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

the fold test in palaeomagnetism summary the fold test suggested by mcelhinny is shown to be inappropriate more correctly one should determine whether the mean direction of a group of sites from one limb of a fold may be distinguished statistically from the mean direction of a group of sites from another limb details of the application of this test are given with examples it is shown that the test indicates significance under far less severe folding than does the test suggested by mcelhinny

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

mechanism of backarc opening in the japan sea role of asthenospheric injection abstract the process of backarc extension in the japan sea includes two modes of spreading the parallel and rotational or fan shape opening data from paleomagnetism for rocks on the northeast japan arc radiometric age determination for dredged volcanic rocks and heat flow in the basin have suggested that the parallel opening of the japan sea took place earlier than the rotational opening at 21 14 ma the mechanism of opening of the japan sea has been examined based on the secular change in the geometry of the subducted slab and in the volcanic activity on the northeast japan arc mechanisms of backarc rifting including mantle diapirism mantle flow induced by the slab subduction or interaction of the over and the underriding plates cannot reasonably explain the process of opening of the japan sea the most plausible mechanism operating in the spreading process of the japan sea is injection of the asthenosphere into the mantle wedge that forcefully pushed the subducted slab to cause both steepening of the slab and the backarc extension the asthenospheric current towards the northeast japan arc trench system may have been associated with the large scale upwelling of the asthenosphere beneath the northeastern china region that caused intra plate volcanism and produced continental rift systems in the region the present mechanism can also explain the active backarc rifting in the okinawa trough in the western pacific and the taupo depression in north island new zealand

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

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

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

shallow marine turbiditic sandstones juxtaposed with deep marine ones at the eastern margin of the niigata neogene backarc basin central japan abstract at the eastern margin of the niigata neogene backarc basin two kinds of turbiditic sandstones were concurrently deposited i e deep marine turbiditic sandstones in the west area and shallow marine turbiditic sandstones in the east area the deposition took place in the upper part of the lower pliocene kawaguchi formation data based on sedimentological and heavy mineral analyses revealed that these sandstones were both supplied from the east but from different source rocks through different routes sedimentary structures of the east shallow marine sandstones mimic those of the bouma sequence except for the poor development of the massive sandstones and the existence of finer grained thin parallel lamination in the lowermost part of some sandstone beds these sandstones were supplied from the coastal area and transported offshore through many small feeder channels sedimentary structures of these sandstones seem to suggest the existence of shallow marine turbidity currents such turbidity currents might have occurred under such specific paleogeographic or tectonic conditions as backarc basins with relatively steep and narrow shelves

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

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

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

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

opening of the sea of japan and kuroko deposit formation the opening of the sea of japan began with the bulge at the margin of the asian continent at 21 ma crustal subsidence and basaltic eruption prior to kuroko deposit formation appear to represent the opening event between 16 5 to 15 5 ma was the most probable time of the counterclockwise rotation of ne japan kuroko ore formation is believed to have taken place between 15 4 and 15 2 ma based on geological evidence and fossil ages it is proposed that kuroko deposits formed in a weakly extensional environment at the waning stage of opening of the sea of japan this may be the only model which can account for the occurrence and simultaneous formation of kuroko deposits in japan the process can be ascribed to decreasing porosity of the crust due to the stress change most commonly from extensional to compressional

