ridge torques and continental collision in the indian australian plate the excess gravitational potential energy of a collisional orogen that may be supported by focusing distributed ridge torques in a plate depends on the ratio l of the effective lengths of ridge systems and collisional fronts in the indian australian plate the net torque due to the asymmetric distribution of mid ocean ridges and young oceanic lithosphere along the southern boundary is 8 5 10 25 n m and is capable of supporting an excess potential energy of the himalayan collision zone of up to 7 5 10 12 j m 2 the concept that collision in the indian australian plate may be sustained by ridge torques suggests that collisional driving stresses may be sensitive to l in the indian australian plate l changed significantly during progressive collision from 55 to 45 ma during amalgamation of the indian and australian plates at 45 ma and during the development of the new guinea orogen through the late oligiocene and miocene such changes in l may help explain the termination of spreading between india and australia following the onset of the himalayan collision and the early miocene normal faulting in the high himalaya

global crustal stress pattern based on the world stress map database release 2008 the world stress map wsm project is a global compilation of information on the contemporary crustal stress field from a wide range of stress indicators the wsm database release 2008 contains 21 750 stress data records that are quality ranked using an updated and refined quality ranking scheme almost 17 000 of these data records have a c quality and are considered to record the orientation of maximum horizontal compressional stress sh to within 25 as this is almost a triplication of data records compared with the first wsm database release in 1992 we reinvestigate the spatial wave length of the stress patterns with a statistical analysis on a global 0 5 grid the resulting smoothed global stress map displays both the mean sh orientation that follows from the maximum smoothing radius for which the standard deviation is b25 and a countour map that displays the wave length of the stress pattern this smoothed global map confirms that long wave length stress patterns n2000 km exist for example in north america and ne asia these have been used in earlier analyses to conclude that the global stress pattern is primarily controlled by plate boundary forces that are transmitted into the intraplate region however our analysis reveals that rather short wave length of the stress pattern b200 km are quite frequent too particularly in western europe alaska and the aleutians the southern rocky mountains basin and range province scandinavia caucasus most of the himalayas and indonesia this implies that local stress sources such as density contrasts and active fault systems in some areas have high impact in comparison to plate boundary forces and control the regional stress pattern

the background concentrations of 13 soil trace elements and their relationships to parent materials and vegetation in xizang tibet china abstract the background concentrations of 13 soil trace elements copper cu lead pb zinc zn cadmium cd nickel ni chromium cr mercury hg arsenic as selenium se cobalt co vanadium v manganese mn and fluorine f from approximately 205 pedons in tibet china are reported here for the first time the 13 trace element concentrations follow an approximately log normal distribution while the mean concentrations of hg and se are lower and as is higher than the average concentration for all of china concentrations of the other trace elements are similar to the national average trace element concentrations are related to vegetation and human activity also played a notable role on the contents of trace elements in tibet the parent material relationship for all 13 soil trace element concentrations follows the pattern shale sandstone igneous rock limestone alluvial sediment glacial deposits lake sediments while for vegetation and human activity the concentration pattern is farmland shrub forests meadow prairie marsh and others the soil trace element concentrations on the tibetan plateau are related primarily to the parent material but were also affected by vegetation and human activity

