detection of miocene saucer shaped sills offshore senegal via integrated interpretation of seismic magnetic and gravity data the atlantic margin offshore senegal has been explored by seismic reflection and grav mag surveys high amplitude laterally transgressive seismic reflectors are found to coincide with gravimetric and magnetic highs once seismic data are integrated with potential fields modelling these reflectors can be safely interpreted as saucer shaped igneous sills up to some hundreds of metres thick some km wide the occurrence of hydrothermal vent complexes and forced folds in the stratigraphic sequence above the sills constrain the intrusion age to the miocene field observations and in situ magnetic susceptibility measurements of oligocene miocene and quaternary igneous rocks emplaced in coastal senegal support this interpretation

lithospheric thickness beneath the dabie shan central eastern china from s receiver functions summary p and s receiver functions obtained from a portable array of 34 broad band stations in east central china provide a detailed image of the crust mantle and lithosphere asthenosphere boundaries lab in the dabie shan and its adjacent areas clear s to p converted waves produced at the lab show a thin lithosphere beneath the whole study area based on our results the thickest lithosphere of 72 km is observed beneath the southern part of the area within the yangtze craton whereas beneath the north china platform the lithosphere is only 60 km thick s receiver functions also reveal in good agreement with p receiver functions a maximum depth of the moho beneath the dabie shan orogen at approximately 40 km furthermore we interpret the structural difference at 32 latitude as the probable location of

a 3d shear wave velocity model of the upper mantle beneath china and the surrounding areas we present a three dimensional model of shear wave velocity for the upper mantle of china and the surrounding region by analyzing 50 338 vertical component multi mode rayleigh wave seismograms recorded at 144 permanent and more than 300 temporary broadband stations in and around china the procedure involves combination of 1 d path average models obtained by modeling each rayleigh waveform up to the 4th higher mode in a tomographic inversion scheme the dense station network and the use of multi mode analysis help to achieve a lateral resolution of a few hundred kilometers down to 400 km depth the seismic lithosphere as it is defined by the crust and the high velocity mantle lid is to the first order thin in east china and thick in the west with a high velocity lid extending down to about 200 km depth beneath much of the tibet pamir plateau beneath india the thickness of the seismic lithosphere gradually increases from 100 km in south india to more than 150 km in north india where it underthrusts the tibetan plateau to approximately the jinsha river suture high velocity lid extending down to 100 150 km depth is also observed in the tarim basin sichuan basin and ordos block in the eastern part of the north china craton the seismic lithosphere is probably close to or thinner than 70 km adjacent to these areas the high velocity lid in the eastern yangtze craton and south china fold system extends down to 70 80 km depth a large scale subhorizontal high velocity body is observed at depths of 150 350 km beneath the entire east china cratonic areas this high velocity body might be the remnant of a delamination process which resulted in the decratonization of the north china and the yangtze cratons

west east transition from underplating to steep subduction in the india tibet collision zone revealed by receiver function profiles abstract closely spaced receiver function profiles in the east central india tibet collision zone reveal drastic west east changes of the crustal and upper mantle structure west of 91 5 e we show the indian crust mantle boundary moho extending subhorizontally from 50 km depth below sea level under the high himalaya to 90 km under the central lhasa terrane further north this boundary transitions to become the top of the indian lithospheric mantle and becoming faint but still observable it can be tracked continuously to 135 km depth near 31 5 n the top of the indian lithospheric mantle is clearly beneath the tibetan moho that is also a conspicuous boundary undulatory at 60 75 km depth from the central lhasa terrane to the north end of our profile at 34 n this geometry is consistent with underthrusting of indian lower crust and underplating of the indian plate directly beneath southern tibet in contrast east of 91 5 e the indian moho is only seen under the southernmost margin of the tibetan plateau and eludes imaging from 50 km south of the yarlung zangbo suture to the north the indian lower crust thins greatly and in places lacks a clear moho this is in contrast to our observation west of 91 5 e that the indian lower crust thickens northwards a clear depression of the top of the indian lower crust is also observed along west east oriented profiles centered above the region where the indian moho is not imaged our observations suggest that roll back of the indian lithospheric mantle has occurred east of 91 5 e likely due to delamination associated with density instabilities in eclogitized indian lower crust with the center of foundering beneath the southern lhasa terrane slightly east of 91 5 e

imaging the mantle lithosphere below the china cratons using s to p converted waves abstract we used s to p converted waves from over a thousand seismic stations of the permanent chinese national seismic network to study the large scale structure of the mantle lithosphere beneath the cratons in china to avoid possible sidelobes of the moho caused by the deconvolution in the s receiver function method we skipped the deconvolution and used the sv onset time as reference time instead of the time of the deconvolution spike with this new method the lithosphere asthenosphere boundary lab is observed near 80 km depth below both the north and the south china cratons however at the north eastern margin of the tibetan plateau the lab is observed at about 160 km depth smoothly shallowing towards the west and abruptly ending at the western end of the north china craton this structure is not visible in traditional s receiver function data because it is overwhelmed by moho sidelobes there is no indication of a deeper near 200 km cratonic lithosphere asthenosphere boundary in the other parts of the china cratons as it is observed in other cratons we hypothesise that at the north eastern margin of the tibetan plateau the original thickening of the lithosphere towards the craton is preserved whereas in most parts of the north and south china cratons the lower part of the lithosphere is removed by some mechanism the moho is well observed it deepens from 30 to 40 km at the sea shores in the east to 60 70 km below eastern tibetan plateau

