late oligocene early miocene birth of the taklimakan desert as the world s second largest sand sea and one of the most important dust sources to the global aerosol system the formation of the taklimakan desert marks a major environmental event in central asia during the cenozoic determining when and how the desert formed holds the key to better understanding the tectonic climatic linkage in this critical region however the age of the taklimakan remains controversial with the dominant view being from 3 4 ma to 7 ma based on magnetostratigraphy of sedimentary sequences within and along the margins of the desert in this study we applied radioisotopic methods to precisely date a volcanic tuff preserved in the stratigraphy we constrained the initial desertification to be late oligocene to early miocene between 26 7 ma and 22 6 ma we suggest that the taklimakan desert was formed as a response to a combination of widespread regional aridification and increased erosion in the surrounding mountain fronts both of which are closely linked to the tectonic uplift of the tibetan pamir plateau and tian shan which had reached a climatically sensitive threshold at this time

eocene tibetan plateau remnants preserved in the northwest himalaya in the northwest himalaya deeply incised mountain ranges contrast with high elevation low relief areas such as the deosai plateau in northern pakistan thermochronology data from the deosai plateau together with thermal history modelling suggest that this plateau along with other morphologically similar surfaces is a remnant of an eocene southwestern tibetan plateau

bounds on the viscosity coefficient of continental lithosphere from removal of mantle lithosphere beneath the altiplano and eastern cordillera 1 the rapid rise of the central andean plateau between 10 and 6 8 ma implies that mantle lithosphere including eclogitized lower crust was removed from beneath the region in that time interval we infer from that removal that the average viscosity coefficient of mantle lithosphere was quite low when removal occurred using scaling laws for the growth of perturbations to the thickness of a dense layer over an inviscid substratum rayleigh taylor instability we place bounds on the average viscosity coefficient for central andean lithosphere when compared with laboratory measurements of flow laws for olivine and eclogite the allowed range of viscosity coefficients yields bounds on the temperature of 500 800 c at the moho beneath this region and suggests that mean stresses across mantle lithosphere during continental deformation are less than 50 mpa this range of temperature is comparable with if a slightly lower than we might expect for lithosphere approximately doubled in thickness and not yet equilibrated with the doubled crustal radioactivity the mean deviatoric stress is comparable to that associated with stresses that drive plates and hence shows that lithospheric material is not too strong to prevent removal of its mantle part

a laboratory study on the thermomechanical behaviour of clayey soils this paper presents the findings of a laboratory investigation of the thermomechanical behaviour of clayey soils the tests were performed on reconstituted and natural clayey soils using triaxial cells modified to control temperature the range of temperatures that were investigated is from 20 to 60 c the thermal behaviour of the clays and the influence of temperature on their mechanical behaviour were investigated separately the analysis of the experimental results obtained in drained and undrained nonisothermal tests and during thermal consolidation made it possible to identify a link between the thermal and the time dependent behaviour of the soil skeleton therefore as for creep processes the response of the soil to thermal loading is not unique but among other factors depends strongly on the overall and recent stress history experienced by the soil in the range of temperatures investigated the deformability and strength of the soil were found to be relatively independent of temperature stiffe

thermal expansion and pore pressure generation in oil sands the prediction of stress changes and deformations arising from ground heating requires the coupled solution of the heat transfer and consolidation equations heat consolidation as a class of problems is distinct from other thermally induced consolidation problems involving processes such as frost heave and thaw consolidation in that it involves heating to elevated temperatures well above normal ground temperatures two of the important parameters required in analyses of heat consolidation problems are thermal expansion coefficients and a coefficient of thermal pore pressure generation relationships describing thermal expansion behaviour and thermal pore pressure generation in oil sands are presented both drained and undrained thermal expansion coefficients for athabasca oil sand were determined by means of heating experiments in the temperature range 20 300 c the thermal pore pressure generation coefficient was evaluated in undrained heating experiments under constant total confining stresses and under

