regional strike slip in the eastern continental margin of korea and its tectonic implications for the evolution of ulleung basin east sea sea of japan air gun profiles delineate a large scale strike slip fault system along the eastern continental margin of korea this system comprises two right stepping curvilinear master faults hupo and yangsan faults that are closely associated with en echelon nominal faults and reverse faults and folds these structural features indicate two phases of regional deformation in the earlier phase late oligocene to early miocene the margin experienced extensional shear strain with dextral strike slip movements along the hupo and yangsan faults that guided a pull apart opening of the pohangyoungduk basin the later phase deformation late miocene to early pliocene was governed by shortening strain that induced a convergent strike slip reactivation of the hupo fault as well as widespread contractile faulting and folding the regional deformation in the eastern korean continental margin reflects three stages of tectonic evolution of the ulleung basin 1 pull apart opening stage from late oligocene to early miocene 2 rotational opening stage with a differential rotation of the japanese arc in middle miocene and 3 back arc closing stage from the end of middle miocene to present

japan sea a pull apart basin abstract recent field work in the hokkaido central belt and marine geology studies along the eastern margin of japan sea in addition to previously published data lead us to propose a new model of opening of the japan sea the synthesis of both on land and offshore structural data gives new constraints about the structural evolution of the system the rhombohedral shape of the japan basin and the particular tectonic behaviour of the margins on both east and west sides can be explained by an early eo oligocene rifting of a pull apart basin accommodated along two large right lateral shear zones east of korea and west of northeast japan and sakhalin it is followed during upper oligocene lower miocene by the main opening of the japan basin as a mega pull apart then a back arc spreading probably related to the subduction process induced the creation of the yamato and tsushima basins at the end of lower miocene and in middle miocene clockwise rotation of southwest japan larger than 20 or major bending of honshu mainland deduced from paleomagnetic studies is unlikely at this time since 1 or 2 my b p to present compression prevails along the eastern margin of the japan sea the generation of marginal basins as pull apart basins along intracontinental strike slip faults is a mechanism which has been proposed by other authors concerning the south china sea the question then is whether the fragmentation of the asiatic continent is an intracontinental deformation related process as proposed here or a subduction related one

hillslope storage boussinesq model for subsurface flow and variable source areas along complex hillslopes 2 intercomparison with a three dimensional richards equation model 1 the boussinesq equation for subsurface flow in an idealized sloping aquifer of unit width has recently been extended to hillslopes of arbitrary geometry by incorporating the hillslope width function w x into the governing equation where x is the flow distance along the length of the hillslope troch et al 2003 introduction of a source sink term n allows simulation of storm interstorm sequences in addition to drainage processes while a function sc x representing the maximum subsurface water storage can be used to account for surface saturation response in variable source areas activated by the saturation excess mechanism of runoff generation the model can thus simulate subsurface flow and storage dynamics for nonidealized more realistic hillslope configurations in this paper we assess the behavior of this relatively simple one dimensional model in a series of intercomparison tests with a fully three dimensional richards equation model special attention is given to the discretization and setup of the boundary and initial conditions for seven representative hillslopes of uniform convergent and divergent plan shape drainage and recharge experiments are conducted on these hillslopes for both gentle 5 and steep 30 bedrock slope angles the treatment and influence of the drainable porosity parameter are also considered and for the uniform idealized hillslope case the impact of the unsaturated zone is examined by running simulations for different capillary fringe heights in general terms the intercomparison results show that the hillslope storage boussinesq model is able to capture the broad shapes of the storage and outflow profiles for all of the hillslope configurations in specific terms agreement with the richards equation results varies according to the scenario being simulated the best matches in outflow hydrographs were obtained for the drainage experiments suggesting a greater influence of the unsaturated zone under recharge conditions due to transmission of water throughout the hillslope in the spatiotemporal water table response a better match was observed for convergent than divergent hillslopes and the bedrock slope angle was not found to greatly influence the quality of the agreement between the two models on the basis of the intercomparison experiments we make some suggestions for further development and testing of the hillslope storage model

late quaternary transpressional earthquakes on a long lived intraplate fault a case study of the southern yangsan fault se korea abstract the nne ssw striking yangsan fault in southeastern korea has been regarded as one of the most prominent seismogenic structures in the korean peninsula on the basis of instrumental and historical seismicity and paleoseismic records along the fault zone its seismic behavior is however still uncertain due to long recurrence intervals of strong earthquakes and insufficient historical and geologic records we conducted a detailed paleoseismic investigation including an 8 m deep excavation in order to understand recent earthquake faulting events along the southern yangsan fault our observations indicate that at least two and possibly four dominantly strike slip surface faulting events have occurred along a subsidiary fault in the eastern boundary of the fault valley since the late pleistocene stratigraphic features and osl ages of the stratigraphy exposed in the trenches indicate that the most recent rupture s occurred after 29 2 1 4 ka and the timing of earlier ruptures are constrained to between 70 0 3 7 ka and 29 2 1 4 ka using a contact of the oldest dated unit that has at least 7 5 m of vertical separation we calculate a minimum vertical slip rate during the late quaternary of 0 11 mm yr the minimum horizontal slip rate is presumed to be two times that of the vertical slip rate based on striations observed on clasts next to the fault core we also propose that the late quaternary earthquake faulting kinematics along the yangsan fault expressed as contractional dextral slip with east side up geometry is strongly dependent on a pre existing fault zone architecture with a strike of n10 20 e that dips to the east and the direction of neotectonic maximum horizontal stress ene wsw to e w

