significance of soft sediment clasts in glacial outwash puget sound usa abstract soft sediment clasts composed of silt and clay are contained within glacial outwash sands in the puget sound washington state usa the outwash was deposited during ice retreat of the cordilleran ice sheet around 17 cal kyr bp the soft sediment clasts have a distinctive and consistent morphology and disposition within the sand beds the sedimentology sedimentary structures and presence of soft sediment clasts suggest sand was deposited as proglacial outwash with silts and clays deposited in meltwater pools following drying out of the pools and subaerial cracking lumps of silt and clay were excavated by meltwater and transported distally as soft sediment clasts within high density flows the most likely final depositional setting is as a salisbury type delta in which subaqueous outwash grades distally into deeper water this interpretation shows the power of soft sediment clasts to inform on past processes and palaeogeography for which there is often little evidence in the geologic record

zircon u pb geochronology of the nyainqentanglha group from the lhasa terrane new constraints on the triassic orogeny of the south tibet abstract the studied amphibolite facies metamorphic rocks typical of the nyainqentanglha group occur as a tectonic block in the central lhasa terrane south tibet and consist mainly of gneiss amphibolite quartz schist and marble the amphibolite has a similar mineral paragenesis of plagioclase garnet biotite amphibole quartz muscovite and were metamorphosed under p t conditions of 0 63 0 66 gpa and 588 622 c zircons from three dated samples include two types first type of zircons has an inherited magmatic core and a metamorphic rim whereas the other type of zircons is of metamorphic origin la icp ms in situ u pb analysis indicates that the inherited magmatic cores of zircons yield widely variable 206 pb 238 u ages ranging from 1857 ma to 743 ma recording the magmatic thermal events of the material source of the metamorphic rock protoliths in contrast the metamorphic zircons yield concordant 206 pb 238 u ages of 225 213 ma the results indicate that the nyainqentanglha group has been subjected to triassic metamorphism instead of the precambrian metamorphic basement of the lhasa terrane as considered traditionally these rocks together with the coeval sumdo eclogite and pana blueschist form a permian to triassic metamorphic belt with an e w extension of up to 300 km therefore this work provides new constraints on the late paleozoic to early mesozoic orogeny in the central lhasa terrane south tibet

the middle cretaceous 110 94 ma evolution of tangza basin in the western tibetan plateau and implications for initial topographic growth of northern lhasa

late paleozoic and mesozoic evolution of the lhasa terrane in the xainza area of southern tibet abstract models for the mesozoic growth of the tibetan plateau describe closure of the bangong ocean resulting in accretion of the lhasa terrane to the qiangtang terrane along the bangong nuijiang suture zone bnsz however a more complex history is suggested by studies of ophiolitic melanges south of the bnsz within the lhasa terrane one such melange belt is the shiquanhe namu co melange zone snmz that is coincident with the geren co namu co thrust gnt to better understand the structure age and provenance of rocks exposed along the snmz we conducted geologic mapping sandstone petrography and u pb zircon geochronology of rocks straddling the snmz the gnt is north directed and places paleozoic strata against the yongzhu ophiolite and cretaceous strata along strike a gabbro in the yongzhu ophiolite yielded a u pb zircon age of 153 ma detrital zircon age data from permian rocks in the hanging wall suggests that the lhasa terrane has affinity with the himalaya and qiangtang rather than northwest australia the late jurassic early cretaceous rila formation overlies the ophiolite and shows signatures of syntectonic deposition detrital zircon ages from the cretaceous duoni formation in the footwall of the gnt suggest that the provenance is mainly early cretaceous igneous rocks in the northern lhasa terrane and paleozoic strata in the hanging wall of the gnt our results support the interpretation that the snmz is a late jurassic early cretaceous suture zone that resulted from closure of a small jurassic ocean basin thereby suggesting more jurassic shortening than conventional models the gnt developed along the snmz in the early cretaceous and facilitated exhumation of paleozoic strata which became a source for cretaceous basins along the gnt early cretaceous igneous rocks in northern lhasa are interpreted to be a consequence of subduction of ocean lithosphere along the snmz and its later delamination

radiolarian age constraints on mesotethyan ocean evolution and their implications for development of the bangong nujiang suture tibet radiolarian discoveries indicate that deep marine conditions prevailed in central tibet from the early middle jurassic until well into the early cretaceous late hauterivian early aptian 131 121 ma and help to constrain the temporal extent of oceanic conditions along the bangong nujiang suture these new fossils occur in chert or siliceous mudstone blocks associated with the lagkor tso ophiolitic melange basin inversion associated with closure of mesotethys between the qiangtang and lhasa terranes was accompanied by melange formation and predated regionally widespread deposition of overlying shallow marine late aptian albian orbitolinid limestones

metamorphic rocks in central tibet lateral variations and implications for crustal structure insights about lateral variations in the age composition and structure of the central tibetan crust are provided by geologic investigations of metamorphic rocks in the qiangtang terrane previous studies have shown that a tectonic melange of triassic age with blueschist and eclogite bearing blocks within a greenschist facies matrix is exposed over an e w distance of 600 km in the central qiangtang terrane new mapping shows that the melange extends over a n s distance of 150 km nearly to the trace of the early mesozoic jinsha suture in the north the melange exposed structurally beneath upper paleozoic to mesozoic strata in the footwalls of early mesozoic normal faults is composed mostly of paleozoic metasedimentary and crystalline rocks these findings support the hypothesis that a large part of the central and northern qiangtang terrane crust is composed of supracrustal rocks the duguer range

lhasa terrane in southern tibet came from australia the u pb age and hf isotope data on detrital zircons from paleozoic metasedimentary rocks in the lhasa terrane tibet defi ne a distinctive age population of ca 1170 ma with e hf t values identical to the coeval detrital zircons from western australia but those from the western qiangtang and tethyan himalaya terranes defi ne an age population of ca 950 ma with a similar e hf t range the ca 1170 ma detrital zircons in the lhasa terrane were most likely derived from the albany fraser belt in southwest australia whereas the ca 950 ma detrital zircons from both the western qiangtang and tethyan himalaya terranes might have been sourced from the high himalaya to the south such detrital zircon connections enable us to propose that the lhasa terrane is exotic to the tibetan plateau system and should no longer be considered as part of the qiangtang greater india tethyan himalaya continental margin system in the paleozoic reconstruction of the indian plate as current models show rather it should be placed at the northwestern margin of australia these results provide new constraints on the paleogeographic reconstruction and tectonic evolution of southern tibet and indicate that the lhasa terrane evolved as part of the late precambrian early paleozoic evolution as part of australia in a different paleogeographical setting than that of the qiangtang greater india tethyan himalaya system

provenance and tectonic setting of upper triassic turbidites in the eastern tethyan himalaya implications for early stage evolution of the neo tethys abstract upper triassic turbidites in the eastern tethyan himalaya preserve archives for early stage evolution of the neo tethys however their provenance and tectonic setting remain controversial here we report petrographic detrital zircon u pb and trace element data of newly discovered sandstones interbedded with layered basalts and whole rock geochemical data of basalts from the yumen melange within upper triassic turbidites and the same types of published data from associated terranes in the eastern gondwana are compiled for comparison the interbedded sandstones are dominated by quartz lithics fragments and plagioclase grains resembling those of the langjiexue group all samples display similar ranges of detrital zircon u pb ages with significant age populations at 280 210 ma 650 450 ma and 1200 800 ma comparable to those of the langjiexue group and coeval turbidites from western myanmar and partly from central tethyan himalaya implying they should share identical provenance trace elements of 300 200 ma aged zircons indicate they were derived from a continental magmatic arc geochemical data from interbedded basalts display typical within plate basalt affinity synthesized with regional geological setting the above observations suggest upper triassic turbidites from eastern and part of central tethyan himalaya and western myanmar were deposited in a marine rift basin of indian passive continental margin with their primary provenance from lhasa however other contemporaneous turbidites from part of central tethyan himalaya and australia had distinct primary sources from indian continent and west papua respectively indicating they may have separate deposystems

u pb zircon geochronology of major lithologic units in the eastern himalaya implications for the origin and assembly of himalayan rocks models for the origin and deformation of himalayan rocks are dependent upon geometric and age relationships between major units we present field mapping and u pb dating of igneous and detrital zircons that establish the lithostratigraphic architecture of the eastern himalaya revealing that 1 the south tibet detachment along the bhutan china border is a top to the north ductile shear zone 2 late triassic and early cretaceous sedimentary samples from the northern indian margin show a similar age range of detrital zircons from ca 3500 ma to ca 200 ma but the late triassic rocks are distinguished by a significant age cluster between ca 280 and ca 220 ma and a well defined age peak at ca 570 ma 3 an augen gneiss in the south tibet detachment shear zone in southeast tibet has a cambrian ordovician crystallization age 4 main central thrust hanging wall paragneiss and footwall quartzites from the far western arunachal himalaya share similar provenance and late proterozoic maximum depositional ages and 5 main central thrust footwall metagraywacke from the central western arunachal himalaya has a paleoproterozoic maximum depositional age indicated by a single prominent age peak of ca 1780 ma recent work in the eastern himalaya demonstrates that in the early middle miocene the himalayan crystalline core here was emplaced southward between two subhorizontal shear zones that merge to the south a proposed subsequent middle miocene brittle low angle normal fault accomplishing exhumation of these rocks along the range crest can be precluded because new and existing mapping demonstrates only a ductile shear zone here the ca 280 220 ma detrital zircons of the late triassic strata are derived from an arc developed along the northern margin of the lhasa terrane detritus from this arc was deposited on the northern margin of india during india lhasa rifting along strike heterogeneity in main central thrust footwall chronostratigraphy is indicated by detrital zircon age spectrum differences from central western to far western arunachal nonetheless the late proterozoic rocks in the main central thrust hanging wall and footwall in far western arunachal can be correlated to each other and to previously analyzed rocks in the south tibet detachment hanging wall to the west and in the indian craton to the south these findings are synthesized in a reconstruction showing late triassic india lhasa rifting and cenozoic eastern himalayan construction via in situ thrusting of basement and cover sequences along the north indian margin

