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1.
Abstract— Chicxulub and Sudbury are 2 of the largest impact structures on Earth. Research at the buried but well‐preserved Chicxulub crater in Mexico has identified 6 concentric structural rings. In an analysis of the preserved structural elements in the eroded and tectonically deformed Sudbury structure in Canada, we identified ring‐like structures corresponding in both radius and nature to 5 out of the 6 rings at Chicxulub. At Sudbury, the inner topographic peak ring is missing, which if it existed, has been eroded. Reconstructions of the transient cavities for each crater produce the same range of possible diameters: 80–110 km. The close correspondence of structural elements between Chicxulub and Sudbury suggests that these 2 impact structures are approximately the same size, both having a main structural basin diameter of ?150 km and outer ring diameters of ?200 km and ?260 km. This similarity in size and structure allows us to combine information from the 2 structures to assess the production of shock melt (melt produced directly upon decompression from high pressure impact) and impact melt (shock melt and melt derived from the digestion of entrained clasts and erosion of the crater wall) in large impacts. Our empirical comparisons suggest that Sudbury has ?70% more impact melt than does Chicxulub (?31,000 versus ?18,000 km3) and 85% more shock melt (27,000 km3 versus 14,500 km3). To examine possible causes for this difference, we develop an empirical method for estimating the amount of shock melt at each crater and then model the formation of shock melt in both comet and asteroid impacts. We use an analytical model that gives energy scaling of shock melt production in close agreement with more computationally intense numerical models. The results demonstrate that the differences in melt volumes can be readily explained if Chicxulub was an asteroid impact and Sudbury was a comet impact. The estimated 70% difference in melt volumes can be explained by crater size differences only if the extremes in the possible range of melt volumes and crater sizes are invoked. Preheating of the target rocks at Sudbury by the Penokean Orogeny cannot explain the excess melt at Sudbury, the majority of which resides in the suevite. The greater amount of suevite at Sudbury compared to Chicxulub may be due to the dispersal of shock melt by cometary volatiles at Sudbury.  相似文献   

2.
Abstract As part of the ICDP Chicxulub Scientific Drilling Project, the Yaxcopoil‐1 (Yax‐1) bore hole was drilled 60 km south‐southwest of the center of the 180 km‐diameter Chicxulub impact structure down to a depth of 1511 m. A sequence of 615 m of deformed Cretaceous carbonates and sulfates was recovered below a 100 m‐thick unit of suevitic breccias and 795 m of post‐impact Tertiary rocks. The Cretaceous rocks are investigated with respect to deformation features and shock metamorphism to better constrain the deformational overprint and the kinematics of the cratering process. The sequence displays variable degrees of impact‐induced brittle damage and post‐impact brittle deformation. The degree of tilting and faulting of the Cretaceous sequence was analyzed using 360°‐core scans and dip‐meter log data. In accordance with lithological information, these data suggest that the sedimentary sequence represents a number of structural units that are tilted and moved with respect to each other. Three main units and nine sub‐units were discriminated. Brittle deformation is most intense at the top of the sequence and at 1300–1400 m. Within these zones, suevitic dikes, polymict clastic dikes, and impact melt rock dikes occur and may locally act as decoupling horizons. The degree of brittle deformation depends on lithology; massive dolomites are affected by penetrative faulting, while stratified calcarenites and bituminous limestones display localized faulting. The deformation pattern is consistent with a collapse scenario of the Chicxulub transient crater cavity. It is believed that the Cretaceous sequence was originally located outside the transient crater cavity and eventually moved downward and toward the center to its present position between the peak ring and the crater rim, thereby separating into blocks. Whether or not the stack of deformed Cretaceous blocks was already displaced during the excavation process remains an open question. The analysis of the deformation microstructure indicates that a shock metamorphic overprint is restricted to dike injections with an exception of the so called “paraconglomerate.” Abundant organic matter in the Yax‐1 core was present before the impact and was mobilized by impact‐induced heating and suggests that >12 km3 of organic material was excavated during the cratering process.  相似文献   

