Subsurface deformation of experimental hypervolcity impacts in nonporous targets

Introduction: During hypervelocity impact the crater subsurface experiences pervasive deformation. Fracture propagation and the localization behavior of damage depend on mechanical properties of the rock, i.e., dynamic strength behavior, which in turn must be constrained by the mechanical properties of the rock-forming minerals phases. These patterns also correspond to the variation of the stress field that occurs during the propagation of the shock wave. Where pressures in the order of a several GPa above the Hugoniot Elastic Limit (HEL) are achieved, non-porous rocks fail under compression, due to pervasive shearing under differential stresses [1]. When pressures decrease to around the HEL, more localized, tensile deformation can occur in the target, e.g. radial fractures [2]. Our goal is to determine how impact-induced deformation varies between different mineralogically homogeneous, nonporous rocks and address their potential influence on the resulting crater. Methods: Two hypervelocity impact experiments into quartzite and marble were conducted, using a spherical 2.5 mm steel and iron meteorite projectile, respectively, with densities between 7.8 – 8.1 g/cm3 and an impact velocity of ca. 5.0 km/s. The crater surface topography was measured with a 3D laser scanner. Craters were sawn in half and thin sections of the crater subsurface were made to analyze subsurface deformation. Orientations of the deformation features were mapped to infer the deformation mechanisms. Results: Subsurface analysis of the two craters shows that a common feature of both is the development of subconcentric tensile fractures directly beneath the crater floor due to dilatancy upon pressure release. The maximum depth of tensile failure below the crater floor varies with target material: quartzite 2.2 dp, marble 1.5 dp (projectile diameter). The quartzite target additionally shows localized deformation along discrete, 25 to 180 μm wide zones with subradial orientation relative to the impact point. Target material within these zones has a mean grain size of ~ 1.3 - 2.6 μm and thus is highly comminuted. Additionally, they are commonly surrounded by areas with large fractures. Outside of these fault zones the quartzite target suffered only the formation of narrow radial fractures down to a depth of at least 8.5 dp. In comparison, the marble target experienced intensive and pervasive intra- and intergranular fracturing, but did not develop the localized fracture zones seen in the quartzite target. The intragranular fractures show a stong correlation to the natural cleavage of calcite and a high percentage of the intragranular fractures is crystallographically orientated. In combination with the intergranular and tensile fractures they led to a much stronger overall comminution in the marble subsurface than in the quartzite. A further impact-induced deformation feature is microtwinning along crystallographic planes in calcite minerals and results in minor crystal-plastic behavior of the calcite. Close to the crater floor several sets of twins per grain developed, but with increasing distance to the crater floor their abundance decreases to a depth of ~2 dp. The depth of observable deformation features in quartzite extends down to ~ 11.4 dp, compared to only 3.6 dp in the marble target. In both targets, the deformation in the most proximal, highly comminuted area underneath the crater floor seems to be controlled by shear deformation. Such a dominance of one deformation mechanism over others cannot be established in the deeper regions of the crater subsurface. Discussion: The compressive and subsequent tensile stress fields generated in the shock wave are demonstrated by distinct deformation featuers in both target materials. The strong grain comminution in the localized deformation zones in quartzite indicates compressive failure due to shearing under differential stresses. The apparent absence of deformation in the neighboring areas of the shear zones is probably attributed to the lack of cleavage in quartz. The radial fractures are suggested to form due to hoop stresses in the elastic decay regime [2]. The less localized and more pervasive deformation in the marble target on the other hand can be attributed to the weaker crystal strength of calcite. The excellent rhombohedral cleavage and the possibility of twin formation led to a stronger absorbtion of impact energy by the formation of cleavage fractures and twins. Thus, the shock wave is more effectively dampened than in the quartzite. Conclusions: First results of SEM microscopy of impacted quartzite and marble target subsurfaces reveal great differences in impact induced deformation mechanisms between the two non-porous target materials. The origin of these differences seems mostly to be found in the dynamic mechanical properties of the main rock-forming minerals. Further investigations will be performed to validate these preliminary results

Asymmetric shock deformation at the Spider impact structure, Western Australia

The distribution of shock deformation effects, as well as the structural expression of an impact structure, can be asymmetric, depending on target rock lithologies (e.g., layered versus homogenous), porosity of target rock, and angle of impact. Here, we present a detailed study of shock-deformed quartz and zircon in silicified sandstones from the asymmetric Spider impact structure in Australia. We utilize optical microscopy and electron backscatter diffraction (EBSD) techniques in order to determine the spatial distribution of shock-deformed zircon along a downrange transect across the central uplift of the structure, with the goal of constraining the physical distribution of shock effects created by an oblique impact. A total of 453 zircon grains from 12 samples of shatter cone-bearing quartzite and breccia within the structure were surveyed for shock deformation by EBSD in situ within thin sections. Nineteen zircon grains contain {112} twins, including one grain with three twin orientations. Quartz grains from five samples along the transect were also surveyed using a universal stage in order to determine orientations of planar deformation features, planar fractures, and feather features, and to provide a baseline for comparison of data from zircon. The distribution of shocked zircon with {112} twins within the samples surveyed appears to be asymmetric relative to the center of the structure, in contrast to quartz, thus providing the first accessory mineral-based evidence that supports an asymmetric distribution of shock deformation as a function of impact obliquity. Our results are an example where the highest intensity of observed shock deformation does not correspond to the geographic center of the structure, and may serve as a guide for field studies aimed at documenting the distribution of shock effects at other sites interpreted to result from oblique impacts

New shock microstructures in titanite (CaTiSiO5) from the peak ring of the Chicxulub impact structure, Mexico

Accessory mineral geochronometers such as apatite, baddeleyite, monazite, xenotime and zircon are increasingly being recognized for their ability to preserve diagnostic microstructural evidence of hypervelocity-impact processes. To date, little is known about the response of titanite to shock metamorphism, even though it is a widespread accessory phase and a U–Pb geochronometer. Here we report two new mechanical twin modes in titanite within shocked granitoid from the Chicxulub impact structure, Mexico. Titanite grains in the newly acquired core from the International Ocean Discovery Program Hole M0077A preserve multiple sets of polysynthetic twins, most commonly with composition planes (K1) = ~ { 1 ¯ 11 } , and shear direction (η1) = , and less commonly with the mode K1 = {130}, η1 = ~ . In some grains, {130} deformation bands have formed concurrently with the deformation twins, indicating dislocation slip with Burgers vector b = can be active during impact metamorphism. Titanite twins in the modes described here have not been reported from endogenically deformed rocks; we, therefore, propose this newly identified twin form as a result of shock deformation. Formation conditions of the twins have not been experimentally calibrated, and are here empirically constrained by the presence of planar deformation features in quartz (12 ± 5 and ~ 17 ± 5 GPa) and the absence of shock twins in zircon (< 20 GPa). While the lower threshold of titanite twin formation remains poorly constrained, identification of these twins highlight the utility of titanite as a shock indicator over the pressure range between 12 and 17 GPa. Given the challenges to find diagnostic indicators of shock metamorphism to identify both ancient and recent impact evidence on Earth, microstructural analysis of titanite is here demonstrated to provide a new tool for recognizing impact deformation in rocks where other impact evidence may be erased, altered, or did not manifest due to generally low (< 20 GPa) shock pressure