late cretaceous volcanic rocks and associated granites in gyeongsang basin se korea their chronological ages and tectonic implications for cratonic destruction of the north china craton abstract the gyeongsang basin is the largest exclusively non marine sedimentary basin in korean peninsula located in southeastern part of the peninsula its sedimentary setting magma source tectonic nature and formation age are important to understand the mesozoic tectonic evolution of not only the sino korea craton but also the entire northeast asia the gyeongsang supergroup the basin fill is divided into the sindong hayang and yucheon groups in ascending order the sindong and hayang groups are composed of sandstone shale minor amounts of conglomerate and marl deposited in a non marine environment whereas the yucheon group is characterized by the dominance of volcanic rocks precise isotopic ages and geochemical analyses are scarce for the volcanic elements of this basin restricting our understanding of its geodynamic setting and evolution in this study we present new u pb zircon ages from the volcanic rocks of the yucheon group and associated granites in the gyeongsang basin thirteen volcanic samples yielded ages ranging from 94 4 to 78 4 ma and four granite samples give ages 72 ma these data confirm that a major magmatic event occurred during late cretaceous and the gyeongsang volcano sedimentary complex represents a rock association representing a late cretaceous island arc in comparison with the sambagawa metamorphic belt the jurassic chichibu accretionary complex ac with cretaceous strike slip basin sediments and the cretaceous shimanto melange zone in southwest japan it is likely that above motioned complexes constitute the trace of a continuous subduction related island arc system in se korea extending from sw japan although the peak ages of the magmatic rocks in the eastern part of the north china craton ncc are 130 10 ma indicating an important time of continental lithospheric thinning under the ncc the ages of the gyeongsang magmatic rocks are markedly younger late cretaceous magmatism was very weak in the eastern ncc therefore it is probable that early middle mesozoic lithospheric thinning beneath the eastern ncc is triggered by multiple tectonic factors not only by the subduction of the pacific plate suggested by some previous studies in addition 20 inherited xenocrystic zircons from these volcanic rocks give archean ages ranging from 2 5 to 2 6 ga suggesting the existence of late archean rocks in the yeongnam massif the yeongnam massif is predominated by the latest archean crustal growth at 2 50 ga consistent with the characteristics of the north china craton

evolution of the sanbagawa and shimanto high pressure belts in sw japan insights from k ar ar ar geochronology abstract the sanbagawa belt sensu lato in japan is one of the world s classical high pressure hp metamorphic belts and has long been considered to be a coherent hp belt of the cretaceous age however recent progress in detrital zircon geochronology together with the accumulation of abundant phengites k ar ar ar ages revealed that the belt is comprised of two petrotectonic belts sanbagawa sensu stricto hp belt and shimanto hp belt these two belts have distinctly different ages of peak metamorphism and different p t conditions of metamorphism the sanbagawa schists sensu stricto were metamorphosed in the conditions of the pumpellyite actinolite facies through the epidote blueschist to epidote amphibolite facies and up to the eclogite facies during a prograde stage in a subduction zone at 120 115 ma phengite k ar geochronology revealed that the subsequent exhumation after the peak metamorphism took placed in a manner that the higher grade rocks exhumed more rapidly in comparison with the lower grade rocks this is supported by a positive correlation between age and apparent metamorphic gradient that have formed during the post metamorphic peak hydration recrystallization moreover the schists have experienced intense ductile deformation and long term exhumation it took longer than 31 m y to reset the phengite k ar system in contrast the shimanto hp schists were metamorphosed in the epidote blueschist greenschist transitional facies to the epidote amphibolite facies and have experienced a short term deformation for less than 13 m y this short deformation formed a negative correlation between k ar age and apparent metamorphic gradient these two contrasting age temperature relationships suggest different exhumation processes between the two hp belts most likely due to a change in subducting oceanic plates in the cretaceous along the paleo japanese convergent margin

