intensified aridity in the qaidam basin during the middle miocene constraints from ostracod stable isotope and weathering records the thick and continuous cenozoic successions in the qaidam basin provide an excellent paleoclimate archive here we focus on the ostracod fauna stable isotope records and paleoweathering indice

paleomagnetic evidence for a mid miocene clockwise rotation of about 25 of the guide basin area in ne tibet abstract ten sections of neogene molasse type sediments were sampled in the guide basin of northeastern tibet for magnetostratigraphy x m fang m d yan r van der voo d r rea c song j m pares j gao j nie s dai late cenozoic deformation and uplift of the ne tibetan plateau evidence from high resolution magnetostratigraphy of the guide basin qinghai province china geol soc america bull 107 2005 1208 1225 1 but they also yield seven well dated formation mean directions that reveal changing declinations as rotations occurred in response to crustal deformation north of the india asia collision zone three formations are of early miocene and oligocene age as indicated by fossils and magnetic reversal records whereas four younger formations yield late miocene and pliocene ages the dual polarity magnetizations are typically antipodal but reveal inclinations that are too shallow most likely because of post depositional inclination flattening the late miocene and younger directions show formation mean declinations between 354 and 7 whereas three early miocene and late oligocene mean declinations range from 31 to 44 this indicates that a clockwise rotation of 25 1 4 6 took place during the middle part of the miocene best estimate 11 17 ma no rotations appear to have occurred during that time in the xining lanzhou linxia and jingning basins longzhong basin to the northeast and east of the guide basin however a rotation of similar magnitude has been documented by dupont nivet and colleagues for pre miocene 29 ma time in these areas collectively these results show that the basins in ne tibet have had independently evolving structural histories

late oligocene miocene mid latitude aridification and wind patterns in the asian interior the asian interior has the largest mid latitude arid zone in the northern hemisphere and so has become increasingly attractive for studying the initiation and the past extent of aridification in this zone given the enormousness of the asian interior it remains unclear how old and extensive the eolian deposits might have been and what wind regimes have been responsible for the formation of the mid latitude arid zone here we report new eolian records of widespread tertiary eolian deposits in a region far from the chinese loess plateau the giant junggar inland basin of northwestern china our results demonstrate that the earliest eolian deposition initiated ca 24 ma we interpret that the tertiary eolian dust in the junggar basin was transported by westerly winds possibly from areas in kazakhstan the dust differs from the airborne dust transported by winter monsoon winds from the deserts of mongolia and northern china that accumulated on the loess plateau these results further reveal that the climate pattern similar to that of the present has prevailed at least since the latest oligocene in central asia

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

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

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

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

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

magnetostratigraphy of the dahonggou section northern qaidam basin and its bearing on cenozoic tectonic evolution of the qilian shan and altyn tagh fault article i nfo the timing of uplift of the tibet plateau has a central role in the development of tectonic models for the tibet plateau and cenozoic global climate change a detailed magnetostratigraphic study of the dahonggou section northern qaidam basin reveal that the section spans from 34 to 8 5 ma and the ages of the shang ganchaigou xia youshashan and shang youshashan formations are from 34 to 22 20 ma 22 20 to 13 ma and 13 to 8 5 ma respectively variations in lithofacies sedimentation rate and magnetic susceptibility k suggest that the southern qilian shan was tectonically inactive and didn t respond to the rapid slip on the altyn tagh fault at 30 ma in contrast the similar sedimentary records in the dahonggou section the xishuigou section along the altyn tagh fault and even more localities along much of the qilian range front imply that the qilian shan and the altyn tagh fault were synchronously tectonically active at about 12 ma the lower k between 12 ma and 8 5 ma in the sediments of the dahonggou section is interpreted to be due to long distanced oxidation and sorting which cause not only that magnetite was oxidated to hematite but also that magnetic minerals are enriched in fine grained sediments and coarse grained sediments bear few magnetic mineral

significance of weathering himalayan metasedimentary rocks and leucogranites for the sr isotope evolution of seawater during the early miocene changes in 87 sr 86 sr ratios of seawater may reflect varying rates of chemical weathering or variations in the isotopic compositions of continental rocks undergoing weathering the unusually high dissolved sr concentrations and 87 sr 86 sr ratios that characterize the ganges and brahmaputra tsangpo river systems are a consequence of weathering early proterozoic age metasedimentary rocks and anatectic miocene granites exposed on the southern slopes of the himalayas on average the metasedimentary rocks are characterized by abundances of 87 sr about twice those of the derived granites the rapid rise in 87 sr 86 sr ratios of seawater during the early miocene was virtually coincident with metamorphism and anatexis of himalayan metasedimentary rocks model calculations indicate that 50 75 of the overall change in 87 sr 86 sr ratios for seawater between 20 and 15 ma could have resulted from exposure of himalayan metasedimentary rocks to chemical weathering rapid tectonically controlled exhumation of the metasedimentary rocks is required to expose these lithologies by 20 18 ma to generate the increase in the 87 sr flux of the trans himalayan rivers during the early miocene

