crustal deformation along the altyn tagh fault system western china from gps we collected gps data from the southern tarim basin the qaidam basin and the western kunlun shan region between 1993 and 1998 to determine crustal deformation along the altyn tagh fault system at the northern margin of the tibetan plateau we conclude from these data that the altyn tagh is a left lateral strike slip fault with a current slip rate of 9 mm yr in sharp contrast with geological estimates of 20 30 mm yr this contrast poses an enigma because the gps data cover a wider region than the geologic data they might be expected to reveal somewhat more slip we also find that the tarim and qaidam basins behave as rigid blocks within the uncertainty of our measurements rotating clockwise at a rate of 11 and 4 5 nrad yr respectively with respect to the eurasia plate the rotation of the tarim basin causes convergence across the tian shan increasing progressively westward from 6 mm yr at 87 e to 18 mm yr at 77 e our data and other gps data suggest that the indo asia collision is mainly accommodated by crustal shortening along the main himalayan thrust system 53 and the tian shan contractional belt 19 eastward extrusion of the tibetan plateau along the altyn tagh and kunlun faults accommodates only 23 of the indo asia convergence

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

electrically conductive crust in southern tibet from indepth magnetotelluric surveying the crust north of the himalaya is generally electrically conductive below depths of 10 to 20 km this conductive zone approaches the surface beneath the kangmar dome dipping north and extends beneath the zangbo suture a profile crossing the northern yadong gulu rift shows that the high conductivity region extends outside the rift and its top within the rift coincides with a bright spot horizon imaged on the indepth cmp common midpoint profiles the high conductivity of the middle crust is atypical of stable continental regions and suggests that there is a regionally interconnected fluid phase in the crust of the region

dynamic topography produced by lower crustal flow against rheological strength heterogeneities bordering the tibetan plateau summary dynamic stresses developed in the deep crust as a consequence of flow of weak lower crust may explain anomalously high topography and extensional structures localized along orogenic plateau margins with lubrication equations commonly used to describe viscous flow in a thin gap geometry we model dynamic stresses associated with the obstruction of lower crustal channel flow due to rheological heterogeneity dynamic stresses depend on the mean velocity u viscosity and channel thickness h uniquely through the term u h 2 these stresses are then applied to the base of an elastic upper crust and the deflection of the elastic layer is computed to yield the predicted dynamic topography we compare model calculations with observed topography of the eastern tibetan plateau margin where we interpret channel flow of the deep crust to be inhibited by the rigid sichuan basin model results suggest that as much 1500 m of dynamic topography across a region of several tens to a hundred kilometres wide may be produced for lower crustal material with a viscosity of 2 10 18 p asfl owing in a 15 km thick channel around a rigid cylindrical block at an average rate of 80 mm yr 1

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

continental subductions beneath tibet and the himalayan orogeny a review argand 1924 and later holmes 1965 proposed that the himalayan orogeny and tibet uplift were the result of large underthrusting of the indian continental crust beneath tibet since then plate motion studies have yielded new constraints on the relative displacements of india and asia and a large number of geological and geophysical investigations have helped to determine the structure of the himalayas and tibet the best natural laboratory for understanding mountain belts the aim of this paper is to review briefly the various models of himalayan mountain building in light of recent geological and geophysical data and to estimate the role of continental subductions

structure of the crust in the vicinity of the banggong nujiang suture in central tibet from indepth magnetotelluric data 1 magnetotelluric data from a 150 km long profile crossing the banggong nujiang suture bns central tibet acquired as part of the international deep profiling of tibet and the himalaya indepth project have been examined for crustal and upper mantle structure strike and dimensionality analyses demonstrate that regional scale electrical structures are two dimensional and oriented approximately parallel to surface geological strike as seen elsewhere in tibet the double thickness crust is generally characterized by resistive upper crust hundreds to thousands of ohm meters overlying conductive middle and lower crust tens to hundreds of ohm meters but in detail there are lateral variations at all levels regionally a northward transition from thick 45 km to thin 15 km resistive upper crust coincides with 1 the surface trace of the bns 2 a prominent strand of the karakorum jiali fault system 3 northward decrease in upper mantle seismic velocities and increase in attenuation and 4 pronounced northward onset of seismic polarization anisotropy the latter two seismological features have been taken to mark the northern limit of indian mantle lithosphere thrust beneath southern tibet on the basis of our electrical model we speculate that 1 the resistive upper crustal root beneath the neogene lunpola and duba basins was produced by crustal shortening localized along the northern edge of the lhasa terrane 2 the low midcrustal resistivity beneath the bns reflects enhanced neogene melting and or metamorphic dewatering of relatively fertile subduction zone complex rocks 3 observed steep upper crustal low resistivity anomalies are produced by hydrothermal fluids within active faults localized within and adjacent to the bns and 4 these strike slip and extensional fault arrays are surface manifestations of lithosphere penetrating shear localized along the northern edge of the underthrust indian plate

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

two stage evolution model for the altyn tagh fault china the northeastern extension of the altyn tagh fault china beyond the apparently truncated qilian shan has been controversial since the recognition of this large left slip fault we propose that the altyn tagh fault may have been contiguous with the east northeast to northeast trending currently inactive but apparently active during the cenozoic alxa east mongolia fault the latter truncated the beishan orogen to the northwest and the inner mongolia orogen to the southeast the beishan orogen can be correlated with the inner mongolia orogen on the basis of the recognition of three offset petrotectonic units and two offset sutures this correlation requires 400 50 km of left lateral offset along the alxa east mongolia fault identical to the slip along the northern segment of the altyn tagh fault inferred by p molnar and p tapponnier we suggest a two stage model for fault evolution during the first stage 400 km of displacement separated the inner mongolia orogen from the originally contiguous beishan orogen along a continuous altyn tagh alxa east mongolia fault during the second stage the alxa east mongolia fault became inactive and additional offset along the altyn tagh fault has been mainly accommodated by shortening of the qilian shan and the qaidam basin we infer that the first stage of movement along the altyn tagh alxa east mongolia fault system may have begun around oligocene time the region transformed to the second stage of fault movement ca 13 16 ma probably because of thinning of the lithospheric mantle beneath northern tibet the proposed model suggests that the derbur fault in northernmost china and beyond is an extension of the alxa east mongolia fault and has a history of strike slip movement the nearly 400 km of offset on the alxa east mongolia fault may have been accommodated mainly by subduction in the sea of okhotsk

