crustal spreading on venus evidence from topography morphology symmetry and map patterns abstract evidence for numerous features similar to terrestrial divergent plate boundaries and spreading centers are present in the topography and morphology of western aphrodite terra venus head and crumpler 1987 linear discontinuities cutting across the strike of western aphrodite terra cross strike discontinuities or csds were examined in further detail it was found that 1 1 they represent the most prominent regional structural trend in western aphrodite and are readily distinguishable from linear patterns caused by ground tracks in the pioneer venus orbiter data 2 2 individual csds are either deep troughs or steep regional scarps and steps 3 3 the spacing between the more prominent csds ranges from about 500 to 1500 km average spacing is 850 km and 4 4 there is a distinctive structural trend orthogonal to the strike of the csds additional smaller scarps and slopes with characteristics of known csds occur in the arecibo altimetric profiles at spacings of as little as 50 km in agreement with previous suggestions of sotin et al 1989 that other smaller csds with less offset may occur in domains between the more prominent ones previously mapped the detailed characteristics of the csds detailed trough and scarp like form association with ridge offsets linear terminations linear slope traces parallelism great length are all very similar to the characteristics of transform faults and fracture zones on the earth s seafloor and this additional analysis supports the earlier interpretation that csds represent analogs to terrestrial oceanic fracture zones assessment of bilateral symmetry of topography and other elements across western aphrodite terra indicate that 1 1 an accurate prediction of the large and small scale symmetrical elements in both the altimetry data and radar images within each domain on one flank of the highlands may be made by substitution of a mirror image of the opposite flank and 2 2 smaller wavelength symmetrical elements are revealed in the altimetric profiles by subtracting the background regional symmetry and examining the residuals residual altimetry profiles obtained in this manner show that individual altimetry features several hundred kilometers wide and several hundred meters high are frequently located at the same distance from the axis of regional symmetry on the north and the south flanks of the highlands topography and geometric features such as ridge crests and transforms have specific predictable relationships in a crustal spreading environment tests were made in western aphrodite for such organized relationships predicted to occur in association with ridge crest offsets at transform faults and fracture zones it was found that 1 a topographic stepdowns occur across csds in accordance with the general consequences of juxtaposing lithospheres of different relative ages 2 b the correct direction of stepdown down toward older crust and lithosphere occurs relative to that predicted from ridge crest offsets and 3 c the observed amount of stepdown is similar in many cases to that predicted from the observed ridge crest offsets these analyses of cross strike discontinuities topographic and image symmetry across aphrodite and predictions of topography based on ridge crest and fracture zone geometric relationships further support the earlier interpretation that the 7500 km extent of western aphrodite terra represents the site of crustal spreading and displays many of the characteristics of terrestrial divergent plate boundaries

palaeo moisture evolution in monsoonal central asia during the last 50 000 years abstract the late quaternary climate history of monsoonal central asia was inferred from 75 palaeoclimatic records which provide information about moisture conditions in the last 50 ka or part of this period wet conditions occurred during middle and late marine isotope stage 3 while the last glacial maximum lgm was characterized by dry climate conditions in the region a stepwise climate amelioration is suggested by the climate records following the lgm several climate signals of this period which were reported from high latitude ice core records are preserved in archives from monsoonal central asia as well during the early holocene high effective moisture was inferred from most records from the area dominated by the indian monsoon e g the tibetan plateau suggesting that holocene optimal climate conditions occurred there during this period in contrast areas which are dominated by the south east asian monsoon se monsoon and the westerlies in north western and north central china mongolia do not uniformly show an early holocene climate optimum for this area optimal conditions prevailed during the mid holocene these apparent contradictions can possibly be explained by the regional uplift and descent of air masses in the holocene during the early holocene strengthened insolation possibly caused an enhanced low level convergence over the tibetan plateau which led to the intensification of the summer monsoon the strong air uplift caused intensified precipitation and air divergence in the upper troposphere over the tibetan plateau the areas adjacent to the north therefore experienced an intensified descent of air masses and consequently increased aridity the majority of the palaeoclimatic records suggest reduced effective moisture since the late holocene in the region

organic geochemistry of lower cretaceous sediments from northwestern pacific guyots odp leg 143 organic geochemical data of lower cretaceous shallow water sediments from two sites 865 and 866 drilled during odp leg 143 are presented the organic matter is mainly terrestrial at the bottom of the sedimentary column at site 865 whereas algal and or bacterial organic matter is dominant at site 866 this is the first evidence of shallow water deposition of organic matter during the early cretaceous in the northwestern pacific the lower aptian organic carbon rich layers from the shallow water sediments of site 866 are geochemically similar to coeval mid water sediments of site 463

venus trough and ridge tessera analog to earth oceanic crust formed at spreading centers one type of tessera terrain on venus the trough and ridge tessera exhibits a distinctive morphology composed of throughgoing generally parallel linear valleys and shorter orthogonal valleys and ridges the similarity of this pattern to oceanic crustal topography and morphology is examined oceanic crust on earth is characterized by a distinctive orthogonal pattern with transforms and fracture zones in one direction and linear abyssal hills developed parallel to the rise crest in the other similarities between the components of the tessera terrain and terrestrial seafloor fabric and structure are found in terms of linearity parallelism length width spacing orthogonality and general morphology a difference is that trough and ridge tessera tend to occur as plateau like regions at intermediate elevations average 2 km above the surrounding plains while terrestrial seafloor ranges in elevation from that observed at rise crests and oceanic plateaus such as iceland down to depths of several kilometers on the abyssal floors where the orthogonal pattern is often masked by sedimentation on the basis of the similarities it is proposed that trough and ridge tessera may have originated through processes analogous to those responsible for the ocean floor fabric on earth forming at a rise crest similar to divergent boundaries on the earth s seafloor and evolving to its present morphology and configuration through processes of crustal spreading the plateau like nature of the trough and ridge tessera distribution could be the result of localized crustal thickening in the spreading process producing elevated iceland like plateaus whose texture is preserved from subsequent lowland volcanic flooding some regions of more complex tessera patterns may result from deformation of the basic trough and ridge pattern of tessera this hypothesis can be tested with global high resolution imaging and altimetry data and gravity data from the magellan mission