equatorial convergence of india and early cenozoic climate trends india s northward flight and collision with asia was a major driver of global tectonics in the cenozoic and we argue of atmospheric co2 concentration pco2 and thus global climate subduction of tethyan oceanic crust with a carpet of carbonate rich pelagic sediments deposited during transit beneath the high productivity equatorial belt resulted in a component flux of co2 delivery to the atmosphere capable to maintain high pco2 levels and warm climate conditions until the decarbonation factory shut down with the collision of greater india with asia at the early eocene climatic optimum at 50 ma at about this time the india continent and the highly weatherable deccan traps drifted into the equatorial humid belt where uptake of co2 by efficient silicate weathering further perturbed the delicate equilibrium between co2 input to and removal from the atmosphere toward progressively lower pco2 levels thus marking the onset of a cooling trend over the middle and late eocene that some suggest triggered the rapid expansion of antarctic ice sheets at around the eocene oligocene boundary

step wise change of asian interior climate preceding the eocene oligocene transition eot abstract understanding the global climate change from greenhouse to icehouse conditions at the eocene oligocene transition eot 34 million years ago requires climatic records from oceanic as well as continental realms of the key late eocene doubthouse period preceding this switch here we report integrated stratigraphic results from well dated late eocene continental mudflat to saline lake paleoenvironments of the xining basin northeastern tibetan plateau western china recording regional and global change cyclostratigraphic analysis strongly suggests continuous dominance of the 41 kyr obliquity cycle in the whole late eocene interval down to the base of polarity chron c18n 2n at 39 ma with additional input of the 100 kyr eccentricity cycle up to the base of chron c13r at 34 7 ma this might imply that high latitude climates dominated the area long before the eot probably related to incipient ice volume fluctuations furthermore our results reveal two paleoenvironmental deterioration steps preceding the eocene oligocene transition the first step occurs in the top of chron c17n 1n at 36 6 ma this age closely corresponds to 1 the high altitude pollen appearance in chron c16 2r at 36 4 ma in the same section 2 the recently dated final retreat of the tarim sea in western china and 3 a shift from precession to obliquity dominance in the atlantic ocean this near co occurrence suggests global change at this time we hypothesize this change is related to an increase in incipient ice sheet volume leading to passing threshold conditions for the high altitude pollen appearance and tarim sea retreat finally leading to decreased moisture availability in the xining basin at the second step in the base of chron c13r at 34 7 ma a substantial increase in clastic sedimentation rates is observed this might relate to increased climate variability preceding the greenhouse to icehouse transition at the eot that prevented landscapes to attain equilibrium configurations

magnetostratigraphy and depositional history of the miocene wushan basin on the ne tibetan plateau china implications for middle miocene tectonics of the west qinling fault zone abstract based on field mapping section measurement and magnetostratigraphy 1700 m of sedimentary rocks have accumulated in the wushan basin between 16 ma and 6 ma basin geometry sedimentation characteristics and the early syn depositional deformation along the northern margin of the basin indicate that formation of the wushan basin was related to tectonic deformation along the west qinling fault zone during the middle miocene a series of basins of similar age to the wushan basin were generated along and to the south of the west qinling fault zone while basalts also erupted in this region at this time we suggest that the middle miocene 16 ma may represent a change in kinematics and deformation style in the region along and to the south of the west qinling fault zone at this time there was a transition from nne ssw compressional deformation that dominated the region since the late paleogene to the development of wnw ese and or e w trending strike slip movement and associated transpressional and transtensional activity that continues today the miocene wushan basin may have developed in association with transpression along the west qinling fault zone whether this transition was related to the onset of strike slip along the east kunlun fault and related deformation transfer lower crustal flow or removal of mantle lithosphere the middle miocene provides direct evidence for a change in the kinematic style along the plateau margin

high resolution shallow seismic experiments in sand part ii velocities in shallow unconsolidated sand we conducted a shallow high resolution seismic reflection and refraction experiment on a sandy beach the depth of investigation was about 2 m we interpret the data using the hertz mindlin contact theory combined with gassmann s equation these were used to obtain the vertical velocity profile then the profile was computed from seismic data using the turning rays approximation the normal moveout nmo velocity at the depth of 2 m matches the velocity profile as a result we developed a method to invert measured velocity from first arrivals i e velocity versus distance into velocity versus depth using only one adjustable parameter this parameter contains all the information about the internal structure and elasticity of the sand the lowest velocity observed was about 40 m s it is noteworthy that the theoretical lower bound for velocity in dry sand with air is as low as 13 m s we find that modeling sand as a quartz sphere pack does not quantitatively agree with the measured data however the the

shallow dynamic overshoot and energetic deep rupture in the 2011 mw 9 0 tohoku oki earthquake strong spatial variation of rupture characteristics in the moment magnitude mw 9 0 tohoku oki megathrust earthquake controlled both the strength of shaking and the size of the tsunami that followed finite source imaging reveals that the rupture consisted of a small initial phase deep rupture for up to 40 seconds extensive shallow rupture at 60 to 70 seconds and continuing deep rupture lasting more than 100 seconds a combination of a shallow dipping fault and a compliant hanging wall may have enabled large shallow slip near the trench normal faulting aftershocks in the area of high slip suggest dynamic overshoot on the fault despite prodigious total slip shallower parts of the rupture weakly radiated at high frequencies whereas deeper parts of the rupture radiated strongly at high frequencies

