Overthrust shear folding and contrasting deformation styles in a multiple decollement setting, Gibraltar Arc external wedge

Abstract

The deformation style distribution and related finite strain exhibited by the tectonic units of the Western Gibraltar Arc depict a multiple decollement setting, which was developed during Miocene westward Arc migration. In the external wedge, alternating ductile and brittle–ductile deformation bands occur, which are mainly controlled by differences in primary rheological properties and structural position. In the northern branch of the Gibraltar Arc, the metamorphic units of the hinterland (Alboran Domain) fully overthrust their peripheral non-metamorphic Dorsal units, producing NW-vergent folds. Similar structures (foliated NW-vergent chevron folds) developed on the Paleomargin units as a result of the overriding Flysch Trough unit emplacement. This scenario could be explained by overthrust shear produced by an overriding mass, which generated a progressively higher shear strain of the Paleomargin units towards the hinterland. This model is supported by (a) changes in axial surface attitude, which becomes steeper towards the foreland; (b) tighter folds towards the hinterland, and (c) stronger evidence of fold flattening towards more internal positions. In contrast, coeval thrust systems shortened and displaced the overriding Flysch Trough units under quite brittle conditions. This indicates that they were not sheared below the Alboran Domain, but essentially deformed by a push-from-behind mechanism, where the Alboran Domain acted as a relatively rigid backstop.