constraints on the moho in japan and kamchatka abstract this review collects and systematizes in one place a variety of results which offer constraints on the depth and the nature of the moho beneath the kamchatka peninsula and the islands of japan we also include studies of the izu bonin volcanic arc all results have already been published separately in a variety of venues and the primary goal of the present review is to describe them in the same language and in comparable terms for both regions we include studies using artificial and natural seismic sources such as refraction and reflection profiling detection and interpretation of converted mode body waves receiver functions surface wave dispersion studies in kamchatka and tomographic imaging in japan the amount of work done in japan is significantly larger than in kamchatka and resulting constraints on the properties of the crust and the uppermost mantle are more detailed japan and kamchatka display a number of similarities in their crustal structure most notably the average crustal thickness in excess of 30 km typical of continental regions and the generally gradational nature of the crust mantle transition where volcanic arcs are presently active

nd and sr isotope systematics of miocene to holocene volcanic rocks from southwest japan volcanism since the opening of the japan sea isotope nd and sr and geochemical data for twelve volcanic rocks from the shimane oki san in region of southwest japan give some insight into the miocene recent volcanic history of southern japan since rifting from the asian mainland the oldest rocks examined dolerites and ushikiri formation were erupted during the opening of the japan sea at about 15 ma and comprise arc tholeiites derived from a depleted source end 0 esr 0 interpreted in terms of a depleted mantle source modified by a slab derived fluid volcanics of oki dozen island in the japan sea are alkalic derived from an enriched mantle plume isotopically distinct from nearly oki dogo island eruption of the calc alkaline wakurayama andesite early pliocene and both arc and within plate type basalts 1 ma show that magmas were tapped from a variety of sources taking into account plate tectonics and seismic data the subduction influence until the quaternary was the pacific plate with philippines sea plate subduction affecting quaternary and younger volcanics

origin of the absence of magnetic lineations in the yamato basin of the japan sea magnetic properties of mafic rocks from ocean drilling program hole 794 d we investigated magnetic properties of mafic rocks recovered from ocean drilling program hole 794 d in the yamato basin of the japan sea in contrast to other back arc basins in the western pacific lineated magnetic anomaly patterns have not been recognized in the yamato basin our objective is to clarify the reason for the absence of lineated magnetic anomalies by direct magnetic measurements of crustal rocks the dominant magnetic mineral of hole 794 d is titanomaghemite as commonly observed for typical oceanic basalts but the grain size is much larger greater than several tens of microns remanence properties i e low natural remanent magnetization intensities 1 5 a m low koenigsberger ratios 1 0 high viscous remanent magnetization and mixed magnetic polarities indicate that the mafic rocks cannot be a source of lineated magnetic anomalies while curie temperature saturation magnetization and susceptibility values are not significantly different from those of typical oceanic basalts magnetic properties related to stability are greatly reduced the large grain size of the mafic rocks from hole 794 d plays a primary role in differentiating the magnetic properties from those of typical oceanic basalts and is caused by the formation of sill sediment complexes beneath the yamato basin sill sediment complexes rather than extrusive basalts in typical oceanic crust are responsible for the absence of lineated magnetic anomalies in the yamato basin

elastic strength of the slave craton at 1 9 gyr and implications for the thermal evolution of the continents the secular cooling of the earth since its formation together with the law of radioactive decay imply that the earth s internal heat generation and heat loss must have been two to three times higher in archaean and proterozoic times than today but this inference is difficult to test directly here we present a flexural analysis of the proterozoic kilohigok basin in the archaean slave craton of canada which shows that 1 9 gyr ago the craton had an effective elastic thickness of only 12 4 km because the present effective elastic thickness of the region is 100 25 km these results imply a two to fourfold decrease in the thermal gradient beneath the slave craton since 1 9 gyr and indicate that at 1 9 gyr 700 myr after the accretion of the continental lithosphere forming the craton the thick continental root beneath this part of the canadian shield had not yet formed

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

paleomagnetic evidence of the rotation of central japan and the paleoposition of japan abstract paleomagnetic studies of the daishoji the echigo yuzawa and the kataimogawa areas revealed complex intrablock tectonic rotations in the central honshu block which is sandwiched in between the southwest and northeast honshu blocks paleogeographic maps of 16 15 and 14 ma are reconstructed by using paleomagnetic declinations obtained so far in japan

