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Neotectonics of the Roer Valley Rift System, the Netherlands
Global and Planetary Change 27 Ž2000. 131–146 www.elsevier.comrlocatergloplacha
Neotectonics of the Roer Valley Rift System, the Netherlands R.F. Houtgast, R.T. van Balen Faculty of Earthsciences, Vrije UniÕersiteit Amserdam, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands
Abstract The Roer Valley Rift System ŽRVRS. is located in the southern part of the Netherlands and adjacent parts of Gemany and Belgium. The last rifting episode of the RVRS started in the Late Oligocene and is still ongoing. The present-day seismic activity in the rift system is part of that last rifting episode. In this paper, the Quaternary tectonics of the RVRS are studied using the detailed stratigraphic record. Subsidence analyses show that three periods of subsidence can be discriminated during the Quaternary. A phase of rapid subsidence took place from the beginning of the Quaternary to the Upper Tiglian Ž; 1800 ka.. This was followed by a phase of slow subsidence lasting until the Late Quaternary Ž; 500 ka.. An acceleration in subsidence at the end of the Quaternary occurred in the central and northern parts of the RVRS Ži.e. the Roer 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 along these fault zones vary between 5 and 80 mmrka. Periods of high and low displacement rates along faults can be discriminated. The magnitude of the subsidence rate in the central part of the RVRS, which in theory is caused by a combination of processes like faulting, cooling of the lithosphere and isostasy, is within the range of the rate of displacement along the major fault 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 largest displacement rate shifts during the Quaternary, whereas the Peel Boundary Fault zone, which is narrow and has a straight structure, is more stable in this respect. The present-day fault displacement rates inferred by geodetic 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–stress interactions. The stratigraphic record has preserved average displacement rates. Flexural analyses shows that the pattern of geodetically determined displacements is in accordance with the fault spacing in the fault zone. The NW–SE directed fault system active during the Quaternary and the Tertiary is inherited from the late stage of the Variscan orogeny. This fault system was also dominantly active during the Mesozoic and Early Cenozoic evolution 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 Cenozoic. The fact that Paleozoic fault systems are reactivated during Quaternary and Tertiary indicates that these faults are fundamental weakness zones. q 2000 Elsevier Science B.V. All rights reserved. Keywords: neotectonics; quaternary; faults; subsidence; Roer Valley Graben; Peel Boundary fault zone; Feldbiss fault zone
E-mail address: [email protected] ŽR.F. Houtgast.. 0921-8181r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 8 1 8 1 Ž 0 1 . 0 0 0 6 3 - 7
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1. Introduction Ahorner Ž1962. proposed a pattern of earthquake distribution in northwest Europe and inferred a correlation between earthquake activity and Quaternary structures. Identification of active faults, structures that have an established record of activity in the Late Quaternary, is an important consideration for estimating the seismic hazard assessment for northwest Europe ŽCamelbeeck and Meghraoui, 1998.. In this paper, we present the Quaternary tectonics of the Roer Valley Rift System and revealed their spatial and temporal patterns. The Roer Valley Rift System ŽRVRS. is an active rift system, located in the southern part of the Netherlands, the northeastern part of Belgium and adjacent parts of Germany ŽFig. 1.. During Quater-
Fig. 1. The Cenozoic Western European Rift System Žafter Ziegler, 1992..
nary, the tectonic movements have affected the courses and the incision–aggradation behavior of river systems Žsee, for example, Bogaart and Van Balen, 2000-this volume; Van Balen et al., 2000b-this volume.. The faulting also influences the morphology: Fault scarps occur along the major fault zones and small streams are aligned to the faults. The seismicity in the area is concentrated at the major fault zones ŽCamelbeeck and Meghraoui, 1998.. For example, the moderate earthquake of Roermond is located along the northern boundary fault zone of the RVRS ŽVan Eck and Davenport, 1994.. In this paper, extensional tectonics of the RVRS are studied using the Quaternary stratigraphic record of the RVRS. The spatial variations in tectonic behavior are assessed, as well as the evolution in time of basin subsidence and faulting behavior. Large-scale subsidence is analyzed using the backstripping method. Fault movements are quantified using the detailed Quaternary stratigraphy. The exceptionally detailed Quaternary geological record of the RVRS provides a unique opportunity to study fault motions on a 100 ka time-scale. On a large scale, the RVRS system is part of a rift system in the southeastern part of the North Sea Basin, the Lower Rhine Embayment, which itself is part of a Cenozoic mega-rift system crossing western and central Europe ŽFig. 1; Ziegler, 1992.. The RVRS has a complex Mesozoic and Cenozoic tectonic history, comprising several extension and inversion phases. The current extension phase started during the Late Oligocene ŽGeluk et al., 1994.. The RVRS comprises the Campine Block in the south, the Roer Valley Graben in the center, and the Peel Horst and the Venlo Block in the northeast ŽFig. 2.. The Roer Valley Graben is separated from the adjoining blocks by the Feldbiss Fault zone in the south and the Peel Boundary Fault zone in the north, which are the most active fault zones during the Quaternary Že.g. Ahorner, 1962; Paulissen et al., 1985.. The central Roer Valley Graben has subsided approximately 1000 to 1200 m since the Late Oligocene, whereas the Peel Horst subsided only up to 200 m ŽGeluk et al., 1994.. In the first part of the paper, the subsidence behavior of different tectonic units within the RVRS is studied, using the borehole database of the Dutch geological survey ŽNITG TNOr National Geologi-
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Fig. 2. The Roer Valley Rift System tectonic units.
cal SurÕey .. Subsequently, the timing of fault activity and quantification of fault displacement during the Quaternary are assessed using the same database. Next, the influence of tectonic activity on the present-day morphology of the RVRS is analyzed using a high-resolution digital elevation model. Finally, the influence of tectonic activity of the RVRS on the evolution of the Maas river is discussed.
2. Quaternary tectonic evolution of the Roer Valley Rift System The Quaternary tectonic activity in the RVRS is part of a rifting episode which started in the Late
Oligocene. Therefore, the Quaternary subsidence is the result of active faulting and of thermal subsidence due to Miocene, Pliocene and Quaternary crustal thinning Že.g. Zijerveld et al., 1992.. The subsidence of the Roer Valley Graben created accommodation space for marine and fluvial deposits of the Maas and Rhine rivers and smaller, local Belgian rivers. The lithostratigraphic boundaries between these deposits are used for the tectonic analysis. During the first part of the Quaternary, lasting from Pretiglian to Early Eburonian ŽTable 1., the graben was filled with Maas and Rhine deposits of the Tegelen Formation in the eastern part of the graben; while in the western part of the rift, marine
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Table 1 Lithostratigraphy and chronostratigraphy in the RVRS ŽZagwijn and van Staalduinen, 1975; Bisschops et al., 1985; Zagwijn, 1989, 1996. Lithostratigraphic formation
Start of deposition Žka.
End of deposition Žka.
Nuenen group Veghel-A Sterksel Kedichem Tegelen Kiezelooliet ¨ Maassluis
Elsterian Ž350. Upper Cromerian Ž650. Menapian Ž1200. Lower Eburonian Ž1800. Lower Tiglian Ž2400. Lower Pretiglian Ž2900. Lower Pretiglian Ž2500.
Late Weichselian Ž15. Holsterian Ž240. Middle Cromerian Ž650. Lower Menapian Ž1200. Upper Tiglian Ž1800. Upper Pretiglian Ž2200. Upper Tiglian Ž1800.
