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

exxon global cycle chart an event for every occasion the basic premise of the recent exxon cycle chart that there exists a globally correlatable suite of third order eustatic cycles remains unproven many of the tests of this premise are based on circular reasoning the implied precision of the exxon global cycle chart is not supportable because it is greater than that of the best available chronostratigraphic techniques such as those used to construct the global standard time scale correlations of new stratigraphic sections with the exxon chart will almost always succeed because there are so many exxon sequence boundary events from which to choose this is demonstrated by the use of four synthetic sections constructed from tables of random numbers a minimum of 77 successful correlations of random events with the exxon chart was achieved the existing cycle chart represents an amalgam of regional and local tectonic events and probably also includes unrecognized miscorrelations it is of questionable value as an independent standard of geologic time

floodplain palaeosols of the cenomanian dunvegan formation alberta and british columbia canada micromorphology pedogenic processes and palaeoenvironmental implications

the genesis and recognition of anastomosed fluvial deposits data from the st mary river formation southwestern alberta canada abstract data from modern fluvial systems show that the term anastomosed is best reserved for suspended load systems with multiple channels and very low slope that flow in regions where the hydrograph has a seasonal peak the interchannel areas are topographic lows relative to the channel hanks and are places where crevasse splay marsh and lacustrine sedimentation occurs channel geometry is intrinsically a function of the rate of sediment partitioning the coarsest sediment is deposited in the channels on the levees and in proximal splay complexes whereas the silts and clays are deposited on the rest of the floodplain extrinsic controls on the formation of these systems are geological and climatic they include a source of suspended sediment and a highly seasonal water budget preservat on of anastomosed fluvial deposits requires a mechanism for maintaining very low depositional slopes over geologic time scales data from the st mary river formation in southwestern alberta show that the stratigraphic equivalents of these rivers are ribbon sandstone bodies encased in thin sandstones shales and siltstones the total distribution of facies rather than proof of simultaneous flow in multiple channels can be used to interpret correctly a deposit as anastomosed fluvial although anastomosed fluvial deposits form in a variety of tectonic settings they are apparently most common in foreland basins because these basins provide suitable source rocks and thrusting as a mechanism that can maintain the low depositional slopes over millions of years the mutual interaction among source rock climate and subsidence all of which play critical roles in forming and preserving these distinctive fluvial deposits allows inferences to be drawn regarding paleoclimate and paleoslope that may help constrain models of basin evolution

complex structure of upper mantle beneath the yadong gulu rift in tibet revealed by s to p converted waves abstract the convergence between the indian and eurasian plates has produced the thick crust and uplifted the tibetan plateau since about 50 ma however the deformation of the mantle lithosphere is still a puzzle the geometry of the subducting indian mantle lithosphere beneath the plateau and the thickening or and delamination of the tibetan mantle lithosphere are the keys for understanding the continental collision process and the evolution of the plateau however knowledge has been restricted due to sparse data coverage in tibet in this study s wave receiver functions are calculated using tele seismic waveforms recorded by two broadband arrays in central tibet to image the lithospheric structure mainly the depth variation of the lithosphere asthenosphere boundary lab our results show that the depth of the tibetan lab decreases from 150 km in the west to 120 km in the east across the north south trending yadong gulu rift similarly the lab depth of the subducting indian slab decreases from 270 km in the west to 200 km in the east and the northernmost subducting indian slab can be observed beneath the bangong nujiang suture these observations suggest that the thickness of the tibetan lithosphere and the depth of the underlying indian slab are segmented across the yadong gulu rift in different degrees the abrupt changes imply that the subducting indian slab has been torn which provided an upwelling channel for the asthenosphere contributing to the development of the rift

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

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

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

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

3 d electrical structure across the yadong gulu rift revealed by magnetotelluric data new insights on the extension of the upper crust and the geometry of the underthrusting indian lithospheric slab in southern tibet abstract the approximately north south trending cenozoic yadong gulu rift ygr in the eastern lhasa block is an ideal location to investigate the extensional kinematic mechanism of the upper crust and the deformation characteristics of the indian lithospheric slab in southern tibet the magnetotelluric mt method has been widely used in probing subsurface structures at lithospheric scale and is sensitive to high electrically conductive body conductor a three dimensional 3 d inversion of mt data was conducted to derive the east west electrical structures across the northern segment of the ygr the result reveals that the conductors in the middle crust are not continuous in the east west direction the deep conductor underneath the ygr is interpreted to result from the tearing of the indian lithospheric slab the upper crust to the east of the ygr is significantly intruded by underlying conductors based on the features of the 3 d inversion result from this study and other geophysical observations the formation of the ygr is most likely caused by tearing of the indian lithospheric slab through the pull of mid lower crustal conductors that have locally weak strength beneath the ygr

paleosols spanning the cretaceous paleogene transition eastern montana and western north dakota paleosols occur in exposures of the latest cretaceous hell creek and paleocene tullock ludlow formations in montana and western north dakota these units are composed mainly of interbedded siltstones and sandstones of meandering fluvial origin the paleosols indicate that changes in ancient soil development occurred concomitantly with the better known faunal transition in order of decreasing abundance silty soils sandy soils organic soils and volcanic soils are preserved pedogenic features present in these soils include roots microscopic segregations of amorphous material birefringence fabric and soil structures the features suggest that throughout the cretaceous paleogene transition pedogenic processes in the region produced immature profiles an observation consistent with the unstable fluvial setting in which the ancient soils formed gleization was a dominant process in this setting and podzolization modified some sandy soils the association of the features enables recognition of o a e btg bhs bg bc cg and c horizons during middle hell creek time soils formed in a poorly drained setting that was only stable enough to permit incipient pedogenesis the bulk of the pedogenesis occurred in levee and flood plain deposits soils also occurred on point bar and crevasse splay deposits in topographically depressed regions organic accumulations formed with minimal soil development matted plant debris is the product of this environment gley features and segregations of iron oxides around voids suggest fluctuation of the water table by latest hell creek time the mean level of the water table rose and in the lowest tullock ludlow exposures extensive ponded deposits are preserved vegetation accumulated at a rate sufficient for coal formation the amount of fluctuation apparently was reduced and pedogenesis was further inhibited as indicated by the virtual absence of illuviated clays in the sediments

