ray tracing directly in a 3 d ellipsoidal earth model for multiphase global arrivals seismic ray tracing methods are widely used in seismology with most such algorithms being executed in cartesian or spherical coordinate systems however the actual earth is not a perfect sphere but rather an ellipsoid meaning that results calculated in spherical coordinates may be different from the actual situation the general approach is to first calculate traveltimes in a spherical earth model then apply ellipsoidal time corrections to obtain more accurate traveltime data alternatively one may directly conduct the ray tracing in the ellipsoidal earth model without any time corrections in this paper we extend the functional of the multistage irregular shortest path method previously formulated for a spherical earth model to an ellipsoidal earth model in order to trace multiphase global seismic arrivals the results of two models indicate that the proposed algorithm has high computational accuracy which can be further tuned by decreasing the secondary node spacing comparison tests indicate that the traveltime differences between the ellipsoidal and spherical coordinate ray tracing methods cannot be ignored for direct p and s arrivals reflected pcp and scs arrivals and reflected and converted pcs and scp arrivals the traveltime differences te ts computed by the ellipsoidal and spherical ray tracing methods have different distribution patterns being dependent on the source locations however in general these traveltime differences te ts have relatively large negative values near the polar region and positive values near the equatorial region except for sources located near the polar region for ellipsoidal time correction in a specific case the maximum differences between the traveltimes computed by the ellipsoidal coordinate ray tracing method and the ak135 traveltime table after application of ellipsoidal time corrections are less than 0 1 s meanwhile the maximum differences between the traveltimes predicted by the ellipsoidal coordinate ray tracing method and by the spherical coordinate ray tracing method after application of ellipsoidal time corrections are only 0 075 s for the six stated phases these results indicating that ray tracing could be conducted directly in the ellipsoidal earth model

gravity anomalies and flexure of the brazilian shield beneath the bolivian andes abstract seismological and geological data suggest that the brazilian shield is now underthrusting beneath the sub andes in order to constrain the minimum amount of shortening that results from this underthrusting we analyzed bouguer gravity anomalies over the shield and the bolivian andes assuming that they are due primarily to variations in crustal thickness that in turn are due to the flexure of an effectively elastic plate brazilian shield loaded by the weight of the andean mountain range gravity anomalies show that airy type isostatic equilibrium does not exist the sub andes are overcompensated and the chaco plain to the east is undercompensated the flexure of the plate can help explain both the overcompensation and the undercompensation if the brazilian shield behaves as an essentially elastic plate that extends at least 150 km and probably 200 km beneath the sub andes and the cordillera oriental and if the flexural rigidity of the plate is between 0 1 and 2 10 24 n m the fit of observed and calculated anomalies however is far from perfect and additional sources of short wavelength anomalies appear to be present assuming that the sediments and sedimentary rocks in the sub andes and cordillera oriental were deposited on the brazilian shield and that later they were overthrust eastward onto one another causing the present topography and the deflection of the plate we computed a family of estimates for the minimum amount of crustal shortening these estimates depend strongly on the amount of sedimentary rock deposited before deformation which does not seem to be well constrained by published geologic mapping nevertheless the calculations suggest that at least 100 km of crustal shortening have occurred and probably more implying that the andes may have been built mostly by crustal shortening

tectonic evolution of a continental collision zone a thermomechanical numerical model we model evolution of a continent continent collision and draw some parallels with the tectonic evolution of the himalaya we use a large scale visco plasto elastic thermomechanical model that has a free upper surface accounts for erosion and deposition and allows for all modes of lithospheric deformation for quartz olivine rheology and 60 mm yr convergence rate the continental subduction is stable and the model predicts three distinct phases during the phase 1 120 km or 6 of shortening deformation is characterized by back thrusting around the suture zone some amount of delaminated lower crust accumulates at depth during phase 2 120 km 420 km or 6 22 of shortening this crustal root is exhumed medium to high grade rocks along a newly formed major thrust fault this stage bears similarities with the period of coeval activity of the main central thrust and of the south tibetan detachment between 20 16 myr ago during phase 3 420 km or 22 of shortening the crust is scraped off from the mantle lithosphere and is incorporated into large crustal wedge deformation is localized around frontal thrust faults this kinematics should produce only low to medium grade exhumation this stage might be compared with the tectonics that has prevailed in the himalaya over the last 15 myr allowing for the formation of the lesser himalaya the experiment is conducted at constant convergence rate which implies increasing compressive force considering that this force is constant in nature this result may be equivalent to a slowing down of the convergence rate as was observed during the india asia collision

