ALTERNATIVE INTERPRETATION OF THE ASYMMETRIC SHAPE OF THE TERTIARY HILLS, MOLASSE BASIN, GERMANY

The geomorphology of foreland basins is a recorder of climatic and geodynamic processes. On the regional scale, basin geometry places constraints on tectonic processes, whereas on the local scale, interpretation of the geomorphological record may be dominated by climatically-induced surface processes. One outstanding example of such a complex landscape that is influenced by both, tectonic and climatic processes, is the NE portion of the Molasse foreland basin: the Tertiary Hills region of Bavaria. In this region, foreland-basin subsidence occurred throughout Tertiary time, but has stopped a few million years ago, due to late Tertiary and Quaternary (?) uplift as a result of renewed plate convergence. The goal of our study is to examine the origin of this uplift and whether the region is actively deforming today. We compiled all existing topographic, geological, structural, seismic and geomorphic information and produced a new preliminary neotectonic map of the Tertiary Hills region. The official tectonic map of Bavaria yields several basement faults with a dominant NW-SE orientation. We also identified a large number of asymmetric hill slopes that strike parallel to underlying basement faults. Previous workers had attributed the pronounced asymmetry to aeolian loess deposits. Our preliminary field mapping, geophysical prospecting and drill core analyses is generally consistent with this interpretation but we propose that these asymmetric valleys are influenced by the structural framework of the basement faults.

Our new observations are: (a) Several linear, NW-SE striking, 2 to 9 m high morphologic steps with a length of 0.5 to >10 km are observed 10 km south of the Danube river. (b) Distinct bends of small rivers and drainages occur in the central area of the Tertiary hills and along the southern margin of the Danube river flood plain. (c) Quarry logs reveal disturbed sediment layers containing offset sand lenses and soft sediment deformation features. These geomorphic features are possibly results of local tectonic activity after the late Miocene. These preliminary results show promising correlations with expected surface morphology effects that may have been caused by tectonic activity – providing an alternative interpretation of the asymmetric hillslopes.