anomalous values of gravity and magnetism in the western margin of gondwana abstract this research is based on a joint geological and geophysical study performed in the south central andes region we acquired and processed terrestrial and satellite gravity data as well as terrestrial and aeromagnetic data balanced geological cross sections were constrained by physical properties of rocks densities and magnetic susceptibilities obtained from field samples and well log this study was performed in order to interpret a complex region that is still under debate the location of famatinian magmatic arc and its boundary with the cuyania terrain by means of gravity anomaly we developed direct and inverse models constrained by field data the existence of a major high density geological structure was evidenced from these models located below the vinchina basin and to the east of cerro rajado respectively the existence of such gravity high could be linked to the boundary between the famatinian magmatic arc and the accreted cuyania wedge

paleozoic laurentia gondwana interaction and the origin of the appalachian andean mountain system laurentia the rift bounded precambrian nucleus of north america may have broken out from a neoproterozoic supercontinent between east and west gondwana several lines of evidence suggest that the appalachian margin of laurentia subsequently collided with the proto andean margin of the amalgamated gondwana supercontinent in different relative positions during early and mid paleozoic time in route to final docking against northwest africa to complete the assembly of pangea hence the appalachian and andean orogens may have originated as a single mountain system the overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the paleozoic era laurentia gondwana collisions may help to explain contemporaneous unconformities in the paleozoic sedimentary cover of the laurentian gondwanan and baltic cratons

high resolution deep sea carbon and oxygen isotope records of eocene thermal maximum 2 and h2 eocene thermal maximum 2 etm2 and h2 were two short lived global warming events that occurred 2 m y after the paleocene eocene thermal maximum petm ca 56 ma we have generated benthic foraminiferal stable carbon and oxygen isotope records of four sites along a depth transect on walvis ridge 3 5 1 5 km paleodepth southeast atlantic ocean and one site on maud rise weddell sea to constrain the pattern and magnitude of their carbon isotope excursions cies and deep sea warming at all sites etm2 is characterized by 3 c warming and a 1 4 cie the h2 event that occurred 100 k y later is associated with 2 c warming and a 0 8 cie the magnitudes of the 13 c and 18 o excursions of both events are signifi cantly smaller than those during the petm but their coherent relation indicates that the 13 c change of the exogenic carbon pool was similarly related to warming during these events despite the much more gradual and transitioned onset of etm2 and h2

the disturbed zone around tunnels in jointed rock masses this paper presents an investigation of the effect of joint spacing on the size and shape of the disturbed zone around a tunnel a two dimensional distinct element simulation program udec was used to model the tunnel excavation in a simply jointed rock mass and an analytical method verified the numerical results rock masses were studied ranging from intact rock to heavily jointed rock with rock spacing less than 1 16 of the tunnel diameter four udec models were used to study the effect of joint spacing the disturbed zone was divided into 1 the failure zone where loose rock blocks fall into the tunnel 2 the open zone where joints open and 3 the shear zone numerical results were obtained for disturbed zones for different block sizes boundary conditions stress state and joint orientation an analytical study was made of the shear zones around the tunnel the failure zone is found in the immediate vicinity of the tunnel local block falling occurs there only when the joint spacing is small enough to create loose blocks the open zone enlarges with reducing joint spacing and stress ratio the shear zone around the tunnel can extend a considerable distance from the tunnel boundary the division of the disturbed zone into these zones makes it easier to study short term and long term tunnel instability

magnitude of the carbon isotope excursion at the paleocene eocene thermal maximum the role of plant community change carbon isotope measurements 13 c of leaf wax n alkanes from the paleocene eocene thermal maximum petm in the bighorn basin wyoming reveal a negative carbon isotope excursion cie of 4 5 which is 1 2 larger than that observed in marine carbonate 13 c records reconciling these records requires either that marine carbonates fail to record the full magnitude of the cie or that the cie in plants has been amplified relative to the marine amplification of the cie has been proposed to result from an increase in available moisture that allowed terrestrial plants to increase 13 c discrimination during the petm leaf physiognomy paleopedology and hydrogen isotope ratios of leaf wax lipids from the bighorn basin however all suggest that rather than a simple increase in available moisture climate alternated between wet and dry during the petm here we consider two other explanations and test them quantitatively with the carbon isotopic record of plant lipids the marine modification hypothesis is that the marine carbonate record was modified by chemical changes at the petm and that plant lipids record the true magnitude of the cie using atmospheric co2 13 c values estimated from the lipid record and equilibrium fractionation between co2 and carbonate we estimate the expected cie for planktonic foraminifera to be 6 instead the largest excursion observed is about 4 no mechanism for altering marine carbonate by 2 has been identified and we thus reject this explanation the plant community change hypothesis is that major changes in floral composition during the petm amplified the cie observed in nalkanes by 1 2 relative to marine carbonate this effect could have been caused by a rapid transition from a mixed angiosperm conifer flora to a purely angiosperm flora the plant community change hypothesis is consistent with both the magnitude and pattern of cie amplification among the different n alkanes and with data from fossil plants this hypothesis predicts that the magnitude and pattern of amplification of cies among different n alkanes will vary regionally and systematically depending on the extent of the replacement of conifers by angiosperms during the petm 2007 elsevier b v all rights reserved

borehole breakouts and compaction bands in two high porosity sandstones we compare the shape and mechanism of failure around vertical boreholes drilled in blocks of two high porosity sandstones subjected to unequal far field principal stresses tablerock sandstone has a porosity of 28 and is composed of 55 quartz and 37 weaker feldspar grains grain cementation is substantial through microcrystalline quartz critical far field stresses induce failure around boreholes in the form of v shaped dog eared breakouts the result of dilatant intra and trans granular microcracking subparallel to both the maximum horizontal far field stress and to the borehole wall on the other hand boreholes in mansfield sandstone which has similar porosity 26 but contains mainly quartz grains 90 held together primarily by spot sutured contacts fail by developing fracture like breakouts we show evidence suggesting that mansfield sandstone breakouts are preceded by the development of a narrow several grain diameters localized compaction zone along the direction of the least horizontal stress which is where the maximum compressive tangential stress concentration occurs failure mechanism here is simply the removal by the circulating drilling fluid of mainly intact grains loosened during the formation of the compaction band the type of cementation mineral homogeneity grain strength and sphericity appear to be major factors in the formation of compaction bands some breakout dimensions in both rocks are correlatable to the far field principal stresses and can be potentially used in conjunction with other information as indicators of their magnitudes

a blast of gas in the latest paleocene simulating first order effects of massive dissociation of oceanic methane hydrate carbonate and organic matter deposited during the latest paleocene thermal maximum is characterized by a remarkable 2 5 excursion in delta 13c that occurred over approximately 10 4 yr and returned to near initial values in an exponential pattern over approximately 2 x 10 5 yr it has been hypothesized that this excursion signifies transfer of 1 4 to 2 8 x 10 18 g of ch4 from oceanic hydrates to the combined ocean atmosphere inorganic carbon reservoir a scenario with 1 12 x 10 18 g of ch4 is numerically simulated here within the framework of the present day global carbon cycle to test the plausibility of the hypothesis we find that 1 the delta 13c of the deep ocean shallow ocean and atmosphere decreases by 2 3 over 10 4 yr and returns to initial values in an exponential pattern over approximately 2 x 10 5 yr 2 the depth of the lysocline shoals by up to 400 m over 10 4 yr and this rise is most pronounced in one ocean region and 3 global surface temperature increases by approximately 2 degrees c over 10 4 yr and returns to initial values over approximately 2 x 10 6 yr the first effect is quantitatively consistent with the geologic record the latter two effects are qualitatively consistent with observations thus significant ch4 release from oceanic hydrates is a plausible explanation for observed carbon cycle perturbations during the thermal maximum this conclusion is of broad interest because the flux of ch4 invoked during the maximum is of similar magnitude to that released to the atmosphere from present day anthropogenic ch4 sources

borehole instability in high porosity berea sandstone and factors affecting dimensions and shape of fracture like breakouts abstract stress induced breakouts in vertical boreholes are failure zones caused by excessive compressive stress concentration at the borehole wall along the springline of the least horizontal far field stress wellbores are sometimes drilled into aquifers or oil reservoirs that are weak poorly consolidated and highly porous sandstone formations which are often conducive to breakout formation breakouts are an expression of borehole instability and a potential source of sand production on the other hand the breakout phenomenon can be used advantageously in obtaining an estimate of the in situ stress condition the average orientation of breakouts as identified by borehole geophysical logging is a reliable indicator of in situ stress directions it has also been suggested that breakout dimensions could potentially be used as indicators of in situ stress magnitudes the research reported here has concentrated on the unique type of breakouts observed for the first time in high porosity berea sandstone drilling experiments in rock blocks subjected to critical far field true triaxial stress regimes simulating in situ conditions induced breakouts that were unlike the dog ear ones previously observed in granites limestones and low porosity sandstones the newly observed breakouts were thin tabular and very long resembling fractures that counterintuitively extended perpendicular to the maximum principal stress we found that a narrow zone ahead of a fracture like breakout tip underwent apparent localized grain debonding and compaction in the field such zones have been termed compaction bands and are a source of concern because in oil fields and aquifers they constitute curtains of low permeability that can impede the normal flow of oil or water in order to determine whether a correlation exists between fracture like breakouts and in situ stress we conducted several series of tests in which the minimum horizontal and vertical stresses were held constant and the maximum horizontal stress h was increased from test to test these tests showed strong dependence of the breakout length on far field stress signaling that potentially the ability to assess fracture like breakout length in the field could be used to estimate in situ stress magnitudes in conjunction with other indicators another series of tests revealed that breakout length increased substantially when borehole diameter was enlarged this result suggested that in the field where wellbore size is considerably larger fracture like breakout could extend to sizable distances creating a sand production hazard two series of tests one to evaluate the effect of drill bit penetration rate and the other to verify the drilling fluid flow rate effect on breakout formation and dimensions yielded inconsistent results and showed no unique trends remarkably fracture like breakouts maintained a consistent narrow width of about 5 10 grain diameters irrespective of the test conditions this characteristic supports the suggestion that fracture like breakouts are emptied compaction bands

