U22A-01
The effect of impact angle on the formation of meteorite impact craters: Insight from 3D numerical modeling
Most meteorite impact structures on Earth or any other planetary surface are circular in plan view. This may seem unintuitive given that most impacts occur at an oblique angle of incidence to the target, but it is simply a consequence of the hypervelocity nature of meteorite impacts and the fact that the resulting crater are much smaller than the projectile. Although the oblique angle in the vast majority of impacts does not have a major effect on the shape (for angles >10-15°) it is likely that crater size and other structural characteristics are affected and our understanding of oblique impact is poor. We use numerical models to quantify the effect of impact angle on the cratering process. We have developed a 3D extension to the well-known hydrocode iSALE-2D that is specifically optimized to investigate late-stage crater- formation processes. In contrast to most previous modeling studies, which investigated only the early-stage effects of impact angle or single specific oblique impact scenarios, we have produced a suite of impact models spanning a range of impactor parameters (velocity, angle, size) and target properties (gravity, strength). We demonstrate with our models that crater efficiency (the size of the crater relative to the size of the projectile) decreases with decreasing angle of impact (measured from horizontal). In scaling relations based on experimental studies and theoretical considerations for vertical impacts it is primarily the kinetic energy of the impactor, as opposed to the momentum, that controls crater-size. Our models show that even for impact-angles as low as 30° the crater efficiency is dominated by the kinetic energy of the impactor in strengthless material; however, in material with internal friction momentum becomes more important with decreasing angle of impact. We provide a modified scaling relation that includes the effect of impact-angle and is based on the assumption that the crater dimensions depend only on the vertical component of the impact velocity. Finally, we find structural (subsurface) evidence at the central peak in particular (in case of complex craters) that is characteristic of oblique impact. These observations might be used to determine the direction of impact at real crater structures on Earth.
U22A-02
Is the Chicxulub Crater Asymmetry due to Target Asymmetry or Oblique Impact? Insight From Numerical Modeling
We combine numerical modeling with seismic data interpretation to understand the formation of the Chicxulub impact crater, and, ultimately, the impact's role in the K/P mass extinction. Seismic data across the Chicxulub impact crater reveal that the crater structure varies around the offshore portion of the crater. The most striking azimuthal variation is in the position of the Cretaceous sediments that step down in a terrace from the crater rim to beneath the topographic peak ring (megablock zone). In the NW the megablock zone extends from 78-km to 42-km radius, with an average slope of 10 degrees; at its innermost point it is 9 km below the peak ring. In the NE the megablock zone is narrower (from 58-km to 42-km radius), and shallower (6.5 km below the peak ring) than in the NW. There also appears to be no crater rim in the NE quadrant. The azimuthal variation in crater structure may be a consequence of oblique impact, target asymmetry, or both. An oblique impact at Chicxulub might have released 2-10 times more volatiles into the atmosphere than a near vertical impact, due to focusing of high shock-pressures in near-surface rocks, with drastic consequences for global climate. However, the seismic data also reveal asymmetry in pre-existing geologic features of the target that correlate with the asymmetries in the final crater, which suggests a strong influence of the target on crater formation. In the NW, the broader terrace zone is associated with shallow Cretaceous bathymetry and a sediment thickness of 3-km, while in the NE the narrower terrace zone is associated with a deep, water-filled basin, overlying a thicker layer of sediments at the time of impact. We use 2D and 3D numerical impact models to examine whether impact angle or the observed pre-impact target asymmetry can explain the asymmetries in final crater structure. Our results suggest that the major asymmetries in the rim and megablock zone are a direct consequence of the sediment and water layer thickening from west to east.
