Meteorite Impact Structure Research Articles

Integrated interpretation of physical properties of rocks of the borehole Yaxcopoil‐1 (Chicxulub impact structure)

The borehole Yaxcopoil‐1, drilled within the Chicxulub meteoritic impact structure (Mexico), was completely cored from 404 to 1511 m through postimpact Tertiary limestones underlain by impactites. The impactites comprise impact melt‐rich, suevitic breccia followed by megablocks of Cretaceous limestones, calcarenites, dolomites, and anhydrites. Measurements of porosity, density, and thermal parameters on 450 samples (equidistant sampling, complete depth range) and of ultrasonic velocities and electric resistivity on 80 representative samples are used to investigate the physical properties of carbonate rocks and to study the influence of the impact. Experiments under elevated pressure, calculations using frequency‐dependent Biot‐Gassmann theory, and cross‐checking with borehole logs, where available, show that ultrasonic laboratory and sonic in situ data correspond. Sonic and electric quasi‐continuous logs are obtained from empirical correlations with thermal conductivity, density, and porosity and consideration of mineralogical composition and microstructure. These data give constraints on interpretation and geophysical modeling of, e.g., seismic and gravity data. In the Tertiary postimpact limestone section, the rock fabric (porosity) influences the physical properties. The upper boundary of the impactites is distinctly determined by the high inhomogeneity factor and anisotropy coefficient of thermal conductivity and by the temperature gradient from high‐resolution borehole temperature measurements. All physical properties indicate that the upper part of the suevitic breccia can be distinguished from the lower suevite unit. In the Cretaceous megablocks, a high variability of all properties (particularly, thermal conductivity, density of solid material, and temperature gradient) due to the high variability in the mineral composition (calcite, dolomite, anhydrite) is observed.

Irradiation-induced stabilization of zircon (ZrSiO 4) at high pressure

Abstract Zircon (ZrSiO 4), the most important accessory mineral in the Earth's crust, transforms under high pressure to reidite, a scheelite-structured polymorph. Recently, reidite was found in association with meteorite impact structures. Here, we show that the zircon-to-reidite transition, and thus the amount of reidite produced during high-pressure events, strongly depends on the microstructure of the initial zircon. Our results clearly demonstrate that radiation damage, present in natural zircon due to radioactive decay, dramatically modifies the phase stability of crystalline zircon at high pressure. By simulating this radiation damage with ion beams, we show that zircon, pre-irradiated with 1.47-GeV Xe ions, formed only minor amounts of reidite up to 36GPa; whereas, an unirradiated zircon was almost completely transformed to reidite under the same conditions. By means of Raman scattering, X-ray diffraction, and transmission electron microscopy, we confirmed that the stability field of the irradiated zircon is expanded to higher pressures as a result of the interplay between pressure, ion beam-induced amorphization, and the formation of nanoscale damage domains. These results provide insight into the formation-conditions of reidite in nature and illustrate how pressure-induced phase transitions may be affected by defects, in this case those caused by radioactive decay.

Meteorite impact structures: the good and the bad

Meteorite impact structures are found on all planetary bodies in the Solar System with a solid surface. On many planets, impact craters are the dominant landform. Earth's active geology, however, tends to rapidly erase impact structures from the geological record, although we know currently of 174 confirmed impact sites. Impact events are destructive and have been linked to at least one of the ‘big five’ mass extinctions over the past 540 Ma. But they also provide certain economic benefits, including the formation of metalliferous ore deposits and hydrocarbon reservoirs. Impact structures can also form new biological niches, which can provide favourable conditions for the survival and evolution of life. Despite this, it was only in the past 40 years that the importance of impact cratering as a geological process was recognized and only during the past 15–20 years that the study of meteorite impact structures has moved into the geological mainstream. There is, therefore, still considerable potential for new and exciting advancements.

The Lake Bosumtwi meteorite impact structure, Ghana—Where is the magnetic source?

