Controls of surface erosion on the evolution of the Alps: constraints fromthe 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 evolut ion of the Alpine orogenic wedge allow an evaluation of the controls of ero sion rates on large-scale Alpine tectonic evolution. Volumetric data from t he Molasse Basin and fining-upward trends in the Gonfolite Lombarda indicat e 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 th an crustal uplift rates. As a result, surface uplift occurred. Because of s urface uplift, the drainage pattern of the Alpine hinterland evolved from a n across-strike to the present-day along-strike orientation. Furthermore, t he decrease of average erosion rates at similar to 20 Ma coincides with ini tiation of a phase of thrusting in the Jura Mountains and the Southern Alpi ne nappes at similar to 50 km distance from the pre-20-Ma thrust front. Cou pled erosion-mechanical models of orogens suggest that although rates of cr ustal convergence decreased between the Oligocene and the present, the redu ction 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 Alp s at that time.