present day stress field changes along the baikal rift and tectonic implications intraplate extension in a frame of a global compressional stress field seems linked to local lithospheric perturbations lithospheric thinning or thickening able to modify the resulting state of stress zoback 1992 the baikal rift zone brz siberia is located north of the india asia collision zone and exhibits no direct communication with any oceanic domain it can thus be fully considered as an area of continental extension dominated by the global compressional intraplate stress field resulting from plate driving forces in order to address the problem of its dynamics and kinematics and their links with the india asia collision a comprehensive stress tensor analysis is presented based on 319 focal mechanisms of earthquakes located along the whole baikal rift the stress field is varying at different scales of observation when looking at central asia several thousands kilometers the maximum horizontal stress shmax directions remain rather constant with a fan shape geometry when the tectonic regime goes from compressional himalayas to extensional baikal when observing the baikal rift about 1000 km long clear variations of the stress regime are observed from an extensional regime in the central part of the rift to wrench ones in its northern and southern ends finally at the scale of 100 km systematic shmax reorientations occur close to major rift faults we thus infer that the interaction between collisional processes and inherited structures may have a strong influence on rift dynamics we then use computed stress tensors to predict slip vectors on major rift faults deformation patterns show two distinct parts of the rift the south baikal rift sbr is characterized by a constant trending around n100 e slip vector meanwhile the north baikal rift nbr exhibits a complex block rotation behavior involving at least three crustal blocks we propose to interpret these surficial structures and motions as the result of an interaction between the regional compression coming from the india asia collision and the geometry of the hardly deformable siberian platform this particular setting can explain most of the surficial deformation patterns which suggest a large scale cracking of the lithosphere in the baikal region other possible sources of stress could also be considered like deep mantellic upwelling or trench suction linked to the pacific subduction

stress magnitude and orientation in the potiguar basin brazil implications on faulting style and reactivation 1 we analyzed borehole breakout data and drilling induced tensile fractures derived from resistivity image logs run at 10 oil wells to derive the orientation of the maximum horizontal stress shmax from the potiguar basin in the continental margin of brazil stress magnitudes are derived from density logs for the vertical stress mini frac tests for the minimum horizontal stress shmin and rock strength laboratory analysis to estimate the shmax magnitudes we compared these results with the stress regime and shmax orientation derived from nine earthquake series located in the crystalline basement where seismicity is concentrated and previous breakout data from the basin in the basin the shmax gradient is 20 0 mpa km and the shmax shmin ratio is 1 154 indicating a normal tectonic stress regime from 0 5 to 2 0 km whereas the shmax gradient of 24 5 mpa km and shmax shmin ratio of 1 396 indicate a transition from a normal to strike slip stress regime at 2 5 to 4 0 km the deeper stress regime in the basin is similar to that in the basement at 1 12 km depth this transition of the tectonic stress regime is consistent with an incipient tectonic inversion in the basin we note that the shmax orientation rotates from nw se in the western part of the potiguar basin to e w in its central and eastern parts roughly following the shoreline geometry indicating that local features such as flexural stresses influence the local scale 100 km stress pattern we also conclude that the basement is critically stressed but not the basin

long lived localized magmatism in central eastern part of the pernambuco alagoas domain borborema province ne brazil implications for tectonic setting heat sources and lithospheric reworking magmatic activity circumscribed to a restricted area lasting for tenths of millions of years allows to link changes in magma chemistry to deformation and thus to determine the tectonic evolution of a region here we describe an area of about 180 40 km in eastern borborema province northeastern brazil where several pulses of magmatism occurred in the time interval 656 562 ma the oldest pulses are represented by orthogneisses bearing a flat lying foliation resulting from deformation and metamorphism of granitoids emplaced in three stages 656 645 ma 632 618 ma and 606 597 ma the oldest and most important unit 656 652 ma old has large length width ratio c 10 alkali calcic signature lacks pronounced negative ta anomalies whereas showing positive hf and zr anomalies in normalized trace element diagrams and yields crystallization age similar to the deposition age of surrounding dominantly clastic metasedimentary units these characteristics point to emplacement in an extensional setting and most probably fit in a rift sag basin model with thermal thinning of the lithosphere generating regional subsidence and magmatism rocks related to the 632 618 ma old stage are represented by orthogneisses with petrographic and geochemical characteristics akin to i type granites indicating a shift to an orogenic setting at around 640 630 ma increased radioactive heating during continued thickening promoted anatexis of metasedimentary rocks at mid crustal levels and segregation of small volumes of peraluminous melts at 606 597 ma the subsequent period of magmatic activity at 587 581 ma witnesses the transition to a transcurrent regime with intrusion of large granitic and syenitic batholiths with high k calc alkalic to shoshonitic affinity intrusion of these plutons added to heat resulting from strain heating to produce peraluminous granites at 573 566 ma through partial melting of metasedimentary sources the final magmatic episode at c 562 ma suggests involvement of the lithospheric mantle and crustal sources perhaps related to localized delamination generally younger orthogneisses and plutons contain zircons inherited from previous magmatic events pointing to important crustal reworking together with the geochemistry of the rocks these results imply only modest addition of juvenile material and thus no net crustal growth in eastern borborema province during the 660 560 ma time interval

strain localization at the margins of strong lithospheric domains insights from analog models the lateral variation of the mechanical properties of continental lithosphere is an important factor controlling the localization of deformation and thus the deformation history and geometry of intraplate mountain belts a series of three layer lithospheric scale analog models with a strong domain sd embedded at various depths are presented to investigate the development of topography and deformation patterns by having lateral heterogeneities within a weak continental lithosphere the experiments performed at a constant velocity and under normal gravity indicate that the presence or absence of the sd controls whether deformation is localized or distributed at a lithospheric scale deformation and topography localize above the edges of the sd while the sd region itself is characterized by minor amounts of surficial deformation and topography the depth of the sd within the ductile crust ductile mantle lithosphere or both controls the pattern of deformation and thus the topography the presence of a sd in the ductile crust or in the mantle results in limited surficial topographic effects but large variations in the moho topography strong moho deflection occurs when the sd is in the ductile crust while the moho remains almost flat when the sd is in the mantle when the sd occupies the ductile lithosphere the sd is tilted these analog experiments provide insights into intraplate strain localization and could in particular explain the topography around the tarim basin a lithospheric scale heterogeneity north of the india asia collision zone

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

factors controlling geochemical and mineralogical compositions of coals preserved within marine carbonate successions a case study from the heshan coalfield southern china abstract the late permian coals in the heshan coalfield of southern china are preserved within marine carbonate successions and characterized by super high organic sulfur 5 13 10 82 minerals identified in the coals include quartz kaolinite illite mixed layer illite smectite albite pyrite marcasite calcite and dolomite along with trace amounts of smectite fluorite strontianite rey bearing carbonate minerals jarosite and water bearing fe oxysulfate the coals are very rich in trace elements including f up to 3362 g g v up to 270 g g se up to 24 4 g g mo up to 142 g g u up to 111 g g and to a lesser extent sr y zr nb cd cs heavy rare earth elements hf ta w hg and th previous studies attributed the high organic sulfur and elevated trace elements to the seawater influence or the formation of soil horizons before the accumulation of peat in the basin however mineralogical and geochemical data presented in this study have shown that the sediment source region and multi stage hydrothermal fluids are the dominant influences on the mineralogical composition and elevated trace elements in the coal although seawater influence also contributed to the composition of the mineral matter for example a large proportion of the quartz and clay minerals as well as almost all the albite in both the coal benches and the parting mudstones were derived from detrital materials of terrigenous origin in the yunkai upland high concentrations of lithophile trace elements were also derived from the sediment source region minerals including fluorite calcite dolomite strontianite and rey bearing carbonate minerals were derived from multi stage hydrothermal activities high concentrations of v mo and u that occur through the coal seam sections were probably derived from hydrothermal solutions during peat accumulation or at the early diagenetic stages the hydrothermal fluids also corroded the syngenetically formed minerals quartz albite and pyrite and caused re distribution of lithophile elements from partings to the underlying coal benches resulting in higher key element ratios yb la nb ta and zr hf and more abundant heavy rare earth elements in the coal benches than in the immediately overlying partings

a fresh approach to investigating co2 storage experimental co2 water rock interactions in a low salinity reservoir system the interactions between co2 water and rock in low salinity host formations remain largely unexplored for conditions relevant to co2 injection and storage core samples and sub plugs from five jurassic aged surat basin sandstones and siltstones of varying mineralogy have been experimentally reacted in low salinity water with supercritical co2 at simulated in situ reservoir conditions p 12mpa and t 60 c for 16days 384h with a view to characterising potential co2 water rock interactions in fresh or low salinity potential siliclastic co2 storage targets located in queensland australia co2 water rock reactions were coupled with detailed mineral and porosity characterisation obtained prior to and following reaction to identify changes in the mineralogy and porosity of selected reservoir and seal rocks during simulated co2 injection aqueous element concentrations were measured from fluid extracts obtained periodically throughout the experiments to infer fluid rock reactions over time fluid analyses show an evolution of dissolved concentration over time with most major e g ca fe si mg mn and minor e g s sr ba zn components increasing in concentration during reaction with co2 similar trends between elements reflect shared sources and or similar release mechanisms such as dissolution and desorption with decreasing ph small decreases in concentration of selected elements were observed towards the end of some experiments however no precipitation of minerals was directly observed in petrography sample characterisation on a fine scale allowed direct scrutiny of mineralogical and porosity changes by comparing pre and post reaction observations scanning electron microscopy and registered 3d images from micro computed tomography micro ct indicate dissolution of minerals including carbonates chlorite biotite members and to a lesser extent feldspars quantitative mineral mapping of sub plugs identified dissolution of calcite from carbonate cemented core with a decrease in calcite content from 17vol to 15vol following reaction and a subsequent increase in porosity of 1 1vol kinetic geochemical modelling of the co2 water rock experiments successfully reproduced the general trends observed in aqueous geochemistry for the investigated major elements after coupling experimental geochemistry with detailed sample characterisation and numerical modelling expected initial reactions in the near well region include partial dissolution and desorption of calcite mixed carbonates chloritic clays and annite due to ph decrease followed in the longer term by dissolution of additional silicates such as feldspars dissolution of carbonates is predicted to improve injectivity in the near well environment and contribute to the eventual re precipitation of carbonates in the far field

