paleolatitudes determined from paleomagnetic data from vertical cores most of the paleomagnetic data available from oceanic plates consist of measurements of inclination but not declination from azimuthally unoriented vertical cores similarly the basic paleomagnetic data to be analyzed from crustal blocks that have rotated by varying amounts about vertical axes are a set of paleoinclinations a new method is proposed for determining the paleocolatitude of a sampling site and the associated confidence limits paleomagnetic inclinations are first converted to paleocolatitudes and averaged to produce a mean colatitude after showing that this mean is biased we derive a paleocolatitude correction to remove the bias using a model for geomagnetic secular variation based on the latitude dependence of secular variation during the past 5 million years after testing the validity of the model by applying it to paleomagnetic data from quaternary lavas for which both inclination and declination are known we apply the method to inclination only paleomagnetic data from suiko seamount the most complete data set from a drilling site the method is then applied to paleomagnetic data from all deep sea drilling project sites for the pacific plate to obtain for each site an unbiased mean paleocolatitude and its confidence limits these paleocolatitudes and confidence limits are then used as input for a least squares analysis to obtain early and late cretaceous poles for the pacific plate and 95 confidence limits for these poles it is shown that the scatter of individual site paleocolatitudes about the best fit pole is consistent with the confidence limits at each site as determined by the new method of analysis

extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

structure and evolution of southeast thetis regio 1 the highland terrain in southeast thetis regio consists of a series of thrust faults and conjugate graben that would usually be classified as tesserae unusually this small area of tesserae is younger than the adjacent plains a conclusion strengthened by modification of jumaisat crater a parabolic dark halo crater 75 myr old two structural belts are distinguished the eastern ridged terrain and the central plateau separated by conjugate graben unlike most terrestrial mountain belts the thrusts in southeast thetis are outward dipping implying obduction of the eastern plains onto thetis this most likely results from the buoyancy of the venusian lithosphere which inhibits subduction a simple two stage orogenic model is inferred plains crust driven by extension in diana chasma initially overrides higher standing tesserae crust in southeast thetis through interaction with a plume or perhaps through convective instability of the lower lithosphere a plateau forms that resists further compression and that exhibits extensive volcanism a new area of thrusting in the ridged terrain develops while the plateau starts to collapse along graben the larger of which are filled with lavas the core of thetis regio appears to be surrounded by other tectonic units like southeast thetis indicating a piecemeal construction similar to terrestrial continents

excess 40ar in biotite and hornblende from the noril sk 1 intrusion siberia implications for the age of the siberian traps the siberian traps represent the largest known phanerozoic manifestation of continental flood volcanism the general agreement is now that the volcanism was of short duration 1 m y and was synchronous with the permian triassic boundary discrepancies between different geochronologic results prevent rigorous evaluation of the age and duration of magmatism several alkalic mafic intrusions associated with the siberian traps represent subvolcanic feeders for the trap lavas the noril sk 1 intrusion is intruded into the earliest flows and is chemically and isotopically related to stratigraphically overlying flows biotites from the noril sk 1 intrusion have previously yielded 40ar39ar plateau dates 254 251 ma that are significantly older than seemingly reliable 40ar39ar plateau dates 250 247 ma on plagioclase and whole rock samples from trap basalts that are geologically constrained to be older than the intrusion possible explanations of this discrepancy include 1 plagioclase and whole rock samples are anomalously young due to 40ar loss via alteration or episodic outgassing and or 2 the biotites contain excess 40ar detailed laser heating 40ar39ar analysis of six individual grains of biotite in a total of 214 steps yield data that permit characterization of multiple components of ar through isotope correlation patterns an isochron age of 250 1 1 5 ma 2 and an excess ar component with 40ar36ar 565 39 are obtained by regression of the 115 highest temperature steps step heating of a bulk hornblende sample also reveals excess ar with 40ar36ar 327 10 and an isochron age of 249 3 1 6 ma neglecting uncertainties in the ages of neutron fluence monitors the two dates just overlap at 2 but are distinctly younger than the previous mean plateau date 253 9 1 6 ma for biotite the different initial 40ar36ar for the two phases is interpreted as reflecting the evolution of an initial lower mantle composition towards increasing crustal contamination

on the causes of mass extinctions the temporal link between large igneous province lip eruptions and at least half of the major extinctions of the phanerozoic implies that large scale volcanism is the main driver of mass extinction here we review almost twenty biotic crises between the early cambrian and end cretaceous and explore potential causal mechanisms most extinctions are associated with global warming and proximal killers such as marine anoxia including the early middle cambrian the late ordovician the intra silurian intra devonian end permian and early jurassic crises many but not all of these are accompanied by large negative carbon isotope excursions supporting a volcanogenic origin most post silurian biocrises affected both terrestrial and marine biospheres suggesting that atmospheric processes were crucial in driving global extinctions volcanogenic atmospheric kill mechanisms include ocean acidification toxic metal poisoning acid rain and ozone damage and consequent increased uv b radiation volcanic darkness cooling and photosynthetic shutdown each of which has been implicated in numerous events intriguingly some of the most voluminous lips such as the oceanic plateaus of the cretaceous were emplaced with minimal faunal losses and so volume of magma is not the only factor governing lip lethality the missing link might be continental configuration because the best examples of the lip extinction relationship occurred during the time of pangaea many of the proximal kill mechanisms in lip extinction scenarios are also potential effects of bolide impact including cooling warming acidification and ozone destruction however the absence of convincing temporal links between impacts and extinctions other than the chicxulub cretaceous example suggests that impacts are not the main driver of extinctions with numerous competing extinction scenarios and the realisation that some of the purported environmental stresses may once again be driving mass extinction we explore how experimental biology might inform our understanding of ancient extinctions as well as future crises

