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 (wes tern Alps) consists of the oceanic schistes lustres units and the continent al Grand Paradise (GP) unit. In complement to previous work, tectonometamor phic investigations allow us to establish pressure gaps between the three s tudied units with: the upper schistes lustres (LS) unit, metamorphosed unde r blueschist facies conditions (9.5 +/- 2 kbar, 330 +/- 30 degrees C). The lower schistes lustres (LI) unit, metamorphosed under low temperature eclog ite facies conditions (12.5 +/- 3 kbar, 480 +/- 50 degrees C). The Gran Par adise (GP) unit, metamorphosed under higher pressure and temperature eclo,o ite facies conditions (12 to 20 kbar, 500 +/- 50 degrees C). The whole meta morphic pile recorded a decompression under amphibolite facies conditions f or 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 o f 4 +/- 1 kbar, and 400 +/- 50 degrees C. A finite strain field study revea ls regional scale extensional tectonics beginning under greenschist facies conditions and ending under brittle conditions. Ductile greenschist deforma tion corresponds to dome and basin trajectories of the foliation surface. A partition is observed between domains where a pure sheer regime prevails i n the core of the domes and domains where a simple shear regime prevails, t owards the WNW, at the rim of the domes, at the contact between units. Brit tle deformation corresponds to a continuum of the ductile deformation. Mean while, metamorphic contrasts suggested by pressure estimates cannot be rule d by the relative extension-related displacements. This extension correspon ds to the accommodation of the vertical indentation of the high pressure un its by the stacking of deep crustal slices, in front of the rigid Apulian m antle back-stop, during continuous convergence. (C) 2000 Editions scientifi ques et medicales Elsevier SAS.