the thermal structure of mantle plumes axisymmetric or triple junction summary a mantle plume is probably a complex 3 d thermal structure that possesses approximate axisymmetry as it approaches the base of the lithosphere from below but followed down towards the base of the layer probably consists of a triple junction or quadruple junction of connected hot sheets a relatively weak hot sheet rising only part way through the layer probably connects two neighbouring mantle plumes these conclusions are suggested by numerical experiments on a 3 d constant viscosity plane layer with stress free boundaries which detail the gradational change in the planform of a convecting layer from the top of the layer to its base the planform of a convecting layer is a map in the horizontal plane of the principal thermal anomalies in the layer these anomalies are the main sources of positive for hot fluid or negative for cold fluid buoyancy and therefore they drive the convective flow they may appear in cross section as structures with either axial symmetry columns planar symmetry sheets or some complex asymmetric form when convection is driven at least partially by basal heating the planform near the top of the layer may be described as a network of cold sinking sheets and isolated hot columns while near the base of the layer it appears as a network of hot rising sheets and isolated cold columns the hot columns near the upper surface arise from the vertices or nodes of the network of hot sheets on the lower surface and similarly the cold columns at the base of the layer form below the vertices of the network of cold sheets near the upper surface near the upper surface the apparent planform of this experiment is analogous to that of mantle convection the cold sheets compared to subduction zones and the hot columns compared to mantle plumes the hot plumes impinging on the upper surface produce approximately axisymmetric temperature anomalies surface uplift and extensional stress fields however the relatively minor deviations from axisymmetry of surface observables reflect the deep structure of the mantle plume formed by the junction of three or four hot sheets on the base of the layer it seems likely that the commonly occurring triple junction form of continental rifts may reflect an underlying structure that is implicit in the convective circulation of the mantle beneath

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

subsidence analyses from the north sea triple junction when compared with theoretical subsidence curves calculated from the uniform stretching model water loaded subsidence data from the triple junction region of the north sea suggest that the jurassic cretaceous rifting event caused lithospheric thinning by a factor of c 2 0 although somewhat larger than the stretching factors found elsewhere in the north sea this amount of thinning is anticipated from the overall geometry of the three graben system and is consistent with the observed volume and elemental and isotopic composition of the jurassic forties volcanic province apart from in the southern north sea permo triassic extension is thought to have been relatively minor in comparison to the later jurassic cretaceous phase the anomalously small amount of late jurassic syn rift subsidence in those wells where local fault controlled effects are minimal supports the well known idea of localized relative uplift or doming in the triple junction area c 10 4 km 2 prior to and during the early stages of the jurassic cretaceous rift phase a time dependent differential stretching model in which the lithospheric mantle is initially stretched by a greater amount than the crust is stretched might provide an explanation such a model would require the total amount of stretching integrated over space and time to be the same for the lithospheric mantle and for the crust in order to avoid space problems alternatively the same data could be explained by invoking a small transient thermal anomaly in the asthenosphere

interdecadal change in typhoon genesis condition over the western north pacific the interdecadal changes in typhoon categories 1 3 frequency and its genesis condition over the western north pacific during the period of 1979 2011 are investigated with consideration for discrepancies among best track datasets to tide over data uncertainty a detection produced dataset is utilized as a homogeneous dataset with five available best track datasets typhoon experienced interdecadal changes around the mid 1990s and the mid 2000s in their genesis conditions even under the oceanic warm state typhoon frequency has decreased since the mid 1990s showing a northwestward movement of its genesis location over the main formation region the eastward gradient of vertical wind shear is the most significant factor for the change in typhoon genesis condition in recent decades the vertical wind shear behavior is strongly linked with zonal asymmetry of local sst we demonstrate that a westward gradient of local sst is the most important modulator of the recent typhoon behavior through the movement of favorable genesis location the present results indicate that the horizontal distribution not magnitude of local sst can be a key factor for prediction of future typhoon activity thus contributing to natural disaster mitigation and climate change adaptation strategies

thermal response of a moving lithosphere over a mantle heat source the mid ocean bathymetric swells observed around some volcanic island chains can be modeled using the thermal conduction equations of an oceanic lithosphere moving over a heat source located at the base of the plate some previous studies of this problem have suggested that the heat source must lie well within the lithosphere in order to explain the observed bathymetry and surface heat flow these conclusions are based on an isothermal boundary condition at the base of the lithosphere but this choice requires that the heat source is maintained above this boundary and can actually lead to a flux of heat back into the mantle particularly in the vicinity of the heat source if the lower boundary is insulated insofar as the anomalous temperature field is concerned flux from the heat source back into the mantle is prevented resulting in a more rapid uplift of and in an increased heat flow across the seafloor previous solutions have considered only the steady state solutions whereas the present model also evaluates transient solutions for rapidly moving oceanic plates the transient effects can be neglected provided that the heat source has been active for at least 107 years and that the direction of motion and the intensity of the heat source have remained constant in this interval solutions with the heat source located near the base of the conductive layer of the conduction equations for heat flow bathymetry and geoid heights in the spatial domain reproduce the principal observational characteristics of the hawaiian swell at distances away from the immediate origin of the swell and this part of the swell can be used to estimate the thermal conduction model parameters the model does not wholly explain the observations near the origin and the mismatch may reflect a dynamic component in the support of the swell

large topographic rises on venus implications for mantle upwelling topographic rises on venus have been identified that are interpreted to be the surface manifestation of mantle upwellings these features are classified into groups based on their dominant morphology atla and beta regiones are classified as rift dominated dione western eistla bell and imdr regiones as volcano dominated and themis eastern eistla and central eistla regiones as corona dominated at several topographic rises geologic indicators were identified that may provide evidence of uplifted topography e g volcanic flow features trending upslope we assessed the minimum contribution of volcanic construction to the topography of each rise which in general represents less than 5 of the volume of the rise similar to the volumes of edifices at terrestrial hotspot swells the total melt volume at each rise is approximated to be 104 106 km3 the variations in morphology topography and gravity signatures at topographic rises are not interpreted to indicate variations in stage of evolution of a mantle upwelling instead the morphologic variations between the three classes of topographic rises are interpreted to indicate the varying influences of lithospheric structure plume characteristics and regional tectonic environment within each class variations in topography gravity and amount of volcanism may be indicative of differing stages of evolution the similarity between swell and volcanic volumes for terrestrial and venusian hotspots implies comparable time integrated plume strengths for individual upwellings on the two planets

tectono thermal regimes and lithosphere behaviour in the external dacides in the upper triassic and jurassic tethyan opening romanian carpathians abstract traces of mesozoic basic magmatism occur in the romanian carpathians in two different tectono stratigraphic positions 1 associated with the opening of the tethys and 2 associated with intracontinental basins the latter were generated within three sedimentary basins which are now inverted constituting the external dacides the black flysch ceahlau eastern carpathians and severin south carpathians basins assuming a normal jurassic lithosphere and a pure shear opening mechanism stretching factors of 1 25 and 1 7 have been calculated for the black flysch and ceahlau basins respectively these values were in turn used to determine the theoretical values of the syn rift sediment thickness and compared with those obtained by backstripping techniques the occurrence of basic magmatism in the black flysch basin associated to such a low stretching factor suggests that melt generation was possible due to wet composition of the mantle peridotite the thermal state was different in the ceahlau and severin basins the lithosphere being affected by updoming processes prior to rifting upper triassic and jurassic thermal states of the european lithosphere are related to the coeval opening mechanism of the tethys there was a transition from a simple shear between austro alpine apuseni mountains and the south carpathians to a pure shear mechanism accompanied by strike slip movements between austro alpine and the northern part of the eastern carpathians related to this change in the position of the master detachment fault an astenospheric uprise developed within the european plate it was largest in the severin zone medium in the ceahlau zone and became smaller toward the black flysch zone the dacidian swell developed as a result of this astenospheric rise starting with the lower and middle jurassic the dacidian swell became subject of rifting

mantle plumes and continental tectonics abstract it is now recognised that plumes originate from a thermal boundary layer deep in the mantle a new plume consists of a large head with a diameter of approximately 800 to 1200 km and it is followed by a smaller tail which has a diameter of 100 to 200 km at the top of its ascent the head flattens to form a disk 2000 to 2500 km in diameter that may melt to produce a continental flood basalt or oceanic plateau the tail melts to produce chains of volcanic activity such as the hawaiian emperor chain the ascent of a new plume leads to surface uplift of 500 to 1000 m over a length scale of 2000 km this places the lithosphere under tension but the horizontal forces produced do not result in runaway extension unless the continental lithosphere is already anomalously hot and therefore weak if the lithosphere is cold and strong slow extension leading to a new spreading ridge may postdate plume arrival by 10 to 40 m y the time difference being the time taken to conduct heat from the plume to the overlying lithosphere plume heads may control the timing and position of new ocean ridges but are not the fundamental force that drives plate tectonics the heat from plume tails may weaken the overlying plate and pin the position of spreading the ascent of a mantle plume can also have an important influence on subduction large oceanic plateaus such as the ontong java plateau are more buoyant than normal oceanic crust because they have a thicker basaltic layer and because they are underlain by a 200 km thick layer of hot material from a mantle plume head as a consequence oceanic plateaus are more difficult to subduct than normal oceanic crust this can lead to jamming of plates at subduction zones resulting in a flip in the sense of subduction as appears to have happened in the caribbean region or to obduction of the plateau finally when a plume head is emplaced beneath continental crust heat released from the plume may lead to melting of the overlying crust the granites associated with many greenstone belts and intercontinental magmatic provinces such as the paleozoic lachlan fold belt of eastern australia and the middle proterozoic anorogenic granites of the central and southwestern united states may have formed in this way