structural analysis of a sampling of tesserae implications for venus geodynamics abstract understanding the nature and timing of tessera formation is fundamental to venus tectonic and geodynamic models tesserae are commonly considered to exhibit complex deformation histories to represent the oldest global stratigraphic unit to have formed during a global phase of tesserization and to require weak lithosphere for their formation although these characterizations of tesserae are gaining widespread mention in the literature they are essentially unsupported by an assemblage of data yet they figure prominently in the way we frame and address questions about venus tectonics thus they are quickly becoming accepted paradigms despite a lack of scientific foundation open discussion of these hypotheses or assumptions is crucial because of their implications concerning geodynamic models of the surface evolution and planetary dynamics of venus we examined tessera terrain in ishtar terra crustal plateaus and as inliers within the plains using high resolution magellan radar imagery we describe several types of tessera terrain that record a wide range of structural histories fold and s c terrains are found only in ishtar terra ribbon lava flow and basin and dome terrains reside within the interior of crustal plateaus whereas folded ribbon terrain and extended folded terrain comprise margins of crustal plateaus inliers are divisible into fracture dominated and graben dominated tesserae although some inliers host early contractional fabrics the range of deformation histories recorded by the various tessera types indicates that tesserae should not be considered a single map unit tessera deformation records local to regional surface strain patterns and reflects near surface rheology at the time of deformation progressive deformation fabrics in some tessera terrain record changes in shallow crustal rheology through time thus structural analysis of individual tessera types and tessera provinces will allow us to better understand the tectonic processes responsible for tessera formation tesserae likely formed in several tectonic environments including 1 as a result of subsurface flow in ishtar terra 2 as sequences of surface layer extension and contraction in crustal plateaus 3 as flooded crustal plateaus and 4 as densely fractured surface layers fractured as a result of corona and chasma formation

chemical characteristics of sedimentary soils in the mediterranean environment a comparison of undisturbed and disturbed soils abstract in many ecosystems especially in siliceous soils the mobile anion balance controls the entire ionic equilibrium however the equilibrium of the mobile anion cycle is very fragile and many factors abiotic biotic and above all human constantly disturb it this paper reports a study of the principal anions found in the surhcial layer of soils situated over sedimentary substrates in a continental mediterranean environment primarily in semiarid regions human deforestation and animal activities over decades overgrazing trampling have altered these soils to different degrees recently their use in some cases as landfills for solid waste has become one of the major causes of disturbance comparisons are made in order to highlight the differences among soil types and those between the disturbed sites and surroundings the results show that natural soils are poor in soluble anions it is also possible to distinguish clear differences in the average concentrations found in three different natural substrates finally there are notable elevated concentrations of cl and so4 in landfill soils

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

subsidence of the west siberian basin effects of a mantle plume impact comparison of modeling results with observed subsidence patterns from the west siberian basin provides new insight into the origin of the siberian traps and constrains the temperature size and depth of an impacting mantle plume head during and after the eruption of the siberian traps at the permian triassic boundary 250 ma we compare subsidence patterns from one dimensional conductive heat flow models to observed subsidence from backstripping studies of wells in the basin this results in a best fit scenario with a 50 km thick initial plume head with a temperature of 1500 c situated 50 km below the surface and an initial regional crustal thickness of 34 km in agreement with published values backstripping and modeling results agree very well including a 60 90 m y delay between the rifting phase and the first regional sedimentation regional subsidence patterns indicate that the plume head was present across a minimum area of 2 5 106 km2 these results re emphasize the viability of a mantle plume origin for the siberian traps provide important constraints on the dynamics of mantle plume heads and suggest a thermal control for the subsidence of the west siberian basin

a minimum upb age for siberian flood basalt volcanism abstract establishing an accurate and precise age for siberian flood basalt volcanism is of great importance in evaluating causes for the unequaled mass extinction of flora and fauna at the permian triassic boundary we report a new minimum u pb age obtained from zircon and baddeleyite from the mineralized noril sk i intrusion that cuts the lower third of this rapidly deposited 3500 m thick volcanic sequence near noril sk this 251 2 0 3 2 ma age is within analytical error of the shrimp u pb age for zircon from the permian triassic boundary at meishan south china 251 1 3 6 ma 2 and confirms siberian basaltic volcanism as a possible contributor to the mass extinction

palaeomagnetism of flood basalts in the pilbara craton western australia late archaean continental drift and the oldest known reversal of the geomagnetic field 1 a late archaean circa 2775 2715 ma succession of terrestrial continental flood basalts mafic tuffs felsic volcanic rocks and clastic sedimentary rocks in the nullagine synclinorium and meentheena centrocline of the east pilbara basin western australia has been sampled for a palaeomagnetic study over 500 oriented mostly basalt drill cores were collected from the supracrustal succession and associated dykes thermal and alternating field demagnetization revealed two distinct components positive fold conglomerate and reversal tests confirm that the primary natural remanent magnetization nrm is still preserved the secondary component is interpreted as the record of remagnetization during a major thermal event possibly in the early proterozoic analysis of the primary nrm directions results in a magnetostratigraphy and an apparent polar wander path apwp for the 60 myr interval covered by the sampled succession assuming a geocentric axial dipole during this time interval the apwp shows that the pilbara craton was drifting during the late archaean and that drift rates probably varied significantly in particular a mean 27 2 shift in palaeolatitude is recorded across an unconformity that represents a relatively short time period and that marks a significant change in basalt geochemistry this study suggests that continents moved horizontally during the late archaean and that the rates of movement were significantly faster than in the phanerozoic in addition a reversed polarity interval with a positive reversal test is recorded we argue that it documents the oldest known geomagnetic reversals of the geomagnetic field

an alternative kinematic interpretation of thetis boundary shear zone venus evidence for strike slip ductile duplexes 1 on venus a 1000 km long 50 to 200 km wide anastomosing shear zone system thetis boundary shear zone tbsz separates the eastern ovda and northwestern thetis regiones previous workers mapped a part of this shear zone and interpreted it as a sinistral strike slip shear zone containing an extensional jog in the present study it is reexamined for a detailed structural and kinematic analysis and an alternative kinematic interpretation is provided the tbsz is divided into western central and eastern segments the western segment is narrow and is oriented in a ene wsw direction along its northern boundary the preexisting folds of tessera are cut by the shear zone and the folds are dragged into parallelism with the shear zone in response to dextral strike slip motion both the western and central segments contain a pair of conjugate deformation bands sigmoidal folds and linear ridges which appear similar to s c like structures of large scale ductile shear zones the acute angle between the s and c bands suggests dextral sense of displacement the eastern segment forms the northeastern restraining bend of the tbsz where the shear zone is characterized by a wide branching fan like anastomosing shear zone system with well developed contractional strike slip ductile duplexes the internal fabric defined by the sigmoidal folds consistently shows dextral sense of displacement therefore the tbsz is a dextral strike slip shear zone showing s c like structures and contractional strike slip duplexes similar to the continental scale ductile shear zones on earth

