crustal deformation of the eastern tibetan plateau revealed by magnetotelluric imaging the ongoing collision of the indian and asian continents has created the himalaya and tibetan plateau through a range of deformation processes these include crustal thickening detachment of the lower lithosphere from the plate delamination and flow in a weakened lower crust 1 6 debate continues as to which of these processes are most significant 7 in eastern tibet large scale motion of the surface occurs but the nature of deformation at depth remains unresolved a large scale crustal flow channel has been proposed as an explanation for regional uplift in eastern tibet 6 8 9 but existing geophysical data 10 11 do not constrain the pattern of flow magnetotellurics uses naturally occurring electromagnetic waves to image the earth s subsurface here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20 40 km the channels extend horizontally more than 800 km from the tibetan plateau into southwest china the electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12 13 that permits flow on a geological timescale these findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux our results reveal the previously unknown complexities of these patterns of crustal flow many previous studies of the indiaasia

kinematic model of active deformation in central asia the velocity field of present day deformation in central asia is modelled using a set of four rotating blocks siberia tarim tibet india on a spherical earth a best fit is inverted on the basis of estimated shortening rates across the main thrust zones himalayas tien shan and measured slip rates along the principal strike slip faults altyn tagh and karakorum separating those blocks the fit to the data implies that nearly all the present convergence between india and asia can be accounted for by slip partitioning on these four zones with as much as 50 absorbed by northeastwards extrusion of tibet this suggests that localised deformation governs the present mechanical behaviour of the central asian lithosphere

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

two crustal low velocity channels beneath se tibet revealed by joint inversion of rayleigh wave dispersion and receiver functions abstract competing geodynamic models such as rigid block extrusion continuous deformation and the mid lower crustal flow have been proposed to describe the growth and expansion of eastern tibet however the dynamic processes responsible for plateau evolution and deformation remain poorly understood partly due to resolution limitations of previous models of lithospheric structure on the basis of joint inversion of rayleigh wave dispersion and receiver functions using data from a newly deployed seismic array we have obtained a high resolution 3d image that reveals the distribution of low velocity zones lvzs with unprecedented clarity the prominent feature of our model is two low velocity channels that bound major strike slip faults in se tibet and wrap around the eastern himalaya syntaxis consistent with the clockwise movement of crustal material in this region most large earthquakes in this region occurred in the boundaries of the lvzs we propose that ductile flow within these channels in addition to shear motion along strike slip faults played a significant role in accommodating intensive lithospheric deformation during the eastward expansion of tibet in the cenozoic

radial anisotropy in the crust of se tibet and sw china from ambient noise interferometry 1 we use rayleigh and love wave green s functions estimated from ambient seismic noise to study crustal structure and radial anisotropy in the tectonically complex and seismically active region west of the sichuan basin and around the eastern himalaya syntaxis in agreement with previous studies low velocity zones are ubiquitous in the mid lower crust with substantial variations both laterally and vertically discrepancies between 3 ds hear velocity from either rayleigh vsv o r love vsh waves are examined both in view of non uniqueness of tomographic solutions and radial anisotropy low shear wave speed and radial anisotropy with vsh vsv are most prominent in mid lower crust in area northwest to the lijiang muli fault and around the red river and xiaojiang faults this anisotropy could be caused by sub horizontal mica fabric and its association with low velocity zones suggests mica alignment due to flow in deep crustal zones of relatively low mechanical strength citation huang h h yao andr d vanderhilst 2010 radialanisotropyinthecrust of se tibet and sw china from ambient noise interferometry geophys res lett 37 l21310 doi 10 1029 2010gl044981

