The quest for the Iberia-Africa Plate boundary west of
Gibraltar
SWIM Team: N. Zitellini1, E. Gràcia2, M.A. Gutscher3, L. Matias4, D. Masson5, T.
Mulder6, P. Terrinha7, L. Somoza8, G. DeAlteriis9, J.P. Henriet10, J.J. Dañobeitia2,
R. Ramella11, M.A. Abreu12 and S. Diez2
1
Istituto di Scienze Marine (ISMAR), Via Gobetti 101, 40129, Bologna, Italy
2
Unitat de Tecnologia Marina (CSIC), Centre Mediterrani d’Investigacions Marines
i Ambientals, Pg. Marítim de la Barceloneta 37-49, 08003, Barcelona, Spain
Université de Bretagne Occidentale/ Institut Universitaire Européen de la Mer,
3
UMR 6538 Domaines Océaniques, Place Nicolas Copernic, F-29280, Plouzané,
France
Centro Geofísica da Universidade de Lisboa (CGUL, IDL), Campo Grande C8,
4
1749-016 Lisboa, Portugal
Southampton Oceanography Centre, European Way, Southampton SO14 3ZH,
5
United Kingdom
Département de Géologie et Océanographie, UMR 5805 EPOC, Université
6
Bordeaux 1, Avenue des Facultés, 33405 Talence Cedex, France
National Institute for Engineering, Technology and Innovation (INETI, LATTEX),
7
Departamento de Geologia Marinha, Estrada da Portela, Zambujal, 2721-866
Amadora, Portugal
8
Geología Marina, Instituto Geológico y Minero de España, Rios Rosas 23, 28003
Madrid, Spain
9
Geomare Sud IAMC, CNR, Calata Porta di Massa, 80133, Napoli, Italy
10
Renard Centre of Marine Geology, Dpt. Geology and Soil Science, Gent
University, Krijgslaan 281 s.8, B-9000 Gent, Belgium
1
11
Department for the Development of Marine Technology and Research, Istituto
Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta
Gigante 42/c, 34010, SGONICO (TS)
12
Estrutura de Missão para a Extensão da Plataforma Continental, Rua Borges
Carneiro nº 38 2º Esq., 1200-619 Lisboa, Portugal
Abstract
A new swath bathymetry compilation of the Gulf of Cadiz Area and SW Iberia is
presented. The new map is the result of a collaborative research performed after
year 2000 by teams from 7 European countries and 12 research institutions. This
new dataset allow for the first time to unravel the Iberia-Africa plate boundary
West of Gibraltar and to better constrain the source location of the 1755 Lisbon
Earthquake, the largest earthquake ever occurred in Western Europe during
historical times.
One-sentence summary: The quest for the Iberia-Africa Plate boundary
Keywords: Gulf of Cadiz, Europe-Africa convergence, Diffuse Plate Boundary,
Earthquake, Tsunami, Compressive Deformation, active faulting, seismicity,
swath-bathymetry
Introduction
The present-day convergent plate boundary separating Northwest Africa from
Southwest Europe has been a matter of debate and it is still an unsolved puzzle
since the early plate tectonic reconstructions made by the pioneering work of
McKenzie (1970) (1) and Dewey et al. (1973) (2). As a matter of fact, it is
frequent to see this boundary depicted as a straight or zig-zag line passing
through the Straits of Gibraltar. The Euler pole of rotation of Africa with respect to
2
Eurasia lies at longitude ~20ºW1, between the Azores Triple Junction and the
Gibraltar Arc. The tectonic line that connects these two features is known as the
Azores-Gibraltar Line (AGL, Fig. 1), which roughly trends west-east. The
kinematics of the plate boundary are right-lateral with a divergent component in
the Azores plateau, right-lateral transform in the central segment, the Gloria
Fault, and convergent between this fault and the Gibraltar arc, where the plate
boundary is not well established because deformation is distributed over a broad
elongated area about 200 km wide (3).
