Large-wavelength late Miocene thrusting in the North Alpine foreland: Implications for late orogenic processes

Additional to classical nappe tectonics, the Oligocene to mid-Miocene post-collisional evolution of the Central European Alps was characterized by vertically directed tectonics, with backthrusting along the Insubric Line and the subsequent uplift of the External Crystalline Massifs (ECMs). Thereafter, the orogen experienced axis-perpendicular growth when deformation propagated into its external parts. For the North Alpine foreland between Lake Geneva and Lake Constance, in the past, this has been kinematically and spatially linked to the uplift and exhumation of the ECMs. Based on apatite (U-Th-Sm)/He thermochronometry, we constrain thrusting in the Subalpine Molasse between 12–4 Ma, thus occurring coeval to main deformation in the Jura fold-and-thrust belt (FTB) and late stage exhumation of the ECMs. However, this pattern of tectonic activity is not restricted to areas which are bordered by ECMs, but is consistent along the northern front of the Alps between Geneva and Salzburg. Therefore, late Miocene foreland deformation is not necessarily a consequence of uplift and exhumation of the ECMs. While the local geometry of the Subalpine Molasse results from lateral variations of the mechanical stratigraphy of the foreland basin sediments, we suggest that the large-wavelength tectonic signal is the response to a shift in tectonic forces possibly caused by deep-seated geodynamic processes. This resulted in a change from dominantly vertical to horizontal tectonics and orogen-perpendicular growth of crustal thickening. We constrain the onset of this major tectonic change to ca. 12 Ma in the North Alpine foreland, resulting in thrusting and folding in the Subalpine Molasse west of Salzburg and in the Jura FTB until at least 4 Ma.