[1] Analysis of a strike-slip fault exhumed from midseismogenic depths reveals that the fault experienced progressive strain localization toward a high-strain fault core. We focus on the Ennstal segment of the 400-km-long Salzach-Ennstal-Mariazell-Puchberg (SEMP) strike-slip fault system in the Eastern Alps, which accommodated $60 km of left lateral displacement during Oligo-Miocene time. Macroscopic and microscopic observations reveal a zoned fault featuring a high-strain core at least 10 m wide within a fault zone that is at least 150 m wide. Grain-size distribution analysis shows how the Ennstal segment of the SEMP evolved. Our data reveal a 10-m-wide high-strain fault core (characterized by a power law relationship of grain sizes, D 2 % 2.0) bordered by a 54-m-wide ''transition zone'' where the largest and smallest grains are characterized by two power law relationships (D 2 % 2.0 and 1.6, respectively). This zone is in turn bordered by a region with grain sizes that show a single low-strain power law relationship of D 2 % 1.6. We interpret these relationships to be the result of concentrated shear overprinting an initial low-strain, power law grain-size distribution before strain localized to the core. This is consistent with the theory that faults mature by smoothing geometrical complexities, forming a highly localized, high-strain fault core. The data do not support the idea that damage forms primarily in response to dynamic stresses during seismic rupture, although they do suggest that this mechanism may operate within tens of meters of the fault once it has developed its zoned structure.Citation: Frost, E., J. Dolan, C. Sammis, B. Hacker, J. Cole, and L. Ratschbacher (2009), Progressive strain localization in a major strike-slip fault exhumed from midseismogenic depths: Structural observations from the Salzach-Ennstal-Mariazell-Puchberg fault system, Austria,

[1] The Miocene Salzachtal-Ennstal-Mariazell-Puchberg (SEMP) strike-slip fault in Austria allows study of the internal structure of a fault zone from the near surface to $30 km depth. As it enters the Tauern Window along the Rinderkarsee shear zone, the SEMP fault passes from a dominantly brittle to a dominantly ductile structure. The shear zone consists of three 1-to 100-m-wide zones of brittle-ductile and ductile deformation separated by 500-m-wide zones of less deformed rocks. The southern shear zone is mylonitic, with ductile amphibole and plagioclase; weak crystal preferred orientations imply that the main deformation mechanism was dislocation-accommodated grain boundary sliding. The northern and central shear zones are characterized by discrete millimeter-wide shear zones with ductile quartz, muscovite, and biotite and brittle feldspar. Shear zone nucleation at the grain scale involved dislocation creep and the transformation of plagioclase to muscovite; strain then localized in muscovite-rich grain boundary shear zones that linked to form throughgoing shear zones.

[1] Using the discrete element modeling method, we examine the two-dimensional nature of fold development above an anticlinal bend in a blind thrust fault. Our models were composed of numerical disks bonded together to form pregrowth strata overlying a fixed fault surface. This pregrowth package was then driven along the fault surface at a fixed velocity using a vertical backstop. Additionally, new particles were generated and deposited onto the pregrowth strata at a fixed rate to produce sequential growth layers. Models with and without mechanical layering were used, and the process of folding was analyzed in comparison with fold geometries predicted by kinematic fault bend folding as well as those observed in natural settings. Our results show that parallel fault bend folding behavior holds to first order in these models; however, a significant decrease in limb dip is noted for younger growth layers in all models. On the basis of comparisons to natural examples, we believe this deviation from kinematic fault bend folding to be a realistic feature of fold development resulting from an axial zone of finite width produced by materials with inherent mechanical strength. These results have important implications for how growth fold structures are used to constrain slip and paleoearthquake ages above blind thrust faults. Most notably, deformation localized about axial surfaces and structural relief across the fold limb seem to be the most robust observations that can readily constrain fault activity and slip. In contrast, fold limb width and shallow growth layer dips appear more variable and dependent on mechanical properties of the strata.Citation: Benesh, N. P., A. Plesch, J. H. Shaw, and E. K. Frost (2007), Investigation of growth fault bend folding using discrete element modeling: Implications for signatures of active folding above blind thrust faults,

[1] Structural analysis of two key exposures reveals the architecture of the brittle-ductile transition (BDT) of the subvertical, strike-slip Salzachtal fault. At Lichtensteinklamm, the fault zone is dominantly brittle, with a ∼70 m wide, high-strain fault core highlighted by a 50 m thick, highly foliated gouge zone. In contrast, at Kitzlochklamm, deformation is dominantly ductile, albeit with relatively low strain indicated by weak lattice-preferred orientations (LPOs). The marked contrast in structural style indicates that these sites span the BDT. The close proximity of the outcrops, coupled with Raman spectroscopy indicating similar maximum temperatures of ∼400°C, suggests that the difference in exhumation depth is small, with a commensurately small difference in total downdip width of the BDT. The small strains indicated by weak LPOs at Kitzlochklamm, coupled with evidence for brittle slip at the main fault contact and along the sides of a 5 m wide faultbounded sliver of Klammkalk exposed 30 m into the Grauwacken zone rocks, suggest the possibility that this exposure may record hybrid behavior at different times during the earthquake cycle, with ductile deformation occurring during slow interseismic slip and brittle deformation occurring during earthquakes, as dynamic coseismic stresses induced a strain rate-dependent shift to brittle fault behavior within the nominally ductile regime in the lower part of the BDT. A key aspect of both outcrops is evidence of a high degree of strain localization through the BDT, with high-strain fault cores no wider than a few tens of meters.Citation: Frost, E., J. Dolan, L. Ratschbacher, B. Hacker, and G. Seward (2011), Direct observation of fault zone structure at the brittle-ductile transition along the Salzach-Ennstal-Mariazell-Puchberg fault system, Austrian Alps,