crustal velocity structure of the northeastern tibetan plateau from ambient noise surface wave tomography and its tectonic implications abstract broadband seismic data from the regional seismic network operated by the china earthquake administration and 32 temporary seismic stations are used to image the crustal velocity structure in the northeast tibetan plateau empirical rayleigh and love wave green s functions are obtained from interstation cross correlation of continuous seismic records group velocity dispersion curves for rayleigh and love waves between 10 and 50 s are obtained using the multiple filter analysis method with phase matched processing the group velocity variations of rayleigh and love waves overall correlate well with the major geologic structures and tectonic units in the study region shear wave velocity structures were then inverted from rayleigh and love wave dispersion maps the results show that the songpan ganzi terrane is associated with a low velocity at depth greater than 20 km the northern qilian orogen with higher elevation and thicker crust compared to the southern qilian orogen is also dominated by low velocity at depth greater than 25 km however there is no clear evidence of the low velocity mid to lower crust beneath the southern qilian orogen as the crustal flow model predicts the low velocity zone lvz beneath the northern qilian orogen may suggest that the crustal thickening and surface uplift of the northern qilian orogen are related to the lvz and the lvz may be considered as an intracrustal response to bear the ongoing deformation in the northern qilian orogen online material figures of crustal topography number of group velocity measurements checkerboard tests for nets stations and 1d velocity models

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

shear velocity structure radial anisotropy and dynamics of the tibetan crust geophysical and geological data suggest that tibetan middle crust is a partially molten mechanically weak layer but it is debated whether this low viscosity layer is present beneath the entire plateau what its properties are how it deforms and what role it has played in the plateau s evolution broad band seismic surface waves yield resolution in the entire depth range of the tibetan crust and can be used to constrain its shear wave velocity structure indicative of crustal composition temperature and partial melting and radial anisotropy indicative of the patterns of deformation we measured love and rayleigh wave phase velocity curves in broad period ranges up to 7 200 s for a few tens of pairs and groups of stations across tibet combining in each case hundreds of interstation measurements made with cross correlation and waveform inversion methods shear velocity profiles were then determined by extensive series of non linear inversions of the data designed to constrain the depth dependent ranges of isotropic average shear speeds and radial anisotropy shear wave speeds within the tibetan middle crust are anomalously low and also show strong lateral variations across the plateau the lowest mid crustal shear speeds are found in the north and west of the plateau 3 1 3 2 km s 1 within a pronounced low velocity zone in southeastern tibet crustal shear wave speeds increase gradually towards southeast whereas in the north the change across the kunlun fault is relatively sharp the lateral variations of shear speeds within the crust are indicative of those in temperature a mid crustal temperature of 800 c reported previously can account for the low shear velocities across lhasa in the north the temperature is higher and exceeds the solidus resulting in partial melting that we estimate at 3 6 per cent strong radial anisotropy is required by the data in western central tibet 5 per cent but not in northeastern tibet the amplitude of radial anisotropy in the crust does not correlate with isotropic average shear speed and by inference with crustal rock viscosity or with surface elevation instead radial anisotropy is related to the deformation pattern and is the strongest in regions experiencing extension crustal flattening as noted previously the growth of tibet by the addition of indian crustal rocks into its crust from the south is reflected in the higher crustal seismic velocities and thus lower temperatures in the southern compared to northern parts of the plateau more recently added rocks having had less time to undergo radioactive heating within the thickened tibetan crust gravity driven flattening the basic cause of extension and normal faulting in the southern western and central tibet is evidenced by pervasive radial anisotropy in the middle crust beneath the regions undergoing extension the overall eastward flow of the crust is directed by the boundaries and motions of the lithospheric blocks that surround tibet

brittle ductile transition shear failure and leakage in shales and mudrocks abstract this paper presents an experimental study on brittle ductile transition shear behaviour the formation of shear fractures in shales and mudrocks and the hydraulic properties of shales and mudrocks with shear induced fractures the experimental study is based on triaxial tests on different mudrocks and shales from north sea reservoirs and adjacent areas sheared at different effective confining stresses it is shown that during burial mudrocks behave as normally consolidated materials and exhibit ductile response to increased load however several processes like uplift chemical diagenesis and overpressure build up turn mudrocks into overconsolidated materials which exhibit brittle behaviour during loading triaxial tests with different loading conditions have been performed to establish the brittle to ductile transition of various mudrocks based on the results of the tests it is shown that the brittle to ductile transition can be related to the overconsolidation ratio a relationship between normalized undrained shear strength and overconsolidation ratio is established in combination with this relationship a correlation between compressional wave velocity and apparent pre consolidation stress which accounts for both mechanical and chemical diagenesis may be used as a tool to evaluate possible leakage of hydrocarbon seals

northward channel flow in northern tibet revealed from 3d magnetotelluric modelling abstract the kunlun fault defines one of the major northern tectonic boundaries of the tibetan plateau previous geophysical studies have detected a major change in rheology across this boundary but it is not clear how this is related to models that have invoked crustal flow the lithospheric resistivity structure of the kunlun fault has been investigated by both the indepth iii and iv magnetotelluric mt transects all the mt data were processed using modern statistically robust methods and have been analysed for directionality and dimensionality in order to improve understanding of the anisotropic distribution of melt previously revealed by our remodelling of the indepth iii mt data a variant approach on 3d inversion of 2d profiles was investigated to explore and improve lateral resolution in addition to the apparent surficial deformation associated with the sinistral strike slip kunlun fault the 3d modelling of the indepth mt data reveals that complex deformation processes are occurring at mid crustal depths in northern tibet the 3d mt inversion results supported by synthetic modelling particularly confirm and highlight the presence of separate north south intrusions of conductive material crossing the kunlun fault into the more resistive kunlun qaidam block these north south intrusions are interpreted to be associated with the horizontal channel flow of partially molten songpan ganzi crust into two or more separated channels moving northwards and crossing the surficial trace of the kunlun fault at mid to lower crustal depths

mechanics of fluid driven fracture growth in naturally fractured reservoirs with simple network geometries 1 a numerical model has been developed for fluid driven opening mode fracture growth in a naturally fractured formation the rock formation contains discrete deformable fractures which are initially closed but conductive because of their preexisting apertures fluid flow that develops along fractures depends on fracture geometry defined by preexisting aperture distribution offsets along a fracture path and intersections of two or more fractures the model couples fluid flow elastic deformation and frictional sliding to obtain the solution which depends on the competition between fractures for permeability enhancement the fractures can be opened by fluid pressure that exceeds the normal stress acting on them and by interactions with intersecting closed fractures experiencing coulomb type frictional slip the newtonian fluid is assumed to flow through the conductive fractures according to a lubrication equation that relates the cube of an equivalent hydraulic aperture to fracture conductivity the rock material is assumed to be impermeable and elastic this paper provides the governing equations for the multiple fracture systems and the solution methods used flow distribution and fracture growth in conductive fracture sets are simulated for a range of geometric arrangements and hydraulic properties numerical results show that elastic interaction between fracture branches plays a controlling role in fluid migration although initial apertures can give rise to a preferential fluid flow direction during the early stage in the presence of offsets fracture segments subject to strong compression are difficult to open hydraulically and their resulting smaller permeability can increase overall upstream fracture pressure and opening the patterns of fluid flow become more complicated if fractures intersect each other a portion of injected fluid is lost into closed empty fractures that cut across the main hydraulic fracture and this delays the pressure increases required for fracture growth past the crosscutting fracture the nonlinear fluid loss rate depends on the geometric complexities of the fracture sets and on the fluid viscosity sometimes fracture growth can be accelerated by the fast fluid transport along an intersected relatively conductive natural fracture

growth model for large branched three dimensional hydraulic crack system in gas or oil shale recent analysis of gas outflow histories at wellheads shows that the hydraulic crack spacing must be of the order of 0 1m rather than 1m or 10m consequently the existing models limited to one

