Large Impact Structures Research Articles

Long-term isostatic relaxation of large terrestrial impact structures: structural characteristics inferred from scaled analogue experiments

Crater floor fractures are prominent post-cratering structural vestiges that are known from large impact craters on rocky celestial bodies. Two mechanisms have been proposed to explain the formation of crater floor fractures: emplacement of horizontal igneous sheets below crater floors and isostatic re-equilibration of crust underlying target rocks, i.e., crustal relaxation. Here, we use two-layer analogue experiments to model the deformation of lower and upper crust following crater formation, scaled to the physical conditions on Earth, to explore the structural and kinematic consequences of crustal relaxation. Specifically, the structural evolution of model upper crust was systematically analysed for various initial depths and diameters of crater floors, gleaned from previous numerical models for average continental crust. The analogue modelling results provide quantitative estimates of the duration, geometry and distribution of deformation zones in the upper crust and, for the first time, a quantitative relationship between the diameter, depth and fracture geometry of crater floors. The experiments also show that crater floor uplift is accomplished by long-wavelength subsidence of the crater periphery, which may operate on time scales of hundreds of thousands of years in nature. We conclude that patterns of natural crater floor fractures, including impact melt rock dikes known from the Sudbury and Vredefort impact structures, can be caused by long-term uplift of the crater floor, compensated by lateral crustal flow toward the crater centre.

Giant impact on early Ganymede and its subsequent reorientation

Ganymede has an ancient impact structure called a furrow system. The furrow system is the largest impact structure in the outer solar system, and the impact should have significantly affected Ganymede’s early history; however, its effects are poorly understood. No attention has been given to the center of the furrow system coinciding with Ganymede's tidal axis, indicating that mass redistribution induced by the furrow-forming impact caused a reorientation (true polar wander) of Ganymede. We propose that the impact ejecta created a mass anomaly that reoriented the impact site toward the tidal axis. We found that an impactor with a radius of 150 km and an incidence angle between 60° and 90° most accurately reproduces the current location of the furrow system. We predict that future explorations would reveal remnant topographic profiles or gravity anomalies associated with the furrow-forming impact and reorientation. Additionally, various possible explanations for the reorientation of Ganymede, such as an impactor-origin mascon beneath the basin or a thickness variation in the lithosphere, should be studied.

Cratering and Tectonic History of the Largest Uranian Satellite, Titania: New Insights Enabled by Image Reprocessing

From heavily cratered Umbriel to extensively tectonized Miranda, Titania is an intermediary of the Uranian system: heavily cratered, yet tectonically modified. An outstanding mystery in Titania's crater population is its apparent relative lack of large (>30 km) craters. However, progress has been limited by the coverage and quality of images available. Here, we present a new map of Titania enabled by reprocessing Voyager images to reduce the effects of motion blur. Of note, we identify a network of fractures, a set of lineaments that may represent a large multi-ring impact structure, and newly identified catenae. These findings suggest Titania's crater population is missing large craters due to viscous relaxation, tectonic resurfacing, and/or planetocentric debris, and does not necessarily require cryovolcanic resurfacing. In preparation for future missions to the Uranian system, this work presents foundations for identifying imaging targets that can contribute to furthering our understanding of the history and evolution of the Uranian system in a broader context of icy satellite evolution.

Scaliger Crater Region on the Moon: Implications for the Australe North Basin and magmatism in the region

Australe North (35.5°S, 96°E) is a pre-Nectarian impact basin north of Mare Australe, first identified using the GRAIL data. It does not show clear topographic signatures typical of any large impact structure on the Moon. However, results from GRAIL (Gravity Recovery and Interior Laboratory) mission suggested the presence of a ∼ 880 km wide basin whose boundary does not coincide with the earlier identified Australe Basin. In this study, we investigate the Scaliger Crater region to understand the controls that the proposed Australe North Basin played on the local geology in the region. The location of the Scaliger Crater at the intersection of the rims of two pre-Nectarian Basins, Milne and the proposed Australe North provides a unique geological setting to reconstruct the region's geological history. We provide geological evidence substantiating the existence of the rim of the Australe North Basin, as has been proposed using gravity data. Based upon the spectral signatures and morphometry, we suggest the presence of a mafic pluton and/or lower crustal/mantle rocks in the region and provide constraints on its emplacement timescale. For the first time, Late-stage volcanism has been reported inside the Bowditch Crater and Lacus Solitudinis, emplaced at ∼1.7 Ga and ∼ 2.3 Ga, respectively, suggesting prolonged volcanism at the eastern lunar nearside-farside boundary inside an obscured pre-Nectarian basin.

