a 7 3 1 6 ma grain size record of interaction between anticline uplift and climate change in the western qaidam basin ne tibetan plateau abstract how sediment grain size corresponds to both climate change and tectonics is increasingly the focus of debate the shrinkage and migration of the huge paleo lake in the western qaidam basin wqb ne tibetan plateau tp provide an excellent case study we present a nearly 6 ma well dated high resolution grain size record from the 723 m deep drill core sg 1b located in the jianshan anticline in the wqb to show how sediment grain size responds to anticline growth and climate change the results show that variations in grain size represent three distinct phases phase i 7 3 3 6 ma is characterized by fine sediments with good sorting with a predominance of clay and fine silt during this phase the drilling site was in a deep to semi deep lake environment the jianshan anticline has undergone relatively weak tectonics and the climate was dry but still much wetter than that in the quaternary grain size variation and lacustrine deposition were principally controlled by climate phase ii 3 6 3 3 ma was a transitional period characterized by a rapid shift toward consistently coarser sediments with increased volumes of medium coarse silt and sand during this phase the drilling site underwent a dramatic shift to a shallow lake environment the anticline experienced rapid uplift and the climate was in an episode of rapid drying grain size variation and lacustrine deposition were principally controlled by tectonics in the jianshan anticline phase iii 3 3 1 6 ma was a period exhibiting a long term fining trend in mean grain size relative to phase ii but still much coarser than that of phase i and with a distinct and prolonged increase in very fine sediments accompanied by poor sorting during this phase the drilling site was in a shallow lake environment between 3 3 ma and 1 9 ma and finally became a lakeshore like environment between 1 9 ma and 1 6 ma the anticline experienced rapid and continuous uplift and the climate was in a long term stepwise drying and cooling trend a conceptual model was drawn to show how grain size and lacustrine deposition responded to uplift of the jianshan anticline and climate change in the qaidam basin

paleogene clockwise tectonic rotation of the xining lanzhou region northeastern tibetan plateau 1 to help understand the deformational history of the northeastern tibetan plateau paleomagnetic samples were collected from 177 sites and two magnetostratigraphic sections at 16 localities distributed among upper jurassic lower cretaceous to pliocene sedimentary and subordinate volcanic rocks within the xining lanzhou region 34 37 n 101 105 e a total of 127 sites at 12 localities yielded primary magnetizations confirmed by fold reversal and conglomerate tests age control on sedimentary rocks is provided by regional synthesis of chronostratigraphic data and our own biostratigraphic and magnetostratigraphic analysis presented in the companion paper by horton et al 2004 analysis of paleomagnetic declination combined with results from previous studies yield a remarkably consistent trend of vertical axis tectonic rotations across the studied region whereas 19 0 7 2 to 37 8 10 6 clockwise rotations are recorded consistently in all paleomagnetic localities in lower cretaceous to eocene rocks all paleomagnetic localities in oligocene to pliocene rocks have recorded minor to insignificant rotations indicating that the xining lanzhou region has undergone a wholesale regional clockwise rotation during late paleogene time consistent with regional chronostratigraphic and thermochronologic results this late paleogene tectonic rotation confirms that deformation reached regions of the northern tibetan plateau shortly after the initial collision of india with asia when compared to rotational paleomagnetic results from adjacent regions several mechanisms can be proposed to explain the clockwise rotation on the basis of consistency with geologic data we prefer a model involving clockwise rotation of the xining lanzhou region through right lateral shear and associated shortening related to northward indentation of the qaidam basin index terms 1525 geomagnetism and paleomagnetism paleomagnetism applied to tectonics regional global 8102 tectonophysics continental contractional orogenic belts 9320 information related to geographic region asia 9604 information related to geologic time cenozoic keywords tectonics paleomagnetism tibetan plateau

significant late neogene east west extension in northern tibet field mapping in northern tibet reveals that the normal slip along late cenozoic northsouth trending faults is comparable to that estimated for equivalent structures in southern tibet the orientation of fault striations in two north south trending rifts suggests an eastnortheast west northwest direction of extension in northern tibet which in turn implies that northeast striking active faults in northern tibet have significant left slip components initiation of rifting in northern tibet postdates the early oligocene and possibly occurred after 4 ma the broad similarities in the magnitude of slip and the direction of extension for normal faults in both northern and southern tibet imply that the entire plateau has been extending this precludes significant eastward extrusion of north tibet relative to south tibet and requires a regional boundary condition as the cause of east west extension for the entire tibet plateau

revisiting clay sized mineral and elemental records of the silicate weathering history in the northern tibetan plateau during the late cenozoic the role of aeolian dust

a first palaeomagnetic study of jurassic formations from the qaidam basin northeastern tibet china tectonic implications summary we present a new palaeomagnetic study on upper jurassic red beds collected at nine sites near huatugou 38 46 n 90 75 e in the qaidam basin thermal demagnetization up to 690 c shows both low and high temperature components ltc and htc respectively ltc seems to be a recent overprint htc carried principally by haematite and presenting a sole reverse polarity passes the fold test at the 95 per cent confidence level the palaeomagnetic pole calculated from the tilt corrected overall mean direction ds 220 4 i s 34 8 95 7 5 is situated at 50 1 n 198 0 e dp dm 5 0 8 6 the comparison of this result with coeval palaeomagnetic poles from tarim the north china block ncb and the apparent polar wander path of eurasia reveals a large scale tectonic evolution of the asian continent since the late jurassic this study suggests that the northward convergence of the qaidam block is statistically negligible with respect to tarim 4 5 8 2 since late jurassic times however a significant relative clockwise rotation of the studied area with respect to tarim seems to have occurred 16 2 11 2 whether this rotation is representative of the whole qaidam basin or is of a local character is yet to be confirmed the important feature of this study resides in the difference of northward displacement of the qaidam block with respect to eurasia on one hand and with respect to the ncb on the other hand the overall ns convergence absorbed between qaidam and eurasia is 19 3 9 2 whereas that absorbed between the qaidam and the ncb is 8 0 6 2 we suggest that the latter value is the result of the india asia collision while its difference with respect to the former would be absorbed during the mongol okhotsk ocean closure at the end of the jurassic or at the jurassic cretaceous boundary at the latest

post 4 ma initiation of normal faulting in southern tibet constraints from the kung co half graben the timing of e w extension of the tibetan plateau provides a test of mechanical models of the geodynamic evolution of the india asia convergence zone in this work we focus on the kung co half graben southern tibet china bounded by an active n s normal fault with a minimum vertical offset of 1600 m to estimate the onset of normal faulting we combined high and medium temperature u pb ar ar and low temperature u th he thermochronometry of the kung co pluton a two mica granite of the northern himalayan granitic belt that outcrop in the footwall of the fault biotite and muscovite ar ar ages are close from each other 16 ma 0 2 ms and 15 0 4 ma bt which is typical of fast cooling the zircon and apatite u th he ages range from 11 3 to 9 6 ma and 9 9 to 3 7 ma respectively these he ages are indicative of 1 fast initial cooling from 11 3 to 9 ma gradually decreasing with time and 2 a high geothermal gradient 400 c km close to the surface at 10 ma the kung co pluton was emplaced at about 22 ma u pb on zircon at less than 10 km depth and 520 545 c subsequent to its shallow emplacement the pluton underwent fast thermal re equilibration ending around 7 5 ma followed by a period of slow cooling caused either by the end of the thermal re equilibration or by very slow exhumation 0 02 0 03 mm yr from 7 5 ma to at least 4 ma in either case the data suggest that the exhumation rate increased after 4 ma we infer this increase to be related to the initiation of the kung co normal fault a critical examination of previously published data show that most n s tibetan normal faults may have formed less than 5 ma ago rather than in the miocene as assumed by several authors such a young age implies that e w extension is not related to the neogene south tibetan magmatism 25 to 8 ma consequently models relating e w extension to magmatism such as convective removal of the lower lithosphere may be inappropriate we rather think that this extension is related with local accommodation of boundary forces and displacements 2007 elsevier b v all rights reserved

transient fluvial incision in the headwaters of the yellow river northeastern tibet china 1 we utilize topographic analysis of channel profiles combined with field measurements of erosion rates to explore the distribution of channel incision in the anyemaqen shan a broad mountainous region in the northeastern tibetan plateau tributary channels to the yellow river display systematic downstream increases in channel gradient associated with convex upward longitudinal profiles steep lower reaches of channels are associated with rapid 1 m ka incision rates along the yellow river while upstream reaches are associated with relatively slow 0 05 0 1 m ka erosion of soil mantled uplands covariance between erosion rates and channel steepness indices suggest that channels are adjusted to match long wavelength differential rock uplift across the range geologic constraints indicate that rapid incision downstream of the range is associated with excavation of basin fill driven by changes in relative base level farther downstream the upstream limit of this wave of transient incision is marked by a series of knickpoints that are found at nearly the same elevation throughout the watershed consistent with knickpoint migration as a kinematic rather than diffusional wave tributary channel gradients downstream of knickpoints however display a progressive adjustment to increased incision rates that may reflect the influence of increased sediment flux comparison of observed channel profiles to a stream power model of fluvial behavior reveals that the rate of knickpoint propagation can only be explained if the erosional efficiency coefficient k increases during incision our results thus highlight the utility of channel profile analysis to reconstruct the fluvial response to both active tectonism and external changes in base level

