upper mantle stratification by p and s receiver functions summary seismic stratification of the upper mantle is investigated by applying two complementary techniques to the records of the graefenberg array in southern germany the anisotropic p receiver function technique kosarev et al 1984 vinnik montagner 1996 is modified by using summary seismic events instead of individual events and different weighting functions instead of the same function for the harmonic angular analysis of the sv and t components of the pds phases the summary events provide better separation of the second azimuthal harmonic than the individual events the parameters of the second harmonics of sv and t thus evaluated should be similar if they reflect the effects of azimuthal anisotropy this can be used as a criterion to identify the anisotropy to detect the sdp phases and their azimuthal variations caused by azimuthal anisotropy we have developed a stacking technique which can be termed the s receiver function technique it includes axis rotation to separate interfering p and s arrivals determination of the principal m component of the s wave motion deconvolution of the p components of many recordings by their respective m components and stacking of the deconvolved p components with weights depending on the level of noise and the angle between the m direction and the backazimuth of the event both techniques yield consistent results for the graefenberg array as indicated by the p receiver functions the upper layer of the mantle between the moho and 80 km depth is anisotropic with dvs vs around 0 03 and the fast direction close to 20 clockwise from north the fast direction of anisotropy below this layer is around 110 the boundary between the upper and the lower anisotropic layers is manifested by the detectable pds and sdp converted phases shear wave splitting in sks is strongly dominated by azimuthal anisotropy in the lower layer asthenosphere

the heterogeneous upper mantle low velocity zone abstract the upper mantle low velocity zone lvz is a depth interval with slightly reduced seismic velocity compared to the surrounding depth intervals the zone is present below a relatively constant depth of 100 km in most continental parts of the world both in cratonic areas with high average velocity and tectonically active areas with low average velocity evidence for the low velocity zone arises from controlled and natural source seismology including studies of surface waves and of primary and multiple reflections of body waves from the bounding interfaces calculations of receiver functions and absolute velocity tomography the available data indicates a more pronounced reduction in seismic velocity and q value for s waves than p waves as well as high electrical conductivity in the lvz seismic waves are strongly scattered by the zone which demonstrates the existence of small scale heterogeneity the depth to the base of the lvz is systematically shallower in cold stable cratonic areas than in hot active regions of the world because of its global occurrence below a relative constant depth of 100 km the lvz cannot be explained by metamorphic or compositional variation and rheological changes calculated upper mantle temperatures indicate that the rocks are close to the solidus in an interval with variable thickness below 100 km depth provided that the rocks contain water and carbon dioxide the presence of even small amounts of such fluids in the mantle rocks will lower the solidus by several hundred degrees and introduce a characteristic kink on the solidus curve around 80 100 km depth the seismic velocities and q values are significantly reduced of rocks which are close to the solidus or contain small amounts of partial melt hence the lvz may be explained by upper mantle temperatures being close to the solidus in a depth interval below 100 km assuming that the rocks contain only limited amounts of fluids this mechanism may explain the low velocities q values and resistivity as well as the intrinsic scattering and the characteristic variation in thickness of the low velocity zone

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

the lithosphere asthenosphere boundary in the tien shan karakoram region from s receiver functions evidence for continental subduction 1 significant s to p converted wave energy from the crust mantle and lithosphere asthenosphere boundaries the moho and the lab respectively has been observed in the tien shan and karakoram orogenic belts and used for the determination of the thickness of the crust and the mantle lithosphere the moho lies at depths of 45 65 km in the tien shan and adjacent regions and deepens to 70 km depth in pamir and karakoram the thickness of the lithosphere varies between 90 and 120 km underneath the tien shan and increases to a value of 160 km beneath the tarim basin to the south of the tarim basin the lower boundary of the asian lithosphere can be clearly followed to a depth of 270 km beneath central pamir and karakoram while the indian lithosphere is observed beneath the karakoram dipping from 130 km to 170 km towards the north the observations indicate a scenario of continental collision and subduction consistent with seismicity and the surface wave tomography results

