seismic attenuation tomography in southwestern china insight into the evolution of crustal flow in the tibetan plateau abstract crustal flow in southwestern china is commonly attributed to the continental collision between the indian and the eurasian plates therefore studying crustal flow in this area can provide valuable insight into the collisional process the quality factor q is an indicator of seismic wave attenuation that provides constraints on lithology rheology and geodynamic processes which are vital for interpreting the nature of the crustal flow in a given region in this study we used local seismic waveforms to obtain regional attenuation models for p qp and s qs waves in the sichuan yunnan region syr the emeishan large igneous province elip is characterized by high qp and qs values in the middle yunnan block myb at depths greater than 5 km the qp and qs values in most of the fault zones surrounding the myb were lower than 300 indicating that the material within crustal faults causes high waveform attenuation our results suggest that the crustal flow was diverted into two channels in the northern myb due to resistance from the elip to illuminate the tectonic dynamics of the syr we propose a simple model that explains both the evolution of the crustal flow and the variation in the seismic wave attenuation

eclogite facies relics and inferred ultrahigh pressure metamorphism in the north dabie complex central eastern china mineral assemblages and microstructures of a newly identified retrogressed eclogite sensu lato in the north dabie complex ndc central eastern china indicate early very high pressure metamorphic relics preserved in a dominant amphibolite facies host where no eclogite sensu stricto has been positively identified before the investigated eclogitic rock shows distinct multistage recrystallization with granulite and amphibolite facies assemblages overprinting eclogite facies relics the minimum temperature for the eclogite facies metamorphism is estimated to be 800 820 c a spectacular microstructure of oriented quartz needles 2 20 m wide 5 200 m long in matrix ca na clinopyroxene implies the prior existence of a non stoichiometric supersilicic omphacite stabilized at ultrahigh pressure uhp 25 kbar conditions although no coesite or coesite pseudomorphs have been found in the samples the absence of coesite may be due to the lack of free silica at uhp conditions or the consumption of silica during retrograde reactions the inferred uhp conditions metamorphism is further supported by sm nd ages that are equivalent to triassic metamorphic ages from uhp eclogites in the southeastern dabie mountains this finding expands the uhp terrane northward about 50 km spatial distribution of subduction collision related uhp rocks includes parts of the ndc

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

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

ultrahigh pressure metamorphism in western tianshan china part i evidence from inclusions of coesite pseudomorphs in garnet and from quartz exsolution lamellae in omphacite in eclogites inclusions of coesite pseudomorphs in garnets have been found in type i eclogites interlayered with mafic blueschists and carbonate eclogites and quartz exsolution lamellae in omphacites have been recognized in type ii eclogites with pillow structure in western tianshan china based on mineralogy and petrology the metamorphic evolution of western tianshan eclogites can be divided into three stages 1 pre peak ultrahigh pressure uhp eclogite facies stage 356 443 c 8 10 kb 2 uhp eclogite facies stage 496 598 c 25 7 26 7 1 kbar and 3 retrograde epidote blueschist facies stage 500 530 c 10 12 kbar consequently the eclogites from the western tianshan region of china have undergone unambiguous uhp metamorphism

plate tectonics and orogenic research after 25 years a tethyan perspective abstract orogeny the process by which the earth s prominent mountain ranges are constructed has been a central topic of interest in the earth sciences since at least the end of the 18th century the recognition that strains and displacements of very considerable magnitude occur along all of the three dimensions within an orogenic belt during its evolution has grown gradually during the last two centuries emphasis on primary vertical movements dominated the ideas on the nature of orogeny during the first half of the 19th century whereas compression and consequent uplift across mountain belts were believed to be the main cause for their origin during the subsequent one hundred years or so that were spent under the dominance of the fixist contraction theory mobilist tectonicians realised that continental drift in places also required motion along the trend of orogenic belts but this view did not gain general acceptance recognition of significant strike slip motion parallel or subparallel with mountain ranges evolved independently and mostly within the fixist camp by the 1960 s presence of important motions both along and across mountain belts had become common knowledge but no theoretical basis existed to account for them all

