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

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

behavior of rare earth elements in a paleoweathering profile on granodiorite in the front range colorado usa abstract a paleoweathering profile on the boulder granodiorite in northern colorado provides an opportunity to trace the behavior of rees from parent rock through a weathering profile into unconformably overlying permian sediments with progressive upward weathering of the granodiorite na2o cao sio2 ta hf co th cr sc cr th zr hf la sc zr y and la th decrease al2o3 and fe2o3t increase and tio2 mgo k2o p2o5 rb zr sc cr co hf nb ta y th u ree ti nb and zr nb increase to maximum values and then either level off or decrease lree enrichment is less in the weathering profile than in the parent granodiorite and although the parent does not have an eu anomaly or only a slight positive anomaly all samples from the weathering profile and overlying sediments have significant negative eu anomalies this observation is especially important in that it shows conclusively that a negative eu anomaly can be produced during chemical weathering of granitoids we suggest these eu anomalies are due to relative enrichment of the other rees and partial loss of eu during the breakdown of plagioclase the boulder weathering profile also has a very minor negative ce anomaly that is within error of a ce anomaly in the parent in the unweathered parent 50 of the ree are contained in sphene and in the case of la also in allanite from 10 20 of the ree are contained in apatite and biotite hornblende and from 7 10 of the hrees are in zircon with exception of eu for which feldspars contribute about 8 negligible amounts of rees occur in the feldspars in weathered samples 75 of the rees are contained in clay minerals the crossover between sphene and clay control of rees occurs over a distance of 1 m near the contact with fresh rock except for their small negative eu anomalies the clay minerals have ree patterns very similar to those of the parent rock isocon plots suggest apparent enrichments of many elements in the boulder weathering profile result from losses of na ca and si during plagioclase weathering in addition variable amounts of sr eu ta nb p and ba were lost during weathering although th u zr y th sc zr hf lu hf and ti zr may have been transferred relatively unchanged from granodiorite parent to the bulk weathering profile most other element ratios and ree distributions were significantly changed during weathering this observation implies that caution needs to be exercised when using ree patterns and element ratios to trace sediment provenance the fact that most element ratios and ree distributions also differ between fountain sediments and the bulk weathering profile may be related to one or a combination of four factors listed in order of probable decreasing importance contribution of other sources to the fountain sediments sorting of minerals during sediment deposition remobilization of elements during diagenesis and leaching of elements by water flow through the upper meter of the weathering profile

antarctic continental weathering changes during eocene oligocene cryosphere expansion clay mineral and oxygen isotope evidence we have analyzed clay minerals and oxygen isotopes over the eocene oligocene climate transition 33 8 32 5 ma in ocean drilling program site 689 maud rise antarctica distinct changes in clay mineral assemblages suggest major instability in the east antarctic climate for 0 7 m y during this time of general expansion of the antarctic cryosphere and cooling of the southern ocean increased illite abundance reflects enhanced physical weathering associated with cryospheric development nevertheless continued dominance of smectite indicates that chemical weathering continued to prevail during the early oligocene weathering was much more intense than in modern deglaciated areas the clay mineral data support evidence from marine sediments suggesting that continental and marine climatic conditions during the early oligocene were intermediate between relative eocene warmth and intense neogene cold the clay mineral variations during the transition reflect major changes in continental precipitation and related continental ice accumulation

evidence for northeastern tibetan plateau uplift between 25 and 20 ma in the sedimentary archive of the xining basin northwestern china the growth history of the tibetan plateau provides a valuable natural laboratory to understand tectonic processes of the india asia collision and their impact on and interactions with asian and global climate change however both tibetan plateau growth and asian paleoenvironments are generally poorly documented in pre pliocene times and reflect limited temporal coverage for different parts of the plateau here we present magnetostratigraphic results from the xining basin at the ne margin of the tibetan plateau precisely dating the record between the earliest oligocene 33 ma to the middle miocene 16 ma the pattern of observed paleomagnetic polarity zones is unequivocally correlated to the geomagnetic polarity time scale gpts indicating relatively constant and low sediment accumulation rates 32 m myr except for a peculiar period of unstable accumulation between 25 3 and 19 7ma at the beginning of this interval a marked permanent increase in magnetite content of the sediments is observed and likely relates to a change in provenance we directly relate this unstable period of sediment accumulation and provenance change to the coeval exhumation recently reported by low temperature thermochronology from the laji shan range which subsequently formed the southernmargin of the xining basin evidence for ne tibet tectonismat 25 20 ma can be associated with widespread deformation over the entire himalayan tibetan orogen at this time which may be linked to the coeval appearance of monsoon climate in eastern asia and the onset of central asian desertification

