investigating the role of the itoigawa shizuoka tectonic line towards the evolution of the northern fossa magna rift basin abstract the itoigawa shizuoka tectonic line istl fault system is considered to have one of the highest probabilities for a major inland earthquake occurrence in the whole of japan it is a complex fault system with the dip directions of the local fault segments changing from north to south between an east dipping low angle thrust fault a strike slip fault and a west dipping thrust fault the tectonic relations between the different parts of the fault system and the surrounding geological units are yet to be fully explained this study aims to reveal the juncture of the northern and central parts of the istl and investigate its contribution towards the shaping of the northern fossa magna rift basin we conducted 3 deployments of 1 or 2 linear arrays of seismic stations across the central and northern istl regions and observed local micro earthquakes for a period of 3 years each deployment recorded continuous waveform data for approximately 3 months using arrival times of 1193 local earthquakes we jointly determined earthquake locations and a 3d velocity model applying the tomography method we were able to image the regional crustal structures from the surface to a depth of 20 km with a spatial resolution of 5 km subsequently we used the obtained 3d velocity model to relocate the background local seismicity from 2003 to 2009 the juncture of the northern and central parts of the istl was well constrained by our results the depth extension of the northern parts of the istl fault segments follows the bottom of the miocene northern fossa magna rift basin nfm and forms an east dipping low angle fault in contrast the central parts of the istl fault segments are estimated to lie along the eastern boundary of the matsumoto basin forming an oblique strike slip fault fig 1

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

paleomagnetic study of plio pleistocene sediments in the concentrated deformation zone along the eastern margin of the japan sea abstract paleomagnetic results are presented for a concentrated deformation zone cdz in a convergent plate boundary region along the eastern eurasian margin we sampled late pliocene early pleistocene tuffs and clastic sediments of the sasaoka formation 750 m thick in northeastern honshu japan to test whether the late pliocene quaternary crustal deformation within the cdz along the eastern margin of the japan sea was accompanied by rotation about a vertical axis rock magnetic experiments suggest that the principal magnetic carrier is magnetite in the fine tuffs and magnetic iron sulfide in the fine sandstones pre folding characteristic remanent magnetization was confirmed using a positive bootstrap fold test we obtained 21 acceptable site mean characteristic directions that include our preliminary published results and which cover an interval from ca 2 7 to 1 7 ma on the basis of magnetostratigraphic correlations an updated age depth model is given and this allowed us to make numerical age estimates for key interbedded tuff beds tephras a positive fold test also suggests that the gojome syncline began to develop after 1 7 ma which means the folding began long after the initiation of late pliocene crustal shortening in northeastern honshu after 100 unfolding the overall mean direction d 359 7 i 54 9 95 6 7 is indistinguishable from the geocentric axial dipole field direction indicating that the gojome syncline the most prominent structural element in the study area developed without vertical axis rotation a comparison of our results with plio pleistocene directions reported from other areas reveals no paleomagnetically detectable rotation in or adjacent to the cdz except for local rotation near strike slip faults in central honshu

paleomagnetism of tertiary rocks from the oga peninsula and the rotation of northeast japan we have made a paleomagnetic study of tertiary rocks from the oga peninsula northern honshu island volcanic and sedimentary rocks were sampled from 26 sites in the peninsula the ages of the rocks range from 62 ma to the present the results indicate a counterclockwise rotation of northeast japan with respect to eastern asia around a pole at 58 n 149 e between about 22 ma and 15 ma the amount of the rotation is about 20 before the rotation northeast japan was situated along the east coast of the asian continent

paleomagnetic evidence for the miocene counter clockwise rotation of northeast japan rifting process of the japan arc paleomagnetic results have been obtained from more than 700 oriented samples in igneous rocks of cretaceous to miocene age from northeast japan the remanent magnetizations of welded tuffs with age between 32 ma and 21 ma old from 17 widely distributed sampling sites in northeast japan are fairly well grouped with a mean ofd 41 2 i 56 5 95 7 2 the remanent magnetization of welded tuffs of 14 ma and 11 ma from 4 sites shows a northerly direction d 11 7 i 60 5 95 15 3 these data imply that northeast japan was apparently subjected to a counter clockwise rotation through 47 14 about a vertical axis on the representative point of northeast japan 141 e 39 n during the past 21 ma comparing with the period of the clockwise rotation of southwest japan the differential rotation between northeast japan and southwest japan is concluded to have occurred concurrently during the period between 21 ma and 12 ma a rifting model of the japan arc is proposed northeast japan rotated counter clockwise through 50 about a northern pivot at 146 e 44 n while southwest japan rotated clockwise through 54 about a southern pivot at 129 e 34 n the differential rotation about different rotation pivots gives rise to the opening of a diamond shaped back arc basin the japan sea

sedimentary response to intra arc extension controls on oligo miocene deposition sarcidano sub basin sardinia the oligo miocene sardinian rift is a rare example of a well exposed intra arc basin comprising several independent depocentres synthesis of field observations from one such depocentre the sarcidano sub basin in central sardinia not only allows a new and integrated sedimentological and tectonostratigraphic history for the area but also provides insights into controls on syn and post rift sedimentation in intra arc settings initial rifting during the late rupelian early chattian oligocene occurred on nw se trending normal faults and produced a half graben topography which controlled the dispersal of locally supplied coarse continental clastic sediments latest chattian aquitanian miocene ne sw trending normal faulting defined a more complex block topography that remained underfilled in all areas except those which received voluminous ignimbritic volcanic material subsequent marginal and shallow marine sedimentation progressively infilled the fault topography as sea level rose deposition of coarse clastic sediments eroded from fault scarps was focused in the hanging wall depocentres of the intersecting fault trends whilst platform carbonates became established in topographically higher areas protected from clastic sedimentary supply finally as the relative rise in sea level drowned sediment source areas in the burdigalian deeper marine marlstones dominated sedimentation the results demonstrate that the sarcidano sub basin was dominated by a mixed volcanic siliciclastic carbonate basin fill with abrupt facies changes developed in response to complex fault defined topography relative rise in sea level and an episodic volcanic supply whilst the rift structure largely controlled the provenance supply and accumulation of sediments relative sea level primarily controlled the types of facies that developed

tectonics of short lived intra arc basins in the arc continent collision terrane of the coastal range eastern taiwan the coastal range in eastern taiwan was originated from an oblique collision between the luzon volcanic arc and asian continent since the late neogene in this collision terrane two intra arc basins the pliocene chingpu and pleistocene chengkung basins were developed on the eastern part of the neogene chimei and chengkuangao volcanic islands respectively prior to their accretion to eastern taiwan the tectonic evolution of these neogene volcanic islands and associated intra arc basins is reconstructed by stratigraphic and sedimentological analysis igneous rock geochemistry and comparison with observations in modern collision zone in the regions off southeastern taiwan in the coastal range the intra arc basin sequences are 1 5 10 km wide and 40 km long comparable in size to their modern analogues in the active collision zone the basin axis trends subparallel to the volcanic ridge in both basins deepwater flysch overlies shallow marine reef carbonates which in turn rest on volcanic basement indicating rapid arc collapse minimum rate of 1 km m y soon after the arc continent collision the arc collapse occurred earlier in the north chimei between 5 1 and 3 5 ma and later in the south chengkuangao between 2 9 and 1 8 ma in concert with a southward propagation of the oblique collision sedimentation ended about 2 ma and 1 ma in the chingpu and chengkung basins respectively coeval with rotation of the neogene volcanic islands this suggests that the rotation inverted the intra arc basin into thrusting uplifting and final emergence thus the duration of sedimentation for the intra arc basins spanned only about 0 8 3 1 m y on the basis of land geology offshore observations and a clay model experiment simulating oblique arc continent collision a model for the intra arc basin evolution in eastern taiwan is proposed during the collision strike slip faults would have been developed in the eastern part of volcanic islands to induce transtension movements thus forming pull apart intra arc basins on the collapsed volcanic island this mechanism is believed to be responsible for the formation of the pliocene chingpu and pleistocene chengkung basins as well as the present day offshore intra arc basins found on the lutao and lanhsu volcanic islands the two intra arc basins on lutao and lanhsu are predicted to be short lived as collision continues these two basins together with their underlying northern part of the luzon arc will be rotated thrust and uplifted in the next 1 m y and finally will become part of the southern extension of the coastal range

miocene counterclockwise rotation of the abukuma mountains northeast japan abstract new paleomagnetic data have been obtained from early to earliest middle miocene volcanic rocks in the ryozen area located in the northern part of the abukuma mountains in northeast japan our data show that the mountains have been rotated about 40 counterclockwise with respect to the asian continent between 20 and 16 ma this rotation occurred synchronously with rifting of the lithosphere in northeast japan which implies that the back arc opening of the japan sea was the cause of this rotation combined with previously obtained paleomagnetic results from adjacent areas a counterclockwise rotation of approximately 40 in the southern part of northeast japan is suggested to have occurred between 20 and 18 ma this result implies that a diachronism exists between the clockwise rotation of southwest japan 15 ma and the counterclockwise rotation of northeast japan 20 18 ma