In this easternmost segment, the seismicity is moderate, with only five events
with magnitude M=>6 since 1960, with well constrained hypocenters shallower
than 50 km (4). Focal mechanisms indicate thrust and strike-slip fault solutions
accommodated by NE-SW and E-W trending faults, respectivelly with a main
horizontal compression axis oriented NW-SE (Fig. 1) (5, 6, 7, 8, 9). Plate
convergence is estimated to be 3.7 mm/yr over the last three million years (10),
and geodetic measurements suggest values ranging between 3.8 to 5.6 mm/yr
(11, 12, 13). The eastern termination of the AGL plate boundary is buried by a
thick pile of deformed sediments that resemble an accretionary wedge related to
a subduction process (14) associated with the emplacement of the Gibraltar
orogenic arc., recently hypothesized to be still active (15).
The present-day puzzling Iberia-Africa plate boundary between the Gorringe
Ridge and the Gibraltar arc is now evaluated in this paper (Fig. 1). The definition
of this segment of the plate boundary and its associated deformation is essential
for understanding the large magnitude earthquakes here generated, such as the
28th February, 1969 (M~8.0) whose epicentre was located in the Horseshoe
Abyssal Plain (16) and the 1st November, 1755 Great Lisbon (M~8.7) earthquake
and tsunami (17, 18). This was the largest earthquake ever occurred in historical
times in Western Europe and its source area is still a matter of debate. Unveiling
1
Current GPS measurements allow the splitting of the African plate into two blocks, which are
identified as the Nubia plate, the largest one, and the Somalia plate. The GPS measurements mentioned
in the text are referred to the Nubia sub-plate.
3
the tectonics of this plate boundary is a necessary step for seismic hazard
assessment
and
future mitigation
of the effects of tsunamis and large
earthquakes occurring in the area.
The SWIM swath-bathymetry compilation
The Gulf of Cadiz area has been recently investigated by means of swathbathymetry
not
only
to
identify
the
possible
tectonic
sources
of
large
earthquakes, such as the 1755 Lisbon Earthquake, but also to investigate other
active geological processes, such as the effects of the Mediterranean Outflow
Water (MOW) on the seafloor, the fluid-flow processes associated with mud
volcanism, low-temperature venting and mass-wasting processes. The SWIM
project, “Earthquake and Tsunami hazards of active faults at the SouthWest
Iberian Margin: deep structure, high-resolution imaging and paleoseismic
signature”, funded by the Eurocores Project “EuroMargins” of the European
Science Foundation, promoted a collaborative research agreement to coordinate
marine cruises aiming at a complete bathymetry coverage of the Gulf of Cadiz.
The dataset consists of 19 surveys representing more than 200 days of ship time,
all performed between 2000 and 2006 by teams belonging to,14 research
institutions from 7 European countries. The covered area totals 1.5 times the
surface area of Portugal (Fig. 2 and Plate 1 in SOM). The surveys are of high
quality being, most of them, acquired under the specification established by the
International Hydrographic Organization for an order 3 hydrographic survey. A
larger version of the bathymetric map, at 100 meters grid resolution, is available
online as support material. This map provides for the first time the detailed
morphostructural information at the scale of the whole study area providing new
answers to old questions: What are the structures that take up the tectonic
deformation at present? Where is located the Iberia-Africa plate boundary?
4
Morphotectonics
The main morphological features of SW Iberia formed during the Miocene
compression caused by the northwards movement of Africa with respect to
Eurasia and the westwards migration of the Alboran terrain (19). Some of these
morphotectonic features were known to exist prior to the acquisition of the high
resolution SWIM bathymetric compilation. However, it was difficult to distinguish
between the Miocene and the Quaternary features and processes and it was not
possible to know the lateral extent of the main active faults because a detailed
map of the seafloor was lacking.
