a review of magmatism and deformation history along the ne asian margin from ca 95 to 30 ma transition from the izanagi to pacific plate subduction in the early cenozoic abstract previous plate reconstruction in the paleo pacific domain indicates that the izanagi and pacific plates influenced the ne asian continental margin during the mesozoic and cenozoic respectively however it remains unclear how and when the transition from izanagi to pacific plate subduction occurred because most evidence came from the oceanic crust however the onshore geological records are poorly understood so in order to solve this problem we review recently published data and identify three magmatic episodes a magma gap and four deformation stages from ca 95 to 30 ma margin parallel izanagi pacific ridge subduction is proposed to explain the changing magmatic and deformation signatures and other regional events in the early cenozoic from ca 95 to 65 ma the n or nnw directed subduction of the izanagi plate triggered sinistral shearing along the nne striking faults arc magmas at that time incorporated older crustal materials from ca 65 to 55 ma the nww striking transform faults and ridge segments were subducted developing small slab windows and facilitating the input of juvenile magmas local extension was active above these windows during ca 55 to 45 ma the ridge was totally subducted and a margin wide slab window was opened resulting in an arc magma gap and extension massive asthenosphere upwelling caused the extensive accretion of juvenile materials from ca 45 30 ma the dextral nne trending faults resulted from the w directed subduction of the pacific plate new arc magmas were generated from juvenile materials and thus are isotopically depleted

secondary palaeoremanence of tethyan sediments from the zanskar range nw himalaya summary palaeomagnetic investigations have been carried out on tethyan sediments from the nw zanskar range 33 9 n 76 5 e a total of 455 oriented samples from 41 sites were drilled from four stratigraphic units of middle cretaceous to early eocene age shillakong formation marpo limestone spanboth formation stumpata quartzarenite spanboth formation kong formation a characteristic remanence chrm could be isolated for 27 sites 254 specimens through detailed thermal and alternating field demagnetization and multicomponent analysis all the chrm directions identified are similar irrespective of geological age they fail the fold test and yield an overall in situ mean direction of d 336 6 i 39 6 with 95 5 9 and k 22 9 this corresponds to a palaeopole position at 66 5 n 323 5 e n pole isothermal remanence irm acquisition and thermal demagnetization of a saturation irm identify pyrrhotite as the dominant ferrimagnetic mineral and carrier of the chrm the pyrrhotite remanence is probably a thermoremanent magnetization acquired when the temperature of low grade metamorphism decreased below about 300 c the chrm directions are thus clearly secondary in origin and a negative conglomerate test carried out on the kong formation demonstrates that remanence acquisition post dates the early eocene the mean chrm inclination suggests that the remanence was acquired at about 20 n cooling events in the higher himalayan crystallines indicate an oligocene age for this remanence acquisition the mean chrm declination shows a counterclockwise rotation of 23 4 since remanence acquisition assuming remanence acquisition at about 30 myr ago a slight counterclockwise rotation of about 10 relative to the stable indian plate can be concluded this does not fit with the general pattern of palaeomagnetic results from neighbouring areas within the western syntaxis of the himalaya from which a clockwise rotation relative to the stable indian plate is expected for the zanskar range

cenozoic biostratigraphy of the pogranichnyi region eastern sakhalin russia abstract diatom assemblages from the oligocene through late miocene siliceous and terrigenous formations of the pogranichnyi region of the east sakhalin basin are documented with particular attention given to the recognition of diatom zones from the oligocene and early miocene in the pilengskaya and borskaya formations improvements in the diatom biostratigraphic zonation scheme for the north pacific have made it possible to recognize oligocene rocks that may have previously been reported as unknown or as lower miocene deposits this paper is the first to more fully describe the evidence for more widespread oligocene and lower miocene siliceous deposits in the northwestern pacific than previously thought

sedimentology csfe relationships and stable isotopic compositions in devonian black mudrocks mackenzie mountains northwest territories canada abstract an integrated approach combining c s fe relationships stable isotopic compositions and lithofacies characterization was utilized to constrain the palaeoenvironmental and early diagenetic conditions of middle upper devonian eifelian frasnian mudrocks from the mackenzie mountains northwest territories canada these rocks include the hare indian formation informally subdivided into the lower bluefish member and the hare indian upper member carcajou marker and canol formation the bluefish member is dominated by black laminated organic rich shales toc 0 35 10 34 wt av 5 83 wt with moderate degrees of pyritization dop of 0 34 0 67 av 0 55 these mudrocks were deposited in dysoxic marine bottom waters that became progressively more oxygenated with time variations in toc dop and organic matter 13cpdb values 29 7 to 19 9 av 27 2 are attributed to repeated clastic dilution and increased input of terrestrial organic matter in association with shallowing upward ramp clinothem cycles pyrite 34scdt values 32 7 to 18 8 av 24 9 indicate an open system bacteriogenic seawater sulphate reduction conversely the overlying hare indian upper member characterized by clinothem facies is composed of grey to green mudstone with minor argillaceous limestones and considerably less organic matter contents toc 0 28 2 99 wt 13c 29 5 to 22 5 deposition occurred in oxic to slightly dysoxic waters dop 0 20 0 54 34s 23 0 to 20 9 depending on the palaeotopographic location along the depositional slope a rapid rise in sea level drowned the carbonate ramp member of the ramparts formation and produced the thin organic rich carcajou marker bottom water stagnation that resulted from subdued ramp palaeotopography produced anoxic sea bottom black laminated organic rich shales from the canol formation toc 1 37 6 68 wt are very similar to those of the bluefish member and are likewise basinal sediments however toc dop and organic matter 13cpdb values 29 1 to 20 8 av 26 2 do not show pronounced variations and indicate that low energy quietwater conditions persisted over relatively long uninterrupted periods of time high dop values 0 72 0 93 throughout the canol formation suggest that deposition occurred in anoxic bottom waters but as basin fill conditions continued there was a shift to a dysoxic environment dop 0 55 0 65 which grades into nearshore and offshore sequences of the overlying imperial formation in contrast with the hare indian formation much heavier 34scdt values of pyrite in canol mudrocks 11 1 to 5 3 av 3 1 point to bacterial sulphate reduction in a closed to semi closed system with respect to seawater sulphate

