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Earth and Planetary Science L
Active faulting offshore SE Spain (Alboran Sea): Implications for
earthquake hazard assessment in the Southern Iberian Margin
Eulalia Gracia a,*, Raimon Pallas b, Juan Ignacio Soto c, Menchu Comas c,
Ximena Moreno a,b, Eulalia Masana b, Pere Santanach b, Susana Diez a,
Margarita Garcıa d, Juanjo Danobeitia a
HITS scientific party 1
a Unitat de Tecnologia Marina, Centre Mediterrani d’Investigacions Marines i Ambientals-CSIC, 08003 Barcelona, Spainb RISKNAT Research Group, Dpt. Geodinamica i Geofısica, Facultat de Geologia, Universitat de Barcelona, 08028 Barcelona, Spain
c Instituto Andaluz de Ciencias de la Tierra (CSIC)-Universidad de Granada, 18002 Granada, Spaind Institut de Ciencies del Mar, Centre Mediterrani d’Investigacions Marines i Ambientals-CSIC, 08003 Barcelona, Spain
Received 17 May 2005; received in revised form 30 October 2005; accepted 7 November 2005
Available online 27 December 2005
Editor: V. Courtillot
Abstract
The southern margin of the Iberian Peninsula hosts the convergent boundary between the European and African Plates. The area
is characterised by low to moderate magnitude shallow earthquakes, although large historical events have also occurred. In order to
determine the possible sources of these events, we recently acquired swath-bathymetry, TOBI sidescan sonar and high-resolution
seismic data on the Almerıa Margin (Eastern Alboran Sea). The new dataset reveals the offshore continuation of the NE–SW
trending Carboneras Fault, a master fault in the Eastern Betic Shear Zone, and its associated structures (N150 and NS faults). These
structures are active since they cut the Late Quaternary sedimentary units. The submarine Carboneras Fault zone is 100 km long, 5–
10 km wide, and is divided into two N045 and N060 segments separated by an underlapping restraining stepover. Geomorphic
features typically found in subaerial strike-slip faults, such as deflected drainage, water gaps, shutter ridges, pressure ridges and benechelonQ folds suggest a strike-slip motion combined with a vertical component along the submarine Carboneras Fault. Considering
the NNW–SSE regional shortening axis, a left-lateral movement is deduced for the Carboneras Fault, whereas right-lateral and
normal components are suggested for the associated N150 and NS faults, respectively. The offshore portion of this fault is at least
twice as long as its onshore portion and together they constitute one of the longest structures in the southeastern Iberian Margin.
Despite the fact that present day seismicity in the Almerıa margin seems to be associated with the N150 to NS faults, the
Carboneras Fault is a potential source of large magnitude (Mw ~7.2) events. Hence, the Carboneras Fault zone could pose a
0012-821X/$ - s
doi:10.1016/j.ep
* Correspondin
Marıtim de la B
E-mail addre1 The HITS sc
Alpiste (IACT-C
Spain), P. Blond
P. Terrinha, J. G
etters 241 (2006) 734–749
ee front matter D 2005 Elsevier B.V. All rights reserved.
sl.2005.11.009
g author. Unitat de Tecnologia Marina-CSIC, Centre Mediterrani d’Investigacions Marines i Ambientals (CMIMA). Passeig
arceloneta, 37-49, 08003 Barcelona, Spain. Tel.: +34 93 230 95 36/+34 93 230 95 00; fax: +34 93 230 95 55.
ss: egracia@utm.csic.es (E. Gracia).
ientific party: R. Bartolome, M. Farran, (CMIMA-CSIC, Barcelona, Spain), M. Gomez (ICTJA-CSIC, Barcelona, Spain), M.J.R.
SIC, Granada, Spain), G. Lastras, V. Wilmott (DEPGM-Universitat de Barcelona, Spain), H. Perea (DGG-Universitat de Barcelona,
el, O. Gomez (University of Bath, UK), L. Bullock, C. Jacobs, I. Rouse, D. White, S. Whittle (National Oceanography Centre, UK),
afeira, C. Roque (Universidade de Lisboa, Portugal).
E. Gracia et al. / Earth and Planetary Science Letters 241 (2006) 734–749 735
significant earthquake and tsunami hazard to the coasts of Spain and North Africa, and should therefore be considered in any
hazard re-evaluation.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Alboran Sea; Carboneras Fault zone; strike-slip faulting; submarine channels; seismicity
1. Introduction
The present-day crustal deformation of the south-
eastern Iberian margin, which includes the Iberian
Peninsula and adjacent offshore Mediterranean region,
is driven mainly by the NW–SE convergence (4–5 mm/
yr) between the African and Eurasian plates [1,2] (Fig.
1). This convergence is accommodated over a wide
deformation zone with significant seismic activity
south of the Iberian Peninsula [4,12,13]. Seismicity is
mainly characterised by low to moderate magnitude
events. Nevertheless, large destructive earthquakes
(MSK Intensity IX–X) have occurred in the region, as
has been revealed by historical records [8,14] (Fig. 1).
This may pose significant earthquake and tsunami
hazards to the coasts of Spain and North Africa.
The Neogene and Quaternary faulting activity in the
southeastern Iberian Margin is dominated by a large
left-lateral strike-slip system of sigmoid geometry re-
ferred to as the Eastern Betic Shear Zone (EBSZ)
[15,16]. The active fault system stretches over more
than 450 km from Alacant to Almerıa, and includes,
from north to south, the Carrascoy, Bajo Segura,
Alhama de Murcia, Palomares and Carboneras faults
[e.g. 15,17]. The Carboneras Fault and terminal splays
of the EBSZ extend further into the Almerıa margin in
the Alboran Sea [e.g. 18–20], towards the area where
the submarine epicentres of the historically recorded
earthquakes of Almerıa (1522), Dalıas (1804), and
Adra (1910) are located [8,9,21] (Fig. 1). Despite the
potential contribution of the Carboneras Fault zone to
the earthquake and tsunami hazard, little is known of its
detailed shallow structure and geometry offshore.
