changes in crack density and wave velocity in association with crack growth in triaxial tests of inada granite 1 a nondimensional second rank tensor fij called the crack tensor has successfully been introduced to deal with geometrical aspects of microcracks fabric such as anisotropy and crack density unfortunately however its usage for practical purposes is rather limited because its determination involves tedious and time consuming laboratory work we seek the possibility of using the directional change of longitudinal wave velocities to conquer the difficulty associated with the determination of crack tensors a new second rank tensor vij is introduced such that the directional change in the longitudinal wave velocities is represented in terms of the tensor and the crack tensor fij is then given as a function of vij on the basis of the analyses of the crack tensors for one intact and several damaged samples of inada granite we then discuss how microcracks grow through the whole inelastic process terminating at brittle failure the conclusions are summarized as follows the second rank symmetrical tensor vij or its inversion tensor vij 1 can be determined experimentally with sufficient accuracy from the directional change in the squared longitudinal wave velocity it is found that the tensor changes markedly so as to reflect the fabric of the damaged inada granite formed by open microcracks the principal axes of vij 1 are coaxial with the principal axes of fij so that both tensors are correlated in terms of their principal values fi and vi 1 four successive stages can be distinguished in regard to the crack growth as follows in stage 1 the rock behaves like an elastic solid in stage 2 microcracks start to grow so that inelastic volumetric strain is slowly accumulated along with microcracking however crack growth does not occur globally but rather is limited within some local zones probably in each grain in stage 3 microcracking is considerably accelerated suggesting that the micromechanism leading to crack growth changes substantially at the boundary stress between stages 2 and 3 in stage 4 the crack density as well as the dilatancy increases explosively in association with a drop of a few percent in the differential stress after the peak stress is reached interestingly this explosive increase is always associated with the development of a few fault zones experimental evidence seems to support the postulate that inada granite starts to collapse once the crack density f0 f the first invariant of fij attains a threshold value of 7 8 regardless of the applied confining pressure this can be a failure criterion in terms of the crack density and be an extended expression for the so called critical dilatancy for creep failure suggested by kranz and scholz 1977

shock vaporization of anhydrite and global effects of the k t bolide shock vaporization experiments were carried out for 30 porous anhydrite up to 76 gpa shocked fully or partially vaporized samples interact with overlying lif windows whose velocity histories are monitored using a velocity interferometer to obtain pressure bounds for incipient and complete vaporization for caso 4 experimental data and thermodynamic calculations indicate that these shock pressures are 81 7 and 155 13 gpa for crystal anhydrite and 27 1 and 67 6 gpa for porous anhydrite respectively a one dimensional finite difference code was used to simulate the measured velocity profiles the vaporized products can be described by a simple gruneisen thermal equation of state where the effective gruneisen parameter varies from 1 5 to 0 73 upon release from 76 to 25 gpa using the above criteria and recent lithic models of the impact site the mass of degassed s has been estimated from the chicxulub impact for asteroids of 10 and 20 km in diameter impacting the earth at 20 km s the mass of degassed s in so 2 or so 3 is found to be 0 5 10 17 to 2 10 17 g simple extrapolation of sigurdsson s h sigurdsson assessment of the atmospheric impact of volcanic eruptions in v l sharpton p d ward eds global catastrophes in earth history an interdisciplinary conference on impact volcanism and mass mortality geol soc am spec pap 247 1990 99 110 formula yields a global cooling prediction of 10 c

effects of asteroid and comet impacts on habitats for lithophytic organisms a synthesis asteroid and comet impacts can have a profound influence on the habitats available for lithophytic microorganisms using evidence from the haughton impact structure nunavut canadian high arctic we describe the role of impacts in influencing the nature of the lithophytic ecological niche impact induced increases in rock porosity and fracturing can result in the formation of cryptoendolithic habitats in some cases and depending upon the target material an increase in rock translucence can yield new habitats for photosynthetic cryptoendoliths chasmoendolithic habitats are associated with cracks and cavities connected to the surface of the rock and are commonly increased in abundance as a result of impact bulking chasmoendolithic habitats require less specific geological conditions than are required for cryptoendolithic habitats and their formation is likely to be common to most impact events impact events are unlikely to have an influence on epilithic and hypolithic habitats except in rare cases where for example the formation of impact glasses might yield new hypolithic habitats we present a synthetic understanding of the influence of asteroid and comet impacts on the availability and characteristics of rocky habitats for microorganisms

hydrocode simulation of the chicxulub impact event and the production of climatically active gases we constructed a numerical model of the chicxulub impact event using the chart d squared csq code coupled with the analytic equation of state aneos package in the simulations we utilized a target stratigraphy based on borehole data and employed newly developed equations of state for the materials that are believed to play a crucial role in the impact related extinction hypothesis carbonates calcite and evaporites anhydrite simulations explored the effects of different projectile sizes 10 to 30 km in diameter and porosity 0 to 50 the effect of impact speed is addressed by doing simulations of asteroid impacts vi 20 km s and comet impacts vi 50 km s the masses of climatically important species injected into the upper atmosphere by the impact increase with the energy of the impact event ranging from 350 to 3500 gt for co2 from 40 to 560 gt for s and from 200 to 1400 gt for water vapor while our results are in good agreement with those of ivanov et al 1996 our estimated co2 production is 1 to 2 orders of magnitude lower than the results of takata and ahrens 1994 indicating that the impact event enhanced the end cretaceous atmospheric co2 inventory by at most 40 consequently sulfur may have been the most important climatically active gas injected into the stratosphere the amount of s released by the impact is several orders of magnitude higher than any known volcanic eruption and with h2o is high enough to produce a sudden and significant perturbation of earth s climate

anisotropic changes in p wave velocity and attenuation during deformation and fluid infiltration of granite fluid infiltration and pore fluid pressure changes are known to have a significant effect on the occurrence of earthquakes yet for most damaging earthquakes with nucleation zones below a few kilometers depth direct measurements of fluid pressure variations are not available instead pore fluid pressures are inferred primarily from seismic wave propagation characteristics such as v p v s ratio attenuation and reflectivity contacts we present laboratory measurements of changes in p wave velocity and attenuation during the injection of water into a granite sample as it was loaded to failure a cylindrical sample of westerly granite was deformed at constant confining and pore pressures of 50 and 1 mpa respectively axial load was increased in discrete steps by controlling axial displacement anisotropic p wave velocity and attenuation fields were determined during the experiment using an array of 13 piezoelectric transducers at the final loading steps 86 and 95 of peak stress both spatial and temporal changes in p wave velocity and peak to peak amplitudes of p and s waves were observed p wave velocity anisotropy reached a maximum of 26 transient increases in attenuation of up to 483 db m were also observed and were associated with diffusion of water into the sample we show that velocity and attenuation of p waves are sensitive to the process of opening of microcracks and the subsequent resaturation of these cracks as water diffuses in from the surrounding region symmetry of the orientation of newly formed microcracks results in anisotropic velocity and attenuation fields that systematically evolve in response to changes in stress and influx of water with proper scaling these measurements provide constraints on the magnitude and duration of velocity and attenuation transients that can be expected to accompany the nucleation of earthquakes in the earth9s crust

