modelling plastic deformation of peridotite with the self consistent theory theories for deformation of polycrystals have been substantially refined enabling us to model deformation of metals and minerals with considerable sophistication so far most modelling has been confined to single phase aggregates such as quartzite and limestone we present the first results for a polyphase aggregate peridotite consisting of 70 olivine and 30 enstatite the problem is approached with a viscoplastic self consistent theory satisfying stress equilibrium and strain compatibility for the average polycrystal and taking account of anisotropic neighbor interactions it is assumed that olivine deforms by 010 100 001 100 and 010 001 slip and enstatite deforms by 100 001 slip simulated textures for olivine and enstatite in peridotite resemble simulated textures in the pure phases indicating that for this system and for these volume fractions there is little influence of the different phases upon each other in our model the harder mineral enstatite deforms at a slower rate than olivine interaction between neighboring grains appears to be minimal which may be due to model assumptions predicted pole figures with olivine 010 axes and enstatite 100 axes aligning with the direction of shortening are in good agreement with preferred orientations in naturally and experimentally deformed peridotites

seismic anisotropy of the northeastern tibetan plateau from shear wave splitting analysis abstract we present shear wave splitting results obtained from the analysis of teleseismic sks skks and pks phases recorded by 70 permanent seismographic stations located in the northeastern tibetan plateau we identify a contrast in the splitting pattern complexity beneath different parts of ne tibet in the western and northern part anisotropy observations are well explained by a single layer of anisotropy with a fast anisotropic direction trending nww see or nw se in xining and its adjacent area the anisotropy shows strong azimuthal dependence of splitting parameters that can be modeled by two anisotropic layers the fast direction for the upper layer lies in the n75 95 e range which is consistent with the surface movement direction determined from gps and could be associated with middle to lower crustal flow the fast direction in the lower layer is in the n105 125 e range and similar to the direction observed in the western and northern part where only a single layer is required these nww see or nw se fast feature could be related to the current orogenesis induced from the india eurasia collision or flow in the asthenosphere related to the absolute motion of eurasia comparison between the anisotropy patterns expected from proposed models with our shear wave splitting observation suggests that no unique geodynamic model can reconcile all splitting measurements for such a complex region

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

anisotropy gradients from ql surface waves evidence for vertically coherent deformation in the tibet region abstract the india eurasia continental collision has not only caused the high uplift of the tibetan plateau but also created a broad diffuse deformation zone in central asia we relate the well constrained extent of crustal deformation in the tibet region with underlying mantle deformation by interpreting the quasi love ql surface wave scattering in the legacy data of tibet ql waves are waveform anomalies generated from cross mode coupling of earth s free oscillations mainly through azimuthal anisotropy over 50 events with ql observations are identified using recordings in tibet the predominant frequency content of the ql waves is near 10 mhz and reflects peak sensitivity of anisotropy at 150 km depth in the mantle assuming horizontally oriented symmetry axes by calculating the delay times between the ql waves and the main love waves we back project the scatterers to cluster in areas like se tibet sayan mountain and iran noticeably the spatial distribution of these scatterers borders the crustal deformation pattern quite well especially at the deformation limits of central asia this linkage suggests a vertically coherent boundary condition through the crust and the upper mantle for the india eurasia continental collision in central asia

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

anisotropy and crustal thickness of northern tibet new constraints for tectonic modelling in 1993 a seismological array was installed on a profile along the golmud lhasa highway from the qaidam basin to central tibet a travel time analysis for local and regional earthquakes was performed in order to better constrain the crustal structure obtained by teleseismic tomography more information about the crustal thickness and anisotropy comes from the analysis of moho converted p to sv waves the two methods supplement each other and confirm the existence of crust thinner than expected accross the northern edge of the tibetan plateau and qaidam p to sv splitting analysis yields a striking similarity between the fast polarization direction and the velocity field of present day deformation as inferred from kinematic modelling the two observations the thinning of the crust and the clockwise rotation of the fast polarization direction emphasize the leading role of eastward extrusion for the absorption of the large amount of crustal shortening occuring in central asia

models of seismic anisotropy in the deep continental lithosphere seismological observations sks wave polarizations systematic p residual variations azimuthal dependence of pn and surface wave velocities or a dispersion of surface waves are not consistent with isotropic if laterally heterogeneous upper mantle structure therefore an anisotropy should be considered as an a priori aspect of future large scale studies of mantle structure most studies of anisotropy however have assumed horizontal or vertical axes of symmetry but such orientations cannot explain bipolar patterns of spatial variations of p residuals which we have observed at many seismological stations on the basis of the petrophysical properties of real upper mantle rocks we consider anisotropy formed either by hexagonal or by orthorhombic aggregates composed of olivine orthopyroxene and clinopyroxene rotations of the aggregates about vertical and horizontal axes allow us to find the three dimensional orientations of symmetry axes that fit combinations of both p and s seismological observations in central europe and in western north america the orientations with plunging symmetry axes velocity extremes seem to be consistent across large spatially uniform tectonic units and change abruptly at important suture zones

mantle discontinuity structure beneath china mantle reflectivity profiles obtained from long period recordings of multiple scs reverberations from intermediate and deep focus earthquakes surrounding china reveal remarkably sharp images of china deep structure the tibetan plateau and fold belts to the north are underlain by a midlithospheric low velocity zone lvz evidenced by coincident g lid lvz boundary and l base of the lvz reflectors and a thickened lithosphere consistent with uniform thickening as the dominant mode of north south deformation the mantle beneath northeastern china mongolia and southeastern russia has a typically cratonic reflectivity signature with h hales and deep l lehmann discontinuities the sino korean paraplatform of eastern china is a region of active extensional tectonics as expressed by an oceanic reflectivity pattern at shallow depths a g discontinuity at 80 km but a more continental structure below the southern sea of japan korean peninsula yellow sea and bordering regions of the china mainland are underlain by a 5 4 impedance decrease near 330 km depth that we interpret as the upper limit of dense silicate melt atop the 410 km discontinuity produced at the subduction zones to the east regional inversion of the reflection coefficient of the 410 km discontinuity r 410 reveals a pronounced high beneath mongolia and north to northwestern china which is best explained by increased upper mantle olivine content signifying extensive basalt depletion in a tectosphere that extends below the 410 km discontinuity a low value of r 660 associated with topographic depression of the 660 km discontinuity beneath eastern asia is consistent with a horizontally inclined slab atop the phase transition marking the seismic discontinuity reflection defocusing is an alternative but requires 38 km of intermediate scale topography within the depressed region exceeding the amplitude of the depression by a factor of 1 5 2 either mechanism requires strong heterogeneity far west of seismically active slabs two lower mantle reflectors one beneath southern honshu kyushu and the sea of japan the other below shantung province china and the yellow sea correlate with horizontally elongated velocity anomalies in western pacific seismic tomography models we speculate that they may represent slabs stagnating below the 660 km discontinuity

seismic anisotropy and mantle creep in young orogens summary seismic anisotropy provides evidence for the physical state and tectonic evolution of the lithosphere we discuss the origin of anisotropy at various depths and relate it to tectonic stress geotherms and rheology the anisotropy of the uppermost mantle is controlled by the orthorhombic mineral olivine and may result from ductile deformation dynamic recrystallization or annealing anisotropy beneath young orogens has been measured for the seismic phase pn that propagates in the uppermost mantle this anisotropy is interpreted as being caused by deformation during the most recent thermotectonic event and thus provides information on the process of mountain building whereas tectonic stress and many structural features in the upper crust are usually orientated perpendicular to the structural axis of mountain belts pn anisotropy is aligned parallel to the structural axis we interpret this to indicate mountainparallel ductile i e creeping deformation in the uppermost mantle that is a consequence of mountain perpendicular compressive stresses the preferred orientation of the fast axes of some anisotropic minerals such as olivine is known to be in the creep direction a consequence of the anisotropy of strength and viscosity of orientated minerals in order to explain the anisotropy of the mantle beneath young orogens we extend the concept of crustal escape or extrusion tectonics to the uppermost mantle we present rheological model calculations to support this hypothesis mountain perpendicular horizontal stress determined in the upper crust and mountain parallel seismic anisotropy in the uppermost mantle require a zone of ductile decoupling in the middle or lower crust of young mountain belts examples for stress and mountain parallel pn anisotropy are given for tibet the alpine chains and young mountain ranges in the americas finally we suggest a simple model for initiating mountain parallel creep