metamorphism and granite genesis in the hidaka metamorphic belt hokkaido japan the main zone of the hidaka metamorphic belt is an uplifted crustal section of island arc type the crust was formed during early tertiary time as a result of collision between two arc trench systems of cretaceous age the crustal metamorphic sequence is divided into four metamorphic zones i iv in which zone iv is in the granulite facies a detailed study of the evolution of the hidaka belt based on a revised p t t analysis of the metamorphic rocks notably a newly found staurolite bearing granulite confirms a prograde isobaric heating path after a supposed event of tectonic thickening of accretionary sedimentary and oceanic crustal rocks during the peak metamorphic event c 53 ma the regional geothermal gradient attained 33 40 c km 1 and the highest p t condition obtained from the lowest part of the granulite unit is 830 c 7 kbar in this part xh2o of gt opx cd gneiss is about 0 15 and that of gt cd bt gneiss is 0 4 the p t xh2o condition of the granulite unit is well within a field where fluid present partial melting of pelitic and greywacke metamorphic rocks takes place this is in harmony with the restitic nature of the gt opx cd gneiss in the lowest part of the granulite unit the possibility that partial melting took place in the main zone is significant for the genesis of the peraluminous s type granitic rocks within it the s type granitic rocks in this zone are opx gt bt tonalite in the granulite zone gt cd bt tonalite in the amphibolite zone and cd bt mus tonalite in the bt mus gneiss zone the mineralogical and chemical nature of these strongly peraluminous tonalitic rocks permit them to be regarded as having been derived from s type granitic magma generated by crustal anatexis of pelitic metamorphic rocks in deeper crust

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

late eocene granites in the central sakhalin island russian far east and its implication for evolution of the sakhalin hokkaido orogenic belt abstract the sakhalin hokkaido orogenic belt along the western margin of the okhotsk sea is an important early cenozoic accretionary orogen related to subduction of the paleo pacific oceanic plate in order to study the eocene evolution of this accretionary orogen and related magmatism we carried out zircon u pb dating whole rock elemental and sr nd isotopic analyses on the langeri and val za granitic plutons that intruded the east sakhalin accretionary complex in the central sakhalin island zircon u pb dating results revealed that the langeri and val za plutons were emplaced in the late eocene at 38 36 ma whole rock elemental data indicate that granitoids are mainly peraluminous s type with subordinate i type granites and all granitic samples show arc like geochemical affinity whole rock sr nd isotopic results show consistent low initial 87sr 86sr ratios 0 7042 to 0 7049 positive end t values 1 7 to 3 1 and young nd two stage model ages 611 to 717 ma elemental and isotopic data reveal that granitic rocks of the langeri and val za plutons were generated from remelting of metasediments and amphibolites in the accretionary complex from comprehensive analyses of regional tectonics we concluded that the late eocene granitic rocks were generated in a syn collisional tectonic setting marking the timing for formation of the sakhalin hokkaido orogenic belt the middle late eocene welding of the okhotsk sea plate and the eurasia plate is the most plausible mechanism that caused crustal thickening beneath the sakhalin island and anatexis of metasediments and oceanic crustal materials in the lower crust might be responsible for the formation of late eocene granitic rocks considering the eocene tectonic events in ne asia late paleocene to early eocene subduction of the kula pacific ridge beneath the hokkaido island and eocene initial rift of pull apart basins along strike slip fault system in the hokkaido and sakhalin islands may also contribute to the emplacement of eocene magmatism in the sakhalin hokkaido orogenic belt

mesozoic cenozoic tectonic evolution and dynamics of the songliao basin ne asia implications for the closure of the paleo asian ocean and mongol okhotsk ocean and subduction of the paleo pacific ocean abstract the songliao basin in northeast china in the eastern part of the central asia orogenic belt is the largest accretionary orogen belt in the world however to date previous studies have not extensively examined the dynamic mechanism of its formation and evolution especially the pre cretaceous in this study the mesozoic cenozoic tectonic style stratigraphic records sedimentary time magmatic activity and tectonic evolution in the songliao basin were systematically analysed and summarised we propose that songliao basin experienced multi stage basin superposition processes the foreland compression stage syn rift and post rift stage and structural inversion stage after palaeozoic deposition the basin experienced a strong orogeny with the formation of the triassic middle jurassic foreland basin during the syn rift period three tectonic movements occurred at the end of the shahezi formation late valangnian 135 ma the end of first member of the yingcheng formation late hauterivian 129 ma and the end of the yingcheng formation late barrenian 125 ma in the songliao basin which ended the evolutional history of the rift in the post rift period three tectonic events occurred at the end of the nenjiang formation middle campanian 79 ma the mingshui formation late maastrichtian 66 ma and the yi an formation late chattian 23 ma which determined the deformation characteristics of the middle and shallow strata of the basin such as folding and reverse faults the rudimentary shape formed at the end of the nenjiang formation was finalised at the end of the mingshui formation and formed at the end of the yi an formation the largest depth and width influence of the two events was at the end of the mingshui and yi an formations which played a decisive role in the generation migration and accumulation of oil and gas in the middle and shallow strata of songliao basin after a series of asthenospheric mantle upwelling crustal uplift and extension and volcanic activity songliao basin was eventually formed the formation and structural evolution of the songliao basin were mainly influenced by the paleo asian ocean the mongol okhotsk ocean and the pacific ocean the closure of the paleo asian ocean in the late permian early triassic was influenced by the siberian plate which began to contact and collide with the xingan mongolian block from west to east causing the mongol okhotsk ocean to close in a scissor like structure from west to east in the early middle jurassic the subduction and movement of pacific rim tectonic systems such as the farallon izanagi pacific and philippine plates led to the formation of songliao basin which has experienced a foreland period via compressive pressure since the triassic syn rift period and post rift period via extensional stress and then a structural inversion period

