structure of the north indian continental margin in the ladakh zanskar himalayas implications for the timing of obduction of the spontang ophiolite india asia collision and deformation events in the himalaya the collision of india and asia can be defined as a process that started with the closing of the tethyan ocean that during mesozoic and early tertiary times separated the two continental plates following initial contact of indian and asian continental crust the indian plate continued its northward drift into asia a process which continues to this day in the ladakh zanskar himalaya the youngest marine sediments both in the indus suture zone and along the northern continental margin of india are lowermost eocene nummulitic limestones dated at similar 54 ma along the north indian shelf margin southwest facing folded palaeocene lower eocene shallow marine limestones unconformably overlie highly deformed mesozoic shelf carbonates and allochthonous upper cretaceous shales indicating an initial deformation event during the latest cretaceous early palaeocene corresponding with the timing of obduction of the spontang ophiolite onto the indian margin it is suggested here that all the ophiolites from oman along western pakistan bela muslim bagh zhob and waziristan to the spontang and amlang la ophiolites in the himalaya were obducted during the late cretaceous and earliest palaeocene prior to the closing of tethys

provenance of xigaze fore arc basin clastic rocks cretaceous south tibet the xigaze group of south tibet was deposited in a fore arc basin produced by northward subduction of the neotethys under eurasia during middle cretaceous time clasts of andesite from the gangdese magmatic belt to the north and limestone clasts from a shelf rimming the basin to the north are the near exclusive detrital components in the xigaze group in the eastern part of the studied region and in older albian deposits in the west younger cenomanian coniacian deposits from western and central parts of the xigaze group show increasing amounts of tonalite pebbles in conglomerates and quartz sand these trends reflect differential arc uplift and erosional dissection with higher rates in the west some arenites show evidence for sediment contribution from an additional source probably from an orogen to the north of the lhasa block the presence of this orogen indicates that crustal thickening of tibet is not restricted to cenozoic times the middle cretaceous gangdese belt apparently lacked acid volcanic rocks this suggests a low degree of crustal contamination and or differentiation of primitive magma probably because the crust beneath the arc area was thin a thin crust is in accordance with a low elevation of the arc deduced from continuous carbonate production on and marine excursions beyond the arc magma eruption rates of the arc estimated from the xigaze basin sediment volume were probably low 7 5 km 3 km arc segment per million years but within the range of modern arcs the varying sediment compositions of the xigaze group may be typical of fore arc basin deposits in front of continental magmatic arcs the lack of a correlation of trends in sediment composition with fining upward sedimentary supercycles in the xigaze group is taken as evidence in favor of a eustatic cycle control

petrology of the mafic rocks of the xigaze ophiolite tibet the xigaze ophiolite yarlung zangbo suture zone southern tibet china shows an unusual crustal sequence characterized by a lack of large masses of cumulate gabbros by dolerites intrusive throughout the whole ophiolite sequence and by the injection of dolerites in already serpentinized peridotites the abyssal tholeiitic nature of all the mafic rocks indicates that they have been generated at an oceanic ridge all the geological arguments and petrological and textural data on the mafic rocks point to very low heat production and large heat losses through widespread intensive sea water circulation for the spreading centre in which they have been formed in good agreement with a slow spreading ridge origin

analytical expressions for transient specific yield and shallow water table drainage 1 new closed form expressions are introduced to capture the dependence of specific yield on time and depth to water table the expressions allow the user to convert observations of water table fluctuations to volumes of water released from storage in a shallow water table aquifer whereas a linear relationship between water table fluctuations and released volumes holds for a deep water table aquifer this relationship is nonlinear for shallow water table aquifers the dependence of specific yield on time stems from the slow drainage of soil water from pores above the water table the new expressions allow determination of transient specific yield and time to drain the soil water profile for a given water table fluctuation if the time step in a numerical groundwater model is longer than the time for limiting specific yield then a constant time independent specific yield can be justifiably adopted the new expressions are easy to use and require knowledge of soil hydraulic properties which are readily available from soil water retention data and surveys

metamorphic history and geodynamic significance of high grade metabasites from the ophiolitic melange beneath the yarlung zangbo ophiolites xigaze area tibet abstract blocks of highly foliated amphibolite are locally embedded within a serpentinite melange underlying the yarlung zangbo ophiolites in the xigaze area of southern tibet the ophiolites are remnants of an early cretaceous back arc basin within the permo cretaceous tethys ocean which are exposed along in the yarlung zangbo suture zone yzsz these amphibolites are interpreted as fragments of a dismembered dynamothermal sole three types of amphibolite are present 1 common amphibolite with assemblages of hbl pl ep ap ttn 2 clinopyroxene bearing amphibolite with hbl pl cpx ep ttn qtz ap and 3 garnet clinopyroxene bearing amphibolite characterized by the assemblages hbl cpx grt pl rt and grt hbl pl corona assemblage in all three types plagioclase is pseudomorphed by late albite prehnite retrograde cataclastic veins containing assemblages of prh ab ep chl are also present p t estimates indicate that the amphibolites reached peak metamorphic conditions of 13 15 kbar and 750 875 c partial replacement of pyrope rich up to 35 mole garnet by al tschermakite al2o3 up to 21 wt reflects a high pressure 18 kbar 600 c metamorphic event followed by rapid exhumation soon after exhumation the amphibolites were intruded by very fine grained diabase dykes that were then hydrothermally altered the field relationships and metamorphic history of the amphibolites indicate formation during inception of subduction within a back arc basin prior to obduction of the ophiolites onto the indian passive margin

geochemical investigation of early cretaceous igneous rocks along an east west traverse throughout the central lhasa terrane tibet article i nfo bulk rock major and trace element sr nd isotope zircon u pb and hf isotope data are reported for early cretaceous igneous rocks sampled along an east west traverse through the central lhasa subterrane e80 00 e89 in southern tibet these results offer new insights into the pre cenozoic geology and tectonomagmatic evolution of the lhasa terrane and provide boundary conditions for the origin and evolution of the tibetan plateau our new data together with data from the literature indicate that the early cretaceous magmatism in the central lhasa subterrane lasted for a long period 143 102 ma with a magmatic flare up at 110 ma the latter is accompanied by mafic magmatism in this terrane the silicic rocks are metaluminous to peraluminous and enrichedinrb th and u and depleted inba nb ta sr p and ti withvarying initial 87

the xigaze ophiolite tibet a peculiar oceanic lithosphere the xigaze ophiolite which outcrops along the yarlung zangbo river southern tibet locally displays a complete ophiolitic sequence from marine sedimentary cover over basaltic volcanics to the north to fresh cr diopside rich harzburgites to the south in contrast with other ophiolites the mafic part of the sequence is particularly thin it is almost devoid of cumulate gabbros consisting of a diabase sill complex covered by lava flows or pillow lavas the ultramafic unit is poor in residual harzburgites and dunites and consists dominantly of fresh cr diopside rich harzburgites the origin of this peculiar ophiolite of oceanic crust is discussed

a delineation of regional hydraulic conductivity based on water table fluctuation summary the estimation of regional hydraulic conductivity is essential for the sustainable development and efficient management of groundwater this paper presents a new physical method of estimating hydraulic conductivity based on the water level fluctuation model developed by park and parker park e parker j c 2008 a simple model for water table fluctuations in response to precipitation j hydrol 356 344 349 the developed procedure was validated by comparison with the hypothetical numerical model for a variety of hydraulic conductivities aquifer inclinations and the heterogeneities the model was found to be highly reliable when using water level data far from the hydraulic boundaries the results show that the monitoring data obtained over a range of relative distances of an observation location x to the flow domain w x w 0 2 avoided the boundary effects a correction factor was also proposed based on the deviation analysis of the calibrated k and f n ratio of recharge to precipitation divided by porosity the results from the basal inclinations and the heterogeneities cases suggest that the estimated hydraulic conductivity is representing regional property rather than the local one to confirm the validity of the proposed procedure it was applied to the coastal aquifer of jeju island korea the calibrated hydraulic conductivities were consistent with the results from previous studies these results show that the proposed method is an effective and economical means to estimate hydraulic conductivity of large supporting volumes of subsurface aquifers with the convenient use of recharge and groundwater level data

observations of near bed velocity profiles and seabed roughness in tidal currents flowing over sandy gravels measurements of near bed velocity profiles and bottom roughness in tidal currents flowing over sandy gravels in the west solent england have shown that although the seabed in the immediate vicinity of the study area was generally level and devoid of regular bed forms the observed velocity profiles were consistently curved concave downwards this behaviour may be attributed to the presence of an internal boundary layer which was due to form drag on irregular topography upstream of the measurement position loosely packed gravels of the type found in the west solent had a geometric mean roughness length of about 0 3 cm comparison of the observed ratio of roughness length to mean particle size with laboratory data suggests an equivalent two dimensional roughness element spacing of about three particle diameters in contrast to tidal flows over rippled sand beds the roughness lengths for sandy gravels remained relatively constant throughout the tidal cycle furthermore these measurements provided no systematic evidence of the effects of flow acceleration on profilederived estimates of the friction velocity u and the roughness length z0