eustatic and tectonic control on the evolution of the jurassic north qiangtang basin northern tibet china impact on the petroleum system abstract the qiangtang basin is the largest mesozoic marine hydrocarbon bearing basin in china investigating its tectonic and sedimentary evolution is therefore relevant for hydrocarbon exploration here we present a detailed stratigraphic and sedimentological analysis of five jurassic stratigraphic sections in the north qiangtang basin nineteen facies associations were identified and allowed us to reconstruct changes of water depth the north qiangtang basin experienced two transgressive regressive cycles during the jurassic the first cycle started in the mid jurassic bajocian bathonian with braided river and floodplain sediments passing up section to tidal flat and eventually inner to mid ramp deposits tidal flat deposition resumed during the callovian the second cycle began in the late jurassic oxfordian with transgressive inner to mid ramp sediments followed by a shallowing upward trend during the late oxfordian leading again to tidal flat sedimentation in the kimmeridgian tithonian petrographic analysis and detrital zircon geochronology indicate provenance constantly from the hoh xil songpan ganzi terrane kunlun arc and central qiangtang because jurassic magmatic activity and active syn depositional faults have not been reported and steady detrital supply indicates a stable tectonic environment eustatic change is inferred to represent the principal control on sedimentary evolution in the north qiangtang basin during the jurassic a comparison with the amu darya basin in central asia characterized by similar sedimentological conditions and distribution of reservoir and source rocks emphasizes the importance of eustatic control for the development of the source reservoir seal system of the north qiangtang basin

provenance of late triassic sediments in central lhasa terrane tibet and its implication abstract in southern tibet late triassic sequences are especially important to understanding the assembly of the lhasa terrane prior to indo asian collision we report new data relevant to the provenance of a late triassic clastic sequence from the mailonggang formation in the central lhasa terrane tibet petrographic studies and detrital heavy mineral assemblages indicate a proximal orogenic provenance including volcanic sedimentary and some ultramafic and metamorphic rocks in situ detrital zircon hf and u pb isotope data are consistent with derivation of these rocks from nearby triassic magmatic rocks and basement that comprise part of the newly recognized late permian triassic sumdo cuoqen orogenic belt the new data suggests correlation with the upper triassic langjiexue group which lies on the opposing southern side of indus yarlung ophiolite sediments from both the mailonggang formation and langjiexue group are interpreted to represent formerly contiguous parts of a sequence deposited on the southern flanks of the sumdo cuoqen belt

facies characteristics of a proglacial turbiditic sand lobe at svalbard abstract a shallow marine turbiditic lobe inferred to be associated with an ice contact delta was deposited during the last glaciation weichselian at svalbard the norwegian arctic the postglacially raised sandy lobe is exposed in a 200 m long river section and exhibits six units that reflect an early phase of glaciomarine mud deposition followed by three episodes dominated by deposition from sandy turbidity currents the lobe is characterized by a complex pattern of turbidite filled erosional features from thin scour fills reflecting a highly mobile flow pattern to stacked chute fill deposits showing confined flow the sandy turbidites show rhythmic deposition of massive to graded laminae deposited from suspension alternating with faint laminae or ripple cross lamination deposited from tractional transport semi continuous turbidity currents are thought to have been generated by retrogressive slumping of ice proximal well sorted sandy deposits on the delta front and slope the lateral and vertical changes of lithofacies within the sandy lobe are mainly attributed to normal mechanisms of turbiditic lobe growth with only minor effect from synsedimentary glacier terminus movement and minor changes in the meltwater outflow position this detailed mapping of facies changes allows a fairly accurate reconstruction of the axis of the former discharge outlet

xainxa ultramafic rocks central tibet china tectonic environment and geodynamic significance the xainxa ultramafic rocks 200 km south of the donqiao ophiolite bangong nu jiang suture zone central tibet form a 1 km thick sheet in tectonic contact with paleozoic sediments to the south and with mesozoic sediments to the north through roughly east west south dipping reverse faults the ultramafic rocks consist of depleted harzburgites with minor dunite patches and 1 m sized deposits of podiform chromite mafic rocks represented by a few isotropic gabbros and dolerites occur in only minor amounts these rocks which may have underlined a suture zone within the lhasa block have lithologies and tectonics very similar to those occurring in the donqiao area along the bangong nu jiang suture zone and therefore can be considered as a klippe of this ophiolite the xainxa ultramafic rocks were emplaced in early cretaceous time the main tectonic event observed in that area northward folding and overthrusting occurred after late early cretaceous time

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

crustal seismic reflection measurements across the northern extension of the taupo volcanic zone north island new zealand abstract a crustal seismic reflection profile across the offshore taupo volcanic zone otvz has been reprocessed to improve the lower crust upper mantle image the otvz is underlain by several strong sub horizontal crustal reflections that may correspond to shear zones or mafic sills a reflector interpreted to correspond to moho lies at a depth of about 8 8 5 s twt seconds two way traveltime about 20 km under the western margin of the raukumara peninsula and rises to a depth of about 6 s twt about 16 km under the active eastern part of the otvz and can be traced at about this depth across most of the otvz underlying this inferred moho are two diffuse zones of discontinuous reflectivity one underlying the active eastern otvz and a less well defined one underlying the landward extension of the havre trough they extend from about 15 to 35 km depth within the mantle wedge and are inferred to be imaging a zone of depleted upper mantle associated with crustal extension a localised zone of deep 30 50 km reflectivity and seismicity occurs under the eastern otvz margin and may relate to fluid flow from the hikurangi plateau being subducted further east

the andean type gangdese mountains paleoelevation record from the paleocene eocene linzhou basin abstract paleoelevation reconstruction using oxygen isotopes is making a significant contribution to understanding the cenozoic uplift of the himalayas and the tibetan plateau this paper presents new oxygen and carbon isotopic compositions from well dated tertiary paleosols lacustrine calcareous carbonates and marls from the nianbo 60 54 ma and upper pana formations 51 48 ma of the linzizong group in the linzhou penbo basin the sediments of the nianbo formation which are 180 m thick were deposited in alluvial fans braided rivers fan deltas and on nearshore to offshore lacustrine settings whereas those of the upper pana formation are 100 m thick and are comprised predominantly of proximal alluvial fan and braided river deposits correlations between the lithofacies and stable isotopic compositions suggest that the basin was mainly a hydrologically open environment it is confirmed that the 18 o c and 13 c c values from nianbo and pana formations have not yet been reset by late stage diagenesis based on petrographic examination oxygen isotope of the fossil ostracodes and tectonic deformation of strata the paleoelevations are reconstructed using the corrected most negative paleosurface water 18 o psw values these imply that the linzhou area had attained an elevation of 4500 400 m during the period of the indo asian collision i e achieved a near present elevation and may form an andean type mountain range stretching the gangdese arc before collision the gangdese mountains probably maintained high elevations since at least the paleocene and could play a crucial role in the climate change in the interior of the tibetan plateau during the early cenozoic the paleogeomorphic scenario of the eocene tibet is proposed to exist at two high mountains in excess of 4500 m that sandwiched a low elevation basin

evolution of the bangong nujiang tethyan ocean insights from the geochronology and geochemistry of mafic rocks within ophiolites abstract the bangong nujiang suture zone bnsz is located in the central tibetan plateau and represents the relict of the bangong nujiang tethyan ocean bnto this paper presents zircon u pb ages and whole rock geochemical and sr nd isotope data for the ophiolitic rocks from the rutog dongco dongqiao amdo and dengqen areas from west to east in the bnsz zircon u pb ages obtained from five gabbros and one leucogabbro from the five areas are 169 2 167 2 187 2 184 2 177 3 and 164 2 ma respectively mafic rocks gabbros and basalts in the bnsz can be divided into morb like and oib type based on geochemical data the morb like rocks are tholeiitic and can be further divided into n and e morb subtypes marked by depleted and flat ree patterns respectively all morb like rocks show supra subduction zone ssz geochemical affinities such as mild enrichment in the large ion lithophile elements and depletion in the high field strength elements coupled with high and positive whole rock e nd t values 8 1 to 5 4 the oib type rocks which formed in the early cretaceous 132 108 ma are exposed in the dongco dongqiao and tarenben areas and they consist mostly of alkali basalts with within plate geochemical characteristics and positive e nd t values 5 9 to 4 5 on the basis of these new data combined with information from the literature we further support that the bnto was an intra oceanic arc backarc basin that existed between the north lhasa subterrane nlt and the south qiangtang subterrane sqt during the early middle jurassic this basin may have been active until the early cretaceous when the oib type basalts erupted