The physiography of the Gulf of Cadiz (Figs. 2 and 3) is dominated by i) deep
basins, as the Horseshoe and Seine Abyssal Plains, ii) elongated seamounts
located between these plains, the Gorringe Ridge, the Hirondelle seamount, the
Coral Patch Seamount, the Coral Patch Ridge and the Seine Abyssal hills; iii) the
escarpments and structural highs on the continental slope on the northern part of
the Gulf of Cadiz, such as the Marques de Pombal, Guadalquivir-Portimão,
Horseshoe and Pereira de Sousa Faults; iv) the smooth surface of the South
Iberian continental shelf and upper slope in the north; v) deeply incised valleys
and submarine canyons; vi) the contourite drifts and other bottom current
features caused by the MOW; vii) the wrinkled arcuate surface of the accretionary
wedge on the east and viii) a set of WNW-ESE trending lineaments that extend
from the Hirondelle across the Horseshoe Abyssal Plain, the accretionary wedge
and the Moroccan continental shelf. The full extent of the WNW-ESE trending
lineaments is approximately 600 km and was detected for the first time on the
SWIM bathymetry compilation. Hereafter these lineaments are designated by
SWIM Lineaments.
The Horseshoe and Seine Abyssal Plains correspond to oceanic crust of Late
Jurassic-Early Cretaceous age that resulted from separation of North America
from Eurasia and Africa. The positive reliefs lying at the edges of these abyssal
plains, such as the Gorringe Ridge, the Coral Patch Ridge, and the series of
5
Abyssal hills in the Seine Abyssal Plain, formed due to northwest directed
thrusting during the latest stages of the Iberia-NW Africa convergence, from Late
Cretaceous through Late Miocene times, and they show now minor activity when
inspected on multi-channel seismic profiles (see Plate 2 in SOM). The Hirondelle
seamount and the western part of the Coral Patch Ridge display a well expressed
NE-SW
trending
parallel
series
of
short
wavelength
ridges
and
valleys
corresponding to the seafloor spreading pattern of Early Cretaceous age (see Fig.
3). The Coral Patch Ridge was later intruded by volcanism that overprinted these
oceanic fabrics (red triangles in Fig. 3). While the Horseshoe Abyssal Plain hosts
an important part of the instrumental seismicity and the multi-channel seismic
profiles show active tectonic deformation, the Tagus Abyssal Plain and the Seine
Abyssal Plain are almost aseismic and seismic reflection data shows a major
Miocene discontinuity sealing the main compressive structures.
The plateaus of Marquês de Pombal, Sagres, Portimão and Guadalquivir Bank are
structural highs uplifted on top of basement thrust ramps during Miocene times
as well as the Horseshoe Fault (Fig. 3). These tectonic structures are active in the
present as shown by recent works (e.g. 7, 20, 21, 22). The 100 km long Pereira
de Sousa escarpment corresponds to the exposure of an active extensional fault
associated to continental uplift on top of a N-S trending reactivated main thrust
(23). Uplift of the western part of the Iberia Margin triggered the incision of the
submarine canyons together with the sea level oscillations.
The boundary of the accretionary wedge displays an arcuate shape, sub-parallel
to the Betic-Rif stacked thrusts of the Gibraltar Arc. The SWIM dataset reveals an
irregular, hummocky, rough topography of ridges and troughs, scours, channels,
circular ponds and isolated cones. The large amount of multi-channel seismic
profiles and high resolution seismic data, sidescan sonar and groundtruthing (21,
24, 25, 26, 27, 28) provided new insights into the variety of geological processes
that built up the Gulf of Cadiz accretionary wedge and are shaping its present-day
seafloor morphology. Multi-channel seismic profiles show a chaotic pattern of
6
variable amplitude seismic horizons and a series of imbricated horizons that were
interpreted as stacked thrusts detaching on top of a decollement surface, i.e. a
subduction accretionary wedge complex, located at the top of the Cretaceous
units (15). The arcuate shape of this thrust belt in front of the Rif-Betic orogenic
arc has been associated with a steep subduction slab dipping to the east based on
seismic tomography (15, 29). The accretionary wedge is topped by up to 500 m,
locally 2 000 m (21, 30), thick package of Uppermost Miocene to Quaternary well
stratified mildly to undeformed sediments indicating present-day reduced to nil
level of activity of the subduction processes (31). The widespread existence of
fluid escape features, mud volcanism, gas hydrates and gravitational collapses
account for the irregular and wrinkled appearance of this structure.
