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Tectonophysics 391
Reflection–refraction seismics in the Gulf of Corinth:
hints at deep structure and control of the deep marine basin
Christophe Clementa, Maria Sachpazib, Philippe Charvisc, David Graindorgec,
Mireille Laiglea, Alfred Hirna,*, Giorgios Zafiropoulosd
aLaboratoire de Sismologie Experimentale, Departement de Sismologie, UMR 7580 CNRS, Institut de Physique du Globe de Paris,
4 Place Jussieu, Tour 14, B89, F-75252 Paris cedex 05, FrancebGeodynamics Laboratory, National Observatory of Athens, Lofos Nymfon, Athens, Greece
cUMR Geosciences Azur-IRD, P.O. Box 3, 06235 Villefranche-sur-Mer, FrancedHellenic Petroleum, Maroussi, Athens, Greece
Accepted 3 June 2004
Available online 11 September 2004
Abstract
The Gulf of Corinth is a natural laboratory for the study of seismicity and crustal deformation during continental extension.
Seismic profiling along its axis provides a 24-fold normal-incidence seismic reflection profile and wide-angle reflection–
refraction profiles recorded by sea-bottom seismometers (OBS) and land seismometers. At wide-angle incidence, the land
receivers document the Moho at 40-km depth under the western end of the Gulf north of Aigion, rising to 32-km depth under
the northern coast in the east of the Gulf. Both refraction and normal-incidence reflection sections image the basement under the
deep marine basin that has formed by recent extension. The depth to the base of the sedimentary basin beneath the Gulf,
constrained by both methods, is no more than 2.7 km, with ~1 km of water underlain by no more than ~1.7 km of sediment, less
than what was expected from past modeling of uplift of the south coast in the East of the Gulf. Unlike the flat sea-bottom, the
basement and sedimentary interfaces show topography along this axial line. Several deeps are identified as depocenters, which
suggest that this axial line is not a strike line to the basin. It appears instead to be controlled by several faults, oblique to the
S608E overall trend of the south coast of the Gulf, their more easterly strikes being consistent with the instantaneous direction of
extension measured by earthquake slip vectors and by GPS.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Gulf of Corinth; Aegean region; Seismic refraction; Reflection; Crustal structure; Rift; Basin; Extension
0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2004.07.010
* Corresponding author. Tel.: +33 1 44273914; fax: +33 1
44273894.
E-mail address: [email protected] (A. Hirn).
1. Introduction
The Gulf of Corinth in central Greece (Fig. 1) is
located in the back-arc region above the Hellenic
subduction zone, in the post-Alpine extensional
(2004) 97–108
Fig. 1. The Gulf of Corinth. Location map of profiles A, B, and C shot by N/O Nadir and recorded by OBSs, located at numbered triangles and
at land stations NAF and KLI (squares). The Moho reflection midpoint lines at these stations from shots on profile A are sketched as the two
thick gray lines (offshore north of Aigion and onshore north of the eastern end of the Gulf). Only the axial profile A and its crooked turn in the
West, W, could be recorded as a multifold seismic reflection profile because of safety reasons concerning deployment of the ship’s seismic
streamer. Geology is adapted from Armijo et al. (1996), with earthquakes from Taymaz et al. (1991), Baker et al. (1997), and Bernard et al.
(1997). Inset shows the study region (rectangle) in relation to its surroundings, including the North Anatolian Fault Zone (NAFZ).
C. Clement et al. / Tectonophysics 391 (2004) 97–10898
domain of the Aegean (e.g., Gautier et al., 1993). This
region has also been affected by the North Anatolian
strike-slip fault (Fig. 1) since the Pliocene (Armijo et
al., 1996). Strong seismic activity characterizes this
region, which is extending in a southward direction, at
a rate that exceeds 10 mm/year across the Gulf (Clarke
et al., 1998). The south coast of the western Gulf of
Corinth has experienced destructive earthquakes, such
as the Helike event of 373 B.C. and the Aigion event of
Ms ~7 in 1861. From an early microearthquake study,
Melis et al. (1989) proposed that the southern
bounding faults of the asymmetrical graben forming
the Gulf of Patras and the adjacent western Gulf of
Corinth were listric, flattening northward into a mid-
crustal decollement. In the western Gulf of Corinth,
microearthquakes recorded during temporary seismo-
graph deployments (Rigo et al., 1996) appear dis-
tributed across the Gulf above a cutoff depth that has
been considered as a plane dipping northward at a low
angle of 108, from a depth of 8 km under Aigion. This
plane has been interpreted as an underlying detach-
ment representing the brittle–ductile transition, the
base of the seismogenic layer, or as a fault in the brittle
domain that is slipping through small earthquakes
(Rietbrock et al., 1996). Focal mechanisms constrained
by waveform modeling of earthquakes of larger
magnitude, 5.7 to 6.2 (Taymaz et al., 1991; Baker et
al., 1997) have low-angle north-dipping nodal planes.
