Tectonics of the Iberian continental margins and abyssal plains

ABSTRACT The tectonic map at scale 1:1,750,000 presented in this work shows an update of the existing knowledge of the main tectonic and volcanic structures along the Iberian continental margins and abyssal plains. The morphotectonic trends around Iberia recorded superposed geodynamic processes linked to the opening of the North Atlantic Ocean since the Mesozoic and the evolution of the Mediterranean Sea during the Cenozoic, as well as the reactivation of Hercynian faults and the formation of new tectonic structures during the Alpine orogeny. This map is based on the interpretation of the digital bathymetric model and other geophysical data from our own studies, and those from other authors. The morphotectonic features in this map provides a basis for the recognition of the tectonic control at lithospheric plate scale and the establishment of potential tectonic and volcanic risk zones. HIGHLIGHTS A tectonic map at a scale 1:1,750,000 of the continental margins and abyssal plains of Iberia is presented. This new and complete tectonic map can be the base for further detailed geological and geophysical studies. The compiled tectonic structures shown in the Main Map are available as supplemental online material.


Introduction
The Iberian continental margins constitute the submarine edge of the Iberia continental crust.They extend from the coast up to the adjacent abyssal plains (Figure 1): The Liguro-Provençal to the east (out of our study area); the Algero-Balearic abyssal plain to the southeast; the Iberia, Tagus, Ferradura and Seine abyssal plains, to the west and southwest; and the Biscay abyssal plain, to the north.The morphology of the Iberian margins is the result of the interaction of tectonic, sedimentary and oceanographic processes (Maestro et al., 2013).The main cause of their current configuration is the tectonic activity developed from the end of the Palaeozoic, in the post-Hercynian stage, until Cenozoic, once the activity of the Alpine orogeny had ended (Maestro et al., 2013(Maestro et al., , 2015)).
The aim of this paper is to present a complete and updated tectonic map of the Iberian continental margins and their adjacent abyssal plains.

Description of the mapped tectonic domains
A set of tectonic domains have been identified around the Iberian Peninsula, including the continental margin and its natural prolongation to the abyssal plain, that are characterized by their geographical position, crustal nature and tectonic deformational style related to their geodynamic evolution.The characteristics of the main sectors analyzed in this map (Figure 1 and Main Map) are described below.

Catalan-Levantine thinned continental crust
The Catalan-Levantine thinned continental crust, stretched during upper Oligocene-lower Miocene times in a back-arc context (e.g.Etheve et al., 2016;Maillard et al., 1992), is currently under a general convergent regime (Jolivet & Faccenna, 2000).The tectonic structure of this margin is due to a significant thinning of the continental crust during the propagation of successive rifting events related to the opening of the northwestern Mediterranean Sea, manifested by the development of normal listric faults developing horst and graben structures (e.g.Roca & Deselgaux, 1992).This extensional tectonics originated an elongated NE-SW depression with very high subsidence, known as the Valencia Trough.Two normal faulting systems are identified, one of NE-SW to NNE-SSW direction, of upper Oligocene-lower Miocene age, and the other of NW-SE orientation, of upper Neogene age.There are some volcanic reliefs on the Valencia Trough seafloor, related to two magmatic events developed in the western Mediterranean margin during the Cenozoic (Martí et al., 1992).Morphologies associated with saline dynamic processes exist in the easternmost sector (e.g.Etheve et al., 2018;Maillard et al., 2003).

Balearic continental block
The Balearic continental block is 348 km long and 105 km wide, separating the Valencia Trough and Algero-Balear abyssal plain to the north and south, respectively.It corresponds to the NE prolongation of the Betic External Zone (Azañón et al., 2002), whose southwestern sector is attached to the eastern margin of the Iberian Peninsula.The tectonic evolution of this continental block is complex with the presence of extensional structures, related to the superimposition of rifting episodes during the lower Cenozoic (Fontboté et al., 1990), and subsequent compressional structures (Sàbat et al., 1995).The Balearic promontory is segmented into three sectors by major NW-SE transcurrent faults (e.g.Acosta et al., 2003).These faults have controlled the development of channels between each sector: the Ibiza, Mallorca, and Menorca channels.The current configuration of the Balearic continental block is due to its clockwise rotation proven by paleomagnetic data (e.g.Balanyá & García-Dueñas, 1987).