the geodynamics of lithospheric extension lithospheric extension is a fundamental plate tectonic process controlling the collapse of mountain belts the break up of continents and the formation of new oceanic basins this review summarises major advances on understanding a the nature of the ocean continent transition b the origin and evolution of detachment faults c the dynamic strength of continental lithosphere during extension and d the role of magmatism during extension major steps have been made in mapping out the complexities of the ocean continent transition zone particularly in magma poor rifted margins where extensional structures are not masked by voluminous magmatism the role of detachment faulting during rifting seems to be crucial and explains observations on depth dependent stretching of the lithosphere despite new insights into the dynamic behaviour of the lithosphere its rheological response remains a major uncertainty in modelling the evolution of extensional systems likewise the multiple roles that basaltic magmatism may have in modifying extension remain a key question future insights into the global behaviour of extensional systems will be gained by constraining lithospheric rheology through integrating structural evolution and magmatism in the field geodetic measurements of active extension thermochronology seismic surveys and drilling programs 2008 elsevier b v all rights reserved

finite duration extension and decompression melting abstract decompression melting of the upwelling mantle during extension of continental lithosphere provides a viable mechanism for the magmatism observed in many rifted sedimentary basins and passive volcanic margins a model has been developed in which in addition to the adiabatic temperature decrease syntectonic conductive cooling melt transport and emplacement at crustal levels are accounted for the heat equation including latent heat is solved numerically and the incremental melt fraction is traced to simulate dynamic melting the emplaced magma is considered an integral part of the continental crust we find that relative to an instantaneous rifting model with only adiabatic cooling there is a more than 10 decrease in the melt thickness even when the duration of rifting is as short as 1 m y whether extension takes place with constant horizontal velocity or strain rate has important implications for the amount of melt produced consequently pressure release melting does not only depend on the amount of stretching and duration of the rift episode but also on how extension develops through time

initiation of subsidence in a sedimentary basin underlain by a phase change summary a mechanism of subsidence for axisymmetric sedimentary basins is investigated it is assumed that the subsidence is caused by the super position of the motion of a phase boundary and of the flexure of an elastic lithosphere in response to the loading of the earth s surface by sediment an approximate solution describing the motion of the phase boundary when it is subjected to changes in pressure is used to describe the early stages of the subsidence history of the basin several models have been investigated and are presented to demonstrate how the spatial and temporal characteristics of the sediment deposition affect the suggested mechanism if the basin is wide i e several times the depth to the phase boundary and the sediment deposition is slow in comparison to the phase transition relaxation time the subsidence can proceed at the same rate or even faster than the sedimentation resulting in an acceleration of the sediment accumulation the mechanism investigated does not appear adequate for small scale basins or basins where the sediment accumulated rapidly it is a feasible mechanism for the large cratonic basins where the sediment deposition proceeded at a low rate for a long time

gravitational orogenic collapse vs plate boundary stresses a numerical modelling approach to the permo carboniferous evolution of central europe a two dimensional thermo mechanical finite element model is used to study the permo carboniferous evolution of central europe along a lithosphere scale transect from the variscan internides to the undeformed foreland the study concentrates on a quantitative evaluation of the processes controlling late orogenic extension and destruction of the variscides particularly the gravitational instability of thickened crust modelling results suggest that gravitational forces along cannot reproduce the observed timing and amount of permo carboniferous crustal thinning tensile plate boundary forces are required in addition to gravitation to restore a crustal thickness of approximately 30 km in the variscan internides stresses as little as 10 mpa result in up to 28 extension and a good fit between observed data and model predictions it is concluded that the stephanian to rotliegend evolution in the vicinity of the modelled traverse resulted not from gravitational forces inherited from variscan crustal thickening but was related to a change in orientation of the plate boundary stresses at the end of the westphalian

seismic signature of the collision between the east tibetan escape flow and the sichuan basin abstract gps displacement vectors show that the crust in east tibet is being squeezed in an easterly direction by the northward motion of the indian plate and the sichuan basin is resisting this stream and redirecting it mainly towards indochina the longmen shan containing the steepest rise to the high plateau anywhere in tibet results from the strong interaction between the east tibetan escape flow and the rigid yangtze block sichuan basin but the kinematics and dynamics of this interaction are still the subject of some debates we herein present results from a dense passive source seismic profile from the sichuan basin into eastern tibet in order to study the deep structure of this collision zone using p and s receiver function images we observe a sudden rise of the lithosphere asthenosphere boundary lab from 120 to 150 km beneath the sichuan basin and from 70 to 80 km beneath eastern tibet in contrast the depth of the crust mantle boundary moho increases from 36 to 40 km beneath the sichuan basin and from 55 to 60 km beneath eastern tibet the 410 km discontinuity is depressed below eastern tibet by about 30 km although the 660 remains at nearly the same depth throughout the lms from these observations we conclude that the mode of collision that occurs between tibet and the sichuan basin is very different to that found between india and tibet in southern tibet we observe in essence the subduction of the indian plate which penetrates northwards for several hundred kilometers under central tibet the very thin mantle part of the lithosphere beneath eastern tibet may indicate delamination or removal of the bottom of the lithosphere by hot asthenospheric escape flow this process leads to the exceptionally steep topography at the eastern tibetan margin as a result of gravitational buoyancy this view is supported by the very unusual depression of the 410 km discontinuity beneath eastern tibet which could be caused by the dynamics of the sub vertical downward asthenospheric flow

simulation of collision zone segmentation in the central mediterranean abstract collision processes between the african and european plates in the ionian sea offshore calabria and in sicily take place in several ways to the east normal subduction of oceanic crust underlying the ionian sea occurs underneath the calabrian arc whereas to the west continental collision between the pelagian block and the calabrian arc which extends from calabria to the northeastern tip of sicily and offshore of the island to the north gives rise to the maghrebian thrust belt on mainland sicily this belt consists mostly of stacked basinal and platform carbonates of mesozoic early tertiary age deposited along the africa passive margin no clear subduction characterizes the area north of the hybleandashmalta plateau where it collides with the maghrebian chain the origin of the hybleandashmalta plateau is unclear geophysical data indicate however that its crustal structure is different from that of the surrounding pelagian foreland crustal structure variations at the edge of the subducted african plate may thus cause the observed segmentation of the collisional arc system a computer model of the region was created to learn about the deformation and faulting processes in the region the simulation starts prior to the collision of the pelagian block with the calabrian arc with time the hybleandashmalta plateau collides with the northern crustal block and as a result a new simulated topography a new distribution of velocities and most important a new distribution of fracture zones are created a shear fracture zone is formed east of the hybleandashmalta plateau malta escarpment extending to the subduction zone at the calabrian arc a less active strike slip fracture zone is created west of the hybleandashmalta plateau an additional e w trending active fracture zone is creating in the southern part of the simulated area this feature may correspond to a fault zone which runs from the strait of sicily to the ionian sea

some comments on two layer extensional models for the evolution of sedimentary basins simple extensional models that involve listric faulting in the brittle upper crust and plastic flow in the lower lithosphere have been shown to account for the subsidence history of various continental sedimentary basins and continental shelves in this paper we present structural evidence that extension in the continental lithosphere during rifting is not uniform we review evidence that the rifting process in major intracratonic rift basins is spasmodic rather than continuous that extension and subsidence within the rift during a tectonic phase is generally accompanied by uplift and erosion of the rift flanks that subcrustal heat input during rifting occurs over an area much broader than the rift proper and that there may be broad regional uplift before and or during rifting to explain the uplift of the flanks during rifting and their postrift subsidence we invoke two mechanisms subcrustal thinning by extension below the flanks and tilted major normal faults in the crust between graben and flanks the uplift and erosion of the flanks due to tilted faults probably do not significantly affect estimates of extension from subsidence analysis but uplift due to both sources can in certain cases be incorrectly attributed to regional uplift or doming we present a simplified analysis of the two layer extensional model for the elementary case in which extension is instantaneous and the crust is thinned by a different amount from the subcrustal lithosphere we provide a simple graphical method of data analysis to obtain the parameters that describe thinning of the crust and lithosphere during extension from the subsidence history application of this method to the central part of the pattani trough in the intracratonic gulf of thailand shows that extension and thinning of the lithosphere of a factor of 2 since late eocene can account for much of the subsidence history and the present day heat flow our analysis suggests that thinning of the crust in this area is 20 greater than thinning of the whole lithosphere assuming that none of the lithosphere is removed from the region of extension during rifting we show by simple geometric examples that a process of nonuniform extension implies that much greater thicknesses of sediment can be deposited in the center of a young basin than in the case of uniform extension of both crust and subcrustal lithosphere such an extensional process produces significant uplift of the flanks of the rift and as a result of erosion of the uplifted areas the effective cross sectional area of the basin can be increased by as much as 10 20 depending on the rate of erosion compared to the area that would have been created by a process of uniform extension finally we show that it may ultimately be possible to compute erosion rates from seismic cross sections of basins

