Seismic slip on the west flank of the Upper Rhine Graben (France–Germany): evidence from tectonic morphology and cataclastic deformation bands

Abstract

Abstract The Upper Rhine Graben (URG) is a seismically active tectonic structure in intraplate Europe. Large and moderate earthquakes have occurred along the URG in the past but no coseismic surface faulting has been reported so far. We investigated active faulting along the western edge of the northern URG and identified the 25 km-long linear Riedseltz–Landau normal fault scarp as a major tectonic structure affecting late Pleistocene and Holocene deposits. The fault zone is exposed in the Riedseltz quarry where it affects Pliocene sand and gravels and overlying late Pleistocene (Wurm) units. These units have not been buried deeper than a few tens of metres and yet the fault zone contains cataclastic deformation textures. Cataclasis is demonstrated by spalling and transgranular fractures in quartz grains concentrated in deformation bands with reduced grain size. The observed microstructures suggest multiple phases of deformation with cataclasis followed by emplacement of a prominent Fe-oxide matrix into deformation bands, and later emplacement of a clay-rick matrix into fractures. Previous geological and geophysical studies along the fault show late Pleistocene (Wurm) loess deposits ( c. 24–10 ka before present) and early Holocene sand–silty deposits with individual or cumulative 1.5 and 0.7 m surface slip, respectively. Field observations and previous results from shallow geophysics provide a minimum 0.15 mm a −1 time-averaged slip rate. The Riedseltz fault parameters integrated in a dislocation model suggest a minimum Mw 6.6 earthquake as a plausible scenario in the northern URG. The observations of cataclasis in shallowly buried sediments coupled with observations of the late Quaternary fault scarp call for palaeoseismic studies that may document the occurrence of a larger earthquake on the western edge of URG. Surface faulting of young, shallowly buried sediments associated with cataclasis provides new evidence for assessing the occurrence of large earthquakes and seismic hazard assessment in the northern URG.