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

tectonic implications of 40ar 39ar muscovite dates from the mt haley stock and lussier river stock near fort steele british columbia the mt haley and lussier river stocks are located northeast of cranbrook b c near the south end of the western main ranges of the southern canadian rocky mountains both are multiphase potassium feldspar porphyritic monzonite plutons that intrude lower paleozoic miogeoclinal strata they crosscut and thermally overprint the lussier river fault and the thrust and fold structures in the east flank of the purcell anticlinorium and the west limb of the porcupine creek anticlinorial fan structure muscovite from the mt haley stock yielded a 40ar 39ar plateau age of 108 2 0 7 ma 2 and a single crystal step heating analysis of muscovite from a skarn in the metamorphic aureole adjacent to the lussier river stock gave a plateau date of 108 7 0 6 ma 2 these dates constrain the timing of thrusting and folding in this portion of the western rocky mountains and of the displacement along the lussier river st mary fault to pre middle albian

earthquakes beneath the himalayas and tibet evidence for strong lithospheric mantle eleven intracontinental earthquakes with magnitudes ranging from 4 9 to 6 occurred in the mantle beneath the western himalayan syntaxis the western kunlun mountains and southern tibet near xigaze between 1963 and 1999 high resolution seismic waveforms show that some focal depths exceeded 100 kilometers indicating that these earthquakes occurred in the mantle portion of the lithosphere even though the crust has been thickened there the occurrence of earthquakes in the mantle beneath continental regions where the subduction of oceanic lithosphere ceased tens of millions years ago indicates that the mantle lithosphere is sufficiently strong to accumulate elastic strain

emplacement of the 18 may 1980 lateral blast deposit ene of mount st helens washington facies variations east northeast of mount st helens preserve a record of depositional processes in the 18 may 1980 lateral blast cloud this paper reports new field grain size and component data from the ene sector of the timber blowdown zone and presents a model for blast flow and sedimentation the first erupted ejecta was rich in juvenile components and extends to the distal blowdown limit the last erupted ejecta was rich in accidental lithics and reached no further than a few kilometres from the mountain due to waning discharge the blast cloud was a turbulent stratified flow which transported and deposited sediment in the manner of a high density turbidity current the possibility that the blast was emplaced as a giant shearing fluidised bed is not favoured by compositional zoning patterns depositional conditions were strongly influenced by the rate of suspended load fallout from the blast within about 8 km from vent rapid sedimentation caused deposition under moderate to high concentration conditions and formation of a basal hindered settling zone able to detach gravitationally and drain into local depressions the resulting proximal facies resembles a low aspect ratio ignimbrite fines depletion in the proximal facies is attributed to a combination of residual turbulence and rapid gas escape during particle settling and compaction through the hindered settling zone component data suggest that the blast head played no significant role in the generation of fines depletion in the blast deposit as suggested by previous workers with increasing distance from vent the rate of particle fallout declined and sedimentation took place under increasingly dilute and tractional conditions building up antidune like bedforms wavelengths of these bedforms range from 20 to 1 m and decrease away from vent there is a systematic relationship between antidune migration direction and depositional slope the transition from proximal ignimbrite like to distal surge like facies suggests a possible gradation in transport and deposition processes between conventional pyroclastic surges and high velocity pyroclastic flows

evidence for mechanically coupled lithosphere in central asia and resulting implications the recent dramatic increase in seismic anisotropy and surface global positioning system gps data for central asia permits a comprehensive examination of the mantle s role in mountain building a joint analysis of 178 shear wave splitting and 2000 gps observations using a new technique reveals that the crust and lithospheric mantle deform coherently arguing for crust mantle mechanical coupling during deformation the observed spatial variations in anisotropy refl ect the large scale pattern of lithospheric deformation as well as a change in deformational style from simple shear on the tibetan plateau transitioning to pure shear in surrounding regions

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

weakly coupled lithospheric extension in southern tibet a b s t r a c t west east extension is a prominent tectonic feature of southern and central tibet despite ongoing north south n s convergence between india and eurasia knowledge of deep structure beneath the n s trending rifts is key to evaluating models proposed for their origin including gravitational collapse oblique convergence along the arcuate plate boundary and mantle upwelling we model direct s and moho reflected sspmp phases at teleseismic distances to constrain variations in crustal thickness across the major rifts crossed by a 900 km long w e broadband array in the lhasa terrane crustal thicknesses are 70 80 km however moho depth decreases by 10 km within a horizontal distance of 100 km west of the yadong gulu rift ygr and nyainquentanghla mountains nqtl this moho uplift taken with deep extensional focal mechanisms and reduced seismic velocity in the upper mantle suggests that asthenospheric upwelling has significantly contributed to the pattern of extension across the ygr and nqtl the 100 km separation between surface rift and moho uplift is likely enabled by partial decoupling across a ductile middle crust 2015 the authors published by elsevier b v this is an open access article under the cc by nc nd license http creativecommons org licenses by nc nd 4 0

separating intrinsic and apparent anisotropy abstract seismic anisotropy plays a key role in studies of the earth s rheology and deformation because of its relation to flow induced lattice preferred orientation lpo of intrinsically anisotropic minerals in addition to lpo small scale heterogeneity produces apparent anisotropy that need not be related to deformation in the same way as intrinsic anisotropy quantitative interpretations of observed anisotropy therefore require the separation of its intrinsic and apparent components we analyse the possibility to separate intrinsic and apparent anisotropy in media with hexagonal symmetry typically used in surface wave tomography and sks splitting studies our analysis is on the level of the wave equation which makes it general and independent of specific data types or tomographic techniques we find that observed anisotropy can be explained by isotropic heterogeneity when elastic parameters take specific combinations of values in practice the uncertainties of inferred anisotropy are large enough to ensure that such a combination is always within the error bars it follows that commonly observed anisotropy can always be explained completely by a purely isotropic laminated medium unless all anisotropic parameters are known with unrealistic accuracy most importantly minute changes in the poorly constrained p wave anisotropy and the parameter can switch between the possible or impossible existence of an isotropic equivalent important implications of our study include 1 intrinsic anisotropy over tomographically resolved length scales is never strictly required when reasonable error bars for anisotropic parameters are taken into account 2 currently available seismic observables provide weak constraints on the relative contributions of intrinsic and apparent anisotropy 3 therefore seismic observables alone are not sufficient to constrain the magnitude of mantle flow 4 quantitative interpretations of anisotropy in terms of mantle flow require combined seismic geodynamic inversions as well as the incorporation of additional data such as topography gravity and scattered waves