the boundary between the indian and asian tectonic plates below tibet the fate of the colliding indian and asian tectonic plates below the tibetan high plateau may be visualized by in addition to seismic tomography mapping the deep seismic discontinuities like the crust mantle boundary moho the lithosphere asthenosphere boundary lab or the discontinuities at 410 and 660 km depth we herein present observations of seismic discontinuities with the p and s receiver function techniques beneath central and western tibet along two new profiles and discuss the results in connection with results from earlier profiles which did observe the lab the lab of the indian and asian plates is well imaged by several profiles and suggests a changing mode of india asia collision in the east west direction from eastern himalayan syntaxis to the western edge of the tarim basin the indian lithosphere is underthrusting tibet at an increasingly shallower angle and reaching progressively further to the north a particular lithospheric region was formed in northern and eastern tibet as a crush zone between the two colliding plates the existence of which is marked by high temperature low mantle seismic wavespeed correlating with late arriving signals from the 410 discontinuity poor sn propagation east and southeast oriented global positioning system displacements and strikingly larger seismic sks anisotropy

crustal thickness map of the chinese mainland from teleseismic receiver functions abstract we present a new crustal thickness map of the chinese mainland and adjacent areas based on a compilation of previously published data more than 1900 estimates of crustal thickness from receiver function studies were collected and the weighted averages were used to develop a contour map of crustal thickness beneath the chinese mainland the combined dataset provided good coverage especially over eastern and central china and our analysis revealed crustal thickness variations of the chinese mainland with considerably more detail than in previous models our crustal thickness map shows a large variation from a thickness of less than 30 km in the mesozoic basin of eastern china to over 80 km in the tibetan plateau the thickest crust was found in central tibet while the thinnest continental crust was observed beneath eastern china normal to slightly thick crust 38 51 km was found for the stable precambrian basins and orogenic belts comparison between our model and other models reveals that the models share similar large scale and smaller scale features over large portions of eastern china where the data coverage is good however large differences up to 25 km in crustal thickness occur in areas with complex crustal structure or poor data coverage we also evaluated the quality of previous models by comparing them with our newly compiled point observations of crustal thickness

crustal rheology of the himalaya and southern tibet inferred from magnetotelluric data the collision between the indian and asian continents which began about 70 million years ago involved a convergence of at least 1 400 km and raised the tibetan plateau since crustal material is too buoyant to be transported downwards into the earth s interior the mechanisms at work in plateau formation are important in understanding the evolution of the continents yet still a matter of debate geophysical observations in the himalaya and tibet using ultra low frequency radio waves now reveal that a partially molten layer is present along the entire length of the himalaya this weak layer implies that widespread crustal flow is occurring and explains many features of plateau evolution the cenozoic collision between the indian and asian continents formed the tibetan plateau beginning about 70 million years ago since this time at least 1 400 km of convergence has been accommodated1 by a combination of underthrusting of indian2 and asian lithosphere crustal shortening3 horizontal extrusion4 and lithospheric delamination5 rocks exposed in the himalaya show evidence of crustal melting1 6 and are thought to have been exhumed by rapid erosion and climatically forced crustal flow7 8 magnetotelluric data can be used to image subsurface electrical resistivity a parameter sensitive to the presence of interconnected fluids in the host rock matrix even at low volume fractions here we present magnetotelluric data from the tibetan himalayan orogen from 77 e to 92 e which show that low resistivity interpreted as a partially molten layer is present along at least 1 000 km of the southern margin of the tibetan plateau the inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in southern tibet

