3.2.2. The Rhine Graben system
|Fig. 39. Seismicity in the Upper Rhine Graben area: Instrumental seismicity ML 3–5 (1952 – 2002) and historical events ML 5–6.2 (859 – 1948). Data source: Leydecker (2003).|
The Rhine Graben system forms an integral part of ECRIS, development of which commenced in the late Middle Eocene in response to the build-up of collision-related compressional stresses in the foreland of the Alps and Pyrenees (Ziegler and Dèzes, 2007). At the transition from the Oligocene to the Miocene, the stress field controlling the evolution of ECRIS changed owing to the consolidation of the Pyrenees (Dèzes et al., 2004). Under the present N- to NW-directed compressional stress regime of the European Platform (Müller et al., 1997) the Rhine Graben system is tectonically active (Fig. 39) and the locus of increased seismic hazards, as evidenced by the great Basel ML 6.2 earthquake of October 18th, 1356.
In the area of the Rhine Graben system, the trajectories of the Miocene and present stress field are very similar (Schumacher, 2002). Yet, the stress level apparently increased between 3 and 2.5 My (Dèzes et al., 2004). This is compatible with a subsidence acceleration of the Roer Valley Graben around 2.5 My (Zijerveld et al., 1992; Geluk et al., 1994; Michon et al., 2003). Moreover, the paleo-Aare river that had flowed from 4.2 My onward westwards along the thrust front of the Jura Mountains into the Bresse Graben (Sundgau gravels) and drained into the Mediterranean Sea, was deflected around 2.9 Ma into the Upper Rhine Graben, thus draining into the North Sea (Müller et al., 2002; Giamboni et al., 2004). This may be attributed to a slow-down of up-warping of the Vosges–Black Forest Arch, a lithospheric fold that was uplifted from mid-Burdigalian (18 My) times onward, and the related resumption of tensional subsidence of the southern parts of the Upper Rhine Graben (Dèzes et al., 2004; Ziegler and Dèzes, 2007). Geodetic data show for the Black Forest a pattern of slow uplift of horst and slow subsidence of graben structures at rates rarely exceeding 0.25 mm/yr (Müller et al., 2002). In the area of the Rhenish Massif, volcanic activity shifted during the Pliocene and Quaternary towards the Eifel area (Lippolt, 1983), for which geomorphologic data indicate from 0.8 My onward accelerating uplift rates (Van Balen et al., 2000; Meyer and Stets, 2002) that at present attain rates of up to 1.2 mm/yr (Mälzer et al., 1983).
High-resolution reflection-seismic data, recorded on the river Rhine, and back-stripped well data indicate that the northern parts of the Upper Rhine Graben subsided continuously during Miocene to Quaternary times with some faults extending upward through Quaternary deposits. A minor, base-Quaternary erosional unconformity, evident in the northernmost parts of the Upper Rhine Graben, presumably developed in conjunction with uplift of the Rhenish Massif. This unconformity disappears southward towards the Heidelberg depocentre. Further southward, late Miocene and Pliocene fluvial and lacustrine sediments progressively overstep the intra-Burdigalian unconformity that had developed in conjunction with the doming of the Vosges-Black Forest arch. In the southern parts of the Upper Rhine Graben, where sedimentation resumed only during the late Pliocene and Quaternary, numerous syn-sedimentary extensional faults and local positive flower-structures (Strasbourg transfer zone) were active during its Plio-Quaternary subsidence.