solubility of niobium in the system caco 3 caf 2 nanbo 3 at 0 1 gpa pressure implications for the crystallization of pyrochlore from carbonatite magma experimental data are presented for the solubility of nanbo3 in the ternary system caco3 caf2 nanbo3 or calcite fluorite lueshite over the temperature range 500 1 000 c at 0 1 gpa pressure liquidus to solidus phase relationships are given for the pseudo binary join caco3 60 caf2 40 100 x nanbo3 x 0 x 60 wt these data show that the maximum solubility of nanbo3 in these liquids is about 17 wt or 13 8 wt nb2o5 at approximately 930 c and is represented by the appearance of pyrochlore as the primary liquidus phase the sub liquidus assemblages with decreasing temperature for nanbo3 contents of 20 50 wt are pyrochlore liquid pyrochlore caf2 liquid pyrochlore caf2 caco3 liquid the solidus assemblage is pyrochlore caf2 caco3 at temperatures of approximately 700 c 20 wt nanbo3 and 600 c 40 wt nanbo3 nanbo3 is present only in sub solidus assemblages these data show that in this fluorine bearing anhydrous system pyrochlore is the principal nb hosting supra solidus phase in contrast to fluorine free hydrous melts from which perovskite structured compounds crystallize the crystallization of pyrochlore and or perovskite structured compounds from haplocarbonatite liquids is thus considered to be dependent upon the f oh ratio of the melt

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

early middle miocene topographic growth of the northern tibetan plateau stable isotope and sedimentation evidence from the southwestern qaidam basin abstract this study presents facies analysis and carbonate stable isotopic records of a 4435 m thick oligocene middle miocene 30 to 11 ma stratigraphic section from the southwestern qaidam basin which place constraints on the interplay between topographic growth and climatic hydrologic changes in the northern tibetan plateau three types of carbonate including marl limestone pedogenic carbonate and carbonate cement were collected and went through detailed screening for diagenesis carbonate cements are primary early diagenetic carbonates formed in shallow ground water setting the majority of marls limestones and pedogenic carbonates are primary micritic carbonates whereas a few of them showing alterations were sub sampled to avoid diagenetic phases a negative shift in the 18o values of marls limestones and depositional environment change from palustrine to shallow lacustrine associated with paleohydrological change at 20 ma are likely caused by tectonic activity in adjacent ranges around 15 14 ma a decrease of 1 5 in the most negative 18o values of carbonate cements is interpreted to reflect the topographic growth of surrounding mountain ranges i e altun shan east kunlun range this inference is supported by the sedimentary facies change from marginal lacustrine to fluvial that was accompanied by an abrupt increase in sedimentation rate finally a 1 5 positive shift in the most negative 18o values of carbonate cements and pedogenic carbonates enriched 13c values of pedogenic carbonates as well as sedimentary environment change from fluvial to braided river alluvial fan at 13 12 ma most likely reflect a regional aridification event observed over a large area of the central asia this aridification event may have been associated with continued topographic growth of the northern tibetan plateau to a critical elevation to block moisture e g westerlies from penetrating the interior of central asia

cenozoic structural inversion from transtension to transpression in yingxiong range western qaidam basin new insights into strike slip superimposition controlled by altyn tagh and eastern kunlun faults abstract a cenozoic structural inversion event from transtension to transpression involving salt tectonics has been uncovered in the yingxiong range the western qaidam basin seismic reflection data show that there are two common structural styles in the yingxiong range 1 the positive flower structure 2 the thrust controlled fold at shallow depth and the positive inverted flower structure at deep levels which are separated by a salt layer in the upper xiaganchaigou formation the yingxiong range experienced a first stage of transtension in the eocene induced by the altyn tagh fault and a second stage of transpression from the early miocene to present jointly controlled by the altyn tagh and eastern kunlun faults the eocene transtension produced numerous nw striking right stepping en echelon transtensional normal faults or fractures in the yingxiong range at the same time evaporites and mudstone were deposited in the vicinity of these faults in the early miocene the eocene transtensional normal faults were reactivated in a reverse sense and the thrust controlled folds at shallow depth started to form simultaneously with transpression enhanced in the late cenozoic positive flower structures directly formed in places without evaporites the cenozoic transtension to transpression inversion of the yingxiong range is the result of strike slip superimposition controlled by the altyn tagh and eastern kunlun faults in time and space

progressive evolution of a fault related fold pair from growth strata geometries sant llorenc de morunys se pyrenees abstract new structural stratigraphical mapping constrains the three dimensional kinematics and mechanisms of eocene oligocene growth folding at sant llorenc de morunys ne ebro basin spain a 1 km wide sub vertical panel of syntectonic alluvial gravels passes southwards via a highly asymmetrical growth fold pair to shallowlydipping strata the axial surface of the anticline comprises either continuous or en echelon segments while that of the syncline is concave and usually continuous while converging upwards the axial surfaces do not define growth triangles principal and subsidiary growth unconformities and thickness changes occur across both axial surfaces and the common limb dips within the common limb decrease up stratigraphy and up dip mesostructures indicate that internal deformation was ongoing during folding at all stratigraphical levels and concentration of cleavage in the syncline indicates that this hinge was essentially fixed sequential restoration of three profiles shows that folds amplified principally by limb rotation but incorporated minor passive hinge migration particle movement vectors generated by section restoration are arcuate about a hinterland pinpoint a new trishear model of fault propagation folding involving non rigid limb rotation reproduces the rounded hinge forms thickening geometries and limb dip variations observed simple kink band migration models fixed axis and constant thickness theories do not replicate these features

rock magnetic record of core sg 3 since 1 ma in the western qaidam basin and its paleoclimate implications for the ne tibetan plateau abstract quaternary saline lake deposits are well formed and widely distributed in the qaidam basin on the northeastern tibetan plateau nw china studying these lacustrine salt mineral deposits can improve our understanding of tectonic deformation asian interior aridification and monsoon climate associated with global cooling and tibetan plateau uplift previous paleoclimate records in the qaidam basin are mostly documented by geochemical proxies pollen spectra and sedimentary characteristics however supporting evidence from the rock magnetic index of sediments with salt deposits in the basin has not been well studied here we present a continuous high resolution 1 0 myr rock magnetic record from the well dated drill core sg 3 for typical saline lake deposits in the western qaidam basin the results show that deposits with low magnetic susceptibility are caused by low contents of magnetic minerals with single domain sd grains while those with high have relatively high contents of magnetic minerals with sd and pseudo single domain psd grains in full consideration of the sedimentary environment weathering and erosion associated with the global climate since the middle pleistocene we propose that variations in the closed lake are mainly influenced by erosion intensity and precipitation by combining the changes with pollen and grain size records we infer that the drying process of central asia since 1 ma with intensified aridity at 0 6 ma and 0 13 ma was primarily induced by global cooling

late plio pleistocene humidity fluctuations in the western qaidam basin ne tibetan plateau revealed by an integrated magnetic palynological record from lacustrine sediments abstract deciphering the climatic evolution of the tibetan plateau region during the plio pleistocene is hampered by the lack of continuous archives and proxy datasets indicative of moisture availability here we assess the suitability of magnetic susceptibility measured on lacustrine sediments as a paleohydrological proxy based on material from drill core sg 1 2 69 0 08 ma from the western qaidam basin our assessment is based on directly comparing with the artemisia chenopodiaceae a c pollen ratio which represents a sensitive well established proxy for moisture changes in arid environments we find that higher and lower values represent drier and less dry conditions respectively for the late plio pleistocene less dry phases were likely caused by transiently increased influence of the westerlies and or decreased influence of the asian winter monsoon on glacial interglacial time scales an exception from this relationship is the interval between 1 9 and 1 3 ma when the sg 1 record exhibits a 54 ka cyclicity which may indicate summer monsoon influence on the qaidam basin during that time after 1 3 ma the summer monsoon influence may have ceased due to global cooling with the consequence that the asian winter monsoon and the westerlies exerted a stronger control on the hydrology of the qaidam basin

