neogene rotations in the jiuquan basin hexi corridor china vertical axis rotations of blocks in around the tibetan plateau can be attributed to the india asia collision study of the vertical axis rotations of these blocks will increase our understanding of the mechanisms and kinematics of continent continent collisions we report here a new palaeomagnetic study of rotations using data from four localities five magnetostratigraphy sections in the jiuquan basin our study indicates that the mean declinations of each section are different from each other similar to what has been observed in the other localities in the ne tibetan plateau however using the mean directions of every 100 m of section we observe that the four localities have similar sequential patterns of rotations during the last 13 ma significant continuous counterclockwise before c 8 0 ma insignificant rotations between 8 0 4 0 ma and slight clockwise rotation after 4 0 ma this indicates that rather than being a record of spatially varying declinations it is a temporal variation in the occurrence of regional rotations combined with other geological evidence the rotation patterns may suggest two major tectonic activity phases of the northeastern tibetan plateau during the last 13 ma an eastward extrusion and strike slip dominant phase before 8 0 ma a significant shortening and a rapid uplift dominant phase after 8 0 ma

evidence for enhanced aridity in the tarim basin of china since 5 3 ma abstract thick cenozoic deposits formed from the erosion of the northern evolving edge of the tibetan plateau provide opportunities to study the exhumation history and climatic changes within the region such deposits are important because they are located in an actively deforming mountain belt as well as in a rain shadow of the tibetan plateau multi proxy studies indicate that hyperarid climate has prevailed since 5 3 ma ago within the basin this information together with the geometry analysis of syntectonic deposits of the sanju anticline which indicate tectonic deformation at 5 3 ma enable us to better understand the interlay between climatic changes and tectonic uplift our results suggest that the tectonic uplift of the kunlun mountains at 5 3 ma must have been one of the significant regional tectonic events and the uplifted elevation of the kunlun mountains more effectively blocks the penetration of moisture from the indian monsoon into the tarim basin and thus enhances the rain shadow effect this can account for the increasing aridity as well as the formation of the world s second largest active sand desert the taklimakan desert in the tarim basin initiated at 5 3 ma ago

seismic evidence for a moho offset and south directed thrust at the easternmost qaidam kunlun boundary in the northeast tibetan plateau abstract during 2006 to 2007 a small array of broadband seismic stations was deployed across the eastern qaidam kunlun border receiver functions derived from this data set show a coherent p to s converted phase from the moho at 65 km to 58 km below sea level b s l from south to north beneath the kunlun mountains the moho is observed at 45 km b s l beneath the qaidam basin a sharp offset of 15 km in the moho depth is observed just beneath the qaidam kunlun border the close correlation between the surface topography and the sharp moho offset substantiates the view that the kunlun crust is weaker than that of the qaidam basin and thickens vertically in response to compression between india through the tibetan plateau and the qaidam basin furthermore a strong steeply north dipping converted phase can be seen from 5 to 15 km b s l north of the north kunlun fault which is likely a major south directed thrust fault partially responsible for the thickening of the kunlun crust

is the north altyn fault part of a strike slip duplex along the altyn tagh fault system although the altyn tagh fault system has played an important role in the indo asian collision its geometry and tectonic evolution remain poorly known between 86 and 92 e this system is at least 100 km wide and is bounded to the north and south by the north altyn and altyn tagh faults respectively mapping along the jianglisai reach of the north altyn fault indicates that miocene to pliocene motion was predominantly left to left reverse slip with transport vectors trending n45 60 e map relationships suggest that total offset on the fault is 120 km these results are inconsistent with previous models of the altyn tagh fault system in which oblique convergence along the northern margin of the tibetan plateau is partitioned into thrusting on the north altyn fault and left slip on the altyn tagh fault an alternative hypothesis is that the north altyn fault is the northern boundary of a transpressional strike slip duplex within which the structurally elevated altyn mountains were created our model suggests that transpressional deformation may be restricted to this strike slip duplex and need not characterize the entire margin

