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Research paper
Constraints on the geodynamic evolution of the AfricaeIberia plate
margin across the Gibraltar Strait from seismic tomography
S. Monna
a
,
*
, A. Argnani
b
, G.B. Cimini
a
, F. Frugoni
a
, C. Montuori
a
a
Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy
b
Istituto di Scienze Marine eConsiglio Nazionale delle Ricerche, Bologna, Italy
article info
Article history:
Received 15 November 2013
Received in revised form
24 January 2014
Accepted 6 February 2014
Available online xxx
Keywords:
AfricaeIberia plate margin
Teleseismic tomography
Velocity anomaly
Mantle upwelling
Lithospheric subduction
abstract
Geophysical studies point to a complex tectonic and geodynamic evolution of the Alboran Basin and Gulf
of Cadiz. Tomographic images show strong seismic waves velocity contrasts in the upper mantle. The
high velocity anomaly beneath the Alboran Sea recovered by a number of studies is now a well estab-
lished feature. Several geodynamic reconstructions have been proposed also on the base of these images.
We present and elaborate on results coming from a recent tomography study which concentrates on both
the Alboran and the adjacent Atlantic region. These new results, while they confirm the existence of the
fast anomaly below the Alboran region, also show interesting features of the lithosphere-asthenosphere
system below the Atlantic. A high velocity body is imaged roughly below the Horseshoe Abyssal plain
down to sub-lithospheric depths. This feature suggests either a possible initiation or relic subduction.
Pronounced low velocity anomalies pervade the upper mantle below the Atlantic region and separate the
lithospheres of the two regions. We also notice a strong change of the upper mantle velocity structure
going from south to north across the Gorringe Bank. This variation in structure could be related to the
different evolution in the opening of the central and northern Atlantic oceans.
Ó2014, China University of Geosciences (Beijing) and Peking University. Production and hosting by
Elsevier B.V. All rights reserved.
1. Introduction
The geodynamic evolution of the Gulf of Cadiz/Alboran Basin
region is the result of the complex interaction between central and
northern Atlantic oceanic domains, and the African and Eurasian
plates (Fig. 1). Observations on magnetic anomalies and seismic
data suggest that the central and northern Atlantic oceans opened
at different times, although the precise age of the early oceanic
crust is still debated (e.g., Labails et al., 2010; Bronner et al., 2011;
Sibuet et al., 2012). Spreading occurred in early Jurassic in the
central Atlantic and in early Cretaceous in the northern Atlantic. In
the Iberian margin of North Atlantic mantle exhumation has been
inferred to play a major role in the early opening stage (Bronner
et al., 2011), whereas magnetic anomalies in the central Atlantic
indicate that accretion is markedly asymmetric, with more oceanic
crust produced on the American side (Labails et al., 2010; Sibuet
et al., 2012). Interestingly, this last observation is connected to
the occurrence, on the African side, of the central Atlantic magmatic
province (Labails et al., 2010). The Gibraltar-Newfoundland Frac-
ture Zone (GNFZ) is the northern limit of the central Atlantic, and is
also the zone of transfer of spreading into the Alpine Tethyan
seaway in Jurassic times. In the western Tethyan region conver-
gence between Eurasia (Iberia) and Africa plates started by Eocene,
leading to consumption of the plate margins and to the present
geological setting (Platt et al., 2013 and references therein). At
present the convergence between the two plates is at a rate of
w5 mm/yr (Stich et al., 2006; Serpelloni et al., 2007). The Africa-
Eurasia plate boundary is clearly defined from the Gloria fault to
the Gorringe Bank (McKenzie, 1972; Srivastava et al., 1990; Zitellini
et al., 2009). From the Gorringe Bank proceeding to the east, across
the Strait of Gibraltar, the boundary is diffuse (McKenzie, 1972;
Sartori et al., 1994; Serpelloni et al., 2007) with different locations
having been proposed for it. A narrow band of deformation (SWIM
Fault Zone), is considered as a precursor to the formation of a new
*Corresponding author. Tel.: þ39 (0) 651860404.
E-mail address: stephen.monna@ingv.it (S. Monna).
Peer-review under responsibility of China University of Geosciences (Beijing)
Production and hosting by Elsevier
Contents lists available at ScienceDirect
China University of Geosciences (Beijing)
Geoscience Frontiers
journal homepage: www.elsevier.com/locate/gsf
1674-9871/$ esee front matter Ó2014, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.gsf.2014.02.003
Geoscience Frontiers xxx (2014) 1e10
Please cite this article in press as: Monna, S., et al., Constraints on the geodynamic evolution of the AfricaeIberia plate margin across the
Gibraltar Strait from seismic tomography, Geoscience Frontiers (2014), http://dx.doi.org/10.1016/j.gsf.2014.02.003
transcurrent plate boundary between Iberia and Africa (Zitellini
et al., 2009). The end result of this complex geodynamic evolu-
tion is a strong crustal and mantle scale heterogeneity. This hete-
rogeneity is particularly evident in our restricted study area (Fig. 1).
East of the Gibraltar Strait, the wide amount of geological and
geophysical data led to the proposal of two broad groups of models
for the geodynamic evolution of the Alboran region: (1) The colli-
sion of Europe and Africa led to lithospheric thickening during the
Paleogene. The thickened continental lithosphere was later
(w25 Ma) detached by convective removal (Platt and Vissers, 1989)
or by delamination (Seber et al., 1996; Calvert et al., 2000). The
increased gravitational potential led to the collapse of this thick-
ened orogen, causing extension of the Alboran basin and uplift
around the margin. (2) The subduction of negatively buoyant
oceanic lithosphere caused slab rollback and extension within the
Alboran Basin in the Miocene (Royden, 1993; Lonergan and White,
1997; Bijwaard and Spakman, 2000; Gutscher et al., 2002); the
break-off of the slab has also been proposed (Blanco and Spakman,
1993; Zeck, 1996).
West of the Gibraltar Strait, in the Atlantic side, geophysical data
show: (1) a large-scale low velocity anomaly seen in global to-
mography images west of Africa below the central Atlantic (e.g.,
Simmons et al., 2012); (2) the distribution of heterogeneous
volcanism of varying age along western, southwestern Iberia and
the Gulf of Cadiz (e.g., Duggen et al., 2004; Merle et al., 2006); (3)
low heat flow anomalies in the Gulf of Cadiz (Polyak et al., 1996;
Grevemeyer et al., 2009); (4) a very strong gravimetric high at
the Gorringe Bank (Purdy, 1975), and (5) a very heterogeneous
bathymetry with seamounts and abyssal plains in a restricted area
such as the Gulf of Cadiz (e.g., Zitellini et al., 2009).
An open question is if and how the subduction process in the
Alboran side (east of the Gibraltar Strait) has extended to the
Atlantic side (west of the Gibraltar Strait). Some of the models for
the Alboran also include the Gulf of Cadiz (e.g., Royden, 1993), but
they are not clearly supported. In spite of the numerous clues, our
knowledge of the mantle is limited by the lack of seismic tomog-
raphy models in the Atlantic region at the appropriate scale, as
most of the previous higher resolution studies have concentrated
on the Alboran due to scarce data coverage in the Atlantic region
(e.g., Bezada et al., 2013; Palomeras et al., 2014).
Recently, a first image at upper mantle scale of both the Alboran
and the Atlantic regions has been produced by Monna et al. (2013)
using OBS (Ocean Bottom Seismometer) teleseismic data recorded
in the Gulf of Cadiz in the framework of the NEAREST project
(http://nearest.bo.ismar.cnr.it). In the present study, starting from
this model (hereafter quoted as model WMGC-OBS), we identify
and discuss some features that can be important for the geo-
dynamic reconstruction of the area.
