Unraveling the eroded units of mountain belts using RSCM thermometry and cross-section balancing: example of the southwestern French Alps

Reconstruction of eroded mountain belt structures is crucial to produce valid pre-contractional palinspastic restorations and quantify the amount of shortening. In this study, we combine Raman spectroscopy of carbonaceous material (RSCM) thermometry and cross-section balancing technique to propose a reconstruction of the thermal and deformational evolution of the Subalpine Digne Nappe in the southwestern French Alps. We sampled Mesozoic to Cenozoic sedimentary strata for RSCM thermometry analyses along the ~ 4-km-thick Digne Nappe to calculate maximum temperatures (T RSCM ). T RSCM values range from 243 ± 10 °C to 349 ± 13 °C in the upper Triassic–upper Cretaceous strata and from 255 ± 19 °C to 328 ± 8 °C in the overlying upper Eocene–lower Oligocene strata. T RSCM values show a gradual increase with depth and toward the inner part of the orogen. These results are consistent with a ~ 28 ± 4 °C/km modeled thermal gradient in the ~ 4-km-thick Digne Nappe and suggest post-Oligocene burial of the Digne Nappe under sedimentary/tectonic units that have been removed by erosion. Palinspastic restoration of a NE-trending and 65-km-long cross-section (Digne–Barcelonnette transect) constrained by thermal data supports the interpretation that the Digne Nappe involved a ~ 3-km-thick middle/upper Eocene–Oligocene to lower Miocene foreland basin tectonically overlain by the 3- to 11-km-thick Embrunais–Ubaye flysch nappes. The front of the Embrunais–Ubaye nappes was located ~ 40 km beyond its present erosional front. The amount of southwestward shortening in the Subalpine thrust wedge derived from palinspastic restoration is about 35 km (35%) over the past ~ 16 million years. Removal of a substantial volume of the southwestern Alpine orogen may have significantly influenced the dynamics of the Subalpine thrust wedge and Valensole foreland basin since the Middle Miocene.