rheology and strength of the lithosphere abstract mechanical properties of lithosphere are of primary importance for interpretation of deformation at all spatial and time scales from local scale to large scale geodynamics and from seismic time scale to billions of years depending on loading conditions and time scale lithosphere exhibits elastic brittle plastic or viscous ductile properties as can be inferred from rock mechanics data a large part of the long term lithospheric strength is supported in the ductile or ductile elastic regime while it also maintains important brittle strength yet at short seismic time scale s the entire lithosphere responds in elastic brittle elastic regime even though rock mechanics experiments provide important insights into the rheological properties of the lithosphere their conditions e g time scales strain rates temperature and loading conditions are too far from those of real earth therefore these data cannot be reliably extended to geological time and spatial scales strain rates 10 17 to 10 13 s 1 without additional parameterization or validation based on geological time scale observations of large scale deformation for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes yses were validated by geodynamic scale observations such as the observations of plate flexure however oceanic lithosphere behavior in subduction zones and passive continental margins is strongly conditioned by the properties of the continental counterpart whose rheology is less well understood for continents and continental margins the uncertainties of available data sources are greater due to the complex structure and history of continental plates for example in a common continental rheology model dubbed jelly sandwich the strength mainly resides in crust and mantle while in some alternative models the mantle is weak and the strength is limited to the upper crust we address the problems related to lithosphere rheology and mechanics by first reviewing the rock mechanics data te flexure and ts earthquake data and long term observations such as folding and subsidence data and then by examining the physical plausibility of various rheological models for the latter we review the results of thermo mechanical numerical experiments aimed at testing the possible tectonic implications of different rheology models in particular it appears that irrespective of the actual crustal strength the models implying weak mantle are unable to explain either the persistence of mountain ranges for long periods of time or the integrity of the subducting slabs although there is certainly no single rheology model for continents the jelly sandwich is a useful first order model with which to parameterize the long term strength of the lithosphere it is concluded that dry olivine rheology laws seem to represent well the long term behavior of mantle lithosphere in oceans margins and continents as to the continent and margin crust rheology analysis of the results of thermo mechanical models and of te data based on the most robust variants of flexural models suggests that continental plates with te 30 50 smaller than their theoretical mechanical thickness hm i e te 20 60 km should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between crust and mantle older plates such as cratons are strong due to crust mantle coupling and specific properties of the cratonic mantle lithosphere

land use change and erosional history in a lake catchment system on the canadian prairies abstract the introduction of agriculture in the late 1800s and early 1900s brought about many changes in the natural prairie landscape of western canada the objective of this research project is to evaluate landscape response to land use change by relating observed changes in the sedimentary record of a lake drainage basin to documented land use change that has occurred since the onset of settlement a 52 cm lake sediment core was sectioned into 1 cm layers the sediment was separated into allogenic and authigenic fractions using a wet chemical extraction technique close interval 210pb dating enabled the calculation of sediment and elemental influx rates prior to european settlement erosion rates were low

paleostress field evolution of the australian continent since the eocene 1 although the low order present stress field of most continents is fairly well established information on paleostress fields is generally sparse knowledge of paleostresses is crucial for understanding brittle tectonic reactivation through time the indian australian plate lends itself well to a reconstruction of paleostresses as it has undergone enormous changes in plate driving forces through the tertiary and there is a rich record of fault reactivation from sedimentary basins we reconstruct the plate boundary configuration and age area distribution of ocean crust around australia through time to obtain estimates for ridge push slab pull and collisional forces acting on the indian australian plate since the eocene other model parameters we explore are the effects of the australian antarctic discordance and the mechanical strength of the australian continental margin we apply these constraints to model the orientation of the maximum horizontal compressive stress shmax regime for the present early miocene and early eocene using the commercial software abaqus along with the optimization software nimrod o we use an elastic two dimensional plane stress finite element model with a resolution of 0 2 in both longitude and latitude realistic elastic parameters representing different rock types and geologic provinces for the australian continent have been included to model the stress field of a heterogeneous plate we show that spatially significant rotations of shmax directions can be modeled as a consequence of perturbations of shmax in areas of juxtaposed rigid and compliant rheologies the absence of the collisional papua new guinea boundary in the miocene and reduced ridge push force from the south result in stress directions considerably different from the present stress directions over the northern australian continent in the early miocene in particular show large disparity with present stress directions stress orientations for the australian plate during the early eocene are controlled predominantly by ridge push forces arising from spreading in the wharton basin in the indian ocean and vary substantially with stress directions in the early miocene and the present because of the drastically different plate geometry and boundary configurations fault reactivation histories observed on the northwest shelf of australia and in the bass strait region are consistent with modeled changes in stress directions through time

