seismic moment catalog of large shallow earthquakes 1900 to 1989 we compile a worldwide catalog of shallow depth 70 km and large ms 7 earthquakes recorded between 1900 and 1989 the catalog is shown to be complete and uniform at the 20 sec surface wave magnitude ms 7 0 we base our catalog on those of abe 1981 1984 and abe and noguchi 1983a b for events with ms 7 0 those catalogs however are not homogeneous in seismicity rates for the entire 90 year period we assume that global rates of seismicity are constant on a time scale of decades and most inhomogeneities arise from changes in instrumentation and or reporting we correct the magnitudes to produce a homogeneous catalog the catalog is accompanied by a reference list for all the events with seismic moment determined at periods longer than 20 sec using these seismic moments for great and giant earthquakes and a moment magnitude relationship for smaller events we produce a seismic moment catalog for large earthquakes from 1900 to 1989 the catalog is used to study the distribution of moment released worldwide although we assumed a constant rate of seismicity on a global basis the rate of moment release has not been constant for the 90 year period because the latter is dominated by the few largest earthquakes we find that the seismic moment released at subduction zones during this century constitutes 90 of all the moment released by large shallow earthquakes on a global basis the seismic moment released in the largest event that occurred during this century the 1960 southern chile earthquake represents about 30 to 45 of the total moment released from 1900 through 1989 a frequency size distribution of earthquakes with seismic moment yields an average slope b value that changes from 1 04 for magnitudes between 7 0 and 7 5 to b 1 51 for magnitudes between 7 6 and 8 0 this change in the b value is attributed to different scaling relationships between bounded large and unbounded small earthquakes thus the earthquake process does have a characteristic length scale that is set by the downdip width over which rupture in earthquakes can occur that width is typically greater for thrust events at subduction zones than for earthquakes along transform faults and other tectonic environments

tectonic feedback and the earthquake cycle the occurrence of cyclical instabilities along plate boundaries at regular intervals suggests that the process of earthquake causation differs in some respects from the model of elastic rebound in its simplest forms the model of tectonic feedback modifies the concept of this original model in that it provides a physical interaction between the loading rate and the state of strain on the fault two examples are developed a central chile and b mexico the predictions of earthquake hazards for both types of models are compared

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

ages and petrogenesis of the late triassic andesitic rocks at the luerma porphyry cu deposit western gangdese and implications for regional metallogeny abstract the tibetan plateau is one of the most significant cu poly metallic mineralization regions in the world and preserves important information related to subductional and collisional porphyry cu mineralization this study investigates a new occurrence of cu mineralization related andesitic porphyries in the western domain of the gangdese magmatic belt and assesses its petrologic zircon u pb geochronology whole rock chemistry and sr nd hf pb isotope data zircon u pb dating of three ore related porphyries yields crystallization ages of 212 211 ma these ages are consistent with previous molybdenite re os dating indicating a late triassic magmatic and cu mineralization event in the western gangdese magmatic belt nb ta and ti depletion th and lree enrichment and high la yb and th yb ratios in addition to high u yb ratios from zircons suggest that the magma was generated in an active continental arc setting the porphyries have radiogenic isotopic compositions with 87sr 86sr i 0 70431 0 70473 end t 1 1 to 3 8 207pb 204pb i 15 601 15 622 and 208pb 204pb i 38 450 38 693 as well as high positive zircon ehf t values from 6 2 to 10 6 mean value 8 3 corresponding to model ages tdm ranging from 509 ma to 819 ma mean 646 ma this suggests that the andesitic magmatism was dominantly sourced from depleted mantle materials that were modified by subducted oceanic sediment derived melts during the subduction of the neo tethys ocean the mineralization related porphyries contain amphibole and epidote as well as high whole rock fe2o3 feo and zircon ce4 ce3 ratios suggesting hydrous and highly oxidized parent magmas considering the existing cu mineralization and highly oxidized magma of the well preserved triassic andesitic igneous rocks in the western gangdese belt the subduction related continental arc magma system is favorable for subduction related porphyry cu deposits the existence of luerma porphyry mineralization demonstrates that there are at least five generations of porphyry cu mo au mineralization in the gangdese magmatic belt which advances the timeframe of porphyry mineralization to the late triassic

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

first report of lower permian basalts in south tibet tholeiitic magmatism during break up and incipient opening of neotethys abstract the neo tethys ocean began to form at early permian times when continental flood basalts were emplaced in various areas of the newly formed indian passive margin exposed today in the so called tibetan sedimentary zone of the himalaya lower permian mafic volcanic rocks which have long been known from various himalayan localities from kashmir to arunachal pradesh are here for the first time reported to occur also in south tibet bhote kosi basalts of the gyirong county the basalts unconformably overlie lowermost permian diamictites with locally intervening black shales and debris flow deposits and are followed in turn by chert bearing quartzarenites and silty to phosphatic marls yielding brachiopods of roadian wordian age the age of the lavas can thus be bracketed as late early permian post sakmarian and pre roadian the geochemistry of these subalkalic tholeiites akin to morbs testifies to their similarity not only with the adjacent nar tsum spilites of central nepal but also with the panjal traps and abor volcanics of the western and eastern himalayas respectively the geochemical signature of lower permian volcanic rocks is in fact uniform all along the himalayan range and markedly different from that of basaltic rhyolitic alkalic products sporadically emplaced during the previous rifting stage rift volcanism in the tethys himalaya began in the early carboniferous and came to an end in sakmarian times in the early permian initial submergence of the rift shoulders and sediment starvation were followed by tholeiitic magmatism which is therefore interpreted as following break up and incipient sea floor spreading in the neotethys ocean roughly contemporaneous emplacement of continental flood basalts of similar geochemical signature along a 2000 km long rift axis would in fact suggest extensive mantle melting at the transition from continental rifting to break up and opening of the neotethys between northern gondwana and the peri gondwanian blocks

