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

stratigraphy and remanence acquisition of a palaeomagnetic reversal in alluvial siwalik rocks of pakistan a high resolution record of a palaeomagnetic reversal is documented in miocene alluvial rocks of pakistan we examined lateral variability of lithostratigraphy and palaeomagnetic stratigraphy through the same palaeomagnetic reversal in six correlated sections each section contains one or more palaeomagnetic sites with directions between fully reversed and fully normal the position of the reversal illustrates local relief in the study area and the time transgressive nature of certain stratigraphic units variability in the thickness of the transitional interval indicates contemporaneous variability in sediment accumulation rates different characteristics of palaeomagnetic remanence are associated with the depositional and postdepositional history of these sediments variability in patterns of remanence behaviour is the basis for inferences about post depositional processes we discuss two magnetic parameters that express coherency of palaeomagnetic samples the maximum angle of deviation and the circular standard deviation of particular interest are samples with incoherent palaeomagnetic signals the incoherency of samples is inversely correlated with the thickness of the transitional interval a low rate of sediment accumulation suggested by a thin transitional interval may facilitate a prolonged period of remagnetization through pedogenic or hydrological processes alternatively transition intervals defined by coherent magnetization may be thin as a result of pedogenically induced incoherency

flexural subsidence by 29 ma on the ne edge of tibet from the magnetostratigraphy of linxia basin china abstract this study provides a detailed magnetostratigraphic record of subsidence in the linxia basin documenting a 27 myr long sedimentary record from the northeastern edge of the tibetan plateau deposition in the linxia basin began at 29 ma and continued nearly uninterruptedly until 1 7 ma increasing rates of subsidence between 29 and 6 ma in the linxia basin suggest deposition in the foredeep portion of a flexural basin and constrain the timing of shortening in the northeastern margin of the plateau to late oligocene late miocene time by late miocene early pliocene time a decrease in subsidence rates in the linxia basin associated with thrust faulting and a 10 clockwise rotation in the basin indicates that the deformation front of the tibetan plateau had propagated into the currently deforming region northeast of the plateau

structural evolution of the yushu nangqian region and its relationship to syncollisional igneous activity east central tibet field mapping geochronological analyses and cross section construction reveal a protracted deformation history and a minimum of 61 km of cenozoic ne sw shortening in present coordinates across the yushu nangqian thrust belt in northern tibet cenozoic contraction started prior to 51 ma and was followed first by northwest striking right slip faulting and later by northwest striking left slip faulting renewed ne sw contraction is expressed by folding of neogene strata and thrusting which again was followed by northwest striking left slip faults late neogene deformation is expressed by local north striking normal faults shortening across the yushu nangqian belt appears to be accommodated by thin skinned thrusting which raises the question of how the deformation was accommodated in the lower crustal levels to resolve this problem we perform geochemical analysis of igneous rocks dated as 51 49 and 38 37 ma the rocks exhibit geochemical signatures characteristic of subduction which implies that coeval crustal thickening in northeastern tibet was most likely induced by continental subduction

an early unroofing in northeastern tibet constraints from 40ar 39ar thermochronology on granitoids from the eastern kunlun range qianghai nw china abstract in order to better constrain the mechanisms accommodating the india asia collision along the whole kunlun range northern tibet which is essential to better understand the evolution of the entire deformations area a 40 ar 39 ar thermochronological study has been carried out on a suite of granitoids collected in the vicinity of golmud eastern kunlun range biotite and k feldspar have been analysed and the cooling histories of the latter have been modelled the ranges of 40 ar 39 ar data bt 128 257 ma kf 102 228 ma are similar to previous rb sr and k ar data indicating both a middle paleozoic unit and a mesozoic unit including a triassic series and a jurassic cretaceous series this suggests that the two geological units previously reported in the western kunlun are also present in the eastern part of the range suggesting that eastern and western kunlun ranges might have been left laterally offset by as much as 500 km along a strike slip fault presently parallel to the altyn tagh fault the cooling histories based upon k feldspar modelling show that samples from the southern part of the range recorded an important cooling event 9 15 myr around 30 ma which is thought to reflect an increasing rate of denudation related to the tectonic activity in this area minimum denudation rates of 0 2 0 3 km ma have been estimated in contrast samples from the northern part of the range did not record any variation in their cooling history this heterogeneous denudation across the range is thought to be related to ramp stacking and normal faulting with associated uplift mainly affecting the southern part of the range suggesting that the eastern kunlun underwent an important crustal thickening period 30 ma ago this crustal thickening event is suggested to be related to the motion along the great asian strike slip faults in a way which remains problematic but possibly associated to the cenozoic propagation of the altyn tagh fault finally unroofing in the eastern kunlun range appears to predate that in the western kunlun range where it is suggested to have started around 20 ma ago this diachronous denudation could be related to a higher denudation rate in western kunlun range and it is thus highly probable that the important unroofing event outlined around 20 ma ago in the western kunlun has also affected the eastern part of the range even though the signature is not available at the moment

