Contemporary kinematics of the Upper Rhine Graben: A 3D finite element approach

Abstract

The Upper Rhine Graben (URG), a Cenozoic intra-plate rift situated in the Alpine foreland, is presently characterised by relative slow tectonic deformation and low to medium seismicity. Concurrently, it is a region with a significant amount of ongoing subsidence in two recent depocentres (0.1 to 0.2 mm/a geological, 1 mm/a geodetical rate). In this paper, the recent kinematic behaviour of the URG is simulated using a 3D finite element model, containing three lithospheric layers (upper mantle, lower crust and upper crust) with different rheological properties. First order fault structures (e.g. border faults) are implemented as frictional contact surfaces within the upper crustal layer. The stresses generated by applying lateral displacements over a time period of 10 ka are insufficient to obtain a match between predicted and observed stress magnitudes. Therefore, a technique of “combined pre-stressing” has been developed to avoid unrealistic deformation and unrealistic stress magnitudes within the model. The stress magnitudes and stress directions predicted are calibrated against in-situ stress measurements and stress indicator data. For benchmarking of the modelling results, the vertical surface displacements predicted are compared to surface uplift derived from geological and geomorphological data. Furthermore, predicted fault slip rates are compared to available geological and geodetical data. Parameters derived from the calculated stress tensor, such as fracture potential and the regime stress ratio are also analysed in order to describe the possible kinematic behaviour of the URG. The modelling results suggest that the URG is currently being reactivated as a sinistral strike–slip system with the central segment of the URG forming a restraining bend and the two recent depocentres situated in releasing bend settings. The modelling results suggest that both sinistral shearing and mantle uplift are active mechanisms driving the recent kinematics of the URG and that the recent subsidence within the two depocentres is re-enforced by ongoing mantle uplift additionally.