accelerated extension of tibet linked to the northward underthrusting of indian crust the tibetan plateau is extending numerical simulations suggest that regional scale extension is caused by gravitational collapse of the plateau whereas rapid extension in the south is caused by underthrusting of the indian slab

rates of late quaternary normal faulting in central tibet from u series dating of pedogenic carbonate in displaced fluvial gravel deposits along the main boundary fault of the central tibetan shuang hu graben two well preserved fluvial terrace surfaces are vertically offset by 1 3 m terrace i and 14 8 m terrace ii using thermal ionization mass spectrometry tims we have determined 48 230th 234u 238u ages of small 10 40 mg samples from pedogenic carbonate rinds on clasts in the terrace deposits based on textural microstratigraphic and geochemical criteria we conclude that the u series ages of innermost rind samples provide reliable ages of 16 4 1 9 ka for terrace i and 233 1 9 3 ka for terrace ii this constrains the average rate of vertical displacement along the normal fault to be 0 079 0 011 mm yr during the past 16 kyr and 0 064 0 007 mm yr during the past 233 kyr combining these results with slip rate estimates for other normal faults along the graben margins indicates that the cumulative vertical displacement on all normal faults did not exceed 0 3 mm yr during the late quaternary this new rate from central tibet is distinctly lower than the rate of 1 9 0 6 mm yr inferred for normal faults bounding the yadong gulu graben system in southern tibet consistent with more pronounced and common graben development in southern tibet we therefore suggest that normal faulting in southern tibet is largely controlled by local processes and that the age of its initiation is not a valid proxy for uplift of the tibetan plateau as has been widely assumed our data also place chronologic constraints on the termination of periglacial conditions in central tibet during the last three interglaciations rind formation appears to have been relatively continuous on clasts in terrace i from 16 ka clast rinds from terrace ii in contrast are characterized by three main episodes of rind growth that are widely separated in time the earliest and least well preserved episode started at 233 1 9 3 ka a second episode occurred at 131 8 7 8 ka and a third episode was approximately coeval with rind formation in terrace i after 16 4 1 9 ka in each case the age of onset of carbonate rind accumulation coincides with independently estimated ages of global deglaciation accordingly we conclude that periglacial conditions in the central tibetan plateau terminated synchronously with early stages of global warming following the last three glacial maxima and that fluvial aggradation terrace formation and pedogenic carbonate accumulation in terrace soils ensued rapidly

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

significant late neogene east west extension in northern tibet field mapping in northern tibet reveals that the normal slip along late cenozoic northsouth trending faults is comparable to that estimated for equivalent structures in southern tibet the orientation of fault striations in two north south trending rifts suggests an eastnortheast west northwest direction of extension in northern tibet which in turn implies that northeast striking active faults in northern tibet have significant left slip components initiation of rifting in northern tibet postdates the early oligocene and possibly occurred after 4 ma the broad similarities in the magnitude of slip and the direction of extension for normal faults in both northern and southern tibet imply that the entire plateau has been extending this precludes significant eastward extrusion of north tibet relative to south tibet and requires a regional boundary condition as the cause of east west extension for the entire tibet plateau

indian punch rifts tibet the strikes of late cenozoic normal faults in tibet fan systematically from a northwesterly to northeasterly direction from west to east they exhibit an axis of symmetry that bisects the himalayan arc and trends subparallel to india asia motion the two dimensional stress field inferred from the spatial pattern of normal fault orientations can be explained solely by collisional stresses localized along a southern part of the himalayan arc continued insertion of indian crust into tibet can explain the persistence of extension and may be increasing plateau crustal thickness and elevation coeval with rifting

mode of cenozoic east west extension in tibet suggesting a common origin of rifts in asia during the indo asian collision cenozoic rifts in tibet were traditionally interpreted as a result of topographic collapse of the tibetan plateau reaching the maximum elevation that can be supported by its mechanical strength recent studies have emphasized possible similarities between rifting in tibet and extension in the basin and range of the western united states however when examined in detail one finds that spacing of long 100 km rifts in tibet 100 300 km is significantly greater than that in the basin and range 20 40 km from south to north rift spacing decreases systematically from 191 67 km in the himalaya south of the indus yalu suture to 146 34 km in south tibet between the indus yalu and bangong nujiang sutures and farther north to 101 31 km in central tibet between the bangong nujiang and jinsha sutures instability analysis suggests that the mantle lithosphere must have been involved in east west tibetan extension specifically the widely spaced rifts in the himalaya and tibet may have been related to the presence of a relatively light crust density 2 90 g cm 3 and a strong mantle lithosphere 40 km thick and a factor of 5 stronger than the upper crust the observed systematic decrease in rift spacing can be explained by the known decrease in the crustal thickness in tibet from 70 80 km in the himalaya in the south to 50 55 km in central tibet in the north a regional comparison of rifts in east asia suggests that both the involvement of the mantle lithosphere and the age of rift initiation are similar for tibetan rifts the baikal rift and the shanxi graben this implies that topographic collapse or a convective event in the mantle cannot be the sole cause for the development of the tibetan rifts a regional boundary condition applied throughout east asia must be required

