FAULT STRUCTURE AT DEPTH: DEFORMATION MECHANISMS AND SHEAR ZONE ORGANIZATION WITHIN AND BELOW THE BRITTLE-DUCTILE TRANSITION, SEMP FAULT, AUSTRIAN ALPS

Fault structure and earthquake-nucleation processes are inexorably linked, but fault structure at depth is incompletely understood from a geological perspective. We used the differentially exhumed Miocene strike-slip Salzach-Ennstal-Mariazell-Puchberg (SEMP) fault in Eastern Austria to study fault structure at a variety of depths, concentrating on the brittle-ductile transition zone, and deeper level ductile deformation. The brittle-ductile transition represents a major mechanical discontinuity in the crust and is likely the base of seismogenic zone, thus the mechanical properties of this region have important implications for earthquake nucleation and mid-crustal rheology. In general, the SEMP fault splays into a series of ductile shear zones at paleodepths of ~12-15 km exposed in the Tauern Window. The Operer Shear Zone is a sinistral amphibolite-facies shear zone. It is discontinuous along strike and developed chiefly in biotite-rich paragneiss pendants within the metaplutonic Zentralgneiss. The relatively weak deformation in the Zentralgneiss indicates that main fabric of the Operer Shear Zone predates activity along the SEMP. The Greiner Shear Zone is hosted entirely within a mixed assemblage that ranges from a feldspathic augen gneiss to a micaceous orthogneiss–schist. Shear strain is heterogeneous across strike, ranging from ~1 to ~10. Deformation partitioning is also variable along strike, such that the shear zone is composed of a series of anastomozing strands separated by less-deformed domains. Microstructural observations show that shear-zone minerals deformed primarily by subgrain-rotation recrystallization, and quartz lattice-preferred-orientations indicate that intracrystalline deformation occurred via prism slip in the direction. Outside the meter-scale shear zones, the host gneiss remained relatively undeformed and quartz LPOs are weak to random. These microstructural data indicate that at depth beneath the SEMP fault, deformation was not distributed, but partitioned into discrete shear zones.