effects of topography on the curvature of fold and thrust belts during shortening of a 2 layer model of continental lithosphere abstract we have used analogue experiments to investigate the effects of surface topography on the curvature of fold and thrust belts under conditions of 1 initial relief but no erosion and 2 no initial relief but differential erosion sedimentation and transport in experiments where a 2 layer model lithosphere shortened and thickened in front of an advancing straight piston the geometry of the developing thrust wedge was very sensitive to variations in surface topography in models with an initially flat horizontal surface and in the absence of erosion and sedimentation thrusts were straight propagated forwards and nucleated at buckle folds far in front of an advancing piston around an initial topographic high plateau or cone thrusts tended to be arcuate forming salients towards the foreland initial plateaux and cones tended to behave rigidly while arcuate thrust slices formed around them to accommodate differential slip transfer zones developed on both sides of initial highs fault blocks rotated about vertical axes and thrusts moved in oblique slip within transfer zones in models with initially horizontal surfaces which were subject to differential erosion sedimentation and transport thrusts initially were straight but then progressively rotated around non eroded thickened and stronger areas these worked as indenters in front of which new thrusts nucleated at curved buckle folds these thrusts were also curved their apices being in front of the thickened non eroded areas in nature arcuate structural patterns are to be found around the altiplano of the central andes and around the tromen volcanic ridge in the neuquen basin of northern patagonia we infer that these areas behaved in quasi rigid fashion protected as they were by their high elevations and that differential erosion at the scale of the entire andes may have contributed to oroclinal bending

gravity above subduction zones and forces controlling plate motions 1 short and intermediate wavelength gravity and geoid anomalies are used to provide constraints on the mechanical structure of subduction zones and on the forces involved this study is based on 2d cartesian dynamically self consistent models with newtonian or power law rheologies we show that both strong decoupling of the two convergent plates shear stresses of the order of 107 pa and weakened bending lithosphere are necessary to reproduce the observed geoid and gravity data good fits are found for relatively low failure stresses 30 50 mpa for all models providing reasonable predictions of gravity only a small fraction of the downgoing slab weight is transmitted to the surface plates about 10 of the energy is dissipated in the contact zone between the two plates 10 to 20 in the bending region and more than 70 in the sublithospheric mantle the basal tractions on the order of 1 4 1012 n m induce a net motion of the plates with the subducting lithosphere moving faster than predicted by the no net motion principle a marked positive geoid anomaly is predicted above subduction zones at intermediate wavelength 2000 4000 km in the case of pure whole mantle convection such large geoid highs are not observed introducing partial layering i e mass anomalies hampering mantle flow through the transition zone is necessary to reconcile model predictions and observations for these wavelengths

contribution of gravitational potential energy differences to the global stress field summary modelling the lithospheric stress field has proved to be an efficient means of determining the role of lithospheric versus sublithospheric buoyancies and also of constraining the driving forces behind plate tectonics both these sources of buoyancies are important in generating the lithospheric stress field however these sources and the contribution that they make are dependent on a number of variables such as the role of lateral strength variation in the lithosphere the reference level for computing the gravitational potential energy per unit area gpe of the lithosphere and even the definition of deviatoric stress for the mantle contribution much depends on the mantle convection model including the role of lateral and radial viscosity variations the spatial distribution of density buoyancies and the resolution of the convection model gpe differences are influenced by both lithosphere density buoyancies and by radial basal tractions that produce dynamic topography the global lithospheric stress field can thus be divided into 1 stresses associated with gpe differences including the contribution from radial basal tractions and 2 stresses associated with the contribution of horizontal basal tractions in this paper we investigate only the contribution of gpe differences both with and without the inferred contribution of radial basal tractions we use the crust 2 0 model to compute gpe values and show that these gpe differences are not sufficient alone to match all the directions and relative magnitudes of principal strain rate axes as inferred from the comparison of our depth integrated deviatoric stress tensor field with the velocity gradient tensor field within the earth s plate boundary zones we argue that gpe differences calibrate the absolute magnitudes of depth integrated deviatoric stresses within the lithosphere shortcomings of this contribution in matching the stress indicators within the plate boundary zones can be corrected by considering the contribution from horizontal tractions associated with density buoyancy driven mantle convection deviatoric stress magnitudes arising from gpe differences are in the range of 1 4 tn m 1 a part of which is contributed by dynamic topography the egm96 geoid data set is also used as a rough proxy for gpe values in the lithosphere however gpe differences from the geoid fail to yield depth integrated deviatoric stresses that can provide a good match to the deformation indicators gpe values inferred from the geoid have significant shortcomings when used on a global scale due to the role of dynamically support of topography another important factor in estimating the depth integrated deviatoric stresses is the use of the correct level of reference in calculating gpe we also elucidate the importance of understanding the reference pressure for calculating deviatoric stress and show that overestimates of deviatoric stress may result from either simplified 2 d approximations of the thin sheet equations or the assumption that the mean stress is equal to the vertical stress

forces controlling the present day state of stress in the andes the present day state of stress in the andes is expected to be controlled primarily by two different types of forces 1 the resistive force exerted on the western overriding margin of the south american plate and 2 forces that arise from the thickened crust of the andes i e the effect of topography and its compensating crustal root we have studied these forces on the basis of a model for the dynamics of the entire south american plate in which the andes are embedded in this model a given set of forces is constrained by the criterion that the net torque on the plate should vanish a thin elastic shell representation is used to calculate the intra plate stress field associated with the various force distributions we define a reference model for the present study that incorporates a uniform magnitude for the resistance associated with convergence along the western plate margin fpcr plate contact resistance and does not include the effects of topography subsequently we investigate the effect of lateral variation in the magnitude of fpcr and add the topography related forces the main results are 1 a uniform magnitude of fpcr leads to a better match with the observations than a magnitude that is a function of the dip of the lithosphere subducting below the western plate margin 2 the amount of horizontal compression across the andes found in the case that ridge push is considered to be the only force driving the south american plate is small compared to the value required to sustain the andes

rheological evolution of the ocean crust a microstructural view rheological studies of the oceanic lithosphere have used direct observations of rocks and inferences based on thermal mechanical and experimental models modeling studies help to constrain an average crust and mantle rheology and the deep seated processes that control regional scale strength and stress field variations the smoothing of fine scale variations in such models can obscure many of the geological processes that influence strain localization and deformation partitioning in the ocean crust examination of deformation mechanisms and histories in ocean crust rocks provides a complementary approach to modeling examples of structures and deformation histories in diabases from deep sea drilling project ocean drilling program site 504b the hayes and atlantis fracture zones mid atlantic ridge and the troodos ophiolite cyprus are presented in conjunction with a synthesis of microstructural studies of the ocean crust over the last 25 years a survey of brittle quasi plastic and synmagmatic viscous deformation is used to demonstrate the influence of primary compositional and textural characteristics and variable magmatic and hydrothermal histories on deformation mechanisms and strain localization in the ocean crust geological evidence indicates that hydrothermal fluids strongly influence the nature of deformation and that effective stresses may be low due to fluid overpressures melt distribution will strongly influence strain localization at the base of the crust and synkinematic hydration during crystal plastic deformation plays a key role in the relative strengths of polyphase oceanic lithologies a schematic distribution of failure mechanisms in the ocean crust is used to discuss the controls on variations in lateral and vertical strength profiles and their possible relation to spreading rates although microstructural studies of ocean crust are still in their infancy they provide valuable constraints for rheological models and further insights to explain the distribution of seismicity at spreading centers and acoustic signatures

