Download Gorringe Ridge gravity and magnetic anomalies

Geophys. J. Int. (2003) 153, 586–594
Gorringe Ridge gravity and magnetic anomalies are compatible with
thrusting at a crustal scale
J. Galindo-Zaldı́var,1 A. Maldonado2 and A. A. Schreider3
1 Departamento
de Geodinámica, Universidad de Granada, 18071 Granada, Spain. E-mail: [email protected]
Andaluz Ciencias de la Tierra, CSIC/Universidad de Granada, Facultad de Ciencias, 18002 Granada, Spain
3 P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, 23, Krasikova 117218 Moscow, Russia
2 Instituto
Accepted 2002 November 29. Received 2002 November 11; in original form 2001 December 31
SUMMARY
The main features of the deep structure of the Gorringe Ridge are analysed on the basis of gravity
and magnetic measurements, as well as seismic profiles, drill holes, rock dredges, submersible
observations and seismicity data. The gravity and magnetic models of the Gettysburg and
Ormonde seamounts, which form the Gorringe Ridge, suggest that the Moho is approximately
flat and the upper part of the ridge corresponds to a northwestwards vergent fold. This structure
is the result of a northwestward vergent thrust that deformed the oceanic crust, with a minimum
slip of approximately 20 km. The activity of the thrust probably started 20 Myr, and produced the
recent stages of seamount uplift. The seamount is mainly composed of gabbros of the oceanic
crust, serpentinized rocks and alkaline basalts. The large antiform, located in the hangingwall
of the thrust, is probably deformed by minor faults. This oceanic ridge is a consequence of the
oblique convergence between the African Plate and the overlapping Eurasian Plate.
Key words: crustal deformation, Gorringe bank, gravity anomalies, magnetic anomalies,
oceanic crust, plate boundary.
1 I N T RO D U C T I O N
The Eurasian–African plate boundary, between the Azores triple
junction and the Gibraltar Strait, shows a tectonic regime that
changes from transtension in the west to transpression in the east,
with strike-slip motion in its central segment at the Gloria Fault
(Fig. 1). Seismology reveals that while westwards of Gorringe Ridge
the plate boundary is located in a narrow zone, eastwards from this
area, the plate boundary deformation spreads over a much broader
region, more than 300 km wide (Fig. 1) (Griminson & Chen 1986;
Jimenez-Munt et al. 2001). Plate motion calculations (Argus et al.
1989; Kazmin et al. 1990; DeMets et al. 1994) indicate, moreover,
that in the Gorringe Ridge area, the African and European plates are
converging at a relative rate of 0.25–0.42 cm yr−1 along a N145◦ E
vector.
The Gorringe Ridge drill site 120 (Ryan et al. 1973), the four
holes of Bavenit (Matveenkov et al. 1993) and the bottom sampling and submersible observations (CYAGOR Group 1982, 1984;
Sborchikov et al. 1988; Verzhbitsky et al. 1989; Matveenkov &
Al’mukhamedov 1994, 1996; Girardeau et al. 1998) (Fig. 2) clearly
show the basic and ultrabasic composition and the oceanic nature
of the rocks in this area. Most authors also agree that the Gorringe
Ridge oceanic crust was formed in an extensional setting during
the Early Cretaceous. Girardeau et al. (1998) point out that hightemperature and low-temperature shear zones deformed the peridotites and gabbros, producing interlayering of these rocks before
the deposition of Cretaceous sediments. Le Gall et al. (1997) pro-
586
pose that a former Gorringe Ridge was formed at this time by upward
arching of the crust. Sartori et al. (1994) and Le Gall et al. (1997)
relate the present-day location of the Gorringe Ridge to a former
transform discontinuity. Most of these authors (e.g. Sartori et al.
1994; Le Gall et al. 1997; Girardeau et al. 1998) agree that the Gorringe Ridge was deformed and amplified since Miocene times and
suggest mechanisms that produce upward arching of the crust, such
as folding of lithospherical size (Verzhbitsky et al. 1989; Bergeron
& Bonnin 1991; Sartori et al. 1994; Le Gall et al. 1997; Girardeau
et al. 1998; Hayward et al. 1999) or isostatic upward movement by
serpentinization of peridotites (Girardeau et al. 1998).
Some authors have proposed, in addition, that the present-day
activity in the ridge results from a single reverse fault in the northwestern side (Srivastava et al. 1990; Hayward et al. 1999) or in
the southeastern side (Morel & Meghraoui 1996). Other authors,
in contrast, indicate the existence of two conjugated reverse faults
(Kazmin et al. 1990; Tortella et al. 1997) or a fault zone with a
predominantly northward deformation front (Le Gall et al. 1997).
The main objective of this study is to determine the deep structure
of the ridge on the basis of the geophysical and geological data available for the area: gravity and magnetic fields, bottom topography,
seismicity, seismic data, drilling and dredging data. The analysis
of all of these data allows us to propose a new model for the recent development of the Gorringe Ridge. We also improve upon the
knowledge of the present-day structure of this transitional region
of the Eurasian and African plate boundary in the eastern central
Atlantic, from a narrow to a widespread zone of deformation.