deposits of the Maassluis Formation were laid down ŽTable 1; Bisschops, 1973.. The overlying Kedichem Formation of Eburonian to Menapian age ŽBisschops, 1973; Zagwijn, 1996. is composed of deposits from the Maas and local rivers. During that period, the Rhine was flowing to the north of the RVRS ŽVan den Berg, 1996.. The Sterksel Formation is deposited during the Menapian to Middle Cromerian by the Rhine river with the Maas as a tributary ŽZagwijn, 1996.. Toward the end of the Cromerian, the Rhine changed its course toward the east forced by tectonic movements further upstream ŽBisschops, 1973.. The Maas became the single large river filling the Roer Valley Graben. During this period, the Maas incised in the Sterksel Formation and subsequently deposited the Veghel-A Formation of Late Cromerian to Holsterian age Že.g. Zonneveld, 1949; Zagwijn, 1996.. After the deposition of the Veghel-A Formation, the course of the Maas shifted from the Roer Valley Graben to the Venlo Block ŽVan den Toorn, 1967; Bisschops, 1973.. After the Maas had left the Roer Valley Graben, deposits of local origin filled the graben ŽBisschops, 1973.. In cooperation with the NITG TNOrNational Geological SurÕey, 350 borings were selected along six regularly spaced profiles, perpendicular to the main border faults of the RVRS. The position of the lithostratigraphic boundaries were identified in these borings. In the analysis of tectonic movements, it is assumed that the lithostratigraphic boundaries have one single age, which is obtained from correlation of the lithostratigraphy to the published chronostratigraphy ŽTable 1; Zagwijn and van Staalduinen, 1975; Bisschops et al., 1985; Zagwijn, 1989, 1996.. Due to the gradual transitions the boundaries between important lithostratigraphic units are sometimes diffi-
cult to pinpoint in borehole records. Because of difficulties in dating, the ages of some stratigraphic units are uncertain; for the older stratigraphic units the uncertainty may be in the order of tens of ka. In addition, as the deposits interfinger stratigraphically, the ages applied in the backstripping method Žsee below. depended on the local stratigraphic development. 2.1. Uplift and subsidence rates As shown by Geluk et al. Ž1994., the subsidence patterns of two boreholes located in the Roer Valley Graben are characterized by an increase in subsidence rates with time. Both borehole records show a 25–30% increase in the average subsidence rate from the Neogene to the Quaternary. At the location of the Herten well ŽFig. 2., the central graben subsided on average with a rate of 60 mmrka during the Neogene and 80 mmrka during the Quaternary. Ten kilometers to the southwest, at the position of the Linne well, the graben subsided with an average rate of 40 mmrka during the Neogene. However, the latter rate is probably too high, as the ages of upper Tertiary lithostratigraphic boundaries are now interpreted to be older, in the order of 100 ka ŽM. Van den Berg, pers. commun., 1999.. The subsidence accelerated to 55 mmrka during the Quaternary. Both subsidence curves show hiatuses in the Early Quaternary ŽEburonian and Waalian. and in the Middle Quaternary ŽCromerian.. These hiatuses suggest periods of minor to no subsidence, or even possible periods of uplift. The extent of the hiatuses is unknown. In order to investigate the spatial variation in the subsidence characteristics of the Roer Valley Graben,
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we have studied six additional wells, using the backstripping method. This method is valid in the terrestrial environment of the RVRS, because the upper surface of the RVRS was part of a large flat delta plain controlled by eustatic sea level and tectonic subsidence. Additionally, there is no indication that the basin was underfilled during the Quaternary. Therefore, change in sedimentation rate is related to eustacy or tectonics and not to sedimentological processes. In this analysis, it is assumed that compaction has a minor effect on the subsidence, due to the limited thickness of the Quaternary deposits and their predominately sandy character. The wells are located on three different tectonic units. Three wells are located in the central Roer Valley Graben Ž58A0087, 45D0046, 45G0059., two wells are situated on the Peel Horst Ž58A0058, 52A0146. and one well is positioned on the Campine Block Ž50H0074.. The general trend of the subsidence curves is characterized by three subsidence phases ŽFig. 3; Table 2.. The first phase is a period of relatively rapid subsidence from the beginning of the Quaternary Žstart of deposition of the Tegelen Formation, "2400 ka. until the Late Tiglian Ž"1800 ka.. The second phase is characterized by relatively slow subsidence, lasting until the end of the Cromerian Ž"350 ka.. All subsidence curves located in the
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Roer Valley Graben and on the Peel Horst show an increased subsidence rate toward the end of the Quaternary during the third phase lasting until the Present. This acceleration is not visible in the subsidence curve on the Campine Block. The exact magnitude of the acceleration in subsidence should be regarded with some caution because of the error bar due to the limitation in the chronostratigraphic age control. The depositional age of the base of the uppermost lithological unit Žthe Nuenen Group. is uncertain, it could be of Elsterian age Ž"350 ka. or younger ŽJ. Schokker, pers. commun., 2000; Bisschops, 1973; Zagwijn, 1996.. Even if one would consider the oldest possible age for the base of the Nuenen Group, the approximately twofold increase in subsidence still has taken place. Although all curves show the same general trend, subtle differences between the three tectonic units exist. In Table 2, the results of the subsidence analyses have been used to calculate average subsidence rates of the three tectonic units. The average subsidence rate in the Roer Valley Graben is close to the rate determined by Geluk et al. Ž1994., 60–90 mmr ka throughout the Quaternary. The other two tectonic units, the Campine Block and the Peel Horst, subsided on average with a rate two or three times smaller. In detail the difference in subsidence rates
Fig. 3. Subsidence curves for six wells located on the Campine Block, in the Roer Valley Graben and on the Peel Horst. For location of the wells, see Fig. 2.
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Table 2 Average subsidence rates for three tectonic units during the Quaternary determined from the subsidence curves shown in Fig. 3 Tectonic unit
Campine Block Roer Valley Graben Peel Horst
Average subsidence rate Žmmrka. 2400–1800 ka
1800–650 ka
650–0 ka
80 180 130
10 50 17
7 70 20
between the Roer Valley Graben and the other two tectonic units is occasionally large. During the first subsidence phase, the differences are small. During the second phase, the Roer Valley Graben subsided on average three to five times faster than the neighboring tectonic units. The Roer Valley Graben subsided 4–10 times faster than the Campine Block during the third phase. A regional difference in subsidence behavior regarding the timing of the deceleration at the end of the first phase also exists ŽFig. 3., which is independent of the tectonic units. The three wells located in the northwest of the Roer Valley Rift System Ž50H0074, 45D0046 and 45G0059. seem to decelerate later during the first subsidence phase than the wells located in the southeastern part of the Roer Valley Rift System.
Quaternary average 27 88 46
ment, that in an area of about 400 km2 in the western Espanola ˘ Basin of the Rio Grande rift more than 17 Quaternary faults with displacements in excess of 4 m occur. This number is in the same order of magnitude as the number of faults with a displacement larger than 4 m found in the RVRS. Because this is a tectonically and depositionally similar setting, their research results gives an indication of how many smaller faults can be present in the RVRS. Based on the same power law relationship, it can be inferred that about 800 faults with a displacement of 0.01–1 m can occur in the RVRS.
2.2. Fault pattern The Quaternary geological record enables the detection of active faults. In this paper, a distinction is made between faults and fault zones: a fault zone is a bundle of faults and the total displacement along a fault zone is the sum of displacements along individual faults which make up the fault zone. The borehole data set of the Geological Survey together with the geological maps ŽVan den Toorn, 1967; Bisschops et al., 1985. and geoelectrical results ŽVandenberghe, 1982. provide an opportunity to locate fault zones with an accuracy of about 1 km at best ŽFig. 4.. Due to the uncertainty in recognizing clear boundaries between lithostratigraphic units, only faults with a large displacement Ž) 4 m. are mapped. Probably, many more faults with a smaller displacement are present in the area. For example, Carter and Winter Ž1995. calculated on the base of a power law relationship between the number of faults and displace-
Fig. 4. Quaternary faults in the Roer Valley Rift System. The locations of fault Žzone.s outside the study area are from Ahorner Ž1962., Vandenberghe Ž1982. and Geluk et al. Ž1994..
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The pattern of the faults, which were active during the Quaternary, correlates with the fault pattern in the base of Tertiary deposits ŽZagwijn et al., 1985.. The larger Quaternary fault zones have the same NW–SE orientation as the larger Tertiary fault zones. The most active fault zones during the Tertiary, the Feldbiss and Peel Boundary Fault zones, are also the most active zones during the Quaternary. The NW–SE-oriented faults date back as far as the late stage of the Variscan orogeny ŽVan Wijhe, 1987; Van Balen et al., 2000a.. They have since then been reactivated continuously during the Mesozoic and Cenozoic evolution of the RVRS and nearby basins. They represent, therefore, fundamental lithospheric weakness zones. The most active Quaternary fault zones, the Peel Boundary and Feldbiss Fault zones, both border the Roer Valley Graben. The Peel Boundary Fault zone is a confined zone, where faults are closely spaced ŽFig. 4.. The faults of the Feldbiss Fault zone are locally more widely spaced. Both fault zones were active during the whole Quaternary. 2.3. Morphological expression of faults The main geomorphological expression of tectonic activity in the southern part of the Netherlands
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is the topography: the Peel Horst is a topographic high and the Roer Valley Graben and Venlo Block are topographic lows ŽFig. 5.. The additional geomorphological expression of tectonic activity in the southern part of the Netherlands are lineaments, which represent terrain steps, river courses, etc. Ž1976. analyzed Landsat images for lineaSesoren ¨ ments and found many lineaments with two main orientations, a dominating NW–SE orientation, and a NE–SW orientation. The NW–SE lineaments show a strong correlation with known Tertiary and Quaternary fault patterns. A dominating NW–SE orientation and a NE–SW orientation were also found by Van den Berg et al. Ž1994.; their lineaments are based on abrupt breaks in slope. In our research a digital terrain model ŽMeetkundige dienst, Rijkswaterstaat., which consists of more than 75 data pointsrkm2 regularly spaced, is analyzed for lineaments. Contrary to previous studies, a mathematical approach was used to detect the lineaments. For a detailed description of the method used, see Appendix A. The result of our lineament analyses is shown in Fig. 6. The NW–SE and NE–SW orientations found in previous studies are also present in our results. The results of the three different methods show that the NW–SE orientation of lineaments is dominant in
Fig. 5. Topographic profile across the Roer Valley Graben combined with a line drawing of the southern section of the deep seismic line 8601 ŽGeluk et al., 1994.. See Fig. 2 for location of both profiles.
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stress field, we expect these faults to be strike–slip. The reason why these faults have not been recognized in the subsurface geology is related to the orientation of 2D seismic lines in that area which are approximately parallel to the NE–SW direction ŽC. De Leeuw, pers. commun., 2000.. Additionally, the displacements associated with NE–SW faults can be too small to be detected on seismic sections. The origin of the NE–SW-oriented faults is probably a bit older than the NW–SE-trending faults, having a likely Caledonian age ŽBless et al., 1980..
Fig. 6. Lineaments found in this research in the southern part of the Netherlands.
the southern part of the Netherlands. The results also show that NE–SW-oriented faults have been active during the Quaternary, despite the fact that they have not been recognized as active faults during the Tertiary ŽZagwijn et al., 1985.. Under the present-day
2.3.1. LeÕelling results The Peel Boundary Fault zone has also been the subject of geodetic levelling studies ŽGroenewoud et al., 1991.. The results of these studies show that the vertical displacements decay exponentially away from the fault zone ŽFig. 7. more or less in accordance with theoretical faulting induced flexural motions ŽVan Balen et al., 1998; Appendix B.. As shown in Appendix B, the spacing of faults within the Peel Boundary Fault zone could be controlled by flexural stresses induced by the faulting. The rate of fault displacement found by the levelling is two orders of magnitude larger than the displacement rate documented in the geological record, see below. An explanation for this discrepancy could be 3D fault– stress interactions inducing fault motions, which are highly variable in time and space on a 10 to 100
Fig. 7. Displacement profile across the Peel Boundary Fault determined by geodetic measurements Žafter Groenewoud et al., 1991..