ancient subduction zone in sakhalin island abstract the northern part of sakhalin island is an area of recent intensive tectonic movements and hydrothermal processes as well as a place of accumulation of useful minerals the deep structure of the lithosphere beneath the region of the neftegorsk earthquake of may 27 1995 in north sakhalin which killed residents and caused significant destruction is examined in this paper our geodynamic model shows that north sakhalin consists of the north sakhalin basin deryugin basin and an ophiolite complex located between them the deryugin basin was formed in place of an ancient deep trench after subducting the okhotsk sea plate under sakhalin in the late cretaceous paleogene the north sakhalin basin was formed on the side of the back arc basin at that time the ophiolite complex is fixed in the position of ancient subduction zone that was active in the late cretaceous paleogene approximately in the miocene the subduction of the okhotsk lithosphere apparently ceased the remains of the subduction zone in the form of an ophiolite complex have been identified from geological and geophysical data on the surface the subduction zone is manifested as deep faults stretched along sakhalin it is probable that the neftegorsk earthquake was a result of activation of this ancient subduction zone

geodynamic implications of paleomagnetic data from tertiary sediments in sakhalin russia nw pacific 1 n s trending right lateral strike slip faults which were active in the tertiary transect sakhalin russia while mesozoic forearc and accretionary rocks testify to an earlier period of subduction several kinematic models have been proposed for the region but the details required to constrain these models such as the timing of the transition from subduction to strike slip tectonics in sakhalin are still unknown even first order tectonic features such as the boundaries of the plates with which sakhalin evolved during the tertiary are poorly known paleomagnetic results from around sakhalin were obtained to constrain the geodynamic evolution of the region comparison of paleomagnetic inclination data with the apparent polar wander paths for the eurasian pacific and north american plates suggests that sakhalin probably evolved with the north american plate although a history including the eurasian plate cannot be ruled out paleomagnetic declination data suggest that significant clockwise vertical axis rotation has occurred in sakhalin since the mid paleocene it is likely that this rotational deformation was accommodated by tertiary activity on right lateral strike slip faults which may be associated with the opening of the japan sea tatar strait and kuril basin these data contradict a published kinematic model for eastern sakhalin where counterclockwise vertical axis rotations were predicted for neogene basins in the east sakhalin mountains agreement is better however with published paleomagnetic data from southern sakhalin where clockwise vertical axis rotations were documented

eocene arc continent collision and crustal consolidation in kamchatka russian far east the age and origin of high grade metamorphic rocks of the sredinnyi range kamchatka have been the subject of a long and controversial debate based on geochronologic data and its association with a collage of accreted oceanic terranes leading interpretations argue that the sredinnyi range metamorphic rocks represent an accreted precambrian or mesozoic microcontinent in this contribution we present new data that indicate that these metamorphic rocks were formed from the cretaceous paleocene sedimentary margin of northeast russia when it was overridden during eocene obduction of the olyutorsky arc a far travelled oceanic island arc our data include new mapping and structural observations along the northern and eastern flanks of the sredinnyi range and shrimp zircon and monazite u th pb age data from 15 key samples these new isotopic data demonstrate that paragneissic units were formed from sediments with depositional ages locally no older than late cretaceous to paleocene furthermore the statistical similarity of zircon u pb grain ages from the kamchatka schist with very low grade turbidite sandstones of the ukelayat and khozgon groups indicate that metasediments of the sredinnyi range are upgraded stratigraphic equivalents of northeast russian marginal strata shrimp u pb ages of zircon overgrowths and metamorphic monazite extracted from migmatite and gneiss indicate that peak metamorphism occurred at 52 ma which is synchronous with the onset of the olyutorsky arc continent collision heating and cooling occurred rapidly at rates approaching 80 c m y rapid heating is attributed to syn collisional subduction related magmatism thermochronology and structural observations indicate that exhumation was due to a combination of ductile and brittle thinning of the crust we speculate that this thinning was caused by diapiric ascent of a low density low viscosity continental material beneath a dense structural lid of the obducted island arc

velocity field of around the sea of okhotsk and sea of japan regions determined from a new continuous gps network data to investigate the current crustal movements in and around the sea of okhotsk and sea of japan regions we have established a continuous gps network by the end of 1997 the network had been expanded to include 12 new stations data for the period from july 1995 to november 1997 were analyzed together with data from international gps service for geodynamics igs global stations to fix the estimated coordinates to the terrestrial reference frame the tsukuba igs station was assumed to be moving westward relative to the stable eurasian continent at 2cm yr according to heki s l996 estimate we find that 1 stations in the western margin of the sea of japan have eastward velocity vectors 2 the pole position of the okhotsk plate is located near okha which reconfirms the okhotsk micro plate 3 a plate boundary of the okhotsk and amurian plates between southen sakhalin and hokkaido is suggested