three dimensional velocity field of present day crustal motion of the tibetan plateau derived from gps measurements 1 using the measurements of 750 gps stations around the tibetan plateau for over 10 years since 1999 we derived a high resolution 3 d velocity field for the present day crustal movement of the plateau the horizontal velocity field relative to stable eurasia displays in details the crustal movement and tectonic deformation features of the india eurasia continental collision zone with thrust compression lateral extrusion and clockwise rotation the vertical velocity field reveals that the tibetan plateau is continuing to rise as a whole relative to its stable north neighbor however in some subregions uplift is insignificant or even negative the main features of the vertical crustal deformation of the plateau are the following a the himalayan range is still rising at a rate of 2 mm yr the uplift rate is 6 mm yr with respect to the south foot of the himalayan range b the middle eastern plateau has a typical uplift rate between 1 and 2 mm yr and some high mountain ranges in this area like the longmen shan and gongga shan have surprising uplift rates as large as 2 3mm yr c in the middle southern plateau there is a basin and endorheic subregion with a series of ns striking normal faults showing obvious sinking with the rates between 0 and 3 mm yr d the present day rising and sinking subregions generally correspond well to the cenozoic orogenic belts and basins respectively e at the southeastern corner of the plateau there is an apparent trend that the uplift rate is gradually decreasing from between 0 8 and 2 3 mm yr in the inner plateau to between 0 5 and 1 6 mm yr outside the plateau with the decrease of terrain height

rayleigh wave tomography of china and adjacent regions 1 this paper presents a tomographic study on the s wave velocity structure of china and adjacent regions group velocity dispersions of fundamental rayleigh waves along more than 4000 paths were determined with frequency time analysis the study region was divided into a 1 1 grid and velocities in between grid nodes were calculated by bilinear interpolation the occam s inversion scheme was adopted to invert for group velocity distributions this method is robust and allows us to use a fine grid in model parameterization and thus helps to restore a more realistic velocity pattern checkerboard tests were carried out and the lateral resolution was estimated to be 4 6 in china and its eastern continental shelves the resulting group velocity maps from 10 to 184 s showed good correlation with known geological and tectonic features the pure path dispersion curves at each node were inverted for shear wave velocity structures the three dimensional velocity model indicates thick lithospheres in the yangtze and tarim platforms and hot upper mantles in baikal and western mongolia coastal area and continental shelves of eastern china and indochina and south china sea regions the tibetan plateau has a very thick crust with a low velocity zone in its middle beneath the crust a north dipping high velocity zone mimicking a subducting plate reaches to 200 km in depth and reaches to the kunlun mountains northward in northern tibet a low velocity zone immediately below the moho extends eastward then turns southward along the eastern edge of the plateau until it connects to the vast low velocity area in indochina and the south china sea

present day crustal motion within the tibetan plateau inferred from gps measurements 1 using the measurements of 726 gps stations around the tibetan plateau we determine the rigid rotation of the entire plateau in a eurasia fixed reference frame which can be best described by an euler vector of 24 38 0 42 n 102 37 0 42 e 0 7096 0 0206 ma the rigid rotational component accommodates at least 50 of the northeastward thrust from india and dominates the eastward extrusion of the northern plateau after removing the rigid rotation to highlight the interior deformation within the plateau we find that the most remarkable interior deformation of the plateau is a glacier like flow zone which starts at somewhere between the middle and western plateau goes clockwise around the eastern himalayan syntaxis ehs and ends at the southeast corner of the plateau with a fan like front the deformation feature of the southern plateau especially the emergence of the flow zone could be attributed to an eastward escape of highly plastic upper crustal material driven by a lower crust viscous channel flow generated by lateral compression and gravitational buoyancy at the later developmental stage of the plateau the first order feature of crustal deformation of the northeastern plateau can be well explained by a three dimensional elastic half space dislocation model with rates of dislocation segments comparable to the ones from geological observations in the eastern plateau although gps data show no significant convergence between the eastern margin of the plateau and the sichuan basin a small but significant compressional strain rate component of 10 5 2 8 nstrain yr exists in a relatively narrow region around the eastern margin in addition a large part of the eastern plateau northeast of the ehs is not undergoing shortening along the northeastward convergence direction of the ehs but is stretching

