Alain Demoulin

Testing the tectonic significance of some parameters of longitudinal river profiles: the case of the Ardenne (Belgium, NW Europe)

The longitudinal profiles of 24 rivers of 3rd to 5th order have been analyzed in order to recognize the regional tectonic differentiation pattern of the Ardenne, a Paleozoic massif of NW Europe. Data collected exclusively from 1/25,000 topographic maps were used to define a set of parameters related to the geometry of the profile and proxies for river bedload and drainage area. Validation of the proxy defined for the bedload/discharge ratio demonstrates its potential use for inter-river comparison based on its value at the river mouth; it is however inadequate to estimate the ratio changes along a given stream course. This proxy has been computed to estimate the portion accounted for by hydrodynamic factors in the river bed profiling. Thus, outliers to the correlation function linking it to the river bed gradient have been considered as undergoing other influences, principally of lithologic or tectonic origin. Most indicative of the computed profile parameters are those related to the concavity of the profile. Four more or less regional longitudinal profile types are recognized which are partly determined by differential tectonic movement. The combined analysis of all the parameters confirms the current uplift of the NE Ardenne and points to a subsidence of the Namur area with respect to its SW and SE surroundings. Whether periglacial or tectonic in origin, the Upper Pleistocene lowering of a local base-level has also been identified in the Famenne. Another conclusion of this study is that in order to evidence tectonic influences and to avoid any misinterpretation in a region of complex geologic and geomorphic history and moderate tectonic activity, it is essential to consider a number of parameters and to consider their significance within the framework of detailed geologic setting and morphogenetic history.

Reconciling geodetic and geological rates of vertical crustal motion in intraplate regions

Abstract Tectonic motion rates of individual faults and regional deformation in an intraplate setting are estimated, based on the analysis of ten yearly surveys of a regional levelling network in NE Ardenne (W Europe). Owing to the frequency and number of measurement epochs, much greater than in classical comparisons of general surveys, the tectonic and near-surface components of ground motion are clearly separated. The marked temporal variability in ground motion strongly depends on the amount of precipitation fallen during the six months preceding each survey and the subsequent drying off of the subsoil at the time of the surveys. Moreover, the ground response to this influence varies from place to place, leading to high differential movements at the local scale. Taking into account the percentage of surveyed faults which moved tectonically during the time of the study, I calculate fault motion rates of 0.06–0.09 mm/yr, similar to geological rates. Moreover, the data indicate that one way for intraplate normal faults to accommodate tectonic strain aseismically is intermittent fault creep, with short episodes of a few mm displacement separated by many year-long time intervals of fault quiescence. A flexural deformation shows the superposition of a tilt event on a trend corresponding to a tilt rate of 0.16 μrad/yr for a 2.8-km-long segment.

Fluvial terraces of the Amblève: a marker of the Quaternary river incision in the NE Ardennes massif (Western Europe).

The geomorphological analysis of the terraces of the Ambleve, an Ardennian subtributary of the Meuse, allows us to reconstruct eleven distinct levels (T1 to T11). Like those of most rivers draining the Ardennes massif, the Ambleve terrace profiles also converge in the upstream direction. Moreover, the “Main Terrace” complex, widely recognized in the Rhine and Meuse systems (including their major tributaries draining the Ardennes-Eifel massif), has also been identified in the Ambleve valley. However, the lack of terrace remnants in the ~ 10km-long reach of the Quarreux gorge, combined with the knickpoint observed in the presentday long profile in this reach, leaves some uncertainty in the overall profile reconstruction. Despite the presence of a paleokarst filled by alluvial material in the lower Ambleve and diversely dated between ~ 0,5 and ~ 1 Ma, firm chronological data about the Quaternary evolution of the Ambleve are still lacking. Therefore, we base our discussion of the temporal evolution of the Ambleve incision on geometrical correlations with dated terraces of the Meuse downstream of Liege.

