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

detrital zircon evidence for progressive underthrusting in franciscan metagraywackes west central california we present new u pb ages for detrital zircons separated from six quartzose metagraywackes collected from different franciscan complex imbricate nappes around san francisco bay all six rocks contain a broad spread of late jurassic cretaceous grains originating from the klamath sierra nevada volcanic plutonic arc units young structurally downward consistent with models of progressive underplating and offscraping within a subduction complex the youngest specimen is from the structurally lowest san bruno mountain sheet at 52 ma it evidently was deposited during the eocene none of the other metagraywackes yielded zircon ages younger than 83 ma zircons from both el cerrito units are dominated by ca 100 160 ma grains the upper el cerrito also contains several grains in the 1200 1800 ma interval these samples are nearly identical to 97 ma metasedimentary rock from the hunters point shear zone zircon ages from this melange block exhibit a broad distribution ranging from 97 to 200 ma with only a single pre mesozoic age the albany hill specimen has a distribution of pre mesozoic grains from 1300 to 1800 ma generally similar to that of the upper el cerrito sheet however it contains zircons as young as 83 ma suggesting that it is significantly younger than the upper el cerrito unit the skaggs spring schist is the oldest studied unit its youngest analyzed grains were ca 144 ma and it is the only investigated specimen to display a significant paleozoic detrital component sedimentation and subduction accretion of this tract of the trench complex took place along the continental margin during early to early late cretaceous time and perhaps into eocene time franciscan and great valley deposition attests to erosion of an andean arc that was active over the entire span from ca 145 to 80 ma with an associated accretionary prism built by progressive underthrusting we use these new data to demonstrate that the eastern franciscan complex in the northern and central coast ranges is a classic accretionary prism where younger structurally lower allochthons are exposed on the west and older structurally higher allochthons occur to the east in the heavily studied san francisco bay area

relicts of the early cretaceous seamounts in the central western yarlung zangbo suture zone southern tibet abstract despite more than 30 years studies in the yarlung zangbo suture zone yzsz southern tibet is still a region of discovery and the geological evolution of the neo tethys here remains controversial in this paper we present the new field observation petrography zircon u pb dating and geochemistry of the zhongba mafic rocks in the western segment of the yzsz zircon u pb analyses from a diabase using sims secondary ion mass spectrometry yielded a concordant age of 125 7 0 9 ma all the zhongba mafic rocks exhibit lree enrichment lan ybn 4 7 15 8 without eu anomalies eu 0 97 1 01 their primitive mantle normalized trace element patterns are similar to those of the average oib and hawaii alkaline basalts as well as other oib type rocks already reported in the central western yzsz these observations lead us to suggest the existence of a series of seamounts within the neo tethys during the early cretaceous these oib type rocks could be originated from 1 hotspot mantle 2 near ridge seamounts by melting induced mixing of two distinct mantle sources 3 late stage magmatism fed by melts that originated from an asthenospheric window due to slab delamination combining literature data from the entire yzsz which include studies of the morb and iab type rocks and mantle peridotites we prefer the third explanation that the oib type rocks represent a late stage magmatic activity above an intra oceanic supra subduction zone within the neo tethys

new insights into the india asia collision process from cretaceous paleomagnetic and geochronologic results in the lhasa terrane abstract to further constrain the india asia collisional process a combined paleomagnetic and geochronologic study has been carried out on the upper cretaceous jingzhushan formation redbeds and the lower cretaceous dianzhong formation volcanic rocks dated at 121 117 ma from the cuoqin area in the central lhasa terrane stepwise thermal demagnetization successfully isolated reliable characteristic remanent magnetization chrm directions that include dual polarity and pass positive fold tests at 95 and 99 confidence levels indicating prefolding primary magnetizations the tilt corrected site mean direction for 33 redbed sites is d 316 8 i 30 2 with 95 5 4 corresponding to a paleopole at 49 0 n 344 3 e with a 95 5 3 and the other for 12 volcanic sites is d 350 5 i 25 5 with 95 7 7 corresponding to a paleopole at 70 5 n 292 9 e with a 95 7 4 our new paleomagnetic results together with reliable cretaceous paleomagnetic data obtained from the lhasa terrane demonstrate that the southern margin of asia was located at 15 1 n during the cretaceous comparison with the apparent polar wander paths apwp of india and the cretaceous paleocene paleopoles of the himalayan terrane suggests that the india asia collision was likely a complex process which consists of the collision of the lhasa and himalayan terranes at 54 0 2 1 ma the longtime subduction of an intra oceanic basin between the himalayan terrane and the indian craton from 54 0 to 40 4 ma and the collision of the himalayan terrane and the indian craton at 40 4 4 1 ma comparing with the late cretaceous average pole of the east asia apwp reveals that a latitudinal convergence of 780 240 km has taken place between the lhasa terrane and east asia the hexi corridor since the india asia collision the amount of latitudinal shortening deduced from paleomagnetic data is very consistent with the 600 1000 km accommodated by the cenozoic fold and thrust belts between the lhasa terrane and hexi corridor

