on the nature and rate of resurfacing of venus crater production and obliteration are modeled for the plains of venus using 1 the observed distribution of venus crossing asteroids and comets 2 viscous relaxation of crater topography and 3 erosion and burial by atmospheric volcanic and tectonic processes crater lifetimes are assumed to be proportional to crater depths for both classes of obliterative processes although the individual criteria vary an average crater retention age between 0 4 to 2 0 ga is estimated for plains under the assumption that craters are produced and not removed the range is driven by uncertainty in identifying degraded impact as opposed to volcanic craters on the other hand crater retention ages greater than about 1 6 ga are unlikely if viscous relaxation operates without loading of crater floors by burial our preferred model has plains subject to crater production and obliteration processes that vary over both space and time in some areas radar bright crater ejecta haloes are preserved for long periods of time because volcanism tectonism and weathering occurs at rates 1 km ga viscous relaxation has probably generated numerous shallow craters in these relatively quiescent regions in other areas volcanism and tectonism have resurfaced the terrain at rates greater than several km ga the global coverage and high resolution sar and altimetry data expected from magellan will allow testing of this model based on detailed crater observations diameter depth distributions morphologic criteria surface scattering properties and their association with volcanic and tectonic features

how does venus lose heat the tectonics and volcanism of the terrestrial planets are controlled by the loss of heat from the planetary interior on the earth about 70 of the heat flow through the mantle is attributed to the subduction of cold lithosphere in order to understand the tectonic and volcanic processes on venus it is necessary to understand how heat is transported through its mantle in this paper three alternative end member hypotheses are considered the first is the steady state loss of heat through the mantle to the surface in analogy to the earth however without plate tectonics and subduction on venus a steady state requires either a very high plume flux or very rapid rates of lithospheric delamination the required plume flux would be equivalent to about 80 plumes with the strength of the hawaiian plume the required delamination flux implies a 50 delamination of the entire venus lithosphere every 10 m y neither appears possible so that it is concluded that venus cannot transport heat through its mantle to its surface on a steady state basis the second hypothesis is that there has been a strong upward concentration of the heat producing elements into the crust of venus the heat generated is then lost by conduction surface measurements of the concentrations of the heat producing elements place constraints on this model if everything is favorable this hypothesis might be marginally acceptable but it is considered to be highly unlikely the third hypothesis is that heat is lost by episodic global subduction events followed by long periods of surface quiescence the near random distribution of craters suggests that the last subduction event occurred about 500 ma this model implies a thick thermal lithosphere 300 km at the present time which is consistent with a variety of surface observations lava lakes on the earth are considered as analogies to plate tectonics they also exhibit episodic subduction events

western eisila regio venus radar properties of volcanic deposits 1988 arecibo observatory dual polarization radar images are presented for western eisila regio venus the polarization information and pioneer venus orbiter reflectivity and altimetry data are analyzed for volcanic deposits on two 400 500 km radius constructs sif and gula montes many of the large effusive deposits studied appear to require superposed flows or multiple vents to explain the observed progression of roughness along their length high fresnel reflectivity material may be present along the summit region of gula mons and in an embayed tessera like region to the n radar dark units on the flanks of sif mons are inferred to be pyroclastic deposits but radar dark features near the summits of both edifices are more consistent with very smooth lava flows higher spatial resolution magellan data will be useful in testing these predictions

the rates of plate creation and consumption summary an alternative vector derivation of the expression for the rates of plate creation and consumption is given from which it can be seen that the rates are proportional to the scalar product of the edges of the geohedron and plate polyhedron corresponding to the same boundary the plate polyhedron is defined by the set of triple junction vectors this is used together with a recent determination of the angular velocity vectors and a number of auxiliary plate boundary vectors to revise the rates of plate creation and consumption along sections of plate boundaries all plates which have a substantial proportion of their boundary connected to a downgoing slab are found to be decreasing in area the total rate of plate generation is used in estimates of the area of the ocean floor and the total heat loss from the oceans the rate of consumption within a relatively small area around new guinea implies a large mass flux away from this region knowledge of the rates of plate creation and consumption is used in a number of different areas calculating the contribution of plate creation to the total heat loss of the earth calculating geochemical fluxes out of the mantle such as for the rare gases and other volatiles and return fluxes back into the mantle resulting from reactions between sea water and the ocean crust during hydrothermal circulation calculating the area of the ocean floor as a function of age mass fluxes related to the distribution of the rates of creation and destruction along the plate boundaries are important boundary conditions on the flow pattern in the mantle previous estimates of these rates deffeyes 1970 chase 1972 garfunkel 1975 have applied a simple expression derived for spreading ridges deffeyes 1970 using angular distances from the poles of rotation measured graphically on a globe an alternative derivation of the expression for rates of plate creation and consumption which is independent of assumptions about the nature of the boundary is given below in terms of the framework used to describe plate motions by mckenzie parker 1967 1974 this derivation emphasizes that the rates depend on two sets of vectors one the instantaneous rotation vectors defining the relative plate motions the other a set of triple junction and auxiliary vectors defining the plate boundaries

new evidence for global tectonic zones on venus venera 15 and 16 spacecraft images show clear evidence of major crustal disruptions on venus which have been interpreted to indicate crustal divergence complementary to the divergent zones are mountain belts that border the continent like high terrains the requisite transcurrent motions appear to be manifested as diffuse shear zones the rift zones form an interconnected transpolar system which ties in with previously recognized equatorial disruption zones suggesting a global tectonic network several independent lines of evidence suggest that the tectonism may be geologically young

