crustal characteristic variation in the central yamato basin japan sea back arc basin deduced from seismic survey results abstract the crustal structure of the yamato bank the central yamato basin and the continental shelf in the southern japan sea back arc basin is obtained based on a seismic survey using ocean bottom seismographs and seismic shot to elucidate the back arc basin formation processes the central yamato basin can be divided into three domains based on the crustal structure the deep basin the seamount and the transition domains in the deep basin domain the crust without the sedimentary layer is about 12 13 km thick very few units have p wave velocity of 5 4 6 0 km s which corresponds to the continental upper crust in the seamount and transition domains the crust without the sedimentary layer is about 12 16 km thick the p wave velocities of the upper and lower crusts differs among the deep basin the seamount and the transition domains these results indicate that the central yamato basin displays crustal variability in different domains the crust of the deep basin domain is oceanic in nature and suggests advanced back arc basin development the seamount domain might have been affected by volcanic activity after basin opening in the transition domain the crust comprises mixed characters of continental and oceanic crust this crustal variation might represent the influence of different processes in the central yamato basin suggesting that crustal development was influenced not only by back arc opening processes but also by later volcanic activity in the yamato bank and continental shelf the upper crust has thickness of about 17 18 km and p wave velocities of 3 3 4 1 to 6 6 km s the yamato bank and the continental shelf suggest a continental crustal character

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

late mesozoic and cenozoic volcanism of the east sikhote alin area russian far east a new synthesis of geological and petrological data abstract the geotectonic framework and the evolutionary history of the east sikhote alin area esa need revision in accordance with various geological and petrological evidence gathered in recent years it has been suggested that the investigated region resulted from the subduction of the izanagi kula and pacific oceanic plates that lasted from late cretaceous to late tertiary however only late cretaceous intermediate and silicic volcanics enriched in lile and depleted in hfse can be interpreted as typical subduction complexes changes in the paleogene paleocene to early miocene magmatic style of the esa area reflect variation in the pacific plate eastern eurasian convergence parameter the termination late cretaceous subduction slab tearing and injection of the oceanic asthenosphere of pacific morb type into subcontinental lithosphere the sea of japan opening began in the eocene with formation of small rift basins in the tatar strait which accumulated coastal facies from eocene to early miocene the extension zone affected by oceanic asthenosphere gradually moved eastward i e to the modern sea of japan the effect of oceanic asthenosphere on the continental margin ended in the late miocene after the sea of japan had opened and new subduction initiated east of the japan islands em i signature of the late miocene pliocene within plate basalts is evidence for eastward asthenospheric flow

motion of the pacific plate relative to eurasia and its potential relation to cenozoic extension along the eastern margin of eurasia extension along the eastern margin of eurasia has been regarded commonly as a far field effect of the india eurasia collision however some aspects of the timing and location of this extension make a link between the collision and extension difficult we suggest that some extensional features commonly interpreted as effects of the collision may be related more simply to changes in plate convergence rates along the eastern plate boundary of eurasia an analysis of the motion of the pacific plate relative to eurasia suggests that the rate of pacific eurasia convergence varied significantly during the tertiary period from a late cretaceous convergence rate of 120 140 mm yr the rate declined substantially during early tertiary time and reached a minimum in eocene time of 30 40 mm yr in oligocene to earliest miocene time the average convergence rate increased moderately to 70 95 mm yr then decreased again to 65 70 mm yr during early to middle miocene time from late miocene to the present the rate of convergence increased to an average of 100 110 mm yr the paleocene through middle miocene episode of relatively slow convergence correlates with a period of widespread extension along the eastern margin of eurasia decreased convergence may have been related to a net reduction in horizontal compressional stress transmitted between the pacific and eurasian plates which resulted in widespread extension adjacent to the margin of eurasia

