Terrestrial Impact Craters Research Articles

Paleomagnetism of Lonar impact crater, India

Lonar crater, India, is the best preserved terrestrial impact crater formed in basalt and is a unique terrestrial analogue for small, simple craters on terrestrial planets and the Moon. We investigated the paleomagnetic and rock-magnetic properties of the 1.88 km diameter crater in order to understand the effect of impacts on magnetization in target rocks. The magnetization in the Lonar basalts consists of an original 65 Ma Deccan magnetization and a recent overprint. We constrained the timing of magnetization acquisition at Lonar using a combination of conglomerate tests on ejecta deposit clasts and fold tests on the overturned and jumbled rim fold. In some areas, the recumbent rim fold is preserved and can be approximated as a horizontal cylindrical fold. In other areas, substantial vertical axis rotation may have occurred where tear zones developed during folding. We observed only subtle effects from the impact on the rock-magnetic properties of Lonar materials, which include a slightly elevated coercivity in shocked ejecta blocks. We show that paleomagnetism can provide a constraint on shock heating in the absence of petrographic evidence of shock (in this case, < 187 ± 15 °C). At Lonar, viscous (and/or chemical) remanent magnetization acquired in the < 50 kyr subsequent to crater formation has obscured any evidence of shock remanent magnetization. We also find no evidence of shock demagnetization or the presence of intense impact-induced or impact-amplified transient magnetic fields that have been proposed around larger impact structures.

Cepstrum Analysis of Terrestrial Impact Crater Records

Study of terrestrial impact craters is important not only in the field of the solar system formation and evolution but also of the Galactic astronomy. The terrestrial impact cratering record recently has been examined, providing short- and intermediate-term periodicities, such as, Myrs, Myrs. The existence of such a periodicity has an implication in the Galactic dynamics, since the terrestrial impact cratering is usually interpreted as a result of the environmental variation during solar orbiting in the Galactic plane. The aim of this paper is to search for a long-term periodicity with a novel method since no attempt has been made so far in searching a long-term periodicity in this research field in spite of its great importance. We apply the cepstrum analysis method to the terrestrial impact cratering record for the first time. As a result of the analysis we have found noticeable peaks in the Fourier power spectrum appear ing at periods of Myrs and Myrs, which seem in a simple resonance with the revolution period of the Sun around the Galactic center. Finally we briefly discuss its implications and suggest theoretical study be pursued to explain such a long-term periodicity.

Impact cratering and the Oort Cloud

We calculate the expected flux profile of comets into the planetary system from the Oort Cloud arising from Galactic tides and encounters with molecular clouds. We find that both periodic and sporadic bombardment episodes, with amplitudes an order of magnitude above background, occur on characteristic time-scales ∼25–35 Myr. Bombardment episodes occurring preferentially during spiral arm crossings may be responsible both for mass extinctions of life and the transfer of viable microorganisms from the bombarded Earth into the disturbing nebulae. Good agreement is found between the theoretical expectations and the age distribution of large, well-dated terrestrial impact craters of the past 250 Myr. A weak periodicity of ∼36 Myr in the cratering record is consistent with the Sun's recent passage through the Galactic plane, and implies a central plane density ∼0.15 M⊙ pc−3. This leaves little room for a significant dark matter component in the disc.

Numerical modelling of impact melt production in porous rocks

Melting of rocks by impact is a fundamental geologic process that results from shock wave compression and subsequent release from high pressure. Shock-melted material is found in many terrestrial craters; however, most impact craters in sedimentary targets, which are typically porous, have been reported as containing significantly less melt than craters in nonporous crystalline targets. To investigate the effect of porosity on shock melting we present a numerical modelling approach that combines a porosity compaction model with an equation of state for the solid component. To demonstrate that our model accurately calculates shock pressures in porous materials we compare these results with Hugoniot data for porous quartzite (Coconino sandstone) and calcite. Our results show that the presence of porosity significantly reduces the critical pressure required for melting, which enhances melt production. We show that this effect is reduced but not totally diminished by faster shock wave decay in porous materials, so that, all other things being equal, impact melt production is greater when target porosity is higher. We conducted a series of simulations of impacts into targets of different material composition (quartzite, calcite, dunite) and porosity of up to 50% and varied the impact velocity between 6 and 50 km/s. We found a significant increase in the production of impact melt with increasing velocity (in agreement with several previous studies) and quantify the net increase in impact melt due to target porosity. For the nonporous case our results agree well with estimated melt volumes at terrestrial impact craters over a range in size (7–90 km in diameter). In porous targets our results contradict the widely cited observation of a melt deficiency in sedimentary target craters. However, our model results agree well with recently revised estimates of melt volumes at terrestrial craters formed in sedimentary, or mixed sedimentary and crystalline targets. This further strengthens the view that substantial volumes of impact melt should be produced during impacts into porous sedimentary targets and that the apparent paucity of impact melt in some sedimentary target craters may be due to difficulties in recognizing impact melts derived from sedimentary rocks.