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

uplift of the ou backbone range in northeast japan at around 10 ma and its implication for the tectonic evolution of the eastern margin of asia abstract the late cenozoic uplift history of a sedimentary basin located in the axial part of the ou backbone range northeast japan was studied using detailed mapping fission track dating and basin analysis the subsidence analysis of the basin clarified the more complex stepwise uplift of the ou backbone range three stages of uplift have been recognized and are interpreted to be the result of compressional stress possibly accompanied by basin inversion the three stages are identified as 1 a phase of surface uplift and regional unconformity 12 9 ma 2 a stage of differential uplift and compression 6 5 3 ma and 3 an intense compression stage 3 ma in the first stage the eastern sector of the backbone range uplifted and a notable unconformity was formed at 10 ma the western sector remained submerged suggesting that the eastern sector uplifted earlier than the western sector although the first uplift stage has been regarded as a tectonically quiet period in northeast japan this tectonic event at 10 ma is supposed to have a regional origin because coeval tectonic events took place across all northeast japan as well as on the eastern margin of asia this study thus provides new insights into the neogene tectonic evolution in the eastern margin of asia

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

counterclockwise rotation of northeast japan paleomagnetic evidence for regional extent and timing of rotation early to late miocene welded tuffs have been sampled from northeast honshu island and the oshima peninsula of hokkaido island japan for paleomagnetic study characteristic directions with a high unblocking temperature component above 560 c are isolated from 24 sites westerly declinations d 1 91 are identified in welded tuffs older than 17 ma from these areas when combined with previous paleomagnetic data from welded tuffs the early miocene paleomagnetic poles derived from northeast honshu island and the oshima peninsula are statistically identical and indicate that northeast honshu island and the oshima peninsula were subjected to counterclockwise rotation as a single tectonic block these results establish that the rotating block of northeast japan extends north to the oshima peninsula to the latitude 43 n the dimension of this rotating block is of the order of 500 km the paleomagnetic data provide a best curve for the rotation process with the counterclockwise rotation reaching a climax at 15 0 ma and a net amount of rotation of 46 4 the 500 km long northeast japan block was rotated counterclockwise through more than 45 and accompanied a clockwise rotation of the 600 km long southwest japan block at about 15 ma

rapid intra arc rifting in miocene northeast japan although life times of major continental rifts are tens of millions of years intra arc rifting in miocene ne japan lasted for only about 3 m y before back arc spreading began the rate of initial subsidence was over 1 km m y much faster than that in major continental rifts thus this intra arc rift evolved 10 times as fast as major intracontinental rifts the large temperature dependence of lithosphere rheology and the hot thermal regime of island arc lithosphere account for the rapid rifting less than 1 m y before the rifting spreading transition the inner ne japan arc subsided from shallow marine depths to middle or lower bathyal depths 1 0 2 5 km the average total basement subsidence was 2 3 km during the rifting the rapid submergence occurred during one third of the duration of rifting but accounts for two thirds of the total subsidence during rifting namely the initial subsidence rate accelerated toward the time of breakup of the arc crust at the same time the region along the present pacific coast remained only shallow marine or terrestrial throughout presumably influenced by global eustacy

stratigraphic review of eocene to oligocene successions along the eastern japan sea implication for early opening of the japan sea abstract stratigraphic review of the on land successions exposed on the back arc side of japan and sakhalin combined with supplemental radiometric dating reveals the existence of late eocene or oligocene marine sediments along the eastern coast of the japan sea this finding implies that the origin of the japan sea dates back to the oligocene or earlier time much older than ever supposed the host eocene to oligocene subaerial to shallow water successions distribute immediate west of the volcanic front of that time and contain volcanic rocks the chemistry of which is characteristic of continental rift or volcanic arc they are associated with parallel dike swarms or normal faults and could be infillings of an incipient back arc rift collective evidence suggests that the rift was rapidly subsided and spread during the following early to middle miocene time however a back arc wide unconformity was produced during a short period from the latest oligocene to the earliest miocene and was succeeded by extensive bimodal volcanism with rapid migration of the volcanic front to the fore arc side these short term events could have been induced by upwelling or inflow of hot asthenosphere beneath the arc rift system followed by rapid subsidence and spreading of the japan sea

the tertiary geodynamical evolution of the aegean arc a paleomagnetic reconstruction abstract the paleomagnetic results obtained in the last 5 6 years from tertiary formations in the aegean domain indicate that the lower miocene arc was almost rectilinear with an e w trend and that its curvature has been acquired tectonically in two major phases during the middle miocene a first phase of deformation is characterized by rotations occurring at the two terminations of the arc clockwise in the west epirus anticlockwise in the east southeastern anatolia a second phase of rotation occurring in the last 5 ma about a pole situated in the southern adriatic sea has affected only the northwestern part the data also indicate that the antalya basin has not been involved in the geodynamical evolution of the arc so that the bey daglari represent its easternmost unit a more complex rotational pattern is observed in the internal zones the large rotations measured in evia 48 and skyros 26 probably occurred in the last 5 ma and result from rotation of fault bounded blocks in a shear zone connecting the north aegean trough to the external arc in the izmir region in the karaburun peninsula of western anatolia and in the island of lesbos significant counterclockwise clockwise or null rotations of coherent blocks have taken place during the neotectonic extensional regime furthermore the values of the inclination obtained from both sedimentary and volcanic oligo miocene formations are systematically shallower than expected on the basis of a geocentered dipole field this suggests that the entire studied area has undergone a northward drift of over 1000 km since the middle miocene

relationships between kuroko volcanogenic massive sulfide vms deposits felsic volcanism and island arc development in the northeast honshu arc japan the northeast ne honshu arc was formed by three major volcano tectonic events resulting from late cenozoic orogenic movement continental margin volcanism before 21 ma seafloor basaltic lava flows and subsequent bimodal volcanism accompanied by back arc rifting 21 to 14 ma and felsic volcanism related to island arc uplift 12 to 2 ma eight petrotectonic domains parallel to the ne honshu arc were formed as a result of the eastward migration of volcanic activity with time major kuroko volcanogenic massive sulfide vms deposits are located within the eastern marginal rift zone kuroko rift that formed in the final period of back arc rifting 16 to 14 ma volcanic activity in the ne honshu arc is divided into six volcanic stages the eruption volumes of volcanic rocks have gradually decreased from 4 600 km3 per 1 my for a 200 km long section along the arc of basaltic lava flows in the back arc spreading stage to 1 000 2 000 km3 of bimodal hyaloclastites in the back arc rift stage and about 200 km3 of felsic pumice eruptions in the island arc stage the kuroko vms deposits were formed at the time of abrupt decrease in the eruption volume and change in the mode of occurrence of the volcanic rocks during the final period of back arc rifting in the area of the kuroko rift felsic volcanism changed from aphyric or weakly plagioclase phyric before 14 ma to quartz and plagioclase phyric with minor clinopyroxene 12 to 8 ma to hornblende phyric after 8 ma and hornblende and biotite phyric after 4 ma the kuroko vms deposits are closely related to the aphyric rhyolitic activity before 14 ma the rhyolite was generated at a relatively high temperature from a highly differentiated part of felsic magma seated at a relatively great depth and contains higher nb ce and y contents than the post kuroko felsic volcanism the kuroko vms deposits were formed within a specific tectonic setting at a specific period and associated with a particular volcanism of the arc evolution process therefore detailed study of the evolutional process from rift opening to island arc tectonics is very important for the exploration of kuroko type vms deposits

westward migration of the izu bonin trench northward motion of the philippine sea plate and their relationships to the cenozoic tectonics of japanese island arcs abstract the essential elements in the understanding of the cenozoic island arc tectonics of the japanese islands came from reconstructing the paleo position of the plate boundaries and estimating the change in the philippine sea plate motion by using the 2nd law of the convergence rate of plates it was estimated that the izu bonin trench wandered around 400 km east from its present position during the paleogene and migrated westward thereafter island arc tectonism is related to the convergence rate of plates 1st law of the convergence rate of plates hence the changes in the philippine sea plate motion was examined by compiling paleomagnetic data as a result the main events of the cenozoic tectonics of japan were well explained by the change in the position of the plate boundaries and the change in motion of the philippine sea plate