infiltration on sloping terrain and its role on runoff generation and slope stability abstract a modified green and ampt model is formulated to quantify infiltration on sloping terrain underlain by homogeneous soil wetted by surficial water application this paper s theory for quantifying infiltration relies on the mathematical statement of the coupled partial differential equations pdes governing infiltration and runoff these pdes are solved by employing an explicit finite difference numerical method that yields the infiltration the infiltration rate the depth to the wetting front the rate of runoff and the depth of runoff everywhere on the slope during external wetting data inputs consist of a water application rate or the rainfall hyetograph of a storm of arbitrary duration soil hydraulic characteristics and antecedent moisture and the slope s hydraulic and geometric characteristics the presented theory predicts the effect an advancing wetting front has on slope stability with respect to translational sliding this paper s theory also develops the 1d pde governing suspended sediment transport and slope degradation caused by runoff influenced by infiltration three examples illustrate the application of the developed theory to calculate infiltration and runoff on a slope and their role on the stability of cohesive and cohesionless soils forming sloping terrain

tectonic evolution of the cretaceous gyeongsang back arc basin se korea transition from sinistral transtension to strike slip kinematics abstract the gyeongsang basin the largest cretaceous nonmarine sedimentary basin in korea formed as a continental back arc basin related to nnw directed subduction of the proto pacific plate underneath the eurasia plate the basin can be divided into an earlier formed western back arc depression and a later formed eastern volcanic arc platform we investigated the basin evolution and the associated tectonic settings largely based on an analysis of structures individuated in the field in the back arc region from 127 ma the basin initiated as a nne trending narrow depression bordered by nne striking sinistral faults and then progressively expanded under a transtensional kinematics induced by progressive trench roll back sinistral shearing of inherited nne striking structures played an important role in basin subsidence and secondary wnw to nw striking transverse faults acted as normal faults the nne striking principal displacement zone in the west of the basin runs along the western marginal area of the jinju and daegu domains and passes through the uiseong domain from south to north but most of this zone is now preserved as deep structures volcanic activity starting at ca 115 ma was characterized mainly by episodic basaltic eruptions occurring contemporaneously with back arc deposition of a sedimentary sequence after ca 90 ma a transtensional kinematics changed to a strike slip one and the basin expansion and sedimentation in the back arc region terminated during the strike slip event rhyolitic dacitic volcanism increased in intensity as a large ne trending volcanic arc developed close to subduction zone and its loading caused the stratal flexure in the back arc region and the orientations of the shortening and stretching axes remained nw se and ne sw respectively additionally continuing sinistral shear generated local depressions along the faults located in the west of the back arc region and within the volcanic arc

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

synvolcanic and syntectonic sedimentation of the mixed volcaniclastic epiclastic succession in the miocene janggi basin se korea understanding the variability and complex interplay of volcanism tectonism and sedimentation is crucial for unraveling the evolution of sedimentary basins and volcanotectonic depressions in many settings the miocene janggi basin in se korea formed in association with backarc opening of the east sea sea of japan is filled by abundant volcanic deposits intercalated with fluvio lacustrine sedimentary deposits four lithostratigraphic units units i to iv in ascending order are identified in the southern part of the basin including extensive and voluminous dacitic lapilli tuffs units ii and iv and mixed dacitic volcaniclastic and epiclastic sedimentary deposits units i and iii these units suggest repetitive ignimbrite emplacement onto a braided stream to lacustrine environment paleoflow measurements show that the epiclastic conglomerates were deposited by a north flowing fluvial system that debouched from a drainage area to the south of the basin whereas the ignimbrites were introduced from the east because of the separate source areas the epiclastic and volcaniclastic depositional processes did not interfere with each other resulting in discrete alternations of epiclastic and mostly primary volcaniclastic deposits the studied succession also lacks an aggradation degradation cycle which results from the alternation of periods of rapid volcaniclastic sedimentation and relatively longer periods of fluvial incision the volcaniclastic and epiclastic deposits are therefore interpreted to have accumulated in rapid succession in association with rapid basin subsidence and contemporaneous ignimbrite forming eruptions the studied succession can thus be regarded as a syntectonic sequence in which the stratal geometry was significantly modulated by tectonostratigraphic controls with overprints of syntectonic contemporaneous volcanism

tectonically controlled vent migration during maar diatreme formation an example from a miocene half graben basin in se korea abstract understanding the roles of external controls such as hydrology and tectonics is crucial for interpreting the evolution of a phreatomagmatic volcano and its subsurface structure called the diatreme the yangpori diatreme in a miocene terrestrial half graben basin of korea provides an opportunity to assess the roles of synvolcanic fault movement in forming a maar diatreme volcano the exposed part of the diatreme fill consists of two offset stacked sequences with contrasting lithofacies characteristics a lower sequence early diatreme fill of locally stratified or bedded basaltic lapilli tuff and tuff in the northeastern part and an upper sequence later diatreme fill of dacite clast rich and disorganized tuff breccia in the southwestern part the boundary between these sequences is abrupt suggesting eruption of the volcano in two distinct phases the first phase of eruption was driven by phreatomagmatic explosions mainly within the basin fill some of the diatreme fill was deposited in a local pond by debris flows and suspension settling of fines whereas the rest by fallout from eruption plumes and jets the second phase of eruption was driven by phreatomagmatic explosions at a fracture aquifer within the basement accompanying sudden lowering of the explosion locus the later diatreme fill therefore contains abundant basement derived dacite clasts its deposition was accomplished by en masse fallout of volcanic debris from dense collapsing columns a number of faults and shear fractures were generated within and in the vicinity of the diatreme together with penecontemporaneous basaltic intrusions structural and geochemical analyses show that many of these structures are genetically related with the formation of the diatreme the early diatreme fill was also tilted toward the southwest suggesting southwestward shift of the locus of explosion and diatreme excavation the explosion locus of the yangpori volcano is therefore interpreted to have migrated both vertically and laterally the abrupt lithofacies boundary between the early and later diatreme fills and the proximity of the diatreme to the basin margin suggests that the migration was caused by an abrupt tectonic activity near the basin margin most probably the feeder dike was dislocated by movement along the basin margin fault which created abundant fractures deep within the basement that redirected ascending magma and opened a new hydraulically active zone in the basement synvolcanic faulting is therefore interpreted to have played a crucial role in forming the offset stacked architecture of the yangpori diatreme sequences the faulting is also inferred to have influenced significantly the morphology and internal facies architecture of unpreserved ejecta rimbeds

