Pseudotachylite in impact structures — generation by friction melting and shock brecciation?: A review and discussion

Abstract

Fault-related pseudotachylites are generated as the result of brittle or brittle-ductile deformation related to seismic faulting. It is generally accepted among tectonic workers that, while cataclasis plays a role in pseudotachylite formation, friction melting is involved. In addition, pseudotachylite and “pseudotachylite-like” breccias have been repeatedly described from the basement exposures of impact and cryptoexplosion structures, and as veinlets in shocked meteorites and lunar samples. The relevant literature is reviewed with regard to controversial usage of the term “pseudotachylite”, and attention is drawn to several problematic aspects: (1) The term “pseudotachylite” is currently used both as a purely descriptive term and with genetic implications. (2) No unambiguous criteria for comparison and distinction of tectonically and impact-generated pseudotachylites have been identified. (3) It needs to be considered that, in impact structures, pseudotachylite could be generated by two distinct processes — namely, by shock brecciation/melting and by friction melting. Pseudotachylite in impact structures could be the result of either of these two processes, or of both. (4) It is recommended to adhere more stringently, when describing breccias from impact or cryptoexplosion structures, to nomenclature accepted for the description of fault rocks in tectonic environments. This would help to avoid confusion when discriminating between melt rocks (pseudotachylite, impact melt rock), “fragmental” breccias (cataclasite, fragmental impact breccia) and mylonitic rocks. (5) No criteria for the distinction of melt-bearing impact breccias (impact melt rock) and pseudotachylite have been established either. In order to solve these problems, close interaction between structural geologists and impact workers is warranted. It is suggested that the understanding of breccias in impact structures would be improved by paying particular attention to the temporal relationships between different breccia types. This would facilitate recognition of possibly different formational processes for impact-produced melt and fragmental breccias, and their distinction from pre- and post-shock deformation products.