In the Roer Valley Graben, Alpine detrital components occur for the first time at the Plio-Quaternary transition (2.6 Ma; Boenigk, 2002; Heumann and Litt, 2002), whilst the timing of the first occurrence of Alpine components in the Upper Rhine Graben is still poorly constrained and may range between 2.9-2.6 Ma. Correspondingly, it is uncertain whether during the Late Pliocens (2.9-2.6 Ma) the sedimentary load of the river Aare was effectively trapped in the Upper Rhine Graben. During this time, sediment supply to the Upper Rhine Graben may have been in balance with the development of accommodation space in response to its extensional subsidence and tectonic controls on its erosional base level (uplift of the Rhenish Massif). During the Late Pliocene, the Upper Rhine Graben was presumably drained by a northward flowing, low-energy river (Bingen–Koblenz Rhine) that linked up with the higher energy Moselle River that crossed the Rhenish Massif and debouched into the Roer Valley Graben where the Kieseloolite sands and gravels were deposited (Brunnacker and Boenigk, 1983; Klett. et al., 2002; Sissingh, 2003). With the end-Pliocene capture of the Alpine Rhine by the Aare drainage system, sediment supply to the Upper Rhine Graben apparently exceeded its subsidence rate and the energy and sediment load of the Bingen–Koblenz Rhine increased, thus facilitating the transport of Alpine components across the Rhenish Massif into the Roer Valley Graben. During the Quaternary, the erosional base level in the continuously subsiding Upper Rhine Graben was controlled by the balance between the uplift-rate of the Rhenish Massif and the incision rate of the river Rhine. Presently the erosional base level of the Upper Rhine Graben is located 80 m above MSL at Bingen where the Rhine canyon starts to cut across the Rhenish Massif.
In the southern parts of the Upper Rhine Graben, Plio-Quaternary tectonic activity is documented by folding of the Pliocene Sundgau gravels along the Jura Mountains thrust front (Giamboni et al., 2004), by faults extending through Quaternary deposits of the graben fill, and by the seismicity of the area. In this context it is noteworthy that earthquakes occur almost down to the Moho but are absent below it. Earthquake focal mechanisms indicate deformation of the upper crust by strike-slip to reverse faulting whilst the lower crust is subjected to extension (Plenefisch and Bonjer, 1997; Deichmann et al., 2000). Transpressional deformation of the upper crust can be attributed to collision-related stresses that are transmitted from the Alps above an incipient mid-crustal detachment level. By contrast, lower crustal extension may be related to folding of the mantle-lithosphere, controlling uplift of the Vosges-Black Forest Arch (Dèzes et al., 2004). The effects of Cenozoic rifting are still evident in the lower crust, as visualized by relative P-wave velocity images (Lopes Cardozo and Granet, 2003; Lopes Cardozo et al., 2005; Lopes Cardozo and Granet, 2005).
Moderate Pliocene and Quaternary extension across the Bresse and Upper Rhine Grabens presumably gave rise to sinistral movements along the seismically still active Burgundy transfer zone that links them. Geodetic data indicate horizontal displacement rates across the Upper Rhine Graben of 0.8 mm/yr (Rozsa et al., 2005) and for the French Jura Mountains shortening rates of 1 mm/y (Walpersdorf et al., 2006) to perhaps as much as 3 mm/y (Jouanne et al., 1995). From about 4 Ma onward, compressional deformation of the Jura Mountains was no longer exclusively thin skinned, but involved also the basement, as indicated by intra-crustal earthquakes (Roure et al., 1994; Becker, 2000).
In the framework of the EUCOR-URGENT (Upper Rhine Graben: Evolution and Neotectonics) and TOPO-EUROPE projects, ongoing studies address (i) management of water resources hosted in the Pliocene and Quaternary aquifers of the Upper Rhine Graben that are endangered by pollution owing to intensive agricultural activities (EU-INTERREG III Project MoNit) and (ii) earthquake microzonation of the greater Basel area, the city of Mulhouse, and the Fessenheim nuclear power plant (EU-INTERREG III Project Microzonation). Regarding the evolution of the Alps-Rhine-North Sea source-sink system, the interrelation between the Quaternary uplift of the Rhenish Massif and sediment accumulation in the Upper Rhine Graben is addressed on the basis of geomorphologic studies and high-resolution river seismic data. In this context, a 400 m deep core-hole is currently being drilled in the Quaternary Heidelberg depocentre, with the objective to analyse the Quaternary climate record and its repercussions on sediment transport. Fission-track studies, addressing the uplift and denudation history of the Vosges-Black Forest Arch and its thermal regime, continue to be pursued and will be supported by river-gradient analyses. The neotectonics of the Rhine-Bresse Transfer Zone and their effect on the evolution of the drainage systems is currently being analyzed with special attention on the reactivation of Permo-Carboniferous crustal weakness zone and related seismotectonics. GPS measuring campaigns, involving stations covering the entire Rhine rift zone, are repeated at intervals.