keeping that youthful look sean c solomonthe record of impact cratering onvenus recently revealed from high resolution radar imaging by the magel lan spacecraft has been interpreted asindicating that the planet underwenteatastrophic global resurfadng about 500million years ago although other inter pretatiom of the crater characteristicshave been suggested2 the possibility ofgeologically rapid global resurfacing onthe planet that most resemblesour own interms of mass density and bulk compo sition has generated widespread interest what mechanisms could cause such acatastrophe several ideas were discus sed at the fall meeting of the americangeophysical union last month the average age of venus s surfacehas been estimated from the number oflarger impact craters the planet s thickatmosphere hastended to protect it fromthe smaller bodies that would give cra ters smaller than 30 km in diameter andfrom the cratering rate on earth and themoon or from estimates of the numberof asteroids whose orbits cross that ofvenus the answer 500 million years z 2 a tenth of the age of the solar system is not in itself remarkable for a planetwhich like the earth has a hot dyna mic inter or despite the absence ofsignificant erosion what is remarkableis that the craters at all sizes are indis tinguishable from a random population that is spatial variations in craterdensity are not larger than those due tochance and almost none has been signi ficantly modified by tectonic strain or by12volcanic flows beyond the crater nm fig 1 even though magellan imagesshow volcanic and tectonic features to benearly ubiquitous on venus the interpretation of sehaber andfig 1 magellanradar image ofan impactcrater onvenus approximately70 km in diameter radar bright rough texturedejecta extendingup to 60 km fromthe crater rimand the remarkablebright flow features extending morethan 300 kmfromthe craterwallsare thoughtto datefromthe impactz and have not been modifiedto anysignificantdegreebylaterdeformationor volcanismoutsidethe crater althoughthe generally radar dark smooth floor of thecrater maycontainyoungervolcanicdeposits 114othersi is that most of the surface datesfrom the end of a global resurfacingevent that ceased about so0 million yearsago and that the small fraction of cra ters volcanically embayed or modified bydeformation indicate that volcanic andtectonic activity has since been at muchlower levels phillips and colleagues have argued that a correlation of loca tions of modified craters with areas oflow crater density and an inverse cor relation between crater density andmagellan radar backscatter a quantityelevated in regions of high topographyand high roughness both thought to besignatures of comparative geologicalyouth indicate that the venus surfaceexhibits a spectrum of ages the paucityof small craters however hampers theuse of crater density in determining therelative ages of geological units amethod used with the other solid planetsand satellites phillips et al have de scribed an alternative model in whichresurfacing occurs episodically in smallpatches a few hundred kilometres inextent with a characteristic time be tween events of the order of i0s years fig 2 nonetheless the catastrophic hypoth esis and its possible mechanisms havecaptured the lion s share of the atten tion at the meeting d l turcotte cornell university proposed thatglobal lithospheric overturn operatesepisodically on venus and that for thepast 500 million years the lithosphere hasbeen cooling and mechanically stable although the mechanism for stabiliza tion of the lithosphere is not an explicitcomponent of turcotte s scheme heargues that a cool and thick lithosphereis in better agreement than one insteady state conductiveequilibrium with the largevalues obtained fromtopographic profiles for therigidity of the lithosphereat sites of presumed litho spheric bending 4 and withthe large ratio oflong wavelength gravityanomaly to long wave length topographic relief athighland areas parmentier and hess 6 following stevenson andbittker 7 have recentlysuggested that lithosphericstabilization may occur onvenus because of a de crease in the density oflithospheric mantle follow ing melt extraction butthey add that the coolingpmmgl ng page blank not filmed

late cenozoic uplift of mountain ranges and global climate change chicken or egg the high altitudes of most mountain ranges have commonly been ascribed to late cenozoic uplift without reference to when crustal thickening and other tectonic processes occurred deep incision and recent denudation of these mountain ranges abundant late cenozoic coarse sediment near them and palaeobotanical evidence for warmer climates where high mountain climates today are relatively cold have traditionally been interpreted as evidence for recent uplift an alternative cause of these phenomena is late cenozoic global climate change towards lower temperatures increased alpine glaciation a stormier climate and perturbations to humidity vegetative cover and precipitation

