Very low-grade metamorphism in the para-autochtonous sedimentary cover of the Pelvoux massif (Western Alps, France)   总被引：1，自引：0，他引：1  

Sébastien Potel  Ghislain Trullenque 《Swiss Journal of Geoscience》2012,105(2):235-247

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Photoelastic and numerical investigation of stress distributions around fault models under biaxial compressive loading conditions     

Ghislain de Joussineau  Jean-Pierre Petit  Bertrand D. M. Gauthier 《Tectonophysics》2003,363(1-2):19-43

This paper presents the results of a photoelastic and numerical study of stress distributions (contours and trajectories) around fault models of various geometries, submitted to a biaxial compressive load. It aims to describe typical biaxial stress behaviours and emphasize the existing differences with the well-known uniaxial compressive load case. Stress trajectories are sometimes shown by joint sets acting as markers of a paleostress field, and they can be interpreted by particular shallow tension–compression situations. At depth fractures can be reactivated, or can dilate under conditions of triaxial compression, and behaviour is essentially controlled by a high stress ratio (high σ₃/σ₁ ratio). In spite of the potential importance of such stress states on fracture permeability and fluid flow, and although they are frequently found at depth in a reservoir context, such stress conditions are poorly investigated, particularly in terms of stress perturbations.The presented analogue experiments consisted of compression tests done on polymethylmethacrylate (PMMA) plates; this material has mechanical properties comparable to those of brittle rocks in the upper crust, and presents birefringence. The samples contained open defects acting as faults, and the stress trajectories around these faults were investigated using a photoelastic device. Comparable numerical experiments were realised with a finite-element code (Franc 2D), using frictionless fault models.First, the effect of an increasing biaxial compressive load ratio σ₃/σ₁ on stress trajectories around an isolated open defect was explored. It was shown that the stress trajectories were drastically modified when σ₃/σ₁ increased from 0.2 to 0.4, this result being consistent with previous studies. In particular, when σ₃/σ₁ was superior or equal to 0.4, external isotropic points around where trajectories diverged (called repulsive isotropic points) appeared near the tips, but away from the defects. They tended to move away from the defects towards the main load direction when σ₃/σ₁ increased. The described isotropic points were points of stress trajectory divergence, i.e. points where stress decreased, implying that zones around them were strongly unfavourable to shear reactivation.Second, stress trajectories around fault models of various geometries (oblique isolated defect, dilational jog, compressive jog, and complex patterns) were studied, the applied biaxial compressive load ratio being 0.7. These biaxial stress trajectories were compared with similar uniaxial stress trajectories in order to explore the existing differences between the two regimes. It enabled new stress trajectory geometries to be described and interpreted. In particular, numerous external repulsive isotropic points were observed, and defect tips were shown to be zones of high convergence of stress trajectories. Furthermore, in contrast to the uniaxial compressive load case, stress trajectories and stress contours were geometrically similar within dilational and compressive jogs under biaxial compressive load. For both jog types, the centre of the overlapping zone, and the areas along the wall of a jog-defining fault and facing the overlapping tip of the other fault, proved to be zones of low mean stress, implying that fluids may migrate towards them from the tips of the faults, in response to mean stress gradients. Furthermore, the centre of the jogs exhibiting a high differential stress was a favourable area in terms of fracture reactivation, which may facilitate fluids transfer and storage.It was also observed that for both the uniaxial and biaxial compressive loads, isotropic zones were localised at the acute angle between branching defects.  相似文献   

Typology of Natural Hazards and Assessment of Associated Risks in the Mount Bambouto Caldera (Cameroon Line,West Cameroon)     

Ghislain T. ZANGMO  Armand D. KAGOU  David G. NKOUATHIO  Pierre WANDJI 《《地质学报》英文版》2009,83(5):1008-1016