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1967; Bisschops et al., 1985.: The thicknesses of sedimentary units vary over the different tectonic units, and sedimentary units are displaced by faults. To calculate the displacement rate of a fault zone, two Žsets of. boreholes are used, located on both sides of a fault zone ŽFig. 8.. The displacement rate during deposition is calculated by dividing the difference between the displacement at the base and the top of the lithostratigraphical unit by the age difference of the base and top of the unit. The displacement rate after deposition is calculated by dividing the displacement at the top of the lithostratigraphic unit by its age. This approach assumes a continuous sedimentary history. In most cases, this is not exactly true. Most of the contacts between sedimentary units are erosional and even within a sedimentary unit erosional surfaces are likely to occur, because these sedimentary units are deposited by fluvial systems, which incise, aggradate and migrate. The hiatus within one sedimentary unit does not affect the outcome of our analyses, because we only consider the base and top of the sedimentary units. The
Fig. 8. The method used for calculating the average displacement rates for faults from well data.
years time-scale, including cessation and possibly a reversal of motions. Alternatively, fault activity maybe also be spasmodic in nature ŽCrone et al., 1997.. On a geological time-scale, these motions are largely averaged, although some variability is still preserved as shown by the fault displacements in time and space presented below. 2.4. Fault displacements The Quaternary deposits in the RVRS have a maximum thickness of about 200 m in the center of the Roer Valley Graben. The tectonic influence on the deposition of these sediments can be observed in several published geological profiles ŽVan den Toorn,
Fig. 9. Average displacement rates for several faults in the Roer Valley Rift System Rijen Fault during the Quaternary Žafter Vandenberghe, 1982. Vessem Fault, side branch of the Feldbiss Fault Žafter Bisschops et al., 1985. Viersen Fault Žafter Ahorner, 1962.. Location of faults indicated in Fig. 2.
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calculated displacement rate is an average for the time period during which the deposition of the sedimentary unitŽs. took place. The hiatus between sedimentary units does influence our analyses. For example, erosion of the top of a sedimentary unit leads to an underestimation of the age of the top of the sedimentary unit Žthe age assigned to the top is too young.. Due to possible erosion of the top of the sedimentary unit, the displacement rates are minimum values. Several profiles across fault zones in the RVRS have been published ŽAhorner, 1962; Van den Toorn, 1967; Verbraeck, 1970, 1984; Bisschops, 1973; Vandenberghe, 1982, 1990; Bisschops et al., 1985; Paulissen et al., 1985.. Using the method outlined before, the average displacement rates for several time periods are calculated from the faults described in the profiles ŽFig. 10.. The Rijen-Beringen and the Tegelen Fault were active during the Early and Middle Quaternary and later activity is not documented in the sedimentary record. The Rijen-Beringen Fault divides the Campine Block into a western and eastern part. Detailed morphological and geoelectric investigation of the Rijen-Beringen Fault zone revealed a complex fault zone, which extends
into the Belgian part of the RVRS ŽVandenberghe, 1982.. Most faults of that fault zone were active during the Middle Pleistocene ŽVandenberghe, 1990.. The Tegelen Fault, which separates the Peel Block and the Venlo Block, had its main period of activity during the Early Quaternary ŽBisschops et al., 1985.. The rates vary from 5 up to 80 mmrka. These rates are one order of magnitude less than the Quaternary fault displacement rates found by Geluk et al. Ž1994., i.e. 800 mmrka. However, according to M. Geluk Žpers. commun., 2000. this is a mistake in their paper, it should be 80 mmrka. A fault displacement rate of 80 mmrka leads to a thickness of Quaternary deposits of about 160 m, which is in accordance with the observed maximum thickness of 200 m of the Quaternary deposits. This implies that the subsidence of the Roer Valley Graben is mainly fault controlled. 2.5. Lateral Õariations in aÕerage displacement rates along fault zones In Fig. 9, displacement rates of several faults are compared. Fault zones, which extend over tens of kilometers, do not behave in a uniform way along
Fig. 10. Lateral variations in average displacement rates along the Rurrand Fault ŽGermany. during the Quaternary Žafter Ahorner, 1962..
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the whole fault zone. Ahorner Ž1962. documented the displacements of the Rurrand Fault, the southeastward extension of the Peel Boundary Fault ŽFig. 10., using terraces of the Rhine river. A variation through time and a spatial trend could be distinguished. At every location along the Rurrand Fault, the lowest average displacement rate is found for the
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Eburonian–Cromerian period and the highest for the Cromerian–Present. The spatial trend shows a gradual increase in average displacement rates in a northwest direction along the Rurrand Fault. Using the borehole database, the lateral variations in average displacement rates along the two main fault zones bordering the Roer Valley Graben, the
Fig. 11. Lateral variations in average displacement rates along the Peel Boundary Fault zone and the Feldbiss Fault zone during the Quaternary.
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Peel Boundary Fault zone and the Feldbiss Fault zone are analyzed. The length of both fault segments is approximately 50 km ŽFig. 2.. The results are presented in Fig. 11. Along the Feldbiss Fault zone, the position of the largest fault displacements shifts from the southeast ŽDf and Ef. to the northwest ŽAf, Bf, Cf and Df. and back again to the southeast. Along the Peel Boundary Fault zone, the pattern is less clear. The fault activity seems to be fixed in space. The largest cumulative fault displacements along the Feldbiss Fault zone are located in the center of the segment Žlocation points Cf and Df.. The largest cumulative fault displacement along the Peel Boundary Fault zone during the Quaternary is concentrated at points Bp. and Cp and further to the southeast at point Fp. In between these locations of high displacement rates along the Peel Boundary Fault zone, a segment of lower displacement rates ŽDP and Ep.. is located. The base Miocene depth map ŽGeluk et al., 1994. shows two depocentres ŽFig. 2.. The larger of the two depocentres is situated in the northwestern part of the graben, the smaller depocentre is located in the southeastern part. The position of the largest displacement rates along the Feldbiss Fault zone is adjacent to the largest depocentre ŽCf and Df.. The position along the Peel Boundary Fault zone of the largest displacement rates ŽBp, Cp and Fp. also coincides with the position of the two depocentres in the Roer Valley Graben. The pattern in fault displacements along these two border faults indicates that the Roer Valley Graben is not one structural unit, but is divided in two or more subgrabens.
3. Discussion The pattern of active faults during the Quaternary and of the present-day active faults inferred from lineament analyses and the morphology is dominated by NW–SE-oriented faults. These faults originate from the late stage of the Variscan orogeny. They have been reactivated continuously during the Mesozoic and Cenozoic tectonic evolution ŽVan Wijhe, 1987; Van Balen et al., 2000a.. The other important active set of faults has NE–SW to N–S directions. These faults are probably from the Caledonian orogeny Ži.e. Bless et al., 1980.. These faults have
apparently not been reactivated during the Mesozoic and early Cenozoic evolution of the RVRS. The reason why the NE–SW faults have only been reactivated recently could be explained by the lack of favorable maximum horizontal stress orientations required to reactivate them during the Mesozoic and Cenozoic. Both fault sets are present in the Paleozoic basement to the south of the RVRS ŽLondon–Brabant Massif and the Rhenish Massif.. The distribution of displacements along the fault zones bordering the Roer Valley Graben is related to the position of the Quaternary depocentres ŽFig. 2.. Both the base Tertiary map ŽZagwijn et al., 1985. and the base Miocene map ŽGeluk et al., 1994. show depocentres at the same locations, bordered by those segments of the fault zones, which have the largest displacement rates. These depocentres also coincide with the depocentres observed in the Nuenen Group ŽJ. Schokker, pers. commun., 2000.. Therefore, a causal relationship between the Tertiary and the Quaternary depocentres could exist. An obvious explanation is that the same faults are active during Quaternary and Tertiary. However, the origin of the Quaternary depocentres could also be delayed compaction of the Tertiary clays in the Roer Valley Graben induced by loading by Quaternary sediments. Kooi and De Vries Ž1998. have shown that the response time of compaction-controlled subsidence is in the order of 10 5 –10 7 years. Therefore, the subsidence due to compaction of Lower Tertiary clays in the Roer Valley Graben could still be an ongoing process, with maxima at the Tertiary depocentres. In that case, the fault activity could be the response to subsidence due to compaction of Lower Tertiary strata. Depending on the assumed permeability of Tertiary clays, the subsidence rate due to compaction of Tertiary clays in the coastal zone of the Netherlands is calculated to be 80 to 120 mmrka ŽKooi and De Vries, 1998.. Although these rates will be slightly smaller in the RVRS due to the smaller thickness of the clayey strata, these rates are about the same as the Quaternary subsidence rates determined for the RVRS. However, compaction is a response to sediment loading. Therefore, when accommodation space is created by extensional faulting, an extra load on the Lower Tertiary clays will be caused by sediments filling in the accommodation space. Therefore, the delayed compaction can have
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contributed to the general subsidence by amplifying the fault controlled subsidence. The spatial distribution of the Tertiary clays in the Roer Valley Graben is saucer-shaped ŽGeluk, 1990.; therefore, we expect a similar shape in subsidence patterns. Further research is required to elucidate the exact contribution of compaction of Lower Tertiary clay to the Quaternary subsidence of the RVRS. The position of the largest displacement rates along the Peel Boundary Fault zone remained fixed throughout time. Along the Feldbiss Fault zone, the position of the largest displacement rates shifts from the southeast to the northwest and back again to the southeast during the Quaternary. In this respect, the Peel Boundary Fault zone seems more stable than the Feldbiss Fault zone. The origin of this relative stability could be the different structure of both fault zones ŽFig. 4.. The Feldbiss Fault zone is wider and has a more staggered pattern compared to the Peel Boundary Fault zone, which is more confined in space and has a straight pattern. Because of the difference in structure, the response of the fault zones to the same change in stress level is probably different. The tectonic control on the Maas river is evident in its morpho-sedimentological expression. The relative uplift of the Campine Block forced the Maas river to incise and form a flight of river terraces. In the subsiding Roer Valley Graben accommodation space was created for the Maas river to deposit a thick stack of sedimentary deposits ŽVan den Berg, 1996.. Subsidence analysis of several wells in the Roer Valley Graben revealed a period of no subsidence or erosion during the Middle Quaternary. From the paleogeographic and spatial distribution of the fluvial deposits in the Roer Valley Graben, Late Middle Pleistocene uplift can be inferred. Deep incision of the Veghel-A Formation in the Sterksel Formation can be explaned by an episode of tectonic uplift, which is also documented in the Ardennes ŽVan Balen et al., 2000b-this volume.. The same uplift is also responsible for the northward diversion of the Rhine river and the shift of the Maas river from the Roer Valley Graben to the Venlo Block. The resulting northeastward tilting of the RVRS forced the Maas river onto the Peel Horst and further onto the Venlo Block ŽBisschops, 1973.. At present, the inci-
143
sion–aggradation pattern of the Maas river reveals a tectonic control: the Maas river incises on the Campine Block, aggradates in the Roer Valley Graben and incises again on the Peel Horst Žsee also Van Balen et al., 2000b-this volume..