palaeomagnetic and palaeoenvironmental study of two parallel sections of late cenozoic strata in the central taklimakan desert implications for the desertification of the tarim basin abstract due to marine recession and stepwise uplift of the tibetan plateau during the cenozoic era an extremely arid environment and large deserts developed progressively in the interior of asia the tarim basin is a typical region that experienced this type of palaeoenvironmental evolution in this study we apply high resolution palaeomagnetic dating esr dating and palaeoenvironmental reconstruction to two parallel sections from an almost 1000 m thick sequence of terrestrial sediments in the central tarim basin in order to characterize and date the process of drying and desertification our results indicate that the western tarim basin started to accumulate terrestrial sediments in a dry land environment at the latest by 4 2 ma and that the initial desert was formed by 3 4 ma from fluvial sands originating from the tibetan plateau when the climate became particularly dry in the interior of asia due to the uplift of the tibetan plateau variations of the stratum properties including colour index and magnetic susceptibility indicate that the aridification trend was significantly intensified at 2 6 ma coincident with the onset of the northern hemisphere glaciations and consequently forming the present desert regime

aridification recorded by lithofacies and grain size in a continuous pliocene quaternary lacustrine sediment record in the western qaidam basin ne tibetan plateau abstract the pliocene quaternary aridification of the asian interior is key to understanding the impacts of global cooling and tibetan plateau uplift and the potential linkage to north pacific ocean biochemical processes and global changes however there is a lack of detailed continuous pliocene quaternary paleoclimatic records from the asian interior because most climatic records have been obtained from discontinuous coarse sediment outcrops around the rims of inland basins here we provide a continuous high resolution 3 1 myr record of grain size from the well dated sg 3 borehole in the western qaidam basin on the northeastern tibetan plateau nw china the results reveal a long term upward coarsening trend in grain size that can be divided into three phases i 3 1 1 1 ma laminated fine sediments fine to medium silts containing scattered gypsum crystals and interbedded by thin beds of marl ii 1 1 0 15 ma massive medium to coarse silts interbedded by beds of mirabilite iii 0 15 0 ma coarse silts containing thin layers of mirabilite halite bloedite and polyhalite this pattern reflects the upward shallowing and shrinking of the lake from a brackish lake to a salt lake and then to a playa suggesting a long term stepwise drying of the asian interior since the late pliocene probably forced by global cooling and uplift of the tibetan plateau

cenozoic deformation history of the qaidam basin nw china results from cross section restoration and implications for qinghai tibet plateau tectonics abstract the deformation and uplifting history of the qinghai tibet plateau is a controversial issue and so far there has been insufficient evidence for interpretation investigations of the cenozoic deformation in the northeastern edge of this plateau may provide some relevant information in this paper the cenozoic deformation history of the qaidam basin has been studied based on results from the balanced cross section restoration of six geological sections they indicate that 1 the qaidam basin has been undergoing continuous shortening since the beginning of cenozoic with two relatively fast shortening phases the first during 42 8 40 5 ma and the second during 2 8 ma present 2 the shortening rate of the qaidam basin reached its maximum value since 2 8 ma and approximately 32 of the average total cenozoic shortening took place in this period and 3 the shortening of all sections reached approximately 50 of their respective total shortening by miocene the cenozoic deformation history of the qaidam basin shows good synchroneity with uplifting history of the qinghai tibet plateau it is therefore proposed that the deformation and uplifting in the south and north edges of the qinghai tibet plateau was almost synchronous with the altyn tagh fault formed at the beginning of india asia collision accordingly it is postulated that since the india asia collision at the beginning of cenozoic the indian plate moved northward continuously with two relatively fast phases the first during 42 8 40 5 ma and the second during 2 8 ma present approximately 50 of the total cenozoic northward displacement of the indian plate of the total amount of crustal shortening and uplifting of the qinghai tibet plateau and of the total cenozoic strike slip displacement of the altyn tagh fault were probably reached by the early miocene moreover another 30 40 might have taken place during 2 8 ma present the periods during 42 8 40 5 ma and during 2 8 ma present may also be the two fast phases for crustal shortening and uplifting of the qinghai tibet plateau and strike slip displacement of the altyn tagh fault

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

a summary of brunhes paleomagnetic field variability recorded in ocean drilling program cores the ocean drilling program odp has recovered many long sediment sequences from around the world that contain medium to high resolution paleomagnetic records of brunhes age these records have provided an important new global view of geomagnetic field variability during stable magnetic dipole polarity that could not be recovered by conventional terrestrial or deep sea piston coring the odp paleomagnetic records of directional variability can be routinely recovered and used regionally as a very highresolution relative chronostratigraphic tool 200 yr resolution odp paleomagnetic records of brunhes age relative paleointensity have dramatically improved our understanding of global geomagnetic field intensity these records document the global scale pattern of paleointensity and demonstrate its use as a high resolution relative chronostratigraphic tool that under ideal conditions could have a precision of 500 yr odp paleomagnetic records and associated oxygen isotope chronostratigraphies have also greatly improved our understanding of the number and ages of brunhes geomagnetic field excursions and their relationship to normal directional secular variation and paleointensity variability although significant progress has been made though odp paleomagnetic studies of brunhes aged sediments it is clear that more work is needed to further resolve and define the detailed global pattern of brunhes psv and its relationship to excursions and true polarity reversals

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

cenozoic tectonic evolution of the qaidam basin and its surrounding regions part 3 structural geology sedimentation and regional tectonic reconstruction the qaidam basin is the largest topographic depression inside the tibetan plateau because of its central position understanding the tectonic origin of the qaidam basin has important implications for unraveling the formation mechanism and growth history of the tibetan plateau in order to achieve this goal we analyzed regional seismic reflection profiles across the basin and a series of thickness distribution patterns of cenozoic strata at different time slices the first order structure of the basin is a broad cenozoic synclinorium which has an amplitude ranging from 16 km in the west to 48 in the west to 15 s 1 to 1 3 10 17 s 1 the eastward decrease in upper crustal shortening requires a progressive shift in crustal thickening mechanisms across qaidam basin from dominantly upper crustal shortening in the west to dominantly lower crustal shortening in the east although sedimentation began synchronously at 65 50 ma across the entire basin the initiation ages of the southern and northern basin bounding structures are significantly different deformation started at 65 50 ma in the north and at 29 24 ma in the south this information and the existing inference that the uplift of the eastern kunlun range south of qaidam basin began after 30 20 ma imply that the paleogene 65 24 ma qaidam and hoh xil basins on both sides of the eastern kunlun range may have been parts of a single topographic depression 500 km wide in the north south direction between the qilian shan and fenghuo shan thrust belts in the north and south the development of this large paleogene basin in central tibet and its subsequent destruction and partitioning by the neogene uplift of the eastern kunlun range requires a highly irregular sequence of deformation possibly controlled by preexist ing weakness in the tibetan lithosphere

evidence for a change in the periodicity of tropical climate cycles at 2 4 myr from whole core magnetic susceptibility measurements the earth s orbit about the sun is influenced by gravitational interactions with the moon and other planets the resulting orbital perturbations give rise to cyclical variations in orbital eccentricity obliquity and precession with periods of 100 41 19 and 23 kyr respectively1 these variations are climatically important because they affect the seasonal and latitudinal distribution of solar radiation magnetic susceptibility measurements in deep sea sediment cores can be used as a sensitive indicator of temporal variations in the concentration of terrigenous material supplied to the sea bed here we use this approach to study the evolution of cycles of terrigenous sedimentation at ocean drilling program sites in the arabian sea and the eastern tropical atlantic we show from spectral analyses of the two susceptibility time series spanning the past 3 2 3 5 myr that the records are driven strongly by orbital forcing before 2 4 myr before present bp both records are strongly influenced by the 23 and 19 kyr periodicities but after 2 4 myr they are both significantly influenced by the 41 kyr periodicity this shift coincides with the beginning of major northern hemisphere glaciation which indicates for the indian ocean site that the supply of terrigenous sediment carried by monsoonal winds has responded to the rapid increase in ice cover at this time we cannot however exclude possible effects of changes in bottom water circulation on the susceptibility record of the atlantic site

signatures of mountain building detrital zircon u pb ages from northeastern tibet although detrital zircon has proven to be a powerful tool for determining provenance past work has focused primarily on delimiting regional source terranes here we explore the limits of spatial resolution and stratigraphic sensitivity of detrital zircon in ascertaining provenance and we demonstrate its ability to detect source changes for terranes separated by only a few tens of kilometers for such an analysis to succeed for a given mountain discrete intrarange source terranes must have unique u pb zircon age signatures and sediments eroded from the range must have well defi ned depositional ages here we use 1400 single grain u pb zircon ages from northeastern tibet to identify and analyze an area that satisfi es these conditions this analysis shows that the edges of intermontane basins are stratigraphically sensitive to discrete punctuated changes in local source terranes by tracking eroding rock units chronologically through the stratigraphic record this sensitivity permits the detection of the differential rock uplift and progressive erosion that began ca 8 ma in the laji shan a 10 25 km wide range in northeastern tibet with a unique u pb age signature

