Foreland exhumation controlled by crustal thickening in the Western Alps

Abstract

In alpine-type collision belts, deformation of the foreland may occur as a result of forward propagation of thrusting and is generally associated with thin-skinned deformation mobilizing the sedimentary cover in fold-and-thrust belts. Locally, foreland deformation can involve crustal-scale thrusting and produce large-scale exhumation of crystalline basement resulting in significant relief generation. In this study, we investigate the burial and exhumation history of Tertiary flexural basins located in the Western Alpine foreland, at the front of the Digne thrust sheet (southeast France), using low-temperature apatite fission-track and (U-Th)/He thermochronology. Based on the occurrence of partially to totally reset ages, we document 3.3–4.0 km of burial of these basin remnants between ca. 12 Ma and 6 Ma, related to thin-skinned thrust-sheet emplacement without major relief generation. The onset of exhumation is dated at ca. 6 Ma and is linked to erosion associated with significant relief development. This evolution does not appear to have been controlled by major climate changes (Messinian crisis) or by European slab breakoff. Rather, we propose that the erosional history of the Digne thrust sheet corresponds to basement involvement in foreland deformation, leading to crustal thickening. Our study highlights the control of deep-crustal tectonic processes on foreland relief development and its erosional response at mountain fronts.

Badly Behaved Detrital (U‐Th)/He Ages: Problems With He Diffusion Models or Geological Models?

“…Furthermore, a single time temperature path fails to explain the large dispersion of the apatite (U‐Th)/He ages (Schwartz et al, ). Here, we combine all the data together for the Esclangon samples from the “Vélodrôme” recumbent syncline and attempt to explain the overall trends using the recovered time‐temperature path of Schwartz et al (), shown in Figure . The measured apatite (U‐Th)/He ages range from 3 to 60 Ma.…”

“…Natural example from the Western Alps, after Schwartz et al (). (a) Corrected (U‐Th)/He ages of the Esclangon samples from the “Vélodrôme” recumbent syncline (Schwartz et al, ).…”

“…Natural example from the Western Alps, after Schwartz et al (). (a) Corrected (U‐Th)/He ages of the Esclangon samples from the “Vélodrôme” recumbent syncline (Schwartz et al, ). Central fission track ages (not shown) for three samples range from 11 to 62 Ma, with the lowest elevation sample corresponding to the youngest age.…”

“…The slowest cooling source area is likely to be the Massif Central, where the thermal history is complex but rocks have been below 150 °C for approximately 300 Myr (Gautheron et al, ). The fastest cooling source areas are likely to be from the internal parts of the western Alps that were rapidly exhumed in the Late Eocene, resulting in cooling rates that were several 10 °C/Myr (Schwartz et al, ). These predepositional thermal histories do a reasonable job at explaining the spread in the apatite (U‐Th)/He ages and result in simulated AFT ages of 14 and 71 Ma using the annealing parameters of Ketcham et al ().…”

“…There has been considerable work on reconstructing the thermal history of the foreland basin sediments from the Western Alps in terms of regional exhumation in relation to climate or tectonics (e.g., Cederbom et al, 2011;Fox et al, 2015). Recently, Schwartz et al (2017) carried out an apatite fission track and apatite (U-Th)/He analysis on samples that were buried by the Digne Nappe before being incorporated into the Alpine orogen and exhumed. The sedimentary rocks that were sampled are comprised of upper Eocene to lower Oligocene fluvial and Miocene marine molasse deposits, and Schwartz et al (2017) used an inverse approach to constrain the burial and exhumation rate history (Gallagher, 2012).…”