Algero-Balear oceanic crust
In the Algero-Balear oceanic crust, there is a remarkable presence of domes and diapiric ridges originated by halokinetic mobilization of the Messinian evaporite series (Camerlenghi et al., 2009).They have N-S, E-W, NE-SW, and NW-SE orientations, reliefs of less than 40 m, and lengths varying from 2 to 50 km.

Alboran thinned continental crust
The Alboran Basin is floored by thinned continental crust located between the Betic and Rifean ranges, respectively to the north and south (Campillo et al., 1992;Maldonado et al., 1992).The extensional processes are generally associated to a back-arc setting active during the Lower and Middle Miocena, partly coeval with the main activity of the thrust systems in the Betic and Rifean external zones (Balanyá & García-Dueñas, 1988;Comas et al., 1992;Pedrera et al., 2020).Contemporaneously, important volcanic activity took place along the entire region, aligned in NE-SW and NNE-SSW directions (e.g.Auzende et al., 1975), and also the development of diapirs, mainly in the western sector (Campillo et al., 1992;Comas et al., 1992;Talukder et al., 2004).From the upper Tortonian to the present day, a N-S to NW-SE compressional tectonic regime, with orthogonal extension, affects the whole basin (e.g.Comas et al., 1992;Pedrera et al., 2011).Other features related to the tectonic activity are pockmarks, carbonated mounds generated by methane expulsion and oxidation along fracture planes, and mud volcanoes (e.g.Ballesteros et al., 2008).

African thinned continental crust
The most relevant tectonic feature in this sector is the Alboran Ridge.This structure is constituted by an ancient volcanic axis that locally outcrops on Alboran Island, together with uplifted basement blocks (e.g.Maestro-González et al., 2008;Martínez-García et al., 2011, 2013).The establishment of a compressional stress regime from the upper Tortonian to the present day in a N-S to NW-SE direction (e.g.Comas et al., 1992) leads to reactivation of the directional faults.The northern boundary of this sector is defined by the Yusuf System, which consists of a morphological escarpment and a narrow basin in the central part of the escarpment.This system extends for 210 km in a WNW-ESE to NW-SE direction, between the Alboran Ridge and the Algerian western margin.The system is the product of the dynamics of the Yusuf Transcurrent Fault System, with a NW-SE direction and dextral movement, so that the Yusuf Basin has been defined as a pull-apart basin (Mauffret et al., 1987).

Gibraltar domain and Gulf of Cadiz accretionary prism
The Gibraltar domain and Gulf of Cadiz accretionary prism lie at the boundary between the African and Eurasian plates in this region of the Atlantic Ocean (a sub-parallel right-lateral strike-slip faults group, named SWIM Fault System by Zitellini et al., 2009), and the westernmost part of the Mediterranean Alpine orogenic chain, represented by the Gibraltar Arc, which is the western front of the Betic-Rifean collision orogen.This sector represents a large transpression zone due to the current NW-SE direction of convergence between the African and Eurasian plates (e.g.Bartolomé et al., 2012).The westward drift and collision of the Alboran Domain with the margins of North Africa and the South Iberia in the lower-middle Miocene caused: (i) the development of the Rifean and Betic orogens, and (ii) the radial emplacement of the Gulf of Cadiz accretionary prism (e.g.Medialdea et al., 2004).These allochthonous units are now interpreted as an accretionary prism composed of a stacking of west-vergent thrusts (Gutsher et al., 2002;Iribarren et al., 2007), intersected by the SWIM Fault System.There are also diapiric and salt migration processes developed since the late Cretaceous until the present-day affecting the Gulf of Cadiz accretionary prism (Maestro et al., 2003).

Gulf of Cadiz oceanic crust
The Gulf of Cadiz oceanic crust was developed during the Mesozoic extension stage, probably in the late Jurassic, in relation to the opening of the Central Atlantic (e.g.Ramos et al., 2017), and was later mostly subducted during the emplacement of the Betic-Rif orogen (e.g.Pedrera et al., 2020;Vergés & Fernàndez, 2012).Westwards, it borders the west Iberia continental margin exhumed mantle zone (e.g.Granado et al., 2021) through a system of thrusts with NE-SW direction and NW vergence.The southern sector borders African oceanic crust through the westward continuation of the SWIM Fault System (Zitellini et al., 2009).

