Reply on RC3

Abstract

Conceptual models of orogenic accretionary prisms assume the increase in peak temperatures (Tmax) towards the internal domains as crustal rocks are accreted from the lower to the upper plate. Yet, 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 Upper Cretaceous strata exposed along the Digne Nappe, at the front of the SW Alps. Our results highlight two groups of depth-dependent temperatures: (1) a regionally extensive constant Tmax up to 300–330 °C measured in the Jurassic succession and (2) regionally variable lower temperature (<150 °C) recorded either in the upper Mesozoic, the nappe stack or the syn-orogenic sequence. Modelling shows that the highest paleotemperatures were achieved during the Early Cretaceous (~130 Ma), during the Valaisan-Vocontian rifting, while the lowermost ones reflect syn-orogenic burial in the Alpine foreland basin. This study provides a striking new example where mid-crustal paleotemperatures measured in lower plate sediments now accreted at the thrust front are inherited. Estimated peak thermal gradient of 80–90 °C/km requires crustal thickness of ~15 km during the Early Cretaceous, hence placing new 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 paleothermal data when they are used to identify past collisional events. Where details of basin evolution are lacking high-temperature record may be misinterpreted as syn-orogenic, which can in turn lead to overestimating both orogenic thickening and horizontal displacement in mountain belts.