The study of the geology of multiring impact basins on the Moon over the past ten years has given us a rudimentary understanding of how these large structures have formed and evolved on the Moon and other bodies. Two-ring basins on the Moon begin to form at diameters of about 300 km; the transition diameter at which multiple (more than two) rings appear is uncertain, but it appears to be between 400 and 500 km in diameter. Inner rings tend to be made up of clusters or aligned segments of massifs and are arranged into a crudely concentric pattern; scarp-like elements may or may not be present. Outer rings are much more scarp-like and massifs are rare to absent. Basins display textured deposits, interpreted as ejecta, extending roughly an apparent basin radius exterior to the main topographic rim. Ejecta may have various morphologies, ranging from wormy and hummocky deposits to knobby surfaces; the causes of these variations in morphology are not known, but may be related to the energy regime in which the ejecta are deposited. Outside the limits of the textured ejecta are found both fields of satellitic craters (secondaries) and light plains deposits. Impact melt sheets are observed on the floors of relatively unflooded basins. Samples of impact melts from lunar basins have basaltic major-element chemistry, characterized by K, rare-earth elements (REE), P, and other trace elements of varying concentration (KREEP); ages are between 3.8 and 3.9 Ga. These lithologies cannot be produced through the fusion of known pristine (plutonic) rock types, suggesting the occurrence of unknown lithologies within the Moon. These melts were probably generated at middle to lower crustal levels. Ejecta compositions, preservation of pre-basin topography, and deposit morphologies all indicate that the excavation cavity of multiring basins is between about 0.4 and 0.6 times the diameter of the apparent crater diameter. Basin depths of excavation can be inferred from the composition of basin ejecta; this evidence strongly suggests that basin excavation was limited to upper crustal levels and that effective excavation was from levels no deeper than about 0.1 times diameter of the excavation cavity. A variety of mechanisms has been proposed to account for the formation of basin rings but none of them are entirely plausible. Mechanisms can be divided into two broad groups: (1) forcible uplift due to fluidization of the target; (2) concentric, brittle, fracturing and failure of the target, on regional (megaterraces) to global scales (lithospheric fracturing). Most basin rings are spaced at a constant factor on all planets, namely the famous √2 relation, first observed between adjacent rings of the lunar Orientale basin. Because geological evidence supports divergent ring-forming models, it may be that the ring- locating mechanism is different from the ring- forming mechanism. Thus, large-scale crustal foundering (megaterracing) could occur along concentric zones of weakness created by some type of resonant wave mechanism (fluidization and uplift); such immediate crustal adjustment could then be followed by long-term adjustment of the fractured lithosphere.

On the Earth there is no firm evidence that impacts can induce volcanic activity. However, the Moon does provide a very likely example of volcanism induced by an immense impact: the Imbrium basin-forming event was immediately succeeded by a crustal partial melting event that released basalt flows characterized by K, rare-earth elements (REE), P, and other trace elements (KREEP) over a wide area creating the Apennine Bench Formation. Impact total melting is inconsistent with the chemistry and petrography of these Apollo 15 KREEP basalts, which are quite unlike the impact melts recognized at Taurus-Littrow as the products of the Serenitatis impact. The Imbrium impact and the KREEP volcanic events are indistinguishable in radiometric age, and thus the volcanism occurred less than about 20 Ma later than the impact (less than about 0.5% of lunar history). The sample record indicates that such KREEP volcanism had not occurred in the region prior to that time, and demonstrates that it never occurred again. Such coincidence in time implies a genetic relationship between the two events, and impact-induced partial melting or release appears to be the only feasible process. Nonetheless, the characteristics of the Apollo 15 KREEP basalts suggest large-degree crustal melting that is not easy to reconcile with the inability of lunar pressure release alone to induce partial melting unless the source was already almost at its melting point. The earliest history of the surface of the Earth, at a time of greater internal heat production and basin-forming impacts, could have been greatly influenced by impact-induced melting.