Abstract: Mount Bambouto is a polygenic stratovolcano of the Cameroon Volcanic Line, built between 21?Ma and 4.5?Ma. It is situated approximately 200?km NE of Mount Cameroon, between 09°55′ and 10°15′ longitude east and, 05°25′ and 05°50′ latitude north. The volcano covers an area of 500?km2 and culminates at 2740?m at Mélétan dome and bears a collapsed caldera at the summit (13?×?8?km). Mount Bambouto is characterized by several natural hazards of different origins: meteorological, such as landslides and rock falls; anthropogenic, such as bushfires, tribal wars and deforestation; and volcanological, such as volcanic eruption. The thematic map shows that 55–60% of the caldera has high probability of occurrence of mass movement. The caldera has a high population density (3000 inhabitants), which increases the level of risk, evaluated at approximately $US3.8 million for patrimony, 3000 civilian deaths and destruction of biodiversity.  相似文献   

Can tensile stress develop in fractured multilayers under compressive strain conditions?     

Ghislain de Joussineau  Jean-Pierre Petit 《Tectonophysics》2007,432(1-4):51-62

To contribute to the understanding of how opening-mode fractures (joints) form and open or close at depth in layered rocks, we present a 2D numerical study aiming to determine whether tensile stress can develop in pre-fractured elastic multilayers submitted to biaxial compressive strain conditions.First, we investigate the role of the elastic and geometrical properties of the layers on the development of tensile stress in models with five bonded layers and containing one open fracture in the central layer. Our results indicate that, in absence of elastic contrast (in Young's modulus) between the layers, no tensile stress develops in the models. However, when the fractured layer is stiffer than the two adjacent layers directly above and below, a lobe of horizontal tensile stress develops centered on the pre-existing fracture. The creation of this tensile stress is contingent upon the partial closing of the fracture. The levels of tensile stress and the thickness of the lobe of tensile stress increase logarithmically with an increase in the elastic contrast and are systematically larger for a larger Soft/Stiff ratio (ratio of the total thickness of the soft layers with the total thickness of the stiff layers).Second, we investigate the role of fracture interaction in the development of tensile stress in models containing a pair of open fractures. We observe that the levels of tensile stress in the region between the fractures are systematically higher than those observed in identical models containing a single fracture. This increase in tensile stress is very large for small elastic contrasts between the layers but diminishes when the elastic contrast increases. Furthermore, the spacing between the pre-existing fractures plays an important role in the stress distribution in the region between them. When the fracture spacing is equal to or lower than 1.15 times the height of the fractured layer for the experimental conditions chosen, the lobes of tensile stress centered on the fractures coalesce. This results in the formation of vast areas of tensile stress in models under remote compressive loading conditions. Such tensile areas are likely to allow the initiation and propagation of subsequent opening-mode fractures.The results obtained provide new insights into the formation of joints in layered rocks in compressive environments, with important consequences on fluid flow.  相似文献   

The sedimentary coastal basin of Togo: example of a multilayered aquifer still influenced by a palaeo-seawater intrusion   总被引：3，自引：0，他引：3  

Ari Akouvi  Martial Dray  Sophie Violette  Ghislain de Marsily  Gian Maria Zuppi 《Hydrogeology Journal》2008,16(3):419-436

In Togo, the hydrogeology of the sedimentary coastal aquifers along the Gulf of Guinea has been studied for the last three decades to define the recharge processes and the origin and evolution of the salinity. Isotope hydrology and fluid geochemistry suggest that the current recharge of all aquifers, both confined and unconfined, occurs through the crystalline basement and the Plio-Quaternary deposits. Two main groundwater mineralization processes are observed: the first one, in recharge areas, is due to farming, village and city life and concerns unconfined aquifers (crystalline basement, Continental Terminal and Quaternary sediments); the second one is a mixing process between recent freshwater and fossil saline water still present in the deep confined aquifers inland, several kilometers away from the coast. Brackish water was trapped in low-permeability lenses of confined aquifers (Eo-Palaeocene and Maastrichtian) during the Quaternary, in periods of low recharge, notably during the last glacial maximum (LGM), and has not yet been flushed out. Hydrodynamic simulations indicate that, at that time, the aquifers experienced a maximum seawater intrusion as far as 20–22 km inland, depending on the palaeo-recharge value at the outcrops.  相似文献