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

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

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

why is the ligurian basin mediterranean sea seismogenic thermomechanical modeling of a reactivated passive margin 1 the seismic activity of the ligurian basin the northeastern termination of the western mediterranean basin is larger than in surrounding regions even though recent geodetic studies attest that this area is subject to very low levels of deformation this basin is an example of a type of passive margins that cannot be considered solely as inert sites of sedimentation and of progressive subsidence and that are reactivated in a compressive pattern other examples include the kwanza basin angola and the brazil margin we investigated by means of 2 d thermomechanical modeling the structural and rheological heterogeneities that can lead to concentration of strain in this marginal basin we deduced that the deformation of the basin is due to its particular geometric features narrow and with a thick surrounding continental crust related to its position at the southern termination of the alps this sharp transition in terms of both geometry and rheological contrast is a main factor in explaining the weakness of the margin we discuss the importance of buoyancy forces versus tectonic forces as well as thermal effects on the observed reactivation influence of contrast in rheology between an oceanic type crust and continental crust is also studied geodynamical implications are proposed for the region the good agreement between the predicted localized deformation and the observed seismicity distribution should help improve seismic hazard assessment in the region

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

dissected paleotopography and base level changes in a triassic fluvial sequence at one stratigraphic level in petrified forest national park in arizona fluvial mudrocks of the chinle formation are truncated by areally extensive deep scours that are filled dominantly by pedogenically modified mudrocks color differences between paleosols developed on scoured and scour fill deposits facilitate recognition of the dissected interval the mudstone filled scour system records a period of channel incision and widespread gullying caused by lowered base level although it is difficult to document subsidence rates in the chinle basin the tectonic setting suggests that the enormous scour system could be the result of fluctuations in regional thermal activity during late triassic time similar scour and fill should occur in other fluvial sequences however because they both truncate and are filled by mudrocks identification can be difficult recognition of them is important because they are indicators of base level fluctuations and thus are useful in unraveling the depositional history of a basin

a two dimensional hybrid method for modeling seismic wave propagation in anisotropic media national science foundation of china 40504006 40434012 90814002 ear 0609717 chinese academy of sciences

tibetan plate overriding the asian plate in central and northern tibet at the southern boundary between india and tibet the indian tectonic plate subducts northwards beneath the tibetan plateau seismic imaging shows that at the northern boundary of tibet the asian plate is also subducting southwards beneath the plateau and the tibetan lithosphere is separate

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

landscape and depositional controls on palaeosols of a distributive fluvial system upper cretaceous brazil abstract the stratigraphic record of distributive fluvial systems is commonly characterised by frequent and complex interstratification of palaeosols among channel and overbank deposits however current models focus primarily on sedimentation and pay only limited attention to palaeopedogenesis thereby failing to incorporate important palaeoenvironmental and stratigraphic information this study proposes a pedosedimentary model for distributive fluvial systems that depicts and accounts for two palaeopedogenetic trends one downdip in relation to distality from the fan apex and one along strike in relation to distance from active channel belts palaeosols are reported in detail from an upper cretaceous succession of the bauru basin southeastern brazil through the application of macro micromorphological and geochemical studies combined with facies and architectural element analyses of sediments in the downdip palaeopedogenetic trend the proximal zone of the depositional system is characterised by a dominance of well drained inceptisols that develop on amalgamated channel fills in the medial zone inceptisols occur interlayered with overbank deposits containing entisols and poorly drained vertisols the distal zone preserves more mature and poorly drained inceptisols developed on deposits of overbank and sporadic distal channel fills these pedotypes show an increase in maturity and hydromorphism moving away from the apex to the fan toe this is likely linked to i the progressive approach of the topographic surface to the water table and ii the average increase in distance to an active channel belt in distal locations the along strike palaeopedogenetic trend culminates in poorly developed palaeosols in floodplain regions that correspond to topographic depressions located between channel belts and which were subject to recurrent floods because palaeopedogenesis in the floodplain region is penecontemporaneous to sedimentation pedotypes show an increase in maturity bioinduced calcification and hydromorphism with distance from the active channels they pass laterally from entisols and inceptisols near active channels to vertisols away from active channels conversely following avulsion abandoned channel belts remain as topographically elevated alluvial ridges located at some distance from the newly active channels and positioned above the water table and this leads to the development of better drained and better developed inceptisols relative to pedotypes of the floodplain region overall both palaeopedogenetic trends demonstrate the overriding controls of topography sedimentation rate and parent material on pedogenesis with only minor climatic influence this work offers a novel pedosedimentary model for distributive fluvial systems and highlights the palaeoenvironmental significance of palaeosol trends providing new constraints for the recognition of distributive fluvial systems in the rock record

architecture of vertically stacked fluvial deposits atane formation cretaceous nuussuaq central west greenland excellent exposures of thick multistorey fluvial deposits from the deltaic atane formation on south east nuussuaq central west greenland show the architecture of up to 100 m thick continuously aggrading fluvial depositional complexes the succession comprises vertically stacked channel belt sandstones separated by thin floodplain deposits with little to no incision between storeys architectural elements and palaeocurrent patterns of channel deposits indicate deposition in large relatively stable low sinuosity rivers probably located within an incised valley gradual transitions from channel to floodplain deposits accompanied by a gradual change from floodplain to spillover sand suggest avulsion on the floodplain as a possible mechanism for the vertically alternating channel and floodplain deposits despite its relative proximity to contemporaneous sea level ca 35 km upstream from the palaeo shoreline the depositional complex is entirely non marine the aggrading nature of the deposits suggests a continuously rising base level coupled with a high and steady sediment supply vertical alternations between floodplain and channel deposits may be forced by subtle interruptions in this balance or autocyclic mechanisms on the floodplain this study provides an example of aggrading lowstand non marine transgressive systems tract deposits

