Exhumation of the Dora Maira ultrahigh-pressure unit by buoyant uprise within a low-viscosity mantle oblique-slip shear zone

The Brossasco-Isasca subunit (BIU) of the Dora Maira massif is currently the only known continental crustal ultrahigh-pressure (UHP) unit in the Western Alps. The peak pressure/temperature conditions are 3.5-4.5 GPa/similar to 730 degrees C; exhumation from similar to 3.5 GPa to similar to 1 GPa occurred within 2.2 +/- 1.8 Ma, but the exhumation mechanism is incompletely understood. We present a conceptual model for the buoyancy-driven exhumation of the BIU inside a low-viscosity, dense mantle shear zone weakened by increased strain rates due to simultaneous strike-slip and subduction (oblique-slip) of the European plate. Two-dimensional thermo-mechanical models simulate such a buoyant uprise of an ellipse inside an inclined layer. Simulations (i) show the feasibility of the conceptual model, (ii) fit the pressure/temperature/time record and (iii) constrain effective viscosities. The model is compatible with the (i) small volume of continental crustal UHP rock in the Western Alps, (ii) minor erosion during exhumation and (iii) strike-slip deformation during the exhumation period.