assessing sedimentary records of paleohurricane activity using modeled hurricane climatology patterns of overwash deposition observed within back barrier sediment archives can indicate past changes in tropical cyclone activity however it is necessary to evaluate the significance of observed trends in the context of the full range of variability under modern climate conditions here we present a method for assessing the statistical significance of patterns observed within a sedimentary hurricane overwash reconstruction to alleviate restrictions associated with the limited number of historical hurricanes affecting a specific site we apply a recently published technique for generating a large number of synthetic storms using a coupled ocean atmosphere hurricane model set to simulate modern climatology thousands of overwash records are generated for a site using a random draw of these synthetic hurricanes a prescribed threshold for overwash and a specified temporal resolution based on sedimentation rates observed at a particular site as a test case we apply this monte carlo technique to a hurricane induced overwash reconstruction developed from laguna playa grande lpg a coastal lagoon located on the island of vieques puerto rico in the northeastern caribbean apparent overwash rates in the lpg overwash record are observed to be four times lower between 2500 and 1000 years b p when compared to apparent overwash rates during the last 300 years however probability distributions based on monte carlo simulations indicate that as much as 65 of this drop can be explained by a reduction in the temporal resolution for older sediments due to a decrease in sedimentation rates periods of no apparent overwash activity at lpg between 2500 and 3600 years b p and 500 1000 years b p are exceptionally long and are unlikely to occur above 99 confidence under the current climate conditions in addition breaks in activity are difficult to produce even when the hurricane model is forced to a constant el nino state results from this study continue to support the interpretation that the western north atlantic has exhibited significant changes in hurricane climatology over the last 5500 years

seismic signature of the collision between the east tibetan escape flow and the sichuan basin abstract gps displacement vectors show that the crust in east tibet is being squeezed in an easterly direction by the northward motion of the indian plate and the sichuan basin is resisting this stream and redirecting it mainly towards indochina the longmen shan containing the steepest rise to the high plateau anywhere in tibet results from the strong interaction between the east tibetan escape flow and the rigid yangtze block sichuan basin but the kinematics and dynamics of this interaction are still the subject of some debates we herein present results from a dense passive source seismic profile from the sichuan basin into eastern tibet in order to study the deep structure of this collision zone using p and s receiver function images we observe a sudden rise of the lithosphere asthenosphere boundary lab from 120 to 150 km beneath the sichuan basin and from 70 to 80 km beneath eastern tibet in contrast the depth of the crust mantle boundary moho increases from 36 to 40 km beneath the sichuan basin and from 55 to 60 km beneath eastern tibet the 410 km discontinuity is depressed below eastern tibet by about 30 km although the 660 remains at nearly the same depth throughout the lms from these observations we conclude that the mode of collision that occurs between tibet and the sichuan basin is very different to that found between india and tibet in southern tibet we observe in essence the subduction of the indian plate which penetrates northwards for several hundred kilometers under central tibet the very thin mantle part of the lithosphere beneath eastern tibet may indicate delamination or removal of the bottom of the lithosphere by hot asthenospheric escape flow this process leads to the exceptionally steep topography at the eastern tibetan margin as a result of gravitational buoyancy this view is supported by the very unusual depression of the 410 km discontinuity beneath eastern tibet which could be caused by the dynamics of the sub vertical downward asthenospheric flow

constraints on the interpretation of s to p receiver functions summary we present results from forward modelling to study the feasibility of using s to p converted phases to image the seismic discontinuity structure of the crust and upper mantle we show that a significant level of p wave energy arriving before the direct s wave arrival can interfere with the s to p converted phases of interest and may result in sp receiver function phases that do not represent true earth structure the source of this p wave energy is attributable to a number of phases including those that have undergone multiple reflections off the earth s surface for deep focus earthquakes 300 600 km deep a significant amount of p wave energy is observed from pppp ppppp and spppp phases and arrives within the same time window as predicted for s to p converted phases from the direct s phase arrival furthermore for earthquakes at all depths interfering p wave energy arrives within the same time window as predicted for s to p converted phases from the sks phase arrival limiting the usefulness of sksp receiver functions for upper mantle imaging to isolate true sp receiver function phases from contamination due to other p wave phases we find it necessary to stack receiver functions from a range of epicentral distances and depths in order to aid the suppression of noise and other unwanted phases we provide constraints on the noise levels to be expected as a function of epicentral distance and earthquake depth we find that the lowest noise levels are achievable by restricting epicentral distance to less than 75 degrees and the depth of earthquakes used to less than 300 km

