Does the structural inventory control tunnel-valley formation? – Insights from the North German Basin

Tunnel valleys are impressive erosional landforms and may attain extreme depths of almost 600 m. Open and buried tunnel valleys have been mapped in many formerly glaciated sedimentary basins. Characteristics of tunnel valleys include undulating basal profiles, abrupt terminations and steep flanks, all indicative of subglacial incision by pressurised meltwater discharge. Tunnel-valley formation is primarily controlled by climatic and glaciological factors. However, the structural inventory, such as faults and salt structures, have been invoked as controlling the location and orientation of tunnel valleys. To identify correlations that may indicate such a structural control, we compare the distribution and orientations of buried Pleistocene tunnel valleys in the North German Basin to the regional structural inventory.  

Our analysis shows that deep tunnel valleys are restricted to areas with thick erodible Cenozoic deposits. The correlation between the trends of tunnel valleys, faults and salt structures varies between the analysed structural regions. The orientations of tunnel valleys commonly follow the trends of faults and salt structures in regions where the structural trend is NNW-SSE to E-W and ice-flow directions were approximately parallel to this trend. However, correlations are rarely observed if the regional structural trend is NW-SE to WNW-ESE and ice advances occurred thus normal or oblique to the regional fault trend. Faults active under the present-day stress field typically are NNW-SSE to NE-SW trending normal faults. Therefore, the strikes of neotectonically active faults were commonly favourable for tunnel-valley incision and may have promoted subglacial erosion. No clear correlation between the orientations of tunnel valleys and elongated salt structures can be identified.

A major motivation for this study was the potential impact of future glaciations and tunnel-valley incision on the long-term safety of radioactive waste repositories. Our results demonstrate that the presence and orientations of faults and salt structures, however, do not provide consistent indicators for future tunnel-valley incision.