ambient noise tomography of north tibet limits geological terrane signature to upper middle crust 1 we use ambient noise tomography to map regional differences in crustal rayleigh wave group velocities with periods of 8 40 s across north tibet using the international deep profiling of tibet and the himalaya phase iv arrays 132 stations deployed for 10 24 months for periods of 8 24 s sensitive to midcrustal depths of 5 30 km we observe striking velocity changes across the bangong nujiang and jinsha suture zones as well as the kunlun qaidam boundary from south to north we see higher velocities beneath the lhasa terrane lower velocities beneath the qiangtang higher velocities in the songpan ganzi and kunlun terranes and the lowest velocities beneath the qaidam basin maps at periods of 34 and 40 s sensitive to the middle and lower crust at depths of 30 60 km do not show evidence of changes across those boundaries any differences between the tibetan terrane lower crusts that were present at accretion have been erased or displaced by cenozoic processes and replaced almost ubiquitously by uniformly low velocities

indepth iii seismic data from surface observations to deep crustal processes in tibet 1 during the summer of 1998 active source seismic data were collected along a transect running 400 km nnw sse across the central tibetan plateau as the third phase of project indepth international deep profiling of tibet and the himalaya the transect extends northward from the central lhasa block across the jurassic bangong nujiang suture bns at 89 5 e to the central qiangtang block a seismic velocity model for the transect to 25 km depth produced by inversion of p wave first arrivals on 3000 traces shows 1 a 50 km wide region of low velocity at least 5 less than surrounding velocities extending to the base of the model at the bns 2 sedimentary cover for the southern qiangtang block that is 3 5 km thick 3 a distinct interface between sedimentary cover and qiangtang basement or underplated jurassic melange in the central qiangtang block and 4 evidence that the bangoin granite extends to a depth of at least 15 km the bns has little geophysical signature and appears unrelated to the 5 km northward shallowing of the moho which is associated with the bns in central tibet geophysical data along the main indepth iii transect show little evidence for widespread crustal fluids in contrast to the seismic bright spots found in southern tibet and to magnetotelluric evidence of fluid accumulations in eastern tibet a comparison between the global average and tibetan velocity depth functions offers constraints for models of plateau uplift and crustal thickening taken together with the weak geophysical signature of the bns these velocity depth functions suggest that convergence has been accommodated largely through pure shear thickening accompanied by removal of lower crustal material by lateral escape likely via ductile flow although we cannot resolve the details we believe lateral lower crustal flow has overprinted or destroyed evidence in the deep crust for the earlier assembly of tibet as a series of accreted terranes

deep structure of southern tibet inferred from the dispersion of rayleigh waves through a long period seismic network our understanding of the tectonic framework for the formation of tibet and the collision of india with eurasia1 depends on our knowledge of the regional structure of the crust and upper mantle such basic information can be derived from seismic studies but lateral inhomogeneities cause problems with interpretation of external observations owing to the lack of stations inside a hardly accessible landmass in 1982 as part of the french chinese cooperation programme a network of long period seismic stations was installed for the first time in southern tibet itself with the object of obtaining local pure path seismic data previously with the exception of ref 2 pure path tibet dispersion curves were extracted from mixed path observations3 9 which required assumptions about neighbouring regions and focal mechanisms in this study local rayleigh phase velocity dispersions from the network were inverted to yield s wave velocity models as the period range of the data extends beyond 120 s the information so obtained constrains deep seated structures down to 200 km the crust is shown to be very thick 65 70 km in accordance with previous estimates2 9 and is divided into an upper part with a velocity of 3 2 km s 1 and a lower part with a steep velocity gradient the underlying mantle has a high velocity lid vs 4 7 km s 1 beneath which the structure resembles that of western europe with a relatively weak velocity minimum compared with active tectonic regions of 4 36 km s 1 at a depth of 150 km

stress dependent permeability of a de mineralised fracture in shale abstract the paper presents the results of laboratory direct shear tests on the hydro mechanical behaviour of an extensional fracture in shale the tests were conducted on fracture specimens obtained by mechanically splitting blocks of a naturally fractured shale along the cemented fracture and dissolving the calcite fracture filling with a strong acid the experiments consisted of radial fluid flow tests from a single fluid source along the fracture surface fracture samples were tested under different levels of effective normal stress and shear displacement the experimental results show significant reduction of fracture permeability during increasing contact normal stress across the fracture and after shearing of the fracture under constant high normal stress however the tested fractures never completely closed even under normal stresses close to or higher than the unconfined compressive strength of the intact shale the fracture permeability remained much higher than the intact shale or matrix permeability even when the fracture surface has undergone some gouge formation from local asperity failure during shearing the results indicate that fractures once created are difficult to close by mechanical loading in tight formations like shales fractures will always be conduits for fluid flow unless closed by cementation currently it is widely assumed that fractures in sedimentary basins stay open only when there is an overpressure which could keep the fractures hydraulically parted however the results presented here indicate that given a sufficient hydraulic gradient fluid can still flow along a fracture even in the absence of large overpressures

effects of rock mineralogy and pore structure on stress dependent permeability of shale samples we conducted pulse decay permeability experiments on utica and permian shale samples to investigate the effect of rock mineralogy and pore structure on the transport mechanisms using a non adsorbin

crustal and upper mantle structure of northern tibet imaged with magnetotelluric data 1 magnetotelluric mt data were collected in northern tibet along the amdo to golmud highway during the 1995 and 1999 project indepth international deep profiling of tibet and the himalaya surveys broadband and long period mt data were collected and the te mode tm mode and vertical magnetic field data were inverted to yield a minimum structure two dimensional resistivity model the model obtained from inverting all responses simultaneously shows that a pervasive midcrustal conductor extends from the kunlun shan to the bangong nuijiang suture the vertically integrated conductivity conductance of this crustal layer is greatest in the northern qiangtang terrane at latitude 34 n the electrical resistivity of the upper mantle is constrained by the mt data to be in the range of 10 30 m across the songpan ganze and qiangtang terranes this is lower than would be expected if asian lithosphere underthrusts northern tibet as far as the qiangtang terrane the mt responses are more consistent with a model in which asian lithosphere extends as far south as the kunlun shan and the upper mantle beneath the songpan ganze and qiangtang terranes is sufficiently hot to contain a small fraction of interconnected partial melt

a basal stress parameterization for modeling landfast ice current large scale sea ice models represent very crudely or are unable to simulate the formation maintenance and decay of coastal landfast ice we present a simple landfast ice parameterization representing the effect of grounded ice keels this parameterization is based on bathymetry data and the mean ice thickness in a grid cell it is easy to implement and can be used for two thickness and multithickness category models two free parameters are used to determine the critical thickness required for large ice keels to reach the bottom and to calculate the basal stress associated with the weight of the ridge above hydrostatic balance a sensitivity study was conducted and demonstrates that the parameter associated with the critical thickness has the largest influence on the simulated landfast ice area a 6 year 2001 2007 simulation with a 20 km resolution sea ice model was performed the simulated landfast ice areas for regions off the coast of siberia and for the beaufort sea were calculated and compared with data from the national ice center with optimal parameters the basal stress parameterization leads to a slightly shorter landfast ice season but overall provides a realistic seasonal cycle of the landfast ice area in the east siberian laptev and beaufort seas however in the kara sea where ice arches between islands are key to the stability of the landfast ice the parameterization consistently leads to an underestimation of the landfast area