tectonic evolution of the japanese island arc system abstract growth of the japanese arc system which has mainly taken place along the continental margin of asia since the permian is the result of subduction of the ancient pacific ocean floor backarc basin formation in the tertiary shaped the present day arc configuration the neotectonic regime which is characterized by strong east west compression has been triggered by the eastward motion of the amur plate in the quaternary the tectonic evolution of the japanese arc system includes formation of rock assemblages common in most orogenic belts because the origin and present day tectonics of these assemblages are better defined in the case of the japanese arc system study of the system provides useful insight into orogenesis and continental crust evolution

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

a method for computing infiltration and redistribution in a discretized moisture content domain 1 a new one dimensional infiltration and redistribution method is proposed as an alternative to the richards equation re for coupled surface and subsurface models the proposed method discretizes soil water content into hypothetical hydraulically interacting bins the entry and propagation of displacement fronts in each bin are simulated by means of explicit infiltration and drainage approximations based on capillary and gravitational driving forces wetting front advances within bins create water deficits that are satisfied by capillary driven interbin flow the method inherently provides numerical stability by precluding the need to directly estimate nonlinear gradients through numerical schemes comparisons of the performance of this method against re solutions for theoretical laboratory and field data in both well drained and high water table conditions are presented the new method produces infiltration flux estimates with errors less than 10 compared to a widely used re solution in tests on multiple soil textures with and without high water table conditions while providing unconditionally guaranteed conservation of mass

ponded infiltration into soils bounded by a water table a simple model of ponded infiltration into homogenous soils bounded by and at steady state equilibrium with a water table is presented the model parameterizes the wetting front profile as a tension saturated zone above a zone in which the moisture content decreases linearly with depth application of the model requires numerical evaluation of an integral relating the time since the onset of ponding to the infiltration rate or wetting front position wetting fronts predicted by the simplified model compare favorably with laboratory measurements of vachaud and thony 1971 and for a wide range of soil types with those predicted by finite element integration of the governing flow equations the presence of a shallow water table causes the infiltration rate to decrease faster than it would under semi infinite conditions with uniform initial moisture content infiltration equations for soils bounded by water table may be used in distributed hydrologie models and other applications where the shallow position of the water table significantly affects the partitioning of precipitation forcing during storms

tectonic and sedimentary evolution of the korean peninsula a review and new view abstract this review focuses on the tectonics and sedimentation of major sedimentary basins and orogenic belts late proterozoic neogene in the korean peninsula the korean peninsula is part of the amuria plate and represents an important link between continental blocks of north and south china and the island arcs of japan the basement rocks exposed in the kyonggi and yongnam massifs consist of 2 7 to 1 1 ga high grade gneiss and schist these massifs are separated by the okchon fold belt which comprises metasedimentary rocks and bimodal meta volcanic rocks the stratigraphy of the okchon group is unclear at present the okchon basin was probably initiated as an intraplate rift prior to the late proterozoic the hwanggangri formation clast bearing phyllite most likely represents deposition by subaqueous debris flows in slope environments of an enclosed basin the stratigraphic relationship between the okchon group and the choson supergroup cambro ordovician of the taebaeksan basin is poorly constrained the choson supergroup unconformably overlies the yongnam massif and consists mainly of carbonate sequence that formed mostly in shallow marine and tidal environments reflecting numerous sea level fluctuations the sequence is disconformably overlain by siliciclastic sequence of pyongan supergroup carboniferous triassic which formed most likely in shallow marine deltaic and fluvial environments the imjingang belt is an east trending fold and thrust zone and consists of metasedimentary rocks and volcaniclastics devonian carboniferous underlain unconformably by proterozoic basement rocks late proterozoic amphibolites of oceanic affinity were metamorphosed at about 8 13 kbar and 630 740 c during the late permian to the early triassic the south vergent contraction and top down to the north normal faulting are suggestive of a suture belt between the north china block sino korea craton and the south china block yangtze craton an extension of sulu belt across the yellow sea entire peninsula experienced strong deformation and metamorphism during the suturing event namely songrim orogeny during this orogenic event the kyonggi massif and the okchon basin accreted to the yongnam massif and taebaeksan basin along the south korean tectonic line running northeast southwest a series of northward trending thrust formed along the boundary zone to the east kaktong and kongsuwon thrusts and others piggyback basins locally developed along the thrust faults forming the taedong group the crustal deformation resumed in the early to late jurassic daebo event under contractional setting dextral ductile shearing associated with thrusting and folding continued in the mid southern part of the peninsula it was due to orthogonal northwestward subduction of the izanagi plate under the asian continent the yongnam massif experienced continuous dextral offset along the honam shear zone in the early cretaceous the izanagi plate began to subduct northward and caused formation of strike slip basins in retroarc setting i e kyongsang basin in the southeastern part and a number of small scale basins in the mid southwestern part of the peninsula small scale alluvial fans and fluvial channel networks formed in the basin margin and were transitional to ephemeral lacustrine systems under semiarid to arid conditions extensive intrusion of granitoids occurred from triassic to early tertiary with a gap between 160 and 100 ma representing continental magmatic arc in the tertiary the southeastern margin of the korean peninsula experienced back arc opening pull apart basins formed in the miocene bounded by the yangsan and hupo faults the yonil group sedimentary fill of the pohang basin comprises more than 1 km thick siliciclastic sequence which represents deposition in fan delta systems on the hanging wall of the yangsan fault thick more than 10 km thick sediments in the ulleung basin margin were deformed in the late miocene due to the northward movement of kyushu block quarternary volcanic events in cheju island represent intraplate hot spots

