The distribution of Moho depths beneath the Arabian plate and margins

Tectonophysics 609 (2013) 234–249

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Review Article

The distribution of Moho depths beneath the Arabian plate and margins J. Mechie a,⁎, Z. Ben-Avraham b, M.H. Weber a, c, H.-J. Götze d, I. Koulakov e, A. Mohsen a, f, M. Stiller a a

Deutsches GeoForschungsZentrum - GFZ, Section “Geophysical Deep Sounding”, Telegrafenberg, 14473 Potsdam, Germany Department of Geophysical, Atmospheric and Planetary Sciences, Tel-Aviv University, Israel c Institut für Geowissenschaften, Universität Potsdam, Germany d Institute of Geosciences, Christian Albrechts University, Otto-Hahn-Platz 1, 24118 Kiel, Germany e Institute of Petroleum Geology and Geophysics SB RAS, Prosp. Akademika Koptyuga, 3, 630090, Novosibirsk, Russia f An-Najah National University, Nablus, Palestine b

a r t i c l e

i n f o

Article history: Received 5 June 2012 Received in revised form 23 October 2012 Accepted 19 November 2012 Available online 30 November 2012 Keywords: Moho depths Arabian plate Red Sea Velocity models Receiver functions Satellite gravity data

a b s t r a c t In this study three new maps of Moho depths beneath the Arabian plate and margins are presented. The first map is based on the combined gravity model, EIGEN 06C, which includes data from satellite missions and ground-based studies, and thus covers the whole region between 31°E and 60°E and between 12°N and 36°N. The second map is based on seismological and ground-based gravity data while the third map is based only on seismological data. Both these maps show gaps due to lack of data coverage especially in the interior of the Arabian plate. Beneath the interior of the Arabian plate the Moho lies between 32 and 45 km depth below sea level. There is a tendency for higher Pn and Sn velocities beneath the northeastern parts of the plate interior with respect to the southwestern parts of the plate interior. Across the northern, destructive margin with the Eurasian plate, the Moho depths increase to over 50 km beneath the Zagros mountains. Across the conservative western margin, the Dead Sea Transform (DST), Moho depths decrease from almost 40 km beneath the highlands east of the DST to about 21–23 km under the southeastern Mediterranean Sea. This decrease seems to be modulated by a slight depression in the Moho beneath the southern DST. The constructive southwestern and southeastern margins of the Arabian plate also show the Moho shallowing from the plate interior towards the plate boundaries. A comparison of the abruptness of the Moho shallowing between the margins of the Arabian plate, the conjugate African margin at 26°N and several Atlantic margins shows a complex picture and suggests that the abruptness of the Moho shallowing may reflect fundamental differences in the original structure of the margins. © 2012 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Data sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Controlled-source seismic profiles . . . . . . . . . . . . . . . . . . . . 2.2. Receiver functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Regional earthquake tomography . . . . . . . . . . . . . . . . . . . . 2.4. Ground-based gravity studies . . . . . . . . . . . . . . . . . . . . . . 3. Map of Moho depths derived from the combined gravity field model, EIGEN 06C . . 4. Maps and cross-sections of Moho depths based on seismic and ground-based gravity 5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The Arabian plate, with an area of 0.12 steradian or about 4,800,000 km 2, is one of the 14 large plates whose motion was ⁎ Corresponding author. Tel.: +49 331 288 1237; fax: +49 331 288 1266. E-mail address: [email protected] (J. Mechie). 0040-1951/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2012.11.015

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described by the NUVEL-1A poles (Bird, 2003). The plate is notable in that it is bounded on its various sides by major representatives of all three types of plate boundaries, namely constructive, conservative and destructive. For example, on its southwestern and southeastern sides it is bounded by the constructive margins of the Red Sea and the Gulf of Aden, which are two of the youngest examples of sea-floor spreading (Fig. 1). On its western side it is bounded by the

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height in metres 4000 36˚

d

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7 4 c b

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c 14

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Gulf of Aden 48˚

50˚

52˚

−4500 54˚

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58˚

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Fig. 1. Location map of the Arabian plate and margins. Blue dots, with numbers tied to locations and references in Table 1, show points for which Moho depths have been obtained from seismic wide-angle reflection/refraction and near-vertical incidence reflection profiles. Black crosses, with letters tied to locations and references in Table 1, show points for which Moho depths have been obtained from receiver functions. Red lines show the locations of the wide-angle reflection/refraction profiles for which data are shown in Fig. 2. Green line across the Dead Sea Transform (DST) shows the location of the near-vertical incidence reflection profile for which a migrated section is shown in Fig. 3. Red triangles show the locations of two seismic stations for which receiver functions are shown in Fig. 4. Plate boundaries (black lines) are from model PB2002 (Bird, 2003). Boundary between Arabian shield and Arabian platform (black dashed line) is from Al-Amri and Gharib (2000).

Dead Sea Transform (DST), which is one of the classical examples of a conservative, transform plate boundary. Finally, its northern side forms part of the destructive Alpine–Himalayan collisional belt, represented in the study region by the Zagros mountains (Fig. 1). Whereas the DST and the recent constructive margins of the Red Sea and the Gulf of Aden have attracted quite a lot of attention in terms of crustal structure studies, this has not been the case for the interior of the Arabian plate and its northern margin. Lazar et al. (2012) and Stern and Johnson (2010) present recent overviews of geophysical and geological data and studies in the region of the Arabian plate. The lack of attention for large regions of the plate interior can be exemplified by the ground-based gravity studies (see Fig. 2 in Lazar et al., 2012). This lack is perhaps due to a large extent to the relative inaccessibility of the interior of the plate due to its being mainly a desert environment and to the fact that the northern margin is on the one hand not as spectacular as the Himalaya–Tibet system and on the other hand not as accessible as the European Alps. The aim of this study is to collate and describe information on Moho depths, the single best known crustal parameter (Molinari and Morelli, 2011), from controlled-source seismic profiles and receiver function studies throughout the region (Fig. 1), a regional earthquake tomography study in the northwest part of the region and ground-based gravity studies mainly towards the edges of the study region and, as a result of this collation, construct two maps of Moho depths for the region. One of these maps includes the ground-based gravity studies and the other does not. As these maps do not unfortunately cover the entire region, a map of Moho depths derived from

the combined gravity field model, EIGEN 6C (Förste et al., 2011), which includes data from satellite missions will also be presented. Additionally, cross-sections of Moho depths along profiles crossing the whole region where data exist more often than not or the plate margins where Moho depths change rapidly over short lateral distances will be presented. In the discussion, a comparison will be made between the maps and cross-sections derived in this study on the one hand and, on the other hand, the CRUST2.0 global model (Bassin et al., 2000) and previous crustal thickness maps for the region (Baranov, 2010; Molinari and Morelli, 2011; Pasyanos and Nyblade, 2007; Seber et al., 1997, 2000; Segev et al., 2006). 2. Data sources 2.1. Controlled-source seismic profiles The first controlled-source seismic profiles in the region were offshore refraction profiles carried out in the 1950s and 1960s in the Gulf of Aden and the Red Sea. Whereas Laughton and Tramontini (1969) were able to determine Moho depths and upper mantle velocities from some of the lines in the Gulf of Aden, neither Drake and Girdler (1964) nor Tramontini and Davies (1969) were able to determine either Moho depths or upper mantle velocities with any certainty from the lines in the Red Sea. The earliest profiles with an onshore component were actually carried out on the African side of the Red Sea in the southernmost corner of the Arabian plate in 1971 and 1972 (Fig. 1, Table 1 and Berckhemer et al., 1975; Ruegg, 1975). The