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

rapid exhumation at 8 ma on the liupan shan thrust fault from apatite fission track thermochronology implications for growth of the northeastern tibetan plateau margin abstract the liupan shan mountain is one of the outermost ranges in northeastern tibetan plateau the onset of its uplift provides insight on whether the plateau grew sequentially outward or broad areas of the plateau deformed simultaneously the apatite fission track method can be used to date rapid cooling as the result of unroofing of rocks in response to tectonically induced vertical motion fission track data from apatite in early cretaceous sedimentary strata exposed on the hanging wall of liupan shan thrust fault in northeastern tibet suggest that a late cenozoic rapid cooling events occurred at 7 3 8 2 ma or 8 ma this age apparently dates fault activity of the liupan shan thrust in response to northeastward growth of the tibetan plateau uplift and lateral growth which continues today numerous studies of the tibetan plateau suggest that the onset of deformation in eastern and northern margin of tibetan plateau occurred in the latest miocene

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

timing of tibetan uplift constrained by analysis of volcanic rocks the tibetan plateau has had a central role in the development of recent models for the mechanics of mountain belts1 2 and cenozoic global climate change3 the present elevation and extensional deformation of the plateau probably result from uplift owing to convective thinning of the underlying lithospheric mantle1 2 an age for the uplift would provide a valuable constraint on these models but because recently proposed indicators of uplift are all climate dependent they are equivocal possibly reflecting global cooling rather than regional uplift4 here we present new geochemical data on post mid miocene lavas from the plateau which show that the lavas were derived from the lithospheric mantle simple thermal arguments indicate that the generation of such magmas also necessitates thinning of the lithospheric mantle thus volcanism is coincident with uplift providing a climate independent means of dating the onset of uplift dating by the laser 40ar 39ar technique places the beginning of this volcanism and therefore the time of uplift of the tibetan plateau at 13 myr ago

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

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

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

an evaluation of geochemical weathering indices in loess paleosol studies abstract applying geochemical proxies as measure for the weathering intensity of paleosols and sediments such as loess the quaternary scientist is confronted with various element ratios that have been proposed in literature this paper gives an overview on the principle of geochemical weathering indices different types of indices are evaluated with respect to the suitability for loess paleosol sequences regarding the special characteristics of this type of sediments and paleosols case examples in this study are key sections in southeastern and eastern europe the loess paleosol sequences batajnica stari slankamen serbia mircea voda romania and stary kaydaky ukraine which represent archives of the late and mid pleistocene climate change of the region considering element behavior during weathering or diagenesis the chemical proxy of alteration cpa i e the molar ratio al2o3 al2o3 na2o 100 is proposed as the most appropriate index for silicate weathering the cpa was evaluated against commonly used weathering indices including the chemical index of alteration cia the chemical index of weathering ciw the plagioclase index of alteration pia the index b of kronberg and nesbitt and the ba sr and rb sr ratio depth profiles of sr type indices e g ba sr rb sr are likely to be influenced by the dynamics of secondary carbonate on the other hand common na type indices e g cia pia ciw may suffer from uncertainties in separating carbonate ca from silicate ca or from biases due to k fixation illitization the cpa is insensitive against such effects additionally using the cpa as with other na type indices provides the possibility to evaluate the homogeneity of the parent material regarding the relevant host minerals via the a cn k diagram