models of passive margin inversion implications for the rhenohercynian fold and thrust belt belgium and germany abstract positive tectonic inversion is related to the transmission of compressional stresses along a decollement into the foreland of an orogenic zone this stress and strain concentration in regions remote from the main orogenic front is commonly related to the presence of pre existing rheological heterogeneities such as normal syn depositional faults during inversion these pre existing normal faults are reactivated as reverse faults tectonic inversion in the rhenohercynian fold and thrust belt during the variscan orogeny shows that inversion is likely synchronous with the onset of collision in the hinterland here we present the results of a simplified thermo mechanical model stm which allows one to study strain partitioning between two orogenic zones we show that if the two orogenic zones have the same mechanical properties the viscosity of the decollement which links them controls the initial strain partitioning during subsequent finite shortening erosional processes determine the partitioning of strain rate the presence of a weak structure in the inverted zone and of a low viscosity decollement leads to initial strain concentration in the inverted track rather than in the collision zone and a progressive decrease in strain partitioning between the two orogenic zones the stm results are in good agreement with results of a 2d finite element model we conclude that in the western part of the rhenohercynian massif simultaneous uplift and deformation within the mid german crystalline rise the main collision zone and the ardenne anticlinorium the inverted zone lead to interpreting this orogenic event as a case of vice tectonic rather than the propagation of a wave of folding towards the variscan front as suggested by previous authors

role of topography induced gravitational stresses in basin inversion the case study of the pannonian basin numerical stress models suggest that gravitational body forces associated with elevated topography around sedimentary basins can significantly influence the stress and strain pattern in basin interiors in the absence of tectonic forces basins surrounded by high altitude mountain ranges experience net horizontal compression owing to gravitational forces pointing from areas of high gravitational potential energy to subsided basin areas further lithospheric extension can eventually terminate leading to a gradual late stage inversion of the entire basin system modeling results suggest that the state of recent stress in the pannonian basin particularly in its western part is controlled by the interplay of plate boundary forces i e the counterclockwise rotation and northward indentation of the adriatic microplate against the alpine dinaric belt and buoyancy forces associated with the elevated topography and related crustal thickness variation of the alpine dinaric belt model calculations show that uplifted regions surrounding the basin system can exert compression on the thinned pannonian lithosphere of 40 60 mpa that is of the order of the assumed far field tectonic stress magnitudes the combined analysis of stress sources of tectonic and gravitational origin helps estimating the magnitude of maximum horizontal compression high levels of compressional stresses up to 100 mpa are concentrated in the elastic core of the lithosphere consistent with the ongoing structural inversion of the pannonian basin system

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

the mechanical state of the lithosphere in the altiplano puna segment of the andes abstract information about topography the shape of the geoid seismicity neogene deformation and volcanism in the region of altiplano puna of western south america is used to analyse the state of stress across the convergent plate margin in terms of the effects of topography and simple models of its compensation an average elevation near 4 km is consistent with compensation by a yet unresolved combination of crustal root and hot uppermost mantle producing a geoid high of 22 27 meters average horizontal compressive stress in excess of a reference sea level lithostatic value of 390 bars in a 150 km thick lithosphere and an average shear stress of 170 bars along a 30 dipping interplate boundary the basis for these estimates is evidence for a neutral to extensional stress regime within the high plateau contrasted with a compressional regime on the eastern slopes and along the interplate boundary itself comparison with other plateaus in a convergent plate tectonic setting suggests an evolutionary sequence from compressional to extensional tectonics as elevation of the plateau increases

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

driving forces for limited tectonics on venus abstract the very high correlation of geoid height and topography on venus along with the high geoid topography ratio can be interpreted as local isostatic compensation and or dynamic compensation of topography at depths ranging from 50 to 350 km for local compensation within the lithosphere the swell push force is proportional to the first moment of the anomalous density since the long wavelength isostatic geoid height is also proportional to the first moment of the anomalous density the swell push force is equal to the geoid height scaled by g2 2 g because of this direct relationship the style i e thermal airy or pratt compensation and depth of compensation do not need to be specified and can in fact vary over the surface phillips 1990 showed that this simple relationship between swell push force and geoid also holds for dynamic uplift by shear traction on the base of the lithosphere caused by thermal convection of an isoviscous infinite half space mantle thus for all reasonable isostatic models and particular classes of dynamic models the geoid height uniquely determines the magnitude of the swell push body force that is applied to the venusian lithosphere given this body force and assuming venus can be approximated by a uniform thickness thin elastic shell over an inviscid sphere we calculate the present day global strain field using equations given in banerdt 1986 areas of positive geoid height are in a state of extension while areas of negative geoid height are in a state of compression the present day model strain field is compared to global strain patterns inferred from magellan derived maps of wrinkle ridges and rift zones wrinkle ridges which are believed to reflect distributed compressive deformation are generally confined to regions with geoid of less than 20 m while rift zones are found primarily along geoid highs moreover much of the observed deformation matches the present day model strain orientations suggesting that most of the rifts on venus and many of the wrinkle ridges formed in a stress field similar to the present one in several large regions the present day model strain pattern does not match the observations this suggests that either the geoid has changed significantly since most of the strain occurred or our model assumptions are incorrect e g there could be local plate boundaries where the stress pattern is discontinuous since the venusian lithosphere shows evidence for limited strain the calculation also provides an estimate of the overall strength of the lithosphere in compression and extension which can be compared with rheological models of yield strength versus depth at the crests of the major swells where evidence for rifting is abundant we find that the temperature gradient must be at least 7 k km

steady state secondary convection beneath lithospheric plates with temperature and pressure dependent viscosity we have constructed a thermomechanical model for upper mantle convection such that the thickness and structure of the lithosphere are determined self consistently by the heat transported by convection in this study of the interaction between the lithosphere and upper mantle strongly temperature and pressure dependent rheologies for both newtonian and non newtonian creep mechanisms are employed for a strictly temperature dependent rheology an insignificant amount of heat less than 12 5 mw m2 can be transported convectively for an interior viscosity 0 1021 pas compatible with postglacial rebound on the other hand for similar values of the interior viscosity steady heat fluxes between 20 and 40 mw m2 are produced by introducing pressure dependence into the rheology for the temperature and pressure dependent flow law the horizontally averaged interior temperature displays very little variation with the amount of heat evacuated once all of the rheological parameters are fixed this finding may have important ramifications for parameterized convection we employ both the single mode mean field approximation and the complete two dimensional equations using finite elements to obtain solutions for the various types of rheologies from evaluating the geophysically relevant observables such as topography free air gravity anomalies surface heat flow and stress fields in the lithosphere we find that the lateral variations of these quantities predicted by a non newtonian rheology are much smaller than those derived from a linear rheology these results suggest that surface variations of geophysical observables are more compatible with a non newtonian rheology in the upper mantle

