Direct observation of fault zone structure at the brittle‐ductiletransition along the Salzach‐Ennstal‐Mariazell‐Puchberg faultsystem, Austrian AlpsErik Frost,1,2James Dolan,1Lothar Ratschbacher,3Bradley Hacker,4and Gareth Seward4Received 21 May 2010; revised 27 October 2010; accepted 9 December 2010; published 17 February 2011.[1] Structural analysis of two key exposures reveals the architecture of the brittle‐ductiletransition (BDT) of the subvertical, strike‐slip Salzachtal fault. At Lichtensteinklamm, thefault zone is dominantly brittle, with a ∼70 m wide, high‐strain fault core highlighted by a50 m thick, highly foliated gouge zone. In contrast, at Kitzlochklamm, deformation isdominantly ductile, albeit with relatively low strain indicated by weak lattice‐preferredorientations (LPOs). The marked contrast in structural style indicates that these sites spanthe BDT. The close proximity of the outcrops, coupled with Raman spectroscopyindicating similar maximum temperatures of ∼400°C, suggests that the difference inexhumation depth is small, with a commensurately small difference in total downdip widthof the BDT. The small strains indicated by weak LPOs at Kitzlochklamm, coupled withevidence for brittle slip at the main fault contact and along the sides of a 5 m wide fault‐bounded sliver of Klammkalk exposed 30 m into the Grauwacken zone rocks, suggest thepossibility that this exposure may record hybrid behavior at different times during theearthquake cycle, with ductile deformation occurring during slow interseismic slip andbrittle deformation occurring during earthquakes, as dynamic coseismic stresses induced astrain rate–dependent shift to brittle fault behavior within the nominally ductile regime inthe lower part of the BDT. A key aspect of both outcrops is evidence of a high degree ofstrain localization through the BDT, with high‐strain fault cores no wider than a fewtens of meters.Citation: Frost, E., J. Dolan, L. Ratschbacher, B. Hacker, and G. Seward (2011), Direct observation of fault zone structure at thebrittle‐ductile transition along the Salzach‐Ennstal‐Mariazell‐Puchberg fault system, Austrian Alps, J. Geophys. Res., 116,B02411, doi:10.1029/2010JB007719.1. Introduction[2] Knowledge of the structure and mechanics of faultzones at the brittle‐ductile transition is vital to our under-standing of the rheological characteristics of the base of theseismogenic zone, where many large earthquakes nucleate.Field and experimental studies have shown that, rather thanbeing a simple transition from brittle faulting to ductilecreep, this part of the crust can deform by a wide range ofmechanisms dependent upon pressure, temperature, strainrate, grain size, fluid activity, mineralogy, phase transfor-mations, and microstructure [e.g., Tullis and Yund, 1977,1992; Carter and Kirby, 1978; Sibson, 1980, 1982;Passchier, 1982; Hobbs et al., 1986; Rutter, 1986; Janeckeand Evans, 1988; Scholz, 1988; Shimamoto, 1989; Hackerand Christie, 1990; Chester, 1995; White, 1996; Hacker,1997; Montesi and Hirth, 2003; Shigematsu et al., 2004;Lin et al., 2005].[3] In one widely accepted model of the structure andmechanics of faulting in the crust [Scholz, 2002], deforma-tion mechanisms change across the brittle‐ductile transitionfrom cataclastic flow to crystal‐plastic flow concurrent withthe onset of interseismic quartz plasticity at ∼300°C. Theresulting interpretation is that the seismogenic crust deformsvia frictional sliding, with strength increasing into themiddle of the brittle‐ductile transition (BDT). In the mid‐BDT to lower BDT, strength begins to decrease as the crustdeforms via temperature‐controlled flow. Assuming a con-stant lithology, this model also suggests that faults narrowdownward toward the brittle‐ductile transition, and thengradually widen in the lower brittle‐ductile transition asdiscrete slip surfaces disappear. Solution creep is expectedto operate at all depths [Brace and Kohlstedt, 1980; Sibson,1984; Chester, 1995], reducing fault strength compared tothe traditional two‐mechanism model.1Department of Earth Sciences, University of Southern Calif ornia,Los Angeles, California, USA.2Now at Fugro William Lettis and Associates, Valencia, California,USA.3Tektonophysik– Institut für Geowissenschaften, TechnischeUniversität Bergakademie Freiberg, Freiberg, Germany.4Department of Earth Science, University of California, Santa Barbara,California, USA.Copyright 2011 by the American Geophysical Union.0148‐0227/11/2010JB007719JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, B02411, doi:10.1029/2010JB007719, 2011B02411 1of15[4] This study presents direct observations of a strike‐slipfault exhumed from the brittle‐ductile transition which,together with companion studies at shallower and deeperexhumation levels, allows insights that refine models offault behavior throughout the crust [Cole et al., 2007; Frostet al., 2009]. The unique opportunity to directly observedepth‐dependent changes on a single fault is afforded by theSalzach‐Ennstal‐Mariazell‐Puchberg (SEMP) fault zone incentral Austria. The SEMP fault zone has been differentiallyexhumed along strike, exposing a range of exhumationdepths from near‐surface conditions in the Vienna basin inthe east to fully ductile middle crust in the western TauernWindow [Ratschbacher et al., 1991a, 1991b]. The differ-ential exhumation along strike resulted from westwardincreasing north‐south shortening across the Eastern Alps;combined with subsidence in the easternmost Eastern Alpsand the Pannonian basin, the entire SEMP fault zone wastilted eastward. Fault structures transition eastward fromdominantly ductile to dominantly brittle around the north-east corner of the Tauern Window, providing a naturallaboratory in which to evaluate models of fault behavioraround the brittle‐ductile transition.2. Geologic Setting[5] The SEMP fault is primarily a sinistral strike‐slip faultzone that extends for 400 km across the Eastern Alps (Figure 1)[Ratschbacher et al., 1991a, 1991b; Linzer et al.,2002].Fromwest to east, the Salzachtal fault forms the northern boundaryof the Tauern Window (exposing European and oceanic(Penninic) units beneath African (Austroalpine) units), theEnnstal fault forms the boundary between the central partof the Northern Calcareous Alps and the basement of theAustroalpine unit, and the Mariazell‐Puchberg fault cutsacross the southern margin of the eastern Northern