lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

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

contemporary crustal deformation in east asia constrained by global positioning system measurements global positioning system gps measurements collected since the early 90 s allow us to derive geodetic velocities at 16 permanent stations in east asia and 68 campaign mode sites in north china the resulting velocity field shows the following 1 contrary to the early inferences that the shanxi rift has accommodated significant right slip motion our results suggest that the rift system at least in its northern part in north china is under ese wnw extension at a rate of 4 2 mm yr the velocity field also suggests that an ese wnw trending left lateral shear zone deforming at a rate of 2 1 mm yr may exist along the north rim of north china at the latitude of 40 n separating actively extending north china in the south from relatively stable mongolia in the north 2 central and east china move at a rate of 8 11 mm yr east southeast with respect to siberia implying that the overall east southeastward motion is the dominant mode of deformation in east china 3 the india plate moves at a rate of 6 1 mm yr slower than the nuvel 1a model prediction relative to the eurasia plate 4 significant eastward motion 20 2 mm yr with respect to siberia occurs in southeastern tibet about half of this eastward motion 11 mm yr is absorbed by structures along the eastern boundary of the tibetan plateau

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

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

organic walled microfossil assemblages from glacial and interglacial neoproterozoic units of australia and svalbard before the onset of the neoproterozoic snowball earth glaciations eukaryotes had begun diversifying and in their aftermath macroscopic life including both animals and macroalgae became abundant and widespread although glacially driven mass extinctions have been hypothesized little is known about the biosphere during and between these glaciations here we present new data from organic walled microfossil assemblages from five successions in australia and svalbard that collectively span the first sturtian glaciation and interglacial interval and integrate them with data derived from a critical evaluation of the literature to produce a new estimate of eukaryotic diversity from 850 to 650 ma these new glacial and interglacial assemblages consist of only smooth walled spheroids leiosphaerids aggregates of cells and filaments in contrast to the much more diverse organic walled microfossil assemblages found in early neoproterozoic rocks this contrast is not attributed to biases in deposition or preservation but is instead interpreted as reflecting an interval of lowered eukaryotic diversity that spanned the glaciations and that may have begun millions of years prior to their onset

interactions between thin and thick skinned tectonics at the western sichuan basin china abstract this study investigates the geometry kinematics and interaction of thin and thick skinned tectonics at the western sichuan basin adjacent to the eastern margin of the tibetan plateau a two level detachment system comprising a shallow triassic and basal precambrian detachment was identified three dimensional 3 d structural modeling was applied to demonstrate the geometry of the shallow detachment that developed in the sichuan foreland basin the shallow detachment forming material is an evaporite sequence deposited in the middle triassic which was subsequently buried beneath the foreland basin and later subsided rapidly during the flexural loading process during the late triassic in the cenozoic this set of evaporite rocks developed into an effective shallow detachment transferring geologic slip from the longmen shan range front to the sichuan basin the structure of this shallow detachment exhibits an asymmetrical characteristic with its deepest location consistent with the mesozoic foreland depression this relationship indicates that foreland subsidence in the late mesozoic is the main factor controlling the geometry of the detachment in the southern cenozoic foreland basin the development of thrust faults above the precambrian basal detachment resulted in a local deformation and uplift of the shallow detachment resulting in a lower basal slope 0 5 1 of the sedimentary basin taper and a high internal distribution in contrast the sedimentary basin taper in the north has a high basal slope that mainly translates along the shallow detachment lacking thrusting related structures this study reveals that the pre existing stratigraphic architecture of the sedimentary basin can influence the subsequent structural deformation of the foreland basin this has important implications for other foreland basins with multistage crustal shortening processes

co seismic ground shortening structures produced by the 2008 mw 7 9 wenchuan earthquake china abstract we observed ground shortening structures produced by the 2008 m w 7 9 wenchuan earthquake along the 285 km long wenchuan co seismic surface rupture zone the structures are characterized by co seismic thrusting and folding structures that include thrust fault scarps flexure slip folds and mole tracks fold structures are generally developed within alluvial deposits river channel deposits and sealed road surfaces mainly restricted to within a

topography reveals seismic hazard the devastating earthquake in the chinese province of sichuan struck an area that was not expected to suffer seismic activity of such magnitude yet topographic analyses of the region indicate active deformation suggesting a way of refining maps of earthquake risk elsewhere

contemporary crustal deformation around the southeast borderland of the tibetan plateau 1 we derive a detailed horizontal velocity field for the southeast borderland of the tibetan plateau using gps data collected from the crustal motion observation network of china between 1998 and 2004 our results reveal a complex deformation field that indicates that the crust is fragmented into tectonic blocks of various sizes separated by strike slip and transtensional faults most notably the regional deformation includes 10 11 mm yr left slip across the xianshuihe fault 7 mm yr left slip across the anninghe zemuhe xiaojiang fault zone 2 mm yr right slip across a shear zone trending northwest near the southern segment of the lancang river fault and 3 mm yr left slip across the lijiang fault deformation along the southern segment of the red river fault appears not significant at present time the region south and west of the xianshuihexiaojiang fault system whose eastward motion is resisted by the stable south china block to the east turns from eastward to southward motion with respect to south china resulting in clockwise rotation of its internal subblocks active deformation is detected across two previously unknown deformation zones one is located 150 km northwest of and in parallel with the longmenshan fault with 4 6 mm yr right slip and another is continued south southwestward from the xiaojiang fault abutting the red river fault with 7 mm yr left slip while both of these zones are seismically active the exact locations of faults responsible for such deformation are yet to be mapped by field geology comparing our gps results with predictions of various models proposed for tibetan plateau deformation we find that the relatively small sizes of the inferred microblocks and their rotation pattern lend support to a model with a mechanically weak lower crust experiencing distributed deformation underlying a stronger highly fragmented upper crust

geodetic measurement of crustal motion in southwest china global positioning system measurements performed over a 2 4 yr period confirm the xianshuihe xiaojiang fault system as one of the primary active structures in southwest china stations southwest of these faults show southerly motions of 5 15 mm yr relative to the western sichuan basin and stations in a 200 km geodetic network located northwest of the basin move at only 0 5 mm yr these results are consistent with clockwise rotation of southwestern sichuan and western yunnan about the eastern tibetan syntaxis accommodated by left lateral slip of 12 4 mm yr on the xianshuihe xiaojiang fault system the results imply that if eastward extrusion of crustal material from the plateau occurs at present it is not accommodated by east west shortening along the margin of the plateau and must involve wholesale eastward motion of low lying regions to the east

present day kinematics at the india asia collision zone the collision of the indian subcontinent with asia drives the growth and evolution of the greater tibetan plateau region fault slip rates resulting from the relative motion between crustal blocks can provide a kinematic description of the distribution of presentday deformation i construct a three dimensional regional scale elastic block model of the india asia collision zone that is consistent with geodetic observations of interseismic deforma tion mapped fault system geometry historical seismicity and the mechanics of the earthquake cycle this mechanical model of the elastic upper crust yields a set of kinematically consistent fault slip rates and block motions that may serve to constrain dynamic models of continental crustal dynamics

