paleomagnetic evidence for block rotations in central hokkaido south sakhalin northeast asia abstract the eocene to lower miocene strata in the bykov region of south sakhalin island far east russia show a mean paleomagnetic direction of dec 28 7 inc 55 7 95 6 7 and n 22 and a paleomagnetic pole at 66 3 n 245 7 e and a95 8 8 which shows that south sakhalin rotated 25 15 clockwise with respect to northeast asia since the early miocene this pole agrees with that of the eocene to middle miocene sedimentary rocks in central hokkaido japan it suggests that both south sakhalin and central hokkaido have rotated clockwise with respect to northeast asia since the early miocene these clockwise rotations can be interpreted as a vertical axis rotation of the blocks distributed in the north south direction more than 1000 km along the dextral hokkaido sakhalin shear zone considering the development of an en echelon structure of the basins in the shear zone we recognized five blocks each of which had rotated since the early miocene we conclude that this dextral shear is responsible not only for these block rotations but also for the opening of the kuril basin and the japan sea and that the shear motion was caused by oblique convergence between the eurasian and the okhotsk plates since the early miocene

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

thermal and exhumation history of sakhalin island russia constrained by apatite u pb and fission track thermochronology abstract sakhalin island represents a key locality to study the tectonic evolution of the western pacific the island is located at the amur okhotsk plate margin and records a complex thermotectonic history apatite double dating u pb and fission track and thermal history modelling were applied to three late eocene granitoid massifs within central and southern sakhalin the aniva okhotsk and langeri complexes apatite u pb results yield consistent late eocene 40 37 ma ages suggesting rapid post magmatic cooling apatite fission track results reveal bimodal age distributions with late eocene early oligocene 38 33 ma and early miocene 20 17 ma age populations that can be correlated with variations in uranium and chlorine concentrations thermal history modelling translates the aft age bimodality into two phase cooling histories the timing of the early cooling phase 38 33 ma corresponds with the apatite u pb ages indicating rapid cooling to at least 100 c during the late oligocene the second cooling phase at 20 17 ma cooled the samples to near surface temperatures both cooling phases correspond with regional unconformities and subsequent accelerations in sedimentation rate suggesting that cooling was a response to rapid exhumation in addition our data suggests that the studied terranes record differential exhumation with respect to the structural architecture the miocene exhumation pulse is coeval with the timing of transpressional fault displacement and the subsequent opening of the kuril basin

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

block rotation by strike slip faulting structural and paleomagnetic evidence geometric analysis shows that motion of sets of strike slip faults should cause block rotation otherwise the distorted fault domain would not fit with its surrounding the sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults all of which can be obtained from structural data the predictions of the geometric model can then be independently tested by paleomagnetic measurements the model was tested in northern israel structural data reveal several domains of contemporaneous right and left lateral strike slip faults paleomagnetic measurements show that in a domain of nnw trending left lateral faults blocks rotated 23 3 8 2 clockwise and in a domain of right lateral faults blocks rotated 22 4 9 0 anticlockwise these results are in agreement with the prediction of the geometric analysis of the structure the combined results of the heterogenous deformation of the area resembles a pure shear which allows n s extension by about 1 3 in other domains rotations of 34 6 9 1 and 53 1 11 0 were found in their original orientation the left and right lateral faults intersected at angles of 60 70 enclosing the principal axis of shortening in accordance with theories of brittle failure as a result of block rotations this angle is now larger and may reach 110 the results demonstrate large block rotations in different senses in domains of strike slip faults the independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model northern israel is considered to be a good model for this efficient mechanism of intraplate deformation which is expected to occur in similar tectonic settings elsewhere