the subduction accretion history of the bangong nujiang ocean constraints from provenance and geochronology of the mesozoic strata near gaize central tibet abstract the mesozoic strata within the bangong nujiang suture zone in central tibet recorded critical information about the subduction accretion processes of the bangong nujiang ocean prior to the lhasa qiangtang collision this paper reports detailed field observations petrographic descriptions sandstone detrital zircon u pb ages and hf isotopic analyses from an accretionary complex preserved as mugagangri group and the unconformably overlying shamuluo formation near gaize the youngest detrital zircon ages together with other age constraints from literature suggest that the mugagangri group was deposited during late triassic early jurassic while the shamuluo formation was deposited during late jurassic early cretaceous based on the differences in lithology age and provenance the mugagangri group is subdivided into the upper middle and lower subunits these units are younging structurally downward southward consistent with models of progressive off scrapping and accretion in a southward facing subduction complex the upper subunit comprising mainly quartz sandstone and siliceous mud shale was deposited in abyssal plain environment close to the qiangtang passive margin during late triassic with sediments derived from the southern qiangtang block the middle and lower subunits comprise mainly lithic quartz sandstone and mud shale containing abundant ultramafic ophiolitic fragments the middle subunit of late triassic early jurassic age records a transition in tectono depositional setting from abyssal plain to trench wedge basin with sudden influx of sediments sourced from the central qiangtang metamorphic belt and northern qiangtang magmatic belt the appearance of ultramafic ophiolitic fragments in the middle subunit reflects the subduction initiation the lower subunit was deposited in a trench wedge basin during early jurassic with influx of jurassic aged zircons originating from the newly active southern qiangtang magmatic arc the lower subunit records the onset of arc magmatism related to the northward subduction of the bangong nujiang ocean the shamuluo formation comprising mainly lithic feldspar sandstone with limestone interlayers was deposited in a post collisional residual sea or pre collisional trench slope basin with sediments derived entirely from the qiangtang block

late cretaceous to middle tertiary basin evolution in the central tibetan plateau changing environments in response to tectonic partitioning aridification and regional elevation gain located in the bangong suture zone between the lhasa and qiangtang terranes of central tibet the nima basin records cretaceous late miocene sediment accumulation during a period of drastically changing paleogeography and paleoelevation the bangong suture zone originally formed during late jurassic early cretaceous time as the lhasa terrane collided with the qiang tang terrane during early to middle cretaceous time the region lay at northern near equatorial paleolatitudes near sea level by aptian time the nima basin was above sea level and was strongly influenced by nearby volcanic activity and crustal shortening in the reactivated bangong suture zone in the southern nima basin an 50 m y late cretaceous eocene depositional hiatus correlates with major crustal shortening and ensuing voluminous ignimbrite eruptions in the lhasa terrane in the northern nima basin deposition continued during latest cretaceous time recording arid paleoclimate in evaporitic lacustrine and eolian dunefield deposits by oligocene time the nima basin comprised two independent depocenters that accumulated coarse grained alluvial fluvial lacustrine evaporitic and fan delta deposits in close association with reactivated thrusts in the bangong suture zone carbon and oxygen isotope data from oligocene paleosol carbonate reported elsewhere indicate that regional paleoelevation during the late oligocene was 4 6 km as it is today overall the depositional record of the nima basin combined with ongoing structural and geochronologic studies demonstrates that the bangong suture zone was reactivated during middle cretaceous and middle tertiary time that the intervening 50 m y interval was a time of regional upper crustal shortening in the lhasa terrane followed by regional ignimbrite eruptions and that arid paleoclimate and high paleoelevation were established by the late cretaceous and late oligocene respectively within the context of other data sets from the lhasa terrane the record of deposition in the nima basin is consistent with low angle subduction of neotethyan oceanic lithosphere and reactivation of the bangong suture zone during the early cretaceous followed by shortening within the gangdese retroarc and northern lhasa terrane thrust belts during the late cretaceous lithospheric delamination dripping and regional magmatic flare up during latest cretaceous through early tertiary time and underthrusting of indian lower crust and lithosphere as far north as the bangong suture zone during late oligocene time

nyainqentanglha shan a window into the tectonic thermal and geochemical evolution of the lhasa block southern tibet 1 in the nyainqentanglha nqtl massif southern tibet a late cenozoic se dipping normal fault exhumed an oblique section of crust in its footwall u th pb dating of zircon and monazite from footwall exposures reveals a collage of felsic intrusions including cretaceous early tertiary and miocene granitoids ages of the latter span 10 m y suggesting semicontinuous or episodic miocene magmatism geochemical and isotopic analyses show a gangdese arc affinity indicating significant mantle heat and mass transfer in their formation and semicontinuous calc alkaline magmatism throughout the cenozoic indo asian collision the undeformed nature of the footwall cretaceous and miocene granitoids suggests that mesozoic cenozoic lhasa block deformation was thin skinned being concentrated in supracrustal assemblages this coupled with the lack of migmatites exposed in the nqtl implies the exposed crust was not a partial melt zone nor involved in large scale channel flow some 40ar 39ar thermochronologic studies of footwall k feldspars reveal that samples collected within several kilometers below the normal fault cooled prior to emplacement of young leucogranites indicating little perturbation of the background thermal structure since 15 ma this plus high melting temperatures and the lack of penetrative granitoid deformation requires that the melts formed at lower crustal levels and were emplaced rapidly to the midcrust seismic reflection results showing high bright spot anomalies in the midcrust along the nqtl rift may have imaged the youngest magmatic episode or its associated hydrothermal system

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

tectonic environment and geodynamic significance of the neo cimmerian donqiao ophiolite bangong nujiang suture zone tibet the bangong nujiang suture zone is underlain by a discontinuous and roughly ew ophiolite belt around donqiao gyanco ophiolitic and metasedimentary palaeozoic series form nappes thrusted over jurassic flysch series all these series have undergone a late symmetrical folding accompanied by generally northward reverse faulting although strongly dismembered a complete ophiolitic sequence comprising harzburgites with large podiform type chromite deposits wehrlites cumulate gabbros sheeted dolerite dykes foliated garnet bearing amphibolites and pillow lavas has been found

cretaceous tertiary structural evolution of the north central lhasa terrane tibet 1 in the north central lhasa terrane of tibet two distinct structural levels of an east west striking thrust system are exposed along the north trending late cenozoic xiagangjiang rift upper paleozoic strata deformed by the south directed langgadong la thrust and cretaceous strata involved in variably north and south directed thrusting characterize these lower and upper structural levels respectively these two structural levels are separated by the tagua ri passive roof thrust balanced cross section restoration suggests that the thrust system accommodated 103 km 53 shortening the 40ar 39ar results together with an interpretation of synthrust deposition of upper cretaceous strata suggest that the majority of shortening occurred during the late cretaceous paleocene cretaceous strata lie unconformable on permian rocks volcanic tuffs directly above the unconformity yield u pb zircon ages of 131 ma upper cretaceous strata record a change from shallow marine to nonmarine deposition indicating uplift above sea level during this time the overall south directed vergence of the thrust belt is consistent with substantial crustal thickening in central tibet by large scale northward underthrusting of lhasa terrane basement beneath the qiantang terrane prior to the indo asian collision the documented decoupling of contractional deformation at shallow crustal levels appears to be a regional characteristic of tibet from at least the bangong suture in the north to the tethyan himalaya to the south this style of deformation explains the absence of basement exposures and major denudation in this region despite substantial crustal shortening

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

the india asia collision in north pakistan insight from the u pb detrital zircon provenance of cenozoic foreland basin the northernmost exposures of sub himalayan cenozoic strata in the hazara kashmir syntaxial region of north pakistan comprises the paleocene eocene marine strata in the lower part and oligocene miocene nonmarine strata in the upper part this study provides the detrital zircon u pb geochronology of the cenozoic strata in this area the strong resemblance of u pb age spectra of paleocene hangu lockhart and patala formations with those of himalayan strata indicate an indian plate provenance the first appearance of 100 ma detrital zircon u pb ages within the lower most part of the early eocene margalla hill limestone indicates a shift from an indian to asian provenance geologic mapping shows the existence of a disconformity between the lower and upper most part of the patala formation which is interpreted to have been formed by the migration of a flexural forebulge through this region we consider the upper most part of the patala formation to have been deposited within the distal foredeep of the foreland basin the indian to asian provenance shift and the presence of a possible foreland basin forebulge provide strong evidence that india asia collision was underway in northern pakistan at ca 56 55 ma

structural sequence and geochronology of the qomo ri accretionary complex central qiangtang tibet implications for the late triassic subduction of the paleo tethys ocean abstract the late paleozoic qomo ri accretionary complex qrac in the central qiangtang region of tibet lies at the southern margin of the longmu co shuanghu suture qrac is tectonically linked with the nierong and jitang metamorphic complexes in eastern tibet and the yunling and lancang complexes in western yunnan province sw china this metamorphic complex is mainly composed of late paleozoic passive continental marginal strata of the south qiangtang basin which was underthrust beneath north qiangtang in the late triassic at least three stages of deformation d1 d2 and d3 with two corresponding metamorphic events m1 and m2 were identified within the qrac d1 is characterized by a penetrative foliation s1 bearing a dextral shear and a corresponding m1 mineral assemblage of phengite garnet and quartz this deformation was interpreted to be the result of the northwestward oblique subduction of the paleo tethys ocean and dated at 211 219 ma by phengite 40ar 39ar ages d2 a subsequent compression deformation followed d1 immediately and exhibits flexural folds and pervasive axial crenulation in foliation s2 deformations from d1 to d2 indicate that an oblique oceanic subduction was immediately followed by a collision between the north and south qiangtang blocks along the longmu co shuanghu suture the d3 recorded by minor folds and divisional axial cleavage s3 is related to a giant arc structure with an extension of 50 km

