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

joint inversion of ambient noise and earthquake data in the trans north china orogen on going lithospheric modification and its impact on the cenozoic continental rifting abstract situated in a transition zone between the ordos basin and the huabei basin the trans north china orogen tnco or the central north china craton evolved as an extensional orogen during the late mesozoic to cenozoic with fenhe weihe rift fwr developing in it in the cenozoic in this study we construct a high resolution lithospheric model of tnco by jointly interpreting the broadband surface wave dispersions derived from ambient noise and teleseismic surface wave data the most significant features of the lithospheric mantle are the two separated low velocity anomalies beneath the northern and southern tnco respectively considering that the pacific slab subducted to the mantle transition zone with a gap under the datong volcanic zone and large scale low velocity mantle materials are ascending from the lower mantle through the slab gap which is evidenced by teleseismic p wave tomography we propose here that the northern tnco was affected by the mantle upwelling from the lower mantle on the contrary the south tnco was mainly modified from the dehydration reaction process of the stagnant pacific slab in the lower crust we observe significant low velocities beneath the datong volcanic field which are interpreted as the recent magmatic underplating resulted from the ongoing mantle upwelling in the northern tnco in addition the mantle upwelling has dominated the rifting process in the northern segment of the fwr although the passive rifting of the fwr propagated from south to north the north fwr is under an active lithosphere thinning regime now

tectonic evolution of a composite collision orogen an overview on the qinling tongbai hong an dabie sulu orogenic belt in central china abstract the formation of collisional orogens is a prominent feature in convergent plate margins it is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision the paleozoic convergence between the south china block scb and the north china block ncb is associated with a series of tectonic processes such as oceanic subduction terrane accretion and continental collision resulting in the qinling tongbai hong an dabie sulu orogenic belt while the arc continent collision orogeny is significant during the paleozoic in the qinling tongbai hong an orogens of central china the continent continent collision orogeny is prominent during the early mesozoic in the dabie sulu orogens of east central china this article presents an overview of regional geology geochronology and geochemistry for the composite orogenic belt the qinling tongbai hong an orogens exhibit the early paleozoic hp uhp metamorphism the carboniferous hp metamorphism and the paleozoic arc type magmatism but the three tectonothermal events are absent in the dabie sulu orogens the triassic uhp metamorphism is prominent in the dabie sulu orogens but it is absent in the qinling tongbai orogens the hong an orogen records both the hp and uhp metamorphism of triassic age and collided continental margins contain both the juvenile and ancient crustal rocks so do in the qinling and tongbai orogens in contrast only ancient crustal rocks were involved in the uhp metamorphism in the dabie sulu orogenic belt without involvement of the juvenile arc crust on the other hand the deformed and low grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late permian to early triassic along the northern margin of the dabie sulu orogenic belt and it was developed on the passive oceanic margin during subduction in the early paleozoic along the northern margin of the qinling orogen three episodes of arc continent collision are suggested to occur during the paleozoic continental convergence between the scb and ncb the first episode of arc continent collision is caused by northward subduction of the north qinling unit beneath the erlangping unit resulting in uhp metamorphism at ca 480 490 ma and the accretion of the north qinling unit to the ncb the second episode of arc continent collision is caused by northward subduction of the prototethyan oceanic crust beneath an andes type continental arc leading to granulite facies metamorphism at ca 420 430 ma and the accretion of the shangdan arc terrane to the ncb and reworking of the north qinling erlangping and kuanping units the third episode of arc continent collision is caused by northward subduction of the paleotethyan oceanic crust resulting in the hp eclogite facies metamorphism at ca 310 ma in the hong an orogen and low p metamorphism in the qinling tongbai orogens as well as crustal accretion to the ncb the closure of backarc basins is also associated with the arc continent collision processes with the possible cause for granulite facies metamorphism the massive continental subduction of the scb beneath the ncb took place in the triassic with the final continent continent collision and uhp metamorphism at ca 225 240 ma therefore the qinling tongbai hong an dabie sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc type magmatism to arc continent and continent continent collision