applications of sediment pick up function two applications are presented of a recently proposed empirical sediment pick up function the first application deals with bed load transport which is computed as the product of the pick up rate and the saltation length comparison with measured bed load transport rates shows encouraging results the second application deals with suspended load transport the classical problem of sediment concentration profiles developing in a steady uniform flow which is initially free of sediment is studied

miocene adakitic intrusions in the zhongba terrane implications for the origin and geochemical variations of post collisional adakitic rocks in southern tibet abstract post collisional adakitic rocks 26 9 ma have been recognized in southern tibet for almost 20 years however their origins and geodynamics remain highly controversial this paper reports geochronological and geochemical data for the lasa intrusion exposed in the zhongba terrane and proposes implications for the origins of the miocene adakitic rocks in southern tibet the lasa intrusion consists of granite and granodiorite porphyries with zircon u pb ages of ca 16 ma the porphyries have high sio2 65 14 67 61 wt al2o3 15 13 16 10 wt k2o 2 87 3 54 wt and sr 727 1046 ppm contents and low y 9 13 ppm and yb 0 82 1 19 ppm contents they are enriched in light rare earth elements lrees and depleted in heavy rare earth elements hrees the porphyries display relatively low mgo contents 1 39 1 98 wt high sr y 65 98 and la yb n 18 8 25 5 ratios initial 87sr 86sr i ratios of 0 70912 0 70805 end t values of 6 4 to 7 9 206pb 204pb t ratios of 18 7108 18 7665 207pb 204pb t ratios of 15 6837 15 7118 and 208pb 204pb t ratios of 39 1600 39 3034 these signatures indicate that the lasa porphyries are adakitic rocks derived from the partial melting of a thickened lower crust the miocene adakitic rocks in the zhongba terrane have similar sr nd isotopic compositions to those of western lhasa and the himalaya terrane which differ from coetaneous adakitic rocks of the eastern lhasa terrane in terms of sr nd isotopic compositions based on the spatial distributions and isotopic features of the post collisional adakitic rocks in southern tibet we suggest that all of the adakitic rocks were derived from the partial melting of thickened lower crust and that their geochemical variations are mainly caused by the heterogeneity of lower crust

the xigaze forearc basin evolution and facies architecture cretaceous tibet abstract the mid cretaceous to eocene flysch deposits of the xigaze forearc basin in southern tibet were investigated in a 120 km segment along the indus yarlung suture zone the basin evolved south of the magmatic arc gangdise belt of the lhasa block on top of trapped oceanic or transitional crust remnants of shelf carbonates are preserved along the northern edge of the basin the mapped segment was shortened by about 65 metamorphism reached low grade conditions along the northern margin of the basin the flysch sequence reaches a thickness of at least 5 km and consists to a large degree of volcaniclastic andesitic to dacitic material shed from the magmatic arc gangdise belt particularly in the western part of the study area plutonic and sedimentary rocks from deep erosion levels and or more distal sources contributed to the basin fill rivers from the lhasa block acted as point sources and fed five major deep sea channel systems turbidity currents in the channels were directed towards the growing accretionary wedge of the subduction zone thus indicating that the basin was continuously filled up to outer ridge level and gradually shallowing the forearc flysch is subdivided into at least three megasequences which begin with wide up to several km incised 10 to 50 m coarse grained channel fills and their associated fan deposits the upper parts of the megasequences contain hemipelagic dark shales and marls deposited above the calcite compensation depth lateral channel migration channel lobe switching but also volcanic pulses generated a predominantly fining upward high frequency cyclicity after continental collision the marine sedimentation in the forearc basin was replaced by fluvial deposits of the eocene oligocene qiuwu formation which is time equivalent to the kailas and indus molasses farther west and rich in coarse gravel derived from the gangdise belt both forearc flysch and qiuwu formation were deformed simultaneously during the miocene we assume that the molasse type qiuwu formation represents the final continental facies of the forearc basin filling

late devonian oib alkaline gabbro in the yarlung zangbo suture zone remnants of the paleo tethys article i nfo the yarlung zangbo suture zone yzsz is believed to be composed of material largely derived from the destruction of the neo tethys that occurred from early mesozoic to early cenozoic we report here geochronological and petrological data obtained for newly discovered alkaline gabbro blocks embedded in a melange zone of the western yzsz single zircon u pb analyses from one representative gabbro sample by sims secondary ion mass spectrometry yielded a combined crystallization age of about 363 7 1 7 ma 1 in situ hf isotopic analyses yielded ehf t values of 2 6 to 5 5 suggesting an enriched mantle source all of the gabbro samples show typical ocean island basalt oib affinity with little or no continental crust contamination they also display strong geochemical similarities with the hawaii basalts and the xigaze seamount basalts suggestive of their intra oceanic setting these observations in combination with the early carboniferous layered gabbros reported at luobusa indicate that these rocks could represent remnants of the paleo tethys we propose that a branch ocean separating the western qiangtang terrane and the lhasa terrane from the gondwana continent might have been present during the late devonian and the early carboniferous providing new constrains on the configuration of paleo tethys in tibetan plateau during early late paleozoic

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

discovery of middle jurassic trench deposits in the bangong nujiang suture zone implications for the timing of lhasa qiangtang initial collision abstract the mesozoic trench deposits in the bangong nujiang suture zone in central tibet can provide critical information for reconstructing the paleotectonic evolution of the bangong nujiang ocean and constraining the timing of the lhasa qiangtang initial collision in this paper the gamulong formation which is well exposed in the dong co area western bangong nujiang suture central tibet was studied the sedimentological analysis shows that the gamulong formation was deposited in a submarine fan environment the youngest detrital zircon age and fossils from limestone pebbles within the gamulong formation combine to determine a middle jurassic age the pebbles in the conglomerate are dominated by limestones and sandstones which is consistent with the sandstone petrology showing limestone and sandstone fragments as the main framework components detrital zircon u pb ages from the sandstones yield age populations of 164 178 ma 200 500 ma 700 1000 ma 1700 2100 ma and 2500 ma the corresponding zircon ehf t values are distributed between 23 5 and 11 1 these zircon features are similar to those from the qiangtang terrane accordingly the provenance results imply that the gamulong formation was recycled from the accretionary complex mugagangri group melange in the bangong nujiang suture zone and the forearc basin deposits lower middle jurassic sewa shaqiaomu jiebuqu formations in the southern qiangtang subterrane the gamulong formation is characterized by sedimentary recycling in the trench likely in response to a tectonic event possibly the continental collision together with the previous studies this work on the occurrence of the gamulong formation indicates that the lhasa qiangtang initial collision could have occurred in middle jurassic time 166 ma

the yarlung zangbo paleo ophiolite southern tibet implications for the dynamic evolution of the yarlung zangbo suture zone the yarlung zangbo suture zone a major geological structure in tibet is well known as the locus of tectonic emplacement of the tethyan ophiolites current models propose that most of the east tethyan oceanic lithosphere was subducted within a single subduction zone active during the middle or late cretaceous which was completed during the paleogene collision between india and asia the early cretaceous sedimentary giabulin formation in southern tibet includes conglomeratic members that contain ultramafic and mafic plutonic pebbles as well as radiolarian chert clasts that record the erosion of oceanic lithosphere involved in a subduction event which occurred earlier than previously believed geochemical analyses mineral chemistry stratigraphic chronology and sedimentary analysis including source provenance suggest that the pebbly conglomerate was formed through erosion of an unknown ophiolitic source that was geochemically distinguishable from the xigaze ophiolites within the yarlung zangbo suture zone southern tibet we infer the existence of an older ophiolitic source termed the yarlung zangbo paleo ophiolite that was dismembered and eroded during an earlier subduction stage not taken into account in current models q 2000 elsevier science ltd all rights reserved

comparisons between acoustic measurements and predictions of the bedload transport of marine gravels continuous detailed records of marine gravel transport have been obtained acoustically and compared with bedload transport rates qb predicted by five bedload transport equations using measurements of the near bed turbulent current flow when mean flow data are used in these equations total qb estimates are similar to those measured however when instantaneous flow data are used total qb is over estimated by approximately one order of magnitude based on the acoustic measurements an empirical equation has been obtained that gives accurate estimates of total qb over a tidal cycle and simulates well the intermittent characteristics of marine bedload transport

modeling the development of wave cut shore platforms abstract a mathematical model was used to investigate the development of wave cut shore platforms with constant sea level the model considered the effects of deep water wave height spectra period and wavelength breaker height and depth breaker type the width and bottom roughness of the surf zone the gradient of the submarine slope an erosional threshold related to the strength of the rocks the number of hours each year in which the water level is at each intertidal elevation and the amount and persistence of the debris at the cliff foot intertidal erosion rates were calculated according to the force of the surf reaching the shoreline whereas submarine erosion rates were determined using a depth decay variable each of the two hundred and fifty model runs consisted of 300 000 iterations platform width increased and gradient progressively decreased through model runs and states of static equilibrium were attained by the end of one hundred and nine runs when the wave induced force at each point in the intertidal zone had became too weak to continue eroding the rock runs with mesotidal spring tidal range 3 m generally produced subhorizontal platforms with cliff platform junctions at the mean low water neap tidal level a greater variety of profiles was produced in runs with macrotidal spring tidal ranges 9 m although almost 30 of these runs produced gently sloping platforms with average gradients of only 0 3 about two thirds of the simulated profiles had average gradients ranging from about 1 to 3 simulated platform width increased with tidal range and decreased with the rate of submarine erosion rock resistance the roughness or irregularity of the platform surface the amount and persistence of the cliff foot debris and wave period because higher waves break in deeper water than lower waves there was no consistent relationship between simulated platform width and wave height

