accelerated extension of tibet linked to the northward underthrusting of indian crust the tibetan plateau is extending numerical simulations suggest that regional scale extension is caused by gravitational collapse of the plateau whereas rapid extension in the south is caused by underthrusting of the indian slab

geochronology petrogenesis and tectonic significance of the jitang granitic pluton in eastern tibet sw china abstract the jitang granitic pluton which is situated in the southern margin of the north qiangtang block in eastern tibet may provide critical information about the source and derivation of the north qiangtang block during gondwana breakup and dispersion in this paper we report relevant data such as zircon u pb age whole rock major and trace element abundances and sr nd isotopes for the jitang pluton the major rock types of the pluton are granodiorite and biotite granite whole rock major element data reveal that the jitang pluton is a peraluminous s type granitic pluton the u pb age of zircons from the pluton is 219 1 1 7 ma which is 10 ma younger than the age of high pressure metamorphism in the longmu co shuanghu collisional suture between the north and south qiangtang blocks the jitang granitoids show pronounced negative ba eu sr anomalies high initial 87sr 86sr ratios from 0 7266 to 0 7389 and low end t values for from 11 1 to 13 2 which are remarkably similar to the gneisses and meta sedimentary rocks from the indian craton as well as the north qiangtang block the results from this study indicate that the jitang granitoids formed by melts derived from a crustal source with sr nd isotopic compositions similar to those of the indian cratonic crust we concur with the previous interpretation based on detrital zircon records that the north qiangtang block was derived from the indian gondwana

late triassic paleolatitude of the qiangtang block implications for the closure of the paleo tethys ocean abstract to better constrain the late triassic paleolatitude of the qiangtang block and the closure of the paleo tethys ocean a combined paleomagnetic and zircon u pb geochronological study has been conducted on the upper triassic jiapila formation volcanic rocks on the northern edge of the qiangtang block of central tibet 34 1 n 92 4 e these rocks are dated to 204 213 ma progressive thermal or alternating field demagnetization successfully isolated stable characteristic remanent magnetizations chrm that pass both the fold and reversal tests consistent with a primary magnetization these are the first volcanic based paleomagnetic results from pre cretaceous rocks of the qiangtang block that appear to average secular variation well enough to yield a reliable paleolatitude estimate based on our new paleomagnetic data from upper triassic lavas we conclude that the late triassic pole of the qiangtang block was located at 64 0 n 174 7 e with a 95 6 6 n 29 we compile published paleomagnetic data from the qiangtang block to calculate a late triassic latitude for the qiangtang block at 31 7 3 0 n the central paleo tethys ocean basin was located between the north china ncb and tarim blocks to the north and the qiangtang block to the south during late paleozoic early mesozoic a comparison of published early triassic paleopole from the qiangtang block with the coeval paleopoles from the ncb and tarim indicates that the paleo tethys ocean could not have closed during the early triassic and that its width was approximately 32 38 latitude 3500 4200 km however the comparison of our new combined late triassic paleomagnetic result with the late triassic poles of the ncb and tarim as well as numerous geological observations indicates that the closure of the paleo tethys ocean at the longitude of the qiangtang block most likely occurred during the late triassic

indepth iii seismic data from surface observations to deep crustal processes in tibet 1 during the summer of 1998 active source seismic data were collected along a transect running 400 km nnw sse across the central tibetan plateau as the third phase of project indepth international deep profiling of tibet and the himalaya the transect extends northward from the central lhasa block across the jurassic bangong nujiang suture bns at 89 5 e to the central qiangtang block a seismic velocity model for the transect to 25 km depth produced by inversion of p wave first arrivals on 3000 traces shows 1 a 50 km wide region of low velocity at least 5 less than surrounding velocities extending to the base of the model at the bns 2 sedimentary cover for the southern qiangtang block that is 3 5 km thick 3 a distinct interface between sedimentary cover and qiangtang basement or underplated jurassic melange in the central qiangtang block and 4 evidence that the bangoin granite extends to a depth of at least 15 km the bns has little geophysical signature and appears unrelated to the 5 km northward shallowing of the moho which is associated with the bns in central tibet geophysical data along the main indepth iii transect show little evidence for widespread crustal fluids in contrast to the seismic bright spots found in southern tibet and to magnetotelluric evidence of fluid accumulations in eastern tibet a comparison between the global average and tibetan velocity depth functions offers constraints for models of plateau uplift and crustal thickening taken together with the weak geophysical signature of the bns these velocity depth functions suggest that convergence has been accommodated largely through pure shear thickening accompanied by removal of lower crustal material by lateral escape likely via ductile flow although we cannot resolve the details we believe lateral lower crustal flow has overprinted or destroyed evidence in the deep crust for the earlier assembly of tibet as a series of accreted terranes

