Submarine Active Faults and Morpho-Tectonics Around the Iberian Margins: Seismic and Tsunamis Hazards

Abstract

The aim of this work is to make a synthesis at regional scale focused on the geophysical characterization of submarine faults around the Iberian margin to identify active structures and analyze their development in the framework of the present plate organization. Most of these submarine faults show seabed morphological expressions mapped with high-resolution swath bathymetry data, high-resolution parametric sub-bottom profiles and multichannel seismic profiles. Present active tectonics, deformation, seismicity, and tsunami-affected coastal areas is mainly focused on south Iberia at the Eurasian and Nubia plate boundary. Submarine active faults in these areas are represented by long strike-slip fault systems and arcuate fold-thrust systems. Their development takes place in response to present NW-SE convergence between the Eurasian and Nubia plates. We propose a strain partitioning model of the plate boundary into simple and pure shear zones to explain the distribution and mechanisms of active submarine faults along the Gulf of Cádiz, Gibraltar Arc and Alborán Sea in response to the present-day shear stress orientation. Nevertheless, deformation is also focused in the NW Iberian margin. Thus, along the Galician and Portuguese margin, several submarine faults mapped as thrust fault systems with high-seismic activity along the Iberian ocean-continent transition reflect the re-activation of former structures. We suggest that submarine active faults in the NW and W Iberia are also the response to the eastwards transfer of short-offset transform faults of the Mid Atlantic Ridge into the oceanic Iberian along a weakness as the former plate boundary between the oceanic Iberia and Eurasia domains. The distribution and activity of submarine faults mapped in this work from geophysical and bathymetric data are in good agreement with geodetic data and focal mechanisms.

Full text

Submarine Active Faults and
Morpho-Tectonics Around the Iberian
Margins: Seismic and Tsunamis
Hazards
Luis Somoza
1
*, Teresa Medialdea
1
, Pedro Terrinha
2
, Adrià Ramos
1
and
Juan-Tomás Vázquez
3
1
Geological Survey of Spain (IGME, CSIC), Madrid, Spain,
2
Portuguese Institute for Sea and Atmosphere IPM, Lisbon, Portugal,
3
Spanish Institute of Oceanography (IEO, CSIC), Málaga, Spain
The aim of this work is to make a synthesis at regional scale focused on the geophysical
characterization of submarine faults around the Iberian margin to identify active structures and
analyze their development in the framework of the present plate organization. Most of these
submarine faults show seabed morphological expressions mapped with high-resolution
swath bathymetry data, high-resolution parametric sub-bottom profiles and multichannel
seismic profiles. Present active tectonics, deformation, seismicity, and tsunami-affected
coastal areas is mainly focused on south Iberia at the Eurasian and Nubia plate boundary.
Submarine active faults in these areas are represented by long strike-slip fault systems and
arcuate fold-thrust systems. Their development takes place in response to present NW-SE
convergence between the Eurasian and Nubia plates. We propose a strain partitioning model
of the plate boundary into simple and pure shear zones to explain the distribution and
mechanisms of active submarine faults along the Gulf of Cádiz, Gibraltar Arc and Alborán Sea
in response to the present-day shear stress orientation. Nevertheless, deformation is also
focusedintheNWIberianmargin.Thus,alongthe Galician and Portuguese margin, several
submarine faults mapped as thrust fault systems with high-seismic activity along the Iberian
ocean-continent transition reflect the re-activation of former structures. We suggest that
submarine active faults in the NW and W Iberia are also the response to the eastwards transfer
of short-offset transform faults of the Mid Atlantic Ridge into the oceanic Iberian along a
weakness as the former plate boundary between the oceanic Iberia and Eurasia domains. The
distribution and activity of submarine faults mapped in this work from geophysical and
bathymetric data are in good agreement with geodetic data and focal mechanisms.
Keywords: submarine faults, tsunami and earthquake assessment, geodynamic activity, Iberian, plate tectonics,
marine geophysical data, Atlantic-Mediterranean margins
INTRODUCTION
The Iberian Peninsula is bounded by a Cenozoic convergent margin between the Eurasia and former
Iberian plates along their northern edge (North and Northwest Iberian Margin), and by a complex
transform boundary (Gulf of Cádiz and Alborán Sea) to the south, between the Eurasia and Nubia
plates (Figure 1). The changes in stress direction from N-S during the early Cenozoic to NW-SE
since late Neogene have triggered the development of new tectonic structures together with the
reactivation of older structures around the plate boundaries of the Iberian Peninsula. A
Edited by:
Hector Perea,
Complutense University of Madrid,
Spain
Reviewed by:
Satish Chandra Singh,
UMR7154 Institut de Physique du
Globe de Paris (IPGP), France
Jacques Deverchere,
Université de Bretagne Occidentale,
France
*Correspondence:
Luis Somoza
l.somoza@igme.es
Specialty section:
This article was submitted to
Structural Geology and Tectonics,
a section of the journal
Frontiers in Earth Science
Received: 14 January 2021
Accepted: 15 June 2021
Published: 30 June 2021
Citation:
Somoza L, Medialdea T, Terrinha P,
Ramos A and Vázquez J-T (2021)
Submarine Active Faults and Morpho-
Tectonics Around the Iberian Margins:
Seismic and Tsunamis Hazards.
Front. Earth Sci. 9:653639.
doi: 10.3389/feart.2021.653639
Frontiers in Earth Science | www.frontiersin.org June 2021 | Volume 9 | Article 6536391
ORIGINAL RESEARCH
published: 30 June 2021
doi: 10.3389/feart.2021.653639

FIGURE 1 | (A) Summary of tectonic structures affecting the Iberian Peninsula. Offshore data modi fied from Maldonado et al. (1999),Somoza et al. (2019). Onshore
data comes from Vegas et al. (2008). Location of regional figures is also located. FZ: Fracture Zone. GoC: Gulf of Cádiz; GA: Gibraltar Arc; Alb: Alborán Sea. (B)
Distribution of earthquakes of magnitude Mw >4 around the Iberia Peninsula. Source: Seismic Hazard Harmonization in Europe (SHARE), Giardini et al. (2013).(C) Alpine
tectonic structures in Iberian in response to the convergence between Iberia, Africa, and Eurasia. Modified from Somoza et al. (2019);Terrinha et al., (2019),and
Terrinha et al. (2020). Background bathymetry from http://www.geomapapp.org (Ryan et al., 2009).
Frontiers in Earth Science | www.frontiersin.org June 2021 | Volume 9 | Article 6536392
Somoza et al. Submarine Active Faults Around Iberia

