An integrated multi-proxy approach to characterize the southern part of the Al Idrissi strike-slip fault system, Alboran sea

In the Alboran Sea, oblique convergence between the African and Eurasian plates led to establishing the active Al Idrissi sinistral strike-slip fault system 1 Ma ago. Several moderate magnitude earthquakes (Mw > 6) have been recorded on different segments of this fault system.

The objective of this study is to analyse the dynamics of this nascent plate boundary by studying the seismic events recorded in sedimentary series. We focused on a key transtensive fault transect, namely the Bokkoya fault system, shifting the small Al Idrissi volcano. This fault has a lateral extent of 11km along strike. Sedimentation is strongly affected by the circulation of deep Mediterranean water masses resulting in contouritic deposits, and likely mass movement during seismic events.

We used a panel of geological, geophysical, geotechnical and geochemical tools acquired during the ALBACORE oceanographic campaign (R/V Pourquoi pas? 2021). This work is part of the ANR ALBANEO project, which aims to understand the dynamics of this new plate boundary and to assess the hazards in this area of the western Mediterranean Sea. The data analysed are derived from (i) 4 sediment calypso cores (ALB_CL26, ALB_CL54, ALB_CL53 and ALB_CL52) from 10m to 16m (analysed with a multi-sensor core logger – MSCL and X-Ray Fluorescence-XRF), (ii) piezocone tests (CPTU) with the Ifremer Penfeld as well as (iii) multibeam bathymetry data and (iv) seismic reflection/sub-bottom profiles. This multi-proxy dataset provided detailed lithological and geophysical stratigraphy, calibrated with the picked seismic horizons, and sediment cores dating along a transect perpendicular to the Bokkoya fault system.

Isotopic analysis of 3 cores provided 𝛿¹⁸O evolution curves, identifying a thermal anomaly in each of them, and in particular in the one penetrating the fault plane. Oxygen isotopic curves were calibrated using ¹⁴C radiocarbon analysis, enabling sedimentary series to be dated up to 40 ka. Accordingly dated, representing the first 16 m of sediment cores: the cold stadials, with the Younger Dryas; the Heinrich Stadials 1 and the Last Glacial Maximum. The sedimentation rate is about 30 cm/kyr in the depression zone whereas on either side of the contourite drift, it is about 20-25 cm/kyr.

The recognition of seismic events in the past is attempted by comparing sedimentary successions in different fault compartments. The active Bokkoya fault appears to offset the sedimentary series with a normal component and a vertical throw of 1 m, evaluated between the seabed and the dated YD reflector.

The results from the different datasets allow us to identify (1) syn-tectonic deposits that may be associated with past co-seismic events (2) intense erosional events that may be associated with localized water masses currents (3) a thermal anomaly whose origin is to be determined. This dataset highlights the complex interaction between tectonics and sedimentation/erosion along this segment of the Bokkoya fault over at least 60 ka.