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

precise radiolarian age constraints on the timing of ophiolite generation and sedimentation in the dazhuqu terrane yarlung tsangpo suture zone tibet well preserved abundant radiolarians provide high precision biostratigraphic age constraints on the timing of the eruption of ophiolitic basalts exposed along the yarlung tsangpo suture zone in southern tibet dazhuqu terrane ophiolites were generated in an intra oceanic supra subduction zone setting within a relatively short

the birth of the xigaze forearc basin in southern tibet abstract the stratigraphic succession of a forearc basin provides crucial information on the history of a convergent plate margin in particular it helps to establish the origin of the underlying ophiolites and to unravel the earliest evolutionary stage of arc trench systems which remain poorly understood the xigaze forearc basin in southern tibet is one of the best examples of a fossil forearc basin this study illustrates detailed stratigraphic and high precision sims u pb zircon geochronological and hf isotopic data from the chongdui formation representing the very base of the xigaze forearc basin succession and reconstructs when and how the basin was formed the chongdui formation includes tuffaceous chert and siliceous mudrocks deposited directly on top of pillow basalts of the xigaze ophiolite and conformably overlain by volcaniclastic turbidites tuff layers are interbedded throughout the unit and their u pb zircon ages range from 119 to 113 ma in the lower member and from 113 to 110 ma in the upper member broadly consistent with the established radiolarian biostratigraphy u pb ages and hf isotope signatures of zircons contained in both tuff layers and turbiditic sandstones indicate clear affinity with magmatic rocks of the lhasa terrane direct depositional and chronostratigraphic relationship with the underlying oceanic crust dated between 131 and 124 ma proves that the xigaze ophiolite is the basement of the xigaze forearc basin after an initial prolonged stage of starved siliceous sedimentation influx of terrigenous detritus began at 113 110 ma reflecting the onset of topographic growth and erosion of the lhasa terrane in response to intense magmatic activity formation of the ophiolitic basement during the early stage of subduction and the subsequent topographic growth of the arc source induced by subduction related magmatism are thus two critical factors for the birth of the xigaze forearc basin similar stratigraphies were identified in the great valley and luzon central valley forearc basins suggesting that the initial geodynamic evolution of the xigaze forearc basin may be common to many other forearc basins worldwide

an assessment and calibration of formulations for bagnold s bedload equation abstract bagnold s bedload equation in the form q u ucr 3 has frequently been employed to predict sediment mass transport rates in tidal flows and to assist with the interpretation of ancient tidal deposits however physical considerations suggest that the form q u2 u2cr u is more correct this latter formulation is therefore calibrated with experimental data and the coefficient of proportionality is shown to be strongly dependent on the sediment grain size

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

mesozoic and cenozoic volcanic rocks from central and southern tibet 39ar 40ar dating petrological characteristics and geodynamical significance field relations radiometric ages and mineralogical and chemical characteristics of the orogenic volcanic rocks from the lhasa block tibet are studied volcanic activity started before the deposition of the albian aptian sedimentary units takena formation and xigaze group volcanism is volumetrically limited in the northern part of the lhasa block basalts andesites and dacites are present while ignimbrites are scarce radiometric ages of these rocks range between 110 and 80 ma conversely in the southern part of the lhasa block volcanism is largely developed gangdese belt there paroxysmal activity occurred later than in the north between 60 and 50 ma lingzizong formation this volcanism is mainly represented by andesites and ignimbrites a younger episode at about 15 10 ma took place in this southern region the mineralogical and geochemical characteristics indicate that as a whole the volcanic rocks from the lhasa block belong to a calc alkaline continental margin series enriched in k20 and other incompatible elements subordinate potassic alkaline lavas are present in the northern margin of the gangdese belt some basalts exhibit a tholeiitic affinity daquin area southern region the geodynamic significance of these cenozoic volcanic rocks lies in the relative motion of india and the lhasa block after accretion of the latter to the northern qantang block a geodynamic model based on changes of the type of subduction of the indian oceanic crust beneath the lhasa block for the period 120 40 ma is proposed to account for the abundance age and magmatic affinity of the volcanic products

in situ acoustic measurements of marine gravel threshold and transport measurements of the nearbed turbulent current flow and the bedload transport of marine gravel have been made over three tidal cycles the turbulence in the bottom boundary layer was measured using two electromagnetic current meters and the gravel transport was measured using a passive acoustic system which monitored the interparticle collision noise of locally mobile material visual estimates of bedload were also obtained with an underwater tv camera the acoustic technique unlike a conventional bedload sampler has allowed estimates of transport to be obtained with a temporal resolution comparable with the turbulence data collected this has enabled a detailed comparison to be made between the turbulent flow and the sediment response to the instantaneous flow conditions the results of the study show that of the turbulent bursting events which contribute towards the reynolds stress only the sweeps and outward interactions play a significant role in the transport of coarse sedimentary material the measurements show that it is the instantaneous increases in the horizontal turbulent velocity fluctuations that generate excess shear stresses which drive the transport process

early tertiary deformation of the zhongba gyangze thrust in central southern tibet abstract as the boundary thrust between india and asia in southern tibet the zhongba gyangze thrust zgt emplaced the yarlung zangbo suture zone yzsz units in the hanging wall southward onto tethyan himalaya sequences ths of the northern indian continental margin in the footwall detailed field investigation electron backscatter diffraction ebsd analysis detrital zircon u pb geochronology and 40 ar 39 ar thermochronology were conducted to understand the evolution of the zgt in sangsang area central southern tibet the shear zone of zgt is located within the sedimentary matrix melange of yzsz that is mainly composed of matrix of blueschist with meta pelagic hemipelagic siliceous and siliciclastic rocks and blocks of basalt limestone and sandstone penetrative f1 foliation and kink band structure were recorded within the matrix both on outcrop and under microscope strong lattice preferred orientation lpo fabric initiated by the low temperature 350 450 c 010 001 slip system was detected by ebsd in the sodic amphiboles of the blueschist the 40 ar 39 ar ages of the phengites from blueschist and sericites from the phyllite in the shear zone indicate that the activity of zgt occurred between 71 and 60 ma in the ths a newly documented younger unit preserving detrital zircons from the southern asian margin lies above the triassic cretaceous sequences that carries only detrital zircons from the indian continent this unit is dated to be 61 ma by the detrital zircon ages similar to the sangdanlin formation and representing the yarlung zangbo foreland basin yzfb system the zgt had probably been active due to the initial india asia collision and acted as the frontal thrust controlling the development of yzfb

lead isotopic study of the xigaze ophiolite tibet the problem of the relationship between magmatites gabbros dolerites lavas and tectonites harzburgites abstract the pb isotopic study of the xigaze ophiolite tibet shows that the tectonic harzburgites and magmatic gabbros dolerites lavas units bear different isotopic signatures the harzburgitic rocks show much higher207pb 204pb values than the samples from the magmatic unit analyses of primary magmatic clinopyroxenes separated from the harzburgites confirmed that this difference is real and not due to secondary alteration processes u pb analysis allowed determination of the crystallization age of the magmatites at 120 10 m y this age is compatible with paleontological age determinations and thus demonstrates that u pb behaved more or less as a closed system since formation of the ophiolite if an exchange with seawater has occurred it must necessarily have happened shortly after crystallization of the xigaze oceanic crust dolerite dykes crossing the harzburgites show similar pb isotopic composition as the magmatites indicating that they were emplaced when the ultramafic rocks were already brittle however the time difference of the emplacement was probably not larger than 280 m y this makes it evident that two sources with different isotopic signatures one similar to normal morb the other with much higher207pb 204pb ratios were active in the immediate neighborhood one possible environment which can account for this association is a propagating ridge

dynamics of a river channel confluence with discordant beds flow turbulence bed load sediment transport and bed morphology 1 river channel confluences play a major role in the dynamics of all fluvial systems and yet our understanding of bed load routing at these sites is very sparse the dynamics of confluences are a function of the momentum ratio between the combining flows and the three dimensional geometry of the junction recent experiments have shown that discordance in bed height between the confluent rivers increases turbulence intensity and enhances upwelling of flow within the confluence however the significance of these flow characteristics on sediment transport is still unknown to examine the relations between flow and sediment transport we have measured near bed flow turbulence bed load transport rates and changes in bed morphology for eight different flow conditions at a sand bed discordant confluence detailed analysis of the near bed flow patterns reveals that within the shear layer low mean flow velocities are combined with the highest values of reynolds shear stresses and that turbulence generation is associated with intense upward movements of flow high sediment transport rates are found at the edges of the shear layer region where horizontal vertical cross stresses uw are high these patterns match changes in bed morphology where erosion occurs along the shear layer the relation between the shear layer and sediment transport confirms the role of bed discordance on the dynamics of the confluence migration of the shear layer into the confluence as a result of a change in momentum ratio modifies local near bed flow characteristics sediment transport rates and the spatial distribution of deposition and erosion zones