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

a revised chronology for tertiary sedimentation in the sikouzi basin implications for the tectonic evolution of the northeastern corner of the tibetan plateau abstract the space time pattern of development of high topography associated with the tibetan plateau remains incomplete due in large part to sparse proxy data that allow estimates of the growth of individual mountain ranges here we present the results of a stratigraphic investigation of a 3100 m section of tertiary fluvio lacustrine strata in the sikouzi basin located at the northeastern most corner of the tibetan plateau results of magnetostratigraphic analysis coupled with a newly discovered miocene in situ fossil assemblage suggest that the section preserves a nearly complete record extending from initial basin formation at 29 myr until 0 5 myr sediment accumulation rates increase from relatively slow accumulation during the oligocene miocene 9 cm kyr to rapid accumulation in the pliocene 22 cm kyr apparently occurring in two sustained pulses at 10 5 ma and 5 ma respectively the increase in accumulation rates starting at 10 5 ma is interpreted as heralding the initial uplift of the liupan shan consistent with cooling and exhumation of the range zheng et al 2006 the pliocene increase in accumulation rate is accompanied by a distinct change in lithofacies the appearance of coarse detritus with provenance from the madong shan and up section changes in bedding orientation that are interpreted as growth strata collectively our data suggest that the generation of relief in the madong shan began at 5 4 ma moreover analysis of paleomagnetic declinations from the sikouzi section reveals a clockwise rotation of the sikouzi basin during the time period 24 5 4 ma that was followed by a subsequent counterclockwise rotation 5 4 0 5 ma our study suggests that activity along the liupan shan haiyuan fault system probably initiated at 10 5 ma but that the present day configuration of the fault system and associated ranges near the fault tip dates to pliocene time

neogene extension and volcanism in the kunlun fault zone northern tibet new constraints on the age of the kunlun fault 1 the geology of the jingyu basin north tibet provides constraints on the relationship between localized e w extension in the qiangtang block and strike slip motion along the kunlun fault eocene to oligocene sw ne compression formed topographic relief in the present western kunlun shan pull apart basins started forming in the western kunlun ranges around 15 ma possibly coincident with the initiation of subduction of the tarim qaidam lithosphere under the kunlun shan extensional tectonism is still active and the jingyu basin is extending as a normal fault connected with the kunlun fault was recently active localized extension allows tertiary and quaternary shoshonitic magmas to reach the surface from crustal depths of 50 to 60 km miocene strike slip faulting indicates that the initiation of the kunlun strike slip fault is early neogene in age and that the total amount of horizontal movement that occurred along the fault has to be reconsidered

magnetostratigraphy of the suerkuli basin indicates pliocene 3 2 ma activity of the middle altyn tagh fault northern tibetan plateau abstract the left lateral strike slip altyn tagh fault atf forming the northern boundary of the tibetan plateau accommodates parts of the overall convergence between the colliding indian and eurasian plates precise dating of the atf activity is essential for understanding possible mechanisms of tibetan plateau deformation and uplift here we report a magnetostratigraphic study of the suerkuli basin deposits recording depositional changes during the atf activity field investigations reveal a remarkable and widespread change in depositional environment in the suerkuli basin i e a transformation from low energy lacustrine deposits grayish green mud siltstone and brown mud siltstone into high energy alluviul fan deposits poorly sorted gray pebble and cobble conglomerates detailed magnetostratigraphy of the 390 m thick daban section at the southeastern margin of the suerkuli basin 38 43 09 n 90 58 84 e shows that this change in depositional facies occurred at 3 2 ma accompanied by a remarkable increase in sediment accumulation rate we attribute this depositional change to the piocene tectonic activity of the middle atf although the contribution of the pliocene global climate deterioration cannot be excluded

evidence for mesozoic shear along the western kunlun and altyn tagh fault northern tibet china the strike slip faults of north tibet accommodate part of the cenozoic convergence between india and asia along the karakax valley south of yecheng and near the xidatan trough south of golmud the active traces of the altyn tagh and kunlun faults follow narrow belts of metamorphic rocks the deformation recorded in those mylonites is sinistral strike slip rb sr and 40ar 39ar ages of deformation from syntectonic fabrics formed at 350 400 c 120 ma argon loss suggests that deformation was associated to a 250 300 c thermal pulse that lasted 5 20 ma after the onset of movement unroofing occurred much later around 25 ma when sudden cooling suggests a component of thrusting or more likely normal faulting the cretaceous shear may be related to collision between the qiantang and the lhasa blocks the karakax and xidatan shear zones may have formed a unique continuous boundary in the cretaceous which was later reused by the tertiary strike slip faults leading to potentially calculable offsets along the altyn tagh fault

injection of tibetan crust beneath the south qaidam basin evidence from indepth iv wide angle seismic data 1 the international deep profiling of tibet and the himalaya phase iv indepth iv active source seismic profile in northeast tibet extends 270 km roughly north south across the songpan ganzi terrane the predominantly strike slip north kunlun fault along the kunlun suture the east kunlun mountains and the south qaidam basin refraction reflection and gravity modeling provide constraints on the velocity and density structure down to the moho the central qaidam basin resembles average continental crust whereas the songpan ganzi terrane and east kunlun mountains exhibit thickened lower velocity crust also characteristic of southern tibet the crustal thickness changes from 70 km beneath the songpan ganzi terrane and east kunlun mountains to 50 km beneath the qaidam basin this jump in crustal thickness is located 100 km north of the north kunlun fault and 45 km north of the southern kunlun qaidam boundary farther north than previously suggested ruling out a moho step caused by a crustal penetrating north kunlun fault the qaidam moho is underlain by crustal velocity material 6 8 7 1 km s for 45 km near the crustal thickness transition the southernmost 10 km of the qaidam moho are underlain by a 70 km reflector that continues to the south as the tibetan moho the apparently overlapping crustal material may represent songpan ganzi lower crust underthrusting or flowing northward beneath the qaidam basin moho thus the high tibetan plateau may be thickening northward into south qaidam as its weak thickened lower crust is injected beneath stronger qaidam crust