evidence of impact melting and post impact decomposition of sedimentary target rocks from the steen river impact structure alberta canada abstract hypervelocity bolide impacts deliver vast amounts of energy to the earth s near surface this crustal process almost universally includes sedimentary target rocks however their response to impact is poorly understood in part because of complexities due to layering pore space and the presence of volatiles that are difficult to model the response of carbonates to bolide impact remains contentious yet whether they melt or decompose and liberate gases by the reaction caco3 s cao s co2 g has significant implications for post impact climatic effects we report on previously unknown carbonate impact melts at the steen river impact structure canada and the first description of naturally shocked barite baso4 carbonate melts are preserved as groundmass supported calcite rich clasts sampled from an up to 164 m thick continuous sequence of crater fill polymict breccias electron microscopy reveals fluidal and ocellar textured calcite and barite intimately associated with silicate melt consistent with these phases being in the liquid state at the same time raman spectroscopy and electron backscatter diffraction ebsd mapping confirm the presence of high pressure phases reidite and coesite within some steen river carbonate melt bearing breccias these minerals attest to the strong shock provenance of the breccia and provide constraints on their shock history preservation of reidite lamellae in zircon indicates a shock pressure 30 gpa

experimental determination of elastic constants of oshima granite barre granite and chelmsford granite employing polyhedral specimens longitudinal and shear wave velocities were measured in various directions of propagation and polarization sound velocities showed orthorhombic elasticity in all of the rocks with the assumption of orthorhombic elasticity the nine stiffness constants of all samples were determined by the sound velocities under atmospheric pressure and kelvin christoffel s equation twenty one stiffness constants of oshima granite determined without assuming any symmetry also showed orthorhombic features directions of the symmetry axes agreed well with the orientation of the preexisting cracks akaike s information criterion showed that the orthorhombic model with nine nonzero elastic stiffnesses was better than the model having 21 nonzero elastic stiffnesses for oshima granite the polyhedrons of two granitic rocks were loaded under hydrostatic pressure all components of the stiffness constants increased with pressure under pressure of more than 120 mpa two granitic rocks were approximately isotropic the results show that oriented microcracks are mainly responsible for the orthorhombic elasticity of the granitic rocks and also indicate that dry oriented cracks can not be a cause for the anisotropic elasticity of granites at depths of more than 6 8 km

crustal thickness and support of topography on venus united states national aeronautics and space administration nasa planetary geology and geophysics program grant

re evaluating the age of the haughton impact event we have re evaluated the published age information for the haughton impact structure which was believed to have formed 23 ma ago during the miocene age and report new ar ar laser probe data from shocked basement clasts this reveals an eocene age which is at odds with the published miocene stratigraphic apatite fission track and ar ar data we discuss our new data within this context we have found that the age of the haughton impact structure is 39 ma which has implications for both crater recolonization models and post impact hydrothermal activity future work on the relationship between flora and fauna within the crater and others at high latitude may resolve this paradox

seismic velocities in fractured rocks an experimental verification of hudson s theory flow of fluids in many hydrocarbon reservoirs aquifers is enhanced by the presence of cracks and fractures these cracks could be detected by their effects on propagation of compressional and shear waves through the reservoir several theories including hudson s claim to predict the seismic effects of cracks although hudson s theory has already been used to calculate crack densities from seismic survey s the predictions of the theory have not yet been tested experimentally on rocks containing a known crack distribution this paper describes an experimental verification of the theory the rock used carrara marble was chosen for its uniformity and low porosity so that the effect of cracks would not be obscured by other influences cracks were induced by loading of laboratory specimens velocities of compressional and shear waves were measured by ultrasound at 0 85 mhz in dry and water saturated specimens at high and low effective pressures the cracks were then counted in polished sections of the specimens in dry specimens with both dry and saturated cracks hudson s theory overpredicted observed crack densities by a constant amount that is attributed to the observed value being systematically underestimated the theory made poor predictions for fully saturated specimens shear wave splitting caused by anisotropy duemore to both crystal and crack alignment was observed cracks were seen to follow grain boundaries rather than the direction of maximum compression due to loading the results demonstrate that hudson s theory may be used in some cases to determine crack and fracture densities from compressional and shear wave velocity data less

evidence for shock induced anhydrite recrystallization and decomposition at the unam 7 drill core from the chicxulub impact structure

surface features of uniaxial tensile fractures and their relation to rock anisotropy in inada granite abstract uniaxial tensile fractures parallel to the rift grain and hardway planes were formed in inada granite most of the fractures split the rock forming mineral grains as transgranular paths under microscopic observations the surface topography of the fractures was measured by the stereo photogrammetric technique and is different in different orientations the rift parallel uniaxial fracture has the smoothest surface and the grain parallel fractures are smoother than hardway parallel fractures the mineral compositions are also different on the failed surfaces of different directions the rift parallel fractures have almost the same constituent mineral ratio as the parent rock however the grain parallel fractures pass preferentially through feldspar grains and contain less quartz grains as compared to the bulk composition of the parent rock in addition it is remarkable on hardway parallel fractures it is considered that the differences of surface topography and mineral compositions are caused by preferred orientations and spatial distributions of pre existing microcracks in inada granite

the effect of noise on estimates of the elastic thickness of the continental lithosphere by the coherence method 1 we model the lithosphere as a uniform elastic plate overlying an inviscid fluid and loaded with both surface and subsurface fractal loads to generate synthetic topography and gravity data to simulate data having low topographic signal to gravity noise ratio we use an algebraically larger exponent for the subsurface load in the spectral synthesis fractal algorithm the gravity power spectrum then decays less rapidly than that of topography the spectra resembling those for central australia we find that the coherence method using multitaper spectral estimation yields significant underestimates of plate thickness for both low and normal signal to noise ratio data unless the data window is larger than several times the true flexural wavelength we quantify this bias for the parameters used here and apply it as a correction to an effective elastic thickness estimate for central australia obtaining a value of 115 25 km index terms 8110 tectonophysics continental tectonics general 0905 8159 tectonophysics rheology crust and lithosphere 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 citation swain c j and j f kirby the effect of noise on estimates of the elastic thickness of the continental lithosphere by the coherence method geophys res lett 30 11 1574 doi 10 1029 2003gl017070 2003

an effective elastic thickness map of australia from wavelet transforms of gravity and topography using forsyth s method 1 we have developed a wavelet version of forsyth s coherence method for estimating the effective elastic thickness te of the lithosphere from gravity and topography we have previously shown that the fan wavelet is particularly well suited to making localised coherence estimates and here we progress the method by showing how the coherence can be modelled by using the wavelet transforms of gravity and topography to infer the initial loads that flex the plate the effectiveness of the method is demonstrated on synthetic data from a model having a rectangular area of large te embedded in a uniform low te surrounding and a load ratio that increases with wave number the results for the australia region show a core of high te under north and central australia with much lower values to the east and also unexpectedly under the western parts of the yilgarn and pilbara cratons

impact induced hydrothermal activity within the haughton impact structure arctic canada generation of a transient warm wet oasis field studies and analytical scanning electron microscopy indicate that a hydrothermal system was created by the interaction of water with hot impact generated rocks following formation of the 24 km diameter 23 ma haughton impact structure hydrothermal alteration is recognized in two settings within polymict impact breccias overlying the central portion of the structure and within localized pipes in impact generated concentric fault systems the intra breccia alteration comprises three varieties of cavity and fracture filling a sulfide with carbonate b sulfate and c carbonate these are accompanied by subordinate celestite barite fluorite quartz and marcasite selenite is also developed particularly in the lower levels of the impact breccia sheet the fault related hydrothermal alteration occurs in 1 7 m diameter subvertical pipes that are exposed for lengths of up 20 m the pipes are defined by a monomict quartz carbonate breccia showing pronounced fe hydroxide alteration associated sulfides include marcasite pyrite and chalcopyrite we propose three distinct stages in the evolution of the hydrothermal system 1 early stage 200 c with the precipitation of quartz vapor phase dominated 2 main stage 200 100 c with the development of a two phase vapor plus liquid zone leading to calcite celestite barite marcasite and fluorite precipitation and 3 late stage 100 c with selenite and fibroferrite development through liquid phase dominated precipitation we estimate that it took several tens of thousands of years to cool below 50 c following impact during this time haughton supported a 14 km diameter crater lake and subsurface water system providing a warmer wetter niche relative to the surrounding terrain the results reveal how understanding the internal structure of impact craters is necessary in order to determine their plumbing and cooling systems

