James Dolan (USC), Brad Hacker (UCSB), Charlie Sammis (USC), Ann Blythe (USC), and their German collaborator Lothar Ratsbacher (Frieberg) are studying the relationship between earthquake mechanics and fault-zone structure through multi-disciplinary analysis of the exhumed, Miocene-aged Salzach-Ennstal-Mariazell-Puchberg [SEMP] fault zone in Austria. One of their major goals is to understand whether seismic ruptures always occur on a well-defined planar structure or whether the complexity observed in exhumed fault zones plays an important role in the nucleation and propagation of earthquakes. Stated another way, is earthquake mechanics fundamentally a problem in granular mechanics, or should it be viewed primarily in terms of frictional sliding along a single slip surface? These issues are critical for the question of scaling laboratory experiments to the Earth. A related issue is the role of fault-zone structure in the long- and short-term transport of fault-zone fluids, and their role in the faulting process. Although much has been learned about these issues from structural studies of exhumed faults, seismic studies of active fault zones, and laboratory studies of the mechanics of earthquake nucleation, this overall set of issues has proven extremely difficult to address in a systematic fashion on a single fault. In their analysis of the SEMP fault zone, the PI's are exploiting a unique example of a major strike-slip fault that has been tilted during exhumation, such that a continuum of exhumation levels-from the near-surface down into the lower crust-are now exposed along strike. Moreover, the SEMP fault system has participated in its own exhumation to a limited extent, reducing potential structural overprints related to younger faulting. Their research will characterize the geometry and internal structural architecture of the SEMP fault zone throughout the entire depth range of the seismogenic crust, with a focus on the recognition of depth-dependent changes in fault-zone structure. Field studies are focusing on structural transects across the SEMP fault zone at exhumation levels ranging from the near-surface at the eastern end of the fault (Vienna pull-apart basin), within the seismogenic crust (central Austria), and down into the ductile lower crust exposed in the Tauern window of western Austria. In addition to detailed field mapping of structural fabrics, fluid-rock interactions, relative timing relationships, and variations in fault geometry, the PI's are conducting detailed analyses of fault-zone rocks designed to explore deformation at a wide range of scales using petrographic microscopy, cathodoluminescence microscopy, fluid-inclusion studies, scanning electron microscopy, and transmission/analytical electron microscopy.
