ANATOMY OF A WRENCH RIFT REVISITED: TOWARDS A 3D STRUCTURAL MODEL OF THE UPPER RHINE GRABEN

The Upper Rhine Graben (URG) is a Cenozoic rift and wrench basin about 300 km long and 35 to 50 km wide, with an Eocene to Quaternary sedimentary fill up to 3.5 km thick. Its complex geological history presents multiple opportunities for the use of its geopotentials, which have increasingly attracted professional and public interest. A detailed knowledge of the deep geological structure of the URG is needed to use this potential successfully and safely. The EU project GeORG (Geopotentials of the deep Upper Rhine Graben) aims at the construction of a transnational database allowing statements about deep subsurface geopotentials, e.g. deep geothermal energy, thermal and mineral waters, possibilities of CO₂ sequestration and storage of compressed air. The basis is a 3D Gocad model parameterized with hydrogeological and geothermal properties based on the interpretation of about 5400 km of reprocessed seismic lines (3900 km in Germany and 1500 km in France), and a database of more than 2500 wells, mainly from the oil and mining industry. It is the first time that such an amount of subsurface data is gathered, studied and modeled in the URG.

In this presentation, we put our emphasis on the inventory of the various observed structural features (e.g. main border faults, normal and strike-slip faults, salt domes), and their implication regarding the structural evolution the URG. We then focus on the predominant role of the Miocene-to-present NNE-SSW strike-slip regime in the present-day 3D geometry of the URG. This wrench system is mainly characterized by the development of transtensional faults and negative flower structures, but also includes local transpression and inversion on older normal fault planes. The most striking result is a segmentation of the URG into several tectonic domains, separated from each other by first order transverse faults. At least some of these domain boundary faults, seen in Cenozoic rocks, seem to extend from segments of reactivated Paleozoic basement faults. These reactivated segments, however, have been cut and offset from their prolongation outside the basin by Neogene shear zones. Thus, Neogene strike-slip deformation tends to obliterate the initial rift structure as well as its basement structural heritage and gives only a distorted view of pre-Miocene structural styles.