geodetic evidence for a low slip rate in the altyn tagh fault system the collision between india and asia has been simulated with a variety of computational models that describe or predict the motions of the main faults of east asia geological slip rate estimates of 20 30 mm yr 1 suggest that the largest of these faults the 2 000 km long altyn tagh fault system on the northern edge of the tibetan plateau absorbs as much of the indo asian convergence signal as do the himalayas partly by oblique slip and partly by contraction and mountain growth however the predictions of dynamic models for asian deformation and the lower bounds of some geological slip rates estimates 3 9 mm yr 1 refs 7 8 suggest that the altyn tagh system is less active here we report geodetic data from 89 91 degrees e that indicate left lateral shear of 9 5 mm yr 1 and contraction of 3 1 mm yr 1 across the altyn tagh system this result combined with our finding that at 90 degrees e tibet contracts north south at 9 1 mm yr 1 supports the predictions of dynamic models of asian deformation

locomotive implication of a pliocene three toed horse skeleton from tibet and its paleo altimetry significance the tibetan plateau is the youngest and highest plateau on earth and its elevation reaches one third of the height of the troposphere with profound dynamic and thermal effects on atmospheric circulation and climate the uplift of the tibetan plateau was an important factor of global climate change during the late cenozoic and strongly influenced the development of the asian monsoon system however there have been heated debates about the history and process of tibetan plateau uplift especially the paleo altimetry in different geological ages here we report a well preserved skeleton of a 4 6 million y old three toed horse hipparion zandaense from the zanda basin southwestern tibet morphological features indicate that h zandaense was a cursorial horse that lived in alpine steppe habitats because this open landscape would be situated above the timberline on the steep southern margin of the tibetan plateau the elevation of the zanda basin at 4 6 ma was estimated to be 4 000 m above sea level using an adjustment to the paleo temperature in the middle pliocene as well as comparison with modern vegetation vertical zones thus we conclude that the southwestern tibetan plateau achieved the present day elevation in the mid pliocene

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

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

mineral magnetic properties of loess paleosol couplets of the central loess plateau of china over the last 1 2 myr 1 we have conducted a multiparameter investigation of 15 loess paleosol couplets s0 l1 to s14 l15 from the jiaodao section in the central loess plateau of china using environmental magnetic approaches coupled with soil science techniques the magnetic parameters display systematic variations that seem to be closely related to paleoclimate variations and intensity of pedogenesis high temperature susceptibility curves of paleosols show a generally decreasing trend in reversibility from the base of the lishi formation to the holocene black loam possibly indicating a decrease in weathering intensity this may reflect a long term increase in aridity and or a general long term cooling trend of the interior of the asian continent from 1 2 ma to the present several samples display wasp waisted hysteresis loops these are most pronounced in moderately enhanced paleosols less pronounced in the practically unaltered loess and subdued in the well developed paleosols but wasp waistedness reappears in the most developed paleosols this wasp waistedness sequence suggests that the composition concentration and grain size of magnetic minerals all contribute to the hysteresis behavior of samples from the studied loess paleosol sequence but each factor has a different effect at different stages of pedogenesis

morphometry of late pleistocene holocene faulting and volcanotectonic relationship in the southern andes of colombia diffuse seismicity and the active galeras volcano characterize the southern andes of colombia whose inhabitants are prone to a high natural risk we used field geological structural and quantitative morphological data to define the geometry and kinematics of late quaternary faults and the volcanotectonic relationships in this area the main ne sw buesaco fault has a late pleistocene holocene 29 km rupture length offset of dated landforms yields 188 13 m of horizontal right lateral long term cumulative component of slip the vertical component of uplift of the nw block being 67 12 m the long term net slip rate is 1 48 0 12 mm yr the other main structure here named aranda fault also strikes ne sw and is 13 km long offset of landforms yields 160 10 m of horizontal right lateral long term component of slip and 3 9 1 9 m of vertical component of uplift of the nw block the long term net slip rate is 1 19 0 08 mm yr on the galeras edifice e w to nw se faults have normal motions and slip rates from 0 02 to 0 23 mm yr both the galeras and a late pleistocene pyroclastic cone lie along the ideal extension of the buesaco fault magma uprising in the crust can occur along the ne striking fractures when magma pressure mp is greater than the normal to fault component of tectonic 1 magma uprising in the uppermost crust i e in the cone can occur by rock cracking along e w planes with mp 3 up to nw se planes with mp greater than shear stress component of the horizontal greatest principal stress hmax