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

co seismic ruptures of the 12 may 2008 ms 8 0 wenchuan earthquake sichuan east west crustal shortening on oblique parallel thrusts along the eastern edge of tibet abstract the m s 8 0 wenchuan earthquake which devastated the mountainous western rim of the sichuan basin in central china produced a surface rupture over 200 km long with oblique thrust dextral slip and maximum scarp heights of 10 m it thus ranks as one of the world s largest continental mega thrust events in the last 150 yrs field investigation shows clear surface breaks along two of the main branches of the ne trending longmen shan thrust fault system the principal rupture on the nw dipping beichuan fault displays nearly equal amounts of thrust and right lateral slip basin ward of this rupture another continuous surface break is observed for over 70 km on the parallel more shallowly nw dipping pengguan fault slip on this latter fault was pure thrusting with a maximum scarp height of 3 5 m this is one of the very few reported instances of crustal scale co seismic slip partitioning on parallel thrusts this out of sequence event with distributed surface breaks on crustal mega thrusts highlights regional ew directed present day crustal shortening oblique to the longmen shan margin of tibet the long rupture and large offsets with strong horizontal shortening that characterize the wenchuan earthquake herald a re evaluation of tectonic models anticipating little or no active shortening of the upper crust along this edge of the plateau and require a re assessment of seismic hazard along potentially under rated active faults across the densely populated western sichuan basin and mountains

static coulomb stress changes on faults caused by the 2008 mw 7 9 wenchuan china earthquake abstract the 12 may 2008 mw 7 9 wenchuan earthquake has changed the regional tectonic stress field significantly it is important to know how such a change affects the tectonic loading processes of the faults and therefore the seismic potential of the region utilizing the slip distribution of the wenchuan earthquake as the driving source we compute the changes of coulomb failure stress cfs on the neighborhood faults assuming a friction coefficient of 0 4 our results show that the cfss have increased on average 1 6 104 and 1 8 104 pa at the sw and ne ends of the longmen shan 2 6 104 pa at the western segment of the qinling southern frontal 2 0 104 pa at the northernmost part of the longriba 1 4 104 pa at the southernmost part of the xianshuihe 1 3 104 pa at the southeast end of the east kunlun 9 0 103 pa at the southernmost part of the minjiang 2 9 103 pa at the baiyu and 3 5 103 pa at the longxian baoji faults recurring times of these faults are advanced about 39 and 230 years for the sw and ne segments of the longmen shan 5 5 years toward the east end of the east kunlun and 1 6 years for the se segments of the xianshuihe faults the quake also relaxed the cfs significantly on most part of the minjiang southwest segment of the longriba northwest segment of the xianshuihe southeast segment of the yushu maqu north part of the xiaojinhe daliangshan and anninghe faults with the cfs reductions ranging on average of 1 8 104 2 5 104 7 1 103 2 4 103 1 0 103 1 0 103 and 1 6 103 pa respectively recurring times of these faults are delayed up to 185 years for the south segment of the minjiang up to 18 years for the northeast of the longriba up to 9 8 years for the west qinling northern frontal and up to 2 7 years for the northwest segment of the xianshuihe faults respectively we test sensitivity of the changes of cfs to variations of apparent fiction coefficient and find that the senses of cfs change are almost invariant with except at the minjiang longriba and qinling southern frontal faults where large normal stresses on faults induced by the wenchuan earthquake could play an important role in manifesting cfs change when is large the cfs changes vary slowly along seismogenic depth on most of the receiver faults

structural setting of the 2008 mw 7 9 wenchuan china earthquake the 2008 m w 7 9 wenchuan earthquake ruptured an imbricate thrust system in the longmen shan range which forms the eastern boundary of the tibetan plateau we use seismic reflection data and surface geology to construct geologic cross sections and a three dimensional 3d fault model in the range front the rupture was complex with slip on both the steeply dipping beichuan fault and the more shallowly dipping pengguan fault which appear to merge at depth in contrast only the beichuan fault ruptured in the north with the transition near a lateral fault offset thus the earthquake involved multiple thrust splays and breached a significant segment boundary to investigate the tectonic setting of the earthquake we extend our previous measurements of crustal shortening hubbard and shaw 2009 into the interior of the range and show that shortening correlates with topography this suggests that upper crustal shortening can produce the high topography of the longmen shan without calling on other uplift mechanisms based on this understanding we model the thrust belt as a critical taper wedge to assess what rock and fault strengths are required to explain the topographic slopes within the range front and basin the low taper within the basin is consistent with a weak detachment f 0 1 while a slightly stronger f 0 3 0 4 detachment is necessary to explain the higher taper of the range front this difference may relate to the localization of the detachment within a triassic evaporite sequence within the basin while the faults beneath the range front are rooted in precambrian metasedimentary or igneous rocks critical taper theory implies that shortening in the range front where the 2008 wenchuan earthquake occurred is many times greater than in the basin consistent with our shortening measurements we examine the implications of these findings for seismic hazard assessment in the region and in other active mountain belts around the world

shallow seismic imaging of folds above the puente hills blind thrust fault los angeles california 1 high resolution seismic reflection profiles image discrete folds in the shallow subsurface 600 m above two segments of the puente hills blind thrust fault system los angeles basin california the profiles demonstrate late quaternary activity at the fault tip precisely locate the axial surfaces of folds within the upper 100 m and constrain the geometry and kinematics of recent folding the santa fe springs segment of the puente hills fault zone shows an upward narrowing kink band with an active anticlinal axial surface consistent with fault bend folding above an active thrust ramp the coyote hills segment shows an active synclinal axial surface that coincides with the base of a 9 m high scarp consistent with tip line folding or the presence of a backthrust the seismic profiles pinpoint targets for future geologic work to constrain slip rates and ages of past events on this important fault system

