apatite fission track evidence for the cretaceous cenozoic cooling history of the qilian shan nw china and for stepwise northeastward growth of the northeastern tibetan plateau since early eocene abstract apatite fission track aft data from hinterland of the qilian shan at the northeastern margin of the tibetan plateau suggest this range has experienced northeastward propagation of surface uplift since early eocene and that crustal shortening occurred in the qilian shan before the late miocene thermochronometry data indicate that the qilian shan experienced a three stage cooling history including 1 rapid initial cooling during cretaceous 2 a stage of slow cooling during late cretaceous early eocene and 3 rapid stepwise cooling in a southwestern northeastern orientation since early eocene cretaceous rapid cooling may be a record of the lhasa block and eurasian collision early cretaceous denudation was followed by tectonic and quasi isothermal quiescence that continued until early eocene early eocene rapid cooling in the south qilian shan may be the first far field response in the qilian shan to the collision and convergence of the indian and eurasian continents from late eocene to middle miocene crustal shortening propagated into the central qilian shan and north qilian shan and produced surface uplift of the entire qilian shan region before the late miocene this study provides a better understanding of the tectonic evolution of the qilian shan and when the far field stress from the india eurasia collision into the northeastern tibetan plateau began

magnitude of post middle jurassic bajocian displacement on the central altyn tagh fault system northwest china the altyn tagh fault system a key structural feature in the tectonic collage of central asia is an active left lateral strike slip fault system however its age of initiation kinematic history and total magnitude of displacement are not well known middle jurassic nonmarine sedimentary rocks that crop out along the central segment of the altyn tagh fault were deposited in a contiguous tarim qaidam foreland style basin that was geodynamically linked to mesozoic contraction in the tian shan bei shan qilian shan and kunlun shan this study documents the best evidence of a piercing point to date consisting of an offset lacustrine shoreline demarcated by aalenian bajocian open lacustrine strata to the west and fluvial and alluvial strata of comparable age to the east detrital compositions palynology paleocurrents and lithostratigraphy support correlation of these sections restoration of this jurassic facies boundary indicates 400 60 km of post bajocian left lateral separation on the altyn tagh fault consistent with this result restoration of the jurassic shoreline aligns felsic plutons across the western segment of the altyn tagh fault system that may have been left laterally offset by 360 km these two pairs of offset features suggest that the current best estimate for net magnitude of post bajocian left lateral separation on the altyn tagh fault is about 360 km

deep structures and surface boundaries among proto tethyan micro blocks constraints from seismic tomography and aeromagnetic anomalies in the central china orogen abstract the qinling qilian orogen preserves the records of early paleozoic convergence among the proto tethyan micro blocks in this study we analyze the seismic velocity structure and the aeromagnetic anomalies in the qinling qilian orogen and its adjacent areas showing that the northernmost boundaries of these proto tethyan micro blocks are defined by the guyuan longshoushan fault in the qilian orogen and by the luonan luanchuan fault in the qinling orogen respectively the lithosphere north of the qinling qilian orogen subducted southward under the qinling qilian orogen the boundaries of the micro blocks in the qinling qilian orogen of the proto tethys domain are mostly ew or ne trending combined with early paleozoic geological records our results indicate that the south dipping low velocity anomaly under the alxa block and the south dipping high velocity ones under the north qilian orogen and the north china block might indicate that the alxa block the north qilian oceanic slab and the north china block subducted southward the south dipping high velocity anomaly under the north qilian orogen is interpreted as the remnant of the proto tethys ocean in that area however the north dipping low velocity anomaly under the south qinling and the north qinling orogens the qaidam block and the north dipping high velocity one under the yangtze and the bikou blocks suggest that the south qinling and the north qinling orogens the qaidam the yangtze and the bikou blocks subducted northward the present spatial framework of the qinling qilian orogen is related to early paleozoic convergence among the proto tethyan micro blocks

postglacial uplift neotectonics and seismicity in fennoscandia abstract fennoscandia has experienced major uplift in postglacial time which is assumed to reflect a glacial isostatic process connected to the melting of the last ice sheets extensive modelling of the isostatic movements show that the applied deglaciation and uplift model fit the observations well there are however areas with significant deviations between uplift measurements and regional model predictions the misfit between observations and the isostatic uplift modelling is interpreted here to reflect a tectonic component of the uplift the objective of the present investigation is to isolate this tectonic uplift component interestingly enough the areas found partly correspond to areas with pronounced seismic activity and the assumption that the postglacial rebound is responsible for much of the observed onshore seismicity is substantiated we conclude that there seems to be present day deformation along the shoreline of mid norway southern norway shoreline and mountain areas and along the swedish east coast with the centre northeast of the gulf of bothnia that cannot be explained by glacial isostasy not all of the deformations in these areas are necessarily co seismic the study suggests that such vertical deformations are small in magnitude and overprint the glacial rebound the deformations may be a consequence of the plio pleistocene erosional pattern which is of glacial origin

crustal structure across northeastern tibet from wide angle seismic profiling constraints on the caledonian qilian orogeny and its reactivation abstract the northeastern tibetan plateau results from the superposition of the caledonian qilian orogenic belt and cenozoic reactivation following continental collision between the indian and asian plates in order to identify the constraints on the qilian orogenic mechanism and the expansion of the plateau we carried out a 430 km long wide angle seismic experiment between jingtai and hezuo we herein present the crustal p wave velocity structure model resulting from the interpretation of this wide angle seismic dataset the principal characteristics of the crustal velocity model include 1 thinning of the crust from south to north within a range of about 48 54 km with some undulation of the moho beneath the northern qilian block 2 the thickness of the sedimentary layer and its average p wave velocity exhibit the obvious tectonic division of the qaidam kunlun west qinling belt the central and northern qilian and the alax blocks 3 the lower crustal layer has a p wave velocity of 6 6 6 8 km s beneath the qaidam kunlun west qinling belt northern qilian orogenic belt and alax block but has a very low value of 6 4 6 5 km s beneath the central qilian block and 4 the central qilian block shares a similar crustal p wave velocity depth relationship with the sierra nevada arc or the accretionary crust the qaidam kunlun west qinling and the northern qilian orogenic belts have a crustal p wave velocity depth relationship characteristic of the crust of the global average continent the particular vp depth relationship beneath the central qilian orogenic belt may result from a caledonian accretionary orogeny or from the delamination of the ecologized lower crust during the arc continental collisional orogeny the lower velocity layer lvl exists at the bottom of the upper crust to the north of the middle of the central qilian block we infer that this lvl plays an important role as an intracrustal decollement to expand the plateau