multiscale spatial distribution of crustal seismic anisotropy beneath the northeastern margin of the tibetan plateau and tectonic implications of the haiyuan fault abstract the haiyuan fault is a large strike slip fault in the northeastern ne margin of the tibetan plateau using new data recorded by a temporary seismic array this study obtains crustal seismic anisotropy measurements in different scales across the haiyuan fault by analyzing splitting of shear waves from local earthquakes parameters of shear wave splitting sws indicate clear zoning characteristics in seismic anisotropy in the upper crust the study area is divided into two different anisotropic subzones by the haiyuan fault the dominant orientation of fast polarizations is nne or ne north of the faults and wnw to ew south of the fault this pattern of spatial variation indicates the controlling impact of haiyuan fault to observed seismic anisotropy with a zone of dozens of kilometers from the haiyuan fault the dominant orientations of fast polarizations are wnw generally coincidental with the strike of the haiyuan fault except for several stations it indicates that the influence of the fault on stress could reach to about 10 km from the fault distance spatial pattern of anisotropy indicates multiple effects by stress faults and local tectonics it suggests different anisotropic mechanisms including stress induced anisotropy and tectonic or structural anisotropy the dominant orientation of fast polarizations can indicate in situ maximum horizontal principal compressive stress although there is always influence of tectonics the normalized time delays are higher around the haiyuan fault than those away from the fault suggesting stronger crustal seismic anisotropy in a narrow belt around the strike slip fault zone the sws parameters suggest that the haiyuan fault is the actual crustal boundary of ne tibetan plateau about 200 km north of the reported boundary of the tibetan plateau block combining with other studies in the crust and the lithosphere this paper infers possible two layer crustal anisotropy beneath the yinchuan graben north of the haiyuan fault the dominant orientation of fast polarizations are nne or ne in the upper crust probably originated from crack induced anisotropy but become wnw or nw in the middle lower crust possibly originated from deformation anisotropy

three dimensional conductivity model of crust and uppermost mantle at the northern trans north china orogen evidence for a mantle source of datong volcanoes abstract while the eastern block of north china craton ncc had experienced significant lithospheric destruction in the mesozoic the western block of ncc and the trans north china orogen tnco have undergone localized lithospheric modification since the cenozoic the northern tnco is highlighted by the cenozoic magmatic activities including hannuoba basalts and datong volcanoes and is a seismically active region in this study 3 d electrical conductivity model of the crust and uppermost mantle is derived by the 3 d inversion technique using data from 72 broadband magnetotelluric mt stations the final model shows that a 15 km thick resistive layer of about 3000 m dominates the upper crust which may represent the intact archean and paleoproterozoic terrains whereas in the mid crust there are marked high conductivity anomalies of about 10 m beneath shanxi rifting basin which may result from the interconnected saline fluid of 0 2 to 6 volume fraction the most important finding is that one significant conductor extended into the mantle is located between hannuoba field and datong volcanoes and it connects with the mid crust conductor beneath the datong volcanoes we suggest that this could be the mantle source partial melting region for the quaternary volcanic activities of datong volcanoes and the melt fraction is estimated as 6 6 its location inside the western block suggests that the volcanic activities at datong volcanoes are irrelevant to the tectonic process to the east of tnco it is likely to be related to the mantle flows from the tibetan plateau around the ordos block which converges at the northeastern corner of the ordos block and local upward flow along the slope of the thinning lithosphere resulted in decompression partial melting and the melt percolated upward through the crust to feed the lava eruptions at the datong volcanoes to the east finally large crustal earthquakes in this region are generally located in resistive zones with high conductivity anomalies beneath the fluids brought up by hot mantle material may play an important role in fault strength weakening and rupture nucleation at mid crust depths

further observations of a decreasing atmospheric co2 uptake capacity in the canada basin arctic ocean due to sea ice loss 1 using data collected in 2009 we evaluated the potential for the southeastern canada basin arctic ocean to act as an atmospheric co2 sink under the summertime ice free conditions expected in the near future beneath a heavily decayed ice cover we found surprisingly high pco2sw 290 320 atm considering that surface water temperatures were low and the influence of ice melt was strong a simple model simulating melt of the remaining ice and exposure of the surface water for 100 days revealed a weak capacity for atmospheric co2 uptake mean flux 2 4 mmol m 2 d 1 due largely to warming of the shallow mixed layer our results confirm a previous finding that the canada basin is not a significant sink of atmospheric co2 under summertime ice free conditions and that increased ventilation of the surface mixed layer due to sea ice loss is weakening the sink even further

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

teleseismic p wave tomography and mantle dynamics beneath eastern tibet we determined a new 3 d p wave velocity model of the upper mantle beneath eastern tibet using 112 613 high quality arrival time data collected from teleseismic seismograms recorded by a new portable seismic array in yunnan and permanent networks in southwestern china our results provide new insights into the mantle structure and dynamics of eastern tibet high velocity high v anomalies are revealed down to 200 km depth under the sichuan basin and the ordos and alashan blocks low velocity low v anomalies are imaged in the upper mantle under the kunlun qilian and qinling fold zones and the songpan ganzi qiangtang lhasa and chuan dian diamond blocks suggesting that eastward moving low v materials are extruded to eastern china after the obstruction by the sichuan basin and the ordos and alashan blocks furthermore the extent and thickness of these low v anomalies are correlated with the surface topography suggesting that the uplift of eastern tibet could be partially related to these low v materials having a higher temperature and strong positive buoyancy in the mantle transition zone mtz broad high v anomalies are visible from the burma arc northward to the kunlun fault and eastward to the xiaojiang fault and they are connected upward with the wadati benioff seismic zone these results suggest that the subducted indian slab has traveled horizontally for a long distance after it descended into the mtz and return corner flow and deep slab dehydration have contributed to forming the low v anomalies in the big mantle wedge our results shed new light on the dynamics of the eastern tibetan plateau