rapid rotation of southwest japan palaeomagnetism and k ar ages of miocene volcanic rocks of southwest japan summary miocene volcanic rocks have been sampled from the san in district in the central part of southwest japan reliable palaeomagnetic direction and k ar dating data have been obtained from 28 localities a clockwise deflection in declination d 38 5 is observed in the kawai formation having an age of 16 1 1 4 ma whereas a northerly declination is observed in formations younger than 14 2 0 6 ma these data indicate that southwest japan had ceased rotational motion by about 14 ma when compared with the amount of rotation estimated from cretaceous palaeomagnetic data it is apparent that more than 80 per cent of the overall clockwise rotation occurred between about 16 and 14 ma we conclude that the angular velocity of southwest japan about a rotation pole at 129 e 34 n reached 20 myr 1 at about 15 ma the corresponding rate of linear motion of the eastern part of southwest japan was 21 cm yr 1

bent structural trends of japan flexural slip folding related to the neogene opening of the sea of japan the trend of the pre miocene structures of japan is bent in kyushu and central japan on land geologic features such as the trend of synmetamorphic jurassic lineations neocomian basins and late cretaceous strike slip faults have distorted trends as a result of the miocene opening of the sea of japan accentuation of the bending in central japan could be due to the pliocene quaternary collision of japan with the izu ridge a preopening reconstruction of japan is proposed the bent structural trends of kyushu and central japan along with neogene fault motions are explained by a two dimensional model of broad scale flexural slip folding

bio geochemical evolution and critical element mineralization in the cretaceous cenozoic coals from the southern far east russia and northeastern china abstract a total of 23 coal and two carbonaceous shale samples of the early cretaceous miocene collected from different basins in the primorye and sakhalin regions of russia and the hunchun basin of northeastern china were used in this study for determination of major oxides by aes and xrf trace elements by icp ms sulfur carbon carbon isotopes and major components of extractable organic matter wet chemical and gas chromatographic analyses interpretation of the obtained data was intended to elucidate geodynamic and other geological processes controlling bio geochemical compositions and critical element mineralization of the studied coals the data suggest that inorganic matter in the studied coals was derived mainly from the paleozoic granites and the cenozoic felsic and intermediate tuffs the early eocene and in particular oligocene epochs represent the two major mineralization events in the regions resulting in enrichment of ge w be mo sb sr ba rey rare earth elements and y th and u in the coals the two events were related respectively to the indo eurasian collision followed by plate reorganization in the pacific and the opening of the sea of japan the geodynamic changes were probably associated with mantle derived volcanic activity and intensive degassing which enhanced the capability of surficial and ground waters to leach and redistribute trace metals organic matter played a significant role in the system of mineralization starting from weathering of source rocks and ending at the coal diagenetic stage organic acids significantly accelerated leaching and transportation of the metals while they together with sulfate reducing bacteria kerogen alcohols ketones and n alkanols in particular played an important role in metal precipitation mainly through reactions of cation exchange and adsorption

k ar age and geochemistry of the sw japan paleogene cauldron cluster implications for eocene oligocene thermo tectonic reactivation abstract systematic k ar dating and geochemical analyses of paleogene cauldrons in the sanin belt of sw japan have been made to explore the relationship between the timing of their formation and the paleogene subduction history of sw japan documented in the shimanto accretionary complex we also examine the magma sources and tectonics beneath the backarc region of sw japan at the eastern plate boundary of eurasia fifty eight new k ar ages and 19 previously reported radiometric age data show that the cauldrons formed during middle eocene to early oligocene time 43 30 ma following a period of magmatic hiatus from 52 to 43 ma the hiatus coincides with absence of an accretionary prism in the shimanto belt resumption of the magmatism that formed the cauldron cluster in the backarc was concurrent with voluminous influx of terrigenous detritus to the trench as a common tectono thermal event within a subduction system the cauldrons are composed of medium k calc alkaline basalts to rhyolites and their plutonic equivalents these rocks are characterized by lower concentrations of large ion lithophile elements lile including k2o ba rb th u and li lower la yb n ratios lower initial sr isotopic ratios 0 7037 0 7052 and higher end t values 0 5 to 3 5 relative to late cretaceous to early paleogene equivalents there are clear trends from enriched to depleted signatures with decreasing age from the late cretaceous to the paleogene the same isotopic shift is also confirmed in lower crust derived xenoliths and is interpreted as mobilization of pre existing enriched lithospheric mantle by upwelling depleted asthenosphere relatively elevated geothermal gradients are presumed to have prevailed over wide areas of the backarc and forearc of the sw japan arc trench system during the eocene to oligocene newly identified late eocene low silica adakites and high mg andesites in the sanin belt and early eocene a type granites in the sw korea peninsula probably formed due to upwelling of hot asthenosphere and subduction of a young plate the backarc region was an extensional tectonic setting and some paleogene rift basins and sanin belt cauldrons occur in linear arrays the eocene oligocene sanin se korea continental arc lies on the ne extension of the east china sea basin the initial stage of which probably formed by continental arc rifting this rifting may have been triggered by upwelling of hot asthenosphere into the wedge space created by rollback of the subducted slab in response to decreased convergence rate between the pacific and eurasian plates