lithology dependent minimum horizontal stress and in situ stress estimate abstract based on the generalized hooke s law with coupling stresses and pore pressure the minimum horizontal stress is solved with assumption that the vertical minimum and maximum horizontal stresses are in equilibrium in the subsurface formations from this derivation we find that the uniaxial strain method is the minimum value or lower bound of the minimum stress using anderson s faulting theory and this lower bound of the minimum horizontal stress the coefficient of friction of the fault is derived it shows that the coefficient of friction may have a much smaller value than what it is commonly assumed e g f 0 6 0 7 for in situ stress estimate using the derived coefficient of friction an improved stress polygon is drawn which can reduce the uncertainty of in situ stress calculation by narrowing the area of the conventional stress polygon it also shows that the coefficient of friction of the fault is dependent on lithology for example if the formation in the fault is composed of weak shales then the coefficient of friction of the fault may be small as low as f 0 2 this implies that this fault is weaker and more likely to have shear failures than the fault composed of sandstones to avoid the weak fault from shear sliding it needs to have a higher minimum stress and a lower shear stress that is the critically stressed weak fault maintains a higher minimum stress which explains why a low shear stress appears in the frictionally weak fault

distinct element modeling of hydraulically fractured lac du bonnet granite 1 the aim of this study is to better understand the mechanics of fracture development and propagation during hydraulic fracturing this paper presents some development and applications of discrete particle modeling of this problem a discontinuum modeling approach idealizes the material as separate particles bonded together at their contact points and utilizes the breakage of individual structural units or bonds to represent damage the numerical models are correlated with existing hydrofracture laboratory experiments which are presented in other publications a simulation of a laboratory scale hydrofracture experiment and the acoustic emission ae data from the experiment is used to validate the synthetic aes produced in the hydrofracture model this technique has been used to examine the mechanics of fracture initiation and time and spatial distributions of ae the modeling results demonstrate that the mechanism of hydraulically induced fracture in the lac du bonnet ldb granite core sample is predominantly tensile failure and that the shear cracks recorded in the hydrofracture experiment were due to slip on preexisting fractures numerical modeling of hydrofracture on homogeneous and heterogeneous synthetic samples seems to capture much of the behavior observed in the laboratory hydrofracture experiments

palaeoenvironmental reconstruction and evidence of marine influence in permian coal bearing sequence from lalmatia coal mine rajmahal basin jharkhand india a multi proxy approach abstract lalmatia coal bearing sequences belong to the barakar formation permian of lower gondwana the well developed coal bearing sequences approx 52 m exposed along the mine section have been studied to understand the depositional environment along with this a cuticle study was performed to understand the palaeoclimate with the help of the stomatal index the distribution of n alkanes n c15 to n c31 suggests major higher plant and algal dominant source input although variations are seen in the relative input steranes identified include c29 regular sterane and 4 methylsteranes the pr ph ratio varied from 0 43 to 4 26 warm temperate to subtropical palaeoclimate with fresh to brackish water bodies and reducing oxygen poor environment were inferred from the rb sr sr cu th u sr ba v v ni mean 13c value measured is 23 0 0 60 for coal and 22 6 for shale these values are well correlated with typical 13c values of the permian coal and shale the stomatal index of glossopteris leaves 280 million years was taken into account and it showed a value of 10 7 that assumes a higher level of atmospheric co2 during the barakar formation artinskian age the result also shows that permian taxon glossopteris can be used for co2 proxy during permian age the geochemical evidence suggests that towards the middle part middle coal seam of the studied section the depositional milieu was under a marine influenced marginal condition the geochemical studies and carbon isotopic ratios unequivocally suggest that fluctuating climatic conditions existed during the deposition of the lalmatia coal bearing sequence

paleoweathering and paleoenvironmental change recorded in lacustrine sediments of the early to middle eocene in fushun basin northeast china deciphering the long term interaction among continental silicate weathering global climate and atmospheric co2 concentrations is helpful in understanding the mechanisms of the cenozoic climate change and accessing the future climatic and environmental response to anthropogenic carbon emissions the eocene which is characterized by the early eocene climatic optimum eeco and the following global cooling represents an ideal test case here we generate geochemical data of the eocene lacustrine sediments from the fushun basin northeast china to explore the regional climatic response to the global climate change the chemical index of alteration cia and plagioclase index of alteration pia consistently show a gradual long term decrease indicating a climatic transition from warm and humid to relatively cold and arid during the eocene in the fushun basin this climatic trend is broadly coincident with the global cooling and decreasing co2 concentration implying that the regional climate is closely correlated with the global climate change over geological time scales additionally the extreme silicate weathering and high lake productivity as reflected by relatively positive 13c values of lacustrine organic matter are associated with the eeco this consistency may demonstrate that enhanced continental weathering and lake productivity had served as effective sinks to lower atmospheric co2 across the eeco collectively our new geochemical data add supporting evidence for a long term close coupling among continental silicate weathering climate and global carbon cycle during the eocene

multiple early eocene hyperthermals their sedimentary expression on the new zealand continental margin and in the deep sea the paleocene eocene thermal maximum petm ca 55 5 ma was a geologically brief interval characterized by massive influx of isotopically light carbon extreme changes in global climate and profound variations in earth system processes an outstanding issue is whether it was an isolated event or the most prominent example of a recurring phenomenon recent studies of condensed deep sea sections support the latter but this finding remains uncertain here we present and discuss lithologic and carbon isotope records across two lower eocene outcrops on south island new zealand the petm manifests as a marl rich horizon with a significant negative carbon isotope excursion cie above in sediment deposited between 54 and 53 ma are four horizons with similar though less pronounced expressions marl beds of all five horizons represent increased terrigenous sedimentation presumably linked to an accelerated hydrological cycle five corresponding clay rich horizons and cies are found in deep sea records although the lithologic variations represent carbonate dissolution rather than siliciclastic dilution the presence of five intervals with similar systemic responses in different environments suggests a mechanism that repeatedly injected large masses of 13 c depleted carbon during the early eocene

thermohydromechanics of partially saturated geological media governing equations and formulation of four finite element models this paper presents the general governing equations for coupled thermohydromechanical thm processes in saturated and unsaturated geologic formations and reviews four finite element codes for modeling of such system three of the codes are developed for the special purpose of analyzing coupled thm processes in unsaturated porous and fractured geological media and the fourth is a commercial code that has been used in its standard version with a few adaptations for this specialized problem the basic assumptions and fundamental equations for coupled thm processes in unsaturated porous fractured rock are presented and formulations of the four finite element models are compared

dual porosity poroelastic analyses of wellbore stability this paper presents a numerical solution of dual porosity poroelastic formulations that couple solid deformations with fluid flow in both matrix and fracture systems of naturally fractured reservoirs the finite element numerical method was applied to solve the inclined wellbore problem in which mud weight was considered for both permeable and impermeable boundary conditions failure criteria relevant to tensile collapse and shear failure modes were introduced into the numerical model and resulting failure areas around boreholes were subsequently defined several related applications including the stability of inclined and horizontal wellbores under various in situ stress regimes were evaluated upper and lower mud weight bounds and the most stable borehole orientations were determined the best trajectory selections for horizontal boreholes were also investigated

assessment of the influence of hydraulic and mechanical anisotropy on the fracture initiation pressure in permeable rocks using a complex potential approach abstract this work aims at studying the fracture initiation pressure of a horizontal wellbore drilled in an anisotropic poroelastic medium based on an analytical model using the complex potential approach the closed form solution of stresses in the surrounding formation is deduced as a function of the orientation in respect with stratification and the pressure inside wellbore the analytical solution is developed for the steady state flow in a permeable formation due to the difference between pressures applied at well wall and the pore pressure at the far field once the stress distribution on the borehole wall is obtained it is possible by using a known anisotropic tensile strength criterion for the rock to infer the explicit expression of the fracture initiation pressure as a function of different parameters such as the anisotropic poroelastic properties the anisotropic initial stresses the bedding dip angle and the anisotropic tensile strength of the formation the validation of the analytical is conducted by comparing it with the numerical results using the finite element method finally through the parametric study we highlight the influence of different factors on the fracture initiation pressure

stable carbon isotopes of wood a clue to palaeoclimate abstract detailed stable carbon isotope and molecular investigations were undertaken on a number of archaeological and fossil wood specimens to provide insights into their use as rigorous independent palaeoclimatic and palaeoenvironmental parameters the isotope data revealed large differences amongst the material with the oldest specimens being least depleted in 13 c although natural variation could account for some of the observed differences the isotope values of the archaeological and tertiary specimens are most probably related to the absolute abundance of polysaccharides present and the degree of lignin alteration the molecular data based on pyrolysis of the antarctic cretaceous conifer specimens revealed only transformed lignin with virtually no intact lignin building blocks 2 methoxyphenols preserved and no evidence of polysaccharides this degree of chemical alteration is suggested here to be one of the main causes for the 13 c enriched values of these conifer specimens these results show the importance of combining detailed molecular information on individual wood components with stable carbon isotope data for palaeoclimatic and palaeoenvironmental studies