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

generation of leucogranites via fractional crystallization a case from the late triassic luoza batholith in the lhasa terrane southern tibet abstract leucogranites are important in understanding intracrustal differentiation and regional tectonic evolution but how these rocks form remains a matter of much debate the luoza batholith in the central lhasa subterrane provides an important opportunity to address these issues as this batholith consists of normal calc alkaline s type granodiorites group 1 and highly fractionated s type leucogranites group 2 laser ablation inductively coupled plasma mass spectrometry zircon u pb dating indicates that these rocks were emplaced at ca 221 205 ma the group 1 samples ca 205 ma have low sio2 content 64 67 wt and low differentiation index values di 75 76 and are slightly depleted in ba p sr and ti compared to the group 2 samples ca 221 ma and 213 ma which have high sio2 content 74 75 wt high differentiation index values 91 94 and are significantly depleted in ba p sr and ti samples from group 1 and group 2 exhibit similar zircon ehf t values 15 6 to 1 4 both of which are significantly higher than melts from the pure ancient basement in the central lhasa subterrane 20 5 to 10 6 the group 1 samples can be interpreted as having been derived from the partial melting of metagreywacke within the ancient basement in the central lhasa subterrane with contributions from mantle or juvenile crust derived components whereas the group 2 samples may have formed via varying degrees of fractional crystallization of different minerals e g plagioclase k feldspar biotite zircon allanite monazite titanite and apatite from the parental magmas represented by the granodioritic samples in group 1 in shallow crustal magma chambers in combination with sedimentary records the high temperature and low pressure conditions indicated by the group 1 samples can be geodynamically linked to a back arc extensional setting in response to the southward subduction of the bangong nujiang tethys ocean seafloor our work implies that fractional crystallization is likely a feasible mechanism for the development of leucogranites and for explaining the compositional diversity of the granite during intracrustal differentiation

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

cenozoic volcanism and tectonic evolution of the tibetan plateau abstract cenozoic volcanism on the tibetan plateau which shows systematic variations in space and time is the volcanic response to the india asia continental collision the volcanism gradually changed from na rich k rich to potassic ultrapotassic adakitic compositions along with the india asia collision shifting from contact collision i e soft collision or syn collision to all sided collision i e hard collision the sodium rich and potasium rich lavas with ages of 65 40 ma distribute mainly in the lhasa terrane of southern tibet and subordinately in the qiangtang terrane of central tibet the widespread potassic ultrapotassic lavas and subordinate adakites were generated from 45 to 26 ma in the qiangtang terrane of central tibet subsequent post collisional volcanism migrated southwards producing ultrapotassic and adakitic lavas coevally between 26 and 8 ma in the lhasa terrane then potassic and minor adakitic volcanism was renewed to the north and has become extensive and semicontinuous since 20 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations provide important constraints on the geodynamic processes that evolved at depth to form the tibetan plateau these processes involve roll back and break off of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root and consequently northward underthrusting of the indian lithosphere the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder and detach owing to the persistently northward push of the underthrust indian lithosphere

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

implications of aplite dykes for mineralization in the late cretaceous vein type xiuwacu w mo deposit in the southern yidun terrane se tibetan plateau abstract the xiuwacu w mo deposit with 24 000 t of mo and 23 000 t of wo3 is located in the southern yidun terrane syt sw china being the only quartz vein type w mo deposit in the region in the deposit aplite dykes are spatiotemporally associated with w mo orebodies however their genetic relationship remains unclear here we present the result of a study of the petrography zircon u pb dating whole rock geochemistry sr nd pb isotopes zircon trace elements and hf isotopes of the aplite dykes the xiuwacu aplites are emplaced at 78 7 1 9 ma and have high silica alkali rich and metaluminous peraluminous compositions they are enriched in light rare elements lrees large ion lithophile elements liles u rb th and depleted in heavy rare earth elements hrees high field strength elements hfses nb p ti and with strong negative eu anomalies eu 0 09 0 58 average 0 36 the aplites have an average estimated crystallization temperature of 700 c and relatively low magmatic oxygen fugacity log fo2 33 2 to 16 1 average 26 1 which is substantially lower than the late triassic biotite granite but similar to the late cretaceous porphyritic monzogranite in the deposit all the aplite samples have varying 87sr 86sr i ratios 0 701240 0 719258 negative end t values 10 33 to 3 22 negative ehf t values 5 3 to 3 3 and ancient two stage hf depleted mantle model ages tdm2 hf 1886 2070 ma these geochemical characteristics demonstrate that the aplite dykes were formed by the partial melting of the ancient lower crust under an extensional setting which was triggered by the nw se ward subduction of the neo tethys ocean slab during the late cretaceous the ancient lower crust is interpreted to have provided the main source of w and mo for the xiuwacu deposit the enrichment of w and mo by fractional crystallization of the aplite magmas played an important role in concentrating w mo during the formation of the deposit aplite dykes have also been found in several late cretaceous mo and or cu deposits in the syt considering their common occurrence within the orebodies the aplite dykes could be an effective prospecting indicator