2. Geological setting
Within the AfricaeEurasia convergence the Iberia plate shows
complex kinematics, as evident from Atlantic sea-floor magnetic
anomalies. According to previous studies (Srivastava et al., 1990),
Iberia was part of Africa from late Cretaceous to early Oligocene,
whereas from late Oligocene to present it became part of Europe,
following the Pyrenean collision. Recent reviews of the magnetic
anomalies suggest a possible alternative (Vissers and Meijer, 2012),
which allows for some 50e70 km convergence between Africa and
Iberia for the late Cretaceousemiddle Eocene time span. Altogether,
the NeS convergence between Africa and Iberia is in the order of
200e250 km, a figure that can hardly account for the westward arc
propagation of the Betic-Rif fold-and-thrust belt and the width and
shape of the east-dipping subducted slab imaged by high-
resolution seismic tomography (e.g., Bezada et al., 2013;
Palomeras et al., 2014). In fact, several tectonic models imply a
substantial westward subduction/rollback in order to account for
the geological and tomographic evidence (e.g., Gutscher et al.,
2002; Rosenbaum et al., 2002; Faccenna et al., 2004).
The complex geodynamic evolution described in section 1is
well represented by the heterogeneous geological setting which is
synthetically depicted in Fig. 2. Outwards thrusting in the external
Betics and external Rif is ca. coeval to the extension in the Alboran
basin, from 20 to 18 Ma (Platt and Vissers, 1989; Platt et al., 2013).
The units involved belong to the continental margins of Iberia and
North Africa, and are facing an oceanic region to the south and
north, respectively. It is implicitly considered to be the same ocean,
i.e., Alpine Tethys, connected to the central Atlantic. However,
although the water masses of the central Atlantic and Alpine Tethys
were certainly connected, the paleogeography of the Gibraltar-
Alboran seaway, and the nature of the underlying crust in partic-
ular, are still not properly constrained.
The extensional tectonics that affected the internal domains
consists of two major events and is closely related to volcanic ac-
tivity (e.g., Platt et al., 2013). The early extensional event (ear-
lyemiddle Miocene) is related to the collapse of the Alboran
Domain and to the exhumation of the Alpujarride units. Large
vertical-axis rotations, documented by paleomagnetic studies,
make it difficult to infer the direction of extension. The later event
(middleelate Miocene) is characterized by SW-directed extension
and basin formation in the internal Betics.
The Alboran Sea basin formed in the Neogene as a result of this
extensional tectonics, and is currently floored by thinned conti-
nental crust, 13e20 km thick (Watts et al., 1993). Moreover, thermal
models for metamorphic units from the floor of the Alboran Basin
are consistent with post-collisional rapid exhumation and associ-
ated heating (e.g., Platt et al., 2013).
Magmatic products cover a ca. 200 km-wide belt that extends
from the eastern Rif to the eastern Betics, crossing the Alboran Sea,
with NNEeSSW direction.Volcanic activity spans from early Miocene
to Pleistocene and presents mainly an orogenic magmatic affinity.
Only in the eastern part of the magmatic belt, and particularly in the
Figure 1. Map of the study region with the main structural features and geographical
names. Colored circles represent subcrustal seismicity at different depth intervals (red
circles, 35e70 km; yellow circles, 70e300 km; blue circles, >300 km). Hypocentral
locations are from http://www.02.ign.es/ign/layoutIn/sismoFormularioCatalogo.do
(Instituto Geográfico Nacional, Spain). GS ¼Gibraltar Strait; CSV ¼Cape St. Vincent;
GC ¼Gulf of Cadiz; GB ¼Gorringe Bank; SAP ¼Seine Abyssal Plain; HAP ¼Horseshoe
Abyssal Plain; TAP ¼Tagus Abyssal Plain; AGFZ ¼Azores-Gibraltar Fracture Zone.
Magenta thick curve is the EurasiaeAfrica plate boundary (Grange et al., 2010). Black
thick curve with triangles is the external limit of the Gulf of Cadiz accretionary wedge
(redrawn from Zitellini et al., 2009).
S. Monna et al. / Geoscience Frontiers xxx (2014) 1e102
Please cite this article in press as: Monna, S., et al., Constraints on the geodynamic evolution of the AfricaeIberia plate margin across the
Gibraltar Strait from seismic tomography, Geoscience Frontiers (2014), http://dx.doi.org/10.1016/j.gsf.2014.02.003
NE Betics, volcanics of anorogenic affinity are present, including
lamproites (e.g., Wilson and Bianchini, 1999). In general, the older
volcanicterms are orogenic, and are followed byanorogenic volcanics.
Geochemical and geochronological data show that this transition
from post-collisional subduction-related to intraplate-type magma-
tism occurred between 6.3 and 4.8 Ma (Duggen et al., 2005). This
change in magmatic character has been related to either removal of
lithospheric root (Turner et al., 1999) or westward slab rollback
(Duggen et al., 2004). Such magmatic evolution recalls that of the
northern Tyrrhenian-Tuscany volcanic province, where the occur-
rence of continental delamination has been inferred (e.g., Serri et al.,
1993; Argnani, 2002). The Alboran region is characterized by the
pronounced arcuate shape of the Betics and Rif fold-and-thrust belts,
which mostly originated in the Mioceneat the expense of the external
units, during the westward migration of the internal units (Alboran
domain).Paleomagnetic data from the Rif and western Betics indicate
that most of the rotation and arc formation was accomplished by the
earlyPliocene (e.g., Platzmanet al., 1993; Krijgsman and Garces, 2004;
Cifelli et al., 2008).
The SW Iberian margin, which faces the Cadiz Gulf, has been
extensively investigated through seismic reflection and refraction
surveys and high-resolution swath bathymetry acquisitions (e.g.,
Zitellini et al., 2009; Gutscher et al., 2012). The margin originated
during the break up of Pangea and the opening of the central
Atlantic Ocean in TriassiceJurassic. The continental crust of SW
Iberia has been thinned and stretched, as shown by tilted fault
blocks along the margin (e.g., Tortella et al., 1997). Recent seismic
refraction experiments have shown the occurrence of oceanic crust,
of presumably Jurassic age in the western part of the Gulf of Cadiz
(Sallarès et al., 2011). This data has been used to infer the occur-
rence of a continuous oceanic subduction that goes from the
Atlantic to beneath the Alboran basin (Gutscher et al., 2012), an
inference that has also strong implications for Jurassic
palaeogeography (e.g., Frizon de Lamotte et al., 2011). As shown
later, our data brings a potentially significant contribution to this
last issue.
The presence of thick oceanic lithosphere of late Jurassiceearly
Cretaceous age is also inferred from the depth of seismicity (down
to 60 km; Geissler et al., 2010) in the eastern part of the Horseshoe
Abyssal Plain. This deep oceanic basin is bounded to the NW by a
large topographic feature known as Gorringe Bank (Fig. 1). Perido-
tites, gabbros and other oceanic floor rocks have been dredged and
drilled from this 200 km long and very shallow bank, which has
been interpreted as a thrust of oceanic lithosphere (e.g., Mauffret
et al., 1989; Hayward et al., 1999 and references therein). The large
Tagus Abyssal Plain is located north of the Gorringe Bank and rep-
resents the oceanic portion of the Iberia continental margin, with a
transition to continental crust which is inferred to occur in the
eastern half of the deep sea plain (Pinheiro et al., 1992). An alter-
native view, implying that the basin is floored by exhumed mantle
rocks, has been recently proposed (Bronner et al., 2011). However,
this point is still debated (e.g., Tucholke and Sibuet, 2012).