evidence for mechanically coupled lithosphere in central asia and resulting implications the recent dramatic increase in seismic anisotropy and surface global positioning system gps data for central asia permits a comprehensive examination of the mantle s role in mountain building a joint analysis of 178 shear wave splitting and 2000 gps observations using a new technique reveals that the crust and lithospheric mantle deform coherently arguing for crust mantle mechanical coupling during deformation the observed spatial variations in anisotropy refl ect the large scale pattern of lithospheric deformation as well as a change in deformational style from simple shear on the tibetan plateau transitioning to pure shear in surrounding regions

generation of pyroclastic flows and surges by hot rock avalanches from the dome of mount st helens volcano usa several hot rock avalanches have occurred during the growth of the composite dome of mount st helens washington between 1980 and 1987 one of these occurred on 9 may 1986 and produced a fan shaped avalanche deposit of juvenile dacite debris together with a more extensive pyroclastic flow deposit laterally thinning deposits and abrasion and baking of wooden and plastic objects show that a hot ash cloud surge swept beyond the limits of the pyroclastic flow plumes that rose 2 3 km above the dome and vitric ash that fell downwind of the volcano were also effects of this event but no explosion occurred all the facies observed originated from a single avalanche erosion and melting of craterfloor snow by the hot debris caused debris flows in the crater and a small flood that carried juvenile and other clasts north of the crater a second broadly similar event occured in october 1986 larger events of this nature could present a significant volcanic hazard

plate interactions crustal deformation and magmatism along the eastern margins of the tibetan plateau abstract in order to investigate the deep structure and dynamics of the triangular colliding system of the eastern tibetan plateau tp as well as the yangtze platform and the qinling dabie fold belt a large number of p and s wave arrival times from both local and teleseismic earthquakes were used to generate high resolution tomographic images beneath the eastern margins of tp the results of this study show that the western margin of the minjiang fault is characterized by low velocity low v anomalies in sharp contrast to the high velocity high v anomalies that are derived from the eastern part this means that the minshan uplift zone is imaged as high v and acts as a barrier to the eastward escape crustal flow which leads to obvious segmentation of the longmenshan fault lmsf zone between velocity belts that are concatenated into the minjiang fault because high v anomalies are clearly imaged in this study to extended depths of ca 450 km we consider the yangtze platform containing the sichuan basin to comprise craton like lithosphere this study reveals the presence of a low v belt extending to a depth of ca 250 km between the sichuan basin and the ordos block which probably represents the migration pathway of upwelling asthenosphere materials from tibet at the same time low v zone with a downdip depth of ca 500 km accompanied by an approximately 400 km rectangular high v body is imaged under the tengchong volcano we interpret the high v body and the low v zone as evidence for the subducted indian plate and the volcanic magma source respectively the results of this study reveal a number of new features of structural heterogeneities relative to deep coupling between the tibet and the yangtze platform as well as the relationship between the sichuan basin and the ordos block and the original source of the tengchong volcano the results presented here enable us a better understanding of the plate interaction crustal deformation and magmatism of the tengchong volcano along the eastern margins of tp

the 2002 denali fault and 2001 kunlun fault earthquakes complex rupture processes of two large strike slip events we studied the source processes of two large continental earthquakes the 3 november 2002 denali fault earthquake and the 14 november 2001 kunlun fault earthquake associated with strike slip faulting along ancient sutures we inverted teleseismic p waveforms using a pulse stripping method for multiple time windows with different focal mechanisms and derived composite source models according to our results the 2002 denali fault earthquake began with initial thrusting m w 7 3 along a 40 km long segment of the north northwest dipping susitna glacier fault and later ruptured a 300 km long segment along the denali and totschunda faults with a right lateral strike slip mechanism m w 7 7 in contrast the 2001 kunlun fault earthquake nucleated near an extensional step over with a subevent pair consisting of 30 km long strike slip m w 6 9 event and 40 km long normal m w 6 8 faulting event and later ruptured a 350 km long segment along the kunlun fault with a left lateral strike slip mechanism m w 7 7 both earthquakes propagated primarily unilaterally to the east and released most of their energy along slip patches asperities far from the hypocenter locations we find that both the denali fault and kunlun fault earthquakes had high average rupture velocities of 3 2 km sec and 3 4 km sec respectively we also compared the source properties of these two earthquakes with other strike slip earthquakes for scaling purposes large strike slip earthquakes were classified as interplate oceanic intraplate or continental intraplate events by using this classification the denali fault and kunlun fault earthquakes have an interplate signature that suggests overall weak faulting

mantle deformation and tectonics constraints from seismic anisotropy in the western united states abstract we have examined shear wave splitting in teleseismic shear waves sks skks s from 15 stations in the western united states based on analysis of 123 records from 67 events the varied past and present tectonic styles in this region subduction transform faulting extension and stable domains are expected to cause a wide variety of anisotropic behavior and therefore make it an excellent natural laboratory fast polarization azimuths vary from e w to nearly n s and time delays t range from being barely detectable less than 0 6 s to 1 6 s most stations yielded consistent measurements independent of station event geometry the exceptions were stations situated very close to the san andreas fault which yielded well constrained but inconsistent splitting parameters these have been successfully modeled by two anisotropic layers with different horizontal symmetry axes the upper layer has a fast direction parallel to the fault and t of about 1 s the lower layer with t from 0 6 to 0 9 s is oriented e w near the san francisco bay area and ne sw in the mojave desert other measurements of e w fast are observed as far east as western nevada with large delay times of 1 3 1 5 s but disappear to the north stations in the northwestern basin and range have values of oriented at about 70 with delay times ranging from 0 7 to 1 2 s varies in other regions of the basin and range from 70 in eastern nevada to 20 in the transition zone between the colorado plateau and the basin and range with t of 1 0 s measurements are geographically coherent and in many cases changes correspond to geologic or tectonic boundaries the e w fast feature extends across several tectonic provinces but disappears near the southern boundary of the gorda plate and may therefore be related to the dynamics of plate interactions regional variations in the fast directions of anisotropy within the basin and range suggest that the present extension has not been strong enough to align the fast directions over a great depth extent fast polarization directions within the transition zone between the basin and range and colorado plateau are nearly parallel to the geologic and geophysical boundaries between the two regions and to other geophysical boundaries the size of the delay times suggests that in many of the areas studied either the mantle is almost twice as anisotropic as we estimate or the anisotropy and therefore mantle deformation extends into the sublithospheric mantle we used the two layer method to examine the effect of a postulated lower layer with fast direction parallel to the absolute motion of the north american plate and found the best estimate of contribution from a uniform layer is 0 1 s of t although a contribution of up to 0 7 s cannot be ruled out a value as low as 0 1 s suggests either that olivine is not strongly aligned in a horizontal direction in this layer or that the layer thickness is of the order of 10 km

global anisotropy in the upper mantle inferred from the regionalization of phase velocities a data set of 2600 paths for rayleigh waves and 2170 paths for love waves was selected in order to resolve azimuthal anisotropy of surface waves in the period range 70 250 s the epicenters were chosen so that the paths provide uniform spatial and azimuthal coverage of earth a synthetic seismogram is computed for each path and compared to the observed seismogram in order to infer the phase velocity along each path to process such a large data set a new tomographic technique is designed a good correlation is found with surface tectonics in the whole period range for love waves and at shorter periods for rayleigh waves the directions of maximum velocities are also well correlated with plate tectonics motions for the 2 azimuthal term of rayleigh waves when the power spectrum of heterogeneities is computed it is found that for love waves the even orders are usually larger than the odd orders for rayleigh waves the degrees 2 and 6 dominate only at long periods suggesting a deep origin for these components

