Active Shortening Simultaneous to Normal Faulting Based on GNSS, Geophysical, and Geological Data: The Seismogenic Ventas de Zafarraya Fault (Betic Cordillera, Southern Spain)

Abstract

The central Betic Cordillera, southern Spain, is affected by an uplift related to the NNW–SSE Eurasia-Nubia convergence and shallow ENE–WSW orthogonal extension accommodated by the extensional system of the Granada Basin. The combination of geophysical, geodetic, and geological data reveals that the southwestern boundary of this extensional system is a seismically active compressional front extending from the W to the SW of the Granada Basin. The near-field Global Navigation Satellite System data determine NNE–SSW shortening of up to 2 mm/yr of the compressional front in the Zafarraya Polje. In this setting, the normal Ventas de Zafarraya Fault developed as a result of the bending-moment extension of the Sierra de Alhama antiform and was last reactivated during the 1884 Andalusian earthquake (Mw 6.5). The uplift in the central Betic Cordillera together with the subsidence in the Western Alborán Basin may facilitate a westward to southwestward gravitational collapse through the extensional detachment of the Granada Basin. The heterogeneous crust of the Betic Cordillera would generate the compressional front, which is divided into two sectors: thrusting to the west, and folding associated with buttressing to the south. Our results evidence that basal detachments, linking extensional fault activity with compressional fronts, may determine the activity of local surface structures and the geological hazard in densely populated regions.

A combination of geological, geophysical, and geodetic methods is used to understand the evolution in the frontal area of the extensional system of Granada Basin, near the Zafarraya Polje. In particular, this approach improves our knowledge of the Ventas de Zafarraya Fault (VZF) that hosted the 1884 Andalusian earthquake. Seismic data provide information on the distribution of stress and deformation in the region and made it possible to identify a compressional front linked to the extensional system of the Granada Basin. Global Navigation Satellite System (GNSS) data accurately locate the sites of the Zafarraya GNSS network surrounding the VZF, and repeated measurements provide deformation rates of the area that suggests shortening. Thus, we locate the VZF within the former compressional setting. Meanwhile, electrical-resistivity tomography images sub-surface structures given its electrical properties and, together with the field geological observations, suggests the extensional behavior of the fault. Therefore, it is considered to be a fold-related fault formed due to the extension of the outer arc of the Sierra de Alhama antiform. As in the research presented here, a joint interpretation of data from different methods makes it possible to propose active seismic tectonic extensional models in a compressional setting applicable to other regions.