cenozoic volcanism in western senegal and its relationship to the opening of the central atlantic ocean abstract the westernmost part of the senegalese basin in west africa is characterized by an extensive magmatic event exposed in the form of sills and dykes mainly during the quaternary but beginning in the tertiary 35 0 1 ma miocene to 0 8 0 1 ma pleistocene the main exposures are located in the cap vert peninsula and thies plateau to the east of dakar this magmatic event postdated the opening of the central atlantic and displays chemical similarities to the tertiary quaternary magmatic centres within the african plate two main facies are distinguished among the flows microporphyric and doleritic textured olivine rich alkaline basalts the former often containing spinel lherzolite mantle inclusions the alkaline sodic affinity of the volcanic series and the ti k na nb p2o5 and zr content of the lavas are typical of magmas generated in a within continent plate tectonic setting this volcanism appears to have resulted from an active mantle hotspot over which the african plate migrated as the central atlantic ocean opened using reactivated faults contemporaneous with central atlantic opening as magmatic pathways

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

formation of cratonic mantle keels by arc accretion evidence from s receiver functions 1 delineating mantle interfaces can provide important clues for understanding the formation of continents we use s wave receiver functions to investigate lithospheric structure along a transect extending from vancouver island to baffin island observed sp converted waves allow for interpretation of boundaries in the depth range expected for tectonic plates receiver functions show a distinct negative amplitude feature interpreted as the lithosphere astbenosphere boundary at shallow depths beneath british columbia 85km deepening abruptly at the eastern edge of the cordillera to 260km beneath the canadian shield dipping mid lithospheric discontinuities extend beneath several giant ca 1 8 ga epicontinental magmatic arcs consistent with formation of cratonic lithosphere by arc accretion this model provides a plausible explanation for global mid lithospheric discontinuities within cratons and aids in understanding their formation

thickness of the central and eastern european lithosphere as seen by s receiver functions summary s receiver functions obtained from seismograms of teleseismic events recorded at 78 european permanent broad band stations are used to estimate the thickness of the european lithosphere our results provide new independent information about the lithospheric thickness beneath the precambrian platform of eastern europe and the phanerozoic platform of central europe detailed high resolution images of the lithosphere asthenosphere boundary lab reveal indications for a typical continental lithosphere of about 100 km thickness beneath a majority of stations within central europe whereas in the vicinity of the trans european suture zone tesz the lithosphere thickens to about 130 km a relatively thin lithosphere of 80 km was found beneath the upper rhine graben region suggesting that the cenozoic extension affects the whole lithosphere no clear signal from the lab was detected beneath the alps and carpathians the lab sp phase might be disturbed by complicated structure due to ongoing collision subduction in these regions or the data are not yet sufficiently dense a relatively thicker lithosphere of about 120 km was found beneath the sw part of the bohemian massif that was formed during the variscan orogeny we found an lab depth of about 190 km near a single station located in the vrancea area eastern carpathians which is characterized by the occurrence of intermediate deep earthquakes beneath the stations located in the precambrian platform of eastern europe the lab deepens to approximately 200 km even though the converted phase from the lab is not as sharp as found beneath other stations located in central europe or even is missing

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

an automatic phase picker for local and teleseismic events an automatic detection algorithm has been developed which is capable of time p phase arrivals of both local and teleseismic earthquakes but rejects noise bursts and transient events for each signal trace the envelope function is calculated and passed through a nonlinear amplifier the resulting signal is then subjected to a statistical analysis to yield arrival time first motion and a measure of reliability to be placed on the p arrival pick an incorporated dynamic threshold lets the algorithm become very sensitive thus even weak signals are timed precisely during an extended performance evaluation on a data set comprising 789 p phases of local events and 1857 p phases of teleseismic events picked by an analyst the automatic picker selected 66 per cent of the local phases and 90 per cent of the teleseismic phases the accuracy of the automatic picks was ideal i e could not be improved by the analyst for 60 per cent of the local events and 63 per cent of the teleseismic events

introduction to tectonics of china abstract the continental crust of china is a mosaic of cratonic blocks and orogenic belts containing small cratons and terranes with various tectonic settings they have diverse origins and complex histories of amalgamation and often suffered repeated reworking after multiple episodes of amalgamation in the last three decades extensive geological geochemical and geophysical investigations have been carried out on these cratonic blocks and intervening orogenic belts producing an abundant amount of new data and competing interpretations this provides important insights into understanding the formation and evolution of the chinese continents the papers assembled in this volume present a timely and comprehensive overview on major advancements and controversial issues related to the formation and evolution of continental crust in china complex tectonic histories were experienced not only by the large scale cratonic blocks and orogenic belts but also by small scale terranes and orogens between and inside these blocks nevertheless our understanding of lithotectonic units and geological processes has been greatly advanced by recent studies of zirconology and geochemistry for various rock types from major petrotectonic units in china it has been further advanced from integrated interpretations of geochemical and petrological data for petrogenesis of magmatic rocks an overview of these observations and interpretations provides new insights into understanding the continental plate tectonics and the chemical geodynamics of subduction zones

