The internal structures of central uplifts of impact craters are among the most complex geologic features within Earth's crust. Upheaval Dome, Utah, is used as a reference and case study to display the internal geometry of a central uplift and to deduce mechanisms of uplift formation in impact craters within a sedimentary, siliciclastic target. Geological and structural data gained from our high-resolution mapping of the central part of the structure were combined with topographic data in an ArcGIS database. A three-dimensional visualization of the geometry of faults and strata within the central uplift is presented and interpreted with respect to their deformation history. Central uplift formation is induced by an inward and upward directed convergent flow of the crater floor during gravity-driven collapse of the transient crater cavity. Radial folds and a concentric stacking of imbricated thrust slices are prominent deformation features and result from a constrictive strain pattern. The arrangement of structural elements in the inner part of the Upheaval Dome roughly displays some bilateral symmetry, trending northwest. The dominance of northwest-dipping reverse faults indicates a material transport of top to the southeast, which may be caused by an oblique impact. Fault planes commonly dip steeply and are bent due to a passive distortion after activation. The macroscopic coherence of large target units and blocks and the anisotropy of the layered target cause remarkable deviations from an ideal convergent flow field. Stratified siliciclastic rocks are commonly deformed by localized brittle faulting, and massive sandstones are deformed by a distributed cataclastic flow. During crater collapse, pervasively crushed sandstones will flow locally as a granular medium, resulting in the formation of dikes. Acting as lubricants, they accommodate the complex mesoscale folding and faulting of the neighboring strata. A standard numerical model of impact cratering was designed for comparison with the observed structures and to estimate impact parameters like initial crater size, amount of erosion, and the time of impact. The best fit between model and field data is found when the White Rim Sandstone is buried ∼2000 m beneath the target surface. This most likely corresponds to an Upper Cretaceous age of Upheaval Dome during deposition of the Mancos shales. The initial diameter of the Upheaval Dome impact crater would have been ∼7–8 km.

ABSTRACT The earliest ejection process of impact cratering involves very high pressures and temperatures and causes near-surface material to be ejected faster than the initial impact velocity. On Earth, such material may be found hundreds to even thousands of kilometers away from the source crater as tektites. The mechanism yielding such great distances is not yet fully understood. Hypervelocity impact experiments give insights into this process, particularly as the technology necessary to record such rapid events in high temporal and spatial resolution has recently become available. To analyze the earliest stage of this hypervelocity process, two series of experiments were conducted with a two-stage light-gas gun, one using aluminum and the other using quartzite as target material. The vertical impacts of this study were recorded with a high-speed video camera at a temporal resolution of tens of nanoseconds for the first three microseconds after the projectile’s contact with the target. The images show a self-luminous, ellipsoidal vapor cloud expanding uprange. In order to obtain angle-resolved velocities of the expanding cloud, its entire front and the structure of the cloud were systematically investigated. The ejected material showed higher velocities at high angles to the target surface than at small angles, providing a possible explanation for the immense extent of the strewn fields.

ABSTRACT The rare earth element–bearing phosphate xenotime (YPO 4 ) is isostructural with zircon, and therefore it has been predicted that xenotime forms similar shock deformation microstructures. However, systematic characterization of the range of microstructures that form in xenotime has not been conducted previously. Here, we report a study of 25 xenotime grains from 10 shatter cones in silicified sandstone from the Spider impact structure in Western Australia. We used electron backscatter diffraction (EBSD) in order to characterize deformation and microstructures within xenotime. The studied grains preserve multiple sets of planar fractures, lamellar {112} deformation twins, high-angle planar deformation bands (PDBs), partially recrystallized domains, and pre-impact polycrystalline grains. Pressure estimates from microstructures in coexisting minerals (quartz and zircon) allow some broad empirical constraints on formation conditions of ~10–20 GPa to be placed on the observed microstructures in xenotime; at present, more precise formation conditions are unavailable due to the absence of experimental constraints. Results from this study indicate that the most promising microstructures in xenotime for recording shock deformation are lamellar {112} twins, polycrystalline grains, and high-angle PDBs. The {112} deformation twins in xenotime are likely to be a diagnostic shock indicator, but they may require a different stress regime than that of {112} twinning in zircon. Likewise, polycrystalline grains are suggestive of impact-induced thermal recrystallization; however, in contrast to zircon, the impact-generated polycrystalline xenotime grains here appear to have formed in the solid state, and, in some cases, they may be difficult to distinguish from diagenetic xenotime with broadly similar textures.

