extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

deep structures and surface boundaries among proto tethyan micro blocks constraints from seismic tomography and aeromagnetic anomalies in the central china orogen abstract the qinling qilian orogen preserves the records of early paleozoic convergence among the proto tethyan micro blocks in this study we analyze the seismic velocity structure and the aeromagnetic anomalies in the qinling qilian orogen and its adjacent areas showing that the northernmost boundaries of these proto tethyan micro blocks are defined by the guyuan longshoushan fault in the qilian orogen and by the luonan luanchuan fault in the qinling orogen respectively the lithosphere north of the qinling qilian orogen subducted southward under the qinling qilian orogen the boundaries of the micro blocks in the qinling qilian orogen of the proto tethys domain are mostly ew or ne trending combined with early paleozoic geological records our results indicate that the south dipping low velocity anomaly under the alxa block and the south dipping high velocity ones under the north qilian orogen and the north china block might indicate that the alxa block the north qilian oceanic slab and the north china block subducted southward the south dipping high velocity anomaly under the north qilian orogen is interpreted as the remnant of the proto tethys ocean in that area however the north dipping low velocity anomaly under the south qinling and the north qinling orogens the qaidam block and the north dipping high velocity one under the yangtze and the bikou blocks suggest that the south qinling and the north qinling orogens the qaidam the yangtze and the bikou blocks subducted northward the present spatial framework of the qinling qilian orogen is related to early paleozoic convergence among the proto tethyan micro blocks

flexural subsidence by 29 ma on the ne edge of tibet from the magnetostratigraphy of linxia basin china abstract this study provides a detailed magnetostratigraphic record of subsidence in the linxia basin documenting a 27 myr long sedimentary record from the northeastern edge of the tibetan plateau deposition in the linxia basin began at 29 ma and continued nearly uninterruptedly until 1 7 ma increasing rates of subsidence between 29 and 6 ma in the linxia basin suggest deposition in the foredeep portion of a flexural basin and constrain the timing of shortening in the northeastern margin of the plateau to late oligocene late miocene time by late miocene early pliocene time a decrease in subsidence rates in the linxia basin associated with thrust faulting and a 10 clockwise rotation in the basin indicates that the deformation front of the tibetan plateau had propagated into the currently deforming region northeast of the plateau

rapid exhumation at 8 ma on the liupan shan thrust fault from apatite fission track thermochronology implications for growth of the northeastern tibetan plateau margin abstract the liupan shan mountain is one of the outermost ranges in northeastern tibetan plateau the onset of its uplift provides insight on whether the plateau grew sequentially outward or broad areas of the plateau deformed simultaneously the apatite fission track method can be used to date rapid cooling as the result of unroofing of rocks in response to tectonically induced vertical motion fission track data from apatite in early cretaceous sedimentary strata exposed on the hanging wall of liupan shan thrust fault in northeastern tibet suggest that a late cenozoic rapid cooling events occurred at 7 3 8 2 ma or 8 ma this age apparently dates fault activity of the liupan shan thrust in response to northeastward growth of the tibetan plateau uplift and lateral growth which continues today numerous studies of the tibetan plateau suggest that the onset of deformation in eastern and northern margin of tibetan plateau occurred in the latest miocene

40ar 39ar dating of a tephra layer in the pliocene seneze maar lacustrine sequence french massif central constraint on the age of the reunion matuyama transition and implications on paleoenvironmental archives the sedimentary sequence cored in the seneze maar velay france is very favourable for magnetostratigraphy of the late pliocene and for studying the paleoenvironmental changes that occurred during that period the magnetostratigraphic studies revealed that the sediments from this lacustrine sequence were deposited during the matuyama reverse period except for the upper part where a short normal polarity episode was recorded a tephra layer interbedded in the sediments contemporaneous with this normal polarity event is dated at 2 10 0 01 ma by the 40ar 39ar method using this chronological marker it can be inferred that the normal polarity episode recorded in the seneze sequence corresponds to the reunion event and that the age of the normal reverse transition closing this event is ca 2 09 ma these chronological constraints also permit to 1 demonstrate that the villafranchian mammal fauna found in the seneze maar is younger than 2 09 ma and 2 correlate the climatic events recorded in the seneze sequence to the marine d18 o records this suggests that the seneze pollen sequence 5 120 m depth ranges from isotopic stage 85 to 76