A study of the 1995 Ms 6.2 event, which caused
damage at Aigion on the south coast but had its focus at
depth beneath the northern margin of the Gulf,
suggested a 338 northward dip for its fault plane,
although aftershocks form a cluster elongated with a
smaller apparent dip (Bernard et al., 1997). In the east
of the Gulf, the 1981 sequence of three large MsN6
earthquakes provides the main data source. These are
typical high-angle normal-fault events. Such a steep
fault and a thick elastic plate have been used to model
the uplift of the Plio–Quaternary marine terraces on the
south coast of the eastern Gulf (e.g., Jackson et al.,
1982; Keraudren and Sorel, 1987, Armijo et al., 1996;
Westaway, 1996, 2002).
The recent geological evolution of the Gulf, of
which this earthquake activity is a present instanta-
neous expression, has accumulated finite extension,
forming a morphological rift structure floored by a
deep and flat marine basin elongated toward N1208E(i.e., S608E), cutting across the strike of Alpine
structures. The mode of seismic energy release, on
low-angle or high-angle normal faults, as well as the
C. Clement et al. / Tectonophysics 391 (2004) 97–108 99
mechanism of extension of the continental crust on the
geological time scale are debated for this structure that
may be the world’s fastest-extending graben in
continental crust. The structure of its marine infill
and of the underlying basement are obvious markers
of its style of extension. Previous attempts at
deciphering this record of deformation from the
structure of this basin were based on single-channel
shallow-penetration seismics (e.g., Brooks and Fer-
entinos, 1984; Higgs, 1988). In order to acquire data
to help constrain the evolution of this Gulf, we
undertook seismic imaging using modern methods in
order to provide observations of its structure, to
introduce into this discussion a different point of
view, complementary to onshore geological observa-
tions and their modeling, and studies of seismicity.
2. Methods and survey
During the Franco–Greek SEISGRECE seismic
survey in January 1997, the oceanographic vessel N/O
Le Nadir of IFREMER shot a 2900 cubic inch array of
14 air guns operated in single-bubble mode (Avedik et
al., 1996) every 50 m in the part of the Gulf to the east
of Aigion (Fig. 1; Sachpazi et al., 1998). This mode of
shooting an air gun source provides with maximum
efficiency a signal that is peaked in the rather low-
frequency 12–20 Hz band but has a duration short
enough for acceptable resolution for normal-incidence
seismic reflection. This frequency band is high
enough to correspond to that typically observed for
reflections returned by the lower crust even in surveys
using sources that also generate higher frequencies,
like the more usual tuned arrays of air guns. This low-
frequency band also provides signal propagation to
the large offset wide-angle reflections, despite attenu-
ation. The shots in the Gulf of Corinth were recorded
by six ocean-bottom seismometers (OBSs), operated
on the bottom of the Gulf, as well as by seismometers
at station sites on land (Fig. 1). The 96-channel, 2400-
m long seismic streamer could be deployed from the
vessel for part of the survey. This allowed recording
of the normal-incidence reflection profile A in Fig. 1
with 24-fold coverage, suitable for advanced process-
ing with pre-stack depth migration. However, this was
only possible for a 50-km distance along the axis of
the Gulf because operation was impeded by the worst
winter storm in the last 15 years. Acquisition could be
maintained for a tie line, W in Fig. 1, at the western
end of the Gulf, until crosspoint E (Fig. 1), but with a
low signal-to-noise ratio and inadequate geometry as
it was acquired along an arc of a circle with the ship
turning. Although the strength of this seismic source
has allowed us to image at normal incidence the
whole crust elsewhere in the Aegean (Sachpazi et al.,
1997), here the noise of extremely strong sideways
reflection of water waves off the coast of the narrow
Gulf dominates at times when reflections that could be
returned from deeper than 15 km are expected.