late eocene sea retreat from the tarim basin west china and concomitant asian paleoenvironmental change the paleogene sediments of the southwest tarim basin along the west kunlun shan in western china include the remnants of the easternmost extent of a large epicontinental sea this shallow sea once extended across the eurasian continent before it retreated westward and eventually separated as the paratethys sea climate modeling results suggest that this sea retreat is an equally important forcing mechanism as the tibetan plateau uplift in the aridification of the asian continental interior and the intensification of the asian monsoon system the age and paleogeography of the retreat are poorly constrained hindering the understanding of its cause and paleoenvironmental impacts this study reports litho and biostratigraphic results from two sections recording the last major regression out of the tarim basin that is expressed by a regional transition from marine clastics and limestones to continental red beds rich micro and macrofossil assemblages including benthic foraminifera ostracods bivalves calcareous nannofossils and organic walled dinoflagellate cysts dinocysts indicate a shallow proximal and marine environment strong similarity to assemblages known from central asia and europe confirms that surface ocean connections extended across eurasia from the tarim basin to the western tethys during the latest eocene moreover the recovered fossil associations date the last marine sediments as earliest priabonian in age 37 ma overlap between dinoflagellate mps interval zone and calcareous nannofossil zone cp 14 the retreat of the sea from the tarim basin is time equivalent with the sea level lowstand at the bartonian priabonian boundary but pre dates both the oligocene miocene regional uplift of the pamir mountains and kunlun shan and the major eustatic sea level falls of the eocene oligocene transition 34 ma and mid oligocene 30 ma which are usually held responsible for the sea retreat furthermore a concomitant and significant aridification step occurs at 36 6 ma top of chron c17n 1n as recorded by regional sedimentary records of the xining basin along the northeastern tibetan plateau suggesting that the tarim sea served as a significant moisture contributor for the asian interior

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

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

recovery of groundwater in the sanriku region contaminated by the tsunami inundation from the 2011 tohoku earthquake many wells in the sanriku region used as sources for water supply systems were heavily contaminated by the tsunami of the 2011 great tohoku earthquake on march 11 in 2011 to better understand the nature of the groundwater contamination by the tsunami inundation and to clarify the recovery process of contaminated groundwater at the study wells groundwater monitoring has been conducted once or twice yearly since early summer in 2011 high and abnormal values of electric conductivity ec chloride ion concentration cic na ca heavy metal ions and heavier isotopes of the contaminated groundwater were also obtained in april and june 2011 the chemical elements have rapidly and exponentially decreased as a result of effective pumping of the contaminated groundwater from the study wells and because of abundant rainfall in 2011 in april 2015 about 4 years after the tsunami inundation the cic and ec of the contaminated groundwater of two study wells in minamisanriku town had reached pre inundation values the estimated residence times of groundwater of the two study wells were 105 118 days in the full day pumping stage and 910 1000 days in the daytime only pumping stage

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

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

cenozoic pulsed deformation history of northeastern tibetan plateau reconstructed from fission track thermochronology abstract the synorogenic basin deposits and bedrocks of their source terranes within and along the tibetan plateau contain fundamental information regarding the spatiotemporal evolution of the largest orogenic plateau on earth the guide xining region is located on the northeastern portion of the tibet and its eocene early pleistocene basin succession is well preserved by integrating apatite fission track thermochronology from sedimentary and basement samples with heavy minerals and paleocurrent data we decipher an almost complete sequence of exhumation and depositional events during the cenozoic our data indicates that the initial deformation along the guide xining region occurred since the eocene with the reorganization of the regional tectonomorphology and the formation of a broad basin thereafter this single large basin was disrupted by multiple episodes of exhumation and deformation our study illuminate that the multiple stage active processes occurred at 49 42 36 32 23 19 16 13 and 8 4 ma work together to produce the current ne tibetan plateau

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

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

magnetic polarity stratigraphy of siwalik group sediments of karnali river section in western nepal the remanent magnetization of siltstones and sandstones sampled at 476 levels sites throughout a 3560 m thick molasse sequence belonging to the siwalik group 0 2015 m lower 2015 3560 m middle has been studied by stepwise thermal demagnetization this section is exposed along the karnali river in nepal the natural remanent magnetization nrm usually consists of two components a viscous or thermoviscous component of recent field origin and an ancient characteristic component chrm the former component is of normal polarity and resides either in goethite unblocking temperature 150 c resistant to afd up to 150 mt or in maghemite unblocking temperature 150 400 c goethite contributes up to 90 per cent of the total intensity in the finer variegated muddy samples belonging to the lower half of the section maghemite content is significant in the grey mud free lithologies from the upper half of the section the main component unblocked in the high temperature range commonly 610 680 c and believed to reside in haematite presumably of mostly detrital origin represents a characteristic remanence chrm the tilt corrected chrm directions at individual sites show antipodal clusters ratio of normal to reverse polarity sites 0 62 and yield mean inclinations recording significant inclination shallowing a feature well recorded in the siwaliks this chrm is interpreted to represent a largely primary detrital remanence the chrm data from 430 sites yield the karnali river magnetic polarity sequence whose correlation with the geomagnetic polarity timescale cande kent 1995 suggests a depositional age of 16 ma younger than chron c5cn 1n to 5 2 ma around the top of chron c3r for the 3560 m section sampled hence the karnali river exposes the oldest part of the siwalik group in nepal estimates of the sediment accumulation rate sar average to 32 9 cm kyr 1 for the 10 8 myr time span of deposition