quantification of the late quaternary throw rates along the yadong rift southern tibet abstract southern tibet is characterized by seven main n s trending rifts bounded by normal faults which accommodate e w extension quantifying the throw and extension rates along these rifts is essential to understand their activity onset timing regional seismic hazard and kinematic role in the tectonic evolution of the tibetan plateau the yadong gulu rift ygr near lhasa is the longest and most prominent rift in southern tibet and one of the few that crosses the himalayas however it remains poorly studied and understood here we focus on the yadong rift i e the southern section of the ygr south of the yarlung zangbo suture we determine the late quaternary holocene throw rates at four sites where the active yadong normal fault vertically offsets glacial and fluvial geomorphic surfaces using terrestrial lidar uav and kinematic gps we precisely measured vertical offsets up to 17 m and constrained the age of abandoned surfaces using 32 10be cosmogenic surface exposure ages 10 20 ka one optically stimulated luminescence age and two travertine samples dated using u series dating the offset age reconstructions yield uniform throw rates of 0 9 0 3 mm yr along the entire yadong rift taking a fault dip of 50 as measured in the field we obtain late quaternary e w extension rates of 0 8 0 3 mm yr which is similar to but at the lower end of those along other rifts in southern tibet 1 2 mm yr

focal depths and fault plane solutions of earthquakes and active tectonics of the himalaya synthetic seismograms were compared with long period body waves for nine earthquakes with epicenters in the himalayan arc to determine depths of foci and to improve fault plane solutions focal depths are shallow 10 20 km inferred slip vectors are locally perpendicular to the mountain range they plunge very gently about 10 deg in the eastern sections of the range and more steeply about 25 deg in western sections assuming india to be a rigid plate the radially oriented slip vectors imply that southern tibet extends at about half the rate of underthrusting in the himalaya and therefore probably at about 5 10 mm yr the shallow depths and gentle dips of the fault planes at least for the events in the eastern half of the range are consistent with coherent underthrusting of the indian plate beneath at least the lesser himalaya the steeper dips of fault planes in the western part of the arc might reflect deformation of the overriding thrust plate or simply a steepening of the main underthrusting zone beneath the greater himalaya

rare earth element complexation behavior in circumneutral ph groundwaters assessing the role of carbonate and phosphate ions rare earth element ree concentrations were determined in circumneutral ph 7 ph 9 groundwaters from south central nevada and the mojave desert in eastern california u s a the inorganic speciation of the rees in these water were evaluated primarily to assess the relative importance of carbonate lnhco32 lnco3 and ln co3 2 and phosphate lnh2po42 lnhpo4 ln hpo4 2 and lnpo40 complexes other ree complexes with sulfate hydroxyl chloride and fluoride ions were also assessed as was the significance of the free metal ion species i e ln3 our speciation calculations indicate that ree carbonate complexes dominate and typically account for more than 99 of each ree in solution moreover carbonato complexes lnco3 were predicted to be the dominant species for the light rees and dicarbonato complexes ln co3 2 were predicted to increase in importance with increasing atomic number all other complexes were predicted to be negligible furthermore the combined percentage of all ree phosphate complexes never accounted for more than 0 1 of the dissolved rees and generally much less than 0 1 phosphate complexes can not compete with carbonate complexes in these groundwaters because of the low po43 concentrations 0 3 1 58 mol kg the much lower concentrations of the free phosphate ion i e po43 f 10 9 10 6 mol kg due to ion pair formation with ca2 and mg2 and the much higher free carbonate ion concentrations co32 f 0 32 87 mol kg

metal speciation assessments in natural waters comment on rare earth element complexation behavior in circumneutral ph ground waters assessing the role of carbonate and phosphate ions by kevin h johannesson klaus j stetzenbach vernon f hodge and w berry lyons