magnetotelluric evidence for thick skinned tectonics in central taiwan taiwan is the type example of an arc continent collision numerous tectonic models have been proposed for this orogen and include both thin skinned and thick skinned lithospheric deformation these models predict very different structures at middle and lower crustal depths but insuffi cient geophysical data exist to unequivocally distinguish between them long period magnetotelluric mt data were collected in central taiwan in 2006 2007 to constrain the crustal resistivity structure a two dimensional inversion of these mt data revealed a prominent electrical conductor that extends across the decollement predicted by the thin skinned model this feature is interpreted to be due to 1 2 saline fl uids and is inconsistent with the thin skinned model in contrast the thick skinned model predicts this feature since fl uids are generated in the crustal root through metamorphism quantitative correlation of the resistivity and seismic velocity models supports small volume high salinity fl uids in a thickened crust as the cause of this conductor

electrical anisotropy of south african lithosphere compared with seismic anisotropy from shear wave splitting analyses abstract electrical anisotropy in southern africa inferred from the analysis of magnetotelluric mt data recorded as part of the southern african mt experiment samtex is compared with seismic anisotropy inferred from an sks shear wave splitting study in the same region given the vastly varying penetration depths in the survey area electrical anisotropy is derived in terms of approximate depth rather than frequency electrical anisotropy directions for crustal depths 45 km are not consistent with the fast axis directions inferred from the sks analyses upper mantle electrical results appear to be mostly a consequence of the geometry of large scale geological structures and provide evidence that some crustal structures are distinct at depth while others seem to be confined to the crust our results indicate that the causative region for the seismic anisotropy in the lithospheric mantle has either a correspondingly weak electrical anisotropic signature or is more prominent at greater lithospheric depths than those we investigate here

electromagnetic constraints on strike slip fault geometry the fraser river fault system magnetotelluric data from four profiles crossing the eocene strike slip fraser river fault in southwestern british columbia suggest that it penetrates the entire crust this conclusion is supported by seismic reflection observations of a 2 3 km step in the moho slightly to the east of the surface expression of the fault but is at variance with an interpretation of the seismic data in which the fault soles into mid crustal reflectors that seem to be continuous across the fault trace a crustal penetrating geometry supports the proposal that the fraser river fault forms part of a 2500 km long intracontinental transform fault system in northwestern north america modeling studies resolve a thin highly conducting mid crustal zone that is connected electrically to the conducting lower crust beneath the coast belt low 13 c values close to the fraser fault system suggest that the electromagnetic signature of this zone may be due to the presence of organic carbon

dissipation of fast strike slip faulting within and beyond northeastern tibet structural patterns global positioning system gps velocities and quaternary fault slip rates in northeastern tibet indicate a transfer of left lateral slip from the kunlun fault northeast to the haiyuan fault and minor crustal shortening and rotation within a 200 km wide stepover zone related deformation also continues at least a few hundred kilometers north of the haiyuan fault into a region of diffuse extensional shear or rotation underlain by average thickness crust fast localized slip along the central kunlun fault transforms into distributed deformation across a 500 km wide zone where the lower crust is weak the distribution of fault parallel gps velocities across this region suggests a decrease in fault slip toward eastern fault tips and progressive dissipation of slip to the north rather than east of the tibetan plateau as previously suggested

geodetic evidence for a low slip rate in the altyn tagh fault system the collision between india and asia has been simulated with a variety of computational models that describe or predict the motions of the main faults of east asia geological slip rate estimates of 20 30 mm yr 1 suggest that the largest of these faults the 2 000 km long altyn tagh fault system on the northern edge of the tibetan plateau absorbs as much of the indo asian convergence signal as do the himalayas partly by oblique slip and partly by contraction and mountain growth however the predictions of dynamic models for asian deformation and the lower bounds of some geological slip rates estimates 3 9 mm yr 1 refs 7 8 suggest that the altyn tagh system is less active here we report geodetic data from 89 91 degrees e that indicate left lateral shear of 9 5 mm yr 1 and contraction of 3 1 mm yr 1 across the altyn tagh system this result combined with our finding that at 90 degrees e tibet contracts north south at 9 1 mm yr 1 supports the predictions of dynamic models of asian deformation

on two dimensional interpretation of magnetotelluric soundings summary the paper presents the views of the 2 d interpretation of magnetotelluric mt data that are characteristic of the russian magnetotelluric school discussing the strategy of 2 d interpretation of mt data we have to answer three questions in decreasing order of importance these questions are as follows 1 which field mode is more sensitive to the near surface and deep structures that are the targets of mt surveys 2 which field mode is more robust to the 3 d evects caused by real geological bodies 3 which field mode is more susceptible to the static shift induced by nearsurface inhomogeneities we examine the transverse magnetic tm and transverse electric te modes of the 2 d magnetotelluric field and show that they satisfy the principle of information complementarity 1 while the tm mode is more sensitive to the near surface structures the te mode may be more sensitive to the deep structures 2 while the tm mode is more robust to the 3 d evects caused by conductive structures the te mode may be more robust to the 3 d evects caused by resistive structures and 3 while the tm mode is more susceptible to the static shift the te mode may be almost undistorted thus the gaps left by one mode can be filled by another mode if so the most comprehensive and reliable information on the conductivity of the earth s interior can be obtained using both modes i e the transverse and longitudinal mt curves the general scheme of this bimodal mt inversion is rather simple the transverse curves provide details of near surface structures e g in the sediments and allow one to evaluate the lithosphere resistance and outline the deep conductive faults while the longitudinal curves help one to detect the conductive zones in deep layers of the lithosphere and in the asthenosphere an eycient two level algorithm for the bimodal mt inversion realizing this scheme is suggested as an illustration the paper presents the geoelectrical model of the kirghiz tien shan constructed by means of the bimodal mt inversion

did the altyn tagh fault extend beyond the tibetan plateau abstract the pre miocene northeastern termination of altyn tagh fault is a critical outstanding problem for understanding the mechanics of cenozoic deformation resultant from the indo asian collision and mechanisms of tibetan plateau formation structures beyond the widely accepted ne end of the altyn tagh fault near the town of yumen are needed in order to accommodate strike slip deformation related to plate like lateral extrusion tectonics but structures with the necessary slip magnitudes and histories have not been identified we report on a series of newly recognized and documented e to ene striking faults within the alxa block ne of the tibetan plateau that are visible on remotely sensed images and confirmed by field studies these structures are demonstrably left lateral faults based on offset geology and kinematic indicators such as striae and s c fabrics in fault gouge the faults have post cretaceous offsets of at least tens to possibly 150 km but limited post miocene displacement constrained by offset sedimentary basins these characteristics suggest that strike slip faults of the alxa region have a similar structural history as the central eastern altyn tagh fault and can provide a mechanism for accommodating oligocene early miocene extrusion along the altyn tagh fault