the sedimentary structure of linear sand dunes linear sand dunes dunes that extend parallel to each other rather than in star like or crescentic forms are the most abundant type of desert sand dune1 but because their development and their internal structure are poorly understood they are rarely recognized in the rock record2 models of linear dune development2 3 4 5 6 have not been able to take into account the sub surface structure of existing dunes but have relied instead either on the extrapolation of short term measurements of winds and sediment transport or on observations of near surface internal sedimentary structures from such studies it has not been clear if linear dunes can migrate laterally2 7 8 here we present images produced by ground penetrating radar showing the three dimensional sedimentary structure of a linear dune in the namib sand sea where some of the world s largest linear dunes are situated these profiles show clear evidence for lateral migration in a linear dune moreover the migration of a sinuous crest line along the dune produces divergent sets of cross stratification which can become stacked as the dune height increases and large linear dunes can support superimposed dunes that produce stacked sets of trough cross stratification these clear structural signatures of linear dunes should facilitate their recognition in geological records

lithospheric flexure at fracture zones bathymetric profiles across six major fracture zones fz s in the north pacific are used to demonstrate the absence of vertical slip on the fossil fault planes the scarp heights on these fz s are constant with age and equal to the initial vertical offsets at the ridge transform fault fz intersections because of the frozen in scarp and the differential subsidence of lithosphere far from the fz the lithosphere bends in the vicinity of the fz this flexure results in a characteristic ridge trough topographic fz signature the flexural amplitude which is the difference between the scarp height and the overall change in depth across the fz increases with age good fits to the bathymetric profiles across the mendocino and pioneer fz s are obtained by modelling the topography as the flexure of a thin elastic plate with an age dependent effective elastic thickness results of the modelling indicate that the base of the elastic lithosphere is approximately defined by the 450 c isotherm maximum bending stresses at fz s are on the order of 100 mpa substantially less than the stresses encountered at subduction zones because the mendocino and pioneer fz s are separated by less than a flexural wavelength they are elastically coupled

elongation and migration of sand dunes two distinct processes are known to act on dynamic dunes the process of migration by erosion on the windward side and deposition on the lee side typical for transverse dunes and the process of elongation typical for linear dunes these two processes are determined by wind direction relative to the dune alignment this article reviews the assertion that linear dunes experience lateral displacement in addition to elongation fieldwork on vegetated linear dunes vlds and gis work on seif dunes indicates no lateral migration for these dunes linear dunes can shift laterally only when a slip face formed on the lee side reaches the plinth of the dune the winds from both sides of the seif dune are never symmetric usually winds from one direction are more dominant and effective the outcome is the formation of peaks and saddles along the dune the strongest winds create a slip face on the lee side of the peak segments of the dune oblique to the dune alignment which reaches the base of the dune and displaces the peak downwind along the dune alignment the internal structure of the seif dune is formed mostly by this dominant wind direction and gives the impression that the dune has shifted laterally on the other hand there are cases in which the wind directions relative to dune alignment fall between those of transverse and seif dunes in such cases both processes act on the dune which subsequently experiences migration as well as elongation

osl chronology of sand deposits and climate change of last 18 ka in gurbantunggut desert northwest china the development of the gurbantunggut desert is mainly controlled by the westerly wind and at present has little influence from indian and southeast asian monsoons a combined study using optically stimulated luminescence osl dating ground penetrating radar gpr surveys and climatic proxies analysis was carried out in the southern part of the desert the chronology extends back to 18 ka and is constructed based on 16 osl ages from boreholes in the linear dune body and the inter dune area the chronology suggests that sand deposition in the last 18 ka experienced two rapid accumulation phases at 11 and 2 5 ka ago which were also evident from the gpr surveys these periods relate to increased aridity in the region five climate phases are identified from the last 18 ka based on the osl chronology and climatic proxy analysis with grain size and magnetic susceptibility measurements the deglacial period 18 3 10 4 ka is characterized by climate instability and possible glacial melting events the holocene optimum peaked 8 5 ka and terminated 3 6 ka ago when the regional climate became arid osl samples from the dune body cluster around 2 5 ka which indicates rapid advance extension of dune bodies at this time copyright 2011 john wiley sons ltd

venus gravity anomalies and their correlations with topography this report provides a summary of the high resolution gravity data obtained from the pioneer venus orbiter radio tracking data gravity maps covering a 70 latitude band through 360 of longitude are displayed as line of sight and vertical gravity topography converted to gravity and bouguer gravity maps are also shown in both systems topography to gravity ratios are made over several regions of the planet there are markedly different ratios for the aphrodite area as compared to the beta and atla areas

depth and age in the north pacific the concept of the oceanic lithosphere as a cooling thermal boundary layer on top of a viscous upper mantle accounts for the increase in depth with age of the ocean floor analyses of the exact nature of this relation provide constraints on the rheology and structure of this layer in past studies problems with uncertainties of age and in sediment thickness have limited the value of this relation on older ocean floor in this study we evaluate different tectonic histories for the quiet zones and construct a revised isochron chart for the north pacific we check and modify the current sediment thickness charts using new deep sea drilling data and revised ideas concerning reflectors in the southwestern north pacific averaging the synbaps digital data base into half by half degree elements and correcting for sediment load we plot basement depth versus age for the entire north pacific because of the skewness in the data associated with seamounts we present the data as contours about the mode and find that two thirds of the observations lie within 300 m of this value on crust between 0 and 80 ma the ocean floor subsides with the square root of age on ocean floor older than 100 ma the depths in general flatten with age with areally correlateable swells and valleys the hawaiian and line island swells correlate positively and the mariana basin valley negatively with geoid height the flattening in the depths indicates that the thermal boundary layer has reached equilibrium conditions under older ocean floor further the correlation of the swells and valleys with geoid height presents evidence for organized convection in the upper mantle beneath older lithosphere in the north pacific

evidence for divergent plate boundary characteristics and crustal spreading on venus detailed examination of the topography and morphology of western aphrodite terra reveals numerous features that are similar to terrestrial divergent plate boundary characteristics individual domains between fracturezone like discontinuities contain a variety of bilaterally symmetrical topographic elements that suggest that topographic features have been created at rise crests rifted and separated and moved laterally to their present symmetrical positions the topographic and morphologic similarities together with strikingly similar mirror image map patterns on both sides of the rise axis suggest that western aphrodite terra shares the characteristics of oceanic divergent plate boundaries and is the site of crustal spreading on venus topographic profiles are consistent with spreading rates of the order of several centimeters per year

