Crescent-like large-scale structures in the external zones of the western Gibraltar Arc (Betic–Rif orogenic wedge)

Abstract

The external wedge of the Gibraltar Arc orogenic system (western Mediterranean) is a natural case of a fold-and-thrust salient. Although at the scale of the arc a swing of the structural trend can be observed, the presence of closed, crescent-like structures, some of them more than 25 km long and scattered over an area of 3500 km 2 , characterize the Gibraltar Arc external wedge. This feature makes it very different from a ‘classical’ arc-shaped fold-and-thrust belt. Detailed structural maps of representative crescent-like structures are presented. Synorogenic sediments reveal that they probably formed during a short time interval, most probably during a single shortening event. The crescent-like structures are localized in front of a recess formed by the internal zones that acted as backstop. This backstop shape and a viscous substrate (essentially Triassic evaporites below the fold-and-thrust external wedge) constrain the experimental setting of an analogue model in which we were able to reproduce structures that developed originally with a highly non-cylindrical shape during a single, straight convergence. Moreover, the tectonic transport directions along the thrusts are broadly comparable in the natural case and the model.