| Outcrops of the Salzach-Ennstal-Mariazell-Puchberg (SEMP) fault system exhumed from depths of ∼4--17 km allow for the direct observation of fault zone structures throughout the crust, and provide insights into the way this fault, and perhaps others, distributes strain in three dimensions. At Gstatterboden, exhumed from ∼4--8 km, grain size distributions and small fault data reveal the presence of a 10-m-wide high-strain core towards which strain localized during fault evolution. Brittle fracture was accommodated via constrained comminution, which only occurs in strain-weakening rheologies and favors localization. Exposures of the SEMP at Lichtensteinklamm and Kitzlochklamm, exhumed from ∼12 km depth, bracket the brittle ductile transition. At these outcrops, the SEMP is characterized by a ∼70-m-wide, cataclastic fault core that has been altered to clays that transitions downward into a wide, ductile shear zone that has accommodated only minor amounts of strain, placing the majority of displacement on the razor-sharp fault contact. Deformation mechanisms transition from cataclasis and minor amounts of dislocation creep in calcite, to dislocation creep in quartz and calcite occurring against a background of fault-normal solution mass transfer. The ductile/ductile-brittle Rinderkarsee shear zone, exhumed from ∼17 km, marks the SEMP's continuation into the Tauern Window and is composed of three distinct shear zones. The southern, 100-m-wide shear zone has accommodated the most strain, and shows evidence for creep-accommodated grain boundary sliding in feldspar and quartz, while incipient shear zones contain ductile quartz and brittle-feldspars that undergo dislocation creep as fluids alter Kspar to muscovite, which localizes strain along felspar grain boundaries, encouraging ductility. These findings are compared to results from other faults exhumed from similar depth ranges, highlighting fundamental fault zone structures and characteristics. |