receiver function imaging of crustal suture steep subduction and mantle wedge in the eastern india tibet continental collision zone abstract to understand the along strike variation of crustal deformation and tectonic processes in the india tibet continental collision zone we deployed a linear array of broadband seismic stations along 92 e to image lithospheric structure our receiver function cross section reveals a prominent negative converter dipping 20 north from 10 55 km depth below sea level almost through the whole crust beneath the southern lhasa terrane we interpret it to be a manifestation of the yarlung zangbo suture zone yzs separating the continental crust of the indian and eurasian plates this implies the hypothesized channel flows of indian middle crust extruding southwards from tibet are limited at this longitude to the southernmost portion of the lhasa terrane a positive converter consistent with previous suggestions of eclogite formation is seen about 10 15 km above the moho and continuing 50 km north of the 20 dipping yzs converter we image this positive converter continuously from 60 km south of the surface trace of the yzs to the vicinity of the jiali fault supporting the interpretation of sub horizontal underplating of tibetan crust by indian crust to 31 n at 85 e on the hi climb transect however we also show a negative mantle converter sub parallel to the crustal yzs converter from the northern limit of the underplating indian lower crust to at least 140 km depth that we interpret as the base of tibetan lithosphere overlying an asthenospheric mantle wedge based on the lithospheric structure observed in this and other studies we infer that indian mantle lithosphere currently detaches from indian lower crust at the mantle suture that is nearly 50 km south of the surface trace of the yzs at 92 e south of the mantle suture suggested by indepth transect beneath the surface trace of the yzs at 90 e and far south of the mantle suture suggested to be at the 31 n northern limit of underthrusting indian lower crust suggested by hi climb transect at 85 e a change from underplating in the west to steep subduction in the east can reconcile all these observations

crustal fabric in the tibetan plateau based on waveform inversions for seismic anisotropy parameters 1 the tibetan plateau has the thickest continental crust on earth and fabrics within the crust that are anisotropic to seismic waves may provide clues to how it reached such extreme proportions and how it is currently deforming waveform modeling using a global minimization inversion technique applied to receiver functions computed from 11 stations spanning the north south length of the eastern plateau has yielded a suite of crustal models that include anisotropy these models suggest that the tibetan crust contains 4 14 anisotropy at different depths that is likely a result of both fossil fabrics and more recent deformation all models contain anisotropy in the surface layer and for most stations the alignment of the slow symmetry axis suggests a relationship with crustal fabrics associated with e w trending thrust faults or sutures middle to lower crustal anisotropy is present at most stations with a fast axis trending n s to nw se in the south nearly e w in the central plateau and n s to ne sw in the northern plateau this pattern appears consistent with recent ductile deformation due to both topographically induced flow and to boundary forces from subducting lithosphere at the northern and southern margins of the plateau the orientations of crustal anisotropy determined for most stations in this study are significantly different from shear wave splitting fast polarization directions implying distinct deformation in the crust and mantle

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

may 18 1980 mount st helens deposits in south coldwater creek washington beginning at 0832 pacific daylight time pdt mount st helens washington produced a rockslide debris avalanche and a lateral explosion blast that generated a laterally directed pyroclastic surge called here a blast surge the resulting deposits are thickest in the east west trending valley of north fork toutle river immediately north of the volcano but are almost as thick in south coldwater creek a small tributary of the toutle river in the lee of an east west trending ridge johnston ridge just north of the north fork toutle river the avalanche surmounted johnston ridge 300 m relief and flowed down the valley of south coldwater creek closely following the avalanche and in part contemporaneously with it the blast surge left thin up to 0 5 m deposits on johnston ridge and thick up to 15 m deposits in south coldwater creek these deposits have a lower layer layer a1 which is friable massive poorer in fines particles 1 16 mm and more poorly sorted than the upper layer layer a2 layer a2 is less friable because it contains more silt and clay sized material beneath layer a1 is a zone containing a small percentage of debris from the blast surge mixed with products eroded from the underlying surface this zone designated as layer a0 is a poorly mixed layer containing schlieren of soil shattered volcanic fragments and considerable quantities of organic material layer a1 is strongly sorted and fines depleted and is interpreted to have been deposited from just behind the turbulent head of the blast surge layer a2 is interpreted to have been deposited from the body of the blast surge which is believed to have been less turbulent than the head layer a0 is interpreted to have formed by shearing at the base of the head of the blast surge in the region between the front and the back of the head of the blast surge it is concluded that a strongly fines depleted basal layer of a pyroclastic flow or surge deposit underlain by a thin sheared zone is the signature of an extremely high velocity particulate density current

lithic breccia and ignimbrite erupted during the collapse of crater lake caldera oregon abstract the climactic eruption of mount mazama 6845 y b p vented a total of 50 km3 of compositionally zoned rhyodacitic to basaltic magma from a a single vent as a plinian pumice fall deposit and the overlying wineglass welded tuff and b ring vents as ignimbrite and coignimbrite lithic breccia accompanying the collapse of crater lake caldera new field and grain size data for the ring vent products are presented in this report the coarse grained poorly bedded clast supported lithic breccia extends as far as 18 km from the caldera center like the associated ignimbrite the breccia is compositionally zoned both radially and vertically and silicic mixed and mafic types can be recognized based on the proportion of rhyodacitic pumice matrix fractions in silicic breccias are depleted of fines and are lithic and crystal enriched relative to silicic ignimbrite due to vigorous gas sorting during emplacement ignimbrite occurs as a proximal veneer deposit overlying the breccia a medial 8 to 25 km from the caldera center compositionally zoned valley fill as much as 110 m thick and an unzoned distal 20 km facies which extends as far as 55 km from the caldera breccia within 9 km of the caldera center is interpreted as a coignimbrite lag breccia formed within the deflation zone of the collapsing ring vent eruption columns expanded pyroclastic flows of the deflation zone were probably vertically graded in both size and concentration of blocks as recently postulated for some turbidity currents an inflection in the rate of falloff of lithic clast size within the lithic breccia at 9 km may mark the outer edge of the deflation zone or may be an artifact of incomplete exposure the onset of ring vent activity at mt mazama was accompanied by a marked increase in eruptive discharge pyroclastic flows were emplaced as a semicontinuous stream as few ignimbrite flow unit boundaries are evident as eruption from the ring vents progressed flow runout distance and the extent of breccia deposition decreased due to a greater internal flow friction and b decreasing eruption column heights effect b probably resulted from a progressive decrease in magmatic gas content and discharge rate waning discharge may have been promoted by the tapping of more viscous crystal rich magma collapse of conduit walls and declining caldera collapse rate