imaging lithospheric structure of the eastern himalayan syntaxis new insights from receiver function analysis 1 we employ the p and s receiver function technique to data from the 44 seismic stations deployed in the eastern himalayan syntaxis to investigate the crustal thickness the average poisson s ratio and the depth of the lithosphere asthenosphere boundary lab the observed crustal thickness exhibits an overall ne deepening trend varying from 55 to 75 km two anomalous areas lie in the west and east of the namche barwa syntaxis characterized by thinner and thicker crust respectively the average poisson s ratios within the study area are low in the north and moderate elsewhere with some high values in the south consistent with felsic and intermediate rocks forming the crust our migrated images reveal that 1 the lab of the tibetan plate exists at relatively shallow depths 110 km and exhibits a gap beneath the namche barwa syntaxis which may have formed by the delamination of mantle lithosphere due to local mantle upwelling and 2 the lab of the asian plate is observed at a depth of 180 km which implies that the asian plate has advanced southward to about 30 n under the lhasa terrane our results provide new insights into the understanding of continental subduction and lithospheric deformation of the eastern himalayan syntaxis

a sharp cratonic lithosphere asthenosphere boundary beneath the american midwest and its relation to mantle flow abstract beneath the american midwest s to p sp converted wave imaging and multi mode surface wave tomography identify a north trending transition in seismic structure at 150 250 km depth to the east of this american midwest transition amt the lithosphere asthenosphere boundary lab is imaged as a 1 2 sp sv amplitude arrival at 200 240 km depth consistent with the depth of negative shear velocity and azimuthal anisotropy gradients imaged by surface wave tomography to the west of the amt sp conversions are much shallower at 150 190 km depth and are much weaker 0 7 or absent azimuthal anisotropy constrained by surface wave tomography also changes across the amt with stronger anisotropy to the east of the transition beneath the thicker lithospheric root we suggest that the seismic changes across the amt can be explained by considering the effects of asthenospheric flow beneath the leading edge of the thick lithospheric root the mantle flow is dominantly driven by the drift of the north america plate locally higher flow velocities are expected where the asthenosphere is forced to flow beneath the thicker root this mantle underflow could create a sharper seismic lab east of the amt via two effects first the local increase in flow velocities could steepen the thermal gradient at the base of the lithosphere and hence the isotropic velocity contrast second the increased strain rate along edge of the lithosphere could enhance the magnitude of azimuthal anisotropy our results suggest that seismically detectable lab sharpness variations could be used to constrain geographic variations in coupling between plates and mantle convection a secondary result is the image of a mid lithospheric discontinuity arrival at 80 110 km depth that is found primarily to the east of the amt this arrival is interpreted as produced by a layer of low velocity metasomatic minerals that have accumulated since the 1 8 ga creation of the lithosphere

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

origin of precursors to teleseismic s waves abstract in the literature there are descriptions of precursors to teleseismic s waves that are polarized as teleseimic p they are commonly regarded as phases converted from sv to p underneath the seismographic stations we present observations of precursors at the broadband digital geoscope stations in the distance range from 45 to 95 and at periods around 10 sec these precursors are polarized as the teleseismic p but cannot be interpreted in terms of conversion underneath the station we propose that they are formed by conversion and scattering from s to p at the free surface and scattering of the resulting p in the lithosphere of the region between the source and the receiver the apparent velocity of s in the region of scattering is usually around 7 km sec which implies a long wavepath of the converted p in the continental crust apparently s to p and p to p scattering play an important role in forming teleseismic wave fields even in the relatively long period band around 10 sec a possibility of observing true s p converted phases originating in the lithosphere underneath the station is certainly not ruled out however our analysis is a warning that scattered waves sometimes can be erroneously taken for converted phases

seismic evidence for stratification in composition and anisotropic fabric within the thick lithosphere of kalahari craton 1 based on joint consideration of s receiver functions and surface wave anisotropy we present evidence for the existence of a thick and layered lithosphere beneath the kalahari craton our results show that frozen in anisotropy and compositional changes can generate sharp mid lithospheric discontinuities mld at depths of 85 and 150 200 km respectively we found that a 50 km thick anisotropic layer containing 3 s wave anisotropy and with a fast velocity axis different from that in the layer beneath can account for the first mld at about 85 km depth significant correlation between the depths of an apparent boundary separating the depleted and metasomatised lithosphere as inferred from chemical tomography and those of our second mld led us to characterize it as a compositional boundary most likely due to the modification of the cratonic mantle lithosphere by magma infiltration the deepening of this boundary from 150 to 200 km is spatially correlated with the surficial expression of the thabazimbi murchison lineament tml implying that the tml isolates the lithosphere of the limpopo terrane from that of the ancient kaapvaal terrane the largest velocity contrast 3 6 4 7 is observed at a boundary located at depths of 260 280 km beneath the archean domains and the older proterozoic belt this boundary most likely represents the lithosphere asthenosphere boundary which shallows to about 200 km beneath the younger proterozoic belt thus the kalahari lithosphere may have survived multiple episodes of intense magmatism and collisional rifting during the billions of years of its history which left their imprint in its internal layering