new evidence from seismic imaging for subduction during assembly of the north china craton comment the frozen in information in the crust plays an important role in improving our understanding of cratonic formation and evolution and plate tectonics in the precambrian the trans north china orogen is a continent continent collision belt generated by the assembly of the north china craton the mechanism and modality of the collision are disputed here we present a seismic image of the western block and the trans north china orogen of the north china craton derived using receiver function analysis of the teleseismic records from a dense array a low velocity zone extending from the middle crust to the moho is interpreted as a remnant of upper middle crustal material associated with westward dipping subduction beneath the western block of the north china craton crustal uplift and magmatic underplating resulting from subsequent tectonic events were responsible for modifying the remaining subduction architecture the western boundary of the trans north china orogen is located west of the boundary earlier identified by surface investigation the results combined with previous seismic imaging in the eastern north china craton provide insight into the amalgamation of the eastern and western blocks and the subsequent tectonic deformation of the north china craton

seismic evidence of on going sublithosphere upper mantle convection for intra plate volcanism in northeast china abstract a 3 d crustal and upper mantle s wave velocity model of ne china is constructed by inversion of phase velocity dispersion curves at 6 140 s periods from ambient noise tomography and two plane surface wave tomography the seismic data used in this study are collected from 120 china earthquake administration cea permanent stations and 127 portable stations of necessarray we observe strong low s wave velocity beneath the changbaishan volcano in the upper mantle to at least 200 km depth which is interpreted as a mantle upwelling beneath the changbaishan volcano that is consistent with the body wave tomographic image the songliao basin is dominated by a high velocity extending to at least 200 km depth built upon the observed velocity anomalies we propose a sub lithosphere mantle convection model for ne china in which the upwelling of upper mantle materials from the mantle transition zone to the changbaishan volcano could induce a local sub lithosphere convection in the upper mantle and the strong high velocity of the upper mantle beneath the songliao basin corresponds to the downwelling limb of this convection cell the downwelling beneath the songliao basin could also induce secondary local convection in the asthenosphere to the west leading to local asthenospheric upwelling beneath the abaga and halaha volcanoes in the xing an mongolia orogenic belt

large contrasts in crustal structure and composition between the ordos plateau and the ne tibetan plateau from receiver function analysis article i nfo article history we analyzed thousands of receiver function data recorded by 154 national and regional seismic stations to study the crustal structure beneath the ordos plateau and the ne margin of the tibetan plateau moho depth and average crustal vp vs ratio were measured at each station the ordos plateau and the trans north china orogen east of the ordos are underlain by a moderately thick crust of 42 km the weihe graben lying at the southern edge of the ordos plateau has a thin crust of 30 km while its southern neighbor the qinling orogenic belt shows a thick crust extending to as much as 45 km deep the moho depth beneath the ne margin of the tibetan plateau varies from 55 to 65 km we found a remarkable contrast between the tibetan and ordos plateaus in the measured poisson s ratio the ordos plateau is featured by a high poisson s ratio while the tibetan margin has a very low poisson s ratio in general mafic lower crustal rocks have a higher poisson s ratio than felsic ones the measured low poisson s ratio beneath the ne margin of the tibetan plateau thus indicates that the crustal column beneath the margin is rather felsic which seems to be inconsistent with a scenario of an inflated crust due to extrusion of lower crust material from the tibetan plateau to the margin

crustal structure of hubei province of china from teleseismic receiver functions evidence for lower crust delamination abstract western hubei province is at the southern end of the 3000 km long north south oriented xing anling taihangshan wulingshan topographic step in china which separates high rising plateaus and mountain ranges in the west from low elevation plains in the east we calculated teleseismic p receiver functions of 32 permanent broadband seismic stations in hubei province and estimated crustal thicknesses under them using the h method we also obtained detailed crustal structural images along three profiles using the ccp stacking method the results show an east west crustal thickness increase in the study area from 30 35 km to 45 50 km in less than 20 km of horizontal distance most likely in a step wise fashion the thin crust beneath the nanxiang and jianghan basins in eastern hubei extends into the interior of the wuling uplift and the huangling massif in western hubei the lack of mirror symmetry between the moho and surface topography suggests that part of the mountain ranges in western hubei is either compensated by non airy type isostasy models or is not in isostatic equilibrium but supported by the strength of the lithosphere the brittle or localized ductile deformation in the lower crust uppermost mantle as indicated by the abrupt moho steps seems to be decoupled with brittle deformation in the upper crust the ccp images also reveal an apparent double moho beneath the wudang mts which is interpreted to be due to a partially eclogitized lower crust after the original cratonic mantle lithosphere was replaced by warm and hydrated mantle materials in eastern china in the late mesozoic the moho steps were formed when a segment of eclogitized lower crust became gravitationally unstable and foundered into the mantle

