“Pseudotachylites” in Large Impact Structures

Abstract

The catastrophic processes associated with meteorite impact are capable of producing dark-matrix breccias by a variety of mechanisms during both the shock compression and crater modification phases. Distinguishing the origins of these breccias provides vital clues to understanding the cratering process. Unfortunately, the criteria by which these distinctions can be made are not always unequivocal. This has led to unfettered use of terminology, which must change if the science is to move forward. Principal among this is the problematic relating to the origin of so-called “pseudotachylite / pseudotachylite-like / pseudotachylitic breccia” in impact crater settings. The purpose of this discussion is to emphasize the problematics concerning the definition and recognition of such material. Ill-defined nomenclature coupled with insufficient field and laboratory investigations have led to confusion with respect to recognition of bona fide pseudotachylite = friction melt (according to its conventional geological terminology, adhered to by these authors), and separation from shock melt and other impactogenic or tectonically produced breccia types. It is essential to deal with these materials as carefully as possible, as they do have the potential to contribute to the understanding of the impact cratering process and have implications regarding the recognition of impact structures and understanding of their formation. It is categorically rejected to apply blanket terminology — such as indiscriminate usage of the term “pseudotachylite” — as label for (ultra) cataclasites, (ultra) mylonites, or dike (veinlet) breccia in impact crater floors of unknown or unrecognizable (e.g., because of thorough metamorphism or alteration of breccia matrix) genus, or the term “tagamite” for dense, black, finest-grained or aphanitic melt rock that could represent impact melt rock or bona fide pseudotachylite, or any of the other breccia types listed herein. Also, the use of simplifying terms such as A- or B-type, “S (shock)-type” or “E (endogenic)-type pseudotachylite”, without discriminating by rock nature or distinctive criteria, in order to contribute to fully understand the genesis of such materials, is discouraged. Recent work at Vredefort has shown that much, if not all of what has conventionally been termed “Vredefort pseudotachylite” could represent shock melt, in contrast to previous thinking where much of the so-called Vredefort “pseudotachylite” was considered to have formed relatively late (in the impact cratering process) during decompression or fault-related melting related to the modification phase of cratering. It is discussed that current A, B or S, E “pseudotachylite” classifications are insufficient and unsatisfactory.