Miocene-Quaternary Strain Partitioning and Relief Segmentation Along the Arcuate Betic Fold-and-Thrust Belt: A Field Trip Along the Western Gibraltar Arc Northern Branch (Southern Spain)

Abstract

AbstractThe Western Gibraltar Arc (WGA), located at the western end of the Mediterranean Alpine orogenic belt, was built up by the westward emplacement of the Alboran hinterland domain upon the South Iberian and Maghrebian paleomargin domains. This guide is focused on the strain partitioning modes developed in the fold-and-thrust belt (FTB) of the northern branch of the WGA. The proposed itineraries aim to show the distinctive strain partitioning modes of this arcuate FTB from the hinge (itinerary 1) to the lateral (itinerary 2) zones of the WGA. We focus our observations on the structural evolution from the late Miocene onwards. This short-term (a few My) temporal frame favours that the structures accommodating strain partitioning may greatly control a number of first-order topographic traits, for instance, relief segmentation. Itinerary1, located in the northern WGA hinge zone, allows one to visit a representative area of the inner FTB characterized by a set of parallel mountain ranges enclosing the Ronda intermontane basin. In this area, the topography is conditioned by late Miocene to Pliocene km-scale, upright or doubly vergent, NE-SW trending folds and associated reverse faults. These structures, parallel to the orogenic grain, gave rise to a conformable relief. Additionally, first-order along-strike relief segmentation controlled by normal faults has accommodated arc-parallel extension. The itinerary includes stops in the two main normal fault zones located at the margins of the Ronda basin, the most important intermontane basin of the northern WGA. Itinerary2 runs along the lateral, eastern end of the WGA, marked by a km-scale, E-W oriented brittle-ductile shear zone, the Torcal Shear Zone (TSZ), which is formed by a set of en-echelon morphostructural highs that define a 70 km long topographic alignment. The itinerary starts at the TSZ western end and continues eastwards to the main structural domains of its central part: the Valle de Abdalajís Massif and the Torcal de Antequera Massif. This permits us to visit the main structures that characterize this highly partitioned shear zone: NE–SW folds and reverse faults, NW–SE normal faults, and E–W dextral strike-slip faults, especially localized along the northern and southern walls of the TSZ. The overall kinematics corresponds to dextral transpression with a nearly vertical significant extrusion and associated surface uplift.