cenozoic multiple phase tectonic evolution of the northern tibetan plateau constraints from sedimentary records from qaidam basin hexi corridor and subei basin northwest china an integrated research of sedimentology stratigraphy and provenance analysis on eleven sedimentary sections from the qaidam basin hexi corridor and subei basin representing 36 km cenozoic strata provides a detailed record of the northern tibetan plateau growth since the early eocene sections are divided into three groups based on age geological similarities and geographical locations group one includes three early eocene late miocene sections from the northern qaidam basin group two contains four sections along the altyn tagh fault atf which preserve a complete stratigraphic record from the oligocene to late miocene group three contains the four youngest sections investigated from the northeastern qaidam basin and hexi corridor which preserve a record since the middle miocene to quaternary together the sections reveal a multiple phase tectonic history of the northern tibetan plateau the punctuated history can be divided into four phases 1 the eocene lulehe formation from the northern qaidam basin is interpreted as a synorogenic conglomerate deposited by high gradient depositional systems strong unimodal paleocurrent towards the southwest coarse lithology and distinct recognizable clast types constrain the sediment source within the north qaidam and south qilian terranes indicating activity on inferred thrust faults within the north qaidam and south qilian terranes in response to the initial india eurasia collision the activity on these thrust faults continued through the late eocene 2 the early oligocene conglomerate from group two formed in response to the sinistral transpression related to motion on the atf suggesting inception of substantial slip on the atf in the early oligocene in order to accommodate the continuing indentation of india into eurasia 3 oligocene early miocene fine grained fluvio lacustrine sediments from groups one and two formed as the result of development of internal drainage systems in the qaidam basin in response to the large amplitude slip motion on the atf paleocurrents collected from the oligocene early miocene strata of group one are northwest directed pointing towards the atf consistent with the pre existing subsurface data showing the shift of depocenter from along north qaidam and south qilian terranes toward the atf 4 all eleven sections preserve a post early miocene upward coarsening sequence consistent with the extensive crustal shortening and topographic growth across the northern tibetan plateau

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

a pedogenic model for the magnetic enhancement of late miocene fluvial lacustrine sediments from the xining basin ne tibetan plateau

magnetostratigraphy of an early middle miocene loess soil sequence in the western loess plateau of china 1 stratigraphy and magnetic susceptibility of the quaternary eolian deposits at different sites in china can be closely correlated confirming their spatial coherence and temporal continuity the extent to which these conclusions are true for the miocene loess soil sequences remains to be addressed here a 231 9 m eolian section qa iii is geomagnetically dated yielding an age span of 21 4 11 4 ma with the portion older than 19 6 ma being partially water reworked the magnetozones litholigic structure and magnetic susceptibility of the sequence are highly correlative with the qa i and qa ii sections 30 km west of qa iii these sequences demonstrate that the miocene loess deposits are also spatially correlative in stratigraphy present a near continuous sequence of paleoenvironment changes and that magnetic susceptibility is a useful tool for stratigraphic correlation the basal age of qa iii confirms again the onset of asian inland deserts and monsoon dominate climates by the early miocene

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

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

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

tectonic uplift in the northern tibetan plateau since 13 7 ma ago inferred from molasse deposits along the altyn tagh fault abstract in response to the collision of india with asia thick molasse deposits were shed off the high evolving edge of the northern tibetan plateau providing great potential for understanding the relationships between mountain building rock denudation and sediment deposition a precise knowledge of the initial accumulation of the molasse deposits is important for understanding uplift history along the northern margin of tibet field investigations indicate that thick neogene sediments in the piedmont depression zones of the altun mountains are strongly deformed in a fold and thrust system indicating crustal shortening and thickening in the late cenozoic detailed magnetostratigraphic studies of the molasse deposits along the altyn tagh fault show that the accumulation of coarse conglomerate began ca 13 7 ma ago the source material of the deposits changing at the same time together with the remarkable increase in the sedimentation rate after 13 7 ma we attribute the accumulation of the conglomerates to the tectonic uplift along the northern edge of the tibetan plateau a process that continued until at least 9 ma ago