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

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

timing and evolution of jurassic cretaceous granitoid magmatisms in the mongol okhotsk belt and adjacent areas ne asia implications for transition from contractional crustal thickening to extensional thinning and geodynamic settings abstract the mongol okhotsk belt and adjacent areas are key areas to study the relationship between the okhotsk and paleo pacific tectonic regimes and their superposition on the older paleo asian regimes during late mesozoic times this paper summarizes the spatial temporal evolution of late mesozoic jurassic cretaceous granitoids and related intrusions in these areas and interprets the magmatic evolution in terms of a transition from contractional crustal thickening to extensional thinning according to 407 published zircon ages these granitoids were mainly emplaced during the intervals 200 180 ma 180 165 ma 165 145 ma 145 135 ma and 135 100 ma jurassic granitoids 200 145 ma predominately occur in the baikal ne mongolia bnem and great xing an range early cretaceous 145 100 ma granitoids are mainly occur in the great xing an range and display a southward younging migration significantly early cretaceous granitoids also extend into the trans baikal area across the mongol okhotsk suture far away from the paleo pacific plate margin in ne china thus they were more plausibly related to post orogenic collapse of the mongol okhotsk orogen from the late jurassic to early cretaceous the granitoids evolved compositionally from calc alkaline and high k calc alkaline i type with some adakite like features to high k calc alkaline and shoshonitic highly fractionated i transitional i a or a type characterized by a significant decrease in their sr y ratios this evolution coincided with a tectonic transition from contractional crustal thickening to extensional thinning combined with regional geology we speculate that the jurassic granitoids were likely derived from melting of the deep seated thickened lower continental crustal lcc sources whereas the cretaceous granitoids produced through crustal melting from an extensional thinning setting our results provide a case study demonstrating that the petrogenesis of granitic magmatism was closely associated with crustal tectonics early jurassic granitoids along the okhotsk belt formed in a subduction collision setting related to closure of the mongol okhotsk ocean whereas late jurassic granitoids in the great xing an range and in the northern north china craton may have formed in a syn or post collisional setting superposed by far field affects of subduction of the paleo pacific plate early cretaceous granitoids in these areas formed in response to post orogenic extensional collapse of the mongol okhotsk belt coupled with back arc extension related to paleo pacific plate subduction

changes in paleostress state along a subduction zone preserved in an on land accretionary complex the yokonami melange in the cretaceous shimanto belt kochi southwest japan a change in paleostress along a subduction zone plate interface in the shallow portion of a seismogenic zone was detected in an on land accretionary complex the yokonami melange in southwest japan using the microfault inversion method microfaults were classified into two groups based on location those occurring throughout the yokonami melange and those occurring in the goshikinohama fault zone which is considered to be a fossil seismogenic fault and is located at the northern end of the yokonami melange stresses obtained for these classified microfaults indicate that two different stress states exist for each deformation feature one is subhorizontal 1 and subvertical 3 with a smaller stress ratio and the other is subvertical 1 and subhorizontal 3 with a larger stress ratio the difference between these stress states could be related to the horizontal stress change by stress drop after large earthquakes that were recently observed after the tohoku oki great earthquake

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

palaeoecological effects of back arc spreading on miocene benthic communities sw japan arc abstract during the early to middle miocene rapid development of backarc basins now known as the sea of japan occurred in the ne margin of the asian continent then the entire sw japan arc the arc was rotated clockwise very rapidly such an event was associated with the opening of another backarc basin located to the south of the arc immediately after this opening the very young hot oceanic plate began to subduct underneath the arc this subduction caused rapid uplifting and enhanced the denudation of the arc in the post early miocene the lower to middle miocene misaki group is a 3 000 m thick siliciclastic forearc basin fill consisting of wave dominated shallow marine and overlying braid plain deposits its defining features are the low ichnodiversity and paucity of ichnofabrics for instance up to 96 of the total thickness of the measured offshore deposits are non or very slightly bioturbated despite the other normal offshore deposits are generally highly bioturbated although the less bioturbated sediments were previously considered as anoxic deposits anoxic water was unlikely to develop within the highly open wave dominated shallow marine settings the highly active tectonics associated with the spreading of the two backarc basins and the forced subduction most likely caused sediment mass production in the hinterland as evidenced by the sedimentological features of the coeval kuma group and over sedimentation highly frequent and rapid sedimentation that was taken place across the entire misaki basin this over sedimentation supported by a series of evidence such as the pervasive soft sediment deformation across the group estimated rate of net aggradation more than 136 to 230 cm kyr etc most likely induced the colonisation of depauperated fauna and diluted their ichnofabrics across the entire basin the extremely active tectono sedimentation played an important role in the development of the basin wide anomalous benthic ecosystems and ichnofabrics during the early to middle miocene

paleomagnetism of mid cretaceous red beds in west central kyushu island southwest japan paleoposition of cretaceous sedimentary basins along the eastern margin of asia abstract a paleomagnetic study was carried out on the mid cretaceous sedimentary strata in west central kyushu island southwest japan to elucidate the origin of sedimentary basins along the asian continental margin in the cretaceous we collected paleomagnetic samples from a total of 34 sites of the mid cretaceous goshonoura group shallow marine clastic deposits in west central kyushu and characteristic remanent magnetizations were recognized from 18 horizons of red beds thermal demagnetization has revealed that the red beds contain three magnetization components with low