representative case is the accretionary wedge of the Gulf of Cadiz
which appears affected by a later complex system of long
submarine strike-slip faults (Medialdea et al., 2004;Rosas
et al., 2009;Zitellini et al., 2009).
Several works have aimed to determine the source of the
famous Mw 8.5–8.7, 1755 Lisbon tsunami earthquake event in the
Gulf of Cádiz, Southwest Iberian Margin (e.g, Terrinha et al.,
2003;Zitellini et al., 2009). The source of the aforementioned
tsunami in the cities of Cádiz and Lisbon was initially attributed
to the NE-SW Marques of Pombal Fault (e.g., Zitellini et al., 1999;
Gràcia et al., 2003b)(Figure 1). However, scaling the source
characteristics of the February 12th
,
2007, Mw 6.0 Horseshoe
earthquake, it was suggested another fault with a length of
230–315 km as potential source (Stich et al., 2006). This led to
relate the potential source of the 1755-Lisbon tsunami to large-
scale WNW-ESE dextral strike-slip faults affecting the
sedimentary cover over the continental and the oceanic
basements. These structures were identified on multichannel
seismic profiles by Medialdea et al. (2009b) and seabed
mapping as SWIM (South West Iberian Margin) lineaments
on the compilation of multibeam bathymetry made by Zitellini
et al. (2009) in the Gulf of Cádiz.
Otherwise, the North and Northwest Iberian Margin have
been affected by subduction of the oceanic lithosphere of the
Bay of Biscay as a consequence of the early Cenozoic collision
between the Eurasian and Iberian plates (e.g., Le Pichon et al.,
1971;Malod et al., 1993). The oblique convergence between the
Eurasian and Iberian plates since the late Cretaceous caused the
formation of the Alps-Pyrenees intracontinental collisional
orogen to the east (e.g., Srivastava et al., 1990;Sibuet et al.,
2004), that progressed westwards to a continent-ocean collision
with subduction of the Bay of Biscay oceanic lithosphere
beneaththeNorthIberianMargin(Sibuet and Collette,
1991). Afterwards the stress field transmitted to the Iberian
Peninsula changed from N-S to NW-SE from late Miocene to
present-day (e.g., Andeweg et al., 1999). In addition, the West
and Northwest Iberia margins are also affected by the
propagation of stress generated by the spreading of the Mid-
Atlantic Ridge (MAR) and the Bay of Biscay Ridge during the
Cenozoic (Figure 1). At present, the spreading of the Mid-
Atlantic Ridge north of the Azores Triple Junction has been
estimated to be at rated of 24 and 26 mm/yr (Argus et al., 1989;
Miranda et al., 2014)(Figure 1).
The aim of this work is to make a synthesis at regional scale
focused on the geophysical characterization of submarine faults
around the Iberian margins to identify offshore active structures,
with seismogenic and tsunamigenic potential. In this work faults
are considered to be active if they show, at least, one of these
characteristics: 1) seafloor expression on high-resolution
bathymetry; 2) Deformation and/or displacement of the sea
floor and the most recent sediments on ultra-high resolution
seismic sections; 3) prominent fault scarps uncovered by recent
sediments; 4) location of swarms of earthquakes near the
submarine fault trace.
Following these criteria, we review and map the main active
submarine faults around the Iberian Margins, which are mainly
concentrated along the southern Eurasia-African plate boundary
(Figure 1B), along with re-activated tectonic structures related to
the former Cenozoic subduction in the northern Iberian Margin.
Finally, based on our new synthesis map, we present a model of
distribution of the present active submarine faults linked to
seismicity around Iberia and analysed the available geodetic
data. We propose that this distribution is the result of the
remnant NW-SE convergence between the Eurasian and Nubia
plates, but also by the propagation of stress from the ocean
spreading of MAR to the west Iberian margin.
GEOLOGICAL SETTING
The South Iberian Margin: Oblique
Convergence Between the Eurasia and
African Plates
The Gulf of Cádiz region, located to the west of the Gibraltar Arc,
offshore SW Iberia and NW Morocco, has been increasingly
recognized as a critical site for tectonics related to the Africa
(Nubia)-Iberia plate boundary (e.g., Sartori et al., 1994;
Maldonado et al., 1999,Gutscher et al., 2002,2009;Medialdea
et al., 2004;Terrinha et al., 2003;Rosas et al., 2009;Zitellini et al.,
2009;Martínez-Loriente et al., 2014;Ramos et al., 2017a,b,c,
2020). This plate boundary extends along the Gloria fault zone to
the Azores Triple Junction to the west (Miranda et al., 2014)
(Figure 1A). In the Gulf of Cádiz the average direction of the
Maximum Horizontal Compressive Stress (S
hmax
) deduced from
earthquake focal mechanisms is N45W (Ribeiro et al., 1996;Stich
et al., 2006;Pedrera et al., 2011). Present-day rate of
approximately 4–5mm yr
−1
of oblique convergence between
Africa and Iberia has been reported at this plate boundary
(e.g., Nocquet and Calais, 2004;Stich et al., 2006)(Figure 1A).
The bathymetric map shows a huge lobe occupying almost the
entire Gulf of Cádiz, which is attributed to an accretionary wedge
emplaced in the late Miocene times associated with an east-
dipping subduction zone close to the Gibraltar Arc (Maldonado
et al., 1999;Gutscher et al., 2002). The accretionary wedge is
represented in seismic profiles by the Allochthonous Unit of the
Gulf of Cádiz (AUGC), characterized by a chaotic seismic
signature and covered by late Miocene to Quaternary
sediments (Maldonado et al., 1999;Medialdea et al., 2004).
The Late Miocene to present-day NW–SE convergent
movement of Africa with respect to Iberia generates
deformation in the Gulf of Cádiz through strain partitioning
accommodated by strike-slip faults and shear zones (Terrinha
et al., 2009) and thrust reactivation along the Southwest Iberian
Margin, offshore Algarve Basin (south Portugal and southwest
Spain; Ramos et al., 2017a). Moreover, this thrust system presents
an associate set of oblique ramps with NW-SE orientation, the
location of which is inherited from the Mesozoic extensional
transfer zones of the passive margin (Ramos et al., 2020).
Numerous authors have also documented the domain of the
Algarve Basin and the Gulf of Cádiz as tectonically active (Gràcia
et al., 2003a;Gràcia et al., 2003b;Duarte et al., 2009;Duarte et al.,
2010;Terrinha et al., 2009;Zitellini et al., 2009;Martínez-Loriente
et al., 2013;Martínez-Loriente et al., 2014). These major
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Somoza et al. Submarine Active Faults Around Iberia

