Syn-convergence extension, vertical pinching and contrasted metamorphic units on the western edge of the Gran Paradise massif (French-Italian Alps)

The Piemontese zone, at the western border of the Gran Paradise massif (western Alps) consists of the oceanic schistes lustres units and the continental Grand Paradise (GP) unit. In complement to previous work, tectonometamorphic investigations allow us to establish pressure gaps between the three studied units with: the upper schistes lustres (LS) unit, metamorphosed under blueschist facies conditions (9.5 +/- 2 kbar, 330 +/- 30 degrees C). The lower schistes lustres (LI) unit, metamorphosed under low temperature eclogite facies conditions (12.5 +/- 3 kbar, 480 +/- 50 degrees C). The Gran Paradise (GP) unit, metamorphosed under higher pressure and temperature eclo,oite facies conditions (12 to 20 kbar, 500 +/- 50 degrees C). The whole metamorphic pile recorded a decompression under amphibolite facies conditions for the LI unit and in the greenschist facies conditions for the LS unit and GP unit, i.e, with a slight and late temperature increase. Fluid inclusion studies on quartz in late cracks and shear planes (C') show that the three units were juxtaposed and deformed together under metamorphic conditions of 4 +/- 1 kbar, and 400 +/- 50 degrees C. A finite strain field study reveals regional scale extensional tectonics beginning under greenschist facies conditions and ending under brittle conditions. Ductile greenschist deformation corresponds to dome and basin trajectories of the foliation surface. A partition is observed between domains where a pure sheer regime prevails in the core of the domes and domains where a simple shear regime prevails, towards the WNW, at the rim of the domes, at the contact between units. Brittle deformation corresponds to a continuum of the ductile deformation. Meanwhile, metamorphic contrasts suggested by pressure estimates cannot be ruled by the relative extension-related displacements. This extension corresponds to the accommodation of the vertical indentation of the high pressure units by the stacking of deep crustal slices, in front of the rigid Apulian mantle back-stop, during continuous convergence. (C) 2000 Editions scientifiques et medicales Elsevier SAS.