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The Red Sea – 50 years of geological and geophysical research
Khalid A. Almalki, Peter G. Betts, Laurent Ailleres
PII: S0012-8252(15)00079-3 DOI: doi: 10.1016/j.earscirev.2015.05.002 Reference: EARTH 2114
To appear in: Earth Science Reviews
Received date: 18 December 2013 Accepted date: 4 May 2015
Please cite this article as: Almalki, Khalid A., Betts, Peter G., Ailleres, Laurent, The Red Sea – 50 years of geological and geophysical research, Earth Science Reviews (2015), doi: 10.1016/j.earscirev.2015.05.002
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The Red Sea – 50 years of geological and geophysical research
Khalid A. Almalki 12 (Corresponding author)
Assistant Professor
P: +61 3 9905 4886 f: +61 3 9905 4903 [email protected]
Peter G Betts 1
Associate Professor p: +61 3 9905 4150 f: +61 3 9905 4903
Laurent Ailleres 1
Senior Research Fellow p: +61 3 9905 1526 f: +61 3 9905ACCEPTED 4093 MANUSCRIPT
1School of Geosciences Monash University, PO Box 28E, Wellington Road, Clayton, VIC
3800, Australia
2King Abdulaziz City for Science and Technology, PO Box 6086, Riyadh 11442, Kingdom of Saudi Arabia
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Abstract
The Red Sea displays lithosphere that is in transition from rifting to drifting and therefore provides enormous opportunity to understand incipient seafloor spreading development.
Despite more than 50 years of extensive geological and geophysical research, there remains significant conjecture concerning the timing of sea-floor spreading initiation, the extent of spreading along the axis of the Red Sea, and the geodynamic processes responsible for the onset of crustal extension. Red Sea tectonic models based on geological data are dominated by single stage rift models involving protracted stretching of continental crust followed by sea floor spreading at ca 5 Ma, and include both asymmetric and symmetrical extension models. Geophysical data suggest that an oceanic crust occurs beneath the Red Sea shelves, although exactly how this crust has evolved remains undecided. Equally, arguments have been made in favor of the shelves forming on extended continental crust, and that oceanic crust is restricted to the present day Red
Sea axial valley. However, our synthesis shows that none of the models proposed so far is applicable to the entire Red Sea basin. The distribution and timing of the Red Sea crustal extension and sea-floor spreading may have been influenced by subduction-related far field forces due to complex plate interactions along the northern edge of the Arabian plate during Early ACCEPTEDMiocene. Our reassessment MANUSCRIPT suggests that following the arrival of the Afar plume the southern Red Sea underwent rapid extension culminating in the transition from rifting to seafloor spreading, which stalled during the early Miocene in the central Red
Sea due to either slowing of the convergence between Arabia and Eurasia or the onset of the Arabian plate passive margin collision with Eurasia. Off axis magmatism is focused on the Arabian margin and Afar depression and has been episodically active since the
Miocene. We consider a formation of the Red Sea as a hybrid process involving both
“active and passive” rifting.
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Keywords: Red Sea; Plume; Sea floor spreading; Ocean basin initiation; Extensional geodynamics; Magnetic stripes.
1.1 Introduction
There is an extensive documentation regarding the continental record of structures that are related to new accretionary plate boundaries located above hot spots in the mantle
(Ghebrab, 1998). The relatively narrow small rifts and oceans such as the Red Sea and
Gulf of Aden at the boundary between the African and Arabian plates represent examples of this process (Fig. 1). Such rifts have been considered the closest modern analogs to continental lithosphere rifting and rupturing and to seafloor spreading initiation, and thus, they are an integral part of our understanding of plate tectonic theory. The Red Sea rift belongs to a rift system that includes the East African, Afar and the Gulf of Aden rifts in the south and the Gulf of Suez and Aqaba in the north (e.g. Guiraud et al., 1985). The Red
Sea and Gulf of Aden rifts converge with the East African rift at the Afar triple junction.
The Red Sea represents a natural laboratory to understand processes responsible for early rift-related magmatic systems and ocean basin formation influenced by both active and passive riftingACCEPTED processes. The processes MANUSCRIPT responsible for the transition from continental rifting to oceanic spreading have been inferred by the study of the ancient geological record of shelf sedimentary successions (e.g., Skipwith 1973; Beydoun and Sikander,
1992), numerical simulations (e.g., Girdler and Underwood 1985; Gerya, 2012), igneous geochemistry (e.g., Coleman et al., 1979; Bosworth et al., 2005), and geophysical data
(e.g., Girdler and Styles, 1974; Cochran and Karner, 2007; Ligi et al., 2015).
This paper represents a summary of the available geological and geophysical observations of the Red Sea and adjacent regions that have shaped and influenced interpretation of the
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evolution of this juvenile ocean basin system. We re-assess these data to establish the extent and distribution of continental, transitional and ocean crust along the Red Sea and how this crust relates to shelf sedimentation and the adjacent continental Arabian-African shields.
1.2 Geologic Record
1.2.1 Red Sea
Drilling in the Red Sea region dates back over half a century, yet some of the information presented here has not been available to the wider geological community (Fig. 2). The earliest exploration drilling is on record as having been carried out between 1927 and
1930 on the Arabian Shelf (De Golyer and MacNaughton, 1953). Three holes were drilled by the Red Sea Petroleum Company (Zifaf-1; 2 and 3; Table1). Exploration continued until 1938 in this area with eight more holes with the deepest hole recorded in Segid-1.
All of these holes intersected limestone, shale, gypsum and anhydrite before terminating in halite (Table 1). In 1967, Auxerap Pty Ltd drilled their first exploration well onshore
(Mansiyah-1) in southwestern Saudi Arabia (Ahmed, 1972; Table 1).
Tenneco Pty Ltd joint ventured with the Auxerup Pty Ltd exploration program and together theyACCEPTED focused their exploration MANUSCRIPT effort to oil seeps within their northernmost concession in the northern Red Sea near the coast of Saudi Arabia. Offshore drilling commenced in 1969 with the Barqan-1 well being drilled (MPMR, 1986) (Table 1; Fig.2).
