The thrust system that affects the morphology of the SW Iberian margin locally controls the present-day bathymetry and consequently the pathway of the Mediterranean Outflow Water as it flows along the Gulf of Cádiz middle continental slope from the Strait of Gibraltar (Figure 3). The Mediterranean Outflow Water has determined the development of a complex contourite depositional system during Pliocene-Quaternary times (Hernández-Molina et al., 2003). Moreover, the main clusters of seismic events in the northern Gulf of Cadiz (Ribeiro et al., 1996; Pedrera et al., 2011) lay parallel and aligned to the thrust faults interpreted by Ramos et al. (2017a) (Figures 3C,D). The focal mechanisms measured in the margin are coherent with N-S to NW-SE directed shortening (e.g., Palano et al., 2013). Thrust fault solutions are compatible with the presence of the E-W o WSW-ENE trending basement-involved thrust faults affecting the Algarve Basin, while the location of the strike-slip fault solutions is consistent with the N-S to NW-SE trending lateral ramps and tear faults of the thrust system. The main seismic cluster sits on the most representative bathymetric highs (Guadalquivir and Portimão banks), which correspond to inverted Cenozoic structures associated to the activity of the most southern thrust (Figure 3). These highs related to neotectonic activity can control in turn the activity of the bottom currents and genesis of contourite features. South of the Algarve Basin however, the generalized absence of seismic activity could indicate that the deformation is shallow (Sallarès et al., 2011) and related to the gravitational effects of shale and evaporite mobility (Medialdea et al., 2004).

On seismic profiles, the AUGC lies folded directly on the southern flank of the Guadalquivir (Figure 3) and Portimão banks, demonstrating that these uplifted structures acted as physiographical barriers to the progression of the AUGC to the NW, and therefore, attesting a more complex inversion history of the margin. Although these structures are active nowadays, at least four main stages of shortening were documented by interpreting the seismic data present in the Gulf of Cadiz: late Cretaceous to early Paleogene, late Paleogene to early Miocene, middle to late Miocene, and late Pliocene to present day (Ramos et al., 2017a).

The opposing dip between the Mesozoic extensional faults and the south-verging thrust system (Figure 3) is interpreted by the reactivation of low-angle thrusts and cleavage within the Paleozoic basement during the Cenozoic inversion, in contrast with the south-dipping extensional faults accommodating extension towards the south. The main inversion structure (the southern thrust) coincides in orientation and location with the necking domain of the margin (Figure 3). This suggests that the thrust fault would have taken advantage of the north-tilted continental crust and Moho during the Mesozoic extensional necking phase (Ramos et al., 2017b).

Along the Gulf of Cádiz, a major system of linear and sub-parallel strike-slip faults has been reported based on the SWIM compilation of multibeam bathymetry (Zitellini et al., 2009). This system has a clear reflection on the seafloor morphology. The angular relationship between en-echelon fold axes affecting the surface sediments and the SWIM faults indicates a dextral strike-slip movement (Rosas et al., 2009). Four SWIM faults were described (Zitellini et al., 2009): The SWIM-1 and SWIM-3 bounds the Coral Patch Ridge (CPR), whereas the SWIM-2 is located further north (Figure 2). In contrast, Rosas et al. (2009) show a system of major bathymetric lineaments termed as L1 to L4 from south to north. L1 and L2 coincides with two of the faults already mapped by Medialdea et al. (2004), Medialdea et al. (2009a) in the Gulf of Cádiz. SWIM-1 and SWIM-2 from Zitellini et al. (2009) coincides with L2 and L4 lineaments from Rosas et al. (2009), but L1 and L3 are intercalated between SWIM faults (Figure 2).

Interpretation of the seismic profile MOUNDFORCE-06 shows that SWIM-1 (L2), L1 and SWIM-3 lineaments linked in depth and belong to the same flower-like structure (Figure 4). Mud volcanoes as Porto and Soloviev (Pinheiro et al., 2003; Medialdea et al., 2009b) appear to be closely related to the SWIM-1 and SWIM-3 strike-slip faults respectively (Figure 2). This points to active fluid expulsion along the transpressive faults. This structure corresponds to a mega-shear zone rooted into the basement and affecting both the autochthonous Mesozoic oceanic sequence (Upper Jurassic-Lower Aptian) and the AUGC. The mega-shear zone is up to 33 km in width reaching a depth up to 10 s two-way travel time (TWT) into the basement and extends to the African margin where it links with the South Moroccan Arc (Figure 2).

