HR: 0800h
AN: T21B-0462    [Abstracts]
TI: Fault-Zone Deformation and Strain Partitioning at the Brittle-Ductile Transition, SEMP Fault, Austrian Alps.
AU: * Cole, J N
EM: jcole@umail.ucsb.edu
AF: University of California Santa Barbara, Webb Hall, Santa Barbara, CA 93106 United States
AU: Hacker, B R
EM: hacker@geol.ucsb.edu
AF: University of California Santa Barbara, Webb Hall, Santa Barbara, CA 93106 United States
AU: Ratschbacher, L
EM: lothar@geo.tu-freiberg.de
AF: Tektonophysik - Institut f�r Geowissenschaften Technische Universit�t Bergakademie Freiberg, Bernhard-von-Cottastr. 2, Freiberg/Sachsen, D-09596 Germany
AU: Dolan, J F
EM: dolan@usc.edu
AF: University of Southern California, USC Earth Sciences Zumberge Hall of Science (ZHS) 3651 Trousdale Pkwy, Los Angeles, CA 90089-0740 United States
AU: Frost, E
EM: efrost@usc.edu
AF: University of Southern California, USC Earth Sciences Zumberge Hall of Science (ZHS) 3651 Trousdale Pkwy, Los Angeles, CA 90089-0740 United States
AU: Barth, N
EM: ncbarth@umail.ucsb.edu
AF: University of California Santa Barbara, Webb Hall, Santa Barbara, CA 93106 United States
AB: The differentially exhumed Miocene strike-slip Salzachtal-Ennstal-Mariazell-Puchberg (SEMP) fault in Eastern Austria allows one to study fault structure from Earth's surface to ~30km depth, simply by moving along strike from the Vienna basin to the Tauern Window. Coincident with its entry into the Tauern Window, the SEMP fault passes from a dominantly brittle to a dominantly ductile structure. It is these kinds of brittle-ductile transitions that represent major mechanical discontinuities in the crust and may represent the base of the seismogenic zone. The Tauern segment of the SEMP fault therefore represents a key location for studying earthquake nucleation and mid-crustal rheology. Previous studies (e.g. Behrmann, 1990; Ratschbacher et al., 1991a; Linzer et al., 2002) suggested that the SEMP fault splayed down-section into the Tauern Window into a series of ductile shear zones, including the Olperer, Griener, and Ahrntal shear zones. At each of these locations, however, outcrop-scale structures (e.g. cross-cutting dikes) demonstrate that the main shear-zone fabrics are pre-Alpine, and thus largely unrelated to the SEMP. In contrast, a brittle-ductile shear zone in the northeastern edge of the Tauern Window (near Rinderkarsee south of Krimml) is a probable deeper level portion of the SEMP. The Rinderkarsee shear zone is localized along the contact between the tonalitic Zentral Gneiss and the metasedimentary/meta-igneous rocks of the Habach Group, but extends at least 1300m into the Zentral Gneiss. Shear strain is partitioned into separate discrete zones of sinistral and dextral shear, and both structures have the same slip plane mineralogy and are thus interpreted as coeval. Generally, sinistral shear zones are subvertical and strike NE-SW, while dextral shear zones are steeply dipping and strike NW-SE. At Rinderkarsee there exists a continuum of deformation from high to low temperature. High-temperature deformation shows dominantly sinistral amphibolite-facies deformation with amphibole and plagioclase ductility, while low-temperature deformation is defined by discrete brittle-ductile mm-scale sinistral and conjugate dextral shear zones with ductile biotite, muscovite, and quartz, but brittle feldspar deformation. Finally, there are late greenschist-facies tension gash arrays that may serve as nucleation zones for discrete sinistral shear zones. This shear zone organization suggests that strain is accommodated in localized domains as opposed to being distributed, and that the lithologic contrast between the Zentral Gneiss and the Habach Group is responsible for localizing SEMP-related strain. Although shear zone deformation at Rinderkarsee is significant, the degree of strain cannot represent the entirety of SEMP deformation, implying that SEMP deformation is either partitioned into multiple shear zones, or a combination of shear zones and areas of distributed strain.
DE: 3625 Petrography, microstructures, and textures
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8010 Fractures and faults
DE: 8031 Rheology: crust and lithosphere (8159)
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: Fall Meeting 2005