active tectonics of the beichuan and pengguan faults at the eastern margin of the tibetan plateau the steep high relief eastern margin of the tibetan plateau has undergone rapid cenozoic cooling and denudation yet shows little evidence for large magnitude shortening or accommodation generation in the foreland basin we address this paradox by using a variety of geomorphic observations to place constraints on the kinematics and slip rates of several large faults that parallel the plateau margin the beichuan and pengguan faults are active dominantly dextral slip structures that can be traced continuously for up to 200 km along the plateau margin both faults offset fluvial fill terraces that yield inheritance corrected cosmogenic 10be exposure ages of 15 kyr indicating latest pleistocene activity the pengguan fault appears to have been active in the holocene at two sites along strike latest quaternary apparent throw rates on both faults are variable along strike but are typically 1 mm yr 1 rates of strike slip displacement are likely to be several times higher probably 1 10 mm yr 1 but remain poorly constrained late quaternary folding and dextral strike slip has also occurred along the western margin of the sichuan basin particularly associated with the present day mountain front these observations support models for the formation and maintenance of the eastern plateau margin that do not involve major upper crustal shortening they also suggest that activity on the margin parallel faults in eastern tibet may represent a significant seismic hazard to the densely populated sichuan basin

distribution of active rock uplift along the eastern margin of the tibetan plateau inferences from bedrock channel longitudinal profiles 1 current models of long term river incision into bedrock suggest that the local rate of differential rock uplift should exert a primary control on the gradient of channel longitudinal profiles however discrimination of this effect from the influence of variations in substrate erodibility sediment flux precipitation and transient changes in profile shape has proved difficult in practice here we investigate the controls on the spatial distribution of bedrock channel gradients adjacent to the sichuan basin in an effort to assess the degree and nature of active deformation along this margin of the tibetan plateau analysis of river longitudinal profiles utilizing a channel steepness index a measure of profile gradient normalized for drainage area reveals a zone of anomalously steep channels adjacent to the topographic front of the plateau margin channel profiles are systematically less steep in their headwater reaches on the plateau and in their lower reaches east of the plateau margin comparison of steepness indices to mapped lithologic variations reveals that lithology has only a limited influence on channel gradient in this field area we observe no systematic relationship between steepness indices and upstream drainage area channels of all size are steeper near the plateau margin we argue that these systematic changes are not readily explained as a consequence of increased sediment flux or of orographic precipitation we are led to conclude that steep channel profiles along the topographic front of the plateau reflect active differential rock uplift between this region and the foreland

present day crustal deformation in china constrained by global positioning system measurements global positioning system gps measurements in china indicate that crustal shortening accommodates most of india s penetration into eurasia deformation within the tibetan plateau and its margins the himalaya the altyn tagh and the qilian shan absorbs more than 90 of the relative motion between the indian and eurasian plates internal shortening of the tibetan plateau itself accounts for more than one third of the total convergence however the tibetan plateau south of the kunlun and ganzi mani faults is moving eastward relative to both india and eurasia this movement is accommodated through rotation of material around the eastern syntaxis the north china and south china blocks east of the tibetan plateau move coherently east southeastward at rates of 2 to 8 millimeters per year and 6 to 11 millimeters per year respectively with respect to the stable eurasia

estimating surface rupture length and magnitude of paleoearthquakes from point measurements of rupture displacement we present a method to estimate paleomagnitude and rupture extent from measurements of displacement at a single point on a fault the variability of historic ruptures is summarized in a histogram of normalized slip then scaled to give the probability of finding a given displacement within a rupture for any magnitude considered the histogram can be inverted assuming any magnitude earthquake is as likely as another yielding probability density functions of magnitude and rupture length for any given displacement measurement to improve these distributions we include a term to account for the probability that the earthquake would cause ground rupture and two alternative distributions of earthquake magnitude the gutenberg richter magnitude distribution predicts shorter rupture lengths and smaller magnitudes than does a uniform distribution where any magnitude earthquake is considered equally likely longer ruptures and larger magnitudes than the uniform model are predicted by an alternative magnitude distribution designed to return site average displacement this model is a generalization of the characteristic earthquake model and reasonably describes paleoseismic findings on the southern san andreas fault where slip is accommodated by average displacements of a few meters and earthquake recurrence times of 100 250 years our results should increase the value of paleoseismic displacement measurements for hazard assessment in particular they quantify probability estimates of earthquake magnitude and rupture length where point observations of rupture displacement are available and so can contribute to probabilistic seismic hazard analyses online material rupture profiles used to sample rupture variability

puente hills blind thrust system los angeles california we describe the three dimensional geometry and quaternary slip history of the puente hills blind thrust system pht using seismic reflection profiles petroleum well data and precisely located seismicity the pht generated the 1987 whittier narrows moment magnitude m w 6 0 earthquake and extends for more than 40 km along strike beneath the northern los angeles basin the pht comprises three north dipping ramp segments that are overlain by contractional fault related folds based on an analysis of these folds we produce quaternary slip profiles along each ramp segment the fault geometry and slip patterns indicate that segments of the pht are related by soft linkage boundaries where the fault ramps are en echelon and displacements are gradually transferred from one segment to the next average quaternary slip rates on the ramp segments range from 0 44 to 1 7 mm yr with preferred rates between 0 62 and 1 28 mm yr using empirical relations among rupture area magnitude and coseismic displacement we estimate the magnitude and frequency of single m w 6 5 6 6 and multisegment m w 7 1 rupture scenarios for the pht manuscript received 16 november 2001