African oceanic crust
The African oceanic domain is marked by a predominantly NE-SW structural trend, defined by the Ampere and Coral Patch seamounts (Hayward et al., 1999).These seamounts separate the Ferradura and Seine abyssal plains.The northern border of the Coral Patch seamounts is defined by a NW verging thrust (e.g.Duarte et al., 2013).

Atlantic-Iberian thinned continental crust
This sector is characterized by the presence of a thinned continuous continental crust that transitions westward to a serpentinized mantle with isolated continental blocks below the sediments (e.g.Granado et al., 2021).From south to north, five sectors bounded by four first-order tectonic structures can be differentiated: the Gulf of Cadiz sector, including the Algarve Basin; the sector of the Alentejo Basin and Gorringe Bank; the southern Lusitanian Basin and the Estremadura Spur sector; the northern Lusitania and Peniche basins sector; and the sector including the Porto and Galicia Interior basins and the seamounts region.This margin sectorization is the consequence of the succession of different extensional tectonic pulses related to the northwards diachronic propagation of the Atlantic Ocean opening during the Mesozoic (e.g.Murillas et al., 1990).This extensional tectonics and the subsequent Cenozoic compressional phase related to the collision between Iberia and Africa (e.g.Terrinha et al., 2009) are responsible for the present-day margin relief.The strike of the extensional structures ranges from E-W to N-S, as controlled from Tethyan-dominated extension in the south to Atlantic-dominated extension in the west, respectively.Compressive structures are inverted extensional structures, following their same trends, and newly formed NE-SW to E-W thrusts, mainly to the north.

Western Iberian oceanic crust
The northwestern part of this domain is characterized by the existence of two submarine reliefs called Finisterre and Coruña seamounts separated by the Theta Gap channel, which connects the Biscay and Iberia abyssal plains.The Finisterre Seamount has an elongated morphology in a NNE-SSW to NE-SW direction.Its genesis has been related to a system of northwest-vergent thrusts developed during the Cenozoic (Vázquez et al., 2009).The Coruña Seamounts present two main structural alignments, E-W towards the Biscay Abyssal Plain, and NE-SW towards the Iberian Abyssal Plain.Evidence of compressional tectonics rejuvenating the relief are noticeable in this area (Druet et al., 2018;Maestro et al., 2018).In the central part of this sector, there is a series of volcanic reliefs, which constitute the Tore Seamounts, separating the Iberia Abyssal Plain from the Tagus Abyssal Plain.The formation of these volcanic reliefs has been dated in between 88-80 Ma (Merle et al., 2006) in relation to an extensional stage that could facilitate the rise of the magmas towards the surface (Sibuet et al., 2007).Finally, in the southernmost sector, there is a volcanic relief, which is the morphological prolongation of Gorringe Bank, forming an ESE-WNW oriented continuous ridge that separates the Ferradura Abyssal Plain from the Tagus Abyssal Plain.The southern sector of this ridge marks the Eurasia-Africa plate boundary in the Atlantic Ocean.

North Iberian thinned continental crust
This sector has an E-W structural orientation related to its tectonic evolution from the Jurassic to the present day.The onset of continental rifting during Upper Jurassic (Lepvrier & Martínez-García, 1990) continued during the early Cretaceous and culminated with the formation of the Cantabrian oceanic crust (Sibuet et al., 2007).The subsequently convergence between the Iberian and Eurasian plates from the late Cretaceous to the Oligocene (Álvarez-Marrón et al., 1996;Pedrera et al., 2017) partially closed the Bay of Biscay, causing the inversion of extensional faults and the formation of compressive structures (Pulgar et al., 1999).Thus, part of the oceanic and transitional crust of the Bay of Biscay subducted beneath the northern Iberian continental crust.Therefore, the margin is partitioned by extensional tectonic structures, giving rise to the development of marginal platforms such as those of Ortegal, Castro and Pardo Bazán in the westernmost sector, and the Landes Plateau in its easternmost part.Moreover, compressional structures, mainly north verging thrusts, uplift the Le Danois Bank, the Jovellanos Seamount and the Cantabria Seamount (Maestro et al., 2015).Also noteworthy is the presence of a N-S elongated spur, the Santander Promontory, that is part of a first order cortical structure that marks the separation of two structural domains, the Basque-Parentis Domain and the Cantabrian Domain, to the east and west respectively (Maestro et al., 2015).The Santander Promontory orientation is controlled by the existence of extensional transfer faults of Lower Cretaceous age that have been subsequently obliquely reversed during the Pyrenean shortening (e.g.Roca et al., 2011).