anisotropy of magnetic susceptibility of the jingou river section implications for late cenozoic uplift of the tian shan better constraints on the uplift history of tian shan will increase our understanding of both mountain building processes and deformation patterns related to the india asian collision anisotropy of magnetic susceptibility ams holds great potential to track variations in tectonic strain which is ultimately responsible for mountain uplift here we present an ams record spanning the interval of 28 4 ma from the fluviolucastrine jingou river section in the tian shan foreland rock magnetic measurements show that the ams are dominated by antiferromagnetic hematite and some paramagnetic minerals both of which have a crystallographic preferred orientation and therefore the ams sequence may be applied to reconstruct tectonic strain although the principal minimum k min direction is nearly perpendicular to the bedding plane indicative of a predominantly sedimentary fabric for the section the tightly grouped principal maximum k max parallel to the fold axis suggests that the section was subject to an embryonic deformation at least since the late oligocene the k max direction is most likely associated with the n s strain caused by the india asian collision during the intervals of 23 3 20 0 ma and 16 5 14 0 ma the strain markedly increased as evidenced by more tightly grouped k max directions and k min largely distributed within a clear n s girdle these two intervals are accompanied by changes in sedimentary facies higher energy conditions and doubled sedimentation rates and are interpreted as the periods when the tian shan experienced significant uplift a conceptual model is tentatively proposed to reconcile various timings for the tian shan uplift

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

a thermochronological perspective on the morphotectonic evolution of the southeastern tibetan plateau a large part of the southeastern tibetan plateau tp is characterized by low relief surfaces at high elevations 3 5 km the origin of these landscapes and their geodynamic implications with regard to evolution of the southeastern tp has been the subject of considerable debate focusing on this topic this study utilizes fission track and u th he thermochronology to reconstruct the thermal history of the late triassic daocheng granite on which a low relief high elevation landscape has developed results suggest that the plateau surface experienced widespread late jurassic to early cretaceous cooling at a rate of 1 2 c ma followed by minimal regional cooling 0 3 c ma and early miocene enhanced cooling 2 3 c ma along deep valleys at the southern edge of the landscape given the subdued late jurassic to early cretaceous crustal deformation in the study area we relate the coeval cooling to crustal refrigeration and exhumation triggered by northward subduction of the meso tethys ocean along the bangong suture subsequent late cretaceous to cenozoic minimal cooling supports arguments for the formation of a low elevation landform prior to uplift of the southeastern tp the enhanced cooling commenced at the early miocene 15 22 ma along deep valleys and heralded the onset of river incision and surface uplift of the southeastern tp this finding does not support previous proposals claiming that high elevations of the southeastern tp were formed in late miocene time resulting from lower crustal flow but rather supports a model highlighting continental subduction and extrusion along the large tanggula yushu batang red river fault system

convective removal of the tibetan plateau mantle lithosphere by 26 ma abstract during the late oligocene early miocene there were several major geological events in and around the tibetan plateau tp first crustal shortening deformation ceased completely within the tp before 25 ma and instead adakitic rocks and potassic ultrapotassic volcanics were emplaced in the lhasa terrane since 26 25 ma several recent paleoelevation reconstructions suggest an oligocene early miocene uplift of 1500 3000 m for the qiangtang qt and songpan ganzi sg terranes although the exact timing is unclear as a possible response to this uplift significant desertification occurred in the vicinity of the tp at 26 22 ma and convergence between india and eurasia slowed considerably at 26 20 ma subsequently e w extension was initiated no later than 18 ma in the lhasa and qt terranes in contrast the tectonic deformation around the tp was dominated by radial expansion of shortening deformation since 25 22 ma the plateau wide near synchroneity of these events calls for an internally consistent model which can be best described as convective removal of the lower mantle lithosphere geophysical and petrochemical evidence further confirms that this extensive removal occurred beneath the qt and sg terranes the present review concludes that other than plate boundary stress the internal stress within the tp lithosphere could have contributed to rapid wholesale uplift and a series of concomitant tectonic events accompanied by major aridification since 26 ma

relationship of p wave seismic attributes azimuthal anisotropy and commercial gas pay in 3 d p wave multiazimuth data rulison field piceance basin colorado this case history is one of three field projects funded by the us department of energy as part of its ongoing research effort aimed to expand current levels of drilling and production efficiency in naturally fractured tight gas reservoirs the original stated goal for the 3 d p wave seismic survey was to evaluate and map fracture azimuth and relative fracture density throughout a naturally fractured gas reservoir interval at rulison field this interval is the cretaceous mesaverde approximately 2500 ft 760 m of lenticular sands silts and shales three dimensional full azimuth p wave data were acquired for the evaluation of azimuthal anisotropy and the relationship of the anisotropy to commercial pay in the target interval the methodology is based on the evaluation of two restricted azimuth orthogonal source receiver azimuth 3 d p wave volumes aligned with the natural principal axes of the azimuthal anisotropy as estimated from velocity analysis of multiazimuth prestack gathers the dix interval