tectonic evolution of the early mesozoic blueschist bearing qiangtang metamorphic belt central tibet this is the published version copyright 2003 american geophysical union all rights reserved

mesozoic and cenozoic tectonic evolution of the shiquanhe area of western tibet 1 in the shiquanhe area of far western tibet mid cretaceous strata lie unconformable on ophiolitic melange and jurassic flysch associated with the bangong nujiang suture zone on the basis of our mapping and geochronologic studies we suggest that these cretaceous strata were shortened by 57 over a north south distance of 50 km during late cretaceous early tertiary time the late cretaceous narangjiapo thrust placed permian strata 20 km over ophiolitic melange and cretaceous strata north of the narangjiapo thrust 40 km of shortening was accommodated by the late cretaceous early tertiary south directed jaggang thrust system that involves jurassic flysch and cretaceous strata and roots into a decollement within ophiolitic melange the most recent shortening was accommodated to the south of the narangjiapo thrust along the north dipping shiquanhe thrust the shiquanhe thrust cuts flat lying 22 6 0 3 ma volcanic rocks and underlying folded tertiary nonmarine strata in its footwall and was likely active during slip along the oligocene gangdese thrust system of southern tibet ophiolitic melange and structurally overlying jurassic flysch near shiquanhe are interpreted to represent remnants of a subduction accretion complex and forearc basin respectively that were obducted southward onto the margin of the lhasa terrane during late jurassic early cretaceous closure of the bangong nujiang ocean subsequent imbrication of the obducted sheet could have produced the two east west trending belts of ophiolitic melanges separated by 100 km in western tibet late cretaceous early tertiary thin skinned shortening may have been accommodated in the deeper crust by northward underthrusting and duplexing of lhasa terrane rocks beneath the obducted ophiolitic melange and the qiangtang terrane to the north

tectonic evolution of the qinghai tibet plateau abstract the qinghai tibet plateau composed of several continental slivers within the eastern tethyan domain is one of the pivotal sites to examine to better understand the theory of plate tectonics and the orogenic evolution on earth this plateau is generally inferred to be a collage of several continental blocks that rifted from gondwanaland and subsequently accreted to the asian continent however recent recognition of over twenty ophiolite melange zones and their associated island arcs indicates that the traditional model of tectonic evolution requires revision based on 177 recently finished 1 250 000 scale geological maps and related studies we summarize the main tectonic context of the qinghai tibet plateau and propose a new integrated model to account for the new findings the complex orogen of the immense qinghai tibet plateau consisting of multiple island arc basin systems that developed at different stages while surrounded by the north china yangtze tarim and indian plates is emphasized the entire orogen surrounded by suture zones that mark the locations of oceanic closure is investigated by examining i the first order tectonic units and ophiolitic melanges including arc arc continent collision zones and ii their internally enclosed blocks as the second order tectonic units therefore the qinghai tibet plateau is divided into three major orogenic systems namely from northeast to southwest the early paleozoic qinling qilianshan kunlunshan qin qi kun the late paleozoic triassic qiangtang sanjiang and the late paleozoic to cenozoic gangdese himalaya orogenic systems which are separated by the kangxiwa muzitagh maqin mianxian and the bangong shuanghu changning menglian sutures respectively we propose that the formation and evolution of the qinghai tibet plateau to have been intrinsically related to those of the eastern tethys recorded by the longmu co shuanghu ophiolite melange zone the southern qiangtang paleozoic accretionary arc basin system the bangong nujiang suture zone and their associated composite island arc basin systems the present day bangong shuanghu changning menglian suture system marks the final closure of the tethyan ocean the yarlung zangbo ocean opened as a back arc basin in response to the southward subduction of the tethyan ocean lithosphere in the middle triassic and closed as a result of the india asia collision at the end of cretaceous followed by the northward indention of the indian plate that resulted in significant intra continental deformation and plateau uplift in the cenozoic

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

zircon u pb and hf isotope constraints on the mesozoic tectonics and crustal evolution of southern tibet the first in situ hf and u pb isotope analyses of zircon separates from mesozoic granites in southern tibet identify a significant previously unknown stage of magmatism igneous zircons n 34 from a granite within the gangdese batholith show a weighted mean 206 pb 238 u age of 188 1 1 4 ma and e hf t the parts in 10 4 deviation of initial hf isotope ratios between the zircon sample and the chondritic reservoir values between 10 4 and 16 8 suggesting predominantly early jurassic intrusive activity with a juvenile mantle contribution of 40 inherited zircons from two cretaceous s type granites in the northern magmatic belt 23 delineate a slightly older 206 pb 238 u age cluster between 188 and 210 ma these zircons have e hf t values from 3 9 to 13 7 yielding crustal hf model ages from ca 1 4 to 2 1 ga suggesting a major episode of crustal growth in proterozoic time and remelting of this crust in early jurassic time combining these with literature data we interpret the jurassic gangdese magmatism as an early product of the neo tethyan subduction that played a long lasting role in the tectonic evolution of southern tibet prior to the india asia collision

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

triassic continental subduction in central tibet and mediterranean style closure of the paleo tethys ocean the qiangtang metamorphic belt qmb in central tibet is one of the largest and most recently documented high pressure hp to near ultrahigh pressure near uhp belts on earth lu hf ages of eclogite and blueschist facies rocks within the qmb are 244 223 ma indistinguishable from the age of uhp metamorphism in the qinling dabie orogen results of a u pb detrital zircon study suggest that protoliths of the qmb include upper paleozoic qiangtang continental margin strata and sandstones that were derived from a paleozoic arc terrane that developed within the paleo tethys ocean to the north we attribute qmb hp metamorphism to continental collision between the qiangtang terrane and a paleo tethys arc terrane this collision and the coeval south china north china collision may have slowed convergence between laurasia and gondwana derived terranes and initiated mediterranean style rollback and backarc basin development within much of the remnant paleo tethys ocean realm

provenance analysis of the mesozoic hoh xil songpan ganzi turbidites in northern tibet implications for the tectonic evolution of the eastern paleo tethys ocean 1 mesozoic strata of the hoh xil songpan ganzi complex in northern tibet are exposed in a vast 370 000 km2 triangle shaped orogenic belt bound by the longmen shan thrust belt in the east the kunlun terrane and north china block in the north and the qiangtang terrane and yidun arc in the south these strata consist of middle upper triassic submarine fan and deep marine facies rocks that were deposited in the paleo tethys ocean late triassic early jurassic contractional deformation in the eastern hoh xil songpan ganzi complex marks the demise of the paleo tethys ocean basin and the accretion of the gondwana derived qiangtang terrane to eurasia we conducted geological mapping regional stratigraphic analyses and u pb geochronology of detrital zircons n 4128 on the mesozoic sequences exposed in the hoh xil songpan ganzi complex kunlun terrane and qiangtang terrane we identify for the first time marine silciclastic sandstone and shale of jurassic age in the northwestern hoh xil songpan ganzi complex that unconformably overlie upper triassic turbidites zircon age data indicate that the middle upper triassic marine gravity flow deposits of the hoh xil songpan ganzi complex were shed from the north and south china blocks and middle late triassic ultrahigh pressure qinling dabie orogenic belt as well as the kunlun and qiangtang terranes in addition the detrital zircon results suggest vast sediment source to sink distances 1500 km for the middle upper triassic hoh xil songpan ganzi strata which is consistent with tectonic models for the paleo tethys ocean basin that incorporate significant components of horizontal tectonic transport like opening of large back arc basins in response to oceanic slab rollback

petrogenesis of early to middle jurassic granitoid rocks from the gangdese belt southern tibet implications for early history of the neo tethys abstract the gangdese belt tibet records the opening and closure of the neo tethyan ocean and the resultant collision between the indian and eurasian plates mesozoic magmatic rocks generated through subduction of the tethyan oceanic slab constitute the main component of the gangdese belt and play a crucial role in understanding the formation and evolution of the neo tethyan tectonic realm u pb and lu hf isotopic data for tonalite and granodiorite from the xietongmen nymo segment of the gangdese belt indicate a significant pulse of jurassic magmatism from 184 ma to 168 ma the magmatic rocks belong to metaluminous medium k calc alkaline series characterized by regular variation in major element compositions with sio2 of 61 35 73 59 low to moderate mgo 0 31 2 59 with mg of 37 45 these magmatic rocks are also characterized by lree enrichment with concave upward trend in mree on the chondrite normalized ree patterns and also lile enrichment and depletion in nb ta and ti in the primitive mantle normalized spidergrams these rocks have high zircon ehf t values of 10 94 to 15 91 and young two stage depleted mantle model ages tdm2 of 192 ma to 670 ma the low mgo contents and relatively depleted hf isotope compositions suggest that the granitoid rocks were derived from the partial melting of the juvenile basaltic lower crust with minor mantle materials injected in combination with the published data it is suggested that northward subduction of the neo tethyan slab beneath the lhasa terrane began by the late triassic which formed a major belt of arc related magmatism

