Backstripping the Briançonnais (Western Alps) to test Alpine rifting models

During the Mesozoic, the relative movement of African and Eurasian plates caused the opening of the Tethys Ocean. The rifting phase is well charted by the stratigraphic sequence of Western Alps, which provide an exceptional record of continental margin evolution. The Briançonnais domain occupies a pivotal place for testing various rifting models. This domain contains a remarkably uniform succession of very shallow-water carbonates of Triassic age, capped by Middle-Jurassic shallow-water carbonates or by non-deposition before passing abruptly up into deep-water facies. Here we show that the back-stripped Mesozoic tectonic evolution of the Briançonnais block can be applied to investigate models of lithospheric stretching. Applying the Airy correction, we found that the Triassic is characterised by a constant tectonic subsidence rate of 17 m/Ma. If this is the result of “post-rift” thermal re-equilibration of upper mantle after late Palaeozoic rifting, this rift phase occurred with a stretching factor of c 1.4. That this thermal subsidence was modulated by differential uplift and erosion of the Briançonnais in the early Jurassic implies significant mantle thinning, reducing net density of the Briançonnais lithosphere. The subsidence of more than 3000m during Bathonian-Callovian stages are too rapid to be explained by thermal reequilibration: it suggests substantial crustal thinning. Our results demonstrate that a uniform stretching model is not able to explain the Jurassic isostatic movement of the Briançonnais domain. It is consistent with two-stage, depth-variable stretching of the Briançonnais lithosphere during the Jurassic. Our study represents a starting point for more sophisticated and developed numerical models, to explain rapid vertical movements in hyper-extended continental margins.