Low-temperature thermochronology shows distinct Late Pleistocene cooling peak in valley bottom samples from the Dent-Blanche Nappe (Austroalpine, Aosta valley, Italy)

The potential role of tectonic and climatic change as mechanisms governing the Late Cenozoic tectonic and topographic evolution of the Western Alps has been strongly debated. There, the Neogene climate cooling effect expressed through glacial erosion and sediment mobilization has been interpreted to produce high rates of isostatically-driven rock uplift. However, these inference remains challenging to test, and data confirming this relationship are spatially confined. Furthermore, the role of glacially-driven erosion at high elevation, compared to erosion of the landscape in the valley bottom where more sediments are mobilized, and major Neogene fault systems occur, remains elusive.

Here, we aim to quantify the relative contribution of tectonically- and climatically-driven erosion on the present-day landscape of the Austroalpine Dent-Blanche Nappes and surrounding Penninic units in the Western Alps. We sampled two ~NW-SE oriented transects crossing the Dent-Blanche (sinsu stricto), Mont Mary and Valpelline units in Aosta Valley (Italy) with sample elevations between ~800 m and 3000 m. We analyzed 18 samples with apatite and zircon (U-Th-Sm)/He thermochronology (ZHe and AHe). We will complement the analysis with ⁴⁰Ar/³⁹Ar dating from muscovite grains (MAr) collected from the same samples.

Preliminary AHe and ZHe ages span from ~60 to ~1.8 Ma. A Late Pleistocene age is found in a lower elevation sample in Valpelline units in both AHe and ZHe. In contrast, Pliocene to Miocene ages are found in samples in the Dent-Blanche and Mont Mary units at similar elevations. These spatial differences in cooling ages do not agree with the idea of a uniform increase of relief due to post-glacial rebound in the Western Alps. If confirmed by further analysis, it seems to suggest episodic pulses of spatially confined exhumation driven by crustal wedging and glacial erosion at the valley bottoms. Such locally confined processes post-date the ~30 Ma collision and subsequent European slab break-off under the western Alps as imaged by high-resolution tomography (e.g., Kästle et al., 2020). To assess the evolution of the topography of the Dent-Blanche nappe and surrounding areas in the Cenozoic, we will apply an inverse numerical thermal-kinematic model with the new and published data coupled with a landscape evolution model.  

 

References:

Kästle, E.D., Rosenberg, C., Boschi, L., Bellahsen, N., Meier, T., El-Sharkawy, A., 2020, Slab break‑offs in the Alpine subduction zone, In: International Journal of Earth Sciences, pp. 1-17.