The Alboran Basin developed over a collapsed Tertiary orogen and is limited onshore by the Betic–Rif belt (Fig. 1) (Comas et al., 1999). The formation of the Alboran Basin has been linked to Early Miocene forearc extension (Booth-Rea et al., 2007; Faccenna et al., 2001; Gómez de la Peña et al., 2018; Jolivet et al., 2008, 2009; Jolivet and Faccenna, 2000). Several Miocene strike-slip shear zones cut the entire basin from the Iberian to the Moroccan margins and accommodate the upper-plate deformation that forms a broad shear zone called the Trans-Alboran shear zone (TASZ; Fig. 1) (Leblanc and Olivier, 1984). Following the westward slab retreat, the TASZ acted as a left-lateral fault zone accommodating the extension of the Alboran Basin. The Africa–Eurasia NW–SE oblique convergence led to a tectonic reorganization during the Late Miocene (Comas et al., 1999; Do Couto et al., 2016). Due to ongoing Africa–Eurasia convergence, the TASZ underwent an oblique positive inversion starting around 8 Ma in the Sorbas Basin of the Betic margin (Do Couto et al., 2014; Martínez-García et al., 2017). The compression has migrated westward since approximately 7–8 Ma from the Spanish and Algerian margins to the Alboran Ridge and since ca. 5 Ma on the Al-Idrissi Fault (Figs. 1 and 2) (Giaconia et al., 2015).

In the southern margin of the Alboran Sea, the Alboran Ridge corresponds to a tectonic high that developed since the Late Miocene (Bourgois et al., 1992; Do Couto, 2014). The Alboran Ridge and the Yusuf Fault divide the Alboran Basin into three different subbasins: the West (WAB), South (SAB) and East (EAB) Alboran Basin (Fig. 1). Conjugate to the Alboran Ridge, the right-lateral Yusuf Fault zone has been active since the Miocene (Fig. 1) (Martínez-García et al., 2013, 2017). The Al-Idrissi Fault divides the Alboran Ridge into the North (NAR) and South Alboran (SAR) ridges (Fig. 1). The SAR corresponds to a series of NE–SW striking submarine highs culminating around -110 m (Xauen Bank, Petit Tofiño Bank, Tofiño Bank, Ramon Margalef High, Eurofleet High and Francesc Pagès Bank in Fig. 3).

Sedimentary processes, volcanism and tectonics shaped the morphology of the Alboran Ridge. Above the Messinian Erosional Surface (MES) (Estrada et al., 2011; Garcia-Castellanos et al., 2011), the deep sedimentation in the Alboran Sea is driven by contouritic processes that has also shaped the seafloor since 5.33 Ma) (Ercilla et al., 2016; Juan et al., 2016). On both flanks of the Alboran Ridge, contourite deposits produce significant thickness variations in the Quaternary depositional units that are pinched and thinned toward the foot of the submarines highs (Juan et al., 2016). Submarine erosion can occur at the moat of the contouritic systems, generally at the foot of the slopes, whereas deposition occurs at deepest locations (Ercilla et al., 2016; Juan et al., 2016). The 70 km long SAR corresponds to a series of faults and folds and to volcanoes affecting the Pliocene–Quaternary depositional sequences (Fig. 3) (Bourgois et al., 1992; Chalouan et al., 1997; Gensous et al., 1986; Martínez-García et al., 2013; Muñoz et al., 2008; Tesson et al., 1987). To the south, the SAR flanks a NE–SW syncline called the South Alboran Trough and to the north, the Alboran Channel and the WAB (Fig. 3). The SAR marks the southward transition from thinned to thickened continental crust (Díaz et al., 2016). It is an inherited Early Miocene extensional structure that has undergone compressive deformation since 8 Ma (Fig. 1) (Do Couto et al., 2016). In the WAB, a syn-rift sequence is dated late Aquitanian–Burdigalian to Langhian (Do Couto et al., 2016). At the base of the sedimentary column of the SAR, the seismic reflection data show Early to mid-Miocene under-compacted shales deposited during the extensional period (Do Couto, 2014; Do Couto et al., 2016; Soto et al., 2008). Pre-Messinian deposits are exposed at the seafloor in the cores of the anticlines of the Alboran Ridge (Chalouan et al., 2008; Do Couto et al., 2016; Juan et al., 2016; Tesson et al., 1987). Local occurrences of volcanism in the Francesc Pagès Bank and the Ras Tarf are of Miocene and Pliocene age (Figs. 1 and 3). The volcanism in the Francesc Pagès Bank is not accurately dated (Gill et al., 2004). Basaltic rocks are dated between 9.6 and 8.7 Ma in the same area by Duggen et al. (2004). In the Ras Tarf (Fig. 3), the volcanism ends around 9 Ma (El Azzouzi et al., 2014). Samples of the Ibn Batouta Seamount contain 5 Ma old gabbro (Duggen et al., 2008).

