Search

River terrace deposits are excellent archives of paleoenvironmental conditions. They reflect the tectonic and climatic settings of their time of formation. For this reason, Late-Pleistocene and Holocene terraces have been previously studied in detail because of their good state of preservation and age control. However, less is known about the Middle- and Early-Pleistocene terraces. The Lower Meuse River, a major tributary of the Rhine River, located in the Southern Netherlands, exhibits a well-preserved terrace staircase which, for decades, has been intensely investigated. Age constraints are available, which are mainly based on correlations to the marine isotope record. As of late, the availability of numerical ages of these terraces have been increasing, allowing for a better determination of the boundaries of the Early, Middle and Late Pleistocene terraces. In order to better understand the effects of the Mid-Pleistocene Transition (MPT; 1.2 – 0.8 Ma) in the Meuse River, we improved the spatial and temporal resolution of the terraces of the Lower Meuse. For the spatial resolution, we used a dense borehole database to characterize key geometrical and compositional parameters of the different terrace levels. For the temporal resolution, we used cosmogenic-nuclide geochronological methods, relying on the measurements of the paired isotopes 26Al-10Be, allowing for the estimation of terraces burial age. In this work we outline general spatial trends of geometrical and compositional parameters of terraces formed pre-, syn-, and post-MPT. These results are displayed in an improved time framework that relies upon the new burial ages results from the cosmogenic nuclides concentration measured for specific terrace levels. We present three new isochron-burial ages from Main Terrace levels, and thirteen new simple-burial ages from Middle-, Main-, and East Meuse-Terraces. The results allow us to better understand the signal propagation generated by the MPT. A general increasing trend in the gravel content and terrace thickness have been observed from older to younger terraces. The ages are mostly in good agreement (within uncertainty) with previously proposed age models, especially the ages for the East Meuse Terraces, which have been confirmed to be Early Pleistocene.

The Lower Rhine Graben (LRG) is an area of slow intra-plate extension in north-western Europe. Located in a densely populated area, this rift, with moderate but rather continuous seismic activity, poses significant seismic hazard. The LRG NW-trending fault system is 200-km long and accommodates a total extension of ~0.1 +/- 0.03mm/yr. While the major active faults are well known, the activity of this complex system as a whole remains poorly understood. This is partly due to the fact that the tectonic signal issued from such low strain rates deformation is often overprinted by other natural or anthropogenic processes. Thus, previous fault models do not integrate minor structures associated with limited deformation and remain elusive about precise fault geometry and branching. A high-resolution DEM, created from Lidar-based DEMs recently available in the surrounding countries, allows us to retrieve detailed tectonic information and refine the fault traces and scarp geometry. We thus present, for the entire region, a revised and homogeneous fault map, based on morphological observations of fault scarps and offset alluvial terraces, complemented by external information from paleoseismological surveys and geophysical profiles. The high-resolution topography shows a clear difference in fault morphological expression between the eastern and western sides of the graben, with clear scarps and sharp boundaries along the eastern side and smoother cumulative scarps in the west, suggesting contrasting fault behavior across the graben. Based on this detailed mapping, we propose a new active faults model for the whole LRG, reflecting the uncertainties in fault geometry. This is compiled in a database, including several levels of fault mapping (traces, fault sections, faults, main faults), where the fault traces are ranked according to the certainty of their identification and location.Another limitation for seismic hazard assessment in the area is the relative scarcity of fault-displacement data compared to the large number of structures. In the southern part of the graben, a well-developed terrace allows us to estimate the activity of most faults over the Quaternary, but such an extended marker is missing in the northern part of the LRG, resulting in only few localized data. To complement these offset observations, we use several 3D-geological models. After a selection of the most representative geological layers, we automatically retrieve their offsets at several locations along each fault, to obtain the spatial slip distribution at different timescales. We observe that along individual faults, the slip profile evolves laterally and in time, showing some fault linkage, while at the scale of the graben borders the total slip does not show significant lateral variations. Moreover, although the surface-expression differs between the two sides of the graben, the total slip rates are fairly equivalent on both sides, suggesting a symmetrical extension, at least for the northern area.All offset measurements available for different marker horizons are also included in the new LRG fault database, thus providing an integrated tool which allows the user to choose the most relevant timescale and degree of geometrical complexity for advanced seismic hazard assessment.

Understanding and modelling the properties of the stochastic variability -- often referred to as noise -- in geodetic time series is crucial to obtain realistic uncertainties for deterministic parameters, e.g., long-term velocities, and helpful in characterizing non-modelled processes. With the ever-increasing span of geodetic time series, it is expected that additional observations would help better understanding the low-frequency properties of the stochastic variability. In the meantime, recent studies evidenced that the choice of the functional model for the time series may bias the assessment of these low-frequency stochastic properties. In particular, the presence of frequent offsets, or step discontinuities, in position time series tends to systematically flatten the periodogram of position residuals at low frequencies and prevents the detection of possible random-walk-type variability. In this study, we investigate the ability of frequently-used statistical tools, namely the Lomb-Scargle periodogram and Maximum Likelihood Estimation (MLE) method, to correctly retrieve low-frequency stochastic properties of geodetic time series in the presence of frequent offsets. By evaluating the biases of each method for several functional models, we demonstrate that neither of these tools is reliable for low-frequency investigation. By assessing alternative approaches, we show that using Least-Squares Harmonic Estimation and Restricted Maximum Likelihood Estimation (RMLE) solves part of the problems reported by previous works. However, we evidence that, even when using those optimal methods, the presence of frequent offsets inevitably blurs the estimated low-frequency properties of geodetic time series by increasing low-frequency stochastic parameter uncertainties more than that of other stochastic parameters.2.14.0.02.14.0.0

Les massifs paléozoïques d’Europe nord-occidentale ont quasi tous connu, à des degrés divers, une surrection au cours du Pliocène et/ou du Quaternaire. Quoiqu’on ait proposé d’expliquer ces soulèvements, soit par un plissement lithosphérique de la plateforme européenne au front de l’arc alpin, soit par la présence d’une série de panaches mantelliques au sein du manteau supérieur, aucun accord n’existe quant à une cause d’ensemble des mouvements, en particulier par manque d’information temporelle précise sur leur évolution. Nous abordons ici la question par l’analyse morphométrique des bassins de rivière sur la base d’un indice composite quantifiant le degré d’avancement de la réponse régionale par incision des systèmes de drainage à un signal tectonique de soulèvement. Appliquée à 7 478 bassins de toute taille, cette mesure effectuée à partir des données altimétriques SRTM 3’’ identifie une vaste zone de la plateforme où les valeurs d’indice évoluent de façon cohérente, témoignant d’un soulèvement d’ensemble dont l’axe se propage depuis la marge de la chaîne alpine, où il se manifeste dès 2,5 – 2 Ma, vers le NNO et le NO, où on l’observe récemment au NO du Bassin parisien et au NNO du Massif schisteux rhénan. Dans le contexte de l’évolution géodynamique de l’arc alpin au Néogène et au Quaternaire et de ses répercussions sur son avant-pays, nous interprétons ce soulèvement plio-quaternaire d’une partie de la plateforme à l’avant des Alpes centre-occidentales comme une manifestation tardive du transfert de contraintes de compression de la chaîne vers son avant-pays au niveau de la croûte supérieure, manifestation prolongeant et propageant de façon atténuée la tectonique de couverture qui, de 12 à 4 Ma, a plissé la chaîne du Jura et soulevé, plus au nord, le plateau du Jura. Cette interprétation a pour conséquence de reléguer à de seconds rôles l’influence d’une part d’instabilités diapiriques dont l’impact est limité localement (Eifel, Massif central), d’autre part du plissement lithosphérique, dont l’influence dans la région étudiée fut la plus sensible au Miocène et dont l’analyse morphométrique ne laisse plus soupçonner un rôle actuel qu’au sud du Bassin parisien.