automated detection extraction and measurement of regional surface waves our goal is to develop and test an effective method to detect identify extract and quantify surface wave signals for weak events observed at regional stations we describe an automated surface wave detector and extractor designed to work on weak surface wave signals across eurasia at intermediate periods 8 s 40 s the method is based on phase matched filters defined by the rayleigh wave group travel time predictions from the broadband group velocity maps presented by ritzwoller and levshin 1998 and ritzwoller et al 1998 and proceeds in three steps signal compression signal extraction or cleaning and measurement first the dispersed surface wave signals are compressed in time by applying an anti dispersion or phase matched filter defined from the group velocity maps we refer to this as the compressed signal second the surface wave is then extracted by filtering noise temporally isolated from the time compressed signal this filtered signal is then redispersed by applying the inverse of the phase matched filter finally we adaptively estimate spectral amplitude as well as group and phase velocity on the filtered signal the method is naturally used as a detector by allowing origin time to slide along the time axis we describe preliminary results of the application of this method to a set of nuclear explosions and earthquakes that occurred on or near the chinese lop nor test site from 1992 through 1996 and one explosion on the indian rajasthan test site that occurred in may of 1998

extension in the graben systems around the ordos china and its contribution to the extrusion tectonics of south china with respect to gobi mongolia the graben basins around the ordos block are major tectonic features of late cenozoic extension in north china this paper gives a synthetic view of the basic characteristics of the graben basins and of the kinematics of the graben bounding faults this view is based on the available geological data spot imagery analysis field observations and focal mechanisms of earthquakes from this region these graben basins have been grouped into two major systems the yinchuan hetao graben system along the northern and northwestern margins of the ordos and the weihe shanxi graben system along the southern and eastern margins of the ordos crustal deformation along these graben systems involves normal slip on ne sw striking graben bounding faults normal right lateral and normal left lateral slip on nne ssw and wnw ese striking faults respectively this fault pattern is consistent with nw se trending extension the amount and rate of deformation resulting from this extension have been estimated from geological data using a cross sectional area balance and the extensional factor method deduced from the tilt of faulted blocks the results of these estimates show that the uppermost pliocene quaternary extension rate in a nw se direction is in the order of 1 6 mm yr 1 across the weihe graben 0 5 mm yr 1 across the central shanxi graben on the southeastern margin of the ordos block and 3 1 mm yr 1 across the linhe graben on the northwestern margin of the ordos block the extension across these graben systems is accompanied with counterclockwise rotation of continental blocks in north china ordos taihang shan north china plain a kinematic model is proposed in which the late cenozoic extension and block rotation in north china are interpreted as the effect of the eastward termination of the left lateral slip haiyuan fault and kunlun taibai fault system the ordos block rotates counterclockwise as a result of the push of tibet through the left lateral strike slip haiyuan fault and the liupan shan thrust and fold belt the extension along the weihe shanxi graben system may represent the effect of the left lateral slip on the kunlun taibai qinling fault system the rough estimates of the extensional deformation around the ordos block indicate that this contributes to about 5 2 mm yr 1 of left lateral movement in a n125 e direction between south china and gobi mongolia this motion adds to the left lateral slip on the qinling fault zone about 7 2 mm yr 1 in a n90 e direction between the qinling shan and the hua shan north china thus the eastward motion of south china with respect to gobi mongolia is partitioned between normal faulting around the ordos and strike slip faulting in the qinling shan it is estimated to be in the order of 12 4 mm yr 1 in a n105 e direction during the quaternary

high resolution 3 d crustal structure beneath ne china from joint inversion of ambient noise and receiver functions using necessarray data abstract we used ambient noise data recorded by the necessarray a portable seismic array deployed in northeast china to construct rayleigh wave phase velocity dispersion curves and obtain phase velocity maps for periods from 6 to 40 s s wave velocity structure beneath ne china is obtained by joint inversion of the phase velocities and receiver functions our results show good correlation between crustal velocity structure and geological features at shallow depths the songliao basin is separated into two small sub basins by a fast velocity anomaly corresponding to a southwest uplifted horst and the southern sub basin is associated with the kailu depression a low velocity anomaly extending from the lower crust to uppermost mantle beneath the changbaishan mountain range is consistent with p and s wave tomography models the mantle low velocity has been interpreted as a local mantle upwelling attributed to the subducting pacific slab induced return flow finally the s wave velocity in the mid and lower crust is lower beneath the xing an mongolia orogenic belt xmob than beneath the changbaishan mountain range this was not expected since the changbaishan mountain range has experienced more extensive volcanic activity in the cenozoic including the currently active changbaishan and jingpohu volcanoes than the xmob we infer that the mid and lower crust of the xmob is mainly felsic in composition and propose that the low shear wave velocity of the mid and lower crust beneath the xmob could be the consequence of removal of deep mafic crust through a continental accretion and subsequent delamination event in the paleozoic or via convection removal induced by the rollback of the flat subducted paleo pacific slab during the middle mesozoic