attenuation tomography of the western united states from ambient seismic noise 1 we show that the spatial coherency of the ambient seismic field can be used for attenuation tomography in the western united states we evaluate the real portion of the spatial coherency with an elastic geometric spreading term a bessel function and a distance dependent decay an attenuation coefficient in order to invert the spatial coherency a weight stack inversion technique is applied we recover phase velocity and attenuation coefficient maps at periods of 8 32s which correspond to the elastic and anelastic structure at crustal and upper mantle depths the phase velocity maps obtained by this method are of similar resolution to more standard two station methods the attenuation results provide an important complement to the information extracted from earthquake based tomography several geological features are easily identifiable in the attenuation coefficient maps such as the highly attenuating sedimentary basins along the west coast of the united states and the highly attenuating yellowstone region and the boundaries of the snake river plains

equations for determining the gas and brine volumes in sea ice samples on a etabli des equations utilisables pour determiner le taux de gaz inclus dans la glace de mer a partir de mesures de densite de salinite et de temperature dans des gammes de temperatures entre 2 c et 30 c inversement ces relations peuvent etre utilisees pour calculer la densite de la glace de mer en fonction de la temperature et de la salinite en prenant en compte a la fois la presence de gaz et de cristaux de sel dans la glace on donne aussi des equations qui permettent le calcul du volume de gaz et d eau salee dans la glace a des temperatures autres que celle pour laquelle la densite a ete determinee

injection of tibetan crust beneath the south qaidam basin evidence from indepth iv wide angle seismic data 1 the international deep profiling of tibet and the himalaya phase iv indepth iv active source seismic profile in northeast tibet extends 270 km roughly north south across the songpan ganzi terrane the predominantly strike slip north kunlun fault along the kunlun suture the east kunlun mountains and the south qaidam basin refraction reflection and gravity modeling provide constraints on the velocity and density structure down to the moho the central qaidam basin resembles average continental crust whereas the songpan ganzi terrane and east kunlun mountains exhibit thickened lower velocity crust also characteristic of southern tibet the crustal thickness changes from 70 km beneath the songpan ganzi terrane and east kunlun mountains to 50 km beneath the qaidam basin this jump in crustal thickness is located 100 km north of the north kunlun fault and 45 km north of the southern kunlun qaidam boundary farther north than previously suggested ruling out a moho step caused by a crustal penetrating north kunlun fault the qaidam moho is underlain by crustal velocity material 6 8 7 1 km s for 45 km near the crustal thickness transition the southernmost 10 km of the qaidam moho are underlain by a 70 km reflector that continues to the south as the tibetan moho 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 stronger qaidam crust

a model for the consolidation of rafted sea ice rafting is one of the important deformation mechanisms of sea ice this process is widespread in the north caspian sea where multiple rafting produces thick sea ice features which are a hazard to offshore operations here we present a one dimensional thermal consolidation model for rafted sea ice we consider the consolidation between the layers of both a two layer and a three layer section of rafted sea ice the rafted ice is assumed to be composed of layers of sea ice of equal thickness separated by thin layers of ocean water results show that the thickness of the liquid layer reduced asymptotically with time such that there always remained a thin saline liquid layer we propose that when the liquid layer is equal to the surface roughness the adjacent layers can be considered consolidated using parameters representative of the north caspian the arctic and the antarctic our results show that for a choice of standard parameters it took under 15 h for two layers of rafted sea ice to consolidate sensitivity studies showed that the consolidation model is highly sensitive to the initial thickness of the liquid layer the fraction of salt release during freezing and the height of the surface asperities we believe that further investigation of these parameters is needed before any concrete conclusions can be drawn about rate of consolidation of rafted sea ice features

penetration of crustal melt beyond the kunlun fault into northern tibet the transition between the weak lithosphere of the tibetan plateau and the surrounding rigid crustal blocks has a key role in the ongoing collision between india and asia a reanalysis of existing magnetotelluric data suggests that crustal melt penetrates north from the tibetan plateau beyond the kunlun fault and weakens the crust beneath the kunlun shan

rayleigh wave tomography of china and adjacent regions 1 this paper presents a tomographic study on the s wave velocity structure of china and adjacent regions group velocity dispersions of fundamental rayleigh waves along more than 4000 paths were determined with frequency time analysis the study region was divided into a 1 1 grid and velocities in between grid nodes were calculated by bilinear interpolation the occam s inversion scheme was adopted to invert for group velocity distributions this method is robust and allows us to use a fine grid in model parameterization and thus helps to restore a more realistic velocity pattern checkerboard tests were carried out and the lateral resolution was estimated to be 4 6 in china and its eastern continental shelves the resulting group velocity maps from 10 to 184 s showed good correlation with known geological and tectonic features the pure path dispersion curves at each node were inverted for shear wave velocity structures the three dimensional velocity model indicates thick lithospheres in the yangtze and tarim platforms and hot upper mantles in baikal and western mongolia coastal area and continental shelves of eastern china and indochina and south china sea regions the tibetan plateau has a very thick crust with a low velocity zone in its middle beneath the crust a north dipping high velocity zone mimicking a subducting plate reaches to 200 km in depth and reaches to the kunlun mountains northward in northern tibet a low velocity zone immediately below the moho extends eastward then turns southward along the eastern edge of the plateau until it connects to the vast low velocity area in indochina and the south china sea

partial melt in the upper middle crust of the northwest himalaya revealed by rayleigh wave dispersion article i nfo seismic shear wave velocities are sensitive to the partial melts that should be present in the himalayan orogen if low viscosity channel flow is active at the present day we analyzed regional earthquakes in the western himalaya and tibet recorded on 16 broadband seismometers deployed across the nw indian himalaya from the indian platform to the karakoram range we used a multiple filter technique to calculate the group velocity dispersion of fundamental mode rayleigh waves and then inverted the dispersion records to obtain separate one dimensional shear wave velocity models for five geologic provinces the tibetan plateau ladakh arc complex indus tsangpo suture zone tethyan himalaya and himalayan thrust belt our velocity models show a low velocity layer lvl with 7 17 velocity reduction centered at 30 km depth and apparently continuous from the tethyan himalaya to the tibetan plateau this lvl shows good spatial correspondence with observations of low resistivity from magnetotelluric studies along the same profile of the possible explanations for low velocity and low resistivity in the mid crust only the presence of melts or aqueous fluids or both satisfactorily explains both sets of observations elevated heat flow observed in the nw himalaya implies that if aqueous fluids are present in the mid crust then the mid crust is well above its solidus comparison of our results with laboratory measurements and theoretical models suggests 3 7 melt is present in a channel in the upper middle crust of the nw himalaya at the present day and the physical conditions to enable active channel flow may be present

introduction energy and the subsurface this theme issue covers topics at the forefront of scientific research on energy and the subsurface ranging from carbon dioxide co2 sequestration to the recovery of unconventional shale oil and gas resources through hydraulic fracturing as such the goal of this theme issue is to have an impact on the scientific community broadly by providing a self contained collection of articles contributing to and reviewing the state of the art of the field this collection of articles could be used for example to set the next generation of research directions while also being useful as a self study guide for those interested in entering the field review articles are included on the topics of hydraulic fracturing as a multiscale problem numerical modelling of hydraulic fracture propagation the role of computational sciences in the upstream oil and gas industry and chemohydrodynamic patterns in porous media complementing the reviews is a set of original research papers covering growth models for branched hydraulic crack systems fluid driven crack propagation in elastic matrices elastic and inelastic deformation of fluid saturated rock reaction front propagation in fracture matrices the effects of rock mineralogy and pore structure on stress dependent permeability of shales topographic viscous fingering and plume dynamics in porous media convection this article is part of the themed issue energy and the subsurface

regional earthquakes in northern tibetan plateau implications for lithospheric strength in tibet 1 a total of 400 regional earthquakes were located in northern tibetan plateau from data recorded by indepth iv and pku eastern kunlun arrays from may 2007 to june 2009 the distribution of these earthquakes is compatible with a continuously deforming tibetan lithosphere most earthquakes occur at a depth range of 0 15 km but no event is deeper than 30 km this observation strongly supports the existence of a hot and weak lower crust beneath the northern tibet the crustal seismogenic zone appears slightly thicker beneath the northern tibet than in the southern plateau possibly reflecting a difference in the rheological dry vs wet structure of the crust the absence of lower crustal and uppermost mantle earthquakes in northern tibet is consistent with a localized asthenospheric upwelling under the qiangtang and songpan ganze terranes finally the lack of mantle earthquakes should be fully addressed in any models of subduction in northern tibet

tectonic evolution of the early mesozoic blueschist bearing qiangtang metamorphic belt central tibet this is the published version copyright 2003 american geophysical union all rights reserved

