Peelrand fault.
Composite source NLCS001 corresponds to the west-dipping Peelrand fault. This is the boundary fault between the Roer Valley Graben (RVG) and the Peel horst. With a length of ~125 km, it is the longest source in the LRG. In the northwest, the Peelrand fault gradually disappears north of the confluence area of the Meuse and Rhine Rivers (Figure S1), whereas in the southeast (Figure S3), the fault bifurcates into the Rurrand fault (DECS001) and the Lövenich/Kast fault (DECS002). Although relatively simple at first sight, the Peelrand fault is a complex fault zone with many secondary faults both west and east of the main fault. As offset data are scarce, it is difficult to infer the hierarchy of those faults. We have included only those fault traces in the AFF database table for which important offset has been demonstrated. We can discern five sections, mainly based on bifurcations and changes in strike of the main fault. These have been labeled, from north to south: Rhine-Meuse delta section Uden section, Peel section, Roermond section, and Meinweg section. In the Uden section (Figure S1), the main Peelrand fault is parallel to the overall trend of the source. East of the main fault an important secondary fault occurs, the 2nd Peel fault. The maximum distance between both faults is ~3.5 km, and the faults merge at both ends of the fault section. Michon and Van Balen (2005) reported topographic offsets for the 2nd Peel fault that are comparable to those of the main Peelrand fault (see below). The Peel section (Figure S2) has a more north-northwesterly trend, and is accompanied by an important secondary fault to the west, the Veghel fault. The map of Ahorner (1962) (which is based on a.o. Zonneveld, 1947) shows significant offset for the Veghel fault, decreasing to the north. Zonneveld, 1947 describes the Peelrand/Veghel faults as a “splintered” fault, this is a fault where decreasing displacement in one direction is compensated by development of a parallel fault with increasing displacement, such that the total displacement over the fault zone is constant. The Roermond section (Figure S2) is again parallel to the main fault trend, and traverses the Meuse River. The Meinweg section (Figure S3) corresponds to the Meinweg fault. This fault partly overlaps with the main Peelrand fault SE of the city of Roermond, but extends further southeast into Germany. Ahorner (1962) reckoned this section as part of the Rurrand fault (DECS001), but we restrict this name for the part of the boundary fault south of the bifurcation with the Lövenich/Kast fault (DECS002). The Meinweg section is the most irregular section of the Peelrand fault, showing strong changes in strike, from N-S to almost E-W. The latter is similar to the trend of the Lövenich/Kast fault, which could indicate that the southern half of the Meinweg fault is continuous with that fault. The limits between the different sections of the Peelrand fault cannot be confidently assigned as segment boundaries, but on the other hand, considering its complexity, it is not very likely that the Peelrand fault could rupture in a single segment.
| Fault section | Trench site | Year | Institute | Reference |
|---|---|---|---|---|
| Roermond section | Neer | 1999 | TNO/KNMI/ROB | van den Berg et al. (2002); Frechen and van den Berg (2002); Miedema and Jongmans (2002) |
The position of Quaternary depocenters (Figure 3 in manuscript) indicates that the Peelrand fault is one of the most active faults in the LRG. This is also confirmed by the offset data. Ahorner (1962) reported offsets of 140 – 175 m on the Meinweg section and the Roermond section for the base of the Upper Main Terrace Sequence. The depth map of the base Quaternary (essentially the same horizon as used by Ahorner, 1962) in Worum et al. (2004) shows a similar offset of ~180 m on the central Peelrand section. Considering an age of 2.58 Ma for the horizon, this corresponds to a vertical displacement rate of 0.054 – 0.070 mm/yr. In spite of the relatively large rate of fault activity, the Meuse River flows seemingly unaffected in a relatively straight course across the Roermond section, from the RVG onto the uplifting Peel Block. Michon and Van Balen (2005) report vertical topographic offsets of 0.9 – 1.2 m for the top of the Pleniglacial terrace, and 0.7 – 1.1 m for the top of the Allerød terrace (12.9 cal. ka BP) in the Roermond area. From these offsets, they inferred a displacement rate of ~0.065 mm/yr. Although some care should be taken with these values, mainly because the considered time span is close to the duration of the seismic cycle, and likely includes incomplete cycles, they are in line with the long-term rates. Further north, on the Uden section, Michon and Van Balen (2005) observed intriguingly large topographic offsets at the top of a late Pleniglacial sand sheet (16 – 14.4 cal. ka BP): 1.9 – 3.3 m on the Peelrand fault, and ~4 m on the 2nd Peel fault. Although part of the offset on the 2nd Peel fault may correspond to a remnant offset (pre-existing offset not entirely leveled by the sandsheet deposit), these values are much higher than could be expected for such a young surface. They correspond to vertical displacement rates of 0.37 – 0.51 mm/yr, almost an order of magnitude faster than the long-term rate. Similar observations have nowhere been made in the LRG, and an explanation has not been offered so far. On the other hand, the gradient map of the base Quaternary (Worum et al, 2004) in this area is not much different from the Peelrand fault farther south, and a study further north in the Rhine-Meuse delta by Cohen et al. (2002) found again lower values for the displacement of the top of the Late Pleniglacial terrace: 1.4 m (directly on the Peelrand fault) to 2.3 m (including regional warping), corresponding to a vertical displacement rate of 0.09 – 0.15 mm/yr. Considering the above-mentioned problem with deformation rates inferred from features with an age comparable to the length of the seismic cycle, we adopted the long-term vertical displacement rate for the Peelrand fault.
| ID | Fault section | Type of evidence | Offset (m) | Time period | Deformation rate (mm/yr) | Reference |
|---|---|---|---|---|---|---|
| 29 | Roermond section | Displacement of base of “Ältere Hauptterrasse” of the Rhine R. | 140 – 175 | Since 2.58 Ma | 0.054 – 0.068 | Based on data from Ahorner (1962) |
| 30 | Roermond section | Topographic offset over top of Pleniglacial terrace of Meuse R. | 0.9 – 1.2 | Since 16 cal. kyr BP | 0.056 – 0.075** | Michon and Van Balen (2005) |
| 35 | Roermond section | Offset of Beuningen gravel bed | 0.9 (1.50 – 1.75 including flexure) | Since 15.8 cal. kyr BP | 0.057 – 0.111** | van den Berg et al. (2002) |
| 31 | Roermond section | Topographic offset over top of Allerød terrace | 0.85 | Since 12.9 cal. kyr BP | 0.066** | Michon and Van Balen (2005) |
| 32 | Peel section | Displacement of base Quaternary | ~160 | Since 2.58 Ma | ~0.062 | Based on data from Worum et al. (2004) |
| 33 | Uden section | Topographic offset over late Pleniglacial sand sheet | 1.9 – 3.3 + ~4 | Since 16.0 – 14.4 cal. kyr BP | 0.37 – 0.51** | Michon and Van Balen (2005) |
| 34 | Uden section | Displacement of top of Late Pleniglacial terrace of Meuse R. | 1.38 – 2.28 | Since ~15 cal. kyr BP | 0.09 – 0.15** | Cohen et al. (2002) |
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