venus tectonics an overview of magellan observations the nearly global radar imaging and altimetry measurements of the surface of venus obtained by the magellan spacecraft have revealed that deformational features of a wide variety of styles and spatial scales are nearly ubiquitous on the planet many areas of venus record a superposition of different episodes of deformation and volcanism this deformation is manifested both in areally distributed strain of modest magnitude such as families of graben and wrinkle ridges at a few to a few tens of kilometers spacing in many plains regions as well as in zones of concentrated lithospheric extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that even many local features reflect a crustal response to mantle dynamic processes ridge belts and mountain belts which have characteristic widths and spacings of hundreds of kilometers represent successive degrees of lithospheric shortening and crustal thickening the mountain belts of venus as on earth show widespread evidence for lateral extension both during and following active crustal compression venus displays two principal geometrical variations on lithospheric extension the quasi circular coronae 75 2600 km diameter and broad rises with linear rift zones having dimensions of hundreds to thousands of kilometers both are sites of significant volcanic flux but horizontal displacements may be limited to only a few tens of kilometers few large offset strike slip faults have been observed but limited local horizontal shear is accommodated across many zones of crustal stretching or shortening several large scale tectonic features have extremely steep topographic slopes in excess of 20 30 over a 10 km horizontal scale because of the tendency for such slopes to relax by ductile flow in the middle to lower crust such regions are likely to be tectonically active in general the preserved record of global tectonics of venus does not resemble oceanic plate tectonics on earth wherein large rigid plates are separated by narrow zones of deformation along plate boundaries rather tectonic strain on venus typically involves deformation distributed across broad zones tens to a few hundred kilometers wide separated by comparatively undeformed blocks having dimensions of hundreds of kilometers these characteristics are shared with actively deforming continental regions on earth the styles and scales of tectonic deformation on venus may be consequences of three differences from the earth 1 the absence of a hydrological cycle and significant erosion dictates that multiple episodes of deformation are typically well preserved 2 a high surface temperature and thus a significantly shallower onset of ductile behavior in the middle to lower crust gives rise to a rich spectrum of smaller scale deformational features 3 a strong coupling of mantle convection to the upper mantle portion of the lithosphere probably because venus lacks a mantle low viscosity zone leads to crustal stress fields that are coherent over large distances the lack of a global system of tectonic plates on venus is likely a combined consequence of a generally lesser strength and more limited horizontal mobility of the lithosphere than on earth

reports venus identification of banded terrain in the mountains of ishtar terra high resolution images obtained with the arecibo radar system at a wavelength of 12 6 centimeters reveal numerous parallel 10 to 20 kilometer wide bands of high radar backscatter situated on and oriented parallel to the major mountain ranges of the ishtar terra region of venus geometric and morphological characteristics suggest that the bands represent earth like tectonic deformational features such as folds and faults

the existence of a thin low viscosity layer beneath the lithosphere the horizontal temperature gradient at the base of the lithosphere at an oceanic fracture zone where plate of different ages is juxtaposed is expected to drive a local circulation the characteristics of which can be constrained by the amplitude wavelength and age dependence of the geoid two dimensional numerical models of convection in a fluid layer overlain by a solid conducting lid have been used to generate theoretical geoid profiles at right angles to the fracture zone only a thin low viscosity layer provides a reasonable fit to the data the best model so far obtained has a fluid layer 150 km thick with viscosity 1 5 1019 pa s under a 75 km lid such a layer which is incapable of transmitting strong horizontal shear stresses could provide the decoupling mechanism between plate and deep mantle flow required to balance the forces on the plates

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

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

venus crater distributions at low northern latitudes and in the southern hemisphere from new arecibo observations arecibo high resolution radar data covering approximately 23 of the surface of venus was analyzed for circular features with crater like appearances overlapping venera 15 16 radar data with its more normal to the surface viewing geometry was used to verity crater diameters measured from the arecibo data a comparison of the diameter frequency distribution of the possible impact features in the arecibo data for northern latitudes south of 40 n and for the southern hemisphere with the distribution from the venera 15 16 data for the northernmost quarter of the planet indicated similar distributions for large crater like features with diameters approximately 30 km but a deficiency of smaller craters in the arecibo data the average crater density measured for all craters in the northernmost quarter of the planet from the venera 15 16 data was 1 27 per 106 km2 while the average for the 106 l06km2 covered by the arecibo data was 0 95 per 106 km2 for craters with diameters less than 22km the densities were 0 59 per million square kilometers for the venera data compared with 0 33 per 106 km2 for the average of the two arecibo data regions all circular features in the arecibo data with a crater like appearance that could not be excluded as being impact features were used in this study so the result represents an upper limit on the crater densities in the size interval considered one possible process responsible for the density differences of small craters is a difference in the rate of volcanic resurfacing an increase in the resurfacing rate of approximately 50 would explain the lower density of small craters in the area covered by the arecibo data compared with the venera data but would not account for the differences being confined to the 30 km crater diameter range

geological correlations with the interior density structure of venus a model has been constructed by stipulating that venusian long wavelength topography and geoid elevations are explained by the sum of density anomalies associated with mantle convection and lithospheric compensation mechanisms for convenience the latter are represented by a single airy surface because individual lithospheric components cannot be resolved at the wavelengths considered in this paper mantle convection is represented at a single depth by mass perturbations that drive viscous flow we invert spherical harmonic representations of the topography and the geoid to create global representations of the mantle convection pattern and near surface density anomalies the mantle convection pattern matches well with previous numerical simulations of mantle convection that show a pattern of isolated upwellings amidst an interconnected network of downwellings the near surface density anomalies show no particular pattern the inversion results have been compared with a global synthesis of magellan data impact crater density does not correlate with the mantle convection pattern or the near surface density anomalies large volcanic structures are associated with upwellings planitiae are associated with downwellings coronae are anticorrelated with high amplitude upwellings and downwellings and apparently are passive features associated with rifting that is related to mantle convection in a complex fashion tesserae are associated with negative lithospheric density anomalies interpreted as areas of thickened crust tesserae show no definitive relationship to the present mantle convection pattern

