continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

the crustal structure from the altai mountains to the altyn tagh fault northwest china 1 we present a new crustal section across northwest china based on a seismic refraction profile and geologic mapping the 1100 km long section crosses the southern margin of the chinese altai mountains junggar accretional belt and eastern junggar basin easternmost tianshan mountains and easternmost tarim basin the crustal velocity structure and poisson s ratio which provide a constraint on crustal composition were determined from p and s wave data despite the complex geology the crustal thickness along the entire profile is nearly uniform at 50 km the thickest crust 56 km occurs at the northern end of the profile beneath the altai mountains and the thinnest 46 km crust is beneath the junggar basin beneath surficial sediments the crust is found to have three layers with p wave velocities vp of 6 0 6 3 6 3 6 6 and 6 9 7 0 km s respectively the southern half of the profile including the eastern tianshan mountains and eastern margin of the tarim basin shows low p wave velocities and 0 25 to a depth of 30 km which suggests a quartz rich granitic upper crustal composition the northern half of the profile below the altai mountains and junggar accretional belt has a higher poisson s ratio of 0 26 0 27 to a depth of 30 km indicative of an intermediate crustal composition the entire 1100 km long profile is underlain by a 15 30 km thick high velocity 6 9 7 0 km s 0 26 0 28 lower crustal layer that we interpret to have a bulk composition of mafic granulite at the southern end of the profile a 5 km thick midcrustal low velocity layer vp 5 9 km s 0 25 underlies the tianshan and the region to the south and may be indicative of a near horizontal detachment interface pn velocities are 7 7 7 8 km s between the tianshan and the junggar basin and 7 9 8 0 km s below the altai mountains and eastern margin of the tarim basin we interpret the consistent three layer stratification of the crust to indicate that the crust has undergone partial melting and differentiation after paleozoic terrane accretion the thickness 50 km of the crust appears to be related to compression resulting from the indo asian collision

deep structure of the southern ural mountains as derived from wide angle seismic data p and s wave modelling of the data obtained during the seismic refraction wide angle reflection experiment of the urseis95 project demonstrate the presence of a 15 18 km thick crustal root beneath the magnitogorsk tagil zone in the central part of the urals orogen however the centre of this crustal root is displaced by 50 80 km to the east of the present day maximum topography also beneath the magnitogorsk tagil island arc zone an upper crustal body with a high p wave velocity of 6 3 km s 1 at 4 9 km depth can be interpreted as consisting of mafic and or ultramafic rocks this in turn would help to explain the positive bouguer gravity anomaly and the surface heat flow minimum associated with the zone and would also be consistent with the known surface geology of the zone another major feature of the seismic model is the presence of high p and s wave velocities 7 5 and 4 2 km s 1 respectively at the base of the crustal root if the deeper parts of the thickened crust also have high densities small density contrast of about 0 1 g cm 3 with respect to the uppermost mantle then this helps to explain the absence of a pronounced gravity minimum associated with the root these high velocities and densities can be most easily explained by mafic rocks or a mix of mafic and ultramafic rocks within the structural framework of berzin et al 1996 these rocks would belong to the lower russian plate which was being subducted beneath the siberian plate during the uralian orogeny it is possible that the crustal root is formed from the remnants of oceanic crust or a mix of oceanic crust and mantle attached to the russian plate this would mean that little or no continental crust has been subducted or that subduction and hence the uralian orogeny stopped when there was no more oceanic crust or when an attempt was made to subduct lighter continental crust

northward channel flow in northern tibet revealed from 3d magnetotelluric modelling abstract the kunlun fault defines one of the major northern tectonic boundaries of the tibetan plateau previous geophysical studies have detected a major change in rheology across this boundary but it is not clear how this is related to models that have invoked crustal flow the lithospheric resistivity structure of the kunlun fault has been investigated by both the indepth iii and iv magnetotelluric mt transects all the mt data were processed using modern statistically robust methods and have been analysed for directionality and dimensionality in order to improve understanding of the anisotropic distribution of melt previously revealed by our remodelling of the indepth iii mt data a variant approach on 3d inversion of 2d profiles was investigated to explore and improve lateral resolution in addition to the apparent surficial deformation associated with the sinistral strike slip kunlun fault the 3d modelling of the indepth mt data reveals that complex deformation processes are occurring at mid crustal depths in northern tibet the 3d mt inversion results supported by synthetic modelling particularly confirm and highlight the presence of separate north south intrusions of conductive material crossing the kunlun fault into the more resistive kunlun qaidam block these north south intrusions are interpreted to be associated with the horizontal channel flow of partially molten songpan ganzi crust into two or more separated channels moving northwards and crossing the surficial trace of the kunlun fault at mid to lower crustal depths

shear velocity structure radial anisotropy and dynamics of the tibetan crust geophysical and geological data suggest that tibetan middle crust is a partially molten mechanically weak layer but it is debated whether this low viscosity layer is present beneath the entire plateau what its properties are how it deforms and what role it has played in the plateau s evolution broad band seismic surface waves yield resolution in the entire depth range of the tibetan crust and can be used to constrain its shear wave velocity structure indicative of crustal composition temperature and partial melting and radial anisotropy indicative of the patterns of deformation we measured love and rayleigh wave phase velocity curves in broad period ranges up to 7 200 s for a few tens of pairs and groups of stations across tibet combining in each case hundreds of interstation measurements made with cross correlation and waveform inversion methods shear velocity profiles were then determined by extensive series of non linear inversions of the data designed to constrain the depth dependent ranges of isotropic average shear speeds and radial anisotropy shear wave speeds within the tibetan middle crust are anomalously low and also show strong lateral variations across the plateau the lowest mid crustal shear speeds are found in the north and west of the plateau 3 1 3 2 km s 1 within a pronounced low velocity zone in southeastern tibet crustal shear wave speeds increase gradually towards southeast whereas in the north the change across the kunlun fault is relatively sharp the lateral variations of shear speeds within the crust are indicative of those in temperature a mid crustal temperature of 800 c reported previously can account for the low shear velocities across lhasa in the north the temperature is higher and exceeds the solidus resulting in partial melting that we estimate at 3 6 per cent strong radial anisotropy is required by the data in western central tibet 5 per cent but not in northeastern tibet the amplitude of radial anisotropy in the crust does not correlate with isotropic average shear speed and by inference with crustal rock viscosity or with surface elevation instead radial anisotropy is related to the deformation pattern and is the strongest in regions experiencing extension crustal flattening as noted previously the growth of tibet by the addition of indian crustal rocks into its crust from the south is reflected in the higher crustal seismic velocities and thus lower temperatures in the southern compared to northern parts of the plateau more recently added rocks having had less time to undergo radioactive heating within the thickened tibetan crust gravity driven flattening the basic cause of extension and normal faulting in the southern western and central tibet is evidenced by pervasive radial anisotropy in the middle crust beneath the regions undergoing extension the overall eastward flow of the crust is directed by the boundaries and motions of the lithospheric blocks that surround tibet