paleogene clockwise tectonic rotation of the xining lanzhou region northeastern tibetan plateau 1 to help understand the deformational history of the northeastern tibetan plateau paleomagnetic samples were collected from 177 sites and two magnetostratigraphic sections at 16 localities distributed among upper jurassic lower cretaceous to pliocene sedimentary and subordinate volcanic rocks within the xining lanzhou region 34 37 n 101 105 e a total of 127 sites at 12 localities yielded primary magnetizations confirmed by fold reversal and conglomerate tests age control on sedimentary rocks is provided by regional synthesis of chronostratigraphic data and our own biostratigraphic and magnetostratigraphic analysis presented in the companion paper by horton et al 2004 analysis of paleomagnetic declination combined with results from previous studies yield a remarkably consistent trend of vertical axis tectonic rotations across the studied region whereas 19 0 7 2 to 37 8 10 6 clockwise rotations are recorded consistently in all paleomagnetic localities in lower cretaceous to eocene rocks all paleomagnetic localities in oligocene to pliocene rocks have recorded minor to insignificant rotations indicating that the xining lanzhou region has undergone a wholesale regional clockwise rotation during late paleogene time consistent with regional chronostratigraphic and thermochronologic results this late paleogene tectonic rotation confirms that deformation reached regions of the northern tibetan plateau shortly after the initial collision of india with asia when compared to rotational paleomagnetic results from adjacent regions several mechanisms can be proposed to explain the clockwise rotation on the basis of consistency with geologic data we prefer a model involving clockwise rotation of the xining lanzhou region through right lateral shear and associated shortening related to northward indentation of the qaidam basin index terms 1525 geomagnetism and paleomagnetism paleomagnetism applied to tectonics regional global 8102 tectonophysics continental contractional orogenic belts 9320 information related to geographic region asia 9604 information related to geologic time cenozoic keywords tectonics paleomagnetism tibetan plateau

late cenozoic tectonic evolution of the ningxia hui autonomous region china part of the transition from active crustal shortening and strike slip faulting in northwestern china to active extension in northeastern china takes place within the ningxia autonomous region four arcuate zones of both strike slip faults and thrust faults with associated ramp anticlines dominate the structures in southern ningxia deformation in these zones indicates that a component of left slip displacement is transferred into crustal shortening on north south trending folds and thrust faults the average quaternary slip rate along the haiyuan liupan shan fault zone is 5 10 mm yr and that along the tianjin shan mibo shan fault zone is about 1 5 2 7 mm yr the amount of offset and rate of slip along the yanton shan and the niushou shan daluo shan fault zones are unknown but the topography of the mountains suggests that the rates of slip along these zones is lower than that of the haiyuan liupan shan fault zone deformation in northern ningxia is dominated by normal faulting and extension the helan shan rise about 2 000 m above the yinchuan basin reportedly filled with a few kilometers of cenozoic sediments the average rate of vertical separation in quaternary time along the east helan shan fault is estimated to be at least 0 5 0 8 mm yr opening of the yinchuan graben is probably partly related to left lateral slip on the niushou shan and daluo shan fault zone the northeastern margin of tibetan plateau is probably being elevated by the irregular growth of convergent and left slip structural zones the evolution of deformation along the haiyuan liupan shan structural zone probably foreshadows the future deformation in the ranges north of it left slip and shortening within the ningxia region appears to accommodate less than 20 25 km of east or northeast displacement of crustal fragments of the northern tibetan plateau large magnitude lateral transport of crust with respect to the area to the east has not taken place within the part of the ningxia region covered by this study