a revised chronology for tertiary sedimentation in the sikouzi basin implications for the tectonic evolution of the northeastern corner of the tibetan plateau abstract the space time pattern of development of high topography associated with the tibetan plateau remains incomplete due in large part to sparse proxy data that allow estimates of the growth of individual mountain ranges here we present the results of a stratigraphic investigation of a 3100 m section of tertiary fluvio lacustrine strata in the sikouzi basin located at the northeastern most corner of the tibetan plateau results of magnetostratigraphic analysis coupled with a newly discovered miocene in situ fossil assemblage suggest that the section preserves a nearly complete record extending from initial basin formation at 29 myr until 0 5 myr sediment accumulation rates increase from relatively slow accumulation during the oligocene miocene 9 cm kyr to rapid accumulation in the pliocene 22 cm kyr apparently occurring in two sustained pulses at 10 5 ma and 5 ma respectively the increase in accumulation rates starting at 10 5 ma is interpreted as heralding the initial uplift of the liupan shan consistent with cooling and exhumation of the range zheng et al 2006 the pliocene increase in accumulation rate is accompanied by a distinct change in lithofacies the appearance of coarse detritus with provenance from the madong shan and up section changes in bedding orientation that are interpreted as growth strata collectively our data suggest that the generation of relief in the madong shan began at 5 4 ma moreover analysis of paleomagnetic declinations from the sikouzi section reveals a clockwise rotation of the sikouzi basin during the time period 24 5 4 ma that was followed by a subsequent counterclockwise rotation 5 4 0 5 ma our study suggests that activity along the liupan shan haiyuan fault system probably initiated at 10 5 ma but that the present day configuration of the fault system and associated ranges near the fault tip dates to pliocene time

late cenozoic tectonic evolution of the southern chinese tian shan structural sedimentological magnetostratigraphic and 40ar 39ar thermochronological investigations were conducted in the southern chinese tian shan on the basis of our own mapping and earlier investigations in the area the late cenozoic southern tian shan thrust belt may be divided into four segments based on their style of deformation from west to east they are 1 kashi aksu imbricate thrust system 2 the baicheng kuche fold and thrust system 3 the korla right slip transfer system and 4 the lop nor thrust system the westernmost kashi aksu system is characterized by the occurrence of evenly spaced 12 15 km imbricate thrusts the baicheng kuche and korla systems are expressed by a major north dipping thrust the kuche thrust that changes its strike eastward to become a nw striking oblique thrust ramp the korla transfer zone the lop nor system in the eastern most part of the southern chinese tian shan consists of widely spaced thrusts all involved with basement rocks geologic mapping and cross section construction suggest that at least 20 40 km of crustal shortening with a horizontal shortening strain of 20 30 has occurred in the southern chinese tian shan during the late cenozoic these estimates are minimum because of both conservative extrapolation of the thrust geometries and partial coverage of the thrust belt by the cross sections the timing of initial thrusting is best constrained in the kuche basin where crustal shortening may have occurred at 21 24 ma the time of a major facies transition between lacustrine and braided fluvial sequences constrained in general by biostratigraphy and in detail by magnetostratigraphy this estimate represents only a minimum age as development of thrusts in the southern chinese tian shan may have propagated southward toward the foreland thus the sedimentary record only represents the southernmost and therefore youngest phase of thrusting if our estimate of timing for the thrust initiation 21 24 ma is correct using the estimated magnitude of shortening 20 40 km and shortening strain 20 30 the averaged rates of late cenozoic horizontal slip and shortening strain are 1 1 9 mm yr 1 and 2 9 4 5 10 16 s 1 respectively our reconnaissance 40ar 39ar thermochronological analysis in conjunction with earlier published results of apatite fission track analysis by other workers in the chinese tian shan suggests that the magnitude of cenozoic denudation is no more than 10 km most likely less than 5 km we demonstrate via a simple airy isostasy model that when the thermal effect on changes in surface elevation is negligible determination of the spatial distribution and temporal variation of both horizontal shortening strain and denudation becomes a key to reconstructing the elevation history of the tian shan using this simple model the loosely constrained magnitude of crustal shortening strain and denudation in the southern chinese tian shan implies that it may have been elevated 1 0 2 0 km since the onset of cenozoic thrusting

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

a 20 ma pollen record of east asian summer monsoon evolution from guyuan ningxia china abstract pollen analysis was conducted on the fluviolacustrine sediments from the east side of the liupan mountains a relatively monsoon sensitive region and provided for the first time a continuous evolutionary history of vegetation and climate in east china during the neogene the pollen record spanning the last 20 ma indicates that the vegetation and thus east asian summer monsoon evolution can be divided into three stages 20 13 14 25 ma 14 25 11 35 ma and 11 35 0 08 ma in spite of general dominance of steppe throughout the neogene during the period 20 13 14 25 ma the pollen assemblage was characterized by high abundance of artemisia and small amounts of temperate to subtropical deciduous broad leafed trees such as juglans carya quercus and betula suggesting that the east asian summer monsoon was generally strong the interval from 14 25 to 11 35 ma was marked by a significant decline in the east asian summer monsoon as indicated by artemisia gradually replaced by humulus and halophytic chenopodiaceae nitraria and ephedra also reached high values in this period since 11 35 ma ago percentages of halophytes dominated by chenopodiaceae and tamaricaceae maintained a high mean value of about 40 revealing a weak intensity of east asian summer monsoon the general weakening of the monsoon circulation during the neogene and the significant monsoon decline during the late middle miocene in particular might link with the global cooling trend probably through two mechanisms cooling and ice sheet expansion over the polar region caused an increase in meridional temperature gradients leading to the southward retreat of the monsoon circulation the global cooling may have also reduced the amount of water vapor held in the atmosphere and led to both additional cooling and further weakening of the east asian summer monsoon the cause for a short lived resumption during the period 1 0 0 8 ma is under investigation