an early unroofing in northeastern tibet constraints from 40ar 39ar thermochronology on granitoids from the eastern kunlun range qianghai nw china abstract in order to better constrain the mechanisms accommodating the india asia collision along the whole kunlun range northern tibet which is essential to better understand the evolution of the entire deformations area a 40 ar 39 ar thermochronological study has been carried out on a suite of granitoids collected in the vicinity of golmud eastern kunlun range biotite and k feldspar have been analysed and the cooling histories of the latter have been modelled the ranges of 40 ar 39 ar data bt 128 257 ma kf 102 228 ma are similar to previous rb sr and k ar data indicating both a middle paleozoic unit and a mesozoic unit including a triassic series and a jurassic cretaceous series this suggests that the two geological units previously reported in the western kunlun are also present in the eastern part of the range suggesting that eastern and western kunlun ranges might have been left laterally offset by as much as 500 km along a strike slip fault presently parallel to the altyn tagh fault the cooling histories based upon k feldspar modelling show that samples from the southern part of the range recorded an important cooling event 9 15 myr around 30 ma which is thought to reflect an increasing rate of denudation related to the tectonic activity in this area minimum denudation rates of 0 2 0 3 km ma have been estimated in contrast samples from the northern part of the range did not record any variation in their cooling history this heterogeneous denudation across the range is thought to be related to ramp stacking and normal faulting with associated uplift mainly affecting the southern part of the range suggesting that the eastern kunlun underwent an important crustal thickening period 30 ma ago this crustal thickening event is suggested to be related to the motion along the great asian strike slip faults in a way which remains problematic but possibly associated to the cenozoic propagation of the altyn tagh fault finally unroofing in the eastern kunlun range appears to predate that in the western kunlun range where it is suggested to have started around 20 ma ago this diachronous denudation could be related to a higher denudation rate in western kunlun range and it is thus highly probable that the important unroofing event outlined around 20 ma ago in the western kunlun has also affected the eastern part of the range even though the signature is not available at the moment

common pb systematics of precambrian granitic rocks of the nubian shield egypt and tectonic implications we report the pb isotopic compositions for alkali feldspars separated from 34 granitic rocks from the late proterozoic assemblages of the eastern desert and the sinai peninsula egypt in the nubian segment of the arabian nubian shield the eastern desert shows a range of initial pb isotopic compositions 206 pb 204 pb 17 375 19 176 207 pb 204 pb 15 462 15 629 208 pb 204 pb 37 023 38 349 that extend from model mantle toward upper crustal values initial pb isotopic compositions do not define geographic provinces except at aswan and surrounding areas where the most radiogenic values were obtained aswan 207 pb 204 pb 15 611 gebel el hudi 207 pb 204 pb 15 629 and wadi mariya 207 pb 204 pb 15 615 samples from these areas are separated from the less radiogenic samples to the east by trails of serpentinites that we interpret from landsat thematic mapper images as the extension of the allaqi heiani ophiolitic belt our data support previous interpretations of the belt as a suture location and suggest that its western margin might coincide with the boundary between the nubian shield and the old african continent we interpret the pb isotopic compositions from the eastern desert and the previously identified group i and ii pb stacey and others 1980 from the arabian nubian shield to indicate various degrees of mixing between mantle derived juvenile oceanic component s and pre pan african crustal material on 207 pb 204 pb 208 pb 204 pb plots the feldspar and galena data display variations characteristic of modern island arc magmas consistent with models that relate shield assembly to accretion of island arcs the crustal pb isotopic signature of sample from aswan and surrounding areas is best explained by interaction of arc magmas with an overriding crustal plate whereas the less radiogenic nature of the eastern desert pb9s is probably related to subduction of sediments derived from adjacent continent or deposition of these sediments in near arc basins accounting for recycled pre pan african crustal material in the shield results in normal growth rates 0 44 km 3 yr using reasonable assumptions for initial pb isotopic compositions and concentrations

growth of lithosphere scale fault system in ne tibet numerical modeling constrained by high resolution seismic reflection data abstract the east kunlun fault is an important strike slip shear zone to understand continental deformation in ne tibet the recent high resolution deep seismic reflection profiling across the kunlun fault reveals a thrust fault system and two decollements in the crust along with another thrust fault system which reaches to the depth of upper mantle and cuts off the moho the mechanism of the growth of these shear zones at lithosphere scale which reveals the type of deep deformation is investigated using the finite element method with an elastic plastic constitutive relationship the results show that the thrust fault system in the crust may be explained by the conjugated plastic deformation belts under compression from the indian plate the pre existing fault in the depth may develop to cut the moho toward two new directions rather than along the original direction the vertical and lateral heterogeneity of material frictional property and geometry of the models all affect the feature of the fault growth the growth of the thrust fault system on both sides of the kunlun fault is only located in the crust it means that the kunlun fault does not reach to the moho depth

ambient noise tomography of north tibet limits geological terrane signature to upper middle crust 1 we use ambient noise tomography to map regional differences in crustal rayleigh wave group velocities with periods of 8 40 s across north tibet using the international deep profiling of tibet and the himalaya phase iv arrays 132 stations deployed for 10 24 months for periods of 8 24 s sensitive to midcrustal depths of 5 30 km we observe striking velocity changes across the bangong nujiang and jinsha suture zones as well as the kunlun qaidam boundary from south to north we see higher velocities beneath the lhasa terrane lower velocities beneath the qiangtang higher velocities in the songpan ganzi and kunlun terranes and the lowest velocities beneath the qaidam basin maps at periods of 34 and 40 s sensitive to the middle and lower crust at depths of 30 60 km do not show evidence of changes across those boundaries any differences between the tibetan terrane lower crusts that were present at accretion have been erased or displaced by cenozoic processes and replaced almost ubiquitously by uniformly low velocities

contemporary crustal deformation of northeast tibet from geodetic investigations and a comparison between the seismic and geodetic moment release rates abstract we processed the data recorded by 32 continuous gps stations from the crustal movement observation network of china and combined them with published gps observations in a common geodetic reference frame to better depict the ongoing crustal velocities and deformation field of northeast tibet the derived velocity field indicates that the deformation of northeast tibet is characterized mainly by crustal shortening coupled with the eastward extrusion of crustal material and clockwise block rotation we further used the gps velocity field to evaluate the strain rate tensors by the method of sandwell and wessel 2016 the strain rate field shows an obvious transition of the contraction rate from ne sw to nee sww and a decrease in the maximum shear strain rate from west to east was observed along the east kunlun fault and haiyuan fault indicating that the block rotation and lateral extrusion therein may be partitioned by ne sw oriented compression to some extent the inhomogeneity of the rotation strain rate implies that northeast tibet does not behave like a single rigid block but instead resembles a left lateral shear zone supporting the hypothesis that the northward transfer of motion along the east kunlun fault may play a significant role in the deformation style of this area the dilatation rate verified that most of northeast tibet experiences crustal thickening which coincides with the average uplift rate of 1 mm y revealed by gps velocities and channel steepness indices we also estimated the geodetic moment rates and compared them with the seismic moment rates estimated using the truncated gutenberg richter earthquake distribution the ratio of the seismic strain rate to the geodetic strain rate in the regions near the altyn tagh fault the qaidam basin adjoining the qilian mountains the haiyuan fault and the central part of the east kunlun fault are near unity indicating that the deformation in these areas is dominated by seismic release and may represent fully coupled seismogenic zones the occurrence of large earthquakes in the past estimated magnitudes m 6 coupled with a high geodetic deformation rate in these regions could indicate overdue m 6 earthquakes however other factors including the occurrence of creep and the long recurrence time of mw 8 0 earthquakes should be taken into consideration along the haiyuan fault by contrast in other regions including the southern and eastern parts of the haiyuan fault along the elashan fault and the northern qilian mountains the geodetic moment rate is higher than the seismic moment rate by a factor between 3 and 9 we propose that this phenomenon is related to the undersampling of long term earthquake rates within the seismic catalogue long term aseismic crustal deformation or a combination of these factors our study has implications for better comprehending the current seismotectonic pattern of northeast tibet

