Peak Ring Research Articles

Post‐impact structural crater modification due to sediment loading: An overlooked process

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.

Tectonics of complex crater formation as revealed by the Haughton impact structure, Devon Island, Canadian High Arctic

The results of a systematic field mapping campaign at the Haughton impact structure have revealed new information about the tectonic evolution of mid-size complex impact structures. These studies reveal that several structures are generated during the initial compressive outward-directed growth of the transient cavity during the excavation stage of crater formation: (1) sub-vertical radial faults and fractures; (2) sub-horizontal bedding parallel detachment faults; and (3) minor concentric faults and fractures. Uplift of the transient cavity floor toward the end of the excavation stage produces a central uplift. Compressional inward-directed deformation results in the duplication of strata along thrust faults and folds. It is notable that Haughton lacks a central topographic peak or peak ring. The gravitational collapse of transient cavity walls involves the complex interaction of a series of interconnected radial and concentric faults. While the outermost concentric faults dip in toward the crater center, the majority of the innermost faults at Haughton dip away from the center. Complex interactions between an outward-directed collapsing central uplift and inward collapsing crater walls during the final stages of crater modification resulted in a structural ring of uplifted, intensely faulted (sub-) vertical and/or overturned strata at a radial distance from the crater center of ∼5.0-6.5 km. Converging flow during the collapse of transient cavity walls was accommodated by the formation of several structures: (1) sub-vertical radial faults and folds; (2) positive flower structures and chaotically brecciated ridges; (3) rollover anticlines in the hanging-walls of major listric faults; and (4) antithetic faults and crestal collapse grabens. Oblique strike-slip (i.e., centripetal) movement along concentric faults also accommodated strain during the final stages of readjustment during the crater modification stage. It is clear that deformation during collapse of the transient cavity walls at Haughton was brittle and localized along discrete fault planes separating kilometer-size blocks.

Geological overview and cratering model for the Haughton impact structure, Devon Island, Canadian High Arctic

Abstract— The Haughton impact structure has been the focus of systematic, multi-disciplinary field and laboratory research activities over the past several years. Regional geological mapping has refined the sedimentary target stratigraphy and constrained the thickness of the sedimentary sequence at the time of impact to ˜1880 m. New 40Ar–39Ar dates place the impact event at ˜39 Ma, in the late Eocene. Haughton has an apparent crater diameter of ˜23 km, with an estimated rim (final crater) diameter of ˜16 km. The structure lacks a central topographic peak or peak ring, which is unusual for craters of this size. Geological mapping and sampling reveals that a series of different impactites are present at Haughton. The volumetrically dominant crater-fill impact melt breccias contain a calcite-anhydrite-silicate glass groundmass, all of which have been shown to represent impact-generated melt phases. These impactites are, therefore, stratigraphically and genetically equivalent to coherent impact melt rocks present in craters developed in crystalline targets. The crater-fill impactites provided a heat source that drove a post-impact hydrothermal system. During this time, Haughton would have represented a transient, warm, wet microbial oasis. A subsequent episode of erosion, during which time substantial amounts of impactites were removed, was followed by the deposition of intra-crater lacustrine sediments of the Haughton Formation during the Miocene. Present-day intra-crater lakes and ponds preserve a detailed paleoenvironmental record dating back to the last glaciation in the High Arctic. Modern modification of the landscape is dominated by seasonal regional glacial and niveal melting, and local periglacial processes. The impact processing of target materials improved the opportunities for colonization and has provided several present-day habitats suitable for microbial life that otherwise do not exist in the surrounding terrain.

