Geological analysis of paleozoic large-scale faulting in the south-central Pyrenees

Abstract

Detailed structural and sedimentological analysis reveals the existence of an east-west directed fundamental fault zone in the south-central Pyrenees, which has been intermittently active from (at least) the Devonian on. Emphasis is laid on the stUdy of fault-bounded post-Variscan (StephanoPermian) sedimentary basins, and the influence of Late Paleozoic faulting on the underlying Variscan basement. The present structure of the basement is rather complex as it results from multiple Variscan and younger deformation. The nature and extent of post-Variscan basement faulting can therefore only be appreciated with the aid of a comprehensive analysis of older structures. The oldest preserved record of Paleozoic deformation in the Pyrenees is related to Devonian basin development in an east-west divergent oblique-slip zone of right-lateral sense. At the onset of the Variscan orogeny, open macroscopic north-south and east-west folds were formed, later to be overprinted by two generations of tight east-west cleavage folds. The end of the Variscan orogenic cycle is marked by the occurrence of minor north-south folds and compressive kinkbands. Post-Variscan right-lateral shearing in the basement along pre-existing east-west cleavage (anisotropy) planes is reflected by the consistent eastward bending (from south to north) of Late Variscan north-south axial planes. This shearing, which took place at an estimated depth of 4-5 km below the Variscan unconformity, is of the same age as post-Variscan brittle faulting and basin development at the surface. It must be concluded that the occurrence of two different types of faulting reflects a change in deformation mechanism with depth within the same regional stress-field. Stephanian and Permian alluvial sediments were deposited in an elongated fault-bounded basin in an overall right-lateral setting, as witnessed by the distribution of sedimentary facies, sequential arrangement of lithotypes, and paleocurrent directions. Fault movements ceased at the end of the Paleozoic. No evidence for Mesozoic deformation has been found in the studied area. In the Paleogene, north-south shortening in the Alpine mobile belt caused the emplacement of large nappes, both in the Variscan basement and the cover. In the Neogene, the fundamental fault zone in the south-central Pyrenees was reactivated as a left-lateral shear zone.