controls of the altyn tagh fault on the early middle miocene sedimentation in the honggouzi area qaidam basin western china abstract the altyn tagh fault atf has important controls on the sedimentation in the qiadam basin in this study detailed sedimentological and geochemical analyses were presented to deduce the impact of the atf on deposition in the honggouzi area during the early middle miocene which is a significant period for atf strike slip movements the early middle miocene strata of the honggouzi area are deposits of a fan delta and show retrogradation by sedimentary characteristic of drilled wells the elementary geochemical analysis indicates that the early middle miocene deposits of the honggouzi area was mainly composed of felsic rocks formed in the continental island arc or active continental margin setting and derived from a segment of the altyn tagh mountains atm in addition the seismic data suggests that 1 the nearly nw se directed anticlines which were caused by the flexural deformation and uplift of the piedmont area in the northwestern qaidam basin occurred simultaneously during the early middle miocene and 2 retrogradational sedimentation was caused by greatly increased accommodation during the early middle miocene when the subsidence amount of flexural deformation was larger than the uplift amount the flexural deformation uplift of the piedmont area of the northwestern basin and uplift of the atm were all controlled by the atf strike slip movements and therefore were the ways in which the atf strike slip movements affected the early middle miocene deposition of the honggouzi area

the evolution of the complex anticlinal belt with crosscutting strike slip faults in the central tarim basin nw china integrated 2 d and 3 d seismic data sets reveal that a wnw trending complex anticlinal belt the tazhong uplift was cut roughly perpendicularly by a series of nne oriented strike slip faults in the central tarim basin nw china through detailed interpretation of the internal architecture of the tazhong uplift and the structural characteristics of the strike slip faults the timing of their movements was determined based upon their geometric relationship and coeval movement we propose an integrated model for the evolution of both the tazhong uplift and the strike slip fault system initially the tazhong uplift formed perpendicular to the nne oriented compression that responded to the collision between the tarim block and the western kunlun terrane in the latest middle ordovician meanwhile regional joints formed parallel to the maximum compression that is perpendicular to the tazhong uplift in the latest ordovician northward compression caused the uplifting climax of the tazhong uplift in response to the collision between the tarim block and the coherent eastern kunlun altyn tagh qaidam qilian terrane under such a northward compression the resolved shear stress on the nne trending joints formed the transpressional strike slip faults afterward the tazhong uplift tilted northward during the ongoing subduction from the kunlun ocean which lasted until the latest middle devonian the negative flower structures formed along the preexisting transpressional strike slip faults due to the dragging of the overlying covers in the latest middle devonian the data and evolution model not only unveil the formation mechanism of the tazhong uplift and the associated strike slip faults but also provide further constraints on the evolutionary age of the proto tethys ocean

sandstone provenance of north western sectors of the intracontinental cenozoic qaidam basin western china tectonic vs climatic control abstract sandstone samples from all cenozoic formations of the nonmarine internally drained qaidam basin have been collected for provenance analysis to monitor possible changes in sandstone composition driven either by tectonics or climate provided an appropriate hinterland biotite contents may be a reasonable indicator for aridity moderately sorted sandstones with subangular to subrounded grains from the northwestern part of the qaidam basin are immature indicated by a high proportion of feldspars and lithic fragments nevertheless they have relatively high quartz contents of 29 65 averaging between 40 and 45 the average framework compositions of the formations show only a slight variation over time limited to the oldest and youngest cenozoic formations contents of white mica and biotite are significantly higher in upper portions of the section white mica contents increases from 1 0 to 3 7 and biotite from 0 1 to 1 6 respectively this change can be explained by increasing aridity during neogene and surface uplift of altyn mountains and qimantagh which led to higher relief when plotted on basic variation diagrams total quartz feldspar lithic fragments there is a slight shift from continental block field to orogenic field which might reflect the motion of the altyn tagh fault since oligocene times although climate changed during the cenozoic from arid over semiarid back to arid we could not find any distinct changes in the framework mineral compositions over time that could be related to climatic events few selected mudstones from eocene to lower miocene formations have been geochemically analysed by icp ms all samples display uniform high enrichment in light rees in chondrite normalized diagrams with one exception the ree concentrations are decreasing in a stepwise manner although insignificantly from formation to formation with time due to a diluting effect by carbonate

formation and evolution of strike slip faults rifts and basins during the india asia collision an experimental approach the processes which have governed the formation and evolution of large tertiary strike slip faults during the penetration of india into eastern asia are investigated by means of plane strain indentation experiments on layered plasticine models the steady state deformation of plasticine obeys a power creep flow law e c t n the stress exponent n is between 6 and 9 at 25 c uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening a process observed for strain rates ranging from 3 5 10 5 to 3 6 10 3 s 1 fault or shear zones form after only 7 10 bulk strain subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine a series of nine plane strain indentation experiments shows the influence of boundary conditions as well as that of the internal structure of the plasticine model on the faulting sequence the ubiquity of strain softening in experimental deformation of a variety of rocks as well as the widespread occurrence of shear zones in nature suggest that long term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain the deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision induced strikeslip faulting in asia particularly to total offsets and rates of movements on the faults the experiments simulate the evolution of the western ends of the strike slip faults which have probably been analogous to trench fault fault triple junctions the experiments also illustrate mechanisms for the formation of extensional basins such as the south china sea north china basin and andaman sea near active continental margins the basins which appear to absorb terminal offsets along major strike slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts the existence of distinct phases of strike slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous non steady state deformation of the continental lithosphere