relations between fluid inclusion trails and regional stress field a tool for fluid chronology an example of an intragranitic uranium ore deposit northwest massif central france abstract episyenites within the uranium ore bearing le bernardan granite northwest french massif central were formed through the dissolution of quartz by hydrothermal solutions these solutions may now be found fossilized as fluid inclusions f i the f i form rectilinear trails along microcracks which trapped the percolating fluids simultaneous study of the orientations of the trails and the chemistry of the inclusions shows that 1 1 the trails may be grouped according to direction each related to successive regional stress fields 2 2 the fluids of each group are compositionally distinct marking different stages of the hydrothermal event this example shows that the geometric relations between secondary f i trails and regional stress fields may be used to establish the chronology of both during a region s geological evolution

latest marine horizon north of qomolangma mt everest implications for closure of tethys seaway and collision tectonics the newly discovered marine horizon at zhepure syncline north of qomolangma revises timing of the closure of the eastern neo tethys seaway in the central himalayas the marine pengqu formation conformably overlies a lutetian shallow water carbonate platform and comprises shales interbedded with sandstones deposited in a neritic shelf environment the strata are dated by nannofossils and foraminifera as late early lutetian to late priabonian age np15 np20 deposited 47 34 ma indicating that the final closure of the tethys seaway in this region occurred at 34 ma the newly discovered strata provide evidence about emplacement of a major thrust sheet south of southern tibet prior to 37 ma which affected the regional climate

the tethys himalayan passive margin from late triassic to early cretaceous south tibet the mesozoic succession of south tibet both in lithologies and stratigraphic thicknesses compares much more closely with that of central nepal than has been reported in the literature facies distribution from relatively proximal environments in the south to more distal settings in the north reflects the paleogeography of the tethys himalaya passive margin representing the southern edge of neotethys as in central nepal and nw himalaya accumulation rates increased greatly in the latest triassic when very thick shelfal siliciclastics and locally volcaniclastics tarap formation were followed by coastal sandstones zhamure formation contrary to reports in the literature this latter unit reaches well into the liassic at least in proximal southern sectors the base of the overlying kioto group yielding rich faunal associations characterized by benthic foraminifers and lithiotis similar to those of western tethys carbonate platforms is dated as middle liassic most probably late pliensbachian the monotonous growth of carbonate ramps during the middle liassic to early dogger was interrupted by two siliciclastic episodes related to major paleogeographic changes in the toarcian middle part of the kioto group and in the bajocian bathonian laptal formation an early callovian flooding event ferruginous oolite formation widespread all along the tethys himalaya was locally followed by deposition of a markedly lenticular pelagic unit previously reported only from the thakkhola graben dangar formation the overlying spiti shale is reduced to only 20 m in southern sectors where it is not younger than the oxfordian whereas in northern sectors the unit is several tens of m thick and mainly ascribed to the tithonian volcaniclastic deposition wolong formation reported herein for the first time probably began as early as the kimmeridgian early tithonian and lasted until aptian times when it was replaced by sedimentation of relatively deep water marlstones this magmatic episode related to tectonic extension associated with the detachment of india from gondwanaland took place earlier in south tibet and progressively later in central nepal and northern india as in nepal volcanic products changed with time from predominantly basaltic composition to intermediate and bimodal character the early cretaceous sedimentary evolution of south tibet matches that of offshore nw australia

