volcanic plutonic parity and the differentiation of the continental crust the continental crust is central to the biological and geological history of earth however crustal heterogeneity has prevented a thorough geochemical comparison of its primary igneous building blocks volcanic and plutonic rocks and the processes by which they differentiate to felsic compositions our analysis of a comprehensive global data set of volcanic and plutonic whole rock geochemistry shows that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction zone settings but are divergent in continental rifts offsets in major and trace element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics in both tectonic settings our results indicate that fractional crystallization rather than crustal melting is predominantly responsible for the production of intermediate and felsic magmas emphasizing the role of mafic cumulates as a residue of crustal differentiation a global geochemical data set of volcanic and plutonic rocks indicates that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction zone settings but are divergent in continental rifts brenhin keller and co authors present a global geochemical dataset of the two fundamental building blocks of the continental crust volcanic externally erupted and plutonic internally solidified rocks their results indicate that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction zone settings but divergent in continental rifts offsets in major and trace element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics this work indicates that in both tectonic settings fractional crystallization rather than crustal melting is predominantly responsible for the production of intermediate and felsic magmas

residual mean solutions for the antarctic circumpolar current and its associated overturning circulation abstract residual mean theory is applied to the streamwise averaged antarctic circumpolar current to arrive at a concise description of the processes that set up its stratification and meridional overturning circulation on an f plane simple solutions are found in which transfer by geostrophic eddies colludes with applied winds and buoyancy fluxes to determine the depth and stratification of the thermocline and the pattern of associated residual meridional overturning circulation

rationalizing the spatial distribution of mesoscale eddy diffusivity in terms of mixing length theory observations and theory suggest that lateral mixing by mesoscale ocean eddies only reaches its maximum potential at steering levels surfaces at which the propagation speed of eddies approaches that of the mean flow away from steering levels mixing is strongly suppressed because the mixing length is smaller than the eddy scale thus reducing the mixing rates the suppression is particularly pronounced in strong currents where mesoscale eddies are most energetic here a framework for parameterizing eddy mixing is explored that attempts to capture this suppression an expression of the surface eddy diffusivity proposed by ferrari and nikurashin is evaluated using observations of eddy kinetic energy eddy scale and eddy propagation speed the resulting global maps of eddy diffusivity have a broad correspondence with recent estimates of diffusivity based on the rate at which tracer contours are stretched by altimetric derived surface currents finally the expression for the eddy diffusivity is extrapolated in the vertical to infer the eddy induced meridional heat transport and the overturning streamfunction

new empirical polyaxial criterion for rock strength abstract a new criterion is presented for prediction of intact rock and rock mass failure under polyaxial state of stresses a comprehensive database of the results of uniaxial triaxial and polyaxial tests on intact rock was utilized for evaluation of the new criterion and comparison of its accuracy with the most accurate and frequently used criteria analysis of 195 individual data groups of brittle failure in the form of 3 1 for twelve different rock types showed that the proposed criterion lead to determination coefficients higher than 0 99 in most cases it also gave the lower values of root mean squared errors relative to mohr coulomb and hoek brown criteria in fitting the normalized strength data for each rock type the criteria were used to fit a typical series of triaxial strength data including brittle and ductile behavior it showed that the new criterion can maintain its accuracy over a wider range of stresses in the absence of rock mass strength data applicability of the new criterion for rock mass was verified by fitting it to typical hoek brown failure envelopes regression analysis of the polyaxial strength data in the form of 3 2 1 for six rock types showed that the new criterion predict the strength more accurately than the modified wiebols cook and you criteria in all cases

a 3 d study of the effects of thermomechanical loads on fracture slip in enhanced geothermal reservoirs heat extraction from enhanced geothermal systems egs can greatly affect the behavior of joints and other discontinuities in the reservoir fracture permeability can change in response to fluid injection extraction rock cooling variations of stress field and mineral dissolution precipitation the reduction in effective stress caused by pore pressure increase can cause the slippage of discontinuities thus inducing seismicity studies have shown that thermal stresses generated by cold water injection have a similar effect in order to assess the influence of thermal stresses on fracture opening and slippage a 3 d coupled heat extraction thermal stress elastic displacement discontinuity model is used in this study the effects of each mechanism on fracture slip is estimated with particular reference to the coso geothermal field the results indicate that under typical field conditions a substantial increase in fracture slip is observed when thermal stresses are taken into account the temporal evolution of the thermal stresses suggests that the rock mass deformation will not cease upon stoppage of water injection which can be a cause of delayed seismic activity

isopycnal eddy diffusivities and critical layers in the kuroshio extension from an eddying ocean model abstracthigh spatial resolution isopycnal diffusivities are estimated in the kuroshio extension ke region 28 40 n 120 190 e from a global parallel ocean program pop simulation the numerical float tracks are binned using a clustering approach the number of tracks in each bin is thus roughly the same leading to diffusivity estimates that converge better than those in bins defined by a regular geographic grid cross stream diffusivities are elevated in the southern recirculation gyre region near topographic obstacles and downstream in the ke jet where the flow has weakened along stream diffusivities which are much larger than cross stream diffusivities correlate well with the magnitudes of eddy velocity the ke jet suppresses cross stream mixing only in some longitude ranges this study estimates the critical layer depth both from linear local baroclinic instability analysis and from eddy phase speeds in the pop model using the radon transform the latter is a better predictor of large mixi

transient thermo poroelastic finite element analysis of borehole breakouts abstract a thermo poroelastic model which is fully coupled to conductive and convective transport processes is employed to analyze wellbore stability and borehole breakout using the two dimensional finite element method stress distribution around an open borehole subjected to non isothermal non hydrostatic loading is investigated four strength criteria mohr coulomb drucker prager modified lade and mogi coulomb are utilized to the stability of boreholes borehole breakout propagation is assumed to occur as successive spalling of thin layers of rock caused by stress concentration and redistribution around boreholes and breakouts stabilization of breakouts is examined by deploying mogi coulomb failure criterion for different wellbore fluid pressures and temperatures to evaluate the influence of various prominent factors on borehole breakouts a parametric study is conducted results indicate that mohr coulomb criterion offers the highest breakout depth so it underestimates rock strength and requires the highest value of the least mud weight necessary for wellbore stability and drucker prager provides the lowest estimation of the breakout depth borehole breakout stabilization is concluded to occur as consequences of reduction in effective tangential stress and increase in confinement effect of radial stress around breakout periphery moreover confinement effect of borehole fluid appears to be more influential in borehole stability than pore pressure effect parametric analysis reveals that drilling a borehole with small radii applying overbalanced fluid pressure condition and maintaining drilling fluid temperature lower than rock temperature boost the creation of a favorable borehole stability condition

spatial gap between lhasa and qiangtang blocks inferred from middle jurassic to cretaceous paleomagnetic data abstract we present paleomagnetic results from middle jurassic red sandstones of the sangba formation orientated samples were collected at 14 sites on north side of the basu 30 1 n 96 9 e northeastern border of the lhasa block after stepwise thermal demagnetization high temperature component with unblocking level of 680 c is isolated from 7 sites primary nature of this magnetization is ascertained through positive fold and reversal tests at 95 confidence level combined with previously reported paleomagnetic results tilt corrected mean direction of d 353 4 i 15 1 95 14 1 n 8 is calculated indicating that during middle jurassic the study area was located at 7 7 n 7 4 comparison with the jurassic paleomagnetic data reported from eastern tibet indicates that the lhasa block was located south of the qiangtang block at a latitudinal difference of 31 11 compilation of paleomagnetic data further suggest that until the cretaceous large spatial gap of more than 1200 km remained open between the lhasa block and the qiangtang block of eastern himalayan syntaxis and eventually vanished during tertiary cretaceous paleolatitude corresponding to this gap 13 0 n 31 0 n is almost identical to that reported from shan thai block 25 4 n 5 2 furthermore the permo carboniferous sequences of gondwanic affinity are distributed in western part of the shan thai block as well as in the lhasa and qiangtang blocks keeping in view these similarities we propose a new pre collision india vs asia reconstruction model in which the lhasa shan thai and qiangtang blocks are shown in their respective order from south to north along southern margin of the asian continent the aftermath of gigantic india asia collision however introduced north south compressional regime in the asian continent that eventually played a role in eastward extrusion of the shan thai block from an area between the qiangtang and lhasa blocks

delamination and detachment of a lithospheric root abstract orogens can be formed by shortening of the lithosphere on time scales much shorter than that of thermal diffusion for time scales of a few ma the major part of the uplift of an orogen which may occur during the transition from crustal shortening to extension may be due to delamination of the mantle lithosphere rather than to convective thinning the negative buoyancy forces of a lithospheric root may cause lateral shortening while the lithospheric root delaminates peels off from the crust detaches and is replaced by hot upwelling asthenosphere this provides a large input of heat into the crust geological observations show that the late orogenic phase may be characterized by volcanic activity high temperature low pressure metamorphism extension high heat flow and a rapid within a few ma increase of the topography we present dynamically and thermally consistent numerical models of the evolution of an orogenic root mantle rheology is based on laboratory data of olivine while models with the rheology of olivine do not allow for delamination due to its very high viscosity the incorporation of a quasi brittle rheology is necessary for modelling delamination of the mantle lithosphere successful delamination models are always associated with a rapid change in topography we examined the parameter range of our models and found different regimes of behaviour of the orogenic root of the mantle lithosphere no delamination delamination delamination and full detachment for a fixed viscosity of the upper crust of 1023 pa s delamination occurs for a range of viscosities of 1023 pa s to 1020 pa s for the lower crust depending on the thickness of the lithospheric root but full detachment of the delaminated lithospheric slab only occurs if the viscosity of the lower crust is greater than approximately 1021 pa s hot asthenosphere is welling up into the gap opening during delamination further implications of our models are lithospheric roots or unsupported slabs of at least 100 170 km depth extent are needed to provide sufficient negative buoyancy to allow delamination and detachment in all delamination models significant amounts of lower crustal material are subducted into the mantle in applying our models to selected orogens we argue that mantle delamination may have played an important role in the variscan and the himalayan orogenies