thermally induced stresses in boulders on airless body surfaces and implications for rock breakdown abstract this work investigates the macroscopic thermomechanical behavior of lunar boulders by modeling their response to diurnal thermal forcing our results reveal a bimodal spatiotemporally complex stress response during sunrise stresses occur in the boulders interiors that are associated with large scale temperature gradients developed due to overnight cooling during sunset stresses occur at the boulders exteriors due to the cooling and contraction of the surface both kinds of stresses are on the order of 10 mpa in 1 m boulders and decrease for smaller diameters suggesting that larger boulders break down more quickly boulders 30 cm exhibit a weak response to thermal forcing suggesting a threshold below which crack propagation may not occur boulders of any size buried by regolith are shielded from thermal breakdown as boulders increase in size 1 m stresses increase to several 10 s of mpa as the behavior of their surfaces approaches that of an infinite halfspace as the thermal wave loses contact with the boulder interior stresses become limited to the near surface this suggests that the survival time of a boulder is not only controlled by the amplitude of induced stress but also by its diameter as compared to the diurnal skin depth while stresses on the order of 10 mpa are enough to drive crack propagation in terrestrial environments crack propagation rates in vacuum are not well constrained we explore the relationship between boulder size stress and the direction of crack propagation and discuss the implications for the relative breakdown rates and estimated lifetimes of boulders on airless body surfaces

reflection transmission coefficients and azimuthal anisotropy in marine seismic studies summary introduce two methods for inferring the direction of vertical fractures from marine seismic data first we derive the necessary reflection and transmission coefficients for an interface between a liquid and an azimuthally anisotropic solid next we show that multicomponent ocean bottom seismometer data from surface airgun sources along two perpendicular shot lines can be rotated into the principal directions of azimuthal anisotropy to determine the orientation of vertical fractures finally we show that p wave amplitude versus offset avo depends on the orientation of the shot line with respect to the vertical fractures thus p wave avo can also be used to determine fracture orientation

a case study of azimuthal avo analysis with anisotropic spreading correction geomechanical properties of tight low porosity reservoirs are largely governed by natural fracture networks hence reliable estimation of fracture density and orientation is extremely important for cost effective hydraulic completion and hydrocarbon production direct information about fracturing can be obtained using borehole methods such as image log analysis which provide estimates of fracture counts and orientations on various scales the main shortcoming of borehole measurements is that they are sensitive only to formation properties in the immediate vicinity of the well in some cases the spatial distribution of fractures can be inferred from fault maps obtained by coherence analysis of surface seismic data however the correlation between areas of high fracture density and fault locations is not always straightforward

fracture prediction from wide azimuth land seismic data in se algeria there is evidence from core image log and well test data that the tiguentourine gas condensate field in the illizi basin of southeast algeria contains fractures that provide a positive permeability assist in 2004 the first ever wide azimuth p wave seismic survey in algeria was acquired over a small part of the field to test the ability of surface seismic data to accurately predict the p wave anisotropy resulting from the preferential orientation of open fractures there appears to be a correlation between velocity anisotropy derived from the seismic data with image log data from the wells thus providing confidence that future wells can be located to directly target areas with high concentrations of oriented fractures that are anticipated to provide significant permeability assist and boost well productivity

acoustic reflectivity from variously oriented orthorhombic media analogies to seismic responses from a fractured anisotropic crust the variations in the strength of seismic reflections with both angles of incidence and azimuth are being used to constrain the orientations and degrees of mechanical anisotropy of geological formations this information is often used in turn to infer the directions of fracturing and stress usually assuming simplified geological structures that in many cases are not realistic in order to further understand this problem the reflectivities from a set of variously titled samples cut from a single block of a common anisotropic composite material are measured with both angles of incidence and azimuth in the laboratory each sample is analogous to a formation in which the fracture sets increasingly dip the observed responses differ from plane wave theory due to the finite nature of the experiment however the deviations are well accounted for by modeling the acoustic reflectivity using its bounded pulse as expected the reflectivities vary with the sample tilt the angle of incidence and the azimuth at which the measurements are made however some of these responses such as the pre critical reflectivities do not appear to strongly vary with azimuth for a given sample critical angle phenomena too must be used cautiously as the observed reflectivity peaks do not lie at the exact critical angle these results suggest that reflectivities could be an important source of information with regards to the spatial orientation of anisotropic layer