evolution of the west siberian basin the west siberian basin is one of the largest intracratonic basins of the world and an important hydrocarbon province of russia perhaps the most important geologic event in siberia was the emplacement of basalts around 250 ma i e permo triassic boundary covering an area of about 5 10 6 km 2 this volcanism may be responsible for a mass extinction that occurred around permian triassic time the prebasaltic rifting event was limited to the north northeastern sector of the basin initial basin wide subsidence took place in the jurassic as a result of which the western part of siberia became the west siberian basin bounded by uplifts to the east and to the west one of the surprising aspects of the west siberian basin is the abundance of subvertical faults believed to be result of strike slip movement while intraplate inversions and fault reactivation structures have been observed in many cratons subvertical faults observed in the west siberian basin are unique because of their geometries and abundance the differentiation between the effects of tectonics and eustasy in cratonic basins is simple the global eustatic signal is basin wide with regional and local tectonics playing an overprinting role thus the middle jurassic turonian first second and third order cycles in the west siberian basin were primarily driven by eustasy the middle jurassic turonian series can be subdivided into two second order and 16 third order transgressive regressive cycles within dataset extent fourth order cycles appear to be controlled by delta shifting although extensively studied a number of fundamental questions regarding the origin and evolution of the west siberian basin remain unresolved or poorly documented in the literature

the timing and extent of the eruption of the siberian traps large igneous province implications for the end permian environmental crisis we present new high precision 40ar 39ar ages on feldspar and biotite separates to establish the age duration and extent of the larger siberian traps volcanic province samples include basalts and gabbros from noril sk the lower tunguska area on the siberian craton the taimyr peninsula the kuznetsk basin vorkuta in the polar urals and from chelyabinsk in the southern urals most of the ages except for those from chelyabinsk are indistinguishable from those found at noril sk cessation of activity at noril sk is constrained by a 40ar 39ar age of 250 3 1 1 ma for the uppermost kumginsky suite the new 40ar 39ar data confirm that the bulk of siberian volcanism occurred at 250 ma during a period of less than 2 ma extending over an area of up to 5 million km2 the resolution of the data allows us to confidently conclude that the main stage of volcanism either immediately predates or is synchronous with the end permian mass extinction further strengthening an association between volcanism and the end permian crisis a sanidine age of 249 25 0 14 ma from bed 28 tuff at the global section and stratotype at meishan china allows us to bracket the p tr boundary to 0 58 0 21 myr and enables a direct comparison between the 40ar 39ar age of the traps and the permo triassic boundary section younger ages 243 ma obtained for basalts from chelyabinsk indicate that volcanism in at least the southern part of the province continued into the triassic

linear and planar structure in ordered multivariate data as applied to progressive demagnetization of palaeomagnetic remanence summary consider a sequence of say 10 to 20 vector observations in threedimensional space it is suspected that a few subsets of consecutive observations are made up of collinear points the purpose of this paper is to construct a statistically based algorithm to find such linear segments and to assess their accuracy a similar assessment is made for coplanar sets of points this algorithm is applied here to palaeomagnetic data and is claimed to be superior to previous methods of palaeomagnetic analysis in terms of completeness and balance of analysis treatment of measurement errors and other sources of scatter criteria for identification of linear and planar sets of points and statistical rigour stability spectra with statistically based confidence limits are obtained as a by product 1 palaeomagnetic analysis background to the geometrical problem the natural remanent magnetization nrm in rocks is known in general to have originated in one or more relatively short time intervals during their formation and subsequent history i e during rock forming or metamorphic events during which nrm is frozen in stabilized by falling temperature grain growth etc the nrm acquired during each such event is a single vector magnetization parallel to the then prevailing geomagnetic field and is called a component of nrm rocks which have suffered more than one event of this kind are therefore liable to carry multi component nrm in the laboratory these components can be identified by demagnetizing the rock step by step usually by thermal alternating field or chemical demagnetization which involves the analysis of the distribution of blocking rzmperature coercivity and chemical leaching respectively resistance to these treatments is called stability of remanence and the range of treatment over which a particular component is removed is called the stability range of the component the various discrete

regional uplift associated with continental large igneous provinces the roles of mantle plumes and the lithosphere the timing and duration of surface uplift associated with large igneous provinces provide important constraints on mantle convection processes here we review geological indicators of surface uplift associated with five continent based magmatic provinces emeishan traps 260 million years ago ma siberian traps 251 ma deccan traps 65 ma north atlantic phase 1 61 ma and phase 2 55 ma and yellowstone 16 ma to recent all five magmatic provinces were associated with surface uplift surface uplift can be measured directly from sedimentary indicators of sea level in the north atlantic and from geomorpholocial indicators of relative uplift and tilting in yellowstone in the other provinces surface uplift is inferred from the record of erosion in the deccan north atlantic and emeishan provinces transient uplift that results from variations in thermal structure of the lithosphere and underlying mantle can be distinguished from permanent uplift that results from the extraction and emplacement of magma transient surface uplift is more useful in constraining mantle convection since models of melt generation and emplacement are not required for its interpretation observations of the spatial and temporal relationships between surface uplift rifting and magmatism are also important in constraining models of lip formation onset of surface uplift preceded magmatism in all five of the provinces biostratigraphic constraints on timing of uplift and erosion are best for the north atlantic and emeishan provinces where the time interval between significant uplift and first magmatism is less than 1 million years and 2 5 million years respectively rifting post dates the earliest magmatism in the case of the north atlantic phase 1 and possibly in the case of siberia the relative age of onset of offshore rifting is not well constrained for the deccan and the importance of rifting in controlling magmatism is disputed in the emeishan and yellowstone provinces in these examples rifting is not a requirement for onset of lip magmatism but melting rates are significantly increased when rifting occurs models that attempt to explain emplacement of these five lips without hot mantle supplied by mantle plumes often have difficulties in explaining the observations of surface uplift rifting and magmatism for example small scale convection related to craton or rift boundaries edge driven convection cannot easily explain widespread 1000 km scale transient surface uplift emeishan deccan north atlantic and upper mantle convection initiated by differential incubation beneath cratons the hotcell