the dependence of southern ocean meridional overturning on wind stress an eddy resolving numerical model of a zonal flow meant to resemble the antarctic circumpolar current is described andanalyzedusing the framework of j marshallandt radko in additionto windand buoyancy forcing at the surface the model contains a sponge layer at the northern boundary that permits a residual meridional overturning circulation moc to exist at depth the strength of the residual moc is diagnosed for different strengths of surface wind stress it is found that the eddy circulation largely compensates for the changes in ekman circulation the extent of the compensation and thus the sensitivity of the moc to the winds depend on the surface boundary condition a fixed heat flux surface boundary severely limits the ability of the moc to change an interactive heat flux leads to greater sensitivity to explain the moc sensitivity to the wind strength under the interactive heat flux transformed eulerian mean theory is applied in which the eddy diffusivity plays a central role in determining the eddy response a scaling theory for the eddy diffusivity based on the mechanical energy balance is developed and tested the average magnitude of the diffusivity is found to be proportional to the square root of the wind stress the moc sensitivity to the winds based on this scaling is compared with the true sensitivity diagnosed from the experiments

enhancement of mesoscale eddy stirring at steering levels in the southern ocean abstract meridional cross sections of effective diffusivity in the southern ocean are presented and discussed the effective diffusivity keff characterizes the rate at which mesoscale eddies stir properties on interior isopycnal surfaces and laterally at the sea surface the distributions are obtained by monitoring the rate at which eddies stir an idealized tracer whose initial distribution varies monotonically across the antarctic circumpolar current acc in the absence of observed maps of eddying currents in the interior ocean the advecting velocity field is taken from an eddy permitting state estimate of the southern ocean sose a three dimensional advection diffusion equation is solved and the diffusivity diagnosed by applying the nakamura technique on both horizontal and isopycnal surfaces the resulting meridional sections of keff reveal intensified isopycnal eddy stirring reaching values of 2000 m2 s 1 in a layer deep beneath the acc but rising toward the surface on the equatorward flank

eddy transport and mixing in a wind and buoyancy driven jet on the sphere abstract the baroclinically unstable wind and buoyancy driven flow in a zonally reentrant pie shaped sector on a sphere is numerically modeled and then analyzed using the transformed eulerian mean tem formalism mean fields are obtained by zonal and time averaging performed at fixed height the very large latitudinal extent of the basin 50 7 s latitude to the equator allows the latitude variation of the coriolis parameter to strongly influence the flow persistent zonal jets are observed in the statistically steady state reynolds stress terms play an important role in redistributing zonal angular momentum convergence of the lateral momentum flux gives rise to a strong eastward jet with an adjacent westward jet equatorward and weaker multiple jets poleward an equally prominent feature of the flow is a strong and persistent eddy that has the structure of a kelvin cat s eye and generally occupies the zonal width of the basin at latitudes 15 10 s a strongly mixed surface diabatic zone overlies the

early cretaceous adakitic granites in the northern dabie complex central china implications for partial melting and delamination of thickened lower crust abstract to date few adakitic rocks have been reported in direct association with contemporary intra continental extensional structures which has cast doubt on genetic models involving partial melting of the lower crust this study presents early cretaceous 143 129 ma new sensitive high resolution ion microprobe shrimp zircon u pb ages adakitic granites which are directly associated with a contemporary metamorphic core complex i e the northern dabie complex in the dabie area these granites exhibit relatively high sr contents negligible to positive eu and sr anomalies high la yb and sr y ratios but very low yb and y contents similar to subducted oceanic crust derived adakites they are also characterized however by very low mgo or mg and ni values and nd sr isotope compositions end t 14 6 to 19 4 and 87sr 86sr i 0 7067 0 7087 similar to triassic continent derived eclogites subducted in the dabie sulu orogen additionally late granitic dikes in the adakitic intrusions exhibit low sr contents clearly negative eu and sr anomalies low la yb and sr y ratios but relatively high yb and y contents similar to 118 105 ma granites in the northern dabie complex based on composition and geochronology data of neoproterozoic amphibolites and orthogneisses triassic high to ultra high pressure metamorphic rocks and early cretaceous mafic ultramafic intrusive rocks and the constraints provided by experimental melt data for tonalites metabasaltic rocks and eclogites we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile bearing eclogitic lower crust as a consequence of triassic middle jurassic subduction and thrusting the late dikes probably originated from plagioclase bearing intermediate granulites moreover we suggest that late mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of early cretaceous mafic ultramafic intrusive rocks in the dabie area and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite asthenospheric upwelling in response to post collisional delamination of lithospheric mantle was likely to have provided the heat source for the cretaceous magmatism

late cretaceous high mg granitoids in southern tibet implications for the early crustal thickening and tectonic evolution of the tibetan plateau abstract this study presents new major and trace element plus sr nd and zircon u pb isotope data for the zhongcang granitic plutons which are located to the south of the yongzhu asuo ophiolite belt within the northwestern part of the central lhasa subterrane tibetan plateau these data provide new insights into the late cretaceous tectonic evolution of southern tibet the zhongcang plutons are dominated by granodiorites and granites that yield zircon u pb emplacement ages of 94 88 ma they can be further divided into metaluminous and peraluminous subtypes the metaluminous rocks have adakite like geochemical signatures including high sio 2 al 2 o 3 and sr concentrations and low yb and y concentrations and high sr y and la yb n ratios these rocks also have negative end t values 3 17 to 0 17 variable initial 87 sr 86 sr i ratios 0 705927 0 707668 and high k 2 o and th concentrations suggesting that they were not derived from the partial melting of subducted oceanic crust in an arc setting the zhongcang adakitic rocks have higher mgo and cr concentrations and mg values than do contemporaneous intrusive rocks derived from a region of thickened lower crust within the central lhasa subterrane these data suggest that the zhongcang adakitic rocks were generated by the partial melting of a delaminated thickened lower crust within a late cretaceous continental setting in comparison with the zhongcang adakitic rocks the peraluminous rocks have significant negative eu and sr anomalies and lower end t values 4 06 to 6 64 this combined with their high mg values and cr concentrations suggests that the peraluminous units formed from primitive magmas similar to those that formed the zhongcang adakitic rocks but modified by contamination with ancient crustal material and by fractional crystallization of plagioclase and apatite during uprising and or emplacement the zhongcang high mg granitoids provide robust evidence for late cretaceous crustal thickening prior to india asia collision

borehole stability analysis in a thermoporoelastic dual porosity medium the problem of diffusion and mass transfer in dual porous media is considered in a threephase framework the solid phase is assumed to contain two distinct cavities filled with fluid the porous mixture is composed of two overlapping media the porous blocks and the fissure network the fluid can transfer between the cavities due to fluid pressure difference in addition hydraulic and thermal diffusions take place through the mixture a global understanding of mass transfer diffusion and deformation is provided the governing equations associated with these phenomena are presented for a mixture in thermal equilibrium the finite element approximation of the governing equations is formulated and applied to the stability analysis of a vertical borehole a parametric analysis is carried out to evaluate the influence of mass transfer on the pressure profiles of the fluids around the borehole permeable and a semipermeable boundary conditions are compared to predict the potential for failure of the wellbore under drained and partially undrained conditions

parameterization of mixed layer and deep ocean mesoscales including nonlinearity abstractin 2011 chelton et al carried out a comprehensive census of mesoscales using altimetry data and reached the following conclusions essentially all of the observed mesoscale features are nonlinear and mesoscales do not move with the mean velocity but with their own drift velocity which is the most germane of all the nonlinear metrics accounting for these results in a mesoscale parameterization presents conceptual and practical challenges since linear analysis is no longer usable and one needs a model of nonlinearity a mesoscale parameterization is presented that has the following features 1 it is based on the solutions of the nonlinear mesoscale dynamical equations 2 it describes arbitrary tracers 3 it includes adiabatic a and diabatic d regimes 4 the eddy induced velocity is the sum of a gent and mcwilliams gm term plus a new term representing the difference between drift and mean velocities 5 the new term lowers the transfer of mean potential energy to mesoscales 6 th

transverse drilling induced tensile fractures in the west tuna area gippsland basin australia implications for the in situ stress regime abstract drilling induced tensile fractures ditfs have been interpreted on image logs from vertical wells in the gippsland basin offshore southeastern australia interpreted axial vertical ditfs have previously been well described worldwide we also interpret transverse horizontal ditfs which are horizontal fractures that are electrically conductive non planar bimodal and constrained to the tensile region of the wellbore elasticity theory predicts formation of both transverse and axial drilling induced tensile fractures ditfs in vertical wells depending on the magnitude of the principal in situ stresses pore pressure and mudweight drilling induced tensile fractures initiate in very specific stress environments axial ditfs can closely constrain a lower bound to the maximum horizontal stress s h max magnitude where the minimum horizontal s h min stress is known if transverse ditfs are observed they can constrain a lower bound to maximum and minimum horizontal stress magnitudes the observation of transverse ditfs on image logs can constrain the stress field to one on the border of strike slip and reverse faulting s h max s h min s v without requiring knowledge of the s h min or s h max magnitude the observation of transverse ditfs in the west tuna area combined with wireline log data leak off tests and pore pressure data are used to constrain the in situ stress tensor the interpreted in situ stress tensor lies on the border of a strike slip and reverse faulting regime s h max 40 5 mpa km s h min s v 21 mpa km interpreted data from leak off tests in the west tuna area confirm that s h min s v