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

seismic polarization anisotropy beneath the central tibetan plateau sks and skks shear waves recorded on the indepth iii seismic array deployed in central tibet during 1998 1999 have been analyzed for the direction and extent of seismic polarization anisotropy the 400 km long nnw trending array extended south to north from the central lhasa terrane across the karakoram jiali fault system and banggong nujiang suture to the central qiangtang terrane substantial splitting with delay times from 1 to 2 s and fast directions varying from e w to ne sw was observed for stations in the qiangtang terrane and northernmost lhasa terrane no detectable splitting was observed for stations located farther south in the central lhasa terrane the change in shear wave splitting characteristics occurs at 32 n approximately coincident with the transcurrent karakoram jiali fault system but 40 km south of the surface trace of the banggong nujiang suture this location is also near the southernmost edge of a region of high sn attenuation and low upper mantle velocities found in previous studies the transition between no measured splitting and strong anisotropy 2 2 s delay time is exceptionally sharp 15 km suggesting a large crustal contribution to the measured splitting the e w to ne sw fast directions are broadly similar to the fast directions observed farther east along the yadong golmud highway suggesting that no large scale change in anisotropic properties occurs in the east west direction however in detail fast directions and delay times vary over lateral distances of 100 km in both the n s and e w direction by as much as 40 and 0 5 1 s respectively the onset of measurable splitting at 32 n most likely marks the northern limit of the underthrusting indian lithosphere which is characterized by negligible polarization anisotropy taken in conjunction with decades of geophysical and geological observations in tibet the new anisotropy measurements are consistent with a model where hot and weak upper mantle beneath northern tibet is being squeezed and sheared between the advancing indian lithosphere to the south and the tsaidam and tarim lithospheres to the north and west resulting in eastward flow and possibly thickening and subsequent detachment due to gravitational instability in northern tibet crustal deformation clearly follows this large scale deformation pattern

formation and evolution of strike slip faults rifts and basins during the india asia collision an experimental approach the processes which have governed the formation and evolution of large tertiary strike slip faults during the penetration of india into eastern asia are investigated by means of plane strain indentation experiments on layered plasticine models the steady state deformation of plasticine obeys a power creep flow law e c t n the stress exponent n is between 6 and 9 at 25 c uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening a process observed for strain rates ranging from 3 5 10 5 to 3 6 10 3 s 1 fault or shear zones form after only 7 10 bulk strain subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine a series of nine plane strain indentation experiments shows the influence of boundary conditions as well as that of the internal structure of the plasticine model on the faulting sequence the ubiquity of strain softening in experimental deformation of a variety of rocks as well as the widespread occurrence of shear zones in nature suggest that long term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain the deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision induced strikeslip faulting in asia particularly to total offsets and rates of movements on the faults the experiments simulate the evolution of the western ends of the strike slip faults which have probably been analogous to trench fault fault triple junctions the experiments also illustrate mechanisms for the formation of extensional basins such as the south china sea north china basin and andaman sea near active continental margins the basins which appear to absorb terminal offsets along major strike slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts the existence of distinct phases of strike slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous non steady state deformation of the continental lithosphere

petrological constraints on seismic anisotropy the purpose of this paper is to investigate the correlations between anisotropic parameters for different orientations and mineralogies for realistic mineralogical and petrological models of the upper mantle such correlations make it possible to reduce the number of independent parameters required in seismic modelling of the upper mantle the variation with depth and with degree of random orientation is also investigated it is shown that the anisotropic combinations involved in an equivalent transversely isotropic medium azumithally averaged velocities with a vertical symmetry axis are strongly correlated but the combinations involved in azimuthal anisotropy are more weakly correlated comparison with available seismic data does not yet allow discrimination based on anisotropy between different petrological models it is shown that even if a proportion of random orientation is introduced it should be possible to achieve petrological discrimination from seismic data for some orientation configurations if the inverse problem is well posed i e takes account of the correlations between anisotropic parameters these correlations are incorporated in an inversion process to show that it is possible to explain the dataset used to derive the prem by alternative anisotropic models for the uppermost 220 km

central and eastern anatolian crustal deformation rate and velocity fields derived from gps and earthquake data abstract we present a new strain rate and associated kinematic model for the eastern and central parts of turkey in the east a quasi n s compressional tectonic regime dominates the deformation field and is partitioned through the two major structural elements of the region which are the conjugate dextral strike slip north anatolian fault zone nafz and the sinistral strike slip east anatolian fault zone eafz the observed surface deformation is similar to that inferred by anisotropy studies which sampled the region of the mantle closer to the crust i e the lithospheric mantle and the moho and is dependent on the presence or absence of a lithospheric mantle and of the level of coupling between it and the overlaying crust the areas of the central and eastern parts of turkey which are deforming at elevated rates are situated above areas with strong gradients in crustal thickness this seems to indicate that these transition zones situated between thinner and thicker crusts promote more deformation at the surface the regions that reveal elevated strain rate values are 1 the elazig bingol segment of the eafz 2 the region around the karliova triple junction including the yedisu segment and the varto fault 3 the section of the nafz that extends from the erzincan province up to the nafz ezinepazari fault junction and 4 sections of the tuz golu fault zone other regions like the adana basin a significant part of the central anatolian fault zone cafz the aksaray and the ankara provinces are deforming at smaller but still considerable rates and therefore should be considered as areas well capable of producing damaging earthquakes between m 6 and 7 this study also reveals that the central part of turkey is moving at a faster rate towards the west than the eastern part turkey and that the wedge region between the nafz and the eafz accounts for the majority of the counter clockwise rotation between the eastern and the central parts of turkey this change in movement rate and direction could be the cause of the extensional deformation and respective crustal thinning with the resulting upwelling of warmer upper mantle observed in tomographic studies for the region between the iskenderun bay and the cafz the partitioning of deformation into an extensional regime could be the cause of the relatively low levels of strain rate in the south west part of the eafz and the northern part of the dead sea fault zone finally using this new compilation of gps data for the central eastern part of turkey we obtained a new anatolia eurasia rotation pole situated at 2 01 w and 31 94 n with a rotation rate of 1 053 0 015 ma

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

depth of investigation in electromagnetic sounding methods the time or frequency at which the electromagnetic em response of a buried inhomogeneity can first be measured is determined by its depth of burial and the average conductivity of the overlying section it is relatively independent of the type of source or receiver and their separation the ability to make measurements at this time or frequency however depends on the sensitivity and accuracy of the instrumentation the signal strength and the ambient noise level these factors affect different em sounding systems in surprisingly different ways for the magnetotelluric mt method it is possible to detect a buried half space under about 1 5 skin depths of overburden the maximum depth of investigation is virtually unbounded because of high signal strengths at low frequencies transient electromagnetic tem soundings on the other hand have a limited depth of penetration but are less affected by static shift errors for tem a buried inhomogeneity can be detected under about one diffusion depth of overburden for conventional near zone sounding in which induced voltage is measured impulse response the depth of investigation is proportional to the 1 5 power of the source moment and ground resistivity by contrast if the receiver is a magnetometer step response system the depth of investigation is proportional to the 1 3 power of source moment and is no longer a function of resistivity magnetic field measurements may therefore be superior for exploration in conductive areas such as sedimentary basins far zone or long offset tem soundings are traditionally used for deep exploration the depth of investigation for a voltage receiver is proportional to the 1 4 power of source moment and resistivity and is inversely proportional to the source receiver separation magnetic field measurements are difficult to make at long offsets because instrumental accuracy limits the measurement of the very slow decay of the magnetic field frequency domain controlled source systems are ideally suited for sounding at the very shallow depths needed for engineering archaeological and groundwater applications because of the relative ease of extending the measurements to arbitrarily high frequencies and also because geometric soundings can be made at low induction numbers

quaternary and active faulting observed on the 1985 academia sinica royal society geotraverse of tibet active and recent faulting along the main north south road in tibet is dominated by normal faulting occurring on northerly trending planes and by strike slip faulting both of which reflect an east west extension of the plateau normal faulting is prevalent in the southern half of the plateau but we saw no evidence for any major graben in the northern half strike slip faulting on roughly easterly trending structures is m ore prevalent in the northern half but conjugate faulting with right lateral slip on northwesterly trending planes and left lateral slip on northeasterly trending planes is common in the southern half in two areas we also observed components of thrust faulting apparently in association with young strikeslip faulting our most important results are bounds on the rates of slip on the two main strands of the kunlun strike slip fault system which trends east w est through the kunlun range ground moraine containing boulders of pyroxenite is separated by 30 km from the nearest outcrop of such rock implying that amount of displacement in the last 1 5 to 3 m a therefore the average rate of slip during the quaternary period has been between 10 and 20 mm a with a likely value of 13 mm a abundant fresh tension cracks and mole tracks imply continued slip on the main strand the xidatan tuosuohu maqu fault and the likely occurrence of a major earthquake in the last few hundred years consistent offsets of gullies and dry stream channels of about 10 m may reflect slip of that amount during such an earthquake and possible multiple offsets at one site suggest that slip may occur by large displacements of 10 m during infrequent great earthquakes along the other strand the kunlun pass fault offsets of roughly 50 to 150 m of apparently post glacial valleys and of one glacier and its terminal moraine suggest a holocene rate of slip between 5 and 20 mm a and most likely about 10 mm a on this fault these rapid rates of displacement imply that tibet is being extruded rapidly eastward at a rate com parable to the rate at which india is penetrating into eurasia and therefore that at present a substantial fraction of this penetration is being absorbed by the eastward extrusion of tibet

a quantitative evaluation of the contribution of crustal rocks to the shear wave splitting of teleseismic sks waves abstract the seismic elastic properties of the crust have been modelled using hypothetical polycrystals with typical crustal compositions and commonly measured petrofabrics this modelling allows us to better evaluate and quantify the crustal contribution to splitting of teleseismic sks waves mafic rocks such as anorthosite or pyroxene bearing gabbros have complex s waves properties i e with small shear wave anisotropies