shear wave anisotropy of the godavari rift in the south indian shield rift signature or apm related strain abstract shear wave splitting analysis using sks skks and s waveforms recorded at eight broadband seismic stations within and in the vicinity of the proterozoic godavari rift system in the south indian shield yielded 104 new measurements of azimuthal anisotropy our results reveal that the delay times lie between 0 70 and 1 20 s similar to the previous measurements from the indian shield the orientations of fast polarization directions at most of the stations within and beyond the rift are generally aligned orthogonal to the rift axis consistent with asthenospheric flow in the extension direction absence of clear evidences for a preferentially thinner lithosphere beneath the godavari rift or anomalously high heat flow values coupled with the fact that the ne oriented fast polarization directions are also close to the apm direction of the indian plate result in an ambiguity in interpreting the results as being due to strain related to plate motion or signatures of rifting a thin lithosphere beneath the rift suggests that the source of anisotropy is likely to be in the asthenosphere in such a scenario either the mechanism of 1 active rifting where the asthenospheric flow drives the rifting or 2 passive rifting where mantle flow responds to lithospheric thinning caused by far field stresses or 3 present day apm related strain appear to be the plausible mechanisms for forging the observed anisotropy

application of gaussian beam migration to multiscale imaging of the lithosphere beneath the hi climb array in tibet abstract in this study we apply gaussian beam gb migration of scattered teleseismic p waves to image the crust and upper mantle beneath tibet using data from the hi climb experiment we use teleseismic p waves from three groups of earthquakes to the southeast northeast and northwest of the hi climb array each within a narrow range of azimuth and distance to obtain stacked radial receiver functions which we then use to image the lithosphere by gb migration we produced images at several different frequency bands in order to constrain the multiscale scattering properties of the lithosphere for each frequency band three gb images are generated each from earthquake sources in a distinct back azimuth group the three images are then stacked to form a composite image the imaged moho is generally strong and continuous under much of the lhasa terrane in southern tibet corresponding to a well defined moho a major disrupted zone in moho scattering extending over 200 km in length is evident in the vicinity of the bangong nujiang suture where there is also increased crustal scattering the disrupted zone marks the diffuse subsurface join between stable portions of mantle lithosphere under southern and central tibet respectively at the northern end of the profile in the qiangtang terrane there is an increase in moho reflectivity but at a shallower depth than under the lhasa terrane comparable length scales of about 200 km between regions with disrupted and smooth varying moho suggest that the mechanically strong mantle lithosphere and the crust respond differently to collision with the upper crust currently undergoing pervasive strain over the entire plateau

mechanics of v shaped conjugate strike slip faults and the corresponding continuum mode of continental deformation v shaped conjugate strike slip faults occur widely on earth venus and mars in the solar system they commonly lie at 60 75 in map view from the maximum compressive stress 1 direction this fault pattern cannot be explained directly by the coulomb fracture criterion which predicts the formation of x shaped shear fractures at 30 from the 1 direction possible explanations of this odd fault geometry include rotation of early formed coulomb fractures or reactivation of preexisting weakness here we show that none of these mechanisms is feasible for the formation of a late cenozoic conjugate strike slip fault system in central tibet instead its initiation can be best explained by distributed deformation during the formation of two parallel and adjoining shear zones that have opposing senses of shear our suggestion is based on the current global positioning system gps velocity field in tibet which can be divided into two east trending shear zones a northern left slip zone consisting of active ene striking left slip faults and a southern right slip zone consisting of active wnw striking right slip faults the correlation between the gps strain field and the fault pattern suggests that the central tibet conjugate faults may have initiated as two sets of riedel shears in the two parallel but separate shear zones because the two east trending shear zones also experience north south contraction we refer to this mechanism of conjugate fault formation as paired general shear pgs deformation assuming a newtonian fluid the observed tibetan gps velocity field requires the paired shear zones to have formed either by gravitational spreading of the tibetan lithosphere or a horizontal shear at the base of the upper crust or mantle lithosphere we demonstrate the feasibility of the two inferred mechanisms for the formation of v shaped conjugate faults using simple sandbox experiments our paired general shear pgs model implies that the combined effect of the state of strain and the state of stress favors only one set i e riedel shear of coulomb conjugate shear fractures under general shear flow it also requires continuum deformation rather than discrete extrusion tectonics as the most dominant mode of deformation during the late cenozoic development of the central tibetan plateau

depth variant azimuthal anisotropy in tibet revealed by surface wave tomography azimuthal anisotropy derived from multimode rayleigh wave tomography in china exhibits depth dependent variations in tibet which can be explained as induced by the cenozoic india eurasian collision in west tibet the e w fast polarization direction at depths 100 km is consistent with the accumulated shear strain in the tibetan lithosphere whereas the n s fast direction at greater depths is aligned with indian plate motion in northeast tibet depth consistent nw se directions imply coupled deformation throughout the whole lithosphere possibly also involving the underlying asthenosphere significant anisotropy at depths of 225 km in southeast tibet reflects sublithospheric deformation induced by northward and eastward lithospheric subduction beneath the himalaya and burma respectively the multilayer anisotropic surface wave model can explain some features of sks splitting measurements in tibet with differences probably attributable to the limited back azimuthal coverage of most sks studies in tibet and the limited horizontal resolution of the surface wave results

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

3d imaging of subducting and fragmenting indian continental lithosphere beneath southern and central tibet using body wave finite frequency tomography abstract we perform a finite frequency tomographic inversion to image 3d velocity structures beneath southern and central tibet using teleseismic body wave data recorded by the tibet 31n passive seismic array as well as waveforms from previous temporary seismic arrays high velocity bodies dip 40 northward beneath the himalaya and the lhasa terrane we interpret these high velocity anomalies as subducting indian continental lithosphere icl the icl appears to extend further north in central tibet than in eastern tibet reaching 350 km depth at 31 n along 85 e but at 30 n along 91 e low p and s wave velocity anomalies extend from the lower crust to 180 km depth beneath the tangra yum co rift yadong gulu rift and the cona rift suggesting that rifting in southern tibet may involve the entire lithosphere the anomaly beneath tangra yum co rift extends down to about 180 km whereas the anomalies west of the yadong gulu rift and east of the cona rift extend to more than 300 km depth the low velocity upper mantle west of the yadong gulu rift extends furthest north and appears to connect with the extensive upper mantle low velocity region beneath central tibet thus the northward subducting indian plate is fragmented along north south breaks that permit or induce asthenospheric upwellings indistinguishable from the upper mantle of northern tibet