a revised chronology for tertiary sedimentation in the sikouzi basin implications for the tectonic evolution of the northeastern corner of the tibetan plateau abstract the space time pattern of development of high topography associated with the tibetan plateau remains incomplete due in large part to sparse proxy data that allow estimates of the growth of individual mountain ranges here we present the results of a stratigraphic investigation of a 3100 m section of tertiary fluvio lacustrine strata in the sikouzi basin located at the northeastern most corner of the tibetan plateau results of magnetostratigraphic analysis coupled with a newly discovered miocene in situ fossil assemblage suggest that the section preserves a nearly complete record extending from initial basin formation at 29 myr until 0 5 myr sediment accumulation rates increase from relatively slow accumulation during the oligocene miocene 9 cm kyr to rapid accumulation in the pliocene 22 cm kyr apparently occurring in two sustained pulses at 10 5 ma and 5 ma respectively the increase in accumulation rates starting at 10 5 ma is interpreted as heralding the initial uplift of the liupan shan consistent with cooling and exhumation of the range zheng et al 2006 the pliocene increase in accumulation rate is accompanied by a distinct change in lithofacies the appearance of coarse detritus with provenance from the madong shan and up section changes in bedding orientation that are interpreted as growth strata collectively our data suggest that the generation of relief in the madong shan began at 5 4 ma moreover analysis of paleomagnetic declinations from the sikouzi section reveals a clockwise rotation of the sikouzi basin during the time period 24 5 4 ma that was followed by a subsequent counterclockwise rotation 5 4 0 5 ma our study suggests that activity along the liupan shan haiyuan fault system probably initiated at 10 5 ma but that the present day configuration of the fault system and associated ranges near the fault tip dates to pliocene time

fast late pleistocene slip rate on the leng long ling segment of the haiyuan fault qinghai china 1 fieldwork along the western haiyuan fault where it cuts young glacial landforms in the leng long ling range qinghai northeastern tibet yields new constraints on its long term left slip rate our mapping near 101 85 e based on aerial photographs 1 50 000 topographic maps and spot images shows glacial valley edges and lateral moraines offset 200 m by the fault quartz rich cobbles were sampled on top of one moraine for in situ 10be and 26al cosmogenic nuclide dating among 12 dated samples the 6 oldest show ages clustering at 10 300 339 years this age probably reflects the time of last reshaping of the moraine before the valley glacier withdrew south of the fault around the end of the younger dryas 11 000 years b p assuming that the 200 40 m moraine offset started to be recorded after glacial retreat across the fault constrains the late pleistocene slip rate on the leng long ling segment of the haiyuan fault to be 19 5 mm yr

three dimensional segmentation and different rupture behavior during the 2012 emilia seismic sequence northern italy describing the slip behavior of an active fault system is central to understanding the seismic potential of seismogenic areas different elements control the nature and the extent of the coseismic and post seismic ruptures including the geometry of faults the nature of faulted rocks and the stress changes caused by the mainshocks in may june 2012 a severe seismic sequence struck a portion of the po plain northern italy where a thick blanket of plio quaternary sediments hides a number of seismogenic sources corresponding to the external thrust systems of the northern apennines we used deep seismic reflection data to reconstruct the geometry of the faults responsible for the sequence these faults exhibit significant non planarity due to tectono stratigraphic heterogeneities inherited from a complex pre thrusting extensional tectonic phase a comparison of the fault parameters derived from our geological analysis and the evidence supplied by seismological aftershock distributions and geodetic data insar allowed to identify the causative fault segments of the two mainshocks we then modeled the coulomb stress changes produced by two mainshocks to analyze on and off fault seismicity discrepancies between the magnitude of the earthquakes and the extent of active faults suggest that the mainshocks did not rupture the entire thrust planes we contend that seismogenic ruptures were confined in the mesozoic carbonates and were stopped by lithological changes and or mechanical complexities of the fault planes both along dip and along strike our findings highlight that along the active structures of the po plain slip tends to be seismogenic where faults are located in mesozoic carbonate rocks

present day crustal thinning in the southern and northern tibetan plateau revealed by gps measurements gps measurements from sites within the tibetan plateau show not only east southeast west northwest extension but also more importantly horizontal dilation throughout the interior of the plateau assuming conservation of volume vertical thinning strain rates equal horizontal dilation rates and they 8 9 0 8 nanostrain a 1 and 7 4 1 2 nanostrain a 1 in northern and southern tibet and 12 0 3 2 nanostrain a 1 in its southwestern part suggest no measureable difference principal extensional strain rates also are similar in magnitude and orientation if crustal thinning began at 10 15 ma and the current rates of horizontal dilation applied both to the entire crust and to that period the crust should have thinned by 5 5 8 5 km if isostatic equilibrium applied the mean elevation of the plateau would have dropped 1 km the similar rates for northern southern and southwestern tibet suggest that the processes dictating crustal extension normal faulting and crustal thinning in the three regions differ little from one another