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

annular modes in a multiple migrating zonal jet regime the authors investigate the dynamics of zonal jets in a semihemisphere zonally reentrant ocean model the forcings imposed in the model are an idealized atmospheric wind stress and relaxation to a latitudinal temperature profile held constant in time while there are striking similarities to the observed atmospheric annular modes where the leading mode of variability is associated with the primary zonal jet s meridional undulation secondary weaker jets emerge and systematically migrate equatorward the model output suggests the following mechanism for the equatorward migration while the eddy momentum fluxes sustain the jets the eddy heat fluxes have a poleward bias causing an anomalous residual circulation with poleward equatorward flow on the poleward equatorward flanks by conservation of mass there must be a rising residual flow at the jet from the thermodynamics equation the greatest cooling occurs at the jet core thus creating a tendency to reduce the baroclinicity on the poleward flank while enhancing it on the equatorward flank consequently the baroclinic zone shifts perpetuating the jet migration

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

the influence of the intermediate principal stress on rock failure behaviour a numerical study abstract the influence of the intermediate principal stress on rock strength has been studied comprehensively by previous researchers however the reason why rock strength firstly increases and subsequently decreases with the increase of intermediate principal stress is still unclear in this paper the mechanism of the intermediate principal stress effect on rock failure behaviour is revealed through a numerical method using the epca3d system elasto plastic cellular automaton in this study both homogeneous and heterogeneous rocks are considered the heterogeneity of a rock specimen is modelled by introducing weibull s statistical distribution two criteria i e the drucker prager and mohr coulomb models are used to determine whether a meso scopic element in the rock specimen is in a failure state or not during the polyaxial stress loading process the epca3d simulation reproduces the typical phenomenon of the intermediate principal stress effect that occurs in some rock experiments by studying the epca3d simulated acoustic emission and complete stress strain curves illustrating failure initiation propagation and coalescence in the failure process of rocks the essence of the intermediate principal stress effect is tracked it is concluded that the heterogeneous stress distribution induced by the natural heterogeneity of rocks and the effect of the loading platen are two of the reasons producing the intermediate stress effect studies indicate that a moderate intermediate principal stress delays the onset of local failure which in turn leads to an increase in the rock strength however once the intermediate principal stress reaches a certain value local failure will be formed through the application of the intermediate principal stress it is the number of failed elements in the pre peak region that determines whether the rock strength decreases or not the extent of rock strength reduction when the intermediate principal stress reaches a certain value is lessened with the increase in the minimum principal stress

comparison of rock failure criteria in predicting borehole shear failure abstract selection of the appropriate rock failure criteria is one of the key steps in determining minimum required mud weight in wellbore stability analysis numerous failure criteria have been used for rock failure analysis but there is no common agreement of which failure criterion to select in this paper thirteen failure criteria used in predicting borehole shear failure were evaluated for four field cases in a comparison of the results with actual field failure cases tresca von mises and inscribed drucker prager overestimated the rock breakout and predicted the highest required minimum required mud weight for all cases also the results of these criteria are significantly higher than the actual borehole shear failure circumscribed drucker prager underestimated the rock breakout and predicted the lowest bound of the minimum required mud weight in most cases which is mainly less than actual onset of borehole breakout the minimum required mud weights determined by modified lade modified wiebols cook and mogi coulomb is above but close to the onset of breakout based on the field reported failure cases this means that using of any of these three criteria in wellbore stability analysis could be a safe approach furthermore modified lade modified wiebols cook and mogi coulomb provided similar results for all studied cases so these failure criteria may be used interchangeably

evidence in cerro pampa volcanic rocks for slab melting prior to ridge trench collision in southern south america late miocene approx 12 ma hornblende bearing andesitic to dacitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage wasysym pagestyle empty declaremathsizes 10 9 7 6 begin document mathrm 63 to 68 sio 2 1 2 to 1 9 k 2 o end document adakite flows at the small cerro pampa center in patagonia 47 55 s 71 25 have some of the clearest slab melt geochemical signatures yet seen in a phanerozoic center on continental crust these magmas formed in response to melting of the hot thin slab that was subducting beneath south america prior to the collision of the chile rise at approx 6 ma or at approx 10 ma their n morb like 87 sr 86 sr 0 70285 0 70309 and low 206 pb 204 pb 18 44 18 59 ratios show that they could have been generated by

enhancement in vertical fluxes at a front by mesoscale submesoscale coupling oceanic frontal instabilities are of importance for the vertical exchange of properties in the ocean submesoscale o 1 rossby number dynamics are particularly relevant for inducing the vertical and lateral flux of buoyancy and tracers in the mixed layer but how these couple with the stratified pycnocline is less clear observations show surface fronts often persist beneath the mixed layer here we use idealized three dimensional model simulations to show how surface fronts that extend deeper into the pycnocline invoke enhanced vertical fluxes through the coupling of submesoscale and mesoscale instabilities we contrast simulations in which the front is restricted to the mixed layer with those in which it extends deeper for the deeper fronts we examine the effect of density stratification on the vertical coupling our results show deep fronts can dynamically couple the mixed layer and pycnocline on time scales that increase with the peak stratification beneath the mixed layer eddies in the interior generate skew fluxes of buoyancy and tracer oriented along isopycnals thus providing an adiabatic pathway for the interior to interact with the mixed layer at fronts the vertical enhancement of tracer fluxes through the mesoscale submesoscale coupling described here is thus relevant to the vertical supply of nutrients for phytoplankton in the ocean a further implication for wind forced fronts is that the vertical structure of the stream function characterizing the exchange between the interior and the mixed layer exhibits significant qualitative differences compared to a linear combination of existing parameterizations of submesoscale eddies in the mixed layer and mesoscale eddies in the interior the discrepancies are most severe within the mixed layer suggesting a potential role for ekman layer dynamics absent in existing submesoscale parameterizations

tectonic evolution of the qinghai tibet plateau abstract the qinghai tibet plateau composed of several continental slivers within the eastern tethyan domain is one of the pivotal sites to examine to better understand the theory of plate tectonics and the orogenic evolution on earth this plateau is generally inferred to be a collage of several continental blocks that rifted from gondwanaland and subsequently accreted to the asian continent however recent recognition of over twenty ophiolite melange zones and their associated island arcs indicates that the traditional model of tectonic evolution requires revision based on 177 recently finished 1 250 000 scale geological maps and related studies we summarize the main tectonic context of the qinghai tibet plateau and propose a new integrated model to account for the new findings the complex orogen of the immense qinghai tibet plateau consisting of multiple island arc basin systems that developed at different stages while surrounded by the north china yangtze tarim and indian plates is emphasized the entire orogen surrounded by suture zones that mark the locations of oceanic closure is investigated by examining i the first order tectonic units and ophiolitic melanges including arc arc continent collision zones and ii their internally enclosed blocks as the second order tectonic units therefore the qinghai tibet plateau is divided into three major orogenic systems namely from northeast to southwest the early paleozoic qinling qilianshan kunlunshan qin qi kun the late paleozoic triassic qiangtang sanjiang and the late paleozoic to cenozoic gangdese himalaya orogenic systems which are separated by the kangxiwa muzitagh maqin mianxian and the bangong shuanghu changning menglian sutures respectively we propose that the formation and evolution of the qinghai tibet plateau to have been intrinsically related to those of the eastern tethys recorded by the longmu co shuanghu ophiolite melange zone the southern qiangtang paleozoic accretionary arc basin system the bangong nujiang suture zone and their associated composite island arc basin systems the present day bangong shuanghu changning menglian suture system marks the final closure of the tethyan ocean the yarlung zangbo ocean opened as a back arc basin in response to the southward subduction of the tethyan ocean lithosphere in the middle triassic and closed as a result of the india asia collision at the end of cretaceous followed by the northward indention of the indian plate that resulted in significant intra continental deformation and plateau uplift in the cenozoic

transformed eulerian mean theory part ii potential vorticity homogenization and the equilibrium of a wind and buoyancy driven zonal flow the equilibrium of a modeled wind and buoyancy driven baroclinically unstable flow is analyzed using the transformed eulerian mean tem approach described in part i within the near adiabatic interior of the flow ertel potential vorticity is homogenized along mean isopycnals a finding readily explained using tem theory given the geometry of the domain the equilibrium zonal mean buoyancy structure at the surface is determined entirely by a balance between imposed surface fluxes and residual mean and eddy buoyancy transport within a surface diabatic layer balance between these same processes and the wind stress determines the stratification and hence potential vorticity immediately below this layer ertel potential vorticity homogenization below then determines the mean buoyancy structure everywhere accordingly the equilibrium structure of this flow can be described and quantitatively reproduced from knowledge of the eddy mixing rates within the surface diabatic zone and the depth of this zone together with potential vorticity homogenization beneath these results emphasize the need to include near surface buoyancy transport as well as interior pv transport in eddy parameterization schemes they also imply that in more realistic models the surface buoyancy balances may be impacted by processes in remote locations that allow diapycnal flow