static and dynamic pressure sensitivity anisotropy of a calcareous shale optimizing the productivity of nonconventional low permeability shale reservoirs requires detailed knowledge of the mechanical properties of such materials these rocks elastic anisotropy is acknowledged but usually ignored due to difficulties in obtaining such information here we study in detail the dynamic and static elastic properties of a suite of calcareous mudstones from the nonconventional duvernay reservoir of alberta canada the complete set of transversely isotropic elastic constants is obtained from strategically oriented ultrasonic transducers to confining pressures of 90 mpa wave speed anisotropies of up to 35 are observed at even the highest confining pressures furthermore the stress sensitivity of the wave speeds and hence moduli is itself highly dependent on direction with speeds taken perpendicular to the bedding plane being highly nonlinearly dependent on pressure whereas those along the bedding plane show unexpectedly nearly no pressure dependence these observations are in qualitative agreement with the preferentially oriented porosity and minerals seen in scanning electron microscope images these results may be significant to the interpretation of sonic logs and azimuthal amplitude versus offset for principal stress directions for the concentration of stress within such formations and for estimation of static engineering moduli from sonic log wave speeds

permeability of granite including macro fracture naturally filled with fine grained minerals information on the permeability of rock is essential for various geoengineering projects such as geological disposal of radioactive wastes hydrocarbon extraction and natural hazard risk mitigation it is especially important to investigate how fractures and pores influence the physical and transport properties of rock infiltration of groundwater through the damage zone fills fractures in granite with fine grained minerals however the permeability of rock possessing a fracture naturally filled with fine grained mineral grains has yet to be investigated in this study the permeabilities of granite samples including a macro fracture filled with clay and a mineral vein are investigated the permeability of granite with a fine grained mineral vein agrees well with that of the intact sample whereas the permeability of granite possessing a macro fracture filled with clay is lower than that of the macro fractured sample the decrease in the permeability is due to the filling of fine grained minerals and clay in the macro fracture it is concluded that the permeability of granite increases due to the existence of the fractures but decreases upon filling them with fine grained minerals

geological overview and cratering model for the haughton impact structure devon island canadian high arctic the haughton impact structure has been the focus of systematic multi disciplinary field and laboratory research activities over the past several years regional geological mapping has refined the sedimentary target stratigraphy and constrained the thickness of the sedimentary sequence at the time of impact to 1880 m new 40ar 39ar dates place the impact event at 39 ma in the late eocene haughton has an apparent crater diameter of 23 km with an estimated rim final crater diameter of 16 km the structure lacks a central topographic peak or peak ring which is unusual for craters of this size geological mapping and sampling reveals that a series of different impactites are present at haughton the volumetrically dominant crater fill impact melt breccias contain a calcite anhydrite silicate glass groundmass all of which have been shown to represent impact generated melt phases these impactites are therefore stratigraphically and genetically equivalent to coherent impact melt rocks present in craters developed in crystalline targets the crater fill impactites provided a heat source that drove a post impact hydrothermal system during this time haughton would have represented a transient warm wet microbial oasis a subsequent episode of erosion during which time substantial amounts of impactites were removed was followed by the deposition of intracrater lacustrine sediments of the haughton formation during the miocene present day intracrater lakes and ponds preserve a detailed paleoenvironmental record dating back to the last glaciation in the high arctic modern modification of the landscape is dominated by seasonal regional glacial and niveal melting and local periglacial processes the impact processing of target materials improved the opportunities for colonization and has provided several present day habitats suitable for microbial life that otherwise do not exist in the surrounding terrain

organic geochemical characterization of a miocene core sample from haughton impact structure devon island nunavut canadian high arctic extremely well preserved sedimentary deposits in the haughton impact structure his provide a unique record of the post impact miocene lacustrine depositional environment detailed organic geochemical characterization of a hydrocarbon impregnated band found in a core from the crater lake sedimentary infill reveals a complex source history for the hydrocarbons these include contributions from eroded pre and post impact formations together with inputs from contemporaneous flora and fauna a deep possibly lower paleozoic petroleum as well as a contribution from hydrothermally altered organic biopolymers geochemical data coupled with paleolimnology and geology proves to be a valuable tool for studying the provenance of hydrocarbons associated with the his and can give insights not only into the post impact geology but also into processes that may contribute to the generation of petroleum fluids at other impact sites

influences of electrolyte concentration on subcritical crack growth in sandstone in water abstract information on subcritical crack growth in rock is essential to ensure the long term stability of structures in a rock mass subcritical crack growth in rock is known to be affected by its surrounding environment in most cases rock found underground is saturated by water the underground water can be fresh water or salt water with variable electrolyte concentrations however the influence of electrolyte concentration on subcritical crack growth in rock has not been fully elucidated in this study subcritical crack growth in berea sandstone and shirahama sandstone was measured when saturated with distilled water and with sodium chloride solutions of differing concentrations crack velocity in berea sandstone changed very little with different electrolyte concentrations by contrast in the shirahama sandstone crack velocity became lower with increased electrolyte concentrations up to 1 0 mol l with increasing electrolyte concentration the width of the electric double layer on the surface of mineral grains decreased which caused a decrease in the repulsive force acting on the crack surface leading to a decrease in crack velocity up to concentrations of 1 0 mol l however when the electrolyte concentration was higher than 1 0 mol l crack velocity increased due to the nucleation of microscopic defects on the boundary between clays and stiff mineral grains thus electrolyte concentration affects subcritical crack growth by decreasing the width of the electric double layer and the condensation of clay minerals

tectonics of complex crater formation as revealed by the haughton impact structure devon island canadian high arctic the results of a systematic field mapping campaign at the haughton impact structure have revealed new information about the tectonic evolution of mid size complex impact structures these studies reveal that several structures are generated during the initial compressive outward directed growth of the transient cavity during the excavation stage of crater formation 1 sub vertical radial faults and fractures 2 sub horizontal bedding parallel detachment faults and 3 minor concentric faults and fractures uplift of the transient cavity floor toward the end of the excavation stage produces a central uplift compressional inward directed deformation results in the duplication of strata along thrust faults and folds it is notable that haughton lacks a central topographic peak or peak ring the gravitational collapse of transient cavity walls involves the complex interaction of a series of interconnected radial and concentric faults while the outermost concentric faults dip in toward the crater center the majority of the innermost faults at haughton dip away from the center complex interactions between an outward directed collapsing central uplift and inward collapsing crater walls during the final stages of crater modification resulted in a structural ring of uplifted intensely faulted sub vertical and or overturned strata at a radial distance from the crater center of 5 0 6 5 km converging flow during the collapse of transient cavity walls was accommodated by the formation of several structures 1 sub vertical radial faults and folds 2 positive flower structures and chaotically brecciated ridges 3 rollover anticlines in the hanging walls of major listric faults and 4 antithetic faults and crestal collapse grabens oblique strike slip i e centripetal movement along concentric faults also accommodated strain during the final stages of readjustment during the crater modification stage it is clear that deformation during collapse of the transient cavity walls at haughton was brittle and localized along discrete fault planes separating kilometer size blocks