tectonics and evolution of venus the global tectonics of venus differs significantly from that of earth most markedly in that the surface is covered predominately by gently rolling terrain there apparently are no features like ocean rises the gravity is positively correlated with topography at all wavelengths and the few highlands are estimated to be supported or compensated at a depth of approximately 100 kilometers the surface of venus appears to be covered mainly by an ancient crust the high surface temperature making subduction difficult it seems likely that well over 1 billion years ago water was destabilized at the surface and soon after plate tectonics ceased the highlands appear to be actively supported presumably as manifestations of long enduring hot spots

hot spot evolution and the global tectonics of venus the global tectonics of venus may be dominated by plumes rising from the mantle and impinging on the lithosphere giving rise to hot spots global sea floor spreading does not take place but direct convective coupling of mantle flow fields to the lithosphere leads to regional scale deformation and may allow lithospheric transport on a limited scale a hot spot evolutionary sequence comprises i a broad domal uplift resulting from a rising mantle plume ii massive partial melting in the plume head and generation of a thickened crust or crustal plateau iii collapse of dynamic topography and iv creep spreading of the crustal plateau crust on venus is produced by gradual vertical differentiation with little recycling rather than by the rapid horizontal creation and consumption characteristic of terrestrial sea floor spreading

implications of oceanic intraplate seismicity for plate stresses driving forces and rheology abstract recent studies of earthquakes occurring within the oceanic lithosphere provide valuable information about stresses in the lithosphere plate tectonic driving forces and the rheology of the lithosphere asthenosphere and mantle focal mechanisms indicate that oceanic lithosphere older than 35 million years is almost entirely in deviatoric compression both compressional and extensional events occur in younger lithosphere in young lithosphere mechanisms generally indicate compressive stress in the spreading direction or extension oblique to the spreading direction extension in the spreading direction is not observed using the constraint of compression in the spreading direction in old lithosphere models of the stresses produced by a combination of ridge push and basal drag forces require the magnitude of the drag be less than a few bars for rapidly moving plates and a few tens of bars for slow moving plates assuming that basal drag results from mantle return flow the upper limits on drag can be converted to constraints on the viscosity structure of the asthenosphere and mantle using simple two dimensional models if return flow occurs in a mantle of viscosity 10 22 poise comparable to the results from glacial rebound studies the predicted basal drag is too high unless a thin asthenosphere with viscosity less than 10 19 to 10 20 poise depending on flow depth is present the intraplate stress data thus are consistent with the idea of oceanic plates largely decoupled from the underlying mantle the strength of the lithosphere is constrained by the maximum depth of oceanic intraplate seismicity which increases with lithospheric age and appears to be bounded by a 700 800 c isotherm this limiting depth is approximately equal to the flexural elastic thickness of the lithosphere and is consistent with experimental olivine rheologies which predict rapid weakening at high temperatures similar phenomena are important for estimating the fraction of seismic and aseismic slip on transform faults and in determining the extent of rupture for large trench normal faulting events

three dimensional spherical models of convection in the earth s mantle three dimensional spherical models of mantle convection in the earth reveal that upwelling cylindrical plumes and downwelling planar sheets are the primary features of mantle circulation thus subduction zones and descending sheetlike slabs in the mantle are fundamental characteristics of thermal convection in a spherical shell and are not merely the consequences of the rigidity of the slabs which are cooler than the surrounding mantle cylindrical mantle plumes that cause hotspots such as hawaii are probably the only form of active upwelling and are therefore not just secondary convective currents separate from the large scale mantle circulation active sheetlike upwellings that could be associated with mid ocean ridges did not develop in the model simulations a result that is in agreement with evidence suggesting that ridges are passive phenomena resulting from the tearing of surface plates by the pull of descending slabs

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

distribution of tessera terrain on venus prediction for magellan tessera terrain is the dominant tectonic unit in the northern hemisphere of venus and is characterized by complex sets of intersecting structural trends and distinctive radar properties due to a high degree of meter and sub meter scale 5 cm to 10 m roughness based on these distinctive radar properties a prediction of the global distribution of tessera can be made using pioneer venus pv reflectivity and roughness data where available venera 15 16 and arecibo images and pv diffuse scattering data were used to evaluate the prediction from this assessment the authors conclude that most of the regions with prediction values greater than 0 6 out of 1 are likely to be tessera and are almost certain to be tectonically deformed lada terra and phoebe regio are very likely to contain tessera terrain while much of aphrodite terra is most likely to be either tessera or a landform which has not yet been recognized on venus this prediction map will assist in targeting magellan investigations of venus tectonics

variations in effective compensation depth across aphrodite terra venus aphrodite terra is the largest elevated terrain on venus and it serves as a focal point in current discussions of global tectonic style using the topography and gravity data acquired by the pioneer venus orbiter pvo the authors have estimated an effective depth of airy compensation for each of 75 orbital arcs that provide fairly uniform areal coverage of the entire province the most pronounced pattern that emerges is a general increase in compensation depth to the east the most rapid change occurs near 135 degree the average west of there is 70 km while the average to the east is 230 km superimposed on this larger trend are five distinctive regional patterns four well defined peaks and one interval of widely scattered and poorly constrained depths the maxima in compensation depth are well correlated with regional topographic highs while these observations are easily reconciled with the general notion that aphrodite is a region of crustal divergence the great depth of compensation is difficult to accord with the more specific suggestion that aphrodite is a terrestrial type divergent plate margin the alternative suggestion that aphrodite and the other equatorial highlands on venus are surface manifestations of hot upwelling mantle plumes ismore consistent both with the great depths of compensation and with the pattern of regional peaks and intervening troughs the broader scale increase in effective depth of compensation from west to east is more enigmatic but it might reflect an age progression of the plumes less

