middle miocene ocean climate transition high resolution oxygen and carbon isotopic records from deep sea drilling project site 588a southwest pacific high resolution stable isotopic records are presented for the epi benthic foraminifer cibicidoides the inferred shallow dwelling planktonic globigerinoides quadrilobatus and the inferred deep dwelling planktonic globoquadrina dehiscens from the middle miocene 16 12 ma of deep sea drilling project site 588a lord howe rise southwest pacific high resolution multiple species oxygen and carbon isotopic data define the timing and character of the well known middle miocene climatic oceanographic transition with a resolution comparable to quaternary records the benthic foraminiferal 18o record is marked by several large fluctuations from 16 to 14 8 ma followed by a series of rapid 50 kyr 18o increases that suggest a new state of the ocean climate system after 14 8 ma the total middle miocene benthic oxygen isotopic increase of 1 2 is largely incorporated in two steps an increase of 0 8 from 14 5 to 14 0 ma and a second increase of 0 7 from 13 45 to 12 45 ma each step is comprised of a series of marked 18o increases indicative of rapid east antarctic ice sheet growth and contemporaneous deepwater cooling a strong covariance of 0 7 between the benthic and deep dwelling planktonic species from 14 5 to 14 0 ma including a rapid increase from 14 1 to 14 05 ma suggests a 0 7 increase in the 18o composition of seawater 18osw because of east antarctic ice sheet growth comparison of the 18o record of gs quadrilobatus suggests that surface waters warmed at this site by 3 c from 14 1 to 13 6 ma carbon isotopic time series for each species generally covary throughout the early to middle miocene interval 16 12 ma confirming that 13c variations in this interval largely represent reservoir changes high resolution 13c data allow improved resolution of the latter five of six 13c maxima within the well known early to middle miocene carbon isotopic excursion the monterey carbon isotopic excursion from 17 0 to 13 5 ma this is useful for global correlation the last of these maxima ends with a 1 decrease centered from 13 9 to 13 7 ma 300 kyr after the 18o increase considered to reflect east antarctic ice growth covariance between benthic 18o and 13c from 16 to 13 8 ma suggests a sensitive relation between global carbon cycling and the ocean climate system prior to 13 8 ma episodic increases in organic carbon burial may have contributed to deep sea benthic 13c maxima and synchronous global cooling the positive relationship ended at 13 8 ma indicative of changing relations between global carbon cycling and the ocean climate system brought on by the increased stability of the east antarctic ice sheet after a major growth phase from 14 5 to 14 0 ma

what controlled mid late miocene long term aridification in central asia global cooling or tibetan plateau uplift a review abstract debate continues over whether global cooling or uplift of the tibetan plateau provided the first order driver for the aridification moisture levels of central asia throughout the mid late miocene between about 17 and 5 ma this review attempts to throw new light on the relations between the aridification and these two key factors this paper examines the evolution of miocene climate both moisture and temperature within five separate regions of eurasia to help understand the large scale controls of long term moisture in central asia the five regions include 1 europe 2 high latitude asia 3 the east asian monsoon region 4 the south asian monsoon region and 5 central asia itself because moisture reaching central asia has to firstly cross at least one of the other four regions temperature proxy data compiled from these five regions correlate with ocean temperatures from the global deep sea oxygen isotope records furthermore compiled moisture proxy data from the four regions surrounding central asia co vary and correlate with each other this can be explained by positive feedbacks between drying and cooling and supports the assumption that global cooling provides a dominant driving factor for the drying of eurasia global cooling reduces the amount of water vapor held in the atmosphere and thereby can cause terrestrial drying however in central asia the moisture evolution shows less similarity with its surroundings the uplift of the tibetan plateau including the tianshan mountains could provide a possible explanation for this difference the changing topography resulting from uplift events over time and space strongly influenced the moisture patterns in central asia during miocene times future research on the spatial timing and amplitude of tibetan plateau uplift should be useful to understand the moisture processes in central asia during the miocene