seismological evidence of an active footwall shortcut thrust in the northern itoigawa shizuoka tectonic line derived by the aftershock sequence of the 2014 m 6 7 northern nagano earthquake abstract a destructive m 6 7 earthquake struck northern nagano prefecture on november 22 2014 the main shock occurred on the kamishiro fault segment of the northern itoigawa shizuoka tectonic line istl we used data recorded at 41 stations of the local seismographic network in order to locate 2118 earthquakes that occurred between november 18 and november 30 2014 to estimate hypocenters we assigned low vp models to stations within the northern fossa magna nfm basin thus accounting for large lateral crustal heterogeneities across the kamishiro fault in order to further improve accuracy the final hypocenter locations were recalculated inside a 3d velocity model using the double difference method we used the aftershock activity distribution and focal mechanism solutions of major events in order to estimate the source fault area of the main shock our analysis suggests that the shallow part of the source fault corresponds to the surface trace of the kamishiro fault and dips 30 45 se while the deeper part of the source fault corresponds to the downdip portion of the otari nakayama fault a high angle fault dipping 50 65 se that formed during the opening of the nfm basin in the miocene along its surface trace the otari nakayama fault has been inactive during the late quaternary we verified the validity of our model by calculating surface deformation using a simple homogeneous elastic half space model and comparing it to observed surface deformation from satellite interferometry assuming large coseismic slip in the areas of low seismicity and small coseismic slip in the areas of high seismicity shallowing of the source fault from 50 65 to 30 45 in the upper 4 km in the areas where both surface fault traces are visible is a result of footwall shortcut thrusting by the kamishiro fault off the otari nakayama fault

timing of rotational motion of southwest japan inferred from paleomagnetism abstract paleomagnetic field directions for the period younger than 35 ma are obtained from igneous rocks distributed in the san in district inner zone of southwest japan the remanent magnetization of samples between 30 and 35 ma old are fairly well grouped with a mean direction of d 65 9 i 48 6 and 95 6 5 this result establishes that southwest japan rotated clockwise 56 12 during the past 30 m y a declination value of about 60 is observed in the rocks of 28 ma d 52 2 i 33 5 and 21 ma d 69 9 i 49 5 comparing this with results from dacitic rocks with an age of 15 ma in other areas of southwest japan suggests that rotational motion did not occur possibly until 15 ma these results require that the rotation of southwest japan occurred just after the shikoku basin had been created

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

active forearc shortening in tohoku japan constraints on fault geometry from erosion rates and fluvial longitudinal profiles abstract convexities in the longitudinal profiles of actively incising rivers are typically considered to represent the morphologic signal of a transient response to external perturbations in tectonic or climatic forcing distinguishing such knickzones from those that may be anchored to the channel network by spatial variations in rock uplift however can be challenging here we combine stream profile analysis 10be watershed averaged erosion rates and numerical modeling of stream profile evolution to evaluate whether knickzones in the abukuma massif of northeast japan represent a temporal or spatial change in rock uplift rate in relation to forearc shortening knickzones in channels that drain the eastern flank of the abukuma massif are characterized by breaks in slope area scaling and separate low gradient alluvial upper channel segments from high gradient deeply incised lower channel segments average erosion rates inferred from 10be concentrations in modern sediment below knickzones exceed erosion rates above knickzones by 20 50 although profile convexities could be interpreted as a transient response to an increase in rock uplift rate associated with slip on the range bounding fault geologic constraints on the initiation of fault slip and the magnitude of displacement cannot be reconciled with a recent spatially uniform increase in slip rate rather we find that knickzone position stream profile gradients and basin averaged erosion rates are best explained by a relatively abrupt spatial increase in uplift rate localized above a flat ramp transition in the fault system these analyses highlight the importance of considering spatially non uniform uplift in the interpretation of stream profile evolution and demonstrate that the adjustment of river profiles to fault displacement can provide constraints on fault geometry in actively eroding landscapes

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

relationship between tectonics volcanism sedimentation and basin development late cenozoic central part of northern honshu japan abstract a pronounced interplay between tectonics volcanism and sedimentation can be established in the volcanic arc region of the central part of northern honshu during the early miocene to quaternary interval four stages are recognized with the type of volcanism and sedimentation varying according to the tectonic stress regime 1 1 rifting stage 22 15 ma ns trending half grabens were formed under an extensional stress regime related to the rifting of the sea of japan basins were rapidly filled by intermediate to felsic volcanic products which are often associated with subaerial fluvial and lacustrine sediments 2 2 back arc basin opening stage 15 13 ma with more advanced stagess of rifting subsidence from crustal stretching resulted in marked marine incursion and an overall transgressive succession developed during this stage the back arc side of northern honshu subsided rapidly to middle bathyal environments at the same time associated with the opening of the sea of japan large volumes of regionally extensive mafic lavas and associated hyaloclastites were erupted within submarine grabens in contrast in the present backbone ranges felsic volcanism occurred within bathyal to shallow marine environments igneous rock types in this stage display a bimodal mafic felsic chemistry 3 3 transitional stage 13 2 4 ma a thermal subsidence substage 13 8 ma an intermediate stress regime developed which included phases of both weak extension and compression consequently grabens along the sea of japan ceased to form later lithospheric cooling eventually led to thermal subsidence of the back arc region and hemipelagic sediments mantled the previously active mafic volcanic apron volcanism at this time was mainly restricted to a narrow zone along the present backbone ranges where felsic to intermediate volcanic products accumulated within shallow marine environments b incipient compressional substage 8 2 4 ma hemipelagic sedimentation continued on the back arc side of northern honshu along the present backbone ranges the intermediate stress regime continued with formation of many large caldera structures associated with subaerial felsic volcanism 4 4 shortening deformation stage 2 4 ma present the change of the absolute direction of motion of the pacific plate at the early to middle pliocene changed the regional stress regime of the volcanic arc from intermediate to compression movement of back arc thrusts along the sea of japan coast began 2 3 ma and led to uplift of the volcanic arc continued compression in the volcanic arc produced narrow uplifted zones with associated basins bounded by reverse faults these basins were filled rapidly by terrestrial sediments containing upward coarsening sequences under this highly compressional tectonic stress regime volcanism became intermediate mafic in composition with large stratovolcanoes becoming prominent

hot regions and the origin of marginal basins in the western pacific abstract volcanic provinces showing no genetic relationship to plate boundaries may be classified into two categories hot spots which are a few hundred kilometers across and may be nearly fixed in position with respect to deep mantle and hot regions which are a few thousand kilometers across and can migrate more freely the mantle upwellings which presumably give rise to them may originate at different depths by different causes it is observed that the marginal basins older than 15 ma of the western pacific become progressively younger northward with the exception of the south fiji basin this leads to the hypothesis that the spreading of marginal basins was caused by the mantle upwelling associated with a migrating hot region and that a hot region which started from the australia region in the late cretaceous migrated northward successively creating the tasman coral sea caroline south china parece vela shikoku and japan basins through paleocene eocene oligocene and early miocene time this hot region presently underlies northern china mongolia and adjacent areas another hot region is assumed to have caused the spreading of the south fiji basin and all the currently spreading basins in the southwestern pacific spreading of individual marginal basins occurred during the short period when a hot region was passing the pertinent area the main merit of this hypothesis is that it can explain the relationship between the location and age as well as the short duration of spreading of western pacific marginal basins marginal basins are formed commonly though not always in the overriding plates near subduction zones because in such a setting a spreading marginal basin is readily accommodated either by an oceanward movement of the nearby subduction zone or by a continentward movement and subduction of the overriding oceanic plate there are a few marginal basins that were formed in other tectonic settings and accommodated by some other mechanism of compensative subduction presumably the splitting of the supercontinent pangea was also caused mainly by the effect of the mantle upwelling associated with the same hot region that later migrated northward creating marginal basins in the western pacific the main basis for this presumption is the fact that the splitting of pangea was completed in the australia region in middle cretaceous time whereas the spreading of marginal basins in the western pacific began in the same australia region in the late cretaceous immediately following

mesozoic arc magmatism along the southern peruvian margin during gondwana breakup and dispersal abstract a high resolution u pb zircon geochronological study of plutonic units along the south peruvian margin between 17 and 18 s allows the integration of the geochemical geodynamic and tectonic evolution of this part of the andean margin this study focuses on the composite jurassic early cretaceous ilo batholith that was emplaced along the southern peruvian coast during two episodes of intrusive magmatism a first period between 173 and 152 ma with a peak in magmatic activity between roughly 168 and 162 ma and a second period between 110 and 106 ma emplacement of the jurassic part of the composite ilo batholith shortly post dated the accumulation of the volcanosedimentary succession it intruded chocolate formation which allows to estimate a subsidence rate for this unit of 3 5 km ma the emplacement of the main peak of jurassic plutonism of the ilo batholith was also closely coeval with widespread and repeated slumping during deposition of the cachios formation in the back arc region suggesting a common causal link between these phenomena which is discussed in the context of an observed 100 km trenchward arc migration at 175 ma and the relation with extensional tectonics that prevailed along the central andean margin during pangaea break up