co2 ecbm field tests in the ishikari coal basin of japan abstract the feasibility of extracting gas from coal seam while storing carbon dioxide underground was evaluated in japan a co2 ecbm project had begun near the town of yubari on the island of hokkaido in northern japan the primary coal seam of interest was a 5 6 m thick yubari coal seam located at the depth of 900 m a micro pilot test with a single well and multi well co2 injection tests involving an injection and production wells were carried out in the period between may 2004 and october 2007 there were a variety of tests conducted in the injection well including an initial water injection fall off test and a series of co2 injection and fall off tests although gas production rate was obviously enhanced by co2 injection water production rate was not clearly affected by co2 injection several injection tests suggested that injectivity of co2 into the virgin coal seam saturated with water was eventually increased as the water saturation near the injector was decreased by the injected co2 it was estimated that low injectivity of co2 was caused by the reduction in permeability induced by coal swelling n2 flooding test was performed in 2006 to evaluate the effectiveness of n2 injection on improving well injectivity the n2 flooding test showed that daily co2 injection rate was boosted but only temporarily moreover the permeability did not return to the initial value after co2 and n2 were repeatedly injected it was also indicated that the coal matrix swelling might create a high stress zone near to the injection well

occurrence of minerals within fractures and matrix of selected bowen and ruhr basin coals abstract late permian age samples of black coal from the coeval bandanna formation and baralaba coal measures of queensland s bowen basin as well as carboniferous coal from germany s ruhr basin were investigated this paper presents scanning electron microscope sem analysis of the mode of occurrence of minerals within these coals and characterisation of the natural fractures present within the samples the coal samples located proximal to orogenic fronts in both basins are structurally complex with multiple generations of mineralisation evident stable isotope analyses of carbonates from selected samples indicate that methanogenesis related co2 reduction has occurred at two of the bowen basin sampling locations ankerite precipitated from a magmatic fluid or basinal brine is associated with significant sulphide mineralisation within brecciated ruhr coal proximal to major fault intersections with anticlines and has been meteorically altered to ferroan calcite and sulphates in coal from immediately below the carboniferous cretaceous unconformity similar alteration has occurred within a bowen basin coal sample close to the present day land surface variation of mineralisation within both the cleats and the coal matrix occurs on a fine scale with crystals of multiple mineral species often being present within micrometres of each other

thermally softened continental extensional zones arcs and rifts as precursors to thickened orogenic belts abstract intra continental deformation of soft zones during continental collision requires weak continental lithosphere which is able to be shortened across considerable width during later convergence this enables significant thickening with formation of an orogenic root we have examined models with a history of lithospheric thinning by pure shear during an earlier phase of intra continental extension with associated heating geologically this situation is appropriate to intra continental rifts and back arc basins if thinning of elevated thermal structure is decoupled from the thinning of lithology then a weak soft lower crust and sub arc rift mantle result this weak structure has a favoured rheology for subsequent convergent thickening while the lithosphere is still hot these regions are associated with formation of granulites and metamorphic assemblages typical of high temperature low pressure ht lp if convergence starts while the heat input is still active then the failed rifts and arcs are shut off by lateral wedging of the hard lower crust and upper mantle of shoulder regions into the softened arc rift domain such sites are ideal for the formation and for the exhumation of metamorphic core complexes subsequent thickening during convergence leads to ht eclogites when the previous arc rift was hot and to medium t eclogites for a thickened standard geotherm these p t paths are counterclockwise and their shapes are strongly dependent on the amount of previous thinning and type of initial geotherm if the compression starts long after cessation of the extensional event and associated thermal anomaly then the geotherm of the extended area relaxes and the whole region hardens in this case no homogeneous thickening occurs and deep continental roots cannot form

the role of a heterogeneous inclusion during continental collision abstract we study a simple model of the deformation due to collision between two continental plates the model is patterned after the geometry of the india asia collision but it may be applicable to more general situations the collision is modelled by the indentation of a viscoplastic plate asia by a rigid punch india a finite element approach capable of dealing with finite deformation heterogeneity and non linear rheologies is used to study the role of an inclusion of increased rigidity on the viscoplastic flow of the plate the heterogeneous zone was chosen to approximate the shape and position of the tarim basin behind tibet the lateral boundaries of the plate were defined so as to approximate those that are believed to apply for the asian plate i e fixed boundaries to the west and north free boundaries to the west and south several geometries and contrasts of rheology were analyzed to verify the general validity of our results the purpose was to determine the main kinematic features of the flow not to model the particular case of tarim it is found that the heterogeneity affects the flow in a drastic way even for small contrasts of rigidity and for small size heterogeneities the main effect of the heterogeneity is to create two important shear zones or internal boundary layers the most important is in front of the inclusion separating the main flow from the inclusion as the flow proceeds the inclusion rotates and the shear zone progressively bends in qualitative agreement with the bending of the large strike slip faults like the altyn tagh that separates the tarim from tiber our model of continuous deformation does not allow for the formation of faults or fractures but the shear zones may be understood as the initial stages for the generation of fractures or velocity discontinuities another shear zone develops to the left of the inhomogeneity near the fixed boundary and in the wake of the punch the role of these shear zones is to isolate the heterogeneity from the main flow to facilitate its rigid rotation inside the flow of the softer plate a final observation is that in the presence of the heterogeneity the lower left hand corner rotates almost like a rigid block producing a sharp bend on the left hand side of the plate as the inclusion rotates pushed by the punch this rigid block rotates clockwise to open up the space necessary for the homogeneity

ductile duplexing at a bend of a continental scale strike slip shear zone example from ne brazil abstract during the pan african orogeny the borborema province in ne brazil developed a continental scale shear zone system that comprises ne and ew trending ductile strike slip shear zones remote sensing and structural mapping has revealed a pattern of arcuate anastomosing strike slip shear zones separating sigmoidal lenses of less deformed material located at the western end of the ew trending patos shear zone which is one of the largest shear zones of the province this structure of imbricate shear zones was initiated under high temperature deformation conditions it is interpreted as a ductile strike slip duplex and may represent a kinematic pattern for strain accommodation in response to a bend of a ductile mega shear zone

paleoproterozoic basement cover infolding and thick skinned thrusting in hearne domain nunavut canada intracratonic response to trans hudson orogen enveloped by orogenic belts during the assembly of laurentia 2 0 1 7 ga the rae and hearne domains of the western churchill province experienced a complex sequence of tectonic events historically referred to as the hudsonian orogeny in hearne domain diverse thick skinned intraplate structures developed in response to accretionary processes in trans hudson orogen these structures include infolds of archean basement and paleoproterozoic cover and broken foreland thrusts in central hearne domain an intracratonic basin succession of continental to shallow marine strata hurwitz group 2 45 1 91 ga is exposed in outliers that constitute the keels of doubly plunging basement cover synclinoria defining a fold train with wavelengths of 35 45 km with the exception of local out of syncline shear zones basement and hurwitz group are welded formed under greenschist facies conditions the basement cover infolds are open concentric and commonly display complex structures in their interiors in the poorfish windy outlier southwestern hearne domain continental rift deposits of the kiyuk group unconformably overlie the hurwitz group in contrast to basement cover infolding to the north and east archean basement the hurwitz group and the kiyuk group are imbricated by thick skinned northwest vergent thrusts and folds thrusting in the poorfish windy belt was completed by 1815 10 ma as indicated by new u pb data from hydrothermal metamorphic titanite recovered from recrystallized cements at the top of the kiyuk group we interpret that shortening from shallow level basement cover infolding is balanced at depth by intrabasement faulting and distributed ductile strain and that the geometric link between central hearne basement cover infolding and poorfish windy thrusting is via intrabasement faults that cut to the surface the main controls on thick skinned deformation were the high competency of the lower hurwitz group previous thermal weakening basement heterogeneities and possibly shallow subduction preceding terminal collision in trans hudson orogen 2002 elsevier science b v all rights reserved

reinterpretation of the tectonic context of high temperature metamorphism in the broken hill block nsw and implications on the palaeo to meso proterozoic evolution abstract the origin of high temperatures during regional low pressure granulite facies metamorphism within the proterozoic broken hill block australia has been reinterpreted to be the result of burial of anomalously hot rock packages for which the lithospheric geothermal gradient was initially elevated during early rifting ca 1 71 1 67 ga and then maintained during a ca 1 62 ga short lived mid crustal extensional event mid crustal extension at ca 1 62 ga was associated with amphibolite facies metamorphism and elevated lithospheric geothermal gradients and occurred prior to activity along d2 high temperature shear zones peak low pressure granulite facies metamorphism ca 1 60 ga and crustal shortening during the olarian orogeny ca 1 60 1 59 ga this reinterpretation places the broken hill block within an environment in which multiple episodes of transient extension ca 1 71 1 62 ga were followed by a switch to shortening at ca 1 60 ga the interpreted tectonic environment is a continental back arc setting located in the over riding plate of a subduction zone along the southern margin of palaeoproterozoic australia

seismic evidence for a fossil mantle plume beneath south america and implications for plate driving forces a teleseismic travel time study reveals the presence of a fossil plume in the deep upper mantle beneath brazil which has apparently remained geographically fixed with respect to the overly ing continent despite thousands of kilometres of plate motion this result implies that the upper mantle and lithosphere beneath south america have remained coupled since the breakup of gondwanaland and may provide an answer to the long standing question of what forces drive continental plates