In order to investigate the offshore prolongation of
the Carboneras Fault and associated seismogenic struc-
tures, we carried out a high-resolution marine
geophysical survey of the Almerıa Margin in the NE
Alboran Sea (Fig. 1). The main objectives of this study
are 1) to image and characterise the seafloor geomor-
phology and the geometry of active tectonic structures
identified offshore Almerıa (Spain), such as the sub-
marine continuation of the Carboneras Fault zone; 2) to
study how these structures affect the present-day phys-
iography of the Almerıa Margin, such as the drainage
system of the Almerıa Canyon; 3) to investigate how
they accommodate the present-day strain regime along
the Eurasian-African plate convergence; and 4) to high-
light the relevance of the Carboneras Fault to seismic
hazard assessment of south Iberia, bearing in mind that
high magnitude earthquakes with long recurrence inter-
vals (104 years) have been documented in the EBSZ
[11].
2. Geological and geodynamical setting of the
southeastern Iberian Margin
The Betic and Rif Cordilleras linked by the Gibraltar
Arc constitutes the westernmost end of the Mediterra-
nean Alpine ranges at the boundary between the Afri-
can and European plates (Fig. 1). The Betic Cordillera
is an ENE–WSW–trending fold-and-thrust belt com-
posed of the External Zone, Mesozoic to Tertiary
rocks corresponding to the Iberian Plate paleomargin,
sedimented on top of the Variscan basement, and the
Internal Zone (or Alboran domain), consisting of a
thrust stack of metamorphic complexes (Fig. 1) [e.g.
16,22. Superimposed onto the previous structure, Neo-
gene to Quaternary sediments fill the intramontane
basins, limited by E–W and NE–SW faults [23]. Middle
Miocene to Pleistocene calc-alkaline through K-rich
volcanic rocks crop out in the Cabo de Gata area and
also in submarine structures, such as the Alboran Ridge
[24] (Fig. 1).
Active deformation in the southeastern Iberian Mar-
gin is revealed by earthquake mechanisms derived from
moment tensor inversion, covering both onshore and
offshore regions [4–7] (Fig. 1). In the central Betics and
southern Spain, focal mechanisms are heterogeneous
and faulting style ranges from normal to reverse, where-
as along the EBSZ and Alboran Sea, strike-slip faulting
dominates [6] as corroborated by geological and geo-
physical data [11,15,17–19,25,26] (Fig. 1). Regional
seismicity in the Ibero-Maghrebian region is diffuse
and does not clearly delineate the present-day Europe-
an-African plate boundary [4,13]. In the southeastern
Iberian Margin, instrumental seismicity is characterized
by continuous, shallow seismic events of low to mod-
erate magnitude (Mwb5.5) [4,6,8].
Although large earthquakes (MSK IntensityNVIII)
are relatively infrequent, they have occurred in the
Fig. 1. Regional bathymetric map of the southeastern Iberian Margin (contour interval: 200 m) constructed from the predicted bathymetry of [3].
Fault plane solutions of small to moderate earthquakes are from [4–7]. The largest historical earthquakes (MSK IntensityN IX) that occurred in the
region are located and depicted by a white star [8]. Location and moment tensor solution obtained for the 1910 Adra Earthquake (mechanism plotted
in gray) is based on [9]. Oppositely pointing grey arrows show the direction of convergence between the Eurasian and African plates from NUVEL1
model [1,2]. Geological domains of the eastern Betic Cordillera with the main Neogene faults are summarized from [10,11]. Indicated fault
movements correspond to late Neogene. CRF: Crevillente Fault, AMF: Alhama de Murcia Fault, PF: Palomares Fault, CAFZ: Corredor de las
Alpujarras Fault Zone, CF: Carboneras Fault. The black outlined rectangle depicts the study area corresponding to Fig. 2. Inset: Plate tectonic
setting and main geodynamic domains of the south Iberian Margin along the boundary between the Eurasian and African Plates. The black outlined
rectangle corresponds to the area depicted in this figure.
E. Gracia et al. / Earth and Planetary Science Letters 241 (2006) 734–749736
region [8,11,21]. Historical and archaeological records
of events suggest that this region has been affected by
at least 50 destructive earthquakes over the past 2,000
years, providing evidence of a significant seismic haz-
ard [21]. The town of Almerıa was devastated by earth-
quakes in 1487, 1522 (IX MSK), 1658 (VIII MSK), and
1804 (IX MSK), and Vera was destroyed in 1518 (IX
MSK) and 1863 (VII MSK) [4,8,21] (Fig. 1). These
events have been attributed to motion along the Carbo-
neras and Palomares fault systems [27]. On the other
hand, the 1910 Adra Earthquake (Mw=6.1, MSK
I0=VIII in Adra) occurred offshore (Fig. 1) probably
generated by N120–N130 trending faults consistent
with moment tensor calculations [9].
E. Gracia et al. / Earth and Planetary Science Letters 241 (2006) 734–749 737
At a regional scale, the Carboneras Fault system is
straight and continuous: its northern end links with the
NS trending Palomares Fault [15,17,18,27,28], and its
southern end, to the east of the town of Almerıa and near
Cabo de Gata, enters the Mediterranean Sea [15,18,20]
(Fig. 1). The Carboneras Fault is probably the largest
brittle structure in the EBSZ, and it could link with other
faults to form the Trans-Alboran Shear Zone, connecting
the Betics with the North African Rif [18]. The exposed
portion of the Carboneras Fault zone is about 50 km long
with a width ranging from 1.4 to 2.5 km [27,29]. Three
main segments (northern, central, and southern) have
been distinguished along the Carboneras Fault on land
based on relationships between geomorphic and tectonic
features [30]. Paleostress analysis indicates that the Car-
boneras Fault has been affected by a rotation in the
maximum horizontal stress direction from NW–SE to
N–S, which took place at the end of the Miocene [31].
