Molasse Basin: constraints from forward tectonic modelling Flexure and ‘unflexure’ of the North Alpine German-Austrian Geological Society, London, Special Publications

We present the results of forward modelling of the Northern Alpine German-Austrian Molasse Basin, which forms part of the Northern Alpine Foreland Basin (NAFB) extending from Lake Geneva in the west to Lower Austria in the east. The observed deflection of the European plate under the NAFB has been modelled along five profiles perpendicular to the basin axis. Models treating the deflection of the NAFB as a flexural response to loading only, require a set of loading parameters (bending moment, vertical force) which would imply bending stresses exceeding the strength of the subsiding plate. Moreover, this approach would not take into account the significant post-Molasse uplift experienced by the Alpine chain and its northern foreland basins. We modelled the defection as the response to two distinct processes: (1) flexure of an elastic plate with lateral variations (EET-values of 7-26 km from east to west) loaded with surface and limited subsurface loads and (2) the Late Cenozoic (post-molasse) phase of 'unflexing', significant uplift observed in the Alps and its northern foreland. Our study provides a first attempt to separate these two processes in order to model the deflection by an elastic platemodel, adopting flexural parameters that will not exceed the strength of the lithosphere. The Alpine mountain chain is a classic continent-continent collision zone. During its evolution the African continental plate overthrusted the European margin and stacked slices of the European plate on to the European plate. This acted as a (topographic) load translating over the underthrusting plate. Due to this loading a flexural deflection developed in front of the thrust belt. As a result of the in general low flexural rigidity of the downgoing lithosphere this led to a deep and narrow basin, just as the Ebro (Zoetemeijer et al. 1990; Millan et aI. 1995), Apennine (Royden 1993; Kruse & Royden 1994), Carpathian (Matenco et al. 1997) and Swiss Molasse basins (Sinclair et al. 1991). The Northern Alpine Foreland Basin (NAFB, see Fig. 1) is considered to be a classical example of this type of basins, featuring a strongly asymmetrical geometry, deepening towards the Alpine mountain front and present along all of the northern part of the Alps and the Carpathians. Although its overall shape appears to be primarily the result of the emplacement of the Alpine nappes, remarkable lateral variations in geometry and kinematics of the NAFB can be observed that distort the simple theoretic asymmetrical shape. The Basin is developed or preserved best in its German part where it is 150 km wide and contains a sedimentary infill of upto almost 5000m (Lemcke 1988; Bachmann & Muller 1992). To the east, in direction of Austria, the Basin narrows to only 15 km and becomes shallower where the crystalline Bohemian Massif is close to the frontal thrusts of the Alps and broadens again to the Carpathian foreland. To the west, in Switzerland, the Basin narrows as well, probably related to the considerable Cenozoic uplift of up to 2500 m (Lemcke 1988) that has destroyed part of its original extent. In this respect it might not be coincidence that this highly uplifted part is situated at the southern termination of the Rhine Graben (Gutscher 1995) and related to local uplift leading to the Jura Mountains.These lateral variations from Switzerland to Austria are probably the product of a combination of processes such as variations in strength of the downgoing European lithosphere, pre-subsidence geological setting, lateral inhomogeneties of the basement (basement faults) and recent significant laterally varying uplift. Modelling the flexure of the European plate underlying the NAFB has been the subject of several studies over the last decades. Most of the authors conclude that the geometry of the NAFB, the asymmetric flexural depression that developed in front of migrating thrust loads of the Alps, can be modelled by an elastic plate overlying an inviscous fluid loaded by the orogenic thrust wedge. The resultant values for the effective elastic thickness of the downflexing European plate show, however, a very wide range: from 35-50 km to only 7.5 km. A source of controversy between most of the studies is the concept of additional subsurface loads; applying a bending moment (Mo) and/or a vertical shear forces (Vo) at the free end of the plate to enhance deflection. In order to simulate the ANDEWEG, B. & CLOETINGH, S. 1998. Flexure and 'unflexure' of the North Alpine German-Austrian Molasse Basin: constraints from forward tectonic modelling. In: MASCLE, A., PUIGDEF,kBREGAS, C., LUTERBACHER, H. P. & FERNANDEZ, M. (eds) Cenozoic Foreland Basins of Western Europe. Geological Society Special Publications, 134, 403-422. 404 B. ANDEWEG & S. CLOETINGH