constant elevation of southern tibet over the past 15 million years the uplift of the tibetan plateau an area that is 2 000 km wide to an altitude of about 5 000 m has been shown to modify global climate and to influence monsoon intensity mechanical and thermal models for homogeneous thickening of the lithosphere make specific predictions about uplift rates of the tibetan plateau but the precise history of the uplift of the plateau has yet to be confirmed by observations here we present well preserved fossil leaf assemblages from the namling basin southern tibet dated to approximately 15 myr ago which allow us to reconstruct the temperatures within the basin at that time using a numerical general circulation model to estimate moist static energy at the location of the fossil leaves we reconstruct the elevation of the namling basin 15 myr ago to be 4 689 895 m or 4 638 847 m depending on the reference data used this is comparable to the present day altitude of 4 600 m we conclude that the elevation of the southern tibetan plateau probably has remained unchanged for the past 15 myr

provenance of neoproterozoic and early paleozoic siliciclastic rocks of the tepla barrandian unit bohemian massif evidence from u pb detrital zircon ages abstract new la icp ms u pb detrital zircon ages from ediacaran and paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the tepla barrandian unit tbu in the centre of the bohemian massif central europe czech republic the samples taken span the period from 635 ma to 385 ma and permit recognition of provenance changes that reflect changes in geotectonic regime detrital zircon age spectra of two ediacaran one lower cambrian and three upper ordovician samples resemble the ages known from the nw african proportion of gondwana particularly the trans saharan belt while three rocks from higher lower cambrian to lowermost ordovician strata contain detritus that may have been derived exclusively from local sources the age spectrum of the devonian rock is a combination of the nw gondwanan and local features these new findings in combination with a wide range of published data are in agreement with a neoproterozoic subduction related setting at the margin of gondwana followed by a cambrian early ordovician rifting stage and an ordovician passive margin setting furthermore the data are in favour of a position of the tbu at the gondwanan margin throughout pre variscan times

eocene north south trending dikes in central tibet new constraints on the timing of east west extension with implications for early plateau uplift east west extension has been a prominent feature of cenozoic tectonics in central southern tibet and the onset of this extension has been interpreted as indicating the surface uplift of the tibetan plateau to a critical level previous studies suggested that extension started in the neogene between 18 13 ma and 8 ma based on dating results from north south trending normal faults grabens or rifts and dikes we report here the discovery of widespread north south trending diabase and andesitic porphyry dikes in and around the shuang hu graben central tibet where crustal extension has previously been argued to start from 13 5 ma based on mineral ages from graben bounding faults our results show that dikes in this area were mainly formed in the eocene 47 38 ma geochemical and nd sr isotopic data suggest that the diabase dikes originated from partial melting of an enriched lithospheric mantle source metasomatized by melts from subducted continental crust and the andesitic porphyry dikes were probably derived from interactions between the subducted continental crust derived melts and the mantle given that north south trending eocene dikes are found widely dispersed in central tibet we argue that the onset of east west extension and likely regional uplift of the tibetan plateau began much earlier than previously thought our study implies that a large part of the tibetan plateau had already attained near maximum elevation in the eocene and is consistent with recent investigations regarding the middle late eocene 47 34 ma initiation of aridification in asia evidence from contemporary global oceanic o sr isotope curves and temperatures and the carbon cycle 2010 elsevier b v

the vast proto tibetan plateau new constraints from paleogene hoh xil basin abstract the surface uplift of the tibetan plateau is the key boundary condition in many cenozoic geological events ranging from global cooling to changes of asian environments during cenozoic however poorly constrained timing for the uplift of tibetan plateau makes these interpretations highly debatable here we report results from sedimentology detrital zircon u pb and lu hf isotopic compositions and paleomagnetic signatures from both the eastern and western hoh xil basins of north central tibetan plateau sedimentary lithofacies and facies associations analyzed in the western hoh xil basin indicate they were deposited in a braided fluvial system and alluvial fan similar with the fenghuoshan group eastern hoh xil basin provenance analyses from conglomerate clast compositions paleocurrent orientations and detrital zircon u pb and lu hf isotopic compositions document sediments in both western and eastern basins were derived from the qiangtang and lhasa blocks these observations in combination with comparative paleomagnetic results imply that hoh xil basin was a single wide basin during paleogene the period of hoh xil basin deposition was coeval with significant period of the early cenozoic uplift and erosion of the qiangtang and lhasa blocks these observations not only reinforce the suggestion that the qiangtang and lhasa blocks were uplifted during eocene to form a proto tibetan plateau but also imply that the proto tibet plateau is vast in areal extent the large dimension and high elevation of the proto tibetan plateau probably contributed to the global cooling during the early eocene

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

palaeomagnetism and 40ar 39ar geochronology of upper palaeogene volcanic rocks from central tibet implications for the central asia inclination anomaly the palaeolatitude of tibet and post 50 ma shortening within asia summary palaeomagnetic results from upper palaeogene volcanics from three localities in the qiangtang terrane of central tibet record a maximum likelihood palaeolatitude of 28 7 3 7 n these are the first volcanic based palaeomagnetic results from the plateau interior that appear to average secular variation well enough to yield a reliable palaeolatitude estimate this palaeolatitude estimate is significantly lower than the 36 3 n predicted for central tibet by late palaeogene eurasia reference poles consistent with the so called central asia inclination anomaly but it is consistent with values predicted from upper palaeogene reference poles for mongolia the mean inclination of coeval sedimentary rocks from central tibet is 15 5 shallower than our volcanic results similar to the magnitude of sedimentary inclination shallowing estimated from elongation inclination analysis of remanent directions in clastic rocks from the same region we conclude that most of the inclination anomaly observed in volcanic rocks and that which remains after removal of sedimentary flattening in clastic rocks probably is due to the combined effect of reference poles that are not representative of stable asia and by cenozoic intracontinental shortening within central asia on the order of several hundred not more than 1000 kilometers our compilation of palaeomagnetic data from the qiangtang terrane further suggest that the southern margin of the lhasa terrane the southern margin of pre india asia was located as far south as 20 n throughout eocene time these palaeolatitudes in conjunction with the upper crustal shortening record for asia indicate that only approximately one third of the post 50 ma india asia convergence is partitioned in asian lithosphere low latitudes also imply that much of the proto tibetan plateau occupied southerly tropical to subtropical latitudes which has important implications for the weathering and erosion of the tibetan plateau palaeoaltimetry proxies and early cenozoic climate dynamics

evidence for tibetan plateau uplift before 14 myr ago from a new minimum age for east west extension important changes in south asian climate occurred in the late miocene epoch 8 myr ago 1 2 and these have been attributed by some researchers to uplift of the tibetan plateau at about the same time3 5 unfortunately this link has been difficult to test because the timing of plateau uplift remains poorly constrained by independent evidence one way to determine the minimum age of uplift is to establish the initiation age of the north striking normal fault systems in southern tibet that are widely regarded6 10 as being related to gravitational collapse of the tibetan plateau here we report an40ar 39ar age of 14 myr for hydrothermal mica from an extensional fracture belonging to such a fault system in north central nepal this age implies that east west extension began before 14 myr ago in at least some parts of the tibetan plateau suggesting that the plateau attained its high mean elevation well before late miocene time

metamorphic rocks in central tibet lateral variations and implications for crustal structure insights about lateral variations in the age composition and structure of the central tibetan crust are provided by geologic investigations of metamorphic rocks in the qiangtang terrane previous studies have shown that a tectonic melange of triassic age with blueschist and eclogite bearing blocks within a greenschist facies matrix is exposed over an e w distance of 600 km in the central qiangtang terrane new mapping shows that the melange extends over a n s distance of 150 km nearly to the trace of the early mesozoic jinsha suture in the north the melange exposed structurally beneath upper paleozoic to mesozoic strata in the footwalls of early mesozoic normal faults is composed mostly of paleozoic metasedimentary and crystalline rocks these findings support the hypothesis that a large part of the central and northern qiangtang terrane crust is composed of supracrustal rocks the duguer range

