Morphotectonics and tectonic evolution of the Northern Markgräflerland, southern Upper Rhine Graben, Germany

Abstract

The Upper Rhine Graben (URG) represents a synorogenic intracontinental rift system in the Alpine foreland. The present-day complex fault pattern in the URG area is the result of the superposition of faults formed during various phases of its crustal stress history. The most-recent stress field is characterised by an approximately NW–SE orientation of the principle stress axis σ1, which is responsible for sinistral transtentional movements of the eastern Upper Rhine Graben boundary fault (EURGF). In the southern segment of the URG (SW Germany), seismic activity indicates that neotectonic processes are ongoing, shaping the present-day relief. However, this area has hardly been investigated. Here we examine the tectonic evolution of this region since the onset of the main URG rifting in the Early Oligocene, with emphasis on neotec-tonic processes. We studied the morphotectonic inventory of the Northern Markgräflerland, south of the city of Freiburg, utilising 1 m and 10 m resolution digital elevation models (DTMs based on LiDAR data), fieldwork, and geophysical measurements (GPR, ERT). This area comprises parts of the Variscan basement of the Black Forest, the EURGF as well as the terraced foothill zone (Vorbergzone). We document the following characteristics indicative of neotectonic activity: (i) triangular facets, (ii) terrain steps in alluvial Quaternary river deposits, (iii) shifts and bends of tributary creek beds, and (iv) asymmetric hillslope drainage indicative of divide migration. The uplifted Variscan basement adjacent to the EURGF is segmented into kilometre-sized blocks that are bordered by NW–SE, N–S and NE–SW striking faults. A northeasterly tilt of two of these blocks can be inferred from the asymmetric hillshape of the fault blocks, which are characterised by southward-shifted crestlines and larger local catchments on the northern with respect to the southern hillslopes. Triangular facets can be observed along several valley slopes in the Variscan basement. They are aligned along both NW–SE and NE–SW trending valley sides. They are most notable at the western entrance of the Lower Münstertal near Staufen. Here, the distinct triangular facets in the crystalline basement indicate an active transtensional normal faulting system. In Quaternary sediments, we found evidence of neotectonic motion along the sinistral transtensional EURGF, such as Riedel-shear fault patterns, bent and shifted creek beds as well as conspicuous terrain steps in alluvial deposits of the Staufen Bay that run parallel to the EURGF. Faults have been (re-)activated with different kinematics at different times due to repeated changes of the regional stress field since the onset of the main rift phase of the URG in Early Oligocene.