crustal and upper mantle structure of northern tibet imaged with magnetotelluric data 1 magnetotelluric mt data were collected in northern tibet along the amdo to golmud highway during the 1995 and 1999 project indepth international deep profiling of tibet and the himalaya surveys broadband and long period mt data were collected and the te mode tm mode and vertical magnetic field data were inverted to yield a minimum structure two dimensional resistivity model the model obtained from inverting all responses simultaneously shows that a pervasive midcrustal conductor extends from the kunlun shan to the bangong nuijiang suture the vertically integrated conductivity conductance of this crustal layer is greatest in the northern qiangtang terrane at latitude 34 n the electrical resistivity of the upper mantle is constrained by the mt data to be in the range of 10 30 m across the songpan ganze and qiangtang terranes this is lower than would be expected if asian lithosphere underthrusts northern tibet as far as the qiangtang terrane the mt responses are more consistent with a model in which asian lithosphere extends as far south as the kunlun shan and the upper mantle beneath the songpan ganze and qiangtang terranes is sufficiently hot to contain a small fraction of interconnected partial melt

injection of tibetan crust beneath the south qaidam basin evidence from indepth iv wide angle seismic data 1 the international deep profiling of tibet and the himalaya phase iv indepth iv active source seismic profile in northeast tibet extends 270 km roughly north south across the songpan ganzi terrane the predominantly strike slip north kunlun fault along the kunlun suture the east kunlun mountains and the south qaidam basin refraction reflection and gravity modeling provide constraints on the velocity and density structure down to the moho the central qaidam basin resembles average continental crust whereas the songpan ganzi terrane and east kunlun mountains exhibit thickened lower velocity crust also characteristic of southern tibet the crustal thickness changes from 70 km beneath the songpan ganzi terrane and east kunlun mountains to 50 km beneath the qaidam basin this jump in crustal thickness is located 100 km north of the north kunlun fault and 45 km north of the southern kunlun qaidam boundary farther north than previously suggested ruling out a moho step caused by a crustal penetrating north kunlun fault the qaidam moho is underlain by crustal velocity material 6 8 7 1 km s for 45 km near the crustal thickness transition the southernmost 10 km of the qaidam moho are underlain by a 70 km reflector that continues to the south as the tibetan moho the apparently overlapping crustal material may represent songpan ganzi lower crust underthrusting or flowing northward beneath the qaidam basin moho thus the high tibetan plateau may be thickening northward into south qaidam as its weak thickened lower crust is injected beneath stronger qaidam crust

acoustic behavior of sedimentary rocks geologic properties versus poisson s ratios velocity transforms are essential for the extraction of geologic information from seismic reflection data sets and or predicting porosity from wireline logging data most commonly used transforms like the wyllie time average equation the raymer modified time average equation and the gardner equation are all based on empirical relationships extracted from relatively small clean mono and bimineral and mostly siliciclastic data sets and generally fail to explain the acoustic relationships within multimineral data sets to extract globally significant relationships between geologic and acoustic parameters more than 2800 plug measurements were collected more than two thirds of the data were our own measurements the others were from the literature this data set covers nearly all sedimentary environments and mixtures of those as well as the entire range in petrophysical properties

crustal structure of central tibet as derived from project indepth wide angle seismic data summary in the summer of 1998 project indepth recorded a 400 km long nnw sse wide angle seismic profile in central tibet from the lhasa terrane across the banggong nujiang suture bns at about 89 5 e and into the qiangtang terrane analysis of the p wave data reveals that 1 the crustal thickness is 65 5 km beneath the line 2 there is no 20 km step in the moho in the vicinity of the bns as has been suggested to exist along strike to the east based on prior fan profiling 3 a thick high velocity lower crustal layer is evident along the length of the profile 20 35 km thick 6 5 7 3 km s 1 and 4 in contrast to the southern lhasa terrane there is no obvious evidence of a mid crustal low velocity layer in the p wave data although the data do not negate the possibility of such a layer of modest proportions combining the results from the indepth iii wide angle profile with other seismic results allows a cross section of moho depths to be constructed across tibet this cross section shows that crustal thickness tends to decrease from south to north with values of 70 80 km south of the middle of the lhasa terrane 60 70 km in the northern part of the lhasa terrane and the qiangtang terrane and less than 60 km in the qaidam basin the overall northward thinning of the crust evident in the combined seismic observations coupled with the essentially uniform surface elevation of the plateau south of the qaidam basin is supportive of the inference that northern tibet until the qaidam basin is underlain by somewhat thinner crust which is isostatically supported by relatively low density hot upper mantle with respect to southern tibet

penetration of crustal melt beyond the kunlun fault into northern tibet the transition between the weak lithosphere of the tibetan plateau and the surrounding rigid crustal blocks has a key role in the ongoing collision between india and asia a reanalysis of existing magnetotelluric data suggests that crustal melt penetrates north from the tibetan plateau beyond the kunlun fault and weakens the crust beneath the kunlun shan

pb pb dating and pb isotope geochemistry of neoproterozoic carbonate rocks from the sao francisco basin brazil implications for the mobility of pb isotopes during tectonism and metamorphism abstract a u pb study was carried out on carbonate rocks from the neoproterozoic bambui group in the southern part of the sao francisco basin brazil pb isotopic compositions and u and pb concentrations were determined on more than 90 samples from different parts of the basin these samples were found to contain four distinct types of pb here called types i ii iii and iv type i pb was found in samples with low pb concentrations and relatively high u concentrations it represents in situ growth of radiogenic pb and has a large enough variation to define pb pb isochron ages type ii pb is present in samples with relatively high pb concentrations and low u concentrations it is nonradiogenic crustal pb that could either represent average crustal pb at the time of deposition or as interpreted here at the time of deformation of these rocks type iii pb is also found in samples with high pb concentrations and low u concentrations but it apparently is radiogenic crustal pb from the archean or paleoproterozoic basement that was incorporated into the carbonates during the 600 ma brasiliano orogeny type iv pb is intermediate in composition between type iii and type i pb it represents a mixture of these two types pb pb isochron ages obtained from mesoscopically undeformed carbonates containing type i pb range from 686 69 to 520 53 ma the older age is the minimum depositional age for carbonate rocks from the sete lagoas formation during the interval from 690 to 500 ma the pb isotope system of carbonates from the sao francisco basin was disturbed and in some areas it was totally reset type iii pb compositions define a straight line type ii pb falls on the lower end of this line which intercepts the stacey and kramers s k pb growth curve at about 520 and 2100 ma this line is interpreted as representing a stage of crustal pb evolution beginning about 2100 ma and ending about 550 to 500 ma when variably radiogenic pb from the basement was incorporated into these carbonates the ages determined in this study are in agreement with most published ages for brasiliano fold belts marginal to underlying sao francisco craton showing that isotopic systems of sao francisco basin rocks were largely affected by brasiliano tectonism

crustal structure of the eratosthenes seamount cyprus and s turkey from an amphibian wide angle seismic profile abstract in 2010 project cococo incipient continent continent collision recorded a 650 km long amphibian n s wide angle seismic profile extending from the anatolian plateau across southern turkey and cyprus to just south of the eratosthenes seamount esm the aim of the project is to reveal the impact of the transition from subduction to continent continent collision of africa with anatolia arrival picking finite differences ray tracing and inversion of the offshore and on offshore data produced a tomographic model across southern cyprus the accretionary wedge and the esm the main features of this model are 1 crustal p velocities predominantly lower than 6 5 km s beneath the esm 2 crustal thickness between 28 and 37 km 3 an upper crustal reflection at 5 km depth beneath the esm 4 the likely presence of oceanic crust south of the esm and a transform margin north of it and 5 a 12 km thick ophiolite sequence on cyprus land shots on turkey also recorded on cyprus gravity data and geological and previous seismic investigations allow to derive a layered velocity model beneath anatolia and the northern part of cyprus the main features of this model are 1 moho depths of 38 45 km beneath the anatolian plateau 2 an upper and lower crust with large lateral changes in velocity and thickness 3 a north dipping subducting plate below cyprus with a steepening of the dip angle of the plate at about 45 km depth thus the wide angle seismic and gravity data provide detailed insights into the 2 d geometry and velocity structures associated with the cyprus arc collision zone finally integrated analysis of the geophysics and geology allows a comprehensive interpretation of the crustal structure related to the collision process