3.
We present and interpret results of petrographic, mineralogical, and chemical analyses of the 1511 m deep ICDP Yaxcopoil‐1 (Yax‐1) drill core, with special emphasis on the impactite units. Using numerical model calculations of the formation, excavation, and dynamic modification of the Chicxulub crater, constrained by laboratory data, a model of the origin and emplacement of the impact formations of Yax‐1 and of the impact structure as a whole is derived. The lower part of Yax‐1 is formed by displaced Cretaceous target rocks (610 m thick), while the upper part comprises six suevite‐type allochthonous breccia units (100 m thick). From the texture and composition of these lithological units and from numerical model calculations, we were able to link the seven distinct impact‐induced units of Yax‐1 to the corresponding successive phases of the crater formation and modification, which are as follows: 1) transient cavity formation including displacement and deposition of Cretaceous “megablocks;” 2) ground surging and mixing of impact melt and lithic clasts at the base of the ejecta curtain and deposition of the lower suevite right after the formation of the transient cavity; 3) deposition of a thin veneer of melt on top of the lower suevite and lateral transport and brecciation of this melt toward the end of the collapse of the transient cavity (brecciated impact melt rock); 4) collapse of the ejecta plume and deposition of fall‐back material from the lower part of the ejecta plume to form the middle suevite near the end of the dynamic crater modification; 5) continued collapse of the ejecta plume and deposition of the upper suevite; 6) late phase of the collapse and deposition of the lower sorted suevite after interaction with the inward flowing atmosphere; 7) final phase of fall‐back from the highest part of the ejecta plume and settling of melt and solid particles through the reestablished atmosphere to form the upper sorted suevite; and 8) return of the ocean into the crater after some time and minor reworking of the uppermost suevite under aquatic conditions. Our results are compatible with: a) 180 km and 100 km for the diameters of the final crater and the transient cavity of Chicxulub, respectively, as previously proposed by several authors, and b) the interpretation of Chicxulub as a peak‐ring impact basin that is at the transition to a multi‐ring basin.  相似文献   

4.
We use hydrocode modeling to investigate dynamic models for the collapse of the Chicxulub impact crater. Our aim is to integrate the results from numerical simulations with kinematic models derived from seismic reflection and wide-angle velocity data to further our understanding of the formation of large impact craters. In our simulations, we model the collapse of a 100-km diameter, bowl-shaped cavity formed in comprehensively fractured crustal material. To facilitate wholesale collapse, we require that the strength of the target be significantly weakened. In the present model, we achieve this using acoustic fluidization, where strong vibrations produced by the expanding shock wave cause extreme pressure fluctuations in the target. At times and positions where the overburden pressure is sufficiently counteracted, the frictional resistance is reduced, enabling the rock debris to flow. Our simulations produce a collapsed crater that contains most of the features that we observe in the seismic data at Chicxulub. In particular, we observe a topographic peak ring, formed as material that is originally part of the central uplift collapses outward and is thrust over the inwardly collapsing transient crater rim. This model for peak-ring generation has not been previously demonstrated by numerical simulations and predicts that the peak ring is composed of deeply derived material and that the stratigraphy within the peak ring is overturned.  相似文献   

5.
Abstract— The Chicxulub Scientific Drilling Project (CSDP), Mexico, produced a continuous core of material from depths of 404 to 1511 m in the Yaxcopoil‐1 (Yax‐1) borehole, revealing (top to bottom) Tertiary marine sediments, polymict breccias, an impact melt unit, and one or more blocks of Cretaceous target sediments that are crosscut with impact‐generated dikes, in a region that lies between the peak ring and final crater rim. The impact melt and breccias in the Yax‐1 borehole are 100 m thick, which is approximately 1/5 the thickness of breccias and melts exposed in the Yucatán‐6 exploration hole, which is also thought to be located between the peak ring and final rim of the Chicxulub crater. The sequence and composition of impact melts and breccias are grossly similar to those in the Yucatán‐6 hole. Compared to breccias in other impact craters, the Chicxulub breccias are incredibly rich in silicate melt fragments (up to 84% versus 30 to 50%, for example, in the Ries). The melt in the Yax‐1 hole was produced largely from the silicate basement lithologies that lie beneath a 3 km‐ thick carbonate platform in the target area. Small amounts of immiscible molten carbonate were ejected with the silicate melt, and clastic carbonate often forms the matrix of the polymict breccias. The melt unit appears to have been deposited while molten but brecciated after solidification. The melt fragments in the polymict breccias appear to have solidified in flight, before deposition, and fractured during transport and deposition.  相似文献   