Based on a first interpretation of the SWIM compilation of swath bathymetry, a prominent arc was identified in the Atlantic Moroccan margin (Zitellini et al., 2009). This arc had been previously observed on side scan sonar data (Ivanov et al., 2000).

Here we present a 3d model of the South Moroccan Arc (SMA) combining high-resolution swath bathymetry data and seismic reflection profiles from several cruises as MOUNDFORCE-2007, MVSEIS-2008 and SUBVENT-2014 (Somoza, 2007; Somoza and UTM-CSIC, 2018; Somoza et al., 2019) (Figure 5A). The northern boundary of the South Moroccan Arc corresponds to a major ESE-striking fault termed as the Larache fault (Figures 2, 5). This major fault splits to the west into an arcuate deformation front. The Larache fault is composed by several segments showing local pull-apart basins. This major ESE strike-slip fault was considered as the prolongation of the SWIM lineaments (Zitellini et al., 2009). However, our bathymetric model shows that the easternmost SWIM lineaments are not aligned with the Larache fault and the South Moroccan Arc is superimposed over the eastern prolongation of the SWIM faults (Figure 2).

Multichannel seismic profiles crossing the deformation front of the South Moroccan Arc show that it is composed of a series of fold-thrust systems linked to the main strike-slip faults (f1 to f3 system in Figure 5B). The thrusts are rooted on the AUGC unit and deform the uppermost sedimentary units including the seafloor, forming prominent diapiric ridges (Figure 2). A strong discontinuity is observed within the fold-thrust system as the beginning of the deformation (Figure 5B).

The generation of the SMA deformation front is associated with the right-lateral movement of the southern side of the Larache strike-slip fault (Figure 6). The seabed expression of this major fault is a narrow valley, 2.2 km width and 75 m depth, that splits into two elongated ridges, 45 m in height (Figure 6A). The elongated ridges are separated by a 2.6 km width depression. Pull-part mini basins are also associated with the right-lateral movements of this fault (Figure 6A).

The subsurface expression of the Larache strike-slip fault, as seen in multichannel seismic profiles (Figure 6B), shows a complex flower-type structure composed of a northern transpressional edge and an eastern transtensional zone. The lower part of this flower-type structure, down to 2 s TWT, is transformed into major thrust system that affect the AUGC units forming subsidiary thrust systems towards the north (Figure 6B). These thrust systems affect the southernmost mud volcanoes of the Moroccan mud volcano province as the Ginsburg, Rabat and Almanzor mud volcanoes (Figure 6B) (Ivanov et al., 2000; Medialdea et al., 2009a; León et al., 2012).

The age of initiation of the Larache strike-slip fault activity can be inferred from the definition of the main unconformities associated with its movement (Figure 6B). A main unconformity marking a pronounced basin at the southern edge of the Larache fault allows to determine the beginning of the fault activity. We correlate this discontinuity with the base of Quaternary Discontinuity (BQD, ∼2.6 My) reported by Toyos et al. (2016) and associated with the development of the Ginsburg MV. An overlying major unconformity is interpreted as the Mid-Pleistocene Discontinuity (MPD, ∼0.9 My). Finally, a third unconformity can be dated as Late Pleistocene. On ultra high-resolution sub-bottom profiles, these seabed ridges related to the Larache fault deform the sea-floor sediments indicating recent activity, at least, from Late Pleistocene times.

Based on this correlation, we estimate the onset of the Larache strike-slip fault activity at the beginning of the Quaternary (∼2.6 My), even though, main activity has taken place during Mid and Late Pleistocene times. This major fault triggered the generation of the South Moroccan Arc structure overlapping the former AUGC unit, estimated to be emplaced in the Gulf of Cadiz during the Late Tortonian (Maldonado et al., 1999).This strike-slip fault and the associated thrust-fold system of the South Moroccan Arc show a total rupture length of 200 km, 80 km of the Larache strike-slip fault plus 120 km of the thrust-folds system, and therefore reach the required scale to be a potential source candidate for large earthquakes (Stich, 2007). Moreover, vertical displacements of the seabed up to 75 m have been observed along the Larache fault and up to 100 m high in the fold-thrust system (Figures 5, 6). Therefore we propose that these related vertical components are potential sources candidates for large tsunamis in the Gulf of Cádiz.

These fault systems are concentrated in the central sector of the Alborán Sea basin, where at least five NNE-SSW and one NE-SW trending fault zones were identified from seafloor morphotectonic deformation (Figure 7).

