The 66 million year ago asteroid impact event that formed the Chicxulub Crater, Mexico, marks the end of the Mesozoic Era of Earth history and has been attributed as the cause of the contemporaneous mass extinction. Numerical impact simulations combined with geophysical investigation of subsurface structure have constrained the kinetic energy of the impact under the simplifying assumption of a vertical trajectory. The trajectory angle and direction of the Chicxulub impact are not known, but a near-vertical impact is unlikely. Only one quarter of impacts occur at angles between 60° and the vertical and only 1 in 15 impacts is steeper than 75°.
For constant impactor size and speed, a shallower impact angle produces a smaller crater, a more asymmetric dispersal of ejected material and partitions more impact energy at shallower depths. As a result, impact direction and trajectory angle to the target plane are important impact parameters that determine, among other things, the direction of most severe environmental consequences and the volume and depth of origin of vaporised target, as well as ejecta and crater asymmetries.
Since the discovery of the Chicxulub impact structure based on diagnostic evidence of shock metamorphism and geophysical anomalies, several asymmetries in the geophysical character of the crater have been noted, which may result from oblique impact and/or impact in a heterogeneous target. Among the most obvious of these are radially oriented gravity lows to the south and northeast, and a radial gravity high to the northwest. However, these large-scale, peripheral features are all likely to be pre-existing features, unrelated to the impact. As models of the subsurface based on potential field data have inherently poor resolution and suffer from non-uniqueness, the most robust evidence of asymmetry comes from seismic reflection and refraction data. High-resolution seismic reflection images along a concentric arc outside the crater rim clearly show that the northeastern gravity low in the offshore half of the crater occurs in an area where Cretaceous and Cenozoic sedimentary rocks are particularly thick, and the northwestern gravity high occurs where this sedimentary sequence is thinnest and basement rocks are closer to surface. Given the observed correlation between the gravity signature and depth to basement outside the crater, the thickness of the sedimentary sequence is the most likely control on the offshore gravity anomaly. There is therefore no evidence that the azimuthal asymmetry in the outer gravity signature is impact related.
A nominal geographical centre of the crater is defined by the geometric centre of the crater rim demarcated by both a circular high in horizontal gravity gradient and the prominent cenote ring. Relative to this point, the centre of both the central gravity high, attributed to the uplift of dense lower-crustal rock, and the surrounding annular gravity low, which underlies the inner edge of the peak ring, are shifted several km to the west-southwest. In contrast, three-dimensional seismic velocity data indicate that the maximum uplift of the mantle beneath the crater occurs about 10 km to the north-northeast of the crater centre.
The south-westerly offset of the central gravity high relative to the crater centre was previously interpreted as indicating impact from the southwest, on the premise that central uplift motion would be directed uprange. An alternative interpretation, of a trajectory from the southeast, was proposed on the basis of a northwest–southeast elongation of the central gravity high and magnetic anomaly, and the northwest truncation of the annular gravity low. However, seismic reflection and refraction data reveal that the zone of structural uplift is not elongated towards the northwest and that the truncation of the gravity low in the northwest is a pre-impact feature of the regional anomaly caused by the shallow depth to basement in this direction. The short-wavelength component of the magnetic anomaly shows a slight (10%) elongation in the northwest–southeast direction, but is also offset to the southwest of the crater centre. The short wavelength and steep gradients of this anomaly both suggest a shallow source, probably related to the melt sheet and impact breccia and not to structural crater asymmetry. On the other hand, the long-wavelength component of the magnetic anomaly is a magnetic high elongated and offset along a direction southwest of the crater centre, consistent with a zone of uplifted basement rocks southwest of the crater centre.
In a paper published in the journal Nature Communications on 26 May 2020, Gareth Collins, Narissa Patel, and Thomas Davidson of the Department Earth Science and Engineering at Imperial College London, Auriol Rae, also of the Davidson of the Department Earth Science and Engineering at Imperial College London, and the Institute of Geology at the University of Freiburg, Joanna Morgan again of the Department Earth Science and Engineering at Imperial College London, and Sean Gulick, of the Institute for Geophysics and Department of Geological Sciences at the University of Texas at Austin, use 3D numerical modelling to examine the relationship between impact angle and structural crater asymmetries in a Chicxulub-scale peak-ring crater in a flat-layered target without lateral pre-impact asymmetry.
