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

paleoelevation reconstruction of the paleocene eocene gonjo basin se central tibet abstract the topography evolution of the southeastern tibetan plateau provides a constraint for evaluating various geodynamic models of plateau formation we reconstruct the paleocene to eocene paleoelevation of the cenozoic gonjo basin in the qiangtang terrane using oxygen and carbon stable isotopic results from pedogenic carbonates of the lower ranmugou formation lithofacies associations indicate that the lower ranmugou formation was deposited in alluvial fan and fluvial floodplain environments u pb dating of volcanics within the middle ranmugou formation constrains the deposition of the lower ranmugou formation as prior to 43 2 ma paleoelevations are calculated using both a thermodynamic model and an empirical relationship the empirical relationship of elevation 18 o is determined from a series of modern water samples calculated paleoelevations indicate that the gonjo basin attained a minimum average elevation of 2100 2500 m in the early eocene together with recent paleoaltimetry studies in the region it can be concluded that the crust of southeastern tibetan plateau was already thickened by that time the calculation is based on the section in northern gonjo basin where 18 o values of paleosol nodules appear to be unaltered but we cannot rule out the possibility that 18 o values of the pedogenic carbonates had been partially reset our preliminary results favor tectonic models compatible with pre miocene uplift of the se central tibetan plateau

extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

stable isotope paleoaltimetry and the evolution of landscapes and life abstract reconstructing topography of our planet not only advances our knowledge of the geodynamic processes that shape the earth s surface equally important it adds a key element towards understanding long term continental moisture transport atmospheric circulation and the distribution of biomes and biodiversity stable isotope paleoaltimetry exploits systematic decreases in the oxygen 18o or hydrogen d isotopic composition of precipitation along a mountain front when the interaction of topography and advected moist air masses induces orographic precipitation these changes in 18o or d can be recovered from the geologic record and recent geochemical and modeling advances allow a broad range of proxy materials to be evaluated over the last 10 yr stable isotope paleoaltimetry has witnessed rapidly expanding research activities and has produced a broad array of fascinating tectonic and geomorphologic studies many of which have concentrated on determining the elevation history of continental plateau regions these single site studies have greatly expanded what used to be very sparse global paleoaltimetric data the challenge now lies in disentangling the surface uplift component from the impact of climate change on 18o and d in precipitation the robustness of stable isotope paleoaltimetry can be enhanced when high elevation 18o or d data are referenced against low elevation sites that track climate modulated sea level 18o or d of precipitation through time approach analysis of central andean paleosols documents that differences in 18o of soil carbonate between the subandean foreland and the bolivian altiplano are small between 11 and 7 ma but rise rapidly to ca 2 9 after 7 ma corroborating the magnitude of late miocene change in 18o on the altiplano future advances in stable isotope paleoaltimetry will greatly benefit from addressing four key challenges 1 identifying topographically induced changes in atmospheric circulation and associated teleconnections in the global climate system that affect 18o or d of precipitation 2 evaluating on a case by case basis if temporal and spatial changes in isotope lapse rates influence interpretations of paleoelevation 3 interfacing with phylogenetic techniques to evaluate competing hypotheses with respect to the timing of surface uplift and the diversification of lineages 4 characterizing feedbacks between changes in surface elevation and atmospheric circulation as these are likely to be equally important to the diversification of lineages than changes in surface elevation alone tackling these challenges will benefit from the accelerating pace of improved data model comparisons and rapidly evolving geochemical techniques for reconstructing precipitation patterns most importantly stable isotope paleoaltimetry has the potential to develop into a truly interdisciplinary field if innovative tectonic paleoclimatic and evolutionary biology phylogenetic approaches are integrated into a common research framework it therefore opens new avenues to study the long term evolution of landscapes and life

geochronological and geochemical constraints on mesozoic suturing in east central tibet 1 this paper reports isotopic major and minor element geochemistry of igneous and metamorphic rocks from the kokoxili and yushu regions of central and eastern tibet the first region lies along the kunlun suture which separates the bayan har songpan ganze songpan terrane from the tarim and qaidam blocks two kokoxili granitoids yield u pb zircon dates of 217 10 and 207 3 ma late triassic which represent the time of emplacement and rb sr isochron dates of 195 3 and 190 3 ma early jurassic which are interpreted as cooling ages the geochemical signatures of these granitoids suggest that they are related to subduction continuing into the late triassic in the yushu area three samples help constrain the age of the jinsha suture which separates the songpan terranes from the qiangtang blocks a leucocratic granite and an orthogneiss in the suture zone yield u pb zircon dates of 206 7 and 204 1 ma respectively and a paragneiss south of it a u pb monazite date of 244 4 ma the existence of coeval magmatism in both the jinsha and kunlun sutures suggests that the two subduction zones were simultaneously active combining isotopic dating with structural evidence on subduction polarity and paleomagnetic reconstructions we propose that the kunlun and qinling block boundaries which were distinct in the permian subsequently formed a continuous late triassic northward subducting plate margin our data suggest that the jinsha suture correlates with the benzilan and nan uttaradit sutures which together belong to a major late triassic subduction zone

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

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

variations in soil carbonate formation and seasonal bias over 4 km of relief in the western andes 30 s revealed by clumped isotope thermometry abstract carbonate clumped isotope thermometry provides a new method for investigating long standing questions regarding seasonal biases in soil carbonate formation and the relationship between soil carbonate formation temperatures recorded by clumped isotopes t 47 and surface temperatures we address these questions by comparing c o and clumped isotope data from holocene soil carbonates to meteorological and in situ soil monitoring data along a 170 km transect with 4 km of relief in chile 30 s this arid transect experiences a winter wet season and a 20 c range in mean annual air temperature we test the hypothesis that regardless of soil moisture conditions soil carbonates from arid regions record warm season biases and form in isotopic equilibrium with soil water and soil co2 below 3200 m precipitation falls as rain and soil carbonate t 47 values at depths 40 cm resemble summer soil temperatures above 3200 m precipitation falls as snow and t 47 values resemble mean annual soil temperatures soil carbonates from the highest site yield anomalous 18 o 13 c and t 47 values indicative of kinetic isotope effects consistent with cryogenic carbonate formation our findings 1 demonstrate that soil carbonate t 47 values from shallow 40 cm 3 highlight the role of the soil moisture regime in modulating the timing of soil carbonate formation which affects the resulting carbonate t 47 values and 4 show that soil carbonates from high elevation or high latitude sites may form under non equilibrium conditions these findings underscore the importance of understanding past soil moisture conditions when attempting to reconstruct paleotemperatures using carbonate clumped isotope thermometry

cenozoic paleoaltimetry of the se margin of the tibetan plateau constraints on the tectonic evolution of the region abstract an improved understanding of the elevation history of the tibetan plateau is crucial in discriminating among the various tectonic models for the evolution of the india asia continental collision we reconstruct the paleoelevation history for three cenozoic sedimentary basins from se tibet and yunnan china to provide more constraints on the tectonic processes for raising the se margin of the tibetan plateau the results presented here together with those of previous studies indicate that 1 the plateau margin of nw yunnan was near its elevation 2 6 km by the latest middle eocene 40 ma 2 the plateau margin of se yunnan reached its current elevation 1 6 km by the middle miocene 13 ma interpretations of the tectonic processes responsible for this inferred surface uplift of the region are made in the context of well documented surface geology we conclude that high landscape 2 6 km elevation in nw yunnan may represent the remnants of the eocene tibetan plateau that originally formed in the northeastern qiangtang block by crustal thickening associated with the india asia continental collision the near modern elevation of se yunnan since 13 ma probably reflects the initiation of lower crustal flow in this area by at least that time collectively our paleoaltimetric interpretations disagree with previously proposed models of middle miocene to pliocene crustal flow acting as a sole tectonic process for raising the se margin of the plateau but support a protracted history of surface uplift that most likely involved crustal thickening during the eocene southeastward extrusion of a portion of eocene tibetan plateau during the oligocene to early miocene and lower crustal flow beneath this region since at least the early miocene

