Ongoing Inversion of a Passive Margin: Spatial Variability of Strain Markers Along the Algerian Margin and Basin (Mediterranean Sea) and Seismotectonic Implications

Abstract

Subduction initiation is an important but still poorly documented process on Earth. Here, we document one of a few cases of ongoing transition between passive and active continental margins by identifying the geometrical and structural signatures that witness the tectonic inversion of the Algerian continental margin and the deep oceanic domain, located at the northern edge of the slow-rate, diffuse plate boundary between Africa and Eurasia. We have analyzed and tied 7900 km of deep seismic reflection post-stacked data over an area of ∼1200 km long and ∼120 km wide. The two-way traveltime lines were converted into depth sections in order to reconstruct and map realistic geometries of seismic horizons and faults from the seafloor down to the acoustic basement. Along the whole length of this young transitional domain, we identify a clear margin segmentation and significant changes in the tectonic signature at the margin toe and in the deep basement. While the central margin depicts a typical thick- and thin-skinned tectonic style with frontal propagation of crustal thrust ramps, the central-eastern margin (Jijel segment) reveals a higher strain focusing at the margin toe together with the largest flexural response of the oceanic lithosphere. Conversely, strain at the margin toe is limited in the western margin but displays a clear buckling of the oceanic crust up to the Spanish margin. We interpret these contrasting, segmented behavior as resulting from inherited heterogeneities in (1) the geometry of the Algerian continental margin from West to East (wrench faulting in the west, stretched margin elsewhere) and (2) the Miocene thermal state related to the diachronous opening of the Algerian basin and to the magmatic imprint of the Tethyan slab tearing at deep crustal levels. The narrow oceanic lithosphere of the Western Algerian basin is assumed to favor buckling against flexure. From the dimension and continuity of the main south-dipping blind thrusts identified at the margin toe, we reassess seismic hazards by defining potential lengths for ruptures zones leading to potential magnitudes up to 8.0 off the central and eastern Algerian margins.

Contractional salt deformation in a recently inverted basin: Miocene to current salt deformation within the central Algerian basin

“…To the north‐east, the convergence rate across the Algerian margin decreases from 3–4 to 1.5 mm/yr (Bougrine et al, 2019; Strzerzynski et al, 2010). However, even though the convergence rate is higher on the south‐western segment of the margin, the displacement is mostly accommodated onshore within the Tell Atlas, while shortening along offshore reverse faults is minimal (Arab et al, 2016; Déverchère et al, 2005; Domzig et al, 2006; Leffondré et al, 2021; Yelles et al, 2009). The thick‐skinned thrust faults bordering the Khair al Din bank (Figure 4a) may accommodate most of the shortening and focus compressional deformation internally at the margin toe (Figures 1 and 3a,b).…”

“…Consequently, the displacement loading applied offshore to the salt and overburden may be lower and the deformation milder in the south-west. By contrast, even though the overall convergence rate is lower to the east of the Thenia fault, it concerns a much broader area and includes a component of strike slip (Domzig al., 2006;Leffondré et al, 2021;Yelles et al, 2009). Regional shortening is more widely distributed in the outer domain, resulting in a stronger tectonic loading accommodated by the salt and its overburden (Figure 3c).…”

“…After the inception of the Algerian margin inversion, during late Miocene (from ~5.97 to ~5.33 Ma), the Messinian Salinity Crisis (MSC) led to the deposition of the MSG (Hsü et al, 1973;Krijgsman et al, 1999;CIESM, 2008;Manzi et al, 2013), resulting in the deposition of 500 m to 1.5 km of salt in the western Algerian basin (Haq et al, 2020). During the Plio-Quaternary, significant tectonic shortening along the Algerian margin generated a set of north-verging transpressional underthrust fronts, combining thin-skinned and thick-skinned tectonic styles (Déverchère et al, 2005;Domzig et al, 2006;Frizon de Lamotte et al, 2000;Leprêtre, 2012;Recanati et al, 2019, p. 0;Leffondré et al, 2021;Strzerzynski et al, 2021). These thrusts likely signal early-stage subduction of the Algerian oceanic crust below Africa (Gueguen et al, 1998;Billi et al, 2011;Roure et al, 2012;Hamai et al, 2018;Recanati et al, 2019;Leffondré et al, 2021).…”

“…During the Plio-Quaternary, significant tectonic shortening along the Algerian margin generated a set of north-verging transpressional underthrust fronts, combining thin-skinned and thick-skinned tectonic styles (Déverchère et al, 2005;Domzig et al, 2006;Frizon de Lamotte et al, 2000;Leprêtre, 2012;Recanati et al, 2019, p. 0;Leffondré et al, 2021;Strzerzynski et al, 2021). These thrusts likely signal early-stage subduction of the Algerian oceanic crust below Africa (Gueguen et al, 1998;Billi et al, 2011;Roure et al, 2012;Hamai et al, 2018;Recanati et al, 2019;Leffondré et al, 2021). GPS-derived velocities show that today, the displacement rate increases westward but is highly variable laterally, with an average shortening of 1.5 ± 0.5 mm year −1 at the central Algerian Basin (Serpelloni et al, 2007;Bougrine et al, 2019).…”

“…Along the Algerian margin, south of the Algerian foredeep, this quaternary shortening is thick-skinned, with north-verging thrust ramps delineating uplifted perched (piggyback) basins (Figure 3; Déverchère et al, 2005;Yelles et al, 2009;Leprêtre et al, 2013;Leffondré et al, 2021;Strzerzynski et al, 2021).…”