neogene stress field in sw japan and mechanism of deformation during the sea of japan opening we present new structural data from sw japan and discuss the mechanism of deformation during the opening of the sea of japan we studied the miocene basins at the southern margin of the sea of japan northern coast of sw japan and of the median tectonic line mtl by means of fault slip analysis to determine stress field directions during the opening of the sea of japan in early and middle miocene time on the southern margin the stress field was extensional or transtensional with a nw se trending extension direction the mtl was a normal fault in shikoku during early middle miocene and probably a normal fault with a left lateral component in central japan before having been strained by the collisions of the tanzawa and izu blocks with central japan we combine our results with published data for the sea of japan area and show that early and middle miocene stress field directions are remarkably consistent on the eastern southern and western margins of the sea of japan with hmax trending ne sw and hmin trending nw se the stress regime is transpressional or transtensional on the eastern and western margin and almost purely extensional on the southern margin this stress field distribution is in agreement with the model of the opening of the sea of japan in an extensional transfer zone between two n s right lateral strike slip zones proposed by jolivet et al 1991 analogue modelings and kinematic reconstructions showed that this model accounts for 20 to 30 of clockwise rotation in sw japan while paleomagnetic rotations reached 40 to 50 during the opening we present structural data showing that sw japan was strained during the rotation and did not behave as a rigid block our observations suggest that rotations of small blocks may have occurred we propose that sw japan has been sheared between the right lateral strike slip zones which bound the sea of japan and that the missing 10 to 30 of rotation are to be found in its internal deformation in our model the mtl is a normal fault with a strike slip component rotating in a right lateral shear zone

on the possibility of a widespread remagnetization of pre oligocene rocks from northeast japan and the miocene rotational opening of the japan sea abstract we present new paleomagnetic results from the well dated miyako cretaceous sediments 100 110 ma from northeast japan these results combined with those of tosha 1 yield an in situ characteristic directiond 321 i 54 5 95 4 5 n 14 sites reduced to a reference point at 40 n 142 e this direction is found to coincide with that of most older plutonic and sedimentary rocks of devonian to lower cretaceous age it is also identical with the westerly pre folding direction which is preserved in many oligocene 20 40 ma formations from northeast japan 1 2 in contrast all recent formations 0 17 ma have been magnetized in the direction of the present axial dipole field only the oligocene and miocene results appear to be primary or at least pre folding the miyako sulfide bearing sediments and lower cretaceous 110 125 ma magnetite bearing granites could either still bear a primary magnetization or be completely remagnetized by a low temperature chemical event evidence for such events is now found in many places and as close as south korea available data constrain the oligo miocene history of northeast japan and indicate at least20 30 counterclockwise rotation with respect to mainland asia during the opening of the sea of japan on the other hand the pre 40 ma history of northeast japan is not well constrained and three models are proposed which are compatible with various interpretations of the data none of them can presently document pro oligocene motion of northeast japan with respect to asia the most economical model implies widespread remagnetization we conclude that because of the scarcity of well tested primary magnetization directions the classical bending of the japanese islands rests on weaker grounds than generally realized and that no pre 40 ma apparent polar wander path of the japanese islands can safely be proposed

mesozoic episodic magmatism in south korea and its tectonic implication 1 ne trending granitoid batholiths in south korea are part of the huge phanerozoic granitoid belt in east asia the crystallization ages of granitoid plutons show three episodes of mesozoic magmatism in south korea triassic 248 210 ma jurassic 197 158 ma and late cretaceous early tertiary 110 50 ma with a late jurassic early cretaceous 50 m y hiatus the granitoid ages and kinematic analyses of the mesozoic structures in south korea suggest that the middle jurassic 180 170 ma dextral strike slip ductile shear zones formed by the northwest orthogonal subduction of the izanagi plate whereas the cretaceous sinistral faults formed by the northward oblique subduction of the same plate which generally agree with proposed reconstruction of plate motions albeit uncertain around east asia among the models proposed to explain magmatic lull in a continental arc shallow angle subduction associated with high convergence rate and subduction of oceanic plateaus might be the probable cause for the magmatic hiatus in south korea and japan the huge width 500 km of the east asian granitoid belt may be explained by oceanward retreat of subduction zone in the cretaceous in association with change in the movement direction of the subducting plate