high resolution 40ar 39ar dating of the oldest oceanic basement basalts in the western pacific basin 1 we report new 40ar 39ar ages for the oldest pacific oceanic floor at ocean drilling program site 801c in the pigafetta basin and site 1149d close to the izu bonin subduction zone in the nadezhda basin these ages were determined by applying high resolution incremental heating experiments including 15 30 heating steps to better resolve the primary argon signal from interfering alteration signatures in these low potassium ocean crust basalts combined with previous results from pringle 1992 for site 801b and 801c we arrive at a multistage history for the formation of the pigafetta ocean crust the oldest part of the pacific plate was formed at the spreading ridges at 167 4 1 4 3 4 ma n 2 2 internal absolute error offering an important calibration point on the geological reversal timescale grts since it represents the old end of the mesozoic magnetic anomalies this mid ocean ridge basalt sequence however is overlain by more tholeiites and alkali basalts that were formed 7 3 1 5 myr later around 160 1 0 6 ma n 7 2 internal error the older age group is confirmed independently by radiolarian ages ranging from late bajocian to middle bathonian 167 173 ma bartolini and larson 2001 and by profound differences in the structural characteristics of this basement section pockalny and larson 2003 thin layers comprising hydrothermal deposits separate these sequences which in addition to the difference in isotopic age show distinct major and trace element compositions this indicates that key volcanic and hydrothermal activity took place 400 600 km away from the spreading ridges on the basis of a jurassic 66 km myr half spreading rate in the pacific it remains unclear if these processes were active continuously after the initial formation of the pacific oceanic crust but all our observations seem to point to an episodic history site 1149d gives another important calibration point on the grts of 127 0 1 5 3 6 ma n 1 2 internal absolute error for anomaly m12 that is slightly younger when compared to current timescale compilations 134 2 2 1 ma gradstein et al 1995 this might suggest that the dated basalt from site 1149d does not represent the age of the ocean crust formed at its ridge axis it may also be part of the early cretaceous intraplate events that have produced dolerite sills in the pacific crust at sites 800 and 802 around 114 126 ma

geometric model of conjugate faulting in the gyeongsang basin southeast korea 1 the nne trending dextral yangsan fault system and the wnw trending sinistral gaum fault system comprise a conjugate set that cuts the cretaceous gyeongsang basin along the se margin of the korean peninsula these faults evidently formed under compressional stress accompanied by a progressive 12 block rotation calculated from a geometric model incorporating angles between conjugate fault sets widths of fault domains and measured fault displacements the angle between paleomagnetic declinations in the set of conjugate fault domains increases with age suggesting incremental block rotation that included overall rotation of the entire gyeongsang basin or the korean peninsula with respect to eurasia the model accounts for yucheon group volcanism subsequent caldera subsidence in the uiseong subbasin and volcanism in the miryang subbasin all produced by regional tension the tectonic regime of the gyeongsang basin apparently evolved from compression to extension at 50 ma reflecting india eurasia collision and opening of the sea of japan east sea

late paleozoic mesozoic tectonic evolution of sw japan a review reappraisal of the accretionary orogeny and revalidation of the collisional model abstract this paper makes a review of the interpretations of the tectonic evolution of sw japan during the last three decades in the late 1970s the dominant model was the so called pacific type orogeny emphasizing the purported absence of nappes and the contrast with the alpine chains and interpreting the evolution as due to a steady oceanic subduction since the paleozoic time in the 80s the discovery of the actual structure made of a pile of large thrust sheets led authors to propose collisional models involving the intermittent underthrusting of buoyant blocks like micro continents at the same time the use of high resolution biostratigraphy allowed several authors to recognize ancient accretionary wedges with a reconstructed ocean plate stratigraphy of individual accreted units especially in the tanba and shimanto zones also precise radiometric dating permitted the distinction of metamorphosed units especially in sanbagawa and shimanto belts as a result of these new data since the 1990s the plate tectonic interpretation of the history of the japanese islands was revised by japanese scientists and presented again in terms of accretionary processes linked to a steadily oceanic subduction with an episodic ridge subduction the so called miyashiro type orogeny the review of different data leads to the following conclusions the structure of sw japan is made of a pile of sub horizontal nappes polydeformed with a geometry similar to the one encountered in collisional orogens the geodynamic mechanisms advocated for the tectonic building within the accretionary orogeny concept miyashiro type orogeny are inappropriate a permanent oceanic subduction with the intermittent collision actually subduction of an active ridge or seamount chain is unable to build such structures as this process induces in fact an acceleration of the tectonic erosion and collapse of the upper plate the underthrusting of a micro continent or mature arc is likely needed the exhumation story of sanbagawa hp schists suggests the setting of a continental subduction the petrological and new geochemical data from the literature strongly support the existence beneath the nappes of accretionary complexes of continental bodies showing affinities with south china from which they were once separated the episodic collision underthrusting of such blocks was responsible for the tectonic piling tectonic erosion plaid likely a major role in removing material during the intervening subduction stages a revised geodynamic model implying the collision of the honshu south kitakami kurosegawa and shimanto blocks is proposed for explaining the three orogenic crises which took place respectively at around 240 130 and 80 60 ma ago in sw japan the paleogeographic position and affinity of the hida block with surrounding units in the hinterland are still unclear more work is needed to solve this question