late cenozoic tectonic evolution of the northwestern tien shan new age estimates for the initiation of mountain building the tien shan are the quintessential intracontinental range situated more than 1000 km north of the suture between india and asia their initiation and growth in the cenozoic however remain poorly understood in this study we present stratigraphic detrital fission track and magnetostratigraphic results that provide a basis for reconstructing the cenozoic tectonic evolution of the kyrgyz range and adjacent chu basin in the northwestern tien shan detrital fission track thermochronology indicates that the northwestern tien shan was tectonically quiescent for much of the cenozoic prior to uplift and exhumation in the late miocene the kyrgyz range was buried by sediments shed from highlands to the south and or east paired bedrock fission track and u th he ages from a sampling transect of 2 4 km relief demonstrate that rapid exhumation commenced at ca 11 ma initial thrusting in the hinterland was followed by evaporite accumulation 0 4 km m y which coincided with erosion of the pre 11 ma strata that mantled the kyrgyz range between 10 and 3 ma bedrockexhumation rates decreased to 0 3 km m y while sedimentation rates decelerated initially to 0 25 km m y before accelerating to 0 4 km m y at 4 5 ma detrital fission track results indicate that by 4 5 ma

onset timing of significant unroofing around qaidam basin northern tibet china constraints from 40ar 39ar and ft thermochronology on granitoids 40ar 39ar and fission track thermochronological results from two granitoid suites in the mountain ranges along the northern and southern edges of qaidam basin constrain the timing of a tertiary denudation event and their tectonic association in northern tibet the cooling histories based upon 40ar 39ar and fission track ages suggest a rapid cooling event 7 5 10 7 c ma during oligocene early miocene time this cooling event is thought to reflect an increasing denudation rate associated with the rapid uplift and denudation of the north qaidam and kunlun shan mountain ranges with consideration to the field evidence sedimentation rate of the qaidam basin and the propagation rate along the altyn tagh fault the rapid uplifting and denudation are probably due to crustal thickening associated with cenozoic propagation of the altyn tagh fault in accommodating the collision of india with asia

palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record interest in palustrine carbonates and calcretes has increased over the last 20 years since they contain significant environmental information much of the work performed in this area has focused on either of two types of terrestrial carbonate palustrine carbonates or calcretes pedogenic and groundwater yet their simultaneous study shows there may be a gradual transition from one form to the other revealing the interplay between pedogenic sedimentary and diagenetic processes three main factors control the formation of these carbonates the position of the water table the host rock and the period of sub aerial exposure in pedogenic calcretes precipitation of carbonate takes places mostly in the vadose zone above the water table and within a previous host rock or sediment in groundwater calcretes the precipitation of carbonate also occurs within a previous host rock and around the groundwater table in palustrine carbonates however the precipitation of lime mud occurs in a lacustrine water body palustrine carbonates necessarily form on previous lacustrine mud whereas both types of calcretes may form on any type of sediment or soil the sub aerial exposure time needed to form palustrine carbonates may by relatively short even a season whereas pedogenic calcretes need more time several years to millions of years groundwater calcretes do not form on the topographic surfaces so there is no need of sub aerial exposure however stable surfaces favour the development of thick groundwater calcretes small fluctuations in the water table cause gradual transitions of these three types of terrestrial carbonates and the subsequent mixture of their characteristic features causing difficulties in the interpretation of these carbonates the formation of these carbonates is controlled by palaeoenvironmental factors both commonly form in semi arid climates arid climates are also suitable for calcretes but sub humid conditions are more suitable for palustrine carbonates more indications of climatic conditions may be obtained through the analysis of the d18o content of both calcretes and palustrine carbonates and from the depth of the horizon containing carbonate nodules in pedogenic calcretes vegetation is also important in the formation of these types of carbonates data on the prevailing vegetation can be obtained from the analysis of the micro and macrofabric as well as from the d13c signal of the primary carbonates which in pedogenic carbonates has also been used to estimate atmospheric pco2 during the phanerozoic these terrestrial carbonates are widely distributed on floodplains and distal areas of alluvial basins their presence and characteristics can be used as indicators of aggradation subsidence or accommodation rates and therefore as indicators of different tectonic regimes even though the study of these carbonates has notably increased in recent years much less is known about them than about marine carbonates presently there is much emphasis on obtaining a general model for sequence stratigraphy in terrestrial basins with a need to include the carbonates analysed in this paper