magnetostratigraphy and sedimentologically derived geochronology of the quaternary lacustrine deposits of a 3000 m thick sequence in the central qaidam basin western china the qaidam basin is a large basin in western china where the thickness of quaternary deposits has a range of 2000 3000 m at its subsiding depocentre this study investigated the chronology of the quaternary deposits including palaeomagnetics biostratigraphic correlation rates of sediment accumulation seismic reflection surveys and electrical property correlations the results give a full interpretation of ages for the sequence from bottom to top the oldest sequence age was defined by the first appearance of microlimnocythere sinensis huang and was placed in the gauss epoch estimated at approximately 3 ma by extrapolation of accumulation rates the m g boundary occurs at the base of the third fossil zone of ostracoda the first appearance of qinghaicypris crassa huang or at the marker layer k 10 of the electrical log in the sebei anticline area considered here to be 2 48 ma the olduvai event in the fifth fossil zone spanning electrical layers k 4 and k 3 in the sebei area occurs at reflection layer t 0 1 of seismic stratigraphy which could be stratigraphically correlated in the whole basin the boundary of early middle pleistocene such as the brunhes and matsuyama b m boundary falls within the eighth fossil zone and coincides with electrical marker k 0 1 two boundaries corresponding to middle late pleistocene and pleistocene holocene occur at the bases of the 11th and 12th biozones respectively neither of which can be correlated with seismic stratigraphy and electrical logs in the basin due to the lack of widely distributed lacustrine deposits since the middle pleistocene the ages of the boundaries can be tested by deposition rates of different facies derived from the varves and synchronous deposits as a result geochronological sequences of subsurface stratigraphy could be established within a basin wide area on the synchronology of seismic reflectors and correlations of electrical logs as well as biozones c 1998 elsevier science b v all rights reserved

a late neogene framework and transpressional system within the yingxiongling range western qaidam basin northeast tibetan plateau insights from seismic reflection profiles and active tectonics abstract the yingxiongling range is a topographically unique and active tectonic belt in the western qaidam basin that has recorded the structural evolution and interactions between itself and surrounding ranges throughout the late cenozoic although a large scale fault fold system within this region accommodates transpressive deformation the overall kinematic pattern remains highly debated regional seismic reflection profiles earthquake distributions focal mechanisms and displacement vector results are used in this study to rebuild a northern verging thrust and wedge tectonic system in a perpendicular direction interpretations of newly acquired three dimensional seismic data the timing of growth strata and the kinematics of active structures strongly support lateral structural deformation propagation and evolution of the southern yingxiongling range in the late neogene a novel analysis of active tectonic and seismic profiles enables us to rebuild the framework kinematics and evolutionary pattern of the yingxiongling range we also provide a new interpretation of lateral differentia along the yingxiongling range based on critical taper theory and suggest that this overall structure is an integrated part of the qimen tagh wedge since the late neogene rapid uplift in this area occurred in response to northward propagation of the qimen tagh wedge

stratigraphic and paleomagnetic evidence of mid pleistocene rapid deformation and uplift of the ne tibetan plateau abstract the ne tibetan plateau has become an important region for deciphering the cenozoic uplift of the plateau previous studies have verified that the quaternary uplift in this region had a multi stage character however geologists have not established which event constituted the main tectonic phase fortunately stratigraphic and paleomagnetic evidence from the jiuquan basin gansu province china now confirm that the laojunmiao movement represented as an angular unconformity occurred between 0 9 and 0 8 ma and an almost synchronous tectonic event has been well recorded within and around the plateau moreover many proxies indicate that the global climate changed more rapidly after 0 9 ma these proxies include eolian loess and vermiculated red soils caused by the asian monsoon the eolian sedimentation rate the stilostomella extinction changes in the global thermohaline circulation and the increased 13 c values recorded in the marine sediments based on the uplift driven climate change the mid pleistocene transition mpt could be the result of the laojunmiao movement certainly the laojunmiao movement and mpt had the significant impact on the quaternary earth we conclude that the laojunmiao movement was likely the main tectonic phase of the quaternary plateau uplift

high resolution magnetostratigraphy of the neogene huaitoutala section in the eastern qaidam basin on the ne tibetan plateau qinghai province china and its implication on tectonic uplift of the ne tibetan plateau abstract the closed inland qaidam basin in the ne tibetan plateau contains possibly the world s thickest 12 000 m continuous sequence of cenozoic fluviolacustrine sedimentary rocks this sequence contains considerable information on the history of tibetan uplift and associated climatic change however work within qaidam basin has been held back by a paucity of precise time constraints on this sequence here we report on a detailed paleomagnetic study of the well exposed 4570 m huaitoutala section along the keluke anticline in the northeastern qaidam basin where three distinct faunas were recovered and identified from the middle miocene through pliocene constrained by these faunal ages the observed thirty three pairs of normal and reversed polarity zones can be readily correlated with chrons 2n 5br of the gpts this study assigns the age of the section to the interval between ca 15 7 ma to 1 8 ma in addition the widely used stratigraphic units the xia youshashan shang youshashan the shizigou and qigequan formations were formed at 15 3 ma 15 3 8 1 ma 8 1 2 5 ma and

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

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

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

geometries of syntectonic sediments associated with single layer detachment folds abstract three kinematic models have been proposed to account for the geometry and kinematics of detachment folds involving a homogeneous competent layer detached over a ductile unit model 1 variable limb dip constant limb length model 2 constant limb dip variable limb length and model 3 variable limb dip variable limb length because the same fold shape can be generated by any of the above mechanisms fold kinematics are best determined by the geometries of syntectonic sediments in single layer detachment folds growth strata patterns are controlled by axial surface activity limb rotation limb lengthening fold uplift rates sedimentation rates and deformation mechanisms of the syntectonic sediments asymmetric kink folds have been modelled and compared with natural examples from the southern spanish pyrenees under conditions of high sedimentation rates model 1 folds produce characteristic fanning growth stratal wedges that initially onlap and then progressively overlap the detachment anticline whereas at low sedimentation rates anticlines with fanning growth wedges on both limbs are formed model 2 folds under conditions of high syntectonic sedimentation rates form anticlines with growth strata largely parallel to the pre growth units and thinned over the fold crest in contrast at low sedimentation rates model 2 folds show offlapping growth strata onto both limbs model 3 growth folds under conditions of high syntectonic sedimentation rates form anticlines with intermediate features from model 1 and model 2 at low sedimentation rates anticlines are formed with offlapping growth structures

characteristic grain size component a useful process related parameter for grain size analysis of lacustrine clastics abstract lacustrine sediments are important archives for paleoclimate reconstructions the application of grain size analysis as palaeoclimatic proxy in lacustrine clastics is valuable but also difficult because the typical polymodal grain size distribution in these clastics to better understand the grain size distribution of lacustrine clastics and to promote the application of grain size in paleoenvironmental interpretation this study investigates lacustrine clastics from northern and southern china the grain size distribution of these sediments was decomposed by log normal distribution function fitting method based on the results and drawing upon the concept of paleomagnetic demagnetization and characteristic remnant magnetization from paleomagnetism a conceptual system has been established and defined for grain size distribution analysis the system is composed of four components i characteristic grain size component chgsc ii affiliated grain size component iii meaningful grain size component and iv combination feature of grain size components cfgscs based on the proposed system chgsc and cfgscs were used to detect the grain size distribution of clastics from the different lake zones investigated our results show the number modal size and percentage of chgsc s in grain size distributions are sensitive to changes in the lacustrine environment the chgsc s mirrors the dominant depositional process and hydrodynamic conditions the modal size of chgsc s is more sensitive to hydrological conditions than the widely used mean grain size approach thus the chgsc s provide a useful process related parameter for paleoenvironmental reconstructions to test this promising application we applied this approach to a deep drill core from the qaidam basin in the northeastern tibetan plateau

global geochemical cycles of carbon sulfur and oxygen time resolved data on the carbon isotopic composition of carbonate minerals and the sulfur isotopic composition or sulfate minerals show a strong negative correlation during the cretaceous carbonate minerals are isotopically heavy during this period while sulfate minerals are isotopically light the implication is that carbon is being transferred from the oxidized carbonate reservoir to the reservoir of isotopically light reduced organic carbon in sedimentary rocks while sulfur is being transferred from the reservoir of isotopically light sedimentary sulfide to the oxidized sulfate reservoir these apparently oppositely directed changes in the oxidation state of average sedimentary carbon and sulfur are surprising because of a well established and easy to understand correlation between the concentrations of reduced organic carbon and sulfide minerals in sedimentary rocks rocks rich in reduced carbon are also rich in reduced sulfur the isotopic and concentration data can be reconciled by a model which invokes a significant flux of hydrothermal sulfide to the deep sea at least during the cretaceous