Meteoritic Impacts and Climatic Changes in Pliocene–Pleistocene Epoch

During the Pliocene–Pleistocene epoch, covering last ∼5.2 Ma of Earth’s history, altogether 34 terrestrial meteoritic impact craters are known. Most of these craters (29) have diameter ≤10 km, among which 11 craters fall in 1,000 to 100 m range, and 7 are still smaller in dimension and of recent age. The age versus impact-frequency plot shows that the meteoritic impacts during this time period occurred in discrete intervals but have a periodicity that shows the best possible coincidence with the ∼425 Ky climatic cycles observed by Fourier analysis and FFT filtering of composite high resolution benthic foraminiferal δ18O record. This observation is also supported by Monte Carlo test with 71% success where meteoritic impact(s) shows coincidence with climatic cooling within our error limit. The newly observed climatic–meteoritic cycle may be same with the ∼400 Ky Milankovitch cycle or it is a different newly understood cycle relating both the climatic variation and meteoritic impact events.

The geochronology of large igneous provinces, terrestrial impact craters, and their relationship to mass extinctions on Earth

The relationship between sudden catastrophic events and sudden mass extinctions on Earth has attracted a great deal of research effort, including measurements of the absolute and relative ages of these events. The result has been a rapid improvement in the precision and accuracy of radiometric dates for large igneous provinces (LIPs), for which there is now a consensus on the peak eruption times over the last 250 Ma. In parallel there has been a more gradual improvement in the radiometric dates for craters formed by hypervelocity impacts (HVIs) of asteroids and comets with Earth. The absolute ages of the largest craters are known to within a few million years but the ages of many smaller craters are poorly constrained. In addition, whereas relatively few LIPs have formed in the last 250 Ma, at least one HVI crater larger than 20 km diameter has formed on Earth every million years (only 25 such craters have been discovered), so coincidence between the ages of HVI crater formation and ages of mass extinctions is related as much to crater preservation as to coincidence of the events. The difference in quality of geochronology between HVIs and LIPs is the result of differing availability of samples: LIPs cover large areas and present an array of basalt samples suitable for dating, whereas HVI craters are often poorly preserved and present few datable samples. The radiometric data for preserved HVIs and LIPs are now sufficiently precise and accurate for us to be confident that there is no one-to-one correlation between either LIPs or HVIs and global mass extinctions of life on Earth. However, there is a close association between a significant number of LIPs and sudden environmental change, and a very close association between at least one massive HVI crater formation and a mass extinction. Further, there is no apparent correlation between HVIs and LIPs and thus no reason to suggest a causative relationship. Finally, the radiometric dating of HVI craters indicates several clusters in the geological record, but the correlation with sudden environmental change is no better than for large craters.

An international and multidisciplinary drilling project into a young complex impact structure: The 2004 ICDP Bosumtwi Crater Drilling Project—An overview