Since the Late Miocene, deformation has migrated from the eastern Betic margin toward the SAR in the southwest (Fig. 1) Giaconia et al. (2015). The SAR has been inverted during the Plio-Quaternary along NE–SW trending faults (Fig. 1) (Chalouan et al., 1997). Seismic reflection profiles and well data show that the folding continued until the Quaternary in the Francesc Pagès Bank and highlight several erosion periods during Pliocene–Quaternary time (Galindo-Zaldivar et al., 2018; Tesson et al., 1987). Unconformities and increasing accumulation rates indicate three tectonic phases: phase 1 dated from 5.33 to 4.57 Ma, phase 2 from 3.28 to 2.45 Ma and phase 3 between 1.81 and 1.19 Ma (Martínez-García et al., 2013). More recently, Martínez-García et al. (2017) suggested that the uplift along the Alboran Ridge culminated around 2.45 Ma in response to shortening.

The most recent deformation involves NNW–SSE sinistral transtension from the frontal indentation of the northern part of the Alboran Ridge to the transtensive Nekor Basin via the AIF, across the NAR and the SAR at the NE tip of the Francesc Pagès Bank (Dillon et al., 1980; Estrada et al., 2018; Lafosse et al., 2017). The Nekor Basin accommodates the present-day deformation of the southern Alboran margin (Figs. 2 and 3). Bathymetric and seismic reflection data show that the deformation along the AIF is accommodated by a series of sinistral NNE–SSW strike-slip faults segments (Figs. 1 and 2) (Ballesteros et al., 2008; Martínez-García et al., 2011). The AIF propagated southward during the Quaternary (Ballesteros et al., 2008; Gràcia et al., 2006; Martínez-García et al., 2011, 2013), connecting to the NNE–SSW active strike-slip Boussekkour–Bokoya Fault zone (Fig. 3) (d'Acremont et al., 2014; Calvert et al., 1997; Lafosse et al., 2017).

East of the TASZ, the SAB and the Oriental External Rif behave as the African block (Koulali et al., 2011; Vernant et al., 2010). GPS kinematics show a WNW–ESE convergence rate of 4.6 mm yr-1 between the Africa and Eurasia plates (Nocquet and Calais, 2004). Maximum present-day extrusion rates of 5.5–6 mm yr-1 in the Alboran tectonic domain are measured between the Jebha and Nekor faults and indicate a southwestward lateral escape (Fig. 2b) (Koulali et al., 2011; Vernant et al., 2010).