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

regional earthquakes in northern tibetan plateau implications for lithospheric strength in tibet 1 a total of 400 regional earthquakes were located in northern tibetan plateau from data recorded by indepth iv and pku eastern kunlun arrays from may 2007 to june 2009 the distribution of these earthquakes is compatible with a continuously deforming tibetan lithosphere most earthquakes occur at a depth range of 0 15 km but no event is deeper than 30 km this observation strongly supports the existence of a hot and weak lower crust beneath the northern tibet the crustal seismogenic zone appears slightly thicker beneath the northern tibet than in the southern plateau possibly reflecting a difference in the rheological dry vs wet structure of the crust the absence of lower crustal and uppermost mantle earthquakes in northern tibet is consistent with a localized asthenospheric upwelling under the qiangtang and songpan ganze terranes finally the lack of mantle earthquakes should be fully addressed in any models of subduction in northern tibet

present day crustal deformation and strain transfer in northeastern tibetan plateau abstract the three dimensional present day crustal deformation and strain partitioning in northeastern tibetan plateau are analyzed using available gps and precise leveling data we used the multi scale wavelet method to analyze strain rates and the elastic block model to estimate slip rates on the major faults and internal strain within each block our results show that shear strain is strongly localized along major strike slip faults as expected in the tectonic extrusion model however extrusion ends and transfers to crustal contraction near the eastern margin of the tibetan plateau the strain transfer is abrupt along the haiyuan fault and diffusive along the east kunlun fault crustal contraction is spatially correlated with active uplifting the present day strain is concentrated along major fault zones however within many terranes bounded by these faults intra block strain is detectable terranes having high intra block strain rates also show strong seismicity on average the ordos and sichuan blocks show no intra block strain but localized strain on the southwestern corner of the ordos block indicates tectonic encroachment

cenozoic tectonic evolution of the qaidam basin and its surrounding regions part 3 structural geology sedimentation and regional tectonic reconstruction the qaidam basin is the largest topographic depression inside the tibetan plateau because of its central position understanding the tectonic origin of the qaidam basin has important implications for unraveling the formation mechanism and growth history of the tibetan plateau in order to achieve this goal we analyzed regional seismic reflection profiles across the basin and a series of thickness distribution patterns of cenozoic strata at different time slices the first order structure of the basin is a broad cenozoic synclinorium which has an amplitude ranging from 16 km in the west to 48 in the west to 15 s 1 to 1 3 10 17 s 1 the eastward decrease in upper crustal shortening requires a progressive shift in crustal thickening mechanisms across qaidam basin from dominantly upper crustal shortening in the west to dominantly lower crustal shortening in the east although sedimentation began synchronously at 65 50 ma across the entire basin the initiation ages of the southern and northern basin bounding structures are significantly different deformation started at 65 50 ma in the north and at 29 24 ma in the south this information and the existing inference that the uplift of the eastern kunlun range south of qaidam basin began after 30 20 ma imply that the paleogene 65 24 ma qaidam and hoh xil basins on both sides of the eastern kunlun range may have been parts of a single topographic depression 500 km wide in the north south direction between the qilian shan and fenghuo shan thrust belts in the north and south the development of this large paleogene basin in central tibet and its subsequent destruction and partitioning by the neogene uplift of the eastern kunlun range requires a highly irregular sequence of deformation possibly controlled by preexist ing weakness in the tibetan lithosphere

lithospheric structures of and tectonic implications for the central east tibetan plateau inferred from joint tomography of receiver functions and surface waves

limited southward underthrusting of the asian lithosphere and material extrusion beneath the northeastern margin of tibet inferred from teleseismic rayleigh wave tomography the northeastern margin of tibet is of key importance in understanding the uplift and expansion of tibetan plateau in this research we perform rayleigh wave tomography from 20 s to 167 s using the data recorded by china digital seismic network and several portable seismic arrays in northeastern tibet and adjacent areas our resultant 3 d shear wave velocity model exhibits strong lateral variations in the lithosphere high shear wave velocities are observed in the mantle of the cratonic ordos and yangtze blocks as well as the east qinling orogen the qilian orogen is underlain by a 120 km thick lithosphere of intermediate velocities and a low velocity layer of asthenospheric mantle below the depth of 120 km low velocities are characteristic in the middle to lower crust of the songpan ganzi terrane and the west qinling orogen possibly representing the crustal flow penetrating the kunlun fault an uppermost mantle low velocity zone is found in the same location indicating either the asthenosphere upwelling or the transitional crust mantle boundary layer our observations have two tectonic implications first the large scale eastward tibetan material extrusion into eastern china through the east qinling orogen is unlikely to be an ongoing process above 200 km depth second the southward subduction of the north china craton toward northeastern tibet may not have fully developed in this region furthermore we propose that the difference in the lithospheric strength between the qaidam block and the west qinling orogen is a critical element to control the tectonics in the northeastern margin of tibet

injection of indian crust into tibetan lower crust a two dimensional finite element model study we propose a model in which the stronger indian crust injects into the weaker lower crust of tibet if the viscosity of the tibetan lower crust is below a critical value the continuous advancing of the indian crust beneath tibet would produce negligible variation in stresses on the bottom of the stronger brittle tibetan upper crust and would result in a spatially uniform uplift and a virtually undisturbed tibetan plateau surface this process is analogous to that of a piston rising in a hydraulic jack as additional fluid is forced into the chamber we find that if the viscosity of the tibetan lower crust is assumed to be constant throughout the computed critical viscosity is 6 1018 pa s for the present day lower crustal thickness of 58 km in addition to uniform viscosity two hypothetical depth dependent viscosities are also tested which give rise to essentially the same distribution of stress along the bottom of brittle upper crust our model also interprets the 4 km elevation contrast between the tibetan plateau and the tarim block a 1 160 viscosity contrast between the tibetan and tarim lower crust is required in order to maintain the current thickening rate of the tibetan crust 1 2 mm yr while keeping the thickness of the tarim crust unchanged

a model of the crust and mantle structure down to 700 km depth beneath the lhasa to golmud transect across the tibetan plateau as derived from seismological data abstract a 700 km deep seismic velocity cross section beneath the lhasa to golmud transect across the tibetan plateau is presented in contrast to the first version of this cross section which comprised an 800 km wide swath centred on the lhasa to golmud transect due to the recent proliferation of publications concerning the mantle structure beneath tibet this study is based only on seismic profiles which either run along or cross the transect and arrays or studies which at least partly cover the transect the results from the recent indepth iv project indicate that the crustal thickness change from 70 km beneath the songpan ganzi terrane and kunlun mountains to 54 km beneath the qaidam basin is located about 100 km north of the kunlun fault and almost 45 km north of the north kunlun thrust the qaidam basin moho is underlain by crustal velocity material for almost 45 km and the apparently overlapping crustal material may represent songpan ganzi lower crust underthrusting or flowing northward beneath the qaidam basin moho thus the high tibetan plateau may be thickening northward into south qaidam as its weak thickened lower crust is injected beneath the stronger qaidam crust beneath the crust high velocity dense cold indian lithospheric mantle extends northwards until about the banggong nujiang suture northwards asian lithospheric mantle is overlain by a low velocity less dense warm tibetan plate consisting of an upper lithospheric and a lower asthenospheric part the apparent northwards deepening of the 410 and 660 km discontinuities by about 20 km implies that the upper mantle beneath north tibet is slower less dense and warmer than under south tibet in agreement with the observed uppermost mantle velocities this in turn could provide some of the isostatic support for the high elevations in the north where the crust is somewhat thinner than in the southern plateau