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

mesozoic and cenozoic tectonics of the northern edge of the tibetan plateau fission track constraints abstract fission track analysis on zircons and apatites yields new information about the timing of deformation of the northern tibetan plateau ages on zircons ranging from 221 22 to 96 4 ma are indicative of a general late triassic early jurassic cooling probably driven by the collision between the qiantang and kunlun blocks mid jurassic slow cooling is recorded also in the apatites in regions not affected by later cenozoic deformation this jurassic denudation was followed by a period of sedimentation during the cretaceous except along the altyn tagh fault atf zone and in some restricted areas of the western and eastern qilian shan this long and relatively quiet period ended at about 40 10 ma along the major altyn tagh and kunlun strike slip fault zones which were activated by the india asia collision this first movement along lithospheric faults resulted in the eastward extrusion of the tibet plateau which was followed in late oligocene miocene times by a major compression event initiating the formation of the high relief of north tibet a final compressional event took place at 9 5 ma and is well correlated with high sedimentation rates in the basins of this region this compression induced continental subduction in the kunlun ranges the altun shan belt and possibly the qilian shan belt

formation of internally drained contractional basins by aridity limited bedrock incision 1 low internal relief aridity and internal drainage characterize the puna altiplano plateau and the tibetan plateau and the tarim basin structurally these areas are reverse fault bounded terrains with intervening wedge top basins that store thick accumulations of sediment for millions of years orographic barriers along the margins of these basins are oriented normal to moisture bearing winds resulting in regional aridity the combination of this aridity rapid shortening and uplift rates and resistant exposed lithologies coincides with the location of these internally drained basins we hypothesize that these large internally drained areas persist indefinitely when uplift overwhelms the fluvial systems and defeats the channel network to test this hypothesis we developed models of channel defeat to examine the threshold conditions required to fragment the channel network the results of these models suggest that low erodibility rocks moderate to high uplift rates and low precipitation favor defeat of orogen traversing channels most coupled tectonic landscape development models prevent the development of internal drainage and so may overestimate the voracity of erosion in regimes that favor internal drainage profoundly influencing predicted orogenic growth patterns

post orogenic tectonic evolution of the qinling belt central china insights from a magnetostratigraphic study of a cretaceous intra mountain basin sedimentary succession abstract the huicheng basin straddling the west and east qinling belt in central china is a key feature linking the ne tibetan plateau and the rifted basins in east china and its sedimentary sequences provide important archives to elucidate tectonic evolution of the qinling belt and possibly the eastward expansion of the tibetan plateau the age of upper red fluvial lacustrine facies sequence known as the chengxian group has been previously assigned to neogene in spite of the lack of age definitive fossils which seems problematic in order to better constrain the age of this group and to establish a chronostratigraphic framework we conducted a magnetostratigraphic study of about 420 m thick composite section and collected 738 paleomagnetic samples progressive thermal demagnetization of these samples reveals both single and two component magnetizations characteristic remanent magnetizations chrms are mostly isolated at or above 350 and show northeasterly downward directions indicating unanimously normal polarities through the composite section positive fold and tilting tests suggest that the chrms are likely primary and that this sequence was possibly deposited during the cretaceous normal superchron instead of the previously assigned neogene by correlating with other cretaceous basins developed in west and east qinling and taking into account the polyphase tectonic overprints from structural analysis we conclude that the huicheng basin was formed during the post orogenic tectonic regime in response to far field effects caused by the northward subduction of the neo tethys ocean and the subsequent india eurasia collision

magnetostratigraphy of tertiary sediments from the hoh xil basin implications for the cenozoic tectonic history of the tibetan plateau summary we conducted an integrated palaeomagnetic and stratigraphic study on a 5452 8 m thick sedimentary sequence of the hoh xil basin in northern qinghai tibet plateau to obtain a chronostratigraphic framework for these sediments a total of 966 individual oriented palaeomagnetic samples spaced at stratigraphic intervals were collected from six measured sections in the hoh xil basin magnetic directions in these samples were obtained by progressive thermal mainly and alternating field demagnetization experiments most samples exhibit two components of magnetization the lower unblocking temperature component is an overprint resembling the present day geocentric axial dipole field direction at the sampling locality the most stable characteristic remanence chrm appears to be an early chemical remanent magnetization residing mainly in haematite the positive results of fold and reversal tests indicate that the chrm is a record of the palaeomagnetic field close to the time of formation of these sediments further evidence for the magnetization of these sediments acquired close to their time of deposition is the fact that patterns of magnetic reversals can be matched with the established polarity timescale on the basis of the distinct interval of magnetic reversal zones and biostratigraphic datums 13 magnetozones can be recognized at the hoh xil basin that range from chrons c11 to c23 30 1 51 0 ma the age of the fenghuoshan group is palaeomagnetically dated as 51 31 ma early eocene middle early oligocene and the age of the yaxicuo group is between 31 and 30 ma middle early oligocene late early oligocene the new palaeomagnetic data from the fenghuoshan group suggest that it has undergone no significant rotation since the oligocene in contrast declination data from the yaxicuo group in wudaoliang area imply a vertical axis clockwise rotation 29 1 8 5 since the late oligocene the tertiary palaeomagnetic pole position of the hoh xil basin implies a significant northward convergence of the hoh xil basin 1600 km with respect to eurasia siberia since early eocene late oligocene time our results are consistent with the pattern of disturbingly low palaeolatitudes derived from a large number of high quality palaeomagnetic studies of tertiary rocks from sites that reach all the way from eastern china to kyrgyzstan future work is needed to separate the influences of sedimentary inclination shallowing and tectonic shortening