constraints on the seismic wave velocity structure beneath the tibetan plateau and their tectonic implications we combine observations of group and phase velocity dispersion of rayleigh waves of the waveform of a long period pl phase of pn and sn velocities from unreversed refraction profiles using earthquakes and of teleseismic s p travel time residuals to place bounds on the seismic wave velocity structure of the crust and upper mantle under tibet from surface wave measurements alone the tibetan crustal thickness can be from 55 to 85 km with corresponding uppermost mantle shear wave velocities of about 4 4 to 4 9 km s respectively the pn and sn velocities were determined to be 8 12 0 06 and 4 8 0 1 km s respectively using travel time data at lhasa from earthquakes in and on the margins of tibet combining these results the crustal thickness is most likely to be between 65 80 km with an average shear wave velocity in the upper crust less than 3 5 km s a synthesis of one pl waveform does not provide an additional constraint on the velocity structure but is compatible with the range of models given above in contrast to observations obtained for eight earthquakes in the himalaya measurements of both teleseismic s and p wave arrival times for nine earthquakes within tibet show unusually large intervals between p and s compared with the jeffreys bullen tables thus the pn and sn velocities apparently do not reflect high velocities in the mantle to a great depth beneath tibet from the dependence of the seismic velocities of olivine on pressure and temperature and from the similarity of the measured pn and sn velocities beneath tibet and beneath shields and platforms the velocities at the moho beneath tibet are compatible with the temperature being 250 300 higher than beneath shields and platforms i e 750 c if the temperature beneath the platforms is close to 500 c such a temperature could reach or exceed the solidus of the lower crust simple one dimensional heat conduction calculations suggest that the volcanic activity could be explained by the recovery of the geotherm maintained by a mantle heat flux of about 0 9 hfu at the base of the crust if the distribution of radioactive heat production elements were not concentrated at the top of the crust radioactive heating could also contribute significantly to the recovery of the geotherm and thus lower the required mantle heat flow thus the idea of a thickened crust in response to horizontal shortening is compatible both with these data and with these calculations appendices are available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 20009 document j81 004 1 00 payment must accompany order

a genetic linkage between subduction and collision related porphyry cu deposits in continental collision zones the genesis of continental collision related porphyry cu deposits pcds remains controversial the most common hypothesis links their genesis with magmas derived from subduction modified arc lithosphere however it is unclear whether a genetic linkage exists between collision and subduction related pcds here we studied jurassic subduction related cu au and miocene collision related cu mo porphyry deposits in south tibet the jurassic pcds occur only in the western segment of the jurassic arc which has depleted mantle like isotopic compositions e g 87 sr 86 sr i 0 7041 0 7048 e nd t as high as 7 5 and e hf t as high as 18 by contrast no jurassic pcds have been found in the eastern arc segment which is isotopically less juvenile e g 87 sr 86 sr i 0 7041 0 7063 e nd t 4 5 and e hf t 12 these results imply that incorporation of crustal components during underplating of jurassic magma induced copper accumulation as sulfides at the base of the eastern jurassic arc inhibiting pcd formation at this time miocene pcds are spatially confined to the jurassic arc and the giant miocene pcds cluster in its eastern segment where no jurassic pcds occur this suggests that the arc segment barren for subduction related pcds could be fertile for collision related pcds miocene ore forming porphyries have young hf model ages and sr nd hf isotopic compositions overlapping with those of the jurassic rocks in the eastern segment whereas contemporaneous barren porphyries outside the jurassic arc have abundant zircon inheritance and crustlike sr nd hf isotopic compositions these data suggest that remelting of the lower crustal sulfide bearing cu rich jurassic cumulates triggered by cenozoic crustal thickening and or subsequent slab break off led to the formation of giant miocene pcds the spatial overlap and complementary metal endowment between subduction and collision related magmas may be used to evaluate the mineral potential for such deposits in other orogenic belts

