Complex Impact Crater Research Articles

An Image Retrieval Method for Lunar Complex Craters Integrating Visual and Depth Features

In the geological research of the Moon and other celestial bodies, the identification and analysis of impact craters are crucial for understanding the geological history of these bodies. With the rapid increase in the volume of high-resolution imagery data returned from exploration missions, traditional image retrieval methods face dual challenges of efficiency and accuracy when processing lunar complex crater image data. Deep learning techniques offer a potential solution. This paper proposes an image retrieval model for lunar complex craters that integrates visual and depth features (LC2R-Net) to overcome these difficulties. For depth feature extraction, we employ the Swin Transformer as the core architecture for feature extraction and enhance the recognition capability for key crater features by integrating the Convolutional Block Attention Module with Effective Channel Attention (CBAMwithECA). Furthermore, a triplet loss function is introduced to generate highly discriminative image embeddings, further optimizing the embedding space for similarity retrieval. In terms of visual feature extraction, we utilize Local Binary Patterns (LBP) and Hu moments to extract the texture and shape features of crater images. By performing a weighted fusion of these features and utilizing Principal Component Analysis (PCA) for dimensionality reduction, we effectively combine visual and depth features and optimize retrieval efficiency. Finally, cosine similarity is used to calculate the similarity between query images and images in the database, returning the most similar images as retrieval results. Validation experiments conducted on the lunar complex impact crater dataset constructed in this article demonstrate that LC2R-Net achieves a retrieval precision of 83.75%, showcasing superior efficiency. These experimental results confirm the advantages of LC2R-Net in handling the task of lunar complex impact crater image retrieval.

New geophysical and zircon data support an impact hypothesis for the Velingara structure (Senegal)

On Earth, impact structures are rare in intertropical zones. Here we evaluate the 35–40 km diameter Velingara depression in Senegal as a candidate impact structure. The depression has a topographic relief of only 50 m and is essentially buried under modern sediments. For the first time, potential-field ground observations and a microstructural survey of detrital zircons for shock features were conducted at Velingara. A 15 mGal gravity low in a 10 km diameter central anomaly was found, while significant (100 nT) amplitude magnetic field anomalies were observed in the depression. Forward modeling was used to explain the crustal sources of the anomalies, arguing in favor of the presence of a buried impact structure. An electron backscatter diffraction survey of circa 5,000 detrital zircons from a sample of modern alluvium identified a single granular zircon; orientation data for the grain does not record the former presence of reidite, but does closely resemble grains with highly dispersed neoblast orientations that have only been reported from impact settings. Age determination by LA-ICP-MS indicates the unusual granular zircon recorded an age of ca. 550 Ma. Results of the geophysical data and zircon microstructural survey reported here are consistent with, but do not yet prove, an impact origin hypothesis for the Velingara structure. The difference in amplitude between the diameter of the depression, and that of the gravity anomaly remains puzzling. Several impact scenarios are considered, which include the subsequent evolution via surface processes of a 10-km crater into a larger depression, or a large-scale (d = 40 km) complex impact crater with a peculiar geophysical anomaly limited to its central region.

Meteorite impact at Ramgarh, India: Petrographic and geochemical evidence, and new geochronological insights