holocene palaeoenvironments from the direndall tufa luxembourg reconstructed from the molluscan succession and stable isotope records this paper reports the results of new malacological analyses from a thick tufa sequence at direndall luxembourg the study is temporally contextualised with radiocarbon dates and an age depth mod

cretaceous deformation history of the middle tan lu fault zone in shandong province eastern china abstract based on field analysis of fault slip data from different rock units of the cretaceous basins along the middle part of the tan lu fault zone shandong province eastern china we document polyphase tectonic stress fields and address the changes in sense of motion of the tan lu fault zone during the cretaceous the cretaceous deformation history of the tan lu fault zone can be divided into four main stages the first stage during the earliest cretaceous was dominated by n s extension responsible for the formation of the jiaolai basin we interpret this extension to be related to dextral strike slip pull apart opening guided by the tan lu fault zone the second stage during the middle early cretaceous was overwhelmingly rift dominated and characterized by widespread silicic to intermediate volcanism normal faulting and basin subsidence it was at this stage that the tan lu parallel yi shu rift was initiated by e w to wnw ese extension the tectonic regime then changed during the late early cretaceous to nw se oriented transpression causing inversion of the early cretaceous rift basin and sinistral slip along the tan lu fault zone during the late cretaceous dextral activation of the tan lu fault zone resulted in pull apart opening of the zhucheng basin which was subsequently deformed by ne sw compression this deformation chronology of the tan lu fault zone and the associated cretaceous basins allow us to constrain the regional kinematic models as related to subduction along the eastern margin of asia or related to collision in the tibet region

crustal structure of southwest japan revealed by the integrated seismic experiment southwest japan 2002 abstract a multi purpose seismic experiment named the 2002 integrated seismic experiment southwest japan was conducted in 2002 along a more than 240 km long line across southwest japan from the pacific coast to the japan sea coast its profile provides the first crustal scale cross section across the japanese island arc which highlights a number of significant points related to the structural development of the arc major outstanding points are that the japanese island arc is composed of two completely different crusts juxtaposed by the median tectonic line mtl and that the mtl started its activity associated with lower crustal thinning and formation of an upper crustal scale half graben in late cretaceous

architecture of basin mountain systems and their influences on gas distribution a case study from the sichuan basin south china abstract the basin mountain systems in the sichuan basin exert major control on today s oil gas distribution chiefly through their controlling influence on petroleum reservoirs preservation the thickness of deposited terrestrial formations and degree of uplift and denudation the composite basin mountain system in the sichuan basin and its peripheral mountains can be classified as two types margin plate systems and interior plate systems the margin plate basin mountain systems include the sichuan basin and its surrounding longmen mountains micang mountains and daba mountains which are not only the western and northern marginal area of the sichuan basin but also represent the western margin of the yangtze plate south china block the margin plate basin mountain systems with binary units of large scale thrust belts and foreland basins have different deep lithospheric structures abrupt boundaries and great contrast in today s geomorphology between the basin and the mountains on the contrary the interior plate basin mountain systems comprised by the sichuan basin and its adjacent interior plate qiyue mountains dalou mountains and daliang mountains represent the eastern and southern marginal area of the sichuan basin located within the yangtze plate they have similar lithospheric structure and gradual boundaries between the basin and the adjacent mountains lacking foreland basins today s medium large scale natural gas accumulations in the sichuan basin are mainly distributed in the areas influenced by the margin plate basin mountain systems and are located especially in the foreland basins

