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

Shipton, Zoe K. and Meghraoui, Mustapha and Monro, Louise (2016) Seismic slip on the west flank of the Upper Rhine Graben (France-Germany) : evidence from tectonic morphology and cataclastic deformation bands. Geological Society Special Publications, 432 (1). pp. 147-161. ISSN 0305-8719

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Abstract

Intraplate large and moderate earthquakes have occurred along the Upper Rhine Graben (URG) in the past but no coseismic surface faulting has been reported so far. We investigate the 25-km-long linear Riedseltz-Landau normal fault scarp affecting late Pleistocene and Holocene deposits of the western edge of the northern URG. The fault zone with cataclastic deformation textures is exposed in the Riedseltz quarry where it affects Pliocene and late Pleistocene (Wurm) units. 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 Fe-oxide matrix into deformation bands, and later emplacement of a clay-rick matrix into fractures. Previous studies along the fault show late Pleistocene (Wurm) loess deposits and early Holocene sand-silty deposits with 1.5 m and 0.7 m surface slip, respectively. New and previous results provide a minimum 0.15 mm/yr slip rate. A dislocation model suggests a minimum Mw 6.6 earthquake as a plausible scenario in the western edge of northern URG. Surface faulting in young sediments associated with cataclasis provides new evidence for assessing the occurrence of large earthquakes and seismic hazard assessment in the northern URG.