TOPO-EUROPE plans to model the Rhine catchments from source (Alps) to sink (Atlantic Ocean, North Sea Basin), as a function of climate and tectonics. Important parameters to be addressed are the sediment production, transfer and storage rates. Understanding the time lags between sediment production events caused by for example climate changes, tectonic events, or river captures, and the resulting sedimentation events in the basins are of prime importance for interpreting the stratigraphic record. Secondly, the relative importance of (temporal) sediment storage in glacial and rifted basins along the course of the Rhine (Upper and Lower Rhine Graben) is at present unknown. Important constraints for this study have been put forward by cosmogenic isotope studies and morphological research.
Active Tectonics in the Upper Rhine Graben (URG)
Rifting of the URG started in the late Middle Eocene, approximately contemporaneously with an important phase of the Alpine orogeny. Changes in the stress regime during the evolution of the graben have resulted in different subsidence and uplift phases (e.g. Illies, 1975; Ziegler, 1992; Sissingh, 1998; Schumacher, 2002; Dèzes et al., 2004). The large thickness variations of Pliocene and Quaternary sediments in the graben imply syn-depositional tectonic movements and suggest an average subsidence rate for the Quaternary of 0.1 – 0.2 mm/y. Precision levelling across major faults in the northern URG shows contemporary movements of 0.4 – 1 mm/y. The most obvious topographic feature of the graben is the significant height difference between the graben shoulders and the alluvial plain of the river Rhine (up to ~ 1,000 m), corresponding to a clear morphological signature of the border faults. This suggests that the latter were tectonically active during the Quaternary, as evidenced also by high-resolution seismic lines recorded on the river Rhine and its tributaries.
|Fig. 40. Location of trench studies in the Upper Rhine Graben along its Western Border Fault (WBF). Right panel: shaded relief map, surface trace of the WBF (dashed white line) as mapped by morphology (after Peters et al., 2005).|
|Fig. 41. a) Photograph of Trench 1; b) Interpretation of Trench 1; c) Interpretation of Trench 2. The fault zone has a basal vertical displacement of 0.7 m and at the top of unit 1B a net displacement of 0.4 m. Unit 2B is offset by a minimum of 0.5 m. Due to lateral variations, the red marker horizon cannot be correlated across the main fault and its strands (fault core, thick black lines). For location see Fig. 40 (modified after Peters et al., 2005).|
Although several authors have addressed the recent tectonics of the URG, Quaternary tectonic activity has only locally been documented in its northern part where trenches were opened across its western border fault (WBF) to investigate evidence of surface deformation in young sediments (Peters et al., 2005). The investigated segment of the WBF is associated with a 20 km long and 50-100m high linear scarp (Fig. 40). Integration of shallow geophysics and paleo-seismological and structural analyses of trench walls permitted to identify and characterize near-surface deformation structures along the WBF, at the base of the southern end of the scarp. The results of 3 trenches point to extensional faulting producing a consistent, conjugate set of 015°-striking faults, paralleling the WBF, with maximum vertical displacements in the order of 0.5 m (Fig. 41). Thermoluminescence dating of the deformed sediments shows that the paleoseismic displacement occurred between 19 and 8 ka and may have been caused by a single seismic event with a moment magnitude of 6.5. It is important to note that creep movements cannot be entirely excluded. Reconstruction of the sequence of events at the trench site speaks for local-scale interplay between tectonic activity on the WBF and fluvial erosional processes. This suggests a mixed origin of the 20 km long WBF scarp, involving regional uplift, localized tectonic activity on the WBF and fluvial dynamics of the River Rhine.
Analyses of river terraces showed that the northern part of the URG is affected by the uplift of the Rhenish Massif, although the timing of uplift may slightly differ. This uplift also affected stream gradients and valley cross-sections (Peters and Van Balen, 2007b). The present-day morphology of the drainage system shows clear-cut evidence for active tectonic control, with the possibility of locating individual faults by terrace analysis (Peters and Van Balen, 2007a).