two phase growth of high topography in eastern tibet during the cenozoic high topography in eastern tibet is thought to have formed in response to weak lower crust flowing towards the plateau margin thermochronologic analyses of rocks exposed at the eastern plateau margin record periods of mountain growth early in the indo asian collision implying that crustal flow alone could not have created the high topography

empirical relationships between elevation and the stable isotope composition of precipitation and surface waters considerations for studies of paleoelevation change a compilation of 68 studies from throughout many of the world s mountain belts reveals an empirically consistent and linear relationship between change in elevation and change in the isotopic composition of precipitation along altitudinal transects the isotopic composition of precipitation decreases linearly with increasing elevation in most regions of the world except in the himalayas and at elevations 5000 m there are no significant differences in isotopic lapse rates from most regions of the world 0 28 permil 100 m except at the extreme latitudes where isotopic lapse rates are higher given information on past changes in the isotopic composition of precipitation preserved in pedogenic or authigenic minerals this global isotopic lapse rate can be used to place numerical constraints on the topo graphic development of some ancient mountain belts or plateaus there are many complicating factors that can confound interpretation of paleoel evation change based on stable isotopes and many of these are unique to specific mountain belts or time periods relevant to all stable isotope based paleoelevation change studies is the temperature dependent isotope fractionation between a pedo genic or authigenic mineral and the water from which it forms in cases where isotopic proxy minerals are sampled from localities where temperature will change simulta neously with elevation change the apparent change in the isotopic composition of precipitation may be dampened by several permil this suggests that samples taken from the rainshadow side of an emerging orographic barrier may be more likely to preserve isotopic changes resulting from mountain uplift than samples taken from atop a rising mountain range or plateau

a thermochronological perspective on the morphotectonic evolution of the southeastern tibetan plateau a large part of the southeastern tibetan plateau tp is characterized by low relief surfaces at high elevations 3 5 km the origin of these landscapes and their geodynamic implications with regard to evolution of the southeastern tp has been the subject of considerable debate focusing on this topic this study utilizes fission track and u th he thermochronology to reconstruct the thermal history of the late triassic daocheng granite on which a low relief high elevation landscape has developed results suggest that the plateau surface experienced widespread late jurassic to early cretaceous cooling at a rate of 1 2 c ma followed by minimal regional cooling 0 3 c ma and early miocene enhanced cooling 2 3 c ma along deep valleys at the southern edge of the landscape given the subdued late jurassic to early cretaceous crustal deformation in the study area we relate the coeval cooling to crustal refrigeration and exhumation triggered by northward subduction of the meso tethys ocean along the bangong suture subsequent late cretaceous to cenozoic minimal cooling supports arguments for the formation of a low elevation landform prior to uplift of the southeastern tp the enhanced cooling commenced at the early miocene 15 22 ma along deep valleys and heralded the onset of river incision and surface uplift of the southeastern tp this finding does not support previous proposals claiming that high elevations of the southeastern tp were formed in late miocene time resulting from lower crustal flow but rather supports a model highlighting continental subduction and extrusion along the large tanggula yushu batang red river fault system

tertiary deformation and metamorphism se of tibet the folded tiger leap decollement of nw yunnan china the yulong haba xue shan range in the northwestern part of yunnan china is a large n s antiform that folds the paleozoic series of the yangzi platform the upper yangzi river jinsha jiang has cut a 3500 m deep valley hu tiao gorge across this antiform thus exposing folded bedding parallel ductile shear zones decollements with transport toward the ssw in the present geographical coordinates the large finite shear strain implies tens of kilometers of transport pointing to the regional significance of these decollements rb sr radiometric dating of phlogopites that crystallized in marbles within the foliation planes yields the age of the metamorphic and deformation event 35 9 0 3 2 ma the age derives from an internal rb sr isochron made on different size fractions of the same mineral which provides a novel demonstration of the feasibility of such plots transport on the decollement and related shortening occurred prior to or at the onset of extrusion of indochina along the ailao shan red river shear zone 80 km west of the yulong shan the 39ar 40ar age spectra of k feldspar from the core of the yulong shan suggest uplift by antiformal folding around 17 ma as indochina s extrusion came to an end we infer that other large scale cenozoic decollements such as that exhumed in the yulong shan underlie some of the vast folded areas that surround the eastern himalayan syntaxis transport on such decollements first toward the south and then toward the east and folding above them might have occurred during two principal shortening phases whose ages bracket indochina s escape toward the se

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

laboratory experiments and field study for the detection and monitoring of potential seepage from co2 storage sites abstract potential co2 seepages from geological storage sites or from the injection rig may affect the surrounding environment to develop reliable detection techniques for such seepages a laboratory rig was designed that is composed of three vertical plexiglas columns the columns can be filled with sediments and water co2 can be injected from the bottom two columns are used to simulate the impact of co2 on soils while the third one which is larger in size simulates co2 seepage in aquatic environments the main results of the laboratory experiments indicate that increased levels of co2 generate a quick drop in ph once the seepage is stopped a partial recovery towards the initial values of ph is recorded the outcomes of the laboratory experiments on the aquatic seepage are compared with observations from a submarine natural emission of co2 in this natural underwater seepage multi parametric probes and laboratory analysis were used to analyze the composition and the chemical effects of the emitted gas basic acoustic techniques were tested as tools for the prompt detection of co2 bubbles in water

stable isotopes reveal high southeast tibetan plateau margin since the paleogene abstract the evolution of the topography of eastern tibet reflects the dynamic interaction between processes at earth s surface and the lower crust reconstruction of its spatio temporal development provides constraints for our understanding of the geodynamics of continental deformation this study examines the topographic evolution of the southeast margin of the tibetan plateau throughout the cenozoic using stable isotope paleoaltimetry of pedogenic carbonates preserved in cenozoic basins extending from the northern to the southern portions of the se margin of the tibetan plateau yunnan province china paleoelevations are calculated using a monte carlo simulation of the normally distributed uncertainties in each parameter used to calculate elevations from the isotopic value of pedogenic carbonate isotope elevation relationships are determined from a modern sampling transect along the modern lancang mekong river and an estimated eocene rayleigh fractionation model that was randomly resampled calculated elevations indicate the se margin of tibet was at its present elevation as early as the late eocene with elevations of 3 3 km as far south as the northern third of china s yunnan province this result expands the extent of the eocene tibetan plateau more than 1000 km east of that previously known and precludes surface uplift via lower crustal flow in the area of sw sichuan and northern yunnan as a driver for drainage reorganization and an observed middle to late miocene pulse of rapid exhumation elevation estimates from the southern two thirds of the study area parallel to the red river fault suggest approximately 1 km of post late miocene surface uplift consistent with estimates from previous studies