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

the composition and history of breccia 67015 from north ray crater we have examined feldspathic fragmental breccia 67015 from the rim of north ray crater and found it to be the most diverse and complex of the apollo 16 ffbs it contains a wider range of clast types than any of the others and is not dominated by any single lithology we have identified several crystalline lithologies occurring in minor amounts that we believe represent highland plutonic rocks plagioclase fragments are the most abundant breccia constituent followed by clasts of impact melt rocks and granulites the impact melts are of two main types feldspathic fragment laden melts and mafic kreepy melt rocks of vha basalt composition textural evidence indicates that some of the vha melts were hot and plastic when they accumulated into the breccia 40ar 39ar dates show that both types of melts solidified about 3 9 109 years ago magnetic measurements on clasts show a scatter of nmr directions making it impossible to read a consistent imprint of an ancient dipolar magnetic field previous studies of apollo 16 stratigraphy suggested that kreepy melt rocks were introduced to the site as imbrium impact ejecta about 3 8 106 years ago our results indicate that the highlands crust contained kreep components as early as 3 9 109 years ago

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

late quaternary left lateral slip rate of the haiyuan fault northeastern margin of the tibetan plateau 1 slip rates of major active strike slip faults within and around the tibetan plateau provide constraints for understanding the dynamics of continental deformation in general because large slip rates can be taken to imply localized deformation between rigid blocks and low slip rates on faults are more consistent with distributed deformation several major strike slip faults have been studied during the last 3 decades as one of important active strike slip faults in and around the tibetan plateau the slip rate of the haiyuan fault has been discrepantly estimated to be high by proponents of escape tectonics or low by others and gps observations to better constrain its slip rate and then to better understand the mechanics of intracontinental deformation we try to more carefully consider offset geomorphic features and age constraints along the haiyuan fault in this paper we select three sites where both upper and lower bounds of slip rate can be obtained we find that slip rates are 4 2 0 8 mm yr at the shaomayin 4 5 0 7 mm yr at gaowanzi and 5 0 2 5 mm yr at huangliangtan site combinations of these rates with those published previously yield an average slip rate of 4 5 1 0 mm yr on the haiyuan fault this rate agrees with the present day slip rate of 4 3 1 5 mm yr measured by gps 4 2 8 mm yr by interferometric synthetic aperture radar and 5 mm yr through paleoseismological studies the temporal consistency suggests a steady state process of strain accumulation and release along the haiyuan fault the low slip rate suggests that the haiyuan fault similar to the altyn tagh fault does not transfer a significant portion of the convergence between india and asia out of india s path into eurasia but merely redistributes crustal thickening

seismogenic structures of the 1999 mw 7 6 chi chi taiwan earthquake and its aftershocks abstract high resolution 3 d vp and vs velocity models around the source region of the mw7 6 chi chi taiwan earthquake show significant lateral and vertical variations the mainshock occurred within a narrow low velocity zone along the chelungpu fault a sudden increase of velocity and seismicity took place across the shuilikeng fault to the east most aftershocks were located in areas of high vp and vs a sharp east dipping zone extending from the shuilikeng fault on the surface to a depth of about 15 km separates the low velocity region in the west from the high velocity region in the east an apparent high angle west dipping seismic zone lies at depths of 15 30 km beneath the western central range numerous aftershocks with normal faulting were located west of the middle segment of the chelungpu fault

