DEFORMATION MIGRATION IN AN OROGEN-SCALE SHEAR ZONE ARRAY - AN EXAMPLE FROM THE BASAL BRIANCONNAIS THRUST, INTERNAL FRANCO-ITALIAN ALPS

Combined structural, geochemical and isotopic studies have allowed an understanding of the timing and nature of an orogen-scale fault array. The results indicate that the deformation loci within the internal we stern Alps, during the Alpine collision, occurred as a foreland propag ating thrust sequence. The east to west deformation migration within t he internal zones is apparently in-sequence in relation to the externa l zones. Rb-Sr white mica dating of syn-kinematic greenschist-facies m ineral assemblages from the Basal Brianconnais Thrust indicate that th rusting ceased between 27 and 32 Ma, several million years after shear ing in the hinterland and several million years prior to shearing in t he foreland. The Brianconnais Domain, which constitutes the hanging wa ll to the Basal Brianconnais Thrust, preserves two major shearing epis odes. The first, with a top-to-the-northwest overshear, has been tenta tively dated at 45 Ma. The second, a very pervasive, east-west orienta ted, greenschist-facies event was previously dated at 34 Ma on the hin terland margin of the Brianconnais Domain and has know been dated at 2 7-32 Ma on the foreland margin of the Brianconnais Domain. The period between 34 and 27 Ma apparently dates the migration of deformation thr ough the relict European passive margin, represented by the Brianconna is Domain. This is believed to be in response to overthrusting of Adri a/Africa and its associated subduction complex. Structural mapping ind icates that the present Basal Brianconnais Thrust in the Col du Petit St Bernard region, France-Italian Alps, is a break-back thrust which c uts through an already imbricated pile. Geochronological evidence sugg ests that the early imbrication of the Brianconnais stratigraphy occur red prior to full interaction of the European and Adria/African plates , that is, during subduction, docking and escape from the subduction c omplex under Adria. Therefore, although the present Basal Brianconnais Thrust is a break-back thrust in terms of local structural geometries , it is an in-sequence foreland-propagating structure. Geochronologica l, micro-structural and micro-chemical data indicate that the Briancon nais Domain in the Col du Petit St Bernard zone is formed from granito id material which intruded and cooled at approximately 320 Ma. During the Alpine event, deformation and metamorphism were insufficient to af fect the Sr isotopic system. This suggests that this portion of the Br ianconnais Domain was probably subducted to much shallower depths and underwent much less pervasive deformation than the other internal Euro pean basement material.