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Table 1 Datasets used to construct the maps of Moho depths in Figs. 8–10. Number or letter in Fig. 1

Location

References

Controlled-source seismic profiles 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Gulf of Aden Southern corner of Arabian plate Southern corner of Arabian plate Dead Sea Transform (DST) valley & western flank Eastern flank of DST Eastern Mediterranean Eastern Mediterranean Eastern Mediterranean Profile across southern DST Profile across southern DST Profile across southern Dead Sea basin Northern Red Sea Northern Red Sea Northern Red Sea & Gulf of Suez Southern Red Sea Saudi Arabia Eastern margin of Arabian plate Eastern margin of Arabian plate Gulf of Aden Zagros Mountains Cyprus

Laughton and Tramontini (1969) Berckhemer et al. (1975) Ruegg (1975) Ginzburg et al. (1979a,b) El-Isa et al. (1987a,b) Makris et al. (1983a) Ben-Avraham et al. (2002) Netzeband et al. (2006) Mechie et al. (2005); Weber et al. (2004, 2009) ten Brink et al. (2006) Mechie et al. (2009); Weber et al. (2009/2010) Makris (1980) Hosney (1985); Makris (1982, 1983); Makris et al. (1983b); Rihm et al. (1991) Gaulier et al. (1988) Egloff et al. (1991) Mechie et al. (1986); Mooney et al. (1985); Prodehl (1985) Whitmarsh (1979) Barton et al. (1990) Leroy et al. (2010); Watremez et al. (2011) Giese et al. (1984) Mackenzie et al. (2006)

Southern DST region Southern Dead Sea basin region Eastern Mediterranean & DST region Eastern flank of northern DST Eastern Arabian plate Southeastern Arabian plate Southern corner of Arabian plate Eurasian plate Southern Zagros Mountains NW profile across Zagros Mts. SE profile across Zagros Mts other stations in Arabian plate, often worked on by more than one of the listed group of authors

Mohsen et al. (2005) Mohsen et al. (2011) Hofstetter and Bock (2004); Sandvol et al. (1998b) Brew et al. (2001) Al-Lazki et al. (2002) Tiberi et al. (2007) Hammond et al. (2011) Afsari et al. (2011); Sodoudi et al. (2009) Hatzfeld et al. (2003) Paul et al. (2006) Paul et al. (2010) Al-Amri and Gharib (2000); Al-Damegh et al. (2005); Gök et al. (2008); Hansen et al. (2007); Kumar et al. (2002); Pasyanos et al. (2007); Sandvol et al. (1998a); Tkalčić et al. (2006)

50

Eastern Mediterranean & DST region

Koulakov and Sobolev (2006a)

Ground-based gravity studies A B C D E F G H I J K L M

Eurasian plate Southern corner of Arabian plate Profiles across Zagros Mountains SE profile across Zagros Mts. Eastern Arabian plate profile Eastern Arabian plate margin profiles Profile across central Red Sea Profiles across Red Sea Profiles across Arabian plate Profiles across & along DST Cyprus profile Profiles across eastern flank of northern DST Eastern Mediterranean profiles

Dehghani and Makris (1984) Makris et al. (1975) Snyder and Barazangi (1986) Molinaro et al. (2005) Al-Lazki et al. (2002) Whitmarsh (1979) Izzeldin (1987) Makris et al. (1991) Hansen et al. (2007) Götze et al. (2007) Shelton (1993) Brew et al. (2001) Ergün et al. (2005)

Receiver function studies a b c d e f g h i j k

Number or letter in Fig. 6 Regional earthquake tomography

first profiles in the vicinity of the Dead Sea Transform (DST) were carried out in 1977 along the western side of the DST valley and the western flanks of the DST valley (Ginzburg et al., 1979a,b). The Dead Sea proved to be a very efficient place to execute shots and observations could be made from shots in the Dead Sea out to distances of about 400 km along the DST valley to the south (red line along the DST in Fig. 1). As a data example from wide-angle reflection/ refraction profiles in the region, the record section (Fig. 2a) from these observations shows very clear first arrivals from the Pn phase, the refracted phase through the uppermost mantle just below the Moho. The Pn phase overtakes the Pg phase, the refracted wave through the crust, as the first arrival at about 150 km distance, beyond which it travels with an apparent velocity of about 8 km s −1. Beyond

about 100 km distance the PmP phase, the reflected wave from the Moho, can also be observed as a prominent secondary arrival. With such observations the depth to the Moho can be well determined and a crustal thickness of 30–35 km was obtained beneath the southern Dead Sea (Ginzburg et al., 1979a,b). Additionally, the high energy at the outer end of the PmP phase at about 230 km distance and 10 s reduced time (Fig. 2a) led Ginzburg et al. (1979b) to propose that beneath the DST valley the lower crust is separated from the upper mantle by a 5 km thick transition zone in which the velocity increases rapidly and smoothly. The profiles in 1977 were followed in 1984 by profiles on the eastern flank of the DST valley (El-Isa et al., 1987a,b), but it was not until 2000 that the first of three profiles now crossing the DST was completed (green line in Fig. 1, Mechie et al., 2005 and

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18 17 16 15 14

Time−Distance/8.0 [s]

13 12 11 10 9 8 7 Pn

6 5 Pg

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3 2 1 0 0 N

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350

Distance [km]

400 S

15 14 13

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12 11 10 9 8 7

PmP

6

Pn

5 4 Pg

3 2 1 0 0 NE

50

100

150

200

Distance [km]

250

300

350 SW

Fig. 2. Seismic wide-angle reflection/refraction data from a) the shot-point in the Dead Sea recorded to the S along the Dead Sea Transform valley in 1977 and b) a shot-point in the interior of the Arabian plate recorded to the SW in 1979 (red lines in Fig. 1). The record sections show the vertical component of P-wave motion, reduced with a velocity of 8 km s−1. Each trace is normalised individually and band-pass filtered from 2 to 15 Hz. Phases are identified following, for example, Ginzburg et al. (1979a) in the case of the shot-point in the Dead Sea and Mechie et al. (1986) in the case of the shot-point in the interior of the Arabian plate. Key: Pg — first arrival through the crust, PmP — Moho reflection, Pn — first arrival through the uppermost mantle.