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

pedogenic and diagenetic influences on void coating formation in lower cretaceous paleosols of the mill creek formation southwestern alberta canada abstract pedogenic and diagenetic influences on void coating formation were studied in two paleosols from the lower cretaceous albian mill creek formation in southwestern alberta canada using detailed micromorphological mineralogical and geochemical methods the paleosols consist of alternating decimetre scale beds of red grey green and variegated mudstone intermediate horizons red and grey green variegated mudstones contain associations of pale yellow clay coatings dark red clay coatings fe depletion hypo coatings and fe oxide quasi coatings sem edx ray analyses indicate that the dark reddish coatings are dominated by si al and fe with subordinate amounts of k na ca mg and ti pale yellow clay coatings have a higher si content and lower al k na ca and fe contents relative to reddish coatings within the paleosol profiles the distribution of clay minerals indicates 1 an increase in discrete clay minerals downward 2 a decrease in mixed layer illite smectite downward and 3 an increase in chlorite upward x ray diffraction indicates that the pale yellow and dark red clay void coatings are polymineralic pale yellow clay coatings have a higher proportion of mixed layer illite smectite and kaolinite less chlorite and are less crystalline than dark reddish coatings despite a diagenetic overprint including development of diffusion gleyans degradation of organic matter burial reddening and probably some smectite illitization both compositional and morphological gradients exist in the paleosol profiles and void coatings these gradients are most likely related to depositional variations and to pedogenic development in the original soils under variable eh ph conditions in the presence of organic matter therefore although the void coatings are altered diagenetic features their morphology mineralogy and perhaps even some of their microchemistry is attributable to the presence of pedogenic precursors initially formed in soils similar to modern temperate alfisols and glossic soils

comparison of the rainy ridge analcime phonolite sill and the crowsnest volcanics alberta canada a 5 7 m thick analcime phonolite sill occurs in the middle of the proterozoic gateway formation in southwest alberta the sill consists of sanidine aegirine augite magnesian hastingsite melanite with hydrogrossular rims titanite and minor biotite apatite and opaque minerals mineralogical and chemical similarities to the analcime rich phases of the cretaceous crowsnest formation found some 20 km to the north suggest a genetic relationship major differences are the presence of amphibole and hydrogrossular minerals not reported in the crowsnest formation the presence of amphibole as a primary hydrous phase in the rainy ridge sill indicates crystallization from a hydrous magma microprobe studies indicate a progressive enrichment of sodium in amphiboles and pyroxenes an apparent difference in chemical composition and alteration behavior of primary analcime phenocrysts and groundmass analcime is interpreted to reflect crystallization of analcime from a hydrous melt at depth followed by rapid trans

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

internal deformation of lithosphere beneath central tibet we use a large number of high quality p wave arrival time data recorded by the hi climb project to determine a 3 d model of azimuthal anisotropy tomography beneath central tibet in the himalayan block variations of fast velocity orientation fvo are revealed from the crust to the upper mantle in contrast the fvo in the lhasa block exhibits only a slight difference between the lower crust and upper mantle reflecting a coherent deformation there different fvos are revealed near the bangong nujiang suture which may reflect anisotropies in different parts of the underthrusting indian plate in the upper mantle beneath the qiangtang block a strong anisotropy is revealed in the shallower part whereas a weak anisotropy appears in the deeper part suggesting that a two layer anisotropy model is applicable there a layered lithosphere is detected in the eastern part of the lhasa block whereas a consistent fvo is revealed in its western part our results indicate that strong deformation has occurred in both the indian and eurasian lithospheres

evidence for pyroclastic emplacement of the crowsnest formation alberta the middle cretaceous crowsnest formation west of coleman alberta is composed of bedded alkaline volcanic deposits containing heterolithic volcanic rock fragments and crystal clasts comparison with modern examples of subaerial pyroclastic rocks suggests that pyroclastic flows surges fallout of material from vertical eruption columns and minor mud flows emplaced the deposits textural evidence in the form of plastically deformed volcanic fragments chilled deposit margins baked rock fragment margins recrystallization and the presence of charred wood and charred wood molds indicate emplacement at elevated temperature massive deposits containing a fine grained basal zone are interpreted as the product of pyroclastic flows whereas deposits characterized by a block rich base overlain by a thin layer of block depleted stratified material are interpreted as the product of density stratified surges deposits exhibiting pronounced stratification were emplaced by ash cloud surges thickly bedded breccias

quaternary extension in southern tibet field observations and tectonic implications we summarize evidence for quaternary and active faulting collected in the field during three sino french expeditions to southeastern tibet 1980 1982 detailed mapping of quaternary and active faults as well as microtectonic measurements indicate that normal faulting has been the dominant tectonic regime north of the himalayas in the last 2 0 5 m y the maximum horizontal principal stress in south tibet appears to be only the intermediate principal stress 2 1 being vertical south of the chord joining the eastern and western syntaxes of the himalayan arc extensional strains are principally localized within seven regularly spaced rift zones three of which have been studied in some detail the extension direction is determined to be n96 e 7 mainly from statistical averaging of strikes of newly formed normal faults throw rates on normal faults are evaluated for different time spans 2 0 5 m y 60 40 kyr and 10 2 kyr b p using structural and topographic reliefs as well as synglacial and postglacial vertical offsets the rate of quaternary extension is about 1 m y 1 along an 1100 km long ese traverse across south tibet this corresponds to a spreading rate of 1 0 6 cm yr this rate and the divergent horizontal projections of slip vectors of earthquakes along the himalayan front constrain the rate at which rigid india underthrusts southern tibet to be 2 1 cm yr although most of the normal faults appear to be independent of and nearly orthogonal to the e w mesozoic tertiary tectonic fabric the yadong gulu rift appears to be guided for over 130 km by the older oblique ne sw nyainqentanglha range and fault zones along it this reactivated zone is the most prominent left lateral strike slip fault system in se tibet excepting this zone and the vicinity of the se extremity of the karakorum fault quaternary strike slip faulting is rare in south tibet i e south of the chord between the syntaxes of the himalayan arc north of the chord the tectonic style is different there minor conjugate strike slip faulting is widespread and appears to control quaternary normal faulting which is more diffuse and subdued than in south tibet along the chord the presence of a major zone of active right lateral en echelon strike slip faults karakorum jiali fault zone probably reflects the greater facility of eastward extrusion in north central tibet in response to the northward push of india the eastern himalayan syntaxis may be an obstacle to such extrusion movements south of the chord