the middle palaeolithic site of birzgane tebessa algeria rock magnetic property characterisation and past rainfall reconstruction abstract a new middle palaeolithic site was discovered at birzgane south of the city of tebessa in eastern algeria the archaeological study revealed richness in archaeological artefacts belonging probably to the aterian culture but an absence of pedunculates the aterian archaeological layer was assigned to the end of the middle pleistocene by uranium thorium dating the environmental conditions at the time were assessed by means of magnetic proxy data such as hysteresis parameters magnetic susceptibility and remanent magnetisation the magnetic fraction of the alluvial sediment contains mainly oxidised magnetite and smaller amounts of haematite and goethite indicated by thermal demagnetisation curves of isothermal remanent magnetisation magnetic properties such as enhanced susceptibility anhysteretic remanent magnetisation and frequency dependent susceptibility indicate that the upper part of the section was subjected to pedogenesis after deposition a conceptual magnetic enhancement model was applied to reconstruct past rainfall quantities in the upper part taking into account present day temperatures and rainfall water saturation soil moisture and magnetic susceptibility a precipitation quantity of 560 40 mm was estimated for the transition period from mis 5 to 4 corresponding to the saharan pluvial an extended humid climate period in northern africa between 100 000 and 70 000 years before present

surface formation rates and impact crater densities on venus impact crater density has been used to estimate relative ages of major venus geologic units typically based on the assumption of short globally synchronous formation periods the total number density of craters is the only parameter which may be quantified for most venus subregions in contrast to the size frequency analysis often used for other planetary studies this paper examines the statistical limits on such crater counts and the predicted crater density for various possible time varying surface formation rate functions these results show that 1 the true mean crater density is poorly constrained due to the limited sample size provided by aggregate geologic units and 2 any given crater density can be accommodated by a very short pulse of unit formation at some age t gaussian distributed formation functions of mean age t exponential functions with initiation ages between t and 2t or a host of other more complicated rate functions these results when coupled with the unknown rate of crater removal by tectonic or volcanic activity indicate that the crater counts for specific landforms on venus offer little constraint on the relative timing or duration of their development the hypothesis of globally synchronous formation of landforms such as volcanoes or coronae cannot be validated with the available crater data

palaeomagnetism and magnetostratigraphy of the permian triassic northwest central siberian trap basalts abstract a detailed palaeomagnetic and magnetostratigraphic study of the permian triassic siberian trap basalts stb in the noril sk and abagalakh regions in northwest central siberia is presented thermal th and alternating field af demagnetisation techniques have been used and yielded characteristic magnetisation directions the natural remanent magnetisation of both surface and subsurface samples is characterised by a single component in most cases occasionally a viscous overprint can be identified which is easily removed by th or af demagnetisation the resulting average mean direction after tectonic correction for the 95 flows sampled in outcrops is d 93 7 i 74 7 with k 19 and 95 3 3 the corresponding pole position is 56 2 n 146 0 e unoriented samples from four boreholes cores in the same regions have also been studied they confirm the reversed normal succession found in outcrops the fact that only one reversal of the earth s magnetic field has been recorded in the traps can be taken as evidence for a rather short time span for the major eruptive episode in this region however there is evidence elsewhere that the whole volcanic activity associated with the emplacement of the stb was much longer and lasted several million years

chemical dieferentiation of a convecting planetary interior consequences for a one plate planet such as venus partial melting to generate the crust of a planet creates compostionally buoyant residual mantle in the absence of mantle flow associated with plate tectonics this buoyant refractory layer may collect at the top of the mantle with important implications for the evolution of the interior and surface in this study models of the thermal and chemical evolution of a planetary interior demonstrate the possible consequences of a chemically buoyant depleted mantle layer as the depleted layer thickens the melting temperature at the top of the underlying convecting mantle also increases and the degree of partial melting of mantle added to the depleted layer decreases as less depleted mantle with less positive compositional buoyancy is added negative thermal buoyancy of the layer eventually exceeds its positive compositional buoyancy the depleted layer then sinks into and mixes with the convecting interior the top of the convecting mantle then moves to a shallower depth larger degrees of melting resume and a new depleted layer accumulates this accumulation and instability of the depleted layer occurs repeatedly over a substantial portion of the planet s evolution with a period of 300 500 myr on venus the population of impacts craters is indistinguishable from a random distribution over the surface and give a surface age of about 500 myr we speculate that the mechanism described above may explain this episodic global resurfacing of venus

origin and emplacement of ophiolites ever since their recognition ophiolites have been a source of controversy although all workers have agreed upon their importance a number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers an expanded version of the penrose ophiolite definition is proposed herein called the ophiolite association taking into account rocks which bear upon the origin and significance of ophiolite complexes an ideal ophiolite association includes the following units from bottom to top 1 a crystalline basement and shallow water sedimentary sequence 2 a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and or melange 3 a metamorphic unit as much as a few hundred meters thick generally with higher grade rocks over lower grade ones 4 ultramafic tectonite unit composed dominantly of multiply deformed peridotite dunite and minor chromitite 5 a cumulate complex ultramafic at the base grading to mafic or intermediate at the top 6 a noncumulus unit of varitextured gabbro and minor trondhjemite 7 a sheeted dike complex 8 an extrusive section of massive and pillowed flows and intercalated sediments 9 an abyssal or bathyal sediment sequence which may include radiolarian chert red pelagic limestone metalliferous sediments breccias and or pyroclastic deposits and 10 postemplacement deposits of laterite reef limestone or shallow marine or subaerial sediments this expanded ophiolite association allows greater precision in inferring original origin and emplacement some ophiolite complexes particularly in the tethys herein designated tethyan type exhibit units 1 2 3 and 4 whereas others particularly in the cordilleran belt herein termed cordilleran type lack units 1 2 3 and in places 4 the emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone emplacement of the cordilleran type generally is not clear cut the nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5 9 than by chemistry or mineralogy some ophiolite complexes lack complete sections and may have formed along transform faults others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone such conversion may result from normal plate evolution or from major plate reorganization major times of ophiolite emplacements appear to correspond with global plate reorganizations

wind interaction with falling ejecta origin of the parabolic features on venus unusual parabolic features associated with impact craters have been observed by magellan on venus a strong correlation exists between the orientation of the features and the zonal winds on venus we propose a quantitative model in which the parabolic features are produced by the interaction of the zonal winds with material ejected ballistically from the impact crater as the ejecta particles fall through the atmosphere the winds transport them downwind from their entry point smaller particles being transported a greater distance since the ejecta distribution is initially axially symmetric and smaller particles are thrown farther from the crater the winds blow the particles on the upwind side back upon one another leading to a pile up of material on the downwind side the winds disperse the ejecta particles and no pile up occurs the resulting thickness distribution on the venusian surface matches the observed parabolic features closely the dual parabolic feature associated with the crater carson is also explained by this model

