uplift changes in basin subsidence rate and sediment supply resulting from tectonics and climate change in the basin and the provenance strongly affected characteristics of the depositional systems and sequences the change in basin subsidence also created a basinwide depositional cycle whereas eustasy affected relative sea level changes created higher order depositional sequences during relatively stable phases of tectonics

speculations on the mesozoic plate tectonic evolution of eastern china several orogenic belts transecting eastern china are the sites of former convergent plate margins although there have been varying views on the collisional framework of individual continental blocks styles of convergence at these zones and the timing of respective collisions a tectonic study of eastern china mongolia and the southern soviet far east indicates the collision of the south china block with a combined north china northeast china fold zone block in the late triassic early jurassic their collective suturing to eurasia in the late jurassic early cretaceous followed by the sikhote alin japan block in the mid to late cretaceous the evidence is as follows 1 a linear belt of late triassic early cretaceous granites and granodiorites trends east from the qinlingshan through the dabieshan to the huaiyang massif ophiolites flysch subduction zone melange a paired metamorphic belt indicating north dipping subduction and marine strata of carboniferous to late triassic age from the qinlingshan define the suture between the north and south china blocks 2 a sinuous belt of ultramafics blueschists silicic to intermediate magmatism and west and north vergent folds and thrusts trend from the west margin of the ordos basin through central inner mongolia and along the east great khingan range to the amur river coupled with a mid jurassic early creataceous unconformity a suturing of eastern chinese blocks to eurasia along this zone is suggested 3 a fold and thrust belt with ultramafics flysch blueschists and subduction zone melange along the ussuri river in northeast china indicates the suturing of the sikhote alin japan block to eurasia along a west dipping subduction zone in the mid to late cretaceous similarly a tectonic study of southern china and southeast asia has revealed a complex regional mosaic of suture bounded terrains which nucleated about the eastern western and southern margins of the yangtze craton during the late triassic and early jurassic the evidence is as follows 4 a north south trending belt of ophiolites blueschists calc alkaline volcanics and subduction zone melange including granites granodiorites and strongly deformed marine strata all of late triassic age exposed in the longmenshan of sichuan merge with the kekexilishan ophiolite zone into the ailaoshan tengtiaohe ophiolite and blueschist belt in central yunnan along which the songban ganzi complex and the shan thai malaya block join the craton and 5 a southeastern prolongation of the ailaoshan tengtiaohe belt bifurcates into the southeast trending konvoi zone of northern vietnam and the north south trending pak lay luang prabang zone of laos and eastern thailand zones of ophiolite calc alkaline volcanics and strong late triassic deformation they separate the indosinia and shan thai malaya blocks from the craton respectively these findings differ significantly from previous interpretations of a late paleozoic consolidation of south eastern asia as well as disputing the existence of a true pangea

incipient subduction and deduction along the eastern margin of the japan sea the eastern margin of the japan sea has been under convergent tectonics since sometime after the pliocene the suture line is on the ocean continent boundary between the japan yamato basins and the tohoku northeast japan arc incipient subduction and obduction of the oceanic and sub oceanic crust are observed along the suture line with the occurrence of numerous n s trending fault bounded ridges and troughs focal mechanism solutions of several compressional type earthquakes with magnitudes of m 6 9 to 7 7 which have occurred along the zone are consistent with the observation of geological thrust faults the convergent stresses in the japan sea may be due to india eurasia collision and its associated intra plate or inter microplate movements in east asia the eastward movement of the amurian plate causes baikal extension along its western margin and the japan sea convergence along its eastern margin

geology of the idonnappu belt central hokkaido japan evolution of a cretaceous accretionary complex the cretaceous idonnappu belt located along the western hidaka mountains of central hokkaido island in japan records evidence of west to northwest directed underthrusting of oceanic crust the idonnappu fault divides the idonnappu belt into two subbelts the western oku niikappu on subbelt and the eastern koiboku ko subbelt the on subbelt is dominated by a melange facies it includes various thicknesses of pillow basalts bedded radiolarian cherts limestones and greenish siliceous shales all of which are intermixed with a highly sheared shaley matrix that displays a scaly cleavage the ko subbelt is dominated by a thick flysch sequence and alternations of sandstone and shale with thin tectonic melange the stratigraphic sequences within these belts usually young toward the west and display westward vergent structures outcrop to microscopic scale structures in the melange zone of the idonnappu belt however suggest eastward vergence detailed biostratigraphic studies show that structural packages young toward the east ranging in age from lower to upper cretaceous these observations are consistent with an accretionary prism model in which oceanic crust is underthrust toward the west or north west in the middle miocene a change to westward vergence was caused by uplift of the hidaka mountains

pn travel time tomography of the paleo continental collision and rifting zone around korea and japan abstract a recent dense deployment of seismic stations in south korea and japan allows regional seismic imaging of the far east asian region that experienced continental collisions and riftings we perform seismic imaging based on a mantle lid p wave pn travel time tomography to exploit the tectonic imprints in the lithosphere the average pn velocity in the region is estimated to be 7 95 0 03 km sec the inverted pn velocities illuminate the tectonic structures high velocities of 8 15 km sec are observed in the precambrian massif regions while low velocities of 7 8 km sec are associated with the fold belt and sedimentary basin regions in the southern korean peninsula the pn velocity is estimated to be low in the backarc basins including the ulleung and the yamato basins while it is estimated to be higher on the continental fragments including the south korea plateau and the oki bank the high velocity structures along the japanese islands support the separation of the japanese province from the eurasia plate the high velocity anomaly along the east coast of the korean peninsula around the hupo bank suggests intrusion and solidification of high density material in the lower crust and mantle lid

ancient subduction zone in sakhalin island abstract the northern part of sakhalin island is an area of recent intensive tectonic movements and hydrothermal processes as well as a place of accumulation of useful minerals the deep structure of the lithosphere beneath the region of the neftegorsk earthquake of may 27 1995 in north sakhalin which killed residents and caused significant destruction is examined in this paper our geodynamic model shows that north sakhalin consists of the north sakhalin basin deryugin basin and an ophiolite complex located between them the deryugin basin was formed in place of an ancient deep trench after subducting the okhotsk sea plate under sakhalin in the late cretaceous paleogene the north sakhalin basin was formed on the side of the back arc basin at that time the ophiolite complex is fixed in the position of ancient subduction zone that was active in the late cretaceous paleogene approximately in the miocene the subduction of the okhotsk lithosphere apparently ceased the remains of the subduction zone in the form of an ophiolite complex have been identified from geological and geophysical data on the surface the subduction zone is manifested as deep faults stretched along sakhalin it is probable that the neftegorsk earthquake was a result of activation of this ancient subduction zone

characteristics of sediments and their dispersal systems along the shelf and slope of an active forearc margin eastern hokkaido northern japan abstract this study revealed sediment distributions and their dispersal systems in a shelf slope setting along an active forearc margin off eastern hokkaido northern japan where the pacific plate is being subducted beneath the okhotsk north american plate at the 7 km deep kuril trench the findings were based on analyses of the textures and structures of surface sediments components of the sampled sand fractions high resolution acoustic reflection records and sediment budgets the studied margin has a 20 30 km wide shelf and steep 5 10 slope marked by faults and folds associated with transpressional tectonics the shelf sediments are classified into gravelly coarse to fine sand from the shelf edge to the uppermost slope 110 1000 m water depth gravel and coarse sand on the shelf off hanasaki fine sand on the middle outer shelf off kiritappu and very fine sand on the shelf off akkeshi the coarse sediments on the eastern shelf off hanasaki and the shelf margin are interpreted to represent lag deposits from coastal erosion associated with a post glacial transgression across topographic highs the two depocenters of shelf sediments subsequent to the last glacial maximum are situated in the middle outer shelf off kiritappu and in the inner middle shelf off akkeshi these are interpreted as transgressive highstand sequences whose locations were controlled by ridges on the shelf margin and topographic highs on the shelf the shelf sediment thickness reaches 20 m and the total volume is estimated at ca 6 km3 the slope sediments are grouped into medium fine sand on the uppermost and upper slopes 150 2000 m water depth and mud dominated sediments on the upper to middle slope 1000 3000 m water depth the sandy sediments on the slope at water depths greater than 1000 m are regarded as being derived from the shelf break by gravitational redistribution the occurrence of diamictite like sediments around gullies that incise into the slope indicates that mass movement and sediment gravity flows play an important role in the redistribution of sediments upon the slope on the upper middle slope hemipelagic fallout and advection of turbid water from the shelf edge provide a supply of suspended material the main contributor of sediment supply to the area is coastal erosion 1 81 mt y 1 along with a lesser contribution from fluvial input 0 10 mt y 1 during the latest quaternary the rates of sediment accumulation upon the shelf are estimated to have been 0 47 mt y 1 the remainder of the material transported into the sea is probably redistributed to deeper parts of the slope via gravitational sediment flows or transported outside of the study area by alongshore and tidal currents the studied shelf that kept less than a quarter of the mass from land is comparable with other active margin shelves of new zealand and california although they are fed by high sediment laden rivers

arc deformation and marginal basin opening japan sea as a case study we discuss the opening mechanism of the japan sea in miocene time using 1 tectonic and published paleomagnetic data along the eastern margin from the north of hokkaido island to sado island 2 a mechanical model which is tested by small scale physical modeling and 3 crustal structure and bathymetric features in the japan sea which constrain our kinematic model and preopening reconstructions our main conclusions are the following the eastern margin of the japan sea was as a whole a dextral shear zone about 100 km wide this conclusion is supported by the existence of a ductile dextral shear zone in central hokkaido hidaka mountains and associated brittle deformation in western hokkaido and northeastern honshu the stress field during the opening which ended about 12 ma ago at the end of the middle miocene changes from right lateral transpression in the north to right lateral transtension in the south the western margin along the korean peninsula during the same period also was an active dextral shear zone paleomagnetic results indicate that clockwise rotations occurred in the south during the opening and counterclockwise rotations in the north we propose a model of right lateral pull apart deformation with clockwise rotations of rigid blocks in the southern transtensional domain and counterclockwise rotations in the transpressional one small scale physical models show that the clockwise rotation in transtension is possible provided that the eastern boundary pacific side is free of stress the opening stopped and compression subsequently began about 12 ma ago finally we show that the dextral shear which is distributed over the whole japan sea area is accommodated by n s trending right lateral faults and rotation of blocks located between these right lateral faults