two modes of back arc spreading back arc spreading is initiated with rifting along the volcanic zone of an island arc under extensional tectonics the width of the volcanic zone of the arc which is closely related to the dip angle of the subducting slab should cause a variation in the style of initial rifting and subsequent back arc spreading two modes of back arc spreading single rift type and multirift type are introduced from the comparative study of the bonin arc and the japan sea divergent tectonics in an island arc above a steeply dipping slab will cause a single rift system in a narrow volcanic zone but will cause a multirift system in a broad volcanic zone which is related to a shallow dipping angle of subduction

the opening processes in the deep tyrrhenian basins of marsili and vavilov as deduced from magnetic and chronological evidence of their igneous crust abstract shapes and chronology of magnetic anomalies of the mafic crust flooring the marsili and vavilov abyssal plains in the central tyrrhenian sea are discussed in the marsili deep basin the possible 3 phase evolution of a magmatic tectonic process of spreading is considered the early magmatic phase apparently took place at the time of the olduvai magnetic event 1 87 to 1 67 ma b p site 650 of the ocean drilling project leg 107 it appears to consist of spreading manifestations of a low magnetic intensity and a round shape forming the igneous basement around the marsili volcano the late phase was characterized by a spreading process exhibiting a linear configuration and high intensity of the magnetic anomalies organized spreading this phase of the brunhes epoch has given rise to the great marsili volcano emplaced in the basin s central part the intermediate phase shows reversed magnetic polarity and slightly elongated shapes it appears to have developed in late matuyama times from 1 67 to 0 73 ma ago the intensities of the magnetic anomalies in the basin of vavilov are on the whole lower than those present in the magnetic field of the marsili also in the vavilov deep basin the magnetic anomalies associated with the early volcanic manifestations which are of late miocene age about 7 5 ma are characterized by low intensity and a round shape whereas those associated with the effusions on the vavilov volcano whose age ranges from late pliocene to late quaternary from 2 4 to less than 0 12 ma exhibit a high intensity and a linear shape the two bathyal plains have inter arc structural settings from west to east the vavilov bathyal plain is bordered by the orogenic volcanic belt of the island of sardinia and maybe a central tyrrhenian arc the latter and the arc of the eolie which is active in the present time border the marsili plain the ages of the early spreading processes in the bathyal plains respectively from about 10 to 6 ma and from 1 9 to 1 3 ma appear to represent distinctive temporal markers in fact they may be coincident with the periods of quiescence of the volcanic activity that affected the adjoining arcs

submarine basaltic fountain eruptions in a back arc basin during the opening of the japan sea abstract basaltic rock generated during the middle miocene opening of the japan sea is widely distributed on the back arc side of the japanese archipelago few studies have investigated on submarine volcanism related to opening of the japan sea the present study aimed to reconstruct details of the subaqueous volcanism that formed the back arc basin basalts babb during this event and to discuss the relationship between volcanism and the tectonics of back arc opening using facies analyses based on field investigation the study area of the southern dewa hills contains well exposed basalt related to the opening of the japan sea five types of basaltic rock facies are recognized 1 coherent basalt 2 massive platy basalt 3 jigsaw fit monomictic basaltic breccia 4 massive or stratified coarse monomictic basaltic breccia with fluidal clasts and 5 massive or stratified fine monomictic basaltic breccia the basaltic rocks are mainly hyaloclastite based on facies distributions we infer that volcanism occurred along fissures developed mainly at the center of the study area given that the rocks contain many fluidal clasts submarine lava fountaining is inferred to have been the dominant eruption style the basaltic rocks are interpreted as the products of back arc volcanism that occurred by tensional stress related to opening of the japan sea which drove strong tectonic subsidence and active lava fountain volcanism

thermal modeling of the geometry of subduction with implications for the tectonics of the overriding plate the path which subducting lithosphere follows can be broken into three zones zone i a subhorizontal region of nearly constant dip near the trench zone ii a region of rapidly increasing dip and zone iii a region generally deeper than 125 km again of nearly constant dip we have modeled the thermal evolution of slabs as they pass through these three zones using the method of lines a numerical technique well suited to the solution of time dependent partial differential equations by use of appropriate initial conditions and time dependent boundary conditions we can simulate subduction with variations in age of subducting lithosphere slab velocities and paths followed by the slab thermal modeling results indicate that the geometry of the slab in region ii can be empirically related directly to the thermal structure of the slab in that region and thus indirectly to age and velocity it follows that low angle subduction can extend several hundred kilometers inland only in cases of high convergent velocities 15 cm yr 1 and old 120 m y lithosphere we further suggest that rapid variations in the convergence velocities can provide an enabling mechanism for back arc spreading while slow changes in convergence velocities and or age variations in the subducting lithosphere lead to migration of volcanic arcs

the polygenetic kahouanne seamounts in the northern lesser antilles island arc evidence for large scale volcanic island subsidence abstract remotely operated vehicle rov explorations of the kahouanne seamounts located 25 km se of montserrat in the northern lesser antilles island arc have discovered the occurrence of honeycomb scalloped erosional features on volcanic and limestone outcrops at depths of up to 600 m below sea level mbsl these features combined with the flat topped morphology of the seamounts the occurrence of shallow water carbonates rhodoliths benthic foraminifera and the presence of oxidized highly vesicular volcanic fragments suggest that the seamounts were once subaerial islands and have subsequently subsided to their present depth the subsidence is likely to have been in response to a combination of 1 graben development southeast of the island of montserrat where faulting and extension are driven by the accommodation of slip convergence associated with oblique subduction along the lesser antilles volcanic arc and 2 regional subsidence inferred from studies of nearby carbonate platforms 40ar 39ar dating of basalt andesite lavas from the central seamount indicates active volcanism around 5 4 ma shallow water carbonates recovered from an apparent wave cut terrace at 600 mbsl in the same area were deposited approximately 3 ma based on sr isotope stratigraphy indicating significant subsidence of the complex since that time the total subsidence is in line with displacements in the kahouanne valley graben and regional subsidence rates inferred from carbonate platform depths around the islands of guadeloupe and martinique implying that intra arc subsidence has been a significant process shaping the present bathymetry and topography of this island arc

subsidence of the shikoku back arc basin abstract sources of basaltic tephras found in miocene cores at dsdp sites 443 and 444 in the eastern side of shikoku basin are likely to be the kinan seamounts which were emergent during middle to early late miocene abnormally high vesicularity of dredged and dsdp pillow basalts together with existence of a thin layer of limestone or chalk overlying the basalts seem to indicate that the axial zone of shikoku basin was much shallower than the present depth immediately after solidification of the basement and that it subsided quickly shallow water depths of the back arc basins are characteristic of the present lau basin and okinawa trough whereas the mariana trough possesses water depths about 1000 m deeper than normal oceans of the same age most of inactive back arc basins have depth age relations consistent with that of mariana trough very shallow water depths of the axial zone of back arc basins similar to those of the shikoku basin axis during their opening period appear to be due to non isostatic uplift caused by excess mass and to isostatic elevation by buoyancy of hot partially molten materials accumulated above the subducting slab

effects of finite rifting times on the development of sedimentary basins abstract most thermo mechanical models for the development of sedimentary basins have assumed that the rifting responsible for the formation of the basin occurred instantaneously and have examined the post rift development of the basin this assumption greatly simplifies the mathematical treatment but is not in accord with what is found in nature where 10 to 50 m y rifting events commonly accompany the formation of sedimentary basins and continental margins the effects of a finite rifting time on the development of sedimentary basins are examined using an analytic technique which allows an arbitrary rifting history in both time and space and which considers the effects of both vertical and horizontal heat transfer this technique allows the thermal structure of the lithosphere to be calculated throughout the rifting event and thus permits the subsidence history and surface heat flow of the developing basin to be traced the effect of a finite duration extension event is that heat is lost during rifting increasing the syn rift subsidence at the expense of the post rift lateral heat flow which was not included in previous studies of the effect of finite rifting times has a significant effect on the subsidence history distribution of sediments and thermal history in particular the post rift subsidence is decreased by more than 25 for a 20 m y rifting event and by more than 10 15 for a rifting event as short as 10 m y this will significantly decrease the subsidence rates in the post rift stage and implies that inferences concerning the structure development and thermal history of the basin derived from using curves to interpret backstripped subsidence can be greatly in error variations in syn rift sediment accumulation and lithospheric thermal structure at the end of rifting resulting from different rifting histories can interact with other factors such as the flexural response of the lithosphere to sediment loading to affect the final width of the basin the total amount of sediments that accumulate and the basin stratigraphy