latest cretaceous himalayan tectonics obduction collision or deccan related uplift abstract diverging interpretations and incompatible scenarios have been proposed for the early stages of himalayan history numerous researchers have postulated that northern india was involved in ophiolite obduction arc continent or continent continent collision during the late cretaceous or early paleocene but firm geological evidence was never produced in this article we argue against orogenic events predating the late paleocene when the neotethys ocean was still open the tethys himalayan sedimentary record testifies to anorogenic evolution primarily controlled by dynamic uplift of the passive margin prior to the massive outburst of deccan lavas and eventually followed by thermal subsidence major stratigraphic gaps in pelagic sediments suggest that such tectono magmatic episode started to affect the base of the indian plate in the campanian or possibly even in the santonian 10 to 20 ma before the climax of deccan flood basalt eruptions the abrupt increase in siliciclastic supply and accumulation rates recorded in sedimentary basins all around the indian subcontinent during the maastrichtian was followed by progradation of coastal quartzarenites along the northern indian margin in the early paleocene sandstones derived from the rejuvenated craton and uplifted inner continental margin in the south are dominantly but not exclusively quartzose felsitic volcanic rock fragments and cr spinels many of which with the same geochemical fingerprint as deccan spinels and newly found throughout the maastrichtian to danian succession resisted the combined effect of subequatorial weathering and subsequent diagenesis and testify that detritus from deccan basalts reached the indian passive margin as far as south tibet at the close of the early paleocene india drifted away from the seychelles block and thermal subsidence led to widespread carbonate deposition along the tethys himalaya this article illustrates how mega events of magmatic upwelling followed by lithospheric cooling may control passive margin sedimentation and stratigraphic patterns as occurred in northern india first in the early cretaceous and next in the latest cretaceous paleocene

the indus yarlung zangbo ophiolites from nanga parbat to namche barwa syntaxes southern tibet first synthesis of petrology geochemistry and geochronology with incidences on geodynamic reconstructions of neo tethys the purpose of this first synthesis is to summarize findings on the yarlung zangbo suture zone yzsz ophiolites and discuss still remaining problems the yzsz studied for almost 30 years and is the youngest of the sutures recognized on tibet plateau it is now acknowledged that the yzsz is a complex assemblage of sedimentary metamorphic and igneous rocks created during and shortly after the collision between india and eurasia the ages of the various lithologies span a time interval from the jurassic to the middle miocene with some permian and devonian exotic blocks from melange zone the yzsz is characterized by ophiolitic complexes and ophiolitic melange the ophiolites are of two types non dismembered and dismembered sections the non disturbed sections although tectonically reworked are observed along the segment from dazhuqu to jiding in xigaze area and spontang ophiolite the dismembered sequences are found in various locations such as nidar kiogar jungbwa saga sangsang xigugabu and luobusa the incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins the ophiolites are distributed into two groups of ages the luobusa zedang and kiogar sequences being jurassic lower cretaceous whereas all other sequences are of lower cretaceous age compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 my from their initial genesis to obduction which occurred around 70 90 my ago ophiolites differ in terms of petrological and geochemical aspects however they were all generated in suprasubduction zone and more specifically in arc few fore arc and back arc settings synthesis of more than 700 geochemical analyses show variable mixing of components from n morb type to iat cab and to oib end members the jurassic ophiolites show the maximum of arc component while the lower cretaceous ones show little to strong mixing in addition most ophiolites were created in short lived 30 my basins and generated close to the eurasiatic continental margin we propose that ladakh tibet ophiolites were generated in a suprasubduction context similar to mariana arc interarc and back arc or tonga lau system the variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system

the ophiolites of the tibetan geotraverses lhasa to golmud 1985 and lhasa to kathmandu 1986 ophiolite belts are found in tibet along the zangbo banggong and jinsha river sutures and in the anyemaqen mountains the eastern extension of the kunlun mountains where studied the zangbo suture ophiolites are characterized by apparently thin crustal sequences 3 3 5 km an abundance of sills and dykes throughout the crustal and uppermost mantle sequences common intraoceanic melanges and unconformities and an n morb petrological and geochemical composition the ophiolites probably formed within the main neo tethyan ocean and the unusual features may be due to proximity to ridge transform intersections rather than to genesis at very slow spreading ridges as the current consensus suggests the banggong suture ophiolites have a supra subduction zone petrological and geochemical composition although at least one locality in the ado massif shows morb characteristics however it is also apparent that the dykes and lavas show a regional chemical zonation from boninites and primitive island arc tholeiites in the south of the ophiolite belt through normal island arc tholeiites in the central belt to island arc tholeiites transitional to n morb in the north the ophiolites could represent fragments of a fore arc island arc back arc complex developed above a jurassic northward dipping subduction zone and emplaced in several stages during convergence of the lhasa and qiangtang terranes the ophiolites of the jinsha river suture have a n morb composition where analysed but more information is needed for a proper characterization the anyemaqen ophiolites where studied have a within plate tholeiite composition and may have originated at a passive margin it is not however certain whether true oceanic lithosphere as opposed to strongly attenuated continental lithosphere existed in this region

remnants of a cretaceous intra oceanic subduction system within the yarlung zangbo suture southern tibet extensive field investigations along the yarlung zangbo suture zone in southern tibet reveal the presence of now fragmented remnants of a south facing intra oceanic subduction system this system developed within tethys during the cretaceous the associated arc forearc ophiolite and subduction complex were emplaced onto the leading edge of india at the end of the cretaceous rapid sedimentation in oblique slip basins and disruption of water saturated sediments into melange was widespread and concomitant with ophiolite emplacement we describe the tectonic entities that developed during this previously unrecognized phase of tethys tibet evolution and present a new model for the evolution of this portion of tibet

the indus clastics forearc basin sedimentation in the ladakh himalaya india abstract the mid cretaceous to early tertiary clastic sediments of the indus group represent the forearc basin succession of the transhimalayan arc trench system in the late albian to early eocene a shallowing upward megasequence tar turbidites to nummulitic limestones and alluvial to deltaic sediments basgo and temesgam formations were deposited respectively on the southern and northern flanks of the basin clastic petrography points to provenance from the largely undissected volcanic cover of the massif with more important plutonic detritus recorded in the northern alluvial units after collision with the indian passive margin in the early eocene sedimentation in the whole basin was characterized by huge alluvial fans nurla formation and then by lacustrine deposits formed in an intermontane depression nimu formation a rapid change in petrographic and geochemical composition is recorded in post early eocene units with sharp increase in plutonic detritus and trace elements typically associated with ophiolites testifying to the uplift of both arc massif and subduction complex sources after collision with the indian passive margin the petrography of the indus basin clastics and the evolution of detrital modes compare well with volcano plutonic suites deposited in modern and ancient forearc basins the composition of sandstone units contained within the accretionary prism instead suggests provenance from an undissected magmatic arc and deposition in abyssal plain and trench settings nindam volcaniclastic turbidites reworking of arc derived sandstones took place in small basins perched on the accretionary trench slope the asian continental margin of the neotethys ocean was thus an active pacific type arc trench system until the eocene when it collided with india underwent polyphase fold thrust deformation and was finally uplifted several kilometers to form the indus suture zone of the himalayas

an intercomparison between visual and acoustic detection of seabed gravel movement abstract an intercomparison between acoustic and visual estimates of sediment transport rates is presented specifically two hydrophones were mounted at 24 cm above a seabed composed of gravel these were used to passively monitor the movement of bedload material by measuring the acoustic intensity generated due to the collision of moving gravel particles with one another and the bed simultaneous underwater tv observations were conducted by directing a low light camera fixed at 50 cm above the bed on to the area of seabed directly below the hydrophones the tv data has been used as a coarse reference scale to which the performance of the acoustic system could be compared high temporal resolution estimates of mass transport rates were derived from the visual and acoustic data the similarity of these two measurement sets has been examined the results show that the measurement of gravel movement by acoustic monitoring is feasible

the zedong terrane a late jurassic intra oceanic magmatic arc within the yarlung tsangpo suture zone southeastern tibet an overturned sequence of igneous and volcaniclastic rocks crops out along the yarlung tsangpo suture zone ytsz in southeastern tibet these rocks are remnants of an intra oceanic island arc the zedong terrane that once lay between india and asia 40ar 39ar dating and u pb ion microprobe analyses reported here reveal that this arc was active during the jurassic u pb dating of zircon from a dacite breccia from the middle portion of the zedong terrane yields an age of 161 2 3 ma 1 40ar 39ar dating of hornblende from a cross cutting andesite dyke yields the youngest age of 152 2 3 3 ma 1 40ar 39ar results from hornblende from andesite dykes and an andesite breccia from the upper portion yield a mean age of 156 0 3 ma 1 additional u pb and 40ar 39ar dating of zircon and hornblende from quartz diorite yields a mean age of 156 8 0 8 ma 1 geochronological data reported here and other published work indicate that the intra oceanic subduction system to which the zedong terrane belonged was active from at least late jurassic to early to mid cretaceous