new early cretaceous paleomagnetic data from volcanic and red beds of the eastern qaidam block and its implications for tectonics of central asia abstract in order to better understand anomalously low paleomagnetic inclinations in the cretaceous rocks of central asia a combined geochronologic and paleomagnetic investigation has been performed on cretaceous basalt sequences intercalated with red beds from zeku area in the eastern qaidam block northeastern tibetan plateau potassium argon k ar dating indicates that the basalt sequences are of early cretaceous age 89 9 2 8 to 112 1 2 4 ma isothermal remanent magnetization irm thermal demagnetization of a three component irm and curie point experiment suggest that magnetite and hematite dominate in red beds magnetite in basalt flows stepwise thermal demagnetization successfully isolated stable characteristic remanent magnetization chrm from 7 lava flows and 12 red beds sites the chrms of red bed and basalt sites pass fold test and likely represent primary remanent magnetization the tilt corrected mean direction from red bed sites basalts plus baked red bed site is d s 18 4 i s 55 3 s 69 1 95 5 5 n 11 sites d s 16 1 i s 52 0 s 42 6 95 8 6 n 8 corresponding to a pole at 75 2 n 182 3 e with a 95 7 07 76 9 n 194 9 e with a 95 10 0 respectively the consistent inclination recorded in red beds and basalts implies no significant inclination shallowing caused by deposition compaction in red beds from the studied area compared with the early cretaceous poles from the north china block ncb or eurasia insignificant post early cretaceous northward motion may have occurred between the qaidam block and the ncb on the basis of the existing regional paleomagnetic data the qaidam block has not experienced wholescale vertical axis rotation since the early cretaceous with respect to ncb

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

magnetostratigraphy of deep drilling core sg 1 in the western qaidam basin ne tibetan plateau and its tectonic implications abstract the qaidam basin is the largest intermontane basin of the northeastern tibetan plateau and contains a continuous cenozoic sequence of lacustrine sediments a 1000 m deep drilling sg 1 with an average core recovery of 95 was carried out in the depocenter of the chahansilatu playa sub depression in the western qaidam basin aimed to obtain a high resolution record of the paleoenvironmental evolution and the erosion history stepwise alternating field and thermal demagnetization together with rock magnetic results revealed a stable remanent magnetization for most samples carried by magnetite the polarity sequence consisted of 16 normal and 15 reverse zones which can be correlated with chrons 1n to 2an of the global geomagnetic polarity time scale magnetostratigraphic results date the entire core sg 1 at 2 77 ma to 0 1 ma and yielded sediment accumulation rate sar ranging from 26 1 cm ka to 51 5 cm ka maximum sars occurred within the intervals of 2 6 2 2 ma and after 0 8 ma indicating two episodes of erosion which we relate to pulse tectonic uplift of the ne tibetan plateau with subsequent global cooling

new paleomagnetic constraints on central asian kinematics displacement along the altyn tagh fault and rotation of the qaidam basin received 23 july 2001 revised 9 january 2002 accepted 26 february 2002 published 5 october 2002 1 in order to better understand the tectonic evolution of central asia under the influence of the india asia collision we carried out a paleomagnetic study of 1500 cores from 106 sites along the altyn tagh fault in the qaidam and tarim basins and on the tibetan plateau samples were mainly collected from jurassic to neogene siltstones and sandstones in most cases stepwise thermal demagnetization unblocks low and high temperature components carried by magnetite and hematite low temperature components are north and down directed and lie close to the recent geomagnetic field high temperature components from 10 of 13 age locality groups pass fold and or reversal tests and likely represent primary remanent magnetizations the ten overall mean directions display a complex pattern of vertical axis block rotations that are compatible with a tectonic model of clockwise rotation of the qaidam basin and concomitant left lateral slip on the altyn tagh fault two of the ten localities are rotated significantly counterclockwise they lie adjacent to the altyn tagh fault zone consistent with the idea that left lateral strike slip motion occurred along it the age of counterclockwise rotation near the eastern extremity of the fault was dated as younger than 19 ma three widely spread areas within the qaidam basin exhibit similar and significant clockwise rotations on the order of 20 with respect to the north china block tarim and eurasia the mean of the three values is thought to represent the total rotation of qaidam because the youngest rocks displaying clockwise rotations are oligocene the main phase of qaidam basin rotation and hence shear on the altyn tagh fault took place after or near the end of the oligocene 24 ma upper neogene strata located on the qaidam basin are not significantly rotated thus tectonic deformation acting since the upper neogene 5 ma is not resolvable by paleomagnetic methods given a 20 5 clockwise rotation of the qaidam basin with respect to the tarim basin the maximum left lateral displacement on the altyn tagh fault since 24 ma is 500 130 km index terms 1525 geomagnetism and paleomagnetism paleomagnetism applied to tectonics regional global 1527 geomagnetism and paleomagnetism paleomagnetism applied to geologic processes 8105 tectonophysics continental margins and sedimentary basins

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

the topographic controls on the decadal scale erosion rates in qilian shan mountains n w china abstract the relationships between climate topography and erosion are significant in understanding landscape evolution in order to study this relationship in a tectonically active landscape the details of 11 drainage basins were collected from qilian shan mountains decadal scale erosion rates including the mechanical load and solute load contributions are estimated in natural conditions the calculated erosion rates show that the average erosion rate of qilian shan mountains is about 0 08 mm yr while the variation of annual erosion rates within each basin is significant the changes of topography and climate which potentially control erosion rates are also employed in this paper for correlating analyses correlation analyses indicate that erosion rates are more closely correlated with topographic variables such as mean local relief and mean slope than all of the climatic variables and mean local relief and decadal scale erosion rates show a linear relationship in these tectonically active mountains however some topographic variables like basin area and elongation ratio exert limited influence on erosion rates while others such as basin elevation basin relief and basin roughness show poor correlation the results indicate that topographic control like aspects of the local terrain steepness plays the most important role in spatial distribution of decadal scale erosion rates throughout qilian shan mountains under topographic control some climatic variables like discharge and runoff however could account for the significant variation of annual erosion rates in individual basin when comparing erosion rates on different timescales we found that the decadal scale erosion rates are lower than the long term river incision rates as well as the exhumation rates during early and middle miocene the change to more arid climatic condition since the middle miocene combining with tectonic uplift should attribute to the inconsistent erosion rates over different timescales in qilian shan mountains

preliminary magnetostratigraphy of dabusan lake qaidam basin central asia abstract sediment samples from a 528 m deep borehole located about 60 km north of golmud on the northern side of dabusan lake have been used to determine the change in magnetic inclination with depth normal polarity brunhes inclination values are present throughout the section but there are six departures from this value five with negative inclinations and one shallow inclination an attempt has been made to date the core using the magnetic inclination departures together with a limited number of 14c dates

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

gyromagnetic remanence acquired by greigite fe3s4 during static three axis alternating field demagnetization summary a magnetic study was carried out on lacustrine sediments from the zoigebasin tibetan plateau in order to obtain a better understanding of palaeoclimatic changes there gyromagnetic remanence grm acquisition is unexpectedly observed during static three axis alternating field af demagnetization in about 20 per cent of a large number of samples x ray divraction xrd analysis on a magnetic extract clearly shows that greigite is the dominant magnetic mineral carrier scanning electron microscopy sem reveals that the greigite particles are in the grain size range of 200 300 nm possibly in the single domain state greigite clumps of about 3 mm size are sealed by silicates fitting of xrd peaks yields a crystalline coherence length of about 15 nm indicating that the particles seen in the sem are polycrystalline grm intensities of most samples are of the same order as the nrm while others show much stronger grm although their magnetic properties are similar variation of the demagnetization sequence confirms that grm is mainly produced perpendicular to the af direction the anisotropy direction can be derived from grm but more systematic studies are needed for detailed conclusions an attempt to correct for grm failed due to high grm intensities and because smaller grm acquisition was also found along the demagnetization axis behaviours of acquisition and af demagnetiz ation of grm are comparable with those of nrm arm irm indicating fine grain sizes of remanence carriers

magnetostratigraphy of drill core sg 1b in the western qaidam basin ne tibetan plateau and tectonic implications the qaidam basin is an ideal archive to study long term climate and erosion histories at the ne tibetan plateau we present a magnetostratigraphic study of the 723 m deep drill core sg 1b of lacustrine sediments at the jianshan anticline in the western qaidam basin the polarity sequence shows 18 normal and 19 reverse polarity zones which can be readily correlated with chrons c1n c3br of the geomagnetic polarity time scale 2004 gpts 2004 dating the core at about 7 3 1 6 ma the resulting mean sediment accumulation rate sar between polarity boundaries ranges from 6 5 to 30 4 cm ka 1 high sars occur within the intervals of 7 3 6 0 5 2 4 2 and 3 6 2 6 ma indicating three episodic phases of higher erosion from the derived variation of sars and previous results we conclude that growth strata at the jianshan anticline started to develop at similar to 1 6 ma by limb rotation all this we relate to pulse tectonic uplift of the ne tibetan plateau and fault propagation folding in the qaidam basin