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

evolution of the sea of japan back arc and some unsolved issues abstract the sea of japan back arc system provides an exceptional opportunity to study a virtually intact continent ocean back arc system in which recent tectonic inversion has exposed entire sequences of back arc structure on land moreover japan s dense seismic geodetic monitoring networks deployed country wide as well as moveable pools of ocean bottom seismometers provide a rich data set through which to investigate deep back arc structure earlier investigations have produced an initial understanding of back arc opening timing structural evolution temporal spatial patterns of magmatic activity many questions remain among them the mechanism of back arc opening pull apart or trench rollback the dynamics of interacting plates location of the philippine sea plate with time the origin of the anomalously thick japan sea ocean crust and possible influences of far field forces india asia collision given existing high resolution geophysical data sets and extensive on land exposures of back arc sequences and structures the sea of japan back arc is a promising context in which to address both local and more universal questions of how back arc systems evolve here we review the tectonic setting and geological evolution of the sea of japan based our own and others work and briefly discuss outstanding questions that invite further investigation

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

subdivision of the hida metamorphic complex central japan and its bearing on the geology of the far east in pre sea of japan time abstract petrological and structural characteristics of the regional metamorphic rocks in south west japan and in the korean peninsula make it possible to speculate on the geological correlation between japan and the asian continent prior to the opening of the sea of japan a typical marginal sea the hida metamorphic complex situated on the sea of japan side of southwest japan is subdivided into two distinct geological units the hida gneisses and the unazuki schists the hida gneisses are polymetamorphosed precambrian rocks while the unazuki schists occurring on its eastern and southern sides are low to medium grade metamorphic rocks originating from upper paleozoic deposits sedimentary facies and other geological features suggest that the hida gneisses were the basemen of the unazuki schists consequently the geotectonic framework of southwest japan is revised from north to south as follows hida gneiss region precambrian massif unazuki zone late permian intermed p t metamorphic belt circum hida tectonic zone mid to late paleozoic melange zone mino tanba terrain late paleozoic to mesozoic geosynclinal region and so on the unazuki zone is similar to the okcheon zone in the korean peninsula in respect of age lithology and biofacies of sedimentary rocks as well as the age and type of regional metamorphism furthermore the hida gneisses and the gyeonggi polymetamorphosed precambrian gneisses in the korean peninsula are similar in the apparent baric type of metamorphism radiometric ages and the relationship with the overlying metamorphosed formations the similarity of the geotectonic frameworks of southwest japan and the korean peninsula suggests that the unazuki zone and the okcheon zone once formed a continuous geotectonic unit thus we have a new geological coordinate in reconstructing the paleogeography prior to the opening of the sea of japan

