1. Introduction

1.1. Geological Setting

The Lower Rhine Graben is a branch of the Rhine graben system which belongs to the reactivated Cenozoic rift in western Europe. The strong subsidence of this region during the last 150,000 years (Geluk et al., 1994), the Quaternary faults and associated morphology along the flanks of the graben, and the 0.8 mm/yr to 2 mm/yr vertical rate of deformation obtained from the comparison of levellings during the last 100 years (Van den Berg et al., 1994; Mälzer et al., 1983) combined with the seismic activity are the most significant elements for recent and present-day crustal deformation.

Based on the main geological structures and Cenozoic subsidence data, the Lower Rhine graben can be divided into several tectonic units (Fig. 1):

1. the Krefeld Block which borders the subsiding area to the northeast,
2. the Venlo and Peel Blocks which have an intermediate subsidence,
3. the Roer Graben and the Erft Block which correspond to central valleys and sites of strong subsidence, and
4. the Campine and the South Limburg Blocks of intermediate subsidence and the Brabant Block, bordering the subsiding area to the southwest.

The Roer Graben is bordered by two main NNW-SSE trending Quaternary normal fault systems, the Peelrand Fault (Rurrand fault in Germany) to the east and the Feldbiss Fault to the west. The Roer Graben subsided from the Late Oligocene (± 28 My ago) onward, and up to 2000 m of sediments accumulated in it since. Subsidence rates markedly increased during the Quaternary, resulting in a fill reaching 200 m in thickness.

Fig. 1 - Seismotectonic map of the Lower Rhine Graben System.