C
2003 RAS
Gorringe Ridge deep structure
The ridge is approximately 200 km in length and 80 km in width.
It consists of two seamounts: the Gettysburg Seamount in the west,
with a minimum depth of 40 m, and the Ormonde Seamount in the
east, with a minimum depth of 60 m (CYAGOR Group 1982, 1984;
Kazmin et al. 1990).
The petrologic composition of the Gorringe Ridge is heterogeneous, as shown by the available boreholes (Ryan et al. 1973;
Matveenkov et al. 1993) and by samples collected from the sea
bottom by dredging and by means of submersibles (Auzende et al.
1982; CYAGOR Group 1982, 1984; Sborchikov et al. 1988; Verzhbitsky et al. 1989; Matveenkov & Al’mukhamedov 1994, 1996;
Girardeau et al. 1998). On the steep southeastern slope of the Ormonde seamount an ophiolitic association is exposed (Fig. 2). Alkaline basalts, trachytes and phonolitic lavas are recognized near the
top of the seamount and on the lower part of the slope. In the depression between the two seamounts, the upper part of the section is
made up of 246 m of Early Cretaceous to Pliocene sediments, below
which metabasalt and metagabbro are found, representing part of the
igneous oceanic crust (layers 2 and 3) (Ryan et al. 1973). The top
of the Gettysburg Seamount consists of serpentinized rocks of the
upper mantle, represented principally by harzburgites (Matveenkov
et al. 1993; Sborchikov et al. 1988). Gabbros were also obtained in
the lower part of the seamount (CYAGOR Group 1982, 1984).
Absolute K–Ar age determinations on gabbros dredged from the
slopes of the Gettysburg Seamount give values of 135 ± 3 Ma
(Prichard & Mitchell 1979; Prichard & Cann 1982), or 143 Ma
4
40ºN
2
IBERIA
4
4
EURASIAN
PLATE
GF
GR
BC
4
2
GS
RC
4
32ºN
AFRICAN
PLATE
AFRICA
4
2
20ºW
4ºW
12ºW
Figure 1. Plate boundaries in the Azores–Gibraltar area, modified from Le
Gall et al. (1997) and Hayward et al. (1999). Dots indicate areas of distributed
deformation. GF, Gloria Fault. GR, Gorringe Ridge. GS, Gibraltar Strait.
BC, Betic Cordillera. RC, Rif Cordillera. Contour lines in km. Black arrow
indicates the direction of shortening.
2 GEOLOGICAL AND GEOPHYSICAL
SETTING
The Gorringe Ridge is a SW–NE elongated elevation, rising approximately 5 km above the Horseshoe and Tagus basin plains (Fig. 2).
12°W
11°W
10°W
30
00
N
500
0
W
587
E
40
00
IN
S
S
S
BA
GU
TA
G2
G1
37°N
3000
5000
2000
1000
DSDP120
.
.
4000
M1
00
40
0
400
36°N
4000
M3
E
NG
RI GE
R
GO RID
0
0
200
00
30
300
1000
.BAV5
M2
0
GETTYSBURG
.
20
BAV3,4
ORMONDE
200
BAV6
E
HO
S
E
RS
O
H
SIN
BA
Figure 2. Simplified bathymetric chart of the study area. Modified from Mauffret et al. (1989). Bathymetry in metres. Thick lines indicate the location of
gravity (G1 and G2) and magnetic (M1, M2 and M3) model profiles in Figs 4–7 . The location of boreholes of Glomar Challenger (DSDP) and Russian drill
ship Bavenit (BAV) is also indicated.
C
2003 RAS, GJI, 153, 586–594
588
J. Galindo-Zaldı́var, A. Maldonado and A. A. Schreider
NW
SE
0
2
GORRINGE
TAGUS
BASIN
TWT (s)
Acoustic
RIDGE
HORSESHOE
BASIN
basement
4
6
8
?
?
top of oceanic crust
MOHO
Lithospheric Mantle
?
10
0
MOHO
30 km
12
Figure 3. Representative NW–SE section crossing the Gorringe Ridge at the depression between Gettysburg and Ormonde seamounts (simplified from Sartori
et al. 1994).
(Féraud et al. 1986). The age of the volcanics on the top of
the Ormonde Seamount is 60–67 Ma (Féraud et al. 1982, 1986;
Matveenkov & Al’mukhamedov 1994, 1996). The change from deep
sea facies of sediments to shallow water facies in DSDP site 120
began approximately 20 Ma (Ryan et al. 1973).
