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

upper cretaceous paleomagnetic data from shikotan island kuril arc implications for plate kinematics abstract maastrichtian tuffaceous sandstones and siltstones were sampled from four sites in the northern part of shikotan island in the lesser kuril islands a characteristic component chrm isolated from most samples passes the reversal and fold tests and is most probably a primary remanence the chrm mean direction d 334 i 56 k 36 95 3 2 is 14 3 5 steeper than the pacific reference direction and 13 4 more gentle than the eurasian reference direction we assume that the lesser kuril islands together with their southwestern extension into the nemuro peninsula hokkaido were originally an island arc situated at about 36 n in the central west pacific soon afterwards probably by the end of the late cretaceous the nemuro shikotan island arc became inactive and started moving with the pacific plate in the miocene about 15 m y ago the nemuro shikotan island arc collided with the eurasian plate overrode the subduction zone and occupied its present day position on the pacific side of the kuril island arc although this scenario fits the available paleomagnetic data and kinematics of the pacific plate itself it disagrees somewhat with the kinematic models for the north pacific as a whole in particular by the end of the late cretaceous the northern margin of the pacific plate together with the extinct nemuro shikotan island arc should have been in an area close to the transform or ridge type kula pacific boundary the available paleomagnetic data from northeast asia however indicate that active islands arcs existed in mid northern latitudes of the modern pacific ocean in the cretaceous

diagenetic barium cycling in black sea sediments a case study for anoxic marine environments high resolution sedimentary records of major and minor elements al ba ca sr ti total organic carbon toc and profiles of pore water constituents view the mathml sourceso42 ch4 ca2 ba2 mg2 alkalinity were obtained for two gravity cores core 755 501 m water depth and core 214 1686 m water depth from the northwestern black sea the records were examined in order to gain insight into the cycling of ba in anoxic marine sediments characterized by a shallow sulfate methane transition smt as well as the applicability of barite as a primary productivity proxy in such a setting the ba records are strongly overprinted by diagenetic barite baso4 precipitation and remobilization authigenic ba enrichments were found at both sites at and slightly above the current smt transport reaction modeling was applied to simulate the migration of the smt during the changing geochemical conditions after the holocene seawater intrusion into the black sea based on this sediment intervals affected by diagenetic ba redistribution were identified results reveal that the intense overprint of ba and baxs ba excess above detrital average strongly limits its correlation to primary productivity these findings have implications for other modern and ancient anoxic basins such as sections covering the oceanic anoxic events which ba is frequently used as a primary productivity indicator our study also demonstrates the limitations concerning the use of baxs as a tracer for downward migrations of the smt due to high sedimentation rates at the investigated sites diagenetic barite fronts are buried below the smt within a relatively short period thus relict barite fronts would only be preserved for a few thousands of years if at all

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

chemistry of waters from furnas volcano sao miguel azores fluxes of volcanic carbon dioxide and leached material abstract the well constrained hydrology of the furnas caldera permits the quantification of the precipitation evapotranspiration runoff and the groundwater discharge out of the volcanic structure chemical composition of the discharge in furnas has been stable for at least a century a hydrogeochemical survey and discharge measurement of the springs and runoff in furnas allow estimates of chemical fluxes from the volcanic structure a tectonically controlled lineament of high carbon dioxide flux has been identified across the caldera floor three water types are encountered in furnas hydrothermal carbonated and cold waters the hydrothermal waters originate from shallow aquifers which derive their heat from local intrusions at shallow depth 100 200 m at about 160 c the carbonated waters some of which are steam heated by discharge from underlying thermal aquifers occurs above the carbon dioxide anomaly across the caldera floor extensive leaching in some cases stoichiometric of glassy volcanic rocks by these mildly acid carbonated waters supplies most of the dissolved solids in furnas waters chemical flux estimates for the volcano show that carbon dioxide 9358 tons yr leached silica 3994 tons yr and leached sodium 2628 tons yr are the dominating mobile components of the system