Abstract— The Bosumtwi impact structure (Ghana) is a young and well‐preserved structure where a vast amount of information is available to constrain any geophysical model. Previous analysis of the airborne magnetic data and results of numerical simulation of impact predicted a strongly magnetic impact‐melt body underneath the lake. Recent drilling through the structure did not penetrate such an expected impact‐melt rock magnetic source. A new 3‐D magnetic model for the structure was constructed based on a newly acquired higher‐resolution marine magnetic data set, with consideration of the observed gravity data on the lake, previous seismic models, and the magnetic properties and lithology identified in the two International Continental Scientific Drilling Program (ICDP) deep boreholes. The new model contains highly magnetic bodies located in the northeast sector of the structure, not centered onto the drilling sites. As in previous models, higher magnetization than that measured in outcropping impactites had to be assigned to the unexposed source bodies. Integration of the new model with the borehole petrophysics and published geology indicates that these bodies likely correspond to an extension to the south of the Kumasi batholith, which outcrops to the northeast of the structure. The possibility that these source bodies are related to the seismically identified central uplift or to an unmapped impact‐melt sheet predicted by previous models of the structure is not supported. Detailed magnetic scanning of the Kumasi batholith to the north, and the Bansu intrusion to the south, would provide a test for this interpretation.

Physical property measurements: ICDP boreholes LB‐07A and LB‐08A, Lake Bosumtwi impact structure, Ghana

Abstract— Physical rock property measurements provide the primary constraints for any geological models hypothesized from geophysical observations. Previous geophysical models of the Bosumtwi impact structure hypothesized that a highly magnetic and dense impact‐melt sheet might be the source of the observed magnetic anomalies. However, magnetic susceptibility and density measurements made on International Continental Scientific Drilling Program (ICDP) cores LB‐07A and LB‐08A from the interior of the Bosumtwi meteorite impact structure contain no evidence for that. Both density and magnetic susceptibility logs on both boreholes exhibit low‐amplitude contrasts between the uppermost polymict lithic breccia and suevite, the intermediate monomict lithic breccia, and the lowermost bedrock. The depth extent of fracture‐related density reduction is much greater at LB‐08A than at LB‐07A. A total magnetic intensity log from borehole LB‐08A supports the suggestion that magnetic anomalies over Lake Bosumtwi are mainly sourced in undetected and/or covered bedrock intrusions, like the ones outcropping at the northeast and to the southwest of the lake.

The Lake Bosumtwi meteorite impact structure, Ghana—A magnetic image from a third observational level

Abstract— The Bosumtwi impact structure in Ghana is the youngest and best‐preserved medium‐sized impact structure on Earth, and because of the vast amount of prior geophysical and geological data gathered in the area, it constitutes a great natural laboratory to try to develop new geophysical interpretation and modeling techniques.During the 2004 International Continental Scientific Drilling Program (ICDP) drilling campaign at Lake Bosumtwi, we made magnetic field observations at 162 stations around the lake. This study differs from all previous magnetic surveys at Bosumtwi, which only measured the scalar portion of the Earth's magnetic field, in that we measured the full magnetic vector at each station. Acquisition of the full magnetic vector was made possible by innovative use of a borehole deviation probe, which uses a magnetic sensor for absolute orientation reference. Estimates of the magnetic vector orientation and magnitude at each observation station were derived from a series of measurements collected at 50 cm spacing over a depth range of 25 m.In this study, we report a comparison between the scalar total field intensity derived from this new survey approach with the other two previously acquired marine and airborne magnetic data sets. The scalar total magnetic intensity (TMI) computed from the vector data set compares in close agreement with the other two data sets. Some discrepancies between the data sets can be explained by differences in the distances between the sensor and the magnetic sources for the various surveys. The highlight of this study is that we demonstrate that is possible to acquire at least partial vector data with readily available instrumentation.

