textural characteristics and genesis of the aeolian sediments in the kuwaiti desert several types of aeolian deposits have been recognized in kuwait a smooth sand sheets that resemble desert floor sand b immobile sands that include rugged vegetated sand sheets and wadi fill deposits and c mobile sands that form active sand sheets and sand dunes simple size frequency curves illustrate the genetic relationship between the various aeolian sediment types the four size parameters namely mean size sorting skewness and kurtosis were calculated scatter plot diagrams of sorting versus mean size and sorting versus kurtosis are effective in differentiating smooth sand sheet deposits from dune sands active sand sheet deposits can also be recognized because they are usually located between the two end members smooth sand sheets and dune sands size parameters change with location regardless of their types coarsening and positive skewness usually increase downwind mineralogical and textural characteristics of the aeolian deposits in kuwait revealed that they are mostly derived from the lower mesopotamian muddy flood plain deposits the sand fraction of the al dibdibba gravelly deposits and the disintegrated material from calcretic and gypcretic duricrusts distribution of depositional and deflational areas indicates that the northern desert of kuwait is characterized by a positive sand budget whereas the southern desert has a negative sand budget

miocene burial and exhumation of the india asia collision zone in southern tibet response to slab dynamics and erosion the india asia collision zone in southern tibet preserves a record of geodynamic and erosional processes following intercontinental collision apatite fission track and zircon and apatite u th he data from the oligocene miocene kailas formation within the india asia collision zone show a synchronous cooling signal at 17 1 ma which is younger than the ca 26 21 ma depositional age of the kailas formation constrained by u pb and 40 ar 39 ar geochronology and requires heating burial after ca 21 ma and subsequent rapid exhumation data from the gangdese batholith underlying the kailas formation also indicate miocene exhumation the thermal history of the kailas formation is consistent with rapid subsidence during a short lived phase of early miocene extension followed by uplift and exhumation driven by rollback and northward underthrusting of the indian plate respectively significant removal of material from the india asia collision zone was likely facilitated by efficient incision of the paleo indus river and paleo yarlung river in response to drainage reorganization and or intensification of the asian monsoon

quartz arenites of the cambro ordovician kamouraska formation quebec appalachians canada ii eolian sands in deep sea sedimentary gravity flow deposits the kamouraska formation is a quartz arenitic unit of latest cambrian earliest ordovician age in the quebec appalachians that was deposited by hyperconcentrated to concentrated density flows in a meandering submarine canyon on the continental slope bordering the iapetus ocean as outlined in a companion paper detailed petrographic study of the quartz arenites of the kamouraska formation combined with scanning electron microscopy of grain surface textures suggests that the quartz sands are of eolian origin having been derived from an inland desert or less likely a barrier beach dune system transport of the mature quartz arenitic sand onto the shelf and deposition into the deep sea was not accompanied by substantial mixing with material from other sources thus preserving the inherited eolian characteristics a modern analogue for the eolian interpretation of the deep sea quartz arenite beds is as follows thick late pleistocene eolian sand beds on a modern abyssal plain in the east atlantic referred

cenozoic anatexis and exhumation of tethyan sequence rocks in the xiao gurla range southwest tibet article i nfo in order to advance our understanding of the suturing process between continental landmasses a geologic and geochronologic investigation was undertaken just south of the india asia suture in southwestern tibet the focus of this study the xiao gurla range is located near the southeastern terminus of the active right lateral strike slip karakoram fault in southwestern tibet the range exposes metasandstone phyllite schist locally of sillimanite facies calc gneiss and marble paragneiss pyroxene quartzite metagranite and variably deformed leucogranite these metamorphic rocks are exposed in the footwall of a domal top to the west low angle normal detachment fault structurally beneath neogene quaternary basin fill and serpentinized ultramafic rocks of the kiogar jungbwa ophiolite the detachment is interpreted to be the northeastern continuation of the gurla mandhata detachment fault system that bounds metamorphic rocks of the gurla mandhata range 60 km to the southwest u pb geochronology on five detrital zircon samples of schist phyllite and quartzite yielded maximum depositional ages that range from 644 363 ma and age probability distributions that are more similar to tethyan sequence rocks than lesser himalayan sequence rocks a felsic gneiss yielded a metamorphic zircon age of 35 3 0 8 ma with a significant population of early paleozoic xenocrystic core ages the gneiss is interpreted to be the metamorphosed equivalent of the cambro ordovician gneiss that is exposed near the top of the greater himalayan sequence leucogranitic bodies intruding metasedimentary footwall rocks yielded two distinct u pb zircon ages of 23 ma and 15 ma locally rocks exposed in the hanging wall of this fault and of the southward dipping northward verging great counter thrust to the north consist of serpentinite bearing melange and conglomerate of inferred paleogene age dominated by carbonate clasts the melange is intruded by a 44 ma granite and the stratigraphically highest conglomerate unit yielded detrital zircon u pb ages similar to tethyan sequence rocks we attribute the middle eocene magmatism south of the suture to break off of the neo tethyan oceanic slab in addition our observations are consistent with the late eocene shortening and crustal thickening within the tethyan himalayan sequence early middle miocene leucogranite emplacement being related to underthrusting and melting of the greater and possibly lesser himalayan sequences and late miocene arc parallel extension in the hinterland of the southward propagating himalayan thrust belt

controls on the erosion of cenozoic asia and the flux of clastic sediment to the ocean rates of continental erosion may be reconstructed from variations in the rate of accumulation of clastic sediment most of which lies offshore global rates of marine sedimentation are usually considered to have reached a maximum after 3 4 ma driven by enhanced erosion in a variable glacial interglacial climate however a new compilation of seismic data from the marginal seas of asia now shows that only the red river reached its historic peak after 4 ma sediment flux from asia first peaked in the early middle miocene 24 11 ma well before the initiation of a glacial climate indicating that rock uplift and especially precipitation are the key controls on erosion at least over long periods of geologic time reconstructions of weathering in east asia show that faster erosion correlates with more humid warm climates in the early middle miocene changing to less erosive drier climates after 14 ma when antarctic glaciation begins average rates of sedimentation on most east asian continental margins since 1 8 ma are 5 6 times less than the modern fluvial flux implying that the flux to the oceans varies sharply on short timescales and is not always buffered over timescales of 10 4 yr by storage in flood plains

mineralogical composition and potential sources of dust fallout deposits in kuwait northern arabian gulf abstract detailed mineralogical investigations of dust fallout deposits collected at eleven locations in kuwait during april 1979 march 1980 were carried out the mineralogy of five size fractions medium sand fine sand very fine sand coarse silt and clay was determined occurrence frequency and potential sources of the mineral grains that form the various size fractions are discussed and the gross mineralogical composition of the dust fallout calculated it was found that dust fallout deposited in kuwait is quartzitic calcareous sandy silt and is mostly derived from the dry mesopotamian flood plain in the middle and south of iraq the occurrence of sand size particles is attributed to pick up from local recent surface deposits

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

age of crustal melting emplacement and exhumation history of the shivling leucogranite garhwal himalaya we report a u pb monazite age of 23 0 0 2 ma for the shivling leucogranite a tourmaline muscovite biotite leucogranite at the top of the high himalayan slab in the garhwal himalaya north india the shivling bhagirathi leucogranite is a viscous near minimum melt emplaced as a foliation parallel laccolith via a dyke network not far from its source region prograde heating occurred soon after the india asia collision at c 50 ma up to melting at 23 ma and high temperatures 550 c were maintained for at least 15 ma after garnet growth the leucogranite was emplaced at mid crustal depths along the footwall of the jhala fault a large scale low angle normal fault part of the south tibetan detachment system above kyanite and sillimanite grade gneisses the geometry of the leucogranite laccolith shows biaxial extension and boudinage both perpendicular north northeast south southwest and parallel to the strike west northwest east southeast of the mountain range unroofing occurred by underthrusting beneath the high himalayan slab along the main central thrust zone progressively jacking up the leucogranites removal of material above by low angle normal faulting and erosion very rapid cooling at rates of 200 350 c ma between 23 21 ma immediately followed crystallization as tectonic unroofing and erosion removed 24 28 km of overburden during this time k ar muscovite ages are 22 1 0 ma and fission track ages of zircons from 5000 m on the north ridge of shivling are 14 2 2 1 and 8 8 1 2 ma and apatites are 3 5 0 79 and 2 61 0 23 ma slow steady state cooling at rates of 20 30 c ma from 20 1 ma shows that maximum erosion rates and unroofing of the leucogranite occurred during the early miocene this timing coincides with initiation of low angle north dipping normal faulting along the south tibetan detachment system