Barqan-1 intersected reefal limestone and thick evaporites body before passing directly into basement granite. An additional four exploration drill holes in the northern Red Sea intersected similar stratigraphy (e.g., AL Kurmah-1; Table 1), with some variations in sediment thickness. Offshore from the Egyptian coastline basement was reported beneath late Miocene sedimentary successions from Hareed South-3 well and Abu Shiban-2 well
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(Table 1; Fig. 2) (Salah and Alsharhan 1995). In 1972 the Bader-1 well was drilled by
Arabian Sun Oil Ltd in the central Red Sea (Fig. 2). This well intersected evaporites beneath a cover of continental and reefal limestone before the hole passed into basement granite (Table 1). Additional exploration drilling (Durwara-2) in the central Red Sea near the coast of Sudan reported fossiliferous late Oligocene shale (Gillman, 1968) and an
Oligocene basaltic basement (K-Ar dating method; Whiteman, 1968; Brown and Girdler,
1982). The presence of late Oligocene foraminifera within marine shale from the southern
Red Sea offshore of Eritrea was confirmed from the Thio-1 well (Hughes et al., 1991).
Several drilling exploration programs near the Dahlak Islands in the southern Red Sea
(Fig. 2) have reported at least 3500 m of almost entirely halite evaporites above a basaltic basement. However, these holes did not intersect Oligocene sedimentary successions
(MPMR, 1986). Thick successions of Pleistocene evaporitic deposits overly the Miocene evaporites. The age of the evaporites successions are constrained by foraminiferal content
(Lowell and Genik, 1972). In the Danakil depression the thickest evaporites succession consists of 970 m of halite with interbedded layers of gypsum, anhydrite and shale. Two of the wells in this area (Amber and B1; Fig. 2) intersected basaltic basement with K-Ar dates of 36±2 Ma (Amber well) and an average radiogenic date of 30 Ma (B1 well)
(Lowell and Genik,ACCEPTED 1972; Savoyat et al., MANUSCRIPT 1989). These are the only available dates from basement rocks, although these ages may be considered a minimum ages as the K-Ar dating method is susceptible to resetting.
More lithologic and stratigraphic information have been collected by the cruise of the R.
V. Chain of the Woods Hole Oceanographic Institution and the Glomar Challenger ship in 1969. Five drill-sites along the edge of the axial trough zone and one drill-site at the axial trough at the central Red Sea (Fig. 2; Table 1) were able to confirm the extend of the
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sedimentary cover to the axial trough zone. Correlation of stratigraphic units between different sites show that the basin is characterized by middle-late Miocene to Pleistocene deposits with the existence of some anhydrite deposits above recent basaltic volcanic rocks at the axial trough zone (Ross, 1977). The stratigraphic relationship from these wells suggests prominent unconformities between the Miocene and the post-Miocene sediments (Ross and Schlee, 1977).
DSDP site 226 (Fig. 2) from the central Red Sea have penetrated basement rocks of the axial trough zone and provide an evidence of young volcanic rocks (Ross, 1977).
Chemical analysis on these volcanic rocks indicates that they are tholeiitic basalts similar to those observed in other areas of Red Sea sea-floor spreading (Ross, 1977; Jónsson and
Xu, 2015). Recently, rock samples dredged from the central Red Sea axial trough (Thetis and Nereus Deeps) yield dates between 2 to 1.6 Ma, and younger than 0.78 Ma (Ligi et al., 2012). The later date suggests initial generation of oceanic crust within the last million years (Ligi et al., 2012).
The outcomes of the geological studies show that the Red Sea basin dominantly preserves
Early Miocene to Pleistocene sedimentary successions along the entire length of the basin (e.g., Coleman,ACCEPTED 1977). Thick reefal limestoneMANUSCRIPT successions, evident throughout the offshore Red Sea area (e.g., Farasan and Dahlak banks), have been examined in detail and show that all fossils are Pliocene-Pleistocene in age and correlate with fossils found in the
Indo-Pacific area (e.g., Bosence et al. 1998; Plaziat et al. 2008). Drilling during the extensive exploration activity in the Red Sea has also confirmed the wide extent of the
Miocene evaporites formations. Well data suggest that up to 4 km of sediments exist in the Red Sea; 3 km or more of this section in some places consists of Miocene evaporites
(e.g., Ahmed, 1972; Bosence et al., 1998; Hadad and Abdullah, 2015). Drill holes show
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that the Red Sea shelf stratigraphy comprises massive evaporitic successions, which contain intercalated anhydrite and shale layers. These are unconformably overlain by
Quaternary reef limestone (e.g., Skipwith, 1973). Detailed analysis of the basin reveals a complex pattern of development with late Oligocene successions (e.g., shale) restricted to the western Red Sea coast in the central and southern parts of the Red Sea. The absence of pre-Oligocene sediments along the Red Sea provides evidence that the Red Sea depression (in its present form) did not exist during this time. Although pre-Oligocene strata are well-exposed on Zabargad Island (e.g., Bonatti et al, 1981), the formation of the island was associated with extensive continental rifting in Africa (Bosworth et al. 1996) allowing exposure of ultramafic rocks, and thus, the island may represent uplifted portions of the sub-Red Sea mantle (Wernicke, 1985). Pre-Miocene extension of continental crust during incipient rifting was interpreted to create a series of troughs along the depression that extends laterally over a distance in which Miocene sediments accumulated (Ross and Schlee, 1977).