They comprise the Al Idrissi (AIFS), the Motril-Djibouti Marginal Plateau (MDF) and La Serrata-Carboneras (S-CF) fault zones (Figures 7, 8 and Table 1) (Vázquez et al., 2018). The lengths of the fault zones vary between 40 (La Herradura Sound Fault Zone) and 140 km (La Serrata-Carboneras Fault Zone), although the later extends approximately 50 km onland and 90 km on the continental margin, with the Al Idrissi fault being the longest on the continental margin (125 km). The fault zones varies between 0.7 and 4 km in wide and are characterized by intense internal brittle deformation. Locally they have transtensive segments characterized by longitudinal grabens, rhomboidal depressions and transtensional relays (in the cases of The Herradura Sound, Calahonda Sound, Djibouti Ville Seamount, and Djibouti Passage fault zones), specially to the north of the La Herradura and Djibouti Ville seamounts, or with transpressive segments characterized by pressure ridges as in the cases of La Serrata-Carboneras and Al Idrissi faults (Figure 8). These last two structures are the most significant. The onland extension of La Serrata-Carboneras Fault Zone has been described as part of the Eastern Betic Shear Zone (Figure 7) and affects geological units during the last 133 ka (Silva et al., 1993; Bell et al., 1997; Moreno et al., 2015; Masana et al., 2018 Even though there are no large instrumental seismic events concentrations along this fault, several authors suggest that some historical events may be related to this fault (Keller et al., 1995; Gràcia et al., 2006). Finally, the Al Idrissi fault zone connects the Al Hoceima and Adra seismic areas, which are respectively located on the southern and northern margins of the basin and clearly displaces the Alborán Ridge (Galindo-Zaldívar et al., 2018; Gràcia et al., 2019; d’Acremont et al., 2020). This fault is divided at least into three segments: 1) the northeastern one is located on the Motril-Djibouti Marginal Plateau and has a transtensive character (Vázquez et al., 2016, 2018; Gràcia et al., 2019), 2) the central segment extends from the Alborán Trough to the Alborán Ridge towards the SSW; it corresponds to the western boundary of the Alborán Ridge Indenter (Estrada et al., 2018b) and has a transpressive configuration (Martínez-García et al., 2013) constituted by elongated pressure ridges and restraining bends with a set of successive high-angle reverse faults (Galindo-Zaldívar et al., 2018; Gràcia et al., 2019; d’Acremont et al., 2020); 3) the southwestern segment extends from the Alborán Ridge to the Al-Hoceima Bay towards the SSW and has an extensional horsetail splay (d’Acremont et al., 2014). In this SSW area a fault zone of similar characteristics has been defined at crustal levels extended both onshore and offshore regions, which could explain the three main earthquake series between 1994 and 2016 (Galindo-Zaldívar et al., 2018; Gràcia et al., 2019). This fault system has been explained by these authors as the growth of a continental strike-slip fault from the African margin to the north and include the eastern set of faults previously described in the of Motril-Djibouti Marginal Plateau.

The faults of this system are concentrated in the northeastern continental margin of the basin and in the eastern part of the Alborán Sea Basin, where they show NW-SE to WNW-ESE trends and constitute the outstanding Yusuf Fault Zone, that corresponds to the eastern boundary of the Alborán Ridge Indenter (Moreno et al., 2016; Estrada et al., 2018b; Perea et al., 2018).

The Yusuf Fault Zone (YF) is a right-lateral strike-slip fault with a transtensional component (Figure 7). It is 175 km long and 15 km wide and is divided into two main segments separated by a relay zone (Mauffret et al., 1992; Mauffret et al., 2007; Gràcia et al., 2014; Gómez de la Peña et al., 2016). The fault zone is characterized by the development of several strike-slip faults with a general transtensive geometry. Its morphology is characterized by a rectilinear escarpment with a relief ranging from 800 to 2000 m in the western part and an elongated ridge in the eastern one, with the development of a pull-apart basin (20 km in length and 10 km in width). The fault trace in its northern segment bends to the west at the connection with the northern Alborán Ridge Fault (ARF, Figure 7), where it shows a reverse component (Martínez-García et al., 2010). It can also continue to the northwest with the NW-SE Averroes system (Perea et al., 2018).

