Due to the frequently observed disparities between stretching amounts obtained from faults, crustal thickness and tectonic subsidence, the development of intracratonic basins cannot always be explained by a simple model of lithospheric extension. Basin evolution may then be regarded as the result of superimposed and successive processes. The Paris Basin is chosen as a type example for the discussion of European intracratonic basins. The tectonic subsidence, reconstructed using a standard method, is small (maximum of 1600 m. The long-term linear or concave curves are interrupted by periods of short-term acceleration and deceleration. Thus, tectonic subsidence is clearly discontinuous and five phases, with duration of 20-60 Myr, constitute the long-term Meso-Cenozoic Subsidence. The boundaries and pattern of each phase are identical all over the basin. Subsidence is interpreted as the result of several modifications of the lithospheric structure. The uniform stretching model can explain the first Triassic to early 1iassic phase, the only one with a concave-upward trend (deceleration). The Jurassic to Cretaceous subsidence could be explained by superposing (1) a long-term component caused by lower crustal flow and/or underplating and (2) several short-term accelerations (convex-upward trend) related to compressive or transpressive forces. Geophysical control is insufficient to test the first postulate accurately and the generation of sufficiently high compressional stresses during the Jurassic-Cretaceous is questionable for the second. Other Northwest European basins are compared with the Paris Basin. Although similar features can be observed, the overall image is not uniform: ‘Paris Basin’ (intracratonic)as well as ‘Celtic Sea Basin’ (passive margin) signatures, variable long-term trends, lack of synchronism of subsidence phases. This picture necessitates different driving mechanisms for subsidence across Northwest Europe. The variable subsidence patterns and processes result from the ‘remote’ geodynamics of the Atlantic and Tethyan realms, combined with mainly ‘active’ processes similar to those proposed for the Paris Basin
Abstract. This paper presents a discrete-element-based elastoplastic-adhesive model which is adapted and tested for producing hillslope debris flows. The numerical model produces three phases of particle contacts: elastic, plastic and adhesive. A parametric study was conducted investigating the effect of model parameters and inclination angle on flow height, velocity and pressure, in order to define the most sensitive parameters to calibrate. The model capabilities of simulating different types of cohesive granular flows were tested with different ranges of flow velocities and heights. The basic model parameters, the microscopic basal friction (ϕb) and ratio between stiffness parameters k1/k2, were calibrated using field experiments of hillslope debris flows impacting a pressure-measuring sensor. Simulations of 50 m3 of material were carried out on a channelized surface that is 41 m long and 8 m wide. The calibration process was based on measurements of flow height, flow velocity and the pressure applied to a sensor. Results of the numerical model matched those of the field data in terms of pressure and flow velocity well while less agreement was observed for flow height. Those discrepancies in results were due in part to the deposition of material in the field test, which is not reproducible in the model. Results of best-fit model parameters against selected experimental tests suggested that a link might exist between the model parameters ϕb and k1/k2 and the initial conditions of the tested granular material (bulk density and water and fine contents). The good performance of the model against the full-scale field experiments encourages further investigation by conducting lab-scale experiments with detailed variation in water and fine content to better understand their link to the model's parameters.
Does the forest provide protection from landslides? Evidence from the WSL Shallow Landslide Database During strong rainfall events, shallow landslides and debris avalanches (hillslope debris flows, or open-slope debris flows) are triggered and sometimes lead to considerable damage. Analysis of damage-causing events show that there are fewer landslides in forested areas compared to non-forested areas, which indicates the generally positive influence of forest vegetation on slope stability. However, these effects depend on the condition of the forest stand and quantification of the effects is difficult. Event documentation contributes to a better understanding of the relevant processes. The information obtained is not only important for the preparation of hazard maps, but also provides valuable insight for assessing the hazard protection provided by the forest. Data from the landslide database of the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) were used to evaluate the influence of the forest on slope stability. Currently, the database contains information on 734 landslides. Of these, 661 were included in the evaluation – 356 landslides in non-forested areas and 305 in forested areas. In areas with slope angles up to 38°, more landslides per unit area are observed in non-forested areas than in forested areas. In areas with steeper slope angles a stabilizing effect of the forest is no longer recognizable. Statistical analyses show that landslides in forested areas are smaller than in non-forested areas and are more frequent on steeper slopes. In general, the landslides become smaller with increasing slope. Multivariate analyses indicate a positive influence of the forest and also somewhat smaller landslides in well-developed forests. Negative effects are evident in non-forested areas and in areas with overly dense forests. In addition to illustrating the importance of the forest condition for slope stability, the paper also discusses how the forest condition can be described.
Located at the hinge between the stable European continental platform and the northern Alpine Tethys, the Lower Jurassic basin of the northern Helvetic realm (western Switzerland) can be subdivided into several elongate symmetrical WSW-ENE sub-basins. Lateral thickness changes are gradual, and areas of non-deposition act as a major control on facies: Helvetic sandy facies near exposed zones and Dauphine shaley facies in more distal zones.The sediments were deposited in shallow marine offshore and foreshore zones. However, due to Alpine tectonics, preservation of sedimentary structures and fossils is poor. The sections studied display systematic events which can be interpreted as transgressive surfaces, sequence boundaries, condensed sections and shallowing-up sequences. The Lower Jurassic sedimentary record can be subdivided into genetically related strata recording relative base-level changes. Sedimentary structures or facies assemblages related to episodic palaeofault activity are absent. At least two orders of cyclicity (2nd and 3rd) have been recognized, and the same 3rd-order sequences are found in different sub-basins and in the two main facies types. The mechanisms controlling sedimentary sequences are therefore either regional (main-basin scale) or global. However, correlation with global events is recognized only in some situations, and eustasy is therefore not the only control on cyclic sedimentation.In order to investigate the regional processes, the subsidence history has been analysed. The tectonic subsidence curves, with corrections for depositional water depth, compaction, tectonic deformation, erosion, eustasy and Airy compensation, have been compared using three stretching models: uniform extension of the entire lithosphere, crustal and subcrustal extension (both non-uniform or depthdependent discontinuous stretching models). For the study area, the most suitable model is subcrustal stretching, with more extension in the lower lithosphere than in the crust. As predicted using the model, the ratio of initial fault-controlled to thermal subsidence is low and the studied basin is mainly thermally controlled.The results from sequence• and geohistory analyses are integrated. A possible process controlling sequences, and compatible with a mainly thermally controlled basin, is in-plane stress change. This mechanism can explain all observed features, including the asymmetric sedimentary cycles. The stratigraphical record may therefore result from the superposition of both eustasy and in-plane stress changes.The northern Helvetic Lower Jurassic basin was an intracratonic sag-basin and not, at that time, a constituent part of the more fault-controlled North Tethyan margin.
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