mesozoic crustal evolution of southern tibet constraints from the early jurassic igneous rocks in the central lhasa terrane abstract phanerozoic growth of continental crust on our planet is one of the important research themes in earth science here we present the results of a systematic study of newly found and previously reported mesozoic igneous rocks including diorite cumulate granodiorite cumulate mafic magmatic enclaves mme and host granitoids in the central lhasa terrane southern tibet these igneous rocks give zircon u pb crystallization ages of 199 189 ma based on constituent mineral and bulk rock compositions the cumulates are best understood as resulting from amphibole plagioclase and titanite crystallization from a mafic andesitic magma the host granitoids also show compositional systematics consistent with amphibole plagioclase fractional crystallization from andesitic magma the mmes share many characteristics with their host granitoids in common including identical crystallization age similar mineralogy mineral chemistry and zircon isotopic compositions representing earlier cumulate derived from the same magmatic system as their host rocks the magma parental to the studied early jurassic igneous rocks is best explained as resulting from partial melting of hydrated ocean crust together with varying continental material the increasing zircon ehf t values of multiple plutons in the central lhasa terrane with time during 215 170 ma indicate its gradual increase in mantle contribution we present the early mesozoic crustal evolution in the lhasa terrane

india s hidden inputs to tibetan orogeny revealed by hf isotopes of transhimalayan zircons and host rocks abstract to better understand tibetan orogenesis which involves complex tectonic processes here we report the first integrated analysis of hf isotopes of zircon separates and their host rocks from the transhimalayan batholiths the rocks studied include 19 granitoids from different parts of the gangdese batholith the largest transhimalayan intrusive complex and five postcollisional adakites that occurred as plugs or dikes cutting the gangdese batholith in the southern lhasa terrane southern tibet published zircon hf isotope data from other transhimalayan granitoids and the linzizong volcanic rocks are also synthesized the magmatic zircons crystallizing between ca 200 and 15 ma show depleted mantle type hf isotopic characteristics throughout the mesozoic but shift markedly in the paleogene the hf isotopic shift is interpreted as tracking the evolving progress of himalayan sediment subduction driven by the approaching indian continent and thus signals the initiation of the india asia collision that we infer to have occurred by 55 ma our data furthermore indicate that southern tibet underwent significant crustal thickening during ca 45 and 30 ma before emplacement of the postcollisional adakites that exhibit hf nd isotopic systematics suggesting binary mixing of melts from the juvenile gangdese mafic crust and incorporated himalayan sediments in the petrogenesis

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

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

tectonic evolution of the qiangtang block northern tibet during the late cisuralian late early permian evidence from fusuline fossil records abstract the tectonic evolution of the qiangtang block in the qinghai tibetan plateau has been a controversial subject for a long time in this paper the discovery of new stratigraphic and fusuline fossil evidence from the permian sequences qudi and lugu formations of the qiangtang block is reported and the palaeogeographical position and tectonic history of this block during the late cisuralian late early permian are discussed the qudi formation is typified by thick turbidite deposits and contains artinskian fusulines such as pseudofusulina and chalaroschwagerina the fusulines were deposited as grains involved in debris flow deposits suggesting a synchronicity with the depositional time of the turbidites the subsequent lugu formation is dominated by seamount type carbonates with an irregular basalt base fusulines cancellina pseudodoliolina and parafusulina in the base of the carbonates confirm its age as middle kungurian the transition from the turbidite qudi formation to the seamount lugu formation is here interpreted to be a continuous depositional process recording the qiangtang block s separation from the indian plate this separation signaled the opening of the neotethys ocean between the qiangtang block and the indian plate palaeogeographically the qiangtang block s separation is comparable with the baoshan block s separation in the east and central pamir s separation in the west by contrast the ultimate opening of the neotethys ocean by the separation of india pakistan and northern oman is apparently much later than this event recorded in the qiangtang block consequently it is interpreted that the opening of the neotethys ocean in the whole northern gondwanan margin is a diachronous series of events