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

paleocene eocene foreland basin evolution in the himalaya of southern tibet and nepal implications for the age of initial india asia collision siliciclastic sedimentary rocks derived from the southern lhasa terrane sitting depositionally upon rocks of the northern indian passive continental margin provide an estimate of the age of initial contact between the continental parts of the indian and asian plates we report sedimentological sedimentary petrological and geochronological data from upper cretaceous paleocene strata in the sangdanlin section located along the southern flank of the indus yarlung suture zone in southern tibet this is probably the most proximal and therefore the oldest record of the india asia collision these strata were deposited by high density turbidity currents or concentrated density flows and suspension settling of pelagic biogenic debris in a deep marine setting an abrupt change from quartz arenitic to feldspatholithic sandstone compositions marks the transition from indian to asian sediment provenance the abrupt compositional change is accompanied by changes in u pb ages of detrital zircons diagnostic of a sediment provenance reversal from indian to asian sources the timing of the transition is bracketed between 60 ma and 58 5 0 6 ma by detrital zircon u pb ages and zircon u pb ages from a tuffaceous bed in the upper part of the section in the context of a palinspastically restored regional paleogeographic framework data from the sangdanlin section combined with previously published data from the northern tethyan himalaya and the frontal nepalese lesser himalaya and subhimalaya suggest that a flexural wave migrated 1300 km southward across what is now the himalayan thrust belt from paleocene time to the present

paleogene carbonate microfacies and sandstone provenance gamba area south tibet stratigraphic response to initial india asia continental collision abstract the paleogene marine strata in the gamba area of south tibet comprise carbonates of the zongpu formation and siliciclastic rocks of the enba and zhaguo formations documenting the final stages of marine deposition in the tethyan himalaya the 350 m thick zongpu formation was dated as late danian to ypresian based on larger benthic foraminifers thirteen distinct microfacies identify three sedimentary environments mudstone wackestone with udoteacean algae bioclastic peloidal packstone packstone with rotaliids and green algae floatstone with alveolina and orbitolites were deposited in restricted lagoonal environments bioclastic packstone and grainstone with rotaliids were deposited in high energy shoal environments floatstones with nummulitids or alveolinids were deposited in shallow open marine environments the zongpu formation was accumulated on a carbonate ramp it documents two deepening upward sequences separated by an unconformity corresponding to the palaeocene eocene boundary and marked by a conglomerate with limestone clasts the overlying enba formation comprises greenish grey calcareous shales intercalated with litho quartzose sandstones in the upper part and capped by subaerial litho quartzose red beds of the zhaguo formation petrographic analysis detrital zircon geochronology and hf isotopic data indicate that detritus in the enba and zhaguo formations deposited on the indian passive margin was derived from the asian active margin in the north these clastic units were thus deposited after the onset of the india asia continental collision in the early himalayan foreland basin major lithological and paleoenvironmental changes occurred at three stratigraphic levels the jidula zongpu boundary 62 ma the paleocene eocene boundary 56 ma and the zongpu enba boundary 51 ma our provenance study confirms that the india asia collision was already under way during the deposition of the enba member i e by 51 ma and along with facies analysis and general palaeogeographic considerations indicates that neo tethys was still wide open during the early middle paleocene it is thus argued consistently with previous studies that the paleocene eocene disconformity documented in the gamba area as in the northwestern tethyan himalaya is likely to record flexural uplift consequent to initial underthrusting of the indian continental margin beneath asia at or just a little earlier than 56 ma

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

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

geochemistry of sedimentary rocks from melange and flysch units south of the yarlung zangbo suture zone southern tibet abstract the mesozoic yamdrock melange and the triassic flysch lie immediately south of the yarlung zangbo suture zone yzsz ophiolitic belt in southern tibet the whole rock chemical signature of sandstones mostly greywackes red shales and black shales of the yamdrock melange and flysch units do not present significant variations between the different rock types and geological units and is concordant with a continental passive margin source despite fairly important chemical weathering the geochemical signature was not affected by significant sedimentary recycling nor significant heavy mineral accumulation ree patterns show an enrichment in lree typical of shales and indicate that the neo tethyan turbidites mostly derive from a felsic old upper continental crust source slight enrichments in ti and sc indicate a mafic contribution that could originate from mafic blocks of enriched intraplate geochemical affinity found in the sedimentary units sedimentary rocks from both units were thus derived from the old indian continental crust including granitic and intraplate mafic blocks but were deposited in different tectonic settings in the neo tethys ocean the homogeneity of the yzsz data which cover the whole neo tethyan basin and correlation with himalayan data support the formation of all himalayan lithotectonic units along a continuous passive margin of northern india