late cenozoic evolution of the eastern margin of the tibetan plateau inferences from 40ar 39ar and u th he thermochronology high topography in central asia is perhaps the most fundamental expression of the cenozoic indo asian collision yet an understanding of the timing and rates of development of the tibetan plateau remains elusive here we investigate the cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the sichuan basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed temperature time paths inferred from 40 ar 39 ar thermochronology of biotite multiple diffusion domain modeling of alkali feldspar 40 ar release spectra and u th he thermochronology of zircon and apatite imply that rocks at the present day topographic front of the plateau underwent slow cooling 30 50 c m y coincident with exhumation from inferred depths of 8 10 km at denudation rates of 1 2 mm yr samples from the interior of the plateau continued to cool relatively slowly during the same time period 3 c m y suggesting limited exhumation 1 2 km however these samples record a slight increase in cooling rate from 1 to 3 c m y at some time during the middle tertiary the tectonic significance of this change remains uncertain regardless late cenozoic denudation in this region appears to have been markedly heterogeneous with the highest rates of exhumation focused at the topographic front of the plateau margin we infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable sichuan basin and thus marks the onset of deformation related to the development of the tibetan plateau in this region the present margin of the plateau adjacent to and north of the sichuan basin is apparently no older than the late miocene or early pliocene 5 12 ma

co seismic thrusting rupture and slip distribution produced by the 2008 mw 7 9 wenchuan earthquake china field investigations reveal that the mw 7 9 wenchuan china earthquake of 12 may 2008 produced a 285 km long surface rupture zone with dominantly thrusting slip accompanied by a right lateral component along the central northern segments of the zone and left lateral component along the southern segment along the longmen shan thrust belt eastern margin of the tibetan plateau the co seismic ruptures mainly occurred along the pre existing yingxiu beichuan guanxian anxian and qingchuan faults which are the main faults of the longmen shan thrust belt the displacements measured in the field are approximately 0 5 6 5 m in the vertical typically 1 3 m accompanied by an average left lateral component of 2 m along the 50 km long southernmost segment of the rupture zone and an average right lateral component of 1 m along the 150 km long central northern segments the maximum thrust slip amount is estimated to be 10 m accompanied by 9 m of shortening across the rupture zone this finding is consistent with estimates based on seismic data the rupture length and maximum vertical displacement are the largest among all intracontinental thrust type earthquakes reported to date our findings demonstrate that i the wenchuan earthquake occurred upon pre existing active faults of the longmen shan thrust belt thereby controlling the spatial distribution of co seismic surface rupture and displacement and the rupture processes of the earthquake ii the long rupture length and large thrusting slip resulted from compressive stress associated with eastward extrusion of the tibet plateau as it accommodates the ongoing penetration of the indian plate into the eurasian plate and iii present day shortening strain upon the eastern margin of the tibetan plateau is mostly released by seismic slip along thrust faults within the longmen shan thrust belt

longmen shan fold thrust belt and its relation to the western sichuan basin in central china new insights from hydrocarbon exploration the longmen shan fold thrust belt is one of the key regions of demonstrable mesozoic cenozoic tectonic evolution in china and the sichuan basin was the first natural gas producing area in china in this article the structural features of the longmen shan belt are presented using both seismic profiles and field data the complex structures of the northeast trending longmen shan fold thrust belt and its foreland in the western sichuan basin are formed by southeast directed thrusting several eastward verging rootless thrust sheets and imbricates of cambrian triassic rocks have been recognized in the northern longmen shan belt evidence suggests that the northern longmen shan belt experienced at least two major periods of deformation in the late triassic and cenozoic however the southern longmen shan belt is represented by the basement involved thrust structures and klippen and its major periods of deformation were in the latest cretaceous early cenozoic sedimentary features in the western sichuan basin reflect a two phase flexural loading history and illustrate that the late triassic foreland basin extends along the foredeep of the entire length of the longmen shan belt but the uppermost cretaceous paleogene rejuvenated foreland basin is restricted in the southern part of the western sichuan basin structural geometries suggest that prospective traps are mainly developed in the frontal zone of the longmen shan fold thrust belt and in the southern part of the western sichuan basin one of the major contributions of this article is finding preexisting paleozoic rift basins under the cenozoic thin skinned thrust belts which represent a new potential hydrocarbon play

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

coseismic reverse and oblique slip surface faulting generated by the 2008 mw 7 9 wenchuan earthquake china the mw 7 9 wenchuan china earthquake ruptured two large thrust faults along the long menshan thrust belt at the eastern margin of the tibetan plateau this earthquake generated a 240 km long surface rupture zone along the beichuan fault and an additional 72 km long surface rupture zone along the pengguan fault maximum vertical and horizontal offsets of 6 5 m and 4 9 m respectively were measured along the beichuan fault a maximum vertical offset of 3 5 m was measured along the pengguan fault coseismic surface ruptures integrated with aftershocks and industry seismic profiles show that two imbricate structures have ruptured simultaneously resulting in the largest continental thrust event ever documented large oblique thrusting observed during this earthquake indicates that crustal shortening is an important process responsible for the high topography in the region as everywhere along the edge of tibetan plateau

active faulting and block movement associated with large earthquakes in the min shan and longmen mountains northeastern tibetan plateau the min shan and central and southern parts of the longmen mountains form the middle segment of the north trending earthquake zone at the eastern edge of the tibetan plateau the min shan mountains are a cenozoic to quaternary uplift zone that has undergone south directed block movements to produce the 1933 diexi earthquake m 7 5 on the min jiang fault and the 1976 songpan earthquake swarm m 7 2 6 7 7 2 on the huya fault the min jiang fault on the western boundary of the uplift zone is dominated by reverse movement with a right lateral component the huya fault on the eastern margin of the min shan uplift zone is also dominated by reverse movement but is associated with a left lateral component

total exhumation across the beichuan fault in the longmen shan eastern tibetan plateau china constraints from petrology and thermobarometry the deep structure and deformation mechanisms of the longmen shan thrust belt sichuan china at the eastern border of the tibetan plateau were largely debated after the devastating mw 7 9 wenchuan earthquake 2008 recent geophysical studies and field investigations have been focused on the active motion of the major beichuan fault which ruptured during the earthquake however the total exhumation across the fault still remains unclear in the hanging wall of the beichuan fault the south china block is exhumed in the pengguan massif close to the beichuan fault the rocks of the pengguan massif underwent greenschist facies metamorphism associated with brittle ductile deformation no metamorphism is observed in the footwall of the fault in this study we characterize and date the metamorphic history recorded in the hanging wall of the beichuan fault in order to constrain the depth and timing of exhumation of the rocks of the pengguan massif along the fault a high resolution petrological approach involving chemical analyses and x ray maps was used to analyze the micrometric metamorphic minerals the p t conditions of the greenschist facies metamorphic event were estimated by an inverse multi equilibrium thermodynamic approach the results 280 30 c and 7 1 kbar suggest that the rocks of the pengguan massif were exhumed from ca 20 km depth our results underline the importance of the thrusting component in the long term behavior of the beichuan fault and provide a minimal depth at which the fault is rooted in situ laser ablation 40ar 39ar dating of metamorphic white mica revealed that the greenschist overprint occurred at 135 140 ma the pengguan massif was therefore partially thrusted along the beichuan fault during the lower cretaceous long before the eocene miocene exhumation phase which is well constrained by low temperature thermochronology our results provide the first independent depth information for the exhumation history of the pengguan massif and reveal a previously undocumented lower cretaceous tectonic event that marks the onset of the thick skinned deformation in the external domain of the longmen shan east of the wenchuan fault