The Nekor Basin, SAR and AIF are affected by significant crustal seismicity (Bezzeghoud and Buforn, 1999; Stich et al., 2005). The focal mechanisms of three main regional earthquakes show subvertical nodal planes and a left-lateral displacement (Fig. 4) (Bezzeghoud and Buforn, 1999; Biggs et al., 2006; Calvert et al., 1997; El Alami et al., 1998; Hatzfeld et al., 1993; Stich et al., 2005, 2006). At the northern border of the Nekor Basin, earthquakes with Mw=6.3 and 5.9 occurred in 1994 and 2004, respectively (Fig. 4) (Custódio et al., 2016). The NNE–SSW fault tracks identified at the seafloor, in the vicinity of the epicenters, can correspond to the active fault planes deduced from seismological data (d'Acremont et al., 2014; Calvert et al., 1997; Lafosse et al., 2017). On 25 January 2016, a Mw=6.3 earthquake occurred in the vicinity of the AIF (Buforn et al., 2017; Medina and Cherkaoui, 2017; Galindo-Zaldivar et al., 2018). In the deep basin, the earthquake sequence indicates a strike-slip mode of the AIF, with mainly NNE–SSW left-lateral motion (Ballesteros et al., 2008; Buforn et al., 2017; Galindo-Zaldivar et al., 2018; Martínez-García et al., 2011; Medina and Cherkaoui, 2017). Several compressional events with NE–SW nodal planes parallel to the Alboran Ridge indicate that the Alboran Ridge is locally reactivated (Fig. 4). In the Nekor Basin, the deformation is partitioned into a normal component in the center of the basin and a left-lateral component on its borders (Fig. 4) (Lafosse et al., 2017). In the SAR, the style of the deformation is unclear, with focal mechanisms showing strike-slip or normal components indiscriminately (Stich et al., 2010). Below the WAB, deep earthquakes occur at depths>60 km (Fig. 2a) and are associated with the ongoing necking of sinking lithospheric material (Fig. 2a) (Bezada et al., 2013; Ruiz-Constán et al., 2011; Sun and Bezada, 2020; Thurner et al., 2014). This distributed lithospheric tear could have propagate from the Betic to the WAB (Heit et al., 2017; Mancilla et al., 2015), yet the timing and the effect of this tear on the local tectonic is still poorly understood.

The Pliocene–Quaternary sedimentary sequence of the southern Alboran margin has been divided into three Pliocene (Pl1, Pl2 and Pl3) and four Quaternary (Qt1 to Qt4) seismic units (Fig. 5). These units are limited at the base by discontinuity surfaces, with M, P0 and P1 for the Pliocene units and BQD and Q0 to Q2 for the Quaternary units. These discontinuity surfaces are mostly defined by onlap and erosive surfaces; locally, downlap surfaces are identified (Figs. 6 and 7). Subparallel, parallel, oblique and wavy stratified reflections characterize the Pliocene units. Pl1, Pl2 and Pl3 units are pinching toward the structural highs and show aggrading wedge geometries. The Quaternary seismic units (QT1 to QT4) show an aggradational geometry and pinch on the older tilted Pliocene deposits (Figs. 6 and 7) (Juan et al., 2016).

Contouritic deposits and associated sedimentary features, mass transport deposits (MTDs) and volcanic deposits constitute the Pliocene–Quaternary deposits. The plastered drift type is dominant and contributes to cover the structural highs (Juan et al., 2016). Truncations at the foot of topographic highs corresponds to contourite moats and channels on seismic lines (Fig. 7). Sediments show local intercalations of lenticular chaotic or transparent facies that are interpreted as mass-flow deposits, corresponding to scars on the bathymetry (Fig. 3) (Rodriguez et al., 2017). Regarding the volcanic deposits, two buried volcanic edifices are identified on seismic reflection, the Big Al-Idrissi Volcano (Figs. 3 and 8) and the Small Al-Idrissi Volcano (Figs. 3, 6 and  9). Acoustically, they correspond to a seismic facies of poorly continuous, high-amplitude reflectors (Figs. 6, 8 and 9). Pliocene to Quaternary reflectors onlaps onto these seismic bodies (Figs. 8 and 9). They trend NE–SW following the trend of South Alboran Trough (Fig. 10).

The Big Al-Idrissi Volcano corresponds to a conic structure located to the north of the Ras Tarf (Figs. 3 and 8) that has been interpreted as an N–S volcanic ridge (Bourgois et al., 1992). The top of this seismic body merges with the M reflector (Fig. 8). Above, Pliocene–Quaternary seismic units bury this volcano and show prograding to aggrading sigmoid reflectors that characterize the growth of a continental shelf (Fig. 8). On the west side of this seamount, the trajectory of the offlap breaks is concave up, indicating that the rate of progradation decreases progressively with time. Reflectors onlap on the bottomsets and foresets of the prograding seismic units, marking the beginning of a retrogradation after 1.81 Ma (Fig. 8). West-dipping normal faults offset the depositional unit of prograding sigmoid reflectors (Fig. 8). These normal faults correspond to scarps at the seafloor (Figs. 8 and 11). Toward the top of the sequence, a unit of flat-lying reflectors corresponds to the bottomsets of the Late Pleistocene Moroccan shelf offshore of the Ras Tarf (Fig. 8). The flat top of the Big Al-Idrissi Volcano culminates at an approximate depth of 150–200 m below the present-day sea level and corresponds to a toplap surface (Fig. 8).

