Superimposed Variscan and Alpine deformation in the basement rocks of southern Andorra, central Pyrenees

Abstract

Detailed mapping, structural analysis and cross-section constructions yield new data on the geometry and chronology of Variscan and Alpine deformations in the central Pyrenees of southern Andorra. The studied area is characterized by polyphase deformation and strain partitioning due to the presence of an efficient décollement level within Silurian black shales. Variscan macrostructures gave rise to a minimum shortening of ca. 22% for Cambrian-Ordovician and Upper Ordovician units below the décollement level and of ca. 35% for Silurian and Devonian units above it. Variscan structures are cut by out-of-sequence thrusts, presumably developed after late Variscan igneous intrusions, and extensional faulting. Those thrusts cut previous Variscan D 2 (main-phase) folds and originate additive and subtractive contacts. We propose an Alpine age for most of these thrusts which, together with the Variscan structures, are affected by subsequent Alpine deformation, which causes changes in their orientation around the intrusions, and a progressive steepening of the cleavage from south to north. We discuss the usefulness and limitations of balanced cross-sections in polydeformed areas and propose that the technique is valid to construct regional sections and to obtain geologically reasonable pre-deformational configurations.