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

pn uppermost mantle tomography of central tibet implication for mechanisms of n s rifts and conjugate faults abstract the north south convergence between the indian and eurasian continents since cenozoic has produced the tibetan plateau and east west extension structures have also developed since late cenozoic whether these extensional structures are limited to the crust or extend to the deep lithosphere remains controversial the uppermost mantle serves as a link between the crust and deep lithosphere the velocity structure of which provides an important constraint on exploring the mechanism of extensional structures based on pn traveltime data recorded by the sandwich network in central tibet we use the interstation traveltime difference method to obtain the uppermost mantle velocity structure our results show significant velocity differences for the uppermost mantle beneath the conjugated strike slip faults and rifts which indicates their different formation mechanisms the high pn velocity beneath the conjugated strike slip faults support an eastward horizontal shear at the base of the upper crust rather than the lower crust has contributed to the development of the conjugated strike slip faults the low pn velocity beneath the rifts in lhasa may provide evidence for the rifts cutting through the entire crust and reflect the connection between the formation of the rifts and deep tectonic activities the most prominent features in our results are the low pn velocity zones beneath the tangra yum co rift tyr the pumqu xianza rift pxr and the zone between the two rifts and we explain the low pn velocity as the top of the mantle wedge upon the subducting indian lithospheric slab

palaeosols in westphalian coal bearing and red bed sequences central and northern england westphalian a c coal bearing strata and red beds in the pennine basin of central and northern england contain a wide variety of palaeosols in coal bearing facies associations alluvial gley semi gley organic and rare well drained palaeosols reflect the predominance of poor drainage conditions during deposition in red beds more evolved ferruginous and ferallitic palaeosols testify to free drainage conditions during deposition catenary relationships can be inferred between most of the palaeosols in both facies associations providing further information on the palaeogeomorphology of the depositional systems and assisting in the understanding of the environments and conditions of coal seam formation

anatomy and evolution of a lower cretaceous alluvial plain sedimentology and palaeosols in the upper blairmore group south western alberta canada the lower cretaceous albian upper blairmore group is part of a thick clastic wedge that formed adjacent to the rising cordillera in south western alberta regional transgressive intervals are superimposed on the overall regressive succession alluvial conglomerates sandstones and mudstones were deposited in east north eastward draining fluvial systems orientated transverse to the basin axis five facies associations have been identified igneous pebble conglomerate thick sandstone interbedded lenticular sandstone and mudstone thick mudstone with thin sandstone interlayers and fossiliferous sandstone and mudstone the facies associations are interpreted as gravelly fluvial channels sandy fluvial channels sand dominated floodplains mud dominated floodplains and marine shoreline deposits respectively five types of palaeosols are recognized in the upper blairmore group based on lithology the presence of pedogenic features clay coatings root traces ferruginous nodules slickensides carbonate nodules and degree of horizonization the regional distribution of the various types of palaeosols enables a refinement of the palaeoenvironmental reconstruction permitting an assessment of the controls on floodplain evolution in source proximal areas palaeosol development was inhibited by high rates of sedimentation in source distal locations poor drainage resulting from high watertables low topography and lower rates of sedimentation also inhibited palaeosol development the best developed palaeosols containing bt horizons occur in intermediate alluvial plain positions tectonic hinge zone where the floodplains were most stable due to a balance between sedimentation erosion and subsidence rates extrapolating from the upper blairmore group suggests that the tectonic hinge zone of continental foreland basins can be established by palaeosol analysis at the hinge zone soil development is controlled primarily by climate and tectonics and their effect on sediment supply whereas closer to the palaeoshoreline relative sea level fluctuations resulting in poor drainage may have a more significant influence

a model of the crust and mantle structure down to 700 km depth beneath the lhasa to golmud transect across the tibetan plateau as derived from seismological data abstract a 700 km deep seismic velocity cross section beneath the lhasa to golmud transect across the tibetan plateau is presented in contrast to the first version of this cross section which comprised an 800 km wide swath centred on the lhasa to golmud transect due to the recent proliferation of publications concerning the mantle structure beneath tibet this study is based only on seismic profiles which either run along or cross the transect and arrays or studies which at least partly cover the transect the results from the recent indepth iv project indicate that the crustal thickness change from 70 km beneath the songpan ganzi terrane and kunlun mountains to 54 km beneath the qaidam basin is located about 100 km north of the kunlun fault and almost 45 km north of the north kunlun thrust the qaidam basin moho is underlain by crustal velocity material for almost 45 km and the apparently overlapping crustal material may represent songpan ganzi lower crust underthrusting or flowing northward beneath the qaidam basin moho thus the high tibetan plateau may be thickening northward into south qaidam as its weak thickened lower crust is injected beneath the stronger qaidam crust beneath the crust high velocity dense cold indian lithospheric mantle extends northwards until about the banggong nujiang suture northwards asian lithospheric mantle is overlain by a low velocity less dense warm tibetan plate consisting of an upper lithospheric and a lower asthenospheric part the apparent northwards deepening of the 410 and 660 km discontinuities by about 20 km implies that the upper mantle beneath north tibet is slower less dense and warmer than under south tibet in agreement with the observed uppermost mantle velocities this in turn could provide some of the isostatic support for the high elevations in the north where the crust is somewhat thinner than in the southern plateau

westerly derived early cretaceous gold paleoplacers in the western canada foreland basin southwestern alberta tectonic and economic implications albian aged early cretaceous igneous pebble to cobble conglomerates fill multiple northeast southwest oriented subparallel channels in the upper blairmore group upper beaver mines and mill creek formations of the rocky mountain foreland basin southwestern alberta and southeastern british columbia paleocurrent data show that the conglomerate was derived from the west clast petrography implies a provenance that includes granitoids mafic volcanics low grade metamorphic rocks and shallow level ca 7 km depth postmetamorphic quartz veins formed from meteoric fluids the conglomerate was probably derived from the southern omineca belt of british columbia prior to the rise of the rocky mountains the conglomerate contains detrital gold grains up to 150 m in diameter and chemical analyses indicate widespread anomalous gold concentrations up to 910 ppb au in conglomerate matrix gold content in these igneous clast conglomerates excedes that reported from the richest modern placers in alberta les

micromorphology and sequence stratigraphy of the interfluve paleosols from the ganga plains a record of alluvial cyclicity and paleoclimate during the late quaternary