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

the diffuse transition between the zagros continental collision and the makran oceanic subduction iran microearthquake seismicity and crustal structure summary the nature of the transition between the zagros intra continental collision and the makran oceanic subduction is a matter of debate either a major fault cutting the whole lithosphere or a more progressive transition associated with a shallow gently dipping fault restricted to the crust microearthquake seismicity located around the transition between the transition zone is restricted to the west of the jaz murian depression and the jiroft fault no shallow micro earthquakes seem to be related to the nnw sse trending zendan minab palami active fault system most of the shallow seismicity is related either to the zagros mountain belt located in the west or to the ns trending sabzevaran jiroft fault system located in the north the depth of microearthquakes increases northeastwards to an unusually deep value for the zagros of 40 km two dominant types of focal mechanisms are observed in this region low angle thrust faulting mostly restricted to the lower crust and strike slip at shallow depths both consistent with ns shortening the 3 d inversion of p traveltimes suggests a high velocity body dipping northeastwards to a depth of 25 km this high velocity body probably related to the lower crust is associated with the deepest earthquakes showing reverse faulting we propose that the transition between the zagros collision and the makran subduction is not a sharp lithospheric scale transform fault associated with the zendan minab palami fault system instead it is a progressive transition located in the lower crust the oblique collision results in partial partitioning between strike slip and shortening components within the shallow brittle crust because of the weakness of the pre existing zendan minab palami faults

japan sea opening history and mechanism a synthesis the respective tectonic effects of back arc spreading and continental collision in asia are considered either as two independent processes or as closely interrelated extrusion tectonics assumes that the opening of the south china sea and the left lateral motion along the red river fault are geometrically linked in a pull apart manner this model is not accepted by several workers because the structural link between the two processes is not clearly demonstrated in the case of the japan sea we can show without ambiguity that back arc opening was controlled by large intracontinental strike slip faults which can be easily understood as effects of the india asia collision far from the indenter the japan sea opened in the early miocene in a broad pull apart zone between two major dextral strike slip shear zones the first one extends from north sakhalin to central japan along 2000 km it has accommodated about 400 km of finite displacement deformation along it varies from dextral transpression in the north to dextral transtension in the south the second is between korea and sw japan and has accommodated a smaller displacement of about 200 km the extensional domain in between lies in the back arc region of japan distributed stretching of the arc crust resulted in the formation of most of the japan sea while localized oceanic spreading at the southern termination of the eastern transpressional shear zone shaped the japan basin the first oceanic crust was formed in a small triangle based on the eastern shear zone and spreading propagated westward inside the pull apart region timing of oceanic crust formation of formation of the dextral shear zones and of block rotation in between as well as the internal structure of the basins and the geometry of deformation along the master shear zones are used to reconstruct the opening history this evolution is discussed by comparison to other manifestations of the arc and back arc activity such as the history of sedimentation and volcanism the paper then suggests that the collision of india can have tectonic consequences as far north as japan and sakhalin and describes the geometrical relation of back arc opening there and diffuse extrusion

the japan trench and its juncture with the kuril trench cruise results of the kaiko project leg 3 this paper presents the results of a detailed survey combining seabeam mapping gravity and geomagnetic measurements as well as single channel seismic reflection observations in the japan trench and the juncture with the kuril trench during the french japanese kaiko project northern sector of the leg 3 on the r v jean charcot the main data acquired during the cruise such as the seabeam maps magnetic anomalies pattern and preliminary interpretations are discussed these new data cover an area of 18 000 km2 and provide for the first time a detailed three dimensional image of the japan trench combined with the previous results the data indicate new structural interpretations a comparative study of seabeam morphology single channel and reprocessed multichannel records lead to the conclusion that along the northern japan trench there is little evidence of accretion but instead a tectonic erosion of the overriding plate the tectonic pattern on the oceanic side of the trench is controlled by the creation of new normal faults parallel to the japan trench axis which is a direct consequence of the downward flexure of the pacific plate in addition to these new faults ancient normal faults trending parallel to the n65 oceanic magnetic anomalies and oblique to the japan trench axis are reactivated so that two directions of normal faulting are observed seaward of the japan trench only one direction of faulting is observed seaward of the kuril trench because of the parallelism between the trench axis and the magnetic anomalies the convergent front of the kuril trench is offset left laterally by 20 km relative to those of the japan trench this transform fault and the lower slope of the southernmost kuril trench are represented by very steep scarps more than 2 km high slightly south of the juncture the erimo seamount riding on the pacific plate is now entering the subduction zone it has been preceded by at least another seamount as revealed by magnetic anomalies across the landward slope of the trench deeper future studies will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both trenches is it due to the collision of an already subducted chain of seamounts or does it correspond to one of the failure lines of the america eurasia plate boundary

present day heat flow thermal history and tectonic subsidence of the east china sea basin the east china sea basin is located in the convergence area between the eurasian plate the pacific plate and the philippine sea plate and consists of two depressions the taibei depression in the west and the xihu depression in the east heat flow measurements show that the east china sea basin is characterized by present day heat flow around 70 6 mw m 2 varying between 55 and 88 mw m 2 no significant difference in heat flow is observed between the xihu and the taibei depressions thermal history reconstruction using vitrinite reflectance suggests that the thermal history was however different in the taibei and the xihu depressions paleo heat flow values when the pretertiary formations experienced their maximum temperature at the end of the paleocene reached a mean of 82 mw m 2 in the taibei depression much higher than the present day value the lower tertiary sediments in the xihu depression experienced maximum temperatures at the end of oligocene 35 4 23 3 ma and reached a mean paleo heat flow value of 83 mw m 2 tectonic subsidence analysis shows that the timing of the major rifting episode was different across the east china sea basin the rifting occurred from the late cretaceous w65 ma to the early eocene w55 ma in the taibei depression followed by thermal subsidence from the late eocene to the end of miocene 23 3 5 2 ma in contrast in the xihu depression the initial subsidence lasted until the early miocene and thermal subsidence to the end of miocene from pliocene to the present an accelerated subsidence took place all along the west pacific margin of asia significant thicknesses of strata were removed from the unconformities in the basin the mean amount of erosion was 1147 m from the paleocene and 1208 m above the oligocene in the taibei depression and 1409 m from the oligocene in the xihu depression q 2004 elsevier ltd all rights reserved

evolution of the miocene waup basin se korea in response to dextral shear along the southwestern margin of the east sea sea of japan abstract the waup basin se korea is a nonmarine basin created coevally with the back arc opening of the east sea sea of japan during the miocene the basin is bounded by normal faults in the northwestern and southeastern margins and by dextral strike slip faults in the northeastern and southwestern margins a number of intrabasinal faults with dextral strike slip and normal senses of slip are also present delineating three structural domains of the basin i e the ne central and sw blocks the basin fills composed of alluvial to lacustrine deposits and dacitic to basaltic volcanogenic rocks have the geometry of a half graben in the ne and central blocks and a graben in the sw block they are younging from the ne to the sw blocks suggestive of propagating rifting of the basin and associated depocenter migration structural and stratigraphic data suggest that the ne and central blocks were produced by nw se extension during the early miocene 22 17 ma involving northwestward block tilting and subsequent clockwise horizontal block rotation on the other hand the sw block formed at about 17 ma by nw se extension and experienced neither block tilting nor block rotation the reconstructed configuration of the waup basin is that of a pull apart basin produced by nnw directed dextral strike slip stress and associated nw se extensional stress the ages of the basin fills ca 22 to

asymmetric deformation in the backarc region of the kuril arc northwest pacific new insights from analogue modeling 1 the eocene to middle late miocene tectonic evolution of the kuril arc and backarc region has been simulated with analogue experiments the experiments simulate asymmetric deformation in the overriding plate due to anticlockwise rollback of the subducting pacific plate the results show the formation of a n s to ne sw oriented dextral shear zone near the far edge of the retreating boundary analogous to the sakhalin hokkaido dextral shear zone contemporaneously normal faults and grabens form striking parallel to the retreating boundary near the far edge but striking more oblique near the hinge point and away from the retreating boundary this is similar to extensional structures observed in the kuril basin and the sea of okhotsk furthermore the model shows that the amount of extension progressively decreases away from the retreating boundary this appears to have also happened in the kuril okhotsk region as evidenced by crustal thickness variation in the region finally the model results show that extension is increasingly accommodated by the region close to the retreating boundary with progressive deformation this can account for the eocene early miocene extension in the sea of okhotsk followed by miocene spreading in the kuril basin