tomographic evidence for localized lithospheric shear along the altyn tagh fault seismic tomography across the altyn tagh fault at the north edge of the tibetan plateau reveals a low p wave velocity anomaly below the fault down to 140 kilometers this anomaly probably reflects strike slip shear in the lithosphere slip partitioning may also induce a wedge of crust from the tarim basin to plunge into the mantle

magnetostratigraphy of cenozoic deposits in the western qaidam basin and its implication for the surface uplift of the northeastern margin of the tibetan plateau abstract thick cenozoic deposits in the northwestern qaidam basin record erosion of the altyn tagh and high terrain west of that basin and presumably the concurrent growth of the northeastern tibetan plateau a detailed magnetostratigraphic study of the huatugou section northwestern qaidam basin reveals that this section spans the period from 30 to 11 ma magnetostratigraphic and sedimentological studies indicate that the accumulation rate abruptly increased near 15 ma the acceleration in sedimentation rate suggests enhanced tectonic deformation in the qaidam basin since 15 ma that may have begun simultaneously with accelerated deformation along the altyn tagh kunlun and haiyuan faults which contributed to the growth history of the qaidam basin and its surroundings since 15 ma

lower triassic conodont sequence in chaohu anhui province china and its global correlation abstract the early triassic paleogeographical setting of the chaohu section in china was in a deep part of the yangtze platform and this has resulted in a rather complete lower triassic sequence with rich ammonoid and conodont successions three lower triassic sections west pingdingshan north pingdingshan and south majiashan have been sampled in detail for a systematic conodont study eight conodont zones are recognized in ascending order these are the zones of hindeodus typicalis neogondolella krystyni neospathodus kummeli neospathodus dieneri neospathodus waageni neospathodus pingdingshanensis neospathodus homeri and neospathodus anhuinensis these are correlated with the sequences in north america and other areas of the world the succession provides a good basis for the definition of the global stratotype of the induan olenekian boundary gssp which has been proposed at the base of the neospathodus waageni zone in west pingdingshan seven new species are introduced neospathodus chaohuensis n chii n concavus n eotriangularis n pingdingshanensis n posterolongatus n tongi a new subspecies n waageni eowaageni and three morphotypes of n dieneri

nailing down the slip rate of the altyn tagh fault 1 previous estimates of the geodetic and geologic slip rates of the 1500 km long altyn tagh fault bordering the northern edge of the tibetan plateau vary by a factor of five proposed reasons for these discrepancies include poor gps geometry interpretative errors in terrace morphology and changes in fault slip rate over time here we present results from a new dense gps array orthogonal to the fault at 86 2 e that indicates a velocity of 9 0 3 2 4 4 mm yr in close agreement with geomorphologic estimates at the same location our estimated geodetic slip rate is consistent with recent geological slip rates based on terrace offsets the resulting mean combined geological and geodetic slip rate 9 0 4 0 mm yr is remarkably uniform for the central 800 km of the altyn tagh fault significantly lower than early kinematic estimates and consistent with deformation elsewhere in tibet and central asia

lithospheric and upper mantle structure of the northeastern tibetan plateau 1 we use receiver functions calculated for data collected by the indepth iv seismic array to image the three dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern tibet our results indicate an average crustal thickness of 65 to 70 km in northern tibet in addition we observe a 20 km moho offset beneath the northern margin of the kunlun mountains a 10 km moho offset across the jinsha river suture and gently northward dipping moho beneath the qaidam basin a region in the central qiangtang terrane with higher than normal crustal vp vs ratio of 1 83 can be the result of the eocene magmatic event in the qiangtang terrane we observe a significant lithospheric mantle discontinuity beneath the bangong nujiang suture at 80 km depth which dips 10 to the north reaching 120 km depth we interpret this feature as either a piece of lhasa terrane or remnant oceanic slab underthrust below northern tibet we detect a 20 km depression of the 660 km discontinuity in the mantle transition zone beneath the northern lhasa terrane in central tibet which suggests this phase transition has been influenced by a dense and or cold oceanic slab a modest 10 km depression of the 410 km discontinuity located beneath the northern qiangtang terrane may be the result of localized warm upwelling associated with small scale convection induced by the penetration of the sinking indian continental lithosphere into the transition zone beneath the central tibetan plateau