imaging the colliding indian and asian lithospheric plates beneath tibet 1 a high resolution image of the base of the lithosphere from s to p converted seismic waves revealed the collision architecture of the indian and asian continental plates beneath the tibetan plateau the base of the indian lithosphere dips northward from a depth of 160 km beneath the himalayas to a depth of 220 km just south of the bangong suture the base of the asian lithosphere is nearly horizontal at the depth of 160 180 km from central to northern tibet there is a vertical gap of about 50 km between indian and asian lithospheres our observation of a well defined thick lithosphere throughout the entire plateau is not consistent with models of wholesale convective instability of a thickened mantle lithosphere which would predict a very thin asian lithosphere the hypothesized sequential southward subduction of tibetan and asian continental lithospheres leading to the growth of the tibetan plateau if correct cannot be occurring below 180 km depth our results along with available geological and geophysical data strongly support that the plateau is predominantly formed by a relatively coherent north dipping subducted indian continental lithosphere in the south which presently can be traced to the middle of the plateau and a south dipping subducted asian lithosphere in the north at a shallower depth

upper mantle velocity structure beneath the tibetan plateau from pn travel time tomography we inverted 1510 p arrival times from regional distances 333 1600 km in and around the tibetan plateau to map the lateral velocity variation within the uppermost mantle previous studies have placed first order constraints on upper mantle velocities but relied on data recorded almost exclusively at stations outside of the plateau we improve resolution by using 40 events recorded at stations within the tibetan plateau we combine these data with observations obtained from the international seismological centre isc to extend our coverage by including pn arrivals from 85 additional plateau events relocated in previous studies and recorded at stations in and around the tibetan plateau we use synthetic travel time data to evaluate the resolution of our data set the observations provide good resolution to about 1 over most of the plateau and surrounding regions our results show average pn velocities that are about 3 lower in the northern plateau relative to the southern plateau these variations correlate well with major tectonic features and previous geophysical observations in the qiangtang terrane of the northern plateau an area known to be inefficient for sn propagation pn is slow relative to both the plateau south of the banggong nujiang suture and the tectonically stable tarim basin north of the plateau this is strong evidence for the existence of partial melt within the uppermost mantle beneath the northern tibetan plateau however when laboratory estimates of relationships between temperature velocity and attenuation are applied a relatively small temperature variation 240 to 370 c is required to explain our pn velocity observations when combined with geochemical constraints from volcanics in the northern plateau our results strongly suggest that the mantle lid is intact beneath the northern plateau this result would preclude tectonic models involving wholesale delamination of the mantle lithosphere in the northern tibetan plateau

subduction of the indian lithosphere beneath the tibetan plateau and burma abstract subduction of the indian lithosphere under eurasia plays an important role in the tectonic evolution of the tibetan plateau and surrounding regions to improve our knowledge of pertinent mantle structures through tomographic imaging we combine p wave arrival time data from temporary arrays in tibet and stations of the chinese seismograph network with reprocessed data from the international seismological centre the new images reveal considerable spatial variations in mantle structure along the collision zone and the horizontal distance over which presumed continental indian lithosphere slides northward beneath the plateau decreases from west where it underlies the himalayas and the entire plateau to east where no indication is found for present day underthrusting beyond the himalayan block and indus tsangpo suture p wavespeed appears low in the shallow mantle beneath much of central and eastern tibet these observations suggest that indian lithosphere underlies only the southwestern part of the plateau and that the central and northeastern part is underlain by lithosphere of asian origin the continental parts of the indian plate that currently underthrust or subduct below the plateau appear generally detached from the oceanic slab fragments that subducted longer ago and that have been detected deeper in the mantle except perhaps in the central part of the collision zone 80 90 e this suggests that ongoing india eurasia collision must increasingly be driven by other forces such as subduction of the indian plate further west and east e g beneath indochina our images reveal structures associated with eastward subduction along the burmese arc southeast of the eastern himalayan syntaxis the tengchong volcanic complex in southwest china is marked by slow wave propagation to 150 km depth suggesting a causal link to subduction along the burmese arc the low velocities beneath the red river fault region extend to greater depth and may be related to upper mantle processes further southeast

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

high resolution lithospheric structure beneath mainland china from ambient noise and earthquake surface wave tomography abstract we present a new high resolution shear velocity model of the lithosphere down to about 160 km beneath china using rayleigh wave tomography we combined ambient noise and earthquake data recorded at 1316 seismic stations the largest number used for the region to date more than 700 000 dispersion curves were measured to generate group and phase velocity maps at periods of 10 140 s the resolution of our model is significantly improved over previous models with about 1 2 in eastern china and 2 3 in western china we also derived models of the study region for crustal thickness and averaged s velocities for upper and mid lower crust and uppermost mantle these models reveal important lithospheric features beneath china and provide a fundamental data set for understanding continental dynamics and evolution different geological units show distinct features in the moho depth lithospheric thickness and shear velocity in particular the north china craton ncc lithosphere shows strong east west structural variations with thin and low velocity lithosphere in eastern ncc and thick and high velocity lithosphere beneath western ncc and the lithosphere of the ordos block seems to have undergone strong erosion the results support the progressive destruction of the ncc lithosphere from east to west at least partly caused by the thermal chemical erosion of the cratonic lithosphere from the asthenosphere another pronounced feature of our model is the strong lateral variations of the mantle lithosphere beneath the tibetan plateau tp the indian lithosphere beneath the tp shows variable northward advancement with nearly flat subduction in western and eastern tp and steep subduction in central tp with evidence for the tearing of indian lithosphere beneath central tp which may be important for the riftings at the surface in himalayas and southern tp the low velocity zone in northern tp shows strong correlation with the region of the mid miocene to quaternary potassic magmatism suggesting that delamination of lithosphere may have played an important role in the rise of the tp