flexural ridges trenches and outer rises around coronae on venus high resolution altimetry collected by the magellan spacecraft reveals trench and outer rise topographic signatures around major coronae e g eithinoha heng o artemis and latona in addition magellan synthetic aperature radar images show circumferential fractures in areas where the plates are curved downward both observations suggest that the lithosphere around coronae is flexed downward by the weight of the overriding coronal rim or by the negative buoyancy of subducted lithosphere we have modelled the trench and outer rise topography as a thin elastic plate subjected to a line load and bending moment beneath the corona rim the approach was tested at northern freyja montes where the best fit elastic thickness is 18 km in agreement with previously published results the elastic thicknesses determined by modelling numerous profiles at eithinoha heng o artemis and latona are 15 40 37 and 35 km respectively at eithinoha artemis and latona where the plates appear to be yielding the maximum bending moments and elastic thicknesses are similar to those found at the middle america mariana and aleutian trenches on earth respectively estimates of effective elastic thickness and plate curvature are used with a yield strength envelope model of the lithosphere to estimate lithospheric temperature gradients at heng o artemis and latona temperature gradients are less than 10 k km which correspond to conductive heat losses of less than one half the expected average planetary value we propose two scenarios for the creation of the ridge trench and outer rise topography differential thermal subsidence and lithospheric subduction the topography of heng o is well matched by the differential thermal subsidence model however at artemis and latona the amplitudes of the trench and outer rise signatures are a factor of 5 too large to be explained by thermal subsidence alone in these cases we favor the lithospheric subduction model wherein the lithosphere outboard of the corona perimeter subducts rolls back and the corona diameter increases

lithospheric flexure beneath the freyja montes foredeep venus constraints on lithospheric thermal gradient and heat flow analysis of venera 15 and 16 radar images and topographic data from the freyja montes region on venus suggest that this mountain belt formed as a result of a sequence of underthrusts of the lithosphere of the north polar plains beneath the highlands of ishtar terra the freyja montes deformation zone consists south to north of a linear orogenic belt an adjacent plateau a steep scarp separating the plateau from the north polar plains a linear depression at the base of the scarp and an outer rise the topographic profile of the depression and outer rise are remarkably similar to that of a foreland deep and rise formed by the flexure of the under thrusting plate beneath a terrestrial mountain range we test the lithospheric flexure hypothesis and we estimate the effective thickness t e of the elastic lithosphere of the under thrusting portion of the north polar plains by fitting individual topographic profiles to deflection curves for a broken elastic plate the theoretical curves fit the observed topographic profiles to within measurement error for values of flexural rigidity d in the range 0 8 3 x 1022 n m equivalent to t e in the range 11 18 km under the assumption that the base of the mechanical lithosphere is limited by the creep strength of olivine the mean lithospheric thermal gradient is 14 23 k km the portion of the north polar plains to the immediate north of freyja montes stands at a regional elevation very near the modal elevation for the planet so the inferred elastic itho sphere thickness and thermal gradient may be at least approxi mately representative of a significant fraction of the venus lithosphere that the inferred thermal gradient is similar to the value expected for the global mean gradient on the basis of scaling from earth provides support for the hypothesis that simple conduction dominates lithospheric heat transport on venus relative to lithospheric recycling and volcanism

modeling of large scale accretionary wedge deformation instantaneous stress and velocity fields within an active accretionary wedge can be calculated using plastic slip line theory given the assumptions of a perfectly plastic rheology and plane strain conditions after selecting a plastic yield stress and an average density a stress field slip line field can be computed for a wedge of known cross sectional configuration from known stress boundary conditions on the upper surface basal surface tractions are derived without basal boundary conditions the geometry of the computed slip line field can then be used to constrain the lower limit of the yield stress the known velocity boundary conditions are insufficient for calculation of a velocity field because the coupling between the wedge and the subducting plate is unknown consequently basal velocity distributions must be assumed to permit the construction of velocity fields these assumed distributions are constrained by known rates of surface uplift and rates of tilt as well as inferred patterns of instantaneous strain rates within a wedge the sunda accretionary wedge west of central sumatra is used to illustrate the plasticity approach a minimum average yield stress of 20 30 mpa and a weaker basal layer of variable effective strength are indicated for this example surface uplift rates may be affected significantly by regions of high basal strain rate finite velocity discontinuities and underplating the known pattern of uplift rates near nias island located on the outer arc ridge is consistent with the constraint that 90 of the total incremental shortening of the wedge is concentrated within 20 km of the trench given a sufficient number of uplift and tilt rate observations across a particular wedge the relative influence of underplating tectonic erosion and basal strain rate variations could be assessed using the perfectly plastic model

determination of the elemental composition of rocks on venus by venera 13 and venera 14 preliminary results venera 13 and venera 14 have determined for the first time ever the elemental composition of the venusian rocks at the probes landing sites to study venusian rocks rolling upland and flat lowland have been chosen as the most typical of the entire planetary surface venera 13 landed on the rolling upland while venera 14 landed in the region with lower topography the chemical composition of the rock at the venera 13 landing site proved to be similar to the composition of potassium alkaline basalts of the earth s crust rocks which can be found on oceanic islands and in rift zones the chemical composition of the rock at the venera 14 landing site is similar to that of tholeiitic basalts of the oceanic crust of the earth the comparison of new data on the composition of venusian rocks with the composition of the rocks of similar provinces on the earth and the moon shows that the evolution of the surface and crust of venus was not similar to that of the earth and especially not that of the moon