The impact origin of Ramgarh crater in India has been a topic of debate for several decades due to its conspicuous morphology, with recent studies confirming the structure as a complex impact crater. In this work, we present the results from an integrated petrographic and geochemical study to evaluate the impact origin of the crater, in conjunction with introduction of a new geochronological data for the crater through zircon U Pb systematics. Petrographic studies of the target Vindhyan sedimentary rocks show the presence of shock fabrics such as planar fractures, decorated planar deformation features, brecciation, melted grain boundaries, diaplectic glass, and vitrified matrix. Geochemically, the lithounits have high concentrations of Cr, Ni, Cu, and the corresponding chondrite normalized values show notably elevated levels than crustal average. The elevated levels of Cr, Ni and Cu in the target rocks indicate its interaction with an extra-terrestrial object. The positive correlation between Ni and Co, higher concentration of Cr (9–623.35 ppm), and more importantly the average Ni/Cr (~0.006- ~ 1.3), Ni/Co (~1.8 to ~22) and Cr/Co (~57–~368) ratios in the target lithounits of Ramgarh, point to a differentiated achondrite being the possible impactor. The zircon U Pb geochronological data from two samples in this study do not show significant Pb-loss and U enrichment. However, the relatively younger age of 528–395 Ma, depicted by quartzite sample (AN-4), along with the discordance and distinguishable Pb-loss, might hint that the impact event at Ramgarh have occurred between this timeframe. This age is considerably closer to the Precambrian-Cambrian Boundary (543 Ma) and, it also encompasses the age corresponding to the mid-Ordovician Meteor Event (OME) (467.5 ± 0.28 Ma). As this study infers that the potential impactor at Ramgarh is a differentiated achondrite and not a L-chondrite, the OME age can be ignored, further narrowing the age of Ramgarh impact structure. Our study also confirms the Ramgarh structure as an impact crater, and constrains the possible timing of the impact. • Tracking the impact event at Ramgarh Crater, NW India • Shock features in quartz grains mainly includes decorated PDFs, and melting. • Whole rock, trace and REE geochemistry reveals impact event. • Zircon U Pb ages of 528 to 395 Ma, which is the first convincing age for this crater

The Nadir Crater offshore West Africa: A candidate Cretaceous-Paleogene impact structure.

Evidence of marine target impacts, binary impact craters, or impact clusters are rare on Earth. Seismic reflection data from the Guinea Plateau, West Africa, reveal a ≥8.5-km-wide structure buried below ~300 to 400 m of Paleogene sediment with characteristics consistent with a complex impact crater. These include an elevated rim above a terraced crater floor, a pronounced central uplift, and extensive subsurface deformation. Numerical simulations of crater formation indicate a marine target (~800-m water depth) impact of a ≥400-m asteroid, resulting in a train of large tsunami waves and the potential release of substantial quantities of greenhouse gases from shallow buried black shale deposits. Our stratigraphic framework suggests that the crater formed at or near the Cretaceous-Paleogene boundary (~66 million years ago), approximately the same age as the Chicxulub impact crater. We hypothesize that this formed as part of a closely timed impact cluster or by breakup of a common parent asteroid.

3D magnetic characterization of the Aimorés structure (Brazil) aiming to identify interpretation elements under cover: Impact crater or circular intrusion?

The Aimorés intrusive complex (AIC), located in the Araçuaí belt (Brazil), has been characterized for many years as the Aimorés Impact Crater due to its topographic features, such as the circular morphology and the ring-shaped rim. Detailed geologic mapping finds a zoned igneous intrusion composed of granite and charnockite rocks in the outer rims, whereas the central part of the structure is covered by overburden. In the absence of direct evidence under the center of the structure, uncertainty remains and there is ambiguity about the geologic interpretation of the area. For this reason, we have used magnetic data to investigate the AIC structure in the subsurface. We explore the different possibilities of geologic scenarios for this region, namely, a pluton with eroded center, a zoned intrusion, a simple impact crater, and a complex impact crater through a synthetic study. Our synthetic tests revealed zoned intrusion and simple impact crater as possible geologic scenarios. Then, in the next stage, we characterize the AIC in three dimensions to unveil the subsurface geometry of the magnetic rocks. The inverted model found the geometry of a high susceptibility ring-shaped body that extends at 4 km depth, pointing to a circular zoned intrusion as the more likely interpretation. Our study has revealed two new geologic features in the study area: (1) the presence of two distinct zones under the central depression cover of the AIC and (2) a new zoned body at southeast of the AIC, which has not been mapped previously because of the overburden. We find the efficacy of using magnetic data for mapping postcollisional intrusions potentially associated with lithium mineralization in subsurface.

Resolving the age of the Haughton impact structure using coupled 40Ar/39Ar and U-Pb geochronology