the evolution of deformation and topography of high elevated plateaus 1 model numerical analysis and general results a temperature dependent viscoplastic flow model of continental lithosphere is used to investigate the evolution of deformation and topography of high elevated plateaus such plateaus are products of both continent continent collision tibetan plateau and ocean continent collision the altiplano and develop in the overriding continental plate in this study we emphasize the mechanically simpler case of oceanic collision because it does not involve mass transfer between the two plates the lithosphere is deformed in response to tectonic and buoyancy forces the tectonic forces arise from subduction of an oceanic plate or underthrusting of continental lithosphere that horizontally indents and vertically shears the overriding lithosphere the buoyancy forces arise in response to horizontal density variations and tend to relax existing topography or thick crust the time evolution of the deformation and topography is investigated using a finite element technique that solves for the flow field in the overriding lithosphere the model produces dynamically supported near trench topography and inland mountain topography that is isostatically supported by a thick crust a finite region of localized deformation thick crust and high topography develops only if the model includes a horizontal thermal perturbation or an initially thick crust however only thermally perturbed lithosphere generates a plateau topography the shape and size of the calculated plateau depend on the wavelength of the thermal perturbation grashof number and density contrast between the crust and mantle the time evolution of the deformation shows a significant change in the deformation pattern as the high elevated plateau evolves during early stages compressional deformation of the crust and mantle are localized in the thermally perturbed weak zone at later stages as the crust thickens buoyancy forces of larger magnitude resist further thickening of the crust and the locus of compressional crustal deformation migrates inland this migration does not affect the location of the mantle deformation which remains in the thermally weak region but it is accompanied by a significant shear deformation in the weak lower crust the separation of the crustal locus from the mantle locus of deformation emphasizes the importance of vertically dependent deformation in the formation of high elevated plateaus this demonstrates the limitations of models that ignore changes of deformation with depth such as plane stress or thin sheet models

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

mineral occurrence and its impact on fracture generation in selected qinshui basin coals an experimental perspective abstract the effect of mineral matter on the fracture evolution was investigated for lower permian and upper pennsylvanian coal samples from the qinshui basin of china optical microscopy scanning electron microscopy with energy dispersive x ray spectrometry sem eds and x ray computed tomography x ray ct were used to investigate the composition and modes of occurrence of minerals within the coals and to characterize the natural and induced fractures present within the samples the spatial distribution of mineral matter investigated by x ray ct demonstrates that the variation of mineralization within the cleats fractures occurred at fine scales and most of minerals were scattered in coal core samples the original fractures normally were present as two sets one set was almost parallel to the bedding plane and the other was almost perpendicular to the bedding plane for created fractures the difference in density between the matrix and mineral matter may weaken the cohesion of coals and it could explain why the created fractures propagate along the junction of minerals and organic coal matrix the internal fracture porosities varied from 0 06 to 20 69 which indicates a strong internal heterogeneity of the coals after a stress is applied the inclination of the main shear fractures ranges from 55 to 95 using energy analysis the fracture energy appears to represent nearly half of the total dissipated energy which can be correlated with the fracture density and orientation the formation and distribution of shear fractures is related to the spatial distribution of mineral matter and the overall mineral content these investigations serve to characterize hydro fracturing and pulverized coal generation during coalbed methane development

strain localization by shear heating and the development of lithospheric shear zones abstract analogue and numerical models show that strong or weak domains in a deforming ductile material cause stress concentrations that may promote strain localization such domains commonly occur in the lithosphere through variations in composition or mineral fabric here we use a 2d plane stress non newtonian viscous model to explore how strain localization develops from an initial isolated weak inclusion we use a temperature dependent rheological law for which the material weakens as a result of work done by shear converted to heat the progress of strain localization follows a power law growth that is strongly non linear and may be regarded as an instability although this localization mechanism is ultimately limited by thermal diffusion this parametrization permits a robust criterion for the conditions in which localized shear zones can form within the lithosphere shear zones in the lower crust are typically depicted as the downward continuation of faults we argue that the depth extent of narrow shear zones within the lithosphere is limited by the stability criteria that we infer from 2d numerical experiments when applied to the rheological laws for common lithospheric minerals the combination of temperature and stress dependence provides a direct means of predicting the depth below which the localization instability does not occur for an olivine based rheology the maximum depth at which rapid localization is expected is in the range of 20 to 60 km depending on heat flow strain rate and water fugacity we apply our calculations to two major continental strike slip zones the san andreas fault and north anatolian fault and compare our predicted maximum localization depths with published seismological images strain localization in the lower crust requires a dry rheology comparable to plagioclase observations that imply localized strain in the uppermost mantle beneath these fault zones are consistent with the localization criteria and the rheological properties of dry olivine

strain distribution and fabric development modeled in active and ancient transpressive zones 1 a model based on kinematics of transpression allows the measurable internal parameters simultaneous strain fabric and vertical elevation to be simulated in relation to the macroscopically determined external parameters of transpressive zones i e zone width velocity angle of convergence and zone depth we present a diagram of convergence angle against time for homogeneous transpression where isolines of strain intensity d strain symmetry k and vertical elevation of rock samples are superposed however the distribution of internal strain parameters is sensitive to three types of strain partitioning 1 discrete partitioning results in general decrease of finite strain accumulations and in increase of pure shear component 2 ductile partitioning splits the transpressional domain into a pure shear zone where strain accumulations decreases and in a wrench dominated zone where strain accumulations increases and 3 viscosity partitioning is marked by different strain rates in zones of different viscosity and therefore by different strain parameters

thermomechanical evolution of a thinned continental lithosphere under compression implications for the pyrenees in order to demonstrate the interactions between plate boundary forces and preexisting zones of weakness and how those control the localization of continental deformation inside the lithosphere the thermomechanical evolution of an initially thinned and thermally perturbed continental lithosphere in compression was analyzed within the framework of the pyrenees while a concensus exists as to the shallow structures the extent and geometry of deformation at depth have been the subject of great controversy the thermomechanical implications of recent geological hypotheses on the initial state prior to the compression recently derived from balanced cross section reconstructions are here quantified and compared to the recent ecors deep seismic reflection profile across the pyrenees a thermomechanical model based on finite elastoplastic viscoplastic deformations has therefore been formulated using phenomenological constitutive equations derived from experimental studies the thermally coupled problem is solved by an appropriate vertical two dimensional plane strain finite element approximation in those models most of the shortening is accommodated inside a local zone of intense plastic shearing that creates a sharp wedge shaped root progressively sinking deep into the mantle the main thickening takes place inside the lower crust where it can reach a factor 2 this indentation and crustal imbrication process is controlled by a buckling instability occuring inside the uppermost part of the mantle the evolution of the topography can be related to these phenomena lateral conductive heat transfer overtakes the advective transfer in the vicinity of the inflow of colder crustal material inside a warmer mantle the deformation pattern can be explained by a three layer mechanical lithosphere with a soft ductile internal layer of potential decoupling between two more competent layers characterized by a significant elastic stored energy the overall response of this composite plate is not much influenced by smaller scale variations of crustal rheology but is very much dependent on the contrast in rates of viscous dissipation between the lower crust and the upper mantle and on the possibility of singling out a layer of significant elastic stored energy by the balance between rate of loading and rate of dissipation inside the uppermost mantle predicted deformation pattern is found to agree with the seismic reflection and gravity data even though extrapolation to other rates of shortening must be verified by appropriate numerical modelling these physical mechanisms may be regarded as a paradigm for other crustal imbrication phenomena found in the himalayas tibet or the alps

why do continents break up parallel to ancient orogenic belts the frequently observed parallelism between rifts and the pre existing orogenic fabric of continents suggests that the inherited tectonic fabric of the lithosphere influences the rupture of continents we propose that the existence of a pervasive fabric in the lithospheric mantle induces an anisotropie strength in the lithosphere that guides the propagation of continental rifts subcrustal mantle mechanical anisotropy is supported by i the anisotropie strength of olivine ii an ubiquitous tectonic fabric in exposed mantle rocks and iii measurements of seismic and electrical anisotropy during major episodes of continent rifting parallel to orogenic belts ocean opening through rifting and continent break up is frequently related to the occurrence ofhotspots there is howevcr a discrepancy between hot spots acting as pin point sources of heat and the linear extent of rifts over thou sands ofkilometres moreover rifts tend to parallel pre existing orogenic fab

physical changes in the lithosphere associated with thermal relaxation after rifting we define the terms modem rift and paleorift with respect to the thermal state of the lithosphere the former being rifted lithosphere not in thermal equilibrium and the latter being thermally stabilized rifted lithosphere a comparison of the physical characteristics of modern rifts with paleorifts shows that they share many of the same geomorphic structural and magmatic features but have important differences in their geophysical signatures using a simple model for the thermal relaxation of the lithosphere in the transition from modern rift to paleorift we show that differences in upper mantle seismic velocities and density surface heat flow thermal structure and curie point depth and magnetotelluric properties of the lithosphere between modern and paleorifts can be explained by lithospheric cooling cooling alone cannot explain the complex subsidence histories associated with and following rifting however nor the presence of high velocity crustal layers and sometimes thickened crust in paleorifts these features can be explained by consideration of lithospheric flexural strength and magmatic thickening of the crust associated with rifting we speculate that the anomalous upper mantle beneath many modern rifts may be associated with the process of magmatic thickening of the rifted crust