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

a late pleistocene holocene lacustrine record from lake manas zunggar northern xinjiang western china information on the late quaternary palaeoclimate in the zunggar desert northern xinjiang is deduced from the study of three 5 m long cores taken from lake manas analyses including the mineralogical composition of bulk sediments stable isotope contents of carbonates organic matter pollen and diatoms were performed to help reconstruct the environmental evolution of the area the chronology is provided by 9 ams 14c dates done on carbonates or organic matter the record suggests a late pleistocene humid episode radiocarbon dated at 37 000 32 000 yr b p this episode is followed by a period of extreme aridity attributed to the last glacial maximum the early mid holocene record indicates conditions generally wetter than today as previously established for western china however our results show that the holocene period is rather complex after a minor wet warm pulse estimated to be around 12 000 yr b p the most important environmental change is recorded at 10 000 yr b p it led to the establishment of a steppic vegetation cover in the catchment and of a permanent lake which lasted 4000 years fluvial sedimentation took place between about 6000 and 4500 yr b p and it coincided with a decrease in moisture availability for vegetation growth a second lacustrine episode is recorded from about 4500 to 2500 yr b p although the pollen record and a sudden influx in detrital organic matter suggest a short term dry event around 3800 3500 yr b p the last 2500 years show climate unstability and the step wise establishment of hypersaline conditions in lake manas the major post glacial environmental changes appear to be roughly in phase with those observed in tibet

a hotspot model for iceland and the voring plateau the proximity of a hotspot to a spreading center may result in the channeling of atmosphere to the section of rise crest closest to the hotspot this produces more basalt and thicker crust at these locations thus forming a plateau over time the geometric constraints of such a model predict a unique orientation location and age progression for a plateau formed by this mechanism the hotspot will channel material to the closest part of the rise therefore the orientation of the plateau will differ from that of a hotspot track by the component of absolute motion perpendicular to the rise axis the plateau will be symmetric with respect to the location of the rise axis at the time of formation also the age progression of the plateau will be contemporaneous with the age of formation of the seafloor on either side because the plateau is seafloor just with thicker crust a set of reconstructions based upon magnetic isochrones and a fixed hotspot reference frame is presented for the norwegian greenland sea as a means of evaluating the model s predictions by locating the iceland hotspot reconstructing the relative positions of the greenland and european plates and then assuming material would be channeled from the hotspot to the closest section of the rise crest we can trace the tectonic evolution of the greenland faeroe and voring plateaus the model is able to locate the plateaus explain their orientations and predict an age progression that satisfies observed age determinations the analysis demonstrates that both plateaus could have been formed by the iceland hotspot with the greenland faeroe plateau being in effect a continuation of the voring plateau which was cut off due to transform motion between the northern and southern spreading centers

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

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

review of oceanic transform activity and development oceanic transforms generally have a complex morphology and structure and they do not conform to the ideal concept of simple fractures that exactly follow small circles centered on euler poles of relative plate motions it is proposed that the special properties of transforms arise because they are inherently leaky i e new plate area forms along them as transforms have finite widths the adjacent ridge tips are at different distances from the euler poles and the plate boundaries along transforms are slightly oblique to the direction of plate motions such that new plate area forms along them however along transforms the normal process of seafloor spreading is interrupted and new plate area forms by a different process transforms are underlain by crust consisting of variable proportions of a material emplaced in the transform domains and b stretched ridge generated crust the transform structure is further modified by shearing deformation related to irregularities of lateral faults and because of propagation of adjacent ridges transform leakiness is not considered to result merely from changes of plate motions though such changes will influence transform structure but it reflects the segmented structure of mid oceanic ridges the persistence of transforms and their migration in the absolute reference frame with a component perpendicular to their strikes shows that the ridge structure is decoupled from flow in deeper levels thermal anomalies along transforms especially enhanced heat losses stabilize the segmented ridge structure

a mechanism for tectonic deformation on venus in the absence of identifiable physiographic features directly associated with plate tectonics alternate mechanisms are sought for the intense tectonic deformation observed in radar images of venus one possible mechanism is direct coupling into an elastic lithosphere of the stresses associated with convective flow in the interior spectral green s function solutions have been obtained for stresses in an elastic lithosphere overlying a newtonian interior with an exponential depth dependence of viscosity and a specified surface density distribution driving the flow at long wavelengths and for a rigid elastic fluid boundary condition horizontal normal stresses in the elastic lid are controlled by the vertical shear stress gradient and are directly proportional to the depth of the density disturbance in the underlying fluid the depth and strength of density anomalies in the venusian interior inferred by analyses of long wavelength gravity data suggest that stresses in excess of 100 mpa would be generated in a 10 km thick elastic lid unless a low viscosity channel occurring beneath the lid or a positive viscosity gradient uncouples the flow stresses the great apparent depth of compensation of topographic features argues against this however thus supporting the importance of the coupling mechanism if there is no elastic lid stresses will also be very high near the surface providing also that the viscosity gradient is negative

dune activity as a record of late quaternary aridity in the northern kalahari new evidence from northern namibia interpreted in the context of regional arid and humid chronologies abstract sediments from presently vegetated linear dune ridges in the caprivi strip of northern namibia are optically dated to assess the timing of dune building phases and aridity in the late quaternary the dunes have been constructed from sediments likely to have been initially transported into the area by fluvial processes dune sediments do not preserve obvious evidence of hiatuses in deposition in the dated period 48 121 ka but continuous deposition and therefore continuous aridity is considered unlikely given differences in dated phases compared with chronologies from western zimbabwe to the southeast and western zambia to the north that include ages derived from the same equivalent sampling depths laboratory experiments provide evidence that dune sediments have not been bioturbated since deposition such that sample palaeodoses determined from different sampling depths are a true reflection of the duration of sediment burial independent chronologies from cave sediments provide evidence that humid episodes have occurred in the region within the dated range of dune construction it is concluded that dune construction in the late quaternary has been punctuated that the preserved record reflects the attributes of the aeolian processes that affect linear dune development and that the late quaternary record of dune building in the middle and northern kalahari was spatially complex

evidence for variations in magma production along oceanic spreading centers a critical appraisal recent geologic geophysical and geochemical studies of the oceanic lithosphere near fracture zones have resulted in the proposal that the magma budget defined as the amount of magma delivered to magma chambers or conduits beneath a spreading center for a given amount of spreading decreases as fracture zones are approached seismic reflection and refraction studies indicate that the crust becomes anomalously thin near fracture zones but reference must be made to the boundary between residual upper mantle peridotites and overlying cumulate rocks in order to assess potential variations in the magma budget the position of this interface however generally is not constrained by geophysical studies geochemical variations in basaltic glasses collected near fracture zones are consistent with a decrease in partial melting as fracture zones are approached but they could also be produced by variations in open system mag matic processes with no change in the extent of partial melting although a decrease in the magma budget as fracture zones are approached is consistent with these data so are alternative models that incorporate a constant magma budget