Figure 2. Simplified geological map of the stu dy area (after Gutscher et al., 2012; Duarte etal., 2013; Platt et al., 2013). The Alboran domain is indicated by cross hatched pattern. The
External Betics (EB in gray pattern) includes the Subbetic and Prebetic, and the External Rif (ER in gray pattern) includes the Intrarif and Mesor if. The white field with dotted pattern
represents the Flysch belt. The gray dashed line indicates the extent of oceanic crust (oc) inferred from refraction seismic (Sallarès et al., 2011). The approximate location of the Lu-
HVA (at about 400 km depth) and of the GC-LVA (100e200 km depth) is also indicated. A-CP ¼Ampere-Coral Patch seamounts. TAP, GB, HAP, and SAP are as in Fig. 1.
S. Monna et al. / Geoscience Frontiers xxx (2014) 1e10 3
Please cite this article in press as: Monna, S., et al., Constraints on the geodynamic evolution of the AfricaeIberia plate margin across the
Gibraltar Strait from seismic tomography, Geoscience Frontiers (2014), http://dx.doi.org/10.1016/j.gsf.2014.02.003
A remarkably large accretionary prism extends for about
300 km into the Atlantic Ocean, off the Gibraltar Strait (e.g.,
Zitellini et al., 2009), representing the outermost deformation of
the Gibraltar Arc. Although it is difficult to assess whether this
accretionary prism is currently active or not, this information is
critical in establishing whether subduction is still active under the
Gibraltar Arc. Some authors proposed that the sedimentary prism
was mostly emplaced gravitationally before the end of Miocene,
with little or no deformation occurring since, as in several places
late Miocene sediments onlap the prism without being deformed
(e.g., Sartori et al., 1994; Zitellini et al., 2009). Other authors
argued that several hints, including PlioceneeQuaternary sedi-
ments found above the prism and showing undulating folds,
suggest that the prism is still actively accreting (e.g., Gutscher
et al., 2012). Far from being just an academic exercise, these
alternative interpretations lead to different possible locations of
the great 1755 Lisbon earthquake (M
w
8.5e8.7; Stich et al., 2007
and references therein). In fact, the first group of authors points
to a fault located in the area of the Gorringe Bank, whereas the
second group supports a seismogenic source at the Gibraltar Arc
subduction interface.
3. Seismic tomography models
A number of tomographic studies have been performed in the
last decade to image the deep seismic velocity structure of the area.
A“bigger-picture”approach that includes both the Atlantic and the
Alboran regions would greatly benefit our understanding of the
evolution of this area and, more generally, of the western Medi-
terranean. Although global scale tomography works cover both
regions, they do not have the capability to resolve the finer details.
On the other hand, in spite of the increase in available data, the
great majority of the higher resolution (regional scale) tomography
models do not image the mantle below the Atlantic region west of
the Gibraltar Strait due to a lack of station coverage at sea. The
recent tomographic model WMGC-OBS (Monna et al., 2013)over-
comes this limitation thanks to broadband marine data from sen-
sors deployed in the Cadiz Gulf. Figs. 3e5show a series of slices
extracted from the 3D model. Fig. 3 displays the velocity structure
on horizontal layers sampled at depth intervals of 100 km. Fig. 4
shows three vertical slices which help in reconstructing the ge-
ometry of the anomalous bodies. Fig. 5 shows an oblique vertical
slice across the Gorringe Bank.
The main features in WMGC-OBS are:
(1) A high velocity body with slab-like shape under the Alboran
Sea area (Al-HVA).
(2) A high-velocity body under the Horseshoe Abyssal Plain in the
Atlantic region (HAP-HVA).
(3) A diffuse low velocity anomaly (EAt-LVA) present in the Eastern
Atlantic region south of the Gloria fault, down to the mantle
transition zone.
(4) A prominent low velocity anomaly below the Gulf of Cadiz (GC-
LVA, part of EAt-LVA) which separates the two high-velocity
bodies found under the Alboran (Al-HVA) and Atlantic re-
gions (HAP-HVA) (See also Fig. 2).
(5) A deep (>180 km down to the transition zone) high velocity
anomaly (Lu-HVA) in the northern Atlantic, below the Lusita-
nian sedimentary basin off-shore central-north Portugal (See
also Fig. 2).
Regional studies show a high-velocity body under the Alboran
Sea, which has been interpreted as a continuous subducting slab
(Gutscher et al., 2002; Piromallo and Morelli, 2003), as a broken-off
slab (Blanco and Spakman, 1993), and as lithosphere which has
undergone delamination (Calvert et al., 2000). This high-velocity
structure has been imaged down to mantle depths since the first
tomographic studies (Blanco and Spakman, 1993; Bijwaard and
Spakman, 2000; Calvert et al., 2000). The geometry of this struc-
ture has been better defined by more recent works thanks to
improved station coverage and data quality (e.g., Piromallo and
Morelli, 2003). Very recent high-resolution studies (Bezada et al.,
2013; Palomeras et al., 2014) confirm the presence a slab-shaped
high velocity body, seen from the surface down to the bottom of
the transition zone, and interpret it as a slab composed of sub-
ducted Alboran mantle lithosphere and the surrounding Alpine
Tethys ocean lithosphere. The shape and position of Al-HVA (Fig. 3;
Fig. 4, cross-sections AA
0
,CC
0
) are in agreement with the most
recent regional models previously mentioned, in spite of some
minor differences.
Figure 3. Perspective view showing a series of horizontal slices extracted from the
WMGC-OBS velocity model at depth interval of 100 km. Blue colored areas indicate
high velocity anomalies, as in the Betic-Alboran region (Al-HVA) and in the Atlantic SW
Portugal (HAP-HVA). Red colored areas are zones of low velocity anomalies for the
eastern Atlantic (EAt-LVA) and the Gulf of Cadiz (GC-LVA).
S. Monna et al. / Geoscience Frontiers xxx (2014) 1e104
Please cite this article in press as: Monna, S., et al., Constraints on the geodynamic evolution of the AfricaeIberia plate margin across the
Gibraltar Strait from seismic tomography, Geoscience Frontiers (2014), http://dx.doi.org/10.1016/j.gsf.2014.02.003
Lateral heterogeneity in the upper mantle beneath western
Mediterranean and adjacent Atlantic region is also imaged by
global travel time tomography (Bijwaard and Spakman, 2000;
Montelli et al., 2004; Li et al., 2008; Simmons et al., 2012, among
many others). These tomographic models are designed to image
large-scale seismic structure in the whole mantle for the primary
purpose of understanding the convective processes within the
Earth’s interior (Grand et al., 1997). The recent global-scale P wave
velocity model LLNL-G3Dv3 developed by Simmons et al. (2012),
which takes advantage of a more complex representation of the
Earth’s stratification (relative to a purely spherical model) and uses
a multiscale inversion approach (called PMTI) to capture also the
fine details (where data are sufficient), reveals interesting features
in the lithosphere-asthenosphere system of the study region. The
most relevant aspects of model LLNL-G3Dv3 that can be compared
to the features of model WMGC-OBS, are: the pronounced high
velocity anomaly, as large as þ2.5% (with respect to the mean ve-
locity of 8.1 km/s), imaged from the Moho to at least 115 km depth
in the Atlantic region southwest Portugal, and the large-scale low
velocity anomaly (1.0%) between the Atlantic Ridge and north-
west Africa, which extends from 265 km down to the lower mantle
(see Fig. 9, Simmons et al., 2012). The fast structure at 115 km depth
corresponds well to our HAP-HVA below the Horseshoe Abyssal
Plain (Fig. 4, cross-sections AA
0
,BB
0
, and Fig. 5), suggesting that it is
a reliable upper mantle structure. The northerly portion of the
large-scale low velocity anomaly in the central Atlantic is also
captured by the WMGC-OBS model southwest of the western Ibe-
rian margin. The low velocity volume EAt-LVA in our study area is
characterized by a larger velocity contrast (w2%) with respect to
the broad LLNL-G3Dv3 low velocity anomaly.