pattern and kinematic polarity of late mesozoic extension in continental ne asia perspectives from metamorphic core complexes 1 late mesozoic extension in ne asia resulted in the development of a large extensional province metamorphic core complexes mccs are the major features in this province and have 40ar 39ar ages of 130 110 ma for the mylonites and u pb zircon ages of 150 110 ma for the integral granitic intrusions based on this and previous studies this paper summarizes major characteristics of these mccs and recognizes a regional kinematic shear sense most mccs in the transbaikalia region sino mongolia border tract and the northwest central portion of the north china craton ncc show a top to the southeast se shear whereas those in the eastern and southern ncc locally underwent top to the northwest nw shear the three largest basins songliao huabei and ordos in north china are located in the transitional zone between domains of opposing shear sense we interpret the extension in the transbaikalia sino mongolia tract and northwestern part of the ncc to reflect late orogenic collapse of thickened crust following middle late jurassic collision along the okhotsk suture the southeastward extension is probably controlled by crustal scale top to the se tangential shear the transition from contraction to extension is marked by detachment faults that nucleated as extensional crenulation cleavage ecc i e c in sub horizontal ductile shear zones late in orogenic crustal thickening the combined effect of gravitational loading and thermal uplifting is considered to be the origin of the late or post orogenic collapse the top to the nw extension in the ne of the ncc might reflect antithetic sub extensional zones or mesozoic back arc extension as a far field effect of cretaceous pacific plate subduction

late mesozoic compressional to extensional tectonics in the yiwulushan massif ne china and its bearing on the evolution of the yinshan yanshan orogenic belt part i structural analyses and geochronological constraints abstract with a cratonic nucleus the north china craton ncc experienced a complex tectonic evolution with multiphase compressional and extensional events during mesozoic times along the northern part of the ncc the yinshan yanshan fold and thrust belt was a typical intraplate orogen jurassic and cretaceous continental sedimentation magmatism widespread intraplate characterize the yinshan yanshan orogenic belt the geodynamic significance of these tectonic events is still in dispute in the western part of the liaoning province the yiwulushan massif crops out at the eastern end of the yinshan yanshan orogenic belt the yiwulushan massif presents an elliptical domal shape with a ne sw striking long axis the structural evolution of this massif brings new insights for the understanding of the mesozoic plutonic tectonic history of the ncc a multidisciplinary study involving structural geology geochronology anisotropy of magnetic susceptibility ams and gravity modeling have been carried out the presentation of the new results splits into two parts part i this paper deals with field and laboratory structural observations and presents the main geochronological results the ams gravity modeling data will be provided in a companion paper part ii the early compressional deformation d1 corresponds to a late jurassic to early cretaceous southward thrusting the subsequent deformation is related to the early cretaceous exhumation of the yiwulushan massif a detailed structural analysis allows us to distinguish several deformation events d2 d3 and d4 the cretaceous extensional structures such as syntectonic plutons bounded by ductile normal faults metamorphic core complexes and half graben basins are recognized in many places in east asia these new data from the yiwulushan massif constitute a link between transbaikalia mongolia north china and south china indicating that nw se extensional mesozoic tectonics occurred throughout the entire region

intra panthalassa ocean subduction zones revealed by fossil arcs and mantle structure the vast panthalassa ocean once surrounded the supercontinent pangaea subduction has since consumed most of the oceanic plates that formed the ocean floor so classic plate reconstructions based on magnetic anomalies can be used only to constrain the ocean s history since the cretaceous period 1 2 and the triassic jurassic plate tectonic evolution of the panthalassa ocean remains largely unresolved 3 4 geological clues come from extinct intra oceanic volcanic arcs that formed above ancient subduction zones but have since been accreted to the north american and asian continental margins 4 here we compile data on the composition the timing of formation and accretion and the present day locations of these volcanic arcs and show that intra oceanic subduction zones must have once been situated in a central panthalassa location in our plate tectonic reconstructions 5 7 to constrain the palaeoposition of the extinct arcs we correlate them with remnants of subducted slabs that have been identified in the mantle using seismic wave tomographic models 8 9 we suggest that a series of subduction zones together called telkhinia may have defined two separate palaeo oceanic plate systems the pontus and thalassa oceans our reconstruction provides constraints on the palaeolongitude and tectonic evolution of the telkhinia subduction zones and panthalassa ocean that are crucial for global plate tectonic reconstructions and models of mantle dynamics the spatial uncertainty in reconstructions of oceanic basins increases the further we go back in time because oceanic plates subduct plate reconstructions for the triassicjurassic history of the panthalassa ocean once surrounding the pangaea supercontinent are based on the premise that the pacific plate formedinthemiddlejurassicasatriangle 2 originatingfromatriple junction between the farallon phoenix and izanagi plates fig 1 the preserved area of jurassic palaeoplates represents only a small portion 10 of the oceanic crust of the jurassic panthalassa ocean the remaining part along with pre jurassic oceanic plate configurations is entirely unconstrained however the geology of the circum panthalassa continental margins provides evidence that early mesozoic subduction related volcanism occurred in the panthalassa ocean 4 10 on subduction continental fragments volcanic arcs ophiolites and ocean floor sediments collided with or accreted to the continental margins of in particular western north america far east asia and japan marine microfossils in these sediments and palaeomagnetic data constrain their age and palaeolatitude but their palaeolongitudes remain undetermined palaeogeographic reconstructions therefore show uncorrelated intra panthalassa exotic terranes essentially