3. Deep crustal elements from wide-angle
reflections
Land stations, offset at either end of the shot-line
along the axis of the Gulf of Corinth (Fig. 1), recorded
clear seismic waves out to the maximum recording
distance of 105 km, showing the efficiency of signal
generation and reflection. These waves arrive much
later, as much as 5–6 s later at 50-km offset, than the
first arrival Pg-wave refracted in the basement. With a
high-velocity move-out, they thus cannot be inter-
preted as anything other than the PmP-phase: wide-
angle reflections from over 30-km depth, that is, from
the Moho (Fig. 2). This is a rewarding case of a
vertical reflection seismic source allowing one to
obtain wide-angle reflections even in single coverage,
without stacking for the Moho, and paves the way for
the regional mapping of Moho topography using this
shooting strategy with numerous receivers. Unfortu-
nately, several other seismograph stations, which were
probably slightly less protected from the worst winter
storm during acquisition, were dominated by back-
ground noise.
There was no previous seismic measurement of the
Moho depth in this region, only inferences from
gravity and regional tomography (Makris and Stobbe,
1984; Tsokas and Hansen, 1997; Papazachos and
Nolet, 1997). From a teleseismic tomography experi-
ment resulting in the usual horizontally layered
medium with blocks of varying velocity, Tiberi et al.
(2000) discussed lateral variations in average velocity
of their upper blocks in terms of a change in the
proportion of crustal and mantle material, that is, a
variation in Moho depth. The artificial source wide-
C. Clement et al. / Tectonophysics 391 (2004) 97–108100
C. Clement et al. / Tectonophysics 391 (2004) 97–108 101
angle reflection observations in Fig. 2 indeed reveal a
strong difference of 8 km in crustal thickness between
the western tip of the Gulf west of Aigion and its
northeast coast (Sachpazi et al., 1998; Clement, 2000).
The water depth and structure of the underlying
sedimentary basin, determined from our vertical
reflection profiling, are taken into account in the
calculation of this variation in Moho, as reported in
Fig. 2 and its corresponding caption.
This is the simplest possible structural model and
has an average crustal velocity typical for Europe, but
other solutions are also possible. Propagation paths
toward each of the recording stations all sample the
crust under the profile of shots but do not sample the
same Moho segment at midpoint. The condition that
the Moho is continuous and straight, consistent with
the average velocity used, could thus be relaxed. With
this alternate assumption, Moho depths remain ~8 km
larger in the west than the east, but the trade-off
between velocity and depth results in a model space
ranging from slower crustal velocities giving an
overall shallower and convex-upward Moho to faster
velocities giving an overall deeper and concave-
upward Moho. The fact that Moho reflections are
observed at as short an offset as 40 km could lead to
prefer slower crustal velocities to shorten the critical
distance. However, to achieve the amplitudes
observed at small distances, other causes, such as
Moho topography focusing the reflection, are neces-
sary, so these observations alone do not provide a
tighter constraint on crustal velocity.
With data from only two locations, this thickness
difference could simply be regarded as inherited from
the Alpine phase of deformation, because a crustal root
is still present, as attested by the Bouguer gravity low
that cuts north–south at right angle to the Gulf (Tsokas
and Hansen, 1997) to the west of the offshore area
north of Aigion where the reflections to station NAF
have their midpoint. After removing the effect of the
Ionian slab, Tiberi et al. (2001) inverted the residual
Fig. 2. Record sections from land stations NAF and KLI (Fig. 1) at variabl
along the axis of the Gulf. Note on both very late, clear waves, interpreted a
from 3 to 5 s reduced time between 40 and 70 km. West is left and East is
model including the water and sediment layers under the shot line, as deriv
to be 8 km deeper in the west than in the east. In this model, an average 6.25
was assumed to dip at 10% (~5.78) from east to west. The Moho thus dips fr
1 under the north coast of the Gulf and east of the shot line (for shots to KLI
Fig. 1 offshore north of Aigion (for shots to NAF).