paleogeography and andean structural geometry northwest argentina neogene tectonic segmentation of the andean cordillera in northwestern argentina is a function of both the dip of the subducted plate and of major paleogeographic structures and sedimentary basins vergence fault dip and lateral continuity of neogene structures can be related to the character and pre existing geometries and structures of the materials in which they form south of 27 s the thick skinned sierras pampeanas which occur mostly over a flat segment of the subducted nazca plate formed where little sedimentary cover overlay crystalline basement north of 24 s the thin skinned subandean belt overlying an inclined slab segment formed within a thick elongate eastward tapering trough of paleozoic and mesozoic sediments that may represent a complex of pre andean foreland basin deposits the transitional region between 24 27 s the santa barbara system and cordillera oriental is characterized paleogeographically by complex cross trending structures and irregular basin geometries and the neogene structures are equally complex and variable the paleogeographic features of nw argentina are a product of compressional tectonism during much of the paleozoic that resulted in an orogenic belt oriented obliquely to the modern andes this earlier orogen was disrupted by several zones of transverse structures and by cretaceous extensional or pull apart basins these features clearly influence the geometry of the superposed andean structures more intriguing however is the possibility that preexisting structures of the continental lithosphere may in some way help to modify the geometry of the subducted plate

problems in the extrapolation of laboratory rheological data abstract the many types of variables and deformation regimes that need to be taken into account in extrapolating rheological behaviour from the laboratory to the earth are reviewed the problems of extrapolation are then illustrated with two particular cases in the case of divine rich rocks recent experimental work indicates that within present uncertainties of extrapolation the flow in the upper mantle could be either grain size dependent and near newtonian or grain size independent and distinctly non newtonian both types of behaviour would be influenced by the present of trace amounts of water in the case of quartz rich rocks the uncertainties are even greater and it is still premature to attempt any extrapolation to geological conditions except as an upper bound the fugacity and the scale of dispersion of the water are probably two important variables but the quantitative laws governing their influence are not yet clear

venus global gravity and topography a new gravity field determination that has been produced combines both the pioneer venus orbiter pvo and the magellan doppler radio data comparisons between this estimate a spherical harmonic model of degree and order 21 and previous models show that significant improvements have been made results are displayed as gravity contours overlaying a topographic map we also calculate a new spherical harmonic model of topography based on magellan altimetry with pvo altimetry included where gaps exist in the magellan data this model is also of degree and order 21 so in conjunction with the gravity model bouguer and isostatic anomaly maps can be produced these results are very consistent with previous results but reveal more spatial resolution in the higher latitudes

critical taper model of fold and thrust belts and accretionary wedges the fold and thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the earth s upper crust although there is considerable natural variation among the many fold and thrust belts and accretionary wedges that have been recognized and explored several features appear to be universal in cross section fold and thrust belts and accretionary wedges occupy a wedge shaped deformed region overlying a basal detachment or decollement fault the rocks or sediments beneath this fault show very little deformation the decollement fault characteristically dips toward the interior of the mountain belt or in the case of a submarine wedge toward the island arc the topography in contrast slopes toward the toe or deformation front of the wedge deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault bend folding two north american fold and thrust belts that exhibit these features are shown in figure 1 neither of these two examples is tectonically active today the southern canadian fold and thrust belt was active during the late jurassic and cretaceous 150 100 ma whereas the southern appa lachians were deformed during the late carboniferous to permian alle ghenian orogeny 300 250 ma figure 2 shows two examples that are currently active the taiwan fold and thrust belt produced by the sub duction of the eurasian plate beneath the philippine sea plate suppe 1981 1987 and the barbados accretionary wedge produced by the sub

how flat is tibet high resolution digital topography three arc second grid for most of tibet provides new information to characterize the relief of the highest and largest plateau on earth the arid to semiarid central and northern part of the plateau interior has low relief average slopes of over 250 m windows and a mean elevation of 5023 m above sea level at moderate wavelengths of m relief is or less for most of tibet as opposed to the much higher relief of up to 6 km on the plateau edges where glacial and fluvial dissection is greater because of higher levels of precipitation the only faults manifesting significant topographic relief are the relatively small scale generally north trending graben systems primarily in southern tibet and several large scale fault systems near the edges of tibet the flatness of tibet implies that 1 there has been little deformation especially shortening of the uppermost crust north of the graben systems during the late cenozoic and 2 shallow crustal isostatic compensation has been acting to level the surface of the plateau