birth and demise of the bangong nujiang tethyan ocean a review from the gerze area of central tibet abstract reconstructing the evolution of the bangong nujiang tethyan ocean bnto plays an indispensable role in understanding the linkage between gondwana dispersion and tibetan plateau accretion the existence of the bnto is inferred from presence of dismembered ophiolite bodies along the bangong nujiang suture zone which separates lhasa from qiangtang in this paper we present a detailed synthesis of ophiolite metamorphism magmatism lithostratigraphy and tectonism from the western segment gerze area of the lhasa qiangtang collision zone in order to reconstruct the wilson cycle of the bnto our results suggest that the bnto initially opened in the late permian 260 ma as indicated by the earliest record of oceanic crust regionally strong evidence supports a northward subduction of the bnto beneath qiangtang likely initiating in the early jurassic 195 ma as suggested by detrital zircon age spectra from mesozoic arc sourced strata northward oceanic subduction generated three episodes of arc magmatism i e 186 170 ma 169 146 ma and 129 100 ma along the southern qiangtang margin in which the early jurassic magmatism is only sporadically preserved in modern outcrops amdo area meanwhile the recognition of an incomplete arc trench system on the northern lhasa margin supports a southward subduction beneath lhasa as well our new provenance data from lower cretaceous strata on the northern lhasa margin together with available multi disciplinary constraints suggest that final closure of the bnto did not occur until the latest jurassic early cretaceous 150 130 ma therefore the wilson cycle of the bnto is hereby reconstructed providing significant insights into the amalgamation of the tibetan plateau

the late triassic rift related volcanic rocks from eastern qiangtang northern tibet china age and tectonic implications abstract the late triassic nadi kangri volcanic rocks with nearly ew trending outcrops within the qiangtang basin northern xizang tibet china are composed mainly of acid tuff dacite rhyolite and minor basic volcanic rocks there exists a significant depositional hiatus between the nadi kangri volcanic rocks and underlying strata therefore the nadi kangri volcanic rocks represent a new evolution history of the mesozoic qiangtang basin the magma emplacement age of the nadi kangri volcanic rocks in the geladaindong area is 220 4 2 3 ma representing the onset of the mesozoic qiangtang basin the nadi kangri basalts have high nb zr 0 049 0 058 ta hf 0 12 0 15 and zr y 4 95 6 01 ratios in the tectonic discrimination diagrams such as zr vs zr y and th hf vs ta hf the nadi kangri basaltic rocks mostly plot in the within plate setting field the geological background and the geochemical characteristics suggest that the nadi kangri volcanic rocks were formed in a continental rift setting

provenance analysis of cretaceous peripheral foreland basin in central tibet implications to precise timing on the initial lhasa qiangtang collision abstract mesozoic strata along northern margin of the lhasa terrane near dingqing tibet provide a semi continuous record of the bangong nujiang ocean closure and the subsequent lhasa qiangtang collision in this study we present results of sandstone petrographic detrital zircon u pb and cr spinel geochemical data to determine the provenance of the mesozoic strata from the triassic quehala group and the mid jurassic to lower cretaceous xihu lagongtang and duoni formations which are young from 220 ma to 100 ma in this region thereby allowing for interpretation of their tectonic setting the similar 1200 900 age cluster from the lower xihu formation to that of the triassic quehala group and the distinct age peaks at 200 ma and 146 ma from the upper xihu formation suggest a lhasa terrane provenance to the south distinctive age clusters of 300 210 ma and 1800 ma and cr spinel composition analysis of the lagongtang formation indicate a provenance shift from the lhasa terrane to the qiangtang terrane and the bangong nujiang suture zone to the north the lagongtang formation was deposited in a bathyal abyssal to shallow slope environment from north to south that we interpret as the foredeep depozone and distal foredeep of a peripheral foreland basin system that developed due to flexural subsidence related to the lhasa qiangtang collision and terrane accretion the age of the lhasa qiangtang collision is constrained by a 140 ma tuffite u pb age at the base of the foreland basin strata an angular unconformity between the xihu and lagongtang formation which we interpret as the result of the lhasa qiangtang collision our results indicate that the lhasa qiangtang collision initiated around 140 ma in the dingqing region simultaneously with previous determinations 1200 km to the west near gaize area therefore we prefer quasi simultaneous onset of collision along strike to zippering collision models wherein the east collision age is older than the west

blueschist bearing metamorphic core complexes in the qiangtang block reveal deep crustal structure of northern tibet a 500 km long belt of metamorphic exposures in the qiangtang block provides an opportunity to study the internal structure of northern tibetan crust metamorphic rocks exposed at two widely separated areas along this belt consist of blueschist bearing melange and are bounded by late triassic early jurassic domal low angle normal faults we propose that this melange was underplated to the qiangtang block and was subsequently exhumed by detachment faulting both the underplating and the exhumation occurred during early mesozoic southward subduction of oceanic lithosphere along the jinsha suture this model predicts that the deeper crust of much of northern tibet consists of accretionary melange in contrast to the continental crystalline crust of southern tibet and may account for north south variations of cenozoic tectonism in tibet

evolutionary model of the himalaya tibet system geopoem based on new modelling geological and geophysical data a two dimensional thermo mechanical laboratory modelling of continental subduction was performed the subducting continental lithosphere includes a strong brittle upper crust a weak ductile lower crust and a strong upper mantle the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is denser than the asthenosphere the lithospheric layers are composed of material whose strength is sensitive to and inversely proportional to temperature throughout the experiment the model surface was maintained under relatively low temperature and the model base at higher temperature the subduction rate satisfied the peclet criterion modelling confirms that the continental crust can be deeply subducted and shows that slab break off delamination and tectonic underplating are fundamental events with drastic consequences on the subsequent evolution of the convergent system combining these results with previous purely mechanical modelling we elaborate a new evolutionary model for the himalaya tibet convergent system the principal successive stages are 1 subduction of the indian continental lithosphere to 200 250 km depth following subduction of the tethys oceanic lithosphere 2 failure and rapid buoyancy driven uplift of the subducted continental crust from ca 100 km depth to some depth that varies along the mountain belt 20 30 km on average 3 break off of the indian subducted lithospheric mantle with the attached oceanic lithosphere 4 subduction underplating of the indian lithosphere under asia over a few to several hundred kilometers 5 delamination roll back and break off of the indian lithospheric mantle 6 failure of the indian crust in front of the mountain belt formation of the main central thrust and underthrusting of the next portion of indian lithosphere beneath tibet for a few hundred kilometers at the beginning of stage 6 the crustal slice corresponding to the crystalline himalayas undergoes erosion activated uplift and exhumation fl 2000 elsevier science b v all rights reserved

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

permo carboniferous sequences of gondwana affinity in southwest china and their paleogeographic implications abstract in southwest china permo carboniferous depositional sequences of gondwana affinity were first discovered in the himalayan region by the expedition team of acadimia sinica in 1975 by 1983 gondwana affinity permo carboniferous sequences had been identified also in the lhasa block the south qiangtang block in tibet and the tengchong and baoshan blocks in yunnan these sequences are characterised by the presence of successions which are interpreted as glacio marine deposits and by the presence of gondwana affinity biota the distribution of gondwana affinity permo carboniferous indicates that basins whose remnants are now represented by the yarlung zangbo suture and the bangong nujiang suture did not exist at that time and the vast area south of the lungmu shuanghu jinshajiang zone in tibet and west of the changning menglian belt in yunnan formed a part of the northern margin of gondwana

cretaceous volcanic rocks in south qiangtang terrane products of northward subduction of the bangong nujiang ocean the subduction of the bangong nujiang ocean is a long standing problem in the study of the geological evolution of the tibetan plateau based on recent data acquired from the abushan volcanic rocks in the qiangtang terrane we will discuss the relationship between the cretaceous volcanism and the evolution of the bangong nujiang ocean the abushan volcanic rocks are mainly composed of trachy andesites dacites and rhyolites zircon u pb dating constrains the time of emplacement as the middle cretaceous 102 6 1 6 96 1 2 4 ma major element geochemistry shows that the volcanic rocks belong to the high k calc alkaline and calc alkaline series all of the volcanic rocks are enriched in light rare earth elements lree and some large ion lithophile elements lile rb th and u and are depleted in heavy rare earth elements hree and some high field strength elements hfse nb ta and ti the geochemical characteristics of the abushan volcanic rocks are correlated with those of the volcanic arc rocks in the subduction zone the petrogenesis of the abushan volcanic rocks suggests that they were derived from the partial melting of the relict subducted oceanic crust combined with the input of oceanic sediments oceanic crust will melt upon reaching its hydrous solidus in the amphibolite facies after plate collision this induces the formation of the abushan volcanic rocks our data compared with the previous studies on volcanism in the lhasa terrane leads us to propose that the bangong nujiang ocean crust was subducted beneath the qiangtang terrane and underwent bidirectional subduction during the cretaceous

