Geleen/Heerlerheide fault.
Composite source BECS001 (Figure S3) comprises the northern section of the Heerlerheide fault, the Geleen fault, and part of the Grote Brogel fault (Grote Brogel – Bree section). Although these faults have been mapped separately, we consider them as a single seismic source. The maximum distance between the Geleen and Heerlerheide faults is only 2.2 km near the Belgian/Dutch border. NW and SE of this position, the faults converge. In the NW, both faults merge into a single strand near the village of Opitter (BE). In the SE, near the village of Schinnen (NL), the Geleen fault terminates against a short, unnamed, N-S oriented fault, and the Heerlerheide fault steps ~2.5 km to the east (i.e. a left step) along a WNW-ESE oriented bend. The SE section of the Heerlerheide fault lines up more or less with the Geleen fault, but is considered to be a separate seismic source (NLCS003) based on a significant drop in slip rate (see description of NLCS003). All these observations suggest that the step near Schinnen is a segment boundary. Further NW, in the Belgian part of the Meuse River valley near the village of Rotem, Vanneste et al. (2008b) documented a smaller left step of the Geleen fault with a width of ~0.5 km, separating the Bree fault scarp section to the NW from the Rotem – Geleen section to the SE. However, a step of this size is not likely to constitute a segment boundary (e.g., Wesnousky, 2006). In the NW, near the village of Bree (BE), the Geleen fault branches into at least three separate faults, from W to E: the Grote Brogel fault, the Reppel fault, and the Bocholt fault. This is likely a structural segment boundary (dePolo et al., 1991). However, the scarps of the Geleen fault (named Bree fault scarp by Camelbeeck and Meghraoui, 1996) and of the Grote Brogel fault appear rather continuous, the latter gradually disappearing towards the west. Based on this geomorphic observation, we propose to include the ~7.5 km long, WNW-ESE oriented section of the Grote Brogel fault into the BECS001 source. Further west, the Grote Brogel fault bends to the northwest, becoming parallel to the Reppel and Bocholt faults. This section of the Grote Brogel fault could be regarded as another seismic source, but it is presently not included in our model, as it is considerably less active than the other faults, judging from the geomorphic expression. The Geleen fault is the first seismic source in the Lower Rhine Graben (LRG) that was investigated paleoseismologically (Camelbeeck and Meghraoui, 1996), and demonstrated to have experienced surface rupture in the recent geologic past. To date, it is the best studied fault in the LRG, with 8 trenches at 7 sites.
| Fault section | Trench site | Year | Institute | Reference |
|---|---|---|---|---|
| Bree fault scarp section | Bree-1 | 1996 | ROB | Camelbeeck and Meghraoui (1996); Camelbeeck and Meghraoui (1998) |
| Bree fault scarp section | Bree-2 | 1996 | ROB | Camelbeeck and Meghraoui (1996); Camelbeeck and Meghraoui (1998) |
| Bree fault scarp section | Bree-3 | 1997 | ROB | Meghraoui et al. (2000) |
| Bree fault scarp section | Bree-4 | 1998 | ROB | Vanneste et al. (2001) |
| Bree fault scarp section | Bree-1bis | 1999 | ROB | Frechen et al. (2001); Camelbeeck et al. (2007) |
| Rotem-Geleen section | Rotem-2 | 2002 | ROB | Vanneste et al. (2008a) |
| Rotem-Geleen section | Rotem-3 | 2005 | ROB | Vanneste et al. (2008a); Vandenberghe et al. (2009) |
| Rotem-Geleen section | Born | 1999 | VUA | Houtgast et al. (2003) |
| Feldbiss fault | Limbricht | 2000 | VUA | Houtgast et al. (2005) |
From the age-altitude record of a flight of Meuse River terraces SE of the Feldbiss fault zone in the Netherlands, van den Berg (1994) inferred an uplift rate of the graben shoulder of ~0.003 mm/yr since 13 Ma, accelerating to ~0.06 mm/yr from 2.7 – 2.4 Ma onward. These values shed light on the long-term evolution of fault activity, but it should be noted that they are not vertical displacement rates, and may also include non-tectonic components of elevation change between subsequent terraces. More direct information is provided by the displacement of younger, Middle-Late Pleistocene, Meuse River terraces which extend more or less perpendicular to the Geleen/Heerlerheide fault in Belgium and the Netherlands. In Belgium, Beerten et al. (1999) report ~40 m of offset for the base of the Cromerian Zutendaal terrace (c. 780 kyr BP according to Houtgast et al., 2002), implying a vertical displacement rate of ~0.05 mm/yr on the Bree fault scarp section. Further SE on the same fault