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

archean protolith and accretion of crust in kamchatka shrimp dating of zircons from sredinny and ganal massifs abstract we used an ion microprobe shrimp rg for u pb dating of individual zircons in high grade metamorphic basements of the felsic sredinny and mafic ganal massifs in kamchatka thirty percent of zircons from paragneisses of the sredinny massif contain archean 2800 2500 ma early proterozoic 1700 2100 ma and late proterozoic 1000 ma detrital cores in addition a broad and almost uninterrupted range of phanerozoic detrital cores 460 175 ma is present with jurassic to lower cretaceous 150 96 ma zircons forming the youngest detrital age group ubiquitous documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand tex

emplacement ages geochemical and sr nd hf isotopic characterization of mesozoic to early cenozoic granitoids of the sikhote alin orogenic belt russian far east crustal growth and regional tectonic evolution abstract the sikhote alin range of the russian far east is an important accretionary orogen of the western pacific orogenic belt in order to study the formation and tectonic evolution of the orogen we performed zircon u pb dating as well as geochemical and sr nd hf isotopic analyses on 24 granitoid samples from various massifs in the primorye and khabarovsk regions the zircon dating revealed that the granitoids were emplaced from 131 to 56 ma cretaceous to paleogene in the primorye region granitoids in the coastal sikhote alin intruded the cretaceous taukha accretionary terrane from ca 90 to 56 ma whereas those along the central sikhote alin fault zone intruded the jurassic samarka accretionary terrane during ca 110 75 ma the oldest monzogranite 131 ma was emplaced in the lermontovka area of the nw primorye region granitoid massifs along the central sikhote alin fault zone in the khabarovsk region formed from 109 to 58 ma thus the most important tectonothermal events in the sikhote alin orogen took place in the cretaceous geochemical analysis indicates that most samples are i type granitoids they have initial 87sr 86sr ratios ranging from 0 7040 to 0 7083 and initial nd isotopic ratios expressed as end t values from 3 0 to 5 0 mostly 0 to 5 the data suggest that the granitoid magmas were generated by partial melting of sources with mixed lithologies including the subducted accretionary complex hidden paleozoic proterozoic basement rocks based on whole rock nd isotopic data we estimated variable proportions 36 77 of juvenile component mantle derived basaltic rocks in the generation of the granitic magmas furthermore zircon hf isotopic data ehf t 0 to 15 indicate that the zircon grains crystallized from melts of mixed sources and that crustal assimilation occurred during magmatic differentiation the quasi continuous magmatism in the sikhote alin orogen suggests that the paleo pacific plate subduction was very active in the late cretaceous the apparently regular progression of granitic intrusion ages from 80 to 56 ma in the taukha terrane may reflect oblique underflow of the paleo pacific plate beneath the eurasian continental margin subduction was not only manifested by granitic intrusion but also by abundant silicic volcanism the late cretaceous paleo pacific plate motion probably changed from parallel or sub parallel to oblique relative to the continental margin of the sikhote alin leading to the change of magmatic source region and geochemical characteristics of the derived igneous rocks late cretaceous rapid sea floor spreading at ca 100 ma induced highly active subduction and led to voluminous magmatism in the entire circum pacific realm finally the present age and isotopic study lends support to the hypothesis of geologic and tectonic correlation between sikhote alin and sw japan

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

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

discussion of the paper switch of ne asia from extension to contraction at the mid cretaceous a tale of the okhotsk oceanic plateau from initiation by the perm anomaly to extrusion in the mongol okhotsk ocean by kai jun zhang li long yan chen ji

elemental and sr nd isotopic geochemistry of cretaceous to early paleogene granites and volcanic rocks in the sikhote alin orogenic belt russian far east implications for the regional tectonic evolution abstract the sikhote alin orogenic belt in russian far east is an important late mesozoic to early cenozoic accretionary orogen related to the subduction of the paleo pacific plate this belt was generated by successive accretion of terranes made of accretionary prisms turbidite basins and island arcs to the continental margin of northeastern asia represented by the bureya jiamusi khanka block from jurassic to late cretaceous in order to study the tectonic and crustal evolution of this orogenic belt we carried out zircon u pb dating and whole rock elemental and sr nd isotopic analyses on granites and volcanic rocks from the primorye region of southern sikhote alin zircon dating revealed three episodes of granitoid emplacement permian early cretaceous and late cretaceous to early paleogene felsic volcanic rocks mainly rhyolite dacite and ignimbrite that overlay all tectonostratigraphic terranes were erupted during 80 57 ma postdating the accretionary process in the sikhote alin belt the cretaceous paleogene magmatism represents the most intense tectonothermal event in the sikhote alin belt whole rock major and trace elemental data show arc like affinity for granitoids and volcanic rocks indicating that they were likely generated in a supra subduction setting their initial 87sr 86sr ratios range from 0 7048 to 0 7114 and end t values vary from 1 7 to 3 8 mostly

america eurasia plate boundary in eastern asia and the opening of marginal basins abstract the opening of the arctic ocean during the past 55 ma resulted in relative rotation of america with respect to eurasia about a pole located in eastern siberia near the plate boundary the extensional plate boundary enters deep inside the eurasian continent up to the rotation pole on the opposite side of the pole on the pacific side of the plate boundary compressive tectonics are recorded along sakhalin and hokkaido from the oligocene to middle miocene the relative movement was accommodated by strike slip motion along sakhalin and hokkaido although the rotation pole was not located at a significatively different position from now in this paper we explain this by independent motion of the southernmost tip of the american plate towards the pacific margin which behaves as a free boundary this oceanward motion resulted in an extension of the american plate giving rise to the wedge structure of the okhotsk sea the japan sea opened as a pull apart basin along the strike slip boundary finally the increasing extension in the okhotsk sea led to the opening of the oceanic kuril basin