6.
Abstract The Chicxulub crater in Mexico is a nearly pristine example of a large impact crater. Its slow burial has left the central impact basin intact, within which there is an apparently uneroded topographic peak ring. Its burial, however, means that we must rely on drill holes and geophysical data to interpret the crater form. Interpretations of crater structures using geophysical data are often guided by numerical modeling and observations at other large terrestrial craters. However, such endeavors are hindered by uncertainties in current numerical models and the lack of any obvious progressive change in structure with increasing crater size. For this reason, proposed structural models across Chicxulub remain divergent, particularly within the central crater region, where the deepest well is only ?1.6 km deep. The shape and precise location of the stratigraphic uplift are disputed. The spatial extent and distribution of the allogenic impact breccias and melt rocks remain unknown, as do the lithological nature of the peak ring and the mechanism for its formation. The objective of our research is to provide a well‐constrained 3D structural and lithological model across the central region of the Chicxulub crater that is consistent with combined geophysical data sets and drill core samples. With this in mind, we present initial physical property measurements made on 18 core samples from the Yaxcopoil‐1 (Yax‐1) drill hole between 400 and 1500 m deep and present a new density model that is in agreement with both the 3D velocity and gravity data. Future collation of petrophysical and geochemical data from Yax‐1 core, as well as further seismic surveys and drilling, will allow us to calibrate our geophysical models—assigning a suite of physical properties to each lithology. An accurate 3D model of Chicxulub is critical to our understanding of large craters and to the constraining of the environmental effects of this impact.  相似文献   

7.
Abstract— The late Eocene Chesapeake Bay impact structure (CBIS) on the Atlantic margin of Virginia is one of the largest and best‐preserved “wet‐target” craters on Earth. It provides an accessible analog for studying impact processes in layered and wet targets on volatile‐rich planets. The CBIS formed in a layered target of water, weak clastic sediments, and hard crystalline rock. The buried structure consists of a deep, filled central crater, 38 km in width, surrounded by a shallower brim known as the annular trough. The annular trough formed partly by collapse of weak sediments, which expanded the structure to ?85 km in diameter. Such extensive collapse, in addition to excavation processes, can explain the “inverted sombrero” morphology observed at some craters in layered targets. The distribution of crater‐fill materials in the CBIS is related to the morphology. Suevitic breccia, including pre‐resurge fallback deposits, is found in the central crater. Impact‐modified sediments, formed by fluidization and collapse of water‐saturated sand and silt‐clay, occur in the annular trough. Allogenic sediment‐clast breccia, interpreted as ocean‐resurge deposits, overlies the other impactites and covers the entire crater beneath a blanket of postimpact sediments. The formation of chaotic terrains on Mars is attributed to collapse due to the release of volatiles from thick layered deposits. Some flat‐floored rimless depressions with chaotic infill in these terrains are impact craters that expanded by collapse farther than expected for similar‐sized complex craters in solid targets. Studies of crater materials in the CBIS provide insights into processes of crater expansion on Mars and their links to volatiles.  相似文献   

8.
Abstract— The structural, topographic and other characteristics of the Vredefort, Sudbury, and Chicxulub impact structures are described. Assuming that the structures originally had the same morphology, the observations/interpretations for each structure are compared and extended to the other structures. This does not result in any major inconsistencies but requires that the observations be scaled spatially. In the case of Vredefort and Sudbury, this is accomplished by scaling the outer limit of particular shock metamorphic features. In the case of Chicxulub, scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring. The observations/interpretations are then used to construct an integrated, empirical kinematic model for a terrestrial peak‐ring basin. The major attributes of the model include: a set of outward‐directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor, some of which are pre‐existing structures that have been reactivated during transient cavity formation; inward‐directed motions along the same outermost structures and along a set of structures, at intermediate radial distances, during transient cavity collapse; structural uplift in the center followed by a final set of radially outward‐directed thrusts at the outer edges of the structural uplift, during uplift collapse. The rock displacements on the intermediate, inward and innermost, outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift.  相似文献   