structure of the central altyn tagh fault revealed by magnetotelluric data new insights into the structure of the northern margin of the india asia collision abstract the altyn tagh fault atf is a left lateral strike slip fault that forms the northern margin of the tibetan plateau and plays a significant role in accommodating the convergence between the colliding indian and eurasian plates as a part of the fourth phase of the indepth project magnetotelluric mt data were collected across the central segment of the atf to determine the lithospheric scale structure of the fault system dimensionality analyses demonstrated that the mt data can be interpreted using two dimensional approaches but some localized 3 d effects are seen consequently both 2 d and 3 d inversions were carried out and a joint interpretation was made on the basis of these two types of models inversion models revealed two major conductors beneath the qaidam basin qb and altyn tagh range atr respectively the conductive region beneath the qb was interpreted as a ductile layer in the lower crust to upper mantle that might represent flow beneath the western margin of the qb whereas the large scale south dipping conductor beneath the atr is interpreted as a region with high fluid content formed by metamorphism associated with the oblique underthrusting of the tarim block beneath the northern tibetan plateau these fluids migrate upwards through the fault system and have formed serpentinized zones in the crust combining these interpretations a structural model compatible with diverse geophysical observations is proposed in which we suggest the competing end member rigid block model and continuum model are reconcilable with the continuum model locally dominant for the study region as evidenced by a thickened crust

slip rates of the altyn tagh kunlun and karakorum faults tibet from 3d mechanical modeling we use 3 d mechanical modeling representing faults as planar surfaces with frictional properties that obey coulomb failure process to explore the long term slip rates of the altyn tagh fault and kunlun faults in the north tibetan plateau crustal theology is simplified as an elastoplastic upper crust and a viscoelastic lower crust far field gps velocities and late quaternary fault slip rates are used to constrain the model results rheological tests show that effective fault friction lower than 0 1 0 08 leads to high slip rates that fit with geologically and geodetically determined slip rates of the kunlun fault 10 11 7 1 5 mm yr meanwhile the modeled altyn tagh fault reaches slip rates similar to 13 7 mm yr to similar to 7 8 mm yr in its central portion between ranges of the geological slip rates associated with high slip rates our model predicts that central tibet similar to 84 degrees e 95 degrees e from the altyn tagh fault to the north of the himalayan arc accommodates north south shortening and east west extension rates of similar to 10 12 mm yr and similar to 8 10 mm yr respectively we also question the widely accepted idea that interseismic strain is driven at the base of the seismogenic zone by a screw dislocation if this assumption fails the presented model implies that interseismic strain around large strike slip faults could be distributed in a much broader way if the lithosphere deforms as a thin elastic plate rather than an elastic half space with an embedded dislocation if this distributed deformation is ignored and the instantaneous surface deformation field modeled as that resulting from slip on a dislocation below a specified depth embedded in an elastic half space the estimated slip rate will inevitably be lower than the true long term slip rate this appears to explain why geodetic slip rates proposed for the altyn tagh fault 510 mm yr are lower than some of the geological slip rates c 2008 elsevier b v all rights reserved

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

detection of widespread fluids in the tibetan crust by magnetotelluric studies magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north to south width of the tibetan plateau the integrated conductivity conductance of the tibetan crust ranges from 3000 to greater than 20 000 siemens in contrast stable continental regions typically exhibit conductances from 20 to 1000 siemens averaging 100 siemens such pervasively high conductance suggests that partial melt and or aqueous fluids are widespread within the tibetan crust in southern tibet the high conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust in northern tibet the conductive layer is at 30 to 40 kilometers and is due to partial melting zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow

a magnetotelluric study of mount ruapehu volcano new zealand summary mt ruapehu is an active andesite cone volcano which marks the southern termination of the kermadec volcanic arc results from 40 broad band magnetotelluric soundings have been analysed using the phase tensor this approach provides a way of determining dimensionality allowing for distortion removal and visualizing data in a 3 d situation the phase tensor analysis suggests that the shallow resistivity structure is largely 1 d in character but that the deeper structure requires a 3 d interpretation 1 d inversions show that at sites on ruapehu a shallow conductive layer lies between a high resistivity layer of a few hundred metres thickness and higher resistivity layer corresponding to basement greywacke the low resistivity layer is contiguous with the waters of the highly acidic crater lake and thus is believed to be the hydraulically controlled upper limit of a zone of acid alteration overlain by dry volcanic rock and ash to the southwest of the volcano the conductive layer merges with a surface conductor associated with tertiary sediments following initial 2 d inversions the deep resistivity structure has been derived through 3 d inversion of data from 38 sites this indicates the existence of a dyke like low resistivity zone that persists to at least 10 km depth and extends from beneath the summit of ruapehu to the northeast where it appears to connect to a poorly constrained region of high conductivity which lies outside the network of measurement sites the low resistivity dyke like feature may be identified with a volcanic feeder system which also supplies the other volcanoes of the tongariro volcanic centre and marks the conduit by which hot gases and occasionally magma reach the surface

application of the 3d magnetotelluric inversion code in a geologically complex area the wsinv3dmt code makes the implementation of 3d inversion of magnetotelluric data feasible using a single pc audio magnetotelluric data were collected along two profiles in a cu ni mining area in xinjiang china where the apparent resistivity and phase curves the phase tensors and the magnetic induction vectors indicate a complex 3d conductivity structure 3d inversions were carried out to reveal the electrical structure of the area the final 3d model is selected from the inversion results using different initial lagrange values and steps the relatively low root mean square rms misfit and model norm indicate a reliable electrical model the final model includes four types of low resistivity areas the first ones coincide with the known location of an orebody and further forward modelling indicates that they are not in full connectivity to form a low resistivity zone the second ones are not controlled by magnetotelluric sites and embody little information of the observed data they are considered as tedious structures the third one is near to the regional kangguer fault and should be treated carefully considering the effect of the fault the last ones are isolated and existing at a limited level as the first ones they should be paid more attention to

electrical resistivity structures between the northern qilian mountains and beishan block nw china and tectonic implications abstract controversies exist on the tectonic models at the northern margin of the qinghai tibetan plateau new magnetotelluric mt data with a bandwidth of 0 001 5827 5 s were collected along one profile starting from the northern qilian mountains in the southern end crossing the hexi corridor the kuantan shan hei shan uplift the huahai basin and coming into the beishan block two dimensional 2d and three dimensional 3d resistivity models of the data are derived using magnetotelluric inversion codes the 2d models and the electrical structures along the profile extracted from the 3d models are comparable and complementary in defining reliable scales and shapes of anomalies in electrical resistivity the crust upper mantle structures beneath the northern plateau are characterized by two imbricated southward underthrusted blocks of continental crust or lithosphere the northern qilian fault is interpreted as a gently south dipping decollement in the upper crust along the top of a high resistivity body below the northern qilian mountains the two high resistivity bodies below the kuantan shan hei shan uplift are disconnected the upper crustal one may connect with the high resistivity basement below the northern qilian mountains in that the attitudes and deformations of the strata at these areas are coherent whereas the lower crust upper mantle one may be the cold lithospheric materials of the block north to the hexi corridor and that forms a south dipping boundary between the hexi corridor and its northern neighbors the observed high electrical resistivity beneath the beishan block provides further support for the notion that the beishan block should be rigid as a whole only in this manner could it transfer stress from south to north the northern boundary of the qinghai tibetan plateau is below the kuantan shan hei shan fault and underthrusting of continental crust or lithosphere is the main cause for the thickening of the plateau crust in the north