Cantabrian oceanic crust
The anticlockwise rotation of the Iberian Plate in relation to the formation of the North Atlantic produced the opening of the Bay of Biscay and the formation of oceanic crust.This opening was diachronic and occurred from west to east.In its western part, the Cantabrian oceanic crust is slightly later than the lower Aptian, in the central part the opening is upper Aptian and in the eastern sector it is lower Albian (Malod & Mauffret, 1990).The oceanic expansion in the Bay of Biscay ended at the beginning of the Campanian.Due to the convergence of the Iberian and Eurasian plates related to the development of the Alpine Orogen during the Cenozoic, active until the present day, some sectors of the oceanic crust were uplifted following the trends defined by the cortical anisotropy developed during the formation of the oceanic crust (Emery & Uchupi, 1986).These reliefs, bounded on their northern flank by E-W trending and N verging thrusts, are the Charcot and Cantabria seamounts (Medialdea et al., 2009;Muñoz-Martín et al., 2022).

Data compilation
This map has been elaborated based on a compilation of different tectonic features described by other authors in scientific publications, books and PhD theses developed in different sectors of the continental margins of Iberia.The tectonic feature mapping derived from this compilation has been implemented in a Geographic Information System (GIS).In some cases, the structures shown have been redrawn or directly interpreted from the available geophysical information (bathymetry, seismic profiles, and potential fields).
In order to facilitate the interpretation of the offshore tectonic structures mapped in the present work, and to provide continuity to them, the main structural elements and Neogene-Quaternary volcanism, collected from the geological maps of Spain, Portugal and France at a scale of 1:1,000,000 (BRGM, 2019;Rodríguez Fernández & Oliveira, 2015) are shown in the emerged areas.

Redrawing features and new geophysical data interpretation
An important part of the tectonic features included in this Tectonic Map come from previous geological maps, available as vector datasets (BRGM, 2019;Rodríguez Fernández & Oliveira, 2015).Even so, all these structures have been reviewed to verify that they adapt adequately to the seafloor relief if they have a correspondence with morphological features of this surface, redrawing them when necessary.The structures compiled from other works published in the literature usually appear on maps and sketches available only as image format.These images have been adequately georeferenced, and the tectonic structures and other features (diapirs, volcanoes, etc.) have been digitized, always paying special attention to the possible correspondence with the seafloor morphological features.
In some regions, no previous work showing detailed tectonic mapping is available.In these cases, we have made a conservative interpretation from the available geophysical information (mainly bathymetry and high-resolution reflection seismic) to improve the cartography.This has been done, for example, for features with a very clear morphological correspondence, as in the case of the saline diapirs present to the south, east and northeast of the Balearic Promontory.
For the delimitation of the different tectonic domains (Figure 1 and Inset in the Main Map), map images of different sectors of the continental margins, previously published by other authors have been georeferenced.From them, the limits of the different domains have been digitized, checking their correlation with the available geophysical data (mainly potential fields).In regions where previous maps of tectonic domains are not available, this cartography has been extended according to bathymetric and potential field criteria.

Conclusions
The complex geodynamic evolution of Iberia and its continental margins yields the intricate tectonic sketch shown in this map, where the structural inheritance plays an important role in the development of the present appearance of this region.
Different crustal domains have been defined from their continental or oceanic origin and the degree of tectonic deformation affecting them.
This map provides a basis for the recognition of the main morphotectonic features at lithospheric plate scale.Thus, it can be the base for further detailed geological and geophysical studies such as the identification of the tectonic control on the submarine relief, and the establishment of potential tectonic and volcanic risk zones.

Software
The cartography of the main tectonic structures and sectors was performed in ESRI ArcMap.This information was later imported into the Adobe Illustrator software, where the final design and layout work was done.

Open Scholarship
This article has earned the Center for Open Science badge for Open Data.The data are openly accessible at http:// hdl.handle.net/10261/288043.