the interaction of mantle plumes with surface thermal and chemical boundary layers applications to hotspots on venus large volcanic swells on venus are believed to be a manifestation of mantle upwelling or hotspots the study of these regions provides important information on the interior of the planet numerical experiments are carried out to examine the interaction of mantle plumes with the thermal lithosphere and a layer of depleted mantle a product of pressure release melting using an axisymmetric finite element and finite difference code that incorporates temperature dependent viscosity and pressure release melting the lithosphere is defined as a high viscosity lid plumes are initiated and maintained by a prescribed temperature at the base of the computational domain the topographic uplift the geoid to topography ratio and the volume of pressure release melt are compared to estimated values for possible hotspots on venus to constrain the properties of plumes and the lithosphere the effects of lithospheric thickness depleted layer viscosity and thickness mantle temperature and plume temperature and duration on the surface observables are predicted models with a thermal lithospheric thickness of approximately 100 150 km are consistent with observations assuming a mantle temperature of 1300 c a maximum plume temperature of approximately 1500 c and mantle plume durations of 150 250 m y to be consistent with estimated volumes of volcanics at venusian hotspots a significantly thinner thermal lithosphere requires a much cooler mantle a thicker lithosphere requires a hotter mantle models with a depleted layer thickness of 100 250 km in addition to a 100 km thick thermal lithosphere predict the range of parameters found on venus interpretation of large volcanic swells on venus based on the evolutionary sequence predicted here implies that beta atla western eistla and imdr regiones overlie either active or recently active mantle plumes the geoid to topography ratio at beta regio is much larger than values found at other hotspots and may indicate that it is in an early stage of evolution or that the chemical or thermal boundary layers are significantly thicker bell dione and themis regiones appear to represent very late stage possibly now extinct hotspots an estimate of the total plume buoyancy flux for venus is much lower than the value obtained for earth the small areal distribution of volcanism is consistent with a low level of ongoing resurfacing at a few active hotspots

rapid intra arc rifting in miocene northeast japan although life times of major continental rifts are tens of millions of years intra arc rifting in miocene ne japan lasted for only about 3 m y before back arc spreading began the rate of initial subsidence was over 1 km m y much faster than that in major continental rifts thus this intra arc rift evolved 10 times as fast as major intracontinental rifts the large temperature dependence of lithosphere rheology and the hot thermal regime of island arc lithosphere account for the rapid rifting less than 1 m y before the rifting spreading transition the inner ne japan arc subsided from shallow marine depths to middle or lower bathyal depths 1 0 2 5 km the average total basement subsidence was 2 3 km during the rifting the rapid submergence occurred during one third of the duration of rifting but accounts for two thirds of the total subsidence during rifting namely the initial subsidence rate accelerated toward the time of breakup of the arc crust at the same time the region along the present pacific coast remained only shallow marine or terrestrial throughout presumably influenced by global eustacy

multiple rifting pulses and sedimentation pattern in the cameli basin southwestern anatolia turkey abstract the neotectonic development of western anatolia was characterized by the formation of numerous graben type basins which have been well documented by general mapping although the cause and timing of the neogene regional tectonic extension remain controversial previous interpretations of the origin and evolution of these neogene basins were based mainly on regional scale tectonic inferences rather than detailed basin fill analysis the present study of the terrestrial intramontane cameli basin in the western taurides combines detailed facies analysis with biostratigraphic dating mammalian and molluscan fossils and documents three pulses of crustal extension that are reflected in changes in the palaeogeography and sedimentary architecture of the basin development of the cameli graben commenced in the vallesian time early tortonian and is marked by alluvial fan fluvial and lacustrine depositional systems with freshwater molluscan fauna a second pulse of tectonic extension occurred in the late ruscinian time early middle pliocene producing a new normal fault that split the basin longitudinally into two compartments the lake environment expanded and deepened coastal peat forming mires developed and abundant mammal fauna appeared by the early villanian time middle pliocene with the lacustrine deposits onlapping the basin margin and intrabasinal fault escarpments the lacustrine environment subsequently shrank as the progradation of axial river deltas and basin margin fan deltas caused water shallowing and shoreline regression a third pulse of extension occurred at the end of villanian time late pliocene when the development of a new generation of normal faults further split the basin into still narrower half graben compartments the third pulse of rifting is estimated to have accounted for little more than 10 of the sub basinal crustal extension but caused the most striking changes in the basin palaeogeography and drainage pattern the inward development of the successive normal faults indicated a high rate crustal extension this is the first regional case study of a terrestrial neotectonic graben employing detailed sedimentary facies analysis and mammal biostratigraphy and providing a time stratigraphic framework for the rifting pulses in western anatolia

a model for low pressure facies metamorphism during crustal thickening low pressure metamorphic facies i e high t p ratios are widespread in a wide range of tectonic settings explanations offered for the occurrence of these facies include extensional and or magmatic models however these fail to explain that the low p facies metamorphism is commonly coeval with a phase of pervasive crustal thickening with t p ratios increasing during or slightly lagging behind the thickening we propose an alternative explanation based on the approximate synchroneity of crustal thickening and erosion thinning of the mantle lithosphere

some consequences of compressional tectonics for extensional models of basin subsidence changes in intraplate stress levels have important consequences for the stratigraphy of rifted basins and provide a tectonic explanation for the generation of sequence boundaries late stage compressional phases during the post rift evolution of basins produce unconformities with offlap phases that increase in magnitude with the age of the rifted basin late stage compression has not only a strong bearing on the generation of unconformities but also induces significant downbending of the centre of a basin ignorance of the vertical motions of the lithosphere induced by late stage compression during post rift evolution can therefore give rise to substantial errors in the estimates of crustal extension derived from analysis of basement subsidence using stretching models consequently late stage compression can be of great significance in estimates of depth and timing of the hydrocarbon window inferred from extensional models of basin subsidence quantification of the subsidence induced by post rift compression has important implications for integrated models of basin subsidence and hydrocarbon generation

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

rheology of the continental lithosphere inferred from sedimentary basins the steady state flow properties of the continental lithosphere play an important role in a wide range of geological processes1 a complete dynamic description of lithospheric deformation requires information about the magnitude of driving forces and the rheology of the crust and lithospheric mantle about which there is little agreement2 6 here we constrain these properties by analysing variations in strain rate during the extension of continental lithosphere we determine the temporal variation of strain rate from the subsidence curves of a global sample of phanerozoic sedimentary basins the peak strain rate and final strain estimated from these strain rate histories suggest that the cessation of extension is governed by cooling and concomitant strengthening of the underlying lithospheric mantle dynamic modelling of these data indicates that the rheology of the lithosphere is controlled by power law creep with a stress exponent of three and an activation energy of 500kj mol 1 this rheology is consistent with that inferred from laboratory experiments on dry olivine7 extrapolated to lithospheric conditions

impingement of mantle plumes on the lithosphere contrast between earth and venus the penetration ascent melt production cooling and solidification of a mantle plume as it impinges on the base of the lithosphere are modelled as functions of the lithospheric temperature and thickness the models indicate that if mantle and plume temperatures and plume dimensions of venus are similar to those of earth the higher lithospheric temperature of venus causes higher ascent of plumes into the lithosphere and a greater amount of lithospheric melting however the rates of intraplate volcanism on venus and earth appear to be similar indicating that mantle or plume temperatures may be less or plumes may be smaller in venus than in earth

high grade reworking of central australian granulites part 1 structural evolution abstract four distinct deformational events d2 d5 accompanying a granulite facies metamorphic cycle m2 m5 are shown to structurally overprint rework pre existing granulites m1 of the northeast strangways range in the central arunta block the first metamorphic cycle m1 at 1800 ma involved an anticlockwise p t path peaking at 850 950 c at 8 9 kbar m1 metamorphism involved widespread partial melting that produced stromitic migmatites and map scale concordant granitic gneisses isobaric cooling after the peak and accompanying hydration gave rise to a wide variety of coronitic and symplectitic reaction textures that enclose and replace m1 mineral parageneses no kinematic structuring is associated with m1 and the period encompassing the thermal peak and immediately subsequent are thought to have been absent of deviatoric stress the second metamorphic cycle m2 m5 possibly at 1400 1500 ma involved a clockwise p t path with a maximum p of 9 10 kbar and a thermal peak of approximately 800 c this metamorphic cycle accompanied the major ductile deformations recognized d2 d5 and has been labelled the proterozoic reworking the proterozoic reworking has been divided into two thermo barometric and structurally distinct periods d2 d3 involved regionally inclined ene wsw non coaxial shear of high bulk shear strains giving rise to an intense often mylonitic pervasive fabric s2 l2 and isoclinal and sheath folds on all scales f2 f3 s2 l2 is the first recognized kinematic fabric and does enclose and partially recrystallize m1 mineral parageneses including late m1 metamorphic reaction textures d2 d3 deformation was due to crustal shortening and gave rise to crustal over thickening loading accompanying prograde metamorphism m2 d4 d5 involved upright open and asymmetrical folding f4 and e w trending shear zone development s5 within a regionally extensive system of inclined oblique sinistral transpression both d2 d3 and d4 d5 episodes occurred under the same approximately e w directed compressive stress and are considered sequential episodes in the one tectonothermal cycle f4 folding accompanied significant melt formation and the peak of metamorphism of the m2 m5 metamorphic cycle s5 shear zones accompanied 3 4 kbar of decompression with cooling presumably during uplift in isostatic response to crustal over thickening in d2 d3 progression from inclined non coaxial shear to transpression occurred in response to an increase in the relative buoyancy forces limiting crustal thickening as is typical of many compressional orogens however the proterozoic reworking is atypical of simple linear or arcuate mountain belts because late stage shearing s5 is aligned sub parallel not at a high angle to the transport vector of the earlier ductile deformations d2 d3 consequently this region is modelled in terms of crustal shortening directed along the length of an e w trending orogen confined to the north and south by relatively stable crustal blocks when e w shortening could no longer be accommodated by crustal thickening strain was partitioned by sinistral transpression between the north and south bounding crustal blocks