cenozoic intracontinental dextral motion in the okhotsk japan sea region a right lateral shear zone trending northerly along more than 2000 km is recognized from central japan to northern sakhalin it was active mainly during the neogene and has accommodated several hundreds of kilometers of displacement the whole structure of sakhalin is built on this shear zone en echelon sigmoidal folds and thrusts en echelon narrow miocene basins and a major discontinuity which is observed along more than 600 km the tym poronaisk fault characterize the deformation there in hokkaido en echelon folds and thrusts and a ductile shear zone with high temperature metamorphism constitute the southern extension of this transpressional shear zone it continues to the south as a zone of transtensional deformation along the eastern margin of japan sea as en echelon basins and dextral transfer faults observed as far south as noto peninsula and yatsuo basin the style of the shear zone thus evolves from transpressional in the north far from the subduction zone to transtensional in the south in the back arc region strike slip motion along this shear zone was primarily responsible for the dextral pull apart opening of japan sea during the early and middle miocene dextral motion is still active in the north along the tym poronaisk fault in sakhalin as well as on the continental margin of japan sea korea and asia mainland active e w compression replaced the dextral motion along the eastern margin of japan sea in late miocene time and incipient subduction began in the early quaternary

empirical relationships among the convergence rate of plates rollback rate of trench axis and island arc tectonics laws of convergence rate of plates abstract the data on 27 recent arcs which were subducted by normal oceanic lithosphere were compiled among the compression extension rates for island arcs v a compression is defined as positive and the kinematic components of absolute plate motions parallel to the direction of backarc compression extension v on oceanward velocity of the overriding plate v sn continentward velocity of the subducting plate and v tn rollback rate of the trench axis the equations v a v on v sn 7 2 cm y 1 v tn 7 2 cm y v sn 2 are found to hold as for the convergent zones associated with a wadati benioff zone deeper than 200 km equation 1 suggests that backarc spreading and crustal shortening play the role of compensating the deviation of the convergence rate from the critical value of 7 2 cm y the equation 2 means that any subducting oceanic lithosphere has the potentiality of making the trench axis rollback at the rate of 7 2 cm y and that the virtual rollback rate v tn is determined by the balance between the potential rollback rate 7 2 cm y and v sn the present author calls the kinematic relationships represented by equations 1 and 2 laws of convergence rate of plates the convergent zones of plates associated with the wadati benioff zone shallower than 200 km also show a tendency that the greater and smaller values of v on v sn correspond to the backarc compression and extension however the critical value is 3 4 cm y instead of 7 2 cm y

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

low angle detachment origin for the red sea rift system abstract the tectonic and magmatic history of the jizan coastal plain tihama asir southwest arabia suggests a two stage evolution a first stage of extension began during the oligocene and ended with uplift of the arabian graben shoulder which began about 14 ma ago it was followed by a period of approximately 10 ma characterized by magmatic and tectonic quiescence a second stage of extension began roughly contemporaneously with the onset of seafloor spreading in the southern red sea some 4 5 ma ago and is still active today the geometry of faulting in the jizan area supports a wernicke model of simple shear for the development of the southern red sea regional asymmetries of the red sea area such as the distribution of volcanism the marginal topography and asymmetries in the geophysical signatures are consistent with such a model available seismic profiles allow a rough estimate for values of the arabian red sea margin and were used to simulate subsidence history and heat flow of the red sea for classical two layer stretching models neither finite uniform nor finite non uniform stretching models can account for observed subsidence and heat flow data thus two model scenarios of whole lithosphere normal simple shear are presented for the geological history of the southwestern arabian margin of the red sea these models are limited because of the serravallian rearrangement in the kinematics of the red sea

sedimentation and volcanism linked to multiphase rifting in an oligo miocene intra arc basin anglona sardinia three extensional phases can be recognized in the northern anglona area of the oligo miocene sardinian rift during a fifteen million year period which spanned corsica sardinia continental microplate separation and western mediterranean back arc basin opening in response to this multiphase rifting a complex facies architecture involving clastic carbonate and volcanic rocks developed integrated onshore facies and structural analysis dating and offshore seismic data are here used to reconstruct the tectono stratigraphic history of the anglona area initial late oligocene extension created a half graben geometry with syn rift clastic deposits shed locally from fault bounded highs passing laterally to lacustrine marlstones calc alkaline volcanic activity subsequently predominated as volcanic centres developed along one half graben bounding fault voluminous pyroclastic and epiclastic material was supplied to the adjacent half graben accommodation space and was deposited in marginal to marine conditions second phase mid aquitanian early burdigalian extensional faulting recognized from localized clastic syn rift stratal wedges truncated and subdivided the half graben the syn rift sediments were sealed by a regionally correlated ignimbrite that in turn was offset by late second phase faulting third phase extensional fault movement which reactivated the original fault trend then occurred a perched lake developed in the resultant topography coeval with the progressive marine transgression of lower areas as sea level rose during mid burdigalian times reefal carbonates and grainstones developed on fault block highs whilst calcarenites and marlstones were deposited in hangingwall locations initial extension was coeval with the formation of the sardinian proto rift and the initiation of the western mediterranean basin second phase faulting occurred as the corsica sardinia microplate rotated to its present position during western mediterranean back arc basin spreading final extension can be correlated to a second major extension phase along the oligo miocene sardinian rift following back arc basin opening as extension was transferred towards the fore arc in anglona the main influence of multiphase tectonism was on rift topography providing accommodation space and localized uplifted source areas varying relative sea level mainly controlled the broad types of facies belts that developed contemporaneous calc alkaline volcanism played a major role in the supply of basin filling material and in changing the topography locally

the relationship between late cenozoic tectonic events and stress field and basin development in northeast japan the regional tectonic stress field basin development and crustal deformation of the ne japan arc in the interval between 32 ma to the quaternary can be synthesized based on dike vein and fault orientation data as well as on the compilation of the regional geology an extensional stress field became prevalent from 32 ma and major normal faulting started at 25 20 ma which resulted from back arc rifting normal faulting trending nearly parallel to the arc propagated from the present japan sea coast to the forearc side following the trenchward migration of the main volcanic field from 20 to 15 ma normal faults with an oblique trend to the arc developed due to the counterclockwise rotation of ne japan the rapid clockwise rotation of sw japan since 16 ma produced a nw se directed transtensional stress regime in the ne japan arc due to crustal stretching associated with this pull apart movement the back arc side of the ne japan arc subsided rapidly to middle bathyal environments after the termination of the opening of the japan sea at about 14 ma a neutral stress regime prevailed which included phases of both weak extension and compression lithospheric cooling eventually led to thermal subsidence of the back arc region and igneous underplating caused uplift of the axial zone of the volcanic arc the increase in velocity of the westward motion of the pacific plate at around 4 ma produced strong compression across the arc reactivating most of the miocene normal faults and uplifting the volcanic arc the greatest crustal shortening occurred in areas that were stretched the most in the miocene within the volcanic arc which implies tectonic inversion

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

tectonic evolution of the east junggar terrane evidence from the taheir tectonic window xinjiang china abstract the taheir tectonic window is located at the southern part of the yemaquan magmatic arc in east junggar this paper reports detailed petrology mineral chemistry whole rock and sr nd isotope geochemistry hornblende 40ar 39ar dating in situ zircon u pb dating and hf isotope analyses from igneous and meta igneous rocks from this region our new results show that the taheir tectonic window consists of metamorphic and deformed ordovician volcanic rocks and granitic porphyries ordovician silurian granites and undeformed silurian devonian granitic diorite diorites and rhyolitic porphyries the ordovician volcanic rocks and granitic porphyries and ordovician silurian granites in the taheir tectonic window exhibit distinct features of andean type continental arc such as enrichment in pb k and u depletion in nb p and ti negative eu anomalies high la yb th yb and ta yb values a high proportion of dacite rhyolite and andesite of the calc alkaline series massive contemporary granitic intrusions mixtures of the juvenile material and 2 5 ga crust and extensive crystallization differentiation these ordovician volcanic rocks witnessed a series of tectonic events including burial associated with the intrusion of 454 449 ma granitic porphyries underthrusting and subsidence to a depth in the middle crust associated with the intrusion of 443 432 ma granites the formation of albite hornblende schists hornblende albite quartz leptynites and amphibolites the transformation from continental to continental island arc at approximately 432 ma the exhumation associated with the intrusion of 416 406 ma diorites with geochemical signatures of continental island arc and exhumation and erosion between 398 ma and 390 ma are also identified the arc types that are associated with the taheir tectonic window and its host the yemaquan magmatic arc changed from andean type continental arc to continental island arc after the intra arc rifting that began at 432 ma on the basis of the new evidence the tectonic regime of the east junggar terrane is redefined and a new model is proposed it is suggested that the east junggar terrane is related to the southward subduction of the paleo asian ocean plate beneath the junggar continent in the early paleozoic and later shift to intra oceanic subduction