geochemical constraints on the origin of bimodal magmatism at the okinawa trough an incipient back arc basin abstract the magmatism at the axial zone of the middle okinawa trough a young continental back arc basin comprises a bimodal basaltic rhyolitic suite accompanied by minor intermediate rocks we report major and trace element and sr nd isotopie data for the intermediate to silicic suites to provide constraints on their petrogenesis the rhyolites recovered as lava and pumice fall into three geochemical groups type 1 2 and 3 rhyolites type 1 rhyolites have 87sr 86sr 0 7040 0 7042 and 143nd 144nd 0 5128 0 5129 identical to those of associated basalts and are characterized by highly fractionated ree patterns petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt in contrast type 2 rhyolites and andesite have slightly higher 87sr 86sr 0 7044 0 7047 but similar 143nd 144nd 0 5128 compared to those of the basalts the compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization afc processes of the basalt magma quantitative analysis suggests assimilation fractional crystallization ma mc ratios of 0 05 hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present we emphasize that mechanical extension in this part of the okinawa trough involves gabbro ic lower crust that resulted from fractionation of mantle derived basaltic magmas type 3 rhyolite occurs only as pumice which makes its derivation questionable this rhyolite has major and trace element compositions and sr nd isotopie ratios which suggests that it may be derived from volcanic activity on the southern ryukyu volcanic front and arrived in the okinawa trough by drifting on the kuroshio current

geochronology and geochemistry of the c 80 ma rutog granitic pluton northwestern tibet implications for the tectonic evolution of the lhasa terrane the rutog granitic pluton lies in the gangdese magmatic arc in the westernmost part of the lhasa terrane nw tibet and has shrimp zircon u pb ages of c 80 ma the pluton consists of granodiorite and monzogranite with sio2 ranging from 62 to 72 wt and al2 o3 from 15 to 17 wt the rocks contain 2 33 4 93 wt k2o and 3 42 5 52 wt na2o and have na2o k2o ratios of 0 74 2 00 their chondrite normalized rare earth element ree patterns are enriched in lree la yb n 15 to 26 and do not show significant eu anomalies eu 0 68 1 15 on a primitive mantle normalized trace element diagram the rocks are rich in large ion lithophile elements lile and poor in high field strength elements hfse hree and y their sr y ratios range from 15 to 78 with an average of 30 the rocks have constant initial 87 sr 86 sr ratios 0 7045 to 0 7049 and slightly positive end t values 0 1 to 2 3 similar to i type granites generated in an arc setting the geochemistry of the rutog pluton is best explained by partial melting of a thickened continental crust triggered by underplating of basaltic magmas in a mantle wedge the formation of the rutog pluton suggests flat subduction of the neo tethyan oceanic lithosphere from the south crustal thickening may have occurred in the late cretaceous prior to the india asia collision

repeated crustal thickening and recycling during the andean orogeny in north chile 21 26 s posttectonic granitoids 2 lower crustal p wave velocities of 7 3 7 7 km s 1 compatible with underplated mafic crust and 3 results from recent experimental petrologic work showing garnet stability in mafic mineralogies 12 kbar 40 km crustal thickness analogous to older andean magmatic episodes in north chile newly underplated basaltic crust may account for the remaining deficit in neogene crustal thickening similar evolutionary patterns in geochemistry and initial sr and nd isotopic characteristics of andean 200 ma to present magmatic rocks suggest that the andean orogeny in this region evolved by a combination of processes of repeated arc migration tectonic and magmatic crustal thickening and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust index terms 1020 geochemistry composition of the crust 1030 geochemistry geochemical cycles 0330 8159 evolution of the earth rheology crust and lithosphere 9360 information related to geographic region south america keywords andean orogeny continental crust recycling adakite ttg crustal thickening

silicic magma formation in overthickened crust melting of charnockite and leucogranite at 15 20 and 25 kbar 50 60 vol accompanied by settling of residualof crustal source rocks at depths 50 km 2 production of syenite crystals would lead to a magma of restricted compositional rangemagma by partial melting of quartzofeldspathic rocks at pressures with a heavy rare earth element depleted pattern overall the granitic 15 kbar melting experiments at 15 20 and 25 kbar were melt composition obtained in the experiments divers from that ofperformed on archaean biotite bearing charnockite of opx bearing a type granite however at degrees of melting below

westward younging high mg adakitic magmatism in central tibet record of a westward migrating lithospheric foundering beneath the lhasa qiangtang collision zone during the late cretaceous abstract magmatic records with high mg and low y signatures provide important insights into the deep mantle dynamics that dictate the evolution of ancient orogenic belts this paper reports the presence of felsic intrusive dikes with high mg and low y signatures intruding the baingoin batholith central tibet at ca 94 ma these dikes show adakitic signatures including high sr and low y and yb contents and high sr y and la yb n ratios the adakitic dacitic dikes can be grouped into high mg mg 48 52 with positive zircon e hf t 8 3 12 4 and whole rock e nd t 0 27 and low mg mg 37 45 with positive zircon e hf t 7 2 13 5 varieties these distinct geochemical features along with geological observations show that the baingoin high mg adakitic dacitic dikes were most likely derived from the partial melting of a foundering juvenile lower crust and subsequent melt mantle reaction whereas the low mg adakitic dacitic dikes can be interpreted as resulting from the partial melting of a thickened lower crust that did not delaminate the generation of these adakitic rhyolitic dikes can be attributed to hornblende dominated fractional crystallization of the adakitic dacitic melts a decrease in y and heavy rare earth element contents of magmatic records from 120 100 ma to 95 80 ma within the lhasa qiangtang collision zone suggest an increase in crustal thickness with time the volumetrically minor westward migrating high mg adakitic magmatism can readily be interpreted as the consequences of the westward migrating foundering of the lithosphere triggered by rayleigh taylor instabilities along the lhasa qiangtang collision zone during the period 94 80 ma

late cretaceous k rich magmatism in central tibet evidence for early elevation of the tibetan plateau abstract major and trace element sr nd pb and zircon u pb and hf isotope data are presented for the newly discovered abushan volcanic rocks in the southern qiangtang terrane central tibet these results offer new insights into the evolution of the tibetan plateau during cretaceous the abushan volcanic rocks are composed mainly of andesites and trachy andesites zircon u pb dating constrains the timing of emplacement as late cretaceous 79 9 2 7 75 9 0 49 ma major element geochemistry shows that the abushan volcanic rocks belong to high k calc alkaline series all the andesites and trachy andesites are enriched in large ion lithophile elements lile and light rare earth elements lree and depleted in high field strength elements hfse and heavy rare earth elements hree the geochemical features and ages of the abushan volcanic rocks relate them with post collisional setting furthermore the volcanic rocks display negative end t 3 1 to 2 5 relatively constant 87sr 86sr ratios 0 70614 0 70735 and negative ehf t values 5 8 to 2 1 the sr nd pb and hf isotope signature suggests that the andesites and trachy andesites were derived from the anatexis of mafic lower crust by intrusion or underplating of mantle derived basaltic magma the petrogenesis of abushan andesites provides robust evidence for the crustal thickening delamination and early uplifting of central tibet during late cretaceous

early cretaceous foreland like northeastern qaidam basin tibetan plateau and its tectonic implications insights from sedimentary investigations detrital zircon u pb analyses and seismic profiling abstract the early cretaceous collision between the lhasa and qiangtang terranes is one of the critical tectonic events that shaped the southern margin of the eurasian continent and that impacted the formation of the cenozoic tibetan plateau however how far north this collision affected the structural and sedimentary geology of the early cretaceous remains uncertain to address this issue we present results from an integrated study of lower cretaceous strata in the northeastern qaidam basin neqb northern tibetan plateau based on field observations detrital zircon u pb analyses and seismic profile interpretations the lower cretaceous mainly consists of deltaic fluvial and alluvial sandstone and conglomerate with a pattern of orogeny related molasse deposits and a provenance in the nearby south qilian shan to the north a number of south directed reverse faults developed and formed piggy back basins in the early cretaceous these reverse faults remained active and affected erosion during the late cretaceous leaving an unconformity above the cretaceous strata these observations indicate a foreland like neqb due to the uplift and southward thrusting of the south qilian shan in the early cretaceous we further interpreted the neqb and the south qilian shan to be the northern edge of the compressional regime associated with the lhasa qiangtang collision in the early cretaceous which contrasts with the contemporaneous extensional setting in the north qilian shan and further north we also observed that some early cretaceous faults were rejuvenated as major thrusts in the south qilian shan thrust belt in the cenozoic for the first time this study maps the northern boundary of the late mesozoic qiangtang plateau and highlights a protracted influence on tibetan plateau tectonism since the early cretaceous