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

mid silurian back arc spreading at the northeastern margin of gondwana the dapingzhang dacite hosted massive sulfide deposit lancangjiang zone southwestern yunnan china abstract the dapingzhang dacite hosted volcanogenic massive sulfide deposit is located in a late paleozoic collisional zone the sanjiang fold and thrust belt characterized by a collage of gondwana derived terranes at the southern margin of the south china yangtze block this area has experienced repeated paleozoic and mesozoic collisional and extensional events and is overprinted by himalayan strike slip and thrust movements the dapingzhang orebody is strongly sheared into discontinuous ore lenses and blocks and is hosted by a several hundred meter thick volcanic rock sequence of dacite with minor metabasalt spilite rhyolite and chert bulk rock nd isotope data give end 429 ma values of 2 to 5 and indicate a dominantly mantle source or origin from young continental crust tdm 800 1000 ma the dacites have distinctly low abundances of ti nb ta zr th and rees which is typical of subduction related volcanism the base metal mineralization occurs as polymetallic massive sulfide mineralization mainly pyrite sphalerite galena chalcopyrite tetrahedrite tennantite and locally barite and as pyrite chalcopyrite stockworks pervasive hydrothermal alteration is dominantly quartz sericite maximum enrichment in gold bismuth selenium and tellurium is at the interface between the stockwork and massive sulfide mineralization styles deeper parts of the stockwork zone are characterized by elevated molybdenum and rhenium whereas the distal parts to the massive sulfide mineralization have high mercury antimony zinc cadmium arsenic lead and silver the metal association and mineral assemblage are typical of volcanic rock hosted massive sulfides in back arc volcanic settings uranium pb dating on zircon by la icp ms defines an age of 429 3 ma 2 n 19 for the dacite sequence rhenium os isotope data on mo rich bulk ore samples define an isochron of 429 10 ma 2 n 9 mswd 0 21 initial 187os 188os 3 1 1 9 common os is very low and the absolute abundances of 187re and 187os define a model age of 429 4 ma n 9 95 confidence the mid silurian dacite sequence and associated massive sulfide mineralization identify a hitherto unknown early rifting and back arc seafloor spreading event at the northeastern gondwana margin related to the early evolution of the paleotethys ocean

shear localization in transformation induced faulting first order similarities to brittle shear failure abstract transformation induced faulting a leading candidate for the mechanism of deep focus earthquakes leads to catastrophic shear failure in laboratory specimens by the growth and coalescence of microscopic mode i anticracks or packets of denser microcrystalline transformed material we have studied this phenomenon in mg 2 geo 4 olivine undergoing a polymorphic phase transformation to its denser spinel phase at 2 7 gpa and 1200 k under applied stress in order to better understand the details of anticrack growth and coalescence which leads to localized shear zones and macroscopic failure here we report observations that address the early stages of shear localization preserved in one experiment of this series are microstructures typical of anticrack nucleation and growth as well as those involved in the self organization of these features during the shear localization process we have analyzed sem and optical images of individual anticrack reaction interfaces and coalesced structures from this experiment and another experiment conducted under similar conditions but with a different strain history we have isolated the reaction boundaries in these images and have performed fractal analysis of these boundary lines we find that the fractal dimension of isolated uncoalesced anticrack structures is consistently higher than that of structures formed from coalescence and coarsening of fine grained reaction products this result is consistent with the varying effects of stress and thermal feedback at these interfaces and also represents an interesting parallel with the fractal nature of brittle cracks and fault surfaces we also measured the length of the long axis of uncoalesced anticrack structures found in the experiment arrested at failure and find a roughly log normal distribution of sizes very similar to the distribution of microcrack lengths reported in experiments involving brittle shear failure microcracks and microanticracks both also commonly nucleate on grain boundaries intracrystalline flaws and phase boundaries shear zones faults formed by either of these mechanisms also regularly form at 30 to maximum compression from this set of observations we conclude that the general phenomenon of shear failure creates similar features in both systems perhaps in any system which generates a shear instability despite the major differences in the detailed microphysics because of the detailed and unambiguous measurements that are possible with images of transformation induced faulting experiments further studies of this faulting mechanism may provide new insights into brittle failure and shear localization processes

p wave tomography and dynamics of the crust and upper mantle beneath western tibet abstract the continental collision between the indian and asian plates plays a key role in the geologic and tectonic evolution of the tibetan plateau in this article we present high resolution tomographic images of the crust and upper mantle derived from a large number of high quality seismic data from the antilope project in western tibet both local and distant earthquakes were used in this study and 35 115 p wave arrival times were manually picked from the original seismograms geological and geochemical results suggested that the subducting indian plate has reached northward to the lhasa terrane whereas our new tomography shows that the indian plate is currently sub horizontal and underthrusting to the jinsha river suture at depths of 100 to 250 km suggesting that the subduction process has evolved over time the asian plate is also imaged clearly from the surface to a depth of 100 km by our tomography and it is located under the tarim basin north of the altyn tagh fault there is no obvious evidence to show that the asian plate has subducted beneath western tibet the indian and asian plates are separated by a prominent low velocity zone under northern tibet we attribute the low velocity zone to mantle upwelling which may account for the warm crust and upper mantle beneath that region and thus explain the different features of magmatism between southern and northern tibet but the upwelling may not penetrate through the whole crust we propose a revised geodynamic model and suggest that the high velocity zones under lhasa terrane may reflect a cold crust which has interrupted the crustal flow under the westernmost tibetan plateau

