wrinkle ridge assemblages on the terrestrial planets the wrinkle ridge assemblage is a group of associated features that have been observed on the moon mars and mercury members of the wrinkle ridge assemblage are classified as either arches or ridges based on morphology and ridges are subdivided into first second or third order ridges on the basis of dimensions the ridge assemblages described in this study are known to occur in mare basalts on the moon and smooth plains units that may be volcanic in origin on mars and mercury the structures that occur in the continental flood basalts of the columbia plateau in the northwestern united states are morphologically and dimensionally similar to features in the ridge assemblages on the terrestrial planets the anticlinal ridges of the columbia plateau and many first order ridges on the moon mars and mercury are interpreted to be folds with reverse or thrust faulting developed as a result of buckling flexure fracture or buckling as a consequence of reverse or thrust faulting fracture flexure estimates of the unit shortening across first order ridges using simple geometric forms to approximate the ridge morphology are lower than those determined for the anticlines of the columbia plateau where displacements are the results of folding and reverse to thrust faulting evidence of layer parallel extension that often accompanies folds developed by fracture flexure mechanisms i e drape or fault bend folds is not found in the anticlinal ridges of the columbia plateau and has only been observed in one ridge on the moon a kinematic model involving buckling followed by reverse to thrust faulting flexure fracture is favored for the origin of the anticlinal ridges of the columbia plateau and many of the first order ridges on the moon mars and mercury

fault and fold growth of the amenthes uplift implications for late noachian crustal rheology and heat flow on mars abstract determining the rheologic conditions that control growth of compressive structures on mars is an elusive problem one limited by the lack of seismologic and other data commonly available for comparable active uplifts on earth in some instances however the geometry of faults on mars that offset impact craters can be deduced from surface topography alone with this aim construction of a balanced and restorable structural cross section across the late noachian amenthes uplift or fault related fold suggest it forms above a deeply penetrating blind thrust with a gently curved or listric geometry that flattens downward preferred structural solutions suggest the thrust dips between 41 5 and 56 1 at the surface and flattens into a horizontal decollement at depths of 33 48 km the range of values for depth to detachment are greater than previous estimates for the amenthes thrust based on elastic modeling for a range of planar thrust geometries using the inference that the decollement corresponds to the onset of plasticity in the crust the depth to detachment is consistent with surface heat flow of 24 33 mw m 2 based on average values for heat production and the temperature threshold for the transition from brittle faulting to ductile shear on mars this suggests the crust at amenthes during the late noachian may be slightly cooler than previously thought similar to recent estimates derived from studies of lithospheric strength

influences on the carbonate hydrochemistry of mound spring environments lake eyre south region south australia abstract mound spring structures composed of tufa precipitated from near ambient temperature spring waters are rare consequently the hydrochemical evolution of the water and the processes that control carbonate deposition in these systems are not well understood this study analysed the water from 3 mound springs in an arid environment in the vicinity of lake eyre south south australia samples were tested for major ion chemistry and stable isotope ratios of water 2 h 1 h and 18 o 16 o most co 2 degasses from water i before emergence as it ascends in the fracture derived spring conduit system below the spring opening and ii after emergence in the turbulent upper tail environment between the spring vent pool and the tail delta however modern carbonate precipitation was generally restricted to tail delta environments spatially disconnected from the areas of strongest co 2 degassing in addition to co 2 degassing and carbonate precipitation evapotranspiration and heterotrophy were inferred to be processes controlling the chemical evolution of water flowing through the tail delta based on considerations of the geometry of the tail delta and evapotranspiration rates it was inferred that infiltration is an important process in determining the tail delta size and hence the morphology of the mound spring environment a reactive transport model was used to assess the importance of these processes for the evolution of carbonate hydrochemistry the model outcomes showed that water loss via evapotranspiration does not significantly affect tufa precipitation despite the springs being located within an arid environment heterotrophy was considered in the model in order to reproduce observed ph and carbonate concentrations highlighting the role of vegetation in controlling the water chemistry the modelling approach adopted here provides a generic framework for the analysis of calcareous spring deposits and places quantitative constraints on data based process conceptualisation

