Thesis #897

Present-day tectonic activity at the southern end of the Upper Rhine Graben in central Western Europe is evidenced by significant seismicity, which has been documented over hundreds of years. The hazard that is posed by this activity was violently demonstrated in 1356, when an earthquake with an estimated magnitude of ML≈6.5 caused extensive damage to the area of Basel in north-western Switzerland. A sound understanding of the regional tectonic deformation field is a prerequisite for the accurate assessment of this hazard. However, long-term deformation rates in this region are very low. Together with the presence of a network of fault families of different age and orientation, which results from the complex tectonic evolution of this area in the Neogene, this makes the characterisation of the regional deformation field and the identification of active faults difficult. Nevertheless, for a better comprehension of the active tectonic processes in general, and for the assessment of the seismic hazard in this region in particular, an improved understanding of the regional tectonic evolution in the recent geological past is indispensable.
This thesis addresses the recent tectonic history of the Basel area by combining seismological data with an investigation of the geomorphological evidence of tectonic activity. The fact that tectonic activity can be recorded and preserved by the landscape provides an additional source of information that has been little used so far. It offers an opportunity to extend the time-scale of observation from the decades covered by (instrumental) seismologic and geodetic records further into the past. Whereas a wide range of geomorphic features can carry signatures of past tectonic events, the focus in this work is laid on fluvial geomorphology.
The fluvial system in the northern Alpine foreland has been affected by a number of large-scale tectonic events since the late Oligocene. Apart from processes related to the Alpine orogeny and the rifting of the Upper Rhine Graben and Bresse Graben, the evolution of the Jura fold-and-thrust belt, the most external element of the Alpine orogen, dramatically influenced the drainage system. Sedimentary and morphological evidence of former river courses allow further constraining the evolution of the drainage system between the Oligocene and the Quaternary.
The tectonic history in the Quaternary was studied using a quantitative geomorphological approach. On the basis of a digital elevation model, geomorphic indices (steepness and concavity index) were determined to characterise the longitudinal profile for a large number of rivers in the area of the southern Upper Rhine Graben and the eastern Jura fold-and-thrust belt. The spatial
distribution of these indices indicates uplift of a region roughly corresponding to the Jura fold-and-thrust belt, as well as subsidence of the interior parts of the Upper Rhine Graben relative to the Tabular Jura. A morphological analysis of Late Quaternary alluvial terraces in the lower Aare valley, northern Switzerland, was carried out using a high-resolution digital elevation model. The results suggest regional northward tilt during the past 20’000 years, compatible with both a general (isostatic) uplift of the Swiss Molasse basin, and continuing convergence due to ongoing Alpine collision.
The geomorphic data revealed no unambiguous evidence of recent tectonic activity on individual faults. Furthermore, no evidence of recent or ongoing thin-skinned deformation in the Mesozoic sedimentary cover could be identified. The study area at the junction of the Upper Rhine Graben and the Jura fold-and-thrust belt is characterised by a pronouncedly diffuse distribution of deformation, typical for regions where strain is accommodated on inherited tectonic structures. The superposition of large-scale regional uplift and small-scale deformation on individual faults, as well as seismic and aseismic movements, results in a distinct heterogeneity of deformation styles in the northern Alpine foreland.