intraseasonal variations in the southern hemisphere circulation abstract intraseasonal variations in the southern hemisphere atmospheric circulation have been examined through the application of a 10 50 day bandpass filter to daily ecmwf analyses for 1980 88 variations on this time scale contribute more than 40 of the daily variance in 500 hpa geopotential over much of the middle and high latitudes of the southern hemisphere eof analysis of the unnormalized variance for all seasons shows that 49 of this variance can be explained by zonal wave trains centered on the south pacific and southern atlantic indian oceans a high latitude mode of global extent and a wavenumber 3 pattern at midlatitudes these modes are essentially equivalent barotropic but slope westward with height so that the patterns at 100 hpa typically lag those at 1000 hpa by around 1o longitude there appears to be little interaction with the low latitude circulation all of the leading modes propagate eastward but the most consistent movement is shown by the south pacific wave train represented

joint imaging by teleseismic converted and multiple waves and its application in the indepth iii passive seismic array 1 a joint imaging by teleseismic converted and multiple phases is put forward to image the crustal structure beneath the indepth iii passive seismic profile in the central tibet a depth dependent vp vs ratio is adopted to map common conversion point ccp via a vp vs scanning and stacking approach our new crustal image shows that the crust is divided into two sections with different moho depth distributions and amplitudes with the boundary nearby the bangong nujiang suture bns a negative p to s amplitude band is located in the upper to middle crust indicating a low velocity zone possibly caused by partial melting beneath the negative amplitude band the average crustal vp vs ratio estimated by our joint imaging method is higher than previous study but is consistent with a partial melting middle to lower crust implied by many other results

west east variation in crustal thickness in northern lhasa block central tibet from deep seismic sounding data 5 30 km depth 5 0 vp 6 4 km s a middle crust 33 45 km depth 6 5 vp 6 8 km s and a lower crust depths below 48 km 7 0 vp 7 4 km s the west to east increase in crustal thickness is accomplished by a 50 thickening of the middle and lower crust larger vertical velocity gradients separate these three layers and bound them above surficial and sedimentary rocks and below moho transition zone the most notable low velocity zone in the crust lies at the base of the upper crustal layer s wave velocity structure is less well constrained but parallels the p wave structure except that vp vs ratios may decrease from west to east in the lower crust our data suggest considerable variation in structure along strike of the tibetan plateau and show that interpretations of tibet as a purely two dimensional orogen are overly simplistic the west east increase in crustal thickness may occur across the karakorum jiali fault system and be an indicator of lateral tectonic escape of the qiangtang terrane

atmospheric teleconnections involving the southern ocean 1 i review the characteristic patterns of low frequency interannual and subdecadal variation in atmospheric circulation over the southern hemisphere sh extratropics and their climatic associations e g temperature precipitation winds sea surface temperatures ssts and sea ice conditions for the southern ocean the considerable spatial and temporal limitations of the observational database have strongly conditioned the pace of our understanding of sh teleconnections particularly for the pacific sector where the tropical el nino southern oscillation enso exerts a dominant influence an enso signal appears in the intensity interannual variations of the amundsen sea mean low pressure asl jet streams and long waves these variations comprise the so called pacific south america psa teleconnection patterns of low frequency variability in sh circulation that are mostly extratropical in origin but which can interact with enso are dominated by the zonally symmetric high latitude mode or antarctic oscillation aao the aao involves an alternation of atmospheric mass between middle and high southern latitudes a zonally asymmetric mode of wave number 1 is represented by an oscillation in pressure height between australian and south american sectors depicted by a trans polar circulation index a significant wave number 2 oscillation in the coupled atmosphere ocean ice system or antarctic circumpolar wave acw has a similar periodicity to enso and is strongest in the pacific sw atlantic sectors atmospheric teleconnections involving the southern ocean including those to enso show evidence of recent changes and the climatic implications of these are discussed

the sequence stratigraphy of fluvial depositional systems the role of floodplain sediment storage abstract the currently used model for alluvial sequence stratigraphy has been criticised miall 1991 and an alternative model is offered here in this model alluviation mainly takes place during transgressive and not highstand phases when accommodation space allows sediment storage the high capacity for some river systems to store floodplain sediments is a critical consideration the effects of climatic and intrinsic geomorphic changes during fluvial deposition are likely to greatly complicate simple base level controlled sequence models and recognising higher order fourth and fifth order cycles will be particularly difficult

stratigraphic modeling of foreland basins interpreting thrust deformation and lithosphere rheology the authors incorporate the processes of erosion and deposition in a numerical model to predict the stratal geometries and facies patterns produced during episodic thrusting in a nonmarine foreland basin the resultant stratigraphic record is characterized by a stairstepped facies package in which each retrogradation of facies toward the thrust marks the onset of a thrusting event the retrogradation of facies coincides with the migration of the forebulge toward the thrust and the generation of an erosional unconformity in the past changes in basin wavelength during basin evolution have been interpreted to record viscoelastic relaxation of the lithosphere this model suggests that changes in basin wavelength are a natural consequence of the interplay between thrust and sediment loading on an elastic 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

redoximorphic paleosols in alluvial and lacustrine deposits 1 8 ga lochness formation mount isa australia pedogenic processes and implications for paleoclimate abstract paleosols in the lochness formation 1 8 ga australia include both rare non red and abundant strongly reddened varieties that formed at subaerial exposure surfaces in both ephemeral river and lacustrine settings physical processes dominated non red paleosols which were characterized by repeated episodes of desiccation shrinking and cracking alternating with wetting and introduction of sand silt clay and iron oxyhydroxides into planar voids redoximorphic oxidation reduction processes were especially intense for the red paleosols redox depletions of fe and mn hypoalbans occur immediately adjacent to desiccation related macropores and peds whereas redox concentrations of fe and mn quasi coatings occur within paleosol matrix adjacent to redox depletions illuviated cla and fe oxide coatings and hypocoatings are also common in planar macropores and on ped faces redoximorphic features indicate periodic water infiltration and saturation accompanied by development of reducing conditions along planar macropores and ped surfaces in lochness formation paleosols variations in soil saturation were caused by seasonal fluctuations of lake level in lacustrine deposits and by formation of perched saturation zones within floodplain deposits respectively occurrences of red hematitic paleosols in the lochness formation are compatible with previous interpretations of a weakly oxygenated 1 8 ga paleoatmosphere redoximorphic features in the paleosols suggest a warm to cool temperate paleoclimate mean annual soil temperature 5 20 c characterized by seasonal saturation by analogy with quaternary redoximorphic soils a minimal concentration of organic c at least 1 wt possibly of microbial or bacterial origin must have been present in these proterozoic soils to allow for fe reduction

