Long-wavelength late-Miocene thrusting in the north Alpine foreland: implications for late orogenic processes

Solid Earth, 11, 1823–1847, 2020 https://doi.org/10.5194/se-11-1823-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Long-wavelength late-Miocene thrusting in the north Alpine foreland: implications for late orogenic processes Samuel Mock1 , Christoph von Hagke2,3 , Fritz Schlunegger1 , István Dunkl4 , and Marco Herwegh1 1 Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland of Geology and Palaeontology, RWTH Aachen University, Wüllnerstrasse 2, 52056 Aachen, Germany 3 Department of Geography and Geology, University of Salzburg, Hellbrunnerstrasse 34, 5020 Salzburg, Austria 4 Geoscience Center, Sedimentology and Environmental Geology, University of Göttingen, Goldschmidtstrasse 3, 37077 Göttingen, Germany 2 Institute Correspondence: Samuel Mock () Received: 17 October 2019 – Discussion started: 27 November 2019 Revised: 24 August 2020 – Accepted: 25 August 2020 – Published: 13 October 2020 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 thrustdriven 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. 1 Introduction 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). However, these deep-seated signals may be masked by tectonic forcing at upper-crustal levels and by enhanced surface erosion related to climate change (e.g. Champagnac et al., 2007; Chemenda et al., 2000; Ganti et al., 2016; Whipple, 2009; Willett et al., 2006). In nearsurface crustal domains, it is thus 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. This is the case because these basins not only record signals that are related to surface dynamics such as changes in sediment fluxes and eustacy (e.g. Pippèrr and Reichenbacher, 2017; Sinclair and Allen, 1992), but they also preserve information on the tectonic processes at the crustal and possibly mantle scales that operate on long timescales and at long spatial wavelengths (e.g. DeCelles and Giles, 1996; Garefalakis and Schlunegger, 2018; Leary et al., 2016). The north Alpine foreland basin, or Molasse Basin, is particularly 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 (e.g. Burkhard and Sommaruga, 1998; Ganss and SchmidtThomé, 1953; Homewood et al., 1986; Kempf et al., 1999; Published by Copernicus Publications on behalf of the European Geosciences Union. 1824 S. Mock et al.: Long-wavelength late-Miocene thrusting in the north Alpine foreland Pfiffner, 1986; Schlunegger et al., 1996; Sinclair et al., 1991), seismic (Hinsch, 2013; Mock and Herwegh, 2017; Ortner et al., 2015; Sommaruga et al., 2012), and low-temperature thermochronological analyses (Cederbom et al., 2004, 2011; Gusterhuber et al., 2012; von Hagke et al., 2012, 2014b; Mazurek et al., 2006). Studies from the forelands of the European Alps have shown that the most external parts of the orogen were incorporated into the orogenic wedge in Miocene times (e.g. Becker, 2000; Burkhard, 1990; von Hagke et al., 2012, 2014b; Hinsch, 2013; Ortner et al., 2015; Pfiffner, 1986; Schmid et al., 1996; Schönborn, 1992). In the case of the Swiss Molasse Basin, late-Miocene deformation has been kinematically and spatially linked to the uplift and exhumation of the external crystalline massifs (ECMs), which represent basement units derived from the subducting European plate. This late-Miocene deformation stage was also linked to the main deformation in the Jura fold-and-thrust belt (FTB) situated at the northern margin of the Molasse Basin (Figs. 1 and 2; e.g. Boyer and Elliot, 1982; Burkhard, 1990; Burkhard and Sommaruga, 1998; von Hagke et al., 2012; Laubscher, 1961, 1992; Mosar, 1999; Pfiffner, 1986; Sommaruga, 1999). The inferred linkages between the uplift of the ECMs, the imbrication of the proximal molasse deposits, and the main deformation in the Jura FTB are mainly based on a classical scenario of ongoing continent–continent collision, where compressional wedge tectonics and shortening result in crustal thickening of basement units in the hinterland and a propagation of the orogenic wedge towards the foreland, including imbricate thrusting (Pfiffner, 1986; Rosenberg and Berger, 2009; Schmid et al., 1996). New studies from the Aar Massif of the Central Alps challenge this view. Based on geometric, kinematic, metamorphic, and geodynamic arguments, Herwegh et al. (2017, 2020) suggest that in the eastern ECMs (Aar, Mont Blanc, and Aiguilles Rouges massifs) the mode of uplift and consequentially exhumation switches between “vertical” and “horizontal tectonics”. Note that the terms “vertical” and “horizontal tectonics” as used by Herwegh et al. (2017, 2020) are based on geometric and kinematic considerations; i.e. they imply a steeper or less steep orientation of the main faults along which strain is accommodated. Thus, in a compressional framework, these terms describe whether the vertical or the horizontal displacement components play the dominant role at a given point in time. In other words, horizontal shortening also continues during phases of vertical tectonics. The evolution of the Helvet (...truncated)