Controls of surface erosion on the evolution of the Alps: constraints from the stratigraphies of the adjacent foreland basins

The combined information about the stratigraphies from the foreland basins surrounding the Swiss Alps, exhumation mechanisms and the structural evolution of the Alpine orogenic wedge allow an evaluation of the controls of erosion rates on large-scale Alpine tectonic evolution. Volumetric data from the Molasse Basin and fining-upward trends in the Gonfolite Lombarda indicate that at similar to 20 Ma, average erosion rates in the Alps decreased by >50%. It appears that at that time, erosion rates decreased more rapidly than crustal uplift rates. As a result, surface uplift occurred. Because of surface uplift, the drainage pattern of the Alpine hinterland evolved from an across-strike to the present-day along-strike orientation. Furthermore, the decrease of average erosion rates at similar to 20 Ma coincides with initiation of a phase of thrusting in the Jura Mountains and the Southern Alpine nappes at similar to 50 km distance from the pre-20-Ma thrust front. Coupled erosion-mechanical models of orogens suggest that although rates of crustal convergence decreased between the Oligocene and the present, the reduction of average erosion rates at similar to 20 Ma was high enough to have significantly influenced initiation of the state of growth of the Swiss Alps at that time.