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

lamproites in north vietnam a re examination of cocites the term cocite was coined by lacroix for rocks that share with lamproites a number of characteristics among them the presence of k rich minerals and high mgo and k 2 o a re examination of the type specimens indicates the absence of ti rich oxides and k richterite but the presence of ti rich phlogopites in addition to previously recognized olivine clinopyroxene sanidine analcime and chromite magnetite a phase absent from typical lamproites is also present the compositions of constituent phases the mineral assemblage and the trace element contents are all unlike those of lamproites and indicate that cocites are close to but not typical lamproites

crustal structure of atlantic fracture zones iii the tydeman fracture zone summary an extensive seismic survey was performed in march 1982 on two different parts of the tydeman fracture zone this survey consisted of two separate experiments a reversed seismic refraction experiment centred on 36 n 26 w using ocean bottom seismometers and free floating sonobuoys which was shot with explosives and a two ship multichannel experiment of the expanding spread type also shot with explosives at approximately 36 n 23 30 w these experiments are the first and to date only wide angle seismic surveys of fracture zone crust older than 25 myr slope intercept and tau p inversion of the obs sonobuoy travel time data indicates low crustal and mantle seismic velocities within the fracture zone these results are confirmed and refined by a more detailed analysis using raytracing and synthetic seismogram modelling for laterally varying structures which implies velocities for the upper mantle beneath the fracture zone of 7 3 7 5 km s 1 similar analysis of a line perpendicular to the fracture zone reveals the transition from normal mantle velocities to the low values found beneath the fracture zone and suggests that the width of the anomalous region is approximately 12 km the geometry of the obs experiment permitted a delay time analysis and a further simple tomographic inversion of the traveltime data which produced an approximate image of the low velocity mantle region associated with the fracture zone the width of the region of anomalous crust is the same as the width of the sediment filled trough which marks the fracture zone location at the line intersection analysis of the two ship seismic data from a site some 240 km to the east also by synthetic seismogram modelling gave similar results to the obs sonobuoy experiment with a mantle velocity of 7 2 km s 1 detected beneath the crust which possessed lower than normal seismic velocities the close seismogram spacing of less than 0 1 km allowed us to transform the dataset into the tau p domain and enabled us to detect arrivals obscured in the x t domain by the reflected water wave both experiments indicate low crustal seismic velocities within the fracture zone together with unusually low velocities of 7 2 7 5 km s 1 in the upper mantle comparison with other fracture zone surveys reveals that the seismic velocity in the upper mantle beneath fracture zones decreases with increasing age we attribute this to serpentinization of mantle peridotite brought about by hydrothermal circulation to the base of the fractured and faulted crust beneath the fracture zone

how old is the asian monsoon system palaeobotanical records from china the recent discovery of monsoon records in early miocene raised a question of the time when the east asian monsoon system initiated a distinguishing feature of the modern monsoon system is its geographic distribution which disturbs the zonal pattern indigenous to the planetary climate system and the appearance of the monsoonal climate pattern in the geological records should signify the onset of the monsoon system here we present the results of a compilation of palaeobotanical and lithological data from 125 sites over china that has revealed two completely different patterns of climate zones the palaeogene pattern with a broad belt of aridity stretched across china from west to east and the neogene pattern with the arid zone restricted to northwest of china which has persisted until today the reorganization of the climate system around the oligocene miocene boundary provides evidence for the establishment of the modern east asian monsoon since then the neogene has witnessed significant variations of the monsoon system including enhancement of aridity and monsoon intensity at about 15 13 my around 8 my and 3 my the new data do not support the onset of the asian monsoon system around 8 my rather the new data led to a hypothesis that the transition to the monsoon climate system in east asia occurred in the latest oligocene

