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

Description

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 (TRSCM). TRSCM 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. TRSCM values show a gradual increase with depth and towards 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.

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