1.2.1.1 Palaeogeographic/Palaeoenvironmental context
There are two competing palaeogeographic models of the Red Sea basin during the Oligocene andACCEPTED Pleistocene. Meulenkamp MANUSCRIPT and Sissingh (2003) interpreted four major changes in the Red Sea palaeogeography (Fig. 3a). The first involved the development of a shallow basin in the northern Red Sea associated with the developing of a seaway with the Mediterranean during the early Miocene. Shallow-water basin conditions in the southern Red Sea are interpreted to record the opening of a middle Miocene seaway between the Red Sea to the Indian Ocean. The seaway connecting the Red Sea and
Mediterranean closed during the late Miocene, although a seaway with the Indian Ocean was maintained with shallow water condition prevailing. Deep water conditions were
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restricted to the Pliocene-Pleistocene period due to the Red Sea opening (Meulenkamp and Sissingh, 2003). In contrast, Bosworth et al (2005) argued, based on stratigraphic evidence, that the southern Red Sea was first connected to the Indian Ocean during the late Oligocene. Then the entire Red Sea developed and connected to the Mediterranean in the early Miocene (Bosworth et al. 2005).
A palaeogeographic connection between the Red Sea and the Mediterranean Sea has been suggested for the Middle to Late Miocene based on similarities of nanoplankton fauna
(Ross, 1977; Mandur, 2009). Very thick accumulations of anhydrite and halite are interpreted to coincide with restricted sea-water exchange between the Mediterranean and the Red Sea and with no connection to the Indian Ocean at the Miocene time (Hadad and
Abdullah, 2015). Bosworth et al (2005) correlated the unconformities above the evaporites of the Red Sea and the Mediterranean. However, it should be noted that the evaporites of the Red Sea were deposited over a 6 m.y period, whereas the evaporites of the Mediterranean represent a much smaller period of deposition. The deposition of the main evaporites sequence in the middle to late Miocene was probably related to global decrease in eustasy (Evans, 1988). Pliocene uplift in the Gulf of Suez region may isolate the Red Sea from the Mediterranean Sea at this time (e.g., Beydoun and Sikander 1992).
The upper sedimentaryACCEPTED succession containing MANUSCRIPT foraminiferal fauna (including bivalves, corals and gastropods) comparable with fauna observed in the Indian Ocean (Dabbagh et al. 1984) suggests an open seaway with the Indian Ocean during the Plio-Pleistocene.
The palaeoenvironmental context of the basin has been described by Hughes and
Beydoun (1992), Crossley et al., (1992) and Bruggemann et al. (2004) (Fig. 3b). The sedimentary facies was variable across the Red Sea depression (Bruggemann et al., 2004).
Late Oligocene shale is restricted to the western side of the Red Sea and provides
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evidence that deep marine condition existed in this area. Paleogeographic interpretations, based on well samples indicate that during the Middle Miocene most of the Red Sea was moderately deep, with deeper conditions in its southern regions (Crossley et al. 1992).
Moderately deep water environments associated with regional subsidence throughout the
Red Sea is reflected by the wide spread deposition of thick evaporites successions along the basin. Mineralogical evidence and hydrogen and oxygen isotopic data provide evidence of both deep basin to shallow-marine deposition for the Red Sea evaporites
(Manheim, 1974). The lower evaporites succession is stromatolitic with interbedded halites, which indicates sedimentation within the photic zone (~200 m) whereas potassic evaporites, characterized by high Sr/Ca ratios, were interpreted to indicate deposition of the upper parts of the succession occurred in a shallow-marine depositional environment
(Manheim, 1974). Nanofossils and benthonic foraminifera from the shale units suggest an open marine environment of considerable depth (Ross, 1977).
During the early Miocene deep marine conditions prevalent in the northern part of the
Red Sea switched to shallow water as the water supply from the Mediterranean Sea became restricted (Crossley et al., 1992; Rouchy et al., 1995). Deep water conditions extended to the south in the late Miocene with areas of moderately deep water conditions up to shallowACCEPTED water as suggested by theMANUSCRIPT potassic evaporites successions (Hughes and
Beydoun, 1992). During the Plio-Pleistocene shallow marine condition prevailed
(Gillman 1968; Dabbagh et al. 1984). The palaeoenvironmental interpretation of the
Middle to Late Pleistocene coral assemblage along the Red Sea coast of Eritrea at the
Gulf of Zula suggests a reef that formed at shallow to mid-depths in a sheltered environment (Bruggemann et al., 2004).
1.2.2 Rift flanks
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1.2.2.1 Precambrian Shield
The Arabian and African shields consist of Proterozoic crystalline basement that underlie
Paleozoic to Paleogene sedimentary platform sequences that surround and dip gently away from crystalline basement (Bohannon, 1986) (Fig. 1; 2). The Precambrian crystalline basement rocks of the Arabian margin are similar to those preserved on the
African margin of the Red Sea and Danakil Alps in the Afar Region (Bosworth et. al.,
2005). They comprise Precambrian gneissic and Neoproterozoic island arc terranes, granite and volcanic rocks that were accreted during the Pan-African orogeny associated with Gondwana amalgamation (Windley et al., 1996; Robinson et al., 2014).
The Precambrian Arabian and African shields have steep slopes that descend rapidly to about 250m to form irregular coastal plains (McClay et al., 1998; Fig. 1). In Eritrea, the trend of the scarp heads southward oblique to the Red Sea and connects with the Main
Ethiopian Rift system (MER) (Fig. 1). This region is cross-cut by the NE-trending MER, creating the triangular Afar depression bordering the southern Red Sea. The scarp forms the western boundary of the Danakil (Afar) Depression that contains post-Miocene sedimentary and volcanic deposits (e.g., Varet, 1978). Pre-rift Jurassic sandstone and limestone sequences are exposed in the Danakil Alps (e.g., Sagri et al., 1998). The edge of the DanakilACCEPTED Alps is defined by a rapid MANUSCRIPT decrease in topographic elevation due to normal faulting associated with half-grabens and grabens (e.g., Allen et al., 1991).
Bohannon et al. (1989) reported fission track data along the Arabian shield indicating that rift shoulder erosion and uplift began at ca 20 Ma and that at least 2.5 km of uplift has occurred since 14 Ma. Apatite fission track data from the Precambrian crystalline rocks along the western and eastern Red Sea margins provide evidence for two erosional events.