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

seismic imaging of the downwelling indian lithosphere beneath central tibet a tomographic image of the upper mantle beneath central tibet from indepth data has revealed a subvertical high velocity zone from 100 to 400 kilometers depth located approximately south of the bangong nujiang suture we interpret this zone to be downwelling indian mantle lithosphere this additional lithosphere would account for the total amount of shortening in the himalayas and tibet a consequence of this downwelling would be a deficit of asthenosphere which should be balanced by an upwelling counterflow and thus could explain the presence of warm mantle beneath north central tibet

spatially variable extension in southern tibet based on gps measurements 1 we use global positioning system gps data and kinematic block models to study the present day deformation of southern tibet gps data from 33 sites in southern tibet and nepal surveyed between 1991 and 2000 reveal 13 2 mm yr of n110 e extension between lhasa and shiquanhe 80 e 91 e of which 9 7 3 0 mm yr represents permanent extension of the tibetan crust the remaining 3 mm yr results from elastic deformation from the locked curving main himalayan thrust fault this extension is spatially nonuniform one half to two thirds of the permanent extension is concentrated across the yadong gulu rift with an opening rate of 6 5 1 5 mm yr most of the remainder occurs on or near the thakkola graben with little extension across the rifts between them differential velocities of sites north and south of the yarlung zangbo suture in western tibet imply that the suture or an adjacent structure may be active as a strike slip fault a numerical model suggests that right lateral strike slip on the yarlung zangbo suture may extend from the karakorum fault zone in the west at least to the yadong gulu rift in the east with 3 mm yr slip rate accommodating part of the eastward extrusion of tibet the convergence directions inferred from gps are consistent with slip vectors of earthquakes however the rate of slip of india beneath tibet along the himalaya is lower than those previously estimated we estimate a slip rate of 12 4 0 4 mm yr between longitudes 83 e and 88 e and 17 1 and 19 1 mm yr in the western and eastern himalaya respectively the variable slip rate correlates with the variable extension rate in southern tibet and we suggest that it results from variation in the deformation rate of the overriding tibetan crust we infer that the slower convergence rate in the central himalaya is significant

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

field evidence for active normal faulting in tibet crustal thickening does not now occur in southern tibet field observations made during the chinese french expedition of 1980 support an earlier hypothesis based on a combined analysis of satellite images and fault plane solutions of earthquakes immediately north of the highest peaks of the himalayas the tectonic regime is dominated by east west extension we have mapped a large number of north south normal faults which sharply cut the glacial and post glacial morphology and followed several recent earthquake breaks along faults detected on satellite and aerial photographs microtectonic measurements in the quaternary suggest that the maximum principal stress is vertical

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

a note of caution on the use of boulders for exposure dating of depositional surfaces abstract exposure dating of boulders has been widely applied to determine the age of depositional surfaces under the assumption that the pre depositional nuclide component in most boulders is negligible here we present a case study on fluvial terraces at the active mountain front of the eastern andes where this assumption is clearly invalid because sandstone boulders n 13 from terraces at two sites contain a highly variable inherited 10be component and have apparent 10be ages that exceed the age of the respective surface by up to 90 ka likewise boulders from active stream channels n 5 contain a substantial inherited 10be component equivalent to 5 48 ka of exposure the age of the fluvial terraces is well determined by two approaches that allow to correct for the pre depositional nuclide component 10be dating of amalgamated pebbles and 10be depth profiles on sand samples at site 1 three terraces have 10be ages of 3 5 ka t2 11 13 ka t3 and 16 20 ka t4 which are consistent with the terrace stratigraphy the age of terrace t3 is confirmed by a calibrated 14 c age of 12 61 0 20 ka bp obtained from a wood sample at site 2 terrace t3 has a 10be age of 13 16 ka the average inherited 10be concentration of sand grains determined from depth profiles and stream sediments is small and equivalent to 1 3 ka of exposure in contrast the mean inheritance of pebbles and boulders is higher and equivalent to exposure times of 10 ka and 30 ka respectively these differences in the pre depositional nuclide component are related to the different provenance and transport history of sand pebbles and boulders the sand is derived from rapidly eroding miocene sediments exposed near the mountain front whereas the pebbles and boulders originate from triassic sandstones in the internal part of the fold and thrust belt on their way to the mountain front boulders and pebbles were temporarily stored and irradiated in alluvial fans that are currently reworked as sediment deposition in intramontane basins and their subsequent excavation is common in the andes and other fold and thrust belts the presence of pre depositional nuclide components should be evaluated when applying exposure dating at active mountain fronts