early cretaceous exhumation of the qiangtang terrane during collision with the lhasa terrane central tibet the timing of the closure of the bangong ocean between the lhasa and south qiangtang terranes in central tibet and the resulting crustal thickening are still under debate we integrate published apatite fission track and u th he thermochronometer data with new zircon u th he ages from eight samples and with structural profiles to document that the south qiangtang terrane experienced slow exhumation between 200 and 150 ma associated with the opening of the bangong ocean accelerated exhumation around 0 2 0 3 mm a of the south qiangtang terrane was initiated at around 150 ma this exhumation event is interpreted to reflect collision between the lhasa and south qiangtang terranes after closure of the bangong ocean associated with crustal thickening via thick skinned folding and thrusting within the south qiangtang terrane the amalgamation of the lhasa and south qiangtang terranes recorded here may represent the first stage of crustal thickening in the central tibetan plateau this article is protected by copyright all rights reserved

observations of the antarctic circumpolar current over the udintsev fracture zone the narrowest choke point in the southern ocean an up to date map of the antarctic circumpolar current acc fronts is constructed from the latest version of mean dynamic topography mdt from satellite altimetry and reveals the narrowest acc width in the udintsev fracture zone ufz with the strongest concentration of the three major acc fronts within a limited distance as short as 170 km about 40 narrower than that at drake passage at 144 w at the entrance of the ufz which lies between the pacific antarctic ridge par and its eastwardly offset segment offset par segment there is a triple confluence of the subantarctic front polar front and southern acc front downstream of this longitude the subantarctic front progressively meanders northward over the relatively shallow offset par segment before channeling through the eltanin fracture zone thus diverging from the polar front which proceeds through the ufz in situ observations from two recent cruises at 144 w confirm the satellite altimetry derived frontal circulation in the ufz region and yield a baroclinic transport relative to the bottom of 113 10 6 m 3 s comparable to that through drake passage the hydrographic sections show no antarctic bottom water colder than 0 2 c characteristics of major water masses are described and the implications for their potential downstream modifications at drake passage are discussed in terms of the meridional overturning circulation across the acc mesoscale eddy activity with periods shorter than 90 days is predominantly concentrated in the immediate downstream area of the offset par segment suggesting a substantial poleward eddy heat flux there

preliminary conclusions of the royal society and academia sinica 1985 geotraverse of tibet the 1985 chinese british expedition to the tibetan plateau attempted to solve the question of the origin of the very thick crustal rocks in this region continuing northwards movement of the indian plate over the past 38 myr has given rise to severe folding and thrust faulting causing crustal thickening by internal deformation previous collisions of microplate terranes derived from gondwanaland occurred during mesozoic times but the kun lun terrane of northern tibet was already part of laurasia by the carboniferous

magmatism and metallogenic mechanism of the ga erqiong and galale cu au deposits in the west central lhasa subterrane tibet constraints from geochronology geochemistry and sr nd pb hf isotopes abstract the ga erqiong galale ore district in the bangong nujiang metallogenic belt hosts two large skarn cu au deposits associated with voluminous late cretaceous volcanism and plutonism in this study we present new zircon u pb age and hf isotopic data whole rock geochemical and sr nd pb isotopic data to constrain the geodynamic mechanisms of late cretaceous magmatism and metallogenesis within the ga erqiong galale ore district the intrusive rocks in the district include diorite quartz diorite granodiorite granite and dioritic porphyrite zircon u pb analyses suggest those rocks mainly emplaced at early late cretaceous 84 95 ma which coincide with the ore forming age of ga erqiong and galale deposits 87 and 88 ma respectively geochemically the granodiorite and dioritic porphyrite are metaluminous with depletion of nb ta and ti and characterized by high mgo 2 68 4 16 wt mg 55 87 sr 414 616 ppm sr y 30 56 cr 22 5 105 0 ppm and ni 26 6 59 7 ppm low y 10 6 14 0 ppm and heavy rare earth elements hree showing geochemical signatures of high mg adakitic rocks four dioritic porphyrite and three granodiorite samples exhibit homologous sr nd pb isotope compositions and have positive whole rock end t values of 1 1 to 3 0 87sr 86sr i values of 0 7045 0 7048 206pb 204pb t ratios of 18 42 18 73 207pb 204pb t ratios of 15 58 15 63 and 208pb 204pb t ratios of 38 54 38 88 besides 116 analyses from the ga erqiong galale adakitic rocks give positive zircon ehf t values 4 3 to 12 1 the geochemical and isotopic data indicate that the early late cretaceous high mg adakitic rocks in the ga erqiong galale ore district originated from the delaminated juvenile lower mafic crust that has been modified by the previously subduction partial melting of previously subduction modified delaminated juvenile lower crust not only formed the hydrous adakitic melts in highly oxidation state but also provided the ore metals e g cu au mo for the subsequent mineralization such melts reacted with surrounding mantle peridotite to elevate the contents of metallic elements again then the melts experienced varying degrees of fractional crystallization process in the shallow magma chamber to form the dioritic and granodioritic magmas those magmas emplaced into the limestone of duo ai and jiega formation to form the ga erqiong and galale skarn cu au deposits respectively field observation together with geochronological and geochemical data indicate that the early late cretaceous mineralization events in the northern and central lhasa subterranes took place in a transition tectonic setting from collisional orogeny to extension and the input of subduction modified juvenile crust play an important role in the late cretaceous ore forming events