venus volcanism and rift formation in beta regio a new high resolution radar image of beta regio a venus highland area confirms the presence of a major tectonic rift system and associated volcanic activity the lack of identifiable impact craters together with the apparent superposition of the theia mons volcanic structure on the rift system suggest that at least some of the volcanic activity occurred in relatively recent geologic time the presence of topographically similar highland areas elsewhere on venus aphrodite terra dali chasma and diana chasma suggests that rifting and volcanism are significant processes on venus

coronae of mnemosyne regio morphology and origin abstract coronae are circular to elliptical structures 150 600 km across characterized in venera 15 16 and arecibo radar images by an annulus of concentric ridges the interiors of the structures are geologically complex displaying volcanic flows domes and radial concentric and or obliquely trending ridges and are generally distinct from the exterior the mnemosyne regio area to the west of ishtar terra contains seven coronae the largest cluster of coronae identified on venus on the basis of the morphologic and topographic characteristics of the coronae in the mnemosyne region we assess several possible models of corona origin using cross cutting and stratigraphic relationships we have been able to identify a sequence of features and events for this group of coronae and have determined that a variety of processes contribute to the origin and evolution of coronae we find that several processes associated with diapirism or hotspots and gravitational relaxation of high topography are most consistent with observed corona topography and morphology and are thus likely to have been significant in the formation and evolution of coronae on the basis of these observations and stratigraphic relationships the following sequence of events is outlined for the formation and evolution of coronae in this region 1 regional volcanic and tectonic activity 2 localized uplift and volcanism caused by diapirism or hotspots 3 annulus and trough formation related to gravitational relaxation accompanied by further volcanic activity 4 continued local volcanic flooding of the corona annulus and trough differences in morphology between coronae reflect variations in amount and significance of these geologic processes coronae in other regions exhibit the same overall characteristics as the mnemosyne coronae and support a similar origin for the majority of coronae

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

venus geology and tectonics hotspot and crustal spreading models and questions for the magellan mission spacecraft and ground based observations of venus have revealed a geologically young and active surface with volcanoes rift zones orogenic belts and evidence for hotspots and crustal spreading yet the processes responsible for these features cannot be identified from the available data the magellan spacecraft will acquire an unprecedented global data set which will provide a comprehensive and well resolved view of the planet this will permit global geological mapping an assessment of the style and relative importance of geological processes and will help in the understanding of links between the surface geology and mantle dynamics of this earth like planet

geologic evidence of hotspot activity on venus predictions for magellan a number of distinctive types of geologic features have been identified on venus that are interpreted to be related to thermal plumes including domal rises coronae and major composite shield volcanoes we document the basic characteristics of these features as well as their distribution the three types of features have related morphologies and we interpret them to represent a continuum of features formed by mantle plumes at scales from 100 s to over 1000 km the artemis structure located in aphrodite terra is proposed to be a large corona if crustal spreading processes are operating on venus hotspot features should form chains on the surface as seen in terrestrial ocean basins on the basis of current data on hotspot related feature distribution on venus no clear evidence exists for hotspot chains the complete distribution of hotspot features in magellan data will be used to understand better the relationship between interior processes and surface features as well as to provide a test for the crustal spreading hypothesis

limits on modes of lithospheric heat transport on venus from impact crater density we present a formalism by which the size frequency distribution of impact craters on venus may be used to estimate upper bounds on the mean global rates of volcanic resurfacing and lithospheric recycling on that planet over the past several hundred million years the principal assumptions are that an upper bound on the rate of crater production on venus may be estimated and that craters are volcanically obliterated only when flows completely cover their rims the impact crater density reported from venera 15 16 observations if valid for the entire venus surface then indicates a mean volcanic flux no greater than 2 km y corresponding to a maximum average rate of resurfacing of about 4 km b y the fraction of global heat loss due to such a rate of volcanic resurfacing is negligible for the lowest estimated crater retention age of the surface imaged by venera 15 16 if globally representative the rate of lithospheric recycling on venus does not exceed 1 5 km y corresponding to 25 of the global heat loss more likely estimates of the mean surface age limit the contribution to global heat loss of any lithospheric recycling to be less than 10 these results support the hypothesis that simple conduction dominates heat transport at lithospheric levels in the venus interior

