Paper No. 8
Presentation Time: 9:00 AM-6:00 PM


BALANYA, Juan Carlos, EXPOSITO, Inmaculada, JIMENEZ, Alejandro, BARCOS, Leticia and DIAZ-AZPIROZ, Manuel, Dpt. Physical, Chemical and Natural Systems, Pablo de Olavide University, Crtra. Utrera, km 1, Sevilla, E 41013, Spain,

Structural trend-line patterns in orogenic arcs usually depict alternating convex (salients) and concave (recesses) to the foreland segments. Transition zones between these often result in large-scale strike-slip or transpressive shear zones. Natural and analogue modelling studies show multiple factors (depth and strength of detachment levels, indenter shape, tectonic transport direction patterns, etc.) can control the geometric features of these curved segments.

The Gibraltar Arc is a tight curved orogen that closes the Mediterranean Alpine system to the west. Within it, trace patterns form second order curved segments. The hinge zone of the Arc (the Western Gibraltar Arc, WGA), defines itself a major salient that ends at the apex of two recessing zones located in the Betic (northern branch) and Rif (southern branch) chains. Overall kinematic data on both branches suggest a piedmont glacier arc-type, in which radial shortening directions were coeval with arc-parallel extension.

The transition between the WGA and the northern recessing zone corresponds to an E-W dextral transpressive zone (The Torcal Shear Zone, TSZ). The TSZ is 70 km long and 4-5 km wide, and affects cover units belonging to the external thrust and fold belt. The TSZ southern wall separates these units from the internal metamorphic zone. Kinematics within the TSZ shows that it is the result of a triclinic dextral transpression, in which strain was highly partitioned into simple shear and pure shear dominated domains. The WNW-ESE oriented far-field vector responsible for the TSZ is compatible with the arc-related kinematic frame.

The TSZ grades to the west, through a diffuse tip zone characterized by the moderate deflection of structural trend lines and the occurrence of distributed strike-slip faults, into the northern part of the WGA. In this sector, NE-SW trending, NW-SE verging shortening structures were coeval during the Neogene with NW-SE oriented normal faults. The intermontane Ronda Basin formation and marine infill (Upper Miocene) and its later evolution resulted from this mode of strain partitioning. Later stages in the TSZ transpression correspond to deformative and erosive events in de Ronda sector. Overprinting criteria and geomorphic indices suggest deformation has been active up to the Quaternary in both structural domains.

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