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

importance of rock properties on the producibility of gas shales abstract as the development of unconventional gas resources has progressed the heterogeneity and complexity of shales as gas and oil reservoirs have become apparent the production histories from shales both within a sequence of interbedded strata and from adjacent wells commonly exhibit inexplicable variations and predictions from numerical modeling are rarely accurate as a result of the variability in the reservoir and rock parameters of gas shales the complex interaction between the shale properties and the producibility of the reservoir is seldom apparent one of the most difficult parameters to quantify is the fabric this study compares the relative importance of the fabric parameters of gas shales on their producibility using a commercial numerical simulator and field and laboratory determined rock properties the fabric parameters include the stress dependent fracture permeability which controls the gas transport through the fracture network as well as the effective fracture spacing which controls the path length for gas transport through the matrix and the stress dependent matrix permeability which controls the gas transport through the matrix the results of the numerical simulations show that for a wide range of stress dependent fracture permeabilities stress dependent matrix permeabilities and fracture spacings the productivity of a gas shale reservoir is limited by inefficient gas transport through the matrix the matrix permeability below which gas production is subeconomic is not a specific value but varies with the effective fracture spacing and with fracture permeability the matrix permeability and effective fracture spacing have a greater impact on the producibility of strata with larger fracture permeabilities the influence of the effective fracture spacing on production is greater than the influence of the matrix permeability the lower production associated with a large fracture spacing or a small matrix permeability can be offset by a large matrix permeability or a small fracture spacing the production simulations also show the strong dependence on the geomechanical properties of the rock which affect how the gas transport through the matrix and fractures changes with stress the influence of the geomechanical properties on the producibility depends on whether the production is limited by the gas transport through the matrix when the fabric parameters result in a matrix independent production small fracture spacing large matrix permeability small fracture permeability the production is solely controlled by the stress dependent fracture permeability with larger initial fracture permeability larger young s modulus and larger poisson s ratio resulting in higher production in this case young s modulus is much more influential than the poisson s ratio when the fabric parameters result in a matrix limited production the rock mechanics parameter which relates the exponential decline of matrix permeability with effective stress has the strongest influence on the producibility the influence of poisson s ratio on producibility not only varies with the fabric parameters but also with the young s modulus and when the production is matrix limited a smaller poisson s ratio results in a higher production for all cases except when both and young s modulus are small

natural fractures in the barnett shale and their importance for hydraulic fracture treatments gas production from the barnett shale relies on hydraulic fracture stimulation natural opening mode fractures reactivate during stimulation and enhance efficiency by widening the treatment zone knowledge of both the present day maximum horizontal stress which controls the direction of hydraulic fracture propagation and the geometry of the natural fracture system which we discuss here is therefore necessary for effective hydraulic fracture treatment design we characterized natural fractures in four barnett shale cores in terms of orientation size and sealing properties we measured a mechanical rock property the subcritical crack index which governs fracture pattern development natural fractures are common narrow 0 05 mm 0 002 in sealed with calcite and present in en echelon arrays individual fractures have high length width aspect ratios 1000 1 they are steep 75 and the dominant trend is west northwest other sets trend north south the narrow fractures are sealed and cannot contribute to reservoir storage or enhance permeability but the population may follow a power law size distribution where the largest fractures are open the subcritical crack index for the barnett shale is high indicating fracture clustering and we suggest that large open fractures exist in clusters spaced several hundred feet apart these fracture clusters may enhance permeability locally but they may be problematic for hydraulic fracture treatments the smaller sealed fractures act as planes of weakness and reactivate during hydraulic fracture treatments because the maximum horizontal stress trends northeast southwest and is nearly normal to the dominant natural fractures reactivation widens the treatment zone along multiple strands

rayleigh wave phase velocity maps of tibet and the surrounding regions from ambient seismic noise tomography ambient noise tomography is applied to the significant data resources now available across tibet and surrounding regions to produce rayleigh wave phase speed maps at periods between 6 and 50 s data resources include the permanent federation of digital seismographic networks five temporary u s program for array seismic studies of the continental lithosphere passcal experiments in and around tibet and chinese provincial networks surrounding tibet from 2003 to 2009 totaling 600 stations and 150 000 interstation paths with such a heterogeneous data set data quality control is of utmost importance we apply conservative data quality control criteria to accept between 5000 and 45 000 measurements as a function of period which produce a lateral resolution between 100 and 200 km across most of the tibetan plateau and adjacent regions to the east misfits to the accepted measurements among passcal stations and among chinese stations are similar with a standard deviation of 1 7 s which indicates that the final dispersion measurements from chinese and passcal stations are of similar quality phase velocities across the tibetan plateau are lower on average than those in the surrounding nonbasin regions phase velocities in northern tibet are lower than those in southern tibet perhaps implying different spatial and temporal variations in the way the high elevations of the plateau are created and maintained at short periods 20 s very high velocities are imaged in the tarim basin the ordos block and the sichuan basin these phase velocity dispersion maps provide information needed to construct a 3 d shear velocity model of the crust across the tibetan plateau and surrounding regions

understanding hydraulic fracturing a multi scale problem despite the impact that hydraulic fracturing has had on the energy sector the physical mechanisms that control its efficiency and environmental impacts remain poorly understood in part because the length scales involved range from nanometres to kilometres we characterize flow and transport in shale formations across and between these scales using integrated computational theoretical and experimental efforts methods at the field scale we use discrete fracture network modelling to simulate production of a hydraulically fractured well from a fracture network that is based on the site characterization of a shale gas reservoir at the core scale we use triaxial fracture experiments and a finite discrete element model to study dynamic fracture crack propagation in low permeability shale we use lattice boltzmann pore scale simulations and microfluidic experiments in both synthetic and shale rock micromodels to study pore scale flow and transport phenomena including multi phase flow and fluids mixing a mechanistic description and integration of these multiple scales is required for accurate predictions of production and the eventual optimization of hydrocarbon extraction from unconventional reservoirs finally we discuss the potential of co2 as an alternative working fluid both in fracturing and re stimulating activities beyond its environmental advantages this article is part of the themed issue energy and the subsurface

assessment of crack initiation and propagation in rock from explosion induced stress waves and gas expansion by cross hole seismometry and fem dem method abstract single hole blast induced damage in a granitic outcrop has been assessed through both controlled experiments and numerical simulations with a combined finite discrete element method fem dem damage tomographies from decoupled short and long explosive charges in flooded boreholes were obtained through a high resolution cross hole system by measuring pre and post blast seismic velocities around the blastholes damage was assessed through crack density which was calculated by inversion of p wave velocity measurements through an effective medium theory emt method the resulting damage was found to be highly asymmetrical around each blast hole both along the vertical and horizontal planes despite the apparent isotropy and homogeneity of the granitic rock mass experimental results combined with numerical simulations carried out to assess damage from stress waves alone showed that most damage from experiments was caused by the expansion of gases while its magnitude and extension were strongly dependent on confining conditions along the blasthole the difficulty of quantifying the relative contribution of stress waves and gas expansion and the problems inherent in prediction of blast induced damage are described