true polar wander and plate driving forces a net torque on the global lithosphere can be exerted both by ridge push and trench pull forces a net ridge push torque is present if there is an asymmetry in the age distribution of seafloor about one or more ocean ridges a net torque due to trench forces is present if the forces per trench length acting on the subducting and overthrust plates are not equal in magnitude the combined net torque on the lithosphere contributed by ridge and trench forces must be balanced by an opposing torque most likely due to basal shear tractions associated with a global rotation of the lithosphere relative to the underlying mantle such a rotation should be identifiable as true polar wander the lack of significant rotation of the lithosphere with respect to the magnetic dipole axis during the cenozoic therefore places a strong constraint on the nature of torques acting on the lithosphere we suggest that the most likely explanation for negligible true polar wander in the cenozoic is that the net ridge and trench torques nearly cancel one another the net ridge and trench torque vectors are in nearly opposite directions and have been so through the cenozoic so the condition of near cancellation amounts to a restriction on the magnitude of the force imbalance along trench boundaries such a near cancellation of net torques and the resulting negligible true polar wander are likely to be consequences of present plate geometries and need not be representative of earlier eras

predicting plate velocities with mantle circulation models 1 we predict plate motions from a comprehensive inversion of theoretical estimates of tectonic forces in order to evaluate the relative importance of these and the uncertainties of such models plate driving forces from the mantle are calculated using global flow models that are driven by tomography and subduction derived density fields observed and predicted plate velocities agree well for a variety of models leading to varied conclusions about the relative importance of forces the dominance of the subduction related density pattern in the mantle is confirmed it appears that p wave models do not satisfactorily image all of the slab associated anomalies in the upper mantle furthermore lower mantle structure always improves the plate motion fit with respect to models that are based on upper mantle anomalies and lithospheric thickening only we show that the average torques from the lower mantle scale with the radial flow through the 660 km phase transition the amplitude of the lower mantle torques will be significant for a range of models if there is mass flux through 660 km we also evaluate parameterized edge forces and find that the additional inclusion of such torques does not significantly improve the model fit the main reason for the nonuniqueness of the inversions is plate boundary geometry since all plate motions are dominated by the trench ridge system and plates move from ridges to trenches

why does near ridge extensional seismicity occur primarily in the indian ocean abstract the possibility that thermoelastic stresses due to plate cooling contribute significantly to the stress field and seismicity in young oceanic lithosphere has been a subject of considerable recent interest this effect is suggested by three key observations a decrease in seismicity with lithospheric age the fact that focal mechanisms show extension perpendicular to the spreading direction and a depth stratification of mechanism types a difficulty with this idea is that although thermoelastic stresses should be comparable in different regions the intraplate seismicity seems to occur in local concentrations in particular the ridge parallel extensional seismicity occurs preferentially in the central indian ocean region we explore the possibility that much of the data favoring thermoelastic stresses can be interpreted in terms of stresses resulting from individual plate geometry and local boundary effects in particular the dramatic concentration of extensional seismicity in the central indian ocean region is consistent with finite element results for the intraplate stress incorporating the effects of the himalayan collision and the various subduction zones the ridge parallel extensional stresses show a decrease with age similar to that of the seismicity as earthquakes in this area provide a major portion of the data for both ridge parallel extension and depth stratification these effects may be due more to the regional stress we thus propose that thermoelastic stresses provide a low level background in all plates but that the dominant effect is that of individual plate geometry and local boundary effects

the legacy of the european geotraverse abstract the european geotraverse egt created a 4600 km profile across europe from north cape norway to tunisia not only did this produce the first comprehensive cross section to a depth of 450 km of continental lithosphere covering an eighth of the earth s circumference it covered the geological history of europe from the archaean to the present egt built up a detailed knowledge of the crust and upper mantle of europe by integrating geological and geophysical information in a coherent way continuously along a single profile it illuminated the dramatic contrast between the thickness and complexity of the lithosphere of western europe and that of fennoscandia which remain in isostatic equilibrium and elucidated the multilayered elements of thickened alpine crust and lithosphere and the dynamics of the western mediterranean nine years on the legacy of egt is the platform it provided for major scientific advances that have stemmed from it the greatest being through europrobe projects the pancardi project has discovered the subducted slab beneath the carpathian arc in the process of tearing apart from the lithosphere uralides has discovered the urals orogen is almost perfectly preserved since the palaeozoic and the tesz project has elucidated the complex evolution of the transitional region between the baltic shield and the caledonian and variscan crustal terranes of western europe the scientific advances of egt were matched by its achievement in mobilising an international workforce from every discipline of the earth sciences friendships made in that endeavour are another legacy of egt and many of those who started their research careers in egt are now leading europrobe projects and other collaborative ventures we all owe a great debt to professor stephan mueller who founded and led the european geotraverse and was its enduring inspiration

long wavelength geoid the effect of continental roots and lithosphere thickness variations summary the existence of continental roots and the gradual thickening of the cooling oceanic lithosphere give rise to large scale rheological heterogeneities in the uppermost mantle the effect of these heterogeneities on the long wavelength geoid is investigated using a 3 d mantle flow model involving a low viscosity asthenosphere beneath the oceanic lithosphere and the tectonically active continental regions and a thick highly viscous lithosphere beneath ancient continents below 400 km the mantle viscosity is laterally homogeneous with a lower mantle more viscous than the overlying layer the mantle circulation is driven by imposed surface velocities nuvel 1 hs2 gripp gordon 1990 and by the density anomalies inferred from the tomographic models p16b30 and s16b30 masters et al 1996 the geoid heights both due to plate motion and due to internal loading differ by as much as 30 per cent between the models with and without lateral viscosity variations in contrast to what was suggested in previous studies spherical harmonics 2 and 3 are strongly affected by the lateral viscosity variations these differences in the forward problem suggest that the response to the inverse problem that consists of finding the profile of viscosity as a function of depth providing the best fit to the geoid should be considerably affected by the lateral viscosity variations the shear stresses at the base of the plates induced by the imposed surface velocities and those induced by the internal loading are sensitive to the lateral viscosity variations this suggests that the lateral viscosity variations are very important for understanding the stress field or the forces acting on the plates the direction of these shear stresses which should be linked to the anisotropy direction is very different from the plate motion direction

secondary convection and the growth of the oceanic lithosphere abstract this paper presents a study of the evolution of the oceanic lithosphere from a thermomechanical approach we have investigated the finite amplitude development of secondary convection cells beneath the oceanic plates by means of the single mode mean field equations and the fully 2 dimensional convection equations using finite element techniques both newtonian and non newtonian rheologies with highly temperature and pressure dependences and an activation energy of 100 kcal mol 1 have been employed the temperature at the base of the convecting medium governs the final thickness of the lithosphere the mean interior temperature varies only slightly during the temporal evolution non newtonian rheology has the tendency to induce oscillatory time dependent behavior of the flow heat flow topography and gravity are influenced by secondary convection in two ways small scale perturbations with wavelengths of around 600 km arise from the lateral thermal differences between the uprising and descending convective limbs large scale features are also produced as a consequence of lithospheric growth the calculated quantities of the heat flow topography and gravity associated with small scale convection are typically in the range of o hfu o 102m and o 10 mgal the horizontal mean temperature profiles from the convection model are used to calculate long wavelength geophysical observables as a function of age convective processes are found to reduce the rate of lithospheric thickening predictions from our model can fit well the observed data of heat flow ocean floor topography and geoid offsets along fracture zones the last data base exhibiting the most sensitivity to thermal perturbations below the lithosphere our calculations show that the oceanic lithosphere is able to grow continuously up to o 109 y long past the flattening of the seafloor we report here that the thickness of a thermally equilibrated lithosphere could reach 250 km which lies within the appropriate range of values for the continental lithosphere as inferred from studies in seismology flexure observations and secular polar motions