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

tectonics of strike slip restraining and releasing bends restraining and releasing bends are common but enigmatic features of strike slip fault systems occurring in all crustal environments and at regional to microscopic scales of observation regional scale restraining bends are sites of mountain building transpressional deformation and basement exhumation whereas releasing bends are sites of topographic subsidence transtensional deformation basin sedimentation and possible volcanism and economic mineralization because restraining and releasing bends often occur as singular self contained domains of complex deformation they are appealing natural laboratories for earth scientists to study fault processes earthquake seismology active faulting and sedimentation fault and fluid flow relationships links between tectonics and topography tectonic and erosional controls on exhumation and tectonic geomorphology this volume addresses the tectonic complexity and diversity of strike slip restraining and releasing bends with 18 contributions divided into four thematic sections 1 a topical review of fault bends and their global distribution 2 bends sedimentary basins and earthquake hazards 3 restraining bends transpressional deformation and basement controls on development 4 releasing bends transtensional deformation and fluid flow

crustal structure across the altyn tagh range at the northern margin of the tibetan plateau and tectonic implications wepresentnewseismicrefraction wide angle reflection dataacrossthealtyntaghrangeanditsadjacentbasins wefindthatthe crustal velocity structure and by inference the composition of the crust changes abruptly beneath the cherchen fault i e 100 km north of the northern margin of the tibetan plateau north of the cherchen fault beneath the tarim basin a platform type crust is evident in contrast south the cherchen fault the crust is characterized by a missing high velocity lower crustal layer our seismic model indicates that the high topography 3 km of the altyn tagh range is supported by a wedge shaped region with a seismic velocity of 7 6 7 8 km s that we interpret as a zone of crust mantle mix we infer that the altyn tagh range formed by crustal scale strike slip motion along the north altyn tagh fault and northeast southwest contraction over the range the contraction is accommodated by 1 crustal thickening via upper crustal thrusting and lower crustal flow i e creep and 2 slip parallel swdirected underthrusting of only the lower crust and mantle of the eastern tarim basin beneath the altyn tagh range d 2005 elsevier b v all rights reserved

tomographic evidence for localized lithospheric shear along the altyn tagh fault seismic tomography across the altyn tagh fault at the north edge of the tibetan plateau reveals a low p wave velocity anomaly below the fault down to 140 kilometers this anomaly probably reflects strike slip shear in the lithosphere slip partitioning may also induce a wedge of crust from the tarim basin to plunge into the mantle

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

a magnetotelluric investigation of the san andreas fault at carrizo plain california high quality wide band magnetotelluric data were collected along two profiles crossing the san andreas fault at carrizo plain california for crustal imaging as part of the san andreas deep drilling project two dimensional inversions of the data indicate that the upper crustal part of the san andreas fault does not appear here as an anomalously conductive zone additionally a broad resistive zone under the temblor mountains east of the fault suggests resistive crystalline or metamorphic rocks may be present here as opposed to the more conducting franciscan assemblage contradicting the generally accepted geologic model

two dimensional interpretation of three dimensional magnetotelluric data an example of limitations and resolution summary interpretation of magnetotelluric mt data for three dimensional 3 d regional conductivity structures remains uncommon and two dimensional 2 d models are often considered an adequate approach in this paper we examine 2 d interpretation of 3 d data by considering the synthetic responses of a 3 d structure chosen specifically to highlight the advantages and limitations of 2 d interpretation 2 d models were obtained from inversion of the synthetic 3 d data set with different conditions noise and distortion applied to the data we demonstrate the importance of understanding galvanic distortion of the data and show how 2 d inversion is improved when the regional data are corrected prior to modelling when the 3 d conductive structure is located below the profile the models obtained suggest that the effects of finite strike are not significant if the structure has a strike extent greater than about one half of a skin depth in this case the use of only tm mode data determined better the horizontal extent of the 3 d anomaly when the profiles are located away from the 3 d conductive structure the use of only tm mode data can imagine phantom conductive structures below the profile in this case the use of both polarizations produced a better determination of the subsurface structures it is important to note that the main structures are identified in all the cases considered here although in some cases the large data misfit would cause scepticism about features of the models

partial melt or aqueous fluid in the mid crust of southern tibet constraints from indepth magnetotelluric data summary the indepth project has applied modern geophysical techniques to the study of the crustal structure and tectonic evolution of the tibetan plateau in the lhasa block seismic reflection surveys in 1994 detected a number of bright spots at 15 20 km depths that indicate zones of crustal fluids aqueous fluids or partial melt coincident magnetotelluric mt data collected in 1995 detected a major zone of high electrical conductivity at the same depth as the brightspots using constrained inversion the mt data require a minimum crustal conductance of 6000 s this abnormally high electrical conductance can be best explained by a layered model with fluids partial melt aqueous fluids or a combination of partial melt and aqueous fluids the non uniqueness of the mt method means that a wide range of melt fraction thickness combinations for the above models could all explain the 6000 s conductance to distinguish between these three models other geophysical and geological data are required reflection seismic data suggest that a high fluid content 15 per cent is present at the top of the layer the amplitude versus offset data suggest that the top of this layer may be aqueous fluids rather than partial melt passive seismic data imaged a 20 km thick layer of lower fluid content that is probably partial melt petrological studies suggest that concentrations of aqueous fluids above 0 1 per cent at mid crustal depth cannot be sustained taken together these data show that the high conductivity in southern tibet is most probably the result of a relatively thin layer of aqueous fluids 100 200 m overlying a thicker zone of partial melt 10 km