late mesozoic and cenozoic thermotectonic evolution along a transect from the north china craton through the qinling orogen into the yangtze craton central china 1 cretaceous and cenozoic reactivation of the triassic qinling dabie orogen between the north china and yangtze cratons resulted from the combined effects of pacific subduction back arc extension in east china and collisions in west china we report new apatite fission track and apatite and zircon u th he data from east qinling along a 400 km long n s transect from huashan through the qinling orogen to huangling the ages show a general pattern of younging northward three major cooling phases are defined by modeling the multiple thermochronologic data sets the first phase occurred locally in the north and south qinling during the late triassic to early jurassic following heating associated with the triassic yangtze subduction and exhumation of the wudang metamorphic core complex on the cratonal edge a second phase represents regional exhumation between 100 and 60 ma coeval with rifting marked by the late cretaceous eocene k2 e red bed deposition in eastern china and possibly indicating a link with pacific subduction back arc extension in eastern china however it may also have been superimposed by eastward tectonic escape resulting from the lhasa west burma qiangtang indochina collision the third cooling phase was initiated at 45 ma exclusively in the north qinling and in the footwall of the graben bounding normal fault of the weihe graben in the lesser qinling we suggest the third phase was related to reactivation of the qinling fault system as a result of eastward tectonic escape imposed by the india asia collision at 50 ma

margin parallel translation in the western pacific paleomagnetic evidence from an allochthonous terrane in japan abstract a new paleomagnetic study illustrates latitudinal translation of ribbon shaped continental fragments along the western margin of the pacific ocean though tectonic translations have long been recognized as prominent features in the eastern pacific margin i e north american cordillera we present new quantitative evidence for similar margin parallel motion through large scale strike slip faulting in the western pacific margin the kurosegawa terrane in southwest japan is an attenuated tectonic sliver and considered to be allochthonous with respect to the main part of southwest japan late triassic and early cretaceous sedimentary rocks from the terrane yielded paleolatitudes of 4 n and 18 n respectively based on paleomagnetic data of the kurosegawa terrane and plate reconstructions of east asia we suggest that the terrane was the easternmost element of the south china block at least from the late triassic to the early cretaceous the terrane was then translated 1500 km northward from the continental margin to its present position associated with sinistral strike slip movement along the east asian continental margin in mid to late cretaceous times the average displacement rate is calculated to be 7 cm yr and is comparable to rates reported for the eastern pacific margin

upper cretaceous stable carbon isotope stratigraphy of terrestrial organic matter from sakhalin russian far east a proxy for the isotopic composition of paleoatmospheric co2 abstract time stratigraphic patterns of stable carbon isotopic ratios recorded in terrestrial organic matter from cenomanian maastrichtian successions for the russian far east can be correlated to those of carbonate carbon from well studied successions for other parts of the world age indicative biostratigraphy based on regional ammonoids and inoceramid bivalves are well established in japan and can be applied to the whole cretaceous succession except for the upper cenomanian lower turonian and the upper part of the maastrichtian zones lacking macrofossils globally correlative carbon isotopic events previously documented with carbonate carbon from europe and with carbonate carbon and marine organic matter from the u s western interior are recognized with similar magnitude through uppermost cenomanian lower campanian in ascending order these events are positive spike across the cenomanian turonian boundary step like leveled segment followed by negative shift lower middle turonian trough like negative excursion middle upper turonian positive rebound coupled with following broad peak coniacian and positive peak basal campanian followed by modest negative excursion the parallel fluctuation of the 13 c value between the terrestrial organic matter and carbonate suggests carbon isotopic equilibrium between surface seawater and atmospheric co 2 and demonstrates that the isotopic curve of terrestrial organic carbon can be used to monitor carbon isotopic fluctuations of co 2 in the ocean atmosphere system in the upper campanian and maastrichtian a negative 13 c excursion within chron 33r and a following rapid rebound can be correlated with the same distinctive carbon isotopic feature of carbonate observed in the deep sea cores from south and north atlantic the indian and the pacific oceans within this interval the 13 c value of terrestrial organic matter also reflects carbon isotopic fluctuation of the global co 2 reservoir with relatively minor regional local noise