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

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

bounds on the holocene slip rate of the haiyuan fault north central china abstract we measured the offsets of six stream valleys of 30 to 90 m along the northwest southeast trending left lateral haiyuan strike slip fault in north central china minimum ages of these offsets were determined to estimate lower bounds for the holocene slip rate the most reliable bounds are 7 6 1 0 and 6 7 1 0 mm yr with three others that are smaller 3 4 0 7 3 5 0 9 and 4 1 0 4 mm yr and one large value 16 4 5 9 mm yr that we doubt thus the average holocene slip rate of the haiyuan fault is larger than 6 mm yr and probably exceeds 7 mm yr if the average slip rate of 5 to 10 mm yr for the quaternary period is applicable to the holocene epoch the average rate is 8 2 mm yr

faulting associated with large earthquakes and the average rate of deformation in central and eastern asia we determine approximate average rates of deformation in portions of eastern and central asia for the last 80 years these rates are based on estimates of source dimensions average displacements seismic moments and orientations of faults of all large earthquakes m 7 5 and as many other major earthquakes m 7 0 as possible in asia since 1900 a major part of this study is the compilation of these data for separate parts of asia we combine seismic moment tensors to estimate the average rotational strain or deformation tensors for these regions for the last 80 years we estimate that shortening across the tien shan has occurred at 11 mm yr between 5 and 19 mm yr left lateral shear on easterly trending planes in mongolia in northwestern tibet gansu and ningxia and on the xianshuihe fault in western sichuan has been very rapid more than 10 mm yr in each region and apparently as rapid as 40 mm yr in northwestern tibet gansu and ningxia slower rates of a few to 10 mm yr appear to characterize east west extension and conjugate north south shortening of tibet the earthquake history of the himalaya is consistent with rates of underthrusting of more than 10 mm yr but is probably less than the convergence rate of india with eurasia of about 50 mm yr the overall strain field is consistent with a large part of india s penetration into eurasia being absorbed by the extrusion of material out of india s way as a result southeast china moved at about 21 mm yr east southeast with respect to eurasia in the last 80 years with an uncertainty of about a factor of 2 in this estimated rate

insights on the seismogenic layer thickness from the upper crust structure of the umbria marche apennines central italy 1 we reconstruct the subsurface geology in a region of the northern apennines central italy where a protracted extensional sequence occurred in 1997 1998 with maximum magnitude m 6 0 our study is mainly based on the interpretation of three reprocessed seismic reflection profiles crossing the epicentral area which constrain the subsurface geometry to a depth of about 12 km where most of the shallow seismicity occurs comparing the subsurface setting with accurately determined earthquake locations we find that the seismicity is located entirely within the sedimentary cover and does not penetrate the underlying basement this is explained by considering that the sedimentary cover is rather thick and composed of relatively strong lithologies platform carbonates and evaporites while the upper part of the basement consists of weak phyllites and siliciclastic rocks this weak horizon is also evidenced by the low vp values measured in deep wells of the region its effect is to decouple the sedimentary cover from the crystalline basement where only microseismicity occurs our study indicates that local structure and stratigraphy can significantly influence the distribution of seismicity within the upper crust particularly in complex geological environments such as thrust and fold belts

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

continental tectonics in the aftermath of plate tectonics the success of plate tectonics required an acceptance of continental drift and thus a reinterpretation of the large scale geological history of most of the earth but the basic tenet of plate tectonics rigid body movements of large plates of lithosphere fails to apply to continental interiors where buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity

a new osl chronology for dust accumulation in the last 130 000 yr for the chinese loess plateau abstract a sensitivity corrected multiple aliquot regenerative dose protocol has been developed for fine grained quartz osl dating of chinese loess its reliability has been assessed on the basis of the methodology and by dating reference samples of known age close to the transition from the last interglacial paleosol s 1 to the last glacial loess l 1 which corresponds to the marine oxygen isotope stage mis 5 4 transition on the basis of the fine grained quartz osl age estimates for 33 loess samples from the upper part of the luochuan profile a detailed chronostratigraphy of continuous dust accumulation in the past 130 ka has been proposed changes in the accumulation rate occurred during the last glacial period mis 4 to mis 2 unexpectedly high accumulation rates were found in the weakly developed l 1 2 s paleosol of the last interstadial mis 3 rather than in the classic l 1 1 and l 1 3 loess of the cold dry glacial condition mis 2 and 4 the osl ages show some disagreement with the previous numerical chronology for the loess paleosol sequence based on correlation of variations in grain size with sedimentation rate the latter method resulted in an almost constant accumulation rate from 72 to 12 ka