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

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

upper mantle temperature determined from combining mineral composition electrical conductivity laboratory studies and magnetotelluric field observations application to the intermontane belt northern canadian cordillera abstract using laboratory derived temperature dependences of the electrical conductivity of mantle minerals coupled with appropriate mixing laws we determine the bulk conductivity of mantle mineral assemblages for the ternary olivine orthopyroxene clinopyroxene ol opx cpx system we calculate physical property bounds hashin shtrikman bounds as a function of the fraction of different phases present these limits correspond to the extreme situations where the most conducting phase is either fully interconnected or fully disconnected the relationships we present between temperature mineral composition and bulk electrical conductivity allow constraining one of them given the other two we apply this approach to an area of the north american cordilleran intermontane belt in the yukon territory northern canada where xenolith evidence indicates bimodal upper mantle mineral assemblages harzburgite and lherzolite this locality coincides spatially with an upper mantle region of low electrical conductivity determined by long period magnetotelluric data given the mantle mineral composition and the maximum and minimum bounds on the electrical conductivity deduced by non linear model appraisal we determine the permitted extremal temperature bounds of the intermontane belt mantle rocks directly below the moho to a depth of some 80 km we show that the mantle in this region is at a minimum temperature of 820 c and a maximum temperature of 1020 c the latter is some 200 c colder than that suggested in a recent interpretation of an observed collocated low velocity zone from a teleseismic survey

continental transforms a view from the alpine fault abstract continental transform faults are dominantly highly localized strike slip shear zones hundreds of kilometers long that accumulate tens to hundreds of kilometers of displacement from work on the alpine fault we pose the questions what is the deep structure of a continental transform and how does the displacement become localized we review research on the alpine fault and propose a model in which the fault partitions at depth into a steep zone extending into the mantle with largely fault parallel motion and a flat ductile decollement in the lower crust the fault localizes around two thirds of the plate motion within a 100 km wide zone of distributed deformation a review of other active continental fault systems suggests that variation between them may reflect their tectonic origins the nature of the crust in which they develop the presence of a significant oblique component of motion and the displacement rate all however have evidence for the development of a single principal fault zone that carries 50 of the total displacement and extends as a localized zone of shear into the upper mantle we review mechanisms of strain weakening and suggest that localization of a principal fault may be initiated in the seismogenic crust and through a series of positive feedbacks eventually extend through the lower crust into the upper mantle

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

lithospheric structures and precambrian terrane boundaries in northeastern botswana revealed through magnetotelluric profiling as part of the southern african magnetotelluric experiment 1 within the framework of the southern african magnetotelluric experiment a focused study was undertaken to gain improved knowledge of the lithospheric geometries and structures of the westerly extension of the zimbabwe craton zim into botswana with the overarching aim of increasing our understanding of southern african tectonics the area of interest is located in northeastern botswana where kalahari sands cover most of the geological terranes and very little is known about lithospheric structures and thicknesses some of the regional scale terrane boundary locations defined based on potential field data are not sufficiently accurate for local scale studies investigation of the nnw sse orientated 600 km long zim line profile crossing the zimbabwe craton magondi mobile belt and ghanzi chobe belt showed that the zimbabwe craton is characterized by thick 220 km resistive lithosphere consistent with geochemical and geothermal estimates from kimberlite samples of the nearby orapa and letlhakane pipes 175 km west of the profile the lithospheric mantle of the ghanzi chobe belt is resistive but its lithosphere is only about 180 km thick at crustal depths a northward dipping boundary between the ghanzi chobe and the magondi belts is identified and two middle to lower crustal conductors are discovered in the magondi belt the crustal terrane boundary between the magondi and ghanzi chobe belts is found to be located further to the north and the southwestern boundary of the zimbabwe craton might be further to the west than previously inferred from the regional potential field data

active thrusting and folding in the qilian shan and decoupling between upper crust and mantle in northeastern tibet fieldwork south of the city of gaotai gansu province china shows that active shortening of surface sediments in the foothills of the yumu shan a large fore mountain of the qilian shan at the northeastern edge of tibet involves both overthrusting and flexural slip folding north of this mountain we found and mapped a prominent north facing thrust scarp that offsets a holocene fan sloping gently 3 4 to the north part of this scarp appears to be related to the m 7 5 180 a d earthquake that may have led to the demise of the han dynasty city of luo tuo chen in the hexi corridor a set of 10 100 150 m long profiles measured across this scarp 3 2 m high on the average can be made to fit the diffusion degraded morphology of a surface break related to the 180 a d event using a value of about 3 3 m 2 10 3 yr for the mass diffusivity of fanglomerates in this part of gansu province smaller mountain facing scarps on a terrace capped foothill result from bedding slip concurrent with active folding of underlying steeply northdipping plioquaternary sandstone and conglomerate beds holocene uplift rates along the yumu shan which is only one of the qilian shan ranges are estimated to be between 0 4 and 1 9 mm yr which implies that much of the mountain formed in the quaternary the periclinal structure of the plioquaternary envelope under which the paleozoic core of the yumu shan plunges towards the west suggests that the whole 3200 m high mountain is a basement ramp anticline mountains striking parallel to the yumu shan with similar structure and comparable or greater sizes north and south of the hexi corridor probably also correspond to recent crustal ramp anticlines this implies that the wide mountainous upper crustal wedge making the northeastern edge of the tibet qinghai plateau is detached from the underlying lower crust and upper mantle

reflection seismic evidence for a moho offset beneath the walls boundary strike slip fault birps has acquired two deep seismic reflection profiles across the walls boundary fault a major late caledonian strike slip fault that is probably the northern continuation of the great glen fault of scotland where crossed by the shet deep seismic survey north of the shetlands the walls boundary fault appears to be a narrow vertical structure which penetrates the entire crustal thickness and juxtaposes crusts of different thicknesses the seismic data show a moho offset in travel time of 1 0 1 5 seconds and two high amplitude diffractions originating at moho travel times these diffractions are as yet unique among seismic data across strike slip faults and can be used to argue that the moho is offset in depth by 2 3 km over a lateral distance of less than 6 km seismic modelling is used to constrain this interpretation the preservation of this moho topography suggests that the structure and rheology of the lower crust of the shetland platform was not significantly modified during post devonian extension of the north sea and atlantic margin

electrical resistivity structures and tectonic implications of main karakorum thrust mkt in the western himalayas nne pakistan abstract we discovered a conductive zone along main karakoram thrust which could be an indication of flat subduction of kohistan island arc beneath the eurasian plate kohistan island arc collided with the karakoram block of the eurasian plate in the early cretaceous however according to findings of many researchers the subduction ceased about 75 ma ago the presence of the conductive zone is an indication of current magmatism or hydrothermal fluids maximum low frequency band data from fourteen sites with recording periods of 10 2 103 s was acquired along a profile crossing mkt our results reveal the existence of multiple low resistivity zones beneath the region extending from shallow to the depths of more than 100 km these low resistivity zones might be a signature of the ongoing magmatic activities or hydrothermal fluids along the shyok suture zone in addition we discovered another large conductive body towards the south of the study area which could be a result of uprising magmatic plumes generated by the subducting indian plate along the indian suture zone and their entrapment in the overlying kohistan block