monitoring near shore shingle transport under waves using a passive acoustic technique passive acoustic techniques have been used to measure shingle gravel sediment transport in very shallow water near the wave breaking zone on a beach the experiments were conducted at 1 1 scale in the large wave flume grose wellen kanal gwk at hannover germany the frequency spectrum induced by shingle mobilized under breaking waves can be distinguished from other ambient noise and is found to be independent of water depth and wave conditions the inverse relationship between centroid frequency and representative grain size is shown to remain valid in shallow water wave conditions individual phases of onshore and offshore transport can be identified analysis of the acoustic frequency spectrum provides insight into the mechanics of phase resolved shingle transport

measurements of turbulence in the benthic boundary layer over a gravel bed it is now generally recognized that the mathematical description of sediment dynamics will be improved through an understanding of geophysical turbulence in this study therefore the turbulence characteristics of the benthic boundary layer over coarse sediments are examined as a step towards the realization of such a description examination of the turbulence has been conducted using spectral correlation quadrant and statistical analyses and a number of features have been identified mean eddy length scales obtained using spectral and autocorrelation techniques were found to be similar the interpretation of the duration and interval between the intermittent bursting events scaled using outer flow variables was found to be dependent upon the sampling interval autocorrelation analysis revealed a high correlation between bedload transport and the instantaneous horizontal flow component rather than instantaneous kinematic stress from the standpoint of developing prognostic transport models based on physically realistic benthic flow processes it is considered that turbulence measurements will lead to such formulation

bainang terrane yarlung tsangpo suture southern tibet xizang china a record of intra neotethyan subduction accretion processes preserved on the roof of the world the bainang terrane an intra oceanic island arc subduction complex into which tethyan oceanic rocks were accreted during the cretaceous is preserved within the yarlung tsangpo suture zone of tibet the lithostratigraphic succession established from field mapping records a long history of sedimentation in different portions of the central tethyan domain from late triassic to mid cretaceous time these rocks are preserved within a south verging imbricate thrust stack of thin 1 km thick northward younging tectonic slices five lithotectonic units were mapped in the terrane and these units are assigned to two distinct tracts the northern tract which accumulated on the north side of neotethys was probably separated from its southern counterpart by a mid ocean ridge detailed radiolarian biostratigraphy is used to constrain the timing of depositional events within each tract oceanic plate stratigraphy of the northern tract records its northward travel and mid cretaceous late aptian approach towards a south facing intra oceanic subduction zone rocks in the southern tract developed closer to the indian subcontinent and experienced thermotectonic subsidence and mid jurassic basic alkaline intraplate magmatism they were probably accreted late in the cretaceous variations in structural style across the terrane indicate deformation at different depths and vertical growth of the wedge rather than lateral accretion the overall tectonostratigraphy of the terrane reflects its development in a remote intra oceanic setting

eocene neotethyan slab breakoff in southern tibet inferred from the linzizong volcanic record abstract formation of the linzizong volcanic successions in the lhasa terrane southern tibet has long been related to northward subduction of the neotethyan oceanic slab under asia here we report new 40ar 39ar age results for the volcanic rocks recovered from a large area 29 n to 32 n and 85 e to 93 e that together with literature data delineate two discrete stages of volcanism these are a widespread cretaceous stage and an intense but spatially confined paleogene stage the latter occurring only in the southern part of the lhasa terrane resulted in the linzizong volcanic successions our data furthermore suggest southward migration and intensification of the volcanism in the lhasa terrane with magmatic flare ups at ca 50 ma while the volcanic successions consist dominantly of calc alkaline rocks typical of arc lava geochemistry those formed during the flare up period show significant compositional variations from low k tholeiitic through calc alkaline to shoshonitic magma suites these observations enable us to interpret the volcanic southward migration and following flare ups as the consequences of rollback and breakoff of the subducted neotethyan slab that occurred ahead and in the early stage respectively of the india asia collision our interpretation that involves a major eocene tectonomagmatic activity and concomitant topographic uplift in southern tibet is consistent with himalayan metamorphic constraints regional sedimentary records and seismic tomography

geochemistry and geochronology of the metamorphic sole underlying the xigaze ophiolite yarlung zangbo suture zone south tibet abstract strongly foliated amphibolite clasts are found embedded within the ophiolitic melange underlying the xigaze ophiolite near bainang and angren yarlung zangbo suture zone southern tibet these high grade amphibolites are interpreted as remnants of a dismembered subophiolitic metamorphic sole that would have formed during the inception of a subduction they include garnet clinopyroxene amphibolites clinopyroxene amphibolites and common amphibolites petrographic descriptions mineral chemistry and thermobarometry of these rocks can be found in a companion paper guilmette c hebert r dupuis c wang c s li z j 2008 metamorphic history and geodynamic significance of high grade metabasites from the ophiolitic melange beneath the yarlung zangbo ophiolites xigaze area tibet journal of asian earth sciences 32 423 437 the geochemistry of the amphibolites confirms that their protoliths were igneous mafic rocks of basaltic to pyroxenitic composition that were likely part of an upper oceanic crust rare earth elements contents are suggestive of an n morb origin however enriched liles and depleted nb ta ti when compared to n morbs rather suggest a suprasubduction zone influence a large proportion of the overlying ophiolitic mafic rocks share the same geochemical characteristics suggesting the protoliths of the amphibolites might have crystallized in the same environment as the xigaze ophiolitic crust likely in a back arc basin 40 ar 39 ar step heating dating of hornblende concentrates from three samples yielded ages of 123 6 2 9 ma 127 7 2 2 ma and 127 4 2 3 ma these cooling ages are slightly younger or overlapping magmatic and sedimentary ages obtained from the overlying ophiolite all these new data support a model in which the ophiolitic crust and the protolith of the amphibolites were formed along the same spreading center above a subduction zone the demise of the early subduction circa 130 ma forced the inception of a new subduction zone at the ssz spreading axis burying ssz mafic rocks underneath a ssz ophiolitic mantle

coarse bedload transport in a mountain river coarse bedload transport dynamics are investigated utilizing hydrodynamic and sediment transport data obtained in an extensively instrumented study reach located in squaw creek montana usa during 1991 and 1992 a number of discrete bedload transport events associated with the daily rise and fall in stream discharge were investigated data show that initiation of sediment transport was accompanied by a reduction in bed roughness and by changes in bulk hydraulic parameters for larger discharges coarser fractions of the bed material mobilized and bedload transport rates and average hydraulic parameters stabilized as discharge reduced mobile coarse particles became less frequent and deposited fine particles were removed resulting in an increase in bed roughness these observations are attributed to the downstream translation of bar sediments during the passage of a hydrograph bedload pulses were aperiodic but spatially variable flow turbulence and velocity profile data obtained during low flows allowed comparison between average bed shear stress and apparent bed roughness estimates obtained using different approaches 1998 john wiley sons ltd

zircon u pb geochronology and hf isotopic constraints on petrogenesis of the gangdese batholith southern tibet abstract during the mesozoic cenozoic northward neotethyan subduction and subsequent india asia collision gave rise to the extensive transhimalayan magmatism that stretches from burma and western yunnan through southern tibet to the ladakh and kohistan complexes to understand the age distribution and petrogenesis of the gangdese batholith the largest intrusive exposure along the transhimalayan magmatic belt fifty granitic samples were selected for in situ zircon u pb and hf isotopic analyses the u pb data suggest four discrete stages of magmatic activity i e 205 152 109 80 65 41 and 33 13 ma respectively with the 65 41 ma stage being the most prominent the hf isotopic data indicate that the gangdese batholith is overwhelmed by positive e hf t values which are comparable to those of the kohistan ladakh batholiths in the west but differ markedly from those of the chayu burma batholiths in the east most of the gangdese granites show similar and young hf model ages 1000 300 ma indicating their derivation from juvenile crust however those formed in the 65 41 ma stage exhibit more heterogeneous hf isotopic ratios with negative e hf t values being observed in some granites younger than 50 ma suggesting the involvement of old indian continental crust in their petrogenesis this age may thus mark the onset of the india asia collision the newly established zircon u pb age and hf isotope database of the gangdese batholith can be used as a powerful tracer or fingerprint when studying the source to sink relation of the sediments eroded from the southern tibetan plateau

the tibetan side of the india eurasia collision results are reported from the 1980 joint french chinese field expedition in tibet the area covered was from the high himalaya in the south to the region of nagqu 250 km north of yangbajain ophiolites in the zangbo valley represent remnants of the crust of an ocean basin which lay adjacent to the gangdise granodiorite belt in the late mesozoic the ophiolites were thrust to the south onto the cretaceous melange and triassic flysch but the age of this event is unclear in the tertiary sediments of the indian margin and the xigaze basin were folded with steeply dipping cleavage which indicates some north south shortening of the crust however the tertiary volcanic sequence on the lhasa block and further north shows comparatively little folding and thrusting in the quaternary deformation of tibet was characterized by east west extension

a comparison of flow intensities in alluvial rivers characteristics and implications for modelling flow processes this paper compares flow intensity data obtained with different instruments from a variety of fluvial environments it examines associations between the root mean square of longitudinal velocity fluctuations flow intensity local mean velocity relative depth and boundary resistance results indicate systematic differences in the behaviour of flow intensity which scale with respect to position in the boundary layer deep sand bedded rivers boundary grain resistance shallow river environments with coarse beds and possibly form resistance shallower sand bedded rivers preliminary approaches to prediction and modelling of variations in flow intensity are suggested based upon linear regression relationships intensity values are also compared with theoretical and empirical limits to the use of taylor s substitution which allows time and frequency properties of a single point velocity time series to be used to yield a flow length scale in general limits are exceeded in all environments for near boundary flow measurements but are met for y d 0 3 in most cases in sand bed rivers and for y d 0 4 in some gravel bed environments 1998 john wiley sons ltd

