STRUCTURAL ANALYSIS OF THE EXHUMED SEMP FAULT ZONE, AUSTRIA: TOWARDS AN UNDERSTANDING OF FAULT ZONE ARCHITECTURE THROUGHOUT THE SEISMOGENIC CRUST

One of the most exciting frontiers in earthquake science is the linkage between the internal structure and mechanical behavior of fault zones. Little is known about how fault-zone structure varies as a function of depth, yet such understanding is vital if we are to understand the mechanical instabilities that control the nucleation and propagation of seismic ruptures. This has led us to the Oligo-Miocene Salzach-Ennstal-Mariazell-Puchberg [SEMP] fault zone in Austria, a major left-lateral strike-slip fault that has been exhumed differentially such that it exposes a continuum of structural levels along strike.

In order to establish the structure of the fault zone in the seismogenic crust, we are studying exposures of this fault at a variety of exhumation levels, from <1 km near the eastern end of the fault, downward through the seismogenic crust, across the brittle-ductile transition, and into the uppermost part of the lower crust in western Austria. This presentation will focus on the analysis of coseismic deformation (both meso- and microscopic) from outcrops at Gstatterboden and Taxenbach, where the SEMP has experienced 40-60 km of displacement.

The outcrop at Gstatterboden has been exhumed from ~ 2-3 km depth. Here the SEMP juxtaposes limestone of the Wettersteinkalk on the south with dolomite of the Ramsaudolomit on the north. Faulting has produced extremely asymmetric damage, extensively shattering and shearing the dolomite while leaving the limestone largely intact. We interpret this brittle damage using mesoscopic calculations of damage intensity and microscopic grain size distributions, and use the results to infer fault zone architecture. These findings are compared to the brittle-ductile outcrop at Taxenbach, which has been exhumed from depths of up to 10 km. Here, the SEMP juxtaposes Greywacke Zone rocks with carbonate mylonites of the Klammkalk. Outcrop-scale mapping of foliations, shear bands, and veins suggests that deformation is distributed throughout the outcrop, although the first-order observation is that ~ 60 km of displacement has localized along the contact between these units. We complement the outcrop-scale mapping with the analysis of calcite twins, recrystalized grain sizes, and CPO's in order to quantify the strain gradient throughout the outcrop.