Bouguer anomaly in terms of a variation of crustal
thickness that may as well explain the teleseismic
tomography results of Tiberi et al. (2000). They find a
similar difference in Moho depth between the two
regions we have sampled with the wide-angle reflec-
tions (Sachpazi et al., 1998; Clement, 2000). The
Moho topography inverted from gravity shows aMoho
with alternating highs and lows, including a low (i.e.,
thick crust) under the western Gulf and a high (i.e., thin
crust) northeast of the Gulf of Corinth, which agree
with the wide-angle reflection observations, plus
another high south of the eastern end of the Gulf.
The crust in this study region thus appears quite
thick, somewhat unexpectedly given the common
belief associating thin crust with rifts and other
extensional domains. Apart from the Moho reflec-
tions, the wide-angle intracrustal response is very
weak, which cannot be due to a too weak seismic
source, as the response of the underlying Moho is
strong. That the intracrustal reflectivity would be so
weak may also be unexpected in this extensional
region, given that strong lower crustal reflectivity has
been observed in numerous vertical reflection and
wide-angle profiles and attributed to extension (e.g.,
Allmendinger et al., 1987). This interpretation has
also been proposed in western Europe, where this
fabric has been acquired in the past, presumably as a
result of post-Variscan crustal thinning (e.g., Bois and
ECORS Scientific Party, 1990). The profiles that
revealed this intracrustal reflectivity showed further-
more that unlike the layer-interface reflectivity well
known in sediments, it may not be due to specular
reflections on few continuous interfaces between
lithological units. It has instead been considered that
lower crustal reflectivity is caused by the heteroge-
neity of the medium and that this heterogeneity is
related not only to the nature of the rocks but also to
finite deformation or its rate and/or any contribution
of magmatism (e.g., Warner, 1990). The strong active
extension in this study region has evidently not yet
e offset, with 8 km/s reduction velocity, of air gun shots from line A
s Moho reflections, and weak upper crustal refracted arrival on NAF,
right. Superimposed in white are travel-time curves computed for a
ed from OBSs and vertical reflection profiling, which reveals Moho
km/s velocity was used for the crust under the sediments, and Moho
om the 32-km depth sampled at midpoints along the gray line in Fig.
), to the 40-km depth sampled at midpoints shown as the gray line in
C. Clement et al. / Tectonophysics 391 (2004) 97–108102
achieved the crustal image seen in terranes inferred to
have undergone extension long ago. This may indicate
that the important factors governing whether exten-
sion is imprinted into the lower-crustal fabric are the
finite amount of strain and the temperature conditions
for flow.
4. Refraction seismic constraints on basement
depth and sedimentary fill
The velocity layering of the upper crust is clearly
illustrated by the conspicuous branches of the travel-
time curve in the wavefield recorded by OBS 4
located in the eastern widest and deepest part of the
basin, in the hanging wall of the Xylocastro fault (Fig.
3a). The velocity–depth structure is derived from the
split-spread profile recorded by this OBS for shots on
line B, as follows. Branch 1 in yellow is a refraction-
diving wave that constrains a velocity–depth gradient
layer just beneath the sea bottom of sediment with a
low seismic velocity V1 of average 2 km/s. Branch 2
in blue is a precritical reflection on and a diving
refraction into the underlying layer of more compact
sediment with a smaller gradient and V2 ~3 km/s.
Branch 3 in brown is the precritical reflection on top
of and refraction-head wave into a V3N5 km/s
medium, interpreted as the pre-rift basement, the top
of the Hellenic nappes after the Alpine orogeny.
The sediment thickness under OBS 4 is tightly
constrained to 2 km by the slopes and intercept times
of the refracted travel-time curves. On the split-spread
profile reduced with the 4.5 km/s velocity of the
basement (Fig. 3a), the intercept or delay time of the
basement refraction only slightly increases towards
south; hence, the maximum depth of the base of the
sedimentary basin at the foot of this fault is no more
Fig. 3. Examples of OBS data and modeling, NW is to the left, SE to the
deep basin edge just south of OBS 1 and 2. (a) OBS 4 in the eastern part of
with 4.5 km/s reduction velocity shown as a split-spread profile across t
yellow, diving wave in upper sediment layer with velocity increasing with
increasing from 3 to 3.5 km/s; in brown, basement with velocity increa
Xylocastro fault. Record section with 4.5 km/s reduction velocity of data r
time of basement refractions as brown first arrivals on either side, which res
(c) Line B velocity modeling and ray-tracing for OBS 2 and OBS 4. Lowe
computed travel-time curves as black lines, represented with 6 km/s reduct
velocity model containing a water layer, then upper and lower sedimentar
basement with 5 km/s velocity at the top and velocity–depth gradient.