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

a sharp lithosphere asthenosphere boundary imaged beneath eastern north america although a rigid lithosphere about 100 km of crust and uppermost mantle moving over a weaker asthenosphere a few hundred kilometres of more deformable rock is a widely accepted aspect of plate tectonics the properties of the boundary between them are poorly understood when compressional waves p waves from an earthquake encounter an interface at an angle some of the energy is converted to shear waves or s waves these converted waves can be used to construct an image of the interface based on converted waves recorded in eastern north america the lithosphere asthenosphere boundary has been resolved as a sharp gradient that cannot be explained by thermal gradients alone indicating that the boundary is controlled by melt or high water content in the asthenosphere plate tectonic theory hinges on the concept of a relatively rigid lithosphere moving over a weaker asthenosphere yet the nature of the lithosphere asthenosphere boundary remains poorly understood the gradient in seismic velocity that occurs at this boundary is central to constraining the physical and chemical properties that create differences in mechanical strength between the two layers for example if the lithosphere is simply a thermal boundary layer that is more rigid owing to colder temperatures mantle flow models1 2 indicate that the velocity gradient at its base would occur over tens of kilometres in contrast if the asthenosphere is weak owing to volatile enrichment3 4 5 6 or the presence of partial melt7 the lithosphere asthenosphere boundary could occur over a much smaller depth range here we use converted seismic phases in eastern north america to image a very sharp seismic velocity gradient at the base of the lithosphere a 3 11 per cent drop in shear wave velocity over a depth range of 11 km or less at 90 110 km depth such a strong sharp boundary cannot be reconciled with a purely thermal gradient but could be explained by an asthenosphere that contains a few per cent partial melt7 or that is enriched in volatiles relative to the lithosphere3 4 5 6

tibetan plate overriding the asian plate in central and northern tibet at the southern boundary between india and tibet the indian tectonic plate subducts northwards beneath the tibetan plateau seismic imaging shows that at the northern boundary of tibet the asian plate is also subducting southwards beneath the plateau and the tibetan lithosphere is separate

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

lithospheric structure across the northeastern margin of the tibetan plateau implications for the plateau s lateral growth abstract variations of lithospheric structure across the northeastern tibetan plateau and its bounding asian blocks the alxa block to the north and the ordos block to the east are crucial for understanding the rise and lateral growth of the tibetan plateau using waveforms from high density seismic arrays in northeastern tibetan plateau and the surrounding regions we investigated the lithospheric structure with s and p wave receiver functions the results show strong and relatively simple negative velocity gradients in the depth range of mantle lithosphere 70 150 km under the ordos and alxa blocks similar to those under typical stable continental lithosphere in contrast under northeastern tibetan plateau including its marginal regions the velocity gradients are weak and diffusive for the mantle lithosphere which may be explained by elevated temperature and presence of partial melts the changes of lithospheric structures are sharp between the tibetan plateau and the bounding ordos and alxa blocks suggesting that these two blocks have restricted the lateral growth of the tibetan plateau as rigid boundaries however across the northeastern corner of the tibetan plateau to the yinchuan rift the lithospheric mantle structures are similar suggesting a lateral mantle flow from the tibetan plateau to the gap between the ordos and the alxa blocks the crustal structures along this transition show evidence of lateral growth of the tibetan plateau in particular the edge of thickened crust and evidence of moho superposition are found between the haiyuan fault and the tianjin shan fault which may have replaced the haiyuan fault as the front boundary of the laterally growing tibetan plateau in its northeastern corner

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

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

lithospheric thickness of the chinese continent abstract we invert for the upper mantle temperatures of the chinese continent in the depth range of 70 240 km from a recent s velocity model the depth where temperatures intersect a mantle adiabat with a potential temperature of 1300 c is in close correspondence with the top of the seismic low velocity zone for most regions this correspondence implies that seismic lithosphere estimated from short time scale seismic information may be equivalent to the long time scale geodynamical lithosphere defining the 1300 c adiabat as coinciding with the lithospheric base we estimate the seismic thermal lithosphere thickness the estimated thickness shows obvious dependence on the tectonic settings beneath eastern china which mainly belongs to the circum pacific tectonic domain it has a thickness of 100 km and beneath the qinghai tibet plateau and south to the tarim craton which belong to the tethyan tectonic domain it has a thickness of 160 220 km the lithospheric thicknesses of the three large para platforms cratons range from 170 km for the western yangtze 140 km for tarim and 100 km for sino korean the three cratons may have been reshaped by phanerozoic tectonic activities and are thinner than most cratons in other continents

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

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

converted waves reveal a thick and layered tectosphere beneath the kalahari super craton abstract thick and high velocity roots are generally observed beneath the archean cratons inside these high velocity keels velocity contrasts are detected neither by surface nor by body waves tomographies we present here evidences based on the s to p and p to s converted waves for the existence of an irregularly stratified and thick keel beneath the kalahari super craton to satisfy surface wave data and s to p conversions the velocity model should have beneath the moho a 160 km thick anisotropic structure with vertical slow axis and decreasing anisotropic parameters with depth such a structure may stem from the preferred orientation of olivine 100 axis in the horizontal plane under shearing deformation this structure together with the 100 km thick layer underlying it forms a 300 km thick continental root beneath the cratons inside this root the p and s velocities should be higher on average respectively by an amount of 6 and 4 than the ak135 velocity model beneath 300 350 km depth a low velocity zone is clearly detected that may be either the remainder of large magma reservoirs related to cratonic flood basalts or a melted silicate layer created by the transformation just above the 410 km deep discontinuity of wadsleyite assembly to olivine assembly

