Tilting, uplift, volcanism and disintegration of the South German block

Abstract

We present a large set of fault-slip data together with six UPb ages on calcite vein fillings constraining the kinematics and timing of near-surface (uppermost 2–3 km of crust) faulting of the uplifted South German block in the northern foreland of the NW-ward advancing Alps. The structural data confirm earlier studies showing Miocene subhorizontal NE-directed extension with shortening mainly being subvertical but also subhorizontally NW oriented. The UPb ages are between 15.3 ± 0.2 and 2.3 ± 1.1 Ma (2σ analytical uncertainty) and date kinematically coordinated normal and strike-slip faulting in the northern Alpine foreland. The faults in the South German block have limited strike lengths indicating that they do not cut deeply into the crust. However, the faults appear to control volcanic centers, the melts of which originated at the base of the lithosphere. We suggest that the Miocene faults could tap the mantle because they reactivated formerly deep-seated faults. The ages for near-surface faulting are coeval with eruption of the Urach and Hegau volcanics. Published data suggest that Alpine shortening induced large-wavelength lithospheric folding in the foreland since 18–16 Ma. Volcanism in the South German block focussed along a lithospheric antiform which is, in part, superimposed on the foreland bulge of the Alps. We propose a close link between progressive southward tilting, uplift and faulting of the South German block due to lithospheric-scale folding and upward flow of the asthenosphere, flexure of the lithosphere, deep-seated volcanism and the NW-ward advancing Alps.