At least five WNW-ESE fault zones have been identified in the eastern part of the Motril-Djibouti Marginal Plateau (MDF) (Figure 8).The most penetrative is the Averroes Fault Zone (AVF), but at least other four fault zones have been identified to the northeast, sub-parallel to the Averroes Fault Zone and called NAF1 to NAF4 by Perea et al. (2018). These faults separate elongated ridges that have been interpreted as anticlines between faults (Moreno et al., 2016). The Averroes Fault Zone is constituted by at least two high-angle faults of 46 km in length and 2 km in maximum width, made up of at least two segments. The southeastern one displaces the seafloor across the Adra Ridge and the Alborán Channel and ends in the Alborán Ridge, generating a longitudinal escarpment and an elongated ridge (Figure 7). Meanwhile the northwestern segment corresponds to a narrow trough formed by a half-graben structure around 15 km long, with a vertical offset of up to 470 m with a downthrown block to the NE (Figure 7) (Estrada et al., 2018b; Perea et al., 2018) The other four remaining fault zones have similar characteristics to the Averroes Fault Zone: high angle fault surface, affect the Motril-Djibouti Marginal Plateau, the Adra Ridge and the Alborán Channel and generate elongated depressions in the seafloor related to negative flower structure geometries and rectilinear scarps, allowing to define a right-lateral to normal movement. Some of them have several branches and their length approximately ranges between 17 and 36 km (Perea et al., 2018).

In addition, another fault zone of this system has been located in the upper continental slope in front of the Adra region that has been called as the Adra Fault (Gràcia et al., 2012), interpreted as a right lateral strike-slip fault (Figure 7). This fault zone is constituted at least by three different faults, with lengths ranging between 10 and 16 km. It is characterized for producing minor rectilinear changes in the slope gradient and small scarps caused by the normal component of these faults (Vázquez et al., 2014).

These structures include antiformal folding related to the main elongated ENE-WSW ridges and banks, as well as thrust faults. Two main structures are defined: the Northern Alborán Ridge Fault and the Xauen Compressive System (Figure 2).

The Alborán Ridge fault (ARF) has an ENE-WSW direction and is constituted by at least two or three thrusts. It is located north of the ridge and corresponds to the front of the Alborán Ridge Indenter (Estrada et al., 2018a) bounded by the Yusuf faults to the east and the Al Idrissi fault (AIFS) to the west, with an approximate length of 75 km and a width of the fault zone around 6 km, presenting an arcuate geometry on the surface (Figure 7). This structure and its southwest extension are considered as a 165 km long left-lateral strike-slip fault zone, with a compressive component that has been active since late Miocene times (Bourgois et al., 1992; Woodside and Maldonado, 1992; Watts et al., 1993; Comas et al., 1999). However, it had an important uplift phase as a tectonic relief, through folding and reverse faults in the Pliocene-Quaternary (Martínez-García et al., 2013, 2017; Vázquez et al., 2015; Estrada et al., 2018a).

The Xauen Compressive System generates the current relief of the banks located to the west of the Alborán Ridge (Francesc Pages and Xauen banks), that are left-laterally displaced by the Al Idrissi fault with respect to the eastern Alborán Ridge (Figure 7). This system has almost 60 km in length and 30 km in width and is constituted by at least three north-verging thrust faults gently arched and a fourth thrust with southern vergence (Bourgois et al., 1992; d’Acremont et al., 2020). The geometry of the Xauen Bank corresponds to a pop-up structure. Two main thrusts bound this positive relief and reach the seafloor; the northern thrust of this system corresponds to a blind thrust that constitutes the deformation front (d’Acremont et al., 2020).

In addition, three gently ridges with a N50-60 trend and around 20 km long affect the continental margin seafloor in front of the Adra coast. The most prominent feature has two high-angle reverse faults at the top affecting the Upper Pleistocene-Holocene units (Vázquez et al., 2014, 2016). These ridges are interpreted as anticline folds associated with blind thrusts verging to the NW, affecting the Quaternary units and bulging the seafloor (Comas et al., 1992; Vázquez et al., 2008b).

Several normal faults trending N-S to NNW-SSE affect the seafloor in the Alborán Sea region generating sets of rectilinear scarps and longitudinal depressions. They have been described in both northern and southern margins. In the southern margin, they are focused in the southern end of the Al Idrissi fault zone into the Al Hoceima Bay where a homogeneous set of N155 oriented normal faults and close to 10 km in length has been described by Lafosse et al. (2018). Normal faults generate small rectilinear scarps on the seafloor (Figure 7). In the northern margin, faults of this system are better represented along the Motril-Djibouti Marginal Plateau (MDF) (Figure 8), although they are more concentrated in the northern end of the Al Idrissi fault zone, where they are located in a corridor of 5 km in width. Normal faults have trends from N165 to N15 and generate rectilinear scarps and elongated tectonic depressions ranging between 2 and 7 km in length, that affect the seafloor (Vázquez et al., 2014; Vázquez et al., 2016). These faults have been explained as the surficial expression of the NNE propagation of Al Idrissi fault strike-slip system (Vázquez et al., 2014; Gràcia et al., 2019).