kinematics topography shortening and extrusion in the india eurasia collision we examine the problem of partitioning between shortening and extrusion in the india asia collision since 45 ma we compute the amount of shortening expected from the kinematics of india s motion with respect to eurasia using the reconstruction at collision time to put bounds on the possible amounts of surface loss within greater india and within eurasia we then compute the amounts of surface loss corresponding to the thickened crust of tibet and of the himalayas assuming conservation of continental crust the spatial distribution of the topography reveals a large systematic deficit of crustal thickening distributed rather uniformly west of the eastern syntaxis but an excess of shortening east of it this distribution indicates an important eastward crustal mass transfer however the excess mass east of the eastern syntaxis does not account for more than one third to one half of the deficit west of the eastern syntaxis the deficit may be accounted either by loss of lower crust into the mantle for example through massive eclogitization or by lateral extrusion of nonthickened crust a mass budget of the crust of the himalayas indicates that lower crust has not been conserved there but the deficit is so large that local loss in the mantle is unlikely to be the unique cause of the deficit alternatively following zhao and morgan 1985 lower crust may have been transferred below the tibetan crust we conclude that a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion has to account for a minimum of one third and a maximum of one half of the total amount of shortening between india and asia since 45 ma this conclusion leaves open the possibility that the partitioning between extrusion and loss of lower crust into the mantle on the one hand and shortening on the other hand has significantly changed during the 45 m y history of the collision

uplift of the central andean plateau and bending of the bolivian orocline the topography of the central andes can be considered the primary tectonic signal of late cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation the spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data a color coded image of digitized topography between 12 s and 37 s highlights the altiplano puna one of the world s most remarkable plateaus and reveals important physiographic clues about the formation of that major feature the topographic data combined with information on structure magmatism seismicity and paleomagnetism support a simple kinematical model for the late cenozoic evolution of the central andes the model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning the model interrelates andean uplift a changing geometry of the subducted nazca plate and a changing outline in map view of the leading edge of the south american plate crustal shortening has accommodated convergence between the chilean peruvian forearc and the south american foreland the altiplano puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping 20 30 subducted plate the seawardly concave bend of western south america the bolivian orocline was enhanced but not completely produced by an along strike variation in the amount of late cenozoic shortening maximum shortening in bolivia both produced the widest part of the plateau and increased the seaward concavity of the bolivian orocline the along strike variations of shortening are hypothesized to result from corresponding along strike variations in the width of a weakened zone in the overriding plate weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates hence the width of the zone of weakening depends upon the dip of the subducted plate two types of shortening are recognized 1 a widespread basin and range laramide like shortening that characterizes modem activity in the sierras pampeanas and late miocene deformation of the altiplano puna and 2 on the eastern side of the cordilleras and plateau an east verging foreland fold thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust the second type of shortening can be applied to plio quaternary deformations throughout the central andes but with a substantial narrowing of the region of plateau uplift in peru and south of 28 s a proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central andes the monocline is located above the tip of the asthenospheric wedge between the converging plates it is postulated to occur above the western limit of lower crustal thickening in the regions of horizontal subduction the monocline can be associated with a late miocene asthenospheric wedge tip

thermal effects of sediment thickening and fluid expulsion in accretionary prisms model and parameter analysis we investigate the thermal consequences of sediment thickening and fluid expulsion in subduction zone accretionary prisms using a model where the rate of fluid expulsion in a uniformly thickening wedge is obtained analytically and the fluid flow and temperature fields are computed numerically variations landward across the wedge in the porosity depth function are included in the model the most important contribution to the thermal regime arises from the thickening of the wedge which can reduce the heat flow at the seafloor relative to the deep lithospheric heat flow by up to a few tens of percent this effect is countered by an opposite but lesser contribution of the heat advected upward by the fluid expelled from the reconsolidating sediments the total thermal perturbation is sensitive to the prism taper angle the incoming sediment thickness and the convergence velocity anomalously low seismic velocities observed in a zone several tens of kilometers wide near the toe of some accretionary prisms suggest that there is a delay between the initial thickening of the sediment section and reconsolidation toward equilibrium porosity at depth the thermal consequences of retarded fluid expulsion are significant and result in locally depressed seafloor heat flow in this region for the full observed range of accretion parameters the rates of fluid expulsion are not sufficient to cause a detectable depth variation in heat flow on a depth scale of less than 1 km

rare helical spheroidal fossils from the doushantuo lagerstatte ediacaran animal embryos come of age a small quantity of helically coiled spheroidal fossils has been recovered from acid digestion of phosphorite samples from the ediacaran doushantuo formation south china these fossils consist of an internal body enclosed in a sculptured envelope that is very similar to that of doushantuo animal eggs and blastula embryos such as megasphaera ornata a hallmark of these fossils is a three dimensional spiral structure which always consists of three clockwise coils and occurs on both the envelope and the internal body the spiral structure consists of a spiral tunnel or canal flanked by two raised levees and it is punctured by a series of holes some specimens show evidence of uncoiling invagination along the spiral structure or bipectinate furrowing on the band between canals a possible ontogenetic link between these helical spheroidal fossils and megasphaera ornata is suggested by similar size similar envelope sculptures and co occurrence we tentatively interpret these fossils as postblastula embryos related to megasphaera ornata thus they may represent the most advanced embryonic fossils so far known from the ediacaran although their adult morphologies and phylogenetic affinity remain unknown

