early neoproterozoic crustal evolution in northern yili block insights from migmatite orthogneiss and leucogranite of the wenquan metamorphic complex in the nw chinese tianshan abstract the northern part of the yili block located in the northwest of the chinese tianshan extends westward to joint with the aktau junggar domain in kazakhstan and is one of the major continental constituents of the central asian orogenic belt caob the nature and tectonic significance of the basement of the continental domains are important for understanding the continental evolution and geodynamic processes of the caob we investigated the wenquan metamorphic complex wmc that represents the basement and metamorphosed sedimentary cover of the northern yili block the wmc is mainly composed of gneissic s type granite migmatite associated with amphibolite paragneiss micaschist quartzite marble and intruding leucogranitic dykes the migmatites display banded schlieren and ptygmatic structures containing rootless lensoid or layered leucosome and leucocratic sills which are indicative of in situ partial melting of the country rocks and variable degrees of melt migration geochemical data suggest that the protoliths of the orthogneiss and gneissic k granite belong to peraluminous s type granites characterized by wide range of i sr values 0 68324 0 72365 low nd t values 1 9 to 4 4 and two stage nd model ages t md 2 1 56 1 82 ga the s type granites might have derived from both clay poor and clay rich source rocks that are probably meta sedimentary and meta volcanic rocks in the wmc the leucogranites show relatively narrow range of i sr values 0 70273 0 70419 more negative nd t values 6 9 to 9 1 and more consistent t md 2 ages 1 51 1 55 ga the leucosome and leucocratic sills show variable sr and nd isotopic compositions probably due to localized and different degree of partial melting our new zircon shrimp and la icpms u pb ages together with previously published data indicate that the migmatization occurred at 926 909 ma the gneissic s type granites emplaced during 919 862 ma and the leucogranites emplaced in the period of 909 845 ma inherited old zircons were found in all dated samples and may have come from their source rocks association of geochronologically overlapped and genetically linked migmatites gneissic s type granites and leucogranitic rocks is interpreted as the results of anatexis of the upper crustal rocks during an early neoproterozoic tectonic metamorphic event that needs to be further studied in the northern yili block such early neoproterozoic migmatization and s type granitic magmatism are considered as an important episode of reworking and cratonization of the continental crust of the yili and adjacent continental blocks in central asia

first early permian paleomagnetic pole for the yili block and its implications for late paleozoic postorogenic kinematic evolution of the sw central asian orogenic belt we conducted a paleomagnetic study on the early permian volcanic and sedimentary rocks and the neoproterozoic mafic dikes in the yili block nw china magnetite and hematite were proven to be the principal magnetic remanence carriers demagnetizations revealed stable characteristic remanence magnetizations with a sole reversed magnetic polarity the magnetic remanence of only the early permian strata turned out to be primary based on positive fold tests meanwhile the magnetic remanence age of the mafic dikes is ambiguous accordingly the first early permian paleomagnetic pole for the yili block is calculated at 81 5 n 256 5 e n 11 and a95 10 9 comparisons of this new pole with published ones from the yili tarim and south junggar blocks provide new quantitative constraints on late paleozoic kinematic evolution of the sw central asian orogenic belt 1 between the yili and tarim blocks significant relative movement took place along major strike slip faults during the late carboniferous to early permian 580 290 km and the early to late permian 585 340 km and the displacement rate increased from the late carboniferous to early permian 19 3 9 7 mm yr to the early to late permian 29 3 17 0 mm yr 2 a significant relative rotation of 28 3 18 3 in the late permian and a lateral displacement of 630 295 km after the late permian occurred between the yili and south junggar blocks the significant strike slip movements played an important role in the formation of the central asian orogenic belt and should be considered with great attention in tectonic and paleogeographic reconstructions

active thrusting and folding along the northern tien shan and late cenozoic rotation of the tarim relative to dzungaria and kazakhstan we have studied geometries and rates of late cenozoic thrust faulting and folding along the northern piedmont of the tien shan mountain belt west of urumqi where the m 8 3 manas earthquake occurred on december 23 1906 the northern range of the tien shan rising above 5000 m overthrusts a flexural foredeep filled with up to 11 000 m of sediment of the dzungarian basement our fieldwork reveals that the active thrust reaches the surface 30 km north of the range front within a 200 km long zone of neogene quaternary anticlines fault scarps are clearest across inset terraces within narrow valleys incised through the anticlines by large rivers flowing down from the range in all the valleys the scarps offset vertically the highest terrace surface by the same amount 10 2 0 7 m inferring an early holocene age 10 2 kyr for this terrace which is continuous with the largest recent fans of the piedmont yields a rate of vertical throw of 1 0 0 3mm yr on the main active thrust at the surface a quantitative morphological analysis of the degradation of terrace edges that are offset by the thrust corroborates such a rate and yields a mass diffusivity of 5 5 2 5 m 2 kyr a rather fresh surface scarp 0 8 0 15 m high that is unlikely to result from shallow earthquakes with 6 m 7 in the last 230 years is visible at the extremities of the main fold zone we associate this scarp with the 1906 manas earthquake and infer that a structure comprising a deep basement ramp under the range gently dipping flats in the foreland and shallow ramps responsible for the formation of the active fault propagation anticlines could have been activated by that earthquake if so the return period of a 1906 type event would be 850 380 years the small size of the scarp for an earthquake of this magnitude suggests that a large fraction of the slip at depth 2 3 is taken up by incremental folding near the surface comparable earthquakes might activate flat detachments and ramp anticlines at a distance from the front of other rising quaternary ranges such as the san gabriel mountains in california or the mont blanc aar massifs in the alps we estimate the finite cenozoic shortening of the folded dzungarian sediments to be of the order of 30 km and the cenozoic shortening rate to have been 3 1 5 mm yr assuming comparable shortening along the tarim piedmont and minor additional active thrusting within the mountain belt we infer the rate of shortening across the tien shan to be at least 6 3 mm yr at the longitude of manas 85 5 e a total shortening of 125 30 km is estimated from crustal thickening assuming local airy isostatic equilibrium under the same assumption serial n s sections imply that cenozoic shortening across the belt increases westwards to 203 50 km at the longitude of kashgar 76 e as reflected by the westward increase of the width of the belt this strain gradient implies a clockwise rotation of tarim relative to dzungaria and kazakhstan of 7 2 5 around a pole located near the eastern extremity of the tien shan west of hami 96 e 43 5 n comparable to that revealed by paleomagnetism between tarim and dzungaria 8 6 8 7 a 6 mm yr rate of shortening at the longitude of manas would imply a rate of rotation of 0 45 m y and would be consistent with a shortening rate of 12 mm yr north of kashgar taking such values to be representative of late cenozoic rates would place the onset of reactivation of the tien shan by the india asia collision in the early to middle miocene 16 22 9 m y in accord with the existence of particularly thick late neogene and quaternary deposits such reactivation would thus have started much later than the collision roughly at the time of the great mid miocene changes in tectonic regimes denudation and sedimentation rates observed in southeast asia the himalayas and the bay of bengal and of the correlative rapid change in seawater sr isotopic ratio 20 to 15 ma like these other changes the rise of the tien shan might be a distant consequence of the end of indochina s escape

late neoproterozoic paleomagnetic results from the sugetbrak formation of the aksu area tarim basin nw china and their implications to paleogeographic reconstructions and the snowball earth hypothesis abstract in order to better constrain the neoproterozoic paleogeographic reconstruction of continents and to improve the understanding of the snowball earth hypothesis paleomagnetic investigations were carried out in the aksu area of the northwestern tarim basin forty eight sites of samples were collected from the sugetbrak and chigebrak formations twenty four sites of sandstone and volcanic rock from the sugetbrak formation revealed stable characteristic remanent components chrm isolated between 500 and 680 c the computed magnetic directions from these components are consistent and significantly distinguished from those of younger ages both normal and reverse polarities have been observed and a positive fold test is revealed after bedding corrections at 95 confidence level anisotropy of magnetic susceptibility measurements shows primary sedimentary fabrics with weak anisotropy degrees a paleomagnetic pole is calculated at 19 1 n 149 7 e k 11 2 a95 9 3 with n 24 yielding a paleolatitude of 27 n for the sampling area the chemo stratigraphic correlation of this section with reference ones reveals an average age of 595 ma a new paleogeographic reconstruction has been attempted showing general low paleolatitude positions for the australia south china and tarim blocks at about 600 ma this observation provides new evidences for the snowball earth hypothesis

geology petrology and tectonic setting of the late jurassic ophiolite in hokkaido japan abstract the gokurakudaira formation which has a n s zonal distribution within a latest jurassic greenstone belt in hokkaido island japan constitutes the uppermost ultramafic mafic unit of the horokanai ophiolite the following three hypotheses for the origin of the ophiolite have been proposed 1 a mid oceanic ridge 2 an oceanic plateau and 3 an island arc the gokurakudaira formation can be subdivided into four zones extending nnw to sse from east zone i to west zone iv based on lithofacies and areal distribution zones i and iii consist of aphyric tholeiite resembling back arc basin basalt babb while zone ii is characterized by the coexistence of babb like tholeiite along with high mg andesite zone iv has a different lithology from the other zones and is composed mainly of picrite and thick sedimentary sequences of island arc tholeiite iat type andesitic subaqueous pyroclastic deposits and terrigenous sediments these stratigraphic and petrological characteristics of the gokurakudaira formation cannot be explained by the oceanic plateau or mid oceanic ridge models but they could correspond to the marginal sea model as in the lau basin therefore we conclude that the horokanai ophiolite was formed in the late jurassic in a marginal basin above a supra subduction zone on the margin of the asian continent