The inverse gravity (1/g) scaling trend for complex crater depths and the shallow depths reported from terrestrial impact structures led to the general expectation that depths of Venusian craters would be Earth-like and considerably shallower than those of the smaller planets like Mars. Pioneer Venus and Venera 15/16 radar observations seemed to indicate very shallow craters as expected; however, the present analysis shows that any crater depth information reported from these missions should be considered only as minimum bounds. I present new estimates of complex crater depths on Venus derived from cross-track distortions in Magellan radar images that reveal that the freshest craters approach those on Mars in terms of their depth/diameter characteristics. Consequently, strict 1/g scaling does not seem to hold for Venus, indicating that other planetary properties, such as the presence of an atmosphere, may have an overwhelming effect on crater depth. Craters with dark floors are distinctly shallower than fresh Venusian craters, indicating their originally bright crater floors probably have been covered by relatively smooth volcanic deposits. Topographic profiles across four complex craters reveal that these structures have rim heights accounting for as much as 50% of the total crater depth. Because there are no complex craters on Earth which have retained their rim crests, reported depths of terrestrial structures may be underestimated by a comparable amount due to rim removal alone. Venusian complex craters may be more useful for reconstructing the original appearance of eroded craters on Earth than either the lunar craters or the simple 1/g scaling previously used.

Seventy-two unequivocal peak-ring craters and four structures that are interpreted to be multiringed basins are identified on Venus from Earth-based Arecibo, Venera 15/16, and Magellan radar images. These ringed craters are relatively pristine, and so they serve as an important new data set that will further understanding of the mechanics of ringed crater formation as well as the structural and rheological properties of the Venusian surface. They are also the most direct analogues for craters formed on the Earth in Proterozoic and Phanerozoic time, such as Sudbury and Chicxulub. For peak-ring craters on Venus, crater-rim to inner peak-ring diameter, or ring, ratios decrease with increasing crater diameter; the ratios do not follow 2 spacing. The morphology of peak-ring craters, the decrease in ring ratios with increasing crater size, and the general size-morphology progression from complex central-peak to peak-ring crater, including some with both an inner ring and a central peak or peaks, on Venus and the other terrestrial planets suggest a similar process of peak-ring formation: hydrodynamic downward and outward collapse of an unstable central peak to form a ring. The four largest ringed structures on Venus—Klenova, Lise Meitner, Mead, and Isabella—are structurally and morphologically more similar to the Orientale Basin on the Moon, and are probably true multiringed basins. Although the four are smaller than Orientale we suggest that the higher gravity and temperature gradients on Venus, compared with that of the Moon when its basins formed, compensate for their smaller scale and allow a crustal or mantle asthenosphere to form and inward viscous flow to create substantial radial stress in the overlying lithosphere. This stress initiates circumferential normal faulting and outer ring formation.

Three of the principal variables in scaling impact-crater dimensions are the impact velocity, the projectile size, and the gravitational acceleration of the target body. The amount of impact melt generated by an impact, however, is independent of gravity, but will grow in direct proportion to the projectile dimensions and as an increasing function of the impact velocity. Thus, if the impact velocity and gravitational acceleration were held constant and projectiles of increasing size were considered, the amount of melt generated relative to the dimensions of the final crater would grow at a steady rate. Using the Earth and the Moon for comparison, this paper examines the effects of differential scaling on the depth of origin of central-peak material, on the amount of stratigraphic uplift associated with the formation of those peaks, and on the clast contents of impact melts. When craters of similar size are compared, central peaks should be derived from greater depths on Earth because of relatively deeper melting. The amount of stratigraphic uplift, however, should be greater on the Moon. A lunar crater will be larger than its terrestrial counterpart formed by an identical projectile, but the terrestrial crater will be accompanied by substantially more impact melt. As a large fraction of the melt would have lined the transient cavity during the excavation stage of the impact event, a greater fraction of the lunar melt will have been in contact with clastic materials on the cavity wall. Thus, the clast contents of lunar impact melts should be higher than in those in terrestrial craters of similar size.

The distinct appearance of the Crisium basin relative to other multiring basins on the Moon illustrates how two fundamental variations in the impact process at low impact angles can affect the formation of multiring impact structures. First, both laboratory cratering experiments and the planetary cratering record indicate that the shape and profile of a transient impact cavity depend on impact angle. Because the point of deepest penetration and excavation migrates uprange of the cavity center at low impact angles, the center of transient cavity collapse in basin-forming impacts also shifts uprange of the cavity center, thereby offsetting both the central mantle uplift and the centers of basin ring formation from the center of basin-filling volcanism. Second, peak shock pressures decrease at lower impact angles. Consequent variations in the distribution of dynamically displaced material around the transient cavity, therefore, may modify the nature of dynamic rebound during cavity collapse and reduce lithospheric failure outside the transient cavity at lower impact angles. Such reduced cavity collapse can account for both the low relief of the outer basin scarp and the topographically high, unusually broad massif ring at Crisium. Also if the reduction in cavity collapse limits isostatic equilibration of the basin cavity, basin-centered lithospheric stress fields during post-impact isostatic uplift may allow widespread early basin volcanism, further enhancing the structural variations in basin appearance as a function of impact angle.