TOPO-EUROPE research will focus on fault activity documented in the Quaternary and Holocene deposits in the axial part of the graben (Haimberger et al., 2005), using river-seismic data, core holes and geomorphology for paleo-tectonic and paleo-geographic reconstructions, to analyze tectonic imprints. Horizontal and vertical movements inferred from geodesy (InSAR) will be integrated with the geologic/geomorphic record. The Heidelberger Loch research well, which is currently being drilled, will yield detailed chrono- and lithostratigraphic information on Quaternary and Late Pliocene sediments.
The Rhine–Meuse delta system
|Fig. 42. colour-coded relief map of the Netherlands and surroundings (data from GTOPO30). Red lines: faults affecting the Base Tertiary, White dots: earthquake epicenters (data from the ORFEUS data centre) (after Cloetingh et al., 2005b).|
The densely populated coastal low lands of Rhine–Meuse delta system, located at the northern end of the ECRIS (Fig. 42), are highly vulnerable to flooding owing to land subsidence and rising sea levels. Moreover, the neotectonically active Roer Valley graben is characterized by increased seismic hazards, as evidenced by the ML 5.8 Roermond earthquake of April 13th 1992 (Van Eck and Davenport, 1994).
|Fig. 43. Present-day tectonic vertical motions correlated to coastal transgression/regression rates (after Van Balen et al., 2005).|
|Fig. 44. Pattern and magnitude of early Middle Pleistocene to Recent uplift of the Ardennes and Rhenish Massif (after Van Balen et al., 2000).|
Neotectonic deformation of the Netherlands involves gentle uplift of its SE and slow subsidence of its NW coastal parts (Fig. 43; Van Balen et al., 2005). Seismic activity reflects reactivation of fault systems outlining the Lower Rhine-Roer Valley graben system. Continued subsidence of this rift, combined with uplift of the Rhenish Massif and Ardennes (Fig. 44), strongly influenced the Neogene and Quaternary evolution of the Rhine-Meuse river system and its delta (Dirkzwager et al., 2000).
During the last decade NEESDI (Netherlands Environmental Earth System Dynamics Initiative) investigated the interplay between tectonic subsidence and faulting, compaction and sea level changes that affect the Rhine-Meuse delta system. Integration of high-resolution reflection-seismic, 3D seismic data analysis and geomechanical modelling resulted in the development of a new generation of delta models and concepts for tectonic controls on river evolution on subsiding coastal plains (Cloetingh, 2000).
|Fig. 45. Isopach map of Neogene sediments in the Netherlands, showing fault zones active during the Neogene (after Van Balen et al., 2005).|
Integrated analysis of crustal-scale cross sections, and their comparison with isopach maps of Tertiary and Quaternary sequences, demonstrate that faults already active during the Mesozoic exert a strong control on recent differential vertical motions in the Roer Valley Graben and in coastal areas (Fig. 43). Results of 3D gravity back stripping show a clear correlation between positive residual gravity anomalies and the main structural trends of the Lower Rhine-Roer Valley Graben system. Geomorphologic studies indicate that during Miocene-Quaternary times tectonic uplift of the Ardennes amounted to as much as 600 m whilst the Roer Valley Graben subsided by several 100 m (Van Balen et al., 2000; 2005). During this time span, much of the sedimentary load of the river Meuse was deposited in the Roer Valley Graben (Fig. 45). In the coastal zone of the Netherlands, development of Weichselian-Holocene terrace systems can be related to climatic processes and the level of sediment supply from the uplifting Ardennes (Van Balen et al., 2000). In the Ardennes, river incision rates reached a maximum during the last 0.75–0.3 Myr, reflecting their uplift by some 250 m (Meyer and Stets, 1998; Van Balen et al., 2000).
In the Roer Valley Graben, neotectonic fault patterns were defined by geomorphologic studies, supported by industrial seismic data and the results of trenching. Modelling studies, focusing on the contribution of climate changes to terrace formation, indicate a complex relationship between fluvial incision and aggradation that mainly results from changes in river discharge and sediment flux, controlled by the coupling of precipitation and vegetation with climate. Preservation of terraces results from tectonic uplift. The longitudinal profile of the river Meuse evolved in response to the combined effects of Quaternary tectonic uplift of the Ardennes, sea level and climate changes, and related changes in the sedimentary load composition of the river Meuse (Van Balen et al., 2000) .