the andean type gangdese mountains paleoelevation record from the paleocene eocene linzhou basin abstract paleoelevation reconstruction using oxygen isotopes is making a significant contribution to understanding the cenozoic uplift of the himalayas and the tibetan plateau this paper presents new oxygen and carbon isotopic compositions from well dated tertiary paleosols lacustrine calcareous carbonates and marls from the nianbo 60 54 ma and upper pana formations 51 48 ma of the linzizong group in the linzhou penbo basin the sediments of the nianbo formation which are 180 m thick were deposited in alluvial fans braided rivers fan deltas and on nearshore to offshore lacustrine settings whereas those of the upper pana formation are 100 m thick and are comprised predominantly of proximal alluvial fan and braided river deposits correlations between the lithofacies and stable isotopic compositions suggest that the basin was mainly a hydrologically open environment it is confirmed that the 18 o c and 13 c c values from nianbo and pana formations have not yet been reset by late stage diagenesis based on petrographic examination oxygen isotope of the fossil ostracodes and tectonic deformation of strata the paleoelevations are reconstructed using the corrected most negative paleosurface water 18 o psw values these imply that the linzhou area had attained an elevation of 4500 400 m during the period of the indo asian collision i e achieved a near present elevation and may form an andean type mountain range stretching the gangdese arc before collision the gangdese mountains probably maintained high elevations since at least the paleocene and could play a crucial role in the climate change in the interior of the tibetan plateau during the early cenozoic the paleogeomorphic scenario of the eocene tibet is proposed to exist at two high mountains in excess of 4500 m that sandwiched a low elevation basin

carbonate oxygen isotope paleoaltimetry evaluating the effect of diagenesis on paleoelevation estimates for the tibetan plateau carbonate oxygen isotope paleoaltimetry is based on analysis of the d 18 o value of carbonate precipitated from surface water deciphering the diagenetic history is important for establishing whether particular carbonates are accurate recorders of paleosurface waters which reflect paleoelevation this study provides examples from southern east central and northeastern tibet of approaches aimed at evaluating the diagenetic history of lacustrine micrites and pedogenic carbonates the most desirable technique for avoiding erroneous interpretations related to diagenetic overprinting is to analyze carbonates that are known to be primary such as aragonitic shell material in rocks that do not contain shell material we have compared lacustrine micrites and pedogenic carbonates to diagenetic carbonate phases to determine the effects of diagenesis on the isotopic composition of primary carbonates where the potential effects of diagenesis are subtle or ambiguous we have evaluated the fidelity of the carbonate record from systematic trends in c and o isotopes that agree with other interpretations of paleoenvironment such as high frequency covariance in c and o that corresponds with changes in the mg concentration of carbonates using these strategies we have determined that diagenesis has not affected the isotopic composition of carbonates in the late miocene pliocene thakkhola graben in southern tibet and the oligocene to pliocene linxia basin in northeastern tibet in the case of paleogene basins in east central tibet however ambiguity in data precludes the determination of diagenetic effects d 2004 elsevier b v all rights reserved

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

old origin for an active mountain range geology and geochronology of the eastern hindu kush pakistan prior to accretion of the kohistan island arc during the late cretaceous and final suturing of india with asia at ca 50 ma the hindu kush mountains along the border of afghanistan and northwest pakistan were situated on the active southern margin of asia geology and geochronology of the eastern hindu kush range in pakistan demonstrate that localized crustal melting and leucogranite intrusion took place in the gharam chasma area at ca 24 ma more regionally developed and widespread deformation metamorphism and magmatism took place before collision between both india and the kohistan island arc with asia ca 195 ma u pb monazite ages on a deformed leucogranite dike from the upper lutkho valley indicate an early jurassic phase of crustal melting u pb monazite ages of 135 126 ma on a staurolite schist from near gharam chasma are interpreted as a minimum age for staurolite grade metamorphism within the tirich mir fault zone pegmatite dikes crosscut the staurolite schists u pb dating of uraninites from one of these pegmatite dikes reveals an age of 114 2 ma monazites from the same rock give ages of 125 121 ma possibly due to inheritance of older cores these jurassic early cretaceous constraints on metamorphism and magmatism relate to subduction and accretion processes perhaps including the suturing of the karakoram and hindu kush terranes along the tirich mir fault in general high temperature low pressure metamorphism and subduction related granitoid magmatism in the eastern hindu kush suggest a high thermal gradient in an active margin setting from early jurassic to cretaceous time

high times on the tibetan plateau paleoelevation of the thakkhola graben nepal east west extension in the tibetan plateau is generally assumed to have resulted from gravitational collapse following thickening and uplift on the basis of this assumption several studies have dated east west extensional structures to determine when the plateau attained its current high elevation however independent estimates of elevation are needed to determine whether extension occurred before during or after the plateau achieved its current elevation because the isotopic composition of meteoric water decreases with increasing elevation significant change in local elevation throughout the thakkhola graben depositional history should be recorded by change in 18 o values of fluvial and lacustrine carbonates the 18 o values of 16 to 23 of thakkhola graben carbonates reflect meteoric water values similar to modern values and suggest that the southern tibetan plateau attained its current elevation prior to eastwest extension initiation of thakkhola graben extension is constrained between 10 and 11 ma based on magnetostratigraphy of the older tetang formation the 13 c values of soil carbonates suggest an age younger than 8 ma for the base of the thakkhola formation

stable isotope based paleoaltimetry theory and validation paleoaltimetry is the quantitative estimate of the surface height above mean sea level of ancient landforms atmospheric thermodynamic modeling of the behavior of 18o relative to 16o during condensation from water vapor establish the systematic relationship that exists between 18o p and elevation where 18o p is the difference between a low altitude preferably sea level 18o p and a potentially high elevation sample comparison of model predictions with observations suggests that the model captures the first order behavior of 18o p during condensation and precipitation in orographic settings the actual relationship between 18o p and elevation depends sensitively on climate and specifically starting temperature and to a lesser degree relative humidity thermodynamic modeling allows the 18o p and elevation relationship to be explicitly calculated for any given starting climate state this makes the theoretical approach significantly more appropriate than empirically calibrated approaches based typically on quite limited samples presently available in most orographic settings today paleoaltimetry archives derive their isotopic compositions from surface and or ground water and hence it is important to understand the systematic differences between these reservoirs and precipitation surface waters and ground waters integrate not just the change in isotopic composition with altitude but also variations in hypsometry within the drainage basin and precipitation amount as functions of elevation thus these archives should reflect the precipitation amount weighted hypsometric mean elevation of the paleo drainage basin from which they derive their waters analysis of modern data from the himalayan region supports this expectation foreland basin rivers are such integrators and it is shown using an example from the siwaliks that the rivers draining the front of the himalayas in the past had isotopic compositions comparable with modern rivers draining the himalayan front suggesting little net change in himalayan hypsometry over the past 11 million years