constraints on the lithospheric structure of venus from mechanical models and tectonic surface features radar images of the surface of venus show numerous structures that are interpreted as having formed due to horizontal compression and extension of the lithosphere many of these features exhibit characteristic scales widths or spacings of deformation one of 10 20 km and another of 100 300 km for a range of simple models we test the hypothesis that these length scales are controlled by dominant wavelengths arising from unstable compression or extension of the venus lithosphere results show that the existence of tectonic features that exhibit both length scales can be explained if at the time of deformation the lithosphere consisted of a crust that was relatively strong near the surface and weak at its base and an upper mantle that was stronger than or of nearly comparable strength to the upper crust the spacings of these features imply crustal thicknesses in the approximate range 5 30 km and a thermal gradient not greater than 25 k km 1 features that exhibit only the smaller scale of deformation can be explained by either a lithosphere with a thick crust that overlies a weak mantle or a lithosphere with a strong mantle but with small internal strength contrasts for a broad range of parameters the models require not only that smaller scale compressional and extensional features have similar spacings but also that the widths and spacings of larger scale compressional features be greater than those formed in extension this is consistent with observed differences in the characteristic length scales of larger scale surface features as well as the similarities in spacings of observed smaller scale features

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

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

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

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

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

plains tectonism on venus the deformation belts of lavinia planitia high resolution radar images from the magellan spacecraft have revealed the first details of the morphology of the lavinia planitia region of venus a number of geologic units can be distinguished including volcanic plains units with a range of ages transecting these plains over much of the lavinia region are two types of generally orthogonal features that we interpret to be compressional wrinkle ridges and extensional grooves the dominant tectonic features of lavinia are broad elevated belts of intense deformation that transect the plains with complex geometry they are many tens to a few hundred kilometers wide as much as 1000 km long and elevated hundreds of meters above the surrounding plains two classes of deformation belts are seen in the lavinia region ridge belts are composed of parallel ridges each a few hundred meters in elevation that we interpret to be folds typical fold spacings are 5 10 km fracture belts are dominated instead by intense faulting with faults in some instances paired to form narrow grabens there is also some evidence for modest amounts of horizontal shear distributed across both ridge and fracture belts crosscutting relationships among the belts show there to be a range in belt ages in western lavinia in particular many ridge and fracture belts appear to bear a relationship to the much smaller wrinkle ridges and grooves on the surrounding plains ridge morphology tends to dominate belts that lie more nearly parallel to local plains wrinkle ridges and fracture morphology tends to dominate belts that lie more nearly parallel to local plains grooves we use simple models to explore the formation of ridge and fracture belts we show that convective motions in the mantle can couple to the crust to cause horizontal stresses of a magnitude sufficient to induce the formation of deformation belts like those observed in lavinia we also use the small scale wavelengths of deformation observed within individual ridge belts to place an approximate lower limit on the venusian thermal gradient in the lavinia region at the time of deformation

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

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

the role of salt in fold and thrust belts davis d m and engelder t 1985 the role of salt in fold and thrust belts in n l carter and s uycda editors collision tectonics deformation of continental lithosphere tectonophysics 119 67 88 the style of deformation in thin skinned fold and thrust belts is critically dependent upon the resistance to sliding along the detachment between the mass of deforming sediments and the underlying rocks evaporites can provide an extremely weak horizon within which a basal detachment can form and along which only a relatively small shear traction can be supported fold and thrust belts that form atop a salt layer such as the appalachian plateau the franklin mountains in northwestern canada and the jura of the alps among others share several readily observable characteristics as predicted by a simple mechanical model for fold and thrust belts a detachment in salt permits a thrust belt to have an extremely narrow cross sectional taper in addition predicted orientations of the principal stress axes over a salt dtcollement are consistent with the commonly observed lack of a consistently dominant vergence direction of structures within the thrust belt other common attributes of salt basal thin skinned deformation include the presence of several widely but regularly spaced folds and abrupt changes in deformational style at the edge of the salt basin

