Neotectonics of the Roer Valley Rift System, the Netherlands

The Roer Valley Rift System (RVRS) is located in the southern part of the N etherlands and adjacent parts of Gemany and Belgium. The last rifting episo de of the RVRS started in the Late Oligocene and is still ongoing. The pres ent-day seismic activity in the rift system is part of that last rifting ep isode. In this paper, the Quaternary tectonics of the RVRS are studied usin g the detailed stratigraphic record. Subsidence analyses show that three periods of subsidence can be discrimina ted during the Quaternary. A phase of rapid subsidence took place from the beginning of the Quaternary to the Upper Tiglian (similar to 1800 ka). This was followed by a phase of slow subsidence lasting until the Late Quaterna ry (similar to 500 ka). An acceleration in subsidence at the end of the Qua ternary occurred in the central and northern parts of the RVRS (i.e. the Ro er Valley Graben and the Peel Horst) during the last 500 ka. During the Quaternary, the most active fault zones in the RVRS are the Peel Boundary Fault zone and the Feldbiss Fault zone. Average displacements alo ng these fault zones vary between 5 and 80 mm/ka. Periods of high and low d isplacement rates along faults can be discriminated. The magnitude of the s ubsidence rate in the central part of the RVRS, which in theory is caused b y a combination of processes like faulting, cooling of the lithosphere and isostasy, is within the range of the rate of displacement along the major f ault zones of the RVRS, which implies that the subsidence of the RVRS is to a large extent controlled by faulting. Along the wide and staggered Feldbiss Fault zone, the location of the large st displacement rate shifts during the Quaternary, whereas the Peel Boundar y Fault zone, which is narrow and has a straight structure, is more stable in this respect. The present-day fault displacement rates inferred by geode tic measurements are two orders of magnitude larger than the rates inferred from the geological record. Such a large difference can be explained by a high variability of fault movements on a short time-scale due to fault-stre ss interactions. The stratigraphic record has preserved average displacemen t rates. Flexural analyses shows that the pattern of geodetically determine d displacements is in accordance with the fault spacing in the fault zone. The NW-SE directed fault system active during the Quaternary and the Tertia ry is inherited from the late stage of the Variscan orogeny. This fault sys tem was also dominantly active during the Mesozoic and Early Cenozoic evolu tion of the RVRS. Lineament analysis of the topography indicates that apart from the dominant NW-SE-oriented faults, N-S and NE-SW directed faults are also prominent. These faults originate from the Caledonian tectonic phases . They have, however, no large displacements during the Mesozoic and Cenozo ic. The fact that Paleozoic fault systems are reactivated during Quaternary and Tertiary indicates that these faults are fundamental weakness zones. ( C) 2000 Elsevier Science B.V. All rights reserved.