The Haughton Dome located on Devon Island, in the Canadian Archipelago represents a well-preserved, moderate-sized, complex impact crater. Previous age constraints for the 24 km-diameter impact structure have ranged from ca. 21 Ma to ca. 39 Ma. Herein, we present a coordinated microstructural and in situ U-Pb study of zircon and monazite coupled with 40Ar/39Ar laser step heating of shock-melted K-feldspar clasts from shock metamorphosed gneissic fragments collected from the allochthonous impact breccia at Haughton. Moderately shocked zircon and monazite grains yield an age distribution consistent with an Archean protolith metamorphosed at ca. 1.9 Ga, whereas shock recrystallized zircon and monazite yield a lower intercept age of 31.8 ± 1.7 Ma (n = 48, MSWD = 0.58, P = 0.99). Four inverse isochron 40Ar/39Ar ages of shocked feldspar clasts yield a weighted mean age of 31.04 ± 0.37 Ma (MSWD = 0.98, P = 0.40), within uncertainty of the U-Pb lower concordia intercept. Ar diffusion modelling supports the interpretation of the impact age and helps resolve impact-driven age resetting. These results highlight the power of coupling multiple geochronologic techniques for determining impact ages, especially from targets with complex geologic histories. Furthermore, they resolve previous discrepancies in the age of the Haughton Dome and the interpretation of the post impact stratigraphy of the crater fill.

Self-Secondaries Formed by Cold Spot Craters on the Moon

Self-secondaries are a population of background secondaries, and they have been observed on top of impact melt and ballistically emplaced ejecta deposits on various planetary bodies. Self-secondaries are formed by impacts of sub-vertically launched ejecta, but the launch mechanism is not confirmed. The potential threat of self-secondaries to the theoretical and applicable reliability of crater chronology has been noted, but not constrained. Hitherto discovered self-secondaries were located around complex impact craters, but their potential existence around simple craters has not been discovered. Here we report the first discovery of self-secondaries around lunar cold spot craters, which are an extremely young population of simple craters formed within the past ~1 million years on the Moon. Self-secondaries are widespread on layers of cascading flow-like ejecta deposits around cold spot craters. The spatial density of self-secondaries dwarfs that of potential primary craters. The spatial distribution of self-secondaries is highly heterogeneous across the ejecta deposits. With respect to the impactor trajectory that formed cold spot craters, self-secondaries formed at the downrange of the ejecta deposits have the largest spatial density, while those at the uprange have the smallest density. This density pattern holds for all cold spot craters that were formed by non-vertical impacts, but self-secondaries do not exhibit other systematic density variations at different radial distances or at other azimuths with respect to the impactor trajectory. Among known mechanics of ejecting materials to the exterior of impact craters, impact spallation is the most likely scenario to account for the required large ejection velocities and angles to form self-secondaries. The production population of self-secondaries is estimated based on the highly diverse crater size-frequency distributions across the ejecta deposits of cold spot craters. For a better understanding of the impact history on the Moon, a systematic investigation for the effect of self-secondaries on lunar crater chronology is required.

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.

Morphological analysis and mapping of complex craters of Copernican age: Crookes, Ohm and Stevinus

The aim of this study is to analyze and compare morphological characters of three complex lunar impact craters- Crookes (10.3° S, 164.5° W), Ohm (18.4° N, 113.5° W) and Stevinus (32.5° S, 54.2° E), where Crookes and Ohm are located on the far side of the Moon and Stevinus is located on the near side of the Moon. High resolution data of Narrow Angle Camera (NAC) and Wide Angle Camera (WAC), onboard Lunar Reconnaissance Orbiter (LRO) of NASA is used for the morphological observations of all three craters. In this study we identify and compare various geomorphic features of three craters including central peak, central mounds, isolated mounds, diverse floor units: Hummocky, Intermediate and Smooth, melt ponds and cooling cracks. Using ArcGIS software, morphological maps of all three craters are generated where special attention is given to the crater floors, with NAC and WAC data as base-maps. Lunar Orbiter Laser Altimeter (LOLA) Digital Elevation Model (118 ​m) is used to generate elevation graphs for all three craters along four different topographic sections using which, relative elevation measurements of central peak, central mounds and raised crater rims and thickness of wall terraces are obtained. Using the ENvironment for Visualising Images (ENVI) software, 3-dimensional surface view models were generated. These 3D models give enhanced view of ‘highs’ and ‘lows’ of impact craters along with ponded surfaces trapped in between wall-terraces. All three craters are oblique impact craters with varying degrees of impact angles. Crookes exhibits later stage modification characters with an impact direction NNE and an impact angle between 15° and 45°. Ohm exhibits elongated crater planform with zone of avoidance in NNE direction. The impact angle is between 30° and 45°. Stevinus represents an impact angle higher than 15° and lower than 40°. The study reveals that there are striking similarities in the morphological features of these three craters of the same age along with few distinct characters of their own.