water weakening of clinopyroxene in the dislocation creep regime we performed a series of triaxial compressive creep experiments at two different water fugacities to investigate the effect of water on the creep strength of a natural clinopyroxenite samples were deformed under water saturated conditions at temperatures between 1373 and 1473 k confining pressures of 150 and 300 mpa and differential stresses from 34 to 261 mpa strain rates were in the range 10 7 to 10 5 s 1 water fugacity was controlled at either 140 or 280 mpa the creep results yield a stress exponent of 2 7 0 3 and an activation energy of 670 40 kj mol compared to dry clinopyroxene wet samples creep over 100 times faster at a given temperature confining pressure water fugacity and differential stress the creep rate of clinopyroxene is proportional to the water fugacity to the 3 0 0 6 power with an activation volume of 0 m 3 mol one possible water weakening mechanism is an enhancement of the rate of dislocation climb associated with increases in the concentration of jogs and the diffusivity of silicon ions compared to other major minerals in earth s lower crust specifically olivine and plagioclase the water weakening effect is most significant for clinopyroxene under hydrous conditions the strengths of clinopyroxene and anorthite are comparable over the investigated stress range and both phases are weaker than olivine since the mineral assemblages in earth s lower continental crust are often dominated by plagioclase and pyroxene in places where a wet flow law applies the mechanical behavior of clinopyroxene will have a substantial effect on creep strength

seismic evidence for stepwise thickening of the crust across the ne tibetan plateau we performed a receiver function analysis on teleseismic data recorded along two 550 km long profiles crossing the northeastern tibetan plateau results from time to depth migration grid search vp vs determination and simulated annealing inversion of waveforms reveal that the crust thickens from 50 km near the northern edge of the plateau to 80 km south of the jinsha suture in the qiang tang block crustal thickening occurs in staircase fashion with steps located beneath the main reactivated sutures the vp vs ratio close to the global continental average does not suggest widespread partial melting but rather a more usual separation between an upper felsic and a lower mafic part within the northeastern tibetan crust

electrical resistivity structures of the kunlun qaidam qilian system at the northern tibet and their tectonic implications abstract the northern tibetan plateau is characterized by northwest southeast trending basin range systems such as the kunlun shan ranges qaidam basin qilian shan ranges the cenozoic evolution and deformation of the qaidam basin and its neighboring ranges are important indications for the northward growth of the tibetan plateau the latest magnetotelluric data were collected along a profile in n16 5 e from the songpan ganzi block across the kunlun shan ranges east section of the qaidam basin and ending in the qilian shan ranges both two dimensional and three dimensional inversions of the data along the profile were performed and the results provided new evidence for lithospheric structures in the northern tibetan plateau with three dimensional isotropic inversion codes we are able to model the phase roll out of quadrant proq magnetotelluric data which exist at low frequencies in a number of continuous sites at the northern qaidam basin sensitivity studies of the three dimensional models indicate that the qaidam basin with low resistivity upper crust is the major contributor for the proq effect while a south north striking low resistivity gap intersecting with the qaidam basin at its northern part affects the range of the proq effect in the magnetotelluric resistivity models the qaidam basin is not in symmetric structure at upper crust level with its depocenter near the northern qaidam fault at mid lower crust level the compositions of both the kunlun shan ranges and the southern qaidam basin are of generally high resistivity which does not support large scale lower crust materials of the songpan ganzi block flow below the kunlun shan ranges or the qaidam basin instead the unsymmetrical qaidam basin and the shovel shape high resistivity anomalies in crust below the kunlun shan ranges and the southern qiadam basin indicate the regional northward push from the songpan ganzi block the south deepening low resistivity anomalies in the crust of the qilian shan ranges support the existence of an intracrustal detachment which controls the thrusts in northern qaidam basin and the qilian shan ranges the crustal deformation in northern tibetan plateau is consistent with mantle convergence or collision between the kunlun qaidam lithospheric mantle and the asian lithospheric mantle

the role of crustal and mantle sources in the genesis of granitoids of the antarctic peninsula and adjacent crustal blocks abstract magmatic rocks from the antarctic peninsula show marked variations in isotope composition which reflect changes in the geodynamic evolution of the peninsula through time most antarctic peninsula granitoids formed as a result of subduction they fall on well defined trends on plots of nd 207 pb 204 pb and 18 o against 87 sr 86 sr i between a component derived from subduction modified mantle or juvenile basaltic underplate ndi 6 207 pb 204 pb 15 61 18 o 5 5 87 sr 86 sr nd 7 207 pb 204 pb 15 67 18 o 10 87 sr 86 sr 0 709 a small group of granitoids emplaced before or during gondwana break up plot on distinct trends towards high 87 sr 86 sr i compositions reflecting mixing between melts derived from proterozoic lower crust and melts of middle upper crustal rocks ndi 9 207 pb 204 pb 15 64 18 o 10 87 sr 86 sr 0 726 with little or no input of new material derived from the mantle or from juvenile basaltic underplate these granitoids are thought to have formed as a result of crustal attenuation during the initial rifting phase of gondwana break up similar trends are shown by data from granitoids of the adjacent crustal blocks of west antarctica the isotope data suggest that an enriched ferrar karoo type lithosphere was not involved in the genesis of granitoids of the antarctic peninsula or of the ellsworth whitmore mountains crustal block

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

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

geologic evolution of northern tibet results of an expedition to ulugh muztagh a reconnaissance expedition across the northern margin of the tibetan plateau revealed evidence of a late cenozoic northward progression of the locus of crustal shortening and therefore of a northward growth of the area encompassed by the plateau active reverse faults crop out at the foot of the altyn tagh on the northern edge of the plateau and at the bases of several ranges within the altyn tagh and kunlun where the elevations of the neighboring basins are less than 4000 meters farther south where elevations are higher there was no evidence of recent faulting but late cenozoic rock in the ayak kum kol basin has been strongly folded south of this basin ulugh muztagh apparently the highest mountain in the eastern kunlun is underlain by late miocene tourmaline bearing and two mica granite these rocks suggest that thickening of continental crust had begun in this area by late miocene time overlying quartz sanidine welded tuffs of pliocene age imply that uplift and erosion occurred between miocene and pliocene time but with little subsequent erosion in addition we found an east west trending belt of mafic and ultramafic rock that probably marks a suture of a crustal fragment with southern asia in triassic or more recent time

receiver function imaging of the lithosphere at the kunlun qaidam boundary northeast tibet abstract we studied the crustal structure and tectonics in the north tibetan plateau from the songpan ganzi terrane to the qaidam basin using teleseismic receiver function imaging across a major lithospheric boundary the kunlun qaidam boundary where previous studies suggest a 15 20 km change in crustal thickness from thicker crust in the kunlun mountains to thinner crust in the qaidam basin we report p receiver functions for 70 stations largely the international deep profiling of tibet and the himalaya indepth phase iv experiment our most dense station coverage is located along the roughly north south indepth iv active source seismic profile at approximately 95 e longitude azimuthal and geographical changes in the receiver functions reveal significant changes in crustal structure and vp vs from across the study area receiver functions show strong converters that we interpret as the moho at 70 km depth beneath the qiangtang songpan ganzi terranes and kunlun mountains and at 50 km depth beneath the central qaidam basin this large change in crustal thickness occurs 50 km north of the north kunlun strike slip fault on which the 2001 m8 1 kunlun earthquake occurred receiver functions for some of the stations north of the thickness change at the kunlun qaidam boundary also show a deeper 70 km bright converter in addition to the 50 km converter the two converters appear to overlap by up to 30 km in some locations along the south qaidam basin we combine previous results with these new results to discuss implications for mechanisms for crustal thickening in the north tibetan plateau including crustal flow and crustal injection at depths imaged here shallower than 100 km we see no evidence of southward subduction of eurasian lithosphere