Holocene impact craters on Earth

Abstract Impact craters are formed by collisions of cosmic bodies moving with hypervelocity. The formation of these features is not restricted to the distant geological past; new structures are constantly being created and at least 13 confirmed impact craters and crater fields have formed during the Holocene alone. This short review paper: (1) introduces the basics of the impact cratering process to physical geographers and Quaternary geologists; (2) provides a short description of representative examples of such features (Morasko, Kaali, Kamil, Ilumetsa); and (3) discusses the similarities and differences among very small craters, and contrasts these with larger impact structures. This manuscript may be useful to researchers planning to test whether a small Quaternary depression in the ground may be of impact origin.

Deglaciation history and subsequent lake dynamics in the Siljan region, south‐central Sweden, based on new LiDAR evidence and sediment records

AbstractThe Siljan region hosts Europe's largest impact structure. The high‐relief landscape, with a central granite dome bordered by lake basins, contains an array of glacial and shore‐level landforms. We investigated its deglaciation history by mapping and analysing landforms on high resolution LiDAR (light detection and ranging)‐based digital surface models coupled with well‐dated sediment successions from peat and lake sediment cores. The granite dome and bordering areas are characterized by streamlined terrain and ribbed moraine with a streamlined overprint. These suggest an ice‐flow direction from north‐northwest (NNW) with wet‐based thermal conditions prior to deglaciation. During its retreat, the ice sheet was split into thinner plateau ice and thicker basin ice. Sets of low‐gradient glaciofluvial erosion channels suggest intense ice‐lateral meltwater drainage across gradually ice‐freed slopes, while ‘down‐the‐slope’ erosion channels and eskers show meltwater drainage from stagnated plateau ice. Thick basin ice receded with a subaqueous margin across the deep Siljan–Orsasjön Basin c. 10,700–10,500 cal. bp. During ice recession the ingression of the Baltic Ancylus Lake led to diachronous formation of highest shoreline marks, from ~207 m in the south to ~220 m above sea level (a.s.l.) in the north. Differential uplift resulted in shallowing of the water body, which led to the isolation of the Siljan–Orsasjön Basin from the Baltic Basin at c. 9800 cal. bp. The post‐isolation water body – the ‘Ancient Lake Siljan’ – was drained through the ancient Åkerö Channel with a water level at 168–169 m a.s.l. during c. 1000 years. A later rerouting of the outlet to the present course was initiated at c. 8800 cal. bp, which led to a lake‐level lowering of 6–7 m to today's level of Lake Siljan (~162 m a.s.l.). This study shows the strength of an integrated methodological approach for deciphering the evolution of a complex landscape, combining highly resolved geomorphological analysis with well‐dated sediment successions.

Utilization of the Voronoï tessellation for improved planetary age determination: A case study of a large rampart crater in Thaumasia Planum (Mars)