pressure and temperature dependence of acoustic wave speeds in bitumen saturated carbonates implications for seismic monitoring of the grosmont formation abstractrecent time lapse seismic observations in carbonate reservoirs subject to steam assisted enhanced oil recovery display substantial changes in seismic reflectivity due to the combined effects of saturation pressure and temperature understanding these field seismic observations requires knowledge of the effects on the seismic wave speeds in bitumen saturated carbonates we have conducted ultrasonic measurements of p and s wave velocities in bitumen saturated dolomite taken from the grosmont formation in northern alberta wave speeds are measured under a variety of conditions of constant pore pressure constant effective pressure and with varying temperature to map the various controlling factors the temperature dependent declines of 12 and 9 for the p and s wave speeds respectively with temperatures from 10 c to 102 c are most notable unlike oil sands at times the dolomite retains its structure upon removal of the bitumen allowing for measurement of the dry and water saturated frame pr

cretaceous tertiary structural evolution of the north central lhasa terrane tibet 1 in the north central lhasa terrane of tibet two distinct structural levels of an east west striking thrust system are exposed along the north trending late cenozoic xiagangjiang rift upper paleozoic strata deformed by the south directed langgadong la thrust and cretaceous strata involved in variably north and south directed thrusting characterize these lower and upper structural levels respectively these two structural levels are separated by the tagua ri passive roof thrust balanced cross section restoration suggests that the thrust system accommodated 103 km 53 shortening the 40ar 39ar results together with an interpretation of synthrust deposition of upper cretaceous strata suggest that the majority of shortening occurred during the late cretaceous paleocene cretaceous strata lie unconformable on permian rocks volcanic tuffs directly above the unconformity yield u pb zircon ages of 131 ma upper cretaceous strata record a change from shallow marine to nonmarine deposition indicating uplift above sea level during this time the overall south directed vergence of the thrust belt is consistent with substantial crustal thickening in central tibet by large scale northward underthrusting of lhasa terrane basement beneath the qiantang terrane prior to the indo asian collision the documented decoupling of contractional deformation at shallow crustal levels appears to be a regional characteristic of tibet from at least the bangong suture in the north to the tethyan himalaya to the south this style of deformation explains the absence of basement exposures and major denudation in this region despite substantial crustal shortening

stress strain response of cold lake oil sands the stress strain response of cold lake oil sands at confining stresses and temperatures up to 18 mpa and 200 c respectively was studied in a triaxial apparatus using 89 mm full diameter cores

recent advances in determination of fracture strike and crack density from p wave seismic data confidence in the ability of avaz analysis amplitude variation with angle and azimuth to capture information about fractures in a reservoir has steadily increased over the last four years because this technique detected seismic anisotropy in many different reservoirs and the results consistently tied other information about fractures

reflection moveout inversion for horizontal transverse isotropy accuracy limitation and acquisition horizontal transverse isotropy hti is the simplest azimuthally anisotropic model used to describe vertical fracturing in an isotropic matrix assuming that the subsurface is laterally homogeneous and using the elliptical variation of p wave nmo velocity with azimuth measured in at least three different source to receiver orientations we can estimate three key parameters of hti media the vertical velocity anisotropy and the azimuth of the symmetry axis such parameter estimation is sensitive to the angular separation between the survey lines in 2 d acquisition or equivalently to source to receiver azimuths in 3 d acquisition and the set of azimuths used in the inversion procedure the accuracy in estimating the azimuth in particular is also sensitive to the strength of anisotropy while the accuracy in resolving vertical velocity and anisotropy is about the same for any strength of anisotropy to maximize the accuracy and stability in parameter estimation it is best to have the azimuths for the

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

determination of the principal directions of azimuthal anisotropy from p wave seismic data in an azimuthally anisotropic medium the principal directions of azimuthal anisotropy are the directions along which the quasi p and the quasi s waves propagate as pure p and s modes when azimuthal anisotropy is induced by oriented vertical fractures imposed on an azimuthally isotropic background two of these principal directions correspond to the directions parallel and perpendicular to the fractures s waves propagating through an azimuthally anisotropic medium are sensitive to the direction of their propagation with respect to the principal directions as a result primary or mode converted multicomponent s wave data are used to obtain the principal directions apart from high acquisition cost processing and interpretation of multicomponent data require a technology that the seismic industry has not fully developed anisotropy detection from conventional p wave data on the other hand has been limited to a few qualitative studies of the amplitude variation with offset avo for different azimutha