International audience; Monitoring vertical land motions (VLMs) at the level of 0.1 mm/yr remains one of the most challenging scientific applications of global navigation satellite systems (GNSS). Such small rates of change can result from climatic and tectonic phenomena, and their detection is important to many solid Earth-related studies, including the prediction of coastal sea-level change and the understanding of intraplate deformation. Reaching a level of precision allowing to detect such small signals requires a thorough understanding of the stochastic variability in GNSS VLM time series. This paper investigates how the aperiodic part of non-tidal atmospheric and oceanic loading (NTAOL) deformations influences the stochastic properties of VLM time series. Using the time series of over 10,000 stations, we describe the impact of correcting for NTAOL deformation on 5 complementary metrics, namely: the repeatability of position residuals, the power-spectrum of position residuals, the estimated time-correlation properties, the corresponding velocity uncertainties, and the spatial correlation of the residuals. We show that NTAOL deformations cause a latitude-dependent bias in white noise plus power-law model parameter estimates. This bias is significantly mitigated when correcting for NTAOL deformation, which reduces velocity uncertainties at high latitudes by 70%. Therefore, removing NTAOL deformation before the statistical analysis of VLM time series might help to detect subtle VLM signals in these areas. Our spatial correlation analysis also reveals a seasonality in the spatial correlation of the residuals, which is reduced after removing NTAOL deformation, confirming that NTAOL is a clear source of common-mode errors in GNSS VLM time series.

Over the past two decades, numerous studies demonstrated that the stochastic variability in GNSS position time series – often referred to as noise – is both temporally and spatially correlated. The time correlation of this stochastic variability can be well approximated by a linear combination of white noise and power-law stochastic processes with different amplitudes. Although acknowledged in many geodetic studies, the presence of such power-law processes in GNSS position time series remains largely unexplained. Considering that these power-law processes are the primary source of uncertainty for velocity estimates, it is crucial to identify their origin(s) and to try to reduce their influence on position time series. Using the Least-Squares Variance Component Estimation method, we analysed the influence of removing surface mass loading deformation on the stochastic properties of vertical land motion time series (VLMs). We used the position time series of over 10,000 globally distributed GNSS stations processed by the Nevada Geodetic Laboratory at the University of Nevada, Reno, and loading deformation time series computed by the Earth System Modelling (ESM) team at GFZ-Potsdam. Our results show that the values of stochastic parameters, namely, white noise amplitude, spectral index, and power-law noise amplitude, but also the spatial correlation, are systematically influenced by non-tidal atmospheric and oceanic loading deformation. The observed change in stochastic parameters often translates into a reduction of trend uncertainties, reaching up to -75% when non-tidal atmospheric and oceanic loading deformation is highest.

International audience; Despite a wealth of recent studies dealing with the evolution of the drainage network in the uplifted Ardennes massif (E. Belgium), especially from the Middle Pleistocene onwards, the Ardennian landscape evolution and long-term incision rates in the Meuse catchment remain poorly documented over the whole Plio-Quaternary. Alluvium-filled multilevel cave systems represent a relevant setting to unravel the Late Cenozoic history of regional river incision. We present here a dataset of 26 Al/ 10 Be concentration data obtained from fifteen pebble samples washed into the Chawresse system, one of the largest multi-level cave systems of Belgium, which developed in Devonian limestones of the lower Ourthe Valley, the main Ardennian tributary of the Meuse. The sample collection spans an elevation difference higher than 120 m and their depleted 26Al/10Be ratios yield burial ages ranging from ~0.25 to 3.28 Ma. After critical assessment of our dataset for intra-karstic reworking issues, the most striking outcome of the obtained burial ages is the acceleration by a factor five of the incision rates (from ~30to ~150 m/Ma) during the first half of the Middle Pleistocene. Integrating this incision peak and our pre-burial denudation rates, we then revisit the existing framework of Plio-Quaternary denudation and river incision in the Ardennian Meuse catchment. Whilst our 26Al/10Be concentration data shed new light on the temporal andspatial variability of the local river and hillslope system response to coupled tectonic and climatic forcings, it simultaneously highlights sampling issues and the need for further chronological data.

L’activite sismotectonique dans l‘Est de la Belgique : l’interet d’un escarpement majeur et de deux glissements de terrain dans la region de Malmedy. Les marqueurs geomorphologiques, tels que les escarpements, les detournements de rivieres et les ruptures de pente, peuvent etre utilises comme indicateurs indirects de l'activite neotectonique d'une region. Cette etude se concentre sur la region de Malmedy-Beverce, dans l’Est de la Belgique, ou des structures geomorphologiques, autrefois inconnues, ont ete decouvertes recemment, dans le cadre de la derniere campagne de cartographie geologique. La region se caracterise par des pentes douces a localement tres raides le long de la vallee de la Warche, traversant le massif de Stavelot et le graben de Malmedy. Couple a une analyse d'imagerie LiDAR-DEM et drone, la cartographie de terrain a revele un escarpement abrupt s'etendant pres de deux ruptures de pente sur les versants sud de la vallee de la Warche, a proximite du village de Beverce. Les instabilites de pente se sont developpees dans le conglomerat permien, connu sous le nom du Poudingue de Malmedy (ou Formation de Malmedy), qui represente le remplissage sedimentaire fluviatile du Graben de Malmedy. Avec une direction N330°E, l’escarpement en question est quasiment perpendiculaire aux structures connues dans le graben, mais parallele a l’orientation de la Zone de Faille de Hockai sismiquement active qui traverse la region de Malmedy a cet endroit. Dans cet article, nous presentons d’abord le contexte geologique et geomorphologique de l’escarpement et des glissements de terrain. Ensuite, nous montrons les resultats d’une campagne geophysique (comportant deux profils sismiques et deux profils electriques) qui a ete realisee sur ces structures. Les profils geophysiques obtenus montrent un deplacement vertical des couches sismiques ainsi que des changements lateraux des resistivites a travers de l’escarpement. Une zone de faible resistivite a pu etre observee a l’interieur de cet escarpement, ainsi que dans le plus grand des deux glissements de terrain, exactement dans le prolongement de l'escarpement. Toutes ces observations sont indicatrices de la presence d’une faille importante, probablement sismiquement active, faisant partie du bord est de la Zone de Faille de Hockai.