a 40 000 yr pollen and diatom record from lake tritrivakely madagascar in the southern tropics abstract links between southern and northern hemisphere climates during the late quaternary are poorly known partly due to the scarcity of continuous climatic records in the southern tropics pollen and diatom evidence from lake tritrivakely 19 47 s provides information on vegetational and hydrological changes in the central highlands of madagascar over the past 40 000 yr most of the record reflects natural environmental variability since humans arrived on the island ca 2000 yr b p during glacial times the migration of mountain plants toward lower altitudes is consistent with a temperature decrease and with reduced atmospheric co2levels in the lake a positive mean annual hydrologic balance from 38 000 to 36 000 and from 17 500 to 9800 cal yr b p coincided with periods of decreasing summer insolation and preceded by several millennia lake rises in the northern tropics a negative hydrologic budget during periods of maximum seasonal contrast in solar radiation is partly attributed to high summer evaporation rate the last glacial maximum was cool and dry the deglacial warming occurred in two steps the first step accompanied by an increase in wetness occurred abruptly at ca 17 000 cal yr b p about two millennia earlier than in the northern hemisphere it is abundantly documented in southern terrestrial data the second step at 15 000 cal yr b p was in phase with the first major temperature change in the northern hemisphere

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

crustal and upper mantle structure of southeast tibet from rayleigh wave tomography 1 we have applied the two plane wave inversion technique to fundamental mode rayleigh waves in southeast tibet phase velocity variations are obtained at 15 periods from 20 to 143 s and are used to construct three dimensional shear wave velocity structure a low velocity zone is imaged in the middle crust of southeast tibet in which velocity perturbation varies from 4 to 3 the slow area might be associated with high temperature and partial melt and can flow ductilely while the high velocity region could be relatively cold and mechanically strong enough to pass crustal strain implying that coherent lithospheric deformation is possible with the presence of complicated crustal flow strong negative anomalies in the shallow crust are imaged at 92 e correlating with the location of a north south rift zone a fast mantle lid to a depth of 120 km is present beneath most of the study area the lithosphere is significantly slower in the central part of the study area than in the west and east to the east of 96 e a subvertical high velocity zone is imaged at 120 240 km depths under the bangong nujiang suture bns which is more like the subduction of the asian lithosphere we interpret the high velocity anomalies to the south of the bns as the subducted indian lithosphere and those to the north as the asian lithosphere a vertical low velocity column is observed in the central part of the region from the lower crust to the midupper mantle which is probably in part responsible for the rift at 92 e the slow anomaly is likely associated with asthenosphere upwelling following the opening of a slab window that could have developed when the slab was torn apart vertically because of the significant change of subducting direction at the eastern syntaxis

lateral growth of ne tibetan plateau restricted by the asian lithosphere results from a dense seismic profile abstract we present high resolution receiver function images along a 700 km long dense seismic array extending from northern tibetan plateau to the alxa block crossing the entire qilian thrust belt qtb the dense stations with less than 2 km station intervals allow the receiver functions to unveil unprecedented details of crustal structures across the northern frontier of the growing tibetan plateau the migration image shows a thickened and strongly deformed qtb crust with an uneven moho and complex internal structures that are indicative of pure shear shortening the alxa block in contrast has a thinner crust a flat moho and little internal crustal deformation these results suggest that the lateral growth of ne tibetan plateau is restricted by the strong asian lithosphere which shows no visible subduction beneath the tibetan plateau as previously suggested

is the asian lithosphere underthrusting beneath northeastern tibetan plateau insights from seismic receiver functions abstract whether or not the asian lithosphere has underthrusted beneath the tibetan plateau is important for understanding the mechanisms of the plateau s growth using data from the permanent seismic stations in northeastern tibetan plateau we studied seismic structures of the lithosphere and upper mantle across the plateau s northeastern margin using p and s receiver functions the migrated p and s receiver function images reveal a thick crust and a diffuse lithosphere asthenosphere boundary lab beneath the tibetan plateau contrasting sharply with the relatively thin crust and clear sharp lab under the bounding asian blocks the well defined lab under the asian blocks tilts toward but does not extend significantly under the tibetan plateau this is inconsistent with the model of asian mantle lithosphere underthrusting beneath the tibet plateau instead our results indicate limited passive deformation of the bounding asian lithosphere as it encounters the growing tibetan plateau