Collins et al. show that the observed asymmetry in the positions of the central uplift, peak-ring centre and maximum mantle uplift, relative to the crater centre, can be attributed to the angle and azimuth of the impact trajectory. Comparison of our simulation results with geophysically constrained models of the Chicxulub crater structure is used to infer the likely trajectory and angle of the impact. The recent joint International Ocean Discovery Program and International Continental Scientific Drilling Program Expedition 364 recovered about 600m of peak-ring rocks from the Chicxulub crater that provide additional constraints to discriminate between impact scenarios. Our simulations also reveal azimuthal variation in peakring material properties, which provide context for International Ocean Discovery Program-International Continental Scientific Drilling Program Expedition 364 core analysis.
Numerical simulation results. Collins et al. performed a series of 3D simulations of impacts that produce a Chicxulub-scale crater, using the iSALE3D shock physics code. The simulations assumed a flat, two-layer target comprising crust and mantle and considered four different impact angles (90° (vertical), 60°,
45° and 30°) and two impact speeds (12 and 20 km per second). Their simulations provide insight into crater asymmetries diagnostic of impact angle and trajectory in the absence of any target asymmetry.
In Collins et al.'s vertical impact simulation, crater formation is axially symmetric and consistent with previous two-dimensional numerical simulations that employed an axially symmetric geometry. Collision of the asteroid with the target surface generates a detached shockwave that propagates symmetrically from the impact site. In the first minute after impact, an excavation flow initiated by the shockwave produces a deep, bowl-shaped cavity, often termed the transient crater. The material flow depresses the crust-mantle boundary beneath the transient crater, uplifts the crust in the transient crater wall and expels the unvaporized portion of the more than 3-km-thick sedimentary rock sequence from the transient crater as part of the ejecta curtain.
The transient crater is unstable and collapses dramatically to produce a much flatter, broader final crater. In the vertical impact simulation, collapse manifests as uplift of the crater floor and downward and inward collapse of the transient crater rim and a surrounding collar of sedimentary rocks. Floor uplift begins directly beneath the transient crater centre and proceeds vertically upward, overshooting the pre-impact surface to form a large central uplift. At the same time, rim collapse occurs symmetrically at all azimuths, converging towards, and helping to drive up, the central uplift. Finally, the overheightened central uplift of crustal rocks collapses downward and outward, overthrusting the collapsed transient crater rim to form an uplifted ring of crystalline basement, overlying inwardly slumped sedimentary rocks from outside the transient crater. Although the spatial resolution of the numerical simulations is insufficient to resolve. This model of peak-ring crater formation is supported by geophysical data the characteristic sharp-peaked topography of the inner ring observed in extraterrestrial peak-ring craters, Collins et al. were able to identify the position and structure of the material that forms the peak ring in the numerical simulations as a 10-km-wide collar around the central uplift and recent geological drilling at Chicxulub, as well as remote-sensing data from the Schrödinger peak-ring crater on the Moon.
Impacts at progressively shallower angles to the horizontal result in an increasingly asymmetric development of the crater, internally, while the planform of the final impact basin remains approximately circular. As impact angle decreases, the downrange offset of the crater centre from the impact point increases; less uplift of the transient crater rim occurs in the uprange direction; and more uplift occurs in the downrange direction. Relative to the final crater centre, the deepest part of the transient crater (and depressed mantle) also shifts with decreasing impact angle, first to the uprange direction, then back towards the centre. The collapse phase of crater formation is also modified by impact angle. Uplift of the crater floor during crater collapse begins uprange of the crater centre, but has a downrange component such that the central uplift axis is tilted downrange and the centre of the uplift prior to its collapse is downrange of the crater centre. Conversely, downward and outward collapse of the central uplift occurs preferentially in the uprange direction, resulting in enhanced overthrusting of the central uplift on top of transient crater rim in the uprange direction. The net result of the downrange-directed rise and uprange-directed fall of the central uplift is a simulated peak ring with a geometric centre only modestly offset in the downrange direction.