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

tectonics and volcanism in the sado pohang belt from 20 to 14 ma and opening of the yamato basin of the japan sea abstract tertiary volcanism around the japan sea is closely related to the opening of the japan sea paleogene to early middle miocene formations in the pohang yangnam area of the korean peninsula mainly consist of volcanic rocks that are stratigraphically and petrographically similar to similar age rocks on the japan sea side of the japanese islands especially those of sado island we propose that all these early cenozoic rocks formed part of a single tectonic volcanic belt called sado pohang belt it is inferred that this belt was located on the south side of the yamato ridge in paleogene time after the japan basin opened the yamato basin opened during early miocene to the early middle miocene time and part of the sado pohang belt migrated southward the tectonics of the sado pohang belt is considered to support the hypothesis of the two stage opening of the japan sea

constraints on the evolution of the japan sea based on 40ar 39ar ages and sr isotopic ratios for volcanic rocks of the yamato seamount chain in the japan sea 40 ar 39 ar and sr isotope analyses were performed on basalts and andesites dredged from the yamato seamount chain in the japan sea the 40 ar 39 ar plateau ages range from about 11 to 17 ma though most samples show ages between 10 and 14 ma the seamounts seem to have formed within a period of a few million years although some of them might have formed earlier based on the present results together with previously reported radiometric age data it is thought that the yamato basin formed during some period prior to 17 ma and probably later than around 25 ma taking into account the radiometric age data on rocks from the japan basin it is conjectured that the opening of the japan sea might have started almost at this time or a little earlier the observed 87 sr 86 sr ratios range from 0 70357 to 0 70388 suggesting incorporation of some time integrated components enriched in incompatible elements such as continental crustal materials this may indicate that in the japan sea area at least the yamato basin had not developed enough to show the characteristics of typical n type morb source materials without being affected by pre existing continental crustal materials

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

precise k ar 40ar 39ar rb sr and u pb mineral ages from the 27 5 ma fish canyon tuff reference standard the accuracy of ages measured using the 40ar 39ar technique is affected by uncertainties in the age of radiation fluence monitor minerals at present there is lack of agreement about the ages of certain minerals used as fluence monitors the accuracy of the age of a standard may be improved if the age can be measured using different decay schemes this has been done by measuring ages on minerals from the oligocene fish canyon tuff fct using the k ar 40ar 39ar rb sr and u pb methods k ar and 40ar 39ar total fusion ages of sanidine biotite and hornblende yielded a mean age of 27 57 0 36 ma the weighted mean 40ar 39ar plateau age of sanidine and biotite is 27 57 0 18 ma a biotite feldspar rb sr isochron yielded an age of 27 44 0 16 ma the u pb data for zircon are complex because of the presence of precambrian zircons and inheritance of radiogenic pb zircons with 207pb 235u 0 4 yielded a discordia line with a lower concordia intercept of 27 52 0 09 ma evaluation of the combined data suggests that the best age for fct is 27 51 ma

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

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

big mantle wedge anisotropy slabs and earthquakes beneath the japan sea abstract the japan sea is a part of the western pacific trench arc backarc system and has a complex bathymetry and intense seismic activities in the crust and upper mantle local seismic tomography revealed strong lateral heterogeneities in the crust and uppermost mantle beneath the eastern margin of the japan sea which was determined using p and s wave arrival times of suboceanic earthquakes relocated precisely with sp depth phases ambient noise tomography revealed a thin crust and a thin lithosphere beneath the japan sea and significant low velocity low v anomalies in the shallow mantle beneath the western and eastern margins of the japan sea observations with ocean bottom seismometers and electromagnetometers revealed low v and high conductivity anomalies at depths of 200 300 km in the big mantle wedge bmw above the subducting pacific slab and the anomalies are connected with the low v zone in the normal mantle wedge beneath ne japan suggesting that both shallow and deep slab dehydrations occur and contribute to the arc and back arc magmatism the pacific slab has a simple geometry beneath the japan sea and earthquakes occur actively in the slab down to a depth of 600 km beneath the ne asian margin teleseismic p and s wave tomography has revealed that the philippine sea plate has subducted aseismically down to the mantle transition zone mtz 410 660 km depths beneath the southern japan sea and the tsushima strait and a slab window is revealed within the aseismic philippine sea slab seismic anisotropy tomography revealed a nw se fast velocity direction in the bmw which reflects corner flows induced by the fast deep subduction of the pacific slab large deep earthquakes m 7 0 depth 500 km occur frequently beneath the japan sea western margin which may be related to the formation of the changbai and ulleung intraplate volcanoes a metastable olivine wedge is revealed within the cold core of the pacific slab at the mtz depth which may be related to the deep seismicity however many of these results are still preliminary due to the lack of seismic stations in the japan sea the key to resolving these critical geoscientific issues is seismic instrumentation in the japan sea for which international cooperation of geoscience communities in the east asian countries is necessary