Weber et al., 2004 and Weber et al., 2009). The other two profiles crossing the DST were completed in 2004, during which experiment a further axial profile and some limited 3-D crustal coverage were obtained (ten Brink and Flores, 2012; ten Brink et al., 2006) and 2006 (Mechie et al., 2009; Weber et al., 2009/2010). Along the northern and southern profiles crossing the DST, near-vertical incidence reflection data were also obtained and, to serve as a data example, a depth-migrated section for the southern profile (green line in Fig. 1) is shown (Fig. 3). In this section, the Moho can also be identified as a band of loosely linked patches of higher energy at 30–35 km depth. Further west in the study region, onshore–offshore and offshore wide-angle reflection/refraction profiles in the Eastern Mediterranean have been completed in 1978 (Makris et al., 1983a), 1989 (Ben-Avraham et al., 2002) and 2002 (Netzeband et al., 2006). During the latter experiment near-vertical incidence reflection data were also obtained (Netzeband et al., 2006). The only completely onshore wide-angle reflection/refraction profile in

this part of the study region is that completed in 1995, E–W across Cyprus (Mackenzie et al., 2006). More recent experiments carried out in 2010 in the Eastern Mediterranean region will not be discussed here as no results about Moho depths are as yet available. Further south in the Arabian plate, the only long-range profile into the central region of the plate is that completed in 1979 (Fig. 1). This profile was the subject of the 1980 IASPEI — Commission on Controlled Source Seismology workshop and thus several interpretations of the data exist (see e.g. Mechie et al., 1986; Mooney et al., 1985; Prodehl, 1985; Voggenreiter et al., 1988). Along this profile, high quality data were again obtained with the Pn phase in the presented data example (Fig. 2b) being well recorded out to beyond 300 km distance. In this example, the crossover distance between the Pg and Pn phases is at about 180 km and thus the crustal thickness of 40–45 km beneath this part of the profile, is somewhat greater than beneath the DST valley. A feature of the PmP phase observations along this profile was that the smallest

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Arava/Araba Fault km

Fig. 3. Automatic line drawing of the depth-migrated, near-vertical incidence reflection common depth point section along the southern of the three seismic profiles crossing the Dead Sea Transform (DST, green line in Fig. 1). The black arrows indicate the break-off of reflectivity, generally interpreted as the Moho in the near-vertical incidence reflection seismic data. The red line marks the position of the Moho derived from the coincident wide-angle reflection/refraction data. The blue line marks the position of the Moho along the profile, derived from a regional earthquake tomography study. The green circles mark Moho depths and the yellow diamonds mark the depths to a lower crustal layer derived from receiver function analysis from nearby stations. The Arava/Araba Fault marks the position of the DST. Modified after Mohsen et al. (2005).

distances at which the phase could be observed with high energy were often larger than those which could be expected if the Moho were a first order discontinuity with a large velocity contrast. Thus, some models and especially those which took into consideration the amplitude behaviour of the PmP phase, showed a transition zone several kilometres thick, from lower crustal to upper mantle velocities with the Moho being at the base of the transition zone (see e.g. Mechie et al., 1986; Mooney et al., 1985; Prodehl, 1985). Wide-angle reflection/refraction profiles along the constructive southwestern margin of the Arabian plate in the Red Sea began again in the late 1970s and continued with several experiments throughout the 1980s (Egloff et al., 1991; Gaulier et al., 1988; Hosney, 1985; Makris, 1980, 1982, 1983; Makris et al., 1983b; Rihm et al., 1991). Most of these experiments comprised both offshore and onshore components. Along the southeastern margin, offshore experiments were completed in 1975 (Whitmarsh, 1979) and 1986 (Barton et al., 1990). The latest experiment published in the literature concerning the constructive margins of the Arabian plate took place in 2006 along the southeastern margin of the plate (Leroy et al., 2010; Watremez et al., 2011). Again this experiment comprised both an offshore component in the Gulf of Aden and an onshore component and collected both wide-angle reflection/refraction and near-vertical incidence reflection data. To the authors' knowledge, only one wide-angle reflection/refraction profile has been completed in the study area in the Eurasian plate (Giese et al., 1984), providing two estimates of Moho depths at 55.74°E, 30.51°N and 55.97°E, 29.31°N (Fig. 1). All of the wide-angle reflection/refraction data in the region have been collected along profiles and thus the results are nearly always presented in the form of 2-D cross-sections. Only in the experiment in 1984 on the eastern flanks of the DST (El-Isa et al., 1987a) was some limited 3-D coverage of the Moho obtained due to some fan-like profile recordings of the PmP phase (see area of blue dots to the east of the DST in Fig. 1). Thus, in this study, the Moho depths have generally been digitised from the 2-D cross-sections published in the literature every 10 km. Further, in this study the Moho has been taken to be that boundary defined by the authors in the various publications. This creates no problems under the study region, except along the constructive margins in the Red Sea and Gulf of Aden where some interesting situations arise. For example, at the southernmost corner of the plate at 42–44°E and 12–13°N, Ruegg (1975) derived extremely low uppermost mantle velocities of about 6.9 km s −1, in an area close to the spreading axis along the southeastern boundary of the plate. In contrast, Watremez et al. (2011) interpreted velocities