ophiolites of the kunlun mountains china and their tectonic implications abstract three ophiolite belts ranging in age from cambrian to triassic provide valuable data on the tectonic evolution of the kunlun mountains which lie along the northern margin of the tibetan plateau all of the ophiolites contain dismembered but nearly complete assemblages of peridotite mafic and ultramafic cumulates pillow and massive lavas and radiolarian cherts 1 the oldest belt contains ophiolites of cambrian age but the initial spreading of the ocean basin most likely began in the late proterozoic these ophiolites outcrop in the central part of the eastern kunlun extend for at least a few hundred kilometers and are believed to reflect a small ocean basin closure of this ocean led the accretion of south kunlun block to the north kunlun block and tarim craton ophiolitic lavas in this belt have island are affinities 2 a second belt of early carboniferous ophiolites extends nearly 600 km along the northern margin of the western kunlun lavas in this belt are basalt basaltic andesite and andesite all of which have compositions characteristic of volcanic arcs this ocean basin possibly developed on the basis of the early paleozoic oceanic basin closure of this ocean basin by southward subduction in the early permian produced a suite of calc alkaline volcanic rocks in what is now the central part of the western kunlun 3 the third belt extends nearly 1200 km along the southern margin of the eastern kunlun and contains numerous ophiolites of early permian to middle triassic age these ophiolites are highly tectonized containing volcanic rocks with the geochemical characteristics of mid ocean ridges oceanic islands and volcanic arcs this belt is tentatively interpreted as the suture zone between gondwana and eurasia

basin architecture and density structure beneath the strait of georgia british columbia georgia basin is located within one of the most seismically active and populated areas on canada s west coast over the last decade geological investigations have resolved important details concerning the basin s shallow structure and composition yet until recently relatively little was known about deeper portions of the basin in this study new seismic velocity information is employed to develop a 3 dimensional density model of the basin comparison of the calculated gravity response of this model with the observed gravity field validates the velocity model at large scales at smaller scales several differences between model and observed gravity fields are recognized analysis of these differences and correlation with independent geoscience data provide new insights into the structure and composition of the basin fill and underlying basement specifically four regions with thick accumulations of unconsolidated pleistocene and younger sediments which were not resolved in the velocity model are id

protolith of eclogites in the north qaidam and altun uhp terrane nw china earlier oceanic crust an early paleozoic ultrahigh pressure metamorphic belt occurs in the north qaidam altun mountains and was offset about 400 km southwestward by the altyn tagh strike slip fault eclogites in the belt consist of major basaltic and minor picritic rock types and can be subdivided into three groups high tio2 2 5 wt medium tio2 1 2 wt and low tio2 1 wt geochemical evidence shows that the protoliths of the eclogite were basaltic rocks from different tectonic environments including mid ocean ridge basalt island arc basalt and ocean island basalt the nd isotopes of these rocks are similar to those of morb characterized by mainly positive and minor negative end 0 values providing further evidence that the eclogite protoliths were ocean floor basalts to which minor crustal components were added during subduction geochronological data indicate that the uhpm occurred about 500 440 ma ago and that there were two eclogite protoliths with ages of 800 750 and 1000 ma respectively geochemically and isotopically the eclogites are similar to basaltic rocks in luliangshan north qaidam mountains which have sr and nd ages of 768 39 and 780 22 ma the age and composition of these volcanic rocks suggest that they were one type of the protoliths for the eclogites a later series of calc alkaline island arc volcanic rocks was formed at about 500 ma ago essentially contemporaneous with the uhp metamorphism we propose the following tectonic model for the evolution of the north qaidam uhp belt at about 1000 ma ago a number of continents were amalgamated to form the rodinian continent in this general area which contained some oceanic volcanic rocks possibly as ophiolitic fragments this part of rodinia was then rifted at about 800 750 ma ago to form an oceanic basin with a variety of morb and ocean island basalts closure of this ocean basin produced the neoproterozoic ophiolites and granitic gneisses in the north qaidam mountains in the early paleozoic another qilian ocean basin formed and subduction of this oceanic lithosphere formed the island arc volcanic rocks at about 500 ma ago and the subduction related granites 470 450 ma once the oceanic lithosphere was consumed the subsequent continent continent collision led to deep subduction of continental crust which was tectonically mixed with the eclogites to cause the ultrahigh pressure metamorphism