scaling relations of source parameters of earthquakes occurring on inland crustal mega fault systems we examined a new scaling relation between source area s and seismic moment m 0 for large crustal earthquakes on mega fault systems including earthquakes with magnitudes larger than m w7 4 we focused on earthquakes that occurred on inland crustal mega fault systems such as the 2008 wenchuan and 2002 denali earthquakes and compiled the source parameters using 11 inland crustal earthquakes which analyses of source rupture processes by waveform inversion as well as investigation of surface ruptures via geomorphological surveys we found that the maximum surface rupture displacement is two to three times larger than the average slip on the source fault and the length of the surface rupture is equivalent to the length of the source fault furthermore our compiled data shows the displacement of the surface rupture d saturates around 10 m when the length of the surface rupture l reaches 100 km assuming that the average width of the source fault w 18 km for japanese inland crustal earthquakes and the saturated surface displacement d 10 m we found that the scaling relations between rupture area s and seismic moment m 0 have three stages for the first stage s is proportional to m 0 2 3 for earthquakes smaller than m 0 7 5 1018 nm for the second stage s ranges from m 0 1 2 to m 0 2 3 depending on the thickness of the seismogenic zone for the third stage s is proportional to m 0 because of the saturation of the slip on the fault from our compiled data we derived the third scaling relation between source area s and seismic moment m 0 for inland crustal mega fault systems to be s km2 1 0 10 17 m 0 nm where m 0 1 8 1020 nm

the role of crustal strength variations in shaping orogenic plateaus with application to tibet 1 the tibetan plateau is the type example of a large orogenic plateau formed as a result of continent continent collision the morphology of the plateau and its margins suggests that preexisting variations in crustal strength have influenced the growth of the plateau we have developed a three dimensional numerical model of deformation in a viscous crust in order to investigate the effects of lateral heterogeneities on plateau growth the model includes a two layer crust and allows for lateral variation of viscosity in both the upper and lower layers model results indicate that crustal strength variations have a dramatic effect on the morphology and dynamics of a developing plateau a region of strong crust is characterized by a very steep plateau margin that propagates extremely slowly does not accommodate significant shortening strain and is subparallel to local upper and lower crustal velocities a weak crustal region develops a gently sloping margin uplift propagates rapidly across the weak zone and crustal material within the plateau is diverted toward the low strength region with a relatively simple distribution of strength variations corresponding to strong tarim and sichuan basin crust and a weak southeastern corner our model produces a plateau with many similarities to the tibetan plateau including the overall morphology rotation around the eastern syntaxis and e w extension analysis of model results suggests that e w extension of the central plateau may be related to the rapid eastward flow of crustal material into a weak zone in the se corner without significant change in plateau elevation

balance of seismic moment in the songpan ganze region eastern tibet implications for the 2008 great wenchuan earthquake abstract the 12 may 2008 wenchuan earthquake ms 8 0 occurred on the longmen shan fault zone where slip rates are low and earthquakes are infrequent in comparison with other major fault zones in the songpan ganze region eastern tibet we have investigated the evolution of strain energy along major faults in this region by comparing the accumulation and release of seismic moment first we calculated the slip rates on the longmen shan and other major faults in the region using a three dimensional regional scale block model constrained by the latest gps data on the longmen shan fault the predicted right lateral and dip slip rates are respectively 1 7 0 8 mm year and 1 2 1 0 mm year along the southwestern segments and 1 4 1 1 mm year and 3 3 1 3 mm year on the northeastern segments these slip rates are one order of magnitude lower than those on the xianshuihe fault and other faults in the region second using the earthquake catalog we estimated the scalar moment released on major faults in the songpan ganze region between 1879 and 2007 the released seismic moment was 63 of the regional scalar moment accumulated during this period the moment deficits were found mainly on the western xianshuihe and eastern kunlun faults the eastern xianshuihe fault has been a focus of studies because of the high slip rates and frequent earthquakes but a sequence of major quakes there since 1893 has reduced the moment deficit to 3 68 1019 n m barely enough for a mw 7 0 event it takes the longmen shan fault more than 1000 years to accumulate the seismic moment 1 15 1021 n m released during the 2008 great wenchuan earthquake hence we conclude that a repeat of great earthquakes on the ruptured segment of the longmen shan fault is unlikely in the next few hundred years but the unruptured southwestern segment of the longmen shan fault is capable of producing a mw 7 7 earthquake in the next 50 years

structural interpretation of the coseismic faults of the wenchuan earthquake three dimensional modeling of the longmen shan fold and thrust belt 1 the 2008 mw 7 9 wenchuan earthquake is a result of ongoing india tibet collision and reflects the growth of the longmen shan fold and thrust belt in this paper we construct a 3 d structural model of the geometry of the coseismic faults and related structures of the wenchuan earthquake by integrating geological investigations relocated aftershocks and seismic reflection profiles in the 3 d structural model the differences between the southern and northern segments of the rupture are highlighted the structural transition zone between the two segments contains a major geometric segment boundary reflecting differences in the structural configuration of the thrust ramp and the tectonic evolution of the fault system which appears to have localized significant damage from anxian to beichuan within the northern segment we identify a transverse structure across which the beichuan fault plunges under the tangwangzhai syncline this boundary corresponds to a marked change in the nature of the surface rupture and is illuminated by a microearthquake sequence perpendicular to the longmen shan thrust belt in the southern segment our investigations confirm that uplift due to active faulting and folding is largely responsible for the areas of steepest topography on the basis of this association the southwestern segment of the longmen shan south of the wenchuan earthquake is likely active and presents a significant earthquake hazard despite the lack of historical earthquakes in this region this study illustrates the importance of building 3 d models to study active faulting and folding as well as to assess earthquake hazard