Abstract— The Bosumtwi impact crater in Ghana, arguably the best‐preserved complex young impact structure known on Earth, displays a pronounced rim and is almost completely filled by Lake Bosumtwi, a hydrologically closed basin. It is the source crater of the Ivory Coast tektites. The structure was excavated in 2.1–2.2 Gyr old metasediments and metavolcanics of the Birimian Supergroup. A drilling project was conceived that would combine two major scientific interests in this crater: 1) to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago, at a near‐equatorial location in Africa for which very few data are available so far, and 2) to obtain a complete record of impactites at the central uplift and in the crater moat, for ground truthing and comparison with other structures.Within the framework of an international and multidisciplinary drilling project led by the International Continental Scientific Drilling Program (ICDP), 16 drill cores were obtained from June to October 2004 at six locations within Lake Bosumtwi, which is 8.5 km in diameter. The 14 sediment cores are currently being investigated for paleoenvironmental indicators. The two impactite cores LB‐07A and LB‐08A were drilled into the deepest section of the annular moat (540 m) and the flank of the central uplift (450 m), respectively. They are the main subject of this special issue of Meteoritics &amp; Planetary Science, which represents the first detailed presentations of results from the deep drilling into the Bosumtwi impactite sequence. Drilling progressed in both cases through the impact breccia layer into fractured bedrock. LB‐07A comprises lithic (in the uppermost part) and suevitic impact breccias with appreciable amounts of impact melt fragments. The lithic clast content is dominated by graywacke, besides various metapelites, quartzite, and a carbonate target component. Shock deformation in the form of quartz grains with planar microdeformations is abundant. First chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and Mg, but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values. Core LB‐08A comprises suevitic breccia in the uppermost part, followed with depth by a thick sequence of graywacke‐dominated metasediment with suevite and a few granitoid dike intercalations. It is assumed that the metasediment package represents bedrock intersected in the flank of the central uplift. Both 7A and 8A suevite intersections differ from suevites outside of the northern crater rim.Deep drilling results confirmed the gross structure of the crater as imaged by the pre‐drilling seismic surveys. Borehole geophysical studies conducted in the two boreholes confirmed the low seismic velocities for the post‐impact sediments (less than 1800 m/s) and the impactites (2600–3300 m/s). The impactites exhibit very high porosities (up to 30 vol%), which has important implications for mechanical rock stability. The statistical analysis of the velocities and densities reveals a seismically transparent impactite sequence (free of prominent internal reflections). Petrophysical core analyses provide no support for the presence of a homogeneous magnetic unit (= melt breccia) within the center of the structure. Borehole vector magnetic data point to a patchy distribution of highly magnetic rocks within the impactite sequence.The lack of a coherent melt sheet, or indeed of any significant amounts of melt rock in the crater fill, is in contrast to expectations from modeling and pre‐drilling geophysics, and presents an interesting problem for comparative studies and requires re‐evaluation of existing data from other terrestrial impact craters, as well as modeling parameters.

Lithological and structural characteristics of the Lake Bosumtwi impact crater, Ghana: Interpretation of acoustic televiewer images

Abstract— Bosumtwi is a very well‐preserved 1.07 Myr old, complex terrestrial impact crater located in south‐central Ghana, West Africa. The impact structure has a diameter of about 10.5 km and was formed in 2.1–2.2 Gyr Precambrian metasedimentary and metavolcanic rocks. Drilling and logging was carried out during the Lake Bosumtwi Drilling Project (BCDP) which was supported by the International Continental Scientific Drilling Program (ICDP). One of the aims of this project is to achieve detailed information on the subsurface structure and crater fill of one of the best preserved large young impact structures.We interpreted the wireline logs and televiewer images. The physical properties including shallow resistivity, p‐wave velocity, magnetic susceptibility, and borehole diameter of the breccia differ significantly from those of the meta‐graywackes and slate/phyllites. Fractures observed in the televiewer images are interpreted to determine their characteristic structural features. The fracture dip angles are steep (50–70°) and the two main dip directions are southeast and southwest. Most fractures observed in the borehole are open. The indicated main stress direction is north‐south.

