Neotectonic activity in the Upper Rhein Graben : Jura Mountains junction (north-western Switzerland and adjacent France)

This contribution provides evidence for latest Pliocene to recent basement-rooted shortening in the southernmost Upper Rhine Graben and adjacent Jura. NE to ENE trending folds affecting the base of the Pliocene Sundgau gravels are interpreted to have formed by NW-SE shortening. This is in agreement with the NW-SE-orientation of maximum horizontal stresses inferred from seismotectonic studies. The spatial coincidence of the most prominent of these folds with the position of major basement-rooted faults suggests that they formed by thick-skinned reactivation of WSW-ENE and NNE-SSW-striking faults in dextral and sinistral mode, respectively. Dextral transpressive reactivation of the WSW-ENE-striking fault system (Rhine- Bresse transfer zone) dominates. Deflections of recent river courses around the crests of topographically clearly discernible anticlines suggest that this deformation is still ongoing at present. A change from thin-skinned tectonics, which prevailed during the main phase of Jura folding, to very probably still ongoing thick-skinned tectonics is inferred to have occurred in the Late Pliocene. This change might be linked to the incipient inversion of Permo-Carboniferous troughs within the Alpine foreland in general. In particular, such inversion in a dextral transpressive mode along a basement fault, which is part of the Rhine- Bresse transfer zone, might be held responsible to have triggered the 1356 Basel earthquake.