stratified flow in pyroclastic surges stratified flow theory is applied to pyroclatic surges in an effort to gain insight into transport dynamics during explosive eruptions particle transport is assumed to be by turbulent suspension and calculations contained herein show that this is likely for many cases including the 18 may 1980 blast at mount st helens the discussion centers on the rouse number pn which represents a ratio of particle settling velocity to scale of turbulence the brunt vaisala frequency n which is the maximum possible frequency of internal waves the froude number fr representing the ratio of inertial forces to gravitational forces and the richardson number ri a ratio of buoyant restoring forces to turbulent mixing forces the velocity or flow power dependence of bed form wavelength in surge deposits is related to a velocity dependence of wavelength of internal waves in the turbulent surge this produces a decrease in dune wavelength with increasing distance from vent migration direction of bed forms is related tofr as it is defined for a continuously stratified flow proximal to distal facies variations in surge deposits reflect increasingpn andri as the flows move away from their sources this produces the progression from sandwave to massive to planar facies with increasing distance from vent where the long axis of topography is at low angles to the flow direction massive facies in topographic lows may from concurrently with sandwave facies on highs due to the higher particle concentration in the lows where long axis of topography is at high angles to flow direction denser lower parts of the surge may be dammed or blocked blocked material tends to form massive flows that may move down slope independent of the overriding surge a model incorporating turbulent transport stratified flow and time evolution of pyroclastic surges is proposed for deposits which have been attributed to both pyroclastic flow and pyroclastic surge transport by various workers during the initial high energy waxing phase of the eruptive event pn is sufficiently low that only coarse but poorly sorted material is deposited to form relatively coarse bottom layers as the event wanes remaining finer material is deposited through a thin bed load to produce overlying bedded and cross bedded veneer deposits throughout most of the event blocking occurs to produce relatively thick and massive deposits in valley bottoms

shear wave birefringence and current configuration of converging lithosphere under tibet article i nfo new data from west central tibet show that birefringence of s waves has two pronounced increases in magnitude toward the hinterland null birefringence persists to about 75 km north of the indus yarlung suture iys between the indian shield and the lhasa terrane of southern tibet a second rapid increase occurs about 100 km farther north of the bangong nujiang sutures between the lhasa terrane and the qiangtang terrane in central tibet the latter feature is consistently observed along three long transects that collectively span a lateral orogen parallel distance of about 600 km and is likely to mark the northern leading edge of sub horizontally advancing mantle lithosphere of the indian shield the greater india an interpretation consistent with the latest results of finite frequency tomography using both p and s wave travel times previous results of modeling gravity anomalies and a host of other seismic observations similarly complementary constraints indicate that the sudden onset of significant birefringence north of the iys is likely to be the southern termination of eurasian mantle lithosphere curiously the shortest of three transects showed null birefringence through much of the lhasa terrane a pattern inconsistent with those of he isotopes and gravity

tomographic imaging of the underthrusting indian slab and mantle upwelling beneath central tibet abstract to better understand the pattern of convergence between the indian and eurasian plates we determine a high resolution p wave tomography of the crust and upper mantle under southern to central tibet using a large number of high quality data of local earthquakes and teleseismic events collected by the antilope ii and hi climb projects a significant low velocity zone is detected above the northward underthrusting indian slab beneath the indus tsangpo suture which may reflect fault zones or incomplete fragmentation as well as melts and or fluids associated with the dehydration of the underthrusting indian slab the variations in the diving depth and extending distance of the indian slab under the antilope ii and hi climb seismic profiles may be caused by the differences in the viscosity contrast between the indian slab and the surrounding mantle as well as strong structural heterogeneities in the upper mantle under the study region both high and low velocity anomalies are revealed in the mid to lower crust under southern tibet which may reflect the complex pattern of crustal structure the rifts in southern tibet exhibit different velocity images suggesting that different structures exist beneath those rifts

role of oblique convergence in the active deformation of the himalayas and southern tibet plateau noting similarities with subduction along curved oceanic trenches and using a simple block model we show that radial vergence evident in earthquake slip vectors along the himalayan deformation front east west extension on north trending normal faults in the himalayas and southern tibet and right lateral strike slip on the karakorum jiali fault zone can all result from basal shear caused by the indian plate sliding obliquely beneath tibet along a gently dipping arcuate plate boundary within the framework of this mechanism the normal faults in the himalayas and southern tibet are not proxies for the uplift history of tibet the distribution and style of the faults in the himalayas and southern tibet suggest that the basal drag from the underthrusting indian lithosphere extends northward beneath most of southern tibet

use of micromorphology for palaeoenvironmental interpretation of complex alluvial palaeosols an example from the mill creek formation albian southwestern alberta canada abstract field observations are often not sufficient for process based interpretations of palaeosols particularly where they form parts of thick aggradational pedocomplexes within alluvial successions under such conditions micromorphology provides genetic temporal and spatial information on soil forming processes that is critical to an understanding of past environmental conditions thick alluvial successions of the albian mill creek formation contain abundant evidence of pedogenesis but few well developed palaeosol profiles and therefore provide an ideal case study in which to demonstrate the usefulness of micromorphological data for palaeoenvironmental interpretation the micromorphological features of greatest interpretive value are types of clay coatings and ferruginous segregations structure and fabric papules evidence of bioactivity and ferruginous concretions provide information on geomorphic surface stability and assist in reconstructing temporal changes in drainage conditions while individual features can provide some palaeoenvironmental information the relationships of features to one another and assemblages of features provides additional information when analysed hierarchically to establish a sequence of sedimentologic and pedogenic events a common recurring sequence of palaeoenvironmental events subject to local variations can be recognized throughout the mill creek formation the presence of illuvial clay requires that water percolated through the soil and that the soil periodically dried out so that the translocated clay was retained dark reddish clay coatings indicate clay illuviation under freely drained conditions while pale yellow and silty clay coatings suggest that phases of free drainage alternated with phases of poorly drained or saturated soil conditions the presence of iron depletion coatings iron nodules and quasiferrans indicates that these units were at least periodically saturated and the occurrence of multiple overlapping phases within single thin sections demonstrates that redox conditions fluctuated strongly suggesting development in the vadose zone recent soils containing similar assemblages of features develop under warm temperate seasonal climates alternating phases of well drained and saturated conditions on the mill creek floodplains are attributed to changing sediment supply and local palaeogeomorphology rather than to any major regional climate change this type of process based micromorphological analysis should have broad application in other complex pedogenically modified alluvial successions and similar studies would lead to a more detailed understanding of ancient palaeoenvironments