miocene cooling in the northern qilian shan northeastern margin of the tibetan plateau revealed by apatite fission track and vitrinite reflectance analysis apatite fission track and vitrinite reflectance data from phanerozoic rocks of the northern qilian shan and jiuxi basin at the northeastern margin of the tibetan plateau show a complex thermal history with multiple paleothermal events peak paleotemperatures reached in the middle cretaceous suggest that hydrocarbon generation occurred in the basin at that time paleozoic mesozoic samples show evidence of a middle tertiary cooling episode between 20 and 10 ma this thermal fingerprint is significant because stratigraphic evidence for miocene exhumation is overwhelmed by spectacular pliocene pleistocene deformation and erosion we interpret the miocene cooling as recording initial exhumation of the northern qilian shan earlier than other models have proposed i e after 6 5 ma miocene cooling is broadly coincident with cenozoic uplift and exhumation recorded elsewhere in the tibetan tarim region

tertiary crustal shortening and peneplanation in the hoh xil region implications for the tectonic history of the northern tibetan plateau abstract the hoh xil basin is the largest cenozoic sedimentary basin in the hinterland of the tibetan plateau tertiary sedimentary strata 5 8 km thick comprising the fenghuoshan yaxicuo and wudaoliang groups provide compelling evidence concerning the crustal shortening erosion and peneplanation of the northern tibetan plateau the basal fenghuoshan and overlying yaxicuo groups span the eocene early oligocene stratigraphically and have been dated by magnetostratigraphy as 56 30 ma old both groups are composed of terrigenous rocks provenance analysis of sandstones and conglomerates demonstrates that permian and triassic strata in the tanggula orogenic zone in the south were the source for the fenghuoshan group in contrast the carboniferous triassic strata in the tanggula bairizhajia and heishishan gaoshan orogenic zones in the north were the source for the yaxicuo group during the late oligocene northern tibet underwent strong north south crustal shortening 43 and thickening extensive erosion which occurred over the entire plateau surface near the end of the oligocene resulted in development of a peneplain surface the latter is overlain by the early miocene wudaoliang group composed of fresh water limestones these are exposed both on summit surfaces as well as on the valley floors showing that a phase of differential uplift occurred after the deposition of the wudaoliang group this post miocene differential uplift was due to regional extension in a region of overall shortening even though we have not succeeded in obtaining conclusive data about the exact timing of phases of rapid uplift of the tibetan plateau it is most likely that the major phase of uplift occurred during the late oligocene

mesozoic cenozoic evolution of the xining minhe and dangchang basins northeastern tibetan plateau magnetostratigraphic and biostratigraphic results 1 accurate stratigraphic ages are crucial to understanding the deformation history of the tibetan plateau prior to and during the indo asian collision efforts to quantify mesozoiccenozoic ages are hindered by limited fossils and a paucity of volcanic horizons and regionally correlative strata magnetostratigraphic and biostratigraphic results from the xining minhe longzhong basin complex and dangchang basin provide an improved chronology of nonmarine basin development over a large region of the northeastern tibetan plateau 34 37 n 101 105 e analyses of 171 magnetostratigraphic levels and 24 palynological assemblages 120 species indicate late jurassic early cretaceous to mid tertiary deposition although magnetic polarity zonation is incomplete independent palynological age control partially restricts possible correlations to the geomagnetic polarity timescale the sediment accumulation record basin provenance structural geology and published thermochronological data support a history of jurassic exhumation late jurassic early cretaceous fault related basin initiation and cretaceous paleogene reduced accumulation these patterns which are compatible with late jurassic early cretaceous extension and cretaceous paleogene postrift thermal subsidence were disrupted at about 40 30 ma when shortening related to the indo asian collision induced localized range uplift vertical axis rotation and amplified subsidence indexterms 1520 geomagnetism and paleomagnetism magnetostratigraphy 8102 tectonophysics continental contractional orogenic belts 9320 information related to geographic region asia 9604 information related to geologic time cenozoic 9609 information related to geologic time mesozoic keywords tectonics magnetostratigraphy tibetan plateau cenozoic mesozoic sedimentary basins

tibetan uplift prior to the eocene oligocene climate transition evidence from pollen analysis of the xining basin uplift of the tibetan plateau and the himalayas since the onset of the indo asia collision is held responsible for asian aridifi cation and monsoon intensifi cation but may also have gradually cooled global climate leading to the 34 ma eocene oligocene transition to unravel the interplay between tibetan uplift and global climate proxy records of asian paleoenvironments constrained by accurate age models are needed for the paleogene period here we report the 38 ma appearance of high altitude vegetation recovered from palynological assemblages in precisely dated lacustrine sediments from the xining basin of the northeastern tibetan plateau region this result confi rms previous evidence for important regional uplift in the central and northern tibetan plateau regions during the early stage of the indo asia collision this is consistent with the idea that the associated increase in rock weathering and erosion contributed to lowering of atmospheric co 2 leading to the eocene oligocene transition

cenozoic pulsed deformation history of northeastern tibetan plateau reconstructed from fission track thermochronology abstract the synorogenic basin deposits and bedrocks of their source terranes within and along the tibetan plateau contain fundamental information regarding the spatiotemporal evolution of the largest orogenic plateau on earth the guide xining region is located on the northeastern portion of the tibet and its eocene early pleistocene basin succession is well preserved by integrating apatite fission track thermochronology from sedimentary and basement samples with heavy minerals and paleocurrent data we decipher an almost complete sequence of exhumation and depositional events during the cenozoic our data indicates that the initial deformation along the guide xining region occurred since the eocene with the reorganization of the regional tectonomorphology and the formation of a broad basin thereafter this single large basin was disrupted by multiple episodes of exhumation and deformation our study illuminate that the multiple stage active processes occurred at 49 42 36 32 23 19 16 13 and 8 4 ma work together to produce the current ne tibetan plateau

