Gravitational sliding at the southern end of the western Alpine arc

The southernmost segment of the western Alpine arc, strikes E-W along the Mediterranean coast between Nice and Menton (F), and it is bounded by N-S structures on both sides. These abrupt changes in orientation, and the E-W strike as such in an area located southwest of the Adriatic indenter is difficult to reconcile with alpine collisional displacements. Indeed the latter are inferred to be governed by NW-ward movement of the Adriatic Indenter, which is located NE of our study area. E-W -oriented structures in this area are commonly attributed to the Pyrenean (Pyreneo-Provençal) orogeny, which however does not explain their young, Miocene age. As a consequence this E-W Alpine (?) arc segment has been variously interpreted in the past literature (Brunsmann et al., 2024, for review), as the result of Alpine indentation, Pyrenean-Provençal shortening re-activated in post-Burdigalian times, or gravitational sliding (Gèze, 1956).

A 40 km long, NNE-SSW cross-section between the latter localities shows that the tectonic style varies from N to S. The northern sector shows that the Permian and Triassic cover are gently folded above the Argentera crystalline basement. Further south, in the areas of Breil-sur-Roya and Sospel, the Jurassic, Cretaceous and Eocene cover is more tightly folded, showing broadly E-W, steeply-dipping axial planes. However, this entire folded sequence tectonically overlies the very gently-dipping upper Triassic gypsum, forming a subtractive contact, thus a normal-type of displacement. Further south, from Castillon to Cap d’Ail, the entire Triassic to Eocene cover forms a stack of 10 distinct north-dipping thrusts, most of which lying directly above Triassic gypsum.

As shown in map view between Sospel and Gorbio, the same gypsum layer can be continuously followed from its normal-fault position in the north to thrust planes in the south, via a thin strike slip fault located between them. We suggest that not only the thrust faults of Gorbio, but even all the others further south are rooted in the gypsum of the normal fault further north.

This spatial distribution of normal faults at higher topographic altitude, kinematically linked with thrusts at a deeper topographic level, and all localized along gypsum layers, is analogous to what is frequently observed in the sedimentary sequences of passive margins. Cooling of the oceanic lithosphere causes differential subsidence (e.g., Brun and Fort, 2012), hence tilting of the sedimentary sequence, which initiates downward gliding of the post-Triassic beds. In our case study, the contemporaneous Miocene uplift of the Argentera crystalline massif (Bigot-Cormier et al., 2006) and extensional thinning of the Liguro-Provençal domain (Rollet et al., 2004) tilts the entire Mesozoic sequence allowing for a gradient that induces gliding along the low-viscosity upper triassic gypsum. If this interpretation is correct, a large part of this arc segment does not directly result from Alpine collisional convergence.

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Brun, J.-P. and Fort, X. (2011). Marine and Petroleum Geology, 28, 1123-1145.
Brunsmann et al. (2024). Comptes Rendus. Géoscience, 356, 231-263.
Gèze, B. (1956). Comptes Rendus Académie Sciences,  2733-2735.
Rollet et al. (2002). Tectonics, 21, 6-23.