an experimental study on the susceptibility of crusted surfaces to wind erosion a comparison of the strength properties of biotic and salt crusts abstract penetrometry has been employed in a number of studies investigating the strength of crusted sediments as related to their stability to particle impact in aeolian systems despite the early success of this procedure questions remain concerning the effect of the angle of penetration the role of fatigue and the relative importance of the ultimate strength as compared to the elasticity of the crust this paper reports on a series of penetrometer and wind tunnel experiments that consider these effects in the context of both salt and biotic crusts the results demonstrate that the angle of penetration is not important but the density of penetrations is the decreased strength observed with high densities of penetration repeated loading and continual impact in wind tunnel experiments provides further confirmation that sedimentary crusts progressively become weaker during mass transport events the elasticity of the surface appears to be at least as important to consider as the ultimate strength although much stronger the salt crusts were found to break down and erode sooner than biotic crusts in wind tunnel studies the notable elasticity of biotic crusts appears to afford them some protection against abrasion

activation of pacific mantle plumes during the carboniferous evidence from accretionary complexes in southwest japan the identification of ancient activity of large igneous provinces which may be surface manifestations of giant mantle plumes is of primary importance in understanding the evolution of the earth system because such major magmatic redistribution could have profound effects on the global environment the immobile trace element compositions of metamorphosed basaltic rocks called greenstones found in accretionary complexes of southwest japan suggest origins in accreted oceanic lavas at pacific hotspots rather than at mid oceanic ridges furthermore some of these greenstones have geochemical characteristics typical of hotspot lavas in the present superswell region of the south pacific ocean available age data show that the hotspot lavas formed at 350 300 ma greenstones with eruption ages identical to those in southwest japan are widely distributed in the circum pacific region these observations provide evidence for activity of a giant pacific mantle plume during the carboniferous the onset of such giant plume magmatism may have preceded that of the geomagnetic quiet zone

evolution of calc alkaline to alkaline magmatism through carboniferous convergence to permian transcurrent tectonics western chinese tianshan continuous magmatic activity occurred in the western chinese tianshan central asia from the carboniferous to the permian i e before and after the late carboniferous amalgamation of junggar and the yili blocks zircon u pb la icpms and ar ar data reveal a coincidence in time between regional wrench faulting and granitoid emplacement permian post collisional granitoids crop out within or at the margins of large scale dextral strike slip shear zones some of them show synkinematic fabrics the whole rock geochemical features of the early middle permian granitoids indicate an evolution from high k calc alkaline towards alkaline series in other places of the north tianshan alkaline magmatism occurred together with deep marine sedimentation within elongated troughs controlled by wrench faults therefore in contrast with previous interpretations that forwarded continental rift or mantle plume hypotheses the coexistence of diverse magmatic sources during the same tectonic episode suggests that post collisional lithosphere scale transcurrent shearing tightly controlled the magmatic activity during the transition from convergent margin to intraplate anorogenic processes

end permian to mid triassic termination of the accretionary processes of the southern altaids implications for the geodynamic evolution phanerozoic continental growth and metallogeny of central asia the altaids is one of the largest accretionary orogenic collages in the world with the highest rate of phanerozoic continental growth and significant metallogenic importance it is widely accepted that subduction related orogenesis of the altaids started in the late precambrian and gradually migrated southward present coordinates however it is uncertain when and how the building of the altaids was finally completed based on structural geology geochemical geochronological and paleomagnetic data this paper presents late paleozoic to early mesozoic accretionary tectonics of two key areas north xinjiang in the west and inner mongolia in the east together with neighboring mongolia the late paleozoic tectonics of north xinjiang and adjacent areas were characterized by continuous southward accretion along the wide southern active margin of siberia and its final amalgamation with the passive margin of tarim which may have lasted to the end permian to early mid triassic in contrast in inner mongolia and adjacent areas two wide accretionary wedges developed along the southern active margin of siberia and the northern active margin of the north china craton which may have lasted to the mid triassic the final products of the long lived accretionary processes in the southern altaids include late paleozoic to permian arcs late paleozoic to mid triassic accretionary wedges composed of radiolarian cherts pillow lavas and ophiolitic fragments and high pressure ultrahigh pressure metamorphic rocks permian alaskan type zoned mafic ultramafic complexes intruded along some major faults of the tien shan we define a new tarim suture zone immediately north of the tarim craton that is probably now buried below the tien shan as a result of northward subduction of the tarim block in the cenozoic the docking of the tarim and north china cratons against the southern active margin of siberia in the end permian to mid triassic resulted in the final closure of the paleoasian ocean and terminated the accretionary orogenesis of the southern altaids in this part of central asia this complex geodynamic evolution led to formation of giant metal deposits in central asia and to substantial continental growth

late cenozoic tectonic evolution of the southern chinese tian shan structural sedimentological magnetostratigraphic and 40ar 39ar thermochronological investigations were conducted in the southern chinese tian shan on the basis of our own mapping and earlier investigations in the area the late cenozoic southern tian shan thrust belt may be divided into four segments based on their style of deformation from west to east they are 1 kashi aksu imbricate thrust system 2 the baicheng kuche fold and thrust system 3 the korla right slip transfer system and 4 the lop nor thrust system the westernmost kashi aksu system is characterized by the occurrence of evenly spaced 12 15 km imbricate thrusts the baicheng kuche and korla systems are expressed by a major north dipping thrust the kuche thrust that changes its strike eastward to become a nw striking oblique thrust ramp the korla transfer zone the lop nor system in the eastern most part of the southern chinese tian shan consists of widely spaced thrusts all involved with basement rocks geologic mapping and cross section construction suggest that at least 20 40 km of crustal shortening with a horizontal shortening strain of 20 30 has occurred in the southern chinese tian shan during the late cenozoic these estimates are minimum because of both conservative extrapolation of the thrust geometries and partial coverage of the thrust belt by the cross sections the timing of initial thrusting is best constrained in the kuche basin where crustal shortening may have occurred at 21 24 ma the time of a major facies transition between lacustrine and braided fluvial sequences constrained in general by biostratigraphy and in detail by magnetostratigraphy this estimate represents only a minimum age as development of thrusts in the southern chinese tian shan may have propagated southward toward the foreland thus the sedimentary record only represents the southernmost and therefore youngest phase of thrusting if our estimate of timing for the thrust initiation 21 24 ma is correct using the estimated magnitude of shortening 20 40 km and shortening strain 20 30 the averaged rates of late cenozoic horizontal slip and shortening strain are 1 1 9 mm yr 1 and 2 9 4 5 10 16 s 1 respectively our reconnaissance 40ar 39ar thermochronological analysis in conjunction with earlier published results of apatite fission track analysis by other workers in the chinese tian shan suggests that the magnitude of cenozoic denudation is no more than 10 km most likely less than 5 km we demonstrate via a simple airy isostasy model that when the thermal effect on changes in surface elevation is negligible determination of the spatial distribution and temporal variation of both horizontal shortening strain and denudation becomes a key to reconstructing the elevation history of the tian shan using this simple model the loosely constrained magnitude of crustal shortening strain and denudation in the southern chinese tian shan implies that it may have been elevated 1 0 2 0 km since the onset of cenozoic thrusting

late paleozoic arc magmatism in the southern yili block nw china insights to the geodynamic evolution of the balkhash yili continental margin central asian orogenic belt abstract the yili block represents the easternmost segment of the kazakhstan yili microcontinent late paleozoic magmatic rocks are extensively exposed in the yili block and are essential in understanding the tectonic development of the sw central asia orogenic belt caob we conducted field investigations zircon u pb dating whole rock geochemical and sr nd isotopic studies on the magmatic rocks from the wusun range southern yili block the late paleozoic volcanic and plutonic rocks of the wusun range show geochemical features consistent with that of calc alkaline series and they have positive end t values 0 9 to 5 6 neo to mesoproterozoic single stage nd model ages 0 80 1 33 ga and variable initial 87 sr 86 sr ratios 0 7041 0 7059 suggesting that these magmatic rocks are likely derived from partial melting of a moderately depleted mantle wedge with minor involvement of a precambrian continental crust these new data combined with previously published results put constraints on the late paleozoic magmatic and geodynamic evolution of the balkhash yili active continental margin the late paleozoic arc magmatism initiated from 386 ma along the northern yili block and migrated southward to have occurred since 340 ma along its southern margin subsequently the arc related magmatism moved back northward and terminated at 310 ma in the south and at 295 ma in the north the oib like geochemical features of some volcanic rocks indicate incipient marginal rifting occurred along the southern yili block during late carboniferous taking into account the regional structural and tectonic data the carboniferous yili magmatic arc may be laterally correlated with the balkhash yili active margin of the kazakhstan microcontinent which was likely resulted from the southward subduction of the junggar north tianshan ocean

an overview of oceanic island basalts in accretionary complexes and seamounts accretion in the western central asian orogenic belt abstract the central asian orogenic belt caob is one of the largest and long lived accretionary collages worldwide and provides an ideal opportunity to address the fundamental geological processes and continental growth of accretionary orogens however the tectonic history of the caob is still controversial previous studies mainly focused on the process and model of accretion and subduction but did not pay much attention to oceanic intraplate volcanoes in this paper we provide an overview of the salient features of twelve oib hosting accretionary complexes with ages from late neoproterozoic to early carboniferous in western the caob and distinguish rock association of typical seamounts from accretionary complexes and shear zones including pillow basalt volcanic breccia limestone olistostromes and terrestrial clastic rocks combined with unique structural features of seamount the stratigraphic framework of seamount was preliminarily reconstructed in paleo asian ocean but the distribution and scale of seamounts is still not clear geochemically volcanic rocks from seamounts are mainly alkaline basalts with litte tholeiitic basalts the alkaline basalts are characterized by lree enrichment and hree depletion and no obvious nb ta and ti negative anomalies suggesting typical oib affinity the nd sr and pb isotopic features indicate that magma sources have enriched components and show fluid metasomatism in short this is consistent with geochemical features of louisville ontong java and hawaiian seamounts which are related to mantle plumes but do not contain high mg picrite and komatitic basalt based on our observations and in combination with previous work we present an innovative model in which the caob was involved multiple intra oceanic arc with seamounts subduction and accretion which is similar to the present day pacific and can better explain the tectonic evolution of the caob however further studies are needed to document the distribution scale and mantle dynamics of seamounts in the caob

role of basin width variation in tectonic inversion insight from analogue modelling and implications for the tectonic inversion of the abanico basin 32 34 s central andes

