Constraints on the size of the Vredefort impact crater from numerical modeling

Abstract— The Vredefort structure in South Africa was created by a meteorite impact about two billion years ago. Since that time, the crater has been deeply eroded; so to estimate its original size, researchers have had to rely heavily upon comparison to other terrestrial impact structures. Recent estimates of the original crater diameter range from 160 km to as much as 400 km. In this study, we combined the capabilities of both hydrocode and finite-element modeling, using the former to predict where the pressure of an impact-generated shock wave would have been high enough to form planar deformation features (PDFs) and shatter cones and the latter to follow the subsequent displacement of these shock isobars during the collapse of the crater. We established constraints on the sizes of the projectile and the transient crater (and, thus, on the size of the final crater) by comparing the observed locations of PDFs around Vredefort to the results of our simulations of impacts by projectiles of various sizes. These simulations indicate that a rocky projectile with a diameter of ～10 km, impacting vertically at a velocity of 20 km/s generates shock pressures that are consistent with the distribution of PDFs around Vredefort. These projectile parameters correspond to a transient crater ～80 km in diameter or a final crater ～120–160 km in diameter. Allowing for uncertainties in our modeling procedures, we consider final craters 120 to 200 km in diameter to be consistent with the observed locations of PDFs at Vredefort. The shock pressure contour corresponding to the formation of shatter cones is almost horizontal near the surface, making the locations of these features less useful constraints on the crater size. However, they may provide a constraint on the amount of erosion that has occurred since the impact.     

Complex crater formation: Insights from combining observations of shock pressure distribution with numerical models at the West Clearwater Lake impact structure

Large impact structures have complex morphologies, with zones of structural uplift that can be expressed topographically as central peaks and/or peak rings internal to the crater rim. The formation of these structures requires transient strength reduction in the target material and one of the proposed mechanisms to explain this behavior is acoustic fluidization. Here, samples of shock‐metamorphosed quartz‐bearing lithologies at the West Clearwater Lake impact structure, Canada, are used to estimate the maximum recorded shock pressures in three dimensions across the crater. These measurements demonstrate that the currently observed distribution of shock metamorphism is strongly controlled by the formation of the structural uplift. The distribution of peak shock pressures, together with apparent crater morphology and geological observations, is compared with numerical impact simulations to constrain parameters used in the block‐model implementation of acoustic fluidization. The numerical simulations produce craters that are consistent with morphological and geological observations. The results show that the regeneration of acoustic energy must be an important feature of acoustic fluidization in crater collapse, and should be included in future implementations. Based on the comparison between observational data and impact simulations, we conclude that the West Clearwater Lake structure had an original rim (final crater) diameter of 35–40 km and has since experienced up to ~2 km of differential erosion.     

Constraints on the role of impact heating and melting in asteroids

Abstract— Imaging of asteroids Gaspra and Ida and laboratory studies of asteroidal meteorites show that impacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks. However, in recent years, impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids, such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events. The latter were proposed to explain the origin of ureilites, aubrites, mesosiderites, the Eagle Station pallasites, acapulcoites, lodranites, and the IAB, IIICD, and HE irons. We considered fundamental observations from terrestrial impact craters, combined with results from laboratory shock experiments and theoretical considerations, to evaluate the efficiency of impact heating and melting of asteroids. Studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of asteroids will produce two types of impact melts: (1) large-scale whole rock melts (total melts, not partial melts) at high shock pressure and (2) localized melts formed at the scale of the mineral constituents (mineral specific or grain boundary melting) at intermediate shock pressures. The localized melts form minuscule amounts of melt that quench and solidify in situ, thus preventing them from pooling into larger melt bodies. Partial melting as defined in petrology has not been observed in natural and experimental shock metamorphism and is thermodynamically impossible in a shock wave-induced transient compression of rocks. The total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater. Internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids, and impact melt rocks are usually clast-laden. Thermal metamorphism of country rocks by impact is extremely minor. Experimental and theoretical considerations suggest that (1) single disruptive impacts cannot raise the average global temperature of strength- or gravity-dominated asteroids by more than a few degrees; (2) cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter; (3) small crater size, low gravity, and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small (0.01–0.1%) fraction of the total displaced crater volume; (4) total impact melt volume formed during the typical lifetime of an asteroid is a small fraction (<0.001) of the volume of impact-generated debris; and (5) much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and, thus, not retained on the asteroid. The inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above, and we must turn to processes other than impact, such as decay of short-lived radionuclides or electromagnetic induction during an early T-tauri phase of the Sun to explain heating and melting of the parent bodies of these meteorites.     