a case study of impact induced hydrothermal activity the haughton impact structure devon island canadian high arctic the well preserved state and excellent exposure at the 39 ma haughton impact structure 23 km in diameter allows a clearer picture to be made of the nature and distribution of hydrothermal deposits within mid size complex impact craters a moderate to low temperature hydrothermal system was generated at haughton by the interaction of groundwaters with the hot impact melt breccias that filled the interior of the crater four distinct settings and styles of hydrothermal mineralization are recognized at haughton a vugs and veins within the impact melt breccias with an increase in intensity of alteration towards the base b cementation of brecciated lithologies in the interior of the central uplift c intense veining around the heavily faulted and fractured outer margin of the central uplift and d hydrothermal pipe structures or gossans and mineralization along fault surfaces around the faulted crater rim each setting is associated with a different suite of hydrothermal minerals that were deposited at different stages in the development of the hydrothermal system minor early quartz precipitation in the impact melt breccias was followed by the deposition of calcite and marcasite within cavities and fractures plus minor celestite barite and fluorite this occurred at temperatures of at least 200 c and down to 100 120 c hydrothermal circulation through the faulted crater rim with the deposition of calcite quartz marcasite and pyrite occurred at similar temperatures quartz mineralization within breccias of the interior of the central uplift occurred in two distinct episodes 250 down to 90 c and 80 c appreciation of the structural features of impact craters has proven to be key to understanding the distribution of hydrothermal deposits at haughton

mid sized complex crater formation in mixed crystalline sedimentary targets insight from modeling and observation large impact crater formation is an important geologic process that is not fully understood the current paradigm for impact crater formation is based on models and observations of impacts in homogeneous targets real targets are rarely uniform for example the majority of earth s surface is covered by sedimentary rocks and or a water layer the ubiquity of layering across solar system bodies makes it important to understand the effect target properties have on the cratering process to advance understanding of the mechanics of crater collapse and the effect of variations in target properties on crater formation the first bridging the gap workshop recommended that geological observation and numerical modeling focussed on mid sized 15 30 km diameter craters on earth these are large enough to be complex small enough to be mapped surveyed and modelled at high resolution and numerous enough for the effects of target properties to be potentially disentangled from the effects of other variables in this paper we compare observations and numerical models of three 18 26 km diameter craters formed in different target lithology ries germany haughton canada and el gygytgyn russia based on the first order assumption that the impact energy was the same in all three impacts we performed numerical simulations of each crater to construct a simple quantitative model for mid sized complex crater formation in a subaerial mixed crystalline sedimentary target we compared our results with interpreted geological profiles of ries and haughton based on detailed new and published geological mapping and published geophysical surveys our combined observational and numerical modeling work suggests that the major structural differences between each crater can be explained by the difference in thickness of the pre impact sedimentary cover in each case we conclude that the presence of an inner ring at ries and not at haughton is because basement rocks that are stronger than the overlying sediments are sufficiently close to the surface that they are uplifted and overturned during excavation and remain as an uplifted ring after modification and post impact erosion for constant impact energy transient and final crater diameters increase with increasing sediment thickness

chicxulub crater a possible cretaceous tertiary boundary impact crater on the yucatan peninsula mexico we suggest that a buried 180 km diameter circular structure on the yucatan peninsula mexico is an impact crater its size and shape are revealed by magnetic and gravity field anomalies as well as by oil wells drilled inside and near the structure the stratigraphy of the crater includes a sequence of andesitic igneous rocks and glass interbedded with and overlain by breccias that contain evidence of shock metamorphism the andesitic rocks have chemical and isotopic compositions similar to those of tektites found in cretaceous tertiary k t ejecta a 90 m thick k t boundary breccia also containing evidence of shock metamorphism is present 50 km outside the crater s edge this breccia probably represents the crater s ejecta blanket the age of the crater is not precisely known but a k t boundary age is indicated because the crater is in a thick carbonate sequence shock produced co2 from the impact may have caused a severe greenhouse warming

a reassessment of spectral te estimation in continental interiors the case of north america 1 conventional spectral te studies use the real part of the admittance between gravity anomalies and topography or alternatively the square of the magnitude of the coherency i e coherence here we show the utility of treating both the admittance and coherency as complex quantities inverting the real parts to estimate te we use the imaginary parts to tell if the inversion is biased by noise one method inverts the square of the real coherency with the internal to total load ratio f derived as a function of wave number directly from the gravity and topography the other method inverts the real part of the admittance assuming that f is wave number independent we test the methods using synthetic elastic plate models loaded at the surface and moho in such a way that the final relief is the actual north american topography in some of the models we add gravity noise generated by a model having both surface and internal loads such that the final topography is zero and find that both methods are susceptible to noise application of the two methods to north america gives te maps showing substantial agreement except in regions affected by noise but these are not a dominant part of the total area given the suggested mechanisms by which noise might arise it is not surprising that it is not a more widespread feature of the north american craton importantly both methods show that large parts of the canadian shield are characterized by te 100 km

mapping the mechanical anisotropy of the lithosphere using a 2d wavelet coherence and its application to australia abstract we develop a new method for imaging the spatial variations of the anisotropy of the flexural response of the lithosphere and apply it to recent topographic and gravity data sets over australia the method uses two dimensional morlet wavelet transforms superposed in a strictly controlled geometry to estimate the auto and cross spectra of the two data sets in a number of different directions the resulting wavelet coherence is a function of scale or wavelength as well as orientation and is inverted at each spatial location for the three parameters of an anisotropic thin elastic plate model i e maximum and minimum flexural rigidities and the orientation of the maximum extensive tests of the method on synthetic anisotropic but uniform data sets show that it retrieves the amplitude and orientation of the anisotropy with useful accuracy the results for australia west of 143 e show a strong correlation with the shallower layers 75 175 km of a recent model of seismic sv wave azimuthal anisotropy the weak axes i e of minimum flexural rigidity in most cases are approximately at right angles to the fast axes of the seismic anisotropy implying that for precambrian australia they arise from the same source this is most likely deformation resulting from the most recent episode of orogeny

impact metamorphism of caco3 bearing sandstones at the haughton structure canada impact metamorphosed caco3 bearing sandstones at the haughton structure have been divided into 6 classes based to a large extent on a previous classification developed for sandstones at meteor crater class 1a sandstones 10 30 gpa consist almost entirely of lechatelierite a new class of shocked sandstones class 6 consists of sio2 rich melt that recrystallized to microcrystalline quartz calcite within class 4 to 6 sandstones also underwent melting and is preserved as globules and euhedral crystals within sio2 phases demonstrating the importance of impact melting and not decomposition in these caco3 bearing sandstones