Multichannel seismic profiles crossing the Gorringe Ridge show
the structure of the sedimentary layers and the Moho in the Horseshoe Basin (Sartori et al. 1994), but the deep structure of the ridge
is not well observed (Fig. 3). The sedimentary layers in the Tagus
Basin have a thickness of up to 1–2 km. These deposits locally
dip gently towards the northwestern flank of the Gorringe Ridge
(Hayward et al. 1999). Seismic refraction profiles have shown that
the oceanic crust is at least 8 km thick in the Tagus Basin (Purdy
1975). At the southern boundary of the basin there is a large olistostrome body within the Miocene sedimentary sequence, possibly
related to the seamount development (Sartori et al. 1994; Hayward
et al. 1999). The sedimentary sequence in the eastern Horseshoe
Basin exceeds 3 km. The oceanic basement and the sedimentary
cover are deformed by southward and northward vergent reverse
faults found throughout the Horseshoe Basin (Sartori et al. 1994;
Tortella et al. 1997).
In the southern Gorringe area there are also active faults, probably with subvertical surfaces, deforming up to the recent sediments
and having a minimum slip of 100 m (Moskalenko & Kogan 1995).
On the basis of refraction data, the depth of Moho is approximately
14 km (11 s TWT) below sea level in the Horseshoe Basin (Purdy
1975). The ridge seems to overthrust the Tagus Basin by faults that
produced large offsets of the top of the igneous crust (Bergeron &
Bonnin 1991; Sartori et al. 1994). These structures are responsible for the asymmetry of the Gettysburg Seamount, with northern
vergence.
The epicentres are concentrated around the top and the northern
slope of the Gorringe Ridge (Fig. 4). Most of the earthquakes are
shallow, with their depth generally not exceeding 45 km, except for
two intermediate earthquakes (approximately 60–70 km) located
in the Tagus Basin near the footstep of Gorringe Ridge (Souriau
1984); an isolated hypocentre approximately 206 km deep has been
detected in this area (USGS database 2001). Most of the focal mechanisms in the Gorringe area (Griminson & Chen 1986, 1988; Buforn
et al. 1988) show the region to be under NW–SE compression, with
active strike-slip and reverse faults. In the Horseshoe Basin, however, strike-slip earthquake mechanisms with NE–SW compression
trends are also found.
Distribution of the free-air anomalies obtained by satellite altimetry data of the Geosat and ERS-1 (Fig. 4) (Sandwell & Smith 1997)
also highlight the main structures of the area. Gravity models developed for the Gorringe Ridge suggest that Moho depths are quite
variable (Souriau 1984), and most of them consider an uplift of
mantle rocks located directly below the ridge (Purdy 1975; Souriau
1984; Bergeron & Bonnin 1991; Hayward et al. 1999). In contrast,
the model proposed by Souriau (1984), following the interpretation
of Le Pichon et al. (1970) of a nascent subduction with successive
southward and northward vergence and an anomalous mantle located below the ridge, leads to an increase in the Moho depth to
approximately 17 km.
The magnetic field is characterized by local high-intensity shortperiod anomalies, with a span of up to 600 nT and horizontal gradients of more than 100 nT km−1 , and some dipole-like anomalies
(Fig. 5). The dipoles in the Ormonde Seamount are more intense than
those of the Gettysburg Seamount. The main magnetic anomalies
above the Gettysburg seamount are directly associated with massive outcrops of serpentinites (Verzhbitsky et al. 1989), which have
a high remanent magnetization of up to 20 A m−1 and an average
value of 3.2 A m−1 . The Königsberger ratio for these rocks ranges
between 1 and 17. The geochemistry and petrology of the alkaline
basalts are well established by Girardeau et al. (1998). The basalts
dredged on the slopes of the Ormonde Seamount down to the 5000
m isobath have high remanent magnetization (1.9–14.7 A m−1 ) and
a Königsberger ratio of 3.7–72, and are associated with the intense
magnetic anomalies above the seamount.
Serpentinites were drilled by the research vessel ‘Bavenit’ in drill
sites 4 and 5, situated on the Gettysburg Seamount (Fig. 2). The
34 determinations of paleoinclination indicate the existence of a
main mode around 0◦ . The average values for remanent magnetization of drilled specimens are approximately 4.2–4.3 A m−1 , and
the Königsberger ratio exceeds 40 (Verzhbitsky et al. 1989; Popov
et al. 1999).
3 G R AV I T Y A N D M A G N E T I C M O D E L S
Two gravity profiles were modelled through the main seamounts
(Fig. 4): Gettysburg, profile G1 and Ormonde, profile G2. These
seamounts have very well-developed elongations, and 2-D models
represent the main features of their deep structure. The models are
derived from the free-air gravity anomalies, and their geometry is
also based on available seismic data for the areas described above
(Fig. 3). Seismic refraction data further constrain the depth of the
Moho and the mean sediment thickness in the Tagus and Horseshoe
C
2003 RAS, GJI, 153, 586–594
Gorringe Ridge deep structure
589
G2
G1
GR
TP
HS
HP
CPS
AS
SP
mGal
0 - 40 km
40 - 150 km
> 150 km Depth
Figure 4. Satellite-derived free-air gravity anomaly chart of the study area. Contour interval of 10 mGal. Gravity data processed by Sandwell & Smith (1997).