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

correlation of himalayan exhumation rates and asian monsoon intensity although most data suggest that the india eurasia continental collision began 45 55 myr ago the architecture of the himalayan tibetan orogen is dominated by deformational structures developed in the neogene period 23 myr ago the stratigraphic record and thermochronometric data indicate that erosion of the himalaya intensified as this constructional phase began and reached a peak around 15 myr ago it remained high until 10 5 myr ago and subsequently slowed gradually to 3 5 myr ago but then began to increase once again in the late pliocene and pleistocene epochs here we present weathering records from the south china sea bay of bengal and arabian sea that permit asian monsoon climate to be reconstructed back to the earliest neogene these indicate a correlation between the rate of himalayan exhumation as inferred from published thermochronometric data and monsoon intensity over the past 23 myr we interpret this correlation as indicating dynamic coupling between neogene climate and both erosion and deformation in the himalaya although the india eurasia collision initiated 50 myr ago major deformation and exhumation of the himalaya did not begin until the early neogene 23 myr ago this coincides with the increased intensity of the asian monsoons as indicated by weathering records from the south china sea bay of bengal and arabian sea and hints at a dynamic coupling between climate and both erosion and deformation in the himalaya

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

impacts of the 2004 tsunami on groundwater resources in sri lanka the 26 december 2004 tsunami caused widespread destruction and contamination of coastal aquifers across southern asia seawater filled domestic open dug wells and also entered the aquifers via direct infiltration during the first flooding waves and later as ponded seawater infiltrated through the permeable sands that are typical of coastal aquifers in sri lanka alone it is estimated that over 40 000 drinking water wells were either destroyed or contaminated from february through september 2005 a team of united states sri lankan and danish water resource scientists and engineers surveyed the coastal groundwater resources of sri lanka to develop an understanding of the impacts of the tsunami and to provide recommendations for the future of coastal water resources in south asia in the tsunami affected areas seawater was found to have infiltrated and mixed with fresh groundwater lenses as indicated by the elevated groundwater salinity levels seawater infiltrated through the shallow vadose zone as well as entered aquifers directly through flooded open wells our preliminary transport analysis demonstrates that the intruded seawater has vertically mixed in the aquifers because of both forced and free convection widespread pumping of wells to remove seawater was effective in some areas but overpumping has led to upconing of the saltwater interface and rising salinity we estimate that groundwater recharge from several monsoon seasons will reduce salinity of many sandy sri lankan coastal aquifers however the continued sustainability of these small and fragile aquifers for potable water will be difficult because of the rapid growth of human activities that results in more intensive groundwater pumping and increased pollution long term sustainability of coastal aquifers is also impacted by the decrease in sand replenishment of the beaches due to sand mining and erosion

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

multiple origins and interpretations of the magnetic susceptibility signal in chinese wind blown sediments abstract in most chinese loess paleosol sequences high magnetic susceptibility values are found in the soil horizons with low values in the loess layers the susceptibility signal has been widely used as a proxy climatic indicator however both the causes and mechanisms which control susceptibility still remain controversial our recent studies challenge some earlier interpretations of the magnetic susceptibility signal in chinese wind blown sediments first the prevailing hypothesis of pedogenic origin cannot totally account for the susceptibility variations in many sections second in some cases the principal carrier of the magnetization seems not to be the ultrafine pedogenic minerals rather coarse lithogenic magnetic minerals derived from local sources can also contribute significantly to the susceptibility signal finally not all the chinese soils have higher susceptibility values than that of loess or sand layers opposite relationships do exist it is concluded that the source and causes of susceptibility may be different from site to site or even at different times at a given site any single hypothesis cannot completely account for the susceptibility variations in the chinese loess plateau and its surrounding regions besides the previous explanations of carbonate leaching pedogenic processes and organic decomposition the contribution of source materials to magnetic susceptibility must also be taken into account

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

chronological sequence and subdivision of chinese neogene mammalian faunas six land mammal ages are recognized for the neogene of east asia the ages are based upon updated large and small mammal faunas and replace or redefine the land mammal stages erected by li et al 1984 comparisons and correlations are made to faunas in europe northern and southern asia and north america in addition methods of utilizing mammalian faunal data to define and recognize geochronologic ages are discussed

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

cenozoic tectonic and paleoenvironmental evolution of northwestern china evidence from two deep boreholes in the jartai basin abstract to study the cenozoic tectonic and paleoenvironmental evolution of northwestern china we drilled two deep boreholes jlt1 and jlt2 in the jartai basin magnetostratigraphy stratigraphy studies were conducted to establish a chronological framework sedimentary facies were reconstructed using lithology geophysical logs gamma ray gr and resistivity r and measurements of grain size sediment color l a and b and magnetic susceptibility and its frequency dependence the stratigraphic sequence of borehole jlt2 is similar to that of paleogene sediments in basins along the northeastern tibetan plateau which may be the result of the far field effect of the uplift of the tibetan plateau during the paleocene via faulting activity the results of borehole jlt1 indicate that the basin was occupied by alternating desert lacustrine and alluvial fan environments since ca 6 7 ma during ca 6 7 3 6 ma alternations of aeolian sand and lacustrine facies may reflect climatic cooling and drying and at that time the area may have been an important sediment source for the aeolian red clay of the loess plateau during ca 3 6 2 1 ma alluvial fan development also reflected the uplift of the northeastern tibetan plateau and the area acted as a source region for dust supplied to the loess plateau and the shanxi graben the alternations of aeolian sand and lacustrine deposits since ca 2 1 ma may have resulted from glacial interglacial variations of the east asian monsoon since ca 1 7 ma lacustrine deposition developed because of the enormous discharge of the yellow river into the jartai basin a tectonic movement at ca 0 8 ma is reflected by lacustrine red clay at the top of borehole jlt1 after the mid pleistocene transition the modern landscape of the jartai basin and its adjacent regions were formed

