A three-dimensional study of fault zone architecture: Results from the SEMP fault system, Austria.

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 Salzach-Ennstal-Mariazell-Puchberg [SEMP] fault system in Austria, a major left-lateral strike-slip fault that has accommodated ~ 60 km of displacement during Oligo-Miocene time. Differential exhumation of the SEMP has resulted in a fault zone that reveals a continuum of structural levels along strike. This provides us with a unique opportunity to directly observe how fault-zone properties change with depth, from near-surface levels, down through the seismogenic crust, across the brittle-ductile transition, and into the uppermost part of the lower crust in western Austria. Here we present results from four key outcrops and discuss the mechanical implications of these new data. Our brittle outcrop at Gstatterboden has been exhumed from at least 4 km depth. Here the SEMP juxtaposes limestone of the Wettersteinkalk on the south against Rauwacken dolomite to the north. Faulting has produced extremely asymmetric damage, extensively shattering and shearing the dolomite while leaving the limestone largely intact. Measurements of outcrop-scale faults and fractures in the dolomite, combined with analysis of grain-size-distributions, suggest that strain has progressively localized to a zone ~ 10 m wide. These findings are compared to those from two outcrops (Kitzlochklamm and Liechtensteinklamm) that bracket the brittle-ductile transition, exhumed from depths of = 10 km. Here, the SEMP juxtaposes Greywacke Zone rocks on the north against carbonate mylonites of the Klammkalk to the south. We calculate the strain gradient in the ductile Klammkalk rocks by analyzing the lattice preferred orientation (LPO) of calcite grains throughout the outcrop. Deformation in the Greywacke Zone, however, contains a significant component of solution mass transfer, and we therefore estimate the strain in these rocks by calculating the change in bulk volume. These analyses do not find significant levels of strain distributed within the Klammkalk or Greywacke Zone, again revealing a highly localized fault zone. Our investigation of the downward continuation of the SEMP into the Tauern Window indicates that the fault remains discrete at mid-crustal levels, with the majority of strain occurring in a 100-m-wide ductile shear zone (Cole et al., 2007). Combined with the recent work of Rosenberg et al. (2007), who have studied the deepest exposures of the SEMP in the western Tauern Window, these data allow us to present a three-dimensional picture of fault zone architecture and mechanics from the top of the seismogenic zone all the way into the ductile lower crust.