magnetostratigraphy of cenozoic sediments from the xining basin tectonic implications for the northeastern tibetan plateau 1 the xining subbasin of the longzhong basin holds the longest continuous cenozoic stratigraphic record at the margin of the northeastern tibetan plateau despite a rich biostratigraphic content including the xiejia mammal fauna the tectonic evolution of the basin is largely unconstrained in this study we present stratigraphic biostratigraphic and magnetostratigraphic results that provide a basis for reconstructing the cenozoic tectonic evolution of the xining basin with respect to adjacent regions of the northeastern tibetan plateau magnetostratigraphic analysis from three red bed sections in the xining basin indicates continuous deposition at low and nearly constant accumulation rates average 2 2 cm kyr from 52 0 to 17 0 ma we interpret this result to indicate that no major regional tectonic event implying large sediment accumulation variations has affected the xining basin deposition during this considerable time window in detail accumulation rate variations outline a three stage evolution with 1 8 cm kyr from 52 0 ma to 34 5 ma 4 1 cm kyr from 34 5 to 31 0 ma and 2 3 cm kyr from 31 0 to 17 0 ma the second order increase between 34 5 and 31 0 ma can be interpreted to result from either a distal tectonic event or to be of climatic origin although the region was tectonically quiescent for much of the cenozoic tectonic activity occurred during basin initiation or reactivation at circa 55 0 52 5 ma and during intense basin deformation after 17 0 ma

uplift driven climate change at 12 ma a long δ18o record from the ne margin of the tibetan plateau carbonates from fluvial and lacustrine sediments were sampled from multiple measured sections in the linxia basin of western china based on textural and mineralogical evidence lacustrine carbonates are primary precipitates from lake water a 29 million year record of the oxygen isotope composition of meteoric water is inferred from the n 18 o values of these carbonates this inference is based on the most negative n 18 o values in the lake carbonates which represent lake waters that have experienced the least evaporative enrichment carbonate n 18 o values a proxy for rainfall n 18 o are v310 5x throughout the interval of 29 12 ma at 12 ma there is a shift to 39x a value that remains into the pliocene this implies a major reorganization of atmospheric circulation patterns and a shift to more arid conditions at the ne margin of the tibetan plateau with the post 12 ma system similar to that of today the 12 ma event may represent the time at which the tibetan plateau achieves sufficient elevation to block the penetration of moisture from the indian ocean or south pacific into western china the period of greatest aridity is from 9 6 to 8 2 ma a time interval which agrees well with other climate records 2003 elsevier b v all rights reserved

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

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

a pedogenic model for the magnetic enhancement of late miocene fluvial lacustrine sediments from the xining basin ne tibetan plateau

dynamics at an elongated intermediate depth seamount in the north atlantic sedlo seamount 40 20 n 26 40 w abstract observations from a five mooring array deployed in the vicinity of sedlo seamount over a 4 month period together with supporting hydrographic and underway adcp measurements are described sedlo seamount is an elongated intermediate depth seamount with three separate peaks rising from 2200 m water depth to summit peaks between 950 and 780 m depth located at 40 20 n 26 40w currents measured in depth range 750 and 820 m the layer close to the summit depth of the shallowest southeast peak showed a mean anti cyclonic flow around the seamount with residual current velocities of 2 5 cm s 1 significant mesoscale variability was present at this level and this is attributed to the weak and variable background impinging flow stronger more persistent currents were found at the summit mooring as a result of tidal rectification and some weak amplification below 1300 m currents were extremely weak even close to the seabed time series of relative vorticity for the depth layer 750 820 m showed persistent anti cyclonic vorticity except for two periods of cyclonic vorticity a mean relative vorticity of 0 06f f the local coriolis frequency was calculated from a triangle of current meters located at the flanks of the seamount modelling results confirmed that anti cyclonic flow above the seamount was likely due to taylor cone generation driven by a combination of steady impinging and tidally rectified flow the closed circulation pattern over the seamount was found to extend to 150 m above the summit level consistent with simple idealised theory and the supporting hydrographic observations at shallower depths

displacement and timing along the northern strand of the altyn tagh fault zone northern tibet abstract the northern strand of the altyn tagh fault has a late cenozoic left slip offset of 69 90 km near its northeastern end deformation began in this area as early as the oligocene but left slip displacement may be mainly of middle miocene to recent age because the altyn tagh fault zone transfers strike slip displacement into shortening in the qilian shan the total offset changes along the fault but the magnitude of displacement along this part of the fault zone suggests that only small scale eastward movement of crustal rocks occurred in this part of the tibetan plateau during the late cenozoic