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

magmatic affinity of modern and ancient subalkaline volcanic rocks determined from trace element discriminant diagrams when dealing with ancient subalkaline volcanic rocks the alkali total iron magnesium afm diagram is of limited use in assigning a tholeiitic versus calc alkaline affinity because these eleme

characterization and origin of aluminous a type granites from the lachlan fold belt southeastern australia the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are a distinctive group of igneous rocks on the basis of chemical and mineralogical criteria those granites that contain 65 72 sio2 can be distinguished from other types on the basis of higher abundances of fetotal fetotal mg high field strength elements trivalent rare earth elements ga and zn mineralogically they contain fe rich hydrous mafic minerals and primary ilmenite and hence are reduced relative to the ni nio buffer however the extremely felsic a type granites sio2 72 have the same chemical and mineralogical characteristics as felsic fractionated i type granites recent analyses indicate that the lachlan fold belt a type granites have sc f alkali element trace transition element and h2o contents similar to those of other unfractionated i type granites rb sr and nd sm isotopic compositions are highly variable probably reflecting source region heterogeneity the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are distinct from peralkaline rocks in terms of chemical composition petrography and field associations although these rocks have been grouped together as a single type in current classification schemes we propose that the metaluminous to weakly peraluminous a type granites such as those of the lachlan fold belt should be defined as aluminous a type granites and should not be grouped with peralkaline granites the lachlan fold belt aluminous a type granites have relatively high calculated zircon saturation temperatures we suggest that these granites were produced by high temperature partial melting of a felsic infracrustal source

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

eclogites from central qiangtang northern tibet china and tectonic implications abstract eclogites in central qiangtang northern tibet china are characterized by the garnet omphacite rutile phengite quartz assemblage hornblende plagioclase and sphene occur as retrograde amphibolite facies phases in the eclogites pressure temperature estimates for the primary eclogite assemblage are consistent with minimum pressures of 20 25 kbar at 482 to 625 c the retrograde amphibolite facies assemblage suggests conditions of 10 12 kbar and 460 600 c consequently the eclogites from central qiangtang have undergone unambiguous ultrahigh pressure uhp metamorphism the presence of uhp metamorphic rocks implies a likely deep seated continental subduction along the shuanghu suture within the qiangtang terrain the denudation of these uhp metamorphic rocks correlates well with the main interval that the songpan ganzi turbidites accumulated and could have provided a significant source for these sedimentary rocks

cambrian to triassic geodynamic evolution of central qiangtang tibet abstract central qiangtang is a tectonic melange between the northern qiangtang terrane nqt and the southern qiangtang terrane sqt and it preserves a complete cambrian to triassic geological record related to tethyan evolution e g high pressure metamorphic rocks and ophiolites thus this region is the key to understanding the geodynamic processes that occurred during the dispersion of gondwana and the accretion of laurussia given previous studies on sedimentary paleomagnetic and seismic records this paper presents an integrated interpretation of ophiolites magmatism metamorphism and tectonism and reconstructs the cambrian triassic tectonic evolution of central qiangtang as described below 1 following the neoproterozoic early cambrian pan african orogeny and associated postcollisional extension the southward subduction of the proto tethys oceanic plate produced andean type magmatism along the northern margin of gondwana a back arc basin between the nqt and sqt further opened during the late cambrian early ordovician accompanying with trench retreat and overriding plate extension resulting from slab rollback 2 with the middle ordovician late silurian successive incorporation of micro continental terranes e g qaidam kunlun and tarim terranes into the northern margin of gondwana the back arc basin closed through subduction polarity reversal from southward to northward 3 during the late devonian early carboniferous the southward subduction of an oceanic plate to the north of the nqt separated the nqt and sqt through back arc extension consequently the nqt drifted northward rapidly and the back arc basin gradually evolved into a mature ocean basin i e longmu co shuanghu paleo tethys ocean lspto 4 subsequently a late carboniferous early permian continent continent collision between the nqt and the south china block scb resulted in the northward subduction of the lspto and the ongoing subduction probably induced the opening of a back arc basin between the nqt and the scb during the late permian 5 finally diachronous collision between the nqt and sqt closed the lspto during the early middle triassic and the termination of the collisional orogenic event was evidenced by strong late triassic magmatism related to postcollisional extension in the qiangtang block in this paper we propose that the opening and closure of back arc basins controlled the cambrian triassic tectonic evolution of central qiangtang which provides significant insights into the dispersion of gondwana and the accretion of laurussia

detection of the deep crustal structure of the qiangtang terrane using magnetotelluric imaging abstract to determine the deep electrical structure of the qiangtang terrane northern tibetan plateau we reanalysed three broadband magnetotelluric mt profiles collected by china university of geosciences wuhan in 1993 1994 and derived corresponding 2 d resistivity models in these models a remarkable high conductivity layer that was divided into northern and southern parts and that had formed an antiformal zone of high conductivity beneath the central qiangtang terrane was visible this high conductivity layer corresponds well with the trace of the southward subduction of the melange from the jinsha river suture inferred from surface geology according to our resistivity models the crust of the northern qiangtang terrane is mostly composed of melange the underplated melange may be closely related to the formation of the high conductivity layer and provide a reasonable explanation for the cause of special geophysical features in this area in addition the similarity between these resistivity models provides evidence that the qiangtang anticlinorium extends eastward to at least 92 e whereas the differences between them may offer an explanation for the gradual narrowing of the metamorphic belt from west to east in the central qiangtang terrane