argon oxygen and boron isotopic evidence documenting 40 ar e accumulation in phengite during water rich high pressure subduction metasomatism of continental crust abstract the luliang shan area of the north qaidam high pressure hp to ultrahigh pressure uhp metamorphic terrane in northwestern china features thick garnet and phengite rich metasomatic selvages that formed around gneiss hosted mafic eclogite blocks during hp conditions here we present new 40 ar 39 ar 18 o and 11 b results from a previously studied 30 m 18 sample traverse that extends from the host gneiss into a representative eclogite block previous thermobarometry and new mica quartz oxygen isotope thermometry from the traverse reveal that the phengite rich selvage formed at temperatures similar to those recorded by the eclogites at peak pressure quartz and white mica 18 o data from the selvage cannot be explained by simple mixing of gneiss and eclogite and indicate a fluid rock ratio 1 during regional scale infiltration of high 18 o ca 14 fluids heavy 18 o overgrowths of metamorphic zircon over lighter 18 o detrital grains indicate that the gneiss was similarly affected starkly contrasting boron content and 11 b compositions for the host gneiss and the selvage also cannot be explained by local scale devolatilization of the gneiss to form the selvage instead the boron systematics are best attributed to two distinct phases of fluid infiltration 1 low boron selvage phengite with 11 b from 10 to 30 grew under hp conditions and 2 tourmaline and boron rich muscovite with generally positive 11 b crystallized in the host gneiss under subsequent lower pressure epidote amphibolite facies conditions as the luliang shan gneiss terrane was exhumed past shallower portions of the subduction channel consistent with observations made worldwide we were able to identify uptake of excess argon 40 ar e in phengite as a high pressure phenomenon phengite 40 ar 39 ar ages from massive eclogite exceed the ca 490 ma zircon u pb age of eclogite metamorphism by a factor of 1 5 however phengite ages from the more permeable schistose selvage were even older exceeding the time of eclogite formation by a factor of 1 7 in contrast lower pressure retrograde muscovite present within the host gneiss and in discrete shear zones cutting the selvage yield 40 ar 39 ar ages that were younger than the time of hp metamorphism and consistent with regional cooling age patterns our observation of high 40 ar e concentrations in phengite from schistose rocks infiltrated by regionally extensive fluids at hp conditions runs contrary to widely held expectations conventional wisdom dictates that low phengite fluid partition coefficients for argon d phg fluid ar 10 3 to 10 5 coupled with the dry closed systems conditions that are widely reported to characterize hp metamorphism of continental crust explains why high concentrations of 40 ar e partitions are able to accumulate within phengite we alternatively propose that phengite fluid partition coefficients for argon increase linearly with pressure to values as high as 10 2 to allow phengites to accumulate large amounts of 40 ar e from aqueous fluids under hp to uhp conditions

juxtaposition of diverse subduction related tectonic blocks with contrasting metamorphic features and ages in the paleoproterozoic aketashitage orogen nw china implications for precambrian orogeny

northeastward growth of the tibet plateau deduced from balanced reconstruction of two depositional areas the qaidam and hexi corridor basins china we address the problem of late cenozoic uplift erosion and growth of northeastern tibet by reconstructing from isopach maps and drill holes the volumes and masses of sediments deposited in the qaidam and hexi corridor basins since 35 myr ago the mass budget is based on simple geometrical assumptions such as regional similarity of the thickness ratios between strata of different ages in the qaidam where our record extends back to the oligocene the budget shows a huge rise of the accumulation rates after the beginning of the pliocene 5 3 ma the early pliocene seems to be the period of maximum deposition with accumulation rates in excess of 1 mm yr 1 2 7 kg m 2 yr 1 of compacted rocks throughout the basin there also seems to be a southeastward shift of the largest depocenters between the upper pliocene 3 4 1 6 ma and the quaternary in the hexi corridor sedimentation is confined to small foreland flexural depressions associated with the frontal thrusts of the qilian shan and occurs at an average rate one order of magnitude smaller than in the qaidam basin the accumulation rate is maximum in the quaternary the sedimentation history appears to support a plateau building mechanism resulting from the combination of two geologicallly common processes crustal scale thrusting and sedimentary basin infilling the time needed to completely fill the qaidam basin and make its catchment a plateau closely resembling that of the highest part of tibet qangtang is of the order of 9 myr the mechanism now at work north of the kunlun which involves rapid infilling of broad flat areas separated by relatively narrow mountain ranges has thus probably been important in producing the high smooth topography that characterizes much of central tibet

the indo asian continental collision a 3 d viscous model abstract the large scale physical process of the indo asian continental collision and the formation of the himalayan tibetan plateau have been simulated in various viscous thin sheet models but the thin sheet simplification also kept some important issues from being fully explored among these issues are the role of strike slip fault zones in facilitating large scale lateral translation of lithospheric blocks the escaping tectonics during the collision and the speculated lateral flow of the ductile middle lower crust under the tibetan plateau here we present a fully three dimensional finite element model to simulate the indo asian continental collision the model includes major boundary faults to simulate the escaping tectonics and vertically variable rheological structures to model lower crustal flow the collisional process is constrained by the history of the indo eurasian plate convergence and the present crustal thickness and topography of the tibetan plateau our results indicate that the restrictive boundaries of the tibetan plateau including the rigid tarim and south china blocks largely control the spatiotemporal patterns of crustal deformation in the collision zone as the indian indenter moves toward the tarim block higher strain rates and topography developed in the western part of the collision zone than in the eastern part causing the northward migration of the deformation front to gradually change to eastward migration in the past 10 20 myr broadly consistent with the initiation of widespread e w extension in the plateau these restrictive boundary blocks also force the crustal and mantle materials in the collision zone to flow coherently hence providing an alternative explanation for the apparently vertically coherent deformation in tibet assuming that the crust weakens as it thickens our model predicts the lateral expansion of the tibetan plateau an important feature of the tibetan tectonics that is missing in previous models with constant rheology