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

timing and magnitude of displacement on the altyn tagh fault constraints from stratigraphic correlation of adjoining tarim and qaidam basins nw china the altyn tagh fault plays an important role in growth of the north western tibetan plateau but the timing of its inception and magnitude of left lateral displacement are not well constrained this study provides new independent constraints by correlating stratigraphic development and spatial distribution of the neighbouring tarim and qaidam basins it is shown that the tarim and qaidam basins experienced quite similar stratigraphic and sedimentary evolution from late sinian to early palaeogene times this fact indicates that the tarim and qaidam basins were a single block at least until oligocene time which was then offset by the altyn tagh fault early palaeozoic and late triassic suture zones within the east and west kunlun and pinch outs of mesozoic strata are chosen as piercing lines restoration of these lines gives total offset estimates of 350 400 km across the south western and central segments of the altyn tagh fault

late cenozoic deformation and uplift of the ne tibetan plateau evidence from high resolution magnetostratigraphy of the guide basin qinghai province china the cenozoic intramontane gonghe guide basin in qinghai province china is tectonically controlled by the sinistral strike slip framework of the kunlun and altyn tagh south qilian faults in the northeastern tibetan plateau the basin is filled with thick cenozoic clastic sedimentary formations which provide important evidence of the deformation of this part of the plateau although they have long lacked good age constraints detailed magnetostratigraphic and paleontologic investigations of five sections in the guide basin and their lithologic and sedimentary characteristics allow us to divide a formerly undifferentiated unit the guide group into six formations where ages are now magnetostratigraphically well established they are given in parentheses the amigang 1 8 2 6 ma ganjia 2 6 3 6 ma and herjia formations 3 6 to ca 7 0 7 8 ma and the older miocene ashigong garang and guidemen formations these rocks document a generally upward coarsening sequence characterized by increasing accumulation rates increasing gravel content and sizes of its components changes of bedding dips and source rock types and marginal growth faults collectively reflect accelerated deformation and uplift of the ne tibetan plateau after 8 ma punctuated by a sharp increase in sedimentation rate at ca 3 2 ma that reflects the boulder conglomerates of the ganjia formation interestingly much of the vergence of the compressional deformation in the basin is to the south accommodated by a sequence of six thrusts f1 f6 which become active one by one progressively later toward the south undoubtedly contributing to the uplift of this part of the plateau f1 likely initiated the guide basin due to crustal flexure in the oligocene f2 was active in the early miocene f4 and f5 at ca 3 6 ma and f6 was active in the early pleistocene the detailed late miocene and younger magnetostratigraphy allows us to place much improved time constraints on the deformation and hence uplift of northeastern tibet which when compared with ages for events on other parts of the plateau provides important boundary conditions for the geodynamical evolution of tibet

grain size dependence of the rock magnetic properties for a natural maghemite the natural maghemite from the robe river mining district australia is shown to be thermally very stable the maghemite hematite inversion occurs at 650 c for sized fractions 250 m to 5 m enabling determination of t c for maghemite at 610 c the maghemite is intimately intergrown with hematite and j s of the maghemite corrected for the hematite content is 64 am 2 kg j rs h cr and h c range between 7 46 10 18 am 2 kg 16 90 23 30 mt and 6 38 9 00 mt respectively indicating psd maghemite with grain sizes between 1 and 20 m the grain size dependence of the rock magnetic parameters of the maghemite is less prominent than those of magnetite of nominally the same size it is suggested that the lt behavior of arm can be diagnostic of the presence of maghemite upon cooling to 196 c it shows no change upon rewarming a gradual decrease in intensity occurs starting at 120 c

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

oligocene miocene 28 13 ma climato tectonic evolution of the northeastern qinghai tibetan plateau evidenced by mineralogical and geochemical records of the xunhua basin abstract changes in sediment composition in the xunhua basin provide insights into the climatic and tectonic evolution of the northeastern qinghai tibetan plateau during the oligocene and miocene here we analyzed the clay mineralogy i e the proportions of smectite illite and chlorite and clay indicators bulk mineral composition i e the amounts of calcite quartz gypsum k feldspar plagioclase dolomite and halite and trace element indicators i e rb sr and ba sr ratios of sediments deposited from 28 to 13 ma in the xunhua basin to reveal the climate and or provenance changes the results suggest that significant changes in sediment provenance occurred at 25 1 ma and 21 6 ma caused respectively by initial uplift and accelerated uplift of the laji shan the xunhua basin experienced warm and humid conditions at 28 0 25 1 ma cool and dry conditions at 25 1 21 6 ma and somewhat warmer and wetter conditions at 21 6 19 2 ma followed by aridification in two steps at 19 2 ma and 13 9 ma the climate cooling event at 25 1 ma corresponds temporally to a major phase of uplift of central tibet relatively warmer and wetter conditions during the early miocene 21 6 19 2 ma are considered to record the influence of the asian summer monsoon and accelerated uplift of the laji shan the shift toward more arid conditions on the northeastern qinghai tibetan plateau at 19 2 13 9 ma was probably driven by uplift of the tibetan plateau and further aridification since 13 9 ma may reflect a weakening of the east asian summer monsoon and global climatic cooling

synchronous deformation on orogenic plateau margins insights from the arabia eurasia collision abstract the temporal and spatial distribution of deformation during the early phases of plateau formation is poorly known the iranian plateau is developing in response to arabia eurasia convergence and is a potential analog to the early phases of tibetan plateau deformation previous studies document the onset of the iranian plateau growth based on the kinematic history of the zagros to the south and the alborz mountains to the north here we present new detrital apatite fission track aft data from 13 modern rivers of the talesh mountains northwestern iranian plateau these mountains lie immediately south west of the rigid south caspian block a total of 1233 single grain cooling ages were measured and range between 185 and 4 ma deconvolution of grain age data documents three main phases of exhumation including the oligocene 30 23 ma early to middle miocene 18 12 ma and early pliocene 5 ma comparison between cooling ages from the talesh mountains and elsewhere across the iranian plateau indicates widespread plateau formation in the oligocene earlier than previously suggested for the northwestern plateau margin these results highlight deformation on both the northern and southern margins of the plateau over almost the entire history of iranian plateau formation a similar pattern of synchronous deformation on the north northeastern and southern margins of the tibetan plateau has recently been documented and highlights the significance of rigid basement blocks focusing far field deformation for both plateaus