west east variation in crustal thickness in northern lhasa block central tibet from deep seismic sounding data 5 30 km depth 5 0 vp 6 4 km s a middle crust 33 45 km depth 6 5 vp 6 8 km s and a lower crust depths below 48 km 7 0 vp 7 4 km s the west to east increase in crustal thickness is accomplished by a 50 thickening of the middle and lower crust larger vertical velocity gradients separate these three layers and bound them above surficial and sedimentary rocks and below moho transition zone the most notable low velocity zone in the crust lies at the base of the upper crustal layer s wave velocity structure is less well constrained but parallels the p wave structure except that vp vs ratios may decrease from west to east in the lower crust our data suggest considerable variation in structure along strike of the tibetan plateau and show that interpretations of tibet as a purely two dimensional orogen are overly simplistic the west east increase in crustal thickness may occur across the karakorum jiali fault system and be an indicator of lateral tectonic escape of the qiangtang terrane

crustal and uppermost mantle velocity structure along a profile across the pamir and southern tien shan as derived from project tipage wide angle seismic data summary utilizing seismic refraction wide angle reflection data from 11 approximately in line earthquakes 2 d p and s velocity models and a poisson s ratio model of the crust and uppermost mantle beneath the southern tien shan and the pamir have been derived along the 400 km long main profile of the tipage tien shan pamir geodynamic program project these models show that the crustal thickness varies from about 65 5 km close to the southern end of the profile beneath the south pamir through about 73 6 km under lake karakul in the north pamir to about 57 7 km 50 km south of the northern end of the profile in the southern tien shan average crustal p velocities are low with respect to the global average varying from 6 26 to 6 30 km s 1 the average crustal s velocity varies from 3 54 to 3 70 km s 1 along the profile and thus average crustal poisson s ratio varies from 0 23 beneath the central pamir in the south central part of the profile to 0 265 towards the northern end of the profile beneath the southern tien shan the main layer of the upper crust extending from about 2 km below the earth s surface to 27 km depth below sea level b s l has average p velocities of about 6 05 6 1 km s 1 except beneath the south central part of the profile where they decrease to around 5 95 km s 1 this is in contrast to the s velocities which range from 3 4 to 3 6 km s 1 and exhibit the highest values of 3 55 3 6 km s 1 where the p velocity is lowest thus for the main layer of the upper crust is 0 26 beneath the profile except beneath the south central part of the profile where it decreases to 0 22 the low value of 0 22 for under the central pamir the along strike equivalent of the qiangtang terrane in tibet is similar to that within the corresponding layer beneath the northern lhasa and southern qiangtang terranes in central tibet and is indicative of felsic rocks rich in quartz in the state the lower crust below 27 km b s l has p velocities ranging from 6 1 km s 1 at the top to 7 1 km s 1 at the base further for this layer is 0 27 0 28 towards the northern end of the profile but is low at about 0 24 beneath the central and southern parts of the profile which is similar to the situation found in the northeast tibetan plateau the low values can be explained by felsic schists and gneisses in the upper part of the lower crust transitioning to granulite facies and possibly also eclogite facies metapelites in the lower part within the uppermost mantle the average p velocity is about 8 10 8 15 km s 1 and is about 0 26 assuming an isotropic situation then a relatively cool 700 800 c uppermost mantle beneath the profile is indicated this would in turn indicate an intact mantle lid beneath the profile an upper mantle reflector dipping from 104 km b s l 120 km from the southern end of the profile to 86 km b s l 155 km from the northern end of the profile has also been identified the preferred model presented here for the crustal and lithospheric mantle structure beneath the pamir calls for nearly horizontal underthrusting of relatively cool indian mantle lithosphere the leading edge of which is outlined by the pamir seismic zone this cool indian mantle lithosphere is overlain by significantly shortening warm asian crust the moho trough that is a feature seen beneath some other orogenic belts for example the alps and the urals beneath the northern pamir may mark the southern tip of the actively underthrusting tien shan crust along the main pamir thrust

crustal thicknesses and poisson s ratios in the eastern tibetan plateau and their tectonic implications 1 crustal thicknesses and poisson s ratios in the eastern tibetan plateau were estimated using the h k stacking method on teleseismic receiver functions the data came mainly from the regional seismic networks in 2006 2008 results show that the crust thickens from east to west with a thickness difference of 30 km a sharp change of crustal thickness from 41 5 to 52 5 km is observed across the longmen shan crustal thickness predictions based on the airy type isostasy are consistent with the estimations from h k stacking implying that the topography is approximately compensated the study area can be divided into three parts according to the poisson s ratios 1 the northern songpan garze terrane and the western qinling orogenic belt with low ratios 0 26 2 the southern songpan garze terrane the sanjiang fold system and the sichuan basin with high to intermediate ratios 0 27 0 30 and 3 the southwestern yangtze platform with low to intermediate ratios 0 27 joint interpretation of the low average vp values from deep seismic sounding profiles and the low vp vs ratios from receiver function analysis favors the presence of felsic crust rather than significant volumes of partial melts in the northern songpan garze terrane and the western qinling orogenic belt the junction of the longmen shan fault zone the xianshuihe fault zone and the anninghe fault zone has an ultrahigh poisson s ratio 0 30 which could be considered as an evidence of local partial melt we propose that the eastward movement of upper and middle crustal material in the southern songpan garze terrane is obstructed by the more rigid sichuan basin resulting in strain accumulation along the longmen shan fault zone

seismic imaging of upper crustal structure using travel times from the passcal ouachita experiment a first arrival travel time inversion applied to the 1986 passcal ouachita experiment has been used to image upper crustal structure twenty one shot points every 10 km were recorded along two 100 km long segments with 400 seismographs on each segment a splined velocity model parameterization allowed for the inversion of laterally inhomogeneous models but not velocity discontinuities over 800 travel times were used in the inversion with a variable data covariance weighting the multiple source nature of the experiment design allowed the use of reciprocity relationships among 21 shot points to enhance data correlation a succession of models with velocities specified at 30 90 171 and 221 nodes were inverted for whose horizontal node spacing decreased from 100 to 12 5 km in the final 221 node model vertical node spacings of 3 0 and 1 5 km were used the results of the inversions indicate changes in depth of isovelocity lines which are interpreted as triassic fault structure based on nearby well data the final rms travel time residual is 0 045 s resolution using first arrivals deteriorates rapidly below 9 km depth three distinct velocity gradients can be correlated with geologic units surface exposed cretaceous and jurassic sediments with velocities from 3 0 tomore 3 5 km s extend to 1 5 km depth triassic rift fill sediments which do not outcrop have inverted velocities ranging from 3 5 to 4 75 km s between depths of 1 5 3 0 km paleozoic sediments are imaged by a decreased velocity gradient below a depth of 3 0 km lack of resolution at depths greater than 9 km necessitates detailed modeling of wide angle reflected phases to further constrain deeper crustal structure less