ice stream shear margin moraines the distribution and pattern of subglacial bedforms provides valuable insights into the configuration and behaviour of former ice sheets and recent work has begun to address the landform assemblages of palaeo ice streams in this paper we focus on subglacially formed ice stream shear margin moraines these landforms are thought to form in the shear zone between fast flowing stream ice and slow moving sheet ice using satellite imagery and aerial photographs we identify four ice stream shear margin moraines associated with the former m clintock channel palaeo ice stream victoria island arctic canada the moraines range in length from 11 to 22 km maintain fairly constant widths of around 500 m and range in height from 10 to 50 m above the surrounding terrain they are composed of carbonate drift of a similar composition as the ice stream bedforms and have been laid down irrespective of local and regional topography two of the moraines display a lateral offset and are thought to reflect minor migrations of the ice stream margin we explore their possible mode of formation and suggest that they occur when erosion at the ice stream margin provides a surplus of sediment which is smeared out in the downstream direction the identification of other palaeo ice stream shear margin moraines has probably been hampered by their interrupted continuity which can only be appreciated at the large scale it is hoped that the descriptions presented in this paper will help to identify these landforms and increased detection of palaeo ice streams will reveal other ice stream shear margin moraines copyright 2002 john wiley sons ltd

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

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

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

sandstone provenance of north western sectors of the intracontinental cenozoic qaidam basin western china tectonic vs climatic control abstract sandstone samples from all cenozoic formations of the nonmarine internally drained qaidam basin have been collected for provenance analysis to monitor possible changes in sandstone composition driven either by tectonics or climate provided an appropriate hinterland biotite contents may be a reasonable indicator for aridity moderately sorted sandstones with subangular to subrounded grains from the northwestern part of the qaidam basin are immature indicated by a high proportion of feldspars and lithic fragments nevertheless they have relatively high quartz contents of 29 65 averaging between 40 and 45 the average framework compositions of the formations show only a slight variation over time limited to the oldest and youngest cenozoic formations contents of white mica and biotite are significantly higher in upper portions of the section white mica contents increases from 1 0 to 3 7 and biotite from 0 1 to 1 6 respectively this change can be explained by increasing aridity during neogene and surface uplift of altyn mountains and qimantagh which led to higher relief when plotted on basic variation diagrams total quartz feldspar lithic fragments there is a slight shift from continental block field to orogenic field which might reflect the motion of the altyn tagh fault since oligocene times although climate changed during the cenozoic from arid over semiarid back to arid we could not find any distinct changes in the framework mineral compositions over time that could be related to climatic events few selected mudstones from eocene to lower miocene formations have been geochemically analysed by icp ms all samples display uniform high enrichment in light rees in chondrite normalized diagrams with one exception the ree concentrations are decreasing in a stepwise manner although insignificantly from formation to formation with time due to a diluting effect by carbonate

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

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

interpretation of shadows and antishadows on saturn and the evidence against south polar eyewalls abstract cassini spacecraft observations of saturn in 2006 revealed south polar cloud shadows the common interpretation of which was initiated by dyudina et al 2008 science 319 1801 who suggested they were being cast by concentric cloud walls analogous to the physically and optically thick eyewalls of a hurricane here we use radiative transfer results of sromovsky et al 2019 icarus this issue in conjunction with monte carlo calculations and physical models to show that this interpretation is almost certainly wrong because 1 optically thick eyewalls should produce very bright features in the poleward direction that are not seen while the moderately brighter features that are seen appear in the opposite direction 2 eyewall shadows should be very dark but the observed shadows create only 5 10 i f variations 3 radiation transfer modeling of clouds in this region have detected no optically thick wall clouds and no significant variation in pressures of the model cloud layers and 4 there is an alternative explanation that is much more consistent with observations the most plausible scenario is that the shadows near 87 9 s and 88 9 s are both cast by overlying translucent aerosol layers from edges created by step decreases in their optical depths the first in the stratospheric layer at the 50 mbar level and the second in a putative diphosphine layer near 350 mbar with optical depths reduced at the poleward side of each step by 0 15 and 0 12 respectively at 752 nm these steps are sufficient to create shadows of roughly the correct size and shape falling mainly on the underlying ammonia ice layer near 900 mbar and to create the bright features we call antishadows