giant m8 earthquake triggered ice avalanches in the eastern kunlun shan northern tibet characteristics nature and dynamics several giant ice avalanches were initiated by slope failure from ice caps due to strong ground motion during the 14 november 2001 m w 7 9 kokoxili earthquake on the kunlun fault four ice avalanches were identified on the north slope of the burhan budai shan several kilometers east of the kunlun pass and two were identified on the south slope of the eastern yuxi feng which is 50 km west of the kunlun pass these ice avalanches originated from steep sided ice caps and progressed over and past the termini of outlet valley glaciers in the burhan budai shan the ice avalanches comprised ice and snow that reached 2 3 km down valley beyond the snouts of the contemporary glaciers detailed study of the largest ice avalanche b2 shows that the initial movement over the contemporary glacier was turbulent in nature having a velocity 35 ms 1 beyond the contemporary glacier the ice avalanche was confined within steep valley walls and entrenched paraglacial fans before coming to rest this ice avalanche moved as a bingham plastic flow at a velocity of 21 ms 1 these ice avalanches transported little rock debris and it is thus unlikely that they are important in contributing to the landscape evolution of this region yet given the appropriate geologic and climatic conditions ice avalanching may be an important process in the landscape evolution of high mountainous terrains the frequency of such events is unknown but such phenomena may become more common in the future as a consequence of increased glacier and slope instability caused by human induced climate change ice avalanches therefore likely constitute a significant geologic hazard in the near future

accuracy of cosmogenic ages for moraines analyses of all published cosmogenic exposure ages for moraine boulders show an average age range of 38 between the oldest and youngest boulders from each moraine this range conflicts with the common assumption that ages of surface boulders are the same as the age of the landform the wide spread in boulder ages is caused by erosion of the moraine surface and consequent exhumation of fresh boulders a diffusion model of surface degradation explains the age range and shows that a randomly sampled small set of boulders n 3 7 will always yield a lower age limit for the moraine the model indicates that for identical dating accuracy six to seven boulders are needed from old and tall moraines 40 000 100 000 yr 50 100 m initial height but only one to four boulders from small moraines 20 000 100 000 yr 10 20 m by following these guidelines the oldest obtained boulder age will be 90 of the moraine age 95 probability this result is only weakly sensitive to a broad range of soil erosion rates our analysis of published boulder ages indicates that 3 of all moraine boulders have prior exposure and 85 of these boulders predate the dated moraine

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

spatially constant slip rate along the southern segment of the karakorum fault since 200 ka abstract determining the slip rate history along the right lateral karakorum fault kf is fundamental to understanding its present day kinematic role in the deformation of tibet geodetic and geologic studies suggest slip rates of 0 11 mm yr along this structure whether slip rate variability exists along strike and or time or simply results from different measuring techniques timescales remains unknown in order to constrain slip rates within a timescale of 200 ka we studied fluvial and glacial geomorphic features that are right laterally or vertically offset by the fault by varying amounts from 7 1 m to 430 30 m and up to 53 5 m respectively we constrained their ages using 10be surface exposure dating on 141 quartz rich samples collected on 4 lateral moraines and at 3 alluvial sites along the southernmost segment of the kf menshi kailas basin and along the gurla mandhata detachment fault in the pulan graben from the 30 fault bend at baer 80 5 e to mount kailas area the slip rate along the kf is 7 1 3 2 1 7 mm yr at menshi and 7 9 3 2 2 5 mm yr at kailas slip on two parallel fault strands in the pulan graben the normal fault slip rate is 1 6 0 4 0 3 mm yr our data suggest that the quaternary slip rate along the southern kf does not decrease eastward but is constant along strike for at least 200 km from 5 11 mm yr in the gar basin further north to 7 8 mm yr in the menshi kailas basin because no expected along strike slip rate gradient is observed it implies that the kf does not end at the kailas but must extend where the slip rate decreases i e eastward along the yarlung zangbo suture and southward along the gurla mandhata humla fault system

incorporating and reporting uncertainties in fault slip rates 1 quantitative slip rate estimates are essential to understanding crustal deformation processes and assessing seismic hazard computing slip rates requires two fundamental ingredients estimates of the age of an offset landforrn or deposit and displacement along the fault of interest because both of these measures contain uncertainty slip rates are inherently uncertain methods to compute and report slip rates have not been standardized and therefore slip rate data are presented inconsistently and are frequently ambiguous in particular slip rate uncertainty is often insufficiently characterized we present a rigorous probabilistic approach to computing and reporting intermediate to long term fault slip rates additionally we have developed freely available software that provides standard age and displacement uncertainty models we demonstrate the method using recent observations from the neodani and death valley fish lake valley fault zones and we compare slip rates determined using this approach with previously published results