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

subaqueous mass flow origin for lower permian diamictites and associated facies of the grant group barbwire terrace canning basin western australia the intracratonic canning basin is western australia s largest sedimentary basin 400 000 km2 and has experienced repeated episodes of phanerozoic extension and subsidence resulting in deposition of a number of first order megasequences a major phase of basin extension and sedimentation grant group occurred in the late carboniferous early permian when australia lay at high palaeolatitudes facies analysis of 5000 m of drill core from 25 continuously cored wells in grant group strata on the fault bounded barbwire terrace in the northern canning basin identified three facies associations fas these record the predominance of fault generated subaqueous mass flow and sediment reworking the lowest association fa i up to 355 m thick rests unconformably on tilted older strata and consists of coarse grained subaqueously deposited sediment gravity flow facies these include fault generated breccias massive and graded sandstones and conglomerates deposited by turbidity currents and diamictites generated by mixing of different textural populations during downslope remobilization fa i is overlain abruptly by relatively fine grained deposits of fa ii up to 140 m thick which consist of laminated to thin bedded mudstone and sandstone turbidites recording an abrupt increase in relative water depths in turn these facies coarsen upwards and are transitional into shallow water swaley cross stratified and rippled sandstones of fa iii up to 125 m thick the overall stratigraphic succession probably records an initial phase of faulting and accommodation of coarse sediment fa i a subsequent phase of rapid subsidence increasing water depths and sediment underfilling fa ii and finally a regressive phase of shoreface progradation the occurrence of rare striated clasts in fa i suggests reworking of glacial sediment but no direct glacial influence on sedimentation can be identified

provenance analysis of upper cretaceous strata in the tethys himalaya southern tibet implications for timing of india asia collision abstract the provenance of upper cretaceous strata in the tethys himalaya provides critical constraints on the closure time of the neo tethys ocean and the initial india asia collision this paper reports detailed petrographic studies in situ detrital zircon u pb ages and lu hf isotopic analyses whole rock nd isotopes and cr spinel electronic microprobe data from upper cretaceous clastic sedimentary rocks of the tethys himalaya near gyangze southern tibet the berriasian coniacian jiabula formation consists of black mudstone chert and minor quartz arenite and is dominated by detrital zircons with archean cambrian u pb ages which were most likely derived from the indian continent overlying conformably is the santonian maastrichtian chuangde formation which consists of red shale limestone and chert the chuangde formation is in turn overlain by the late maastrichtian late paleocene zongzhuo formation which is composed of black mudstone and lithic sandstone enclosing various olistoliths of sandstone limestone and chert the rilang conglomerate is a lens which is located within the upper part of the zongzhuo formation and consists of an upward thinning and fining succession of volcanic conglomerate sandstone and black mudstone the zongzhuo formation and the rilang conglomerate record an abrupt influx of cretaceous zircon grains with juvenile hf isotopic compositions arc related cr spinels and positive end 0 sediments suggesting an arc and suture zone provenance the change in provenance of upper cretaceous strata from the southern indian continent to a northern arc and suture zone is attributed to either 1 initial collision between the indian plate and lhasa terrane or 2 initial collision between the indian plate and an intra oceanic arc we prefer option 1 that the initiation of india asia collision occurred during maastrichtian 70 65 ma

early cretaceous gangdese retroarc foreland basin evolution in the selin co basin central tibet evidence from sedimentology and detrital zircon geochronology the selin co basin in the northern lhasa terrane includes more than 3000 m of upward coarsening lower cretaceous strata and the sedimentary sequence from the flysch to the molasse indicates the evolution of a foreland basin petrographic analysis shows that sandstones are rich in volcanic and sedimentary lithics and most of them fall into recycled orogen and magmatic arc uranium lead u pb ages were determined for 435 detrital zircons from the lower cretaceous strata in the selin co basin relative probability of detrital zircon ages shows the eshaerbu formation was rich in zircon grains with the age of 125 140 and 160 180 ma and the duoni formation was dominated by one main age cluster of 125 150 ma analysis of the potential provenances suggests the early cretaceous zircon grains were primarily derived from the gangdese magmatic arc to the south the youngest zircon ages in the lowermost exposure of the eshaerbru formation are c 130 ma providing a maximum depositional age of sediments in the selin

assembly of the lhasa and qiangtang terranes in central tibet by divergent double subduction abstract integration of lithostratigraphic magmatic and metamorphic data from the lhasa qiangtang collision zone in central tibet including the bangong suture zone and adjacent regions of the lhasa and qiangtang terranes indicates assembly through divergent double sided subduction this collision zone is characterized by the absence of early cretaceous high grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca 120 110 ma two jurassic cretaceous magmatic arcs are identified from the caima duobuza rongma kangqiong amdo magmatic belt in the western qiangtang terrane and from the along tso yanhu daguo baingoin daru tso magmatic belt in the northern lhasa terrane these two magmatic arcs reflect northward and southward subduction of the bangong ocean lithosphere respectively available multidisciplinary data reconcile that the bangong ocean may have closed during the late jurassic early cretaceous most likely ca 140 130 ma through arc arc soft collision rather than continent continent hard collision subduction zone retreat associated with convergence beneath the lhasa terrane may have driven its rifting and separation from the northern margin of gondwana leading to its accretion within asia

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 of an eclogite belt in the lhasa block tibet a new border for paleo tethys abstract a newly discovered eclogite belt in the eastern part of the lhasa block tibet is about 500 1000 m wide and at least 60 km long in an e w direction the eclogites occur as tectonic slices in garnet bearing mica quartz schist they are generally fresh and form thick massive layers that consist chiefly of garnet grt omphacite omp phengite phe rutile rut quartz qtz p t calculations based on the grt omp phe mineral assemblage yielded peak metamorphic conditions of 2 7 gpa and 730 c close to the phase boundary between coesite and quartz and thus the eclogites can be regarded as part of a very high pressure metamorphic belt petrochemical data suggest that the eclogite protoliths were typical morb basalts derived from depleted mantle shrimp u pb dating of zircons from the eclogite yielded metamorphic ages ranging from 242 15 to 292 13 ma with an average value of 262 5 ma the morb eclogites are interpreted to be remnants of paleo tethyan oceanic lithosphere the eclogites along with permian island arc volcanic rocks to the north are believed to mark a carboniferous permian suture zone dividing the lhasa block into a northern and southern segment this newly identified suture zone suggests that the border of the paleo tethyan ocean jumped southward from north of the bangong nujiang suture to within what is now the lhasa block

new insights into the collision process of india and eurasia evidence from the syntectonic sedimentation induced inclinational divergence of cretaceous paleomagnetic data of the lhasa terrane abstract unidentified cretaceous syntectonic growth strata in the lhasa terrane lt has resulted in inclinations calculated from the paleomagnetic data from the southern limb of folds being substantially higher than those from the northern limb this causes conspicuous inclinational divergences in the cretaceous paleomagnetic data obtained from the lt and results in cluttered restoring of the cretaceous paleolatitude of the southern margin of eurasia contradictory constraining of the collision process of india and eurasia and imprecise estimation of the post collision crustal convergence within southern eurasia after applying a new syntectonic sedimentation correction the inclinational divergences of the cretaceous paleomagnetic data within the lt are effectively eliminated the cretaceous inclinations of the western part of the lt are found to converge within the range of 30 1 30 7 indicating that the western part of the lt was situated at the paleolatitude of 16 9 1 3 n during the cretaceous nevertheless the syntectonic sedimentation correction for the cretaceous paleomagnetic data from the central eastern parts of the lt show alternative cretaceous paleolatitudes of 17 4 1 3 n or 12 9 1 6 n because of lacking paleomagnetic sampling sections across the two limbs of folds when considering the western central and eastern parts of the lt were consistently situated at the paleolatitude of 17 n the lt had an approximate e w extended banded shape in the entire cretaceous this interpretation supports the diachronous collision process of india and eurasia in which the collision first occurred at the western end of the lt and subsequently the counterclockwise rotation of india closed the neo tethys ocean from the west to the east on the other hand the central part of the lt was situated at the paleolatitude of 12 9 1 6 n resulting in a shape of inverted triangle of the lt during the interval of the cretaceous this suggested that the india probably had collided with the lt quasi synchronously at the western and central part of the himalaya terrane and that the neo tethys ocean was closed simultaneously along the western and central segment of the yarlung zangbo suture zone subsequently the indian plate began to experience counterclockwise rotation approximately centered on the western himalaya syntaxis which compressed the lt and resulted in non uniform crustal convergence within the lt the central part of the lt should experience larger amount of crustal convergence than that of the western part of the lt