The first occurred at ca 34 Ma followed by renewed or increased erosion between 25-21
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Ma (Omar and Steckler, 1995). However, it is questionable if these episodes of erosion and uplift are related to the Red Sea evolution. An in-depth discussion of the 34 Ma event is provided in Bosworth and McClay (2001) and Bosworth et al. (2005). Fission track studies along the Yemeni margin (Menzies et al., 1992) showed that exhumation and uplift occurred at ca 17-16 Ma. In addition, apatite fission track dating of basement amphibolite rocks in west Yemen suggest exhumation and erosional unroofing between ca 29 and 16 Ma (Menzies et al., 1992). Along the conjugate margin in Eritrea, fission track data suggest a major crustal cooling event at 20 Ma (Abbate et al., 2002). Ghebreab et al. (2002) identified apatite fission-track thermochronologic cooling ages on the
Eritrean border north of Danakil clustered between ca 23 and 17 Ma. Apatite fission track dates from the Gulf of Suez's western border specify that uplift occurred at 21 ± 2 Ma
(Omar and Steckler, 1995). In addition, four fission track dates from Sinai obtained by
Kohn and Eyal (1981), also indicate cooling ages of ca 22-20 Ma. However, there is no evidence for uplift or subsidence within the Gulf of Aqaba in the Early Miocene
(Bosworth et al., 2005).
1.2.2.2 Late Eocene to Late Oligocene Volcanism
A 300 km wide magmatic belt flanks the Red Sea and extends for more than 3000 km northward, fromACCEPTED the Afar Depression to MANUSCRIPTthe northern Arabian Plate (Davison et al., 1994)
(Fig. 2). Geochronological and geochemical analysis of the volcanism in Ethiopia shows that the maximum of alkaline basalts activity occurred at ~ 31 Ma (Afar Plume) and is characterized by low Al 2O3 and high TiO 2 (e.g., Berhe et al., 1987; Coulie et al., 2003).
This magmatic activity was followed by transitional to tholeiitic basaltic magmatism until
26 Ma (e.g., Civetta et al., 1978; Berhe et al., 1987). Bimodal volcanism is evident in the silica contents of the flood volcanic rocks which contain 50 ± 4 and 72 ± 4 wt% SiO 2
(Baker et al., 1993b). These observations were used to correlate similar age and rock
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types in Yemen and Saudi Arabia (e.g., White and McKenzie, 1989, 1995; Ghebrab,
1998).
Coleman et al (1979; 1983) reported early Oligocene alkaline basalts erupted between 32 and 26 Ma and mafic plutons with Kr-Ar cooling ages between ca 30 and 26 Ma were emplaced throughout southwestern Saudi Arabia. The largest eruption in Saudi Arabia occurred at Harrats Uwaynd between ca 28 and 26 Ma ( 40 Ar/ 39 Ar; Sebai et al., 1991), which is the only available 40 Ar/ 39 Ar dates in this area (Bosworth et al., 2005). Both K-Ar and 40 Ar/ 39 Ar techniques have been applied to basaltic lava samples from Ethiopia and
Yemen and yielded ages of 30.6±0.4 and 30.2±0.4 Ma (Coulie et al., 2003). Basaltic rocks found west of Massawa in Eritrea yield 40 Ar/ 39 Ar dates varying between ca 30 to 18
Ma (Drury et al., 1994). Further north in Sudan (at Derudeb), Kenea et al. (2001) documented 40 Ar/ 39 Ar dates for a felsic tuff and a rhyolite flow at ca 30 Ma. Recently published 40 Ar/ 39 Ar dating from the flood basalts (ankaramitic dolerites) along the western Afar margin places further constraints on this magmatic event between ca 31 and
27 Ma (Rooney et al., 2013).
Trace-elements and isotope geochemistry analyses of the flood-basalt (high 3He/ 4He contents and ACCEPTED mafic compositions up to MANUSCRIPT 12% MgO; e.g., Davison et al., 1994; Rogers, 2006; Rooney et al., 2013), the temperature of the rocks at eruption (1520 °C; e.g., Lin et al., 2005; Rooney et al, 2012) coupled with the extent of the large flood volcanic provinces (Fig. 2) and the resultant topography (Fig. 1) are consistent with the arrival of a mantle plume. Global S-wave velocity models also lead to the conclusion of a plume source under the Afar with a low-velocity zone (4.1 km/s) extending to a depth of 250 km, similar to velocities observed beneath hot spots (e.g., Zhang and Tanimoto, 1992;
Hansen et al., 2008).
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Rooney et al. (2013) estimated the impact of the Afar mantle plume with the base of the
African lithosphere at 31 Ma. This event generated large volume of magma, initiated lithospheric thinning, and promoted melting of the rising depleted mantle. Radiogenic isotopic data reveal that ca 30 to 27 Ma lavas contained significantly elevated enriched plume-related component compared with flood basalt magmas erupted elsewhere on the western Ethiopian plateau. Several researchers have proposed that the East African – Red
Sea – Gulf of Aden rift systems may have been affected by more than one single plume interaction with the African/Arabian lithosphere (e.g., Davison et al., 1994; Rogers,
2006). Lin et al. (2005), for example, proposed that the Kenya plume interacted with the
African lithosphere at 45 Ma beneath the MER and contributed to the initiation of rifting processes, whereas the Afar plume interacted with the continental lithosphere at 30 Ma. A two plume numerical model suggests that the melt generation within the lithosphere was triggered by thermal influence of the Afar plume (Lin et al., 2005). However, there is no independent stratigraphic or structural evidence that supports the Kenya plume playing a role in the evolution of the Red Sea rift.
1.2.2.3 Early Miocene Dikes
Several NW-SE-striking mafic dike swarms are documented along the eastern and western flanksACCEPTED of the Red Sea (e.g., Coleman, MANUSCRIPT 1993; Voggenreiter et al. 1988) (Fig. 2).