ABSTRACT We report on the Cleanskin structure (18°10′00″S, 137°56′30″E), situated at the border between the Northern Territory and Queensland, Australia, and present results of preliminary geological fieldwork, microscopic analyses, and remote sensing. The Cleanskin structure is an eroded complex impact structure of ~15 km apparent diameter with a polygonal outline caused by two preexisting regional fault sets. The structure has a central uplift of ~6 km diameter surrounded by a rather shallow ring syncline. Based on stratigraphy, the uplift in the center may not exceed ~1000 m. The documentation of planar deformation features (PDFs), planar fractures (PFs), and feather features (FFs) in quartz grains from sandstone members of the Mesoproterozoic Constance Sandstone confirms the impact origin of the Cleanskin structure, as proposed earlier. The crater was most likely eroded before the Cambrian and later became buried beneath Cretaceous strata. We infer a late Mesoproterozoic to Neoproterozoic age of the impact event. In this chapter, the Cleanskin structure is compared with other midsized crater structures on Earth. Those with sandstone-dominated targets show structural similarities to the Cleanskin structure.

ABSTRACT Rampart craters are omnipresent features on volatile-rich solid planetary surfaces. This raises the question whether, and how many, rampart craters are present on Earth. We reviewed the terrestrial impact crater record with regard to possible rampart morphologies and present detailed morphological analyses of these terrestrial craters here. Our results show that the Ries crater in Germany, Bosumtwi crater in Ghana, Tenoumer crater in Mauritania, Lonar crater in India, and Meteor crater in the United States are terrestrial rampart craters. The Ries and Bosumtwi craters can be classified as double-layer ejecta (DLE) craters, whereas Tenoumer, Lonar, and Meteor craters can be classified as single-layer ejecta (SLE) craters. Tenoumer and Meteor craters show rampart as well as common lunar-like ejecta characteristics within their ejecta blankets and, thus, appear to be hybrid craters. In addition, we discuss seven crater structures that show at least some morphological or lithological peculiarities that could provide evidence for possible ejecta ramparts. Considering the low number of terrestrial impact craters with well-preserved ejecta blankets, the relatively high proportion of rampart craters is astonishing. Obviously, the formation of layered or rampart craters is a common and not a rare process on Earth.

ABSTRACT During hypervelocity impacts, target rocks are subjected to shock wave compression with high pressures and differential stresses. These differential stresses cause microscopic shear-induced deformation, which can be observed in the form of kinking, twinning, fracturing, and shear faulting in a range of minerals. The orientation of these shear-induced deformation features can be used to constrain the maximum shortening axis. Under the assumption of pure shear deformation, the maximum shortening axis is parallel to the maximum principal axis of stress, σ 1 , which gives the propagation direction of the shock wave that passed through a rock sample. In this study, shocked granitoids cored from the uppermost peak ring of the Chicxulub crater (International Ocean Discovery Program [IODP]/International Continental Drilling Project [ICDP] Expedition 364) were examined for structures formed by shearing. Orientations of kink planes in biotite and basal planar deformation features (PDFs) in quartz were measured with a U-stage and compared to a previous study of feather feature orientations in quartz from the same samples. In all three cases, the orientations of the shortening axis derived from these measurements were in good agreement with each other, indicating that the shear deformation features all formed in an environment with similar orientations of the maximum principal axis of stress. These structures formed by shearing are useful tools that can aid in understanding the deformational effects of the shock wave, as well as constraining shock wave propagation and postshock deformation during the cratering process.

Abstract Hypervelocity impacts are a fundamental and quite common process in the Solar System. Extreme pressures, temperatures and stress and strain rates characterize an impact event. These unique transient physical parameters result in unique geological and mineralogical phenomena that include the formation of (macroscopic) shatter cones, and shock effects at the scale of minerals. Adiabatic pressure release and post-shock heating generate decomposition, melting and vaporization of rocks. In this chapter these shock and post-shock effects are discussed in terms of formation processes and characteristic features. A case study on the Lake Bosumtwi crater illustrates geochemical aspects of impact melt formation. Key facts about the high-pressure mineral phases in strongly shocked meteorites are discussed. Finally, key results of a unique series of meso-scale cratering experiments ('The MEMIN project') provide details of the role of target porosity in cratering efficiency and the ejection process.