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

palaeozoic collisional tectonics and magmatism of the chinese tien shan central asia abstract the chinese tien shan range is a palaeozoic orogenic belt which contains two collision zones the older southern collision accreted a north facing passive continental margin on the north side of the tarim block to an active continental margin on the south side of an elongate continental tract the central tien shan collision occurred along the qinbulak qawabulak fault southern tien shan suture the time of the collision is poorly constrained but was probably in in the late devonian early carboniferous we propose this age because of a major disconformity at this time along the north side of the tarim block and because the youshugou ophiolite is imbricated with middle devonian sediments a younger probably late carboniferous early permian collision along the north tien shan fault northern tien shan suture accreted the northern side of the central tien shan to an island arc which lay to its north the north tien shan arc this collision is bracketed by the middle carboniferous termination of arc magmatism and the appearance of late carboniferous or early permian elastics in a foreland basin developed over the extinct arc thrust sheets generated by the collision are proposed as the tectonic load responsible for the subsidence of this basin post collisional but palaeozoic dextral shear occurred along the northern suture zone this was accompanied by the intrusion of basic and acidic magmas in the central tien shan late palaeozoic basic igneous rocks from all three lithospheric blocks represented in the tien shan possess chemical characteristics associated with generation in supra subduction zone environments even though many post date one or both collisions rocks from each block also possess distinctive trace element chemistries which supports the three fold structural division of the orogenic belt it is unclear whether the chemical differences represent different source characteristics or are due to different episodes of magmatism being juxtaposed by later dextral strike slip fault motions because the southern collision zone in the tien shan is the older of the two the tarim block sensu stricto collided not with the eurasian landmass but with a continental block which was itself separated from eurasia by at least one ocean the destruction of this ocean in late carboniferous early permian times represented the final elimination of all oceanic basins from this part of central asia

postcollisional eastward extrusion and tectonic exhumation along the eastern tianshan orogen central asia constraints from dextral strike slip motion and 40ar 39ar geochronological evidence abstractpostcollisional extrusion and tectonic evolution in the eastern tianshan orogenic belt etob remains poorly known especially the mechanism of dextral strike slip motion and associated tectonic exhumation to better constrain this development a structural and 40ar 39ar geochronological study was carried out on a syndextral strike slip intrusion the jueluotag batholith as well as on other granitic plutons in the etob 40ar 39ar analyses of hornblende biotite k feldspar and plagioclase from quartz mica diorite granodiorite and dioritic porphyry dykes were used to construct cooling histories of the etob hornblendes have cooling ages of 277 272 ma similar to the syntectonic granitic intrusions but biotite ages are 261 254 ma along the syndextral strike slip pluton from east to west the dextral strike slip motion cuts through 268 ma dioritic porphyry dikes as well from these data we conclude that dextral strike slip motion occurred from 270 to 245 ma based on the syntectonic granitic intr

provenance and sedimentary evolution from the middle permian to early triassic around the bogda mountain nw china a tectonic inversion responding to the consolidation of pangea abstract the tectonic attribute and evolution of the bogda mountain are crucial to understand the palaeozoic development of the tian shan and central asian orogenic belt caob in this work we combined sedimentological and geophysical methods such as field section measurements core analyses well logs and seismic sections interpretation to study the sedimentary evolution from the middle permian to early triassic in addition provenance analysis was performed for the sediments from both flanks of the bogda mountain this set of data allows to improve the constraint on the tectonic attribute and evolution in this area the facies studies indicate abrupt changes in depositional environments from the lacustrine alluvial fan to fan delta during the middle to late permian the subsidence centers shifted from the paleo bogda mountain to present bogda piedmonts based on detrital zircon u pb geochronology the north tian shan nts early middle permian post collisional plutons are identified as the main provenances of the middle permian deposits from both flanks the upper permian sample shows a strongly concentrated unimodal age peak of 305 320 ma indicating a single source from the nts and bogda mountain the lower triassic samples in the chaiwopu depression cwd in the south flank of the bogda mountain show obvious sources from the central tian shan cts but these are absent in all samples from the north flank of bogda mountain meanwhile the early permian ages of 280 295 ma which correspond to the bimodal volcanism in the bogda mountain occur in both flanks of this mountain according to the comprehensive geological evidence the deposition of the lower permian middle permian strata probably occurred in a rift extension and expansion setting a tectonic inversion happened at the beginning of the late permian which resulted in initial uplift of the bogda mountain the uplift also occurred in the nts and cts in the late permian this tectonic inversion happened for the whole junggar basin and was an important intracontinental tectonic movement which might result from the consolidation of the pangea supercontinent

constraining the intracontinental tectonics of the sw central asian orogenic belt by the early permian paleomagnetic pole for the turfan hami block in order to better understand the kinematics and geodynamics of the late paleozoic to early mesozoic transition from accretionary to intracontinental tectonics in the southwest central asian orogenic belt we performed a paleomagnetic study on lower permian strata in the turpan hami basin nw china magnetite and hematite are shown to be the principal magnetic remanence carriers a typical sedimentary fabric that has not been modified by postdepositional deformation is recognized via magnetic fabric study stable characteristic remanent magnetizations all of reverse magnetic polarity are revealed by step demagnetizations remanence age is determined to be of early permian affinity based on a fold test remanence behavior of minerals and geochronologic data with inclination shallowing corrections the first early permian paleomagnetic pole for the turpan hami block is acquired as 75 7 n 276 3 e a 95 5 7 and n 10 sites comparisons of this new pole with published ones from neighboring cratonic blocks provide kinematic constraints on the tectonic transition and subsequent intracontinental tectonics of this region the turpan hami and yili blocks have kept their relative positions since the early permian significant latitudinal movement of 14 7 7 2 occurred between the turpan hami and south junggar blocks and could be mainly accommodated by late paleozoic crustal shortening of at least 530 km across an intra arc basin in the bogda belt the turpan hami block has experienced large magnitude relative rotations of 41 1 7 1 a nd 21 2 9 4 with respect to tarim and south junggar since the early permian respectively corresponding to dominant intracontinental tectonics characterized by large magnitude strike slip displacements along megashear zones

accretionary tectonics of back arc oceanic basins in the south tianshan insights from structural geochronological and geochemical studies of the wuwamen ophiolite melange

tectonic evolution of the south tianshan orogen and adjacent regions nw china geochemical and age constraints of granitoid rocks geochemical and geochronological evidence was obtained from granitoids of the south tianshan orogen and adjacent regions which consist of three individual tectonic domains the kazakhstan yili plate the central tianshan terrane and the tarim plate from north to south the central tianshan terrane is structurally bounded by the early paleozoic nikolaev line north nalati fault and late paleozoic atbashy inyl chek south nalati qawabulak fault zones against the kazakhstan yili and tarim plates respectively the meta aluminous to weakly peraluminous granitic rocks which are exposed along the kekesu river and the bikai river across the central tianshan terrane have a tholeiitic calc alkaline or high potassium calc alkaline composition i type geochemical trace element characteristics and the y versus rb nb or y versus nb discrimination diagrams favor a continental arc setting for these granitoid rocks shrimp u pb and la icp ms u pb zircon age data indicate that the magmatism started at about 480 ma continued from 460 to 330 ma and ended at about 275 ma the earlier magmatism 470 ma is considered to be the result of a simultaneous southward and northward subduction of the terskey ocean beneath the northern margin of the tarim plate and the kazakhstan yili plate respectively the later magmatism 460 330 ma is related to the northward subduction of the south tianshan ocean beneath the southern margin of the kazakhstan yili central tianshan plate the dataset presented here in conjunction with previously published data support a late paleozoic tectonic evolution of the south tianshan orogen not a triassic one as recently suggested by shrimp u pb zircon dating for eclogites

middle cambrian to permian subduction related accretionary orogenesis of northern xinjiang nw china implications for the tectonic evolution of central asia abstract middle cambrian to permian subduction related records are widely distributed in northern xinjiang which can be grouped into the chinese altay east junggar eastern tien shan west junggar yili and tarim domains by integrating paleogeographic and geological data we suppose that the chinese altay east junggar eastern tien shan domain was more closely located to siberia while the west junggar and yili domains occupied an intermediate position near the kazakhstan block in the early paleozoic paleoasian ocean distribution of andean type magmatic arcs island arcs accretionary wedges ophiolitic slices and or microcontinents shows an archipelago paleogeography forming a huge accretionary active margin sequences the tarim domain was on the opposite side of the early paleozoic paleoasian ocean remaining passive margin these tectonic units drifted northwards and approached the southern active margin of the siberian craton in the late paleozoic leading to termination of the paleoasian ocean and formation of a complicated orogenic collage between siberian craton and the tarim block between the end permian and triassic these multiple accretion processes significantly contributed to the lateral growth of central asia

the collision between the yili and tarim blocks of the southwestern altaids geochemical and age constraints of a leucogranite dike crosscutting the hp lt metamorphic belt in the chinese tianshan orogen article i nfo a ca 600 m long 0 5 20 m wide nw se trending granite dike crosscuts the high pressure low temperature hp lt tianshan metamorphic belt the foliation of which is parallel to the main ene regional trend in the chinese south tianshan orogen it is mainly composed of plagioclase k feldspar quartz muscovite biotite and secondary chlorite while fluorite zircon and xenotime occur as accessories the immediate country rock is a quartz biotite plagioclase schist which grades several tens of meters away from the granite dike into a chlorite mica albite schist the latter schist is intimately intercalated with blueschist layers and boudins the a cnk value of the granite dike samples varies from 1 15 to 1 27 indicating a strongly peraluminous composition cao na2o ranges from 0 06 to 0 17 and al2o3 tio2 from 240 to 525 similar to the ratios of strongly peraluminous sp granites exposed in high pressure collision zones such as the himalayas a zircon u pb age of 285 ma was obtained for the granite dike thus constraining the upper limit for the age of hp lt metamorphism the petrological and geochemical data suggest that the sp leucogranite dike intruded during the exhumation of overthickened crust in the post collisional setting between the yili central tianshan and tarim blocks the dataset presented here in conjunction with previously published data corroborate that the hp lt metamorphism must have occurred earlier than the permian in the tianshan orogen therefore the collision between the yili central tianshan and tarim blocks and the final amalgamation of the southwestern altaids must have been terminated in late paleozoic and not in triassic times as previously suggested