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

sr isotope evolution of seawater the role of tectonics we use a high resolution seawater sr isotopic evolution curve for the last 100 m y in conjunction with modern riverine sr flux measurements and also geologic tectonic and geochronological data to make the case for a close relationship between seawater sr isotopic composition and the india asia continental collision using a simple seawater sr budget model we begin by showing that the sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater sr isotopic curve of the last 100 m y the flux of dissolved sr carried by rivers originating in the himalaya tibet region on the other hand is presently a significant fraction of the global sr budget we calculate how this riverine flux would have had to change with time in order to match the observed seawater sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of india with asia but then increases very significantly after collision we note that the period of most rapid change in seawater sr isotopic ratio from 20 ma to 15 ma is also a period of exceptionally high erosion in parts of the himalayas and the tibetan plateau as further evidence that sr derived from the collision of india with asia plays a major role in the sr isotopic evolution of seawater we show that the total amount erosion of the himalaya tibetan plateau since collision which we calculate separately represents a total amount of sr that is very nearly the same as the cumulative amount required by the sr isotopic change of seawater since collision the relationship between erosion and riverine sr flux allows us to use the sr isotopic evolution of seawater to reconstruct a history of erosion since collision and we find that the erosion rate accelerates with time since collision with the present having the largest rate when we apply the sr budget model to the entire phanerozoic using a new compilation of deformed continental area versus time we find that we can account for the large scale structure of the seawater sr isotopic curve but fail to reproduce several local maxima and minima especially in the period 100 300 ma the present high s7sr s6sr of seawater and similar highs in the devonian and cambrian do correlate with extensive deformation on the continents

the interplay between climate and tectonics during the upward and outward growth of the qilian shan orogenic wedge northern tibetan plateau abstract mountains grow upward and outward when the accretion of material outpaces its removal mountain building is synergistically influenced by geodynamics internal structures crustal and lithospheric and climate the northward growth of the qilian shan which occupies a transition zone between the 4500 m high central tibetan plateau and the adjacent

the middle eocene to early miocene integrated sedimentary record in the qaidam basin and its implications for paleoclimate and early tibetan plateau uplift detailed analysis of whole rock geochemistry clay minerals sedimentary color and pollen in the dahonggou section northeast of the qaidam basin are investigated and the results suggest an intense weathering in the source area during the middle eocene 48 5 40 5 ma indicating a warm and humid condition the distinct decrease of chemical weathering degree in source regions began at 40 5 ma which is in agreement with the distinct decrease in redness of sedimentary sequences and the disappearance of thermophilic elements in pollen records this 40 5 ma cooling event extent demonstrated evidence for an intensification of central asian aridification which could be attributed to attainment of high elevations in southern central tibet and retreat of the paratethys from central asia in the late eocene reducing moisture transport to the qaidam basin

latest marine horizon north of qomolangma mt everest implications for closure of tethys seaway and collision tectonics the newly discovered marine horizon at zhepure syncline north of qomolangma revises timing of the closure of the eastern neo tethys seaway in the central himalayas the marine pengqu formation conformably overlies a lutetian shallow water carbonate platform and comprises shales interbedded with sandstones deposited in a neritic shelf environment the strata are dated by nannofossils and foraminifera as late early lutetian to late priabonian age np15 np20 deposited 47 34 ma indicating that the final closure of the tethys seaway in this region occurred at 34 ma the newly discovered strata provide evidence about emplacement of a major thrust sheet south of southern tibet prior to 37 ma which affected the regional climate

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

late cretaceous to middle tertiary basin evolution in the central tibetan plateau changing environments in response to tectonic partitioning aridification and regional elevation gain located in the bangong suture zone between the lhasa and qiangtang terranes of central tibet the nima basin records cretaceous late miocene sediment accumulation during a period of drastically changing paleogeography and paleoelevation the bangong suture zone originally formed during late jurassic early cretaceous time as the lhasa terrane collided with the qiang tang terrane during early to middle cretaceous time the region lay at northern near equatorial paleolatitudes near sea level by aptian time the nima basin was above sea level and was strongly influenced by nearby volcanic activity and crustal shortening in the reactivated bangong suture zone in the southern nima basin an 50 m y late cretaceous eocene depositional hiatus correlates with major crustal shortening and ensuing voluminous ignimbrite eruptions in the lhasa terrane in the northern nima basin deposition continued during latest cretaceous time recording arid paleoclimate in evaporitic lacustrine and eolian dunefield deposits by oligocene time the nima basin comprised two independent depocenters that accumulated coarse grained alluvial fluvial lacustrine evaporitic and fan delta deposits in close association with reactivated thrusts in the bangong suture zone carbon and oxygen isotope data from oligocene paleosol carbonate reported elsewhere indicate that regional paleoelevation during the late oligocene was 4 6 km as it is today overall the depositional record of the nima basin combined with ongoing structural and geochronologic studies demonstrates that the bangong suture zone was reactivated during middle cretaceous and middle tertiary time that the intervening 50 m y interval was a time of regional upper crustal shortening in the lhasa terrane followed by regional ignimbrite eruptions and that arid paleoclimate and high paleoelevation were established by the late cretaceous and late oligocene respectively within the context of other data sets from the lhasa terrane the record of deposition in the nima basin is consistent with low angle subduction of neotethyan oceanic lithosphere and reactivation of the bangong suture zone during the early cretaceous followed by shortening within the gangdese retroarc and northern lhasa terrane thrust belts during the late cretaceous lithospheric delamination dripping and regional magmatic flare up during latest cretaceous through early tertiary time and underthrusting of indian lower crust and lithosphere as far north as the bangong suture zone during late oligocene time

exhumation history of the gangdese batholith southern tibetan plateau evidence from apatite and zircon u th he thermochronology abstractto test previously suggested exhumation histories of the gangdese batholith in the central part of the transhimalayan plutonic belt we conducted paired apatite and zircon u th he thermochronological investigations of the yarlung zangbo gorge in the central part of the batholith age elevation relationships and multisystem thermochronometers showed three periods of accelerated exhumation 46 48 22 18 and 11 8 ma combining these data with previously published thermochronological ages and synthesizing these ages with regional geological events provides an entire exhumation history the cretaceous early paleogene exhumation of the gangdese batholith was probably caused by both the northward subduction of the neo tethys and the collision between the lhasa and qiangtang blocks the early miocene rapid exhumation might be a response to shortening caused by the gangdese thrust or erosion driven by dynamic uplift following lithospheric delamination in contrast the late miocene exhumation is coi

peneplain formation in southern tibet predates the india asia collision and plateau uplift the uplift history of tibet is crucial for understanding the geodynamic and paleoclimatologic evolution of asia however it remains controversial whether tibet attained its high elevation before or after india collided with asia 50 m y ago here we use thermochronologic and cosmogenic nuclide data from a large bedrock peneplain in southern tibet to shed light on the timing of the uplift the studied peneplain which was carved into cretaceous granitoids and jurassic metasediments is located in the northern lhasa block at an altitude of 5300 m thermal modeling based on u th he ages of apatite and zircon and apatite fission track data indicate cooling and exhumation of the granitoids between ca 70 and ca 55 ma followed by a rapid decline in exhumation rate from 300 m m y to 10 m m y between ca 55 and ca 48 ma since then the peneplain has been a rather stable geomorphic feature as indicated by low local and catchment wide erosion rates of 6 11 m m y and 11 16 m m y respectively which were derived from cosmogenic 10 be concentrations in bedrock grus and stream sediment the prolonged phase of erosion and planation that ended ca 50 ma removed 3 6 km of rock from the peneplain region likely accomplished by laterally migrating rivers the lack of equivalent sediments in the northern lhasa block and the presence of a regional unconformity in the southern lhasa block indicate that the rivers delivered this material to the ocean this implies that erosion and peneplanation proceeded at low elevation until india9s collision with asia induced crustal thickening surface uplift and long term preservation of the peneplain