quantifying landscape differences across the tibetan plateau implications for topographic relief evolution 1 we quantify the bulk topographic characteristics of the tibet qinghai plateau with specific focus on three representative regions northern central and southeastern tibet quantitative landscape information is extracted from shuttle radar topography mission digital elevation models we find that the morphology of the tibetan plateau is nonuniform with systematic regional differences the northern and central parts of the plateau are characterized by what we suggest to call positive topography i e a topography in which elevation is positively correlated with relief and mean slope a major change from the internally drained central part of tibet to the externally drained part of eastern tibet is accompanied by a transition from low to high relief and from positive to negative topography i e a topography where there is an inverse or negative correlation between elevation and relief and between elevation and mean slope relief in eastern tibet is largest along rivers as they cross an ancient eroded plateau margin at high angle to the major strike slip faults the yalong yulong thrust belt implying strong structural control of regional topography we propose that the evolution of river systems and drainage efficiency the ability of rivers to transport sediments out of the orogen coupled with tectonic uplift is the simplest mechanism to explain systematic regional differences in tibetan landscapes basin filling due to inefficient drainage played a major role in smoothing out the tectonically generated structural relief this mode of smoothing started concurrently with tectonic construction of the relief as most clearly illustrated today in the qilian shan qaidam region of the northeastern plateau in the interior of tibet further passive filling due to internal drainage only continued to smooth the local relief millions of years after the cessation of major phases of surface uplift due to crustal shortening thus diachronous beveling at high elevation produced the low relief surface of the high plateau in southeast tibet headward retreat of external drainages brought back in the global ocean base level first disrupting then interrupting the relief reduction process it produced a transitional topography by dissecting the old remnant plateau surface which introduced younger and steeper incision of this hitherto preserved high base level this provides a unifying mechanism for the formation of the low relief plateau interior and for the origin of the high elevation low relief relict surface in southeastern tibet our analysis brings forth the importance of surface processes in particular drainage efficiency in shaping plateau morphology and landscape relief such key processes appear to have been mostly ignored in numerical models of plateau deformation our results also cast doubt on and provide a more realistic alternative to the fashionable contention that a continuous preuplift low relief surface first formed at low elevation extending all the way to the south china sea shore before being warped upward in the late miocene pliocene by lower crustal channel flow

the stable isotope altimeter do quaternary pedogenic carbonates predict modern elevations stable isotope altimetry is a useful tool for estimating paleoelevation in sedimentary records yet questions remain regarding how source moisture climate and local topography can influence these estimates here we present stable isotope altimetry results on late quaternary pedogenic carbonates of known elevation on both flanks of the andean orogen at 33 s we measured 18 o values of pedogenic carbonates and river water samples from small drainages at regular elevation increments within the rio aconcagua chile and rio mendoza argentina catchments the 18 o values of river waters correlate well with elevation and show similar isotopic gradients between the chilean 3 7 km and argentine 4 8 km sides of the range uncertainties associated with scatter in the river water data and assumptions about the temperature of carbonate formation indicate that elevation estimates have 1 errors of 350 450 m we estimate the isotopic composition of soil water from pedogenic carbonates on both sides of the range by assuming mean annual temperatures based the modern temperature lapse rate from meteorological station data combined our data show that stable isotope altimetry produces reasonable estimates of modern elevation with the majority of our samples 60 within the 1 uncertainties and 77 within 2

reconstruction of the deformed collision zone between india and asia by backward motion of lithospheric blocks 1 on the basis of a synthesis of tectonic data available on the india asia collision we present a first attempt to reconstruct the evolution of the collision zone assuming that the deformation of the lithosphere is localized along narrow shear zones and that the interiors of mantle blocks in between remain relatively undeformed we define block contours from the fault pattern and move back the blocks along their boundary faults along convergent or extensional boundaries the crust is assumed to shorten or stretch coherently step by step we go backward in time to finally reach the collision onset for each time step we find a solution compatible with the data set available and the position of the adjacent blocks for each block the search for compatibility at the scale of the entire collision zone allows for solving the kinematics of regions with fewer data and suggests plausible scenarios for regions where data is lacking for each step we calculate large scale displacement maps and determine euler poles for each block for the most recent time step the map proposed is compared to gps motions the deformation budget implies that extrusion absorbed 30 of the convergence between india and siberia during the entire collision span but varied with time accounting for as little as 3 or as much as 60 of this convergence at different epochs

topographic development of the southern alps recorded by the isotopic composition of authigenic clay minerals south island new zealand abstract the southern alps are developing as a consequence of oblique collision between the pacific and australian plates the southern alps lie on the west side of the south island of new zealand and create a massive rain shadow where greater than 12 m year of rain falls on the west coast and semiarid conditions exist to the east the rain out effect across the mountains causes precipitation west of the southern alps to have d and 18 o values averaging 30 and 5 5 whereas precipitation in the rain shadow to the east is isotopically lighter d 72 and 18 o 9 8 such large differences in the isotopic composition of precipitation would not have existed prior to the development of significant topography we have examined the topographic evolution of the southern alps using oxygen isotope analyses of authigenic kaolinites formed in the rain shadow to the east of the mountains between the cretaceous low topography and the pleistocene changes in the isotopic composition of authigenic clay minerals forming in equilibrium with meteoric water in the stratigraphic sequence record the development of southern alps topography and the resultant rain shadow our oxygen isotope analyses of authigenic kaolinites show a 5 6 decrease in the early pliocene from 18 2 in older rocks to 12 3 in younger rocks in addition smectite is abundant in all samples from the late miocene to recent but is conspicuously absent in most older rocks suggesting a change to a generally drier climate roughly coincident with the isotopic shift in kaolinites this method may be useful in unraveling timing of development of mountain belts elsewhere in the world

role of the westerlies in central asia climate over the cenozoic abstract there remains substantial debate concerning the relative roles of tectonics and of global climate in pacing the evolution of aridity in central asia over the cenozoic tibetan plateau uplift variable monsoonal strength paratethys retreat and reduced moisture transport from the north atlantic have all been hypothesized to drive aridity in central asia distinguishing between these mechanisms requires knowledge of moisture transport pathways to central asia through time presently central asia receives recycled high 18 o moisture that has been transported across eurasia by the westerlies while southern tibet receives low 18 o moisture distilled by passage over the himalaya here we reconstruct the spatial distribution of oxygen isotopes in precipitation since the early eocene using a compilation of 18 o data from 2750 sedimentary carbonate samples across asia the spatial distribution of paleo precipitation 18 o remains remarkably similar through time with low 18 o values in the lee of the himalaya in southern tibet constant high 18 o values in central asia and intermediate values on the central plateau despite independent evidence for substantial changes in both topography and climate through time these results suggest that a long standing topographic feature has continuously blocked southerly moisture and subsequent progressive uplift of the tibetan plateau has had little impact on central asian climate in turn southerly monsoonal moisture has never persistently extended northward of the central plateau as a result the westerlies have remained the dominant moisture source in central asia since at least the early eocene therefore sedimentary aridity indicators in central asia such as loess deposition are controlled by the trans eurasian westerly moisture flux rather than tibetan plateau uplift

mechanical behavior of hydrate bearing pressure core sediments visualized under triaxial compression abstract geomechanical and geotechnical properties are essential for evaluating the stability of deep seabed and subsea production systems for gas hydrate extraction from marine sediments in this study natural gas hydrate bearing sediment was subjected to triaxial compression tests shearing using a newly developed triaxial testing system tactt to investigate the geomechanical behavior of sediments recovered from below the seafloor in the eastern nankai trough where the first japanese offshore production test was conducted in 2013 the sediments were recovered using a hybrid pressure coring system with pressure cores cut using onboard pressure core analysis tools the pressure cores were subsequently transferred to our shore based laboratory and subsampled using pressure core non destructive analysis tools pnats for the tactt system pressure and temperature conditions were maintained within the hydrate stability boundary during coring and laboratory testing an image processing technique was used to capture deformation of the sediment sample within the transparent acrylic test cell and digital photographs were obtained for each 0 1 strain level experienced by the sample during the triaxial compression test analysis of the digitized images showed that sediments with 63 hydrate saturation exhibited brittle failure whereas hydrate free sediments exhibited ductile failure the increase in shear strength with increasing hydrate saturation in natural gas hydrates is in agreement with previous data from sediments containing synthetic gas hydrates