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

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

palaeoclimate interpretation of stable isotope data from lake sediment archives the isotope composition of authigenic and biogenic carbonates and diatom silica are commonly used as palaeoclimate proxies from lake sediments this article reviews the controls on the isotope composition of lacustrine skeletal and non skeletal deposits and illustrates how stable isotope studies contribute to an understanding of changes in temperature precipitation patterns evaporation and the carbon cycle it highlights the differences in the palaeoclimate potential of a wide range of lakes ranging from open to closed lake basins a large number of the case histories but not all are drawn from studies of temperate lakes from europe large closed lake systems in the tropics and elsewhere lose water predominantly through evaporation and contain sediments with variable and generally high 18o values fluctuations in the isotope composition of authigenic or biogenic minerals are mainly a function of long term changes in the precipitation evaporation ratio in contrast small open lakes which have a degree of through flow typically contain sediments with 18o values that vary by no more than a few these variations are generally ascribed to variations in temperature or the isotope composition of precipitation p from which either an annual or seasonally specific signal can be gained these types of lakes are common in northern europe and at high altitudes the interpretation of isotope data from a lacustrine succession requires a knowledge of the local processes that might control and modify the signal their effects need to be quantified and a robust calibration using the modern lake system is necessary to establish the relationship between the measured signal the isotopic composition of the host waters and climate

eocene oligocene transition in central asia and its effects on mammalian evolution the eocene oligocene boundary eob marks a period of dramatic global climatic change correlated with pronounced mammalian faunal change the timing of these events is well constrained in north america and europe but the asian record has yet to produce a synthetic section linking environmental change mammalian fossils and precise geochronological dates here we present the first magnetostratigraphic section for the hsanda gol formation mongolia which yields significant oligocene fossils and also marks a pattern of aridification that is tightly correlated to the eob 33 9 ma supporting a broader pattern of aridification in the central asian plateau across the eob oligocene faunas of asia can now be confidently correlated to those of north america europe and africa these results suggest that mammalian faunal turnover within asia occurred slightly later than similar events within europe and question the influence of asian immigrants on the grande coupure faunal turnover

impacts of tectonic changes on the reorganization of the cenozoic paleoclimatic patterns in china abstract geologic studies have illustrated that the planetary wind dominant climate in the paleogene is changed into the monsoon dominant one near the oligocene miocene boundary in china the evolution is marked by the changes of regional aridity humidity contrasts the contrasts occur between the south and the north part of china in the eocene and then between east china and central asia near the oligocene miocene boundary indicating the onset of monsoon dominant climate in china the impacts of the himalaya tibetan plateau uplift and or the paratethys sea retreat on the asian monsoon have been well demonstrated however whether or not other factors have affected the above reorganization of paleoclimatic patterns remains a question to be addressed additional factors that should be addressed at least include the indian peninsular drift the south china sea expansion and the east china sea transgression here we use the iap agcm to explore their roles in the above paleoclimatic evolution our experiments demonstrate that the south china sea expansion is another major forcing in addition to the important roles of the paratethys retreat and the himalaya tibetan plateau uplift on the contrary the impacts of the indian peninsular drift and the east china sea transgression are relatively subordinate the himalaya tibetan plateau uplift plays a crucial role in the magnification of the aridity humidity contrasts between the south and the north part of china the paratethys retreat the himalaya tibetan plateau uplift and the south china sea expansion coact to cause the formation of the aridity humidity contrasts between east china and central asia the retreat and the uplift favor the dynamic condition and the expansion provides the water vapor condition for the monsoon dominant climate in china