Complex geology of two large impact craters in Tyrrhena Terra, Mars: Detailed analysis using MEX HRSC camera data

Tyrrhena Terra, located just north of the roughly 200 km Hellas impact basin, is a typical region of the ancient cratered southern highlands on Mars. Its base material is a mixture of Hellas ejecta, prebasin remnants, and highland terrain, all later saturated with smaller (<200 km) impact craters. Fluvial and lacustrine deposits, erosion, tectonic movements, volcanic materials and aeolian processes have subsequently modified the region further. In this study we take a closer look at two adjacent unnamed craters in Tyrrhena Terra, located at 24.5°S, 80.8°E (crater “A”) and 23.9°S, 79.3°E (crater “B”). The craters are covered by the ∼20–80 m/pixel Mars Express High Resolution Stereo Color imager (HRSC) multispectral data, which together with MOC, THEMIS, and MOLA data sets allows us to make very detailed analysis of the area. We describe several identified geological and geomorphological units with their individual characteristics (e.g., morphology, color, cratering records, and elevation differences) and interpret their individual evolution. The crater floors show several unique material types of both depositional and erosional origin. Crater A has a large 200 m high central massif and a central peak ring as well as a unique low‐lying terrain type, “honeycomb terrain,” with narrow 50–200 m high ridges and intervening pits. Additionally, both craters exhibit erosional features similar to each other and nearby craters, indicating episodes of deposition of various material types in both craters. We find that the crater pair reveals many characteristics which are typical of the N Hellas rim but are not found elsewhere.

Hypervelocity collisions into continental crust composed of sediments and an underlying crystalline basement: comparing the Ries (∼24 km) and Chicxulub (∼180 km) impact craters

The Chicxulub and Ries impact craters were excavated from layered continental terrains that were composed of carbonate-bearing sedimentary sequences and underlying crystalline silicate basement materials. The Chicxulub and Ries impact events were sufficiently large to produce complex peak-ring impact craters. The walls of transient craters and excavation cavities, with diameters of 12–16 km for the Ries and 90–100 km for Chicxulub, collapsed to form final crater diameters of ∼24 and ∼180 km, respectively. Debris from both the sedimentary and crystalline layers was ejected during crater formation, but the bulk of the melting occurred at depth, in the silicate basement. The volume of melt and proportion of melt among shock-metamorphosed debris was far larger at Chicxulub, producing a central melt sheet ∼3 km in depth. The central melt sheet was covered with melt-bearing polymict breccias and, at the Ries, similar breccias (crater suevites) filled the central cavity. Also at the Ries (and presumably at Chicxulub), large hill-size megablocks of crystalline basement material were deposited near the transient crater rim. Blocks and megablocks of sedimentary lithologies were ejected into the modification zone between the peak ring and final crater rim, while additional material was slumping inward during crater growth, and buried beneath a fallout deposit of melt-bearing polymict breccias. The melt and surviving clasts in the breccias are dominantly derived from the deeper, basement lithologies. At greater distances, however, the ejecta is dominated by near-surface sedimentary lithologies, large blocks of which landed with such high energy that they scoured and eroded the pre-existing surface. The excavation and ejecta pattern produced lithological and chemical variations with radial distance from the crater centers that evolve from basement components near the crater centers to sedimentary components far from the crater centers. In addition, carbonate (and anhydrite in the case of Chicxulub) was vaporized, producing environmentally active gases. The vaporized volume produced by the Ries impact event was too small to dramatically alter the evolution of life, but the vaporized volume produced by the Chicxulub impact event is probably a key factor in the Cretaceous–Tertiary boundary mass extinction event.

Numerical modeling of an impact‐induced hydrothermal system at the Sudbury crater