the neoproterozoic brasiliano orogeny in northeast brazil 40ar39ar and petrostructural data from ceara abstract the borborema province in ne brazil comprises paleoproterozoic basement and its proterozoic cover that experienced a complex tectonometamorphic history during the brasiliano pan african orogeny in ceara four major domains separated by continental scale steeply dipping dextral shear zones have been identified 40 ar 39 ar step heating of mineral separates and laser probe fusion of single grains have been used in conjunction with petro structural observations to delineate the cooling tectonic history of these domains to the north of the senador pompeu shear zone spsz amphibole and biotite pairs from low pressure granulite belts exposed along the granja shear zone and spsz record plateau ages in the range 575 560 ma corresponding to cooling rates close to 20 c m y 40 ar 39 ar laser probe muscovite dates from the chaval augen gneiss point to a similar age for shearing responsible for the exhumation and fast cooling of the granulite belts a muscovite biotite pair from the central ceara high pressure nappes lying between the shear zones documents a lower cooling rate in the interval 534 525 ma related to the decompression and uplift of previously deelly buried rocks to the south of the spsz only moderate crustal thickening occurred within the various schist belts a distinct cooling pattern is indicated for these belts and the western patos shear zone attesting to a different crustal development 40 ar 39 ar dates of amphibole phlogopite muscovite and biotite range from 550 to 522 ma recording a postmetamorphic cooling rate of 6 9 c m y slower cooling rates of 3 4 c m y between amphibole and biotite closure ages in the range 540 500 ma are reported for the eastern part of the palos shear zone system the diachronous cooling recorded along this shear zone is interpreted to reflect the variations in thickness of the high temperature mylonitic belts earlier more rapid cooling occurred where the belts are narrow in the west late slower cooling occurred in wider zones to the east these thermochronological data reveal an age and cooling rate distribution at the scale of the borborema province that provides new constraints on the development of the brasiliano orogeny between 580 and 500 ma

heat producing elements enriched continental mantle lithosphere and proterozoic intracontinental orogens insights from brasiliano pan african belts abstract collisional and accretionary processes have been recognized in several precambrian orogens but the formation of intracontinental ensialic orogens has remained elusive the recognition that modern orogenic belts as for example the mountain belts of central asia may form in response to far field stresses transmitted to the interior of continental plates removed much of the objection against proterozoic intracontinental deformation however the causes responsible for the localization of contractional deformation are still uncertain due to the strong dependence of lithosphere rheology on temperature lithospheric thermal weakening prior to contraction must be a required condition studies of dioritic rocks from borborema province northeastern brazil and damara belt southern africa indicate that they are derived from ancient incompatible elements enriched subcontinental lithosphere the concentrations of the heat producing elements k 0 6 wt u 0 02 0 2 ppm and th 0 15 1 5 ppm estimated for this source allow calculations of heat production between 0 09 and 0 25 mw m 3 these high values of heat generation may cause increases in temperature of several tens to three hundred degrees celsius with respect to cratonic mantle lithosphere leading to great reductions in lithosphere strength consequently continental domains underlain by hot and weak continental mantle may thicken under the action of relatively feeble tectonic forces high rheological contrasts between these domains and adjacent stiff and cold blocks provide an explanation for formation of intracontinental orogens

experimental mineral dissolution in berea sandstone reacted with co2 or so2 co2 in nacl brine under co2 sequestration conditions a comparison of the geochemical changes induced in sandstone by either pure or impure co2 at geological co2 storage conditions was investigated samples of berea sandstone were batch reacted in 1 w v nacl brine saturated with pure co2 or mixed so2 co2 gas for 360 h at 50 degrees c and 10 mpa geochemical analysis of incremental water samples showed increases in the concentrations of elements such as calcium magnesium iron manganese and silicon throughout experiments likely being the products of carbonate and reactive silicate dissolution scanning electron microscope images taken of specific points of interest before and after batch reactions confirmed dissolution of carbonates but showed no reaction for minerals such as k feldspar the magnitude of apparent mineral reaction was higher for the mixed gas so2 co2 brine experiment with geochemical modelling also indicating greater dissolution of reactive silicates such as chlorite and potential precipitation of amorphous silica c 2014 elsevier b v all rights reserved

continental scale rheological heterogeneities and complex intraplate tectono metamorphic patterns insights from a case study and numerical models abstract continental plates are built over long periods of time through successive extensional and compressional cycles they are therefore rheologically heterogeneous this heterogeneity should significantly influence the mechanical response of the continental lithosphere during collision processes the study of the neoproterozoic borborema shear zone system of northeast brazil highlights a systematic link between marked changes in its tectono metamorphic pattern and the pre existing structure of the plate that is characterized by juxtaposition of continental domains either comprising an old basement palaeo to eoproterozoic or accreted during an extensional event between 1 0 and 0 7 ga in neoproterozoic time when the shear zone system was developed these domains displayed different geotherms and lithospheric thicknesses and therefore contrasted rheological behaviours we use numerical models simulating the mechanical evolution of a continental plate comprising multiple thermally induced rheological heterogeneities submitted to compression to investigate how these heterogeneities may affect strain localization and the distribution of deformation regimes and vertical strain within the plate from the very beginning of the deformation weak and stiff heterogeneities induce strain localization due to a lower initial effective viscosity or to stress concentrations at their tips respectively shear zones propagate from the heterogeneities and finally coalesce forming a network of high strain zones bounding almost underformed blocks within this network shear zones transfer strain between the different heterogeneities and model boundaries the evolution of the system depends essentially on the geometrical distribution of heterogeneities and on their strength contrast relative to the surrounding lithosphere the resulting finite strain field is heterogeneous and displays rapid lateral variations in vertical and or rotational deformation such a heterogeneous strain distribution may induce contrasted magmatic metamorphic and uplift evolutions within an orogenic belt as observed in the borborema shear zone system and other collisional belts

termination of a continental scale strike slip fault in partially melted crust the west pernambuco shear zone northeast brazil continental scale strike slip faults rooted in the upper mantle have been recently interpreted in deep seismic profiling these shear zones transect the entire crust and it may be assumed that the deformation pattern associated with their propagation at different depths was strongly influenced by the rheological properties of the rocks the continental scale west pernambuco shear zone provides information on the deformational behavior of a hot and partially melted crust that has undergone large scale shearing this late precambrian shear zone terminates eastward in a fanlike structure passing to a wide transpression zone at a high angle to the shear direction the main fault branches into numerous splay faults over a distance of about 50 km severe perturbation of the mylonitic flow pattern commonly occurs at branch points where subsidiary shear zones diverge from the main zone in these areas problems of strain compatibility are raised by the sharp bending and decrease in dip of the foliation surface we propose that this unusual deformation pattern is a result of the low strength of the continental crust at the considered level of observation

depth extent of cratons as inferred from tomographic studies we address the question of the depth extent of mantle high velocity zones under ancient cratons by using seismic velocity maps from recent mantle tomographic studies we divided old continents into two age provinces 800 1700 ma middle proterozoic and older than 1700 ma archean and early proterozoic the areas included in these age provinces are cross correlated with the tomography at different depths to quantify the global occurrence of high velocity anomaly hva extensions beneath cratons statistically significant hvas underlie the oldest cratons to a depth of 250 km there is no significant consistent correlation between tomography and younger cratons the older cratons have correlated hvas extending to depths varying from 100 to 450 km there is no significant difference in the average velocity anomaly beneath the early proterozoic part and the archean part of the canadian shield whereas the middle proterozoic part does not contain any hva the mantle under early precambrian crust is distinctly different from mantle elsewhere we propose two different hypotheses to explain these results 1 cratons of early proterozoic age or older contain ancient and deep physically attached roots or 2 these cratons contain a small permanent physical root that induces cold downwellings beneath it a root thickness of 200 km can be explained by an feo poor olivine rich mineralogy whereas a requirement of high strength probably demands in addition a volatile poor composition different root depths for cratons of similar age may be related to their location in the various supercontinents in which they were involved and to their subsequent deformational history or they may represent different snapshots in time of transient roots such as thermal boundary layers

late palaeozoic deformation in central africa a result of distant collision seismic reflection profiles from the cuvette centrale central basin of zaire show that a major contractional deformation affected a part of central africa during the late permian and early triassic periods at the same time other regions of central africa experienced extensional and strike slip deformation and associated rift formation the coexistence of such varied styles of tectonic activity over a large continental area resembles the cenozoic tectonics of central asia and northwest europe the intracontinental deformation seen in these latter areas is attributed to collisional processes at a distant continental margin1 3 by analogy we argue here that deformation of central africa in the late palaeozoic period was a direct result of collisional processes some distance away at the southern margin of gondwana

rheological controls on grenvillian intrusive suites implications for tectonic analysis abstract studies of plutonic suites in western grenville along with the recent advances in understanding magma ascent and emplacement and interactions between magma and their country rock provide insights on the use of intrusive rocks to infer the accretionary history of deeply exposed precambrian orogens in the grenvillian central metasedimentary belt of quebec terrane juxtaposition was inferred to be late based on the distribution of 1 08 ga potassic alkaline plutons this view was questioned however upon recognition that magma ascent by dyke propagation can be stalled to enable the formation of plutons upon intersecting rheologically softer units for example grenvillian marble in this context preferential ponding of magmas in marble rather than tectonics can control regional distribution of plutons the spatial association of plutons with marble and their contact relationships are at the local scale obscured by transformation of marble to skarn and by mechanical excision of marble from the paragneiss wall rock sequence in contrast plutons and dykes of an older 1 17 ga less alkaline magma association are shown by field and remote sensing studies to be evenly distributed across the various lithotectonic domains of the belt their sheet like emplacement along the belt boundary constrains grenvillian tectonic assembly in the region to be early contrasting loci type and degree of deformation of intrusive bodies illustrate that host rock rheological differences influence the final characteristics of an intrusive suite consequently the nature of the host is an important variable in inferring timing relationships and tectonic scenarios from intrusive suites in high grade terranes