Hydrocode modeling of the Sierra Madera impact structure

Abstract— We present the first hydrocode simulations of the formation of the Sierra Madera structure (west Texas, USA), which was caused by an impact into a thick sedimentary target sequence. We modeled Sierra Madera using the iSALE hydrocode, and here we present two best‐fit models: 1) a crater with a rim (final crater) diameter of ?12 km, in agreement with previous authors' interpretations of the original structure, and 2) a crater ?16 km in diameter with increased postimpact erosion. Both models fit some of the geologic observational data, but discrepancies with estimates of peak shock pressure, extent of deformation, and stratigraphic displacement remain. This study suggests that Sierra Madera may be a larger crater than previously reported and illustrates some of the challenges in simulating impact deformation of sedimentary lithologies. As many terrestrial craters possess some amount of sedimentary rocks in the target sequence, numerical models of impacts into sedimentary targets are essential to our understanding of target rock deformation and the mechanics of crater formation.     

What metal‐troilite textures can tell us about post‐impact metamorphism in chondrite meteorites

Abstract— Metal‐troilite textures are examined in metamorphosed and impact‐affected ordinary chondrites to examine the response of these phases to rapid changes in temperature. Complexly intergrown metal‐troilite textures are shown to form in response to three different impact‐related processes. (1) During impacts, immiscible melt emulsions form in response to spatially focused heating. (2) Immediately after impact events, re‐equilibration of heterogeneously distributed heat promotes metamorphism adjacent to zones of maximum impact heating. Where temperatures exceed ～850 ° C, this post‐impact metamorphism results in melting of conjoined metal‐troilite grains in chondrites that were previously equilibrated through radiogenic metamorphism. When the resulting Fe‐Ni‐S melt domains crystallize, a finely intergrown mixture of troilite and metal forms, which can be zoned with kamacite‐rich margins and taenite‐rich cores. (3) At lower temperatures, post‐impact metamorphism can also cause liberation of sulfur from troilite, which migrates into adjacent Fe‐Ni metal, allowing formation of troilite and occasionally copper within the metal during cooling. Because impact events cause heating within a small volume, post‐impact metamorphism is a short duration event (days to years) compared with radiogenic metamorphism (>10⁶ years). The fast kinetics of metal‐sulfide reactions allows widespread textural changes in conjoined metal‐troilite grains during post‐impact metamorphism, whereas the slow rate of silicate reactions causes these to be either unaffected or only partially annealed, except in the largest impact events. Utilizing this knowledge, information can be gleaned as to whether a given meteorite has suffered a post‐impact thermal overprint, and some constraints can be placed on the temperatures reached and duration of heating.     

The effect of impact obliquity on shock heating in planetesimal collisions

Collisions between planetesimals in the early solar system were a common and fundamental process. Most collisions occurred at an oblique incidence angle, yet the influence of impact angle on heating in collisions is not fully understood. We have conducted a series of shock physics simulations to quantify oblique heating processes, and find that both impact angle and target curvature are important in quantifying the amount of heating in a collision. We find an expression to estimate the heating in an oblique collision compared to that in a vertical incidence collision. We have used this expression to quantify heating in the Rhealsilvia‐forming impact on Vesta, and find that there is slightly more heating in a 45° impact than in a vertical impact. Finally, we apply these results to Monte Carlo simulations of collisional processes in the early solar system, and determine the overall effect of impact obliquity from the range of impacts that occurred on a meteorite parent body. For those bodies that survived 100 Myr without disruption, it is not necessary to account for the natural variation in impact angle, as the amount of heating was well approximated by a fixed impact angle of 45°. However, for disruptive impacts, this natural variation in impact angle should be accounted for, as around a quarter of bodies were globally heated by at least 100 K in a variable‐angle model, an order of magnitude higher than under an assumption of a fixed angle of 45°.     

