EXOGENIC AND ENDOGENIC BRECCIAS - A DISCUSSION OF MAJOR PROBLEMATICS

Pseudotachylitic breccias, impact melt rock, (ultra)cataclasites, and even ultramylonites may macroscopically be of very similar appearance. Even detailed optical microscopic work may not suffice to identify th e actual type of melt rock studied. The difficulties to distinguish di fferent tectonically produced breccias (fault rocks) and impact brecci as (for which no uniformly accepted nomenclature exists) an examined, and the various formational processes (friction melting or shock melti ng), at the macro- and micro-scales, are discussed. Special emphasis i s placed on the occurrence of different and multiple breccia types wit hin impact structures, where not only impact-induced friction melt and impact melt may occur, but where tectonically produced fault locks th at are not related to the impact event may further complicate the brec cia situation. Melting on shatter cone surfaces and its likely formati on, the proposed origin as distal impact breccia of certain diamictite s, and problems with carbonate breccias and caused by metamorphic over print on breccias are discussed. It is cautioned not too easily to rea ch generalized conclusions, for example with regard to the usage of th e A- and B-type pseudotachylite terminology. Areas in need of further breccia studies - if possible in close collaboration between impact an d tectonic researchers - are emphasized. (C) 1998 Elsevier Science B.V .