The Late Variscan control on the location and asymmetry of the Upper Rhine Graben

Abstract

The NNE-trending Upper Rhine Graben (URG) of the European Cenozoic Rift System developed from c. 47 Ma onwards in response to changing lithospheric stresses in the northwestern foreland of the Alps. The composite graben structure consists of three segments, each c. 100 km long and 30–40 km wide, but flares to c. 60 km near its southern and to c. 80 km near its northern termination. Normal faulting induced a total extension of 5–8 km of the 1–2 km thick Mesozoic sedimentary Franconian platform and underlying Variscan basement rocks. However, distribution of an up to 3.5 km thick sedimentary graben fill and cumulative displacements near Eastern and Western Main Border fault systems suggest that subsidence of the graben floor and shoulder uplift created strong cross-sectional asymmetries. Cumulative W-down displacements >3 km along strongly segmented transfer faults in the east contrast with E-down displacements 600 km long zone that delimits the trailing edge of a SW-moving lithospheric block. In the URG area, NE–SW-oriented seismic anisotropy at sublithospheric depths of c. 60–80 km suggest active mantle flow in this direction as a possible driving force for the reactivation of pre-graben lithospheric shear zones.