The study of drainage network response to uplift is important not only for understanding river system dynamics and associated channel properties and fluvial landforms, but also for identifying the nature of crustal deformation and its history. In recent decades, geomorphic analysis of rivers has proved powerful in elucidating the tectonic evolution of actively uplifting and eroding orogens. Here, we review the main recent developments that have improved and expanded qualitative and quantitative information about vertical tectonic motions (the effects of horizontal deformation are not addressed). Channel long profiles have received considerable attention in the literature, and we briefly introduce basic aspects of the behaviour of bedrock rivers from field and numerical modelling perspectives, before describing the various metrics that have been proposed to identify the information on crustal deformation contained within their steady-state characteristics. Then, we review the literature dealing with the transient response of rivers to tectonic perturbation, through the production of knickpoints propagating through the drainage network. Inverse modelling of river profiles for uplift in time and space is also shown to be very effective in reconstructing regional tectonic histories. Finally, we present a synthetic morphometric approach for deducing the tectonic record of fluvial landscapes. As well as the erosional imprint of tectonic forcing, sedimentary deposits, such as fluvial terrace staircases, are also considered as a classical component of tectonic geomorphology. We show that these studies have recently benefited from rapid advances in dating techniques, allowing more reliable reconstruction of incision histories and estimation of incision rates. The combination of progress in the understanding of transient river profiles and larger, more rigorous data sets of terrace ages has led to improved understanding of river erosion and the implications for terrace profile correlation, i.e., extrapolation of local data to entire profiles. Finally, planform changes in fluvial systems are considered at the channel scale in alluvial rivers and regional level in terms of drainage reorganisation. Examples are given of how numerical modelling can efficiently combine with topographic data to shed new light on the (dis)equilibrium state of drainage systems across regional drainage divides.

Multi-level cave systems record the history of regional river incision in abandoned alluvium-filled phreatic passages which, mimicking fluvial terrace sequences, represent former phases of fluvial base-level stability. In this respect, cosmogenic burial dating of in cave-deposited alluvium (usually via the nuclide pair 26Al/10Be) represents a suitable method to quantify the pace of long-term river incision. Here, we present a dataset of fifteen 26Al/10Be burial ages measured in fluvial pebbles washed into a multi-level cave system developed in Devonian limestone of the uplifted Ardenne massif (eastern Belgium). The large and well-documented Chawresse system is located along the lower Ourthe valley (i.e. the main Ardennian tributary of the Meuse river) and spans altogether an elevation difference exceeding 120 m.The depleted 26Al/10Be ratios measured in four individual caves show two main outcomes. Firstly, computed burial ages ranging from ~0.2 to 3.3 Ma allows highlighting an acceleration by almost one order of magnitude of the incision rates during the first half of the Middle Pleistocene (from ~25 to ~160 m/Ma). Secondly, according to the relative elevation above the present-day floodplain of the sampled material in the Manants cave (per descensum model of speleogenesis generally acknowledged for the regional multi-level cave systems and their abandoned phreatic galleries. In addition to its classical use for inferring long-term incision rates, cosmogenic burial dating can thus contribute to better understand specific and complex speleogenetic evolution.

Determining rainfall thresholds for landsliding is crucial in landslide hazard evaluation and early warning system development, yet challenging in data-scarce regions. Using freely available satellite rainfall data in a reproducible automated procedure, the bootstrap-based frequentist threshold approach, coupling antecedent rainfall (AR) and landslide susceptibility data as proposed by Monsieurs et al., has proved to provide a physically meaningful regional AR threshold equation in the western branch of the East African Rift. However, previous studies could only rely on global- and continental-scale rainfall and susceptibility data. Here, we use newly available regional-scale susceptibility data to test the robustness of the method to different data configurations. This leads us to improve the threshold method through using stratified data selection to better exploit the data distribution over the whole range of susceptibility. In addition, we discuss the effect of outliers in small data sets on the estimation of parameter uncertainties and the interest of not using the bootstrap technique in such cases. Thus improved, the method effectiveness shows strongly reduced sensitivity to the used susceptibility data and is satisfyingly validated by new landslide occurrences in the East African Rift, therefore successfully passing first transferability tests.

Rainfall threshold determination is a pressing issue in the landslide scientific community. While major improvements have been made towards more reproducible techniques for the identification of triggering conditions for landsliding, the now well-established rainfall intensity or event-duration thresholds for landsliding suffer from several limitations. Here, we propose a new approach of the frequentist method for threshold definition based on satellite-derived antecedent rainfall estimates directly coupled with landslide susceptibility data. Adopting a bootstrap statistical technique for the identification of threshold uncertainties at different exceedance probability levels, it results in thresholds expressed as AR = (α±Δα)⋅S(β±Δβ), where AR is antecedent rainfall (mm), S is landslide susceptibility, α and β are scaling parameters, and Δα and Δβ are their uncertainties. The main improvements of this approach consist in (1) using spatially continuous satellite rainfall data, (2) giving equal weight to rainfall characteristics and ground susceptibility factors in the definition of spatially varying rainfall thresholds, (3) proposing an exponential antecedent rainfall function that involves past daily rainfall in the exponent to account for the different lasting effect of large versus small rainfall, (4) quantitatively exploiting the lower parts of the cloud of data points, most meaningful for threshold estimation, and (5) merging the uncertainty on landslide date with the fit uncertainty in a single error estimation. We apply our approach in the western branch of the East African Rift based on landslides that occurred between 2001 and 2018, satellite rainfall estimates from the Tropical Rainfall Measurement Mission Multi-satellite Precipitation Analysis (TMPA 3B42 RT), and the continental-scale map of landslide susceptibility of Broeckx et al. (2018) and provide the first regional rainfall thresholds for landsliding in tropical Africa.

Accurate precipitation data are fundamental for understanding and mitigating the disastrous effects of many natural hazards in mountainous areas. Floods and landslides, in particular, are potentially deadly events that can be mitigated with advanced warning, but accurate forecasts require timely estimation of precipitation, which is problematic in regions such as tropical Africa with limited gauge measurements. Satellite rainfall estimates (SREs) are of great value in such areas, but rigorous validation is required to identify the uncertainties linked to SREs for hazard applications. This paper presents results of an unprecedented record of gauge data in the western branch of the East African Rift, with temporal resolutions ranging from 30 min to 24 h and records from 1998 to 2018. These data were used to validate the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) research version and near-real-time products for 3-hourly, daily, and monthly rainfall accumulations, over multiple spatial scales. Results indicate that there are at least two factors that led to the underestimation of TMPA at the regional level: complex topography and high rainfall intensities. The TMPA near-real-time product shows overall stronger rainfall underestimations but lower absolute errors and a better performance at higher rainfall intensities compared to the research version. We found area-averaged TMPA rainfall estimates relatively more suitable in order to move toward regional hazard assessment, compared to data from scarcely distributed gauges with limited representativeness in the context of high rainfall variability.