U22A-03 INVITED
Imaging the Asymmetric Chicxulub Impact Crater and Plans for Drilling
Formation, release of volatiles, and subsequent collapse of the 65 Ma Chicxulub impact crater are of key interest due to the impact's role in the Cretaceous-Paleocene (K/P) mass extinctions. Seismic data acquired in 1996 and 2005 image the buried and surprisingly asymmetric final crater, and highlight key features that are the target of proposed IODP-ICDP drilling. Gravitational collapse of the transient crater created a terrace zone consisting of faulted slump blocks that reach the greatest depth in the northwest part of the crater. Lying above the terrace zone closer to the center of the crater is the topographic peak ring; a geometry that suggests interaction of the inward slumping terrace zone and the rebounding central uplift is important for the formation of the peak ring. The peak ring rises higher above the crater floor in the west and northwest relative to the east and northeast. A Cenozoic basin overlies the peak ring and crater floor within the inner rim in all imaged azimuths except north and northeast where the inner rim is absent. The K/P surface, defined based on mapped reflections that correlate with the base of the Cenozoic basin, shows significant pre-existing relief on the Cretaceous seafloor. This relief appears to correlate with the observed asymmetries in terrace zone depth, peak ring relief and lack of a crater rim to the north and northeast, and therefore suggests that target heterogeneities strongly influence final crater structure. Reflectivity is present beneath the topographic peak ring along all imaged azimuths that may represent a lithologic base of the peak ring material, or a marker for an extinct hydrothermal system. Asymmetry in the peak ring allows for sampling the lithologies beneath the topographic peak ring at relatively shallow depths in order to explain how the proposed deep crustal material can result in lower velocities and densities than the surrounding impact rocks. Bright, discontinuous reflections to the interior of the topographic peak ring may represent the top of the impact melt sheet that lies beneath potential re-surge deposits and is thought to cap the central uplift observed on seismic and gravity data. A proposed IODP-ICDP drilling transect plans to penetrate and sample the shallowest peak ring and underlying dipping reflectivity in the offshore and the Cenozoic sediments deposits, potential surge deposits, melt sheet, and if possible the central uplift onshore to calibrate existing models for impact crater formation and the mass extinction.
U22A-04
Broad zone of structural uplift beneath the Chicxulub impact structure
Although meteorite impacts are a ubiquitous and fundamental geologic process affecting the terrestrial planets, they are relatively poorly understood. The Earth has comparatively few pristine craters, and only three large (>150 km diameter) impact basins: Chicxulub, Vredefort and Sudbury, of which Chicxulub is the best preserved. Seismic reflection data acquired across the offshore half of the Chicxulub crater in 1996 and 2005 reveal clear images of the target rocks and impact basin. There is no reflection data across the crater center, and therefore central crater structure at Chicxulub, and large impact craters in general, is a matter of some debate. Although we know that large craters possess particular features (structural uplift, impact melt rocks, impact breccias, a peak ring), the precise geometric relationship between these features remains uncertain. Models of Chicxulub constructed from geophysical data are diverse in part due to the lack of terrestrial examples and the inherent ambiguity of geophysical modeling, and also because drill holes within the impact basin have penetrated the uppermost crater deposits only. We have constructed a new model of central crater structure across Chicxulub, based upon inversions of geophysical data. Previous interpretations of the width of structural uplift beneath Chicxulub vary from 50 to 150 km. In 1996 and 2005 we acquired tomographic seismic and gravity data, and have performed both 3D travel-time and joint gravity and travel-time inversions to produce a well-constrained velocity model across the central crater. This model possesses a 15-25 km wide high-velocity-zone near the crater center, where rock velocity is >6.3 km s-1 below 5 km depth and, outside this zone, velocity gradually decreases. We interpret these velocities in terms of a broad 80-km wide zone of structural uplift, in which the central rocks originate from the lower crust, and the surrounding rocks from the mid and upper crust. The new velocity model across the central crater, which incorporates gravity constraints, is a major advance. The resolution of the new 3D tomographic velocity model significantly surpasses that of the 1996 model. Our interpretation of the velocities from the joint travel-time and gravity inversion in terms of lower, mid and upper crustal rocks is supported by regional refraction data, general crustal models, the lithology of basement clasts in Chicxulub impact breccias, impact scaling laws, observations at the similar-sized crater Vredefort, and dynamic models of crater formation.