This chapter is a short review of the experimental studies of the geochemical aspects of impact cratering, conducted at the Vernadsky Institute in collaboration with other institutes in Moscow and Saint Petersburg. The results of these studies have shown that the different elements and chemical compounds composing geologic materials have different volatilities. This makes impact-induced vaporization/condensation a possible factor in geochemical differentiation of planetary materials. The experiments show that fast impact-induced vaporization differs in many ways from slow equilibrium and quasi-equilibrium vaporization. The experiments also indicate that impact-induced vaporization produces materials with a compositional trend toward granite-like compositions. This may have implications for the generation of Early Precambrian granites on Earth.

A deep drill hole in the central mound of the Puchezh-Katunki impact structure, Russia, was finished last year. In this paper, an interpretation of the drilling data is presented using a simplified numerical two-dimensional simulation to outline the main processes of crater formation, including those that need to be investigated more thoroughly in the future. The observed shock pressure gradient in the drill core is consistent with the scaling-law predictions of the initial model impact parameters.

The low projectile component in tektites in contrast to the high projectile component in the Cretaceous-Tertiary (K-T) boundary clay has prompted a study of hypervelocity target-projectile mixing processes. Results from a 6.4-km/sec impact of a Fe-Ni-PGE alloy projectile (90% Fe) into a Mo target indicate that high-angle (55° to 75°), high-velocity (< 6 km/sec) melted ejecta is relatively projectile-rich, whereas low-angle (10° to 40°), low-velocity ejecta (< 1 km/sec) contains less projectile material and is more enriched in the target component. These results support theoretical predictions. Not predicted by theoretical calculation, but observed here, is a break in the compositional trend such that at angles of ejection between 50° and 70°, the projectile/target ratio in the melted ejecta decreases suddenly with increasing angle, only to rise to very high values at higher angles. It appears that for large-body terrestrial impacts, the composition of the high-angle, high-speed ejecta which reaches stratospheric heights will be critical to sudden changes in global climate and the induced environmental stresses. Application of these results to large impacts such as the K-T boundary event, are expected to provide new data pertinent to physical theories of extinction mechanisms.

The objective of this paper is to determine the thickness of the melt layer relative to the crater diameter for simple and complex craters. A numerical code was employed to calculate the amount of melting and the crater geometry. We used the code results and the scaling formalism of Holsapple and Schmidt (1987) to determine the scaling laws for the relative melt layer thickness. Simple crater dimensions are dominated by impact parameters and the planet’s strength, whereas complex crater dimensions are dominated by planetary gravity, strength, and the impact parameters. The volume of melt is proportional to impact energy for impact velocities and melt enthalpies of interest to planetary science. Crater geometry and dimensions scale with an exponent, μ, which is intermediate between momentum (μ = 1/3) and energy (μ = 2/3) scaling. For simple craters, the melt layer thickness/crater diameter, T / D , for a given planetary surface (constant melt enthalpy and mean impact velocity), is independent of the crater size. For complex craters, T / D , for a given planetary surface (constant melt enthalpy, impact velocity, and gravitational acceleration), increases with the size of the crater. For simple craters, at a fixed size, the relative melt layer thickness, T / D , increases slowly with increasing impact velocity, U, according to ∝ U 0.1 ), whereas, for complex craters (∝ U 0.22 ).

Evidence from South China and Western Australia for a Late Devonian extraterrestrial impact in the conodont Early crepida Zone (∼365 Ma ago) of the Famennian stage includes microtektites and elemental anomalies (including Ir) in Hunan, South China, and a strong Ir anomaly with a meteoritic Ru/Ir ratio in the Canning Basin, Western Australia. The temporary disappearance of the benthic community coupled with a brachiopod faunal turnover and a drastic change of the carbonate facies in South China, and the coeval “Strangelove Ocean” δ 13 C excursions in both Chinese and Australian sections indicate that at least a regional-scale (possibly global) extinction may have occurred in eastern Gondwana at the time of the impact. The difference in the abundances of Ir between the Canning Basin and Hunan is probably caused by the presence in the former and absence in the latter of the stromatolite Frutexites which may have concentrated the Ir. The presence of microtektites in Hunan is probably due to its more proximal location to the impact site and local diagenetic conditions that allowed the preservation of tektite glass. This Late Devonian impact event is a separate event which postdates the Frasnian/Famennian (F/F) mass extinction by about 1.5 to 2 Ma. If the F/F extinction was also caused by extraterrestrial impacts, it appears that the Late Devonian was a time of closely spaced extraterrestrial impacts which may have triggered at least two extinctions.