late quaternary linear dune accumulation and chronostratigraphy of the southwestern kalahari implications for aeolian palaeoclimatic reconstructions and predictions of future dynamics abstract the linear dunes of the kalahari now largely inactive have long been identified as having potential palaeoenvironmental significance the application of optically stimulated luminescence osl dating to these dunes in the 1990s provided the first chronology of aeolian accumulation in this region though field methodologies and time consuming multiple aliquot laboratory protocols limited both the depth of sampling in dune bodies and the total number of samples dated in order to permit a more thorough investigation of the potential of these dunes to preserve long chronological records this intensive study presents 71 osl ages from the linear dunes of the southwestern kalahari at witpan south africa sampled with coring equipment at regular and frequent intervals down to bedrock the earliest sand accumulation recorded at witpan is at 104 ka and in spatially discrete locations other evidence of dune activity is recorded at 77 76 57 52 and 35 27 ka although an inherently discontinuous archive the linear dunes of the southwestern kalahari have the potential to record multiple phases of dune construction following the last glacial maximum there is near continuous evidence of dune building with a peak of accumulation recorded from 15 to 9 ka at five individual sites this latter period is generally recognised from other proxy evidence as being unusually arid in this region and such periods of dune activity are likely to be related to intensification of the continental anticyclone during the holocene accumulation has continued at most sites sampled albeit at a lesser intensity this may imply that these dunes are presently not far from thresholds of activation

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

optical dating of dune sands in the northeastern deserts of china abstract optical dating has been used to obtain the ages of fossil stabilized sand dunes from four sections in the northeastern deserts of china our results indicate that the optically stimulated luminescence ages of the four sections correlate well even though the samples were collected from different deserts about 600 km apart our results also indicate that active dune formation in this region lasted from the last glacial maximum to about 10 ka and that the warm climate of the holocene was interrupted by a cold dry dune formation episode about 3 5 1 7 ka the holocene optimum in this region is between 10 and 3 6 ka and a later warm humid dune stabilization phase lasted from at least 1 6 to 1 0 ka the youngest age on the uppermost sand unit yielded an age of only 40 yr supporting the previous argument that the existence of modern active eolian sands in the regions with a mean annual precipitation of up to 450 mm is not mainly due to drought but to extensive land cultivation over historic time from the luminescence properties of the quartz grains it is hypothesized that the sands in most of the sections are probably derived from more than one source with a minor source of quartz having a different thermal history before deposition

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

wind energy environments and dunefield activity in the chinese deserts there are few detailed analyses of the wind energy environments and dunefield activity in the chinese deserts on the basis of wind data and other climatic parameters collected at meteorological stations in and around the deserts this paper considers wind environments and dunefield activity in the chinese deserts in the 1990s the results show that dunes in most of the deserts were developed and controlled by low energy wind regimes although most dunes had the morphology of transverse and linear types which is consistent with wind regimes some deserts did not show close relations between dune morphology and regional wind directional distributions due to other controls although sand transport mainly occurs in spring and early summer there is strong sand transport activity in winter in some deserts dune activity in the chinese deserts has great spatial variations the regions with low dune activity are located in the chaidm mu us hobq horqin and nenjiang deserts due to much higher annual precipitation in these regions the areas with strong dune activity are located in the eastern taklimakan and northern badain jaran deserts due to arid conditions and much stronger winds

application of a soil water balance model to evaluate the influence of holocene climate change on calcic soils mojave desert california u s a abstract we used a process based soil water balance model to simulate the downward flux of soil water under varied conditions of climate vegetation and soil texture to determine the potential impact of episodic periods of wetter pluvial climate during the holocene on calcic soils in the mojave desert that have a bimodal distribution of carbonate daily weather data associated with a relatively wet climate years with extreme increases in annual rainfall 33 cm yr and dry climate historic average annual rainfall 15 cm yr was used to simulate the affects of pleistocene and holocene climate change on soil water balance linkages among atmospheric circulation patterns regional increases in precipitation and historic flooding in the mojave desert california suggest that historic wet years provide an analog for wetter climates that occurred during the last glacial period latest pleistocene and episodically during holocene periods of pluvial activity modeling results indicate that soil water balance for dry and wet years strongly corresponds with the upper and lower zones of carbonate accumulation respectively soil water only reached the lower zone of carbonate during a wet year when extreme increases in winter spring storm activity resulted in a significant increase in precipitation and the downward flux of soil water the linkage between increases in frontal storm activity and pluvial events suggests that the shallow zone of the bimodal distribution of carbonate is a result of periods of significant decreases in winter and spring rainfall and not primarily due to increases in holocene temperature or the development of clay rich horizons calculation of carbonate solubility and accumulation rates suggests that the bimodal distribution of carbonates in soils may have also been impacted by episodic periods of extreme increases in precipitation associated with perennial lakes during the holocene results suggests that much of the carbonate in the upper 75 cm of pleistocene soils may have accumulated during the late holocene rather than throughout the entire holocene

quantitative tests for plate tectonics on venus if plate tectonics exists on venus the rate of topographic dropoff from spreading centers should be about half that on earth due to greater rock fluid density contrast and lower temperature differential between the surface and interior using 1 1 mean elevations 21 000 km of ridge were identified on venus and 33 000 km on earth heights h at distances up to 1000 km to either side of the ridges were analyzed to make least squares determinations of the parameters ho h and n in where s is off crest distance the venus ridges have a much less well defined mode in crest heights ho than the earth ridges and a somewhat smaller exponent n on the average indicating greater concavity fixing n to the theoretical value of 0 5 plate velocities were inferred from the h s using h t from north atlantic and north pacific data on earth these velocity solutions were compared to those from plate rotations which indicate the h t is probably about 40 too high for a global mean the resulting plate creation rates on venus assuming comparable total heat generation and taking into account the density and temperature differences are 0 7 km yr or less and indicate that not more than 15 of venus s energy is delivered to the surface by plate tectonics in contrast to 2 9 km yr and 70 energy delivery on earth