9.
Abstract— The 3.6 Myr old El'gygytgyn impact crater is located in central Chukotka, northeastern Russia. The crater is a well‐preserved impact structure with an inner basin about 15 km in diameter, surrounded by an uplifted rim about 18 km in diameter. The flat floor of the crater is in part occupied by Lake El'gygytgyn, 12 km in diameter, and surrounding terraces. The average profile of the rim is asymmetric, with a steep inner wall and a gentle outer flank. The rim height is about 180 m above the lake level and 140 m above the surrounding area. An outer ring feature, on average 14 m high, occurs at about 1.75 crater radii from the center of the structure. El'gygytgyn crater is surrounded by a complex network of faults. The density of the faults decreases from the bottom of the rim to the rim crest and outside the crater to a distance of about 2.7 crater radii. Lake El'gygytgyn is surrounded by a number of lacustrine terraces. Only minor remnants are preserved of the highest terraces, 80 and 60 m above the present‐day lake level. The widest of the terraces is 40 m above the current lake level and surrounds the lake on the west and northwest sides. The only outlet of the lake is the Enmivaam River, which cuts through the crater rim in the southeast. In terms of structure, El'gygytgyn is well preserved and displays some interesting, but not well understood, features (e.g., an outer ring), similar to those observed at a few other impact structures.  相似文献   

10.
Abstract— This study presents results of platinum group element (PGE) analyses of impactites from the Yaxcopoil‐1 (Yax‐1) and Yucatán 6 drill cores of the 180 km‐diameter Chicxulub crater. These are the main elements used for projectile identification. They were determined by nickel sulfide fire assay combined with inductively coupled plasma mass spectrometry. The concentration of PGE in the samples are low. The concentration patterns of the suevite samples resemble the pattern of the continental crust. We conclude that any meteoritic fraction in these samples is below 0.05%. A syn‐ and post‐impact modification of the PGE pattern from meteoritic toward a continental crust pattern is very unlikely. The globally distributed fallout at the Cretaceous‐Tertiary (K/T) boundary, however, has high PGE concentrations. Therefore, the lack of a significant meteoritic PGE signature in the crater is not an argument for a PGE‐poor impactor. Taking the results of three‐dimensional numerical simulations of the Chicxulub event into account, the following conclusions are drawn: 1) The main fraction of the impactor was ejected into and beyond the stratosphere, distributed globally, and deposited in the K/T boundary clay; and 2) the low amount of projectile contamination in the Yax‐1 lithologies may reflect an oblique impact. However, the role of volatiles in the mixing process between projectile and target is not well‐understood and may also have played a fundamental role.  相似文献   

11.
The lunar Orientale basin and its associated facies formed as a result of impact into lunar highland crustal rocks. The crater rim is shown to be closely represented by the position of the outer Rook Mountain ring, approximately 620 km in diam. The inner Rook Mountains form a central peak ring within the crater. The 900 km diam Cordillera ring is a fault scarp which formed in the terminal stages of the cratering event as a large portion of the crust collapsed inward toward the recently excavated crater, forming a mega-terrace. This collapse pushed the wall of the Orientale crater inward, distorting it and slightly decreasing its radius.A domical facies is almost exclusively developed between the Cordillera and outer Rook rings. The domical facies is interpreted to be radially textured ejecta which was disrupted and modified to a jumbled domical texture by seismic shaking associated with the formation of the mega-terrace. The plains and corrugated facies pre-date the mare fill and lie within the Orientale crater. These facies are interpreted to have been deposited contemporaneously with the cratering event as partial and total impact melts which collected on the floor of the crater during the terminal stages of the event. The plains facies, with an estimated thickness of 1 km and a volume of 75000 km3, represent the most thoroughly impact melted materials which collected and ponded in the central portion of the crater floor. The corrugated facies, with an estimated thickness of 1 km and a volume of 180000 km3, represent impact partial melts mixed with debris. A relatively small volume of mare material was subsequently deposited in the basin (probably less than 25000 km3 in Mare Orientale).There is little evidence that the basin has undergone major structural modifications subsequent to the terminal stages of the cratering event. The striking implication for the Orientale gravity anomaly is that mascon formation may be primarily related to crustal excavation and upwarping of a moho plug, rather than attributable to post-impact mare filling.The plains units on the floor of Orientale are similar to Cayley-like plains in other multi-ringed basins and on smaller crater floors. Impact melt deposits may therefore be a significant source of Cayley-like plains units.The volumes of impact melt associated with the Orientale basin and their mode of deposition have important implications for petrogenetic models. Multi-ringed basin formation provides a mechanism for instantaneously melting large volumes of shallow to intermediate depth lunar crustal material which is emplaced such that the differentiation and crystallization of a variety of igneous rock types and textures may occur.  相似文献   