crustal structure of the paleozoic kunlun orogeny from an active source seismic profile between moba and guide in east tibet china abstract we herein present a new seismic refraction wide angle reflection profile that crosses the songpan ganzi terrane the animaqing suture zone and the eastern kunlun mountains comprised of the south kunlun and middle kunlun blocks separated by the middle kunlun fault the profile is 380 km long and extends from moba to guide in eastern tibet the crustal thickness is about 62 km under the songpan ganzi terrane 62 64 km under the south kunlun and 60 km under the middle kunlun block the songpan ganzi flysch seems to be present up to a depth of 15 km south of the animaqing suture zone and up to a depth of 10 km in the middle kunlun block with thicknesses elsewhere that depend on assumptions about the likely lithologies the profile exhibits clear lateral variations both in the upper and lower crust which are indicative of different crustal blocks juxtaposed by the kunlun fault system whether or not the songpan ganzi flysch was originally deposited on oceanic crust at the longitude of our profile 100 e it is now underlain by continental crust and the presence of continental crust beneath the songpan ganzi terrane and of a continental arc under the south kunlun block suggest paleozoic continent continent arc collision in the eastern kunlun mountains comparison of crustal velocity columns from all wide angle seismic profiles across the eastern kunlun mountains indicates a remarkable west to east change in the moho topography across the kunlun fault system 15 20 km moho step at 95 e but only 2 5 km along our profile at 100 e lower crustal thickness of the kunlun terranes is rather uniform about 35 km from 80 95 e which suggests that similar thrust thickening processes have played a role where the qaidam basin abuts the kunlun fault but thins to 20 25 km at 100 e east of the qaidam basin the increased crustal thickness from 93 to 98 e compared to that at 100 e may be due to the differences in the thickness of the crust of the two plates before their collision and or largely achieved by thickening of the lower crust perhaps indicating a crustal flow mechanism operating more strongly in the western region

crustal thickening and variations in architecture from the qaidam basin to the qang tang north central tibetan plateau from wide angle reflection seismology abstract wide angle reflection refraction profiles at crustal scale have been obtained along a 700 km transect of northeastern tibet from six shotpoints a combination of sampling by broadside fan recordings and in line profiling allows us to image increases in moho depth towards the centre of the plateau these occur with different styles across the two block boundaries between north kun lun bayan har and qang tang as shown also by the changes resolved in the internal architecture and layer velocities in the crust clear s waves are observed together with p waves they bring rare constraints on the composition of crustal units which reveals an abnormally felsic average composition for the whole crust and its lower half along the whole length of the transect across the north kun lun qaidam block variations of crustal thickness are detected with moho depth between 55 and 65 km and an inverse correlation with basement topography in the north of the bayan har block a strong heterogeneity in the middle of the 70 km thick crust is shown as suggested from synthetic seismograms modelling it may result from thin layer inclusion marking the northward tectonic superposition of the bayan har crust on the north kun lun crust as imaged by a fan profile the corresponding upper part could have been transported northwards to thicken also the upper crust of the present north kun lun block in the north of qang tang a model is obtained with a 75 km thick crust and a lower half of the crust with average velocities characteristic of felsic composition from bayan har to qang tang variations of deep architecture are documented across the jinsha suture and xianshuihe fault from recording in line and broadside to several shotpoints together with the felsic composition of the lower part of the crust a moho offset suggests the imbrication of the bayan har crust in the qang tang lithosphere between a south dipping moho above and a north dipping moho below a model of indentation of the qang tang by an originally thinner bayan har crust and lithosphere with part of the qang tang brought to greater depth would also be consistent also with the moho topography across the transition from the qang tang to bayan har as imaged by fan profile

geochemical and sr nd pb isotopic data bearing on the origin of pan africangranitoids in the kid area southeast sinai egypt the kid plutonic suite in southeast sinai egypt is composed of 1 a gabbro diorite complex 2 quartz monzodiorite 3 granodiorite monzogranite association 4 syenogranite major and traceelement analyses and radiogenic isotope data rb sr sm nd and pb pb are presented for these rocks rb sr isochrons indicate ages of 581 11 ma 576 11 ma and 570 4 ma for the quartz monzodiorite granodiorite monzogranite association and syenogranite respectively the gabbro diorite complex is a layered intrusion varying in sio2 from 42 to 59 wt it shows a calc alkaline trend and the trace element characteristics of arc lavas subsequently this magma fractionated to give quartz monzodiorite andsyenogranite magmas the quartz monzodiorite has low initial 87 sr 86 sr ratios 0 703 and high nd values 4 4 0 3 with trace element characteristics of volcanic arc granites the granodiorite monzograniteassociation has higher initial sr ratio 0 7044 lower nd 1 5 2 7 and relatively high 207 pb 204 pb and 208 pb 204 pb significantly different from those of the older rocks isotopic data and ree modelling suggestthat it formed by partial melting of an early pan african amphibolitic lower crust the syenogranite isa typical highly fractionated post orogenic granite with a deep negative eu anomaly hfse and reeenrichment a 87 sr 86 sr initial ratio of 0 7033 nd of 4 5 0 5 and distinct variations in the 206 pb 204 pbratio the isotopic compositions of sr nd and pb indicate that the syenogranite and the quartzmonzodiorite are comagmatic increasing fluorine concentration from the quartz monzodiorite to thesyenogranite suggests that fluid fractionation involving fluorine complexing played an important role inthe genesis of the syenogranite this process led to the enrichment of hfse and hree to extensivefractional crystallization to changes in the u pb ratios and a distinct variation in 206 pb 204 pb

shallow seismic structure of kunlun fault zone in northern tibetan plateau china implications for the 2001 ms8 1 kunlun earthquake summary the shallow seismic velocity structure of the kunlun fault zone klfz was jointly deduced from seismic refraction profiling and the records of trapped waves that were excited by five explosions the data were collected after the 2001 kunlun ms8 1 earthquake in the northern tibetan plateau seismic phases for the in line record sections 26 records up to a distance of 15 km along the fault zone were analysed and 1 d p and s wave velocity models of shallow crust within the fault zone were determined by using the seismic refraction method sixteen seismic stations were deployed along the off line profile perpendicular to the fault zone fault zone trapped waves appear clearly on the record sections which were simulated with a 3 d finite difference algorithm quantitative analysis of the correlation coefficients of the synthetic and observed trapped waveforms indicates that the kunlun fault zone width is 300 m and s wave quality factor q within the fault zone is 15 significantly s wave velocities within the fault zone are reduced by 30 45 per cent from surrounding rocks to a depth of at least 1 2 km while p wave velocities are reduced by 7 20 per cent a fault zone with such pand s low velocities is an indication of high fluid pressure because vs is affected more than vp the low velocity and low q zone in the klfz model is the effect of multiple ruptures along the fault trace of the 2001 ms8 1 kunlun earthquake

