Rift thermal inheritance in the SW Alps (France): insights from RSCMthermometry and 1D thermal numerical modelling

Conceptual models of orogenic accretionary prisms assume that peak temperatures (Tmax) increase towards the internal domains as crustal rocks are accreted from the lower to the upper plate. However, the recognition of pre-orogenic heating events in mountain belts questions the magnitude of thermal overprint during nappe stacking. Using Raman spectroscopy on carbonaceous material (RSCM) to calculate Tmax, we have investigated the thermal record of Lower Jurassic to Eocene strata exposed along six stratigraphic sections at the front of the Digne Nappe (SW Alps), from the Devoluy Massif to the Castellane Arc. Our results highlight two groups of depth-dependent temperatures: (1) a regionally extensive and constant Tmax up to 300-330 & LCIRC;C measured in the Jurassic succession and (2) regionally variable lower temperatures (< 150 & LCIRC;C) recorded either in the Upper Mesozoic or the syn-orogenic sequence. Modelling shows that the highest palaeotemperatures were achieved during the Early Cretaceous (& SIM; 130 Ma), associated with the Valaisan-Vocontian rifting, while the lowest Tmax reflect post-rift thermal relaxation in the Alpine foreland basin. This study provides a striking new example where mid-crustal palaeotemperatures measured in sediments accreted from the downgoing plate are inherited. An estimated peak thermal gradient of 80-90 & LCIRC;C km-1 requires a crustal thickness of 8-10 km during the Early Cretaceous, hence placing constraints for tectonic reconstruction of rift domains and geophysical interpretation of current crustal thickness in the SW Alps. These results call for the careful interpretation of palaeothermal data when they are used to identify past collisional thermal events. Where details of basin evolution are lacking, high-temperature records may be misinterpreted as syn-orogenic, which can in turn lead to an overestimation of both orogenic thickening and horizontal displacement in mountain belts.