Understanding Environmental Effects Associated with Impact Events
The awareness of the devastating consequences of impact events on the terrestrial ecosystem only arose in the 1980s, when the great end-Cretaceous (KP) mass extinction was linked for the first time to a large asteroid impact. Our understanding of the environmental effects of impacts has increased dramatically over the past three decades, yet we are still far from a satisfactory understanding of the problem. Most asteroid and comet impacts cause localized effects, not global-scale extinctions. Studies of nuclear blasts have taught us most of what we know on the localized direct effects of shock waves generated by impact in the atmosphere (blast waves) and at the Earth's surface (earthquakes and tsunami). Minor effects include the production of toxic gases like nitrogen oxides and nitric acid by shock heating of the atmosphere from the entering bolide. It has been estimated that total destruction from a Meteor Crater impact event (impact energy of 20-40 Mt) may extend to about 2 km from the impact point; farther out, casualty rates close to 50% for human size mammals and flattened trees from the blast wave could extend to 15-20 km from the impact. These local effects have little influence on the long-term evolution of the climate. Global scale devastation is commonly associated with large impact events, and extends over months to decades after the impact with profound effects on the environment and climate. Studies of the atmospheric effects from large impacts include the stratospheric loading of small size dust and climatically active gases. One of the most famous effects associated with large impact events involves the frictional heating associated with atmospheric re-entry of ejecta. The interaction of ejecta with the upper atmosphere as it falls back after ballistic ejection produce strong frictional heating of both dust and atmosphere. Recent results related to the KP impact event suggest that for about 30 minutes the resulting infrared (IR) pulse could have been strong enough to kill unsheltered organisms, but not enough to start global wildfires, contrary to previous conclusions. Soot identified at several KP boundary sites is consistent with combustion of fossil organic matter from the impact site. A large amount of soot would be produced upon impact and distributed ballistically with the ejecta. Recent climate modeling have shown that soot is a strong absorber of short-wave (SW) radiation and strongly affects the energy balance at the Earth's surface. Submicron dust and climatically active gases are also important factors in the perturbation of the Earth's radiative balance. The impact release of climatically active gases depends on the characteristics of the target at impact: CO2 and SOx are released from sediments while large amounts of water is released in oceanic impacts. Carbon dioxide is an important greenhouse gas, but it is improbable that the amount of CO2 released by an impact event on thick carbonate sediments could cause dramatic climatic shifts. Massive production of sulfate aerosols may be triggered by impacts on thick evaporite deposits, whenever the impact also releases significant amounts of water vapor. Volcanic eruptions have taught us that the effect of sulfate aerosols is size-dependent: very small (<0.05 μm) and large (>1 μm) particles may cause surface heating, while particles of intermediate size may cause strong surface cooling. Lastly, oceanic impacts can inject large amounts of seawater into the upper atmosphere. Besides water itself (source of free radicals OH and HO2, which participate in the catalytic cycle that destroys ozone), seawater salts would contribute significant amounts of Cl and S to the upper atmosphere, causing strong perturbations of upper atmosphere's chemistry.
Modeling the Formation of the K-Pg Boundary Layer Through Ballistic Transport and Skidding
We have modeled the impact at Chicxulub, as well as the ejection and arrival of material around the globe to form the K-Pg boundary layer. Each particle travelling in the expanding vapour plume is characterized by its individual parameters (mass, density, position, velocity), is subjected to gravity and drag, and exchanges momentum and energy with the surrounding vapour-air mixture. We capture the lithology, state, and velocity of material as it reaches the model borders, and then track it to its final destination on a ballistic path. Some ejecta escapes the Earth, some reaches distal and proximal sites, and some stays close to the crater. We have run a suite of simulations for different impactor diameters, velocities and impact angles. Parameters were selected to produce a final crater diameter of 180-200 km, to match geological and geophysical observations. A subset of our simulations were successful in roughly re-producing: the volume and chemistry of material at proximal sites, as well as the spherule volume and meteoritic composition in the distal K-Pg layer. Neither vertical or low-angle impacts appear to be able to duplicate the observed data, and no simulations re-produced the semi-constant layer thickness (~3 mm) of the distal K-P layer (all showed a decrease in thickness with distance from Chicxulub) and no basement material reached distal sites. The latter result is in accordance with the observation that high-velocity ejecta, such as tektites or meteorites from other planets, originate exclusively from the uppermost target, and not deeper than 1/10 of the projectile diameter. As shocked minerals originate from basement rocks, and these are >2.5 km below surface, our models do not re-produce the presence of shocked quartz in the distal K-Pg layer. We conclude that some ejecta must reach its final destination by non-ballistic means. During the Shoemaker-Levy impact large volumes of ejecta slid sub-horizontally around Jupiter