triassic subduction of the paleo tethys in northern tibet china evidence from the geochemical and isotopic characteristics of eclogites and blueschists of the qiangtang block abstract a 500 km long high pressure metamorphic belt occurs in the qiangtang block of northern tibet the occurrence of high pressure metamorphic rocks eclogite and blueschist has a significant bearing on the paleo tethys subduction and plate suturing processes a few studies have been conducted on petrology mineralogy and geochronology of high pressure rocks in this paper we present the results of a geochemical and isotopic investigation on eclogite and blueschist from this belt and discuss the probable geodynamic implications geochemical analyses show that the two rock types eclogite and blueschist have grossly similar bulk compositions but they can be subdivided into two groups based on their tio 2 contents the low tio 2 2 2 75 wt rocks are more akin to ocean island basalts oib the positive e nd t values ranging from 0 4 to 6 3 for all samples suggest that the basaltic protoliths of the eclogites and blueschists were derived from a long term depleted mantle source however their geochemical characteristics require that the source be metasomatized and enriched in large ion lithophile and light rare earth elements shortly before the formation of these basaltic protoliths like for most oib and e morb of the world the recognition of oceanic basalts suggests that a paleozoic ocean should be present in the central qiangtang block this ocean was hypothesized to be the western extension of the changning menglian paleo tethys ocean in the eastern margin of the tibetan plateau the eclogite and blueschist were formed by northward subduction of this paleozoic ocean in the triassic

detection of southward intracontinental subduction of tibetan lithosphere along the bangong nujiang suture by p to s converted waves teleseismic primary p to secondary s converted waves recorded on the indepth iii seismic array have been used to detect lithospheric scale deformation structures of the central tibetan plateau from the central lhasa terrane to the central qiangtang terrane a south dipping crustal converter is seen from the upper crust near the 500 km long metamorphic core complex exposures in the qiangtang terrane to the lower crust near the bangong nujiang suture at deeper depths a southeast dipping mantle converter is seen extending from 50 km north of the bangong nujiang suture at the depth of the moho to a depth of 180 km 100 km south of the bangong nujiang suture we found the observations to be most consistent with a model of lithospheric deformation involving 1 southward subduction of the tibetan lithospheric mantle along the bangong nujiang suture and 2 thickening of the central tibetan crust through a thick skinned crustal accretionary thrust wedge structure in response to the india9s collision with asia

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

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

carboniferous arc magmatism in the qiangtang area northern tibet zircon u pb ages geochemical and lu hf isotopic characteristics and tectonic implications abstract the longmu co shuanghu suture zone lssz which lies between the southern qiangtang baoshan block and northern qiangtang qamdo block on the tibetan plateau represents remnants of the paleo tethys ocean previous investigations have focused on the ophiolite high pressure zone and metamorphic belt in this zone whereas few studies have considered the arc magmatism the present study examines a suite of carboniferous arc magmatic rocks in the western region of the lssz that consist of acidic to basic volcanic rocks including rhyolite dacite andesite basaltic andesite basalt and pyroclastics zircon u pb dating of three samples two andesites and one dacite by laser ablation inductively coupled plasma mass spectrometry demonstrates that these rocks were emplaced at ca 351 346 ma the basalts have low cr and ni abundances indicating that they were generated by varying degrees of partial melting of an enriched lithospheric mantle that was metasomatized by subduction derived components zircons in the felsic rocks have positive e hf t values of 0 8 14 8 indicating an origin by varying degrees of partial melting of juvenile basaltic lower crust consisting mainly of underplated magmas with similar compositions to the basalts geochemical data show that the volcanic rocks are enriched in light rare earth elements and large ion lithophile elements e g th u and pb but are strongly depleted in high field strength elements e g nb ta and ti have high al 2 o 3 and low zr concentrations and are calc alkaline these magmas are therefore subduction related volcanic arc rocks indicating that paleo tethys subduction began in the early carboniferous

triassic nb enriched basalts magnesian andesites and adakites of the qiangtang terrane central tibet evidence for metasomatism by slab derived melts in the mantle wedge new chronological geochemical and isotopic data are reported for triassic 219 236 ma adakite magnesian andesite nb enriched basaltic rock associations from the tuotuohe area central qiangtang terrane the adakites and magnesian andesites are characterized by high sr y 25 45 la yb 14 42 and na2o k2o 12 49 ratios high al2o3 15 34 18 28 wt and moderate to high sr concentrations 220 498 ppm and end t 0 86 to 1 21 values low enrichments of th rb relative to nb and subequal normalized nb and la contents and enrichments of light rare earth elements combine to distinguish a group of nb enriched basaltic rocks nebs they have positive end t 2 57 to 5 16 values positive correlations between th la and nb and an absence of negative nb anomalies on mantle normalized plots indicate the nebs are products of a mantle source metasomatized by a slab melt rather than by hydrous fluids a continuous compositional variation between adakites and magnesian andesites confirms slab melt interaction with mantle peridotite the spatial association of the nebs with adakites and magnesian andesites define an adakitic metasomatic volcanic series recognized in many demonstrably subduction related environments e g mindanao arc philippines kamchatka arc russia and southern baja california arc mexico the age of the touhuohe suite and its correlation with triassic neb to the north indicates that volcanism derived from subduction modified mantle was abundant prior to 220 ma in the central qiangtang terrane