Rampart craters are typical impact structures of Mars and are characterized by fluidized or lobate ejecta blankets. Such structures may result from impact processes interacting with volatiles in the crust and/or with the atmosphere. Determining the age of their formation, therefore, is potentially important for understanding the evolutionary histories of the hydrosphere and atmosphere of the planet. In this study, we analyzed the morphological characteristics of a large, relatively well-preserved rampart crater (rim-to-rim diameter 6.5 ​× ​6.4 ​km) located in Thaumasia Planum within the equatorial region of Mars. The counting of small impact craters on its ejecta blanket and the construction of crater size-frequency distribution (CSFD) allow absolute dating of this structure and identification of multiple resurfacing events. The dating was achieved by analyzing only primary impact craters; secondary impact craters (SICs) and fragmented single impactor craters (FSICs), instead, were identified and discarded on the basis of their morphological characteristics and distributions. In order not to alter the counting area, a revised approach using the Voronoï tessellation was carried out to define the surfaces relative to SICs/FSICs. This approach enables more precise age estimation in crater counting. We find that determined ages for younger events such as those of post-impact resurfacing are significantly different between two approaches of 1) counting only primary craters and utilizing the Voronoï tessellation and 2) counting primary craters ​+ ​SICs/FSICs and no Voronoï tessellation. Based on the approach 1), this large rampart impact structure is determined to have formed in the Late Noachian (3.86 ​Ga −3.6+0.11), while three post-impact resurfacing events apparently occurred during the Amazonian (1.63 ​Ga −0.66+0.66, 736 ​Ma −130+130, 276 ​Ma −12+12). To verify the proposed approach, a second rampart crater in the NW sector of Thaumasia Planum was analyzed with this procedure. The absolute dating showed an impact structure always formed in the Late Noachian (3.79 ​Ga −3.6+0.11), with two major resurfacing events in the Amazonian (2.48 ​Ga −1.0+0.72, 176 ​Ma −11+11). • Absolute dating of large rampart craters in the equatorial region of Mars. • Construction of a proper crater size-frequency distribution considering only primary impact craters. • Utilization of Voronoi tessellation for the removal of secondary impact craters.

High precision multi-collector 40Ar/39Ar dating of moldavites (Central European tektites) reconciles geochrological and paleomagnetic data

Moldavites (Central European tektites) are genetically related to the meteorite impact event that produced the 24-km diameter Ries crater (Germany) during the Langhian, and representing one of the youngest large impact structures on Earth. Despite the numerous geochronological studies over the last decades and the potential implications for stratigraphic, paleontological and paleoclimatic studies, the age of the Ries impact is still debated. In this study, I investigate in detail moldavite samples by multicollector 40Ar/39Ar laser dating in order to address the age of the Ries impact. Data were obtained relative to the key Fish Canyon sanidine (FCs) and Alder Creek sanidine (ACs) reference materials, over a period of nearly two years, in four irradiations of different duration. Results, completed through the step-heating and the total fusion techniques, demonstrate an excellent intrasample and intersample reproducibility of moldavites and prove that analytical performances of moldavites in terms of uncertainties on the 40Ar*/39ArK ratios are in line with those achievable by the most widely used FCs and ACs reference minerals. Results from total fusion analyses yield a mean RACsMoldavite = 12.4952 ± 0.0038 (±2σ) and a mean RFCsMoldavite = 0.52106 ± 0.00014 (±2σ). Data also allow to define a direct mean RFCsACs = 0.041703 ± 0.000032 (±2σ) and an indirect (using moldavites as intermediary), more precise, mean RFCsACs = 0.041700 ± 0.000019 (±2σ). Using the internally consistent astronomically-calibrated ages for the ACs (1.1848 ± 0.0012 Ma) and the FCs (28.201 ± 0.046 Ma) reference minerals from the literature, the R-values yield indistinguishable ages for moldavites of 14.7495 ± 0.0045 (± 0.016 Ma, including all known source of errors) and 14.7486 ± 0.0039 Ma (± 0.025 Ma), respectively. The most recent astronomically calibrated age for FCs (28.176 ± 0.023 Ma) yields a slightly younger, but more precise, age of 14.7355 ± 0.0039 Ma (± 0.013 Ma). Results place definitively the age of the Ries impact in the reverse polarity chron C5ADr of the ATNTS2022, and suggest that the Ries impact preceded the switch from reversed to normal chron by at least 127 ± 15 ka.