12.
Abstract— The 40 km diameter Mjølnir Crater is located on the central Barents Sea shelf, north of Norway. It was formed about 142 ± 2.6 Myr ago by the impact of a 1–2 km asteroid into the shallow shelf clays of the Hekkingen Formation and the underlying Triassic to Jurassic sedimentary strata. A core recovered from the central high within the crater contains slump and avalanche deposits from the collapse of the transient crater and central high. These beds are overlain by gravity flow conglomerates, with laminated shales and marls on top. Here, impact and post‐impact deposits in this core are studied with focus on clay mineralogy obtained from XRD decomposition and simulation analysis methods. The clay‐sized fractions are dominated by kaolinite, illite, mixed‐layered clay minerals and quartz. Detailed analyses showed rather similar composition throughout the core, but some noticeable differences were detected, including varying crystal size of kaolinite and different types of illites and illite/smectite. These minerals may have been formed by diagenetic changes in the more porous/fractured beds in the crater compared to time‐equivalent beds outside the crater rim. Long‐term post‐impact changes in clay mineralogy are assumed to have been minor, due to the shallow burial depth and minor thermal influence from impact‐heated target rocks. Instead, the clay mineral assemblages, especially the abundance of chlorite, reflect the impact and post‐impact reworking of older material. Previously, an ejecta layer (the Sindre Bed) was recognized in a nearby well outside the crater, represented by an increase in smectite‐rich clay minerals, genetically equivalent to the smectite occurring in proximal ejecta deposits of the Chicxulub crater. Such alteration products from impact glasses were not detected in this study, indicating that little, if any, impact glass was deposited within the upper part of the crater fill. Crater‐fill deposits inherited their mineral composition from Triassic and Jurassic sediments underlying the impact site.  相似文献   

13.
Abstract— Post‐impact crater morphology and structure modifications due to sediment loading are analyzed in detail and exemplified in five well‐preserved impact craters: Mjølnir, Chesapeake Bay, Chicxulub, Montagnais, and Bosumtwi. The analysis demonstrates that the geometry and the structural and stratigraphic relations of post‐impact strata provide information about the amplitude, the spatial distribution, and the mode of post‐impact deformation. Reconstruction of the original morphology and structure for the Mjølnir, Chicxulub, and Bosumtwi craters demonstrates the long‐term subsidence and differential compaction that takes place between the crater and the outside platform region, and laterally within the crater structure. At Mjølnir, the central high developed as a prominent feature during post‐impact burial, the height of the peak ring was enhanced, and the cumulative throw on the rim faults was increased. The original Chicxulub crater exhibited considerably less prominent peak‐ring and inner‐ring/crater‐rim features than the present crater. The original relief of the peak ring was on the order of 420–570 m (currently 535–575 m); the relief on the inner ring/crater rim was 300–450 m (currently ?700 m). The original Bosumtwi crater exhibited a central uplift/high whose structural relief increased during burial (current height 101–110 m, in contrast to the original height of 85–110 m), whereas the surrounding western part of the annular trough was subdued more that the eastern part, exhibiting original depths of 43–68 m (currently 46 m) and 49–55 m (currently 50 m), respectively. Furthermore, a quantitative model for the porosity change caused by the Chesapeake Bay impact was developed utilizing the modeled density distribution. The model shows that, compared with the surrounding platform, the porosity increased immediately after impact up to 8.5% in the collapsed and brecciated crater center (currently +6% due to post‐impact compaction). In contrast, porosity decreased by 2–3% (currently ?3 to ?4.5% due to post‐impact compaction) in the peak‐ring region. The lateral variations in porosity at Chesapeake Bay crater are compatible with similar porosity variations at Mjølnir crater, and are considered to be responsible for the moderate Chesapeake Bay gravity signature (annular low of ?8 mGal instead of ?15 mGal). The analysis shows that the reconstructions and the long‐term alterations due to post‐impact burial are closely related to the impact‐disturbed target‐rock volume and a brecciated region of laterally varying thickness and depth‐varying physical properties. The study further shows that several crater morphological and structural parameters are prone to post‐impact burial modification and are either exaggerated or subdued during post‐impact burial. Preliminary correction factors are established based on the integrated reconstruction and post‐impact deformation analysis. The crater morphological and structural parameters, corrected from post‐impact loading and modification effects, can be used to better constrain cratering scaling law estimates and impact‐related consequences.  相似文献   