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

late triassic batang group arc volcanic rocks in the northeastern margin of qiangtang terrane northern tibet partial melting of juvenile crust and implications for paleo tethys ocean subduction the batang group btg volcanic rocks in the zhiduo area with nw trending outcrops along the northeastern margin of the qiangtang terrane northern tibet are mainly composed of volcaniclastic rocks dacite and rhyolite major and trace element sr and nd isotope zircon u pb and hf isotope data are presented for the btg dacites laser ablation inductively coupled plasma mass spectrometry zircon u pb dating constrains the timing of volcanic eruption as late triassic 221 1 ma major and trace element geochemistry shows that the btg volcanic rocks are classified as calc alkaline series all samples are enriched in large ion lithophile elements and light rare earth elements with negative slightly positive eu anomalies eu eu 0 47 1 15 and depleted in high field strength elements and heavy rare earth elements in addition these rocks possess less radiogenic sr 87sr 86sr i 0 7047 0 7078 much radiogenic nd end t 4 2 to 1 3 and hf ehf t 4 0 6 6 isotopes suggesting that they probably originated from partial melting of a crustal source containing a mantle derived juvenile component the inferred magma was assimilated by crustal materials during ascending and experienced significant fractional crystallization by combining previously published and the new data we propose that the btg volcanic rocks were genetically related to southwestward subduction of the ganzi litang ocean a branch of paleo tethys in the northeastern margin of the qiangtang terrane given the coeval arc affinity magmatic rocks in the region we envisage that the ganzi litang ocean may extend from the zhongdian arc through the yidun terrane to the zhiduo area probably even further northwest to the tuotuohe area

the late quaternary loess record of tokaj hungary reconstructing palaeoenvironment vegetation and climate using stable c and n isotopes and biomarkers abstract large areas of southeast europe are covered by pleistocene loess deposits with interbedded palaeosol layers reflecting interglacials or stadials these loess palaeosol sequences are valuable terrestrial archives for continuous palaeoclimatic and palaeoenvironmental information in recent years the traditional paradigm of treeless full glacial palaeoenvironments has increasingly been called into question as there is palynological anthracological and malacological evidence for the continuous presence of trees from loess palaeosol sequences in the carpathian basin this paper aims at contributing to this discussion by applying biogeochemical geophysical and sedimentological methods to a late quaternary loess palaeosol profile at tokaj ne hungary previous studies at the same location have focused on molluscs and charcoal while this study concentrated on stable c and n isotopes and lipid biomarkers long chain n alkanes c 23 c 33 which form major constituents of plant leaf waxes have successfully been used as biomarkers over the last years and allow for a reconstruction of palaeovegetation based on the differentiation of grassland vs shrubs and trees stable carbon isotopes were used to further differentiate between c 3 and c 4 palaeovegetation and stable n isotopes to reconstruct the n cycle and climatic variations magnetic susceptibility ms organic carbon and grain size distribution which may serve as proxies for e g the intensity of pedegenesis weathering and palaeowind strength were measured to complement the biogeochemical results and to establish a pedo and biostratigraphy although the results do not agree in every detail with previous studies there are surprising matches and similarities in the data alkane and stable n isotopic data indicate that pedogenesis occurred under productive fertile steppe grasslands probably in an intensified seasonal climate and during reduced sedimentation two short treeless steppe periods were found between 26 and 12 ka bp however periods characterised by a cooler climate in which trees and shrubs expanded were also found this interpretation is also supported by previous mollusc data that provide evidence for several periods of favourable conditions for tree growth

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

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

lithospheric and upper mantle structure of the northeastern tibetan plateau 1 we use receiver functions calculated for data collected by the indepth iv seismic array to image the three dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern tibet our results indicate an average crustal thickness of 65 to 70 km in northern tibet in addition we observe a 20 km moho offset beneath the northern margin of the kunlun mountains a 10 km moho offset across the jinsha river suture and gently northward dipping moho beneath the qaidam basin a region in the central qiangtang terrane with higher than normal crustal vp vs ratio of 1 83 can be the result of the eocene magmatic event in the qiangtang terrane we observe a significant lithospheric mantle discontinuity beneath the bangong nujiang suture at 80 km depth which dips 10 to the north reaching 120 km depth we interpret this feature as either a piece of lhasa terrane or remnant oceanic slab underthrust below northern tibet we detect a 20 km depression of the 660 km discontinuity in the mantle transition zone beneath the northern lhasa terrane in central tibet which suggests this phase transition has been influenced by a dense and or cold oceanic slab a modest 10 km depression of the 410 km discontinuity located beneath the northern qiangtang terrane may be the result of localized warm upwelling associated with small scale convection induced by the penetration of the sinking indian continental lithosphere into the transition zone beneath the central tibetan plateau