The Rift of Corinth is a world-class example of young active rifting and, as such, is an ideal natural laboratory of continental extension. However, though much investigated for two decades, several aspects of the mechanisms at work are still poorly understood. The aim of this paper is a detailed morphometric study of the fluvial landscape response to the tectonic uplift of the rift southern shoulder in order to reconstruct the rift's Quaternary evolution, with special attention to timing, location, and intensity of uplift episodes. Based on the use of a large set of catchment and long profile metrics complemented by the new R/SR integrative approach of the regional drainage network, we identified three distinct episodes of uplift of the northern Peloponnese coastal tract, of which the intermediate one, dated around 0.35–0.4 Ma, is only recorded in the topography of the central part of the rift shoulder, and the youngest one appears to have propagated from east to west over the last 10–20 ka. While net uplift remained minimum in the eastern part of the study area during the whole Quaternary, it shows a clear maximum in the central part of the rift shoulder since 0.4 Ma and an eastward shift of this maximum in recent times. Maximum uplift rates calculated from the morphometric data are of > 1.05 and 2–5 mm year− 1 for, the mid-Middle Pleistocene and Holocene uplift episodes, respectively. The morphometric evidence reveals an onshore uplift history remarkably consistent with the rift evolution reconstructed from other data sets. In the long term, it shows a stable pattern of maximum activity in the central part of the rift, confirming previous conclusions about the absence of rift propagation. In the short term, it sheds light on a possible E–W migration of the zone of recent uplift, suggesting that in the near future fault activity and seismic hazard might concentrate in the Heliki–Aegion area, at the western tip of this uplift wave.

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.

International audience; This chapter deals with the long-term geomorphology of the Paleozoic Ardenne–Oesling massif of S Belgium and the landforms that currently attest the very long persistence and high resistance to erosion of landscape elements created as far back in time as the Lower Cretaceous in a region of predominantly continental regime and constant low to moderate elevation since the Permian. We first present recent results regarding the antiquity of kaolinic weathering mantles preserved in four sites on or close to the plateau surfaces characteristic of the Ardennian landscape, showing that they record at least three main weathering phases, namely in the Early Cretaceous (extending in the early Late Cretaceous), the Late Paleocene, and the Early Miocene. Then, having provided a working definition of the erosion surface and listed criteria helping recognize them in the landscape, we focus on the Hautes Fagnes massif, the highest NE part of the Ardenne–Oesling. We describe the geometric and sedimentary evidence that allow one to recognize a succession of four erosion surfaces formed during the Cretaceous and the Cenozoic and reconstruct the morphogenetic evolution of the area that responded to the variable interplay of the marine transgression-regression, climatic, and tectonic controls. Finally, we present the few estimates of long-term denudation rates available for the Ardenne massif, fission track and cosmogenic nuclide studies both suggesting Cenozoic denudation rates in the order of a few 10 m/Myr, and comment on their compatibility with field observations.

The present climate of Belgium and Luxembourg is shown to be oceanic warm-temperate, benefitting from the warming effect of the North Atlantic Drift. Mean annual air temperatures are around 10 ℃ and vary spatially mainly as a function of elevation. Annual temperature amplitudes are in the 13–17 °C range. Annual rainfall depths vary from ~700 mm in western Belgium to 1300–1400 mm in the wettest areas of NE and SW Ardenne. Belgium and Luxembourg are located in the zone of seasonal shift of the north polar front and the associated mid-latitude, or polar front jet stream.

Tectonic landforms are generally modest in stable plate interiors characterized by low strain rates and rare earthquakes. Nevertheless, specific investigations identified such landforms in Belgium, which is located in the most seismically active region of stable Europe northwest of the Alps. Here, we present two active fault zones among the best documented in continental Europe, and whose geomorphology is related to earthquake activity in eastern Belgium. The 12-km-long Bree fault scarp is readily identified in the flat alluvial landscape of the Campine. It results from the activity since the Middle Pleistocene of the Geleen fault bounding the Roer Valley Graben (RVG) to the west. At its base, a ~1-m-high scarplet corresponds to the surface rupture associated with a Holocene large normal-faulting earthquake. Combined geomorphological and paleoseismic investigations allowed evaluating the average slip rate and the return period of large earthquakes on the Geleen fault during the Late Pleistocene. Extending across a more animated landscape in NE Ardenne, owing also to lower slip rates, the Hockai Fault Zone (HFZ) is morphologically less conspicuous. Microseismicity, geology and geomorphology provide consistent evidence of recent activity of this fault zone. The location of the M = 6 ¼ 18 September 1692 earthquake, the largest historical earthquake recorded in this part of Europe, on the northern HFZ suggests that the fault morphology could have resulted from sporadic bursts of large earthquakes.

The Ambleve valley at Fonds de Quarreux displays one of the most spectacular incised landscapes of the Ardenne massif. Cut into highly resistant Cambrian quartzites and surrounded by steep wooded hillslopes, a 3-km-long gorge is cluttered with hundreds of huge quartzite boulders up to several metres in size. These boulders originate from the locally outcropping Cambrian Venne Formation and reached the riverbed through periglacial slope processes. Up to 2-m-large boulders have been transported by the river over distances of several kilometres, some of them being observed 90 km downstream in the Meuse terraces of the Campine Plateau. We show that such transport of very large boulders over long distances cannot occur through purely fluvial processes and conclude that ice rafting was the most likely transportation mode, active during Pleistocene cold periods and probably supplemented by other ice breakup-related processes. We also briefly present nearby sites of geomorphological value within the Ambleve catchment.

As a response to Late Cenozoic tectonic uplift of the Ardenne massif, the Meuse River and its drainage system deeply incised the landscapes of Southern Belgium. Well-preserved terrace staircases flanking the main Ardennian valleys show a break at the height of the Main Terrace Complex, whose younger level (YMT) marks the transition towards the steep-sided lower part of valley transverse profiles. Numerical 10Be/26Al dates obtained for the YMT in various places along the lower Meuse—lower Ourthe—Ambleve line yield an age around 620 ka for the pulse of tectonic uplift responsible for increased incision rates and fast valley deepening. They also show diachronic abandonment of the YMT, indicating that post-YMT erosion invaded the drainage system through the migration of knickpoints originating in the ~20 m base level lowering initially created at the margin of the en-bloc uplifted region. A study of knickpoints in modern long profiles of Ardennian rivers fully confirms this view. Assimilation of the new data imposes a revised model be proposed for river incision in the Ardenne. Integrating the climatic control with the various modes of tectonic control over river incision, this conceptual model offers a sounder frame of the Quaternary river system evolution in the Ardenne, also explaining observed cases of stream piracy. Basin average denudation rates published for the Ardenne are also briefly compared with valley incision rates.