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

megasource time domain electromagnetic sounding methods the colorado school of mines time domain electromagnetic em sounding system makes use of a grounded length of cable powered with high amplitude current square waves to generate an em field for probing the earth the vertical component of magnetic induction is detected at a sounding site located at a relatively large distance compared to the desired depth of investigation with a source moment of a million ampere meters or greater offset distances of several tens of kilometers can be achieved easily providing depths of investigation of up to 10 km the recorded induction field versus time curves are routinely interpreted by comparison with computer generated theoretical curves for a layered earth megasource em surveys have been carried out at the geysers in northern california and near yakima in central washington providing apparently meaningful information on the electrical structure in these areas at depths as great as 10 km

a study of azimuthal p residuals and shear wave splitting across the kunlun range northern tibetan plateau abstract the kunlun fault system is an east west transcurrent fault bordering the tibetan plateau that has experienced left lateral motion since 3 ma a linear array of one component and three component seismological stations was deployed across the kunlun range for 4 months p travel time residuals define three basic units along the profile the north kunlun unit the south kunlun unit and the bayan har unit p residuals are a function of azimuth p residuals are fast in the southern stations for southern azimuths and in the northern stations for northern azimuths measurements of shear wave splitting of the s scs and sks phases have been performed on deep events delay times and fast polarization directions have been computed by finding the maximum of the cross correlation between the two horizontal components rotated at different angles delay times from 0 40 s to 2 52 s remain in the range of those found by mcnamara et al 1994 j geophys res 99 13655 13665 at two sites along the same profile p residual travel times and the splitting of s waves are discussed in relation to models of the geological evolution of this region the northern tibetan plateau appears as a juxtaposition of different terranes the azimuthal p residual pattern implies that p wave anisotropy is about 3 in the lithosphere on both sides of the kunlun fault and that fast velocities are oriented outwards from the kunlun fault plane s wave splitting is observed far from the fault and implies that the lithosphere beneath northern tibet has undergone an important deformation

confrontation of mantle seismic anisotropy with two extreme models of strain in central asia although most authors agree that convergence is accommodated by a large shortening in asia two radically different modes of shortening have been put forward in one approach the mechanism of shortening is considered to be homogeneous producing diffuse deformation in the crust and the mantle in the second one motions are localized along faults which are supposed to extend into deep lithospheric shear zones inducing a heterogeneous deformation pattern seismic anisotropy is one observable manifestation of large scale deformation large scale seismic anisotropy field are inferred from the two extreme competing models 3d mapping of anisotropy inferred from surface waves reveals a better coherence on average with filtered heterogeneous strain models between 100 and 200 km depths it suggests a strong coupling between surface and deep motions down to at least 200 km at larger depths the coherence with homogeneous strain models suggests that mantle flow becomes more homogeneous

correlated crust and mantle strain fields in tibet vertical to subvertical planes of shear within the active crustal deformation field in tibet align with fast directions of shear wave polarization this observation suggests that the present day velocity gradient tensor field within the crust of tibet correlates with the lithospheric mantle velocity gradient tensor field beneath tibet this inference requires the following 1 the 100 axes of olivine are aligned within established zones of lithospheric mantle fabric or anisotropy built up through finite strain 2 present day shear in the mantle lithosphere is occurring within these established zones of fabric parallel to the direction of olivine 100 axis alignment 3 the zones of mantle lithospheric fabric or zones of mantle shear are aligned with zones of crustal shear faults the correlation of crustal and mantle strain fields most simply results from the fact that both crust and mantle lithosphere are under the influence of similar velocity boundary conditions furthermore the observations confirm the distributed nature of lithospheric mantle deformation beneath east central tibet and suggest that left lateral shear has been the dominant component of finite shear there

deep long offset transient electromagnetic surveys for crustal studies in the u s a abstract controlled source electromagnetic methods are often thought of as being used for detailed studies of earth conductivity at shallow depths such as is appropriate for minerals exploration however for the past quarter of a century the colorado school of mines has placed considerable effort into the development of similar techniques for exploration deeper into the earth s crust the main difference between these techniques and shallow methods is primarily that the source moment is generally one to two orders of magnitude greater for the former these deep electromagnetic sounding studies allow for a considerable flexibility in survey design so that optimum results may be obtained in diverse geological settings four time domain electromagnetic sounding surveys in different geological settings across continental u s a are reviewed the surveys on the u s west coast in the pacific northwest new york state and southern colorado provide insight into not only the diversity of the technique but also its ability to gather knowledge of deep electrical structures

interpretation of sks waves using samples from the subcontinental lithosphere abstract the seismic properties of five south african kimberlite nodules have been calculated from petrofabric measurements these garnet lherzolite and harzburgite composition nodules 56 80 olivine are known from previous studies to have originated at depths of 120 170 km in the subcontinental lithosphere their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature 900 1050 c and pressure 3 0 3 5 gpa conditions the average of the five samples has a maximum s wave anisotropy v s of 3 7 which is sufficiently high to explain previously reported teleseismic sks delay times t in continental shield areas of up to 1 7 s the biggest delay time is observed when the foliation is vertical the lineation horizontal and the fastest s wave polarized parallel to the lineation i e an orientation for transcurrent motion delay times of over 1 s can only be generated by this orientation a strong mantle crust mechanical coupling is suggested for such situations the potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine the most abundant phase 70 in the upper mantle the v p and v s seismic anisotropy increases with fabric strength and finite strain recrystallization tends to reduce fabric strength and seismic anisotropy resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50 60 strain or 8 9 v s for pure olivine aggregates a survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum s wave anisotropy for the olivine component of 9 or about 8 for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account the ophiolite samples are twice as anisotropic as the kimberlite nodules if an average anisotropy value is representative of the lithosphere then the sks delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal the magnitude of s wave anisotropy is less sensitive than v p to variations in olivine volume fraction in range 50 100 and to deformation or fabric intensity further suggesting that s wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness for smaller delay times there is some trade off between structural orientation and thickness

differential rotation between lithosphere and mantle a consequence of lateral mantle viscosity variations the description of plate motions in the so called hotspot reference frame introduces a global rotation of the lithosphere with respect to the mantle this rotation called toroidal field of degree 1 is roughly westward it reaches an amplitude of about 2 cm yr and has been consistently found in the different generation of plate tectonic models various authors have tried to relate this observation to the deceleration of the earth s rotation to polar wander or to tidal drag however these different physical mechanisms cannot explain the requested amplitude in this paper we compare the values of this rotation vector using different relative plate motion models expressed in the hotspot reference frame in a model earth with lateral viscosity variations a differential rotation is predicted the observed net lithospheric rotation is consistent with the dynamics of a model earth where the asthenospheric viscosity below the oceans is at least one order of magnitude lower than underneath the continents this relative westward drift of the lithosphere may account for the significant structural differences between east or west dipping subduction zones

