PSEUDOTACHYLITE IN IMPACT STRUCTURES - GENERATION BY FRICTION MELTINGAND SHOCK BRECCIATION - A REVIEW AND DISCUSSION

Fault-related pseudotachylites are generated as the result of brittle or brittle-ductile deformation related to seismic faulting. It is gene rally accepted among tectonic workers that, while cataclasis plays a r ole in pseudotachylite formation, friction melting is involved. In add ition, pseudotachylite and ''pseudotachylite-like'' breccias have been repeatedly described from the basement exposures of impact and crypto explosion structures, and as veinlets in shocked meteorites and lunar samples. The relevant literature is reviewed with regard to controvers ial usage of the term ''pseudotachylite'', and attention is drawn to s everal problematic aspects: (1) The term ''pseudotachylite'' is curren tly used both as a purely descriptive term and with genetic implicatio ns. (2) No unambiguous criteria for comparison and distinction of tect onically and impact-generated pseudotachylites have been identified. ( 3) It needs to be considered that, in impact structures, pseudotachyli te could be generated by two distinct processes - namely, by shock bre cciation/melting and by friction melting. Pseudotachylite in impact st ructures could be the result of either of these two processes, or of b oth. (4) It is recommended to adhere more stringently, when describing breccias from impact or cryptoexplosion structures, to nomenclature a ccepted for the description of fault rocks in tectonic environments, T his would help to avoid confusion when discriminating between melt roc ks (pseudotachylite, impact melt rock), ''fragmental'' breccias (catac lasite, fragmental impact breccia) and mylonitic rocks. (5) No criteri a for the distinction of melt-bearing impact breccias (impact melt roc k) and pseudotachylite have been established either. In order to solve these problems, close interaction between structural geologists and i mpact workers is warranted. It is suggested that the understanding of breccias in impact structures would be improved by paying particular a ttention 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.