late holocene activity and historical earthquakes of the qiongxi thrust fault system in the southern longmen shan fold and thrust belt eastern tibetan plateau abstract the 12 may 2008 wenchuan earthquake m w 7 9 generated a 285 km long surface rupture zone along the longmen shan fold and thrust belt lsftb on the eastern margin of tibetan plateau the wenchuan earthquake did not rupture into the southwestern longmen shan along which there is no evidence for large paleo or historical earthquakes seismic reflection profiles and field investigations reveal that the 50 km long qiongxi thrust fault qtf of the southern lsftb is currently active the qtf consists of three west dipping ramp segments overlain by fault bend folds rooted in a regional detachment that transfers shortening from the mountain belt into the sichuan basin trench investigations coupled with interpretations of seismic reflection profiles and radiocarbon results show that a recent surface rupturing earthquake occurred on the qtf during the late ming to qing dynasty between ad 1600 and 1800 in addition seismic reflection profile and topographic analysis indicate the presence of a subtle topographic produced by kink band migration folding above a fault bend at about 5 km depth these findings confirm that the qtf is a significant seismic hazard and that it should be incorporated into current regional seismic hazard models for the densely populated sichuan basin

late cenozoic uplift of southeastern tibet the age of surface uplift in southeastern tibet is currently unknown but the initiation of major river incision can be used as a proxy for the timing of initial uplift the topographically high eastern plateau and gently dipping southeastern plateau margin are mantled by an elevated low relief relict landscape that formed at a time of slow erosion at low elevation and low tectonic uplift rates prior to uplift of the eastern tibetan plateau thermochronology from deep river gorges that are cut into the relict landscape shows slow cooling between ca 100 and ca 10 20 ma and a change to rapid cooling after ca 13 ma with initiation of rapid river incision at 0 25 0 5 mm yr between 9 and 13 ma a rapid increase in mean elevation of eastern tibet beginning at this time supports tectonic climate models that correlate the lateral eastern expansion of high topography in tibet with the late miocene intensification of the indian and east asian monsoons

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

quaternary activity of the range front thrust system in the longmen shan piedmont china revealed by seismic imaging and growth strata reliable estimates of quaternary or cenozoic upper crustal shortening in the longmen shan fold and thrust belt are rare in this paper we report on our use of high resolution 2 d and 3 d seismic reflection profiles at various scales together with borehole data to investigate the structural geometry of the longmen shan piedmont the results reveal a thrust system beneath the longmen shan termed the range front thrust system which consists of the range front blind thrust and its upward splay faults moreover on these faults we identified growth strata that provide an excellent opportunity for assessing the activity of this thrust system analyses of the growth strata reveal early to late pleistocene activity on the range front blind thrust with minimum dip slip and horizontal shortening rates of 1 1 0 2 mm yr and 1 mm yr accordingly the maximum accumulated slip on the range front blind thrust is calculated to be 7 5 0 3 km in the longmen shan using the new horizontal shortening rate and other published data we also estimated that the long term shortening rate across the longmen shan fold and thrust belt is 1 3 mm yr which is comparable to the short term gps rate the similarity of these rates suggests that the longmen shan attained a steady state condition over the past 2 myr an additional highlight of our results is that we show quaternary activity around the tibetan plateau to have been nearly synchronous in different regions including in the longmen shan the himalayas the western kunlun shan and the northern qilian shan

3 d seismic discontinuity for faults and stratigraphic features the coherence cube seismic data are usually acquired and processed for imaging reflections this paper describes a method of processing seismic data for imaging discontinuities e g faults and stratigraphic features one application of this nontraditional process is a 3 d volume or cube of coherence coefficients within which faults are revealed as numerically separated surfaces figure 1 compares a traditional 3 d reflection amplitude time slice with the results of the new method to our knowledge this is the first published method of revealing fault surfaces within a 3 d volume for which no fault reflections have been recorded

an unpredictably violent fault chinese researchers placed a dense array of seismometers around a dangerous looking seam in the rocks of sichuan only to be blindsided by the true killer

emplacement of the longmen shan thrust nappe belt along the eastern margin of the tibetan plateau abstract the topographic descent from the eastern margin of the tibetan plateau to the sichuan basin passes through the longmen shan thrust nappe belt which is sub divided by six nw dipping major listric thrusts with accompanying duplexes and imbricate fans into five large scale nappes each nappe has its own stratigraphic and deformational features in the inner longmen shan longmen mountains allochthonous nappe units have incorporated both mesoproterozoic basement and sinian neoproterozoic to triassic cover sequences as thick skinned horses whereas in the frontal longmen shan sinian cretaceous cover sediments have been stripped from the basement as thin skinned fold and thrust sheets including the extensively distributed klippen structures pre thrusting extension during devonian to middle late triassic times resulted in syn depositional normal faults structural inversion of these faults initiated the peng xian guan xian basement complex jiuding shan and tangwangzhai nappes during the early episode of the indosinian orogeny norian to rhaetian this was followed by episodic thrusting during latest triassic to early cretaceous times to develop the guan xian an xian and southeastern marginal nappes that have incorporated sediments from the neighbouring foreland basin into the frontal part of the thrust nappe belt during a late miocene reactivation of the pre existing thrusts differential thrusting occurred across the thrust nappe belt

osl and ams dating of the penultimate earthquake at the leigu trench along the beichuan fault longmen shan in the northeast margin of the tibetan plateau paleoseismic studies conducted along the surface rupture of the wenchuan earthquake of 12 may 2008 the 5 12 event provide important information regarding the earthquake reccurrence intervals and slip rates of the longmen shan fault zone the leigu trench was excavated along the middle segment of the surface rupture of the yingxiu beichuan fault zone based on the syntectonic sedimentary structure two earthquake events including the 5 12 event were identified at this site this study utilizes optically stimulated luminescence osl dating of samples collected from the leigu trench section using smar sensitivity corrected multiple aliquot regenerative dose protocols ams graphic 1 dating was also carried out on charcoal from the same sediments osl ages are generally consistent with the calibrated ams graphic 2 ages calibrated year b p for the same units the last major earthquake event in this region on the scale of the 5 12 event occurred between 2 1 0 2 ka and 1 1 0 2 ka 1 embed inline graphic 3 gif 2 embed inline graphic 4 gif

