Deformation migration in an orogen-scale shear zone array: an example from the Basal Briançonnais Thrust, internal Franco-Italian Alps

Abstract

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 western Alps, during the Alpine collision, occurred as a foreland propagating thrust sequence. The east to west deformation migration within the internal zones is apparently in-sequence in relation to the external zones. Rb-Sr white mica dating of syn-kinematic greenschist-facies mineral assemblages from the Basal Briançonnais Thrust indicate that thrusting ceased between 27 and 32 Ma, several million years after shearing in the hinterland and several million years prior to shearing in the foreland. The Briançonnais Domain, which constitutes the hanging wall to the Basal Briançonnais Thrust, preserves two major shearing episodes. The first, with a top-to-the-northwest overshear, has been tentatively dated at 45 Ma. The second, a very pervasive, east-west orientated, greenschist-facies event was previously dated at 34 Ma on the hinterland margin of the Briançonnais Domain and has know been dated at 27-32 Ma on the foreland margin of the Briançonnais Domain. The period between 34 and 27 Ma apparently dates the migration of deformation through the relict European passive margin, represented by the Briançonnais Domain. This is believed to be in response to overthrusting of Adria/Africa and its associated subduction complex. Structural mapping indicates that the present Basal Briançonnais Thrust in the Col du Petit St Bernard region, Franco-Italian Alps, is a break-back thrust which cuts through an already imbricated pile. Geochronological evidence suggests that the early imbrication of the Briançonnais stratigraphy occurred prior to full interaction of the European and Adria/African plates, that is, during subduction, docking and escape from the subduction complex under Adria. Therefore, although the present Basal Briançonnais Thrust is a break-back thrust in terms of local structural geometries, it is an in-sequence foreland-propagating structure. Geochronological, micro-structural and micro-chemical data indicate that the Briançonnais Domain in the Col du Petit St Bernard zone is formed from granitoid material which intruded and cooled at approximately 320 Ma. During the Alpine event, deformation and metamorphism were insufficient to affect the Sr isotopic system. This suggests that this portion of the Briançonnais Domain was probably subducted to much shallower depths and underwent much less pervasive deformation than the other internal European basement material.

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

“…However, according to our own observations the dated tectonites formed during top SE normal faulting (Fügenschuh et al 1999). Since the formation ages of Freeman et al (1998) are compatible with coeval cooling in the footwall of this ductile normal fault, as indicated by the data of Cannic (1996), we suggest that this normal fault was active during the Early Oligocene. This indicates that thrusting along the RT, which post-dates this normal fault, cannot have been active before mid-Oligocene times.…”

“…St. Bernard unit in the immediate contact to the Zone Houillre from the hangingwall of this normal fault. Freeman et al (1998) interpreted the Rb±Sr white mica ages around 32 Ma in terms of formation ages related to NW-directed thrusting of the zone Houillre over the Valaisan. However, according to our own observations the dated tectonites formed during top SE normal faulting (Fügenschuh et al 1999).…”

“…These ages have been interpreted by Cannic as cooling ages. Freeman et al (1998) performed Rb±Sr dating of white micas from sheared rocks of the Pt. St. Bernard unit in the immediate contact to the Zone Houillre from the hangingwall of this normal fault.…”

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

“…This is also in agreement with stratigraphic constraints indicating postPriabonian thrusting along the Roselend thrust (Ceriani et al 2001). In the light of these findings, the few Ar/Ar and Rb/Sr phengite ages available from the Valais units within our area and ranging between 33 and 27 Ma (Cannic 1996;Freeman et al 1998) are also best explained as cooling ages induced by erosion of the hangingwall during D3 thrusting. The age of the top-N D1-nappe stacking that was probably immediately followed by top-N near-isoclinal folding of the nappe stack is only indirectly defined by correlation with the immediately adjacent and more internal Briançonnais-derived units where new geochronological data have been obtained .…”

Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study

“…This multi-stage movement zone, referred to as 'Briançonnais Movement Zone' hereafter, accommodated both extensional and shortening deformation (e.g. Cannic et al, 1996;Freeman et al, 1998). It separates the so-called Zone Houillèr, which underwent low grade Alpine metamorphism (P = 0.6 ± 0.2 GPa and T = 280-300°C; Lanari et al, 2012), from the underlying Valaisan nappe system, which underwent Alpine metamorphism up to P = 1.5-1.7 GPa and T = 350-400°C ( Fig.…”