paleocene eocene syncontractional sedimentation in narrow lacustrine dominated basins of east central tibet sedimentologic stratigraphic compositional and structural data from four elongate basins 30 km long in the nangqian yushu region of east central tibet near the headwaters of the mekong and yangtze rivers indicate nonmarine sedimentation synchronous with paleocene eocene northeast southwest shortening sedimentation in the nangqian niuguoda xialaxiu and shanglaxiu basins involved 1 mud and carbonate accumulation in offshore to nearshore lacustrine environments and 2 gravel and sand deposition in fan delta to alluvial fan environments localized near basin margins growth strata in both fine and coarse grained deposits primarily in upper levels of basin fill provide evidence for sedimentation contemporaneous with motion on fold thrust structures provenance data collected from 25 measured stratigraphic sections including 1500 paleocurrent measurements and 150 conglomerate compositional analyses show that each basin was fed sediment from several directions by proximal source areas composed of carboniferous triassic rocks the source area proximity and a stratigraphic variability among the basins suggest that each basin evolved independently and was filled by sediment derived from relatively small drainage networks 103 km2 age control for basin fill is based on paleogene fossils 38 37 ma 40ar 39ar ages from volcanic rocks interbedded with uppermost strata of the nangqian basin and 51 49 ma 40ar 39ar ages from igneous rocks that intrude and unconformably overlie strata of the shanglaxiu basin strata containing middle cretaceous palynomorph and ostracod assemblages are present only locally in the lowermost part of the nangqian basin although the tectonic setting for cretaceous sedimentation is unclear early tertiary basin development was controlled by thin skinned fold thrust deformation we interpret the narrow widths of paleocene eocene basins to be a result of thrust spacing which in turn may have been controlled by the depth to the decollement 5 km deep according to our balanced cross section from which imbricate thrusts ramped up through the carboniferous triassic section sedimentologic and provenance evidence for internal drainage limited unroofing and relatively low average sediment accumulation rates in these syncontractional plateau interior basins indicates generally small drainage systems short main stem rivers shallow regional slopes and limited denudation in east central tibet during early tertiary time such geomorphic conditions which are similar to the modern low relief interior of the tibetan plateau suggest that the deeply incised mekong and yangtze rivers of eastern tibet were not established until after the termination of paleogene basin development in the region

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

tectonic uplift in the northern tibetan plateau since 13 7 ma ago inferred from molasse deposits along the altyn tagh fault abstract in response to the collision of india with asia thick molasse deposits were shed off the high evolving edge of the northern tibetan plateau providing great potential for understanding the relationships between mountain building rock denudation and sediment deposition a precise knowledge of the initial accumulation of the molasse deposits is important for understanding uplift history along the northern margin of tibet field investigations indicate that thick neogene sediments in the piedmont depression zones of the altun mountains are strongly deformed in a fold and thrust system indicating crustal shortening and thickening in the late cenozoic detailed magnetostratigraphic studies of the molasse deposits along the altyn tagh fault show that the accumulation of coarse conglomerate began ca 13 7 ma ago the source material of the deposits changing at the same time together with the remarkable increase in the sedimentation rate after 13 7 ma we attribute the accumulation of the conglomerates to the tectonic uplift along the northern edge of the tibetan plateau a process that continued until at least 9 ma ago

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

high resolution topography along surface rupture of the 16 october 1999 hector mine california earthquake mw 7 1 from airborne laser swath mapping in order to document surface rupture associated with the hector mine earthquake in particular the area of maximum slip and the deformed surface of lavic lake playa we acquired high resolution data using relatively new topographic mapping methods we performed a raster laser scan of the main surface breaks along the entire rupture zone as well as along an unruptured portion of the bullion fault the image of the ground surface produced by this method is highly detailed comparable to that obtained when geologists make particularly detailed site maps for geomorphic or paleoseismic studies in this case however for the first time after a surface rupturing earthquake the detailed mapping is along the entire fault zone rather than being confined to selected sites these data are geodetically referenced using the global positioning system thus enabling more accurate mapping of the rupture traces in addition digital photographs taken along the same flight lines can be overlaid onto the precise topographic data improving terrain visualization we demonstrate the potential of these techniques for measuring fault slip vectors

northeastward growth and uplift of the tibetan plateau magnetostratigraphic insights from the guide basin 1 most of the evidence for the uplift of the tibetan plateau comes from its southern and central parts although the northern rim has been less studied it may greatly contribute to the understanding of the mechanism and timing of the uplift recent studies on the northeastern part of the plateau suggest that the uplift can largely be explained by cenozoic thrusting and folding linked to the movement of the altyn tagh and kunlun faults and that diffuse shortening might be a realistic way to explain the thickening of the continental interior nevertheless the age of sediments that record the tectonic processes is poorly known the terrigenous formations in the guide gonghe basin ne tibet show several phases of deformation and include thick deposits of conglomerates interbedded with sandstones and siltstones that are thought to be the result of an increased sediment flux dating of these gravel deposits with magnetostratigraphic and faunal studies has permitted the precise timing of the basin infilling and its tectonic evolution to be established our results indicate that the onset of conglomerate accumulation occurs in the pliocene an observation that is in agreement with those of the linxia basin just east of the plateau if the high sedimentation rates 22 cm kyr in the guide basin are mostly due to tectonism then significant uplift occurred as late as pliocene