4. Conclusions Subsidence analyses revealed three phases of subsidence during the Quaternary. A phase of rapid subsidence from the beginning of the Quaternary to the Upper Tiglian Ž; 2200 ka., followed by a second phase of slow subsidence until approximately 500 ka. An acceleration in subsidence toward the end of the Quaternary is recognized in the Roer Valley Graben and the Peel Horst. The relative uplift rate for the Campine Block found by Van den Berg Ž1994. Ž60 mmrka; 1994., based on the Maas incision record, matches the relative subsidence rate of 61 mmrka of the Roer Valley Graben with respect to the Campine Block. This could imply that the uplift of the Campine Block inferred by Van den Berg is an uplift relative to the graben. The most active fault zones in the RVRS during the Quaternary are the Peel Boundary Fault zone and the Feldbiss Fault zone, which border the subsiding Roer Valley Graben. Average fault displacements during the Quaternary along these fault zones vary from 5 to 80 mmrka. A fault displacement rate of 80 mmrka, combined with the observed maximum thickness of about 200 m of the Quaternary deposits, implies that the subsidence of the Roer Valley Graben is mainly fault controlled. The displacement rates vary laterally along fault zones and the position of the largest displacement rates coincide with the position of Miocene–Present depocentres. The structure of the fault zones influences their tectonic behavior: The wider and more staggered Feldbiss Fault zone has a more variable spatial and temporal pattern in displacement rates; whereas the more confined Peel Boundary Fault zone is more stable. The pattern of active faults during the Quaternary and of the present-day active faults is dominated by NW–SE-oriented faults. The other important active set of faults has NE–SW to N–S directions. As both
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faults sets are active at present, they must both represent fundamental lithospheric weakness zones. The reason why the NE–SW faults, recognized in the morphology, have only been reactivated recently could be explained by the lack of favorable maximum horizontal stress orientations required to reactivate them during the Mesozoic and Cenozoic.
Acknowledgements The research presented in this paper is a contribution to the Netherlands Environmental Earth System Initiative ŽNEESDI. programme, funded by the Netherlands Organization for Scientific Research ŽNWO.. We thank H. Pagnier, W.Westerhoff, P. Kiden and W. Dobma of the NITG TNO for their cooperation; Prof. J. Vandenberghe, F.W. Van der Wateren and P.W. Bogaart for their discussions and constructive criticism. We thank R. Pelzing ŽGeologisches Landesamt Nordrhein-Westfalen., K. Vanneste ŽRoyal Observatory Belgium. and M.W. Van den Berg ŽNITG-TNO. for their constructive and valuable comments on the manuscript.
Appendix A. Filter techniques, GIS conversions and criteria First, a regularly spaced DTM was made of all the available data points, using the kriging interpolation method. In this exercise, four filters ŽNWSE, NESW, NS, EW. were used, which enhance the terrain steps in a certain direction and suppress the areas with no or minor terrain steps in that direction, see Table 3. Four maps were created with terrain steps Žboth negative and positive, depending on which direction the higher area lies. and these were converted to
maps with only positive terrain steps. These were then converted to a classified map. The end result were four maps with classified terrain steps visible in a certain direction. Several lineaments were detected. To distinguish the height jumps of possible tectonic origin from others Žriver dunes, aeolian dunes., the following criteria have been applied: Ž1. A length of at least 5 km Žuninterrupted or shorter segments clearly in line with each other.; Ž2. No correlation with other geomorphological features like aeolian dunes and river dunes visible on various geomorphological and topographical maps. It is assumed that the appearance of these features has no correlation with faults.
Appendix B From the general solution of the elastic thin-plate flexure equation ŽHetenyi, 1946., the flexural profile ´ induced by faulting can be derived ŽSpadini and Podladchikov, 1996.. The deflection is given by: w s ucos Ž a x . eŽy a x . ,
Ž 1.
where w s deflection, u s fault displacement, a s flexural parameter and x s distance. The fiberstress resulting from the deflection of the thin-plate is given by the second derivative of Eq. Ž1.: wY s 2 a 2 usin Ž a x . eŽy a x . .
Ž 2.
In the deflection profile given by the geodetic measurements of Groenewoud et al. Ž1991; Fig. 8., the horizontal distance between the two positive deflection maxima is about 15 km. Therefore:
a s 2pr Ž 15 = 10 3 . . In Fig. 8, the theoretical flexural profile is compared to the measured profile. The comparison shows
Table 3 Filters used NWSE filter ŽNESW filter is the reflected version. 0 2r22 1r22 0 0
y2r22 0 2r22 1r22 0
y1r22 y2r22 0 2r22 1r22
0 y1r22 y2r22 0 2r22
NS filter ŽEW filter is the reflected version. 0 0 y1r22 y2r22 0
1r30 1r30 1r30 1r30 1r30
2r30 2r30 2r30 2r30 2r30
0 0 0 0 0
y2r30 y2r30 y2r30 y2r30 y2r30
y1r30 y1r30 y1r30 y1r30 y1r30
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a reasonable fit for the footwall of the faultblock, but a bad fit for the hanging wall. The bad fit could be explained by the fact that the geodetic displacement is partly co-seismic, whereas the theoretical profile represents the sum of co-seismic and post-seismic movements. Spadini and Podladchikov Ž1996. propose that the locations of maximum fiberstress are the best places for new faults in an evolving fault system. The fiberstresses are superimposed on the far field stresses, which cause the rift development. Therefore, the actual stress situation at a fault, which determines whether the fault surface is going to rupture, is determined by the farfield stress and the stress perturbation by the flexural motions. According to Eq. Ž2., the position of the first maximum is located at x s Žpr2.ra s 15 = 10 3r4 m. Therefore, the theoretical spacing between faults close to the Peel Boundary Fault zone is about 4 km, which is in accordance with the spacing depicted in Fig. 4.
References Ahorner, L., 1962. Untersuchungen zur Quartaren ¨ Bruchtektonik der Niederrheinischen Bucht. Eiszeitalter Ggw. 13, 24–105. Bisschops, J.H., 1973. Toelichtingen bij de geologische kaart van Nederland 1: 50.000 blad Eindhoven Oost Ž51O.. Rijks Geologische Dienst, Haarlem. Bisschops, J.H., Broertjes, J.P., Dobma, W., 1985. Toelichtingen bij de geologische kaart van Nederland 1: 50.000 blad Eindhoven West Ž51W.. Rijks Geologische Dienst, Haarlem. Bogaart, P.W., van Balen, R.T., 2000. Numerical modeling of the response of alluvial rivers to Quarternary climate change. Global Planet. Change 27, 147–163. Bless, M.J.M., Bosum, W., Bouckaert, J., Durbaum, H.J., Kockel, ¨ F., Paproth, E., Querfurth, H., van Rooyen, P., 1980. Geophysikalische Untersuchungen am ost-rand des Brabanter Massivs in Belgien, den Niederlanden un der Bundesrepublik Deutschland. Meded. Rijks Geol. Dienst. 32 Ž17., 313–343. Camelbeeck, T., Meghraoui, M., 1998. Geological and geophysical evidence for large palaeo-earthquakes with surface faulting in the Roer Graben Žnorthwest Europe.. Geophys. J. Int. 132, 347–362. Carter, K.E., Winter, C.L., 1995. Fractal nature and scaling of normal faults in the Espanola Basin, Rio Grande rift, New ˘ Mexico: implications for fault growth and brittle strain. J. Struct. Geol. 17 Ž6., 863–873. Crone, A.J., Machette, M.N., Bowman, J.R., 1997. Episodic nature of earthquake activity in stable continental regions revealed by paleoseismicity studies of Australian and North American Quaternary faults. Aust. J. Earth Sci. 44, 203–214.