a fluid model for the shape of accretionary wedges in this paper the accretionary wedges landward of most oceanic trenches are modelled assuming a newtonian fluid rheology it is shown that lubrication theory is applicable and that the addition of material to the wedges can be neglected in determining the shape of the wedges the predicted shape of the wedges is in good agreement with bathymetric profiles across the kurile ryukyu and aleutian wedges from the shape of the above wedges it is deduced that accretionary sediments have viscosities between 1018 and 1019 poise

formation of mountain belts on venus evidence for large scale convergence underthrusting and crustal imbrication in freyja montes ishtar terra the freyja mottles venus deformation zone consists of a distinctive linear orogenic belt freyja montes an adjacent plateau a steep scarp separating the plateau from the north polar plains lying about 3 km below the plateau and a volcanically flooded foredeep and rise at the base of the scarp on the basis of mapping of radar images and interpretation of the geologic units and topography the freyja montes deformation zone is interpreted to be the site of large scale crustal convergence and imbrication involving the underthrusting of the north polar plains beneath ishtar terra this deformation is interpreted to have begun with north south regional compressional deformation causing flexure buckling and underthrusting of the crust and lithosphere of the north polar plains beneath the relatively higher proto ishtar lakshmi further convergence and underthrusting caused crustal thickening and uplift and created compressive stresses in the upper part of the underthrusting layer failure of this layer led to a new site of underthrusting outboard of the rise and a new detachment surface the detached layer and the wedge of volcanic flows in the intervening foredeep were further deformed and incorporated sutured onto the plateau and mountain range this crustal underthrusting and imbrication caused crustal thickening and corresponding crustal uplift the continuing process of convergence underthrusting detachment and imbrication produced large scale crustal thickening uplift and the formation of the plateau behind the asymmetrical front of freyja montes the present configuration of topography and units and the distinctive elevation of the scarp and freyja montes suggest that the process continues to operate at the present time

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

metamorphism transient mid crustal rheology strain localization and the exhumation of high grade metamorphic rocks 1 we present a series of three dimensional numerical models investigating the effects of metamorphic strengthening and weakening on the geodynamic evolution of convergent orogens that are constrained by observations from an exposed mid crustal section in the new england appalachians the natural mid crustal section records evidence for spatially and temporally variable mid crustal strength as a function of metamorphic grade during prograde polymetamorphism our models address changes in strain rate partitioning and topographic uplift as a function of strengthening weakening in the middle crust as well as the resultant changes in deformation kinematics and potential exhumation patterns of high grade metamorphic rock results suggest that strengthening leads to strain rate partitioning around the zone and suppressed topographic uplift rates whereas weakening leads to strain rate partitioning into the zone and enhanced topographic uplift rates deformation kinematics recorded in the orogen are also affected by strengthening weakening with complete reversals in shear sense occurring as a function of strengthening weakening without changes in plate boundary kinematics

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

lithosphere rheology a note of caution abstract it has become common practice to use laboratory determined low temperature frictional sliding data together with power law equations for steady state creep of rocks at high temperatures to construct inferred profiles of rock strength and mode of failure with depth in the lithosphere in some cases this may involve unwarranted extrapolation of rock mechanics behaviour beyond the region of its validity the form of transition from frictional behaviour to intracrystalline plastic flow in the earth is much more complex than is suggested by the above model within the regime of plastic flow substantial changes in flow strength and mode of failure whether flow becomes localized into shear zones may accompany microstructural changes developed over large strains when deformation is accompanied by metamorphic changes existing flow laws for rocks are likely to be wholly inapplicable until a clearer understanding has emerged of the full range of expression of the rheological behaviour of rocks existing models of lithosphere rheology should be treated with caution

