Download Upper mantle anisotropy of southeast Arabia passive margin [Gulf of

Arab J Geosci (2012) 5:925–934
DOI 10.1007/s12517-011-0477-2
ORIGINAL PAPER
Upper mantle anisotropy of southeast Arabia passive margin
[Gulf of Aden northern conjugate margin], Oman
Ali Al-Lazki & Cindy Ebinger & Michael Kendall &
George Helffrich & Sylvie Leroy & Christel Tiberi &
Graham Stuart & Khalfan Al-Toobi
Received: 6 August 2011 / Accepted: 2 November 2011 / Published online: 20 November 2011
# Saudi Society for Geosciences 2011
Abstract In this study, we used data recorded by two
consecutive passive broadband deployments on the Gulf of
Aden northern margin, Dhofar region, Sultanate of Oman.
The objective of these deployments is to map the young
eastern Gulf of Aden passive continental margin crust and
upper mantle structure and rheology. In this study, we use
shear-wave splitting analysis to map lateral variations of upper
mantle anisotropy beneath the study area. In this study, we
found splitting magnitudes to vary between 0.33 and 1.0 s
A. Al-Lazki (*)
Department of Earth Sciences, College of Science,
Sultan Qaboos University,
P.O. Box 36, Postal Code 123, Alkhodh, Sultanate of Oman
e-mail: [email protected]
C. Ebinger
Department of Earth and Environmental Sciences,
University of Rochester,
Rochester, NY, USA
M. Kendall : G. Helffrich
Department of Earth Sciences, University of Bristol,
Bristol, UK
S. Leroy
ISTEP UPMC CNRS Paris06,
Paris, France
C. Tiberi
Geosciences Montpellier,
Montpellier, France
G. Stuart
School of Earth Sciences, University of Leeds,
Leeds, UK
K. Al-Toobi
Earthquake Monitoring Center, Sultan Qaboos University,
Alkhodh, Sultanate of Oman
delay times, averaging about 0.6 s for a total of 17 stations
from both deployment periods. Results show distinct abrupt
lateral anisotropy variation along the study area. Three
anisotropy zones are identified: a western zone dominated
by NW–SE anisotropy orientations, an eastern zone dominated with NE–SW anisotropy orientations, and central zone with
mixed anisotropy orientations similar to the east and west
zones. We interpret these shorter wavelength anisotropy zones
to possibly represent fossil lithospheric mantle anisotropy. We
postulate that the central anisotropy zone may be representing
a Proterozoic suture zone that separates two terranes to the east
and west of it. The anisotropy zones west and east were being
used indicative of different terranes with different upper
mantle anisotropy signatures.
Keywords Gulf of Aden . Arabia . SKS . Shear-wave
splitting . Anisotropy . Passive continental margin
Introduction
This study is part of an international collaboration aimed at
studying the evolution and deformation style of the Gulf of
Aden from its northern passive margin in the Dhofar region
to its southern passive margin in Socotra including the Gulf
of Aden basin and ridge (Leroy et al. 2010b). The aim of
the Gulf of Aden research collaboration is to better
understand crust and mantle structure and rheology and
their changes from continental realms in both Arabia
(Dhofar) and the Somalia plate (Socotra) to its oceanic
realms in the Gulf of Aden. This presented research is
focused on studying the upper mantle anisotropy inland on
the continental margin of Arabia, Dhofar region, Oman. It
is aimed at deciphering upper mantle anisotropy in Dhofar
region to further understanding of the eastern Gulf of Aden
926
rifting process. The general approach of this study is aimed
at the analysis of shear-wave splitting data from Dhofar
region and interpreting the results of this study in the
context of both local and regional scale, previous studies’
results and interpretations.
Geodynamic setting and evolution of the Arabian
lithosphere
The formation of present-day Arabia is the result of
complex tectonic events, which took place in different
times in the geologic history of the Arabian plate. These
tectonic events varied between compressional and
extensional events since the Precambrian times. Terrane
accretion in the Precambrian period (c. 715 to c.