crustal recycling of metamorphic basement late palaeozoic granitoids of northern chile 22 s implications for the composition of the andean crust upper palaeozoic silicic magmatism is widespread in the central introduction andes but its origin is poorly constrained we investigated wholethe magmatic arc in the puna altiplano plateau of the rock chemical and isotopic composition of upper palaeozoic grancentral andes rests on a continental crust of a maximum itoids and their early palaeozoic high grade country rocks in thickness of 70 km crustal thickening is mainly atthe chilean coastal cordillera and precordillera at 22 s in tributed to shortening of the crust and not to juvenile comparison with an upper cretaceous granitoid the age of the magmatic additions from the subduction zone see review late palaeozoic granitoids from a rb sr isochron of 300 ma by allmendinger et al 1997 this implies a major is consistent with k ar cooling ages of hornblende and biotite contribution of pre mesozoic material to the present similar major and trace element patterns as well as nd and pb thick crust because mesozoic juvenile additions to the isotopic composition of upper palaeozoic granitoids and gneisses crust are focused in the mesozoic magmatic arcs mainly point to a source of the granitoids that is similar to the gneisses at in the chilean coastal cordillera fig 1 e g scheuber outcrop sr isotope ratios of the upper palaeozoic granitoids are et al 1994 lucassen et al 1996a which is not involved less radiogenic than those in many of the gneisses we propose a in the crustal thickening the last major reorganization of stratification of the early palaeozoic crust with a rb deficient the crust before the cenozoic was in the early palaeozoic granulitic mid lower crust resulting in less radiogenic sr compared orogeny between 500 ma the age of peak metawith the upper crust based on the interpretation of the p t t morphism and 400 ma the age of the final exhumation history and isotopic composition of the lower palaeozoic meta damm et al 1990 1994 lucassen et al 1996b unmorphic basement and of the isotopic composition of the late published data 1998 becchio et al 1997 the widepalaeozoic granitoids and younger magmatic rocks nd isotopic spread upper palaeozoic granitoid magmatism starting composition is identical in lower and upper crust and in crustal at 300 ma was assigned to arc magmatism e g brown melts from the late palaeozoic to recent the cretaceous granitoid 1991 bahlburg herve 1997 or to melting of the evolved from partial melts of a mantle derived source with concrust at a passive margin breitkreuz zeil 1984 and siderable contamination by the old crustal component the crust references therein the metamorphic history of the prethat formed in the early palaeozoic is the major source of material mesozoic crust was studied in detail before lucassen et for the cenozoic tectonic thickening of the andean crust al 1999a unpublished data 1998 summary of previous

blob tectonics a prediction for western aphrodite terra venus it is proposed that the western aphrodite terra region of venus was formed by four diapirs of heated mantle material or blobs rising to the surface in a region of convective upwelling the four blobs are in different stages of evolution producing different topography gravity anomalies and morphology we show that this model is consistent with available data and make specific predictions that can be tested with magellan radar data

mantle flow tectonics and the origin of ishtar terra venus it is proposed that western ishtar terra formed due to compression and crustal thickening above a cylindrical mantle downwelling a model for crustal deformation due to downwelling successfully reproduces many observed characteristics of western ishtar although axisymmetric downwellings occur in numerical models of constant viscosity mantle convection there is no evidence for their existence in earth s mantle where downwellings are sheet like either modes of downflow in venus and earth are fundamentally different or differences in near surface conditions and material behavior selectively emphasize surface expressions of the different downwelling modes

a mantle plume model for the equatorial highlands of venus the equatorial highlands of venus consist of four main structures atla beta ovda and thetis regiones each has a circular to oval shaped planform and rises 4 5 km above the mean planetary radius these highlands are associated with long wavelength geoid highs with amplitudes ranging from 35 m at ovda to 120 m at atla they also contain topographic valleys interpreted as extensional rift zones and beta is known to contain shield volcanoes these characteristics are all consistent with the equatorial highlands being formed by mantle plumes an alternative model in which ovda and thetis are interpreted as spreading centers analogous to terrestrial mid ocean ridges fails to explain most of the observed geoid anomalies and topography in these regions some smaller highlands such as bell regio eistla regio and the hathor innini ushas region may also be plume related but most coronae are unlikely to be the direct result of plume activity we have modeled plumes using a cylindrical axisymmetric finite element code and a depth dependent newtonian rheology we compare our model results with profiles of geoid and topography across atla beta ovda and thetis our best model fits are for beta and atla assuming whole mantle convection and that earth and venus have similar mantle heat flows venus must lack an earth like low viscosity zone in its upper mantle in order satisfy the observed geoid and topography for these features this conclusion is consistent with the long wavelength admittance spectrum of venus and with the observed differences in the slopes of the geoid spectra for the two planets one explanation for the different viscosity structures of the two planets could be that the mantle of venus is drier than earth s mantle

magellan observations of alpha regio implications for formation of complex ridged terrains on venus magellan images of alpha regio reveal previously undetected structures and details of the morphology of this region of complex ridged terrain we examine the complex ridged terrain of alpha regio using morphology and crosscutting relationships between structures to derive a sequence of tectonic events structures include broad 10 20 km wide linear and arcuate ridges fine scale 3 km wide ridges linear disruption zones ldzs up to several kilometers wide and numerous grabens 5 km wide and associated scarps and troughs based on their morphology we interpret the broad and fine scale ridges as compressional structures possibly folds ldzs appear to be due to small amounts of lateral shear which most commonly disrupts the older ridge fabric graben and associated structures are interpreted as extensional features they crosscut ridges and ldzs and thus appear to be the youngest structures in alpha regio this sequence of events and information on the orientation of these various structures are compared to the predictions of two models for the formation of complex ridged terrain and highlands on venus in general a hotspot model and a coldspot model the presence of compressional features along and parallel to the margins of alpha regio and the lack of any high elevation ring of extensional features are more consistent with a coldspot or roughly axisymmetric mantle downwelling mantle downwelling appears to be the most likely mode of formation of the upland of alpha regio and is likely to be important in other highland regions such as ovda and thetis regiones which are also dominated by complex ridged terrain

tectonics of lakshmi planum venus tests for magellan the origin of lakshmi planum and its surrounding mountain belts remains an important unresolved element in the global tectonic framework of venus from the perspective of gravity signature and potential driving forces the mantle upwelling model is the simplest and its principal failure that it cannot produce radial shortening on the uplift periphery may be resolved if the lithosphere is laterally heterogeneous our preferred model consists of a hot mantle plume rising beneath a pre existing block of tessera the lithosphere is weakened at this hotter and presumably thicker crust and the outward near surface flow is attenuated at the peripheral discontinuity in lithospheric strength crustal thickening and mountain belt formation occur there we propose several criteria to test this tessera plume model together with its competitors at the higher resolution in both imaging and gravity afforded by the magellan mission