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

estimation of relative depth of burial using vitrinite reflectance implications for a sedimentary basin formation by basement tilting abstract the kinetics of coalification are still debatable the role of time has not been conclusively assigned if accepting the importance of time we have not reached consensus in formulation of the kinetics here we show that estimation of burial depth is possible under some conditions using vitrinite reflectance neither with computation of a specific kinetic equation of organic metamorphism nor with postulating plausible geothermal gradients of the past the present technique is based only on empirical linear relations between vitrinite reflectance and burial depth and is demonstrated to be effective for investigation of syn depositional tectonics the authors applied the technique to miocene coals of the chichibu basin central japan and this study indicates that the sedimentary basin was formed by tilting of the basement rocks

spherical shell tectonics on the buckling of the lithosphere at subduction zones abstract the buckling phenomena of the subducting lithosphere due to the sphericity of the earth are studied by a nonlinear finite element method fem and the effects of geometrical nonlinearity are examined the subduction of the lithosphere is modelled by a hemispherical shell squeezed at its circumferential edge in scaling the geometry of the subducting lithosphere two parameters are employed one is the thickness of the hemispherical shell i e the thickness of the lithosphere and the other is the length of its deformable portion i e the length of the descending slab to simulate the bending of the lithosphere at the trench uniform inward load is applied to the free circumferential edge of the deformable portion which corresponds to the leading edge of the subducting lithosphere at the opposite edge of the deformable portion i e at the trench a built in boundary condition is imposed no displacements and no rotations are allowed with a load greater than some critical value the deformable portion of the shell buckles and undulates azimuthally the investigation of this buckling phenomenon can be summarized as follows 1 1 buckling with a shorter wavelength is associated with a thinner and shorter shell mechanical irregularities simulating lithospheric tearing or seamount collision do not alter the buckling wavelength very much after appropriate scaling the lengths of subduction zones coincide with the buckling wavelength calculated by f e m 2 2 sharp cusps pointing outward from the shell appear during the postbuckling undulation without any irregularities in shell property and in the applied load the shape of this cuspidal undulation is quite similar to that actually observed at an arc arc junction 3 3 buckling can take place in the stress environment likely to exist in the earth these results strongly suggest that the arcuate shapes of subduction zones are in fact generated by buckling of the subducting lithosphere

seismic structure and seismogenesis off sanriku region northeastern japan summary in 1997 a seismic experiment using an airgun array and ocean bottom seismographs obss was performed in the forearc region of the northeastern japan nej arc the objectives of this experiment were to clarify seismic structures of the forearc region off sanriku japan using airgun obs data and to understand the nature of the japan trench seismogenic zone fundamental features of the structure are as follows i the subduction angle becomes steeper from east to west from 3 to 8 to 11 with the top of the plate located at depths of 11 km 40 km west from the trench 21 km 120 km west from the trench and 28 km 140 km west from the trench respectively ii the nej moho beneath the forearc is defined at a depth of approximately 20 km iii p wave velocity of the island arc upper mantle is approximately 8 km s 1 iv the p wave velocity of the cretaceous sedimentary layer exhibits significant lateral variability 4 0 5 5 km s 1 i nthe e w direction v the upper crust of nej forearc also has the significant lateral heterogeneity 5 0 6 3 km s 1 i nthe e w direction but not in the n s direction vi the middle crust defined on land extends in some locations 40 km east of the coastline vii lateral variability in the p wave velocity of the lower crust 6 7 7 0 km s 1 i ssmall compared with that of the upper crust according to these results it is suggested that the rupture areas of large earthquakes m7 in this region are limited within the contact zone between the nej arc crust and the subducting oceanic crust although a thin layer with low velocity is probably present within this contact zone the serpentinization of uppermost mantle wedge of the nej arc by reaction between the mantle materials and the water from the thin layer beneath forearc region is limited this might indicate that a significant volume of water and sediments subduct with the slab to depths of over 20 km

maturation based geodynamic geochemical classification of sedimentary basins abstract geodynamic principles and processes are combined with continental margin type basin position on a plate and crustal type to classify sedimentary basins in a tectonic mode tectonic basin classifications however have not improved the rate of discovery of hydrocarbons or mineral deposits in sedimentary basins to improve prediction of commercial hydrocarbon occurrence and sedimentary hosted ore deposits it is recommended that the proposed geodynamic basin classification could be combined with organic petroleum or inorganic metals geochemical classification systems organic geochemical basin classification discriminates basin types using three criteria charge factors migration drainage factors and impedance factors demaison and huizinga 1989a b a similar approach is applicable to mineral deposits an inorganic geochemical classification is proposed that recognizes three criteria of inorganic maturation metal solubility factor migration drainage factor and environmental potential for mineral precipitation this combined geodynamic and geochemical maturation approach to basin classification should enhance resource exploration efficiency

structural control on late miocene to quaternary volcanism in the ne honshu arc japan 1 volcanological and structural field data are used to define the tectonic control on the n s volcanic arc of ne honshu japan since late miocene during late miocene pliocene bimodal products were mainly erupted from along arc and ne sw aligned and elongated calderas the deformation pattern mostly consisted of n s dextral faults and subordinate ne sw extensional structures produced by ne sw compression this pattern because of the indentation of the kuril sliver is similar to that of oblique convergence settings magma rose and extruded along ne sw areas of localized extension created by the dextral faults these extensional areas were uncoupled with regard to those e w trending inferred to have focused the rise of melts from the subducting slab in the mantle during quaternary a larger amount of andesite was mainly erupted from along arc and e w aligned and elongated stratovolcanoes the deformation pattern mostly consisted of n s thrust faults and subordinate e w extensional structures produced by e w compression resulting from orthogonal convergence due to the variation in the absolute motion of the pacific plate the e w extensional structures are the shallowest expression of e w trending hot mantle fingers suggesting mantle crust coupling for the rise of magma such a coupling ensures 1 higher extrusion and 2 mixing between a deeper mafic and a shallower felsic magma generating the andesites the significantly larger volumes ma 1 200 km 1 of length of the arc of magma erupted during quaternary show that pure convergence conditions do not necessarily hinder the rise and extrusion of magma

backarc deformation along the eastern japan sea margin offshore northern honshu abstract the structural evolution of the eastern japan sea margin offshore northern honshu is discussed on the basis of single channel seismic profiles backarc structures evolved in response to late oligocene middle miocene extension which was generated at high angles to the arc trench system the evolution of a late miocene recent compressional plate tectonic regime resulted in widespread basement inversion the eastern japan sea margin is characterized by variable inverted half graben systems which are obliquely oriented to the island arc the area is underlain by three major structural blocks which are separated by a nw and a wnw trending structural discontinuity these blocks experienced a northward increase in extension the two southern blocks contain nne ne oriented reverse faults while the northern block is underlain by n nnw trending reverse faults the orientation and geometry of basement faults compares well with faults mapped onshore honshu the evaluation of the eastern japan sea margin can be explained by early miocene ese directed extension followed by a middle miocene sse directed extension pliocene recent east dipping reverse faults extend along the base of the continental slope and form a discontinuous thrust front which is characterized by minor fault offsets these reverse faults are locally associated with small active arc volcanoes

40ar 39ar and k ar dating on the volcanic rocks in the songliao basin ne china constraints on stratigraphy and basin dynamics precise 40ar 39ar and k ar ages were determined on sanidine and whole rock separates of volcanic rocks in the songliao basin sb the volcanism is characterized by a series of continual and episodic eruptions in the late jurassic 147 157 ma and early cretaceous 113 136 ma a fairly accurate chronostratigraphic column was first established based on those data the volcanic activities happened synchronously with closure of the mongolia okhotsk ocean north to the sb structures of the volcanogenic successions are s dipping or se dipping fault blocks coupled with n dipping or nw dipping major detachment systems distribution of the volcanics revealed by high resolution seismic reflections is neither restricted within nor in agreement with the framework of the overlying sedimentary sequence striking nne this new evidence fits the overlap model proposed in the present paper that formation of the volcanic successions in the sb is related to the subduction of the mongolia okhotsk plate under the mongolia north china block during the jurassic and early cretaceous whereas the overlying sedimentary sequence unconformably on the volcanics is tectonically controlled by the oblique subduction of the pacific plate under the eurasian plate since late aptian in addition the coal bearing epiclastic sediments intercalating the volcanics were deposited at the subduction quiescence episodes when extensional collapse occurred

