Tectonometamorphic evolution of the Schistes Lustres Complex; implications for the exhumation of HP and UHP rocks in the Western Alps

Abstract

We present new structural and metamorphic data on the Schistes Lustres complex which occupies a central position in the western Alps between the external LP and the internal UHP domains (e.g., the Dora Maira massif). Metamorphic conditions are shown to increase progressively from west to east from ca. 12-13 kbar/300-350 degrees C to 20-21 kbar/450-500 degrees C close to the Dora Maira massif. Two distinct exhumation episodes are recognized: (1) A pervasive east-vergent ductile D2 event, with a large component of vertical shortening, took place under low blueschist-facies to greenschist-facies conditions. This event is responsible for most of the exhumation of the Schistes Lustres complex as well as for the preservation of carpholite occurrences at its front, and took place during the period 50-40 Ma. (2) A west-vergent ductile-to-brittle, highly non-coaxial, extensional D3 event subsequently developed, with a deformation intensity decreasing from east to west. This event took place at subgreenschist-facies conditions and is coeval with (and probably derives from) the west-vergent greenschist deformation taking place in the Dora Maira massif and other internal domains by ca. 40-35 Ma.