fault propagation folds induced by gravitational failure and slumping of the central costa rica volcanic range implications for large terrestrial and martian volcanic edifices long sublinear ridges and related scarps located at the base of large volcanic structures are frequently interpreted as normal faults associated with extensional regional stress in contrast the ridges bordering the central costa rica volcanic range ccrvr are the topographic expression of hanging wall asymmetric angular anticlines overlying low angle thrust faults at the base of the range these faults formed by gravitational failure and slumping of the flanks of the range due to the weight of the volcanic edifices and were perhaps triggered by the intrusion of magma over the past 20 000 years the thrust fault underlying alajuela ridge rises from a depth of 700 m at a dip of 15 over a distance of a few kilometers it ramps up at 600 m depth on a 40 slope and terminates in the axis of the forward syncline 400 m below the surface the hanging wall of this fault has been thrusted about 200 m away from the volcanic range axis thrusting on the lower flanks of the ccrvr is compensated by extension on the range axis with formation of summit grabens shallow seismicity in the volcanic range suggests that thrusting is still active we postulate that at kilauea volcano hawaii the gravity and magma induced slumping of the southern flank on low angle thrust faults generates hanging wall anticlines similar to those observed in costa rica these anticlines are hypothesized to occur along the base of the volcano where the thrust faults ramp up toward the sea bottom ridges and scarps between 2000 and 5000 m below sea level are interpreted as the topographic expression of these folds we further suggest that the scarps of the ccrvr and kilauea are valid scaled terrestrial analogs of the perimeter scarp of the martian volcano olympus mons we suggest that the crust below olympus mons has failed under the load of the volcano triggering the radial slumping of the flanks of the volcano on basal thrusts the thrusting would have in turn formed the anticlinal ridges and scarps that surround the edifice the thrusting seems to be preferential toward the northwest and southeast and a northeast southwest rift zone seems to have developed perpendicular to the regional topographic gradient the thrust faults may extend all the way to the base of the martian crust about 40 km and they may have been active until almost the end of the volcanic activity we suggest that gravitational failure and slumping of the flanks of volcanoes is a process common to most large volcanic edifices in the ccrvr and at kilauea this slumping of the flanks is a slow intermittent process but it could evolve to rapid massive avalanching leading to catastrophic eruptions thus monitoring of uplift and displacement of the folds related to the slump tectonics could become an additional effective method for mitigating volcanic hazards

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

a 16 ka oxygen isotope record from genggahai lake on the northeastern qinghai tibetan plateau hydroclimatic evolution and changes in atmospheric circulation moisture source history and changes in atmospheric circulation are the most important attributes for portraying past climate changes and for estimating possible future trends however few climate records reflecting these attributes are available from the marginal zones of the asian summer monsoon here we present a record of the oxygen isotopic composition of authigenic carbonates delta 18 ocarb of sequential sediments from genggahai lake in the northeastern qinghai tibetan plateau qtp isotopic analyses were performed on the fine grained carbonate fraction 38 mu m mollusc shells and stem encrustations from submerged plants the stratigraphic variations of the delta o 18 record from the different carbonate components exhibit a remarkably similar pattern probably reflecting the fact that delta 18 ocarb variability was controlled primarily by changes in the oxygen isotopic composition of the lake water delta 18 olw disequilibrium effects and water temperature are precluded as major factors affecting the delta 18 ocarb variations genggahai lake is hydrologically open and characterized by a rapid discharge rate as indicated by analysis of the hydrological setting of the lake system and by the observed significant positive correlation between delta o 18 lw and the oxygen isotopic composition of the inflowing water delta o 18 1 under such hydrological conditions we argue that the isotopic signals of different moisture sources should be reflected in the carbonate isotopic composition furthermore placing the delta 18 ocarb record in the context of regional palaeoclimate archives we found that the isotopic signals particularly the negative shifts from the average values cannot be interpreted consistently despite a process of evaporative enrichment at the lake surface during the early to mid holocene low delta 18 ocarb values during 10 6 9 4 and 7 4 63 ka were associated with higher lake levels and thus the record may have been significantly affected by a strengthened asian summer monsoon at those times and hence by a positive moisture balance isotopic depletions with similar magnitudes occurred at 15 14 5 13 8 13 3 12 5 11 4 5 3 4 8 3 7 3 4 2 8 2 3 1 7 1 3 and 0 6 1 a i e within the lateglacial and the late holocene these negative shifts in delta 18 ocarb corresponded to lower lake level phases and most likely were a response to intensification of the mid latitude westerly circulation that may have transported significantly more o 18 depleted moisture to the region compared to that from the asian summer monsoon overall our results suggest that the alternating influence of the asian summer monsoon and the westerlies played a significant role in determining the pattern of atmospheric circulation on the northeastern qtp c 2017 elsevier ltd all rights reserved