compressional structures are interpreted as south-verging
basement-involving blind thrusts, being responsible for the
southeastward stair-stepped geometry of the SW Iberian
margin observable in the basement, the base of the Miocene
unconformity and the present-day bathymetry, as the case of the
uplift and tilting of the Guadalquivir Bank. Their orientation is
perpendicular to the orientation of the present-day convergence
between Africa and Eurasia (e.g., Olaiz et al., 2009).
A distributed deformation in the Alborán Sea region, to the
East of the Gibraltar Arc, has been classically defined as diffuse in
relation to the seismicity pattern (Vegas, 1991;Jímenez-Munt
et al., 2001;Buforn et al., 2015;Grevemeyer et al., 2015;Palano
et al., 2015), however recent geophysical and geological studies
points to concentration of crustal deformation along several
striking NE-SW and NW-SE fault zones in this region
(Negredo et al., 2002;Serpelloni et al., 2007;Neres et al.,
2016). The current plate boundary between Nubia (West
Africa) and Eurasia is related to the evolution of the Betic-Rif
orogenic system which was generated by the westward drift of the
Alborán Crustal Domain in relation to the westward retreat of a
subduction slab (Lonergan and White, 1997;Jolivet and
Faccenna, 2000;Pedrera et al., 2011;Molina-Aguilera et al.,
2019)(Figure 1C). This orogen is characterized by an arcuate
front (Gibraltar Arc) and the development of the Alborán Basin
in the backarc region between two main cordilleras, respectively
in the south of Iberia (Betic Ranges) and northern Africa (Rif
Ranges). During the late Tortonian, the regional change of
convergence from N-S to NW-SE between the main plates
caused a general inversion of the region. As a consequence an
indenter deformation has been developed in the area, the Alborán
Ridge constituted by an African crustal domain (Gómez de la
Peña et al., 2018) works as an indenter towards the northern
margin of the Alborán Basin (Estrada et al., 2018a) producing a
conjugated system of left lateral (NE-SW to NNE-SSW) and
right-lateral strike-slip faults (Figure. 2). The left-lateral strike-
slip family correspond to the Trans Alborán Shear Zone currently
represented by the Al Idrissi fault zone (Galindo-Zaldívar et al.,
2018;Gràcia et al., 2019;Vázquez et al., 2021a). This fault zone
connects southwards with the active onshore faults of the Al
Hoceima region and the southwestern Rif deformation
(Chalouan et al., 2014;d’Acremont et al., 2014;Galindo-
Zaldívar et al., 2015;Lafosse et al., 2018), and northwards with
the active faults of the Adra region in southern Iberia (Gràcia
et al., 2012;Galindo-Zaldívar et al., 2013) and the Eastern Betic
Shear Zone by means of La Serrata-Carboneras Fault (Gràcia
et al., 2006;Borque et al., 2019). The right-lateral strike-slip
family corresponds to the Yusuf Fault (Mauffret et al., 2007;
Martínez García et al., 2017), connected to the east with the
Algerian compressive region, which accomodates the current
deformation between Nubia and Eurasia (Vázquez et al., 2021b).
The North and Northwest Iberian Margin: A
Cenozoic Convergent Plate Boundary
Between Iberia and Eurasia
The North Iberian Margin was deformed by subduction of the
oceanic lithosphere of the Bay of Biscay due to the early Cenozoic
convergence between the Eurasian and Iberian plates (e.g., Le
Pichon et al., 1971;Malod et al., 1993). The oblique convergence
between the Eurasian and Iberian plates progressed to a
continent-ocean collision with subduction of the Bay of Biscay
oceanic lithosphere beneath the North Iberian margin, thus
FIGURE 2 | Synthetic map of active submarine faults in the Gulf of Cadiz and the Alborán Sea along the Iberia-Africa plate boundary and location of figures. The
plate boundary is partitioned into the main stress areas: (i) the SW Iberian Margin simple shear zone; (ii) the Gulf of Cadiz pure shear zone; (iii) the South Moroccan
compressional arc; and (iv) the Eastern Betic pure shear zone. Abbreviations of the submarine active faults are listed in Table 1. CS: Calahonda Sound Fault; DP:Djibouti
Passage Fault; DVS: Djibouti Ville Fault: HS: Herradura Sound Fault; XCS: Xauen Compressive System; AB: Algarve Basin; Units of the Betic-Rifian arc (modified
from Medialdea et al., 2009a) are also shown: Ff: Flysch Units front; SBf: Subbetic front; MDf: Mud diapiric front, AUGCf: Boundary of the Allochthonous Unit of the Gulf of
Cádiz; IB:Ibn-Batouta Bank LHB: La Herradura Bank Bathymetry from SWIM compilation (Zitellini et al., 2009) and EMODnet project (http://www.emodnet.eu). Present-
day stress fields from Pedrera et al. (2011).
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Somoza et al. Submarine Active Faults Around Iberia

provoking uplift and deformation of the Cantabrian range
(Figure 1C; e.g., Le Pichon and Sibuet, 1971;Boillot et al.,
1979;Pulgar et al., 1996;Cadenas et al., 2018). The North
Iberian margin was modeled with a south or southeast dipping
oceanic crust beneath the outer part of the margin (Boillot et al.,
1979), thereby forming an accretionary prism (Figure 1C). The
estimated amount of underthrusting of the southern Bay of
Biscay varies from a maximum of 120 km to a minimum of
40 km (e.g., Pulgar et al., 1996;Gallastegui and Pulgar, 2002).
Since Oligocene times, the compressional deformation was
transferred to the south through the Iberian plate, resulting in
the development of several mountain ranges. Convergence
between Eurasia (Iberia being incorporated) and African plates
through the Alborán microplate transmitted to the Iberian
Peninsula changed from N-S to NW-SE from late Miocene to
present-day (e.g., Dewey et al., 1989;Andeweg et al., 1999),
defining the current location of the Eurasian-African plate
boundary in south Iberia (Figure 1A).
GENERAL METHODOLOGY AND DATA
This study is mainly based on a large array of data including
multichannel seismic (MCS), ultra high-resolution parasound
sub-bottom profilers (SBP) and high-resolution multibeam
data (MBES) acquired during the MOUNDFORCE-2007
cruise aboard the RV L´Atalante (Somoza, 2007), MVSEIS-
2008 cruise aboard the RV Hespérides (Somoza and UTM-
CSIC, 2018), and SUBVENT-2 cruise aboard the RV
Sarmiento de Gamboa (Somoza et al., 2019) in the Gulf of
Cádiz and West Moroccan margin and BREOGHAM-2005,
aboard RV Hespérides in the NW Iberian Margin (Somoza
et al., 2005). A complete list of data used in this work with
detail information on the cruises, methods and configurations is
provided in the Supplementary Table S1.
The following MBES have been used to make 3d bathymetric
images from the Galicia margin and the Gulf of Cadiz: Simrad
EM-12S, 13 kHz, Konsberg EM-120 (12 kHz), KONSBERG EM-
12 dual (12 kHz), KONSBERG EM-120 (13 kHz), and Atlas DS
1x1 14–16 kHz (12 kHz).
In the N and NW Iberian Margin (Galicia and Cantabrian
regions), we use a MBES dataset acquired for the Spanish
Exclusive Economic Zone (EEZ) and Extended Continental
Shelf (ECS) mapping programs of the Galicia region at 150 m
resolution (Somoza et al., 2005;Somoza et al., 2019). In the Gulf
of Cádiz, we use the SWIM bathymetric compilation as
multibeam background data at an average resolution of 250 m
(Zitellini et al., 2009). For background bathymetry in other areas,
the EMODnet project data (http://www.emodnet.eu/bathymetry)
and Marine Geoscience Data Systems (MGDS), Global Multi-
Resolution Topography data (GMRT, Ryan et al., 2009) were
used. Multi-resolution DTMs was used to generate regional sun-
shaded image renders, perspective views and to extract margin-
wide bathymetric profiles using Fledermaus™software in order
to interpret the submarine landscapes. It was also used to generate
derivative products such as slope angle maps by means of
ArcGIS™.
Two main types of parasound sub-bottom profilers (SBP) has
been used to acquire ultra-high resolution profiles: TOPAS
(Topographic Parametric Echosounder) and CHEOPS. System
details are summarized in Supplementary Table S1.
Multichannel seismic profiles were acquired during two
cruises. The BREOGHAM-2005 survey in the Galicia Margin
and Celtic Sea used a seismic source of six BOLT™guns (1500 LL
model) and two SLEEVE™guns with a total volume of 22.85 L
and 50 m shooting interval. The acquisition consisted of an
analogical TELEDYNE streamer composed of 24 sections with
a total length of 2,400 m. The MOUNDFORCE-2007 in the Gulf
of Cádiz and western Moroccan Margin used as a source an array
of 14–16 guns G.I. GUN and BOLT with a total volume of 56.1 L
and 50–75 m shooting interval. The acquisition was performed
with a SERCEL streamer composed of 360 channels with a length
of active sections of 4,500 m, and a total length of 5.000 m.
Kingdom Suite software has been used to perform seismic
images both of the SBP and MCS profiles. For the SW Iberian
margin, we took into account 2D and 3D multichannel reflection
seismic data (i.e., Ramos et al., 2017a). The seismic interpretation
was calibrated with 72 wells in the area, both offshore and
onshore.
RESULTS
Mapping Quaternary Active Submarine
Faults Along the Africa-Eurasian Plate
Boundary
A new map with a synthesis of the submarine faults between the
Gulf of Cádiz and Alborán Sea has been made in this work
(Figure 2). This map has been constructed on the basis of high-
resolution multibeam bathymetry of the Gulf of Cádiz and
Alborán Sea, combined with multichannel and high-resolution
seismic data (Medialdea et al., 2009b;Zitellini et al., 2009). This
map allows linking the main faults in the Gulf of Cádiz and in the
Alborán Sea along the Africa-Eurasian plate boundary (Figure 2).
Quaternary Active Submarine Faults in the
Gulf of Cádiz
The Case of the SW Iberian Margin: Active Inversion of
a Passive Margin
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
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Somoza et al. Submarine Active Faults Around Iberia