630 Ma) is responsible for the buildup of the Arabian
shield that is partly outcropping in central Saudi Arabia,
Yemen, and Oman (Stoeser and Camp 1985). This was
followed by two extensional events during the periods
Late Devonian–Mid Permian and Mid Permian–Late
Cretaceous. The latter two events led to the formation of
the northeast Arabian margin and the formation of the
neo-Tethys Ocean. A compressional event in the Late
Cretaceous was responsible for the obduction of the
Semail Ophiolites along the northeast boundary of Arabia,
Oman (Glennie et al. 1973; Coleman 1981). Finally, the
latest is an extensional event that led to the opening of the
Gulf of Aden and the Red Sea in Middle Cenozoic times
(Leroy et al. 2010a; Hempton 1987).
The Gulf of Aden exemplifies an oblique rift. Its initial
rifting began along its eastern portion in the Oligocene and
Arab J Geosci (2012) 5:925–934
continued until Early Miocene (Cochran 1981; Bellahsen et
al. 2006). Bellahsen et al. (2006) argue for a two-phase
rifting process oriented first along 020° E then 160° E. In
the first phase (020°E), reactivation of Mesozoic fault
structures was dominant. The Gulf of Aden oceanic crust
age is 17.6 Ma up to Shukra el Sheikh Fracture Zone
(Leroy et al. 2004, 2010a).
The Gulf of Aden separates the Arabian Plate from the
Somalian Plate, where Yemen and southern Oman represent
the northern conjugate margin, while Somalia and Socotra
Island represent the southern conjugate margin (Fig. 1). The
young ocean basin is wider in the east and narrows towards
the west as it approaches the Afar Triple Junction. The
eastern Gulf of Aden exemplifies a non-volcanic margin,
while the western Gulf of Aden exemplifies a volcanic
margin where the age of volcanism is between 30 and
16 Ma (Leroy et al. 2004, 2010a).
Our study area is located on the continental margin of
Arabia, north of a portion of the East Sheba Ridge system
(Fig. 1). This ridge segment is bounded in the west by the
Alula-Fartak fault zone and in the east is bounded by the
Socotra-Hadbeen fault zone (Fig. 1). The latter two zones
represent transform accommodation zones, oriented N25°E,
parallel to the direction of opening of the Gulf of Aden
(Leroy et al. 2010a; d’Acremont et al. 2005). The study
area is located in the southernmost region of Oman known
as the Dhofar region (Fig. 1). The Dhofar region is a raised
platform that is part of an east–west-oriented Qara Arch
(Fig. 1). The Qara Arch is part of a sequence of Mesozoic
arches and troughs that occur along the northern margin of
the Gulf of Aden from Yemen to Oman (Beydoun 1996).
These arch-through structures are the result of Jurassic and
Fig. 1 Gray scale topography and bathymetry map of the region showing the study area location. The map also shows important tectonic features
in the region. Solid lines are used to indicate important fracture zones. Arrows indicate the Gulf of Aden current direction of opening
Arab J Geosci (2012) 5:925–934
Cretaceous rifting structures, east–west and northwest–
southeast-oriented occurring along both the northern and
southern Gulf of Aden margins (Beydoun 1996).
Previous work
Although coarsely sampled over the study area, azimuthal
anisotropy derived from surface wave tomography (Debayle et
al. 2005) showed a range of fast anisotropy orientations ranging
from N–S, E–W, and NW–SE (at 50 to 150 km depth) that
coincided within and around the boundaries of this study area.
Larger magnitude N–S anisotropy seemed to occupy the Gulf
of Aden at 50 to 100 km depths. These anisotropy orientations
rotate clockwise in the direction towards the NE–SW with
increasing depth. Smaller magnitude E–W to NW–SE
anisotropy orientations seemed to reduce in magnitude
below 100 km depth (Debayle et al. 2005).
Determination of anisotropy orientation using core-phase
shear-wave splitting methodology was conducted in three
main studies in and surrounding the Arabian plate. Consistent
N–S anisotropy orientations were observed throughout the
western Arabian shield from Asir Terrane, north of Yemen, to
Gulf of Aqabah north, and east in Riyadh (Hansen et al. 2006).