stratigraphy structure and tectonic evolution of the himalayan fold thrust belt in western nepal regional mapping stratigraphic study and 40ar 39ar geochronology provide the basis for an incremental restoration of the himalayan fold thrust belt in western nepal tectonostratigraphic zonation developed in other regions of the himalaya is applicable with minor modifications in western nepal from south to north the major structural features are 1 the main frontal thrust system comprising the main frontal thrust and two to three thrust sheets of neogene foreland basin deposits 2 the main boundary thrust sheet which consists of proterozoic to early miocene lesser himalayan metasedimentary rocks 3 the ramgarh thrust sheet composed of paleoproterozoic low grade metasedimentary rocks 4 the dadeldhura thrust sheet which consists of medium grade metamorphic rocks cambrian ordovician granite and granitic mylonite and early paleozoic tethyan rocks 5 the lesser himalayan duplex which is a large composite antiformal stack and hinterland dipping duplex and 6 the main central thrust zone a broad ductile shear zone the major structures formed in a general southward progression beginning with the main central thrust in late early miocene time eocene oligocene thrusting in the tibetan himalaya north of the study area is inferred from the detrital unroofing record on the basis of 40ar 39ar cooling ages and provenance data from synorogenic sediments emplacement of the dadeldhura thrust sheet took place in early miocene time the ramgarh thrust sheet was emplaced between 15 and 10 ma the lesser himalayan duplex began to grow by 10 ma simultaneously folding the north limb of the dadeldhura synform the main boundary thrust became active in latest miocene pliocene time transport of its hanging wall rocks over an 8 km high footwall ramp folded the south limb of the dadeldhura synform thrusts in the subhimalayan zone became active in pliocene time the minimum total shortening in this portion of the himalayan fold thrust belt since early miocene time excluding the tibetan zone is 418 493 km the variation depending on the actual amounts of shortening accommodated by the main central and dadeldhura thrusts the rate of shortening ranges between 19 and 22 mm yr for this period of time when previous estimates of shortening in the tibetan himalaya are included the minimum total amount of shortening in the foldthrust belt amounts to 628 667 km this estimate neglects shortening accommodated by small scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

sedimentology of cold climate aeolian sandsheet deposits in the askja region of northeast iceland active aeolian sandsheets cover an area of 270 km2 in the askja region of ne iceland the sandsheet is divided into i a deflationary upwind part that is characterised by deflation lags ventifacted boulder fields and restricted aeolian sand patches in the lee of obstacles ii an accumulating central part that is characterised by continuous aeolian sand cover up to 10 m thick where three scales of aeolian ripples are evident along with larger scale forms including longitudinal zibars and iii a downwind part that is currently subject to aeolian bypass with only localised accumulation occurring in topographic hollows between basaltic lava fields trains of aeolian granule megaripples are a characteristic landform of the downwind part of the sandsheet these sinuous crested oblique bedforms attain heights of 0 35 m and downwind spacings of 6 m they are composed principally of low density pebble grade pumice clasts and other reworked volcaniclastic detritus derived from the nearby askja volcanic centre the distribution of landforms across the sandsheet reflects a regional aeolian sediment budget that is controlled by dynamic interactions between glacial ice margin fluvial aeolian and volcaniclastic processes under the influence of a distinctive climatic regime sediment supply at the upwind sandsheet margin adjacent to the dyngjujokull glacier increases during summer periods when rates of glacial ablation are at a maximum longer term controls on sediment supply rate are determined by episodes of glacier surging and retreat and by periodic eruptions of volcaniclastic debris from the askja volcanic centre the availability of sediment for aeolian transport is controlled by the level of the groundwater table the depth and continuity of the permafrost layer in the proglacial area the degree of development of armoured pebble lags and freezing of the sandsheet surface during winter months the transport capacity of the wind varies on a seasonal basis with mean wind velocities and wind gustiness reaching a maximum during winter periods both winter and summer mean wind velocities have been steadily increasing since formal records began in 1969 the suite of aeolian landforms developed within the askja region as defined by sediment texture and composition and bedform scale morphology and spatial distribution reflects a special set of controlling circumstances which mean that the askja sandsheet deposits differ significantly from those described from elsewhere

transtensional deformation in the central himalaya and its role in accommodating growth of the himalayan orogen 1 field mapping structural analysis and geochronologic data from northwestern nepal reveal major normal right slip motion along a previously unrecognized west northwest striking system of shear zones that we term the gurla mandhata humla fault system gmh the gmh obliquely cuts across the greater himalayan crystalline sequence and into the lesser himalayan imbricate thrust belt via two right step over structures the average slip direction on the gmh parallels the strike of the himalayan orogen motion along this fault system has resulted in an apparent left separation of the south tibet detachment main central thrust zone and lesser himalayan imbricate thrust belt along a north striking segment of the fault system we estimate a minimum of 21 km of net slip on the southern branch of the gmh by restoring the trace of the main central thrust zone parallel to the average slip direction on the fault taking into account slip estimates from the northern branch of the gmh yields a minimum net slip estimate of 24 4 to 32 4 km for the gmh the 232th 208pb ion microprobe monazite ages from leucogranite bodies indicate that motion on the gmh occurred after 15 ma its initiation immediately followed crustal thickening between the main central thrust zone and indus yalu suture zone motion on the gmh is contemporaneous with arc normal contraction in the southernmost himalayan orogen these observations can be explained by a model that involves foreland propagating structural systems facilitating arc normal contraction in the foreland and arc parallel extension in the hinterland that work together to maintain the arcuate shape of the himalayan orogen

geochemical mineralogical and sedimentological studies on the taklimakan desert sands major element composition mineral composition and grain size distribution have been studied for quaternary aeolian sediments from the taklimakan desert north western china together with the variation of chemical and mineralogical compositions of different grain size fractions aeolian sediments from the taklimakan desert have higher ratios of feldspar quartz and calcite quartz finer grain size poorer roundness of quartz and feldspar grains and lower abundances of frosted quartz than found in aeolian sediments from other deserts such as the saudi arabian desert in spite of these immature mineralogical and sedimentological features the aeolian sediments from the taklimakan desert show low regional variations in major element and mineral compositions and are homogenized these observations confirm that two processes glacial activity within surrounding mountains and aeolian activity at the tarim basin are important in the homogenization of the taklimakan desert sands taklimakan desert sediments are constantly and effectively supplied from basement rocks in the surrounding mountains by glacial erosion the supplied sediments are further homogenized by aeolian activity in the desert and are partly blown away serving as the source of chinese aeolian loess compositional differences are observed between loess mainly 10 40 m particles and the 45 m fraction of the taklimakan desert sediments as well as between loess and whole rock of the taklimakan desert sediments these observations provide constraints for precise modelling of loess formation and for assessment of the chemical composition of the upper continental crust based on the chemical composition of aeolian loess

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

revision of the cretaceous paleogene stratigraphic framework facies architecture and provenance of the xigaze forearc basin along the yarlung zangbo suture zone abstract new field investigation structural analysis review of biostratigraphy stratigraphic framework facies architecture provenance and flow direction of the cretaceous paleogene sedimentary strata in southern tibet resulted in reinterpretation of the evolution of the xigaze forearc basin xfb the xfb is comprised by flysch dominant xigaze group consisting of ngamring chongdoi and sangzugang formations and of the shallow marine cuojiangding group padana qubeiya quxia and gyalaze formations with their ages herein revised as the albian coniacian and the santonian ypresian respectively the deep sea flysch represented by the ngamring formation constitutes the early sedimentary fill of the xfb it was subdivided into five megasequences ms1 ms5 in order the early submarine fan facies stage channel submarine fan stage the mature submarine fan stage the highstand submarine fan stage and outer fan pelagic stage the paleocurrent flows were eastwards and westwards during the early and late stages of basin development and northwards and southwards during its middle stage indicating typical forearc basin sedimentation with longitudinal supply and lateral filling provenance studies point to multiple sources with andesitic debris sourced from the jurassic lower cretaceous yeba formation and sangri group formed in the volcanic island arc chains in the southern gangdese the four periods of the xfb development are 1 the formation of the xfb basement by the xigaze ophiolite and completed before the aptian the coeval deposits on the shelf are the sangzugang formation carbonate 2 the earliest xfb deposits of the earliest aptian age ca 125 120 ma represented by the chongdoi formation related to the residual forearc 3 composite forearc developed represented by the late albian late cenomanian flysch megasequences ms1 ms2 and the turonian coniacian flysch megasequences ms3 ms5 respectively and 4 the final stage of the xfb development started the terminal forearc stage represented by the late late cretaceous early paleogene shallow marine cuojiangding group and terminated during middle ypresian when the gyalaze formation was deposited