The WNW-ESE trending SWIM Lineaments occur from the Hirondelle Seamount,
cutting across the Horseshoe Abyssal Plain, the continental rise and accretionary
wedge through the Morocco shelf (Figs. 3 and 4). These lineaments are
distributed within a WNW-ESE striking, 600 km long x 80 km wide area, i.e. subparallel to the present day movement of Africa with respect to Iberia, as taken
from GPS measurements (11, 12, 13, 32). This area, which is interpreted here as
a zone of concentration of dextral strike.slip deformation, i.e. a shear zone, is
shown as a grey stripe in Fig. 3. This narrow stripe approximately coincides with
the trajectory of a small circle centered on the present-day Euler pole of rotation
of the Africa1 plate with respect to Iberia (Figs. 1 and 3, (11)). The SWIM
Lineaments occur on the seafloor as a sharp scarp on the Hirondelle-Gorringe
seamounts, as a series of aligned hills on the Horseshoe Abyssal Plain, as an
aligned series of ridges and troughs and associated en echelon folds on the slope,
aligned mud volcanoes and crests and valleys on the accretionary wedge surface,
and as scarps and valleys on the Morocco shelf edge (Fig. 4). Locally, the lack of
continuity of the lineaments in the Horseshoe Abyssal Plain is due to the fact that
the sedimentation rate overcomes the rate of fault slip (Fig. 5).
7
Detailed work on these lineaments in the Horseshoe Abyssal Plain and slope,
based on bathymetry analysis, multi-channel seismic profiles interpretation and
analogue deformation has shown that the SWIM Lineaments correspond to
dextral strike-slip faults (the SWIM Faults) cutting across the accretionary wedge
of the Gulf of Cadiz and that they are active during the last 2 My (31, 33).
Seismicity
The seismic strain in the Gulf of Cadiz is accommodated by structures located
north of the SWIM Fault Zone. The maximum values coincide with the active
morphotectonic features and faults shown in Figs. 3 and 6.
The Guadalquivir Bank- Portimao area concentrates most of the seismic strain of
the northern part of the Gulf of Cadiz. It is bound by an oblique-slip
southeastwards directed thrust fault that establishes the contact between the
inverted rifted continental South Iberia margin and the accretionary wedge,
underlain by thin crust (34).
Seismic strain also clusters around the south of the Gorringe Ridge and the
eastern part of the Horseshoe Abyssal Plain. Inspection of seismic reflection
profiles
and
multibeam
bathymetry
shows
that
the
Gorringe
Ridge
northwestwards directed frontal thrust has dramatically diminished its activity
since Late Miocene times and vertical WNW-ESE trending active faults developed
on its southern flank and in the Horseshoe Abyssal Plain, as shown in Plate 2 of
SOM. Among these, stand out the SWIM Faults (Fig. 4) that deform the seafloor
and the blind NE-SW trending thrusts that generated the 28th February 1969
(M~8) and 12th February, 2007 (M~6) events (9, 35).
Conclusion
Iberia-Africa: Two engines for a transitional plate boundary
8
The emplacement of the Betic-Rif orogen and the formation of the Gibraltar Arc
by means of roll-back of a subducting slab is a process that is probably
approaching to its end, since only minor thrusting is observed in the accretionary
wedge to affect the Pliocene-Quaternary cover. Most of the present-day
deformation offshore SW Iberia is now accommodated by westward thrusting on
the NE-SW to NNE-SSW faults, by oblique-slip on the E-W faults and by dextral
strike-slip on the WNW-ESE SWIM faults (Fig. 3). The SWIM Faults form a set of
parallel lineaments that cut across the whole region from the Hirondelle seamount
to the Moroccan shelf controlling the location of various mud volcanoes along
which deep seated fluidized sediments are extruded, whose origin can be at the
level of the accretionary wedge or at the Triassic evaporites (26, 31). The SWIM
Fault Zone also constitutes the southern edge of the main active tectonics and
seismicity of the Gulf of Cadiz, as mentioned.
