Low temperature thermochronological constrains on the late exhumation of the Alpine foreland (Digne nappe, France).

Abstract:

The frontal part of the southwestern Alpine belt is characterized by important compressional deformation marked by the emplacement of the Digne nappe and the formation of the Valavoire thrust-sheet. The final emplacement of this nappe is dated Late Miocene thanks to Tertiary continental molasses of the foreland basin that are involved in the famous Vélodrôme recumbent syncline and exposed in erosional windows. The stratigraphic series of the Digne nappe is made of ~5000 m thick Liassic to Eocene deposits, which overthrust the vélodrôme syncline. We performed a low temperature apatite fission tracks (AFT) and (U-Th)/He (AHe) study on detrital grains of Tertiary molasses in order to (i) characterize the thermal conditions during burial and exhumation and to (ii) propose a late tectonic evolution in the front of the European Alpine foreland. Tertiary molasses were sampled in two sites of the erosional windows at different elevations. Samples present dispersed AHe and AFT ages due to an incomplete resetting of both thermochronometers, expected for the lowest elevation samples. In detail, AHe ages ranges from 2±0.2 to 60.4±5.4 Ma, whereas central AFT ages range from 11±1 to 67±16 Ma. On both sites, the total and partial reset of the thermochronological data suggests a heating event after the sediment deposition. Using QTQt inverse modeling and He damage codes, we determined the samples thermal history. The results implied a common burial temperature at 110±5°C during ~5Ma and a similar exhumation starting at 6±1 Ma. From these results, we conclude that the thermal conditions during burial associated with the Digne nappe thrusting were sufficient to reset the detrital apatites. Using mean surface temperature of 10°C and typical thermal gradient from 25°C/km, our new data show that the Digne Nappe reached at least 4.5 to 3.6 km-thick on both sites before further erosion. We propose that the late exhumation occurred at ~6 Ma ago, before the Messinian incision, is due to tectonic processes compensated by erosion. The link between exhumation and erosion is compatible with the outward propagation of the alpine deformation in the frontal part of the European foreland. This deformation results of sedimentary cover stacking, and / or of deep-seated basement shortening.