a high resolution magnetotelluric survey of the iapetus suture zone in southwest scotland summary magnetic variation mv surveys show a 40 km wide zone of high electrical conductivity in the middle crust extending across the southern uplands of scotland from the north sea to galloway a new magnetotelluric mt survey across the conductor in galloway was designed to improve the resolution of structure in the depth range 1 15 km and at the same time assess the improvement in resolution that could be achieved in a practical situation by optimizing data acquisition processing analysis and modelling a 40 km long profile was measured doubled for part of its length at a separation of 1 km with a typical site spacing of 2 km or less it was supplemented by shorter parallel and orthogonal lines giving a total of 40 sites galvanic distortion models involving regional 2 d structures gave poor fits to the impedance tensors both for the array as a whole and for individual sites confirming the picture from induction arrow maps that many sites were influenced by 3 d structures on scales of a few kilometres and upwards other indicators of the electrical strike regional and local mv measurements the spatial structure of the groom bailey regional impedance phase and of the rotated off diagonal phase gave values more in agreement with the geological strike of n52 e which was adopted as the most appropriate coordinate system into which to resolve the data the mt impedances were accordingly rotated into directions n38 w tm and n52 e te and inverted using the 2 d code of rodi mackie the investigation explored the sensitivity of the outcome to a wide range of starting model resistivities and roughness parameters and to different data subsets the te mode data were insensitive to structural detail and the best fit achieved was unacceptable suggesting the influence of structure outside the plane of the section the tm mode data could be fitted satisfactorily by varying the model roughness and gave the best control on the geometry of the conductive bodies forward modelling showed that the model generated from the tm mode also fitted the mv data adequately the inversion resolved the conductive zone into distinct blocks with edges matching faults mapped in the surface geology the leadhills fardingmullach and orlock bridge faults and the moffat valley lineament in the resistivity image the faults are vertical structures disrupting other features to a depth of 15 km which agrees with the view that although they originated as thrusts they were reactivated by strike slip motion during oblique closure of the iapetus ocean within some of the blocks are highly conductive regions no more than 4 km wide the most likely explanation is graphite or other metallic mineralization localized in shear zones

geoelectric response of archean lithosphere in the western superior province central canada abstract magnetotelluric mt soundings were made at more than 230 sites in the western superior province in central canada as part of the lithoprobe western superior transect investigations the superior province forms the nucleus of the north american continent its western part contains a regional pattern of east west trending 100 200 km wide granite greenstone and metasedimentary belts or subprovinces the objectives of the mt study are to define the electrical resistivity structure of the crust of the subprovinces and their boundaries and to determine if there exists a signature in the upper mantle related to the formation of the craton the exceptionally large mt database for the western superior province provides a three dimensional sampling of the resistivity structure in this paper the primary mt responses from the data set including geoelectric strike apparent resistivity and phase are used to define first order geoelectrical structures in the region apparent resistivity responses indicate the ca 3 0 ga north caribou terrane contains extremely resistive crust crust to the south accreted at ca 2 7 ga is significantly more conductive with resistivities similar to in the southeast superior province the results suggest that the supracrustal component of the north caribou terrane provides minimal enhancement of the crustal conductivity at the southern margin of the former north caribou superterrane induction arrows define a conductivity anomaly that extends in an east west direction for at least 600 km and causes a reversal of arrows in a region 100 200 km wide this anomaly is interpreted to be caused by a conductive component of the 2 7 ga metasedimentary rocks of the english river subprovince the observations demonstrate the ability of electromagnetic methods to map some neoarchean continental margins in the southwest superior province geoelectric strike azimuths for periods corresponding to signal penetration into the upper mantle are aligned with the trend of subprovince boundaries and are interpreted to be related to fossil structure in the mantle lithosphere this result provides support to the interpretation that the lithospheric mantle of the western superior province has remained intact since the 2 7 ga accretion of the subprovinces the mt responses from the western superior province have thus defined a number of large scale resistivity structures providing information on tectonic structure and history and confirming the value of electromagnetic imaging of archean lithosphere

lithospheric structure of the arabia eurasia collision zone in eastern anatolia magnetotelluric evidence for widespread weakening by fluids eastern anatolia is the location of a young continent continent collision between the arabian and eurasian plates long period magnetotelluric data have been used to image the electrical resistivity of the crust and upper mantle in this region the anatolian block is being extruded to the west and is characterized by a low resistivity fluid rich lower crust underlain by relatively normal upper mantle structure the anatolian plateau has a lower crust that contains pockets of very low resistivity that may indicate local accumulations of melt this is underlain by an upper mantle with an anomalously low resistivity that can be accounted for by an asthenosphere containing a few percent partial melt the presence of fluids may weaken the crust and mantle sufficiently to permit lateral flow and may also allow a decoupling of the upper and lower portions of the lithosphere the lithospheric structure of the anatolian plateau is similar to that of the northern tibetan plateau with zones of elevated fluid content however low resistivity in the anatolian crust is found in isolated pockets rather than the widespread regions observed in tibet

early palaeozoic north qaidam uhp metamorphic belt on the north eastern tibetan plateau and a paired subduction model an early palaeozoic uhp metamorphic belt was recently discovered in the north qaidam mountains in the north eastern part of the tibet plateau the belt contains abundant eclogite with lesser amounts of garnet peridotite and gneiss the paragneiss contains abundant zircon with coesite inclusions whereas the eclogite has garnet with coesite pseudomorphs and omphacite with exsolved quartz the calculated peak metamorphic conditions for the eclogite are t 730 c and p 2 8 gpa the garnet peridotites are magnesium rich with mg s 100 mg mg fe up to 92 olivine in the peridotites is highly magnesian fo 91 5 and tio2 poor and is typical of low t garnet peridotites calculated peak metamorphic conditions for the peridotites are t 837 c and p 2 5 gpa u pb and sm nd isotope dating indicate that continental subduction started during the early ordovician 495 ma the north qaidam uhp together with the north qilian hp belt about 300 km to the north constitute a pair of genetically related early palaeozoic subduction zones along the northern margin of the tibet plateau

electrical resistivity structures northeast of the eastern kunlun fault in the northeastern tibet tectonic implications abstract only sparse deep geophysical data have been obtained in the region to the north of the eastern kunlun fault eklf in the tibetan plateau which leads to inconsistent and ambiguous descriptions of the area in various geodynamic models the detailed geophysical issues in the region including the northeast boundary of the plateau and the crust mantle coupling mode northeast of the eklf are under dispute to resolve these disputes we have collected magnetotelluric mt data along five profiles in the region to the north of the eklf since 2009 the interpretation of data from the latest profile labeled l5 which starts from the eastern margin of the qaidam basin crosses the qilian mountain and the hexi corridor and finally reaches the southern alashan block is the focus of this paper these new results are comparable with our previously determined structures the two dimensional 2d resistivity model supports three divisions of the qilian mountain a complex northern qilian a relatively simple sandwiched southern qilian and a transitional central qilian in between them our data indicate a relatively low resistivity layer at the middle lower crust below the eastern margin of the qaidam basin and the southern qilian mountain this result indicates a weak crustal zone wcz existing northeast of the eklf and is interpreted as a large north dipping thrust at this depth the presumed imbricated thrust in the crust in the northeastern plateau resulting from the indenting of the mantle wedge from the south is the primary process in the crustal thickening in the region the mt results indicate that the northeastern boundary of the tibetan plateau is not below the eklf instead we suggest that the northeastern boundary of the outgrowth of the tibetan plateau is along and below the northern rim of the hexi corridor