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

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

hydraulic fracturing fluid migration in the subsurface a review and expanded modeling results understanding the transport of hydraulic fracturing hf fluid that is injected into the deep subsurface for shale gas extraction is important to ensure that shallow drinking water aquifers are not contaminated topographically driven flow overpressured shale reservoirs permeable pathways such as faults or leaky wellbores the increased formation pressure due to hf fluid injection and the density contrast of the hf fluid to the surrounding brine can encourage upward hf fluid migration in contrast the very low shale permeability and capillary imbibition of water into partially saturated shale may sequester much of the hf fluid and well production will remove hf fluid from the subsurface we review the literature on important aspects of hf fluid migration single phase flow and transport simulations are performed to quantify how much hf fluid is removed via the wellbore with flowback and produced water how much reaches overlying aquifers and how much is permanently sequestered by capillary imbibition which is treated as a sink term based on a semianalytical one dimensional solution for two phase flow these simulations include all of the important aspects of hf fluid migration identified in the literature review and are performed in five stages to faithfully represent the typical operation of a hydraulically fractured well no fracturing fluid reaches the aquifer without a permeable pathway in the presence of a permeable pathway 10 times more fracturing fluid reaches the aquifer if well production and capillary imbibition are not included in the model

natural hydraulic cracking numerical model and sensitivity study abstract natural hydrofracturing caused by overpressure plays an important role in geopressure evolution and hydrocarbon migration in petroliferous basins its mechanism is quite well understood in the case of artificial hydraulic fracturing triggered by high pressure fluid injection in a well this is not so for natural hydraulic fracturing which is assumed to initiate as micro cracks with large influence on the permeability of the medium the mechanism of natural hydraulic cracking triggered by increasing pore pressure during geological periods is studied using a fracturing model coupled to the physical processes occurring during basin evolution in this model the hydraulic cracking threshold is assumed to lie between the classical failure limit and the beginning of dilatancy fluid pressure evolution is calculated iteratively in order to allow dynamic adjustment of permeability so that the fracturing limit is always preserved the increase of permeability is interpreted on the basis of equivalent fractures it is found that fracturing is very efficient to keep a stress level at the rock s hydraulic cracking limit a fracture permeability one order of magnitude larger than the intrinsic permeability of the rock would be enough observations reported from actual basins and model results strongly suggest that natural hydraulic cracking occurs continuously to keep the pressure at the fracturing limit under relaxed stress conditions

receiver function imaging of the lithosphere at the kunlun qaidam boundary northeast tibet abstract we studied the crustal structure and tectonics in the north tibetan plateau from the songpan ganzi terrane to the qaidam basin using teleseismic receiver function imaging across a major lithospheric boundary the kunlun qaidam boundary where previous studies suggest a 15 20 km change in crustal thickness from thicker crust in the kunlun mountains to thinner crust in the qaidam basin we report p receiver functions for 70 stations largely the international deep profiling of tibet and the himalaya indepth phase iv experiment our most dense station coverage is located along the roughly north south indepth iv active source seismic profile at approximately 95 e longitude azimuthal and geographical changes in the receiver functions reveal significant changes in crustal structure and vp vs from across the study area receiver functions show strong converters that we interpret as the moho at 70 km depth beneath the qiangtang songpan ganzi terranes and kunlun mountains and at 50 km depth beneath the central qaidam basin this large change in crustal thickness occurs 50 km north of the north kunlun strike slip fault on which the 2001 m8 1 kunlun earthquake occurred receiver functions for some of the stations north of the thickness change at the kunlun qaidam boundary also show a deeper 70 km bright converter in addition to the 50 km converter the two converters appear to overlap by up to 30 km in some locations along the south qaidam basin we combine previous results with these new results to discuss implications for mechanisms for crustal thickening in the north tibetan plateau including crustal flow and crustal injection at depths imaged here shallower than 100 km we see no evidence of southward subduction of eurasian lithosphere

surface wave array tomography in se tibet from ambient seismic noise and two station analysis ii crustal and upper mantle structure summary we determine the 3 d shear wave speed variations in the crust and upper mantle in the southeastern borderland of the tibetan plateau sw china with data from 25 temporary broad band stations and one permanent station interstation rayleigh wave phase velocity dispersion curves were obtained at periods from 10 to 50 s from empirical green s function egf derived from ambient noise interferometry and from 20 to 150 s from traditional two station ts analysis here we use these measurements to construct phase velocity maps from 10 to 150 s using the average interstation dispersion from the egf and ts methods between 20 and 50 s and estimate from them with the neighbourhood algorithm the 3 d wave speed variations and their uncertainty the crust structure parametrized in three layers can be well resolved with a horizontal resolution about of 100 km or less because of the possible effect of mechanically weak layers on regional deformation of particular interest is the existence and geometry of low shear velocity layers lvls in some regions prominent lvls occur in the middle crust in others they may appear in the lower crust in some cases the lateral transition of shear wave speed coincides with major fault zones the spatial variation in strength and depth of crustal lvls suggests that the 3 d geometry of weak layers is complex and that unhindered crustal flow over large regions may not occur consideration of such complexity may be the key to a better understanding of relative block motion and patterns of seismicity

experimental investigation into the sealing capability of naturally fractured shale caprocks to supercritical carbon dioxide flow geological storage of co2 is considered a solution for reducing the excess co2 released into the atmosphere low permeability caprocks physically trap co2 injected into underlying porous reservoirs injection leads to increasing pore pressure and reduced effective stress increasing the likelihood of exceeding the capillary entry pressure of the caprocks and of caprock fracturing assessing on how the different phases of co2 flow through caprock matrix and fractures is important for assessing co2 storage security fractures are considered to represent preferential flow paths in the caprock for the escape of co2 here we present a new experimental rig which allows 38 mm diameter fractured caprock samples recovered from depths of up to 4 km to be exposed to supercritical co2 scco2 under in situ conditions of pressure temperature and geochemistry in contrast to expectations the results indicate that scco2 will not flow through tight natural caprock fractures even with a differential pressure across the fractured sample in excess of 51 mpa however below the critical point where co2 enters its gas phase the co2 flows readily through the caprock fractures this indicates the possibility of a critical threshold of fracture aperture size which controls co2 flow along the fracture

crustal shear wave velocity structure of northeastern tibet revealed by ambient seismic noise and receiver functions abstract the tibetan plateau is formed by the persistent convergence between the indian and eurasian plates the northeastern tibetan plateau is undergoing young deformation that has been noticed for a long time we conduct a passive source seismic profile with 22 stations in ne tibet in order to investigate the crustal shear wave velocity structure and its relationship with tectonic processes in this paper we obtain the rayleigh wave phase velocity dispersion data among all station pairs within the period bandwidth of 5 20 s from the method of ambient noise cross correlations phase velocity variations correlate well with surface geological boundaries and tectonic features for instance low phase velocity beneath the songpan ganzi block and the guide basin we also compute p wave receiver functions based on the selected teleseismic events with similar ray parameters and perform the joint inversion of surface wave dispersion data and receiver functions to obtain the 2 d crustal shear wave velocity structure along the profile the inversion results show that low shear wave velocities beneath the songpan ganzi block are widespread in the middle to lower crust in together with high crustal vp vs ratios and high temperature suggested by the p wave velocities obtained from the active source seismic study we suggest that the low velocity zone beneath the songpan ganzi block is probably attributed to partial melting across the north kunlun fault there is no crustal lvz found beneath the kunlun block this structural difference may have already existed before the collision of the two blocks or due to limit of the northward extension for the crustal lvz across the north kunlun fault