Results from the first geologic traverse on the topographic rim of a complex impact crater, Endeavour Crater, Mars

AbstractA geologic traverse along the rim of the 22-km-diameter Endeavour Crater by the Opportunity Mars rover has provided the first field geologic observations of outcrop-scale structure and stratigraphy at a complex impact crater, characteristics that were previously undocumented due to erosion of similar-size craters on Earth. Two findings of note are (1) the attitude of sheets, foliations, and contacts between rim impact breccias and pre-impact substrate is antiformal, the limbs dipping inward toward the center of the crater inside the crater rim and outward exterior to the crater rim; and (2) coherent blocks of crust segment the rim topographically and structurally into a series of right- and left-stepping elongate rises of variable size and orientation. These segments experienced differing magnitudes of uplift during crater formation along identified vertical scissors faults. Brecciation along the faults bounding rim segments created zones of enhanced subsurface fluid transport through the crater rim, potentially responsible for localized areas of aqueous alteration identified in outcrops near segment boundaries.

The Lomonosov Crater Impact Event: A Possible Mega‐Tsunami Source on Mars

AbstractRecent research suggests that major meteorite impact events into a Late Hesperian/Early Amazonian ocean likely produced a mega‐tsunami that would have resurfaced coastal areas in northwestern Arabia Terra. The orientations of the associated lobate deposits, a conspicuous type of landforms called Thumbprint Terrain, suggests that if an impact event triggered the mega‐tsunami, the most likely location of the source crater is within the northern plains regions situated north of Arabia Terra. This study focuses on the identification of impact craters that impacted into the ocean and are likely to have produced the tsunami. We selected 10 complex impact craters, based on their diameters, location, and geomorphic characteristics. Of those, the Late Hesperian ~120‐km‐diameter Lomonosov crater exhibits a unique topographic plan view asymmetry (compared to other similar‐sized and similar‐aged craters in the northern plains such as Micoud, Korolev, and Milankovic). We attribute its broad and shallow rim, in part, to an impact into a shallow ocean as well as its subsequent erosion from the collapsing transient water cavity. The likely marine formation of the Lomonosov crater, and the apparent agreement in its age with that of the Thumbprint Terrain unit (~3 Ga), strongly suggests that it was the source crater of the tsunami. These results have implications for the stability of a late northern ocean on Mars.

Geologic Analyses of the Causes of Morphological Variations in Lunar Craters Within the Simple‐to‐Complex Transition

AbstractThe diameter range of 15 to 20 km is within the transition from simple to complex impact craters located on the Moon. This range spans roughly a factor of 3 in impact energy for the same impactor speed, composition, and trajectory angle. We analyzed the global population of well‐preserved craters in this size range in order to assess the effects of target and impactor properties on crater shapes and morphologies. We observed that within this narrow diameter range, simple craters are confined to the highlands, and complex craters are more abundant in the mare. We found unusually deep craters around the highlands‐mare boundaries and favor the hypothesis that they form by impact cratering on high‐porosity terrain. We infer that target properties primarily contribute to the observed morphological variations in the craters. Simple crater formation is favored by a terrain that is more homogeneous in strength and topography, while transitional and complex crater formation is aided by heterogeneity in lithology, topography, or strength, or a combination of these parameters. Clearly visible impact melt deposits in a small percentage of simple craters, and two cases of craters differing in morphologies from their nearest neighbors in similar geologic settings, suggest that variation in impactor properties such as impact velocity and impactor size may have some role in causing morphological differences between similar‐sized craters.

Shocked quartz in polymict impact breccia from the Upper Cretaceous Yallalie impact structure in Western Australia