Thick lines indicate the location of gravity model profiles in Fig. 6. Stars correspond to earthquake epicentre from USGS database (2001). AS, Ampere
Seamount. CPS, Coral Patch Seamount. HP, Horseshoe Plain. HS, Hirondella Seamount. GR, Gorringe Ridge. SP, Seine Plain. TP, Tagus Plain.
basins (Purdy 1975). The main crustal structures below the ridge,
however, are unknown from these previous studies.
We propose simple models for the region, using density values
based on the mean densities of rocks forming each one of the main
bodies (Telford et al. 1990) and on the drill data. The densities
considered in the two models are: sea water (1.03 g cm−3 ), sediments
(2.35 g cm−3 ), layer 2 of oceanic crust (2.72 g cm−3 ), layer 3 of
oceanic crust (2.95 g cm−3 ) and mantle (3.32 g cm−3 ). Layer 3 of
oceanic crust in this region is considered to consist of gabbros and
mixed peridotites, sometimes serpentinized, which were structured
before the Mid-Cretaceous (Girardeau et al. 1998). The alkaline
basalts differentiated in magnetic modelling of the Gorringe Ridge
are integrated in layer 3 for gravity modelling, taking into account
the similar density of these rocks.
The Moho depth below the Tagus and Horseshoe abyssal plains,
has been determined as 13.5–14 km, and modelling with a flat Moho
below both the Gettysburg and the Ormonde seamounts is compatible with the observed gravity anomaly (Fig. 6). The Gorringe Ridge
would be formed by the uplift of the crystalline basement of oceanic
crust. The structure of the ridge is asymmetrical, and, according
to Girardeau et al. (1998), the anomalous bodies approximately
C
2003 RAS, GJI, 153, 586–594
follow the sea bottom topography. The sediment thickness, 1 km
thick on the northwestern slope of the ridge, increases northwestward towards the Tagus Basin. The sediments on the southern slope
are practically absent from the top of the ridge to water depths of
3.5 km.
Calculations of mass load anomaly derived for the gravity models
clearly indicate that the area of non-isostasy compensation does not
exceed 80 km in width (Fig. 6), although anomaly values are variable
in this area and the region of maximum values is approximately
20 km in width. The anomalous load of the Gorringe Ridge for the
two profiles is approximately 1.1 × 107 kg m−2 . The mass anomaly
values are steeper for the northern slope.
Three magnetic models were calculated on the basis of the magnetic anomalies observed over the Gorringe Ridge area (Fig. 7). All
three models have a NW–SE trend, orthogonal to the ridge elongation, though not located in the same transects of the gravity models.
Models in 2.5 dimensions, taking into account the length of anomalous bodies orthogonal to the profile, were developed for the main
magnetic dipoles (Figs 5 and 7).
The present-day magnetic field of the region has total intensities of approximately 42 000 nT, declination of –7◦ and
J. Galindo-Zaldı́var, A. Maldonado and A. A. Schreider
11°20’ W
36°
50’ N
W
400
E
S
00
30
0
BA
1
36°
50’ N
1
1
GU
TA
0
20
00
N
SI
S
0
0
100
500
0
20
00
N
11° W
0
11°40’ W
0
590
0
-1
1
2
1
0
0
0
-1
-2
2
1
0
1
4
2
3
3
0
2
3
5
1
3000
-1
2
1
1
-1
36°
30’ N
1
1
0
00
40
1
200
500
0
1 0
10
M3
00
2
2
36°
30’ N
M2
0
M1
4000
IN
00
OE
20
H
ES
00
RS
HO
30
11°40’ W
S
BA
11°20’ W
11° W
Figure 5. Magnetic anomaly chart (thick lines, contour intervals, ×100 nT) of Gorringe Ridge top area (Verzhbitsky et al. 1989) over simplified bathymetry
(thin lines, depth in metres). The trace of magnetic model profiles is shown.
inclination of 51◦ . The magnetic models are developed allowing
for remanent magnetization, bearing in mind the high Königsberger
ratios for most of the dredged and drilled rocks of the area
(Verzhbitsky et al. 1989; Popov et al. 1999). In the Gettysburg
seamount (models M1 and M2, Fig. 7) the magnetic properties
of drilled rocks indicate an inclination of remanent magnetization
near 0◦ . In addition, the dipole has an approximate N135◦ E orientation, and the drilled rocks show mean remanent magnetization of
4.2–4.3 A m−1 . These data were incorporated into our magnetic
models. The dredged serpentinites from the Ormonde seamount
are more weathered, however, and have lower remanent magnetization. Alkali basalts—mainly phonolites—constitute the top of the
seamount, with remanent magnetization between 1.9 and 14.7 A
m−1 . Although the trend of the dipole is also NW–SE, there are no
direct data on the magnetic inclination of these rocks. Considering
the age of 60–67 Ma for these rocks (Féraud et al. 1986; Matveenkov
& Al’mukhamedov 1994, 1996) and the paleogeographic maps of
Dercourt et al. (1993) for this time period that indicate a roughly
25◦ N latitude for the Gorringe Ridge, the dipole field inclination
can be estimated at approximately 45◦ . This value for inclination
was used to determine the remanent magnetization in model M3
(Fig. 7).