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

current microseismicity and generating faults in the gyeongju area southeastern korea abstract a study of microseismicity in a 15 20 km2 subregion of gyeongju southeastern korea establishes a direct link between minor earthquakes and known fault structures the study area has a complex history of tectonic deformation and has experienced large historic earthquakes with small earthquakes recorded since the beginning of modern instrumental monitoring from 5 years of continuously recorded local seismic data 311 previously unidentified microearthquakes can be reliably located using the double difference algorithm these newly discovered events occur in linear streaks that can be spatially correlated with active faults which could pose a serious hazard to nearby communities at risk infrastructure includes the largest industrial park in south korea nuclear power plants and disposal facilities for radioactive waste the current work suggests that the southern segment of the yeonil tectonic line and segments of the seokup and waup basin boundary faults are active for areas with high rates of microseismic activity reliably located hypocenters are spatially correlated with mapped faults in less active areas earthquake clusters tend to occur at fault intersections microearthquakes in stable continental regions are known to exist but have been largely ignored in assessments of seismic hazard because their magnitudes are well below the detection thresholds of seismic networks the total number of locatable microearthquakes could be dramatically increased by lowering the triggering thresholds of network detection algorithms the present work offers an example of how microearthquakes can be reliably detected and located with advanced techniques this could make it possible to create a new database to identify subsurface fault geometries and modes of fault movement which could then be considered in the assessments of seismic hazard in regions where major earthquakes are rare

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

evolution of the miocene waup basin se korea in response to dextral shear along the southwestern margin of the east sea sea of japan abstract the waup basin se korea is a nonmarine basin created coevally with the back arc opening of the east sea sea of japan during the miocene the basin is bounded by normal faults in the northwestern and southeastern margins and by dextral strike slip faults in the northeastern and southwestern margins a number of intrabasinal faults with dextral strike slip and normal senses of slip are also present delineating three structural domains of the basin i e the ne central and sw blocks the basin fills composed of alluvial to lacustrine deposits and dacitic to basaltic volcanogenic rocks have the geometry of a half graben in the ne and central blocks and a graben in the sw block they are younging from the ne to the sw blocks suggestive of propagating rifting of the basin and associated depocenter migration structural and stratigraphic data suggest that the ne and central blocks were produced by nw se extension during the early miocene 22 17 ma involving northwestward block tilting and subsequent clockwise horizontal block rotation on the other hand the sw block formed at about 17 ma by nw se extension and experienced neither block tilting nor block rotation the reconstructed configuration of the waup basin is that of a pull apart basin produced by nnw directed dextral strike slip stress and associated nw se extensional stress the ages of the basin fills ca 22 to

dips and extension plots of dip measurements taken from syntectonic deposits in extended terranes provide a simple and useful technique that can help to unravel complex deformation histories in the castaneda hills signal area of west central arizona dip analysis has enabled us to identify an unconformity that is at best equivocally displayed in the field this unconformity marks the inception of extension that formed the highly extended terranes het the technique also highlights a later unconformity that marks the onset of basin range br extension and one that formed because extension ceased and through flowing drainage developed northeastward decrease of dips related to het type extension is evidence for a decrease in deformation toward the colorado plateau conversely a mild increase of dips of younger basin fill deposits in that direction suggests that the locus of strongest deformation shifted toward the colorado plateau in br time the variability in amount and timing of tilting displayed by the dip data supports listric and rolling hinge models of extensional tectonism more than the domino model of upper plate het type normal faults dip data and stratigraphic data enable us to construct a sequence of events for middle tertiary extension in the castaneda hills signal area the first phase started ca 27 ma and consisted of upwarp of the area now occupied by the buckskin rawhide metamorphic core complex no rotation of blocks or volcanism accompanied this phase the second phase 19 to 13 ma is characteristic of het type extension and was marked by movement on a basal detachment fault and upper plate normal faults rotation of blocks along near horizontal axes and widespread basalt rhyolite volcanism the third phase about 13 to 7 5 ma is characteristic of br extension and was marked by movement on high angle normal faults minor rotation of blocks and basaltic volcanism the fourth and final phase ca 7 5 to 0 ma saw the waning of tectonism the inception of through flowing drainage related to the colorado river and the effusion of thin megacryst bearing basalt flows the early uplift probably dome like and thermally driven the areal variability of rotation and the migration of deformation toward the northeast do not support the concept that the extended terranes result from simple shear along a low angle fault of crustal dimensions or from a rolling hinge mechanism a more attractive mechanism is a thermal welt in the upper mantle and lower crust that moves northeast with time in a manner analogous to that proposed for the tyrrhenian sea apennine system

