Eocene‐Oligocene tectonics and kinematics of the Rhine‐Saone Continental Transform Zone (eastern France)

Abstract

A tectonic analysis of brittle deformation was performed in the Rhine‐Saône transform in order to decipher and understand the deformation processes in a typical continental transfer zone. Cenozoic major and minor fault patterns are described, and brittle structures are interpreted in terms of paleostress orientations. The Eocene N‐S compression related to Africa‐Eurasia convergence is marked mainly by strike‐slip faults. It was followed by a major phase of crustal extension, mainly Oligocene in age, related to the west European rifting. This extensional phase is expressed by normal faults, tension gashes, and extensional strike‐slip faults which are especially abundant in the transform zone. These two events are found to be quite different in terms of distribution of horizontal stress trajectories. The trend of σ1 axes is very homogeneous for the N‐S compression, whereas σ3 orientations associated with the major extension vary from E‐W in most of the Rift system to NW‐SE within the transform zone. Finally, the Mio‐Pliocene tectonic emplacement of the Jura fold‐and‐thrust belt is marked in the field by strike‐slip faults associated with a well‐defined trend of σ1 oriented W‐NW‐E‐SE to NW‐SE. The preexisting basement fracture pattern between the grabens was also considered in order to estimate which faults were likely to have been reactivated during the Oligocene transform kinematics, and the theoretical shear motion on these inherited fault surfaces, when submitted to E‐W extension, was calculated. Left‐lateral/extensional reactivation of a preexisting 60°N trending fault system in the brittle upper crust is likely to have accommodated the shear strain arising between the rift segments in response to crustal stretching. Two independent numerical modelings, based on distinct element and finite element analyses, respectively, enabled us to verify and to refine the hypothesis of transform kinematics related to E‐W extension and inducing regional perturbations of extensional stress trajectories. The overall mechanism and the transtensional kinematics of the Rhine‐Saône transform zone are thus tightly constrained. The inherited crustal anisotropy, the large‐scale perturbations of extensional stress trajectories, and the development of distributed brittle deformation within the sedimentary cover are found to play a major role in the tectonic evolution of continental rift‐transform systems.