two phase stratigraphic model of foreland basin sequences application of flexural models to nonmarine foreland basin evolution indicates that two different stratigraphic styles of basin fill may develop over time basin subsidence is most rapid during times of thrust load emplacement associated sedimentation is coarse grained immediately adjacent to the thrust front and grades rapidly into fine grained deposits that cover most of the basin the distal part of the basin may also contain deposits derived from streams that flow from beyond the basin toward the thrust belt subsequent removal of the thrust load by erosion and other processes results in flexural rebound of the thrust belt and adjacent foreland basin during this postorogenic phase of adjustment a regional unconformity develops in the proximal part of the foreland basin proximal deposits along with thrust derived sediment are redeposited in the distal foreland basin and beyond this two phase model of foreland sedimentation predicts that coarsening upward sequences in the proximal and distal parts of the basin have reciprocal significance the proximal sequence represents thrust belt advance whereas the distal sequence represents thrust belt cessation

siliciclastic sequence stratigraphic patterns in foreland ramp type basins stratal patterns within shelf depositional sequences are dependent on tectonically controlled subsidence rates and their regional patterns in active tectonic basin settings regional subsidence patterns can be very different from passive continental margin settings resulting in substantial modifications of the basic sequence stratigraphic model in ramp type foreland basins on the tectonically active side subsidence rates decrease seaward in contrast with passive continental margins where the opposite subsidence pattern exists analysis of the interplay between eustasy and subsidence suggests the existence of two tectono stratigraphic zones occurring proximally and distally with respect to the basin margin zone a is defined as the region within which the rate of subsidence always exceeds the rate of eustatic fall consequently relative sea level rises continuously during a eustatic cycle albeit at varying rates zone b is defined as the region within which the rate of eustatic fall periodically exceeds the rate of subsidence resulting in an interval of relative sea level fall during a eustatic cycle on the tectonically active sides of foreland basins zone a lies on the landward side of the basin margin proximal to the orogenic belt and zone b lies seaward off zone a away from the orogenic belt where subsidence rates are lower on passive continental margins because of the opposite subsidence patterns zone a lies seaward of zone b where subsidence rates are greater the location of the shoreline relative to these zones determines the stacking patterns and stratal discontinuities within a depositional sequence if the shoreline remains in zone a then only type 2 sequence boundaries will occur if sufficient sediment flux is available allowing the shoreline to prograde into zone b then type 1 sequence boundaries can occur in foreland basins these type 1 sequence boundaries would become type 2 sequence boundaries updip in zone a the pattern of seaward decreasing subsidence on the tectonically active sides of foreland basins results in characteristic longitudinal facies and stratal patterns when the two zones occur the updip region of the basin proximal to the orogenic belt is characterized by nearly continuous nonmarine deposition albeit at varying rates the downdip region is characterized by forced regressions and deposition primarily of lowstand and transgressive systems tracts a transitional region containing deposits of all three systems tracts commonly occurs between the updip and downdip areas

joint inversion of surface waves and teleseismic body waves across the tibetan collision zone the fate of subducted indian lithosphere our study has included data from gsn including ic iu and ii china digital seismograph network geoscope iris ida pacific 21 kyrgyz digital network kyrgyz seismic telemetry network and iris usgs permanent seismic networks and the manas tien shan continental dynamics tibetan plateau broadband experiment indepth ii indepth iii indepth iv ascent himnt bhutan nanga parbat pakistan and ghengis passcal temporary seismic deployments we thank iiees and lgit for seismic data from iran and also seisuk for provision and assistance with instruments operated in northeast india cn was supported by a natural environment research council studentship grant ne h52449x 1 with case funding from awe blacknest we thank nick rawlinson and an anonymous reviewer for their constructive and helpful reviews figures were prepared with generic mapping tools gmt software wessel smith 1998

possible influence of subducting ridges on the himalayan arc and on the ruptures of great and major himalayan earthquakes abstract subduction of bathymetric features such as ridges seamounts fractures etc on the subducting plate influences the arc morphology and earthquake ruptures we analyse their effect on the development of the arcuate shape of the himalayan arc and on the ruptures of great and major himalayan earthquakes besides the two most prominent ridges in the indian ocean namely the chagos laccadive deccan ridge and the 90 e ridge which are assumed to extend up to the himalayan arc at least three major subsurface ridges have been mapped on the underthrusting indian plate under the indo gangetic plains it appears that the subduction of the two most prominent ridges contributed to the development of the arcuate shape of the himalayan arc the interaction and subduction of the other subsurface ridges probably influenced the himalayan arc morphology by causing a localised cusp in the frontal topography also these ridges probably acted as barriers to the ruptures of the major and great himalayan earthquakes

crustal structure across the san andreas fault southern california from teleseismic converted waves crustal structure along a 140 km long profile across the san andreas fault saf in southern california was imaged by stacking teleseismic p s converted phases recorded by a dense short period seismic array the crust mantle discontinuity moho is visible as a continuous feature at a depth around 30 km but is offset 6 to 8 km beneath the saf a small moho disruption can also be seen under the eastern california shear zone ecsz these results suggest that the saf and ecsz extend through the entire crust the moho upwarp under the san gabriel mountains indicates that the mountain ranges were lifted en masse as a result of crustal buckling under horizontal compression

tectonic controls on dispersal of gold into a foreland basin an example from the western canada foreland basin abstract the western canada foreland basin and overlying post eocene succession preserve a nearly continuous succession of sediment derived from the rising canadian cordillera from the jurassic to now initially sediment was derived from the middle of the uplifting omineca belt central cordillera exposing progressively deeper rocks including plutono metamorphic complexes with continued uplift the drainage divide stepped eastward and only shallower level eastern omineca material was shed to the foreland basin laramide uplift beginning in the late cretaceous exposed more igneous material to be shed into the foreland basin but an eastward shift of the drainage divide to the rocky mountain belt in the early tertiary cut off the supply of omineca debris gold is present in four intervals f the foreland basin succession gravels of modern rivers draining the rocky mountain belt middle to upper tertiary conglomerates derived from the rocky mountains upper cretaceous to lower tertiary sandstones and conglomerates and a distinctive lower cretaceous igneous clast conglomerate in the blairmore group the latter two occurrences contain up to 90 igneous clasts derived from the gold bearing omineca belt gold in the former two occurrences was derived from quartz veins in upper proterozoic to lower cambrian low grade greenschist subgreenschist metasediments these quartz veins were formed and exposed during cretaceous tertiary uplift of the rocky mountains windows of gold deposition exist in a foreland basin succession between exposure of primary gold deposits due to uplift and cutoff of source by lateral stepping of the drainage divide the western canada foreland basin contains three such gold deposition windows the last of which is still open