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

detrital zircon provenance comparison between the paleocene eocene nangqian xialaxiu and gongjue basins new insights for cenozoic paleogeographic evolution of the eastern tibetan plateau abstract paleogeographic reconstructions of terranes can greatly benefit from the provenance analysis of sediments a series of cenozoic basins provide key sedimentary archives for investigating the growth of the tibetan plateau yet the provenance of the sediments in these basins has never been constrained robustly here we report sedimentary petrological and detrital zircon geochronological data from the paleocene eocene nangqian xialaxiu and gongjue basins sandstone detrital modes and zircon morphology suggest that the samples collected in these two basins were sourced from recycled orogen detrital zircon geochronology indicates that sediments in the nangqian xialaxiu basin are characterized by two distinct age populations at 220 280 ma and 405 445 ma in contrast three predominant age populations of 207 256 ma 423 445 ma and 1851 1868 ma and two subordinate age populations of 50 ma and 2500 ma are recognized in the gongjue basin comparison with detrital zircon ages from the surrounding terranes suggests that sediments in the nangqian xialaxiu basin come from the neighboring thrust belts whereas sediments from the gongjue basin are predominantly derived from the distant songpan ganzi terrane with minor contribution from the surrounding areas a three stage cenozoic evolution of the eastern tibetan plateau is proposed during the paleocene the nangqian xialaxiu basin appeared as a set of small intermontane sub basins and received plentiful sediments from the neighboring mountain belts during the eocene the gongjue basin kept a relatively low altitude and was a depression at the edge of a proto plateau since the oligocene the tibetan plateau further uplifted and the marginal gongjue basin was involved in the tibetan interior orogeny indicating the eastward propagation of the tibetan plateau

mesozoic and cenozoic tectonic evolution of the shiquanhe area of western tibet 1 in the shiquanhe area of far western tibet mid cretaceous strata lie unconformable on ophiolitic melange and jurassic flysch associated with the bangong nujiang suture zone on the basis of our mapping and geochronologic studies we suggest that these cretaceous strata were shortened by 57 over a north south distance of 50 km during late cretaceous early tertiary time the late cretaceous narangjiapo thrust placed permian strata 20 km over ophiolitic melange and cretaceous strata north of the narangjiapo thrust 40 km of shortening was accommodated by the late cretaceous early tertiary south directed jaggang thrust system that involves jurassic flysch and cretaceous strata and roots into a decollement within ophiolitic melange the most recent shortening was accommodated to the south of the narangjiapo thrust along the north dipping shiquanhe thrust the shiquanhe thrust cuts flat lying 22 6 0 3 ma volcanic rocks and underlying folded tertiary nonmarine strata in its footwall and was likely active during slip along the oligocene gangdese thrust system of southern tibet ophiolitic melange and structurally overlying jurassic flysch near shiquanhe are interpreted to represent remnants of a subduction accretion complex and forearc basin respectively that were obducted southward onto the margin of the lhasa terrane during late jurassic early cretaceous closure of the bangong nujiang ocean subsequent imbrication of the obducted sheet could have produced the two east west trending belts of ophiolitic melanges separated by 100 km in western tibet late cretaceous early tertiary thin skinned shortening may have been accommodated in the deeper crust by northward underthrusting and duplexing of lhasa terrane rocks beneath the obducted ophiolitic melange and the qiangtang terrane to the north

combined paleomagnetic and geochronological study on cretaceous strata of the qiangtang terrane central tibet abstract a combined paleomagnetic and geochronological investigation has been performed on cretaceous rocks in southern qiangtang terrane 32 5 n 84 3 e near gerze central tibetan plateau a total of 14 sites of volcanic rocks and 22 sites of red beds have been sampled our new u pb geochronologic study of zircons dates the volcanic rocks at 103 8 0 46 ma early cretaceous while the red beds belong to the late cretaceous rock magnetic experiments suggest that magnetite and hematite are the main magnetic carriers after removing a low temperature component of viscous magnetic remanence stable characteristic remanent magnetization chrm was isolated successfully from all the sites by stepwise thermal demagnetization the tilt corrected mean direction from the 14 lava sites is d 348 0 i 47 3 k 51 0 95 5 6 corresponding to a paleopole at 79 3 n 339 8 e a 95 5 7 and yielding a paleolatitude of 29 3 5 7 n for the study area the chrm directions isolated from the volcanic rocks pass a fold test at 95 confidence suggesting a primary origin the volcanic data appear to have effectively averaged out secular variation as indicated by both geological evidence and results from analyzing the virtual geomagnetic pole vgp scatter the mean inclination from the late cretaceous red beds however is 13 1 shallower than that of the 100 ma volcanic rocks after performing an elongation inclination analysis on 174 samples of the red beds a mean inclination of 47 9 with 95 confidence limits between 41 9 and 54 3 is obtained which is consistent with the mean inclination of the volcanic rocks the site mean direction of the late cretaceous red beds after tilt correction and inclination shallowing correction is d 312 6 i 47 7 k 109 7 95 3 0 n 22 sites corresponding to a paleopole at 49 2 n 1 9 e a 95 3 2 yielding a paleolatitude of 28 7 3 2 n for the study area the chrm of the red beds also passes a fold test at 99 confidence indicating a primary origin comparing the paleolatitude of the qiangtang terrane with the stable asia there is no significant difference between our sampling location in the southern qiangtang terrane and the stable asia during 100 ma and late cretaceous our results together with the high quality data previously published suggest that an 550 km n s convergence between the qiangtang and lhasa terranes happened after 100 ma comparison of the mean directions with expected directions from the stable asia indicates that the gerze area had experienced a significant counterclockwise rotation after 100 ma which is most likely caused by the india asia collision

from alluvial fan to playa an upper jurassic ephemeral fluvial system neuquen basin argentina abstract the upper jurassic fill of the back arc neuquen basin includes a lowstand wedge known as the tordillo formation the studied deposits crop out along a n s oriented belt that runs parallel to the andean magmatic arc they are limited to the south by the east west oriented positive structure of the huincul arch formed as a result of upper jurassic tectonic inversion the tordillo deposits were formed in an arid fluvial dominated system characterised by systematic downstream changes in architectural style a gravelly and sandy bedload fluvial system is recognised in the southern upstream sector the reduced thickness and the coarse grain size suggest steep gradients excess of bedload supply and a low subsidence rate thicker and finer grained deposits prevail to the north and northeast they were formed under arid conditions in a wadi sand flat playa fluvial system this distal facies association indicates increased accommodation owing to high rates of subsidence relative to coarse siliciclastic sedimentation rates these low gradient deposits are characterised by cyclic alternations of mud dominated and sand dominated packages interpreted as high and low accommodation systems tracts the overall fining upward stacking pattern of the tordillo formation suggests a change towards higher accommodation rates this is accompanied by frequent development of soil horizons and darker primary and reworked pyroclastic deposits these attributes indicate a stronger explosive volcanic activity associated with increased precipitation and high water table emplacement towards the end of the tordillo lowstand wedge