New observations on shatter cones in the Vredefort impact structure, South Africa, and evaluation of current hypotheses for shatter cone formation

Abstract— Shatter cones have been described from many meteorite impact structures and are widely regarded as a diagnostic macroscopic recognition feature for impact. However, the origin of this meso‐ to macroscopic striated fracture phenomenon has not yet been satisfactorily resolved, and the timing of shatter cone formation in the cratering process still remains enigmatic. Here, previous results from studies of shatter cones from the Vredefort impact structure and other impact structures are discussed in the light of new field observations made in the Vredefort Dome. Contrary to earlier claims, Vredefort cone fractures do not show uniform apex orientations at any given outcrop, nor do small cones show a pattern consistent with the previously postulated “master cone” concept. Simple back‐rotation of impact‐rotated strata to a horizontal pre‐impact position also does not lead to a uniform centripetal‐upward orientation of the cone apices. Striation patterns on the cone surfaces are variable, ranging from the typically diverging pattern branching off the cone apex to subparallel‐to‐parallel patterns on almost flat surfaces. Striation angles on shatter cones do not increase with distance from the center of the dome, as alleged in the literature. Instead, a range of striation angles is measured on individual shatter cones from a specific outcrop. New observations on small‐scale structures in the collar around the Vredefort Dome confirm the relationship of shatter cones with subparallel sets of curviplanar fractures (so‐called multipli‐striated joint sets, MSJS). Pervasive, meter‐scale tensile fractures cross‐cut shatter cones and appear to have formed after the closely spaced MSJ‐type fractures.The results of this study indicate that none of the existing hypotheses for the formation of shatter cones are currently able to adequately explain all characteristics of this fracturing phenomenon. Therefore, we favor a combination of aspects of different hypotheses that includes the interaction of elastic waves, as supported by numerical modeling results and which reasonably explains the variety of shatter cone shapes, the range of striation geometries and angles, and the relationship of closely spaced fracture systems with the striated surfaces. In the light of the currently available theoretical basis for the formation of shatter cones, the results of this investigation lead to the conclusion that shatter cones are tensile fractures and might have formed during shock unloading, after the passage of the shock wave through the target rocks.

Late Quaternary seismo-stratigraphy of Lake Wanapitei, Sudbury, Ontario, Canada: Arguments for a possible meteorite impact origin

Lake Wanapitei (132.75 km 2) fills what has been identified as an Eocene (c. 37 Ma) meteorite impact basin in the Canadian Shield near Sudbury, Ontario, Canada. The area was glaciated many times during the Pleistocene and the basin lies immediately north of the prominent Cartier Moraine built during the last glaciation by the retreating Laurentide Ice Sheet some 11,000 years ago. Study of the deeper geophysics of the basin using magnetic and gravity techniques, and confirmation of its origin, is hampered by lack of data regarding water depths, the form of the bedrock surface and the thickness and character of glacial and postglacial sediment. To this end, more than 300 km of high-resolution single channel seismic chirp and 200 kHz bathymetric data were collected from the basin in the summer of 2002. Water depths reach a maximum of 118 m and acoustic basement is defined by a glacially scoured bedrock surface. The overlying Pleistocene sediment fill exceeds 35 m in thickness and consists of a lowermost late-glacial succession of rhythmically laminated silty clays deposited when the basin was flooded by a deep and regionally extensive ice dammed water body (Glacial Lake Algonquin). Truncation of the upper surface of this succession across large parts of the lake floor records the drainage of Lake Algonquin and the isolation of Wanapitei Lake as a separate water body. Overlying Holocene sediment is up to 10 m thick but is markedly discontinuous and commonly occurs as mounded ‘drifts’ reflecting strong bottom currents and low inputs of modern sediment. The presence of apparently undisturbed Precambrian bedrock below large portions of the lake basin places significant constraints on the dimensions of any meteorite impact structure.