impactites of the haughton impact structure devon island canadian high arctic contrary to the previous interpretation of a single allochthonous impactite lithology combined field optical and analytical scanning electron microscopy sem studies have revealed the presence of a series of impactites at the haughton impact structure in the crater interior there is a consistent upward sequence from parautochthonous target rocks overlain by parautochthonous lithic monomict breccias through allochthonous lithic polymict breccia into pale grey allochthonous impact melt breccias the groundmass of the pale grey impact melt breccias consists of microcrystalline calcite silicate impact melt glass and anhydrite analytical data and microtextures indicate that these phases represent a series of impact generated melts that were molten at the time of and following deposition impact melt glass clasts are present in approximately half of the samples studied consideration of the groundmass phases and impact glass clasts reveal that impactites of the crater interior contain shock melted sedimentary material from depths of 920 to 200 to 760 m and are interpreted as remnants of the continuous ejecta blanket the pale grey impact melt breccias although similar to the impact melt breccias of the crater interior are more carbonate rich and do not appear to have incorporated clasts from the crystalline basement thus the spatial distribution of the crater fill impactites at haughton the stratigraphic succession from target rocks to allochthonous impactites the recognition of large volumes of impact melt breccias and their probable original volume are all analogous to characteristics of coherent impact melt layers in comparatively sized structures formed in crystalline targets

evidence for active hotspots on venus from analysis of magellan gravity data abstract the 500 myr average crater retention age for venus has raised questions about the present day level of tectonic activity in this study we examine the relationship between the gravity and topography of four large volcanic swells beta atla bell and western eistla regiones for clues about their stage of evolution the magellan line of sight gravity data are inverted using a point mass model of the anomalous mass to solve for the local vertical gravity field spectral admittance calculated from both the local gravity inversions and a spherical harmonic model is compared to three models of compensation local compensation a flexural model with local and regional compensation of surface and subsurface loads and a hotspot model of compensation that includes top loading by volcanoes and subsurface loading due to a deep low density mass anomaly the coherence is also calculated in each region but yields an elastic thickness estimate only at bell regio in all models the long wavelengths are compensated locally the long wavelength estimates of the deep compensation depth for bell atla western eistla and beta regiones are 125 175 200 and 225 km respectively with an error of approximately 35 km the flexural model is rejected primarily because it gives values of effective elastic thickness of approximately 100 km which is at least a factor of two larger than estimates of effective elastic thickness obtained from the coherence spectra at bell regio and from other studies that model the topographic expression of flexure at bell regio and in other tectonic settings globally assuming a crustal thickness of 30 km the fit to the hotspot model at atla regio gives an effective elastic thickness of 30 5 km atla regio is interpreted as an active hotspot because of the deep compensation depth and a strong subsurface loading signature at bell regio effective elastic thickness is 30 5 km at short wavelengths and 50 5 km at long wavelengths using a crustal thickness of 10 km the 30 km value is interpreted as the effective elastic thickness at the time when the volcanoes are emplaced the 50 km value is believed to reflect the present day effective elastic thickness bell regio has a relatively shallow compensation depth 125 km and a weak bottom to top loading ratio determined from the coherence spectra these results may indicate a relatively old possibly inactive plume the data at beta and western eistla regiones are of relatively poor quality and do not permit detailed interpretations these areas are interpreted as active hotspots because of their large compensation depths greater than typical of the rest of the planet

geochemical constraints on source region of cretaceous tertiary impact glasses the 50 cm thick cretaceous tertiary boundary layer at beloc in haiti contains spherules of silicic black glass 1 8 mm in diameter which have been attributed to impact fusion of continental crust1 calcic yellow glass up to 30 wt cao forms a coating on and streaks within some black glass spherules and has been attributed to melting of carbonate rich sediment1 or organic rich or pyritic limestone2 here we present new trace element and stable and radiogenic isotope data which show that the silicic glass is derived from continental crust of andesitic composition whereas the high ca glass formed by melting of evaporite rich sediment this is confirmed by melting experiments with evaporite and andesite at 1 200 1 400 c which approximately reproduce the high ca glass the temperature dependent variation of sulphur content in synthetic high ca glasses indicates a formation temperature of 1 300 c for the haiti glasses the geology of the impact site inferred from the geochemistry of the haiti glasses matches the lithologies found in the 180 km chicxulub structure which occurs in cretaceous evaporite deposits in mexico the high sulphur content of the calcic glasses suggests that the impact may have generated significant emissions of sulphur dioxide to the atmosphere causing short term global cooling

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

evidence for the shock melting of sulfates from the haughton impact structure arctic canada abstract sulfate minerals can form an important component of the matrix to carbonate rich impact melt breccias at the 24 km diameter 23 ma haughton impact structure arctic canada the textural and chemical features of the matrix forming sulfates indicate that these phases in addition to co existing carbonates and silicates crystallized directly from an impact generated melt evidence for this includes 1 the matrix supported nature of the crater fill lithologies 2 sulfate carbonate silicate liquid immiscible textures 3 possible quench textures in anhydrite and 4 flow textures developed between anhydrite and silicate rich glasses further supporting evidence includes the presence of si al and mg in the anhydrite structure which were probably trapped by quenching from a melt irregular blebs and globules of shock melted carbonates within anhydrite also suggest a common origin for the two phases field studies reveal that clasts of anhydrite bearing target material are also present in the crater fill deposits several clasts of anhydrite quartz lithologies exhibit evidence for incipient shock melting in both phases previous assumptions about the response of sulfates and carbonates to hypervelocity impact i e lack of melting may therefore be incorrect

compositional heterogeneity of impact melt rocks at the haughton impact structure canada implications for planetary processes and remote sensing connecting the surface expression of impact crater related lithologies to planetary or regional subsurface compositions requires an understanding of material transport during crater formation here we use imaging spectroscopy of 6 clast rich impact melt rock outcrops within the well preserved 23 5 ma 23 km diameter haughton impact structure canada to determine melt rock composition and spatial heterogeneity we compare results from outcrop to outcrop using clasts groundmass and integrated clast groundmass compositions as tracers of transport during crater fill melt rock formation and cooling supporting laboratory imaging spectroscopy analyses of 91 melt bearing breccia and clast samples and microscopic x ray fluorescence elemental mapping of cut samples paired with spectroscopy of identical surfaces validate outcrop scale lithological determinations results show different clast rich impact melt rock compositions at three sites kilometers apart and an inverse correlation between silica rich sandstone gneiss and phyllosilicate rich shales and gypsum rich rocks that suggests differences in source depth with location in the target stratigraphy gypsum is primarily sourced from 1 km depth while gneiss is from 1 8 km depth sandstone from 1 3 km and shales from 1 6 1 7 km observed heterogeneities likely result from different excavation depths coupled with rapid quenching of the melt due to high content of cool clasts results provide quantitative constraints for numerical models of impact structure formation and give new details on melt rock heterogeneity important in interpreting mission data and planning sample return of impactites particularly for bodies with impacts into sedimentary and volatile bearing targets e g mars and ceres

hyperspectral mapping of alteration assemblages within a hydrothermal vug at the haughton impact structure canada meteorite impacts on earth and mars can generate hydrothermal systems that alter the primary mineralogies of rocks and provide suitable environments for microbial colonization we investigate a calcite marcasite bearing vug at the 23 km diameter haughton impact structure devon island nunavut canada using imaging spectroscopy of the outcrop in the field 0 65 1 1 m and samples in the laboratory 0 4 2 5 m point spectroscopy 0 35 2 5 m major element chemistry and x ray diffraction analyses the mineral assemblages mapped at the outcrop include marcasite marcasite with minor gypsum and jarosite fibroferrite and copiapite with minor gypsum and melanterite gypsum fe3 oxides and jarosite and calcite gypsum clay microcline and quartz hyperspectral mapping of alteration phases shows spatial patterns that illuminate changes in alteration conditions and formation of specific mineral phases marcasite formed from the postimpact hydrothermal system under reducing conditions while subsequent weathering oxidized the marcasite at low temperatures and water rock ratios the acidic fluids resulting from the oxidation collected on flat lying portions of the outcrop precipitating fibroferrite copiapite that assemblage then likely dissolved and the changing chemistry and ph resulting from interaction with the calcite rich host rock formed gypsum bearing red coatings these results have implications for understanding water rock interactions and habitabilities at this site and on mars