the geology and geomorphology of the venus surface as revealed by the radar images obtained by veneras 15 and 16 a region by region condensed description of almost all of the area that was radar photographed by veneras 15 and 16 is presented using some generalizations the diversity of terrain was reduced to a discrete set from which a geological morphological map was constructed the predominant type of terrain of the studied area is a plain that was tentatively subdivided into five morphological types ridge and band patchy rolling plain dome and butte plain smooth plain and high smooth plain stratigraphically the ridge and band plains are the oldest and the smooth plains are the youngest the stratigraphic position of the other types is yet to be determined large sections of the plains show similarities to the mare type basaltic plains of the moon mercury and mars other types of terrain are combinations of ridges and grooves in various patterns linear parallel orthogonal diagonal or chevron like and chaotic in some places the ridge and groove terrain is stratigraphically below the plain material but in other places it appears to be plain material that has been subsequently deformed near the eastern and western boundaries of ishtar terra large several hundred kilometers in diameter ring like features can be seen that are named coronae or ovoids evidence of tectonic deformation and the presence of flow like patterns support their designation as volcano tectonic features beta regio seems to be an uplifted plain showing evidence of rifting and volcanism all types of terrain are sparesely peppered with craters of obvious impact morphology their average density gives the plain an age range of 0 5 to 1 109 years the fact that many impact craters are still in the pristine state indicates a very low rate of surface reworking at least for the last 0 5 to 1 109 years no evidence for water erosion sedimentation processes has been found the tectonic activity of venus has no equivalent on the moon mercury or even mars and can be compared only with that of the earth intensive horizontal deformation previously known only on earth occurs on venus but in a characteristic venusian style

styles of tectonic deformations on venus analysis of venera 15 and 16 data analysis of the radar images obtained by veneras 15 and 16 leads to the conclusion that the ridge and groove structures on the surface of venus are the result of tectonic deformation although the mechanism of such deformation cannot yet be unequivocally deduced several styles of deformation can be described areal deformation occurs where horizontal stresses have operated over large areas shear deformation appears in bands showing differential longitudinal deformation transversal stresses operating over long and relatively narrow areas have produced belt deformation circular deformation is related to a specific locus of the stresses the absence of densely cratered areas indicates that the terrains were deformed after the period of heavy bombardment the origin of the stresses could be drag of the lithosphere by asthenospheric currents or gravity induced spreading of surface material over upwellings a general conclusion is that in the surveyed area of venus neither terrestrial plate tectonics nor lunar highland type terrain exists

constraints on geological strain rates arguments from finite strain states of naturally deformed rocks a summary of known finite strain states is presented longitudinal strains 1 e as measured in many rocks often range from 1 to 40 and 1 to 0 025 the time span available to produce such measureable strains in young orogenic zones seems to be less than 10 m y possibly less than 1 m y which constrains conventional strain rates into the range of 10 sup 13 s sup 1 to 10 sup 15 s sup 1 for both pure and simple shear the most efficient way and much less efficient way to accumulate incremental strains respectively the ellipticity of the finite strain ellipse increases in a nonlinear manner finite strain variations in adjacent layers which give rise to features such as cleavage refraction arise with only slight differences in the strain rates within these layers

