MICROSCALE TO MACROSCALE INTERACTIONS BETWEEN DEFORMATIONAL AND METAMORPHIC PROCESSES, TAUERN-WINDOW, EASTERN ALPS

Rocks from the western Tauern Window preserve a complex record of defo rmational and metamorphic processes at all scales. Syn- and post-kinem atic fabrics can be combined with P-T-t data to reconstruct the histor y of N-S continental convergence and subsequent E-W extension from at least Paleocene through Miocene time. Petrologic data from the Lower S chieferhulle (LSH) indicate progressive metamorphism along a clockwise P-T-t path that reached P greater-than-or-equal-to 10 kbar and T is s imilar to 550-degrees-C during collision. The overlying Upper Schiefer hulle (USH) followed a similar path, but reached pressures of less-tha n-or-equal-to 7 kbar and T is similar to 450-500-degrees-C. The approx imately 3 kbar difference in P(max) conditions attained by the units i mplies approximately 10 km of structural separation of the LSH and USH at depth, yet the sampled localities are now less-than-or-equal-to 2 km apart in the field. P-T paths of garnet growth from the LSH and USH imply that garnets grew simultaneously in the two units during the ea rly stages of unroofing. Rb/Sr dating of garnet segments and matrix ma terial in both units confirms this interpretation (CHRISTENSEN et al., 1991). However, garnets are postkinematic in the LSH and synkinematic in the USH, implying that the two units responded very differently to the same tectonothermal event. This observation, combined with synmet amorphic shear indicators and evidence for thinning of the section aft er P(max), can be accounted for by significant W-directed normal shear beginning at or prior to 35 Ma, with most of the strain initially acc ommodated by extensional ductile shearing of the USH. Ductile fabrics in the USH were subsequently overprinted by increasingly brittle fabri cs with the same sense of shear towards the west; these fabrics grade into the low-angle Brenner Line normal fault zone that unroofed the we st end of the window in the Miocene. Strain heterogeneities in the wes tern part of the window profoundly affected metamorphic development in a variety of ways. Within the Greiner shear zone, Si-scavenging fluid s transformed granodiorite into aluminous schist at approximately 40 k m depth, thereby producing assemblages that were sensitive monitors of P-T history. Shearing elsewhere in this zone may have contributed to formation of hornblende garbenschiefer horizons by a combination of ex treme grain-size reduction, diffusion creep, and rapid grain-boundary diffusion processes at approximately 35 km depth. Shearing along the B renner Line resulted in channelized fluid flow and alteration of the U SH over a depth interval from approximately 15 to 5 km. Initial fluid flow along this zone at depth may have subsequently controlled the loc ation of the brittle Brenner Line normal fault. Localized deformation at depths of 5-40 km thus affected bulk chemistry, fluid migration pro perties, and development of P-T-sensitive assemblages, all of which co ntribute to our ability to read the tectonometamorphic rock record.