Applications of twin analysis to studying meteorite impact structures

This paper describes a technique, twin analysis of dolomite and calcite, which was used to estimate the level of erosion of an impact crater. If the age of impact is known, the level of erosion gives the amount of sediments present at that time. The estimate of the level of erosion, and the presence or absence of tectonic strains in calcite constrains the age of impact. This technique might also be useful in identifying deeply eroded impact structures. Twin analysis gives the greatest shortening direction, ε 1. Tilt corrected ε 1 directions converge at or above the present day surface, if a shallow gas explosion or meteorite impact explains the structure, rather than a deep gas or cryptovolcanic explosion. The energy density recorded by twinning distinguishes an impact from a shallow gas explosion for structures ≤ 100 m diameter. At Serpent Mound, a well-established impact structure, ε 1 directions converge at 1400 ± 390 m (95% confidence interval of the mean) above the present day structure and the sample closest to the center of the structure records an energy density of 1.7 × 10 7 J/m 3. These results are consistent with previous studies showing that Serpent Mound is an impact (e.g., [R.W. Carlton, C. Koeberl, M.T. Baranoski, G.A. Schumacker, Discovery of microscopic evidence for shock metamorphism at the Serpent Mound structure, south–central Ohio: confirmation of an origin by impact, Earth and Planet. Sci. Lett. 162 (1998) 177–185]). The level of erosion and other data suggests that Serpent Mound formed between 290 and 256 Ma and was originally 8.5 to 11 km in diameter. Dolomite twinning is not observed at Serpent Mound suggesting that this technique may only be useful for studying larger, more deeply eroded structures.

Emplacement of the Whistle Dike, the Whistle Embayment and Hosted Sulfides, Sudbury Impact Structure, Based on Anisotropies of Magnetic Susceptibility and Magnetic Remanence

The Sudbury impact structure is one of the few multi-ring meteorite impact structures recognized on Earth. The meteorite impact occurred ca. 1850 Ma, resulting in the production of impact melts derived from the crust, possibly with a mantle component. The impact melts formed the Sudbury Igneous Complex and also were emplaced as radial and concentric dikes within the target rocks. The Whistle dike, one of the radial “offset” dikes in the Sudbury impact structure, is connected to the Sudbury Igneous Complex by the Whistle embayment structure, which is composed of noritic cumulates. We present a study of the emplacement flow patterns in the Whistle dike and the embayment, using magnetic anisotropy methods to measure the petrofabrics that record emplacement. Petrographic and scanning electron microscope observations, combined with thermal demagnetization measurements, and tests of the acquisition and demagnetization of anhysteretic magnetic remanence indicate that the primary sources of the magnetic signals in the sample suite are coarse-grained, soft coercivity magnetite and pyrrhotite that are interpreted to be primary igneous phases. Some samples also contain hard coercivity phases of possible metamorphic origin. The anisotropy of magnetic susceptibility (AMS), the anisotropy of an-hysteretic remanent magnetization (AARM) and the anisotropy of partial anhysteretic remanent magnetization (ApARM) were measured for the samples collected at 20 sites within the Whistle dike and the embayment structure, including an exposed chalcopyrite-rich massive sulfide body. The AMS method provided well-defined, interpretable fabrics for many of the sample sites. However, the AMS and AARM methods were unsuccessful in providing interpretable fabric measurements at some sample sites due to contributions from the hard coercivity ferromagnetic minerals. For those samples, the ApARM method was successful in isolating the sub-fabric defined by the primary magnetite ± pyrrhotite crystals. The fabric maps are consistent with lateral (horizontal) injection of the Whistle offset dike and subsequent sinking of molten massive sulfides within the still ductile dike. Sinking of the massive sulfides is recorded by steeply plunging magnetic lineations. The magnetic fabrics in the cumulate norites of the embayment structure show that the norites were deposited in a dynamic environment that resulted in a well-oriented lineation, which may record flow of gravity or thermal currents, channelled along the embayment axis. In the embayment, the lineations plunge toward a massive sulfide body, suggesting the flow that led to the formation of the lineation may also have led to emplacement of the massive sulfide ore. It is suggested that steeply plunging lineations elsewhere in the Whistle offset dike, and possibly in other Sudbury offset dikes, might indicate the trails of sunken massive sulfides in the subsurface. It is also suggested that magnetic fabrics might be useful to locate massive sulfide bodies hosted in embayment structures.