Chromium isotopic studies of terrestrial impact craters: Identification of meteoritic components at Bosumtwi, Clearwater East, Lappajärvi, and Rochechouart

Chromium isotopic compositions and elemental abundances in impact melt rock and impact glass samples from four terrestrial impact craters were measured to verify the presence of an extraterrestrial component and to identify the meteorite type of the impactor. All meteorite classes have Cr isotopic signatures that are different from those of terrestrial rocks; thus, precise measurements of Cr isotopic abundances can unequivocally distinguish terrestrial from extraterrestrial materials. For all four studied craters — Bosumtwi (Ghana), Clearwater East (Canada), Lappajärvi (Finland), and Rochechouart (France) we found positive 53Cr excesses that eliminate carbonaceous chondrite projectiles (because those would show apparent negative excesses) and enstatite chondrites (because of the magnitude of the excess). In all four cases, ordinary chondrites have been identified as the best fit for the data; in the case of Lappajärvi interelement correlations together with the Cr isotope data make an H-chondrite the most likely projectile, whereas in the case of Clearwater East both L or H chondrites are possible. For Bosumtwi and Rochechouart the high indigenous contents of the siderophile elements, and disturbances of the elemental abundances by weathering and hydrothermal alteration, respectively, do not allow further constraints to be placed on the type of ordinary chondrite involved in the impact.

Ejecta range: A simulation study of terrestrial impacts

In this work the meteorite and target mass partition into high-speed ejecta during the formation of terrestrial impact craters is investigated. Multi-material hydrocode calculations are carried out through the entire excavation phase, and the mass of each material moving upwards with velocities inside a range of intervals is obtained. Impact of a 10 km diameter stony asteroid with 20 km s - 1 into the continental crust is compared for the cases of a single layer of granite, taken to be representative for the crust, and of a two-layer crust with a 3 km thick sedimentary cover of limestone on top of granite basement, more appropriate for the Chicxulub crater. The proportion of meteorite and crustal material in high-speed ejecta is found as a function of velocity and time, and maximum distances to the crater can be estimated. The resulting distal ( > 7000 km ) ejecta mass for vertical impact is less than a percent of the impactor mass, assuming ballistic transport. Simulations of oceanic impact of a 1 km-sized stony asteroid into 5.5 km deep sea are also presented. Here, ejection of meteorite material initially is delayed, but finally it leaves the ocean in a cloud of steam and water. The velocities of meteorite material are much lower compared with the continental impact, insufficient to reach large distances on ballistic trajectories.

Trace element and isotopic evidence for Archean basement in the Lonar crater impact breccia, Deccan Volcanic Province

The Lonar impact crater in the Deccan Traps of the Indian peninsula provides unique opportunities to study physical and chemical processes of impact cratering on basaltic targets, because terrestrial impact craters on basalts are extremely rare. Such studies are needed for determining provenance and other parameters of the excavated rocks and the cratering phenomenon that may have implications for similar crater formations in Lunar, Martian and other basaltic targets in the solar system. Considering some of these objectives, we analyzed trace elements and Nd, Sr, Pb-isotopes of impact breccia rocks and target basalts collected from the Lonar crater. Chondrite-normalized Rare Earth Element (REE) patterns in the target basalts and breccia rocks show similar light REE-enriched patterns, although in detail, the impact breccia are more fractionated in La/Sm compared to the target basalts. The target basalts also show much lower concentrations of Rb, Ba, Th, U and Pb compared to the breccia and are characterized by Rb, Ba and Pb depletions with respect to the primitive mantle-normalized Th, U, Nb, Ta and the REE contents. The breccia rocks are significantly enriched in Rb, Ba and Pb, and to a lesser extent in Th and U, compared to the target rocks. The Nd, Sr and Pb-isotopic compositions of the Lonar target basalts can be correlated with those of the Poladpur suite, one of the mid-section volcano-stratigraphic units of the Deccan traps. In contrast to the host basalts, the impact breccia rocks show more radiogenic Sr, less radiogenic Nd and higher Rb/Sr and lower Sm/Nd ratios, indicating an additional component, other than the target basalt, that must have been derived from beneath the basaltic target rocks at the impact site. The Deccan traps in western India are underlain by Archean to mid-Proterozoic cratonic rocks. The overall geochemical signatures of the impact breccia rocks, specifically, the trace element concentrations, negative ε Nd values, radiogenic Sr isotopic composition as well as the high 207Pb/ 204Pb at low 206Pb/ 204Pb indicate that a major component of the Lonar impact breccia was derived from melting of Archean basement rocks. We argue that the Archean component in the breccia cannot be from the incorporation of paleosols that are weathering products of the target basalts, or from the inter-trappean sediments that are most commonly cherts and limestones of Mesozoic age. Similarly, the possible role of eolian sediments in causing the Archean Pb-isotopic signature, identical to those of the Deccan basement, in the breccia rocks can be excluded. The basement beneath the Lonar region is believed to be similar to the Dharwar craton of peninsular India. Based on their similar Pb-isotopic compositions with the breccia rocks, we suggest the Archean Chitradurga Group of rocks of this craton to be present in the basement beneath the Deccan lavas of the Lonar region. The thickness of the basaltic target rocks at the crater-site (∼ 400 m) and the inferred crater depth (350 m–610 m), based on depth to diameter ratios in simple planetary craters (0.2–0.33), are consistent with our conclusion regarding melting and incorporation of these ancient basement rocks in the impact breccia of the Lonar crater. Using Pi-group scaling relations, the observed crater diameter, and density of the basaltic target rocks, we have estimated with reasonable approximation the diameter of the Lonar impactor to be either 70 m, 86 m or 120 m, assuming the bolide to be an iron meteorite, stony-iron meteorite, or an ordinary chondrite, respectively.