paleostress reconstructions and geodynamics of the baikal region central asia part 2 cenozoic rifting abstract investigations on the kinematics of rift opening and the associated stress field present a renewed interest since it has recently been shown that the control of the origin and evolution of sedimentary basins depends to a large extent on the interplay between lithospheric strength and applied stresses it appears that changes of stress field with time are an important factor that either controls or results from the rifting process the object of this paper is to study the changes of fault kinematics and paleostress field with time in the baikal rift system during the cenozoic reduced paleostress tensors were determined by inversion from fault slip data measured in the central part of the rift and its southwestern termination between 1991 and 1995 results show that the stress field varies as well in time as in space two major paleostress stages are determined corresponding broadly to the classical stages of rift evolution late oligocene early pliocene and late pliocene quaternary the first paleostress stage is related to the rift initiation and the second to the major stage of rift development similarities between the recent paleostress field and the present day stress field inverted from focal mechanisms indicate that the second paleostress stage is still active therefore we propose to use proto rift for the late oligocene early pliocene stage and active rift for the late pliocene quaternary stage of rift development during the proto rift stage the stress field was characterized by a compressional to strike slip regime a progressive change from transpression to transtension is suspected for the central part of the rift baikal and barguzin basins during this period in the western termination of the rift sayan massif tunka depression a strongly compressional stress field with oblique thrusting kinematics is well constrained in the late miocene early pliocene interval the active rift stage was initiated by a marked change in fault kinematics and stress regime in the late pliocene in the central part of the rift the stress regime changed into pure extension while in the southwestern extremity it changed into pure strike slip fault kinematics suggests that rifting was initiated by an extrusion mechanism due to the interaction of far field compressional stress on a mechanically heterogeneous crust with the southwards pointing wedge of the siberian craton acting as a passive indentor the cenozoic time space evolution of the stress field is believed to reflect the increasing influence of locally generated buoyancy extensional stresses associated with density anomalies of the lithosphere on intraplate stresses generated by the india eurasia convergence and the west pacific subduction

joint seismic geodynamic mineral physical modelling of african geodynamics a reconciliation of deep mantle convection with surface geophysical constraints recent progress in seismic tomography provides the first complete 3 d images of the combined thermal and chemical anomalies that characterise the unique deep mantle structure below the african continent we present a tomography based model of mantle convection that provides an excellent match to fundamental surface geodynamic constraints on internal density heterogeneity that includes both compositional and thermal contributions where the latter are constrained by mineral physics the application of this thermochemical convection model to the problem of african mantle dynamics yields a reconciliation of both surface gravity and topography anomalies to deep seated mantle flow under the african plate over a wider range of wavelengths than has been possible to date on the basis of these results we predict flow in the african asthenosphere characterised by a clear pattern of focussed upwellings below the major centres of late cenozoic volcanism including the kenya domes hoggar massif cameroon volcanicline cape verde and canary islands the flow predictions also reveal a deep seated large scale active hot upwelling below the western margin of africa under the cape verde islands that extends down to the core mantle boundary the scale and dynamical intensity of this west african superplume is comparable to the south african superplume that has long been assumed to dominate the large scale flow dynamics in the deep mantle under africa we evaluate the plausibility of the predicted asthenospheric flow patterns through a comparison with seismic azimuthal anisotropy derived from independent analyses of african shear wave splitting data

altered granitic rocks nusab el balgum area southwestern desert egypt mineralogical and geochemical aspects of rees abstract gabel nusab el balgum is located in the northern part of bir safsaf area southwestern desert egypt about 600 km southwest of aswan city and covers an area of about 40 km 2 a few workers have investigated the nusab el balgum area from the mineralogical and geochemical point of view the area consists mainly of devonian sandstone permo triassic volcanic rocks altered granitic rocks and post granitic dykes the altered granitic rocks silicified and kaolinized have undergone several hydrothermal alteration processes and were subjected to different types of alteration hematitization carbonatization kaolinization sericitization and silicification and are characterized by extreme enrichment in rare metals such as zr nb y and rees bastnaesite parisite fergusonite columbite hydrothermal zircon monazite and xenotime are common in the silicified and kaolinized granites the silicified granite commonly contains more rare metals than the kaolinized granite the rees in nusab el balgum granites were introduced by hydrothermal fluids during alteration and occur in carbonate minerals such as bastnaesite lree co 3 f and parisite ca lree 2 co 3 3 f 2 the ree chondrite normalized patterns of altered granites show an m type tetrad effect and indicate higher lrees negative ce anomaly due to alteration by hydrothermal fluids and negative eu anomaly