impact generated carbonate melts evidence from the haughton structure canada evidence is presented for the melting of dolomite rich target rocks during formation of the 24 km diameter 23 ma haughton impact structure on devon island in the canadian high arctic field studies and analytical scanning electron microscopy reveal that the s 200 m thick crater fill deposit which currently covers an v60 km 2 area in the center of the structure comprises fragmented target rocks set within a carbonate silicate matrix the silicate component of the matrix consists of si al mg rich glass the carbonate component is microcrystalline calcite containing up to a few wt si and al the calcite also forms spherules and globules within the silicate glass with which it develops microtextures indicative of liquid immiscibility dolomite clasts exhibit evidence of assimilation and may show calcite and rare dolomite overgrowths some clasts are penetrated by calcite and silicate injections along with the carbonate silicate glass textures the presence of pigeonite and spinifex textured diopside suggests that the matrix to the crater fill deposit was originally molten and was rapidly cooled this indicates that the impact event that generated haughton caused fusion of the predominantly dolomitic target rocks it appears that the ca mg component of the dolomite may have dissociated with mg entering the silicate melt phase while the ca component formed a caco3 dominant melt the silicates were derived by the fusion of lower paleozoic sandstones siltstones shales and impure dolomites evidence for melting is corroborated by a review of theoretical and experimental work which shows that caco3 melts at s 10 gpa and s 2000 k instead of dissociating to release co2 this work indicates that carbonate rich sedimentary targets may also undergo impact melting and that the volume of co2 released into the atmosphere during such events may be considerably less than previously estimated fl 2001 elsevier science b v all rights reserved

shock induced vaporization of anhydrite and global cooling from the k t impact discovery of abundant anhydrite caso 4 and gypsum caso 4 2h 2o in the otherwise carbonate sediment comprising the upper 3 km thick layer of the target rock at the k t impact site has prompted research on these minerals evaluation of the severity of the proposed extinction mechanism involving injection of impact generated so 2 and so 3 into the stratosphere entails determination of criteria for shock induced vaporization of these minerals in the present work we present new data on the vaporization criteria of anhydrite these are based on the reanalysis of the shock wave experiments of yang and ahrens earth planet sci lett 156 1998 125 140 conducted on material with 30 porosity in which the shock fully or partially vaporized sample interacts with an overlying lif window the velocity histories monitored using a velocity interferometer are compared with numerical simulations employing an improved equation of state for porous anhydrite and its vaporization products we also employ the entropy criterion for vaporization of material under shock compression the values of the entropies of incipient and complete vaporization for anhydrite are determined to be 1 65 0 12 and 3 17 0 12 kj kg k 1 respectively and the corresponding pressures for incipient and the complete vaporization along the hugoniot for the solid material are 32 5 2 5 and 122 13 gpa respectively as compared with 81 7 and 155 13 gpa previously reported by yang and ahrens along with these criteria the use of the recent estimate of diameter 100 km for the chicxulub transient crater o keefe and ahrens j geophys res 104 e11 1999 27091 27104 morgan et al nature 390 1997 472 476 that is smaller than previously assumed along with ivanov et al s geol soc am spec pap 307 1996 125 142 2 d hydrodynamic simulation to determine the shock attenuation and pope et al s j geophys res 102 e9 1997 21645 21664 radiative transfer model yields the maximum decrease in the average global surface temperature of 12 19 k for 9 0 9 5 years at the k t boundary thus the global cooling is inferred to have been less severe than that indicated by the upper limit of the range of 5 31 k decrease lasting for 12 years calculated by pope et al because ambient global surface temperatures at k t time were 18 20 c warmer than present values this cooling event produced cold but not freezing conditions at the earth s surface

source analysis of acoustic emissions in aue granite cores under symmetric and asymmetric compressive loads the effect of stress anisotropy on the brittle failure of granite is investigated under uniaxial compression non standard asymmetric compression tests are performed on cores of aue granite diameter 52 mm length 100 mm in which 20 per cent of the core top surface remains unloaded the edge of the asymmetric steel loading plate acts as a stress concentrator from where a shear rupture is initiated the propagation of the fracture related process zone from top to bottom of the core is mapped by microcrack induced acoustic emissions compared to standard uniaxial tests with symmetric loading in the asymmetric tests both a greater quantity and more localized distributions of emission event hypocentres are observed the maximum event density doubles for asymmetric 20 events per 10 6 m3 compared to symmetric tests the cluster correlation coefficient a measure of strain localization in the faulting process reaches 0 15 for symmetric and 0 30 for asymmetric tests the clustering of events however is found post failure only three different amplitudes are used to determine b values discussed as a possible failure precursor focal amplitudes determined at a 10 mm source distance and maximum amplitudes measured at eight piezoceramic sensors lead to b values that drop before rock failure first pulse amplitudes automatically picked from emission wavelets show no anomaly first motion polarity statistics of amplitudes indicate that a shear crack type radiation pattern is responsible for 70 per cent of the failure of granite irrespective of stress boundary conditions for type s events with an equal percentage of dilatational and compressional first motions focal mechanisms are determined by fitting measured first pulse amplitudes to an assumed double couple radiation pattern while hypocentres of large type s events align parallel to the later fracture plane their fault plane solutions show no coherent pattern spatial views of fracture planes reconstructed from x ray computed tomograms reveal local small scale changes in fracture plane orientation nodal planes from average fault plane solutions of the microscopic acoustic emission events coincide with the overall orientation of the macroscopic fracture plane azimuth strike angle determined from thin sections and tomograms

potential anomalies on a sphere applications to the thickness of the lunar crust a new technique for calculating potential anomalies on a sphere due to finite amplitude relief has been developed we show that by raising the topography to the nth power and expanding this field into spherical harmonics potential anomalies due to topography on spherical density interfaces can be computed to arbitrary precision using a filter for downward continuing the bouguer anomaly we have computed a variety of crustal thickness maps for the moon assuming both a homogeneous as well as a dual layered crust the crustal thickness maps for the homogeneous model give plausible results but this model is not consistent with the seismic data petrologic evidence and geoid to topography ratios all of which suggest some form of crustal stratification several dual layered models were investigated and it was found that only models with both upper and lower crustal thickness variations could satisfy the gravity and topography data these models predict that the entire upper crust has been excavated beneath the major nearside multiring basins additionally significant amounts of lower crustal material was excavated from these basins especially beneath crisium this model also predicts that mantle material should not have been excavated during the south pole aitken basin forming event and that lower crustal material should be exposed at the surface in this basin