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Geluk, M.C., 1990. The Cenozoic Roer Valley Graben, Southern Netherlands. Meded. Rijks Geol. Dienst. 44 Ž4., 66–72. Geluk, M.C., Duin, E.J., Dusar, M., Rijkers, R., Van den Berg, M.W., van Rooijen, P., 1994. Stratigraphy and tectonics of the Roer Valley Graben. Geol. Mijnbouw 73 Ž2–4., 129–141. Groenewoud, W., Lorenz, G.K., Brouwer, F.J.J., Molendijk, R.E., 1991. Geodetic determination of recent land subsidence in the Netherlands. In: Johnson, A.I. ŽEd.., Land Subsidence ŽProceedings of the 4th International Symposium on Land Subsidence, Houston, TX, 12–17 May 1991.. IAHS, pp. 463–471 Publ. No. 200. Hetenyi, M., 1946. Beams on Elastic Foundation. Univ. Michigan ´ Press, Ann Arbor, MI, p. 255. Kooi, H., de Vries, J.J., 1998. Land subsidence and hydrodynmic compaction of sedimentary basins. Hydrol. Earth Syst. Sci. 2 Ž2–3., 159–171. Paulissen, E., Vandenberghe, J., Gullentops, F., 1985. The Feldbiss fault in the Maas Valley bottom ŽLimburg, Belgium.. Geol. Mijnbouw 64, 79–87. Sesoren, A., 1976. Lineament analysis from ERTS ŽLandsat. ¨ images of the Netherlands. Geol. Mijnbouw 55 Ž1–2., 61–67. Spadini, G., Podladchikov, Y., 1996. Spacing of consecutive normal faulting in the lithosphere; a dynamic model for rift axis jumping ŽTyrrhenian Sea.. Earth Planet. Sci. Lett. 144 Ž1–2., 21–34. Van Balen, R.T., Podladchikov, Y.Y., Cloetingh, S.A.P.L., 1998. A new multilayerd model for intraplate stress-induced differential subsidence of faulted lithosfere, applied to rifted basins. Tectonics 17 Ž6., 938–954. Van Balen, R.T., Van Bergen, F., De Leeuw, C., Pagnier, H., Simmelink, H., Van Wees, J.D., Verweij, J.M., 2000a. Modelling the hydrocarbon generation and migration in the West Netherlands Basin, the Netherlands. Neth. J. Geosci. Geol. Mijnbouw 79, 29–44. Van Balen, R.T., Houtgast, R.F., van der Wateren, F.M., Vandenberghe, J., Bogaart, P.W., 2000b. Sediment budget and tectonic evolution of the Meuse catchment in the Ardennes and Roer Valley Rift System. Global Planet. Change 27, 113–129. Van den Berg, M.W., 1994. Neotectonics of the Roer Valley rift system. Style and rate of crustal deformation inferred from syn-tectonic sedimentation. Geol. Mijnbouw 73, 143–156. Van den Berg, M.W., 1996. Fluvial sequences of the Maas—a 10 Ma record of neotectonics and climate change at various time-scales, PhD thesis, Agricultural University, Wageningen. Van den Berg, M.W., Groenewoud, W., Lorenz, G.K., Lubbers, P.J., Brus, D.J., Kroonenberg, S.B., 1994. Patterns and velocities of recent crustal movements in the Dutch part of the Roer Valley rift system. Geol. Mijnbouw 73, 157–168. Vandenberghe, J., 1982. Geoelectric investigations of a fault system in Quaternary deposits. Geophys. Prospect. 30, 879– 897. Vandenberghe, J., 1990. Morpological effects of pleistocene faulting in unconsolidated sediments ŽCentral Graben, Netherlands.. Z. Geomorphol. 34 Ž1., 113–124. Van den Toorn, J.C., 1967. Toelichtingen bij de geologische kaart van Nederland 1: 50.000 blad Venlo West Ž52W.. Rijks Geologische Dienst, Haarlem.
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Proc. SEQS Cromer Symposium, Balkema, Rotterdam. pp. 145–172. Zagwijn, W.H., van Staalduinen, C.J., 1975. Toelichting. Toelichting bij Geologische Overzichtskaarten van Nederland-Haarlem: RGD, 197. Zagwijn, W.H., Beets, D.J., Van den Berg, M., van Montfrans, H.M., van Rooijen, P., 1985. Atlas van Nederland in 20 delen: deel 13 Geologie. Staats Uitgeverij’s-Gravehage. p. 23. Ziegler, P.A., 1992. European Cenozoic rift system. Tectonophysics 208, 91–111. Zijerveld, L., Stephenson, R., Cloetingh, S., Duin, E., Van den Berg, M.W., 1992. Subsidence analysis and modelling of the Roer Valley Graben ŽSE Netherlands.. Tectonophysics 208, 159–171. Zonneveld, J.I.S., 1949. Het Kwartair van het Peelgebied en de naaste omgeving. Meded. Geol. Sticht. Ser. C, VI Ž3., 223 pp.
Neotectonics of the Roer Valley Rift System, the Netherlands R.F. Houtgast, R.T. van Balen Faculty of Earthsciences, Vrije UniÕersiteit Amserdam, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands
Abstract The Roer Valley Rift System ŽRVRS. is located in the southern part of the Netherlands and adjacent parts of Gemany and Belgium. The last rifting episode of the RVRS started in the Late Oligocene and is still ongoing. The present-day seismic activity in the rift system is part of that last rifting episode. In this paper, the Quaternary tectonics of the RVRS are studied using the detailed stratigraphic record. Subsidence analyses show that three periods of subsidence can be discriminated during the Quaternary. A phase of rapid subsidence took place from the beginning of the Quaternary to the Upper Tiglian Ž; 1800 ka.. This was followed by a phase of slow subsidence lasting until the Late Quaternary Ž; 500 ka.. An acceleration in subsidence at the end of the Quaternary occurred in the central and northern parts of the RVRS Ži.e. the Roer 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 along these fault zones vary between 5 and 80 mmrka. Periods of high and low displacement rates along faults can be discriminated. The magnitude of the subsidence rate in the central part of the RVRS, which in theory is caused by a combination of processes like faulting, cooling of the lithosphere and isostasy, is within the range of the rate of displacement along the major fault 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 largest displacement rate shifts during the Quaternary, whereas the Peel Boundary Fault zone, which is narrow and has a straight structure, is more stable in this respect. The present-day fault displacement rates inferred by geodetic 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–stress interactions. The stratigraphic record has preserved average displacement rates. Flexural analyses shows that the pattern of geodetically determined displacements is in accordance with the fault spacing in the fault zone. The NW–SE directed fault system active during the Quaternary and the Tertiary is inherited from the late stage of the Variscan orogeny. This fault system was also dominantly active during the Mesozoic and Early Cenozoic evolution 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 Cenozoic. The fact that Paleozoic fault systems are reactivated during Quaternary and Tertiary indicates that these faults are fundamental weakness zones. q 2000 Elsevier Science B.V. All rights reserved. Keywords: neotectonics; quaternary; faults; subsidence; Roer Valley Graben; Peel Boundary fault zone; Feldbiss fault zone
E-mail address: [email protected] ŽR.F. Houtgast.. 0921-8181r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 8 1 8 1 Ž 0 1 . 0 0 0 6 3 - 7
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1. Introduction Ahorner Ž1962. proposed a pattern of earthquake distribution in northwest Europe and inferred a correlation between earthquake activity and Quaternary structures. Identification of active faults, structures that have an established record of activity in the Late Quaternary, is an important consideration for estimating the seismic hazard assessment for northwest Europe ŽCamelbeeck and Meghraoui, 1998.. In this paper, we present the Quaternary tectonics of the Roer Valley Rift System and revealed their spatial and temporal patterns. The Roer Valley Rift System ŽRVRS. is an active rift system, located in the southern part of the Netherlands, the northeastern part of Belgium and adjacent parts of Germany ŽFig. 1.. During Quater-
Fig. 1. The Cenozoic Western European Rift System Žafter Ziegler, 1992..
nary, the tectonic movements have affected the courses and the incision–aggradation behavior of river systems Žsee, for example, Bogaart and Van Balen, 2000-this volume; Van Balen et al., 2000b-this volume.. The faulting also influences the morphology: Fault scarps occur along the major fault zones and small streams are aligned to the faults. The seismicity in the area is concentrated at the major fault zones ŽCamelbeeck and Meghraoui, 1998.. For example, the moderate earthquake of Roermond is located along the northern boundary fault zone of the RVRS ŽVan Eck and Davenport, 1994.. In this paper, extensional tectonics of the RVRS are studied using the Quaternary stratigraphic record of the RVRS. The spatial variations in tectonic behavior are assessed, as well as the evolution in time of basin subsidence and faulting behavior. Large-scale subsidence is analyzed using the backstripping method. Fault movements are quantified using the detailed Quaternary stratigraphy. The exceptionally detailed Quaternary geological record of the RVRS provides a unique opportunity to study fault motions on a 100 ka time-scale. On a large scale, the RVRS system is part of a rift system in the southeastern part of the North Sea Basin, the Lower Rhine Embayment, which itself is part of a Cenozoic mega-rift system crossing western and central Europe ŽFig. 1; Ziegler, 1992.. The RVRS has a complex Mesozoic and Cenozoic tectonic history, comprising several extension and inversion phases. The current extension phase started during the Late Oligocene ŽGeluk et al., 1994.. The RVRS comprises the Campine Block in the south, the Roer Valley Graben in the center, and the Peel Horst and the Venlo Block in the northeast ŽFig. 2.. The Roer Valley Graben is separated from the adjoining blocks by the Feldbiss Fault zone in the south and the Peel Boundary Fault zone in the north, which are the most active fault zones during the Quaternary Že.g. Ahorner, 1962; Paulissen et al., 1985.. The central Roer Valley Graben has subsided approximately 1000 to 1200 m since the Late Oligocene, whereas the Peel Horst subsided only up to 200 m ŽGeluk et al., 1994.. In the first part of the paper, the subsidence behavior of different tectonic units within the RVRS is studied, using the borehole database of the Dutch geological survey ŽNITG TNOr National Geologi-
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Fig. 2. The Roer Valley Rift System tectonic units.
cal SurÕey .. Subsequently, the timing of fault activity and quantification of fault displacement during the Quaternary are assessed using the same database. Next, the influence of tectonic activity on the present-day morphology of the RVRS is analyzed using a high-resolution digital elevation model. Finally, the influence of tectonic activity of the RVRS on the evolution of the Maas river is discussed.
2. Quaternary tectonic evolution of the Roer Valley Rift System The Quaternary tectonic activity in the RVRS is part of a rifting episode which started in the Late
Oligocene. Therefore, the Quaternary subsidence is the result of active faulting and of thermal subsidence due to Miocene, Pliocene and Quaternary crustal thinning Že.g. Zijerveld et al., 1992.. The subsidence of the Roer Valley Graben created accommodation space for marine and fluvial deposits of the Maas and Rhine rivers and smaller, local Belgian rivers. The lithostratigraphic boundaries between these deposits are used for the tectonic analysis. During the first part of the Quaternary, lasting from Pretiglian to Early Eburonian ŽTable 1., the graben was filled with Maas and Rhine deposits of the Tegelen Formation in the eastern part of the graben; while in the western part of the rift, marine
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Table 1 Lithostratigraphy and chronostratigraphy in the RVRS ŽZagwijn and van Staalduinen, 1975; Bisschops et al., 1985; Zagwijn, 1989, 1996. Lithostratigraphic formation
Start of deposition Žka.
End of deposition Žka.
Nuenen group Veghel-A Sterksel Kedichem Tegelen Kiezelooliet ¨ Maassluis
Elsterian Ž350. Upper Cromerian Ž650. Menapian Ž1200. Lower Eburonian Ž1800. Lower Tiglian Ž2400. Lower Pretiglian Ž2900. Lower Pretiglian Ž2500.