Skeletonization of aeromagnetic data

Skeletonization is a syntactic pattern‐recognition method that is applied to gridded data to produce an automatic line drawing, with an associated event catalog. Previous implementations of skeletonization have been tailored for seismic data. Here, we modify that technique to render it more suitable for other types of gridded data, with particular emphasis on aeromagnetic maps. A modification from previous schemes is the use of a two‐pass approach, to reduce the effects of an otherwise problematic directional bias that discriminates against events oriented parallel to columns of the grid. The method can be used effectively for filtering aeromagnetic data on the basis of strike direction, event linearity, event amplitude, and polarity. It is based on the delineation of peak‐trough pairs (cycles), which are traced throughout the grid to form contiguous events. Cycles and events are characterized by attributes that include amplitude, polarity, and pulse width. Events are further characterized by length, average strike direction, and linearity. The event attributes are stored in a catalog, thus enabling one to perform attribute‐based analysis and data filtering. We illustrate our algorithm using two regional aeromagnetic examples from different parts of the Canadian Shield. The first, from the Great Slave Lake shear zone, is dominated by linear anomaly trends produced by faults and mafic dikes. The second, from the Manicouagan region of northeastern Quebec, contains abundant subcircular and arcuate anomaly patterns caused by large intrusive complexes and a meteorite impact structure.

The Rock Elm meteorite impact structure, Wisconsin: Geology and shock-metamorphic effects in quartz

The Rock Elm structure in southwest Wisconsin is an anomalous circular area of highly deformed rocks, ∼6.5 km in diameter, located in a region of virtually horizontal undeformed sedimentary rocks. Shock-produced planar microstructures (PMs) have been identified in quartz grains in several lithologies associated with the structure: sandstones, quartzite pebbles, and breccia. Two distinct types of PMs are pres ent: P1 features, which appear identical to planar fractures (PFs or cleavage), and P2 features, which are interpreted as possible incipient planar deformation features (PDFs). The latter are uniquely produced by the shock waves associated with meteorite impact events. Both types of PMs are oriented parallel to specific crystallographic planes in the quartz, most commonly to c (0001), ξ{11\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\overline{2}\) \end{document}2}, and r / z {10\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\overline{1}\) \end{document}1}. The association of unusual, structurally deformed strata with distinct shock-produced microdeformation features in their quartz-bearing rocks establishes Rock Elm as a meteorite impact structure and supports the view that the presence of multiple parallel cleavages in quartz may be used independently as a criterion for meteorite impact. Preliminary paleontological studies indicate a minimum age of Middle Ordovician for the Rock Elm structure. A similar age estimate (450–400 Ma) is obtained independently by combining the results of studies of the general morphology of complex impact structures with estimated rates of sedimentation for the region. Such methods may be applicable to dating other old and deeply eroded impact structures formed in sedimentary target rocks.

Jackpine Creek magnetic anomaly: Identification of a buried meteorite impact structure

The meteorite impact process is quite a remarkable natural phenomenon. When it occurs in a sedimentary basin, it can create a trap-generating structure (rim and central uplift) and reservoir rocks (ejecta piles and breccia infill), both at the same time.