forcing of late cenozoic northern hemisphere climate by plateau uplift in southern asia and the american west geologic evidence indicates that net vertical uplift occurred on a large kilometer scale and at accelerating rates during the middle and late cenozoic in plateaus of southern asia and the american west based on this evidence general circulation model sensitivity tests were run to isolate the unique effects of plateau uplift on climate the experiments simulated significant climatic changes in many places some far from the uplifted regions the basic direction of most of these simulated responses to progressive uplift is borne out by changes found in the geologic record winter cooling of north america northern europe northern asia and the arctic ocean summer drying of the north american west coast the eurasian interior and the mediterranean winter drying of the north american northern plains and the interior of asia and changes over the north atlantic ocean conducive to increased formation of deep water the modeled changes result from increased orographic diversion of westerly winds from cyclonic and anticyclonic surface flow induced by summer heating and winter cooling of the uplifted plateaus and from the intensification of vertical circulation cells in the atmosphere caused by exchanges of mass between the summer heated and winter cooled plateaus and the mid latitude oceans disagreements between the geologic record and the model simulations in alaska and the southern rockies and plains may be related mainly to the lack of narrow mountain barriers in the model orography taken together the observed regional trends comprise much of the pattern of late cenozoic climatic deterioration in the northern hemisphere that culminated in the plio pleistocene ice ages the success of the uplift sensitivity experiment in simulating the correct pattern and sign of most of the observed regional climatic trends points to uplift as an important forcing function of late cenozoic climatic change in the northern hemisphere at time scales longer than orbital variations however the modest amplitude of the uplift induced cooling simulated at high latitudes indicates a probable need for additional climatic forcing

calibration of the clumped isotope geothermometer in soil carbonate in wyoming and nebraska usa implications for paleoelevation and paleoclimate reconstruction abstract recent work has shown that soil carbonates typically form as the soil dries after seasonal rainfall and therefore record seasonal aspects of climate rather than mean annual conditions because soil carbonate formation is closely related to the timing of local rainfall and drying it is necessary to understand the formation seasonality and temperature and soil water o 18 values recorded in modern soil carbonate before accurate estimates of stable isotope based paleoelevation and paleoclimate can be made here we study carbonate clumped isotope 47 and oxygen isotope o 18 compositions of modern soil carbonates and seasonal variations of soil moisture and temperature of nearby climate stations in wyoming and western nebraska usa to understand the seasonality of soil carbonate formation in semi arid to arid temperate montane settings we find that soil carbonate clumped isotope temperatures t 47 are 3 5 c higher than mean summer air temperature and are similar to or higher than average summer soil temperature at depths 40 cm soil moisture dramatically decreases in early summer following the cessation of spring rains and snowmelt and shows only brief increases after major mid late summer rain events soil water o 18 values calculated using carbonate o 18 and t 47 values are similar to the o 18 values of local mean summer precipitation these lines of evidence suggest that soil carbonates in our study area formed during times of soil dewatering in early summer and after major summer storm events in mid late summer and in or near equilibrium with mean summer precipitation o 18 values t 47 values of modern soil carbonates are inversely correlated with elevation with a lapse rate 4 0 c km similar to the modern air and soil temperature lapse rates calculated soil water o 18 values also are inversely correlated with elevation with a lapse rate of 3 7 km sample elevations can be reconstructed using both t 47 values and calculated soil water o 18 values between 900 m and 2600 m in our studied area with an accuracy of 0 5 km or less however o 18 values of precipitation and soil water reflect the complex interaction of climatic environmental and topographic conditions prior to carbonate formation whereas t 47 values may be influenced by site specific conditions we suggest that both soil carbonate t 47 values and calculated soil water o 18 values should be used to corroborate paleoelevation reconstruction and the seasonal nature of soil carbonate formation should be considered when soil carbonate t 47 values and o 18 values are applied for paleoclimate reconstruction

a new approach to stable isotope based paleoaltimetry implications for paleoaltimetry and paleohypsometry of the high himalaya since the late miocene the change in oxygen isotopic composition of precipitation is modeled using equilibrium fractionation during rayleigh distillation linked to the thermodynamics of atmospheric ascent and water vapor condensation the primary controllers of the vertical variation in isotopic composition with elevation are the low elevation temperature and relative humidity as these control the vertical distribution of condensation an empirical fit of precipitation versus model condensation based on alpine stations is derived this fit is represented in the model as the weighted mean composition of condensation within a 1000 m thick air parcel 1500 500 m above the ground surface and is used for all other regions comparison of model versus observed modern precipitation reveals a close fit particularly of more highly elevated sites comparison of modern waters in the himalayas and southern tibet with model predictions particularly as revealed by comparison of observed and predicted hypsometry provides additional support to the validity of the model finally application of this model to estimates of paleo waters in the himalayas and southern tibet reveals that this region had already achieved its present hypsometry by the late miocene about 10 ma ago fl 2001 published by elsevier science b v

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

injection of indian crust into tibetan lower crust a two dimensional finite element model study we propose a model in which the stronger indian crust injects into the weaker lower crust of tibet if the viscosity of the tibetan lower crust is below a critical value the continuous advancing of the indian crust beneath tibet would produce negligible variation in stresses on the bottom of the stronger brittle tibetan upper crust and would result in a spatially uniform uplift and a virtually undisturbed tibetan plateau surface this process is analogous to that of a piston rising in a hydraulic jack as additional fluid is forced into the chamber we find that if the viscosity of the tibetan lower crust is assumed to be constant throughout the computed critical viscosity is 6 1018 pa s for the present day lower crustal thickness of 58 km in addition to uniform viscosity two hypothetical depth dependent viscosities are also tested which give rise to essentially the same distribution of stress along the bottom of brittle upper crust our model also interprets the 4 km elevation contrast between the tibetan plateau and the tarim block a 1 160 viscosity contrast between the tibetan and tarim lower crust is required in order to maintain the current thickening rate of the tibetan crust 1 2 mm yr while keeping the thickness of the tarim crust unchanged

new cretaceous and early tertiary paleomagnetic results from xining lanzhou basin kunlun and qiangtang blocks china implications on the geodynamic evolution of asia we present the results of a paleomagnetic study of 360 cores drilled at three different areas from the tibetan plateau 13 sites from lower cretaceous red beds in the xining lanzhou basin around xining and lanzhou cities southwest of the qilian mountains 36 2 n 103 5 e 13 sites from cretaceous red beds in the kunlun block near maqin 34 5 n 100 1 and 9 sites from paleocene lower eocene red beds in the qiangtang block near fenghuoshan 34 5 n 92 8 e thermal demagnetization of the samples allowed us to isolate a high temperature component which passes both positive reversal and fold tests for all formations the corresponding paleopoles lie at 50 3 n 195 5 e a95 4 6 for xining lanzhou 80 1 n 281 2 e dp 7 8 dm 12 7 for maqin and 62 6 n 210 5 e dp 3 9 dm 6 8 for fenghuoshan we discuss these in the frame of a new paleogeographic reconstruction of the cretaceous paleoposition of the blocks forming the asian mosaic we conclude that the xining lanzhou area could not be part of the north china block but rather was associated with the tarim qaidam assemblage paleomagnetic data argue in favor of a qaidam kunlun tarim junggar assemblage in the cretaceous significantly to the south of its current position with respect to the asian continent siberia mongolia and north china the large n s convergence 800 500 km implied since the cretaceous appears to be far larger than could be absorbed in the altay range to the north and qilian shan to the east of the order of 300 km part of this motion could have occurred along a large left lateral strike slip fault system which may connect with the mongol okhotsk suture to the northeast verifying this hypothesis will require new geologic and paleomagnetic data from these remote regions

hot or not impact of seasonally variable soil carbonate formation on paleotemperature and o isotope records from clumped isotope thermometry quantifying the timing and temperature of soil carbonate formation is important for interpreting isotopic records of earth surface temperature and soil water composition from paleosols pedogenic carbonates typically have been assumed to form at mean annual temperature but recent work suggests warm season bias in carbonate formation could impact the interpretation of d 18 o values and carbonate