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

the nature and timing of crustal thickening in southern tibet geochemical and zircon hf isotopic constraints from postcollisional adakites abstract rising as the roof of the world the tibetan plateau is now underlain with the thickest continental crust on earth how and when was this crust formed which would have exerted pivotal controls to the formation of the plateau has long been an issue of hot debates this paper reports zircon u pb ages and hf isotope data for postcollisional 30 9 ma adakites in the southern lhasa terrane southern tibet a comparative analysis of whole rock rare earth element geochemistry and zircon hf isotopes between the adakites and associated gangdese igneous rocks suggests that the tibetan crust underwent a major phase of tectonic thickening between ca 45 and 30 ma in the region the lower part of the thickened crust consisted prevailingly of mafic lithologies which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the late cretaceous and eocene time related to the neotethyan subduction processes including breakoff of the subducted slab at ca 50 ma in the early stage of the india asia collision these processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern lhasa terrane the indentation of india consequently caused distributed lithospheric thickening with formation of an orogenic root beneath southern tibet root foundering during the oligocene gave rise to the adakitic magmatism regional topographic uplift and onset of northward underthrusting of the indian plate that has since played a key role in forming the entire tibetan plateau

geology of the cenozoic indus basin sedimentary rocks paleoenvironmental interpretation of sedimentation from the western himalaya during the early phases of india eurasia collision this study reassesses the stratigraphy sedimentology and provenance of the indus basin sedimentary rocks deposited within the indus tsangpo suture zone itsz during the early phases of india eurasia collision using field observations biostratigraphy and petrographic and isotopic analyses we create a paleodepositional reconstruction within the paleotectonic setting of the early phases of india eurasia collision we then re examine existing constraints to the timing of india eurasia collision previously interpreted from the earliest occurrence of mixed indian and eurasian derived detritus in the succession from mid cretaceous to early paleocene times the jurutze and sumda formations were deposited within an arc bounded marine basin between the dras and kohistan ladakh island arcs the 51 ma aged deltaic chogdo formation then filled the basin until deposition of the 50 8 49 4 ma aged nummulitic limestone during a marine incursion before continental facies developed in an evolving intermountain basin with the deposition of the paleogene indus group within these systems sediment was sourced from the eurasian margin to the north and was transported southward into the suture zone in this section we see no unequivocal evidence of indian plate input to the sedimentary succession and thus no evidence of mixed indian eurasian derived detritus indicative of india asia collision until the upper stratigraphic horizons of the indus group when facies are representative of an axial northwesterly flowing river system we suggest that the paleo indus river was initiated within the itsz during late oligocene early miocene times sedimentation of the indus group continued until the late miocene

measurement of bedload gravel transport the calibration of a self generated noise system self generated noise sgn is a passive acoustic technique for the measurement of bedload transport through the acoustic detection of intergranular collisions using a submerged hydrophone system sgn is of interest because of the possibilities it offers to the measurement of transport parameters at the frequencies of turbulent or oscillating current scales this paper describes the background of the technique and reports the results from laboratory experiments on the relationship between sgn and the absolute transport rate

bed load transport under waves and currents the data from the flume experiments of kalkanis 1964 and abou seida 1965 giving the rates of accumulation of sand in recessed traps in oscillating sand beds have been reassessed in contrast to the work of madsen and grant 1976 the accumulation rate is interpreted as a measure of the average concentration of bed load per unit area during the period of oscillation for which the shields number t exceeds the threshold shields number th for the experimental runs where the reynolds number 35 the volume concentration of bed load c t cm3 cm 2 was found to be directly proportional to the excess shields number ex t t th c t 0 09 0 03 ext and the volume transport rate q t under instantaneous flow velocity u t was therefore q t 0 09 0 03 ex t u t for t th the rate of transport predicted by this equation was tested against the data of manohar 1955 and good agreement found showing a closer match between calculated and observed transport at high rates than found by the equation of madsen and grant 1976 the form of the transport rate equation proposed here q t u2 t u2th u t is compared with the form of the bedload transport equation for unidirectional flow and is found to have a firmer physical basis than some other wave transport equations presently in use

the lhasa terrane record of a microcontinent and its histories of drift and growth article i nfo the lhasa terrane in southern tibet has long been accepted as the last geological block accreted to eurasia before its collision with the northward drifting indian continent in the cenozoic but its lithospheric architecture drift and growth histories and the nature of its northern suture with eurasia via the qiangtang terrane remain enigmatic using zircon in situ u pb and lu hf isotopic and bulk rock geochemical data of mesozoic early tertiary magmatic rocks sampled along four north south traverses across the lhasa terrane we show that the lhasa terrane has ancient basement rocks of proterozoic and archean ages up to 2870 ma in its centre with younger and juvenile crust phanerozoic accreted towards its both northern and southern edges this finding proves that the central lhasa subterrane was once a microcontinent this continent has survived from its long journey across the paleo tethyan ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the qiangtang terrane to the north and with the indian continent to the south zircon hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks e g 50 90 0 70 and 30 100 to the mesozoic magmatism in the southern central and northern lhasa subterranes respectively we suggest that much of the mesozoic magmatism in the lhasa terrane may be associated with the southward bangong nujiang tethyan seafloor subduction beneath the lhasa terrane which likely began in the middle permian or earlier and ceased in the late early cretaceous and that the significant changes of zircon ehf t at 113 and 52 ma record tectonomagmatic activities as a result of slab break off and related mantle melting events following the qiangtang lhasa amalgamation and india lhasa amalgamation respectively these results manifest the efficacy of zircons as a chronometer u pb dating and a geochemical tracer hf isotopes in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics

discovery and significance of cretaceous fossils from the xigaze forearc basin tibet abstract the xigaze cretaceous forearc basin located between the lhasa block and indus yarlung suture was filled with flysch deposits during the mid cretaceous as the tethys oceanic crust was being subducted underneath eurasia the flysch comprises the xigaze group dominated by the angren formation well preserved newly discovered foraminifera within the angren formation constrain its age to late albian to late coniacian radiometric dating of the ophiolite belt impinging on the southern edge of the forearc basin yielded an age of 120 10 ma which implies that the deposition in the forearc basin occurred shortly after initiation of the subduction of the neotethys

late cretaceous evolution of the coqen basin lhasa terrane and implications for early topographic growth on the tibetan plateau the tectonic evolution of the lhasa terrane southern tibetan plateau played a fundamental role in the formation of the tibetan plateau however many uncertainties remain with regard to the tectonic and paleogeographic evolution of the lhasa terrane prior to the india asia collision to determine the early tectonic processes that controlled the topographic evolution of the lhasa terrane we analyze the cretaceous strata exposed in the coqen basin northern lhasa subterrane which comprises the langshan and daxiong formations the langshan formation unconformably overlies the volcanic rocks of the lower cretaceous zelong group and consists of similar to 80 m of orbitolina bearing limestones which were deposited in a low energy shallow marine environment micropaleontological analysis indicates that the langshan formation in the coqen basin was deposited from late aptian to early cenomanian times ca 113 96 ma the overlying daxiong formation similar to 1700 m thick consists of conglomerate coarse sandstone and siltstone with interbedded mudstone and represents deposits of alluvial fans and braided rivers the daxiong formation was deposited after the early cenomanian ca 96 ma and accumulated until at least ca 91 ma indicating accumulation rates of greater than 0 3 km m y 1 by combining paleocurrent data sandstone petrology detrital zircon u pb ages and hf isotope analysis we demonstrate that the daxiong formation was derived from lower cretaceous volcanic rocks and pre cretaceous strata in the northern lhasa subterrane during late cretaceous time two thrust systems with opposite vergence were responsible for transforming the northern lhasa subterrane into an elevated mountain range this process resulted in the evolution from a shallow marine environment langshan formation into a terrestrial depositional environment daxiong formation on the southern margin of the northern lhasa subterrane given the regional paleogeographic context we conclude that the daxiong formation in the coqen basin records local crustal shortening and flexure resulting in foreland basin development on the southern margin of the northern lhasa subterrane which implies early topographic growth of the northern lhasa subterrane in southern tibet prior to the india asia collision