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

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

climatic variation in the linxia basin ne tibetan plateau from 13 1 to 4 3 ma the stable isotope record the 13 c and 18 o values of carbonates and 13 corg values c and c n ratios of organic matter from lacustrine and fluvial sediments were measured from two stratigraphic sections in the linxia basin on the northeastern margin of the tibetan plateau diagenesis when present is early limited to small spatial scales and restricted stratigraphic intervals a strong correlation exists between dolomite content and the 18 o values of micritic carbonate suggesting that carbonates are primary precipitates from lake water and that 18 o values reflect changes in the lake environment with more positive values derived from evaporated and more saline lake water the first part of this record is characterized by strong oscillations between dry and wet conditions accompanied by changes in the lake system between hydrographically closed and open states 13 1 8 0 ma the most severe aridity occurred from 9 6 to 8 5 ma after 8 0 ma a more stable and less arid climate dominated the region and the drainage system was open 8 0 5 3 ma after 5 3 ma climate became gradually drier and or cooler 5 3 4 3 ma the organic matter preserved in the linxia basin is most likely a mixture of terrestrial c3 plant matter and lake algae a correlation between c n and 13 corg indicates that a significant percentage of the organic matter is derived from terrestrial sources the organic matter has undergone selective degradation during which the c n ratios and organic carbon percentage decreased the 13 corg values are relatively stable throughout the 13 1 to 4 3 ma interval ranging from 24 to 29 vpdb suggesting that c4

monitoring cenozoic climate evolution of northeastern tibet stable isotope constraints from the western qaidam basin china carbon and oxygen stable isotopic composition of cenozoic lacustrine carbonates from the intramontane qaidam basin yields cycles of variable length and shows several distinct events driven by tectonics and climate changes from eocene to oligocene the over all trend in the 13c composition of lacustrine carbonates shows a shift toward higher values possibly related to higher proportions of dissolved inorganic carbon transported to the lake or lower input of soil derived co2 at the same time the 18o composition of lacustrine carbonates is decreasing in accordance with the global cooling trend and northwards drifting of the whole region during the miocene distinct isotopic events can be recognized although their interpretation and linkage to a certain tectonic event remains difficult these events may be related to uplift in the himalayas to the strongest phase of uplift in the altyn mountains to pronounced subsidence of the qaidam basin or to the expansion of c4 plants on land generally cold highly evaporative conditions can be deduced from enrichment of 18o isotopic compositions during pliocene and quaternary times

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

dominant control of the south asian monsoon by orographic insulation versus plateau heating heat emitted from the tibetan plateau as dry heat and water vapour has long been assumed to be the main driver of the south asian summer monsoon but new work suggests that in fact it is the neighbouring mountains that are the major influence william boos and zhiming kuang use an atmospheric model to show that flattening the tibetan plateau has little effect on the monsoon so long as the himalayas and surrounding mountain ranges remain the plateau does boost rainfall locally along its southern edge but it is the build up of hot moist air over india insulated from colder drier air by the himalayas that drives large scale monsoon circulation the elevation of the tibetan plateau is thought to cause its surface to serve as a heat source that drives the south asian summer monsoon potentially coupling uplift of the plateau to climate changes on geologic timescales here however an atmospheric model is used to show that flattening of the tibetan plateau has little effect on the monsoon provided that the narrow orography of the himalayas and adjacent mountain ranges is preserved the tibetan plateau like any landmass emits energy into the atmosphere in the form of dry heat and water vapour but its mean surface elevation is more than 5 km above sea level this elevation is widely held to cause the plateau to serve as a heat source that drives the south asian summer monsoon potentially coupling uplift of the plateau to climate changes on geologic timescales1 2 3 4 5 observations of the present climate however do not clearly establish the tibetan plateau as the dominant thermal forcing in the region peak upper tropospheric temperatures during boreal summer are located over continental india south of the plateau here we show that although tibetan plateau heating locally enhances rainfall along its southern edge in an atmospheric model the large scale south asian summer monsoon circulation is otherwise unaffected by removal of the plateau provided that the narrow orography of the himalayas and adjacent mountain ranges is preserved additional observational and model results suggest that these mountains produce a strong monsoon by insulating warm moist air over continental india from the cold and dry extratropics these results call for both a reinterpretation of how south asian climate may have responded to orographic uplift and a re evaluation of how this climate may respond to modified land surface and radiative forcings in coming decades