for hours after impact. It was suggested that re-entering debris heated the atmosphere, which could expand upwards and outwards, and redistribute the debris (a mechanism originally proposed by Colgate and Petschek in 1985). We performed a pilot study using a narrow beam of ejecta and have demonstrated that "skidding" is a plausible mechanism for redistributing low-velocity ejecta (e.g. shocked quartz) around the globe, and may also lead to a more uniform layer thickness. We are now developing our model to make it more realistic. We will model the ejecta arriving at all distances across the model, traveling with a range of particle velocities, and different masses per unit area. The dispersion process should be more intensive than observed in our pilot study, as the later arriving ejecta will enter an atmosphere that has already been disturbed by the initial, faster-arriving ejecta. Thus, we predict that particles suspended in the expanding plume, as well as proximal ejecta with velocities of 2-3 km/s, may be efficiently re-distributed by this mechanism to much larger distances, and within a comparable time interval, compared with pure ballistic transport. Results from our new models will be presented at the meeting.
Are Seismites Where we Should Look for Distal Impact Ejecta?
On Earth distal ejecta (>5.0 crater radii from impact) have rarely been found. With thicknesses of millimeters to centimeters they are difficult to spot in the field. To find them one needs to look for an associated feature which is larger. A seismite is such a feature. They are caused by liquefaction of water-saturated sediments and are meters thick. Good places to form seismites are in the shallow (<200 m) eperic seas, which periodically covered the continents. These areas are sites of deposition, so ejecta and seismites are likely to be preserved. Liquefaction of sediments will occur if the sediments are subjected to shear stress. Meteorite impacts produce almost no s-waves, thus shear is caused by surface-waves. The slowest surface-waves move at 3.5 km/s. For ejecta shot from an impact at >45° in a vacuum, the surface-waves outrun the ejecta. Thus, a plausible scenario is for a surface-wave to pass through water-saturated sediments, liquefying them, and then later for ejecta to be deposited on top. The choice of considering only distal ejecta is that, closer to the crater; tsunamis determine what sedimentary structures form. Tsunamis travel at speeds of 140 to 260 m/s, so the ejecta are likely to arrive before the tsunami. The tsunami will stir up thick sequences of sediments, mixing them with the ejecta. With water depths <200 m and >5.0 crater radii, bottom topography will determine which regions have seismites or tsunamites. For ballistic trajectories of 65° and earthquake durations of three times crater formation time, the lag time between the end of seismic shaking and ejecta arrival at 1000 km is 320 s and at 4500 km is 1050 s. Using Stokes settling to simulate atmosphere suggests even longer lag times. To calculate the maximum distance seismites are found from the impact, the equations of Wang et al. (2005) and Shishkin (2007) were combined. For an impact crater the size of Chicxulub the maximum distance is between 1650 km and 4300 km for a wide range of mechanical properties of the target. This distance is consistent with the maximum observed distance to seismites at Chicxulub of 2500 to 3000 km. Further work characterized the ejecta in terms of ejecta layer thickness, average grain size and maximum size of shocked quartz.
Impact-triggered Flow of Deep Crust Beneath the Witwatersrand Basin Identified With Zircon Strain Chronometry
Numerical modeling of large >100km impact craters predicts widespread ductile modification of the deep crust following crater formation. Evidence of this process has recently been proposed to have affected the Martian crust but, until recently, has never been recognized on Earth. This is due partly to a rarity of samples to test the hypothesis but also to the absence of a technique with which to directly date the ubiquitous crystal plastic flow fabrics of the deep crust and age-correlate to the surface impact record. We have combined in situ and single-zircon U-Pb isotope, micro-XRD, EBSD and colour SEM-CL analyses to investigate the age of mylonitization of granulite-facies mafic xenoliths exhumed in Jurassic kimberlite within the Vredefort impact basin of South Africa. We show that zircons have been plastically deformed within high strain fabric domains, and that low angle grain boundaries within deformed zircons are sites of up to 100 per cent out-diffusion of radiogenic Pb. Using the high chemical resolution ID-TIMS technique and high spatial resolution SIMS (GSC- SHRIMPII) spot analyses we have been able to directly date the crystal-plastic deformation fabric in these Archean xenolith samples of the African crust-mantle transition at 2023+/-15 million years, indicating deep level flow coeval with the 2020+/-3 Ma Vredefort impact event. Assuming a radial distribution of the deformation beneath the crater, the area of the lower crust that could carry this fabric is >20,00km2. Hence, large impacts have the capacity to generate regional fabrics that are 'tectonic' in appearance, a scenario perhaps more common on the early Earth. LREE enrichment of the deformed and isotopically disturbed ductile zircon zones suggests fluid participation in mylonitization, allowing speculation regarding a causal link between regional post-impact flow beneath the 'Witwatersrand impact basin' and the hydrothermal pulse that caused gold remobilization in the region around the time of Vredefort impact. Our work establishes a new method for strain chronometry of planetary materials while offering a first view into impact-triggered processes at the unexposed crust- mantle transition of continental lithosphere.