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).
Simple Shear Zone: Deep-Rooted Right-Lateral
Strike-Slip Faults
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
FIGURE 3 | Cross-sections through the Algarve Basin showing the major ontractional structures responsible for the inversion of the SW Iberian margin. (A)
Interpreted seismic section through the central Algarve Basin, including interpretation of the onshore basin. (B) Interpreted seismic section through the western Algarve
Basin. Pairs of black dots represent salt welds. See Figure 2 for location. Modified after Ramos et al. (2017a). See this reference for further discussion on the seismic
interpretation and supplementary data for seismic sections. (C) Location of earthquake epicentres in the SW Iberian margin in relation with the thrust fault system
described in the text in more detail. (D) Location of focal mechanisms compiled from several earthquakes: red for strike-slip faulting, black for thrust faulting, and blue for
normal faulting in relation with the thrust faults of SW Iberia. Modified after Palano et al. (2013) and Ramos et al. (2017a). GB: Guadalquivir Bank; PB: Portimão Bank.
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Somoza et al. Submarine Active Faults Around Iberia

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)appeartobeclosely
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).
The Fold-Thrust System of the South Moroccan Arc
and Larache strike-Slip Fault: A Recent Major Seafloor
Deformation
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
FIGURE 4 | Multichannel seismic profiles MOUNDFORCE-06 showing the SWIM lineaments that belong to a flower structure rooted in the basement between the
African and Iberian Margins. The flower structure comprises the SWIM-1, SWIM-3, and L1-faults large-scale right-lateral strike slip faults. The relationship between Faults
and mud volcanoes as Soloviev and Porto MVs are clearly observed in the seismic profile The accretionary wedge is represented in the seismic profile by the
Allochthonous Unit of the Gulf of Cádiz (AUGC). See Figure 2 for location of seismic profile.
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Somoza et al. Submarine Active Faults Around Iberia

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
FIGURE 5 | (A) 50 ×60 km 3 days image of the multibeam bathymetry of the South Morocca nArc -SMA [data from Somoza (2007),Somoza et al.(2008)]. f1 to f3
are splays of the Larache right-lateral strike-slip fault. (B) Multichannel seismic profile MOUNDFORCE-01 crossing the SMA. The Larache strike-slip fault is split to the
west into several fold-thrusts systems (f1 to f3) which deform the seabed in the South Moroccan Arc (Inset location in Figure 2).
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Somoza et al. Submarine Active Faults Around Iberia

related vertical components are potential sources candidates for
large tsunamis in the Gulf of Cádiz.
Quaternary Active Submarine Faults in the
Alborán Sea
The Alborán Sea region has been highly deformed during the
Pliocene and Quaternary as a consequence of the indentation of
the Alborán Ridge block to the north and the generation of two
main families of strike-slip faults in this process, left-lateral
transcurrent NNE-SSW to NE-SW faults and right-lateral
transcurrent WNW-ESE to NW-SE faults, together with the
uplifting of several ENE-WSW compressive structures mainly
focused in the Alborán Ridge (Figure 2). In addition, minor N-S
to NNW-SSE normal faults have also played an important role in
the northern margin. This general fault system explains the main
crustal seismic activity in the region (Stich et al., 2006;
Grevemeyer et al., 2015;Buforn et al., 2017;Peláez et al., 2018;
Stich et al., 2020) and its development strongly affects the seafloor
morphology (Estrada et al., 1997;Gràcia et al., 2006;Martínez
García et al., 2013,2017;Estrada et al., 2018a;Galindo-Zaldívar
et al., 2018;Perea et al., 2018;Soumaya et al., 2018;Gràcia et al.,
FIGURE 6 | (A) Upper left: 3 days image of the swath bathymetry showing seafloor expression of the ESE strike-slip Larache fault: nR and sR north and south
ridges; Upper right: Ultra high-resolution profiles across the fault. See location in Figure 2.(B) Multichannel seismic section (Moundforce 02) showing the subseafloor
expression of the Larache fault composed by a flower-like structure dividing two main domains: Iberian and Africa. Unconformities are taken from Toyos et al. (2016).
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Somoza et al. Submarine Active Faults Around Iberia