Shock‐metamorphosed zircon in terrestrial impact craters

Abstract— To ascertain the progressive stages of shock metamorphism of zircon, samples from three well‐studied impact craters were analyzed by optical microscopy, scanning electron microscopy (SEM), and Raman spectroscopy in thin section and grain separates. These samples are comprised of well‐preserved, rapidly quenched impactites from the Ries crater, Germany, strongly annealed impactites from the Popigai crater, Siberia, and altered, variably quenched impactites from the Chicxulub crater, Mexico. The natural samples were compared with samples of experimentally shock‐metamorphosed zircon. Below 20 GPa, zircon exhibits no distinct shock features. Above 20 GPa, optically resolvable planar microstructures occur together with the high‐pressure polymorph reidite, which was only retained in the Ries samples. Decomposition of zircon to ZrO₂ only occurs in shock stage IV melt fragments that were rapidly quenched. This is not only a result of post‐shock temperatures in excess of ?1700 °C but could also be shock pressure‐induced, which is indicated by possible relics of a high‐pressure polymorph of ZrO₂. However, ZrO₂ was found to revert to zircon with a granular texture during devitrification of impact melts. Other granular textures represent recrystallized amorphous ZrSiO₄ and reidite that reverted to zircon. This requires annealing temperatures >1100 °C. A systematic study of zircons from a continuous impactite sequence of the Chicxulub impact structure yields implications for the post‐shock temperature history of suevite‐like rocks until cooling below ?600 °C.     

The clay mineralogy of sediments related to the marine Mjølnir impact crater

Abstract— The 40 km diameter Mjølnir Crater is located on the central Barents Sea shelf, north of Norway. It was formed about 142 ± 2.6 Myr ago by the impact of a 1–2 km asteroid into the shallow shelf clays of the Hekkingen Formation and the underlying Triassic to Jurassic sedimentary strata. A core recovered from the central high within the crater contains slump and avalanche deposits from the collapse of the transient crater and central high. These beds are overlain by gravity flow conglomerates, with laminated shales and marls on top. Here, impact and post‐impact deposits in this core are studied with focus on clay mineralogy obtained from XRD decomposition and simulation analysis methods. The clay‐sized fractions are dominated by kaolinite, illite, mixed‐layered clay minerals and quartz. Detailed analyses showed rather similar composition throughout the core, but some noticeable differences were detected, including varying crystal size of kaolinite and different types of illites and illite/smectite. These minerals may have been formed by diagenetic changes in the more porous/fractured beds in the crater compared to time‐equivalent beds outside the crater rim. Long‐term post‐impact changes in clay mineralogy are assumed to have been minor, due to the shallow burial depth and minor thermal influence from impact‐heated target rocks. Instead, the clay mineral assemblages, especially the abundance of chlorite, reflect the impact and post‐impact reworking of older material. Previously, an ejecta layer (the Sindre Bed) was recognized in a nearby well outside the crater, represented by an increase in smectite‐rich clay minerals, genetically equivalent to the smectite occurring in proximal ejecta deposits of the Chicxulub crater. Such alteration products from impact glasses were not detected in this study, indicating that little, if any, impact glass was deposited within the upper part of the crater fill. Crater‐fill deposits inherited their mineral composition from Triassic and Jurassic sediments underlying the impact site.     

Characterization and morphological reconstruction of the Terny impact structure,central Ukraine

The Terny impact structure, located in central Ukraine, displays a variety of diagnostic indicators of shock metamorphism, including shatter cones, planar deformation features in quartz, diaplectic glass, selective melting of minerals, and whole rock melting. The structure has been modified by erosion and subsequently buried by recent sediments. Although there are no natural outcrops of the deformed basement rocks within the area, mining exploration has provided surface and subsurface access to the structure, exposing impact melt rocks, shocked parautochthonous target rocks, and allochthonous impact breccias, including impact melt‐bearing breccias similar to suevites observed at the Ries structure. We have collected and studied samples from surface and subsurface exposures to a depth of approximately 750 m below the surface. This analysis indicates the Terny crater is centered on geographic coordinates 48.13° N, 33.52° E. The center location and the distribution of shock pressures constrain the transient crater diameter to be no less than approximately 8.4 km. Using widely accepted morphometric scaling relations, we estimate the pre‐erosional rim diameter of Terny crater to be approximately 16–19 km, making it close in original size to the well‐preserved El'gygytgyn crater in Siberia. Comparison with El'gygytgyn yields useful insights into the original morphology of the Terny crater and indicates that the amount of erosion Terny experienced prior to burial probably does not exceed 320 m.     