imaging the lithosphere asthenosphere boundary of the indian plate using converted wave techniques 1 the lithospheric structure of the indian plate has been investigated using converted wave techniques p and s receiver functions and a novel stacking analysis technique without using deconvolution applied to a large seismological data set from permanent and temporary broadband seismic stations we observe coherent energy from at least two seismic discontinuities i e the crust mantle moho and lithosphere asthenosphere boundary lab in the uppermost mantle here we provide a novel seismic image of the indian lithosphere showing definitive evidence of its flexure which is interpreted to be primarily caused by the hard collision at 55 myr resulting in the world s highest mountain chain the himalayas and the tibetan plateau results from geoidal and gravity studies do suggest postcollisional flexuring of the indian plate however the flexure lacks observational constraints the observed wavelength of the flex is 1000 km with the thickness of the indian shield lithosphere varying from 70 km to 140 km such a low value for a continent implies that the indian plate has been reworked in the past the plate deepens in the himalayan region to a depth of 170 km further the converted phases are interpreted to be resulting from the bottom of the lithosphere we clearly demonstrate that these are distinct and different from the midlithospheric discontinuity for a large number of stations the midlithospheric discontinuity and lab are clearly separated in depth our observations suggest that the archaean lithosphere is no longer intact and is prone to deformation

miocene to holocene exhumation of metamorphic crustal wedges in the nw himalaya evidence for tectonic extrusion coupled to fluvial erosion the himalayan crystalline core zone exposed along the sutlej valley india is composed of two high grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded as a consequence of the foreland directed propagation of crustal deformation in the indian plate margin the high himalayan crystalline sequence hhcs is composed of amphibolite facies to migmatitic paragneisses metamorphosed at temperatures up to 750 c at 30 km depth between eocene and early miocene during early miocene combined thrusting along the main central thrust mct and extension along the sangla detachment induced the rapid exhumation and cooling of the hhcs whereas exhumation was mainly controlled by erosion since middle miocene the lesser himalayan crystalline sequence lhcs is composed of amphibolite facies para and orthogneisses metamorphosed at temperatures up to 700 c during underthrusting down to 30 km depth beneath the mct the lhcs cooled very rapidly since late miocene as a consequence of exhumation controlled by thrusting along the munsiari thrust and extension in the mct hanging wall this renewed phase of tectonic extrusion at the himalayan front is still active as indicated by the present day regional seismicity and by hydrothermal circulation linked to elevated near surface geothermal gradients in the lhcs as recently evidenced in the himalayan syntaxes active exhumation of deep crustal rocks along the sutlej valley is spatially correlated with the high erosional potential of this major trans himalayan river this correlation supports the emerging view of a positive feedback during continental collision between crustal scale tectono thermal reworking and efficient erosion along major river systems

generalized dix equation and analytic treatment of normal moveout velocity for anisotropic media despite the complexity of wave propagation in anisotropic media reflection moveout on conventional common midpoint cmp spreads is usually well described by the normal moveout nmo velocity defined in the zero offset limit in their recent work grechka and tsvankin showed that the azimuthal variation of nmo velocity around a fixed cmp location generally has an elliptical form i e plotting the nmo velocity in each azimuthal direction produces an ellipse and is determined by the spatial derivatives of the slowness vector evaluated at the cmp location this formalism is used here to develop exact solutions for the nmo velocity in anisotropic media of arbitrary symmetry for the model of a single homogeneous layer above a dipping reflector we obtain an explicit nmo expression valid for all pure modes and any orientation of the cmp line with respect to the reflector strike the contribution of anisotropy to nmo velocity is contained in the slowness components of the zero offset ray along with the derivatives of the vertical slowness with respect to the horizontal slownesses quantities that can be found in a straightforward way from the christoffel equation if the medium above a dipping reflector is horizontally stratified the effective nmo velocity is determined through a dix type average of the matrices responsible for the interval nmo ellipses in the individual layers this generalized dix equation provides an analytic basis for moveout inversion in vertically inhomogeneous arbitrarily anisotropic media for models with a throughgoing vertical symmetry plane i e if the dip plane of the reflector coincides with a symmetry plane of the overburden the semi axes of the nmo ellipse are found by the more conventional rms averaging of the interval nmo velocities in the dip and strike directions modelling of normal moveout in general heterogeneous anisotropic media requires dynamic ray tracing of only one zero offset ray remarkably the expressions for geometrical spreading along the zero offset ray contain all the components necessary to build the nmo ellipse this method is orders of magnitude faster than multi azimuth multi offset ray tracing and therefore can be used efficiently in traveltime inversion and in devising fast dip moveout dmo processing algorithms for anisotropic media this technique becomes especially efficient if the model consists of homogeneous layers or blocks separated by smooth interfaces the high accuracy of our nmo expressions is illustrated by comparison with ray traced reflection traveltimes in piecewise homogeneous azimuthally anisotropic models we also apply the generalized dix equation to field data collected over a fractured reservoir and show that p wave moveout can be used to find the depth dependent fracture orientation and to evaluate the magnitude of azimuthal anisotropy