crustal structure of the eastern qinling orogenic belt and implication for reactivation since the cretaceous a high resolution crustal model of the eastern qinling belt and central north china craton central ncc is obtained along a n s trending profile corridor by joint inversion of surface wave and receiver function the ncc is one of the oldest cratons on earth and the qinling belt is the suture zone between the ncc and south china block scb the qinling belt is characterized by low crustal velocity 1 8 the forward gravity modeling supports the presence of a high density layer 3 05 g cm3 at the base of the crust beneath the central ncc we propose that the high velocity in the lowermost crust beneath the central ncc is most likely due to the repeated mafic underplating which also results in high crustal vp vs ratio and is responsible for the rapid crustal uplift during the late mesozoic

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

seismic anisotropy beneath the southern ordos block and the qinling dabie orogen china eastward tibetan asthenospheric flow around the southern ordos abstract sks wave splitting analysis is performed to estimate the seismic anisotropy in the upper mantle using teleseismic data recorded by a temporary seismic array of 180 stations called sosarray deployed in the southern ordos block and the qinling dabie orogen the most important finding is that large delay times with nw se fast polarization directions in the northeastern tibet are continuous across the boundary into the southwestern part of the ordos block where the sks wave splitting results are significantly different from those in the rest of the ordos block based on our sks wave splitting results in addition to the results from previous studies we propose an asthenospheric flow model for the eastward extrusion of the tibetan upper mantle the model consists of two corner flows around the southwestern corner and the southeastern corner of the ordos block and the eastward flow along the weihe graben and the qinling dabie orogen for the escaping tibetan upper mantle finally the clockwise turning flow of the asthenosphere around the southwestern corner of ordos block has currently extended laterally into the interior of the ordos block suggesting that the thick cold lithospheric root of the southwestern ordos block there is currently being replaced with hot tibetan asthenosphere at depths that is we observed an on going process of thermal erosion of a cratonic lithosphere by lateral hot asthenospheric flow

the regional tectonic fabric of the east pacific rise from 12 50 n to 15 10 n seamarc ii backscatter data for the east pacific rise are used to create structural maps of ridge parallel fabric on the crest and flanks of the rise from 12 50 n to 15 10 n the structural data are statistically analyzed to describe the spacing density azimuth facing direction and length of faults results of the statistical studies are compared with predictions for the width of the zone of active fault formation models for the generation of abyssal hills plate kinematic predictions models of along strike segmentation and observations of the asymmetric subsidence of oceanic crust on the cocos and pacific plates comparisons of the number of faults with distance from the rise crest and examination of the stratigraphic relationship between seamounts and the ridge parallel tectonic fabric illustrate that the zone where new faults are created is located within a few kilometers of the ridge crest fault density data reveal that fault distributions do not resemble periodic processes examination of fault density and spacing data along strike indicates that there is a high probability that the mean number of faults per crustal block differs significantly from north to south changes in along strike statistics correlate well with the occurrence of three overlapping spreading centers located within the survey area additionally fault spacing s determined from seamarc i and seamarc ii backscatter imagery demonstrate that quantitative analyses of seafloor fabric are dependent upon the instrument used to collect the data analysis of inward and outward facing faults indicates that especially in the northern portion of the survey area half graben models are better predictors of abyssal hill morphology than full graben models within the survey area inward facing faults are more abundant and affect more of the oceanic crust than outward facing faults although the cocos and pacific plates subside at different rates this asymmetry is not reflected in the tectonic component of morphology fault azimuth is observed to vary as a function of crustal age although the overall trend of the change in azimuthal values agrees with the trend predicted by relative poles of opening for the pacific and cocos plates the variability in azimuthal data suggests that other processes contribute to the orientation of lineations formed near the axis of the east pacific rise finally the statistical analyses demonstrate that all fault parameters are better correlated about the ridge axis than along strike of the axis the differences between the northern and southern portions of the survey area are reflected in the seamarc ii bathymetric data which depict a continuous narrow ridge crest in the southern region and an irregularly shaped ridge crest in the northern region that shoals and deepens every 30 to 40 km this along strike variability observed over distances of less than 100 km suggests that large scale plate stresses are not the only processes responsible for generating the tectonic fabric observed on the flanks of the east pacific rise

cenozoic exhumation and deformation of northeastern tibet and the qinling is tibetan lower crustal flow diverging around the sichuan basin apatite fission track thermochronology data elucidate the cooling exhumation history of the qinling qin mountains which contain a paleozoic mesozoic orogenic collage north of the sichuan basin and northeast of the tibetan plateau in particular we examine the extent to which the qinling were affected by the rising plateau the northern and eastern qinling show continuous cooling and slow exhumation since the cretaceous in contrast in the southwestern qinling rapid cooling initiated at 9 4 ma a few million years later than in the eastern tibetan plateau a compilation of major cenozoic faults in the eastern tibetan plateau and the qinling and their kinematic and dynamic characterization shows that deformation in the qinling has predominantly been strike slip active sinistral and dextral strike slip faults delineate an area of eastward rock flow and bound the area of rapid late cenozoic cooling outlined by apatite fission track thermochronology these data can be interpreted to indicate that lower crustal flow has been diverted around the longmen shan and beneath the southwestern qinling causing active plateau uplift in this area alternatively northeastern tibet may be growing eastward faster in the western qinling than the entire south china block is extruding to the east