Impact melt sheet formation on Mars and its implication for hydrothermal systems and exobiology

Hydrothermal deposits are a prime exobiological research target on Mars and impact craters are potentially a significant source for these deposits. Key factors in the longevity of terrestrial impact crater hydrothermal systems are the volume of shock melt produced and the degree to which this melt is quenched by mixing with un-melted clasts. Mixing occurs when the shock melt is smeared along the crater floor, and the degree of mixing is inversely proportional to the thickness of this smeared layer. To investigate the potential development of thick melt sheets on Mars, we apply an analytical impact model to asteroid and long-period (LP) comet impacts that produces martian craters with final diameters of 30–640 km. We combine our model shock melt volumes with estimates of the surface area of the transient cavity to estimate initial shock melt thickness, which we use as a proxy measure of the amount of mixing between shock melt and un-melted clasts. We calculate this shock melt thickness parameter for a range terrestrial craters with and without thick melt sheets, from which we infer the melt sheet characteristics for the model Mars craters. The results demonstrate that most asteroid impacts on Mars do not produce thick, clast-free melt sheets. Only asteroid impact craters ⩾ 580 km in diameter on Mars produce shock melt thicknesses comparable to Earth's 200-km-diameter Sudbury crater, and martian craters ⩽ 65 km in diameter produce shock melt thicknesses comparable to terrestrial craters that lack melts sheets entirely. LP comet impacts on Mars are more likely to produce thick melt sheets. LP comet impact craters ⩾ 280 km in diameter on Mars produce shock melt thicknesses comparable to that of the Sudbury crater. Given that LP comet impacts on Mars are rare compared to asteroid impacts, we conclude that large, long-lived impact hydrothermal systems are likewise rare on Mars. Comparisons of the Mars asteroid impact rate with the potential longevity of impact hydrothermal systems indicate that such systems were active < 1 % of the time in the last 3.5 Ga, which raises questions about the exobiological importance of impact crater hydrothermal systems on Mars after the Noachian.

Shock‐metamorphosed zircon in terrestrial impact craters

Abstract— To ascertain the progressive stages of shock metamorphism of zircon, samples from three well‐studied impact craters were analyzed by optical microscopy, scanning electron microscopy (SEM), and Raman spectroscopy in thin section and grain separates. These samples are comprised of well‐preserved, rapidly quenched impactites from the Ries crater, Germany, strongly annealed impactites from the Popigai crater, Siberia, and altered, variably quenched impactites from the Chicxulub crater, Mexico. The natural samples were compared with samples of experimentally shock‐metamorphosed zircon. Below 20 GPa, zircon exhibits no distinct shock features. Above 20 GPa, optically resolvable planar microstructures occur together with the high‐pressure polymorph reidite, which was only retained in the Ries samples. Decomposition of zircon to ZrO2 only occurs in shock stage IV melt fragments that were rapidly quenched. This is not only a result of post‐shock temperatures in excess of ˜1700 °C but could also be shock pressure‐induced, which is indicated by possible relics of a high‐pressure polymorph of ZrO2. However, ZrO2 was found to revert to zircon with a granular texture during devitrification of impact melts. Other granular textures represent recrystallized amorphous ZrSiO4 and reidite that reverted to zircon. This requires annealing temperatures &gt;1100 °C. A systematic study of zircons from a continuous impactite sequence of the Chicxulub impact structure yields implications for the post‐shock temperature history of suevite‐like rocks until cooling below ˜600 °C.