Structure and impact indicators of the Cretaceous sequence of the ICDP drill core Yaxcopoil‐1, Chicxulub impact crater,Mexico

Abstract As part of the ICDP Chicxulub Scientific Drilling Project, the Yaxcopoil‐1 (Yax‐1) bore hole was drilled 60 km south‐southwest of the center of the 180 km‐diameter Chicxulub impact structure down to a depth of 1511 m. A sequence of 615 m of deformed Cretaceous carbonates and sulfates was recovered below a 100 m‐thick unit of suevitic breccias and 795 m of post‐impact Tertiary rocks. The Cretaceous rocks are investigated with respect to deformation features and shock metamorphism to better constrain the deformational overprint and the kinematics of the cratering process. The sequence displays variable degrees of impact‐induced brittle damage and post‐impact brittle deformation. The degree of tilting and faulting of the Cretaceous sequence was analyzed using 360°‐core scans and dip‐meter log data. In accordance with lithological information, these data suggest that the sedimentary sequence represents a number of structural units that are tilted and moved with respect to each other. Three main units and nine sub‐units were discriminated. Brittle deformation is most intense at the top of the sequence and at 1300–1400 m. Within these zones, suevitic dikes, polymict clastic dikes, and impact melt rock dikes occur and may locally act as decoupling horizons. The degree of brittle deformation depends on lithology; massive dolomites are affected by penetrative faulting, while stratified calcarenites and bituminous limestones display localized faulting. The deformation pattern is consistent with a collapse scenario of the Chicxulub transient crater cavity. It is believed that the Cretaceous sequence was originally located outside the transient crater cavity and eventually moved downward and toward the center to its present position between the peak ring and the crater rim, thereby separating into blocks. Whether or not the stack of deformed Cretaceous blocks was already displaced during the excavation process remains an open question. The analysis of the deformation microstructure indicates that a shock metamorphic overprint is restricted to dike injections with an exception of the so called “paraconglomerate.” Abundant organic matter in the Yax‐1 core was present before the impact and was mobilized by impact‐induced heating and suggests that >12 km³ of organic material was excavated during the cratering process.     

Combining shock barometry with numerical modeling: Insights into complex crater formation—The example of the Siljan impact structure (Sweden)

Siljan, central Sweden, is the largest known impact structure in Europe. It was formed at about 380 Ma, in the late Devonian period. The structure has been heavily eroded to a level originally located underneath the crater floor, and to date, important questions about the original size and morphology of Siljan remain unanswered. Here we present the results of a shock barometry study of quartz‐bearing surface and drill core samples combined with numerical modeling using iSALE. The investigated 13 bedrock granitoid samples show that the recorded shock pressure decreases with increasing depth from 15 to 20 GPa near the (present) surface, to 10–15 GPa at 600 m depth. A best‐fit model that is consistent with observational constraints relating to the present size of the structure, the location of the downfaulted sediments, and the observed surface and vertical shock barometry profiles is presented. The best‐fit model results in a final crater (rim‐to‐rim) diameter of ~65 km. According to our simulations, the original Siljan impact structure would have been a peak‐ring crater. Siljan was formed in a mixed target of Paleozoic sedimentary rocks overlaying crystalline basement. Our modeling suggests that, at the time of impact, the sedimentary sequence was approximately 3 km thick. Since then, there has been around 4 km of erosion of the structure.     