finite frequency tomography in southeastern tibet evidence for the causal relationship between mantle lithosphere delamination and the north south trending rifts 1 while several mechanisms have been suggested to explain the evolution of the tibetan plateau observational constraints on the deep lithospheric processes have been sparse and previous seismic studies were mostly along profiles perpendicular to the collision front of the indian and eurasian plates in this study we show tomographic evidence for the delamination of the mantle lithosphere beneath southeastern tibet a process in which the entire mantle lithosphere peels away from the crust along the moho and thus is a mechanism for rapid thinning of the lithosphere our p and s wave velocity models show the presence of a low velocity anomaly in the crust and upper mantle down to 300 km depth beneath a north south trending rift zone in southeastern tibet this low velocity anomaly is situated above a tabular high dipping angle high velocity anomaly that extends into the upper mantle transition zone the vp vs ratio of this high velocity anomaly suggests that temperature variations are not the only cause of the anomaly and a highly melt depleted mantle is required these observations suggest a causal relationship between the delamination of mantle lithosphere and the formation of the north south trending rift in southeastern tibet

mineral chemistry of igneous melanite garnets from analcite bearing volcanic rocks alberta canada the mineral chemistry of melanite garnets from the crowsnest volcanic rocks of sw alberta canada has been investigated by using electron microprobe scans quantitative analyses and multivariate statistical analysis the garnets occur with aegirine augite sanidine analcite and rare plagioclase as phenocrysts in trachyte and phonolite flows agglomerates and tuffs wavelength dispersive microprobe scans reveal complex zonation patterns both normal and oscillatory the results of fifty quantitative analyses were subjected to r mode factor analysis to delineate the chemical exchanges producing the zonation the chemical zonation of the garnets may be attributed to four independent binary exchanges al fe3 si ti ca mn and mg fe2 the stoichiometry of these garnets based on microprobe and wet chemical fe analyses combined with the strongly antithetic behavior of si and ti lead us to infer that the ti in these garnets is dominantly tetravalent it is clear from this study that quantitative modelling of the processes of crystal growth and zonation of melanite garnets in alkaline undersaturated igneous rocks should be aimed at simulating the four chemical exchanges listed above

thinning and flow of tibetan crust constrained by seismic anisotropy intermediate period rayleigh and love waves propagating across tibet indicate marked radial anisotropy within the middle to lower crust consistent with a thinning of the middle crust by about 30 the anisotropy is largest in the western part of the plateau where moment tensors of earthquakes indicate active crustal thinning the preferred orientation of mica crystals resulting from the crustal thinning can account for the observed anisotropy the middle to lower crust of tibet appears to have thinned more than the upper crust consistent with deformation of a mechanically weak layer that flows as if confined to a channel

the 1982 eruptions of el chichon volcano mexico 3 physical properties of pyroclastic surges two major pyroclastic surges generated during the 4 april 1982 eruption of el chichon devastated an area of 153 km2 with a quasi radial distribution around the volcano the hot surge clouds carbonized wood throughout their extent and were too hot to allow accretionary lapilli formation by vapor condensation field evidence indicates voidage fraction of 0 99 in the surge cloud with extensive entrainment of air thermal calculations indicate that heat content of pyroclasts can heat entrained air and maintain high temperatures in the surge cloud the dominant bed form of the surge deposits are sand waves shaped in dune forms with vertical form index of 10 20 characterized by stoss side erosion and lee side deposition of 1 10 cm reversely graded laminae a systematic decrease in maximum lithic diameter with distance from source is accompanied by decrease in wavelength and amplitude modal analysis indicates fractionation of glass and pumice from the surge cloud relative to crystals resulting in loss of at least 10 25 of the cloud mass due to winnowing out of fines during surge emplacement greatest fractionation from the 1 0 0 0 grain sizes reflects relatively lower pumice particle density in this range and segregation in the formative stages of the surge cloud extensive pumice rounding indicates abrasion during bed load transport flow of pyroclastic debris in the turbulent surge cloud was by combination of bed load and suspended load transport the surges are viewed as expanding pyroclastic gravity flows which entrain and mix with air during transport the balance between sedimentation at the base of the surge cloud and expansion due to entrainment of air contributed to low cloud density and internal turbulence which persisted to the distal edge of the surge zone

imaging the indian subcontinent beneath the himalaya new seismic images reveal the geometry of the indian plate as it passes beneath the himalayan plateau several important subsurface features of vanished india are revealed including a shear zone above the base of the indian crust beneath the himalaya and conversion of the deep indian crust into denser rock the rocks of the indian subcontinent are last seen south of the ganges before they plunge beneath the himalaya and the tibetan plateau they are next glimpsed in seismic reflection profiles deep beneath southern tibet1 yet the surface seen there has been modified by processes within the himalaya that have consumed parts of the upper indian crust and converted them into himalayan rocks2 3 the geometry of the partly dismantled indian plate as it passes through the himalayan process zone has hitherto eluded imaging here we report seismic images both of the decollement at the base of the himalaya and of the moho the boundary between crust and mantle at the base of the indian crust a significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths north of the himalaya the lower indian crust is characterized by a high velocity region consistent with the formation of eclogite a high density material whose presence affects the dynamics of the tibetan plateau

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

refinement of mps method for practical application to snow avalanches this paper considers the possible commercial viability of applying the moving particle semi implicit mps method to avalanches the mps method is a powerful tool for reproducing the flow phenomenon with large scale surface deformation in order to apply this method to snow avalanches we modified the original model to introduce constitutive equations of bingham fluid dilatant fluid and the erosion deposition process the modified model was applied to some cases and evaluated through comparison with experimental results and observed data

deep structure of japan subduction zone as derived from local regional and teleseismic events we have determined a detailed three dimensional p wave velocity structure of the japan subduction zone to 500 km depth by inverting local regional and teleseismic data simultaneously we used 45 318 p wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the japan islands the arrival times are recorded by the japan university seismic network which covers the entire japan islands densely and uniformly we also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern japan in comparison with the previous results obtained from only local and regional events the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes in the crust and uppermost mantle low velocity zones are clearly visible beneath active volcanoes in the mantle wedge the low velocity zones generally parallel with the slab and exist continuously to a depth of about 200 km which is consistent with the petrological geochemical and geodynamic studies we consider that the existence of volcanism related low velocity anomalies in the mantle wedge is a general seismological characteristic of subduction zones in light of all the available tomographic results for many subduction zones in the world the pacific slab beneath japan is imaged more clearly than in previous studies as a high velocity zone with a thickness of 80 90 km and a p wave velocity 4 6 higher than the normal mantle lower velocity anomalies are found in the mantle below the slab