imaging the colliding indian and asian lithospheric plates beneath tibet 1 a high resolution image of the base of the lithosphere from s to p converted seismic waves revealed the collision architecture of the indian and asian continental plates beneath the tibetan plateau the base of the indian lithosphere dips northward from a depth of 160 km beneath the himalayas to a depth of 220 km just south of the bangong suture the base of the asian lithosphere is nearly horizontal at the depth of 160 180 km from central to northern tibet there is a vertical gap of about 50 km between indian and asian lithospheres our observation of a well defined thick lithosphere throughout the entire plateau is not consistent with models of wholesale convective instability of a thickened mantle lithosphere which would predict a very thin asian lithosphere the hypothesized sequential southward subduction of tibetan and asian continental lithospheres leading to the growth of the tibetan plateau if correct cannot be occurring below 180 km depth our results along with available geological and geophysical data strongly support that the plateau is predominantly formed by a relatively coherent north dipping subducted indian continental lithosphere in the south which presently can be traced to the middle of the plateau and a south dipping subducted asian lithosphere in the north at a shallower depth

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

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

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

the s receiver functions synthetics and data example summary recently the s receiver function method has been successfully developed to identify upper mantle interfaces s receiver functions have the advantage of being free of s wave multiple reflections and can be more suitable than p receiver functions for studying mantle lithosphere however because of specific ray geometry and interference of diverse phases the s receiver function method has some technical difficulties and limitations we use synthetic seismograms to demonstrate the feasibility and limitations of s receiver functions for studying mantle structures full wavefield seismograms were calculated using the reflectivity method and processed to generate synthetic s receiver functions for s sks and scs waves results show that s receiver functions can be obtained from waveforms of s sks and scs waves the synthetic s receiver functions for these incident waves show s to p converted phases at all discontinuities in the crust and upper mantle useful ranges of epicentral distances for calculation of s receiver functions are 55 85 for s 85 for sks and 50 75 for scs waves we apply both the s and p receiver function methods to data recorded at broadband station ykw3 in northwest canada the study shows that there is significant agreement among different receiver function methods and demonstrates the usefulness of s receiver functions for imaging the mantle lithosphere

electrical resistivity variations of lithospheric mantle beneath the northern tibetan plateau with tectonic implications abstract the northern tibetan plateau including the areas north of kunlun shan is characterised by a strapped basin range system and major strike slip faults however the surficial deformation differs laterally the collection of magnetotelluric mt data covering the northern tibetan plateau has made it possible to apply a three dimensional isotropic inversion of these data for the entire study area the plane area covered by mt sites is more than 700 700 km2 and the longest periods of most of these sites approximate or exceed 10 000 s all of these aid in distinguishing the weak responses generated by deep anomalies in the lithospheric mantle the three dimensional inversion is applied to a subset of mt data and it is observed that the full impedance data of most mt sites well fit the responses of the final three dimensional model two modes of relationship among the lithospheric mantles in the northern tibetan plateau are proposed the moderate low resistivity corridor indicates that the lithospheric mantle has a low viscosity which is favourable for shortening at shallow crustal levels under a regional compression environment furthermore the low resistivity anomalies in the west qaidam basin and southwest qilian shan form a passage that may connect with weak materials in the lower crust of songpan ganzi terrane through the qiman tagh range the high resistivity lithospheric mantle of east kunlun shan east qaidam basin and alashan is dislocated according to the surface boundaries indicating a face to face subduction below qilian shan this represents the major dynamic model in the northern tibetan plateau