active faults and magnitudes of left lateral displacement along the northern margin of the tibetan plateau abstract the northern margin of the tibetan plateau nmtp is a major intracontinental cenozoic transpressional zone that comprises a series of active strike slip faults and thrust faults it is important to document cumulative horizontal displacements along the nmtp in order to understand quantitatively strain partitioning in east asia since the india eurasia collision based on an analysis of horizontal slip along major active faults the total amount of horizontal displacements is estimated up to 700 km between the tibetan plateau and the tarim basin since the convergence of india and eurasia along the western and middle segment of the altyn tagh fault to the northern margin of the qaidam basin there are abundant evidence that show that the net displacement is 400 km since 40 35 ma and along the shulenan shan and southeast of middle qilian shan since 25 17 ma the amount of offset is 150 km the largest horizontal slip in qilian shan hexi corridor to the northeast of the altyn tagh fault is also 150 km since late oligocene to early miocene it decreases to only 60 km along the haiyuan fault since late miocene and to 25 km along the zhongwei tongxin fault since the pliocene about 5 3 3 4 ma at the northeast margin of the tibetan plateau this clearly implies northeastward diminishing of the total horizontal displacement and temporal getting younger of the fault slip along the nmtp however this tendency is very complicated at different times and different segments as a result of the uplift growth and rotation of different segments of the nmtp at different stages during the convergence of india and eurasia

electrical resistivity structure at the northern margin of the tibetan plateau and tectonic implications 1 the ene wsw striking altyn tagh fault atf and the wnw ese trending western qilian mountains define the northern margin of the tibetan plateau new magnetotelluric data were collected along three profiles crossing the eastern section of the atf and the southern qilian mountains the basic sounding bandwidth ranged from 0 003 to 7000 s the transverse electric and transverse magnetic mode data sets were inverted into resistivity sections using a conventional two dimensional 2d inversion code and data at periods of 0 3333 5464 s were inverted using a commonly employed three dimensional 3d code the 3d results constrain the interpretation of the 2d models and the final interpretative resistivity models show that lithospheric structures are spatially variable along the atf and across southern qilian in profiles across the atf the main fault is imaged as a vertical resistivity boundary and the high resistivity body in the western profile extends about 15 km deeper than the corresponding body in the eastern profile positive flower structures are apparent in the western profile but are only weakly visible in the eastern profile these observations suggest that the depth of the atf is spatially variable mantle resistivity images indicate a relatively cold and rigid tarim lithosphere which is consistent with a geodynamic model of oblique subduction of the tarim basin below the atf a high resistivity body in the crust of southernmost qilian indicates a huge thick skinned structure low resistivity bodies at the crust mantle boundary south of the main atf are presumed to form a weak layer beneath northern tibet

inescapable slow slip on the altyn tagh fault 1 gps surveys conducted in 1994 1998 and 2002 on the central portion of the altyn tagh fault confirm left lateral slip rates of 9 4 mm yr significantly slower than geological estimates of slip rate for the past several thousand years the measurements were obtained on a 300 km long profile between 88 and 91 e within one fault dimension of the mw 7 6 and mw 7 8 kunlun earthquakes of 1997 and 2001 the inferred elastic displacements from these events are incorporated into the interpreted 1994 2002 velocity field the recent serendipitous occurrence of these two large earthquakes on a sub parallel fault system suggests that slip on the kunlun fault may have recently accelerated thereby reducing the slip rate on the altyn tagh fault however geodetic data from northern tibet prior to the 2001 earthquake indicate present day rates similar to long term rates this suggests that systematic errors may be present in published geologic slip rate estimates for the altyn tagh fault

rapid active thrusting along the northwestern range front of the tanghe nan shan western gansu china the western part of the tanghe nan shan range southwest of subei western gansu china is presently growing on thrust ramps splaying from the left lateral altyn tagh fault late cenozoic thrusting has folded and sliced oligocene miocence red beds into an imbricate wedge capped by warped and uplifted quaternary terraces that form a 2 to 5 km wide ledge north of the steeply faceted range front seismic scarps 1 5 to 4 5 m high cut young fans along the outer thrusts carbon 14 dating of organic remains collected on strath terraces constrains the chronology of deposition and incision by the streams most of the fans and terraces in the southern part of the subei basin appear to have been emplaced after the last glacial maximum particularly during the early holocene optimum 9 5 ka measurements of the shapes of the warped terraces constrain minimum and maximum throws on the outer thrusts the minimum vertical throw of 34 2 m of a surface dated at 8411 530 years b p at one site provides a minimum vertical uplift rate of 4 1 0 5 mm yr the maximum possible uplift 115 15 m of the oldest terrace surface whose probable age is 15 to 18 ka places an upper bound on the uplift rate of 7 2 mm yr the thrust geometry at depth and the cumulative shortening 10 20 km deduced from balancing sections logged across the imbricate thrust wedge are consistent with a shortening rate of about 5 mm yr and an onset of thrusting at about 4 2 ma such a shortening rate implies a significant northward decrease in slip rate along the altyn tagh fault the recent growth of the western qilian mountain ranges thus appears to be intimately coupled with sinistral motion on the altyn tagh fault and the extrusion of 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

structure of the altyn tagh fault and daxue shan from magnetotelluric surveys implications for faulting associated with the rise of the tibetan plateau magnetotelluric measurements in the foreland of the daxue shan have imaged low angle 20 thrust faults that extend to a depth of 3 km the surface projections of the faults at depth coincide with the mapped traces of the chang ma m 7 6 1932 and north hills thrusts minimum cumulative offsets of 7 km on the chang ma thrust and 14 km on the north hills thrust are estimated from the horizontal extent of underthrust sediment assuming that regional thrusting began at 5 6 ma this corresponds to minimum convergence rates of 1 3 mm yr and 2 5 mm yr on the chang ma and north hills thrusts respectively these slip rates correspond to 3 mm yr of sinistral slip on the altyn tagh fault between 96 e and 97 e this is consistent with the 4 2 mm yr of slip estimated by geological studies assuming comparable slip rates and similar fault geometry in the neighboring ranges this requires a minimum of 11 mm yr of shortening parallel to the altyn tagh fault between the dang he nan shan and qilian shan both the style of thrusting and rate of shortening are in agreement with geologic studies that favor a relatively high rate of slip on the altyn tagh fault this in turn implies that lithospheric extrusion contributes significantly in accommodating the ongoing convergence between india and asia farther west the altyn tagh fault is imaged on four magnetotelluric profiles as a vertical resistivity contrast extending to a depth of at least 8 km two strands of the north altyn tagh fault are imaged east of the asperity near subei 39 30 n 95 e