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

the apparent polar wander path for the north china block since the jurassic summary a palaeomagnetic study was carried out on sedimentary rocks from datong shanxi province and on volcanic rocks from chifeng inner mongolia and zhangjiakou hebei province china all of these sites belong to the stable part of the north china block ncb after detailed demagnetization experiments reliable palaeomagnetic directions were obtained for middle jurassic late cretaceous and late tertiary these data define the apparent polar wander path for the ncb since the jurassic free from the possible effect of the movement along tanlu fault since the mesozoic comparisons with reliable palaeomagnetic poles obtained in siberia and south china indicate that the middle jurassic poles for north and south china are almost the same while the siberian pole is displaced from them to the west by about 18 13 on the other hand the cretaceous poles for the three areas are not significantly different from each other consequently it is suggested that the amalgamation of north china and south china was almost complete in the middle jurassic and that their accretion to siberia continued until the late cretaceous

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

paleomagnetotectonics of east asia in the proto tethys ocean abstract seven paleomagnetic poles are obtained from early ordovician late carboniferous permo triassic jurassic early cretaceous miocene and quaternary rocks in korea these poles define the apparent polar wander path for the southern part of korean peninsula the comparisons of apwps from the north china block yangtze block and korea reveals that the east asia is comprised of two blocks the north sino korean and south sino korean which were parts of gondwana during the paleozoic time the north sino korean block was migrated northwards in late permian and accreted with laurasia in triassic time during middle late triassic time the north sino korean block collided with the south sino korean and subsequently divided into two the western part of the north sino korean block suffered counter clockwise rotation while the eastern part rotated clockwise until jurassic time complicated shear senses of the early mesozoic fault systems in east asia are well matched by this scenario

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

can the okhotsk plate be discriminated from the north american plate the plate geometry in northeast asia has been a long standing question with a major issue being whether the sea of okhotsk and northern japanese islands are better regarded as part of the north american plate or as a separate okhotsk plate this question has been difficult to resolve because earthquake slip vectors along the kuril and japan trenches are consistent with either pacific north america or pacific okhotsk plate motion to circumvent this difficulty we also use slip vectors of earthquakes along sakhalin island and the eastern margin of the japan sea and compare them to the predicted eurasia okhotsk and eurasia north america motions for a model with a separate okhotsk plate we invert 10 eurasia okhotsk and 255 pacific okhotsk slip vectors with pacific north america and eurasia north america nuvel 1 data alternatively for a model without an okhotsk plate those eurasia okhotsk and pacific okhotsk data are regarded as eurasia north america and pacific north america data respectively the model with an okhotsk plate fits the data better than one in which this region is treated as part of the north american plate because the improved fit exceeds that expected purely from the additional plate the data indicate that the okhotsk plate can be resolved from the north american plate the motions on the okhotsk plate s boundaries predicted by the best fitting euler vectors are generally consistent with the recent tectonics the eurasia okhotsk pole is located at northernmost sakhalin island and predicts right lateral strike slip motion on the nne striking fault plane of the may 27 1995 neftegorsk earthquake consistent with the centroid moment tensor focal mechanism and the surface faulting along the northern boundary of the okhotsk plate the north america okhotsk euler vector predicts left lateral strike slip consistent with the observed focal mechanisms on the nw boundary of the okhotsk plate the eurasia okhotsk euler vector predicts e w extension discordant with the limited focal mechanisms and geological data this misfit may imply that another plate is necessary west of the magadan region in southeast siberia but this possibility is hard to confirm without further data such as might be obtained from space based geodesy

late jurassic early cretaceous intra arc sedimentation and volcanism linked to plate motion change in northern japan the sorachi group composed of upper jurassic ophiolite and lower cretaceous island arc volcano sedimentary cover provides a record of late jurassic early cretaceous sedimentation and volcanism in an island arc setting off the eastern margin of the asian continent stratigraphic changes in the nature and volume of the sorachi group volcanic and volcaniclastic rocks reveal four tectonic stages these stages resulted from changes in the subduction direction of the pacific oceanic plate stage i in the late jurassic was characterized by extensive submarine eruptions of tholeiitic basalt from the back arc basin slab roll back caused rifting and sea floor spreading in the supra subduction zone along the active asian continental margin stage ii corresponded to the berriasian and featured localized trachyandesitic volcanism that formed volcanic islands with typical island arc chemical compositions at the beginning of this stage movement of the pacific oceanic plate shifted from northeastward to northwestward during stage iii in the valanginian submarine basaltic volcanism was followed by subsidence the pacific oceanic plate motion turned clockwise and the plate boundary between the asian continent and the pacific oceanic plate changed from convergent to transform during stage iv in the hauterivian barremian in situ volcanism ceased in the sorachi yezo basin and the volcanic front migrated west of the sorachi yezo basin