triassic continental subduction in central tibet and mediterranean style closure of the paleo tethys ocean the qiangtang metamorphic belt qmb in central tibet is one of the largest and most recently documented high pressure hp to near ultrahigh pressure near uhp belts on earth lu hf ages of eclogite and blueschist facies rocks within the qmb are 244 223 ma indistinguishable from the age of uhp metamorphism in the qinling dabie orogen results of a u pb detrital zircon study suggest that protoliths of the qmb include upper paleozoic qiangtang continental margin strata and sandstones that were derived from a paleozoic arc terrane that developed within the paleo tethys ocean to the north we attribute qmb hp metamorphism to continental collision between the qiangtang terrane and a paleo tethys arc terrane this collision and the coeval south china north china collision may have slowed convergence between laurasia and gondwana derived terranes and initiated mediterranean style rollback and backarc basin development within much of the remnant paleo tethys ocean realm

facies analysis and depositional systems of cenozoic sediments in the hoh xil basin northern tibet abstract a sedimentary succession more than 5800 m thick including the lower eocene to lower oligocene fenghuoshan group the lower oligocene yaxicuo group and the lower miocene wudaoliang group is widely distributed in the hoh xil piggyback basin the largest cenozoic sedimentary basin in the hinterland of the tibetan plateau the strata of the fenghuoshan and yaxicuo groups have undergone strong deformation whereas only minor tilting has occurred in the wudaoliang group we analyze their sedimentary facies and depositional systems to help characterize continental collision and early uplift of the tibetan plateau the results indicate fluvial lacustrine and fan delta facies for the fenghuoshan group fluvial and lacustrine facies for the yaxicuo group and lacustrine facies for the wudaoliang group development of the hoh xil basin underwent three stages 1 the fenghuoshan group was deposited mainly in the fenghuoshan hantaishan sub basin between 56 0 and 31 8 ma ago 2 the yaxicuo group was deposited mainly in the wudaoliang and zhuolai lake sub basins between 31 8 and 30 0 ma ago and 3 the wudaoliang group was deposited throughout the entire hoh xil basin during the early miocene the fenghuoshan and yaxicuo groups were deposited in piggyback basins during the early eocene to early oligocene whereas the wudaoliang group was deposited in a relatively stable large lake the hoh xil basin underwent two periods of strong north south shortening which could have been produced by the collision between india and asia and the early uplift of the tibetan plateau the study suggests the hoh xil region could reach a high elevation during the late oligocene and the diachronous uplift history for the tibetan plateau from east to west

the late triassic rift related volcanic rocks from eastern qiangtang northern tibet china age and tectonic implications abstract the late triassic nadi kangri volcanic rocks with nearly ew trending outcrops within the qiangtang basin northern xizang tibet china are composed mainly of acid tuff dacite rhyolite and minor basic volcanic rocks there exists a significant depositional hiatus between the nadi kangri volcanic rocks and underlying strata therefore the nadi kangri volcanic rocks represent a new evolution history of the mesozoic qiangtang basin the magma emplacement age of the nadi kangri volcanic rocks in the geladaindong area is 220 4 2 3 ma representing the onset of the mesozoic qiangtang basin the nadi kangri basalts have high nb zr 0 049 0 058 ta hf 0 12 0 15 and zr y 4 95 6 01 ratios in the tectonic discrimination diagrams such as zr vs zr y and th hf vs ta hf the nadi kangri basaltic rocks mostly plot in the within plate setting field the geological background and the geochemical characteristics suggest that the nadi kangri volcanic rocks were formed in a continental rift setting

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

eocene to late oligocene history of crustal shortening within the hoh xil basin and implications for the uplift history of the northern tibetan plateau the timing and magnitude of deformation across the northern tibetan plateau are poorly constrained but feature prominently in geodynamic models of the plateau s evolution the fenghuoshan fold and thrust belt located in the hoh xil basin provides a valuable record of the cenozoic deformation history of the northern tibetan plateau here we integrate fault gouge geochronology low temperature thermochronology geologic mapping and a balanced cross section to resolve the deformation history of hoh xil basin chronologic data suggest that deformation initiated in the mid eocene continued until at least 34 ma and ceased by 27 ma the balanced cross section resolves 34 12 km upper crustal shortening 24 9 we explore whether the observed cenozoic shortening can account for the modern elevation and lithospheric thickness in the northern tibetan plateau for a range of reasonable preshortening conditions we conclude that the observed shortening alone cannot achieve modern crustal and mantle lithospheric thicknesses or modern elevation without either the removal of lithospheric mantle the influx of lower crustal material or some combination of these processes our results along with previous studies suggest that crustal shortening propagated into the northern tibetan plateau shortly after the onset of the indo asian collision the small magnitude of shortening and the late oligocene cessation of deformation in the northern tibetan plateau raise questions of how and where the remaining indo asian convergence was accommodated between eocene to mid miocene time prior to the approximately late miocene establishment of the deformation patterns observed in the present day

middle miocene paleoaltimetry of southern tibet implications for the role of mantle thickening and delamination in the himalayan orogen the stable isotope composition of pedogenic and early diagenetic carbonates from the oiyug basin of southern tibet allows model estimates of the paleoaltimetry of the tibetan plateau for the middle miocene pedogenic calcium carbonate nodules have average d 18 occ values of 219 6 whereas nodular lacustrine dolomites range in composition from 27 6 to 25 5 the most negative of the carbonate isotope values can be used to constrain the oxygen isotope composition of paleoprecipitation from which model estimates of paleoaltimetry can be made model results indicate that the southern tibetan plateau achieved elevations of 5200 11370 2605 m by at least 15 ma our results are identical within uncertainty to previous workers paleoelevation estimates based on oiyug basin fossil floral physiognomy this is the first time that two paleoaltimeters have been directly compared and are in accord collectively these data strongly support tectonic models in which thickening of mantle lithosphere beneath the domain of crustal thickening and subsequent detachment of the mantle lithosphere plays an indiscernible role in the elevation history of this part of the himalaya tibet orogenic system

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

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

cretaceous to cenozoic evolution of the northern lhasa terrane and the early paleogene development of peneplains at nam co tibetan plateau abstract highly elevated and well preserved peneplains are characteristic geomorphic features of the tibetan plateau in the northern lhasa terrane north northwest of nam co the peneplains were carved in granitoids and in their metasedimentary host formations we use multi method geochronology zircon u pb and u th he dating and apatite fission track and u th he dating to constrain the post emplacement thermal history of the granitoids and the timing and rate of final exhumation of the peneplain areas la icp ms u pb geochronology of zircons yields two narrow age groups for the intrusions at around 118 ma and 85 ma and a third group records paleocene volcanic activity 63 58 ma in the nam co area the low temperature thermochronometers indicate common age groups for the entire nam co area zircon u th he ages cluster around 75 ma apatite fission track ages around 60 ma and apatite u th he ages around 50 ma modelling of the thermochronological data indicates that exhumation of the basement blocks took place in latest cretaceous to earliest paleogene time by middle eocene time the relief was already flat documented by a thin alluvial sediment sequence covering a part of the planated area the present day horst and graben structure of the peneplains is a late cenozoic feature triggered by e w extension of the tibetan plateau the new thermochronological data precisely bracket the age of the planation to early eocene i e between ca 55 and 45 ma the erosional base level can be deduced from the presence of early cretaceous zircon grains in eocene strata of bengal basin the sediment generated during exhumation of the nam co area was transported by an early cenozoic river system into the ocean suggesting that planation occurred at low elevation