Large impact events, like the one that formed the Sudbury crater in Ontario, Canada, at 1.85 Ga, significantly increase the temperature of target rocks. The heat sources generated by such an impact event can drive the circulation of groundwater, establishing a hydrothermal system. We report on the results of numerical modeling of postimpact cooling with and without the presence of water at the Sudbury crater. A hydrothermal system is initiated in the annular trough between the peak ring and final crater rim, perhaps venting through faults that bound blocks of the crust in the modification zone of the crater. Although circulation through the overlying breccias may occur in the center of the crater, the central melt sheet is initially impermeable to circulating fluids. By ∼105 years the central melt sheet crystallizes and partially cools, allowing fluids to flow through it. Host rock permeability is the main factor affecting fluid circulation and lifetimes of hydrothermal systems. High permeabilities lead to a rapid system cooling, while lower permeabilities allow a steady transport of hot fluids to the surface, resulting in high surface temperatures for longer periods of time than cooling by conduction alone. The simulations presented in this paper show that a hydrothermal system at a Sudbury‐sized impact crater can remain active for several hundred thousand to several million years, depending on assumed permeability. These results suggest that a hydrothermal system induced by an impact event can remain active for sufficiently long periods of time to be biologically significant, supporting the idea that impact events may have played an important biological role, especially early in Earth's history.

Structure and impact indicators of the Cretaceous sequence of the ICDP drill core Yaxcopoil‐1, Chicxulub impact crater, Mexico

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.

Chicxulub central crater structure: Initial results from physical property measurements and combined velocity and gravity modeling

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.

Impact lithologies and their emplacement in the Chicxulub impact crater: Initial results from the Chicxulub Scientific Drilling Project, Yaxcopoil, Mexico

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.

Tectonic influences on the morphometry of the Sudbury impact structure: Implications for terrestrial cratering and modeling

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.

Empirical and theoretical comparisons of the Chicxulub and Sudbury impact structures

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.

Emplacement of Sulfide Deposits in the Copper Cliff Offset Dike during Collapse of the Sudbury Crater Rim: Evidence from Magnetic Fabric Studies

Quartz diorite offset dikes in the country rocks surrounding the 1.85 Ga Sudbury impact structure were emplaced within fractures formed during hypervelocity impact, providing conduits for the injection of quartz diorite impact melts. The offset dikes host major Ni-Cu-PGE deposits, with the radial Copper Cliff dike hosting ~15 percent of the known Sudbury ore. The anisotropy of magnetic susceptibility of the Copper Cliff dike was studied to determine the nature of the fabrics that developed during emplacement of the quartz diorite and the associated sulfide bodies. The anisotropy of magnetic susceptibility reveals a predominantly northwest-southeast-striking, steeply dipping foliation and a steeply northwest- to vertically plunging lineation. The foliation along both the eastern and western sides of the dike is counterclockwise-oblique to the north-south to north-northwest–south-southeast-striking, vertically dipping dike margins. The magnetic fabrics in the dike are consistent with a component of strike-slip displacement during emplacement of the quartz diorite melt. The data are interpreted to indicate that the Copper Cliff dike acted as a melt-lubricated dextral transfer fault during collapse of the inferred inner rim of the central peak ring to accommodate ultra-high strain rate displacements on previously documented pseudotachylite-rich normal fault zones. Collapse may have caused injection of sulfide-rich melts into the Copper Cliff dike leading to the formation of Cu-Ni-PGE deposits. Collapse of the inner rim of the central peak ring is thought to have occurred too soon after the impact to allow for fractionation of the sulfides from the impact melt sheet. Therefore, it is suggested the sulfide-rich impact melts were derived principally from the target rocks.

Hubble Space Telescope/WFPC2 Images of the GG Tauri Circumbinary Disk

We present the first visible wavelength images of the GG Tau circumbinary ring obtained with the Hubble Space Telescope's (HST) Wide Field and Planetary Camera 2. Scattered light from the ring is detected in both V- and I-band images. The images show that the ring is smooth, except for a small gap that could be a shadow caused by material between the stars and the ring. The spokes seen extending from the stars to the ring in ground-based adaptive optics images are not seen in our data, which suggests that they may be image artifacts. The nearside/farside surface brightness ratio is 6.9 in the I band, consistent with forward scattering by small dust grains. The azimuth of the peak ring surface brightness appears offset by 13° from the azimuth closest to us, as seen in previous near-IR HST observations. This may indicate that the ring is warped or somehow shadowed by the circumstellar disks. The color of the ring is redder than the combined light from the stars as observed by HST, confirming previous measurements that indicate that circumstellar disks may introduce extinction of light illuminating the ring. We detect a bright, compact arc of material 03 from the secondary star at an azimuth opposite the primary. It appears to be too large to be a circumstellar disk and is not at the expected location for dust trapped at a Lagrange point.