jurassic to cenozoic exhumation history of the altyn tagh range northwest china constrained by 40ar 39ar and apatite fission track thermochronology

crustal structure of the northeastern tibetan plateau from the southern tarim basin to the sichuan basin china abstract we present a new crustal cross section of the northeastern tibetan plateau based on active source seismic data recorded along a 1600 km long profile crossing the southern tarim basin the western flank of the south qilian shan the northeastern margin of qaidam basin east kunlun shan songpan ganzi terrane and sichuan basin the crustal p and s wave velocity structure and poisson s ratio outline the seismic characteristics of the crust and provide constraints on the crustal composition the derived crustal cross section shows several significant features 1 the crustal thickness varies considerably along this profile from 48 km to 70 km 2 north of the kunlun fault variations in total crustal thickness are mainly caused by variations in lower crustal thickness whereas south of the kunlun fault they are caused by thickness variations throughout the crust 3 north of the kunlun fault we detect a mid crustal low velocity zone that is not apparent south of the fault 4 the kunlun fault seems to act as a compositional boundary for the lower crust with a poisson s ratio of 0 29 north of the fault kunlun qaidam terrane and 0 26 south of the fault songpan ganzi terrane measured poisson s ratio and p wave velocity values suggest that the lower crust throughout the tibetan plateau south qilian shan margins of the qaidam basin east kunlun shan songpan ganzi terrane is of intermediate composition thus the ne tibetan plateau along our profile is missing a mafic lower crustal layer

structural anatomy of the early paleozoic laohushan ophiolite and subduction complex implications for accretionary tectonics of the proto tethyan north qilian orogenic belt northeastern tibet

tectono magmatic rejuvenation of the qaidam craton northern tibet abstract ascertaining the enigmatic characteristics of the crystalline basement of the qaidam basin the deepest intracontinental basin yet located in the highest orogenic plateau with the thickest continental crust has broad implications for the cenozoic basin evolution in particular and intracontinental tectonics of asia in general we carried out a comprehensive investigation on the ages and geochemical isotopic characteristics of the qaidam and suganhu basement rocks obtained from drill cores petrological observations and u pb zircon dating la icp ms of granitoid samples from drill cores indicate that early neoproterozoic early paleozoic and late paleozoic mesozoic intrusions are largely represented suggesting that the qaidam and suganhu blocks were involved into three tectono magmatic episodes similar whole rock geochemistry and sr nd isotopic characteristics between the early neoproterozoic granitoids beneath the qaidam basin and the contemporaneous basement rocks in the surrounding orogenic belts confirm the existence of pervasive early neoproterozoic metamorphism and magmatism along the present day northern tibetan plateau by comparing the trace and rare earth elements patterns as well as the sr nd compositions of the ordovician arc related granitoids and silurian devonian post collisional granitoids beneath the qaidam and suganhu basins with the corresponding granitoids from the surrounding orogenic belts we assert that the proto tethys subduction events that were recorded within the eastern kunlun altyn tagh and the qilian shan orogenic belts were different they generated distinctive subduction related arc magmas whereas the closure of the oceanic basins resulted in the amalgamation of the orogenic belts inducing subsequent extensive post collisional magmatism within the qaidam block the widespread permian and triassic granitoids beneath the northwestern northern qaidam basin mainly involved a lower crust or a crust mantle source including mesoproterozoic crust they demonstrate that the influence of the paleo tethys evolution penetrated the qaidam block northwards reaching to the northern edges of the block we conclude that the qaidam block is not a craton as typical as the rigid tarim block but pertains to a tectono magmatic rejuvenated craton which is not mechanically stronger than its surrounding regions however trapped between the lithospheric left lateral strike slip altyn tagh fault system to the northwest and the qilian shan transpressional system to the northeast the qaidam block forms a relatively stable domain that escaped from significant cenozoic deformation

rate of left lateral movement along the easternmost segment of the altyn tagh fault east of 96 e china summary field work carried out in gansu province and complemented with analysis of spot panchromatic scenes allows us to characterize the deformations along the eastern segment of the altyn tagh fault and to place bounds on its holocene left slip rate east of 96 e the long term left lateral offset of stream channels alluvial fans and terrace edges is about 50 m these offsets are most probably of holocene age 12 2 ka and imply that the corresponding derived slip rate is 4 2 mm yr 1 this observation is consistent with a north eastward along strike decreasing slip rate on the altyn fault due to partitioning of slip on multiple more easterly trending splays

early carboniferous paleomagnetic results from the northeastern margin of the qinghai tibetan plateau and their implications abstract we conducted paleomagnetic investigations on limestone from the lower carboniferous huaitoutala formation in the qaidam basin near delingha city qinghai province china the characteristic remanent magnetization d 5 8 i 25 7 k 114 3 95 4 8 passes a fold test and indicates a paleopole position of 39 2 n 90 4 e and a paleolatitude of 13 5 n for the qaidam block for the early carboniferous based on global tectonic reconstructions and paleontological evidence we suggest that the qaidam block was adjacent to but independent from the north china south china alashan hexi and tarim blocks at this time this result suggests that pre carboniferous sutures reported around the qaidam basin represent collisional events within gondwana rather than the final sutures that gave rise to the present tectonic configuration

paleozoic to cenozoic deformation along the altyn tagh fault in the altun shan massif area eastern qilian shan northeastern tibet china