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

venus limited extension and volcanism along zones of lithospheric weakness three global scale zones of possible tectonic origin are described as occurring along broad low rises within the equatorial highlands on venus lat 50 n to 50 s long 60 to 310 the two longest of these tectonic zones the aphrodite beta and themis atla zones extend for 21 000 and 14 000 km respectively several lines of evidence indicate that beta and atla regiones located at the only two intersections of the three major tectonic zones are dynamically supported volcanic terranes associated with currently active volcanism rift valleys south of aphrodite terra and between beta and phoebe regiones are characterized by 75 to 100 km widths raised rims and extensions of only a few tens of kilometers about the same magnitudes as in continental rifts on the earth horizontal extension on venus was probably restricted by an early choking off of plate motion by high crustal and upper mantle temperatures and the subsequent loss of water and an asthenosphere

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

fracture zones in the north atlantic morphology and a model a morphology is presented for the typical cross section of an inactive fracture zone in the atlantic inactive fracture zones typically consist of an asymmetric valley to the young side of the fracture zone plane and a high wall or scarp to the old side large offset fracture zones may be accompanied by a marginal valley on the other side of the high wall this topography is superimposed on the depth age step due to lithospheric cooling a model is developed which accounts for this morphology the model relates the inactive fracture zone morphology to the topography found at present day intersections of the spreading axis with transform faults the asymmetry of the median valley near fracture zones plays an important role in explaining the typical fracture zone morphology the existence of a median valley is related to the viscous delay of the upwelling mantle material at the spreading axis its asymmetry near fracture zones can be accounted for by modelling the viscous drag exerted by the lithosphere on the asthenosphere if the viscosity is low as under reykjanes ridge and the east pacific rise no median valley develops and a different morphology may be expected the occurrence of marginal valleys is interpreted as the result of lithospheric warping when the graben walls in the transform domain which are caused by tension due to horizontal thermal contraction are excessively high

multiple episodes of aridity in southern africa since the last interglacial period there is generally a dearth of evidence of the nature of quaternary climate change within desert systems which has limited previous interpretations of past environmental change at low latitudes the last glacial maximum has previously been identified as the peak of late quaternary aridity when desert systems expanded to five times their present extent1 2 3 and low latitude aridity has been described for previous glaciations4 but little evidence has been derived directly for large desert basins particularly southern africa here we report new chronological optical dating evidence of arid episodes recorded in aeolian sediments from the mega kalahari sand sea episodic aeolian activity is recorded at the northeastern desert margin whereas more sustained activity is evident from the southwestern desert core several significant arid events are apparent since the last interglacial period with dune building arid phases at 95 115 41 46 20 26 and 9 16 kyr before present existing atmospheric general circulation model simulations and independent palaeoclimate data indicate that the changes in aridity are related to changes in the northeast southwest summer rainfall gradient which are in turn related to sea surface temperatures in the southeastern atlantic ocean

support of long wavelength loads on venus and implications for internal structure a thin shell formulation for computing stress distributions on the surfaces of one plate planets is developed the lithospheric model includes laterally varying density anomalies at two depths corresponding to undulations on a crust mantle boundary and variations in upper mantle density for a given set of model parameters values for the vertical displacement crustal thickness anomaly and mantle density anomaly required to satisfy the topographic and gravitational boundary conditions are computed along with the resulting stress field the computed stresses can then be compared to stress directions inferred from observed tectonic features this formulation is used to study loads on venus s lithosphere and it is found that most highland areas appear to have been formed in a state of isostasy or by uplift with a significant portion of their support probably derived from dynamic processes in the mantle a possible exception is ishtar terra whose circumferential mountain belts are most consistent with a radially compressive stress regime generated by flexural loading reasonable lithospheric models predict low density mantle beneath atla beta eisila thetis and tethus regiones and thick crust accompanied by higher density mantle under ishtar terra and ovda regio thus western aphrodite terra appears to be more closely related to ishtar than to the rest of the equatorial highlands these results favor a hot spot model of venus tectonics in which beta eastern aphrodite eisila and tethus are expected to be relatively young and thermally active while ovda and ishtar should be older and less active

bilateral topographic symmetry patterns across aphrodite terra venus western aphrodite terra venus is characterized by a series of parallel linear structural discontinuities 2000 4000 km in length and 100 200 km wide which strike at high angles to the general topographic trend of the aphrodite terra highlands the broad characteristics of the cross strike discontinuities csds are similar to both strike slip fault zones and terrestrial oceanic fracture zones in an effort to distinguish between these two hypotheses topographic profiles were taken across aphrodite terra to test for bilateral symmetry of the type associated with thermal boundary layer topography at divergent plate boundaries on earth in addition to a broad bilateral symmetry at a range of angles across aphrodite terra detailed bilateral symmetry is observed within domains between linear discontinuities in directions generally parallel to the strike of the discontinuities in addition within a domain the centers of symmetry of several profiles define a linear rise crest that is oriented normal to the bounding csds and terminates against them furthermore linear rise crests are offset in a right lateral sense along the csds between domains by 200 1200 km if the discontinuities represent strike slip movement then retrodeformation of the axes of symmetry along the csds to form a straight line should form a broadly symmetrical topographic high instead when the topography is retrodeformed the edges of the highlands are characterized by distinctive misfits of topography at the edge of the rise where it intersects the csds the nature and configuration of the csds the characteristics of the topography and topographic patterns including bilateral topographic symmetry generally parallel to csds within domains the existence of linear rise crests defined by the axes of bilateral symmetry within domains and the offsets of these linear rise crests between domains all suggest that western aphrodite terra on venus is similar to terrestrial oceanic divergent plate boundary environments and that the cross strike discontinuities are analogous to oceanic fracture zones rather than strike slip faults