Hydrocode Simulations of Chicxulub Crater Collapse and Peak-Ring Formation

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.

Shallow seismic velocity structure of the Chicxulub impact crater from modelling of Rg dispersion using a genetic algorithm

A 20 element seismic array was deployed onshore across the 65 Ma. Chicxulub impact crater, Yucatan, Mexico as part of an integrated onshore–offshore seismic experiment. Fundamental and higher mode group velocity dispersion curves have been calculated through the application of multiple filter and phase match techniques to high frequency (0.2–5 Hz) Rayleigh waves recorded on the array and originating from local quarry blasts. Several 1-D shear wave velocity-depth models across the crater have been produced through the optimisation of these dispersion curves using a genetic algorithm. These models are resolved to a depth of circa 3.5 km and provide information on the velocity structure of the upper crater sequences and post-impact sedimentation, placing constraints on crater morphology and post impact sedimentation patterns. Shear wave velocities of 0.8–2.2 km s −1 are modelled within the interpreted Tertiary sedimentary sequence. An inverted velocity gradient is observed over the upper 300 m of the sediments with the exception of a circular region across the crater centre, suggesting the crater influenced sedimentation throughout the Tertiary. The base of the Tertiary sequence is modelled at circa 1–1.5 km depth where a thin layer with velocities up to 30% higher than the overlying sediments is observed. Immediately below this high velocity layer a circa 200 m thick low velocity zone with velocities of 1.0–2.3 km s −1 is interpreted as a layer of suevitic impact breccia. A shallowing to the base of this layer to circa 1.3 km depth between 30 and 40 km radius from the crater centre would be consistent with the existence of a peak ring occurring as a topographic high above the crater floor.

Spaceborne imaging radar-C (SIR-C) observations of groundwater discharge and wetlands associated with the Chicxulub impact crater, northwestern Yucatan Peninsula, Mexico

Analyses of spaceborne imaging radar-C (SIR-C) data and field data from the northwestern Yucatan Peninsula, Mexico, demonstrate that spaceborne multifrequency polarimetric radars are excellent tools for characterizing patterns of wetland flooding. Seasonal flooding can be detected in most types of forest and marsh in the radar backscatter magnitude and phase data of both L and C band. Field observations made in the wet and dry seasons concurrent with the space missions and chemical analyses of floodwaters confirm that flooding is the product of discharge from the Yucatan aquifer, which consists of a fresh-water lens floating on seawater. This discharge controls the distribution of wetlands. Therefore, vegetation and flooding patterns, mapped with SIR-C imagery, provide valuable information on the hydrogeology of the region. Radar-image maps of wetlands and flooding indicate that there are three major zones of groundwater discharge that correlate with structures of the buried Chicxulub crater—zone 1 with the peak ring, zone 2 with the crater rim, and zone 3 with the exterior ring. Zone 1 has sulfate-poor discharge, unlike the sulfate-rich discharge in zones 2 and 3. The highest discharge is in zone 3, where the buried crater is closest to the surface. This groundwater-discharge pattern can be explained by tidal pumping of fresh water to the surface through high permeability zones developed in the Tertiary carbonates overlying crater faults and escarpments.

Peak-ring rim collapse accommodated by impact melt-filled transfer faults, Sudbury impact structure, Canada

Quartz diorite dikes in the footwall of the 1.85 Ga Sudbury impact structure were emplaced within fractures formed during hypervelocity impact. The anisotropy of magnetic susceptibility of the radial Copper Cliff dike reveals a steeply dipping foliation that is counterclockwise oblique along both the eastern and western sides of the dike, and a steeply northwest plunging lineation. The data suggest that the Copper Cliff dike acted as a melt-filled dextral transfer fault during collapse of the inner rim of the central peak ring of the impact crater to accommodate ultrahigh-strain-rate displacements. Peak-ring rim collapse may have caused sulfide-rich melts to be injected into the Copper Cliff dike, leading to the formation of large Cu–Ni–platinum group element deposits.