AbstractYallalie is a ~12 km diameter circular structure located ~200 km north of Perth, Australia. Previous studies have proposed that the buried structure is a complex impact crater based on geophysical data. Allochthonous breccia exposed near the structure has previously been interpreted as proximal impact ejecta; however, no diagnostic indicators of shock metamorphism have been found. Here we report multiple (27) shocked quartz grains containing planar fractures (PFs) and planar deformation features (PDFs) in the breccia. The PFs occur in up to five sets per grain, while the PDFs occur in up to four sets per grain. Universal stage measurements of all 27 shocked quartz grains confirms that the planar microstructures occur in known crystallographic orientations in quartz corresponding to shock compression from 5 to 20 GPa. Proximity to the buried structure (~4 km) and occurrence of shocked quartz indicates that the breccia represents either primary or reworked ejecta. Ejecta distribution simulated using iSALE hydrocode predicts the same distribution of shock levels at the site as those found in the breccia, which supports a primary ejecta interpretation, although local reworking cannot be excluded. The Yallalie impact event is stratigraphically constrained to have occurred in the interval from 89.8 to 83.6 Ma based on the occurrence of Coniacian clasts in the breccia and undisturbed overlying Santonian to Campanian sedimentary rocks. Yallalie is thus the first confirmed Upper Cretaceous impact structure in Australia.

Malvinas (Falkland) Plateau structure versus Mjølnir crater: Geophysical workflow template for proposed marine impact craters

AbstractA diagnostic geophysical‐based template, supported by modelling, is suggested to be used prior to, or in combination with geological/drilling data, when proposing a marine impact crater. The latter refers to impacts occurring in a marine setting and resulting in structures that are currently partially or totally underwater. The methodology is based on the well‐documented Mjølnir crater in the Barents Sea. The template has been developed in conjunction with the recently proposed and debated impact crater on the Malvinas (Falkland) Plateau in the South Atlantic. Despite their different sizes, their comparison adds to the ambiguous nature of the Malvinas structure and shows that the integrated analysis of seismic and potential field data and modelling is crucial for any interpretation of a marine impact crater without relevant geological information. The proposed workflow template utilizes all available geophysical data and is composed of a series of iterative steps, including a range of alternative nonimpact interpretations that must be discussed and accounted for. Subsequently, further iterative geophysical modelling is required to support and decipher the impact related processes. A more complex impact crater model and additional impact crater features can be resolved by physical property modelling. In all cases, a close spatial correspondence of the defined impact structure with potential field anomalies is a necessity to establish a causal relationship. We suggest that the diagnostic workflow template provides a methodology to be applied to future studies of the Malvinas structure, as well as to proposed marine (and, with minor adaptions, to nonmarine) impact craters in general.

Remote Sensing Identification of Possible Meteorite Impact Crater on Ciletuh, West Java

Ciletuh is a geopark area in West Java that has a semicircle-shaped cliff called mega-amphitheater. Morphogenesis of the amphitheater is still unknown. One possible cause is a meteorite impact that triggered a mega slump. This study aims to collect the evidence of meteorite impact. It has significant value for the geology of Indonesia since this region not yet has confirmed impact crater. In this research, image analysis from satellite remote sensing is used for identification of meteorite impact geomorphology. An analysis based on DEM unveils a ring morphology, a possible clue of an impact crater. The depression zone surrounded by a circular shape with an uplift area in the middle resembles characteristics of the complex impact crater. Meandering Ciletuh river indicates a compression due to a slump towards the southwest. Reconstruction of possible position toward the northeast (before slump deformation occurred) shows a clearer possible meteorite impact crater. A ground checking of identified structure proves that a bowl-shaped morphology found at the possible impact crater. Some steep dip structures have found, these might be due to an impact that causes the orientation of the structure to be upright. A stone that resembles the shatter-cone is found in the melange area. The melange is believed as a product of a sea trench million years ago. By using meteorite impact theory, it is also possible that the melange is a kind of impact breccia. This feature adds uniqueness to Ciletuh as a geopark.

Microstructural dynamics of central uplifts: Reidite offset by zircon twins at the Woodleigh impact structure, Australia