cenozoic tectonic evolution of asia a preliminary synthesis abstract asia has been a major testing ground for various competing models of continental deformation due to its relatively well understood plate boundary conditions in the cenozoic exceptional exposure of active structures and strain distribution and widespread syn collisional igneous activity as a proxy for the thermal state of the mantle and crust two cenozoic orogens dominate the continent the himalayan tibetan orogen in the east induced by the india asia collision and the turkish iranian caucasus orogen in the west induced by the arabia asia collision the development of the two orogens was accomplished by shortening in the early stage followed by strike slip faulting and extension in the late stage in the himalayan tibetan orogen shortening across two discrete thrust belts at 55 30 ma in southern and northern tibet created a large intracontinental basin the paleo qaidam basin in between subsequent crustal thickening and a possible thermal event in the mantle e g convective removal of central tibetan mantle lithosphere may have raised the elevation of this early intra plateau basin up to 2 3 km to its current height collision between india and asia also caused lateral extrusion of southeast asia between 32 ma and 17 ma the latest stage of the india asia collision was expressed by north trending rifting and the development of trench facing v shaped conjugate strike slip faults in central mongolia central tibet eastern afghanistan and southeast asia in the turkish iranian caucasus orogen early crustal thickening in the orogenic interior began at or prior to 30 20 ma this style of deformation was replaced by strike slip faulting at 15 5 ma associated with further northward penetration of arabia into asia westward extrusion of the anatolia turkey block and rapid extension across the sea of crete and sea of aegean the late stage extension in both orogens was locally related to extensional core complex development the continental margin extension of east asia was developed in two stages initially in a widely distributed zone that has an east west width of 500 800 km during 65 35 ma which was followed by localized extension and opening of back arc basins associated with the development of spreading centers at 32 17 ma e g japan sea or east korea sea bohai bay and south china sea opening of the back arc basins could be induced by 1 rapid eastward migration of the western pacific trench system or 2 oblique subduction of pacific plate beneath asia that had produced a series of en echelon right slip primary shear zones linking with back arc spreading centers oriented obliquely to the strike of the nearby trench since 15 ma the eastern margin of asia became contractional in the east west direction as indicated by the collapse of back arc basins in the western pacific and the development of fold thrust belts along the eastern continental margin coeval with the contraction is widespread east west extension in siberia north china and the tibetan plateau the above observations can be explained by a change in boundary condition along the eastern margin of asia that allowed the thickened asian continent to spread eastward causing east west extension in its trailing edge and east west compression in its leading edge in west asia continental margin extension started at about 25 20 ma in the aegean and cretan regions which was associated with a rapid southward retreat of the hellenic arc the complex evolution of cenozoic deformation in asia may be explained by a combined effect of temporal changes in plate boundary conditions thermal evolution of the upper mantle perturbed by collisional tectonics and the built up of gravitational energy through crustal thickening and thermal heating although the past research in asia has treated the india asia and arabia asia convergence as separate collisional processes their interaction may have controlled the far field cenozoic deformation in asia the most pronounced result of this interaction is the creation of a northeast trending 300 400 km wide and 1500 km long zone of northwest striking right slip faults which extends from the zagros thrust belt in the south to western mongolia in the north and links with the active tian shan and altai shan intracontinental orogens cenozoic deformation and coeval igneous activity spatially overlap with one another in the himalayan tibetan and turkish iranian caucasus orogens a large cenozoic magmatic gap exists between tibet in the south and mongolia in the north where cenozoic deformation has not been associated with any coeval igneous activity finally cenozoic igneous activity is always associated with jurassic cretaceous magmatic arcs suggesting a causal relationship between the early arc magmatism and later syn collisional magmatism

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

early cenozoic faulting of the northern tibetan plateau margin from apatite u th he ages models to explain the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary yet a growing body of evidence in the tibetan orogen suggests that deformation occurred at the far northern extent of the modern plateau early in the orogen s history and thus our current mechanical understanding of orogenic plateau development is incomplete new apatite u th he ages from four elevation transects document periods of rapid exhumation related to erosion pulses in hanging wall rocks of major thrust structures accelerated erosion is used as a proxy of fault timing and is interpreted in a larger context of structural data and sediment accumulation in adjacent foreland basins helium results are synthesized with published geologic thermochronometric and sedimentologic data from which a growing picture of regional compressional deformation in middle to late eocene time in northern tibet emerges we relate the early cenozoic period of deformation to the initiation of collision between india and eurasia despite the fact that the plate boundary was located 3000 km to the south regardless of whether or not high topography was built simultaneously as a result of this deformation early cenozoic strain signifies that the modern limit of the orogen has been relatively stationary since continental collision began and that deformation has not significantly propagated farther away from the plate boundary in time