The authors proposed that these anisotropy orientations are
the result of combined NE absolute Arabian plate motion
and NW-oriented flow associated to channelize Afar upwelling along the Red Sea. Similarly, Wolfe et al. (1999) showed
a coherent anisotropy orientation on nine stations mostly
located on central parts of the Arabian shield in Saudi
Arabia. However, they credited the almost N–S anisotropy
orientation to be the result of, either dominantly east–west
Proterozoic terrane accretion of Arabia, or perhaps to
representing the mantle flow of the Afar plume.
A single-station receiver function analysis, located at the
eastern edge of the Arabian Shield, conducted by Levin and
Park (2000) concluded that the observed anisotropy orientation varied between about 5° and 176° azimuth from north
were perhaps the result of two layers anisotropy levels that
occupied the zone below Moho and the Hale’s discontinuity
at about 70 km depth, and that fossil anisotropy, the remnant
of the Proterozoic continent–continent collision, may represent
the measured N–S anisotropy orientation.
A more recent study by Kaviani et al. (2009) focused on
the republic of Iran showed that anisotropy signature tended
to form regional zones. Null anisotropy measurements are
found to coincide with the Zagros, Sanandaj-Sirjan zone,
Alborz, western Kopeh-Dagh, and Binalud in Iran. A
regional band of NW–SE (phi=141±16, del=1.12±0.36 s)
fast anisotropy orientation coincided with the Central Iranian
microplate (CIMC) and Lut Block. Other trends of fast
anisotropy orientations (phi=49±10°; del 1.2±0.32 s) in
eastern Kopeh-Dagh and northern CIMC. Reported anisotropy
927
orientations by Kaviani et al. (2009) study were interpreted to
be associated to fossil anisotropy inherited in the fabric of the
lithospheric mantle.
Sandvol et al. (2003) presented results of broadband
temporary deployment in eastern Turkey and across Arabia–
Eurasia Bitlis suture zone. His anisotropy results showed
generally an average orientation about N45°E throughout the
Eastern Anatolian plateau and across the Bitlis suture. He
attributed the relatively uniform anisotropy to represent a net
anisotropy caused by the northeasterly oriented dominant
asthenospheric mantle flow and fossil lithospheric mantle
anisotropy. The observed anisotropy ranged between 0.7 and
2 s delay times caused by mantle anisotropy.
A travel time tomography study conducted on south and
western Arabia plate shows hot mantle centralized beneath
the western Gulf of Aden ridge, but unfortunately does not
cover our study area (Chang et al. 2011).
Data and methodology
In the first broadband deployment, 11 three-component
broadband (Guralp CMG-40TD) stations were deployed in
the period March 2003 until March 2004 (Fig. 2), recording
at 50 samples per second. This was followed by a second
deployment of 19 broadband stations in the period
September 2005 to August 2006 (Fig. 2). In the second
deployment, seismometers were a mix of Guralp CMG
6TD and 40TD, recording at 50 samples per second.
Stations in both deployments were powered by solar panels,
and placed in remote locations away from traffic and
human noise. The second deployment was intended to
cover larger area to compliment the first deployment. The
Dhofar region topography consists of the flat coastal plains
of Salalah; mountainous areas covered with trees such as
Jabal Samhan, Jabal Qamar, and Jabal Qara; and the semiflat plains that extend to northwest desert areas of south
Oman. This variation in topography prohibited even
distribution of station throughout the study area, and
resulted in semi-concentrated station in the eastern, central,
and western parts of the study area (Fig. 2).
In this study, we use the modified version of the method of
Silver and Chan (1991) and Teanby et al. (2004) to analyze
shear-wave splitting of SKS, PKS, and SKKS of the transverse
component of waves arriving from epicentral distances between
85° and 140°. For more details on the methodology, refer to
Silver and Chan (1991) and Teanby et al. (2004).
Results
We analyzed events of magnitude 5.5 and above, based on
the USGS-NEIC catalogs for the periods March-2003 to
928
Arab J Geosci (2012) 5:925–934
Fig. 2 Elevation map of the study area showing stations locations in
black color-filled triangles representing both March 2003 March 2004
deployment (e.g., S10) and September 2005–September 2006 deploy-
ments (e.g., SAH). Approximate known mountain locations are also
indicated in this map (e.g., Jabal Samhan)
March-2004 and September-2005 to August-2006. During
the two deployment periods, of the 147 possible events
with magnitudes 5.5 and larger, only 19 events recorded by
28 stations yielded usable records for study (Fig. 3). The
majority of these analyzed events were from the east,
mostly from the western Pacific sources, from about 100°
backazimuth direction. Few recorded events were located
along the South America–Nazca plate’s boundary and in the
South Sandwich Sea (Fig. 3).