phase speed spectra and the recent poleward shift of southern hemisphere surface westerlies 1 the poleward shift of the southern hemisphere surface westerlies in recent decades is examined in reanalysis data and in the output of coupled atmosphere ocean and uncoupled atmospheric models the space time spectra of the eddy momentum fluxes in the upper troposphere reveal a trend that marks an increase in the eastward phase speed of the tropospheric eddies accompanied by a poleward displacement of the region of wave breaking in the subtropics a dynamical mechanism is suggested that may help explain the connections among the lower stratospheric wind anomalies the increased eastward propagation of tropospheric eddies and the poleward shift of the tropospheric circulation

the experimental deformation of dunite deformation experiments have been carried out on two dunites anita bay of 100 m grain size and aheim of 900 m grain size at strain rates from 10 3 to 10 6 s 1 and temperatures from 1000 c to 1300 c in a gas medium deformation apparatus at 300 mpa confining pressure most of the tests were under wet conditions defined by the presence of small amounts of water from hydrous minerals initially present constant strain rate and relaxation experiments covering ranges of flow stress down to about 70 mpa and 7 mpa respectively show that there is a change in flow law in going below about 100 mpa differential stress and that the coarser grained rock is stronger than the finer grained one power law parameters above the transition are n 4 48 0 31 and q 498 38 kj mol 1 for aheim dunite and n 3 35 0 17 and q 444 24 kj mol 1 for anita bay dunite while below the transition relaxation tests on anita bay dunite give n 2 44 0 18 and q 386 27 kj mol 1 it is concluded that there is a weakening effect of water that this effect is mainly in the grain boundaries and that grain boundary sliding is probably a significant deformation mechanism at the lower stresses under wet conditions

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

shear zone controlled magma emplacement or magma assisted nucleation of shear zones insights from northeast brazil abstract in the borborema province of northeast brazil neoproterozoic granitoids and large scale transcurrent shear zones are spatially associated suggesting a genetic link between magma bodies and shear zones in some cases magma emplacement was clearly favored by shear zone activity but for several plutons this model is not satisfactory in these plutons pre full crystallization strike slip deformation evidenced by parallelism of magmatic foliations and lineations with the solid state mylonitic fabric and by a transition from magmatic to solid state flow is restricted to the vicinity of the shear zones evidence of shear zone activity prior to magma emplacement is lacking and the magmatic foliation away from the shear zones is in most cases shallowly dipping and concordant with the slightly older gently dipping regional gneissic foliation field and anisotropy of magnetic susceptibility mapping together with petrographic and geochemical studies performed in one of the magmatic complexes of the borborema province have revealed a structure and a magmatic fabric incompatible with the shear zone controlled emplacement model away from the shear zones this complex has retained a stratification inherited from the mixing of crystal poor magmas of contrasting composition and a magmatic fabric characterized by low to moderate dip magmatic foliations bearing a nw trending lineation which contrast with vertical foliations bearing ne or e w trending stretching lineations in the shear zones and indicates that crystallization started prior to shear zone development based on evidence that magma emplacement predated strike slip shearing and on information about the transition from magmatic to solid state deformation observed in the studied plutons we suggest that incompletely solidified plutons within the crust represent rheological heterogeneities that may induce strain localization and favor shear zone nucleation we propose that in the studied cases deformation first concentrated within the plutons where shear zones nucleated and grew subsequently as the solid phase component increased the crystal mush began to behave as a solid and the shear zones propagated into the country rocks

rheology of some continental lower crustal rocks abstract to gain insight into the rheology of selected feldspar bearing rocks from the continental lower crust clc three granulites from exposed lower crustal terrains and a microgabbro have been deformed to ca 10 strain at confining pressures of 0 8 1 0 gpa temperatures of 600 900 c and constant strain rates of 10 4 10 7 s 1 the most extensive work was done on the mafic pikwitonei and felsic adirondack granulites the pikwitonei mg hornblende plag an70 2 px has only a limited t range of apparent steady state behavior and strain softens at t 800 c at 10 5 10 6s 1 apparently associated with developing shear zones this strain softening is pressure insensitive from pc 0 8 1 0 gpa at t 850 c 10 6s 1 even though the dominant strain accommodation mechanisms change from semibrittle cataclastic to crystal plastic including dynamic recrystallization with increasing pressure plagioclase appears to bear more of the creep strain than amphibole at most experimental conditions except in those runs where both plagioclase and amphibole recrystallization occurs kapuskasing plag cpx gt granulite deformed at 800 c and at 850 c 10 6 s 1 has deformed in the steady state at stress levels only slightly greater than the pikwitonei the stillwater microgabbro and adirondack granulite are appreciably weaker and the latter exhibits homogeneous semibrittle steady state behavior over the entire range of conditions investigated for the adirondack 2 felds 2 px gt qtz granulite quartz 21 appears to control the creep rate at these low strains and steady state mechanical data fit well a creep power law with a 8 10 3 mpa ns 1 qc 243 kj mol and n 3 1 extrapolation of this flow law and of preliminary power law creep relations for the pikwitonei and microgabbro to a range of physical conditions assumed to encompass those at the continental moho supports geological and geophysical inferences that a mechanical discontinuity occurs at this transition zone with the lower crust generally being the weaker material in addition the stain softening exhibited by the pikwitonei may bear directly on the development of detachment zones at mid to lower crustal depths during prograde metamorphism

assessing the geochemical and tectonic impacts of fluid rock interaction in mid crustal shear zones a case study from the intracontinental alice springs orogen central australia the reynolds anmatjira ranges central australia form part of a high grade basement terrane dissected by intensely metasomatized transpressional shear zones active during the ordovician carboniferous alice springs orogeny unlike typical retrograde structures associated with discrete fluid flow the mid crustal setting and intracontinental nature of these shear systems present significant problems for the source and ingress mechanism of the fluid involved in their rehydration to address these issues we describe two detailed traverses across deformed and metasomatized basement rocks in this region and interrogate their record of fluid rock interaction from various perspectives both traverses combine structural and petrological observations with zr in rutile and ti in quartz thermobarometry oxygen and hydrogen stable isotope analysis and major trace and rare earth element mobility trends each technique is critically evaluated for its utility in this study and its more widespread applicability to alternative field areas providing a strategic framework for the general investigation of fluid affected shear zones ultimately the integrated data sets specify pressure temperature conditions of 530 c and 4 5 kbar implying average apparent thermal gradients of 29 36 c km 1 and depths of 14 18 km other characteristic features to emerge include strongly variable element mobilities and pronounced isotopic depletion fronts consistent with the alteration effects of an externally derived non equilibrium fluid this is confirmed by calculated fluid compositions indicative of contributions from a fluid of meteoric origin with estimated 18o and d values as low as 2 3 and 59 8 respectively we propose that these surficial fluid signatures are imposed on the mid crust by the prograde burial and dehydration of hydrothermally altered fault panels produced during pre orogenic basin formation progressive fluid release with continued subsidence then leads to the accumulation of increasing fluid volumes in the vicinity of the brittle ductile transition promoting extensive hydration metasomatism and reaction softening at the locus of stress transmission from plate boundary sources the sustained injection of externally derived fluids into refractory crustal material may thus stimulate a critical reduction in the long term strength of the lithosphere providing strong impetus for the initiation and advancement of intracontinental orogenesis

role of lithospheric strength heterogeneities in the tectonics of tibet and neighbouring regions unlike rigid oceanic plates continents show diffuse deformation for example the tibetan plateau that has accommodated much of the convergence of india with asia we interpret the tertiary deformation of asia as the three dimensional strain of a non linear viscous continental lithosphere lateral strength contrasts modify this deformation stress concentration around the relatively strong tarim basin gives rise to local crustal thickening in the tien shan to the north and to strike slip in the altyn tagh along its southern margin

intraplate seismicity in se brazil stress concentration in lithospheric thin spots summary intraplate seismicity has generally poor correlation with surface geological patterns except for major extensional features such as aborted continental rifts which may act as weak zones it is usually difficult to find simple geology based models to explain differences in seismic activity in stable continental regions seismicity in brazil is clearly not uniform and a few areas of higher activity have been identified however the seismic areas show almost no correlation with the main geological provinces which is typical of other intraplate settings a recent uppermantle tomography study in se and central brazil using approximately 8500 p wave and 2000 pkp wave arrivals recorded in 59 sites since 1992 has mapped p wave velocity anomalies from lithospheric depths down to 1300 km in this region higher seismic activity occurs preferentially in areas with low p wave velocities at 150 250 km depth the low p wave velocities are interpreted as shallower asthenosphere in such areas a hotter geotherm will reduce the strength of the lithospheric upper mantle causing most of the intraplate forces to be concentrated in the brittle upper crust the low velocity anomalies coincide with late cretaceous provinces of alkaline intrusions the proposed ponding of the trindade plume head beneath lithospheric thin spots is consistent with our tomography results suggesting that plume effects may have helped to preserve lithosphere asthenosphere topography although other factors are also important the present data show that stress concentrations resulting from lithosphere asthenosphere topography should play an important role in explaining the intraplate seismicity in the brazilian platform

phase speed spectra of transient eddy fluxes and critical layer absorption abstract tropospheric zonal mean eddy fluxes of heat and momentum and the divergence of the eliassen palm flux are decomposed into contributions from different zonal phase speeds data analyzed are ecmwf operational global analyses covering 1980 87 eastward moving medium scale waves zonal waves 4 7 dominate the spectra of lower tropospheric heat fluxes in both hemispheres and all seasons upper tropospheric wave flux spectra are similar to the low level spectra in midlatitudes but shift to slower zonal phase speeds as low latitudes are approached the cause of this shift is the selective absorption of faster moving components in midlatitudes as the waves propagate meridionally latitude phase speed distributions of eddy fluxes are constructed and compared to the zonal mean wind structure these results demonstrate that upper tropospheric eddies break and decelerate the zonal mean flow approximately 10 20 in latitude away from their critical line where phase speed equals zonal wind speed compari