Geophysics and origin of the Deniliquin multiple-ring feature, Southeast Australia

Major geophysical elements of the Deniliquin multiple-ring feature, a spatially distinct buried structure in mainland southeast Australia, suggest it represents the deep-seated root zone of a large impact structure from which the upper levels, including the original crater, central uplift and associated breccia, have been eroded. Principal features include (A) a multiple ring total magnetic intensity (TMI) pattern; (B) a central quiet magnetic zone; (C) circular Bouguer gravity patterns; (D) an underlying mantle Moho rise about 10 km shallower than under the adjacent Tasman Orogenic Belt; (E) radial faults associated with magnetic and demagnetized anomalies. The minimum radius of the TMI ring is ~260 km, including a central circular quiet magnetic zone about ~120 km in radius. In the east the feature is faulted against the Early Paleozoic Lachlan Orogenic belt whereas in the west it deflects the northwest and northeast trending Early Cambrian ~525–514 Ma Kanmantoo Group. An interpretation of the Deniliquin multiple-ring feature in terms of an orocline is inconsistent with the discontinuity between the feature and surrounding regional structural trends. Limits on the age of the Deniliquin feature are defined by the onset of the Adelaide fold belt at 514 +/− 5 Ma (Foden et al., 1999) and the ~427–417 Ma age of intrusive Silurian granites. Basement drill cores within the Deniliquin multiple-ring feature show Boehm lamellae but did not reveal shock metamorphic textures. Based on its multiple-ring structure, structural distinction from surrounding regional structural trends, occurrence of a central quiet magnetic zone, an underlying shallower Moho and radial faults associated with magnetic bodies, we suggest the Deniliquin multiple-ring feature may represent the deep seated root zone of a large impact structure.

The origin of the potassium‐rich annular zones at the Bosumtwi impact structure, Ghana, investigated by field study, radiometric analysis, and first cosmogenic nuclide data

AbstractThe 10.5‐km‐diameter, 1 Ma Bosumtwi impact structure in Ghana is one of the youngest, large impact structures known on Earth. The preservation of the morphology of its ejecta deposits, with an annular moat and outer ridge resembling those of rampart impact craters on Mars, makes Bosumtwi a remarkable impact structure on the African continent. An airborne radiometric survey of the southwestern part of Ghana reveals enigmatic circular feature enriched in potassium, coinciding with the crater rim and an outer ejecta ridge at Bosumtwi. The goal of this study is to investigate possible origins of these features, by impact processes (shock metamorphic effects, impact‐induced hydrothermal systems) or postimpact surficial processes (erosion, weathering). The origin of these features is discussed here based on field observations, ground‐based radiometric measurements, and first cosmogenic nuclide analyses (10Be). The data indicate that the rim and outer ridge were eroded more rapidly than the rest of the impact structure. Accordingly, the downward advance of the weathering fronts in the annular moat, after ejecta emplacement, are responsible for leaching of K from the lateritic residual observed at the surface. The Bosumtwi impact structure is, therefore, a valuable natural laboratory to investigate the factors controlling erosion and weathering processes in the Ashanti belt since impact 1 Ma ago. Simulations of vertical profiles of 10Be concentration further constrain local variations of the erosion rate. In light of this study, circular K anomalies in radiometric surveys might be indicative of potential impact structures in tropical regions.

Gamma-ray spectrometry of the Araguainha impact structure, Brazil: Additional insights into element mobilization due to hydrothermal alteration.

We present the analysis of airborne and ground gamma-ray spectrometry signatures of the Araguainha impact structure, located in central Brazil, the largest impact structure in South America with ~ 40 km diameter. The airborne data are total gamma-ray counts per second collected along flight lines spaced 1 km apart. The ground gamma-ray data are concentrations of potassium, uranium, and thorium isotopes calculated from radiations measured in three individual channels. The objectives are to distinguish lithologies within the structure, which have naturally distinctive radiogenic signatures, and identify potential post-impact hydrothermal alteration zones, as indicated by high K concentrations. Based on results obtained by numerical modeling of the crater formation, we infer the locations of potential occurrences of target rocks that may have undergone hydrothermal alteration as a result of the impact. The deviations from the background potassium concentration show significant anomalous K values at the center and in the northwestern crater rim, where high concentrations of U are also observed. The numerical model shows that ideal temperature conditions for hydrothermal fluid circulation were attained right after pos-impact gravitational stabilization.