multiple environmental change at the time of the modern human passage through the middle east first results from geoarcheological investigations on upper pleistocene sediments in the wadi sabra jordan upper pleistocene sediments at wadi sabra ma an district jordan were geoarcheologically investigated from 2008 2010 the wadi sabra valley system situated east of the dead sea rift and south of the ancient nabataean capital of petra has preserved sedimentary deposits which are connected with several upper paleolithic sites first results from sedimentological and geochemical analysis supported by archeological and radiometric dating provide evidence of fluvial and fluvio eolian sedimentation during marine isotope stage mis 3 and 2 from at least 38 ka to 18 ka followed by at least one erosional event that finally determined the recent valley morphology several calcretes buried surfaces and initial soil horizons indicate phases of landscape stability and potentially moister climatic conditions than today inferred favorable environment of the time allowed for human occupation of the wadi sabra attested throughout the cultural stages of the initial early and evolved upper paleolithic and the early epipaleolithic thus illustrating the occurrence of homo sapiens in the near east field work at wadi sabra is expected to further elucidate the out of africa ii movement of our species leading modern humans to europe possibly via the middle east and the balkan peninsula both regions now under comparative investigation by the cologne aachen b1 project group of the crc 806

upper oligocene lower miocene gangrinboche conglomerate in the xigaze area southern tibet implications for himalayan uplift and paleo yarlung zangbo initiation abstract the gangrinboche conglomerate exposed along the yarlung zangbo suture zone records a crucial stage of the himalayan orogeny the type section of these strata in the xigaze area southern tibet including the qiuwu and dazhuka formations was studied by integrated stratigraphic sedimentologic petrographic and geochemical techniques palynological data and detrital zircon u pb ages indicate that the qiuwu formation was deposited during the latest oligocene to the earliest miocene most probably 23 ma while the overlying dazhuka formation was deposited during 23 18 ma the qiuwu formation was deposited in a deltaic setting and detritus was entirely derived from the gangdese magmatic arc in the north the dazhuka formation in contrast was deposited in mainly braided river environments and contains clasts derived from both the gangdese arc in the north and the himalayan orogen in the south clasts derived from the south first occur at the base of the dazhuka formation and increase in abunda

post collisional exhumation of the indus yarlung suture zone and northern tethyan himalaya saga sw tibet abstract the cretaceous eocene sedimentary sequences of the sangdanlin section at saga sw tibet within the india asia collisional zone have been studied in detail for constraining the timing of initial collision however the post depositional history has not been investigated here we report new low temperature thermochronology data along the classic sangdanlin section to reveal the cooling exhumation history of the indus yarlung suture zone iysz and northern tethyan himalaya samples from the iysz all yield latest oligocene miocene ages with most zircon u th he ages ranging between 18 and 27 ma and apatite fission track aft ages of 12 9 ma thermal history modelling indicates a consistent middle late miocene cooling episode related to post great counter thrust gct incision of the yarlung river to the south samples from the northern tethyan himalaya mesozoic sequences yield older zhe ages of 32 36 ma signifying activity of the tethyan himalayan fold thrust zone while two granites linked to formation of the north himalayan gneiss domes yield late miocene aft ages the regional low temperature thermochronology data reveal different cooling patterns for tributaries on the northern and southern sides of the yarlung river this points to the strong influence of early miocene tectonism along the gct as a major controlling factor on river drainage reorganization at this time

textural characteristics of dust fallout and potential effect on public health in kuwait city and suburbs the textural and mineralogical characteristics of the dust fallout along with the potential effect on human health were investigated at 12 locations in kuwait city and suburbs during march 2006 february 2007 mineralogically the dust comprises mostly calcite and quartz with grain size ranging from 1 to 25 m chemically elements such as ca fe mg and al are the dominant ones with the highest amount of average percentage concentration 12 89 for ca results showed that humans in the 12 sites are being exposed to different ranges of average percentage concentrations of minerals such as calcite 15 00 51 80 quartz 26 40 66 9 albite 6 30 17 30 dolomite 1 20 10 20 and gypsum 0 00 6 4 where site k3 recorded the lowest average concentrations for calcite dolomite and gypsum and the highest for quartz and albite the elements ca fe mg al k zn na cr and cd recorded ranges of 5 845 12 890 0 946 3 141 0 752 2 513 0 545 2 261 0 059 0 307 0 004 0 792 0 003 0 018 0 001 0 005 and 0 0004 respectively inhalation of such concentrations of calcite particles may cause coughing sneezing and nasal irritation and in cases of chronic exposure to excessive oral doses of calcite may produce alkalosis and hypercalcemia quartz can have potentially serious respiratory effects following long term exposure 1 year or more which is the case here in our study comparing with international and local standards it is highly likely that harmful and hazardous effects on the human body are being experienced by long term exposure to dust fallout in kuwait

mass accumulation rates in asia during the cenozoic summary this work establishes estimates of mass accumulation rates in 18 mostly offshore sedimentary basins in asia since the beginning of the cenozoic 66 ma the estimates were derived from isopach maps cross sections and drill holes or stratigraphic columns assuming regional similarity of the strata average solid phase volumes and accumulation rates were calculated for nine epochs approximately corresponding to geological periods palaeocene 66 58 ma eocene 58 37 ma oligocene 37 30 and 30 24 ma miocene 24 17 17 11 and 11 5 ma pliocene 5 2 ma and quaternary 2 0 ma these rates shed new light on the geological history of asia since the onset of the collision of india with asia 50 ma the overall average accumulation rates curve for asian sedimentary basins since the beginning of the tertiary shows an exponential form with slow accumulation rates less than 0 5 106 km3 myr 1 until the beginning of the oligocene more than 15 myr after the onset of the collision from the oligocene onwards rates increase quickly in an exponential manner reaching their maximum values in the quaternary more than 1 5 106 km3 myr 1 from these observations we suggest that extrusion and crustal shortening are complementary processes that have been successively dominant throughout the india eurasia collision history at smaller scales one may distinguish between independent histories at the subcontinental and basin scales this permits a comparison of the relative importance of tectonic and climatic erosion processes affecting the different mountain belts of asia during the cenozoic

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

assessing the microped size distributions of desert soils erodible by wind the microped size distribution of erodible soils is one of the main factors controlling the emissions of mineral aerosol to date the microped size distributions of desert soils in natural field conditions are not well known and thus the existing dust source models cannot be applied to natural sites based on dry sieving and elemental analysis of 26 soil samples collected in arid and semiarid regions it is shown that the microped size distributions of these desert soil samples involve a combination of four major mass size populations these four size populations correspond to salty soils fine quartz sand coarse dune sand and aluminosilicate silt we show that the microped size distributions of the various desert soils according to their mineralogical characteristics can be calculated by a mixture involving two or three of these four populations the main field of application for this scheme is the characterization of the erodible soil size distributions for modelling the atmospheric dust cycle

factors controlling threshold friction velocity in semiarid and arid areas of the united states a physical model was developed to explain threshold friction velocities u t for particles of the size 60 120 m lying on a rough surface in loose soils for semiarid and arid parts of the united states the model corrected for the effect of momentum absorption by the nonerodible roughness for loose or disturbed soils the most important parameter that controls u t is the aerodynamic roughness height z0 for physical crusts damaged by wind the size of erodible crust pieces is important along with the roughness the presence of cyanobacteriallichen soil crusts roughens the surface and the biological fibrous growth aggregates soil particles only undisturbed sandy soils and disturbed soils of all types would be expected to be erodible in normal wind storms therefore disturbance of soils by both cattle and humans is very important in predicting wind erosion as confirmed by our measurements

cenozoic thermo tectonic evolution of the gangdese batholith constrained by low temperature thermochronology abstract gangdese batholith in the southern lhasa block is a key location for exploring the tibetan plateau uplift and exhumation history we present the new low temperature thermochronological data from two north south traverses in the central gangdese batholith to reveal their cooling histories and corresponding controls zircon fission track ages show prominent clusters ranging from 23 7 to 51 6 ma apatite fission track ages from 9 4 to 36 9 ma apatite u th he ages between 9 5 and 12 3 ma and one zircon u th he age around 77 8 ma these new data and thermal modeling in combination with the regional geological data suggest that the distinct parts of gangdese batholith underwent different cooling histories resulted from various dynamic mechanisms the late eocene early oligocene exhumation of northern gangdese batholith coeval with the magmatic gap might be triggered by crust thickening followed by the breakoff of neotethyan slab while this stage of exhumation in southern gangdese batholith cannot be clearly elucidated probably because the most of plutonic rocks with the information of this cooling event were eroded away since then the northern gangdese batholith experienced a slow and stable exhumation while the southern gangdese batholith underwent two more stages of exhumation the late oligocene early miocene rapid cooling might be a response to denudation caused by the gangdese thrust or related to the regional uplift and exhumation in extensional background by the early miocene the rapid exhumation was associated with localized river incision or intensification of asian monsoon or north south normal fault