The analyzed shear-wave splitting events occurred at
two depth ranges 10–212 km depth and 535–622 km depth
(Fig. 4a). A gap of events with depths between 212 and
535 km depth is observed on the analyzed shear-wave
splitting events (Fig. 4a). Also, it was observed consistently
that shallow depth events with magnitudes smaller than 6,
gave low signal-to-noise ratio, resulting into poor splitting
analysis. The analyzed events were arriving from the entire
range between 85° and 135° epicentral distances, with
events clustering at 85°, 100°, 120°, and 130° epicentral
distances (Fig. 4b). Most of the events occurred at about
100° backazimuth direction relative to study area (Fig. 5a,
b). Only four events are coming from 31°, 208°, 256°, and
302° backazimuth directions (Fig. 5a, b). In order to assess
the possibility of a double-layer anisotropy beneath the
station array MDY, HAY, AYD, and S11, backazimuth
versus delay and fast orientation plots were made for each
station (Fig. 6). Periodicity of delays or fast orientations is
used indicative for the presence of more than one
anisotropy layer beneath the station array MDY, HAY,
AYD, and S11 (Fig. 6). However, no clear relationship
could be drawn between anisotropy magnitudes or orientation
with events backazimuth for the array of MDY, HAY, AYD,
and S11 stations (Fig. 6).
The observed anisotropy magnitudes, delay times, were
stacked for a narrow back-azimuth window weighted by their
signal-to-noise ratio. Delay times varied between a minimum
0.15 s and a maximum 1.05 s throughout the study area,
averaging about 0.63 s delay (Table 1). The analysis of
mostly SKS and some PKS and SKKS shear-wave splitting
resulted into three anisotropy zones, western, central, and
eastern zones (Fig. 7). The western domain anisotropy zone
consistently showed NW–SE oriented anisotropy, while the
eastern zone is characterized by sparsely distributed NE–SW
anisotropy orientations. The central zone seemed to act as a
transitional zone between the eastern and western zones, where
its anisotropy signature contained mixed NW–SE as well as
Arab J Geosci (2012) 5:925–934
929
NE–SW anisotropy orientations. The western zone represented
by a north–south-oriented array of stations, MDY, HAY, AYD,
and S11 (Fig. 7). These stations consistently showed NW–SE
00
30
60
anisotropy orientation, averaging about −34° (anti-clockwise
orientation), with an average delay times of about 0.64 s
(Table 1). The eastern zone included the stations DMT, HAD,
700
a
Depth (Kilometers)
600
500
400
300
200
100
0
0
50
100
150
200
Back azimuth (degrees)
250
300
100
Orientation (Degrees)
Fig. 4 a A plot of station-event
backazimuth versus events
depth. Notice events depth gap
at about 250–500 km. b a plot of
station-event epicentral distance
versus stations anisotropy
orientations. Notice the cluster
of events about 100° and
120–130° epicentral distances
90
120
150W
Fig. 3 Azimuthal-equidistant
map showing events analyzed
in this study. Illumination of the
study area is dominantly from
the ESE. The study area is
indicated by the green-filled
triangle
350
b
50
0
−50
−100
80
90
100
110
Distance (degrees)
120
130
140
930
4
Delay time (seconds)
Fig. 5 a A plot of station-event
backazimuth versus delay time.
b Plot of station-event
backazimuth versus fast
anisotropy orientation. The
marker is scaled relative to the
error size. Note the clustering
about 100° backazimuth
Arab J Geosci (2012) 5:925–934
a
3
2
1
0
Orientation (degrees)
100
b
50
0
−50
−100
0
50
SOO, SAH, S09, and TQH stations, that showed consistent
NE–SW anisotropy orientations, averaging about 34° (clockwise rotation), with an average of about 0.42 s delay time.