The impact simulations employ an impact speed of 12 km per second, only slightly larger than the minimum possible speed, Earth’s escape velocity of 11.2 km per second. While these results are likely to be representative of the about 25% of all impacts that occur at speeds below 15 km per second, Collins et al. also conducted another suite of simulations with a more probable impact speed of 20 km per second (close to Earth’s mean and median asteroid impact speed) to examine the sensitivity of our results to impactor speed. The higher-speed impacts produced similar offsets in mantle-uplift centre and simulated peak-ring centre, and the same trends in offsets with impact angle.
An important consequence of higher impact speed is enhanced melt production caused by higher shock pressures close to the impact site. The larger melt volume complicates the interpretation of peak-ring structure in the 20 km per second simulations as the dynamics of the melt are not expected to be well captured, given the 500-m spatial resolution of the 3D simulations, and would likely continue long after the simulation end time. Nevertheless, the lateral distribution of the melt material relative to the peak-ring material at the end of the simulations. suggests that below an impact angle of 45° there is a high concentration (thick sheet) of surficial melt in the downrange quadrant of the crater, which is likely to hinder or prevent formation of a topographic peak ring at these azimuths. Our results therefore support the idea that horse-shoe shaped peak-ring planforms are indicative of shallow-angle impacts, with the gap in the peak ring diagnostic of the downrange direction.
Asymmetry in crater development produces differences in central crater structure in the uprange and downrange directions. While the centre of the simulated peak ring appears to be consistently offset downrange of the crater centre by about 5% of the crater diameter in the three oblique impacts, the centre of the mantle uplift is offset uprange of the crater centre in the 60° impact and, to a lesser extent, the 45° impact; and is offset downrange in the 30° impact. This pattern of mantle-uplift offset relative to the final crater rim is a consequence of the corresponding change in the offset of the deepest part of the transient crater relative to the centre of the final crater. Geophysical observations at Chicxulub suggest the peak-ring and mantle-uplift centres are offset in different, approximately opposite directions from the crater centre. Uncertainty in the precise locations of the centres of the crater, peak ring and mantle uplift, as well as uncertainty in the crater diameter, contribute to an approximate uncertainty of 26% and 48% for the relative offset of the peak ring and mantle uplift, respectively. Comparison of these observations with our simulation results suggests that the observed configuration is most similar to the 60° impact simulations (or possibly the 45° impact simulation at 20 km per second).
Tracer particles that track the history of material in the simulation afford analysis of the provenance of peak-ring materials and their variation with azimuth. The mean depth of origin of peak-ring materials is 10–12 km for the 45°, 60° and 90° impacts, only dropping significantly, to about 8 km, in the 30° impact. In the 30° scenario a significant fraction of the simulated peak ring originates from the sedimentary sequence in the uprange direction; the presence of significant amounts of sedimentary material in the simulated peak ring is not consistent with geophysical interpretations or results from Expedition 364.
Collins et al. also observe a systematic change in the up/downrange difference in subsurface structure of simulated peak rings with impact angle. Similar to the situation in a vertical impact, at 60° the simulated peak ring is formed of overthrusted granitic crustal rocks from the central uplift above down-slumped sedimentary rocks from the transient crater wall, in all directions. However, the sedimentary rocks are deeper and extend farther beneath the simulated peak ring in the uprange direction compared with the downrange direction. At 45° and 30° this difference is more pronounced: on the downrange side of the crater, the inwardly slumped sedimentary rocks do not extend under the simulated peak ring owing to enhanced transient crater rim uplift in this direction. This downrange configuration is inconsistent with geophysical interpretations at Chicxulub, which suggest sedimentary slump blocks lie beneath the outer portion of the peak ring at all azimuths offshore. However, pre-impact asymmetries in sediment thickness, water depth, particularly in the northeast part of the crater (and potentially in the crust), may also affect structure beneath the peak ring.