Timescales of impact melt sheet crystallization and the precise age of the Morokweng impact structure, South Africa

Impact cratering was a fundamental geological process in the early Solar System and, thus, constraining the timescales over which large impact structures cool is critical to understanding the thermal evolution and habitability of early planetary crusts. Additionally, impacts can induce mass extinctions and establishing the precise timing of the largest impacts on Earth can shed light on their role in such events. Here we report a high-precision zircon U–Pb geochronology study of the Morokweng impact structure, South Africa, which appears to have a maximum present-day diameter of ∼80 km. Our work provides (i) constraints on the cooling of large impact melt sheets, and (ii) a high-precision age for one of Earth's largest impact events, previously proposed to have overlapped the ca. 145 Ma Jurassic–Cretaceous (J–K) boundary. High-precision U–Pb geochronology was performed on unshocked, melt-grown zircon from five samples from a borehole through approximately 800 m of preserved impact melt rock. Weighted mean 206Pb/238U dates for the upper four samples are indistinguishable, with relative uncertainties (internal errors) of better than 20 ka, whereas the lowermost sample is distinguishably younger than the others. Thermal modeling suggests that the four indistinguishable dates are consistent with in situ conductive cooling of melt at this location within 30 kyr of the impact. The younger date from the lowest sample cannot be explained by in situ conductive cooling in line with the overlying samples, but the date is within the ∼65 kyr timeframe for melt-present conditions in footwall rocks below the impact melt sheet that is indicated by our thermal model. The Morokweng impact event is here constrained to 146.06 ± 0.16 Ma (2σ; full external uncertainty), which precedes current estimates of the age of the J–K boundary by several million years.

Chicxulub museum, geosciences in Mexico, outreach and science communication – built from the crater up

Abstract. The Chicxulub science museum is special, in that it is built around an event in geological time representing a turning point in the planet's history and which brings together the Earth system components. Studies on the Chicxulub impact, mass extinction and Cretaceous–Paleogene boundary provide an engaging context for effective geoscience communication, outreach and education. The museum is part of a research complex in Yucatán Science and Technology Park in Mexico. Natural history museums with research components allow for the integration of up-to-date advances, expanding their usefulness and capabilities. The impact ranks among the major single events shaping Earth's history, triggering global climatic change and wiping out ∼76 % of species. The ∼200 km Chicxulub crater is the best preserved of three large terrestrial multi-ring impact structures, being a natural laboratory for investigating impact dynamics, crater formation and planetary evolution. The initiative builds on the interest that this geological site has for visitors, scholars and students by developing wide-reaching projects, a collaboration network and academic activities. The Chicxulub complex serves as a hub for multi- and interdisciplinary projects on the Earth and planetary sciences, climate change and life evolution, fulfilling a recognized task for communication of geosciences. After decades of studies, the Chicxulub impact remains under intense scrutiny, and this programme with the core facilities built inside the crater will be a major player.

Shock-deformed zircon from the Chicxulub impact crater and implications for cratering process

Large impact structures with peak rings are common landforms across the solar system, and their formation has implications for both the interior structure and thermal evolution of planetary bodies. Numerical modeling and structural studies have been used to simulate and ground truth peak-ring formative mechanisms, but the shock metamorphic record of minerals within these structures remains to be ascertained. We investigated impact-related microstructures and high-pressure phases in zircon from melt-bearing breccias, impact melt rock, and granitoid basement from the Chicxulub peak ring (Yucatán Peninsula, Mexico), sampled by the International Ocean Discovery Program (IODP)/International Continental Drilling Project (IODP-ICDP) Expedition 364 Hole M0077A. Zircon grains exhibit shock features such as reidite, zircon twins, and granular zircon including “former reidite in granular neoblastic” (FRIGN) zircon. These features record an initial high-pressure shock wave (>30 GPa), subsequent relaxation during the passage of the rarefaction wave, and a final heating and annealing stage. Our observed grain-scale deformation history agrees well with the stress fields predicted by the dynamic collapse model, as the central uplift collapsed downward-then-outward to form the peak ring. The occurrence of reidite in a large impact basin on Earth represents the first such discovery, preserved due to its separation from impact melt and rapid cooling by the resurging ocean. The coexistence of reidite and FRIGN zircon within the impact melt–bearing breccias indicates that cooling by seawater was heterogeneous. Our results provide valuable information on when different shock microstructures form and how they are modified according to their position in the impact structure, and this study further improves on the use of shock barometry as a diagnostic tool in understanding the cratering process.