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.

Following a few words about the historical context of geomorphological research in Belgium and Luxembourg, the main geomorphic regions of the two countries are presented. The dominant controls exerted on morphogenic processes by lithology and elevation (i.e., tectonic background) lead to distinguish northern Belgium, corresponding to the low-elevation Cenozoic Belgian basin where sedimentation generally prevails over erosion, and the Ardennian Paleozoic massif of southern Belgium and northern Luxembourg, uplifted and strongly incised and eroded in the Plio-Quaternary. In between are the transitional plateaus of Middle Belgium. South of the Ardenne, Belgian Lorraine and Luxembourgian Gutland pertain to the homoclinal landscapes of the Paris basin. Finally, a brief overview is presented of the processes and landforms treated in the successive chapters.

Most landslides in Belgium, and especially the largest features, do not occur in the Ardenne, where the relief energy and the climate conditions seem most favourable. They appear in regions located mainly north of them where the lithology consists primarily of unconsolidated material. They develop on slopes that are relatively smooth, and their magnitude is pretty large with regard to that context. An inventory of more than 300 pre-Holocene to recent landslides has been mapped. Twenty-seven percent of all inventoried landslides are shallow complex landslides that show signs of recent activity. The remaining landslides are deep-seated features and rotational earth slides dominate (n > 200). For such landslides, the average area is 3.9 ha, but affected areas vary from 0.2 to 40.4 ha. The exact age of the deep-seated landslides is unknown, but it is certain that during the last century no such landslides were initiated. Both climatic and seismic conditions during the Quaternary may have triggered landslides. The produced landslide inventory is a historical inventory containing landslides of different ages and triggering events. Currently, only new shallow landslides or reactivations within existing deep-seated landslides occur. The focus on the Hekkebrugstraat landslide in the Flemish Ardennes allows us to understand the recent dynamics of a large reactivated landslide. It shows the complexity of the interactions between natural and human-induced processes. The focus on the Pays the Herve allows for a deeper understanding of landslide mechanisms and the cause of their origin in natural environmental conditions. These two examples are among the best-studied landslides and their analysis allows us to highlight the main processes at play and to better unravel their interactions.

Once thought to be of human origin, ramparted circular or elongate depressions that densely cover many, but not all, summit surfaces of the Hautes Fagnes area, NE Ardenne, have been recognized since the 1950s to represent relics of periglacial mounds. We examine first the regional distribution of these periglacial ramparted depressions (PRDs), whose location seems controlled mainly by elevation and lithologic factors. Geomorphological characteristics of the PRDs and trenching data obtained from several ramparts consistently show that these landforms initially developed as lithalsas through accumulation of segregation ice in the mineral soil, whose later thaw produced the presently observed ramparted depressions. 14C dating of, and pollen data from, peat layers pre- and postdating the formation of the periglacial mounds, further supported by the presence of the 12.9-ka-old Laacher See tephra in the underlying peat, point to a period of rapid formation during the second half of the Younger Dryas. Mean annual air temperature and mean temperature of July derived for the Hautes Fagnes area (~600 m asl) during the Younger Dryas from a compilation of NW European pollen data are in excellent agreement with the temperature values observed in the western Labrador Peninsula where lithalsas are currently active, showing that PRDs are valuable paleoclimate indicators.

Much research has been devoted to the development of numerical models of river incision. In settings where bedrock channel erosion prevails, numerous studies have used field data to calibrate the widely acknowledged stream power model of incision and to discuss the impact of variables that do not appear explicitly in the model's simplest form. However, most studies have been conducted in areas of active tectonics, displaying a clear geomorphic response to the tectonic signal. Here, we analyze the traces left in the drainage network 0.7 My after the Ardennes region (western Europe) underwent a moderate 100–150 m uplift. We identify a set of knickpoints that have traveled far upstream in the Ourthe catchment, following this tectonic perturbation. Using a misfit function based on time residuals, our best fit of the stream power model parameters yields m = 0.75 and K = 4.63 × 10-8 m-0.5y-1. Linear regression of the model time residuals against quantitative expressions of bedrock resistance to erosion shows that this variable does not correlate significantly with the residuals. By contrast, proxies for position in the drainage system prove to be able to explain 76% of the residual variance. High time residuals correlate with knickpoint position in small tributaries located in the downstream part of the Ourthe catchment, where some threshold was reached very early in the catchment's incision history. Removing the knickpoints stopped at such thresholds from the data set, we calculate an improved m = 0.68 and derive a scaling exponent of channel width against drainage area of 0.32, consistent with the average value compiled by Lague for steady state incising bedrock rivers. Copyright © 2014 John Wiley & Sons, Ltd.

It is still disputed whether very old archaeological and palaeontological remains found in the Belle-Roche palaeocave (eastern Belgium) pertain to the Early (∼1 Ma) or Middle (∼0.5 Ma) Pleistocene. Here, in situ-produced cosmogenic 10Be concentrations from a depth profile in nearby sediments of the Belle-Roche terrace (Ambleve Main Terrace level) are used as an indirect solution of this chronological issue. The distribution of 10Be concentrations in the upper 3 m of this profile displays the theoretically expected exponential decrease with depth. Assuming a single exposure episode, we obtain a best fit age of 222.5±31 ka for the time of terrace abandonment. However, below 3 m, the 10Be concentrations show a marked progressive increase with depth. This distinctive cosmogenic signal is interpreted as the result of slow aggradation of the fluvial deposits over a lengthy interval. Modelling of the whole profile thus suggests that the onset of the terrace formation occurred at around 550 ka, with a sediment accumulation rate of ∼20 mm ka−1. Based on two slightly different reconstructions of the geomorphic evolution of the area and a discussion of the temporal link between the cave and Main Terrace levels, we conclude that the fossil-bearing layers in the palaeokarst pertain most probably to MIS 14–13, or possibly MIS 12–11. This age estimate for the large mammal association identified in the Belle-Roche palaeokarst and the attribution to MIS 14–13 of a similar fauna found in the lowermost fossiliferous layers of the Caune de l'Arago (Tautavel) are in mutual support. Our results therefore confirm the status of the Belle-Roche site as a reference site for the Cromerian mammal association in NW Europe.