self indentation of a heterogeneous continental lithosphere the neoproterozoic ribeira aracuai belt stretches along the southeastern edge of the sao francisco craton of southeastern brazil and extends south of the cratonic domain for 1000 km the termination of the craton is spatially correlated with a significant modification of the deformation pattern in the belt 1 the structural trend bends from due north to northeast 2 the dominant tectonic flow shifts from orogen transverse to orogen normal and 3 the metamorphic conditions of deformation decrease southwestward from high to medium grade finite element modeling suggests that the presence of a craton within a continent favors strain localization initiation of continental scale shear zones and differential vertical deformations the southward termination of the sao francisco craton may have triggered the development of the complex deformation pattern that characterizes the ribeira aracuai belt

comparative leaching experiments for trace elements in raw coal laboratory ash fly ash and bottom ash abstract samples of raw coal fly ash and bottom ash have been collected from a chinese power plant together with laboratory ash obtained by ashing of the coal under 850 c comparative leaching experiments were carried out on each fraction under various ph conditions a mathematical model for leaching of trace elements has been developed and leaching intensity il has been calculated for each element the results show that ph of the solution leaching time and particularly properties and occurrence of the elements have a strong influence on leaching behavior

interpretation of recent southern hemisphere climate change climate variability in the high latitude southern hemisphere sh is dominated by the sh annular mode a large scale pattern of variability characterized by fluctuations in the strength of the circumpolar vortex we present evidence that recent trends in the sh tropospheric circulation can be interpreted as a bias toward the high index polarity of this pattern with stronger westerly flow encircling the polar cap it is argued that the largest and most significant tropospheric trends can be traced to recent trends in the lower stratospheric polar vortex which are due largely to photochemical ozone losses during the summer fall season the trend toward stronger circumpolar flow has contributed substantially to the observed warming over the antarctic peninsula and patagonia and to the cooling over eastern antarctica and the antarctic plateau

the intraplate character of supercontinent tectonics abstract for several decades geoscientists have recognised intraplate tectonic activity far from plate margins both from modern and ancient examples this apparent disconnect with the drivers of plate tectonics does not necessarily imply unconnected processes but rather an uncertainty in understanding exactly how these systems operate are the driving forces derived locally or do they propagate from plate margins how do these forces interact with a complex tectonic inheritance to generate the observed tectonism furthermore what novel approaches have been applied to understand these processes here we review the general literature and the contents of this special issue to develop some partial answers to these questions key observations include the critical importance of local lithospheric heterogeneities as a control on the mode of orogenesis and also the role of locally derived forces from mantle upwelling or from depositing thick piles of magmatic or sedimentary rocks these processes happen within the overarching tectonic setting provided by far field plate margins which show intimate links in both time and observed processes with many intraplate regions these insights would not be possible without a growing arsenal of geological geophysical and numerical methods that can be applied to intraplate regions and this is reflected in the varied approaches within the issue we envisage that this special issue will provide a stimulus for further progress in understanding intraplate tectonics

geothermal structure in a seismoactive region of central india the geothermal structure beneath a seismoactive region of central india has been determined by using two independent geophysical parameters i e p n velocity and curie point depth which are temperature dependent p n velocity has been determined by reanalyzing deep seismic sounding dss records obtained for this region surface heat flow has been computed using the p n velocity data from this region and found to be 55 5 mw m 2 curie point depths have been derived from the magnetic satellite data in this region and reveal an identical value of heat flow of around 56 4 mw m 2 we estimated temperatures of 600 700 c at a moho depth of 39 km by using surface heat flow ranging from 50 to 60 mw m 2 and a simple 1 d geothermal model the present study suggests the presence of a brittle ductile transition high strength zone ranging in depth from 20 to 35 km where the temperatures range from 350 to 550 c we also suggest that the nucleation of earthquakes in this region took place in a relatively high strength zone this is supported by the fact that the focal depths of all the recent earthquakes from 1978 to 2000 of magnitude 3 5 lie in this zone of high strength

on the thermo mechanical evolution of continental lithosphere elastic thickness of continental lithosphere is modified by both mechanical thickening and thinning events and thermal evolution a simple thermo mechanical model can account for the processes of tectonic alteration and subsequent thermal reequilibration of lithospheric thermal structure mechanical thinning or thickening of the lithosphere directly affects its thickness and flexural rigidity after a tectonic event thermal reequilibration proceeds at a rate dependent upon the amount of thinning or thickening and total lithospheric thickness previous tectonic history and the heat flux at the base of the lithosphere heat flux at the base of the lithosphere is controlled by the efficiency of convective mantle heat delivery rapid heat delivery to the base of the lithosphere retards lithospheric thickening whereas slow conductive heat delivery allows lithospheric thickening at the maximum rate the relative importance of thermal effects to mechanical alterations of lithospheric elastic thickness depends on the time scale of concern very thin lithosphere undergoes rapid thermal evolution such that cooling between tectonic events significantly thickens the lithosphere for thick lithosphere conductive heat transport is so slow that only minor changes in thermal structure and no significant changes in elastic thickness take place between tectonic events model results with infinite stretching agree with observations of oceanic lithospheric thickness evolution showing a divergence from a cooling half space at about 70 m y model results also agree with independent estimates of continental thermal evolution in the absence of tectonic events on the longest time scales approaching the age of the earth thermal reequilibration is attained and any tectonic effects are overprinted the time scale at which the transition between mechanically and thermally dominated elastic thickness evolution occurs depends on the heat transport efficiency of the convecting mantle for high nusselt numbers thermal effects take longer to become important even for lower nusselt numbers mechanical effects still dominate on time scales of 108 years crustal radiogenic heat production reduces the equilibrium lithospheric thermal and elastic thickness it also introduces a dependence of equilibrium thickness on tectonic crustal thinning or thickening model results indicate that on time scales of thrust sheet and sedimentary loading in foreland basins mechanical effects dominate the evolution of elastic thickness except for very thin lithosphere the model can be applied to the lithosphere beneath foreland basins so that observed basin geometry can be used to constrain tectonic history prior to loading by thrust sheets and basin sediments

how preexisting weaknesses control the style of continental breakup continental breakup is a highly variable process differences occur in the relative timing and extent of volcanism uplift and graben formation as well as in the mode and amount of continental extension before breakup we propose a model which reconciles this variability with the previously recognized tendency for breakup to occur along preexisting weak trends continental lithosphere is viewed as a composite material composed of two strong layers one in the upper mantle and one in the middle crust finite element simulation indicates that extensional failure at weaknesses in the mantle causes concentrated extension in the mantle and diffuse extension in the crust this leads to early melt segregation and volcanism margin uplift during the late stages of the extension process and relatively narrow symmetrical extended margins in contrast failure at weaknesses in the crustal strong zone causes focused extension in the crust and diffuse extension in the mantle this produces initial graben formation cooling in the lower crust and upper mantle and broad asymmetrical extended margins volcanism only occurs late in the process failure at laterally offset weaknesses within both strong layers leads to a deformation pattern dominated simple shear thus differences in the prerift configuration of the continental lithosphere can control the overall style of continental breakup

evolution of deformation styles at a major restraining bend constraints from cooling histories mae ping fault zone western thailand the c 500 km long mae ping fault zone trends nw se across thailand into eastern myanmar and has probably undergone in excess of 150 km sinistral motion during the cenozoic a large c 150 km long restraining bend in this fault zone lies on the western margin of the chainat duplex the duplex is a low lying region dominated by north south trending ridges of mesozoic and palaeozoic sedimentary metamorphic and igneous rocks flanked by flat post rift basins of pliocene recent age to the north and south a review of published cooling age data plus new apatite and zircon fission track results indicates that significant changes in patterns of exhumation occurred along the fault zone with time oldest uplift and erosion eocene occurred in the umphang gneiss region west of an inferred thrust dominated restraining bend setting from 36 ma to 30 ma exhumation was strongest north of the duplex along the nw se trending segment of the fault zone at the northern exiting bend of the chainat duplex this region of the fault zone is characterized by a mid crustal level shear zone 5 6 km wide lan sang gneisses that passes to the nw into an apparent strike slip duplex geometry the deformation is interpreted to have occurred during passage around the northern restraining bend which resulted in vertical thickening uplift erosion and extensional collapse of the northern side of the shear zone this concentration of deformation at the bends at the ends of the restraining bend is thought to be a characteristic of strike slip dominated restraining bends following late oligocene early miocene extension there is apatite fission track evidence for 22 18 ma exhumation in the chainat duplex that coincides with a phase of inversion in the phitsanulok basin to the north the miocene recent history of the chainat duplex is one of minor sinistral and dextral displacements related to a rapidly evolving stress field influenced by the numerous tectonic reorganizations that affected se asia during that time

thermo mechanical role of a cambro ordovician paleorift during the variscan collision the ne margin of the bohemian massif abstract the stare m sto sm belt ne margin of the bohemian massif represents a preserved example of an intracontinental cambro ordovician rift that has been shortened during the variscan orogeny the rifted sequence consists of gabbroic intrusions and a leptyno amphibolite complex of cambro ordovician protolith age the latter suffered medium pressure granulite facies cambro ordovician metamorphism associated with extension in a continental protorift the variscan tectonometamorphic event is manifested by convergent orogenesis with syn convergent intrusion of a carboniferous tonalitic sill high temperature medium pressure compressional deformation of gabbros and high temperature metamorphism of peridotites numerical thermal and rheological modelling suggests that the sm paleorift after 140 ma of cooling represented a stiff heterogeneity between an easterly situated cadomian continent and a westerly lying variscan orogenic root zone carboniferous underplating of the paleorift by tonalitic magma was probably responsible for the thermal weakening of both the upper mantle and the mafic lower crust and the subsequent uplift of the remaining rigid part of the cambro ordovician crust all the units were later deformed in transpressional heterogeneous greenschist facies regime at a supracrustal level