oblique convergence of the crust driven by basal forcing implications for length scales of deformation and strain partitioning in orogens summary we investigate the effects of oblique convergence on crustal thickening by modelling the crust as a thin viscous sheet crustal deformation is driven by basal velocity boundary conditions representing the kinematics of the underlying convergent mantle lithosphere which is assumed to detach and subduct the crustal sheet is coupled to the basal velocity field by a weak simple shear layer which exerts a traction on the base of the sheet the use of basal velocity boundary conditions with a thin sheet model is shown to be valid provided coupling with the base is weak and deformation extends over a horizontally wide zone in comparison to the thickness of the sheet the effect of the mantle lithosphere on crustal deformation can be shown to depend on the non dimensional ampferer number am which represents the relative strengths of the crust and the weak basal shear zone analytical and numerical model results for small amounts of normalized convergence show that the ratio of horizontal length scales for normal and transcurrent crustal deformation is 2 for both linear and non linear viscous rheologies the length scales depend on the relative strength of the crust and the basal shear zone convolved with a forcing length scale from the boundary conditions for obliquely incident velocities the length scale ratio is between 2 and 1 the exact value depending on the power law exponent of the rheology and the relative magnitude of the normal and transverse velocity components driving deformation the model results for the ratio of normal to transcurrent deformation length scales are different from those found by using a side driven viscous thin sheet model england houseman sonder 1985 sonder england houseman 1986 we propose that this difference may be used as a test to distinguish the primary driving mechanism for large scale orogenic deformation if the deformation mechanism corresponds to pure shear of the whole lithosphere without significant basal tractions we expect the deformation length scale to be determined by the scale of the applied boundary conditions and buoyancy forces in the crust the normal transcurrent length scale ratio for this case in early stages of deformation will be approximately 4 alternatively if the results presented here are more appropriate then we expect the deformation length scale to be determined by the intrinsic relative strengths of the crust and underlying basal shear zone the corresponding normal transcurrent length scale ratio for this case in early stages of deformation will be between 2 and 1 on the basis of a comparison of the predicted length scales of deformation with the length scale over which the velocity boundary conditions are applied we suggest that long narrow linear orogens such as the andes are controlled by basal boundary conditions whereas the himalayan tibetan orogen has dimensions consistent with either basally or side driven deformation

constraining effective rheology through parallel joint geodynamic inversion abstract the dynamics of crust and lithosphere is to a large extent controlled by its effective viscosity unfortunately extrapolation of laboratory experiments indicates that viscosity is likely to vary over many orders of magnitude additional methods are thus required to constrain the effective viscosity of the present day lithosphere using more direct geophysical observations here we discuss a method which couples 3d geodynamic models with observations surface velocities and gravity anomalies and with a bayesian inversion scheme on massively parallel high performance computers we illustrate that the basic principle of a joint geodynamic and gravity inversion works well with a simple analytical example in a next step we test our approach using a synthetic 3d model of salt tectonics with erosion and sedimentation and check how much noise conditions model resolution and sparse data coverage affect the resolving power of the method results show that it is possible to constrain the effective viscosity and density of layers that contribute to the large scale dynamics provided that those layers are numerically well resolved the properties of thin layers that do not contribute much to the overall dynamics cannot be constrained but noise or sparse data sampling does not significantly affect the inversion results this thus illustrates that a joint geodynamic and gravity inversion is a potentially powerful method to constrain the dynamics of the crust and lithosphere having better constraints on the structure of the present day crust and lithosphere will help to narrow the parameter space for models that aim to unravel lithosphere dynamics on a geological time scale

