Multi-proxy evidence of activity of the Bokkoya fault system during the Last Glacial Maximum (LGM), Alboran sea

The Alboran Basin is transected from southern Spain to northern Morocco by the active left-lateral Al Idrissi Fault Zone, whose southern termination corresponds to the Bokkoya fault system. These faults accommodate the oblique convergence between the African and Eurasian plates and the extrusion of the Betic–Rif block, generating recurrent seismicity. The Bokkoya Fault Zone lies between offshore segments that ruptured during the 1994–2004 seismic crises (Mw 5.9 and 6.3) and the 2016 and 2021 events (Mw 6.4 and 5.5). The ANR-funded ALBANEO project aims to constrain the long-term behaviour of this currently low-seismicity segment by reconstructing its activity over the last ~120 ka, with implications for regional seismic hazard assessment.

This study integrates a multi-proxy dataset from the ALBACORE marine campaign (https://doi.org/10.17600/18001351), including multibeam bathymetry, seismic reflection and sub-bottom profiles, piezocone penetration tests (CPTu), and sediment cores. Data were collected along ~20 km of the Bokkoya fault segment, from the Small Al Idrissi Volcano to Al Hoceima Bay.

Deformation is distributed across localized and diffuse fault segments with both vertical and horizontal offsets. Fault architecture evolves from north to south, controlled by relay zones and step-overs, up to the Moroccan coastline where the fault system terminates. Individual segments are on average ~5 km long, with maximum cumulative horizontal offsets of ~3 km over 1 Ma and vertical offsets of up to 32 m over the last 120 ka.

Paleoseismological analysis highlights major tectonic events during the Last Glacial Maximum (LGM). In the Bokkoya fault system, seismic reflection data calibrated with sediment cores and CPTu measurements indicate late- to post-LGM fault sealing on some segments, as well as in-situ disrupted seismic facies dated to the LGM. This facies is interpreted as the result of seismically induced soft-sediment deformation.

Moreover, a chaotic sedimentary facies observed between 8 and 10 m depth in core ALB_CL56 correlates with increased sediment strength derived from CPTu data and is dated between 20.9 and 20.3 ka. This facies extends over ~30 km² on sub-bottom profiles and is interpreted as a mass-transport deposit (MTD), likely triggered by a coeval seismic event. The source area is identified on the eastern shelf of the Bokkoya fault system, where submerged headscarps are observed. During the LGM (~18–24 ka), sea level was approximately 120 m lower, exposing the shelf by up to ~40 m.

The MTD and the in-situ disrupted seismic facies likely represent paleoseismic archives, consistent with recent studies documenting LGM-aged seismic events on the Bokkoya fault (Vidil et al., 2025). However, disentangling climatic forcing (sea-level changes and post-LGM warming) from tectonic triggering remains challenging. The spatial distribution of seismic clusters and paleo-fault activity suggests an immature segmentation of the plate boundary, with important implications for regional seismic hazard.