Research Article| September 27, 2018 Microstructural dynamics of central uplifts: Reidite offset by zircon twins at the Woodleigh impact structure, Australia Morgan A. Cox; Morgan A. Cox * 1Space Science and Technology Centre (SSTC), School of Earth and Planetary Science, Curtin University, Perth, Western Australia 6102, Australia *E-mail: morgan.cox@student.curtin.edu.au Search for other works by this author on: GSW Google Scholar Aaron J. Cavosie; Aaron J. Cavosie 1Space Science and Technology Centre (SSTC), School of Earth and Planetary Science, Curtin University, Perth, Western Australia 6102, Australia Search for other works by this author on: GSW Google Scholar Phil A. Bland; Phil A. Bland 1Space Science and Technology Centre (SSTC), School of Earth and Planetary Science, Curtin University, Perth, Western Australia 6102, Australia Search for other works by this author on: GSW Google Scholar Katarina Miljković; Katarina Miljković 1Space Science and Technology Centre (SSTC), School of Earth and Planetary Science, Curtin University, Perth, Western Australia 6102, Australia Search for other works by this author on: GSW Google Scholar Michael T.D. Wingate Michael T.D. Wingate 2Geological Survey of Western Australia, Department of Mines, Industry Regulation and Safety, Perth, Western Australia 6004, Australia Search for other works by this author on: GSW Google Scholar Geology (2018) 46 (11): 983–986. https://doi.org/10.1130/G45127.1 Article history received: 09 May 2018 rev-recd: 10 Sep 2018 accepted: 11 Sep 2018 first online: 27 Sep 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Morgan A. Cox, Aaron J. Cavosie, Phil A. Bland, Katarina Miljković, Michael T.D. Wingate; Microstructural dynamics of central uplifts: Reidite offset by zircon twins at the Woodleigh impact structure, Australia. Geology 2018;; 46 (11): 983–986. doi: https://doi.org/10.1130/G45127.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Impact cratering is a dynamic process that is violent and fast. Quantifying processes that accommodate deformation at different scales during central uplift formation in complex impact structures is therefore a challenging task. The ability to correlate mineral deformation at the microscale with macroscale processes provides a critical link in helping to constrain extreme crustal behavior during meteorite impact. Here we describe the first high-pressure-phase–calibrated chronology of shock progression in zircon from a central uplift. We report both shock twins and reidite, the high-pressure ZrSiO4 polymorph, in zircon from shocked granitic gneiss drilled from the center of the >60-km-diameter Woodleigh impact structure in Western Australia. The key observation is that in zircon grains that contain reidite, which forms at >30 GPa during the crater compression stage, the reidite domains are systematically offset by later-formed shock deformation twins (∼20 GPa) along extensional planar microstructures. The {112} twins are interpreted to record crustal extension and uplift caused by the rarefaction wave during crater excavation. These results provide the first physical evidence that relates the formation sequence of both a high-pressure phase and a diagnostic shock microstructure in zircon to different cratering stages with unique stress regimes that are predicted by theoretical and numerical models. These microstructural observations thus provide new insight into central uplift formation, one of the least-understood processes during complex impact crater formation, which can produce many kilometers of vertically uplifted bedrock in seconds. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Diversity of basaltic lunar volcanism associated with buried impact structures: Implications for intrusive and extrusive events

Relatively denser basalt infilling and the upward displacement of the crust–mantle interface are thought to be contributing factors for the quasi-circular mass anomalies for buried impact craters in the lunar maria. Imagery and gravity observations from the Lunar Reconnaissance Orbiter (LRO) and dual Gravity Recovery and Interior Laboratory (GRAIL) missions have identified 10 partially or fully buried impact structures where diversity of observable basaltic mare volcanism exists. With a detailed investigation of the characteristics of associated volcanic landforms, we describe their spatial distribution relationship with respect to the subsurface tectonic structure of complex impact craters and propose possible models for the igneous processes which may take advantage of crater-related zones of weakness and enable magmas to reach the surface. We conclude that the lunar crust, having been fractured and reworked extensively by cratering, facilitates substance and energy exchange between different lunar systems, an effect modulated by tectonic activities both at global and regional scales. In addition, we propose that the intrusion-caused contribution to gravity anomalies should be considered in future studies, although this is commonly obscured by other physical factors such as mantle uplift and basalt load.