2019;d’Acremont et al., 2020;Gómez de la Peña et al., 2020;
Lafosse et al., 2020;Vázquez et al., 2021b), where several
penetrative morphotectonic features such as linear scarps,
ridges, elongated pressure push-up swells, and longitudinal or
rhomb-shaped depressions show the contemporary variety of
Quaternary tectonics (Ballesteros et al., 2008;Vázquez et al.,
2008b;Gràcia et al., 2012;d’Acremont et al., 2014;Giaconia et al.,
2015;Gómez de la Peña et al., 2016;Vázquez et al., 2016;Lafosse
et al., 2018). The seismicity event distribution appoints that the
NNE-SSW to NE-SW left lateral strike-slip fault system assumes
most of the regional deformation (Figure 2). Secondarily,
earthquakes epicenters are focused in the WNW-ESE to NW-
SE right lateral strike-slip conjugate fault system and in the ENE-
WSW compressive structures.
Left-Lateral Strike-Slip Fault Systems
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 7,8and Table 1)(Vázquez et al., 2018).
The lengths of the fault zones vary between 40 (La Herradura
FIGURE 7 | Three days multibeam bathymetry of the Alborán Sea showing the main active submarine faults. See location in Figure 1A. AF: Adra Fault; AIFS: Al
Idrissi Fault System; ABR: Alboran Ridge Fault; AVF: Averroes Fault; CSF: Calahonda Sound Fault; DVSF: Djibouti Seamount Sound Fault; DPF: Djibouti Passage Fault;
HSF: Herradura Sound Fault, MD-2 secondary fault zones, S-CF: Serrata-Carboneras Fault, YF: Yussuf Fault. Nomenclature from Vázquez et al. (2018).
FIGURE 8 | High-resolution multichannel seismic profiles crossing the set of NNW-SSE left-lateral strike-slip faults that composes the Motril-Djibouti Marginal
Plateau (MDF): MD-2 and MD-5 secondary fault zones, CSF: Calahonda Sound Fault, DVSF: Djibouti Ville Seamount Fault; DPF and DPF-2: Djibouti Passage Fault and
AIFS: Al Idrissi Fault System. See location in Figures 2,7.
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TABLE 1 | List of active submarine faults and parameters.
Zones Single structures faults
and abbreviations
Trace
length
(km)
Shear
zone
width
(km)
Depth
(TWT)
Seabed
vertical
shift (m)
Crustal domains Characteristics
Southern
GoC
L3 192 33 >1 s Oceanic crust African-Iberian
boundary domain
ESE-WNW right-lateral strike-
slip shear zoneSWIM-1 L2 205
L1 90
SWIM-3 130
Northern
GoC
SWIM-2 314 5 >1 s Iberian continental-oceanic crust
boundary (?) African-Iberian
boundary domain
ESE-WNW right-lateral strike-
slip fault
Shear zone
(NGoC)
South
moroccan
arc
Larache fault (LF) 80 2.5 5 s 125 Continental basement African-
Iberian boundary domain
ESE-WNW right-lateral strike-
slip fault
South moroccan arc (SMA) 100 120 3 s 45 E-W thrust and fold belt
Alborán sea Motril-Djibouti marginal
plateau fault system (MDF)
Alborán domain Shear zone composed of NNE-
SSW left-lateral strike-slip faults
La Herradura sound
fault (HS)
40 1.4
Calahonda sound fault (CS) 79 4
Djibouti Ville seamount
fault (DS)
68 1.7
Djibouti passage 45 1–3.5
Al idrisi fault (AIFS) 125 1.4–5 Western boundary between the
Alborán indenter and the Alborán
domain
NE-SW left-lateral strike-slip fault
La Serrata-Carboneras
fault (S-CF)
140 1.4 Onshore and offshore Alborán
domain
NNE-SSW left-lateral strike-slip
fault
Alborán ridge fault (ARF) 75 6 Northern boundary between the
Alborán indenter and the Alborán
domain
ENE-WSW thrust fault
Xauén compressive system 60 20 Alborán domain ENE-WSW thrust fault
Yusuf fault (YF) 175 15 Eastern boundary between Alborán
identer and the eastern Alborán
basin
WNW-ESE right-lateral strike-
slip fault
Averroes fault (AVF) 46 2 Alborán domain WNW-ESE right-lateral strike-
slip fault
Adra fault (AF) 16 5 Alboran domain
SW Iberian
margin
Nazaré fault (NzF) ? Tore seamount-Estremadura spur NE-SW thrust fault
Marques de pombal
fault (MPF)
80 Re-activation of west Iberian passive
margin
NE-SW thrust fault
Arrabida fault (ArrF) 40–50 Re-activation of west Iberian passive
margin
NE-SW thrust fault
Horseshoe fault (HF) 75–110 40–50 m Eastern boundary of the horseshoe
abyssal plain
NE-SW thrust fault
Portimao fault (PF) 110 Re-activation of south iberian
passive margin
ENE-SWS thrust fault
Guadalquivir bank
fault (GBF)
62 Re-activation of south iberian
passive margin
ENE-SWS thrust fault
NW and N
Iberian
margin
Coruña seamount fault
(CRSF)
70 Re-activation of north atlantic
oceanic crust
Re-activate oceanic ridges.
Earthquake swarms Mw >5
Finisterre seamount
fault (FSF)
75 22 >10 s Re-activation of cenozoic
subduction structures at the former
Arcuate landward dipping thrust
fault system. Earthquakes
Mw >5Iberian-eurasia plate boundary
Burato shear zone (BSZ) 112 28 Re-activation of former mesozoic
structures
NW-SE tensional gashes and
craters. High density of
earthquakes. 5 >Mw >2
Castelao fault (CSF) 130 10 s Re-activation of former mesozoic
structures of the galicia interior basin
NNW-SSE normal to right-lateral
strike-slip faults. Earthquakes
5>Mw >2
East Galicia bank fault
(EGBF)
110
Ortegal fault (OF) 133 40 Re-activation of cenozoic strike-slip
faults
NW-SE right-lateral strike-slip
fault split into three
Branches.Submarine canyons
Malpica fault (MLF) 105
Coruña fault (CRF) 60
Ferrol fault (FRF) 35
Estaca de bares fault (EBF) 83 34 Re-activation of cenozoic strike-slip
faults
NW-SE right-lateral strike-slip
fault split into branches
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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.
Right-Lateral Strike-Slip to Normal Faults System
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;GómezdelaPeñaetal.,
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).
Compressive ENE-WSW Structures
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).
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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).
FIGURE 9 | Structure map of the Galicia margin showing the main Alpine-Pyrenes structures along North Iberia subduction margin (yellow lines) as the buried front
of the accretionary wedge segmented by NW-SE large right-lateral strike-slip faults. CSm: Coruña Seamount; HGD, Half-Graben Domain; DGM: Deep Galicia Margin;
FSm: Finisterre Seamount; GB:Galicia Bank; TZ, Transition Zone; GIB: Galicia Interior Basin; JSm: Jean Charcot Seamount; PR: Peridotite Ridge; TP:Theta Passage;
MO: Magnetic anomaly MO.
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N-S to NNW-SSE Normal Faults
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).
Quaternary Reactivation of Submarine
Faults Along the Northwest Iberian Margin
In the Galicia region (Figure 9) Three main zones with active
submarine faults related to seismicity have been identified in the
NWIberianmargin(Figure 10): 1) The arcuate Finisterre thrust fault
(FSF) to the NW of the Galicia Bank; 2) The Burato Shear Zone,
located between the eastern Galicia Bank and the NNW-SSE Castelao
Fault (CSF); 3) The NW-SE Ortegal Fault, split onshore into the
Meirama and As Pontes strike-slip faults (e.g., Andeweg, 2002).
The Finisterre Thrust System: Reactivation of the
Former Cenozoic Subduction Zone
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).
FIGURE 10 | Main active submarine faults (red lines) and distribution of earthquakes in the Northwestern Iberia Margin. These active submarine faults are related to
re-activation of former structures in response to present-day NW-SE convergence. (A) Reactivation of the Finisterre thrust fault (FSF); (B) The Burato Shear Zone (BSZ),
an area of tension gashes and craters bounded by NNW-SSE normal to strike-slip faults (CSF: Castelao Fault); and (C) NW-SE strike-slip faults: Ortegal Fault (OF) split
into three strike-slip faults in the margin (MLF: Malpica Fault, CRF: Coruña Fault and FRF: Ferrol Fault) and two main faults onshore (As Pontes and Meirama faults);
and Estaca de Bares Fault (EBF). Onshore faults: Padrón-Vigo Fault (PVF) and Becerreá Fault (BCF) a highly seismic zone. Earthquakes location taken from the online
database of the Instituto Geográfico Nacional (IGN). Faults are also listed in Table 1. Other labels same than Figure 9.
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Somoza et al. Submarine Active Faults Around Iberia