than 2.7 km. Further south, the basement travel-time
curve shows a significant kink, visible in Fig. 3b
around 3.5-km offset, with early arrivals due to
traversing the master fault into the higher footwall
block. Data from OBS 2 (not shown) on the same line
reverse this profile, for which the 2D model is
controlled by ray-tracing in Fig. 3c. The record
section of a parallel line of shots, C, through OBS
1, located closest to the offshore continuation of the
Xylocastro fault farther east is shown in Fig. 3b. The
basement wave on its split-spread profile is corre-
spondingly very asymmetric. It yields a value of 2.5–3
km for the maximum basement depth.
Such in situ refraction measurements are consid-
ered to give the best geophysical constraint on
basement depth. The value thus measured for the
sediment thickness is unexpected. Indeed, Armijo et
al. (1996) used a general model of a thick plate and a
single high-angle fault, which accounted for the
footwall uplift they constrained from the glacio-
eustatic markers of the Corinth marine terraces. The
corresponding sediment thickness on the subsiding
hanging wall resulted as 5 km from this modeling.
The much smaller value that we measure may enter as
a constraint in further modeling attempts. Westaway
(2002) has assumed such a small value of sediment
thickness and discussed rheological concepts that
allow him to model that realistic value.
5. Imaging of sediments, basement topography,
and intra-basement structure by multichannel
reflection
The 24-fold stack of multichannel reflection profile
A along the axis of the Gulf (Fig. 4a) allows reduction
of the strong sea-bottom multiples that, along with the
right. Xylocastro fault is assumed to continue eastward forming the
the basin, in the hanging wall of the Xylocastro fault. Record section
he fault for the shot line B. Travel time branches correlated are: in
depth from 2.0 to 2.2 km/s; in blue, lower sediments with velocity
sing from 5.3 to 6 km/s. (b) OBS 1 located just basinward of the
ecorded for the parallel shot line C. Note the difference in the arrival
olves the greater depth of the basement under the deep marine basin.
r frame displays the arrival times picked for the observations and the
ion velocity. Upper frame shows the corresponding two-dimensional
y layers at average 2 and 3 km/s with velocity–depth gradients and
C. Clement et al. / Tectonophysics 391 (2004) 97–108 103
C. Clement et al. / Tectonophysics 391 (2004) 97–108104
C. Clement et al. / Tectonophysics 391 (2004) 97–108 105
different strengths of the seismic sources, limited
single-channel penetration. The western, crooked part
is only a partial brute stack due to the effect on
streamer noise and the geometry of the ship’s turning
and is shown for the sake of completeness. Fig. 4a
shows a series of strong reflections with echo times of
up to 2–3 s, indicating a basin with stratified
sediments. In order to preserve the characteristic
signal frequency response of interfaces with depth,
this profile is shown as a time section (restretched to
time from the result of pre-stack depth migration)
rather than the depth section of Fig. 4b, which
stretches the waveforms within the high-velocity
basement and thus gives a longer apparent signal
period, so that the true longer-period character of
these seismic waves could not be assessed. In the time
section in Fig. 4a, the clear signature in the time-
domain signal waveform is real, marking the usual
low-frequency response of the top of basement that
represents a former land surface. This high-velocity
layer can thus be interpreted here as the pre-rift
basement, indicating the subaerial land surface that
developed at the top of the sequence of nappes that
were emplaced during the Alpine orogenic evolution
of the Hellenides.