evolution of stress induced borehole breakout in inherently anisotropic rock insights from discrete element modeling the aim of this study is to better understand the mechanisms controlling the initiation propagation and ultimate pattern of borehole breakouts in shale formation when drilled parallel with and perpendicular to beddings a two dimensional discrete element model is constructed to explicitly represent the microstructure of inherently anisotropic rocks by inserting a series of individual smooth joints into an assembly of bonded rigid discs both isotropic and anisotropic hollow square shaped samples are generated to represent the wellbores drilled perpendicular to and parallel with beddings at reduced scale the isotropic model is validated by comparing the stress distribution around borehole wall and along x axis direction with analytical solutions effects of different factors including the particle size distribution borehole diameter far field stress anisotropy and rock anisotropy are systematically evaluated on the stress distribution and borehole breakout propagation simulation results reveal that wider particle size distribution results in the local stress perturbations which cause localization of cracks reduction of borehole diameter significantly alters the crack failure from tensile to shear and raises the critical pressure rock anisotropy plays an important role on the stress state around wellbore which lead to the formation of preferred cracks under hydrostatic stress far field stress anisotropy plays a dominant role in the shape of borehole breakout when drilled perpendicular to beddings while a secondary role when drilled parallel with beddings results from this study can provide fundamental insights on the underlying particle scale mechanisms for previous findings in laboratory and field on borehole stability in anisotropic rock

rapid carbon sequestration at the termination of the palaeocene eocene thermal maximum the palaeocene eocene thermal maximum warm event about 56 million years ago was caused by the release of large amounts of carbon to the ocean and atmosphere estimates of the rate of recovery from the event suggest that about 2 000 pg of the carbon released was sequestered as organic carbon

tempo and scale of late paleocene and early eocene carbon isotope cycles implications for the origin of hyperthermals abstract the upper paleocene and lower eocene are marked by several prominent 1 carbon isotope 13c excursions cie that coincide with transient global warmings or thermal maxima including the paleocene eocene thermal maximum petm the cie which are recorded mainly in marine sedimentary sequences have also been identified in continental sequences occurred episodically and yet appear to be paced or triggered by orbital forcing to constrain the timing and scale of the cie relative to long term baseline variability we have constructed a 4 52 million year myr long high resolution 3 kyr bulk sediment carbon isotope record spanning the lower eocene to upper paleocene c25r c24n from a pelagic sediment section recovered at odp site 1262 in the southeast atlantic this section which was orbitally tuned utilizing high resolution core log physical property and geochemical records is the most stratigraphically complete upper paleocene to lower eocene sequence recovered to date time series analysis of the carbon isotope record along with a high resolution fe intensity record obtained by xrf core scanner reveal cyclicity with variance concentrated primarily in the precession 21 kyr and eccentricity bands 100 and 400 kyr throughout the upper paleocene lower eocene in general minima in 13c correspond with peaks in fe i e carbonate dissolution both of which appear to be in phase with maxima in eccentricity this covariance is consistent with excess oceanic uptake of isotopically depleted carbon resulting in lower carbonate saturation during periods of high eccentricity this relationship includes all late paleocene and early eocene cie confirming pacing by orbital forcing the lone exception is the petm which appears to be out of phase with the 400 kyr cycle though possibly in phase with the 100 kyr cycle reinforcing the notion that a mechanism other than orbital forcing and or an additional source of carbon is required to account for the occurrence and unusual scale of this event

early eocene carbon isotope excursions evidence from the terrestrial coal seam in the fushun basin northeast china a series of transient global warming events between 56 and 50 ma are characterized by a pronounced negative carbon isotope excursion cie however the documents of these hyperthermals such as eocene thermal maximum 2 and h2 events have come chiefly from marine sediments and their expression in terrestrial organic carbon is still poorly constrained here we yield a high resolution carbon isotope record of terrestrial organic material from the fushun basin which displays four prominent cies with magnitudes larger than 2 5 based on age constraint and comparisons with deep sea records our data provide the first evidence of the four hyperthermals in coal seams and suggest a global significance of these events moreover the difference of cie magnitudes between marine and terrestrial records shows a significant linear correlation with the marine carbonate cie implying that these events are likely attributable to recurring injections of 13c depleted carbon from submarine methane hydrates and or permafrost

multiporosity multipermeability approach to the simulation of naturally fractured reservoirs this paper presents an array of deformation dependent flow models of various porosities and permeabilities relevant to the characterization of naturally fractured reservoirs a unified multiporosity multipermeability formulation is proposed as a generalization of the porosity or permeability oriented models of specific degree some new relationships are identified in the parametric investigation for both single porosity and dual porosity models a formula is derived to express skempton s constant b by biot s coefficient h and relative compressibility it is found that the recovery of the original expression for skempton s constant b is largely dependent on the choice of representing relative compressibility the dual porosity dual permeability model is evaluated through an alternative finite element approximation the deformation dependent fracture flow mechanism is introduced where the rock matrix possesses low permeability and fracture flow is dominant a preliminary study of the reservoir simulation identifies the strong coupling between the fluid flow and solid deformation

flow to a heated borehole in porous thermoelastic rock analysis exact solutions are obtained for fluid flow induced by the heating of a borehole the rock is modeled as a fluid saturated porous thermoelastic medium the temperature and pore pressure fields are governed by a pair of diffusion equations which are coupled through a source term in the pressure equation proportional to the temperature rate the pressure profile exhibits a maximum that grows in magnitude and propagates away from the borehole for a constant heat flux applied as an instantaneous step the fluid flux to the borehole takes a finite initial value and decays monotonically when the heat flux exhibits a finite rise time the fluid flux is initially zero rises to a maximum and then decays at late time the inverse of the fluid flux is linear in ln t this observation can be exploited to estimate the permeability and fluid diffusivity of low permeability rock sample calculations are shown for westerly granite

crustal stress determination from boreholes and rock cores fundamental principles abstract obtaining quantitative information on the state of stress in the crust can only be accomplished by accessing the zone of interest by drilling the borehole cavity however disrupts the virgin stress state by concentrating stresses in predictable patterns with a number of consequences at sufficiently low stress magnitudes the concentrated stresses amplify the elastic anisotropy azimuthally around the borehole because the elastic properties of most rocks depend nonlinearly on stress at higher levels the stresses damage then fracture the rock near the wellbore wall or within a growing core stub indeed almost all of the borehole techniques indirectly measure stress through these manifestations of the concentrated stresses this contribution reviews at a fundamental level the concentration of stresses by the borehole the effects on the materials in the borehole s vicinity and how these are used to infer stress states stress concentrations applied to rocks which are generally nonlinear elastic materials induce both radial and azimuthal variations in elastic wave speeds near the borehole that can be used to infer stress directions from advanced acoustic logging methods hydraulic fractures initiate propagate and remain propped open by measured pressures that can be interpreted with knowledge of the stress concentrations to obtain quantitative stress magnitudes at higher stress levels the rock fails producing borehole breakouts or drilling induced tensile fractures both of which are indicative of stress directions and can be used to constrain stress magnitudes similarly the various styles of drilling induced core fractures indicate both stress directions and the faulting environment encountered unfortunately no method currently allows for complete determination of the stress tensor using a number of different but complementary techniques is necessary to best constrain in situ stress states

cuyania an exotic block to gondwana review of a historical success and the present problems abstract a review of the early history of the cuyania terrane and the numerous pioneering works of the past century provides the present robust framework of evidence supporting a derivation from laurentia travel towards gondwana as an isolated microcontinent and final amalgamation to the protomargin of western gondwana in middle to late ordovician times the major remaining uncertainties and inconsistencies such as the time of deformation and collision with gondwana the lack of evidence of famatinian derived zircons the effects of strike slip displacements proposed along the suture as well as the potential sutures defined by ophiolite assemblages are discussed the precise boundary along the northern and southern limits is not yet well defined the most suitable hypothesis based on present data is that cuyania originated as a conjugate margin of the ouachita embayment south of the appalachian platform during early cambrian times the subsequent travel toward the gondwana protomargin is clearly depicted by the changing faunal assemblages in the carbonate platform new geochemical and age data on k bentonites presented by several authors reinforce the strong connection between cuyania ash fall tuffs and famatina volcanics by 468 470 ma indicating cuyania and gondwana were in close proximity at that time extension related to flexural subsidence preceded by the drowning of the carbonate platform in early llanvirnian times is recorded by abrupt facies changes in the sedimentary cover during late llanvirnian and early caradocian times this episode marked the beginning of contact between cuyania and gondwana the subsequent evolution of the foreland basin indicates that deformation lasted until latest silurian early devonian times the time of collision is tracked by the cessation of arc related magmatic activity in the upper plate gondwana protomargin at about 465 ma in western sierras pampeanas and ages around 454 ma corresponding to syncollisional and postcollisional magmatism the age of the collision is also preserved in the lower plate cuyania where both angular unconformities in the sedimentary cover and the ages of peak of regional metamorphism in the basement rocks point to 460 ma as the most probable age for the beginning of the collision evidence from the upper plate is essentially identical with an age of 463 ma thermal gradients along this suture vary from 13 c km in the lower plate to 18 c km in the fore arc upper plate reaching more than 30 c km along the famatinian arc based on these data a llandelian caradocian age for the collision can be postulated on firm grounds deformation continued through most of the early paleozoic until amalgamation of the chilenia terrane by the late devonian