magnetostratigraphy of deep drilling core 15yzk01 in the northwestern qaidam basin ne tibetan plateau tectonic movement salt deposits and their link to quaternary glaciation abstract the thick cenozoic sediments in the qaidam basin contain considerable information about the uplift of tibetan plateau climate change and salt deposits in this paper a palaeomagnetic study was performed on the 1400 m long drilling core 15yzk01 from the yiliping playa in the northwestern qaidam basin to obtain a timescale documenting this information the polarity sequence consisted of 17 pairs of normal and reversed zones and can be correlated well with subchrons c2ar c1n of the geomagnetic polarity timescale gpts 2012 from ca 4 1 to 0 002 ma three periods of high sedimentation rates occurred at ca 4 1 3 207 ma 2 581 1 945 ma and 0 781 0 002 ma these 3 periods correspond to 4 episodes of tectonic activity on the tibetan plateau the qingzang movement 3 6 ma the b phase of the qingzang movement 2 6 ma the kunlun huanghe movement 1 1 0 7 ma and the gonghe movement 0 15 ma the dips of the formations in the drilling core indicate that the yiliping area was uplifted at ca 3 236 ma after this uplift the sedimentation rate sharply decreased from 39 1 cm ka to 19 2 cm ka in the pliocene the yiliping area mainly featured a freshwater environment and the earliest gypsum sediment formed at ca 3 530 ma after the qingzang movement after the b phase of the qingzang movement the yiliping area alternated between freshwater and semi saline lake environments and 20 layers of gypsum bearing strata began to appear since 2 103 ma after the kunlun huanghe movement the yiliping area alternated between semi saline and saline lake environments and salt deposits began to form at 0 733 ma or 0 746 ma in the middle pleistocene four salt forming periods in the yiliping area can be correlated with quaternary glaciations on the tibetan plateau the s1 deposit corresponds to the wangkun glaciation the s3 and s4 deposits and the mirabilite bearing stratum between them correspond to the penultimate glaciation and s5 began to form in the last glaciation the calculated formation age of s2 has to be considered with caution and it is very close to the timing of the wangkun glaciation a reduction in the water supply of the salt lakes during the quaternary glaciations was an important driving factor in the formation of salt deposits in the qaidam basin and the quaternary glaciations were closely related to the tectonic activity of the tibetan plateau

cenozoic qaidam basin china a stronger tectonic inversed extensional rifted basin qaidam basin is a composite sedimentary basin developed on typical continental crust it comprises a jurassic foreland basin and a cenozoic extensional basin the geologic history of the cenozoic qaidam basin can be divided into two megastages on the basis of stratigraphy and structure the first megastage from latest cretaceous to oligocene consisted of two periods of rifting due to upwelling of the hot upper mantle this is demonstrated by two tectono sequences formed by the rifting the synrifting magmatic thermal activity and the control of protobasin development by syndepositional faults the second megastage comprises three tectono sequences that developed in the miocene and pliocene and was a period of structural inversion that consisted of compressive downwarping and reverse faulting this tectonic inversion may be ascribed to the long distance propagation of compressive stress caused by the collisions of the indian and eurasian plates the thick sequence of primary oil source rocks was deposited in the major rifted depression and the restricted drainage graben of the rifted protobasin during the tectonic inversion megastage they were subject to deep burial and prolonged heating a major and a minor oil generating basin have developed the tectonic inversion processes produced several structural features that may contain potential hydrocarbon reservoirs and traps

evidence for a new geomagnetic reversal from lava flows in idaho discussion of short polarity reversals in the brunhes and late matuyama polarity chrons k ar ages and paleomagnetic data for basalt samples from a new core hole site e at the idaho national engineering laboratory inel indicate that the age of the reversed polarity event recorded in snake river plain lavas is older than 465 50 ka 1000 years before present reported previously by champion et al 1981 nine basalt flows eight with normal polarity and one with reversed polarity were recognized in the site e core hole the flows above and below the reversed flow have ages of 491 80 ka and 580 93 ka respectively the inclination of the paleomagnetic field direction of the reversed flow at site e agrees with the inclination of reversed flows elsewhere at inel which have an age of 565 14 ka these reversed flows were previously thought to be correlative with the emperor event we suggest that this polarity event is an older event which we name the big lost reversed polarity subchronozone and subchron a review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the brunhes and two besides the jaramillo in the late matuyama these 10 short subchrons begin to indicate the many short events that cox 1968 hypothesized must exist if polarity interval lengths have a poisson distribution these events are true subchrons not excursions and may or may not have low associated paleointensities although low field strengths might explain why the reversal process aborts the mean sustained polarity interval length since late matuyama chron time is 90 000 years the similarity of this number with the 105 year period of the earth s orbital eccentricity suggests anew that linkage between geomagnetic paleoclimatic and possible underlying earth orbital parameters should be evaluated

a study of gyroremanent magnetisation grm and rotational remanent magnetisation rrm carried by greigite from lake sediments summary further studies of gyroremanent magnetisation grm and rotational remanent magnetisation rrm have been conducted on lake sediments from the zoige basin in the eastern tibetan plateau in which greigite is the main carrier of gyroremanent magnetisation greigite has the greatest effective gyrofield bg of all magnetic minerals studied so far being several hundred t for a peak af of 80 mt this high bg value has the potential to be used as an indicator for greigite the grm produced during static alternating field af demagnetisation became close to its maximum at a peak af of 150 mt attempts to extract the natural remanent magnetisation nrm by algebraic elimination of the grm were unsuccessful above fields of about 30 mt because the grm became much larger than the remaining nrm the grm of a crushed sample was much reduced because of the destruction of the sample s anisotropy although as expected both rrm and bg were similar before and after crushing thus demonstrating that rrm and bg are independent of anisotropy measurement of the anisotropy of two samples from different depths showed that the deeper sample which acquired the higher grm also had the higher anisotropy presumably as a result of greater sediment compaction study of anisotropy of grm may help to elucidate the preferred alignment of greigite within the sample which is difficult to ascertain by other means

geometric analysis of hybrid fault propagation detachment folds abstract fault propagation folds and many detachment folds may be analyzed using a single geometric model a modification of a previously published approach allows the interpreter to quantitatively predict thrust fault trajectory and displacement from the geometry of an associated fault propagation detachment anticline because geological and geophysical data typically constrain the mesoscopic and macroscopic forms of folds more completely and more accurately than they constrain faults this technique offers a useful way to rapidly construct balanced structural cross sections in detached fold thrust systems

paleomagnetism indicates no neogene rotation of the qaidam basin in northern tibet during indo asian collision paleomagnetic data were obtained from tertiary red sedimentary rocks at two locations separated by several hundred kilometers within the qaidam basin in the east central part of the basin 30 sites from the lower pliocene youshashan formation yielded characteristic remanent magnetization chrm directions with intermediate unblocking temperatures 100 600 8c chrm with high unblocking temperatures to 680 8c was isolated from 14 sites in the same area chrm directions were obtained from six sites within the oligocene lower gancaigou formation characteristic magnetization was also determined from 16 sites within the lower gancaigou formation exposed in the e bo liang range of the north central qaidam basin when compared with equivalentage expected directions for eurasia the mean paleomagnetic directions indicate no neogene vertical axis rotation of the qaidam basin or the altyn tagh fault the qaidam basin may act as an indentor translating without rotation toward the north china block and driving clockwise vertical axis rotations by differential shortening within the nan shan fold and thrust belt

stable isotopic constraints on the tectonic topographic and climatic evolution of the northern margin of the tibetan plateau abstract this study presents oxygen and carbon isotopic records from lacustrine paleosol alluvial and fluvial carbonate sampled in thirteen cenozoic sedimentary sections that span the northern margin of the tibetan plateau isotopic analyses of 1475 carbonate samples yield 18 o values that range from 13 1 to 38 9 smow and 13 c values range from 11 5 to 3 7 pdb based on these analyses we see two isotopic trends regionally we interpret a paleogene decrease in average oxygen isotope values to result from an increase in the mean elevation of the study region s drainages as they tapped waters from a growing tibetan plateau to the south of the study area we interpret a neogene increase in both carbon and oxygen isotope values to result from the initiation of local mountain building in the study area and the withdrawal of marine waters which increased basin isolation aridity and temperatures the timing of inferred initiation of uplift in this region is consistent with structural thermochronological and sedimentological evidence for a tectonic transition in the early and middle miocene from accommodation of india eurasia convergence through extrusion along the altyn tagh fault to accommodation of convergence through active shortening and mountain building in this region this study also supports tectonic models for the differential uplift of the tibetan plateau in that mountain building in this study region significantly postdates estimates for the attainment of high elevation from isotopic studies of the central tibetan plateau

