Richard Fabre

Analogue modelling of fault reactivation: tectonic inversion and oblique remobilisation of grabens

Abstract Analogue models of sand above a silicone layer were examined to determine the effects of normal fault reactivation. Models were first subjected to extension, which lead to the formation of two linear grabens. Each model was then subjected to a second phase deformation, either parallel or oblique to the previous initial extension, and either extensional or contractional. The influence of sedimentation has been evaluated using experiments with and without sedimentation. In cases of oblique secondary deformation, all newly formed faults were parallel to the older grabens, thus they were oblique to the direction of the principal stress directions during the second deformation phase. In experiments without sedimentation, all older faults were reactivated, whereas in experiments with sedimentation, some of the older faults were not reactivated. In the case of an oblique compressional secondary deformation phase with post rift sedimentation, strain partitioning occurred between reactivated older normal faults and new reverse faults. σ 2 was vertical on reactivated normal fault planes, whereas σ 3 was vertical on reverse fault planes. In the case of oblique compressional secondary deformation phase without post rift sedimentation, no strain partitioning was observed. In this model, σ 3 was vertical on every fault plane. It is therefore concluded that sedimentation within grabens induces a variation of stress orientation and strain partitioning.

Découverte de turbidites du Crétacé supérieur métamorphisées au contact d'intrusions d'ophites dans les Pyrénées occidentales (vallée d'Aspe, France). Vers une révision de l'âge des ophites pyrénéennes

Abstract On the French side of the Pyrenees, in the Bedous area of the Aspe Valley, fine-grained turbidites classically dated as Carboniferous and Triassic are attributed to the Upper Cretaceous. They contain fucoids and Globotruncanidae assemblages of the Dicarinella concavata Zone (Upper Turonian–Lower Santonian) and are metamorphosed at the contact of ophites intrusions. This discovery implies a new interpretation of the regional stratigraphy and palaeogeography. Moreover, it questions the age of the ophites emplacement in the region as well as the structural relationships between the ‘Bedous basin’ and the North Pyrenean Zone to the north and the Palaeozoic High Chain to the south. To cite this article: C. Desreumaux et al., C. R. Geoscience 334 (2002) 197–203.

Les glissements rocheux du versant sud du Layens (Vallee d'Aspe, Pyrenees occidentales)

Detailed geological surveys carried out recently on the Layens have permitted to redefine the lithostratigraphy and to establish the structure of its south slope where several rockslides have been precisely located. This south slope is composed of a carbonaceous Mesozoic series ranging from the Triassic up to the Lower Cretaceous which is located on the reverse flank of a large overfolded north north-eastern syncline. The lithological and structural heritage, together with its special geodynamic setting, generate many brittle zones which favour the mechanical instability of the slope. On this basis the morphostructural position of the rockslides has been established, some of which are unstable while others are stable. Two stages in the evolution of the rockslides have been deduced from the geomorphology and mapping. These field observations have enabled us to distinguish three different rockslides (A, B, C) which are either deep or superficial. The analysis of the causes of their instability have been considered by taking into account the orientation of the major mechanical discontinuities, inherited from the structural context, as well as the sensitivity to the slipping of Triassic clay formations. The natural causes of the destabilization of the south slope of the Layens have been established for the most active rockslide : the geological surveys permit an evaluation of the volume and the total displacements.

Shrinkage and swelling of clay soil: Comparison between laboratory and in situ measurements

ABSTRACT This paper presents the results of the shrinkage and swelling behavior of clay soils in a geological formation in the laboratory and in situ. Free swelling and drying tests on undisturbed samples, from an experimental site in the Gironde department (33) in France, have allowed theoretical displacements to be computed, starting from the free swelling parameter ϵg (%) and the linear shrinkage factor Rl (-). The computed displacements are of the order of millimeters in swelling and centimeters in shrinkage, whereas on the experimental site, they are of the order of millimeters during an annual cycle. However, following a second year of in situ measurements, we observed a cumulative settlement effect of the order of one centimeter. By comparing the results obtained, we revealed the influence of various factors, such as lithological variabilities at different scales, and repetitive shrinkage-swelling cycles, on the behavior of clay soils.

3D NUMERICAL SIMULATIONS OF WAVES GENERATED BY SUBAERIAL MASS FAILURES: APPLICATION TO LA PALMA CASE

Three-dimensional (3D) waves generated by landslides are simulated using a three-fluid Navier-Stokes VOF model. With this approach, the interaction between slide and water is implicitly solved. The model capabilities are first tested for benchmark cases featuring rigid body motion. Results are good in two dimensions (2D) and encouraging in 3D. Wave generation by a potential collapse of the Cumbre Vieja Volcano, on La Palma island, is then studied. Stability analyses show that the Cumbre Vieja flank is currently highly stable and that potential slide volumes are likely to be closer to 100 km, rather than the 500 km predicted in earlier studies. Results of the Navier-Stokes model show that waves generated are highly dependent upon the details of slide mechanism and kinematics. In our worst 3D scenario (assuming an inviscid fluid), the initial wavelength is 20 km and the wave height decrease due to lateral spreading is high.