Models M1 and M2 of the Gettysburg Seamount consider a body
that has a 4 A m−1 magnetization and a half-length of 2 km (Fig. 7).
The magnetization of the body is equivalent to that of the drilled
serpentinites. In both profiles the anomalous body is elongated, with
low dips. The body crops out in the southern part of the ridge and has
an irregularly shaped bottom. On the northern slopes it is located
approximately 1 km below the relief. The general shape of the body
is asymmetrical, similar to the bathymetric profile. The dip is greater
on the northern slopes. This geometry may correspond to a folded
layer of serpentinites with northern vergence, probably cut by many
faults, thereby producing the irregular bottom.
Model M3 of the Ormonde Seamount takes into account an
8 A m−1 remanent magnetization and a 2 km half-length for the
anomalous bodies. The value of magnetic remanence is approximately in the middle of the range of values determined for alkaline
basalts (Verzhbitsky et al. 1989). This body, with a thickness that
reaches 700 m on the top of the seamount, approximately follows
the bottom relief and may constitute a folded layer in the seamount,
probably made up of basalts.
4 DISCUSSION
The gravity model of Hayward et al. (1999), located along a profile
on the Ormonde Seamount, implies the emplacement of a body of
mantle rocks below the Gorringe Ridge, some 6 km deep and 25 km
wide. The intrusion is characterized by very steep walls and nearly
vertical Moho slopes, rising from 12 to 14 km in depth, which
are difficult to interpret geologically. In contrast, our two gravity
profiles (G1 and G2, Fig. 6) cutting across Gettysburg and Ormonde
seamounts, can also be explained with an approximately flat Moho.
The calculated crustal thickness data (Jimenez-Munt et al. 2001)
suggest a crust approximately 13 km thick in the area of Gorringe
Ridge, with a sea bottom near sea level, and approximately 8 km
thick in the Horseshoe Basin, with a sea bottom depth of 5 km.
C
2003 RAS, GJI, 153, 586–594
Gorringe Ridge deep structure
591
Figure 6. 2-D gravity models for Gettysburg (G1) and Ormonde (G2) seamounts and load-mass anomaly profiles. 1, 3.2 g cm−3 (Mantle). 2, 2.95 g cm−3
(layer 3, oceanic crust, in Gorringe Ridge including gabbros, serpentinites, peridotites and alkaline basalts). 3, 2.72 g cm−3 (layer 2, oceanic crust). 4, 2.35 g
cm−3 (sediments of layer 3, oceanic crust). 5, 1.03 g cm−3 (Sea water). Location in Figs 2 and 4.
These data support our model of a nearly flat Moho in the area,
with a mean depth of approximately 13 km. Our interpretation also
contrasts with the model of Bergeron & Bonnin (1991), who propose
an upward arching of the Moho below the Gorringe Ridge undulated
on a regional scale, with the final stage of the model proposed by
Le Gall et al. (1997), and with the model proposed by Souriau
(1984). Although the latter model considers Gorringe Ridge to be
the result of overprinted thrust structures, it requires the presence
of an anomalous mantle and irregular Moho beneath the ridge.
The deep structures of the two seamounts have similar features
in our models. Layer 3 is thicker in the seamounts than in the basins
C
2003 RAS, GJI, 153, 586–594
(Fig. 8). This geometry is produced by recent deformations and, at
least partially, by the interlayered gabbros and peridotites with variable degrees of serpentinization which, according to submersible
observations, were formed during the initial stages of the oceanic
crust development (Girardeau et al. 1998). Moreover, the thickness
of the body mainly corresponding to layer 3, appears to have been enlarged by the addition of alkaline basalt flows. As the gravity models
do not allow us to specify the geometry of the interlayered rocks,
they are considered as a single anomalous body. The low-density
body (profiles G1 and G2, Fig. 6) as well as the low-magnetized
body (profile M1 and M2, Fig. 7) located in the northern slope
592
J. Galindo-Zaldı́var, A. Maldonado and A. A. Schreider
Calculated
Observed
M-1
200
nT
100
0
-100
0
5
15
10
20
25
NW
30
Distance (km)
40
35
SE
Depth ( km )
0
2
4
6
nT
Calculated
Observed
M2
200
100
0
-100
-200
1
0
5
10
NW
15
Distance (km)
20
SE
2
Depth ( km )
0
2
4
3
6
Calculated
Observed
M-3
400
nT
200
0
0
NW
5
10
15
20
Distance (km)
25
SE
Depth ( km )
0
2
4
6
Figure 7. 2.5-dimensional magnetic models for Gettysburg (M1, M2) and Ormonde (M3) seamounts. Only remanent magnetization was considered on the
basis of the rocks magnetic properties (see text). Half-length of bodies, 2 km. 1, Host rock. 2, Body with 4 A m−1 remanent magnetization (attributed to
serpentinites). 3, Body with 8 A m−1 remanent magnetization (attributed to alkaline basalts). The terminations of calculated profiles were eliminated because
of ‘edge’ effects. Location in Figs 2 and 5.
correspond to the olistostroms detected in this area by seismic profiles (Sartori et al. 1994; Hayward et al. 1999).