asthenospheric upwelling beneath northeastern margin of ordos block constraints from rayleigh surface wave tomography abstract phase velocities at periods of 20 143 s are inverted from rayleigh surface waves recorded at 201 stations of permanent and temporary seismic network deployed in the northern ordos block we obtained the shear wave velocities in the upper mantle from phase velocities our velocity models clearly reveal a striking low velocity anomaly zone in the upper mantle beneath the northeastern margin of the ordos block the low velocity zone at periods of 50 100 s extends from the north end of the luliangshan mountain to the north border of the western north china craton the minimum shear wave velocity is approximately 4 2 km s and the minimum lithospheric thickness is approximately 80 km in the low velocity anomaly zone our shear wave velocity results indicate that lithospheric thinning asthenospheric upwelling and partial melting may occur in the region during the cenozoic high temperature and thinned lithosphere may be significant factors that cause partial melting in the asthenospheric mantle during the cenozoic asthenospheric upwelling may be associated with the lithospheric stretching caused by the extension stress in the northeastern margin of the ordos block the extension stress may be contemporaneous with the regional extension of various stages which is related to the pacific subduction and the far field effect of the indian eurasian collision in the cenozoic

reactivation of an archean craton constraints from p and s wave tomography in north china 1 the unusual reactivation of the north china craton ncc challenges the classical views concerning the strength and stability of cratonic lithosphere by using teleseismic body waves recorded at 250 seismic stations this paper presents high resolution north china models of p and s wave velocity based on finite frequency kernel tomography both p and s wave velocity models reveal that 1 an obvious n s trending narrow low velocity region is located at the base of the lithosphere beneath the central block cb of the ncc which extends to more than 500 km depth 2 a region of high velocity extends to more than 250 300 km depth beneath the western block in contrast to the much shallower high velocity zones beneath the cb and shallower high velocities beneath the eastern block these features suggest that warm mantle material with a source at least as deep as the transition zone possibly a mantle plume may be responsible for the reactivation of the ncc the central block may have behaved as a sublithospheric corridor for the warm mantle material due to its pre existing weakness

upper mantle seismic velocities and anisotropy in china determined through pn and sn tomography 1 we have obtained velocity images of the uppermost mantle beneath china by performing tomographic inversion of both pn and sn traveltimes from the annual bulletin of chinese earthquakes 99 139 pn arrivals and 43 646 sn arrivals were selected pn anisotropy was also obtained simultaneously with pn velocity average pn and sn velocities are 8 05 and 4 55 km s respectively and maximum velocity perturbations are about 3 4 the pn and sn velocities are low in eastern china and high in western china particularly high velocities are associated with old basins for example tarim junggar turpan hami qaidam and sichuan and stable craton for example ordos low sn velocities are found mainly throughout north china in addition velocities are relatively low beneath the central tibetan plateau and the north south seismic zone along 103 e in tarim and central china where we observe strong anisotropy the fast pn velocity directions are consistent with the directions of maximum principal compressive stress as well as directions of crustal movement determined from global positioning system beneath the india eurasia collision zone the pn anisotropy direction is parallel to the collision arc and nearly perpendicular to both the direction of maximum compression and crustal movement resulting from pure shear deformation both the velocity variations and anisotropy indicate that the tibetan plateau was extruded and the mantle material beneath the plateau has flowed around the east himalaya syntax while the remaining material has diverted northwestward beneath the tarim basin

mountain building at northeastern boundary of tibetan plateau and craton reworking at ordos block from joint inversion of ambient noise tomography and receiver functions abstract we have obtained a high resolution 3 d crustal and uppermost mantle velocity model of the ordos block and its surrounding areas by joint inversion of ambient noise tomography and receiver functions using seismic recordings from 320 stations the resulting model shows wide spread low velocity zone vs 3 4 km s in the mid to lower crust beneath northeastern tibet plateau which may favor crustal ductile flow within the plateau however our model argues against the eastward crustal ductile flow beneath the qinling belt from the tibetan plateau we find high velocities in the middle part of qinling belt which separate the low velocities in the mid to lower crust of the eastern qinling belt from the low velocity zone in eastern tibetan plateau more importantly we observe significant low velocities and thickened lower crust at the liupanshan thrust belt as the evidence for strong crustal shortening at this boundary between the northeastern tibetan plateau and ordos block the most important finding of our model is the upper mantle low velocity anomalies surrounding the ordos block particularly the one beneath the trans north china craton tnco that is penetrating into the southern margin of the ordos block for 100 km horizontally in the depth range of 70 km and at least 100 km we propose an on going lithospheric mantle reworking at the southernmost boundary of the ordos block due to complicated mantle flow surrounding the ordos block that is the eastward asthenospheric flow from the tibet plateau proposed by recent sks study and mantle upwelling beneath the tnco from mantle transition zone induced by the stagnant slabs of the subducted pacific plate