vegetation development in the middle euphrates and upper jazirah syria turkey during the bronze age vegetation changes are reconstructed based on more than 51 000 charcoal fragments of more than 380 samples from nine bronze age sites in northern syria and southern turkey in addition to fragment proportions special attention was paid to the frequency of pistacia relative to quercus and populus salix relative to tamarix fruit tree ubiquity and riverine diversity in order to gain an improved understanding of the human versus climatic impact on the vegetation the results indicate that human impacts first took place within the riverine forest this phase was followed by land clearing within the woodland steppe especially in the northern portion of the study area in the south near emar the woodland steppe probably disappeared by the late bronze age it is uncertain whether this was caused by aridification and or human clearing the northward shift of the pistacia woodland steppe is very likely a result of climatic drying that occurred throughout the entire period under investigation although increased deforestation is evident through time the small proportions of imported wood indicate that local resources were still available

continental plate collision p t t z conditions and unstable vs stable plate dynamics insights from thermo mechanical modelling there was much debate recently on the mechanisms of continental convergence and related pressure temperature p t conditions both in modeling and petrologic community depending on the mechanisms of convergence subduction collision folding or rt instability one can argue about the possibility of large scale deviations of pressure and temperature in the accretion prism and below it from reference i e lithostatic conditions commonly used for petrologic reconstructions of evolution of exhumed metamorphic rocks these deviations can be caused for example by tectonic overpressure that as suggested in some studies may be responsible for formation of uhp ultra high pressure rocks however overpressure in exhumation zones can be built only in specific collision scenarios associated with plate coupling pure shear folding in this study we analyze conditions that define various mechanisms of convergence and consequently of exhumation these mechanisms can be represented as a superposition of 1 simple shear subduction 2 pure shear collision 3 folding and 4 rayleigh taylor instability we study these scenarios using a thermo mechanical model that accounts for brittle elastic ductile rheology surface processes and metamorphic changes it appears that stable oceanic type subduction may occur in the case of cold lithospheres tmohob550 c and relatively high convergence rates n3 5 cm yr depending on the lower crustal rheology strong or weak either the whole upper and lower crust or only the lower crust can be involved in subduction in case of weak metamorphic rheologies phase changes improve chances for stable subduction pure shear becomes a dominant mechanism when tmohon550 c or convergence rates are lower than 3 cm yr large scale folding is favored in case of tmoho 500 650 c and is more effective in the case of mechanical coupling between crust and mantle e g strong lower crust gravitational rayleigh taylor instabilities overcome other mechanisms for very high values of tmoho n800 c and may lead to development of subvertical cold spots however it is reasonable to assume that in most cases continental collision is initiated at oceanic subduction rate which is rarely slower than 5 cm yr this rate is sufficient to drive continental subduction during the first several myr of collision in this case the subduction channel is characterized by nearly lithostatic pressure conditions large scale zones of tectonic overpressure may be built outside the channel but do not affect the exhumed rocks overpressure may be built inside the channel in the short moment of its closure we suggest that most continental orogenic belts could have started their formation from continental subduction this evokes small tectonic overpressures and thus deep origin of the uhp rocks that may be brought to the surface via the suggested multi level mechanism of exhumation

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

passive margins getting squeezed in the mantle convection vice 1 passive margins often exhibit uplift exhumation and tectonic inversion we speculate that the compression in the lithosphere gradually increased during the cenozoic as seen in the number of mountain belts found at active margins during that period less clear is how that compression increase affects passive margins in order to address this issue we design a 2 d viscous numerical model wherein a lithospheric plate rests above a weaker mantle it is driven by a mantle conveyor belt alternatively excited by a lateral downwelling on one side an upwelling on the other side or both simultaneously the lateral edges of the plate are either free or fixed representing the cases of free convergence and collision or slab anchoring respectively this distinction changes the upper mechanical boundary condition for mantle circulation and thus the stress field between these two regimes the flow pattern transiently evolves from a free slip convection mode toward a no slip boundary condition above the upper mantle in the second case the lithosphere is highly stressed horizontally and deforms for a constant total driving force compression increases drastically at passive margins if upwellings are active conversely if downwellings alone are activated compression occurs at short distances from the trench and extension prevails elsewhere these results are supported by earth like models that reveal the same pattern where active upwellings are required to excite passive margins compression our results substantiate the idea that compression at passive margins is in response to the underlying mantle flow that is increasingly resisted by the cenozoic collisions

pseudotachylyte generated in the semi brittle and brittle regimes bench canyon shear zone central sierra nevada pseudotachylyte cuts cretaceous plutonic and volcanic rocks in the bench canyon shear zone central sierra nevada california as documented by optical and scanning electron microscopy the pseudotachylyte displays evidence for a friction melt origin including vesicles amygdules crystallites flow fabrics and embayed crystal fragments based on fault related rock association vein morphology and inferred crustal level of generation two distinct types of pseudotachylyte are documented in the shear zone pseudotachylyte associated with cataclasite was generated at shallow crustal levels 1 10 km via unstable velocity weakening abrasive wear friction pseudotachylyte associated and interlayered with mylonite was generated at deeper crustal levels 10 15 km most likely via stable velocity strengthening adhesive wear friction this pseudotachylyte underwent ductile deformation in the solid state interlayered pseudotachylyte and mylonite records cyclic aseismic seismic slip at a transitional crustal level between the semi brittle field of quartzofeldspathic rocks and the base of the seismogenic zone the cyclicity is attributed to strain rate fluctuation effected by strain resistant phacoids pore fluid pressure variation ductile instabilities or downward fault propagation the mode of deformation within the shear zone progressed from i production of mylonite during amphibolite to greenschist facies conditions to ii generation of pseudotachylyte by intermittent seismic faulting within a background of aseismic shear in the upper realm of the semi brittle field 300 400 c to iii generation of pseudotachylyte through seismic events in the brittle 300 c during uplift exhumation and cooling some of the latter may have been generated at shallow enough crustal levels 4 km to allow vesicle formation the exhumation of a crustal level exposing such an assemblage provides a rare opportunity to study the enigmatic nature of the transitional change from semi brittle flow to seismogenic rupturing

surface motions and continental deformation in the indian plate and the india eurasia collision zone the collision of the indian plate with eurasia has played a major role in controlling the dynamics of central asia leading to the world s largest continental deformation zone in order to study the deformation within the indian plate as well as the india eurasia collision zone we model the lithospheric stress field by calculating the two primary sources of stress one arising due to topography and shallow lithospheric structure estimated by gravitational potential energy differences and the other arising from basal tractions derived from density driven mantle convection we use several tomography models to calculate horizontal tractions using the convection code hc for two radially varying viscosity structures we also take into account lateral viscosity variations in the lithosphere model arising from stiff cratons weak plate boundaries and strength variations due to old and young oceanic lithosphere we do a quantitative comparison of our predicted deviatoric stresses strain rates and plate velocities with surface observables and find that the regional tomography model of singh et al 2014 embedded in the global s wave model s40rts does a remarkable job of fitting the observations of gps velocities and strain rates as well as intraplate stress field from the world stress map