an analytical model of coal wellbore stability based on block limit equilibrium considering irregular distribution of cleats abstract an analytical model of coal wellbore stability is established based on the block limit equilibrium method taking the irregular distribution of cleats into account because coal seam is cut into discontinuous blocks with different shapes and sizes block slip can be treated as the symbol of coal wellbore instability according to the geometric characteristic of blocks blocks surrounding the wellbore can be divided into 3 categorizes type i type ii and type iii to evaluate the block slip the stress distribution near the wellbore is determined based on the continuous medium mechanics and the fracture mechanics using mohr coulomb criterion and limit equilibrium method the critical conditions of block slip are derived by defining the failure coefficient including type i type ii and type iii the proposed model is verified by comparison of the obtained results from the proposed analytical method with the discrete element method as well as the actual drilling it can be proved that the proposed model can predict accurately block slip additionally the effect of drilling fluid pressures on the wellbore stability is specially investigated the results show that with the increasing of drilling fluid pressure the degree of wellbore collapse may be more severe meanwhile the analysis method of reasonable drilling fluid pressure for coal drilling is proposed based on the interval theory combining the slip area of blocks finally the parametric study is focused on investigating the effects of the internal friction angle of face cleats the cohesion of face cleats and the tensile strength of butt cleats on the failure coefficients of blocks

atmospheric pco2 reconstructed across five early eocene global warming events abstract multiple short lived global warming events known as hyperthermals occurred during the early eocene 56 52 ma five of these events the paleocene eocene thermal maximum petm or etm1 h1 or etm2 h2 i1 and i2 are marked by a carbon isotope excursion cie within both marine and terrestrial sediments the magnitude of cie which is a function of the amount and isotopic composition of carbon added to the ocean atmosphere system varies significantly between marine versus terrestrial substrates here we use the increase in carbon isotope fractionation by c3 land plants in response to increased pco2 to reconcile this difference and reconstruct a range of background pco2 and peak pco2 for each cie provided two potential carbon sources methane hydrate destabilization and permafrost thawing organic matter oxidation although the uncertainty on each pco2 estimate using this approach is low e g median uncertainty 23 18 this work highlights the potential for significant systematic bias in the pco2 estimate resulting from sampling resolution substrate type diagenesis and environmental change careful consideration of each of these factors is required especially when applying this approach to a single marine terrestrial cie pair given these limitations we provide an upper estimate for background early eocene pco2 of 463 248 131 ppmv methane hydrate scenario to 806 127 104 ppmv permafrost thawing organic matter oxidation scenario these results which represent the first pco2 proxy estimates directly tied to the eocene hyperthermals demonstrate that early eocene warmth was supported by background pco2 less than 3 5 preindustrial levels and that p co 2 1000 ppmv may have occurred only briefly during hyperthermal events

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

palaeoclimatic evolution during eocene and its influence on oil shale mineralisation fushun basin china the fushun basin is a small explored coal and oil shale bearing cenozoic fault basin in the liaoning province northeast china the basin mainly consists of eocene swamp to lacustrine deposits of the guchengzi to xilutian formation and contains the biggest opencast oil shale mine in asia this mine has provided an ideal opportunity to undertake palaeoclimate reconstruction in this basin based on a single geological profile and the analyses of 93 samples using various approaches namely field geological observation clay mineralogical and geochemical sr ba sr cu stable c and o isotope analyses all of which were compared with palaeobotanical data the eocene climate of fushun basin evolved from warm temperate to north subtropical and generally changed from warm humid to subhumid semiarid paleoclimatic and geochemical parameters shows that the very warm and humid climate during jijuntun formation increased the initial productivity of lake water and caused a steady stratification of the lake water then caused oxygen lack in the bottom of water productivity of the lake provides the mean origin of organic matters for oil shale formation and steady anoxic environment is beneficial for the conservation of organic matters

early gondwanan connection for the argentine precordillera terrane abstract the precordillera of argentina is widely accepted as an exotic terrane of laurentian north american affinity newly acquired u pb ages on individual detrital zircons from lower cambrian and upper ordovician quartz sandstone beds in the argentine precordillera indicate a gondwanan provenance not associated with any known part of laurentia accordingly the precordillera terrane is likely underlain by basement rock of gondwanan affinity in addition detrital zircons from the upper ordovician sandstone bed provide no evidence for a mid ordovician position against the inboard famatina arc these results demand critical re evaluation of widely held assumptions regarding the paleogeography of the argentine precordillera

warming the fuel for the fire evidence for the thermal dissociation of methane hydrate during the paleocene eocene thermal maximum dramatic warming and upheaval of the carbon system at the end of the paleocene epoch have been linked to massive dissociation of sedimentary methane hydrate however testing the paleocene eocene thermal maximum hydrate dissociation hypothesis has been hindered by the inability of available proxy records to resolve the initial sequence of events the cause of the paleocene eocene thermal maximum carbon isotope excursion remains speculative primarily due to uncertainties in the timing and duration of the paleocene eocene thermal maximum we present new high resolution stable isotope records based on analyses of single planktonic and benthic foraminiferal shells from ocean drilling program site 690 weddell sea southern ocean demonstrating that the initial carbon isotope excursion was geologically instantaneous and was preceded by a brief period of gradual surface water warming both of these findings support the thermal dissociation of methane hydrate as the cause of the paleocene eocene thermal maximum carbon isotope excursion furthermore the data reveal that the methane derived carbon was mixed from the surface ocean downward suggesting that a significant fraction of the initial dissociated hydrate methane reached the atmosphere prior to oxidation

application of drill monitoring for chargeability assessment in sublevel caving abstract currently the charging procedure for sublevel caving mining is carried out with no prior information of the rock mass condition thus engineers are blindsided to unexpected rock conditions and ill prepared to address issues associated with collapsing boreholes this results on charging problems and as consequence bad fragmentation of the rock after blasting which difficult ore loading and transportation as the gravity flow of the rock is reduced this paper builds up the work done by ghosh et al ijrmms 2018 to classify the geotechnical rock condition into five classes solid rock fractured rock cave in minor and major cavity from it two applications based on the measure while drilling mwd technique have been developed one for geotechnical rock condition of orebodies and the other for predicting the risk of collapse in boreholes the work of ghosh et al has been improved into a geotechnical rock condition block model to simplify the quantitative assessment and automatic recognition of rock trends a thorough correction of the mwd parameters has been also applied to minimize external influences other than the rock mass from it the risk of borehole collapses model has been developed by comparing different combinations of the geotechnical rock condition block model with the charging length of 102 production fan holes the assessment of the number of collapsed and non collapsed blastholes and the charging length blasthole length ratio has been used to assign high medium or low risk of collapse to each combination the results predict collapses in the first half of the fan holes for the high risk collapses in the second half of the fan hole for the medium risk and no collapses along the hole for the non risk holes the two models have been applied in large scale for two orebodies in the malmberget mine sweden which comprises 20 drifts and 5060 fan shape long holes

early eocene warming events and the timing of terrestrial faunal exchange between india and asia the timing of initiation of continent continent collision between asia and india is controversial but this major tectonic event is generally thought to have occurred in the early eocene ca 50 ma new and independent data from strontium isotopes stable carbon isotopes microfossil biostratigraphy and mammal fossils from an early eocene marginal marine sequence cambay shale at the vastan lignite mine of western india indicate that terrestrial faunal exchanges and therefore continental collision between asia and the indian subcontinent took place before 53 7 ma this age coincides with the second eocene thermal maximum etm2 a short lived warming episode that followed the paleocene eocene thermal maximum petm ca 55 5 ma our data also document for the first time a clear record of the etm2 in terrestrial organic material from a low latitude site which is represented by a 3 4 carbon isotope excursion cie in lignite and dispersed organic carbon 13c values the magnitude of the cie at this location closely matches that observed in marine cores from the arctic ocean which supports an interpretation that this hyperthermal event though of lower magnitude was similar in character to that of the petm being a global phenomenon that affected both terrestrial and marine ecosystems

borehole stability analysis accounting for anisotropies in drilling to weak bedding planes abstract borehole instabilities pose significant challenges to drilling and completion operations particularly in regions with weak bedding planes and pre existing fractures where formations have strong anisotropies the bedding planes rock anisotropy and their impacts on horizontal stresses are considered in the proposed model to improve borehole stability modeling this improved model enables to calculate borehole failures and minimum mud weight along borehole trajectories with various drilling orientations versus bedding directions laboratory test data of rock compressive strengths are analyzed and a new correlation is developed to allow for predicting uniaxial compressive strengths in weak rocks from sonic velocities time dependent rock compressive strength is also examined to analyze the wellbore failure evolution with time the slip failure gradient in the weak planes is derived which can be used to model wellbore sliding shear failure in the planes of weakness the mud weight applied to prevent borehole shear failures in both intact rocks and ones with weak bedding planes can be obtained from the proposed model

effect of confining pressure unloading on strength reduction of soft coal in borehole stability analysis underground borehole drilling usually causes instability in the surrounding coal due to in situ stress redistribution including stress concentration and stress release however the mechanisms of unloading induced coal strength reduction are still poorly understood the primary objective of this study is to investigate the effect of confining pressure unloading on soft coal strength reduction for borehole stability analysis a series of mechanical tests were conducted on both the traditionally and newly reconstituted coal samples under two different experimental stress paths including conventional uniaxial triaxial compression and triaxial compression with confining pressure unloading the unloading stress path was obtained by analyzing the stress redistribution around a borehole to capture a more accurate coal mechanical response according to our experimental results plastic deformation generated before failure under the unloading stress path is smaller than that generated under the conventional loading stress path furthermore the cohesion of the traditionally and newly reconstituted samples diminishes approximately by 44 77 and 29 66 respectively with confining pressure unloading indicating that there is a significant reduction in coal strength due to confining pressure unloading the mechanism for unloading induced coal strength reduction comes from confining pressure unloading induced increase in shear stress on the fracture surface and a decrease in shear strength this effect increases the shear slipping potential whose driving force generates tension fractures at both ends of the preexisting fractures