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

late cretaceous to early paleogene paleomagnetic results from sikhote alin far eastern russia implications for deformation of east asia abstract welded tuffs in the bogopol and sijanov groups were sampled at 27 sites from 12 caldera formations in the sikhote alin mountain range around kavalerovo 44 3 n 135 0 e for chronological and paleomagnetic studies k ar age dates show that the welded tuffs erupted between 66 ma and 46 ma all sites yield reliable paleomagnetic directions with unblocking temperatures higher than 560 c the high temperature component at 12 sites and the medium temperature component at 3 sites in the bogopol group show reversed polarity d 193 7 i 57 6 95 8 1 the high temperature component at 11 sites in the sijanov group showed both reversed and normal polarities and its mean direction reveals no detectable deflection from north d 2 9 i 59 6 95 11 2 the combined paleomagnetic direction of the two groups yields a paleomagnetic pole of 250 5 e 84 1 n a 95 8 8 which falls near cretaceous paleomagnetic poles from outer mongolia inner mongolia the north china block and the south china block the sikhote alin area appears not to have been subjected to detectable motion with respect to east asia since about 50 ma this implies that the sikhote alin area behaved as an integral part of east asia during the opening of the japan sea at about 15 ma however significant separation between the paleomagnetic poles of east asia and europe during the jurassic paleogene implies a major relative movement between these two blocks since the paleogene

tectonic analysis and the evolution of a curved collision belt the hsuehshan range northern taiwan abstract detailed studies of fracture patterns and paleostress reconstructions based on analyses of fault slip data sets allow identification of four main patterns of compressional paleostress orientations in the cenozoic rocks of northern taiwan these patterns correspond to five successive tectonic events dominated by 1 n s 2 wsw ene 3 wnw ese 4 nw se and 5 wnw ese directions of compression the first two events are characterized by relatively minor structures in the foreland fold and thrust belt the first event probably reflects major left lateral shear along nne ssw trends which occurred in the adjacent eastern metamorphic belt during the middle late miocene these first two events do not affect the plio pleistocene in the localities studied in contrast the last three events created the major compressional structures of the foreland belt during the plio pleistocene collision in taiwan preliminary studies suggest that the magnetic fabric of the rocks has been significantly influenced by compressional deformation along wnw ese and nw se trends especially in the slate belt thus confirming the importance of these last three events on the basis of paleomagnetic work in progress the northernmost segment of the taiwan belt has been affected by a clockwise rotation of about 20 taking into account the distribution of successive compressional paleostress patterns we conclude that of the 55 angle of the sharply curved belt of northern taiwan about 20 probably correspond to actual clockwise rotation during the plio pleistocene whereas the remaining 35 reflect the initial shape of the curved plio quaternary belt the paleostress history of northern taiwan is examined in terms of geodynamic evolution and kinematic reconstructions the plio quaternary development of the collision between the luzon arc and eurasia followed a late miocene predominantly transform motion from south to north of the philippine sea plate relative to eurasia the succession of plio pleistocene paleostress patterns in northern taiwan is found to be consistent with the longitudinal shift from northeast to southwest and the transverse migration from the inner to the outer zones of compressional phenomena related to the evolution of oblique collision some major changes in compressional trends are also accounted for by variations in the kinematics of plate convergence the most recent charge comprises a counterclockwise change in the direction of philippine sea plate eurasia convergence

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

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

a late cretaceous paleomagnetic pole from koto rhyolite southwest japan implications for eastern margin deformation of asia 1 a paleomagnetic pole was obtained from late cretaceous rhyolitic ash flow tuffs and associated ring dikes which are collectively called the koto rhyolite in the eastern part of southwest japan characteristic remanent magnetizations were isolated from 33 sites through thermal demagnetization normal and reversed polarity magnetizations were found and are mutually antipodal although the magnetization directions become more dispersed after tilt correction the clearly defined polarity sequence and the positive contact test between ash flow tuffs and ring dikes ensure that the characteristic magnetizations of ash flow tuffs predate the intrusion of associated ring dikes since no regional tilting is expected we regard the in situ magnetization directions to represent the late cretaceous 70 ma paleomagnetic field d 84 6 i 57 5 95 5 5 in eastern southwest japan the paleomagnetic pole is not significantly different from the coeval poles of western southwest japan this result gives a further support to a microplate model for the miocene clockwise rotation of southwest japan by canceling out the effect of the miocene rotation we were able to obtain a paleomagnetic pole of 70 ma 219 7 e 63 0 n 95 12 2 for southwest japan when the japan arc was incorporated in the eastern margin of asia paleomagnetic poles of late cretaceous age from korea south china coast and restored southwest japan are statistically indistinguishable from each other but significantly differ from that of the continental interior of east asia the observed rotation of about 20 probably results from shear deformation exerted by northeast trending strike slip faults along the eastern continental margin of asia