Vertical distribution of Galactic disk stars

We used red clump stars to measure the surface mass density of the Galactic disk in the solar neighbourhood. High resolution spectra of red clump stars towards the NGP have been obtained with the ELODIE spectrograph at OHP for Tycho-2 selected stars, and nearby Hipparcos counterparts were also observed. We determined their distances, velocities, and metallicities to measure the gravitational force law perpendicular to the Galactic plane. As in most previous studies, we applied one-parameter models of the vertical gravitational potential. We obtained a disk surface mass density within 1.1kpc of the Galactic plane, Sigma_{1.1kpc}=64+/-5 Msun_pc^{-2}, with an excellent formal accuracy, however we found that such one-parameter models can underestimate the real uncertainties. Applying two-parameter models, we derived more realistic estimates of the total surface mass density within 800pc from the Galactic plane, Sigma_{0.8kpc}=57-66 Msun pc^{-2}, and within 1.1kpc, Sigma{1.1kpc}=57-79 Msun pc^{-2}. This can be compared to literature estimates of \sim40 Msun pc^{-2} in stars and to 13 Msun pc^{-2} in the less accurately measured ISM contribution. We conclude that there is no evidence of large amounts of dark matter in the disk and, furthermore, that the dark matter halo is round or not vey much flattened. A by-product of this study is the determination of the half period of oscillation by the Sun through the Galactic plane, 42+/-2Myr, which cannot be related to the possible period of large terrestrial impact craters \sim 33-37Myr.

About this title

This Special Publication has 24 papers with an international authorship, and is prefaced by an introductory overview which presents highlights in the field. The first section covers the acceptance by science of the reality of the falls of rock and metal from the sky, an account that takes the reader from BCE (before common era) to the nineteenth century. The second section details some of the world's most important collections in museums - their origins and development. The Smithsonian chapter also covers the astonishingly numerous finds in the cold desert of Antarctica by American search parties. There are also contributions covering the finds by Japanese parties in the Yamato mountains and the equally remarkable discoveries in the hot deserts of Australia, North Africa, Oman and the USA. The other seven chapters take the reader through the revolution in scientific research on meteoritics in the later part of the twentieth century, including terrestrial impact cratering and extraordinary showers of glass from the sky; tektites, now known to be Earth-impact-sourced. Finally, the short epilogue looks to the future. The History of Meteoritics and Key Meteorite Collections should appeal to historians of science, meteoriticists, geologists, astronomers, curators and the general reader with an interest in science.

The period dichotomy in terrestrial impact crater ages

Impact cratering on the Earth during the past 250 Myr has occurred with either of two apparent periodicities, ∼30 or ∼35 Myr, depending on the set of impact crater ages that is adopted. When the craters are segregated by size and the possible age errors are explicitly taken into account in the analysis, only the longer periodicity survives, and does so only in the case of the largest craters (diameters ≥ 35 km). Smaller craters exhibit no robust periodicity. Despite their relative abundance, the inclusion of data points for the small craters merely degrades, without shifting or destroying, the periodic signal of the largest craters when all of the craters are analysed together. The possible consequences for quasi-periodic Galactic perturbations of the Oort comet cloud are briefly discussed.

Experimental shock synthesis of diamonds in a graphite gneiss

Abstract— The occurrence of diamonds in terrestrial impact craters and meteorites is related to dynamic shock loading during hypervelocity impacts. To understand the mechanism of impact diamond formation in natural rocks, shock‐recovery experiments with graphite gneiss were carried out at shock pressures between 35 and 79 GPa. This is the first report on the successful shock synthesis of microdiamonds in a natural rock. Micrometer‐size diamonds and a wide range of intermediate, presently unclassified, amorphous, and disordered carbon phases were observed within vesiculated biotite melts in the vicinity of relic graphite grains using microRaman spectrometry. We explain these findings by jetting mechanisms of carbon and graphite clusters, originating at the edges of graphite grains, into the very hot and volatile rich biotitic melt veins during shock loading. This environment enabled the thermally activated crystallization of diamonds during shock compression in a period of less than 0.5 μsec. Regraphitization of diamonds during pressure release was widespread and caused the formation of the amorphous to disordered carbon phases recorded frequently with microRaman spectroscopy. The surviving diamonds must have cooled down to 2000 K during the compression phase at local thermal sinks and cooler interfaces to avoid regraphitization.