4. Discussion
Seismic tomography gives us a snapshot of the current velocity
heterogeneity which can help us answer some open problems
regarding the geodynamic evolution of this area. One of the key
questions concerns the present relation between the Atlantic and
Alboran regions. Recent studies based on extensive deployment of
temporary land seismic arrays, both on the Iberian Peninsula and
North Africa (Topo-Iberia, http://www.ictja.csic.es/gt/rc/LSD/PRJ/
indexTOPOIBERIA.html; PICASSO, https://earth.usc.edu/research/
picasso/home), are able to focus with high resolution on the
Alboran region. The shape of the high velocity body below the
Alboran region has been well imaged in a recent paper (Bezada
et al., 2013) where the slab appears disconnected (torn off) from
the Iberian plate under the Betics. The Al-HVA of our model is
consistent with the Alboran slab imaged by these high resolution
studies, pointing to a former oceanic domain now entirely sub-
ducted. These land-based studies, however, do not give information
on the transition towards the Atlantic region. Model WMGC-OBS is
afirst attempt to image this transition at upper-mantle scale based
on the integration of marine (OBS) and land data. A major result
evident from the model is that there is not a continuity between the
fast structures imaged in the two regions. In fact, the high-velocity
body (Al-HVA) under the Alboran Sea area is interrupted at its
western side by a pronounced low-velocity anomaly (GC-LVA;
Figs. 2 and 4, cross-section AA
0
). Fig. 6 is a 3D rendition of the high
(A) and low (B) upper mantle velocity heterogeneity. These plots
suggest a more articulate reconstruction than one based on east-
ward subduction underneath the Alboran of a single oceanic slab
connected to the Atlantic domain (e.g., Royden, 1993; Gutscher
et al., 2002).
What more can we say about the Atlantic region? HAP-HVA is
imaged in an area roughly underlying the Horseshoe Abyssal Plain,
where the plate boundary passes from a linear transform to a
diffuse convergent margin, and where the occurrence of Jurassic
oceanic lithosphere is inferred (Hayward et al., 1999; Zitellini et al.,
2009; Geissler et al., 2010). The thickness of HAP-HVA
(w80e150 km in cross sections AA
0
eBB
0
;Fig. 4) does agree with
values proposed in literature for old (w140 Ma) oceanic lithosphere
(McKenzie et al., 2005; Conrad and Lithgow-Bertelloni, 2006). The
presence of oceanic crust, of presumably Jurassic age, has also been
supported by a seismic refraction profile in the Gulf of Cadiz
(Sallarès et al., 2011). The HAP-HVA is interrupted tothe east by the
well resolved GC-LVA seen in model WMGC-OBS.
The position and geometry of HAP-HVA, dipping in the south-
east direction within the upper mantle, suggest the presence of
subducted lithosphere (Fig. 5). The Gorringe Bank has been iden-
tified by Gurnis et al. (2004), from geodynamic modeling based on
geophysical and geological information, as an incipient margin that
could develop into a subduction zone. In this view the Gorringe
Bank is an early stage system undergoing a forced style of subduc-
tion driven by compression. The shortening accommodated across
the Gorringe Bank and adjacent structures has been estimated to
range from w50 km (Hayward et al., 1999) to a minimum of 20 km
(Jiménez-Munt et al., 2010) on the basis of flexural isostatic models.
In spite of possible vertical smearing of the velocity anomalies,
our images show a continuous, narrow and steep slab which rea-
ches at least 350 km depth, suggesting a developed subduction
process well beyond the early stage. The occurrence of a thickened
oceanic lithosphere which has accommodated the shortening
caused by AfricaeIberia convergence (Jiménez-Munt et al., 2011),
does not seem appropriate to explainthe HAP-HVA given the depth
extent of the high velocity body. In fact, assuming a 120 km thick
oceanic lithosphere, the subducted slab is about 250 km long,
suggesting that AfricaeIberia convergence has been accommo-
dated in this structure since some millions of years. Future seis-
mological investigations with larger instrumental coverage of this
area need to be carried out for a better definition of the geometry of
the subducted slab.
Subduction initiation in this area may be due to the convergence
component rather than foundering of a negatively buoyant litho-
sphere, as no evidence of extensional tectonics has been reported in
the upper plate of the Horseshoe Abyssal Plain, where compres-
sional earthquake focal mechanisms are found (e.g., Stich et al.,
2007). Furthermore, our tomographic results indicate that sub-
duction is limited to the Gorringe Bank structure and it may be
related to a lateral change from strike slip to transpression along
the Gloria fault (Serpelloni et al., 2007).
The tomographic evidence of limited subduction underneath
the Horseshoe Abyssal Plain, physically separated by the larger
Alboran slab (Al-HVA), points to a different scenario with respect to
those proposed by Duarte et al. (2013) for the active tectonics of the
region: the Gibraltar subduction is mostly inactive, as suggested by
geodetic data which show small to none differential motion across
the Gibraltar Strait (Stich et al., 2006; Serpelloni et al., 2007). On the
other hand, AfricaeIberia convergence is active and it is accom-
modated by significant shortening in the Gorringe Bank-Horseshoe
region (Jiménez-Munt et al., 2011).
Interestingly, most of the subcrustal seismicity located in this
area is included in HAP-HVA (Fig. 5). Furthermore, two recent
strong earthquakes have been located below the Horseshoe abyssal
Plain (Fukao, 1973; Stich et al., 20 07) and are found at the top of the
descending lithosphere (Fig. 5). Our tomographic results support
the interpretation that the Gorringe Bank-Horseshoe region could
be the possible source area of the great 1755 Lisbon tsunamigenic
earthquake, as suggested by other authors (e.g., Stich et al., 2007
and references therein), and contrast with the hypothesis of a
source located at the subduction interface underneath the Gibraltar
Arc (e.g., Gutscher et al., 2012 and references therein).
S. Monna et al. / Geoscience Frontiers xxx (2014) 1e10 5
Please cite this article in press as: Monna, S., et al., Constraints on the geodynamic evolution of the AfricaeIberia plate margin across the
Gibraltar Strait from seismic tomography, Geoscience Frontiers (2014), http://dx.doi.org/10.1016/j.gsf.2014.02.003
Figure 4. Vertical slices displaying the velocity anomalies of model WMGC-OBS along a west to east profile crossing the Gibraltar Arc (AA0), and along two south to north profiles
crossing the Atlantic Ocean (BB0) and the Alboran Sea (CC0). SAP, HAP, and TAP as in Fig. 1. Blue triangles and red dots indicate the seismic stations and the subcrustal seismicity
located within a 100 km wide zone centered on the profile. In profile CC0, hypocenters deeper than 60 0 km represent the deep events below the Granada region. In the geographic
map, colored symbols indicate seismic stations of the Portuguese (green circles), Spanish (blue squares), Moroccan (red triangles), and NEAREST-OBS (magenta stars) networks used
in the tomography. Topography/bathymetric profiles are from the Global-Integrated-Topo/Bathymetry Grid (GINA) (Lindquist et al., 2004). Moho topography is from the European
Moho depth map (Grad et al., 2009).
Please cite this article in press as: Monna, S., et al., Constraints on the geodynamic evolution of the AfricaeIberia plate margin across the
Gibraltar Strait from seismic tomography, Geoscience Frontiers (2014), http://dx.doi.org/10.1016/j.gsf.2014.02.003
Another important feature seen in the Atlantic region is EAt-
LVA, the diffuse low velocity anomaly found at sub-lithospheric
depths below the Atlantic region (down to 600 km depth; Figs. 3
and 4, cross-sections AA
0
eBB
0
and Fig. 6B), which is the northern
part of a larger low velocity anomaly imaged in global models
(Simmons et al., 2012). The low values of EAt-LVA could be asso-
ciated to a hot upper mantle. In fact, although other important
factors, such as the phase (i.e., the presence or absence of partial
melt) and composition (lithology, mineralogy and chemistry),
should be considered (Anderson, 2007), at these depths tempera-
ture can play a first order role in determining lateral seismic ve-
locity heterogeneity (e.g., Ranalli, 1996). This global low velocity
anomaly has been explained as a sheet-like region of upper-mantle
upwelling linked to the Eastern Atlantic Magmatic Province
(Hoernle et al., 1995), or as part of the Azores-Canary-Cape Verde
mantle plume extending into the lower mantle (Montelli et al.,
2004). It has also been inferred that the Mid Atlantic Ridge (MAR)
decoupled from the hotspots represented by this wide low velocity
anomaly after 70 million years ago (Anderson et al., 1992). In this
case the low velocity anomaly, of which our EAt-LVA is part, could
represent a previous location of the MAR (a “ghost”ridge).