In the South Alboran Trough, the Small Al-Idrissi Volcano has a 4–5 km wide conic structure and trends roughly NNE–SSW (Figs. 9, 10 and 11). This seismic body intercalated within the Pl1 seismic unit pinches abruptly toward the west (Fig. 9). This body corresponds to a rounded high at the seafloor (Fig. 11). The top of the Pl1 seismic unit rests unconformably on this seismic body indicating an Early Pliocene age (Fig. 9). In the Francesc Pagès Bank, a seismic body with similar facies is present at the core of an NNE–SSW striking anticline (Fig. 6), truncated by the M reflector (Fig. 6).

North of the Nekor Basin, the shelf records an Early Quaternary regression (Fig. 12). We follow the Q1 surface northeastward toward the top of the submerged shelf surrounding the Big Al-Idrissi Volcano (Fig. 8). The Q0 reflector corresponds to an unconformity at the bottom of prograding oblique reflectors. This depositional unit displays the geometry of continental shelf deposits. The most distal offlap break shows the maximum extent of the Pleistocene continental shelf north of the Nekor Basin. It indicates that the retrogradation of the shoreline starts before 1.12 Ma and after 1.81 Ma (Q1 reflector, Fig. 12). The most distal offlap break near Al Hoceima is located around 312±30 mstwt (milliseconds two-way travel time), corresponding to a depth of 188±5 m below sea level (Fig. 12). In the distal part of the shelf, we interpret a seismic body of poorly continuous wavy reflectors deposited above an erosional surface as a local mass transport complex, which could mark an early-Quaternary destabilization of the shelf.

Truncated folds (Fig. 6f) indicate that shortening started in the South Alboran Ridge before the Messinian Salinity Crisis (MSC) (Do Couto et al., 2016). The lateral and vertical stratal pattern of the Plio-Quaternary units shows that the shortening occurs mostly during the Pliocene. The overall geometry of deformation in the SAR shows the development of a N065∘ shear zone that partitioned the deformation in imbricated folds and thrusts and left-lateral shear (Fig. 6). The change in stacking pattern of the Pliocene deposits along the folds suggests a diachronous growth during the Pliocene with lateral variation in the uplift rates (Figs. 6 and 7). The intra-Pliocene unconformities, the tilting of the Pliocene units and the aggradation of Quaternary contourite deposits indicate a relative quiescence of the folding during the Quaternary after 2.6 Ma (Juan et al., 2016).

The Pliocene deformation is locally contemporaneous with volcanism. The lateral continuity of the highly reflective facies from west to east suggests that the Small and Big Al-Idrissi volcanoes are part of a volcanic structure that is offset by local extensional faults during the Pleistocene (Figs. 8 and 10). This highly reflective material triggers the acoustic masking of the reflections below (Fig. 9), as observed in debris-avalanche deposits elsewhere (Le Friant et al., 2002, 2009). The intercalation of this volcanic material toward the top of the Pl1 unit dates the Small Al-Idrissi Volcano between 4.5 and 5.33 Ma (Fig. 9). The NE–SW distribution of the volcanic material suggests a syn-folding infill of the syncline of the South Alboran Trough (Fig. 10). The local volcanism is contemporaneous with the volcanic activity occurring to the north of the Alboran Ridge, dated between 6 and 4.5 Ma (Duggen et al., 2008). This volcanism occurred above a thinned continental lithosphere (Duggen et al., 2008). The local volcanism suggests that the SAR could have accommodated westward thinning of the crust in the West Alboran Basin from Late Miocene to Pliocene. This extension could be linked to the transition from slab rollback to delamination as proposed in Petit al. (2015).