structural sequences and styles of subsidence in the michigan basin subsidence in the michigan basin produced 5 km of sedimentation over a period of more than 200 m y during paleozoic time utilizing well log correlations and constrained by compaction corrections and estimates of paleobathymetry we recognize four different styles of subsidence in the basin trough shaped regional tilting narrow basin centered and broad basin centered subsidence began as a trough shaped northerly extension of the illinois basin during late cambrian to early ordovician time this was followed by narrow basin centered subsidence in early to middle ordovician time basin centered subsidence ceased for 30 m y during long wavelength 1000 km eastward tilting in middle to late ordovician time a pattern incompatible with thermal contraction subsidence models basin centered subsidence resumed in silurian time but with a broader distribution a second episode of narrow basin centered subsidence occurred in latest silurian through middle devonian time and was replaced by broad basin centered subsidence at the end of middle devonian time the geometry of upper devonian and younger paleozoic deposits suggests another eastward tilting event but results remain inconclusive due to erosion of strata and uncertainties in their paleobathymetry in addition to these subsidence patterns two distinct unconformity styles are present basin wide and marginal erosion there is no evidence for significant basin centered unconformities as predicted by purely thermal mechanisms a history of episodic subsidence reactivations is interpreted as the result of a stress induced crustal weakening mechanism for the narrow basin centered subsidence whereas broad basin centered subsidence is interpreted as thermal contraction related to lower crustal attenuation during the narrow subsidence episodes recently proposed dynamic topography related to initiation of ordovician subduction provides a driving mechanism for long wavelength eastward tilting together with a temporal correlation to appalachian tectonism these mechanisms provide a plate tectonic framework for the history of the michigan basin

relative importance of strain rate and rheology for the mode of continental extension summary both extension rate and rheology have been shown to influence the style of continental rifting so far their effect on the dynamics of extension has been discussed separately in the literature leading to diverging conclusions the rate of extension mostly affects the dynamics of rifting through the occurrence of syn rift cooling at low extension rates this cooling results in an increase of strength if the lithosphere has a temperature dependent rheology england 1983 first suggested that this effect will limit the amount of extension that an area may undergo kusznir park 1987 argued more generally that it will induce a widening of the rift and concluded that extension rate is the major control on wide versus narrow rifting later studies bassi 1991 bassi keen potter 1993 however have demonstrated the importance of rheological layering within the lithosphere for the mode of continental extension by showing that a plasticity dominated rheology will lead to rapid and narrow necking while a creep dominated rheology results in wider rifts and margins gravitational stresses arising from density anomalies due to lithosphere thinning also affect the style of rifting buck 1991 in this paper we examine the combined effect of these different parameters in order to reconcile earlier studies and assess the importance of syn rift cooling for the process of lithospheric necking the approach introduced in a previous study bassi 1991 uses a finite element technique to simulate extension of a vertical cross section of the lithosphere the effect of cooling on the necking pattern depends strongly on the rheology of the thinned lithosphere when it starts to cool if heat diffusion always induces strengthening and delays rupture in time it only modifies significantly the geometry of the rift when the upper mantle is weak and viscous in this case the locus of maximum strain rate migrates laterally once the central area starts to cool and harden as hypothesized by kusznir park 1987 eventually strain concentrates at the transition between deformed and undeformed lithosphere we predict that this pattern will induce asymmetric breakup and produce a very narrow margin and a wide uniformly thinned conjugate margin in all other cases i e when the upper mantle is initially in the low plasticity regime or if it reaches the yield stress during extension the rate of extension appears to have little effect on the rifting pattern

back arc rifting initiated with a hot and wet continental lithosphere abstract how back arc rifting is initiated is studied by inferring the tectonic force and strain rate regimes from published subsidence data for nine back arc basins in the european tethyan belt and in the western pacific the results show that rifting only begins after some particular magnitude of tectonic force is reached this implies that subduction has already progressed to a point such that the total negative buoyancy of the slab becomes so large that the relevant tectonic force exceeds its bending strength once back arc rifting has been initiated the force required to maintain it gradually decays this indicates that the processes governing back arc extension are fast enough that lithosphere weakening due to an increase in geothermal gradient exceeds strengthening due to the thermal relaxation and crustal thinning this is achieved given currently accepted estimates for the maximum force 7 tn m if the thickness of the thermal lithosphere is significantly less than 125 km and if the lithosphere has a wet rheology meaning that a weak continental lithosphere is required for the initiation of back arc rifting the presence of significant strength in the uppermost mantle is not precluded but it is not required

an active thermoelastic rift mechanism finite element solutions for stresses and displacements during rift development are obtained for a variable thickness elastic lithosphere plate subjected to thermoelastic loading by conductive heating from an axial line source in the asthenosphere the asthenosphere is modelled as a fluid foundation the prerift lithosphere is modelled as an infinite continuous plate a semi infinite plate is used to characterize the rifted lithosphere horizontal restraints are removed at the rift axis to model the transition from a continuous to separated lithosphere plate during rifting lithosphere strength envelope curves derived from olivine flow laws are used to determine the elastic lithosphere depth in the solutions conductive heating of a variable thickness continuous lithosphere plate by a 2000 w m line source at 60 km depth produces high stresses oriented orthogonal to the forming rift axis and small vertical displacements the maximum uplift obtained in the continuous lithosphere plate models subjected to thermoelastic loading was 0 12 km at a distance of 120 km from the forming rift axis this small uplift is accompanied by shallow tensile stresses of 80 mpa at a distance of 5 km from forming rift axis which decrease to 6 mpa at a distance of 210 km shallow horizontal thermoelastic tensile stresses are over 5 times greater than the horizontal stresses produced by thermal buoyancy loads near the forming rift axis the transition from a continuous to rifted lithosphere is accompanied by an increase in lithosphere uplift at the rift axis from 0 11 to 0 42 km and a reduction in shallow horizontal stress magnitude by a factor of 30 thermoelastic loading may be a source of shallow tensile stress in the lithosphere and a contributing factor for the initiation of rifting thermal loading may also produce postrifting topography changes thermoelastic loading of the lithosphere is proposed as a possible driving mechanism for rift formation which produces separation and the transition from a continuous to rifted lithosphere plate

postextensional mantle healing and episodic extension in the canning basin regions of the lithosphere that undergo finite crustal extension but are subsequently allowed to thermally relax during a period of reduced tectonic activity may become regions of relative strength and therefore resist any further extension the strengthening is a consequence of the temperature dependence of rock rheology and the local thinning of the crust bringing strong upper mantle olivine rich rocks closer to the surface where they replace weaker lower crustal plagioclase feldspar amphibole bearing rocks this concept which i call postextensional mantle healing is illustrated by the predictions of a numerical model although the model is based on the results of rock testing in the laboratory the results do not depend on a particular choice of creep parameters for the lower crust the palaeozoic tectonic history of the canning basin northwestern australia is characterized by a series of extensional episodes separated by periods of relative quiescence in this paper i suggest that postextensional mantle healing is responsible for the continuous shifting through time of tectonic activity between the several major depocenters the tectonic development of the east african rift system follows a scenario similar to that of the canning basin extensional structures inherited from two bursts of extension in the early mesozoic and cretaceous have not been reactivated during the tertiary rifting which is responsible for the present day rift morphology

brittle failure in the upper mantle during extension of continental lithosphere i present a model for the stress required to extend or stretch continental lithosphere the lithosphere is considered to have four rheological fields quartz in the brittle field quartz in the ductile field olivine in the brittle field and olivine in the ductile field as continental lithosphere extends the required stress initially decreases due to nearly isothermal thinning of the strong upper mantle olivine near the brittle ductile transition the stress required to continue extending the crust may then increase due to cooling of the upper mantle however the required stress can increase to no more than about 1 3 times the stress required to start extension this contrasts with an earlier study that required exponentially increasing stress to continue extension my model improves on this earlier study by considering the effects of brittle failure in the upper mantle the rheology of the continental lithosphere should be no barrier to its continued extension regardless of the rate of extension

genesis of flood basalts from eclogite bearing mantle plumes on the basis of the geochemistry of flood basalts we investigate the hypothesis that the source material for these magmas includes a significant eclogite component derived from ancient subducted oceanic crust entrained into mantle plumes at the core mantle boundary we present finite element models of the melting in starting plumes plume heads in both continental and oceanic regions starting plume models in which the plume heads are composed solely of pyrolite mantle are incapable of generating sufficient volumes of melt or any melt at all for reasonable plume head temperatures 1400 1600 c or lithosphere ages 6 25 100 m y this result is in contrast to the results of farnetani and richards 1994 whose models require higher plume head temperatures 1600 1700 c and a weak mobile lithosphere in order to generate volumes of melt similar to flood basalts plumes composed of 15 wt eclogite yield volumes of melt comparable to flood basalts in both oceanic and continental settings the maximum extent of melting in these models is about 10 30 this agrees with the model of hofmann and white 1982 who argued that ocean island basalts are derived from subducted oceanic crust if the embedded eclogite thermally reequilibrates with the surrounding plume material then melt volumes and total extents of melting increase by a additional factor of 2 3 a simple two dimensional rifting model for a slow spreading ocean basin underlain by a lens of hot plume material shows that plume related melting occurs over 50 60 m y a much longer period than for flood basalts