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

modification of the lithospheric stress field by lateral variations in plate mantle coupling 1 the presence of deeply penetrating continental roots may locally increase the magnitude of basal shear tractions by up to a factor of 4 compared to a layered viscosity structure here we examine how these increases in mantle lithosphere coupling influence stress patterns in the overlying elastic lithosphere by coupling a mantle flow model to a model for the elastic lithosphere we show that the amplification of mantle tractions beneath cratons increases elastic stress magnitudes by at most a factor of only 1 5 in a pattern not correlated to local basal traction changes this disconnect is explained by the transmission of elastic stresses across large distances which makes them sensitive to regionally averaged changes in basal tractions but not local variations our results highlight the importance of regional variations in lithospheric strength which could allow stress patterns to more closely match regional changes in basal shear

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

relevance of a lithogeochemical database to epidemiological studies in central saskatchewan canada abstract archived drill cuttings of cretaceous sedimentary rocks from 96 wells were analyzed to provide a geochemical database and maps of a 100 000 km 2 area of bedrock in central saskatchewan a dark shale the white speckled shale is enriched in ca cd mo se u and v and to a lesser degree in as cu ni sb sr and zn when compared to other geological strata in the district this shale subcrops beneath glacial drift near henribourg where there is a coincidence of structural weakness and formation water discharge because groundwater chemistry is controlled mostly by rock and sediment chemistry the possibility was examined that the enrichment of some elements in the white speckled shale may have a bearing on the high incidence of childhood homes of multiple sclerosis patients at henribourg the lithogeochemical study provides basic data that may be of use in epidemiological studies

redistribution of recent collision push and ridge push in central europe insights from fem modelling summary 2 d elastic finite element models of the recent stress field of central europe are built to evaluate the loads exerted on the continental boundary and the magnitude of tectonic stresses within the continental part of the plate the models comprise 24 tectonic blocks their stiffness is either constant throughout the model or varies from block to block 16 fault zones and 12 geologically significant boundary segments we have obtained a relatively unique balance of external tectonic forces by 1 careful adjustment of calculated stress directions and regimes to complex pattern of stress from data and 2 by calibration with gravitational potential energy a high level of compression ca 9 10 12 nm 1 exerted to the short ionian side of the adriatic indenter is crucial for the stress field pattern in central europe the adria microplate rotates due to eccentricity between the africa push from the south and the alpine buttress to the north a free boundary of the apennines does not contribute significantly to this motion kinematics of this indenter is controlled by friction on the dinaric suture which in turn is decisive for strain energy distribution between the alpine and the pannonian domains the predicted pronounced extension in the greece aegean segment 2 5 10 12 nm 1 implies active pull transferred from the hellenic subduction zone this extension releases stress in the balkan pannonian region and enables the eastward escape of tectonic blocks in front of advancing adria significant changes of tectonic push trends are found along the black sea caucasus boundary segment and at the european passive margin from the north sea to the arctic ocean differential stresses in central europe are estimated in the range of 10 60 mpa when averaged over the 30 80 km thick mechanically heterogeneous lithosphere the maximum stiffness contrast across the model is predicted to be of one order of magnitude apparent friction coefficients of fault zones differ between the north european part of the plate 0 4 0 7 the pannonian region 0 15 0 25 and the dinaric suture 0 55

african stress pattern from formal inversion of focal mechanism data abstract the kinematic models and the associated orientation of extensional stress of the east african rift system have been subjected to much debate since a long time in the past decades the proposed models relied on the interpretation of the overall rift geometry geological fault slip data and the few focal mechanisms available these models generally suffer of a poor time control and an underestimation of the possible changes in the stress field and geodynamic regime with time and space in the recent years there has been a significant increase in the number of focal mechanisms available for the entire rift system and it is now possible to estimate the present day stress field in relative detail based on seismotectonic data alone we compile 347 focal mechanism data from the global harvard cmt catalogue and various other sources and grouped 332 of them in 24 distinct regions boxes on the basis of their geographical proximity kinematic homogeneity and tectonic setting for each box and for the same data set reduced stress tensors have been obtained by formal stress inversion using both the tensor program delvaux sperner 2003 and the slick method michael 1984 1987 both inversion methods show in comparable results in terms of horizontal stress axes orientations and tectonic stress regimes which stem for the robustness of the approach the obtained stress pattern reflects a complex interaction between 1st order effects as different driving forces including plate boundary forces and 2nd and 3rd order effects as gravitational potential of topography intra lithospheric processes and the influence of structural heterogeneities of the rift structures the evidence present in the stress orientations of the 2nd and 3rd order stress pattern as the variations in the horizontal stress axes along the axis of the rift are of particular interest as they were not yet captured or shown in earlier numerical models of the stress field additional sources of tectonic forces in supplement to the gravitational potential energy forces as considered in the earlier models are necessary to explain the observed patterns