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

opening of the sea of japan back arc basin by asthenospheric injection abstract the position of a volcanic front on an arc is governed by dehydration reactions of amphibole chlorite in down dragged hydrated peridotite at the base of the mantle wedge as these reactions are essentially pressure dependent ca 3 5 gpa the paleo position of a volcanic front offers an estimate for the depth of the subducted oceanic lithosphere beneath it this paper evaluates volcanism in the northeast japan arc since the oligocene and discusses the mass transportation in the mantle wedge during the episode of back arc extension in the sea of japan with reference to the migration of the volcanic front the volcanic front during the oligocene ca 30 ma was located along the western coast of the present northeast japan are and migrated about 200 km trenchwards at 23 ma before the opening of the sea of japan back arc basin it follows that the angle of subduction became steeper during the period of 30 23 ma triggered by the injection of the asthenosphere into the mantle wedge the back arc extension of the sea of japan between 21 and 14 ma is a manifestation of the dynamic process of the lithosphere due to the asthenospheric injection the temperature of the injecting asthenospheric mantle was higher than that of the surrounding upper mantle or lithosphere this caused the characteristic volcanism at 23 22 ma 1 high magnesian andesite volcanism took place in the near trench region 2 across arc compositional variation was not observed 3 the volume of volcanic materials increases towards the back arc side of volcanic arc the volcanic front has migrated over a short distance inwards less than 100 km since 15 ma possible as a result of cooling in the mantle wedge because both amphibole and chlorite break down at slightly higher pressures at lower temperatures the upper mantle materials which injected into the mantle wedge and caused back arc spreading are identical to those of the mid oceanic ridge basalt source in sr nd isotopic compositions because volcanic rocks in the back arc region of the northeast japan arc show secular variation in isotopic compositions from enriched to depleted isotopic characteristics during the last 30 ma

tsunamis caused by offshore active faults and their deposits abstract this paper reviews the variable features of coastal tsunamis caused by offshore active faults along the eastern margin of the japan sea emjs based on the historical records and modern observations of earthquakes and tsunamis tsunami deposits discovered along the emjs coast were also introduced and categorized depending on their sedimentary environments the offshore active faults which are densely distributed along the emjs and generate tsunamis are originated from the opening of the japan sea back arc basin during the late oligocene to miocene period the magnitudes of the earthquakes that caused large tsunamis in the emjs were under 8 0 the size of the fault plane approximately equal to the tsunami source area was approximately 10 100 and several tens of kilometers in length the slip distance of the reverse type faults was less than 20 m the sedimentary characteristics and depositional processes of tsunami deposits were obtained from deposits in various environments such as coastal lowlands marine terraces coastal lagoons and inland ponds certain unique approaches used to identify tsunami deposits were reviewed such as gravel layers that intercalate inorganic soils and a holocene marine terrace dinoflagellate cysts and foraminiferal linings which are potentially derived from the sea and chemical features e g the ca2 mg2 ratio as indicators of seawater inflow into a coastal lagoon the spatial distribution and recurrence of earthquakes and tsunamis in the emjs were found based on previous paleo tsunami studies the difference between tsunami deposits caused by offshore active faults and those by megathrust earthquakes only manifests itself in the horizontal and vertical directions but not in the sedimentary environments and structures the data thus inferred can be used for the estimation of offshore active fault models and validation of previous tsunami disaster reconstructions in the future