These strike parallel to the Red Sea axis, dip to the NE or SW and are preserved on the coastal plains and the escarpments inland 20–25 km from the Red Sea coast (Ghebreab,
1998)(Fig. 2). Outcropping tholeiitic basaltic dikes in the southwestern Saudi Arabia coastal plain (Tihama Asir Magmatic Complex) have been dated at ca 22-24 Ma
(Ar 40 /Ar 39 cooling ages: Sebai et al., 1991) and are overlain by Quaternary sedimentary rocks (Coleman et al. 1979; Schmidt et al. 1982). In Yemen, dikes are found most abundantly along the 20–30 km wide escarpment zone (Ghebreab, 1998). These dikes
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yield Kr-Ar dates of 22–25 Ma suggesting that they could be a continuation of the
Tihama Asir Magmatic Complex (e.g., Coleman et al., 1979; Bohannon and Eittreim,
1991).
On the African side, basaltic flows dated at 23 to 20 Ma ( 40 Ar/ 39 Ar method; Coulie et al.,
2003) occurred at the edge of the East African Rift System (Alem Ketema felsic units,
Addis Ababa). Silicic volcanic rocks were reported by Ukstins et al. (2002) further east at
Robit along the western escarpment. These volcanic rocks yield 40 Ar/ 39 Ar dates of ca 19
Ma. Localized magmatic activity is represented by closely spaced basaltic dikes restricted to Desse-Combolcha area (at the western Afar margin). These dikes are dated at ca 24 to 22 Ma ( 40 Ar/ 39 Ar method; Rooney et al., 2013).
Trace elements and isotope geochemistry suggest that the Afar dikes are sourced from the
Afar plume (Rooney et al., 2013). Rooney et al. (2013) noted that dikes are characterised by temporal isotopic heterogeneities indicating that the Pan-African lithosphere may also be a chemical reservoir. The dikes show little evidence for a depleted mantle source and this may be explained by limited mantle upwelling related to a small degree of lithosphere thinning during this period.
1.2.2.4 MiddleACCEPTED Miocene to Pliocene volcanism MANUSCRIPT
Alkali olivine basaltic volcanism spread in the northern part of the Arabian plate from
Harrat ash Shamah (13 Ma; Ilani et al., 2001) southward to the younger Harrats Uwaynd
(12 Ma), Khaybar (11 Ma), and Rahat (10 Ma) (K–Ar dates; Robson, 1971; Coleman,
1993) (Fig. 2). Basaltic magmatism with similar 40 Ar/ 39 Ar dates between 12.51 Ma to
8.31 Ma have been reported recently along the western Afar margin (Rooney et al., 2013).
Furthermore, Barberi et al. (1975) documented volcanic rocks in the Afar depression with
K–Ar dates from 4.4 Ma to 7.4 Ma with magmatic hiatus’ in the MER between 6.5 Ma
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and 3.2 Ma (Keranen et al., 2004). 40 Ar/ 39 Ar dating of Somali basalts yielded ages between ca 3 and 2.7 Ma (Audin et al., 2004).
The absence of high 3He/ 3He ratios in the basaltic lavas (< 12 Ma) suggests that the compositional effects of the Afar plume does not extend beyond the topographic influence of the plume (Rogers, 2006). Rooney et al. (2013) suggest that temporal decrease in plume activity contributes to the depletion of the upper mantle. This depletion became an important melt generation mechanism at this period and is supported by isotope geochemistry (low radiogenic Pb). Furthermore, maturation of the western Red
Sea margin (at Afar Depression) at 8 Ma led to a more established magmatic plumbing system at the rift edge of the margin, leading to a diminished lithospheric contribution for magmas (Rooney et al., 2013).
1.2.2.5 Quaternary volcanism
Alkali-olivine basalts were extruded during the Quaternary (0.3 - 0.9 Ma; Dabbagh et al.,
1984) near the coast of southeastern Saudi Arabia; close to the Tihama Asir Magmatic
Complex (e.g., Voggenreiter et al 1988) (Fig. 2). In addition, basaltic flows and alkaline to peralkaline silicic rocks were erupted in the Afar Depression over the past 1 Ma (e.g., Varet, 1978).ACCEPTED Pleistocene to Holocene MANUSCRIPT alkaline volcanism is also reported in Yemen (Davison et al., 1994). These volcanic successions were generated by strombolian and phreatomagmatic eruptions with a hawaiite composition that is similar to the Middle
Miocene to Pliocene volcanism along the Saudi Arabian flank of the Red Sea (Camp and
Roobol, 1989).
1.2.2.6 Recent Dikes
Dike emplacement was still occurring in the Afar Depression in 2006 (e.g., Keir et al.,
2009) and along the eastern flanks of the central Red Sea in 2009 (e.g., Pallister et al.,
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2010). In Saudi Arabia these intrusions are parallel to the Red Sea rift axis (Pallister et al.,
2010) and parallel with older dyke swarms in the region (Fig. 2). Dike emplacement is marked by more than 30,000 earthquakes swarm and correlates with geodetic data (e.g.,
Keir et al., 2009; Pallister et al., 2010). Seismic activity generated low and high frequency ground motion components that may result from active extension (allowing dike intrusion) and fracturing of the basement rocks (Ebinger and Belachew, 2010). Several recent studies on these dikes provide an evidence of off-axis magmatism near the rift margin in the MER and the Afar Rift (e.g., Kendall et al., 2005; Rooney et al., 2011).
1.3 Geophysical Record
1.3.1 Seismic Reflection
During R. V. Chain cruise in 1970, 34 shallow-penetration seismic reflection profiles were obtained along the axial trough zone of the southern and central sectors of the Red
Sea (Ross and Schlee, 1977) (Fig. 4). Seismic velocities between 1.5 to 4.9 km/s on both sides of the axial zone have been associated with sedimentary layers in grabens with downthrown sides toward the axial trough; this was also confirmed by drilling (Ross and
Schlee, 1977). Phillips and Ross (1970) suggest that the most prominent seismic reflector in the upper sedimentaryACCEPTED succession represents MANUSCRIPT an unconformity at the Miocene-Pliocene transition (reflector “S”), in agreement with well data collected by Glomar Challenger (Ross, 1977). This strong reflector reaches a depth of up to 500 m below seafloor and it has been linked to the top of the evaporites sequence (average velocity of 4.6 km/s) and several folds and diapir structures have been interpreted along these profiles (Ross and
Schlee, 1977) (Fig. 5). The salt domes interpreted from the seismic sections generally follow the NNW to NW trend of the Red Sea basin. The R.V. Chain survey confirms the absence of these sediments in some parts of the axial trough and this was interpreted as
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being the result of post-Miocene sea floor spreading (Phillips and Ross, 1970). However,
Girdler and Whitmarch (1974) interpreted at least some parts of the axial trough have been covered by salt that had flowed laterally from the adjacent marine shelves, as recently evidenced in Mitchell et al. (2010) and Augustin et al. (2014).