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

volcaniclastic sedimentation in a shallow water marginal basin the early miocene koura formation sw japan abstract the early miocene koura formation sw japan is composed mainly of reworked volcaniclastics and andesite to rhyolite pyroclastics and constitutes part of the volcanic zone of sw japan of that time reworked volcaniclastic sequences are thick and mostly coarse grained and based on conventional facies concepts are interpreted as shoreface to beach deposits turbidites and debris flow deposits thick acid tuffs intercalated in the volcaniclastic sequences individually show internal features characteristic of either subaerial or subaqueous ash flow emplacement or in some cases both andesite pyroclastic facies include surge and fall deposits facies relationships together with the fossil record indicate deposition in a shallow lacustrine environment the deposition rate of the koura formation was probably high enough to rapidly fill the shallow lake but sedimentation was balanced by rapid subsidence of the lake similar to lakes present within the modern volcanic zones of taupo new zealand and hohi japan the koura formation grades upward into marine deposits and may represent the initial volcanism and volcaniclastic sedimentation during the early stages of back arc or intra arc rifting

gravity map and heat flow measurements in indonesia abstract a detailed gravity map 1 5 000 000 in colour of indonesia has been prepared by green et al 1979 a regional base station network was established throughout indonesia and gravity measurements from each of the local island surveys have been incorporated in total some 100 000 gravity values have been used in the production of the map

spatial and temporal evolution of the subducting pacific plate structure along the western pacific margin 1 tomographic images of the subducting pacific plate beneath the izu bonin mariana arc illustrate a progression of geometries from shallow dipping to vertical from north to south along the arc recent advances in technology and inversion techniques have improved resolution of slab structure beneath the western pacific island arcs but reasons for the variation in geometry and morphology are still poorly understood by comparing high resolution tomographic images of the western pacific and updated paleogeographic reconstructions we are able to link the spatiotemporal evolution of the subducting pacific plate back to the mid miocene we have reconstructed tectonic motions along the kurile japan izu bonin mariana arc system and the ryukyu arc to provide an independent additional interpretation of the subducting pacific plate using the most current plate motion data we then investigate the plausibility of our model and three other proposed models based on the interpreted slab structure from tomographic images the new reconstruction agrees with the basic characteristics of former trench retreat models but illustrates the importance of the collision of the ogasawara plateau with the trench in the mid miocene and its subsequent effect on the slab structure at depth and the impact of other aseismic ridge collisions along the plate boundary the combination of evidence in changed physical properties imaged with tomography and the current interpreted slab morphology can be analyzed with the past plate motions to understand subduction zone processes

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

preserved collisional crustal structure of the southern urals revealed by vibroseis profiling the vibroseis reflection profiling component of the urseis 95 experiment provides a high resolution crustal scale image of the unextended southern uralide orogen a marked lateral and vertical variation of reflectivity throughout the entire crust differentiates the former margin of the east european craton to the west from accreted terranes to the east between these regions is a less reflective zone corresponding to the magnitogorsk volcanic arc and the crustal root zone continuous reflections and reflective domains that correlate at the surface with major tectonic features define a bivergent orogen in which the paleozoic collisional structure has been largely preserved

kinematic model for the opening of the sea of japan and the bending of the japanese islands we have analyzed paleomagnetic data from japan reported over the past 25 years in combination with marine geophysical and on land geologic data to reconstruct the opening of the sea of japan and the bending of honshu from these data we propose that the sea of japan opened by a two stage back arc spreading event 1 in the early to middle tertiary japan began to rift off the asian coast as a single rigid block rotated clockwise approximately 25 and formed the japan basin 2 in the miocene the yamato and tsushima basins opened and shortly thereafter 19 ma the japan arc collided with another island arc in central hokkaido at this stage honshu began to behave as two rigid blocks southwest honshu and northeast honshu southwest honshu continued to rotate clockwise approximately 45 as the yamato and tsushima basins continued to open northeastern honshu rotated counterclockwise approximately 40 as hokkaido was held fixed by the arc collision while the yamato basin continued to open these two events the opening of the tsushima and yamato basins and the arc collision in hokkaido resulted in the early to middle miocene bending of the japan arc observed in the paleomagnetic data after 12 ma back arc spreading ceased