Valley downcutting in the Ardennes (W Europe): Interplay between tectonically triggered regressive erosion and climatic cyclicity

While climatic models of valley downcutting discuss the origin of terrace staircases in valleys of middle Europe within the frame of alternating cold and temperate periods of the Quaternary, other models, starting from a base level fall imposed by an initial tectonic signal, describe the response of the drainage network mainly as the propagation of an erosion wave from the place of base level fall (the margin of the uplifted region) toward the headwaters, the two types of model being rarely confronted. In the Ardennes (West Europe), cosmogenic 10 Be and 26 Al ages have recently been calculated for the abandonment of the Younger Main Terrace (YMT) level, a prominent feature at mid-height of the valleysides marking the starting point of the mid-Pleistocene phase of deep river incision in the massif. These ages show that the terrace has been abandoned diachronically as the result of a migrating erosion wave that started at 0.73 Ma in the Meuse catchment just north of the massif, soon entered the latter, and is still visible in the current long profiles of the Ardennian Ourthe tributaries as knickpoints disturbing their upper reaches. At first glance, these new findings are incompatible with the common belief that the terraces of the Ardennian rivers were generated by a climatically triggered stepwise general incision of the river profiles. However, several details of the terrace staircases (larger than average vertical spacing between the YMT and the next younger terrace, varying number of post-YMT terraces in trunk stream, tributaries and subtributaries) show that a combination of the climatic and tectonic models of river incision is able to satisfactorily account for all available data. The cosmogenic ages of the YMT also point out a particular behaviour of the migrating knickpoints, which apparently propagated on average more slowly in the main rivers than in the tributaries, in contradiction with the relation that makes knickpoint celerity depend directly on drainage area. We tentatively suggest a process accounting for such anomalies in migration rates.

Cenozoic tectonics on the Hautes Fagnes plateau (Belgium)

Abstract Morphological features, under which deformations of Oligocene surfaces are included, demonstrate the varied tectonic activity of the Hautes Fagnes plateau during Cenozoic times. Beyond the general uplift of the region, a local movement raised the Baraque Michel massif by about 100 m above the nearby crest of “La Vecquee”. Moreover, several zones of radial NW-SE faulting are active on the plateau. They are related to the opening of the Lower Rhine Embayment and represent the ultimate SW extension of the fault system associated with this opening.

Quaternary tectonics in the northern Ardennes, Belgium

Abstract A number of NW-SE or NNW-SSE trending radial faults are recognized on the northern slope of the Ardenne massif. They have been active since the Oligocene, as depicted by deformations of planation surfaces and other morphological features. The study of river terraces demonstrates their Quaternary activity, with vertical movements ranging from 4 to 10 m during the late Quaternary. This fault activity is related to the opening of the Lower Rhine Embayment.

The Flemish Valley: Response of the Scheldt drainage system to climatic and glacio-eustatic oscillations

Beyond its focus on the climatic and eustatic controls of the Flemish Valley evolution, this chapter explores more broadly the Plio-Quaternary history of the Belgian part of the Scheldt basin, in which the Flemish Valley is essentially a buried incised-valley system that once drained the Scheldt area. The Scheldt is typically a lowland river flowing across a subdued hilly topography in its middle course and on the flat plain chiefly made of its own alluvial deposits in the lower course. We first describe the terraces located on the interfluves and valley sides of the Middle Scheldt region, providing also a tentative chronology of their development. Then, the morphology and the sedimentary infill of the Flemish Valley are described in relation to alternating erosional and aggradational episodes since the Saalian. The transition from stepped terraces to cut-and-fill processes at the Flemish Valley level is attributed to the waning of the tectonically driven uplift of the area during the Middle Pleistocene and the bedrock incision episode that lastly created the Flemish Valley is tentatively related to the base level fall induced by the catastrophic breaching of the Dover Strait dam during the Late Saalian. Finally, the Late Glacial and Holocene fate of the Flemish Valley is evoked, emphasizing the role of the Late Pleniglacial/Late Glacial coversand morphology in deflecting the river’s former NNW-striking course to the east and over Antwerp and pointing to the first impact of human activities on the Scheldt behaviour in the last third of the Holocene.

