Tectono-thermal evolution of the External Western Alps (France): evidence for rift-related thermal event

Raman Spectroscopy on Carbonaceous Material (RSCM) approach is commonly used to calculate thermal peaks recorded by rocks. A first calibration of the RSCM was developed to measure temperatures ranging from 330°C to 640°C (Beyssac et al., 2002). Its applicability was later expanded to lower temperatures from 200°C to 350°C (Lahfid et al., 2010). Here, we apply the RSCM approach to Digne Nappe area - thrust front of the SW Alps - in order to evaluate the thermal evolution of the sub-Alpine domain from rift to the present. About 150 temperatures have been obtained along seven continuous stratigraphic sections sampled along the strike the whole Digne thrust sheet. The base of the Digne thrust sheet (i.e. the Lias syn-rift carbonates) shows temperatures ranging from 250°C to 330°C. These temperatures are 200-240°C in post-rift marls dated to Callovian-Oxfordian and rapidly drop upsection in the Kimmeridgian-Tithonian carbonates and younger Cretaceous rocks to temperatures below 100-120°C. While these temperatures are seen to decrease from bottom to top we note the lack of well-defined apparent geothermal gradients. One of our section (Serre-Ponçon) structurally positioned beneath the Embrunais-Ubaye thrust sheets is remarkable because the temperatures are rather homogeneous, ranging between 300°C and 350°C along the whole 5km-thick sedimentary pile from the Lias until the Eocene. The regional dataset suggests that the thermal history of the sub-Alpine fold-and-thrust belts varies along-strike and requires the succession of several thermal events. The drop of temperatures observed in the Late Jurassic sediments is interpreted to be related to increasing heat flow during crustal thinning associated to the formation of the Alpine Tethys rifted margin.  The high temperatures observed along the Serre-Ponçon specifically indicate a burial beneath the Embrunais-Ubaye thrust sheets during Alpine orogeny, possibly combined with high geothermal gradients inherited from the Mesozoic Alpine Tethys thinning phase.