accommodating large scale intracontinental extension and compression in a single stress field a key example from the baikal rift system the baikal rift system in southern siberia is one of the main intracontinental extensional features on earth the rift system represents the northwestern boundary of the amuria plate and in that respect can be considered as an evolving plate boundary the baikal rift system has been widely studied both in terms of geology and geophysics and many models have been proposed for its formation and evolution however the age of the initiation of deformation and the mechanism driving this deformation are still largely debated while major extension has occurred since the late miocene pliocene the onset of extension seems older than the india asia collision implying that several driving mechanisms may have acted together or in relay through time in this work we review the available data and models for deformation in an area encompassing the baikal rift system the sayan ranges to the west and the transbaikal to the east using a synthesis of this data and our own field and mapping observations we show that the baikal rift system along with transpressional deformation in the sayan ranges and transtension in the transbaikal area can be explained through major left lateral strike slip systems the deformation is strongly controlled by inherited crustal and lithospheric structures and is distributed over a wide area within the western amuria plate that consequently cannot be considered as a rigid block such distributed deformation is likely to have a strong effect on the structure of the future continental margin if extension evolves towards the formation of oceanic crust

cenozoic pulsed compression of da an dedu fault zone in songliao basin ne china and its implications for earthquake potential evidence from seismic data abstract the da an dedu fault zone ddfz is a major tectonic feature cutting through the songliao basin from south to north in ne china pulsed compression deformation of ddfz during the cenozoic implies a complex geodynamic process and the latest stage of which occurred in the quaternary directly influences the present seismicity of the interior basin although most of the evidence for quaternary deformation about the songliao basin in the past decades was concentrated in marginal faults all five earthquake swarms with magnitudes over 5 0 along the buried ddfz with no surface expression during the past 30 years suggest it is a main seismogenic structure with seismic potential which should deserve more attention of geologists however limited by the coverage of the quaternary sedimentary and absence of strong historic and instrumental earthquakes records m 7 the geometric pattern quaternary activity and seismic potential of the ddfz remain poorly understood thus unlike previous geophysical studies focused on crust mantle velocity structure across the fault and the aim of exploring possible mineral resources in the basin in this study we have integrated a variety of the latest seismic data and drilling holes from petroleum explorations and shallow depth seismic reflection profiles to recognize the cenozoic pulsed compression deformation of the ddfz and to discuss its implication for earthquake potential the results show that at least four stages of compression deformation have occurred along the ddfz in the cenozoic 65 ma 23 ma 5 3 ma and 1 8 ma respectively although the geodynamic process behind which still in dispute the results also imply that the tectonic style of the ddfz fits well with the occurrence of modern seismic swarms moderate earthquake potential m 7 0 is suggested along the ddfz

new insights into the distribution and evolution of the cenozoic tan lu fault zone in the liaohe sub basin of the bohai bay basin eastern china abstract as the largest strike slip fault system in eastern china the northeast trending tan lu fault zone tlfz is a significant tectonic element contributing to the mesozoic cenozoic regional geologic evolution of eastern asia as well as to the formation of ore deposits and oilfields because of the paucity of data its distribution and evolutionary history in the offshore liaohe sub basin of the northern bohai bay basin bbb are still poorly understood investigations of the strike slip fault system in the western portion of the offshore liaohe sub basin via new seismic data provide us with new insights into the characteristics of the cenozoic tlfz results of this study show that cenozoic dextral strike slip faults occurred near the center of the liaoxi graben in the offshore liaohe sub basin these strike slip faults connect with their counterparts to the north the western part of the onshore liaohe sub basin and have similar characteristics to those in other areas of the bbb in terms of kinematics evolutionary history and distribution consequently these faults are considered as the western branch of the tlfz all strike slip faults within the liaoxi graben merge at depth with a central subvertical basement fault induced by the reactivation of a pre existing strike slip basement fault the pre cenozoic tlfz data suggest that the tlfz across the whole liaohe sub basin comprises two branches and that the cenozoic distribution of this system was inherited from the pre cenozoic tlfz this characteristic distribution might be possessed by the whole tlfz thus the new understandings about the distribution and evolutionary model of the tlfz in this study can be inferred in many research fields along the whole fault zone such as regional geology ore deposits petroleum exploration and earthquake hazard

regional seismicity and on land deformation in the ryukyu arc implications for the kinematics of opening of the okinawa trough the stress field evolution and the kinematics of opening of the okinawa trough are investigated on the basis of earthquake focal mechanisms and structural data in the ryukyu arc and the okinawa trough focal mechanisms show that the crust underlying the arc and the trough undergoes extension along two suborthogonal directions a regional arc perpendicular extension and a local arc parallel extension both extensions are concurrent and related to the same regional stress field characterized by permutating horizontal 2 and 3 axes earthquake slip vectors reveal a southward motion of the ryukyu arc with respect to the south china block the current pole of opening of the okinawa trough is located around 16 n and 50 e fracture analysis in okinawa island allows identification of three episodes of extension a late miocene n40 w to n20 e extension episode i a late pliocene to early pleistocene n20 e extension episode ii and a latest pleistocene to present day n20 w extension episode iii episodes ii and iii are characterized by permutations between the two horizontal 2 and 3 axes by synthesizing regional deformation data and by comparing the geometry of the deformation with analogue models of oblique rifting we reconstruct the kinematics of opening of the okinawa trough since the late miocene the direction of divergence of the ryukyu arc has rotated clockwise from n150 e in the late miocene to nearly n s today

lithification of biogenic siliceous sediments evidence from neogene diatomaceous sequences of northeast japan abstract biogenic siliceous deposits are a significant component of neogene stratigraphic sequences of the northeast japan arc field observations demonstrate that the siliceous rocks are classified into five lithologic types diatomite opaline porcelanite opaline chert quartzose porcelanite and black shale stratigraphic data suggest that the siliceous formations were deposited continuously from the middle miocene to early pliocene diatomite occurs throughout the sedimentary sequences and changes laterally into chert and porcelanite multivariate statistical analysis of geochemical data indicates that lithification and mineralogical transformation of silica rich deposits was governed not only by compaction and cementation during burial but also by sorting of grains by biological sediment seawater mixing diatomite is composed of frustules without fine ornamentations and delicate structures fine and delicate tests were more susceptible to dissolution than coarse and solid ones and dissolved in the course of biological sediment disturbance ubiquitous occuerences of diatomite in the stratigraphic sequences are attributed to residual frustules which were more resistant to the diagenetic dissolution precipitation reaction of silica biogenic silica is the main source of silica to form chert diagenetic precipitation of silica in diatomaceous beds through progressive sediment burial formed porcelanitic rocks with opaline porcelanite being the precursor of quartzose porcelanite chert was not subjected to biological sediment reworking and fragile shells were preserved during accumulation silicification of precursor deposits of chert was promoted by intraformational dissolution precipitation reaction of silica with increasing sediment compaction and dehydration

post 19 ka b p eruptive history of ulleung island korea inferred from an intra caldera pyroclastic sequence ulleung island is a quaternary volcanic island located in the mid western part of the east sea sea of japan back arc basin which has erupted from the pliocene until the late holocene this study focuses on reconstructing the latest eruptive history of the island by describing the sedimentological and stratigraphic characteristics of the most recent trachytic phonolitic pyroclastic sequence named the nari tephra formation this formation is preserved as a succession of unwelded pyroclastic and epiclastic deposits within an embayed margin of the nari caldera the embayment acted as a topographic trap for proximal pyroclastic deposits and contains a complete record of the past 19 000 years of eruption history the formation includes evidence for five separate eruptive episodes member n 1 to n 5 with intervening weathered and or soil horizons indicating hundreds to thousands of years of repose between each eruption eruption styles and depositional mechanisms varied between and during individual episodes reflecting changing dynamics of the magma plumbing system magmatic gas coupling and a variable role of external water extra caldera sequences show that only a few of these eruptions generated sustained eruption columns or pyroclastic density currents pdcs large enough to overtop the caldera wall thus tephra sequences outside the caldera provide an underestimate of eruption frequency and care needs to be taken in the interpretation and correlation to distal tephra sequences recognized in marine and terrestrial records in addition topographic effects of caldera structures should be considered for the assessment of pdc related hazards in such moderately sized pyroclastic eruptions

the latest cretaceous early paleogene rapid growth of accretionary complex and exhumation of high pressure series metamorphic rocks in northwestern pacific margin episodic growth of accretionary complexes along the northwestern pacific margin is well documented from detailed biostratigraphy and radiometric dating of metamorphic rocks the latest cretaceous to early paleogene rapid growth of the accretionary complexes observed in hokkaido and sakhalin islands was caused by the supply of a large amount of terrigenous sediments from an intracontinent collision zone the mongolia okhotsk zone in the northern asian continent rapid growth of the accretionary prism resulted in episodic tectonic events in the forearc and magmatic arc rapid underplating caused uplift and exhumation of previously accreted complexes along with high pressure series metamorphic rocks now exposed as the kamuikotan complex in hokkaido and the susunai complex in sakhalin the forearc basin shallowed as a result of the uplift the uplift collapsed the old accretionary complex and supplied olistoliths onto the forearc region magmatic activity along the continental margin was also active at the same time subduction of a large amount of terrigenous sediments into the mantle may have contributed to active magmatism at that time a large supply of water trapped in the subducted sediments entered the mantle and increased partial melting of the mantle wedge