of 7.6–7.8 km s −1 to belong to an intermediate velocity body with the Moho occurring at the base of this body. 2.2. Receiver functions In the receiver function technique, teleseismic body waveforms are used to image the crustal structures underneath isolated seismic stations. The theoretical background of the technique used to analyse the waveforms has been described in many publications (see e.g. Ammon et al., 1990; Farra and Vinnik, 2000; Kumar et al., 2005; Langston, 1979; Owens et al., 1984; Vinnik, 1977; Yuan et al., 1997). Basically the method takes advantage of the P to S (Ps) or S to P (Sp) conversion which takes place at significant discontinuities e.g. the Moho beneath the seismic station. These Ps or Sp converted phases and the crustal multiples PpPs and PpSs + PsPs contain a wealth of information concerning average crustal properties such as Moho depth and Vp/Vs (P velocity/S velocity) ratio (see e.g. Zhu and Kanamori, 2000). In the study region, especially in the interior of the Arabian plate, the analyses of Al-Amri and Gharib (2000), Al-Damegh et al. (2005), Al-Lazki et al. (2002), Gök et al. (2008), Hansen et al. (2007), Kumar et al. (2002), Pasyanos et al. (2007), Sandvol et al. (1998a), Tiberi et al. (2007), and Tkalčić et al. (2006) have proved to be very useful in obtaining Moho depths (see black crosses in Fig. 1). The studies of Gök et al. (2008), Pasyanos et al. (2007), and Tkalčić et al. (2006) involved the joint inversion of receiver functions and surface wave group velocity dispersions. In the northeastern part of the study region in the Eurasian plate, the receiver function analyses of Afsari et al. (2011), Hatzfeld et al. (2003), Paul et al. (2006, 2010), and Sodoudi et al. (2009) have provided constraints on Moho depths. Of particular value were the two receiver function profiles across the Zagros mountains as they provided clear images of the Moho along approximately 600 km long transects with an average station spacing of about 15 km (Paul et al., 2006, 2010). The study of Hammond et al. (2011) has provided estimates of Moho depths in the southernmost corner of the Arabian plate and the neighbouring region of the African plate. In the vicinity of the Dead Sea Transform (DST), the studies of Brew et al. (2001), Hofstetter and Bock (2004), Sandvol et al. (1998b) and two studies by Mohsen et al. (2005, 2011) have provided estimates of Moho depths. From one of these studies (Mohsen et al., 2005), examples of receiver functions from two stations are shown (Fig. 4). The left of the two examples (southern red triangle in

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Fig. 4. Two examples of receiver functions at stations just east of the Dead Sea Transform (red triangles in Fig. 1). The summed trace is shown at the top. Key: LCM — lower crustal multiple, from which the depth to the top of a lower crustal layer is derived (yellow diamonds in Fig. 3).

Fig. 1) shows the data from a station situated on the basement east of the DST. In this example the signal from the Moho at about 4 s delay time is the only strong primary P to S (Ps) conversion which can be observed. Behind it, signals identified as multiples from the Moho and the lower crust (LCM) can be recognised. The right of the two examples (northern red triangle in Fig. 1) shows the data from a station situated on sediments east of the DST. In this example, in addition to the Ps conversion from the Moho at about 5 s delay time a Ps conversion from the base of the sediments can be observed at about 1 s delay time. From such clear data Moho depths can be well determined and a comparison between Moho depths determined from receiver functions and other seismic methods along the southern of the three profiles crossing the DST (green line in Fig. 1) shows good agreement between the various seismic methods (Fig. 3).

model. Further relative corrections of sources were based on a double-difference approach proposed by Waldhauser and Ellsworth (2000). The inversion was performed simultaneously for 3-D P and S velocity anomalies in the crust and uppermost mantle and 2-D Moho depth variations, as well as for source parameters (coordinates and origin times). The models were parameterised with nodes distributed according to the ray density. No nodes were installed in areas where

2.3. Regional earthquake tomography For the northwestern part of the study region in the eastern Mediterranean Sea region a P and S velocity model of the crust and uppermost mantle has been constructed (Koulakov and Sobolev, 2006a) based on regional earthquake tomography. One of the main features of this model is that it includes the Moho as a specific discontinuity (Fig. 5, see also Fig. 9 in Koulakov and Sobolev, 2006a). To compute this model, data from regional earthquakes having both crustal (Pg and Sg) and mantle (Pn and Sn) phases were selected from the ISC catalogue in the time period from 1964 to 2001. Note that the separate consideration of crustal and mantle phases does not provide any constraints on the Moho depth. Only their joint processing allows separating the different parameters. About 3000 events in a circle of 6° radius centred on the Dead Sea basin with about 82,000 P and S phases were used for this study. In order to be selected for processing, the events had to have at least 25 records and an azimuthal gap of not more than 180°, which enabled robust source locations. Moho depths in the initial model were based on those from the CRUST2.0 global model (Bassin et al., 2000). 1-D reference velocities in the crust and uppermost mantle in the initial model were estimated using the information from controlled-source studies in the region. All the data from the ISC catalogue were initially reprocessed using an algorithm described in Koulakov and Sobolev (2006b). The sources were re-located using a grid-search method with simultaneous determination of outliers which were rejected from the dataset. At this stage, the Moho variations were taken into account as time corrections added to tabulated travel times computed in the 1-D

Mediterranean Sea

Dead Sea Transform

Moho depth (km) Fig. 5. Moho depth map of the eastern Mediterranean Sea region as derived from regional earthquake tomography. Modified after Koulakov and Sobolev (2006a).

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part of the region, north of 37°N, the Moho occurs at depths of more than 40 km. To the east of the Dead Sea Transform (DST), except at the latitude of the Dead Sea itself (31–32°N), the Moho is at 33–36 km depth, which corresponds to regular values for continental crust. Offshore, Moho depths of 19–25 km are observed beneath the Mediterranean Sea and the Red Sea. Along the eastern coast of the Mediterranean Sea at 34°N, the Moho occurs at 19 km depth, which is the smallest value in the region. It is of interest to note that the difference in Moho depths between the onshore and offshore regions in the final model appears to be stronger and sharper than in the initial CRUST2.0 model. Along the DST, between 29.5°N and the southern end of the Dead Sea at 31°N, a local linear deepening of the Moho to 33–35 km depth is observed in some places. To the east of the Dead Sea itself at 31– 32°N, an area in which the Moho shallows to 27–29 km depth can be seen. Two other features, which were not in the initial model, but which were derived as a result of the inversion, are a local deepening of the Moho to 33 km depth at 33.5°E, 35°N beneath Cyprus and a local deepening to more than 27 km depth at 33°E, 33.5°N in the vicinity of the Eratosthenes seamount in the Mediterranean Sea. This is consistent with a number of observations suggesting continental crust in this region (e.g. Ben-Avraham et al., 2002; Garfunkel, 1998; Netzeband et al., 2006). The latter local deepening seems to persist to the south, but by 32.5°N an area of no resolution is reached.

the ray coverage was insufficient. The simultaneous inversion for different types of parameters was performed using the LSQR method (van der Sluis and van der Vorst, 1987). Defining appropriate weights and damping coefficients for different parameters was based on results of synthetic tests with realistic patterns and noise levels. After the inversion, all the rays were traced in the updated velocity and Moho models for the corrected source locations. The final model was computed after five iterations. Moho depth values have been sampled from this final model at an interval of 0.1° (see Fig. 6 for coverage) which is approximately the interval used to sample the wide-angle reflection/refraction profiles and, in fact, it is the model sampled at an interval of 0.1° which is presented here (Fig. 5). A lot of different tests reported in Koulakov and Sobolev (2006a) were performed to show the reliability of the final model. To check the sensitivity of the results with respect to the parameters in the initial model, several models were computed based on different Moho distributions (flat, simple gradient and CRUST2.0). In all models, the results appeared to be similar for the central part, where the resolution was sufficient to separate the velocity and Moho depth parameters. Similar tests were performed for models with different initial velocity distributions. Another test consisted in reconstruction of irregularly shaped synthetic patterns in the crustal and mantle velocities and in the Moho topography. The reconstruction results show that all these parameters can be robustly separated and recovered. To assess the role of random noise, a test with independent inversions of two subsets of the data (e.g. with odd and even numbers of events) was performed. The resulting Moho depth distribution (Fig. 5) shows a clear differentiation between the thicker continental crust beneath the onshore regions and the thinner crust beneath the offshore regions of the eastern Mediterranean Sea and the northern Red Sea. In the northern