Kgagodi Basin: The first impact structure recognized in Botswana

Abstract— The 3.4 km wide, so‐called Kgagodi Basin structure, which is centered at longitude 27°34.4′ E and latitude 22°28.6′ S in eastern Botswana, has been confirmed as a meteorite impact structure. This crater structure was first recognized through geophysical analysis; now, we confirm its impact origin by the recognition of shock metamorphosed material in samples from a drill core obtained close to the crater rim. The structure formed in Archean granitoid basement overlain and intruded by Karoo dolerite. The crater yielded a gravity model consistent with a simple bowl‐shape crater form. The drill core extends to a depth of 274 m and comprises crater fill sediments to a depth of 158 m. Impact breccia was recovered only between 158 and 165 m depth, below which locally brecciated basement granitoids grade into fractured and eventually undeformed crystalline basement, from ∼250 m depth. Shock metamorphic effects were only found in granitoid clasts in the narrow breccia zone. This breccia is classified as suevitic impact breccia due to the presence of melt and glass fragments, at a very small abundance. The shocked grains are exclusively derived from granitoid target material. Shock effects include multiple sets of planar deformation features in quartz and feldspar; diaplectic quartz, and partially and completely isotropized felsic minerals, and rare melt fragments were encountered. Abundances of some siderophile elements and especially, Ir, in suevitic breccia samples are significantly elevated compared to the contents in the target rocks, which provides evidence for the presence of a small meteoritic component.Kgagodi is the first impact structure recognized in the region of the Kalahari Desert in southern Africa. Based on lithological and first palynological evidence, the age of the Kgagodi structure is tentatively assigned to the upper Cretaceous to early Tertiary interval. Thus, the crater fill has the potential to provide a long record of paleoclimatic conditions.

The Bosumtwi meteorite impact structure, Ghana: A magnetic model

Abstract— A magnetic model is proposed for the Bosumtwi meteorite impact structure in Ghana, Africa. This relatively young (∼1.07 Ma) structure with a diameter of ∼10.5 km is exposed within early Proterozoic Birimian—Tarkwaian rocks. The central part of the structure is buried under postimpact lake sediments, and because of lack of drill cores, geophysics is the only way to reveal its internal structure. To study the structure below and beyond the lake, a high‐resolution, low altitude (∼70 m) airborne geophysical survey across the structure was conducted, which included measurements of the total magnetic field, electromagnetic data, and gamma radiation. The magnetic data show a circumferential magnetic halo outside the lakeshore, ∼12 km in diameter. The central‐north part of the lake reveals a central negative magnetic anomaly with smaller positive side‐anomalies north and south of it, which is typical for magnetized bodies at shallow latitudes. A few weaker negative magnetic anomalies exist in the eastern and western part of the lake. Together with the northern one, they seem to encircle a central uplift. Our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact‐melt rock or melt‐rich suevite bodies.Petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites. Suevites have a higher magnetization and have low densities and high porosities compared to the target rocks. In suevites, the remanent magnetization dominates over induced magnetization (Koenigsberger ratio > 3). Preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the Jaramillo normal polarity epoch. This interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake. The reverse polarity remanence component, superimposed on the normal component, is probably acquired during subsequent reverse polarity events.

Traces of Catastrophe: A Handbook of Shock‐Metamorphic Features in Terrestrial Meteorite Impact Structures

Once the subject of scientific curiosity by an elite few, the study of impact events and their consequences is receiving greater attention by the larger scientific community and the public. Among the reasons for this in‐creased sensitivity are the scientific debate over the involvement of a major impact event at the Cretaceous‐Tertiary boundary, the witnessing of the impact of the fragments of comet Shoemaker‐Levey 9 on Jupiter in 1994, and the accompanying spate of made‐for‐television and Hollywood blockbuster disaster movies involving the potential demise of humanity because of a wayward asteroid or comet. As noted, however, by the author of Traces of Catastrophe, despite the growing importance of meteorite impact phenomena in terrestrial geology, the topic is not widely addressed in general geoscience textbooks. Although a large body of literature exists, it is, in general, specialized and scattered.The book, subtitled A Handbook of Shock‐Metamorphic Features in Terrestrial Meteorite Impact Structures, is an attempt to fill this gap. It is not intended for the specialist but rather for general geoscientists and academics. It is divided into eight chapters of which only three deal exclusively with shock metamorphic features.The others provide background and details of such topics as cratering mechanics, impact flux, and crater morphology. Each chapter is designed to stand alone. Technical terms are highlighted when they are first defined in the text. The text is simply written and very readable. It is well illustrated, mostly with photographs of shocked lithologies and microscopic thin sections. At the back of the book is an extensive reference list for the reader who wishes to delve further into this fascinating subject.