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

crustal shear s velocity and poisson s ratio structure along the indepth iv profile in northeast tibet as derived from wide angle seismic data summary from the s wave data collected along a 270 km long profile spanning the kunlun mountains in ne tibet 14595 sg phase arrivals and 21 sms phase arrivals were utilized to derive a whole crustal s velocity model and together with a previously derived p velocity model a poisson s ratio model beneath the profile the final tomogram for the upper 10 15 km of the crust reveals the lower velocities associated with the predominantly neogene quaternary sediments of the qaidam basin to the north and the higher velocities associated with the predominantly palaeozoic and mesozoic upper crustal sequences of the songpan ganzi terrane and kunlun mountains to the south this study finds no evidence that the kunlun mountains are involved in large scale northward overriding of the qaidam basin along a shallow south dipping thrust the in the upper 10 15 km of the crust are often lower than 0 25 indicating a preponderance of quartz rich rocks in the upper crust beneath the profile below 10 15 km depth the remainder of the crust down to the moho has an average of 0 24 beneath the songpan ganzi terrane and kunlun mountains and 0 25 below the qaidam basin these low are similar to other low found along other profiles in the northeastern part of the plateau assuming an isotropic situation and no significant variation in between 10 15 km depth and the moho then the lower crust between 25 30 km depth below sea level and the moho with p velocities varying from 6 6 km s 1 at the top to around 6 9 km s 1 at the base and of 0 24 0 25 should comprise intermediate granulites in the upper part transitioning to granulite facies metapelites in the lower part as the pre cenozoic qaidam basin crust has probably not lost any of its lower crust during the present himalayan orogenic cycle in the cenozoic and only has a of 0 245 0 25 then it appears that the pre cenozoic qaidam basin crust involved in the collision is more felsic and thus weaker and more easily deformable than normal continental crust with a global average of 0 265 0 27 and the tarim and sichuan basin crusts this situation then probably facilitates the collision and promotes the formation of new high plateau crust at the ne margin of tibet south of the qaidam basin the crust of the songpan ganzi terrane and kunlun mountains has an even lower average crustal of 0 23 0 24 and is thus presumably even weaker and more easily deformable than the crust beneath the qaidam basin this then supports the hypothesis of karplus et al that the high tibetan plateau may be thickening northward into south qaidam as its weak thickened lower crust is injected beneath stronger qaidam crust

normal faulting from simple shear rifting in south tibet using evidence from passive seismic profiling across the yadong gulu rift abstract the tibetan plateau is undergoing north south shortening accompanied by west east extension as evidenced by the widespread development of north south trending normal faults grabens and rifts while the mode of the north south shortening has been the main focus of most international studies knowledge of the deep structure beneath south tibet is required for understanding the mechanism of the west east extension the onset of the north south trending normal faulting is commonly taken as an indicator that the tibetan plateau was uplifted to a near maximum elevation before entering a collapsing stage here we report on the receiver functions of a seismological experiment across the northern segment of the yadong gulu rift ygr one of the youngest rifts in south tibet the migrated receiver function images reveal that the ygr is a high angle normal fault characterized by a 5 km moho rise from its western to eastern flank together with distinct differences in the crustal structure and intracrustal seismic conversion patterns between the two flanks this highly asymmetric lithospheric structure suggests whole crustal extension controlled by a simple general shear rifting mechanism this simple general shear rifting in the ygr is attributed to an eastward horizontal shear at the base of the upper crust as evidenced by the observed tibetan gps velocity field and our observation of shear wave splitting discrepancy among the upper crust lower crust and lithospheric mantle we propose that in the ygr simple shear rifting accommodates the northward injection of the indian lithosphere which may suggest that the onset of the north south normal faulting does not indicate gravitational collapse of the tibetan lithosphere

crustal radial anisotropy across eastern tibet and the western yangtze craton 1 phase velocities across eastern tibet and surrounding regions are mapped using rayleigh 8 65 s and love 8 44 s wave ambient noise tomography based on data from more than 400 program for array seismic studies of the continental lithosphere and chinese earthquake array stations a bayesian monte carlo inversion method is applied to generate 3 d distributions of vsh and vsv in the crust and uppermost mantle from which radial anisotropy and isotropic vs are estimated each distribution is summarized with a mean and standard deviation but is also used to identify highly probable structural attributes which include 1 positive midcrustal radial anisotropy vsh vsv across eastern tibet spatial average 4 8 1 4 that terminates abruptly near the border of the high plateau 2 weaker 1 0 1 4 negative radial anisotropy vsh vsv in the shallow crust mostly in the songpan ganzi terrane 3 negative midcrustal anisotropy 2 8 0 9 in the longmenshan region 4 positive midcrustal radial anisotropy 5 4 1 4 beneath the sichuan basin and 5 low vs in the middle crust 3 427 0 050 km s of eastern tibet midcrustal vs 3 4 km s perhaps consistent with partial melt is highly probable only for three distinct regions the northern songpan ganzi the northern chuandian and part of the qiangtang terranes midcrustal anisotropy provides evidence for sheet silicates micas aligned by deformation with a shallowly dipping foliation plane beneath tibet and the sichuan basin and a steeply dipping or subvertical foliation plane in the longmenshan region near vertical cracks or faults are believed to cause the negative anisotropy in the shallow crust underlying tibet

intracrustal detachment within zones of continental deformation geologic mapping of active to recently active geologic structures in panamint valley western united states and in the haiyuan region and northern tibetan plateau china suggests detachment in the middle and lower crust on a scale of tens of kilometres to at least several hundred kilometres detachment occurs similarly in predominantly extensional panamint valley and in predominantly compressional china environments it involves structures with displacements of more than 10 km and displacement rates of more than 3 mm yr perhaps more than 10 mm yr the steeply dipping strike slip faults present in all three areas hunter mountain fault haiyuan fault and altyn tagh fault zone terminate in zones of extension or compression and geometric relations indicate that all structures including strike slip faults are thin skinned and restricted to the upper crust in panamint valley and in the haiyuan region deformation within these systems can be reconstructed in three dimensions displacement on the strike slip faults is absorbed by extension or compression occurring at the termination of the faults so strike slip displacement is roughly equal to and in the same direction as shortening or extension we propose that left slip on the altyn tagh fault zone in northern tibet is similarly absorbed by shortening southeast of the fault zone within the qaidam basin and the nan shan

explosion seismic p and s velocity and attenuation constraints on the lower crust of the north central tibetan plateau and comparison with the tethyan himalayas implications on composition mineralogy temperature and tectonic evolution p and s velocity and attenuation estimates in the lower crust are obtained from a set of wide angle reflection refraction profiles in the region of active tectonics at the ne edge of the tibetan plateau and discussed together with respect to similar data at its himalaya south tibet edge the quality factor is estimated in the lower half of the crust by accounting for the differential effect on amplitude frequency observed between waves of different penetrations and both in p and s modes attenuation values allow to exclude a significant proportion of partial melt and to estimate the homologous temperature ratio of in situ to solidus absolute temperatures the latter depend on the physical conditions being of dry wet or dehydration melting which are found different among the regions of the northern bayan har and northern qang tang boundaries between blocks as well as the tethyan himalayas south of the indus tsangpo suture their in situ temperatures differ also as estimated from their different vp for a similar felsic composition joint measurement of several parameters vp vs qp and qs reveals the composition mineralogy temperature and hydration conditions of the lower half of the thickened crust of tibet that may be discussed in terms of evolution the material presently in the thickened crust even its lower part has a felsic composition upper to middle crustal lithology and the temperature conditions estimated suggest that basic material that could have underlain it could be eclogitized and not appear anymore above the seismic moho under northern qang tang the felsic material in the lower half of the crust appears as hot and dry its burial may have occurred earlier or may have been moderate in the postcollisional phase this is consistent with a model of indentation of the qang tang crust by an originally thinner bayan har crust to bring part of its crust to greater depth suggested from imaging the crustal architecture under northern bayan har the material in the lower half of the crust appears as felsic at low temperature and not dry conditions this is evidence that it has been transported from a shallower depth and this recently enough not to be yet dehydrated and temperature equilibrated in a conductive geotherm it supports a model of recent overriding of the middle crust of the north kun lun block to the north independently suggested from the image of crustal architecture the tethyan himalayas case appears bracketed by these two cases in northern tibet for vp and temperature conditions but shows highest attenuation in the lower crust that is colder but less dry than under northern qang tang