age geochemistry and tectonic implications of a late paleozoic stitching pluton in the north tian shan suture zone western china the central asian orogenic belt is the largest tectonic assembly of continental and oceanic terranes on earth due to closure of the paleo asian ocean in the phanerozoic among major suture zones in the north xinjiang region of western china the north tian shan suture zone because of collision between the yili terrane in the south and the junggar terrane in the north contains the youngest ophiolitic rocks and may represent the terminal stage of development of the central asian orogenic belt in western china but the timing of the suture zone remains poorly constrained a sensitive high resolution ion microprobe shrimp zircon u pb age of 316 3 ma i e the beginning of the late carboniferous from the undeformed sikeshu pluton which crosscuts the suture zone places a crucial upperage bound for the time of collision between the yili and junggar terranes this event occurred later than or nearly concurrent with other accretion collision events in the north xinjiang region implying that fi nal terrane amalgamation was completed in the late carboniferous the sikeshu pluton shares geochemical characteristics of the widespread late carboniferous to permian postcollisional a type and i type granitoids with depleted mantle like sr nd isotopic signatures in the north xinjiang region they all occurred during a protracted ca 320 270 ma episode of postcollisional magmatism that may have been induced by basaltic under plating due to either slab breakoff or delamination of thickened mantle lithosphere beneath the central asian orogenic belt the same postcollisional magmatism also generated cu ni sulfi de bearing mafi c ultramafi c magmatic complexes adakites and porphyry type coppermolybdenum bearing magmatic rocks in the north xinjiang region

signature of remnant slabs in the north pacific from p wave tomography summary a 3 d ray tracing technique was used in a global tomographic inversion in order to obtaintomographicimagesofthenorthpacific thedatareportedbythegeophysical surveyofrussia 1955 1997 wereusedtogetherwiththecataloguesoftheinternational seismologicalcenter 1964 1991 andtheusgeologicalsurveynationalearthquake information center 1991 1998 and the recompiled catalogue was reprocessed the final data set used for following the inversion contained 523430 summary ray paths the whole of the earth s mantle was parametrized by cells of 2 2 and 19 layers the large and sparse system of observation equations was solved using an iterative lsqr algorithm asubhorizontalhigh velocityanomalyisrevealedjustabovethe660kmdiscontinuity beneaththealeutiansubductionzone thishigh velocityfeatureisobservedatlatitudes of up to 70 n and is interpreted as a remnant of the subducted kula plate which disappeared through ridge subduction at about 48ma a further positive velocity perturbation feature can be identified beneath the chukotka peninsula and okhotsk sea extending from 300 to 660km depth and then either extending further down to 800km chukotka or deflecting along the 660km discontinuity okhotsk sea this high velocity anomaly is interpreted as a remnant slab of the okhotsk plate accreted to siberia at 55ma

paleo pacific subduction accretion evidence from geochemical and u pb zircon dating of the nadanhada accretionary complex ne china the nadanhada terrane located along the eastern margin of eurasia contains a typical accretionary complex related to paleo pacific plate subduction accretion the yuejinshan complex is the first stage accretion complex that consists of meta clastic rocks and metamafic ultramafic rocks whereas the raohe complex forms the main parts of the terrane and consists of limestone bedded chert and mafic ultramafic rocks embedded as olistolith blocks in a weakly sheared matrix of clastic meta sedimentary rocks geochemical data indicate that the yuejinshan metabasalts have normal mid ocean ridge basalt n morb affinity whereas the raohe basaltic pillow lavas have an affinity to ocean island basalts oib sensitive high resolution ion microprobe shrimp u pb zircon analyses of gabbro in the raohe complex yield a weighted mean 206pb 238u zircon age of 216 5 ma whereas two samples of granite intruded into the complex yield weighted mean 206pb 238u zircon ages of 128 2 and 129 2 ma laser ablation inductively coupled plasma mass spectrometry la icpms u pb zircon analyses of basaltic pillow lava in the raohe complex define a weighted mean age of 167 1 ma two sandstone samples in the raohe complex record younger concordant zircon weighted mean ages of 167 17 and 137 3 ma these new data support the view that accretion of the raohe complex was between 170 and 137 ma and that final emplacement of the raohe complex took place at 137 130 ma the accretion of the yuejinshan complex probably occurred between the 210 and 180 ma suggesting that paleo pacific plate subduction was initiated in the late triassic to early jurassic

mesozoic evolution of northeast asia and the collision of the okhotsk microcontinent abstract tectonic models of northeast japan generally involve the progressive subduction of the paleo pacific oceanic crust during the mesozoic and collision of the okhotsk microcontinent at the end of the cretaceous with eurasia however these models are not based on a precise knowledge of the overall structure and succession of deformations in the collision zone our survey leads us to propose structural maps and cross sections of the hokkaido central belt and a tectonic description of each tectonic stage it appears that at the end of the jurassic until the early cretaceous an important event of nappe emplacement occurred we interprete this event as the collision of the okhotsk block with eurasia much earlier than in classical models during this collision the oceanic crust of a marginal basin was obducted with an east vergence and partly metamorphosed contemporaneously with the collision an important volcanism linked with the pacific subduction started and continued during the whole of the cretaceous period the continental margin was affected by strike slip faults running parallel to the margin with a left lateral motion the main fault was the tanlu fault which accommodated the northward movement of the eastern part of the asian continent which led to the closing of an oceanic domain between siberia and the okhotsk block this tectonic scenario ended 70 m y ago contemporaneously with the cessation of the volcanism in the okhotsk uda margal volcanic belt from the eocene to the middle miocene a right lateral strike slip motion along sakhalin and hokkaido gave rise to the hidaka shear zone and finally led to the opening of the sea of japan