gravity and magnetic field interpretation to detect deep buried paleobasinal fault lines contributing to intraplate earthquakes a case study from pohang basin se korea

development of the tanakura strike slip basin in japan during the opening of the sea of japan constraints from zircon u pb and fission track ages abstract the tanakura basin was formed by strike slip movement of the tanakura fault during the opening of the sea of japan in the early to middle miocene in this study we conducted u pb and fission track ft double dating of pyroclastic rocks from two stratigraphic units in the tanakura basin using these ages we constructed a detailed chronostratigraphy and discuss the evolution of the tanakura basin with respect to regional tectonics and the opening of the sea of japan we obtained new ages of 17 1 0 6 ma for the middle part of the asakawa formation and 17 4 0 9 ma for the lower part of the nawashiroda formation based on a comparison of the ages from this study and previously published data we clarify the ages of each formation of the miocene basin fill we suggest that the tanakura basin was formed by sinistral movement on the tanakura fault between 17 2 and 16 7 ma followed by the development of lakes and the deposition of alluvial fans within the basin rapid tectonic subsidence and submarine dacite volcanism occurred just prior to 16 7 ma the basin then began to uplift due to dextral strike slip movement on the tanakura fault at ca 15 7 15 3 ma

structural evolution of the southwestern margin of the ulleung basin east sea japan sea and tectonic implications abstract structural mapping and restoration of depth converted seismic profiles from the southwestern ulleung basin in the east sea japan sea back arc basin reveal the structural evolution of the area since the middle miocene the back arc closure began in the earliest middle miocene ca 15 ma and caused compressional deformation that has propagated southwestward from the dolgorae to the gorae structures and further to the unnamed inversion structure immediately adjacent to se korea the accelerated convergence of the young

paleomagnetism of tertiary rocks on the korean peninsula tectonic implications for the opening of the east sea sea of japan abstract two different distributions of declinations with dual polarities are found in the tertiary rocks of the yeonil basin and yangnam basin along the yangsan fault in the southeastern part of the korean peninsula these are northward declinations in the middle miocene yeonil group and strong eastward declinations in the early miocene yangbuk group and in the eocene volcanic rocks this is interpreted by invoking 1 the yangsan fault as a dextral wrench fault system by spreading in the uleung basin of the east sea sea of japan and 2 a clockwise vertical rotation caused by the movement of the fault at around 17 3 ma just prior to the sedimentation of the yeonil group the timing of the deflection found in korea is distinguished from that in southeastern and northwestern japan at around 15 ma two stages of spreading are thus constrained in the east sea area parallel at 23 15 ma and pivotal at around 15 ma crustal thinning also occurred a reliable pole position 84 9 n 292 6 e d p d m 3 3 4 8 for the miocene about 15 ma in korea is obtained from the yeonil group

early neogene biochemostratigraphy of pohang basin a paleoceanographic response to the early opening of the sea of japan east sea abstract the location of the pohang basin near the korea strait in the southwest of the sea of japan east sea makes this area appropriate for providing a record of paleoenvironmental biotic changes associated with the early neogene 16 5 ma opening of this gateway to the pacific ocean stable isotopic and planktonic foraminiferal records are presented that assist with the understanding of paleoenvironmental changes in the sea of japan resulting from the opening of the korea strait the oldest sediments of early neogene age of the pohang basin overlie cretaceous basement and are early miocene 16 5 ma shallow estuarine facies containing a benthic foraminiferal assemblage dominated by ammonia beccarii l the oldest early neogene planktonic foraminiferal assemblages in the basin are 16 5 ma in age latest early miocene foraminiferal zone n8 the migration of these planktonic assemblages to the sea of japan at that time appears to have resulted from the initial opening of the korea strait the overlying early neogene marine sequence of the pohang basin extends from 16 5 ma zone n8 through 14 ma zone n10 change in the oxygen isotopic record of the pohang basin sequence suggests strong local paleoenvironmental control related to the early opening of the korea strait early middle miocene isotopic temperatures of planktonic foraminifera are relatively cool at 15 ma at a time when global temperatures were high in the middle low latitude regions it was not until 14 8 ma that isotopic temperatures of planktonic foraminifera increased markedly this distinct warming is inferred to reflect the major intrusion of the kuroshio current into the sea of japan probably as a result of further critical opening and deepening of the korea strait at this time planktonic foraminifera increased in abundance reflecting expanding oceanic influence a cooling that followed at 14 5 ma when the strait was well open is unlikely to reflect local tectonic control on the paleoceanography but global cooling during the early middle miocene associated with the expansion of the east antarctic ice sheet

dip analysis as a tool for estimating regional kinematics in extensional terranes abstract in basin analyses that rely almost solely on seismic reflection data to configure the underlying structure it has become common practice to use extensional fault geometries to infer kinematic histories this procedure relies on the existence of fault geometries predicted by ideal orthogonal extension and assumes the structure is well constrained in areas where data are sparse or of equivocal quality this often leads to circular analysis that is ideal orthogonal extension is assumed to have been operative then under constrained fault geometries are mapped according to model predictions fault geometries so derived are subsequently used as evidence that orthogonal extension was operative and that the direction of extension is determinable we develop an alternative tool for inferring kinematics which is independent of fault geometries the method relies on the empirical evidence that the dip of the hangingwall s is systematic and predictable in extensional systems even when fault geometries are non ideal although not without significant limitations dip analysis can provide additional constraints for structural and kinematic interpretations especially where seismic reflection data are the main source of information and dip domains define discrete crustal blocks