paleoaltimetry of the tibetan plateau from d h ratios of lipid biomarkers abstract the past elevation of the land surface provides a unique constraint on the underlying lithospheric structure during mountain and plateau formation development of new paleoaltimetry techniques that can be applied to a wide variety of sample types is therefore of continuing importance this study evaluates organic substrates that preserve the d ratio of surface waters as a new approach to reconstruct paleoaltimetry we measured the hydrogen isotope composition of n alkanes from epicuticular plant waxes preserved in lacustrine deposits to reconstruct the d of precipitation in cenozoic basins that have been elevated as part of the tibetan plateau n alkane d and carbonate 18 o inferred water compositions from the eocene miocene lunpola basin and miocene hoh xil basin plot near or at enriched values relative to the global meteoric water line as expected for evaporative lakewater and leafwater systems that have the same precipitation source n alkane d based water compositions are nearly identical to the minimum carbonate 18 o based values demonstrating that plant wax d is minimally affected by evaporation compared to lacustrine calcite 18 o this agreement strongly supports the presence of similar precipitation isotopic compositions in both archives despite different isotope systems source water reservoirs archive materials modes of incorporation and diagenetic processes paleoelevations for each basin and time period were calculated from precipitation isotope ratios using the isotope altitude relationship derived from both a simple thermodynamic model and modern precipitation sampling from the plateau region our new results from the hoh xil basin suggest 1700 to 2600 m of uplift may have occurred some time between the late eocene and early miocene the timing of this uplift is consistent with late oligocene compressional deformation of the hoh xil basin and northward growth of the tibetan plateau however the calculated uplift is not a unique solution from the paleoisotope data because of uncertainties in eocene and miocene moisture sources and isotope gradients for the northern plateau our results demonstrate the utility of lipid based estimates of paleoelevation and expand the types of deposits amenable to paleoaltimetry analysis

provenance analysis of upper cretaceous strata in the tethys himalaya southern tibet implications for timing of india asia collision abstract the provenance of upper cretaceous strata in the tethys himalaya provides critical constraints on the closure time of the neo tethys ocean and the initial india asia collision this paper reports detailed petrographic studies in situ detrital zircon u pb ages and lu hf isotopic analyses whole rock nd isotopes and cr spinel electronic microprobe data from upper cretaceous clastic sedimentary rocks of the tethys himalaya near gyangze southern tibet the berriasian coniacian jiabula formation consists of black mudstone chert and minor quartz arenite and is dominated by detrital zircons with archean cambrian u pb ages which were most likely derived from the indian continent overlying conformably is the santonian maastrichtian chuangde formation which consists of red shale limestone and chert the chuangde formation is in turn overlain by the late maastrichtian late paleocene zongzhuo formation which is composed of black mudstone and lithic sandstone enclosing various olistoliths of sandstone limestone and chert the rilang conglomerate is a lens which is located within the upper part of the zongzhuo formation and consists of an upward thinning and fining succession of volcanic conglomerate sandstone and black mudstone the zongzhuo formation and the rilang conglomerate record an abrupt influx of cretaceous zircon grains with juvenile hf isotopic compositions arc related cr spinels and positive end 0 sediments suggesting an arc and suture zone provenance the change in provenance of upper cretaceous strata from the southern indian continent to a northern arc and suture zone is attributed to either 1 initial collision between the indian plate and lhasa terrane or 2 initial collision between the indian plate and an intra oceanic arc we prefer option 1 that the initiation of india asia collision occurred during maastrichtian 70 65 ma

the paleoaltimetry of tibet an isotopic perspective stable isotopes provide a valuable perspective on the timing of elevation change of the tibetan plateau we begin our paper by looking in depth at isotopic patterns in modern tibet we show that the 18 o value of surface waters decreases systematically up the himalayan front in central nepal by about 2 8 km in agreement with the patterns documented and modeled by previous research on the tibetan plateau itself there is no apparent correlation between elevation and the 18 o value of flowing surface waters both surface waters and soil carbonates display a northward increase in 18 o values of about 1 5 north of the himalayan crest even though elevation increases modestly the isotopic increase with latitude reduces the isotope elevation gradient for water in the northernmost plateau to 1 to 2 km carbonates in both soils and lakes form at higher temperatures than assumed by previous studies on the plateau temperature estimates from clumped isotope 47 analyses of modern soil carbonates significantly exceed mean annual air t and modeled maximum summer soil temperatures by 15 8 2 8 and 9 7 2 5 c respectively similarly elevated temperatures best account for the 18 o values observed in modern soil and lake carbonates we recalculated paleoelevations from previous studies on the plateau using both higher formation temperatures and latitude corrected isotopic values with one notable exception our revised model produces paleoelevation estimates very close to previous estimates the exception is the reconstruction from late eocene age deposits at xoh xil for which we calculate elevations that are higher and much closer to the current elevation than previously reconstructed therefore there is no evidence for northward progression through time of tibetan elevation change instead the available but admittedly very scanty evidence suggests that much of tibet attained its modern elevation by the mid eocene a truly robust test of the various geodynamic models of uplift await expansion and replication of isotopic records all across tibet especially in the center and north and for 15 ma

did the indo asian collision alone create the tibetan plateau it is widely believed that the tibetan plateau is a late cenozoic feature produced by the indo asian collision however because tibet was the locus of continental accretion and subduction throughout the mesozoic crustal thickening during that time may also have contributed to growth of the plateau this portion of the geologic history was investigated in a traverse through the central lhasa block southern tibet together with earlier studies our mapping and geochronological results show that the lhasa block underwent little north south shortening during the cenozoic rather our mapping shows that 60 crustal shortening perhaps due to the collision between the lhasa and qiangtang blocks occurred during the early cretaceous this observation implies that a significant portion of southern tibet was raised to perhaps 3 4 km elevation prior to the indo asian collision

late cretaceous evolution of the coqen basin lhasa terrane and implications for early topographic growth on the tibetan plateau the tectonic evolution of the lhasa terrane southern tibetan plateau played a fundamental role in the formation of the tibetan plateau however many uncertainties remain with regard to the tectonic and paleogeographic evolution of the lhasa terrane prior to the india asia collision to determine the early tectonic processes that controlled the topographic evolution of the lhasa terrane we analyze the cretaceous strata exposed in the coqen basin northern lhasa subterrane which comprises the langshan and daxiong formations the langshan formation unconformably overlies the volcanic rocks of the lower cretaceous zelong group and consists of similar to 80 m of orbitolina bearing limestones which were deposited in a low energy shallow marine environment micropaleontological analysis indicates that the langshan formation in the coqen basin was deposited from late aptian to early cenomanian times ca 113 96 ma the overlying daxiong formation similar to 1700 m thick consists of conglomerate coarse sandstone and siltstone with interbedded mudstone and represents deposits of alluvial fans and braided rivers the daxiong formation was deposited after the early cenomanian ca 96 ma and accumulated until at least ca 91 ma indicating accumulation rates of greater than 0 3 km m y 1 by combining paleocurrent data sandstone petrology detrital zircon u pb ages and hf isotope analysis we demonstrate that the daxiong formation was derived from lower cretaceous volcanic rocks and pre cretaceous strata in the northern lhasa subterrane during late cretaceous time two thrust systems with opposite vergence were responsible for transforming the northern lhasa subterrane into an elevated mountain range this process resulted in the evolution from a shallow marine environment langshan formation into a terrestrial depositional environment daxiong formation on the southern margin of the northern lhasa subterrane given the regional paleogeographic context we conclude that the daxiong formation in the coqen basin records local crustal shortening and flexure resulting in foreland basin development on the southern margin of the northern lhasa subterrane which implies early topographic growth of the northern lhasa subterrane in southern tibet prior to the india asia collision