The recently discovered 6-km-diameter impact structure Jebel Waqf as Suwwan of Jordan (31°02.9′N, 36°48.4′E) has a prominent outer rim and a well-exposed central uplift of ~1000 m diameter, which provides a section through the entire target stratigraphy. The impact occurred into sedimentary rocks of considerable competence contrast. The innermost area of the central uplift exposes Lower Cretaceous sandstones, the oldest strata of the crater. Limestones and marly limestones surround this core and are dismembered into competent blocks that are internally folded. The limestone blocks, in turn, are encircled by a sequence of incompetent marls and chalks. These weak beds accommodated space incompatibilities during block deformation of the competent beds beneath and above. Thick chert beds form the prominent outer collar of the central uplift. Radial folding and faulting are the most conspicuous structural attributes of this sequence. In the southwestern part of the collar, normal layering dominates, and fold axes plunge outward, whereas overturning of strata and fold axes is the rule in the northeastern part. This indicates a top-to-NE shearing component that is explained by an oblique impact scenario with an impact from the southwest. The inferred trajectory runs parallel to the SW-NE axis of symmetry of the central uplift defined by the exposure of strata. Block sizes in limestones and cherts of the central uplift increase with increasing radial distance; however, block sizes are also influenced by the different strength properties of limestone and chert. Shatter cones are abundant throughout the Waqf as Suwwan central uplift, but they also occur prominently along its periphery. Other shock features, such as planar deformation features, planar fractures, and feather features, occur exclusively in Lower Cretaceous sandstones; limestone and microcrystalline chert—the dominant lithologies—are devoid of such effects. The moat between the central uplift and crater rim is largely covered by alluvial wadi sediments. The crater rim is composed of white marls and massive chert beds of Eocene age, the youngest strata of the crater, which also provide a maximum age for the cratering event. Both antithetic and synthetic block slumping are common along the uplifted crater rim.

The International Continental Scientific Drilling Program (ICDP)–U.S. Geological Survey (USGS) Eyreville boreholes through the annular moat of the Chesapeake Bay crater recovered polymict impact breccias and associated rocks from the depth range of 1397–1551 m. These rocks record cratering processes before burial beneath resurge deposits. Quantitative analyses of clast sizes, matrix contents, and distribution of impact melt reveal a shock metamorphic gradient in these impactites. The reason for the low estimated quantity of impact melt in the crater (~10 km 3 ) remains elusive. Possible causes may relate to increased excavation efficiency due to a high ratio of water column and sedimentary target to depth of excavation, an oblique impact, or a buried melt sheet at depth. A plausible petrogenetic scenario consists of a lower block-rich section that slumped from an outer region of the transient cavity into the annular moat ~1.5 min after impact. This blocky debris was mixed with the remains of the excavation flow, which contained a pod of melt entrained in ground-surge debris on top. Subsequently, melt-rich suevites were emplaced that record interaction of the expanding ejecta plume with fallback material related to the evolving central uplift. A clast-rich impact melt rock that likely shed off the central uplift covers these suevites. Incipient collapse of the ejecta plume is recorded in the uppermost subunit, but the arrival of resurge flow terminated its continuous deposition ~6–8 min after impact. Limited thermal annealing allowed preservation of glassy melt and high-pressure polymorphs. Mild hydrothermal overprint in the central crater was likely driven by the structural uplift of ~100 °C warmer basement rocks.

The combination of petrographic analysis of drill core from the recent International Continental Scientific Drilling Program (ICDP)–U.S Geological Survey (USGS) drilling project and results from numerical simulations provides new constraints for reconstructing the kinematic history and duration of different stages of the Chesa-peake Bay impact event. The numerical model, in good qualitative agreement with previous seismic data across the crater, is also roughly consistent with the stratigraphy of the new borehole. From drill core observations and modeling, the following conclusions can be drawn: (1) The lack of a shock metamorphic overprint of cored basement lithologies suggests that the drill core might not have reached the parautochthonous shocked crater floor but merely cored basement blocks that slumped off the rim of the original cavity into the crater during crater modification. (2) The sequence of polymict lithic breccia, suevite, and impact melt rock (1397–1551 m) must have been deposited prior to the arrival of the 950-m-thick resurge and avalanche-delivered beds and blocks within 5–7 min after impact. (3) This short period for transportation and deposition of impactites may suggest that the majority of the impactites of the Eyreville core never left the transient crater and was emplaced by ground surge. This is in accordance with observations of impact breccia fabrics. However, the uppermost part of the suevite section contains a pronounced component of airborne material. (4) Limited amounts of shock-deformed debris and melt fragments also occur throughout the Exmore beds. Shard-enriched intervals in the upper Exmore beds indicate that some material interpreted to be part of the hot ejecta plume was incorporated and dispersed into the upper resurge deposits. This suggests that collapse of the ejecta plume was contemporaneous with the major resurge event(s). Modeling indicates that the resurge flow should have been concluded some 20 min after impact; hence, this also likely marked the end of the major episode of deposition from the ejecta plume.