thermal fatigue as a driving mechanism for activity on asteroid bennu many boulders on 101955 bennu a near earth rubble pile asteroid show signs of in situ disaggregation and exfoliation indicating that thermal fatigue plays an important role in its landscape evolution observations of particle ejections from its surface also show it to be an active asteroid though the driving mechanism of these events is yet to be determined exfoliation has been shown to mobilize disaggregated particles in terrestrial environments suggesting that it may be capable of ejecting material from bennu s surface we investigate the nature of thermal fatigue on the asteroid and the efficacy of fatigue driven exfoliation as a mechanism for generating asteroid activity by performing finite element modeling of stress fields induced in boulders from diurnal cycling we develop a model to predict the spacing of exfoliation fractures and the number and speed of particles that may be ejected during exfoliation events we find that crack spacing ranges from 1 mm to 10 cm and disaggregated particles have ejection speeds up to 2 m s exfoliation events are most likely to occur in the late afternoon these predictions are consistent with observed ejection events at bennu and indicate that thermal fatigue is a viable mechanism for driving asteroid activity crack propagation rates and ejection speeds are greatest at perihelion when the diurnal temperature variation is largest suggesting that events should be more energetic and more frequent when closer to the sun annual thermal stresses that arise in large boulders may influence the spacing of exfoliation cracks or frequency of ejection events

fracture characterization at valhall application of p wave amplitude variation with offset and azimuth avoa analysis to a 3d ocean bottom data set the delineation and characterization of fracturing is important in the successful exploitation of many hydrocarbon reservoirs such fracturing often occurs in preferentially aligned sets if the fractures are of subseismic scale this may result in seismic anisotropy thus measurements of anisotropy from seismic data may be used to delineate fracture patterns and investigate their properties here fracture induced anisotropy is investigated in the valhall field which lies in the norwegian sector of the north sea this field is a chalk reservoir with good porosity but variable permeability where fractures may significantly impact production e g during waterflooding to investigate the nature of fracturing in this reservoir p wave amplitude variation with offset and azimuth avoa is analyzed in a 3d ocean bottom cable obc data set in general 3d ocean bottom seismic obs acquisition leads to patchy coverage in offset and azimuth and this must be addressed when considering such data to overcome this challenge and others associated with 3d obs acquisition a new method for processing and analysis is presented for example a surface fitting approach which involves analyzing azimuthal variations in avo gradients is used to estimate the orientation and magnitude of the fracture induced anisotropy this approach is also more widely applicable to offset azimuth analysis of other attributes e g traveltimes and any data set where there has been true 3d data acquisition land or marine using this new methodology we derive high resolution maps of p wave anisotropy from the avoa analysis for the top chalk reflection at valhall these anisotropy maps show coherent but laterally varying trends synthetic avoa modeling using effective medium models indicates that if this anisotropy is from aligned fracturing the fractures are likely liquid filled with small aspect ratios and the fracture density must be high furthermore we show that the fracture normal direction is parallel to the direction of most positive avo gradient in other situations the reverse can be true i e the fracture normal direction can be parallel to the direction of the most negative avo gradient effective medium modeling or comparisons with anisotropy estimates from other approaches e g azimuthal variations in velocity must therefore be used to resolve this ambiguity the inferred fracture orientations and anisotropy magnitudes show a degree of correlation with the positions and alignments of larger scale faults which are estimated from 3d coherency analysis overall this work demonstrates that significant insight may be gained into the alignment and character of fracturing and the stress field variations throughout a field using this high resolution avoa method

