Multi-proxy investigations into the active tectonics of the Bokkoya strike-slip fault, southern Alboran Sea

Abstract

In the Alboran Sea, oblique convergence between African and Eurasian plates led to the formation of the active NNE-SSW Al Idrissi sinistral strike-slip fault system ∼1 Ma ago, generating several moderate-magnitude earthquakes (Mw > 6). This study investigates the dynamics of this emerging plate boundary by analysing seismic events recorded in sedimentary sequences over the past 120 kyr. A multi-proxy approach combining sediment cores, piezocone tests, multibeam bathymetry, seismic reflection and sub-bottom profiles was applied for the first time to investigate and quantify tectonic activity along the Bokkoya fault. This NNE-SSW-oriented fault system segments the southern part of the Small Al Idrissi Volcano and locally forms a local transtensive basin. These proxies have allowed the identification and dating of faulted sedimentary sequences and major erosive events. An intense erosional period in the western contouritic drift between 90 and 60 kyr is responsible for the strong thickness asymmetry observed between the two blocks of the Bokkoya fault. The vertical displacement history of the north Bokkoya segment reveals a succession of four possible paleoearthquake events preserved since 120 kyr. These are the sum of coseismic events and potential post-seismic slips related to fault creeping or aseismic creep. The studied segment exhibits transtensional deformation and shows a cumulative vertical offset of 33 m, with distinct phases of vertical displacement: a higher rate (0.30 mm yr-1) between 90 and 40 kyr, followed by a general slowdown to 0.17 mm yr-1 since 40 kyr. From a detailed study at the core scale, at least three preserved paleoearthquake events are identified, dated at 42, 21.5, and 17.5 kyr. This study presents the seismic and aseismic behaviour of a segment from the Bokkoya strike-slip fault, providing insights into the mechanisms governing fault activity during the evolution of a nascent plate boundary.