organic geochemistry of impactites from the haughton impact structure devon island nunavut canada organic matter in impactites from the 24 km wide and 39 ma old haughton impact structure canadian high arctic is a mixture of fossil and modern biological components the fossil component represents a conventional oil that was generated from lower palaeozoic marine source material before impact and permeates bedrock dolomites biomarker maturity parameters record the thermal effect of the mid tertiary impact maturity influenced sterane rearranged hopanoid and triaromatic steroid ratios all increase towards the centre of the impact structure where thermal alteration was greatest the heating was probably dominated by an impact related hydrothermal system as such systems last long enough for kinetically based thermal alteration to occur kinetically related biomarker data suggest that the hydrothermal heating lasted for c 5000 years biomarkers are also preserved in dolomite clasts within impact melt breccia and indicate strong thermal alteration modern biological contamination of the rocks is responsible for the superposition of two geochemical signatures which could be cyanobacteria non marine algae or higher plant matter onto the fossil component but they can be recognized and distinguished the data show that the impact structure system holds a record of both the pre impact organic signature and the thermal signature of the impact and thereby indicates that organic geochemistry is a valuable tool in documenting the response of rocks to impacts

lithospheric structure of venus from gravity and topography abstract there are many fundamental and unanswered questions on the structure and evolution of the venusian lithosphere which are key issues for understanding venus in the context of the origin and evolution of the terrestrial planets here we investigate the lithospheric structure of venus by calculating its crustal and effective elastic thicknesses tc and te respectively from an analysis of gravity and topography in order to improve our knowledge of the large scale and long term mechanical behaviour of its lithosphere we find that the venusian crust is usually 20 25 km thick with thicker crust under the highlands our effective elastic thickness values range between 14 km corresponding to the minimum resolvable te value and 94 km but are dominated by low to moderate values te variations deduced from our model could represent regional variations in the cooling history of the lithosphere and or mantle processes with limited surface manifestation the crustal plateaus are near isostatically compensated consistent with a thin elastic lithosphere showing a thickened crust beneath them whereas the lowlands exhibit higher te values maybe indicating a cooler lithosphere than that when the venusian highlands were emplaced the large volcanic rises show a complex signature with a broad range of te and internal load fraction f values finally our results also reveal a significant contribution of the upper mantle to the strength of the lithosphere in many regions

recent hotspot volcanism on venus from virtis emissivity data the questions of whether venus is geologically active and how the planet has resurfaced over the past billion years have major implications for interior dynamics and climate change nine hotspots areas analogous to hawaii with volcanism broad topographic rises and large positive gravity anomalies suggesting mantle plumes at depth have been identified as possibly active this study used variations in the thermal emissivity of the surface observed by the visible and infrared thermal imaging spectrometer on the european space agency s venus express spacecraft to identify compositional differences in lava flows at three hotspots the anomalies are interpreted as a lack of surface weathering we estimate the flows to be younger than 2 5 million years and probably much younger about 250 000 years or less indicating that venus is actively resurfacing

isostatic compensation of equatorial highlands on venus abstract spherical harmonic models for venus global topography and gravity incorporating magellan data are used to test isostatic compensation models in five 30 30 regions representative of the main classes of equatorial highlands the power spectral density for the harmonic models obeys a power law scaling with spectral slope 2 brown noise for the topography and 3 kaula s law for the geoid similar to what is observed for earth the venus topography spectrum has lower amplitudes than earth s which reflects the dominant lowland topography on venus observed degree geoid to topography ratios gtrs on venus are significantly smaller than degree gtrs for uncompensated topography indicative of substantial compensation assuming a global airy compensation most of the topography is compensated at depths greater than 100 km suggesting a thick lithosphere on venus for each region considered we obtain a regional degree of compensation c from a linear regression of bouguer anomaly versus bouguer gravity data geoid anomaly n versus topography variation h data for each sample were compared in the least squares sense to theoretical correlations for pratt airy and thermal thinning isostasy models yielding regional gtr zero elevation crustal thickness h and zero elevation thermal lithosphere thickness y l 0 respectively we find the regional compensation to be substantial c 52 90 and the h n data correlations in the chosen areas can be explained by isostasy models applicable on the earth and involving variations in crustal thickness airy and or lithospheric thermal thinning thickness however a thick crust and lithosphere y l 0 300 km must be assumed for venus compensation mechanisms are distributed as follows for the high plateaus of ovda gtr 9 m km and thetis gtr 11 m km mainly airy isostasy with h 50 60 km for the chasmata area sample gtr 15 m km airy with perhaps a thermal component finally a very strong thermal component for the broad swells of beta gtr 32 m km and atla gtr 25 m km with the lithosphere thinned to 100 km aphrodite terra which shows a marked eastward increase in pratt and airy model parameter values could be a candidate for thermal isostasy decay from east to west

relation of fracture resistance to fabric for granitic rocks double torsion specimens of two granitic rocks were prepared in several directions with reference to microcracks fabric even for the same rock and at the same stress levels the observed crack velocities in two granitic rocks were dependent on both the propagation direction and the opening direction the maximum difference by several orders of magnitude was found for both rocks the highest crack velocity was observed when the subcritical crack was parallel to most of the preexisting cracks the maximum critical stress intensity factor was about twice as high as the minimum one in different directions an analysis for a thin plate having anisotropic elasticity under torsional load showed that the observed difference in the crack velocity and the critical stress intensity factor was not an error due to conventional equations derived on the assumption of isotropic elasticity but the true material s property as the preferred orientation of microcracks has been pointed out for many granitic rocks we can conclude that the anisotropic nature of the fracture resistance of the two granitic rocks used in this study was not exceptional a region of a transport limited velocity was not found for rocks even at the velocity of 10 2 m s that is almost equal to the theoretical limit of the stress corrosion cracking

the cretaceous tertiary extinction a lethal mechanism involving anhydrite target rocks the chicxulub crater yucatan mexico is a leading contender as the site for the impact event that caused the cretaceous tertiary k t extinctions a considerable thickness of anhydrite caso4 forms part of the target rock high temperatures resulting from impact would drive so2 off from the anhydrite hundreds of billions of tonnes of sulfuric acid aerosol would thus enter the stratosphere and cause considerable cooling of the earth s surface decrease photosynthesis by orders of magnitude deplete the ozone layer and permit increased uv radiation to reach the earth s surface finally the aerosol would fall back to earth as acid rain and devastate land and some lacustrine biota and near surface marine creatures the presence of anhydrite in the chicxulub target rock may thus help explain the many extinctions observed at the k t boundary

major and trace element characteristics of impactites from the yaxcopoil 1 borehole chicxulub structure mexico approximately 100 m of impactites were retrieved from the icdp borehole yaxcopoil 1 yax 1 located 60 km south southwest from the center of the chicxulub impact crater on the yucatan peninsula of mexico here we characterize and discuss this impact breccia interval according to its geochemical characteristics chemical analysis of samples from all five recognized breccia units reveals that the impactites are of heterogeneous composition with regard to both major and trace elements at the single sample 8 16 cm 3 scale this is primarily due to a strong mixing relationship between carbonate and silicate fractions however averaged compositions for suevitic units 1 to 3 are similar and the silicate fraction after removal of the carbonate component indicates thorough mixing and homogenization analysis of the green melt breccia horizon unit 4 indicates that it contains a distinct mafic component large brown melt particles in units 2 3 and 4 represent a mixture of feldspathic and mafic components with high cao abundances unit 5 shows the greatest compositional diversity with highly variable abundances of sio2 cao and mgo inter sample heterogeneity is the result of small sample size combined with inherent heterogeneous lithological compositions highly variable particle size of melt and lithic components and post depositional alteration in contrast to samples from the y6 borehole from closer to the center of the structure yax 1 impactites have a strong carbonate component elevated loss on ignition rb and cs contents in the upper two impactite units indicate strong interaction with seawater the contents of the siderophile elements including ni co ir and cr do not indicate the presence of a significant extraterrestrial component in the yax 1 impactites