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

mantle plume influence on the neogene uplift and extension of the u s western cordillera espite its highly extended and thinned crust much of the western cordillera in the united states is elevated more than 1 km above sea level therefore this region cannot be thought of as thick crust floating isostatically in a uniform mantle rather the lithospheric mantle and or the upper asthenosphere must vary in thickness or density across the region utilizing crustal thickness and density constraints we modeled the residual mass deficit that must occur in the mantle lithosphere and asthenosphere beneath the western cordillera a major hot spot broke out during a complex series of cenozoic tectonic events that included lithospheric thickening backarc extension and transition from a subduction to a transform plate boundary we suggest that many of the characteristics that make the western cordillera unique among extensional provinces can be ttributed to the mantle plume that created the yellowstone hot spot

magnetostratigraphy of the lower triassic volcanics from deep drill sg6 in western siberia evidence for long lasting permo triassic volcanic activity summary the deep drill hole sg6 in western siberia 66 n 78 5 e penetrated 1 1 km of lower triassic basalts which are possibly an extension of the central siberian permo triassic flood basalt province about 300 samples of these basalts were progressively demagnetized and measured principal component analysis often shows multiple magnetizations carried by haematite and magnetite the corrected mean inclinations are 77 and 77 for the haematite component a magnetostratigraphic scale was derived and showed a n r n r n succession this is quite diverent from the noril sk and taimyr typical polarity scale r n the basalts found in the sg6 deep drill hole are slightly younger than those of central siberia and taimyr they correspond to middle upper induan age whereas the noril sk and taimyr sections correspond to an uppermost permian and lower induan age altogether they indicate that after a high output rate of volcanic material near the permo triassic boundary this activity slowed down drastically on the siberian platform and taimyr but persisted for several million years in western siberia

a reconnaissance 40ar39ar geochronologic study of ore bearing and related rocks siberian russia abstract 40 ar 39 ar age spectra of biotite from a mineralized vein in the ore bearing noril sk i intrusion and from picritic like gabbrodolerite from the weakly mineralized lower talnakh intrusion show that these bodies were emplaced at 249 2 ma which is not significantly different from the age of the permian triassic boundary the ore bearing intrusions postdate the lower third of the flood basalt sequence in the noril sk area and on the basis of geochemistry can best be correlated with lavas slightly younger than those which they cut thus flood basalt was erupted at the time of the permian triassic mass extinction event although its role in this event is as yet ill defined additional new 40 ar 39 ar age data for a group of intrusive and extrusive rocks on the western margin of the siberian craton indicate that mafic magmatism extended over a period of several tens of million years whereas paleomagnetic data suggest that the bulk of the siberian flood basalt sequence near noril sk has been erupted in only a million years or so 40 ar 39 ar ages of plagioclase from early flood basalt flows are about 2 younger than those obtained for biotite from the crosscutting noril sk i intrusion probably because of slight alteration and argon loss from the plagioclase

on the estimation of the true mean inclinations when declinations are unknown paleomagnetists dealing with deep drill samples cannot use the classical fisher statistics when they don t know the declinations of the magnetization of their samples the arithmetical mean of the inclinations gives a strongly biased estimation of the true mean inclination when these inclinations are steep and scattered by fitting cumulative distribution curves we show that we can obtain an unbiased or at least less biased estimation of the true mean inclination and of the k value the method is tested on artificial and real cases

late permian to early triassic magnetostratigraphy a late permian to early triassic magnetostratigraphic reference section is presented the lower triassic part is based on results from marine limestone sections in south china published earlier 1 2 reliable new permian data are added here which have been collected in the nammal gorge salt range northwest pakistan where marine sediments have been deposited quasi continuously with occasional minor hiatuses during the late palaeozoic to early mesozoic about 50 of the permian samples from the nammal section contain hidden beneath a strong recent or tertiary overprint a characteristic remanent magnetization chrm which is very likely of permian age this component which was imprinted on the southern hemisphere has normal as well as reversed polarity with a normal mean direction decl 289 3 incl 50 3 95 4 3 n 113 which is in close agreement with the palaeofield direction expected for a site belonging to the indian plate as part of gondwanaland during the permian in the lower upper permian several normal polarity zones are recognized this contradicts the current assumption that rocks of this age belong to the long reversely polarized kiaman hyperzone the kiaman interval must end and the illawarra hyperzone of mixed polarity must begin in or prior to the lowermost upper permian the permian triassic boundary at nammal as well as in the chinese sections is situated very close to a transition from a reversed to a normal polarity zone the upper permian at nammal together with the lower triassic south china sections is estimated to cover about 20 ma nearly 30 polarity changes are observed which result in an average reversal frequency very similar to that observed during the early tertiary the reversal rate after the end of the long lasting reversed kiaman hyperchron apparently increases in a manner similar to that after the end of the cretaceous long normal superchron only a few polarity zones are found in the lower upper permian while progressively more frequent reversals follow in the lower triassic