climatic and topographic controls on the style and timing of late quaternary glaciation throughout tibet and the himalaya defined by 10be cosmogenic radionuclide surface exposure dating abstract temporal and spatial changes in glacier cover throughout the late quaternary in tibet and the bordering mountains are poorly defined because of the inaccessibility and vastness of the region and the lack of numerical dating to help reconstruct the timing and extent of glaciation throughout tibet and the bordering mountains we use geomorphic mapping and 10 be cosmogenic radionuclide crn surface dating in study areas in southeastern gonga shan southern karola pass and central western nyainqentanggulha shan and tanggula shan tibet and we compare these with recently determined numerical chronologies in other parts of the plateau and its borderlands each of the study regions receives its precipitation mainly during the south asian summer monsoon when it falls as snow at high altitudes gonga shan receives the most precipitation 2000 mm a 1 while near the margins of monsoon influence the karola pass receives moderate amounts of precipitation 500 600 mm a 1 and in the interior of the plateau little precipitation falls on the western nyainqentanggulha shan 300 mm a 1 and the tanggula shan 400 700 mm a 1 the higher precipitation values for the tanggula shan are due to strong orographic effects in each region at least three sets of moraines and associated landforms are preserved providing evidence for multiple glaciations the 10 be crn surface exposure dating shows that the formation of moraines in gonga shan occurred during the early mid holocene neoglacial and little ice age on the karola pass during the lateglacial early holocene and neoglacial in the nyainqentanggulha shan date during the early part of the last glacial cycle global last glacial maximum and lateglacial and on the tanggula shan during the penultimate glacial cycle and the early part of the last glacial cycle the oldest moraine succession in each of these regions varies from the early holocene gonga shan lateglacial karola pass early last glacial western nyainqentanggulha shan and penultimate glacial cycle tanggula shan we believe that the regional patterns and timing of glaciation reflect temporal and spatial variability in the south asian monsoon and in particular in regional precipitation gradients in zones of greater aridity the extent of glaciation has become increasingly restricted throughout the late quaternary leading to the preservation of old 100 ka glacial landforms in contrast in regions that are very strongly influenced by the monsoon 1600 mm a 1 the preservation potential of pre lateglacial moraine successions is generally extremely poor this is possibly because lateglacial and holocene glacial advances may have been more extensive than early glaciations and hence may have destroyed any landform or sedimentary evidence of earlier glaciations furthermore the intense denudation mainly by fluvial and mass movement processes which characterize these wetter environments results in rapid erosion and re sedimentation of glacial and associated landforms which also contributes to their poor preservation potential

middle to late miocene extremely rapid exhumation and thermal reequilibration in the kung co rift southern tibet 1 the kung co rift is an approximately nnw striking wsw dipping normal fault exposed in southern tibet and is part of an extensive network of active approximately ns striking normal faults exposed across the tibetan plateau detailed new and published u th he zircon and apatite thermochronometric data from the footwall of the early miocene kung co granite provide constraints on the middle miocene to present day exhumation history of the footwall to the kung co fault inverse modeling of thermochronometric data yield age patterns that are interpreted as indicating 1 initiation of normal fault slip at 12 13 ma and rapid exhumation of the footwall between 13 and 10 ma 2 acceleration of normal fault slip at rates of 21 9 6 9 mm yr at 10 ma 3 rapid thermal reequilibration between 10 and 9 ma and 4 slow exhumation and or quiescence from 9 ma to the present day hanging glacial valleys in the footwall and fault scarps that cut late quaternary till and moraine deposits indicate that fault slip continues today middle to late miocene initiation of extension across the kung co rift is broadly the same as the documented initiation of ew extension across the south central tibetan plateau eastward flow of middle or lower crust from beneath tibet accommodated by northward underthrusting of indian crust beneath tibet provides a plausible explanation for the onset of ew extension across the tibetan plateau

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

lhasa block and bordering sutures a continuation of a 500 km moho traverse through tibet a north south wide angle fan profile through the yarlung zangbo suture across the lhasa block and through the bangong nujiang suture shows several significant and sharp changes in crustal thickness both sutures appear as vertical 20 km steps in the moho and may have been the loci for eastward strike slip motion of the tibetan lithosphere