sandstone provenance and tectonic evolution of the xiukang melange from neotethyan subduction to india asia collision yarlung zangbo suture south tibet abstract the xiukang melange of the yarlung zangbo suture zone in south tibet documents low efficiency of accretion along the southern active margin of asia during cretaceous neotethyan subduction followed by final development during the early paleogene stages of the india asia collision here we present integrated petrologic u pb detrital zircon geochronology and hf isotope data on different types of sandstone blocks in the xiukang melange three groups of sandstone blocks with different provenance and depositional setting are distinguished by their petrographic geochronological and isotopic fingerprints blocks of turbiditic quartzarenite originally sourced from the indian continent were deposited in pre cretaceous time on the northernmost edge of the indian passive margin and eventually involved into the melange at the early stage of the india asia collision two distinct groups of volcaniclastic sandstone blocks were derived from the central lhasa block and gangdese magmatic arc one group was deposited in the trench and or on the trench slope of the asian margin during the early late cretaceous and the other group in a syn collisional basin just after the onset of the india asia collision in the early eocene the largely erosional character of the asian active margin in the late cretaceous is indicated by the scarcity of off scraped trench fill deposits and the relatively small subduction complex developed during limited episodes of accretion the xiukang melange was finally structured in the late paleocene eocene when sandstone blocks of both indian and asian origin were progressively incorporated tectonically in the suture zone of the nascent himalayan orogen

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

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

upb geochronology of gangdese transhimalaya plutonism in the lhasa xigaze region tibet abstract a series of different plutons from the lhasa xigaze segment of the gangdese transhimalaya belt has been studied by high resolution u pb analyses of zircon using zircon fractions of 5 100 grains each selected upon specific grain characteristics for two diorites located east of xigaze dazhuka the zircons yield concordant ages of 93 4 1 0 and 94 2 1 0 m y respectively also concordant ages of 41 1 0 4 and 41 7 0 4 m y have been obtained for two granodiorites collected southwest of lhasa qushui the precision on the ages of two granites from the xigaze and lhasa area is limited by two factors the presence of inherited radiogenic lead and the occurrence of subsequent lead loss however some concordant zircons detected in both granites define approximate ages of about 67 and 53 m y respectively the inherited lead components show that melting of precambrian material was involved in magma genesis the u pb ages substantiate a magmatic activity lasting from mid cretceous cenomanian to eocene lutetian time such a period of plutonism at the southern margin of eurasia as well as the occurrence of magma generation from continental crust suggest that the gangdese range results from the subduction of tethys oceanic lithosphere indian plate underneath eurasia eurasian plate if this model is true the collision of india with eurasia along the lhasa xigaze sector postdates the emplacement of the 41 m y old gangdese granodiorites i e the collision occurred after lutetian time

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

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

the middle devonian shoreline in north devon england the hollowbrook formation is an early middle devonian regressive succession of sandstones and heterolithic facies marking a passage from shallow marine shelf muds through a sandy shoreface to continental ephemeral stream sheet sands ten facies are recognized arranged in two sequence types the shelf sequence consists of alternations of bioturbated muddy sandstone fair weather sediment and shelly concentrates or parallel laminated fine sandstone storm deposits the nearshore and foreshore sequences coarsen upwards and pass from bioturbated muddy sandstone shelf to lenticular beds inner shelf then either wavy and flaser beds fair weather lower shoreface or hummocky beds and multistorey parallel laminated sands with bioturbated tops lower shoreface storm shoals upper shoreface wave ripple cross laminated sandstones follow then parallel and planar cross laminated sandstones foreshore and parallel laminated lithic sandstone continental sheet flood the lower parts of the shoreface sequence and inner shelf are burrowed by skolithos arenicolites and chondrites the sequence differs from the high energy 9oregon coast9 shore in the absence of a megarippled facies and presence of much bioturbation but closely matches the sequence found in modern low energy wave influenced sandy shores such as sapelo island georgia

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