Pleistocene northwards fold propagation of the Jura within the southern Upper Rhine Graben: seismotectonic implications

On the basis of the geophysical (seismic profiles and electric tomography), geomorphic and geological data, we re-evaluate the post-Pliocene structural interpretation of the southern Upper Rhine graben (Basel–Mulhouse area): we demonstrate a Plio–Pleistocene northward propagation of the Jura thrust and fold belt up to Mulhouse proceeding from a succession of four 10 km apart ramps (from north to south Ferrette, Muespach, Magstatt and Rixheim) rooted within the late Triassic evaporitic marls acting as a decollement. This domain was previously considered as having undergone an on-going continuous extension (horst of Mulhouse bounded by the Quaternary Sirentz and Dannemarie grabens). The Quaternary activity of this thin-skinned tectonics induces the growth of a sedimentary wedge whose regional slope, which comprised between 1.4° and 1° to the north, also attests to a low friction basal detachment. More into details, these ramps correspond to 40–50-m high jumps within the forward topographic slope. Pleistocene activity is suggested just above the Muespach ramp by the presence of a 5–10-m north-facing scarp corresponding in depth to a 3-m vertical offset of early Pleistocene alluvial deposits. Farther to the north, a stronger incision of the Rhine Würm terrace can be interpreted as the result of the growth of the Mulhouse–Rixheim frontal ramp. This northward propagation of the Jura thrust and fold belt is strongly controlled by the Oligocene structural inheritage. The development of the frontal ramp in Mulhouse has to be related to the Oligocene significant vertical offset of the Triassic evaporite along the Mulhouse Railway Station fault preventing a propagation of the decollement farther to the north. In the same way, the fold propagation is laterally segmented by the N20°E trending Oligocene fabrics (from East to West, Rhine Valley flexure fault, Allschwil–Istein fault system and Illfurth fault) which acts above the decollement as lateral ramps. To the west, the development of a shallow anticline along the Illfurth fault suggests that the thin-skinned propagation is oblique with respect to the Oligocene fabrics. It results in spacial contrast between a left-lateral-reverse and a right-lateral–normal shallow kinematics along the western and eastern lateral ramps, respectively. In depth to the east, it also induces a vertical contrast between shallow (right-lateral–normal) and deep (left-lateral given by fault plane solutions) kinematics along the Istein–Allschwill–Rhine Valley fault system. Few arguments supporting a nucleation of the Basel-1356 earthquake, the strongest event in NW Europe in the last thousand years, onto the Rhine Valley fault system beneath the decollement have been given. However, we emphasize that the above mentioned coeval thin (aseismic)- and thick (seismic)-skinned tectonics along the Istein–Allschwill–Rhine Valley fault system would make difficult both the identification and the interpretation of the surface rupture of the Basel-1356 earthquake.