early cretaceous subduction related adakite like rocks of the gangdese belt southern tibet products of slab melting and subsequent melt peridotite interaction abstract the limited geochronology and geochemistry data available for the early cretaceous igneous rocks of the southern gangdese belt southern tibet has resulted in the proposal of conflicting geodynamic models for the generation of the widespread cretaceous igneous rocks in the middle and northern parts of the belt to explore this issue we present shrimp u pb zircon data and geochemical and sr nd pb hf isotopic data for the mamen andesites from the southern margin of the gangdese belt the mamen andesites emplaced at 136 5 ma are sodic na2o k2o 1 2 2 3 and have geochemical characteristics typical of adakites i e high al2o3 high la yb ratios and sr contents low y and hree contents and positive eu anomalies except for high cr ni and mgo contents the andesites have initial 87sr 86sr t ratios of 0 70413 0 70513 positive end t values of 3 7 5 8 and 206pb 204pb t ratios of 18 37 18 51 207pb 204pb t ratios of 15 59 15 65 and 208pb 204pb t ratios of 38 43 38 72 in situ hf isotopic analyses of zircons that had previously been dated by shrimp yielded positive initial ehf t values ranging from 11 0 to 15 5 a model calculation using trace element and sr nd pb isotopic data indicates that several percent of subducted sediment is required to generate the mamen andesites which were derived via the partial melting of subducted neo tethyan slab morb sediment fluid and subsequently hybridized by peridotite in the mantle wedge our data indicate that the neo tethyan oceanic crust was subducted northward beneath the gangdese belt during the early cretaceous at a high angle our results are inconsistent with a tectonic model that advocates the low angle or flat slab subduction of neo tethyan oceanic crust in generating the widespread cretaceous magmatism recorded in the gangdese belt

petrology and texture of the ultramafic rocks of the xigaze ophiolite tibet constraints for mantle structure beneath slow spreading ridges abstract the xigaze ophiolite southern tibet exhibits an unusual ophiolite sequence characterized by 1 1 the limited thickness less than 3 km of its crustal unit which is mostly composed of dolerite intrusives the gabbros being very scarce 2 2 the scarcity of harzburgites and dunites the ultramafic rocks consisting mostly of cr diopside bearing harzburgites and iherzolites although there are some textural and chemical variations from the basal iherzolites to the upper harzburgite and dunites no sharp discontinuity exists between the peridotite series of the xigaze ophiolite which all belong to the same peridotite suite the peridotites have been equilibrated at relatively low temperatures and pressures 1155 c 0 5 0 6 gpa for the cr diopside bearing harzburgites and 1255 c 1 0 1 1 gpa for the stratigraphically 2 km deeper iherzolites indicative of a low thermal gradient at the xigaze paleo ridge accordingly the peridotites have locally been deformed under low stress 0 2 gpa conditions and at decreasing temperatures from up to 1000 c to 900 800 c as evidenced by the activation of the 010 100 ok1 100 and 001 100 olivine glide systems the equilibrium temperatures and pressures estimated for the peridotites the inferred shape of the melting zone beneath the xigaze paleo ridge the low temperature low stress conditions inferred for the deformation of the peridotites and the pattern of the mantle structure determined on the basis of structural and microstructural data are in good agreement with a slow spreading ridge origin for the xigaze ophiolite

outward growth of the tibetan plateau during the cenozoic a review abstract the surface uplift history of the tibetan plateau tp offers a key testing ground for evaluating models of collisional tectonics and holds important implications for processes ranging from global cooling to the onset of the asian monsoon various models have been proposed to reveal the surface uplift history of the tp but controversies remain we evaluate these models using data from sedimentology and stratigraphy structural geology magmatism exhumation and paleoaltimetry studies structural analyses indicate that thrust belts which spread from the central tp outward toward its surrounding margins accommodated most of the india asia convergence and facilitated crustal shortening and thickening in the central tp eocene adakitic rocks located in the qiangtang and the lhasa blocks likely were generated by partial melting of an eclogitic source paleogene 50 30 ma potassic rocks only occur in the qiangtang block whereas late oligocene late miocene 26 8 ma potassic rocks occur both in the qiangtang and lhasa blocks low temperature thermochronologic ages in the central tp are older than 40 35 ma whereas those in the margins are younger than 20 ma mostly late miocene and pliocene pleistocene in age independent paleoaltimetry estimates suggest that the lhasa and qiangtang terranes attained their current elevations during the eocene most likely due to the initial collision between india and lhasa whereas the hoh xil basin area to the north and himalayas to the south were still low even below sea level in the latter case we argue for an inside out growth pattern for the tibetan plateau the tp grew southward and northward from a nucleus of high topography and is likely to continue expanding along the mazar tagh fault to the northwest the kuantai shan hei shan longshou shan to the northeast the longquan shan to the east and the shillong plateau to the south if the northward force of india would not diminished

origin of the xigaze ophiolite yarlung zangbo suture zone southern tibet abstract the xigaze ophiolite tibet displays unusual lithological petrological textural and structural characteristics there are no large masses of cumulate gabbros but dolerite intrusives throughout the whole ophiolite sequence some of which were intruded into already serpentinized peridotites only minor residual harzburgites and dunites in dominantly iherzolitic peridotites equilibrated at low temperatures and pressures and relatively low temperature deformation structures in the uppermost peridotites these features suggest a very low heat flow at the spreading center where the xigaze ophiolite was formed in good agreement with a discontinuous and slowly accreting spreading center origin however this ophiolite does not represent a typical mid oceanic ridge ophiolite rather it was formed in a small basin located at the southern margin of eurasia hence within a preexisting oceanic lithosphere the opening of the nearly n s xigaze paleo ridge resulted from the w e drift of africa relative to eurasia from 180 to 110 ma the n s emplacement onto the continent of the xigaze ophiolite formed 120 to 110 ma ago can be correlated to changes in direction of motion of the african and indian plates a primary intra oceanic thrusting event probably occurring at 110 or 85 ma and the final obduction near 50 ma during the india eurasia collision

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

modeling the erosion of cohesive clay coasts a model was developed to study the erosion of cohesive clay coasts in macro to non tidal environments the model shares some of the characteristics of previous models including the erosion of bare clay surfaces by wave generated bottom shear stresses and of mobile sediment covered surfaces by abrasion it differs from previous models however in several important ways the morphodynamics of beaches with clay foundations under different wave conditions are based on a previously developed model for beaches on rocky shore platforms sediment thickness along a beach profile is calculated at regular intervals and compared with the maximum thickness that could be moved at that location under prevailing wave conditions wave friction factors are determined where necessary according to the occurrence and morphology of ripples on the bottom in addition to abrasion and the effect of wave induced shear stresses on the clay bottom erosion by stresses generated by wave impact at the bluff foot and on the intertidal platform is calculated using an expression derived from hard rock coastal models tides are represented by their computed tidal duration values the amount of time each year that the water level falls within each 0 1 m vertical interval water depths are modified by wave setup and set down conditions several preliminary model runs were made the profiles were concave in the submarine zone and roughly linear in the intertidal zone equilibrium profiles developed which were maintained as they migrated landwards

methane derived authigenic carbonates of mid cretaceous age in southern tibet types of carbonate concretions carbon sources and formation processes abstract methane derived authigenic carbonates with distinctive structures and morphologies have been documented worldwide but they are rarely found from ancient strata in the eastern tethys ocean the methane derived authigenic carbonates found in southern tibet are developed in calcareous or silty shales of mid cretaceous age in the xigaze forearc basin and in the tethyan himalaya tectonic zone the morphology mineralogy elemental geochemistry and composition of carbon and oxygen isotopes of these carbonates are studied in detail the carbonates have nodular tubular and tabular morphologies they are primarily composed of carbonate cement that binds and partly replaces host sediment grains host siliciclastic sediments are composed mainly of quartz and plagioclase feldspar a few foraminifers and framboidal or subhedral to euhedral pyrite carbonate cements dominantly are micritic calcite with minor contribution of dolomite nodular concretions are characterized by depleted 13c values commonly ranging from 30 to 5 the 13c values show a gradual decrease from the periphery to the center and the cao sio2 fe2o3 al2o3 k2o and tio2 contents generally show a gradual change these features indicate that the nodular concretions grew from an early formed center toward the periphery and that the carbon source of the nodular concretions was derived from a mixture of methane methanogenic co2 and seawater dissolved inorganic carbon the tubular concretions are characterized by 13c values of 8 85 to 3 47 in the shangba section and 27 37 to 23 85 in the upper gamba section unlike the nodular concretions the tubular concretions show central conduits which are possible pathways of methane rich fluids suggesting that the cementation of tubular concretions begins at the periphery and proceeds inward moreover the tubular concretions show morphological similarity with the methane derived carbonate chimneys pipes and slabs reported in present day cold seep settings we suggest that the carbon source of the tubular concretions was derived from a mixture of seawater dissolved inorganic carbon and oxidized methane formed by released hydrate the tabular concretions are characterized by 13c values of 21 87 to 6 67 in the xiege sections these depleted 13c values suggest that the carbon of the tabular concretions was derived at least in part from aom the tabular concretions are characterized by 13c values of 28 81 to 12 99 in the gamba section according to the 13c values and field observation we infer that their carbon source was more likely to be a mixture of the oxidized methane formed by released hydrate and seawater dissolved inorganic carbon