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

an upper mantle s wave velocity model for east asia from rayleigh wave tomography abstract we present a new shear velocity model of the upper mantle beneath the east asia region derived by inverting rayleigh wave group velocity measurements between 10 and 145 s combined with previously published rayleigh wave phase velocity measurements between 150 and 250 s rayleigh wave group velocity dispersion curves along more than 9500 paths were measured and combined to produce 2d dispersion maps for 10 145 s periods the group velocity maps benefit from the inclusion of new data recorded by the china national seismic network and surrounding global stations the increase in available data has resulted in enhanced resolution compared with previously published group velocity maps the horizontal resolution across the region is about 3 for the periods used in this study the new shear wave velocity models indicate varying velocity structure beneath eastern china which yields estimates of a lithosphere asthenosphere boundary depth from around 160 km beneath the yangtze block to approximately 140 km beneath the western part of the north china craton ncc up to depths of 70 100 km beneath the eastern ncc northeast china and the cathaysia block the models reveal the subduction of two opposite facing continental plates under the southern and northern margin of tibet an obvious low velocity anomaly appears in the top 200 km of the upper mantle beneath northern tibet which is inconsistent with the presence of subducted asian or indian mantle lithosphere beneath northern tibet the cenozoic volcanism fields in the mongolian plateau are characterized by an obvious upper mantle negative anomaly but no signature of deep seated plume was observed

structure of the southern senegalo mauritanian basin west africa from geoelectrical studies abstract the southern part of the senegalo mauritanian basin is geologically featureless and little is known about the division of the sedimentary sequence and regional geological structure because of the scarcity of deep drillholes a magnetotelluric survey was therefore carried out in an attempt to obtain details concerning the electrical characteristics thickness and structure of the mesozoic cainozoic senegalo mauritanian basin from the surface down the interpretation of magnetotelluric data shows several distinct electrical boundaries the general trend shows a decrease in near surface resistivity from east to west the basin is characterized by water saturated unconsolidated low resistivity sediments with a westward sloping monoclinal style the combination of borehole and geological data with a geoelectrical cross section provides a simplified model of the geological electrical units of the basin this shows the distribution of strata above and below the base of the senonian conductive material 10 30 ohm m at depth in the central part of the profile may represent a northern extension of the palaeozoic sediments of the bove sequence which lies immediately south of the magnetotelluric line significant electrical contrasts between the sediments and basement rocks allow mapping of the top of the high resistivity basement which is characterized by progressively deeper burial to the west

receiver function structures beneath the deep large faults in the northeastern margin of the tibetan plateau using the teleseismic p and s wave receiver functions of the dense linear temporary seismic array the crust and uppermost mantle structures beneath the deep large faults in the northeastern margin of the tibetan plateau were imaged the images of the first converted wave and the multiples indicated that the north fault zone of west qinling nwq mountain and diebu lueyang dbl faults cut the mohorovicic moho discontinuity and cause an obvious difference feature for the moho in the two sides of the faults the higher vp vs ratio and lower velocity layer is found beneath the west portion of the array near the tibetan plateau which implies a lower crust channel flow coming from the tibetan plateau the weak moho and higher vp vs ratio beneath the eastern portion of the array near the ordos suggest the upwelling of the hot mantle material the results also indicate an obvious deformation in the upper crust with the lower vp vs ratio beneath the middle of the array such upper crust deformation is closely related to the topography of the surface therefore we deduce that the deformation of the brittle upper crust is accompanied by the formation of the local topography during the uplift of the tibetan plateau which is also the primary reason for the active seismicity in the study region the deformation of the lithosphere asthenosphere boundary lab can also be associated with the formation of the diapir caused by the upwelling hot materials in the upper mantle due to the uprising of the thrusting plate caused by the subduction of the india plate the existence of the lower crust channel flow the crust shortening and the mantle diapir in the local region simultaneously implies that the elevation and formation of the tibetan plateau cannot be explained with a single model the higher resolution results for the crust and the mantle especially beneath the block boundary region are necessary to construct the completed geodynamic model to understand the formation of the tibetan plateau

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

structure of the crust and mantle down to 700 km depth beneath the east qaidam basin and qilian shan from p and s receiver functions

the lithosphere asthenosphere boundary beneath the western united states summary s receiver functions from 67 broad band seismic stations in the western united states clearly reveal the existence of a mantle discontinuity with velocity reduction downward which we interpret as the lithosphere asthenosphere boundary lab the average depth of the lab is 70 km the boundary is relatively sharp with an overall sharpness of less than 20 km the boundary is more prominent south of the mendocino triple junction where the farallon plate has completely subducted this may indicate partial melts at the base of the lithosphere caused by the upwelling of the asthenospheric flow through the slab window a double low velocity zone is observed at base of the lithosphere beneath southern sierra nevada implying a second melting zone at a depth of 100 km well correlated with previous studies of lithospheric delamination in the area