a cretaceous eocene depositional age for the fenghuoshan group hoh xil basin implications for the tectonic evolution of the northern tibet plateau the fenghuoshan group marks the initiation of terrestrial deposition in the hoh xil basin and preserves the first evidence of uplift above sea level of northern tibet the depositional age of the fenghuoshan group is debated as are the stratigraphic relationships between the fenghuoshan group and other terrestrial sedimentary units in the hoh xil basin we present new radiometric dates and a compilation of published biostratigraphic data which are used to reinterpret existing magnetostratigraphic data from the fenghuoshan group from these data we infer an 85 51 ma depositional age range for the fenghuoshan group u pb detrital zircon age spectra from this unit are compared to age spectra from tibetan terranes and mesozoic sedimentary sequences to determine a possible source terrane for fenghuoshan group strata we propose that these strata were sourced from the qiangtang terrane and may share a common sediment source with cretaceous sedimentary rocks in nima basin field relationships and compiled biostratigraphic data indicate that the fenghuoshan and tuotuohe groups are temporally distinct units we report late oligocene ages for undeformed basalt flows that cap tilted fenghuoshan group strata together our age constraints and field relationships imply exhumation of the central qiangtang terrane from the late cretaceous to earliest eocene followed by eocene oligocene deformation and shortening of the northern qiangtang and southern songpan ganzi terranes crustal shortening within the hoh xil basin ceased by late oligocene time as is evident from flat lying basaltic rocks which cap older deformed strata

temporal constraints and pulsed late cenozoic deformation during the structural disruption of the active kashi foreland northwest china 1 in response to the ongoing indo asian collision structural deformation has encroached into the tian shan foreland in western china since the early miocene in order to reconstruct a detailed history of foreland deformation along the southern margin of the tian shan we have synthesized extensive mapping analysis of seismic sections magnetostratigraphy of the foreland fill and associated growth strata apatite fission track dating changes in sediment accumulation rates and geodetic surveys of deformed fluvial terraces across an area exceeding 6500 km2 that spans the kashi foreland in the northwestern tarim basin we use these data to place temporal constraints on individual structures that delineate at least 4 distinct stages of deformation stage 1 involved initial uplift of hinterland structures beginning at 20 25 ma stage 2 began at 16 3 ma when the basement involved deformation front stepped south to the kashi basin thrust that bounds the foredeep strata stage 3 occurred from 13 5 to 4 ma as the deformation front migrated south in episodic steps above thin skinned detachment and ramp flat folding finally stage 4 began after 4 ma with growth of the keketamu anticline followed by rapid southward migration of the deformation front to the atushi and then kashi detachment anticlines on the basis of the deformed tertiary strata and available seismic data calculations of the minimum magnitude of shortening yield totals ranging from 10 to 32 km since the early miocene of which 7 12 km occurred since 4 ma although average shortening rates since 16 3 ma were 0 8 1 3 mm a rates were faster from 16 3 to 13 5 ma 1 1 3 2 mm a after which they decreased to 0 5 mm a between 13 5 and 4 ma before increasing to 2 2 2 7 mm a after 4 ma similarly the rate of southward propagation of the deformation front was initially 3 7 mm a for the first 10 ma of basin development slowed to 1 5 mm a between 13 5 and 4 ma and increased to more than 10 mm a since 4 ma these changes in shortening and structural migration rates over time suggest that the loci of orogenic deformation shifted in and out of the foreland during the neogene surveyed stream terraces suggest that although most current deformation is located along the southern front of the foreland modest deformation and shortening continue in the hinterland structural style within the foreland has also changed over time from imbricate thrust faults with hanging wall folds along the northern margin to ramp flat faults and detachment folds along the southern margin where the basement rocks are previously deformed such as in the hinterland of the kashi foreland thick skinned fault ramps and related fault propagation folds dominate the structural style as syntectonic sedimentation within the foreland produced thick subhorizontal stratigraphy the structural style evolved to detachment and fault bend folding above these footwall flats within the neogene strata