topography boundary forces and the indo australian intraplate stress field the relative contribution of topographic e g ridge push continental margins and elevated continental crust and plate boundary e g subduction and collisional forces to the intraplate stress field in the indo australian plate iap is evaluated through a finite element analysis two important aspects of the iap intraplate stress field are highlighted in the present study 1 if substantial focusing of the ridge push torque occurs along the collisional boundaries i e himalaya new guinea and new zealand many of the first order features of the observed stress field can be explained without appealing to either subduction or basal drag forces and 2 it is possible to fit the observed shmax maximum horizontal stress orientation and stress regime information with a set of boundary conditions that results in low tectonic stress magnitudes e g tens of megapascals averaged over the thickness of the lithosphere throughout the plate this study therefore presents a plausible alternative to previous studies of the iap intraplate stress field which predicted very large tectonic stress magnitudes hundreds of megapascals in some parts of the plate in addition topographic forces due to continental margins and elevated continental material were found to play an important role in the predicted stress fields of continental india and australia and the inclusion of these forces in the modeling produced a significant improvement in the fit of the predicted intraplate stresses to the available observed stress information in these continental regions a central focus of this study is the relative importance of the boundary conditions used to represent forces acting along the northern plate margin we note that a wide range of boundary conditions can be configured to match the large portion of the observed intraplate stress field and this nonuniqueness continues to make modeling the iap stress field problematic while our study is an important step forward in understanding the sources of the iap intraplate stress field a more complete understanding awaits a better understanding of the relative magnitude of the boundary forces acting along the northern plate margin

role of mantle flow in nubia somalia plate divergence present day continental extension along the east african rift system ears has often been attributed to diverging sublithospheric mantle flow associated with the african superplume this implies a degree of viscous coupling between mantle and lithosphere that remains poorly constrained recent advances in estimating present day opening rates along the ears from geodesy offer an opportunity to address this issue with geodynamic modeling of the mantle lithosphere system here we use numerical models of the global mantle plates coupled system to test the role of present day mantle flow in nubia somalia plate divergence across the ears the scenario yielding the best fit to geodetic observations is one where torques associated with gradients of gravitational potential energy stored in the african highlands are resisted by weak continental faults and mantle basal drag these results suggest that shear tractions from diverging mantle flow play a minor role in present day nubia somalia divergence

the african plate a history of oceanic crust accretion and subduction since the jurassic we present a model for the jurassic to present evolution of plate boundaries and oceanic crust of the african plate based on updated interpretation of magnetic gravity and other geological and geophysical data sets location of continent ocean boundaries and age and geometry of old oceanic crust jurassic and cretaceous are updated in the light of new data and models of passive margin formation a new set of oceanic palaeo age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and can be used as input for quantifying the plate boundary forces that contributed to the african plate palaeo stresses and may have influenced the evolution of intracontinental sedimentary basins as a case study we compute a simple model of palaeo stress for the late cretaceous time in order to assess how ridge push slab pull and horizontal mantle drag might have influenced the continental african plate we note that the changes in length of various plate boundaries especially trenches do not correlate well with absolute plate motion but variations in the mean oceanic crust age seem to be reflected in acceleration or deceleration of the mean absolute plate velocity