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

clockwise tectonic rotation of tertiary sedimentary basins in central hokkaido northern japan paleomagnetic data from tertiary sedimentary strata in the yubari and haboro areas of central hokkaido indicate consistently clockwise deflected magnetic declinations the mean declination of middle middle miocene rocks deviates 21 5 from the eurasian reference direction and that of the oligocene to lower middle miocene rocks deviates 30 21 this suggests that these sedimentary basins have rotated 10 30 clockwise about a vertical axis since the middle miocene these clockwise rotations can be explained by a right lateral displacement of a shear zone in central hokkaido this study demonstrates that central hokkaido is not simply a single tectonic unit of northeast japan characterized by counterclockwise rotations from early to middle miocene time

the latest cretaceous early paleogene rapid growth of accretionary complex and exhumation of high pressure series metamorphic rocks in northwestern pacific margin episodic growth of accretionary complexes along the northwestern pacific margin is well documented from detailed biostratigraphy and radiometric dating of metamorphic rocks the latest cretaceous to early paleogene rapid growth of the accretionary complexes observed in hokkaido and sakhalin islands was caused by the supply of a large amount of terrigenous sediments from an intracontinent collision zone the mongolia okhotsk zone in the northern asian continent rapid growth of the accretionary prism resulted in episodic tectonic events in the forearc and magmatic arc rapid underplating caused uplift and exhumation of previously accreted complexes along with high pressure series metamorphic rocks now exposed as the kamuikotan complex in hokkaido and the susunai complex in sakhalin the forearc basin shallowed as a result of the uplift the uplift collapsed the old accretionary complex and supplied olistoliths onto the forearc region magmatic activity along the continental margin was also active at the same time subduction of a large amount of terrigenous sediments into the mantle may have contributed to active magmatism at that time a large supply of water trapped in the subducted sediments entered the mantle and increased partial melting of the mantle wedge

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

palaeomagnetism of the west sakhalin basin evidence for northward displacement during the cretaceous summary a significant margin parallel translation of terranes is postulated by all models for tectonic evolution of the east asian continental margin although the timing and magnitude of displacements of individual elements are poorly constrained as yet the west sakhalin basin a forearc basin associated with the cretaceous volcanic arcs is one of the displaced tectonic elements our palaeomagnetic study of the basin rocks provides the first quantitative constraints on its cretaceous history we identified a characteristic magnetization in the berriasian valanginian basalt representing the oceanic basement of the basin and four characteristic magnetizations in a continuous sequence of the albian to maastrichtian siliciclastic fill of the basin a combination of the positive fold conglomerate and reversals tests supports the primary origin of the remanence in the sediments palaeomagnetic data indicate a gradual shift of the west sakhalin basin from subequatorial latitudes during the early cretaceous to about 40on by the late cretaceous the main phase of the margin parallel migration took place during the early cretaceous and ended by the cenomanian the continuous sedimentary sequence records 50 degrees of progressive clockwise rotation during the late cretaceous