A cluster of earthquakes Mw > 5 is associated with the Finisterre thrust system (FSF, Figure 10). This system consists of an array of thrust faults that deform the recent sedimentary sequence at the northern area of the Finisterre Seamount (Figure 11). The Theta Passage is a deep passage that separates the Iberian continental margin from the Atlantic oceanic ridges of the Coruña Seamount (Figure 11A). (e.g., Vázquez et al., 2009a; Somoza et al., 2019). The Finisterre Seamount fault system (FSF) is interpreted as landward-dipping thrusts which presently deforms the seafloor forming a serie of ridges (Figure 11B). This zone has been considered as a zone of obduction of the serpentinized mantle and Atlantic oceanic crust during the Alpine-Pyrenean compression due to the outcropping lherzholites that were collected at the foot of the NW slope of the Galicia Bank (Boillot et al., 1979; Malod et al., 1993). The Finisterre external deformation front has a length of 75 km, at the seabed, extending from the SW to NE (Figure 11).

The second zone with high seismic activity is the so-called Transitional Zone (Murillas et al., 1990) located between the Galicia Bank and the Galicia Interior Basin (Vázquez et al, 2009c). This area extends over 75 km and is bounded by two main NW-SE trending faults from the Galicia Interior Basin (Castelao Fault, CSF) and the Galicia Bank (Eastern Galicia Bank Fault, EGBF) (Figure 12). This area is characterized by an elongated WNW-ESE dome-like morphology at water depths from 1,500 to 2,500 m with gentle slopes of 1.2° (Figures 12, 13). The seafloor floor exhibits an array of en-echelon NW-SE to N-S trending ridges with lengths up to 10 km and circular depressions (Figure 12). The largest depression, named as the “Burato Hole” (Vázquez et al., 2009c), is a 3 km in diameter crater-like depression of 300 m deep with average flank slopes of 12° (Figure 13).

The Burato Shear Zone shows a prominent elongated NW-SE trending ridge bounded by a dense network of tensional gashes (Figure 12). High-resolution multichannel seismic profiles show that the Burato shear zone is characterized by a dense array of sub-vertical faults identified as tension gashes on the multibeam bathymetry (Figure 13).

The third structure associated with relatively moderate activity of earthquakes MW > 5 is the NW-SE Ortegal right lateral strike-slip fault (OF in Figure 10). This type of NW-SE structure was formed, at least, during the Cenozoic Alpine-Pyrenean orogeny, dissecting the accretionary wedge of the North Iberian Margin.

The Ortegal Fault (OF) is the westernmost of a system of large Cenozoic strike-slip faults as Estaca de Bares (EBF) or Ventaniella (VF) (Figure 10), developed from the Northwest Iberian margin to the Pyrenees (Figure 1). These faults were formed, at least, during the Pyrenean phase 9 in response to Paleogene-early Neogene N-S and NE-SW compression between Eurasia and Iberia (Figure 1C) (e.g., Andeweg, 2002). The Ortegal Fault offsets the Mesozoic oceanic ridges of the Jean Charcot Seamounts and the buried front of the accretionary wedge formed along the N Iberia margin (Figure 9). The high-resolution bathymetry shows that this fault is split into several branches when enter into the continental margin, controlling the development of deep submarine canyons named as Ferrol and La Coruña canyons (Figure 9). We have termed, each of these branches affecting the continental margin from south to north, as the Malpica (MLF), La Coruña (CRF) and Ferrol (FRF) faults (Figure 10). The activity of these faults is revealed by the occurrence of earthquakes Mw > 5 on the upper margin, especially along the central branch (CRF) (Figure 10). The onshore prolongation of these strike-slip faults (Figure 10) is constituted by the As Pontes and Meirama strike-slip faults and the Padrón-Vigo Fault (PVF) (de Vicente, 2009), that form the main deep-intracontinental Tertiary basins in the Galicia region (e.g., Andeweg et al., 1999). The Becerreá Fault (BCF in Figure 10) holds a remarkable concentration of earthquakes within the Galicia Region (e.g., López-Fernández et al., 2012) This high-seismic onshore lineament might be linked to the offshore front of the Galicia Bank and would represent the crustal boundary between the former convergent N Iberian Margin and the hyperextended-rifting W Iberian Margin (Somoza et al., 2019) (Figure 10).