zircon u pb geochronology and geochemistry of early jurassic granodiorites in sumdo area tibet constraints on petrogenesis and the evolution of the neo tethyan ocean abstract the gangdese magmatic belt is oriented e w and located between the indus yarlung tsangpo and bangong nujiang suture zones in tibet it records the subduction of the neo tethyan oceanic crust indian eurasian collision and uplift of the tibetan plateau here we present new zircon u pb geochronology hf isotopic compositions and whole rock geochemistry of the granodiorites in the sumdo area of the gangdese belt our aim was to identify the petrogenesis of granitoid rcoks and to clarify the early stage evolution of the neo tethyan ocean during the late triassic to early jurassic the granodiorites yield zircon u pb ages of 193 201 ma indicating an early jurassic magmatic event they are metaluminous with medium to high k calc alkaline geochemistry and show characteristics of i type granitoids they show enrichment in light rare earth elements lrees and depletion in high field strength elements hfses in primitive mantle normalized spider diagrams indicating that their initial magma was generated in a subduction related active continental margin they have negative ehf t values 11 3 to 4 1 and crustal model ages tc dm of 1 50 1 95 ga hf isotopes and whole rock geochemistry indicate that the granodiorites were derived from partial melting of ancient lower crust material mixed with mantle material our data in combination with published geochronological and geochemical data show that the late triassic early jurassic arc magmatism in the gangdese magmatic belt was formed by the northward subduction of the neo tethyan oceanic crust beneath the lhasa terrane

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

early middle jurassic paleogeography reconstruction in the western qaidam basin insights from sedimentology and detrital zircon geochronology abstract the western qaidam basin wqb is situated at the northeastern margin of the tibetan plateau it is adjacent to the altyn tagh range to the west and is bordered at the south and north by the eastern kunlun range and southern qilian range respectively sporadic jurassic outcrops are exposed along the altyn tagh range although subsurface data have revealed that the jurassic rocks also developed in the basin interior little is known about their distribution and depositional characteristics hydrocarbon exploration has revealed that the lower middle jurassic strata are important source rocks and reservoirs in the wqb however due to post depositional uplifting exhumation or deep burial the sediments provenance paleogeography and the basin properties during the early middle jurassic in the whole wqb remain less understood in this study we examine the sedimentary records combined with detrital zircon u pb geochronology based on an analysis of post depositional modification to reveal the source to sink relationship between the wqb and surrounding potential source areas and to further reconstruct the early middle jurassic paleogeography of the wqb during the early jurassic there were a series of small scale separated rift basins which were characterized by proximal sedimentation and rapid facies shift in the wqb specifically the lower jurassic sediments in the southern wqb mainly originated from the eastern kunlun range with minor supply from the paleozoic intrusions that might have been exposed in the hinterland of the qaidam block the provenance in the northern wqb was dominated by the uplift area to the west there was a broader depression basin in the southern wqb the eastern kunlun range and the ancestral altyn mountains served as the two dominant source areas for the clastic material of the southern wqb during the middle jurassic integrated analysis on sedimentology and provenance reveals that the southeastern tarim basin and the wqb were likely to have been interconnected or adjacent during middle jurassic period we thus infer that no large scale strike slip displacement along the altyn tagh fault occurred during that period combining regional tectonic settings we tentatively suggest that the extensional setting following the orogenic movement in the eastern part of the paleo tethys ocean could have been responsible for the formation of the early jurassic rift basins by the early middle jurassic the bangonghu nujiang tethys ocean and the mongol okhotsk ocean were subducted northward beneath the qiangtang terrane and siberian continent respectively these led to the presence of an extensional stress field and the formation of a broader depression basin in the southern wqb during the middle jurassic paleogeography reconstruction results indicate that the northern wqb should have much potential for hydrocarbon exploration