U22A-05
Chicxulub Crater Infilling and Yucatan Carbonate Platform Development: Implications for the Evolution of Large Terrestrial Impact Craters
Stratigraphic analysis of the Yaxcopoil-1 core and seismic analysis of offshore 2D seismic data provide insight into the Tertiary infilling history of the Chicxulub impact basin. High-resolution logging of the carbonate-dominated rocks in Yax-1 provides details of the litho- and bio-stratigraphy and permits a preliminary Tertiary sequence stratigraphic analysis of the basin. Seismic analysis provides a broader view of Tertiary basin infilling and the history of Yucatan carbonate platform development. We have identified 10 lithofacies and 5 lithostratigraphic units based on data from the core and examination of 150 thin sections. Lithofacies are categorized as either redeposited or background facies. Redeposited facies include a wide variety of coarse and finer-grained gravity flow deposits while fine-grained background facies were deposited mainly from suspension. Depositional environments range from a steep, bathyal slope inside the Chicxulub crater inner rim to a neritic, outer carbonate ramp setting once the YucatÃÆ' n platform prograded seaward. We have identified 5 depositional sequences based on preliminary sequence stratigraphic analysis of the Tertiary succession in the core. Equivocal biostratigraphic data indicate that sequences 1-3 range from Early Paleocene to Early Eocene in age. The bases of sequences 1-4 contain redeposited carbonates interpreted as lowstand deposits. Sequences 4 and 5 consist mainly of background and fine-grained redeposited facies. By the top of sequence 4 it appears that the Yucatan platform had prograded over the position of the Yax-1 core. Seismic analysis identified at least 6 seismic units, the lower 5 of which appear to roughly correlate with the 5 lithostratigraphic units in the Yax-1 core based on available P-wave velocity data. The seismic stratigraphic analysis shows two major patterns of post-impact deposition within the basin. The earlier one is controlled by the underlying crater morphology where major accumulation (units A and B) is on both sides of the peak ring and infilled the western and central parts of the basin first. Later sedimentation is potentially controlled by changes in sea level as suggested by the deposition of clinoforms (unit C) and regional unconformities (top units C, D, E). The timing of the switch from impact-dominated to sea level-dominated sedimentation occurs within seismic unit C. If Chicxulub is representative, large marine impacts in tectonically quiescent regions may dominate local depositional environments for millions to tens of millions of years before returning control to eustasy. These successions will likely record shoaling and reduction of slope as the instantaneously created impact basin is filled over geologic timescales. Remarkably, most of the topographic scar along the southern Chicxulub crater margin appears to have been annealed within 10-15 Ma after the impact.
U22A-06
Geophysical detection of eroded and buried impact structures in layered sedimentary and sedimentary-igneous terrains: Foelsche impact structure and relatives, Australia
Weakly deformed Proterozoic and early Paleozoic basins of Australia contain perhaps the worlds best record of old impact structures, many being of Proterozoic age. Early discoveries resulted from identification of anomalous circular features on geological maps and aerial photographs. Recently, high quality aeromagnetic surveys are revealing circular and concentric magnetic anomalies in these basins. In most cases such anomalies occur in areas where sedimentary successions contains interbedded magnetic volcanic units or sills. Subsequent investigation of several sites has revealed evidence for impact. The best example, Foelsche impact structure, is used here as a model for impact-induced circular aeromagnetic anomalies in flat-lying, layered, sedimentary- igneous terrains. Foelsche lies within flat-lying Paleoproterozoic to Mesoproterozoic rocks of the McArthur Basin, Northern Territory (16 40 S, 136 47 E). It was discovered after an aeromagnetic survey revealed a prominent concentric magnetic anomaly. An outer ring 5.5 km in diameter surrounds a 4 km in diameter annular magnetic trough. An inner somewhat discontinuous annular magnetic ridge 2.5 km in diameter surrounds a central magnetic low. Most of the structure is buried beneath a sub-circular hill of post-impact Neoproterozoic sandstone, with relics of deformed Mesoproterozoic sandstone around the edge, interpreted as remnants of the collapse rim. The structure is interpreted as a partly buried complex impact crater with an original rim diameter about 6 km. The lowest exposed levels of infilling sediments comprise conglomeratic sandstone bearing common shocked clasts (planar fractures and planar deformation features - PDFs - in quartz), and clasts of possible melt rock. This material may be redeposited from fall-back breccia, or eroded from the central uplift. On stratigraphic grounds, Foelsche is inferred to be Neoproterozoic, was probably buried soon after impact, and only recently partly exhumed. The aeromagnetic anomaly, which was instrumental in discovery of the site, is mostly due to the disruption, displacement and/or local removal of thin magnetic dolerite sills in the target stratigraphy, but inner parts of the anomaly may include a contribution from buried melt rocks.
