Retrodeformation of the southern Upper Rhine Graben: new insights on continental oblique rifting

Surface flattening retrodeformations of the southern Upper Rhine Graben (URG) are presented in this paper. The two models presented cover the southwestern and southeastern parts of the URG (Colmar, France, and Freiburg, Germany, areas respectively). Results from these models indicate that the URG resulted from a sinistrally oblique extension for a significant part of the rifting. Early extension was along a nearly E-W (i.e. orthogonal) direction and concentrated on the main border faults. Deformation then propagated toward the graben interior as stretching direction rotated counter clockwise of 20-40°. Systematic along-strike variation of cumulated heave, throw and slip is also shown. It is suggested that this along-strike variation of fault displacement could be a characteristic feature of oblique continental rifting in general. Offset analyses of selected geological horizons indicate that some fault segments (Vosgian fault, near Selestat, France) were possibly active prior to deposition of early Tertiary sediments. Finally, close spatial coincidence between major fault segments and recent earthquakes hypocenters indicates that part of the fault system, at least in the Freiburg area, is still active.