long offset transient em sounding north of the rhine ruhr coal district germany1 during the past two decades electromagnetic methods have been increasingly applied to problems in hydrocarbon exploration it has been shown that em methods achieve satisfactory results which can be supplementary to results of the seismic method we extend the application of the long offset transient electromagnetic sounding lotem to exploration problems in the coal mining industry a survey was conducted north of the rhine ruhr coal district using a grounded dipole source and receivers laid out along several profile lines giving areal coverage the strong noise from cultural as well as natural electromagnetic sources was reduced using prestack data processing techniques resulting in reliable data with a good signal to noise ratio the interpretation is self consistent and matches the well logs and the geological information of the area quite well the high station density permitted the mapping of the top of the carboniferous formation to a depth of between 800 and 1000 m below surface

a theory for plastic deformation and textural evolution of olivine polycrystals seismic anisotropy in the upper mantle is due primarily to preferred orientation of olivine crystals induced by progressive deformation in order to understand better the origin of seismic anisotropy we present a simple theory for plastic deformation and textural evolution of olivine polycrystals each crystal in the aggregate is assumed to deform by intracrystalline slip on three major slip systems whose hardnesses and stress exponents are known from experiments the evolving grain orientation distribution in the aggregate is calculated by minimizing the difference between the local crystal deformation and the global aggregate deformation subject to the constraint of global strain compatibility the axial compression texture predicted by our model agrees well to first order with that determined experimentally by nicolas et al 1973 although there are significant discrepancies in the details our results suggest that the orientation texture developed during progressive plane strain deformation is a nearly unique function of the finite strain such that the crystallographic axes 100 010 and 001 are concentrated around the finite strain axes a c and b respectively a b c this result may allow the state of finite strain at depth to be estimated from observations of seismic anisotropy

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

a 3 d kinematic model of fabric development in polycrystalline aggregates comparisons with experimental and natural examples abstract a 3 d generalization of the approach previously developed by etchecopar for the simulation of fabrics in polycrystalline aggregates is presented the model is based on a geometric minimization of gaps overlaps and boundary displacements between deformed neighbouring cells each cell may deform using a small number

the magnitude symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites summary seismic anisotropy within the upper mantle originates from the preferred orientation of highly anisotropic single crystals the symmetry and magnitude of anisotropy depend upon 1 the volume percentages of the minerals constituting the upper mantle 2 the degree and symmetry of preferred orientation of each mineral and 3 the alignment of the minerals crystallographic axes relative to one another the nature of upper mantle anisotropy can be examined by studying mineral orientations within ultramafic rocks which were once part of the mantle petrofabric data for olivine and pyroxene have been used to obtain velocity anisotropy patterns over large regions of ultramafic rocks from the samail ophiolite oman the troodos ophiolite cyprus the bay of islands ophiolife newfoundland the twin sisters ultramafic washington usa the dun mountain ophiolite new zealand the red hills ophiolite new zealand and the red mountain ophiolite new zealand the compressional wave anisotropy calculated for these massifs ranges from 3 to 8 per cent in excellent agreement with observed seismic anisotropy in the upper continental and oceanic mantle the symmetry varies from orthorhombic to axial with the axial symmetry axis corresponding to the olivine a axes maxima and subparallel to spreading directions in oceanic upper mantle pyroxene a b and c axes maxima generally parallel olivine b c and a axes respectively and anisotropy decreases with increasing pyroxene content shear wave splitting is predicted for all propagation directions within the upper mantle symmetry is also orthorhombic or axial with the minimum difference in velocity between the two shear waves parallel to the maximum compressional wave velocity

p wave anisotropic tomography of the alps the first tomographic images of p wave azimuthal and radial anisotropies in the crust and upper mantle beneath the alps are determined by joint inversions of arrival time data of local earthquakes and teleseismic events our results show the south dipping european plate with a high velocity high v anomaly beneath the western central alps and the north dipping adriatic plate with a high v anomaly beneath the eastern alps indicating that the subduction polarity changes along the strike of the alps the p wave azimuthal anisotropy is characterized by mountain chain parallel fast velocity directions fvds in the western central alps and ne sw fvds in the eastern alps which may be caused by mantle flow induced by the slab subductions our results reveal a negative radial anisotropy i e vph vpv in the low velocity mantle wedge which may reflect the subvertical plate subduction and its induced mantle flow the results of anisotropic tomography provide important new information on the complex mantle structure and dynamics of the alps and adjacent regions

anisotropic convection with implications for the upper mantle on the basis of polycrystalline theory describing the plasticity in olivine and enstatite the flow in a convection cell has been simulated using a finite element formulation the spatial variations in anisotropic properties are computed from the textures that evolve with the flow a kinematically constrained equilibrium based assumption is used to partition the macroscopic deformation among crystals within an aggregate we model the convection for one specific cell geometry and two sets of boundary conditions a complete map of textures throughout the cell is obtained the textured convection cell is structurally very heterogeneous and textures along streamlines do not correlate with the finite strain the results of the simulations indicate that during up welling a strong texture develops rapidly it convects during spreading and is attenuated during subduction results are compared with features of the upper mantle in our predictions the pattern of preferred orientation during spreading is inclined to the flow coordinates due to deformation by simple shear this is contrary to hess 1964 intuition that 001 slip planes of olivine orient themselves parallel to the flow planes yet the pattern is consistent with natural fabric data significant differences are observed as a function of depth within the cell the variations in the p wave velocities in this textured model mantle are analyzed and correspond well with observed seismic data

significant and vertically coherent seismic anisotropy beneath eastern tibet 1 shear wave splitting measurements using teleseismic sks and skks phases recorded by the indepth iv arrays has revealed a strong upper mantle anisotropic fabric in northeastern tibet with large delay times of up to 2 2 s suggesting that anisotropy exists in both the lithospheric and asthenospheric mantle the coherence among fast polarization orientations of split core phases and the left lateral slip on eastern striking southern striking faults in eastern tibet and the surface deformation fields calculated from both gps observations and quaternary fault slip rates support the idea that left lateral shear strain is the predominant cause of the orientation of the upper mantle petrofabrics we suggest the bending of the eastern himalayan syntaxis around the foundering burma andaman sumatra slab also contributes to the observed seismic anisotropy in the eastern himalayan syntaxis region two plausible competing processes are proposed for the flow of asthenosphere in eastern tibet in the first the deforming lithosphere glides over the passive asthenosphere inducing flow in the asthenospheric mantle in the second the asthenosphere beneath northeastern tibet is squeezed between the advancing indian continental lithosphere and the thick tarim and qaidam lithospheric blocks to the north a westward retreat of the burma slab from eurasia may induce flow that is toroidal and located exclusively around the northern edge of the slab the rotation of fast orientations for stations in the eastern himalayan syntaxis region are consistent with the toroidal flow pattern as well as the rotational deformation of the overlying lithosphere

crust and upper mantle tomography in tibet using surface waves we have obtained maps of lateral variations of structure in the crust and lithosphere in tibet and surrounding regions by inversion of single station measurements of fundamental mode rayleigh wave phase velocities in the period range 25 100 sec for earthquakes located in central asia and observed at teleseismic digital stations in eurasia phase velocity maps are obtained using a tomographic method without a priori constraints on region boundaries the lateral variations of s velocity reveal a symmetric pattern on each side of crustal low velocity zone centered on the chang tang region north central tibet this zone is bounded by rather steep velocity gradients to the south and east about 250 and 400 km north and west of the frontal thrusts of the himalayas and lungmen shan respectively the northern boundary of the zone crosses the northern limit of the tibet plateau altyn tagh and kunlun extending into the southwest tarim and qaidam basins the low crustal velocities are consistent with a thick hot crust in the central part of tibet consistent with quaternary volcanism in chang tang the steep gradients towards higher velocities in the south and in the north are compatible with the documented continental subduction in the south and suggest that a similar situation might exist in the north

variation of shear wave residuals and splitting parameters from array observations in southern tibet a tight array of seismographs spanning a 500 km traverse of southern tibet resolved anisotropy from sks with a spatial variation of its direction and an increase northward of the splitting delay as well as of its first arrival residual both waves split by velocity anisotropy are slow relative to p and their waveform analysis may be interpreted to suggest attenuation anisotropy the array here provides examples of residuals and splitting of other s waves which do not tightly conform to the anisotropy assumed in the simplest model of olivine of transverse isotropy with horizontal symmetry axis s waves are also split with parameters which vary along the array and hence are relevant to near receiver structure like those of sks their splitting delay for non vertical incidence and polarization appears larger than that for sks residuals of s wave first arrivals and splitting delays increase less northwards for s than sks anisotropy with a slow vertical axis may account for these observations its origin may be related to horizontal shear or flow in low velocity layers