effect of advective flow in fractures and matrix diffusion on natural gas production although hydraulic fracturing has been used for natural gas production for the past couple of decades there are significant uncertainties about the underlying mechanisms behind the production curves that are seen in the field a discrete fracture network based reservoir scale work flow is used to identify the relative effect of flow of gas in fractures and matrix diffusion on the production curve with realistic three dimensional representations of fracture network geometry and aperture variability simulated production decline curves qualitatively resemble observed production decline curves the high initial peak of the production curve is controlled by advective fracture flow of free gas within the network and is sensitive to the fracture aperture variability matrix diffusion does not significantly affect the production decline curve in the first few years but contributes to production after approximately 10 years these results suggest that the initial flushing of gas filled background fractures combined with highly heterogeneous flow paths to the production well are sufficient to explain observed initial production decline lastly these results also suggest that matrix diffusion may support reduced production over longer time frames

heterogeneity and anisotropy of the lithosphere of se tibet from surface wave array tomography 1 understanding the geotectonic evolution of the southeastern tibetan plateau requires knowledge about the structure of the lithosphere using data from 77 broadband stations in sw china we invert rayleigh wave phase velocity dispersion curves from ambient noise interferometry t 1 0 40 s and teleseismic surface waves t 2 0 150 s for 3 d heterogeneity and azimuthal anisotropy in the lithosphere to 150 km depth our surface wave array tomography reveals 1 deep crustal zones of anomalously low shear wave speed and 2 substantial variations with depth of the pattern of azimuthal anisotropy upper crustal azimuthal anisotropy reveals a curvilinear pattern around the eastern himalayan syntaxis with fast directions generally parallel to the main strike slip faults the mantle pattern of azimuthal anisotropy is different from that in the crust and varies from north to south the tomographically inferred 3 d variation in azimuthal anisotropy helps constrain the source region of shear wave splitting south of 26 n off the high plateau most of the observed splitting can be accounted for by upper mantle anisotropy but for stations on the plateau proper with thick crust crustal anisotropy cannot be ignored on long wavelengths the pattern of azimuthal anisotropy in the crust differs from that in the mantle this is easiest explained if deformation varies with depth the deep crustal zones of low shear wave speed and presumably mechanical strength may represent loci of ductile deformation but their lateral variation suggests that in se tibet localized crustal channel flow and motion along the major strike slip faults are both important

anelastic earth structure from the coherency of the ambient seismic field 1 cross correlation of the ambient seismic field is now routinely used to measure seismic wave travel times however relatively little attention has been paid to other information that could be extracted from these signals in this paper we demonstrate the relationship between the spatial coherency of the ambient field and the elastodynamic green s function in both time and frequency domains through measurement of the frequency domain coherency as a function of distance we sequentially recover phase velocities and attenuation coefficients from these measurements we generate 1 d shear wave velocity and attenuation models for southern california the ambient field measurements of attenuation and the exceptional path coverage that results from the many possible interstation measurements allow us to extend q estimates over a range of frequencies that has previously been difficult to analyze using earthquake data measurements from paths that cross major sedimentary basins show both lower wave speeds and lower quality factors than other paths as expected our results indicate that there is a wealth of information available in the spatial coherency of the ambient seismic field

ambient noise surface wave tomography of the iberian peninsula implications for shallow seismic structure 1 we have obtained high resolution group velocity maps of rayleigh waves at periods from 8 to 25 seconds across the iberian peninsula by cross correlating four months of ambient noise data recorded by 40 permanent broadband stations group velocity maps accurately image the main structural elements of the iberian upper crust including the iberian massif alpine orogens and major sedimentary basins the pyrenees and the iberian chain are imaged as relatively high group velocities in contrast with the betic cordillera which is characterized by low velocities we explain these low velocities in the betics by the presence of widespread intramontane basins created in an extensional episode simultaneous with north south convergence between the african and eurasian plates the most prominent low velocity anomalies in the iberian peninsula are related to the guadalquivir basin the flysch units of the campo de gibraltar and the sediments of the gulf of cadiz

ambient noise rayleigh wave tomography in western sichuan and eastern tibet abstract we perform rayleigh wave tomography in western sichuan and eastern tibet by applying the ambient noise method to broadband data recorded at 49 stations from the sichuan digital seismic network cross correlations of vertical component ambient noise data are computed in one hour segments and stacked over 7 months from june to december 2007 then rayleigh wave group dispersion curves from 6 to 48 s were measured using the multiple filter analysis method group velocity maps at periods from 8 to 44 s were inverted the resulting group velocity maps generally show good correlations with tectonic features the sichuan basin is clearly resolved with lower group velocities at the short periods 8 to 20 s due to thick sedimentary layers and a prominent low velocity zone exhibits in the middle crust beneath the eastern margin of the tibetan plateau the longmen shan thrust belt marks not only the rapid lateral velocity variation between the topographically high tibetan plateau and low elevation sichuan basin but also the transition from the thick crust beneath the tibetan plateau to the normal continental crust beneath the sichuan basin

new constraints on crustal structure and moho topography in central tibet revealed by sinoprobe deep seismic reflection profiling abstract from october 2009 to may 2010 through the support of the sinoprobe project a series of deep seismic reflection profiles were conducted in order to image deep crustal structure of central tibet these profiles extend from the northern lhasa terrane to the qiangtang terrane by crossing the bangong nujiang suture bns the moho depth varies from 75 1 km beneath the northmost lhasa terrane to 68 9 km beneath southmost qiangtang terrane there is a 6 2 km sharp moho offset across the bns within 25 km to the north of the bns the moho rises smoothly to 62 6 km distinct moho reflector lies at 62 6 67 3 km beneath the qiangtang terrane the boundary of the middle and upper crusts and that between the middle and lower crusts interpreted from indepth velocity model correspond to the strong reflection horizons at about18 8 km and 31 3 km depth beneath south qiangtang terrane respectively a north dipping series of reflection packages in the mid to lower crust may mark subduction of the lhasa terrane beneath the qiantang terrane the central portion of the reflection profile exhibits an antiformal structure at 14 1 km in the upper crust which corresponds with the blueschist bearing metamorphic belt

crustal structure of northern and southern tibet from surface wave dispersion analysis 1 group and phase velocities of fundamental mode rayleigh waves in the period range of 10 to 70 s are obtained for southern and northern tibet significant variations in crustal velocity structure are found the group velocity minimum for tibet occurs at 33 s and the minimum is 0 12 km s lower for southern tibet than for northern tibet at periods greater than 50 s however group velocities are up to 0 2 km s faster in southern tibet the group and phase velocities are inverted for layered s wave models the dispersion observations in southern tibet can only be fit with a low velocity layer in the middle crust in contrast the velocity models for northern tibet do not require any lowvelocity zone in the crust the s wave velocity of the lower crust of southern tibet is 0 2 km s faster than the lower crust of northern tibet in southern tibet the sub moho velocity increases with a positive gradient that is similar to a shield while there is no velocity gradient beneath northern tibet the high velocity lower crust of southern tibet is consistent with the underthrusting of indian continental lithosphere the most plausible explanation of the mid crustal low velocity zone is the presence of crustal melt resulting from h2o saturated melting of the interplate shear zone between the underthrusting indian crust and overflowing asian crust the lack of a pronounced crustal low velocity zone in northern tibet is an indication of a relatively dry crust the low s wave velocity in the lower crust of northern tibet is interpreted to be due to a combination of compositional differences high temperatures presumably caused by a high mantle heat flux and possibly small amounts of partial melt combined with all available observations in tibet the new surface wave results are consistent with a hot and weak upper mantle beneath northern tibet index terms 7205 seismology continental crust 1242 7218 seismology lithosphere and upper mantle 7255 seismology surface waves and free oscillations 8102 tectonophysics continental contractional orogenic belts keywords tibet crustal velocity structure surface wave rayleigh waves continental collision