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

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

weathering and global denudation a negative correlation between sediment yield and weathering history as measured by the chemical alteration cia of the suspended sediment is observed for many of the world s major rivers and other regions of denudation the weathering history is a first order control on the sediment yield of such areas termed equilibrium denudation regions for other areas data scatter with either apparent increases or decreases of sediment yield for given cia values these areas are termed nonequilibrium denudation regions low sediment yields can be attributed to moderated erosion either natural or human induced and or the incorporation of unweathered glacial debris accelerated erosion resulting in high sediment yield is primarily human induced and results from cultivation and other land use each of these effects has a profound influence on global sediment discharge from the continents pre human suspended sediment discharge from the continents is estimated to be 12 6 times 10 15 g yr or about 0 6 the

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

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

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

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

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

middle miocene reorganization of deformation along the northeastern tibetan plateau temporal variations in the orientation of cenozoic range growth in northeastern tibet define two modes by which india asia convergence was accommodated thermochronological age elevation transects from the hanging walls of two major thrust fault systems reveal diachronous miocene exhumation of the laji jishi shan in northeastern tibet whereas accelerated growth of the wnw trending eastern laji shan began ca 22 ma rapid growth of the adjacent north trending jishi shan did not commence until ca 13 ma this change in thrust fault orientation refl ects a middle miocene change in the kinematic style of plateau growth from long standing nne ssw contraction that mimicked the plate convergence direction to the inclusion of new structures accommodating east west motion this kinematic shift in northeastern tibet coincides with expansion of the plateau margin in southeastern tibet the onset of normal faulting in central tibet and accelerated shortening in northern tibet together these phenomena suggest a plateau wide reorganization of deformation

reconstruction of the altyn tagh fault based on u pb geochronology role of back thrusts mantle sutures and heterogeneous crustal strength in forming the tibetan plateau 1 reconstructing deformation along the northwestern margin of the tibetan plateau is critical for evaluating the relative importance of microplate versus continuum models of the indo asian collision questions regarding this margin s evolution are as follows 1 what is the total offset along the sinistral altyn tagh strike slip system 2 how has that offset been absorbed in the western kunlun shan 3 why does the n s width of the plateau vary along strike ion microprobe u pb zircon apparent ages of 17 plutons from nw tibet together with regional geologic observations define a discrete e w trending boundary between two tectonic belts that has been offset along the altyn tagh system by 475 70 km kinematic arguments indicate that this offset cannot be the result of north directed thrusting in the western kunlun shan therefore we propose that south directed faulting in the tianshuihai thrust belt both offset the tectonic boundary and produced the asymmetry in the plateau shortening appears to have been absorbed in the upper crust by thin skinned thrusting in the middle lower crust by east directed ductile flow and or subduction and in the mantle by north dipping subduction factors controlling the formation of the south directed thrust system appear to be the contrast between the rigid tarim and the weaker songpan ganzi flysch belt and a mantle suture inherited from late paleozoic subduction the evolution of western tibet leads to a view of continental deformation that integrates elements of the microplate model e g plate like mantle and crust mantle decoupling with aspects of the continuum model weak crustal flow beneath the plateau

magnetostratigraphy of the neogene sikouzi section at guyuan ningxia china abstract the thick widely distributed lacustrine sediments in northern china may have recorded abundant information on the asian monsoon evolution during the late cenozoic however reconstruction of such a history with these materials has been hampered by the paucity of reliable chronological constraints in this study systematic paleomagnetic measurements were conducted on a well exposed 2880 m thick fluviolacustrine sequence on the east side of the liupan mountains a relatively monsoon sensitive region correlation of the resultant magnetic polarity column to the geomagnetic polarity timescale gpts suggests that this sequence spans a time interval of 20 13 0 07 ma and is generally continuous the reliability of this correlation was further supported by three lines of independent evidence i e hipparion fossils pollen assemblages and color changes of the sediments measurements of redness of the sedimentary succession suggest a remarkable decrease in air temperature over northern china which may be closely associated with the global mid miocene cooling caused by the extension of the antarctic ice sheets this event may have had an important influence on the evolution of the asian monsoon circulation