geochemical evaluation of fenghuoshan group lacustrine carbonates north central tibet implications for the paleoaltimetry of the eocene tibetan plateau abstract a sedimentologic petrologic and geochemical evaluation of lacustrine carbonates from the eocene oligocene fenghuoshan group of north central tibet provides insight into the paleoenvironmental and paleolimnological setting of the hoh xil basin during the mid tertiary fenghuoshan lacustrine rocks consist primarily of carbonate mudstones and wackestones these carbonates are generally less than 3 m in thickness and are intercalated with siliciclastic lacustrine and fluvial alluvial plain deposits individual limestone beds coarsen and contain increasing amounts of siliciclastic material upward fenghuoshan carbonates also contain abundant ostracode bivalve and charaphyte fossils sedimentologic evidence suggests that carbonate deposition occurred in shallow relatively short lived lacustrine systems that were subsequently filled by alluvial deposits of coeval fluvial systems stable c and o isotopic analysis of fenghuoshan carbonates show 18o values ranging from 11 7 to 10 3 vienna pee de

paleogene high elevations in the qiangtang terrane central tibetan plateau abstract the timing of uplift of the tibetan plateau remains controversial with estimates varying from eocene to more recent than pliocene in particular the paleoaltimetry of the qiangtang terrane on the central tibetan plateau is completely unknown here we present new stable isotope results of fluvial lacustrine carbonate cement pedogenic carbonate and marl from the kangtuo and suonahu formations deposited between 51 and 28 ma in the heihuling bamaoqiongzong area of the northern qiangtang terrane the lithofacies associations indicate that the kangtuo formation was deposited in alluvial fan and fluvial floodplain environments and the suonahu formation was deposited in near shore lacustrine playa lake and channelized fluvial environments carbon and oxygen isotope values coupled with the sedimentary facies interpretations point to evaporation and low respiration rates in the eocene oligocene paleosols suggesting an arid climate in the high qiangtang area at the time the 18opsw values of paleo surface water reconstructed from the lowest i e the least evaporated 18oc pdb values of the unaltered authigenic carbonates are used to make the minimum estimates of the average paleoelevation of the drainage basin the paleoelevation of the northern qiangtang terrane is reconstructed as above 5000 m by at least the middle oligocene 28 ma similar to the present elevation in this area the aridity and the positive shift in oxygen isotope values of surface waters in our study area may suggest that the high lhasa terrane established by the middle oligocene blocked the northward transport of tropical moisture maintenance of high elevation 5000 m from at least the oligocene to the present suggests that the qiangtang crust was already thickened by that time and that underthrusting of india beneath asia since then has continued to provide additional material while at the same time driving asian lower crust eastward by crustal flow

numerical modeling of eastern tibetan type margin influences of surface processes lithospheric structure and crustal rheology the eastern tibetan margin is characterized by a steep topographic gradient and remarkably lateral variations in crustal lithospheric structure and thermal state gps measurements show that the surface convergence rate in this area is strikingly low how can such a mountain range grow without significant upper crustal shortening in order to investigate the formation mechanism of the eastern tibetan type margins we conducted 2d numerical simulations based on finite difference and marker in cell techniques the numerical models were constrained with geological and geophysical observations in the eastern tibetan margin several major parameters responsible for topography building such as the convergence rate the erosion sediment rate and the presence of partially molten crust were systematically examined the results indicate that the presence of partially molten material in the middle lower crust can make a positive contribution to the formation of steep topography but it is not a necessary factor a steep topographic gradient may be a characteristic feature when a thin lithosphere with thick crust converges with a thick lithosphere with thin crust in the context of a high erosion rate the longmen shan range still gains and maintains its steep high topography to the present this could be explained by exerting a large push force on tibet side our numerical experiments suggest that topographic characteristic across the eastern tibetan type margins is mainly derived from isostatic equilibration forces and intensive convergence between two continental lithospheres with totally different rheological properties

outward growth of the tibetan plateau during the cenozoic a review abstract the surface uplift history of the tibetan plateau tp offers a key testing ground for evaluating models of collisional tectonics and holds important implications for processes ranging from global cooling to the onset of the asian monsoon various models have been proposed to reveal the surface uplift history of the tp but controversies remain we evaluate these models using data from sedimentology and stratigraphy structural geology magmatism exhumation and paleoaltimetry studies structural analyses indicate that thrust belts which spread from the central tp outward toward its surrounding margins accommodated most of the india asia convergence and facilitated crustal shortening and thickening in the central tp eocene adakitic rocks located in the qiangtang and the lhasa blocks likely were generated by partial melting of an eclogitic source paleogene 50 30 ma potassic rocks only occur in the qiangtang block whereas late oligocene late miocene 26 8 ma potassic rocks occur both in the qiangtang and lhasa blocks low temperature thermochronologic ages in the central tp are older than 40 35 ma whereas those in the margins are younger than 20 ma mostly late miocene and pliocene pleistocene in age independent paleoaltimetry estimates suggest that the lhasa and qiangtang terranes attained their current elevations during the eocene most likely due to the initial collision between india and lhasa whereas the hoh xil basin area to the north and himalayas to the south were still low even below sea level in the latter case we argue for an inside out growth pattern for the tibetan plateau the tp grew southward and northward from a nucleus of high topography and is likely to continue expanding along the mazar tagh fault to the northwest the kuantai shan hei shan longshou shan to the northeast the longquan shan to the east and the shillong plateau to the south if the northward force of india would not diminished

facies analysis and basin architecture of the neogene subandean synorogenic wedge southern bolivia foreland sedimentation in the subandean zone of south central bolivia spans from the upper oligocene to present it records sediment dispersal patterns in an initially distal and later proximal retroarc foreland basin and thereby contains stratigraphic information on the tectonic evolution of the adjacent andean fold thrust belt within the neogene orogenic wedge individual siliciclastic dominated depositional systems formed ahead of an eastward propagating deformation regime we defined described and interpreted eight architectural elements and 24 lithofacies from 15 outcrop locations representing the neogene foreland basin in the subandean zone and the chaco plain these are combined to interpret depositional settings the up to 7 5 km thick neogene wedge is subdivided in five stratigraphic units on the basis of facies associations and overall architecture 1 the basal oligocene miocene up to 250 m thick petaca formation consists dominantly of calcrete reworked conglomeratic pedogenic clasts and fluvial sandstone and mudstone this unit is interpreted to represent extensive pedogenesis under an arid to semiarid climate with subordinate braided fluvial processes 2 the overlying upper miocene up to 350 m thick yecua formation records numerous small scale transgressive regressive cycles of marginal marine tidal and shoreline facies of sandstone ooid limestones and varicoloured mudstone 3 the upper miocene up to 4500 m thick tariquia formation principally consists of sandstone with interbedded sandstone mudstone couplets representing frequent crevassing in anastomosing streams with an upsection increasing degree of connectedness 4 the up to 1500 m thick lower pliocene guandacay formation represents braided streams and consists principally of granule to cobble conglomerate interbedded with sandstone and sandy mudstone 5 the upper pliocene up to 2000 m thick emborozu formation consists predominantly of alluvial fandeposited cobble to boulder conglomerate interbedded with sandstone and sandy mudstone the coarsening and thickening upward pattern and eastward progradation coupled with the variable proportions of overbank facies channel size and degree of channel abandonment in the tariquia guandacay and emborozu formations reflect a distal through proximal fluvial megafan environment this long lived megafan grew by high sedimentation rates and a northeast through southeast radial paleoflow pattern on large coarse grained sediment lobes the marked overall

