REEXAMINING AN ARCHETYPAL THRUST FAULT: NORMAL MOTION ON THE AUSTROALPINE 'OVERTHRUST' DURING EXHUMATION OF THE PENNINE ZONE, C. 35-20 MA

A recent compilation of 340 published K/Ar, Ar/Ar, zircon and apatite fission track, and zircon and apatite U/Th-He cooling ages, combined with 55 new white mica Ar/Ar, zircon fission track, and zircon and apatite U/Th-He cooling ages, in the Central Alps of Switzerland-Austria-Italy shows that cooling rates of c. 4°C/M.y. persisted from the Cretaceous to the Miocene in the Austroalpine hanging wall, which finally cooled to below the apatite-helium closure temperature (~60-70°C) by early Miocene time (~22-17 Ma). In contrast, Pennine zone footwall rocks exhibit cooling rates of c. 10°C/M.y. between Eocene (~44-38 Ma) peak metamorphism (c. 450°C, >1 GPa) and Miocene (c. 14-8 Ma) apatite fission track closure (~110°C). Cooling in the Pennine footwall, particularly in the Tambo-Suretta-Schams-Grava nappes, occurred almost synchronously across a c. 60-km-north-south zone, from the most southerly extent not thermally peturbed by complexities of the Lepontine dome and 'Southern Steep belt' to the latitude of the Landquart valley. This suggests that the entire Pennine stack east of the Lepontine dome and the northern Adula nappe cooled rapidly as a large, coherent slab. Peak pressure estimates for this same regional extent are ≥1 GPa based on blueschist-facies metamorphism, including the common occurrence of carpholite in the Grava nappe. Published estimates for the pre-Alpine cumulative thickness of the Lower East Alpine nappes is ≤10 km, which was thinned during the middle of the Mesozoic. Juxtaposition of the structurally-thick (20-25 km), high-pressure Pennine footwall rocks against the thin (≤10 km), relatively cold Austroalpine hangingwall rocks was achieved by top-S shear of the crystalline Briançonnais basement (Tambo-Suretta), their cover rocks (Schams), and the adjacent Valaisan trough sediments (Grava) during the period c. 35-20 Ma. Juxtaposition of hotter footwall rocks against colder hanging wall rocks at, i.e., Turba mylonite zone and Martegnas-Gurgaletsch shear zones, requires that the Austroalpine overthrust be a normal fault during most of the time of Penninic exhumation. Numerous post-30 Ma zircon U-Th/He cooling ages in the Pennine zone indicate that normal-sense movement on the Austroalpine 'overthrust' and allied structures continued even after the intrusion of the Bergell pluton.