geologic constraints on crustal plateau surface histories venus the lava pond and bolide impact hypotheses 1 venusian crustal plateau formation is hotly debated crustal plateaus are large 1500 2500 km quasi circular 0 5 to 4 km high plateaus that host distinctive tectonic fabrics called ribbon tessera debate centers on plateau support and fabric formation detailed geologic mapping of an 360 000 km2 region eastern ovda regio provides critical clues for plateau evolution ribbon tessera fabrics record broadly synchronous layer contraction extension and flooding as an initially strong thin layer 100 m developed across ovda as the layer underwent ductile shortening and brittle extension subsurface lava flooded structural lows with progressive deformation the layer thickened leading to formation of progressively longer wavelength structures early formed shorter wavelength structures and lava filled valleys were carried piggyback on longer wavelength folds this history indicates an extremely large strength contrast that is inconsistent with a conductive geotherm reflecting instead the presence of a near surface molten layer i propose the lava pond hypothesis ribbon tessera fabrics represent progressively solidified scum of a huge lava pond leading to crustal plateau formation i further propose that individual lava ponds formed as a result of large bolide 20 30 km diameter impact with ancient thin lithosphere causing massive partial melting in the upper mantle melt rose buoyantly to form huge 1500 2000 km diameter lava ponds completely resurfacing the local area in the upper mantle depleted residuum would be compositionally more buoyant and stronger than adjacent undepleted mantle isostatic adjustment resulted in pond uplift and plateau formation subsequent secular cooling could freeze a plateau in place or mantle convection could strip away the residuum root leading to plateau collapse

flow patterns in the siberian traps deduced from magnetic fabric studies summary the siberian traps rank among the largest flood basalt provinces in the world major magmatic activity is restricted to several short periods of less than 1 myr around the permo triassic boundary we measured the anisotropy of magnetic susceptibility ams of 183 basaltic samples from 28 individual lava flows belonging to the northwestern part of the flood basalt province in order to test the reliability of such data in structural interpretation these preliminary ams data show a remarkably consistent pattern of magnetic lineations that likely reflect the primary flow related texture of the basalt our results also emphasize that ams data are consistent with the well documented petrological and geochronological data the inferred flow directions are also consistent with flow directions expected from isopach maps our results suggest that the entire 2 km thick lava pile was fed by fissure eruptions located in the rifted basin that bound the siberian platform and was controlled by topographical features particularly major rift zones

volcanism impact and mass extinctions incredible or credible coincidences massive continental volcanism and or bolide impacts are considered by many authors to have caused three major mass extinction events during the last 300 million years the end permian end cretaceous and end triassic extinctions however reevaluation of the frequency of bolide impacts and plume related flood basalt provinces indicates that both types of event occur much more frequently than mass extinctions and so in isolation may not be responsible for the largest extinctions furthermore the kill mechanisms associated with either flood basalts or impacts do not appear to be sufficiently powerful to cause worldwide collapse of ecosystems leading to the largest mass extinctions contemporaneous flood basalts and bolide impact may be prerequisites for the largest mass extinctions we present a statistical analysis of the probability of coincidence between volcanism and impact and show that three random coincidences of these events in the last 300 m y are likely no causal relationship between impact and volcanism is necessary the lesser mass extinctions on the other hand may not require juxtaposition of two such catastrophic events such coincidences occurring on more than three occasions during the last 300 m y become increasingly unlikely d 2004 elsevier b v all rights reserved

venus crater distribution and plains resurfacing models detailed analysis of the distribution of craters on venus using mth nearest neighbor analysis coupled with models based upon surface morphology constraints indicates that the hypothesis of complete spatial randomness csr cannot be rejected but is not a unique model of the observed crater distribution based on morphologic mapping the extensive volcanic plains can be divided into four units that have a spread in age of the order of 0 5t the mean surface age of the planet this four unit plains model along with its derivatives produce test statistics that indicate such models also cannot be rejected further the probability of obtaining a result at least as extreme as the observed test statistic given that the null hypothesis model corresponds to venus is true is lowest for the csr model there is no particular reason to pick a csr model along with its implications for catastrophic resurfacing as a constraint on the evolution of venus and there are geological reasons to choose the multiage models we find that we cannot distinguish statistically among models that have two three or four distinct production ages within the plains however the hypothesis that the variation in crater density within all of the plains is due to a single random process can be rejected for two reasons first the binomial probability that such a process could exist within each of the plains units is 0 05 except the smallest and youngest unit pl1 second using a chi squared statistic to test the hypothesis that four plains units have the same age gives a p value of 10 4 indicating confident rejection of the hypothesis thus csr cannot be used as a constraint on models of resurfacing or planetary evolution of venus because of the non uniqueness in matching such a model to the observed crater distribution and the strong indication of distinct ages within the plains with a significant spread in age geological and geophysical constraints provide our best clues for understanding venus

the geologic history of venus a stratigraphic view on the basis of regional and global stratigraphic analyses we outline the major events in the geologic history of venus determined by photogeological study of surface features because the morphological signatures of terrain emplaced prior to the time of tessera formation are not preserved the stratigraphic record presented comprises only the last 10 20 of the total history of venus the estimated range of the mean crater retention age of the surface from 200 to 1600 million years leads us to describe the timing and duration of different events in terms of fractions of the mean surface age t the beginning of the observed history of venus was characterized by intensive tectonic deformation of global or semi global scale which formed the tessera terrain termination of the compressional stage is estimated to have occurred at about 1 4t while the tensional stage lasted for another 0 1 0 2t after tessera formation several stages of extensive volcanism occurred burying vast areas of tessera and forming what are now observed as regional plains the combined duration of the emplacement of these plains is estimated to be about 0 2 0 3t with an implied average global rate of volcanism of a few cubic kilometers per year regional plains forming materials can be subdivided and are separated from each other and from underlying and overlying units by unconformities these unconformities are formed from oldest to youngest by tessera forming deformation dense fracturing broad ridging and finally wrinkle ridging these tectonic episodes are interpreted to be generally globally synchronous and to represent successive episodes characterized by the dominance of compression then tension then again compression and finally tension the last global scale tectonic episode extensive wrinkle ridging happened at about time t which was very close in time to the emplacement of the most areally abundant plains unit this marked the transition to the present stage of the history of venus which is characterized by a predominance of regional rifting and related volcanism this stage appears to have lasted from about time t to the present making it the longest time duration among the stratigraphic units considered although the resulting tectonic and volcanic features and deposits cover only 10 20 of the surface of venus these observations mean that the general intensity of tectonics and the flux of volcanism a few tenths of a cubic kilometer per year in this latest period were much lower than those in earlier times in summary the morphologically observable part of the history of venus was characterized by two key characteristics that stand in contrast to the comparable period of earth history approximately the phanerozoic when global geodynamic processes were dominated by plate tectonics 1 venus shows no signature of plate tectonics instead its global tectonic environment passed from an initial dominance of compression through tension then again compression and finally tension with the density of deformational structures and the strain rate declining with time 2 in the beginning of this period of time on venus plains forming volcanism occurred at a rate comparable to volcanism at mid ocean ridges but was emplaced in an entirely different style the predominant component of volcanism on earth during this time was the extrusive volcanism at mid oceanic ridges for the last few hundred million years venus has been dominated primarily by rift associated volcanism emplaced at a production rate comparable to or even lower than present intraplate volcanism production rates on earth