amount and style of late cenozoic deformation in the liupan shan area ningxia autonomous region china the structures of the liupan shan area are characterized by numerous active thrust and strike slip faults that suggest thin skinned deformation the structural history of this area can be divided into three phases that probably overlap one another in time and are parts of a single protracted deformation the oldest cenozoic deformational phase occurred probably between late pliocene and early quaternary time and produced some of the folds and thrust faults in the liupan shan and yueliang shan during this phase deformation was the result of approximately n50 e shortening and the amount of shortening seems to have been about 1 2 km the second phase of deformation was dominated by left lateral strike slip faulting left slip on the n60 w striking haiyuan fault zone and shortening on north south trending structures shortening was associated with a transfer of the left slip displacement on the haiyuan fault zone to shortening in areas farther east shortening occurred by thrust faulting in the liupan shan and xiaoquan shan and by folding in the madong shan during this phase the orientation of shortening changed to n60 w the average amount of shortening on the north south trending folds in the madong shan is about 6 3 7 8 km most of the shortening on the liupan shan and xiaoguan shan thrust faults also occurred during this phase and amounted to a minimum of 4 8 6 3 km and 6 6 7 6 km respectively also with an orientation of n60 w during the third phase of deformation about 1 1 5 km of late pleistocene to recent left slip occurred on the xiaokou fault which was transferred into oblique left slip thrusting in the liupan shan at this time deformation in the madong shan and xiaoguan shan ceased or was reduced to a very slow rate the present active left slip on the haiyuan fault zone is accommodated by shortening in the liupan shan area the total displacement along the haiyuan fault is essentially the same as the total amount of shortening in the liupan shan area the sequence and interaction of strike slip and thrust faults in the liupan shan area seem to apply to the folds and thrust faults farther north in the southern ningxia region thus the northeastern margin of the tibetan plateau appears to grow by shortening oriented northeast and by left slip faulting that transfers material from farther to the west the total left slip in the entire southern ningxia region probably is less than 20 25 km and may be absorbed by shortening within this region thus the eastward translation of crustal fragments in the northern part of the tibetan plateau may not extend east of southern ningxia and if large scale eastward displacement has occurred it must lie farther south

crustal structures across the western weihe graben north china implications for extrusion tectonics at the northeast margin of tibetan plateau the stable ordos plateau extensional weihe graben and qinling orogenic belt are located at the northeast margin of the tibetan plateau they have been thought to play different roles in the eastward expanding of the tibetan plateau peking university deployed a linear seismic array across the western end of the weihe graben to investigate the crustal structures of the tectonic provinces of this structure receiver function analyses revealed low to moderate poisson s ratios and anticorrelations between poisson s ratios and topography beneath the qinling orogen these features may indicate a tectonic thickening of the felsic upper crust by folding and thrusting within the qinling orogen we observed a strong horizontal negative signal at the midcrust beneath the ordos plateau which may indicate a low velocity zone this observation would suggest the stable cratonic ordos plateau had been modified due to the compression between the tibetan plateau and the ordos plateau we also observed an abrupt 4 km moho offset across the weihe fault changing from 44 km beneath the ordos plateau to 40 km beneath the qinling orogen we conclude that the weihe fault is a lithosphere scale fault shear zone which extends into the upper mantle beneath the weihe graben it acts as the major boundary separating the stable ordos plateau and the active qinling orogen

crust and upper mantle structure of the north china craton and the ne tibetan plateau and its tectonic implications abstract understanding the mesozoic cenozoic tectonic evolution of the north china craton ncc and the ne tibetan plateau tp requires detailed knowledge of the lithospheric structure using dense regional networks and temporary deployments as well as updated reference models we obtain the crust and upper mantle structure to 120 km depth our tomographic results show several major features which have particular implications for the weihe shanxi rift system wsrs deformation of the ne tp and lithospheric evolution of the ncc beneath the wsrs the crust gradually thickens from south to north the lithospheric mantle gradually becomes slower and the mid lower crustal velocities are lower in the weihe rift where rifting of the wsrs initiated we suggest that along strike variations of the lithospheric structures of the wsrs have played an important role in its multistage evolution a low velocity zone lvz in the mid crust beneath the qilian orogen is characterized by relatively higher velocities compared to lvzs in other parts of the tp thus coherent lithospheric deformation may occur due to the high viscosity of the lvz during early plateau growth causing strong anisotropy to develop the western ncc including the ordos block and part of the alashan block shows a high velocity cratonic root extending to the base of our model in contrast the lithosphere of the eastern ncc appears to have been completely modified during the mesozoic through cenozoic and presents a thin lithosphere of relatively low velocities underlain by hot asthenosphere we observed significant upper mantle heterogeneities in the ncc which may reflect its diachronous lithospheric modification