effect of the brittle ductile transition on the topography of compressive mountain belts on earth and venus the coulomb critical taper model has been very successful in explaining the large scale topography of a number of terrestrial accretionary wedges however this model is limited to cases of purely brittle frictional deformation in this paper we extend the range of applicability of the critical taper model by explicitly including the effects of temperature dependent ductile deformation the new model includes temperature dependent power law flow an assumed velocity field and linear thermal gradients in the atmosphere and within the crust flexural isostasy is also incorporated so that the decollement geometry is computed as a response to the applied load of the wedge material we assume that ductile deformation within the decollement zone is controlled primarily by diffusion flow whereas ductile deformation within the wedge itself is controlled by dislocation creep the topographic profiles predicted by the model are very similar to those of a number of fold and thrust belts on both earth and venus a typical wedge profile includes three distinctive topographic regions a narrow taper toe where both the wedge and the decollement zone deform in a brittle frictional manner a region of relatively steep slope where the wedge base deforms ductilely and the decollement zone is still frictional and a flat plateau region where both the wedge base and the decollement zone are deforming by ductile flow we have applied the model to two fold and thrust belts on venus maxwell montes and uorsar rupes and to the andes on earth and we find good agreement between observed and predicted topography using reasonable parameter values the model accounts for the observed positive correlation between relief and elevation of venusian fold and thrust belts on the basis of different thermal environments at different elevations it is also able to explain the first order differences between terrestrial and venusian fold and thrust belts fundamentally this difference is due to a combination of the lower temperatures and the presence of water on earth

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

diffusion creep in feldspar aggregates experimental evidence a combination of mechanical and microstructural results for experimentally deformed albite aggregates of 2 10 m grain size indicates a regime of grain boundary diffusion creep that depends upon the presence of water temperature and strain rate at 900 c and 10 5 s 1 deformation occurs dominantly by recrystallization accommodated dislocation creep when the aggregates contain 0 2 wt water but by grain boundary diffusion creep when the aggregates contain 0 9 wt water experiments at other temperatures and strain rates indicate that fine grained aggregates with added water may go directly from cataclastic flow to grain boundary diffusion creep with increasing temperature or decreasing strain rate bypassing the field of dislocation creep microstructural evidence for grain boundary diffusion creep in the wet fine grained aggregates includes the development of rectangular grains rather than polygonal grains grain growth a very low average dislocation density open grain boundaries including pores and channels and the presence of overgrowths of a different composition on grains in a two plagioclase an1 and an79 aggregate additional evidence for diffusion creep in the wet fine grained aggregates is indicated by their similar microstructures and strengths when deformed at 600 and 1500 mpa in contrast dry feldspar aggregates deforming by dislocation creep show an increase in strength and a transition to semi brittle behavior at lower confining pressures grain boundary diffusion creep of feldspars may be common in naturally deformed rocks due to the ease of grain size reduction by both cataclasis and dynamic recrystallization

the effects of brittle plastic transitions in basement involved foreland belts the central southern alps case n italy abstract the central southern alps csa a 100 km wide basement involved fold and thrust belt formed during the europe africa convergence from the cretaceous onward two main phases of deformation have been recognized in the central southern alps the first is a pre late oligocene thrusting of pre alpine basement units upon triassic cover sequences which is sealed by the adamello intrusion 42 30 ma in the northern sector of the belt the folds and thrust faults of this phase are locally overprinted by a post adamello deformation phase possibly corresponding to the middle late miocene 16 6 ma milano phase recorded by the oligo miocene sediments buried under the plio pleistocene padane plain several lines of evidence indicate that compressive deformations originating from the current relative convergence between eurasian and adriatic plates are still occurring within the southalpine wedge which can therefore be modelled as a critical wedge a simple brittle rheology is not adequate to explain the slope and plateau topographic features of the belt which compare more favourably with those predicted by a critical taper model including the effects of temperature dependent deformations low angle surface slopes typical of frontal sectors of wedges deforming with a purely brittle behaviour are interrupted by relatively steeper slopes where the wedge base crosses the brittle plastic b p transition and starts creeping while the basal decollement deforming at higher strain rates still behaves frictionally further to the rear where both the wedge base and the decollement are plastic the surface slope flattens plateau calculated brittle plastic transition depths along the basal decollement fit well with observed topographic changes the location of slope to plateau topographic changes in the csa is strongly influenced by surface heat flow distribution and the occurrence within the wedge and in particular at the wedge base of rock types characterised by different plastic rheological parameters is responsible for repeated topographic changes each related to a different b p transition the transition from brittle to plastic behaviour at the base of the wedge is responsible for a significant weakening within the basal decollement which in turn has dramatic consequences on the dynamics and hence on the kinematics of the foreland fold and thrust belt the maintenance of the critical state of a wedge with a weak plastic basal decollement necessitates a lower amount of internal deformation than that required by completely brittle wedges moreover these deformations are strongly localised in the lower weak portions of the wedge and do not pervasively affect the upper brittle portions these characteristics have been ascertained in the rear part of the csa b p transitions commonly develop not only within wide mountain belts e g the andes but also in narrow ones e g csa this means that b p transitions must be taken into account when modelling the mechanics of foreland fold and thrust belts at all scales the occurrence of b p transitions must be tested particularly when topographic features suggest their existence since only an appropriate rheological behaviour can account for the structural evolution and kinematics derived from field studies