The perceptible angular departure of the plate kinematic velocity of Africa with
respect to Iberia from the last 3 My to present day GPS velocities (Fig. 1) is
coherent with the development of the SWIM Faults in Pliocene-Quaternary times.
The clear difference between the NW-SE oriented maximum horizontal stress
inferred from the focal mechanisms and the WNW-ESE oriented present-day
direction of convergence derived from GPS measurements together with the low
of seismicity south of the SWIM Fault Zone is a strong indication of impingement
of the Africa plate into Iberia. This is causing propagation of deformation into SW
Iberia that is being partitioned on pre-existent NE-SW and E-W trending
structures, as thrusts and oblique-slip faults, respectively. This is in accordance
with the existence of a lithospheric vertical mechanical decoupling discontinuity,
the SWIM Fault zone.
It is thus concluded that the WNW-ESE trending SWIM Fault Zone is in the
process of connecting the Gloria Fault with the north Africa plate boundary along
the Rif-Tell and consequently will constitute a new plate boundary (see Fig. 6).
The new plate boundary is a consequence of a second engine taking over a
9
previous one, i.e. to the WNW-ESE Eurasia-Africa plate oblique convergence
taking over the roll-back of a subducted slab.
This scenario of plate boundary re-definition has important implications on the
understanding of the occurrence of large earthquakes and destructuive tsunamis,
such as the Lisbon 1755 event. First, it implies important fault growth in localized
areas, second, because during the transition from one plate boundary to a new
one, associated with the progressive switch of the subduction slab roll-back to the
oblique plate convergence, the two active fault systems will interact in a
unpredictable manner. Fault growth associated with a plate boundary propagation
implies non-steady state seismic rupture and the substitution of one plate driving
mechanism by a new one implies complex interactions.
Acknowledgements
We thank the captains, crews and technical staff on board research vessels for
their assistance throughout all the cruises that allowed to collect the data
presented here. We thank A. Kopf for allowing the use of the GAP bathymetric
data acquired during Sonne Cruise SO-175, Nov./Dec. 2003. We acknowledge
financial support from the ESF EuroMargins Program, contract n. 01-LEC-EMA09F
and from EU Specific Programme “Integrating and Strengthening the European
Research Area”, Sub-Priority 1.1.6.3, “Global Change and Ecosystems”, contract
n. 037110 (NEAREST), and national funding from MEC in Spain (SWIM REN200211234MAR, IMPULS, REN 2003-05996MAR and EVENT CGL2006-12861-C02-02
projects), Portugal (MATESPRO, PDCTM/P/MAR/15264/1999).. We
thank Vasco
Valadares for his valuable help on figures preparation. ISMAR-BO contribution
n.XXX
10
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Figure captions:
Fig.1- Shaded bathymetry map of Iberia, northwest Africa and Central Atlantic
(36) with sketches of the main elements of plate boundaries: AGL: AzoresGibraltar Line; GC: Gulf of Cadiz; GF: Gloria Fault; MAR: Mid-Atlantic Ridge; TR:
Terceira Ridge. Solid yellow line: plate boundaries from Bird, 2003. Box for Fig. 2:
location of the study area. Small red circles: epicentres, from ISC, M>4, 1964 to
12
Present, http://www.ISC.ac.uk. Focal mechanisms from CMT catalogue for whole
area,
http.//www.seismology.harvard.edu;
between
20º
W
–
5º
W
data
completed from various sources, see SOM material. Arrows at right bottom corner
show the relative movement of Nubia with respect to Eurasia at the centre of the
Gulf of Cadiz, according to different authors. Black arrows deduced from
geological indicators (10; 37) and red arrows from GPS (11, 12, 13, 32). Inset:
location of the Euler pole and the relative movement of Nubia with respect to
Eurasia after Fernandes et al., 2003 (11).