surface wave array tomography in se tibet from ambient seismic noise and two station analysis i phase velocity maps summary empirical green s functions egfs between pairs of seismographs can be estimated from the time derivative of the long time cross correlation of ambient seismic noise these egfs reveal velocity dispersion at relatively short periods which can be used to resolve structures in the crust and uppermost mantle better than with traditional surface wave tomography we combine rayleigh wave dispersion estimates from egfs and from traditional two station ts analysis into a new approach to surface wave array tomography with data from dense receiver arrays we illustrate the methodology with continuous broad band recordings from a temporary seismographic network on the southeastern part of the tibetan plateau in sichuan and yunnan provinces sw china the egfs are robust under temporal changes in regional seismicity and the use of either ambient noise approximated by records without signal from events with magnitude mb 5 or 4 or surface wave coda produces similar results the egfs do not strongly depend on the presence of large earthquakes but they are not reciprocal for stations aligned in the n s direction this directionality reflects the paucity of seismicity to the north of the array using a far field representation of the surface wave green s function and an image transformation technique we infer from the egfs the rayleigh wave phase velocity dispersion in the period band from 10 30 s a classical ts approach is used to determine rayleigh wave phase velocity dispersion between 20 120 s together they constrain phase velocity variations for t 10 120 s which can be used to study the structure from the crust to the upper mantle beneath se tibet short and intermediate period 10 80 s phase velocities are prominently low suggesting that the crust and upper mantle beneath se tibet is characterized by slow shear wave propagation

frequency dependent seismic anisotropy in fractured rock to examine the effects of multiple aligned fractures in rock we have developed a two dimensional elastic finite difference code for fractured media fractures are incorporated into the model explicitly as displacement discontinuity boundary conditions the wavefield is computed using a 4th order staggered grid scheme simulations were performed for a broadband explosion point source center frequency 374 hz located at the center of the model the model consisted of 90 horizontal fractures spaced approximately 1 8 of a wavelength apart the normal and shear fracture stiffnesses were selected such that the normal incidence transmission coeficient is 0 6 the simulations show strong scattering attenuation of the p wave in the vertical direction to the fractures and channeling of guided waves in the horizontal direction ii to the fractures the same code was also used to model wave propagation in an anisotropic medium with equivalent effective moduli for the 90 fracture system significant differences between the amplitudes velocities and frequency content of the waves in the explicit and equivalent medium fracture models were observed these differences result from frequency dependent time delays and filtering across each fracture and channeling along fractures that are not included in the zero frequency effective medium description these effects are especially interesting because they illustrate that the dynamic properties of fractured rock include significant amplitude anisotropy that may prove useful in the characterization of fractured rock

continuous deformation of the tibetan plateau from global positioning system data global positioning system velocities from 553 control points within the tibetan plateau and on its margins show that the present day tectonics in the plateau is best described as deformation of a continuous medium at least when averaged over distances of 100 km deformation occurs throughout the plateau interior by ese wnw extension and slightly slower nne ssw shortening relative to eurasia material within the plateau interior moves roughly eastward with speeds that increase toward the east and then flows southward around the eastern end of the himalaya crustal thickening on the northeast ern and eastern margins of the plateau occurs over a zone 400 km wide and cannot be the result of elastic strain on a single major thrust fault shortening there accommodates much of india s penetration into eurasia a description in terms of movements of rigid blocks with elastic strain associated with slip on faults between them cannot match the velocity field

thinning and flow of tibetan crust constrained by seismic anisotropy intermediate period rayleigh and love waves propagating across tibet indicate marked radial anisotropy within the middle to lower crust consistent with a thinning of the middle crust by about 30 the anisotropy is largest in the western part of the plateau where moment tensors of earthquakes indicate active crustal thinning the preferred orientation of mica crystals resulting from the crustal thinning can account for the observed anisotropy the middle to lower crust of tibet appears to have thinned more than the upper crust consistent with deformation of a mechanically weak layer that flows as if confined to a channel