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

chronology of sanbagawa metamorphism by collating age data based on the fossil age of the protoliths radiometric dating of the metamorphic minerals and sedimentary records of erosion at the earth s surface the history of the sanbagawa metamorphism can be summarized as follows 1 the pre metamorphic sedimentary rocks carboniferous jurassic early cretaceous became mixed and formed a thickened packet in the vicinity of an ancient trench through a variety of subduction related tectono sedimentary processes probably in early cretaceous time c 130 120 ma 2 the subducted protoliths underwent progressive metamorphism reaching a maximum depth of c 30 km in late early cretaceous time c 116 10 ma 3 the high p t metamorphic rocks began to rise toward the surface during the interval 110 50 ma with minimum estimates for the average cooling rate around 9 12 c ma and an average uplift rate around 0 4 0 5 mm year 4 finally at some stage after reaching the erosional surface the high p t metamorphic rocks were covered unconformably by the middle eocene c 50 42 ma kuma group on the basis of the present chronological summary of the sanbagawa metamorphism the areal extent of the sanbagawa metamorphism is also discussed with respect to the weakly metamorphosed subduction accretion complex of the next tectonic belt to the south the northern chichibu belt

tectonics of the qinling central china tectonostratigraphy geochronology and deformation history abstract the qinling orogen preserves a record of late mid proterozoic to cenozoic tectonism in central china high pressure metamorphism and ophiolite emplacement songshugou ophiolite assembled the yangtze craton including the lower qinling unit into rodinia during the 1 0 ga grenvillian orogeny the lower qinling unit then rifted from the yangtze craton at 0 7 ga subsequent intra oceanic arc formation at 470 490 ma was followed by accretion of the lower qinling unit first to the intra oceanic arc and then to the sino korea craton subduction then imprinted a 400 ma andean type magmatic arc onto all units north of the northern liuling unit oblique subduction created silurian devonian wnw trending sinistral transpressive wrench zones e g lo nan shang dan and late permian early triassic subduction reactivated them in dextral transpression lo nan shang xiang shang dan and subducted the northern edge of the yangtze craton exhumation of the cratonal edge formed the wudang metamorphic core complex during dominantly pure shear crustal extension at 230 235 ma post collisional south directed shortening continued through the early jurassic cretaceous reactivation of the qinling orogen started with nw se sinistral transtension coeval with large scale early cretaceous crustal extension and sinistral transtension in the northern dabie shan it presumably resulted from the combined effects of the siberia mongolia sino korean and lhasa west burma qiangtang indochina collisions and pacific subduction regional dextral wrenching was active within a ne sw extensional regime between 60 and 100 ma an early cretaceous andean type continental magmatic arc with widespread early cretaceous magmatism and back arc extension was overprinted by shortening related to the collision of yangtze indochina block with the west philippines block strike slip and normal faults associated with eocene half graben basins record paleogene nne ssw contraction and wnw ese extension the neogene is characterized by normal faults and nne trending sub horizontal extension pleistocene quaternary nw se extension and ne sw contraction comprises sinistral strike slip faults and is part of the nw se extension imposed across eastern asia by the india asia collision

shrimp u pb zircon dating of quartz bearing eclogite from the sanbagawa belt south west japan implications for metamorphic evolution of subducted protolith in order to decipher the origin of eclogite in the high p t sanbagawa metamorphic belt shrimp u pb ages of zircons from quartz bearing eclogite and associated quartz rich rock metasandstone were determined one zircon core of the quartz rich rock yields an extremely old provenance age of 1899 79 ma suggesting that the core is of detrital origin eight other core ages are in the 148 134 ma range and are older than the estimated age for trench sedimentation as indicated by the youngest radiolarian fossil age of 139 135 ma from the sanbagawa schists ages of metamorphic zircon rims 132 112 ma from the quartz rich rock are consistent with metamorphic zircon ages from the quartz bearing eclogite indicating that eclogite facies metamorphism peaked at 120 110 ma these new data are consistent with both the iratsu eclogite body and surrounding highest grade sanbagawa schists undergoing coeval subduction zone metamorphism and subsequent re equilibration under epidote amphibolite facies conditions during exhumation