The diameter of an impact crater is one of the most basic and important parameters used in energy scaling and numerical modeling of the cratering process. However, within the impact and geological communities and literature, there is considerable confusion about crater sizes due to the occurrence of a variety of concentric features, any of which might be interpreted as defining a crater's diameter. The disparate types of data available for different craters make the use of consistent metrics difficult, especially when comparing terrestrial to extraterrestrial craters. Furthermore, assessment of the diameters of terrestrial craters can be greatly complicated due to post-impact modification by erosion and tectonic activity. We analyze the terminology used to describe crater geometry and size and attempt to clarify the confusion over what exactly the term “crater diameter” means, proposing a consistent terminology to help avert future ambiguities. We discuss several issues of crater-size in the context of four large terrestrial examples for which crater diameters have been disputed (Chicxulub, Sudbury, Vredefort, and Chesapeake Bay) with the aim of moving toward consistent application of terminology.

Prominent magnetic anomalies over large impact craters are attributed to remanent magnetization as thermal effects induce extremely high Koenigsberger values (remanent to induced magnetization ratio, Q). Magnetization of impact melt rocks, breccias, and the rocks underneath the crater floor is related to the thermal evolution of large impact craters, from a single heat pulse to long-lived hydrothermal processes and associated alteration and mineral deposits. The magnetic signature observed on large impact structures can be primarily the aggregate of three effects: (1) composition and properties of target rocks, (2) modification of magnetic carriers due to high pressure-temperature ( P-T ) conditions, and (3) natural remanent magnetization (NRM). Numerical modeling is used to predict the pressure and temperature distribution for varying size craters ranging from a 1.5-km-diameter simple crater to a 90-km-diameter complex crater. We first compare the results of two hydrocodes: the Simplified Arbitrary Lagrangian Eulerian code, version B (SALEB), to produce the final crater shape after a vertical impact; and the Solid, Vapor, Air (SOVA) code to model the initial stage of an oblique impact and to evaluate differences in the volumes of highly shocked materials within the crater created by a vertical impact versus an oblique one. After defining the accuracy of the numerical modeling and reliability of the resulting P-T conditions predicted, numerical modeling is applied for 1–90 km diameter impact structures. The P-T distributions of the target material at its final position obtained with the SALEB code are used for geophysical predictions. The calculated maximum P-T values and their initial and final spatial distributions can be linked with geological and physical and/or chemical processes that lead to different geophysical signatures. For a 5 km diameter crater, brecciation ( P > 1 GPa) affects all material in a 2.5 km hemisphere. Brecciation and fluid circulation trigger a hydrothermal system, which introduces chemical remanent magnetization (CRM) and modify magnetic carriers due to alteration. Pressures larger than 30 GPa induce a secondary component of remanent magnetization (shock remanent magnetization, SRM). These P-T conditions are generally restricted to a radius of <0.5 km and depth <0.8 km. Therefore, SRM and melting are confined to small regions, occasionally not even discernible by a regional airborne magnetic survey, and easily removed by erosive processes.

Lake Wanapitei, located within the Southern Province of Ontario, Canada, provides the setting for a unique study of an impact crater situated within a shield environment. Evidence for the 7.5-km-diameter Wanapitei impact includes a circular Bouguer gravity low centered over the central area of the lake and features of shock metamorphism in samples of glacial drift found on the southern shores. Geophysical studies of craters in hard-rock environments are often limited by the lack of markers used for exploration; this may be overcome with the use of the large igneous dike swarms that characterize Archean terrains. The 1.2 Ga Sudbury dike swarm predates the impact that is suggested to have generated Lake Wanapitei and provides the setting for a study to constrain the size and location of the impact crater. The swarm is clearly visible on aeromagnetic maps as high amplitude, linear features, suggesting they could be used as vertical markers indicative of structural changes having an effect on target rock susceptibilities. To fully establish the size of the crater, a total field magnetic map was produced to trace the Sudbury dikes through the proposed crater center. A gap in their signature, expressed as a 100 nT low, 2–3 km in width, constrains the size of the crater to <5 km. Numerical modeling suggests that a crater of this size will demagnetize target rocks, producing a low in the total magnetic field, up to a maximum diameter of 3 km. Dikes