NE–SW thrust faults distribute the deformation between the Nekor and Jebha Fault and the Alboran Ridge and accommodate the strike-slip motion during the Pliocene. The 20∘ angle between the N065∘ trend of the SAR and the N085∘ trend of internal folds evidence a N–S maximum horizontal shortening direction in the SAR (in the present-day structural framework) and indicates left-lateral transpression during the Pliocene folding (e.g., Fossen et al., 1994; Fossen and Tikoff, 1998). It reflects the oblique shortening direction relative to the NE–SW basement faults during the Pliocene. In the SAR, the Pliocene folds show a left-lateral deflection of their hinge lines from the E–W to NE–SW, drawing an overall sigmoidal shape (Figs. 6 and 10). Comparison of the structures in the SAR with analogue models of fold-and-thrust belt (e.g., ter Borgh et al., 2011; Koyi et al., 2016; Storti et al., 2007) suggests reactivation of basement faults and vertical-axis rotation of the faults (Fig. 16a–c). The development of E–W faults and thickness variation in the sedimentary cover, resulting in non-cylindrical thrust wedges, lateral escape of frontal thrust sheets and vertical-axis block rotations demonstrate the influence of a viscous layer. In the SAR, such a weak layer corresponds to the Early Miocene, under-compacted shales at the base of the sedimentary cover (Soto et al., 2008, 2012). Such a weak layer can explain why the deformation is distributed in the SAR, whereas it appears to be more localized in the NAR.

As the direction of relative plate motion between Africa–Eurasia has been approximatively constant since 6 Ma (DeMets et al., 2015), left-lateral transpression in the SAR in the present-day framework is unlikely. Instead, it suggests a progressive rotation of basement faults relative to the regional shortening direction since the Pliocene and a progressive change from left-lateral transpressive to more purely compressive deformation (Fig. 16a and b). This model is in accordance with the bookshelf model, which assumes 2–3∘ Ma-1 progressive vertical-axis rotation of basement faults since the Pliocene (Meghraoui and Pondrelli, 2013).

The offshore Pliocene oblique compression is equivalent to transpressive tectonics in the Rif onshore (Fig. 15), where the area between the Nekor and Jebha Fault accommodates distributed deformation (Fig. 16a). The passive infilling of paleo-rias indicates relatively low vertical motion (Romagny et al., 2014) (Fig. 15). After 3.8 Ma, a transition from compression to radial extension (Benmakhlouf et al., 2012) causes NE–SW normal faulting and tectonic tilting of the Moroccan margin (Fig. 16b) (Romagny et al., 2014). Toward the southeast, the Nekor Fault has acted as a transpressive fault zone accommodating the shortening (Ait Brahim et al., 2002; Aït Brahim and Chotin, 1990). The offshore extensional faults prolonging the Nekor Fault were sealed by Pliocene deposits and inverted as blind thrust faults during the Plio-Quaternary (Watts et al., 1993). In the external Rif, in the southwestward continuity of the Nekor Fault, field studies demonstrate NE–SW compression (Roldán et al., 2014). Interpretation of 2D seismic reflection lines indicates thick-skin tectonic from Tortonian–early Messinian to Pliocene times, causing the uplift of intra-mountainous basins around the Nekor Fault (Fig. 16a) (Capella et al., 2016).