deep structure of lithospheric fault zones 1 we calculate the cumulative width w of ductile shear zones accommodating plate motion in continental lithosphere based on the assumptions that 1 the flow stress is controlled by the yield strength of intact rock at any given depth 2 the yield strength profile through the crust can be constrained from observations in exhumed shear zones and 3 strain localization is primarily caused by grainsize reduction leading to a switch to grainsize sensitive creep we use a mid crustal stress temperature profile measured in the whipple mountains california and calculate stress profiles at depth from published flow laws for feldspar and olivine we conclude thatwfor a plate boundary shear zone accommodating 50 mm yr displacement comparable to the san andreas transform may reach 180 km in the quartz rich mid crust depending on water fugacity and thermal gradient it narrows to a few meters in feldspathic lower crust and in the uppermost mantle and then widens rapidly with depth in the lower lithosphere we explore the effects of differences in crustal thickness and composition thermal gradient and water activity

stress gradients around porphyroclasts palaeopiezometric estimates and numerical modelling abstract based on strain gradients commonly found around porphyroclasts the mechanical interaction of porphyroclasts and recrystallized mylonite matrix in a retrograde amphibolite facies shear zone of the ivrea verbano zone italy was investigated differential stresses inferred from recrystallized grain size and dislocation density indicate stress concentrations at the porphyroclast matrix interface of about 1 7 2 0 with respect to the average in the matrix the palaeopiezometric stress data are compared to the results of a finite element model composed of two rheologically different phases in this model the local distribution and concentration of stresses around porphyroclasts mostly depend on clast matrix coupling and the effective viscosity contrast of the two phases the bulk strength and work hardening of the composite system increases with interface coupling both the modified stress field and the reduced average grain size around porphyroclasts may result locally in instabilities and promote strain localization and a change in the dominant deformation mechanism

exhumation of the dahinggan mountains ne china from the late mesozoic to the cenozoic new evidence from fission track thermochronology abstract zircon and apatite fission track ft analyses were used to reveal the exhumation of phanerozoic granitoid intrusions in the dahinggan mountains northeastern china zircon ft ages ranging from 84 5 to 97 8 ma and apatite ft ages from 47 3 to 67 4 ma with mean track lengths between 12 3 2 3 and 13 3 2 2 m respectively were obtained for the phanerozoic granitoids sampled along a south south west to north north east transect across the dahinggan mountains three cooling stages from the late mesozoic to the cenozoic have been identified in the studied region based on ft results and modeling stage one was a rapid cooling period from the closure temperature of zircon ft to the high temperature part of the apatite ft partial annealing zone 210 110 c during 90 57 ma stage two was a relatively slow cooling period 110 60 c occurred between 57 ma and 20 ma suggesting that the granitoids had been exhumed to depths of 1 2 km the final stage was a cooling period 60 20 c occurred since the miocene which bring studied samples to the earth s surface at an accelerated exhumation rate the estimated maximum exhumation is 5 km assuming a steady state geothermal gradient of 35 c km our results indicate that this exhumation is possibly led by the pacific plate subduction combined with intracontinental orogeny associated with asthenospheric upwelling

gravel bearing radiolaria as tracers for erosional events a review of the status of recent research in sw japan and korea abstract the authors introduce a case study of research on the provenance of radiolaria bearing gravels and clasts of the monomictic chert pebble conglomerates in the lower cretaceous monobegawa group a paralic molasse in the outer zone of southwest japan reconstruction of oceanic plate stratigraphy from gravels and clasts in monomictic chert pebble conglomerates based on their lithology and radiolarian ages is possible and effective for the provenance research in the case of the monobegawa group a review of provenance studies of radiolaria bearing gravels in mesozoic orogenic sediments in sw japan and korea is included the paper concludes with the discussion of the erosional events of the jurassic and pre jurassic accretionary complexes in sw japan with reference to the late mesozoic geological history in the eastern margin of asian continent