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

rapid late miocene rise of the bolivian altiplano evidence for removal of mantle lithosphere abstract the oxygen isotopic compositions of carbonates deposited in the northern altiplano demonstrate a rapid change in late miocene time which we attribute to an increase in elevation rainfall samples over an elevation transect adjacent to the altiplano of northern bolivia display a systematic decrease in mean annual 18 o values and define a local 18 o vs altitude gradient r gonfiantini m a roche j c olivry j c fontes g m zuppi the altitude effect on the isotopic composition of tropical rains chem geol 181 2001 147 167 which we apply to paleo meteoric water values determined from carbonates age constraints from magnetostragraphy and paleoelevation estimates suggest surface uplift of 2 5 to 3 5 km occurred between 10 3 ma and 6 8 0 4 ma when the altiplano rose to its current elevation only the removal of dense eclogitic lower crust and mantle lithosphere can generate a change of this magnitude and rapidity this change coincides with an 30 decrease in the rate of convergence between the south america and nazca plates and the propagation of deformation into the eastern subandean zone which we infer to have resulted from the increase in the force per unit length applied by the elevated andean plateau to the surrounding lowlands

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

crenulation cleavage formation evolving diffusion deformation and equilibration mechanisms with increasing metamorphic grade abstract an integrated microstructural and chemical study of crenulation cleavages developed in chlorite to garnet zone schists from the wenchuan maowen shear zone central china has resulted in the recognition of the deformation and equilibration mechanisms operating during their formation at a range of temperatures deformation during crenulation cleavage formation at chlorite to biotite zone temperatures 520 c is essentially a solution deposition process in which transport occurred by a combination of localized advection and grain boundary diffusion recrystallization and hence equilibration of muscovite in discrete cleavages is recognized by distinct compositional differences between muscovite from different domains crenulation cleavages developed in garnet zone schist are characterized by homogeneous muscovite compositions across domains and microstructures related to intracrystalline deformation indicating an increasing importance of volume diffusion and diffusive equilibration of mica at temperatures greater than 520 c

stable isotopic results from paleosol carbonate in south asia paleoenvironmental reconstructions and selective alteration article i nfo we measured 18 o and 13 c values of paleosol carbonate nodules from a variety of sedimentary units in southern asia in order to reconstruct paleoenvironmental conditions and to evaluate for possible diagenetic alteration paleosol carbonate nodules were collected from lower to upper cretaceous strata exposed in two locations in southern tibet and from lower to middle miocene strata in northern india and nepal additional samples were collected from stratigraphically adjacent marine carbonate units to test for possible resetting of stable isotope values based on the implausibly low 18 o values 13 of most of the marine carbonate samples we interpret the 18 o values of the associated paleosol carbonate nodules as being reset and therefore unreliable for paleoenvironmental or paleoelevation reconstruction importantly the 18 o values of 13 c values of marine carbonate rocks fall within the expected primary

influence of vegetation type and site to site variability on soil carbonate clumped isotope records andean piedmont of central argentina 32 34 s abstract the clumped isotope geothermometer estimates the formation temperature t 47 of carbonates and has great potential to enhance the extraction of environmental data from pedogenic soil carbonate in the geologic record however the influence of vegetation type and site specific conditions on carbonate formation processes and t 47 records remains poorly understood this study examines the potential for variability in t 47 data between nearby same elevation sites with different c 3 c 4 biomass pedogenic carbonates undercoatings and nodules were collected from five modern soil pits in the semi arid eastern andean piedmont of argentina under a summer precipitation regime three pits were instrumented with temperature and moisture sensors to 1 m depth and a fourth was instrumented with additional soil co 2 and atmospheric temperature relative humidity insolation and rainfall sensors t 47 values mean 30 6 c 1se are invariant with depth and are statistically indistinguishable between the four instrumented sites though a 10 c difference between our t 47 values and those of a nearby peters et al 2013 epsl study suggests the potential for significant site to site variability likely due to local soil hydrology the results of this study suggest that deeper 40 cm t 47 values are consistent with carbonate formation during the early part of soil drying immediately after large mid summer rainstorms carbonate formation 40 cm depth may be biased to soil drying after small frequent precipitation events occurring throughout the spring summer and fall months averaging to shallow summer t 47 values and resulting in a near isothermal t 47 profile

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

late cenozoic uplift of southeastern tibet the age of surface uplift in southeastern tibet is currently unknown but the initiation of major river incision can be used as a proxy for the timing of initial uplift the topographically high eastern plateau and gently dipping southeastern plateau margin are mantled by an elevated low relief relict landscape that formed at a time of slow erosion at low elevation and low tectonic uplift rates prior to uplift of the eastern tibetan plateau thermochronology from deep river gorges that are cut into the relict landscape shows slow cooling between ca 100 and ca 10 20 ma and a change to rapid cooling after ca 13 ma with initiation of rapid river incision at 0 25 0 5 mm yr between 9 and 13 ma a rapid increase in mean elevation of eastern tibet beginning at this time supports tectonic climate models that correlate the lateral eastern expansion of high topography in tibet with the late miocene intensification of the indian and east asian monsoons

paleoelevation reconstruction using pedogenic carbonates paleoelevation reconstruction using stable isotopes although a relatively new science is making a significant contribution to our understanding of the recent growth of the world s major orogens in this review we examine the use of both light stable isotopes of oxygen and the new clumped isotope 47 carbonate thermometer in carbonates from soils globally the oxygen isotopic composition 18 o of rainfall decreases on average by about 2 8 km of elevation gain this effect of elevation will in turn be archived in the 18 o value of soil carbonates and paleoelevation can be reconstructed provided 1 temperature of formation can be estimated 2 the effects of evaporation are small 3 the effects of climate change can be accounted for and 4 the isotopic composition of the carbonate is not diagenetically altered we review data from modern soils to evaluate some of these issues and find that evaporation commonly elevates 18 o values of carbonates in deserts an effect that would lead to underestimates of paleoelevation some assessment of paleoaridity using qualitative indicators or carbon isotopes from soil carbonate is therefore useful in evaluating the oxygen isotope based estimates of paleoelevation sampling from deep 50 cm in paleosols helps reduce the uncertainties arising from seasonal temperature fluctuations and from evaporation the new clumped isotope 47 carbonate thermometer expressed as 47 offers an independent and potentially very powerful approach to paleoelevation reconstruction in contrast to the use of 18 o values nothing need be known about the isotopic composition of water from which carbonate grew in order to estimate of temperature of carbonate formation from 47 values using assumed temperature lapse rates with elevation paleoelevations can thereby be reconstructed case studies from the andes and tibet show how these methods can be used alone or in combination to estimate paleoelevation in both cases the potential for diagenetic alteration of primary carbonate values first has to be assessed clear examples of both preservation and alteration of primary isotopic values are available from deposits of varying ages and burial histories 47 values constitute a relatively straightforward test since any temperature in excess of reasonable surface temperatures points to diagenetic alteration for 18 o values preservation of isotopic heterogeneity between different carbonate phases offers a check on diagenesis results of these case studies show that one area of south central tibet attained elevations comparable to today by the late oligocene whereas 2 7 0 4 km of uplift occurred in the bolivian altiplano during the late miocene