late mesozoic tectonic evolution and growth of the tibetan plateau prior to the indo asian collision abstract the elevation of the tibetan plateau is a revolutionary event in the earth history which resulted in change not only of cenozoic regional and global climate but also of monsoon intensity a critical aspect for the development of the tibetan plateau that remains open to intense debate is whether the collision among the tibetan continental blocks or the subduction of the tethys prior to the indo asian collision in the cenozoic contributed to the plateau growth we propose here that an andean type orogen could have been present in southern tibet from the jurassic to early cretaceous but collapsed during the middle cretaceous based on a synthesis of existing structural sedimentary geochronological and geochemical data the orogeny is marked by north verging jurassic early cretaceous thrusting in the lhasa block a result of flat subduction of the yarlung zangpo neo tethys that is recorded by a narrow belt of 205 174 ma adakitic rocks along the southernmost margin of the lhasa block denudation of the orogen produced a thick up to 15 km sequence of jurassic turbidites and molasse type sediments on the northern lhasa block widespread middle cretaceous 135 100 ma postorogenic strongly peraluminous to calc alkaline magmatism and extensional deformation in the lhasa block and an extensive marine transgression in tibet indicate that the orogen broke down due to extension resulting from lithosphere delamination and asthenosphere upwelling meanwhile the north dipping subduction of the bangong meso tethys produced a wide jurassic middle cretaceous magmatic arc along the southern margin of the qiangtang block findings of 132 108 ma aged ophiolites rich with middle cretaceous radiolarians in central tibet indicate that the bangong meso tethys did not close until the late cretaceous repeated flat slab subduction of the yarlung zangpo neo tethys occurred during the late cretaceous 90 78 ma as indicated by the adakites in the southernmost lhasa block which together with the ongoing qiangtang lhasa collision could have contributed to the growth of the tibetan plateau

did the indo asian collision alone create the tibetan plateau it is widely believed that the tibetan plateau is a late cenozoic feature produced by the indo asian collision however because tibet was the locus of continental accretion and subduction throughout the mesozoic crustal thickening during that time may also have contributed to growth of the plateau this portion of the geologic history was investigated in a traverse through the central lhasa block southern tibet together with earlier studies our mapping and geochronological results show that the lhasa block underwent little north south shortening during the cenozoic rather our mapping shows that 60 crustal shortening perhaps due to the collision between the lhasa and qiangtang blocks occurred during the early cretaceous this observation implies that a significant portion of southern tibet was raised to perhaps 3 4 km elevation prior to the indo asian collision

tectonostratigraphy and provenance of an accretionary complex within the yarlung zangpo suture zone southern tibet insights into subduction accretion processes in the neo tethys abstract accretionary complexes record critical information about the history of subduction and accretion along the southern margin of asia prior to the india asia collision this paper presents detailed field mapping petrographic and detrital zircon u pb data from an accretionary complex within the yalung zangpo suture zone southern tibet from structurally higher to lower levels the accretionary complex consists of a serpentinite melange the chert matrix tangga melange the mud matrix pomunong melange and the coherent rongmawa formation the tangga melange consists of late triassic early cretaceous abyssal chert and siliceous shale with blocks of chert and mafic to ultra mafic rocks the tangga melange was accreted beneath ophiolitic rocks during the aptian the pomunong melange consists of a late jurassic early cretaceous hemipelagic siliceous shale and chert matrix with blocks from early permian seamounts and from late cretaceous trench fill sandstones it was accreted beneath the tangga melange after the aptian but prior to 71 ma structurally beneath these melanges to the south the uppermost cretaceous rongmawa formation consists of turbiditic sandstone pelagic chert and siliceous shale and records a transition in depositional setting from lower abyssal plain to upper trench detrital zircons from sandstone blocks of the pomunong melange and the coherent rongmawa formation display similar u pb age spectra and are dominated by peaks at 71 231 ma 481 693 ma and 701 1372 ma these age peaks overlap with igneous crystallization ages and detrital zircon ages from sedimentary strata in the lhasa terrane our data indicate that the serpentinite melange tangga melange pomunong melange and rongmawa formation comprise a southward younging accretionary complex that developed during the northward subduction of neo tethyan oceanic lithosphere beneath the south margin of the lhasa terrane and that all exposed ophiolitic and accretionary complex assemblages were accreted to the asian margin prior to the india asia continental collision

evidence for early 44 ma himalayan crustal thickening tethyan himalaya southeastern tibet abstract comprehensive understanding of himalayan orogenesis is limited in part by a poor knowledge of the eohimalayan episode a phase of tectonic activity predating the better understood neohimalayan crustal thickening associated with coaxial deformation along the main central thrust the cambo ordovician to tertiary metasedimentary sequences outcropping within the tethyan himalaya are the structurally highest units of the himalayan fold and thrust belt they and are inferred to have been the first to have accreted to asia a compilation of data from transects along the length of the main himalayan arc shows that the tethyan sequences have experienced at least five deformation events although the timing of these episodes is poorly constrained emplacement of a series of undeformed granitoid bodies following the second of these events which accounts for the majority of crustal thickening in the tethyan himalayan units is constrained by u pb zircon dating to be older than 44 1 1 2 ma thus significant crustal thickening had occurred along the length of the proto himalayan arc by the mid eocene or within 10 to 20 myr of the initiation of himalayan orogenesis

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

age and petrogenesis of a type granites in the middle segment of the bangonghu nujiang suture tibetan plateau abstract during geological investigations conducted over the past several years the authors found a group of a type granites in the middle segment of the bangonghu nujiang suture on the tibetan plateau these granitic plutons occur as stocks with exposed areas of less than 1 km 2 and lithologically include biotite granite and granodiorite porphyry which had intruded into cretaceous sedimentary rocks petrogeochemically the a type granites contain high sio 2 levels varying from 68 62 75 36 wt moreover they contain relatively high fe 2 o 3 t 0 86 5 39 wt and low al 2 o 3 12 76 15 54 wt levels and are weak peraluminous to metaluminous a trace element n morb normalized spider diagram showed apparent enrichment of the large ion lithophile elements lile rb th u k and pb with marked depletions of ba and sr the high field strength elements hfse nb ta and ti were sharply depleted with relative enrichment of zr and hf these signatures are characteristic of a type granite the granite samples had high rare earth element ree contents with ree 122 37 291 19 ppm light ree were rich relative to heavy ree with lree hree 4 89 9 58 and negative eu anomalies were sharp with eu varying from 0 14 0 5 and a mean value of 0 34 their chondrite normalized rare earth element distribution patterns appear as gently right dipping v types zircon u pb laicpms dating for three granitic plutons yielded weighted mean ages of 109 6 1 4 ma mswd 3 7 112 3 0 51 ma mswd 0 21 and 113 73 0 5 ma mswd 0 21 indicating that they formed in the late stage of the early cretaceous epoch the nd sr and pb isotopic results showed that the granites were rich in radiogenic isotopes the 87 sr 86 sr and i sr ratios varied from 0 719842 0 786395 and 0 706900 0 710378 respectively the 143 nd 144 nd ratios were from 0 512123 0 512392 with relatively large negative e nd values 3 37 to 10 34 the 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb ratios ranged from 18 703 19 070 15 680 15 732 and 39 121 39 576 respectively exhibiting isotopic signatures typical of crustal sources all of the data demonstrated unanimously that these granites were derived from partial melting of arc basaltic rocks underplated in the lower crust during oceanic crust subduction the high temperature heat source for melting of anhydrous basaltic rocks of the lower crust was from upwelling asthenosphere through slab windows formed as a result of slab break off during post collisional extension of the orogenic zone

provenance of lower cretaceous sedimentary rocks in the northern margin of the lhasa terrane tibet implications for the timing of the lhasa qiangtang collision abstract placing precise constraints on the timing of initial lhasa qiangtang collision is essential to understand the assembly processes and uplift mechanism of the tibetan plateau prior to the cenozoic india asia collision along the over 2000 km long bangong nujiang suture zone the timing of initial lhasa qiangtang collision remains highly controversial with estimates ranging from middle jurassic to late cretaceous spanning nearly 100 myr provenance analysis of clastic rocks has been proved as an effective approach to pinpoint the timing of initial continental collision by documenting the provenance exchange from one plate to another this study focuses on the lower cretaceous duoni formation in the gerze area of northern lhasa margin the youngest detrital zircon u pb ages along with other age constraints in literature indicate that the duoni formation was deposited during the late early cretaceous 119 122 7 ma a combined sandstone petrographic detrital zircon and cr spinel provenance analysis demonstrates that the duoni formation records unambiguously suture zone and qiangtang derived detritus from the north such as the bangong nujiang ophiolites and southern qiangtang arc based on structural geology stratigraphy and provenance analysis the lower cretaceous duoni formation on the northern lhasa margin was most likely deposited in a peripheral foreland basin in response to the lhasa qiangtang collision these data provide undisputable evidence that the lhasa qiangtang collision was well underway and the lhasa terrane had been welded to the southern margin of eurasia by at least the late early cretaceous 119 122 7 ma

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

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

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

gangdese retroarc thrust belt and foreland basin deposits in the damxung area southern tibet abstract geologic mapping and u pb detrital zircon geochronologic studies of meta sedimentary rocks in the damxung area 90 km north of lhasa of the southern lhasa terrane in tibet provide new insights into the history of deformation and clastic sedimentation prior to late cenozoic extension cretaceous nonmarine clastic rocks 10 km southeast of damxung are exposed as structural windows in the footwall of a thrust fault the damxung thrust that carries paleozoic strata in the hanging wall to the north of damxung in the southern part of the northern nyainqentanglha range nnqtl metaclastic rocks of previously inferred paleozoic age are shown to range in depositional age from late cretaceous to eocene the metaclastic rocks regionally dip southward and are interpreted to have been structurally buried in the footwall of the damxung thrust prior to being tectonized during late cenozoic transtension along the northern flank of the nnqtl lower eocene syncontractional redbeds were deposited in a triangle zone structural setting all detrital zircon samples of cretaceous eocene strata in the damxung area include early cretaceous grains that were likely sourced from the gangdese arc to the south we suggest that the that newly recognized late cretaceous to early eocene meta clastic deposits and thrust faults represent the frontal and youngest part of a northward directed and propagating gangdese retroarc thrust belt and foreland basin system that led to significant crustal thickening and elevation gain in southern tibet prior to india asian collision