The eastern zone anisotropy magnitude is about 26% smaller
than that of the western zone. For simplicity purpose, the
remainder stations BAN, DAH, NNM, ASH, MAD, RAH,
AYU, S01, S02, S03, S04, S05, and S06 were considered part
of the central zone. The latter stations showed mixed
anisotropy orientations (Fig. 7).
100
150
200
Backazimuth (degrees)
250
300
350
Discussion
Anisotropy magnitude and orientation results for this study
differ from the N–S regional anisotropy (magnitude average
~1.6 s) structure observed beneath most of the Arabian
shield and along the eastern margin of the Red Sea and the
Gulf of Aqaba reported by earlier studies (Wolfe et al.
1999; Levin and Park 2000; Hansen et al. 2006). This study
showed smaller average anisotropy magnitudes (~0.6 s) and
Fig. 6 Shows backazimuth versus delay and fast orientations plots for each station for the array MDY, HAY, AYD, and S11. The marker size is
indicative of the error size (also, see Table 1)
Arab J Geosci (2012) 5:925–934
Table 1 Includes final stacked
anisotropy results for each
station
stcode Station code, stlat station
latitude, stlon station longitude,
fast the orientation of the fast
anisotropy, tlag anisotropy time
delay (second), dfast fast
orientation calculated error
bound, dlag anisotropy delay
time calculated error bound,
Stack the number of single
anisotropy values used in the
stacking calculations
931
stcode
stlat
stlon
fast
AYD
16.99
53.36
−44
3.75
0.675
0.0313
6
BAN
DMT
17.69
17.73
54.44
55.07
−2
41
1.75
2.75
0.8
0.725
0.0525
0.0437
2
3
tlag
dtlag
Stack
4
0.575
0.0625
4
2.35
3.75
0.775
0.5
0.0375
0.0125
9
6
0.375
0.05
7
0.875
0.325
0.1063
0.1188
3
4
HAD
17.22
55.19
28
HAY
MDY
17.18
17.46
53.34
53.36
−32
−35
NNM
17.36
54.25
21
RAH
SAH
17.06
17.11
53.81
54.68
−68
34
SOO
17.08
54.88
62
TQH
17.06
54.43
46
S01
S02
17.03
17.13
54.11
54
−76
−64
S03
17.25
54.08
86
0.75
0.0938
5
S04
S04
17.44
17.44
54.04
54.04
49
−27
6.26
15.75
0.5
0.625
0.075
0.2188
6
1
S05
S06
S09
S10
17.35
17.62
16.99
17.5
53.97
54.05
54.7
54.2
15
−27
26
−52
1.25
4.75
2
3.5
1.05
0.65
0.75
0.825
0.0113
0.0313
0.0813
0.1688
2
1
8
4
S11
16.82
53.47
−25
6.26
0.6
0.0525
8
4
2.25
17.5
9
15.25
0.925
3.5
4
0.4
0.0375
6
0.15
0.0375
4
0.925
0.475
0.0688
0.1188
3
7
As the observed anisotropy does not seem to conform to
the observed regional anisotropy trend observed mostly in
western parts of the Arabian plate, it may be expected that
the measured anisotropy orientations reflect either partially
or entirely some association with the Gulf of Aden opening
process. The observed anisotropy orientations represent a
continental realm anisotropy structure beneath all stations.
Tiberi et al. (2007) showed that stations located along the
coast including S11, S01, and S09 had crustal thicknesses
of about 27 km, and the thickness of the crust increased
showed abrupt lateral anisotropy variations (Fig. 8). Within
the width of the study area (2.5°) three zones of distinct and
consistent anisotropy orientations are observed (Fig. 8).
This variation of upper mantle anisotropy is similarly
reported in Iran, where anisotropy signature is found to
abruptly change beneath known terranes (Kaviani et al.
2009). The latter study, showed that the Zagros Fold and
Thrust Belt upper mantle is isotropic compared to a NW–
SE anisotropic upper mantle beneath Central Iranian
microplate (Kaviani et al. 2009).