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

modelling the oxygen isotope distribution of ancient seawater using a coupled ocean atmosphere gcm implications for reconstructing early eocene climate one of the motivations for studying warm climates of the past such as the early eocene is the enhanced understanding this brings of possible future greenhouse conditions traditionally climate information deduced from biological or chemical proxies have been used to test computer model simulations of past climatic conditions and hence establish some of the uncertainties associated with model based predictions however extracting climate information from proxies is itself an interpretative process and discrepancies between climate information inferred from different types of proxy undermines the assumption that model data conflicts automatically mean that the model is inherently flawed a new approach which both acknowledges and reduces the uncertainties associated with both model and data is required although the oxygen isotopic ratio 18o preserved in calcareous marine fossils has been used to reconstruct past seawater temperature for several decades significant uncertainties associated with this method persist these include assumptions about past seawater 18o for which no proxy exists and which is a key control on the temperature inferred from fossil carbonate here we present the results of an early eocene simulation made using a state of the art general circulation model gcm hadcm3 with co2 set at six times pre industrial values and which has oxygen isotopes incorporated into the full hydrological cycle and hence simulates the 18o of past seawater this allows us to explore the implications of the different seawater 18o correction factors commonly used for 18o based temperature reconstruction it also allows us to focus model data comparison on 18o rather than interpret ocean temperature an approach that reduces uncertainties in model data comparison since the effects of both the temperature and the isotopic composition of ocean water on 18o of carbonate are accounted for the good agreement between model and data for both modern and well preserved early eocene carbonate increases confidence in climate reconstructions of this time

mesozoic cooling across the yidun arc central eastern tibetan plateau a reconnaissance 40ar 39ar study abstract 40 ar 39 ar dating of hydrous minerals from igneous and metamorphic rocks was undertaken with the aim of characterizing the thermal history of the yidun arc eastern tibetan plateau regional metamorphism of palaeozoic rocks of the western yidun arc occurred during the early triassic collision with the qiangtang block along the jinsha jiang suture hornblende muscovite and biotite 40 ar 39 ar ages from syn tectonic igneous rocks within this collision zone suggest that the region had cooled to below 300 c by the late triassic muscovite samples from mica schists within this collision zone yield variably discordant age spectra with generally triassic 40 ar 39 ar ages consistent with cooling of the region to below muscovite closure temperatures by the close of the triassic hornblende and biotite 40 ar 39 ar ages from granite and granodiorite plutons in the eastern yidun arc suggest post emplacement cooling of these plutons occurred rapidly consistent with these granites being high level intrusions in sum these results suggest that by the beginning of the jurassic the yidun arc as a whole had cooled to below 300 c two cretaceous granites in the central yidun arc also experienced rapid cooling suggesting they too were emplaced at shallow crustal depths post triassic deformation of the region must have occurred at or below the brittle ductile transition within the crustal levels now exposed

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

thermochronologic evidence for plateau formation in central tibet by 45 ma the timing of tibetan plateau development remains elusive despite its importance for evaluating models of continental lithosphere deformation and associated changes in surface elevation and climate we present new thermochronologic data biotite and k feldspar 40 ar 39 ar apatite fi ssion track and apatite u th he from the central tibetan plateau lhasa and qiangtang terranes the data indicate that over large regions rocks underwent rapid to moderate cooling and exhumation during cretaceous to eocene time this was coeval with 50 upper crustal shortening suggesting substantial crustal thickening and surface elevation gain thermal modeling of combined thermochronometers requires exhumation of most samples to depths of 3 km between 85 and 45 ma followed by a decrease in erosional exhumation rate to low values of 0 05 mm yr the thermochronological results when interpreted in the context of the deformation and paleoaltimetric history are best explained by a scenario of plateau growth that began locally in central tibet during the late cretaceous and expanded to encompass most of central tibet by 45 ma

stable isotope paleoaltimetry a critical review stable isotope paleoaltimetry makes use of systematic trends in the distribution and isotopic composition of modern precipitation with climate and topography and of the potential to estimate the isotopic composition of paleoprecipitation from authigenic in situ formed minerals to illustrate the usefulness as well as potential limitations of this method we review 1 processes controlling the isotopic composition of modern precipitation 2 stable isotope data from modern precipitation across regions of high topography and 3 stable isotope data from authigenic minerals that have been used to infer paleotopography from this we conclude that stable isotope studies of authigenic minerals can permit useful inferences on paleotopography with uncertainties that critically depend on a detailed understanding of local to global scale paleoclimate during the time interval of interest

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

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

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

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

evolution of early cenozoic marine temperatures the equator to high southern latitude sea surface and vertical temperature gradients are reconstructed from oxygen isotope values of planktonic and benthic foraminifers for the following five time intervals late paleocene early eocene early middle eocene late eocene and early oligocene paleotemperatures are calculated using standard oxygen isotope temperature equations with adjustments to account for 1 variations in sea water 18o related to changes in global ice volume over time and 2 latitudinal gradients in surface water 18o these reconstructions indicate that sea surface temperatures sst of the southern oceans in the early eocene were as high as 15 c whereas temperatures during the late paleocene and early middle eocene reached maximum levels of 10 12 c by the late eocene and early oligocene high latitude sst had declined to 6 and 4 c respectively for most of the early paleogene low latitude sub tropical temperatures remained constant and well within the range of holocene temperatures 24 ndash 25 c but by the late eocene and early oligocene declined to values in the range of 18 to 22 c the late paleogene apparent decline in tropical temperatures however might be artificial because of dissolution of near surface foraminifera tests which biased sediment assemblages toward deeper dwelling foraminifera moreover according to recent plate reconstructions it appears that the majority of sites upon which the late eocene and early oligocene tropical temperatures were previously established were located either in or near regions likely to have been influenced by upwelling global deepwater temperature on average paralleled southern ocean sst for most of the paleogene we speculate based on the overall timing and character of marine sea surface temperature variation during the paleogene that some combination of both higher levels of greenhouse gases and increased heat transport was responsible for the exceptional high latitude warmth of the early eocene

a mechanistic analysis of early eocene latitudinal gradients of isotopes in precipitation we use a one dimensional reactive transport model of isotopes in precipitation 18o to investigate the physical mechanisms controlling global meridional isotope profiles under early eocene hothouse conditions simulations of early eocene precipitation isotopes display reduced meridional gradients relative to the modern climate with the largest increases in 18o occurring at high latitudes matching proxy data these reduced gradients are controlled primarily by polar amplification that increases high latitude length scales of specific humidity and match our compilation of proxy based reconstructions comparing eocene general circulation model simulations run with pco2 of 2240 and 4480 ppm we find that meridional isotopic profiles are insensitive to the associated 5 c change in global temperatures due to the relative lack of polar amplification finally we hypothesize that observed negative d anomalies in precipitation during peak warming of early eocene hyperthermal events are the result of a theorized reduction in the strength of midlatitude transient eddies