radar images of asteroid 4179 toutatis delay doppler images of the earth crossing asteroid 4179 toutatis achieve resolutions as fine as 125 nanoseconds 19 meters in range and 8 3 millihertz 0 15 millimeter per second in radial velocity and place hundreds to thousands of pixels on the asteroid which appears to be several kilometers long topographically bifurcated and heavily cratered the image sequence reveals toutatis to be in an extremely slow non principal axis rotation state

origin and water depth of a newly identified seep carbonate and paleoecology of bathymodiolus in the miocene taishu group southwestern japan abstract the fukuzaki limestone is a small limestone lens embedded in siliciclastic taishu group rocks on tsushima island in the southwestern japan sea a fossil assemblage in this limestone consisting of the bivalve mollusc bathymodiolus sp bathymodiolinae was previously inferred on the basis of the limestone s isolated lenticular occurrence and the chemosynthetic nature of modern bathymodiolinae to rely on chemosynthetic production but conclusive evidence was not provided carbon nitrogen oxygen and sulfur isotope ratios in the limestone and its host rock mudstone were measured to clarify the paleoecology of the fossil assemblage and the formation environment of the limestone the low 13c 39 8 to 31 7 and 18o 13 8 to 11 9 values of carbonate of the limestone lens strongly suggest that the limestone is composed of authigenic carbonate rock precipitated from bicarbonate derived from anaerobic oxidation of biogenic or thermogenic methane the fossil assemblage in the limestone lens is consistent with a chemosynthesis based animal assemblage harboring thioautotrophic sulfur oxidizing symbionts and the low 13c and 34s values of total organic carbon and sulfide sulfur respectively in the limestone lens and mud layers suggest that they reflect chemosynthetic production using hydrogen sulfide derived from anaerobic methane oxidation by microbes furthermore the stratigraphic position of the lens in the upper part of the lower unit of the taishu group indicates that it was precipitated at a water depth 800 m deeper than the lower middle bathyal zone these findings indicate that a deep sea basin locally supplied with methane had already formed in the southwestern japan sea by rapid subsidence during the opening stage of the japan sea 16 17 ma

variability in coal facies as reflected by organic petrological and geochemical data in cenozoic coal beds offshore shimokita japan iodp exp 337 abstract this study reports on cenozoic coal seams recovered at integrated ocean drilling program iodp site c0020 during expedition 337 iodp site c0020 is located in a forearc basin formed by the subduction of the pacific plate off shimokita peninsula japan hole c0020a penetrated 14 coal layers between 1825 and 2466 mbsf eleven of them were investigated within the frame of this paper investigated seams show a slight maturity increase with depth from lignite to sub bituminous coal in order to detect temporal changes in maceral and molecular composition and to relate them to changes in vegetation and depositional environment macro and micropetrographic data bulk geochemical parameters biomarker analysis stable isotope geochemistry and vitrinite reflectance measurements were performed results were also compared with palynological data obtained from 9 coal samples elevated sulfur contents and high ash yields occur in the upper seams cores 14r to 18r whereas low sulfur contents and varying ash yields were determined for the lower coal seams 24r to 30r the maceral composition and biomarker ratios of the uppermost seams argue for coal formation in a paralic environment and brackish alkaline water conditions in contrast lignite samples from the lower part of unit iii point to a limnic fluviatile deposition conifers contributed significantly to peat formation in the uppermost seams from cores 15r to 22r and in the lowermost lignite seam in all other samples angiosperms are considered as the major peat forming plants the pollen and spore floras indicate rich angiosperm vegetation however significant contributions from pinaceae and taxodiaceae are evident for all coals sporophytes have no dominant influence on the coal flora microbial activity in the peat is suggested for instance by higher concentrations of hop 17 21 ene with increasing contents of hopanes or by low 13 c values of hop 17 21 ene the revealed changes in the environment during coal deposition highlight the importance of combined organic petrography organic geochemistry and palynology to reconstruct palaeoenvironmental conditions

