Cross-propagation of the western Alpine orogen from early to late deformation stages: Evidence from the Internal Zones and implications for restoration

Abstract

The internal zones of the Western Alps arc are derived from an oceanic and continental subduction wedge developed beneath the Adria plate during its paleogene northward drift. Exhumation of the internal zones proceded from early Oligocene onwards due to westward extrusion of the Adria plate. The prominent fold-and-thrust structures which follow the arc shape, either forward or backward verging, postdate the initial nappe stacking and overprint differently oriented older deformations which are relevant to proper retoration of this arcuate orogen to minimise overlap problems. We document this early stacking phase through outcrop-scale structural analysis at 55 sites between the Maurienne and Ubaye valleys, along with larger-scale examples of early structures. They consistently show an initial N- to NW tectonic transport, whose kinematic indicators are overprinted by either forward (W- to SW-directed) or backward (E- to NE-directed) deformation associated with post-nappe transport along the Penninic thrust. Accordingly, restoring the Briançonnais fold/thrust system must incorporate reconstruction of the nappe stack along the initial top N-NW direction of orogenic propagation, with careful consideration of their paleogeographic origin towards the S-SE. This stack was built during the Eocene Adria-Iberia collision, and overthrust the Subbriançonnais-Valaisan trough to the NW before involving the Dauphiné-Helvetic foreland. It includes different types of Paleozoic units, either Permo-Carboniferous sediments towards its base, or polymetamorphic basement above, which can be explained by inversion of a late Variscan basin and of its southern shoulder, whereas the uppermost Prepiedmont units result from inversion of the Tethyan margin toe. Mixed breccia, locally preserved close to the tectonic contact between the latter units and the overlying "Schistes Lustrés" oceanic nappes, are interpreted as olistostromes fed by both units in a very early collision stage. ³⁹Ar/⁴⁰Ar dating suggests that these shallow tectono-sedimentary formations were involved in the subduction wedge during the early Eocene, whereas younger (late Eocene) equivalent olistostromes mark the propagation of the Briançonnais stack over the external (Dauphiné/Helvetic) foreland. The Eocene orogenic wedge was rapidly exhumed during Oligocene westward indentation and radial spreading, in a markedly different tectonic context driven by extrusion around an Adriatic upper mantle indenter, which controlled development of the Western Alps arc in relation with the Ligurian sea opening.

Introduction

Despite being one of the most extensively studied mountain ranges in the world, the Western Alps are a very specific part of the Alpine orogen whose kinematic evolution is markedly different from the rest of the chain. Whereas the Alps trend approximately E-W from Austria to Switzerland, a shape easily understandable considering N-S Africa-Europe convergence during the Cenozoic (Rosenbaum et al., 2002), the western arc shows a 180° shift across western Switzerland, SE France and N Italy. This shape was partly inherited from the Mesozoic rifting stage, and mainly developed progressively from the late Eocene onwards (Caby, 1996; Ford et al., 2006; Vignaroli et al., 2008; Dumont et al., 2012; Malusà et al., 2015), as an accommodation of westward extrusion, possibly combined with oblique convergence (Laubscher and Bernoulli, 1982; Ricou, 1984) and anticlockwise rotation of the northern part of the Adria plate (Laubscher, 1988, Laubscher, 1991; Malusà et al., 2009; Eva et al., 2020). This non-cylindrical propagation produced a complex and polyphase internal deformation of the subduction wedge preserved between the Adria plate and the European foreland, including fast exhumation and changes in tectonic transport direction through time (Platt, 1986; Ramsay, 1989), which are both characteristic of the Western Alps.

This complex 3D and polyphase deformation history makes the initial architecture of the precursor continental margin, presently involved in the western Alpine arc difficult to restore, particularly because the most prominent structures which define the present-day trend of the arc probably postdate the initial contractional features as they overprint the evidence of the earliest orogenic propagation developed during Eocene times. Moreover, the Adria plate first collided with terranes connected to the eastern Iberia plate, such as the Briançonnais domain (Stampfli et al., 2002; Handy et al., 2010), so that an early part of the convergence history is likely to have been accommodated by oblique contraction and reactivation along the original eastern extent of the Pyrenean orogen, resulting in complex interference between pre-existing Pyrenean structures and newly evolving Alpine deformation (Lacombe and Jolivet, 2005; Schreiber et al., 2011; Balansa et al., 2022). Finally, the Alpine structures have experienced more recent fragmentation, in the southern part of the arc, through the development of the Ligurian and Tyrrenian breakups and by the growth of the Apenninic chain, driven by the complex lithospheric motion of various lithosphere slabs (Jolivet et al., 2008; Zhao et al., 2016; Salimbeni et al., 2018).

The surface geology of the Western Alps arc gives a misleadingly simple expression of this history. Radial transects have been regarded as more or less equivalent and comparable with little regard for their relative orientation. However, this approach does not incorporate consideration of the magnitude of oblique to lateral transfer and tectonic transport oblique or parallel to the modern orogenic trend, which were potentially of major importance considering the evidence for oblique-slip motions in the southern part of the western Alpine arc (Butler et al., 1986; Ricou and Siddans, 1986; Laubscher, 1991; Malusà et al., 2009). This current work is focused on deciphering the structural and tectono-sedimentary features related to the early Alpine orogenic stages, which were active before the development of the present-day arcuate shape, and which consist of multi-scale evidence for different tectonic transport directions through time, and possible interference structures. Since the early orogenic propagation is also characterised by surficial interactions between relief, gravity and flexural basin distribution, the potential link between selected tectono-sedimentary breccias and the major tectonic contacts is also examined.