thrust initiation and propagation during shortening of a 2 layer model lithosphere analogue modelling was used to investigate thrust initiation and propagation in the lithosphere the model comprised a top layer of light sand model brittle crust overlying a layer of heavy silicone putty model ductile lower crust both resting upon higher density and lower viscosity natural honey model ductile mantle the experiments show the following 1 thrust initiation may occur in two main modes fold first or fault first they seem to be the result of presence or absence of an initial perturbation close to the piston respectively the fault first mode eventually goes into a fold first mode after a certain amount of deformation 2 in both modes thrusts localized around the inflection points of both fore and back limbs of buckles as a result of tectonic horizontal gravitational vertical and elastic forces 3 thrust propagation in the fault first mode occurred by thrust wedge thickening and downward flexure due to isostatic compensation which led to forward formation of bulges and subsequent amplification by buckling continued shortening increased the stresses that reached the brittle yield of sand and new thrusts originated 4 bulging ahead of thrusts tended to develop into asymmetric buckles with the shorter limb back limb steeper towards the piston 5 as a whole thrust initiation and propagation seem to depend mostly on how stresses evolve and propagate through the model and how material strength varies with progressive deformation the main stresses at play are born at the moving piston model horizontal tectonic force from thickening and isostatic compensation in the model gravitational forces and from elastic resistance to buckling

stresses and plate boundary forces associated with subduction plate margins primary tectonic stress in the lithosphere is predominantly caused by lateral density variations within the earth and associated topographical loading when such a stress system caused by major sublithospheric density anomalies is intersected by a weak zone which cuts across the lithosphere plate boundary forces develop and modify the plate interior stresses in this paper unite element analysis is used to model the stresses and plate boundary forces associated with subduction plate boundaries with dipping and vertical slabs extending to about 270 and 400 km depths having the subduction fault both locked and unlocked in such regions several types of horizontal deviatoric stress may occur including 1 local compression in the trench arc region caused by the dense sinking slab and the associated surface downflexure 2 plate interior tension which occurs when this compression is intersected by a weak subduction fault 3 local tension associated with thickening of the crust at the arc and elsewhere 4 local tension in the back arc region produced by the underlying low density upper mantle and 5 downbending stresses in the subducting slab thermal stresses and transmitted ridge push which are not included in the modelling here a gradient from compression in the forearc to tension in the back arc can be modelled in terms of these stresses when the fault is partially locked it is however the intersection of the local compression 1 by an unlocked or partially locked subduction fault that modifies the plate interior stresses and gives rise to the slab pull and trench suction plate boundary forces the state of stress in the interior of the overriding plate is also crucially influenced by back arc spreading where this occurs plate boundary forces have been evaluated for each of the models it is shown that slab pull and trench suction may be of comparable magnitude

the formation of the south tharsis ridge belt basin and range style extension on early mars the south tharsis ridge belt strb is located along the northeastern edge of terra sirenum and partially surrounds the southwestern part of tharsis in an arc it consists of 29 large ridges separated by distances 130 to 260 km with average relief of 1 5 km above the surrounding plains because the strb is among the oldest tectonic features associated both spatially and developmentally with tharsis it may provide key information on the early evolution of tharsis and possibly pre tharsis processes earlier studies concluded that the ridges formed through compressional tectonism however the shape size and separation of the ridges support the interpretation that the strb resembles the extensional basin and range province on earth both regions are characterized by series of parallel mountain ranges separated by broad valleys in this study we evaluate both extensional and compressional hypotheses for the origin of the ridges using evidence from topography deformed craters crustal thickness models and strain modeling though neither interpretation explains all aspects of the ridges the topography of the ridges and crustal thinning associated with the western part of the ridge belt support an extensional origin strain models predict that basin and range style wide rifting would be expected for early martian conditions this extension may have been initiated by plume induced uplift in the early stages of tharsis formation but the large amount of extensional strain inferred in the western strb must have been accommodated by compression elsewhere possibly in the heavily deformed craters of western terra sirenum

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

the origin of tectonic stress in the lithosphere abstract the sources of lithospheric stress and their distinctive features are briefly reviewed it is suggested that there are two main categories of lithospheric stress renewable stress which persists despite continuing stress relaxation and non renewable stress which can be dissipated by relief of the initial strain the two most important types of renewable stress arise from plate boundary forces and from isostatically compensated loads non renewable stress systems include bending stresses membrane stresses and thermal stresses an important phenomenon generating large stresses at shallow depth is stress amplification caused by lower lithospheric creep this applies to renewable stresses but not to the non renewable type it is suggested that only renewable stresses contribute significantly to tectonic activity however bending and thermal stresses are locally important in subducting lithosphere

thermoelastic stress how important as a cause of earthquakes in young oceanic lithosphere we present a series of exploratory models which test the hypothesis that thermoelastic stress is a significant contributor to the state of stress in young oceanic lithosphere less than 35 m y in age in support of this hypothesis is the concentration of seismicity in lithosphere younger than 15 m y where cooling rates are relatively high most near ridge earthquakes have focal depths between the moho and the depth of the 800 c isotherm thrust and strike slip events dominate the shallowest seismicity while the focal mechanisms of the deeper events are generally characterized by normal faulting to assess the importance of thermal stress in the generation of near ridge earthquakes we model the lithosphere as an elastic half space in which the response to cooling is computed using the method of thermoelastic displacement potentials a key assumption in the models is that stress is relieved on time scales generally short compared with the age of the lithosphere a further assumption in some models is that material will not contribute thermal stress until it has cooled below an elastic blocking temperature normal faulting is predicted throughout the lithosphere in thermal stress models based on simple half space cooling models in which a cooled surface layer is incorporated to simulate the effect of hydrothermal circulation on shallow thermal structure predict stresses that can match the locations of both thrust and normal faulting earthquakes near mid ocean ridges the addition of a ridge push stress to the computed thermal stress enhances the likelihood of thrust faulting in the uppermost lithosphere at ages greater than about 15 m y but probably contributes little to the stress field at younger ages though these simple models are limited by several simplifying assumptions particularly in the immediate vicinity of the ridge axis they support the hypothesis that thermoelastic stress can play a major role in the tectonics of young oceanic lithosphere

eocene oligocene tectonics and kinematics of the rhine saone continental transform zone eastern france a tectonic analysis of brittle deformaton was performed in the rhine saone transform in order to decipher and understand the deformation processes in a typical continental hansfer zone cenozoic major and minor fault patterns are described and brittle structures are interpreted in terms of paleostress orientations the eocene n s compression related to africa eurasia convergence is marked mainly by strike slip faults it was followed by a major phase of crustal extension mainly oligocene in age related to the west european rifting this extensional phase is expressed by normal faults tension gashes and extensional strike slip faults which are especially abundant in the transform zone these two events are found to be quite different in terms of distribution of horizontal shess trajectories the trend of ol axes is very homogeneous for the n s compression whereas o3 orientations associated with the major extension vary from e w in most of the rift system to i v se within the transform zone finally the mio pliocene tectonic emplacement of the jura fold and thrust belt is marked in the field by strike slip faults associated with a well defined trend of ol oriented w iiw e se to nw se the preexisting basement fracture pattern between the grabens was also considered in order to estimate which faults were likely to have been reactivated during the oligocene transform kinematics and the theoretical shear motion on these inherited fault surfaces when submitted to e w extension was calculated left laterauextensional reactivation of a preexisting 60 n trending fault system in the brittle upper crust is likely to have accommodated the shear strain arising between the rift segments in response to crustal stretching two independent numerical modelings based on distinct element and finite element analyses respectively enabled us to verify and to refine the hypothesis of transform kinematics related to e w extension and inducing regional perturbations of extensional stress trajectories the overall mechanism and the transtensional kinematics of the rhine saone transform zone are thus tightly conshained the inherited crustal anisotropy the large scale perturbations of extensional stress trajectories and the development of distributed brittle deformation within the sedimentary cover are found to play a major role in the tectonic evolution of continental rift transform systems