u pb zircon age geochemical and lu hf isotopic constraints of the southern gangma co basalts in the central qiangtang northern tibet abstract a recent study suggests that the central qiangtang is a key locality to investigate the evolution of the opening and closure of the paleo tethys ocean basalts are commonly associated with supercontinent fragmentation and they have the potential to indicate the tectonic environment into which they were erupted in this study laser ablation inductively coupled plasma mass spectrometry la icp ms u pb zircon dating whole rock geochemistry and zircon hf isotope analysis were used to study newly discovered basalts in the southern gangma co area of central qiangtang dating of magmatic zircons from three basalt samples indicates that the basalts erupted in the late devonian to early carboniferous early tournaisian at 360 350 ma the basalts are geochemically similar to within plate basalts and formed from a depleted mantle source we suggest that the basalts may have been a product of the breakup of the northern margin of gondwana in the late devonian to early carboniferous 360 350 ma which may be linked to the initial rifting and opening of the paleo tethys ocean in the qiangtang area

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

an alternative plate tectonic model for the palaeozoic early mesozoic palaeotethyan evolution of southeast asia northern thailand burma abstract an alternative model for the geodynamic evolution of southeast asia is proposed and inserted in a modern plate tectonic model the reconstruction methodology is based on dynamic plate boundaries constrained by data such as spreading rates and subduction velocities in this way it differs from classical continental drift models proposed so far the different interpretations about the location of the palaeotethys suture in thailand are revised the tertiary mae yuam fault is seen as the emplacement of the suture east of the suture we identify an indochina derived terrane for which we keep the name shan thai formerly used to identify the cimmerian block present in southeast asia now called sibumasu this nomenclatural choice was made on the basis of the geographic location of the terrane eastern shan states in burma and central thailand and in order not to introduce new confusing terminology the closure of the eastern palaeotethys is related to a southward subduction of the ocean that triggered the eastern neotethys to open as a back arc due to the presence of late carboniferous early permian arc magmatism in mergui burma and in the lhasa block south tibet and to the absence of arc magmatism of the same age east of the suture in order to explain the presence of carboniferous early permian and permo triassic volcanic arcs in cambodia upper triassic magmatism in eastern vietnam and lower permian middle permian arc volcanites in western sumatra we introduce the orang laut terranes concept these terranes were detached from indochina and south china during back arc opening of the poko song ma system due to the westward subduction of the palaeopacific this also explains the location of the cathaysian west sumatra block to the west of the cimmerian sibumasu block

early ordovician granites from the south qiangtang terrane northern tibet implications for the early paleozoic tectonic evolution along the gondwanan proto tethyan margin abstract early ordovician granites occur in the gemuri and bensong co areas of the south qiangtang terrane northern tibet and la icp ms zircon u pb dating reveals that the magmatism took place between 486 and 480 ma the geochemical features of these early ordovician granites are comparable to those of high k calc alkaline s type granites they exhibit negative values of zircon ehf t 8 5 to 1 7 and whole rock end t 8 1 to 0 5 as well as old zircon hf model ages tcdm 1349 1992 ma and whole rock nd model ages tcdm 1145 1804 ma these early ordovician granites can be attributed to partial melting of the paleoproterozoic metasedimentary rocks of the south qiangtang terrane while minor amounts of mantle derived components were introduced into their magma sources taking into account previous data we propose that the early ordovician granites developed in a post collisional tectonic setting and that they were related to the collisional accretion of asian microcontinental fragments such as the south china north china tarim and qaidam kunlun terranes that took place as a result of oceanic subduction along the gondwana proto tethyan margin during the early paleozoic

origin and pre cenozoic evolution of the south qiangtang basement central tibet abstract the qiangtang terrane in central tibet with its high pressure rocks is a key area to unravel the evolution of the paleo tethys we present a new 1 50 000 map of the rongma area in the central qiangtang terrane of which relatively little is known so far we show that the qiangtang metamorphic belt can be separated in a paleozoic autochthonous basement and an overlying allochthonous thrust stack of subduction melange that contains high pressure rocks and permian sediments detrital zircon ages youngest peak at 591 ma and an 470 ma granite intrusion age constrain the age of the qiangtang basement to be between the late precambrian to middle ordovician this is in agreement with the observed unconformity between basement and overlying mid late ordovician strata this unconformity and the zircon age spectra are comparable to the himalaya area the qiangtang terrane must thus have been part of the gondwana supercontinent during the early paleozoic the occurrence of the late triassic eclogite and glaucophane bearing schists in the central qiangtang terrane indicates the existence of a suture zone between the north and south qiangtang terranes before the late triassic this suture zone resulted from closure of the paleo tethys between the north and south qiangtang terranes late triassic syn collisional granites and rapidly exhumed high pressure rocks resulted from the closure of this ocean and final amalgamation of the re united qiangtang terranes with eurasia collision of the lhasa and qiangtang terranes is correlated with north dipping reverse faults in the south qiangtang terrane which were probably related to exhumation of the qiangtang basement and formation of the qiangtang culmination