the growth of northeastern tibet and its relevance to large scale continental geodynamics a review of recent studies recent studies of the northeastern part of the tibetan plateau have called attention to two emerging views of how the tibetan plateau has grown first deformation in northern tibet began essentially at the time of collision with india not 10 20 myr later as might be expected if the locus of activity migrated northward as india penetrated the rest of eurasia thus the north south dimensions of the tibetan plateau were set mainly by differences in lithospheric strength with strong lithosphere beneath india and the tarim and qaidam basins steadily encroaching on one another as the region between them the present day tibetan plateau deformed and its north south dimension became narrower second abundant evidence calls for acceleration of deformation including the formation of new faults in northeastern tibet since 15 ma and a less precisely dated change in orientation of crustal shortening since 20 ma this reorientation of crustal shortening and roughly concurrent outward growth of high terrain which swings from nne ssw in northern tibet to more ne sw and even ene wsw in the easternmost part of northeastern tibet are likely to be in part a consequence of crustal thickening within the high tibetan plateau reaching a limit and the locus of continued shortening then migrating to the northeastern and eastern flanks these changes in rates and orientation also could result from removal of some or all mantle lithosphere and increased gravitational potential energy per unit area and from a weakening of crustal material so that it could flow in response to pressure gradients set by evolving differences in elevation

present day crustal vertical movement inferred from precise leveling data in eastern margin of tibetan plateau abstract we collect precise leveling data observed in the period of 1970 to 2012 around the eastern margin of the tibetan plateau and process the data to acquire present day crustal vertical velocity field vertical rates of 9 gps sites are employed as a priori constraints to define the reference frame the velocity field shows that most of the regions in the eastern margin of the tibetan plateau are undergoing uplift the gongga shan and west qinling are uplifting at maximum rates of 5 8 1 0 mm a and 6 0 1 3 mm a respectively a 2 d buried dislocation model is used to infer normal or reverse dip slip rates the result indicates that the reverse slip rate along the longmen shan fault is 4 2 0 5 mm a followed by the daliangshan and longriba faults with the rates of 3 7 0 5 mm a and 2 8 0 3 mm a respectively thezemuhe and northern section of the xiaojiang fault have the normal slip rates of 2 3 1 0 mm a and 2 9 0 4 mm a respectively combining vertical velocity field with gps derived horizontal velocity field we investigate the characteristics of crustal movement and dynamic mechanism the 3d velocity field indicates that the helan shan is uplifting and the yinchuan graben is subsiding the 3d crustal movement of the liupan shan region is segmented with the area in the vicinity of the northern liupan shan fault uplifting and the area in the vicinity of southern liupan shan fault subsiding respectively the subsidence of the central and southern sichuan yunnan terrain is caused by the near east west extension the budget of horizontal material influx suggests that movements of the upper and lower crust are rather different in most of the region excessive material influx into the lower crust upper mantle underneath most of the sub blocks seems required to maintain the uplift rates inferred from leveling providing evidence for the existence of lower crustal flow in this region

seismic evidence for the north china plate underthrusting beneath northeastern tibet and its implications for plateau growth abstract lithospheric deformation of the tibetan plateau is caused by subduction of the indian northward and asian southward plates the effects of this interaction on inner reaches of the asian continent between the tibetan plateau and the north china craton ncc for example remain uncertain due to poor geophysical data coverage in northeastern tibet ne tibet we provide here detailed knowledge of the lithospheric structure beneath ne tibet as determined from a dense broadband seismic profile traversing ne tibet to the westernmost ncc receiver function imaging reveals several significant features including a north dipping intracrustal converter nc moho offset overlap beneath major fault zones and a low velocity layer lvl in the middle lower crust the lithosphere asthenosphere boundary lab is clearly defined and appears as a south dipping interface that runs continuously from the alxa interior to the qilian orogen interpretation of these observations combined with other seismic evidence implies that the ncc lithospheric mantle has been persistently underthrust beneath the qilian orogen this process forms the thick skinned crustal accretionary wedges which develop above a middle lower intracrustal decollement our results provide further deep geophysical constraints on the cenozoic post collisional evolution of the convergent boundary between ne tibet and the ncc and help clarify the mechanism of plateau growth in this boundary area

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

lithospheric velocity structure of the northeast margin of the tibetan plateau relevance to continental geodynamics and seismicity abstract we provide teleseismic constraints on the continental collision zone along the sw ne profile from the tibetan plateau to the alxa block traversing lenglong ling a topographic boundary of the northeastern tibetan plateau our seismic profile crosses the qilian shan thrust belt a zone of intense earthquake activity that includes a pair of mw 5 9 earthquakes in 1986 and 2016 with a 30 year time interval occurring at the city of menyuan near lenglong ling a high degree of similarity between the two waveforms and their similar hypocenters indicate that the two events are repeating earthquakes both p and s wave receiver functions show contrasting lithospheric structure and crustal seismic properties across the repeating earthquake region suggesting that lenglong ling can be the crustal boundary between the tibetan plateau and the alxa block high frequency p receiver functions clearly show a sw dipping underthrusted upper crust beneath the foreland of the qilian shan thrust belt a strong low velocity feature above the intracrustal underthrust beneath southwest lenglong ling indicates a mechanically weak zone developed coeval with the main compressional deformation during the last major tectonic event our observed localized upper crustal deformation beneath lenglong ling can be direct evidence of the lateral growth of the northeastern tibetan plateau and can control the seismicity of the qilian shan thrust belt including the menyuan repeating earthquakes