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

thick skinned tectonics and basement involved fold thrust belts insights from selected cenozoic orogens defining the structural style of fold thrust belts and understanding the controlling factors are necessary steps towards prediction of their long term and short term dynamics including seismic hazard and to assess their potential in terms of hydrocarbon exploration while the thin skinned structural style has long been a fashionable view for outer parts of orogens worldwide a wealth of new geological and geophysical studies has pointed out that a description in terms of thick skinned deformation is in many cases more appropriate this paper aims at providing a review of what we know about basement involved shortening in foreland fold thrust belts on the basis of the examination of selected cenozoic orogens after describing how structural interpretations of fold thrust belts have evolved through time this paper addresses how and the extent to which basement tectonics influence their geometry and their kinematics and emphasizes the key control exerted by lithosphere rheology including structural and thermal inheritance and local regional boundary conditions on the occurrence of thick skinned tectonics in the outer parts of orogens

critical taper wedge mechanics of fold and thrust belts on venus initial results from magellan fold and thrust belts exist on venus at the margins of crystal blocks such as plateaus tesserae and coronae and as ridge belts within the plains these fold belts display a number of key features that are consistent with their formation by critical taper wedge mechanics a mechanics that is well known for fold and thrust belts and accretionary wedges on earth for example an analysis of fold geometry at the toe of the artemis chasma fold belt indicates fault bend folding above a regionally extensive decollement horizon at a depth of about 1 5 km near surface deformation on venus is interpreted to be brittle and is anticipated to be dominated by cohesive strength in the upper 1 2 km critical taper wedge mechanics under anticipated venus conditions suggests that brittle wedges should have maximum surface slopes in the range 10 20 which is similar to some estimated slopes in the steep parts of the fold belts the low taper toes of fold belts may be cohesion dominated toes on either brittle or plastic decollement horizons once the base of the wedge undergoes the brittle plastic transition the surface slope is expected to flatten to near horizontal in qualitative agreement with many topographic profiles of fold and thrust belts on venus the estimated depth of the brittle plastic transition is uncertain based on rock mechanics data but is expected to be close enough to the surface to be affected by the atmospheric temperature gradient the relief of fold belts measured between the toe of the wedge and the flat crest displays a remarkable linear dependence on absolute elevation figure 15 ranging from 6 km for maxwell montes at an elevation of 10 km to a few hundred meters at the lowest planetary elevations 0 to 2 km this remarkable phenomenon appears to reflect an absolute elevation dependence of the depth of the brittle plastic transition possibly controlled by an isostatic coupling of elevation lithospheric thickness and geothermal gradient

the role of tectonic grain size reduction in the rheological stratification of the lithosphere with the aid of published constitutive flow laws for common crustal and upper mantle rock types it is possible to construct curves showing variation of rock strength with depth in the continental lithosphere assuming a particular crustal stratification of rock types and a particular geothermal gradient such exercises lead to the conclusion that marked changes in strength can occur at major petrological interfaces such as the moho discontinuity which are therefore likely to be associated with a tendency for major sub horizontal detachments to form constitutive flow laws used for such purposes are usually assumed to represent steady state flow at constant rock microstructure

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

the siwaliks of western nepal ii mechanics of the thrust wedge abstract comparison between numerical models and structural data is used for a better understanding of the evolution of the siwalik thrust belt of western nepal the numerical model involves discontinuities within a critical wedge model a kinematic forward model of serial cross sections and a linear diffusion algorithm to simulate erosion and sedimentation in western nepal large piggy back basins duns are located above thick thrust sheets that involve more than 5500 m of the neogene siwalik group whereas piggy back basin sedimentation is less developed above thinner thrust sheets 4300 m thick numerical model results suggest that thrust sheet thickness and extension of wedge top basins are both related to an increase of the basal decollement dip beneath the duns the west dang transfer zone wdtz is a n ne trending tectonic lineament that limits the westward extent of the large piggy back basins of mid western nepal and is linked to a thickening of the himalayan wedge eastward the wdtz also affects the seismotectonics pattern the geometry of the thrust front the lateral extent of lesser himalayan thrust sheets and the subsidence of the foreland basin during middle siwalik sedimentation numerical models suggest that the individualisation of the piggy back basins at the transition between the middle siwalik and upper siwaliks followed the deposition of the middle siwaliks that induced a geometry of the foreland basin close to the critical taper as wdtz induces an e w thickning of the himalayan wedge it could also induce a northward shift of the leading edge of the ductile deformation above the basal detachment in greater himalayas of far western nepal field data locally suggest episodic out off sequence thrusting in the frontal thrust belt of western nepal whereas numerical results suggests that episodic out off sequence reactivation could be a general characteristic of the himalayan wedge evolution often hidden by erosion