hot spot heat transfer its application to venus and implications to venus and earth gross similarities between earth and venus suggest that both planets might be expected to lose heat by the same mechanism available data do not resolve plate tectonics on venus however and other heat loss mechanisms such as hot spot heat loss have been suggested using a model which gives a relationship among surface elevation lithospheric thickness and heat flux we test the hot spot heat loss mechanism for venus and determine that it can easily explain the predicted heat loss of the planet with a modest number of hot spots of the order of 35 approximately 93 of the mapped topography of venus can be explained solely on the basis of lithospheric thickness variations additional compensation is required for topography above a radius of 6053 km and this can be effected by incorporating a variable thickness crust into the model if crust is assumed to be generated on the crests of the hot spots probably by processes associated with volcanism the model is consistent with almost 99 of the mapped venus topography the model is also basically consistent with available gravity data and interpretations which indicate compensated topography and great depths of compensation 100 1000 km for the mid latitudes of the planet the remaining approximately 1 of the topography not explained by hot spot crustal generation is thought to be compensated at a shallower depth primarily by variations in crustal thickness which are not directly related to hot spot volcanism the models relating surface elevation to lithospheric thickness and heat flow are grossly consistent with either a plate tectonic or hot spot heat loss mechanism for venus the range of lithospheric thicknesses implied by the range of elevations on venus implies that if plate tectonics is the dominant heat loss mechanism on venus the mean plate velocities are slower than on earth slow spreading ridges should be resolvable by the available venus topographic data but are not apparent comparison and analysis of the hypsometric curves for earth and venus suggest basic differences between the curves the skewed terrestrial ocean floor curve being consistent with plate tectonics but the relatively symmetrical venusian curve indicates that either plate tectonics differs on venus from the terrestrial analogue or that plate tectonics does not dominate the topographic expression of venus tectonism the venusian curve is consistent with a hot spot heat loss mechanism a hot spot crustal genesis mechanism is proposed which would probably result in crust very similar to the granite greenstone terranes of many terrestrial archaean cratons we speculate that hot spot heat loss may have preceded plate tectonics as the dominant terrestrial heat loss mechanism

palaeoclimatic evidence from sand seas abstract sand seas can be an important source of information on the quaternary history of many deserts because sand seas appear to react sensitivity to climatic changes the quality of palaeoenvironmental information obtained from them is frequently much better than that from adjacent rocky desert areas patterns of dune alignments may be used to reconstruct the wind directions that formed currently inactive or fixed dunes and thus can provide information on palaeocirculation patterns the climatic conditions under which these dunes formed can be examined using palaeoclimatic models to explore the combinations of wind velocity rainfall and evaporation rates required to reactivate them choice of appropriate values for these parameters may be hampered by the problems of dating episodes of dune activity in many sand seas the best palaeoclimatic information comes from studies of interdune lacustrine or playa deposits which may reflect increased local rainfall or higher groundwater discharge or greater penetration of the sand sea by rivers with headwaters outside the desert caution should be taken in the interpretation of such deposits for juxtaposition of aquatic and aeolian environments can occur even in modern hyperarid and seas the record of climatic changes preserved in the deposits of modern sand seas suggests that periods of dune formation may have been interrupted by long periods of stability which may also have included deposition of interdune lacustrine deposits for example within the past 40 000 years active dune formation in many sand seas appears to have been restricted to the interval between about 20 000 and 12 000 yr b p and to the late holocene approximately 1 4 to 1 3 rd of the time

a comparison of the regional slope characteristics of venus and earth implications for geologic processes on venus the range of 3 o by 3 o regional slopes of the earth and venus is similar approximately 0 0o 2 4o however the surface distribution of these values differs significantly on earth cratonic and abyssal plains form extensive regions of 0 0 o slope within these regions a variety of features mid ocean ridges volcanic island chains subduction zones and folded mountains have regional slope characteristics influenced by seafloor spreading and plate recycling as well as an active weathering regime continental margins form laterally continuous zones of relatively high slope passive margins up to 1 9o active margins up to 2 4o the plains provinces of venus are much more rugged than earth s plains and are marked by numerous closely spaced circular and linear features 0 1o 0 2 o regional slope concentrated into broad linear zones of global extent although venus highlands are bounded by narrow zones of relatively steep slope the margins of aphrodite terra and beta regio are not as steep as earth s continental margins and appear to be best developed parallel to the trends of major chasmata within these regions linear trends formed by some highland margin segments extend into the plains provinces as rugged belts densely populated with circular and linear features ishtar terra s margins are significantly steeper and more continuous than other highland margins and are comparable to passive margins on earth the venus highlands do not contain appreciable smooth flat interior regions implying that highland topography is not significantly modified by erosion or deposition systematic variations in the density of plains features elongate planitia highland margin trends and aligned highland topography form several major great circle like patterns oriented at generally less than 45 o to the equator and differing in character from both the mosaiclike patterns of terrestrial lithospheric plates and the subdued tectonic fracture grids of the smaller terrestrial one plate planets the moon and mercury

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

convection beneath young oceanic lithosphere implications for thermal structure and gravity small scale convection under the oceanic lithosphere which begins in the first 5 m y of cooling can produce a gravity signal with the amplitude and wavelength observed for large areas of the central pacific and southern indian oceans using seasat altimeter data the trend of the observed anomalies is parallel to the direction of plate motion as might be expected if they were produced by small scale convection models predict that the wavelengths of gravity anomalies increase more rapidly with age than is observed the persistence of short relatively uniform wavelength anomalies 200 km to crustal ages of 50 ma may indicate that they were produced when the lithosphere was very young and thin and were frozen in as cooling thickened the elastic lithosphere small scale convection which begins under very young lithosphere does not violate other geophysical data such as the rate of seafloor subsidence and variations of geoid height with age after convection has begun the subsidence due to thermal contraction within the lithosphere varies linearly with age in the absence of mantle heat sources although the rate of change of these quantities is affected by convection much of the variation of the geoid height across fracture zones can be fit by a model which includes small scale convection

geologic evolution of hess rise central north pacific ocean cores from four deep sea drilling project dsdp sites 310 464 465 and 466 and seismic reflection profiles provide data that are used to interpret the geological evolution and paleoenvironments of hess rise a prominent oceanic plateau in the central north pacific ocean hess rise apparently formed in the southern hemisphere along the western flank of the pacific farallon ridge 110 to 100 m y b p core stratigraphies and lithologies show the response of sedimentation to subsidence and northward movement of hess rise on the pacific plate oceanic islands which crowned hess rise during its early evolution were eroded and subsequently subsided below sea level major structural trends include three northwest trending 327 arms or ridges and an east northeast trending southern hess rise that parallels the mendocino fracture zone 060 normal faults offset basement as much as 3 000 m along the southern edge and 1 500 m on the western flank of hess rise many faults were active during sedimentation tholeiitic basalt from the base of hole 464 trachyte from the base of hole 465a and alkalic basalt clasts within sediment of hole 466 show the diversity of rock types that constitute the igneous basement a major rock unit is middle cretaceous limestone chalk and minor chert that form the basal sedimentary unit some limestone samples rich in organic carbon reflect accumulation above the carbonate compensation depth ccd within a mid water oxygen minimum zone the organic carbon rich sediments probably were deposited on the submarine slopes of islands and banks that were at upper bathyal depths as hess rise crossed the wide equatorial divergence where increased upwelling and biogenic productivity contributed to high accumulation rates the source of organic matter was mostly lipid rich autochthonous marine organic matter analyses of sediment samples from across the cretaceous tertiary boundary at site 465 show that there was a significant decrease in surface water temperature and biological productivity an abrupt increase in transition metals and iridium suggests that an outside source perhaps extraterrestrial was the cause for many of the sudden oceanographic geochemical and biological changes at the boundary

