Abstract
Abstract. In this paper, we present new exhumation ages for the imbricated proximal molasse, i.e. Subalpine Molasse, of the northern Central Alps. Based on apatite (U-Th-Sm)/He thermochronometry, we constrain thrust-driven exhumation in the Subalpine Molasse between 12 and 4 Ma. This occurs synchronously to the main deformation in the adjacent Jura fold-and-thrust belt farther north and to the late stage of thrust-related exhumation of the basement massifs (i.e. external crystalline massifs) in the hinterland. Our results agree with other findings along the north Alpine foreland. While site-specific variations in the mechanical stratigraphy of the molasse deposits influence the pattern of thrusting at the local scale, we observe that late-Miocene thrusting is a long-wavelength feature occurring along the north Alpine foreland roughly between Lake Geneva and Salzburg. The extent of this thrusting signal as well as the timing suggests that late-Miocene thrusting in the north Alpine foreland coincides with the geometries and dynamics of the attached Central Alpine slab at depth. Interestingly, this implies that the slab geometry at depth does not coincide with the boundary between the Eastern and Central Alps as observed in the surface geology. Using this observation, we propose that thrusting in the Subalpine Molasse and consequently also the late stage of thrust-related exhumation of the external crystalline massifs, as well as the main deformation in the Jura fold-and-thrust belt are at least partly linked to changes in slab dynamics.
Highlights
Deep crustal processes and slab dynamics have been considered to influence the evolution of mountain belts (e.g. Davies and von Blanckenburg, 1995; Molnar et al, 1993; Oncken et al, 2006)
We propose that thrusting in the Subalpine Molasse and the late stage of thrust-related exhumation of the external crystalline massifs, as well as the main deformation in the Jura fold-and-thrust belt are at least partly linked to changes in slab dynamics
Looking at the pattern over a longer wavelength, the general disappearance of the external crystalline massifs (ECMs) towards the east and the decrease in foreland shortening are in good correlation (Fig. 6). Combining these observations with the aforementioned data about the timing and kinematics of ECM exhumation and foreland deformation, we suggest that late-Miocene thrusting is a long-wavelength feature which occurs along the entire northern Central Alps, encompassing both the ECMs and the foreland (i.e. Subalpine Molasse and Jura fold-and-thrust belt (FTB); Fig. 8c)
Summary
Deep crustal processes and slab dynamics have been considered to influence the evolution of mountain belts (e.g. Davies and von Blanckenburg, 1995; Molnar et al, 1993; Oncken et al, 2006) These deep-seated signals may be masked by tectonic forcing at upper-crustal levels and by enhanced surface erosion related to climate change It is challenging to isolate the exhumation signal related to slab dynamics In this context, foreland basins offer suitable archives as they potentially bear information that allows us to resolve the influence of deep-seated processes on mountain building. Foreland basins offer suitable archives as they potentially bear information that allows us to resolve the influence of deep-seated processes on mountain building This is the case because these basins record signals that are related to surface dynamics such as changes in sediment fluxes and eustacy The north Alpine foreland basin, or Molasse Basin, is suited to constrain the geodynamic evolution of the collisional Alpine orogen because the history of this sedimentary trough has been well established through numerous magneto- and tectonostratigraphic
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