Central Peak/Uplift Formation on Mars: Analogous to Terrestrial Complex Craters
Central peaks in complex craters on Earth contain uplifted target rocks that have been deformed by the geologic processes associated with the three primary stages of impact. Resulting deformation fabrics (microfractures, microfaults, folds, major faults, and fault breccias) occur in a predictable sequence in relation to pre-existing textures, definite shock metamorphic features, and post-impact fabrics. Previous work has demonstrated that this sequence occurs in smaller (<13 km) complex impact structures and preliminary field investigations indicate that this sequence may as well be expressed in larger terrestrial impacts (e.g. Charlevoix, 54 km; Vredefort, 300 km). It then follows that, because impact cratering is a ubiquitous process throughout the solar system, this petrogenetic sequence should be expressed on other terrestrial planets. With the improved spatial resolution of visible imagers orbiting Mars, it is now possible to discern deformation fabrics in the central uplifts of Martian impact craters. High resolution imagery from the Mars Orbiter Camera (MOC) on board Mars Global Surveyor (MGS) and from the High Resolution Imaging Science Experiment (HiRISE) on board Mars Reconnaissance Orbiter (MRO) provides visible images of 1.5 m and 25 cm/pixel respectively. Of the hundreds of central uplifts examined, most (>90%) of these are dust-covered or the impact occurred in regolith, precluding examination of deformation fabrics. Four to six of the craters have MOC or HiRISE coverage suitable for study of the layered bedrock exposed in these complex craters. Observations reveal that, similar to Earth, Martian central peaks are comprised of large megablocks of target strata that are separated by major faults that contain fault breccias. Also similar to terrestrial observations, megablocks are coherent, but internally folded and deformed with fractures that superficially resemble microfractures and microfaults. While not all deformation fabrics are visible at the resolution of the MOC and HiRISE images, cross-cutting relationships between visible fabrics are identical to those in terrestrial impacts, as predicted. This indicates that the geologic processes responsible for rock fracturing, crater floor displacement, and rise and collapse of central uplifts operate on all solid bodies of the solar system.
Central Uplift Emplacement at Manicouagan: Clues From a Pseudotachylite-Bearing Fault Zone
The Manicouagan impact crater, located at 51°30'N, 68°30'W in eastern Quebec, was formed by meteorite impact at 215±1 Ma. This well-preserved, undeformed structure is a ∼90 km diameter complex crater formed in predominantly crystalline rocks of the ∼1 Ga Grenville province. Weathering and episodic glacial scouring have eroded and exposed its internal structure, which makes Manicouagan a valuable source of data. The rock types, level of preservation, and good outcrop at Manicouagan have the potential to facilitate comparisons with other planetary bodies; in particular the Moon and Mars. The intention of this study is to establish how the central uplift at Manicouagan was emplaced. Impact scaling laws suggest it has undergone 10-15 km of rapid structural uplift. Four months of field work investigating the central uplift reveals a core of metamorphosed anorthositic rocks, which constitute Mont de Babel (957 m) and Maskelynite Peak (945 m), and associated high-grade, granitic and metabasic gneisses. These rocks retain their overall coherency (i.e., are not breccias), but are interspersed with pseudotachylite veins typically 1 mm to 15 cm thick. Sporadically distributed anastamosing, multi-vein pseudotachylite systems, 1 to 2 m wide, are interconnected by more pervasive subsidiary veins that occur at angles to these multi-vein zones. Where observed, offset associated with the pseudotachylite zones is typically 20-30 cm. Initial results indicate that these pseudotachylite systems have facilitated movement within the host rocks, but are unlikely to have enabled kilometer-scale displacements necessary for central uplift formation. Discovery of less common, high-angle and low-angle pseudotachylite veins suggest possible mechanisms for larger-scale movement within the central uplift (in both vertical and horizontal directions). The high-angle pseudotachylite veins have been observed in association with ball-fracture breccias as ∼50m wide fault zones dissecting portions of Maskelynite Peak. The 10-15 cm thick low-angle pseudotachylite veins have been observed