Multistage shortening in the Dauphiné zone (French Alps): the record of Alpine collision and implications for pre-Alpine restoration

Abstract

Three-dimensional modelling tools are used with structural and palaeomagnetic analysis to constrain the tectonic history of part of the Dauphiné zone (external Western Alps). Four compressive events are identified, three of them being older than the latest Oligocene. Deformation D1 consists of W-SW directed folds in the Mesozoic cover of the study area. This event, better recorded in the central and southern Pelvoux massif, could be of Eocene age or older. Deformation D2 induced N-NW-oriented basement thrusting and affected the whole southern Dauphiné basement massifs south of the study area. The main compressional event in the study area (D3) was WNW oriented and occurred before 24 Ma under a thick tectonic load probably of Penninic nappes. The D2-D3 shift corresponds to a rapid transition from northward propagation of the Alpine collision directly driven by Africa-Europe convergence, to the onset of westward escape into the Western Alpine arc. This Oligocene change in the collisional regime is recorded in the whole Alpine realm, and led to the activation of the Insubric line. The last event (D4) is late Miocene in age and coeval with the final uplift of the Grandes Rousses and Belledonne external massifs. It produced strike-slip faulting and local rotations that significantly deformed earlier Alpine folds and thrusts, Tethyan fault blocks and Hercynian structures. 3D modelling of an initially horizontal surface, the interface between basement and Mesozoic cover, highlights large-scale basement involved asymmetric folding that is also detected using structural analysis. Both, Jurassic block faulting and basement fold-and-thrust shortening were strongly dependent on the orientation of Tethyan extension and Alpine shortening relative to the late Hercynian fabric. The latter's reactivation in response to oblique Jurassic extension produced an en-échelon syn-rift fault pattern, best developed in the western, strongly foliated basement units. Its Alpine reactivation occurred with maximum efficiency during the early stages of lateral escape, with tectonic transport in the overlying units being sub-perpendicular to it.

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

“…Hence, a similar change in the thrusting direction from a more northerly towards a more westerly direction is observed in the Dauphinois units. An evolution from top-NNW to top-WNW thrusting is also reported from the Helvetic units of western Switzerland (Ramsay 1989) and the Dauphiné units near the Pelvoux massif (Dumont et al 2008). …”

Structural and sedimentary records of the Oligocene revolution in the Western Alpine arc

“…There is no evidence for compressional reactivation of the extensional boundary faults. The first-order wavelength of compressional structures is of the same order of magnitude than the major tilted blocks width: i.e., the Grandes Rousses basement anticline is superimposed on a 10 km-wide tilted block (Dumont et al, 2008).…”

“…3; Lemoine et al, 1986). They result from reactivation of the Hercynian fabric and are subparallel to it, but they were sollicitated obliquely by Tethyan synrift extension (NW-SE ; Lemoine et al, 1989 ;Dumont et al, 2008). Alpine inversion consists mainly of buttressing in the hangingwall of master rift faults and shortcuts in their footwall.…”

“…Basement shortening involves both folding and thrusting. In the Argentera massif, maximum burial is estimated at 4.8 to 7 kb (Sanchez et al, 2011b), but in the Pelvoux massif the basement deformation processes are still debated considering the more moderate burial setting (Ford, 1996;Dumont et al, 2008). Compressional deformation increases eastwards, approaching the Crustal Pennine Thrust.…”

“…The olistostrome representing the very top of the Paleogene sedimentary sequence is locally dated early Oligocene (Mercier de Lépinay & Feinberg, 1982). The D2, top-to-the N to NW deformation of the Dauphiné foreland is related with the emplacement of the earliest Alpine nappes (Embrunais Nappes, Dumont et al, 2008), whose remnants are found immediately to the SE of the Pelvoux massif ( fig. 1, 2, 4, 5).…”