The Burato Shear Zone and the Castelao Fault System:
Seabed Tension Gashes and Deep Craters
The second zone with high seismic activity is the so-called
Transitional Zone (Murillas et al., 1990)locatedbetweenthe
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).
Inherited Right-Lateral Strike-Slip Faults Split Into the
Iberian Margin and Forming Deep-Incised Submarine
Canyons
The third structure associated with relatively moderate activity of
earthquakes MW >5 is the NW-SE Ortegal right lateral strike-slip
FIGURE 11 | (A) Multibeam bathymetry of the NW Iberian Margin showing the location of the Finisterre Thrust Fault interpreted as the former boundary between the
North Atlantic plate and Iberia. (B) Multichannel seismicline BREOGHAM-08showing the Finisterre thrust systemdeforming the uppermost seafloor. See location in Figure 1.
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Somoza et al. Submarine Active Faults Around Iberia

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
FIGURE 12 | Slope gradient map of the high-resolution bathymetry of the Burato Shear Zone (BSZ) between the Galicia Bank (GB) and the Galicia Interior Basin (GIB).
Seismic profiles shown in Figure 13 are also displayed. This shear zone is located between two main systems of NNW-SSE fault systems acting as right-lateral strike-slip
system, i.e., the Castelao fault (CSF) and the East Galicia Bank fault (EGBF). Other labels same than in Figure 9. See also Figure 10 for location of the Burato Shear Zone.
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Somoza et al. Submarine Active Faults Around Iberia

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).
DISCUSSION AND CONCLUSION
Synthesis Map of Active Submarine Faults
Around Iberia
A synthesis map of the main active submarine faults around the
Iberian Peninsula has been carried out. In this synthesis map
(Figure 14), we only include those active tectonics structures that
show the following characteristics: 1) The submarine fault/fold
shows seafloor morphological expression on high-resolution
MBES images with resolution <250 m; 2) The structure shows
deformation and/or displacement of the seabed affecting the most
recent sediments, at least, since the late Quaternary unconformity
identified on ultra-high SBP sections with vertical resolution
<1 m; 3) The seafloor expressions of fault/fold surface has to
be linked to deep-seated structures on MCS profiles, affecting at
least Cenozoic units; 4) The fault/fold structures or surface shear
zones have to be linked to swarms of Mw <5 earthquakes or
single Mw >5 earthquakes hypocenters.
FIGURE 13 | High-resolution multichannel seismic profiles crossing the Burato Shear Zone (BSZ) bounded by the Castelao Fault (CSF in Figure 12). The Burato
Shear Zone shows a prominent elongated WNW-ESE trending dome bounded by a dense network of sub-vertical faults. Location of profiles is shown in Figure 12.
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Somoza et al. Submarine Active Faults Around Iberia

Shear Zones, Submarine Faults and
Potential Tsunamigenic Sources Along the
Plate Boundary Between Nubia and Eurasia
Along the southern boundary, the oblique convergence between
Africa and Iberia is not a well-defined plate boundary but
constituted by, at least, five main shear zones (Vegas et al.,
2008). Thus, in the Gulf of Cadiz, we define three main shear
zones (Figure 2): 1) A pure-shear zone along the northern margins;
2) a simple shear zone along the central Gulf of Cadiz; and 3) a
convergence zone along the SMA South Moroccan Arc (Figure 14).
The pure shear zone along the southern Iberian margin in the
northern Gulf of Cadiz is characterized by SW-NE thrusts as the
Gorringe (GF), Marques de Pombal (MPF), Horseshoe (HF),
Portimao (PF), and Guadalquivir (GF) faults (e.g., Terrinha et al.,
2003;Medialdea et al., 2004;Martínez-Loriente et al., 2018)
(Figures 2,12).
The simple shear zone along the central Gulf of Cadiz is
formed by 200–315 km length, 33 km width, ENE-WSW shear
zones composed of arrays of large right lateral strike-slip faults,
which are linked at 10 s TWT depth assumed as the oceanic crust.
Two major simple shear structures are identified (Figure 2): 1)
the southern shear zone composed by a set of seafloor trace faults
like the L-3, SWIM-1 (L-2), L-1 and SWIM-3 right-lateral
faults.(e.g., Medialdea, 2007;Rosas et al., 2009;Zitellini et al.,
2009;Bartolome et al., 2012) and the northern shear zone
composed mainly by the SWIM-2 (L-4) fault (e.g. Medialdea
et al., 2004;Terrinha et al., 2009;Zitellini et al., 2009). Best
documented offshore historical large earthquake events (I
max
>
VIII) around Iberia Peninsula have occurred in 1,531, 1,356, 881,
FIGURE 14 | Synthesis map of the main active submarine faults around Iberia with location of the main Mw >5 earthquakes (yellow stars). Active submarine faults
by regions; (A) Gulf of Cádiz. ArrF: Arrabida Fault; GBF: Gualdalquivir Bank Fault; GF: Gorringe Fault; HF: Horseshoe Fault; LF: Larache Fault; MPF: Marques de Pombal
Fault; SGoCs: South Gulf of Cadiz Shear Zone; SMA: South Moroccan Arc. (B) Alborán Sea. AIFS: Al Idrisi Fault System; ARF: Alborán Ridge Fault; AVF: Averroes Fault;
JF: El Jebha Fault; MF: Maro-Nerja Fault; MDF: Motril-Djibouti Fault System; S-CF: La Serrata Carboneras Fault; YF: Yusuf Fault. (C) SE and E Iberia. AA: Aguilas
Arc; AMF: Alhama de Murcia Fault; AvrF: Abubacer volcanic ridge Fault; TF: Torrevieja Fault; EME: Emile Baudot Escarpment. (D) N and NW Iberia Margin. CRF: Coruña
Fault; CSF: Castelao Fault; CRSF: Coruña Seamount Fault; EGBF: East Galicia Bank Fault; FRF: Ferrol Fault; FSF: Finisterre Seamount Fault; MLF: Malpica Fault; OF:
Ortegal Fault; VF: Ventaniella Fault. (E) Iberia intraplate active fault lineations (from Vegas et al., 2008). BT Bajo Tagus; SFV Vilarica Fault System; LG: Guadiana
Lineaments. (F) Onshore faults. APF: As Pontes Fault; BF: Becerreá Fault; MRF: Meirama. Geodetic GPS velocities and 95 per cent confidence ellipses data from Gárate
et al. (2015) and Palano et al. (2015).S
hmax
directions from Andeweg et al. (1999),Fernández-Ibáñez et al. (2007) and Pedrera et al. (2011). Earthquake distribution taken
from the Instituto Geográfico Nacional (www.ign.es) and SHARE database (Giardini et al., 2013). Tsunamis from Lario et al. (2011). Background bathymetry from Ryan
et al. (2009). Abbreviations are also listed in the Supplementary Table S2.
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Somoza et al. Submarine Active Faults Around Iberia