Unexpectedly, in contrast to the flat sea bottom
along this axial line, this reflector at the base of this
basin fill and the internal sediment layering both show
significant variations in topography. They should
instead be flat and horizontal if this axial line were a
strike line to the extensional basin. The OBS refraction
observations on profile A are consistent with this
topography and the corresponding velocity and depth
estimates of the pre-stack depth migration, for which a
depth section is displayed in Fig. 4b. Along this
reflection seismic section, three distinct basement
Fig. 4. (a) Right-hand part: profile A, shot with a 2900 cubic inch array of 1
into a 96-channel, 2.4-km-long streamer, giving 24-fold coverage. This t
restretched to time to illustrate the reflective character of the basement (w
dips opposite to that expected for Alpine nappes and could be younger fa
semicircular turn, forming the westward continuation of profile A. It is a lo
the ship’s turning. For orientation, letters indicate positions in Fig. 1; in par
crosspoint of profiles A and W in Fig. 1. The outline shaded in gray may be
pre-stack depth migration. Vertical exaggeration is approximately 6:1. Sedim
which imaging artefacts due to noise dominate as a result of increasing dep
subdued reflectivity and velocity 3.3 km/s, upper unit subdivided betwee
tentatively sketched as black lines along disruptions of layers.
deeps are revealed. They can each be identified as
causing a local depocentre because there is neither a
succession of horizontal nor of equal thickness layers
over these basement deeps that would indicate filling or
draping of a preexisting depression. The fact that there
are three depocenters as imaged here requires that
extensional evolution of this basin has been controlled
by several distinct normal faults in the basement.
Overall extension has thus occurred at an oblique strike
to the axis of the Gulf, with segmentation at a shorter
scale than the length spanned by this axial seismic
profile. This is consistent with the observed geometry
of faults to the south of the Gulf, which pass onshore to
offshore from west to east (Fig. 1).
This image does not support the view that the
subsidence of this basin was controlled by a single
normal fault striking parallel to the average N1208Etrends of the south coast and basin axis. However,
profile A is thus not perpendicular to the faults
controlling these localized depocenters either, so it
does not allow a straightforward analysis of the
geometry of sedimentation and deformation. Infer-
ences can nonetheless be drawn on the number and
approximate location of controlling faults, as well as
on the possible interpretation of the reflectors that are
suggested in the basement in Fig. 4a. The southeastern
depocenter in Fig. 4a is obviously in the hanging wall
of the Xylocastro fault. Although these sediments are
controlled by this fault, they do not show it, nor does
the basement surface. This is because this profile is
oblique to this fault and remains in its hanging wall.
This profile does not cross the trace of this fault,
which was instead imaged by line B (Fig. 3a).
The western depocenter on profile A is in the
hanging wall of faults that could correspond to the
prolongation 15 km eastward into the basin of the
4 guns in single-bubble mode (Avedik et al., 1996) at 50 m spacing,
ime section was obtained from a pre-stack depth-migrated section,
hich is outlined). Possible features in the basement (sketched) have
ults. To the left, profile W (Fig. 1) is the crooked line, including a
wer-quality brute stack time section of partial data, due to noise and
ticular, point E labeled on both parts of this figure corresponds to the
the base of the sediments. (b) Depth section of profile A obtained by
ent layers are coloured, above the basement left black and white, in
th relative to streamer length. Lower sedimentary unit in brown, with
n main reflectors, layer velocities from 1.7 to 2. 2 km/s. Faults are
C. Clement et al. / Tectonophysics 391 (2004) 97–108106
Helike fault documented on land (Fig. 1) and whose
footwall at Cape Akrata has uplifted marine terraces
deposited during Pleistocene interglacial marine high-
stands (e.g., Armijo et al., 1996; Westaway, 1996,
2002). The middle depocenter provides evidence for
control by another fault in between. We suggest that
such a fault, extending down into the basement,
corresponds to the deep expression of the feature
previously mapped from the surface into the upper
kilometer of the basin fill and interpreted as a listric
growth fault in the sediments from a grid of single-
channel seismic lines (Brooks and Ferentinos, 1984;
Higgs, 1988). From its strike mapped at shallow
depth, this deep-reaching fault appears to be the
eastward prolongation of a fault along the locally
north-facing coast segment north of Derveni (Sorel,
2000), which passes onshore south of Cape Akrata
(Fig. 1). Our reflection line also suggests the presence
of intra-basement reflective structures. However, with
a single profile, their identification and interpretation
remain speculative. Nonetheless, westward-dipping
reflectors that crop out at the basement–sediment
interface, as sketched in a preliminary way on Fig. 4a,
cannot be inherited Alpine nappe structures, as these
would dip in the opposite direction, but they may be
candidates for low-angle normal faults active during
the recent evolution of the basin.
