Detrital high-pressure/low-temperature minerals in a late Turonian flysch sequence of the eastern Alps (western Austria): Implications for early Alpine tectonics

Along the northern margin of the eastern Alps, structurally intercalated between the main Austroalpine thrust and the Pennine Rhenodanubic flysch nappe, a thin, tectonically complex zone occurs that comprises a series of tectonic melanges and imbricates of oceanic (South Pennine) and distal continental margin (Austroalpine) origin. This zone is the lateral continuation of the ophiolite-bearing Arosa Zone of the Swiss Alps; we refer to it as the Walsertal Zone.

In a broken flysch formation of the Walsertal Zone, turbiditic sandstones yield detrital grains of the high-P/low-T metamorphic minerals lawsonite and glaucophane (crossite, ferroglaucophane). The sandstones are dated as late Turonian to earliest Coniacian, about 87 to 89 Ma. This age falls into the time span of radiometric ages of subduction-related high-P/low-T metamorphism along the South Pennine/Austroalpine convergent margin, and of coeval cooling ages of medium-P/medium-T metamorphism in the middle Austroalpine nappes. If the high-P/low-T minerals are related to mid-Cretaceous high-P/low-T metamorphism as we think, the high-P metamorphic rocks must have been brought up from about 20-km depth within about 10 m.y. or even less. As isostatic uplift alone would have been too slow, alternative tectonic mechanisms such as “corner flow” must be responsible for rapid uplift and erosion in a fore-arc environment.