241, and 245 BC along these shear structures in the Gulf of Cadiz
(Udias, 2015).
The South Moroccan margin in the south of the Gulf of Cadiz
is composed of a fold-thrust system migrating westwards and
bounded to the north by the Larache right lateral strike-slip fault
and to the south by the offshore prolongation of the El Jebha left
lateral strike-slip fault (Figure 2). Therefore, we suggest that this
fault might have acted as a link between the active submarine
structures of the Gulf of Cádiz and Alborán Sea. A slight change
in the orientation of the S
hmax
to ENE-WSW identified onshore
Morocco from focal mechanisms (Pedrera et al., 2011) could
explain the present inactivity of the El Jebha Fault as a left lateral
strike-slip fault onshore (Figure 2).
In the Alborán Sea, the convergence between Africa and Iberia
is assumed by an oblique pure shear regime forming a conjugated
system (e.g., Vegas and Vázquez, 2000; Vegas et al., 2008). In the
Iberian margin, the eastern Betic pure shear zone is formed by the
NE-SW La Serrata-Carboneras left lateral strike-slip fault (S-CF
in Figure 7) (e.g., Rutter et al., 2012;Gràcia et al., 2014), the
conjugated NW-SE Maro-Nerja right lateral strike-slip fault (MF
in Figure 14)(Fernández-Ibáñez et al., 2007) and the Motril-
Djibouti shear zone (MDF in Figure 14)(Vázquez et al., 2018),
composed by an array of near N-S dextral strike-slip faults with
tensional components due to the N-S direction (Vázquez et al.,
2008c).
Towards the north, the active submarine La Serrata-
Carboneras fault continues onshore into the Eastern Betics
through the NNE-SSW Palomares and Alhama de Murcia left
lateral strike-slip faults (AMF), forming the Aguilas Arc in the
eastern Betics (Figure 14) (e.g., Silva et al., 1993;Somoza, 1993).
In the African margin, the conjugated system of this pure
shear system, which accommodate most of convergence, is
formed by the NNE-SSW left lateral strike-slip Al Idrisi Fault
System (AIFS in Figure 14)(Gràcia et al., 2019;d’Acremont et al.,
2020) and the WNW-ESE Yusuf right lateral strike-slip fault (YF
in Figure 14). These two conjugated faults are linked through the
Alborán ridge (ARF in Figure 14), bounded by a thrust fault as
response to a near NNW-SS convergence in the region (Vázquez
et al., 2021a;Vázquez et al., 2021b).
Re-Activation of the Inherited Faults Along
the Northwest and Southwest Iberian
Margin
Besides the high record of earthquakes and tsunamis along the
southern boundary of Iberia, a remarkable concentration of
earthquakes can be observed, onshore and offshore, along the
NW margin of Iberia. In this region, the present-day stress
field caused by the convergence between Africa and Iberia is
modified by the opening of the Mid-Atlantic Ridge with near
E-W spreading rates up to 26 mm/yr at 43°N decreasing
southwards to 24 mm/yr at 42°N(Figure 14). Short-offset
transforms of the Mid-Atlantic Ridge as the Kurchatov,
MARNA and Moytirra are almost 30°oblique to the
direction of plate motion showing secondary strike-slip
faults with N°120 trend (Figure 14)(Somoza et al., 2019).
For the oceanic Iberia, especially for the northern part, the
incorporation of these forces leads to a dramatic change in the
stress field orientation and magnitude (Andeweg et al., 1999).
Therefore, the local stress data derived from focal mechanism
solutions, active faults and bore-hole breakout analysis (e.g.,
Stich et al., 2006;Pedrera et al., 2011;Custódio et al., 2016)of
the Iberian Peninsula show a certain anticlockwise deviation of
the S
hmax
direction from the south to north Iberia margins
(Figure 14). This would explain the inversion and re-
activation as thrust faults of northern oceanic seamounts as
the North Jean-Charcot Seamounts (Medialdea et al., 2009b),
the Coruña Seamount Fault (CRSF) and the Finisterre
Seamount (FSF), which show occurrence of earthquakes
reaching magnitudes between 5.6 and 5.8 (Figure 14). The
northernmost system, the North Charcot structure, connects
with the Ortegal strike-slip fault that splits as it enters into the
continental margin forming deep incised submarine canyons
and continues onshore with major active faults during the
Tertiary (Meirama and As Pontes faults) (e.g, Andeweg, 2002).
Both later faults are aligned with the main intraplate
lineaments as the Iberic Range (LG in Figure 14). At the
same time, the Cenozoic deformational structures of the
Galicia Bank region (Vázquez et al., 2008a) seem to be re-
activated by two faults as the East Galicia Bank Fault (EGBF)
and Castelao Fault (CSF), which form a vast shear zone
expressed on the seafloor as craters and tension gashes.
This northern domain is bounded by the Nazaré Fault
(NzF) which runs near E-W from the Estremadura Spur to
the Tore Seamount. This fault shows earthquakes with Mw
between 5.3 and 5.8 and strike-slip fault solutions. This fault-
plane mechanism could be explained by the relative movement
propagated westwards from the Mid-Atlantic Ridge segment
north of Kurchatov short-offset transform fault (Figure 14).
Southwards of this boundary along the SW Iberian margin, the
northwards propagation of the deformation takes place
between major thrusts as the Arrabida Fault (ArrF),
Gorringe (GB), Marques de Pombal (MPF) and Horseshoe
Faults (HS), and along the westward prolongation of the
southern Gulf of Cádiz shear zone, linked to the Gloria
Fault (Figure 14).
Submarine Faults and Geodetic Constraints
The dense spatial coverage of geodetic velocities in the Iberian
Peninsula and North Africa, comprising over 380 stations
(Gárate et al., 2015;Palano et al., 2015), helps to understand
the dynamic of the offshore submarine faults (Figure 14).
Geodetic data reveals that significant deformation occurs
prevailing along the southern margins of the Iberian
Peninsula, from the Alborán Sea to the Gulf of Cádiz,
including the Gibraltar Arc, and to a lesser extent along its
W and NW margins, while on the inner parts of the Peninsula,
the crustal deformation occurs locally at rates <15 nano strain/
year (Palano et al., 2014). Stations located in central and
northern Portugal move northwards with rates of ∼1 mm/yr
(Palano et al., 2015). Along the NW margin, geodetic data
evidences an E-W oriented contraction up to 55 nanostrain/
year. In contrast, along the Gibraltar Arc, ∼2–5mm/yearWSW
motion can be detected (Palano et al., 2014). The Al-Hoceima
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Somoza et al. Submarine Active Faults Around Iberia