Peak-ring formation in large impact craters: geophysical constraints from Chicxulub

A seismic reflection and three-dimensional wide-angle tomographic study of the buried, ∼200-km diameter, Chicxulub impact crater in Mexico reveals the kinematics of central structural uplift and peak-ring formation during large-crater collapse. The seismic data show downward and inward radial collapse of the transient cavity in the outer crater, and upward and outward collapse within the central structurally uplifted region. Peak rings are formed by the interference between these two flow regimes, and involve significant radial transport of material. Hydrocode modeling replicates the observed collapse features. Impact-generated melt rocks lie mostly inside the peak ring; the melt appears to be clast-rich and undifferentiated, with a maximum thickness of 3.5 km in the center.

Ringed structural zones with deep roots formed by the Chicxulub impact

The Chicxulub meteorite/comet impact of 65 Ma occurred on the present northeast coast of the Yucatan peninsula of Mexico and is considered by many to have caused a mass extinction at the end of the Cretaceous period. Impact craters are traditionally described in terms of ring structures. Deeply rooted faults underlying four previously recognized ring zones of the Chicxulub impact crater, as revealed on new deep seismic reflection profiles, demonstrate the catastrophic and diverse deformations that produced these zones. The innermost annular zone at Chicxulub coincides with a central peak ring. The seismic data show that the peak ring is composed of low‐velocity (<5 km s−1), chaotically reflective material, interpreted as impact breccia, lying on top of blocks of pre‐impact strata that were downdropped 4–6 km. Normal faults within the Cretaceous strata and deep, inward dipping reflections characterize two deformation zones at radial distances of 55–65 and 85–98 km. These normal faults accommodated the collapse, during the modification stage of crater development, of large (radial width of 5–25 km) blocks of pre‐impact strata uplifted during the excavation stage. The structures root in the lower crust at depths of 15–30 km. Blind thrusts indicated by monoclines in pre‐impact sedimentary rocks and deep, inward dipping reflections within the crystalline basement mark the outermost ring of deformation. These monoclines correlate with small topographic changes on the Cretaceous‐Tertiary boundary layer at radial distances of 120–135 km. Palinspastic restoration of coherent blocks of downdropped pre‐impact strata enabled improved estimates of the size and shape of the hole formed by the impact in a prominent reflector at 3‐ to 4‐km depth. This hole is slightly elliptical along a SE‐NW major axis (radius of 68 versus 63 km).

Chicxulub: The third dimension of a multi-ring impact basin

The buried 65 Ma Chicxulub impact structure in Mexico is the largest well-preserved impact crater known on Earth. Seismic reflection data have revealed Chicxulub to be a multi-ring basin—it has the morphology of the largest impact craters in the solar system. We use these seismic data to relate surface morphology and near-surface structure to deeper deformation within the crust to provide the first high-resolution look into the third dimension of a multi-ring impact basin. We observe three distinct topographic ring types: crater rims, peak rings, and outer rings; each is associated with a different style of deep deformation. Crater rims are the head scarp of the terrace zone formed during the collapse of the transient cavity. The peak ring at Chicxulub appears to have formed when the central uplift collapsed, overthrusting and overriding the terrace zone. The impact has affected the whole crust; the outer rings at Chicxulub are linked to whole-crustal deformation in which the middle crust and lower crust have moved inward and downward. Strong reflections that dip craterward at ∼35° cut the entire crust and connect normal faulting in the sedimentary section with zones of downthrown Moho at a crater radius of ∼35–55 km. Weakly developed exterior rings appear as thrust faults with small offsets; these appear to be the progenitors of the more significant, normally faulted outer rings in multi-ring basins.