early palaeozoic orogenesis along the indian margin of gondwana tectonic response to gondwana assembly abstract shrimp u pb dating of zircons from a peralkaline s type lesser himalayan granite from the kathmandu region nepal indicate an age of emplacement of 475 ma the granites along with metasedimentary xenoliths show a similar signature of inherited detrital zircon ages ranging from archaean to early palaeozoic with prominent late mesoproterozoic grenvillian and neoproterozoic pan african age peaks and have a maximum age of 500 ma based on the youngest detrital grains deformation structures in xenoliths are truncated by the granite and along with the granites are assigned to a cambro ordovician orogenic event herein termed the bhimphedian orogeny that can be traced across the himalaya from pakistan to the eastern himalaya and possibly extends west into afghanistan we interpret the orogeny as being related to andean type orogenic activity on the northern margin of the indian continent following gondwana assembly the magmatic arc was associated with andesitic and basaltic volcanism and was active from c 530 to 490 ma the arc activity overlaps with and is succeeded by regional deformation crustal melting and s type granite emplacement that extended to 470 ma orogenic activity was driven by coupling across the plate margin either during on going subduction or through accretion of microcontinental ribbons possibly represented by the lhasa and qiangtang blocks it represents the termination of an orogenic cycle termed the north indian orogen that commenced with neoproterozoic rifting and passive margin development and terminated with the bhimphedian orogeny this was succeeded by a return to passive margin setting along the northern indian margin of gondwana which continued until the cenozoic himalayan orogeny

zircon u pb hf isotopes and whole rock geochemistry of gneissic granites from the jitang complex in leiwuqi area eastern tibet china record of the closure of the paleo tethys ocean abstract previous investigations have indicated that the changning menglian cmsz and longmu co shuanghu lssz suture zones represent the paleo tethys ocean of northern tibet and western yunnan of china respectively however our understanding of the paleo tethys suture zone in eastern tibet seems to be incomplete the north lancangjiang suture zone nlsz has been hypothesized to mark the paleo tethys ocean in eastern tibet the present study reports new zircon u pb ages and hf isotope data and whole rock major and trace element data from the gneissic granites from the jitang complex in the leiwuqi area of the nlsz in order to verify this hypothesis six samples of gneissic granites yielded weighted mean 206 pb 238 u ages of 249 237 ma the geochemical data show that the gneissic granites are characterized by high sio 2 70 79 77 75 wt k 2 o 2 00 6 13 wt low a cnk values hf t values from 2 7 to 2 5 and t c dm model ages 1447 1009 ma indicate that they were generated by the partial melting of the meso proterozoic juvenile crust these gneissic granites were interpreted to have formed in the post collisional tectonic setting related to the closure of the paleo tethys ocean by combining previously published and the new data we propose that the nlsz connects the lssz and cmsz in the tibetan plateau and marks a paleo tethys ocean basin in eastern tibet

a review of permian stratigraphy palaeobiogeography and palaeogeography of the qinghai tibet plateau the permian marine lithostratigraphy biostratigraphy and faunal compositions among all the major tectonic blocks in the qinghai tibet plateau are synthesised based on published literature authors unpublished data and field observations palaeobiogeographically the permian marine faunas in the qinghai tibet plateau can be assigned to four biotic provinces cathaysian indoralian cimmerian and himalayan provinces as well as a broad south transitional zone for parts of the permian the qamdo block clearly belongs to the cathaysian province as it is characterised by a lack of glacial deposits and a dominance of warm water faunas during the whole permian the qiangtang block lhasa block and the himalaya tethys zone in southern tibet can be grouped into the south transitional zone during the cisuralian which is characterised by the combination of the bivalve eurydesma the brachiopods cimmeriella bandoproductus and the fusuline monodiexodina the south transitional zone gave birth to the cimmerian province as shown in the qiangtang and lhasa blocks from roadian to capitanian during which its faunas were characterised by an admixture of cold and warm water elements supplemented by a limited number of endemic taxa such as the fusuline eopolydiexodina the non fusuline foraminifer shanita hemigordiopsis assemblage and the coral thomasiphyllum the formation and development of the south transitional zone and the cimmerian province is interpreted to be closely linked to and driven by the northward drift of the cimmerian continent superimposed with a gradual global warming in the wake of gondwanan deglaciation the consequently combined effects of these events forced the climate of the drifting peri gondwana blocks to ameliorate and its marine faunas to transform from an essentially cold water gondwanan type in the early cisuralian through a transitional stage to a predominantly warm water cathaysian type by the lopingian the himayala tethys zone in southern tibet is assigned to the himalayan province from late guadalupian to lopingian and identified by persistent occurrences and dominance of cold water brachiopods and corals both the permian sedimentary sequences and faunal successions are strongly different between the qamdo block and the qiangtang block this is taken to suggest that the longmu co shuanghu lancangjiang suture between the two blocks most likely represents the main branch of the palaeotethys suture in tibet a comparison and correlation of permian sequences and faunas between tibetan blocks and adjacent blocks in western yunnan indicates that the qiangtang block should be aligned with the baoshan block whereas the lhasa block more closely related to the tengchong block further comparison and correlations of permian stratigraphy and faunas of all major peri gondwana blocks suggest that they probably constituted three different continental slices rather than one as perceived in some previous literature the first slice is composed of central pamir the qiangtang block and the baoshan block the second slice consists of south pamir karakorum south afghanistan central iran and turkey these two continental slices are both characterised by a continuously active rifting event from artinskian to guadalupian by contrast the third slice is interpreted to consist of the lhasa block the tengchong block and the sibumasu block none of which has late cisuralian rift related basalts the baoshan block and the lhasa block are thought to have initially early cisuralian intersected probably at a triple junction point close to western australia and were then rifted off from gondwana at different times during the permian