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

late silurian early devonian transpressional rift origin of the quebec reentrant northern appalachians constraints from geochemistry of volcanic rocks silurian devonian rocks in the northwestern mainland appalachians form part of an overstep sequence deposited across the vestiges of iapetus after the collision of laurentia and gondwana they occur in three synclinoria gasl connecticut valley merrimack aroostook and fredericton synclinoria separated by two anticlinoria munsungun pennington boundary mountain bronson hill and miramichi silurian rocks in gasp6 connecticut valley and merrimack aroostook synclinoria consist of shelf and trough sediments derived mainly from laurentia to the northwest with some local contributions from an ephemeral landmass appalaclfia and from the miramiclfi highlands in contrast early devonian rocks in all three synclinoria are mainly flysch derived froxn the southeast in avalonia interbedded volcanic rocks are predominantly late silurian early devonian in age they are generally bimodal with the mafic rocks exhibiting a change from transitional alkalic tholeiitic to tholeiitic around the siluro devonian boundary broadly coincident with the salinic disturbance in the gaspe peninsula the geochemical characteristics of the basalts indicate that they were erupted in a continental intraplate environment with melting migrating upward through time across the garnet to spinel phase boundary as stretclfing increased thermal uplift during rifting is inferred to have led to erosion associated with the salinic disturbance the start of the volcanism appears to coincide with a switch from sinistral to dextral transpression along the orogen that may be related to the change from clockwise to anticlockwise rotation of laurentia relative to gondwana the effects of dextral transpression vary with the trend of the orogen intense deformation and metamorplfism occurred in the central mobile belt opposite the new york and st lawrence promontories whereas rifting developed in the quebec reentrant leading to tlfinning of the crest up welling of the asthenosphere melting and magmatism

upper mantle anisotropy of the borborema province ne brazil implications for intra plate deformation and sub cratonic asthenospheric flow abstract the geological record of the borborema province northeast brazil documents tectonic events that characterised the precambrian formation and mesozoic breakup of gondwana large scale shear zones and associated granitic plutons that developed during the neoproterozoic brasiliano pan african orogeny and major sedimentary basins of mesozoic age indicate significant deformation across the region however whether or not the shear zones resulted from precambrian terrane accretion or are simply the result of episodes of subsequent intra plate deformation is debated also poorly understood is the effect of the thick sao francisco mantle keel on present day asthenospheric flow to address these issues we have performed a teleseismic shear wave splitting study of mantle seismic anisotropy from a new broadband seismograph network centred on the borborema province shear wave splitting parameters t reveal a lack of plate scale anisotropic fabrics associated within the continental interior perhaps supporting models of formation and evolution of the borborema province involving minimal deformation of the lithospheric mantle delamination of anisotropic lithosphere during the development of cenozoic volcanism that eroded older fossil lithospheric fabrics is unlikely because the widespread cenozoic magmatism required to achieve this is absent in the geological record instead the apparently low levels of seismic anisotropy observed in the interior of the borborema province may simply reflect depth dependent anisotropy nulls low t observations may be the subtractive result of orthogonal fast directions in the lithosphere and asthenosphere towards the brazilian coast t 1 s and fast directions are sub parallel to stretching fabrics formed during the opening of the south atlantic this may imply that the mantle lithosphere was deformed but not completely destroyed during gondwana breakup however a more complete backazimuthal coverage of splitting measurements across the region is needed to confirm or refute these hypotheses unambiguously

thermal modeling of extensional tectonics application to pressure temperature time histories of metamorphic rocks one and two dimensional finite difference models are used to generate theoretical pressure temperature time ptt paths for rocks uplifted from deep and intermediate crustal levels during extension via ductile stretching of the entire lithosphere pure shear and via movement of crustal blocks along rooted large scale low angle normal faults simple shear the temperature time paths of rocks uplifted by these pure shear and simple shear mechanisms have similar morphologies although rocks from pure shear settings reach their final depth at higher temperatures than do rocks uplifted by simple shear and experience a greater amount of posttectonic cooling the depth temperature paths of rocks uplifted from deep crustal levels 30 km depth via pure shear extension are often characterized by a period of nearly isothermal high dp dt uplift during the early stages of extension a modified pure shear extension model that includes enhanced heating in the lower lithosphere during extension generally produces ptt paths characterized by a more protracted period of nearly isothermal uplift in contrast simple shear uplift along low angle normal faults produces almost linear depth temperature paths with moderate dp dt and isothermal uplift is not observed for a given initial geotherm the single factor controlling the ptt paths of rocks unroofed below gently dipping normal faults that dip less than 30 is the rate at which unroofing takes place unroofing rate is defined as the rate of uplift relative to the surface and the syntectonic ptt paths are insensitive to fault dip or horizontal displacement rate current geothermometric and geobarometric techniques yield typical uncertainties in real pressure temperature and temperature time data of 1 kbar and 50 c thus it will be difficult to distinguish between these various extensional mechanisms on the basis of ptt data derived from a single sample or from a single structural level although use of data from samples collected from a broad range of structural levels may improve resolution ultimately the best prospect for distinguishing between simple shear and pure shear mechanisms through petrologic work lies in improving the precision of metamorphic pressure and temperature measurements to 0 5 kbar and 25 c

assessing the tectonic significance of a large scale transcurrent shear zone system the pernambuco lineament northeastern brazil abstract the pernambuco lineament borborema province northeastern brazil is traditionally considered a continuous 700 km long dextral e w strike slip shear zone it is interpreted as a fundamental tectonic element of the brasiliano pan african orogeny and proposed to continue into west africa as the sanaga or amadoua faults recent structural work reveals that the pernambuco lineament is segmented into two branches separated by more than 100 km this study shows that the eastern branch the east pernambuco shear zone system consists of two high temperature amphibolite facies and several low temperature greenschist facies mylonitic belts which are laterally and transversely discontinuous the main protoliths of the mylonites are magmatic rocks country rocks proximal to the shear zones are little affected by strike slip shearing either at high or low t conditions the high t mylonitic belts are located at the southern border of a major granitoid batholith and the low t belts form a series of overlapping right stepping shear zones to the south and east of them these observations strongly indicate that the pernambuco lineament is not a transcontinental structure that it was not responsible for significant displacements or for emplacement of large magmatic bodies and consequently that its tectonic role during the brasiliano event was secondary

complex 3d strain patterns in a synkinematic tonalite batholith from the aracuai neoproterozoic orogen eastern brazil evidence from combined magnetic and isotopic chronology studies abstract this work combines structural and geochronological data to improve our understanding of the mechanical behaviour of continental crust involving large amount of magma or partially melted material in an abnormally hot collisional belt we performed a magnetic and geochronological u pb study on a huge tonalitic batholith from the neoproterozoic aracuai belt of east brazil to determine the strain distribution through space and time anisotropy of magnetic susceptibility combined with rock magnetism investigations supports that the magnetic fabric is a good proxy of the structural fabric field measurements together with the magnetic fabrics highlight the presence in the batholith of four domains characterized by contrasted magmatic flow patterns the western part is characterized by a gently dipping orogen parallel ns magmatic foliation that bears down dip lineations in agreement with westward thrusting onto the sao francisco craton eastward the magmatic foliation progressively turns sub vertical with a lineation that flips from sub horizontal to sub vertical over short distances this latter domain involves an elongated corridor in which the magmatic foliation is sub horizontal and bears an orogen parallel lineation finally the fourth narrow domain displays sub horizontal lineations on a sub vertical magmatic foliation oblique n150 e to the trend of the belt u pb dating of zircons from the various domains revealed homogeneity in age for all samples this together with the lack of solid state deformation suggests that 1 the whole batholith emplaced during a magmatic event at 580 ma 2 the deformation occurred before complete solidification and 3 the various fabrics are roughly contemporaneous the complex structural pattern mapped in the studied tonalitic batholith suggests a 3d deformation of a slowly cooling large magmatic body and its country rock we suggest that the development of the observed 3d flow field was promoted by the low viscosity of the middle crust that turned gravitational force as an active tectonic force combining with the east west convergence between the sao francisco and congo cratons

strike slip faults in an intraplate setting and their significance for landform evolution in the kachchh peninsula western india abstract the island belt fault zone ibz and wagad upland that are associated with past and recent earthquakes have been investigated to ascertain the role of tectonics in landform evolution towards this we used data from fill and strath terraces employing conventional geomorphology sedimentology and optical chronology the study suggests episodic accelerated crustal deformation along the ne sw and nw se trending transverse strike slip faults that are genetically related to the regional n s directed compression the deformation events are optically dated to 61 ka 47 ka 38 ka 29 ka 15 ka and 2 ka uplift rate has been estimated from the height and age of the strath terraces along the river valleys of ibz and wagad upland the study observed a significantly lower uplift rate 0 2 0 06 mm year compared to the kachchh mainland 1 mm this is ascribed to the strain partitioning caused due to the presence of the kmf and khf located in the south of the ibz further it is hypothesized that the restraining and releasing bends along the strike slip faults could act as barriers to earthquake propagation and might act as nucleation sites for major earthquakes in the intraplate setting thus the study calls for the identification of such locations in order to assess the severity of the future earthquakes in the region