2.4. Ground-based gravity studies Moho depths derived from 13 ground-based gravity studies in the region have been utilised in this study (Table 1). The first of these studies in the Eurasian plate in the northeastern part of the region

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resulted in a map of Moho depths (Dehghani and Makris, 1984). From the second of these studies in the southernmost corner of the Arabian plate and the southern Red Sea a map of Moho depths was also derived (Makris et al., 1975). In both these studies existing seismic wide-angle reflection/refraction profiles in the region or just outside the region were taken into account when constructing the gravity-based Moho depth maps. From both maps values of Moho depths were read at an interval of 0.5° and re-gridded using bi-linear interpolation to an interval of 0.1° (see Fig. 6 for coverage). The third and fourth studies comprised three profiles across the northern boundary of the Arabian plate and the Zagros mountains (Molinaro et al., 2005; Snyder and Barazangi, 1986). The fifth study was a profile close to the northern boundary of the Arabian plate at about 57°E (Al-Lazki et al., 2002). The sixth study comprised two profiles in the study region at 18°N and 20°N across the northern portion of the southeastern Arabian plate margin (Whitmarsh, 1979). The seventh and eighth studies consisted of four profiles across the Red Sea at about 20°N (Izzeldin, 1987) and at about 14.5°N, 16.5°N and 27.5°N (Makris et al., 1991). The ninth study consisted of two profiles from the Red Sea into the interior of the Arabian plate and, in one case, almost all the way to the northern plate boundary (Hansen et al., 2007). The tenth study (Götze et al., 2007) included three profiles across and one profile along the southern Dead Sea Transform (DST). This study was based to a large extent on a data base of approximately 97,700 gravity stations in the region of the southern DST compiled by Hassouneh (2003). The last three studies comprised six profiles in the northwestern part of the region (Brew et al., 2001; Ergün et al., 2005; Shelton, 1993). In all these studies, where available, existing seismic wide-angle reflection/refraction profiles and receiver function studies in the region were taken into account when interpreting the gravity data. Moho depths were taken every 10 km along all the profiles comprising the third to 13th studies (Fig. 6).

3. Map of Moho depths derived from the combined gravity field model, EIGEN 06C In order to have a map of Moho depths with complete coverage in the study region and also to have an alternative map of the Moho interface for comparison, a map of Moho depths was derived by inverting combined satellite and ground-based gravity data. Today the new generation of gravity fields/models based on recent satellite missions such as GRACE and GOCE provides excellent opportunities to calculate Moho depths based also on satellite gravity fields. In former times it was seldom possible to invert ground-based gravity data for this purpose due to large gaps in the onshore and offshore gravity data base. The compilation in this study is based on the latest GRACE/GOCE combined gravity model, EIGEN 06C (GFZ, International Centre for Global Earth Models (ICGEM) services: http://icgem.gfz-potsdam.de/). ICGEM is one of six centres of the International Gravity Field Service (IGFS) of the International Association of Geodesy (IAG). The centre collects all existing gravity field models and provides on-line interfaces to download and visualise these models and to calculate different functionals from these models on grids chosen by the user. All models are available in a unified format. For the Moho inversion the grid spacing was 0.5°×0.5° in both E–W and N–S directions. This equals approximately 50×50 km, which is more than enough to calculate/invert for the Moho interface. A smaller grid spacing of 0.25° ×0.25° was not successful because the field contained too many short wavelengths which caused rather artificially short wavelengths in the Moho. The satellite derived Bouguer anomaly is calculated at the geoid and corrected for the effects of topography with heights above the geoid (Barthelmes, 2002). To find a simple approximation for the correction which is consistent with the results for the “Bouguer plate” one can define the potential of a spherical cap of constant thickness H (height), or a Gaussian bell-shaped “mountain” with height H and

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an extent which produces the attraction of the flat Bouguer plate (Barthelmes, 2002). In the ICGEM files, coordinates of the gravity grid are given in geographical coordinates. For further processing, the data had to be transformed into UTM coordinates which is a rectangular system and enables the application of fast Fourier transform (FFT) techniques. Transformations (e.g. from degrees to UTM) were done by the Christian-Albrechts-University (CAU), Kiel in-house software, TRANSKO, for applying FFT on a grid. To avoid numerical edge effects (Gibb's phenomena) the EIGEN 06C gravity model was downloaded in an extended area between 25°E, 41°N (upper left corner) and 65°E, 5°N (lower right corner). Generally, an unconstrained inversion was performed with the CAU in-house software, INTERP, which is based on the Parker algorithm (e.g. Lahmeyer, 1989; Parker and Oldenburg, 1973). There are some critical input parameters for the inversion. Firstly, there is the density contrast at the Moho interface, which was here set to 300 kg m −3. Secondly, there is the reference crustal thickness which, in this case, was chosen to be 30 km (Götze et al., 2007; Mechie et al., 2009). It turned out that this parameter does not influence the results significantly. Of crucial impact for the inversion results is the selection of both filter lengths, namely the filter length (W) and effective filter length (Weff., see Lahmeyer, 1989). After a series of attempts, the following filter parameters, W = 600 km and Weff. = 300 km were selected for the calculation of the Moho interface based on the EIGEN 06C gravity model (Fig. 7). This selection results in a smooth interface and keeps edge effects to a minimum. Numerical experiments with wavelengths smaller than the ones which were chosen for the inversion, resulted in unrealistically shallow Moho depths in the offshore area of the southeastern margin of the Arabian plate. The smallest Moho depths, 6–10 km, derived from the EIGEN 06C gravity model in the study region (Fig. 7) occur in the southeast in the offshore area of the southeastern margin of the Arabian plate and in the northwestern corner. The Moho depths in the Red Sea show an elongated trend, focused on the axis in the centre with values between 22 and 25 km. Moho depths of 30–35 km characterise a large area in the interior of the Arabian plate, especially towards the northeast. In contrast, towards the southwest along the southwestern coast of the Arabian plate larger values of around 40 km are often attained. The largest Moho depths in the study region, reaching values of about 50 km, are attained in the northeastern corner beneath the Eurasian plate and beneath the African plate margin at 37–40°E. In summary, although the inversion of the EIGEN 06C gravity field is unconstrained, the field is good enough in terms of resolution and errors to use it for calculations of Moho depths. Such gravity fields, incorporating data from satellite missions have the capacity to replace the inversion of ground-based gravity data, especially if these are out of date, of bad quality and contain erroneous positioning information and processing. They also have the obvious advantage over ground-based geophysical methods in that they offer continuous and complete coverage. 4. Maps and cross-sections of Moho depths based on seismic and ground-based gravity data 21,436 data points have been used to construct the Moho depth map of the Arabian plate and margins based on seismological and ground-based gravity data (Fig. 8). For the map based on seismological data only (Fig. 9), 7572 data points were employed. In constructing the maps the GMT software (Wessel and Smith, 1991) has been utilised. Firstly, the average depth value in each 0.1° × 0.1° cell was calculated. Then a surface was fitted through the points using bi-cubic splines with a low tension. This results in the maps having many small-scale maxima and minima. Finally a mask was created around each point, so that any location in the maps which is