origin and pre cenozoic evolution of the south qiangtang basement central tibet abstract the qiangtang terrane in central tibet with its high pressure rocks is a key area to unravel the evolution of the paleo tethys we present a new 1 50 000 map of the rongma area in the central qiangtang terrane of which relatively little is known so far we show that the qiangtang metamorphic belt can be separated in a paleozoic autochthonous basement and an overlying allochthonous thrust stack of subduction melange that contains high pressure rocks and permian sediments detrital zircon ages youngest peak at 591 ma and an 470 ma granite intrusion age constrain the age of the qiangtang basement to be between the late precambrian to middle ordovician this is in agreement with the observed unconformity between basement and overlying mid late ordovician strata this unconformity and the zircon age spectra are comparable to the himalaya area the qiangtang terrane must thus have been part of the gondwana supercontinent during the early paleozoic the occurrence of the late triassic eclogite and glaucophane bearing schists in the central qiangtang terrane indicates the existence of a suture zone between the north and south qiangtang terranes before the late triassic this suture zone resulted from closure of the paleo tethys between the north and south qiangtang terranes late triassic syn collisional granites and rapidly exhumed high pressure rocks resulted from the closure of this ocean and final amalgamation of the re united qiangtang terranes with eurasia collision of the lhasa and qiangtang terranes is correlated with north dipping reverse faults in the south qiangtang terrane which were probably related to exhumation of the qiangtang basement and formation of the qiangtang culmination

structural evolution of the footwall of the indus suture in malakand n pakistan during the himalayan collision abstract we report new data on the geometry and kinematics of structures in the rocks of the indian plate below the indus suture in the area of malakand n pakistan the study area forms part of the exposed northwestern promontory of the indian plate indenting eurasia to form the himalayan orogenic belt earlier widespread structures i e regional foliation and lineation are consistent with an approximate n s convergence direction in coincidence with the movement direction of the indian plate younger structures such as nne trending open folds that laterally become syntaxes i e besham or nanga parbat and relatively minor e w dextral strike slip shear zones are related to a regional tectonic shortening direction subparallel to the trend of the suture these folds are radial to the northwestern corner of the indian plate and contemporaneous with the general bulk n s convergence it is only very recently in the structural record that the irregular outline of the indian plate played a role in determining the orientation of new structures during its indentation the structural pattern presents similarities to arcuate megastructures in ancient collisional orogens such as the ibero armorican arc of the variscan belt in europe where indentation tectonics as well as post orogenic oroclinal folding are invoked to explain the present curvature

from sea level to high elevation in 15 million years uplift history of the northern tibetan plateau margin in the altun shan approximately 1300 in of oligocene miocene clastic strata are exposed along the miran river in the southeastern tarim basin where the adjacent altun shan form the topographic escarpment of the northern tibetan plateau the sedimentary section is faulted against proterozoic rocks of the altun shan in the footwall of the south dipping oblique reverse northern altyn tagh fault oligocene lower miocene strata consist of fine grained rocks that record low gradient depositional systems mid miocene and younger rocks consist of coarse conglomerate derived from the altun shan and deposited by high gradient depositional systems the change to coarse high gradient depositional systems with detrital source areas coincident with the modern miran river drainage is interpreted to mark the onset of uplift of the altun shan on the northern altyn tagh fault and its erosional exhumation the age of the change from pre orogenic to synorogenic sedimentation is constrained by a foraminifera assemblage at the base of the conglomeratic section that includes early middle miocene planktonic foraminifera this interpretation is also supported by apatite fission track and u th he ages and thermal models that indicate rapid miocene cooling and hence rapid exhumation of the altun shan in addition to defining the age of the synorogenic section the foraminifera assemblage contains planktonic taxa indicating a connection to open marine waters and benthic assemblages typical of brackish to near sea level paleobathymetry thus micropaleontologic evidence demonstrates that the miran river locality now at 1400 in elevation was at sea level approximately 15 million years ago thus in addition to constraining the age of surface uplift and exhumation of the altun shan the principal mountain range of the tibetan plateau in this region as 15 to 16 ma the foraminifera assemblage indicates that the se tarim basin off the northern edge of the plateau had an average surface uplift rate of nearly 100 m m y for the past 15 million years these results suggest that shortening in the altun shan and uplift of the range significantly post dated the initiation of large scale strike slip on the altyn tagh fault and that regional surface uplift mechanisms operated in the tarim basin beyond the margins of the tibetan plateau

provenance of the upper cretaceous eocene deep water sandstones in sangdanlin southern tibet constraints on the timing of initial india asia collision abstractthe first arrival of sedimentary material from asia onto the indian continental margin provides a minimum constraint on the timing of initial india asia collision a combination of petrology detrital cr spinel geochemistry and zircon u pb dating of the upper cretaceous to eocene deep water succession at the sangdanlin section southern tibet provides evidence for major provenance change within the strata the upper cretaceous paleocene denggang formation quartzarenites contain zircons with dominant proterozoic ordovician u pb ages with an additional age peak of early cretaceous which we interpret to be derived from the northern indian margin by contrast the lithic sandstones of the early to middle eocene sangdanlin and zheya formations are dominated by zircons younger than 200 ma showing one major peak at 80 125 ma and two subdominant peaks at 54 70 and 180 196 ma comparable to those from the gangdese magmatic arc cr spinels in the sangdanlin and zheya formations are abundant and char

vast early miocene lakes of the central tibetan plateau lacustrine strata of the wudaoliang group demonstrate that two vast lake complexes covered the central tibetan plateau during early miocene time the wudaoliang group which is characterized by dolostone limestone and marl as thick as 310 350 m lies horizontally or with a slight dip above oligocene reddish brown sandstone and is covered by upper miocene and pliocene mudstone and sandstone the extensive distribution of this group outlines interconnected lake basins of 5000 15 000 km 2 or larger in the north central tibetan plateau such as the wudaoliang beiluhe tuotuohe tongtianhe and east wenquan basins and lake basins 2000 km 2 or larger in the south central plateau such as the ando naqu bangoin lunpola and shuanghu basins these lake basins separated by mountain ranges and islands were linked by water passages in early miocene time when they apparently formed a huge lake complex that covered as much as 100 000 km 2 in the north central tibetan plateau and another one 50 000 km 2 in the south central part of the plateau such immense lakes existed for several million years between ca 23 5 and ca 13 5 ma after the tibetan plateau rose and their interconnected basins define the topography during early miocene time the fossils contained in these basins indicate a change from the warm dry climate of the oligocene to moderately cool and wet conditions at the beginning of the early miocene followed by progressive cooling and drying which suggests continuous uplift during deposition of the wudaoliang group

palaeo altimetry of the late eocene to miocene lunpola basin central tibet the elevation history of the tibetan plateau provides direct insight into the tectonic processes associated with continent continent collisions here we present oxygen isotope based estimates of the palaeo altimetry of late eocene and younger deposits of the lunpola basin in the centre of the plateau which indicate that the surface of tibet has been at an elevation of more than 4 kilometres for at least the past 35 million years we conclude that crustal but not mantle thickening models combined with plate kinematic solutions of india asia convergence are compatible with palaeo elevation estimates across the tibetan plateau