Keller et al. [27] suggested that since the Burdigalian,
left-lateral strike-slip motion has been in the range of 30–
40 km and vertical offset in the range of 5–6 km.
The study area is located in the Adra-Almerıa Mar-
gin, comprising the offshore continuation of the Carbo-
neras Fault and associated structures to the NE Alboran
Sea (Fig. 1). The Almerıa continental shelf is narrow
(4–6 km) along much of this margin. Its main geomor-
phologic features have been recently revealed by swath-
mapping in the frame of the Program of Spanish Con-
tinental Shelf Cartography (ESPACE) [32]. This margin
is characterised by deeply incised canyons with a com-
plex pattern of tributary valley systems [33]. The
Almerıa Canyon drains from the shelf edge to the
deep-sea fan deposits in the Alboran Basin, which
exceeds 2000 m in depth [34,35]. The lowermost part
of the Almerıa Canyon was insonified using the MAK-
1 sidescan sonar system disclosing individual elements
of its channel morphology [36]. The Plio-Quaternary
sedimentary architecture of the NE Alboran Sea, based
on sparse single-channel seismic profiles, shows a cy-
clic pattern of deposition with alternating terrigenous
input into the basin related to sea-level oscillations
[34,35]. Deep multichannel seismic reflection data to-
gether with commercial wells and ocean drilling
revealed the regional tectonic-sedimentary evolution
and crustal structure of the margin and the eastern
Alboran Basin [19,20,26,37–39].
3. Data and methods: The HITS cruise
The present study results from an integration of dif-
ferent types of data: deep-towed sidescan sonar back-
scatter, swath-bathymetry, and high-resolution seismic
profiling, acquired during the HITS cruise (bHigh Res-
olution Imaging of Tsunamigenic Structures of the
Southern Iberian MarginsQ) on board the Spanish RV
Hesperides (Fig. 2). High-resolution sidescan sonar data
were collected using the Towed Ocean Bottom Instru-
ment (TOBI) [40] from the National Oceanography Cen-
tre, Southampton (UK) as part of the programme
European Access to Seafloor Survey Systems (EASSS
III). The TOBI sidescan survey consisted of six parallel
tracks, 6-km wide swath insonifying most of the area
bounded by latitude 36817V–36840VN and longitude
3805V–2810VW (Fig. 2b). The acoustic response (intensi-
ty of the backscattered signal) depends on the incidence
angle, roughness/topography and on the nature of the
seafloor. Reflective surfaces, such as high relief areas,
rock outcrops and landslides are depicted as light-grey to
white, whereas less reflective surfaces, such as low-relief
and fine-sediment covered areas, are dark-grey. Acoustic
shadows are black. The TOBI sidescan data, with an
along-track resolution of 6 m [40] enabled us to recog-
nise the seafloor superficial features in detail.
The swath-bathymetric data were collected by using
the Simrad EM12-S multibeam system covering an area
of about 35�100 km (Fig. 2a). Data gaps have been
filled using the Simrad EM300 bathymetric grid ac-
quired by the bInstituto Espanol de OceanografıaQ forthe Spanish Fisheries Office. Water depth ranges from
80–100 m at the top of the banks and shelf break down to
more than 1800m at the base of the slope. Simultaneous-
ly acquired and amounting to about 450 km of profiles,
high-resolution (1–5.5 kHz) Simrad TOPAS (TOpo-
graphic PArametric Sonar) seismic profiler provides de-
tailed stratigraphic information on the uppermost tens of
metres below the seafloor (50 to 80 m at an assumed
sediment velocity of 1.5 km/s). These data provide new
insights into the control of neotectonic structures over
the Plio-Quaternary sedimentary architecture of the mar-
gin. The seismic units recognised in the high-resolution
profiles correspond to Quaternary-age sediments based
on correlation with oil exploration wells and single-
channel seismic reflection profiles of the Almerıa Mar-
gin [19,20,35,36]. Similar high-resolution methodolo-
gies proved to be useful in exploring submarine
earthquake geology in other active areas along the Eur-
asian-African plate boundary, such as the Marmara Sea
[41] and the southwestern Iberian Margin [42].
4. Results
The main morphostructural elements of the Almerıa
Margin are identified in Fig. 2a. They correspond to 1)
the Almerıa Canyon and Channel, a meandering turbi-
Fig. 2. (a) Colour shaded relief bathymetric map of the Almerıa Margin surveyed during the HITS 2001 cruise on board the RV Hesperides. Contour
interval is 50 m. Main morphostructural features are identified. (b) High-resolution TOBI sidescan sonar of the same area. The white outlined boxes
depict the areas presented in Figs. 3a and 4a.
E. Gracia et al. / Earth and Planetary Science Letters 241 (2006) 734–749738
dite system confined between large topographic highs,
2) a dense tributary valley network, 3) a N045–N060
prominent lineation, which corresponds to the subma-
rine continuation of the Carboneras Fault, and 4) the
Cabo de Gata, Sabinar, and Chella Banks.
4.1. The Almerıa Canyon and Channel
A high-resolution map of the northern part of the
Almerıa Canyon is illustrated in the TOBI image (Fig.
3a). The northern Almerıa Canyon is more than 3 km
wide (talweg width of 380 m) and shows abrupt west
flanks (up to 300 m high), well developed meanders
and terraces (Fig. 3a). Some portions along its path
show high reflectivity, probably due to the presence
of coarse-grained talweg deposits. The net direction of
the Almerıa Canyon is roughly parallel to the subma-
rine continuation of the Carboneras Fault, located fur-
ther west (Fig. 3a). To the west and to the east of the
Almerıa Canyon there is a network of tributaries,
Fig. 3. (a) TOBI sidescan sonograph of the northern part of the Carboneras Fault zone and Almerıa Canyon. The canyon talweg deepens from 550 to 1050 m from the top to the bottom of the image,
with a mean slope of 2% and sinuosity of 1.51. Thick dashed line depicts location of (b). TVS: Tributary Valley System. (b) High-resolution seismic profile TA6 across the northern part of the
Almerıa Canyon and Carboneras Fault. Vertical scale in m is calculated by applying a velocity of sound propagation in water of 1500 m/s. Vertical exaggeration ~40.