neogene strike slip faulting in sakhalin and the japan sea opening we describe structural data from a 2000 km n s dextral strike slip zone extending from northern sakhalin to the southeast corner of the japan sea satellite images field data and focal mechanisms of earthquakes in sakhalin are included in the interpretation since miocene time the deformation in sakhalin has been taken up by n s dextral strike slip faults with a reverse component and associated en echelon folds narrow en echelon neogene basins were formed along strike sup faults and were later folded in a second stage of deformation we propose a model of basin formation along extension al faults delimitating dominos between two major strike slip faults and subsequent counterclockwise rotation of the dominos in a dextral transpressional regime basins becoming progressively oblique to the direction of maximum horizontal compression and undergoing shortening the association of both dextral and compressional focal mechanisms of earthquakes indicates that the same transpressional regime still prevails today in sakhalin we present fault set measurements undertaken in noto peninsula and yatsuo basin at the southern end of the sakhalin east japan sea strike slip zone early and middle miocene formations recorded the same transtensional regime as observed along the west coast of ne honshu during the early and middle miocene the strike slip regime was transpressional to the north in sakhalin and hokkaido and transtensional to the south along the west coast of ne honshu as far as noto peninsula and yatsuo basin dextral motion accommodated the opening of the japan sea as a pull apart basin with the tsushima fault to the west the opening of the japan sea ceased at the end of the middle miocene when transtension started to change to e w compression in the japan arc subduction of the japan sea lithosphere under the japan arc started 1 8 ma ago the evolution of the stress regime from transtensional to compressional in the southern part of the strike slip zone is related to the inception of the subduction of the young philippine sea plate lithosphere under the japan arc during the late miocene subduction related extension is a necessary condition for the opening of the japan sea two possible mechanisms can account for dextral shear in this area 1 counterclockwise rotation of crustal blocks due to the collision of india with asia 2 extrusion of the okhotsk sea block squeezed between the north america and eurasia plates

stratification effects in the turbulent boundary layer beneath a melting ice shelf insights from resolved large eddy simulations abstractocean turbulence contributes to the basal melting and dissolution of ice shelves by transporting heat and salt toward the ice the meltwater causes a stable salinity stratification to form

collision rotation and back arc spreading in the region of the okhotsk and japan seas the india eurasia collision caused extensive deformation in the eurasia continent since late eocene time we propose that back arc spreading in the okhotsk and japan seas was related to the movement of microplates relevant to the india eurasia collision the kuril basin of the okhotsk sea and the basins of the japan sea were formed as back arc basins simultaneously their late oligocene to middle miocene age is constrained by sediment stratigraphy basement depth and heat flow data the movements of two microplates the okhotsk and amuria plates are significant for the tectonics of the okhotsk and japan seas simultaneous retreat of both microplates from trench hinge lines caused back arc spreading in the kuril basin and the japan sea the amuria plate moved north northeastward due to the india eurasia collision this caused pull apart opening of the baikal rift along a transform boundary between the siberia and amuria plates and a collision along the stanovoy range at its northern margin this movement triggered a clockwise rotation of the okhotsk plate with a component of dextral collision between the okhotsk and amuria plates which is well observed along the central zone of the sakhalin hokkaido islands between the two back arc plates the clockwise rotation of the okhotsk plate is suggested by the southwestward fan shaped opening of the kuril basin the kuril basin narrows to the northeast and terminates south of the kamchatka peninsula where a relative rotation pole between the okhotsk and kuril forearc plates is located the kamchatka peninsula was a zone of collision on the other side of the clockwise rotating okhotsk plate during the opening of the kuril basin the sredinny range in the kamchatka peninsula is one of the results of the collision between the okhotsk plate and the forearc plate of the kuril arc the bending of the japanese islands occurred in association with the back arc spreading of the yamato basin of the japan sea

analytical solutions for whirling groundwater flow in two dimensional heterogeneous anisotropic aquifers analytical solutions are derived for steady state groundwater flow in a heterogeneous anisotropic semiconfined aquifer the aquifer consists of a number of horizontal layers while each layer consists of a number of homogeneous cells with different hydraulic conductivity tensors an exact solution for an arbitrary number of layers and cells is derived using a multilayer approach hypothetical models are used to study how flow is affected by heterogeneity in the anisotropy when the major principal direction of the transmissivity tensor varies between layers streamlines have the shape of spirals bundles of spiraling streamlines rotating in the same direction are called groundwater whirls analytical and numerical models of hypothetical cases produce the same flow patterns of groundwater whirls projected streamlines may be represented by stream function contours which allows for easy interpretation of the main characteristics of complex whirl patterns typical whirl patterns may be identified in relatively simple models clockwise and counterclockwise whirl axes occur at the interfaces of adjacent layers one or more clockwise whirls may occur within one counterclockwise whirl and vice versa the practical consequence of groundwater whirls is that the exchange of water between aquifer layers is intensified this may have a significant impact on contaminant spreading