basin and range large scale tectonics constraints from gravity and reflection seismology two geophysical informations constrain the mechanical state of the lithosphere in the basin and range province cocorp reflection seismic structure across nevada shows a rather smooth reflector at about 30 km depth which can be interpreted as indicative of a rather flat moho on the other hand the bouguer map exhibits two marked troughs in the same region and filtering techniques reveal a predominant 200 km wavelength of the deep compensating masses of the topography this paper attributes these deep heterogeneities to lateral thermal variations created by boudinage type instabilities of the stretched mantle lithosphere

ongoing tectonic reactivation of the outer carpathians and its impact on the foreland results of borehole breakout measurements in poland abstract the direction of present day maximum horizontal compression shmax tectonic stress was interpreted for 62 wells in poland using the method of borehole breakout analysis of 4 arm and 6 arm dipmeter logs the study area covers complex tectonic junction of the carpathian orogen and its foreland which comprises the east european craton eec divided by the teisseyre tornquist zone ttz from the palaeozoic platform of western poland for this area frequent deviation of the shmax direction from nw se characteristic for the atlantic ridge push has been interpreted in terms of the alcapa tectonic push in the upper silesian segment of the polish outer carpathians poc nne ssw oriented shmax in the accretionary wedge differs significantly from nnw sse shmax in the autochthonous basement the above discrepancy points to uncoupled type of the suture in this segment of the poc in this scenario the alcapa push involves the nappes and is compensated in the top of the basement which is expressed by systematic shmax rotations in the accretionary wedge of the eastern malopolska segment shmax follows the trend perpendicular to the strike of nappes it is in general agreement with nne ssw oriented shmax in the autochthonous basement that also parallels the alcapa push direction similarity in stress orientation between these structural levels implies coupled suture zone in this segment of the poc further to the north alcapa push is transmitted into the foreland plate where it causes n s orientation of shmax as determined for sedimentary cover of the eec within the baltic portion of the eec further shmax rotation towards the intermediate nnw sse position suggests balance between the alcapa push and the ridge push components within the ttz common shmax rotations from n s to nw se indicate structurally controlled accommodation of the alcapa push in the palaeozoic platform of western poland mesozoic complex of the fore sudetic monocline reveals nne ssw directed shmax that differs from nw se oriented shmax in the variscan accretionary wedge here mechanical decoupling along zechstein evaporates is likely presented set of breakout data from poland shows that recent compressive reactivation of the carpathians exerts strong impact on the stress field of the foreland plate at a distance of 700 km from the suture due to this effect the fore carpathian stress domain within the north european stress province can be discriminated

predicting the lithospheric stress field and plate motions by joint modeling of lithosphere and mantle dynamics the way in which basal tractions associated with mantle convection couples with the lithosphere is a fundamental problem in geodynamics a successful lithosphere mantle coupling model for the earth will satisfy observations of plate motions intraplate stresses and the plate boundary zone deformation we solve the depth integrated three dimensional force balance equations in a global finite element model that takes into account effects of both topography and shallow lithosphere structure as well as tractions originating from deeper mantle convection the contribution from topography and lithosphere structure is estimated by calculating gravitational potential energy differences the basal tractions are derived from a fully dynamic flow model with both radial and lateral viscosity variations we simultaneously fit stresses and plate motions in order to delineate a best fit lithosphere mantle coupling model we use both the world stress map and the global strain rate model to constrain the models we find that a strongly coupled model with a stiff lithosphere and 3 4 orders of lateral viscosity variations in the lithosphere are best able to match the observational constraints our predicted deviatoric stresses which are dominated by contribution from mantle tractions range between 20 70 mpa the best fitting coupled models predict strain rates that are consistent with observations that is the intraplate areas are nearly rigid whereas plate boundaries and some other continental deformation zones display high strain rates comparison of mantle tractions and surface velocities indicate that in most areas tractions are driving although in a few regions including western north america tractions are resistive citation ghosh a w e holt and l m wen 2013 predicting the lithospheric stress field and plate motions by joint modeling of lithosphere and mantle dynamics

isostasy and the ambient state of stress in the oceanic lithosphere two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium the first is that the stress is hydrostatic below some depth the level of compensation and the second is that above that depth there are no vertical shear stresses in the case of a planar two dimensional mid ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge but to determine that component an additional assumption is required it is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid ocean ridges the deviatoric stress in the oceanic lithosphere is a minimum the unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid ocean ridges with a magnitude in old ocean basins of a few hundred bars in agreement with oceanic intraplate earthquake mechanisms at any given distance from the ridge the stress is maximum at the seafloor and it decreases with depth at about the same rate as the cooling of the lithosphere so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

dynamic fault rupture and stress recovery processes in continental crust under depth dependent shear strength and frictional parameters abstract mikumo t 1992 dynamic fault rupture and stress recovery processes in continental crust under depth dependent shear strength and frictional parameters in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 201 222 the dynamic rupture process of large earthquakes in continental crust and the post seismic stress recovery process due to visco plastic behaviour are investigated by three and two dimensional numerical modelling taking into account the possible depth variations of the shear strength tectonic stress expected stress drop and of the critical weakening slip which are suggested by recent laboratory experiments on frictional parameters it was found that dynamic rupture can initiate at any depth in a brittle zone if high stress is concentrated with a dimensionless stress ratio s less than a certain limit but that the rupture cannot nucleate in a semi brittle zone with negative stress drop this suggests that the boundary between the fully brittle and semi brittle zones delineates the base of the seismogenic zone of background seismicity and aftershock distribution however the dynamic rupture of large earthquakes originating from the brittle zone can propagate into the semi brittle zone after some temporary arrest or with decelerated velocities which could generate low frequency seismic waves the depth extent of dynamic faulting and the maximum fault displacement are affected by the depth to the brittle semi brittle boundary and hence by the geothermal state within the crust the stress recovery due to visco plastic behaviour in the semi brittle and plastic regimes suggests that the rupture for eventual earthquakes tends to be nucleated near the upper boundary of the semi brittle zone

ridge push and associated plate interior stress in normal and hot spot regions abstract the stress distribution and plate boundary force produced by ridge push are modelled by elastic viscoelastic finite element analysis in oceanic lithosphere and in an adjacent continent separated by a passive margin models are presented for both a normal slow spreading ridge and for one underlain by an anomalously low density upper mantle associated with a hot spot which gives rise to 1 km extra elevation of the sea floor the weak zone at the ridge crest is simulated by the viscoelastic elements extending up to the surface beneath the immediate crest using a standard thermal and rheological model of the oceanic lithosphere a normal ridge gives rise to horizontal deviatoric compression of about 40 mpa in the 25 30 km elastic layer beneath older ocean floor a much smaller modelled compression of 9 mpa occurs in the 20 km thick elastic layer of the adjacent upper continental crust as a result of the superimposed tension produced by the thick low density continental crust in reality this compression may be up to 30 mpa higher if the sub continental mantle is denser than that beneath old ocean floor as the geoid anomalies possibly suggest a fourfold increase in spreading rate has an insignificant effect on the stresses much larger compressions develop adjacent to the modelled hot spot ridge in normal oceanic and especially continental lithosphere these reaching 100 mpa beneath the old ocean floor and 90 mpa in the adjacent continent the stresses determined by the modelling are comparable in magnitude but generally slightly lower than values estimated using the density moment function the ridge push force referenced to 90 ma old oceanic lithosphere is estimated to be 2 5 1012 n m for the modelled normal ridge and 6 2 1012 for the modelled hot spot ridge the modelling shows that the ridge push is produced by the high pressure and shear drag exerted by the asthenosphere on the lithosphere the high pressure effect predominates at normal ridges where the plates are forced apart by gravitational wedging the shear drag effect contributes substantially at hot spot ridges the modelling helps to explain the prominent compressional stress in the continents bordering the north atlantic in terms of the hot spot activity beneath the mid atlantic ridge