the lithosphere of north china craton from surface wave tomography abstract fundamental mode rayleigh wave group velocity dispersion data were used in a tomographic study to reconstruct the crust and upper mantle shear velocity structure in east asia this article expounds the features of lithosphere structure in different sub regions of north china and discusses their implications for the tectonic evolution and present tectonics of north china craton ncc the study revealed that strong earthquakes took place in the low velocity zones and along the boundaries between high and low velocities in the middle and lower crust the active nw trending zhangjiakou bohai seismic belt corresponds to a prominent low velocity zone in the crust and upper mantle lid in addition to the well known structural difference between western and eastern ncc we are able to resolve clearly the lithospheric features of sub regions in eastern ncc the southern north china basin and north yellow sea are characterized by a high velocity crust and upper mantle lid down to 70 80 km depth while the northern north china basin has a low velocity crust and the velocity below moho to about 130 km is rather uniform and low there is also a general trend of velocity decrease in the upper mantle from west to east the velocity images bear the imprints of subduction and collision that took place in the periphery of ncc since mesozoic and shed some light on the mechanism of mesozoic lithospheric thinning of eastern ncc

inhomogeneous lithospheric thinning in the central north china craton zircon u pb and s he ar isotopic record from magmatism and metallogeny in the taihang mountains abstract the large scale mesozoic magmatism and related metallogeny in the taihang mountains tm provide important clues for the lithospheric thinning of the north china craton ncc among the ore deposits the vein gold mineralization of shihu in the fuping region and the skarn ore deposit of xishimen in the wu an region represent typical mesozoic metallogeny in the tm in the shihu gold mine the mapeng batholith is dominantly composed of monzogranite and granodiorite whereas the wu an pluton in the xishimen iron mine mainly comprises monzonite and diorite here we present zircon la icp ms u pb data from 8 samples which reveal the timing of magmatism in the tm as ca 130 ma which is contemporaneous with the large scale metallogeny in the margins of the ncc the 34s values recorded in the sulfide minerals from the shihu gold deposit and the xishimen skarn iron deposit show a range of 2 2 5 0 and 11 6 18 7 respectively helium isotopic compositions of fluid inclusions in pyrite from the shihu gold deposit vary from 0 12 to 1 98 ra where ra is the 3he 4he ratio of air 1 39 10 6 with calculated mantle helium values of 1 4 25 whereas those of the xishimen skarn iron deposit range from 0 06 to 0 19 ra with calculated mantle helium of 0 7 2 2 the s he ar isotopic data suggest a lower crustal origin for the ore forming components with variable inputs of mantle source the large population of inherited zircons in our samples with 207pb 206pb ages ranging between 2500 ma and 1800 ma also supports crustal participation our data reveal that the shihu gold deposit witnessed greater mantle input than the xishimen skarn iron deposit suggesting that the continental lithosphere is markedly thinner under the fuping region than that under the wu an region our interpretation is also supported by published data from two ultra broadband high precision magnetotelluric sounding profiles across the tm region showing a variation in the lithosphere thickness from 155 km to 70 km while moving from the south wu an region to the north fuping region our study suggests that inhomogeneous lithospheric thinning in the central ncc occurred at least as early as ca 130 ma ago

seismic images of the biggest crash on earth in plate tectonics the upper layer of earth the lithosphere consists of rigid plates that shift over geological time scales data from global positioning surveys have estimated that the entire indian subcontinent has moved over the past 50 years about 2 m to the north diving slowly underneath tibet this giant collision has been ongoing for 50 million years and has thrown up the highest mountains as well as the largest and highest plateau on earth not only is the world climate strongly influenced by this massive plateau average elevation of 5000 m but the collision also causes catastrophic earthquakes in southern central and eastern asia

the lithospheric structure beneath southeast tibet revealed by p and s receiver functions abstract yunnan is located at the margin of southeast tibet where dramatic tectonic activities occur in this study we calculated the p and s receiver functions by the deconvolution of three component seismic data from 48 permanent broad band stations deployed in yunnan region in order to improve signal noise ratios of the receiver functions we move out corrected receiver functions to a reference epicentral distance of 67 and then stacked them to one trace for each station finally the stacked traces were converted to the depth domain to obtain the crustal and lithospheric thicknesses beneath each station in southwestern yunnan at the west side of the jinshajiang red river fault the crustal thicknesses from the p wave receiver functions prfs and from the s wave receiver functions srfs are in the ranges of 30 36 km and of 33 39 km respectively but in northwestern yunnan the crustal thicknesses from prfs and srfs are from 66 to 69 km and from 63 to 66 km respectively our results show that the crustal thicknesses in yunnan from prfs and srfs are consistent with a maximum deviation of 3 km and increase gradually from 30 km in the south to 68 km in the northwest besides the lithospheric thickness from prfs is also similar to that from srfs with the largest difference of 15 20 km in southeastern yunnan at the west side of the jinshajiang red river fault in western yunnan it is only about 80 100 km and increases to 140 150 km regionally in northern and southeastern yunnan the thinned lithosphere extends eastward from western yunnan to eastern yunnan we attribute the thinned lithosphere to the upwelling of hot upper mantle materials associated with the eastward subduction of the indian plate