the takena formation of the lhasa terrane southern tibet the record of a late cretaceous retroarc foreland basin our understanding of the processes involved in the indo asian collision and the construction of the tibetan plateau are in part predicated on our understanding of the tectonic setting and crustal conditions of southern tibet in the time period immediately preceding the indo asian collision late cretaceous several hypotheses have been proposed that describe the middle to late cretaceous tectonic and paleogeographic evolution of southern tibet each with different implications for when and how the tibetan plateau was uplifted we examined the mid upper cretaceous takena formation of the lhasa terrane in southern tibet in order to reconstruct the middle to late cretaceous tectono sedimentary history of this area and test the competing tectonic hypotheses the takena formation consists of 2 km of sedimentary strata that include a lower marine limestone member penbo member and an upper member of clastic red beds lhunzhub member the aptian albian penbo member consists of 250 m of orbitolinid limestone beds that were deposited in a shallow marine seaway during a time when there was little regional subsidence the overlying lhunzhub member 1500 m consists primarily of fl strata that were deposited during a period of increased subsidence overall the lhunzhub member coarsens upward from meandering and anastomosing stream deposits interbedded with numerous paleosols near the base to multistory multilateral braided stream deposits near the top the fl uvial sandstone units are lithic rich and contain abundant plagioclase and volcanic grains and paleocurrent data record northwest directed transport these data indicate that the sediment was derived from the gangdese volcanic arc that developed along the southern margin of the lhasa terrane following deposition but prior to ca 70 ma the beds of the takena formation were folded and partially eroded the sedimentary and stratigraphic characteristics of the takena formation are most consistent with deposition in a retroarc foreland basin setting the limestone beds of the penbo member were deposited in a shallowmarine seaway that was eventually infi lled by clastic sediment derived principally from the gangdese volcanic arc the low subsidence rate recorded in the lowermost strata of the takena formation including the penbo member and the paleosol rich interval of the lhunzhub member is interpreted to be associated with the passage of a fl exural forebulge the overlying upward coarsening fl uvial strata were deposited in progressively more proximal locations within the foredeep of a foreland basin the late cretaceous folding of the takena formation indicates that the foreland basin strata were eventually incorporated into a fold and thrust belt the upper cretaceous sedimentary strata of the lhasa terrane of southern tibet predict that a north verging fold and thrust belt existed along the southern margin of the lhasa terrane during late cretaceous time prior to the indo asian collision collectively the evidence indicates that the southern margin of the lhasa terrane had thickened crust and was likely to have been at high elevations immediately before the indo asian collision

sea bed noises reveal role of turbulent bursting phenomenon in sediment transport by tidal currents in many engineering and sedimentological applications it is usual to relate sediment movement to the time averaged bed shear stress however the shear stress exerted on the sea bed by tidal currents is intermittent in nature due to the turbulent bursting phenomenon1 2 sediment movement at the sea bed also occurs in bursts3 4 however attempts to link these two processes have failed because of the poor temporal resolution of existing sediment transport measurement techniques recent experimental advances now make this possible and we describe here the first detailed observations of sediment movement and fluid turbulence in tidal currents flowing over sandy gravels these results show that high stress values are a necessary but not sufficient condition for bed load sediment movement and that form drag on individual particles may have greater dynamical significance than the stress this work also suggests that it may now be possible to describe sediment movement at a level which is deterministic in terms of the turbulence and at very much shorter timescales than have been used previously

the eastern iranian orocline abstract the eastern iranian range is seen as a large scale india eurasia collisional event and the subsequent amalgamation of the afghan and lut micro continents including the neo tethyan accretionary complexes and associated island arcs revised examination of the nature of tectono stratigraphic units distributed in both sides of the n s elongated eastern iranian orogen reveals two remarkable facts first from a general perspective it is structurally a giant parallel antiform in profile view second lithologically the most time equivalent comparable tectonic units from both northern and southern domains comprising the eastern iranian orogen are connectable follow the strike of hinge and limbs zones of the antiform and a m shaped disharmonic pattern in its core these characters are thought to be formed in response to two principal deformational phases resulting from two perpendicular stress directions the first was n s associated with the north dipping subduction of the tethyan oceanic lithosphere beneath the roughly ribbon shaped cimmerian supercontinent including the afghan and lut blocks before late eocene afterward the second was e w shortening originated by the westward escape of the afghan block as a consequence of the indian plate indentation into eurasia this orogen parallel stress was correspondingly accommodated by tens of degrees counter clockwise rotation of the lut block around a vertical axis the result of such shortening was oroclinal buckling and concomitant retreating of the subduction zone and closure of the sistan ocean a remaining wedge shaped part of the neotethys ocean formed by the ribbon curvature the development of the eastern iranian orogen already included the sistan suture zone an enormous mass of folded eocene flysch deposits with a general n s axial surface and some cretaceous ophiolitic remnants the plate scale buckling which was formed by this late stage orogenic event is hereby considered as an orocline call eastern iranian orocline lithosphere buckling around a vertical axis caused a large scale crustal folding coeval with the bending of its mantle part increasing the lithospheric thickness of the orocline compared to adjacent regions the syn orocline deformation associated delamination and resulting magmatic events took place in eastern iran during a short period of about 10 to 20 ma in the late paleogene the orocline hinge zone is occupied by the khorasan outer arc and the birjand core while its limbs are composed of the baluch inner arc and the southwestern and southeastern terminations of the lut and afghan blocks the eastern iranian orocline seems to be the middle segment of a triple bending belt with n s two opposite facing bend including afghan lut and sanandaj sirjan blocks this large curvature is possibly an example of secondary oroclines which was formed by collision related escape tectonics although the limited available kinematic data show a kind of progressive orocline

geochemical and geochronological constraints on the origin and emplacement of the yarlung zangbo ophiolites southern tibet

palynological indications of environmental changes during the late creataceous eocene on the southern continental margin of laurasia xizang tibet abstract a palynostratigraphic and palynofacies study of a geological section at cuojiangding in zhongba county has provided a basis for discussing the palaeoenvironmental evolution of the southern continental margin of laurasia in xizang tibet towards the end of the cretaceous period and during the paleogene it is the only record of fossil spores and pollen grains from collision and convergence related sediments close to the major yarlung zangbo suture deposits of the padana formation at the base of the section through the qubeiya quxia and jialazi formations to the rujiao zangbo conglomerate at the top are considered to range in age from santonian to eocene or possibly oligocene during this period the india plate collided with and continued to push against laurasia plate two spore and pollen zones and three palynofacies types are recognized these are correlated with two transgressive regressive cycles associated with the collision periods of geologically rapid uplift being reflected by the molasse like sediments of the upper quxia formation and the rujiao zangbo conglomerate which accumulated during the late paleocene and eocene oligocene respectively the first regression led to intermittent subaerial exposure and erosion of the cuojiangding area the second ended the marine history of the area and led first to the development of swamps in a subtropical climate now preserved in the coal bearing qiuwu formation and later to the development of mountainous terrain with a cooler climate at higher elevations

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

comparisons of measured and predicted sediment transport rates in tidal currents abstract this paper describes bedload transport measurements in a large scale sedimentary system on the british continental shelf steady flow sediment transport theories are applied to oscillatory tidal currents and various equations are compared with measured transport rates from tracer dispersion experiments the effects of waves on both measured and predicted rates are briefly considered and the predicted directions of sediment movement compared with those inferred from sedimentological data

delineation of recharge rate from a hybrid water table fluctuation method 1 the concept of the hybrid water table fluctuation wtf method for recharge rate estimation was revisited to estimate the recharge rate a physically based wtf equation was established the concept of transient fillable porosity was proposed and computed with unsaturated hydraulics models the developed model is tested by applying to the water table fluctuation data from hongcheon korea in the applications the recharge and fillable porosity estimates were found to be most sensitive to nonlinearity in the unsaturated water content profile and permeability also the water table level drift which does not originate from precipitation serves as a major source of estimation error

geochemistry detrital zircon geochronology and hf isotope of the clastic rocks in southern tibet implications for the jurassic cretaceous tectonic evolution of the lhasa terrane abstract in order to reconstruct tectonic evolution history of the southern margin of asia i e lhasa terrane before the india asia collision here we present a comprehensive study on the clastic rocks in the southern lhasa terrane with new perspectives from sedimentary geochemistry detrital zircon geochronology and hf isotope clasts from the jurassic early cretaceous sedimentary sequences i e yeba and chumulong formations display high compositional maturity and experienced moderate to high degree of chemical weathering whereas those from the late early late cretaceous sequences ngamring and shexing formations are characterized by low compositional maturity with insignificant chemical weathering our results lead to a coherent scenario for the evolution history of the lhasa terrane during the early middle jurassic 192 168ma the lhasa terrane was speculated to be an isolated geological block the yeba formation is best understood as being deposited in a back arc basin induced by northward subduction of the neo tethys ocean with sediments coming from the interiors of the lhasa terrane the middle jurassic early cretaceous lhasa qiangtang collision resulted in the formation of a composite foreland basin with southward flowing rivers carrying clastic materials from the uplifted northern lhasa and or qiangtang terranes during the late early late cretaceous 104 72ma the gangdese magmatic arc was uplifted rapidly above the sea level forming turbidites ngamring formation in the xigaze forearc basin and fluvial red beds shexing formation on the retro arc side at the end of late cretaceous the lhasa terrane was likely to have been uplifted to high elevation forming an andean type margin resembling the modern south america before the india asia collision