the tectonics of the okhotsk sea abstract the northern and central parts of the okhotsk sea form an epimesozoic platform the hetero aged acoustic basement is represented by deformed geosynclinal rocks from cretaceous to precambrian in age the slightly deformed sedimentary cover levelled the uneven surface of the acoustic basement and this upper paleogene neogene cover filled up the system of the structural basins the general nw se and w e extensions of the taphrogenic horsts and grabens of the acoustic basement were formed due to extension and subsidence of the earth s crust during the late paleogene neogene

america eurasia plate boundary in eastern asia and the opening of marginal basins abstract the opening of the arctic ocean during the past 55 ma resulted in relative rotation of america with respect to eurasia about a pole located in eastern siberia near the plate boundary the extensional plate boundary enters deep inside the eurasian continent up to the rotation pole on the opposite side of the pole on the pacific side of the plate boundary compressive tectonics are recorded along sakhalin and hokkaido from the oligocene to middle miocene the relative movement was accommodated by strike slip motion along sakhalin and hokkaido although the rotation pole was not located at a significatively different position from now in this paper we explain this by independent motion of the southernmost tip of the american plate towards the pacific margin which behaves as a free boundary this oceanward motion resulted in an extension of the american plate giving rise to the wedge structure of the okhotsk sea the japan sea opened as a pull apart basin along the strike slip boundary finally the increasing extension in the okhotsk sea led to the opening of the oceanic kuril basin

cenozoic evolution of the tan lu fault zone east china constraints from seismic data abstract the tan lu fault zone tlfz is a continental scale strike slip fault zone in the east china that contains rich and significant information about the mesozoic cenozoic evolution of eastern asia it experienced a long term and complex superimposed deformation characterized by mesozoic sinistral and cenozoic dextral motions the fault has a well documented mesozoic sinistral displacement history but its cenozoic history is poorly known in this study based on the observations of the interrelationships of fault kinematics fault geometry and concomitant sedimentation the cenozoic evolution of the tlfz is constrained by abundant seismic data from oil fields numerous bends along the cenozoic master fault sensitively record changes in the stress field during strike slip motion taking into account the cenozoic evolution of the bohai bay basin east china the evolutionary history of these strike slip bends is investigated and the results indicate that dextral motion along the tlfz began at about 40 ma furthermore the cenozoic evolutionary history of the tlfz is suggested to include an extensional deformation with a possible weak sinistral shear from 65 to 40 ma from 40 to 0 ma dextral motion occurred due to a change in the subduction direction of the pacific plate relative to the eurasian plate causing the fault to experience dextral shear with a component of extensional deformation dextral motion was most intense from 40 to 25 ma and then the dextral direction may have suddenly changed from a nearly ne sw trend to a nearly n s trend at around 25 ma dextral motion was relatively enhanced again after 12 ma and was particularly strong after 5 ma episodic and ephemeral thrusting deformation might have occurred during the quaternary the cumulative cenozoic dextral displacement of the tlfz is far smaller than its mesozoic sinistral strike slip displacement may be no more than 21 km

the 12 july 1993 hokkaido nansei oki japan earthquake effects of source complexity on surface wave radiation abstract we present an analysis of the data dependency of surface wave inversions for source parameters of the 12 july 1993 hokkaido nansei oki japan earthquake this dependency is demonstrated by differences between source mechanisms obtained from different data sets particularly from rayleigh waves or love waves although inversions of various data sets yield unanimously thrust fault mechanisms for the inversion of rayleigh waves the mechanism is characterized by a shallow dipping west plunging nodal plane which is consistent with p wave first motion data and the distribution of aftershocks close to the hypocenter however for the inversion of both rayleigh and love waves the mechanism is characterized by a shallow dipping east plunging nodal plane several numerical experiments were carried out to test a hypothesis that the data dependency is attributed to the effects of source complexity on the surface wave radiation with the source complexity being characterized by tempo spatial changes in focal mechanism and moment release during the rupture process the experiments show that source parameters obtained from inversions of surface waves with periods as long as a few hundred seconds can be largely affected by using various models of source complexity proposed previously for the hokkaido earthquake and the effects on love waves tend to be systematically different from those on rayleigh waves our preferred source model is characterized by significant tempo spatial changes in focal mechanism the experiments based on this model provide a good explanation for the data dependency although our analysis cannot completely reject the alternative explanation that attributes the dependency to the manifestation of errors in modeling the surface wave propagation and or inversion instability the model is consistent with the data of body waves and long period surface waves for a very broad frequency band implying the existence of weak zones with various fault geometries along the eastern margin of the japan sea

backarc extension and collision an experimental approach to the tectonics of asia summary the deformation of the eastern asian lithosphere during the first part of the india asia collision was dominated by subduction related extension interacting with far effects of the collision in order to investigate the role of large scale extension in collision tectonics we performed analogue experiments of indentation with a model of lithosphere subjected to extension we used a three layer rheological model of continental lithosphere resting upon an asthenosphere of low viscosity and strained along its southern boundary by a rigid indenter progressing northward the lithosphere model was scaled to be gravitationally unstable and to spread under its own weight so that extension occurred in the whole model the eastern boundary was free or weakly confined and always allowed eastward spreading of the model we studied the pattern of deformation for different boundary conditions the experimental pattern of deformation includes a thickened zone in front of the indenter a major northeast trending left lateral shear zone starting from the northwest corner of the indenter antithetic north south right lateral shear zones more or less developed to the east of the indenter and a purely extensional domain in the southeastern part of the model in this domain graben opening is driven by gravitational spreading whereas it is driven by gravitational spreading and indentation in the northeastern part where grabens opened along strike slip faults the results are compared with the oligomiocene deformation pattern of asia consecutive to the collision of india our experiments bring a physical basis to models which favour distributed deformation within a slowly extruded wide region extending from the baikal rift to the okhotsk sea and to southeast asia and indonesia in this large domain the opening of backarc basins japan sea okinawa trough south china sea and continental grabens north china grabens have been associated with approximately north south trending right lateral strike slip faults which accommodated the northward penetration of india into eurasia

miocene tectonic evolution of the basins and fault systems se korea dextral simple shear during the east sea sea of japan opening crustal deformation of se korea caused by back arc opening of the east sea sea of japan commenced locally in the late oligocene intense deformation occurred during the early miocene accompanying extension of parallelogram shaped pull apart basins between nnw striking principal displacement zones clockwise rotation and northwestward tilting of blocks and southwestward propagating rifting at about 17 ma the crustal deformation suddenly expanded westward and northward owing to activation of the yeonil tectonic line resulting in a dramatic change of depositional environment and the subsidence of wedge shaped pull apart basins the yeonil tectonic line and western border faults of the pohang basin are the westernmost limit of the miocene crustal deformation all the features of crustal deformation in se korea support the pull apart model for the east sea opening however they indicate that the nnw striking faults such as the yeonil tectonic line acted as the principal displacement zones rather than the nne striking yangsan fault under a consistent dextral simple shear at about 16 ma the collision of the philippine sea plate with the japanese islands caused a tectonic inversion this inversion resulted in a compositional change of basaltic magma at about 15 ma and crustal uplift in se korea which caused the cessation of sedimentation at about 10 ma

late cretaceous cenozoic deformation of northeast asia abstract the plate tectonic paradigm implies rigid plates and narrow plate boundaries in contrast diffuse plate boundaries are common both in the oceans and continents r g gordon annu rev earth planet sci 26 1998 615 642 and their history is difficult to constrain especially in remote tectonically complex areas such as northeast asia m e chapman s c solomon j geophys res 81 1976 921 930 here we show how extensive north atlantic marine magnetic r macnab et al eos 76 1995 449 458 and gravity data d t sandwell w h f smith j geophys res 102 1997 10039 10054 can be used to unravel with tight confidence limits successive periods of deformation over 80 million years along the diffuse continental eurasian north american plate boundary a period of compression in the late cretaceous 14 mm yr in the laptev sea to 20 mm yr in kamchatka led to thrusting in the verkhoyansk mountains and was followed by extension from 68 to 40 ma when 400 km of extension was accommodated by the formation of a series of grabens including the moma rift system since 40 ma time varying compression and transpression along the moma rift system created strike slip faults thrusts and folds at rates up to 6 3 mm yr in the laptev sea region 600 km of extension from latest late cretaceous to present created the laptev sea and lena rift systems the deformation predicted by our model fits most geological features formed in the laptev sea and central northeast asia during late cretaceous cenozoic times the most recent deformation late miocene pliocene is not very well constrained since our model lacks data younger than 11 ma the deformation that occurred in kamchatka reflects a complex tectonic setting and our model s predictions are only tentative