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

tectonic history of ulleung basin margin east sea sea of japan a geologic history analysis of an exploratory well drilled on the southern margin of the ulleung back arc basin east sea western part of the sea of japan reveals that tectonic force and sedimentary loading played a major role in the subsidence of the paleo ulleung basin where a thick clastic sequence was deposited during the neogene initial subsidence was most likely caused by a tectonic force related to the back arc rifting and spreading associated with convergent processes the southern margin underwent regional deformation uplift faulting and folding toward the late miocene this event was probably due to back arc deformation caused by changes both in plate motions and in subduction mode at the convergent zones

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

remote measurement of river discharge using thermal particle image velocimetry piv and various sources of bathymetric information abstract although river discharge is a fundamental hydrologic quantity conventional methods of streamgaging are impractical expensive and potentially dangerous in remote locations this study evaluated the potential for measuring discharge via various forms of remote sensing primarily thermal imaging of flow velocities but also spectrally based depth retrieval from passive optical image data we acquired thermal image time series from bridges spanning five streams in alaska and observed strong agreement between velocities measured in situ and those inferred by particle image velocimetry piv which quantified advection of thermal features by the flow the resulting surface velocities were converted to depth averaged velocities by applying site specific calibrated velocity indices field spectra from three clear flowing streams provided strong relationships between depth and reflectance suggesting that under favorable conditions spectrally based bathymetric mapping could complement thermal piv in a hybrid approach to remote sensing of river discharge this strategy would not be applicable to larger more turbid rivers however a more flexible and efficient alternative might involve inferring depth from thermal data based on relationships between depth and integral length scales of turbulent fluctuations in temperature captured as variations in image brightness we observed moderately strong correlations for a site aggregated data set that reduced station to station variability but encompassed a broad range of depths discharges calculated using thermal piv derived velocities were within 15 of in situ measurements when combined with depths measured directly in the field or estimated from field spectra and within 40 when the depth information also was derived from thermal images the results of this initial proof of concept investigation suggest that remote sensing techniques could facilitate measurement of river discharge

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

seismic structure of the extended continental crust in the yamato basin japan sea from ocean bottom seismometer survey abstract we present the result of a seismic experiment conducted using ocean bottom seismometers and an airgun in the yamato basin of the japan sea the japan sea is one of the most well studied back arc basins in the western pacific the japan sea is believed to have been formed by back arc opening however the timing and formation processes of the opening of individual basins in and around the japan sea are not clear to reveal the crustal structure of the yamato basin it is important to consider the formation process of the japan sea therefore we conducted a seismic survey and estimated the p wave seismic velocity structure beneath the 170 km profile using a 2 d ray tracing method a layer with a p wave velocity of 3 4 4 0 km s underlies the sedimentary sections which is thought to consist of a sill and sediment complex the upper crust below the profile varies greatly in thickness the thickness of the upper crust is 3 5 km in the thinnest part and 7 km in the thickest part the thickness of the lower crust is approximately 8 km and is relatively constant over the profile the total thickness of the crust is approximately 15 km including the sedimentary layer the distribution of p wave velocities and the thickness indicate that the crust in the yamato basin is neither a typical continental nor a typical oceanic crust from the point of view of seismic velocity the obtained structure is more similar to a continental crust than to an oceanic crust the large lateral thickness variation in the upper crust and the uniform thickness of the lower crust suggest that the crust in the study area was formed by rifting extension of continental crust during the opening of the japan sea the margins of the continent or of island arcs can be divided into two types volcanic rifted margins and non volcanic rifted margins volcanic rifted margins are normally classified by the presence of a high velocity body in the lower crust at the volcanic rifted margin the high velocity body is formed in the lower crust we did not observe such a high velocity layer at depth on our profile in contrast the acoustic basement consists of a sill and sediment complex therefore we infer that the study area was affected by volcanism during its formation however the magmatism was not significantly massive