millennial recurrence of large earthquakes on the haiyuan fault near songshan gansu province china the haiyuan fault is a major active left lateral fault along the northeast edge of the tibet qinghai plateau studying this fault is important in understanding current deformation of the plateau and the mechanics of continental deformation in general previous studies have mostly focused on the slip rate of the fault paleo seismic investigations on the fault are sparse and have been targeted mostly at the stretch of the fault that ruptured in the 1920 m 8 6 earthquake in ningxia province to investigate the millennial seismic history of the western haiyuan fault we opened two trenches in a small pull apart basin near songshan in gansu province the excavation exposes sedimentary layers of alternating colors dark brown silty to clayey deposit and light yellowish brown layers of coarser grained sandy deposit the main fault zone is readily recognizable by the disruption and tilting of the layers six paleoseismic events are identified and named ss1 through ss6 from youngest to oldest charcoal is abundant yet generally tiny in the shallowest parts of the trench exposures thirteen samples were dated to constrain the ages of paleoseismic events all six events have occurred during the past 3500 3900 years the horizontal offsets associated with these events are poorly known however events ss3 to ss6 appear to be large ones judging from comparison of vertical separations and widths of fault zones the youngest event ss1 instead seems to be a minor one probably the 1990 mw 5 8 earthquake thus four large events in 3500 3900 years would imply a re currence interval of about 1000 years three events ss2 to ss4 prior to 1990 occurred sometime during 1440 1640 a d shortly after 890 1000 a d and 0 410 a d re spectively we tentatively associate them with the 1514 a d 1092 a d and 143 or 374 a d historical earthquakes taking 10 2 m of slip for large events ss3 and ss4 comparable to the 1920 m 8 haiyuan earthquake their occurrence times would be consistent with the long term 12 4 mm yr estimate of lasserre et al 1999 however a more realistic evaluation of slip rate and its possible change with time requires a more rigorous determination of coseismic slip amounts of past earth quakes

transformation of displacement between strike slip and crustal shortening in the northern margin of the tibetan plateau evidence from decadal gps measurements and late quaternary slip rates on faults abstract studies of late quaternary geological slip rates and the gps decadal slip rates along the major strike slip faults in the northern margin of the tibetan plateau show that slip rates remain relatively constant along the middle sections of each fault and decrease rapidly toward the ends our recent studies suggest that the slip rates are low

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

late pleistocene left lateral slip rates of the gulang fault and its tectonic implications in eastern qilian shan ne tibetan plateau china abstract the late pleistocene deformation pattern in the eastern qilian shan remains controversial study of the geometry and slip rate of the fault holds crucial value for better understanding the regional deformation pattern the active e w trending gulang and tianjing shan fault zone is located in the southern boundary of the tengger desert which plays a significant role in tectonic deformation and slip transfer in the ne tibetan plateau geomorphic mapping of the active fault traces reveal that the gulang fault is composed of two segments separated by the guanjia tai pull apart basin offset geomorphic features indicate that the gulang fault appears to be primarily sinistral strike slip with no reverse component based on offset gullies terraces and optically stimulated luminescence dating of offset related sediments we obtain a consistent late pleistocene slip rate of 2 5 mm a along the western and eastern segments of the gulang fault meanwhile the slip vectors indicate the gulang fault redistributes strain and transforms displacement along the haiyuan fault accommodating the westward motion of the gobi alashan block moreover a devastating earthquake of ms 8 0 struck gulang on 23rd may 1927 geomorphically fresh fault scarps and surface rupture zone are observed at gulang fault we modify the modified mercalli intensities mmi and revise the isoseismal map for the gulang earthquake combining with the high resolution deep seismic reflection data the gulang fault may be the source of the earthquake regionally gps velocities indicate that faulting crustal shortening and basin block rotation accommodate deformation of the plateau s northern margin in conclusion the fault geometry kinematics and regional gps velocities reflect that this region experiences eastward movement of the northeastern tibetan plateau relative to the gobi alashan