This global anomaly also corresponds to the passive continental
margin of Morocco, where extensive lava emissions took place
during the opening of the central Atlantic Ocean (190 Ma), and
originated seaward dipping reflectors (Menzies et al., 2002). This
magmatism is part of the vast central Atlantic magmatic province
(Knight et al., 2004). Magmatic activity continued through Creta-
ceous to present, leading to the onset of alkaline magmatism along
aNeNE trend in the eastern Atlantic margin and Europe (Oyarzun
et al., 1997). In fact, isotopic and geochemical studies indicate that a
long-lived thermal anomaly is the most plausible explanation for
the alkaline magmatism of western Portugal and the adjacent
Atlantic region (Merle et al., 2006; Martins et al., 2009; Miranda
et al., 2009; Grange et al., 2010).
It is noteworthy that EAt-LVA is mostly confined in its northern
part by the prolongation of the Gloria transform (Fig.1 and Fig. 4-BB
0
;
see also Simmons et al., 2012). The position of the main part of EAt-
LVA can be explained by the early opening stage of the central
Atlantic Ocean. The edge of EAt-LVA is to a limited extent present as a
strong anomalybelow the Gorringe Bank and the Tagus AbyssalPlain
(Figs. 3 and 5), whichcould be the result of a late expansion of the EAt-
LVA following the initial stage of opening of the northern Atlantic
Ocean (Sibuet et al., 2012). This strong sub-lithospheric mantle
anomaly underlies the lithospheric density anomalyobserved below
Gorringe Bank/Tagus Abyssal Plain and interpreted as serpentinized
mantle (Jiménez-Munt et al., 2010; Salláres et al., 2013).
The low velocity anomaly below the Gulf of Cadiz, GC-LVA ex-
tends from the top of the model down to about 300 km, inter-
rupting to the west the continuity of the Alboran subducted slab.
Given its areal extent, this low velocity anomaly may likely repre-
sent a fragment of thinned continental lithosphere which origi-
nated during the Mesozoic opening of the central Atlantic and
Alpine Tethys (Dewey et al., 1989; Rosenbaum et al., 2002; Vissers
and Meijer, 2012). This piece of continental lithosphere may
Figure 6. Three-dimensional perspective view (azimuth 20 0, elevation 30) of the
high velocity (A) and low velocity (B) isosurfaces extracted from the 3D model. Iso-
surfaces are drawn at 0.07 km/s velocity perturbations relative to the reference
values. Note the complex morphology of the Alboran subduction zone moving along
the pronounced arcuate shape of the Betics and Rif mountain belts. West of Gibraltar
Strait, the 3-D visualization shows well-developed slow structures which interrupt the
lateral continuity with the Atlantic high velocity anomaly.
Figure 5. Vertical slice crossing the Gorringe Bank and adjacent abyssal plains from
NW to SE. The image shows a slab-like high velocity anomaly dipping southeastward
down to the top of transition zone. Red dots as in Fig. 4. The green stars indicate the
locations of the February 28th 1969, M
w
7.8, and February 12th, 2007, M
w
6.0,
Horseshoe earthquakes. Hypocentral depths are about 22 km (Fukao, 1973) and 45 km
(Stich et al., 2007), respectively.
S. Monna et al. / Geoscience Frontiers xxx (2014) 1e10 7
Please cite this article in press as: Monna, S., et al., Constraints on the geodynamic evolution of the AfricaeIberia plate margin across the
Gibraltar Strait from seismic tomography, Geoscience Frontiers (2014), http://dx.doi.org/10.1016/j.gsf.2014.02.003
indicate that the Tethyan margin between Africa and Iberia was a
continental transform that linked the oceanic crust of the Horse-
shoe and Seine Abyssal plains to the oceanic Alpine Tethys. The
most relevant implication for this paleogeographic interpretation is
that this buoyant continental lithosphere may have caused the
extinction of the westward rollback of the Alpine Tethys oceanic
lithosphere, while entering subduction. This interpretation agrees
with paleomagnetic data that show that oroclinal bending ended in
early Pliocene or earlier (e.g., Platt et al., 2003,Cifelli et al., 2008)
and also with the present day GPS velocity filed, which shows that
the Betics and Rif units in the Gibraltar Strait are currently moving
together with the African region located north of the Atlas (e.g.,
Serpelloni et al., 2007).
Finally, in the resolved part of our model below the European
plate we find a deep high velocity anomaly, Lu-HVA (Figs. 2 and 4,
profile BB
0
) which resides in the mesosphere (400e650 km depth),
the region where slabs may accumulate (Anderson et al., 1992). This
high velocity body is located below the Variscan belt of Portugal.
The Variscan orogeny took place from about 350 to 300 Ma (e.g.,
Matte, 2001), contributing to the assembly of Pangea. The high
velocity body could therefore represent a relic subducted slab.
Similar relics of Variscan subduction are interpreted to occur un-
derneath the Paris Basin, where a high velocity anomaly is present
in the upper mantle (Averbuch and Piromallo, 2012). Pre-Variscan
palaeoreconstructions suggest that western Iberia had a different
evolution with respect to NW Africa, (e.g., Frizon de Lamotte et al.,
2013). Interestingly, the transform zone (GNFZ) that separated the
Variscan belt of Europe from the Appalachian-Mauritanian belt is
inferred to run more or less along the present day Gloria Fault. This
long-term evolution may have originated mantle heterogeneity
that varies north and south of the Gloria Fault. The subsequent
impinging of the eastern Atlantic magmatic activity may have also
overprinted any pre-existing mantle features on the African plate.
5. Conclusions
The tomographic images of the upper mantle below the
southwestern Iberian margin-Alboran region presented in this
study can help us better understand the lithosphere-asthenosphere
system across the Gibraltar Strait. Two clear high velocity anoma-
lies of oceanic nature have been imaged in the Atlantic (HAP-HVA)
and Alboran (Al-HVA) regions. The passage from the Atlantic to the
Alboran is characterized by the pronounced low velocity anomaly
GC-LVA which separates the high velocity lithospheres (HAP-HVA
and Al-HVA), thus excluding the existence of a single slab sub-
ducting from the Cadiz Gulf under the Alboran region. Velocity
contrasts between the fast/slow structures are 3 to 5%. In our
interpretation GC-LVA represents a thinned continental lithosphere
which separated the Alboran and Atlantic oceanic domains.
The geometry of HAP-HVA below the Horseshoe abyssal plain,
is consistent with the presence of a limited subduction zone (of ca.
250 km) underlying the Horseshoe Abyssal Plain, driven by plate
convergence. This evidence suggests that compression in the re-
gion comprising the Gorringe Bank-Horseshoe Abyssal Plain
possibly started earlier than expected on the basis lithospheric
thickening only.
A diffuse volume of low velocity, EAt-LVA, extends from north-
west Africa to southwest Iberia and from the surface down to the
bottom of our model. This EAt-LVA is mostly confined in its
northern part by the prolongation of the Gloria transform, and it
may have originated in the Jurassic, during the opening of the
central Atlantic Ocean.