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

active thrusting and folding in the qilian shan and decoupling between upper crust and mantle in northeastern tibet fieldwork south of the city of gaotai gansu province china shows that active shortening of surface sediments in the foothills of the yumu shan a large fore mountain of the qilian shan at the northeastern edge of tibet involves both overthrusting and flexural slip folding north of this mountain we found and mapped a prominent north facing thrust scarp that offsets a holocene fan sloping gently 3 4 to the north part of this scarp appears to be related to the m 7 5 180 a d earthquake that may have led to the demise of the han dynasty city of luo tuo chen in the hexi corridor a set of 10 100 150 m long profiles measured across this scarp 3 2 m high on the average can be made to fit the diffusion degraded morphology of a surface break related to the 180 a d event using a value of about 3 3 m 2 10 3 yr for the mass diffusivity of fanglomerates in this part of gansu province smaller mountain facing scarps on a terrace capped foothill result from bedding slip concurrent with active folding of underlying steeply northdipping plioquaternary sandstone and conglomerate beds holocene uplift rates along the yumu shan which is only one of the qilian shan ranges are estimated to be between 0 4 and 1 9 mm yr which implies that much of the mountain formed in the quaternary the periclinal structure of the plioquaternary envelope under which the paleozoic core of the yumu shan plunges towards the west suggests that the whole 3200 m high mountain is a basement ramp anticline mountains striking parallel to the yumu shan with similar structure and comparable or greater sizes north and south of the hexi corridor probably also correspond to recent crustal ramp anticlines this implies that the wide mountainous upper crustal wedge making the northeastern edge of the tibet qinghai plateau is detached from the underlying lower crust and upper mantle

discussions on the neoproterozoic diorites in central tarim basin a comment on geochronology and geochemistry of deep drill core samples from the basement of the central tarim basin by guo et al journal of asia earth sciences 2005 vol 25 45 56

magmatic history of the northeastern tibetan plateau 1 the northeastern margin of the tibetan plateau is underlain by the qaidam and qilian terranes which consist primarily of mid proterozoic through lower paleozoic oceanic and arc type assemblages that have been accreted to the southern margin of the tarim sino korean craton most previous models suggest that these assemblages formed along a northeast dipping subduction system constructed along the margin of the tarim sino korean craton during early paleozoic time the main components are interpreted to have formed either as an archipelago of volcanic arcs and back arc basins or as a broad expanse of accretionary complexes our geochronologic data support a model suggested by sobel and arnaud 1999 in which the qaidam and qilian terranes are separated from the tarim sino korean craton by a mid paleozoic suture that closed along a southwest dipping subduction zone the basement to these terranes consists of oceanic assemblages that were amalgamated into a coherent crustal fragment prior to emplacement of 920 930 ma granitoids early paleozoic arc type magmatism occurred between 480 and 425 ma apparently sweeping southwestward across much of the qilian and qaidam terranes accretion related magmatism along the inboard margin of the qilian terrane occurred between 423 ma and 406 ma following silurian devonian accretion the region has experienced late paleozoic and mesozoic uplift and erosion and has been severely overprinted by tertiary thrusting uplift and strike slip motion along the altyn tagh fault correlation of geologic features and magmatic histories between the altun shan and the nan shan suggests that the eastern altyn tagh fault has a total left lateral offset of 375 km

the geodynamic setting of dulan eclogite type rutile deposits in the north qaidam orogen western china abstract abundant eclogite type rutile deposits have recently been discovered along the north qaidam ultrahigh pressure uhp metamorphic belt in china the identified titanium reserves exceed 3 million tons and range in grade from 1 0 to 4 5 the rutile deposits of economic interest are found within two types of eclogite however the origin and original geodynamic setting of the two types of rutile bearing eclogites remain controversial this study therefore investigates ore and mineralization bodies belonging to the two types by way of detailed field work and geochronological geochemical and sr nd hf isotopic compositional analysis taking the dulan eclogite type rutile deposit as its case study type 1 rutile bearing eclogites enclosed in granitic gneisses are found to have neoproterozoic protolith ages of 845 4 ma and an eclogite facies metamorphic age of ca 433 ma their high tio2 tfe2o3 and p2o5 contents cannot have been produced by rutile formation during uhp metamorphism but could have arisen through the accumulation of ilmenite and apatite in a high ti protolith they have ocean island basalt oib like trace element patterns with enrichment in light rare earth elements lrees and high field strength elements hfses nb ta zr and hf the hf isotopic compositions of the residual magmatic zircon cores in type 1 rutile bearing eclogites ehf 845 1 4 to 6 0 tdm 1 2 1 5 ga indicate that their parental magmas were derived from the proterozoic mantle however they show initial 87sr 86sr values of 0 71059 0 71326 and end 845 values of 1 5 to 2 5 suggesting significant crustal contamination thus the type 1 rutile bearing eclogites may have originated through partial melting of an oib type mantle plume source during the neoproterozoic rifting of the rodinia supercontinent in contrast the type 2 rutile bearing eclogites which are hosted in a mafic ultramafic ophiolitic complex have protolith ages of 449 3 ma and eclogite facies metamorphic ages of ca 435 433 ma correlations between the whole rock mgo and the major and trace element contents of type 2 rutile bearing eclogites suggest that they are derived from cumulates with alternating olivine and pyroxene rich and ilmenite rich layers they have mid ocean ridge basalt morb and oib like trace element patterns indicating extensive fractional crystallization and the possibility that there were multiple injections of magma during the formation of the protolith meanwhile the 0 6 to 4 3 end 449 values and 1 8 to 11 8 ehf 449 values of the type 2 rutile bearing eclogites indicate that the protoliths may have been generated by partial melting of a mantle wedge source metasomatized by slab derived components the short interval ca 14 my between magma generation and eclogite facies metamorphism in combination with the type of rock assemblage within which they are found indicates that the type 2 rutile bearing eclogites originated in a supra subduction zone ssz ophiolite type 1 and type 2 rutile bearing eclogites have similar eclogite facies metamorphic ages of ca 435 433 ma indicating that the type 1 and ssz related type 2 protoliths were incorporated into a subduction melange that was dragged down the subduction channel and underwent eclogite facies metamorphism via subduction of the qaidam block eventually forming eclogite type rutile deposits

