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.

References Allen PA (2007) The Huqf Supergroup of Oman: basin development and context for Neoproterozoic glaciation. Earth Sci Rev 84:47. doi:10.1016/j.earscirev.2007.06.005

934 Bellahsen N, Fournier M, d’ Acremont E, Leroy S, Daniel JM (2006) Fault reactivation and rift localization: northeastern Gulf of Aden margin. Tectonics 25. doi:10.1029/2004TC001626 Beydoun Z (1996) Sedimentary basins of the Republic of Yemen: their structural evolution and geological characteristics. Rev Inst Fr Petrole Geol 51(51):13 Chang S-J, Merino M, Lee SVd, Stein S, Stein CA (2011) Mantle flow beneath Arabia offset from the opening Red Sea. Geophys Res Lett 38(L04301):5. doi:10.1029/2010GL045852 Cochran JR (1981) The Gulf of Aden: structure and evolution of a young ocean basin and continental margin. J Geophys Res 86:25 Coleman RG (1981) Tectonic setting for ophiolite obduction in Oman. J Geophys Res 86(B4):12 d’Acremont E, Leroy S, Beslier M-O, Bellahsen N, Fornier M, Robin C, Maia M, Gente P (2005) Structure and evolution of the eastern Gulf of Aden conjugate margin from seismic reflection data. Geophys J Int 160:22. doi:10.111/j.1365-246X.2005.02524.x Debayle E, Kennett BL, Priestly K (2005) Global azimuthal seismic anisotropy and the unique plate motion deformation of Australia. Nature 433:4. doi:10.1038/nature Glennie KW, Boeuf MGA, Clarke MWH, Moody-Stuart M, Pilaar WFH, Reinhardt BM (1973) Late Cretaceous nappes in Oman mountains and their geologic evolution. AAPG Bulletin 57:22 Hansen S, Schwartz S, Al-amri A, Rogers A (2006) Combined plate motion and density-driven flow in the asthenosphere beneath Saudi Arabia: evidence from shear-wave splitting and seismic anisotropy. Geology 34(10):4. doi:10.1130/G22713.1 Hempton MR (1987) Constraints on Arabian plate motion and extensional history of the Red Sea. Tectonics 6:9 Husseini MI (1988) The Arabian Infra-Cambrian extensional system. Tectonophysics 8:11 Kaviani A, Hatzfeld D, Paul A, Tatar M, Priestley K (2009) Shearwave splitting, lithospheric anisotropy, and mantle deformation beneath the Arabia-Eurasia collision zone in Iran. Earth Planet Sci Lett 286. doi:10.1016/j.epsl.2009.07.003 Leroy S, Gante P, Fournier M, d’Acremont E, Patriat P, Beslier MO, Bellahsen N, Maia M, Blais A, Perrot J, Al-Kathiri A, Merkouriev S, Fleury JM, Ruellan PY, Lepvrier C, Huchon P (2004) From rifting to spreading in the eastern Gulf of Aden: a geophysical survey of a young oceanic basin from margin to margin. Terra Nova 16:8

Arab J Geosci (2012) 5:925–934 Leroy S, d’Acremont E, Tiberi C, Basuyau C, Autin J, Lucazeau F, Sloan H (2010a) Recent off-axis volcanism in the eastern Gulf of Aden: implication for plume-ridge interaction. Earth Planet Sci Lett 293:14. doi:10.1016/j.epsl.2010.02.036 Leroy S, Lucazeau F, d’Acremont E, Watremez L, Autin J, Rouzo S, Bellahsen N, Tiberi C, Ebinger C, Beslier M-O, Perror J, Razin P, Rolandone F, Sloan H, Stuart G, Al-Lazki A, Al-Toubi K, Bache F, Bonneville A, Goutorbe B, Huchon P, Unternehr P, Khanbari K (2010b) Contrasted styles of rifting in the eastern Gulf of Aden: a combined wide-angle, multichannel seismic, and heat flow survey. GC Res Lett 11(7):14. doi:10.1029/2009GC002963 Levin V, Park J (2000) Shear zones in the Proterozoic lithosphere of the Arabian Shield and the nature of the Hales discontinuity. Tectonophysics 323:18 Loosveld R, Bell A, Teerken J (1996) The tectonic evolution of Interior Oman. Geo Arab 1:24 Sandvol E, Turkelli N, Zor E, Gok R, Bekler T, Gurbuz C, Seber D, Barazangi M (2003) Shear wave splitting in a young continentcontinent collision: an example from Eastern Turkey. Geophys Res Lett 30(24):4. doi:10.1029/2003GL017390 Silver PG, Chan WW (1991) Shear wave splitting and subcontinental mantle deformation. J Geophys Res 96(B10):25 Stern RJ (1994) Arc assembly and continental collision in the Neoproterozoic East African Orogen: implications for the consolidation of Gondwana land. Ann Rev Earth Planet Sci 22:33 Stern RJ, Johnson P (2010) Continental lithosphere of the Arabian Plate: a geologic, petrologic, and geophysical synthesis. Earth Sci Rev 101:39. doi:10.1016/j.earscirev.2010.01.002 Stoeser DB, Camp VE (1985) Pan-African microplate accretion of the Arabian Shield. Geol Soc Am Bull 96(7):10 Teanby N, Kendall J-M, Jones RH, Barkved O (2004) Stressinduced temporal variations in seismic anisotropy observed in microseismic data. Geophys J Int 156:8. doi:10.1111/j.1365246X.2004.02212.x Tiberi C, Leroy S, d’ Acremont E, Bellahsen N, Ebinger C, Al-Lazki A, Pointu A (2007) Crustal geometry of the northern Gulf of Aden margin from receiver function. Geophys J Int 168:14. doi:10.1111/j.1365-246X.2006.0329.x Wolfe CJ, Vernon FL III, Al-amri A (1999) Shear-wave splitting across western Saudi Arabia: the pattern of upper mantle anisotropy at a Proterozoic shield. Geophys Res Lett 26(6):4