U22A-07
Maximum Velocity of a Boulder Ejected From an Impact Crater Formed on a Regolith Covered Surface
We investigate the effect of regolith depth on boulder ejection velocity. A "boulder" refers to an apparently intact rock or rock fragment lying on a planetary surface, regardless of emplacement mechanism. Boulders appear in planetary images as positive relief features --- bright, sun-facing pixels adjacent to dark, shadowed pixels. We studied 12 lunar craters in high resolution (1~m) photographs from Lunar Orbiter III and V. Local regolith depth was measured using the method of small crater morphology. Ejection velocities of boulders were calculated assuming a ballistic trajectory to the final boulder location. A plot of regolith depth/crater diameter vs.\ maximum boulder ejection velocity shows that craters formed in deeper regolith (with respect to crater size) eject boulders at lower velocities. When ejection velocity (EjV) is in m/s, and regolith depth (Dr) and crater diameter (Dc) are in meters, the data fit the relation Dr / Dc = 1053 × EjVmax-2.823. To explain the data, we turn to impact cratering theory. An ejected particle will follow a streamline from its place of origin to its ejection point (the Z-model), and then follow a ballistic trajectory. Material ejected along more shallow streamlines is ejected at greater velocities. If shallow regolith covers the surface, the most shallow (greatest velocity) streamlines will travel only through the regolith. Boulders, however, must be ejected from the bedrock below the regolith. Thus, the boulder ejected with the greatest velocity originates just below the regolith, along the most shallow streamline through the bedrock. If the regolith is deeper, the most shallow streamline through the bedrock will be deeper, and the maximum velocity of an ejected boulder will be lower. Hence, the regolith depth and maximum ejection velocity of a boulder are correlated: greater boulder ejection velocities correspond to thinner regolith. We observe this correlation in the data.
U22A-08
Impact-Induced Deglaciation of the Snowball Earth?
Observational evidence indicates that the Precambrian Earth's history had episodes of total ice coverage of the planet. The Snowball Earth hypothesis states that the Sturtian (about 710 Ma) and Marinoan glaciations (about 635 Ma) were of global extent and lasted for several million years each. A variation of this hypothesis, called the Slushball Earth, requires milder conditions without substantial equatorial sea ice. The Snowball Earth glaciations would have ended abruptly in a greenhouse environment, whereas the Slushball would have experienced a slower deglaciation. Not only is the cause of a possible glaciation unclear, but the cause and mechanism of deglaciation is also debated. The goal of our study is to investigate if it is conceivable that a large-scale impact event might have triggered the deglaciation. The problem of the climatic effects of large impact events is not clear, as previously a Chicxulub-scale impact was suggested to induce global freezing. In terms of cratering rates, it is statistically plausible that the impact of a ~5 km diameter asteroid occurs during a "snowball period" with a duration of several million years. Most probably is an impact into the ice-covered ocean. In such a case a vapor plume with a total mass of n×1015 kg will rise up and then collapse over the atmosphere, creating a transient "hot spot". The more indirect consequences may include a global enrichment of the upper atmosphere with water vapors, dust and sea salt particles (in the case of an impact into ocean). Photochemical reactions should be taken into account for a further climatic modeling. At this point our simulations do not allow a conclusion if an impact of a realistic magnitude could cause deglaciation of a Snowball Earth.