Late Quaternary evolution of the Feldbiss Fault (Roer Valley Rift System, the Netherlands) based on trenching, and its potential relation to glacial unloading

The most important tectonic structure in the southern part of the Netherlands is the Roer Valley Rift System (RVRS). The main border fault zones of the RVRS, the Peel Boundary and the Feldbiss Fault zones, have been active during the Quaternary and are still active today, as indicated by the occurrence of earthquakes. In this paper we present the results of a trench across the Feldbiss Fault, part of the Feldbiss Fault zone. This trench provided no evidence for earthquakes along this segment of the fault zone; fluidization structures were not encountered. A sudden increase in the average fault displacement rate was inferred for the period around 15 ka B P in a trench nearby, across the Geleen Fault of the same fault zone. The (limited) age control cannot exclude that such an acceleration also occurred at the Feldbiss Fault. During the second half of the 1990's several paleoseismological trenching studies have been carried out in the RVRS. The compilation of the results of these studies reveals that the main border faults possibly experienced an increase in fault activity around 10-15 ka B P. The timing of this increased activity and results from glacial rebound modelling indicate that glacial unloading of the crust can have triggered this increase in fault activity.