COMPRESSION VERSUS EXTENSION IN THE EXHUMATION OF THE DORA-MAIRA COESITE-BEARING UNIT, WESTERN ALPS, ITALY

Several recent papers suggest that uplift and exhumation of high-pressure, low-temperature (HP-LT) metamorphic rocks are due to crustal extension, coeval with and/or post-dating the contraction responsible for the metamorphism. A detailed survey of the coesite-bearing unit of the southern Dora-Maira massif offers an opportunity to discuss the relative importance of extension and compression in the exhumation tectonics of ultra-high-pressure (UHP) rocks. This UHP unit consists of pre-Alpine basement rocks, which equilibrated at a depth of 100 km, probably 120-100 Ma ago. It is overlain by two units with a similar lithology and Eoalpine metamorphic age and high P/T gradients, which equilibrated at 60 and 50 km, respectively. This Eoalpine basement complex is intercalated between blueschist facies units. The whole pile was affected by a Mesoalpine greenschist facies mylonitic deformation, during which the early contacts were reactivated as low-angle shallow-dipping shear zones. Extension is documented only during the Mesoalpine collisional period, and is referred to the gravity collapse of a thickened orogenic wedge, with an exhumation rate of about 1 mm/yr. The most important part of the exhumation of the UHP rocks occurred earlier, at a higher rate, close to 4 mm/yr. A forced return flow, or extrusion tectonics of imbricate slices, in an on-going subduction setting seems appropriate to account for this early stage. The fact that the omission of intermediate pressure rocks between juxtaposed units is an unreliable criterion for demonstrating extension is discussed.