stress driven relaxation of heterogeneous upper mantle and time dependent afterslip following the 2011 tohoku earthquake stress driven relaxation of heterogeneous upper mantle and time dependent afterslip following the 2011 tohoku earthquake yan hu 1 roland burgmann 1 naoki uchida 2 paramesh banerjee 3 and jeffrey t freymueller 4 university of california berkeley berkeley ca united states graduate school of science tohoku university japan earth observatory of singapore nanyang technological university singapore singapore university of alaska fairbanks fairbanks ak united states corresponding author yan hu earth and planetary science university of california berkeley 307 mccone hall berkeley ca 94720 usa yhu seismo berkeley edu this article has been accepted for publication and undergone full peer review but has not been through the copyediting typesetting pagination and proofreading process which may lead to differences between this version and the version of record please cite this article as doi 10 1002 2015jb012508 2015 american geophysical union all rights reserved

constraints of sea beam data on crustal fabrics and seafloor spreading in the south china sea abstract the south china sea is a mid late tertiary marginal basin the magnetic anomaly lineations in the eastern part of the basin trend approximately east west 1 2 suggesting a north south direction of spreading in the spring of 1985 two cruises on the french research vessel jean charcot provided sea beam coverage seismic reflection magnetic and gravity profiles the sea beam data exhibit two major structural trends scarps striking n50 e 15 interpreted as normal faults and scarps striking n140 e 15 interpreted as fracture zones this fabric implies a northwest southeast direction of spreading up to about 100 km north and south of the inferred spreading axis 3 dense sea beam coverage of a roughly 1 square area northwest of the scarborough seamounts chain shows that the emplacement of these seamounts was at least in its initial stage controlled by faulting in two orthogonal directions n50 e and n140 e magnetic and gravimetric maps of the same area also reveal anomalies trending roughly n50 e which are disrupted by transform zones striking n140 e this detailed study indicates that the fracture zones may be closely spaced less than 20 30 km east of macclesfield bank furthermore magnetic anomalies identified as 6 and 6a 20 myr along two north south profiles located at both edges to the north of this detailed study area may be correlated with the n80 e trend characteristic of such anomalies in the eastern part of the south china sea the east west trend of magnetic anomalies 6 and 6a south of scarborough seamount chain recognised by taylor and hayes 1983 1 is incompatible with the trend of the fault scarps observed on sea beam data we infer that progressive right lateral offsets of the ridge across closely spaced discontinuities may account for the nearly east west average trend of some of the magnetic anomalies and of the scarborough seamount chain which represents the location of the relict spreading axis in spite of a n50 e spreading direction the whole central part of the basin east of 115 e may have formed in this way since n130 140 e striking fracture zones are observed on the sea beam swaths this model may correspond to the second of two successive spreading phases with n s and nw se directions of extension respectively as was presented by the authors in a previous paper pautot et al 1986 3 alternatively we may assume that the direction of extension did not change radically during the opening history 3 implying that oblique spreading occurred along n80 e trending ridge segments creating the n80 e trending scarps and magnetic anomalies observed to the north and to the south of the study area such a direction of extension is compatible with mid late tertiary left lateral movements along large n130 150 e strike slip faults such as the red river and wang chao faults in south china and sundaland

tectonic sedimentary and volcanic evolution of a back arc basin in the east sea sea of japan abstract this paper focuses on the tectonic sedimentary and volcanic evolution of a unique back arc basin ulleung basin in the southwestern part of the east sea sea of japan the basin consists of thick extended continental or transitional crust and an overlying sedimentary succession 4 8 km thick with interlayered volcanic flows and sills defining a number of seismic units of variable reflection characteristics the northern margin is bounded by faulted continental blocks south korea plateau with isolated basement lows and sub basins with intruded and extruded volcanics whereas the southern margin is underlain by deep seated basement with a thick 8 km thick sedimentary succession the western margin is bounded by a series of strike slip and normal faults produced by nnw sse directed dextral movements these structural features suggest a south southeastward drift of the southwestern japanese arc away from the south korea plateau during the early to middle miocene involving large scale right lateral strike slip deformation along the western margin akin to pull apart basin formation during the back arc opening the thinned continental crust was largely modified by intrusive volcanics under tensional stress regime the volcanics also extruded across the axis of extension as well as along the boundary faults on the west in the middle to late miocene major faults in the southern and western margins were inverted forming partial closure of the basin especially in the southern part although the plateau in the northern margin experienced continuous subsidence the ulleung basin thus provides an example of an immature back arc basin which experienced a rather brief episode of rifting and extension followed by closure due to tectonic inversion

alternate senses of displacement along the tsushima fault system during the neogene based on fracture analyses near the western margin of the japan sea abstract the western margin of the japan sea is characterized by the existence of a n20 e trending transcurrent fault system in the korea strait between korea and kyushu this fault system called here the tsushima fault system played a major role during the opening of the japan sea during the tertiary in order to understand its displacement history we analysed the deformation recorded by neogene rocks exposed on the korea strait islands tsushima goto and hirado islands a synthesis of published stratigraphical and structural data together with our field observations reveals that two tectonic events occurred in succession in the study area an early miocene nw se oriented extension event i and a middle miocene nw se oriented shortening event ii two distinct families of fractures with contrasted stratigraphic occurrences have been recognized in the neogene rocks exposed on the korea strait islands and allow us to define and date the stress fields characterizing each tectonic event stress field i is of intermediate type the vertical axis is either 1 or 2 with a n45 e hmax and n135 e hmin and was active between 22 and 16 ma stress field ii is compressional the vertical axis is 3 with a n135 e hmax and a n45 e hmin and was active between 15 and 10 ma the trends of stress field i principal axes imply that the displacement along the tsushima fault system was dextral during the early miocene conversely the trends of stress field ii principal axes imply that the displacement along the same fault system was sinistral during the second half of the middle miocene the inferred displacement history agrees with 1 a pull apart model for the opening of the japan sea during the early miocene and 2 transition from tensional to compressional stress fields in the japanese arc and back arc region at the end of the middle miocene

mesozoic evolution of northeast asia and the collision of the okhotsk microcontinent abstract tectonic models of northeast japan generally involve the progressive subduction of the paleo pacific oceanic crust during the mesozoic and collision of the okhotsk microcontinent at the end of the cretaceous with eurasia however these models are not based on a precise knowledge of the overall structure and succession of deformations in the collision zone our survey leads us to propose structural maps and cross sections of the hokkaido central belt and a tectonic description of each tectonic stage it appears that at the end of the jurassic until the early cretaceous an important event of nappe emplacement occurred we interprete this event as the collision of the okhotsk block with eurasia much earlier than in classical models during this collision the oceanic crust of a marginal basin was obducted with an east vergence and partly metamorphosed contemporaneously with the collision an important volcanism linked with the pacific subduction started and continued during the whole of the cretaceous period the continental margin was affected by strike slip faults running parallel to the margin with a left lateral motion the main fault was the tanlu fault which accommodated the northward movement of the eastern part of the asian continent which led to the closing of an oceanic domain between siberia and the okhotsk block this tectonic scenario ended 70 m y ago contemporaneously with the cessation of the volcanism in the okhotsk uda margal volcanic belt from the eocene to the middle miocene a right lateral strike slip motion along sakhalin and hokkaido gave rise to the hidaka shear zone and finally led to the opening of the sea of japan

plates and their motions the last reviews which covered research in plate motions were those of herron and pitman 1975 on marine magnetic anomalies and dewey 1975 on plate tectonics both reporting on research for the period 1971 1974 in the last 12 years there has been considerable refinement of relative motion models as well as further development of global models also major effort has gone into defining reference frames for the study of plate motions and discriminating between them more recently the role of errors in individual plate rotations and the associated uncertainties in reconstructions have received attention in this report i concentrate on work published in the last 4 years 1983 1986 citing a minimum of publications from previous years however some papers in press for early 1987 have been included this report first reviews work on relative plate motions for the present and past and reconstructions and then models for these motions global ones for driving forces and local ones determining subduction history and back arc spreading the problem of reference frames both hotspot and paleomagnetic is discussed next i end with a review of work on the errors in plate motions

neogene strike slip faulting in sakhalin and the japan sea opening we describe structural data from a 2000 km n s dextral strike slip zone extending from northern sakhalin to the southeast corner of the japan sea satellite images field data and focal mechanisms of earthquakes in sakhalin are included in the interpretation since miocene time the deformation in sakhalin has been taken up by n s dextral strike slip faults with a reverse component and associated en echelon folds narrow en echelon neogene basins were formed along strike sup faults and were later folded in a second stage of deformation we propose a model of basin formation along extension al faults delimitating dominos between two major strike slip faults and subsequent counterclockwise rotation of the dominos in a dextral transpressional regime basins becoming progressively oblique to the direction of maximum horizontal compression and undergoing shortening the association of both dextral and compressional focal mechanisms of earthquakes indicates that the same transpressional regime still prevails today in sakhalin we present fault set measurements undertaken in noto peninsula and yatsuo basin at the southern end of the sakhalin east japan sea strike slip zone early and middle miocene formations recorded the same transtensional regime as observed along the west coast of ne honshu during the early and middle miocene the strike slip regime was transpressional to the north in sakhalin and hokkaido and transtensional to the south along the west coast of ne honshu as far as noto peninsula and yatsuo basin dextral motion accommodated the opening of the japan sea as a pull apart basin with the tsushima fault to the west the opening of the japan sea ceased at the end of the middle miocene when transtension started to change to e w compression in the japan arc subduction of the japan sea lithosphere under the japan arc started 1 8 ma ago the evolution of the stress regime from transtensional to compressional in the southern part of the strike slip zone is related to the inception of the subduction of the young philippine sea plate lithosphere under the japan arc during the late miocene subduction related extension is a necessary condition for the opening of the japan sea two possible mechanisms can account for dextral shear in this area 1 counterclockwise rotation of crustal blocks due to the collision of india with asia 2 extrusion of the okhotsk sea block squeezed between the north america and eurasia plates