Late Weichselian Ž15. Holsterian Ž240. Middle Cromerian Ž650. Lower Menapian Ž1200. Upper Tiglian Ž1800. Upper Pretiglian Ž2200. Upper Tiglian Ž1800.
deposits of the Maassluis Formation were laid down ŽTable 1; Bisschops, 1973.. The overlying Kedichem Formation of Eburonian to Menapian age ŽBisschops, 1973; Zagwijn, 1996. is composed of deposits from the Maas and local rivers. During that period, the Rhine was flowing to the north of the RVRS ŽVan den Berg, 1996.. The Sterksel Formation is deposited during the Menapian to Middle Cromerian by the Rhine river with the Maas as a tributary ŽZagwijn, 1996.. Toward the end of the Cromerian, the Rhine changed its course toward the east forced by tectonic movements further upstream ŽBisschops, 1973.. The Maas became the single large river filling the Roer Valley Graben. During this period, the Maas incised in the Sterksel Formation and subsequently deposited the Veghel-A Formation of Late Cromerian to Holsterian age Že.g. Zonneveld, 1949; Zagwijn, 1996.. After the deposition of the Veghel-A Formation, the course of the Maas shifted from the Roer Valley Graben to the Venlo Block ŽVan den Toorn, 1967; Bisschops, 1973.. After the Maas had left the Roer Valley Graben, deposits of local origin filled the graben ŽBisschops, 1973.. In cooperation with the NITG TNOrNational Geological SurÕey, 350 borings were selected along six regularly spaced profiles, perpendicular to the main border faults of the RVRS. The position of the lithostratigraphic boundaries were identified in these borings. In the analysis of tectonic movements, it is assumed that the lithostratigraphic boundaries have one single age, which is obtained from correlation of the lithostratigraphy to the published chronostratigraphy ŽTable 1; Zagwijn and van Staalduinen, 1975; Bisschops et al., 1985; Zagwijn, 1989, 1996.. Due to the gradual transitions the boundaries between important lithostratigraphic units are sometimes diffi-
cult to pinpoint in borehole records. Because of difficulties in dating, the ages of some stratigraphic units are uncertain; for the older stratigraphic units the uncertainty may be in the order of tens of ka. In addition, as the deposits interfinger stratigraphically, the ages applied in the backstripping method Žsee below. depended on the local stratigraphic development. 2.1. Uplift and subsidence rates As shown by Geluk et al. Ž1994., the subsidence patterns of two boreholes located in the Roer Valley Graben are characterized by an increase in subsidence rates with time. Both borehole records show a 25–30% increase in the average subsidence rate from the Neogene to the Quaternary. At the location of the Herten well ŽFig. 2., the central graben subsided on average with a rate of 60 mmrka during the Neogene and 80 mmrka during the Quaternary. Ten kilometers to the southwest, at the position of the Linne well, the graben subsided with an average rate of 40 mmrka during the Neogene. However, the latter rate is probably too high, as the ages of upper Tertiary lithostratigraphic boundaries are now interpreted to be older, in the order of 100 ka ŽM. Van den Berg, pers. commun., 1999.. The subsidence accelerated to 55 mmrka during the Quaternary. Both subsidence curves show hiatuses in the Early Quaternary ŽEburonian and Waalian. and in the Middle Quaternary ŽCromerian.. These hiatuses suggest periods of minor to no subsidence, or even possible periods of uplift. The extent of the hiatuses is unknown. In order to investigate the spatial variation in the subsidence characteristics of the Roer Valley Graben,
R.F. Houtgast, R.T. Õan Balen r Global and Planetary Change 27 (2000) 131–146
we have studied six additional wells, using the backstripping method. This method is valid in the terrestrial environment of the RVRS, because the upper surface of the RVRS was part of a large flat delta plain controlled by eustatic sea level and tectonic subsidence. Additionally, there is no indication that the basin was underfilled during the Quaternary. Therefore, change in sedimentation rate is related to eustacy or tectonics and not to sedimentological processes. In this analysis, it is assumed that compaction has a minor effect on the subsidence, due to the limited thickness of the Quaternary deposits and their predominately sandy character. The wells are located on three different tectonic units. Three wells are located in the central Roer Valley Graben Ž58A0087, 45D0046, 45G0059., two wells are situated on the Peel Horst Ž58A0058, 52A0146. and one well is positioned on the Campine Block Ž50H0074.. The general trend of the subsidence curves is characterized by three subsidence phases ŽFig. 3; Table 2.. The first phase is a period of relatively rapid subsidence from the beginning of the Quaternary Žstart of deposition of the Tegelen Formation, "2400 ka. until the Late Tiglian Ž"1800 ka.. The second phase is characterized by relatively slow subsidence, lasting until the end of the Cromerian Ž"350 ka.. All subsidence curves located in the
135
Roer Valley Graben and on the Peel Horst show an increased subsidence rate toward the end of the Quaternary during the third phase lasting until the Present. This acceleration is not visible in the subsidence curve on the Campine Block. The exact magnitude of the acceleration in subsidence should be regarded with some caution because of the error bar due to the limitation in the chronostratigraphic age control. The depositional age of the base of the uppermost lithological unit Žthe Nuenen Group. is uncertain, it could be of Elsterian age Ž"350 ka. or younger ŽJ. Schokker, pers. commun., 2000; Bisschops, 1973; Zagwijn, 1996.. Even if one would consider the oldest possible age for the base of the Nuenen Group, the approximately twofold increase in subsidence still has taken place. Although all curves show the same general trend, subtle differences between the three tectonic units exist. In Table 2, the results of the subsidence analyses have been used to calculate average subsidence rates of the three tectonic units. The average subsidence rate in the Roer Valley Graben is close to the rate determined by Geluk et al. Ž1994., 60–90 mmr ka throughout the Quaternary. The other two tectonic units, the Campine Block and the Peel Horst, subsided on average with a rate two or three times smaller. In detail the difference in subsidence rates
Fig. 3. Subsidence curves for six wells located on the Campine Block, in the Roer Valley Graben and on the Peel Horst. For location of the wells, see Fig. 2.
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Table 2 Average subsidence rates for three tectonic units during the Quaternary determined from the subsidence curves shown in Fig. 3 Tectonic unit
Campine Block Roer Valley Graben Peel Horst
Average subsidence rate Žmmrka. 2400–1800 ka
1800–650 ka
650–0 ka
80 180 130
10 50 17
7 70 20
between the Roer Valley Graben and the other two tectonic units is occasionally large. During the first subsidence phase, the differences are small. During the second phase, the Roer Valley Graben subsided on average three to five times faster than the neighboring tectonic units. The Roer Valley Graben subsided 4–10 times faster than the Campine Block during the third phase. A regional difference in subsidence behavior regarding the timing of the deceleration at the end of the first phase also exists ŽFig. 3., which is independent of the tectonic units. The three wells located in the northwest of the Roer Valley Rift System Ž50H0074, 45D0046 and 45G0059. seem to decelerate later during the first subsidence phase than the wells located in the southeastern part of the Roer Valley Rift System.
Quaternary average 27 88 46
ment, that in an area of about 400 km2 in the western Espanola ˘ Basin of the Rio Grande rift more than 17 Quaternary faults with displacements in excess of 4 m occur. This number is in the same order of magnitude as the number of faults with a displacement larger than 4 m found in the RVRS. Because this is a tectonically and depositionally similar setting, their research results gives an indication of how many smaller faults can be present in the RVRS. Based on the same power law relationship, it can be inferred that about 800 faults with a displacement of 0.01–1 m can occur in the RVRS.
2.2. Fault pattern The Quaternary geological record enables the detection of active faults. In this paper, a distinction is made between faults and fault zones: a fault zone is a bundle of faults and the total displacement along a fault zone is the sum of displacements along individual faults which make up the fault zone. The borehole data set of the Geological Survey together with the geological maps ŽVan den Toorn, 1967; Bisschops et al., 1985. and geoelectrical results ŽVandenberghe, 1982. provide an opportunity to locate fault zones with an accuracy of about 1 km at best ŽFig. 4.. Due to the uncertainty in recognizing clear boundaries between lithostratigraphic units, only faults with a large displacement Ž) 4 m. are mapped. Probably, many more faults with a smaller displacement are present in the area. For example, Carter and Winter Ž1995. calculated on the base of a power law relationship between the number of faults and displace-
Fig. 4. Quaternary faults in the Roer Valley Rift System. The locations of fault Žzone.s outside the study area are from Ahorner Ž1962., Vandenberghe Ž1982. and Geluk et al. Ž1994..
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The pattern of the faults, which were active during the Quaternary, correlates with the fault pattern in the base of Tertiary deposits ŽZagwijn et al., 1985.. The larger Quaternary fault zones have the same NW–SE orientation as the larger Tertiary fault zones. The most active fault zones during the Tertiary, the Feldbiss and Peel Boundary Fault zones, are also the most active zones during the Quaternary. The NW–SE-oriented faults date back as far as the late stage of the Variscan orogeny ŽVan Wijhe, 1987; Van Balen et al., 2000a.. They have since then been reactivated continuously during the Mesozoic and Cenozoic evolution of the RVRS and nearby basins. They represent, therefore, fundamental lithospheric weakness zones. The most active Quaternary fault zones, the Peel Boundary and Feldbiss Fault zones, both border the Roer Valley Graben. The Peel Boundary Fault zone is a confined zone, where faults are closely spaced ŽFig. 4.. The faults of the Feldbiss Fault zone are locally more widely spaced. Both fault zones were active during the whole Quaternary. 2.3. Morphological expression of faults The main geomorphological expression of tectonic activity in the southern part of the Netherlands
137
is the topography: the Peel Horst is a topographic high and the Roer Valley Graben and Venlo Block are topographic lows ŽFig. 5.. The additional geomorphological expression of tectonic activity in the southern part of the Netherlands are lineaments, which represent terrain steps, river courses, etc. Ž1976. analyzed Landsat images for lineaSesoren ¨ ments and found many lineaments with two main orientations, a dominating NW–SE orientation, and a NE–SW orientation. The NW–SE lineaments show a strong correlation with known Tertiary and Quaternary fault patterns. A dominating NW–SE orientation and a NE–SW orientation were also found by Van den Berg et al. Ž1994.; their lineaments are based on abrupt breaks in slope. In our research a digital terrain model ŽMeetkundige dienst, Rijkswaterstaat., which consists of more than 75 data pointsrkm2 regularly spaced, is analyzed for lineaments. Contrary to previous studies, a mathematical approach was used to detect the lineaments. For a detailed description of the method used, see Appendix A. The result of our lineament analyses is shown in Fig. 6. The NW–SE and NE–SW orientations found in previous studies are also present in our results. The results of the three different methods show that the NW–SE orientation of lineaments is dominant in
Fig. 5. Topographic profile across the Roer Valley Graben combined with a line drawing of the southern section of the deep seismic line 8601 ŽGeluk et al., 1994.. See Fig. 2 for location of both profiles.