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

constraints on the late quaternary glaciations in tibet from cosmogenic exposure ages of moraine surfaces abstract this contribution provides new constraints on the timing of tibetan glacial recessions recorded by the abandonment of moraines we present cosmogenic radionuclide 10 be inventories at 17 sites in southern and western tibet 32 crests 249 samples and infer the range of permissible emplacement ages based on these analyses individual large embedded rock and boulder samples were collected from the crests of moraine surfaces and analyzed for 10 be abundance we consider two scenarios to interpret the age of glacial recession leading to the moraine surface formation from these sample exposure ages 1 erosion of the moraine surface is insignificant and so the emplacement age of the moraines is reflected by the mean sample age and 2 erosion progressively exposes large boulders with little prior exposure and so the oldest sample age records the minimum moraine emplacement age we found that depending on the scenario chosen the moraine emplacement age can vary by 50 for 100 ka old samples we consider two scaling models for estimating the production rates of 10 be in tibet which has an important although lesser effect on inferred moraine ages while the data presented herein effectively increase the database of sample exposure ages from tibet by 20 we find that uncertainties related to the interpretation of the 10 be abundance within individual samples in terms of moraine emplacement ages are sufficient to accommodate either a view in which glacial advances are associated with temperature minima or precipitation maxima that are recorded by independent paleoclimate proxies a reanalysis of published data from moraines throughout tibet shows that the variation we observe is not unique to our dataset but rather is a robust feature of the tibetan moraine age database thus when viewed in a similar way with other samples collected from this area uncertainties within moraine exposure ages obscure attribution of tibetan glacial advances to temperature minima or precipitation maxima our work suggests that more reliable chronologies of tibetan glaciations will come from improvements in production rate models for this portion of the world as well as a better understanding of the processes that form and modify these geomorphic surfaces

the october 6 2008 mw 6 3 magnitude damxung earthquake yadong gulu rift tibet and implications for present day crustal deformation within tibet abstract a mw 6 3 magnitude earthquake occurred on october 6 2008 in southern damxung county within the n s trending yangyi graben which forms the northern section of the yadong gulu rift of south central tibet the earthquake had a maximum intensity of ix at the village of yangyi also yangying 29 43 3 n 90 23 6 e and resulted in 10 deaths and 60 injured in this sparsely populated region field observations and focal mechanism solutions show normal fault movement occurred along the nne trending western boundary fault of the yangyi graben in agreement with the felt epicenter pattern of the isoseismal contours and distribution of aftershocks the earthquake and its tectonic relations were studied in detail to provide data on the seismic hazard to the nearby city of lhasa the damxung earthquake is one of the prominent events along normal and strike slip faults that occurred widely about tibet before and after the 2008 mw 7 9 magnitude wenchuan earthquake analysis of these recent m 5 0 earthquake sequences demonstrate a kinematic relation between the normal strike slip and reverse causative fault movements across the region these earthquakes are found to be linked and the result of eastward extrusion of two large structural blocks of central tibet the reverse and oblique slip surface faulting along the longmenshan thrust belt at the eastern margin of the tibetan plateau causing the wenchuan earthquake was the result of eastward directed compression and crustal shortening due to the extrusion prior to it east west extensional deformation indicated by normal and strike slip faulting events across central tibet had led to a build up of the compression to the east the subsequent renewal of extensional deformational events in central tibet appears related to some drag effect due to the crustal shortening of the wenchuan event unraveling the kinematical relation between these earthquake swarms is a very helpful approach for understanding the migration of strong earthquakes across tibet

normal faulting in central tibet since at least 13 5 myr ago tectonic models for the evolution of the tibetan plateau interpret observed east west thinning of the upper crust to be the result of either increased potential energy of elevated crust1 or geodynamic processes that may be unrelated to plateau formation2 3 4 5 6 a key piece of information needed to evaluate these models is the timing of deformation within the plateau the onset of normal faulting has been estimated to have commenced in southern tibet between about 14 myr ago7 and about 8 myr ago8 and in central tibet about 4 myr ago9 here however we report a minimum age of approximately 13 5 myr for the onset of graben formation in central tibet based on mineralization ages determined with rb sr and 40ar 39ar data that post date a major graben bounding normal fault these data along with evidence for prolonged activity of normal faulting in this and other tibetan grabens support models that relate normal faulting to processes occurring beneath the plateau thinning of the upper crust is most plausibly the result of potential energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath tibet this is supported by recent geophysical and geological data10 11 12 13 14 15 16 17 which indicate that spatial heterogeneity exists in both the tibetan crust and lithospheric mantle