flexural isostasy and loading sequence of the dokdo seamounts on the ulleung basin in the east sea sea of japan abstract the isostatic compensation and loading sequence of the dokdo seamounts on the ulleung basin in the east sea are studied by a flexure model using gravity and bathymetric data the dokdo seamounts comprise dokdo dok island lying as a cluster of emerged small islets surmounting a large submerged volcanic edifice and two voluminous tablemounts simheungtaek and isabu in spite of their similarity in topographic expression a large difference of about 50 mgal between the observed gravity anomaly peaks over dokdo and the isabu tablemount implies different amounts of compensation a comparison between the observed and calculated gravity anomalies of the seamounts shows that the flexural compensation investigated in this study predicts well the observed data the flexure model suggests that the dokdo seamounts might be composed of high density volcanic rocks modeling results show that the flexural rigidity or effective elastic thickness of the lithosphere under dokdo is stronger or thicker than that under the isabu tablemount it implies that the age of lithosphere at the time of loading for isabu was younger than that for dokdo the effective elastic thicknesses associated with the dokdo seamounts approximately follow 200 400 c isotherms in the cooling plate model the results of this study and the former age dating of dokdo suggest that the isabu tablemount was formed first followed by dokdo after the opening of the east sea ceased

collision rotation and back arc spreading in the region of the okhotsk and japan seas the india eurasia collision caused extensive deformation in the eurasia continent since late eocene time we propose that back arc spreading in the okhotsk and japan seas was related to the movement of microplates relevant to the india eurasia collision the kuril basin of the okhotsk sea and the basins of the japan sea were formed as back arc basins simultaneously their late oligocene to middle miocene age is constrained by sediment stratigraphy basement depth and heat flow data the movements of two microplates the okhotsk and amuria plates are significant for the tectonics of the okhotsk and japan seas simultaneous retreat of both microplates from trench hinge lines caused back arc spreading in the kuril basin and the japan sea the amuria plate moved north northeastward due to the india eurasia collision this caused pull apart opening of the baikal rift along a transform boundary between the siberia and amuria plates and a collision along the stanovoy range at its northern margin this movement triggered a clockwise rotation of the okhotsk plate with a component of dextral collision between the okhotsk and amuria plates which is well observed along the central zone of the sakhalin hokkaido islands between the two back arc plates the clockwise rotation of the okhotsk plate is suggested by the southwestward fan shaped opening of the kuril basin the kuril basin narrows to the northeast and terminates south of the kamchatka peninsula where a relative rotation pole between the okhotsk and kuril forearc plates is located the kamchatka peninsula was a zone of collision on the other side of the clockwise rotating okhotsk plate during the opening of the kuril basin the sredinny range in the kamchatka peninsula is one of the results of the collision between the okhotsk plate and the forearc plate of the kuril arc the bending of the japanese islands occurred in association with the back arc spreading of the yamato basin of the japan sea

opening mode of the japan sea inferred from the palaeomagnetism of the japan arc the western edge of the pacific plate is marked by a chain of back arc basins stretching from the aleutians through japan and the philippines to new zealand most of the basins lie behind volcanic island arcs far from continents and are the result of extension of the oceanic lithosphere1 but the japan sea is bounded on one side by continental crust the sikhote alin and korean peninsula of the asian continent and on the other by an active arc with continental fragments it can thus be said to be associated more with continental rifting1 4 than with oceanic processes we report here palaeomagnetic data from the japan arc which indicate that since the early miocene north east japan has rotated counter clockwise through 47 around a vertical axis while south west japan has rotated clockwise through 56 refs 5 8 we attribute this differential rotation which took place concurrently between 21 and 11 myr ago to the back arc opening of the japan sea8 with a double door mode that is there are two fan shaped spreading basins facing each other such a fan shaped opening is probably peculiar to the geological phenomenon of rifting of a continental fragment from thick continental lithosphere

seismic reflection study of acoustic basement in the south korea plateau the ulleung interplain gap and the northern ulleung basin volcano tectonic implications for tertiary back arc evolution in the southern east sea abstract in order to understand the volcano tectonic evolution history of the east sea back arc basin this study focuses on the seismic characteristics of acoustic basement based on the multi channel seismic reflection data acquired from the south korea plateau the northern ulleung basin and the ulleung interplain gap according to the seismic reflection characteristics the acoustic basement in the study area can be classified into three types type a acoustic basement is a remnant of rifted continental crust comprising relatively shallow seated continental margin platforms elongated ridges and equidimensional blocks in the south korea plateau and its vicinities type b acoustic basement is volcanic sills flows sediments complexes infilling the rift controlled basement lows in the south korea plateau the ulleung interplain gap and the northern ulleung basin type c acoustic basement is a cluster or chain of volcanic edifices which shows internal facies variation from main body to apron slope facies volcanic activities identified in the types b and c acoustic basements can be classified into three stages based on the volcano stratigraphic correlation with the existing stratigraphic framework stage 1 volcanism is characterized by a fissure type eruption represented by scattering volcanic cones and mounds type c and volcanic sills and lava flows interlayering with syn rift sedimentary units type b early stage continental rifting prior to the middle miocene most likely controlled this volcanism stage 2 volcanism is characterized by ene wsw trending chain of volcanic edifices type c along the northern margin of the ulleung basin and ulleung interplain gap during the middle miocene the weakening of back arc extension might have resulted in limited volcanic eruption which formed elongate volcanic fields across the ulleung and yamato basins stage 3 volcanism is represented by highly peaked volcanic islands and seamounts type c showing vertically stacked apron slopes in seismic cross sections this vertical stacking pattern is indicative of multiple eruption events from a single eruption center at least from the late miocene to the quaternary expanded volcanic age control based on volcano stratigraphic and isotopic age dating reveals that the volcanic activities in the southwestern and southeastern east sea show similar spatio temporal variations this implies that the entire southern east sea has experienced almost same volcano tectonic evolution during the cenozoic back arc evolution especially the stage 2 volcanic chain along the northern ulleung basin and the yamato seamount chain in the yamato basin matches in their eruption timing ca 12 ma and geomorphic properties i e direction of the volcanic chains and distances from subduction zone which strongly suggests the existence of regional scale chain volcanism across the southern east sea considering the chemical properties of volcanic rock samples from the volcanic chain in association with the suggested tectonic frameworks during the tertiary the remnant magma source derived from the waning mantle convection most likely fed the volcanic chain in the terminal stage of back arc opening

interpretations of a middle miocene and late quaternary steady dextral transpression in sw japan and the opening tectonics for the japan sea abstract an internal rotation hypothesis for the deformation of sw japan for middle miocene and recent times is studied rotations are known to have occurred from paleomagnetic data for the earlier case two geodetic flow field estimates as well as another tectonic study suggest that the rotations are also taking place at present a further examination of the geodetic flow models suggests that the rotations contained in these flow models are intrinsic to the deformation of sw japan which occurs in an e w dextral transpressive simultaneous pure and simple shear tectonic regime maps of rotation rates one half of vorticity and shear gradients the n s variation of the e w velocity field constructed from the geodetic flow models resemble one another in pattern the estimates of the directions of the principal strain rates from the geodetic models are on the whole n s contractional and or e w extensional which match the geodetic reference coordinate system used in the adjustment of the geodetic data these observations suggest that the vortical flow contained in the geodetic flow models is a frame independent shear induced vorticity as the material is rotating with respect to the principal directions of the strain rates and that the oblique motion of the philippine sea plate is partitioned between shear and arc normal convergence components the rotation rate is 10 12 ma furthermore the kinematical vorticity number estimates from the geodetic data correlate well with the first order tectonic fabric patterns in sw japan super simple shear deformation for large parts of sw japan where the median tectonic line and other large strike slip faults are found is also indicated in the internal kinematical vorticity number estimates a new model for the formation of the japan sea is proposed extrusion of japan due to the okhotsk eurasia collision this model agrees with the fundamental observations of the early middle miocene deformation in the region the application of an indentation extrusion model to the okhotsk eurasia collision results in a better agreement between the predicted and observed structures and leads to simpler more reasonable and coherent interpretations for the origin and development of the structures compared with the achievements of the previous models the nne trending early middle miocene rifts lying along the continental margin of western hokkaido formed as pull apart basins along a sinistral fault related to the collision of the okhotsk plate which segmented in a left handed sense because the collision was oblique the n s trending rifts and the normal faults found in the back arc domain of the ne honshu arc formed by eastward extrusion under the influence of the mariana type subduction in the japan trench these rift basins were subsequently left laterally displaced by movement along nw trending sinistral strike slip faults the shikoku basin opened from the east toward the west and dextral shear was imparted to the sw margin from the nankai trough leading to clockwise rotations in the middle miocene in sw japan the paleomagnetic rotations are accommodated as fully internal structural rotations in the new model for the origin of the japan sea the current episode of deformation in sw japan began 1 5 million years ago after a hiatus during a 15 5 ma interval and is likely a continuation of the middle miocene style of tectonics