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

survey of 2011 tohoku earthquake tsunami inundation and run up 1 at 14 46 local time on march 11 2011 a magnitude 9 0 earthquake occurred off the coast of northeast japan this earthquake generated a tsunami that struck japan as well as various locations around the pacific ocean with the participation of researchers from throughout japan joint research groups conducted a tsunami survey along a 2000 km stretch of the japanese coast more than 5300 locations have been surveyed to date generating the largest tsunami survey dataset in the world on the sendai plain the maximum inundation height was 19 5 m and the tsunami bore propagated more than 5 km inland along the ria coast from about 50 to 200 km north of sendai the narrow bays focused the tsunami waves generating the largest inundation heights and run ups the survey data clearly show a regional dependence of tsunami characteristics

oligo miocene magnetostratigraphy and rock magnetism of the xishuigou section subei gansu province western china and implications for shallow inclinations in central asia magnetostratigraphy of 222 remanent directions together with late oligocene to early miocene mammal and charophyte paleontology suggest that 2179 m of the xishuigou section subei gansu province china were deposited from 26 to 19 ma stratigraphic patterns of bulk susceptibility anisotropy of magnetic susceptibility parameters and natural and anhysteretic remanent magnetization intensities demonstrate that 1 faulting does not significantly affect the record 2 sediment deposition was relatively continuous 3 sediment source changed around 23 ma and 4 rapid uplift near subei occurred at 21 ma subei rotated 27 5 counterclockwise with respect to the 20 ma pole from the eurasian synthetic apparent polar wander path folding and rotation of the section took place after 19 ma the paleolatitude of subei is 14 less than at present and 19 3 less than predicted from the reference pole both rock magnetic and paleomagnetic data sets suggest that the unusually low paleolatitude is the result of synsedimentary inclination shallowing a phenomenon which has likely affected other paleomagnetic data from central asia

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

tectonic evolution of the central annapurna range nepalese himalayas the metamorphic core of the himalayan orogen or greater himalayan sequence is a northward tapering prism bound at the bottom by a n dipping family of thrust faults the main central thrust system and at the top by a n dipping family of normal faults the south tibetan detachment system research in the central annapurna range of nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the early miocene the main central thrust system is represented here by a 2 to 3 km thick zone of high strain that developed during two or more episodes of movement most of its displacement was concentrated along the chomrong thrust a sharp late metamorphic discontinuity that places middle amphibolite facies rocks of the greater himalayan sequence on top of lower amphibolite facies rocks of the lesser himalayan sequence the earliest demonstrable movement on this thrust system occurred 22 5 ma the most recent movement may be as young as pliocene the oldest element of the south tibetan detachment system in this area is the deorali detachment which appears to have been active at the same time as the earliest shortening structures of the main central thrust system fabrics related to the deorali detachment are disrupted by a previously unrecognized sw vergent thrust structure the modi khola shear zone the effect of this structure which is constrained to be between 22 5 and 18 5 ma was to shorten rock packages that had been extended previously during movement on the deorali detachment transition back to a local extensional regime after 18 5 ma was marked by development of the machhupuchhare detachment and related splays geologic evidence for rapid two way transitions between contraction and extension in the annapurna range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle it also appears to be an important factor in mountain range development

tertiary structural evolution of the gangdese thrust system southeastern tibet structural and thermochronological investigations of southern tibet xizang suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost tibetan plateau since late oligocene two thrust systems are documented in this study the north dipping gangdese system gts and the younger south dipping renbu zedong system rzt west of lhasa the gangdese thrust juxtaposes the late cretaceous forearc basin deposits of the lhasa block the xigaze group over the tethyan sedimentary rocks of the indian plate whereas east of lhasa the fault juxtaposes the late cretaceous eocene andean type arc the gangdese batholith over tethyan sedimentary rocks near zedong 150 km southeast of lhasa the gangdese thrust is marked by a 200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks a major south dipping backthrust in the hanging wall of the gangdese thrust puts the xigaze group over tertiary conglomerates and the gangdese plutonics north of xigaze and west of lhasa a lower age bound for the gangdese thrust of 18 3 0 5 ma is given by crosscutting relationships the timing of slip on the gangdese thrust is estimated to be 27 23 ma from 40 ar 39 ar thermochronology and a displacement of at least 46 9 km is indicated near zedong the age of the gangdese thrust gt is consistent with an upper age limit of 24 ma for the initiation of movement on the main central thrust in places the younger renbu zedong fault is thrust over the trace of the gt obscuring its exposure the rzt appears to have been active at circa 18 ma but had ceased movement by 8 1 ma the suture between india and asia has been complexly modified by development of the gts rzt and locally strike slip and normal fault systems