Although the Gorringe Ridge is considered to have developed
since the Cretaceous (Le Gall et al. 1997), the present-day structure
may be mainly related to the recent (since Miocene) deformation.
The magnetic models suggest that the structure of the upper part of
both the Gettysburg and the Ormonde seamounts consists of a largescale antiformal, north-vergent fold that evokes the fold proposed by
Le Gall et al. (1997). However, the observed gravity profiles can be
modelled using an approximately flat Moho below the seamounts.
This geometry may be formed by the development of the northwestward thrust, deforming the oceanic crust of the Gorringe Ridge
(Fig. 8), which folded the layers of serpentinites included in the
oceanic crust and the alkali basalt body. These structures create the
present-day bathymetry of the area. Although it is possible that a
backthrust deformed the base of the southern slope, as proposed by
Kazmin et al. (1990) and Tortella et al. (1997), the gravity models
suggest that backthrust development has only limited importance.
In our models of the Gorringe Ridge area, the African Plate overlies the Eurasian Plate. The most important active structure is a
northwest early verging thrust plane dipping shallowly to the SE
that may produce the seismicity observed in the Horseshoe Basin.
Thrust dip should be low, and probably subparallel to the southern
slopes, because steeply dipping reverse faults will cut the Moho at
high angles, and may produce large local variations in Moho depth
that are in contradiction with the results of the gravity models. According to the gravity models (Figs 6 and 8), the thrust slip of the
African Plate above the Eurasian Plate in the Gorringe Ridge area
is approximately 20 km, although the fault surface has more than
80 km in length in the analysed sections. Given the relative motion
of the two plates in this region described, for example, by the Nuvel
1A model of 4 mm yr−1 in a N120◦ trend (DeMets et al. 1994),
C
2003 RAS, GJI, 153, 586–594
Gorringe Ridge deep structure
593
Figure 8. Tectonic sketches of the Gorringe Ridge recent evolution based on all the available data. Not to scale. Legend: 1, Mantle (M) (a, emplaced in
Cretaceous time; b, undifferentiated); 2, layer 3 of oceanic crust, mainly gabbros over 135 Myr old, mixed with bodies of mantle rocks according to Girardeau
et al. (1998) (L3 ); 3, layer 2 of oceanic crust, mainly volcanic rocks over 135 Myr old (L2 ); 4, volcanic rocks interlayered with sediments over 60 Myr old; 5,
layer 1 of oceanic crust, mainly sediments (L1 ); 6, sea water; 7, trend of compression; 8, extension; 9, earthquake hypocentre; 10, main reverse fault.
and assuming whole plate boundary deformation concentrated in
the thrust structure, it would have taken approximately 5 Myr for
such a thrust structure to develop. However, the onset of uplift indicated by the shallow water sediments of site 120 is 20 Myr old (Ryan
et al. 1973). These data suggest that nearby structures may have also
accommodated some part of the deformation of the plate boundary,
for instance the thrusts located in Horseshoe Basin (Fig. 3). The
deformation is mainly concentrated in the Gorringe Ridge in recent
times, however, as indicated by the seismicity distribution (Fig. 4).
There is no clear evidence of a Benioff zone related to subduction
despite the compressive tectonics and the intermediate earthquakes
occurring in the Gorringe Ridge area. The intermediate seismicity
is probably related to heterogeneities that are not connected with the
shallow structure determined by the gravity and magnetic surveys.
The focal mechanisms with NW–SE extension or compression axes
(Hayward et al. 1999) may be a consequence of the large fold that developed in the oceanic crust in association with the development of
the seamount. Nevertheless, this topography is not necessarily compensated isostatically on a local level, as shown by the mass-load
anomaly that features an 80 km wide area of mass excess (Fig. 6). In
addition, this structure does not show downbending flexure, probably because it is active and uncompensated.
The plate boundary has a predominantly E–W orientation and a
dextral transpressive character, highlighted by the earthquake focal
mechanism (Griminson & Chen 1986, 1988; Buforn et al. 1988).
The development of the Gorringe Ridge includes northwestward
thrusting of the African Plate over the Eurasian Plate, accompanied
by the thrusting of a block belonging to the African crust. Thus,
the E–W dextral transcurrent faults west of the Gorringe Ridge area
are replaced by NE–SW-striking thrusts, orthogonal to the trend of
compression, in a setting with a more variable local stress field.
5 C O N C LU S I O N S
The comprehensive analysis of available geological and geophysical information provides new insights as to the present knowledge
of the African–Eurasian plate boundary in the Gorringe Ridge area.