wave planation surfaces in the mid western east sea sea of japan indicators of subsidence history and paleogeographic evolution of back arc basin abstract this study focuses on revealing the origin of deep marine wave planation surfaces and its geomorphic and tectonic implications for the back arc evolution based on multi channel seismic reflection profiles acquired from the mid western east sea sea of japan on seismic reflection profiles the wave planation surfaces are recognized as continuous and high amplitude single reflectors outlining the flattened top of submarine terraces or isolated topographic highs the wave planation surfaces are classified into four types depending on the geomorphic characteristics and origins of substratal landmasses i e continental margin terrace type 1 rifted continental fragment type 2 volcanic edifices type 3 and uplifted sedimentary successions type 4 which have common origin of wave erosion near the sea level but different subsidence history the mean subsidence rate of wave planated substratum is calculated by dividing the maximum depth of a wave planation surface below modified datum sea level d e by the time elapsed since the onset of subsidence t information of local subsidence history enables reconstruction of paleogeographic evolution of mid western east sea during the neogene quaternary time 1 incipient continental rifting and onset of marine incursion ca 23 18 ma 2 progressive continental rifting and onset of wave planation ca 18 11 ma 3 final eruption of post rift volcanoes ca 11 ma 4 uplifting of shelf margin banks and onset of the pliocene holocene volcanic activities ca 11 2 5 ma 5 change in the locus of volcanic eruption and cessation of contractile uplifting in the basin margin ca 2 5 0 ma

paleoposition of southwest japan at 16 ma implication from paleomagnetism of the miocene ichishi group abstract new paleomagnetic data from shallow marine sediments of the ichishi group suggest a clockwise tectonic rotation of southwest japan in the middle miocene samples have been collected from mud or tuff layers at 17 sites stability of remanent magnetization has been examined by using alternating field and thermal demagnetization the polarity sequence composed of four normal and seven reversed polarity sites is correlated to polarity epoch 16 15 2 17 6 ma based on micropaleontological assignment of the upper ichishi group to blow s zone n8 the mean paleomagnetic direction of the 11 sites shows an anomalous declination toward the northeast this result suggests that southwest japan was subjected to a clockwise rotation through 45 since 16 ma the clockwise rotation can be explained by the drift of southwest japan associated with the spreading of the japan sea during the middle miocene

lateral structural variation across a collision zone in central hokkaido japan as revealed by wide angle seismic experiments the crustal evolution in the central part of hokkaido island japan has been dominated by a series of accretion and collision processes from the late jurassic to the present this region is divided into three main tectonic belts the sorachi yezo hidaka and tokoro belts from west to east formed in different cretaceous subduction systems since the miocene the sorachi yezo belt and the western part of the hidaka belt the hidaka metamorphic belt hmb have suffered severe deformation during two tectonic events the dextral oblique collision between the north american okhotsk and eurasian plates and the arc arc collision of the kuril forearc sliver with the western part of hokkaido the ne japan arc a thrust belt along the western margin of the sorachi yezo belt is considered to represent the miocene to pliocene eastward subduction of the ne japan arc due to the compressional stress regime this subduction jumped to the eastern margin of the japan sea in pleistocene time the reanalysis of wide angle seismic data collected in central hokkaido has provided a new image of crustal scale structural variation and deformation in the tectonic environments mentioned above the western part of the sorachi yezo belt is characterized by a 2 4 5 km thick forearc sedimentary layer with a velocity of 3 0 3 9 km s 1 beneath which a 3 9 5 8 km s 1 body forms an eastward dipping boundary to the upper unit this boundary may be the eastward extension of the thrust belt representing the miocene to pliocene plate boundary a 5 3 5 7 km s 1 body almost crops out beneath the eastern part of the sorachi yezo belt where ophiolite sequences and high p metamorphic rocks are exposed our rather low velocity of 5 3 5 7 km s 1 may suggest that the original oceanic crustal rocks or the metamorphics were fragmented and merged with low velocity accretionary prism rocks plausible tectonic events responsible for this process are fast uplift of a large amount of underplated materials in the cretaceous subduction system of this region and or severe deformation by collision tectonics since the miocene the hmb is characterized by high t metamorphic rocks almost equivalent to those of continental or arc crust the seismic velocity in the upper 5 7 km of crust in this part increases westwards from 5 6 to 5 85 km s 1 showing a good correlation with the change in metamorphic grade of the exposed rocks the velocity structure in the eastern part of the hidaka belt is composed of 15 20 km thick upper crust overlying 6 6 6 7 km s 1 lower crust the crystalline part of the upper crust is divided into a 6 7 km thick 5 6 6 0 km s 1 layer and an 8 5 11 km thick 6 2 6 35 km s 1 layer in the depth range 25 45 km there exist crustal laminations probably forming a ductile and sheared zone good agreement of this velocity structure with physical properties of the metamorphics exposed in the hmb indicates that a 25 26 km thick brittle part of the hidaka crust was obducted towards the west to form the hmb under the collision framework in this region major layer boundaries east of the hmb show a gentle eastward dip which might have acted as a decollement at the time of the obduction the tokoro belt which is composed of accreted jurassic seamounts is marked by a high upper crustal velocity 6 1 km s 1 in our model this velocity represents relatively mafic rock compositions of the seamounts and or oceanic crust trapped in this belt

electrical resistivity of substitutionally disordered hcp fe si and fe ni alloys chemically induced resistivity saturation in the earth s core abstract the thermal conductivity of the earth s core can be estimated from its electrical resistivity via the wiedemann franz law however previously reported resistivity values are rather scattered mainly due to the lack of knowledge with regard to resistivity saturation violations of the bloch gruneisen law and the matthiessen s rule here we conducted high pressure experiments and first principles calculations in order to clarify the relationship between the resistivity saturation and the impurity resistivity of substitutional silicon in hexagonal close packed hcp iron we measured the electrical resistivity of fe si alloys iron with 1 2 4 6 5 and 9 wt silicon using four terminal method in a diamond anvil cell up to 90 gpa at 300 k we also computed the electronic band structure of substitutionally disordered hcp fe si and fe ni alloy systems by means of korringa kohn rostoker method with coherent potential approximation kkr cpa the electrical resistivity was then calculated from the kubo greenwood formula these experimental and theoretical results show excellent agreement with each other and the first principles results show the saturation behavior at high silicon concentration we further calculated the resistivity of fe ni si ternary alloys and found the violation of the matthiessen s rule as a consequence of the resistivity saturation such resistivity saturation has important implications for core dynamics the saturation effect places the upper limit of the resistivity resulting in that the total resistivity value has almost no temperature dependence as a consequence the core thermal conductivity has a lower bound and exhibits a linear temperature dependence we predict the electrical resistivity at the top of the earth s core to be 1 12 10 6 m which corresponds to the thermal conductivity of 87 1 w m k such high thermal conductivity suggests high isentropic heat flow leading to young inner core age

geodynamics of the baikal rift zone and plate tectonics of asia the baikal rift zone stretches from the southern termination of lake baikal as far as 118 e and includes the baikal basin and the barguzin upper angara and muysk grabens that form its easterly continuation coincident with the rift zone is a narrow seismic belt which extends further east along the stanovoy range to the okhotsk sea this belt can be considered as the boundary between the eurasian and amurian lithospheric plates earthquake focal mechanism solutions that show a consistent orientation of extensional axes across the strike of the rift zone indicate the behaviour of the eurasian and amurian plates as rigid bodies from 83 focal mechanism solutions the pole of rotation of the eurasian and amurian plates was computed to lie at the point 56 95n 117 45e the angular rate estimated from the amount of baikal opening is about 1 10 7 deg year the position of the pole at the eastern termination of the rift zone accounts for the dying out of the rift zone and for the appearance east of the pole of compressive structures in the stanovoy range to the southwest the baikal rift zone is replaced by the mosaic of microplates in central asia that occupy the space between the eurasian and indian lithospheric plates prominent are the mongolian dzungarian alashan tarim tibetan and pamir microplates earthquakes in central asia are dispersed although higher concentrations of earthquakes associated with microplate boundaries are prominent the tectonics of central asia are considered to be due to the interaction of rigid microplates assuming that plastic deformations occur within the relatively wide zones along the microplate boundaries by considering the strike slip faults to define the trajectories of microplate motions the parameters of relative movements of the microplates have been computed the microplate mosaic of central asia is moving northeasterly as a result of the collision and continued convergence of india and eurasia the mosaic acts as a wedge splitting off the amurian plate from eurasia causing it to rotate counterclockwise so opening the baikal rift

baikal rift active or passive comparison of the baikal and kenya rift zones abstract the comparison of geological and geophysical data from the kenya and baikal rift zones permits us to consider the formation of these zones to be associated with the development of wide asthenospheric upwellings beneath them in this sense both rifts are active differences between them in amounts of volcanics and geological features may be explained by a higher degree of partial melting of asthenospheric material beneath the kenya rift