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

a jackknife for magnetostratigraphy we investigate the problem of evaluating the reliability of a magnetostratigraphic section using a jackknife resampling scheme we define a parameter j as the slope relating the percentage of polarity zones of a sampled section remaining after random deletion of sites increasing in proportion to a total of 20 of the initial collection so defined j quantifies the dependence of a magnetostratigraphic record on the distribution of sampling sites based on simulated magnetostratigraphic sections j was found to be an indicator of the approximate percentage of polarity zones present in the original reference sections that were recovered in the sampled section j will perhaps be most helpful as a guide to investigators as to whether further sampling would provide useful information as well as to the non paleomagnetist user of black and white interpretive polarity logs as an estimate of the reliability of a given section

development and growth of basement involved structural wedges in the northwestern qaidam basin china structural wedges contain two connected fault segments a fore thrust and back thrust that bound a triangular or wedge shaped fault block coeval displacements on both faults drive the wedge into surrounding rock causing distinctive patterns of uplift and folding we identified a series of thick skinned structural wedges in northwestern qaidam basin china where syntectonic strata record deformation timing and kinematics the qaidam basin is located at the northern margin of the tibetan plateau and contains more than 100 large anticlinal structures many of which exhibit characteristics of structural wedges we interpreted and modeled these structures using seismic reflection data well logs surface geological exposures remote sensing images and digital elevation data these wedge structures are distinguished by having elevated and deformed strata in the footwalls of the back thrusts and synclinal folds that extend upward from the wedge tips with the constraints of fold shape growth strata structural relief and shallow fault geometry we developed a series of forward models that describe the geometry and kinematic evolution of these wedge structures the analysis suggests that some of the wedge structures reactivate preexisting normal faults our results provide a better understanding of how to identify thick skinned wedge systems and model their geometry and kinematic evolution which has implications for studying the tectonic history of the basin understanding the geometry and kinematics of the wedge structures is also crucial for petroleum exploration because both hanging wall and footwall traps constitute an important component of current exploration targets

concentration of crustal displacement along a weak altyn tagh fault evidence from paleomagnetism of the northern tibetan plateau 1 like many regional structures in the northern tibetan plateau the tula syncline changes strike by 40 from nw se to nearly ew as it approaches the altyn tagh fault from the south to test whether this strike curvature is due to oroclinal bending we analyzed paleomagnetic samples from 109 sites collected from late jurassic to paleogene red sedimentary strata of the tula syncline fold and reversal tests suggest a primary origin for the characteristic remanent magnetization from nine sites in the eastern half and 41 sites in the western half of the syncline the observed 13 3 8 8 declination difference between the two halves of the tula syncline is far less than the 40 difference predicted for oroclinal bending instead the arc shape of the syncline is an original configuration produced by transport above an arcuate thrust ramp along with paleomagnetic data from the northern qaidam basin these results from the tula syncline indicate that crustal displacement between the tarim basin and the northern tibetan plateau is accommodated by strike slip motion on the altyn tagh fault rather than distributed shear within the northern tibetan plateau

oligo miocene magnetostratigraphy and rock magnetism of the xishuigou section subei gansu province western china and implications for shallow inclinations in central asia magnetostratigraphy of 222 remanent directions together with late oligocene to early miocene mammal and charophyte paleontology suggest that 2179 m of the xishuigou section subei gansu province china were deposited from 26 to 19 ma stratigraphic patterns of bulk susceptibility anisotropy of magnetic susceptibility parameters and natural and anhysteretic remanent magnetization intensities demonstrate that 1 faulting does not significantly affect the record 2 sediment deposition was relatively continuous 3 sediment source changed around 23 ma and 4 rapid uplift near subei occurred at 21 ma subei rotated 27 5 counterclockwise with respect to the 20 ma pole from the eurasian synthetic apparent polar wander path folding and rotation of the section took place after 19 ma the paleolatitude of subei is 14 less than at present and 19 3 less than predicted from the reference pole both rock magnetic and paleomagnetic data sets suggest that the unusually low paleolatitude is the result of synsedimentary inclination shallowing a phenomenon which has likely affected other paleomagnetic data from central asia

slowing extrusion tectonics lowered estimate of post early miocene slip rate for the altyn tagh fault abstract determination of long term slip rate for the altyn tagh fault is essential for testing whether asian tectonics is dominated by lateral extrusion or distributed crustal shortening previous slip history studies focused on either quaternary slip rate measurements or pre early miocene total offset estimates and do not allow a clear distinction between rates based on the two the magmatic and metamorphic history revealed by shrimp zircon dating of clasts from miocene conglomerate in the xorkol basin north of the altyn tagh fault strikingly matches that of basement in the southern qilian shan and northern qaidam regions south of the fault this match requires that the post early miocene long term slip rate along the altyn tagh fault cannot exceed 10 mm year supporting the hypothesis of distributed crustal thickening for post early miocene times this low long term slip rate and recently documented large pre early miocene cumulative offset across the fault support a two stage evolution wherein asian tectonics was dominated by lateral extrusion before the end of early miocene and since then has been dominated by distributed crustal thickening and rapid plateau uplift