riser diachroneity lateral erosion and uncertainty in rates of strike slip faulting a case study from tuzidun along the altyn tagh fault nw china 1 at late quaternary timescales offset fluvial terrace risers are among the most common landforms used to determine rates of strike slip faulting although diachroneity in the age of riser segments on opposite sides of the fault has been noted previously an unexplored source of uncertainty associated with deriving slip rates from these markers centers on quantifying the size of the displacement uncertainty such diachroneity introduces to evaluate the impact of riser diachroneity we investigated the tuzidun site 37 73 n 86 72 e along the cherchen he reach of the active central altyn tagh fault the east bank of the channel is flanked by a left laterally offset terrace riser while the measured offset is 54 3 m geochronologic measurements and analysis of riser topography indicate that the downstream riser segment formed between 6 0 0 8 ka and 5 7 0 4 ka while the upstream riser segment may have been laterally refreshed as recently as 0 5 0 2 ka a valley wall on the west bank of the channel places a maximum limit of 38 6 m on the amount of possible lateral erosion of the upstream riser this bound in turn limits the total offset since formation of the downstream riser to range from 54 3 to 89 7 m together these observations bracket the millennial altyn tagh fault slip rate to range from 9 0 1 3 to 15 5 1 7 mm a 1 more generally this investigation shows that the observed riser displacement does not necessarily correlate with the age of either riser segment downstream or upstream of the fault in cases where one segment is displaced while the other is subjected to lateral erosion if this diachroneity goes undetected erroneous slip rate measurements are likely to result

ore genesis and fluid evolution of the kaladawan south zn pb cu ore field eastern altyn mountains nw china evidence from fluid inclusions h o isotopes and geochronology abstract the newly discovered kaladawan south zn pb cu orefield in eastern altyn mountains is composed of five zn pb cu deposits with the largest one containing 414 4 thousand tonnes kt zn 265 2 kt pb and 149 2 kt cu stratiform lenticular orebodies are hosted by the ordovician volcanic sedimentary sequences subjected to greenschist facies metamorphism two major ore forming stages with significant cross cutting relationship are identified at kaladawan south stage i of the earlier massive banded disseminated barite rich sulfide ores and stage ii of the later polymetallic sulfide quartz veins with economically less importance fluid inclusions fis locked in quartz of stage i massive sulfide ores are homogenized completely at 226 354 c with calculated seawater like salinities of 2 7 4 2 wt nacl equiv these characteristics of ore forming fluids coupled with ore textures of typical vms deposits are interpreted to be originated from the modified seawater by sea floor volcanic hydrothermal activities in compared the stage ii fis yielded lower homogenization temperatures ranging 251 c 357 c with higher salinities of 0 8 14 4 wt nacl equiv laser raman analyses identified large amounts of co 2 ch 4 n 2 in the hydrothermal fluids of stage ii the 18 o h2o values 0 5 5 7 and d h2o values of 68 1 to 32 3 of stage ii quartz are plotted in the range of metamorphic fluids in the d o isotopic diagram which indicating that the co 2 rich and mesothermal fluids are metamorphic derived the phlogopite paragenetic in the stage ii polymetallic vein yielded a 40 ar 39 ar plateau age of 435 4 3 9 ma similar to the age of the metamorphic muscovite 451 0 4 1 ma from the ore hosting schist based on the ore deposit geology mineral assemblages and fluid geochemistry the kaladawan south zn pb cu mineralization is attributed to be an ordovician vms type deposit overprinted by the silurian metamorphic fluids

early paleozoic to devonian multiple accretionary model for the qilian shan nw china abstract the qilian shan located in the northern tibetan plateau of nw china has an excellent record of early to middle paleozoic subduction accretion that resulted from convergence between the alax and qaidam blocks but there is no consensus about its detailed tectonic history this paper summarizes its tectonic divisions and discusses its tectonic evolution from the cambrian to devonian the belt has the following tectonic divisions in the far north the southern passive margin of the alax block is juxtaposed against an early cambrian to ordovician marianan type intra oceanic arc north qilian the north qilian arc is separated from a japanese type arc central qilian to the south by a high pressure metamorphic belt composed mostly of ophiolitic slices and oceanic crusts the central qilian is bounded to the south by a wide melange zone south qilian consisting of ophiolitic slices and continental margin sequences farther to the south lies the oulongbuluk microcontinent that is separated from the qaidam block and farther south by an ultrahigh pressure metamorphic belt tectonostratigraphic analysis together with geochemical geochronological and geophysical data indicates a complex evolution by subduction accretion processes from the cambrian to the devonian before final amalgamation and docking to the northern alax block this model solves the long lasting discussion on the polarity of subduction in paleozoic time this multiple subduction accretion history sheds light on the continuity of paleozoic sutures along the qilian shan and the nature of the altyn tagh fault and thus contributes to an improved understanding of the tectonic architecture of central asia

a possible middle paleozoic suture in the altyn tagh nw china preliminary 40ar 39ar thermochronologic results from two transects across the altyn tagh at range allow new interpretations for the genesis of the range and provide constraints for its subsequent displacement recent reports of middle to upper ordovician pillow lavas in the ne portion of the range as well as the first reports of high pressure metamorphic rocks within the range are interpreted as the remnants of a lower paleozoic ocean subducted within the at muscovite and biotite ages from schist gneiss and granodiorite document a significant metamorphic and intrusive event throughout the range at 435 20 ma this fabric is crosscut by undeformed postorogenic microgranite plutons dated at 383 7 ma by muscovite these data suggest that an ocean basin closed after the early silurian and prior to the middle devonian forming the herein named lapeiquan suture the lack of similary aged plutons to the north and the suggestion of a synchronous arc to the south implies that subduction was south dipping present geography if the altyn tagh contains a middle paleozoic suture lateral continuations should be identifiable possibilities to the west include the kudi suture in the western kunlun shan comparisons of lithologies and radiometric data reveal significant similarities between the kudi suture and the altyn tagh suggesting that the at originated as part of the same middle paleozoic orogenic belt reconstruction of the paleozoic structure of northern tibet is complicated by cenozoic deformation particularly along the altyn tagh fault although this could be used as a measure of the finite post devonian deformation a more thorough understanding of paleozoic geodynamics is a prerequisite to unraveling subsequent displacements