near surface eddy heat and momentum fluxes in the antarctic circumpolar current in drake passage abstractthe authors present new estimates of the eddy momentum and heat fluxes from repeated high resolution upper ocean velocity and temperature observations in drake passage and interpret their role in the regional antarctic circumpolar current acc momentum balance the observations span 7 yr and are compared to eddy fluxes estimated from a 3 yr set of output archived from an eddy resolving global parallel ocean program pop numerical simulation in both pop and the observations the stream averaged cross stream eddy momentum fluxes correspond to forcing consistent with both a potential vorticity flux into the axis of the subantarctic front saf and a sharpening of all three main acc fronts through drake passage further the pop analysis indicates that the mean momentum advection terms reflect the steering of the mean acc fronts and are not fully balanced by the eddy momentum forcing which instead impacts the strength and number of acc fronts the comparison between pop and observed eddy heat fluxe

middle miocene paleoaltimetry of southern tibet implications for the role of mantle thickening and delamination in the himalayan orogen the stable isotope composition of pedogenic and early diagenetic carbonates from the oiyug basin of southern tibet allows model estimates of the paleoaltimetry of the tibetan plateau for the middle miocene pedogenic calcium carbonate nodules have average d 18 occ values of 219 6 whereas nodular lacustrine dolomites range in composition from 27 6 to 25 5 the most negative of the carbonate isotope values can be used to constrain the oxygen isotope composition of paleoprecipitation from which model estimates of paleoaltimetry can be made model results indicate that the southern tibetan plateau achieved elevations of 5200 11370 2605 m by at least 15 ma our results are identical within uncertainty to previous workers paleoelevation estimates based on oiyug basin fossil floral physiognomy this is the first time that two paleoaltimeters have been directly compared and are in accord collectively these data strongly support tectonic models in which thickening of mantle lithosphere beneath the domain of crustal thickening and subsequent detachment of the mantle lithosphere plays an indiscernible role in the elevation history of this part of the himalaya tibet orogenic system

estimates and implications of surface eddy diffusivity in the southern ocean derived from tracer transport abstract near surface effective diffusivities associated with geostrophic eddies in the southern ocean are estimated by numerically monitoring the lengthening of idealized tracer contours as they are strained by surface geostrophic flow observed by satellite altimetry the resulting surface diffusivities show considerable spatial variability and are large 2000 m2 s 1 on the equatorward flank of the antarctic circumpolar current and are small 500 m2 s 1 at the jet axis regions of high and low effective diffusivity are shown to be collocated with regions of respectively weak and strong isentropic potential vorticity gradients the maps of diffusivity are used along with climatological estimates of surface wind stress and air sea buoyancy flux to estimate surface meridional residual flows and the relative importance of eulerian and eddy induced circulation in the streamwise averaged dynamics of the antarctic circumpolar current

a model of the upper branch of the meridional overturning of the southern ocean a zonal average model of the upper branch of the meridional overturning circulation of the southern ocean is constructed and used to discuss the processes wind buoyancy eddy forcing and boundary conditions that control its strength and sense of circulation the geometry of the thermocline wedge set by the mapping between the vertical spacing of buoyancy surfaces the stratification on the equatorial flank of the antarctic circumpolar current and their outcrop at the sea surface is seen to play a central role by setting the interior large scale potential vorticity distribution it is shown that the action of eddies mixing this potential vorticity field induces a residual flow in the meridional plane much as is observed with upwelling of fluid around antarctica northward surface flow and subduction to form intermediate water

initiation pressure location and orientation of hydraulic fracture the hubbert and willis 1957 1 fracture criterion has been widely adopted to predict longitudinal fracture initiation in vertical or horizontal wellbores current transverse fracture criteria predict fracture by comparing the axial stress in the wellbore wall with the tensile strength of rock it is shown in this paper that the axial stress is not a good predictor of transverse fracture initiation because it remains constant during hydraulic pressurization the magnitude of the most tensile principal stress in the wellbore wall should always be compared with the tensile strength of the rock to predict hydraulic fracture initiation based on elastic theory special cases have been found in which the most tensile principal stress reaches the tensile strength of the rock simultaneously at all point on the circumference of the wellbore it is proposed that transverse fractures are initiated in those cases

modified mohr coulomb criterion for non linear triaxial and polyaxial strength of jointed rocks abstract rocks encountered in civil and mining engineering fields are invariably jointed and act under triaxial or polyaxial stress conditions the mohr coulomb shear strength criterion is the most widely used criterion for jointed rocks in its present form there are two major limitations of this criterion firstly it considers the strength response to be linear and secondly the effect of the intermediate principal stress on the strength behaviour is ignored a modified non linear form of mohr coulomb strength criterion has been suggested in this study to overcome these limitations barton s concept of critical state for rocks has been imbibed in the linear mohr coulomb criterion to deduce a semi empirical expression for non linear criterion however the shear strength parameters of the conventional mohr coulomb criterion are used in the proposed criterion the proposed criterion is a simple and rational nonlinear polyaxial strength criterion for anisotropic jointed rocks in an earlier publication 1 the applicability of the criterion was evaluated for intact rocks in present paper the criterion is extended to jointed rocks which are anisotropic in nature the applicability of the proposed criterion has been verified by applying it to extensive experimental data on triaxial and polyaxial test results on jointed rocks available from literature applicability of the criterion to explain rock burst conditions for some indian rocks is also demonstrated

cretaceous to cenozoic evolution of the northern lhasa terrane and the early paleogene development of peneplains at nam co tibetan plateau abstract highly elevated and well preserved peneplains are characteristic geomorphic features of the tibetan plateau in the northern lhasa terrane north northwest of nam co the peneplains were carved in granitoids and in their metasedimentary host formations we use multi method geochronology zircon u pb and u th he dating and apatite fission track and u th he dating to constrain the post emplacement thermal history of the granitoids and the timing and rate of final exhumation of the peneplain areas la icp ms u pb geochronology of zircons yields two narrow age groups for the intrusions at around 118 ma and 85 ma and a third group records paleocene volcanic activity 63 58 ma in the nam co area the low temperature thermochronometers indicate common age groups for the entire nam co area zircon u th he ages cluster around 75 ma apatite fission track ages around 60 ma and apatite u th he ages around 50 ma modelling of the thermochronological data indicates that exhumation of the basement blocks took place in latest cretaceous to earliest paleogene time by middle eocene time the relief was already flat documented by a thin alluvial sediment sequence covering a part of the planated area the present day horst and graben structure of the peneplains is a late cenozoic feature triggered by e w extension of the tibetan plateau the new thermochronological data precisely bracket the age of the planation to early eocene i e between ca 55 and 45 ma the erosional base level can be deduced from the presence of early cretaceous zircon grains in eocene strata of bengal basin the sediment generated during exhumation of the nam co area was transported by an early cenozoic river system into the ocean suggesting that planation occurred at low elevation

slip tendency analysis fault reactivation potential and induced seismicity in a deep geothermal reservoir abstract a slip tendency analysis is used to assess the reactivation potential of shear and dilational fractures in a deep geothermal reservoir in the northeast german basin based on the notion that slip on faults is controlled by the ratio of shear to normal stress acting on the plane of weakness in the in situ stress field the reservoir rocks composed of lower permian sandstones and volcanics were stimulated by hydraulic fracturing a surprisingly low microseismic activity was recorded with moment magnitudes mw ranging from 1 0 to 1 8 the slip tendency analysis suggests a critically stressed reservoir exists in the sandstones whereas the volcanic rocks are less stressed rock failure first occurs with an additional pore pressure of 20 mpa presumed failure planes form a conjugate set and strike nw and ne slip failure is more likely than tensional failure in the volcanic rocks because high normal stresses prevent tensional failure these results from slip tendency analysis are supported by the spatial distribution of recorded microseismicity source characteristics indicate slip rather than extension along presumed ne striking failure planes this suggests that slip tendency analysis is an appropriate method that can be used to understand reservoir behavior under modified stress conditions

the nature and timing of crustal thickening in southern tibet geochemical and zircon hf isotopic constraints from postcollisional adakites abstract rising as the roof of the world the tibetan plateau is now underlain with the thickest continental crust on earth how and when was this crust formed which would have exerted pivotal controls to the formation of the plateau has long been an issue of hot debates this paper reports zircon u pb ages and hf isotope data for postcollisional 30 9 ma adakites in the southern lhasa terrane southern tibet a comparative analysis of whole rock rare earth element geochemistry and zircon hf isotopes between the adakites and associated gangdese igneous rocks suggests that the tibetan crust underwent a major phase of tectonic thickening between ca 45 and 30 ma in the region the lower part of the thickened crust consisted prevailingly of mafic lithologies which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the late cretaceous and eocene time related to the neotethyan subduction processes including breakoff of the subducted slab at ca 50 ma in the early stage of the india asia collision these processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern lhasa terrane the indentation of india consequently caused distributed lithospheric thickening with formation of an orogenic root beneath southern tibet root foundering during the oligocene gave rise to the adakitic magmatism regional topographic uplift and onset of northward underthrusting of the indian plate that has since played a key role in forming the entire tibetan plateau

eddy induced diapycnal fluxes and their role in the maintenance of the thermocline abstract high resolution numerical experiments are diagnosed to study the integral effects of geostrophic eddy fluxes on the large scale ocean circulation three characteristic large scale flows are considered 1 an anticyclonic single gyre 2 a double gyre and 3 an unblocked zonal flow a simple analog of the antarctic circumpolar current it is found that buoyancy and potential vorticity budgets in the presence of eddies are dominated by a balance between vertical advection into the control volume by ekman pumping and eddy transfer across the density surfaces achieved by diapycnal eddy fluxes with small scale mixing making only a minor contribution possible oceanographic implications of the results are discussed