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

odp leg 107 in the tyrrhenian sea insights into passive margin and back arc basin evolution leg 107 of the ocean drilling program drilled a west northwest east southeast transect of seven sites across the tyrrhenian sea the youngest of the sub basins of the mediterranean sea sites 654 653 652 and 656 document the rifting and subsidence of the sardinia passive continental margin on the upper margin site 654 we cored a classic transgressive sequence subaerial conglomerates overlain by oyster bearing sands overlain by marine marl comparison between the recovered lithologies and seismic reflection profiles suggests that the synrift sediments on the upper margin are tortonian late miocene to messinian latest miocene in age whereas synrift sediments on the lower margin are messinian to pliocene in age during the messinian desiccation of the mediterranean sites 654 and 653 now on the upper sardinian margin apparently occupied a basinal setting where they received nannoplankton bearing clays interbedded with laminated gypsum sites 656 and 652 now on the lower sardinia margin were apparently higher standing during the desiccation event their messinian facies are subaerial and lacustrine respectively we infer from these lines of evidence that tilting and subsidence occurred more than a million years earlier on the upper margin than on the lower margin such diachroneity can be interpreted in terms of migration of the zone of maximum extension above a rolling back subduction zone or in terms of extension of continental crust by shear along a deep detachment fault sites 655 651 and 650 were drilled into two small basalt floored basins of the central and eastern tyrrhenian emplacement of basaltic crust in the central tyrrhenian vavilov basin apparently began more than a million years before emplacement of basaltic crust in the eastern tyrrhenian marsili basin this observation is compatible with previous suggestions that the tyrrhenian has grown southeastward in response to rollback of the down going slab that currently dips northwestward under the toe of italy at the easternmost site high vesicularity of the basalt and benthic foram assemblages in the oldest sediments imply that the basalt erupted in water shallower than 2 500 m it has apparently subsequently subsided to its present depth of 4 100 m below sea level nearly three times as fast as normal subsidence of crust formed at a mid ocean ridge

sedimentary processes caused by felsic caldera forming volcanism in the late miocene to early pliocene intra arc aizu basin ne japan arc abstract six large late miocene to quaternary calderas 10 km in diameter cluster together with several medium to small calderas and stratovolcanoes in a 60 30 km area of the aizu volcanic field southern ne japan arc these caldera volcanoes were built on a wnw ese trending highland coincident with a local uplifted swell since late miocene the flare up of felsic volcanism occurred synchronously along the ne japan arc pyroclastic flow sheets from the calderas spread over the surrounding intra arc basins and are interstratified with various sediments geochronological data indicates that the large caldera eruptions have occurred six times since 8 ma at intervals of 1 to 2 million years late miocene to early pliocene extra caldera successions in the basin consist of nine sedimentary facies associations 1 primary pyroclastics 2 lahars 3 gravelly fluvial channels 4 sandy fluvial channels 5 floodplains 6 tidal flats 7 delta fronts 8 pro delta slopes and 9 pro delta turbidites the distribution of facies associations show westward prograding of volcaniclastic aprons made up of braid delta braidplain pyroclastic flow sheet and incised braided river deposits the extra caldera successions record 1 an increase in felsic volcanism with an associated high rate of volcaniclastic sediment supply at about 10 ma prior to catastrophic caldera forming eruptions and 2 progradation of volcaniclastic aprons toward the back arc side in response to the succeeding caldera forming eruptions and sea level changes until about 3 ma

landward migration of active folding based on topographic development of folds along the eastern margin of the japan sea northeast japan abstract the eastern margin of the japan sea northeast japan lies in a strongly compressional area and contractional deformation in the region is ongoing we reconstructed the temporal and spatial evolution of contractional deformation across the tohoku district on the eastern margin of the japan sea by using surface geologic and geomorphologic data we measured the distance from the fold hinge to the topographic ridge line in antiforms developed by surface uplift associated with fold growth under the e w to wnw ese compressional stress regime the topographic development of fold structures developed in the region since the pliocene is consistent with the activity of the folds when the folding that generates the uplift becomes inactive the topographic contrasts between hinge and ridge location can be remarkable spatial variations in this parameter are consistent with the systematic eastward migration of fold growth in the area the documentation of fold related topography thus provides important information for understanding the dynamics of folding along the eastern margin of the japan sea

k ar geochronology of a middle miocene submarine volcano plutonic complex in southwest japan a volcano plutonic complex in the susa area southwest japan consists of the yamashima andesites the koyama gabbros and syn plutonic porphyrite dykes derived from a common basaltic andesite magma the complex is closely associated with middle miocene turbidite deposits the yamashima andesites are composed mainly of basaltic andesite feeder dykes massive submarine lavas with hyaloclastites and their reworked deposits the lavas and deposits immediately overlie turbidite deposits indicating submarine volcanic activity the koyama gabbros formed hornfels by contact metamorphism of the surrounding turbidites and andesites highly purified clinopyroxene and plagioclase mineral separates from the yamashima andesites were dated by a k ar method using an ultra low blank k analysis procedure ages obtained from duplicate analyses are 16 5 1 5 15 2 1 4 15 8 1 7 and 16 5 2 0 ma for clinopyroxene and 14 2 0 8 15 2 0 9 and 15 6 0 9 ma for plagioclase the clinopyroxene and plagioclase data define a mineral isochron age of 14 7 0 9 1 ma with an initial 40 ar 36 ar ratio of 297 3 2 4 1 suggesting that clinopyroxene has no excess argon and can be reliably dated by k ar most of the groundmass ages are considerably younger 12 1 14 6 than the isochron age perhaps due to argon loss during alteration the gabbros give ages of 14 2 0 3 and 14 1 0 3 ma for biotite and 13 7 0 3 and 13 7 0 7 ma for green hornblende the porphyrite dyke yields an age of 12 5 0 3 ma for the groundmass and the pelitic hornfels gives a biotite age of 14 8 0 3 ma our new k ar ages together with previous studies show that a series of geological events took place in the susa area between 16 and 13 ma conglomerates and sandstones were deposited in the beginning of marine transgression subsequent abrupt deepening led to deposition of a thick black shale unit turbidite deposits and large scale submarine channel fill deposits coeval igneous activity formed the volcano plutonic complex the magmato tectonic event was synchronous with the opening of the japan sea and the associated clockwise rotation of the southwest japan arc sliver recording a unique tectonic setting

deep seismic reflection profiling across the ou backbone range northern honshu island japan knowledge of the crustal structure especially the geometry of seismogenic faults is key to understanding active tectonic processes and assessing the size and frequency of future earthquakes to reveal the relationship between crustal structure and earthquake activity in northern honshu island common midpoint cmp deep reflection profiling and earthquake observations by densely deployed seismic stations were carried out across the active reverse faults that bound the ou backbone range the 40 km long cmp profiles portray a relatively simple fault geometry within the seismogenic layer the reverse faults merge at a midcrustal detachment just below the base of the seismogenic layer producing a pop up structure that forms the ou backbone range the top of the reflective middle to lower crust 4 5 s in travel time twt nearly coincides with the bottom of seismogenic layer the p wave velocity structure and surface geology suggest that the bounding faults are miocene normal faults that have been reactivated as reverse faults

high resolution reconstruction of quaternary crustal movement based on sedimentary facies analysis an example from the oga peninsula northern japan abstract quantitative evaluation of the changes in mode and rate of quaternary crustal movement is important but is generally difficult to reconstruct with high time resolution it is also difficult to evaluate to crustal subsidence in this study we demonstrate a method to reconstruct vertical crustal movement during the middle to late quaternary on the basis of sedimentary facies analysis the method is applicable to the cyclothemic shallow marine to nonmarine sequences such as those in the oga peninsula northern japan whose ages have been estimated in detail assuming eustatic sea level is known for the last 450 ky we estimate the temporal variation in vertical crustal movement using reconstructed temporal changes in depositional depth and accumulated sediment thickness in the study area a change in mode of vertical crustal movement from subsidence to uplift occurred at approximately 130 ka combining the results of reconstruction of vertical crustal movement at two neighboring sites crustal movement is reconstructed which involves a combination of tilting and uplift in the studied case subsidence was caused by simple crustal tilting with a fixed tilting hinge associated with folding perhaps since ca 450 ka rapid regional uplift started ca 130 ka all over the northeast oga peninsula including the study area this case study demonstrates that 1 using this method crustal subsidence is reconstructed with certainty 2 this method is especially useful to reconstruct change in mode of vertical crustal movement from subsidence to uplift and 3 combining results of reconstruction it is possible to separate vertical movement from tilting combining this method and conservative methods using marine terraces and raised coral reefs crustal movement in quaternary orogenic zones will be understood more systematically