late quaternary evolution of the kumkol basin at the northeastern margin of the tibetan plateau revealed by tectonic geomorphology and the analysis of in situ cosmogenic nuclides abstract the tibetan plateau was formed by the collision between the indian and eurasian plates and it continues to grow laterally while keeping its elevation constant the mechanism of the lateral expansion at the northeastern margin of the plateau however is highly debated owing to the scarcity of tectonic research we conducted tectonic geomorphological analyses in the kumkol basin where lateral growth is ongoing this basin encloses a large active anticlinorium the kumkol anticlinorium ka which would provide a quantitative constraint on the lateral growth of the plateau if the tectonically deformed landforms were dated our geomorphological mapping revealed six terrace steps designated t1 to t6 in descending order along the pitileke river in situ cosmogenic radionuclide concentrations in subsurface and surface samples from t1 t2 t3 t4 and t6 showed their formation ages to be 252 24 ka 140 160 ka 103 23 ka 98 11 ka and 0 23 ka respectively thus t1 formed during marine isotope stage mis 8 t2 during mis 6 and t3 and t4 during mis 5 our results are quantitative evidence that degradation aggradation cycles in the basin are correlated with global climatic fluctuations to estimate the geometry of faults contributing to the evolution of the anticlinorium we used a dislocation fault model and the overall surface deformation across the ka the results suggest that the ka has been produced principally by slip on a detachment fault at 15 to 10 km depth which is within or just above the brittle ductile transition zone we then determined the maximum uplift rate to be 1 mm yr and the horizontal shortening rate to be 2 5 3 2 mm yr from the formation ages of the terraces and their deformation these results suggest that there is a large scale detachment fault below a basement involved fold and thrust belt and that the concentration of stress at the plateau margin likely facilitated the development of crustal scale deformation underlying the kumkol basin and this deformation then accelerated the lateral growth of the tibetan plateau

the role of shortwave radiation in the summer decay of a sea ice cover heat balance models of ice leads and the underlying water column were used to investigate the role of shortwave radiation in the summer decay of a sea ice cover because of the potential importance of positive feedback between decreasing ice concentration and increasing solar input to the ocean particular emphasis was given to the treatment of heat transfer and lateral melting in leads the rate of heat loss to floe edges was calculated from lead temperatures using an empirical boundary layer parameterization coefficients for this parameterization were obtained from field measurements in the dynamically active ice cover of the greenland sea during the 1984 marginal ice zone experiment and in the static ice of mould bay northwest territories field coefficients were found to be considerably larger than those previously obtained in the laboratory suggesting that heat transfer rates near floe edges were controlled more by winds and currents than by local density differences model studies of individual leads indicate that heat losses to the atmosphere cause lateral melt rates to asymptotically approach a constant value as lead size increases thus although very wide leads transmit considerable energy to the underlying ocean they have relatively little effect on lateral melting other results provide information on the relative importance of horizontal heat transfer rates ice thickness floe perimeter atmospheric forcing and oceanic heat flux in general the calculations indicate that much of the shortwave radiation entering the ocean is absorbed below the ice where it contributes to bottom ablation rather than lateral melting this suggests that previous work has seriously overestimated the magnitude of the feedback between ice concentration and incident shortwave radiation

the nature and cause of polarization anomalies of surface waves crossing northern and central eurasia summary results of a pilot experiment preliminary to a systematic study of surface wave polarization across eurasia are presented long period fundamental rayleigh and love waves recorded by the broad band seismic stations kiv kislovodsk and obn obninsk of the iris ida network deployed in the former soviet union fsu were analysed from 14 events to the north east east and south east of these stations in search of particle motion anomalies using a technique called frequency time polarization analysis ftpan all anomalies indicate deviations of wave propagation paths to the north relative to the great circle paths polarization anomalies at obn are uniformly small 5 significant frequency dependent polarization anomalies 5 20 are found for rayleigh and love waves arriving at kiv from teleseismic events at a wide range of backazimuths 26 103 polarization measurements are repeatable both for a number of nearly degenerate events and for reciprocal paths and vary smoothly and continuously as epicentral location is moved consequently such measurements are robust and can provide useful structural information the systematics of the frequency dependence of the polarization anomalies as epicentral position is moved from north to south display sensitivity to structures at a number of length scales results from synthetic experiments using coupled normal mode and gaussian beam synthetics reveal that 1 recently constructed long wavelength aspherical models produce polarization anomalies that are significantly smaller 5 at kiv but of the same sign negative as the observed anomalies and 2 the frequency dependence of the observed anomalies must result from a combination of structures of differing wavelengths with scale lengths ranging from regional to global thus polarization measurements provide new information about currently unmodelled structures a model that fits the polarization anomalies observed at kiv includes a regional scale low velocity feature near to kiv in order to fit the large magnitude short period polarization anomalies and a smaller magnitude continent scale increase in upper mantle velocities and or the reduction of crustal thickness from south to north in central eurasia in order to fit the broad band longer period anomalies the small scale low velocity feature is a model of the sedimentary basin of the sub caspian depression we conclude that measurements of polarization anomalies can be obtained accurately that they are reproducible that they contain currently unmodelled information and that they should prove to be useful in combination with velocity and amplitude information in future tomographic inversions especially to help focus global scale models to regional wavelengths

midcrustal low velocity layer beneath the central himalaya and southern tibet revealed by ambient noise array tomography 1 ambient noise tomography has been becoming an important tool to image the shallow lithospheric structure of the earth using 2 months of ambient noise data from 20 stations of the himalayan nepal tibet seismic experiment we investigate the upper and middle crustal structure in the central himalaya and southern tibet about 120 interstation rayleigh wave empirical green s functions with sufficient signal to noise ratio are obtained and used for group velocity dispersion analysis in the period range 6 25 s using frequency time analysis technique the obtained dispersion data are then used to construct 2 d group velocity maps at the short periods from 9 to 15 s the distribution of rayleigh wave velocities delineates several distinct low and high velocity zones separated mainly by geological boundaries the high group velocity zone is located mainly around regions with plutonic rocks and the low group velocity zone is located around regions with sedimentary or metasedimentary rocks finally we invert for the shear velocity structure in the upper and middle crust along a n s trending cross section at the longitude 86 5 e we observe a clear low velocity layer in the middle crust about 10 25 km depth distributed on both sides of the indus yarlung suture zone the existence of this midcrustal low velocity zone suggests a mechanically weaker middle crust beneath the central himalaya and southern tibet which might decouple the upper crustal deformation from that of the lower crust in the tibetan himalayan orogenic processes

characterization of the morphometry of impact craters hosting polar deposits in mercury s north polar region 1 earth based radar images of mercury show radar bright material inside impact craters near the planet s poles a previous study indicated that the polar deposit hosting craters pdcs at mercury s north pole are shallower than craters that lack such deposits we use data acquired by the mercury laser altimeter on the messenger spacecraft during 11 months of orbital observations to revisit the depths of craters at high northern latitudes on mercury we measured the depth and diameter of 537 craters located poleward of 45 n evaluated the slopes of the northern and southern walls of 30 pdcs and assessed the floor roughness of 94 craters including nine pdcs we find that the pdcs appear to have a fresher crater morphology than the non pdcs and that the radar bright material has no detectable influence on crater depths wall slopes or floor roughness the statistical similarity of crater depth diameter relations for the pdc and non pdc populations places an upper limit on the thickness of the radar bright material 170 m for a crater 11 km in diameter that can be refined by future detailed analysis results of the current study are consistent with the view that the radar bright material constitutes a relatively thin layer emplaced preferentially in comparatively young craters

