Continental subduction and exhumation: an example from the Ulten Unit, Tonale Nappe, Eastern Austroalpine

Some exhumed complexes in collisional belts consist of continental basement containing slivers of mafic and ultramafic material showing evidence of UHP metamorphism (P c. 3 GPa). Their PTt history can be interpreted in terms of subduction of continental material to depths ≥ 100 km and subsequent exhumation. This type of tectonic history is illu strated by the Late Palaeozoic evolution of the Ulten Unit, Tonale Nappe, Eastern Austroalpine. The upper crustal felsic component (c. 80% by volume) incorporated mafic material at the trench, and peridotitic material at deeper levels in the subduction zone. The peridotites show evidence of a P-increasing, T-decreasing path before incorporation in the felsic material, compatible with flow in the mantle wedge above the subducting slab. After emplacement of the peridotites, which occurred at or near peak metamorphic conditions (P ≥ 2.7 GPa, T ≥ 850 °C), the complex underwent a two-stage pre-Alpine exhumation path: a first, fast stage (c. 0.1-1 cm a-1), lasting c. 30 Ma and bringing rocks from depths ≥ 100 km to approximately 25 km; and a second, slow stage (c. 0.01-0.1 cm a-1), lasting c. 100 Ma and bringing rocks to depths <20 km. The subduction of felsic material to the required depths can be modelled by analysing the time-evolution of negative buoyancy, which confirms that relatively light continental upper crust can be subducted to depths > 200 km if attached to a mature oceanic slab that does not break-off during the early stages of continental subduction. The first exhumation stage can be accounted for by buoyancy-driven tectonic extrusion of continental slices along the subduction channel during continuing subduction. A force balance analysis shows that such a mechanism is compatible with the rheology of felsic and intermediate rocks at high temperature. The second exhumation stage is compatible with isostatic rebound and tectonic denudation following slab break-off. The conclusion that fast exhumation occurs during continuing subduction and before slab break-off is in accordance with the observed rates, which show fast movement of the rising slices with respect to the surrounding material. Slab break-off, on the other hand, generates a long-wavelength gentle upwarping of the overlying region, which is more compatible with later and slower exhumation rates. © The Geological Society of London 2005.