Between the middle and late Miocene, the Central Alps of Switzerland and northern Italy underwent a major phase of lateral crustal growth as recorded by the formation of the southern Alps and the Jura fold-and-thrust belt. This period of dominantly outward-directed deformation differs from the late Oligocene and early Miocene deformation style, which was characterized mainly by rapid vertical exhumation. Sediment budgets from circum-Alpine sedimentary basins indicate a decrease in the erosional efficiency in the Alpine hinterland several million years before lateral orogen growth was initiated, and decreasing magnitudes of sediment discharge thereafter. This decrease in erosional efficiency of the hinterland coincides approximately with widespread exposure of the crystalline core in the Alpine hinterland as indicated by clast types in conglomerates of synorogenic deposits, and with a contemporaneous increase in the continental influence in the paleoclimate, as suggested by the fossiliferous plant record. We suggest that the observed decrease in erosional efficiency caused gravitational forces to increase relative to tectonic forces driving the orogenesis, which led to a transition from dominantly vertical to horizontally directed extrusion. The data thus can be interpreted to indicate an active link between surface erosion and orogenic evolution.

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