Crustal structure and dynamics in the Rhine Graben and the Alpine foreland

Abstract

SUMMARY The Rhine Graben rift stretches for 300 km from Basel to Frankfurt. The axis of maximum subsidence switches from west in the southern graben, to east in the northern graben. Subsidence in the south began in the upper Eocene and was interrupted in the mid-Miocene by a broad uplift of 1–1.5 km in the Vosges-Schwarzwald (Black Forest) region, whereas subsidence in the north proceeded continuously from the lower Oligocene to the present. A combined geophysical and geological interpretation of the Rhine Graben is presented in the form of a 3-D gravity model and 2-D flexural plate modelling from the Alps across the Alpine foreland to the Rhenish Massif. Modelling is based on crustal structure revealed by geologic, gravity and seismic data, including ECORS, DEKORP and EGT profiles. The 3-D gravity modelling supports a modest density contrast of 0.3 g cm−3 at the crust-mantle boundary. Since there is no evidence of a large-scale upper-mantle anomaly, a dense lower crust (2.9-3.0 g cm−3) is proposed as the cause of this modest density contrast. The presence of a dense, mafic, lower crust in south-west Germany is corroborated by lower-crustal xenoliths, elevated P-wave velocities (6.7-7.2 km s−1) and high Poisson's ratios (0.26-0.29). Residual gravity and magnetic data reveal strong crustal heterogeneities, particularly from the dense, magnetic Saxothuringian Terrane to the light, non-magnetic Moldanubian Terrane. The polarity of the asymmetric Rhine Graben reverses at this Variscan tectonic boundary. Light graben fill can account for nearly the entire 20–40 mGal, short-wavelength gravity minimum, but the presence of intrusive bodies ranging from granitic to mafic composition can also strongly influence the local gravity field. 2-D modelling of the flexure of the European lithosphere along seven 1000 km long profiles indicates a flexural bulge of 1–1.5 km in the Alpine foreland of Southern Germany, which corresponds to an ENE-trending Moho uplift of the same magnitude and to a band of free-air gravity highs. Longer flexural wavelengths in the east indicate increasing lithospheric rigidity and increasing elastic thickness towards the Bohemian Massif. The Vosges-Schwarzwald dome is interpreted as the intersection of the Oligocene Rhine Graben rift and associated rift flank uplift with the mid-Miocene flexural bulge.