plate motions and stresses from global dynamic models delineating the driving forces behind plate motions is important for understanding the processes that have shaped earth throughout its history however the accurate prediction of plate motions boundary zone deformation rigidity and stresses remains a difficult frontier in numerical modeling we present a global dynamic model that produces a good fit to such parameters by accounting for lateral viscosity variations in the top 200 kilometers of earth together with forces associated with topography and lithosphere structure as well as coupling with mantle flow the relative importance of shallow structure versus deeper mantle flow varies over earth s surface our model reveals where mantle flow contributes toward driving or resisting plate motions furthermore subducted slabs need not act as strong stress guides to satisfy global observations of plate motions and stress

new extended thin sheet approximation for geodynamic applications ii two dimensional examples the potential of the new extended thin sheet approximation etsa has been investigated by application to a representative range of 2 d problems the system of governing equations presented by medv

large scale lithospheric stress field and topography induced by global mantle circulation stresses in the lithosphere are one indication of processes in the earth interior here we present a calculation of largescale lithospheric stresses caused by global mantle circulation the mantle flow field is calculated based on density structures inferred from global seismic tomography predicted principal stress directions are compared to interpolations based on observed stresses agreement between predictions and observations is often good in regions where lithospheric stresses and mantle tomography are well constrained predicted magnitudes of scalar stress anomalies vary more strongly than predicted stress directions for various tomographic models hotspots preferentially occur in regions where calculated stress anomalies are tensile or slightly compressive results do not strongly depend on radial mantle viscosity structure lithospheric rheology viscous or elastic or plate motion model the model also predicts the directions of motion well for most plates misfits in the predicted magnitudes can be explained qualitatively stress anomalies due to causes within the lithosphere oceanic cooling with age variations in crustal thickness topography isostatically compensated at subcrustal levels are also computed predicted stress directions in the absence of mantle flow can explain observations almost as well as mantle flow nevertheless current models of mantle flow are largely in accord with interpolations of observed principal stress directions and the observed plate motions fl 2001 elsevier science b v all rights reserved

cenozoic dynamics of the african plate with emphasis on the africa eurasia collision we investigate the forces acting on the african plate from about 56 ma to the present day we aim 1 to establish a basis for quantification of the overall dynamics of the mediterranean middle east region and 2 to understand how africa reacts to temporal variation in the forces associated with the africa eurasia collision we use the assumption of dynamical equilibrium which implies that at any given time the net torque of forces on africa s northern margin must have been balanced by the other torques acting on the plate moreover temporal variation in the forces on africa s northern margin must have been balanced by contemporaneous changes in one or more of the other torques by quantifying those other torques we may derive a constraint on the forces at the northern margin we expect the africa eurasia collision to have been associated with changes in the associated forces and examine how these changes are balanced using alternative parameterizations we calculate the net torque of ridge push transform fault resistance and basal drag we also quantify changes in the torque of forces associated with the continent ocean transition the results constitute a framework for detailed dynamic models of the africa eurasia convergence zone temporal changes in the ridge push torque prove to be small the torque of transform resistance does change but these changes do not relate to forces at the northern margin evidence for a relation with collision is found only for the case of basal drag the results point to a mechanism where the plate maintains its dynamical equilibrium mainly through collision induced changes in absolute motion

the origins of the intraplate stress field in continental australia the ridge push force acting on the indo australian plate exerts a significant torque 8 5 1025n m about a pole at 30 3 n 34 5 e the angular difference between this torque pole and the observed pole of rotation for the plate 19 2 n 35 6 e is less than 12 and suggests that the ridge push force plays an important role in the dynamics of the indo australian plate we have used an elastic finite element analysis to study the predicted intraplate stress field in continental australia for four models which employ different boundary conditions to balance the ridge push torque acting on the plate the modeling indicates that a number of important features of the observed stress field within the australian continent can be explained in terms of balancing the ridge push torque with resistance imposed along the himalaya papua new guinea and new zealand collisional boundaries segments these features include n s to ne sw oriented compression in the northern australia and e w oriented compression in southern australia our analysis also shows that subduction processes along the northern and eastern boundaries provide only second order controls on the intraplate stress field in continental australia