The extensional system of the Granada Basin in the frame of the Betic CordilleraStructure, activity and geohazard implications

Résumé

The Granada Basin is the main intramontane basin in the Betic Cordillera. It is located in its central region and bounded to the east by the highest area of the cordillera, Sierra Nevada. Its formation since the early to middle Miocene is attributed to the activity of extensional detachments that exhumed the metamorphic complexes. Currently, its development is mainly related to the activity of NW-SE faults that generate recurrent seismicity, making it the region with the highest seismic hazard in the Iberian Peninsula. The evolution of the Granada Basin is framed within an extensional context that affects the central Betic Cordillera almost entirely. This region contrasts with the surrounding areas, which are primarily influenced by shortening. On one hand, the eastern Betic Cordillera and the central Alboran Sea undergo NNW-SSE shortening due to the push of two tectonic indentors related to the Eurasia-Nubia convergence. On the other hand, the western Betic Cordillera moves towards the west, generating shortening in the westernmost front. In the complex framework of the current development of the western Mediterranean, the central Betic Cordillera in general, and particularly the Granada Basin, constitute an excellent natural laboratory for studying the causes of extensional processes at convergent plate boundaries. Thus, this PhD thesis combines geological, geophysical, and geodetic techniques to study the main active structures of the region. It aims to enhance the understanding of the processes that influence the extension of the central Betic Cordillera and the ongoing development of the Gibraltar Arc. The eastern Granada Basin, next to Sierra Nevada, is affected by a WSW-ENE extension of up to 1.3 mm/year according to GNSS data. Analysis of the main structures and earthquake focal mechanisms suggests that this extension is accommodated through a set of NW-SE high-angle normal faults rooted in a low-angle normal fault beneath the basin. These structures constitute the extensional system of the Granada Basin. Field geological data indicate a structural connection between the high-angle normal faults and the Miocene extensional detachments that currently outcrop in western Sierra Nevada. Therefore, the active low-angle normal fault may represent the westward continuation of ancient extensional detachments. In this context, high-precision levelling lines calculate an average vertical displacement of 0.73 mm/year along the Granada Fault, which is the main high-angle normal fault in the system. Hence, it could accommodate between one-third and potentially the entire extension in the region, if this displacement is transmitted to the low-angle normal fault. In the extensional system of the Granada Basin, seismic sequences like the Granada 2021 are recurrent. This sequence exhibits characteristics of both a mainshock-aftershock sequence and a swarm. The relocation of seismicity reveals a `chimney-shape geometry. Geological and gravimetric analysis points to the interaction between an active set of NW-SE faults and another set of NE-SW faults acting as barriers, segmenting the former and confining the seismicity. As a result, the length and rupture area of the activated segments are reduced, limiting the maximum magnitude of earthquakes to Mw 4.5-5.0. In this way, the accumulated stresses in the area need to be released through the recurrence of several earthquakes of similar magnitude. This gives the `swarmy¿ characteristic to the sequence and decreases the seismic potential of the faults. Regional GNSS data suggest that the western and southwestern edges of the Granada Basin constitute a compressional front. Geological and geophysical observations reveal that this front is characterized by N-S blind thrusts in the northern region, affecting the External Zones, and the development of large WNW-ESE folds in the southern region, within the Internal Zones. The relocation of seismicity indicates a structural relationship between this front and the extensional detachment of the Granada Basin. The Ventas de Zafarraya normal Fault constitutes the main seismogenic structure in the region, most likely responsible for the 1884 Andalusian earthquake. Data from the Zafarraya GNSS network, located near the External/Internal Zones boundary, indicate that the activity of the Ventas de Zafarraya Fault is linked to the development of the W-E Sierra de Alhama antiform within a context of local NNE-SSW shortening of up to 2 mm/year. Consequently, it is a normal fault accommodating the bending-moment extension of the Sierra de Alhama antiform in its western segment. Extension in the central Betic Cordillera is probably associated with an extensional collapse from the highest region in the Betic Cordillera, Sierra Nevada, towards the more subsided regions to the west, southwest, and south. This collapse activates the extensional system of the Granada Basin at the western edge of Sierra Nevada, determining the current development of the basin. The formation of the compressional front in the west and southwest of the Granada Basin is probably due to the presence of a dense body embedded in the crust acting as a buttress, as indicated by a strong positive gravity anomaly. To the south, the system is constrained by the active indentation in the central Alboran Sea. The uplift in the Sierra Nevada region could result from isostatic rebound related to slab tearing, as well as the formation of large E-W folds due to the Eurasia-Nubia convergence. Whereas the subsided regions to the west, southwest, and south would result from both back-arc extension related to rollback and subsidence in the western Alboran Basin associated with active subduction in the western Betic Cordillera. The proposed active geodynamic model of the central Betic Cordillera, carried out by means of this multidisciplinary study, contributes to the characterization of geological hazards in the southern Iberian Peninsula that also may apply to other similar geodynamic contexts.