ridge belts evidence for regional and local scale deformation on the surface of venus we evaluate models for the formation of the ridge belt fan assemblage on venus through consideration of the orientation spatial distribution topographic expression and wavelengths of observed tectonic surface features we favor a compressional mechanism for long wavelength deformation corresponding to the spacing of ridge belts 300 400 km however short wavelength ridges and grooves 10 20 km that are contained within belts and trend parallel to them are likely to be compressional and extensional and to reflect both regional and local stress fields we hypothesize that large scale ridge spacing early stage mantle downwelling is the source of regional compression responsible for the establishment of the long wavelength of deformation

styles of volcanism on venus new arecibo high resolution radar data arecibo high resolution 1 5 to 2 km radar data of venus for the area extending from beta regio to western eisila regio provide strong evidence that the mountains in beta and eisila regiones and plains in and adjacent to guinevere planitia are of volcanic origin recognized styles of volcanism include large volcanic edifices on the beta and eisila rises related to regional structural trends plains with multiple source vents and a mottled appearance due to the ponding of volcanic flows and plains with bright features surrounded by extensive quasi circular radar dark halos the high density of volcanic vents in the plains suggests that heat loss by abundant and widely distributed plains volcanism may be more significant than previously recognized the low density of impact craters greater than 15 km in diameter in this region compared to the average density for the higher northern latitudes suggests that the plains have a younger age

consequences of a hydrous mantle for venus tectonics predictions for magellan we will consider the consequences of a venus mantle enriched in water relative to the earth if the moon forming impact hypothesis is correct earth may have lost substantial volatiles venus however experienced no such impact and is therefore likely to be relatively more water rich we demonstrate that a higher abundance of water in the venus mantle has the effect of inhibiting the rise of crustforming melts and volatiles to the surface which results in a thin stable crust a dry stiff upper mantle and restricted magmatism conduction through a shallow thermal lithosphere dominates the removal of heat from the interior and flow in the mantle is strongly coupled to the surface deformation current observations of venus are consistent with this model a review of the consequences of this model allows us to make predictions for the magellan radar images these include 1 no evidence for plate tectonic processes 2 younger apparent surface ages and enhanced crater relaxation for highland areas 3 limited quantity of hot spot volcanism and 4 occasional evidence of localized magmatism diatremes associated with a volatile rich mande source the model has implications to the earth as well in the absence of the moon forming impact crustal formation in the water rich earth mantle may have been arrested and plate recycling would have ceased early in the planet s history

the evolution of impact basins viscous relaxation of topographic relief we evaluate the hypothesis that viscous relaxation has been an important process for modifying the topographic profiles of ancient large impact basins on the moon we adopt a representative topographic profile of the orientale basin the youngest large impact basin on the moon as an estimate of the initial topography of older basins of similar horizontal dimensions and we predict the topographic profiles that would result from viscous relaxation according to a number of simple analytical representations of the rheological response of the moon to surface topography at wavelengths greater than the thickness of a high viscosity lithosphere both a decrease in viscosity with depth and the partial to complete isostatic compensation of topographic relief have pronounced effects on the wavelength dependent relaxation times and must be considered in modeling viscous relaxation for features as large as impact basins the effect of a decrease in viscosity with depth is to enhance the rate of viscous relaxation at long wavelengths while the rate of relaxation decreases substantially for the fraction of long wavelength topography which is isostatically compensated as an initial condition these models are applied to two pre nectarian basins on the moon the tranquillitatis basin on the lunar nearside and the much larger and older south pole aitken basin on the farside the topographic profile of tranquillitatis after correcting for the effect of mare basalt fill is consistent with significant viscous relaxation of relief prior to the oldest episode of mare volcanism preserved as a surface unit the large topographic relief of the larger farside basin on the contrary is not consistent with significant viscous relaxation and implies a mean crustal viscosity at least a factor of 10 higher than for the central nearside over the time interval during which substantial viscous relaxation of older nearside basins such as tranquillitatis probably occurred a difference in typical crustal temperature profiles between the farside and nearside is the most likely explanation of such a viscosity difference such a nearside farside asymmetry in near surface temperature may date from the time of crust mantle differentiation and may have persisted because of the different histories of late stage impact basin formation and mare volcanism on the two hemispheres

gravity driven deformation of the crust on venus intense horizontal deformation features are observed on the surface of venus despite the apparent absence of terrestrial style plate tectonics high surface temperature low erosion rate and regions of elevated terrain suggest that gravity spreading may be responsible for some of these features gravity spreading on the earth is known to cause extensive deformation in certain thin skinned tectonic regions where the uppermost layers become detached along a decollement while the high pore pressures and soft sedimentary rocks frequently observed along decollements on the earth are not plausible for venus temperature induced regions of ductile deformation may exist simple viscous models of gravity spreading above a ductile region of the crust are developed to examine the effects of rheology thermal gradient surface slope and crustal thickness on deformation for a reasonable range of parameters the models show that surface slope driven ductile deformation at relatively shallow depths in the crust is possible and may be capable of detaching the uppermost rigid portion of the crust resulting in horizontal translation

venus ishtar gravity anomaly ishtar has been detected as a positive gravity anomaly verification of this result has been made with two independent data sets isostatic compensation of the ishtar topography using an airy model is achieved at approximately 150km