Fig. 7 Anisotropy map of the
study area. Open black line
triangles represent station
location. Red lines represent null
orientations for each station, and
their length is fixed. Blue lines
represent anisotropic
orientations, with their length
reflecting the average anisotropy
delay time (second), magnitude,
for each station
dfast
18.0°
anisotropy orientation
null orientation
DMT
BAN
0.725 second
S06
S10
MDY
DAH
HAS
S04
S05
AYU
S03
HRN
ASH
SHI
RAH
HAD
MAD
S02
AYD
SAH
S01
TQH
SOO
S09
17.0°
MUG
S11
50 km
53.5°
54.0°
17.5°
NNM
54.5°
55.0°
932
Fig. 8 The dashed lines are
interpretation of the observed
anisotropy in the study area. The
central local zone is interpreted
to represent a suture zone
separating the west and east
anisotropy zones
Arab J Geosci (2012) 5:925–934
anisotropy orientation
Western Zone
Central Zone
18.0°
Eastern Zone
null orientation
DMT
BAN
0.725 second
S06
DAH
S10
MDY
HAS
S04
S05
S03
ASH
SHI
MAD
S02
RAH
AYD
HAD
SAH
S01
TQH
SOO
S09
MUG
53.5°
landward beneath the remainder stations to about 34 km
(Fig. 2). Additionally, a study based on the interpretation of
offshore seismic reflection profiles by d’Acremont et al.
(2005) indicates that this study area stations occupy what
they considered proper continental crust. From the above,
we infer that the initial rifting stages did not seem to have
left a strong coherent anisotropy imprint that can be traced
along the northern Gulf of Aden margin. Instead, the
observed anisotropy in this study showed strong lateral
anisotropy variations that does not reflect a single source
for upper mantle deformation (Fig. 8).
Furthermore, since initial rifting did not seem to have left a
strong anisotropy signature, it is then perhaps that later stage
Gulf of Aden opening processes might have affected the upper
mantle anisotropy orientations. We observe that the anisotropy
zoned boundaries parallel the on-land projection of both
Alula-Fartack and Socotra-Hadbeen transform fault zones
(Fig. 8). The Socotra-Hadbeen oceanic transform fault zone
bounds the eastern anisotropy zone from the east side, while
the Alula-fartack transform fault zone on-land projection
falls on both the border zone western-central anisotropy
zones and southern stations of the western array (Fig. 1). On
the one hand, we find that the average anisotropy orientation
of the eastern zone is ~34°±7°, which is subparallel to the
orientation of Socotra-Hadbeen transform fault zone (~25°
clockwise). This may be indicative of possible influence of
the Socotra-Hadbeen transform fault on the margin eastern
zone upper mantle anisotropy. While on the other hand, the
Alula-Fartak on-land projection does not seem to have
influenced the observed anisotropy orientations on either
side, central, or western zones. The western anisotropy zone
forms an acute angle (~60°) with the Alula-Fartak transform
fault projection, but does not align with it. From the above
observation, it is difficult to attribute direct influence of
either fracture zones on the northern Gulf of Aden
Soc
50 km
54.0°
17.0°
otra
-
S11
projection of
Alula-Fartak
ault
Zon
e
HRN
Had
bee
nF
AYU
17.5°
NNM
54.5°
55.0°
continental margin. Furthermore, the Socotra-Hadbeen inland northern projection falls outside the current study area.
Hence, Socotra-Hadbeen in-land influence could not be
inferred from this study yet.
Owing to the complex tectonic history of the region
(Stern 1994; Loosveld et al. 1996; Husseini 1988), the
small-scale localized anisotropy zones could potentially be
reflecting lithospheric fossil anisotropy (Fig. 8). Theses
localized anisotropy observations are smaller in magnitudes
than the regional Arabia plate observations, which may be
considered indicative of perhaps shallower, lithospheric
mantle, anisotropy source. The Arabian Plate terranes
accreted in the Proterozoic period (715–610 Ma) outcrop
in central Saudi Arabia and western parts of Yemen.
Similarly, based on a wealth of subsurface well data,
potential-field data, and seismic reflection data from the
hydrocarbon exploration industry, different basement terranes were mapped beneath the sedimentary cover in
Oman. Furthermore, a major, Cambrian, Western Deformation Front (WDF) is mapped to bound the western side of
South Oman Salt Basin (Stern and Johnson 2010) (Fig. 9).