3 d 40ar39ar geochronology in the parana continental flood basalt province new 40ar39ar analyses of borehole samples from the parana etendeka continental flood basalt province provide critical evidence of eruption rates in unexposed regions when combined with surface samples the new data clarify the duration of this major volcanic province new ages from both surface and borehole samples confirm a 10 12 million year duration of magmatism which post dates the tithonian mass extinction and provide a unique 3 dimensional picture of the spatial and temporal eruption of the lava pile chemically defined magma types are diachronous and the onset of magmatism occurred 500 1000 km inland migrating southeast towards the incipient south atlantic ocean calculated eruption rates increased from 0 03 km3 yr 1 between 138 and 135 ma through 0 13 km3 yr 1 between 135 and 133 ma to 0 21 km3 yr 1 between 133 and 131 ma supporting the notion that the rate of magmatism increased with time and proximity to the developing south atlantic after 131 ma the continental eruption rates dropped dramatically to 0 01 km3 yr 1 presumably because rifting had occurred and eruption centred in the oceanic rio grande rise with eruption rates of around 0 3 0 5 km3 yr 1 given the available constraints on the potential temperature of the plume thickness of the continental lithosphere and degrees of extension through time predicted melt production rates within the plume seriously underestimate the calculated eruption rates on the continent however the eruption rates can be reconciled if melting occurred by conductive heating of volatile enriched mantle the diachronous nature of the magma types supports the idea that melting occurred over a wide area and that the different magma types reflect different source regions rather than the temporal evolution of magmas from a single source given that 10 million years is appropriate to the timescales for conductive heating above the mantle plume it is suggested that these distinct source regions were located within the lithospheric mantle consistent with geochemical studies

synchrony and causal relations between permian triassic boundary crises and siberian flood volcanism the permian triassic boundary records the most severe mass extinctions in earth9s history siberian flood volcanism the most profuse known such subaerial event produced 2 million to 3 million cubic kilometers of volcanic ejecta in approximately 1 million years or less analysis of 40 ar 39 ar data from two tuffs in southern china yielded a date of 250 0 0 2 million years ago for the permian triassic boundary which is comparable to the inception of main stage siberian flood volcanism at 250 0 0 3 million years ago volcanogenic sulfate aerosols and the dynamic effects of the siberian plume likely contributed to environmental extrema that led to the mass extinctions

tessera terrain on venus a survey of the global distribution characteristics and relation to surrounding units from magellan data the tessera terrain on venus comprised of areas of high radar backscatter complex deformation patterns relative to other units and topography standing higher than surrounding plains covers 35 33 106 km2 about 8 of the surface of venus and is nonrandomly distributed being preferentially located at equatorial and higher northern latitudes with a distinct paucity below about 30 s individual tessera occurrences range in area from the lower limits of our measurements about 200 km2 up to the largest tessera ovda with an area of about 8 6 106 km2 or about 2 of the surface area of venus the size frequency distribution of tessera patches is strongly unimodal and skewed toward smaller sizes reflecting the great abundance of small tessera fragments modes of occurrence include 1 large clusters e g aphrodite terra and ishtar terra 2 arc like segments which may extend for thousands of kilometers and are either concave inward toward the major tessera cluster development or away from it 3 areas where tesserae are rare or absent which occur both as low lying plains e g guinevere planitia and as elevated regions e g atla regio tessera terrain has a bimodal elevation frequency distribution with the main peak at about 0 1 km and an additional peak at about 3 km above mean planetary radius in terms of number of occurrences however tesserae do not display a correlation with elevation at the global scale since small tessera patches commonly occupy low lying regions although tessera exhibit a range of gravity signatures many occurrences are interpreted to represent relatively shallow crustal levels of compensation tessera boundaries include type i sinuous embayed dominated by adjacent lava plains embaying tessera massifs 73 of total tesserae boundaries and type ii linear tectonic only a small percentage of the length of all boundary types show no lava embayment and could be interpreted as tectonically active for long periods subsequent to initial tessera formation occurrence of broad slopes of post tessera embayed plains away from tessera boundaries suggests that regional tilting occurred subsequent to final tessera deformation in some places several lines of evidence suggest the possibility that a widespread tessera like basement comprising at least 55 of the surface of venus is buried under a cover of lava plains a few hundred meters to as much as 2 4 km thick a wide variety of deformational structures and patterns is observed within the tessera including those representing extension compression shear and transpression in some cases the apparently complex patterns can be resolved into single event kinematic interpretations involving noncoaxial deformation e g itzpapalotl while in other cases polyphase deformation is more likely e g central ovda and thetis where relations can be determined stratigraphically earliest deformation within the tessera is primarily related to crustal shortening and compression phase i followed by pervasive extensional deformation commonly oriented normal to the strike of phase i features generally along the same principal stress direction phase ii evidence also exists for the contemporaneous formation of these distinctive deformation patterns lava plains within and adjacent to the tessera embay both of these fabrics but sometimes overlap in time with phase ii extensional deformation and with regional tilting tessera terrain as a geologic unit occupies the lowest portion of the stratigraphic column in all areas that we have observed an observation consistent with many other mapping studies we see no evidence for transitional stages between tessera and volcanic rises and or lowlands that might represent a long term sequence of upwelling or downwelling followed by crustal deformation and tessera formation no impact craters deformed by phase i deformation have yet been observed on tessera suggesting that phase i tessera deformation sufficiently intense to eradicate earlier impact craters ceased relatively abruptly somewhat before 300 500 m y ago however the starting time and thus duration of tessera formation is unknown on the basis of the very small number of on tessera craters deformed by phase ii extensional deformation this period probably did not last more than several tens of millions of years after the cessation of phase i little observable deformation of the tessera terrain appears to have taken place in the last several hundred million years during which time the vast volcanic plains were emplaced although tilting of early plains along some tessera margins is observed building on the global synthesis presented here future analyses of individual tessera occurrences will provide the detailed descriptions kinematic interpretations and strain histories necessary to assess and distinguish between the several catastrophic and uniformitarian models for tessera formation