14.
Abstract— An approximately 0.4 km diameter elliptical structure formed in Devonian granite in southwestern Nova Scotia, herein named the Bloody Creek structure (BCS), is identified as a possible impact crater. Evidence for an impact origin is based on integrated geomorphic, geophysical, and petrographic data. A near‐continuous geomorphic rim and a 10 m deep crater that is infilled with lacustrine sediments and peat define the BCS. Ground penetrating radar shows that the crater has a depressed inner floor that is sharply ringed by a 1 m high buried scarp. Heterogeneous material under the floor, interpreted as deposits from collapse of the transient cavity walls, is overlain by stratified and faulted lacustrine and wetland sediments. Alteration features found only in rim rocks include common grain comminution, polymict lithic microbreccias, kink‐banded feldspar and biotite, single and multiple sets of closely spaced planar microstructures (PMs) in quartz and feldspar, and quartz mosaicism, rare reduced mineral birefringence, and chlorite showing plastic deformation and flow microtextures. Based on their form and crystallographic orientations, the quartz PMs consist of planar deformation features that document shock‐metamorphic pressures ≤25 GPa. The age of the BCS is not determined. The low depth to diameter ratio of the crater, coupled with anomalously high shock‐metamorphic pressures recorded at its exposed rim, may be a result of significant post‐impact erosion. Alternatively, impact onto glacier ice during the waning stages of Wisconsinian deglaciation (about 12 ka BP) may have resulted in dissipation of much impact energy into the ice, resulting in the present morphology of the BCS.  相似文献   

15.
The 455 Ma old Lockne crater in central Sweden is a well-preserved and accessible instance of marine impact crater. The process of formation of the over 7 km wide crater (referred to as inner crater) in crystalline Proterozoic basement is numerically modeled under the assumption of a 45° oblique impact of an asteroid-like impactor. The 3D version of the SOVA multi-material hydrocode is used to model the shock wave propagation through the target, transient crater growth, material ejection in water and basement target, and water and fragmented rock ejecta expansion. The model results in a crater formation with the greatest ejection and melting transferred in the downrange direction. The model reproduces the growth of the water crater accompanied by the growth of a “wall” of ejected water at its outer margin. The basement ejecta are mostly trapped in this transient “water wall”. Only the largest ejected rock fragments could break through this water wall and thus reach distances farther than about 6 km from the center of the target. The model predicts approximately of impact melt formation, less than 10% of which is ejected outside of the inner (basement) crater, whereas the rest is reckoned to have remained within the inner crater. We assume that most of the ejected melt occurs as sand-sized fragments in the resurge sediments that formed subsequent to the collapse of the water crater that resulted in the powerful backflow of water. The model results are in accordance with several important details of the known geology of the crater. The model also outlines the difference in the marine crater formation processes in contrast to a crater with similar size formed on land.  相似文献   

