POST-EOCENE DENUDATION OF THE ALPS (EUROPE): TEMPORAL VARIATIONS FORCED BY TECTONICS

Until 6 Ma, the dominant controlling factor of sediment discharge from the Alps is regional tectonics, superimposed on regional climate change. A duplication of sediment discharge around 30 Ma is attributed to isostatic adjustment after a thermal reorganisation of the lithosphere related to slab breakoff, or/and to crustal thickening as continental crust became subducted. Between 28 and 6 Ma, erosion rates in the Alps varied due to short-lived tectonic events. These forced an increase of erosion rates between 23-21 Ma and 18-16.4 Ma, and a decrease between 20.5-18 Ma and 16.4-12 Ma (Eastern Alps) or 16.4-6 Ma (Western Alps). From 20.5 to 12 Ma, extension and relief collapse in the entire Alps triggered changes of erosion rates. Lateral extrusion in the Eastern Alps is interrupted by a period of updoming between 18-16.4 Ma, coeval with massive paleogeographic reorganisations. An ongoing period of uplift started at 6 Ma in the Western Alps and at 4 Ma in the Eastern Alps, doubling erosion rates. The time lag of 2 Myrs is attributed to deep-seated lithospheric reorganisations which are prograding eastward.

A steady state between uplift and erosion is likely, but both uplift and erosion rates varied through time, in response to changes of the tectonic setting. Between 21 and 12 Ma, plate convergence was mostly absorbed by lateral extrusion (growth along strike) and minor erosion, whereas before it was absorbed by constructive growth across strike and major erosion. Since 6 Ma, plate convergence is totally consumed by erosion in the Western Alps, whereas in the eastern Eastern Alps constructive growth both across and along strike still plays a minor role.

The response time of steady state erosion to tectonic events is in the range of 2-3 Myrs at average erosion rates between 0.25 and 0.6 mm/a in the Western Alps.