eastward migration of the qaidam basin and its implications for cenozoic evolution of the altyn tagh fault and associated river systems the qaidam basin is the largest cenozoic intermontane basin within the tibetan plateau bounded by the qilian shan range to the north the east kunlun range to the south the altyn tagh range to the west and the ela shan range to the east its deposits consist of nonmarine sedimentary rocks with a maximum thickness exceeding 13 400 m modern sedimentation mainly takes place along the marginal areas of the basin where alluvial fans are formed along a series of rivers that originate from the surrounding mountains nevertheless drilling sedimentary and seismic data reveal that the depocenter of the basin during much of cenozoic time was not developed as foreland basins of the qilian shan belt to the north and the east kunlun belt to the south but instead the depocenter was in the center of the western part of the basin and had a northwest southeast trend that shifted eastward 380 km during oligocene to quaternary time this indicates that most of the basin sediments were not derived from the surrounding mountain ranges the results of the fi ssion track age dating reveal that the rapid uplift of the yousha shan anticline bounding the depocenter to the southwest initiated at ca 31 ma indicating that the southeastward shift of the depocenter began at that time yielding a migration rate for the depocenter of 10 2 mm yr the northwestern margin of the depocenter of the qaidam basin lies to the east of the westward prolongation of the qaidam basin the tula trough a 300 km long narrow trough fl anked by the altyn tagh range along the altyn tagh strikeslip fault on the north and the east kunlun thrust belt on the south the sedimentary and deformational evidence from this region indicates that the tula trough was formed as a syncline along which a paleoriver fl owed to the east into the qaidam basin to the southwest this river valley merges with the trace of the altyn tagh fault the latter and its westward continuation are interpreted to have extended to the northern margin of the pamir folded belt north of the western syntaxis of the himalaya we propose that most of the sediments within the depocenter of the qaidam basin were transported from the northern margin of the pamir folded belt along a progressively lengthening 2000 kmlong longitudinal river that developed along the left lateral altyn tagh fault and its western continuation as the eastward extrusion of the tibetan plateau took place in oligocene time both the crustal uplift and continuous southeastward aggradations of the sediments carried by the paleo kunlun river are interpreted to be the cause for southeastward migration of the depocenter of the qaidam basin the longitudinal river disappeared in late cenozoic time 2 4 ma due to stream capture and climate change

miocene to holocene exhumation of metamorphic crustal wedges in the nw himalaya evidence for tectonic extrusion coupled to fluvial erosion the himalayan crystalline core zone exposed along the sutlej valley india is composed of two high grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded as a consequence of the foreland directed propagation of crustal deformation in the indian plate margin the high himalayan crystalline sequence hhcs is composed of amphibolite facies to migmatitic paragneisses metamorphosed at temperatures up to 750 c at 30 km depth between eocene and early miocene during early miocene combined thrusting along the main central thrust mct and extension along the sangla detachment induced the rapid exhumation and cooling of the hhcs whereas exhumation was mainly controlled by erosion since middle miocene the lesser himalayan crystalline sequence lhcs is composed of amphibolite facies para and orthogneisses metamorphosed at temperatures up to 700 c during underthrusting down to 30 km depth beneath the mct the lhcs cooled very rapidly since late miocene as a consequence of exhumation controlled by thrusting along the munsiari thrust and extension in the mct hanging wall this renewed phase of tectonic extrusion at the himalayan front is still active as indicated by the present day regional seismicity and by hydrothermal circulation linked to elevated near surface geothermal gradients in the lhcs as recently evidenced in the himalayan syntaxes active exhumation of deep crustal rocks along the sutlej valley is spatially correlated with the high erosional potential of this major trans himalayan river this correlation supports the emerging view of a positive feedback during continental collision between crustal scale tectono thermal reworking and efficient erosion along major river systems

cenozoic shortening budget for the northeastern edge of the tibetan plateau is lower crustal flow necessary 1 two of the most popular mechanisms for thickening the crust beneath the tibetan plateau are 1 pure shear with faulting and folding in the upper crust and horizontal shortening below and 2 flow of lower or middle crust without significant shortening of the upper crust to help discriminate between the relative contributions of these two mechanisms well constrained estimates of upper crustal shortening are needed here we document the cenozoic shortening budget across the northeastern tibetan plateau margin with several balanced cross sections that exploit thermochronological and magnetostratigraphic constraints these sections indicate 11 2 1 east west shortening since middle miocene time and 9 2 3 nne ssw shortening between middle eocene and middle miocene times with little subsequent shortening of this orientation shortening rates accelerate fivefold after middle miocene time given the present day crustal thickness of 56 4 km in northeastern tibet crustal restorations that remove cenozoic shortening suggest that the northeastern tibetan crust was 45 5 km thick prior to the india asia continental collision this precollision thickness estimate is equivalent to average continental crustal thicknesses both adjacent to the tibetan plateau 44 4 km and globally 41 6 km and suggests that pure shear alone may account for cenozoic crustal thickening in northeastern tibet in contrast to eastern tibet where in the absence of significant shortening structures crustal flow has been invoked to explain the addition of crustal material since middle miocene time our results may obviate lower crustal flow as a necessary crustal thickening agent in northeastern tibet