petrogenesis of the majiari ophiolite western tibet china implications for intra oceanic subduction in the bangong nujiang tethys abstract the majiari ophiolite lies in the western bangong nujiang suture zone which separates the qiangtang and lhasa blocks in central tibet the ophiolite consists of peridotite gabbro diabase and basalt zircon u pb dating yielded an age of 170 5 1 7 ma for the gabbro whereas 40 ar 39 ar dating of plagioclase from the same gabbro yielded ages of 108 4 2 6 ma plateau age and 112 2 ma isochron age indicating that the ophiolite was formed during the middle jurassic and was probably emplaced during the early cretaceous zircons from the gabbro have e hf t values ranging from 6 9 to 10 6 and f lu hf values ranging from 0 92 to 0 98 mafic lavas plot in the tholeiitic basalt field but are depleted in nb ta and ti and enriched in rb ba and th in the n morb normalized trace element spider diagram these lavas have whole rock e nd t values of 5 9 to 6 6 suggesting that they were derived from a depleted mantle source which was probably modified by subducted materials the majiari ophiolite probably formed in a typical back arc basin above a supra subduction zone ssz mantle wedge intra oceanic subduction occurred during the middle jurassic and collision of the lhasa and south qiangtang terranes likely occurred in the early cretaceous thus closure of the bangong nujiang tethys ocean likely occurred before the early cretaceous

the exhumation history of collision related mineralizing systems in tibet insights from thermal studies of the sharang and yaguila deposits central lhasa abstract the large newly discovered sharang porphyry mo deposit and nearby yaguila skarn pb zn ag mo deposit reside in the central lhasa terrane northern gangdese metallogenic belt tibet multiple mineral chronometers zircon u pb sericite 40 ar 39 ar and zircon and apatite u th he reveal that ore forming porphyritic intrusions experienced rapid cooling 100 c ma during a monotonic magmatic hydrothermal evolution the magmatic hydrothermal ore forming event at sharang lasted 6 0 myr 1 8 myr for cooling from 900 to 350 c and 4 0 myr for cooling from 350 to 200 c whereas cooling was more prolonged during ore formation at yaguila 1 8 myr from 900 to 500 c and a maximum of 16 myr from 900 to 350 c all porphyritic intrusions in the ore district experienced exhumation at a rate of 0 07 0 09 mm yr apatite he ages between 37 and 30 ma combined with previous studies this work implies that uplift of the eastern section of the lhasa terrane expanded from central lhasa 37 30 ma to southern lhasa 15 12 ma at an increasing exhumation rate all available geochronologic data reveal that magmatic hydrothermal exhumation activities in the sharang yaguila ore district occurred within four periods of magmatism with related mineralization significant porphyry type mo mineralization was associated with late cretaceous eocene felsic porphyritic intrusions in the central lhasa terrane resulting from neotethyan oceanic subduction and india asia continental collision

reconstructing in space and time the closure of the middle and western segments of the bangong nujiang tethyan ocean in the tibetan plateau when and how the bangong nujiang tethyan ocean closed is a highly controversial subject in this paper we present a detailed study and review of the cretaceous ophiolites ocean islands and flysch deposits in the middle and western segments of the bangong nujiang suture zone bnsz and the cretaceous volcanic rocks late mesozoic sediments and unconformities within the bnsz and surrounding areas our aim was to reconstruct the spatial temporal patterns of the closing of the middle and western segments of the bangong nujiang tethyan ocean our conclusion is that the closure of the ocean started during the late jurassic and was mainly complete by the end of the early cretaceous the closure of the ocean involved both longitudinal diachronous closure from north to south and transverse diachronous closure from east to west the spatial temporal patterns of the closure process can be summarized as follows the development of the bangong nujiang tethyan oceanic lithosphere and its subduction started before the late jurassic after the late jurassic the ocean began to close because of the compressional regime surrounding the bnsz along the northern margin of the bangong nujiang tethyan ocean collisions involving the arcs back arc basins and marginal basins of a multi arc basin system first took place during the late jurassic early early cretaceous resulting in regional uplift and the regional unconformity along the northern margin of the ocean and in the southern qiangtang terrane on the northern side of the ocean however the closure of the bangong nujiang tethyan ocean cannot be attributed to these arc arc and arc continent collisions because subduction and the development of the bangong nujiang tethyan oceanic lithosphere continued until the late early cretaceous the gradual closure of the middle and western segments of bangong nujiang tethyan ocean was diachronous from east to west starting in the east in the middle early cretaceous and being mainly complete by the end of the early cretaceous the bnsz and its surrounding areas underwent orogenic uplift during the late cretaceous