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

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

climatic and tectonic uplift evolution since 7 ma in gyirong basin southwestern tibet plateau clay mineral evidence climate evolution associated with the uplift in gyirong southern margin of the tibetan plateau was investigated based on the clay mineralogy of the gyirong sediments using x ray diffraction and scanning electron microscopy methods the chlorite illite kaolinite smectite ratio and the illite crystallinity index suggest the trend of paleoclimate evolution in gyirong basin during 7 1 7 ma bp with a warm and humid climate in 7 0 5 8 ma a relative cool and dry climate in 5 8 5 5 ma a relative warm and humid climate in 5 5 2 5 ma and a much warmer climate in 2 5 1 7 ma respectively the fluctuating kaolinite smectite index and calcite content and the frequent occurrence of gypsum in the episode of 6 7 5 5 ma suggest intense climatic fluctuations between humid and more seasonal humid conditions five peaks exhibit at 5 8 4 9 4 3 3 6 and 2 5 ma in the curves of illite and chlorite content and illite crystallinity the intense uplift and tectonic forced cooling at 5 8 and 4 3 ma were indicated by the high illite crystallinity index and illite and chlorite content a remarkable increase in illite and chlorite content associated with only little increase in illite crystallinity at 3 6 ma indicates a rapid uplift but no significant climate change and a sharp increase in illite crystallinity while small change in illite and chlorite content at 2 5 ma suggests that no remarkable relief was produced by the uplift which may take place in the whole south tibet plateau while cooling at 4 9 ma is probably a record of the global climate cooling

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

late cenozoic central asian drying inferred from a palynological record from the northern tian shan abstract sediments contained in basins adjacent to great mountains provide opportunities to reconstruct the aridification history of central asia here we use a palynological record from the fluviolucustrine jingou river section collected from the northern tian shan nw china to reconstruct aridification in central asia during the late cenozoic biogeomagnetic results show that the section was continuously deposited through the interval 28 4 2 ma the palynological record with the auxiliary of the principal component analysis and the diversity index indicates that a late oligocene wet condition existed in central asia and shifted to dry conditions at 23 8 23 3 ma the dry condition remained until 17 3 ma and subsequently ameliorated to a relatively wet stage to 16 2 ma but then began to increase once again and reached a peak at 13 5 ma that last throughout the late miocene and the early pliocene comparing the aridification process to the global temperature trend and history of tectonics we suggest that the long term drying trend in central asia is dominated by late cenozoic cooling while the dry events that occurred at 23 8 23 3 ma and 16 2 13 5 ma are more likely associated with regional mountain building the land sea redistribution further complicated the drying processes

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

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

late oligocene early miocene initiation of shortening in the southwestern chinese tian shan implications for neogene shortening rate variations abstract the cenozoic tian shan is one of the preeminent examples of an intracontinental orogen however there remains a significant controversy over when deformation related to the india asia collision commenced and therefore how shortening within the mountains has been partitioned over time one approach has been to look at the modern shortening rate as measured by geodetic studies combined with estimates of the total shortening across the range and extrapolate backwards this approach suggests that the onset of range construction was ca 10 ma k y abdrakhmatov s a aldazhanov b h hager m w hamburger t a herring k b kalabaev k b kalabayev v i makarov p molnar s v panasyuk m t prilepin r e reilinger i s sadybakasov b j souter y a trapeznikov v y tsurkov a v zubovich relatively recent construction of the tien shan inferred from gps measurements of present day crustal deformation rates nature 384 6608 1996 450 453 an alternate method is to determine the age of the onset of exhumation using thermochronology we present 19 new apatite fission track aft results from the southwestern chinese portion of the belt this region represents the first area exhumed during the late tertiary along a transect at ca 76 e exhumation commenced at the oligocene miocene boundary 24 ma along the maidan and muziduke thrusts which bound the southern side of the kokshaal range subsequently deformation propagated ca 20 km south to the kashi basin bounding thrust kbt which was exhumed by no earlier than 18 9 3 3 ma three detrital aft samples from plio pleistocene strata deposited ca 20 km farther south contain fission track grain age peaks that young monotonically upwards from 20 9 7 0 5 3 ma to 15 9 5 4 4 0 ma with a fairly constant lag time of 16 to 18 ma these ages combined with structural data suggest that both the hanging wall and the footwall of the kbt experienced a renewed episode of exhumation during the latest cenozoic the discrepancy between the late oligocene miocene initiation of significant exhumation shown herein and the 10 ma initiation estimate from geodesy suggests that the tian shan has undergone a complex late cenozoic shortening history assuming that the present shortening rate could account for the total amount of cenozoic shortening in 10 ma and realizing that shortening initiated at least 15 myr earlier we conclude that the shortening rate must have varied over time possibly in pulsed southward migrating events and that the present rate may not reflect the average rate since initiation of range uplift