stable isotopes reveal southward growth of the himalayan tibetan plateau since the paleocene abstract original stable isotope compositions of carbonates representing conditions in the latest oligocene early miocene kailas and qiabulin areas both in southern tibet record oligocene miocene paleoelevations of the gangdese arc and the himalayan orogen and provide constraints on the formation of the himalayan tibetan plateau oxygen isotope compositions of bivalve shell paleosol and lacustrine carbonates indicate the preservation of unaltered isotopic signals of paleometeoric waters in the kailas and qiabulin areas whereas the oxygen isotopic compositions of liuqu eocene paleosols were likely altered by paleometeoric waters paleoelevation estimates using oxygen isotopes indicate the kailas area was at 4 9 km during 20 19 ma and the qiabulin basin was at 2 0 km during 24 21 ma but rose rapidly to 4 1 km between 21 and 19 ma these results suggest a steep south facing flank on the proto tibetan highland prior to the onset of the india asia collision the himalayan orogen began to be built against the pre existed high 4 5 km gangdese mountains in the early eocene and obtained elevations close to those of the present by the early miocene we propose that the southernmost tibetan plateau and himalayan orogen are the expression of an early eocene to early miocene southward migration of the locus of deformation early stage uplift is linked to the crustal thickening in the early eocene but the dramatic elevation gain in the early miocene may have been caused by indian slab rollback break off and coeval renewed underthrusting behind which late miocene to present east west crustal extension took place

sedimentological characteristics and morphology of the aeolian sand dunes in the eastern part of the uae a case study from ar rub al khali abstract the umm az zimul al wijan area is located in the southeastern part of the uae and covers about 1600 km 2 it is bounded to the east by the oman mountains and to the south and west by the extensive dune area of ar rub al khali field investigation revealed the common occurrence of parallel to sub parallel sand ridges with a general east west orientation also present but less frequent are barchan barchanoid and star dunes interdune areas are classified into four types that include dolomite mixed ancient sabkha and ophiolitic rock fragments mixed sand sheet and ophiolitic rock fragments and recent sabkha the textural characteristics of the sand forming the dunes are unimodal with a modal class in fine sand size moderately to very well sorted positively skewed and mesokurtic quartz grains are mainly subrounded to subangular the similarity in roundness properties in different dune types in the eastern province of the uae seems not to be influenced by dune form based on the heavy mineral suites and the angularity of quartz grains it is believed that the sand sediments are mostly derived from the ultrabasic rocks forming the ophiolitic sequence of the oman mountains acidic rocks of iran and from the coastal area

the evolution of yarlung tsangpo river constraints from the age and provenance of the gangdese conglomerates southern tibet abstract cenozoic conglomerates are exposed discontinuously along the length of the yarlung tsangpo suture zone on the southern margin of the gangdese arc these conglomerates the gangdese conglomerates herein record a crucial stage in the uplift and erosion histories of the southern tibet after the initial india asia collision in the mt kailas area the gangdese conglomerates strata consist of multiple sedimentary cycles and each cycle is a fining upward sequence that was deposited by alluvial fan braided river and delta systems whereas in the xigaze area the gangdese conglomerates strata comprise a coarsening upward sequence that was deposited by delta braided river and alluvial fan systems based on the detrital and igneous zircon u pb ages the depositional ages of the gangdese conglomerates are late oligocene to early pliocene ca 26 5 ma in the mt kailas area late oligocene to middle miocene ca 26 15 ma in the xigaze area and late oligocene to early miocene ca 26 19 ma in the zedong area paleocurrent measurements and provenance data i e conglomerate clast composition sandstone petrology and detrital zircon age indicate that the initial detritus of the gangdese conglomerates were entirely derived from the north mainly from the gangdese arc sediment resulting from denudation to the south the xigaze forearc basin the yarlung tsangpo suture zone and the northern margin of the indian plate first appeared by the early miocene ca 19 ma and subsequently increased in abundance gradually our new results together with previous data from the xigaze area reveal 3 major stages in the evolution of the yarlung tsangpo river system 1 the southward flowing stage ca 26 19 ma featured southward draining transverse rivers that transported materials from the gangdese arc southward southward paleocurrents in the gangdese conglomerates indicate a northern source 2 the westward flowing stage ca 19 15 ma developed due to the uplift of the suture zone and tethys himalaya to the south northward draining rivers began to develop and lakes resembling a string of beads formed and finally connected together initiating the westward flowing paleo yarlung tsangpo river westward paleoflows were recorded in the gangdese conglomerates 3 the eastward flowing stage ca 15 ma present resulted from differential uplift and denudation of the southern tibet which reversed the direction of the young yarlung tsangpo river by ca 15 ma the deposition of the gangdese conglomerates was controlled by eastward paleoflows at this point the modern eastward flowing yarlung tsangpo river system was established

tertiary deformation history of southeastern and southwestern tibet during the indo asian collision geologic mapping and geochronological analysis in southwest kailas area and southeast zedong area tibet reveal two major episodes of tertiary crustal shortening along the classic indus tsangpo suture in the yalu river valley the older event occurred between ca 30 and 24 ma during movement along the north dipping gangdese thrust the development of this thrust caused extensive denudation of the gangdese batholith in its hanging wall and underthrusting of the xigaze forearc strata in its footwall examination of timing of major tectonic events in central asia suggests that the initiation of the gangdese thrust was approximately coeval with the late oligocene initiation and development of north south shortening in the eastern kunlun shan of northern tibet the nan shan at the northeastern end of the altyn tagh fault the western kunlun shan at the southwestern end of the altyn tagh fault and finally the tian shan north of the tarim basin such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the eurasian plate and the indian and pacific plates the younger thrusting event along the yalu river valley occurred between 19 and 10 ma along the south dipping great counter thrust system equivalent to the locally named renbu zedong thrust in southeastern tibet the backthrust system in south central tibet and the south kailas thrust in southwest tibet the coeval development of the great counter thrust and the north himalayan granite gneiss dome belt is consistent with their development being related to thermal weakening of the north himalayan and south tibetan crust due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between india and asia or frictional heating along major thrusts such as the main central thrust beneath the himalaya

clay mineral accumulation rates in the arabian sea during the late quaternary abstract the regional variations in clay mineral concentrations in 39 turbidite free and undisturbed deep sea sediment cores from the arabian sea are compared with distribution patterns of clay mineral accumulation rates during the holocene and the last glaciation dust plumes from the red sea area arabia mesopotamia iran and pakistan can be traced by accumulation rates of mineral specific dust loads during the late quaternary the major proportions of aeolo marine sediments in the western arabian sea were derived from arabia during the summer season the clay mineral assemblage in these dust plumes consists of illite smectite kaolinite chlorite and palygorskite in decreasing order of abundance the distribution pattern of palygorskite in particular reveals that dust transport from southern arabia occurs mainly with northwesterly to westerly winds in the mid troposphere dust plumes from iran and pakistan which are characterized by high chlorite content are dispersed by northwesterly winds over the northwestern arabian sea dust contributions from the red sea area which are rich in smectite and characterized by the occurrence of amphiboles are transported to the gulf of aden the amount of dust related to northeast monsoon winds during the winter season appears to be negligible when compared to the proportions borne on other dust bearing winds in addition to aeolian contributions we distinguish two fluvial contributions a chlorite illite kaolinite and quartz rich assemblage from oman and pakistan into the gulf of oman and a smectite rich assemblage which is dominant off southern india originating from rivers that drain the indian subcontinent the significant difference between the holocene and glacial accumulation rates of wind transported clay minerals consist in the higher abundance of quartz illite and chlorite in the northwestern arabian sea during the last glaciation this pattern indicates intensified dust contributions by northwesterly winds from the persian gulf area which was dry during glacial times