seismic evidence for the north china plate underthrusting beneath northeastern tibet and its implications for plateau growth abstract lithospheric deformation of the tibetan plateau is caused by subduction of the indian northward and asian southward plates the effects of this interaction on inner reaches of the asian continent between the tibetan plateau and the north china craton ncc for example remain uncertain due to poor geophysical data coverage in northeastern tibet ne tibet we provide here detailed knowledge of the lithospheric structure beneath ne tibet as determined from a dense broadband seismic profile traversing ne tibet to the westernmost ncc receiver function imaging reveals several significant features including a north dipping intracrustal converter nc moho offset overlap beneath major fault zones and a low velocity layer lvl in the middle lower crust the lithosphere asthenosphere boundary lab is clearly defined and appears as a south dipping interface that runs continuously from the alxa interior to the qilian orogen interpretation of these observations combined with other seismic evidence implies that the ncc lithospheric mantle has been persistently underthrust beneath the qilian orogen this process forms the thick skinned crustal accretionary wedges which develop above a middle lower intracrustal decollement our results provide further deep geophysical constraints on the cenozoic post collisional evolution of the convergent boundary between ne tibet and the ncc and help clarify the mechanism of plateau growth in this boundary area

far field lithospheric deformation in tibet during continental collision 1 crustal deformation along the present day northern margin of the tibetan plateau has occurred since mid to late eocene time soon after india collided with eurasia assuming that on these distance and time scales the lithosphere can be approximated using a thin viscous sheet we show that far field lithospheric deformation caused by an indenting boundary is expected to start shortly after collision begins when a strong region analogous to cratonic lithosphere like that of the tarim basin is included in the calculations shear and vertical strain rates concentrate along the southern edge of the strong region resulting in enhanced deformation there in calculations scaled to the dimensions of india and the tibetan plateau shear and vertical strain rates on the northern edge of the plateau increase soon after collision begins crustal thickening accelerates later in the calculations we propose that the apparent increased exhumation and perhaps surface uplift around 10 ma may signal the arrival of the thickening front

neogene uplift of the tian shan mountains observed in the magnetic record of the jingou river section northwest china the tian shan mountains constitute central asia s longest and highest mountain range understanding their cenozoic uplift history thus bears on mountain building processes in general and on how deformation has occurred under the influence of the india asia collision in particular in order to help decipher the uplift history of the tian shan we collected 970 samples for magnetostratigraphic analysis along a 4571 m thick section at the jingou river xinjiang province china stepwise alternating field and thermal demagnetization isolate a linear magnetization component that is interpreted as primary from this component a magnetostratigraphic column composed of 67 polarity chrons are correlated with the reference geomagnetic polarity timescale between 1 ma and 23 6 ma with some uncertainty below 21 ma this correlation places precise temporal control on the neogene stratigraphy of the southern junggar basin and provides evidence for two significant stepwise increases in sediment accumulation rate at 16 15 ma and 11 10 ma rock magnetic parameters also undergo important changes at 16 15 ma and 11 10 ma that correlate with changes in sedimentary depositional environments together with previous work we conclude that growth history of the modern tian shan mountains includes two pulses of uplift and erosion at 16 15 ma and 11 10 ma middle to upper tertiary rocks around the tian shan record very young 5 ma counterclockwise paleomagnetic rotations on the order of 15 to 20 which are interpreted as because of strain partitioning with a component of sinistral shear that localized rotations in the piedmont

seismic anisotropy in exploration and reservoir characterization an overview recent advances in parameter estimation and seismic processing have allowed incorporation of anisotropic models into a wide range of seismic methods in particular vertical and tilted transverse isotropy are currently treated as an integral part of velocity fields employed in prestack depth migration algorithms especially those based on the wave equation we briefly review the state of the art in modeling processing and inversion of seismic data for anisotropic media topics include optimal parameterization body wave modeling methods p wave velocity analysis and imaging processing in the p domain anisotropy estimation from vertical seismic profiling vsp surveys moveout inversion of wide azimuth data amplitude variation with offset avo analysis processing and applications of shear and mode converted waves and fracture characterization when outlining future trends in anisotropy studies we emphasize that continued progress in data acquisition technology is likely to spur transition from t

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

an analysis of avo inversion for postcritical offsets in hti media abstractazimuthal variations of wavefield characteristics such as traveltime or reflection amplitude play an important role in the identification of fractured media a transversely isotropic medium with a horizontal symmetry axis hti medium is the simplest azimuthally anisotropic model typically used to describe one set of vertical fractures there exist many techniques in industry to recover anisotropic parameters based on moveout equations and linearized reflection coefficients using such a model however most of the methods have limitations in defining properties of the fractures due to linearizations and physical approximations used in their development thus azimuthal analysis of traveltimes based on normal moveout ellipses recovers a maximum of three medium parameters instead of the required five linearizations made in plane wave reflection coefficients pwrcs limit the amplitude versus offset avo analysis to small incident angles and weak contrast interfaces inversion based on azimuthal