shear wave anisotropy beneath the tibetan plateau eleven broadband digital seismic stations were deployed across the central tibetan plateau in the first extensive passive source experiment attempted within the tibetan plateau one year of recording resulted in 186 event station pairs which we analyze to determine the characteristics of shear wave splitting in the upper mantle beneath the array measurements of the fast polarization direction and delay time t for sks and direct s arrivals reveal systematic variations along the north south oriented array in the north central region of the plateau very large delay times are observed at three stations the largest of which is budo with of t 2 4 s however at tunl which is off the northern edge of the plateau and 110 km from budo and at sites in the south central plateau t decreases by nearly a factor of 3 we also observe systematic rotation of from about 45 ne to 90 e w from south to north along the array a previously identified zone of inefficient sn propagation correlates well with our region of large t observations the large delay times suggest that a relateively high number of anisotropic crystals are preferentially alligned within the mantle lid beneath the north central portion of the tibetan plateau in most cases fast polarization directions appear to be parallel to surface geologic features suggesting as much as 200 km of the upper mantle has been involved in the collisional deformation that has produced the tibetan plateau

comparison of shear wave splitting and finite strain from the india asia collision zone we investigate whether observations of shear wave splitting in asia may be related to distributed strain of the lithosphere or to strain caused by motion of the asian lithosphere over the deep mantle the hs2 nuvel angular velocity for eurasia and that of minster and jordan 1978 predict absolute plate motions that are a factor of at least 4 slower than the nne relative motion of india with respect to stable eurasia shear of the upper mantle beneath the south asian lithosphere is therefore likely to be dominated by the velocity field caused by the internal deformation of asia rather than by plate motion if that shear causes sks splitting fast polarization directions in tectonic asia should be aligned approximately in the direction of the motion of asian lithosphere relative to the deep mantle which for southern asia is nne in general however fast polarization directions are not aligned with this direction and in some cases are nearly orthogonal to it we calculate finite deformation of the asian lithosphere in response to its indentation by india and from the deformation predict the orientations of fast polarization directions aligned with the principal axes of elongation these predictions agree with the gross features of the anisotropy in tibet and the tien shan orientations of anisotropy in the baikal region of siberia are consistent with the orientations of the local principal axes of active strain orientations of anisotropy in mongolia however are inconsistent both with the azimuth of the velocity of this region with respect to the deeper mantle and with the orientation of the calculated maximum elongation of the region we conclude that sks splitting observations from asia agree better with those predicted by calculations of internal lithospheric deformation due to indentation by india than with strain predicted from motion of the lithosphere over the deeper mantle these results place constraints on the depth range where shear wave splitting occurs which we propose lies predominantly within the lithospheric mantle

late cenozoic right lateral strike slip faulting in southern tibet field observations collected during the sino french himalaya tibet cooperation project 1980 1982 show that right lateral faulting between southern and northern tibet roughly along the chord joining the eastern and western syntaxes of the himalayan arc is a major component of the active tectonics north of the himalayas just north of the well developed rifts of southern tibet we studied three right lateral 100 200 km long strike slip faults forming an en echelon array along that chord evidence for earthquake surface breaks was found on each of these faults for the m 8 beng co or damxung strike slip earthquake of november 18 1951 rupture parameters could be constrained to be 8 2 m and l 90 km offset glacial and postglacial morphological markers 105 years and 104 years b p imply displacement rates of the order of 1 2 cm yr on the three faults the en echelon fault zone appears to extend westward 2000 km across the plateau to the karakorum fault and is thus referred to as the karakorum jiali fault zone kjfz fast rates of slip on it and on faults along the northern rim of tibet appear to contrast with the slower rates of deformations in the interior of the plateau our estimates of slip rates on the strike slip and normal faults of southeastern tibet suggest that eastward extrusion of north tibet between the right lateral kjfz and the left lateral altyn tagh and kun lun faults absorbs at least 30 of the present convergence between india and asia we propose an instantaneous velocity model of the active tectonics of asia compatible with such faulting rates and geometries evidence for tertiary right lateral strike slip movements in southern tibet and around the eastern syntaxis of the himalayas is also found in the structural record offsets of various large scale geological features imply that these movements have been large

no net rotation model of current plate velocities incorporating plate motion model nuvel 1 we present nnr nuvel1 a model of plate velocities relative to the unique reference frame defined by requiring no net rotation of the lithosphere while constraining relative plate velocities to equal those in global plate motion model nuvel 1 demets et al 1990 differences between nnr nuvel1 and no net rotation plate motion model am0 2 minster and jordan 1978 are as large as 15 mm yr in nnr nuvel1 the pacific plate rotates in a right handed sense relative to the no net rotation reference frame at 0 67 m y about 63 s 107 e this rotation nearly parallels but is 32 slower than the pacific hotspot euler vector of hs2 nuvel1 which is a global model of plate hotspot velocities constrained to consistency with nuvel 1 gripp and gordon 1990 at hawaii the pacific plate moves relative to the no net rotation reference frame at 70 mm yr which is 25 mm yr slower than the pacific plate moves relative to the hotspots differences between nnr nuvel1 and hs2 nuvel1 are described completely by a right handed rotation of 0 33 m y about 49 s 65 e the 99 confidence ellipsoids for plate hotspot motion in model hs2 nuvel1 exclude the corresponding euler vectors from model nnr nuvell thus the no net rotation reference frame differs significantly from the hotspot reference frame if the difference between reference frames is caused by motion of the hotspots relative to a mean mantle reference frame then hotspots beneath the pacific plate move with coherent motion towards the east southeast alternatively the difference between reference frames may show that the uniform drag no net torque reference frame which is kinematically equivalent to the no net rotation reference frame is based on a dynamically incorrect premise possible exceptions to the assumption of uniform drag include a net torque on the lithosphere due to attached subducting slabs and greater resistance to plate motion beneath continents than beneath oceans

resistivity structure of unzen volcano derived from time domain electromagnetic tdem survey abstract time domain electromagnetic tdem surveys were carried out around unzen volcano shimabara peninsula south west japan in 2001 and 2002 in the eastern part of the peninsula the surveys were a completion of the previously interpreted tdem data collected from surveys in 1995 in the western part of the peninsula interpretation of the recorded transient magnetic fields resulted in a general feature of the resistivity structure which are composed of three main layers a resistive surface layer a conductive layer below 10 m from about sea level to 2 5 km below the surface and a third resistive layer the conductive layer is considered to be a complex of water saturated and altered layers which spreads widely beneath shimabara peninsula the spatial conductance distribution shows a w e trending pattern of high conductance from tachibana bay to the summit of unzen fugen dake this evidence indicates that volcanic gas was supplied to shallow layer in the western part of the peninsula which is in accordance with the path of magma migration derived from seismic and deformation studies another high conductance zone was also found near mayu yama in the eastern part of shimabara peninsula indicating magma degassing process due to the possibility of the presence of a magma system beneath it

shear wave splitting beneath the central tien shan and tectonic implications 1 shear wave splitting analyses were performed at 30 seismic stations in the central tien shan and its vicinity fast orientation at most of the stations on the tien shan range is ene wsw parallel to the strike of the tien shan it largely reflects a coherent deformation of the lithosphere in response to the ns shortening anisotropy with nne ssw fast axes is observed within adjacent basins around the issyk kul and near the northwestern tarim where the lithosphere is presumably strong less deformed the nne ssw orientation which coincides with the surface velocity of the central tien shan can be best explained by shear between the lithosphere and asthenosphere on a regional scale rather than small scale convection as proposed in previous studies the dextral talasso fergana fault seems to have little effect on anisotropy at its nearby stations suggesting this fault is probably confined to the shallow part of the lithosphere