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further than 0.7° from a data point is left white. The value of 0.7° was chosen as this is the value which is required so that a map of Moho depths in Germany created from seismic wide-angle reflection/refraction profiles alone has no white areas in it. Due to the small cell size of 0.1° × 0.1° and the low tension of the splines, the traces of some of the profiles and the locations of some of the isolated seismic stations can unfortunately be seen in the maps, in regions where e.g. Moho depths derived from the seismological data on the one hand and gravity data on the other hand are not in good agreement. This is particularly true in the northeast portion of the maps. The Moho depths shown vary from 4 to 59 km (Fig. 8) or alternatively from 4 to 67 km (Fig. 9). The smallest values are along the plate boundaries associated with the constructive margins of the Red Sea and Gulf of Aden. The largest values are north of the plate boundary associated with the destructive margin, in the Eurasian plate. Beneath the Zagros mountains Moho depths exceed 50 km. However, seismological data (Paul et al., 2006, 2010) tend to suggest that the deepest Moho occurs somewhat further north than the gravity data would suggest (Fig. 7 and Dehghani and Makris, 1984). This is the cause of the most prominent profile trace mentioned above at 31°N, 53°E in the map including ground-based gravity data (Fig. 8). Where the Moho depth values beneath the interior of the Arabian plate have been determined they seem to concentrate between 35 and 45 km. At the western plate boundary where the Dead Sea Transform (DST) occurs, there is a general shallowing of the Moho from about 100 km east of the DST towards the Mediterranean Sea. The distribution of Moho depths in the vicinity of the southern DST is shown in more detail (Fig. 10) as this area has been well studied relative to the rest of the study region. Cross-sections of Moho depths (Fig. 11) have been constructed through the maps (Figs. 7–8 and 10). The first is along the southern of the three seismic wide-angle reflection/refraction profiles crossing

the DST and its offshore extension (white line in Fig. 10). The second runs from 40.16°E, 12.04°N to 50.76°E, 35.96°N and partly coincides with the long-range seismic wide-angle reflection/refraction profile penetrating into the interior of the Arabian plate (Fig. 6). Finally, a comparison between the southwestern and southeastern Arabian passive continental margins and the conjugate African passive margin at 26°N on the one hand and the U.S. Atlantic continental margin (Grow and Sheridan, 1988), the Newfoundland Atlantic continental margin (Funck et al., 2003; Lau et al., 2006; Van Avendonk et al., 2009) and the Namibian continental margin (Bauer et al., 2000) on the other hand is presented (Fig. 11, bottom). The first cross-section is an extension of the section shown in Fig. 3. Firstly, it should be noted that all methods described in this study, including wide-angle reflection/refraction, near-vertical incidence reflection, receiver functions, local earthquake tomography and gravity data show an overall deepening of the Moho from NW to SE along this cross-section. The cross-section shows a steady deepening of the Moho from 21 to 23 km at the northwestern end of the cross-section in the southeastern Mediterranean Sea to almost 40 km at the southeastern end of the cross-section beneath the highlands of the Arabian plate east of the DST. A further feature of note is a small depression of the Moho beneath the DST, seen most prominently in the regional earthquake tomography study (Fig. 5, see also Fig. 9 in Koulakov and Sobolev, 2006a). As the western portion of this cross-section also traverses a Mesozoic Neo-Tethys passive margin, the Levant margin (see e.g. Garfunkel, 2004; Netzeband et al., 2006; Robertson and Dixon, 1984), for comparison, Moho depths across the U.S. Atlantic continental margin (Grow and Sheridan, 1988) and the Namibian continental margin (Bauer et al., 2000) are shown and will be discussed further below. The second cross-section shows thin crust beneath the southern Red Sea with respect to both Africa and Arabia, although again the

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curve based on the EIGEN 06C gravity model shows more muted trends than the other curves. The transition beneath the Arabian side of the Red Sea is particularly sharp. Beneath the interior of the Arabian plate the Moho varies in depth between 32 and 45 km along the cross-section or between 35 and 45 km if one excludes the EIGEN 06C gravity model. North of the northern plate boundary the Moho deepens to around 50 km beneath the Zagros mountains. 5. Discussion A comparison between the map of Moho depths based on the EIGEN 06C gravity model (Fig. 7) and the CRUST2.0 global model (Bassin et al., 2000) shows that differences in Moho depths are less than 5 km for 61% of the 180 2°× 2° tiles in the region. For 21 of the tiles differences in excess of 10 km exist, with the largest difference of about 16 km occurring for the tile centred at 57°E, 25°N. From the map of Moho depths derived from seismological and ground-based gravity data (Fig. 8), values can be determined for 117 of the 180 2°× 2° tiles in the region. Again, a comparison with the CRUST2.0 global model shows that differences in Moho depths are less than 5 km for 79% of these 117 tiles. For seven of the 117 tiles differences in excess of 10 km exist. The largest difference is almost 14 km along the Arabian Gulf of Aden margin for the tile centred at 47°E, 13°N. This tile spans both the offshore region of the Gulf of Aden and the onshore region of the highlands of Arabia. The large difference is due to the fact that a depth of about 23 km has been assigned for this tile in the CRUST2.0 model, whereas in this study information only exists about the offshore region of the Gulf of Aden where a Moho depth of about 9 km has been obtained. It is perhaps not surprising that for 79% of the 117 tiles, differences in Moho depths are less than 5 km, as both the map of Moho depths derived from seismological and

