Ancient and recent rifting in the Rhinegraben

Abstract

The Rhinegraben is part of a western European rift system. Although of different ages and trend, these rifts are situated on pre-existing basement weakness zones. Signs of Rhinegraben subsidence occur in mid-Jurassic, but true rifting began in mid-Eocene. Boundary faults are complex systems of normal faults, dipping about 65°, perhaps flattening with depth. "Horizontal stylolite" orientations indicate an Eocene palaeostress field (related to Alpine plate collision) with its minimum component normal to the graben, i.e. ideal for rifting on the basement weakness zone. Rifting continued until Lower Miocene, but the depocentre shifted north with continued development of a mantle bulge in the south. Then, although crustal upwarping and volcanism continued, graben formation ceased - the stress field was no longer appropriately oriented. Graben activity restarted in Upper Pliocene and continues today. In-situ stress measurements indicate a sinistral shear component parallel to the graben, producing different tectonic reactions in the three slightly differently oriented graben segments. This shear motion is ascribed to continuing Alpine uplift and extension pushing northwestward the block east of the graben. The Rhinegraben and Lower Rhine Embayment are connected by a continuous seismotectonic belt where rifting is controlled by pre-existent basement fractures and regional stress,