The current state of knowledge about shatter cones: Introduction to the special issue

Abstract

Shatter cones are a fracture phenomenon that is exclusively associated with shock metamorphism and has also been produced in the laboratory in several shock experiments. The occurrence of shatter cones is the only accepted meso‐ to macroscopic recognition criterion for impact structures. Shatter cones exhibit a number of geometric characteristics (orientation, apical angles, striation angles, sizes) that can be best described as varied, from case to case. Possible links between geometric properties with impact or crater parameters have remained controversial and the lack of understanding of the mechanism of formation of shatter cones does not offer a physical framework to discuss or understand them. A database of shatter cone occurrences has been produced for this introduction paper to the special issue of Meteoritics and Planetary Science on shatter cones. Distribution of shatter cones with respect to crater size and lithology suggests that shatter cones do not occur in impact craters less than a few kilometers in diameter, with a few, currently questionable exceptions. All pertinent hypotheses of formation are presented and discussed. Several may be discarded in light of the most recent observations. The branching fracture mechanism and the interference models proposed, respectively, by Sagy et al. (2002) and Baratoux and Melosh (2003) require further evaluation. New observations, experiments, or theoretical considerations presented in this special issue promise an important step forward, based on a renewed effort to resolve the enigmatic origin of these important features.

The Agoudal (High Atlas Mountains, Morocco) shatter cone conundrum: A recent meteorite fall onto the remnant of an impact site

“…Extrapolating observations at other impact structures, and considering that the currently known area of shatter cone occurrence at Agoudal corresponds to buried layers belonging to the central part of a single buried impact structure, the distribution of shatter cone locations suggests that the original diameter of the Agoudal crater would have been somewhere between 1000 (deduced and estimated from the approximate distance between most distant shatter cone outcrops of 700 m) and 3000 m. Although the lower limit is a robust constraint, the upper limit may be revised if new shatter cone exposures are discovered, extending further the area of shatter cone occurrence. The range is lower but close to the smallest craters associated with unequivocal shatter cones, although it is of note that less than 20% of known impact structures of less than 3 km diameter are actually associated with (unambiguous) observations of shatter cones (Baratoux and Reimold ).…”

“…, ) were also first identified from the presence of shatter cones. All known occurrences of shatter cones can be correlated with an estimated diameter of the impact crater inferred from geological or geophysical observations (Baratoux and Reimold ). This site at Agoudal is an exception to this rule, as no other observation related to the impact event, besides the presence of shatter cones, may be used to give a hint at the diameter of the former structure.…”

“…, ; Baratoux and Melosh ; Wieland et al. ; Baratoux and Reimold ; Kenkmann et al. ), there is consensus that these fractures are formed as extensional cracks during the propagation of shock waves, and require pressure levels greater than 1 GPa.…”

“…Shatter cone‐bearing outcrops occur within a largely gravel‐covered terrain as, generally, < 1 m 2 occurrences. Shatter cones are well developed: curved surfaces of various orientations and with diverging striations occur in the rock offering unequivocal evidence for hypervelocity impact deformation (Baratoux and Reimold ). No meteorites were found in the main shatter cone area.…”

Microcomputed tomography and shock microdeformation studies on shatter cones

“…). Ferrière and Osinski () stated that shatter cones have been reported from at least half of the confirmed impact structures on Earth; an up‐to‐date summary of shatter cone occurrences is provided by Baratoux and Reimold (). These authors also review the previous hypotheses attempting to explain the processes on shatter cone generation.…”