thermal stress and the spacings of transform faults bathymetric charts are used with satellite altimeter profiles to locate major ridge transform intersections along five spreading ridges the ridges are the mid atlantic ridge the east pacific rise the chile rise the pacific antarctic rise and the southeast indian ridge analysis of these data show spacings between transform faults w increase linearly with spreading rate w 6 28 m y this linear correlation is explained by a thermoelastic model of a cooling strip of lithosphere spreading at a rate the traction free boundaries of the thin elastic strip simulate cracks in the lithosphere at transform faults a two dimensional thermoelastic solution for the in plane stress shows the largest stress component is tensional and parallel to the ridge stresses are zero at the ridge and increase as age to a maximum value at an age of w 4 all stress components are small for ages greater than w when the transform spacing is large compared with the spreading rate w 100 m y thermal stresses exceed the strength of the lithosphere for ages between 0 and 30 ma the observed maximum ratio of transform spacing to spreading rate w 10 m y results in low thermal stresses that only exceed the strength of the lithosphere for ages less than 1 ma thus transform faults relieve most of the thermal stress model predictions also agree with earthquake studies showing that normal faults in young lithosphere have tensional axes aligned with the ridge moreover oceanic intraplate earthquakes rarely occur in lithosphere older than 30 ma as predicted by the model these and other geophysical observations confirm turcotte s hypothesis that transform faults are thermal contraction cracks

holocene moisture evolution in arid central asia and its out of phase relationship with asian monsoon history we synthesize palaeoclimate records from the mid latitude arid asian region dominated today by the westerlies arid central asia aca to evaluate spatial and temporal patterns of moisture changes during the holocene sediment records from i i takes with reliable chronologies and robust proxies were selected to reconstruct moisture histories based on a five class ordinal wetness index with assigned scores from the driest to wettest periods at individual sites for 200 year time slices the proxies used in these records include pollen and diatom assemblages sediment lithology lake levels and geochemistry mainly isotope data the results of our synthesis show that aca as a whole experienced synchronous and coherent moisture changes during the holocene namely a dry early holocene a wetter less dry early to mid holocene and a moderately wet late holocene during the early holocene most of the lakes experienced very low water levels and even dried out before ca 8 ka 1 ka 1000cal a bp hence the effective moisture history in aca is out of phase with that in monsoonal asia as documented by numerous palaeoclimate records in monsoonal asia a strong summer monsoon and humid climate characterized the early holocene and a weakened summer monsoon and drier climate prevailed during the late holocene which were mainly controlled by changes in low latitude summer insolation in contrast we propose that the pattern of holocene effective moisture evolution in the westerly dominated aca was mainly determined by north atlantic sea surface temperatures ssts and high latitude air temperatures that affect the availability amount and transport of water vapor also topography of the tibetan plateau and adjacent asian highlands could have contributed to the intensification of dry climate in aca during the early holocene as a result of strengthening the subsidence of dry air masses associated with stronger uplift motion on the plateau by intense heating under a stronger summer insolation summer insolation might have played a key role in directly controlling moisture conditions in aca but only after the northern hemisphere ice sheets had disappeared in the mid and late holocene c 2007 elsevier ltd all rights reserved

age and dynamics of linear dunes in the namib desert bristow c s duller g a t lancaster n 2007 age and dynamics of linear dunes in the namib desert geology 35 6 555 558 sponsorship american chemical society

multiple closely spaced transform faults in fast slipping fracture zones the 400 km long offset of the east pacific rise spreading axis between the quebrada fracture zone lat 3 5 s and the gofar fracture zone lat 4 6 s has been surveyed using the long range sidescan sonar gloria the offset is accomplished in three distinct fracture zones each of which contains up to four actively slipping transform faults separated by short spreading centers transform spacings range from 5 to 16 km each transform is associated with its own transverse valley but at quebrada fracture zone and to some extent at the other two these valleys merge into one large slightly oblique fracture zone valley the morphological trends of fracture zone valleys are therefore not necessarily reliable for estimating directions of relative plate movement multitransform fracture zones appear to be much commoner on the fast spreading east pacific rise than on slower spreading ridges the tectonic spreading fabric which comprises closely spaced normal faults formed near the spreading axes and striking subparallel to them is modified to a sigmoidal pattern between the closely spaced transforms even midway between transforms these faults remain oblique to the spreading direction the degree of obliquity increases as the transform separation decreases showing progressive rotation of the stress ellipse as transforms are approached

a dynamic model of venus s gravity field unlike earth long wavelength gravity anomalies and topography correlate well on venus venus s admittance curve from spherical harmonic degree 2 to 18 is inconsistent with either airy or pratt isostasy but is consistent with dynamic support from mantle convection a model using whole mantle flow and a high viscosity near surface layer overlying a constant viscosity mantle reproduces this admittance curve on earth the effective viscosity deduced from geoid modeling increases by a factor of 300 from the asthenosphere to the lower mantle these viscosity estimates may be biased by the neglect of lateral variations in mantle viscosity associated with hot plumes and cold subducted slabs the different effective viscosity profiles for earth and venus may reflect their convective styles with tectonism and mantle heat transport dominated by hot plumes on venus and by subducted slabs on earth convection at degree 2 appears much stronger on earth than on venus a degree 2 convective structure may be unstable on venus but may have been stabilized on earth by the insulating effects of the pangean supercontinental assemblage

regional linear cross strike discontinuities in western aphrodite terra venus western aphrodite terra venus is characterized by a series of parallel linear structural discontinuities 2000 4000 km in length and 100 200 km wide which strike at high angles to the general topographic trend of the highlands the detailed characteristics of these features are similar to terrestrial oceanic fracture zones and strike slip fault zones either interpretation implies the presence of large scale horizontal tectonic movement on venus