Shock-induced kelyphite formation in the core of a complex impact crater

We present a compositional and textural analysis of shock-induced microtextures in garnet porphyroblasts in migmatitic garnet–cordierite–biotite paragneisses from the centre of the Vredefort impact structure, South Africa. Detailed imaging and major element analysis of deformation features in, and adjacent to, the garnet porphyroblasts record a complex, heterogeneous distribution of shock effects at the microscale. As the most competent silicate mineral in the assemblage, with the highest Hugoniot Elastic Limit and a wide pressure–temperature stability field, the porphyroblastic garnet preserves a more diverse shock deformation response compared to minerals such as quartz and feldspar, which underwent more comprehensive shock metamorphism and subsequent annealing. The garnet porphyroblasts display pre-impact fractures that are overprinted by later intra-granular Hertzian and distinctive planar fractures associated with the impact event. Shock-induced strain localization occurred along internal slip planes and defects, including pre-existing fractures and inclusion boundaries in the garnet. Symplectitic (kelyphitic) coronas commonly enclose the garnet porphyroblasts, and inhabit intra-granular fractures. The kelyphite assemblage in fractures with open communication beyond garnet grain boundaries is characterized by orthopyroxene—cordierite—sapphirine. Conversely, the kelyphite assemblage in closed-off intra-granular fractures is highly variable, comprising spatially restricted combinations of a secondary garnet phase with a majoritic component, Al-rich orthopyroxene, sapphirine and cordierite. The impedance contrast between garnet porphyroblasts and their inclusions further facilitated the formation of shock-induced features (Al-rich orthopyroxene coronas). Together, the textural and mineralogical data suggest that these features provide a record of oscillatory shock perturbations initiated under confining pressure beneath the transient crater floor. This occurred as the shocked rock volume underwent post-shock expansion, forming the core of the central uplift, and was followed by variable textural re-equilibration. This study thus provides a microtextural and mineralogical perspective of the shock regime within confined crust immediately prior to and during central uplift formation.

The structural inventory of a small complex impact crater: Jebel Waqf as Suwwan, Jordan

AbstractThe investigation of terrestrial impact structures is crucial to gain an in‐depth understanding of impact cratering processes in the solar system. Here, we use the impact structure Jebel Waqf as Suwwan, Jordan, as a representative for crater formation into a layered sedimentary target with contrasting rheology. The complex crater is moderately eroded (300–420 m) with an apparent diameter of 6.1 km and an original rim fault diameter of 7 km. Based on extensive field work, IKONOS imagery, and geophysical surveying we present a novel geological map of the entire crater structure that provides the basis for structural analysis. Parametric scaling indicates that the structural uplift (250–350 m) and the depth of the ring syncline (<200 m) are anomalously low. The very shallow relief of the crater along with a NE vergence of the asymmetric central uplift and the enhanced deformations in the up‐range and down‐range sectors of the annular moat and crater rim suggest that the impact was most likely a very oblique one (~20°). One of the major consequences of the presence of the rheologically anisotropic target was that extensive strata buckling occurred during impact cratering both on the decameter as well as on the hundred‐meter scale. The crater rim is defined by a circumferential normal fault dipping mostly toward the crater. Footwall strata beneath the rim fault are bent‐up in the down‐range sector but appear unaffected in the up‐range sector. The hanging wall displays various synthetic and antithetic rotations in the down‐range sector but always shows antithetic block rotation in the up‐range sector. At greater depth reverse faulting or folding is indicated at the rim indicating that the rim fault was already formed during the excavation stage.

A matter of minutes: Breccia dike paleomagnetism provides evidence for rapid crater modification

Abstract During an impact event, a crater’s transient structure adjusts gravitationally. Within medium-sized complex craters, a central uplift rises and collapses resulting in large-scale rotations of the target rock. Estimated crater modification rates from numerical models indicate that complex impact craters modify to a structurally stable state within tens of seconds to several minutes after excavation. However, there is little direct geologic evidence constraining these rates. We show how paleomagnetic measurements of lithic breccia dikes emplaced during crater excavation can be used to constrain the rate of crater modification within the central uplift of the ∼34-km-diameter Slate Islands impact structure, Ontario, Canada. The uniformity and linearity of paleomagnetic directions among the clasts and matrix of breccia dikes throughout the impact structure indicate that breccia dikes were frictionally heated above the magnetite Curie temperature (580 °C) during their emplacement and subsequently cooled in situ through magnetic blocking temperatures. The tight grouping of these paleomagnetic directions implies that these breccia dikes cooled and locked in magnetic remanence over a time interval in which the impact structure was not experiencing structural rotations and had already reached a stable state. Conductive cooling of the thinnest sampled breccia dike would have led to the recording of magnetic remanence approximately six minutes after emplacement. This constraint necessitates a stable crater structure only minutes after impact and presents a rare case in which a geological process can be resolved on such a short time scale.