assembly of the lhasa and qiangtang terranes in central tibet by divergent double subduction abstract integration of lithostratigraphic magmatic and metamorphic data from the lhasa qiangtang collision zone in central tibet including the bangong suture zone and adjacent regions of the lhasa and qiangtang terranes indicates assembly through divergent double sided subduction this collision zone is characterized by the absence of early cretaceous high grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca 120 110 ma two jurassic cretaceous magmatic arcs are identified from the caima duobuza rongma kangqiong amdo magmatic belt in the western qiangtang terrane and from the along tso yanhu daguo baingoin daru tso magmatic belt in the northern lhasa terrane these two magmatic arcs reflect northward and southward subduction of the bangong ocean lithosphere respectively available multidisciplinary data reconcile that the bangong ocean may have closed during the late jurassic early cretaceous most likely ca 140 130 ma through arc arc soft collision rather than continent continent hard collision subduction zone retreat associated with convergence beneath the lhasa terrane may have driven its rifting and separation from the northern margin of gondwana leading to its accretion within asia

the lhasa terrane record of a microcontinent and its histories of drift and growth article i nfo the lhasa terrane in southern tibet has long been accepted as the last geological block accreted to eurasia before its collision with the northward drifting indian continent in the cenozoic but its lithospheric architecture drift and growth histories and the nature of its northern suture with eurasia via the qiangtang terrane remain enigmatic using zircon in situ u pb and lu hf isotopic and bulk rock geochemical data of mesozoic early tertiary magmatic rocks sampled along four north south traverses across the lhasa terrane we show that the lhasa terrane has ancient basement rocks of proterozoic and archean ages up to 2870 ma in its centre with younger and juvenile crust phanerozoic accreted towards its both northern and southern edges this finding proves that the central lhasa subterrane was once a microcontinent this continent has survived from its long journey across the paleo tethyan ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the qiangtang terrane to the north and with the indian continent to the south zircon hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks e g 50 90 0 70 and 30 100 to the mesozoic magmatism in the southern central and northern lhasa subterranes respectively we suggest that much of the mesozoic magmatism in the lhasa terrane may be associated with the southward bangong nujiang tethyan seafloor subduction beneath the lhasa terrane which likely began in the middle permian or earlier and ceased in the late early cretaceous and that the significant changes of zircon ehf t at 113 and 52 ma record tectonomagmatic activities as a result of slab break off and related mantle melting events following the qiangtang lhasa amalgamation and india lhasa amalgamation respectively these results manifest the efficacy of zircons as a chronometer u pb dating and a geochemical tracer hf isotopes in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics

analysis of the microseismicity induced by fluid injections at the egs site of soultz sous forets alsace france implications for the characterization of the geothermal reservoir properties in june july 2000 a hydraulic stimulation experiment took place at the geothermal egs site of soultz sous forets alsace france in order to enhance the permeability of the fractured granitic massif at 5 km depth as it is well known that fluid injections tend to induce microseismic events a downhole and a surface seismological network have been installed to monitor the seismic activity during the stimulation test 23400 m3 of fluid have been injected in the rock volume through the open hole section 4400 m 5000 m of the well gpk2 at increasing rates of 30 l s 1 40 l s 1 and then 50 l s 1 more than 7200 microseismic events in the magnitude range 0 9 to 2 6 have been precisely located through a simultaneous inversion of the seismic velocity structure and location parameters the analysis of the behavior of the seismicity relative to the hydraulic parameters gives important information about the geothermal reservoir it appears that the evolution of the seismicity strongly depends on the variations of the injection rate an increase or a decrease leads to changes of the velocity structure the number and magnitude of microseismic events this involves different hydro mechanical processes between the fluid flow and the fracture planes which will control the final shape of the microseismic cloud moreover the study of the variations of the b value with time suggests that the stimulation experiment produces a large proportion of small earthquakes but records of events of magnitude higher than 2 indicate that fluid injection could reactivate structures whose dimensions allow the generation of such earthquakes

rapid changes in mixed layer stratification driven by submesoscale instabilities and winds 1 submesoscale eddies generated by baroclinic instability of upper ocean fronts lead to rapid restratification of the mixed layer on a time scale of days this restratification can be opposed by a down front wind stress acting in the direction of the geostrophic velocity that drives a surface ekman flow from the dense side to the light side of the front to arrest the slumping of isopycnals a scaling diagnostic is suggested to determine whether the effect of eddies or wind dominates under different conditions using a numerical model we investigate the juxtaposition of submesoscale eddies and down front winds acting on the mixed layer by estimating the eddy induced overturning stream function in the mixed layer we separate the along and cross isopycnal fluxes of buoyancy associated with submesoscale mixed layer eddies and demonstrate the need for parameterization of the advective along isopycnal flux though the cross front transport of buoyancy induced by the down front component of the wind opposes restratification by mixed layer eddies it becomes diminished as the eddies and growth of the frontal instability disrupt alignment between the wind and frontal axis

late mesozoic tectonic evolution and growth of the tibetan plateau prior to the indo asian collision abstract the elevation of the tibetan plateau is a revolutionary event in the earth history which resulted in change not only of cenozoic regional and global climate but also of monsoon intensity a critical aspect for the development of the tibetan plateau that remains open to intense debate is whether the collision among the tibetan continental blocks or the subduction of the tethys prior to the indo asian collision in the cenozoic contributed to the plateau growth we propose here that an andean type orogen could have been present in southern tibet from the jurassic to early cretaceous but collapsed during the middle cretaceous based on a synthesis of existing structural sedimentary geochronological and geochemical data the orogeny is marked by north verging jurassic early cretaceous thrusting in the lhasa block a result of flat subduction of the yarlung zangpo neo tethys that is recorded by a narrow belt of 205 174 ma adakitic rocks along the southernmost margin of the lhasa block denudation of the orogen produced a thick up to 15 km sequence of jurassic turbidites and molasse type sediments on the northern lhasa block widespread middle cretaceous 135 100 ma postorogenic strongly peraluminous to calc alkaline magmatism and extensional deformation in the lhasa block and an extensive marine transgression in tibet indicate that the orogen broke down due to extension resulting from lithosphere delamination and asthenosphere upwelling meanwhile the north dipping subduction of the bangong meso tethys produced a wide jurassic middle cretaceous magmatic arc along the southern margin of the qiangtang block findings of 132 108 ma aged ophiolites rich with middle cretaceous radiolarians in central tibet indicate that the bangong meso tethys did not close until the late cretaceous repeated flat slab subduction of the yarlung zangpo neo tethys occurred during the late cretaceous 90 78 ma as indicated by the adakites in the southernmost lhasa block which together with the ongoing qiangtang lhasa collision could have contributed to the growth of the tibetan plateau

did the indo asian collision alone create the tibetan plateau it is widely believed that the tibetan plateau is a late cenozoic feature produced by the indo asian collision however because tibet was the locus of continental accretion and subduction throughout the mesozoic crustal thickening during that time may also have contributed to growth of the plateau this portion of the geologic history was investigated in a traverse through the central lhasa block southern tibet together with earlier studies our mapping and geochronological results show that the lhasa block underwent little north south shortening during the cenozoic rather our mapping shows that 60 crustal shortening perhaps due to the collision between the lhasa and qiangtang blocks occurred during the early cretaceous this observation implies that a significant portion of southern tibet was raised to perhaps 3 4 km elevation prior to the indo asian collision

age and petrogenesis of a type granites in the middle segment of the bangonghu nujiang suture tibetan plateau abstract during geological investigations conducted over the past several years the authors found a group of a type granites in the middle segment of the bangonghu nujiang suture on the tibetan plateau these granitic plutons occur as stocks with exposed areas of less than 1 km 2 and lithologically include biotite granite and granodiorite porphyry which had intruded into cretaceous sedimentary rocks petrogeochemically the a type granites contain high sio 2 levels varying from 68 62 75 36 wt moreover they contain relatively high fe 2 o 3 t 0 86 5 39 wt and low al 2 o 3 12 76 15 54 wt levels and are weak peraluminous to metaluminous a trace element n morb normalized spider diagram showed apparent enrichment of the large ion lithophile elements lile rb th u k and pb with marked depletions of ba and sr the high field strength elements hfse nb ta and ti were sharply depleted with relative enrichment of zr and hf these signatures are characteristic of a type granite the granite samples had high rare earth element ree contents with ree 122 37 291 19 ppm light ree were rich relative to heavy ree with lree hree 4 89 9 58 and negative eu anomalies were sharp with eu varying from 0 14 0 5 and a mean value of 0 34 their chondrite normalized rare earth element distribution patterns appear as gently right dipping v types zircon u pb laicpms dating for three granitic plutons yielded weighted mean ages of 109 6 1 4 ma mswd 3 7 112 3 0 51 ma mswd 0 21 and 113 73 0 5 ma mswd 0 21 indicating that they formed in the late stage of the early cretaceous epoch the nd sr and pb isotopic results showed that the granites were rich in radiogenic isotopes the 87 sr 86 sr and i sr ratios varied from 0 719842 0 786395 and 0 706900 0 710378 respectively the 143 nd 144 nd ratios were from 0 512123 0 512392 with relatively large negative e nd values 3 37 to 10 34 the 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb ratios ranged from 18 703 19 070 15 680 15 732 and 39 121 39 576 respectively exhibiting isotopic signatures typical of crustal sources all of the data demonstrated unanimously that these granites were derived from partial melting of arc basaltic rocks underplated in the lower crust during oceanic crust subduction the high temperature heat source for melting of anhydrous basaltic rocks of the lower crust was from upwelling asthenosphere through slab windows formed as a result of slab break off during post collisional extension of the orogenic zone

isopycnal diffusivities in the antarctic circumpolar current inferred from lagrangian floats in an eddying model average 750 250 m 2 s 1 around the polar frontal zone the results imply that parameterizations that only use eddy kinetic energy to parameterize the diffusivities are incomplete we suggest that dominant correlations of lagrangian eddy diffusivities with eddy kinetic energy found in previous studies may have been due to the use of too short time lags in the integration of the velocity autocovariance used to infer the diffusivities we find evidence that strong mean flow inhibits cross stream mixing within the acc but there are also areas where cross stream diffusivities are large in spite of strong mean flows for example in regions close to topographic obstacles such as the kerguelen plateau