WOS: 000329766400099, While the Plio-Quaternary uplift of the Kazdag mountain range (Biga Peninsula, NW Turkey) is generally acknowledged, little is known about its detailed timing. Partly because of this lack of data, the cause of this uplift phase is also debated, being associated either to back-arc extension in the rear of the Hellenic subduction zone, to transpression along the northern edge of the west-moving Anatolian microplate, or to extension driven by gravitational collapse. Here, we perform a morphometric study of the fluvial landscape at the scale of the Biga Peninsula, coupling the recently developed R/S-R analysis of the drainage network with concavity and steepness measures of a set of 29 rivers of all sizes. While the dependence of profile concavity on basin size confirms that the landscape of the peninsula is still in a transient state, the spatial distribution of profile steepness values characterized by higher values for streams flowing down from the Kazdag massif shows that the latter undergoes higher uplift rates than the rest of the peninsula. We obtain a S-R value of 0.324 +/- 0.035 that, according to the relation established by Demoulin (2012), yields an age range of 0.5-1.3 Ma and a most probable value of 0.8 Ma for the time of the last tectonic perturbation in the region. In agreement with the analysis of knickpoint migration in a subset of rivers, this suggests that a pulse of uplift occurred at that time and, corroborated by sparse published observations in the Bayramic and Canakkale depressions, that the peninsula was uplifted as a whole from that time. This uplift pulse might have been caused by transient compressive conditions in the Anatolian plate when the Eratosthenes seamount came to subduct beneath the Cyprus arc around the early-to-mid Pleistocene transition (Schattner, 2010). (C) 2013 Elsevier B.V. All rights reserved.

Model parameterization through adjustment to field data is a crucial step in the modeling and the understanding of the drainage network response to tectonic or climatic perturbations. Using as a test case a data set of 18 knickpoints that materialize the migration of a 0.7-Ma-old erosion wave in the Ourthe catchment of northern Ardennes (western Europe), we explore the impact of various data fitting on the calibration of the stream power model of river incision, from which a simple knickpoint celerity equation is derived. Our results show that statistical least squares adjustments (or misfit functions) based either on the stream-wise distances between observed and modeled knickpoint positions at time t or on differences between observed and modeled time at the actual knickpoint locations yield significantly different values for the m and K parameters of the model. As there is no physical reason to prefer one of these approaches, an intermediate least-rectangles adjustment might at first glance appear as the best compromise. However, the statistics of the analysis of 200 sets of synthetic knickpoints generated in the Ourthe catchment indicate that the time-based adjustment is the most capable of getting close to the true parameter values. Moreover, this fitting method leads in all cases to an m value lower than that obtained from the classical distance adjustment (for example, 0.75 against 0.86 for the real case of the Ourthe catchment), corresponding to an increase in the non-linear character of the dependence of knickpoint celerity on discharge.

Geomorphometry may be a powerful tool to describe the characteristics of the landscape's response to tectonic signals, but the meaning of morphometric indices is often obscured by the interplay between the many variables controlling the geomorphological evolution. Moreover, although the so-called hypsometric integral refers to the basin scale, most indices are generally derived from the river long profiles and thus focus mainly on the short-term response of a drainage network to base level change, providing limited information in regions of older and/or moderate uplift. Here, using the Rhenish shield (western Europe), an area of moderate Quaternary uplift, as a test case, I attempt to build an index yielding a comprehensive view of the stage attained by the landscape's response and, indirectly, an evaluation of the timing of the triggering base level change. This index, called R 1 , is a ratio of differences between the three integrals linked respectively to the classical basin's hypsometric curve, to the main river's long profile, and at the intermediate level, to a ‘drainage network's hypsometric curve’. While its ratio form minimizes the lithological effect on R 1 , this index is strongly correlated with basin size (regional correlation coefficients are in the range 0.88–0.93), reflecting the way an erosion wave propagates from the outlet of a basin toward its headwaters. Therefore, it is not directly usable as a proxy for relative uplift age. However, one can show that the relation between R 1 and basin size is theoretically expected to change with time. Following uplift, the slope S r of the linear relation R 1 = f (ln A ) first increases rapidly but briefly, then it gradually diminishes over several million years. This is fully confirmed by the analysis of R 1 and S r in the study area. Once its initial increase is completed (assumedly in a few ten thousand years), S r appears to be a reliable indicator of relative uplift (or any other cause of base level lowering) age.

article i nfo The area of the Pays de Herve in eastern Belgium is strongly affected by recent landslide activity. Dormant landslides are widespread in the region and some of these dormant landslides have been reactivated by human activity such as highway construction, suburban development, or building of industrial sewage pipes. The investigated landslide was reactivated by heavy rainfall events in the late 1990s. During this reactivation an existing industrial sewage pipe was damaged and the chemically loaded waste water drained into the landslide mass. The current landslide movement is assumed to be heavily influenced by the additional water supply as well as by the chemical activity of univalent ions in the clay fraction leading to a decreased shear resistance. Consequently, the movement-triggering rainfall thresholds are reduced and result in a decreased overall slope stability. This study describes a general methodology of field investigation, data collection and laboratory testing for an exploratory study. Influences of chemical properties of soil and soil water and their effects on landslide stability are analysed, discussed and related to findings of other studies. Although no clear empirical evidence for a dependency between slope stability and ionic loaded soil water was found in this study, we suggest the existence for such a relationship. We conclude that this particular landslide would not have moved based on current environmental conditions. The leaking waste water might favour current landslide movement.

It is still debated whether the primary control on the middle Pleistocene denudation of the uplifted Ardennes massif (western Europe) is tectonic or climatic. Here, based on geomorphological observations, we calculate the amount of river incision and interfluve denudation in the Meuse basin upstream of Maastricht since 0·7 Ma and we show that the main response to tectonic forcing was incision. This allows us to provide first-order estimates of the tectonic and climatic contributions to the denudation of the Ardennes. From a dataset of 71 remnants of a terrace level dated ∼0·7 Ma, we first derive a basin-scale functional relationship linking incision with distances to the regional base level (Lc) and to the source (Ls) in the Ourthe basin (pertaining to the Ardennian part of the Meuse basin). Expressed as I = I0*(1 – a*Lcb/Lsc), I0 being the incision measured at the basin outlet, this relationship calculates that river incision has removed 84 km3 of rock in the Meuse basin upstream of Maastricht since 0·7 Ma. In the same time, 292 km3 were eroded from the interfluves. A comparison of these volumes shows that the tectonically forced river incision accounts for ∼22% of the total post-0·7 Ma denudation. Furthermore, the mean denudation rate corresponding to our geomorphological estimate of the overall denudation in the Meuse basin since 0·7 Ma amounts to 27 mm/ky, a figure significantly lower than the ∼40 mm/ky mean rate derived from 10Be studies of terrace deposits of the Meuse (Schaller et al., 2004). This suggests that, taken as a basin average, the 10Be-derived rate is overestimated, probably due to an overrepresentation of the erosion products of the rapidly incising valleys in the alluvial deposits. Copyright © 2009 John Wiley & Sons, Ltd.

article i nfo A good evaluation of the Quaternary uplift of the Rhenish shield is a key element for the understanding of the Cenozoic geodynamics of the western European platform in front of the alpine arc. Previous maps of the massif uplift relied on fluvial incision data since the time of the rivers' Younger Main Terrace to infer a maximum post-0.73 Ma uplift of ~290 m in the SE Eifel. Here, we propose a new interpretation of the incision data of the intra-massif streams, where anomalies in the terrace profiles would result from knickpoint retreat in the tributaries of the main rivers rather than from tectonic deformation. We also use additional geomorphological data referring to (1) deformed Tertiary planation surfaces, (2) the history of stream piracy that severely affected the Meuse basin in the last 1 Ma, and (3) incision data outside the Rhenish shield. A new map of the post-0.73 Ma uplift of the Rhenish shield is drawn on the basis of this enlarged dataset. It reduces the maximum amount of tectonic uplift in the SE Eifel to ~140 m and modifies the general shape of the uplift, namely straightening its E-W profile. It is also suggested that an uplift wave migrated across the massif, starting from its southern margin in the early Pleistocene and currently showing the highest intensity of uplift in the northern Ardennes and Eifel. These features seem to favour an uplift mechanism chiefly related to lithospheric folding and minimize the impact on the topography of a more local Eifel plume.