synrift stratigraphic geometry in a transfer zone coarse grained delta complex miocene pohang basin se korea the gojusan fan delta is a coarse grained delta complex on the western margin of the miocene pohang basin se korea the deposits consist of five facies associations fa alluvial fan conglomerates fa i shallow marine mouthbar sandstones fa ii fluvial and mouth bar conglomerates fa iii gilberttype foreset conglomerates fa iv and hemipelagic mudstones fa v different facies associations characterize the northern central and southern parts of the delta complex to the north fa iv is laterally juxtaposed with fa i or the basement with scarp derived breccias along the contact centrally and to the south fa i is laterally juxtaposed with fa ii with an abrupt facies change and local inliers of basement rocks along the contact early alluvial fan and mouth bar deposits are overlain by the topset fa iii and truncated by the foreset fa iv of a gilbert type delta in the central part of the fan delta complex whereas fa ii passes transitionally upward into fa iii in the south with the latter extending basinward into a gently inclined shoal water delta front gilbert type and shoal water geometries are therefore developed in the same delta complex the composite delta geometry is interpreted as reflecting 1 its development near an extensional transfer zone where the hangingwall relief was variable over short distances along strike and 2 the operation of intrabasinal faults this interpretation contrasts with previous studies that viewed the delta complex as having formed along a pull apart basin margin

diagenetic study on the neogene sedimentary basin as paleoenvironmental proxy data for an offshore co2 storage project in pohang basin south korea abstract as the importance of reducing greenhouse gas emissions has attracted much attention pohang basin has been identified as a co2 geological sequestration test site in korea this study aims to provide a better understanding of the diagenetic history of pohang basin through mineralogical and microstructural analyses with a focus on tectonic evolution in the reservoirs offshore of the pohang basin during the east sea sea of japan opening in the miocene we focused on two sections in the core that are considered to be target layers for co2 storage samples were analyzed for texture observation and chemical analysis the major mineral components include quartz feldspar illite chlorite kaolinite and calcite and the minor mineral components include pyrite siderite iron oxide and titanium oxide the detrital grains are composed of quartz and feldspar and the primary pore is cemented with kaolinite i calcite pyrite and chlorite the analyzed chlorites are differentiated into five types by their textural characteristics such as pore filling pf pore filling with kaolinite pf k pore lining pl replaced rd and detrital da chlorites and they were all categorized as either brunsvigite diabantite or ripidolite generally the dissolution of feldspar is observed for all samples the diagenetic temperature is estimated by chlorite geothermometry to be 170 c on average which is higher than the present temperature of pohang basin 55 60 c considering all results together the target layers have undergone diagenetic processes from early diagenesis eogenesis to late diagenesis mesogenesis and uplift related diagenesis telogenesis after sedimentation of the pohang basin kaolinite i calcite and pyrite were cemented in the pore and detrital mica was replaced with chlorite as the burial depth increased the dissolution of feldspar advanced the high temperature resulting from the chlorite geothermometry could be influenced by the burial heat as source also the neogene volcanism and related tectonics known as eoil orogeny could become a heat source capable of high temperatures and a high geothermal gradient and tectonically compressional environment may have influenced fluid flow after the uplift kaolinite ii was precipitated in both an intergranular pore and intragranular pore as a result of dissolution the effects of diagenesis such as cementation and dissolution in co2 reservoir rocks could be an important factor in assessing the appropriacy of predicting potential charge after injection in carbon capture and storage project

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

cenozoic intracontinental dextral motion in the okhotsk japan sea region a right lateral shear zone trending northerly along more than 2000 km is recognized from central japan to northern sakhalin it was active mainly during the neogene and has accommodated several hundreds of kilometers of displacement the whole structure of sakhalin is built on this shear zone en echelon sigmoidal folds and thrusts en echelon narrow miocene basins and a major discontinuity which is observed along more than 600 km the tym poronaisk fault characterize the deformation there in hokkaido en echelon folds and thrusts and a ductile shear zone with high temperature metamorphism constitute the southern extension of this transpressional shear zone it continues to the south as a zone of transtensional deformation along the eastern margin of japan sea as en echelon basins and dextral transfer faults observed as far south as noto peninsula and yatsuo basin the style of the shear zone thus evolves from transpressional in the north far from the subduction zone to transtensional in the south in the back arc region strike slip motion along this shear zone was primarily responsible for the dextral pull apart opening of japan sea during the early and middle miocene dextral motion is still active in the north along the tym poronaisk fault in sakhalin as well as on the continental margin of japan sea korea and asia mainland active e w compression replaced the dextral motion along the eastern margin of japan sea in late miocene time and incipient subduction began in the early quaternary

miocene tectonic evolution of the basins and fault systems se korea dextral simple shear during the east sea sea of japan opening crustal deformation of se korea caused by back arc opening of the east sea sea of japan commenced locally in the late oligocene intense deformation occurred during the early miocene accompanying extension of parallelogram shaped pull apart basins between nnw striking principal displacement zones clockwise rotation and northwestward tilting of blocks and southwestward propagating rifting at about 17 ma the crustal deformation suddenly expanded westward and northward owing to activation of the yeonil tectonic line resulting in a dramatic change of depositional environment and the subsidence of wedge shaped pull apart basins the yeonil tectonic line and western border faults of the pohang basin are the westernmost limit of the miocene crustal deformation all the features of crustal deformation in se korea support the pull apart model for the east sea opening however they indicate that the nnw striking faults such as the yeonil tectonic line acted as the principal displacement zones rather than the nne striking yangsan fault under a consistent dextral simple shear at about 16 ma the collision of the philippine sea plate with the japanese islands caused a tectonic inversion this inversion resulted in a compositional change of basaltic magma at about 15 ma and crustal uplift in se korea which caused the cessation of sedimentation at about 10 ma