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

chinese loess and the paleomonsoon abstract this paper presents a general review of the recent research advances of the east asian paleomonsoon based mainly on studies of the loess soil sequences in the chinese loess plateau in the last 2 6 million years the paleomonsoonal history may be divided into about 166 events on a time scale of the earth s orbital variations during the last glacial period millenial scale oscillations of the monsoon system were prominent which can be fairly well correlated with the dansgaard oeschger cycles recognized in the greenland ice cores the monsoonal rainfall belt has experienced a wide repeated advance retreat change during the glacial interglacial cycles of the pleistocene both temporal and spatial changes of the monsoon system in the quaternary could have been linked closely to global ice volume variations

interlaboratory standardization and calibration of day mineral crystallinity and crystallite size data clay mineral crystallinity and crystallite domain size data determined by x ray diffraction xrd are methods extensively used in the characterization of very low grade metamorphic conditions however the lack of sufficient interlaboratory standardization has made comparisons between different research groups unreliable due to significant variations in numerical results obtained a consequence of the different machine conditions measurement methods and sample preparations used during analysis a calibration approach to the standardization of data using rock chip standards is presented which allows data sets produced by different research groups to be directly and quantitatively compared a standardized scale the crystallinity index standard cis is proposed with illite crystallinity anchizonal boundary limits of 0 25d 2 and 0 42d 2 and equivalent illite crystallite sizes of 52 and 23 nm respectively determined by the warren averbach method calibrating both old and new data will enable more reliable comparisons between similar and contrasting geological environments and should improve the accuracy and reliability of correlations made between xrd data and other indicators of very low grade metamorphism hence increasing the value of such clay mineral studies

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

a 25 m y isotopic record of paleodiet and environmental change from fossil mammals and paleosols from the ne margin of the tibetan plateau we use the carbon and oxygen isotopic compositions of fossil tooth enamel and paleosols to reconstruct the late cenozoic history of vegetation and environmental change in the linxia basin at the northeastern margin of the tibetan plateau the d 13 c values of fossil enamel from a diverse group of herbivores and of paleosol carbonate and organic matter indicate that c4 grasses were either absent or insignificant in the linxia basin prior to 2 3 ma and only became a significant component of local ecosystems in the quaternary this is in striking contrast to what was observed in pakistan nepal africa and the americas where c4 plants expanded rapidly in the late miocene as indicated by a positive d 13 c shift in mammalian tooth enamel and paleosols the d 18 o results from the same herbivore species show several significant shifts in climate in the late cenozoic most notably a positive d 18 o shift after 7 ma indicates a shift to warmer and or drier conditions and is comparable in timing and direction to the d 18 o shift observed in paleosol carbonates in pakistan and nepal this late miocene climate change observed in the indian sub continent and in the linxia basin however seems to be a regional manifestation of a global climate change the lack of evidence for c4 plants in the linxia basin prior to 2 3 ma suggests that the east asian summer monsoon which brings precipitation into northern china during the summer and creates optimal conditions for the growth of c4 grasses was probably not strong enough to affect this part of china throughout much of the neogene this implies that the himalayantibetan plateau may not have reached the present day elevation across its vast extent to support a strong east asian monsoon system before 2 3 ma our data also suggest that regional climatic conditions played an important role in controlling the expansion of c4 plants d 2005 elsevier b v all rights reserved