50 million year long record of glacial to postglacial marine environments preserved in a carboniferous lower permian graben northern perth basin western australia abstract the perth basin is an intracratonic rift in western australia and contains a carboniferous to cretaceous infill in excess of 12 km thick some 2 km of mid carboniferous to lower permian glacially influenced marine strata spanning the serpukhovian to kungurian about 50 my form one of the longest late paleozoic cold climate successions preserved anywhere in gondwana and record the transition from full glacial to postglacial conditions at high 70 paleolatitudes the great thickness of the succession reflects an abundant supply of glaciclastic sediment from the adjacent ice covered yilgarn craton and continued subsidence along the darling urella fault system the oldest part of the basin fill comprises a 1 5 km thick succession of massive and laminated marine mudstones with striated dropstones nangetty formation deposited in a fiord like environment from plumes of suspended sediment dilute turbidity currents and icebergs meltwater inputs are recorded by erosionally based multistory channels filled with coarse grained gravity flow facies wicherina member deposited on a fan delta along the steep faulted basin margin the nangetty formation passes upward into a regressive succession of postglacial mudstones holmwood shale with bioclastic calcarenites and marine macrofossils capped by sandstone and mudstone facies high cliff sandstone deposited in a storm influenced inner shelf setting on the outer margins of coalesced fan deltas along the basin margin the overlying irwin river coal measures contain a flora that indicates colonization of fan deltas and the yilgarn craton by ferns and lycopods an overlying thick transgressive shale with ice rafted debris carynginia formation records renewed subsidence but continuing seasonally cool climates where coarse debris was rafted by ice into a marine embayment the sedimentology of the perth basin highlights the key role of glacial meltwaters in transporting coarse and fine grained clastic sediment to marine basins rather than direct glacial processes per se the equivalent terrestrial record of glaciation and climate change across the yilgarn craton at this time is very limited

late triassic paleogeographic reconstruction along the neo tethyan ocean margins southern tibet abstract sandstone petrographic and u pb detrital zircon analyses of upper triassic sedimentary rocks from the northern margin of india tethyan himalaya sequence and southern margin of eurasia lhasa terrane provide new constraints on the mesozoic paleogeography of neo tethyan ocean basins the upper triassic nieru formation of the tethyan himalaya sequence ths near lazi city 29 n 87 5 e is dominated by indian affinity precambrian detrital zircons which are typical of the majority of the ths however the upper triassic langjiexue formation of the ths exposed to the east at 90 93 e longitude includes significant populations of permian to early jurassic 291 184 ma detrital zircons for which there is no known indian source in addition the upper triassic nieru formation near kangma town 28 5 n 90 e located 200 km to the southeast of lazi city yielded detrital zircon age spectra that are similar to those of langjiexue formation based on detrital zircon age spectra comparisons we propose that both the langjiexue and nieru formations were derived from continental crustal fragments that were adjacent to the northwestern margin of australia furthermore we suggest that these ths units and age equivalent strata in northwest australia west sulawesi timor and west papua comprised a late triassic submarine fan along the northern australian shelf the upper triassic mailonggang formation in the southern lhasa terrane 35 km northeast of lhasa city 30 n 91 5 e is dominated by permian detrital zircons which were likely derived from proximal lhasa terrane sources the mailonggang formation differs from all age equivalent strata in the tethyan himalaya therefore we interpret that it was separated from greater india by the neo tethyan ocean

northward subduction of bangong nujiang tethys insight from late jurassic intrusive rocks from bangong tso in western tibet abstract new zircon la icpms u pb age data whole rock major and trace elements and zircon hf isotopic data of intrusive rocks from larelaxin and caima plutons in the southern margin of the western qiangtang subterrane provide important evidence of northward subduction of the bangong nujiang tethyan ocean lithosphere host granitoids including quartz diorites granodiorites and syenogranites and associated mafic varieties including gabbroic enclaves dioritic dikes and enclaves and quartz dioritic enclaves are investigated in this study five host granitoid samples are dated at 163 160 ma and one dioritic dike and three dioritic enclave samples are dated at 162 158 ma which indicates that these rocks were contemporaneous the quartz diorites and granodiorites are normal calc alkaline i type granitoids the syenogranites are characterized by high sio 2 74 77 wt and differentiation index di 92 97 and marked depletion in ba nb ta sr p ti and eu thus indicating that they are highly fractionated i type granitoids the host granitoids exhibit uniform zircon e hf t values 1 4 to 1 9 considering the current data and their negative whole rock e nd t values 4 7 to 3 5 reported in recent studies these rocks can be interpreted to have resulted from the partial melting of ancient mafic lower crust with varying contributions from mantle derived or juvenile crust derived components one gabbroic enclave sample exhibits high al 2 o 3 18 3 wt low mgo 3 3 wt high tio 2 1 4 wt and high nb 24 ppm and is geochemically similar to high alumina basalt hab and high nb basalt hnb this indicates that the gabbroic enclave originated from the partial melting of mantle wedge peridotite that was metasomatized by slab melting and subsequently experienced significant fractional crystallization of olivine and clinopyroxene the dioritic dike and enclave samples yield e hf t values 1 6 to 3 1 similar to those of the host granitoids they most likely originated from magma mixing between mantle derived and crust derived melts at or close to the moho the intrusive rocks with geochemical diversity of the larelaxin and caima plutons from bangong tso in western tibet can be interpreted to have resulted from melting assimilation storage and homogenization a mash process above subduction zones thus indicating that the bangong nujiang tethyan ocean lithosphere was subducted northward beneath the western qiangtang subterrane during the late jurassic

volcanic rocks of the 1985 tibet geotraverse lhasa to golmud volcanic rocks encountered during the tibet geotraverse have been studied in the field in thin section and by major and trace element geochemistry in order to determine their most probable original eruptive environment rocks from a total of eleven distinct volcanic provinces were studied in this way they provide evidence for an active continental margin or post collision province of probable devonian early carboniferous age in the northern kunlun mountains an active continental margin of late carboniferous age in the southern lhasa terrane permian continental rifts in the central qiangtang and central kunlun terranes triassic volcanic arcs in the southern lhasa and northern qiangtang terranes a triassic active continental margin dyke swarm in the northern kunlun mountains a jurassic post collision or back arc rifting province in the southern qiangtang terrane a jurassic island arc in the northern lhasa terrane a cretaceous post collision province in the northern lhasa terrane possibly extending into the southern qiangtang terrane and a palaeogene active continental margin in the southern lhasa terrane an oligocene trachyte plug in the northern qiangtang terrane was the only evidence encountered during the geotraverse of volcanism post dating the palaeogene india eurasia collision however the composition of this plug coupled with new and published analyses from miocene volcanics in the southern lhasa terrane and from the pliocene recent volcanic province of northwest tibet places important constraints on models for post collision underplating of tibet by continental lithosphere any underplating is likely to have been a much later than the start of collision b directed beneath tibet from the north as well as the south and c limited in extent

timing and mechanism of bangong nujiang ophiolite emplacement in the gerze area of central tibet abstract the bangong nujiang suture zone bnsz separates the lhasa terrane from the qiangtang terrane and contains remnants of the bangong nujiang oceanic lithosphere ophiolites despite decades of research when and how the bangong nujiang ophiolites were emplaced remains enigmatic in the gerze area western segment of the bnsz the geochemistry and provenance discrimination of chromian spinels cr spinels from the pre collisional subduction complex mugagangri group and syn collisional peripheral foreland basin succession wuga formation can help us solve this fundamental problem in the bnsz evolution this study compares the geochemistry of cr spinels from the mugagangri group and wuga formation with those from the bangong nujiang ophiolites cr spinels in the bangong nujiang ophiolites have either low tio2 0 01 0 15 and low al2o3 11 74 26 76 indicating an ssz peridotite origin or high al2o3 45 28 49 15 indicating a morb peridotite origin cr spinels from the ultramafic fragments within the mugagangri group have extremely low tio2

sedimentology palaeoecology and palaeoenvironmental evolution of the 1985 lhasa to golmud geotraverse vertical and horizontal measurements of almost 30 km of sections were made along the geotraverse route at 113 localities ranging in age from ordovician to tertiary over 280 palaeocurrent measurements were taken and 200 thin sections were studied ordovician strata occur only in the kunlun terrane w here thick metamorphosed sequences of elastics and carbonates occur these are tentatively interpreted as platform margin and slope deposits during carboniferous times in the kunlun terrane transgressive late dinantian marine limestones with tropical to subtropical eurasian reefoidal faunas overlie fluviatile redbeds derived from an unroofed orogenic belt the lhasa terrane contains shelf basin elastics with low diversity faunas succeeded by thick late carboniferous early permian glacio marine mixtites deposited by iceberg meltout permian carbonate ramp and shelf facies with reefoidal developments occur over both the lhasa and qiangtang terranes with coal bearing elastics and fluviatile redbeds also occurring in the latter

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 origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved