1 Geomorphic and geophysical reconnaissance of the Reppel and Bocholt faults, NE Belgium Koen VERBEECK 1 , Hilde BEATSE 2 , Kris VANNESTE 1 , François RENARDY 3 , Henk VAN DER MEER 4 , Kabir ROY-CHOWDHURY 4 & Thierry CAMELBEECK 1 1 Royal Observatory of Belgium, Ringlaan 3, B-1180 Brussel, Belgium E-mail: koen.verbeeck@oma.be 2 University of Gent - Renard Centre of Marine Geology, Krijgslaan 281 S8, B - 9000 Gent, Belgium 3 University of Liège - L.G.I.H., Sart-Tilmann, B - 4000 Liège, Belgium 4 University of Utrecht - Instituut voor aardwetenschappen, Budapestlaan 4, 3584 CD Utrecht, the Netherlands. ABSTRACT Results of an investigation of the activity of the Reppel and Bocholt Faults at the southwestern border of the Roer Valley Graben are presented. These show clearly that the small geomorphic scarps (± 2 m high) coincide with a shallow geological contrast and with a geophysical anomaly on GPR, electrical, electrical tomography and shallow seismic reflection profiles. We thus found evidence for recent surface faulting that most probably was caused by large earthquakes. KEYWORDS Active fault, fault scarp, Roer Valley Graben, paleoseismology. Introduction To the NW of the town of Bree, the Neeroeteren Fault which is part of the Feldbiss Fault Zone at the southwestern border of the Roer Valley Graben separates into three branches. From west to east these are the Grote Brogel, Reppel and Bocholt Faults (fig. 1). The latter two were studied in more detail. These faults have been recognised on intermediate resolution seismic reflection profiles performed by the Belgian Geologic Survey on land (Demyttenaere and Laga, 1988) and on those done by the Renard Centre of Marine Geology on the Campine Canals (De Batist & Versteeg, 1999). The long term slip rates from the upper Miocene onwards (table 1) were derived from these seismic lines. Neeroeteren Gr. Brogel Reppel Bocholt slip rate mm/yr 0.06 0.02 0.01 0.033 height m 20 9 2 2 Table 1 - Average slip rates since Upper Miocene and average scarp heights for the different faults (Béatse, 1999) This table shows how the slip of the single Neeroeteren Fault is distributed between the different fault branches. When extended to the surface, the faults on the seismic profiles correspond with subtle morphologic scarps (fig. 1) that are described in Paulissen, 1997. Geomorphology The fault scarps of the Reppel and Bocholt Faults are much smaller than that of the Neeroeteren Fault (table 1) probably due to the distribution of fault slip. On the field the scarps are expressed as low angle linear slopes of ± 2 m height in the more or less flat Bocholt Plain. At many places however anthropic action or river erosion modified the scarp. This could clearly be seen on detailed DEM’s of the sites REP-1 and BOCH-2. Several sites where the scarps were better preserved where selected for further investigation in order to confirm the tectonic nature of the scarps and to find favourable sites for trenching. Theodolite profiles were made perpendicular to the scarp. With one of these we modelled the scarp degradation back in time (Camelbeeck et al., this volume). The result indicates that the last tectonic event at the Reppel Fault likely occurred during the last 20 kyr and produced a surface offset of ± 0.5 m. Geomorphic arguments for a shallow surface rupturing faults were also found in differences in meander sinuosity and incision of small rivers that cross the faults. To the NW the scarps gradually become less evident and eventually they disappear. The larger height of the Grote Brogel Scarp is probably partly due to differential erosion because it forms the edge of the outcrop of Main Terrace Gravel on the Campine Plateau. The Grote Brogel Fault is not one sharp plane but a zone of fault branches as can be seen on seismic profiles. As a consequence the total fault scarp is much broader. Due to these difficulties the Grote Brogel Fault was excluded from this study.