high resolution lithospheric structure beneath mainland china from ambient noise and earthquake surface wave tomography abstract we present a new high resolution shear velocity model of the lithosphere down to about 160 km beneath china using rayleigh wave tomography we combined ambient noise and earthquake data recorded at 1316 seismic stations the largest number used for the region to date more than 700 000 dispersion curves were measured to generate group and phase velocity maps at periods of 10 140 s the resolution of our model is significantly improved over previous models with about 1 2 in eastern china and 2 3 in western china we also derived models of the study region for crustal thickness and averaged s velocities for upper and mid lower crust and uppermost mantle these models reveal important lithospheric features beneath china and provide a fundamental data set for understanding continental dynamics and evolution different geological units show distinct features in the moho depth lithospheric thickness and shear velocity in particular the north china craton ncc lithosphere shows strong east west structural variations with thin and low velocity lithosphere in eastern ncc and thick and high velocity lithosphere beneath western ncc and the lithosphere of the ordos block seems to have undergone strong erosion the results support the progressive destruction of the ncc lithosphere from east to west at least partly caused by the thermal chemical erosion of the cratonic lithosphere from the asthenosphere another pronounced feature of our model is the strong lateral variations of the mantle lithosphere beneath the tibetan plateau tp the indian lithosphere beneath the tp shows variable northward advancement with nearly flat subduction in western and eastern tp and steep subduction in central tp with evidence for the tearing of indian lithosphere beneath central tp which may be important for the riftings at the surface in himalayas and southern tp the low velocity zone in northern tp shows strong correlation with the region of the mid miocene to quaternary potassic magmatism suggesting that delamination of lithosphere may have played an important role in the rise of the tp

application of scale selective data assimilation to tropical cyclone track simulation tropical cyclone track is strongly controlled by the large scale environmental circulation in limited area models lams driven by global analyses or forecasts through a conventional lateral boundary nesting approach the global analyses are often distorted by the use of sponge zone or interpolation when they are passed into the lam in this study a dynamical downscaling approach based on scale selective data assimilation ssda is applied to a limited area numerical weather prediction model with emphasis on tropical cyclone track simulation the idea of the ssda approach is to drive the lam not only from the lateral boundary but also from the model domain interior the large scale flow from global analyses or forecasts is assimilated into the regional model using 3 d variational data assimilation the large scale features in the lam are thus constrained to follow the global analyses while allowing the regional model itself to develop the regional and small scale characteristics the results from the case study of hurricane katrina 2005 show that both large and small scale flows in the regional model benefited from the ssda approach leading to an improvement in the accuracy of storm track simulation when provided with an accurate large scale circulation from global analyses in addition the ssda procedure is shown to be an effective method to construct a nested grid regional modeling system that reduces model sensitivity to model domain geometry and location

extent of underthrusting of the indian plate beneath tibet controlled the distribution of miocene porphyry cu mo au deposits miocene igneous rocks in the 1 600 km long e w gangdese belt of southern tibet form two groups separated at longitude 89 e the eastern group is characterized by mainly intermediate felsic calc alkaline plutons with relatively high sr y ratios 23 to 342 low 87sr 86sr i ratios 0 705 to 0 708 and high endi values 5 5 to 6 1 in contrast the western group is characterized by mainly potassic to ultrapotassic volcanic rocks with relatively high th and k2o contents low sr y ratios 11 to 163 high 87sr 86sr i ratios 0 707 to 0 740 and low endi values 4 1 to 17 5 the eastern plutonic group is associated with several large porphyry cu mo au deposits whereas the western group is largely barren we propose that the sharp longitudinal distinction between magmatism and metallogenic potential in the miocene gangdese belt reflects the breakoff of the greater india slab and the extent of underthrusting by the indian continental lithosphere at that time magmas to the east of 89 e were derived by partial melting of subduction modified tibetan lithosphere mostly lower crust triggered by heating of hot asthenospheric melt following slab breakoff these magmas remobilized metals and volatile residual in the crustal roots from prior arc magmatism and generated porphyry cu mo au deposits upon emplacement in the upper crust in contrast magmas to the west of 89 e were formed by smaller volume partial melting of tibetan lithospheric mantle metasomatized by fluids and melts released from the underthrust indian plate they are less hydrous and oxidized and did not have the capacity to transport significant amounts of metals into the upper crust

