Ronald T. van Balen

Numerical modeling of the response of alluvial rivers to Quaternary climate change

Abstract A numerical model, which simulates the dynamics of alluvial river channels on geological (Quaternary) time scales, is presented. The model includes water flow, channel dimensions, sediment transport and channel planform type. A number of numerical experiments, which investigate the response of an alluvial river to imposed sequences of water and sediment supply, with special emphasis on the time lags between these controlling variables, as well as a downstream discharge increase, are presented. It is found that the influence of the time lags can be substantial, having major implications for the reconstructions of palaeo climate based on river channel behavior documented in the geological record. The model is further applied to both a conceptual warm–cold–warm cycle and a reconstructed evolution of the river Meuse, the Netherlands, during the Late Glacial–Holocene warming. Results show that the model is capable of explaining the response of this river, although better validation against palaeoenvironmental data remains necessary.

Slip tendency analysis as a tool to constrain fault reactivation: A numerical approach applied to three‐dimensional fault models in the Roer Valley rift system (southeast Netherlands)

We describe a new numerical approach to constrain the three‐dimensional (3‐D) pattern of fault reactivation. Taking advantage of the knowledge of the tectonic stress field, the ratio of the resolved shear and normal stresses (slip tendency) as well as the direction of the shear stress is calculated at every location on the faults modelled by triangulated surfaces. Although the calculated contact stresses represent only a first order approximation of the real stresses, comparison of the 3‐D pattern of slip tendency with the frictional resistance of the fault can provide useful constraints on the probability of fault reactivation. The method was applied to 3‐D geometrical fault models in the Roer Valley Rift System (southeast Netherlands) which is presently characterized by pronounced tectonic activity. The input stress tensors were constrained by published stress indicators. The analysis demonstrated that the observed fault activity could be explained within a reasonable range of frictional parameters and input stress magnitudes. In addition a fairly good correlation was found between the predicted slip directions and the focal mechanisms of local earthquakes. This suggests that in the study area, fault models being valid in the uppermost part of the crust are suitable to constrain fault reactivation even in the deeper part of the seismogenic layer. The analysis further demonstrated that fault hierarchy and the regional tectonic contexts of the fault system are important factors in fault reactivation. Therefore they always should be taken into account during evaluation of the calculated slip tendency and slip direction patterns.

Hydrogeological aspects of fault zones on various scales in the Roer Valley Rift System.

Abstract The impact of faults on the groundwater flow system in the Roer Valley Rift System (RVRS) is demonstrated with examples from outcrop scale to regional scale. Faults in the RVRS can form strong barriers to horizontal groundwater flow as well as enhanced vertical groundwater flow paths at the same location. The strongly anisotropic hydraulic conductivity distribution within fault zones has important implications for the modeling of groundwater flow in sedimentary aquifer systems that are cut by faults. In this study, the hydraulic behavior of fault zones is studied at different scales. An outcrop study over the Geleen Fault zone shows deformation mechanisms as particulate flow and clay smearing in great detail. Qualitative and quantitative image analysis allows for an estimate of the micro-scale variation of the hydraulic properties within a fault zone. Additional core-plug measurements indicate that the damage zone around fault zones may form preferential flow paths. On a larger scale, observations over the Peel Boundary fault near the village of Uden also indicate that vertical groundwater flow close to the fault is enhanced, which results in a discharge of the underlying aquifers at the location of the fault zone. Finally, on a regional scale, hydraulic head patterns around the lignite mining areas in Germany show the importance of faults and the variation of their hydraulic properties to regional groundwater flow patterns.

The effect of inplane force variations on a faulted elastic thin‐plate, Implications for rifted sedimentary basins

Studies of flexural motions of lithosphere commonly apply a differential equation based on the thin-plate approach. In this approximation, the flexural response to a changing horizontal inplane force depends only on the curvature of the midplane of the thin-plate representing the mechanical behaviour of the lithosphere. However, in cases where an abrupt change of the geometry of the lithosphere occurs the midplane of the thin-plate is offset. We demonstrate that the combination of the offset with an inplane horizontal force produces an additional, rheology independent moment at the position of the geometry change. This effect has been overlooked by previous studies of lithosphere deflections, and thin-plate problems in general. In the presented analysis, the thin-plate with an abrupt change in plate geometry represents lithopshere with a mechanically healed, inactive fault. However, the derived analytical solutions are general and can be used to study problems with similar abrupt geometry changes.

Fluvial terraces of the northwest Iberian lower Miño River

A new fluvial terrace map with a tectonic framework for the northwest Iberian lower Miño River is presented. It is the first integrated map to cover the entire lower, 67-km reach of the Miño River, and to cover both the Spanish and Portuguese side of the river. The map is presented at a scale of 1:200,000, although its features were mapped at a scale of 1:5000. Various map layers can be viewed, such as a digital elevation model (DEM), fluvial sediment thickness layers, a palaeoflow direction layer, a lineament and fault layer, and two terrace and tectonic basin layers, showing up to 10 fluvial terraces and a floodplain level. Interpretation of the map shows that next to regional tectonic uplift and glacioeustacy, local basin subsidence and small-scale block movement are very important for the fluvial network, localised fluvial terrace formation, and preservation.

Climatic and tectonic controls on the fluvial morphology of the Northeastern Tibetan Plateau (China)

The geomorphological evolution of the Northeastern Tibetan Plateau (NETP) could provide valuable information for reconstructing the tectonic movements of the region. And the considerable uplift and climatic changes at here, provide an opportunity for studying the impact of tectonic and monsoon climate on fluvial morphological development and sedimentary architecture of fluvial deposits. The development of peneplain-like surface and related landscape transition from basin filling to incision indicate an intense uplift event with morphological significance at around 10–17 Ma in the NETP. After that, incision into the peneplain was not continuous but a staircase of terraces, developed as a result of climatic influences. In spite of the generally persisting uplift of the whole region, the neighbouring tectonic blocks had different uplift rates, leading to a complicated fluvial response with accumulation terraces alternating with erosion terraces at a small spatial and temporal scale. The change in fluvial activity as a response to climatic impact is reflected in the general sedimentary sequence on the terraces from high-energy (braided) channel deposits (at full glacial) to lower-energy deposits of small channels (towards the end of the glacial), mostly separated by a rather sharp boundary from overlying flood-loams (at the glacial-interglacial transition) and overall soil formation (interglacial). Pronounced incision took place at the subsequent warm-cold transitions. In addition, it is hypothesized that in some strongly uplifted blocks energy thresholds could be crossed to allow terrace formation as a response to small climatic fluctuations (103–104 year timescale). Although studies of morpho-tectonic and geomorphological evolution of the NETP, improve understanding on the impacts of tectonic motions and monsoonal climate on fluvial processes, a number of aspects, such as the distribution and correlation of peneplain and the related morphological features, the extent and intensity of tectonic movements influencing the crossing of climatic thresholds, leading to terrace development, need to be studied further.