16.
Abstract— Impact structures developed on active terrestrial planets (Earth and Venus) are susceptible to pre‐impact tectonic influences on their formation. This means that we cannot expect them to conform to ideal cratering models, which are commonly based on the response of a homogeneous target devoid of pre‐existing flaws. In the case of the 1.85 Ga Sudbury impact structure of Ontario, Canada, considerable influence has been exerted on modification stage processes by late Archean to early Proterozoic basement faults. Two trends are dominant: 1) the NNW‐striking Onaping Fault System, which is parallel to the 2.47 Ga Matachewan dyke swarm, and 2) the ENE‐striking Murray Fault System, which acted as a major Paleoproterozoic suture zone that contributed to the development of the Huronian sedimentary basin between 2.45–2.2 Ga. Sudbury has also been affected by syn‐ to post‐impact regional deformation and metamorphism: the 1.9–1.8 Ga Penokean orogeny, which involved NNW‐directed reverse faulting, uplift, and transpression at mainly greenschist facies grade, and the 1.16–0.99 Ga Grenville orogeny, which overprinted the SE sector of the impact structure to yield a polydeformed upper amphibolite facies terrain. The pre‐, syn‐, and post‐impact tectonics of the region have rendered the Sudbury structure a complicated feature. Careful reconstruction is required before its original morphometry can be established. This is likely to be true for many impact structures developed on active terrestrial planets. Based on extensive field work, combined with remote sensing and geophysical data, four ring systems have been identified at Sudbury. The inner three rings broadly correlate with pseudotachylyte (friction melt) ‐rich fault systems. The first ring has a diameter of ?90 km and defines what is interpreted to be the remains of the central uplift. The second ring delimits the collapsed transient cavity diameter at ?130 km and broadly corresponds to the original melt sheet diameter. The third ring has a diameter of ?180 km. The fourth ring defines the suggested apparent crater diameter at ?260 km. This approximates the final rim diameter, given that erosion in the North Range is <6 km and the ring faults are steeply dipping. Impact damage beyond Ring 4 may occur, but has not yet been identified in the field. One or more rings within the central uplift (Ring 1) may also exist. This form and concentric structure indicates that Sudbury is a peak ring or, more probably, a multi‐ring basin. These parameters provide the foundation for modeling the formation of this relatively large terrestrial impact structure.  相似文献   

17.
Abstract— The Vredefort structure in South Africa was created by a meteorite impact about two billion years ago. Since that time, the crater has been deeply eroded; so to estimate its original size, researchers have had to rely heavily upon comparison to other terrestrial impact structures. Recent estimates of the original crater diameter range from 160 km to as much as 400 km. In this study, we combined the capabilities of both hydrocode and finite-element modeling, using the former to predict where the pressure of an impact-generated shock wave would have been high enough to form planar deformation features (PDFs) and shatter cones and the latter to follow the subsequent displacement of these shock isobars during the collapse of the crater. We established constraints on the sizes of the projectile and the transient crater (and, thus, on the size of the final crater) by comparing the observed locations of PDFs around Vredefort to the results of our simulations of impacts by projectiles of various sizes. These simulations indicate that a rocky projectile with a diameter of ～10 km, impacting vertically at a velocity of 20 km/s generates shock pressures that are consistent with the distribution of PDFs around Vredefort. These projectile parameters correspond to a transient crater ～80 km in diameter or a final crater ～120–160 km in diameter. Allowing for uncertainties in our modeling procedures, we consider final craters 120 to 200 km in diameter to be consistent with the observed locations of PDFs at Vredefort. The shock pressure contour corresponding to the formation of shatter cones is almost horizontal near the surface, making the locations of these features less useful constraints on the crater size. However, they may provide a constraint on the amount of erosion that has occurred since the impact.  相似文献   