Traces of catastrophe: a handbook of shock-metamorphic effects in terrestrial meteorite impact structures

This handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures emphasizes terrestrial impact structures, field geology, and particularly the recognition and petrographic study of shock-metamorphic effects in terrestrial rocks. Individual chapters include: 1) Landscapes with Craters: Meteorite Impacts, Earth, and the Solar System; 2) Target Earth: Present, Past and Future; 3) Formation of Impact Craters; 4) Shock-Metamorphic Effects in Rocks and Minerals; 5) Shock-Metamorphosed Rocks (Impactities) in Impact Structures; 6) Impact Melts; 7) How to Find Impact Structures; and 8) What Next? Current Problems and Future Investigations.

The significance of the Vredefort Dome for the thermal and structural evolution of the Witwatersrand Basin, South Africa

The Vredefort Dome represents an area of significant (∼ 10 km) structural uplift within the central parts of the economically important Witwatersrand Basin. Its rocks experienced higher grades of metamorphism than the equivalent stratigraphic horizons exposed around the periphery of the basin. Recent studies of this medium- to high-grade metamorphism, as well as new evidence concerning the origin of the dome, have contributed to a metamorphic model for the Witwatersrand Basin as a whole. This evidence shows that the gold-bearing strata experienced at least two metamorphic events at ca. 2 Ga. The unusually high strain rate and shock deformation features exposed in the rocks of the dome rule out an endogenous origin by tectonic or diapiric processes. Recent work on these features has shown that the dome is best explained as the central uplift of a large, 250–300 km diameter, 2023±44 Ma old meteorite impact structure, the extent of which closely correlates with the present-day limits of the Witwatersrand Basin. Impact-related deformation features in the Vredefort rocks facilitate the separation of metamorphic textures developed during a pre-impact event associated with the 2.05–2.06 Ga Bushveld magmatism, and textures developed during a slightly lower-grade, post-impact, static overprint. The post-impact overprint decreases in intensity outwards from the dome. It is attributed to the massive disturbance of the thermal structure of the crust by impact-induced exhumation, and to shock heating of the rocks as a consequence of the impact event.

Hydrocarbons in meteorite impact structures: Oil reserves in the ames feature

In the quest to meet the world’s growing energy needs, an understanding of the role of hydrocarbons in meteorite impact structures may become increasingly important. This can only happen with a clearer understanding of impact structures in general and the conditions necessary to form petroleum systems within them. Identifying impact structures that meet these conditions is difficult, but the results of such efforts hold the possibility of significant economic returns.

Sudbury meteorite-impact structure modeling with LITHOPROBE high-resolution seismic refraction results

The Sudbury Structure is well known but is still being investigated extensively because the origin and formation history have many questions. Geologically it is believed to be a meteorite-impact structure and is composed of the Sudbury Igneous Complex along the border, young sedimentary rocks in the Basin, and Levack Gneiss Complex outside the north range of the basin structure. The 1992 high-resolution refraction seismic survey, carried out as a part of the LITHOPROBE Abitibi—Grenville Transect experiments, included two in-line profiles in cross pattern and two fan-shot profiles perpendicular to them. Scientific objectives of these profiles includ 2-D and 3-D imaging of the crustal structures beneath the Sudbury Structure and the surrounding areas. Results of forward and inverse modelings, tomographic inversion, and preliminary interpretation of the refraction seismic data indicate that there is a distinctive high-velocity layer with a lenticular shape below the Sudbury Basin structure at a depth range of approximately 4.5–9.0 km. Even after 2 billion years of deformation, including the truncation by erosion and several stages of subsequent geological evolution, the concave-upward shape of the velocity anomaly conforms to the proposed original meteorite-impact model and has close correlation.