crustal shear s velocity and poisson s ratio structure along the indepth iv profile in northeast tibet as derived from wide angle seismic data summary from the s wave data collected along a 270 km long profile spanning the kunlun mountains in ne tibet 14595 sg phase arrivals and 21 sms phase arrivals were utilized to derive a whole crustal s velocity model and together with a previously derived p velocity model a poisson s ratio model beneath the profile the final tomogram for the upper 10 15 km of the crust reveals the lower velocities associated with the predominantly neogene quaternary sediments of the qaidam basin to the north and the higher velocities associated with the predominantly palaeozoic and mesozoic upper crustal sequences of the songpan ganzi terrane and kunlun mountains to the south this study finds no evidence that the kunlun mountains are involved in large scale northward overriding of the qaidam basin along a shallow south dipping thrust the in the upper 10 15 km of the crust are often lower than 0 25 indicating a preponderance of quartz rich rocks in the upper crust beneath the profile below 10 15 km depth the remainder of the crust down to the moho has an average of 0 24 beneath the songpan ganzi terrane and kunlun mountains and 0 25 below the qaidam basin these low are similar to other low found along other profiles in the northeastern part of the plateau assuming an isotropic situation and no significant variation in between 10 15 km depth and the moho then the lower crust between 25 30 km depth below sea level and the moho with p velocities varying from 6 6 km s 1 at the top to around 6 9 km s 1 at the base and of 0 24 0 25 should comprise intermediate granulites in the upper part transitioning to granulite facies metapelites in the lower part as the pre cenozoic qaidam basin crust has probably not lost any of its lower crust during the present himalayan orogenic cycle in the cenozoic and only has a of 0 245 0 25 then it appears that the pre cenozoic qaidam basin crust involved in the collision is more felsic and thus weaker and more easily deformable than normal continental crust with a global average of 0 265 0 27 and the tarim and sichuan basin crusts this situation then probably facilitates the collision and promotes the formation of new high plateau crust at the ne margin of tibet south of the qaidam basin the crust of the songpan ganzi terrane and kunlun mountains has an even lower average crustal of 0 23 0 24 and is thus presumably even weaker and more easily deformable than the crust beneath the qaidam basin this then supports the hypothesis of karplus et al that the high tibetan plateau may be thickening northward into south qaidam as its weak thickened lower crust is injected beneath stronger qaidam crust

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

influence of fracture scale heterogeneity on the flow properties of three dimensional discrete fracture networks dfn while permeability scaling of fractured media has been so far studied independently at the fracture and network scales we propose a numerical analysis of the combined effect of fracture scale heterogeneities and the network scale topology the analysis is based on 2 106 discrete fracture network dfns simulations performed with highly robust numerical methods fracture local apertures are distributed according to a truncated gaussian law and exhibit self affine spatial correlations up to a cutoff scale lc network structures range widely over sparse and dense systems of short long or widely distributed fracture sizes and display a large variety of fracture interconnections flow bottlenecks and dead ends at the fracture scale accounting for aperture heterogeneities leads to a reduction of the equivalent fracture transmissivity of up to a factor of 6 as compared to the parallel plate of identical mean aperture at the network scale a significant coupling is observed in most cases between flow heterogeneities at the fracture and at the network scale the upscaling from the fracture to the network scale modifies the impact of fracture roughness on the measured permeability this change can be quantified by the measure a2 which is analogous to the more classical power averaging exponent used with heterogeneous porous media and whose magnitude results from the competition of two effects i the permeability is enhanced by the highly transmissive zones within the fractures that can bridge fracture intersections within a fracture plane ii it is reduced by the closed and low transmissive areas that break up connectivity and flow paths citation de dreuzy j r y meheust and g pichot 2012 influence of fracture scale heterogeneity

seismic anisotropy from sks splitting beneath northeastern tibet the northeastern boundary of the tibetan high plateau is marked by a 2 km topographic drop and a coincident rapid change in crustal thickness surface tectonics are dominated by the kunlun strike slip fault system and adjacent kunlun concealed thrust the main objective of the current study is to map lateral variations of seismic anisotropy parameters in this region along the linear indepth iv array in order to investigate the link between surface and internal deformation in the context of crust and mantle structure to achieve this aim we performed minimum transverse energy based sks splitting measurements using 23 stations of the indepth iv array deployed across the northeastern margin of tibet average fast polarization directions and splitting time delays are obtained by averaging stacked misfit surfaces of all analyzed events at each station the agreement of fast directions with the strikes of major active strike slip faults and strike slip focal mechanisms but not with fossil structures such as the jinsha suture implies that the anisotropy records lithospheric petrofabric formed by recent deformation within the lithosphere rather than representing frozen in anisotropy or shear within the asthenosphere due to absolute plate motion the distribution of large splitting delays throughout the northern plateau suggests that deformation is distributed rather than focused onto narrow shear zones associated with the kunlun strike slip faults the drop in splitting delays toward the qaidam is then a natural consequence of the much lower degree of deformation there

thermodynamics of slush and snow ice formation in the antarctic sea ice zone snow over antarctic sea ice is often flooded by brine or seawater particularly in spring forming slush and snow ice here we evaluate the representation of the thermodynamics of slush and snow ice formation in large scale sea ice models using laboratory experiments nacl solutions poured into grated ice in an isolated container scaling analysis highlights latent heat as the main term of the energy budget the temperature of the new sea ice immediately after flooding is found very close to the saltwater freezing point whereas its bulk salinity is typically 20 g kg large scale sea ice models faithfully represent such physics yet the uncertainty on the origin of flooding saltwater impacts the calculated new ice temperature because of the different salinities of seawater and brine the laboratory experiments also suggest a potential limitation to the existing physical representations of flooding for brine fractions 60 ice crystals start floating upon saltwater natural sea ice observations suggest that the isolated system assumption holds for a few hours at most after which rapid heat and salt exchanges mostly destroy the initial flooding signature on temperature and salinity a small footprint on ice salinity remains however natural snow ice is found 3 5 g kg more saline than other forms of sea ice

consolidation of first year sea ice ridges 1 measurements of spatial and temporal temperature development geometry morphology and physical properties in three first year sea ice ridges at spitsbergen and in the gulf of bothnia have been performed the corresponding thickness and the physical properties of the surrounding level ice were also measured the thickness of the consolidated layer was examined through drilling and temperature measurements the temperatures gave a ratio of the thickness of the consolidated layer to the level ice thickness from 1 39 to 1 61 whereas the drillings indicated a ratio of 1 68 1 85 the measured consolidated layer appeared to be 28 thicker when based on drillings in comparison to temperature thus the result depended on the method of investigation the drillings included a partly consolidated layer however the measured growth of the consolidated layer did not depend on the method of investigation the scatter of the physical properties in the consolidated layer was higher than that of the level ice the consistency of the unconsolidated rubble differed markedly at the two sites it was soft and slushy at spitsbergen and harder in the gulf of bothnia three possible explanations for these differences are discussed surrounding currents different keel shapes and difference in salinity

a synoptic view of the distribution and connectivity of the mid crustal low velocity zone beneath tibet 1 based on 1 2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around tibet rayleigh wave phase velocity maps are constructed from 10 s to 60 s period a 3 d vsv model of the crust and uppermost mantle is derived from these maps the 3 d model exhibits significant apparently inter connected low shear velocity features across most of the tibetan middle crust at depths between 20 and 40 km these low velocity zones lvzs do not conform to surface faults and significantly are most prominent near the periphery of tibet the observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear zones rather than being localized horizontally near the edges of rigid blocks the prominent lvzs are coincident with strong mid crustal radial anisotropy in western and central tibet and probably result at least partially from anisotropic minerals aligned by deformation which mitigates the need to invoke partial melt to explain the observations irrespective of their cause in partial melt or mineral alignment mid crustal lvzs reflect deformation and their amplification near the periphery of tibet provides new information about the mode of deformation across tibet

surface wave tomography of eastern and central tibet from two plane wave inversion rayleigh wave and love wave phase velocity mapssurface wave tomography of eastern and central tibet from two plane wave inversion rayleigh wave and love wave phase velocity maps