dip domain method applied to the mesozoic connecticut valley rift basins this paper presents a new approach to structural analysis of u s east coast mesozoic basins a contoured compilation of numerical values of dip data for the connecticut valley rift basins reveals distinct domains related to their broader tectonic development additional more standard geologic constraints are then used to interpret the dip domain patterns most of the tilt deformation involved and hence postdated early jurassic basaltic flow units and overlying jurassic thick alluvial deposits normal drag characterizes the attitude of mesozoic strata along the western border fault of the main hartford basin whereas reverse drag is common along its larger listric eastern border fault strata within the basin mostly along its eastern margin include several subgrabens and or strongly tilted zones associated with early subbasins marked by local changes in strike of the early border fault the meriden cross graben may be a similar structure that extended much farther southwestward across the basin within the hartford basin linear domains of steeper dips are interpreted as surface expressions of the edges of a 5 to 15 km wide late stage central graben that trends n15 e diagonally across the basin possibly a reflection of very late strike slip stress systems

alternate senses of displacement along the tsushima fault system during the neogene based on fracture analyses near the western margin of the japan sea abstract the western margin of the japan sea is characterized by the existence of a n20 e trending transcurrent fault system in the korea strait between korea and kyushu this fault system called here the tsushima fault system played a major role during the opening of the japan sea during the tertiary in order to understand its displacement history we analysed the deformation recorded by neogene rocks exposed on the korea strait islands tsushima goto and hirado islands a synthesis of published stratigraphical and structural data together with our field observations reveals that two tectonic events occurred in succession in the study area an early miocene nw se oriented extension event i and a middle miocene nw se oriented shortening event ii two distinct families of fractures with contrasted stratigraphic occurrences have been recognized in the neogene rocks exposed on the korea strait islands and allow us to define and date the stress fields characterizing each tectonic event stress field i is of intermediate type the vertical axis is either 1 or 2 with a n45 e hmax and n135 e hmin and was active between 22 and 16 ma stress field ii is compressional the vertical axis is 3 with a n135 e hmax and a n45 e hmin and was active between 15 and 10 ma the trends of stress field i principal axes imply that the displacement along the tsushima fault system was dextral during the early miocene conversely the trends of stress field ii principal axes imply that the displacement along the same fault system was sinistral during the second half of the middle miocene the inferred displacement history agrees with 1 a pull apart model for the opening of the japan sea during the early miocene and 2 transition from tensional to compressional stress fields in the japanese arc and back arc region at the end of the middle miocene

miocene pleistocene volcanism and tectonics in southern korea and their relationship to the opening of the japan sea abstract miocene pleistocene volcanism and tectonics in southern korea are discussed the miocene volcanism and related tectonics are represented by the eoil orogeny which occurred in the yangnam and pohang basins in the southeastern corner of the korean peninsula the yangnam and pohang basins represent the uplifted central part of the pohang ulsan graben which was rifted between the proto korean peninsula and paleo ulleung land probably in late oligocene early miocene times the eoil orogeny is correlated with the green tuff orogeny in japan evidence of pliocene volcanism is observed on ulleung island located in the western part of the central japan sea volcanic and tectonic events represented on ulleung island are in chronological order 1 volcanic eruption 2 subsidence and marine transgression 3 dome like uplift and 4 volcanic eruption evidence of pleistocene volcanism is present on cheju island located at the southwestern entrance to the korea tsushima strait volcanic and tectonic events represented on cheju island are in chronological order 1 volcanic eruption 2 subsidence and marine transgression 3 uplift and volcanic eruption and 4 doming and volcanic eruption three grabens namely the pohang ulsan graben the ulleung basin and the yamato basin were formed in the southern half of the japan sea which was above sea level in early miocene times paleo ulleung land was present between the pohang ulsan graben and the ulleung basin in miocene pliocene times these three grabens are active rifts and the tectonic events that occurred in this region originated from the action of rising magma the extension and deepening of the southern half of the japan sea resulted from the tectonic movements of these active rifts and their surrounding regions thus it is suggested that the opening of the japan sea progressed by diapiric tectogenesis