Post-Impact Faulting of the Holfontein Granophyre Dike of the Vredefort Impact Structure, South Africa, Inferred from Remote Sensing, Geophysics, and Geochemistry

Better characterization features borne from long-term crustal modification processes is essential for understanding the dynamics of large basin-forming impact structures on Earth. Within the deeply eroded 2.02 Ga Vredefort Impact Structure in South Africa, impact melt dikes are exposed at the surface. In this study, we utilized a combination of field, remote sensing, electrical resistivity, magnetic, petrographical, and geochemical techniques to characterize one such impact melt dike, namely, the Holfontein Granophyre Dike (HGD), along with the host granites. The HGD is split into two seemingly disconnected segments. Geophysical modeling of both segments suggests that the melt rock does not penetrate below the modern surface deeper than 5 m, which was confirmed by a later transecting construction trench. Even though the textures and clast content are different in two segments, the major element, trace element, and O isotope compositions of each segment are indistinguishable. Structural measurements of the tectonic foliations in the granites, as well as the spatial expression of the dike, suggest that the dike was segmented by an ENE–WSW trending sinistral strike-slip fault zone. Such an offset must have occurred after the dike solidified. However, the Vredefort structure has not been affected by any major tectonic events after the impact occurred. Therefore, the inferred segmentation of the HGD is consistent with long-term crustal processes occurring in the post-impact environment. These crustal processes may have involved progressive uplift of the crater floor, which is consistent with post-impact long-term crustal adjustment that has been inferred for craters on the Moon.

Estimating Venusian thermal conditions using multiring basin morphology

Despite their critical roles in Venus’s geological evolution, neither heat flow through the Venusian lithosphere nor the corresponding tectonic regime in its geological past is well constrained. However, because impact basin formation is sensitive to thermal conditions at depth, studying large basin development can provide crucial insights into the past geological conditions of a planet. Here we model the formation of Mead Basin, the largest impact structure on Venus, and its two ring faults at approximately 190 km and 270 km diameter, to determine the thermal conditions in Venus’s crust and upper mantle at the time of impact. For present-day surface temperatures, we find that lithospheric thermal gradients no higher than 14 K km−1, corresponding to surface heat fluxes of 28 mW m−2, are required to reproduce the morphology of Mead Basin. These values are less than half of what is expected for an active lid planet, implying that Venus may have had a stagnant lid when Mead Basin formed, between 0.3 and 1 billion years ago. Modelling of Mead Basin, the largest impact feature on Venus, shows that it could only have got its shape, with the two ring faults at the correct position, if Venus were in a stagnant lid regime at the time of Mead Basin formation, between 0.3 and 1 billion years ago.

Imaging the Chicxulub Central Crater Zone from Large-Scale Seismic Acoustic Wave Propagation and Gravity Modeling

Large impact structures are characterized by peak ring and central uplifts with lateral/vertical mass transport during late formation stages. Here we investigate the Chicxulub crater, which has been surveyed by an array of marine, aerial and land-borne geophysical methods. Seismic reflection surveys in its central sector have shown lack of resolution, making it difficult to image the central uplift. We develop an integrated seismic and gravity model for the structural elements, imaging the central uplift and melt and breccia units. The 3D velocity model built from interpolation of seismic data is validated using perfectly matched layer seismic acoustic wave propagation modeling, optimized at grazing incidence using the shift in the frequency domain. Modeling shows that lack of illumination relates to seismic energy that remains trapped in an upper low-velocity zone corresponding to the carbonate sediments, upper melt/breccias and surrounding faulted blocks. After conversion of seismic velocities into a volume of density values, we apply parallel forward gravity modeling to constrain the size and shape of the central uplift, which has a ~ 40 km diameter concave upwards top lying at ~ 3.5–4.5 km depth. The preferred model provides a high-resolution image of crater units and structure. The gravity response of modeled units shows asymmetries in structure and the distribution of breccias, melt and target carbonates. Finally, we apply an adjoint reverse time migration approach for seismic imaging using the density and velocity models built for the acoustic wave propagation and gravity modeling, which allows improved modeling of the crater structure.