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

strengthening of paleo typhoon and autumn rainfall in taiwan corresponding to the southern oscillation at late holocene the purpose of this study is to improve prehistoric knowledge of paleo typhoon and winter monsoon activity in the western north pacific after 8 cal ka bp we examine two sediment cores derived from lakes located to the south and north of the ilan plain in northeastern taiwan seasonal and typhoon rainfall data over the past several decades are collected for this study region we also use mineral indicators k rb ratios and grain size to estimate sediment sources intensity of chemical weathering and rainfall intensity the results show that the chemical weathering index k rb ratio of meihua lake indicates the continuing effects of autumn rainfall while coarse grain sizes of dahu lake sediments suggest increased intensity in typhoon rainfall grain size results reveal that increasing el nino southern oscillation intensity in the tropical pacific has intensified the activity of super typhoons in the western north pacific finally we find that frequent activity of super paleo typhoons occurred in taiwan during the la nina like state and the strengthening winter monsoon activity and higher sea surface temperature enhanced plentiful autumn rainfall in el nino like state after comparing our results with the paleo typhoon data in japan and south china we suggest that the tracks of super typhoon are mainly affected by variations in the warm pool and subtropical northwestern pacific high in the western pacific copyright 2012 john wiley sons ltd

distinct lateral variation of lithospheric thickness in the northeastern north china craton abstract a detailed knowledge of the thickness of the lithosphere in the northeastern north china craton ncc is important for understanding the significant tectonic reactivation of the craton in the mesozoic and cenozoic time we achieve this goal by applying the newly proposed wave equation based migration technique to the s receiver functions recently collected in the region distinct negative signals are identified below the moho in all the s receiver function migrated images and stacks which we interpret as representing the s to p conversions from the lithosphere asthenosphere boundary lab the imaged lab is as shallow as 60 70 km in the southeast basin and coastal areas and deepens to no more than 140 km in the northwest mountain ranges and continental interior these observations indicate widespread lithospheric thinning in the study region in comparison with the 180 km lithospheric thicknesses typical of most cratonic regions the revealed topography of the lab generally agrees with the lateral variation in upper mantle seismic anisotropy previously measured through sks splitting analysis in particular a sharp lab step of 40 km is detected at the triple junction of the basin and mountains at almost the same place where an abrupt change from nw se to ne sw in fast polarization direction of shear waves was found these findings suggest a close correlation between the seismic anisotropy and hence deformation of the upper mantle the lithospheric thickness and the surface tectonics of the northeastern ncc while the thinned lithosphere and the nw se fast shear wave polarizations in the east areas probably are related to the dominant nw se tectonic extension in the late mesozoic cenozoic time the thicker lithosphere and the ne sw fast polarization direction in the west mountain ranges may reflect earlier contractional deformations of the region synthetic tests indicate that the lab beneath the northeastern ncc is a well defined zone 10 20 km thick combined with seismic tomography results and geochemical and petrological data this suggests that complex modification of the lithosphere probably accompanied significant lithospheric thinning during the tectonic reactivation of the old craton

lithospheric thickness thinning subduction and interaction with the asthenosphere beneath china from the joint inversion of seismic s wave train fits and rayleigh wave dispersion curves abstract a joint inversion method of simultaneously inverting regional multimode surface waveforms and fundamental mode surface wave dispersion curves is used to better constrain a 3 d s wave velocity model for the upper mantle beneath china thousands of broadband vertical component seismograms were processed and 16 500 rayleigh wave group velocity measurements were retrieved at a period of 20 s with fewer measurements at shorter and longer periods in addition 4947 regional rayleigh waveforms were obtained for paths passing mainly through the chinese mainland the joint inversion of this dataset provides new s wave velocity structures at upper mantle depths down to 400 km in particular the new model is resolved to greater depths than the approximately 200 km depth obtainable in most fundamental mode surface wave tomographic studies and is better resolved at shallow lithospheric depths than that possible in teleseismic body wave tomographic studies because the upper mantle s wave velocity is controlled mainly by temperature rather than by composition thermal structures estimated from the tomographic s wave velocity model are used to estimate the thickness of the lithosphere beneath china the results indicate that in eastern china the north china craton and the yangtze craton are characterized by pronounced variations in lithospheric thickness with the lithosphere thinning from west to east the underlying low velocity layer and deeper level high velocities provide important evidence in support of the idea that lithospheric thinning beneath eastern china is related to the westward subduction of oceanic lithosphere to the east and related thermal and dehydration processes beneath western china thickened lithosphere is imaged beneath the qinghai tibetan plateau except for beneath qiangtang in the northern part of the plateau which is underlain by low velocities the present model generally supports the interpretation of low velocities beneath qiangtang as mantle material squeezed out by convergence between the indian and asian lithospheres moreover two separate segments of the subducting indian lithosphere are imaged indicating that the deeper portion of the subducting slab may have started to break off from the shallower portion as early as 20 ma