Diaplectic changes in minerals result from passage of high-pressure shock waves through rocks. These changes are an important source of information concerning impact processes. To the author’s knowledge, the decomposition of mafic minerals due to shock has not been the subject of detailed studies until recently. Aggregates that form as the result of such changes of mafic minerals include feldspar (alkali feldspar or plagioclase, more rarely both of them), orthopyroxene with elevated alumina content, a mineral of the spinel group (magnetite or ilmenite or hercynite), amphibole, and clinopyroxene. The study of biotite, staurolite, garnet, amphibole, and clinopyroxene in impactites of the Janisjarvi, Popigay and Puchezh-Katunky astroblemes shows that new minerals are formed during selective melting. During such a process the rock, as a whole, remains solid. Recrystallization is practically instantaneous, as the secondary aggregates are very Fine grained (a few micrometers in size). It has been established, by using a scanning electron microscope, microprobe analyses, and the Mössbauer spectroscopy, that this process was accompanied by intensive shock diffusion of material, resulting in an exchange of chemical components between new minerals and the host rocks. This process is accompanied by an increase in oxygen activity and iron oxidation. Superheated vapor-water fluid is very important in this process. The temperature of crystallization varies considerably from one mineral to another and within one grain the temperature may have exceeded the average residual temperature of the whole specimen.

Tektites are natural glasses that occur on earth in four distinct strewn fields (North American, Central European, Ivory Coast, and Australasian). Geochemical arguments have shown that tektites have been derived by hypervelocity impact melting from terrestrial upper crustal rocks, most likely sediments. The contents of Be-10 in tektites are evidence for a derivation of tektites from surface rocks, thus precluding an origin from greater depth in the crater. For two of the four tektite strewn fields (Ivory Coast, Central European), a possible connection to impact craters (Bosumtwi, and Ries, respectively) has been suggested on the basis of chemical, isotopic, and age data. No clear crater identifications have been made for the North American or Australasian strewn fields, although there are good candidates for both. Even though the geochemistry of tektites is in unequivocal favor of an origin by impact melting of terrestrial rocks, the unambiguous demonstration of the presence of an extraterrestrial contribution to the chemistry of tektites remains a problem. However, recent osmium isotope studies have shown that there is a clear meteoritic signature in at least some tektites. The exact mechanism of tektite formation is still not obv3ious, although some facts become increasingly clear. Tektite production requires specific impact conditions—otherwise there would be many more tektite strewn fields connected to the 150 or so known impact craters. Tektites are produced by nonequilibrium shock melting of surficial rocks, and the superheated melt may be subjected to a plasma phase during which they are subjected to partial reduction. They are then lofted through the atmosphere (probably in the wake of the expanding vapor cloud), quenched, and distributed over a geographically extended area—the strewn field. Some tektites solidify in a near-vacuum and re-enter the atmosphere. During the re-entry they melt again and form ablation-shaped tektites. Larger tektites, from a lower part of the target stratigraphy, are only distributed closer to the source crater. Many of them are more inhomogeneous melts and show a layered structure; they are called Muong Nong–type tektites. The study of tektites and the identification of possible new strewn fields provide important contributions toward the understanding of impact cratering.