experimental observations of the mechanics of borehole failure in porous sandstone abstract to investigate borehole failure under hydrostatic stress in sandstone of varying porosity experiments were conducted on three sandstone varieties with 2 5 mm bores tennessee darley dale and penrith sandstone types were selected to represent most of the spectrum of sandstone porosity variations respectively 0 07 0 12 and 0 28 confining pressures ranged up to 500 mpa and pore fluid volumometry was used to detect bore failure failed samples were studied microstructurally using optical and scanning electron microscopy each rock type failed with the development of oriented breakout features normally seen to develop around boreholes loaded under non hydrostatic stress and in tennessee sandstone at least these developed in a consistent orientation with respect to an external reference frame dip direction of foreset beds all of the rock types showed strength and or elastic anisotropy thus the formation of oriented breakouts under hydrostatic loading is attributed to the effects of anisotropy two modes of breakout development were observed in darley dale and penrith sandstones a combination of intergranular shear and extensional fracture produced broad and shallow features with breakout width unaltered during growth in tennessee sandstone a purely extensional mode of intragranular fracture was observed leading to deeper breakout features the samples tested showed higher failure pressures for smaller bore sizes when failure pressure is normalized with respect to grain crushing pressure p i and bore diameter with the product of porosity with grain size all data lie on a single master curve with bore failure pressure approximately 0 15 p i in the regime of bore size independent behaviour

lithospheric and upper mantle structure of central chile and argentina summary the tectonics of central chile and argentina have been greatly affected by the shallow dips of the subducting nazca plate which controlled patterns of magmatism and deformation nearly 1000 km away from the plate boundary we calculate receiver functions from data recorded by the charge array which transected the andes and sierras pampeanas in central chile and argentina to better constrain the crustal structure of this region beneath the northern transect of the charge array where the nazca slab flattens near 100 km we find the crust is over 60 km thick beneath the andes and thin to the east the thick crust however extends 200 km to the east of the high elevations estimates of vp vs obtained from receiver functions vary along ancient terrane boundaries exhibiting higher values to the west interestingly we observe that the amplitude of the phase corresponding to the moho on receiver functions diminishes to the west complicating our images of crustal structure we proposed that the observations presented here of thickened crust within a region of low elevations diminished receiver function arrivals and reports of high shear wave speeds atop of the mantle wedge overlying the shallowly subducted nazca slab can be explained by partial eclogitization of the lower crust the moho appears simpler across the southern transect where it can be identified near 50 km depth at its deepest point beneath the andes and shallows eastwards volcanism remains active near the latitudes of our southern transect and we observe multiple crustal lowvelocity zones indicative of regions of partial melt near the centres of volcanism signals related to the nazca slab remain more elusive suggestive of a small impedance contrast between the slab and overlying mantle

gravity characterization of the la rioja valley basin argentina thelariojavalleyisasedimentarybasininthepampean ranges a region of basement uplifts and broad valleys in the andean foreland the present study advances understanding ofthegenesisofthebasinbyprovidingsubsurfacedataonits geometry and characteristics gravity and topographic data including a newly completed gravity survey were integrated tocreateacompletebougueranomalymap eulerandwerner deconvolution techniques sum with traditional potentialfieldsmethodsandsparseindependentdatafromseismicand magnetotelluric methods to test and constrain the subsurface geophysical characterization of the la rioja valley basin the residual anomaly obtained through upward continuation was inverted to obtain the best 3d gravity model of the la riojavalley basin which shows the first order geometry ofthebasin thisrevealsthatthebasinisasymmetric withup to 6000 m of low density basinfill in a depocenter located in thewest centralsectorofthevalley second orderfeaturesof thebasinincludeasoutheast trendingbasementhigh likelya fault zone that subdivides the basin which has been revealed by a suite of analytical methods 3d model of depth to basement a 2d inverse model and werner deconvolution solutions

goce satellite derived gravity and gravity gradient corrected for topographic effect in the south central andes region summary global gravity field models derived from satellite measurements integrated with terrestrial observations provide a model of the earth s gravity field with high spatial resolution and accuracy the earth gravity model egm08 a spherical harmonic expansion of the geopotential up to degree and order 2159 has been used to calculate two functionals of the geopotential the gravity anomaly and the vertical gravity gradient applied to the south central andes area the satellite only field of the highest resolution has been developed with the observations of satellite goce up to degree and order 250 the topographic effect a fundamental quantity for the downward continuation and validation of satellite gravity gradiometry data was calculated from a digital elevation model which was converted into a set of tesseroids this data is used to calculate the anomalous potential and vertical gravity gradient in the southern central andes region the geological structures are very complex but not well resolved the processing and interpreting of the gravity anomaly and vertical gradients allow the comparison with geological maps and known tectonic structures using this as a basis a few features can be clearly depicted as the contact between pacific oceanic crust and the andean fold and thrust belt the seamount chains over the oceanic nazca plate and the famatinian and pampean ranges moreover the contact between the rio de la plata craton and the pampia terrain is of great interest since it represents a boundary that has not been clearly defined until now another great lineament the valle fertil desaguadero mega lineament an expression of the contact between cuyania and pampia terranes can also be clearly depicted the authors attempt to demonstrate that the new gravity fields can be used for identifying geological features and therefore serve as useful innovative tools in geophysical exploration

investigations of water inrushes from aquifers under coal seams in many coal mines limestone confined aquifers underlie coal seams during coal extraction from these mines water inrushes occur frequently with disastrous consequences this paper introduces the hydrogeological conditions of the coal mines and the potential water inrush disasters from aquifers under coal seams it then presents the water inrush mechanism the main factors which control water inrushes include strata pressure mining size geologic structures and the water pressure in the underlying aquifer analysis shows that reduction of confinement due to mining is the major cause of the water conducting failure in the floor strata the depth of the failure zone is strongly dependent on the mining width this paper also presents field observation results of the water conducting failure in the floor strata and applies the finite element method coupled with stress dependent permeability to analyze hydraulic conductivity enhancement due to coal extraction finally theoretical and empirical methods to predict water inrushes are given and technical measures for improving mine design and safety for coal extraction over aquifers are presented these measures include fault and fracture grouting and mining method modification such as changing long wall to short wall mining

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

early paleozoic orogenic belt of the andes in southwestern south america result of laurentia gondwana collision the late precambrian to early paleozoic age rock units of the pampean ranges the puna and the north patagonian massif of southwestern south america constitute the famatinian orogenic belt they are interpreted as an ordovician collisional belt between the occidentalia terrane and the gondwana craton they include mafic and ultramafic belts of neoproterozoic to early paleozoic age an intense tectonothermal event resulted from the collision syntectonic granitoids represent crustal melting in that collision syntectonic to late tectonic foreland basins developed the recently proposed juxtaposition of laurentia and east antarctica australia in the neoproterozoic raises the possibility that laurentia and western south america were close together in the early paleozoic and therefore that the famatinian belt resulted from laurentia gondwana collision occidentalia which is bordered by a cambrian carbonate platform similar to that of eastern north america may be a sliver detached from laurentia during late ordovician time

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

thick skinned deformation observed on deep seismic reflection profiles in western argentina one of the first deep seismic reflection profiles in south america has resulted from reprocessing nearly 90 km of 96 channel vibroseis data that were originally collected by ypf the argentine national oil company across the sierra de la huerta in the sierras pampeanas of western argentina reprocessed sections show the following features in the upper 20 km of crust 1 prominent layering within carboniferous to quaternary strata and 2 a complex suite of faint reflections which are interpreted to define both extensional and compressional faults the mesozoic strata are bounded by one or more west dipping normal faults one or two east dipping thrust faults shallow with depth a geometry similar to the rocky mountain foreland of the western u s a one thrust has a clear surface trace although lacking evidence of surface scarps or clear fault plane reflections panels of constant dip domain seen in the seismic data justify the application of fault bend folding techniques to suggest that a second deeper fault becomes nearly flat at 15 20 km cumulative cenozoic offset on the thrusts beneath the sierra de la huerta is about 13 km a well located earthquake hypocenter lies within the footwall block of a major thrust and is presumed to represent susidiary thrust motion along a secondary high angle fault gravity models suggest that the thrust ramps may nucleate at an anomalous dense mass in the middle or lower crust interpretation of lower crustal structure is hampered by the lack of clear reflections presumably due to small scale and low amplitude impedance contrast variations of lithologic units at those depths consideration of recorrelation limitations and signal penetration constraints provide likely rationale for the scarcity of clear reflections in the basement rocks of the region but low reflectance of mafic metamorphic rocks remains a possible explanation

shallow gravity driven flow in a poro elastic layer by combining biot s theory of poro elasticity with standard shallow layer scalings a theoretical model is developed to describe axisymmetric gravity driven flow through a shallow deformable porous medium motivated in part by observations of surface uplift around co2 sequestration sites the model is used to explore the injection of a dense fluid into a horizontal deformable porous layer that is initially saturated with another less dense fluid the layer lies between a rigid base and a flexible overburden both of which are impermeable as the injected fluid spreads under gravity the matrix deforms and the overburden lifts up the coupled model predicts the location of the injected fluid as it spreads and the resulting uplift of the overburden due to deformation of the solid matrix in general the uplift spreads diffusively far ahead of the injected fluid if fluid is injected with a constant flux and the medium is unbounded both the uplift and the injected fluid spread in a self similar fashion with the same similarity variable r t1 2 the asymptotic form of this spreading is established results from a series of laboratory experiments using polyacrylamide hydrogel particles to create a soft poro elastic material are compared qualitatively with the predictions of the model