formation and re orientation of the suerkuli basin within the altyn tagh in northeastern tibetan plateau since late miocene abstract the altyn tagh fault atf had formed in eocene time however the temporal evolution of slip along the fault remains elusive despite its important role in governing growth of the tibetan plateau tp and evaluating models of continental lithosphere deformation here we perform a high resolution magnetostratigraphic study on neogene sediments from the suerkuli basin a small basin located between two active strands of the atf results show that both depositional environment and the sedimentation rate changed at about 5 4 and 6 ma respectively lithofacies shift from conglomerates and sandstones with parallel and cross beddings to mudstones with horizontal bedding suggesting a sedimentary fluvial to lacustrine depositional environment shift even aridification increased over northern china from the tarim basin to the qaidam basin during late miocene to early pliocene times lacustrine deposition had started in the suerkuli basin still we argue that an increase in slip rate along the atf triggered concurrently forming of the intermontane basin the inferred timing of increased slip rate on the atf coincides with a variation in movement of the indian plate relative to eurasian plate since late miocene early pliocene time our work suggests that a re orientation of crustal shortening and deformation in the northern tp began since late miocene and supports the idea that the modern boundary of the tp orogen had rejuvenated to adjust deformation in plate boundary conditions

far field lithospheric deformation in tibet during continental collision 1 crustal deformation along the present day northern margin of the tibetan plateau has occurred since mid to late eocene time soon after india collided with eurasia assuming that on these distance and time scales the lithosphere can be approximated using a thin viscous sheet we show that far field lithospheric deformation caused by an indenting boundary is expected to start shortly after collision begins when a strong region analogous to cratonic lithosphere like that of the tarim basin is included in the calculations shear and vertical strain rates concentrate along the southern edge of the strong region resulting in enhanced deformation there in calculations scaled to the dimensions of india and the tibetan plateau shear and vertical strain rates on the northern edge of the plateau increase soon after collision begins crustal thickening accelerates later in the calculations we propose that the apparent increased exhumation and perhaps surface uplift around 10 ma may signal the arrival of the thickening front

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

spatial variations of flexure parameters over the tibet quinghai plateau we investigate the tibet quinghai plateau and the tarim basin in terms of spatial variations of the elastic thickness te in the frame of the thin plate flexure model the method of investigation makes use of a convolutive method which allows high spatial resolution of the flexure properties and overcomes some of the problems tied to the spectral admittance coherence methodologies we study the relation between the topographic and subsurface loads and the observed crust mantle interface cmi undulations the latter having been obtained from gravity inversion the gravity data used for the inversion are a unique set of high quality data available over the chinese part of the plateau and constitute the highest resolution grid today available in this impervious area the gravity inversion is constrained by results from the study of the propagation of seismic waves the two extensive sedimentary basins the tarim and the qaidam basins are modeled by forward gravity modeling the oscillations of the cmi obtained from the gravity inversion agree well with those expected by loading the thin plate model of spatially variably elastic thickness with the surface and subsurface loads it is found that the modeling of the sedimentary basins is essential in the flexure analysis the spatial variations of elastic thickness correlate with the extensions of the different terrains that constitute the plateau most of the tibet plateau has low te varying in the bounds 10 30 km with lower values in the qiangtang terrain where the te reaches 8 km the tarim and the qaidam basins precambrian platforms overlain by sediments are rigid and have a te of up to 110 km and 70 km respectively the flexural analysis distinctly discerns the tibet plateau with thick crust part of which is molten from the cratonic areas the tarim and qaidam basins which though of thinner crust act as undeformable rigid blocks

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

large scale drainage capture and surface uplift in eastern tibet sw china before 24 ma inferred from sediments of the hanoi basin vietnam current models of drainage evolution suggest that the non dendritic patterns seen in rivers in se asia reflect progressive capture of headwaters away from the red river during and as a result of surface uplift of eastern asia mass balancing of eroded and deposited rock volumes demonstrates that the red river catchment must have been much larger in the past in addition the nd isotope composition of sediments from the hanoi basin vietnam interpreted as paleo red river sediments shows rapid change during the oligocene before similar to 24 ma we interpret this change to reflect large scale drainage capture away from the red river possibly involving loss of the middle yangtze river reorganization was triggered by regional tilting of the region towards the east this study constrains initial surface uplift in eastern tibet and southwestern china to be no later than 24 ma well before major surface uplift and gorge incision after 13 ma

stratigraphic classification of ancient lakes balancing tectonic and climatic controls lakes and lake deposits present two fundamental paradoxes 1 modern lakes are vastly complicated but the rock records of lakes are relatively simple extensive observations reveal three distinct facies associations of common and widespread occurrence these are referred to here as fluvial lacustrine fluctuating profundal and evaporative facies associations 2 most explanations of modern and ancient lakes attribute their nature to climate but neither modern lake parameters lake size depth and salinity nor the character of ancient lake strata thickness extent lithology correlate with measured or inferred climatic humidity we propose that it is the relative balance of rates of potential accommodation mostly tectonic with sediment water fill mostly a function of climate that controls lake occurrence distribution and character lake basins may be termed overfilled balanced fill or underfilled depending on the balance between these rates we conclude that climate and tectonics exert coequal influence on lake deposits at both mesoscales 1 m to hundreds of meters and macroscales hundreds to thousands of meters