The early impact histories of meteorite parent bodies

We have developed a statistical framework that uses collisional evolution models, shock physics modeling, and scaling laws to determine the range of plausible collisional histories for individual meteorite parent bodies. It is likely that those parent bodies that were not catastrophically disrupted sustained hundreds of impacts on their surfaces—compacting, heating, and mixing the outer layers; it is highly unlikely that many parent bodies escaped without any impacts processing the outer few kilometers. The first 10–20 Myr were the most important time for impacts, both in terms of the number of impacts and the increase of specific internal energy due to impacts. The model has been applied to evaluate the proposed impact histories of several meteorite parent bodies: up to 10 parent bodies that were not disrupted in the first 100 Myr experienced a vaporizing collision of the type necessary to produce the metal inclusions and chondrules on the CB chondrite parent; around 1–5% of bodies that were catastrophically disrupted after 12 Myr sustained impacts at times that match the heating events recorded on the IAB/winonaite parent body; more than 75% of 100 km radius parent bodies, which survived past 100 Myr without being disrupted, sustained an impact that excavates to the depth required for mixing in the outer layers of the H‐chondrite parent body; and to protect the magnetic field on the CV chondrite parent body, the crust would have had to have been thick (approximately 20 km) to prevent it being punctured by impacts.     

Kamenetsk—A new impact structure in the Ukrainian Shield

The Kamenetsk impact structure is a deeply eroded simple crater that formed in crystalline rocks of the Ukrainian Shield. This study presents structural, lithologic, and shock metamorphic evidence for an impact origin of the Kamenetsk structure, which was previously described as a paleovolcano. The Kamenetsk structure is an oval depression that is 1.0–1.2 km in diameter and 130 m deep. The structure is deeply eroded, and only the lower part of the sequence of lithic breccia has been preserved in the deepest part of the crater to recent time, while the predominant part of impact rocks and postimpact sediments was eroded. Manifestations of shock metamorphism of minerals, especially planar deformation features in quartz and feldspars, were determined by petrographic investigations of lithic breccia that allowed us to determine the impact origin of the Kamenetsk structure. The erosion of the crater and surrounding target to a minimal depth of 220 m preceded the deposition of the postimpact sediments. The time of the formation of the Kamenetsk structure is bracketed within a wide interval from 2.0 to 2.1 Ga, the age of the crystalline target rocks, to the Late Miocene age of the sediments overlaying the crater. The deep erosion of the structure suggests it is probably Paleozoic in age.     

Numerical modeling of oblique hypervelocity impacts on strong ductile targets

Abstract– The majority of meteorite impacts occur at oblique incidence angles. However, many of the effects of obliquity on impact crater size and morphology are poorly understood. Laboratory experiments and numerical models have shown that crater size decreases with impact angle, the along‐range crater profile becomes asymmetric at low incidence angles, and below a certain threshold angle the crater planform becomes elliptical. Experimental results at approximately constant impact velocity suggest that the elliptical threshold angle depends on target material properties. Herein, we test the hypothesis that the threshold for oblique crater asymmetry depends on target material strength. Three‐dimensional numerical modeling offers a unique opportunity to study the individual effects of both impact angle and target strength; however, a systematic study of these two parameters has not previously been performed. In this work, the three‐dimensional shock physics code iSALE‐3D is validated against laboratory experiments of impacts into a strong, ductile target material. Digital elevation models of craters formed in laboratory experiments were created from stereo pairs of scanning electron microscope images, allowing the size and morphology to be directly compared with the iSALE‐3D craters. The simulated craters show excellent agreement with both the crater size and morphology of the laboratory experiments. iSALE‐3D is also used to investigate the effect of target strength on oblique incidence impact cratering. We find that the elliptical threshold angle decreases with decreasing target strength, and hence with increasing cratering efficiency. Our simulations of impacts on ductile targets also support the prediction from Chapman and McKinnon (1986) that cratering efficiency depends on only the vertical component of the velocity vector.     