sedimentological modal analysis and geochemical studies of desert and coastal dunes altar desert nw mexico sedimentological compositional and geochemical determinations were carried out on 54 desert and coastal dune sand samples to study the provenance of desert and coastal dunes of the altar desert sonora mexico grain size distributions of the desert dune sands are influenced by the colorado river delta sediment supply and wind selectiveness the desert dune sands are derived mainly from the quartz rich colorado river delta sediments and sedimentary lithics the dune height does not exert a control over the grain size distributions of the desert dune sands the quartz enrichment of the desert dune sands may be due to wind sorting which concentrates more quartz grains and to the aeolian activity which has depleted the feldspar grains through subaerial collisions the desert dune sands suffer from little chemical weathering and they are chemically homogeneous with chemical alteration indices similar to those found in other deserts of the world the desert sands have been more influenced by sedimentary and granitic sources this is supported by the fact that ba and sr concentration values of the desert sands are within the range of the ba and sr concentration values of the colorado river quartz rich sediments the sr values are also linked to the presence of ca bearing minerals the zr values are linked to the sedimentary sources and heavy mineral content in the desert dunes the golfo de santa clara and puerto penasco coastal dune sands are influenced by long shore drift tidal and aeolian processes coarse grains are found on the flanks whereas fine grains are on the crest of the dunes high tidal regimens long shore drift and supply from colorado delta river sediments produce quartz rich sands on the beach that are subsequently transported into the coastal dunes outcrops of quaternary sedimentary rocks and granitic sources increase the sedimentary and plutonic lithic content of the coastal dune sands the chemical index of alteration cia values for the desert and coastal dune sands indicate that both dune types are chemically homogeneous the trace element values for the coastal dune sands are similar to those found for the desert dune sands however an increase in sr content in the coastal dune sands may be due to more caco3 of biogenic origin as compared to the desert dune sands correlations between the studied parameters show that the dune sands are controlled by sedimentary sources e g colorado river delta sediments since heavy minerals are present in low percentages in the dune sands probably due to little heavy mineral content from the source sediment grain sizes in the dune sands are coarser than those in which heavy minerals are found and or the wind speed might not exert a potential entrainment effect on the heavy mineral fractions to be transported into the dune a cluster analysis shows that the el pinacate group is significantly different from the rest of the dune sands in terms of the grain size parameters due to longer transport of the sands and the long distance from the source sediment whereas the puerto penasco coastal dune sands are different from the rest of the groups in terms of their geochemistry probably caused by their high caco3 content and slight decrease in the cia value copyright 2006 john wiley sons ltd

modeling of the temperature sensitivity of the apatite u th he thermochronometer apatite u th he apparent ages will generally reflect residence for extended periods at temperatures where helium is neither quantitatively retained nor lost by diffusion to characterize the response of apatite he ages to thermal histories involving partial he retention we explored solutions to the he production diffusion equation under thermally static conditions the analytical solution to this equation coupled with published diffusivity data demonstrates that the zone of partial he retention extends from about 40 c to 85 c this zone lies at temperatures 35 c cooler than the analogous fission track partial annealing zone he ages within the partial retention zone ultimately achieve a balance between he production and loss yielding a steady state age both the ultimate age and the time it takes to achieve this age are temperature dependent for example an apatite held at 75 c equilibrates to an age of 2 ma after 17 myr regardless of whether equilibrium is approached from a higher or a lower initial he age for representative dynamic thermal histories we evaluated apatite he ages using a numerical solution to the ingrowth diffusion equation the results illustrate the sensitivity of he ages to various geologic histories and are useful for understanding he age elevation relationships and for testing time temperature paths derived from apatite fission track length distributions in addition although he diffuses rapidly from apatite at shallow crustal temperatures modeling of ambient temperature fluctuations indicates that he ages are nearly unaffected by surficial processes

apatite fission track evidence for miocene denudation history in the gangdese conglomerate belt and yarlung tsangpo river implications for the evolution of southern tibet abstract low temperature thermochronology is used widely in the tibet plateau uplift some researches however have defined the time of rapid denudation as simply rock uplift and have neglected the fact that the rock denudation recorded by fission track ft data was controlled by both surface incision and rock uplift the incision of the yarlung zangbo river had a significant influence on uplift history inversion in southern tibet this paper simulated the bedrock denudation and river incision histories using apatite fission track aft data sampled from the gangdese conglomerate belt which is located in the middle of southern tibet and analyzed the geological meaning of the aft age of each sample the results showed the following 1 in the early miocene 22 16 ma both the value of the denudation rate and the incision rate were high 0 56 mm yr and 0 24 mm yr 2 in the middle late miocene the incision rate 0 12 mm yr was similar to the denudation rate 0 09 0 11 mm yr 3 the historical model between river incision and bedrock denudation revealed a significant difference in the denudation rate during the period ca 8 6 ma combining these data with previously published thermochronological ages and synthesizing these ages with regional geological we arrived at the following conclusions 1 in the early miocene the denudation event probably was caused by a combined result of indian plate rollback and the incision of the yarlung zangbo river 2 in the middle late miocene the denudation rate was consistent with the incision rate which suggested that the denudation episode was caused by climate change associated with asian monsoon intensification 3 after 8 ma the stable and slow incision rate indicated that regional drastic uplift had ceased the paleo elevation of the research area had approached and even exceeded the present day elevation in the late miocene

mass independent fractionation of mercury isotopes in the environment 1 the toxicity of mercury s methylated species and its biomagnification in aquatic food chains and global dispersion by the atmosphere are the cause of worldwide health problems recent reports have observed natural mass dependent fractionation in mercury isotopes and recent theoretical work has demonstrated that isotopic separation in mercury is due primarily to nuclear field shifts nuclear volume effect and the magnetic spin effect gives rise to mass independent fractionation mif of odd neutron number isotopes now we present analytical evidence of mass independent isotopic variations in mercury produced by both nuclear volume and magnetic isotope processes even mass number isotopes exhibit a pattern indistinguishable from that produced by mass dependent fractionation with both positive and negative 199hg and 201hg anomalies mif is easier to reliably determine in hg isotopes than mass dependent fractionation alone and thus it provides a potentially important key in constraining models of mercury sources and pathways in the environment

distinguishing different sedimentary facies in a deltaic system abstract an attempt has been made to differentiate sedimentary facies in a modern deltaic system by means of grain size characteristics of the ganga alluvial plain of west bengal india three main energy environments marine mixed and riverine comprising the delta were considered sand samples were collected from rivers both tidal and non tidal coastal dunes beaches and tidal flats of the deltaic plain the grain size distribution patterns were compared with the two model distributions of log normal and log skew laplace different sedimentary facies were identified by discriminant functions the analytical results indicate that the energy gradient of the different sedimentary facies of the deltaic system is well reflected by the grain size characteristics of the individual facies while critically analyzing the role of different textural parameters in discriminating the individual facies associations it is observed that the mean size alpha slope of coarser fractions of laplace model and skewness have greater potential to distinguish different sedimentary facies of the deltaic system the results of discriminant analysis might be applicable to paleo environmental interpretation of a deltaic system by distinguishing the individual facies associations

the zedong window a record of superposed tertiary convergence in southeastern tibet determining the timing magnitude and location of deformation due to the indo asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision a puzzling result of geological investigations of the lhasa block over the past 2 decades has been the apparent lack of significant tertiary deformation there perhaps the most important structural feature of the lhasa block is the south directed gangdese thrust system which developed along its southern edge the thrust system which separates the andean type batholith of southern asia from rocks of indian affinity is obscured at most locations across southeastern tibet by back thrusts of the younger north directed renbu zedong thrust system the best documented site where both thrusts are exposed occurs near zedong zedong window systematic geochronologic analyses were conducted in this area u pb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid the yaja granodiorite cut by the gangdese thrust indicates a crystallization age of 30 4 0 4 ma 2 thus placing an upper bound on the initiation of the thrust u pb zircon dating of granitoid samples structurally higher in the gangdese hanging wall yields emplacement ages of 42 5 1 0 ma and 48 9 0 8 ma 2 similar to other magmatic complexes within the gangdese arc geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the gangdese batholith suggesting a significant post collisional input of juvenile heat the 40ar 39ar thermochronology of samples from a vertical section through the hanging wall within the yaja granodiorite coupled with results of a numerical thermal model indicate an average slip rate along the gangdese thrust of 7 mm yr between 30 and 23 ma and a minimum displacement of 50 km farther east in the zedong window thermal effects produced by later north directed thrust sheets of the renbu zedong system appear to have obscured thermal history signatures in the gangdese hanging wall related to earlier south directed thrusting thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 ma consistent with previous estimates