The southern extent of this deformation front, WDF, falls
nicely on the proposed suture zone of the central anisotropy
zone in our study area (Fig. 9). Allen (2007) proposes a
potential megasuture zone of Late Precambrian-Cambrian
collision-related thrusting coincident with the WDF
(Fig. 9). Hence, we conclude that this study finding
supports and complements the latter two researchers
finding, of the possibility that the central zone is representative of a suture zone, which may perhaps explain the
random anisotropy signature observed in the central
anisotropy zone. Therefore, we propose an interpretation
of the observed distinct anisotropy orientation in the
localized western and eastern zones to possibly represent
two different terranes or cratons that are separated by the
Arab J Geosci (2012) 5:925–934
933
30°N
Paleogene Volc.
Neogene Volc.
Ophiolites
Basement
Iran
Arabian Gulf
25
Oman Sea
UAE
tal fossil anisotropy. This means that the Gulf of Aden
rifting process did not affect the fabric of the lithospheric mantle in this part of the margin. The latter
argument may be supported by the absence of volcanism in the eastern portion of the Gulf of Aden Margin,
where effects of rifting may be subdued at this end of
the Gulf of Aden rifted zone. The location of S11 is
about 20 km inland from where proper oceanic lithosphere
occurs in the eastern Gulf of Aden (d’Acremont et al. 2005).
This may indicate that the rifting deformation event was
focused within a 40-km zone, representing twice the width of
the average transitional zone mapped by d’Acremont et al.
(2005).
Oman
Fr
on
t
line
rm
arder
W
m. suture
es
te
rn
De
ate bo
xim
Appro
fo
20
Conclusions
at
ion
Saudi Arabia
Yemen
Arabian Sea
Study area
0
15
50
55
100
200
300
400
500 km
°
60 E
Fig. 9 Simplified map of Oman showing in dashed line the
approximate subsurface extension of the Western Deformation Front
(after Allen 2007) and this study mapped lithospheric suture zone (m.
suture). Map boundaries do not represent any political boundary
central suture zone (Figs. 8 and 9) and are also supported
by Allen (2007) and Stern and Johnson (2010) findings.
Furthermore, it is natural to pose the following question. To
what extent, laterally, did rifting deform the continental
lithosphere structure? The answer to the latter question could
be deduced from the observation of the three distinct localized
anisotropy zones along the study area. Had the margin been
affected by the rifting process it may be expected that the
lithospheric mantle would have shown homogenous anisotropy
signature along the entire length of the study area, where all
localized anisotropy zones east, central, and west would have
shown similar anisotropy imprint.
The above stated observations could shed some light
on the rifting process along the eastern portion of the
Gulf of Aden. We could deduce that the eastern Gulf of
Aden lithosphere deformation during the rifting process
is focused and perhaps crustal and mantle deformation
is coincident along a restricted or focused stripe. This is
evident from the preserved localized zones of continen-
In this study, we find that recorded anisotropy along the
northern Gulf of Aden margin changes abruptly along the
margin length. It is difficult to confidently attribute the
observed anisotropy to processes related to the opening of
the eastern Gulf of Aden. The observed anisotropy zoning
could be reflecting, perhaps, lithospheric mantle anisotropy
resulting from earlier tectonic events that have affected the
region. The observed anisotropy could not be attributed to a
particular tectonic event. A possible interpretation is that
the observed anisotropy of the eastern and western zones is
representative of older accreted terranes anisotropy, while
the central zone is representing a suture zone. This study
brings more evidence and supporting earlier studies findings on the nature of the Western Deformation Front and its
possibly representing a Precambrian–Cambrian megasuture
zone (Stern and Johnson 2010; Allen 2007) that coincides
with the central anisotropy zone.
We also conclude that the observed anisotropy may reflect
the deformation extent for the entire Gulf of Aden rifting
process, as its upper mantle influence dies out towards the
eastern end of the Gulf of Aden. This speculation is supported
by the almost absent volcanic activity on the eastern portion of
the Gulf of Aden margin. This may indicate that the rifting
process of crust and upper mantle was concurrent and focused
within a narrow zone about 40 km wide.
Acknowledgments The authors are grateful to all who have
contributed directly or indirectly to this work. This work was
supported by NERC grant NE/C514031/1 and a Royal Society travel
grant to the first author. GDR Margins contribution.
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