absorption of p and s waves above subduction zones we investigate local variations of absorption above the subduction zones of the northwest pacific teleseismic broad band bb recordings at german arrays from 59 earthquakes in this area allow the determination of the quality factor qp and qs the method to determine q is the phase pair method here for the first time p wave bb recordings at dense arrays are used extending the frequency range studied and increasing the reliability of the qp determination for the northwest pacific 1 d earth models have typical qp values of 200 to 800 qs 100 to 400 in the upper 200 km qp under the kuriles the kurile basin and japan is about 100 i e significantly lower than average qs under japan is also only about 1 3 of the value of 1 d models no frequency dependence of qp and qs could be detected for the broad frequency range studied 0 05 to 1 hz

crustal scale strike slip deformation in hokkaido northern japan abstract field surveys in hokkaido reveal that the entire tectonic belt in northern japan was a right lateral shear zone from eocene to middle miocene time this zone of deformation can be followed northwards in sakhalin for more than 2000 km and formed at the same time as the opening of the japan sea a thrusting component during the strike slip deformation and later deformation stages led to the uplift of the deep parts of the shear zone exposing a complete section through the upper crust the stress field inferred from the analysis of fault sets is consistent with the non coaxial deformation observed in the deeper ductile parts of the shear zone there is a transition from west to east across the width of the belt 100 km from en echelon folds thrusts and nappes at upper structural levels through the brittle ductile transition with an incipient vertical schistosity into ductile deformation with a vertical foliation and metamorphic recrystallization in the amphibolite facies deep parts of the crust granulites were uplifted and retrogressed during the strike slip movement fault set analysis together with data concerning the direction and sense of nappe emplacement imply that thrusting west of the ductile deformation zone occurred during the strike slip deformation minor flat lying structures coexist with vertical pure strike slip structures this study and previous ones reveal a crustal scale half flower structure with superficial thrusting associated with a deep narrow ductile zone with a high gradient of deformation and temperature

cenozoic deformation history of the area around yangnam yangbuk se korea and its tectonic significance abstract detailed mapping and structural analysis were carried out to establish the cenozoic deformation history of the yangnam yangbuk area se korea all structural elements were analyzed to establish the chronological sequence based on the classification of the fracture systems and the ages of the rocks two detailed grid maps represent the deformation history based on the cross cutting relationship between deformation fabrics we established the cenozoic deformation history in the studied area by synthesizing the analysis of structural elements in rocks of different ages and observing cross cutting relationships on grid maps results using these two approaches with two independent data sets are consistent the deformation history is comparable to that proposed from previous studies restoration of the structural elements suggests tilting and rotation in this area there is clear evidence of tectonic inversion which was observed in a previous trench study of the eupchon fault this implies that normal faults related to the right lateral yangsan fault associated with opening of the east sea japan sea may have been reactivated as thrust faults in the quaternary period

paleomagnetic evidence for the clockwise rotation of southwest japan abstract over 500 oriented samples of felsic rocks of cretaceous to middle miocene age were collected along the go river in the central part of southwest japan in an attempt to detect the process of tectonic rotation of southwest japan from the paleomagnetic view point thermal demagnetization was successful in isolating characteristic directions from the remanent magnetization of samples reliability of the paleomagnetic direction is ascertained through the agreement of directions from different kinds of rocks as well as the presence of both normal and reversed polarities the paleomagnetic results establish that southwest japan began to rotate clockwise through58 14 later than 28 ma and ceased its motion by about 12 ma southwest japan has undergone no detectable north south translation since 28 ma these results imply that southwest japan was rotated about the pivot around 34 n 129 e between 28 ma and 12 ma in association with the opening of the japan sea

the role of the east asian active margin in widespread extensional and strike slip deformation in east asia east asia is a region of widespread deformation dominated by normal and strike slip faults deformation has been interpreted to result from extrusion tectonics related to the india eurasia collision which started in the early eocene in east and se china however deformation started earlier than the collision latest cretaceous to palaeocene suggesting that extrusion tectonics is not the only driving mechanism for east asia deformation it is suggested that the east asian active margin has influenced deformation in east asia significantly along the margin cenozoic back arc extension took place behind several adjoining arcs implying eastward rollback of the subducting slab and collapse of the overriding plate towards the retreating hinge line we show that extension took place along a c 7400 km long stretch of the east asian margin during most of the cenozoic physical models are presented simulating overriding plate collapse and back arc extension the models reproduce important aspects of the strain field in east asia for geometrical and rheological conditions scaled to represent east asia modelling shows that the active margin can be held responsible for deformation in east asia as far west as the baikal rift zone located c 3300 km from the margin

gravity influenced brittle ductile deformation and growth faulting in the lithosphere during collision results from laboratory experiments 1 because of the competition between brittle and ductile rheologies and their interplay with tectonic and buoyancy forces lithospheric deformation results in very contrasting styles in continental collision especially with unconfined boundaries deformation can be either homogeneously distributed or localized on complex fault patterns and different deformation modes such as contraction extension and strike slip interfere using scaled lithospheric analog experiments made of dry sand silicone putties and dense honey we investigate the mechanical parameters that control the deformation style in colliding systems with a particular focus on the roles of buoyancy and brittle ductile coupling the analysis of tens of experiments shows that the principal deformation features depend on two main parameters a brittle to ductile strength ratio g which controls deformation localization at the largest scale and a buoyancy to strength ratio ar which fixes the relative amount of contractional extensional and strike slip structures strain localization occurs only for g larger than a critical value 0 5 and the range of g values over which the transition from nonlocalized to localized deformation occurs is small the three main deformation regimes contraction strike slip and extension which coexist in most of the collision experiments occur in relative proportions that depend mainly on ar and on the nature of the boundary conditions

opening of the kuril basin deduced from the magmatic history of central hokkaido north japan abstract the kuril basin is a fan shaped back arc basin in the northwestern pacific rim the basin is separated by the kuril arc from the pacific ocean to the southeast and is bounded by the subsided okhotomorsk block to the north and by the late eocene to early miocene hidaka magmatic arc to the west the clastic sediments constituting the late cretaceous to early eocene proto kuril arc which underlies the modern kuril arc were transported from the okhotomorsk block between them the kuril basin now intervenes this suggests that the kuril basin had not opened and that the proto kuril arc had been located in the southern margin of the okhotomorsk block at least until early eocene time it also indicates that the kuril basin was not entrapped by the proto or modern kuril arc the tertiary hidaka magmatic arc in central hokkaido was formed along a n s trending subduction zone where the pacific plate was subducting beneath the eurasian plate therefore the kuril basin was not yet present on the east of central hokkaido during the period when the hidaka magmatism was taking place based on the fact that the hidaka arc magmatism ceased at 17 16 ma and coevally along its n s trending extension it is suggested that the kuril basin opened just after 16 ma by the southward retreat of the proto kuril arc trench system in central hokkaido an extensional stress regime during the middle miocene is inferred from the occurrence of a type magmatism and graben like sedimentary basins this stress field can be attributed to subsidence of the pacific slab covered by the retreated proto kuril arc during the opening of the kuril basin the occurrence of an undeformed and horizontal sedimentary cover on the rugged basement of the kuril basin is consistent with rather rapid opening at 16 15 ma during the middle miocene the kuril basin and japan basin were formed by a similar process i e rotation and oceanward retreat of an arc trench system opening of the two back arc basins along different plate boundaries i e the pacific north american boundary for the kuril basin and the pacific eurasian boundary for the japan basin may have been caused by the migration of a hot region across the overriding plate boundary

on the evolution of marginal basins this paper presents a critique of the models proposed to explain the formation of back arc basins those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related several marginal basins are not generically back arc but rather are related to the initiation of subduction continental rifting or plate boundary readjustments models of back arc basin formation must be consistent with the observed characteristics of back arc basins which include the following 1 the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar implying that the processes of crustal accretion in both tectonic environments are also similar 2 arc trench systems without currently spreading back arc basins are in the majority 3 although petrographically similar to and within the compositional range of midocean ridge basalts morb back arc basin basalts babb show consistent geochemical differences from n type morb and are in many respects transitional toward island arc tholeiites iat 4 distinct babb and iat mantle sources may be in extremely close proximity during the initial rifting of an island arc 5 rifting of the volcanic arc may occur on either side of the line of active volcanoes 50 km and may vary from one side to the other along strike 6 a remnant arc is not always developed 7 back arc spreading may be initiated above subducted lithosphere but with time the center of spreading migrates away from the arc and no longer overlies a benioff zone and 8 there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins the three main classes of models proposed to explain back arc basin formation are mantle diapirism induced aesthenospheric convection and global plate kinematics because the driving mechanism for the first two classes of models is provided by the local subducting slab these models fail to explain the temporal and spatial distribution of back arc basins the third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line the latter being observed to move only in a seaward direction with respect to a hot spot reference frame however the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions for example some dynamic back arc force seems required to move the new hebrides tonga and south sandwich forearc plates seaward neither the local nor the global models adequately specify the necessary and or sufficient conditions for back arc basin formation a better understanding of the processes associated with the initiation of rifting and of subduction is required arc rifting and back arc spreading are both secondary phenomena to subduction but they are also primary extensional phenomena fundamentally analogous to continental rifting and the initial spreading of major ocean basins

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