Thesis #856

This thesis was carried out within the framework of the international EUCOR-URGENT project and represents a joint study (co-tutelle) between the Universities of Basel and Besançon. It is devoted to the structural evolution and present-day tectonic activity of the region of Franche Comté in Eastern France, a region that is located at the intersection between the Jura fold-and-thrust belt representing the front of the Alpine orogen and the immediately adjacent northwestern foreland, characterized by the intra-continental Rhine-Bresse Transfer Zone (RBTZ). The formation of the RBTZ, that forms a central segment of the Paleogene European Cenozoic Rift System, involved the extensional reactivation of the Late Paleozoic Burgundy Trough System. Substantial extension occurred in Eo-Oligocene times and was achieved by ENE-WSW striking normal faults that are highly oblique to the dominant strike of the Rhine and Bresse grabens. Fission track data suggest that basement reactivation also contributed to the Eo-Oligocene exhumation of the Late Paleozoic La Serre Horst in the context of rifting. Basement reactivation in connection with the formation of the RBTZ resulted in complex fault patterns and local stress field perturbations, especially in the surroundings of the pre-existing Paleozoic horst structure. During Late Miocene to Early Pliocene times the northwestern most segment of the thin-skinned Jura fold-and-thrust belt, the Besançon Zone, encroached onto the RBTZ. Moreover, NW-SE directed foreland compression caused thick-skinned, transpressive reactivation of the RBTZ that started by Late Pliocene times at the earliest. Present-day seismicity in the RBTZ indicates that thick-skinned tectonics are still active and may reflect ongoing tectonic underplating in the northwestern Alpine foreland. Post-Pliocene relative rock uplift is recorded along the RBTZ by differential erosion of the Middle Pliocene Sundgau-Forêt de Chaux Gravels on a regional scale. Uplifted remnants of this gravel plain, identified by heavy mineral analyses, permitted to determine a latest Pliocene to recent minimum regional rock uplift rate of 0.05 +/- 0.02 mm/yr. A reconstruction of the Plio-Pleistocene drainage basin evolution of the Ognon and Doubs Rivers revealed that this relative rock uplift is still ongoing and most likely partly driven by the inversion of the RBTZ. Pleistocene folding near Besançon is evident from differentially up warped paleo-meanders along the Citadelle Anticline. This deformation is apparently enhanced by the bedrock incision of the Doubs River and associated with higher local uplift rates of 0.17 +/- 0.03 mm/yr. The results of this study vividly illustrate the dynamic processes that control the progressive evolution of continental collisional forelands. It appears that while the evolution of the RBTZ was largely controlled by the reactivation of pre-existing structures, its present tectonic activity is to some extent also controlled by surface processes and involves positive feedbacks between large-scale uplift, enhanced erosion and active deformation.