tests of crustal divergence models for aphrodite terra venus aphrodite terra the largest highland region of venus is a likely site of mantle upwelling active volcanism and extensional tectonics we examine three alternative kinematic models for the interaction of mantle convection and the surface one in which little horizontal surface displacement results from mantle flow vertical tectonics one in which shear strain from large horizontal displacements is accommodated only in narrow zones of deformation plate divergence and one in which strain from large horizontal motions is broadly accommodated distributed deformation in support of a divergent plate model for aphrodite head and crumpler have cited the presence of an organized system of lineaments held to be analogues to oceanic fracture zones small scale bilateral symmetry of topographic elements hypothesized to be rifted and separated relief analogous to some terrestrial oceanic plateaus and subsidence of topography proportional to the square root of distance consistent with that expected for a divergent thermal boundary layer we undertake quantitative tests of each of these assertions and we compare the results with those from similar tests of the mid atlantic ridge we find that apart from the long wavelength symmetry of broadly elevated regions there is no convincing evidence for regional bilateral symmetry of features several hundred kilometers in size on either planet the fit of the topography of aphrodite to that of a thermal boundary layer is in general much poorer than for the earth and so other mechanisms such as dynamic uplift or crustal thickness variations must dominate the topography the broad saddle shaped region between thetis and atla regiones shows the best fit to the root distance relation yielding apparent spreading half rates of a few centimeters per year but the goodness of fit and apparent spreading rates are quite sensitive to the distance range analyzed calculation of a single pole of relative motion for the entire postulated system of transform faults shows that the inferred fracture zone traces are not consistent with a simple two plate model regardless of past pole motions a multiple plate geometry is therefore called for and one or more of the lineaments must act as a plate boundary along nearly its full length if the plate divergence hypothesis is correct such a boundary would be distinguishable from other lineaments by geological evidence for recent nontransform motion in forthcoming magellan radar images in the absence of such evidence as long as the existence and distribution of the lineaments are verified then broad disruption of a thin lithosphere is favored in such a model lineaments may be surface manifestations of mantle convective flow a model dominated by primarily vertical tectonics is tenable only if the lineaments are not confirmed by magellan

effects of lateral resolution on the identification of volcanotectonic provinces on earth and venus topographic data sets for the continental united states and the north pacific have been degraded and displayed in the same manner as the pioneer venus altimeter data for venus results for the u s data show that the cordillera orogenic zone would be seen as a broad swell with a height of 2 km and a wavelength of about 2500 km individual structural elements within the cordillera are reduced to mild height about 700 m wavelength about 500 km swells that are superimposed on the broad cordillera rise for the pacific data the hawaiian swell the e w trending fractures in the eastern pacific the increase in depth away from the east pacific rise and most trenches can still be discerned the venusian data do not show the integrated trench transform and ridge system evident in the degraded pacific data these features would be discernible in the degraded pacific data even after correcting depths for a the higher surface temperature on venus and b the lack of loading due to ocean water thus a plate tectonic regime similar to earth s does not appear to currently exist on venus unfortunately as shown by the degraded cordillera data the pv altimetry data are not of sufficient quality to discern whether or not the highlands of venus preserve evidence for orogenic events related to an ancient epoch of plate tectonics

grain size distribution function of polymodal sediments in hydraulic and aeolian environments and numerical partitioning of the sedimentary components most continental sediments are polymodal composed of overlapping components of which the grain size generally obeys some type of natural distribution the grain size components and their function types can be determined from frequency and cumulative curve plots in order to define the function formula of the grain size distribution the function parameters can be estimated by fitting a defined function formula to the measured grain size data of the sample which simultaneously achieves numerical partitioning of the sedimentary components genetic analysis of grain size components of hydraulic and aeolian sediments demonstrates the following environmental implications fluvial sediment is composed of isolated saltation and suspension components the sediments in closed lake basins are dominated by a suspension silt clay component with a small proportion of saltation sand the fine sand component makes up the majority of desert sand overlapping with a small proportion of fine dust aeolian loess is composed of two overlapping components a short suspension time silt component and a long suspension time fine component aeolian material in the north pacific deep sea sediments is dominated by long suspension time fine dust the fine component in aeolian sediments shows a consistent grain size distribution and genetic connection from the desert sand loess of northern china to the north pacific ocean which is mainly transported by westerly winds and is dispersed in the atmosphere forming a background dust c 2002 elsevier science b v all rights reserved

terrestrial spreading centers under venus conditions evaluation of a crustal spreading model for western aphrodite terra abstract assessment of terrestrial spreading centers under venus environmental conditions indicates that the major difference between earth and venus would be the influence of the enhanced surface temperature on upper mantle temperatures on venus and the resulting increase in crustal thickness and elevation of isostatically compensated topography crust on venus produced at average spreading centers would be about 15 km thick in contrast to the average crustal thickness on earth of about 5 km we have used this model and the pioneer venus altimetry and gravity data to test the hypothesis of head and crumpler 1987 6 that western aphrodite terra represents the location of crustal spreading on venus the topographic data are processed such that all points within equally spaced 100 km wide bins parallel to the proposed axis are stacked together and the average value of topography in each bin is calculated providing average profiles out to a distance of 2000 km on either side of the axis the topography is shown to be very symmetric about the axis with data points less than 1000 km from the axis providing a fit to thermal boundary layer topography comparable to that seen in spreading centers on earth at 1000 km on either side of the axis steep topographic slopes exist this change in topography could be explained by variations of about 15 km in crustal thickness with the crust being thicker in the central domain by a ridge jump or by mechanisms unrelated to crustal spreading for processing line of sight los acceleration data we first show that static and dynamic calculations give the same results for track numbers 427 449 this allows us to determine a synthetic orbit track parallel to the cross strike dis continuities csd s and to compare it with observed los accelerations gravity data for this domain were then processed in a manner similar to that for topography models give a reasonable fit to the data for half spreading rates between 0 3 and 0 5 cm yr we conclude that a spreading center model for ovda regio could account for the observed topography and los gravity anomalies if it had the following characteristics a central domain with a 30 km thick crust that has been spreading at about 0 5 cm yr for the last 200 myr over a mantle whose temperature is about 1600 c at about 200 myr ago a change in upper mantle temperature of about 100 c is required prior to this time the decreased upper mantle temperature would have produced a thinner crust averaging about 15 km at the spreading center this thinner crust would now lie on the flanks of ovda regio at distances greater than 1000 km from the spreading center and would be in excess of 200 myr old on the basis of our analysis we conclude that the hypothesis that ovda regio in western aphrodite terra is the site of crustal spreading on venus is consistent with the topography and gravity data