E.Gracia
etal./Earth
andPlaneta
ryScien
ceLetters
241(2006)734–749
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E. Gracia et al. / Earth and Planetary Science Letters 241 (2006) 734–749740
corresponding to the Dalıas and Gata Tributary Valley
Systems [33], respectively. The denser Dalıas Tribu-
tary Valley System is composed of tens of linear
gullies and channels, which depart from the shelf
edge, mostly following the straight-line slope of the
margin, trending N150 to N170. At their intersection
with the Carboneras Fault lineation, the tributaries
show abrupt changes in stream direction (e.g. gullies
A to D, Fig. 3a).
A high-resolution seismic profile crossing the
northern part of the TOBI image illustrates the shallow
structure and architecture of the upper stratigraphic
units (Fig. 3b). To the west, the Dalıas Tributary
Valley System depicts channel-levee systems and
gullies showing different degrees of incision, ranging
from a few metres to 45 m deep. The profile shows a
gentle upwarp which is bounded to the west by the
Carboneras Fault. It also shows how the Carboneras
Fault cuts and folds the sedimentary succession up to
the most recent units. A thick (~18 m) mass-wasting
deposit characterised by chaotic seismic facies is lo-
cated at the base of the surface break, and is thinly
Fig. 4. (a) TOBI sidescan sonograph of the southern part of the Almerıa C
Canyon, the talweg deepens from 1030 to 1220 m with a gentle slope of 1.8%
sinuosity of 1.95, with the talweg deepening from 1220 m to more than 1630
dashed lines depict the locations of (b) and (c). (b) High-resolution seismic p
is imaged. C-L: Channel-levee system. (c) High-resolution seismic profile
applying a velocity of sound propagation in water of 1500 m/s. Vertical ex
draped by the uppermost sedimentary units (Fig. 3b).
To the east are the northern Almerıa Canyon and the
Gata Tributary Valley System, depicting well stratified
reflectors incised by channel-levee complexes.
The southern part of the Almerıa Canyon runs from
36831V to 36826.5VN, as revealed by the TOBI sidescan
sonar mosaic and the swath-bathymetric map (Figs. 2a
and 4a). The southern Almerıa Canyon is narrower
(average width of 1.5 km) and rectilinear, showing a
net N038 trend. Several terraces are identified on the
western flank of the canyon. The lower part of the
Dalıas Tributary Valley System drains into the southern
Almerıa Canyon, where some sections of the gullies are
rectilinear, developed along N150 trending lineations
(Fig. 4a). One of these lineations is visible at depth on a
high-resolution seismic profile crossing the southern
part of the Almerıa Canyon (Fig. 4b). The lineation
corresponds to a transparent-facies high-angle zone
which separates areas where the stratified units show
contrasting thicknesses, dipping attitudes and local an-
gular relationships. This lineation is interpreted as a
NE-dipping normal fault which deforms the whole
anyon and the Almerıa Channel. In the southern part of the Almerıa
, yielding a low sinuosity of 1.05. The Almerıa Channel shows a high
m and an average slope of 3.8%. TVS: Tributary Valley System. Thick
rofile TA3 across the Almerıa Canyon, where the ~N150 trending fault
TA1 across the Almerıa Channel. Vertical scale in m is calculated by
aggeration ~40.
E. Gracia et al. / Earth and Planetary Science Letters 241 (2006) 734–749 741
sedimentary succession up to the surface, favouring the
development of a rectilinear channel of the Dalıas
Tributary Valley System (Fig. 4b). The prolongation
of this fault towards the southeast reaches the Almerıa
Canyon, and probably facilitates the sharp and straight
morphology of one of its meanders. The faults striking
N150, where Dalıas Tributary Valley System gullies
develop, are designated by the general term of bN150faultsQ (Figs. 3a and 4a).
The Almerıa Channel is the tight meandering agra-
dational channel-levee turbidite system [34–36] located
downstream from the canyon, characterised by a rela-
tively high acoustic backscatter (Fig. 4a). The new full
coverage high-resolution dataset reveals the following
features. Four large abandoned meanders are observed
on the eastern flank of the channel, while narrow
terraces and outside meander furrows are identified on
its western flank (Fig. 4a). South of 36822VN, the
Almerıa Channel changes orientation from a general
NS to N120 trend and varies in sinuosity, general
morphology and infill (Fig. 4a). Towards the lower
part of the image, the channel narrows, is less deeply
incised, and shows an asymmetric transverse profile,
with higher levees on its western flank (Fig. 4b, c).
Incised into well-stratified, non-disturbed sedimentary
units, chaotic facies at the axis of the channel are
interpreted as the effect of coarse-grained bedload
(Fig. 4b). At the eastern end of the profile, transparent
irregular deposits overlie the stratified units of the
Almerıa Channel, corresponding to a landslide deposit
sourced southwest of the Sabinar Bank (Fig. 4c).
4.2. The Carboneras Fault zone
The swath-bathymetric data allow us to identify a
more than 70 km long and 5 km wide lineation striking
N045–N060, which corresponds to the surface expres-
sion of the Carboneras Fault (Figs. 2a and 5a). We
distinguish two main segments along the lineation:
the northern (33 km long, N045-trending), and the
southern (26 km long, N060-trending). Between these
two segments of the Carboneras Fault there is an area of
complex morphology formed by a narrow (b1 km)
slightly up-slope concave arcuate ridge (referred to as
ridge AR) which defines an elongated depression to the
northwest and is flanked by an abrupt escarpment to the
southeast (Figs. 2a and 5b profile TA3).
North of 36836VN, the northern segment of the
Carboneras Fault is rectilinear showing a NW-facing
escarpment evidenced by the sharp backscatter contrast
on the TOBI sonograph, up to 10 m high (Fig. 3a).
