3D stratigraphic and structural synthesis of the Dannemarie basin (Upper Rhine Graben)

Research areas:
The Dannemarie basin is the southwestern most 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 ``Fosse
rhenan{''}, 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 I D 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 oil the Vosges fault Zone is about 1400 in 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 in).
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 oil 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.