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

lithospheric controls on melt production during continental breakup at slow rates of extension application to the north atlantic rifted margins form from extension and breakup of the continental lithosphere if this extension is coeval with a region of hotter lithosphere then it is generally assumed that a volcanic margin would follow here we present the results of numerical simulations of rift margin evolution by extending continental lithosphere above a thermal anomaly we find that unless the lithosphere is thinned prior to the arrival of the thermal anomaly or half spreading rates are more than 50mmyr 1 the lithosphere acts as a lid to the hot material the thermal anomaly cools significantly by conduction before having an effect on decompression melt production if the lithosphere is thinned by the formation of extensional basins then the thermal anomaly advects into the thinned region and leads to enhanced decompression melting in the north atlantic a series of extensional basins off the coast of northwest europe and greenland provide the required thinning this observation suggests that volcanic margins that show slow rates of extension only occur where there is the combination of a thermal anomaly and previous regional thinning of the lithosphere

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

intraplate stresses and the subsidence history of the sirte basin libya abstract results of quantitative analysis of subsidence data from 20 wells covering six major basement blocks from the sirte basin libya are presented following the late cretaceous onset of extension and associated collapse of the sirte arch the tectonic history post dating this event can be separated into four distinct phases 1 1 minor subsidence during the late cretaceous 2 2 differential fault tectonics and reactivation of basement faults during latest cretaceous to paleocene 3 3 regional uplift and subsidence during the eocene 4 4 minor subsidence during the early oligocene and early miocene a late cretaceous paleocene stretching phase of finite duration can explain the overall patterns of observed tectonic subsidence during the syn and post rift evolution of the sirte basin strong short term deviations from thermally induced long term subsidence observed during the eocene can be explained by intraplate compression and subsequent stress release related to changes in atlantic flowline geometry modelling of the observed tectonic subsidence for the sirte basin yields estimates of extension in excess of estimates inferred from displacements along high angle normal faults the effect of late stage intraplate compression on tectonic subsidence may cause part of the observed discrepancy between estimates of crustal extension

bowl shaped basin related to low angle detachment during continental extension the case of the controversial neogene siena basin central italy northern apennines abstract this paper deals with the tectonic evolution of a bowl shaped neogene basin siena basin located in the inner northern apennines and developed during an articulated tectonic evolution this basin was controversially interpreted by different authors as graben half graben or thrust top basin structural and stratigraphic data from the basin substratum and filling sediments integrated with the interpretation of reflection seismic profiles and borehole logs highlight that the siena basin consists of a tectonic depression with a complex internal architecture such a basin developed on a multilayered substratum deeply affected by extensional tectonics since the middle miocene extensional detachments produced a widespread thinning of the basin substratum and superimposed on contractional structures developed during the northern apennines collisional events extensional detachments accommodated within weak horizons and gave rise to the lateral segmentation of most competent stratigraphic horizons sediments filling the siena basin mainly deposited on the ligurian units the topmost tectonic units forming the tectonic pile in the gap between two laterally segmented geological bodies made up of the triassic siliciclastic succession of the verrucano group such a lateral segmentation produced a synformal structural depression not delimited by faults filled by late serravallian late messinian marine to continental sediments which deposited during the extensional faulting from the latest early pliocene to the middle pliocene the architecture of the basin was modified by an high angle normal fault system which affected the eastern side of the basin and produced new accommodation space for marine sediments in sum the siena basin consists of a sin tectonic basin developed on the hanging wall of staircase extensional detachments later affected by high angle normal to oblique faults the latter faults defined a half graben during the latest evolution of the basin the tectonic evolution of the siena basin made it possible to point out some differences with respect to the types of bowl shaped basins related to extensional detachments and named as supradetachment basins

the role of rheology in extensional basin formation modelling the rheology of the lithosphere determines its deformation under given initial and boundary conditions this paper presents a critical discussion on how rheological properties are taken into account in extensional basin modelling since strength envelopes are often used in models we review the uncertainties in temperature and rheological parameters and assumptions in type of rheology and mode of deformation involved in their construction models of extensional basins are classified into three groups kinematic kinematic with rheological constraints and dynamic rheology enters kinematic models only implicitly in the assumption of an isostatic compensation mechanism we show that there is a critical level of necking that reconciles local isostasy with the finite strength of the lithosphere which requires a flexural response kinematic models with rheological constraints make use of strength envelopes to assess the initial lateral variations of lithospheric strength and its evolution with time at the site of extension dynamic models are the only ones to explicitly introduce rheological constitutive equations usually in plane strain or plane stress they usually however require the presence of an initial perturbation thickness variations pre existing faults thermal inhomogeneities rheological inhomogeneities the mechanical boundary conditions kinematic and dynamic and the thermal boundary conditions constant temperature or constant heat flux at the lower boundary of the lithosphere may result in negative positive feedbacks leading to cessation acceleration of extension we conclude that while kinematic models with rheological constraints if possible are very successful in accounting for the observed characteristics of sedimentary basins dynamic models are necessary to gain insight into the physical processes underlying basin formation and evolution

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

crustal evolution of the eastern canadian cordillera the cordillera is not like the himalayas the himalayas are supported by the indian continent that was thrust under the asian continent the eastern cordillera is supported not by imbricated continents but by imbricated continental margin of the ancient western north america that ancient margin was compressed and imbricated as it was confined between the north american craton and the minicontinent of stikinia in the eastern canadian cordillera tectonic thickening is in the collision zone and not on either side of it as for example in the tibetan plateau where the underthrust indian continent has duplicated the thickness of the crust in the eastern canadian cordillera jurassic and early cretaceous thrusts and faults are directed away from the collision zone toward the converging blocks the oppositely directed thrust faults outline wedge shaped sheets of rock sedimentary and volcanic rocks cover the crystalline rocks on which they were deposited basement and the underlying upper mantle may have been detached from each other during the compression the basement therefore would have imbricated internally between the cover and the mantle as a duplex structure confinement of the ancient margin s basement between the colliding blocks requires that the volume of basement under the margin be the same as that presently between the colliding blocks the possibility of such an equality has been tested and confirmed in cross section the cross section of the ancient western margin from that test has many features comparable to cross sections recently drawn through the eastern continental margin of canada

tectonic evolution of the north sea basin crustal stretching and subsidence summary the lithospheric stretching model for the formation of sedimentary basins was tested in the central north sea by a combined study of crustal thinning and basement subsidence patterns a profile of crustal structure was obtained by shooting a long range seismic experiment across the central graben the main axis of subsidence a seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin the data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure and the final model showed substantial crustal thinning beneath the graben subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central north sea since jurassic times changes in water depth were quantified using foraminiferal assemblages where possible and observed basement subsidence paths were corrected for sediment loading compaction and changes in water depth through time the seismic model is shown to be compatible with the observed gravity field and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the central graben during the mid jurassic to early cretaceous extensional event this extension appears to have occurred over crust already slightly thinned beneath the graben and the seismic data suggest that total extension since the early permian may have been more than 100km the data presented here may all be explained using a simple model of uniform extension of the lithosphere

from a mountain belt collapse to a sedimentary basin development 2 d thermal model based on inversion of stratigraphic data in the paris basin abstract the thermal mineralogical and mechanical behaviour of the lithosphere determines the genesis and evolution of sedimentary basins subsidence amplitude geometry of sediment bodies sedimentary processes using the large stratigraphic database available for the paris basin we constrain the lithosphere thermal evolution thickness and rigidity as well as the mechanism that gave birth to this wide intracontinental basin the thermal subsidence calculated with a two dimensional thermal model is adjusted to the estimated tectonic subsidence by optimization of three parameters extension profile crustal thickness profile and average mantle heat flow the best fits for the paris basin lithosphere are obtained for a mantle heat flow of 23 mw m 2 and for an elastic thickness lower than 20 km preferred value te 5 km two mechanisms for the origin of the basin subsidence have been tested a pure extensional collapse of the variscan belt and a delamination of the mantle lithosphere at the end of carboniferous it appears that in both cases inverted extension factors are large in the range 1 2 1 85 in the first case and in the range 1 0 1 85 in the second case both cases involve the presence of a thick variscan crustal wedge and a mountain belt elevation of 1500 3500 m located in the paris basin centre the maximum extensional rates are obtained in the centre part of the basin in addition lithospheric delamination at around 300 ma allows for both crustal and mantle melting in agreement with the widespread existence of permian bimodal magmatism in france at the end of extension the basin centre kept a high elevation of about 1000 m before subsiding around 40 my later below sea level

some remarks on lithospheric forces and decompression magmatism abstract during lithospheric extension the upwelling mantle melts if it crosses the solidus to model this process we have used a dynamic model where strain rate varies with time as a function of a constant tensile force for forces larger than 6 12 tn m there is after a protracted period with a nearly linear increase in the stretching factor a short period with rapidly increasing strain rates and adiabatic conditions are effectively realized the stretching factor at which this is achieved is about 1 1 1 3 increasing with decreasing force the model could explain the observation of late stage voluminous magmatism in some rifts and at divergent margins that have evolved for several tens of millions of years