reconstruction of subduction and back arc spreading in the nw pacific and aleutian basin clues to causes of cretaceous and eocene plate reorganizations the eocene 50 45 ma major absolute plate motion change of the pacific plate forming the hawaii emperor bend is thought to result from inception of pacific plate subduction along one of its modern western trenches subduction is suggested to have started either spontaneously or result from subduction of the izanagi pacific mid ocean ridge or from subduction polarity reversal after collision of the olyutorsky arc that was built on the pacific plate with ne asia here we provide a detailed plate kinematic reconstruction of back arc basins and accreted terranes in the northwest pacific region from japan to the bering sea since the late cretaceous we present a new tectonic reconstruction of the intraoceanic olyutorsky and kronotsky arcs which formed above two adjacent oppositely dipping subduction zones at 85 ma within the north pacific region during another pacific wide plate reorganization we use our reconstruction to explain the formation of the submarine shirshov and bowers ridges and show that if marine magnetic anomalies reported from the aleutian basin represent magnetic polarity reversals its crust most likely formed in an 85 to 60 ma back arc basin behind the olyutorsky arc the olyutorsky arc was then separated from the pacific plate by a spreading ridge so that the 55 to 50 ma subduction polarity reversal that followed upon olyutorsky ne asia collision initiated subduction of a plate that was not the pacific hence this polarity reversal may not be a straightforward driver of the eocene pacific plate motion change whose causes remain enigmatic

composition of lipids from coal deposits of the far east relations to vegetation and climate change during the cenozoic abstract twenty one samples of sub bituminous coal and 1 sample of coaly shale have been collected from different coal seams representing the cenozoic coal basins widely distributed in the region of primorye and sakhalin russia and ne china hunchun deposit the seams belong to lower miocene upper oligocene sineutesovsky oligocene upper eocene pavlovsky upper eocene ust davydovsky lower eocene uglovsky and upper paleocene kivdinsky local chronostratigraphic units terpenoid biomarker compositions indicate the contribution of conifers and angiosperms to peat formation the results are in agreement with paleobotanical data from sedimentary successions closely associated with the seams highlighting abundant conifers of the taxodioid cupressaceae and pinaceae families as well as betula alnus fagus and ulmus during cenozoic in the far east in contrast to previous findings about changes in peat forming vegetation in central europe the terpenoid biomarker ratios revealed increased contributions of angiosperms during the oligocene and gymnosperm domination during eocene and miocene the different evolution may be related to differences in temperature and precipitation distribution as well as in geomorphology and landscape evolution a positive relationship between the di di tri terpenoid biomarker ratios and 13c of oligocene samples indicates the role of varying gymnosperm angiosperm influences on the 13c of coal differences in vegetation and climate are most probably responsible for the different trends in 13c of terpenoid biomarkers over the cenozoic beside their general variation coupled to the evolution in 13c of atmospheric co2 lower 2h values of plant derived lipids during the oligocene were most probably caused by lower temperatures during coal formation this conclusion is supported by the results of recent paleobotanical studies yielding lower mean annual temperatures mat during the oligocene but comparable values for mean annual precipitation map during the eocene and oligocene in the primorye region pimarane type diterpenoids are significantly depleted in 2h the results are in agreement with previous studies identifying different sources of hydrogen and or different biosynthetic pathways during their formation

first images of asteroid 243 ida the first images of the asteroid 243 ida from galileo show an irregular object measuring 56 kilometers by 24 kilometers by 21 kilometers its surface is rich in geologic features including systems of grooves blocks chutes albedo features crater chains and a full range of crater morphologies the largest blocks may be distributed nonuniformly across the surface lineaments and dark floored craters also have preferential locations ida is interpreted to have a substantial regolith the high crater density and size frequency distribution 3 differential power law index indicate a surface in equilibrium with saturated cratering a minimum model crater age for ida and therefore for the koronis family to which ida belongs is estimated at 1 billion years older than expected