earthquake hazards of active blind thrust faults under the central los angeles basin california we document several blind thrust faults under the los angeles basin that if active and seismogenic are capable of generating large earthquakes m 6 3 to 7 3 pliocene to quaternary growth folds imaged in seismic reflection profiles record the existence size and slip rates of these blind faults the growth structures have shapes characteristic of fault bend folds above blind thrusts as demonstrated by balanced kinematic models geologic cross sections and axial surface maps we interpret the compton los alamitos trend as a growth fold above the compton ramp which extends along strike from west los angeles to at least the santa ana river the compton thrust is part of a larger fault system including a decollement and ramps beneath the elysian park and palos verdes trends the cienegas and coyote hills growth folds overlie additional blind thrusts in the elysian park trend that are not closely linked to the compton ramp analysis of folded pliocene to quaternary strata yields slip rates of 1 4 0 4 mm yr on the compton thrust and 1 7 0 4 mm yr on a ramp beneath the elysian park trend assuming that slip is released in large earthquakes we estimate magnitudes of 6 3 to 6 8 for earthquakes on individual ramp segments based on geometric segment sizes derived from axial surface maps multiple segment ruptures could yield larger earthquakes m 6 9 to 7 3 relations among magnitude coseismic displacement and slip rate yield an average recurrence interval of 380 years for single segment earthquakes and a range of 400 to 1300 years for multiple segment events if these newly documented blind thrust faults are active they will contribute substantially to the seismic hazards in los angeles because of their locations directly beneath the metropolitan area

partitioning of localized and diffuse deformation in the tibetan plateau from joint inversions of geologic and geodetic observations abstract the spatial complexity of continental deformation in the greater tibetan plateau region can be defined as the extent to which relative motion of the indian and asian plates is partitioned between localized slip on major faults and distributed deformation potency rates provide a quantitative metric for determining the magnitudes of diffuse and on fault crustal deformation which are proportional to strain rates within crustal micro plates and fault slip rates respectively we simultaneously estimate micro plate rotation rates interseismic elastic strain accumulation fault slip rates on major structures and strain rates within 24 tectonic micro plates inferred from active fault maps in the greater tibetan plateau region using quasi static block models constrained by interseismic surface velocities at 731 gps sites and 9 holocene late quaternary geologic fault slip rates the joint geodetic geologic inversion indicates that geologic slip rates are kinematically consistent with differential micro plate motions estimated left lateral slip rates on the altyn tagh west central kunlun and xianshuihe faults are relatively homogeneous along strike 11 5 10 5 and 11 mm yr respectively while segmentation of the eastern kunlun fault by the intersecting elashan and riyueshan faults results in a decreased slip rate consistent with geologic observations the fraction of total potency rate associated with intrablock strain uncorrected for observational noise ranges from 0 27 in the himalayan range block to 0 91 in the aksai chin block monte carlo simulations are used to quantify the likelihood that internal deformation is statistically distinguishable from the uncertainties in geodetic velocities these simulations indicate that internal block deformation is statistically significant only within the himalayan range front where internal deformation accounts for id 0 11 of block potency rate budget west central plateau id 0 68 ganzi yushu xianshuihe 0 58 burma 0 09 jiali 0 38 and aksai chin 0 68 blocks in the other 18 tectonic micro plates within the plateau region estimated internal block potency is not currently distinguishable from the expected contribution of observational noise to residual velocities of the total potency budget within the tibetan plateau 85 is taken up by slip on major faults with the remaining 15 accommodated by internal processes at sub block scale distinguishable from observational noise the localization of the majority of plate boundary activity is also supported by the spatial distribution of modern and historical crustal earthquakes sixty seven percent of the total moment released by earthquakes in the cmt catalog and 90 of historical moment since 1900 have been released within 25 km of the major faults included in the block model representing only 12 of the characteristic half block length scale of 215 km the localization of deformation inferred from geologic geodetic and seismic observations suggests that forces applied to tectonic micro plates drive fault system activity at the india asia collision zone over decadal to quaternary time scales

paleoseismic evidence and repeat time of large earthquakes at three sites along the longmenshan fault zone abstract in historical records no earthquake of magnitude comparable with that of the wenchuan mw 7 9 earthquake has ever been reported in chengdu and longmenshan regions it is important therefore to assess if there is any evidence of prior large earthquakes in the recent geological record in order to explore this problem 3 trenches were excavated separately at 3 sites along the surface rupture zones in the middle segment of the beichuan yingxiu fault and jiangyou guanxian fault of the longmenshan fault zone and the offsets of geomorphic surfaces were measured in the trenches the basic facts of recurrence of large earthquake on the seismogenic fault of the wenchuan earthquake are demonstrated in this paper on the basis of the accumulated deformation amounts of old and young geomorphic surfaces as well as the paleoearthquake events revealed in the trenches the document that at leigu along the beichuan yingxiu fault at bailu along the jiangyou guanxian fault and xiaoyudong on a nw trending fault the height of fault scarp on the second terrace has a multiple relationship to that on the first terrace after the wenchuan earthquake of 12 may the elevation difference of the marker horizon the yellow sandy clay bed of the second terrace revealed in the trenches on both sides of the fault is nearly twice or three times larger in bailu of that produced by the wenchuan earthquake these observations suggest the occurrences of paleoearthquake events with surface deformation comparable to that produced by the wenchuan mw 7 9 earthquake according to the dating data and the fact that there is no such slip of comparable paleoearthquakes in the 2300 year long historic literature in longmenshan region we suggest that the penultimate event occurred between 2300 a bp and 3300 a bp