18.
Lonar Crater is a young meteorite impact crater emplaced in Deccan basalt. Data from 5 drillholes, a gravity network, and field mapping are used to reconstruct its original dimensions, delineate the nature of the pre-impact target rocks, and interpret the emplacement mode of the ejecta. Our estimates of the pre-erosion dimensions are: average diameter of 1710 m; average rim height of 40 m (30–35 m of rim rock uplift, 5–10 m of ejected debris); depth of 230–245 m (from rim crest to crater floor). The crater's circularity index is 0.9 and is unlikely to have been lower in the past. There are minor irregularities in the original crater floor (present sediment-breccia boundary) possibly due to incipient rebound effects. A continuous ejecta blanket extends an average of 1410 m beyond the pre-erosion rim crest.In general, fresh terrestrial craters, less than 10 km in diameter, have smaller depth/diameter and larger rim height/diameter ratios than their lunar counterparts. Both ratios are intermediate for Mercurian craters, suggesting that crater shape is gravity dependent, all else being equal. Lonar demonstrates that all else is not always equal. Its depth/diameter ratio is normal but, because of less rim rock uplift, its rim height/diameter ratio is much smaller than both fresh terrestrial and lunar impact craters. The target rock column at Lonar consists of one or more layers of weathered, soft basalt capped by fresh, dense flows. Plastic deformation and/or compaction of this lower, incompetent material probably absorbed much of the energy normally available in the cratering process for rim rock uplift.A variety of features within the ejecta blanket and the immediately underlying substrate, plus the broad extent of the blanket boundaries, suggest that a fluidized debris surge was the dominant mechanism of ejecta transportation and deposition at Lonar. In these aspects, Lonar should be a good analog for the fluidized craters of Mars.  相似文献   

19.
Abstract— Large impact crater formation is an important geologic process that is not fully understood. The current paradigm for impact crater formation is based on models and observations of impacts in homogeneous targets. Real targets are rarely uniform; for example, the majority of Earth's surface is covered by sedimentary rocks and/or a water layer. The ubiquity of layering across solar system bodies makes it important to understand the effect target properties have on the cratering process. To advance understanding of the mechanics of crater collapse, and the effect of variations in target properties on crater formation, the first “Bridging the Gap” workshop recommended that geological observation and numerical modeling focussed on mid‐sized (15–30 km diameter) craters on Earth. These are large enough to be complex; small enough to be mapped, surveyed and modelled at high resolution; and numerous enough for the effects of target properties to be potentially disentangled from the effects of other variables. In this paper, we compare observations and numerical models of three 18–26 km diameter craters formed in different target lithology: Ries, Germany; Haughton, Canada; and El'gygytgyn, Russia. Based on the first‐order assumption that the impact energy was the same in all three impacts we performed numerical simulations of each crater to construct a simple quantitative model for mid‐sized complex crater formation in a subaerial, mixed crystalline‐sedimentary target. We compared our results with interpreted geological profiles of Ries and Haughton, based on detailed new and published geological mapping and published geophysical surveys. Our combined observational and numerical modeling work suggests that the major structural differences between each crater can be explained by the difference in thickness of the pre‐impact sedimentary cover in each case. We conclude that the presence of an inner ring at Ries, and not at Haughton, is because basement rocks that are stronger than the overlying sediments are sufficiently close to the surface that they are uplifted and overturned during excavation and remain as an uplifted ring after modification and post‐impact erosion. For constant impact energy, transient and final crater diameters increase with increasing sediment thickness.  相似文献   

20.
Depending on their sizes, impact craters have either simple or complex geometries. Peak‐ring craters such as the Chicxulub impact structure possess a single interior ring of peaks and hills and a flat interior floor. The exact mechanisms leading to the formation of a morphological peak‐ring are still a matter of debate. In this study, analog modeling was used to study the flow field of a collapsing central uplift. A 3‐D‐printed cast was used to bring the analog material in the shape of an overheightened central uplift that was based on numerical modeling. The cast was then quickly removed and the central peak collapsed, forming a flattened broad mound that spread out onto the annular moat of the crater cavity. A subwoofer was used to fluidize the granular target material. The kinematics of the collapse were analyzed with the aid of particle image velocimetry, revealing a downward and outward collapse of the central uplift. This mode of collapse is partly in agreement with numerical models, in particular for the initial and middle phases. The overthrusting of the collapsing central peak onto the inward moving crater floor predicted by numerical modeling was observed, though to a lesser degree. A peak‐ring, however, could not be reproduced since the collapse came to a halt before the central peak was completely leveled. Nevertheless, the method provides qualitative insights into the kinematics of collapse phenomena. This experimental study provides independent support of the theory of acoustic fluidization, in addition to numerical simulations.  相似文献   

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