jurassic early cenozoic tectonic inversion in the qilian shan and qaidam basin north tibet new insight from seismic reflection isopach mapping and drill core data

amphibolite facies metamorphism and geochronology of the paleoproterozoic aketashitage orogenic belt northwestern china abstract garnet amphibolite and semi pelitic gneiss preserving three stages of metamorphic mineral assemblages i e the prograde metamorphic inclusion assemblage m1 preserved in the garnet porphyroblast metamorphic peak assemblage m2 and retrograde symplectic assemblage m3 have been recognized in the aketashitage area northwestern china geothermobarometric computation shows that the m1 m2 and m3 assemblages record the metamorphic p t conditions of 600 700 c 4 2 5 3 kbar 730 790 c 6 3 7 1 kbar and 770 c 5 6 kbar respectively these three metamorphic stages define clockwise p t paths including nearly isothermal decompression itd process indicating typical orogenic metamorphism the peak metamorphic p t conditions reach the transition zone between the high amphibolite facies and granulite facies and belong to intermediate p t facies series the shrimp and la icp ms u pb dating of metamorphic zircon implies that the subduction might occur as early as 1 97 ga and last as late as 1 84 ga combined with literature data it is concluded that the aketashitage area was a paleoproterozoic orogenic belt which is possibly related to the assembly of the basements of the columbia supercontinent

mesozoic northeast qaidam basin response to contractional reactivation of the qilian shan and implications for the extent of mesozoic intracontinental deformation in central asia

detrital zircon geochronology of the northeastern tibetan plateau u pb geochronologic analyses have been conducted on 413 detrital zircon grains collected from 16 samples in the altun shan nan shan and qilian shan the samples come primarily from quartz arenites and metaturbidites of middle to late proterozoic age and from feldspathic and volcanic clast rich sandstones of early paleozoic age zircon grains in proterozoic strata resting on tarim basement yielded mainly 2 0 1 9 ga ages whereas proterozoic strata of the qaidam and qilian terranes yielded mainly ca 930 820 ma and ca 1 9 1 1 ga ages the younger grains were apparently shed from local igneous rocks whereas the grains older than 1 1 ga were shed from an undetermined continental source grains in the lower paleozoic strata are mainly ca 500 430 ma and were shed from nearby plutonic and possibly volcanic rocks that formed in a magmatic arc setting our detrital zircon ages are consistent with a model first proposed by e r sobel and n arnaud in which early paleozoic magmatism occurred within a single northeast facing magmatic arc that was constructed across an assemblage of middle to late proterozoic accretionary complexes remnants of magmatic arcs and shallow marine strata this arc system was accreted to the tarim and sino korean cratons during silurian devonian time the resulting suture has been reactivated as tertiary thrust faults that currently define the structural and topographic margin of the tibetan plateau our data also provide two new estimates for the offset along the eastern altyn tagh fault a belt of middle proterozoic shallow marine strata is offset by 400 km whereas a belt of 490 480 ma magmatic arc rocks is offset by 370 km these values are generally similar to the 350 400 km offset reported in most previous studies

record of multiple stage channelized fluid and melt activities in deeply subducted slab from zircon u pb age and hf o isotope compositions abstract geochemical features of island arc rocks require two slab derived components of a melt and a fluid added to their sources these two liquids may unite to an intermediate aqueous melt at the base of volcanic arc direct evidence of this process has been scarce yet here we present an integrated study of in situ u pb age trace element and o hf isotope for zircons from a suite of a quartz vein a felsic vein their host eclogite and surrounding gneiss at the xitieshan uhp terrane of the north qaidam metamorphic belt nqmb oscillatory zoned zircons from the quartz vein are enriched in u li nb and ree with a formation age of 442 6 ma consistent with the timing of the peak uhp metamorphism this demonstrates there is no temporal gap between the uhp metamorphism and fluid flow in the xitieshan terrane in particular the oscillatory zoned zircons of the quartz vein have similar hf isotope compositions to zircons from the surrounding gneiss but variable o isotope compositions between the host eclogite and surrounding gneiss the fluid from surrounded gneiss was thus an aqueous melt with high transport ability for lu and hf and o while the fluid from the host eclogite was an aqueous fluid only with the ability to transport o according to the similar behaviors of granitic rock and sediment it is inferred that a channelized hydrous melt can be formed in the downgoing sediment layer where an aqueous fluid can be formed from underlying basalt and or peridotite layers at the sub arc depth the two types of fluid are fed to the overlying mantle wedge from subducting slab to form island arc rock by contrast the zircons in the felsic vein yield a weighted mean 206pb 238u age of 420 4 ma which can be regard as the timing of partial melting during the exhumed stage for the xitieshan uhp rocks the zircon o and hf isotopic features of the felsic vein are between the country gneiss and host eclogite compatible with their emanating from hydrous melt of both the eclogite and gneiss during the exhumation stage of the uhp rocks this indicates that hydrous melt would be released from both the sediment and basalt or peridotite layers of the subducting slabs at relatively shallow depth

provenance of eclogitic metasediments in the north qilian hp lt metamorphic terrane western china geodynamic implications for early paleozoic subduction erosion abstract in this contribution we present new petrological geochemical zircon u pb and hf isotopic data for eclogitic metasediments from the north qilian orogenic belt in which early paleozoic ophiolitic sequences and hp lt metamorphic rocks have been previously recognized the studied metasediments contain eclogite facies assemblages reflecting p t conditions of 450 560 c and 1 9 2 4 gpa consistent with those of adjacent eclogites ree compositions of the eclogitic metasediments overlap those of average upper continental crust the shrimp and la mc icpms u pb data of zircons from metasediments demonstrate input from sources with major age components about 500 ma and 1600 1900 ma peak at 1800 ma with a smaller population at 660 800 ma peak at 750 ma and minor 1 9 ga zircon ages the youngest detrital zircons suggest a maximum depositional age of ca 475 ma and combined with p t conditions and previously determined metamorphic age suggest a burial rate of 0 6 0 7 cm yr zircon hf isotopic analyses show that 500 ma zircons have positive ehf t mainly between 8 and 16 by contrast meso to paleoproterozoic detrital zircons show a broad spectrum of ehf t mainly between 5 and 10 with t dm hf of 1800 2500 ma these data suggest that eclogitic metasediments are derived from a mixture of proterozoic continent crust and juvenile early paleozoic oceanic and or island arc crust and their protoliths likely were deposited in a terrigenous dominated continental margin basin rather than a pelagic oceanic crust environment the new results are consistent with subduction erosion of the active continental margin during south dipping subduction as these sediments formed in a fore arc environment close to the qilian block to the south were transported in the subduction zone to 60 70 km depth prior to their exhumation

