The last erosional stage of the Molasse Basin and the Alps

Abstract

We present a synoptic overview of the Miocene-present development of the northern Alpine foreland basin (Molasse Basin), with special attention to the pattern of surface erosion and sediment discharge in the Alps. Erosion of the Molasse Basin started at the same time that the rivers originating in the Central Alps were deflected toward the Bresse Graben, which formed part of the European Cenozoic rift system. This change in the drainage direction decreased the distance to the marine base level by approximately 1,000 km, which in turn decreased the average topographic elevation in the Molasse Basin by at least 200 m. Isostatic adjustment to erosional unloading required ca. 1,000 m of erosion to account for this inferred topographic lowering. A further inference is that the resulting increase in the sediment discharge at the MiocenePliocene boundary reflects the recycling of Molasse units. We consider that erosion of the Molasse Basin occurred in response to a shift in the drainage direction rather than because of a change in paleoclimate. Climate left an imprint on the Alpine landscape, but presumably not before the beginning of glaciation at the Pliocene-Pleistocene boundary. Similar to the northern Alpine foreland, we do not see a strong climatic fingerprint on the pattern or rates of exhumation of the External Massifs. In particular, the initiation and acceleration of imbrication and antiformal stacking of the foreland crust can be considered solely as a response to the convergence of Adria and Europe, irrespective of erosion rates. However, the recycling of the Molasse deposits since 5 Ma and the associated reduction of the loads in the foreland could have activated basement thrusts beneath the Molasse Basin in order to restore a critical wedge. In conclusion, we see the need for a more careful consideration of both tectonic and climatic forcing on the development of the Alps and the adjacent Molasse Basin.

Fluid flow through the sedimentary cover in northern Switzerland recorded by calcite–celestite veins (Oftringen borehole, Olten)

“…13 indicate that the veins formed at elevated temperatures (average values of 56-68°C) and therefore at substantial depth, possibly around the time of maximum burial in the region (10-5 Ma, see Mazurek et al 2006;Schlunegger and Mosar 2010;Willett and Schlunegger 2010). This time interval also coincides with the main stage of deformation in the Folded Jura (Becker 2000), and this deformation event may have triggered fracturing and permeability that finally led to the downward migration of Molasse water into the underlying Jurassic units.…”

High magnitude and rapid incision from river capture: Rhine River, Switzerland

“…The results suggest that drainage reorganization can account for a significant amount of Pliocene-modern incision within the Aare-Rhine River basin in the Alpine Foreland (Table 2) [Mazurek et al, 2006;Cederbom et al, 2011;Schlunegger and Mosar, 2011]. The model predictions are consistent with a number of observations in the region including (1) rapid pulses of incision and the abandonment of regional terrace levels [Graf, 1993;Graf, 2009;Kock et al, 2009;Preusser et al, 2011], (2) the magnitude of erosion through the Pliocene-Pleistocene in the Rhine and Danube basins [Mazurek et al, 2006;Willett and Schlunegger, 2010], and (3) the pattern of erosion following terrace abandonment [Preusser et al, 2011] (e.g., rather uniform upstream of the capture and decaying downstream of capture).…”

“…Various mechanisms have been proposed to drive drainage reorganization in the Alps [Ziegler and Fraefel, 2009;Schlunegger and Mosar, 2011] and elsewhere [Bishop, 1995;Brocard et al, 2012]. Such physical mechanisms include tectonic activity, ice sheets, landslides, and groundwater processes.…”

“…This pulse of erosion is also propagated upstream through both the captured and original river basins. Few studies have quantified the rates and magnitudes of fluvial erosion associated with river capture events [e.g., Gunnell and Harbor, 2010;Schlunegger and Mosar, 2011;Prince et al, 2011;Andrews et al, 2012;Brocard et al, 2012], and we are not aware of studies that have explored the implications of drainage capture for landscape evolution with a numerical model.…”

“…The magnitude of the incision generated by this drainage reorganization is unknown but likely greater than 250 m [Schlunegger and Mosar, 2011]. Recent interest in quantifying climate and tectonic controls on erosion in the Alps and its foreland [Kuhlemann et al, 2002;Champagnac et al, 2007;Norton et al, 2008;Vernon et al, 2009;Willett, 2010;Schlunegger and Mosar, 2011] motivates a quantitative understanding of the magnitude and evolution of erosion occurring from drainage reorganization events. Understanding the controls on past erosion in the region is relevant to evaluating the impacts of future erosion on potential radioactive waste disposal sites in northern Switzerland.…”