Moreover, Cochran (1983) and Martinez and Cochran (1988) interpreted two seismic reflection profiles (61-3 and 61-4 from R. V. Chain survey; Fig. 5) showing that extensional normal faults affect the underlying continental basement. Seismic reflection lines (from R. V. Chain survey; Fig. 4) between 21 o N and 25 o N in the central Red Sea provide evidence for at least 800 m of sediments over the axial zone (Cochran, 1983;
Martinez and Cochran, 1988). This would imply that sea-floor spreading was relatively recent compared with the southern region. The discontinuous geometries of the axial troughs in the northern Red Sea are interpreted as a region that is in transition from a diffuse mode of extension to an organized Mid Ocean Ridge (Cochran, 1983; Martinez and Cochran, 1988).
In 1976, the Saudi Sudanese Commission collected over 4000 km of deep-water marine and 645 km of shallow marine seismic profiles at the central Red Sea area (Nawab, 1984; Izzeldin, 1987)ACCEPTED (Fig 4). The survey covered MANUSCRIPT the areas between 18 o N and 24 o N and consisted of 16, 170 km-long lines oriented approximately perpendicular to the coast. The lines are spaced 40 km apart and linked by 5 additional lines oriented parallel to the coast. Profiles oriented perpendicular to the coast show similar structures with an average velocity for the sedimentary cover ranging between 1.7 and 4.6 km/s, sedimentary thicknesses up to 3 km over the main trough area, thinning towards the axis (Izzeldin,
1987). Oceanic crust extends for about 80 km (at 19 o N) on each side of the rift axis and the lithosphere beneath the shelves and the marginal parts of the main trough is
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continental, though transitional zones separate the oceanic from the continental domain
(Izzeldin, 1987).
From 1981 to 1984 the RV CONRAD cruise recorded four seismic profiles (PI, PV, PIV and PIII) throughout the northern Red Sea (Rihm et al 1991) (Fig. 4). Profiles PV, PIV and PIII illustrate a sedimentary cover with a thickness ranging from 2 to 5 km, and seismic velocities between 3.5 and 4 km/s (Fig. 6). The top reflector in the lower crust was identified at depths of 10 to 12 km. Below this reflector, velocities range between
5.8 and 6.2 km/s. The upper crust thickness is estimated at between 5 and 7 km along PV and PIII, whereas along PIV it ranges between 6 and 8 km. Crustal thickness everywhere along profiles PV and PIV exceeds 15 km. The lower crustal seismic velocity is estimated at 6.7 km/s (Rihm et al., 1991). Rihm et al., (1991) suggest that there is no evidence for an ocean-continent transition and thus the eastern flank of the Northern Red
Sea differs significantly from that of the western flank. Based on these results, and those of Gaulier et al., (1988) from offshore Egypt, Rihm et al., (1991) interpreted significant asymmetry in the northern Red Sea with stretching and attenuation of a continental crust on the eastern side and ocean crust formation beneath its western flank. Some authors interpret strike-slip motion along NNE-striking faults affected the early evolution of the
Red Sea (e.g.,ACCEPTED Makris and Rihm, 1987). MANUSCRIPT
Thirteen wide angle seismic reflection lines were acquired during 1986 from the shore line of Egypt to the axial trough (Gaulier et al., 1988) (Fig. 4). The profiles are characterised by crustal velocities between 6.0 and 6.2 km/s, which decrease to 5.8 km/s toward the axial trough of the northern Red Sea (Fig. 7a). However, some of the profiles close to the shore line indicate a crustal velocity of up to 6.7 km/s. Gaulier et al. (1988) suggested the crust characterised by 6.0 – 6.2 km/s seismic velocities had continental
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affinities, although the higher velocity of 6.7 km/s could be attributed to correspond to transitional crust or oceanic type crust near to the coast of Egypt.
In 1988 three seismic lines collected from offshore Sudan (SI; SII; and SIII) and two lines collected offshore from Yemen (YV and YVI) (Fig. 4) image the central and southern
Red Sea (Egloff et al. 1991). The data show a thick sedimentary pile of up to 5 km thickness, thinning toward the axial trough to less than 1 km thick (Fig. 6). These sedimentary layers have seismic velocities ranging between 3.5 and 4.4 km/s. Egloff et al. (1991) interpreted the 3.5 km/s layer to be associated with the pre-evaporites successions based on well data, whereas the 4.4 km/s velocities were interpreted to reflect evaporites successions. These profiles support the occurrence of stretched continental crust at the flanks of the Red Sea, although Egloff et al. (1991) proposed the presence of old oceanic crust in between the axial trough and the stretched continental crust over a length of 150 to 200 km and a width of 15 to 40 km. This interpretation is based on higher seismic velocities (up to 6.5 km/s, observed along the profiles SI and SIII) compared with those observed closer, towards the African margin (up to 6.0 km/s). Profiles YV and YVI show thinner sedimentary units (up to 3 km) than those observed offshore Sudan. The axial trough crustal layer in profile YV has a seismic velocity of 6.7 km/s. A similar velocity for theACCEPTED axial trough has not been MANUSCRIPT observed over profile YVI, possibly indicating the absence of oceanic crust (Egloff et al., 1991).