analysis of borehole breakouts boreholes drilled into rock which is subjected to stresses that amount to a significant fraction of the strength of the rock may cause the rock to fail adjacent to the borehole surface often this results in the elongation of the cross section of the borehole in the direction of the minimum principal compressive stress orthogonal to the borehole axis such breakouts are valuable indicators of the direction of the minimum compressive stress orthogonal to the axis of the borehole their shapes may provide information about the magnitudes of both the maximum and minimum stresses relative to the strength of the rock borehole breakouts also may be impediments to drilling and to in situ measurement techniques such as hydraulic fracturing observations and analyses of borehole breakouts raise three important questions first how does the shape of the borehole breakout evolve second why are breakout shapes stable despite the very high compressive stress concentrations that they produce third how is the shape of the breakout related to the magnitudes of the stresses in the rock in this paper extensile splitting of rock in unconfined plane strain compression is assumed to be the process of rock failure adjacent to the circumference of the borehole by which a breakout forms to simulate the evolution of a borehole breakout this process is combined with a numerical boundary element analysis of the stresses around a borehole as its cross section evolves from the originally circular shape to that of a stable breakout the tangential stresses around a stable breakout cross section are found to be everywhere less than the unconfined plane strain tensile or compressive strength of the rock the stresses outside the stable breakout are found to be everywhere less than the limiting values of shear strength given by a mohr coulomb criterion in the regions of great stress concentrations at the ends of a breakout cross section which have a pointed shape the state of stress approaches that of equal biaxial compression in plane strain as it does ahead of a mathematical crack or notch the fact that the stresses around a breakout are less than the relevant strength establishes both the stability of the final breakout cross section and the appropriateness of an elastic analysis of the stresses according to this model the cross sectional shapes of stable breakouts are not related uniquely to the state of stress and the strength of the rock for example stable breakouts created instantly in rock already subjected to stress are much larger than stable breakouts created in the same rock with a preexisting borehole by subsequently increasing the stresses to the same values the results of drilling into an actual rock probably lie between these extremes modest changes in borehole cross section as a result of breakout do not alter significantly the minimum tangential tensile stress around a borehole with internal pressure from that given by the kirsch solution for a circular hole subjected to the same stresses therefore hydraulic fracturing interpretations based on the kirsch solution give the correct values for the far field stresses

dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the paleocene isotopic records across the latest paleocene thermal maximum lptm indicate that bottom water temperature increased by more than 4 c during a brief time interval 104 years of the latest paleocene 55 6 ma there also was a coeval 2 to 3 excursion in the 13c of the ocean atmosphere inorganic carbon reservoir given the large mass of this reservoir a rapid 13c shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere however a direct consequence of warming bottom water temperature from 11 to 15 c over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic ch4 hydrate at locations with intermediate water depths in terms of the present day oceanic ch4 hydrate reservoir thermal dissociation of oceanic ch4 hydrate during the lptm could have released greater than 1 1 to 2 1 1018 g of carbon with a 13c of approximately 60 the release and subsequent oxidation of this amount of carbon is sufficient to explain a 2 to 3 excursion in 13c across the lptm fate of ch4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the lptm

consolidation of fissured clays a numerical model for consolidation of fissured clays on the basis of the double porosity concept is presented as applications of the model two numerical examples are analysed a one dimensional soil column and a strip loading on a two dimensional medium of a fissured clay it is shown that in comparison to an equivalent single porosity model the settlement in a double porosity medium is delayed due to the slow dissipation of excess fluid pressure in the clay matrix in addition three distinct time periods are identified in the consolidation of a fissured clay the characteristic of each period are discussed and the parameters affecting these periods are highlighted moreover it is shown that the mandel cryer effect which is often discussed in relation to consolidation of soils treated as single porosity material may not develop in fissured clays nous presentons un modele numerique pour la consolidation des argiles fissurees sur la base du concept de double porosite en applications du mod acu

multiscale insights into borehole instabilities in high porosity sandstones this paper presents a multiscale analysis of the classical borehole instability problem in high porosity sandstones using a hierarchical multiscale approach a rigorous two way message passing coupling of finite element method and discrete element method is employed where the finite element method is employed to solve a boundary value problem and the constitutive material responses required at each of its integration points are derived by the discrete element method solution to an embedded representative volume element instead of by using an assumed phenomenological model we employ this multiscale approach to examine the successive failure of a borehole subjected to gradually decreased support stresses at the inner borehole wall or increased far field stresses to reproduce the material behavior of high porosity sandstone representative volume elements with a high porosity structure and interparticle bonds are generated it is found that stress concentration triggers the initial failure at the borehole wall and the subsequent failure in form of deformation bands is driven by further stress concentration ahead of the band tips the failure pattern around the borehole varies with initial stress state and global loading path the comparison of the local stress paths of the initial failure points in various cases reveals that the change of failure pattern from shear failure to mixed mode failure and to compaction failure is dominated by the increased mean stress cross scale parametric studies show that the failure mode changes from clear compaction bands to multiple arrays of shear bands by changing the high porosity specimen to a low porosity one the increase in cohesion strength expands the yield locus and enlarges the critical mean stress between different failure patterns hence the failure mode may change from compaction failure to mixed mode failure or from mixed mode failure to shear failure due purely to the increased cohesion strength the qualitative comparisons indicate greater length larger area and more severe damage of the diametrically opposite failure with the increase in minor principal stress 0 and principal stress ratio the diametrically opposite failure mode is found a good indicator for 0 direction but it may also occur under hydrostatic far field stress due to material anisotropy

closed form solution of stress state and stability analysis of wellbore in anisotropic permeable rocks abstract this work aims to develop a closed form solution of stress state around wellbore drilled in the anisotropic permeable rocks taking into account the transient effect of fluid flow for this purpose a simplified method was adopted to solve the anisotropic hydraulic diffusion equation knowing the explicit transient evolution of pore pressure the hydraulic and hydro mechanical potentials were proposed through which stress state around wellbore are determined thanks to the use of the well known complex potential approach the validity of this closed form solution was verified by comparing with the result obtained from the numerical simulation by combining the analytical solution of stresses on well wall with a chosen anisotropic failure criterion of rock mass the stability analysis of wellbore was carried out through the parametric study we elucidated the influence of different factors on the safe mud pressure window notably the anisotropic poro elastic properties the anisotropic strength of the rock masses as well as the anisotropic initial stress state and the inclined bedding plane of formation

a laurentian grenville age oceanic arc back arc terrane in the sierra de pie de palo western sierras pampeanas argentina

a mathematical model for consolidation in a thermoelastic aquifer due to hot water injection or pumping a mathematical model is developed for the areal distribution of fluid pressure temperature land subsidence and horizontal displacements due to hot water injection into thermoelastic confined and leaky aquifers the underlying assumption is that the aquifer is thin in relation to the horizontal distances of interest and hence all dependent variables of interest are average over the thickness values the solid matrix is assumed to be thermoelastic following the development of three dimensional conservation of mass and energy equations and equilibrium equations in terms of horizontal and vertical displacements the mathematical model is derived by averaging the three dimensional model over the vertical thickness of the aquifer subject to conditions of plane total stress the effects of viscous dissipation and compressible work have been included in the formulation the resulting averaged coupled equations are in terms of pore water pressure temperature and vertical and horizontal displacements which are functions of x y and t only the equations are nonlinear and have to be solved simultaneously due to the coupling that exists among them equations and appropriate boundary conditions in radial coordinates have also been presented for an example of a single injecting or pumping well

major and trace elements in mid eocene lacustrine oil shales of the fushun basin ne china concentration features and paleolimnological implications abstract mid eocene tectonic subsidence of the pull apart fushun basin ne china accommodated an up to 300 m thick oil shale succession within the lacustrine jijuntun formation lutetian based on sedimentary records and bulk element geochemistry accumulation of this oil shale succession is modeled to evaluate the principal controls on enrichment patterns of organic matter om and trace metals in lacustrine organic facies the jijuntun formation consisting primarily of massive oil shale and interbedded siltstone and sandstone that grade rapidly upward into well laminated oil shale records an overall evolution from lacustrine deltaic facies associations to profundal facies associations sedimentological characteristics of the oil shale succession display a gradual change of moderately overfilled to balanced fill lake fill mode consistent with the up section drop in siliciclastic inputs reflected by detrital proxies like decreased si al and k na ratios variations in the sedimentary fill states are accompanied by increasing hydrographic restriction of the jijuntun paleo lake in agreement with low mo toc consistently

do the large carbon isotopic excursions in terrestrial organic matter across paleocene eocene boundary in india indicate intensification of tropical precipitation abstract five distinct transient warming hyperthermal events paleocene eocene thermal maximum petm h1 etm2 elmo h2 i1 and i2 marked by negative carbon isotope excursions cies occurred between late paleocene and early eocene 56 to 52 ma interval however not many records of either the petm or definitive early eocene hyperthermals eehs are yet available from terrestrial realm in the tropics except two neo tropical sections of colombia and venezuela jaramillo et al 2010 therefore response of the tropical biosphere to these warming events is not very well known here we report high resolution carbon isotope 13c chemostratigraphy biomarker calcareous nannofossils and pollen data from the cambay shale formation of western india paleolatitude 5 s which show complete preservation of all the above cie events including the petm hitherto unknown from tropical terrestrial record comparatively larger magnitudes of cies for all the hyperthermal events the petm and eehs point towards a possible intensification of precipitation during the petm and all the early eocene hyperthermal cie events this inference is supported by data of lignin phenols and presence of tropical rain forest elements spanning the entire time period 56 52 ma and suggest that higher organic burial and soil erosion favored deposition of thick lignitic seams as a consequence of high tropical precipitation