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

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

crustal thickening versus lateral expulsion in the indian asian continental collision since the beginning of the continental collision between india and asia there has been about 2500 km of convergence and the northward movement of india has been accommodated by major internal deformation of the asian lithosphere the crustal thickening in and around the tibetan plateau is clearly a direct consequence of this collision but there is considerable debate as to whether a large fraction of the indentation has been accommodated by eastward motion of the lithospheric blocks of southeastern asia and southern china numerical experiments described here test this hypothesis for a range of indentation geometries and rheological models of the lithosphere we employ a thin viscous sheet model of the lithosphere with a depth averaged nonlinear viscous rheology described by a stress strain rate exponent n and including gravitational buoyancy forces scaled by the dimensionless argand number ar the eastern boundary for the collision region is described as a lithostatic boundary the precollision normal stress is determined by static balance and that constant stress is applied throughout the collision the experiments show that during collision the eastern boundary is smoothly displaced to the east at a rate about 1 4 of the indentation rate with only minor variation due to geometry or rheology crustal thickening rates in the region of the plateau are reduced by between 10 and 25 of the corresponding rates determined for similar experiments with a rigid eastern boundary to generalize from the results of these experiments the total north south shortening strain produced by the collision is partitioned between crustal thickening and eastward displacement in the ratio of at least 3 1 and more probably 4 1

tibetan basement rocks near amdo reveal missing mesozoic tectonism along the bangong suture central tibet the u pb and 4 0 ar 3 9 ar studies of a unique exposure of crystalline basement along the jurassic early cretaceous bangong suture of central tibet reveal previously unrecognized records of mesozoic metamorphism magmatism and exhumation the basement includes cambrian and older orthogneisses that underwent amphibolite facies metamorphism coeval with extensive granitoid emplacement at 185 170 ma the basement cooled to 300 c by 165 ma and was exhumed to upper crustal levels in the hanging wall of a south directed thrust system during early cretaceous time we attribute jurassic metamorphism and magmatism to the development of a continental arc during bangong ocean subduction and early cretaceous exhumation to northward continental underthrusting of the lhasa terrane beneath the qiangtang terrane we speculate that a jurassic arc extended regionally along the length of the bangong suture but in all other places in tibet has been buried either depositionally or structurally beneath supracrustal assemblages

mantle contributions to crustal thickening during continental collision evidence from cenozoic igneous rocks in southern tibet the tibetan crust is twice as thick as average continental crust crustal compression and shortening as a result of indian asian collision isoftenconsideredtobetheprimarycauseforthecrustalthickening inthispaper weshowthatmagmaticcontributionisalsoimportant we come to this conclusion by documenting the paleogene linzizong volcanic succession lvs its coeval granitoid batholiths and the miocene adakitic rocks along the gangdese magmatic belt in southern tibet it has been widely accepted that the indian asian collision proceeded from a soft phase at 65 70 ma to a hard phase at 45 40 ma followed by continued post collisional convergence to the present in response to the collision and post collision convergence are a series of tectono magmatic events recorded in the gangdese magmaticbelt theseinclude 1 thesyn collisionallvsvolcanism 65 40ma andtheemplacementofsoutherngangdesebatholiths a peakageof 50ma 2 aperiod 40mato25ma thatismagmaticallyquiescent yettectonicallydominatedbyactivecompressionand crustal shortening and 3 the emplacements of post collisional adakitic rocks 25 12 ma potassic ultrapotassic volcanics 25 10 ma and peraluminous muscovite bearing granites between 24 and 18 ma these three major events contribute in different ways to the crustal thickness phase i formation of the lower juvenile crust from 65 ma to 50 ma with crustal thickening largely concentrated at 50 40maviainputofmantle derivedmagmas phaseii crustalthickeningbytectonicshorteningat 40 25ma andphaseiii retaining crustal thickness but thinning of the lithospheric mantle since 25 ma in response to crustal extension and upwelling and lateral flow of asthenosphericmantle weemphasizethatcollision inducedcrustalthickeningtookplacemainlyintheperiodof 50 40maand 25ma i e the period between the late stage of the lvs volcanism and the beginning of the adakitic rock emplacement most of the lvs rocks and the collision related granitoids in southern gangdese have end 0 attesting to the significance of mantle input most likely through remelting of mantle derived basaltic rocks including the subducted neo tethyan ocean crust the petrologic and geochemical characteristics of the miocene potassic adakitic rocks support the idea that the lower portion of the thickened tibetan crust is mafic and is genetically associated with the earlier lvs magmatism we estimate that the mantle material input contributed about 30 of the total thickness of the present day tibetan crust by assuming a pre collision crustal thickness of 35 km then the tectonic contribution would be about 20 km