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.

The study of small and/or slow reactivations of landslides requires describing their displacements over decades, which may be done with accurate multi-temporal digital terrain models (DTMs). We applied aerial stereophotogrammetry to build the historical topographies of old deep-seated landslides close to Oudenaarde in the Flemish Ardennes (West Belgium) at different dates. Three precise aerotriangulations (1996, 1973, 1952) were carried out. After capturing the ground data manually from the stereomodels, 2 m-resolution DTMs were interpolated by kriging, with a final accuracy ranging between ~ 45 cm and ~ 65 cm. Another DTM was interpolated with an accuracy of ~ 30 cm from airborne LIDAR data acquired in 2002. Differential DTMs were produced to identify vertical and horizontal ground displacements over the 1952–2002 period. We describe here the kinematics of a particularly active landslide with a well-documented recent activity. Until the first half of the 90 s, little activity of the landslide was detected. In February 1995 a reactivation event caused vertical displacements of up to − 7 m along the main scarp and up to + 4 m in the accumulation zone. Horizontal movements of 4 to 10 m are also inferred. These topographic changes correspond to reactivated slip along the rotational basal shear surface. In the same time, the main scarp retreated by up to 20 m. The reactivation, favoured by several anthropogenic factors (e.g. loading, impeded drainage), was triggered by intense rainfall. Between 1996 and 2002, the observed displacements correspond to limited scarp retreat (≤ 4 m) and compaction of the slipped mass, partly enhanced by artificial drainage.

Fluvial terraces are a powerful tool for unraveling the combined tectonic and climatic conditions that controlled, directly or indirectly, the Quaternary incision of rivers. Terrace long profiles are usually retrieved from sparse traces of ancient floodplains preserved in the present topography. However, when these traces classically collected from topographic maps, aerial photographs, and field analyses are too few, the inferred profiles may be questionable. Yet the now available high quality and high resolution digital elevation models (DEMs) offer an opportunity to increase greatly the quantity of information usable to reconstruct terrace profiles. Therefore, the purpose of this study was to develop a new DEM-based method of terrace recognition in order to create a larger database and better constrain the profile reconstruction. Moreover, particular procedures of image and numerical processing were defined to fully automate the analysis. Basically, our method relies on the production of bivariate scatter plots depicting the relation between slope and relative altitude (i.e., the altitude above the current alluvial plain) for all pixels of successive sections of the valley. For each scatter plot, the curve of the lowest slope values observed at every relative altitude is smoothed and its minima are assumed to locate the altitudes of the “terrace” elements preserved in the section. We successfully tested this method in the Vesdre valley, incised in the NE Ardenne massif (E Belgium), notably identifying fault deformation of the profiles. The main advantages of our approach are its objectivity, exhaustiveness, and rapidity, allowing fast and coherent analysis of many rivers over extended regions.

A landslide susceptibility map is proposed for the Pays de Herve (E Belgium), where large landslides affect Cretaceous clay outcrop areas. Based on a Bayesian approach, this GIS-supported probabilistic map identifies the areas most susceptible to deep landslides. The database is comprised of the source areas of ten pre-existing landslides (i.e. a sample of 154 grid cells) and of six environmental data layers, namely lithology, proximity to active faults, slope angle and aspect, elevation and distance to the nearest valley-floor. A 30-m-resolution DEM from the Belgian National Geographical Institute is used for the analysis. Owing to the small size of the sample, a special cross-validation procedure of the susceptibility map is performed, which uses in an iterative way each of the landslides to test the predictive power of the map derived from the other landslides. Four different sets of variables are used to produce four susceptibility maps, whose prediction curves are compared. While the prediction rates associated with the models not involving the “proximity to active fault” criterion are comparable to those of the models considering this variable, strong weaknesses inherent in the fault data on which the latter rely suggest that the final susceptibility map should be based on a model that excludes any reference to fault. This highlights the difference between a triggering factor and determining factors, and in the same time broadens the scope of the produced map. A single reactivated slide is also used to test the possibility of predicting future reactivation of existing landslides in the area. Finally, the need for geomorphological control over the mathematical treatment is underlined in order to obtain realistic prediction maps. © 2007 Elsevier B.V. All rights reserved.

The number of human-induced landslides is increasing worldwide, but information on the impact of human intervention on slope stability is often lacking. Therefore, this study analyses the Hekkebrugstraat landslide, the best-recorded landslide in the Flemish Ardennes (Belgium). Information obtained from local inhabitants, aerial photographs and newspaper articles enabled a 50-year reconstruction of both the landslide history and the land-use changes at or close to the landslide site. The reconstruction suggests that anthropogenic preliminary factors such as: (i) the absence of well-maintained drainage ditches in the affected area; (ii) the elevation of the surface of the road, i.e. a sunken lane, in the affected area; (iii) increased surface runoff from the drainage area; (iv) the creation of ponds; and (v) the removal of the lateral support at the landslide foot have played an important role in the reactivation of the Hekkebrugstraat landslide. After the reactivation of February 1995, landslide movement was observed for more than 5 years and caused damage to houses, and other infrastructure. However, also natural factors, such as the presence of an impermeable clay layer at limited depth, springs and relatively steep slopes (i.e. 0.14 m m )1 ), and above normal antecedent rainfall have contributed to the reactivations. Comparison of our reconstruction of the reactivation with precise Digital Terrain Models (DTMs) of 1952, 1973 and 1996, produced by digital stereophotogrammetry, indicated that the reported movements correspond well with the uplifted and collapsed zones found on the DTMs. Hence, this analysis provides valuable information for land-use planners in areas with old, apparently stable, landslides.

