Multi-stage thrusting at the "Penninic Front" in the Western Alps between Mont Blanc and Pelvoux massifs

Abstract

The different segments of the tectonic boundary between external (European) and internal (Penninic) units in the Western Alps, the so-called Penninic Front (PF), formed at different times and according to different kinematic scenarios. During a first episode (Eocene), the PF corresponds to a transpressive suture zone between Penninic and European units. North-to NNW-trending stretching lineations, found along internal nappe contacts within the Penninic units, are related to this episode. This subduction zone was sealed by the Priabonian flysch of the Aiguilles d'Arves, a detrital trench formation that formed during the final stages of subduction. During a second episode, starting in mid-Oligocene times, the PF, imaged along the ECORS-CROP profile, acted as a WNW-directed thrust. This thrust, the Roselend Thrust (RT), only partially coincides with the PF. South of Moßtiers, the RT propagates into the Dauphinois units, carrying the former Eocene PF (including the Priabonian flysch) passively in its hangingwall. South of the Pelvoux massif the RT finds its continuation along the ªBriançonnais Frontº, an out-ofsequence thrust behind the Embrunais±Ubaye nappes. On a larger scale, our findings indicate oblique (sinistral) collision within the future Western Alps during the Eocene, followed by westward indentation of the Adriatic block.

The Valais units in Savoy (France): a key area for understanding the palaeogeography and the tectonic evolution of the Western Alps

“…2), the mylonitic foliation defining the Roselend thrust systematically becomes steeper. The dips increase from around 35°found in the SW at the Cormet de Roselend, also typical for areas even further SW (Ceriani et al 2001), towards a sub-vertical orientation in the NE (Pyramides Calcaires). This steepening is due to late-stage folding that led to the updoming and erosion-assisted exhumation of the Mt.…”

“…2, 6, 8d). This Roselend thrust (Loprieno 2001;Ceriani et al 2001;Ceriani and Schmid 2004) forms the contact of the Valais units with the underlying Dauphinois units and largely coincides with the Penninic Front, also referred to as Penninic Basal Contact (Ceriani et al 2001). Note that the trace of the Roselend thrust is oblique in respect of the strike of the nappe contacts amongst the different units derived from the Valais palaeogeographical domain (Fig.…”

“…In Savoy, the tectonic units derived from the Valais palaeogeographical domain, first defined by Haug (1909) then revived by Trümpy (1955), mark the front of the Penninic nappes that bend around the northern part of the arc of the Western Alps, connecting with the E-W striking Zone de Sion-Courmayeur in adjacent Switzerland and ending in a N-S striking segment south of our area of investigation (Fügenschuh et al 1999;Ceriani et al 2001). Ophiolitic rocks have been known for a long time to occur within the Valais palaeogeographical domain in two key areas of the Western Alps.…”

“…The tectonic slices represent the root zone of the Helvetic Wildhorn nappe exposed in Western Switzerland (Trümpy 1951(Trümpy , 1963Grasmück 1961;Epard 1990). The top-WNW Roselend thrust, also referred to as Penninic Front (Ceriani et al 2001), forms the tectonic boundary of the Dauphinois with the overlying and more internal Valais units. To the SE, the Valais units are separated from the Zone Houillère by a late-stage normal fault (Fügen-schuh et al 1999) that reactivated a former frontal thrust of the Zone Houillère , i.e.…”

“…1; Ceriani et al 2001). The Cheval Noir unit represents the remnants of a trench formed during the subduction of the Valais units and is made up of a thick series of Tertiary (mainly Priabonian) sediments (wildflysch, breccias and flysch), which rest on a Briançonnais type substratum (Serre et al 1985;Ceriani et al 2001;Ceriani and Schmid 2004).…”