Latest Pliocene to recent thick-skinned tectonics at the Upper Rhine Graben – Jura Mountains junction

The southernmost Upper Rhine Graben and adjacent Jura experienced basement-rooted shortening that occurred after the deposition of the Pliocene fluvial “Sundgau gravels”. Folds affecting the base of these gravels systematically trend NE to ENE. Combined evidence from reflection seismic lines and contour maps of the base-Tertiary and base-Pliocene levels indicates that these folds probably formed by thick-skinned reactivation of both NNE-SSW and WSW-ENE-striking faults. This thick-skinned shortening is NW-SE oriented, i.e. parallel to the maximum horizontal stresses inferred from seismotectonics. NNE-SSW-striking faults (paralleling the Upper Rhine Graben) have been reactivated in sinistral strike-slip mode. However, dextrally transpressive reactivation of the WSW-ENE-trending faults that belong to the Rhine-Bresse Transfer Zone is interpreted to predominate. Deflections of recent river courses around the crests of en-échelon-aligned surface anticlines suggest that the deformation is ongoing at present. Retro-deformation of the folds affecting the base of the Sundgau gravels indicates horizontal displacement rates of about 0.05 mm/a. This corresponds to a minimum strain rate in the order of 2·10 −16 s −1 , given the maximum time span of 2.9 Ma for this deformation, i.e. the biostratigraphically determined minimum age of the gravels. A change from thin-skinned tectonics, that 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. We speculate that this change might be linked to the incipient inversion of Permo-Carboniferous troughs within the Alpine foreland in general. This inversion in dextrally transpressive or purely compressive mode along a WNW-ESE-trending basement fault, that is part of the Rhine-Bresse Transfer Zone, which in turn was prestructured during the formation of the Permo-Carboniferous troughs, could have triggered the 1356 Basel earthquake.