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

late eocene clay boron derived paleosalinity in the qaidam basin and its implications for regional tectonics and climate abstract the qaidam basin located on the northeastern tibetan plateau and containing cenozoic sediments with a maximum thickness of 12 000 m is an ideal place to study the phased uplift of the ne tibetan plateau and regional climate change the estimation of the paleosalinity of sedimentary environments not only helps to evaluate the evolution of lakes in this region but offers insights into contemporaneous climate change we present detailed geochemical and mineralogical investigations from the lacustrine interval of the hongliugou section in the northern qaidam basin to reconstruct salinity fluctuations in the paleolake during the late eocene era 42 0 35 5 ma the clay mineral assemblages mainly contain smectite illite chlorite kaolinite and irregular illite smectite mixed layers clay boron derived paleosalinity estimates equivalent boron content couch s paleosalimeter and b ga ratios along with other proxies sensitive to salinity changes e g rb k ratios and ostracod assemblages collectively indicate an overall brackish sedimentary environment with a higher salinity period at approximately 40 0 39 2 ma this higher salinity period indicates a more arid environment and is probably related to global cooling however the global cooling in late eocene cannot explain the overall stable long term salinity pattern implying that other factors exist we propose that the migration of the yiliping depression depocenter in the northern qaidam and increased orographic rainfall induced by late eocene tectonic activity at the northern margin of the basin might have partly offset the increase in salinity driven by global cooling

vertebrate paleontology biostratigraphy geochronology and paleoenvironment of qaidam basin in northern tibetan plateau abstract qaidam basin is the largest terrestrial basin of the tibetan plateau and has the most continuous sedimentary record in the cenozoic although constituting the first known fossil vertebrates of the tibetan plateau and discovered in early scientific expeditions in the 1930s mammalian fossils from qaidam basin have not played a significant role in studying the basin stratigraphy our explorations in the qaidam basin during the past 8 years have resulted in the first well documented collection of fossil vertebrates from there in addition to being the best organisms for terrestrial biostratigraphy in the cenozoic the new fossil vertebrates offer a unique window into the ancient biotas that chronicle the late cenozoic uplift of the plateau and associated environmental changes here we establish for the first time a faunal sequence of early oligocene to early pliocene age four mammalian faunas are recognized olongbuluk mammal fauna middle miocene tuosu mammal fauna early late miocene shengou mammal fauna early late miocene and huaitoutala mammal fauna early pliocene all four mammal faunas can be directly or indirectly tied to a paleomagnetic section in the olongbuluk huaitoutala area permitting robust calibration of the chronology of these faunas at least four fish faunas are also found lulehe fish fauna late early to early late oligocene shengou fish fauna early late miocene eboliang fish fauna late late miocene to early pliocene and yahu fish fauna early pliocene the fish faunas represent the only records of their respective times in the tibetan plateau and thus by themselves cannot be used for age determination however these fish faunas are either integrated into paleomagnetic sections or directly tied to mammal faunas of known ages permitting independent assessment of their chronology zoogeographically qaidam mammal faunas show increasing endemism through time presumably related to topographic barriers formed by the growth and uplift of the tibetan plateau and to climatic changes triggered by the same tectonic process fishes that were trapped in internally drained basins developed pachyostotic morphologies caused by hypersaline environments which are associated with the increasing aridity of the tibetan plateau since early pliocene

paleoclimatic context of geomagnetic dipole lows and excursions in the brunhes clue for an orbital influence on the geodynamo the hypothesis of an influence of the astronomical precession on the geodynamo energy budget was recently reappraised by theoreticians paleomagnetic indications of such an influence remain controversial because reconstructions of paleointensity variations from sediments are suspected to be contaminated by lithological paleoclimatically induced influences three sets of complementary indicators are however available 1 records of the direction of magnetization in sediments 2 records of magnetic anomalies of the deep sea floor basalts and 3 records of production variations of cosmogenic isotopes from sediment and ice cores these records confirm the genuine geomagnetic origin of paleointensity lows and their narrow link with excursions or short reversals recorded in various materials and often dated by radiometric methods the analysis of these time series and their comparison with 18o records of the paleoclimate suggest that such globally recorded geomagnetic dipole lows have preferentially occurred in the context of interglacial or transitional paleoclimates at the time of low or decreasing obliquity the dominant periods extracted with the complex continuous wavelet transform technique range from 40 to 125 ka further suggesting a link with orbital parameters these results encourage future efforts of research to improve the precision the resolution and the dating of the time series of geomagnetic dipole low in order to better decipher orbital signatures and understand their origin an important implication of this topic is that the next geomagnetic dipole low should be related with the present interglacial

orbitally tuned age model for the late pliocene pleistocene lacustrine succession of drill core sg 1 from the western qaidam basin ne tibetan plateau

intermontane basin development in the past 4 myr in the north west himalaya the through the combined use of magnetic polarity stratigraphy with fission track dating of volcanic ashes a new chronology spanning 4 myr has been developed for the intermontane basin of kashmir in the northwestern himalaya

toward the discrimination of fine grained ferrimagnets by magnetic measurements in lake and near shore marine sediments the magnetic properties of low field susceptibility frequency dependent susceptibility fd and susceptibility of anhysteretic remanent magnetisation arm are used to define sets of natural soil and sediment samples within which by the criteria of king et al 1982 and maher 1988 the modal ferrimagnetic grain diameter is less than 0 07 0 1 m within this sample set two distinct groups with low and high values respectively for the quotients arm and arm fd are defined the first group includes sediment samples from sites where published studies propose a detrital origin for the fine grained ferrimagnetic content where catchment samples are available for comparison they fall within the same envelope as do clays from palaeosol samples within chinese loess this envelope of low quotient values also overlaps with the values for the fine grained synthetic magnetite samples within maher s 1988 new mt series the high quotient envelope of values includes sediments from the adriatic sea and clays from saltmarsh and shallow water marine sediments in nw britain in these cases no catchment source is postulated for the fine grained magnetite the size range of magnetite in this set appears to be almost exclusively stable single domain ssd and a bacterial origin seems likely sample sets from estuarine environments between river inflow sites and the open sea show intermediate values which in the case of the potomac are ordered by distance down river although at this stage magnetic measurements alone cannot discriminate between bacterial and fine grained detrital ferrimagnets in sediments they hold out some promise of doing so provided the distinctions proposed here can be substantiated by transmission electron microscopy

pliocene uplift of the northern tibetan plateau neogene redbeds passing upward into upward coarsening conglomerate and debris flow deposits at the foot of the kunlun mountains record the change in paleoslope related to uplift of the surface of the northern tibetan plateau detailed magnetostratigraphy of a 4 5 km section near yecheng in the western kunlun mountains shows that the change from deposition on distal alluvial plains to proximal alluvial fans occurred during the gilbert reversed chron 4 5 3 5 ma the change in depositional facies was accompanied by an increase in sedimentation rate from an average 0 15 mm yr between the earliest oligocene and the earliest pliocene to 1 4 mm yr in the gauss normal chron 3 6 2 6 ma we interpret the change in depositional facies and increase in sedimentation rate as indicating that the main uplift of the northwestern tibetan plateau began ca 4 5 ma

tibetan plateau aridification linked to global cooling at the eocene oligocene transition the eocene oligocene transition about 33 5 million years ago was a major global climate event the end of the eocene was unusually warm with no significant ice on antarctica but the oligocene saw the arrival of a permanent antarctic ice sheet two papers this week relate to the continental effects of this global change dupont nivet et al examined sedimentary records from the tibetan plateau and find a drop in atmospheric water which caused cooling and aridification coincident with antarctic cooling previous studies attributed this phenomenon to the rapid uplift of the tibetan plateau but this new work suggests that regional tibetan climate was influenced by global events in an unrelated paper on the same climate transition zanazzi et al explore the cooling in north america at the time using stable isotope measurements from fossil teeth and bones to create a proxy temperature record they find a large drop in mean annual temperature of 8 2 c a greater fall than seen in the oceans this continental transition may explain why many cold blooded reptiles and amphibians became extinct whereas mammals able to regulate their body temperature escaped relatively unscathed precisely dated evidence details that aridification on the tibetan plateau occurred at the time of the eocene oligocene transition in the marine records this suggests a direct link between the palaeontological and palaeoenvironmental changes recorded in continental asia at the time and global climate cooling continental aridification and the intensification of the monsoons in asia are generally attributed to uplift of the tibetan plateau and to the land sea redistributions associated with the continental collision of india and asia1 2 3 whereas some studies suggest that past changes in asian environments are mainly governed by global climate4 5 6 the most dramatic climate event since the onset of the collision of india and asia is the eocene oligocene transition an abrupt cooling step associated with the onset of glaciation in antarctica 34 million years ago7 8 9 however the influence of this global event on asian environments is poorly understood here we use magnetostratigraphy and cyclostratigraphy to show that aridification which is indicated by the disappearance of playa lake deposits in the northeastern tibetan plateau occurred precisely at the time of the eocene oligocene transition our findings suggest that this global transition is linked to significant aridification and cooling in continental asia recorded by palaeontological and palaeoenvironmental changes10 11 12 and thus support the idea that global cooling is associated with the eocene oligocene transition13 14 15 we show that with sufficient age control on the sedimentary records global climate can be distinguished from tectonism and recognized as a major contributor to continental asian environments