mobility and fractionation of rare earth elements during supergene weathering and gossan formation and chemical modification of massive sulfide gossan primary massive sulfide gossans msg in the bathurst mining camp bmc new brunswick canada are characterized by relative enrichment of au sb and as formation of jarosite group minerals jarosite plumbojarosite and argentojarosite and little or no fractionation in the rare earth elements ree including preservation of large positive eu anomalies average eu eu nasc 4 14 in msg 6 61 in massive sulfide mineralization 0 60 in host rocks the chemical and mineralogical characteristics of msg e g halfmile lake deposit imply low ph 3 and relatively oxidizing conditions during gossan formation oxidation of a volcanogenic massive sulfide body comprising pyrite pyrrhotite sphalerite galena and chalcopyrite with a falling water table the lack of light ree or heavy ree fractionation and preservation of positive eu anomalies characteristic of the original 465 ma hydrothermal fluid is consistent with relatively large water rock ratios during massive sulfide mineralization oxidation and removal of the ree predominantly as sulfate complexes lnso4 ln so4 2 low ph groundwaters recovered from past producing mines in the bmc display ree patterns reflecting those inferred to have occurred during gossan formation gossan at the restigouche deposit in contrast to the halfmile lake deposit displays mineralogical and chemical evidence for having been chemically reworked since primary gossan formation evidence for chemical reworking includes loss of primary massive sulfide mineralization textures replacement of plumbojarosite with anglesite almost complete removal of jarosite minerals loss of au sb and as and apparent preferential removal of eu resulting in loss of positive eu anomalies for most samples average eu eu nasc 1 21 in the gossan with many displaying strong negative anomalies 3 65 in massive sulfide mineralization 0 54 in host rocks based on geochemical modeling conditions inferred for the chemical reworking of the restigouche deposit include near neutral conditions and either relatively oxidizing conditions with eu2 hosted in a preferentially weathered mineral host possibly through substitution for pb in plumbojarosite and beudantite or cycling between reduced and oxidized conditions during gossan reworking

a deforming block model for the present day tectonics of tibet 1 we use gps data from 45 sites across the tibetan plateau surveyed between 1991 and 2001 to study the distribution of strain in that part of the india eurasia collision zone the plateau is cut by a few major rapidly slipping strike slip fault zones with broadly distributed strain between those zones the gps velocities can be fit well by a simple deforming block model that combines uniform strain with the motion of blocks separated by the major known fault zones within the plateau the boundaries of the four blocks in this model correspond to the major rapidly slipping faults a rigid block model with a smaller number of blocks fits the data poorly and can be rejected we estimate that 5 9 0 7 mm yr of extension in the n69 w direction occurs on the yadong gulu rift south of the plateau localized extension may extend as far north as the nyainqentanglha range we find 7 4 0 7 mm yr of right lateral slip on the karakorum jiali fault zone significantly slower than that previously estimated from offset geologic features our deforming block model and a two dimensional single fault screw dislocation model give lower and upper bounds of 4 4 1 1 and 10 3 0 4 mm yr on the slip rate on the kunlun fault respectively comparable in its highest to the long term slip rate observed geologically the distributed deformation is surprisingly uniform over the tibetan plateau qaidam basin and qilian shan and approximates a combination of pure shear and uniaxial contraction with the same axes of maximum contraction n32 e this strain field is a combination of shortening and extension that produces little net dilatation 1 area loss per million years at present the broadly distributed strain most likely represents slip on many faults each with a relatively low slip rate distributed conjugate strike slip faulting is the most plausible mechanism to produce the observed strains as supported by the record of medium to large earthquakes within the plateau the distributed deformation is just as important in the accommodation of the total india eurasia convergence as is slip on the major faults the eastward motion of central tibet is as large as 50 of the convergence rate between india and eurasia however this eastward motion is not motion of a relatively undeforming block bounded by rapidly moving strike slip faults as suggested by past extrusion models instead much of this eastward extrusion is driven by the internal extension of the plateau our inference is in agreement with the model of f shen et al 2001 that suggested east west extension north south contraction and eastward plateau growth dominate the present tectonics of the tibetan plateau

yadong cross structure and south tibetan detachment in the east central himalaya 89 90 e the yadong cross structure ycs occurring at 89 30 east longitude in the himalaya is the largest across strike discontinuity in the geologic structure and topography of the high himalaya between the himalayan syntaxes it is manifest by a plan view left offset of the topographic crest of the range and a coincident apparent left strike offset of the south tibetan detachment system stds of about 70 km new field mapping indicates that the stds intersects the west side of the ycs at zherger la and further suggests that the ycs is the surface expression of a large north northeast striking west facing lateral ramp in the himalayan thrust system the greater himalayan allochthon is apparently draped across this lateral ramp resulting in a north northeast striking monoclinal flexure of the allochthon manifest in the chomolhari range superimposed steep west northwest directed normal shear appears to have obliterated earlier northerly directed stds shear fabric along along the ycs segment of the tethyan belt greater himalayan belt contact recent seismicity suggests that the greater himalayan allochthon is still moving southward along the lateral ramp u pb monazite dates on leucogranites in the footwall of the stds on opposite sides of the ycs are suggestive of a south to north decrease in the crystallization age of these granites the age pattern is consistent with southward extrusion of the greater himalayan allochthon and suggests the possibility of determining an average slip rate for the stds by dating granites in the immediate footwall of the stds that are substantially separated in north south section an average slip rate of about 7 mm yr between 23 and 12 ma is suggested by the data presented here