quaternary faulting near arco and howe idaho abstract well defined high angle faults were exposed in 1969 by trenching across scarps in alluvial fans along mountain ranges north of arco and howe idaho faulting along both the arco and howe scarps disrupts well developed calcic soils that are considered to be about 30 000 yr old on the other hand both scarps are interrupted by gaps occupied by unfaulted alluvial fans that probably are younger than 15 000 yr old the arco scarp coincides with a zone of closely spaced faults in alluvial fans along the western foot of the lost river range from arco the scarp extends northward about 16 km 10 miles where it merges into an unstudied scarp in bedrock at a trench across the scarp 10 km 6 miles north of arco multiple movements in the fault zone have resulted in aggregate vertical displacement of at least 12 m 40 ft measured offsets of stratigraphic units exposed in the trench indicate at least two episodes of vertical movement on individual faults one episode of movement caused a minimum offset of 4 6 to 6 1 m 15 to 20 ft and another offset was more than 3 m 10 ft the howe scarp coincides with a zone of closely spaced faults in alluvial fans along the western foot of the lemhi range a southern part of the scarp trends northwest along the southern part of the range a distance of 15 km 9 miles and a northern part trends north at least another 6 km 4 miles these parts of the scarp are separated by a bedrock ridge 3 km 2 miles wide in which the scarp is indistinct at a trench aross the scarp about 11 km 7 miles north of howe multiple movements in the fault zone have resulted in aggregate vertical displacement of at least 15 m 50 ft measured offsets of stratigraphic units exposed in the trench indicate five or more episodes of vertical movement on individual faults ranging from 0 3 m 1 ft to more than 3 m 10 ft

late quaternary and present day rates of slip along the altyn tagh fault northern margin of the tibetan plateau 1 both global positioning system gps measurements and studies of late quaternary faulting are consistent with a slip rate of 10 mm yr along the central segment of the altyn tagh fault and a systematic decrease in that rate toward the eastern end of the fault dates of terraces above and below laterally offset terrace risers yield bounds on quaternary slip rates that range from those that agree with gps measurements to values as much as three times faster we argue that offset terrace risers that are protected by topography upstream of them are more closely dated by the age of the upper terrace than by that of the lower terrace in some cases valleys upstream of the fault have been incised into bedrock and few if any terrace risers can be seen within the valleys such streams debouch onto alluviated floodplains or fans that become incised presumably during climate changes to create terrace risers the terrace risers are then displaced so that they lie downslope from bedrock ridges on the upstream side of the fault and thus the risers become protected from further incision in such cases dates of upper terraces should more closely approximate the ages of the risers than those of lower terraces such dates yield slip rates of 10 mm yr in the central segment of the fault and decreasing rates eastward although we cannot with certainty rule out the higher slip rates along the altyn tagh fault our analysis does show that viable interpretations consistent with gps measurements are more likely at least along some segments of the fault not only do these rates support the view that the tibetan plateau deforms internally by slip on a distributed network of faults in the shallow brittle crust and hence behaves as a continuum at depth but the gradual decrease toward the east also shows that the altyn tagh fault does not separate two effectively rigid lithospheric plates correspondingly the relatively low slip rate and the eastward decrease in slip rate suggest that the altyn tagh fault does not transfer a significant portion of the convergence between india and asia into northeastward extrusion of the tibetan plateau thus large scale extrusion of crustal material in india s path into eurasia seems to be limited largely to the confines of the tibetan plateau

magma sources and petrogenesis of the early middle paleozoic backarc granitoids from the central part of the qilian block nw china abstract the petrology geochemistry geochronology and sr nd hf isotopes of the backarc granitoids from the central part of the qilian block are studied in the present work both s and i type granitoids are present in petrographic classification they are granite alkali feldspar granite felsic granite diorite quartz diorite granodiorite and albite syenite the shrimp ages are 402 447 ma for the s type and 419 451 ma for the i type granitoids they are mostly high k calc alkaline granitoids the s type granitoids are weakly to strongly peraluminous and are characterized by negative eu anomalies eu eu 0 18 0 79 the i type granitoids are metaluminous to weakly peraluminous and are characterized mostly by small negative to small positive eu anomalies eu eu 0 71 1 16 the initial 87 sr 86 sr values are 0 708848 0 713651 for the s type and 0 704230 0 718108 for the i type granitoids the e nd 450 ma values are 8 9 4 1 and 9 7 1 9 for the s type and i type granitoids respectively the t dm values are 1 5 2 4 ga for the s type and 1 0 2 3 ga for the i type granitoids for the qilian block the backarc granitoid magmatism took place approximately 60 million years after the onset of the southward subduction of the north qilian oceanic lithosphere and lasted approximately 50 million years partial melting of the source rocks consisting of the neoproterozoic metasedimentary rocks of the huangyuan group and the intruding lower paleozoic basaltic rocks could produce the s type granitoid magmas partial melting of basaltic rocks mixed with lower continental crustal materials could produce the i type granitoid magmas major crustal growth occurred in the late archean and meso paleoproterozoic time for the qilian block the magma generation was primarily remelting of the crustal rocks with only little addition of the mantle materials after 1 0 ga for the qilian block