late paleozoic tectonic amalgamation of northwestern china sedimentary record of the northern tarim northwestern turpan and southern junggar basins sedimentary rocks contained in basins adjacent to the tian shan provide a long and complex record of the late paleozoic continental amalgamation of northwestern china complementing that provided by rocks preserved within the range this record which comprises dramatic changes in sedimentary facies sediment dispersal patterns sandstone provenance and basin subsidence rates broadly supports previous interpretations of a two part evolution of the tian shan late devonian to early carboniferous collision of the tarim continental block with the central tian shan followed by collision of this combined block with island arcs in the north tian shan and bogda shan in late carboniferous early permian times the first collision resulted in widespread angular unconformities within the tarim basin continued convergence following the collision created a long lived flexural foredeep along the northern margin of the tarim block which received at least 2000 m of lower carboniferous through lower permian fluvial and marine sediment derived from the interior of tarim subsequent early permian continental extension of the northern tarim basin resulted in the deposition of interbedded nonmarine siliciclastic sedimentary rocks and mafic to felsicvolcanic rocks sandstone within this interval was derived from the paleo tian shan and is composed predominantly of lithic volcanic grains similar to the rhyolite in contrast to the tarim basin calc alkaline volcanic rocks and volcanogenic sedimentary rocks dominated carboniferous and permian sedimentation in the northern turpan and northwestern junggar basins volcanic arcs remained active in the north tian shan and bogda shan through the early late carboniferous depositing a kilometers thick interval of deep marine sediment gravity flows in the northwestern junggar basin major arc magmatism ceased in the late carboniferous in response to closure of the oceanic basin between the combined tarim central tian shan block and the north tian shan bogda shan arcs upper carboniferous through lower permian rocks in the northwestern junggar basin compose the sedimentary fill of a bathymetric basin of oceanic depth on the northern side of the volcanic arcs culminating in a 1000 m thick marine regressive sequence middle to upper permian sandstones were derived from the uplifted paleo tian shan and bear the distinctive provenance imprint of granitic rocks presently exposed within the range late permian subsidence of the junggar basin accommodated 5 km of nonmarine sediments however the cause of this subsidence and its relationship to regional tectonic events remain controversial

closing history of the southern tianshan oceanic basin western china an oblique collisional orogeny abstract the tianshan tien shan range is an important paleozoic collisional orogenic belt and the key to understand the central asia tectonic evolution this paper integrates our research results with the existing chinese and international literature on sedimentology geochemistry isotopic geochronology paleontonology and paleomagnetism of the tianshan and tarim regions to propose that the oblique collision may have played an important role in the late paleozoic closing of the southern tianshan oceanic basin as a result of the sinian latest proterozoic younger than 800 ma continental extension and rifting process the tarim and yili blocks separated from their parent continent in the late cambrian ordovician the southern tianshan oceanic crusts between the two blocks subducted northward beneath the southern margin of the yili block in the silurian during the devonian early carboniferous the tarim block rapidly drifted to the north and rotated about 46 clockwise this process induced the collision of the yili micro continent with the eastern segment present geographical position of the tarim continent in the late devonian and the southern tianshan oceanic crust evolved to be a west facing remnant oceanic basin during the late carboniferous early permian the tarim block located within an almost constant latitude range rotated about 26 clockwise with respect to the yili micro continent which ultimately closed the remnant oceanic basin in a scissors like manner from east to west and completed the tarim yili collision subsequent a type subduction of the tarim continental crust and lithosphere scale sinistral shearing generated a magmatic arc on the southern margin of the tarim yili suture zone the late permian early triassic clastics deposited in a peripheral foreland basin developed above the arc

deformed continental arc sequences in the south tianshan new constraints on the early paleozoic accretionary tectonics of the central asian orogenic belt abstract the tectonic affinity and structural evolution of the south tianshan are key issues to understand the paleozoic accretionary or collisional orogenesis in the southwestern central asian orogenic belt but remain controversial geological and structural investigations on the early paleozoic low grade metamorphic volcanic sedimentary sequences around the bayinbuluk basin demonstrate that they were prevalently reworked by north vergent ductile deformation and overlain unconformably by devonian to carboniferous carbonates zircon u pb dating indicates that the volcanic rocks erupted during the middle ordovician to early devonian time 460 410 ma and contain neoarchean to neoproterozoic zircon xenocrysts 2 75 ga 2 46 ga 1 60 ga 997 963 ma and 827 ma their whole rock elemental sr nd isotopic end t 5 0 to 3 6 and zircon lu hf isotopic ehf t 13 5 to 11 6 features resemble those of andean type continental arc volcanic rocks the coexisting tuffaceous sedimentary rocks indicate a depositional environment amidst a continental volcanic arc at 421 to 404 ma the newly obtained and available geochronological geochemical and structural data suggest that 1 a wide active continental margin covering the south and central tianshan and northern tarim was formed due to early mid paleozoic southward subduction of the paleo tianshan ocean beneath the tarim craton and 2 a mid paleozoic transition from advancing to retreating accretionary orogeny was followed by opening of the south tianshan back arc basins and break up of the northern tarim continental margin the south tianshan was subsequently subjected to regional deformation during the successive diachronous closure of the major and back arc oceanic basins and locally reworked by the ductile strike slip faulting 307 255 ma

tectonic mode switches and the nature of orogenesis abstract the birth and death of many mountain belts occurs in lithosphere that over rides major subduction zones here the tectonic mode shortening versus extension can abruptly switch even during continuous and otherwise smooth convergence if the hinge line of the foundering slab rapidly retreats i e rolls back the foundering slab creates a gravitational potential well into which the orogen collapses this motion coupled with stress guides can pull the orogen apart a slowing of roll back or of hinge retreat means that the subduction flexure may subsequently begin to be pushed back or be pushed over by the advancing orogen the consequence of such changes in relative motion is that orogenic belts are affected by abrupt tectonic mode switches the change from push to pull leads to a sudden change from horizontal extension to horizontal shortening potentially throughout the entire mass of the orogenic lithosphere that over rides the subducting slab the sequencing of these tectonic mode switches affects the thermal evolution of the orogen and thus fundamentally determines the nature of orogenesis this insight led to us to our quite different views as to how orogens work it is evident that orogens affected by abrupt push pull mode switches are characterized by high pressure metamorphism whereas orogens affected by abrupt pull push mode switches are characterized by high temperature metamorphism magmatism and anatexis

geodynamics asian continental growth the ancient central asian orogenic belt formed during a period of extensive continental crust formation comparison with modern continent building systems suggests that the processes that operate today were already active at that time

paleozoic accretionary and collisional tectonics of the eastern tianshan china implications for the continental growth of central asia this paper deals with the various tectonic units in the chinese eastern tianshan orogenic collage in the central asian orogenic belt and discusses the paleozoic geological history of the several periods of accretion and collision of archipelago systems lying between the tarim and southern angaran continental margins the late ordovician silurian to early devonian eastern tianshan archipelago was characterized by a the harlik dananhu subduction system with a s dipping polarity in the north b a southerly n dipping subduction system beneath the central tianshan arc in the middle and c the south tianshan ocean against tarim in the south during the devonian to early carboniferous n dipping subduction led to the harlik dananhu arc and the kanggurtag forearc basin accretionary complex in the early to mid carboniferous the magmatic front associated with the n dipping subduction beneath the dananhu harlik arc migrated southwards forming the yamansu arc constructed upon the kanggurtag accretionary forearc by the late carboniferous the dananhu harlik arc was attached northwards to the angaran margin resulting in lateral enlargement of the angaran continent in the latest carboniferous to early permian a multiple soft collision left wide suture zones in the south that include the ophiolite strewn aqikkuduk shaquanzi and kumishi accretion collision complexes which were stitched by early permian post collisional plutons by re defining and re interpreting the various tectonic terranes this paper presents a new improved model for the paleozoic evolution of this part of central asia

reverse age zonation of zircon formed by metamictisation and hydrothermal fluid leaching abstract understanding the stability of radiation damaged zircon is crucial to the application of zircon to u pb geochronology especially for zircons rich in radioactive nuclides electron microprobe and laser raman spectral analyses show that zircons from the ejinao sodalite syenite in southeast china are composed of cores without zonation and oscillatory rims the cores are rich in uranium and have experienced increased metamictisation an internal sieved structure and radial cracks were formed by radiation damage and radiation induced volume expansion infrared spectral analyses show that zircon from the ejinao sodalite syenite contains large amounts of oh and h 2 o this finding suggests that the zircon crystals experienced hydrothermal alteration the pb loss of zircons is ascribed to their interaction with a hydrothermal fluid derived from the ejinao syenite radiogenic pb 4 situated in recoil damaged sites has been significantly reduced to pb 2 by the hydrothermal fluid becoming incompatible in the crystal lattice and highly mobile in solution consequently it has been leached out from the zircon this pb loss leads to diverse 206 pb 238 u ages from 134 ma to 147 ma and severe pb loss from the inner core even produces reverse age zonations with cores approximately 5 ma younger than the rims the real crystallisation age for zircons from the ejinao sodalite syenite should be approximately 145 ma considering the entire range of analytical data obtained by both the shrimp and la icpms methods