The TanDEM-X Digital Elevation Model and Terrestrial Impact Structures

We utilized the TanDEM-X digital elevation model (DEM) for investigating the complete record of confirmed terrestrial impact structures with respect to its suitability to support geological analysis. The consistently high resolution and high accuracy of this model is a prerequisite for detailed morphological studies. This DEM represents an interesting repository to aid in preparing and executing fieldwork for the exploration of new impact crater candidates. For a selection of small, mid-sized, and large impact structures, we here compare the TanDEM-X results with those from other DEMs that were derived either with synthetic aperture radar interferometry or from optical stereo pairs. Our analysis includes high-resolution mapping and the generation of detailed elevation cross sections. Only for very small impact craters, when the diameter is in the order of the pixel posting of TanDEM-X of 12 m or when the texture of the local environment does not support radar remote sensing, accurate analysis is hampered. Our results demonstrate that the high horizontal and vertical accuracies of the TanDEM-X DEM, coupled with its dense pixel grid, provide a considerable improvement in space-borne remote sensing of the complete record of simple and complex terrestrial impact structures over a wide range of diameters.

Shock Metamorphic Features in the Archean Simlipal Complex, Singhbhum Craton, Eastern India: Possible Remnant of a Large Impact Structure

ABSTRACT The Simlipal complex in eastern India is an elliptical structure with diameter of ca. 50 km coinciding with an elliptical region of high gravity. It overlies the Archaean basement of the Singhbhum craton and has a ring structure characterized by complex series of concentric ridges with inward dipping slopes. Quartz clasts in melt-breccias from the complex display diagnostic shock metamorphic features such as two to three sets of decorated and annealed planar deformation features (PDFs), spall and concussion micro-fractures, and the presence of coesite. They also preserve microstructures suggestive of crystallographically-controlled melting/amorphization along two or three planar directions, and within concussion micro-fractures. The inferred shock pressures in excess of 40–60 GPa are possible only during bolide impact. The Simlipal structure is possibly the remnant of a large complex impact crater having an original diameter of at least 50 km. Although zircons from an impact melt rock furnish a concordia age of 3107±14 Ma, in the absence of any unambiguous shock metamorphic effects in them, it is difficult to assign the age to the impact event. Therefore, the age of the impact is currently unconstrained.

Thickness of orthopyroxene-rich materials of ejecta deposits from the south pole-Aitken basin

The South Pole-Aitken (SPA) basin is the largest impact structure on the Moon and is believed to have excavated the orthopyroxene (Opx)-rich lower crust and/or upper mantle materials. For the complex craters outside of the SPA transient cavity, the origin of the Opx-rich central peaks is possibly from either the Opx-rich materials of the SPA ejecta deposits or the unexcavated lower crust and/or upper mantle. To estimate the thickness of the Opx-rich materials of the SPA ejecta deposits, this study investigated large complex craters (dozens of kilometers) that have penetrated the Opx-rich materials and exposed deeper mafic-poor crustal material based on spectra extracted from small fresh craters (sub-kilometer scale). The amount of foreign material introduced to these large complex craters by other lunar impact events was estimated to guarantee the least influence on the compositional analysis. The study results suggest that a 56 km-diameter crater at ~640 km northwest of the SPA center is large enough to penetrate the Opx-rich materials of the SPA ejecta deposits, which are thinner than ~4.7 km at this location. This result also indicates that the intense bombardment history of the large craters and basins outside of the SPA transient cavity excavated and redistributed a large amount of crustal material across the basin, possibly resulting in the heterogeneous distribution of mafic-rich and mafic-poor materials on the SPA surface.