coupling effects in double porosity media with deformable matrix 1 the significance of the microscopic coupling effects on the macroscopic response of double porosity media is investigated in reference to the consolidation of fissured clays the basic findings of the investigation are validated through a set of laboratory test data

increased precipitation and weathering across the paleocene eocene thermal maximum in central china global warming during the paleocene eocene thermal maximum petm 55 5 million years ago ma was associated with a massive release of carbon to the ocean atmosphere system as evidenced by a prominent negative carbon isotope excursion cie and widespread dissolution of marine carbonates the paleohydrologic response to the petm warming has been studied worldwide however relevant records of environmental perturbation in asia are lacking so far here we extend the record of this event in central china a subtropical paleosetting through geochemical and mineralogical analyses of lacustrine sediments geochemical indicators of authigenic carbonates including molar mg ca and sr ca ratios suggest an overall increased precipitation across the petm compatible with the disappearance of authigenic dolomite and the appearance of kaolinite in the strata the relatively humid conditions persisted long after the carbon cycle perturbation had stopped implying that the transient hyper greenhouse warming might have forced the regional climate system into a new climate state that was not easily reversed additionally a gradual increase in chemical index of alteration cia and the appearance of kaolinite are associated with the petm indicating an intensified silicate weathering and pedogenesis in the watershed in response to warmer and more humid climate our results corroborate the theory that an accelerated continental chemical weathering served as a negative feedback to sequester carbon and lower the atmospheric greenhouse gas levels during the petm

terrestrial carbon isotope excursions and biotic change during palaeogene hyperthermals the earth s climate between 60 and 50 million years ago was punctuated by several abrupt warming events the largest of these being the palaeocene eocene thermal maximum carbon isotope records from the bighorn basin wyoming document these events in the terrestrial realm and show a consistent scaling between marine and terrestrial records across the three main events

an overview of rock stress measurement methods this paper presents an overview of methods that have been used to estimate the state of stress in rock masses with the emphasis on methods applicable to hard rocks and scandinavia rock stress is a difficult quantity to estimate because the rock stress measuring techniques consist of perturbing the rock measuring displacements or hydraulic parameters and converting the measured quantities into rock stresses there are two main types of method those that disturb the in situ rock conditions i e by inducing strains deformations or crack opening pressures and those that are based on observation of rock behaviour without any major influence from the measuring method the most common methods are briefly described including their application areas and limiting factors

mathematical model for regional land subsidence due to pumping 2 integrated aquifer subsidence equations for vertical and horizontal displacements a mathematical model for regional subsidence due to pumping from an aquifer is developed on the basis of biot s work on coupled three dimensional consolidation following biot s work on threedimensional consolidation with coupling between mass conservation and equilibrium equations a mathematical model for regional subsidence due to pumping from an aquifer is developed by averaging the three dimensional model over the thickness of the aquifer and assuming conditions of plane stress both vertical land subsidence and horizontal displacements as functions of plane coordinates and time can be estimated by solving the model equations for a given confined or leaky confined aquifer an analytical solution is presented for the special case of a single well pumping from an infinite homogeneous isotropic aquifer the solution provides estimates of changes in averaged over the vertical values of piezometric head vertical subsidence and horizontal displacement the results indicate that under the conditions of the studied case of radial flow the solution for piezometric head is identical to the one obtained by noncoupled models furthermore half the volume strain is produced by vertical subsidence while the other half by the horizontal displacement hence the vertical subsidence is only approximately half the value obtained in noncoupled models which neglect horizontal displacement a numerical example demonstrates these conclusions

evolution of permeability in sand injectite systems abstract sand injectite complexes comprise kilometer scale clastic intrusion networks that act as effective conduits for the migration accumulation and then recovery of hydrocarbons and other fluids an equivalent continuum model is constructed to represent a sand injectite reservoir coupling stress and fluid flow in fractured rock using the continuum simulator toughreact coupled with flac3d to follow deformation and fluid flow a permeability model which uses staged percolation models is proposed to improve permeability estimation of fracture networks by accommodating four different levels of fracture connectivity this permeability model is confirmed against field and laboratory data corresponding to the different connectivities of fracture networks the new constitutive permeability model is incorporated into the coupled hydro mechanical simulator framework and applied to sand injectites with the analysis of permeability evolution mechanisms and mechanical sensitivity the results indicate that when the magnitudes of principal stresses increase in a constant ratio normal closure is the dominant mechanism in reducing fracture aperture and thereby permeability conversely the evolution of stress difference can accentuate aperture and permeability due to an increase in shear dilation for critically or near critically oriented fractures also the evolution of aperture and related permeability of fractured rock are more sensitive at lower stress states than at higher stress states due to the hyperbolic relationship between normal stress and normal closure of the fractures

mathematical model for regional land subsidence due to pumping 1 integrated aquifer subsidence equations based on vertical displacement only a mathematical model for regional land subsidence has been developed by employing terzaghi s concept of effective stress and assuming vertical solid compressibility only first the groundwater mass conservation equation in a compressible aquifer is developed for a three dimensional space assuming that the problem justifies an assumption of a thin aquifer relative to horizontal lengths of interest this equation is then integrated over the aquifer s thickness taking into account the conditions on top and bottom surfaces bounding the aquifer the result is the commonly used aquifer flow equation in terms of averaged piezometric head by introducing a relationship between changes in averaged piezometric head and corresponding changes in aquifer thickness a single equation is obtained in terms of land subsidence the development is carried out for a single and a multi layered confined aquifer and for a phreatic one an example is given of the application of the equation in radial coordinates describing land subsidence in a hypothetical six layered aquifer

response of carbon isotopic compositions of early middle permian coals in north china to palaeo climate change abstract to investigate the magnitude to which the carbon isotopic ratio 13c varies in coals in response to their contemporary terrestrial environment the early middle permian huainan coals including coals from the shanxi formation lower shihezi formation and upper shihezi formation in north china were systematically sampled a 2 5 variation range of 13c values 25 15 to 22 65 was observed in huainan coals with an average value of 24 06 as coal diagenesis exerts little influence on carbon isotope fractionation 13c values in coals were mainly imparted by those of coal forming flora assemblages which were linked to the contemporary climate the 13c values in coals from the shanxi and lower shihezi formations are variable reflecting unstable climatic oscillations heavy carbon isotope is enriched in coals of the capitanian upper shihezi formation implying a shift to high positive 13c values of coeval atmospheric co2 notably our study provides evidence of the kamura event in the terrestrial environment for the first time

the famatina complex nw argentina back docking of an island arc or terrane accretion early palaeozoic geodynamics at the western gondwana margin

the enigmatic chad lineament revisited with global gravity and gravity gradient fields abstract the crustal structure of northern africa is puzzling large areas being of difficult access and concealed by the sahara the new global gravity models are of unprecedented precision and spatial resolution and offer a new possibility to reveal the structure of the lithosphere beneath the sahara the gravity gradients correlate better than gravity with geological features such as rifts fold belts and magmatic deposits and intrusions they are an ideal tool to follow geological units e g basement units below a stratigraphic layer of varying density e g sediments we focus on the chad lineament a 1300 km arcuate feature located between the west and central african rift system the gravity fields show differences between the lineament and the west and central african rift system along the centre of the lineament high density rocks must be present which relate to either magmatic or metamorphic rocks this is very different to the lineaments of the western and central west african rift system which are filled with sediments considering present models of rifting and the absence of topography the lineament cannot be coeval to the west and central african rift system and is most likely older we suggest that the lineament is a structural element of the saharan metacraton

carbon isotope behaviour in wood and cellulose during artificial aging abstract modern wood was subjected to an artificial decay process to investigate its behaviour with respect to the stable carbon isotope composition of wood and the corresponding cellulose four different woods were used namely oak quercus robur beech fagus sylvatica pine pinus sylvestris sequoia sequoiadendron giganteum and pure spruce cellulose the process of decay was simulated by heating samples in water at 180 c from minutes to months during the decay period the carbon content of wood increased to almost 70 while the cellulose content decreased gradually to zero the degradation proceeds according to a two stage process an initial very fast break down is followed by a rather slow degradation after a strong discrimination against 13 c during the initial phase the 13c 12c ratio increases again mostly up to its initial value the carbon isotope trend for cellulose shows a similar pattern though it is less pronounced maximum discrimination was 1 relative to the untreated wood 13c values of cellulose decreased by up to 0 3 the 13 c depletion is presumably a consequence of the preferential decomposition of cellulose as compared to lignin data on fossil trees are compared with the results obtained from this treatment of aging the 13 c decrease is in line with observations from these fossil woods but the ensuing 13 c enrichment is at most weakly to be seen

proterozoic early paleozoic evolution in western south america a discussion abstract the hypothesis of exotic terranes in peru bolivia argentina and chile generated discussions on the mode of transfer and extent of accretional events that may have occurred in the southern andes during the late proterozoic early paleozoic initially a tectogenesis based on autochthonous mobile fold belts was discussed following ideas emphasised the fragmentation of the supercontinent rodinia laurentia moving along the west gondwana border and colliding with the gondwana western margin the most important effect of this laurentia gondwana relationship was attributed to the argentine precordillera or cuyania terrane splitting off from laurentia and docking to gondwana in the early paleozoic in this study the most cited arguments for this laurentia precordillera relationship are discussed emphasising paleontological considerations it is shown that these arguments do not exclude a close original vicinity of the precordillera terrane to gondwana the precordillera terrane is suggested to be part of a hypothetical platform which developed between south america africa and antarctica safran platform and which was displaced to its actual position by transcurrent faults the collisional events in the sierras pampeanas ensued from strike slip movements and were responsible for the s and i type transpressional magmatism along the pampean and famatinian terranes the final result of this continent parallel movement of terrane slices is similar to that of a terrane split off from laurentia but the first named way of formation easier explains the general continuity of plate convergence at the western border of gondwana than the laurentia precordillera connection does