contemporary in situ stress determinations at three sites in scotland and northern england abstract during 1987 in situ stress measurements using two overcoring techniques doorstopper and triaxial strain cell were made in england and scotland as part of the former sfb 108 project stress and stress release in the lithosphere at karlsruhe university the results for three of the test sites spittal in caithness gartur in the midland valley of scotland and at burton in cumbria are reported complementary studies on cores from gartur using portable asr equipment are included the maximum horizontal stress orientations agree closely with those seen in the general contemporary stress field in nw europe these results confirm that in favourable circumstances these shallow methods offer effective approaches to the acquisition of reliable contemporary stress information whether derived from boreholes or rock cores

on the gravitational potential of the earth s lithosphere the mean potential energy of the lithosphere is useful for defining the tectonic reference state trs of the earth and can be used to constrain the ambient state of stress in the plates in the absence of external forces applied at the base or along plate boundaries a lithospheric column with the potential energy of the trs would remain undeformed thus the difference between the potential energy of a lithospheric column and the trs determines whether the column is in an extensional joeutral or compressional state of stress we evaluate and intraplate variations about this mean using a simple first order lithospheric density model this model assumed that the continental geotherm is linear and density variations below a depth of 125 km have negligible influence on and is consistent with observed geoid anomalies across continental margins is estimated to be 2 379 1014 n m 1 which is equivalent to the potential energy of both near sea level continental lithosphere 160 to 220 m for an assumed crustal density c in the range 2800 2700 kg m 3 and cooling oceanic lithosphere at a depth of 4 3 km with the exception of eurasia which has anomalously high mean potential energy 2 383 1014 n m 1 the mean potential energies of the continental plates are nearly identical to the global mean the mean potential of the oceanic plates was found to be a strong function of the mean age of the oceanic lithosphere both the global and plate mean potential energies are relatively insensitive to a wide range in c the potential of the mid ocean ridges 2 391 1014 n m 1 is greater than the global mean which is consistent with the divergent nature of the ridges elevated continental lithosphere with a height of about 70 m has an equivalent potential energy to suggesting that in the absence of external forces continental regions will be in a slightly extensional state of stress the importance of our potential energy formulation is substantiated by the strong correlation between the torque poles associated with the potential energy distributions and the observed plate velocity poles for the south american nazca indo australian and pacific plates

mapping high pn velocity beneath the colorado plateau constrains uplift models the massive international seismological centre data set of the past 20 years and the two station method are used to determine pn velocities in the mantle lid beneath the colorado plateau in this method the event is located at distances where pn is the first arrival 2 16 and the path is in or very near the azimuth of the two station pair and crosses the plateau this technique to a large extent minimizes the hypocenter mislocation effect and possible errors due to variations in the crustal structure near the source since only the difference in travel times at the two stations is used however this technique has a few underlying assumptions and possible sources of errors such as the quality of the pn data base and station delays caused by varying crustal structure that require an extremely careful application of the method a detailed study of the source of errors and a methodology of selection of the data are presented application of this method to the colorado plateau using all possible two station pairs from 53 stations located within or along the margin of the plateau yields an average high pn velocity of 8 12 0 09 km s this value is considerably larger than the average value of 7 83 km s based on available but very limited seismic refraction profiles but is remarkably similar to the average value of 8 1 km s for the relatively stable midcontinent region our new pn velocity for the colorado plateau eliminates the paradox in the literature that emphasizes the rather close similarity between average pn velocities beneath the colorado plateau and the basin and range province while their tectonic and magmatic cenozoic history is dramatically different previous models for the structure and evolution of the plateau have used the low pn velocity as an important constraint on density and thermal state of the lithosphere hence such models should be reexamined on the basis of this new uppermost mantle pn velocity determination there are two main models that have been proposed to explain the 2 km uplift of the colorado plateau one is based on a combination of thermal thinning of the lithosphere and crustal thickening and the other involves a combination of the delamination of the subducted subhorizontal farallon oceanic plate from the overriding north american plate and crustal thickening we show that the delamination model is more readily consistent not only with our velocity determination and the elevation of the plateau but also with varied geological observations reported in the literature that concern the cenozoic evolution of western north america

finite element modelling of stress patterns along the mid norwegian continental margin 62 to 68 n the inferred nature of the regional intraplate stress field off the coast of norway is based primarily on borehole breakout information and earthquake focal mechanisms the stress indicators reveal a slightly clockwise rotation of the orientation of the principal horizontal stress sh to the northwest with respect to the dominant e w orientation observed in the northern north sea the stress regime is predominantly strike slip and thrust fault although two normal faulting events have occurred on the shelf and shelf edge within the mid norwegian margin segment 62 to 68 n the tr0ndelag platform exhibits large variations of observed sn orientations this may be due to an almost isotropic horizontal stress state sh sh resulting from the superposition of local and regional stress sources this paper presents the predicted tectonic stresses along the mid norwegian margin based on a 3 d finite element analysis the two principal modelling objectives are to elaborate the effects of 1 changes in the regional stress magnitude sh and 2 variations in rock properties i e young s modulus on the horizontal stress anisotropy hsa of the v0ring basin and the tr0ndelag platform the model incorporates simplified mainland topography the tr0ndelag platform the voring basin the v0ring plateau and the lofoten basin our modelling results indicate that horizontal stress anisotropy hsa and the regional sh magnitude within the various structural units is dependent on their strike with respect to the orientation of the present day regional stress field specifically we find 1 a near isotropic horizontal stress magnitude configuration for the southern part of the v0ring basin where the gradient of topography across the mid norwegian margin is oriented oblique to the regional trend of sh 2 a maximum horizontal stress anisotropy hsa in the southern part of the tr0ndelag platform this local hsa maximum bordering the hsa minimum of the voting basin 3 a narrow band of high hsa at the eastern edge of the tr0ndelag platform imposed by compressive gravitational stresses related to the elevated topography of mainland norway 4 a low hsa at the western margin of the tr0ndelag platform related to spreading stresses associated with the topographic lowering towards the v0ring basin and 5 in case of a far field stress of sh 1 1sh the modelling results predict exchanged principal horizontal stresses sh and sh at the western margin of the tr0ndelag platform