Gravity and magnetic models suggest that the Gorringe Ridge, com
C
2003 RAS, GJI, 153, 586–594
prising the Gettysburg and Ormonde seamounts, is the result of a
northwest-vergent thrust that deformed the oceanic crust. This thrust
is characterized by a gentle southeastward dip, a minimum slip of
approximately 20 km, and an onset of activity approximately 20
Ma. Present-day seismicity in the Tagus Basin probably bears some
relation with the prolongation in depth of this thrust. The hangingwall of the thrust is made up mainly of gabbros of oceanic crust
and serpentinized rocks, structured in Early Cretaceous, and alkaline basalts. The seamount slopes are covered by a thin layer of
sediments, sometimes with outcrops of the crystalline rocks.
Magnetic models show that the internal structure corresponds to a
large antiform, most likely deformed by minor faults, that folded the
serpentinite and alkaline basalt bodies. This antiform would have
developed simultaneously with the thrust structure, as a consequence
of the oblique convergence between the African and Eurasian plates.
The Gorringe Ridge, located in the area of transition from a sharp
westward to a diffuse eastward plate boundary, stands out as an
interesting example of tectonic oceanic seamount development.
AC K N OW L E D G M E N T S
Anatoly Schreider would like to express his appreciation for the
CSIC, Junta de Andalucı́a and the University of Granada grant allowing him to spend periods at the Instituto Andaluz de Ciencias
de la Tierra and the University of Granada, during which time this
study was elaborated. We also acknowledge the comments of Dr
J. Woodside that have helped to improve this paper, and the very
interesting comments of two anonymous referees.
REFERENCES
Argus, D.F., Gordon, R.G., DeMets, C. & Stein, S., 1989. Closure of the
Euarasian–Africa–North America plate motion circuit and tectonic of the
Gloria Fault, J. geophys. Res., 94, 5585–5602.
Auzende, J.M. et al., 1982. The Gorringe Bank: first results of submersible
expeditions CYAGOR II, Terra Cognita, 2, 123–130.
594
J. Galindo-Zaldı́var, A. Maldonado and A. A. Schreider
Bergeron, A. & Bonnin, J., 1991. The deep structure of Gorringe Bank (NE
Atlantic) and its surrounding area, Geophys. J. Int., 105, 491–502.
Buforn, E., Udı́as, A. & Colombás, M.A., 1988. Seismicity, source mechanisms and tectonics of the Azores–Gibraltar plate boundary, Tectonophysics, 152, 89–118.
CYAGOR Group, 1982. The Gorringe Bank: first results of submersible
expedition CYAGOR II, Terra Cognita, 2, 123–130.
CYAGOR Group, 1984. Intraoceanic tectonism on the Gorringe Bank: observations by submersibile, in Ophiolites and Oceanic Lithosphere, pp.
113–120, eds Gass, E.I.C., Lippard, S.J. & Shelton A.W., Spec. Publ. Geol.
Soc., Oxford.
DeMets, C., Gordon, R.G., Argus, D.F. & Stein, S., 1994. Effect of recent
revisions to the geomagnetic reversal time scale on estimate of current
plate motions, Geophys. Res. Lett., 21, 2191–2194.
Dercourt, J., Ricou, L.E. & Verielynck, B., 1993. Atlas Tethys paleoenvironmental maps. Gauthier-Villars, Paris, 14 sheets.
Féraud, G., Gastaud, J., Auzende, J.M., Olivet, J.L. & Cornen, G., , 1982.
40Ar/39Ar for the alkaline volcanism and the basement of Gorringe Bank,
North Atlantic Ocean, Earth. planet. Sci. Lett., 57, 211–226.
Féraud, G., York, D., Mevel, C., Cornen, G., Hall C. & Auzende, J., 1986.
Additional Ar40 –Ar39 dating of the basement and alkaline volcanism of
Gorringe Bank (Atlantic ocean), Earth planet. Sci. Lett., 79, 255–269.
Girardeau, J. et al., 1998. Extensional tectonics in the Gorringe Bank rocks,
Eastern Atlantic ocean: evidence of an oceanic ultra slow mantellic accreting centre, Terra Nova, 10, 330–336.
Griminson, N.L. & Chen, W.P., 1986. The Azores–Gibraltar plate boundary:
focal mechanisms, depth of earthquakes and their tectonic implications,
J. geophys. Res., 91, 2029–2047.
Griminson, N.L. & Chen, W.P., 1988. Source mechanisms of four recent
earthquakes along Azores Gibraltar plate boundary, Geophys. J. Int., 92,
391–401.
Hayward, N., Watts, A., Westbrook, G. & Collier, J., 1999. A seismic reflection and GLORIA study of compressional deformation in the Gorringe
Bank region, eastern North Atlantic, Geophys. J. Int., 138, 831–850.
Jimenez-Munt, I., Fernández, M., Torné, M. & Bird, P., 2001. The transition from linear to diffuse plate boundary in the Azores–Gibraltar region: results from a thin-sheet model, Earth planet. Sci. Lett., 192, 175–
189.
Kazmin, V.G., Marova, N.A., Alekhina, G.N. & Tichonova, N.F., 1990.
Neogene–Quaternary deformation of the oceanic lithosphere in the vicinity of Gorringe bank, Atlantic, Izvestiya AN SSSR, seriya geologicheskaya,
12, 48–56.
