Morphotectonics of the eastern Rhine Graben Boundary Fault (Germany): an active fault within the plate interiors of Central Europe

The eastern Rhine Graben Boundary fault zone (RGBF) constitutes the oriental margin of the Upper Rhine Graben (URG) which forms part of the European Cenozoic Rift System. The URG with low to moderate intraplate seismicity is one of the most seismically active areas in the plate interiors of Central Europe. Assessing seismic hazard in intraplate Europe is challenging as modest lithospheric deformation (<1 mm/yr) resulting from far-field stresses is accommodated by slow-slip faults. The instrumental and historical earthquake catalog of the URG (dating back to 800 AD) is too short to include the complete earthquake history and, for instance, document the occurrence of large earthquakes, potentially leading to underestimate capable faults.

Identifying and characterizing active faults is essential towards a comprehensive seismic hazard assessment in the URG. Several research efforts have been made towards this direction, focusing on the western RGBF and the southern end of the eastern RGBF. However, neotectonic studies integrating the whole eastern RGBF are lacking. As a first step, we here present a study of the neotectonic imprint in the morphology of the eastern margin of the URG based on the 12 m resolution TanDEM-X DEM and the 1 m resolution DEM of Baden-Württemberg derived from LIDAR data together with data from regional geological maps. We performed geomorphological mapping of Quaternary deposits, paleoseismic features, and faults. Besides, we calculated several morphometric parameters, including mountain front sinuosity, basin asymmetry, knickpoints, and hypsometric curve analysis to depict long-term deformation. The eastern RGBF consists of several NNE-SSW parallel fault strands marked by topographic steps that constitute the boundary between the Rhine River plain and the eastern uplifted URG shoulder. We have identified along the fault landforms that appear typical of active tectonic landscapes: a) topographical scarps, b) well-defined triangular facets developed on the hillslope associated with the main fault trace, c) displaced alluvial fans, d) left-lateral channel deflections and beheaded channels, and e) hanging valleys; that allows us to prove the kinematics of the fault as transtensional left-lateral strike-slip which is consistent with the regional stress (S_{H max}). The occurrence of these neotectonic features varies along the 300 km long eastern RGBF fault, which, together with the results from the morphometric analysis, allow us to differentiate areas with differential tectonic activity suggesting fault segmentation. These results point out the seismic potential of the eastern RGBF, are critical to find suitable sites for paleoseismological trenching and are key to later propose plausible rupture scenarios for further PSHA studies.