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

an unrecognized major collision of the okhotomorsk block with east asia during the late cretaceous constraints on the plate reorganization of the northwest pacific abstract interactions at plate boundaries induce stresses that constitute critical controls on the structural evolution of intraplate regions however the traditional tectonic model for the east asian margin during the mesozoic invoking successive episodes of paleo pacific oceanic subduction does not provide an adequate context for important late cretaceous dynamics across east asia including continental scale orogenic processes significant sinistral strike slip faulting and several others the integration of numerous documented field relations requires a new tectonic model as proposed here the okhotomorsk continental block currently residing below the okhotsk sea in northeast asia was located in the interior of the izanagi plate before the late cretaceous it moved northwestward with the izanagi plate and collided with the south china block at about 100 ma the indentation of the okhotomorsk block within east asia resulted in the formation of a sinistral strike slip fault system in south china formation of a dextral strike slip fault system in north china and regional northwest southeast shortening and orogenic uplift in east asia northeast striking mountain belts over 500 km wide extended from southeast china to southwest japan and south korea the peak metamorphism at about 89 ma of the sanbagawa high pressure metamorphic belt in southwest japan was probably related to the continental subduction of the okhotomorsk block beneath the east asian margin subsequently the north northwestward change of motion direction of the izanagi plate led to the northward movement of the okhotomorsk block along the east asian margin forming a significant sinistral continental transform boundary similar to the san andreas fault system in california sanbagawa metamorphic rocks in southwest japan were rapidly exhumed through the several kilometer wide ductile shear zone at the lower crust and upper mantle level accretionary complexes successively accumulated along the east asian margin during the jurassic early cretaceous were subdivided into narrow and subparallel belts by the upper crustal strike slip fault system the departure of the okhotomorsk block from the northeast striking asian margin resulted in the occurrence of an extensional setting and formation of a wide magmatic belt to the west of the margin in the campanian the block collided with the siberian margin in northeast asia at about 77 ma a new oceanic subduction occurred to the south of the okhotomorsk block ending its long distance northward motion based on the new tectonic model the abundant late archean to early proterozoic detrital zircons in the cretaceous sandstones in kamchatka southwest japan and taiwan are interpreted to have been sourced from the okhotomorsk block basement which possibly formed during the late archean and early proterozoic the new model suggests a rapidly northward moving okhotomorsk block at an average speed of 22 5 cm yr during 89 77 ma it is hypothesized that the okhotomorsk east asia collision during 100 89 ma slowed down the northwestward motion of the izanagi plate while slab pull forces produced from the subducting izanagi plate beneath the siberian margin redirected the plate from northwestward to north northwestward motion at about 90 89 ma

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

how far did india drift during the late cretaceous placenticeras kaffrarium etheridge 1904 ammonoidea used as a measuring tape abstract india once a member of the lost supercontinent gondwana broke away from it and made a solitary northward excursion and finally collided with asia during its long voyage india remained isolated for 100 ma and is expected to be characterized by stunning endemic biodiversity but this is not recognized by the terrestrial faunal and floral content and their distribution patterns paint no simple scenario for example the inter trappean vertebrate faunas of india which lived during india in exile do not show any made in india assemblages but rather betray a mixed biota having both gondwanan and laurasian affinities these differential distribution patterns of fauna and flora and their affinities with those of other areas prompted many workers to envisage an array of suggestions regarding the time of india s final separation from gondwana the time of northward drifting and different palaeopositions during its long journey but closer examination of the nature of the vertebrate fossil records reveals that the so called elusive endemicity of indian fauna during its sojourn is in fact a product of taxonomic artefact the majority of the faunas have been described on the basis of poor fossil data and comparisons for biogeographic correlations are made at higher taxonomic levels which perhaps masked india s faunal distinctiveness yet the biological processes that constrain biogeographical distribution operate at the species level in this paper we present our own data to reconstruct the palaeoposition of india in the late cretaceous and to estimate the time of its northward migration the present study is based on a newly recorded ammonite species placenticeras kaffrarium etheridge from the coniacian horizons in bagh central india the species abounds in bagh and represents a complete population structure it resembles significantly the populations described from the coeval horizons of madagascar and zululand south africa p kaffrarium has a stunning display of intraspecific variability represented by at least seven distinct morphotypes reported from zululand and madagascar the indian population significantly matches the latter two populations variant by variant and remarkably also for the sexual dimorphs within each variant geographic range is a biological property of a species and ammonite species have differential distribution patterns depending on the length of larval longevity oceanic circulation patterns and favourable environments placenticeras was a nektobenthic genus living in the shallowest depths of epicontinental seas and could not cross a narrow strait wider than a few hundred kilometers palaeogeographic distribution patterns of all placenticeratid species during the late cretaceous support this view and show the wandering nature of the genus over time however they always exhibit a latitudinally limited distribution and are restricted mainly to the subtropics north and south of the equator thus it appears that the p kaffrarium population of zululand madagascar and bagh constitute a homogenous single breeding population implying geographical proximity of these areas connected by exclusively shallow seas till the coniacian other fossil records of ammonites and geophysical data also support this view shortly after this india started its northward drift by the late maastrichtian india was completely isolated by a wide oceanic gap from the rest of the gondwana fragments and the northern part of india crossed the equator this is evident from strong endemicity of ammonite species assemblages of the northwest and southern indian subcontinent which constitute the most diverse communities known

occurrences of large magnitude earthquakes in the kachchh region gujarat western india tectonic implications abstract moderate to large damaging earthquakes in the peninsular part of the indian plate do not support the long standing belief of the seismic stability of this region the historical record shows that about 15 damaging earthquakes with magnitudes from 5 5 to 8 0 occurred in the indian peninsula most of these events were associated with the old rift systems our analysis of the 2001 bhuj earthquake and its 12 year aftershock sequence indicates a seismic zone bound by two linear trends nnw and nne that intersect an e w trending graben the bouguer gravity values near the epicentre of the bhuj earthquake are relatively low 2 mgal the gravity anomaly maps the distribution of earthquake epicentres and the crustal strain rate patterns indicate that the 2001 bhuj earthquake occurred along a fault within strain hardened mid crustal rocks the collision resistance between the indian plate and the eurasian plate along the himalayas and anticlockwise rotation of the indian plate provide the far field stresses that concentrate within a fault bounded block close to the western margin of the indian plate and is periodically released during earthquakes such as the 2001 m w 7 7 bhuj earthquake we propose that the moderate to large magnitude earthquakes in the deeper crust in this area occur along faults associated with old rift systems that are reactivated in a strain hardened environment