pulsed miocene range growth in northeastern tibet insights from xunhua basin magnetostratigraphy and provenance author s lease ro burbank dw hough b wang z yuan d abstract sedimentary rocks in tibetan plateau basins archive the spatiotemporal patterns of deformation erosion and associated climate change that resulted from cenozoic continental collision despite growing understanding of basin development in northeastern tibet during initial india asia collision as well as in the late miocene holocene surprisingly little is known about the intervening period a time when the plateau may have undergone fundamental tectonic changes to fill this gap we present new magnetostratigraphy from a g2300 m thick fluviolacustrine succession that spans ca 30 9 3 ma an integrated analysis of sedimentology subsidence and provenance from this section reveals the sequential pulsed erosion of multiple ranges bordering the xunhua basin emergence of the wnw trending laji shan is highlighted by a doubling of sediment accumulation rates between 24 and 21 ma and a transition to coarse allu vial facies at 20 3 ma detrital zircon u pb age spectra show that these coarse sediments came from basement terranes within the laji shan together these observations suggest accelerated growth of the laji shan and its coupled foreland basin at ca 22 ma the most rapid accumulation rates in xunhua basin occur within the finest grained strata and suggest an underfilled basin during the fastest interval of laji shan deformation growth of the laji shan occurred northward of the contemporaneous plateau margin which had been defined since ca 45 50 ma by the west qinling lying 60 km farther south hence following 20 25 m y of apparent stability the deformation front in this region jumped 60 km to the north at ca 22 ma subsequently growth of the north trending jishi shan occurred at ca 13 ma and is highlighted by an acceleration in xunhua basin accumulation rates between 12 and 9 ma as well as by a significant change in detrital zircon provenance of nearby linxia basin deposits by 11 5 ma initial growth of the wnw trending laji shan in the early miocene and subsequent growth of the north trending jishi shan 10 m y later support interpretations of a middle mio cene kinematic reorganization in northeastern tibet 2012 geological society of america

paleogene crustal extension in the eastern segment of the ne tibetan plateau abstract the tibetan plateau experienced lateral growth in response to the india eurasia collision but uncertainties remain about when and how its present day margins were formed it was commonly viewed that crustal shortening in the ne tibetan plateau had commenced since the eocene and led to reverse faulting and compressional basins our recent investigations of some cenozoic intermontane basins reveal that synsedimentary normal faults occur widely in the eastern segment of the ne tibetan plateau normal faulting at basin bases obviously controlled basin sedimentation and presumably resulted in footwall uplift and basement exhumation small scale synsedimentary normal faults are very common in basin fill sequences although they decrease in intensity upwards and eventually vanish in the uppermost paleogene sequences measurement of the attitudes of synsedimentary normal faults indicates that crustal stretching was in nw se direction the nw se extension was consistent with paleogene stress field of the whole north china craton which were thought responsible for the ne sw trending rift basins in the interior of the north china craton such as the yinchuan weihe and bohai bay rift basin systems a broad unconformity separates paleogene and neogene sequences in the eastern segment of the ne tibetan plateau contrasting strikingly with conformable oligocene to middle miocene sequences in the neighboring ningnan and yinchuan rift basins it thus follows that the eastern segment of the ne tibetan plateau had not been formed in paleogene when it was basically dictated by crustal extension possibly as a result of far field effect of subduction of the western pacific plate end oligocene termination of normal faulting and the presence of a regional unconformity indicate that the eastern segment of the ne tibetan plateau began rising at the beginning of the miocene

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

resolving eolian volcanogenic and authigenic components in pelagic sediment from the pacific ocean abstract we measured major and trace element concentrations in the operationally defined chemically extracted residual aluminosilicate component of sediment from ocean drilling program sites 1215 and 1256 in the central and eastern equatorial pacific ocean and found that this residual component contains volcanogenic and authigenic aluminosilicates in addition to inferred eolian material while the residual component younger than 20 ma from the central pacific odp site 1215 is similar compositionally to upper continental crust and suggests an increase in the delivery of asian dust material since 20 ma the residual in sediment older than 20 ma indicates significant amounts of volcanogenic and authigenic materials volcanogenic debris comprises as much as 40 of the residual between 23 40 ma which coincides with the mid tertiary ignimbrite flare up that occurred in much of western north america the residual component extracted from the 50 ma biogenic sediment reflects authigenic signatures seawater like negative cerium anomalies and elevated fe si ratios the previously interpreted increase in an andesitic detrital source in north pacific locations may instead be authigenic material presenting significant challenges for many paleoclimate proxies additionally in the eastern pacific odp site 1256 the residual component contains 70 of volcanogenic material most likely originating from central america and also includes refractory barite the ability to separately identify eolian volcanogenic and authigenic materials in the aluminosilicate component of pelagic sediment allows resolution respectively of the climatic geologic and chemical processes contributing to the paleoceanographic archive in this critical oceanic region

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

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

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