a complex tibetan upper mantle a fragmented indian slab and no south verging subduction of eurasian lithosphere due to the non uniform seismic station coverage in tibet critically important questions remain about the existence of southward continental subduction of eurasia beneath northern tibet from north and the nature of the underthrusting indian lithosphere underneath southern tibet from south using differential p and s wave travel times measured from 301 stations of all the temporary experiments deployed throughout tibet we constructed a comprehensive tomographic model the upper mantle of northern tibet consists of a rather homogeneous low velocity zone with no evidence of southward asian continental subduction in contrast the upper mantle from the himalayas to central tibet exhibits laterally variable p and s wave velocities significant low velocity zones are observed that are elongated in a north south direction and extending to at least 150 km depth which we interpret as evidences for fragmentation of the underthrusting indian continental lithosphere

along strike topographic variation of qinghai nanshan and its significance for landscape evolution in the northeastern tibetan plateau abstract cenozoic basins and ranges constitute the high topography of the northeastern tibetan plateau which is derived from the indo asian collision the geomorphological characteristics of the ranges provide direct insight into the uplift processes which are responsible for the patterns of the ranges and the growth of the northeastern tibetan plateau this study analyzed the geomorphological features of qinghai nanshan one of the youngest ranges within the northeastern tibetan plateau including elevation slope distribution and the channel steepness index topographic analysis revealed that elevations and slopes in western and central portions of qinghai nanshan were higher than those in the eastern portion whereas the channel steepness index revealed a step growth mode from west to east considering the topographic features the lateral gradient of precipitation increasing from west to east and the lithology distribution it is concluded that the geomorphological features of qinghai nanshan are influenced mainly by tectonic activities and reflect the growth pattern and or the tectonic setting of the range comparing with previous studies it is shown that the formation mechanism of qinghai nanshan follows a middle to sides growth pattern furthermore the degree of inclination of the erosion surface distributed on the northern flank of the range suggests that qinghai nanshan is controlled jointly by the listric qinghai nanshan thrust fault and the influences from the elashan and riyueshan dextral strike slip faults in addition the northeastern tibetan plateau exhibits an outside in growth process which might reflect the northeastward growth of the entire northeastern tibetan plateau

a low velocity belt beneath northern and eastern tibetan plateau from pn tomography 1 new pn tomography on tibetan plateau tp and neighboring region is conducted with data assembled from various sources the improved pn tomographic image delineates several distinct geophysical regimes that are approximately separated by geological boundaries at surface dense distribution of the stations in the eastern half of the plateau allows us to infer a 3d view of crustal thickness variation of the region the most striking feature is the bended low velocity belt lvb which extends from west to east beneath qiangtang block and songpan ganzi fold belt sgfb in northern tp and then turns consistently southward with sgfb beneath eastern tp and further south to indo china the consistency between geological boundaries on surface and sharp velocity contrast boundaries in uppermost mantle may suggest strong deformation coupling between crust and at least uppermost mantle if not whole lithosphere a north south striking lvb is also observed cutting through the southern tp coinciding with the yadong gulu rift at surface our tomographic results are generally consistent with dynamic models that involve with underthrusting in the southern tp shortening in the northern tp and extrusion to the east and the southeast furthermore our results suggest significant east west variations in the structure and dynamics

surface effects of triggered fault slip on reykjanes peninsula sw iceland three large earthquakes mw 4 5 were triggered within 5 min 85 km west of a mw 6 5 earthquake in the south iceland seismic zone sisz we report on surface effects of these triggered earthquakes which include fresh rupture widespread rockfall disrupted rockslides and block slides field data confirm that the earthquakes occurred along n striking right lateral strike slip faults field data also support the conclusion from modeling of insar data that deformation from the second triggered event was more significant than for the other two a major hydrological effect was the draining of water through an open fissure on a lake bed lowering the lake level by greater than 4 m field relationships suggest that a component of aseismic slip could have been facilitated by water draining into the fault zone d 2003 elsevier science b v all rights reserved