structures in tessera terrain venus issues and answers many workers assume that tessera terrain marked by multiple tectonic lineaments and exposed in crustal plateaus comprises a global onionskin on venus because tesserae are exposed mostly within crustal plateaus which exhibit thickened crust issues of tessera distribution and the mechanism of crustal plateau formation e g mantle downwelling or upwelling are intimately related a review of magellan data indicates that tessera terrain does not form a global onionskin on venus although ribbon bearing tesserae reflect an ancient time of a globally thin lithosphere individual tracts of ribbon bearing tessera terrain formed diachronously punctuating time and space as individual deep mantle plumes imparted a distinctive rheological and structural signature on ancient thin crust across spatially discrete 1600 2500 km diameter regions above hot mantle plumes plume related magmatic accretion led to crustal thickening at these locations resulting in crustal plateaus crustal plateau surfaces record widespread early extension ribbon structures and local minor perpendicular contraction of a thin competent layer above a ductile substrate within individual evolving crustal plateaus the thickness of the competent layer increased with time and broad gentle folds formed along plateau margins and short variably oriented folds formed in the interior late complex graben cut folds local lava flows accompanied all stages of surface deformation in contrast to these conclusions gilmore et al 1998 summarized post magellan arguments in favor of downwelling models for crustal plateau formation in light of this discrepancy we reexamine the regions investigated by these workers and evaluate their arguments against upwelling models we show that gilmore et al 1998 did not differentiate ribbons from graben and therefore their proposed temporal relations are invalid they disregarded shear fracture ribbons thus invalidating their criticism of ribbon models they misunderstood previous radargrammetric work that constrains ribbon geometry and they relied solely on geometrical relations to constrain timing violating kinematic analysis methodology their stratigraphic constraints on ribbon fold temporal relations are invalid because they 1 misinterpreted implications of map relations 2 did not isolate radar artifacts due to local radar slope effects from proposed material units 3 chose a region for analysis that clearly shows the effects of younger tectonism and volcanism and 4 presented map relations that cannot be reproduced their attempts to discount upwelling models of crustal plateau formation fail because they combine fundamentally different pre magellan and post magellan upwelling models these misconceptions about the upwelling model and processes responsible for global warming phillips and hansen 1998 lead to serious errors in gilmore et al s 1998 criticism furthermore we show that the data of gilmore et al 1998 are actually more consistent with upwelling than downwelling models consistent with arguments that tessera terrain is not global in spatial distribution

stagnant lid convection on venus the effect of strongly temperature dependent viscosity on convection in the interior of venus is studied systematically with the help of finite element numerical models for viscosity contrasts satisfying experimental constraints on the rheology of rocks venus is likely to be in the regime of stagnant lid convection this regime is characterized by the formation of a slowly creeping very viscous lid on top of the mantle venusian lithosphere and is in agreement with the tectonic style observed on venus stagnant lid convection explains large geoid to topography ratios on venus by the thermal thinning of a thick lithosphere the thickness of the lithosphere can be as large as 400 550 km for beta regio and 200 400 km on average geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle 10 20 10 21 pa s the convective stresses in the interior 0 2 0 5 mpa the stresses in the lid 100 200 mpa the velocity in the interior 0 5 3 cm yr 1 and the heat flux beneath the lithosphere 8 16 mw m 2 parameterized convection calculations of thermal history of venus are difficult to reconcile with a thick present day lithosphere however a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0 5 b y ago one of the possible explanations for the cessation of venusian plate tectonics is that during the evolution of venus stresses in the lid dropped below the yield strength of the lithosphere this model predicts a drastic drop in the heat flux thickening of the lithosphere and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on venus around 0 5 b y ago

tectonic effects of climate change on venus venusian plains regions are commonly crossed by small strain deformation features such as wrinkle ridges polygonal terrains and gridded terrains of these polygonal terrains are observed to have a relatively uniform spacing and are widely distributed on plains that formed during global resurfacing models of venusian climate that assume resurfacing occurred through massive volcanic events suggest that surface temperatures could have dramatically changed over nearly a billion years propagating thermal stresses into the surface and potentially causing small strain features we investigate this hypothesis by approximating the temperatures predicted by climate models as a step function and employ a fixed plate and strength envelope model to predict the resulting depth of failure and amount of strain our calculations indicate that strains due to temperature changes of 50 100 k which are favored for volcanic resurfacing events of 1 10 km thickness are consistent with the observed spacing of polygonal and gridded terrains as well as some wrinkle ridges the global nature of the climate change event is consistent with the global distribution and uniformity of such features and implies that such terrains may be a global stratigraphic marker

gravitational spreading of high terrain in ishtar terra venus magellan altimetry measurements indicate that the mountain belts and plateau margins of ishtar terra have topographic slopes in the range 2 30 magellan radar images reveal that numerous sets of narrow closely spaced troughs and lineations which we interpret as graben and normal faults are located in many of these regions of high slope the orientation of many of these graben sets perpendicular to the downslope direction suggests that they formed as a result of gravitational spreading in several of the mountain belts the extensional structures are parallel to the apparent direction of shortening consistent with the interpretation that gravitational spreading occurred while mountain building was still active to explore the implications of this spreading for the tectonic evolution of ishtar terra we model the process by means of a finite element algorithm for viscoelastic deformation in a vertical section of the crust at the margin of a plateau or broad mountain range flow in the crust is assumed to be governed by a nonlinear depth dependent rheology we predict brittle failure and relaxation of topographic relief as functions of time for ranges of crustal thickness 10 30 km and thermal gradient 5 25 k km and for observed ranges in topographic elevation 1 6 km and slope 1 30 this parameter range predicts a large spread in the time scales for initial failure and significant relaxation of relief for a crustal thickness greater than 10 km a thermal gradient of 15 k km or more and average values of relief 3 km and slope 3 the topography relaxes to 25 of its original height in less than 10 m y although initial failure in the form of normal faulting on the highlands and margin and occasional shallow thrusting in the lowlands is predicted to occur much earlier values of horizontal surface strain that are likely to be observable 1 do not accumulate until significant relaxation of relief begins given the predicted rate of topographic relaxation either the crust of ishtar terra must be anomalous in some way either much stronger than predicted by standard flow laws or very thin or cold or the topographic relief and high slopes have been actively built and maintained until times significantly younger than the average crater retention age for venus of 500 ma the large apparent depth of compensation the abundance of volcanism and the uncertainty in the local age favor the view that mountain building in ishtar terra has occurred until times at least as recent as 10 ma