fluid present anatexis of metapelites at el joyazo se spain constraints from raman spectroscopy of graphite the garnet biotite sillimanite anatectic xenoliths in the neogene dacite dome of el joyazo also called cerro de hoyazo se spain contain four types of graphite i to iv distinguished on the basis of grain size and texture structural characterization of graphite by laser raman spectroscopy lrs shows systematic differences in the degree of ordering among the four types only type iii is fully consistent with the granulite facies conditions reached by the xenoliths during partial melting the others indicate metamorphic temperatures covering amphibolite facies conditions with only a few examples of granulite grade crystallinity all graphite crystallized before or during the anatectic event indicating that a large fraction of the graphite did not equilibrate at peak temperatures the mm scale coexistence of different types and degrees of ordering in the graphite suggests different origins i e of biogenic derivation and fluid deposited and is explained in terms of fluid melt graphite interaction during the anatectic event disequilibrium behaviour during high temperature metamorphism and anatexis is typical of types i ii and particularly of iv and is attributed to sluggish kinetics of solid state graphitization mainly owing to the limited time of the process and carbon saturation of the intergranular fluid the coexisting well ordered type iii graphite is the product of melting in the presence of a graphite saturated fluid a process that would account for the deposition of new graphite the lrs results together with petrologic observations suggest that it is possible that high melt fractions can be generated by fluid present melting of a metasedimentary protolith also in a closed system although this contradicts the commonly accepted hypothesis that due to limited rock porosities extensive fluid present melting is precluded unless infiltration occurs it is a possible end member model in anatectic settings characterized by rapid heating rates and low grade source rocks

a model of thick plate deformation and its application to the isostatic movements due to surface subsurface and internal loadings summary a model of thick plate deformation is presented without any restriction on stress or strain conditions in the plate an analytical solution is given for the vertical displacement of a thick elastic plate the present method is exact and applicable to 3 d plate deformation problems associated with surface subsurface and internal loadings the conventional thin plate theory which ignores the variation of vertical strain of a plate may not be appropriate to be used as a basic model to study deformation problems of the continental lithosphere since it is shown that the actual surface isostatic adjustment due to surface loading could reach an amount twice as much as predicted by thin plate theory even on a relatively thin plate with 30 km thickness and young s modulus e lo00 kbar and poisson s ratio y 0 25 the isostatic adjustment due to surface load depends on the stress condition at the lower boundary of the elastic lithosphere if there is an active mantle upwelling or downwelling force acting on the boundary then the isostatic adjustment depending on the magnitude of the mantle force is either greatly hindered or amplified to a degree that is beyond the normal prediction of isostatic movements in addition the viscous effect of mantle material on the isostatic movements is shown to be negligible on geological time scales if the mantle viscosity is below ld4poise it is also shown that the airy isostasy of the total sediment load in a surface depression with small wavelength could be achieved only if a considerable period of geological time is allowed for the stress relaxation of viscoelastic or viscous material in the lithosphere the rheological property of the continental lithosphere has a significant effect on basin subsidence caused by sediment loading alone while pure elastic lithosphere restricts surface basin development because of the strong basement support throughout the evolution history

basin migration caused by slow lithospheric extension sedimentary basin migration caused by low lithospheric extension rates is investigated using a two dimensional dynamic numerical model of the lithosphere we find that continental breakup will eventually occur when larger extension velocities are used the duration of rifting prior to continental breakup is dependent on the extension velocity stretching the lithosphere with lower velocities does not lead to breakup instead the locus of maximum extension migrates deformation localizes outside the first formed basin that is in turn uplifted this basin then becomes a cold spot in the area in this case syn rift cooling predominates the lithosphere regains its strength during stretching instead of becoming weaker and the lithosphere necking zone becomes stronger than surrounding regions the transition velocity is for the cases studied about 8 mm yr while the locus of maximum thinning migrates after about 50 60 myr a comparison with observations of the mid norwegian galicia and ancient south alpine margins shows a close resemblance of important features fl 2002 elsevier science b v all rights reserved

why rifts invert in compression abstract the common observation of sedimentary basin inversion in orogenic forelands implies that rifts constitute weak areas of the continental lithosphere when compressed the rifts respond with uplift of the deepest parts and erosion of sediments therein simultaneously syn compressional marginal troughs are formed flanking the inversion zone since rifting and subsequent post rift thermal re equilibration are processes expected to alter the long term mechanical state of the lithosphere the phenomenon of basin inversion is non trivial from a rheological point of view stochastic modelling of the long term thermal structure beneath sedimentary basins indicates that the crustal part of a rift is warmer and hence weaker than the surrounding crustal blocks in contrast the mantle part is cold and strong beneath the basin centre in this paper it is investigated whether the rifting induced strength alterations constitute a sufficient condition for a thermally equilibrated rift to invert by compression numerical experiments with two dimensional dynamic thermo mechanical models are performed in particular the focus is on rifting related mechanical instabilities that reduce the load bearing capacity of the lithosphere in the experiments strain softening behaviour is introduced in the non associated plasticity model representing brittle yielding the result is self consistent large scale fault formation the models predict that the rifting related necking instability induces differential crustal thinning increasing the post rift crustal weakness strain softening and the associated fault formation amplifies the necking instability and introduces zones of structural weakness exposed for compressional re activation under these circumstances basin inversion follows as a natural consequence of rift compression

rifts and swells geophysical constraints on causality abstract analyses are developed to examine whether rifting can generate broad topographic swells and whether swells formed by lithospheric thinning can generate rifts although swells can be supported by deep lithospheric cracks filled with low density intrusions observed topography and gravity constrain how this might occur single deep dikes apparently cannot explain the width of swells the low gravity gradients on swell flanks or the lack of a large negative free air anomaly over swell crests if swells are supported by deep cracks there must be numerous narrow dikes distributed over the full breadth of each swell comparing the hydrostatic pressure of various columns of lithosphere to that of an ocean ridge crest suggests the conditions under which a swell might spread apart oceanic basins are under deviatoric horizontal compression due to ridge push the uplift caused by lithospheric thinning will reduce this compression but because swells never rise higher than ridge crests oceanic swells should not develop deviatoric tensile stresses continental platforms are under less compression so formation of a continental swell can generate significant rifting forces

western united states extension how the west was widened abstract cenozoic extension in the western united states presents a complex interrelation of extension volcanism and plate boundary tectonics that defeats simple notions of active or passive rifting forces driving extension can originate at plate boundaries through basal traction basal normal forces or from buoyancy forces internal to the crust and lithospheric mantle the latter two are most responsible for driving extension where it is observed in the basin and range the complex evolution of the northern basin and range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust in contrast crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 ma to drive extension in the southern basin and range the central basin and range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western united states

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

crustal thinning and subsidence in the north sea we present here the results of a detailed study of subsidence and crystal thinning across the central graben of the north sea basin two independent lines of evidence have been used to test how well the uniform extensional model1 2 can account for the evolution of the north sea basin a long range seismic profile was shot perpendicular to the central graben from edinburgh to southern norway and data were analysed from 11 exploration wells situated close to the seismic line fig 1 the estimates of the increase in surface area due to extension from the crustal structure determined using the seismic data and from the well subsidence patterns are both compatible with a simple uniform extensional mechanism for the post palaeozoic evolution of the north sea

sirte basin north central libya cretaceous rifting above a fixed mantle hotspot the complex pattern of horsts and grabens in the sirte basin may have developed when mesozoic drift of the african plate put north central libya over a fixed mantle hotspot in early cretaceous time 140 to 100 m y ago significant change in the motion of the plate during the prolonged residence above a hypothetical cameroon plume may have produced stress that fragmented thinned and weakened lithosphere successive uplift and subsidence along a reconstructed track of the plume as well as in the sirte basin are compatible with predicted effects of the drift of northern africa over a fixed mantle hotspot this speculation suggests a plausible alternative to the possibility that rifting throughout northern africa in early cretaceous time may have been produced along a wide zone of extension between two african plates when they were at rest relative to underlying plumes

the structural evolution of the northern viking graben and its bearing upon extensional modes of basin formation recent advances in the understanding of rift basin formation coupled with the increasing public availability of seismic and well data across the northern viking graben 60 61 n have enabled a detailed analysis of its development two rifting episodes can be recognized late permian early triassic and bathonian ryazanian major regional unconformities thought to be primarily tectonic rather than eustatic in origin separate and subdivide the rifting episodes the earlier episode involved extension about a n s axis ensuing triassic mid jurassic thermal subsidence was accommodated on steep faults during the later episode a new ne sw fault trend was superimposed on pre existing patterns major block rotation marking active rifting ceased at the end of the ryazanian during the second post rift episode there was a progressive migration of active faulting towards the basin margins and as a result a widening with time of the area undergoing subsidence asymmetric subsidence of the central part of the basin was hinged at the western margin of the horda platform and accommodated to the nw on major faults within the tampen spur factors for the second rifting episode were calculated both by relating subsidence to extension and by measuring observed extension values calculated by both methods increase consistently towards the basin axis for both rifting and thermal subsidence phases but are greater for the latter phase subsidence patterns are similar for both rifting and thermal subsidence episodes so that there is vertical stacking of relatively thick sequences in the axis of the northern viking graben these factors preclude the application of models involving uniform and non uniform stretching and also preclude oblique extension depth dependent stretching is preferred

the stratigraphy and deformation history of the tumut region implications for the development of the lachlan fold belt abstract the tumut region forms the southern part of the tumut synclinorial zone in the southeastern part of the lachlan fold belt lfb the area contains two major stratigraphie packages ordovician to early silurian quartz rich to quartz intermediate flysch and volcanics package a and overlying fossiliferous early late silurian volcanics and flysch package b rhyolite within the older package a yields a u pb zircon age of 428 6 ma both packages were meridionally folded during the siluro devonian bowning orogeny following intrusion of the gocup granite at 411 5 ma an earlier deformation characterised by thrust faulting e w recumbent folding and later local coaxial upright folding is present only in the older flysch and volcanics package a this earlier deformation is part of the benambran orogeny that affects the ordovician metamorphics of the wagga metamorphic belt and other parts of the lfb and is constrained to about 425 ma fold characteristics of this deformation are indicative of thin skinned intraplate transpressional deformation rather than classical collisional tectonics as envisaged by some workers for the benambran orogeny here and elsewhere in the lfb the composition and deformational history of the older flysch indicates that it is part of the ordovician to early silurian molong volcanic belt consequently the older concept of an early silurian tumut trough incorporating both packages of rocks is rejected instead it is proposed that up to 2500 m of early late silurian rocks formed in a pull apart basin the tumut basin thus the silurian depositional history of the tumut region previously considered unique in the lfb is little different from other basins of similar age throughout the lfb