evolution of cambrian and early ordovician arcs in the kyrgyz north tianshan insights from u pb zircon ages and geochemical data abstract geochronological geochemical and structural studies of magmatic and metamorphic complexes within the kyrgyz north tianshan nts revealed an extensive area of early palaeozoic magmatism with an age range of 540 475 ma during the first episode at 540 510 ma magmatism likely occurred in an intraplate setting within the nts microcontinent and in an oceanic arc setting within the kyrgyz terskey zone in the south during the second episode at 500 475 ma the entire nts represented an arc system these two phases of magmatism were separated by an episode of accretionary tectonics of uncertain nature which led to obduction of ophiolites from the kyrgyz terskey zone onto the microcontinent the occurrence of zircon xenocrysts and predominantly negative whole rock nd t values and hf t values of magmatic zircons suggest a continental setting and melting of precambrian continental sources with minor contributions of palaeozoic juvenile melts in the generation of the magmatic rocks the late cambrian to early ordovician 500 475 ma arc evolved mainly on mesoproterozoic continental crust in the north and partly on oceanic crust in the south arc magmatism was accompanied by spreading in a back arc basin in the south where supra subduction ophiolitic gabbros yielded ages of 496 to 479 ma the relative position of the arc and active back arc basin implies that the subduction zone was located north of the arc dipping to the south variably intense metamorphism and deformation in the nts reflect an early ordovician orogenic event at 480 475 ma resulting from closure of the djalair naiman ophiolite trough and collision of the djel tau microcontinent with the northern margin of nts comparison of geological patterns and episodes of arc magmatism in the nts and chinese central tianshan indicate that these crustal units constituted a single early palaeozoic arc and were separated from the tarim craton by an oceanic basin since the neoproterozoic

late cenozoic tectonics of the kepingtage thrust zone interactions of the tien shan and tarim basin northwest china the kepingtage kalpin thrust zone northwest china is an actively deforming part of the india asia collision system it lies south of the tien shan has an area of 16 000 km2 and consists of arcuate emergent imbricates overall vergence is toward the interior of the tarim basin to the south thrust sheets typically expose upper cambrian to permian platformal strata thrusting is largely thin skinned thin upper cambrian evaporites are likely to be the main decollement horizon the kepingtage thrust zone is the only margin of the tarim basin to deform in this style in the cenozoic it is replaced to the east and west by thrust zones which have propagated shorter distances into the interior of the tarim basin thick up to 10 km mesozoic and cenozoic clastic successions are present and deformed in these regions such successions are not present in the kepingtage thrust zone or the adjacent bachu uplift within the tarim basin because of episodic activity on steep northwest southeast trending thrusts which define the margins of the bachu uplift these mesozoic cenozoic strata may have suppressed the ability of regions along strike from the kepingtage thrust zone to deform by thin skinned thrusting utilizing the upper cambrian decollement the along strike variation in active thrusting at the southern margin of the tien shan is an example of syntectonic sedimentation controlling thrust belt deformation style a balanced section across the thrust zone indicates 28 shortening equivalent to 35 km this is equivalent to an average slip rate of 1 8 mm yr 1 and a strain rate of 4 4 10 16 s 1 assuming deformation began at circa 20 ma active deformation is focused along the frontal thrust the kepingtage fault the northern boundary of the kepingtage thrust zone is formed by the south tien shan fault this major north dipping thrust is seismically active with published earthquake focal depths at midcrustal levels 14 and 18 km it juxtaposes upper carboniferous sedimentary rocks of different facies and different paleozoic deformation histories speculatively it represents a reactivation of a late paleozoic thrust which originated at the boundary between the platformal interior of the tarim block and a deeper water foreland basin to its north

biological soil crust development and its topsoil properties in the process of dune stabilization inner mongolia china this study was undertaken at horqin sand land inner mongolia northern china field samples of biological soil crusts bscs and underlying topsoil 0 5 cm under bsc were taken in areas of different dune stabilization stages and their physicochemical properties were analyzed including particle size distribution bulk density organic matter nitrogen phosphorus electrical conductivity ec ph and caco3 content the results revealed that semi mobile dunes semi fixed dunes and fixed dunes had developed a physical crust algae crust and moss crust respectively the thickness hardness water content fine fraction and nutrient contents of bscs were gradually increasing along the dune stabilization gradient meanwhile bsc establishment and development enhanced the bulk density silt and clay content and nutrients of the topsoil under it in an increasing tend from semi mobile dune to fixed dune organic matter concentrations and other nutrients in the 0 5 cm topsoil layer under bscs were significantly higher compared to unconsolidated soil control moreover there were strong significant positive correlations between topsoil and bscs organic matter total nitrogen available nitrogen available phosphorus caco3 and 0 05 mm particle content suggesting that bscs have an influence on some of the properties of the underlying topsoil

early neoproterozoic gneissic granitoids in the southern yili block nw china constraints on microcontinent provenance and assembly in the sw central asian orogenic belt abstract the central asian orogenic belt caob was built by amalgamation of diverse geologic units including several microcontinents the nature and geodynamic evolution of the precambrian basement of these units are essential to constrain their origin and thus the geodynamic and paleogeographic reconstruction of the accretionary system the yili block located in the west of the chinese tianshan orogen is one of these precambrian based continental constituents field investigations zircon u pb dating and lu hf isotopic analysis whole rock geochemical and sr nd isotopic studies were conducted on the gneissic granitoids from the giergelang area southern yili block these rocks yielded early neoproterozoic zircon u pb ages 962 934 ma containing substantial number of paleo to mesoproterozoic inherited zircons 2403 1268 ma their protoliths belong to peraluminous high k granite series displaying right declining ree patterns and eu negative anomalies together with lile k rb u th enrichment and depletions in ba nb sr p ti they have negative end t values ranging from 2 5 to 4 7 and old nd model ages varying from 1650 ma to 1910 ma they also exhibit highly variable zircon ehf t values 12 1 to 4 4 and hf in zircon model ages 1558 2574 ma integrating all these results it is suggested that the primary magma of gneissic granitoids was generated by partial melting of supracrustal rocks with involvement of old crustal materials a comparison of data from the yili block and its neighboring continental units reveals that the neoproterozoic magmatic event is an important episode of crustal reworking for the major continental blocks within the caob similar neoproterozoic magmatic events and comparable upper neoproterozoic to lower paleozoic stratigraphic sequences of the yili chinese central tianshan and tarim blocks suggest that they had a similar crustal evolution and close tectonic and paleogeographic relation during neoproterozoic and were likely derived from one single continental plate

paleozoic multiple accretionary and collisional tectonics of the chinese tianshan orogenic collage abstract subduction related accretion in the junggar balkash and south tianshan oceans paleo asian ocean mainly in the paleozoic gave rise to the present 2400 km long tianshan orogenic collage that extends from the aral sea eastwards through uzbekistan tajikistan kyrgyzstan to xinjiang in china this paper provides an up to date along strike synthesis of this orogenic collage and a new tectonic model to explain its accretionary evolution the northern part of the orogenic collage developed by consumption of the junggar balkash ocean together with paleozoic island arcs northern ili issyk kul and chatkal located in the west which may have amalgamated into a composite arc in the paleozoic in the west and by addition of another two roughly parallel arcs dananhu and central tianshan in the east the western composite arc and the eastern dananhu and central tianshan arcs formed a late paleozoic archipelago with multiple subduction zones the southern part of the orogenic collage developed by the consumption of the south tianshan ocean which gave rise to a continuous accretionary complex kokshaal kumishi which separated the central tianshan in the east and other paleozoic arcs in the west from cratons tarim and karakum to the south cross border correlations of this accretionary complex indicate a general southward and oceanward accretion by northward subduction in the early paleozoic to permian as recorded by successive southward juxtaposition of ophiolites slices of ophiolitic melanges cherts island arcs olistostromes blueschists and turbidites which are mainly paleozoic in age with the youngest main phase being late carboniferous permian the initial docking of the southerly tarim and karakum cratons to this complicated late paleozoic archipelago and accretionary complexes occurred in the late carboniferous early permian in the eastern part of the tianshan and in the late permian in the western part which might have terminated collisional deformation on this suture zone the final stages of closure of the junggar balkash ocean resembled the small ocean basin scenario of the mediterranean sea in the cenozoic in summary the history of the altaids is characterized by complicated multiple accretionary and collisional tectonics

emplacement of the ophiolitic melanges in the west karamay area implications for the late paleozoic tectonic evolution of west junggar northwestern china abstract the darbut and karamay ophiolitic melanges in the west karamay area at the southeastern margin of west junggar southern altaids represent the relict fragments of the paleozoic junggar ocean basin the melanges contain structurally complex tectonic blocks of ultramafic rocks gabbro basalt and chert that are dispersed within an extensively sheared serpentinite matrix this study presents new field observations structural analyses geochronological and geophysical data for the melanges and the overlying carboniferous strata three stages of deformation d1 d3 have been identified d1 is only locally preserved in the melanges and is characterized by thrust imbricated faults recording the subduction and accretion of the paleo oceanic crust in the early carboniferous d2 mainly consists of map scale upright folds f1 in the carboniferous strata and ne sw striking sub vertical right lateral shear zones in the melanges d3 is characterized mainly by ne sw striking transcurrent left lateral strike slip faults and s shaped drag folds f2 the extensive tectonic reworking during d2 and d3 deformations eradicated most of the d1 structures our structural observations combined with geophysical data suggest that the basement of the west karamay area consists of trapped oceanic crust or dismembered ophiolitic fragments and that the darbut and karamay ophiolitic melanges were extruded through right lateral shear zones our results and the published data further suggest that the emplacement of the melanges and the formation of f1 folds represent regional scale amalgamation processes between different tectonic blocks and or plates in the late carboniferous these processes resulted in the closure of the west junggar remnant ocean basin and other sub oceanic basins in the paleo asian ocean leading to the formation of the kazakhstan orocline furthermore the d3 deformation was associated with anticlockwise rotation of the west junggar region in a post collisional setting in the permian