origin of planetary wrinkle ridges based on the study of terrestrial analogs wrinkle ridges linear asymmetric topographic highs are common physiographic features on the moon mars and mercury we describe here some terrestrial features of similar morphology and scale that have formed under compressional stress systems similar to planetary wrinkle ridges all of the terrestrial analogs are produced by the anticlinal folding of rocks above reverse faults that shallow and typically break the surface characteristics common to both terrestrial and planetary ridges include 1 a linear asymmetric antiformal shape 2 tension cracks and grabens along the hinges of the antiform 3 overlapping en echelon lobes 4 reversals of the sense of asymmetry along strike 5 occurrence in compressional environments and 6 similarity to features produced during compression of various materials in laboratory scale models these similarities strongly suggest that planetary wrinkle ridges result from deformation associated with shallow thrust faults that either break or come close to the surface kinematic models capable of explaining the development of wrinkle ridges include 1 anticlinal folding of rocks over a buried thrust fault that may subsequently propagate to the surface and 2 fault bend folding of upper plate rocks over the surface flattening bend of a thrust fault

origin of convex accretionary wedges evidence from barbados the surface slopes of submarine accretionary wedges generally decrease away from the toe or defor mation front toward the arc this paper presents evidence that this characteristic convex cross sectional shape is a direct mechanical response to the lithification of sediments during accretion the recently accreted sediments near the toe which are typically unconsolidated and therefore very weak must deform until a critical taper is attained at which they are stable in the presence of the horizontal tectonic compression this critical wedge taper and thus the bathymetric slope of the accretionary prism is relatively high for the porous weak sediments near the toe in contrast the compacted sediments farther back in the wedge are stronger so they require a relatively modest critical taper and therefore a lower bathymetric slope to be stable under the same tectonic compression the result is a convex cross sectional wedge shape we investigate several possible causes of the increase in sediment strength within the wedge and conclude that the primary cause is an increase in cohesive strength with decreasing porosity a straightforward generalization of the exact critical taper model is combined with empirical cohesion porosity velocity relations to infer the distribution of porosity and p wave velocity within the barbados accretionary wedge we estimate that the porosity decreases from its near toe value of 70 to approximately 30 at a depth of 3 km below the seafloor and a distance of 60 km arcward from the front of the wedge

orogenic belts on venus recent radar observations of venus show evidence for three types of deformational features in the linear mountain belts of ishtar terra linear ridges and troughs oriented parallel to the strike of the mountains and interpreted to be anticlines and synclines broad low arches similar to mare ridge type features seen on the moon and interpreted to represent low angle thrusting and buckling and linear discontinuities that cut across the strike of the ridges and troughs and are interpreted to represent strike slip movement on the basis of the localized concentration of these features their orientations and patterns of distribution and their association with linear mountainous topography we conclude that they mark the location of concentrated horizontal compressional deformation and that akna and freyja montes represent erogenic belts on venus

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

convection driven tectonics on venus an analysis is presented for convective stress coupling to an elastic lithosphere as it applies to tectonic deformation on venus theoretical solutions are introduced for the response of a mathematically thick elastic plate overlying a newtonian viscous medium with an exponential depth dependence of viscosity a green s function solution is obtained for the viscous flow driven by a harmonic density distribution at a specified depth it is shown that during the transient phase of the solution the field quantities stress velocity displacement in both media depend upon the rheology of both media whereas in steady state the solutions become uncoupled due to this uncoupling stresses obtained from a purely viscous solution can be applied as boundary conditions at the base of an isolated elastic plate i show for such a plate that 1 shear stress imparted at the base of the lower boundary can induce large in plane stretching mode forces and 2 these forces can be estimated directly from free air gravity anomalies without detailed knowledge of the density distribution in the underlying flow free air gravity anomalies for venus imply in plane forces of about 2 3 1012 n m in plane forces of this magnitude are capable of extensive disruption of the venusian lithosphere to demonstrate this an elastic plastic analysis is carried out for the deformation of a model venus lithosphere the calculation is based on a lithospheric strength envelope and uses gravity and topography as a guide to the magnitude of stresses imparted by convective flow the results predict that dynamic uplift of venusian topography must be accompanied by extensive brittle failure and viscous flow in the lithosphere geoid anomalies and geoid to topography ratios for venusian topographic features are large compared to values for thermal swells in earth s ocean basins this suggests that dynamic support of topography must be an important process on venus further a low viscosity zone is probably not present beneath the venusian lithosphere a comparative analysis suggests that tectonic deformation of the lithosphere associated with dynamically supported topography on earth is unimportant