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

a regional record of expanded holocene wetlands and prehistoric human occupation from paleowetland deposits of the western yarlung tsangpo valley southern tibetan plateau abstract the asian monsoon which brings 80 of annual precipitation to much of the tibetan plateau provides runoff to major rivers across the asian continent paleoclimate records indicate summer insolation and north atlantic paleotemperature changes forced variations in monsoon rainfall through the holocene resulting in hydrologic and ecologic changes in plateau watersheds we present a record of holocene hydrologic variability in the yarlung tsangpo yt valley of the southern tibetan plateau based on sedimentology and 14 c dating of organic rich black mats in paleowetlands deposits that shows changes in wetlands extent in response to changing monsoon intensity four sedimentary units indicate decreasing monsoon intensity since 10 4 ka bp wet conditions occurred at 10 4 ka bp 9 6 ka bp and 7 9 4 8 ka bp with similar to modern conditions from 4 6 2 0 ka bp and drier than modern conditions from 2 0 ka bp to present wetland changes correlate with monsoon intensity changes identified in nearby records with weak monsoon intervals corresponding to desiccation and erosion of wetlands dating of in situ ceramic and microlithic artifacts within the wetlands indicates epipaleolithic human occupation of the yt valley after 6 6 ka bp supporting evidence for widespread colonization of the tibetan plateau in the early and mid holocene during warm wet post glacial conditions

inland aeolian deposits of the iberian peninsula sand dunes and clay dunes of the duero basin and the manchega plain palaeoclimatic considerations abstract this paper describes the latest research on the geomorphological characteristics formation environment and chronology of the main inland aeolian deposits from the south eastern duero basin db and the manchega plain mp of the iberian peninsula similarities and differences between the aeolian deposits of these two locations are summarised wind deflation from the guadiana and jucar alluvial systems created the aeolian deposits of the mp these deposits are mainly composed of quartz sands however in the san juan alluvial plain mp there is a large extent of clay dunes formed by exposure to prevalent winds of seasonal playa lakes with salt and clay sediments in the db wind remobilisation of the small particles from quaternary terraces and tertiary arkosic sediments left aeolian deposits of quartz feldspar sands textural parameters of the aeolian deposits show large variations depending on the location and the original deposit thus in the db the aeolian sands derived from the deflation of fluvial sediments are better sorted and smaller in grain size than those created by the deflation of arkosic sediments morphologically simple and compound parabolic dunes u v forms hemicyclic lobate and elongate crescentic and linear dunes climbing dunes and blowout dunes have been recognized at both sites barchan and dome dunes are present only in the db while lunette lunette clay dunes are found only in the mp in both locations the large extent of aeolian sand sheets and the predominance of simple and compound parabolic dunes indicates the active role of sparse vegetation cover in the formation of this aeolian system in the db dunes were formed by southwest and west winds while in the mp the aeolian morphologies indicate that the prevalent winds were west and northwest the chronology of the dune deposits is being determined with luminescence tl osl dating and mass spectrometry analysis 14 c ams in this way the aeolian activity and stabilisation stages can be established the latter well marked in the db through soil a horizon development thus the main sand dune formation in the db and the eastern regions of the mp occurred between 13 5 and 7 ka bp during the cold and arid younger dryas episode and the early holocene the clay dunes of the mp accumulated mainly from 29 to 19 ka bp that corresponds with heinrich events he 3 and he 2 and the last glacial maximum however clay dunes were also formed between 13 5 and 7 ka bp in both locations there have been reactivations of some sand deposits in the recent holocene with maximum activity around 5 2 ka bp and 0 5 0 2 ka bp on the other hand three marked stages of stabilisation of the db aeolian system have been established with 14 c ams around 10 2 6 2 and 1 2 ka bp finally the main winds contributing to dune construction were also responsible for the deflation processes with the formation of erosional depressions

india asia convergence insights from burial and exhumation of the xigaze fore arc basin south tibet the composite fore arc syncollisional xigaze basin in south tibet preserves a key record of india asia collision new apatite fission track and zircon u th he data from an n s transect across the preserved fore arc basin sequence near xigaze show a consistent northward late cretaceous to middle miocene younging trend while coexisting apatite u th sm he ages are all miocene corresponding detrital zircon u pb data are also reported for constraining the cretaceous depositional ages of the xigaze basin sequence in the region thermal history modeling indicates that the basin experienced northward propagating episodic exhumation along with a northward migration of the depocenter and a pre existing cenozoic syncollisional basin sequence which had been removed in the southern part fore arc exhumation commenced in the late cretaceous 89 2 ma following transition to a syncollisional basin in the paleocene sedimentation in the central and northern xigaze basin continued until the latest eocene 34 4 ma ongoing folding and thrusting e g great counter thrusts caused by progressive plate convergence during late oligocene early miocene time resulted in regional uplift and considerable basin denudation which fed two fluvial basins along its northern and southern flanks and exposed the basement ophiolite subsequent incision of the yarlung river resulted in miocene cooling in the region different episodes in the exhumation history of the xigaze basin caused by thrusting of an accretionary wedge and ophiolitic basement can be linked to changes in india asia convergence rates and the changing subduction pattern of the indian and neo tethyan slabs

low temperature thermochronology of the indus basin in central ladakh northwest india implications of miocene pliocene cooling in the india asia collision zone the india asia collision zone in ladakh northwest india records a sequence of tectono thermal events in the interior of the himalayan orogen following the intercontinental collision between india and asia in early cenozoic time we present zircon fission track and zircon and apatite u th he thermochronometric data from the indus basin sedimentary rocks that are exposed along the strike of the collision zone in central ladakh these data reveal a postdepositional miocene pliocene 22 4 ma cooling signal along the india asia collision zone in northwest india our zircon fission track cooling ages indicate that maximum basin temperatures exceeded 200 c but stayed below 280 300 c in the stratigraphically deeper marine and continental strata thermal modeling of zircon and apatite u th he cooling ages suggests postdepositional basin cooling initiated in early miocene time by 22 20 ma occurred throughout the basin across zircon u th he partial retention temperatures from 20 10 ma and continued in the pliocene time until at least 4 ma we attribute the burial of the indus basin to sedimentation and movement along the regional great counter thrust the ensuing miocene pliocene cooling resulted from erosion by the indus river that transects the basin an approximately coeval cooling signal is well documented east of the study area along the collision zone in south tibet our new data provide a regional framework upon which future studies can explore the possible interrelationships between tectonic geodynamic and geomorphologic factors contributing to miocene pliocene cooling along the india asia collision zone from nw india to south tibet

the nature and possible origin of mega dunes in liwa ar rub al khali uae abstract the liwa area is located in the south central part of the uae along the northeastern upwind margin of the ar rub al khali sand sea and covers about 3694 km 2 it has one type of dunes a mega barchan that is compound and complex in form characterized by the superimposition of smaller dunes due to the giant size of barchans interdune areas appear as closed depressions that vary in shape from irregular elongate triangular to circular or semi circular these depressions are mostly covered with small barchans or sabkha deposits with patches of gypcrete in their upper parts scattered flat top mounds are also observed in interdune areas the size of dunes in liwa is directly controlled by quaternary climatic changes multiple generations are superimposed on one another separated by calcretic horizons the climate during dune formation alternated between relatively arid to humid the dune sands are unimodal with a modal class of fine sand moderately to very well sorted and near symmetrical and mesokurtic quartz grains are mainly subrounded to subangular based on the heavy mineral suites the sand came mostly from the ultrabasic rocks that form the ophiolitic sequence of the oman mountains the acidic rocks of iran and from the arabian shield

climate induced sediment palaeosol cycles in a late cretaceous dry aeolian sand sheet marilia formation north west bauru basin brazil aeolian sand sheets which are characterized by low relief surfaces that lack dunes are common in arid and semi arid climatic settings the surface of an aeolian sand sheet can either be stable and subject to pedogenetic effects or unstable such that it is affected by deflation or sedimentation the maro lia formation late cretaceous may be interpreted as an ancient aeolian sand sheet area where alternating phases of stability and instability of the accumulation surface have been recorded detailed field studies were carried out in several sections of the maro lia formation where cyclic alternations of palaeosols and aeolian deposits were evident using palaeopedological and facies analysis methods supported in the laboratory by the analysis of rock samples cut and polished in slabs thin sections scanning electron microscope images and x ray diffraction data from the clay minerals the deposits comprise three lithofacies that in order of abundance are characterized by i translatent wind ripple strata ii flood deposits and iii ephemeral river channel deposits palaeosols constitute on average 65 of the vertical succession three types of palaeosols pedotypes are recognized i aridisols ii entisols and iii vertisols erosional surfaces due to aeolian deflation divide the top of the palaeosol profiles from the overlying aeolian deposits the palaeoenvironmental interpretation of the deposits and the palaeosols allows the depositional system of the maro lia formation to be defined as a flat area dominated by aeolian sedimentation with subordinate ephemeral river sedimentation and characterized by a dry climatic setting with occasional rainfall the climate is the main forcing factor controlling the alternation between episodes of active sedimentation and periods of palaeosol development a climate controlled model is proposed in which i the palaeosols are indicative of a stable surface that is developed during the more humid climatic phases and ii the erosional surfaces and the overlying aeolian sediments attest to periods of deflation and subsequent sedimentation thereby increasing the availability of sediment during the drier climatic phases the ephemeral fluvial deposits mark the more humid climatic conditions and contribute to the lagged sediment influx caused during the drier periods by the erosion of previously stored sediment

atmospheric summer circulation and coastal upwelling in the arabian sea during the holocene and the last glaciation accumulation rates of biogenic and lithogenic components were studied in 39 turbidite free well dated sediment cores from the northern indian ocean to define the proportions of fluvial and eolian input and to reconstruct quaternary patterns of coastal upwelling the majority of dust deposited in the western arabian sea during the holocene about 100 106 t yr 1 is advected from arabia by northwesterly winds which overlie the low level southwest monsoon the glacial increase in dust flux to 160 106 t yr 1 culminated in the northern arabian sea most probably due to i entrainment of dust rich in chlorite dolomite and lithogenic carbonate in the then dry persian gulf and ii a southward shift of the mean position of the southwest monsoon during glacial summer this shift is recorded in reduced accumulation rates of biogenic opal and increased rates of marine carbonate off somalia and oman both the terrigenous and biogenic sediment records show that the northwesterly winds and the southwest monsoon persisted over the last 27 000 yr as well as the asian continental summer heat low however the glacial seasonal time span of the southwest monsoon season was much reduced most likely because of a delay in the seasonal onset of the southwest monsoon