A proposed indicator of shallow-angle impact is the truncation of the peak ring in the downrange direction. Collins et al.'s numerical simulations at typical terrestrial impact speeds (20 km per second) are consistent with the production of a gap in the peak ring in the downrange direction for impact angles shallower than 45°. However, a prominent gap in the Chicxulub peak ring that might indicate a shallow-angle impact is not supported by the geophysical data. The topographic expression of the peak ring is clearly resolved in all radial seismic reflection lines through the offshore portion of the crater and is particularly prominent in the northwest seismic reflection line Chicx-B the downrange direction according to shallow-angle impact hypothesis proposed by Peter Schultz and Steven d’Hondt. While the onshore portion of the crater has not been seismically imaged, the annular negative gravity anomaly that has been shown to correlate with peak-ring position offshore is well-pronounced and continuous in this region, with no break that might indicate an abundance of melt or change in the character of the peak ring. The continuity of the geophysical signature of the peak ring therefore also supports a more steeply inclined impact trajectory.
In summary, Collins et al.'s numerical simulations of oblique Chicxulubscale impacts appear to be most consistent with the internal structure of the Chicxulub crater for a steeply inclined impact angle of 45–60° to the horizontal. If the observed asymmetries in the Moho uplift, central uplift and peak ring of the Chicxulub impact structure are attributable to impact trajectory, the implied direction of impact is northeast-to-southwest. This is the opposite direction to that proposed by a team led by Alan Hildebrand in 1998, based on the offset of the central uplift relative to the crater centre. Our results indicate that uplift of the crater floor occurs in a downrange rather than uprange direction, consistent with numerical simulations of complex crater formation and geological interpretation of eroded structural uplifts at terrestrial complex craters.
Analyses of Venusian craters have not shown a clear link between asymmetries in central crater features and direction of impact. A slight tendency for the peak-ring centre to be offset in the downrange direction was observed, but the results were inconclusive, in part owing to the relatively small number of craters used in the study. The magnitude of the offset (0.03–0.07 D) is, however, consistent with Collins et al.'s numerical simulation results. In contrast, there is no correlation between impact trajectory direction and the offset from the crater centre of central peaks in small complex craters. While Collins et al. did not simulate central peak formation in this work, their results provide a possible explanation for the absence of correlation. At steep angles, uplift of the crater floor initiates uprange of the crater centre, while at shallow angles uplift initiates downrange. If central peaks represent frozen central uplifts, offsets in either uprange or downrange direction might therefore be expected at moderately oblique angles 30–60°.
Impacts that occur at a steep angle of incidence are more efficient at excavating material and driving open a large cavity in the crust than shallow incidence impacts. Our preferred impact angle of about 60° is close to the most efficient, vertical scenario, which suggests that previous estimates of impactor kinetic energy based on high-resolution 2D vertical impact simulations do not need to be revised dramatically based on impact angle.
Steeply inclined impacts favour a more symmetric distribution of material ejected from the crater among both proximal and distal ejecta. Asymmetry in the distribution of ejecta was originally used by Peter Schultz and Steven d’Hondt as an argument for a shallow impact angle towards the northwest. This was based on the observation that both the particle size and layer thickness were relatively large in North American Cretaceous/Palaeogene boundry sites. Subsequent work has shown that number and size of shocked quartz grains present in the global ejecta layer decreases with distance from Chicxulub, and is independent of azimuth. In addition, the 1–3-cm-thick double layer in North America is also observed to the south and southeast of Chicxulub in Colombia and the Demerara Rise at equivalent paleodistances from Chicxulub. The global Cretaceous/Palaeogene boundary layer therefore has a more-or-less symmetric ejecta distribution, consistent with our preferred steep impact angle.
Impact angle has an important influence on the mass of sedimentary target rocks vaporised by the Chicxulub impact. Recent complementary numerical simulations of impact vapour production in oblique impacts using the SOVA shock physics code showed that a trajectory angle of 30–60° constitutes the worst-case scenario for the high-speed ejection of carbon dioxide and sulphur by the Chicxulub impact. At this range of impact angles, the ejected mass of carbon dioxide is a factor of two-to-three times greater than in a vertical impact and approximately an order of magnitude greater than a very shallow-angle (15°) scenario. An absence of evaporites in the International Ocean Discovery Program-International Continental Scientific Drilling Program Expedition 364 drill core is consistent with highly efficient vaporisation of sedimentary rocks at Chicxulub. Our simulations therefore suggest that the Chicxulub impact produced a near-symmetric distribution of ejecta and was among the worst-case scenarios for the lethality of the impact by the production of climate-changing gases.
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