active fault related folding beneath an alluvial terrace in the southern longmen shan range front sichuan basin china implications for seismic hazard abstract the devastating 2008 m w 7 9 wenchuan earthquake china demonstrates that the central and northern parts of the longmen shan are currently active evidence for active faulting and folding in the southern longmen shan however remains poorly documented in this paper we define the structural geometry fault kinematics and seismic hazard of the qiongxi thrust fault system qtf along the southern longmen shan range front by integrating deep and shallow seismic reflection data and geomorphic observations the qtf is a 50 km long north south trending set of faults and associated folds that exhibit geomorphic evidence of quaternary surface deformation geomorphic observations and seismic reflection data reveal that these faults dip steeply to the east and merge at depth with a blind west dipping thrust ramp the trend and reverse sense of slip along the qtf indicates that the structure accommodates east west crustal shortening based on uplift of stratigraphic horizons across the fault zone we define a late pliocene to early pleistocene fault slip rate of 0 2 0 3 mm yr and a middle pleistocene to present rate of 0 4 1 2 mm yr on the west dipping thrust ramp this ramp soles to a basal detachment in the triassic section at a depth of 4 5 5 5 km to the west this detachment steps down onto a blind northwest dipping thrust termed the range front thrust a rupture of the qtf in combination with the range front thrust could generate an m w 7 8 earthquake with average displacement of 5 7 m this type of earthquake source poses significant hazards to the adjacent highly populated sichuan basin online material figures detailing cross sections and 3d surfaces additional reflection profiles and dating information

late cenozoic evolution of the central longmen shan eastern tibet insight from u th he thermochronometry 1 this article presents u th he thermochronological data from the longmen shan belt eastern tibet located between the songpan garze terrane and the yangtze craton this mountain range is one of the steepest margins of the tibetan plateau and an important area for the comprehension of the mechanisms that control the dynamics of such plateau borders in terms of spatial distribution of deformation or timing of topographic building we describe several age elevation transects and perform forward modeling of our data to derive quantitative information on the exhumation of the range a major phase of exhumation started at 8 11 ma with an average rate of 0 65 mm a a1 comparison of zircon and apatite ages indicates that the eastern part of the range may have experienced a significant decrease in exhumation since 2 3 ma we use the distribution of finite exhumation across the major faults of the area to quantify their dip slip throw rate over the last 10 ma the beichuan fault which was activated during the 2008 sichuan earthquake is the major active structure of the longmen shan since the late miocene with an average thrusting slip rate between 0 4 and 1 mm a a1 conversely over the same time period only minor dip slip activity occurred on the wenchuan fault zone this distribution in space and time of exhumation and deformation is discussed and compared to the different proposed models for the geodynamical evolution of the eastern tibetan margin it also provides an important long term perspective to put in context the destructive 2008 sichuan earthquake that struck the central longmen shan

stress evolution before and after the 2008 wenchuan china earthquake abstract we examine the distribution of coulomb stress before and following the m 7 9 wenchuan earthquake and model the time dependent components of the stress field into the future our modelling shows that the combined coseismic plus postseismic viscoelastic stress change due to the preceding large events m 6 7 at the hypocentral area of the wenchuan earthquake is negative which may have led to a delay on its occurrence between 16 and 95 yr depending on the rake of the rupture on the other hand coseismic stress modelling of the wenchuan earthquake shows strong interaction particularly with the faults close to its sw and ne ends such interactions have previously been shown to trigger further large earthquakes in addition redistribution of stresses due to continuing postseismic deformation will potentially increase the stress levels not only on these faults but also on the distant faults such as the xianshuihe kunlun and tazang faults postseismic viscoelastic stress load on these distant faults over the next 22 yr since the earthquake could be up to 2 5 times larger than that of the coseismic stress increase alone this would potentially advance these faults in their seismic cycle and thereby encourage future earthquakes resulting in clustering of large earthquakes in the region this behaviour however strongly depends on the stage that these faults are in their seismic cycle which is not known currently given the high density of population in the area with several cities chengdu guangyuan and hanzhong of more than 1 million inhabitants our results indicate a need for detailed paleoseismic studies to help elucidate the state of the stressed faults

differential exhumation in response to episodic thrusting along the eastern margin of the tibetan plateau abstract thermochronological data from the songpan ganze fold belt and longmen mountains thrust nappe belt on the eastern margin of the tibetan plateau in central china reveal several phases of differential cooling across major listric thrust faults since early cretaceous times differential cooling indicated by distinct breaks in age data across discrete compressional structures was superimposed upon a regional cooling pattern following the late triassic indosinian orogeny 40ar 39ar data from muscovite from the central and southern longmen mountains thrust nappe belt suggest a phase of differential cooling across the wenchuan maouwen shear zone during the early cretaceous the zircon fission track data also indicate differential cooling across a zone of brittle re activation on the eastern margin of the wenchuan maouwen shear zone during the mid tertiary between 38 and 10 ma apatite fission track data from the central and southern longmen mountains thrust nappe belt reveal differential cooling across the yingxiu beichuan and erwangmiao faults during the miocene forward modelling of apatite fission track data from the northern longmen mountains thrust nappe belt suggests relatively slow regional cooling through the mesozoic and early tertiary followed by accelerated cooling during the miocene beginning at ca 20 ma to present day regional cooling is attributed to erosion during exhumation of the evolving longmen mountains thrust nappe belt lmtnb following the indosinian orogeny differential cooling across the wenchuan maouwen shear zone and the yingxiu beichuan and erwangmiao faults is attributed to exhumation of the hanging walls of active listric thrust faults thermochronological data from the longmen mountains thrust nappe belt reveal a greater amount of differential exhumation across thrust faults from north to south this observation is in accord with the prevalence of proterozoic and sinian basement in the hanging walls of thrust faults in the central and southern longmen mountains the two most recent phases of reactivation occurred following the initial collision of india with eurasia suggesting that lateral extrusion of crustal material in response to this collision was focused along discrete structures in the lmtnb

crustal structures in the area of the 2008 sichuan earthquake from seismologic and gravimetric data abstract the 12 may 2008 the sichuan earthquake was one of the worst natural disasters in china here we present a detailed picture of the epicentral area inferred geophysical data based on the analysis of teleseismic data acquired by a dense seismic network we highlight an abrupt 20 km moho offset between the sichuan basin and the tibetan plateau and a horizontal discontinuity at 15 km depth which may connect with the ruptured zone of the 2008 event we obtain low mean crustal velocity ratios which suggest the absence of a thick and extensive zone of partial melt within the crust beneath the eastern part of the songpan garze terrane all our data support the idea that the longmen shan range mark a zone of active strain localization due to the rigid yangtze craton resisting eastward displacement of the tibetan plateau