new age constraints on counter clockwise rotation of ne japan summary the miocene basaltic andesitic lavas and cretaceous granites were sampled in the abukuma terrane of northeast honshu for geochronological and palaeomagnetic investigations to determine precise timing of counter clockwise rotation of ne japan characteristic component with unblocking temperature of 560 590 c is isolated from 26 sites of the takadate and ryozen formations and 12 sites of the cretaceous granites concordant northwesterly palaeomagnetic directions d 265 331 are observed in the cretaceous granites of the abukuma terrane the cretaceous mean palaeomagnetic direction from this terrane d 313 5 i 54 6 95 6 3 and n 20 is almost identical to oligo miocene 33 20 ma palaeomagnetic direction reported from the japan sea side of ne japan as well as to the cretaceous palaeomagnetic direction reported from the kitakami terrane this type of behaviour clearly suggests that the abukuma terrane underwent a coherent counter clockwise rotation as a part of ne japan after 20 ma conventional potassium k argon ar dating method performed on the plagioclase grains revealed distinctive ages of 14 44 0 88 and 16 52 0 82 ma for the takadate formation 11 sites and the ryozen formation eight sites respectively this type of behaviour indicates that two independent episodes of volcanic activities were probably responsible for the production of igneous rocks in the takadate and ryozen areas northerly declination is obtained from the takadate formation d 355 3 i 39 4 95 11 6 while slightly westerly deflected declination appears in the ryozen formation d 333 5 i 58 0 and 95 9 6 the here presented results therefore suggest that the province of ne japan was still experiencing a counter clockwise rotation at 16 5 ma however by 14 4 ma the rotational motion has completely ceased compared with the available palaeomagnetic and geochronological data from sw japan rotational motion in both provinces of ne and se japan has already been ceased by 14 ma thus any additional evidence regarding the complete cessation of rotational motions in ne japan will provide a key to understand the timing of opening in the japan sea

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

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

the thickness of the lithosphere from the dispersion of surface waves summary the inversion of regional rayleigh and love wave dispersion data in the period range 30 200 or 250 s leads to an inference that high s wave velocities in the mantle are to be found to depths of the order of 300 km or greater under the ancient precambrian shields of the world under the other parts of the world a low s wave velocity channel is found at depths up to 100 km below the surface the depth to the onset of the low velocity channel is consistent with the onset of partial melting of a mantle of peridotitic composition under the shields there is no evidence for partial melting to depths of the order of 300km no statement can be made from these data about the presence or absence of a low velocity channel in subduction zones if the lithosphere is defined to be that region of the mantle that moves with the same velocity of flow as the surface above it then the pacific and nazca plates have uniform thicknesses of only 100 km for all the other major plates of the world two models can be proposed that cannot be resolved with the present data in the first model all the other major plates including their oceanic parts have uniform thicknesses of at least 300 km and may have thicknesses as great as 700 km in the second model all the other major plates have deep roots under the ancient precambrian shields and an eddy of flow around these roots begins at a depth of about 100 km

p wave velocity structure of the margin of the southeastern tsushima basin in the japan sea using ocean bottom seismometers and airguns abstract the tsushima basin is located in the southwestern japan sea which is a back arc basin in the northwestern pacific although some geophysical surveys had been conducted to investigate the formation process of the tsushima basin it remains unclear in 2000 to clarify the formation process of the tsushima basin the seismic velocity structure survey with ocean bottom seismometers and airguns was carried out at the southeastern tsushima basin and its margin which are presumed to be the transition zone of the crustal structure of the southwestern japan island arc the crustal thickness under the southeastern tsushima basin is about 17 km including a 5 km thick sedimentary layer and 20 km including a 1 5 km thick sedimentary layer under its margin the whole crustal thickness and thickness of the upper part of the crust increase towards the southwestern japan island arc on the other hand thickness of the lower part of the crust seems more uniform than that of the upper part the crust in the southeastern tsushima basin has about 6 km s layer with the large velocity gradient shallow structures of the continental bank show that the accumulation of the sediments started from lower miocene in the southeastern tsushima basin the crustal structure in southeastern tsushima basin is not the oceanic crust which is formed ocean floor spreading or affected by mantle plume but the rifted extended island arc crust because magnitudes of the whole crustal and the upper part of the crustal thickening are larger than that of the lower part of the crustal thickening towards the southwestern japan island arc in the margin of the southeastern tsushima basin high velocity material does not exist in the lowermost crust for that reason the margin is inferred to be a non volcanic rifted margin the asymmetric structure in the both margins of the southeastern and korean peninsula of the tsushima basin indicates that the formation process of the tsushima basin may be simple shear style rather than pure shear style