a case for a comet impact trigger for the paleocene eocene thermal maximum and carbon isotope excursion abstract we hypothesize that the rapid onset of the carbon isotope excursion cie at the paleocene eocene boundary 55 ma may have resulted from the accretion of a significant amount of 12c enriched carbon from the impact of a 10 km comet an event that would also trigger greenhouse warming leading to the paleocene eocene thermal maximum and possibly thermal dissociation of seafloor methane hydrate indirect evidence of an impact is the unusual abundance of magnetic nanoparticles in kaolinite rich shelf sediments that closely coincide with the onset and nadir of the cie at three drill sites on the atlantic coastal plain after considering various alternative mechanisms that could have produced the magnetic nanoparticle assemblage and by analogy with the reported detection of iron rich nanophase material at the cretaceous tertiary boundary we suggest that the cie occurrence was derived from an impact plume condensate the sudden increase in kaolinite is thus thought to represent the redeposition on the marine shelf of a rapidly weathered impact ejecta dust blanket published reports of a small but significant iridium anomaly at or close to the paleocene eocene boundary provide supportive evidence for an impact

discussion on integrating borehole breakout dimensions strength criteria and leak off test results to constrain the state of stress across the chelungpu fault taiwan abstract the in situ stress computations presented by haimson et al 2010 overestimated the maximum horizontal stress in the vicinity of the chelungpu fault taiwan our analyses from extended leak off tests drilling induced tensile fractures and anderson s faulting theory show considerable lower values for the maximum horizontal stress our results suggest that closer to the ruptured fault more s h is released implying a strong earth crest in the shallower section which is far away from the ruptured fault however it would require more detailed and intensive work in order to achieve an accurate interpretation of the in situ stress in this region

well bore breakouts and in situ stress the detailed cross sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data we show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression this work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ

coal strength and young s modulus related to coal rank compressional velocity and maceral composition abstract this paper presents an experimental investigation of coal rank and maceral composition influences on the coal mechanical behaviors the complete stress strain behavior uniaxial compressive strength young s modulus and acoustic compressional velocity were measured and correlated to coal ranks and microstructures the test results show that coal is an elasto brittle geo material and its uniaxial compressive strength and young s modulus increase as coal rank increases this occurs because as vitrinite reflectance or coal rank increases coal has less microporous structure and thus higher uniaxial compressive strength therefore using vitrinite reflectance value instead of vitrinite content is advantageous for correlating coal strength the experimental results also demonstrate that compressive strength and young s modulus have positive exponential correlation even for different types of coal therefore the compressive strength of coal is highly related to its young s modulus the uniaxial compressive strength and acoustic compressional velocity of coal are also correlated but a single correlation does not exist for different coal ranks instead different relationships occur for different types of coal

mode and tempo of the paleocene eocene thermal maximum in an expanded section from the venetian pre alps mode and tempo of an expanded paleocene eocene thermal maximum section in the venetian pre alps

the thermo hydro mechanical behavior of the argillaceous cobourg limestone the cobourg limestone is a low permeability argillaceous rock that forms a part of the paleozoic sedimentary sequence found in southern ontario canada the limestone has a heterogeneous fabric consisting of nodular regions of calcite and dolomite and argillaceous partings of a similar composition but with a low clay content which gives the appearance of nominal stratifications the thermo hydro mechanical thm behavior of the rock is of interest to the proposals for using sedimentary formations as candidate rocks for siting deep geological repositories for the storage of heat emitting nuclear fuel waste the paper presents the results of experiments where thm processes were initiated in an intact cylindrical sample of the cobourg limestone containing a central cylindrical fluid filled cavity biot s classical theory of poroelasticity extended to include thermal effects is used to examine the thm response of the fluid cavity due to boundary heating of the cylinder the rise and decay of thermally induced cavity fluid pressure is used to examine the applicability of the thm modeling the experiments were conducted on cylindrical samples of the cobourg limestone with their axes either along or normal to the nominal planes of the argillaceous partings

generation of tonalitic and dioritic magmas by coupled partial melting of gabbroic and metasedimentary rocks within the deep crust of the famatinian magmatic arc argentina the source regions of dioritic and tonalitic magmas have been identified in a deep crustal section of the famatinian arc sierras pampeanas of western argentina the source zones of intermediate igneous rocks are located at the transition between a gabbro dominated mafic unit and a tonalite dominated intermediate unit in the upper levels of the mafic unit mafic magmas intruded into metasedimentary wall rocks crystallized mainly as amphibole gabbronorite and caused the partial melting of the surrounding metasediments in turn the leucogranitic melts sourced from the metasedimentary rocks intruded into the newly crystallized but still hot mafic layers and catalysed the process of partial melting of the gabbroic plutonic rocks the gabbroic rocks became mafic migmatites comprising amphibole rich pyroxene bearing mesosomes and leucotonalitic veins significantly most of the mafic migmatites have isotopic compositions sr sr t 50 7063 and end t 1 4 0 94 to 2 24 similar to those of the gabbroic rocks and distinct from those of their complementary leucotonalitic veins sr sr t 1 4 0 7075 0 7126 and end t 5 2 65 providing evidence for the idea that melting of the mafic rocks was triggered by the intrusion of leucogranitic anatectic melts sr sr t 1 4 0 715 and end t 1 4 6 21 mass balance calculations show that the model reaction plagioclase amphibole leucogranitic melt leucotonalitic melt clinopyroxene orthopyroxene can better explain the partial melting of the gabbroic rocks based on field observations we argue that the coalescence of leucotonalitic veins in the mafic migmatites led to breakdown of the solid matrix to form melt dominated leucotonalitic pools however the leucotonalitic veins that crystallized before leaving behind the mafic migmatitic rock are chemically elemental and isotopic more evolved than the dioritic and tonalitic rocks we envisage that once detached from their source region the leucotonalitic magmas were able to react commingle and mix with entrained fragments of both mafic and metasedimentary rocks this process gave rise to melts that became tonalitic and dioritic magmas this study concludes that the generation of intermediate magmas is a multistage process with three critical steps 1 influx and emplacement of hydrous mafic magmas into a deep crust containing metasedimentary country rocks 2 physically and chemically coupled melting of mafic and metasedimentary rocks leading to the formation of a leucotonalitic vein and dyke system that coalesces to form leucotonalitic or tonalitic magma bodies 3 retrogression of the leucotonalitic magmas by partially assimilating entrained fragments of their mafic and metasedimentary precursors the dimensions of the

lignite deposits of the kutch basin western india carbon isotopic and palynological signatures of the early eocene hyperthermal event etm2 abstract this study presents new results of combined palynological and stable carbon isotope 13 c investigations carried out in the well known lignite sequence at panandhro district kutch in the gujarat state of western india dinoflagellate cysts and associated spore pollen assemblage assign an early eocene ypresian age to the lignitic succession at panandhro furthermore a pronounced negative carbon isotope excursion cie of about 2 7 correlated to the second eocene thermal maximum 53 7 ma a globally recognized hyperthermal event was discovered in the middle part of the succession consistent with the palynological constraints this is the first record of an eocene hyperthermal event etm2 from the kutch basin our data has regional implications for the age of the lignitic sequences across western india as it demonstrates that there is no significant age difference between the lignite deposits of the kutch and cambay basins our results also support a lutetian age for the previously described vertebrate fossils including whales from the panandhro mine section

numerical assessment of the effects of end restraints and a pre existing fissure on the interpretation of triaxial tests on stiff clays conventional laboratory triaxial tests apply axisymmetric boundary conditions to a cylindrical sample which has an axisymmetric geometry for a homogeneous sample this implies that the deformed sha

u pb th pb and ar ar geochronology from the southern sierras pampeanas argentina implications for the palaeozoic tectonic evolution of the western gondwana margin

coupled thermal hydraulic mechanical phenomena in saturated fractured porous rocks numerical approach the fundamentals of the theory of consolidation and thermoelasticity are recast into the formulation of a phenomenon called thermohydroelasticity subsequently a variational principle and galerkin formulation are combined with the finite element method to develop a new technique to investigate coupled thermal hydraulic mechanical behavior of liquid saturated fractured porous rocks a code to code verification of the method is performed finally the environment of a heater emplaced in hard rock is simulated the effects of the coupled thermal stresses in the fractured rock are evident from the dramatic reduction of permeability due to the deformation of the fractures these results can improve the understanding of observations and displacement measurements made in the in situ experiments at the stripa mine in sweden 46 references

release of methane from a volcanic basin as a mechanism for initial eocene global warming a 200 000 yr interval of extreme global warming marked the start of the eocene epoch about 55 million years ago negative carbon and oxygen isotope excursions in marine and terrestrial sediments show that this event was linked to a massive and rapid approximately 10 000 yr input of isotopically depleted carbon it has been suggested previously that extensive melting of gas hydrates buried in marine sediments may represent the carbon source and has caused the global climate change large scale hydrate melting however requires a hitherto unknown triggering mechanism here we present evidence for the presence of thousands of hydrothermal vent complexes identified on seismic reflection profiles from the voring and more basins in the norwegian sea we propose that intrusion of voluminous mantle derived melts in carbon rich sedimentary strata in the northeast atlantic may have caused an explosive release of methane transported to the ocean or atmosphere through the vent complexes close to the palaeocene eocene boundary similar volcanic and metamorphic processes may explain climate events associated with other large igneous provinces such as the siberian traps approximately 250 million years ago and the karoo igneous province approximately 183 million years ago