intra crater sedimentary deposits at the haughton impact structure devon island canadian high arctic detailed field mapping has revealed the presence of a series of intra crater sedimentary deposits within the interior of the haughton impact structure devon island canadian high arctic coarse grained well sorted pale gray lithic sandstones reworked impact melt breccias unconformably overlie pristine impact melt breccias and attest to an episode of erosion during which time significant quantities of impact melt breccias were removed the reworked impact melt breccias are in turn unconformably overlain by paleolacustrine sediments of the miocene haughton formation sediments of the haughton formation were clearly derived from pre impact lower paleozoic target rocks of the allen bay formation which form the crater rim in the northern western and southern regions of the haughton structure collectively these field relationships indicate that the haughton formation was deposited up to several million years after the formation of the haughton crater and that they do not therefore represent an immediate post impact crater lake deposit this is consistent with new isotopic dating of impactites from haughton that indicate an eocene age for the impact event sherlock et al 2005 in addition isolated deposits of post miocene intra crater glacigenic and fluvioglacial sediments were found lying unconformably over remnants of the haughton formation impact melt breccias and other pre impact target rock formations these deposits provide clear evidence for glaciation at the haughton crater the wealth and complexity of geological and climatological information preserved as intra crater deposits at haughton suggests that craters on mars with intra crater sedimentary records might present us with similar opportunities but also possibly significant challenges

shock induced devolatilization of calcium sulfate and implications for k t extinctions the devolatilization of calcium sulfate which is present in the target rock of the chicxulub mexico impact structure and dispersal in the stratosphere of the resultant sulfuric acid aerosol have been suggested as a possible mechanism for the cretaceous tertiary extinctions we measured the amount of so 2 produced from two shock induced devolatilization reactions of calcium sulfate up to 42 gpa in the laboratory caso 4 sio 2 casio 3 so 3 caso 4 cao so 2 1 2o 2 we found both to proceed to a much lower extent than calculated by equilibrium thermodynamic calculations reaction products are found to be 10 2 times those calculated for equilibrium upon modeling the quantity of sulfur oxides degassed into the atmosphere from shock devolatilization of caso 4 in the chicxulub lithographic section the resulting 9 10 16 to 6 10 17 g in sulfur mass is lower by a factor of 10 100 than previous upper limit estimates the related environmental stress arising from the resultant global cooling and fallout of acid rain is insufficient to explain the widespread k t extinctions

a comparative study of the anisotropic dynamic and static elastic moduli of unconventional reservoir shales implication for geomechanical investigations abstractwe obtained the complete set of dynamic elastic stiffnesses for a suite of shales representative of unconventional reservoirs from simultaneously measured p and s wave speeds on single prisms specially machined from cores static linear compressibilities were concurrently obtained using strain gauges attached to the prism regardless of being from static or dynamic measurements the pressure sensitivity varies strongly with the direction of measurement furthermore the static and dynamic linear compressibilities measured parallel to the bedding are nearly the same whereas those perpendicular to the bedding can differ by as much as 100 compliant cracklike porosity seen in scanning electron microscope images controls the elastic properties measured perpendicular to the rock s bedding plane and results in highly nonlinear pressure sensitivity in contrast those properties measured parallel to the bedding are nearly insensitive to stress this anisotropy to the pressure dependency of the stra

carbonate silicate liquid immiscibility upon impact melting ries crater germany the 24 km diameter ries impact crater in southern germany is one of the most studied impact structures on earth the ries impactor struck a triassic to upper jurassic sedimentary sequence overlying hercynian crystalline basement at the time of impact 14 87 0 36 ma storzer et al 1995 the 350 m thick malm limestone was present only to the south and east of the impact site to the north and west the malm had been eroded away exposing the underlying dogger and lias the largest proportion of shocked target material is in the impact melt bearing breccia suevite the suevite had been believed to be derived entirely from the crystalline basement calcite in the suevite has been interpreted as a postimpact hydrothermal deposit from optical inspection of 540 thin sections of suevite from 32 sites i find that calcite in the suevite shows textural evidence of liquid immiscibility with the silicate impact melt textural evidence of liquid immiscibility between silicate and carbonate melt in the ries suevite includes carbonate globules within silicate glass silicate globules embedded in carbonate deformable and coalescing carbonate spheres within silicate glass sharp menisci or cusps and budding between silicate and carbonate melt fluidal textures and gas vesicles in carbonate schlieren a quench crystallization sequence of the carbonate spinifex textured quenched carbonate separate carbonate spherules in the suevite mineral fragment matrix and inclusions of mineral fragments suspended in carbonate blebs given this evidence of liquid immiscibility the carbonate in the suevite therefore has like the silicate melt a primary origin by impact shock melting evidence of carbonate silicate liquid immiscibility is abundant in the suevites from the southwest to east of the ries crater the rarer suevites to the west to northeast of the crater are nearly devoid of carbonate melts this correspondence between the occurrence of outcropping limestones at the target surface and the formation of carbonate melt indicates that the malm limestones are the source rocks of the carbonate impact melt this correspondence shows that the suevites preserve a compositional memory of their source rocks from the regional distribution of suevites with or without immiscible carbonate melts it is inferred that the ries impactor hit the steep albtrauf escarpment at its toe in an oblique impact from the north

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

fracture toughness measurements and acoustic emission activity in brittle rocks fracture toughness measurements under static loading conditions have been carried out in barre and lac du bonnet granites an advanced ae technique has been adopted to monitor real time crack initiation and propagation around the principal crack in these tests to understand the processes of brittle failure under tension and related characteristics of the resulting fracture process zone the anisotropy of mode i fracture toughness has been investigated along specific directions microcrack density and orientation analysis from thin section studies have shown these characteristics to be the primary cause of the observed variation in fracture toughness which is seen to vary between 1 14 mpa m 1 2 and 1 89 mpa m 1 2 in barre granite the latter value represents the case in which the crack is propagated at right angles to the main set of microcracks the creation of a significant fracture process zone surrounding the propagating main crack has been confirmed real time imaging of the fracture process and formation of fracture process zone by ae techniques yielded results in very good agreement with those obtained by direct optical analysis

effect of volatiles and target lithology on the generation and emplacement of impact crater fill and ejecta deposits on mars impact cratering is an important geological process on mars and the nature of martian impact craters may provide important information as to the volatile content of the martian crust terrestrial impact structures currently provide the only ground truth data as to the role of volatiles and an atmosphere on the impact cratering process recent advancements based on studies of several well preserved terrestrial craters have been made regarding the role and effect of volatiles on the impact cratering process combined field and laboratory studies reveal that impact melting is much more common in volatile rich targets than previously thought so impact melt rocks melt bearing breccias and glasses should be common on mars consideration of the terrestrial impact cratering record suggests that it is the presence or absence of subsurface volatiles and not the presence of an atmosphere that largely controls ejecta emplacement on mars furthermore recent studies at the haughton and ries impact structures reveal that there are two discrete episodes of ejecta deposition during the formation of complex impact craters that provide a mechanism for generating multiple layers of ejecta it is apparent that the relative abundance of volatiles in the near surface region outside a transient cavity and in the target rocks within the transient cavity play a key role in controlling the amount of fluidization of martian ejecta deposits this study shows the value of using terrestrial analogues in addition to observational data from robotic orbiters and landers laboratory experiments and numerical modeling to explore the martian impact cratering record