new age constraints on the metamorphic evolution of the high pressure low temperature belt in the western tianshan mountains nw china abstract the western tianshan high pressure low temperature orogenic belt in nw china contains eclogite facies metavolcanic rocks and omphacite bearing blueschists previous sm nd omphacite garnet glaucophane whole rock and 40ar 39ar crossite dating of eclogite facies rocks has suggested an age of ca 345 ma as the best approximation for the timing of peak metamorphic conditions the samples described here are blueschist facies rocks that formed during or after the transition from the eclogite facies to the epidote blueschist facies and subsequently experienced an incipient greenschist facies overprint by use of white mica geochronology k ar 40ar 39ar rb sr an attempt is made to date postpeak metamorphic stages of the complex pt path rb sr and 40ar 39ar ages range between 313 and 302 ma and 323 and 312 ma respectively but mostly cluster at ca 310 311 ma indicating that the studied samples recrystallized at this time however the 40ar 39ar age spectra show complex release patterns that a

paleozoic tectonic evolution of the tianshan orogen northwestern china abstract the chinese tianshan orogenic belt which extends west east for about 1500 km is one of the most important paleozoic orogenic belts in central asia it includes parts of the tarim yili central tianshan and junggar plates which are microcontinental blocks with precambrian basements and meso cenozoic depositional covers the tarim plate is separated from the yili central tianshan plate by the southern central tianshan suture early carboniferous ophiolitic melanges and blueschists are widespread in this suture and are believed to be produced by the collision between a passive continental margin on the north side of the tarim plate and an active continental margin on the south side of the yili central tianshan plate the collision occurred at the end of early carboniferous the yili central tianshan and the junggar plates are separated by the northern central tianshan suture also considered as an early carboniferous collisional zone along which the middle ordovician ophiolitic melanges blueschists and late devonian early carboniferous ophiolites are exposed in early paleozoic times the subduction of the paleo junggar ocean took place in an active margin on the north side of the yili central tianshan plate along the northern central tianshan suture at the end of early carboniferous the north side of the yili central tianshan plate collided with the north tianshan arc along this suture the paleo oceanic crust in the tianshan and adjacent region was entirely consumed at the end of early carboniferous meanwhile the tarim yili central tianshan and junggar plates were accreted to eurasia epicontinental sediments were then deposited throughout the tianshan region during the middle late carboniferous times in the early permian tianshan began to uplift with accompanying volcanic eruption and granitic intrusion the uplift was accelerated in the late permian and continental molasse was extensively deposited

an early cambrian plume induced subduction initiation event within the junggar ocean insights from ophiolitic melanges arc magmatism and metamorphic rocks abstract the central asian orogenic belt caob is one of the largest accretionary orogens in the world the mechanism of continental growth and tectonic evolution of the caob remain debated here we present an overview of early paleozoic ophiolitic melanges calc alkaline intrusions and metamorphic rocks in west junggar with an aim to provide constraints on the time and mechanism of subduction initiation in the junggar ocean a branch of the southern paleo asian ocean pao the early paleozoic ophiolitic melanges are composed of ultramafic mafic rocks cherts pelagic limestones basaltic breccias and tuffs the mafic rocks from these ophiolitic melanges are divided into morb like and oib like types the morb like rocks were formed in a fore arc setting but the oib like mafic rocks were formed by the intra plate magmatism related to mantle plume activities the early paleozoic intrusions are occurred as small stocks with a dominant composition of diorite trondhjemite and granodiorite these granitoids display high k calc alkaline affinities and have high and positive end t and ehf t values formed in an arc related setting the metamorphic complex is mainly composed of blueschist and amphibolite blocks with metamorphism ages ranging from 500 ma to 460 ma their protoliths are calc alkaline andesite and tholeiitic and alkaline basalts formed in an arc related and seamount setting respectively it is clearly show that the west junggar was under an intra oceanic subduction regime during the early paleozoic and the initial subduction of the southern pao might have occurred in the early cambrian based on our observations and in combination with previous work we propose the plume induced subduction initiation model for the early middle cambrian tectonic evolutionary of the junggar ocean our new model not only shed light on subduction initiation dynamics of the southern pao but also contribute to tectonic evolution of the caob

influence of cryptogamic crust disturbance to wind erosion on sand and loam rangeland soils a portable field wind tunnel was used to assess the sediment flux rates of loam and sand textured soils in the mallee region of southeastern australia three levels of crust disturbance nil moderate and severe simulating stock trampling were investigated the results demonstrated the importance of cryptogamic crusts in binding the soil surface and providing roughness after the soil was moderately disturbed on the loamy soil the crust helped maintain sediment flux rates below the erosion control target to 5 g m 1 s 1 for a 65 km h 1 wind measured at 10 m height once the crust was severely disturbed sediment fluxes increased to 1 6 times the erosion target on the sandy soil even with no crust disturbance the sediment flux was 1 6 times the erosion control target disturbing the crust increased sediment fluxes to a maximum of 6 7 times the erosion control target removal of the crust also decreased the threshold wind velocity that resulted in an increase to the ris k of erosion from 5 per cent to 20 per cent 1998 john wiley sons ltd

structural and geochronological study of high pressure metamorphic rocks in the kekesu section northwestern china implications for the late paleozoic tectonics of the southern tianshan abstract blueschist and eclogite facies high to ultrahigh pressure hp uhp metamorphic rocks occur in the southern tianshan belt their deformation and metamorphic history is important for understanding the paleozoic tectonics of the central asian orogenic belt our study focuses on the structural analysis and geochronology of the hp metamorphic rocks and the surrounding rocks in the kekesu section in the southern chinese tianshan geometric and kinematic analyses indicate three ductile deformation events a top to the north thrusting a top to the south shearing and a dextral wrenching new 40ar 39ar laser probe plateau ages were obtained on white mica from retrograde blueschist documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr

paleozoic accretionary orogenesis in the paleo asian ocean insights from detrital zircons from silurian to carboniferous strata at the northwestern margin of the tarim craton a detrital zircon u pb and lu hf isotopic study was carried out in the middle silurian to late carboniferous sedimentary strata of the northwestern tarim craton in order to understand accretionary processes in the southern part of the central asian orogenic belt detrital zircons from these strata yielded u pb ages clustering around 2 8 2 3 ga 2 0 1 7 ga 1 3 0 9 ga 880 600 ma and 500 400 ma with age populations and hf isotopic signatures matching those of magmatic rocks in the tarim craton and the central tianshan block abundant 500 400 ma detrital zircons most likely reflect deposition in a retroarc foreland basin inboard of an andean type magmatic arc to the north supporting the northern tarim central tianshan connection during early paleozoic time the absence of 380 310 ma zircon population in the carboniferous siliciclastic rocks suggests that the central tianshan block may have been separated from the tarim craton in the early devonian caused by the interarc back arc opening of the south tianshan ocean we propose an accretionary orogenic model switching from advancing to retreating mode during paleozoic time in the southwestern part of the paleo asian ocean this transition most likely occurred coevally with the rifting of southeast asian blocks from the northeastern margin of gondwana

structural overprints of early paleozoic arc related intrusive rocks in the chinese central tianshan implications for paleozoic accretionary tectonics in sw central asian orogenic belts abstract the chinese central tianshan constitutes the centric and vital component of the tianshan orogen it contains important information for understanding the accretionary orogeny of the central asian orogenic belt caob to better constrain the paleozoic evolution of the chinese central tianshan we present new field observations fabric analyses zircon u pb ages zircon lu hf isotopic and whole rock geochemical data of mafic intermediate and felsic intrusive rocks in the baluntai area chinese central tianshan field scale and microscopic structural patterns of the intrusive rocks reveal that the early paleozoic gabbro diorite intrusions underwent multistage structural reworking and generally exhibit gently south or north dipping foliations and microscopic kinematic fabrics indicating top to the north shearing these ductilely deformed intrusive rocks were intruded by granitic sills or dikes that are generally sub parallel or with low angle to the main foliations of the host rocks gentle foliations and top to the north kinematics can be rarely noticed within the granitic intrusions at the sites close to the baluntai fault locally intense mylonitization occurred in the felsic intrusions and deformed mafic intermediate ones where earlier shallow foliations were completely or partially transformed to steep ones bearing sub horizontal stretching lineations and asymmetric kinematic indicators suggest dextral shearing zircon la icp ms u pb datings constrain the emplacement ages of the gabbroic and dioritic intrusive rocks at 452 420 ma and the crystallization age of one mylonitic k granite at 417 ma two samples of granitic sill contain plenty of zircons inherited from the host gabbroic and dioritic rocks and a few such zircons have dark rims probably formed during granitic injection dating on these rims roughly constrains the crystallization ages of the felsic sills at ca 370 360 ma the ehf t values of a gabbro 420 ma two diorite samples 425 and 445 ma and a mylonitic granite 417 ma vary from 15 45 to 2 06 corresponding to two stage crustal model ages tdm c ranging from 2 4 ga to 1 5 ga suggesting variable degrees of involvement of the proterozoic crustal basement and the juvenile mantle components during the early paleozoic magmatic generation the geochemical data of the mafic intermediate plutons exhibit features of magmatic rocks formed in continental arcs the mylonitic granite and one undeformed granite resemble a type granite in terms of geochemical features suggesting that these granites were produced in a post collisional extensional environment considering the previously published data the early paleozoic arc type magmatic rocks within the chinese central tianshan likely resulted from the southward subduction of an oceanic basin located to the north and ceased progressively during 420 400 ma the amalgmation between chinese central tianshan and tarim most likely completed at ca 360 ma following the closure of the south tianshan back arc basin this event reworked the chinese central tianshan arc and formed the top to the north kinematics thereafter the chinese central tianshan was affected by the early permian dextral strike slip faulting due to the regional large scale transcurrent tectonics and overprinted by the permian mafic magmatism

