Géodynamique andine : résumé étendus = Andean geodynamics : extended abstracts

Le système de failles d'Atacama, système de décrochements senestres de la cordillère côtière du nord Chili associé à la zone de subduction, est le siège de deux styles de minéralisations hydrothermales Fe-Cu, qui peuvent être associés à différentes phases tectoniques au sein de l'arc magmatique crétacé. Les minéralisations riches en magnétite sont en général localisées le long de la zone de failles N-S et apparaissent être associées au développement de l'arrière/intra-arc néocomien aux environs de 128-106 Ma. Plus tard, après 80 Ma, le système d'Atacama se marque par la propagation de failles orientées NW-SE à travers l'avant-arc. Le long de ces failles, des brèches et des veines hydrothermales à spécularite sont présentes dans les zones d'ouverture. (Résumé d'auteur

Three-dimensional distinct element modelling of relay growth and breaching along normal faults

Three-dimensional numerical models of neutral (i.e. slip-parallel) relay zones on normal faults that cut massive sandstone host rocks have been constructed using the distinct element method code, Particle Flow Code in 3-D (PFC3D). The models successfully reproduce the geometries, displacement profiles and strains observed in natural relay zones. In contrast to boundary element method simulations, the modelled relay ramps dip towards the hanging wall, consistent with observations of most natural relay zones. The modelling shows that relay zones are stable structures that ‘grow’ by progressive rotation of an approximately planar relay ramp without significant propagation of the relay-bounding faults prior to breaching. Stable growth is terminated when a breaching fault propagates across the top or bottom of the relay ramp. Breaching fault propagation is not instantaneous and the ramp continues to rotate, and therefore transfer displacement between the relay-bounding faults, until they become fully hard linked. Following hard linkage, displacement is accommodated by slip on the through-going fault surface. The modelling results confirm previous conceptual models of relay growth and breaching based on geometric and kinematic analysis of natural relay zones

Geometric and kinematic controls on the internal structure of a large normal fault in massive limestones : the Maghlaq Fault, Malta

The Maghlaq Fault is a large, left-stepping normal fault (displacement >210 m) cutting the Oligo-Miocene pre- to syn-rift carbonates of SW Malta. Two principal slip zones separate the deformed rocks of the fault zone from the undeformed wall rocks. Fault rocks derived from fully lithified, pre- to early syn-rift sediments comprise relatively continuous fine-grained veneers of cataclasite and localised fault-bound lenses of wall rock, occurring over a range of scales, which are commonly brecciated. The lenses result from the linkage of slip surfaces, the inclusion of asperities and the formation of Riedel shears within the fault zone. In contrast, fault rock incorporated from unlithified syn-rift sediments comprise relatively continuous veils of rock that deformed in a ductile manner. Anomalously thick parts of the fault zone with highly complex structure and content are associated with breached relay zones, branch-lines and bends; these structures represent progressive stages of fault segment linkage. The progressive evolution and bypassing of fault zone complexities to form a smoother and more continuous active fault surface, results in complex fault rock distributions within the fault zone. Segment linkage structures have high fracture densities which combined with their significant vertical extents suggest they are potentially important up-fault fluid flow conduits.Irish Research Council for Science, Engineering and TechnologyENI Exploration and Production Divisio