Cenozoic paleo-stress pattern in the Alpine Foreland and structural interpretation in a platform basin

Abstract

Abstract The existence and orientation of a present-day compressive stress field in the European and African-Arabian Alpine Foreland is known from in-situ stress measurements and fault-plane analyses of earthquakes. This stress field is distributed over a zone of up to a few hundred kilometers in width. Methods using micro-tectonic measurements such as tectonic stylolites, tensional joints, slickensides on fault planes or bed-to-bed surfaces, have been developed to determine the state and orientation of paleo-stresses. Compressive paleo-stress orientations measured in the continental platform sedimentary basins are relatively uniform in both time and space or vary gradually, except near some wrench or folded zones, or along collision plate boundaries. These variations could be due to a deflection of the stress field, or to a rotation by deformations during the compressive phase, or a later phase as shown by some paleomagnetic measurements. As with paleomagnetism, a comparison between paleo-stress orientations could detect or confirm rotation due to drifting (e.g., 30°–40° anticlockwise rotation of Sardinia relative to Europe, 15° anticlockwise rotation of Sicily relative to Tunisia). Paleo-stress orientations observed in the Western Europe craton are roughly parallel to the convergent movement of the European-African plate, as deduced from oceanic magnetic anomalies. Synchronism in peaks is observed in the magnitude of the deformations in both the Alpine belt and the foreland platform, but with a magnitude strain decrease. The width of the zone with micro- and macro-structural evidence of compression varied from several thousand kilometers during the Eocene peaks of deformation to only a few thousand kilometers during the Miocene-Pliocene Alpine phases. We often observe, just after compressive phases, a regional extension with parallel normal faults, grabens or rift. These normal faults develop parallel to the preceding σ 1 maximum compressive stress. This phenomenon could be tentatively explained by stress inversion σ 1 → σ 2 , σ 2 → σ 1 , due to the decrease in the magnitude of the tectonic compressive stress, and specific boundary conditions. The late-Cretaceous-Cenozoic evolution of the stress field in the western Alpine Foreland is mostly controlled by EU-AF Plate convergence and continental collision phenomena acting on heterogeneous continental blocks. As shown by field work and seismic surveys, knowledge of the evolution of the stress enables the pattern of tectonic deformations in the platform sedimentary basins to be determined and interpreted.