quantifying landscape differences across the tibetan plateau implications for topographic relief evolution 1 we quantify the bulk topographic characteristics of the tibet qinghai plateau with specific focus on three representative regions northern central and southeastern tibet quantitative landscape information is extracted from shuttle radar topography mission digital elevation models we find that the morphology of the tibetan plateau is nonuniform with systematic regional differences the northern and central parts of the plateau are characterized by what we suggest to call positive topography i e a topography in which elevation is positively correlated with relief and mean slope a major change from the internally drained central part of tibet to the externally drained part of eastern tibet is accompanied by a transition from low to high relief and from positive to negative topography i e a topography where there is an inverse or negative correlation between elevation and relief and between elevation and mean slope relief in eastern tibet is largest along rivers as they cross an ancient eroded plateau margin at high angle to the major strike slip faults the yalong yulong thrust belt implying strong structural control of regional topography we propose that the evolution of river systems and drainage efficiency the ability of rivers to transport sediments out of the orogen coupled with tectonic uplift is the simplest mechanism to explain systematic regional differences in tibetan landscapes basin filling due to inefficient drainage played a major role in smoothing out the tectonically generated structural relief this mode of smoothing started concurrently with tectonic construction of the relief as most clearly illustrated today in the qilian shan qaidam region of the northeastern plateau in the interior of tibet further passive filling due to internal drainage only continued to smooth the local relief millions of years after the cessation of major phases of surface uplift due to crustal shortening thus diachronous beveling at high elevation produced the low relief surface of the high plateau in southeast tibet headward retreat of external drainages brought back in the global ocean base level first disrupting then interrupting the relief reduction process it produced a transitional topography by dissecting the old remnant plateau surface which introduced younger and steeper incision of this hitherto preserved high base level this provides a unifying mechanism for the formation of the low relief plateau interior and for the origin of the high elevation low relief relict surface in southeastern tibet our analysis brings forth the importance of surface processes in particular drainage efficiency in shaping plateau morphology and landscape relief such key processes appear to have been mostly ignored in numerical models of plateau deformation our results also cast doubt on and provide a more realistic alternative to the fashionable contention that a continuous preuplift low relief surface first formed at low elevation extending all the way to the south china sea shore before being warped upward in the late miocene pliocene by lower crustal channel flow

an eocene magmatic belt across central tibet mantle subduction triggered by the indian collision mountain domes rising to 6600 m along tibet s tanggula range herald the eocene intrusion of calcalkaline granites into terranes accreted much earlier together with coeval cogenetic volcanics such intrusives which have similar crystallization and cooling ages may be part of an ese trending belt cutting central tibet in half this magmatic belt may have marked a former northern boundary of the plateau testifying to s directed subduction of the asian mantle such subduction would have developed soon after india s collision but long before the rise of the plateau s present rims along one of central tibet s mesozoic sutures

some methodological developments in the analysis of sediment transport processes using age distribution of floodplain deposits abstract floodplain areas are primary storage sites for river sediment in the saru river ages of floodplain surfaces were examined by tree ring analysis and vertical and horizontal configurations of floodplain deposits were measured by field and aerial photo surveying a flood in 1992 provided a good opportunity to examine depositional and erosional processes of floodplain sediment before and after the flood predominant disturbances were observed in the unconstrained wide reaches where floodplains develop this event indicated that eroded areas of floodplain deposits in each age class linearly increased with sediment volume and that the proportion of the total area eroded decreased exponentially with increase in the age of sediment we constructed basic equations expressing continuity of age distribution in order to analyze river sedimentation in a time series according to the results of the 1992 flood the floodplain disturbance rates determined by this analysis showed similar temporal changes with sediment transport rates monitored at the iwachishi reservoir a sharp increase in sediment discharge was seen after 1962 associated with the historical maximum rainfall in the temporal analysis of floodplain sediment the time series approach presented here is useful for evaluating the speed of sediment waves and the cumulative impact of sedimentation in a river basin