Impactites (suevites, tagamites, impact glasses) are widespread in the Popigai impact crater. The continuous cover of suevite overlying allogenic polymict breccia is more than 1,000 m thick in the central part of the crater. Suevites also occur in form of lenses or irregular bodies within the allogenic breccia. Tagamites crop out predominantly in the western sector of the crater. Most occur as sheet-like bodies, up to 600 m thick, overlying allogenic breccia and as small irregular bodies in suevites. Commonly tagamites have a complex inner structure with distinct horizontal zones distinguished mainly by crystallinity and fragment content. Differentiation of impact melt bodies was not observed. Tagamites and suevites are similar in terms of chemical composition and petrographie characteristics: they correspond to the composition of biotite-garnet gneisses of the crystalline basement. The distribution of Ir and Ni indicates that the impact melt was well homogenized. Concentric zonation in SiO 2 , MgO, Na 2 O distribution and the band-like distribution of other components testify to an inherited compositional heterogeneity in target rocks within the melting zone. The content of high-pressure carbon phases in these impactites varies radially and reflects shock transformation of former graphite and radial ejection of melt. Detailed exploration of impactite bodies and their composition point to complex processes in melt-forming and rock-forming petrogenetic systems vis-à-vis the impact process.

Dikes and pods of pseudotachylite up to one meter thick have been found over an area >50 km 2 in the same area as shatter cones and other possible features of shock metamorphism in the Beaverhead and Tendoy Mountains in southwestern Montana, defining the allochthonous remains of the Beaverhead impact structure (see also Hargraves et al., Chapter 19, this volume). They are not associated with any tectonic feature in the area and have several features uncommon in pseudotachylites formed by tectonic processes (large size, vesicles, pseudotachylite clasts within pseudotachylite), but which have been documented in pseudotachylites from other impact structures. Rare single sets of planar deformation features (PDFs) are found in quartz grains in the pseudotachylites with crystallographic orientations similar to those found in shocked quartz from other impact structures. The major and trace element chemistry of the pseudotachylites is similar to their host rocks, but with some enrichments (Al, Mg, Fe, K, volatiles) and depletions (Si, Na) indicating low-grade metasomatic alteration. 40 Ar/ 39 Ar laser microprobe analyses of pseudotachylites from three localities show a wide spectrum of ages, from Precambrian to Tertiary. The distribution of ages suggests two isotopic signatures, one of Precambrian age (although younger than the age of the protoliths), and the other of Cretaceous age. 29 Si Magic angle spinning (MAS) nuclear magnetic resonance (NMR) studies of the pseudotachylite failed to detect the presence of high-pressure polymorphs of quartz. The evidence suggests that these pseudotachylites were formed by the same event that formed the shatter cones. Related work (see Hargraves et al., Chapter 19, this volume) suggests that the original crater was at least 75 km in diameter and was formed in the late Proterozoic to Cambrian, 20 to 150 km to the west of the present location.

The Vredefort Structure is the type locality for pseudotachylite, a type of clast-laden melt breccia often observed in tectonic zones of brittle or brittle-ductile deformation and generally believed to be the result of frictional melting. Pseudotachylitic breccia is also abundant in the Sudbury Structure and the Roter Kamm impact crater, as well as in the northern part of the Witwatersrand Basin surrounding the Vredefort Dome. In order to facilitate comparison between pseudotachylites from Vredefort and from impact structures and tectonic settings, the existing data base is reviewed and recent field and laboratory observations summarized. Modes of breccia occurrences, petrographic appearance, the distribution and orientation data for Vredefort and Witwatersrand pseudotachylite, as well as the complex temporal relationships between Vredefort pseudotachylite and other deformation phenomena in the structure are reviewed. Geometrically, Vredefort pseudotachylites are generally similar to those in tectonic settings. Comparison of chemical compositions of pseudotachylite and host rock pairs leads to the conclusion that preferential melting of certain minerals (first hydrous ferromagnesian minerals, then feldspar minerals and some quartz) is involved in pseudotachylite formation. This process has also been recognized for the formation of tectonic pseudotachylites and pseudotachylite from the Rochechouart impact structure. The Ar chronological record and field observations for Vredefort and Witwatersrand pseudotachylite strongly suggest multiple pseudotachylite-forming events in the Vredefort Dome. Further quantitative mineralogical, chronological, and structural studies are needed to allow full comparison of—and perhaps discrimination between—pseudotachylites from different geological environments and to fully understand the role that pseudotachylite formation played during the extended evolution of the region of the Witwatersrand Basin.

Roter Kamm is a relatively young 2.5-km-diameter impact crater located in the southern Namib desert. The rocks of the crater rim are fractured and cut by numerous fine-grained, clast-rich veins and dykes that closely resemble pseudotachylytes in the field. Most veins closely match their host rocks in bulk composition and have formed by localized cataclasis of the host rock followed in many cases by partial re-crystallization that has partly obliterated the cataclastic texture in the finest grain size fraction. This thermal annealing is most prominent in the very narrow veinlets, in the margins of the larger veins, and in the most quartz-rich veins. The spatial distribution of the veins and the association of veins in quartz breccias with quartz showing planar elements are consistent with an impact origin. The similarities to pseudotachylyte occurrences at other impact sites, including the type locality at Vredefort, warrants the classification of these veins as pseudotachylytes, despite the absence of evidence for melting.

