Cenozoic block rotation according to a conjugate shear system in central Europe — indications from palaeomagnetic measurements

Abstract

Results of palaeomagnetic measurements in the middle Tertiary volcanic provinces of Siebengebirge and Westerwald (northeastern Rhenish Massif, Germany) indicate that block rotation of the northeastern Rhenish Massif of the order of 10° to 16° has occurred since the Late Oligocene. Contemporaneous subsidence in the Upper Rhine graben, Neuwied basin, and Lower Rhine basin and intensive faulting in the Hessian depression document a new period of intraplate tectonism in central Europe. In the Tertiary, presumably from the Eocene to the Miocene, the axis of maximum horizontal stress rotated from the NNE–SSW to the NNW–SSE and the NW–SE direction, respectively. The new stress conditions may be responsible for the development of a conjugate shear system in central Europe north of the Alps. A model of a conjugate shear system is presented: a sinistral divergent strike-slip fault extends SSW–NNE from the Upper Rhine graben to the north. The fault splits up into two branches in the area of the Hessian depression. A conjugate dextral wrench fault system develops from the Bavarian Pfahl zone (southeastern Germany) to the Rhenish Massif. About 200 km to the northeast, a parallel dextral strike-slip fault is known from the southern Lower Saxony basin to the north of the Osning zone and Harz mountains. The northeastern block of the Rhenish Massif was created between dextral strike-slip faults. The block began clockwise rotation presumably in the Late Oligocene due to small changes in direction of the maximum horizontal compressive stress from NNW to NW and contemporaneous extension of the western European crust to the southwest. It is assumed that the block rotation is responsible for the V-shaped opening of the Lower Rhenish basin, and anticlockwise micro-block rotation within the Hessian depression. Opening between the rotating and nonrotating crust along the recent Rhine river in the central part of the Rhenish Massif formed triangular transrotational basins (basin of Neuwied) due to movements of crustal blocks in the western Rhenish Massif bounded by WNW–ESE dextral faults. Fault-plane solutions from recent earthquakes in the Rhine system support the model of block rotation and related crustal movements. Due to rotation of the northeastern Rhenish Massif the continuation of the Upper Rhine graben development to the north is prevented.