seismic receiver functions and the lithosphere asthenosphere boundary abstract the lower boundary of the lithospheric plates has remained as an enigmatic boundary for seismologists since it is relatively poorly observed by seismic means there is traditionally a broad consensus that the asthenosphere is observable as a low velocity zone by seismic surface waves seismic techniques which use shorter period p to s or s to p converted body waves are now far enough developed to be successful in observing such a low velocity zone with a higher resolution the principle of this technique the so called receiver function technique is that a strong teleseismic mother phase e g p s pp or sks incident from below on any seismic discontinuity beneath a station produces a converted phase ps or sp which indicates its depth and properties we discuss details of this technique a sufficient number of such observations exist already to indicate that the top of the low velocity zone is a globally observable discontinuity and it is sharper than previously thought an intriguing observation is that in some cratons the new seismic data indicate that the low velocity zone exists already at shallower depths than obtained from surface waves this confirms earlier results from controlled source observations thybo and perchuc 1997 we discuss possible interpretations of this shallow low velocity zone in cratonic regions

lithospheric structure variations between the eastern and central north china craton from s and p receiver function migration abstract i imaged the lithospheric structure beneath the bohai bay basin bbb in the eastern north china craton ncc and the taihang mountains tm and areas further to the west in the central ncc by applying the wave equation migration technique to both the s and p receiver functions rfs collected in this region the data images and synthetic modeling results indicate that s rf imaging is more appropriate than p rf imaging for mapping lithospheric discontinuities especially the lithosphere asthenosphere boundary lab in various types of regions even directly applicable to basin areas that are covered by thick sediments the constructed s rf images show that the lithospheric thickness of the study region is highly variable the imaged lab is on average 80 km beneath the bbb and displays obvious undulations from 120 km under the tm to 40 km under the mountain areas in the central ncc consistent with previous p rf imaging results the observed structural differences between the eastern and central ncc are likely associated with different lithospheric tectonics in these two regions while widespread lithosphere reactivation and extension in the eastern ncc during the late mesozoic time was plausibly responsible for both the thinned crust and lithosphere in the bbb lithospheric thinning in the central ncc may be equally heterogeneous but less significant and probably occurred mainly in cenozoic such a process may have given rise to the highly variable present day lithospheric thickness in this region however it probably did not noticeably affect the overlying crust that was reported to have thickened in response to the early magmatic underplating in the mesozoic time the different tectonic processes in the eastern and central ncc may have induced the formation of the nsgl which likely serves as a major tectonic boundary separating the two regions of contrasting lithospheric structures at present

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

progress in deep lithospheric exploration of the continental china a review of the sinoprobe abstract the sinoprobe deep exploration in china is a multidisciplinary earth science research program that aims at revealing the composition structure and evolution of the continental lithosphere in china the sinoprobe 2008 2012 has successfully conducted research and field experiments on determining the crustal and mantle structures using new deep seismic and magnetotelluric mt exploration this has allowed the accumulation of new exciting data that have significantly accelerated china s development on deep lithospheric exploration the new data also led to new understandings on the mesozoic and cenozoic geological evolution of the continental china the main results of the sinoprobe so far include 1 a collection of ca 6000 km long seismic reflection profile data 2 a nation wide geochemical baseline 3 a nation wide 4 4 mt array and regional 1 1 mt arrays in the north china and the qinghai tibet plateau 4 three dimensional 3 d exploration of ore districts in the eastern china 5 several continental scientific drilling holes 6 regional in situ stress monitoring networks 7 geodynamic modeling of the lithosphere underneath the continental china and 8 instrumentation development for deep exploration in china etc for the first time the sinoprobe has obtained deep seismic reflection evidence for the moho surface below the thick crust of the central qinghai tibet plateau it also reveals dipping fabrics in the lithospheric mantle beneath the northeastern china the preliminary results from mt array observation of the sinoprobe show an abnormal electric conductivity structure of the lithosphere beneath the ordos basin providing important evidence for the evolution mechanism of the north china craton ncc generally the sinoprobe has conducted successfully during its initial phase 2008 2012 which has settled a foundation for the next phase of the sinoprobe in the future

inversion of receiver functions without deconvolution application to the indian craton s u m m a r y receiver functions are a powerful tool to isolate and interpret receiver side structure effects in teleseismic seismic records they are easily constructed by deconvolving one component of a seismogram by another deconvolution is the inverse of convolution and hence can be mathematically viewed as an inverse problem it is a numerically unstable procedure that needs to be stabilized i e regularized this points to a recurring problem in geophysical imaging there is a trade off between variance and resolution where the user needs to arbitrarily define a level of compromise here we propose a novel misfit function for inversion of converted phases that avoids deconvolution in this way the choice of regularization parameters e g water level width of a low pass filter is avoided and statistics of data errors can be correctly accounted for we use this misfit measure to construct a likelihood probability function and carry out a transdimensional bayesian inversion for shear wave structure after illustrating the method with a synthetic test a real data application is shown where teleseismic signals recorded at hyb station hyderabad india and surface wave dispersion measurements are jointly inverted to provide a probabilistic 1 d seismic model beneath the station the results help address the debate on the thickness of the lithosphere in this region we show that the sharp negative velocity jump at 110 km that was previously interpreted as the lithosphere asthenosphere boundary lab is actually a mid lithospheric discontinuity the actual lab is seen deeper as a milder gradient between 150 and 200 km