138
R.F. Houtgast, R.T. Õan Balen r Global and Planetary Change 27 (2000) 131–146
stress field, we expect these faults to be strike–slip. The reason why these faults have not been recognized in the subsurface geology is related to the orientation of 2D seismic lines in that area which are approximately parallel to the NE–SW direction ŽC. De Leeuw, pers. commun., 2000.. Additionally, the displacements associated with NE–SW faults can be too small to be detected on seismic sections. The origin of the NE–SW-oriented faults is probably a bit older than the NW–SE-trending faults, having a likely Caledonian age ŽBless et al., 1980..
Fig. 6. Lineaments found in this research in the southern part of the Netherlands.
the southern part of the Netherlands. The results also show that NE–SW-oriented faults have been active during the Quaternary, despite the fact that they have not been recognized as active faults during the Tertiary ŽZagwijn et al., 1985.. Under the present-day
2.3.1. LeÕelling results The Peel Boundary Fault zone has also been the subject of geodetic levelling studies ŽGroenewoud et al., 1991.. The results of these studies show that the vertical displacements decay exponentially away from the fault zone ŽFig. 7. more or less in accordance with theoretical faulting induced flexural motions ŽVan Balen et al., 1998; Appendix B.. As shown in Appendix B, the spacing of faults within the Peel Boundary Fault zone could be controlled by flexural stresses induced by the faulting. The rate of fault displacement found by the levelling is two orders of magnitude larger than the displacement rate documented in the geological record, see below. An explanation for this discrepancy could be 3D fault– stress interactions inducing fault motions, which are highly variable in time and space on a 10 to 100
Fig. 7. Displacement profile across the Peel Boundary Fault determined by geodetic measurements Žafter Groenewoud et al., 1991..
R.F. Houtgast, R.T. Õan Balen r Global and Planetary Change 27 (2000) 131–146
139
1967; Bisschops et al., 1985.: The thicknesses of sedimentary units vary over the different tectonic units, and sedimentary units are displaced by faults. To calculate the displacement rate of a fault zone, two Žsets of. boreholes are used, located on both sides of a fault zone ŽFig. 8.. The displacement rate during deposition is calculated by dividing the difference between the displacement at the base and the top of the lithostratigraphical unit by the age difference of the base and top of the unit. The displacement rate after deposition is calculated by dividing the displacement at the top of the lithostratigraphic unit by its age. This approach assumes a continuous sedimentary history. In most cases, this is not exactly true. Most of the contacts between sedimentary units are erosional and even within a sedimentary unit erosional surfaces are likely to occur, because these sedimentary units are deposited by fluvial systems, which incise, aggradate and migrate. The hiatus within one sedimentary unit does not affect the outcome of our analyses, because we only consider the base and top of the sedimentary units. The
Fig. 8. The method used for calculating the average displacement rates for faults from well data.
years time-scale, including cessation and possibly a reversal of motions. Alternatively, fault activity maybe also be spasmodic in nature ŽCrone et al., 1997.. On a geological time-scale, these motions are largely averaged, although some variability is still preserved as shown by the fault displacements in time and space presented below. 2.4. Fault displacements The Quaternary deposits in the RVRS have a maximum thickness of about 200 m in the center of the Roer Valley Graben. The tectonic influence on the deposition of these sediments can be observed in several published geological profiles ŽVan den Toorn,
Fig. 9. Average displacement rates for several faults in the Roer Valley Rift System Rijen Fault during the Quaternary Žafter Vandenberghe, 1982. Vessem Fault, side branch of the Feldbiss Fault Žafter Bisschops et al., 1985. Viersen Fault Žafter Ahorner, 1962.. Location of faults indicated in Fig. 2.
140
R.F. Houtgast, R.T. Õan Balen r Global and Planetary Change 27 (2000) 131–146
calculated displacement rate is an average for the time period during which the deposition of the sedimentary unitŽs. took place. The hiatus between sedimentary units does influence our analyses. For example, erosion of the top of a sedimentary unit leads to an underestimation of the age of the top of the sedimentary unit Žthe age assigned to the top is too young.. Due to possible erosion of the top of the sedimentary unit, the displacement rates are minimum values. Several profiles across fault zones in the RVRS have been published ŽAhorner, 1962; Van den Toorn, 1967; Verbraeck, 1970, 1984; Bisschops, 1973; Vandenberghe, 1982, 1990; Bisschops et al., 1985; Paulissen et al., 1985.. Using the method outlined before, the average displacement rates for several time periods are calculated from the faults described in the profiles ŽFig. 10.. The Rijen-Beringen and the Tegelen Fault were active during the Early and Middle Quaternary and later activity is not documented in the sedimentary record. The Rijen-Beringen Fault divides the Campine Block into a western and eastern part. Detailed morphological and geoelectric investigation of the Rijen-Beringen Fault zone revealed a complex fault zone, which extends
into the Belgian part of the RVRS ŽVandenberghe, 1982.. Most faults of that fault zone were active during the Middle Pleistocene ŽVandenberghe, 1990.. The Tegelen Fault, which separates the Peel Block and the Venlo Block, had its main period of activity during the Early Quaternary ŽBisschops et al., 1985.. The rates vary from 5 up to 80 mmrka. These rates are one order of magnitude less than the Quaternary fault displacement rates found by Geluk et al. Ž1994., i.e. 800 mmrka. However, according to M. Geluk Žpers. commun., 2000. this is a mistake in their paper, it should be 80 mmrka. A fault displacement rate of 80 mmrka leads to a thickness of Quaternary deposits of about 160 m, which is in accordance with the observed maximum thickness of 200 m of the Quaternary deposits. This implies that the subsidence of the Roer Valley Graben is mainly fault controlled. 2.5. Lateral Õariations in aÕerage displacement rates along fault zones In Fig. 9, displacement rates of several faults are compared. Fault zones, which extend over tens of kilometers, do not behave in a uniform way along
Fig. 10. Lateral variations in average displacement rates along the Rurrand Fault ŽGermany. during the Quaternary Žafter Ahorner, 1962..
R.F. Houtgast, R.T. Õan Balen r Global and Planetary Change 27 (2000) 131–146
the whole fault zone. Ahorner Ž1962. documented the displacements of the Rurrand Fault, the southeastward extension of the Peel Boundary Fault ŽFig. 10., using terraces of the Rhine river. A variation through time and a spatial trend could be distinguished. At every location along the Rurrand Fault, the lowest average displacement rate is found for the
141
Eburonian–Cromerian period and the highest for the Cromerian–Present. The spatial trend shows a gradual increase in average displacement rates in a northwest direction along the Rurrand Fault. Using the borehole database, the lateral variations in average displacement rates along the two main fault zones bordering the Roer Valley Graben, the
Fig. 11. Lateral variations in average displacement rates along the Peel Boundary Fault zone and the Feldbiss Fault zone during the Quaternary.
142
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Peel Boundary Fault zone and the Feldbiss Fault zone are analyzed. The length of both fault segments is approximately 50 km ŽFig. 2.. The results are presented in Fig. 11. Along the Feldbiss Fault zone, the position of the largest fault displacements shifts from the southeast ŽDf and Ef. to the northwest ŽAf, Bf, Cf and Df. and back again to the southeast. Along the Peel Boundary Fault zone, the pattern is less clear. The fault activity seems to be fixed in space. The largest cumulative fault displacements along the Feldbiss Fault zone are located in the center of the segment Žlocation points Cf and Df.. The largest cumulative fault displacement along the Peel Boundary Fault zone during the Quaternary is concentrated at points Bp. and Cp and further to the southeast at point Fp. In between these locations of high displacement rates along the Peel Boundary Fault zone, a segment of lower displacement rates ŽDP and Ep.. is located. The base Miocene depth map ŽGeluk et al., 1994. shows two depocentres ŽFig. 2.. The larger of the two depocentres is situated in the northwestern part of the graben, the smaller depocentre is located in the southeastern part. The position of the largest displacement rates along the Feldbiss Fault zone is adjacent to the largest depocentre ŽCf and Df.. The position along the Peel Boundary Fault zone of the largest displacement rates ŽBp, Cp and Fp. also coincides with the position of the two depocentres in the Roer Valley Graben. The pattern in fault displacements along these two border faults indicates that the Roer Valley Graben is not one structural unit, but is divided in two or more subgrabens.