the regional intraplate stress field in south america a compilation of lithospheric stress directions for continental south america and the inferred major patterns of the regional intraplate stress field are presented stress orientations are based primarily on earthquake focal mechanisms and quaternary fault slip inversion published in the literature four new focal mechanisms based on short period p wave modeling and selected centroid moment tensor solutions published by u s geological survey consistent with p wave first motions at south american and other world wide standard seismograph network stations are also included in the data base the observed patterns of intraplate stresses may be useful in constraining numerical models of the plate driving forces in the south american plate in the andean plateau altitudes greater than 3000 m n s extensional stresses predominate e w compressional stresses are observed in the sub andean and platform regions extending up to about 1000 km east of the andes the maximum horizontal stress shmax is uniformly oriented in the e w direction throughout western south america averages of the shmax direction at grid points spaced 2 5 o were taken as representing the regional field the e w direction of this regional field is not affected by the change of strike of the andean chain nor by the contact of a flat subducted slab underneath the regional shmax direction is oriented 15 o more clockwise than the direction of the nazca plate convergence the difference between the regional shmax orientations and the absolute plate motion may be only about 6 o this may indicate that contact forces with the nazca plate may not be the only major contributor to the intraplate stresses in western south america the eastern limit of this andean stress province seems to coincide with aseismic regions in the upper amazon basin and in the paraml basin in the central amazonian region seismicity and stress data suggest a seismic province with n s compressional stresses the origin of these stresses is not yet clearly understood but could possibly be related to lower crustal loading along the middle amazon basin in northeastern brazil seismicity is characterized by upper crustal strike slip earthquakes bordering the potiguar marginal basin a model is proposed for this region in which the stress field is the result of a superposition of regional e w compressional stresses and local extensional stresses oriented perpendicular to the continental margin possibly related to density contrasts and sediment loading in the continental shelf

deflection of a compressional stress field by large scale basement faults a case study from the tertiary almazan basin spain abstract palaeostresses deduced from brittle mesostructures more than 50 sites and 1500 structures measured in the mesozoic and tertiary rocks of the almazan basin located in the northwestern iberian chain indicate a regional nne sse directed compression that lasted during all the tertiary shortening stage the stress regime corresponding to this compression ranges from thrust to wrench n s directed compression was the responsible for most macrostructures folds and thrusts forming a s vergent system with ne sw to e w and nw se orientations and mesostructures mainly faults stylolytes and impressed pebbles we propose that the stress trajectories are deflected following anderson s model in the western part of the almazan basin by the ne sw striking left lateral strike slip soria fault this basement fault was inherited from the late variscan fracturing stage and controlled the deposition in the mesozoic basins as well as the location of tertiary compressional macrostructures in the eastern border of the almazan basin palaeostress trajectories were deflected both by right lateral strike slip faults and nw se trending thrusts near which the regional n s directed compression becomes perpendicular to them three minor compression directions have also been recorded in several sites ne sw e w and nw se the ne sw directed compression is interpreted as a deviation of the main n s directed compression at a regional scale whereas the nw se directed stress field corresponds to the regional field responsible for the main tertiary shortening stage in the spanish central system the age of compression ranges from eocene to miocene

an episodic hypothesis for venusian tectonics it is suggested that episodic plate tectonics occurs on venus episodes of rapid plate tectonics are separated by periods of surface quiescence for the last 500 200 m y it is postulated that the surface of venus has been a single rigid plate that has been thickening due to conductive cooling a near uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows a lithosphere that has conductively thickened for some 500 m y has a thickness of about 300 km nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss such a thick lithosphere can support the high topography and associated gravity anomalies on venus as well as the unrelaxed craters studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics on the earth 75 90 of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges this process is not operative on venus this paper suggests that episodic plate tectonics on venus constitutes the primary mechanism for mantle heat transport on that planet

a local weakening of the brittle ductile transition can explain some intraplate seismic zones abstract a decrease in the strength of a localized area of the lower crust would decrease the depth to the brittle ductile transition and concentrate stress in the stronger elastic crust around and above the zone of decreased strength deformation within the zone of weakened lower crust would occur through viscous or dislocation creep in response to regional plate stress as well as by elastic failure in small earthquakes two dimensional finite element models of zones of weakness subjected to a regional plate stress predict stress amplification of 10 100 surrounding the local decrease in strength a finite element model for an irregular zone of weakness in the lower crust can generate stress directions and relative magnitudes that satisfy the focal mechanisms and seismicity of southeastern tennessee an analysis of the displacements and stresses in the central area of decreased crustal strength suggests that strike slip faulting should dominate above this zone the strike slip faulting should exhibit a strong thrust component the compression and extension of the crust surrounding the weakened zone at the level of the brittle ductile transition predicts that the dominant strike slip faulting should exhibit components of normal faulting on the edges of the weak zone which are near a line through the weak zone parallel to the regional stress and reverse faulting on the edges which are near a line transverse to the regional stress the seismicity of southeastern tennessee is diffused over a narrow elliptical zone trending ne the greatest concentration of activity is near the center the central zone is characterized by deep focus strike slip events with predominantly n or e striking nodal planes these events are responding directly to compression in the direction of the regional compressive stress and extension perpendicular to the regional compressive stress within the weak zone the area surrounding the central zone is characterized by focal mechanisms with larger components of reverse or normal fault movements events with normal components are dominant on the edges in line with the regional stress as predicted by the stress model for a weak central zone under stress the agreement between observed earthquake focal mechanisms in southeastern tennessee and models of crustal stress surrounding a zone of weakness suggests that these events may be caused by a zone of weakness in the lower crust

tectonic stresses in the lithosphere various types of observables earthquake focal mechanisms in situ measurements and geological deformations give information about the large scale lithospheric stress field the latter has often been explained by postulating appropriate forces acting at the edges and beneath the plates this approach ignores the role of mass heterogeneities within the lithosphere here we analyze the effect of both boundary and internal forces on the stress pattern and show that both contributions are of comparable magnitude the presence of internal sources makes the problem three dimensional we show however that it can be reduced to a two dimensional plane stress formulation whereby the edge forces are expressed by the non hydrostatic stresses and the basal shear is increased by the addition of a term proportional to the gradient of the mean vertical stress for the oceanic lithosphere we derive a compression that increases with age the comparison with geophysical observables yields an upper bound of a few bars on the magniude of the basal drag for the continents we infer the existence of an underlying upper mantle somewhat denser than under oceans

pulsating continents on venus an explanation for crustal plateaus and tessera terrains we propose that tessera terrains on venus represent continental crust that does not participate in the periodic recycling of the lithosphere through global subduction events we have studied the force balance on the boundary of a continental area that survives a global subduction event using an analytical model in the proposed model the ratio between the crustal and lithospheric mantle thicknesses controls the force balance if the crust thickness is less than 2 5 of the lithospheric mantle thickness the continental area will be compressed but if the crustal thickness is higher than 2 5 of the lithospheric mantle thickness the continental area will spread out and collapse consequently if the lithospheric mantle beneath a continental region is delaminated during a global subduction event the continent will collapse generating tessera inliers dominated by extensional tectonics but if a significant portion of lithospheric mantle remains then the continental area will be compressed generating a plateau by crustal shortening the observed plateau heights can be explained by this model a 2 km height plateau can be generated by a lithospheric mantle thickness of 40 km while a 4 km height plateau can be generated by a 90 km thick lithospheric mantle we have modelled this crustal thickening of a continental area by tectonic contraction using a thin viscous sheet approach with a newtonian viscosity for the crust the force from a hot mantle elevated during a global subduction event is enough to build up a plateau by compression in 50 ma using a viscosity for the continental crust of 1021 pa s and 200 ma for 5 1021 pa s during this compressional stage concentric fold and thrust belts are generated in the plateau continent erasing any impact craters that were present the subsequent stabilization of a new crust and lithosphere in the surrounding mantle changes the force balance allowing a moderate gravitational collapse of the plateau continent accommodated by radial grabens the pulsating continent model links for the first time the generation of crustal plateaus and the origin of the volcanic plains predicting the observed equivalent effective crater density for both terrains