timing cause and impact of the late eocene stepwise sea retreat from the tarim basin west china abstract a vast shallow epicontinental sea extended across eurasia and was well connected to the western tethys before it retreated westward and became isolated as the paratethys sea however the palaeogeography and the timing of this westward retreat are too poorly constrained to determine potential wider environmental impacts let alone understanding underlying mechanisms of the retreat such as global eustasy and tectonism associated with the indo asia collision here an improved chronostratigraphic and palaeogeographic framework is provided for the onset of the proto paratethys sea retreat at its easternmost extent in the tarim basin in western china is provided five different third order sea level cycles can be recognised from the cretaceous palaeogene sedimentary record in the tarim basin of which the last two stepped successively westwards as the sea retreated after the maximum third incursion new biostratigraphic data from the fourth and fifth incursions at the westernmost margin of the tarim basin are compared to our recent integrated bio magneto stratigraphic results on the fourth incursion near the palaeodepocentre in the south western part of the basin while the fourth incursion extended throughout the basin and retreated at 41 ma base c18r the last and fifth incursion is restricted to the westernmost margin and its marine deposits are assigned a latest bartonian early priabonian age from 38 0 to 36 7 ma near top c17n 2n to base c16n 2n similar to the fourth the fossil assemblages of the fifth incursion are indicative of shallow marine near shore conditions and their widespread distribution across eurasia suggests that the marine connection to the western tethys was maintained the lack of diachronicity of the fourth incursion between the studied sections across the southwest tarim basin suggests that the sea entered and withdrew relatively rapidly as can be expected in the case of eustatic control on a shallow epicontinental basin however the westward palaeogeographic step between the fourth and fifth incursions separated by several millions of years rather suggests the combined long term effect of tectonism possibly associated with early uplift of the pamir kunlun shan thrust belt the fourth and fifth regressions are time equivalent with significant aridification steps recorded in the asian interior thus supporting climate modelling results showing that the stepwise sea retreat from central asia amplified the aridification of the asian interior

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

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

what triggers the transition of palaeoenvironmental patterns in china the tibetan plateau uplift or the paratethys sea retreat abstract geological research has illustrated the transition of palaeoenvironmental patterns by the earliest miocene from a planetary wind dominant type to a monsoon dominant type indicating that the east asian monsoon became markedly intensified and played a leading role in the east asian climate from a modeling point of view the pioneering research using the reduced number of scenarios had demonstrated that both the tibetan plateau uplift and the paratethys sea retreat were important for understanding the asian monsoon evolution however the sensitivity of the paratethys retreat to the east asian climate still needs further studies based on the more detailed scenarios thirty numerical experiments under the six paratethys sea and the five tibetan plateau conditions illustrate the shifts from zonal climate to the monsoon climate in east asia the results confirm again that both the paratethys retreat and the tibetan plateau uplift play important roles in the formation of the monsoon dominant environmental pattern and show that the paratethys retreat can strengthen the east asian monsoon and greatly increase humidity and aridity respectively in the monsoon areas and northwest china which is similar to the impact of the tibetan plateau uplift on the east asian climate furthermore the fact that the paratethys sea retreats to the turan plate is found to be the key criterion for the palaeoenvironmental patterns transition in china the shrinkage of paratethys sea leads to the reconstructions of the pressure system and the atmospheric circulations which result in the variations of precipitation and the transition of palaeoenvironmental patterns