In this study we presented and discussed new images extracted
from the tomographic model WMGC-OBS. Our model clarifies some
aspects and at the same time opens new questions on the upper
mantle structure and on the plate-tectonic evolution of this area.
We hope that this new information will prompt further discussion
and reinterpretation of the wide amount of acquired geophysical
and geological data.
Acknowledgments
We thank the Guest Editor M. Yoshida for inviting us to partic-
ipate to this special volume. We appreciated the constructive sug-
gestions of the reviewer, which improved the manuscript. The OBS
teleseismic data was collected during the NEAREST EC-project
(coordinator N. Zitellini). We thank the NEAREST Working Group
for making the seismic data available. The figures were produced by
using GMT (Wessel and Smith, 1991). We thank an anonymous
reviewer for helping us improve our paper.
References
Anderson, D.L., 2007. New Theory of the Earth, second ed., p. 408. http://dx.doi.org/
10.2277/0521849594
Anderson, D.L., Tanimoto, T., Zhang, Y., 1992. Plate tectonics and hotspots: the third
dimension. Science 256, 1645e1651.
Argnani, A., 2002. The Northern Apennines and the kinematics of Africa-Europe
convergence. Bollettino della Società Geologica Italiana vol. spec. 1, 47e60.
Averbuch, O., Piromallo, C., 2012. Is there a remnant Variscan subducted slab in the
mantle beneath the Paris basin? Implications for the late Variscan lithospheric
delamination process and the Paris basin formation. Tectonophysics 558e559,
70e83.
Bezada, M.J., Humphreys, E.D., Toomey, D.R., Harnafi, M., Dávila, J.M., Gallart, J.,
2013. Evidence for slab rollback in westernmost Mediterranean from improved
upper mantle imaging. Earth and Planetary Science Letters 368, 51e60.
Bijwaard, H., Spakman, W., 2000. Nonlinear global P-wave tomography by iterated
linearised inversion. Geophysical Journal International 141, 71e82.
Blanco, M.J., Spakman, W., 1993. The P-wave velocity structure of the mantle below
the Iberian Peninsula: evidence for a subducted lithosphere below southern
Spain. Tectonophysics 221, 13e34.
Bronner, A., Sauter, D., Manatschal, G., Péron-Pinvidic, G., Munschy, M., 2011.
Magmatic breakup as an explanation for magnetic anomalies at magma-poor
rifted margins. Nature Geoscience 4 (8), 549e553.
Calvert, A., Sandvol, E., Seber, D., Barazangi, M., Vidal, F., Alguacil, G., Jabour, N.,
2000. Propagation of regional seismic phases (Lg and Sn) and Pn velocity
structure along the AfricaeIberia plate boundary zone: tectonic implications.
Geophysical Journal International 142, 384e408. http://dx.doi.org/10.1046/
j.1365-246x.2000.00160.x.
Cifelli, F., Mattei, M., Porreca, M., 2008. New paleomagnetic data from Oligocene-
eupper Miocene sediments in the Rif chain (northern Morocco): Insights on the
Neogene tectonic evolution of the Gibraltar arc. Journal of Geophysical Research
113, B02104. http://dx.doi.org/10.1029/2007JB005271.
Conrad, C.P., Lithgow-Bertelloni, C., 2006. Influence of continental roots and
asthenosphere on plate-mantle coupling. Geophysical Research Letters 33,
L05312. http://dx.doi.org/10.1029/2005GL025621.
Dewey, J.F., Helman, M.L., Turco, E., Hutton, D.H.W., Knott, S.D., 1989. Kinematics of
the western Mediterranean. In: Coward, M.P., Dietrich, D., Park, R.G. (Eds.),
Alpine Tectonics, Geological Society, London, Special Publication, pp. 265e283.
Duarte, J.C., Rosas, F.M., Terrinha, P., Shellart, W.P., Boutelier, D., Gutscher, M.-A.,
Ribeiro, A., 2013. Are subduction zones invading the Atlantic ? Evidence from
the southwest Iberia margin. Geology. http://dx.doi.org/10.1130/G34100.1.
Duggen, S., Hoernle, K., Van Den Bogaard, P., Harris, C., 2004. Magmatic evolution of
the Alboran Region: the role of subduction in forming the western Mediterra-
nean and causing the Messinian Salinity Crisis. Earth and Planetary Science
Letters 218, 91e108.
Duggen, S., Hoernle, K., Van Den Bogaard, P., Garbe-Schönberg, D., 2005. Post-
Collisional transition from subduction to intraplate-type magmatism in the
westernmost Mediterranean: evidence for continental-edge delamination of
subcontinental lithosphere. Journal of Petrology 46 (6), 1155e1201. http://
dx.doi.org/10.1093/petrology/egi013.
Faccenna, C., Piromallo, C., Crespo-Blanc, A., Jolivet, L., Rossetti, F., 2004. Lateral slab
deformation and the origin of the western Mediterranean arcs. Tectonics 23.
http://dx.doi.org/10.1029/2002TC001488.
Frizon de Lamotte, D., Raulin, C., Mouchot, N., Wrobel-Daveau, J.-C., Blanpied, C.,
Ringenbach, J.-C., 2011. The southernmost margin of the Tethys realm during
the Mesozoic and Cenozoic: initial geometry and timing of the inversion pro-
cesses. Tectonics 30, TC3002. http://dx.doi.org/10.1029/2010TC002691.
Frizon de Lamotte, D., Tavakoli-Shirazi, S., Leturmy, P., Averbuch, O., Mouchot, N.,
Raulin, C., Leparmentier, F., Blanpied, C., Ringenbach, J.-C., 2013. Evidence for
Late Devonian vertical movements and extensional deformation in northern
Africa and Arabia: integration in the geodynamics of the Devonian world.
Tectonics 32, 107e122. http://dx.doi.org/10.1002/tect.20007.
S. Monna et al. / Geoscience Frontiers xxx (2014) 1e108
Please cite this article in press as: Monna, S., et al., Constraints on the geodynamic evolution of the AfricaeIberia plate margin across the
Gibraltar Strait from seismic tomography, Geoscience Frontiers (2014), http://dx.doi.org/10.1016/j.gsf.2014.02.003
Fukao, Y., 1973. Thrust faulting at a lithospheric plate boundary. The Portugal
earthquake of 1969. Earth and Planetary Science Letters 18, 205e216.
Geissler, W.H., Matias, L., Stich, D., Carrilho, F., Jokat, W., Monna, S., IbenBrahim, A.,
Mancilla, F., Gutscher,M.-A., Sallarès, V., Zitellini, N., 2010. Focal mechanisms for
subecrustal earthquakes in the Gulf of Cadiz from a dense OBS deployment.
Geophysical Research Letters 37, L18309. http://dx.doi.org/10.1029/
2010GL044289.
Grad, M., Tiira, T., ESC Working Group, 2009. The Moho depth map of the European
Plate. Geophysical Journal International 176, 279e292. http://dx.doi.org/
10.1111/j.1365-246X.2008.03919.x.
Grand, S.P., Van der Hilst, R.D., Widiyantoro, S., 1997. Global seismic tomography: a
snapshot of convection in the Earth. GSA Today 7, 1e7.
Grange, M., Schärer, U., Merle, R., Girardeau, J., Cornen, G., 2010. Plume-lithosphere
interaction during migration of cretaceous alkaline magmatism in SW Portugal:
evidence from U-Pb ages and Pb-Sr-Hf isotopes. Journal of Petrology 51 (5),
114 3e1170 .
Grevemeyer, I., Kaul, N., Kop, A., 2009. Heat flow anomalies in the Gulf of Cadiz and
off Cape San Vincente, Portugal. Marine and Petroleum Geology 26, 795e804.