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

process of lithospheric delamination beneath the lhasa qiangtang collision orogen constraints from the geochronology and geochemistry of late cretaceous volcanic rocks in the lhasa terrane central tibet abstract the geochemistry of post collisional granitoids can be used to reconstruct crustal thickness and constrain deep geodynamic processes two series of volcanic rocks of the jiangba formation in the xiongmei area central tibet record the process of lithospheric delamination in the lhasa qiangtang collisional orogen the andesites and dacites crystallized at c 87 and 85 ma respectively and have positive zircon ehf t values 2 7 5 4 and 2 7 7 1 respectively they have calc alkaline compositions with relatively high mg 100 molar mg2 mg2 fe2 39 66 sr contents and sr y and la yb n ratios subscript n signifies normalization to chondrite low y and yb contents and no clear negative eu sr or ti anomalies similar to adakites the andesite dacite series likely formed by partial melting of foundering juvenile mafic lower crust with residual garnet followed by the interaction of this melt with peridotite the rhyolite series was emplaced at c 81 ma and has positive zircon ehf t values 1 0 3 5 the rhyolites are characterized by non adakitic signatures and have high k calc alkaline to shoshonite compositions with low mgo contents mg 21 34 and sr y and la yb n ratios coupled with flat hree patterns and marked negative eu sr and ti anomalies geochemical data indicate that these rhyolites were generated by partial melting of a much shallower crustal root with a plagioclase rich residue zircon u pb ages and different compositional trends for the two series indicate that the lithospheric mantle keel was partially removed by small scale mantle convection that triggered the late cretaceous high mg adakitic magmatism in the lhasa qiangtang collisional orogen partial lithospheric delamination increased heat flow at the base of the crust and caused the thinned crust to melt producing the low mg non adakitic rocks

determination of optimal well trajectory during drilling and production based on borehole stability abstract several studies have been done about borehole stability and optimized wellbore direction however the majority of them focused on stability during drilling and there are only a few studies concerns with stability during drilling and production and its problems such as sanding simultaneously this paper presents an analytical model that estimates collapse pressure in stability analysis during drilling and in addition determines maximum drawdown pressure to prevent sand production using the mogi coulomb failure criterion the results show that in different in situ stress regimes the inclination and azimuth have a significant role in wellbore stability during both drilling and production furthermore the results show that the optimum direction for wellbore stability during drilling is also the best direction for stability of a production well the analytical model is applied to field data in order to verify the applicability of the developed model

the geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

a seasonal climatology of effective diffusivity in the stratosphere a 7 year 1992 1998 seasonal climatology of effective horizontal diffusivity is presented for the stratosphere 350 1900 k as in previous studies the diagnosis is based on the equivalent length of a test tracer advected on isentropic surfaces in this case using the van leer flux limiting scheme driven by the united kingdom meteorological office assimilated winds although the magnitude of equivalent length is resolution dependent its structure is shown to be reasonably robust for quantifying the inhomogeneous mixing in the stratosphere the van leer calculation agrees well with a more expensive spectral transform calculation at a comparable resolution suggesting that the diagnostic is not sensitive to the advection scheme or subgrid representation of the models a first attempt is also made at estimating the implicit numerical diffusion of the van leer scheme which is necessary to convert equivalent length to effective diffusivity compared with the spectral calculations with a constant diffusion coefficient the van leer results show notably greater diffusivity within the winter polar vortices despite the comparable equivalent lengths suggesting that the scheme may be overdiffusing in the vicinities of mobile tracer extrema the climatology of equivalent length is then presented and the seasonal evolution is discussed in detail for the lower middle and upper stratosphere the major mixing barriers winter and summer polar vortices and tropical eddy transport barrier are easily identified as well as the strong mixing regions surf zones generally equivalent length tends to be small in regions of strong zonal wind and large in regions of weak wind large values of equivalent length are observed in the summer lower stratosphere and in the middle stratosphere during the breakup of the polar vortices interhemispheric asymmetry is observed both after the winter polar vortex breakup where in the middle stratosphere the southern hemisphere shows much larger equivalent lengths and in the summer tropical region where larger equivalent lengths are observed in the northern hemisphere likely related to the monsoon circulation a monthly averaged equivalent length climatology is presented and implications for two dimensional stratospheric chemistry transport modeling are discussed

poroelastic response of a borehole in a non hydrostatic stress field abstract this paper deals with the solution of the stress displacement and pore pressure field induced by the drilling and or the pressurization of a vertical borehole the rock which is permeated by fluid is assumed to behave as a poroelastic material with compressible constituents following the biot theory the analytical solution is derived in the laplace transform space and is transformed to the time domain using an approximate numerical inversion technique the solution reveals a full range of coupled poroelastic effects which provide potential mechanisms for delayed borehole instability and shear fracture initiation inside the rock

mesoscale eddy buoyancy flux and eddy induced circulation in eastern boundary currents abstracta dynamical interpretation is made of the mesoscale eddy buoyancy fluxes in the eastern boundary currents off california and peru chile based on regional equilibrium simulations the eddy fluxes are primarily shoreward and upward across a swath several hundred kilometers wide in the upper ocean as such they serve to balance mean offshore air sea heating and coastal upwelling in the stratified interior the eddy fluxes are consistent with the adiabatic hypothesis associated with a mean eddy induced velocity advecting mean buoyancy and tracers furthermore with a suitable gauge choice the horizontal fluxes are almost entirely aligned with the mean horizontal buoyancy gradient consistent with the advective parameterization scheme of gent and mcwilliams the associated diffusivity is surface intensified matching the vertical stratification profile the fluxes span the across shore band of high eddy energy but their alongshore structure is unresolved because of sampling limitations in the sur

geochemistry of limestones deposited in various plate tectonic settings abstract limestone a major part of the global sedimentary succession susceptible to post depositional diagenesis studies of limestone geochemistry are essential in the discrimination of tectonic settings of basins in which the limestones were deposited six late mesozoic and one tertiary limestone successions of tibet western china that were deposited in oceanic plateau passive continental margin active continental margin fore arc basin back arc basin and foreland basin and continental inland freshwater basins were analyzed for their major trace and rare earth element ree composition this geochemical dataset in combination with the deep sea drilling project and ocean drilling program dsdp and odp literature geochemical data regarding limestones deposited in open ocean environments permitted delineation of the geochemical characteristics of limestones accumulated in these various plate tectonic settings major elements e g fe2o3 and mno except for cao of these limestone successions show large variations but are positively correlated with al2o3 the ree and trace element abundances for the inland and margin limestones show a distinct positive correlation with al2o3 whereas rees and trace elements of the open ocean limestones are positively correlated with mno there is a systematic increase in the magnitude of ce anomalies of open ocean floor limestones away from spreading ridges to open ocean highs to passive margins and to active margins and inland freshwater basins open ocean limestones display a narrow range of la sm n 0 46 0 96 sm yb n 0 25 1 96 and la yb n 0 23 1 38 but high la ce n 1 5 whereas the inland margins limestones display a much larger range la sm n 0 43 2 18 sm yb n 0 6 2 98 and la yb n 0 7 2 25 but low la ce n

thickened juvenile lower crust derived 90 ma adakitic rocks in the central lhasa terrane tibet abstract the questions of why the late cretaceous magmatism generated and how the nature of the lower crust evolves in central tibet remain poorly constrained in this paper we report the presence of early late cretaceous adakitic rocks from the azhang area northern edge of the central lhasa subterrane central tibet these rocks are rhyodacites dacites in composition and have geochemical characteristics of adakitic rocks e g high sr 554 836 ppm sr y 66 100 and la yb n 20 21 low y 7 96 8 96 ppm and heavy rare earth elements hree in situ zircon u pb dating for two samples yields an early late cretaceous age 90 1 ma and 87 1 ma the low mgo 1 4 1 9 wt contents and compatible element abundances e g cr 22 30 ppm ni 19 25 ppm indicate that these rocks were most likely derived from the partial melting of a garnet bearing amphibolite under a thickened lower crust condition the positive whole rock e nd t 2 5 to 5 6 and zircon e hf t 8 9 to 16 0 values suggest that this thickened lower crust was juvenile the crust beneath the central lhasa subterrane may have been significantly thickened due to tectonic shortening in response to the lhasa qiangtang collision and magma underplating before the emplacement of azhang adakitic rocks 90 ma we argue that regional lithospheric delamination at 90 ma triggered the partial melting of the lowermost garnet bearing crust that is still attached to the middle crust to generate the azhang adakitic rocks the presence of the 90 ma azhang adakitic rocks provides valuable constraints on the origin of the early late cretaceous magmatism in the central lhasa subterrane and on the crustal evolution beneath the lhasa qiangtang collisional zone prior to the cenozoic india asia collision