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

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

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

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

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

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

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

accommodation of transpressional strain in the arabia eurasia collision zone new constraints from u th he thermochronology in the alborz mountains north iran the alborz range of n iran provides key information on the spatiotemporal evolution and characteristics of the arabia eurasia continental collision zone the southwestern alborz range constitutes a transpressional duplex which accommodates oblique shortening between central iran and the south caspian basin the duplex comprises nw striking frontal ramps that are kinematically linked to inherited e w striking right stepping lateral to obliquely oriented ramps new zircon and apatite u th he data provide a high resolution framework to unravel the evolution of collisional tectonics in this region our data record two pulses of fast cooling associated with sw directed thrusting across the frontal ramps at 18 14 and 9 5 7 5 ma resulting in the tectonic repetition of a fossil zircon partial retention zone and a cooling pattern with a half u shaped geometry uniform cooling ages of 7 6 ma along the southernmost e w striking oblique ramp and across its associated nw striking frontal ramps suggests that the ramp was reactivated as a master throughgoing n dipping thrust we interpret this major change in fault kinematics and deformation style to be related to a change in the shortening direction from ne to n nne the reduction in the obliquity of thrusting may indicate the termination of strike slip faulting and possibly thrusting across the iranian plateau which could have been triggered by an increase in elevation furthermore we suggest that 7 6 m y old s directed thrusting predated inception of the westward motion of the south caspian basin

stable isotope records of cenozoic climate and topography tibetan plateau and tarim basin we seek to better understand growth of the tibetan plateau and associated climate change by presenting the first regionally extensive set of oxygen isotope data obtained from ca 40 m y of cenozoic strata from a 700 000 km2 area ranging from the northern edge of the tibetan plateau northward across the entire tarim basin our results reveal a previously unrecognized large positive isotope shift from eocene to oligocene which we suggest results from initial topographic growth of the himalayan tibetan system and attendant reorganization of climate by this growing mountain belt a negative isotopic shift recorded in miocene strata may reflect retreat of the neotethyan sea from the tarim basin as well as enhanced elevation of the himalaya tibetan plateau region and the rise of the tian shan which diverted moisture bearing westerlies and deepened aridity in the tarim basin and the northern tibetan plateau

oligocene miocene tectonics and sedimentation along the altyn tagh fault northern tibetan plateau analysis of the xorkol subei and aksay basins abstract tertiary basins adjacent to the northern edge of the tibetan plateau include both giant intracontinental sedimentary basins such as tarim and qaidam and smaller basins directly related to the evolution of the altyn tagh strike slip fault system and nearby fold thrust belts three examples of these smaller basins that formed during early evolution of the altyn tagh fault system are the xorkol aksay and subei basins the xorkol basin was the site of nonmarine siliciclastic sedimentation in alluvial fan playa and fluvial environments from at least the mid oligocene through the miocene sediment was delivered to the basin from source areas uplifted by basin bounding thrust faults and from the opposite side of the altyn tagh fault the age of the aksay basin is more poorly constrained but it also contains oligocene miocene nonmarine clastic strata that were derived from the southern side of the altyn tagh fault and adjacent thrust fault bounded uplifts the subei basin is characterized by s

magnetostratigraphy of paleogene sediments from northern qaidam basin china implications for tectonic uplift and block rotation in northern tibetan plateau abstract we have carried out a detailed magnetostratigraphic study of tertiary sediments in the northern margin of the qaidam basin northern tibetan plateau a total of 400 paleomagnetic samples with an interval of about 2 m were collected from the shanggancaigou and xiagancaigou formations in the hongsanhan section stepwise thermal demagnetization was used to isolate the magnetic components in these samples a higher temperature component htc was separated between 300 and 680 c from most of samples positive fold and reversal tests suggest a primary origin for the htc a magnetic polarity sequence is established and then compared with the geomagnetic polarity time scale ostracoda assemblages and also a fission track age from an adjacent section suggest a tertiary age of the shanggancaigou and xiagancaigou formations in the hongsanhan area the magnetozones in the xiagancaigou formation may correspond to subchrons c16n c18 ranging from 40 to 35 5 ma mid eocene and the shanggancaikou formation to subchrons c8 2n c15 age from 35 5 to 26 5 ma late eocene to late mid oligocene the accumulation of sandy conglomerate and conglomerate layers on top of the shanggancaikou formation and an abrupt increase in sedimentation rates are probably resulted from the main uplift of the north qaidam at about 30 ma when compared with coeval directions expected from eurasia the mean declinations indicate a slight vertical axis rotation of the northern qaidam basin since the mid eocene sinistral shear strain may be concentrated on the altyn tagh fault atf