palaeomagnetism of east siberian traps and kimberlites two new poles and palaeogeographic reconstructions at about 360 and 250 ma summary we studied basalt sills and dykes and kimberlite pipes from the eastern part of the siberian platform these are spread out over a few hundred kilometres and are divided into two age groups late permian early triassic mir aikhal and olenyek regions and late devonian early carboniferous viluy and markha basins aikhal region the palaeomagnetic poles determined for each sill or dyke are statistically different from each other but averaging all corresponding directions from these sills and dykes with those from kimberlites of different generations yields results with acceptable statistics this is probably due to the averaging out of secular variation and or changes in magnetic polarity the resulting palaeomagnetic poles allow one to reconstruct the palaeopositions of siberia during the two large flood basalt events at about 360 and 250 ma a significant rotation of the siberian platform during the devonian and carboniferous is suggested the new data are in general agreement with some earlier palaeomagnetic poles for the late permian early triassic traps of the tungusska basin and significantly reduce uncertainties for previous late devonian early carboniferous siberian poles which were very scattered together with geological and absolute age data the palaeomagnetic data support the hypothesis that kimberlite magmatism and trap intrusions were both rapid and relatively close in time the 360 ma event is emerging as a magmatic event with considerable extent over the russian and siberian platforms related to incipient continental break up and triple junction formation possibly at or near the time of the frasnian famennian mass extinction

paleomagnetism and magnetostratigraphy of the traps from western taimyr northern siberia and the permo triassic crisis abstract the siberian traps were emplaced near the permo triassic boundary and are among the largest continental igneous provinces a 1750 m thick section of traps from the western taimyr 72 9 n 84 e has been sampled this section is equivalent to the lower and middle part of the total siberian traps section thirty one oriented hand samples were studied and demagnetised by alternating fields and heating cycles a stable characteristic component passes the fold test and is thought to be primary the mean direction is d 84 i 75 with 95 10 n 29 and the corresponding pole is 59 n 150 e the bottom part 660 m is of reversed polarity the middle part 900 m is normal and the uppermost level 150 m is mixed correlations with reference permo triassic magnetostratigraphic timescales are discussed the most probable correlation indicates an age quite close to the permian triassic boundary and a duration for the volcanism of less than 1 m y the data are therefore consistent with the hypothesis that this volcanism triggered the permian triassic crisis

permo triassic boundary superanoxia and stratified superocean records from lost deep sea pelagic cherts of japan and british columbia canada recorded a long term and worldwide deep sea anoxic oxygen depleted event across the permo triassic or paleozoic and mesozoic boundary 251 2 million years ago the symmetry in lithostratigraphy and redox condition of the boundary sections suggest that the superocean panthalassa became totally stratified for nearly 20 million years across the boundary the timing of onset climax and termination of the oceanic stratification correspond to global biotic events including the end guadalupian decline the end permian extinction and mid triassic recovery

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

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels

late early triassic climate change insights from carbonate carbon isotopes sedimentary evolution and ammonoid paleobiogeography the late early triassic sedimentary facies evolution and carbonate carbon isotopemarine record 13ccarb of ammonoid rich outer platform settings show striking similarities between the south china block scb and the widely distantnorthern indianmargin nim the studied sections are located within the triassic tethyshimalayan belt losar section himachal pradesh india and the nanpanjiang basin in the south china block jinya section guangxi province respectively carbon isotopes from the studied sections confirm the previously observed carbon cycle perturbations at a time of major paleoceanographic changes in the wake of the end permian biotic crisis this study documents the coincidence between a sharp increase in the carbon isotope composition and the worldwide ammonoid evolutionary turnover extinction followed by a radiation occurring around the smithian spathian boundary based on recent modeling studies on ammonoid paleobiogeography and taxonomic diversity we demonstrate that the late early triassic smithian and spathian was a time of a major climate change more precisely the end smithian climate can be characterized by a warm and equable climate underlined by a flat pole to equator sea surface temperature sst gradient while the steep spathian sst gradient suggests latitudinally differentiated climatic conditions moreover sedimentary evidence suggests a transition froma humid and hot climate during the smithian to a dryer climate fromthe spathian onwards by analogywith comparable carbon isotope perturbations in the latedevonian jurassic andcretaceouswe propose that high atmosphericco2 levels could have been responsible for the observed carbon cycle disturbance at the smithian spathian boundary we suggest that the end smithian ammonoid extinction has been essentially caused by a warm and equable climate related to an increased co2 flux possibly originating froma short eruptive event of the siberian igneous province this increase in atmospheric co2 concentrations could have additionally reduced the marine calcium carbonate oversaturation and weakened the calcification potential of marine organisms including ammonoids in late smithian oceans

crustal analysis of venus from magellan satellite observations at atalanta planitia beta regio and thetis regio we investigated an improved quantitative method for interpreting attributes of the venusian crust from magellan spacecraft imagery and spherical harmonic topographic and gravity models we used spectral correlation theory to compare gravity effects of topography against free air gravity anomalies for three tectonically disparate crustal regions adjusting terrain gravity effects for correlated free air anomalies yields compensated effects that we used to estimate the moho and crustal thickness variations for atalanta planitia the crust may be less than 10 km thick but thickened peripherally at concentric ridge belts and tessera terrain crustal thickness may increase rapidly between the interior of this lowland plain and the peripheral ridge belts for beta regio the crustal thickness may exceed 40 km of the three regions considered the crust of beta regio may be the most poorly compensated because its free air anomalies are largest relative to corresponding terrain gravity effects for thetis regio crustal thicknesses may exceed 50 km the thickened crust of thetis regio may also have been rifted and embayed by plains forming lavas the lavas appear to embay and cover complexly ridged areas except where these units remained above the reach of the plains forming lavas crosscutting relationships which are evident from magellan imagery together with our crustal thickness estimates suggest that the complexly ridged terrain or tesserae may be the oldest crustal units within the three study regions hence these units may represent relict surfaces that predate emplacement of the plains