Gurnis, M., Hall, C., Lavier, L., 2004. Evolving force balance during incipient sub-
duction. Geochemistry Geophysics Geosystems 5, Q07001. http://dx.doi.org/
10.1029/2003GC000681.
Gutscher, M.-A., Malod, J., Rehault, J.P., Contrucci, I., Klingelhoefer, F.,
MendeseVictor, L., Spakman, W., 2002. Evidence for active subduction beneath
Gibraltar. Geology 30, 1071e1074. http://dx.doi.org/10.1130/0091-7613.
Gutscher, M.-A., Dominguez, S., Westbrook, G.K., Le Roy, P., Rosas, F., Duarte, J.C.,
Terrinha, P., Miranda, J.M., Graindorge, D., Gailler, A., Sallares, V., Bartolome, R.,
2012. The Gibraltar subduction: a decade of new geophysical data. Tectono-
physics 574, 72e91. http://dx.doi.org/10.1016/j.tecto.2012.08.038.
Hayward, N., Watts, A.B., Westbrook, G.K., Collier, J.S., 1999. A seismic reflection and
GLORIA study of compressional deformation in the Gorringe Bank region,
eastern North Atlantic. Geophysical Journal International 138, 831e850.
Hoernle, K., Zhang, Y.-S., Graham, D., 1995. Seismic and geochemical evidence for
large-scale mantle upwelling beneath the eastern Atlantic and western and
central Europe. Nature 374, 34e39.
Jiménez-Munt, I., Fernàndez, M., Vergés, J., Afonso, J.C., Garcia-Castellanos, D.,
Fullea, J., 2010. Lithospheric structure of the Gorringe Bank: insights into its
origin and tectonic evolution. Tectonics 29, TC5019. http://dx.doi.org/10.1029/
2009TC002458.
Jiménez-Munt, I., Fernàndez, M., Vergés, J., Garcia-Castellanos, D., Fullea, J., Pérez-
Gussinyé, M., Afonso, J.C., 2011. Decoupled crust-mantle accommodation of
Africa-Eurasia convergence in the NW Moroccan margin. Journal of Geophysical
Research 116, B08403. http://dx.doi.org/10.1029/2010JB008105.
Knight, K.B., Nomade, S., Renne, P.R., Marzoli, A., Bertrand, H., Youbi, N., 2004. The
Central Atlantic Magmatic Province at the TriassiceJurassic boundary: paleo-
magnetic and
40
Ar/
39
Ar evidence from Morocco for brief, episodic volcanism.
Earth and Planetary Science Letters 228, 143e160.
Krijgsman, W., Garces, M., 2004. Paleomagnetic constrains on the geodynamic
evolution of the Gibraltar Arc. Terra Nova 16, 281e287.
Labails, C., Olivet, J.-L., Aslanian, D., Roest, W.R., 2010. An alternative early opening
scenario for the Central Atlantic Ocean. Earth and Planetary Science Letters 297,
355e368.
Li, C., van der Hilst, R.D., Engdahl, E.R., Burdick, S., 2008. A new global model for P
wave speed variations in Earth’s mantle. Geochemistry Geophysics Geosystems
9, Q05018. http://dx.doi.org/10.1029/2007GC001806.
Lindquist, K.G., Engle, K., Stahlke, D., Price, E., 2004. Global topography and ba-
thymetry grid improves research efforts. Eos Transactions American Geophys-
ical Union 85 (19), 186.
Lonergan, L., White, N., 1997. Origin of the Betic-Rif mountain belt. Tectonics 16,
504e522.
Martins, L.T., Madeira, J., Youbi, N., Munhá, J., Mata, J., Kerrich, R., 2009. Rift-related
magmatism of the Central Atlantic magmatic province in Algarve, Southern
Portugal. Lithos 101, 102e124.
Mauffret, A., Mougenot, D., Miles, P.R., Malod, J.A., 1989. Cenozoic deformation and
Mesozoic abandoned spreading centre in the Tagus Abyssal Plain (west
Portugal): result of a multichannel seismic survey. Canadian Journal Earth
Science 26, 1101e11 23.
Matte, P., 2001. The Variscan collage and orogeny (480e290 Ma) and the tectonic
definition of the Armorica microplate: a review. Terra Nova 13, 122e128. http://
dx.doi.org/10.1046/j.1365-3121.2001.00327.x.
McKenzie, D.P., 1972. Active tectonics of the Mediterranean region. Geophysical
Journal of the Royal Astronomical Society 30, 109e185 .
McKenzie, D., Jackson, J., Priestley, K., 2005. Thermal structure of oceanic and
continental lithosphere. Earth and Planetary Science Letters 233, 337e349.
Menzies, M., Klemperer, S.L., Ebinger, C.,J., Baker, J., 2002. Characteristics of volcanic
rifted margins. In: Geological Society of America Special Paper 362, pp. 1e14.
Merle, R., Schärer, U., Girardeau, J., Cornen, G., 2006. Cretaceous seamounts along
the continent-ocean transition of the Iberian margin: U-Pb ages and Pb-Sr-Hf
isotopes. Geochimica et Cosmochimica Acta 70, 4950e4976.
Miranda, R., Valadares, V., Terrinha, P., Mata, J., do Rosario Azevedo, M., Gaspar, M.,
Kulberg, J.C., Ribeiro, C., 2009. Age constraints on the Late Cretaceous alkaline
magmatism on the West Iberian Margin. Cretaceous Research 30, 575e586.
Monna, S., Cimini, G.B., Montuori, C., Matias, L., Geissler, W.H., Favali, P., 2013. New
insights from seismic tomography on the complex geodynamic evolution of
two adjacent domains: Gulf of Cadiz and Alboran Sea. Journal of Geophysical
Research 118. http://dx.doi.org/10.1029/2012JB009607.
Montelli, R., Nolet, G., Dahlen, F.A., Masters, G., Engdahl, E.R., Hung, S.H., 2004.
Finite-frequency tomography reveals a variety of plumes in the mantle. Science
303 (5656), 338e343.
Oyarzun, R., Doblas, M., López-Ruiz, J., Cebrá, J.M., 1997. Opening of the central
Atlantic and asymmetric mantle upwelling phenomena: implications for long-
lived magmatism in western North Africa and Europe. Geology 25, 727e730.
Palomeras, I., Thurner, S., Levander, A., Liu, K., Villasenor, A., Carbonell, R.,
Harnafi, M., 2014. Finite-frequency Rayleigh wave tomography of the western
Mediterranean: mapping its lithospheric structure. Geochemistry Geophysics
Geosystems 15. http://dx.doi.org/10.1002/2013GC004861.
Pinheiro, L.M., Whitmarsh, R.B., Miles, P.R., 1992. The ocean-continent boundary off
the western continental margin of Iberia-II: crustal structure in the Tagus
Abyssal Plain. Geophysical Journal International 109, 106e124.
Piromallo, C., Morelli, A., 2003. P wave tomography of the mantle under the Alpine-
Mediterranean area. Journal of Geophysical Research 108 (B2), 2065. http://
dx.doi.org/10.1029/2002JB001757.
Platt, J.P., Allerton, S., Kirker, A., Mandeville, C., Mayfield, A., Platzman, E.S., Rimi, A.,
2003. The ultimate arc: differential displacement, oroclinal bending, and ver-
tical axis rotation in the external Betic-Rif arc. Tectonics 3 (22), 1017. http://
dx.doi.org/10.1029/2001TC001321.
Platt, J.P., Behr, W.M., Johanesen, K., Williams, Jason R., 2013. The Betic-Rif Arc and
its orogenic hinterland: a review. Annual Review of Earth and Planetary Sci-
ences 41, 313e357.
Platt, J.P., Vissers, R.L.M., 1989. Extensional collapse of thickened continental lith-
osphere: a working hypothesis for the Alboran Sea and the Gibraltar arc. Ge-
ology 17, 540e543.