Acraman, located in the Gawler Range Volcanics (1592 ± 2 Ma), South Australia, is Australia’s largest known impact structure and the evident source of an ejecta horizon containing shocked volcanic fragments within the Neoproterozoic (about 590 Ma) Bunyeroo Formation 220 to 350 km east of Acraman and in coeval shales 470 km northwest of Acraman. The deeply eroded structure comprises a central uplift area at least 10 km across marked by sparse outcrops of intensely shattered Yardea Dacite, within an inner topographic depression about 30 km in diameter containing the Lake Acraman salina. Rocks of the central uplift exhibit shatter cones and multiple sets of shock lamellae in quartz grains that indicate shock pressures of up to 15 GPa. An apparent ring structure occurs at 85 to 90 km diameter and arcuate surface features are evident at about 150 km diameter. A dike of melt rock in the central uplift area consists mainly of laths of albite and scattered grains of titanomagnetite in a matrix of K-feldspar and finely inter-grown quartz. Both the Na- and K-feldspar phases have virtually pure end-member compositions (Ab 99 , Or 98 ), and evidently are low-temperature authigenic phases that formed by secondary alteration of the melt rock. Matrix K-feldspar and quartz may be devitrification products of glassy material. The melt rock is enriched in potassium but is not anomalous in cosmogenic siderophile elements. A negative gravity anomaly of about 6 mGal amplitude and 30 to 35 km in diameter is centered on the inner topographic depression. This depression is marked also by a subdued aeromagnetic signature, and a dipolar aeromagnetic anomaly indicates that a shallow (approximately 300 m depth) magnetic source occurs in the central uplift area. Paleomagnetic study of the melt rock indicates a stable remanent magnetization and a virtual geomagnetic pole that agrees closely with the paleomagnetic pole determined previously for the Bunyeroo Formation. Apatite fission track analyses and estimated rates of erosion together suggest that as much as 2 km thickness or more of overlying rocks has been eroded from the Acraman region since the impact at about 590 Ma. The transient cavity and excavated area thus may have been as much as 40 km in diameter, 30% greater than the diameter of strongly disrupted bedrock at the present level of erosion. The limits of the final structural rim after gravitational slumping of the crater walls may be marked by the apparent ring feature at 85 to 90 km diameter. Diameters of the uneroded structural features—central uplift, excavated area, and possible final structural rim—therefore may have been ≥10 km, about 40 km, and 85 to 90 km, respectively. Arcurate features at about 150 km diameter may be faults or fractures marking the outer limit of disturbance. Acraman could have been formed by impact with an Earth-crossing chondritic asteroid estimated to be 4.7 km in diameter and of density 3500 kg/m 3 moving at 25 km/s, with kinetic energy of 6 × 10 22 J.

Upward-pointing shatter cones in sandstones of uncertain age (Middle Proterozoic? to Lower Cambrian?) and older crystalline basement rocks are exposed over an area of approximately 25 × 8 km in southwestern Montana. These shatter cones, together with pseudotachylites and breccias of various types (particularly in basement gneisses), are inferred to be products of a meteorite or cometary impact. However, Late Cretaceous contraction and Tertiary extension have contributed to the structural complexity of the area, and distinguishing unequivocally the shock brecciation from that due to younger tectonism is difficult. Stratigraphic constraints suggest the structure is Late Proterozoic or Cambrian in age. The shocked rocks are present in the Cabin thrust plate—one of many in the Late Cretaceous Cordilleran Thrust belt—and hence are allochthonous, having been transported tens of kilometers from the west. They are considered to represent only a piece from the central uplift of an original complex crater at least 75 km in diameter. It is speculated that some of the considerable uplift and erosion inferred to have taken place in Late Proterozoic to early Paleozoic time in east-central Idaho (The Lemhi arch) may be related to the postulated impact event. Furthermore, quasi-circular magnetic and regional gravity anomalies (50 to 75 km diameter) centered south-southeast of Challis, Idaho, may mark the concealed scar of the original impact structure.