Decadal-scale analysis of ground movements in old landslides in western Belgium

Summary. More than 150 large deep-seated landslides have been mapped in the Flemish Ardennes. Slope instabilities that have occurred in this hilly area of western Belgium during the last decades correspond to ground movements within these pre-existing landslides. In order to identify the mechanisms and controlling factors of these ground movements, a good knowledge of their spatial and temporal distribution is critical. 13 landslides affecting two hills were investigated based on several DTMs extracted by aerial stereophotogrammetry and spanning the 1952–1996 period. Vertical ground displacements were measured at each pixel by DTM subtraction with a confidence value of 0.70 m. Horizontal displacements were also estimated within the landslides and along the head scarps through topographical profiles. Most observed movements displayed patterns typical of rotational landslides. Vertical and horizontal displacements vary in magnitude both spatially and temporally, with respective ranges – 7.4 m–+ 3.8 m and 0–14 m. Many displacements are materialized in the field. We distinguished two kinds of slope processes, corresponding to either reactivation at a deeper level or shallower motion. The former re-uses pre-exiting surfaces of rupture located at depths of ~ 15– 20 m and is associated with the largest subsidence and uplift. They are also smaller reactivations confined at the landslide head. The other displacements consist in (1) earth flows occurring in the zone of accumulation sometimes as a consequence of large upslope reactivations, and (2) small failures occurring randomly. While most movements were triggered by intense rainfall, their spatial and temporal distribution is strongly related with the nature of the vegetation cover and the human activity.

Applying Pattern Oriented Sampling in current fieldwork practice to enable more effective model evaluation in fluvial landscape evolution research

Field geologists and geomorphologists are increasingly looking to numerical modelling to understand landscape change over time, particularly in river catchments. The application of landscape evolution models (LEMs) started with abstract research questions in synthetic landscapes. Now, however, studies using LEMs on real-world catchments are becoming increasingly common. This development has philosophical implications for model specification and evaluation using geological and geomorphological data, besides practical implications for fieldwork targets and strategy. The type of data produced to drive and constrain LEM simulations has very little in common with that used to calibrate and validate models operating over shorter timescales, making a new approach necessary. Here we argue that catchment fieldwork and LEM studies are best synchronized by complementing the Pattern Oriented Modelling (POM) approach of most fluvial LEMs with Pattern Oriented Sampling (POS) fieldwork approaches. POS can embrace a wide range of field data types, without overly increasing the burden of data collection. In our approach, both POM output and POS field data for a specific catchment are used to quantify key characteristics of a catchment. These are then compared to provide an evaluation of the performance of the model. Early identification of these key characteristics should be undertaken to drive focused POS data collection and POM model specification. Once models are evaluated using this POM/POS approach, conclusions drawn from LEM studies can be used with greater confidence to improve understanding of landscape change.

Neotectonic implications of a lineament-coplanarity analysis in Southern Calabria, Italy

Abstract A synthesis of the geodynamical interpretations proposed in the literature shows that the respective role of different possible driving mechanisms in the Neogene and Quaternary tectonic evolution of southern Calabria is still under discussion. Owing to different methodological approaches and scales of observation, the same holds true for marine terrace levels recognized in the area of Reggio Calabria. Their number, elevation, spatial correlation and age, as well as their tectonic significance, are still debated, resulting in some uncertainty about the timing and rate of the regional uplift and its local variations. To complete the mapping of fault zones potentially dislocating the marine terraces, a three-dimensional (3D) morphometrical analysis of lineaments has been carried out. From a data-set composed of linear structures recognized on an illuminated digital elevation model (17 km × 18 km in size) centred on Reggio Calabria, the method aims at identifying lineaments pertaining to a single plane. Assuming that faults may be assimilated to planes at the local scale, the coplanar lineaments allow the 3D attitude and location of fault or fracture planes to be determined. The planes retained as tectonically significant (on the dip value and number of coplanar lineaments) have been successfully compared with the existing faults as well as with geomorphological data (long river profiles) of southern Calabria. Beyond mapping the main faults of the study area, the coplanarity analysis highlights the existence of conjugate faults within the NW-SE fault zone of Terreti-Monte Embrisi. It also locates some E-W striking faults which were previously known only at the outcrop scale.