From 36836V to 36831VN, the Carboneras Fault inter-
sects the Dalıas Tributary Valley System producing a
sharp deflection of its channels and gullies (Figs. 2, 3a
and 5a). For instance, Gully D is deflected towards the
southwest at the Carboneras Fault and follows this
lineation for more than 2 km (Fig. 3a). Then, it
bends sharply at a right-angle towards the southeast
following, for more than 4 km, the superficial trace of a
N150 high-angle fault. Similar geometries and trends
are displayed by gullies A, B and C (Fig. 3a). The
southern segment of the Carboneras Fault shows a step-
like morphology to the west (Fig. 5b, profile TA1),
which gradually passes to a subdued irregular ridge to
the east (referred to as ridge SR, profile TA2 in Fig.
5b). East of ridge SR, the seafloor shows a series of
10–15 km long and ~6 km wide undulations consis-
tently trending N120–130 (Fig. 5a). Based on the
TOPAS seismic profiles (Fig. 5b), these are interpreted
as widely spaced folds affecting the sedimentary suc-
cession (Fig. 6).
The succession of six high-resolution seismic pro-
files across the Carboneras Fault lineation, from the
shelf to the base of slope, illustrates how its shallow
structure varies along-strike (Fig. 5b). Near the shelf
area, well stratified units are folded and cut by subver-
tical faults which separate blocks of different stratigra-
phy and structure forming a positive flower structure
(Fig. 5b, profile BA8). In the area intersecting the
Dalıas Tributary Valley System, the distinction of faults
is less clear-cut owing to the gully and channel inci-
sions which mask the structure at depth. However, the
general geometry of the Carboneras Fault zone also
coincides with a seafloor upwarp (Fig. 5b, profiles
TA4 and TA5). Profile TA3 crosses ridge AR where
stratified units are folded in anticline which we interpret
as a pressure ridge. To the west, a small basin is
underlain by a gently folded thick stratified succession.
This basin is separated from ridge AR by tightly spaced
high-angle faults, some of which appear to be reverse
(Fig. 5b, profile TA3). East of ridge AR, a 30 m high
escarpment coinciding at depth with a high-angle fault
limits with gently folded strata. Towards the base of the
slope, profiles TA2 and TA1 also show folded elevated
areas limited by subvertical discontinuities interpreted
as faults (Fig. 5b).
4.3. The Chella and Sabinar banks
Prominent highs, such as the Chella, Sabinar, Pollux
and Cabo de Gata spur dominate the physiography of
the margin (Figs. 2 and 5a). They are composed of a
Neogene volcanic basement topped by carbonate plat-
forms [19,24]. The banks, shallowing up to 80 m, are
Fig. 5. (a) Slope map overlain on top of the 3D swath-bathymetry data of the Carboneras Fault zone, view from the southwest. The white thick lines
depict location of profiles (BA8 to TA8) presented on (b). (b) Succession of high-resolution seismic profiles across the Carboneras Fault zone, from
the shelf to the base of the slope depicting along-strike morphostructural variability. AR: arcuate elevated ridge, SR: subdued ridge. Vertical scale in
m is calculated by applying a velocity of sound propagation in water of 1500 m/s. Vertical exaggeration ~40, except for profile BA8, which is ~55.
E. Gracia et al. / Earth and Planetary Science Letters 241 (2006) 734–749742
Fig. 6. Structural synthesis of the offshore Carboneras Fault zone with the main segments and associated structures identified, overlying a 10 m
contour bathymetric map. See text for explanation.
E. Gracia et al. / Earth and Planetary Science Letters 241 (2006) 734–749 743
tens of kilometres in diametre and several hundred
metres higher than their surroundings, and are easily
identified on the TOBI sidescan image as very high
reflectivity areas (Fig. 2b). On the Chella Bank, the
circular shape of the old-volcanic edifice is distinguish-
able (Fig. 2). Moats mainly developed on the eastern
flank of the Chella Bank and the northern flank of
Sabinar provide evidence of contour currents. Of espe-
cial interest is the presence of deeply carved scars to the
south of the Sabinar and Pollux banks, suggesting
dismantlement of the margin by large and complex
mass movements (Fig. 5a). The scars, up to 200 m of
headscarp height, cover an area of about 20 km2 with
run out distances of more than 10 km. Steep slopes (up
to 25%, Fig. 5a) and friable lithologies of the volcanic
edifices may favour large mass movements along this
part of the margin. Localized individual slide lobes are
observed on the western flank of the Sabinar Bank and
on the southern flank of the Chella Bank. To the west of
the Chella Bank we identify several NS morphological
ruptures on the seafloor, showing prominent normal
fault scarps, based on swath-bathymetry and high-res-
olution seismic profiles (Fig. 5a).
5. Discussion
5.1. Interplay between submarine channels and faults:
Control on the sedimentary architecture of the Almerıa
Margin
According to the bathymetric and high-resolution
seismic data, the superficial expression of the Carbo-
neras Fault zone mainly consists of an upwarped nar-
row area, bounded by subvertical faults at depth. The
elevation of the seafloor associated with this structure is
responsible for the deflection of the Dalıas Tributary
Valley System. Deflections to the southwest are inter-
preted as the result of drainage-blocking produced by
the Carboneras Fault seafloor upwarps, which act as
shutter ridges (see Section 4.1, Fig. 6). As pointed out
by [43], the deflection of streams should be approached
with caution when determining the direction of lateral
displacement along directional faults. Despite the mis-
leading impression of dextral fault bbayonetsQ (offset
drainage), the consistent deflections to the southwest
should be interpreted as resulting from the relationship
between the direction of the Carboneras Fault drainage-
E. Gracia et al. / Earth and Planetary Science Letters 241 (2006) 734–749744
blocking and the general slope of the margin. Along the
Carboneras Fault lineation, water gaps, the place where
drainage cuts through strike-slip faults between shutter
ridges [44], allow deflection of the gullies to the south-
east. In this case, the mode of deflection is conditioned
not only by the general slope of the margin but, in some
cases, by the structural depressions coinciding with
faults trending N150, which locally constitute prefer-
ential paths for the submarine drainage (Figs. 3a and 6).
