Fault mirrors in the process zone of a seismogenic fault (Karawanks, Eastern Alps)

Square centimetres sized polished fault surfaces are exposed in the vicinity of a seismic fault in the Obir Caves (Northern Karawanks, Carinthia, Austria). Microtectonic studies reveal that contrary to published natural and experimental fault mirrors, these small-scale faults record almost no displacement. Based on our findings, we propose a new process, in which during the passage of seismic waves, small shaking movements along the slip surfaces with hardly any offset caused the development of polished slickensides. The Obir Caves in the Hochobir massif are located just north of the seismogenic ESE striking, dextral Periadriatic Fault System, which forms the boundary between the Eastern and the Southern Alps. The cave was partially formed along the sinistral Obir Fault, close to which a high density of centimetres to decimetres long polished slickensides have been observed. The knife-sharp slip surfaces are associated with millimetre to centimetre thin layers of ultracataclasites, in either hardly deformed or protocataclastic rocks. The highly mature ultracataclasites record angular to rounded clasts, which are truncated along the polished slickensides, but clear kinematic indicators are generally missing. Injection of the ultracataclasites into the protocataclastic host rocks along millimetre-wide apophysis can be locally observed. Some of the older cataclasite generations are overprinted by stylolites and calcite veins suggesting dissolution precipitation processes after cataclastic flow. The only several centimetres-long polished slickensides with varying properties and orientations cannot have accumulated large displacements but the observed microstructures as well as the geological position in an area with several active faults suggest that the investigated fault surfaces in the Obir Caves formed during seismic slip.