venus banded terrain tectonic models for band formation and their relationship to lithospheric thermal structure recent radar images of the mountains of ishtar terra venus at approximately 3 km radar resolution show a series of linear bands of alternatively greater and lesser backscatter the bands are generally aligned with the topographic strike of the mountain ranges we test the hypothesis that these band features are tectonic in origin the product either of folding during lithospheric compression or of block faulting during lithospheric extension tested models for folding include uniform elastic and viscous plates overlying inviscid substrates layered plates and a viscous half space in which viscosity decreases exponentially with depth the uniform plate models can produce folds at a dominant wavelength equal to the characteristic spacing between bands on venus about 15 to 20 km if the elastic or high viscosity layer is at most a few kilometers in thickness though the required compressive stresses are several kilobars in magnitude for the case of an exponentially decreasing viscosity the skin depth plays the role of layer thickness and similar results hold layered elastic or viscous plates however can fold at the required wavelength with subkilobar stresses and therefore are favored over uniform plate models tested extensional models include graben and horst formation in a surface elastic brittle layer imbricate normal faulting and necking of a surficial layer that is everywhere in a state of extensional failure the spacing between adjacent extensional features can match the spacing between bands on venus if the surficial brittle layer is no more than a few kilometers in thickness both compressional and extensional models for band formation are therefore consistent with radar images of venus mountain ranges at the image resolution presently available additional evidence however including the linearity and continuity of individual bands the relationship between band trends and topographic contours in some regions near the ends of mountain ranges and the very different manifestations of lithospheric extension in other highland areas of venus leads us to favor the hypothesis that banded terrain formed contemporaneously with the mountain ranges of ishtar terra as a result of horizontal compression of the venus lithosphere independent arguments on the thermal structure of the venus crust and the mechanical behavior of crustal rocks as a function of temperature indicate that the elastic lithosphere of venus is approximately 1 10 km thick we therefore suggest that the surficial layer of elastic brittle or high viscosity behavior required for either the compressional or extensional models may in fact be the elastic lithosphere of the venus highlands at the time of band formation

venus tectonics initial analysis from magellan radar imaging and altimetry data from the magellan mission have revealed a diversity of deformational features at a variety of spatial scales on the venus surface the plains record a superposition of different episodes of deformation and volcanism strain is both areally distributed and concentrated into zones of extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that many features in the plains reflect a crustal response to mantle dynamic processes ridge belts and mountain belts represent successive degrees of lithospheric shortening and crustal thickening the mountain belts also show widespread evidence for extension and collapse both during and following crustal compression venus displays two geometrical patterns of concentrated lithospheric extension quasi circular coronae and broad rises with linear rift zones both are sites of significant volcanism no long large offset strike slip faults have been observed although limited local horizontal shear is accommodated across many zones of crustal shortening in general tectonic features on venus are unlike those in earth s oceanic regions in that strain typically is distributed across broad zones that are one to a few hundred kilometers wide and separated by stronger and less deformed blocks hundreds of kilometers in width as in actively deforming continental regions on earth

a forward modeling strategy for balancing cross sections 1 a strategy to balance cross sections of complex structures is documented and illustrated by the interpretation of a compressional structure in the deep water gulf of mexico the strategy is applicable to structures formed in sedimentary rocks under low temperatures in both compressional and extensional environments and involves the comparison of the observed structure with simple balanced forward models forward models generated using fault related fold theory help in understanding the processes and kinematics involved in the deformation further forward models are completely constrained and easy to balance whereas it is difficult to balance data therefore forward models are useful in evaluating ideas without completely solving the structure models are constructed assuming parallel behavior preservation of layer thickness no net distortion where layers are horizontal and conservation of bed length unmetamorphosed sedimentary rocks are generally observed to deform obeying the assumptions of parallel behavior in field map well and seismic data