3-D Fault-Related Folds in a Rift Setting, Dannemarie Basin, Upper Rhine Graben, France

Ford, M., C. Le Carlier de Veslud, O. Bourgeois, M. Diraison, CRPG-CNRS, Vandoeuvre-les-Nancy, France

In the framework of GeoFrance 3D project Fossé rhénan, a multidisciplinary 3D model of the Dannemarie basin (southwesternmost depocenter of the Upper Rhine Graben) was built in the gOcad 3D modeler, incorporating about 40 seismic cross-sections. The 3D model shows that on both western and eastern borders, the basin subsidence was controlled by syn-sedimentary folds affecting both Mesozoic and Cenozoïc formations, and associated normal faults. The seismic data indicate that these folds were active during Early and Middle Rupelian, a period during which the Rhine Graben was under extension. Thus, these data suggests that these folds are normal fault-propagation folds, similar to those observed in other extensional tectonic contexts, and more particularly on the western border of the Rhine Graben, close to Colmar. It is proposed that during rifting, Jurassic and probably Triassic layers were folded above upward propagating normal faults. Fold locations corre­spond to gravimetric discontinuities attributed to Variscan structures, suggesting a reacti­vation of these structures during rift. From Priabonian to Middle Rupelian, Tertiary deposits were syn-tectonically deformed, in parallel with an upward advance of the faults. The dis­placement on the faults decreases southward so that on both limits on the basin, displace­ment is accommodated principally by folds. On the eastern border, the Illfurth fault zone dis­plays geometries that resemble flower structures. The data suggests that the Illfurth fault zone acted as a normal fault with associated folds during the Rupelian, with a possible sinis­tral strike-slip component due to a post-Miocene NW-SE compressive regional stress field.