Subsidence of the Dutch coastal area, quantified by precision levelling, is largely controlled by compaction of Neogene deltaic series with neotectonics playing an overprinting role. During sediment compaction, lateral fluid flow in shallow and deeper aquifers plays an important role (Kooi, 2000). Coastal regression and transgression patterns are strongly influenced by these subsidence patterns (Fig. 45; Van Balen et al., 2005).
The Rhine-Meuse delta system provides an ideal laboratory for analyzing the effects of natural perturbations of the Earth system on the human environment, owing to the availability of a high-quality database and intense studies by academic and industrial researchers and government organizations. The vulnerability of this delta system to eustatically rising sea levels and a subsidence-controlled relative sea level rise takes priority in developing strategies for optimizing the use and management of the environment. The stability of coastal zones, the dynamics of river systems, and the management and protection of non-contaminated groundwater resources are of greatest societal relevance.
In view of this, TOPO-EUROPE plans to probe the interplay of neotectonics, sea level and climate changes in the lowlands of the Rhine-Meuse delta system. Quantification of the role of neotectonics and their interplay with regional subsidence/uplift and climate changes are of fundamental importance. The magnitude of vertical crustal movements in the Netherlands during the last 2.6 My is illustrated by the thickness of Quaternary sediments that attains values of up to 500 m in on-shore areas, increasing to 1000 m offshore in the North Sea Central Graben. Precision levelling shows a systematic difference between the eastern parts of the Netherlands, which are being uplifted, and its western parts that subside (Fig. 43). The overall pattern suggests tilting of the entire country that is consistent with accelerated Pliocene-Quaternary tectonic subsidence of the North Sea Basin and uplift of the Rhenish Massif-Ardennes. At the same time, important contributions from compaction-driven subsidence and glacio-isostasic uplift are evident.
|Fig. 46. Location of Holocene avulsion nodes in the Rhine-Meuse delta. Avulsion nodes are mainly located on the fault bounding the Peel Block (modified after Berendsen and Stouthamer, 2002).|
Separating the effects of neotectonics from those of eustasy and climate remains a formidable task (Cloetingh, 2000) that requires an improved and more detailed chronostratigraphic subdivision of Plio-Quaternary deposits. Moreover, it is vital to access the shallow parts of industrial 3D seismic data sets and to analyze them with state-of-the-art seismic processing and interpretation techniques in terms of reconstructing the architecture and evolution of the Rhine-Meuse delta. High-resolution river reflection-seismic, a rapidly developing and very important research tool in the study of neotectonics, images fault control on the course of the river Meuse (Fig. 46). Furthermore, trenching permitted to identify fault activity in the Roer Valley Graben that occurred during the last 40 Ky. Detailed reconstruction of river systems in the Roer Valley Graben and in northward adjacent areas has provided evidence for neotectonics affecting fluvial systems and river gradients. A systematic study of the Holocene evolution of the Rhine-Meuse delta system has documented tectonic control on river avulsion (Fig. 46) (Berendsen and Stouthamer, 2002; Cohen et al., 2002). Integration of shallow seismic and borehole data with the results of paleo-seismicity studies will be further pursued, aiming at defining the recurrence time of major earthquakes.
TOPO-EUROPE will extend the geological record by integrating the high-resolution seismic data recorded on rivers into the neotectonic dataset. Secondly, high-precision levelling data (InSAR) and high-precision digital elevation models (AHN) will be used, in combination with the seismic catalogue, to characterize the recent tectonic activity. In a third step, the Middle to Late Pleistocene evolution of the landscape will be addressed by forward modelling of the geomorphology as a function of sediment supply (fluvial and eolean), climate changes, tectonics, and anthropogenic activity. In this, the full 3D geometry of the faults and their displacement rates and slip directions will be taken into account. Special attention will be paid to modelling the effect of faulting on the Rhine and Meuse rivers.