Recent seismic reflection profiles across the central Red Sea axial trough were obtained in 2005 (Ligi et al., 2012; 2015; Fig. 4). The seismic reflection data have a shot interval of 50 m and show two different basinal layers (Fig. 7b). The first layer was interpreted to be associated with the post-evaporites sequence while the deeper layer was attributed to the Miocene evaporites succession (Ligi et al., 2012). Mitchell and Park (2014) compared
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seismic velocity laboratory and field measurements data and determined a ~ 6.9 km/s seismic velocity for the lower crustal layer (Tramontini and Davies, 1969), which was interpreted to indicate mafic, probably oceanic, crust in the central Red Sea around Thetis
Deep.
1.3.2 Seismic Refraction
In 1958, sixteen seismic refraction profiles were collected during the Vema, Atlantis and
Discovery surveys (Fig. 4). The results of these surveys are summarized in table 2. Three sedimentary layers and two types of crust were interpreted from the seismic refraction data (Drake and Girdler, 1964). The three sedimentary layers are assumed to represent the
Pleistocene (1 st layer), Pliocene (2 nd layer) and Miocene (3 rd layer) sedimentary sequences. The overall thickness of these layers varies between 3 and 6 km thick in the shelf areas. These sedimentary layers overlie a crust characterized by seismic velocities varying from 5.5 to 7.3 km/s. The highest velocities occur over the axial trough and are interpreted as basic intrusive rocks (Drake and Girdler, 1964). Drake and Girdler (1964) suggest that velocities between 5.5 and 6.4 km/s are typical of continental basement rocks, whereas velocities between 6.7 and 7.4 km/s are associated with oceanic crust. For the higher basement velocity of 6.97 km/s from beneath the central Red Sea main trough,
Cochran (1983)ACCEPTED and Martinez and Cochran MANUSCRIPT (1988) suggested that this higher velocity could result from either Neogene intrusions or from the presence of a Precambrian or early Paleozoic mafic and ultramafic belt. The seismic refraction data were therefore interpreted to indicate that the basement in the southern Red Sea is represented by continental crust, intruded by dense, high-velocity, mafic igneous rocks (Cochran, 1983;
Martinez and Cochran, 1988).
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Refraction seismic data was also collected by Ray Geophysical Company in southwestern
Saudi Arabia between 1963 and 1966. The refraction line on land shows a 4.5 km/s layer to a depth of 2.5 km inland near Jizan (southwestern Saudi Arabia) (MPMR, 1986). It also shows a basement layer with velocities of ~6.6 km/s at a depth of 5 km (MPMR,
1986). Further, the results of surveys collected by Cambridge University and R.S.S.
Discovery until 1968, suggest a basement characterised by seismic velocities greater than
6.7 km/s under the Red Sea south of 25 o N, indicating a possible oceanic crust (Skipwith,
1973). North of 25 o N basement velocities are usually less than 6.0 km/s.
A survey with 20 seismic refraction lines funded by the Natural Environment Research
Council was acquired in 1970 between 22 o and 23 o N covering part of the axial trough and part of the main trough of the Red Sea (Fig. 4). The result of this survey is shown in table 3. The seismic refraction data interpretation of Davies and Tramontini (1970) suggests that oceanic crust extends beyond the central Red Sea axial trough.
In 1972, geophysical institutes of the Federal Republic of Germany together with the
Geophysical Observatory of Addis Ababa conducted five seismic refraction profiles across the Afar depression (III, V, VI, and IV) and one profile (II) across the Ethiopian plateau (MakrisACCEPTED and Ginzburg, 1987) (Fig. MANUSCRIPT 4). Based on these data (Fig. 8a; b), Mechie and Prodehl (1988) inferred that the MER was underlain by normal continental crust that had undergone minor crustal thinning. However, the crust beneath the northern Afar depression (Danakil Depression) is significantly thinner and may possibly comprise of transitional crust. Berckhemer et al. (1975) argue that if the seismic velocity of 6.0-6.1 km/s of the upper crust across the Afar represents a stretched crystalline basement layer then the ratio of the thickness of this layer and that of the interpreted lower crust is too small for the original continental crust to have been stretched and thinned.
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In 1978, the U.S. Geology Survey carried out a deep seismic refraction study across the
Arabian shield and adjacent shelf (Fig. 4). The general features of the seismic refraction section are shown in figure 8c. Gettings et al. (1986) interpreted that 4 to 5 km of sediments overlie the basement up to the shelf. If this interpretation is correct, basement seismic velocities are greater than estimated by earlier interpretations (Chulick and
Mooney, 2002; Chulick et al., 2013). The basement under to the Farasan Bank has been interpreted as oceanic crust having a thickness of 5-9 km (e.g., Mooney et al., 1985).
Alternate models (e.g., Milkereit and Fluh, 1985; Mechie et al., 1986) interpreted this crust as either oceanic or strongly modified transitional continental crust with a thickness of 11-13.5 km.
A more recent interpretation of seismic profiles across the Afar depression (III, V, VI:
Fig. 4) was used to constrain crustal thickness in the Afar depression (Bastow and Keir,
2011) (Fig. 8d). Bastow and Keir (2011) suggested that the data show the transition from continental rifting in the MER to seafloor spreading northward of the Afar depression.
The seismic refraction model proposed by Bastow and Keir (2011) shows thinned lithosphere and explains the recent basalt flows in the area as an expression of the late stage of continental thinning before the onset of sea floor spreading. Similar evolution models, involvingACCEPTED a transition from continental MANUSCRIPT rifting to sea-floor spreading have been proposed by Makris and Ginzburg (1987) and Mechie and Prodehl (1988).
1.3.3 Gravity Data
Gravity surveys carried out by Vening Meinesz in 1934, established that a broad positive
Bouguer anomaly extend along the axial trough of the Red Sea (Skipwith, 1973).
Subsequent surveys such as those by R.V. Chain (1964-1971), R.R. Discovery (1967) and the Saudi Sudanese Commission (1976) along the central Red Sea axial trough between
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18 o N to 24 o N extended the coverage of gravity data. Through a combination of existing gravity data from the Red Sea and by conducting a number of surveys, Makris et al.,
(1991) produced a Bouguer gravity anomaly map for the entire Red Sea (Fig. 9). Overall, the gravity anomaly along the axis of the Red Sea exhibits positive Bouguer gravity values between +90 to +140 mGal (Izzeldin, 1987). In general, the Bouguer gravity values drop away laterally to almost null values at the margin of the Red Sea (Fig. 9).
