Lateral termination of the north‐directed Alpine orogeny and onset of westward escape in the Western Alpine arc: Structural and sedimentary evidence from the external zone

Abstract

[1] The initial propagation of the Western Alpine orogen was directed northwestward, as shown by basement-involved and Mesozoic sedimentary cover compressional structures and by the early foreland basins evolution. The crystalline basement of the Dauphiné zone recorded three shortening episodes: pre-Priabonian deformation D1 (coeval with the Pyrenean-Provence orogeny), and Alpine shortening events D2 (N-NW directed) and D3 (W-directed). The early Oligocene D2 structures are trending sub-perpendicular to the more recent, arcuate orogen and are interfering with (or truncated by) D3, which marks the onset of westward lateral extrusion. The NW-ward propagating Alpine flexural basin shows earliest Oligocene thin-skinned compressional deformation, with syn-depositional basin-floor tilting and submarine removal of the basin infill above active structures. Gravity enhanced submarine erosion gave birth locally to steep submarine slopes overlain by kilometric-scale blocks slid from the orogenic wedge. The deformations of the basin floor and the associated sedimentary and erosional features indicate a N-NW-ward directed propagation, consistent with D2 in the Dauphiné foreland. The Internal zones represent the paleo-accretionary prism developed during this early Alpine continental subduction stage. The early buildup has been curved in the arc and rapidly exhumed during the Oligocene collision stage. Westward extrusion and indenting by the Apulian lithosphere allowed the modern arc to crosscut the western, lateral termination of the ancient orogen from ∼32 Ma onward. This contrasted evolution leads to propose a palinspastic restoration taking in account important northward transport of the distal passive margin fragments (Briançonnais) involved in the accretionary prism before the formation of the Western Alps arc.Citation: Dumont, T., T. Simon-Labric, C. Authemayou, and T. Heymes (2011), Lateral termination of the north-directed Alpine orogeny and onset of westward escape in the Western Alpine arc: Structural and sedimentary evidence from the external zone, Tectonics, 30, TC5006,

Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology

“…Ma Alpine strike-slip shear zones in the basement (probably reactivating former Pyrenean faults, Simon-Labric et al, 2009;Dumont et al, 2011). In the sediments, a regional cleavage formed before 26 Ma at temperatures around 300°C (Crouzet et al, 2001), which is confirmed by the presence of pyrophyllite and cookeite in the Liassic sediments (280 to 350°C from west to east (Jullien and Goffé, 1993).…”

“…1.; Ford et al, 1996 and references therein) supposedly mark the first Alpine deformation in the Western Alps, but the significance of these folds has been recently challenged by Michard et al (2010) who reinterpreted them as huge gravity structures unrelated to any compressional event. Thus, Alpine deformation probably started with pre-Priabonian E-W thrusts caused by a N-S compression mainly described in the southern Oisans massif (Gidon, 1979;Ford, 1996;Sue et al, 1997;Dumont et al, 2008Dumont et al, , 2011). …”

Geological setting of the southern termination of Western Alps

“…Consequently, the presence of a major kinematic "transfer zone" (Ligurian sinistral transfer, LST in Fig. 19) at the southern termination of the Western Alps arc could be envisaged as a major crustal feature that may have provided the left-lateral "southern disengaging lineament" (Giglia et al 1996) necessary for the Oligocene westward indentation (Dumont et al 2011(Dumont et al , 2012 and counterclockwise rotation (Collombet et al 2002;Rolland et al 2012) of Adria promontory, as invoked in literature during the last decades (see "Structural setting at the southern termination of the Western Alps" section for details), and roughly described as "Stura couloir" or Stura Fault.…”

Dating emplacement and evolution of the orogenic magmatism in the internal Western Alps: 1. The Miagliano Pluton

“…A refined view on the situation sketched by Tommasini et al (2011) would include a plate tectonic boundary between the Western Alps and the central Alps in order to explain the difference in chemical evolution for these two areas. Such a plate tectonic strike slip boundary has been proposed by several authors (e.g., Ricou and Siddans 1986;Handy et al 2010;Dumont et al 2011). This boundary explains the different evolution between the western-and the central-Alps.…”