Localized ductile shear below the seismogenic zone: Structural analysis of an exhumed strike‐slip fault, Austrian Alps

Abstract

[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.

Direct observation of fault zone structure at the brittle-ductile transition along the Salzach-Ennstal-Mariazell-Puchberg fault system, Austrian Alps

“…(a) Schematic reconstruction of the SEMP fault system during Oligo‐Miocene time [after Cole et al , 2007] compared against (b) a generalized shear zone model [after Scholz , 1988]. At our study sites near the brittle‐ductile transition at Taxenbach, we observe the SEMP to be highly localized at outcrops that are both dominantly brittle (Lichtensteinklamm) and dominantly ductile (Kitzlochklamm).…”

“…Sixty kilometers to the west, the Rinderkarsee shear zone southeast of Krimml is a more deeply exhumed part of the Salzachtal fault (Figure 15). Cole et al [2007] estimated that this shear zone has been exhumed from ∼16–17 km depth. Where the SEMP terminates in the Vienna basin, some 300 km to the east of Krimml, the fault has experienced no exhumation [ Ratschbacher et al , 1991a, 1991b].…”

“…The Salzach‐Ennstal‐Mariazell‐Puchberg (SEMP) fault zone and major structural features of the Eastern Alps [after Linzer et al , 2002; after Cole et al , 2007]. Differential exhumation of the fault exposes a range of exhumation depths along strike, from ductile shear zones in the Tauern Window, to brittle‐ductile outcrops near Taxenbach, and finally brittle fault zones of decreasing exhumation depth to the east.…”

“…This study presents direct observations of a strike‐slip fault exhumed from the brittle‐ductile transition which, together with companion studies at shallower and deeper exhumation levels, allows insights that refine models of fault behavior throughout the crust [ Cole et al , 2007; Frost et al , 2009]. The unique opportunity to directly observe depth‐dependent changes on a single fault is afforded by the Salzach‐Ennstal‐Mariazell‐Puchberg (SEMP) fault zone in central Austria.…”

Depth-dependent stress field in and around the Atotsugawa fault, central Japan, deduced from microearthquake focal mechanisms: Evidence for localized aseismic deformation in the downward extension of the fault

“…For an improved understanding of the stress accumulation mechanism, it is important to know the style of deformation, whether it is localized in a narrow zone [e.g., Stuart et al , 1997; Iio and Kobayashi , 2002a] or widely distributed in volume [e.g., Kenner and Segall , 2000]. In geologic studies of exhumed fault zones, there is much evidence for localized ductile shear zones in the lower crust [e.g., Shigematsu et al , 2004; Cole et al , 2007]. In addition, geophysical evidence for a localized slip below the seismogenic zone has recently been found at other faults.…”