mobilisation of elements from coal due to batch reactor experiments with co2 and water at 40 c and 9 5mpa injection of co2 into coal seams with or without enhanced coal bed methane recovery co2 ecbm is one method for sequestering co2 greenhouse gas emissions in the subsurface conventional saline formations that could be used for co2 sequestration also may have coal seams within the seal rock units however relatively little is known concerning the potential interactions between co2 acidified water and coal this geochemical study investigates the short term mobilisation of elements from selected samples of coal exposed to highly reactive conditions expected in the mixing front between the injected co2 plume and groundwater coals were obtained from both the permian bowen basin of queensland australia and the carboniferous ruhr basin of germany within a custom built batch reactor cubes of coal were reacted with supercritical co2 saturated water at 9 5mpa and 40 degrees celsius for 72h carbonate minerals within the coal cleats and matrix were most affected by the co2 water experiments as indicated by both sem images and reaction water chemistry an experiment with n2 saturated water to compare water only mobilisation with co2 water mobilisation was also done for a single sample in each of the seven sets of coals the effluent concentrations of several elements including al ba ca fe mg and mn consistently were greater during the co2 water coal experiments in comparison to water only mobilisation from the same samples whereas elements such as ag b hg k li na and sb were much less likely to be preferentially mobilised by co2 acidified water

initiation and propagation of shear zones in a heterogeneous continental lithosphere continental plates represent rheologically heterogeneous media in which complex finite strain fields may develop due to interaction of plate tectonic processes with intraplate heterogeneities such a deformation pattern is displayed by the borborema shear zone system in northeastern brazil it involves continental scale curvilinear e w trending right lateral transcurrent shear zones that branch off from a major ne trending right lateral strike slip deformation zone formed at the northern termination of the sao francisco craton and that finally terminate in transpressive metasedimentary belts we suggest that this strain field results from the compressional deformation of a highly heterogeneous continental lithosphere composed of a stiff domain craton and rheologically weaker domains basins the effect of a low viscosity domain on the deformation of a continental plate and in particular the perturbation induced by this domain on the development of a shear zone formed at the termination of a stiff block were investigated using numerical modeling the low strength domain induces an enhanced strain localization and the geometry of the shear zone is significantly modified it is either split forming a branched shear zone system in which one branch maintains its original orientation while the other rotates toward the low viscosity domain or completely rotated toward the weak block the perturbation of the finite strain field depends on the ability of the weak domain to accommodate deformation which is controlled by its initial viscosity contrast relative to the surrounding lithosphere its orientation relative to the convergence direction and its distance from the shear zone initiated at the termination of the stiff block the interaction between imposed boundary conditions tectonic forces and plate geometry and the internal structure of the plate may give rise to highly heterogeneous strain fields as exhibited by the borborema shear zone system in which intraplate rheological heterogeneities induce the development of branched or sinuous shear zones a heterogeneous continental plate subjected to a normal convergence may therefore display significant lateral variations in strain intensity with shear zones bordering nearly undeformed blocks and in deformation regimes and vertical strains that would result in contrasting metamorphic and uplift histories

granite fabrics and regional scale strain partitioning in the serido belt borborema province ne brazil 1 fabrics of the brasiliano age granitoid plutons of the serido belt northeastern brazil were organized following the partitioned regional strain field which shaped larger areas of the basement and its metapelitic cover nine plutons located at the central and western portions of the belt have been investigated by means of anisotropy of magnetic susceptibility these plutons emplaced between 580 and 575 ma are mainly potassic calk alcalic magnetite bearing monzogranites the presence of primary coarse grained magnetite as the main magnetic mineral is indicated by optical and electronic microscopy magnetic susceptibilities hysteresis ratios and thermomagnetic curves the plutons show a strong internal coherence of magnetic fabric with foliation poles distributed around a zone axis parallel to the gently plunging mean lineation the magnetic fabric was used to trace the strain partitioning along the belt in the central transpressive shear belt the magnetic lineations are parallel to north trending regional stretching on the other hand in the western basement block a voluminous granitic magmatism was emplaced as a consequence of a transtensional extensional deformation lineations in these granites trend in a ne direction which is parallel to the transport inferred from kinematic indicators along the shear zones a crustal scale east trending dextral shear zone connects both the transtensional and transpressional domains

asymmetric upwarp of the asthenosphere beneath the baikal rift zone siberia in the summer of 1991 we installed 27 seismic stations about lake baikal siberia aimed at obtaining accurately timed digital seismic data to investigate the deep structure and geodynamics of the baikal rift zone and adjacent regions sixty six teleseismic events with high signal to noise ratio were recorded travel time and q analysis of teleseisms characterize an upwarp of the lithosphere asthenosphere boundary under baikal theoretical arrival times were calculated by using the international association of seismology and physics of the earth s interior 1991 earth model and travel time residuals were found by subtracting computed arrival times from observed ones a three dimensional downward projection inversion method is used to invert the p wave velocity structure with constraints from deep seismic sounding data our results suggest that 1 the lithosphere asthenosphere transition upwarps beneath the rift zone 2 the upwarp has an asymmetric shape 3 the velocity contrast is 4 9 in the asthenosphere 4 the density contrast is 0 6 and 5 the p wave attenuation contrast t is 0 1 s

the lithosphere in central europe seismological and petrological aspects abstract the lithosphere thickness in the variscan belt of central europe varies between about 60 and 150 km with typical values of 100 120 km our estimates derived from directionally independent representative p wave residuals are in good agreement with magnetotelluric determinations of a layer with increased conductivity in the upper mantle the large scale anisotropies of the subcrustal lithosphere beneath four seismological stations determined from spatial variations of relative p residuals vary between 6 5 and 15 2 for p velocities the s wave anisotropies determined from sks polarizations vary between 2 2 and 6 7 these values are in reasonable agreement with the anisotropies of peridotites determined in laboratory systematic spatial variations of the directional terms of relative residuals in dependence on azimuths and incidence angles suggest the existence of large dipping anisotropic structures in the subcrustal lithosphere the residual patterns at most stations in the saxothuringicum rhenohercynicum and in the massif central imply northwesterly orientated dips of the anisotropic structures while stations in the moldanubicum the alpine foredeep and most of the alps north of the insubric line suggest southeasterly orientated dips in our interpretation the dipping anisotropic structures may represent paleosubductions which retain olivine preferred orientations originating from an ancient oceanic lithosphere the variscides of central europe may thus represent a collision zone characterized by two systems of paleosubductions divergent relative to the suture between the moldanubicum and the saxothuringicum

concentration and distribution of elements in late permian coals from western guizhou province china abstract with the aim of better understanding geochemistry of coal 71 late permian whole seam coal channel samples from western guizhou province southwest china were studied and 57 elements in them were determined the contents of al ca co cr cu fe ga hf k li mn mo nb ni sn ta ti th u v zr and rees in the late permian coals from western guizhou province are higher than the arithmetic means for the corresponding elements in the us coals whereas as ba br f hg p se and tl are lower compared to common chinese coals the contents of co cr cu ga hf li mn mo ni sc sn ti u v zn and zr in western guizhou coals are higher and as f hg rb sb tl and w are lower five groups of elements may be classified according to their mode of occurrence in coal the first two group a tm yb lu y er ho dy tb ce la nd pr gd sm and group b as sr k rb ba f ash si sn ga hf al ta zr be th na have high positive correlation coefficients with ash yield and they show mainly inorganic affinity some elements from group b such as ba be ga hf and th are also characterized by significant aluminosilicate affinity in addition arsenic also exhibits high sulfide affinity r s fe 0 5 the elements which have negative or lower positive correlation coefficients with ash yield with exceptions of bi cs nb mn se and ti are grouped in other four associations group c cr v mo u cd tl group d hg li sc ti eu nb cs w group e bi sb and group f co ni cu pb zn mg se ca mn s fe the correlation coefficients of some elements including co cr cu fe hg li mo ni p s sc u v and zn with ash yield are below the statistically significant value only cr and cu are negatively correlated to ash yield 0 07 and 0 01 respectively showing intermediate organic and inorganic affinity manganese and fe are characterized by carbonate affinity probably due to high content of epigenetic veined ankerite in some coals phosphorus has low correlation coefficients with any other elements and is not included in these six associations there are five possible genetic types of enrichment of elements in coal from western guizhou province source rock volcanic ash low temperature hydrothermal fluid groundwater and magmatic hydrothermal inputs

thermal control on post orogenic extension in collision belts abstract intracontinental extension develops in two different ways in the rhine graben the baikal or the east african rifts it is localized in a long and narrow zone along a few normal faults which often delineate a single graben in contrast in the basin and range province or the tibetan plateau it is distributed over a much broader area up to more than 1000 km across this difference arises from differences in rheology and applied forces widespread extension probably reflects the thinning of a previously thickened crust geological observations indicate that only wide orogenic belts undergo late or post orogenic thinning a related fact is that anatectic granites seem to be most abundant in wide orogenic belts the general correlation between belt width and occurrence of both extension and anatectic granites suggest that it reflects in part the characteristics of horizontal heat transfer between thickened crust and surroundings in wide orogenic plateaux the large amount of heat produced in the crust by radioactive decay and lack of erosion leads to high temperatures in narrow mountain belts horizontal heat transfer and high erosion rates concur to prevent temperatures from rising to values at which the crust may yield in extension we have tested this hypothesis using transient two dimensional numerical models of orogenic belts focussing on the effects of the shortening rate the final width of the belt and the rate of crustal radiogenic heat production in the range of geological values the shortening rate has little influence on the results crustal temperatures increase as a function of belt width we find that the sensitivity of crustal temperatures to belt width is most pronounced in the 100 300 km width range and for large values of crustal heat generation the pressure temperature paths followed by geological formations depend on the belt width which is consistent with available data the integrated shear strength of the lithosphere decreases with time as the belt is being built and then eroded this evolution is sensitive to belt width if extension occurs when the integrated shear strength drops below a critical value our results yield a framework for comparing different orogenic belts we predict that extension starts late in the history of wide belts and never occurs in narrow belts a final implication of this study is that the average rate of crustal heat production is probably close to 1 1 10 6 w m 3 which is in the upper range of possible values we present a simple and convenient diagram summarizing the thermal characteristics of orogenic belts as a function of width and shortening rate