The two Suvasvesi impact structures,Finland: Argon isotopic evidence for a “false” impact crater doublet

The two neighboring Suvasvesi North and South impact structures in central‐east Finland have been discussed as a possible impact crater doublet produced by the impact of a binary asteroid. This study presents ⁴⁰Ar/³⁹Ar geochronologic data for impact melt rocks recovered from the drilling into the center of the Suvasvesi North impact structure and melt rock from glacially transported boulders linked to Suvasvesi South. ⁴⁰Ar/³⁹Ar step‐heating analysis yielded two essentially flat age spectra indicating a Late Cretaceous age of ~85 Ma for the Suvasvesi North melt rock, whereas the Suvasvesi South melt sample gave a Neoproterozoic minimum (alteration) age of ~710 Ma. Although the statistical likelihood for two independent meteorite strikes in close proximity to each other is rather low, the remarkable difference in ⁴⁰Ar/³⁹Ar ages of >600 Myr for the two Suvasvesi impact melt samples is interpreted as evidence for two temporally separate, but geographically closely spaced, impacts into the Fennoscandian Shield. The Suvasvesi North and South impact structures are, thus, interpreted as a “false” crater doublet, similar to the larger East and West Clearwater Lake impact structures in Québec, Canada, recently shown to be unrelated. Our findings have implications for the reliable recognition of impact crater doublets and the apparent rate of binary asteroid impacts on Earth and other planetary bodies in the inner solar system.     

Progressive deformation of feldspar recording low‐barometry impact processes,Tenoumer impact structure,Mauritania

The Tenoumer impact structure is a small, well‐preserved crater within Archean to Paleoproterozoic amphibolite, gneiss, and granite of the Reguibat Shield, north‐central Mauritania. The structure is surrounded by a thin ejecta blanket of crystalline blocks (granitic gneiss, granite, and amphibolite) and impact‐melt rocks. Evidence of shock metamorphism of quartz, most notably planar deformation features (PDFs), occurs exclusively in granitic clasts entrained within small bodies of polymict, glass‐rich breccia. Impact‐related deformation features in oligoclase and microcline grains, on the other hand, occur both within clasts in melt‐breccia deposits, where they co‐occur with quartz PDFs, and also within melt‐free crystalline ejecta, in the absence of co‐occurring quartz PDFs. Feldspar deformation features include multiple orientations of PDFs, enhanced optical relief of grain components, selective disordering of alternate twins, inclined lamellae within alternate twins, and combinations of these individual textures. The distribution of shock features in quartz and feldspar suggests that deformation textures within feldspar can record a wide range of average pressures, starting below that required for shock deformation of quartz. We suggest that experimental analysis of feldspar behavior, combined with detailed mapping of shock metamorphism of feldspar in natural systems, may provide critical data to constrain energy dissipation within impact regimes that experienced low average shock pressures.     

Comparison of petrophysical properties of impactites for four meteoritic impact structures

We reanalyzed and compared unique data sets, which we obtained in the frame of combined petrophysical and geothermal investigations within scientific drilling projects on four impact structures: the Puchezh–Katunki impact structure (Vorotilovo borehole, Russia), the Ries impact structure (Noerdlingen‐73 borehole, Germany), the Chicxulub impact structure (ICDP Yaxcopoil‐1 borehole, Mexico), and the Chesapeake impact structure (ICDP‐USGS‐Eyreville borehole, USA). For a joined interpretation, we used the following previously published data: thermal properties, using the optical scanning technique, and porosities, both measured on densely sampled halfcores of the boreholes. For the two ICDP boreholes, we also used our previously published P‐wave velocities measured on a subset of cores. We show that thermal conductivity, thermal anisotropy, porosity, and velocity can be correlated with shock metamorphism (target rocks of the Puchezh–Katunki and Ries impact structures), and confirm the absence of shock metamorphism in the samples taken from megablocks (Chicxulub and Chesapeake impact structure). The physical properties of the lithic impact breccias and suevites are influenced mainly by their impact‐related porosity. Physical properties of lower porosity lithic impact breccias and suevites are also influenced by their chemical composition. These data allow for a distinction between different types of breccias due to differences concerning the texture and chemistry and the different amounts of melt and rock clasts.     