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

precise temperature estimation in the tibetan crust from seismic detection of the α β quartz transition in the deep crust temperature which is among the key pa rameters controlling lithospheric dynamics is inferred by extrap olation from the surface using several assumptions that may fail in regions of active tectonics and fluid migration in the rare case that temperatures of 700 8c or higher are exceeded in the upper and middle continental crust composed of quartz rich felsic rocks the a b quartz transition abqt will occur generating a mea surable seismic signature and offering the possibility for precisely estimating temperature from the known abqt phase diagram here it is shown that all expected seismic features of the abqt are met by the boundary between the upper and middle crust be low the indepth iii profile in central tibet this finding implies that a temperature of 700 8c is achieved at a depth of 18 km under the southern qiangtang block which agrees with the depth to the top of a high electrical conductivity anomaly likely representing partially melted crust to the south in the northern lhasa block the abqt is at 32 km depth corresponding to a temperature of 800 8c it thus appears that this seismic boundary representing the abqt is the result of recent geologic processes rather than being a lithologic boundary

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

a synoptic view of the distribution and connectivity of the mid crustal low velocity zone beneath tibet 1 based on 1 2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around tibet rayleigh wave phase velocity maps are constructed from 10 s to 60 s period a 3 d vsv model of the crust and uppermost mantle is derived from these maps the 3 d model exhibits significant apparently inter connected low shear velocity features across most of the tibetan middle crust at depths between 20 and 40 km these low velocity zones lvzs do not conform to surface faults and significantly are most prominent near the periphery of tibet the observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear zones rather than being localized horizontally near the edges of rigid blocks the prominent lvzs are coincident with strong mid crustal radial anisotropy in western and central tibet and probably result at least partially from anisotropic minerals aligned by deformation which mitigates the need to invoke partial melt to explain the observations irrespective of their cause in partial melt or mineral alignment mid crustal lvzs reflect deformation and their amplification near the periphery of tibet provides new information about the mode of deformation across tibet

crustal flow pattern beneath the tibetan plateau constrained by regional lg wave q tomography abstract as a prominent geophysical anomaly unusually high seismic wave attenuation is observed in the crust and upper mantle of the tibetan plateau particularly along its northern area theoretical and laboratory investigations show that the strong seismic attenuation can indicate high temperatures and partial melting which may decrease the viscosity of the material and cause it to flow thus seismic attenuation distribution may provide useful constraints to the crust flows if they exist using lg wave q tomography we construct a 0 05 10 0 hz broadband high resolution crust attenuation model for the tibetan plateau and its surrounding regions the maximum spatial resolution is approximately 1 0 1 0 in well covered areas and for frequencies between 0 05 and 1 5 hz this attenuation model reveals an apparent low qlg belt stretching along the northern and eastern tibetan plateau combining the lg wave q model with other geophysical data two possible crust flow channels are found in the tibetan plateau the main flow channel is from north to east and then turns to southeastern tibet along the western edge of the rigid sichuan basin while a second channel starts from southern tibet and crosses the eastern himalayan syntaxis

eastward expansion of the tibetan plateau by crustal flow and strain partitioning across faults the tibetan plateau is expanding eastwards but the modes of deformation are poorly understood high resolution seismic images from the region identify localized zones of weak crustal rocks as well as deep faults implying that deformation occurs through a combination of crustal flow and movement of rigid blocks of crust