the surface branch of the zonally averaged mass transport circulation in the troposphere the near surface branch of the overturning mass transport circulation in the troposphere containing the equatorward flow is examined in isentropic and geometric coordinates a discussion of the zonal momentum balance within isentropic layers shows that the equatorward flow at a given latitude is confined to isentropic layers that typically intersect the surface at that latitude as a consequence of mass transport within the surface mixed layer much of the equatorward flow occurs in layers with potential temperatures below the mean surface potential temperature in the conventional transformed eulerian mean formulation for geometric coordinates the surface branch of the overturning circulation is represented in an unrealistic manner streamlines of the residual circulation do not close above the surface a modified residual circulation is introduced that is free from this defect and has the additional advantage that its computation unlike that of the conventional residual circulation does not require division by the static stability which may approach zero in the planetary boundary layer it is then argued that cold air advection by the residual circulation is responsible for the formation of surface inversions at all latitudes in idealized gcms with weak thermal damping also included is a discussion of how a general circulation theory for the troposphere must be built upon a theory for the near surface meridional mass fluxes

eddy mixing of potential vorticity versus thickness in an isopycnic ocean model abstract parameterizations of the eddy induced velocity that advects tracers in addition to the eulerian mean flow are traditionally expressed as a downgradient fickian diffusion of either isopycnal layer thickness or large scale potential vorticity pv there is an ongoing debate on which of the two closures is better and how the spatial dependence of the eddy diffusivity should look like to increase the physical reasoning on which these closures are based the authors present a systematic assessment of eddy fluxes of thickness and pv and their relation to mean flow gradients in an isopycnic eddy resolving model of an idealized double gyre circulation in a flat bottom closed basin the simulated flow features strong nonlinearities such as tight inertial recirculations a meandering midlatitude jet pools of homogenized pv and regions of weak flow where h dominates the pv gradient it is found that the zonally averaged eddy flux of thickness scales better with the zonally averaged meridional thickn

porothermoelastic effect on wellbore stability in transversely isotropic medium subjected to local thermal non equilibrium abstract provided that the hydraulic boundary conditions at the wellbore wall are identified as permeable pbc and impermeable impbc and that the assumption of plane strain holds the present paper formulates a newly porothermoelastic analytical solution which is based on the thermodynamic principles in laplace space domain for inclined borehole the adopted linear porothermoelastic model embodies the transient heat transfer process in the presence of local thermal non equilibrium ltne during the derivation the solution is available for the cases where the wellbore axis of inclined wellbore is perpendicular to the isotropic plane of the low permeable transversely isotropic porous medium under non hydrostatic stress field condition the wellbore problem is decomposed into axisymmetric and deviatoric loading cases the numerical results of the time dependent distributions of field variables are obtained by virtue of performing the inversion technique for laplace transforms an integrated geomechanical wellbore stability model is proposed which is not only comprised by the abovementioned new porothermoelastic solution but also associated with the theoretical derivations of strength criteria to evaluate the rock to shear failure which include mogi coulomb criterion and single weak plane criterion when a horizontal borehole is drilled along the direction of in the minimum horizontal in situ stress the numerical comparative analyses in the present paper examine the combined influence of material anisotropy and thermal effect which embody in the ltne and local thermal equilibrium lte models on the thermally induced pore pressure stresses and the shear and tensile failure regions around the wellbore

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

influence of andean uplift on climate and paleoaltimetry estimates article i nfo article history recent elevation reconstructions of the andean plateau suggest a rapid 2 5 1 0 km rise of the central andes between 10 and 6 ma this rapid rise has been attributed to a catastrophic removal of a dense lithospheric mantle root beneath the andes however these findings are based on the assumption that climate did not change during deposition of paleoaltimetry proxies here we evaluate south american climate change due to andean uplift and its influence on interpretations of plateau elevation from climate sensitive paleoaltimetry data a series of experiments are presented using the regcm3 regional general circulation model rcm to characterize changes in andean precipitation amount surface temperature and wind direction vapor source as a function of changing plateau elevation results indicate that south american and andean climate changed significantly in response to plateau growth more specifically rising of the plateau results in up to a 900 mm increase in rainy season december january february precipitation over the plateau plateau uplift also results in a decrease in non adiabatic surface temperature of up to 6 5 c in addition to adiabatic cooling directly related to elevation change through the lapse rate finally the prevailing wind direction and the vapor source for precipitation switches from the south pacific ocean to the equatorial atlantic as plateau elevation increases above of its present day elevation taken together these changes in paleoclimate would have substantially depleted the oxygen isotopic concentration of paleoprecipitation through the cenozoic unless this climatic effect is taken into consideration paleoaltimetry reconstructions based on stable isotope methods may overestimate the rapid rise of the andes by up to several kilometers we conclude that some or all of the apparent rapid rise of the andean plateau from paleoaltimetry data could be an artifact of large changes in paleoclimate

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

early cretaceous palaeogeographic evolution of the coqen basin in the lhasa terrane southern tibetan plateau abstract the lower cretaceous sedimentary strata in the lhasa terrane record the palaeogeographic and tectonic history of the tibetan plateau prior to the india asia collision the lower cretaceous strata in the coqen basin of the northern lhasa subterrane include the duoni and langshan formations the duoni formation is composed of conglomerates sandstones siltstones and mudstones in the south deposited in a fluvial environment and quartzose lithic sandstones siltstones and mudstones in the north deposited in a shelf coastal environment the overlying langshan formation is characterized by abundant orbitolina and rudist bearing wackestones and packstones which were deposited on a low energy carbonate ramp the langshan formation in the coqen basin was deposited between 119 115 ma and 98 ma based on its large benthic foraminiferal assemblages age constraints based on the interbedded tuff and conformably overlying langshan formation indicate that the duoni formation was deposited between 123 and 115 ma sandstone detrital modal compositions further indicate that the duoni formation was primarily derived from magmatic arc and recycled orogen sources detrital zircons from the duoni formation yield a primary age population of 160 110 ma peaking at 130 ma with negative e hf t values 18 7 to 2 7 and additional age ranges of 550 500 1200 900 and 1600 1500 ma thereby indicating that the zenong volcanic rocks and palaeozoic sedimentary bedrocks in the northern lhasa subterrane are the most likely sources the palaeogeography of the northern lhasa subterrane can be approximately described in two stages during the aptian 123 115 ma with the eruption of the zenong volcanic rocks the duoni formation accumulated in fluvial and shelf coastal environments and was sourced from the zenong volcanic rocks and basement rocks from the southern region of the northern lhasa subterrane the enhanced magmatic activities in the northern lhasa subterrane may have influenced the duoni formation deposition during the late aptian early cenomanian 115 98 ma a north south transgression occurred causing the majority of the northern lhasa region to be dominated by shallow marine limestones of the langshan formation

the role of eddy transfer in setting the stratification and transport of a circumpolar current abstract high resolution numerical experiments of a circumpolar current are diagnosed to study how lateral and vertical transfer of buoyancy by geostrophic eddies balances advection by a meridional circulation driven by surface wind stresses and buoyancy fluxes a theory is developed in the framework of the residual circulation to relate the vertical and horizontal stratification set up to the transfer properties of eddies and the patterns of imposed wind and buoyancy forcing simple expressions are found for the depth of penetration stratification baroclinic transport and residual circulation of the current finally the ideas are applied to the antarctic circumpolar current acc and yield predictions for how its properties depend on wind and buoyancy forcing

secular geochemical variations of the lower cretaceous siliciclastic rocks from central tibet china indicate a tectonic transition from continental collision to back arc rifting abstract fifty five lower cretaceous siliciclastic rocks from central tibet were analyzed for major and trace elements in an attempt to evaluate their provenance and the type of tectonic setting in which they were deposited the spatial temporal variation of both sediment maturity indices sio 2 al 2 o 3 la v and zr tio 2 the ratios of immobile trace elements cr th th sc th co la sc and la co and sio 2 k 2 o na 2 o eu eu gd yb n and ferromagnesian contents displays that the berriasian valanginian bv sediment could have been derived from northern orogenic source areas whereas the hauterivian lower barremian hb sediment could have been closely related to the syndepositional volcanism in the northern lhasa block itself the na 2 o al 2 o 3 k 2 o al 2 o 3 relations also place approximately half of hb samples in the field of volcanic rocks of back arc rifting origin whereas little volcanic clasts exist in the bv rocks there are distinct signatures that the mafic ultramafic materials in the bv rocks were derived from oceanic island basalts ophiolites in the bangong nujiang suture therefore the collisional orogen due to the qiangtang lhasa collision should have played a key role in supply of the sediments during bv time bimodal sedimentary basaltic and rhyolitic tuffs with respective distinguished geochemical features exist in the hb sequence therefore a distinct tectonic transition from continental collision to back arc rifting in central tibet is believed to have occurred during early cretaceous time extreme depletion of na of most lower cretaceous samples compared to the upper crust and their high average chemical index alteration cia values and th u ratios point out the source rocks formed in tropical climatic regions with little tectonic uplift although the weathering degree of the source rocks for hb samples could likely have been affected by the syndepositional volcanism during middle cretaceous time the entire southern eurasian margin could have been characterized by a series of back arc rift basins including those in kohistan and ladakh