ground-based gravity data presented here and that derived from the CRUST2.0 global model are based to a large extent on the results from the same wide-angle reflection/refraction profiles in the region. The same is not true when comparing the map of Moho depths derived from seismological and ground-based gravity data presented here and that of Pasyanos and Nyblade (2007). Their map for Africa and Arabia (Fig. 6a in Pasyanos and Nyblade, 2007) has been derived from surface wave dispersion data. In contrast to the maps shown here, it shows a deepening of the Moho from the vicinity of the Dead Sea Transform (DST) to the Mediterranean Sea. It also shows a deeper Moho beneath the region just south of the northern boundary of the Arabian plate south of the Zagros mountains than beneath the Zagros mountains themselves. The region south of the northern boundary of the Arabian plate south of the Zagros mountains is rather poorly constrained in the map of Moho depths derived from seismological and groundbased gravity data presented here. Nevertheless, the map (Fig. 8) indicates that the Moho shallows south of the Zagros mountains and is shallower beneath the region south of the northern boundary of the Arabian plate south of the Zagros mountains than beneath the Zagros mountains themselves. Further, the map of Moho depths derived from the EIGEN 06C gravity model also shows that the Moho shallows south of the Zagros mountains and is shallower beneath the region south of the northern boundary of the Arabian plate south of the Zagros mountains than beneath the Zagros mountains themselves. Seber et al. (1997, 2000) derived maps of Moho depths for essentially the same region as that for which the maps presented in this study are derived. The studies of Seber et al. (1997, 2000) were based primarily on the results from wide-angle reflection/refraction profiles and ground-based gravity studies published until about 2000. The map of Baranov (2010) is more or less identical to that of

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Seber et al. (2000) and seems to be based essentially on the same profiles as used by Seber et al. (2000). The map for the European plate of Molinari and Morelli (2011) which also covers the Arabian plate north of 20°N is based mainly on the map of Baranov (2010) for the Arabian plate. Thus it would appear that the maps presented in this study (Figs. 8–10) are updates essentially of the map presented by Seber et al. (2000). Table 1 reveals most easily which studies were published after 2000. Nevertheless, the maps of Seber et al. (1997, 2000) show small Moho depths beneath the Red Sea and Gulf of Aden, Moho depths of around 40 km beneath much of the interior of the Arabian plate and large Moho depths beneath the Zagros mountains. The maps presented in this study also show these major features. However, for example, they extend further into the eastern Mediterranean Sea and show more details across the DST and some of the margins of the Red Sea and Gulf of Aden. The map of Moho depths of Segev et al. (2006) for the Levant margin and the southern DST is based primarily on the results from wide-angle reflection/ refraction profiles published until 2005. The map of Segev et al. (2006) shows a decrease in Moho depths from the highlands east of the DST to the Mediterranean Sea, in agreement with the maps shown in this study (Figs. 8–10). Al-Damegh et al. (2005) constructed a cross-section of Moho depths based on receiver function results along a SW–NE trending profile across the southwestern Arabian margin at 18–19°N. They compared their cross-section with a cross-section across the U.S. Atlantic continental margin (Grow and Sheridan, 1988) and noted that the southwestern Arabian margin had a much sharper change in Moho depths than the eastern U.S. continental margin. Al-Damegh et al. (2005) further stated that the observed abruptness of the southwestern Arabian margin may be an inherent feature of the process of

rifting and that over a longer period of geological time the abrupt margin may evolve into an extended margin. In this study, this comparison has been extended (Fig. 11, bottom). On the one hand in addition to the U.S. Atlantic continental margin, Moho depths across the Newfoundland (Funck et al., 2003; Lau et al., 2006; Van Avendonk et al., 2009), Namibian (Bauer et al., 2000) and the Mesozoic Neo-Tethys Levant (Fig. 11) continental margins are shown. On the other hand, in addition to the southern section at 17°N across the southwestern Arabian margin, Moho depths for sections across the southwestern Arabian margin at 20.5°N and 26°N, across the African margin at 26°N and across the southeastern Arabian margin at 54°E are shown. The first point to note is that the picture is somewhat more complicated than that presented by Al-Damegh et al. (2005). In agreement with Al-Damegh et al. (2005), the sections through the map based on seismological and ground-based gravity data (Fig. 8) across the southwestern Arabian margin at 17°N and 20.5°N (darker and lighter blue lines in Fig. 11, bottom) do show a sharper change in Moho depths than the U.S. and Namibian Atlantic margins. This is even true for the section at 17°N if one of the smoother interpretations of the data from the long-range wide-angle reflection/refraction profile is considered (Milkereit and Flüh, 1985, short dashed blue line in Fig. 11, bottom). However, this is not true for the Newfoundland Atlantic margin which also shows a sharp change in Moho depths, albeit much further offshore. In contrast, the section across the southwestern Arabian margin at 26°N (red lines in Fig. 11, bottom) shows that the Moho depths run more or less parallel to those across the U.S. and Namibian Atlantic margins and also those across the Mesozoic Neo-Tethys Levant margin (Fig. 11, top). This is true for both the sections derived from the seismological and ground-based gravity data (Fig. 8) and

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the EIGEN 06C gravity model (Fig. 7). On the other hand, the section through the map based on seismological and ground-based gravity data (Fig. 8) across the conjugate African margin at 26°N (green lines in Fig. 11, bottom) again shows a sharp change in Moho depths. This asymmetry across the margins of the northern Red Sea was already noted by, for example, Rihm et al. (1991), who postulated that whereas the Arabian margin is underlain by stretched and thinned continental crust the African margin is underlain by oceanic crust. Cochran and Karner (2007), on the other hand for example, contest that both margins of the northern Red Sea are underlain by stretched and thinned continental crust. Although not as sharp as the sections across the southwestern Arabian margin at 17°N and 20.5°N, the change in Moho depths along the section across the southeastern Arabian margin at 54°E (brown lines in Fig. 11, bottom) appears to be sharper than the U.S. and Namibian Atlantic margins. In this case, the section derived from the EIGEN 06C gravity model (Fig. 7) shows a sharper change in Moho depths than that derived from the seismological and ground-based gravity data (Fig. 8). In summary, the fact that the Newfoundland Atlantic margin is sharper than the U.S. and Namibian Atlantic margins and that all three margins are at least 120 Ma old (Bauer et al., 2000; Funck et al., 2003; Grow and Sheridan, 1988) suggests that sharp passive margins do

not necessarily become smooth with increasing age. Rather, it seems more likely that sharp margins such as the Arabian margins in the southern Red Sea and the Gulf of Aden, the African margin in the northern Red Sea or the Newfoundland Atlantic margin are fundamentally different to smooth margins such as the Arabian margin in the northern Red Sea and the U.S. and Namibian Atlantic margins. Van Avendonk et al. (2009) found a crustal high velocity body bordering the sharp margin along one of the profiles across the Newfoundland Atlantic margin and suggested that the high velocity is due to a more mafic composition than the surrounding material. This more mafic material would then be stronger and the sharp margin might be formed at the edge of such a stronger body. As yet, though, no such similar high velocity bodies have been identified along any of the other transects with sharp margins mentioned in this study. This may be because they are below the resolution of the studies undertaken to date. All interpretations of the long-range wide-angle reflection/refraction seismic profile penetrating the interior of the Arabian plate obtained Pn velocities of 8.0 km s−1 or greater beneath the Arabian shield and platform (see e.g. Mooney et al., 1985). Several interpretations also showed one or more discontinuities in the lithospheric mantle beneath the Arabian shield and platform between 50 and 80 km depth (see e.g.