subduction and accretionary tectonics of the east kunlun orogen western segment of the central china orogenic system abstract the kunlun orogen is generally divided into the east kunlun orogenic belt e kob and the west kunlun orogenic belt w kob by the altyn tagh fault the e kob forms part of the western segment of the central china orogenic system ccos and is considered to have formed by the collision between the qaidam block and qiangtang or bayanhar terrane as a consequence of the closure of the kunlun ocean branch of the paleo tethyan ocean based on a compilation recently published high quality data this contribution provides an overview of the composition nature and ages of the principal tectonic elements including ophiolitic melanges and related volcanic rocks intrusive plutons and sedimentary cover sequences in the e kob according to multiple lines of evidence from these tectonic elements we proposed herewith a paleozoic triassic subduction and accretionary tectonic model to interpret the spatiotemporal tectonic framework plate subduction polarity and tectonic processes from accretion to collision of the e kob three main ophiolitic melange zones are identified in the e kob from north to south they are the qimantagh xiangride ophiolitic melange zone qxm the aqikekulehu kunzhong ophiolitic melange zone akm and the muztagh buqingshan anemaqen ophiolitic melange zone mbam according to these ophiolitic melange zones the e kob is divided into four major tectonic units the north qimantagh belt the central kunlun belt the south kunlun belt and the bayanhar terrane based on several lines of evidence from geology geochemistry and geochronology the south kunlun belt is interpreted as a paleozoic to triassic fore arc and accretionary complex related to northward subduction of the kunlun ocean during the ordovician triassic time the akm mabm and the south kunlun belt constitute a wide accretionary complex along the kunlun suture zone that marks final closure of the major paleo tethyan ocean while the qxm represents the best expression of another suture that records final closure of the qimantagh back arc basin the central kunlun belt as a long lived island arc terrane from ordovician to triassic times rifted from the qaidam block due to the spreading of the qimantagh back arc basin during the period of ca 485 425 ma taken into all the geological geochemical and geochronological lines of evidence together a trench arc back arc basin tectonic system in the e kob was built up and evolved into a protracted and long lived northward subduction and accretion along the kunlun suture during paleozoic and triassic time

geochronology and geochemistry of early cretaceous granitic rocks in the dongqiao area central tibet implications for magmatic origin and geological evolution abstractwe present here new geochronological and whole rock geochemical data for the dongqiao intrusive rocks six samples yield zircon u pb ages of 111 115 ma indicating that the dongqiao granitic rocks are contemporaneous with an early cretaceous 115 110 ma magmatic flare up event in central tibet on the basis of geochemical data granitic rocks from the dongqiao area can be divided into i and a type granites the a type granites contain higher na2o k2o zr hf and zr ce nb y contents than the i type granites the results indicate that i type granites were formed by partial melting of a juvenile mafic lower crustal source while a type granites were derived from a middle crustal source followed by fractional crystallization the formation of a type granites requires broad thermal anomalies indicating that early cretaceous magmatism occurred in a postcollision extensional setting likely related to the upwelling of hot asthenospheric mantle triggered by detachment and sinking of the bango

seismic gap hypothesis ten years after the seismic gap hypothesis states that earthquake hazard increases with time since the last large earthquake on certain faults or plate boundaries one of the earliest and clearest applications of the seismic gap theory to earthquake forecasting was by mccann et al 1979 who postulated zones of high medium and low seismic potential around the pacific rim in the 10 years since there have been over 40 large m 7 0 earthquakes enough to test statistically the earlier forecast we also analyze another forecast of long term earthquake risk that by kelleher et al 1973 the hypothesis of increased earthquake potential after a long quiet period can be rejected with a large confidence the data suggest that contrary to these forecasts places of recent earthquake activity have larger than usual seismic hazard whereas the segments of the circum pacific belt with no large earthquakes in recent decades have remained relatively quiet the clustering of earthquake times does not contradict the plate tectonic model which constrains only the long term average slip rate not the regularity of earthquakes

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

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

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

petrogenesis and tectonic implications of late cretaceous highly fractionated i type granites from the qiangtang block central tibet abstract the tectonic evolution of the lhasa and qiangtang collision zone is poorly understood due to a lack of pivotal magmatic records and complex geology in central tibet in this study we present zircon u pb ages whole rock major and trace elements and sr nd pb isotopic data for the newly discovered chuburi pluton in the southern qiangtang block zircon u pb data reveal that the chuburi pluton was emplaced during the late cretaceous ca 73 ma bulk rock analyses show that this intrusion is characterized by high sio2 na2o and k2o but low mgo cao and p2o5 the granites are enriched with large ion lithophile elements and light rare earth elements with marked eu anomalies and there are few heavy rare earth elements and high field strength elements these geochemical features indicate that the chuburi pluton is a highly fractionated i type granite the nine granitic samples show high 87sr 86sr i ratios 0 7064 0 7089 negative end t values 1 94 to 5 59 and have nd isotopic model ages ranging from 0 76 to 0 97 ga the initial pb isotopic ratios of the chuburi pluton is uniform 206pb 204pb t 18 774 18 884 207pb 204pb t 15 695 15 713 208pb 204pb t 39 362 39 613 based on geochemical characteristics of the chuburi pluton and consideration of the region geological setting we suggest that the parent magma was generated by magmatic mixing of mantle and lower crust and then undergoing extensive fractional crystallization with removal of k feldspar plagioclase mafic mineral and minor accessory mineral the crust beneath the central tibet may have been significantly thickened following the final lhasa qiangtang amalgamation we argue the generation of the chuburi pluton can be interpreted as a result of regional lithospheric delamination