cenozoic tectonic evolution of qaidam basin and its surrounding regions part 1 the southern qilian shan nan shan thrust belt and northern qaidam basin cenozoic qaidam basin the largest active intermountain basin inside tibet figures importantly in the debates on the history and mechanism of tibetan plateau formation during the cenozoic indo asian collision to determine when and how the basin was developed we conducted detailed geologic mapping and analyses of a dense network of seismic reflection profiles from the southern qilian shan nan shan thrust belt and northern qaidam basin our geologic observations provide new constraints on the timing and magnitude of cenozoic crustal thickening in northern tibet specifically our work shows that the southernmost part of the qilian shan nan shan thrust belt and contractional structures along the northern margin of qaidam basin were initiated in the paleocene early eocene 65 50 ma during or immediately after the onset of the indo asian collision this finding implies that stress was transferred rapidly through tibetan lithosphere to northern tibet from the indo asian convergent front located 1000 km to the south the development of the thrust system in northern qaidam basin was driven by motion on the altyn tagh fault as indicated by its eastward propagation away from the altyn tagh fault the eastward lengthening of the thrust system was spatially and temporally associated with eastward expansion of qaidam basin suggesting thrust loading was the main control on the basin formation and evolution the dominant structure in northern qaidam basin is a southwest tapering triangle zone which started to develop since the paleocene and early eocene 65 50 ma and was associated with deposition of an overlying southwest thickening growth strata sequence recognition of the triangle zone and its longevity in northern qaidam basin explains a long puzzling observation that cenozoic depocenters have been located consistently along the central axis of the basin this basin configuration is opposite to the prediction of classic foreland basin models that require the thickest part of foreland sediments deposited along basin edges against basin bounding thrusts restoration of balanced cross sections across the southern qilian shan nan shan thrust belt and northern qaidam basin suggests that cenozoic shortening strain is highly inhomogeneous varying from 20 to 60 both vertically in a single section and from section to section across the thrust belt the spatially variable strain helps explain the conflicting paleomagnetic results indicating different amounts of cenozoic rotations in different parts of qaidam basin the observed crustal shortening strain also implies that no lower crustal injection or thermal events in the mantle are needed to explain the current elevation 3000 3500 m and crustal thickness 45 50 km of northern qaidam basin and the southern qilian shan nan shan thrust belt instead thrusting involving continental crystalline basement has been the main mechanism of plateau construction across northern qaidam basin and the southern qilian shan nan shan region

late quaternary and present day rates of slip along the altyn tagh fault northern margin of the tibetan plateau 1 both global positioning system gps measurements and studies of late quaternary faulting are consistent with a slip rate of 10 mm yr along the central segment of the altyn tagh fault and a systematic decrease in that rate toward the eastern end of the fault dates of terraces above and below laterally offset terrace risers yield bounds on quaternary slip rates that range from those that agree with gps measurements to values as much as three times faster we argue that offset terrace risers that are protected by topography upstream of them are more closely dated by the age of the upper terrace than by that of the lower terrace in some cases valleys upstream of the fault have been incised into bedrock and few if any terrace risers can be seen within the valleys such streams debouch onto alluviated floodplains or fans that become incised presumably during climate changes to create terrace risers the terrace risers are then displaced so that they lie downslope from bedrock ridges on the upstream side of the fault and thus the risers become protected from further incision in such cases dates of upper terraces should more closely approximate the ages of the risers than those of lower terraces such dates yield slip rates of 10 mm yr in the central segment of the fault and decreasing rates eastward although we cannot with certainty rule out the higher slip rates along the altyn tagh fault our analysis does show that viable interpretations consistent with gps measurements are more likely at least along some segments of the fault not only do these rates support the view that the tibetan plateau deforms internally by slip on a distributed network of faults in the shallow brittle crust and hence behaves as a continuum at depth but the gradual decrease toward the east also shows that the altyn tagh fault does not separate two effectively rigid lithospheric plates correspondingly the relatively low slip rate and the eastward decrease in slip rate suggest that the altyn tagh fault does not transfer a significant portion of the convergence between india and asia into northeastward extrusion of the tibetan plateau thus large scale extrusion of crustal material in india s path into eurasia seems to be limited largely to the confines of the tibetan plateau

helium isotopes and tectonics in southern italy geodynamic evolution of southern italy can be understood within the framework of the mediterranean alpine system subduction of a plate along the sicily calabrian forearc under the tyrrhenian sea has been suggested by many geophysicists although it is not yet confirmed and remains somewhat controversial helium isotope ratios provide useful information on the geotectonic structure of the region the authors report here the sup 3 h sup 4 he ratios of terrestrial gas samples from southern italy the observed sup 3 he sup 4 he ratios are relatively high in the eolian volcanic arc region and low in the other areas dichotomous explanations are presented firstly volcanic arc forearc hypothesis suggests the subduction along the sicily calabrian forearc secondly horizontal transport hypothesis is described based on the relationship between the ratios and radial distance from the recent spreading basin in southern tyrrhenian sea

shear heating in continental strike slip shear zones model and field examples summary a two layer crust and upper mantle finite diverence steady state thermomechanical model of a long lived several million years lithospheric strike slip fault is presented and its predictions compared with field observations from various major fault zones in order to estimate the maximum amount of shear heating all mechanical energy is assumed to be dissipated in heat in ductile as well as in brittle layers deformation follows a friction law in the brittle layer s and a power flow law in the ductile one s variations of several independent parameters and their influence on the thermomechanical state of the fault zone and on shear heating are systematically explored shear heating is found to be more important in fault zones avecting an initially cold lithosphere and increases with slip rate friction coeycient and stivness of materials in extreme cases slip rate of 10 cm yr 1 stiv lithosphere shear heating could lead to temperature increases close to 590 c at the moho and 475 c at 20 km depth for more common cases shear heating leads to smaller temperature increases but can still explain high grade metamorphic conditions encountered in strike slip shear zones however modelled temperature conditions often fall short of those observed this could be due to heat transport by mechanisms more eycient than conduction common syntectonic emplacement of granitic melts in ductile strike slip shear zones can be explained by lower crust partial melting induced by shear heating in the upper mantle besides slip rate the possibility of such melting depends mostly on the upper mantle rheology and on the fertility of the lower crust for hard upper mantle and highly fertile lower crust partial melting could occur at rates of 1 cm yr 1 while in most cases it would result from the breakdown of micas for slip rates over 3 cm yr 1 as a result of shear heating partial melting of the upper mantle could occur in the presence of small amounts of fluids rise of magmas and or hot fluids in the shear zone will further enhance the temperature increase in shallower parts of the fault zone in nature shear heating would inevitably cause strain localization in the deeper parts of strike slip faults as is often observed in the field for crustal shear zones