undercutting relatively large blocks on Mont de Babel and Maskelynite Peak, suggesting lateral movement as klippen during central uplift formation. Markers necessary to measure amounts of displacement or slip related to both types of these veins have not yet been observed. These pseudotachylites and their host rocks have been investigated using optical microscopy, scanning electron microscopy (imaging and analysis), and XRF spectrometry. The results are in agreement with previous work indicating the pseudotachylites were produced in high-speed slip zones by frictional comminution and selective melting of wall rock lithologies. This work has implications for understanding central uplift formation in complex craters in general. Moreover, work on well-exposed, undeformed terrestrial craters like Manicouagan can help us better understand the structure of impact craters on other planetary bodies, for which we typically only have topographic information.
Investigation of Impact Breccias at Manicouagan
Impact breccias at Manicouagan include (1) lithified, purely clastic breccias and (2) suevitic breccias, both of which can occur as lenses or sheet-like bodies at the contact of the impact-melt sheet and underlying basement, and as dykes and irregular bodies within the crater floor; (3) pseudotachylyte that typically occurs as anastomosing, cm-thick veins and dykes (up to 10 cm wide) within the crater floor; and (4) impact-melt breccias. This work presents results of a detailed study of the clastic, suevitic and impact-melt breccias and that immediately underlie the impact-melt sheet. The majority of clastic breccias typical of the impact-melt sheet - crater floor contact are characterized by weakly consolidated, green to brown, very fine-grained matrices enclosing angular to rounded lithic and mineral fragments, up to metres in size. They tend to be matrix supported and polymict, with rock and mineral fragments exhibiting varying degrees of shock metamorphism, indicating derivation from disparate sources. The suevites at Manicouagan are identifiable at the outer edge of the impact-melt sheet where they form green to red/brown friable layers, lenses and dykes, typically associated with purely clastic breccias. This is in contrast to the more massive, resistant and blocky weathering impact melt and impact-melt breccia. Interfingering and intermingling of the suevite and basal impact-melt sheet and impact-melt breccia, indicate movement relative to one another. Breccias intersected in drill holes from the centre of the structure may represent hornfelsed suevitic breccias where they underlie thick (>1 km) sections of impact-melt sheet. In the field and in hand specimen, the suevites possess friable, clastic matrices (similar to the clastic breccias) enclosing rounded to angular mineral and rock fragments ranging in size from <mm to metres, which themselves may be brecciated and/or contain pseudotachylyte-like veins and shears. Shatter cones are observed in some of the larger lithic fragments. The suevites are polymict, although in a given area, an abundance of one lithology may dominate. Also entrained within the clastic matrices are blebs, schlieren and larger masses of melt. Many of the larger lithic clasts are rimmed by melt. Preliminary petrographic and analytical scanning electron microscope observations of the suevitic breccias reveal matrices now cemented by clay minerals, with larger monomineralic and lithic fragments exhibiting varying degrees of brecciation, shock metamorphism and melting. Shock metamorphism is evident primarily as planar deformation features in quartz and, to a lesser extent, feldspar. There is some evidence of partial melting of clasts. Discrete melt blebs and schlieren exhibit microcrystalline igneous textures comprising the same mineralogy and compostion as the impact-melt sheet. This supports the field observation that the majority of melt entrained within the suevite is derived from the immediately overyling impact-melt breccia. The fluidal textures and a lack of obvious cooling of melt blebs against the clastic matrix indicates that they were still hot, plastic, and not completely solid, upon incorporation into the suevite. Despite the erosion of the fallback breccia, Manicouagan preserves a variety of impact breccia lithologies. In particular, the excellent exposures of basal/crater suevite at the edge of the central island, as well as in drill core sections, provide new insights into this relatively little- studied variant of suevite, as well as important links to lunar and martian breccias.