Tectonic control of these basement deeps and
depocenters thus occurs by more than a single normal
fault, striking at an oblique angle, 308 counterclock-
wise, to the axis of the Gulf and the average
orientation of its coastline but consistent with the
present N–S direction of extension across the Gulf
from GPS measurements and earthquake slip vectors
(e.g., Baker et al., 1997; Clarke et al., 1998).
6. Discussion and conclusions
The structure of the seismically active and rapidly
extending continental rift in the Gulf of Corinth has
been seismically imaged into the crust using new
marine normal-incidence reflection and OBS wide-
angle reflection–refraction seismics. Although the
middle and lower crustal response cannot be seen on
the vertical reflection profile, as it was hidden in the
very strong and complex water waves reflected off the
shelves and coastlines along either side, the signal
energy was sufficient to penetrate to the Moho and to
be recorded as wide-angle reflections by fixed stations
on land. However, the extreme noise level due to
storm conditions resulted in clear data at only two
stations, beyond the western and northeastern ends of
the shot line along the axis of the Gulf. The Moho
depth is 40 km under the western Gulf north of Aigion
and 32 km under its north coast, north of Corinth.
Surprisingly, there appears to be no significant lower-
crustal reflectivity, as is expected to result from
extension (e.g., Bois and ECORS Scientific Party,
1990). This could suggest that large finite deforma-
tion, or lower-crustal flow, or a time lag after its
occurrence may be needed for such a fabric to be seen.
However, this deduction remains speculative because
the effect of extension on lower-crustal reflectivity has
been generally discussed using the vertical reflection
response, which is here unattainable because of the
extreme amplitude of basin-side water waves. How-
ever, the fact that intracrustal reflections are not even
seen at wide angle may instead be because they are
hidden by noise, as even the Moho reflection has a
low signal-to-noise ratio.
Using OBSs, which recorded our lines of shots as
refraction profiles, the maximum depth of pre-rift
basement in the hanging wall of the Xylocastro fault is
measured as 2.7 km. This is significantly less than the
5 km expected from applying to the whole time period
of sedimentation the model of a single steep fault in a
thick elastic plate used to account for the uplift of the
Plio–Quaternary Corinth marine terraces (Armijo et
al., 1996) but consistently interpreted in the model of
Westaway (2002).
Our multichannel vertical reflection seismic image
reveals that a sedimentary basin, consisting of several
depocenters, underlies the flat sea floor. We suggest
that these depocenters have been controlled by at
least three normal faults, striking oblique to the
seismic profile, which follows the axis of the Gulf.
These faults would thus be oblique to the overall
Gulf axis and the trend of its south coast and would
have roughly east–west strikes, consistent with the
present north–south direction of extension across the
Gulf from GPS and earthquake slip vectors. These
faults could be the along-strike eastward prolonga-
tions of the active faults that crop out on the south
coast of the Gulf. The segmentation of these
structures may relate to the low-angle dipping fault
C. Clement et al. / Tectonophysics 391 (2004) 97–108 107
plane of the recent earthquake at Aigion in 1995, to
the distribution of its aftershocks (Bernard et al.,
1997), and to other possible low-angle normal faults
in this western part of the Gulf of Corinth (Baker et
al., 1997; Rietbrock et al., 1996).
Acknowledgments
N/O Nadir and its multichannel seismic facility,
operated by IFREMER, and R/V Filia (for OBS
deployment) participated in this SEISGRECE cruise.
We acknowledge the support of their masters and
crews. This multichannel processing was initiated at
Centre de Traitement Sismique at the Institut de
Physique du Globe, Strasbourg. Pre-stack depth
migration was facilitated by C. Ranero, GEOMAR,
Kiel, through the Training and Mobility in Research
Program of the European Union, under grant
ERBFMGECT98-0108. R. Nicolich, L. Jolivet, and
anonymous reviewers provided constructive criticism.
We thank R. Westaway for helpful suggestions and
editorial assistance.
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