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 the actual type of melt rock studied. The difficulties to distinguish different tectonically produced breccias (fault rocks) and impact breccias (for which no uniformly accepted nomenclature exists) are examined, and the various formational processes (friction melting or shock melting), at the macro- and micro-scales, are discussed. Special emphasis is placed on the occurrence of different and multiple breccia types within impact structures, where not only impact-induced friction melt and impact melt may occur, but where tectonically produced fault rocks that are not related to the impact event may further complicate the breccia situation. Melting on shatter cone surfaces and its likely formation, the proposed origin as distal impact breccia of certain diamictites, and problems with carbonate breccias and caused by metamorphic overprint on breccias are discussed. It is cautioned not too easily to reach generalized conclusions, for example with regard to the usage of the A- and B-type pseudotachylite terminology. Areas in need of further breccia studies – if possible in close collaboration between impact and tectonic researchers – are emphasized.