Observations at terrestrial impact structures: Their utility in constraining crater formation

Abstract

Abstract-Hypervelocity impact involves the near instantaneous transfer of considerable energy from the impactor to a spatially limited near-surface volume of the target body. Local geology of the target area tends to be of secondary importance, and the net result is that impacts of similar size on a given planetary body produce similar results. This is the essence of the utility of observations at impact craters, particularly terrestrial craters, in constraining impact processes. Unfortunately, there are few well-documented results from systematic contemporaneous campaigns to characterize specific terrestrial impact structures with the full spectrum of geoscientific tools available at the time. Nevertheless, observations of the terrestrial impact record have contributed substantially to fundamental properties of impact. There is a beginning of convergence and mutual testing of observations at terrestrial impact structures and the results of modeling, in particular from recent hydrocode models. The terrestrial impact record provides few constraints on models of ejecta processes beyond a confirmation of the involvement of the local substrate in ejecta lithologies and shows that Z-models are, at best, first order approximations. Observational evidence to date suggests that the formation of interior rings is an extension of the structural uplift process that occurs at smaller complex impact structures. There are, however, major observational gaps and cases, e.g., Vredefort, where current observations and hydrocode models are apparently inconsistent. It is, perhaps, time that the impact community as a whole considers documenting the existing observational and modeling knowledge gaps that are required to be filled to make the intellectual breakthroughs equivalent to those of the 1970s and 1980s, which were fueled by observations at terrestrial impact structures. Filling these knowledge gaps would likely be centered on the later stages of formation of complex and ring structures and on ejecta.

Rimaal: A Sand Buried Structure of Possible Impact Origin in the Sahara: Optical and Radar Remote Sensing Investigation

“…Applying the formula in [7] to the Rimaal structure of a 10.5 km in diameter, yields an apparent depth of 243 m and an actual depth of 412 m. Based on the topographic data, the current depth of the structure is only 45 m, which means that about 80% of the crater's depression is filled by debris and sediments. The deep erosion to the outer rim would have also contributed to the shallowness of the Rimaal structure.…”

“…According to [7] for a complex impact structure, the true depth (the depth to the crater true floor from the top of the outer rim), and the apparent depth (the depth to the top of the breccia lens from the top of the outer rim), can follow the following empirical relationship: (1) where D is the final rim diameter in km, and d a and d t are the apparent and true depths in meters, respectively. Applying the formula in [7] to the Rimaal structure of a 10.5 km in diameter, yields an apparent depth of 243 m and an actual depth of 412 m. Based on the topographic data, the current depth of the structure is only 45 m, which means that about 80% of the crater's depression is filled by debris and sediments. The deep erosion to the outer rim would have also contributed to the shallowness of the Rimaal structure.…”

“…According to [7] for a complex impact structure, the true depth (the depth to the crater true floor from the top of the outer rim), and the apparent depth (the depth to the top of the breccia lens from the top of the outer rim), can follow the following empirical relationship:…”

“…A simple crater is a small bowl-shaped depression, with a structurally uplifted rim, which includes an overturned flap and ejecta. A complex crater, on the other hand, exhibits a structurally complex outer rim with a terraced wall, a topographically lower annular trough, and a central topographic peak [7]. Today, there are about 190 confirmed terrestrial impact craters (simple and complex) listed in the Earth Impact Database (http://www.unb.ca/passc/ImpactDatabase).…”

Petrographic Investigation of Target Rock Transformation under High Shock Pressures from the Colônia Impact Crater, Brazil

“…In some cases, impact melt rocks may display a very close similarity in texture and mineralogical assembly to typical igneous rocks. However, it is important to note that adiabatic decompression melting (i.e., magma generating process) is different to impact melting where melting of the target lithology occurs under different degrees of shock metamorphism, causing phase changes and variable degrees of mechanical mixing of the original target rock (Grieve & Therriault, 2004;). An important aspect of impact lithologies is that they may contain a series of distinctive microscopic deformation features (i.e., impact indicators) from simple fracturing to complete melting, such as planar deformation features in quartz, mechanical twins in pyroxene, kink-band deformation in mica, granular texture in zircon, diapletic glass, and ballen silica (French, 1998;French & Koeberl, 2010;Ferrière & Osinski, 2012).…”