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

new constraints on the india asia collision the lower miocene gangrinboche conglomerates yarlung tsangpo suture zone se tibet abstract lower miocene conglomerates crop out along the length of the yarlung tsangpo suture zone on the southern margin of the lhasa terrane these conglomerates known by various local names are correlated herein as the gangrinboche conglomerates all units exhibit broadly similar stratigraphic histories and a basal depositional contact upon an eroded surface of rocks of the lhasa terrane is ubiquitous at most localities the tops of sections are either removed by erosion or truncated by north directed thrusts these conglomeratic molasse units developed in response to the india asia collision and record aspects of its development in all units initial clast derivation was from the lhasa terrane on the northern margin of the yarlung tsangpo suture zone up section the first appearance of clasts derived from terranes within the suture zone and the northern margin of india all of which lie to the south of any outcrops of gangrinboche conglomerates is observed although these units were previously thought to be eocene analysis of fossil and structural constraints indicates early miocene deposition as development of the gangrinboche conglomerates records a significant phase in the evolution of the india asia collision understanding of their age and stratigraphic evolution has wider implications for regional tectonic models

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

synorogenic extension quantitative constraints on the age and displacement of the zanskar shear zone northwest himalaya structural thermobarometric and geochronological data place limits on the age and tectonic displacement along the zanskar shear zone a major north dipping synorogenic extensional structure separating the high grade metamorphic sequence of the high himalayan crystalline sequence from the overlying low grade sedimentary rocks of the tethyan himalaya a complete barrovian metamorphic succession from kyanite to biotite zone mineral assemblages occurs within the 1 km thick zanskar shear zone thermobarometric data indicate a difference in equilibration depths of 12 3 km between the lower kyanite zone and the garnet zone which is interpreted as a minimum estimate for the finite vertical displacement accommodated by the zanskar shear zone for the present day dip of the structure 20 a simple geometrical model shows that a net slip of 35 9 km is required to regroup these samples to the same structural level because the kyanite to garnet zone rocks represent only part of the zanskar shear zone and because its original dip may have been less than the present day dip these estimates for the finite displacement represent minimum values field relations and petrographic data suggest that migmatization and associated leucogranite intrusion in the footwall of the zanskar shear zone occurred as a continuous process starting at the barrovian metamorphic peak and lasting throughout the subsequent extension induced exhumation geochronological dating of various leucogranitic plutons and dikes in the zanskar shear zone footwall indicates that the main ductile shearing along the structure ended by 19 8 ma and that extension most likely initiated shortly before 22 2 ma

episodic rapid uplift in the himalaya revealed by 40ar 39ar analysis of detrital k feldspar and muscovite bengal fan detrital k feldspar and muscovite samples from ocean drilling program leg 116 cores have been dated by the sup 40 ar sup 39 ar technique and have depositional ages from 0 to 18 ma from 4 to 13 individual k feldspars and 1 to 12 individual muscovites have been dated from 7 stratigraphic levels in every level at least one k feldspar and one muscovite yielded a minimum age identical within uncertainty to the age of deposition these results indicate that a significant portion of the material in the bengal fan is first cycle detritus derived from the himalaya therefore the substantial amount of sediment deposited in the distal fan in early to middle miocene time can be ascribed to a significant pulse of uplift and erosion in the collision zone at this time moreover these data indicate that throughout the neogene some part of the himalayan orogen was undergoing rapid erosion 1 to 10 mm yr this erosion must have been less than or equal to uplift relative to sea level the lack of granulite facies rocks in the eastern himalaya and tibetan plateau suggests to us that very rapid uplift must have been distributed in brief pulses over different parts of the mountain belt these datamore are incompatible with tectonic models in which the himalaya and tibetan plateau are uplifted either uniformly over the past 40 m y or mostly within the past 2 to 5 m y less

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

cretaceous tertiary geology of the gangdese arc in the linzhou area southern tibet abstract knowledge of the cretaceous tertiary history of upper crustal shortening and magmatism in tibet is fundamental to placing constraints on when and how the tibetan plateau formed in the lhasa terrane of southern tibet the widely exposed angular unconformity beneath uppermost cretaceous lower tertiary volcanic bearing strata of the linzizong formation provides an excellent geologic and time marker to distinguish between deformation that occurred before vs during the indo asian collision in the linzhou area located 30 km north of the city of lhasa a 3 km thick section of the linzizong formation lies unconformably on cretaceous and older rocks that were shortened by both northward and southward verging structures during the late cretaceous the linzizong formation dips northward in the footwall of a north dipping thrust system that involves triassic jurassic strata and a granite intrusion in the hanging wall u pb zircon geochronologic studies show that the linzizong formation ranges in age from 69 ma to at least 47 ma and that the hanging wall granite intrusion crystallized at 52 ma coeval with dike emplacement into footwall cretaceous strata 40ar 39ar thermochronologic studies suggest slow cooling of the granite between 49 and 42 ma followed by an episode of accelerated cooling to upper crustal levels beginning at 42 ma the onset of rapid cooling was coeval with the cessation of voluminous arc magmatism in southern tibet and is interpreted be a consequence of either 1 tertiary thrusting in this region or 2 regional rock uplift and erosion following removal of overthickened gangdese arc lower crust and upper mantle or break off of the neo tethyan oceanic slab

onset of mid crustal extensional flow in southern tibet evidence from u pb zircon ages new ion microprobe u pb dates from zircon in deformed orthogneiss and migmatite and an undeformed granite in mabja dome are the first to constrain the timing of peak metamorphism and onset and duration of mid crustal ductile extension in southern tibet at 35 0 0 8 ma and 12 19 million years the structural metamorphic and intrusive his tories in mid crustal rocks exposed in these north himalayan gneiss domes are similar to those recorded in the greater himalayan sequence suggesting that middle crust was continuous from beneath southern tibet southward to the high himalaya strain compatibility indicates that 35 ma ductile extension in mid crustal rocks of southern tibet was accommodated to the south at shallow crustal levels via normal slip along the southern tibetan detachment system the oldest age estimate for slip along this normal fault zone and extrusion of its footwall if gravitational collapse is an additional important process driving extension then southernmost tibet may have been at or near maximum elevation by late eocene early oligocene time

tectonics and structural zonation of southern tibet china the yarlung zangbo suture zone is composed of several thrust sheets with distinct stratigraphies states of deformation and metamorphic grades successive deformations have been produced by persistent north south shortening since obduction of the ophiolites by late cretaceous palaeocene times a possible decoupling of the crust may be associated with the intracontinental thrust system which has formed in the subducted indian plate this article emphasizes the role of shear deformations in the building of a collision belt

rapid early miocene acceleration of uplift in the gangdese belt xizang southern tibet and its bearing on accommodation mechanisms of the india asia collision abstract five samples from a biotite hornblende granodiorite phase of the 42 5 ma quxu pluton gangdese batholith southern tibet have been collected at 250 m vertical intervals biotite from these rocks yields monotonically decreasing40ar 39ar isochron ages with decreasing elevation of 26 8 0 2 23 3 0 5 19 7 0 3 18 4 0 4 and17 8 0 1ma tc 335 c coexisting k feldspars have virtually identical minimum apparent40ar 39ar ages of 17 0 0 4ma tc 285 c these data indicate parts of southern tibet experienced a pulse of uplift in the early miocene with the rate of uplift rising from 0 07 to 4 4 mm year in the interval 20 to 17 ma an apatite fission track age of 9 9 0 9ma from this locality constrains the average uplift rate at this site to about 0 81 mm year between 17 and 9 9 ma and 0 30 mm year from 9 9 ma to present k feldspar from the dagze granite 30 km to the east near lhasa yields a minimum apparent40ar 39ar age of 35 9 0 9ma tc 227 c which indicates an average uplift rate there of 0 21 mm year since then the marked pulse of uplift of the quxu granodiorite and the difference in uplift history between the dagze and quxu plutons suggests southern tibet has experienced discrete pulses of uplift variable in both space and time these data are not consistent with models which require a large proportion of uplift of the tibetan plateau to have occurred in the last 2 ma the data support the suggestion that convergence between india and asia was largely accommodated by tectonic escape during the opening of the south china sea 32 to 17 ma ago and permit distributed shortening as a mechanism for crustal thickening and uplift of this part of the tibetan plateau subsequent to 20 ma

