The collision between the African and Eurasian plates in the western Mediterranean has traditionally been modeled as diffuse strain, characterized by distributed deformation across numerous faults and moderate seismicity. This model, however, lacks robust testing due to the underwater location of most deformation and the poorly characterized nature of the faults involved. The slow convergence rate (4.5 ± 1 mm/yr) raises concerns that large earthquake cycles might exceed instrumental record lengths, leading to an underestimation of seismic hazards. Previous research has documented numerous active onshore and offshore faults, but their slip histories remain largely undocumented, aside from a few exceptions like the Al-Idrissi and Carboneras fault systems. The most prominent seafloor relief in the region is associated with the YFS and ARFS, making them critical for understanding the overall deformation pattern. Focal mechanisms suggest right-lateral strike-slip movement for the YFS and right-lateral oblique thrusting for the ARFS. This study aims to quantify the deformation absorbed by these two systems to assess their role in absorbing plate convergence and their contribution to regional seismic and tsunami hazards.

Existing models describe the western Mediterranean as a diffuse plate boundary, with deformation distributed across a broad region (~300 km wide) encompassing numerous faults. Moderate-magnitude seismicity and complex patterns of crustal block deformation, revealed by GPS data, are cited as support for this model. However, the slow convergence rate raises concerns about the possibility of underestimating hazards due to large earthquake cycles exceeding the instrumental record. While some studies have examined specific fault systems like Al-Idrissi and Carboneras, the majority lack detailed slip history data. This paper focuses on the YFS and ARFS, which are prominent features yet largely overlooked in kinematic models, to refine the understanding of plate boundary dynamics.

The study utilizes pre-stack depth migrated (PSDM) seismic images from the TOPOMED and EVENT-DEEP Leg 1 surveys to characterize the structure and kinematic evolution of the YFS and ARFS. The seismic data processing involved both time and depth domain processing workflows, detailed in the Methods section. Stratigraphic units were interpreted based on established seismostratigraphy, calibrated with ODP Leg 161 data and industry wells. Slip estimation employed multiple approaches: forward modeling using the fbfFOR software to reconstruct fault evolution, an analysis of pull-apart basin formation based on Rodgers’ model to quantify strike-slip along the YFS, the excess-area method for ARFS to determine total slip and detachment depth, and fault-related folding calculations to estimate slip along the ARFS, supplemented by an analysis of brittle-ductile transition depth based on geothermal gradient, rheological behavior, and the Anderson equation. All these methods consider plane strain conditions, though acknowledging the potential influence of transpression.

The YFS, a ~160-km-long dextral strike-slip fault system, displays significant late Miocene to early Pliocene activity. Analysis of the pull-apart basin and modeling of extensional faults within it suggests a minimum cumulative slip of 12 km, potentially reaching 16-30 km considering sub-seismic deformation. The ARFS, a ~130-km-long structure, exhibits up to 2 km of seafloor relief. Analysis using the excess-area method, accounting for potential erosion and uncertainties in measurements, estimates a total slip of 16 ± 4.7 km for the main fault, consistent with the forward modelling and fault-related folding results. Both YFS and ARFS exhibit a significant change in crustal thickness across the fault indicating that both are lithospheric-scale boundaries. The brittle-ductile transition is estimated to lie between 8.5 and 10 km depth beneath the southern flank of the Alboran Ridge. These estimations, despite uncertainties inherent to the methods and data limitations, consistently point to a substantial amount of deformation absorbed by these two systems. The combined slip of at least 24 km (12 km minimum from the YFS and 16 ± 4.7 km from the ARFS) suggests that the YFS-ARFS system may have accommodated at least half, and potentially the entirety of the Plio-Holocene Africa-Eurasia convergence (24 ± 5 km).

The findings significantly challenge the prevailing diffuse-deformation model for the western Mediterranean plate boundary. The substantial slip accommodated by the YFS-ARFS system indicates a more focused, well-defined plate boundary than previously thought. The large-scale nature of these faults, their favorable orientation for absorbing plate convergence, along with the potential for large magnitude earthquakes and tsunamis (estimated Mw 7.4 to 8.8) based on earthquake fault scaling relations for continental regions, highlights a previously underappreciated seismic and tsunami hazard. While other fault systems in the region exist, their cumulative contribution to Plio-Holocene deformation is considered significantly smaller. The formation of the YFS-ARFS along pre-existing crustal domain boundaries may have facilitated deformation focusing. This suggests a need for updated kinematic models and a reassessment of seismic hazard in the region.

This research demonstrates that the YFS and ARFS together form a significant, previously underestimated plate boundary in the western Mediterranean. The large amount of slip accommodated by these structures challenges the traditional diffuse deformation model, suggesting the need for a revised understanding of regional tectonics and seismic hazard. Future studies should integrate these findings into probabilistic seismic hazard analyses to improve risk assessment and consider the possibility of transpression in more advanced modeling.

The slip estimations presented have inherent uncertainties due to limitations in the available data and the methodologies employed. The depth of the fault traces is not fully resolved in the seismic images, and the forward modeling was limited in its ability to fully capture the complexities of deformation, particularly the simultaneous activity of multiple faults at various scales. The excess-area method also carries inherent uncertainty, particularly concerning the detachment depth estimation. The plane strain assumption might not perfectly reflect the regional stress field, which could influence the slip values.