seismic anisotropy and large scale deformation of the eastern alps abstract mountain chains at the earth s surface result from deformation processes within the earth such deformation processes can be observed by seismic anisotropy via the preferred alignment of elastically anisotropic minerals the alps show complex deformation at the earth s surface in contrast we show here that observations of seismic anisotropy suggest a relatively simple pattern of internal deformation together with earlier observations from the western alps the sks shear wave splitting observations presented here show one of the clearest examples yet of mountain chain parallel fast orientations worldwide with a simple pattern nearly parallel to the trend of the mountain chain in the eastern alps the fast orientations do not connect with neighboring mountain chains neither the present day carpathians nor the present day dinarides in that region the lithosphere is thin and the observed anisotropy thus resides within the asthenosphere the deformation is consistent with the eastward extrusion toward the pannonian basin that was previously suggested based on seismicity and surface geology

electromagnetic sounding in the columbia basin yakima washington a controlled source electromagnetic csem survey was conducted near yakima washington to map the thickness and resistivity of a thick volcanic sequence overlying a sedimentary section and to infer structure in the sediments the survey was conducted with a frequency domain system employing loop transmitters 400 500 m on a side and three component squid magnetometer receivers separated from the loop by 1 to 5 km data collected along a 30 km profile orthogonal to regional strike were interpreted initially with 1 d layered models which were then pieced together to make a geoelectric section induction logs in a 5000 m exploration hole at one end of the profile agree very well with the csem soundings made around the hole the geoelectric section reveals a smoothly varying thickness of volcanics with a pronounced anticlinal structure approximately concordant with a surface topographic ridge to assess the validity of inferring lateral structure from 1 d interpretations we made scale models of an anticlinal structure and of a surface inhomogeneity and conducted csem measurements over the scale models layered model inversions of these data show that the anticlinal structure and its location are very well determined by 1 d inversion but its height and width are not accurately determined csem sounding over the surface inhomogeneity model shows that this feature does not significantly degrade the interpretation of a deep target layer in this setting a geoelectric section made up from 1 d interpretations provides good qualitative measures of subsurface structure and also provides excellent starting models for detailed 2 d or 3 d modeling

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

long offset transient electromagnetic lotem depth soundings applied to crustal studies in the black forest and swabian alb federal republic of germany the long offset transient electromagnetics lotem sounding method is a valuable complementary technique for deep seismic exploration as applied to earth crustal investigations during 1986 two lotem surveys were conducted to augment completed seismic investigations for crustal studies in southern west germany east of the rhine graben our studies were motivated by the geoscientific activities within the framework of the german continental deep drilling ktb project the survey objectives were 1 to determine the applicability of the lotem method to crustal geophysics 2 to investigate the correlation of resistivity anomalies with velocity anomalies and 3 to confirm the method and interpretation in an area of known geology with existing geophysical data the black forest and the urach geothermal area survey results exhibit conductive features in the upper 10 km of the crust which were not previously defined by other geophysical data importantly the results demonstrate a very strong correlation

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

field study of a highly active fault zone the xianshuihe fault of southwestern china the xianshuihe fault of western sichuan province china is one of the world s most active faults having produced 4 earthquakes during this century of magnitude 7 along a 350 km length of the fault at least 8 such events have occurred since 1725 in the more limited 150 km long segment including luhuo and daofu major earthquakes in 1904 1923 1973 and 1981 m 7 7 7 6 6 9 were associated with overlapping surficial fault ruptures and with individual left lateral displacements as large as 3 6 m field studies indicate that this high degree of activity is typical of the fault s longer term history the holocene left lateral slip rate on the north western segment of the fault has been 15 5 mm yr decreasing to about 5 mm yr on its southeastern segment based on radiometrically dated offset stream channel and terrace deposits and on offset glacial moraines physiographic features of active faulting are fully as diagrammatic as those of california s san andreas fault mainly because of high altitude preservation and the absence of cultural modification on this eastern margin of the tibetan plateau detailed en echelon tensional and pushup features resulting from surface ruptures in 1973 1955 1923 and 1893 can still be recognized today and new data have been collected bearing on the offsets and fault rupture lengths during these and other events the locations and magnitudes of historic earthquakes suggest that the characteristic earthquake model may apply to the xianshuihe fault obvious geometric segmentation of the fault has controlled the initiation and termination of ruptures in some events whereas segmentation control for others remains obscure based on the historic record repeat times estimated from slip rates and current seismic gaps two segments are particularly likely sites for m 7 earthquakes in the near future the 65 km long segment between daofu and qianning and the 135 km long segment bracketing kangding continuing creep has been documented along some segments of the fault and this together with the high degree of activity and other unique attributes makes the xianshuihe fault one of the most promising sites in the world for earthquake prediction and hazard evaluation studies

teleseismic shear wave splitting and lithospheric anisotropy beneath and across the altyn tagh fault sixteen 3 component 5 sec stations along a profile crossing the altyn tagh fault atf at 90 e were used to measure shear wave splitting beneath tibet s northern edge splitting delay times t of 0 3 to 1 3 sec show a marked increase in a narrow zone centered on the fault implying anisotropy in the lithospheric mantle fast polarization directions fpd swing counterclockwise from a mean azimuth of 110 e in the qaidam basin south of the fault to a mean azimuth of 60 e in the altyn mountains this swing occurs in less than 40 km both the t increase and fpd swing are consistent with ductile sinistral shear along the 40 km wide low p wave velocity anomaly 6 8 imaged to a depth of 140 km beneath the fault by teleseismic tomography this confirms that the atf guides the northeastwards extrusion of the tibet qaidam s lithosphere relative to the tarim s

s p wave travel time residuals and lateral inhomogeneity in the mantle beneath tibet and the himalaya the average s wave travel time residuals from 11 earthquakes each in tibet and the himalaya imply that there are marked lateral variations in structure beneath tibet and the himalaya average s p wave residuals measured with respect to the jeffreys bullen j b tables for each of 11 earthquakes in the himalaya are less than 1 s average j b s p residuals from 10 of 11 earthquakes in tibet however are greater than 1 s even when corrected for the large crustal thickness of tibet the largest values 2 5 3 2 3 8 4 5 and 4 9 s are for five events in central and northern tibet and they imply that the average velocities in the crust and upper mantle of these portions of tibet are 4 10 lower than those beneath the himalaya therefore it is unlikely that a shield structure underlies these parts of tibet unless these s p residuals are due to variations in structure deeper than 250 km s p residuals from two other events each in western and eastern tibet are 1 2 s late and hence suggest that velocities beneath these areas are also somewhat lower than beneath shields the mean s wave velocity seems not only to be different beneath tibet and the himalaya but also seems to increase from a minimum beneath northern tibet to values close to those of the average earth toward the east south and west appendix a is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 010 2 50 payment must accompany order

inversion of long offset tem soundings near the borehole munsterland 1 germany and comparison with mt measurements summary data from a long offset transient electromagnetic lotem survey in northwest germany around the borehole munsterland 1 are interpreted using 1 d inversion special emphasis is given to the fact that the best fitting model is not necessarily the most realistic one we therefore investigate different inversion algorithms in order to obtain a more geologically meaningful result the 1 d inversion results using a second order marquardt algorithm show a three layer structure with a conductor embedded between two more resistive layers this is in good agreement with the known lithology and the well logs profile inversion uses the result from one site as the starting model for the next the application of the technique yields a smoother section than the individual inversions and corresponds more closely to the presumably layered structure in the area if desired using a parameter weighting scheme makes it possible to force consistency at the cost of a larger misfit yielding a smooth section even in a part of the area where the data are influenced by a 3 d anomaly occam inversion is a smoothness constrained technique which is independent of the starting model the result of the occam inversion shows the same features as the marquardt inversion including effects which are attributed to 3 d distortions in the data joint inversions of the lotem data were carried out with magnetotelluric data from the same area in that case the lotem electric field is recognized as crucial for the resolution of the resistive layer in the section

right lateral shear and rotation as the explanation for strike slip faulting in eastern tibet the convergence of two continents can be accommodated by crustal thickening and by lateral transport of crust out of the path of the converging continents1 3 the principal features suggesting that lateral transport is important are major strike slip faults striking roughly orthogonally to the orientation of convergence between the continents displacement on such faults is imagined to allow lateral transport of material with respect to the converging continents possibly accounting for a large fraction of the convergence between the two continents the most spectacular example of such strike slip faulting is in the eastern part of the tibetan plateau where three major left lateral faults with slip rates of 10 mm yr 1 and several smaller roughly parallel faults dominate the regional strain field2 3 cobbold and davy4 suggested that these faults may be rotating in a clockwise fashion here we put bounds on that rate of rotation and conclude that the image of lateral transport on such faults2 known also as continental escape extrusion or expulsion is an illusion and that instead the left lateral slip on east striking planes in eastern tibet is a manifestation of north striking right lateral simple shear if this conclusion is correct the east striking left lateral faults and the crustal blocks between them are rotating clockwise at 1 2 myr 1 the east west dimension of eastern tibet is shortening at 10 20 mm yr 1 and little material is moving eastward out of india s path into eurasia by left lateral simple shear

