FLUID-MEDIATED STRAIN LOCALIZATION DURING ALPINE AGE STRIKE-SLIP DEFORMATION IN THE EASTERN ALPS

The Greiner shear zone in the Tauern Window (Austria/Italy) shifts from a >1 km zone of shear in supracrustal rocks to a series of narrow splays where it enters basement gneisses. Many of these splays are marked by linear drainages 10-30 m wide that trend N70E. Within these drainages, the host granodiorite is transformed into ~qtz-free, chl+bio schists. One ~5 m wide zone that trends S80E shows a transition from granodiorite to bio+chl to a central zone of chl+gar+staur. Mylonitic granodiorite shows dominantly distributed sinistral shear, but the gully-forming schist zones show younger, localized dextral motion.

The dramatic change in mineralogy from granodiorite to chl+bio+/-gar+staur schist requires a significant leaching of Ca, Na, Si; the resulting volume loss concentrates K, Al, Fe, Mg. The metasomatism within the shear splays necessitates large, channelized fluid fluxes. Monazites from the S80E splay yield ages of 24-28 Ma, indicating that metasomatism and dextral deformation were associated with the thermal peak of the Alpine orogeny (600C, 8 kbar).

Stable isotopic data from shear zone samples (garnet d¹⁸O around 5 per mil; chlorite dD around -68 per mil) are not consistent with fluid derivation solely from the host granodiorite and instead indicate a fluid contribution from a mafic/ultramafic source. The shear zone splays are on strike with a series of pre-Alpine serpentinite bodies. We suggest that Alpine metamorphism of the serpentinites released large amounts of low-d¹⁸O, Si-undersaturated fluids that were localized along zones of weakness in the granodiorites. These fluids scavenged Si, Ca, Na from the granodiorite and produced bio+chl. Fluid flow thus promoted dextral strain localization in narrow splays by creating zones rich in phyllosilicate minerals. The S80E splay is in an appropriate orientation to have been an en echelon tension fracture during dextral shear, and may have interacted with more fluid than N70E splays, allowing for a greater degree of metasomatism. Sinistral shear prior to the thermal peak was pervasively distributed in much of the granodioritic portion of the Greiner zone. In contrast, fluid flow during dextral deformation during peak metamorphism transformed strong granodiorite into weaker schists, and hence contributed to strain localization in narrow splays.