presence of an intralithospheric discontinuity in the central and western north china craton implications for destruction of the craton detailed knowledge of lithospheric structure is essential for understanding the long term evolution and dynamics of continents we present an image of lithospheric structure across the central and western north china craton ncc derived using s and p receiver functions from a dense seismic array a negative velocity discontinuity is identified at 80 100 km depth within the thick lithosphere 160 200 km similar to that observed in many other cratonic regions and roughly at the same depth as the base of the lithosphere in the eastern ncc the intralithospheric discontinuity may indicate an ancient mechanically weak layer within the overall strong cratonic lithosphere and probably also existed beneath the eastern ncc before the mesozoic the presence of such a weak layer could have facilitated simultaneous lithospheric modification at the base and the middle of the lithosphere in the eastern ncc especially under the strong influence of the mesozoic pacific subduction leading to the severe lithospheric thinning and destruction recorded in this region the weak layer probably did not strongly affect the stability and evolution of the central and western ncc and other cratonic regions where effects from plate boundary processes were weak our seismic images integrated with geological data provide new insights into structural heterogeneities in the subcontinental lithospheric mantle and their roles in the dynamic evolution of continents

the crust structures and the connection of the songpan block and west qinling orogen revealed by the hezuo tangke deep seismic reflection profiling abstract the songpan block in the northeastern qinghai tibet plateau qtp located at the junction of the e w and n s trending tectonic belts has affected the formation and evolution of the chinese continent since mesozoic and is a tectonic node in the central part of the chinese continent in this study the hezuo tangke deep seismic reflection profiling of the 257 km long was conducted in 2004 which is the first profile across the songpan block and the west qinling orogen we reprocessed the data in 2009 and the results revealed the fine lithosphere structure of the northern margin of the songpan block and the west qinling orogen the results also revealed the connection between the songpan block and east kunlun west qinling orogeny indicating that the songpan block and the west qinling orogen belong to the same stable continental block and that their lower crusts are both characterized by north dipping strong reflectors which provide seismic evidence for subduction of the lower crust of the songpan block beneath the west qinling orogen we found that the sub horizontal moho reflectors beneath the west qinling orogen indicate that it underwent intensive extension after collisional orogeny the results of this study also indicate that the kunlun fault did not cut off the moho actually it terminated at a sub horizontal decollement in the lowermost crust of tibet at a depth of 35 km the data provided through the investigation also are helpful in evaluating prospective oil and gas resources in the songpan block

neogene and quaternary faulting in and along the qinling shan the structure of the qinling shan mountain range shaanxi china involves major east south east strike slip faults field observations made during two expeditions in 1983 and 1984 show that the faults are active and may have undergone several tens of kilometres of post eocene left lateral displacement the faults cut the morphology sharply and offset left laterally small channels their trace is marked by cataclastic to mylonitic non coaxial deformation zones at least 50 100 m wide the qinling faults allow the eastward extrusion of south china and may be considered the easternmost continuation of the altyn tagh fault system they link extensional tectonics in northeastern china to the himalayan collision

lateral variation of crustal structure in the ordos block and surrounding regions north china and its tectonic implications abstract crustal thicknesses and poisson s ratios in the ordos block and surrounding regions were estimated by the use of the h k stacking method on teleseismic receiver functions the data came from 353 temporary and permanent seismic stations in 2006 2011 results show that the crustal thickness and poisson s ratio gently vary within the ordos block with an average of 41 3 km and 0 265 respectively consistent with a felsic composition of the crust crustal thicknesses predicted on the basis of airy isostasy are consistent with the estimated thicknesses implying that the topography is approximately compensated the reactivated portion of the north china craton that has undergone mesozoic cenzoic lithospheric thinning is also characterized by the thinning crust while the ordos block maintains normal crustal thickness and average crustal velocity inferred higher densities in the lower crust and the anti correlation between poisson s ratio and crustal thickness in the ordos block may be the result from underplating of mafic magmas in the precambrian around the ordos block the paleoproterozoic khondalite zone in the northern edge has higher poisson s ratio and thickened crust which is consistent with the lower crust being of more mafic composition the weihe shanxi graben in the southeastern edge has mid high poisson s ratio high heat flow and thinning crust which is consistent with the known transtensional tectonic setting in the liupanshan thrust belt along the southwestern edge of the ordos plateau significant variations in crustal thicknesses and poisson s ratios occur on two sides besides a thickened crust a concaved moho implies horizontal shortening of this edge of the ordos block due to its collision with the northeastern tibetan plateau the structural differences between the eastern and western edges of the ordos block reflect that the ordos block is in tectonic stress environment of the western compression and the eastern extension

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