early jurassic and late cretaceous granites in the tongka micro block central tibet implications for the evolution of the bangong nujiang ocean abstract the tongka micro block located in the eastern section of the bangong nujiang suture and characterized by mesozoic granitoids and high grade metamorphic rocks holds the key to the evolution of the bangong nujiang ocean bno here we present a systematic petrologic geochemical zircon u pb hf and whole rock sr nd pb isotopic investigation on six new granitoids in tongka the results suggest these granitoids yielded two stages of crystallization 171 177 ma and 74 ma and two stages of metamorphism 201 215 ma and 170 175 ma combined with the existing work within bns the magmatism along the bns can be divided into five stages stage 1 240 250 ma stage 2 200 230 ma stage 3 150 190 ma stage 4 100 135 ma and stage 5 73 100 ma both the early jurassic and late cretaceous tongka granitoids in this study are high k calc alkaline series with enrichments in large ion lithophile elements e g cs rb and k and depletions of large ion lithophile elements e g nb ta and ti the variable ehf t values 17 3 to 7 6 end t values 12 08 to 11 15 and ancient tdm2 1 48 2 00 ga for the early jurassic granitoids and weakly negative ehf t values 6 3 to 1 5 end t values 8 75 to 8 60 and old tdm2 ages 1 07 1 33 ga for the late cretaceous granitoids suggest that they are both mainly derived from the melting of ancient crust with minor mantle material input however the formation of the early jurassic and late cretaceous granitoids in tongka is related to different tectonic settings roll back after the tongka micro block subducted beneath the southern qiangtang subterrane and extension collapse after the tongka micro block collided with the northern lhasa subterrane the initial subduction of the bno may start from 190 ma and the collision between the lhasa and qiangtang terranes may continute to the latest cretaceous

moment ration imaging of seismic regions for earthquake prediction an algorithm for predicting large earthquakes is proposed the reciprocal ratio mri of the residual seismic moment to the total moment release in a region is used for imaging seismic moment precursors peaks in mri predict recent major earthquakes including the 1985 michoacan 1985 central chile and 1992 eureka california earthquakes

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

zircon u pb dating and in situ hf isotopic analysis of permian peraluminous granite in the lhasa terrane southern tibet implications for permian collisional orogeny and paleogeography article i nfo the lhasa terrane has long been interpreted as a simple tectonic block rifted from gondwana during the late paleozoic and then drifted northward before finally amalgamating with the qiangtang terrane during the early cretaceous in this paper we document permian peraluminous granites near pikang in the southern margin of the central lhasa terrane close to the recently documented songdo eclogite of permian age zircon shrimp and la icpms u pb dating for a pikang granite sample gives an identical crystallization age of about 263 ma and a wide age range of inherited zircons 283 2141 ma in situ hf isotopic analyses for 20 zircons of 263 ma yielded ehf t values of 4 5 to 1 9 the pikang granites have high a cnk values 1 08 and high normative corundum 1 3 2 0 indicative of peraluminous s type granite they are characterized by moderately negative eu anomalies eu eu 0 48 0 61 and strongly negative ba nb sr p and ti anomalies the granites have high end t values 6 4 to 6 0 and low initial 87 sr 86 sr ratios 0 7082 0 7096 relative to melts derived from mature continental crust these rocks are interpreted to have been generated by mixing between mantle melts and their induced melting of mature crustal materials we interpret the pikang peraluminous granite magmatism the regional angular unconformity between the middle and the upper permian and the eclogite of the same metamorphic age 262 ma from the same geotectonic location to represent different products of a common event in time and space we tentatively term this common event as syncollisional orogeny i e the permian gangdese orogeny we further hypothesize that the orogeny may be genetically associated with the collision between the lhasa terrane and the northern margin of australia following the closure of the paleo tethyan ocean south of the lhasa terrane

thickened juvenile lower crust derived 90 ma adakitic rocks in the central lhasa terrane tibet abstract the questions of why the late cretaceous magmatism generated and how the nature of the lower crust evolves in central tibet remain poorly constrained in this paper we report the presence of early late cretaceous adakitic rocks from the azhang area northern edge of the central lhasa subterrane central tibet these rocks are rhyodacites dacites in composition and have geochemical characteristics of adakitic rocks e g high sr 554 836 ppm sr y 66 100 and la yb n 20 21 low y 7 96 8 96 ppm and heavy rare earth elements hree in situ zircon u pb dating for two samples yields an early late cretaceous age 90 1 ma and 87 1 ma the low mgo 1 4 1 9 wt contents and compatible element abundances e g cr 22 30 ppm ni 19 25 ppm indicate that these rocks were most likely derived from the partial melting of a garnet bearing amphibolite under a thickened lower crust condition the positive whole rock e nd t 2 5 to 5 6 and zircon e hf t 8 9 to 16 0 values suggest that this thickened lower crust was juvenile the crust beneath the central lhasa subterrane may have been significantly thickened due to tectonic shortening in response to the lhasa qiangtang collision and magma underplating before the emplacement of azhang adakitic rocks 90 ma we argue that regional lithospheric delamination at 90 ma triggered the partial melting of the lowermost garnet bearing crust that is still attached to the middle crust to generate the azhang adakitic rocks the presence of the 90 ma azhang adakitic rocks provides valuable constraints on the origin of the early late cretaceous magmatism in the central lhasa subterrane and on the crustal evolution beneath the lhasa qiangtang collisional zone prior to the cenozoic india asia collision