Gullies and channels of the Dalıas Tributary Valley
System in the northern part of the Almerıa Canyon are
seafloor features present throughout the sedimentary
succession on high-resolution seismic profiles. They
maintain their morphology and position over time,
suggesting that these tributary valley systems are stable
features at least during the Quaternary, and that no
lateral migration of the submarine drainage network
is produced where the general slope conditions are
maintained.
5.2. Structural model of the Carboneras Fault zone in
the Alboran Sea: Evidence for strike-slip displacement
There are several lines of evidence supporting that
the submerged portion of the Carboneras Fault de-
scribed here is a strike-slip fault. Swath-bathymetry
and high-resolution seismic profiles show a consider-
able variation in the three-dimensional structural geom-
etry along the strike of the fault (Fig. 5), analogous to
those that have been documented in strike-slip faults
exposed on land [44]. For instance, the submarine
Carboneras Fault displays several geomorphological
characteristics, such as systematically deflected gullies,
shutter ridges, water gaps and incised channels parallel
to the main structure (Fig. 6). The shallow structure of
the Carboneras Fault consists of a series of upward-
splaying subvertical faults defining positive flower
structures in cross section (Fig. 5b), as previously in-
ferred by [20,35]. These high-angle faults probably
coalesce at greater depths and constitute part of a single
large-scale directional fault zone 5 to 10 km wide. This
is a typical geometry found in the shallow structure of
large strike-slip systems [44].
In agreement with evidence found onland [e.g.
15,27,29,30], there are structures which may indicate
that the submarine Carboneras Fault is a strike-slip fault
with a sinistral sense of motion (Fig. 6). Firstly, the
series of widely spaced obliquely trending folds
(N120–N130) observed in the southern segment are
compatible with the structural geometries predicted
under left-lateral shear deformation. Secondly, ridges
AR and SR show a series of undulations with ben
echelonQ arrangement observable in the 10-m contour
bathymetric map (Fig. 6). These elevated and folded
areas correspond to pressure ridges: Ridge AR is
formed at an underlapping restraining stepover between
the southern N060 and northern N045 segments of the
Carboneras Fault, whereas ridge SR is placed in an
overlapping restraining stepover. Both structures
(Ridges AR and SR) reveal a geometrical pattern in
agreement with a left-lateral strike-slip component on
the Carboneras Fault.
It is possible to find elongated hills along pure
strike-slip faults depending on the lateral displacement
of topographic features [44]. Nevertheless, in the case
of the Carboneras Fault, when the general slope of the
margin and the left-lateral movement along strike are
considered, it is not possible to attribute the blocking
and deflection of the Dalıas Tributary Valley System
drainage to pure sinistral strike-slip deformation. This
would bring deeper portions of the margin across the
fault, thereby locally increasing the slope, which would
be inconsistent with the observed drainage-blocking.
This is our explanation for suggesting that the Carbo-
neras Fault is also affected by a vertical slip component,
as has been pointed out for the onland segments by
[21,30].
Along the strike of the Carboneras Fault, we identify
changes in the dip direction of the fault escarpment,
which produces variations in the relative upthrown and
downthrown blocks (Figs. 5b and 6). Thus, along the
northern segment, the fault escarpment faces towards
the NW, opposite to the general gradient of the margin
but following the same arrangement of uplifted blocks
as the Carboneras Fault on land and on the continental
shelf (Fig. 7). In contrast, along ridge AR and the
southern segment, the main fault escarpment faces to
the SE, increasing the margin slope (Fig. 5b). This
arrangement suggests a bscissorsQ structure [44] illus-
trated in map view in Fig. 6. However, the distinction in
the sense of throw between normal and reverse vertical
components together with the detailed structure of the
Carboneras Fault at depth can only be resolved by using
deeper penetration seismic reflection systems.
5.3. The Carboneras Fault zone in the context of the
Eurasia-Africa plate convergence
Deformation associated with Eurasia-African plate
convergence in the Alboran Region is distributed over a
broad area encompassing the EBSZ, Betics, Alboran
Ridge, Moroccan Rif and Tell-Atlas in Northern
Algeria [4,12,13]. The convergence rate estimated by
[1,2] is about 4–5 mm/yr following a NW–SE direction.
Fig. 7. Interpretative structural map of the southeastern Iberian Margin based on the results from the HITS 2001 high-resolution acoustic data. Trace
of the Carboneras Fault on the shelf based on [32]. Faults are depicted as thick black lines, summarized from [11,17,20,25, and this work]. PF:
Palomares Fault; CAFZ: Corredor de las Alpujarras Fault Zone; CF: Carboneras Fault. Seismicity from the bInstituto Geografico NacionalQcatalogue for the period between 1970 and 2000 is depicted [8]. Smaller dots are epicentres of earthquakes for 2.5bmbb3.5, and larger dots are
epicentres of earthquakes for mbN3.5. The thick black outlined rectangle depicts the study area.
E. Gracia et al. / Earth and Planetary Science Letters 241 (2006) 734–749 745
Recently published geodetic models based on GPS
velocities from the western Mediterranean [45,46] pre-
dicted a relatively slower (2.5 to 5 mm/yr) and more
oblique motion (208 counter-clockwise rotation in the
direction of convergence) of the present-day African
and Eurasian plates [45].
The new data presented show that the submarine
portion of the Carboneras Fault and associated faults
are active given that they affect all the sedimentary
sequences, cutting and folding the upper and recent
units, which according to [20,35] are of Late Holocene
age. Studies along the Carboneras Fault onland indicate
5–10 m of vertical offset across Tyrrhenian marine
terraces with no evidence of large lateral offset [30].
The central onland segment, known as La Serrata,
consists of two main parallel fault traces bounding a
1 km wide horst (Fig. 7), where stream-channels older
than 100 kyr are left-laterally offset by 80–100 m [30].