detailed configuration of the underthrusting indian lithosphere beneath western tibet revealed by receiver function images we analyze the teleseismic waveform data recorded by 42 temporary stations from the y2 and antilope 1 arrays using the p and s receiver function techniques to investigate the lithospheric structure beneath western tibet the moho is reliably identified as a prominent feature at depths of 55 82 km in the stacked traces and in depth migrated images it has a concave shape and reaches the deepest location at about 80 km north of the indus yarlung suture iys an intra crustal discontinuity is observed at 55 km depth below the southern lhasa terrane which could represent the upper border of the eclogitized underthrusting indian lower crust underthrusting of the indian crust has been widely observed beneath the lhasa terrane and correlates well with the bouguer gravity low suggesting that the gravity anomalies in the lhasa terrane are induced by topography of the moho at 20 km depth a mid crustal low velocity zone lvz is observed beneath the tethyan himalaya and southern lhasa terrane suggesting a layer of partial melts that decouples the thrust fold deformation of the upper crust from the shortening and underthrusting in the lower crust the sp conversions at the lithosphere asthenosphere boundary lab can be recognized at depths of 130 200 km showing that the indian lithospheric mantle is underthrusting with a ramp flat shape beneath southern tibet and probably is detached from the lower crust immediately under the iys our observations reconstruct the configuration of the underthrusting indian lithosphere and indicate significant along strike variations

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

cenozoic volcanism in tibet evidence for a transition from oceanic to continental subduction geochronological k ar or ar ar major and trace element sr nd pb isotopic and mineral chemical data are presented for newly discovered cenozoic volcanic rocks in the western qiangtang and central lhasa terranes of tibet alkali basalts of 65 45ma occur in the western qiangtang terrane and represent primitive mantle melts as indicated by high mgnumbers 100 mg mg fe 54 65 cr 204 839 ppm and ni 94 218 ppm contents and relatively low ratios of sr sr 0 7046 0 7061 pb pb 18 21 18 89 pb pb 15 49 15 61 and pb pb 38 42 38 89 and high ratios of nd nd 0 5124 0 5127 in contrast younger volcanic rocks in the western qiangtang terrane 30ma and the central lhasa terrane 23 13 and 8ma are potassic to ultrapotassic and interpreted to have been derived from an enriched mantle source they are characterized by very high contents of incompatible trace elements negative ta nb and ti anomalies and radiogenic pb isotopic compositions pb pb 18 43 19 10 pb pb 15 64 15 83 pb pb 39 14 39 67 sr sr 0 7088 0 7092 and nd nd 0 5122 ratios of the western qiangtang terrane potassic lavas are similar to those of 45 29ma potassic volcanic rocks in the north central qiangtang terrane whereas sr sr 0 7167 0 7243 and nd nd 0 5119 ratios of central lhasa terrane lavas are similar to those of 25 16ma ultrapotassic volcanic rocks in the western lhasa terrane the 65 45ma alkali basalts in the western qiangtang terrane along with widespread calc alkaline volcanic rocks of this age in the lhasa terrane may be related to roll back of a previously shallow north dipping slab of tethyan oceanic lithosphere beneath tibet subduction as opposed to convective thinning of continental lithosphere is favored to explain potassic volcanism in tibet because of its occurrence in distinct east west trending belts 45 29ma in the qiangtang terrane 25 17ma in the northern lhasa terrane 16 8ma in the southern lhasa terrane and temporal and spatial relationships with major thrust systems

upper mantle deformation beneath central southern tibet revealed by shear wave splitting measurements abstract seismic anisotropy in the upper mantle has been inferred from shear wave splitting measurements at the temporary seismic array antilope 2 in the central southern tibet from september 2005 to october 2006 teleseismic shear waves mainly sks and skks are used to determine the splitting parameters fast polarization direction and delay time weak splitting with average delay time of 0 3 s is observed at stations in the southern lhasa terrane and northernmost tethyan himalaya significant seismic polarization anisotropy with delay times from 0 8 to 1 5 s and ne oriented fast direction is obtained for stations located farther north in the northern lhasa terrane the transition in shear wave splitting characteristics occurs at 30 5 n and suggests distinct upper mantle deformation from south to north the negligible anisotropy south of this transition indicates that there is no appreciable large scale mantle seismic polarization anisotropy the substantial anisotropy in the north may be caused by the eastward flow in a lithospheric crush zone which is squeezed and sheared between the advancing indian plate to the south and eurasian plate to the north

current slip rates on conjugate strike slip faults in central tibet using synthetic aperture radar interferometry this is the published version copyright 2006 american geophysical union all rights reserved

characteristics of dune bedded pyroclastic surge bedsets abstract the grainsize characteristics of dune bedded pyroclastic surge bedsets are surveyed the variance between coarsest and finest beds ranges from 1 to 6 phi in different surge bedsets and it increases as the grainsize of the coarsest bed increases reflecting an increasing velocity of emplacement deposits of wet surges identified as those which contain accretionary and ash coated lapilli tend to be finer and show less variance this partly because wet ash is cohesive but mainly because wet surges tend to be weaker dry surge bedsets are strongly fines depleted wet ones less so the lack of erosion of underlying ash layers shows that the environment is a strongly depositional one individual bedsets are demarcated by thin intervening fine ash fall layers which are the complementary ash cloud deposits settled or flushed out after the passage and decay of each turbulent surge surge deposits are generally less coarse than the coarsest associated airfall deposits which shows that they are formed by generally weaker events this study helps interpret the dune bedded parts of the landscape mantling may 18th 1980 blast deposit of mount st helens the blast was a very violent event but the variance and the grainsize of the coarsest bed are those of a relatively weak surge this suggests that the dune bedding was produced by a weak effect such as minor turbulence in a thin pyroclastic flow coming to rest in a mountainous terrain roughened by tree stumps and fallen logs

tearing of the indian lithospheric slab beneath southern tibet revealed by sks wave splitting measurements a b s t r a c t shear wave birefringence is a direct diagnostic of seismic anisotropy it is often used to infer the northern limit of the underthrusting indian lithosphere based on the seismic anisotropy contrast between the indian and eurasian plates most studies have been made through several near north south trending passive source seismic experiments in southern tibet to investigate the geometry and the nature of the underthrusting indian lithosphere an east west trending seismic array consisting of 48 seismographs was operated in the central lhasa block from september 2009 to november 2010 splitting of sks waves was measured and verified with different methods along the profile the direction of fast wave polarization is about 60 in average with small fluctuations the delay time generally increases from east to west between 0 2 s and 1 0 s and its variation correlates spatially with north south oriented rifts in southern tibet the sks wave arrives 1 0 2 0 s later at stations in the eastern part of the profile than in the west the source of the anisotropy estimated by non overlapped parts of the fresnel zones at stations with different splitting parameters is concentrated above ca 195 km depth all the first order features suggest that the geometry of the underthrusting indian lithospheric slab in the himalayan tibetan collision zone beneath southern tibet is characterized by systematic lateral variations a slab tearing and or breakoff model of indian lithosphere with different subduction angles is likely a good candidate to explain the observations