origin and age of pore waters in an actively venting gas hydrate field near sado island japan sea interpretation of halogen and 129i distributions abstract a gas hydrate field with highly active venting of methane was recently found near sado island in the eastern japan sea piston cores were collected from active venting sites and nearby locations in the umitaka spur joetsu knoll area during two cruises in 2004 ut04 and 2005 ky05 08 we report here halogen concentrations and 129 i i ratios in pore waters associated with gas hydrates from these expeditions the strongly biophilic behavior of i and to a lesser degree of br together with the presence of the long lived iodine radioisotope 129 i allow evaluation of potential source materials for methane in gas hydrate systems depth profiles of all three halogens particularly the very rapid downward increases of br and i concentrations strongly suggest input of deep fluids enriched in br and i but the profiles also display the effects of gas hydrate formation and dissociation although the 129 i i ratios are modified by 129 i from seawater and sediments at shallow depth likely ratios of the deep fluids are estimated to be between 400 10 15 and 600 10 15 equivalent to a late oligocene to early miocene age ages in the active methane venting sites typically are closer to the old end of this range than those in the reference sites this age range suggests that the methane associated with venting and gas hydrate formation in this area is derived from organic materials accumulated during the initial opening of the japan sea the umitaka spur joetsu knoll gas hydrate field demonstrates the movement of deep fluids associated with the release of significant amounts of methane from the seafloor processes which might be important components of mass transfer and carbon cycle in the shallow geosphere

cambrian plutonism in northeast japan and its significance for the earliest arc trench system of proto japan new u pb zircon ages of the oldest granitoids in the kitakami and ou mountains in order to clarify the early history of japan particularly during the early paleozoic pre devonian granitoids in the south kitakami belt skb ne japan were dated by u pb zircon age by laser ablation inductively coupled plasma mass spectrometry la icp ms two samples of diorite tonalite from the nagasaka area shoboji diorite and two of mylonitic tonalite from the isawa area isawagawa tonalite yielded late cambrian ages 500 490 ma for the primary magmatism these ages newly identify a ca 500 ma late cambrian arc plutonism in central ne japan which has not been recognized previously and has the following geological significance the cambrian granitoids are the oldest felsic plutonic rocks in ne japan which are independent of the previously known ca 450 ma latest ordovician hikami granite in skb the cambrian granitoids are extremely small in size at present but likely had a much larger distribution primarily at least 30 km wide and potentially up to 80 km wide in a cross arc direction their southerly extension was recognized in the hitachi area ca 200 km to the south and in central kyushu ca 1500 km to sw they likely represent remnants of the same mature arc plutonic belt in early paleozoic japan which developed in western panthalassan paleo pacific margin as well as the khanka block in far east russia the extremely small size of the cambrian granitoids at present can be best explained by intermittent severe tectonic erosion since the paleozoic this cambrian arc granitoid belt likely developed from primorye possibly to eastern cathaysia south china via japan

evolution of the pannonian basin system 2 subsidence and thermal history analysis of sedimentation and thermal data indicates that for most parts of the pannonian basin observed heat flow thermal gradients rates of thermal subsidence and vitrinite reflectance are about twice those predicted by assuming a uniform extension model for basin formation in late miocene time a modified extension model whereby large amounts of heat are added to the uppermost mantle during extension roughly equivalent to replacing the entire sub crustal lithosphere by asthenosphere produced results in good agreement with observation and could also account for periods of uplift observed during extension crustal extension beneath most parts of the basin is estimated to have been about 80 to 170 1 8 to 2 7 although in the deepest parts of the basin deeper than 4 km crustal extension could have been much greater analysis of extension within the danube and zala basins indicates that the late miocene extension in these basins involved the entire lithosphere but subsidence and heat flow data are not well enough constrained to determine if extra heating is required during extension on the basis of tectonic and structural data the vienna basin has been interpreted as the result of thin skinned extensional tectonics above a shallow detachment surface within the upper crust royden et al this issue analysis of thermal and subsidence data from the northern vienna basin suggests that little or no heating occurred during basin extension these data are consistent with a thin skinned extensional origin for the vienna basin and are in better agreement with this interpretation than with the assumption of a uniformly extended lithosphere beneath the basin this study emphasizes the need for good thermal subsidence and structural data in making reconstructions of extensional and thermal histories of sedimentary basins when such data are available it is found that a simple uniform extension model does not explain very well the evolution of some sedimentary basins such as the vienna and pannonian basins in the first instance the basin is thought to be the result of upper crustal extension only due to its special tectonic setting in the second instance the extra heating during extension is due to some unknown processes in the lower lithosphere that may be a general feature of lithospheric extension thus lithospheric extension and associated heating may be distributed in a variety of ways not all extensional sedimentary basins form in the same way application of a uniform subsidence model to basins for which good thermal subsidence and structural data are not available or not properly analysed can lead to reconstructions of thermal histories in error by a factor of 2 or more

continental tectonics in the aftermath of plate tectonics the success of plate tectonics required an acceptance of continental drift and thus a reinterpretation of the large scale geological history of most of the earth but the basic tenet of plate tectonics rigid body movements of large plates of lithosphere fails to apply to continental interiors where buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity

accretion of japanese island arcs and implications for the origin of archean greenstone belts the present day region of arc arc collision between the izu bonin arc and mainland japan honshu arc in the western pacific called the izu collision zone or icz provides a useful kinematic model for the development of many archean greenstone belts the compositional assemblage thermal structure and structural style of the crust in the icz are very similar to those inferred for many archean greenstone belts in both cases thick young and flaky arc crust with a high heat flux is in collision with another crustal block the icz has resulted in the imbrication of very thick slices of crust and associated sediments juxtaposed against older accretionary prisms many archean greenstone belts show similar geological features and we suggest that the incremental imbrication of the crust was a common tectonic style in some archean convergent margins

tectonic stress state in ne japan as part of the okhotsk plate abstract an existing geodetic flow velocity model obtained by using an internal free network adjustment technique is used to derive estimates for various strain rates parameters in ne japan the greatest shortening rates of the principal strains trending e w are located in regions where much steady internal frame invariant plastic flow deformation is observed to be taking place the internal geodetic adjustment technique yielded the internal deformation in the tohoku arc most of the intraplate deformations including the much folding deformations observed in the inner zone are produced from within an interseismic transient elastic loading at a strongly coupled locked japan trench would not be needed the observed ongoing extensive ductile folding deformation in the inner zone of tohoku may mean that the geodetic strain rates causing shortening at 2 3 cm yr probably reflect the more correct level of the deformation which is steady permanent in ne japan as compared with seismic faulting data which indicate 0 5 cm yr shortening the calculated principal strain rates are used to make an interpretation for the origin of the deviatoric principal stresses within the greater regional plate tectonic framework the tectonic stress state in ne japan as part of the okhotsk plate could mostly be influenced by the okhotsk plate which is extruding southward to lessen the significant accumulated contractional deformation in ne asia in the verkhoyansk cherskii mountains the principal strain rates are n s extensional essentially everywhere in ne japan as a result of the southerly extrusion except in its southernmost leading edge in the uetsu fossa magna province where the japan alps rampart rises in front of the extrusion here an e w compressional stress state prevails a second e w contractional zone is found in north central tohoku extending from the sanriku province in the outer zone to the inner zone in the japan sea side being more prevalent in the latter zone the calculated rotation rates from the geodetic flow model are clockwise cw in both of the e w contractional regions ne japan extruding southward faces buttresses in 1 the oga ojika line ool and or a crustal weakness zone between the northern and the southern halves of tohoku approx at 38 5 n latitude and especially 2 the japanese alps rampart these obstacles cause the northern and southernmost tohoku to veer to its right and rotate cw thereby setting up the e w trending compressional deformation in their respective inner zones between the ool or the 38 5 n boundary and the kanto tectonic line ktl the sense of the differential rotations is counterclockwise ccw towards the ocean to the se the northern tohoku north of the ool and the southernmost tohoku south of the ktl cannot rotate ccw towards the ocean because of the izu block s collision in the south and the relatively strong coupling along the subduction interface beneath the japan trench in the north off sanriku the relatively stronger long term coupling between the northern tohoku and the pacific plate at the sanriku coast with respect to that in off fukushima is due to a flatter subduction of the pacific slab there increasing the plates interface contact area the flattening of the subduction dip angle was caused by ccw rotation and shifting of the northern tohoku along the dextral honjo matsushima line roughly corresponding to the ool towards the pacific and overriding of the subduction zone during the formation of the japan sea