quantitative mineral resource assessment of undiscovered pge resources in finland abstract most of the known platinum group element pge resources in finland are in contact type and reef type deposits in 2 45 ga mafic ultramafic layered intrusions these intrusions are also estimated to contain the majority of possibly existing yet undiscovered pge resources in finland the undiscovered pt pd au ni and cu resources in contact type and reef type deposits were estimated down to 1000 m depth using a three part quantitative assessment method this included the delineation of 19 contact type and 24 reef type permissive tracts the formulation of a grade tonnage model for the contact type deposits and a separate model for each reef type tract estimation of the number of undiscovered deposits for each permissive tract and monte carlo simulation to estimate the frequency distributions of metal tonnages in the undiscovered deposits the expected number of undiscovered contact type deposits is 29 and that of reef type deposits is 23 at the 50 probability level the total undiscovered pge resources of finland are estimated to be at least 5600 t pt 12 000 t pd 430 t au 4 2 mt ni and 5 7 mt cu at the 90 probability level the corresponding tonnages are 720 t pt 1700 t pd 59 t au 0 87 mt ni and 0 91 mt cu more than 80 of the pt pd in the undiscovered resources are estimated to be in reef type deposits the known pge resources in finland are 259 t pt pd in six contact type deposits plus 69 t pt pd in the atypical kevitsa deposit these figures indicate that the presently known deposits account for approximately 2 of the total assessed pge resources in finland they suggest that significant potential exists for the discovery of additional pge resources in the country

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

new tertiary paleomagnetic poles from mongolia and siberia at 40 30 20 and 13 ma clues on the inclination shallowing problem in central asia we report results of a paleomagnetic study of 490 cores from 59 sites corresponding to 52 distinct basaltic flows from mongolia and siberia khaton sudal 39 4 ma 44 5 n 101 4 e taatsyn gol 1 31 5 ma 45 4 n 101 3 e 2 28 0 ma 45 5 n 101 1 e ust bokson 19 9 ma 52 1 n 100 3 e and taatsyn gol 3 12 7 ma 45 5 n 101 0 e stepwise thermal and alternating field demagnetizations isolated a stable high temperature component htc of magnetization in most specimens which we interpret as the primary magnetization of these basaltic lava flows the four corresponding paleopoles appear consistent with coeval paleopoles from other asian effusive formations however except for the 12 7 ma paleopole the paleopoles are systematically far sided from the european apparent polar wander path apwp with respect to site locations corresponding to anomalously shallow inclinations in tertiary asian effusive formations in the hypothesis of a dipolar magnetic field in the tertiary this indicates a 1000 1500 km position of the siberia craton and amuria block farther south than expected at 40 and 30 ma tectonically this interpretation implies decoupling and relative rotations between the western and eastern parts of eurasia between the cretaceous and present we show that if siberia were located more to the south the 15 20 paleolatitude anomaly generally observed in sedimentary formations from central asia reduces to a more reasonable average value of 7 which could result from the superimposition of shallowing mechanisms due to sedimentary processes and northward motion of asian blocks under the effect of ongoing penetration of india into eurasia in the tertiary

normal faulting in central tibet since at least 13 5 myr ago tectonic models for the evolution of the tibetan plateau interpret observed east west thinning of the upper crust to be the result of either increased potential energy of elevated crust1 or geodynamic processes that may be unrelated to plateau formation2 3 4 5 6 a key piece of information needed to evaluate these models is the timing of deformation within the plateau the onset of normal faulting has been estimated to have commenced in southern tibet between about 14 myr ago7 and about 8 myr ago8 and in central tibet about 4 myr ago9 here however we report a minimum age of approximately 13 5 myr for the onset of graben formation in central tibet based on mineralization ages determined with rb sr and 40ar 39ar data that post date a major graben bounding normal fault these data along with evidence for prolonged activity of normal faulting in this and other tibetan grabens support models that relate normal faulting to processes occurring beneath the plateau thinning of the upper crust is most plausibly the result of potential energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath tibet this is supported by recent geophysical and geological data10 11 12 13 14 15 16 17 which indicate that spatial heterogeneity exists in both the tibetan crust and lithospheric mantle

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

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

multi stage tectono magmatic events of the eastern kunlun range northern tibet insights from u pb geochronology and u th he thermochronology article i nfo to understand the magmatic and tectonic events in the eastern kunlun range we conducted systematic zircon u pb geochronology and paired zircon zhe and apatite ahe u th he themchronology investigations in the central and eastern segments of this range zircon u pb ages show two stage magmatic events of late silurian and latest permian early jurassic zhe and ahe ages and multi system thermochronometers reveal multi stage rapid cooling events from a combination of these data with previously published geochronological and thermochronological ages and regional geological setting we confirm several important phanerozoic tectono magmaticeventsinthisregion whereasthesiluriangranitesmightberelatedtoasmallpaleo oceansub duction and subsequent collision the latest permian early triassic granitoids were produced by northward sub duction of the songpan ganzi paleo tethyan ocean the late triassic earliest jurassic granitoids and late triassic early jurassic exhumation are interpreted as response to the collision between kunlun qaidam and qiangtang samples close to the kunlun fault show rapid late oligocene early miocene exhumation 1 3 1 6 km ma whereas those from the northern part of central segment display extremely low exhumation rates 0 02 0 05 km ma this heterogeneous denudation during 30 20 ma between northern and southern part of the central segment requires that crustal thickening was completed by lower crustal underthrusting with littleex humation or crustal thickening in pre cenozoic times the activity of the kunlun fault and associated normal faulting component were probably responsible for this stage of rapid exhumation in the southern part the com plex tectono magmatic evolution of the eastern kunlun range is controlled by pre cenozoic paleo ocean subduc tions and subsequent continent collisions and paleogene india asia collision

high and dry in central tibet during the late oligocene the time at which the tibetan plateau rose to its present high elevation remains controversial with estimates ranging from 40 ma to more recent than 7 ma new stable isotope analyses of modern and accurately dated ancient paleosol carbonate in the nima basin of central tibet point to an arid climate and high paleoelevation 4 5 5 km comparable to today s setting by 26 ma oxygen isotope values of ancient 26 ma soil carbonate are both very negative and indistinguishable after modest corrections for changes in global climate from the lowest least evaporated oxygen isotope values of modern soil carbonates in the area substantial enrichments in oxygen 18 in paleolacustrine carbonates as well as high carbon isotope values from paleosol carbonates indicate considerable lake evaporation and low soil respiration rates respectively and both are consistent with the present arid climate of the nima area blockage of tropical moisture by the himalaya and perhaps the gangdese shan probably has contributed strongly to the aridity and very negative oxygen isotope values of soil carbonate and surface water in the nima area since at least the late oligocene the maintenance of high elevation since at least 26 ma suggests that any flow of lower crust from beneath central tibet must have been balanced by coeval northward insertion of indian crust beneath the plateau

international geomagnetic reference field the eleventh generation the eleventh generation of the international geomagnetic reference field igrf was adopted in december 2009 by the international association of geomagnetism and aeronomy working group v mod it updates the previous igrf generation with a definitive main field model for epoch 2005 0 a main field model for epoch 2010 0 and a linear predictive secular variation model for 2010 0 2015 0 in this note the equations defining the igrf model are provided along with the spherical harmonic coefficients for the eleventh generation maps of the magnetic declination inclination and total intensity for epoch 2010 0 and their predicted rates of change for 2010 0 2015 0 are presented the recent evolution of the south atlantic anomaly and magnetic pole positions are also examined