40ar39ar ages of igneous rocks recovered from daiichi kashima and erimo seamounts during the kaiko project abstract ten igneous rocks recovered from the daiichi kashima seamount the erimo seamount and near the japan trench with the use of the submersible nautile were dated by the 40 ar 39 ar and k ar methods the daiichi kashima seamount is estimated to have been formed about 120 ma based on the 40 ar 39 ar plateau ages of recovered rocks this result is consistent with the fossil age estimated from the limestone covering the top of the seamount a plateau age of 103 9 ma was obtained for a sample of the erimo seamount comparing the 40 ar 39 ar ages of these seamounts and the ages of the surrounding ocean floors based on magnetic anomaly patterns these seamounts are supposed to have been formed by the off ridge volcanism ages of rocks near the japan trench were determined to be 56 59 and 80 ma considering the reported age data and the circumstances of the sampling site these rocks are inferred to have not extruded at the sampling site

south kitakami region an allochthonous terrane in japan the pre cretaceous geology of the japanese islands may be interpreted as an accretion complex in which many terranes of several origins are juxtaposed and a number of allochthonous blocks are involved the south kitakami and abukuma regions of northeast japan and probably also silurian and devonian blocks in the kurosegawa fault zone and the border fault zone of the hida metamorphic terrane are the allochthonous blocks the stratigraphic sequence and paleobiofacies of the regions are similar to each other and their lithologic characters are of continental affinity but however are quite different from those of the other terranes having oceanic characters the south kitakami and abukuma regions have been fragments of a lost continent less remotely situated to gondwana during the middle paleozoic and may have drifted and incorporated with the other part of the japanese islands before late mesozoic to early tertiary igneous activity

tertiary paleomagnetism of north and south china and a reappraisal of late mesozoic paleomagnetic data from eurasia implications for the cenozoic tectonic history of asia abstract to contribute to the understanding of the cenozoic tectonic history of china and eastern eurasia in general we collected paleomagnetic samples from tertiary rocks in both north and south china our miocene paleomagnetic data for northern china are from four widely separated basalt sections in the inner mongolia autonomous region and shandong province both normal and reversed polarities of the characteristic magnetization are present in the stratigraphie sequence and the mean normal and reversed directions are closely antiparallel to each other the fact that the same characteristic direction was carried by both magnetite and hematite suggests that the hematite was probably formed by auto oxidation processes during primary cooling of the basalts the paleomagnetic data from these four areas also pass a regional generalized fold test the corresponding mean miocene paleomagnetic pole for northern china is longitude 261 6 e latitude 88 3 n a 95 6 4 our early tertiary paleomagnetic results for south china are from two sandstone sections around the city of nanning 22 7 n 108 3 e in guangxi zhuang autonomous region high temperature characteristic directions isolated in 31 samples from five sites pass both tilt and reversal tests the paleomagnetic pole position determined from the five sites lies at longitude 228 0 e latitude 83 6 n a 95 8 5 the tertiary paleomagnetic poles for north and south china are statistically indistinguishable from each other and thus confirm that north and south china were fully assembled at least by the tertiary our new tertiary paleomagnetic data are consistent with the hypothesis that the major continental blocks of north and south china were in the same position as today during the tertiary a critical evaluation and selection of paleomagnetic results from the late jurassic onward for siberia europe and north and south china further corroborate this hypothesis most previously existing jurassic and cretaceous paleomagnetic poles from europe siberia are of uncertain reliability and should not be used for tectonic reconstruction using poles derived from transferring reliable data from other continents we show that the previous paleomagnetic inferences of large eastward displacements between these chinese blocks during the cenozoic are no longer warranted the available paleomagnetic data that meet our acceptance criteria are also of interest concerning the boundary conditions of the propagating extrusion model for asian tectonics additional mesozoic and cenozoic paleomagnetic data from siberia and north and south china are still needed to resolve the smaller second order displacements that appear to characterize the cenozoic tectonic history of this part of asia