reverse modelling of detachment folds application to the pico del aguila anticline in the south central pyrenees spain abstract a simple method to estimate fold amplification and thrust movement rates for detachment folds is documented and illustrated by its application to a symmetrical detachment fold in the southern pyrenees spain the technique provides a complete record of the kinematic evolution of detachment folds and is based on the application of equations for detachment folds involving limb rotation the method uses the stratal pattern of the syntectonic sediments and assumes that these growth strata were deposited horizontally that the folds involve a homogeneous competent unit detached over a ductile horizon and that the folds can be represented by chevronkink bands the procedure is applicable to any detachment fold with associated growth strata that display wedge geometries progressive unconformities indicating limb rotation through time this method can be used for both detachment folds formed with constant limb length or variable limb length and it can also accommodate undecompacted or decompacted growth strata

constraints on the early uplift history of the tibetan plateau the surface uplift history of the tibetan plateau and himalaya is among the most interesting topics in geosciences because of its effect on regional and global climate during cenozoic time its influence on monsoon intensity and its reflection of the dynamics of continental plateaus models of plateau growth vary in time from pre india asia collision e g 100 ma ago to gradual uplift after the india asia collision e g 55 ma ago and to more recent abrupt uplift 7 ma ago and vary in space from northward stepwise growth of topography to simultaneous surface uplift across the plateau here we improve that understanding by presenting geologic and geophysical data from north central tibet including magnetostratigraphy sedimentology paleocurrent measurements and 40ar 39ar and fission track studies to show that the central plateau was elevated by 40 ma ago regions south and north of the central plateau gained elevation significantly later during eocene time the northern boundary of the protoplateau was in the region of the tanggula shan elevation gain started in pre eocene time in the lhasa and qiangtang terranes and expanded throughout the neogene toward its present southern and northern margins in the himalaya and qilian shan

magnetic properties of sedimentary greigite fe3s4 new data are presented here that document the previously poorly known magnetic characteristics of sedimentary greigite fe3s4 several diagnostic magnetic properties can be used in conjunction to rapidly screen sediments to detect the presence of greigite such discriminants are necessary because greigite displays a similar or higher range of coercivities compared to most ferrimagnetic minerals and the commonly used method of alternating field demagnetization cannot be used to distinguish between palaeomagnetic remanence components due to greigite and other ferrimagnetic minerals sedimentary greigite has high ratios of the saturation isothermal remanent magnetization to magnetic susceptibility mrs which are accompanied by hysteresis ratios of mrsms 0 5 and bcr bc 1 5 ms is the saturation magnetization bc is the coercive force and bcr is the coercivity of remanence greigite has no low temperature phase transition whereas the ferrimagnetic iron sulphide pyrrhotite undergoes a major phase transition at about 34 k and unoxidized magnetite displays the verwey transition at 118 k low temperature measurements may therefore prove valuable in distinguishing between magnetite pyrrhotite and greigite although the absence of a low temperature transition cannot be taken as definitive evidence for the presence of greigite greigite also displays characteristic high temperature behaviour with a major drop in magnetization between 270 and 350 c determination of the mrs ratio is an excellent means of rapidly screening sediments to identify stratigraphic intervals that may contain greigite subsequent low and high temperature analyses can then provide unambiguous identification of greigite and enable the evaluation of the presence of other magnetic minerals

cenozoic tectonic evolution of qaidam basin and its surrounding regions part 1 the southern qilian shan nan shan thrust belt and northern qaidam basin cenozoic qaidam basin the largest active intermountain basin inside tibet figures importantly in the debates on the history and mechanism of tibetan plateau formation during the cenozoic indo asian collision to determine when and how the basin was developed we conducted detailed geologic mapping and analyses of a dense network of seismic reflection profiles from the southern qilian shan nan shan thrust belt and northern qaidam basin our geologic observations provide new constraints on the timing and magnitude of cenozoic crustal thickening in northern tibet specifically our work shows that the southernmost part of the qilian shan nan shan thrust belt and contractional structures along the northern margin of qaidam basin were initiated in the paleocene early eocene 65 50 ma during or immediately after the onset of the indo asian collision this finding implies that stress was transferred rapidly through tibetan lithosphere to northern tibet from the indo asian convergent front located 1000 km to the south the development of the thrust system in northern qaidam basin was driven by motion on the altyn tagh fault as indicated by its eastward propagation away from the altyn tagh fault the eastward lengthening of the thrust system was spatially and temporally associated with eastward expansion of qaidam basin suggesting thrust loading was the main control on the basin formation and evolution the dominant structure in northern qaidam basin is a southwest tapering triangle zone which started to develop since the paleocene and early eocene 65 50 ma and was associated with deposition of an overlying southwest thickening growth strata sequence recognition of the triangle zone and its longevity in northern qaidam basin explains a long puzzling observation that cenozoic depocenters have been located consistently along the central axis of the basin this basin configuration is opposite to the prediction of classic foreland basin models that require the thickest part of foreland sediments deposited along basin edges against basin bounding thrusts restoration of balanced cross sections across the southern qilian shan nan shan thrust belt and northern qaidam basin suggests that cenozoic shortening strain is highly inhomogeneous varying from 20 to 60 both vertically in a single section and from section to section across the thrust belt the spatially variable strain helps explain the conflicting paleomagnetic results indicating different amounts of cenozoic rotations in different parts of qaidam basin the observed crustal shortening strain also implies that no lower crustal injection or thermal events in the mantle are needed to explain the current elevation 3000 3500 m and crustal thickness 45 50 km of northern qaidam basin and the southern qilian shan nan shan thrust belt instead thrusting involving continental crystalline basement has been the main mechanism of plateau construction across northern qaidam basin and the southern qilian shan nan shan region

evaluating the role of climate and tectonics during non steady incision of the yellow river evidence from a 1 24 ma terrace record near lanzhou china abstract the competing roles of bedrock uplift and climatic change in the formation of fluvial terraces remain uncertain most of recent studies have attributed terrace formation to climatic changes and held that even in tectonically active settings climate variations control cycles of terrace planation and abandonment based on field investigations of loess paleosol sequences magnetostratigraphy and optically stimulated luminescence osl dating we develop a new chronology for a spectacular flight of terraces along the yellow river near lanzhou china over past 1 24 ma all the terraces are strikingly similar in that they have several meters of paleosol developed directly above fluvial deposits on the terrace treads suggesting that the abandonment of each terrace due to river incision occurs during the transition from glacial to interglacial climates however the ages of terraces cluster in two relatively short time periods 1 24 0 86 ma and 0 13 ma present during the intervening time between 0 86 ma and 0 13 ma terraces either did not form or were not preserved we suggest that this record indicates that rock uplift rates varied through time and influenced terrace formation preservation thus our results demonstrate the utility of deep chronologic records from fluvial terraces for deconvolving the effects of tectonics and climate on fluvial incision

geometry and kinematics of the arlar strike slip fault sw qaidam basin china new insights from 3 d seismic data abstract the right stepping en echelon kunbei fault system consisting of the hongliuquan arlar xiii and kunbei faults features importantly in the tectonic evolution of the southwest qaidam basin the 3d seismic cross section interpretation and key horizon similarity attribute analysis show that the nww striking arlar fault is composed of two steeply dipping faults f1 and f2 in cross sectional view and branches eastward into two west dipping faults f5 and f6 in map view the fact that the strata above t 2 the base of the early miocene xiayoushashan formation become dramatically thinner toward the high points of the arlar fault suggests an early miocene initiation ca 22 0 ma for this fault as indicated by the windowed amplitude related attribute maps the arlar fault and f5 one branch of the arlar fault have cumulative sinistral offsets of 8 6 8 7 km and 5 2 5 4 km respectively these considerably greater strike slip components in conjunction with the respective maximum true dip slip displacements of