zircon u pb and hf isotopic constraints from eastern transhimalayan batholiths on the precollisional magmatic and tectonic evolution in southern tibet abstract before the indian collision with asia northward subduction of the neo tethyan oceanic lithosphere produced an andean type convergent margin in south asia characterized by arc magmatism starting from the early jurassic and lasting until the eocene the magmatic arc is now represented by widespread intrusive bodies or the so called transhimalayan batholiths in the lhasa terrane of southern tibet that have been divided into two main magmatic suites i e the northern plutonic belt and the southern gangdese batholith their temporal distribution concerning how exactly the magmatic suites correlate eastwards and then southeastwards around the eastern himalayan syntaxis however remains poorly constrained here we report the first combined zircon u pb and hf isotopic study of the transhimalayan batholiths from the eastern part of the lhasa terrane 95 97 5 e and 28 5 30 n se tibet zircon u pb dating results of 24 granitoids indicate that the rocks were emplaced principally in the early cretaceous 133 110 ma and subordinately in the paleocene 66 57 ma long after an older granite intrusion in the earliest jurassic 198 ma the zircon ehf t values range from 5 to 20 yielding hf crustal model ages tdmc between 0 8 and 2 4 ga that peak at 1 7 ga suggesting a major episode of crustal growth in the proterozoic and a predominantly or even exclusively crustal source for the granitoid petrogenesis these zircon u pb and hf isotopic constraints together with whole rock geochemical characteristics allow us to correlate them to s type granitoids in the northern plutonic belt rather than to the i type gangdese batholith from the central part of the southern lhasa terrane in contrast to the gangdese magmatism that formed with a significant juvenile mantle input related to the neo tethyan subduction these s type granitoids show geochemical similarities to the north american cordilleran interior batholiths that did not form directly from a subduction setting thus we attribute the petrogenesis of the widespread early cretaceous granitoids in the northern belt and eastern himalayan batholiths to a postcollisional regime due to the late jurassic early cretaceous continental collision between the lhasa and qiangtang terranes under this framework the s type magmatism was generated in response to collision induced crustal thickening which may have capabilities of causing crustal anatexis by itself in the region however continued interplay with the neo tethyan subduction zone processes such as back arc extension could also have played a crucial and long lasting role in the magma generation

internal crustal deformation in the northern part of shan thai block new evidence from paleomagnetic results of cretaceous and paleogene redbeds in order to describe the internal deformation of the northeast part of the shan thai block resulting from the penetration of the indian plate into asia a paleomagnetic study was carried out on the cretaceous and paleogene strata in the southern part of the simao terrane high temperature magnetic components with unblocking temperatures around 680 degrees c were isolated and positive fold test shows its primary nature of magnetization the tilt corrected site mean directions of the lower cretaceous and upper cretaceous from the menglun area are d s 46 2 degrees i s 45 9 degrees k 11 0 alpha 95 10 6 degrees and d s 33 2 degrees i s 30 9 degrees k 68 4 alpha 95 8 2 degrees respectively the site mean directions of the upper cretaceous and lower paleogene for the mengban area are d s 50 5 degrees i s 31 0 degrees k 207 6 alpha 95 6 4 degrees and d s 43 5 degrees i s 23 0 degrees k 26 1 alpha 95 13 4 degrees respectively the results of the lower cretaceous and upper paleogene from the mengla area are d s 46 9 degrees i s 42 2 degrees k 27 3 alpha 95 7 7 degrees and d s 41 8 degrees i s 23 8 degrees k 38 9 degrees alpha 95 5 8 degrees respectively in comparison with previous paleomagnetic data of the simao terrane the rotational deformation can be divided into four stages the cretaceous and paleocene sediments of the simao terrane experienced intensive folding before the early oligocene during the early oligocene to early miocene the shan thai and indochina blocks experienced southeastward extrusion with similar to 20 degrees clockwise rotation relative to the south china block during the middle to late miocene the shan thai blocks experienced more 15 degrees clockwise rotation since the late miocene the activity of strike slip faults surrounding the simao terrane enhanced the internal differential rotations of the terrane these deformation processes finally resulted in a complex internal deformation of the simao terrane c 2013 elsevier b v all rights reserved

tertiary tectonics of the sea of okhotsk russia far field effects of the india eurasia collision a new structural map of the western sea of okhotsk based on grids of multichannel seismic data provides several new insights into the tertiary deformation of asia especially in regard to the extension of extrusion tectonics linked to the india eurasia collision into northeastern asia the sedimentary basins in this offshore region are the result of two regional shear systems in the west the north trending sakhalin hokkaido dextral fault system transects sakhalin island accessory structures closely related to its dextral shear include northeast trending normal faults as well as northwest trending en echelon folds and thrusts the accessory normal faults are predominantly eocene to lower miocene and indicate transtensional shear whereas the folds and thrusts are mostly younger and indicate late miocene and pliocene transpression in the north an east west trending sinistral shear zone enters the sea at its northwest corner this sinistral shear system links farther west to extensional faults of the early tertiary baikal rift extending northeastward from the terminus of this sinistral shear zone are a series of long predominantly northwest dipping listric normal faults that form a large lazy s shaped pull apart basin the shantar liziansky basin slb normal faults in this 500 140 km basin accommodate 15 to 20 extension extension and related sinistral shear appear to be largely eocene to oligocene in age with lesser later activity to the northeast of slb a further extension of the sinistral shear zone appears to bend northeastward transecting pustorets and penzhina basins and following the course of the older mesozoic mongol okhotsk chukotsk active margin and suture it possibly reaches as far as the arctic as suggested by previous workers the baikal rift and its associated sinistral shear zone may be interpreted as by products of the india eurasia collision in eocene time mapping results included here show that this diffuse sinistral shear system extends much farther to the northeast the sinistral shear zone is transected by the dextral sakhalin hokkaido fault zone in the kashevarov region of central slb where a fan of north to northwest striking late tertiary transpressional fault splays marks the northern termination of the dextral fault zone older slb normal faults are locally deformed and half grabens are partly inverted by the slightly younger dextral splays the two regional fault systems are essentially conjugate and the pattern of intersection closely resembles that predicted by recent modelling studies of india eurasia collision tectonics