geochemical discrimination of clastic sedimentary rock sources abstract factors controlling the geochemistry of a clastic sedimentary rock can include 1 composition of source terrain 2 chemical weathering 3 hydraulic sorting 4 diagenesis 5 metamorphism and 6 hydrothermal alteration a linear solution inferring source terrain composition from geochemistry of the sediment is impossible in this multivariate system as several unknowns will commonly be present the use of graphical analysis of element pairs circumvents the problem chemically immobile elements will maintain invariant ratios during rock mass change caused by either addition or depletion of mobile elements this results in chemically immobile element scattergrams exhibiting linear trends along radians extending from the origin if the major mineral phases of the immobile elements have behaved in a hydrodynamically similar manner as chemically mobile element plots will produce a scatter of points this relationship can be used to test chemical immobility and similarity in hydrodynamic sorting history the constraint on analysis is that the source area must be compositionally uniform or the sediment well mixed prior to delivery to the basin a second technique using sio2 plots has also been developed to investigate element mobility and hydrodynamic behaviour of the mineral phases containing the elements sio2 immobile element plots result in a linear arrangement of points extending towards either 0 or 100 sio2 the 0 intercept position occurs for elements with major mineral phases concentrated in sand the 100 intercept for those concentrated in clays plotting chemically mobile elements produces different patterns and this can be used to gain information on alteration and sorting history elemental ratios for chemically immobile elements with similar hydrodynamic behaviour will be the same as those for the weighted average composition of the source material this provides a powerful tool for deducing source terrain from sediment geochemistry techniques outlined above were tested on archaean metasandstones from superior province canada immobile element ratio diagrams for nb al ti and zr al ti indicate that a calc alkaline extrusive intrusive suite lying to the north of the study area was the source and not five other volcanic suites in the region this conclusion agrees with previous clast lithology studies and accentuates the applicability of the geochemical techniques

paleomagnetism indicates no neogene vertical axis rotations of the northeastern tibetan plateau 1 paleomagnetic data were obtained from 108 paleomagnetic sites collected in cretaceous to tertiary red beds from seven localities distributed in three general regions adjacent to the altyn tagh fault at the northern edge of the tibetan plateau in the hexi corridor 12 sites in oligocene strata at yaoquanzi 39 97 n 97 68 e yield a mean paleomagnetic direction i 33 0 d 8 6 95 6 0 with concordant declination and 36 sites in early cretaceous mudstones of the longshou shan 39 09 n 100 50 e provide a concordant paleomagnetic direction i 53 4 d 6 8 95 3 8 across the nan shan fold thrust belt miocene paleomagnetic directions from 39 sites distributed among four localities have a concordant mean declination i 40 6 d 7 2 95 5 8 in the altyn tagh range 21 sites in oligocene strata at xorkoli 38 93 n 91 43 e yield a paleomagnetic direction with concordant declination i 49 7 d 5 0 95 5 4 these results combined with existing regional paleomagnetic data indicate that 1 the hexi corridor along with the north china block has not undergone tectonic vertical axis rotation since at least early cretaceous time and is separated from the xining lanzhou basin and eastern tibet by an important post early cretaceous tectonic boundary 2 the nan shan fold thrust belt and the qaidam basin have not experienced wholesale vertical axis rotation during neogene time and 3 absence of vertical axis rotations in areas adjacent to the altyn tagh fault indicates that sinistral shear strain between the tarim basin and the northern tibetan plateau is concentrated on the fault these results are consistent with asian tectonic models that combine distributed lithospheric deformation and thickening with narrow and weak shear zones

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

stable isotope evidence for topographic growth and basin segmentation implications for the evolution of the ne tibetan plateau lithologic magnetostratigraphic and stable isotope records from the neogene xunhua and linxia basins along the tibetan plateau9s northeastern margin suggest that topography in the intervening jishi shan mountain range began to develop between 16 and 11 ma perturbations to local climate patterns resulting from the evolution of local topography are tracked through comparison of stable isotope compositions of calcareous basin fill materials across the jishi shan similarity of isotopic compositions is interpreted to reflect the presence of integrated basins whereas distinct isotopic compositions reflect unique basin hydrologies divergent isotope trends develop between ca 16 and 11 ma and are indicative of hydrologic separation in the adjacent xunhua and linxia basins and increased aridity in the leeward xunhua basin the development of aridity in the lee of the growing topography along the plateau9s northeast margin highlights the importance of evaporative enrichment in this extremely continental setting and explains the presence of anomalously positive 18 o values in modern rainfall our findings add to a growing body of evidence for deformation along the plateau9s north and northeastern margins in the middle to late miocene

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

effect of orogeny plate motion and land sea distribution on eurasian climate change over the past 30 million years the eurasian climates of today 10 million and 3o million years ago are simulated using an atmospheric general circulation model that incorporates realistic continental geography and epicontinental sea distributions the resulting climates compare well with various palaeoclimate records the retreat of the paratethys an epicontinental sea shifts the central asian climate from temperate to continental conditions and plays as important a role as uplift of the himalayan tibetan plateau in driving the asian monsoon changes