the fate of the indian lithosphere beneath western tibet upper mantle elastic wave speed structure from a joint teleseismic and regional body wave tomographic study abstract we investigate the fate of the indian lithosphere following its descent beneath western tibet by means of tomographic imaging based on arrival times of body waves from regional and teleseismic sources recorded by a portable network deployed in the region from 2007 to 2011 we use a non linear iterative algorithm that simultaneously models absolute regional and relative teleseismic arrival times to obtain a 3 d velocity structure in a spherical segment that extends from 26 n to 37 n from 76 e to 89 e and from the surface to 430 km depth we find that variations in p and s wave speeds in the upper mantle are similar and identify a number of prominent fast anomalies beneath western tibet and the adjacent himalayas we associate these fast anomalies with the mantle lithosphere of india that is likely colder and hence faster than the ambient mantle resolution tests confirm the ability of our dataset to resolve their shapes in the upper 300 km and the lack of significant downward smearing of these features we interpret the presence of faster material below 300 km as being consistent with former indian lithosphere having reached these depths there are two main fast anomalies in our model one resembles a 100 km wide sub vertical column located directly beneath the india asia plate boundary the other anomaly is thinner and has the shape of a dipping slab that spans the north south width of the lhasa block it dips towards the ne starting near the indus yarlung suture and ending north of the bangong nujiang suture at depths in excess of 300 km another finding of our study is the absence of major fast anomalies west of 80 e which our resolution tests show to be significant our results do not support the notion of a continuous body of formerly indian lithosphere being presently underthrust northward and extending all the way to the northern boundary of the plateau rather shapes of fast anomalies in western tibet suggest colder material beneath the northernmost himalaya and the southernmost part of the plateau the presence of multiple relatively small scale anomalies rather than a single large fast body aligned with the india asia boundary rules out subduction of the intact indian lithosphere our preferred scenario for the process that currently removes the mantle lithosphere of india from the collision zone envisages viscous behavior with gravity driven instabilities leading to the formation of multiple drips of different size to reconcile our findings with evidence of past underthrusting of indian lithosphere beneath tibet we postulate a geologically recent change in the mode of lithosphere removal our scenario includes initial underthrusting subsequent gravitational destabilization and removal of the mantle lithosphere of indian plate at present the mantle lithosphere of india that continues to underthrust southern tibet develops a drip like instability that is seen as a fast columnar feature in our model

constraining the extent of crust mantle coupling in central asia using gps geologic and shear wave splitting data abstract we have obtained constraints on mechanical crust mantle coupling for tibet and yunnan indo china by comparing the observed surface deformation field inferred from gps and quaternary fault slip rate data with the mantle deformation field inferred from several sks shear wave splitting data sets we first determined whether the anisotropy is dominantly asthenospheric or lithospheric by testing simple models of both types against the observed values of the fast polarization direction which is assumed to be parallel to the horizontal projection of the direction of maximum shear for asthenospheric flow we solved for a best fitting uniform sub asthenospheric velocity model for eurasia the fit however was not satisfactory rms misfit 20 solving for separate uniform flow fields in each region improves the fit 15 for tibet 11 for yunnan although the resulting flow fields are inconsistent with several geophysical and geological constraints and thus considered unlikely we then considered lithospheric models for tibet vertically coherent deformation i e maximum shear direction from surface deformation is parallel to yields an improved match 11 for left lateral shear both the goodness of fit and the dominance of left lateral surface faulting in tibet argue for a lithospheric source of anisotropy the misfit for yunnan is large for either right 53 or left lateral 49 shear which argues for complete crust mantle decoupling in yunnan we show that the fast polarization directions throughout both the tibet and yunnan region can be fit by a single lithospheric dynamic model in which there is strong coupling between crust and mantle beneath tibet but a complete decoupling between crust and mantle beneath yunnan crust this dynamic model predicts left lateral maximum shear directions within the mantle that align with fast polarization directions in both regions 9 these maximum shear directions within the mantle align with the left lateral maximum shear directions in the crustal deformation field in tibet but are not in yunnan our results have the following implications first the coherence between crust and mantle deformation in tibet implies strong crust mantle mechanical coupling since this is the only way that crustal buoyancy forces required to account for the surface deformation field can be transmitted into the mantle this behavior is consistent with a uniform strength lithosphere or strong crust but not with a substantially weaker crust for example it is inconsistent with the popular jelly sandwich rheology and thus precludes behavior such as large scale lower crustal flow in tibet the crust mantle coherence is also incompatible with mantle delamination second crust mantle decoupling within the yunnan lithosphere argues that mantle deformation there is controlled only by boundary conditions crustal buoyancy forces are not transmitted into the mantle beneath yunnan moreover the dynamic model for the mantle shows that the yunnan crust is moving south with respect to the mantle at rates as high as 30 mm yr third there is a fundamental rheological lithospheric transition between tibet and yunnan that may provide a key to understanding this significant orogen

the layered shear wave velocity structure of the crust and uppermost mantle in china abstract we present a layered shear wave velocity structure of the crust and uppermost mantle of china and the surrounding area we apply an adaptive moving window sun li et al 2004 to construct the s wave velocity model from high quality body wave phase data extracted from the annual bulletin of chinese earthquakes abce more than 350 000 s wave arrivals are used spanning from 1990 to 2004 the study area is represented by a 1 geographic grid consisting of 2338 points at each point 1d s velocity depth profiles are determined independently from the surface to the uppermost mantle this is accomplished by performing a monte carlo random search of optimum layer parameters thickness and velocity via minimizing travel time misfits for fixed earthquake locations each profile contains a four layer crust and a one layer uppermost mantle a final 3d model is obtained by combining and smoothing the 1d models the obtained s wave model has a good correlation with the previously published p wave model using the same method sun li et al 2004 and reveals key tectonic features such as the low velocity crust beneath tibet our s wave model is generally consistent with the existing regional local models constructed from body wave travel time tomography and provides more detailed structure in both horizontal and vertical directions compared with the model derived from surface wave inversion based on our p and s wave models the generated synthetic seismograms fit well with the observed seismograms recorded at broadband stations