a high resolution micromorphological record of the late quaternary paleosols from ganga yamuna interfluve stratigraphic and paleoclimatic implications abstract this study reports the micromorphological record of a continuous set of samples from paleosol bearing intervals of two subsurface cores of 50 m depth representing typical pedosedimentary sequences of the ganga yamuna interfluve in the western ganga plains micromorphology microstructures rhizocretes fe mn features pedogenic carbonate illuvial clay coatings and relict pedofeatures was used to characterize pedogenic processes in the 13 paleosols in the iitk core and the 10 paleosols in the bhognipur core formation of these paleosols took place through incipient to moderate weathering on relatively stable surfaces of the ganga yamuna interfluve over the last 100 ka these paleosols range in character from simple ones having weakly developed pedofeatures to mature paleosols with strongly developed pedofeatures and are similar to modern entisols inceptisols alfisols and vertisols of the ganga plains pedosedimentary evolution in the two interfluve successions differs in terms of stratigraphic stacking of paleosols types of paleosols and heterogeneous nature of sediments in the northern part the iitk interfluve core is marked by a thick mature paleosol with a strongly developed argillic bt horizon at 41 45 m depth and weakly developed paleosols in the remaining part of the core the mature paleosol represents a major discontinuity when prolonged pedogenic activity took place on stable surfaces with little or no deposition on the other hand thin simple thick cumulic paleosols with weakly developed pedogenic features even with some preservation of sedimentary laminations indicate short stratigraphic breaks in rapidly aggrading sediments in the southern part the bhognipur core is marked by the heterogeneous nature of sediments with a predominance of cratonic sediments in the lower part and the himalayan sediments in the upper parts in contrast the iitk core shows a himalayan source throughout the entire interfluve succession in the bhognipur region is marked by rapid sedimentation and short pedogenic intervals of weakly developed paleosols excepting one mature paleosol with well developed argillic bt and vertic bss horizons at 10 14 m depth this mature paleosol possibly formed synchronously with a mature paleosol from the northern part of this interfluve the mature paleosol in both the cores represents a major stratigraphic break of 8000 10 000 years when prolonged pedogenesis occurred over large areas of the ganga yamuna interfluve following regional degradation and local gullying in response to climate change from sub humid to semi arid conditions during the mis 5 4 transition

tibetan and indian lithospheres in the upper mantle beneath tibet evidence from broadband surface wave dispersion broadband seismic experiments over the last two decades have produced dense data coverage across tibet yet the mechanism of the india asia lithospheric convergence beneath it remains a puzzle with even its basic features debated and with very different end member models advocated today we measured highly accurate rayleigh and love wave phase velocity curves in broad period ranges up to 5 200 s for a few tens of pairs and groups of stations across tibet combining in each case hundreds to thousands of interstation measurements made with cross correlation and waveform inversion methods robust shear velocity profiles were then determined by extensive series of nonlinear inversions of the data designed to constrain the depth dependent ranges of isotropic average shear speeds and radial anisotropy temperature anomalies in the upper mantle were estimated from shear velocities using accurate petrophysical relationships our results reveal strong heterogeneity in the upper mantle beneath tibet very large high velocity anomalies in the upper mantle are consistent with the presence of underthrust beneath southwestern tibet and subducted beneath central and eastern tibet indian lithosphere the corresponding thermal anomalies match those estimated for subducted indian lithosphere in contrast to the indian lithosphere tibetan lithosphere and asthenosphere display low to normal shear speeds tibetan lithosphere is thus warm and thin radial anisotropy in the upper mantle is weak in central and strong in northeastern tibet possibly reflecting asthenospheric flow above the subducting indian lithospheric slab

fragmented indian plate and vertically coherent deformation beneath eastern tibet 1 using fundamental mode rayleigh waves from the indepth iv and namche barwa seismic experiments for periods between 20 and 143 s we have investigated the lithospheric structure beneath eastern tibet we have found a 200 km wide high velocity body starting at 60 km depth and roughly centered beneath the bangong nijuang suture which is most likely a piece of the underthrusting indian continental lithosphere the sub horizontal underthrusting of the indian lithosphere beneath eastern tibet appears to be accompanied by its lateral tearing into at least two fragments and subsequent break off of the westernmost portion at 91 e 33 n the uppermost mantle low velocity zone we observe beneath the n qiangtang and songpan ganzi terranes is most probably due to warmer and thinner lithosphere relative to southern tibet we attribute the low velocity zones concentrated along the northern and southern branches of the eastern kunlun fault at lithospheric depths to strain heating caused by shearing the azimuthal fast directions at all periods up to 143 s 200 km peak sensitivity depth beneath the n qiangtang and songpan ganzi terranes are consistent suggesting vertically coherent deformation between crust and uppermost mantle furthermore the low velocity zone below the kunlun shan reaching down to 200 km argues against a present southward continental subduction along the southern margin of qaidam basin