neocene stress field of the japanese arcs and its relation to igneous activity abstract the neogene stress field in the southern part of northeast japan and the eastern part of southwest japan was restored with special reference to its time scale based mainly on k ar ages and strike directions of dike swarms the present study indicates the changes of stress field at about 22 ma b p 15 ma b p 12 ma b p and 8 ma b p in the southern part of northeast japan and at about 15 ma b p in the eastern part of southwest japan the stress changes are also thought to have occurred at about 12 ma b p 9 ma b p and 1 ma b p in the eastern part of southwest japan these results imply that some common causes generated the stress changes in both northeast and southwest japan at about 15 ma b p 12 ma b p and 8 9 ma b p major igneous activity in the japanese arcs seems likely to have taken place almost at same times as the stress changes the history of volcanic activity in the northwest pacific region during the miocene also shows the temporal correlation with the change of the stress regime the tectonic evolution in subduction zones therefore appears to be characterized by the synchronism of stress changes and intensive igneous activity

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

secondary remanent magnetization carried by magnetite inclusions in silicates a comparative study of unremagnetized and remagnetized granites abstract magnetic carriers in remagnetized cretaceous granitic rocks of northeast japan were studied using paleomagnetism rock magnetism optical microscopy and scanning electron microscopy sem by comparison with unremagnetized granitic rocks the natural remanent magnetization nrm of the remagnetized rocks is strong 0 3 1 7 a m and shows a northwesterly direction with moderate inclination nw remanence whereas the unremagnetized rocks preserve weak nrm

amount of clockwise rotation of southwest japan fan shape opening of the southwestern part of the japan sea abstract paleomagnetic measurements have been carried out on welded tuffs ranging in age between 58 ma and 112 ma from yamaguchi and go river areas in the central part of southwest japan the new data together with those of younger igneous rocks published previously define the change of paleomagnetic field direction during the late mesozoic cenozoic period for southwest japan the paleomagnetic direction from this area has pointed 56 3 clockwise from the expected field direction estimated from apwp apparent polar wandering path of the whole of eurasia during the period between 100 and 20 ma in comparison with the expected one from the eastern margin of eurasia korea china siberia the cretaceous field direction of southwest japan shows the clockwise deflection by 44 49 these results establish that while the eastern margin of eurasia including southwest japan was rotated more or less with respect to the main part of eurasia during last 100 ma southwest japan was rotated clockwise through more than 40 with respect to the eastern margin of eurasia since 20 ma the large amount of rotation for southwest japan implies that it is rotated by an opening of the southwestern part of the japan sea which widens northeastward fan shape opening the tectonic feature of southwest japan and the japan sea is analogous to that of corso sardinia and the ligurian sea in the mediterranean indicating that the fan shape opening is a specific feature of the rifting of the continental sliver at the continental rim

origin and motion history of the philippine sea plate abstract the philippine sea plate is the one major plate whose tertiary motion is poorly constrained and whose origin is problematical its southern boundary is the sorong fault system which is part of a major left lateral fault system at the northern margin of the australian plate the southern part of the plate in eastern indonesia has been neglected in most syntheses but includes some of the oldest rocks within the plate which are separated from remnant arcs of the daito ridge province of the northern philippine sea by the west philippine central basin the east indonesian islands of the halmahera waigeo region contain a good mesozoic and tertiary stratigraphic record indicating a long arc history for the southern part of the plate new palaeomagnetic data from these islands define two sub areas an area forming part of the philippine sea plate north of the sorong fault and an area within the sorong fault system the area north of the fault records a long term clockwise rotation history whereas that within the fault zone records local rotations interpreted as due to deformation at the plate edge rocks of philippine sea plate origin within both areas record similar latitudinal shifts the rotation of the area north of the sorong fault is considered to represent the motion of the southern part of the philippine sea plate the new data indicate large tertiary clockwise rotations similar to earlier suggestions for other parts of the plate but record a discontinuous and more complex motion history than previously suggested for the southern part of the plate there was 40 rotation with northward translation between 0 and 25 ma no significant rotation between 25 and 40 ma and there was 50 rotation with southward translation between 40 and 50 ma we show that the new palaeomagnetic data form part of a single set with earlier palaeomagnetic data from elsewhere in the plate the translation history of the southern part of the plate in eastern indonesia can be reconciled with northward motions recorded elsewhere and can be used to determine rotation poles for the plate 15 n 160 e for the interval 5 25 ma and 10 n 150 e for the interval 40 50 ma reconstructions based on these poles predict that at 45 ma the palau kyushu ridge had a wnw ese orientation which is very different from that postulated by many models used to explain the widespread boninite volcanism in the izu bonin marians forearc at this time the long arc history of the southern part of the plate and the reconstructions based on the rotation poles calculated from the palaeomagnetic data favour an origin for the west philippine basin by spreading in a backarc basin