Numerical Modeling of the Largest Terrestrial Meteorite Craters

Multi-ring impact basins have been found on the surfaces of almost all planetary bodies in the Solar system with solid crusts. The details of their formation mechanism are still unclear. We present results of our numerical modeling of the formation of the largest known terrestrial impact craters. The geological and geophysical data on these structures accumulated over many decades are used to place constraints on the parameters of available numerical models with a dual purpose: (i) to choose parameters in available mechanical models for the crustal response of planetary bodies to a large impact and (ii) to use numerical modeling to refine the possible range of original diameters and the morphology of partially eroded terrestrial craters. We present numerical modeling results for the Vredefort, Sudbury, Chicxulub, and Popigai impact craters and compare these results with available geological and geophysical information.

Enhancement of magnetic signatures of impact structures

Aeromagnetic surveys play an important role in the detection and analysis of terrestrial impact structures as large semi-regional aeromagnetic surveys are widely available. Impact craters can be divided into two groups based on morphostructure, namely simple and complex. Simple craters are relatively small bowl-shaped depressions with an upraised and fractured rim whereas complex craters are larger with a central uplift zone. Magnetic signatures of terrestrial impact craters vary greatly, reflecting the target rocks, the impact-related magnetisation and effects of crater fill and post-impact sediments. In basement rocks, the common signature is a magnetic low, ranging in amplitude from a few nT up to a few hundred nT. The central peak or ring uplift of crushed basement may produce strong magnetic highs. The magnetic signature may be due to shock demagnetisation, shock remagnetisation, and thermal and chemical remanent magnetisation effects. Impact craters in sedimentary targets are usually subdued and amplitudes of a few nT up to 10 nT are common. Enhancement of magnetic signatures of impact structures using filtering techniques is an important part of detection and analysis. Derivatives and shaded relief techniques, along with separation filtering, are probably the most used methods. Algorithms for fractional order derivatives and circular shaded relief have dramatically improved filter results. The fractional derivative order can be varied to optimise separation of the impact magnetic signature. Circular shaded relief treats all directions equally unlike the fade-out for features sub-parallel to the shading direction evident in conventional shading. The fractional order derivative and circular shaded relief algorithms are illustrated from impact structures in Australia and Canada in both basement and sedimentary cover rocks.

Remote sensing studies of impact craters: how to be sure?

After previous studies on impact craters in the eastern Sahara, particularly in Chad [2,15] and Libya [1], the paper by Paillou et al. [11] provides a good opportunity to discuss some important aspects of impact craters. They are a dominant landform on our Moon, and on other atmosphereless bodies (including planets and the satellites of planets) in our solar system. On the Earth, impact craters are far less obvious, and only about 170 structures have so far been confirmed to have been formed by hypervelocity impact. The diameters of these terrestrial impact craters range from less than 100 m to about 200 km; a couple of these structures have originally been even larger, probably about 250 to 300 km in diameter. The reason why we do not see more obvious impact craters on the surface is the Earth is intimately connected to the reason why it is such a suitable place for life: the Earth is a geologically active planet. The forces that shape our planet – for example, tectonics, volcanism, erosion, water, and weather – are those that obliterate the traces of even large-scale and devastating impact scars after geologically short time scales. This is probably one of the reasons why it took geologists so long to accept the reality of impact craters on Earth. A clear hiatus in the history of impact-related studies was the realization, around 1980, that CretaceousTertiary (K/T) boundary rocks bear unambiguous evidence for a large-scale catastrophic impact event; this was followed in the early 1990s by the discovery of the ca. 200-km-diameter Chicxulub impact structure, Mexico, as the source of the world-wide impact ejecta.