mantle deformation in sikkim and adjoining himalaya evidences for a complex flow pattern abstract new measurements of azimuthal anisotropy within the himalayan collision zone and southern tibet obtained by applying the sks splitting technique to waveforms from 31 broadband seismic stations reveal significant variations in the strength and orientation of the fast axes stations in the sikkim himalaya show strong anisotropy south of main central thrust mct with delay times 1 s the fast polarization directions are nearly parallel to the strike of the himalaya mountain chain and vary consistently along the profile from 105 in the south to 130 in the north however in the adjacent nepal himalaya a predominance of plate motion related strain is observed at least up to the mct the western portion of southern tibet south of the indus tsangpo suture zone reveals characteristics of e w oriented anisotropy contrary to null anisotropy found in the eastern part the complex flow pattern revealed in the present study may be due a combination of basal shear resulting from plate motion and ductile flow along the collision front due to compression

tearing of the indian lithospheric slab beneath southern tibet revealed by sks wave splitting measurements a b s t r a c t shear wave birefringence is a direct diagnostic of seismic anisotropy it is often used to infer the northern limit of the underthrusting indian lithosphere based on the seismic anisotropy contrast between the indian and eurasian plates most studies have been made through several near north south trending passive source seismic experiments in southern tibet to investigate the geometry and the nature of the underthrusting indian lithosphere an east west trending seismic array consisting of 48 seismographs was operated in the central lhasa block from september 2009 to november 2010 splitting of sks waves was measured and verified with different methods along the profile the direction of fast wave polarization is about 60 in average with small fluctuations the delay time generally increases from east to west between 0 2 s and 1 0 s and its variation correlates spatially with north south oriented rifts in southern tibet the sks wave arrives 1 0 2 0 s later at stations in the eastern part of the profile than in the west the source of the anisotropy estimated by non overlapped parts of the fresnel zones at stations with different splitting parameters is concentrated above ca 195 km depth all the first order features suggest that the geometry of the underthrusting indian lithospheric slab in the himalayan tibetan collision zone beneath southern tibet is characterized by systematic lateral variations a slab tearing and or breakoff model of indian lithosphere with different subduction angles is likely a good candidate to explain the observations

seismic anisotropy in western tibet 1 teleseismic sks data collected by a temporary broadband array across western tibet are analysed to constrain shear wave splitting parameters small delay anisotropy is detected with a rotation of the fast axis azimuth from 40 e in the tarim basin west of yecheng to more easterly values between hotien and shiquanhe a large contribution of crustal anisotropy would be required to link fast axis directions orthogonal to the kunlun and karakorum ranges with crustal shear due to underthrusting the fast axes north of yecheng may instead reflect frozen proterozoic strain in the tarim lithosphere the e w direction south of hotien may be related to sinistral shear along the altyn tagh fault it is possible that fast axes directions south of this fault reflect northeastwards motion of tibet relative to asia

seismic anisotropy beneath tibet evidence for eastward extrusion of the tibetan lithosphere strong seismic anisotropy beneath tibet has recently been reported from the study of sks shear wave splitting the fast split waves are generally polarized in an easterly direction close to the present day direction of motion of the tibetan crust relative to stable eurasia as deduced from holocene slip rates on the major active faults in and around tibet this correlation may be taken to suggest that the whole tibetan lithosphere is being extruded in front of indenting india and that the anisotropic layer is the deforming asthenosphere that accommodates the motion of the tibetan lithosphere relative to the fixed mantle at depth uncertainties about this motion are at present too large to bring unambiguous support to that view assuming that this view is correct however a simple forward model is used to compute theoretical delay times as a function of the thickness of the anisotropic layer the observed delay times would require a 50 100 km thick anisotropic layer beneath south central tibet and an over 200 km thick layer beneath north central tibet where particularly hot asthenosphere has been inferred this study suggests that the asthenospheric anisotropy due to present absolute block motion might be dominant under actively deforming continents

cenozoic rifting and volcanism in eastern china a mantle dynamic link to the indo asian collision abstract the indo asian continental collision is known to have had a great impact on crustal deformation in south central asia but its effects on the sublithospheric mantle remain uncertain studies of seismic anisotropy and volcanism have suggested that the collision may have driven significant lateral mantle flow under the asian continent similar to the observed lateral extrusion of asian crustal blocks here we present supporting evidence from p wave travel time seismic tomography and numerical modeling the tomography shows continuous low velocity asthenospheric mantle structures extending from the tibetan plateau to eastern china consistent with the notion of a collision driven lateral mantle extrusion numerical simulations suggest that at the presence of a low viscosity asthenosphere continued mass injection under the indo asian collision zone over the past 50 my could have driven significant lateral extrusion of the asthenospheric mantle leading to diffuse asthenospheric upwelling rifting and widespread cenozoic volcanism in eastern china

high frequency seismic wave propagation beneath the indian shield himalayan arc tibetan plateau and surrounding regions high uppermost mantle velocities and efficient sn propagation beneath tibet summary this paper reports the regional propagation characteristics of high frequhcy p p s and l waves 0 5 to 2hz and p pg and s velocities for several regions in central asia tibet and india an important result is that s waves propagate efficiently in the uppermost mantle beneath most of the tibetan plateau also s transmission in tibet is not affected by crossing many physiographic and tectonic boundaries such as the indus tsangpo suture these observations are similar to observations of s propagation in stable continental and shield regions efficient s propagation is also observed across the stable tarim basin along the tien shan and himalayan mountains and across the indian shield s is strongly attenuated across the northern tibetan plateau chang thang terrane such strong attenuation suggests the existence of a low q zone in the uppermost mantle beneath that region this interpretation is supported by recent geological observations of widespread quaternary to recent basaltic volcanism in the chang thang terrane l propagates efficiently across the tarim basin the indian shield and along the strike of the himalayan mountains l is also observed although less pronounced for paths crossing large portions of the complicated structures of tien shan karakoram and pamir l is not observed for paths crossing the tibetan plateau the southern boundary of the region of inefficient l propagation in tibet lies in the vicinity of the indus tsangpo suture zone the lack of l can be attributed to attenuation within the tibetan crust however other possibilities including scattering by structural discontinuities at the margins of tibet are equally plausible pg and s are the most prominent features on most short period wwssn seismograms at epicentral distances less than about 4 apparent pg velocity determined for the area of the himalayan mountains is 6 0 0 2 km s l apparent velocities of p and s waves that have at least 50 per cent of their paths in the uppermost mantle beneath the tibetan plateau are 8 42 it 0 10 and 4 73 f 0 06 km s respectively these velocities are quite similar to those determined for the himalayan mountains p 8 45 f 0 08 km s i s 4 74 0 05 km s and the indian shield p 8 40 0 08 km s s 4 69 k 0 04 km s p

integrating long offset transient electromagnetics lotem with seismics in an exploration environment1 the applications of electromagnetics have increased in the past two decades because of an improved understanding of the methods improved service availability and the increased focus of exploration in the more complex reservoir characterization issues for electromagnetic methods surface applications for hydrocarbon exploration and production are still a special case while applications in borehole and airborne research and for engineering and environmental objectives are routine in the past electromagnetic techniques in particular deep transient electromagnetics made up a completely different discipline in geophysics although many of the principles are similar to the seismic one with an understanding of the specific problems related to data processing initially and then acquisition the inclusion of principles learned from seismics happened almost naturally initially the data processing was very similar to seismic full waveform processing the hardware was also changed to include multichannel acquisition systems and the field procedures became very similar to seismic surveying as a consequence the integration and synergism of the interpretation process is becoming almost automatic the long offset transient electromagnetic lotem technique will be summarized from the viewpoint of its similarity to seismics the complete concept of the method will also be reviewed an interpretation case history that integrates seismic and lotem from a hydrocarbon area in china clearly demonstrates the limitations and benefits of the method