Despitetheavailabilityofstudiesonthefrequencydensityoflandslideareasinmountainousregions,frequency–areadistributions of historical landslide inventories in populated hilly regions are absent. This study revealed that the frequency–area distribution derived from a detailed landslide inventory of the Flemish Ardennes (Belgium) is significantly different from distributions usually obtainedinmountainousareaswherelandslidesaretriggeredbylarge-scalenaturalcausalfactorssuchasrainfall,earthquakesorrapid snowmelt. Instead, the landslide inventory consists of the superposition of two populations, i.e. (i) small (b1–2·10 −2 km 2 ), shallow complexearthslides that areat most30yrold, and(ii) large(N1–2·10 −2 km 2 ), deep-seatedlandslides that areolder than100yr. Both subpopulations are best represented by a negative power–law relation with exponents of −0.58 and −2.31 respectively. This study focused on the negative power–law relation obtained for recent, small landslides, and contributes to the understanding of frequency distributions of landslide areas by presenting a conceptual model explaining this negative power–law relation for small landslides in populatedhilly regions.According to the model hilly regions canberelatively stable under the present-dayenvironmental conditions, andlandslidesaremainlytriggeredbyhumanactivitiesthathaveonlyalocalimpactonslopestability.Therefore,landslidescausedby anthropogenic triggers are limited in size, and the number of landslides decreases with landslide area. The frequency density of landslide areas for old landslides is similar to those obtained for historical inventories compiled in mountainous areas, as apart from the negative power–law relation with exponent −2.31 for large landslides, a positive power–law relation followed by a rollover is observed for smaller landslides. However, when analysing the old landslides together with the more recent ones, the present-day higher temporal frequency of small landslides compared to large landslides, obscures the positive power–law relation and rollover. © 2007 Elsevier B.V. All rights reserved.

Although desirable for a reliable hazard assessment, rate estimates of landslide motion rarely combine a good time resolution and a sufficiently long time of observation. Here, both angles are tackled for the Manaihan landslide (East Belgium), dramatically reactivated in September 1998. I monitored the landslide displacements by repeated Global Positioning System (GPS) surveys from 1999 to 2005. Two digital elevation models were also produced, one of the landslide topography in 1999 by GPS and a second by stereophotogrammetry from aerial photographs of 1953. Subtracting one model from the other, I mapped the height changes within the landslide over the 1953–1999 period. All measurements consistently showed that, beyond the sudden ∼1.5 m slip of September 1998, the landslide moved at a mean rate of c. 20 cm yr−1 since 1980. Most displacements occurred around the winter's end, when long-lasting precipitation combined with minimal evaporation and occasional intense daily rainfall. The motions are spatially determined by seepage from a broken sewage pipe inducing local high pore pressures. Terra Nova, 18, 290–298, 2006

The Feldbiss fault is the main active fault bounding to the southwest the Roer Valley Graben of NW Europe. In order to investigate the reasons of the discrepancy between its short- and long-term slip rate estimates, we have repeatedly surveyed a 2.3 km-long line by levelling at Sittard (The Netherlands) from April 2001 to December 2004. Simultaneously, three superposed aquifers were monitored on both sides of the fault. The across-fault differential groundwater variations are shown to be responsible for a seasonal 0.87 mm/m vertical motion of the fault, mainly through the effect of pore pressure changes in confined aquifers. After removal of this effect, I get a residual trend of − 0.60 ± 0.11 mm/yr for the relative motion of the hangingwall. This trend was disturbed in the summer of 2002 by transient millimetre-level up and down motions temporally associated with the small Eschweiler earthquake that occurred on July 22, 2002 on the Feldbiss fault at a distance of 35 km from Sittard. I explain this as a creep event triggered by the passage of trapped seismic waves. The subsisting discrepancy between the long-term slip rate of 0.06 mm/yr and short-term rates of ∼ 1.5 mm/yr (from 1962 to 2001) and 0.6 mm/yr (from 2001 to 2004) is tentatively interpreted as resulting from the current occurrence of a decade-long episode of aseismic slip involving a large part of the fault plane. Occasionally associated with minor seismicity, this aseismic slip event could betray a thickening of the transitional region located below the upper crustal zone of stable sliding. It causes a total fault slip of 5–10 cm in a few tens of years, releasing smoothly all, or a great part of, the strain accumulated during the previous 1–2 ky of fault quiescence.

Traditionally, the long-term landscape evolution of the southern Buenos Aires ranges of east central Argentina has been related to the influence of the Andean orogeny. We describe the large-scale morphological units and associated weathering products in the Tandilia and Ventania ranges. Two main planation surfaces are encountered at varying altitudes in different sectors of these ranges. The lower surface is characterized by roots of kaolinized weathering profiles in the Tandil area and silicified conglomerates around Sierra de La Ventana. In an interpretative model linking the range morphogenesis to the tectonosedimentary evolution of the bordering Salado and Colorado Basins, we suggest that the main morphogenetic stages are related to the late Jurassic-early Cretaceous south Atlantic rifting and Miocene tectonic reactivation induced by the Andean orogeny. Thus, the uplifted surfaces appear much older than commonly believed: pre-Cretaceous and Paleogene. Although they contradict recent results of apatite fission-track studies along the South America and South Africa passive margins, the implied low denudation rates (∼4 m/My) can be explained by the limited Meso-Cenozoic uplift suffered by the southern Buenos Aires ranges. The discussion also shows the limits of the comparison that can be made with the South African planation surfaces.

International audience; The Flemish Ardennes (W Belgium) are known to be affected by deep-seated landslides. The assessment of the landslide reactivation hazard requires understanding the driving processes and delimiting precisely not only the landslide boundaries but especially that of their most active parts. Precise 3D models of 13 landslides were produced by digital stereophotogrammetry using aerial photographs of different dates. Dealing with photographs at the scale 1:25000 or larger, we obtained for each model an accuracy better than 0.5m. As a first result, the main size parameters of the landslides (width, length, depth, volume, ...) are easily computed. Moreover, the obtained DTMs may be subtracted from each other in order to determine the apparent vertical displacement of each pixel during the interval of time considered. Provided that more than 2 epochs are documented, such DTMs not only supply precise information about distribution and style of the landslide activity but may also point to temporal variations in this activity. The subtraction of DTMs allows us to give an estimation of the volume of the "uplifted" and "collapsed" terrains between two epochs.

We present the HARD project of GPS monitoring of vertical ground motion in NE Ardenne and Eifel (western Europe). Its main purposes are to get a better insight into the present-day rates of vertical ground motion in intraplate settings and to identify the various causes of these motions. Since 1999, we have carried out yearly campaigns of simultaneous GPS measurements at 12 sites situated so as to sample the different tectonic subunits of the study area and especially to record potential displacements across the seismogenic Hockai fault zone. Five campaigns (1999–2003) have been processed currently. Key issues of the data processing with the Gamit software are discussed and first results are presented. Though temporally consistent in many cases, the obtained vertical motion rates are spatially highly variable. They are also much too high (several mm/year) to support a tectonic interpretation, and a long-term influence of groundwater level variations is proposed to account for the observed motions. This influence should be distinguished from seasonal variations and from inter-survey variations linked to the varying degree of soil and subsoil drying off during the successive spring surveys.