deformation mode in the frontal edge of an arc arc collision zone subsurface geology active faults and paleomagnetism in southern central hokkaido japan abstract frontal edge of a cenozoic arc arc collision zone in southern central hokkaido is described based on subsurface geology reflection seismic and drilling surveys delineate geologic structure of the n s fold and thrust belt west verging faulted blocks extend as long as 200 km in central hokkaido and the activity of thrust has been propagating westward since the late neogene the cretaceous paleomagnetic data obtained from cores and the affinity in basement lithology implies a connection between the downthrown block of a floor thrust of the collision front and paleo northeast japan arc remanent magnetization of core samples belonging to the upthrown block indicates post oligocene counterclockwise rotation it is opposite to surface rotational motions of the same block then more detail study on fault architecture and block rotation is required to understand the mode of deformation of the fold and thrust belt maturity level of organic matters in a borehole suggests that burial by the thrusting initiated in the quaternary thus the deformation front of the arc arc collision is in an active nascent stage of crustal contraction

crustal scale strike slip deformation in hokkaido northern japan abstract field surveys in hokkaido reveal that the entire tectonic belt in northern japan was a right lateral shear zone from eocene to middle miocene time this zone of deformation can be followed northwards in sakhalin for more than 2000 km and formed at the same time as the opening of the japan sea a thrusting component during the strike slip deformation and later deformation stages led to the uplift of the deep parts of the shear zone exposing a complete section through the upper crust the stress field inferred from the analysis of fault sets is consistent with the non coaxial deformation observed in the deeper ductile parts of the shear zone there is a transition from west to east across the width of the belt 100 km from en echelon folds thrusts and nappes at upper structural levels through the brittle ductile transition with an incipient vertical schistosity into ductile deformation with a vertical foliation and metamorphic recrystallization in the amphibolite facies deep parts of the crust granulites were uplifted and retrogressed during the strike slip movement fault set analysis together with data concerning the direction and sense of nappe emplacement imply that thrusting west of the ductile deformation zone occurred during the strike slip deformation minor flat lying structures coexist with vertical pure strike slip structures this study and previous ones reveal a crustal scale half flower structure with superficial thrusting associated with a deep narrow ductile zone with a high gradient of deformation and temperature

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

the role of the east asian active margin in widespread extensional and strike slip deformation in east asia east asia is a region of widespread deformation dominated by normal and strike slip faults deformation has been interpreted to result from extrusion tectonics related to the india eurasia collision which started in the early eocene in east and se china however deformation started earlier than the collision latest cretaceous to palaeocene suggesting that extrusion tectonics is not the only driving mechanism for east asia deformation it is suggested that the east asian active margin has influenced deformation in east asia significantly along the margin cenozoic back arc extension took place behind several adjoining arcs implying eastward rollback of the subducting slab and collapse of the overriding plate towards the retreating hinge line we show that extension took place along a c 7400 km long stretch of the east asian margin during most of the cenozoic physical models are presented simulating overriding plate collapse and back arc extension the models reproduce important aspects of the strain field in east asia for geometrical and rheological conditions scaled to represent east asia modelling shows that the active margin can be held responsible for deformation in east asia as far west as the baikal rift zone located c 3300 km from the margin

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

oblique subduction and collision forearc tectonics of the kuril arc the tectonics of the southwestern kuril arc are a result of the oblique subduction of the pacific plate at the kuril trench in association with forearc sliver migration caused by the oblique subduction collision tectonics occur at the leading margin of the sliver and tensional tectonics take place at the tapering margin as a result of the collision a deep crustal section of island arc is observed at the leading margin of the forearc sliver tectonics of the kuril arc related to oblique subduction are different from those of the western sunda arc where backarc spreading occurs at the leading margin this difference is due to margin morphology of the oblique subduction zone