Unveiling the crustal structure of the central-eastern Betic Cordillera, Southern Spain: a potential geological hazard?

The Gibraltar Arc, comprising a northern branch in southern Spain and a southern limb in northern Africa separated by the back-arc Alboran Sea basin, is a key sector of the plate boundary between Africa and Eurasia, which represents the most tectonically active region of the western Mediterranean. In this region, plate convergence is accommodated through a complex pattern of deformation involving compressional and extensional regimes. In the central Betic Cordillera, deformation is dominated by WSW-directed extension and normal faulting, producing moderate seismicity in the Granada basin. In contrast, NNW–SSE compression prevails in the northern Alboran Sea. The interaction and mechanical coupling between these contrasting deformation styles occur across a structurally complex zone defined by two major antiforms, the Sierra Nevada to the north and the coastal Sierra de Lújar–Contraviesa–Gádor to the south, separated by a synformal domain, whose roles in stress transfer and seismicity remain poorly understood. Seismicity propagates onshore northward from the marine faults, yet no major surface structures have been identified on land that could account for this activity. To the west, significant seismicity is absent until reaching the southernmost normal faults of the Granada Basin, located approximately 50 km away. In this sector, the Padul fault accommodates most of the WSW extension, characterized by a significant creep component.

In this study, magnetic data have been collected along several N-S profiles, revealing a prominent regional E-W elongated dipole, consistent with existing aeromagnetic data. Moreover, a well-defined N-S dipole suggests the existence of a shallower conductive body. Preliminary processing and modelling of long-period magnetoteluric data from a dense survey of 20 sites further indicate the existence of conductive bodies at multiple crustal depths. Incipient field reconnaissance identifies widely spread NW-SE trending joints affecting marble lithologies, consistent with dominant regional extension NE-SW. Peridotite bodies crop out in the central-western Betic Cordillera, suggesting that similar lithologies and their associated high conductivity may also be present in depth in the study area. Our results point to the existence of a hidden crustal structure, expressed as conductive anomalies at different crustal levels, which localize brittle deformation and act as a mechanical link between these contrasting deformation styles. This hidden structure plays a key role in focusing deformation and controlling the propagation of seismicity onshore, despite the absence of major mapped surface faults, with important implications for seismic hazard assessment in the region.

Acknowledgements

This publication is part of the PID2022-136678NB-I00 project, funded by Spanish Ministry of Science, Innovation and Universities/State Research Agency MICIU/AEI (10.13039/501100011033) and by the European Regional Development Fund (ERDF), EU. In addition, the author V.M.B. gratefully acknowledges the pre-doctoral fellowship associated with grant PREP2022-000591, financed by MICIU/AEI (10.13039/501100011033) and by the European Social Fund (ESF+).