the role of gravitational potential energy in active deformation in the southwestern united states in orogenic belts intrinsic buoyancy and extrinsic boundary forces act on the continental lithosphere determining the relative importance of these forces is fundamental to understanding the dynamics of continental deformation in the western united states estimates of gravitational potential energy show that given available constraints on rheology buoyancy forces are sufficient to produce the wide variety of rates and styles of observed deformation including absence of deformation this underscores the importance of gravitational potential energy for deformation in orogenic belts

damage formation associated with bending under a constant moment abstract deformation within the earth s lithosphere is largely controlled by the rheology of the rock fracture and faulting are characterized by elastic rheologies with brittle mechanisms while folding and flow are characterized by plastic and or viscous rheologies due to ductile mechanisms however it has been recognized that deformation that resembles ductile behavior can be produced within the confines of the brittle lithosphere specific examples are folds that form in the shallow crust steep hinges at subduction zones that are accompanied by seismicity and large scale deformation at plate boundaries in these cases the brittle lithosphere behaves elastically with fracture and faulting yet produces ductile behavior in this paper we attempt to simulate such ductile behavior in elastic materials using continuum damage mechanics engineers utilize damage mechanics to model the continuum deformation of brittle materials we utilize a modified form of damage mechanics that represents a reduction in frictional strength of preexisting fractures and faults we use this empirical approach to simulate the bending of the lithosphere under the application of a constant moment we use numerical simulations to obtain elastostatic solutions for plate bending and where the longitudinal stress at a particular node exceeds a yield stress we apply damage to reduce young s modulus at the node damage is calculated at each time step by a power law relationship of the ratio of the yield stress to the longitudinal stress and the yield strain to the longitudinal strain this results in the relaxation of the material due to increasing damage to test our method we apply our damage rheology to an infinite plate deforming under a constant bending moment we simulate a wide range of behaviors from slow relaxation to instantaneous failure over timescales that span six orders of magnitude using this method stress relaxation produces elastic perfectly plastic behavior in cases where failure does not occur for cases of failure we observe a rapid increase in damage leading to failure analogous to the acceleration of microcrack formation and acoustic emissions prior to failure the changes in the rate of damage accumulation in failure cases are similar to the changes in b values of acoustic emissions observed in triaxial compression tests of fractured rock and b value changes prior to some large earthquakes thus continuum damage mechanics can simulate the phenomenon of ductile behavior due to brittle mechanisms as well as observations of laboratory experiments and seismicity

deformation and seismicity associated with continental rift zones propagating toward continental margins 1 we study the propagation of a continental rift and its interaction with a continental margin utilizing a 3 d lithospheric model with a seismogenic crust governed by a damage rheology a long standing problem in rift mechanics known as thetectonic force paradox is that the magnitude of the tectonic forces required for rifting are not large enough in the absence of basaltic magmatism our modeling results demonstrate that under moderate rift driving tectonic forces the rift propagation is feasible even in the absence of magmatism this is due to gradual weakening and long term memory of fractured rocks that lead to a significantly lower yielding stress than that of the surrounding intact rocks we show that the style rate and the associated seismicity pattern of the rift zone formation in the continental lithosphere depend not only on the applied tectonic forces but also on the rate of healing accounting for the memory effect provides a feasible solution for thetectonic force paradox our modeling results also demonstrate how the lithosphere structure affects the geometry of the propagating rift system toward a continental margin thinning of the crystalline crust leads to a decrease in the propagation rate and possibly to rift termination across the margin in such a case a new fault system is created perpendicular to the direction of the rift propagation these results reveal that the local lithosphere structure is one of the key factors controlling the geometry of the evolving rift system and seismicity pattern

on the stability of isostatically compensated mountain belts the formation of convergent mountain belts is invariably accompanied by an increase in gravitational potential energy due to part of the work done by the forces driving convergence the evolution of potential energy stored in an orogen is dependent on 1 the density structure 2 the thermal evolution and 3 the way convergent deformation is partitioned between crust and mantle lithosphere it is now well recognized that this increase in potential energy associated with the mountain building process raises the possibility that significant extension or collapse may accompany the relaxation of the forces driving convergence provided the lithosphere is thermally weakened e g england 1987 in this paper we evaluate the stability of isostatically compensated mountain belts under the assumption that the strength of continental lithosphere is governed by a combination of frictional sliding and creep processes using the brace goetze model for the rheology of the lithosphere the reference lithosphere defined to be in potential energy and isostatic balance with the mid ocean ridges changes with different thermal parameters of the lithosphere the instantaneous extensional strain rate for thermally mature mountain belts is calculated by balancing the horizontal buoyancy force stored in the mountain belts measured relative to the reference state with the vertically integrated strength of the lithosphere for initial strengths spanning the probable natural range it is shown that horizontal buoyancy forces arising in isostatically balanced mountain belts are sufficiently large to induce the collapse at significant rates greater than a few times 10 16 s 1 and leading to significant finite extension providing the moho temperatures exceed about 650 700 c a condition only likely to be attained if the mantle lithosphere has not been thickened to the same extent as the overlying crust consequently processes that thin the mantle lithosphere as a consequence of convergent deformation such as the convective instability of a thickened lower thermal boundary layer greatly increase the possibility of collapse the calculations presented here suggest that near complete destruction of the mountain system by extensional collapse may be possible if such processes can reduce total lithospheric thickening to less than half the contemporary crustal thickening i e fl fc 2 where fl and fc are the lithospheric and crustal thickening factors

the magnetic quiet zone in the eastern gulf of aden implications for the early development of the continental margin summary the magnetic quiet zone in the eastern gulf of aden is located between the oceanic crust of sheba ridge and the continental crust of arabia and somalia and is separated from both by important structural boundaries the seaward boundary is marked by the end of the seafloor spreading magnetic anomaly sequence and by a basement depth discontinuity the landward boundary is marked by escarpments made up of a series of normal faults these escarpments extend from 2 3 km below sea level to 1500 m above sea level and are equivalent of the hinge zone found at mature continental margins the magnetic field in the quiet zone is flat in some areas and in others is characterized by anomalies of up to several hundred gammas which are correlatable over distances of up to about 20 km and which appear related to basement topography the basement lacks the topographic slope characteristic of mid ocean ridge flanks and is characterized by moderately rough relief the crustal structure appears quite heterogeneous and where the crustal thicknesses have been determined they are slightly greater than those of oceanic crust new heat flow measurements show high values 95 7 123 3 mw m 2 in the quiet zone with values decreasing from sheba ridge toward the coast the unusual structure of the quiet zone and the observations that more opening appears to have occurred between arabia and somalia than can be accounted for by the oceanic crust of sheba ridge leads to the suggestion that the magnetic quiet zone was generated by diffuse extension of continental crust through a combination of rotational listric faulting and dyke injection this possibility is investigated using both a stretching or lithospheric attenuation model and a model in which a portion of the extension occurs through dyke injection it is found that these models can adequately match the observed heat flow and basement depths although very large amounts of extension 4 6 are required in the deep seaward portion of the quiet zone this results in more extension than is compatible with the documented motion between arabia and africa however formation of the magnetic quiet zone occurred over a period of 10 15 myr rather than instantaneously as assumed in the simple models when the effects of a finite length rifting episode are considered less extension is required and the observed geophysical data are consistent with a diffuse extension origin for the magnetic quiet zone

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

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

cenozoic deformation in western yunnan china myanmar border in the present paper we describe ductile and brittle deformation styles in western yunnan and ne myanmar using field data and landsat 7 imagery we show that this complex area located at the northern termination of sunda plate three rivers area was wedged during the tertiary between the left lateral ailao shan chong shan metamorphic belts to the east and the right lateral shan scarp gaoligong metamorphic belt in west this triangular region therefore underwent the effects of these continental size ductile strike slip faults separating major blocks with a dominant ew to ene compression since the late miocene date of the reversal of motion along the rrf the incipient eastward motion of the sunda block and the persisting right lateral motion along its western boundary sagaing fault created n s compression and e w to wnw extension underlined by left lateral transtension along the wanding nanting fault zones at the same time the diangcan shan situated along strike the ailao shan metamorphic belt was slightly impinged by the blocks extruded from the syntaxis and exhumed again from the early pliocene in accordance with this late and still active state of stress q 2004 elsevier ltd all rights reserved

influence of tectonics on permian coal rank patterns in australia abstract coal in the australian permian basins is dominantly subbituminous coal to medium volatile bituminous coal the subbituminous coal and higher rank bituminous coals are generally the result of burial diagenesis localized low volatile bituminous coal to anthracite is commonly associated with igneous activity extensive zones of high rank coal in the eastern part of the bowen basin are due to uplift of deeply buried coal measure sequences during late permian and triassic tectonism high rank coals in the sydney basin are due to uplift of deeply buried permian rocks preceding sea floor spreading in the tasman sea during the late cretaceous and early tertiary although this conclusion is controversial localized semianthracite and anthracite at depths greater than 2 500 m in the cooper basin were formed either by enhanced heat flow from radiogenic basement granites or by vertical movement of hot fluids through the fractured granitic bodies during the tertiary permian coals in the pedirka and arckaringa basins in the center of the continent and the perth carnarvon canning bonaparte and officer basins in the west are generally low in rank due to either shallow depths of burial or slight heat flow or both the fitzroy trough of the canning basin is an exception there significant tectonic activity has occurred since the permian