Platzman, E.S., Platt, J.P., Olivier, P., 1993. Palaeomagnetic rotations and fault kine-
matics in the Rif arc of Morocco. Journal of the Geological Society, London 150,
707e718.
Polyak, B.G., Fernàndez, M., Khutorskoy, M.D., Soto, J.I., Basov, I.A., Comas, M.C.,
Khain, V.Y., Alonso, B., Agapova, G.V., Mazurova, I.S., Negredo, A., Tochitsky, V.O.,
Linde, J. d. l., Bogdanov, N.A., Banda, E., 1996. Heat flow in the Alboran Sea,
western Mediterranean. Tectonophysics 263, 191e218.
Purdy, G.M., 1975. The eastern end of the Azores-Gibraltar plate boundary.
Geophysical Journal of the Royal Astronomical Society 43, 973e1000 .
Ranalli, G., 1996. Seismic tomography and mineral physics. In: Boschi, E.,
Ekström, G., Morelli, A. (Eds.), Seismic Modelling of the Earth Structure. Istituto
Nazionale di Geofisica, Rome, pp. 443e459.
Rosenbaum, G., Lister, G.S., Dubosz, C., 2002. Relative motions of Africa, Iberia and
Europe during Alpine orogeny. Tectonophysics 359, 117e129.
Royden, L.H., 1993. Evolution of retreating subduction boundaries formed during
continental collision. Tectonics 12, 629e638.
Sallarès, V., Gailler, A., Gutscher, M.-A., Graindorge, D., Bartolomé, R., Gràcia, E.,
Díaz, J., Dañobeitia, J., Zitellini, N., 2011. Seismic evidence for the presence of
Jurassic oceanic crust in the central Gulf of Cadiz (SW Iberian margin). Earth
and Planetary Science Letters. http://dx.doi.org/10.1016/j.epsl.2011.09.003.
Sallarès, V., Martínez-Loriente, S., Prada, M., Gràcia, E., Ranero, C.R., Gutscher, M.-A.,
Bartolomé, R., Gailler, A., Dañobeitia, J.J., Zitellini, N., 2013. Seismic evidence of
exhumed mantle rock basement at the Gorringe Bank and the adjacent
Horseshoe and Tagus abyssal plains (SW Iberia). Earth Planetary Science Letters
365, 120e131. http://dx.doi.org/10.1016/j.epsl.2013.01.021.
Sartori, R., Torelli, L., Zitellini, N., Peis, D., Lodolo, E., 1994. Eastern segment of the
AzoreseGibraltar line (centraleeastern Atlantic): an oceanic plate boundary
with diffuse compressional deformation. Geology 22, 555e558.
Seber, D., Barazangi, M., Ibenbrahim, A., Demnati, A., 1996. Geophysical evidence for
lithospheric delamination beneath the Alboran Sea and Rif-Betic mountains.
Nature 379, 785e790.
Serri, G., Innocenti, F., Manetti, P.,1993. Geochemical and petrological evidence of the
subduction of delaminated Adriatic continental lithosphere in the genesis of the
Neogene-Quaternary magmatism of central Italy. Tectonophysics 223, 117e147.
Serpelloni, E., Vannucci, G., Pondrelli, S., Argnani, A., Casula, G., Anzidei, M., Baldi, P.,
Gasperini, P., 2007. Kinematics of the Western Africa-Eurasia plate boundary
from focal mechanisms and GPS data. Geophysical Journal International 169,
118 0 e1200 . http://dx.doi.org/10.1111/j.1365-246X.2007.03367.x.
Sibuet, J.-C., Rouzo, S., Srivastava, S., 2012. Plate tectonic reconstructions and
paleogeographic maps of the central and north Atlantic oceans. Canadian
Journal of Earth Sciences 49, 1395e1415.
Simmons, N.A., Myers, S.C., Johannesson, G., Matzel, E., 2012. LLNL-G3Dv3: global P
wave tomography model for improved regional and teleseismic travel time
prediction. Journal of Geophysical Research 117, B10302. http://dx.doi.org/
10.1029/2012JB009525.
Srivastava, S.P., Roest, W.R., Kovacs, L.C., Oakey, G., Lévesque, S., Verhoef, J.,
Macnab, R., 1990. Motion of Iberia since the late Jurassic: results from detailed
aeromagnetic measurements in the Newfoundland Basin. Tectonophysics 184,
229e260. http://dx.doi.org/10.1016/0040-1951(90)90442-b.
Stich, D., Serpelloni, E., Mancilla, F., Morales, J., 2006. Kinematics of the Iberia-
Maghreb plate contact from seismic moment tensors and GPS observations.
Tectonophysics 426, 295e317.
Stich, D., Mancilla, F., Pondrelli, S., Morales, J., 2007. Source analysis of the February
12th 2007, Mw 6.0 Horseshoe earthquake: implications for the 1755 Lisbon
earthquake. Geophysical Research Letters 34, L12308. http://dx.doi.org/10.1029/
2007GL030012.
Tortella, D., Torne, M., Perez-Estauan, A., 1997. Geodynamic evolution of the Eastern
segment of the Azores-Gibraltar fracture zone: the Gorringe Bank and the Gulf
of Cadiz region. Marine Geophysical Research 19, 211e230.
S. Monna et al. / Geoscience Frontiers xxx (2014) 1e10 9
Please cite this article in press as: Monna, S., et al., Constraints on the geodynamic evolution of the AfricaeIberia plate margin across the
Gibraltar Strait from seismic tomography, Geoscience Frontiers (2014), http://dx.doi.org/10.1016/j.gsf.2014.02.003
Tucholke, B.E., Sibuet, J.-C., 2012. Problematic plate reconstruction. Nature Geo-
science 5, 676e677.
Turner, S.P., Platt, J.P., George, R.M.M., Kelley, S.P., Pearson, D.G., Nowell, G.M., 1999.
Magmatism associated with orogenic collapse of the BeticeAlboran Domain, SE
Spain. Journal of Petrology 40, 1011e1036.
Vissers, R.L.M., Meijer, P. Th, 2012. Iberian plate kinematics and Alpine collision in
the Pyrenees. Earth Science Reviews 114, 61e83.
Watts, A.B., Platt, J.P., Buhl, P., 1993. Tectonic evolution of the Alborán Sea basin.
Basin Research 5, 153e177.
Wessel, P., Smith, W.H., 1991. Free software helps map and display data. Eos,
Transactions American Geophysical Union 72 (41), 441e446.
Wilson, M., Bianchini, G., 1999. Tertiary-quaternary Magmatism within the Medi-
terranean and Surrounding Regions. In: Geological Society, London, Special
Publication 156, pp. 141e168 . http://dx.doi.org/10.1144/GSL.SP.1999.156.01.09.
Zeck, H.P., 1996. Betic-Rif orogeny: subduction of Mesozoic Tethys lithosphere un-
der eastward drifting Iberia, slab detachment shortly before 22 Ma, and sub-
sequent uplift and extensional tectonics. Tectonophysics 254, 1e16.
Zitellini, N., Gràcia, E., Matias, L., Terrinha, P., Abreu,M.A ., De Alteriis, G., Henriet, J.P.,
Dañobeita, J.J., Masson, D.G., Mulder, T., Ramella, R., Somoza, L., Diez, S., 2009.
The quest for the AfricaeEurasia plate boundary west of the Strait of Gibraltar.
Earth and Planetary Science Letters 280, 13e50. http://dx.doi.org/10.1016/
j.epsl.2008.12.005.
S. Monna et al. / Geoscience Frontiers xxx (2014) 1e1010
Please cite this article in press as: Monna, S., et al., Constraints on the geodynamic evolution of the AfricaeIberia plate margin across the
Gibraltar Strait from seismic tomography, Geoscience Frontiers (2014), http://dx.doi.org/10.1016/j.gsf.2014.02.003