tectonic framework and phanerozoic evolution of sundaland abstract sundaland comprises a heterogeneous collage of continental blocks derived from the india australian margin of eastern gondwana and assembled by the closure of multiple tethyan and back arc ocean basins now represented by suture zones the continental core of sundaland comprises a western sibumasu block and an eastern indochina east malaya block with an island arc terrane the sukhothai island arc system comprising the linchang sukhothai and chanthaburi blocks sandwiched between this island arc formed on the margin of indochina east malaya and then separated by back arc spreading in the permian the jinghong nan uttaradit and sra kaeo sutures represent this closed back arc basin the palaeo tethys is represented to the west by the changning menglian chiang mai inthanon and bentong raub suture zones the west sumatra block and possibly the west burma block rifted and separated from gondwana along with indochina and east malaya in the devonian and were accreted to the sundaland core in the triassic west burma is now considered to be probably cathaysian in nature and similar to west sumatra from which it was separated by opening of the andaman sea basin south west borneo and or east java west sulawesi are now tentatively identified as the missing argoland which must have separated from nw australia in the jurassic and these were accreted to se sundaland in the cretaceous revised palaeogeographic reconstructions illustrating the tectonic and palaeogeographic evolution of sundaland and adjacent regions are presented

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

contrasting rift and subduction related plagiogranites in the jinshajiang ophiolitic melange southwest china and implications for the paleo tethys 1 the jinshajiang ophiolitic melange zone in southwest china represents a remnant of the eastern paleo tethys ocean field geochronological and geochemical studies have identified two distinct suites of plagiogranites within the melange the dongzhulin trondhjemite and jiyidu tonalite which represent rift and subduction settings respectively related to opening and closing of the ocean shrimp u pb analysis on zircons extracted from the dongzhulin trondhjemite yields a mean 206pb 238u age of 347 7 ma ree and isotopic characteristics suggest an origin from low pressure partial melting of an amphibolitic protolith highly variable hf isotopic compositions for zircons from this body may indicate a heterogenous source involving both depleted mantle and enriched continental components this together with geologic relations suggests formation near an embryonic spreading center in a continent ocean transition setting the jiyidu tonalite has a u pb zircon age of 283 3 ma and geochemical data indicates high sr y la yb n nb ta and low y and marked heavy ree depletion these signatures suggest derivation from low degree partial melting of subducted slab at pressure high enough to stabilize garnet and rutile a slab melt origin is also supported by in situ hf and o data for zircon that show isotopic compositions comparable with typical altered oceanic crust thus the crystallization age of the jiyidu high sr y tonalite provides a constraint for the subduction of the jinshajiang ocean floor the rift related dongzhulin trondhjemite and subduction related jiyidu high sr y tonalite constrain the timing and setting of opening and closing of this segment of the paleo tethys ocean

petrogenesis and tectonic setting of triassic granitoids in the qiangtang terrane central tibet evidence from u pb ages petrochemistry and sr nd hf isotopes abstract triassic granitoids including the 220 ma shuanghu and 210 ma rongma granitoids studied here are widely distributed around the longmu shuanghu suture and in the qiangtang terrane central tibet the majority of these granitoids can be classified as high k calc alkaline in nature and yield negative ba and sr anomalies on primitive mantle normalized diagrams in addition they are enriched in light rare earth elements lree la yb n 1 61 21 79 strongly enriched in large ion lithophile elements lile e g cs rb and k and depleted in high field strength elements hfse e g nb and ti magma mixing played a role in the genesis of the shuanghu granodiorites as indicated by the occurrence of dioritic enclaves and the wide range in zircon hf compositions ehf t 15 0 to 2 5 the i type shuanghu granodiorites and s type shuanghu and rongma granites might have been derived from melting of southern qiangtang crust given the high initial sr 0 7131 0 7272 low end t 8 9 to 11 1 and zircon ehf t values 15 to 7 2 the granitoids may have formed during melting of southern qiangtang crust heated by upwelling asthenosphere mantle a result of break off and delamination of the paleo tethys slab in a collisional setting

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

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