syntectonic growth strata and implications for late cenozoic tectonic uplift in the northern tian shan china abstract propagation of faults and folds in the foreland basins of tian shan is an important process accommodating cenozoic crustal shortening and mountain building but little is known about the accurate time of the cenozoic tectonic deformation based on growth strata and age determination we show that syntectonic growth strata began to develop in the middle part of tian shan since 6 ma ago geometry analysis indicates that formation of the growth strata is associated with progressive fold limb rotation formation of the growth strata is contemporaneous with the tectonic deformation in the thrusting and folding zones together with the remarkable increase of sedimentation rate as well as the accumulation of coarse molasse deposits we conclude that the late cenozoic crustal shortening and mountain building in the region initiated since about 6 ma and lasted to the early pleistocene as a consequence of intracontinental deformation within the india eurasia convergent system

a cretaceous eocene depositional age for the fenghuoshan group hoh xil basin implications for the tectonic evolution of the northern tibet plateau the fenghuoshan group marks the initiation of terrestrial deposition in the hoh xil basin and preserves the first evidence of uplift above sea level of northern tibet the depositional age of the fenghuoshan group is debated as are the stratigraphic relationships between the fenghuoshan group and other terrestrial sedimentary units in the hoh xil basin we present new radiometric dates and a compilation of published biostratigraphic data which are used to reinterpret existing magnetostratigraphic data from the fenghuoshan group from these data we infer an 85 51 ma depositional age range for the fenghuoshan group u pb detrital zircon age spectra from this unit are compared to age spectra from tibetan terranes and mesozoic sedimentary sequences to determine a possible source terrane for fenghuoshan group strata we propose that these strata were sourced from the qiangtang terrane and may share a common sediment source with cretaceous sedimentary rocks in nima basin field relationships and compiled biostratigraphic data indicate that the fenghuoshan and tuotuohe groups are temporally distinct units we report late oligocene ages for undeformed basalt flows that cap tilted fenghuoshan group strata together our age constraints and field relationships imply exhumation of the central qiangtang terrane from the late cretaceous to earliest eocene followed by eocene oligocene deformation and shortening of the northern qiangtang and southern songpan ganzi terranes crustal shortening within the hoh xil basin ceased by late oligocene time as is evident from flat lying basaltic rocks which cap older deformed strata

temporal constraints and pulsed late cenozoic deformation during the structural disruption of the active kashi foreland northwest china 1 in response to the ongoing indo asian collision structural deformation has encroached into the tian shan foreland in western china since the early miocene in order to reconstruct a detailed history of foreland deformation along the southern margin of the tian shan we have synthesized extensive mapping analysis of seismic sections magnetostratigraphy of the foreland fill and associated growth strata apatite fission track dating changes in sediment accumulation rates and geodetic surveys of deformed fluvial terraces across an area exceeding 6500 km2 that spans the kashi foreland in the northwestern tarim basin we use these data to place temporal constraints on individual structures that delineate at least 4 distinct stages of deformation stage 1 involved initial uplift of hinterland structures beginning at 20 25 ma stage 2 began at 16 3 ma when the basement involved deformation front stepped south to the kashi basin thrust that bounds the foredeep strata stage 3 occurred from 13 5 to 4 ma as the deformation front migrated south in episodic steps above thin skinned detachment and ramp flat folding finally stage 4 began after 4 ma with growth of the keketamu anticline followed by rapid southward migration of the deformation front to the atushi and then kashi detachment anticlines on the basis of the deformed tertiary strata and available seismic data calculations of the minimum magnitude of shortening yield totals ranging from 10 to 32 km since the early miocene of which 7 12 km occurred since 4 ma although average shortening rates since 16 3 ma were 0 8 1 3 mm a rates were faster from 16 3 to 13 5 ma 1 1 3 2 mm a after which they decreased to 0 5 mm a between 13 5 and 4 ma before increasing to 2 2 2 7 mm a after 4 ma similarly the rate of southward propagation of the deformation front was initially 3 7 mm a for the first 10 ma of basin development slowed to 1 5 mm a between 13 5 and 4 ma and increased to more than 10 mm a since 4 ma these changes in shortening and structural migration rates over time suggest that the loci of orogenic deformation shifted in and out of the foreland during the neogene surveyed stream terraces suggest that although most current deformation is located along the southern front of the foreland modest deformation and shortening continue in the hinterland structural style within the foreland has also changed over time from imbricate thrust faults with hanging wall folds along the northern margin to ramp flat faults and detachment folds along the southern margin where the basement rocks are previously deformed such as in the hinterland of the kashi foreland thick skinned fault ramps and related fault propagation folds dominate the structural style as syntectonic sedimentation within the foreland produced thick subhorizontal stratigraphy the structural style evolved to detachment and fault bend folding above these footwall flats within the neogene strata