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or from continental slope break + 150 km for Newfoundland Fig. 11. Moho depth and topography cross-sections across the Levant passive margin and the Dead Sea Transform (top), across the Arabian plate from the African plate to the Eurasian plate (middle) and across the southwestern and southeastern Arabian passive continental margins and the conjugate African margin at 26°N (bottom). For locations of the cross-sections, see Figs. 6 and 10. Top — solid black line marks the Moho from the map shown in Fig. 8, the green line marks the Moho from the map shown in Fig. 7, the blue line marks the Moho from the map shown in Fig. 5, the red lines mark the Moho from wide-angle reflection/refraction and the brown line marks the Moho from ground-based gravity studies. The long dashed black line marks the Moho across the eastern U.S. continental margin (Grow and Sheridan, 1988) and the short dashed black line marks the Moho across the Namibian continental margin (Bauer et al., 2000). Middle — black line marks the Moho from the map shown in Fig. 8, the green line marks the Moho from the map shown in Fig. 7, the red crosses and the red line mark the Moho from wide-angle reflection/refraction, the blue crosses mark the Moho from receiver functions and the brown lines mark the Moho from ground-based gravity studies. Bottom — the solid black line marks the Moho across the eastern U.S. continental margin (Grow and Sheridan, 1988), the long dashed black lines mark the Moho across the Newfoundland continental margin (Funck et al., 2003; Lau et al., 2006; Van Avendonk et al., 2009), the short dashed black line marks the Moho across the Namibian continental margin (Bauer et al., 2000), other solid lines mark the Moho from the map shown in Fig. 8 and long dashed lines mark the Moho from the map shown in Fig. 7. The darker blue lines are across the southwestern Arabian margin at about 17°N, the lighter blue lines are across the southwestern Arabian margin at about 20.5°N, the red lines are across the southwestern Arabian margin at about 26°N, the green lines are across the conjugate African margin at 26°N and the brown lines are across the southeastern Arabian margin at about 54°E. The short dashed blue line marks an alternative interpretation (Milkereit and Flüh, 1985) of the Moho across the southwestern Arabian margin at about 17°N. Key: see Fig. 1 and C — coast, PB — plate boundary and AS/AP — Arabian shield/Arabian platform boundary.

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Mechie et al., 1986; Mooney et al., 1985; Prodehl, 1985). Beneath the western boundary of the Arabian plate in the vicinity of the DST, Pn velocities of 7.8–8.1 km s−1 have generally been obtained (El-Isa et al., 1987a; Ginzburg et al., 1979a,b). Additionally, beneath the valley of the DST a discontinuity in the lithospheric mantle at about 55 km depth has been identified (Ginzburg et al., 1979b). Anomalously low Pn velocities are only thought to occur near the southwestern and southeastern plate boundaries along the constructive margins of the Arabian plate (see e.g. Ruegg, 1975). At the southernmost corner of the plate at 42–44°E and 12–13°N, Ruegg (1975) derived extremely low uppermost mantle velocities of about 6.9 km s−1, in an area close to the spreading axis along the southeastern boundary of the plate. Ruegg (1975) postulated that these anomalously low uppermost mantle velocities were due to high temperatures and partial melts in the area close to the spreading axis. In contrast, Watremez et al. (2011) interpreted velocities of 7.6–7.8 km s−1 beneath the southeastern Arabian margin at 54°E to belong to an intermediate velocity body with the Moho occurring at the base of this body. This interpretation was based on the fact that reflected phases from both the top and the base of this body could be observed, while a refracted phase through this body provided constraints on its velocity and the Pn phase provided constraints on the uppermost mantle velocity. Within the Arabian plate, higher Pn and Sn velocities have been derived beneath the northeastern part of the plate and lower Pn and Sn velocities have been derived under the southwestern part of the plate from body wave tomography for the Pn phase (Simmons et al., 2011) and surface wave tomography for the Sn phase (Pasyanos and Nyblade, 2007). In this respect it is interesting to note that Mooney et al. (1985) also derived a northeastwards increase in Pn velocity along the long-range wide-angle reflection/ refraction profile penetrating into the interior of the Arabian shield. A shear wave splitting analysis using primarily SKS phases revealed N–S oriented fast directions and delay times averaging about 1.4 s over much of the Arabian plate (Hansen et al., 2006). Another shear wave splitting analysis using the SKS phase from one event recorded on a dense array across the DST revealed a narrow, approximately 20 km wide zone in which the fast direction of anisotropy was oriented parallel to the DST (Rümpker et al., 2003). This zone extends throughout the entire mantle lithosphere and accommodates the transform motion between the Arabian and African plates in this region. Summarising, in this study three maps of Moho depths for the Arabian plate and margins have been constructed. One of these maps has been derived from the EIGEN 06C gravity model, which includes gravity fields from satellite missions (Fig. 7). One of the other maps has been derived from seismological and ground-based gravity data (Fig. 8) while the third map has been derived only from seismological data (Fig. 9). All maps show similar trends although that derived from the EIGEN 06C gravity model generally shows more subdued variations in Moho depths than those based on seismological and ground-based gravity data. Beneath the interior of the Arabian plate the Moho occurs between 32 and 45 km depth. Across the northern plate boundary the Moho deepens beneath the Zagros mountains. Across the western plate boundary, the DST, the shallowing of the Moho from SE to NW towards the Mesozoic Neo-Tethys Levant passive margin seems to be modulated by a slight depression beneath the DST. Across the southwestern and southeastern plate margins the Moho shallows significantly but with varying degrees of sharpness towards the constructive plate boundaries in the Red Sea and Gulf of Aden. A comparison between the margins of the Red Sea, the Gulf of Aden and the Levant on the one hand and Atlantic margins on the other hand shows a complex picture and suggests that the different sharpness of the margins reflects fundamental differences in their original structure. The suggestion of Van Avendonk et al. (2009) that sharp margins may form along the edges of strong, high-velocity, mafic bodies, as seen along one of the profiles across the Newfoundland Atlantic margin, requires more effort to try and identify such bodies along other sharp margins.

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Acknowledgements Marco Paschke and Michaela Merz digitised many of the seismic wide-angle reflection/refraction and ground-based gravity profiles used in the study. Trond Ryberg provided the first scripts to create the maps with the GMT software.

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