The Late Miocene–Early Pliocene period in the Rif Belt matches the uplift of the Miocene intra-mountainous basin along the Nekor Fault under compression and left-lateral displacement (Fig. 16a) (Capella et al., 2016). The uplift of those basins corresponds to the change from thin-skin to thick-skin deformation in the external Rif during the inversion of the deep Mesozoic extensional structures (Capella et al., 2016; Martínez-García et al., 2017). Our preferred tectonic scenario consists of a progressive mechanical coupling between the African margin and the Alboran Domain, locking the Nekor Fault in its eastern segment (Fig. 16). This scenario is supported by paleomagnetic data from the External Rif that indicate at least 15∘ of counterclockwise rotation since the Upper Miocene (Crespo-Blanc et al., 2016, and references therein). Progressive vertical-axis rotation associated with the shortening of the Alboran Basin decreases the left-lateral shear and increases the compressive deformation along the Alboran Ridge (Fig. 16b). Eventually, the deformation localized on the AIF during the Early Quaternary and has been decoupling the deformation between the NAR and the SAR with a developing transtensive regime since 1.81 Ma (Fig. 16c). This evolution induced a change in strain partitioning along the TASZ illustrated by the transition from a Pliocene left-lateral shearing and folding of the SAR to a transtensive Quaternary deformation localized on the AIF and the Nekor Basin (Fig. 16). Since 6 Ma, the relative direction of Africa–Eurasia convergence has remained constant (DeMets et al., 2015), with a NNE–SSW direction in an absolute reference frame (Spakman et al., 2018). This prohibits changes in the direction of far-field forces as the cause for changes in tectonic style in the Alboran Basin as suggested by Martínez-García et al. (2013).

Other scenarios consider that delamination has occurred in the Rif since 6 Ma, explaining the structural pattern and extension in the Nekor basin (Bezada et al., 2013; Petit et al., 2015). Extension in the Nekor Basin and strike-slip tectonics along the Al-Idrissi Fault since 1.8–1.12 Ma would then correspond to a reappraisal of mantle delamination. However, this process corresponds to a long-term convective removal of the African lithosphere (Petit et al., 2015). To explain the progressive tectonic reorganization during the Plio-Quaternary, we do not favor this last hypothesis because we do not observe an increase in widespread long-wavelength (>100 km) subsidence that is usually associated with convective thinning of the lithosphere in thermomechanical models (e.g., Le Pourhiet et al., 2006; Valera et al., 2011).

Recent papers (Heit et al., 2017; Mancilla et al., 2015; Sun and Bezada, 2020) suggest that a slab tear propagates from the Betic to the western tip of the Alboran Ridge, with a lithospheric necking distributed from north to south below the West Alboran Basin (Sun and Bezada, 2020). It is unclear how fast this slab tear propagates from the Betic to the study area and how it controls the Plio-Quaternary deformation. Slow uplift and extension since 4 Ma are recorded in the central Rif (Fig. 15) (Romagny et al., 2014) and might mark the inception of the lithospheric necking. Since 1.8–1.12 Ma, vertical motion has been local, however, and associated with the activity of the AIF. The timing is of primordial importance as demonstrated by thermomechanical models. These models show that slab detachment is a fast process that can occur in less than 1 Ma, causing a high-amplitude topographic response (Duretz et al., 2011, 2012). It indicates that the vertical pull due to the sinking lithosphere must be constant during the Plio-Quaternary. It suggests that the necking of the sinking lithosphere is a slow process or is very recent with yet indiscernible effects on the shallower structures in the upper plate.

In this framework, normal strike-slip faulting observed to the north of the NAR (Fig. 1) (Estrada et al., 2018; Giaconia et al., 2015; Gràcia et al., 2012; Grevemeyer et al., 2015) provides additional evidence of indentation by the Africa plate into the Alboran tectonic domain (Estrada et al., 2018; Palano et al., 2015). This indentation is accommodated through the left-lateral AIF and the right-lateral Averroes Yusuf Fault zone (Figs. 1 and 16) in a way similar to that the Palomares Fault zone, transferring the orthogonal shortening of the Iberian margin toward the Carboneras Fault zone and the central Alboran Sea (Estrada et al., 2018; Giaconia et al., 2015). In the SAR and the Nekor Basin, the present-day deformation under the transtensional regime (NNW–SSE to N–S extensional network and NNE–SSW strike-slip faults; Figs. 4 and 16) is limited to the east by the Al-Idrissi Fault. The deformation in the NAR is on the contrary clearly compressive (Estrada et al., 2018; Martínez-García et al., 2017), and the geodetic data indicate similar displacements in the EAB and in the Rifian units east of the Boudinar Basin (Koulali et al., 2011; Vernant et al., 2010). Such a difference in behavior suggests that the AIF may represent the present-day plate boundary between Africa and Alboran Domain.