These authors suggest that the slip history of the onland
Carboneras Fault during the last 100 kyr appears to be
one of vertical uplift rather than strike-slip movement,
with maximum vertical slip rates in the order of 0.05–
0.1 mm/yr [30]. In contrast, based on neotectonic mod-
elling, the onland Carboneras Fault zone might accom-
modate about 0.7 mm/yr of sinistral strike-slip motion
[47,48]. Our findings on the submarine segments of the
Carboneras Fault do not disagree with the above sug-
gestions. However, the lack of reliable chronological
markers prevents us from estimating the vertical and
strike-slip rate components.
A local shortening along an approximate NNW–SSE
axis is suggested by the regional stress field derived
from earthquake focal mechanisms inversions in the
Alboran Sea and southeast Spain [6,7]. This direction
also coincides with the most compressive horizontal
stress orientation and regime predicted by neotectonic
E. Gracia et al. / Earth and Planetary Science Letters 241 (2006) 734–749746
modelling based on thin-sheet finite element [47,48].
Considering the NNW–SSE shortening axis, we can
assume the strain regime of the newly identified struc-
tures: the Carboneras Fault may have a left-lateral
component, the N150 faults would move as right-later-
al, and the NS faults located to the west of the Chella
Bank would have a predominantly normal component
(Fig. 6). The geometrical relationship between the left-
lateral Carboneras Fault and the N150 to NS normal
faults identified in the area is compatible with the
model of block tectonics presented by [25] to explain
the neotectonic structures of the southern Almerıa prov-
ince. In their model, predominantly extensional struc-
tures (such as the NS and N150 faults) accommodate
the deformation produced by squeezing the wedge
located between the dextral strike-slip Corredor de las
Alpujarras and sinistral Carboneras fault zones (Fig. 7).
The combined movements of these large strike-slip
fault zones would induce a westward tectonic escape
[25].
Moment tensor solutions for small and moderate
earthquakes in the Alboran Sea show a predominant
left-lateral strike-slip motion. They mainly correspond
to the Alboran Ridge series of 2003, which follow a
N055 nodal plane [6,7] (Fig. 1). Focal mechanisms and
epicentre locations also indicate that the newly identi-
fied structures to the south of Adra and Campo de
Dalıas (i.e. N150, NS faults) are active and consistent
with the present-day extensional strain pattern of this
area [6,21,25,48] (Figs. 1 and 7).
The Carboneras Fault zone corresponds to the trans-
pressional southern end of the EBSZ. The new marine
geophysical data reveal that the morphological expres-
sion of the Carboneras Fault zone terminates against a
topographic high at 36817.3VN–3803.5VW on the
Alboran Sea (Figs. 5 and 6). This suggests that, in
contrast to previous works [e.g. 18], the Carboneras
Fault may not extend as a large continuous fault zone
connecting the Eastern Betics with the Jebha and
Nekor Faults in the North-African coast, calling into
question the bTrans-Alboran Shear ZoneQ. Instead, wepropose that strain along the offshore Carboneras Fault
zone is transferred to the parallel structure located
about 20 km to the south, which flanks the Alboran
Trough (Figs. 1 and 7).
5.4. Seismicity and active faulting offshore Almerıa
region: Implications for seismic hazard assessment in
the Southern Iberian Margin
Present day seismicity in the southeastern Iberian
Margin shows swarms of small to moderate magnitude
(Mwb5) shallow earthquakes [49] which are more con-
centrated to the north and east of the Chella Bank (Fig.
7). This seismicity seems to be associated with the
secondary N150 to NS faults, suggesting moderate
faulting in the region. Based on epicentre relocation,
these structures also seem to be the source of the
shallow (4.8 to 6.5 km) and moderate (Mw 4.8 to 5.1)
Adra earthquake swarms, the most intense earthquake
activity recorded recently in this region, which occurred
between December 1993 and January 1994 [25,49].
Slope instability features (landslide headscarps, de-
tached blocks, and debris flow deposits), which are
present on the bank flanks, could also be associated
with seismic and paleoseismic activity of this margin.
The unstable nature of the volcanic edifices together
with the moderate magnitude seismic activity recorded
in the area could account for the triggering of subma-
rine landslides observed on the banks, especially the
steep southern flanks of the Sabinar and Pollux Banks
(Figs. 5–7).
As regards the Carboneras Fault zone (onshore and
offshore), only few epicentres are placed along its trace.
However, this does not mean that little seismological
hazard should be attributed to the Carboneras Fault. For
instance, the location of the Almerıa (MSK intensi-
tyN IX) and Adra (MSK intensityNVIII and Mw=6.1
[49]) historical events (Fig. 1) fall relatively close
(considering location uncertainties) to the submerged
trace of the Carboneras Fault as mapped in the present
study (Fig. 7). This suggests that the Carboneras Fault
is a possible source of these events, especially the 1522
Almerıa Earthquake and Tsunami (?). Considering that
tsunamis are not expected from pure strike-slip motion,
the occurrence of a tsunami in 1522 [8] may indicate a
large landslide, but possibly also, a dip-slip component
on the submarine portion of the Carboneras fault, in line
with our interpretations. Calculation of the relationship
between magnitude and intensity for the most important
historical earthquakes in the Ibero-Maghrebian region
assigns a magnitudemb 5.8–6.0 to the 1522 Almerıa
Earthquake [50].
In addition, a detailed macro- and micro-structural
analysis of clay-bearing fault-gouges in the northern
segment of the Carboneras Fault yields some insight
into its mechanical seismic behaviour [29,51]. It shows
a wide heterogeneous deformation zone with low per-
meability layers in the fault gouge, containing lenses of
fractured permeable rocks that may trigger large peri-
odic seismic events against a background of fault creep
in the surrounding materials [29]. The onland Carbo-
neras Fault has been suggested as an analogue for the
San Andreas Fault around Parkfield (California), where
E. Gracia et al. / Earth and Planetary Science Letters 241 (2006) 734–749 747
geophysical observations of the fault suggest a large
zone (1–2 km) of deformation with fault creep and
Mw=6.0 recurrent earthquakes [29].