3. Discussion The pattern of active faults during the Quaternary and of the present-day active faults inferred from lineament analyses and the morphology is dominated by NW–SE-oriented faults. These faults originate from the late stage of the Variscan orogeny. They have been reactivated continuously during the Mesozoic and Cenozoic tectonic evolution ŽVan Wijhe, 1987; Van Balen et al., 2000a.. The other important active set of faults has NE–SW to N–S directions. These faults are probably from the Caledonian orogeny Ži.e. Bless et al., 1980.. These faults have
apparently not been reactivated during the Mesozoic and early Cenozoic evolution of the RVRS. The reason why the NE–SW faults have only been reactivated recently could be explained by the lack of favorable maximum horizontal stress orientations required to reactivate them during the Mesozoic and Cenozoic. Both fault sets are present in the Paleozoic basement to the south of the RVRS ŽLondon–Brabant Massif and the Rhenish Massif.. The distribution of displacements along the fault zones bordering the Roer Valley Graben is related to the position of the Quaternary depocentres ŽFig. 2.. Both the base Tertiary map ŽZagwijn et al., 1985. and the base Miocene map ŽGeluk et al., 1994. show depocentres at the same locations, bordered by those segments of the fault zones, which have the largest displacement rates. These depocentres also coincide with the depocentres observed in the Nuenen Group ŽJ. Schokker, pers. commun., 2000.. Therefore, a causal relationship between the Tertiary and the Quaternary depocentres could exist. An obvious explanation is that the same faults are active during Quaternary and Tertiary. However, the origin of the Quaternary depocentres could also be delayed compaction of the Tertiary clays in the Roer Valley Graben induced by loading by Quaternary sediments. Kooi and De Vries Ž1998. have shown that the response time of compaction-controlled subsidence is in the order of 10 5 –10 7 years. Therefore, the subsidence due to compaction of Lower Tertiary clays in the Roer Valley Graben could still be an ongoing process, with maxima at the Tertiary depocentres. In that case, the fault activity could be the response to subsidence due to compaction of Lower Tertiary strata. Depending on the assumed permeability of Tertiary clays, the subsidence rate due to compaction of Tertiary clays in the coastal zone of the Netherlands is calculated to be 80 to 120 mmrka ŽKooi and De Vries, 1998.. Although these rates will be slightly smaller in the RVRS due to the smaller thickness of the clayey strata, these rates are about the same as the Quaternary subsidence rates determined for the RVRS. However, compaction is a response to sediment loading. Therefore, when accommodation space is created by extensional faulting, an extra load on the Lower Tertiary clays will be caused by sediments filling in the accommodation space. Therefore, the delayed compaction can have
R.F. Houtgast, R.T. Õan Balen r Global and Planetary Change 27 (2000) 131–146
contributed to the general subsidence by amplifying the fault controlled subsidence. The spatial distribution of the Tertiary clays in the Roer Valley Graben is saucer-shaped ŽGeluk, 1990.; therefore, we expect a similar shape in subsidence patterns. Further research is required to elucidate the exact contribution of compaction of Lower Tertiary clay to the Quaternary subsidence of the RVRS. The position of the largest displacement rates along the Peel Boundary Fault zone remained fixed throughout time. Along the Feldbiss Fault zone, the position of the largest displacement rates shifts from the southeast to the northwest and back again to the southeast during the Quaternary. In this respect, the Peel Boundary Fault zone seems more stable than the Feldbiss Fault zone. The origin of this relative stability could be the different structure of both fault zones ŽFig. 4.. The Feldbiss Fault zone is wider and has a more staggered pattern compared to the Peel Boundary Fault zone, which is more confined in space and has a straight pattern. Because of the difference in structure, the response of the fault zones to the same change in stress level is probably different. The tectonic control on the Maas river is evident in its morpho-sedimentological expression. The relative uplift of the Campine Block forced the Maas river to incise and form a flight of river terraces. In the subsiding Roer Valley Graben accommodation space was created for the Maas river to deposit a thick stack of sedimentary deposits ŽVan den Berg, 1996.. Subsidence analysis of several wells in the Roer Valley Graben revealed a period of no subsidence or erosion during the Middle Quaternary. From the paleogeographic and spatial distribution of the fluvial deposits in the Roer Valley Graben, Late Middle Pleistocene uplift can be inferred. Deep incision of the Veghel-A Formation in the Sterksel Formation can be explaned by an episode of tectonic uplift, which is also documented in the Ardennes ŽVan Balen et al., 2000b-this volume.. The same uplift is also responsible for the northward diversion of the Rhine river and the shift of the Maas river from the Roer Valley Graben to the Venlo Block. The resulting northeastward tilting of the RVRS forced the Maas river onto the Peel Horst and further onto the Venlo Block ŽBisschops, 1973.. At present, the inci-
143
sion–aggradation pattern of the Maas river reveals a tectonic control: the Maas river incises on the Campine Block, aggradates in the Roer Valley Graben and incises again on the Peel Horst Žsee also Van Balen et al., 2000b-this volume..
4. Conclusions Subsidence analyses revealed three phases of subsidence during the Quaternary. A phase of rapid subsidence from the beginning of the Quaternary to the Upper Tiglian Ž; 2200 ka., followed by a second phase of slow subsidence until approximately 500 ka. An acceleration in subsidence toward the end of the Quaternary is recognized in the Roer Valley Graben and the Peel Horst. The relative uplift rate for the Campine Block found by Van den Berg Ž1994. Ž60 mmrka; 1994., based on the Maas incision record, matches the relative subsidence rate of 61 mmrka of the Roer Valley Graben with respect to the Campine Block. This could imply that the uplift of the Campine Block inferred by Van den Berg is an uplift relative to the graben. The most active fault zones in the RVRS during the Quaternary are the Peel Boundary Fault zone and the Feldbiss Fault zone, which border the subsiding Roer Valley Graben. Average fault displacements during the Quaternary along these fault zones vary from 5 to 80 mmrka. A fault displacement rate of 80 mmrka, combined with the observed maximum thickness of about 200 m of the Quaternary deposits, implies that the subsidence of the Roer Valley Graben is mainly fault controlled. The displacement rates vary laterally along fault zones and the position of the largest displacement rates coincide with the position of Miocene–Present depocentres. The structure of the fault zones influences their tectonic behavior: The wider and more staggered Feldbiss Fault zone has a more variable spatial and temporal pattern in displacement rates; whereas the more confined Peel Boundary Fault zone is more stable. The pattern of active faults during the Quaternary and of the present-day active faults is dominated by NW–SE-oriented faults. The other important active set of faults has NE–SW to N–S directions. As both
R.F. Houtgast, R.T. Õan Balen r Global and Planetary Change 27 (2000) 131–146
144
faults sets are active at present, they must both represent fundamental lithospheric weakness zones. The reason why the NE–SW faults, recognized in the morphology, have only been reactivated recently could be explained by the lack of favorable maximum horizontal stress orientations required to reactivate them during the Mesozoic and Cenozoic.
Acknowledgements The research presented in this paper is a contribution to the Netherlands Environmental Earth System Initiative ŽNEESDI. programme, funded by the Netherlands Organization for Scientific Research ŽNWO.. We thank H. Pagnier, W.Westerhoff, P. Kiden and W. Dobma of the NITG TNO for their cooperation; Prof. J. Vandenberghe, F.W. Van der Wateren and P.W. Bogaart for their discussions and constructive criticism. We thank R. Pelzing ŽGeologisches Landesamt Nordrhein-Westfalen., K. Vanneste ŽRoyal Observatory Belgium. and M.W. Van den Berg ŽNITG-TNO. for their constructive and valuable comments on the manuscript.
Appendix A. Filter techniques, GIS conversions and criteria First, a regularly spaced DTM was made of all the available data points, using the kriging interpolation method. In this exercise, four filters ŽNWSE, NESW, NS, EW. were used, which enhance the terrain steps in a certain direction and suppress the areas with no or minor terrain steps in that direction, see Table 3. Four maps were created with terrain steps Žboth negative and positive, depending on which direction the higher area lies. and these were converted to
maps with only positive terrain steps. These were then converted to a classified map. The end result were four maps with classified terrain steps visible in a certain direction. Several lineaments were detected. To distinguish the height jumps of possible tectonic origin from others Žriver dunes, aeolian dunes., the following criteria have been applied: Ž1. A length of at least 5 km Žuninterrupted or shorter segments clearly in line with each other.; Ž2. No correlation with other geomorphological features like aeolian dunes and river dunes visible on various geomorphological and topographical maps. It is assumed that the appearance of these features has no correlation with faults.
Appendix B From the general solution of the elastic thin-plate flexure equation ŽHetenyi, 1946., the flexural profile ´ induced by faulting can be derived ŽSpadini and Podladchikov, 1996.. The deflection is given by: w s ucos Ž a x . eŽy a x . ,
Ž 1.
where w s deflection, u s fault displacement, a s flexural parameter and x s distance. The fiberstress resulting from the deflection of the thin-plate is given by the second derivative of Eq. Ž1.: wY s 2 a 2 usin Ž a x . eŽy a x . .
Ž 2.
In the deflection profile given by the geodetic measurements of Groenewoud et al. Ž1991; Fig. 8., the horizontal distance between the two positive deflection maxima is about 15 km. Therefore:
a s 2pr Ž 15 = 10 3 . . In Fig. 8, the theoretical flexural profile is compared to the measured profile. The comparison shows
Table 3 Filters used NWSE filter ŽNESW filter is the reflected version. 0 2r22 1r22 0 0
y2r22 0 2r22 1r22 0
y1r22 y2r22 0 2r22 1r22
0 y1r22 y2r22 0 2r22
NS filter ŽEW filter is the reflected version. 0 0 y1r22 y2r22 0
1r30 1r30 1r30 1r30 1r30
2r30 2r30 2r30 2r30 2r30
0 0 0 0 0
y2r30 y2r30 y2r30 y2r30 y2r30
y1r30 y1r30 y1r30 y1r30 y1r30
R.F. Houtgast, R.T. Õan Balen r Global and Planetary Change 27 (2000) 131–146
a reasonable fit for the footwall of the faultblock, but a bad fit for the hanging wall. The bad fit could be explained by the fact that the geodetic displacement is partly co-seismic, whereas the theoretical profile represents the sum of co-seismic and post-seismic movements. Spadini and Podladchikov Ž1996. propose that the locations of maximum fiberstress are the best places for new faults in an evolving fault system. The fiberstresses are superimposed on the far field stresses, which cause the rift development. Therefore, the actual stress situation at a fault, which determines whether the fault surface is going to rupture, is determined by the farfield stress and the stress perturbation by the flexural motions. According to Eq. Ž2., the position of the first maximum is located at x s Žpr2.ra s 15 = 10 3r4 m. Therefore, the theoretical spacing between faults close to the Peel Boundary Fault zone is about 4 km, which is in accordance with the spacing depicted in Fig. 4.
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