Abstract

The Dannemarie basin is the southwesternmost depocentre of the Upper Rhine Graben, which belongs to the West-European Tertiary rift system. It is bounded to the west by the Vosges mountains, to the south by the folded Jura belt and to the east by the Mulhouse block. The rifting reached its maximum activity during the Priabonian and early Rupelian (35-31 Ma). In the framework of the GeoFrance 3D project “Fossé rhénan”, a 3D geometrical model of the Dannemarie basin was built in the gOcad 3D modeler. It incorporates the BRGM well database and geological maps, and 40 seismic cross-sections. These data are used to study the structure and geological history of the area. Seismic data have been converted from time to depth using a 1D time-to-depth polynomial law deduced from the analysis of the Bellemagny borehole. The Dannemarie basin is bounded to the west by the Vosges fault zone and to the east by the Illfurth fault zone. On both borders, basin subsidence was controlled by normal faults and associated syn-rift flexures. The minimum throw on the Vosges fault zone is about 1400 m to the north of the model, decreasing to the south, where it is replaced by a syn-rift flexure. On the Illfurth fault zone, subsidence is accommodated by faults (with about 1000 m throw) and by a flexure (about 300 m). Stratigraphic data indicate that these flexures were active during Priabonian and early Rupelian extension. These monoclinal flexures are interpreted as fault-propagation folds that developed above upward propagating normal faults in the basement. As displacement accumulates, the fault propagates upwards and cuts the overlying fold. Similar fault-fold geometries have been described on the western border of the Rhine graben, close to Colmar and in other extensional tectonic contexts. In the Colmar area, the Vosges fault zone cuts through the basin margin fold, while further south along the western border of the Dannemarie basin, displacement on the fault decreases and subsidence is accommodated on a major flexure. Flexure locations correspond to gravimetric discontinuities attributed to Variscan structures, suggesting reactivation of deep structures during rifting. The Illfurth fault zone displays upwardly divergent fault geometries that resemble “flower structures”. The data can be interpreted as follows, either that (a) the Illfurth fault zone accommodated a minor sinistral strike-slip component due to a post-Miocene NW-SE compressive regional stress field or (b) these faults developed in association with the fault propagation folds.

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