tectonic evolution of the chinese tianshan orogen from subduction to arc continent collision insight from polyphase deformation along the gangou section central asia the central asian orogenic belt as the largest accretionary orogen on earth is an ideal candidate to study the geodynamics of convergent plate boundaries through a prolonged period the evolution of this orogen has been explained by different tectonic models which incorporated one or a combination of the following mechanisms lateral stacking of arc systems along major shear zones arc amalgamation oroclinal bending and trench migration here we elucidate major mechanisms responsible for the tectonic evolution of the central asian orogenic belt focusing on the chinese tianshan orogen in the southern central asian orogenic belt structural observations from the 50 km long gangou section show evidence of polyphase deformation the earliest episode of orogen parallel sinistral shearing constrained to the early devonian ca 399 ma by syn deformational intrusions was possibly controlled by oblique subduction this was followed by an episode of ne sw contractional deformation dated at ca 356 ma 40ar 39ar age of syn deformational hornblende and likely associated with an episode of trench advance the third stages of deformation during the latest carboniferous and permian involved ne sw contraction orogen parallel extension and dextral transpression our new geochronological data constrain the timing of orogen parallel extension to ca 303 293 ma and confirm that dextral activation along shear zones occurred during the permian the results highlight the role of trench migration oblique tectonics and syn collisional orogen parallel extension in the build up of the central asian orogenic belt and contribute to the pre collisional reconstruction of this orogenic system

ediacaran early ordovician and early silurian arcs in the south tianshan orogen of kyrgyzstan abstract the oldest magmatic rocks from the south tianshan orogen of kyrgyzstan sts are important for better understanding of the neoproterozoic and early palaeozoic evolution of the southwestern central asian orogenic belt bulk rock major and trace element and sm nd isotopic composition and zircon u pb ages of granitoids from melange blocks reveal two previously unknown episodes of arc magmatism in the sts orogen of kyrgyzstan namely the ediacaran ca 624 ma and early ordovician ca 472 ma episodes moderately positive nd t value of 5 8 of the ediacaran granodiorite indicates a mainly juvenile source for this rock negative nd t value of 9 1 for the early ordovician granodiorite indicates melting of the predominantly precambrian crustal material the latter is also supported by the inherited zircon cores with an age range of 1 04 2 7 ga the geochemical signature of the silurian late llandovery and wenlock basalts suggests formation in arc settings arc related origin of the ediacaran granodiorite suggests that mature oceanic basin with subduction system existed in the western sts since the late neoproterozoic the arc also evolved in the ordovician and silurian and was inactive in the middle cambrian to the earliest ordovician ediacaran granodiorite old zircon cores and negative nd t values of the ordovician granodiorite suggest the existence of precambrian microcontinents in the kyrgyz sts these crustal fragments were likely rifted off the tarim and or karakum tajik microcontinents where the rocks with similar ages occur in the palaeozoic they traveled north and were incorporated in the sts orogen in the late carboniferous

dynamics of exhumation and deformation of hp uhp orogens in double subduction collision systems numerical modeling and implications for the western dabie orogen abstract the dynamics of formation and exhumation of high pressure hp and ultra high pressure uhp metamorphic orogens in double subduction collision zones remain enigmatic here we employ two dimensional thermo mechanical numerical models to gain insights on the exhumation of hp uhp metamorphic rocks as well as their deformation during the collision of a micro continent with pro and retro continental margins along two subduction zones a three stage collisional process with different convergence velocities is tested in the initial collisional stage a fold and thrust belt and locally rootless superimposed folds are developed in the micro continent and subduction channel respectively in the second exhumation stage of hp uhp rocks a faster convergence model results in upwelling of the asthenosphere which further leads to a detachment between the crust and lithospheric mantle of the micro continent a slower convergence model results in rapid exhumation of hp uhp rocks along the north subduction channel and a typical piggy back thrusting structure in the micro continent a non convergence model produces a slab tear off leading to the rebound of residual lithosphere of the micro continent in the third and final stage a series of back and ramp thrusts are formed in the micro continent with the pro continent re subducted based on an analogy of our numerical results with the western dabie orogen wdo we suggest that 1 slab tear off results in a rebound of residual lithosphere which controls the two stage syn collisional exhumation process of hp uhp rocks in the wdo and 2 in contrast to the single subduction collision system the exhumation range of the partially molten rocks with lower viscosity and density is restricted to a specific region of the micro continent by the mianlue and shangdan subduction zones which generated the complex deformation features in the wdo

interference between shallow and deep seated structures in the sicilian fold and thrust belt italy abstract the fold and thrust belt in western sicily is characterized by the presence and interference of shallow and deep seated compressional structures which were generated and developed at different structural levels the shallow structures consist of imbricated thrusts and asymmetric folds with a typical wavelength of 2 km involving relatively thin deep water units these units are superimposed on thick platform carbonate units along a wide and originally almost flat floor thrust the axial trend of the folds is variable as multi phase folding often occurred producing a characteristic interference pattern reflecting continuous variations of the apparent transport direction during the emplacement i e rotation of the allochthonous thrust sheets the deep seated structures consist of large double verging pop up structures with a typical wavelength of 5 10 km involving thicker platform carbonate successions the deep seated structures are characterized by large folds with vertical to overturned limbs caused by high angle transpressive ramps that reactivated previous i e mesozoic synsedimentary normal or transtensional faults the floor thrust of the shallow structures was passively deformed by the subsequent growth of the underlying younger deep seated structures large clockwise rotations of the tectonic units occurred during the compressional deformation and the amount of rotation is apparently related to the timing and amount of the tectonic transport

late paleozoic strike slip shear zones in eastern central asia nw china new structural and geochronological data 1 new structural studies and 40ar 39ar dating in northwest china provide information about late paleozoic strike slip motions subsequent to accretional events which built eastern central asia during the paleozoic two principal areas were affected by these large transcurrent motions first in the tianshan range main east west ductile shear zones are dextral and coeval with an eastward decreasing greenschist retrograde metamorphism associated biotites give ages ranging from 290 ma to 245 ma the earlier n110 shearing occurred in western tianshan while the last one dextral in whole tianshan occurred 250 245 myr ago second in the chinese altay region several nw se shear zones structured the area the main motion is sinistral and occurred along the erqishi zone at 280 290 ma it is followed by a complex succession of dextral and sinistral shearing episodes leading to the northwestward structuring dated at 245 ma of a metamorphic zone that was folded during a compressive event

paleozoic northward drift of the north tien shan central asia as revealed by ordovician and carboniferous paleomagnetism abstract three new middle late ordovician and two new early carboniferous paleomagnetic poles have been obtained from the north tien shan zone ntz of the ural mongol belt in kyrgyzstan and kazakhstan paleolatitudes for the carboniferous are unambiguously northerly and average 15 5 n whereas the ordovician paleolatitudes 6 9 and 9 are inferred to be southerly given that a very large 180 rotation of the ntz would be necessary during the middle paleozoic if the other polarity option was chosen thus the ntz drifted northward during much of the paleozoic east west drift cannot be determined as is well known from paleomagnetic data in addition detailed thermal demagnetization analysis reveals two overprints one of recent age and the other of permian age which is a time of strong deformation in the ntz the paleolatitude of the combined permian overprint is 30 5 2 n the paleolatitudes collectively track those predicted for the area by extrapolation from baltica very well but are different from those of siberia for ordovician times this finding is compatible with sengor and natal in s sengor a m c natal in b a 1996 paleotectonics of asia fragments of a synthesis in yin a harrison m eds the tectonic evolution of asia cambridge univ press cambridge pp 486 640 model of tectonic evolution of the ural mongol belt and disagrees with the models of other researchers declinations of the ordovician and early carboniferous results range from northwesterly to northeasterly and are clearly affected by local relative rotations which seem characteristic for the entire ntz because the permian overprint declinations also show such a spread apparently the important latest paleozoic triassic deformation involved shear zone related rotations as well as folding and significant granitic intrusions

petrogenesis of late paleozoic diorites and a type granites in the central eastern tianshan nw china response to post collisional extension triggered by slab breakoff abstract geological evolution of the eastern tianshan in the late paleozoic is significant to understanding the multiple accretions and final formation of the southern central asian orogenic belt here we report new whole rock geochemical and sr nd isotopic data and in situ zircon u pb ages for three typical a type granitic intrusions syenogranite k feldspar granite and monzonitic granite and one dioritic pluton in the central eastern tianshan nw china in order to constrain their petrogenesis tectonic setting and geodynamic evolution zircon la icp ms u pb ages indicate that the syenogranites and k feldspar granites were both formed during a short time interval in the early permian 284 286 ma whereas the diorites were crystallized at 293 6 ma the monzonitic granites were generated at 307 3 ma earlier than the other two granitic intrusions the three kinds of granites syenogranite k feldspar granite and monzonitic granite have high hfse hree and y nb ratios 1 68 2 84 and remarkably low sr and ba contents with the absence of aluminium rich minerals and show metaluminous to weakly peraluminous natures indicating geochemical characteristics of aluminous a2 type granites these granites show negative end t 3 32 to 4 40 values and mesoproterozoic two stage nd model ages tdm2 1 36 to 1 43 ga which suggest a relatively old crustal source for these rocks the sr nd isotopic compositions imply that the mesoproterozoic metamorphic basement of the central tianshan block can be considered as a potential magmatic source for these aluminous a2 type granites in addition they have geochemical characteristics similar to the experimental melt that was derived from partial melting of crustal metaigneous rocks at the depth of middle to lower crust levels therefore the granites were most likely produced by partial melting of metaigneous rocks in a post collisional extensional setting the diorites have low sio2 and high mgo with uniform 87sr 86sr i values 0 7055 negative end t values 4 11 to 4 59 and mesoproterozoic two stage nd model ages tdm2 1 41 to 1 45 ga they have higher ti zr 42 8 46 7 and ti y 280 359 ratios than continental crust and show negative nb ta ti anomalies which can be explained by partial melting of a relatively fertile mantle modified by old crustal materials of the central tianshan block the formation of the dioritic pluton was induced by an upwelling asthenosphere in an extensional geodynamic setting based on the zircon saturation temperatures of these a type granites and the distribution of late carboniferous to early permian mantle derived intrusions a slab breakoff model is suggested to interpret the formation of the studied a type granites and diorites we infer that the slab breakoff was initiated in the late carboniferous and ended in the early permian