reworked precambrian metamorphic basement of the lhasa terrane southern tibet zircon titanite u pb geochronology hf isotope and geochemistry abstract due to the paucity of exposure the formation and evolution of the precambrian basement of the lhasa terrane remain poorly known here we report zircon and titanite in situ u pb ages bulk rock geochemical and zircon hf isotopic data on the orthogneisses from the dongjiu area of the southern lhasa subterrane slt southern tibet geochemical data suggest that the protoliths of the biotite amphibole gneiss and biotite gneiss are granodiorite and granite respectively inherited magmatic zircon cores from these orthogneisses give protolith crystalline ages of 1520 1506 ma whereas the overgrown zircon rims give metamorphic ages of 605 590 ma the mesoproterozoic granitic rocks have bulk rock end t values of 3 6 to 0 1 and zircon core ehf t values of 4 5 to 2 6 which give similar tdm2 ages of 2 35 2 05 ga and 2 54 2 10 ga respectively suggesting their derivation from partial melting of paleoproterozoic crustal material the granitic rocks are also local provenance for the mesoproterozoic detrital zircons in the paleozoic strata in the lhasa terrane titanite in situ u pb ages further indicate that the dongjiu orthogneiss experienced more recent metamorphism at 26 ma the mineral assemblage and thermobarometry calculations indicate that the oligocene metamorphism occurred under medium pressure mp amphibolite facies conditions 5 4 7 2 kbar 691 765 c we propose that the dongjiu gneisses represent the precambrian metamorphic basement of the lhasa terrane but have been intensively reworked by metamorphism in the slt in response to the continued india asia convergence since the collision

femg partitioning between coexisting garnet and phengite at high pressure and comments on a garnet phengite geothermometer abstract the fe mg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite mg 67 in the presence of quartz and water at pressures of 20 35 kb and temperatures of 800 1000 c compositions of coexisting garnet and mica indicate a linear relation between both the inkd fe mg garnet fe mg phengite and temperature and inkd and pressure in the above p t range this fe mg exchange reaction between garnet and phengite is shown to be dependent on the ca content of the garnet and on the mg number of the bulk composition these two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46 and by using a synthetic basaltic composition the overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems respectively however because of non ideality in the exchange reaction this geothermometer should not be used in any practical application outside the composition ranges studied also careful consideration of the presence of fe3 in phengite must be made if the fe3 content of the natural phengite is unknown then the temperatures obtained will be maximum temperatures only

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

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

deep thermal state on the eastern margin of the lhasa gangdese belt and its constraints on tectonic dynamics based on the 3 d electrical model abstract since the mesozoic the lhasa gangdese block has undergone several stages of tectonism magmatism caused by the closure of the new tethys ocean and the collision between the indian and eurasian plates which led to the lithospheric shortening and crustal uplift through the careful processing and analysis of the magnetotelluric data on the eastern margin of the lhasa gangdese block near 92 e a reliable 3d electrical model was obtained to examine the relationship between the lithospheric electrical structure and geodynamics some conductive and resistive layers were obtained which may be formed in some tectonic processes the thermal model of the lithospheric uppermost mantle 75 100 km were calculated using the arrhenius equation and the hashin shtrikman hs boundary condition combined with the terrestrial heat flow point and curie isothermal in the study area the temperature distribution at the 30 100 km depths was calculated the melt fractions of the mid lower crust calculated by the classical archie s law pressure calculated by density and temperature at different depths can be obtained in a meantime the resistor beneath the tethys himalaya terrane represented the subduction of the indian plate with a deep angle the subduction of the indian crust may not exceed the indus yarlung zangbo suture while the indian lithospheric mantle may not reach the luobadui milashan fault additionally our electrical and thermal structure indicated that the indian lithospheric mantle probably detached from the indian crust our study further discussed the underplating of the mantle derived materials and the southern extrusion along the main himalaya trust and offered a subsidiary evidence for the possible south eastward crustal flow in a local area finally the relationship between the electrical structure and the deep metallogenic mechanism was briefly established