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

tectonic structure across the accretionary and erosional parts of the japan trench margin structure sections across the japan trench from prestack depth migrated seismic data define the accretionary prism a wedge shaped seaward end of the continental framework and a tectonized middle slope between them the 10 km wide accretionary prism is underthrust by sediment remaining with the subducted oceanic crust structure beneath the steep middle slope more nearly resembles the adjacent continental margin than the accreted mass this deformed middle slope domain forms a buffer between the compliant accretionary wedge and a more rigid older continental framework along the plate boundary subducted strata thicken beneath the middle and upper slopes and where last imaged are four times the input thickness beneath the accretionary prism this interplate stratified layer appears to control interplate friction because the layer is imaged 45 km landward from the trench axis to 12 km depth and in that expanse few upper plate earthquakes are recorded erosion of the base of the upper plate beneath the middle continental slope is indicated by margin subsidence and material flux this erosion is concurrent with accretion at the front of the margin

cenozoic northward translation of the kitakami massif in northeast japan paleomagnetic evidence abstract paleogene and early cretaceous welded tuffs have been sampled from eleven sites at the omoe peninsula 39 6 n 142 0 e in the kitakami massif of northeast japan for paleomagnetic study characteristic paleomagnetic directions with a high unblocking temperature component above 560 c are isolated westerly declinations with shallow inclination are identified after tilt correction in the 62 71 ma heizaki volcanics d 283 5 i 9 4 95 8 0 and in the 114 119 ma harachiyama formation d 274 2 i 20 4 the shallow inclination which was previously reported from early cretaceous plutonic rocks without tilt correction in the kitakami massif has been discovered in the tilt corrected paleomagnetic directions the presence of both normal and reversed polarities suggests reliability of this shallow and westerly paleomagnetic direction the shallow inclination indicates that the kitakami massif was located at low latitude 5 4 n at 71 62 ma taking into account a reference paleomagnetic pole of the paleocene expected from sikhote alin the kitakami massif was translated northward over 25 in latitude since 65 ma the kitakami massif can be subjected to latitudinal northward motion more than 24 during the last 70 ma when it was transported by push of the moving pacific plate and reached the asian continental margin near sikhote alin later than 30 ma we conclude that the kitakami massif was incorporated into northeast japan at some time between 30 and 22 ma after the cenozoic northward translation

differential rotation of the eastern part of southwest japan inferred from paleomagnetism of cretaceous and neogene rocks paleomagnetic directions have been determined on late cretaceous and miocene rocks in the nohi area in the eastern part of southwest japan in an effort to clarify within block deformation of the rotated part of southwest japan twenty two sites in the area are found to have a reliable primary magnetic component through progressive demagnetization tests using thermal and alternating field methods tiltcorrected paleomagnetic data in the nohi area are divided into three stages based on geologic age late cretaceous early miocene 23 15 ma and middle miocene 12 10 ma the formation mean direction of 10 sites obtained from late cretaceous nohi rhyolite is d 17 0 i 49 8 and 95 6 1 neogene rock units underlain by nohi rhyolite show two distinct formation mean directions d 13 0 i 52 7 and 95 7 2 for eight sites of early miocene and d 2 5 i 55 6 and 95 6 6 for four sites of middle miocene differential rotation is proposed between the eastern part of southwest japan including the nohi area and the central part of southwest japan which is known to have rotated clockwise through about 50 around 15 ma the spatial distribution of contemporaneous paleomagnetic data in southwest japan suggests that two differentially rotated parts are connected in the western neighborhood of the nohi area it is concluded that the nohi area had rapidly rotated counterclockwise through 48 relative to the central part of southwest japan between 15 and 12 ma together with previous paleomagnetic data from early miocene rocks in the kanto area which is situated to the east of the nohi area the present results suggest that differential rotation of the eastern part of southwest japan is attributed to the formation of northward cuspate shapes of pre neogene terranes which constitute the framework of the japan arc one probable explanation of the formation of cusps is a crustal indentation caused by the collision of landmasses on the philippine sea plate against the japan arc in the middle miocene