varied thermo rheological structure mechanical anisotropy and lithospheric deformation of the southeastern tibetan plateau abstract to investigate the lithospheric structure and deformation of the tibetan plateau we present thermo rheological models for two transects across the southeastern tibetan plateau se tibet showing strong lateral heterogeneity the geodynamic implications are also investigated in relation to gps motions seismic anisotropy seismicity distribution and crustal isostatic state the unmatched pattern between pms and sks splittings which even intersect at right angles to the south of 26 n suggests the crust mantle decoupling constrained by a brittle load bearing layer at the uppermost mantle beneath the indochina block the decoupling may occur below the uppermost mantle the strong crust beneath the south china plate and indochina block implied by the seismicity distribution has two thick brittle load bearing layers indicating a coupled system the crust beneath the emeishan large igneous province elip also has two brittle load bearing layers but the brittle deformation is restricted to the topmost 10 km of the upper and lower crust only one brittle load bearing layer resides in the upper crust in the other part of the chuandian block combining with gps and pms splitting we can infer that the weak crust with seismicity restricted into the depth of

rare earth element tetrad effect and negative ce anomalies of the granite porphyries in southern qiangtang terrane central tibet new insights into the genesis of highly evolved granites abstract highly evolved granites are geochemically similar to the mature continental crust and are sometimes associated with w sn nb ta and rare metal mineralizations these rocks have important implications for tracing the compositional evolution of continental crust and magma metallogenesis they were commonly considered to have been produced by partial melting of crustal materials or by high degree fractional crystallization in this study we report on high sio2 granite porphyries in the xiabie co area of southern qiangtang terrane central tibet which provide new insights into the petrogenesis of the highly evolved granites zircon dating results indicate that they were generated in the late early cretaceous 107 1 ma they consist mainly of orthoclase albite and quartz with accessory minerals including zircon fluorite and fe ti oxides geochemically they have high sio2 but low mgo fe2o3t cao and p2o5 and are enriched in rb but depleted in ba sr eu and ti similar to highly evolved granitic rocks they are characterized by distinct rare earth element ree tetrad effects and negative ce anomalies they also have slightly higher end t values 0 99 to 0 36 than those of the adjacent early granites 2 05 to 3 01 and relatively high and variable zircon ehf t values ranging from 0 3 to 5 4 compared to typical highly evolved granites and petrological experiment data we suggest that the xiabie co granite porphyries represent highly evolved melts produced by fractional crystallization the interaction between fractionated melts and f rich aqueous fluids played an important role in the genesis of their ree tetrads effect involvement of meta sedimentary rocks that went through surficial chemical weathering into their source region is the predominant reason for the generation of negative ce anomalies this indicates that in addition to fractional crystallization the melt fluid interactions and partial melting of source materials also play important roles in the generation of highly evolved granites

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

remnants of late permian middle triassic ocean islands in northern tibet implications for the late stage evolution of the paleo tethys ocean abstract in this paper we present new data for the tianquan tq and dabure db ocean islands in the western segment of the longmuco shuanghu lancangjiang suture zone northern tibet including the results of major and trace element analyses zircon u pb dating and hf isotope analyses our aim was to assess the genesis of these ocean islands and to consider the implications for the tectonic evolution of the region as a whole both tq and db retain an ocean island type double layered structure comprising a volcanic basement basalt and andesite and an oceanic sedimentary cover sequence conglomerate limestone and chert the basalts and andesites in the tq and db are enriched in light rare earth elements and high field strength elements nb ta zr hf and ti yielding chondrite normalized ree patterns and primitive mantle normalized trace element patterns that are similar to those of ocean island basalts given the small and generally positive ehf t values of the tq andesites 4 25 to 6 22 and db andesites 0 59 to 1 97 mostly 0 we conclude that the basalts were derived from the partial melting of garnet peridotite in the mantle and that the andesites were formed by fractional crystallization of the mafic parent magma derived from the garnet peridotite mantle the ascending magmas underwent varying degrees of fractional crystallization but were not contaminated by crustal material these features indicate that both tq and db are typical ocean islands that formed in an ocean basin geochemical analyses of cherts from tq and db show that they contain terrigenous material indicating the proximity of a continental margin the andesites of tq contain zircons that yield two u pb ages of 251 ma given that ages of 246 247 and 254 ma had been reported previously we conclude that tq formed during the late permian early triassic the andesites of db contain zircons that yield u pb ages of 242 and 246 ma taking into account the youngest age of 244 ma from the db basalt we conclude that db formed during the middle triassic these data combined with the geological history of the region indicate that the development of the longmuco shuanghu lancangjiang paleo tethys ocean continued after the early permian and that the closure of this ocean was diachronous from east to west the eastern segment of the ocean closed during the early triassic however the western segment remained at least partially open until the middle triassic although the ocean was relatively small at this time the ocean finally closed in the late triassic