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

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

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

internal deformation of sikhote alin volcanic belt far eastern russia paleocene paleomagnetic results abstract we present paleomagnetic results of paleocene welded tuffs of the 53 50 ma bogopol group from the northern region 46 n 137 e of the sikhote alin volcanic belt characteristic paleomagnetic directions with high unblocking temperature components above 560 c were isolated from all the sites a tilt corrected mean paleomagnetic direction from the northern region is d 345 8 i 49 9 95 14 6 n 9 the reliability of the magnetization is ascertained through the presence of normal and reversed polarities the mean paleomagnetic direction from the northern region of the sikhote alin volcanic belt reflects a counterclockwise rotation of 29 from the paleocene mean paleomagnetic direction expected from its southern region the counterclockwise rotation of 25 is suggested from the paleomagnetic data of the kisin group that underlies the bogopol group these results establish that internal tectonic deformation occurred within the sikhote alin volcanic belt over the past 50 ma the northern region from 44 6 to 46 0 n in the sikhote alin volcanic belt was subjected to counterclockwise rotational motion through 29 17 with respect to the southern region the tectonic rotation of the northern region is ascribable to relative motion between the zhuravlevka terrane and the olginsk taukhinsk terranes that compose the basements of the sikhote alin volcanic belt

provenance of the hayang group early cretaceous in the yeongyang subbasin se korea and its bearing on the cretaceous palaeogeography of sw japan abstract the lower cretaceous hayang group in the yeongyang subbasin se korea was deposited in a forearc basin formed by the subduction of the palaeo pacific izanagi plate underneath the east asian continent it is characterized by an alternating sequence of reddish sandstones and mudstones deposited in fluvial plain environments sandstone compositions upsequence reveal that the tectonic regimes of source terrains evolved successively from basement uplift to recycled orogen to magmatic arc and back to recycled orogen characteristically sandstones contain abundant plagioclases but rare k feldspars except for sandstones derived from basement sources sandstones derived from recycled orogens are rich in sedimentary rock fragments especially chert fragments which contain radiolarian faunas similar to those of the jurassic accretionary complex located in southwest japan minor continental arc volcanism contributed volcanic materials to the basin during the middle stage of the basin filling during the deposition of the hayang group strike slip fault movements were active due to the oblique subduction of the izanagi plate and caused strong pulsed uplift of the trench slope break areas resulting in supply of accretionary complex derived sediments considering a direct drainage connection between the uplifted accretionary complex in southwest japan and the yeongyang subbasin current cretaceous palaeogeographic reconstructions of the east asian continental margin need to be reconsidered and it is suggested that the collage of tectonic blocks in southwest japan assembled in post early cretaceous time

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

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

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

evolution of mantle structure beneath the northwest pacific evidence from seismic tomography and paleogeographic reconstructions 1 plate motions and subducting slab morphology are intricately connected and through the integration of seismicity tomographic images and relative plate motions the evolution of mantle structure can be interpreted tomographic images of p wave shear wave speed and bulk sound speed perturbations of the northwest pacific region have been interpreted to define the extent and geometry of the subducting pacific plate we have found that the subducted pacific plate beneath the japan and kurile arcs is coherent but is very complex at the junction of the two arcs near the hokkaido corner as the slab subduction angle decreases from north to south while the slab thickens the pacific slab beneath the japan arc is slightly thinner and subducting through the upper mantle at a less steep angle than the slab beneath the kurile arc as the slab reaches the 660 km discontinuity it becomes stagnant and lies in a horizontal position on top of the transition zone both beneath the izu and japan arcs in contrast the slab beneath the kurile arc is subducting at a steeper angle and penetrates through the transition zone into the lower mantle the difference in slab morphology appears to be due to a combination of change in dip of the subducting plate and the convergence velocity of the pacific plate along the margin a new paleogeographic reconstruction of northeast asia was assembled to evaluate the plate motions that could explain the foundation of the current slab morphology the plate reconstruction was based on euler pole motions of the pacific philippine amurian okhotsk and north american plates relative to stable eurasia plate the new tectonic model illustrates the collision of the japan and kurile arcs the opening of the kurile basin disparity in pacific plate velocities along the arc and different rates of trench retreat along the izu japan and kurile segments of the western pacific margin the plate motion model was interpreted in conjunction with the physical properties of the mantle imaged with the p wave and joint tomography to assess the evolution of the pacific plate morphology since the mid miocene and to provide constraints on the plausible plate motions in the region