pulsed miocene range growth in northeastern tibet insights from xunhua basin magnetostratigraphy and provenance author s lease ro burbank dw hough b wang z yuan d abstract sedimentary rocks in tibetan plateau basins archive the spatiotemporal patterns of deformation erosion and associated climate change that resulted from cenozoic continental collision despite growing understanding of basin development in northeastern tibet during initial india asia collision as well as in the late miocene holocene surprisingly little is known about the intervening period a time when the plateau may have undergone fundamental tectonic changes to fill this gap we present new magnetostratigraphy from a g2300 m thick fluviolacustrine succession that spans ca 30 9 3 ma an integrated analysis of sedimentology subsidence and provenance from this section reveals the sequential pulsed erosion of multiple ranges bordering the xunhua basin emergence of the wnw trending laji shan is highlighted by a doubling of sediment accumulation rates between 24 and 21 ma and a transition to coarse allu vial facies at 20 3 ma detrital zircon u pb age spectra show that these coarse sediments came from basement terranes within the laji shan together these observations suggest accelerated growth of the laji shan and its coupled foreland basin at ca 22 ma the most rapid accumulation rates in xunhua basin occur within the finest grained strata and suggest an underfilled basin during the fastest interval of laji shan deformation growth of the laji shan occurred northward of the contemporaneous plateau margin which had been defined since ca 45 50 ma by the west qinling lying 60 km farther south hence following 20 25 m y of apparent stability the deformation front in this region jumped 60 km to the north at ca 22 ma subsequently growth of the north trending jishi shan occurred at ca 13 ma and is highlighted by an acceleration in xunhua basin accumulation rates between 12 and 9 ma as well as by a significant change in detrital zircon provenance of nearby linxia basin deposits by 11 5 ma initial growth of the wnw trending laji shan in the early miocene and subsequent growth of the north trending jishi shan 10 m y later support interpretations of a middle mio cene kinematic reorganization in northeastern tibet 2012 geological society of america

paleogene crustal extension in the eastern segment of the ne tibetan plateau abstract the tibetan plateau experienced lateral growth in response to the india eurasia collision but uncertainties remain about when and how its present day margins were formed it was commonly viewed that crustal shortening in the ne tibetan plateau had commenced since the eocene and led to reverse faulting and compressional basins our recent investigations of some cenozoic intermontane basins reveal that synsedimentary normal faults occur widely in the eastern segment of the ne tibetan plateau normal faulting at basin bases obviously controlled basin sedimentation and presumably resulted in footwall uplift and basement exhumation small scale synsedimentary normal faults are very common in basin fill sequences although they decrease in intensity upwards and eventually vanish in the uppermost paleogene sequences measurement of the attitudes of synsedimentary normal faults indicates that crustal stretching was in nw se direction the nw se extension was consistent with paleogene stress field of the whole north china craton which were thought responsible for the ne sw trending rift basins in the interior of the north china craton such as the yinchuan weihe and bohai bay rift basin systems a broad unconformity separates paleogene and neogene sequences in the eastern segment of the ne tibetan plateau contrasting strikingly with conformable oligocene to middle miocene sequences in the neighboring ningnan and yinchuan rift basins it thus follows that the eastern segment of the ne tibetan plateau had not been formed in paleogene when it was basically dictated by crustal extension possibly as a result of far field effect of subduction of the western pacific plate end oligocene termination of normal faulting and the presence of a regional unconformity indicate that the eastern segment of the ne tibetan plateau began rising at the beginning of the miocene

danghe area western gansu china biostratigraphy and implications for depositional history and tectonics of northern tibetan plateau the danghe area in western gansu province is at the focal point of interaction of the northeastern end of the left lateral altyn tagh fault and growing ramp thrusts of the danghe nanshan mountains along the northern rim of the qinghai tibetan plateau with a thick sequence of tertiary sediments and associated fossil records the danghe area is one of few places on the tibetan plateau that can offer integrated studies of its tectonic history depositional environment and biological records including vertebrate and plant fossils past studies however have not been able to capitalize on the paleontological data or were often misguided by an outdated notion of regional chronology incorrect age estimates in these studies often have profound effects on tectonic interpretations we present new stratigraphic and paleontologic evidence from the danghe area and demonstrate a new chronologic scheme that indicates a much younger age for the majority of the sediments we recognize three packages of sediments in the danghe area 1 oligocene paoniuquan new name formation basal predominantly fine grained dark purple mudstones and siltstones that include the classic yindirte fauna which forms the basis of the tabenbulukian mammal age 2 early miocene to early late miocene tiejianggou new name formation that contains a new platybelodont proboscidean in a coarsening upward sequence terminating in a massive conglomerate and 3 an unnamed late neogene formation that contains another coarsening upward sequence in light of our new stratigraphic framework we reinterpret a previously published magnetic column for the middle sequence in xishuigou as representing chrons c6n through c4ar corresponding to a span of about 20 9 3 ma much younger than has been realized so far this new chronological framework suggests a depositional history of the danghe nanshan that spans at least the early oligocene through late miocene to possibly pliocene early depositions during the oligocene through middle miocene are dominated by fine grained sediments indicating a distant source of sedimentation paleontological data suggest a relatively dry environment as is typical of northern china today by late miocene around 9 12 ma the danghe area began to receive coarse sediments and was much closer to the mountain front the new stratigraphic framework indicates an earlier onset of sedimentation than has generally been assumed but also suggests the presence of sediments much younger than many have realized c 2003 elsevier science b v all rights reserved