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

interplay between longitudinal fluvial and transverse alluvial fan systems and growing thrusts in a piggyback basin se pyrenees abstract the integration of sedimentological and structural data in the western end of the ripoll syncline allows a clearer understanding of the palaeogeography of the area between the oblique termination of the pedraforca thrust sheet and the frontal vallfogona thrust during middle eocene times in this region a 2 5 km thick clastic succession corresponding to the bellmunt and milany sequences infilled the ripoll syncline this succession evolved from fluvio deltaic to alluvial the long lived and west prograding longitudinal fluvio deltaic system was parallel to the axis of the syncline the fluvio deltaic deposits are found interfingered with others which correspond to transverse alluvial fans of northern provenance which are increasingly important towards the top of the section the persistent and uniform wsw palaeoflow direction of the fluvio deltaic system together with the position of open marine conditions to the south indicate that the southern boundary of the system was constrained by a tectonic barrier which is attributed to the emergence of the vallfogona thrust the ramp of this thrust above the foreland basin strata formed the ripoll piggyback basin from middle lutetian times on several millions years earlier than previously proposed the western end of the ripoll syncline is thus an example of a complex partitioning of a foreland basin over time and space by structures of differing growth duration and different trend the interaction of the oblique system of thrusts corresponding to the eastern termination of the pedraforca thrust sheet and the vilada anticline and the frontal system of the vallfogona thrust controlled the position of the bellmunt river dominated delta which acted as the outlet of the ripoll piggyback basin

a complex tibetan upper mantle a fragmented indian slab and no south verging subduction of eurasian lithosphere due to the non uniform seismic station coverage in tibet critically important questions remain about the existence of southward continental subduction of eurasia beneath northern tibet from north and the nature of the underthrusting indian lithosphere underneath southern tibet from south using differential p and s wave travel times measured from 301 stations of all the temporary experiments deployed throughout tibet we constructed a comprehensive tomographic model the upper mantle of northern tibet consists of a rather homogeneous low velocity zone with no evidence of southward asian continental subduction in contrast the upper mantle from the himalayas to central tibet exhibits laterally variable p and s wave velocities significant low velocity zones are observed that are elongated in a north south direction and extending to at least 150 km depth which we interpret as evidences for fragmentation of the underthrusting indian continental lithosphere

southern limit of mantle derived geothermal helium emissions in tibet implications for lithospheric structure the isotopic composition of helium emitted from geothermal springs in the southern tibetan plateau reported as rc ra rc air corrected sample 3 he 4 he ra air 3 he 4 he ranges from 0 013 to 0 38 and defines two principal domains in southernmost central tibet helium isotope ratios are typical of radiogenic helium production in the crust rc ra 6 0 05 crustal helium domain further north there is a resolvable 3 he anomaly consistent with a mantle contribution r ra s 0 1 mantle helium domain the highest values of 0 27 0 38 ra occur at the southern end of the karakoram fault the boundary between the two domains lies 50 100 km north of the indus zangpo suture zone there seems to be no association between the 3 he anomaly and zones of active normal faulting and litho tectonic crustal units such as the ultramafic rocks of the indus zangpo suture zone and the gangdese intrusive belt although scavenging of mantle derived helium stored in large ultrabasic and basic intrusions in the crust cannot be ruled out entirely we argue that the 3 he anomaly most plausibly reflects degassing of volatiles from young quaternary mantlederived melts intruded into the crust as such it defines the southern limit of recent mantle melting and mantle melt extraction beneath the tibetan plateau the southern limit of the 3 he anomaly coincides with the junction between the indian and asian plates in the region where the indian lithospheric slab steepens and is subducted beneath tibet as suggested by seismic studies recent mantle melting and melt extraction is confined to the asian mantle but the southern limit of the melt zone may have migrated northwards during the last 10 ma as the indian lithosphere has progressively underthrust the himalayas and tibet fl 2000 elsevier science b v all rights reserved

empirical orthogonal functions and multiple flow regimes in the southern hemisphere winter abstract we present a new empirical orthogonal function eof analysis of winter 500 mb geopotential height anomalies in the southern hemisphere an earlier eof analysis by two of the present authors prefiltered the anomalies to exclude wavenumbers 5 and higher we do not the different preprocessing of data affects the results all three distinct planetary flow regimes identified in the winter circulation of the southern hemisphere by a pattern correlation method are captured by the new set of eofs only two of those regimes were captured by the earlier set the new results therefore lend further support to the idea that eofs point to distinct planetary

a low velocity belt beneath northern and eastern tibetan plateau from pn tomography 1 new pn tomography on tibetan plateau tp and neighboring region is conducted with data assembled from various sources the improved pn tomographic image delineates several distinct geophysical regimes that are approximately separated by geological boundaries at surface dense distribution of the stations in the eastern half of the plateau allows us to infer a 3d view of crustal thickness variation of the region the most striking feature is the bended low velocity belt lvb which extends from west to east beneath qiangtang block and songpan ganzi fold belt sgfb in northern tp and then turns consistently southward with sgfb beneath eastern tp and further south to indo china the consistency between geological boundaries on surface and sharp velocity contrast boundaries in uppermost mantle may suggest strong deformation coupling between crust and at least uppermost mantle if not whole lithosphere a north south striking lvb is also observed cutting through the southern tp coinciding with the yadong gulu rift at surface our tomographic results are generally consistent with dynamic models that involve with underthrusting in the southern tp shortening in the northern tp and extrusion to the east and the southeast furthermore our results suggest significant east west variations in the structure and dynamics

deformation of crust and upper mantle in central tibet caused by the northward subduction and slab tearing of the indian lithosphere new evidence based on shear wave splitting measurements abstract shear wave splitting provides insight into geodynamic processes such as lithospheric deformation and upper mantle flow this study presents shear wave splitting parameters determined from xks sks skks and pks and pms phases from receiver functions recorded by the 2d seismic array sandwich deployed in central tibet from november 2013 to april 2016 the xks splitting measurements show a generally strong anisotropy with an average of 1 3 s that even includes 17 stations with delay times no less than 2 0 s interestingly significantly weak splitting is also observed between nam tso and siling tso near 90 e spatial coherence analysis of the splitting parameters indicates an upper asthenospheric source as the primary cause of seismic anisotropy in the study region the regionally dominant ene wsw oriented upper mantle anisotropy can emerge from the corner flow in the overlying mantle wedge induced by the subduction of the indian lithospheric slab the crustal anisotropy obtained through pms splitting analysis might reflect lattice preferred orientation of anisotropic minerals formed by the ene oriented middle lower crustal flow with a minor contribution to the strong anisotropy the weak splitting observations in the east are likely caused by the nearly vertical axis of olivine induced by upwelling asthenosphere at a slab tear in the indian lithosphere