controls on cenozoic exhumation of the tethyan himalaya from fission track thermochronology and detrital zircon u pb geochronology in the gyirong basin area southern tibet the gyirong basin southern tibet contains the record of miocene pliocene exhumation drainage development and sedimentation along the northern flank of the himalaya the tectonic controls on basin formation and their potential link to the south tibetan detachment system stds are not well understood we use detrital zircon zft and apatite aft fission track analysis together with detrital zircon u pb dating to decipher the provenance of gyirong basin sediments and the exhumation history of the source areas results are presented for nine detrital samples of gyirong basin sediments aft zft and u pb two modern river sediment samples zft and aft and six bedrock samples zft from transect across the gyirong fault bounding the basin to the east the combination of detrital zircon u pb and fission track data demonstrates that the gyirong basin sediments were sourced locally from the tethyan sedimentary sequence this provenance pattern indicates that deposition was controlled by the gyirong fault active since 10 ma whose vertical throw was probably 5000 m rather than being controlled by normal faults associated with the stds the detrital thermochronology data contain two prominent age groups at 37 41 and 15 18 ma suggesting rapid exhumation at these times a 15 18 ma phase of rapid exhumation has been recorded widely in both southern tibet and the himalaya a possible interpretation for such a major regional exhumation event might be detachment of the subducting indian plate slab during the middle miocene inducing dynamic uplift of the indian plate overriding its own slab

palaeodrainage evolution of the large rivers of east asia and himalayan tibet tectonics understanding the tectonics that gave rise to the formation of tibet is critical to our understanding of crustal deformation processes the unusual geomorphology of the drainage basins of east asia s major rivers has been proposed to be the result of either 1 distortion and attenuation of antecedent drainages as india indents into asia which can therefore be used as passive strain markers of horizontal shear or 2 due to fragmentation by river captures and flow reversals of an originally continental scale drainage in which the major east asian rivers once flowed into the palaeo red river if the latter hypothesis is correct then it has been proposed that dating the drainage fragmentation constrains the timing of uplift of tibet a number of sedimentary provenance studies have been undertaken in order to determine whether the palaeo red river was once a river of continental proportions into which the upper reaches of the yangtze salween mekong irrawaddy and yarlung drained we have assessed the evidence that the yarlung originally flowed into the palaeo red river and then sequentially into the irrawaddy and brahmaputra connecting to the latter first via the lohit and then the siang for this river system we have integrated our new data from the paleogene recent irrawaddy drainage basin detrital zircon u pb with hf and fission track rutile u pb mica ar ar bulk rock sr nd and petrography with previously published data to produce a palaeodrainage model that is consistent with all datasets in our model the yarlung never flowed into the irrawaddy drainage during the paleogene the yarlung suture zone was an internally drained basin and from neogene times onwards the yarlung drained into the brahmaputra in the bengal basin the central myanmar basin through which the irrawaddy river flows today received predominantly locally derived detritus until the middle eocene the irrawaddy initiated as a through going river draining the mogok metamorphic belt and bomi chayu granites to the north sometime in the late eocene to early oligocene and the river was dominated by a stable mmb dominated drainage throughout the neogene to present day existing evidence does not support any connection between the yarlung and the red river in the past but there is a paucity of suitable palaeo red river deposits with which to make a robust comparison we argue that this limitation also precludes a robust assessment of a palaeo connection between the yangtze salween mekong and the red river it is difficult to unequivocally interpret the recorded provenance changes as the result of specific drainage reorganisations we highlight the palaeo red river deposits of the hanoi basin as a potential location for future research focus in view of the near complete cenozoic record of palaeo red river deposits at this location a majority of previous studies consider that if a major continental scale drainage ever existed at all it fragmented early in the cenozoic such a viewpoint would agree with the growing body of evidence from palaeoaltitude studies that large parts of se tibet were uplifted by this period this then leads towards the intriguing question as to the mechanisms which caused the major period of river incision in the miocene in this region

reconstructing carbonate alteration histories in orogenic sedimentary basins xigaze forearc southern tibet abstract carbonate clumped isotope thermometry t 47 and the oxygen stable isotope record 18 o provide critical constraints on the temperature of carbonate mineral formation and isotopic composition of ancient waters used in reconstructions of past climate tectonics and ecological and environmental change the robust use of these proxies requires that carbonate minerals in the rock record retain primary isotopic compositions through often complex post depositional thermal histories new carbonate clumped isotope thermometry and sedimentology data are paired with existing thermochronology and isotope exchange reaction modeling to provide unique constraints on the alteration history of the carbon and oxygen isotopes in tethyan marine carbonates jialazi fm tso jianding group of the xigaze forearc southern tibet within the framework of the textural and isotopic data the effects of early and late burial alteration on 47 and 18 o values of marine rocks that were buried to 4 km above sea level are modeled the alteration products are considered within two well studied mechanistic frameworks water rock recrystallization and solid state reordering of 13 c 18 o bonds at high temperature the proposed alteration model is as follows 1 shallow early diagenesis with water of similar composition and temperature to formation waters infills primary porespace with microspar 2 high temperature rock buffered water rock exchange and partial solid state reordering at 3 4 km depth and 3 near surface low temperature water rock exchange on the exhumation pathway the 47 heterogeneity within individual samples 0 456 to 0 721 requires significant differential alteration on the retrograde path high to low temperature low water rock ratio in which primary compositions and compositions inherited during early burial diagenesis are overprinted with modern meteoric values a low t 47 value is often assumed to record the primary temperature of the depositional environment however samples yielding low t 47 values in the jialazi fm have undergone extensive high temperature water rock alteration and therefore require a mechanism for increasing 13 c 18 o bond ordering on the retrograde pathway while preserving many primary biogenic structures and micritic textures this study highlights the potential significance of late stage low temperature alteration of carbonates derived from orogenic sedimentary basins for studies of terrestrial paleoenvironments and tectonics this study also provides an example framework for combining observational and analytical data to reconstruct a carbonate alteration history of a buried sedimentary package

spatial and temporal trends in exhumation of the eastern himalaya and syntaxis as determined from a multitechnique detrital thermochronological study of the bengal fan the bengal fan provides a neogene record of eastern and central himalaya exhumation we provide the first detrital thermochronological study apatite and rutile u pb mica ar ar zircon fission track of sediment samples collected during international ocean discovery program iodp expedition 354 to the mid bengal fan our data from rutile and zircon fission track thermochronometry show a shift in lag times over the interval 5 59 3 47 ma the oldest sample with a lag time of 6 m y has a depositional age of 5 59 4 50 ma and the zircon and rutile populations then show a static peak until 12 ma this interval from 5 59 4 50 ma to 12 ma is most easily interpreted as recording passive erosion of the greater himalaya however single grains with lag times of 4 m y but with high analytical uncertainty are recorded over this interval for sediments older than 10 ma these grains were derived from the greater himalaya which was exhuming rapidly until ca 14 ma in sediments younger than 10 ma these grains could represent slower yet still rapid exhumation of the syntaxial antiform to the south of the massif lag times 1 m y are again recorded from 14 5 ma to the base of the studied section at 17 ma reflecting a period of greater himalayan rapid exhumation mica 40ar 39ar and apatite u pb data are not sensitive to syntaxial exhumation we ascribe this to the paucity of white mica in syntaxial lithologies and to high levels of common pb resulting in u pb ages associated with unacceptably high uncertainties respectively

constraints on the exhumation and erosion of the high himalayan slab nw india from foreland basin deposits petrography sr nd isotope compositions and single grain laser 40ar 39ar ages of detrital white mica from early middle miocene molasse of the dharamsala and lower siwalik formations of northern india dated by magnetostratigraphy determine the sediment sources their metamorphic grade and exhumation rates in the himalayan palaeo hinterland deposition of the lower dharamsala member 21 17 ma occurred during the period of rapid isothermal decompression and crustal anatexis 24 18 ma of the metamorphic core this episode of decompression is thought to be coeval with thrusting on the main central thrust and normal faulting on the south tibetan detachment system the sediment composition and detrital mica ages indicate erosion from the rapidly exhumed metamorphic slab of the high himalayan crystalline series deposition of the upper dharamsala member 17 13 ma and basal siwalik group 13 12 5 ma spanned the period in which thrusting transferred south from the main central thrust the sediment composition and detrital mica ages contrast strongly with those of the lower dharamsala indicating erosion from sedimentary and low grade rocks the isotopic composition indicates that these rocks were part of the high himalayan series unaffected by tertiary metamorphism i e from upper structural levels of the high himalayan slab this suggests that a major reorganisation of the orogenic wedge occurred at 17 ma involving forward propagation of the mct and cessation of rapid exhumation of the metamorphic slab