Le Gall, B., Piqué, A., Réhault, J.P., Specht, M. & Malod J., 1997. Structure et
mise en place d’ une ride océanique dans un contexte de limite de plaques
convergentes: le Banc de Gorringe (SW Ibérie), C.R. Acad. Sci. Paris.,
325, 853–860.
Le Pichon, X., Bonnin, J. & Pautot, G., 1970. The Gibraltar end of the
Azores–Gibraltar plate boundary, an example of compressive tectonics.
Upper Mantle Committee Symposium, Flagstaff, A.2.
Matveenkov, V.V. & Al’mukhamedov, A.I., 1994. Consistence of the Gorringe Bank ultrabasites (Atlantic ocean), Geochimia, 4, 616–622.
Matveenkov, V.V. & Al’mukhamedov, A.I., 1996. Alkalic volcanism of the
Gorringe bank, Atlantic ocean, Petrologia, 4, 46–56.
Matveenkov, V.V., Poyarkov, S.G., Dmitrenko, O.V., Al’mukhamedov, A.I.,
Gamsakhurdia, G.R. & Kuznetsov, O.L., 1993. Geological particularities
of the seamounts structure in Azoro-Gibraltar zone (based on results of
Deep Sea Drilling), Oceanologia, 33, 752–762.
Mauffret, A., Mougenot D., Miles P. & Malod J., 1989. Results from Multichannel reflection profiling of the Tagus Abyssal Plain (Portugal)—
comparision with the Canadian Margin, AAPG Memoir, 46, 379–393.
Morel, J.L. & Meghraoui, M., 1996. Gorringe–Alboran–Tell Tectonic
Zone—a transpression system along the Africa–Eurasia plate boundary,
Geology, 24, 755–758.
Moskalenko, V.N. & Kogan, L.I., 1995. Sedimentary cover structure in the
eastern Azores–Gibraltar zone (the Horseshoe basin), Geotectonics, 28,
334–343.
Prichard, H.M. & Cann, J.R., 1982. Petrology and mineralogy of dredged
gabbro from Gettysburg Bank, Eastern Atlantic, Contrib. Mineral. Petrol.,
79, 46–55.
Prichard, H.M. & Mitchell, J.G., 1979. K–Ar data for the age and evolution
of the Gettysburg bank, North Atlantic ocean, Earth planet. Sci. Lett., 44,
261–268.
Popov, K.V., Shcherbakov, B.P. & Tselmovich, V.A., 1999. The analysis of
magnetic properties and results of Paleomagnetic investigation of serpentinites from Gorringe Ridge (Azoro-Gibraltar fracture zone), Oceanologia, 39, 288–297.
Purdy, G.M., 1975. The Eastern End of the Azores–Gibraltar plate boundary,
Geophys. J. R. astr. Soc., 43, 973–1000.
Ryan, W.B.F. et al., 1973. Init. Repts. DSDP, site 120, Washington DC, US
Governement Printing Office., 13, 19–39.
Sandwell, D.T. & Smith, W.H., 1997. Marine gravity anomaly from Geosat
and ERS 1 satellite altimetry, J. geophys. Res., 102, 10 039–10 054.
Sartori, R., Torelli L., Zitellini N., Peis D. & Lodolo E., 1994. Eastern segment of the Azores–Gibraltar line (central-eastern Atlantic): an oceanic
plate boundary with diffuse compressional deformation, Geology, 22,
555–558.
Sborchikov, I.M., Schreider, A.A., Rimsky-Korsakov, N.A. & Yastrebov,
V.S., 1988. The Gorringe Ridge and Azoro-Gibraltar zone tectonics,
Oceanologia, 28, 967–973.
Souriau, A., 1984. Geoid anomalies over Gorringe Ridge, North Atlantic
Ocean., Earth planet. Sci. Lett., 68, 101–114.
Srivastava, S., Roest, W., Kovacs, L., Oakey, G., Levesque S., Verhoef J.
& Macnab R., 1990. Motion of Iberia since the late Jurassic: results
from detailed aeromagnetic measurements in the Newfoundland Basin,
Tectonophysics, 184, 229–260.
Telford, W.M., Geldart, L.P. & Sheriff, R.E., 1990. Applied Geophysics,
p. 770. Cambridge University Press, Cambridge.
Tortella, D., Torne, M. & Perez-Estaun, A., 1997. Geodynamic evolution of
the eastern segment of the Azoro-Gibraltar Zone of Cadiz region, Mar.
Geophys. Res., 19, 211–230.
USGS database, 2001. National Earthquake Information Center,
http://wwwneic.cr.usgs.gov/.
Verzhbitsky, Y., Gorodnitskiy, A., Yemelianov, M., Lobkovskiy, L., Marova,
N., Sborschikov, I., Schreider, A.A. & Yastrebov V., 1989. New Data on
geologic structure and tectonic development of the underwater Gorringe
Ridge (North Atlantic), Geotectonika, 1, 12–22.
C
2003 RAS, GJI, 153, 586–594