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

the late miocene through present paleoelevation history of southwestern tibet recent research using stable isotopes of carbon and oxygen from carbonates and fossil teeth seems to support both a pre and post mid miocene uplift of the southern tibetan plateau we examined this issue by analysis of well preserved fossil mollusks and plant remains from the zhada basin in southwestern tibet which ranges in age from 9 2 to paleoenvironmental modeling and physical evidence shows that the climate in zhada basin was cold and arid indistinguishable from modern the 13cpm values of well preserved vascular plant material increase from 23 4 to 26 8 permil at the base of the zhada formation to as high as 8 4 permil above 250 to 300 m this shift denotes the expansion of c4 biomass in this high arid watershed at 7 ma and thus corresponds to the c3 c4 transition observed in neogene deposits of the northern indian sub continent

stable isotopes of surface water across the longmenshan margin of the eastern tibetan plateau characterization of the stable isotope compositions 18o and d of modern day surface waters traversing mountain ranges and bordering continental plateaus is important for refining climate models and establishing modern isotope elevation gradients along mountain ranges the longmenshan margin of the tibetan plateau is a steep 4 km topographic front situated near the boundaries between westerlies and asian monsoon moisture sources and is previously unexplored with respect to the variation in water isotopic composition with elevation this study reports stable isotope data from 101 water samples collected from streams springs and ponds along the min river minjiang watershed which traverses the longmenshan margin local meteoric water lines d excess values and surrounding precipitation and river water data sets suggest that precipitation across the longmenshan margin is dominated by the east asian summer monsoon the increase in d excess values with increasing catchment elevation breaks down as local moisture recycling becomes important at elevations 3 km a s l along the min river however the 18ow and dw values decrease with increasing catchment elevation which fit second order polynomial curves and are well approximated by a simple rayleigh fractionation processes the temperature corrected oxygen values in authigenic carbonates from the pleistocene zoige basin north of the min river watershed yield elevations equivalent to present by the oxygen isotope elevation relationship of the longmenshan margin

δ18o and δd of streamwaters across the himalaya and tibetan plateau implications for moisture sources and paleoelevation reconstructions abstract this study presents new 18o and d data from 191 streams across the himalaya and tibetan plateau to better constrain the spatial variability of stable isotopes in modern precipitation over this region moisture penetrating into the southeastern tibetan plateau is predominantly derived from monsoonal airmasses originating from the bay of bengal and transported into the eastern himalayan syntaxis along the brahmaputra river progressive rainout during orographic lifting and cooling results in clear relationships between 18o and d and catchment hypsometric elevation on the plateau margin however monsoonal derived moisture is progressively mixed with central asian airmasses in more western and northern parts of the tibetan plateau as a result predicted isotope elevation relationships that are based on empirical lapse rates or thermodynamic models of the isotopic evolution of an airmass produce large 1 3 km misfits between measured and predicted catchment elevations for much of the tibetan plateau including some areas directly north of the central himalayan crest this suggests that changes in the 18o or d of paleoprecipitation on the central and southwestern tibetan plateau may reflect surface uplift along moisture transport pathways or changes in the penetration of monsoonally derived moisture rather than regional surface uplift histories

asian monsoons in a late eocene greenhouse world the strong present day asian monsoons are thought to have originated between 25 and 22 million years myr ago driven by tibetan himalayan uplift however the existence of older asian monsoons and their response to enhanced greenhouse conditions such as those in theeocene period 55 34myrago are unknownbecause of the paucity of well dated records here we show late eocene climate records revealing marked monsoon like patterns in rainfall and wind south and north of the tibetan himalayan orogen this is indicated by low oxygen isotope values with strong seasonality in gastropod shells and mammal teeth from myanmar and by aeolian dust deposition in northwest china our climate simulations support modern like eocene monsoonal rainfall and show that a reinforced hydrological cycle responding to enhancedgreenhouse conditions counterbalanced the negative effect of lowertibetanrelief onprecipitation these strong monsoons later weakened with the global shift to icehouse conditions 34myr ago

denudational response to surface uplift in east tibet evidence from apatite fission track thermochronology east tibet preserves broad areas of low relief that occur at elevations 3000 m and are dissected by major strike slip faults that have been repeatedly reactivated since the late mesozoic apatite fission track samples from these low relief high elevation surfaces indicate cretaceous cooling ca 90 ma fast rock uplift of the 110 c isotherm and a fossilized partial annealing zone began in the plateau interior of east tibet in the miocene low denudation rates in the plateau interior during the cenozoic which are constrained by thermal history modeling and a consistency in ages from apatite fission tracks suggest that dip slip displacement on faults has been minor some localized areas have experienced the effects of structurally enhanced rock uplift and denudation or fault related hydrothermal reheating cooling ages in eocene sediments deposited on the older plateau surface suggest that rapid denudation occurred from the miocene to holocene other evidence of significant cenozoic denudation comes from river valleys in particular the yalong river valley where incision was initiated in the oligocene to miocene 28 12 ma estimated mean denudation in east tibet during the cenozoic was 1 2 km in the low relief high elevation plateau interior and at least 5 km at the high relief plateau margin in the longmen shan the region south of the longmen shan and the sichuan basin was not uplifted with the main tibetan plateau in the miocene but it has undergone enhanced denudation since the eocene due to an orographic effect most of the denudation in the east tibetan plateau interior has occurred in major river valleys incision in the major rivers of the longman shan occurred simultaneously and has kept pace with surface uplift that began in the middle late miocene 12 ma with acceleration of denudation rates in the late miocene 5 ma

cretaceous tertiary shortening basin development and volcanism in central tibet the geologic map pattern of the qiangtang terrane in central tibet defines a 600 km long and up to 270 km wide east plunging structural culmination it is characterized by early mesozoic blueschist bearing melange and upper paleozoic strata in the core and mainly triassic jurassic strata along the limbs in the western qiangtang terrane 84 e the culmination is unconformably overlain by weakly deformed mid cretaceous volcanic flows and tuffs along the bangong suture to the south 32 n 84 e mid cretaceous nonmarine red beds and volcanic rocks lie unconformably on jurassic suture zone rocks these relationships demonstrate that west central tibet was above sea level during the mid cretaceous and experienced significant denudation prior to mid cretaceous time growth of the qiangtang culmination is inferred to have initiated during southward emplacement of a thrust sheet of early mesozoic melange and upper paleozoic strata during the early cretaceous lhasa qiangtang collision the north south width of the inferred thrust sheet provides a minimum slip estimate of 150 km at 84 e decreasing eastward to 70 km at 87 e paleogene deformation in the qiangtang terrane is characterized by widely distributed mainly north dipping thrust faults that cut eocene oligocene red beds and volcanic rocks in their footwall along the bangong suture the north dipping shiquanhe gaize amdo thrust system cuts 64 and 43 m y old volcanic tuffs in its footwall and accommodated 40 km of post mid cretaceous shortening the tertiary south dipping gaize siling co backthrust bounds the southern margin of the bangong suture and marks the northernmost limit of mid cretaceous marine strata in central tibet cretaceous deformation and denudation in central tibet is attributed to northward underthrusting of the lhasa terrane beneath the qiangtang terrane along the bangong suture this model implies that 1 cretaceous strata along the bangong suture and in the northern lhasa terrane were deposited in a flexural foreland basin system and derived at least in part from the qiangtang terrane and 2 the central tibetan crust was thickened substantially prior to the indo asian collision although its magnitude is poorly known tertiary shortening in the qiangtang terrane is more prevalent than in the lhasa terrane this difference may be attributed to the presence of underthrust melange in the deeper central tibetan crust which would have made it weaker than the lhasa terrane during the indo asian collision

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

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