controls and evolution of fan delta systems in the miocene pohang basin se korea abstract the miocene pohang basin southeastern korea occurs in the eastern part of the ne sw trending yangsan fault which has experienced right lateral strike slip deformation since the eocene during the miocene however the strike slip movement was negligible and instead was under extensional regime large scale drainage systems were developed at the junction between the strike slip zone and the oblique transfer faults acting as major sources for coarse grained sediments high rate of sediment supply and rapid subsidence of the hanging wall resulted in the progradation of large scale gilbert type fan deltas doumsan duksung and gohyun systems which all show a radial distributional pattern along the steeply inclined footwall scarps created by ne sw trending faults the drainage systems were relatively small here low rate of sediment supply formed scree apron type malgol and yugye systems and steep faced slope type maesan system fan deltas based on variations in sedimentary facies and distribution architecture and overlapping patterns the fan delta systems in the pohang basin can be divided into four stages of evolution during the early stage early miocene alluvial fans prograded into shallow marine setting in the doumsan and duksung systems high rate of sediment supply and rapid rise of relative sea level resulted in the transformation of shoal water type to gilbert type foresets in the malgol and maesan systems rapid sea level rise accompanied with low rate of sediment supply caused transformation of alluvial fans to scree apron type and steep faced slope type fan deltas respectively in the second stage subsidence of the hanging wall formed large scale truncation surfaces in the doumsan and duksung fan deltas coarse grained sediments were continuously supplied in the doumsan system whereas only fine grained sediments were deposited in the duksung system in the malgol system sediment supply was almost terminated in this stage in the third stage middle miocene sediment supply rate of the doumsan fan delta decreased abruptly and the system was overlain by fine grained gilbert type deposits the duksung fan delta was overlain by the maesan and gohyun systems the maesan system prograded further basinward during the last stage late miocene sediment supply of the doumsan and maesan systems was also terminated and the entire basin was draped by fine grained biogenic materials mainly diatoms

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

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

paleomagnetism of the early miocene kani group in southwest japan and its implication for the opening of the japan sea paleomagnetic directions of a volcanic and sedimentary sequence of the kani basin ranging from 22 to 15 ma show a clockwise declination shift of about 50 degree this result implies that detectable rotation of southwest japan did not occur in this period but after 15 ma the initial opening of the japan sea before 15 ma might have caused a southeastward translation of southwest japan which was then followed by clockwise rotation between 16 and 14 ma

a miocene pull apart deformation zone at the western margin of the japan sea back arc basin implications for the back arc opening mode abstract opening mode of the japan sea is studied from the viewpoint of geologic structure around the western marginal fault zone of the back arc basin latest reflection seismic survey in the fukue basin located at the western end of the rifted sliver of southwest japan delineates an nne ssw transcurrent fault that is an extension of the yangsan fault running along the eastern coast of the korean peninsula as suggested by paleomagnetic studies in korea the yangsan fault was activated with dextral motions during the early miocene rifting event of the japan sea our offshore seismic study indicates that the nne ssw fault zone was simultaneously active with dextral sense because it constitutes a pull apart depression on a right stepping of the fault the fukue basin was under continuous transtensional stress regime during the japan sea rifting accompanied with a clockwise rotation of southwest japan it is discordant from the previous tectonic model of two stage parallel and fan shaped back arc opening which requires compressive regime with sinistral fault motions to the west of rotation pivot of southwest japan in the second stage the southward transportation of the rifted sliver with formation of pull apart basins was the dominant mode along the western margin of the japan sea throughout the opening event the southern part of the marginal fault zone has been immune from post opening tectonic events such as the late miocene strong inversion on the back arc margin of southwest japan or the quaternary e w tectonic stress in the japan sea reactivating the northern sector of the fault zone yangsan fault

wave planation surfaces in the mid western east sea sea of japan indicators of subsidence history and paleogeographic evolution of back arc basin abstract this study focuses on revealing the origin of deep marine wave planation surfaces and its geomorphic and tectonic implications for the back arc evolution based on multi channel seismic reflection profiles acquired from the mid western east sea sea of japan on seismic reflection profiles the wave planation surfaces are recognized as continuous and high amplitude single reflectors outlining the flattened top of submarine terraces or isolated topographic highs the wave planation surfaces are classified into four types depending on the geomorphic characteristics and origins of substratal landmasses i e continental margin terrace type 1 rifted continental fragment type 2 volcanic edifices type 3 and uplifted sedimentary successions type 4 which have common origin of wave erosion near the sea level but different subsidence history the mean subsidence rate of wave planated substratum is calculated by dividing the maximum depth of a wave planation surface below modified datum sea level d e by the time elapsed since the onset of subsidence t information of local subsidence history enables reconstruction of paleogeographic evolution of mid western east sea during the neogene quaternary time 1 incipient continental rifting and onset of marine incursion ca 23 18 ma 2 progressive continental rifting and onset of wave planation ca 18 11 ma 3 final eruption of post rift volcanoes ca 11 ma 4 uplifting of shelf margin banks and onset of the pliocene holocene volcanic activities ca 11 2 5 ma 5 change in the locus of volcanic eruption and cessation of contractile uplifting in the basin margin ca 2 5 0 ma

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

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

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

asymmetrical erosion and morphological development of the central ladakh range northern india abstract variations in erosion were quantified across the topographically and morphometrically asymmetrical central ladakh range in nw india to elucidate erosion and sediment transfer processes across space and time and to gain insight into how mountains erode and evolve morphometric analysis and 10 be cosmogenic nuclide analysis of 14 fluvial sediment samples from active channels in six catchments conducted across the mountain range constrains 100 ka timescale erosion rates for catchments on the northern side of the mountain range and are between 56 12 and 74 11 m ma while catchments on the southern side of the mountain range to between 20 3 and 39 8 m ma for the last 300 ka maximum elevation from swath analysis across the range shows a strong correlation with the elas of 382 contemporary glaciers the higher erosion rate to the north likely relates to tectonic tilting of the central ladakh range and to active rock uplift on the northern side of the range along the karakoram fault morphometric analysis shows that the maximum and average elevations increase at nearly the same rate on a catchment scale across the central ladakh range with higher elevation on the northern side this suggests that greater erosion on the northern side of the range is not keeping pace with rock uplift moreover long term denudational unloading does not play a significant role in the tectonic tilting of the central ladakh range