The Carboneras Fault zone is by far the longest,
continuous fault mapped in the southeastern Iberian
margin (50 km onshore, plus more than 100 km
offshore) (Fig. 7) and, therefore, it would be the
best candidate to generate large magnitude earth-
quakes. For instance, assuming a surface rupture
length of 70 km following a ~N045 trend (this
would include the assumed bnon-gougedQ southern
segment onshore, the shelf, and the northern offshore
segment of the Carboneras Fault, Fig. 7) and using the
empirical relationships of [52], a moment magnitude
of ~7.2 is obtained. The southeastern Iberian Margin
corresponds to the most seismically active and haz-
ardous Spanish region. It is considered to have a
moderate seismic hazard, between 0.8 and 1.6 m/s2
mean peak ground acceleration for a return period of
475 year, i.e., with 10% probability of exceedance in
50 years [53]. This correlates well with the values
obtained in a recent probabilistic seismic hazard as-
sessment in terms of Arias intensity for different soil
conditions in southeast Spain [54]. However, as these
models are mainly based on instrumental and histori-
cal earthquake catalogues, a reevaluation of the previ-
ously calculated seismic and tsunami hazard in the
region taking into account the offshore segments of
the Carboneras Fault could significantly increase the
suggested hazard.
Finally, a paleoseismic approach is required to pro-
vide an assessment of the seismic hazard associated
with the Carboneras Fault, especially when considering
high magnitude earthquakes and probably long recur-
rence intervals (104 years) [11,55]. Future work on the
Carboneras Fault will be devoted to the imaging of
deep structure and to the identification and dating of
geological markers of the fault offshore, and to geo-
physical surveying and trenching onshore. This will be
helpful in determining the past activity and seismic
parametres of the Carboneras Fault (geometry, slip
rate and recurrence interval).
6. Conclusions
1. New marine geophysical data (swath-bathymetry,
deep-towed sidescan sonar TOBI and high-resolu-
tion seismic data) from the Almerıa Margin show
that the superficial expression of the Carboneras
Fault consists of an upwarped 5–10 km wide defor-
mation zone. This is bounded by subvertical faults at
depth that trend N045 to N060 and extend for more
than 100 km. The drainage system of the margin is
deflected by the Carboneras Fault upwarp, and is
locally conditioned by secondary structures trending
N150.
2. The submarine Carboneras Fault displays geomor-
phic features and tectonic structures usually found
on strike-slip faults onland, such as deflected drain-
age, shutter ridges, water gaps, pressure ridges and
positive flower structures. A narrow underlapping
restraining stepover separates the 33 km long north-
ern segment trending N045 from the 26 km southern
segment trending N060. Obliquely trending folds
(N120–N130) and ben echelonQ pressure ridges sug-gest a left-lateral shear deformation along the Car-
boneras Fault. In addition, drainage blocking
produced by the northern segment of submarine
Carboneras Fault indicates a vertical slip component.
3. The submarine portion of the Carboneras Fault and
associated faults cut and fold the most recent sedi-
mentary units of Late Holocene age, showing that
they are active structures. Considering the NNW-
SSE shortening axis, the Carboneras Fault may
have a left-lateral component, the N150 faults
would move as right-lateral, and the NS faults,
located to the west of the Chella Bank would have
a predominantly normal component. These struc-
tures are consistent with the model of block tectonics
suggested by [25]. The morphological expression of
the Carboneras Fault zone ends in a structural high
in the Alboran Sea, in contradiction to the alleged
continuous fault zone connecting South Spain with
North Africa, known as the bTrans-Alboran Shear
ZoneQ.4. Present-day seismicity in the Almerıa margin shows
swarms of small to moderate magnitude shallow
earthquakes [49], which are more concentrated to
the north and east of the Chella Bank and may be
associated with the secondary N150 to NS faults.
This moderate seismicity may have triggered the
submarine landslides observed on the flanks of the
banks. The submerged portion of the Carboneras
Fault is at least twice as long as its subaerial portion.
The onshore and offshore segments together consti-
tute one of the longest faults in the southeastern
Iberian Margin. Despite the low instrumental seis-
micity along its trace, the Carboneras Fault is a
potential source of large (up to moment magnitude
~7.2) events such as the 1522 Almerıa Earthquake.
Seismic and tsunami hazard assessment for southeast
Iberia and African margins would significantly in-
crease if the offshore segments of the Carboneras
Fault were considered.
E. Gracia et al. / Earth and Planetary Science Letters 241 (2006) 734–749748
Acknowledgements
The authors acknowledge the support of the MCYT
Accion Especial HITS (REN2000-2150-E), European
Commission European Access of Seafloor Survey Sys-
tems EASSS-III programme (HPRI-CT99-0047), Span-
ish national Project IMPULS (REN2003-05996MAR),
ESF EuroMargins WESTMED project (01-LEG-
EMA22F and Accion Especial REN2002-11230E-
MAR), and funding by Generalitat de Catalunya, re-
search group SGR2001-0081. We thank the captain,
crew and technical staff on board the R/V Hesperides
and the TOBI Team from the National Oceanography
Centre (Southampton, UK) for their assistance through-
out the data collection. We also thank Juan Acosta
(IEO, Madrid) for providing us with supplementary
bathymetric data from the program bFisheries Cartog-
raphy of the Alboran SeaQ. We are indebted to Miquel
Canals (Univ. Barcelona) for valuable suggestions dur-
ing the preparation of the cruise and Tim Le Bas (NOC,
UK) for assisting two of us (O.G. and J.G.) during the
processing of TOBI data. We benefited from fruitful
discussions during the revision of this article with Ana
Negredo (UC, Madrid) and Daniel Stich (INGV, Bolo-
gna). The latter is also acknowledged for providing
focal mechanisms for Fig. 1. We wish to thank Milene
Cormier (LDEO, USA) and three anonymous reviewers
whose constructive criticism enabled us to improve our
original manuscript.
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