hydrothermal zircon geochronology age constraint on nanling range tungsten mineralization southeast china abstract the nanling range southeast china is well known for its wolframite bearing quartz vein wqv tungsten deposit this study focuses on the geochemistry and geochronology of zircons from the wqv and challenges the current view of the tungsten mineralization in the nanling range the features of the wqv zircons include 1 pale brown murky brown or orange red color and translucence under microscope 2 110 101 type crystal form 3 weak cathodoluminescence 4 enrichment of hf ranging from 1 97 to 7 83 wt hfo2 u ranging from 0 02 to 3 97 wt uo2 th ranging from 0 to 0 65 wt tho2 and p ranging from 0 to 1 82 wt p2o5 and 5 presence of solid hydrothermal and ore minerals and fluid inclusions these features indicate that the wqv zircons crystallized from hydrothermal fluids during tungsten mineralization the in situ la icpms u pb results of the wqv zircons from five different tungsten deposits in the nanling range yield similar ages ranging from 134 4 1 9 ma to 132 9 1 5 ma approximately 20 million years younger than proposed tungsten ore ages 155 5 ma several mineralization characteristics and field observations also cast doubt on the current model nanling range tungsten ore is the result of orthomagmatic processes the zircon characterization method provided in this study could be applied to tungsten metallogenic research in other parts of the world

palaeozoic tectonics of the south western chinese tianshan new insights from a structural study of the high pressure low temperature metamorphic belt the south western chinese tianshan orogenic belt is famous for its omphacite bearing blueschists and associated eclogite facies metavolcanic rocks although numerous petrologic geochemical and geochronological studies are available structural data and interpretations are still rare this paper provides new structural data including bulk geometry of structures and kinematic analyses based on field and laboratory studies along the akyazhi keburt and muzaert rivers the study area is divided into three tectonic units namely 1 a southern unit composed of weakly metamorphosed sedimentary rocks of silurian age 2 a central hp lt unit composed of blueschist eclogite facies metamorphic rocks derived from basalts pelites and volcaniclastic rocks 3 a northern unit which consists of a carboniferous magmatic arc developed upon an amphibolite facies metamorphic continental basement our structural analysis documents a polyphase deformation the main event d1 is reflected by devonian to carboniferous top to the northwest ductile shearing coeval with hp lt metamorphism this is followed by north directed thrusting d2 of the southern unit over the central hp lt unit coeval with retrogression of the high pressure rocks a top to the s se deformation d3 overprints the earliest events and is observed in the northern and central units lastly permian dextral ductile brittle wrenching d4 overprints the older flat lying fabrics d4 is conspicuous along the nalati fault that separates the northern unit from the central hp lt unit the absolute timing of these deformation events is discussed in the light of available radiometric dating the structural metamorphic and geochronological data are integrated into a geodynamic model of the south western chinese tianshan that emphasizes south directed subduction of microcontinents located between tarim and junggar

ordovician 40ar 39ar phengite ages from the blueschist facies ondor sum subduction accretion complex inner mongolia and implications for the early paleozoic history of continental blocks in china and adjacent areas we obtained 453 21 8 ma and 449 41 8 ma 2 laser step heating 40 ar 39 ar plateau ages for phengite from quartzite mylonites from the blueschist facies ondor sum subduction accretion complex in inner mongolia northern china these ages are within error of the inverse isochron ages calculated using the plateau steps and the weighted mean ages of total fusion of single grains the compositional change from glaucophane in the cores to crossite in the rims of blue amphiboles as revealed by electron microprobe analysis points to decompression probably caused by progressive exhumation of the subducted material the late ordovician ages were not affected by excess argon incorporation because in all likelihood the oceanic sediments were wet on arrival at the trench and free of older detrital mica the ca 450 ma ages are hence interpreted as the time of crystallization during mylonitization under high fluid activity at fairly low temperatures this means that accretion of the quartzite mylonite unit occured about 200 ma before final closure of the paleo asian ocean amalgamation of the siberian tarim and north china cratons and formation of the end permian solonker suture zone we argue that the early paleozoic evolution of the ondor sum complex occurred along the northeastern cimmerian margin of gondwana which was composed of micro continents fringed by subduction accretion complexes and island arcs the later evolution took place during the building of the eurasian continent following middle devonian and younger rifting along the east gondwanan margin and northward drift of the detached north china craton an extensive review shows that this type of two stage scenario probably also applies to the geodynamic evolution of other micro continents like south china tarim a number of kazakh terranes alashan qaidam and kunlun as well as south kitakami and correlatives in japan and probably indochina like the north china craton these were bordered by early paleozoic subduction accretion complexes island arcs or contained calc alkaline volcanic mar gins like for example the central tienshan north qinling north qaidam altun north qilian and kunlun belts in china as well as the oeyama and miyamori ophiolites and matsugadaira motai blueschist belt of japan and the dismembered sergeevka ophiolite of the southern part of the russian far east this implies that a vast orogenic system comprising an archipelago of micro continents seems to have existed along the cimmerian margin of east gondwana in early paleozoic time in which the ultrahigh pressure metamorphism that characterizes the early evolution of many of the asian micro continents occurred introduction and aim

paleozoic accretion and cenozoic redeformation of the chinese tien shan range central asia the tien shan range in central asia contains two late paleozoic sutures the older southern suture marks the collision of a passive margin at the north of the tarim block and an active continental margin subduction under the latter was to the north the younger northern suture separates a northern carboniferous island arc from an active continental margin developed over a south dipping subduction zone the subduction direction under the island arc is unknown mesozoic elastics were deposited over the doubly sutured orogen rate and energy of sedimentation waned until deposition of oligocene conglomerates above a regional unconformity interpreted as marking the onset of deformation induced by the india asia collision molasse deposition accelerated in pliocene and quaternary time and deposition continues today as active thrusts generate relief paleozoic structures control the gross divergence of cenozoic thrusts across the orogen

crustal scale structure of south tien shan implications for subduction polarity and cenozoic reactivation based on new structural and petrological investigations we present two crustal scale cross sections of the kyrgyz south tien shan and correlations of main faults and units between kyrgyzstan and china the overall structure corresponds to a doubly vergent mountain belt the kyrgyz and chinese areas exhibit identical structural and metamorphic histories to the west the atbashi range comprises high pressure oceanic and continental units stacked by north verging thrusts above a low metamorphic accretionary prism high pressure hp gneisses are bound to their south by a south dipping detachment exhibiting mantle relicts the high pressure oceanic and continental units underwent similar pressure temperature p t paths with peak conditions of around 500 c 20 kbar followed by rapid exhumation the overall south dipping structure and kinematics indicate a south dipping subduction of the central tien shan ocean at 320 310 ma ending with the docking of the tarim block to the kazakh continent to the east the pobeda massif shows a narrow push up structure a major north vergent thrust exhumes deep crustal level granulites constituting the highest summits which were thrust towards the north onto low grade devonian carboniferous schists the southern part of south tien shan is made up of a south verging thrust stack that formed later during ongoing convergence reactivated throughout post 30 ma phases

continental vertical growth in the transitional zone between south tianshan and tarim western xinjiang nw china insight from the permian halajun a1 type granitic magmatism article i nfo the south tianshan collisional belt stcb and northern margin of the tarim block ntb are key areas for understanding the prolonged tectonic evolution of the central asian orogenic belt caob the halajun region in xinjiang province nw china is located within the tectonic transition zone between stcb and tarim blocks several granitic intrusions and one mafic ultramafic complex piqiang complex are exposed in this region zircon u pb dating whole rock major oxide trace element and nd isotopic data are presented for the huoshibulake tamu kezi ertuo and halajun ii granitic intrusions in this area new la icp ms u pb age for kezi ertuo intrusion coupled with previously published shirmp u pb ages for huoshibulake and halajun ii intrusions and piqiang complex reveals that all the igneous rocks in the halajun region are coeval 275 ma geochemically the four granitic intrusions show high contents of sio2 k 2o and total alkalis and possess trace element patterns characterized by rb nb ta zr and hf enrichment and significantly negative ba sr p eu and ti anomalies these features strongly favor an a1 type affinity for the halajun granitic intru sions among the four intrusions the kezi ertuo tamu and halajun ii intrusions possess positive to slightly negative end t values ranging from 0 9 to 0 6 whereas the huoshibulake intrusion displays less deplet ed end t values of 2 6 to 2 9 our new elemental and isotopic data suggest that the four granitic intru sions were generated by the partial melting of a common neoproterozoic gabbroic source probably as a result of the 275 ma underplating of the asthenosphere mantle derived magmas the variable involvement of the mantle components accounts for the range of end t values after the generation of the parental magma alkali feldspar arfvedsonite biotite fe ti oxides and zircon seem to have fractionated prior to the final emplacement of the granitic magmas in combination with the regional geological history we propose that the 275 ma a1 type granitic magmatism in the halajun region and other areas of the ntb provides a good proxy record for the vertical continental crustal growth in the southern margin of the caob during the permian our study in combination with other geological evidence indicates that these a1 type felsic and oib like mafic ultramafic rocks with ages from 282 ma to 275 ma in the southern margin of the caob are parts of the permian tarim large igneous province and could be genetically related to the tarim mantle plume

palaeozoic late collisional strike slip deformations in tianshan and altay eastern xinjiang nw china in central asia thrusts and shear zones resulting from palaeozoic accretional events were reworked by e w trending ductile strike slip faults during late palaeozoic early mesozoic time in the tianshan range microstructures and quartz c axis fabrics show a main dextral shearing associated with sinistral localized shear zones the relationship between these conjugate structures indicates a nnw sse trending bulk shortening in the chinese altay mountains the existence of type microstructures in an important sinistral mylonitic zone infers high rates of deformation this shear zone is bordered by a late dextral ductile fault synchronous with a granite emplacement field evidence and datings from the literature provide chronological constraints in the late carboniferous the sinistral mylonitic deformation took place in the erqishi irtysh shear zone in the northeastern part of xinjiang and in kazakhstan during the early permian a regional dextral event occurred in the tianshan range and under the whole of northern xinjiang