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 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 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 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

Aeromagnetic surveys are a useful tool in the detection and analysis of terrestrial impact structures. Although gravity anomalies provide clearer and simpler signatures of impact craters, large regional-scale aeromagnetic surveys are more widely available. A combination of many factors, such as the nature of the target rocks, the impact-related magnetization, and effects of crater fill and post-impact cover results in a great variation of magnetic signatures in the terrestrial impact craters. In crystalline basement targets, the most common signature of a complex impact structure is a magnetic low with a central peak or ring uplift magnetic anomaly. Contributions to the magnetic signature include demagnetization, shock remagnetization, and thermal and chemical remanent magnetization effects. Impact craters in sedimentary targets usually are of small magnetic amplitude, depending on the lithology. The origin of the magnetic signatures in sedimentary targets is not well understood. Enhancement of magnetic signatures of terrestrial impact structures using filtering techniques is an important part of detection and analysis. Derivative and derivative-based (such as sunshading) techniques, along with separation filtering, are probably the most used methods. Here we present our new developments of algorithms for fractional order derivatives and circular shaded relief that have dramatically improved filter results. The fractional derivative order can be varied to optimize the separation of the impact magnetic signature. Given a chosen center location, the circular shaded relief algorithm treats all directions equally, thus preventing fade-out of features subparallel to the shading direction evident in conventional shaded relief. Unlike Hough transform based algorithms, the circular sunshading method is not sensitive to the radius of the circular feature being searched for, and no radius parameter is specified during the data processing We illustrate the new fractional derivative and circular shaded relief algorithms using selected Australian and Canadian impact crater data sets involving both crystalline basement and sedimentary targets.

Considering the Ries crater as an example for middle-sized complex impact structures on Earth, we use geophysical data to examine the structure underneath the crater and simulate the formation process by numerical modeling. In contrast to previous investigations, we show by reanalyzing seismic refraction profiles that some clues for structural uplift exist beneath the crater in a range of at least 1 km. We propose that the average P-wave velocity inside the crater is lower than outside the crater from the surface to ∼2.2 km depth, but that below this level, the velocity increases beneath the center, presumably due to uplifted basement rocks. In addition, we utilized magnetotelluric depth sounding to investigate the deep electrical structure beneath the crater. Two-dimensional inversion models of the data show anomalously high conductivity beneath the crater. Our best model features a zone of presumably brine-filled fractures in open pore space to a depth of ∼2 km. Furthermore, our numerical modeling results for the crater formation are consistent with surface and subsurface observations in the vicinity of the crater. In order to explain the structural differences between similarly sized craters and Ries, we investigate the sensitivity of crater shape and subsurface structure to varying target compositions. We show that for a reasonable range of constitutive material and acoustic fluidization parameters the model calculations produce a large variety of different crater shapes, even for the same amount of impact energy. In contrast to the conventional estimate of crater diameters, our results suggest that Ries crater is comparable in size with the Bosumtwi and Zhamanshin crater. Despite their apparent lack of similarity at first look, Ries and Bosumtwi are closely matched in terms of transient crater size (inner ring), aspect ratio, and structural elements, and we conclude that they both represent typical complex crater structures of the terrestrial impact record for their size range.

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.

This study reexamines seven reprocessed (increased vertical exaggeration) seismic reflection profiles that cross the eastern rim of the Chesapeake Bay impact crater. The eastern rim is expressed as an arcuate ridge that borders the crater in a fashion typical of the “raised” rim documented in many well preserved complex impact craters. The inner boundary of the eastern rim (rim wall) is formed by a series of crater-facing, steep scarps, 15–60 m high. In combination, these rim-wall scarps represent the footwalls of a system of crater-encircling normal faults, which are downthrown toward the crater. Outboard of the rim wall are several additional normal-fault blocks, whose bounding faults trend approximately parallel to the rim wall. The tops of the outboard fault blocks form two distinct, parallel, flat or gently sloping, terraces. The innermost terrace (Terrace 1) can be identified on each profile, but Terrace 2 is only sporadically present. The terraced fault blocks are composed mainly of nonmarine, poorly to moderately consolidated, siliciclastic sediments, belonging to the Lower Cretaceous Potomac Formation. Though the ridge-forming geometry of the eastern rim gives the appearance of a raised compressional feature, no compelling evidence of compressive forces is evident in the profiles studied. The structural mode, instead, is that of extension, with the clear dominance of normal faulting as the extensional mechanism.

Previous and recent geological, topographic, drilling, and geophysical data are summarized to acknowledge the 100 km original size of the relatively young and well-preserved multi-ring Popigai impact structure and to estimate the depth of its erosion. The relics of the bedrock rim are traced in the northern and western sectors of the crater, allowing the correct evaluation of the crater diameter. The depth of erosion varies in different crater sectors and concentric zones from 50 to 100 m to 300 m. The most significant erosion occurred on the crater rim. The crater rim has a complex inner structure and is composed of outwardly dragged and uplifted blocks of target rocks overlain by ejecta. During the 35 m.y. that have passed after the crater formation, ∼1000–1200 km 3 of unconsolidated allogenic breccias were eroded from the ejecta blanket on the crater rim and from the upper layers of breccias and suevites of the central depression.

Shuttle Radar Topography Mission (SRTM) data over the Chicxulub impact crater are imaged and compared to previously available topography data. While the two data sets contain different biases related to variations in terrain and vegetation cover, the correspondence of the two sets supports earlier interpretations that the complex structure of the buried crater is expressed in the topography of the northwestern Yucatán Peninsula, México.