Fig.2- The SWIM Multibeam Compilation. Swath Bathymetry map compiled
on behalf the SWIM collaborative research agreement, see SOM for complete list
of contributors. Color scale in meter.
Fig.3 Tectonic Map. It is derived from the swath bathymetry map presented on
Fig.2 with toponimics and structural interpretation. Black thick lines: location of
multi-channel seismic lines shown in SOM; gray stripe: 80 km wide, centered at
the small circle relative to Euler pole of rotation of Africa with respect Eurasia
inferred by Fernandes et al., 2003 (11); red lines with triangles: active reverse
faults; blue lines with triangle: inactive reverse faults; blue lines with rhombs:
axis of inactive anticline; short, close-spaced red lines: lineaments related to
accretion of oceanic crust; violet lines: Cretaceous normal faults; long, WNW-ESE
oriented, red lines: SWIM Lineaments; red triangles: outcrop of volcanic edifices;
red dots: salt diapirs; blue dots: major depressions larger that within the
accretionary wedge; black thin lines: oceanic magnetic lineation with chrons
alongside.
Fig.4- SWIM Lineament. Shaded relief image of the Swath Bathymetry map
showing an example of one of the SWIM Lineaments running from the Horseshoe
Abyssal Plain through the accretionary wedge;
The view is from Horseshoe
13
Abyssal Plain from the WNW and 45 degree elevation; the figure clearly shows
the positive and negative features and scarps arranged along the SWIM
Lineament.
Fig.5 Multi-channel seismic line crossing SWIM Lineament shown in
Fig.4. Blow-up of multi-channel seismic line AR07; location is in Fig.2, the full
seismic line and its interpretation is shown in the SOM. This portion of AR07
crosses the SWIM Lineament in the Horseshoe Abyssal Plain. Despite the fact that
there is no morphological expression of the lineament this profile shows that the
active deformation in the Present continues also in the flat part of the Horseshoe
Abyssal Plain.
Fig.6 Conceptual model of the present Africa-Europe Plate Boundary.
Red line: proposed SW Eurasia-NW Africa plate boundary, as a small circle
centered in Euler pole DEOS2k (11) coincident with the SWIM Lineaments; solid:
where it is accompanied by instrumental seismicity; dashed: where it lacks
seismicity and is in the process of propagation to connect with the Rif-Tell plate
boundary, in black; dotted, connection of the SWIM Fault/plate boundary with the
Gloria Fault (see text for discussion). Colour contours: seismic strain rate
computed from all events, open circles, 3<M<5.5. Star: events M>5.5 since
1964. Bathymetry from Smith and Sandwell, 1997 (36). Solid thin black lines:
SWIM Lineaments.
Supporting online material (SOM):
Plate1 Bathymetry of the Gulf of Cadiz, North-East Atlantic: The SWIM
Multibeam Compilation. Bathymetry map, as in Fig.2 of the paper, designed to
14
be printable at A0 format with a resolution that will allow the recognition and
detection of the features discussed in the text.
Plate 2 Multi-channel seismic lines transect. Selection of multi-channel
seismic lines, AR03, AR07 and AR08, encompassing the whole area with a strike
parallel to the Miocene slip vector of Africa-Europe Plate Motion. Location is in
Fig.2. Interpretation goes along with the data. This figure is designed to be
printable at A1 format with a resolution that will allow the interpretation of the
lines.
File n.1 SWIM digital map at 250 grid-internal resolution on zipped file.
The data in the file are organized as ascii XYZ column allowing reader to print his
own image and or to merge it with his own data.
15