a stretching model for the oslo rift abstract data on the tectonic and magmatic activity crustal thickness seismic velocity and gravity have been integrated to constrain a stretching model for the late carboniferous early permian oslo rift the crustal structure and numerical modelling of temperature and melt generation during rifting suggest that the oslo rift in skagerrak developed in response to crustal stretching with a factor between 1 4 and 1 6 however upper crustal extension by faulting accounts for a stretching factor of only 1 1 1 2 thermal expansion of the lithosphere due to an increased asthenospheric potential temperature the intrusion of melted mantle material into the lower crust and probably a lowering of the erosional base by a fall in sea level all contributed to the erosion of 2 4 km of lower palaeozoic pre rift sediments over large areas the lateral extent of the thermal anomaly reached far beyond the oslo and skagerrak grabens

a note on the troughs in the three gorges channel of the changjiang river china the reaches of the changjiang river between baidicheng and fengjie of sichuan province and nanjinguan and yichang of hubei province collectively called the three gorges channel are about 200 km long more than 90 deep troughs occur in this part of the changjiang river which cumulatively cover 45 of the three gorges channel this study investigated the geomorphological features of the troughs in the channel the troughs were formed by fluvial downcutting along lines of tectonic weakness the deepest points are normally located in tectonic zones of high intensity where joints and fissures longitudinal and transverse to the stream flow direction intersect

the interaction of plume heads with compositional discontinuities in the earth s mantle the effects of compositional discontinuities of density and viscosity in the earth s mantle on the ascent of mantle plume heads is studied using a boundary integral numerical technique three specific problems are considered 1 a plume head rising away from a deformable interface 2 a plume head passing through an interface and 3 a plume head approaching the surface of the earth for the case of a plume attached to a free surface the calculated time dependent plume shapes are compared with experimental results two principal modes of plume head deformation are observed plume head elongation or the formation of a cavity inside the plume head the inferred structure of mantle plumes namely a large plume head with a long tail is characteristic of plumes attached to their source region and also of buoyant material moving away from an interface and of buoyant material moving through an interface from a high to low viscosity region as a rising plume head approaches the upper mantle most of the lower mantle will quickly drain from the gap between the plume head and the upper mantle if the plume head enters the upper mantle if the plume head moves from a high to low viscosity region the plume head becomes significantly elongated and for the viscosity contrasts thought to exist in the earth could extend from the 670 km discontinuity to the surface plume heads that are extended owing to a viscosity decrease in the upper mantle have a cylindrical geometry the dynamic surface topography induced by plume heads is bell shaped when the top of the plume head is at depths greater than about 0 1 plume head radii as the plume head approaches the surface and spreads the dynamic topography becomes plateau shaped the largest stresses are produced in the early stages of plume spreading when the plume head is still nearly spherical and the surface expression of these stresses is likely to be dominated by radial extension as the plume spreads compressional stresses on the surface are produced beyond the edges of the plume consequently extensional features will be produced above the plume head and may be surrounded by a ring of compressional features

did rifting on australia s southern margin result from tectonic uplift subsidence analyses from australia s southern margin are used to investigate the idea that rifting on the margin was caused by an extensional lithospheric stress field induced by a hot thermal plume impinging on the base of the lithosphere from beneath the continental lithosphere is modelled as a thin sheet with depth dependent rheology the model does not explicitly include elastic flexural or thermal stresses the excess buoyancy of a thermal plume pushes the lithosphere upward by as much as 1 km thereby increasing its potential energy if the horizontal components of lithospheric stress are fixed at some distant plate boundary the lithosphere is a stress guide the lithospheric stress field above the plume will produce horizontal extension that may ultimately be followed by continental separation and seafloor spreading the uplift is modelled by a time dependent function which increases smoothly from zero to its peak value and then decreases back to zero using a plausible mantle rheology the amount of extension inferred at well sites on the margin typically 20 to 30 extension is explained by uplift of the lithosphere to approximately 900 m with a subsequent decay of the uplift over a period of approximately 60 m y the rise time of the uplift function is poorly constrained in the uplift model extension occurs in the subaerial environment during the late jurassic early cretaceous the initial stage of rapid subsidence observed at most well sites on the margin is due in large part to decrease of the vertical stress anomaly caused by the decay of the thermal plume or by the lithosphere moving away from above it the rapid subsidence is followed by very gradual continued subsidence attributed to cooling of the extended lithosphere as in previous studies

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

partial delamination of continental mantle lithosphere uplift related crust mantle decoupling volcanism and basin formation a new model for the pliocene quaternary evolution of the southern east carpathians romania abstract a geodynamic model is proposed for the mid miocene to quaternary evolution of the southern east carpathians in order to explain the relationships between shallow and deep geological phenomena that occurred synchronously during late collision tectonics in this area an active volcanic zone cross cuts since 2 my the suture between the overriding tisza dacia and subducting european continental plates mafic calc alkaline and alkaline magmas south harghita and persani volcanoes erupted contemporaneously these magmas were supplied by partial melting of the mantle lithosphere of the subducting and not of the overriding plate in an effort to decipher this geodynamically a typical setting of magma generation the spatial and temporal distribution of shallow and deep phenomena was successively examined in order to establish the degree of their interdependence our model indicates that intra mantle delamination of the subducting european plate is the principal cause of a succession of events it caused upwelling of the hot asthenosphere below a thinned continental lithosphere of the carpathians inducing the uplift of the lithosphere and its internal decoupling at the moho level by isostatic and mostly thermal effects during this uplift the crust deformed flexurally whilst the mantle deformed in a ductile way this triggered decompressional partial melting of the uppermost mantle lithosphere flexural deformation of the crust induced its fracturing allowing for the rapid ascent of magmas to the surface as well as reactivation of an older detachment horizon at the base of the carpathian nappe stack above which the brasov ciuc and gheorghieni hinterland basins formed by extension and gravity spreading the rapid subsidence of the focsani foreland basin is controlled by the load exerted on the lithosphere by the delaminated mantle slab that is still attached to it in this model crust mantle decoupling magma genesis and volcanism local near surface hinterland extension are consequences of uplift induced by asthenospheric upwelling triggered by intra mantle delamination this model enables to conceive that delamination induced decoupling began to be efficient 9 4 my ago at the northern end of the east carpathians when the mantle slab dipped westwards since then intra mantle delamination migrated laterally normal to the slab strike and follows the arcuate shape nw se sw of the carpathians nowadays whereas the mantle slab is still actively foundering below the vrancea seismic zone to the se of the most recent volcanic area south harghita persani a significant southwestward shift of the delamination process can be discerned

plate scale potential energy distributions and the fragmentation of ageing plates abstract geoid anomalies associated with mid ocean ridge systems and a number of continental margins imply that on the scale of individual plates the old ocean lithosphere represents a gravitational potential energy sink since lateral variations in potential energy contribute to deviatoric stresses in the lithosphere the changing potential energy distributions in individual plates associated with the growth and ageing of the oceanic lithosphere may be expected to result in changes in the intraplate stress field analytical models for simple plate geometries using lithospheric density models consistent with small positive 6 m geoid anomalies across continental margins show that the growth of oceanic lithosphere over a period of 200 ma may contribute to a decline in the mean plate potential energy u p of about 1 10 12 n m 1 and thus contribute a mean extensional stress difference zz xx in continental lithosphere of up to about 8 mpa averaged over a 125 km thick lithosphere these estimates are sensitive to the assumed mean continental potential energy u c about which there is some uncertainty for higher u c approaching that of the mid ocean ridges u mor the net decline in u p may be as much as 1 7 10 12 n m 1 whereas for significantly lower u c approaching that of old ocean lithosphere plate growth may increase u p transiently by up to 2 7 10 12 n m 1 leading to compression in the continents in the african and antarctic plates the ageing of the ocean lithosphere since the late jurassic is estimated to have contributed to a decline in u p of about 0 6 10 12 and 0 95 10 12 n m 1 respectively contributing a mean stress difference of about 5 mpa and 7 5 mpa in the respective continents whereas the predicted stress changes associated with ageing of the oceanic lithosphere are significantly smaller than most laboratory based estimates of the extensional strength of the lithosphere they may provide an important contribution to the stress fields that eventually lead to the fragmentation of ageing plates

dynamic modeling of the transition from passive to active rifting application to the pannonian basin we examine a number of first order features of pannonian basin evolution in terms of the feedback relation between passive far field induced extension and active raleigh taylor instable upwelling of the astheno sphere we show that active mantle upwelling following a phase of passive extension are viable mechanisms ex plaining the pannonian basin formation the dynamic interplay between far field driven passive extension and active thinning of the mantle lithosphere by convective upwelling beneath the rift zone is modeled using ther momechanical finite element methods our modeling results predict a first phase of passive lithospheric thin ning which is followed by a second phase of late synrift to postrift active mantle lithosphere thinning due to buoyancy induced flow beneath the rift zone we argue that the pattern of coeval extension in the thinning re gion and compression in the flanking regions may be explained by the buoyancy forces due to lithosphere thinning it is demonstrated that timescales of and stresses generated by both processes are comparable the model appears also to explain the occurrence of late shallow mantle related decompression melts in the pannonian region and late regional doming