section, but in an area where the Heerlerheide fault section has split off, Paulissen et al. (1985) infer that the bases of the Saalian Eisden-Lanklaar terrace (c. 180 kyr BP) and of the Weichselian Maasmechelen terrace (c. 117 kyr BP) are displaced 7-11 and 2-3 m, respectively, corresponding to vertical displacement rates of 0.039 – 0.061 mm/yr and 0.017 – 0.026 mm/yr. Further SE, in the Netherlands, Houtgast et al. (2002) have determined vertical offsets of the base and top of different terraces. They demonstrated that where the Geleen and Feldbiss faults are overlapping, decreasing slip rate along strike on one fault is compensated by increasing slip rate on the other fault. However, as we consider the two faults as a single seismic source where they are overlapping, we need to consider the total vertical displacement rate across both faults, which is 0.042 – 0.051 mm/yr according to their preferred age model, and which has been rather constant throughout the Late Pleistocene. These values are in good agreement with vertical displacement rates obtained from paleoseismic trenching. Camelbeeck et al. (2007) inferred a rate of 0.050 ± 0.036 mm/yr based on the two most recent complete seismic cycles correlated across four trenches along the Bree fault scarp, and a rate of 0.031 ± 0.012 mm/yr based on the four most recent complete seismic cycles in a single trench. We therefore adopt a vertical displacement rate of 0.04 – 0.05 m/yr for source BECS001. We note that the vertical displacement rate obtained by Houtgast et al. (2002) from the displacement of the youngest terrace (Geistingen terrace, abandoned near 11 kyr BP) yields a value (0.100 mm/yr) that is about twice the other values. This is interpreted to be the result of sampling an incomplete seismic cycle, i.e. the offset was produced by a surface-rupturing earthquake sometime after deposition of the Geistingen terrace (in agreement with OSL dates of the most recent paleoearthquake on the Geleen fault in Belgium by Vandenberghe et al., 2009), but the recurrence interval is longer than the age of the offset feature. This stands in contrast to the continuous deformation (i.e. aseismic creep) that is implicitly assumed by Houtgast et al. (2002).
| ID | Fault section | Type of evidence | Offset (m) | Time period | Deformation rate (mm/yr) | Reference |
|---|---|---|---|---|---|---|
| 1 | Bree fault scarp section | Displacement of base Tertiary on seismic reflection profiles | 500 – 550 | Since Late Oligocene (29 Ma) | 0.017 – 0.019 | Based on data from Demyttenaere and Laga (1988) |
| 2 | n.a. | Altitude-age record of Meuse River terraces on graben shoulder | Miocene – Pliocene (13 – ~3 Ma) | 0.003* | van den Berg (1994) | |
| 3 | n.a. | Altitude-age record of Meuse River terraces on graben shoulder | ~3 Ma – present | 0.060* | van den Berg (1994) | |
| 4 | Bree fault scarp section | Displacement of base of Cromerian Main Terrace of Meuse River | ± 40 | Since Middle Pleistocene (780 kyr) | ± 0.050 | Based on data from Beerten et al. (1999) |
| 5 | Step between Bree fault scarp and Rotem-Geleen sections | Displacement of base of Eisden-Lanklaar terrace | 7 – 11 | Since Saalian (180 kyr) | 0.040 – 0.060 | Based on data from Paulissen et al. (1985) |
| 6 | Step between Bree fault scarp and Rotem-Geleen sections | Displacement of top of Eisden-Lanklaar terrace | 2 – 3 | Since Weichselian (117 kyr) | 0.017 – 0.025 | Based on data from Paulissen et al. (1985) |
| 7 | Rotem-Geleen section | Displacement of base of Caberg-1 terrace | 9.0 + 1.7 + 16.1 | Since 530 kyr | 0.051 | Houtgast et al. (2002) |
| 8 | Rotem-Geleen section | Displacement of base of Caberg-2 terrace | 4.4 + 13.1 | Since 420 kyr | 0.042 | Houtgast et al. (2002) |
| 9 | Rotem-Geleen section | Displacement of base of Caberg-3 terrace | 6.4 + 8.5 | Since 330 kyr | 0.045 | Houtgast et al. (2002) |
| 10 | Rotem-Geleen section | Displacement of top of Eisden-Lanklaar terrace | 4.0 + 2.2 | Since 130 kyr | 0.048 | Houtgast et al. (2002) |
| 11 | Rotem-Geleen section | Displacement of top of Geistingen terrace | 0.3 + 0.8 | Since 11 kyr | 0.100** | Houtgast et al. (2002) |
| 12 | Bree fault scarp section | Observed fault displacement in paleoseismic trench (4 cycles) | 2.7 ± 0.5 | Since 101.4 ± 9.6 kyr BP | 0.031 ± 0.012 | Camelbeeck et al. (2007) |
| 13 | Bree fault scarp section | Observed fault displacement in paleoseismic trench (2 cycles, correlated across 4 trenches) | 1.4 ± 0.2 | Over 27.3 ± 15.6 kyr | 0.050 ± 0.036 | Camelbeeck et al. (2007) |
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