tectonic setting of the plutonic belts of yakutia northeast russia based on 40ar 39ar geochronology and trace element geochemistry samples from plutonic belts and individual intrusions in the sakha republic yakutia northeastern russia were analyzed using 40ar 39ar geochronology and trace element geochemistry four phases of activity were identified a subduction event along the southwest margin of the kolyma omolon superterrane with apparent ages of ca 160 140 ma a superposed collisional event dated at 143 138 ma a subduction event along the northern margin of the superterrane dated at 130 123 ma with a slightly older to contemporaneous extensional event to the west and intrusions associated with east west extension between 106 and 92 ma these phases are interpreted to represent 1 subduction under and 2 accretion to asia of kolyma omolon followed by local extension 3 subduction along the northern edge of this superterrane and 4 regional extension following closure of the south anyui suture and or the start of subduction associated with the okhotsk chukotka volcanic belt

origin transport and emplacement of an exotic island arc terrane exposed in eastern kamchatka russia the regional stratigraphy of eastern kamchatka includes an exotic early late cretaceous ophiolite and late cretaceous island arc volcanic sequence integrating the existing geologic and geophysical data we examine the origin transport emplacement and postemplacement deformation of the island arc terrane which is named the olyutorsky island arc results from several paleomagnetic studies consistently indicate that the island arc terrane originated 1000 km to the south of where it is presently exposed although the formative paleolatitudes of the island arc rocks approximately correspond to the location of the izanagi farallon subduction zone the age of the volcanic rocks postdates the cessation of izanagi farallon convergence thus indicating that an unnamed plate or back arc basin existed in the northwest pacific during late cretaceous time we examine two possible models for northward transport of the island arc terrane to kamchatka 1 infra oceanic transport with the pacific or kula plates and 2 coastwise translation of the island arc terrane after accretion to the eurasian margin far to the south of kamchatka for both models the dominant eocene and miocene deformation ages observed in eastern kamchatka are used as two possible age limits for the cessation of northward transport although the observed paleolatitudes from paleomagnetic data correspond best with the infra oceanic transport model the provenance of the paleogene transport stratigraphy indicates a near shore sediment supply our preferred interpretation is that the island arc terrane 1 accreted onto the eurasian margin concurrent with cessation of island arc volcanism maastrichtian danian and 2 underwent northward coastwise translation along a major strike slip fault zone ending by middle late eocene time 43 50 ma it is unclear whether the ophiolite was exposed during arc continent collision or whether the ophiolite was obducted onto the island arc prior to collision a consequence of either infra oceanic transport or coastwise translation is that an open corridor between the western terminus of the aleutian arc and kamchatka must have existed until middle to late eocene time spreading within the komandorsky basin subduction of sea mounts and collision of the aleutian arc with kamchatka are proposed to have instigated the second miocene phase of deformation which uplifted and reexposed the island arc terrane

paleontological view of the ages of the deccan traps the cretaceous tertiary boundary and the india asia collision volcanism in the deccan traps of india occurred over at most three magnetic chrons centered on a main reversed chron paleontological data indicate that this reversed chron must coincide with 29r which contains the cretaceous tertiary k t boundary recent 40 ar 39 ar data may therefore provide one of the best estimates of the absolute age of the k t boundary 65 7 2 0 ma moreover the eurasian character of k t boundary terrestrial faunas in india suggests that this was also close to the time of india asia collision this is significantly earlier than generally recognized and implies about 4000 km of continental shortening in the collision zone probably absorbed jointly by crustal thickening and strike slip extrusion

arc continent collision and subduction reversal in the cenozoic evolution of the northwest pacific an example from kamchatka ne russia abstract the principal new feature in the cenozoic evolution of the northwest pacific is based on the fact that two arc terranes of upper cretaceous paleogene age were incorporated in the kamchatka orogen during the early tertiary and at the end of the miocene respectively these two arc terranes so called achaivayam valaginskaya campanian lower paleocene and kronotskaya coniacian eocene arcs are separated by a suture zone that contains deformed slices of young paleocene lower eocene oceanic crust paleomagnetic data on the original paleolatitudes of the arc positions as well as geochemical data characterized the arcs as intraoceanic originated in the northwest pacific the arcs were separated by the vetlovka oceanic plate suggested to be a trapped fragment of the kula plate in which a new spreading zone developed in the early tertiary the principal event in the reconstructed evolution is the eocene collision of the extended achaivayam valaginskaya arc terrane with the continental margin of asia the margin is interpreted as the southeastern facing the pacific cretaceous accretionary boundary of the okhotsk microplate docked against the asian continent at 55 65 ma the arc continent collision developed progressively northward from the end of the paleocene early eocene in southern kamchatka to the middle eocene in the olutorka region the different stages of the collision occurred simultaneously along strike of the achaivayam valaginskaya arc according to the geodynamic model based on geological data for southern kamchatka collision included 1 the entrance of the continental margin in the subduction zone deformation of the overriding plate and subduction of fore arc block 2 arc obduction on the continental margin blocking of continental margin subduction and 3 subduction reversal with onset of oceanic subduction at the eastern side of the deformed arc this subduction reversal produced deformation and reverse faulting along the eastern side of the arc thus allowing accretion of tectonic slices of young oceanic crust of the vetlovka plate which began to subduct during the subduction reversal the subducted plate dipping to the southeast was broken off and detached at depth under the arc area the subduction polarity reversal the most characteristic feature of the collision model is strongly supported by structural and geologic data of southern kamchatka and explains the main tectonic metamorphic and magmatic events related to and postdating the collision

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