The Pennine Front zone in Savoie (western Alps), a review and new interpretations from the Zone Houillère Briançonnaise

What is the significance of the Pennine Front (PF) and where is it precisely located in the Savoie segment of the alpine chain between the Isère and Arc valleys? The only continuous guide to precisely locating the PF is the Zone Houillère Briançonnaise (ZHB), a lithological unit which may be followed over about 200 km, and outlined along its western side by a continuous belt of gypsum and cargneule, the Briançonnais Front (BF). The PF itself is defined as separating the Pennine domain from the Dauphinois domain i.e. the Mesozoic to Cenozoic parautochtonous to allochtonous cover of the external basement massifs (EBM). In Savoie, the Valaisan units, the most external units of the Pennine domain, die out with the Niélard sub-unit located south of Moûtiers. Toward the south, the Valaisan is relayed by the so-called Sub-Briançonnais units. A sharp contrast in stratigraphy and tectonic evolution seems to exist between Briançonnais (= Mesozoic to Early Cenozoic Pennine formations) and Dauphinois, with the Sub-Briançonnais showing an intermediate, poorly documented, structural evolution. This paper attempts to fit existing map and structural data on both sides of the PF zone with the classical tectonic units which are, until now, mostly defined upon stratigraphical grounds and palaeogeographical interpretations. The apparent correlation of major seismic reflectors shown on the ECORS-CROP profile with gypsum and cargneule-outlined belts, especially in the BF case, pleads for a relatively simple structure, with two parallel east-dipping tectonic contacts corresponding respectively to the PF and the BF, with the Sub-Briançonnais units resting in between. The actual significance of these tectonic contacts, at least for the BF, is interpreted here as the result of a late extensional event. A structural study of the ZHB suggests a new interpretation of the contrast between dominantly gently west-dipping schistosities in the ZHB and east-dipping schistosities in the Sub-Briançonnais. A neoalpine event, including a late extension, leads to the broad refolding of earlier structures behind the BF and may explain the present geometry and the linearity of the seismic reflectors. Preliminary observations of the neotectonic behaviour in the PF zone provide a preliminary test of this interpretation.