region (south Alboran domain, Figure 2)showsanE-W-
oriented elongation up to 90 nanostrain/year, and the
Alboran domain is characterized by elongation strain-rate
axes WSW-ENE oriented coupled with shortening strain-rate
axes of ∼25–40 nanostrain/year. Finally, along the SE Iberian
margin a prevailing NNW-SSE-oriented contraction up to 30
nanostrain/year can be recognized (Palano et al., 2014). Thus,
the main deformation around Iberia takes place along the
southern margin (Alboran Sea-Eastern Betics), SW (Gulf of
Cádiz-S Portuguese margin) and NW Iberian margins (N
Portuguese and Galicia margins).
In the Alborán Sea, the NW motion up to 5 mm/yr of the
stations located in the eastern Rif clearly depicts a contraction of
the Alborán Sea and explain the dynamics of the Alborán
indentation system and westwards escape tectonics (Chalouan
et al., 2014)(Figure 14). Therefore, the Al Idrissi left lateral
strike-slip fault system in the south propagates to the north into
the system of the Motril-Djibouti Marginal Plateau and the La
Serrata-Carboneras Fault in the north. The conjugated right-
lateral system is composed of the Yusuf Fault in the south and the
Averroes Fault and related system to the north (Figure 7). In SE
Iberia (Eastern Betics e.g., Almería–Murcia-Alicante region),
geodetic values show a deformation pattern that strongly
differs from that observed for surrounding areas (Palano et al.,
2015). Thus, geodetic velocities clearly show a NW-to- NE fan-
shaped pattern with rates ranging from ∼3 mm/yr near the
coastline to ∼0.8 mm/yr inland (Figure 14). This pattern could
explain the dynamics of the Palomares and Alhama de Murcia
faults as left-lateral strike-slip faults, and specially the clockwise
rotation from NE to NW of the Aguilas Arc, probably forced by
the confluence with NW-SE faults as the Torrevieja Fault or the
Ibiza-Alicante right-lateral strike-slip faults, that continues into
the South Balearic Basin and the Algerian margin.
The westward motion up to 5 mm/yr of the stations located on
the central sector of the Gibraltar Arc and eastern Gulf of Cádiz
could explain the activity of the South Moroccan Arc (SMA),
interpreted as the most prominent thrust structure with seafloor
expression (Figure 5). Otherwise, the left lateral focal mechanism
of the southernmost shear structure (SGoCs) in the Gulf of Cádiz
is explained by the NW motion with rates of ∼1.1 mm/yr. In
contrast, stations located in the northern Gulf of Cádiz and the
south Portuguese margin show NW motions with rates of
∼3 mm/yr, suggesting the re-activation of thrust faults along
the SW Iberian continental margins such as the Guadalquivir
Bank (GBF), Portimao (PF), Horseshoe (HF), Marques de
Pombal (MPF), or Gorringe (GF) faults.
Northwards along the Atlantic margin, stations located in
central and northern Portugal and Galicia region indicate
northward motion with rates of ∼1mm/yr (Figure 14). This
could explain the right-lateral mechanism of the Nazaré Fault
and the strike-slip mechanism of the Eastern Galicia Bank
(EGBF) and Castelao (CSF) faults in the Galicia Bank region.
In the NW Iberian margin, this motion could also explain the
inversion of former oceanic crust seamounts as the Jean
Charcot, Coruña (CRSF) and Finisterre (FSF) seamounts that
could also be affected by the propagation of tectonic stress from
the Mid-Atlantic Ridge.
Geodynamic Model: Oceanic Vs.
Continental Iberia
Based on geodetic data, Palano et al. (2015), proposed a present-
day large-scale clockwise rotation of Iberia acting as a microplate
with a southern boundary at the Nubia-Eurasia convergent
boundary without describing its north and northwest
boundaries. According to geophysical data, the main weakness
zones along the N and NW Iberia are the former boundaries
between 1) the Eurasia and Iberia oceanic domains, and between 2)
the Eurasia oceanic and Iberian continent domains (Figure 14).
Based on our synthesis of submarine faults, we propose that the
weakness zone between the Iberian and Eurasia oceanic domains is
constituted by the westward prolongation of the Moytirra short
offset-transform fault (SOTZ) into the Kings Trough, Azores
Biscay Rise and Jean Charcot Seamounts (Figure 14). This
boundary shows earthquakes higher than Mw five and links
with the NW-SE Ortegal strike-slip faults. These NW-SE faults
continue onshore and split into two main high seismicity zones in
Galicia with earthquakes higher than Mw five and strike-slip
mechanisms (Figure 14). Otherwise, based on offshore
submarine faults, we suggest that the clockwise rotation of
Iberia proposed by Palano et al. (2015) would only affect the
Iberian Continental domain, e.g., the SW and NW margins.
Westwards, deformation affecting the Iberian oceanic domain is
mainly dominated by stress derived from the spreading of the Mid-
Atlantic Ridge in the segment between the Azores Triple Junction
and the Moytirra SOTZ with a main direction of propagation
trending N-120°(Somoza et al., 2021). Furthermore, we suggest
that a major left-lateral shear-zone results as consequence of the N
motion of the Iberian continental domain and SE motion of the
Iberia oceanic domain along the west Iberian continent-oceanic
transition. This zone is characterized by the occurrence of
serpentinized mantle which probably buffers the magnitude of
earthquakes along this area.
CONCLUDING REMARKS
As the main conclusion of the synthesis map presented in
this work, we propose that the present active submarine
faults and their associated seismicity around the Iberia
margins (Figure 14) can be explained by the present-day,
roughly NNW-SSE compressional stress field related to the
convergence between Eurasia and Africa plates. The
distribution and activity of submarine faults mapped in this
work from geophysical and bathymetric data are in good
agreement with geodetic and seismological observations.
Major deformation is located in the south Iberia margin
along the Nubia-Eurasia plate boundary according to
submarine fault distribution, earthquake distribution pattern
and geodetic data. Nevertheless, deformation is also focused in
the NW Iberian margin. We suggest that deformation in this
area is derived from the westward motion of the Iberian oceanic
domain due to differential spreading rates of the MAR and the
clockwise rotation of Iberian continental domain with respect to
stable Eurasia proposed by Palano et al. (2015).Thisinteraction
takes place over a crustal weakness zone that corresponds to the
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Somoza et al. Submarine Active Faults Around Iberia

transition between the oceanic and the continental Iberian
crusts.
DATA AVAILABILITY STATEMENT
The datasets presented in this study can be found in online
repositories. The names of the repository/repositories and
accession number(s) can be found in the article/
Supplementary Material.
AUTHOR CONTRIBUTIONS
LS: Data acquisition, writing, and overall organizations; TM:
Seismic Interpretation and Writing (Gulf of Cádiz and Galicia
regions); PT: Geological interpretation SW Margin; AR: Seismic
interpretation and writing. JV: Interpretation and writing
Alborán Sea.
FUNDING
This work is funded by the Spanish Minister for Science and
Innovation, projects EXPLOSEA (grant CTM201675947-R) and
FAUCES (CTM 2015-65461-C2-2-R). This study is a
contribution to the EMODNET-Geology project (EASME/
EMFF/2018/1.3.1.8-Lot 1/SI2.811048), the European project
H2020 GeoERA-MINDeSEA (Grant Agreement No. 731166,
project GeoE.171.001), the IEO project RIGEL and the
AGORA PAIDI project.
ACKNOWLEDGMENTS
This study also benefits from the Atlantic Seabed Mapping
International Working Group (ASMIWG) as part of the
Atlantic Ocean Research Alliance Coordination and Support
Action (AORA-CSA). Thanks to the two reviewers, Satish
Chandra Singh and Jacques Deverchere, for their useful
comments and suggestions. Special thanks to the guest editor,
Hector Perea, for his valuable helpful and suggestions.
SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found online at:
https://www.frontiersin.org/articles/10.3389/feart.2021.653639/
full#supplementary-material
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