a rb sr record of the weathering response to environmental changes in westerly winds across the tarim basin in the late miocene to the early pleistocene the long term record of lacustrine deposits in the tarim basin seems to provide a record of climate influenced by westerly winds to characterize the evolution of westerly winds we analyzed concentrations of rb v zn sc and sr for the drill core ls2 from lop nor in the eastern tarim basin the rb sr ratio in lacustrine deposits depends primarily on the sr element activity during weathering after the influence of source provenance has been removed we assume that with such a correction the rb sr ratio is controlled by precipitation which depends on moisture transport by westerly winds to the tarim basin the rb sr ratio displays a large amplitude oscillation that generally follows a pattern similar to that of pollen and ostracod assemblages which support the inference that rainfall amount affects the rb sr ratio and in turn suggests that it may serve as an index of precipitation that was transported by westerly wind all these climatic proxies obviously changed since the late miocene suggesting stronger aridification that may be attributed to a latitudinal shift in westerly winds since the late miocene a southward migration of westerly winds which then transported more moisture to the southern and eastern parts of the tibetan plateau agrees with modeling results and the geological record during that time this event may be causally related to the expansion of the northern hemisphere ice and or regional tectonic uplift of high mountains around the tarim basin

early cenozoic faulting of the northern tibetan plateau margin from apatite u th he ages models to explain the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary yet a growing body of evidence in the tibetan orogen suggests that deformation occurred at the far northern extent of the modern plateau early in the orogen s history and thus our current mechanical understanding of orogenic plateau development is incomplete new apatite u th he ages from four elevation transects document periods of rapid exhumation related to erosion pulses in hanging wall rocks of major thrust structures accelerated erosion is used as a proxy of fault timing and is interpreted in a larger context of structural data and sediment accumulation in adjacent foreland basins helium results are synthesized with published geologic thermochronometric and sedimentologic data from which a growing picture of regional compressional deformation in middle to late eocene time in northern tibet emerges we relate the early cenozoic period of deformation to the initiation of collision between india and eurasia despite the fact that the plate boundary was located 3000 km to the south regardless of whether or not high topography was built simultaneously as a result of this deformation early cenozoic strain signifies that the modern limit of the orogen has been relatively stationary since continental collision began and that deformation has not significantly propagated farther away from the plate boundary in time

the growth of northeastern tibet and its relevance to large scale continental geodynamics a review of recent studies recent studies of the northeastern part of the tibetan plateau have called attention to two emerging views of how the tibetan plateau has grown first deformation in northern tibet began essentially at the time of collision with india not 10 20 myr later as might be expected if the locus of activity migrated northward as india penetrated the rest of eurasia thus the north south dimensions of the tibetan plateau were set mainly by differences in lithospheric strength with strong lithosphere beneath india and the tarim and qaidam basins steadily encroaching on one another as the region between them the present day tibetan plateau deformed and its north south dimension became narrower second abundant evidence calls for acceleration of deformation including the formation of new faults in northeastern tibet since 15 ma and a less precisely dated change in orientation of crustal shortening since 20 ma this reorientation of crustal shortening and roughly concurrent outward growth of high terrain which swings from nne ssw in northern tibet to more ne sw and even ene wsw in the easternmost part of northeastern tibet are likely to be in part a consequence of crustal thickening within the high tibetan plateau reaching a limit and the locus of continued shortening then migrating to the northeastern and eastern flanks these changes in rates and orientation also could result from removal of some or all mantle lithosphere and increased gravitational potential energy per unit area and from a weakening of crustal material so that it could flow in response to pressure gradients set by evolving differences in elevation

osl dating of offset streams across the altyn tagh fault channel deflection loess deposition and implication for the slip rate abstract two deflected streams flowing across the altyn tagh fault atf were investigated at the aksay segment nw china due to strike of the atf stream channels are deflected and elongated by adding new sections along the fault line after subsequent water erosion and fluvial refreshment banks of these deflected streams are overlain by aeolian loess geomorphological features suggest that stream deflections along the fault were developed as a result of faulting activities and displacements were progressively accumulated from multiple faulting cycles each faulting cycle is composed of faulting event triggered elongation of channels creation of stream banks or risers and subsequent aeolian loess deposition on the banks a model is proposed to illustrate the relationship between fault movement and loess deposition which is supported by osl dating of the loess on the offset stream channels our results demonstrated that 1 loess deposits along deflected streams can be used to trace fault slip history given specific geomorphic assumptions discussed in the text and 2 based on stream channel offsets and loess ages the holocene slip rate of the atf is estimated to be 11 2 mm year along the aksay segment