Post‐impact structural crater modification due to sediment loading: An overlooked process

Abstract— Post‐impact crater morphology and structure modifications due to sediment loading are analyzed in detail and exemplified in five well‐preserved impact craters: Mjølnir, Chesapeake Bay, Chicxulub, Montagnais, and Bosumtwi. The analysis demonstrates that the geometry and the structural and stratigraphic relations of post‐impact strata provide information about the amplitude, the spatial distribution, and the mode of post‐impact deformation. Reconstruction of the original morphology and structure for the Mjølnir, Chicxulub, and Bosumtwi craters demonstrates the long‐term subsidence and differential compaction that takes place between the crater and the outside platform region, and laterally within the crater structure. At Mjølnir, the central high developed as a prominent feature during post‐impact burial, the height of the peak ring was enhanced, and the cumulative throw on the rim faults was increased. The original Chicxulub crater exhibited considerably less prominent peak‐ring and inner‐ring/crater‐rim features than the present crater. The original relief of the peak ring was on the order of 420–570 m (currently 535–575 m); the relief on the inner ring/crater rim was 300–450 m (currently ?700 m). The original Bosumtwi crater exhibited a central uplift/high whose structural relief increased during burial (current height 101–110 m, in contrast to the original height of 85–110 m), whereas the surrounding western part of the annular trough was subdued more that the eastern part, exhibiting original depths of 43–68 m (currently 46 m) and 49–55 m (currently 50 m), respectively. Furthermore, a quantitative model for the porosity change caused by the Chesapeake Bay impact was developed utilizing the modeled density distribution. The model shows that, compared with the surrounding platform, the porosity increased immediately after impact up to 8.5% in the collapsed and brecciated crater center (currently +6% due to post‐impact compaction). In contrast, porosity decreased by 2–3% (currently ?3 to ?4.5% due to post‐impact compaction) in the peak‐ring region. The lateral variations in porosity at Chesapeake Bay crater are compatible with similar porosity variations at Mjølnir crater, and are considered to be responsible for the moderate Chesapeake Bay gravity signature (annular low of ?8 mGal instead of ?15 mGal). The analysis shows that the reconstructions and the long‐term alterations due to post‐impact burial are closely related to the impact‐disturbed target‐rock volume and a brecciated region of laterally varying thickness and depth‐varying physical properties. The study further shows that several crater morphological and structural parameters are prone to post‐impact burial modification and are either exaggerated or subdued during post‐impact burial. Preliminary correction factors are established based on the integrated reconstruction and post‐impact deformation analysis. The crater morphological and structural parameters, corrected from post‐impact loading and modification effects, can be used to better constrain cratering scaling law estimates and impact‐related consequences.     

Validation of numerical codes for impact and explosion cratering: Impacts on strengthless and metal targets

Abstract— Over the last few decades, rapid improvement of computer capabilities has allowed impact cratering to be modeled with increasing complexity and realism, and has paved the way for a new era of numerical modeling of the impact process, including full, three‐dimensional (3D) simulations. When properly benchmarked and validated against observation, computer models offer a powerful tool for understanding the mechanics of impact crater formation. This work presents results from the first phase of a project to benchmark and validate shock codes. A variety of 2D and 3D codes were used in this study, from commercial products like AUTODYN, to codes developed within the scientific community like SOVA, SPH, ZEUS‐MP, iSALE, and codes developed at U.S. National Laboratories like CTH, SAGE/RAGE, and ALE3D. Benchmark calculations of shock wave propagation in aluminum‐on‐aluminum impacts were performed to examine the agreement between codes for simple idealized problems. The benchmark simulations show that variability in code results is to be expected due to differences in the underlying solution algorithm of each code, artificial stability parameters, spatial and temporal resolution, and material models. Overall, the inter‐code variability in peak shock pressure as a function of distance is around 10 to 20%. In general, if the impactor is resolved by at least 20 cells across its radius, the underestimation of peak shock pressure due to spatial resolution is less than 10%. In addition to the benchmark tests, three validation tests were performed to examine the ability of the codes to reproduce the time evolution of crater radius and depth observed in vertical laboratory impacts in water and two well‐characterized aluminum alloys. Results from these calculations are in good agreement with experiments. There appears to be a general tendency of shock physics codes to underestimate the radius of the forming crater. Overall, the discrepancy between the model and experiment results is between 10 and 20%, similar to the inter‐code variability.     

Intra‐crater sedimentary deposits at the Haughton impact structure,Devon Island,Canadian High Arctic

Abstract— 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.