western ishtar terra and lakshmi planum venus models of formation and evolotion regional geologic mapping and gravity data reveal a variety of characteristics that must be accounted for in models for the formation and evolution of western ishtar terra and lakshmi planum including 1 high elevation 2 plateau shaped profile 3 abnormally steep bounding slopes 4 foredeeps 5 polygonal outline 6 adjacent orogenic belts 7 volcanic plains 8 plains emplaced synchronously with orogenic belts 9 paterae 10 variable topography of lakshmi 11 tessera like material underlying lakshmi and 12 a large apparent depth of compensation a tessera peripheral deformation model in which a preexisting block of tessera is the locus of convergence of adjacent thinner crust and lithosphere underthrusting mountain building subsurface melting and plateau uplift is interpreted to account for most of the characteristics the apparent depth of compensation is not simply explained by this model and appears to require a second deeper mantle anomaly component such as broad mantle upwelling or a hot spot

the spin and inertia of venus one of the most remarkable rotational features of venus is the presence of a relatively large offset between spin and maximum inertia axis according to the most recent gravity field models this offset may amount to 0 5 considerably larger than the one which characterizes the earth s chandler wobble 0 3 at present it is not clear whether the offset of venus is the result of the limited resolution of the gravity models or alternatively it may be regarded as an evidence in support to the existence of a wobble purpose of this note is to demonstrate on quantitative grounds that an offset between spin axis and maximum inertia axis is to be expected on the time scales of mantle convection due to the peculiar rotational characteristics of venus we also show that the inertia perturbation associated to the rising of hot thermal plumes from the core mantle boundary to the surface is large enough to maintain offsets ranging between 0 1 and 1 of course this does not preclude that other processes such as venusquakes or periodic atmospheric variabilities may contribute significantly to the observed offset on shorter time scales

venus gravity analysis of beta regio radio tracking data acquired over beta regio were analyzed to obtain a surface mass distribution from which a detailed vertical gravity field was derived in addition a corresponding vertical gravity field was evaluated solely from the topography of the beta region a comparison of these two maps confirms the strong correlation between gravity and topography which was previously seen in line of sight gravity maps it also demonstrates that the observed gravity is a significant fraction of that predicted from the topography alone the effective depth of complete isostatic compensation for the beta region is estimated to be 330 km which is somewhat deeper than that found for other areas of venus

tessera deformation and the contemporaneous thermal state of the plateau highlands venus abstract geodynamic modeling of tessera deformation at the ancient venusian plateau highlands implies lithospheric thermal gradients exceeding 17 k km 1 regularly spaced contractional ridges in the highlands manifest the unstable growth of perturbations at a dominant wavelength of 20 km controlled by a mechanical layer of relatively uniform thickness we model the deformation as the shortening of a plastic layer over a viscous substrate a layer thickness of less than 6 km is consistent with the observed ridge spacings assuming an anhydrous diabase crustal rheology and a minimum strain rate of 10 16 s 1 the geotherm was at least 17 k km 1 higher strain rates consequently higher thermal gradients are more probable we rule out the possibility that the plateau highlands were locally anomalously hot at the time ridges formed in the tessera that the highlands are dominantly composed of rock more felsic than diabase or that the crust was weak because it was hydrated therefore the average global lithospheric heat flux when the highlands tesserae were deforming was considerably higher than it has been since

mantle convection and crustal evolution on venus venus is probably similar to the earth in that recycling of basaltic crust has been induced by the development of cratons combinations of siliceous crust with high mg fe mantle venus has less remaining energy sources at depth but still enough to to support the great plateaus and to deliver appreciable heat close to the surface by convection the main differences of venus from the earth arise from its lack of water rather than higher temperatures lack of water plus lower stress levels due to lesser energy makes the upper mantle more viscous and hence venus tectonics more driven by bulk mantle rather than boundary layer characteristics making tectonics difficult to infer from magellan imagery will be shallower and more widespread layers of weakness in crustal rocks obscuring the mantle driven patterns

small mantle upwellings are pervasive on venus and earth observations of venus and africa an analogous terrestrial plate indicate that upwellings a few hundred kilometers across are pervasive in the ambient upper mantle of both venus and earth because of their small size when they reach the lithosphere the upwellings affect the surface only under special circumstances on venus they form volcano tectonic structures called coronae where the lithosphere has been thinned by rifting on earth the small upwellings produce topographic swells where they impinge on a plate stationary with respect to the hot spot reference frame as is africa the small upwellings presence throughout the mantle explains both the corona paradox in which coronae are associated with rifts on venus but do not appear to drive rifting and the basin and swell topography of the african plate the morphology of coronae requires that the small upwellings have some diapiric properties their pervasiveness transient nature and generally small size make them fundamentally different from traditional hawaiian style plumes and they may be detached diapirs within a mantle convecting in the hard turbulence regime