formation and re orientation of the suerkuli basin within the altyn tagh in northeastern tibetan plateau since late miocene abstract the altyn tagh fault atf had formed in eocene time however the temporal evolution of slip along the fault remains elusive despite its important role in governing growth of the tibetan plateau tp and evaluating models of continental lithosphere deformation here we perform a high resolution magnetostratigraphic study on neogene sediments from the suerkuli basin a small basin located between two active strands of the atf results show that both depositional environment and the sedimentation rate changed at about 5 4 and 6 ma respectively lithofacies shift from conglomerates and sandstones with parallel and cross beddings to mudstones with horizontal bedding suggesting a sedimentary fluvial to lacustrine depositional environment shift even aridification increased over northern china from the tarim basin to the qaidam basin during late miocene to early pliocene times lacustrine deposition had started in the suerkuli basin still we argue that an increase in slip rate along the atf triggered concurrently forming of the intermontane basin the inferred timing of increased slip rate on the atf coincides with a variation in movement of the indian plate relative to eurasian plate since late miocene early pliocene time our work suggests that a re orientation of crustal shortening and deformation in the northern tp began since late miocene and supports the idea that the modern boundary of the tp orogen had rejuvenated to adjust deformation in plate boundary conditions

the high k2o volcanism of northwestern tibet geochemistry and tectonic implications cenozoic volcanic rocks sampled during the 1989 sino french kunlun geotraverse have been analyzed to constrain the origin of widespread tertiary to neogene tibetan basic magmatism samples ranging from phonotephrites to trachyandesites are high k lavas k na always greater than 1 which exhibit typical calc alkaline characteristics although they are highly enriched in mobile trace elements and ree 600 times chondrites for la isotopic compositions exhibit radiogenic sr and pb 87sr 86sr 0 708 208pb 204pb 39 39 4 207pb 204pb 15 6 15 7 206pb 204pb 18 7 and unradiogenic nd 143nd 144nd 0 5123 0 5124 constancy of sr and nd isotopic values contrasts with variations in pb isotopes and highlights the problem of crustal contamination afc and simple mixing models are compatible with isotopic variations but were probably buffered by elemental concentration in sr and nd while pb isotopic values for the most primitive magmas are probably uncontaminated isotopic features are therefore proposed to be representative of a mantle source in which heterogeneity could be highlighted in the pb isotopic values such a source highly enriched in trace elements and radiogenic sr and pb is proposed to lay in the subcontinental mantle isolated from the overall convection movements the calc alkaline characteristics can date from the succession of enrichment phases linked to the numerous subduction events that have affected this part of asia while the isotopic characteristics are proposed to be the result of the aging of the concomitant lile enrichments the broad geographical geochemical and timing aspects of recent basic cenozoic magmatism are discussed in the light of our new data and we suggest that two mechanisms are acting together 1 a limited delamination of the base of the lithosphere or a thermal reequilibration of isotherms subsequent to thickening and 2 intracontinental subduction under the kunlun such mechanisms appear the most probable as they respect at the same time recent geophysical data and other geochemical studies

inescapable slow slip on the altyn tagh fault 1 gps surveys conducted in 1994 1998 and 2002 on the central portion of the altyn tagh fault confirm left lateral slip rates of 9 4 mm yr significantly slower than geological estimates of slip rate for the past several thousand years the measurements were obtained on a 300 km long profile between 88 and 91 e within one fault dimension of the mw 7 6 and mw 7 8 kunlun earthquakes of 1997 and 2001 the inferred elastic displacements from these events are incorporated into the interpreted 1994 2002 velocity field the recent serendipitous occurrence of these two large earthquakes on a sub parallel fault system suggests that slip on the kunlun fault may have recently accelerated thereby reducing the slip rate on the altyn tagh fault however geodetic data from northern tibet prior to the 2001 earthquake indicate present day rates similar to long term rates this suggests that systematic errors may be present in published geologic slip rate estimates for the altyn tagh fault

deciphering the rate of mountain growth during topographic presteady state an example from the ne margin of the tibetan plateau 1 we use field investigations topographic profiles and 10be exposure dating to constrain the growth rate of the yumu shan a thrust bounded mountain range at the northeastern margin of tibet wind gaps fault scarps at the mountain front and hanging paleocatchments indicate that the yumu shan is growing both laterally and vertically scarp profiles and 10be ages yield rock uplift rates of 500 and 800 mm ka 1 for the eastern and central parts of the range respectively the fact that the rock uplift rate in the high center of the yumu shan exceeds catchment wide 10be denudation rates which range from 180 to 280 mm ka 1 indicates that the mountain range continues to rise and has not yet reached a topographic steady state combining the total structural relief of the range with the rates of rock uplift and denudation suggests that the growth of the yumu shan started 3 7 0 9 ma ago

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