In 1974, a gravity survey was carried out by the Saudi Arabian Government over the southwestern coastal plain and the Farasan Bank. The survey comprises 145 gravity measurements (Gettings, 1977) and covered an area of 150 x 120 km from the Arabian
Shield in the east and the coastal plain (Tihama) to the Farasan Islands to the west (Fig.
9). The main features of the gravity survey were a steep gradient to a large negative
Bouguer gravity anomaly over the eastern Precambrian rocks (-100 mGal) and a gravity low (almost zero mGal) over the bank. This gravity anomaly low was attributed to thick accumulations of Miocene evaporites successions (Gettings, 1977). Gettings (1977) suggested that discontinuous elongated NW-SE gravity highs (+ 25 to +35 mgal) at the inner edge of the coastal plain are caused by partially exposed layered gabbro and mafic dikes. Finally, the steep gradient from the coastal plain to the large negative gravity values over theACCEPTED Precambrian Shield were MANUSCRIPT considered to be typical values at the boundary between the oceanic crust (beneath Tihama Complex) and the continental crust (Gettings et. al., 1986). Gettings (1977) used gravity profile (B-B`; Fig. 9) to test if the gravity gradient between the Red Sea and the Arabian shield was related to either down faulting of continental crust at the rift margin or if the Arabian shelf was underlain by oceanic crust (Fig. 10a) and concluded the shape and amplitude of the gravity gradients is more consistent with oceanic crust beneath the Arabian shelf.
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At the central Red Sea, Izzeldin (1987) used gravity result from the Saudi Sudanese
Commission survey to calculate a forward model characterized by three main layers (X-
X`; Fig 9): (1) sedimentary successions with an average density of 2.2 g/cm 3; (2) oceanic crust underlying the axial trough and part of the main trough with an average density of
2.9 g/cm 3 and; (3) intermediate crust flanking the oceanic crust with an average density of
2.85 g/cm 3 (Fig. 10b). In this forward model the crustal thickness of the continental shield varied from 40 km to15 km approaching the transitional crust and the oceanic crust.
Bouguer gravity data acquired over the MER, the Afar depression and part of the southern Red Sea by German, Italian and French researchers between 1969 and 1974
(Fig. 9) were forward modelled by Makris and Ginzburg (1987). One profiles was located between Massawa and Jizan across the southern Red Sea, and the second profile was located on the Ethiopian plateau across the Afar depression between Dessi to Assab
(B1 and B2; Fig. 9). Gravity models were constrained by seismic data collected by the
USGS in 1978 from the Farasan Bank to the Arabian shelf, and data collected in 1972 by the Federal Republic of Germany and the Addis Ababa Geophysical Observatory across the Afar depression. Oceanic crust known to be associated with the axial trough of the southern Red Sea has a modeled density of 2.88 g/cm3. Makris and Ginzburg (1987) interpreted thatACCEPTED continental crust extends MANUSCRIPT across the southern Red Sea, however it is a stretched crust intruded by melt from the upper mantle.
Further, Makris et al., (1991) modeled three gravity profiles (Fig 10 c, d, and e) constrained by result of seismic experiments (P1) across the African Shield (off the coast of Egypt) and the northern Red Sea, (P2) across the southern Red Sea at the Farasan and
Dahlak Banks; and (P3) across the southern Red Sea at the Danakil depression (Fig. 9).
These models suggested that on the eastern flank of the northern Red Sea, the crust is
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attenuated continental crust, whereas higher density oceanic crust of early Miocene age occurrs on the western flank. A second profile showed that the oceanic crust is limited to the axial zone flanked by stretched and thinned continental crust on either side. A third model shows stretched continental crust in the southern Red Sea and the Danakil depression. The large positive NW-trending gravity anomaly is interpreted as dense oceanic crust and intruding magmas in stretched continental crust (Makris et al., 1991).
A total of 170,707 surface-ship gravity measurements have been acquired in the northern
Red Sea (Fig. 11a) by the National Science Foundation (Cochran, 2005). The Bouguer gravity anomaly map of the northern Red Sea is characterized by positive gravity anomalies along the axial trough decreasing to negative gravity anomalies towards the coasts on either side of the trough. Cochran (2005) and Cochran and Karner (2007) suggested that the high gravity anomaly at the southeastern end of the survey (Fig. 11a) may represent a transition from continental to oceanic crust, which is consistent with seismic reflection results.
The Red Sea also has recent extensive marine-satellite altimetry gravity coverage available from the Scripps Institution of Oceanography-University of California San Diego (SandwellACCEPTED and Smith, 2009). The MANUSCRIPT latest version (Fig. 14a) has a resolution of 1- minute using global gravity models. The satellite free air gravity data over the Red Sea is characterized by high and low gravity anomalies between -180 to 200 mGal. Once again, gravity highs appear to be associated with the axial trough of the Red Sea but also to some parts of the Red Sea shelves near the margins.
1.3.4 Magnetic Data
The marine and aeromagnetic surveys coverage of the Red Sea and their acquisition parameters are listed in table 4. A total intensity map of the entire Red Sea was prepared
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by Hall et al. (1977) (Al-Subbary et al., 1998) (Fig. 12). The magnetic data revealed three major zones with distinct anomaly patterns. Two of them are clearly associated with the axial trough and the Precambrian shields respectively. The third domain, in between the axial trough and the shield, has a more diffuse signature and less distinct magnetic pattern.
Generally, the central portion of the magnetic zone related to the axial trough, is consistent with the axial zone defined by seismic and gravity data (e.g. Getting et al.
1986; Izzeldin, 1987). The zone is characterized by large amplitude (800 nT) and short wavelength (semi-wavelength of 15 km estimated from amplitude peak to trough) anomalies forming a linear pattern approximately parallel with the Red Sea. Several NE-
SW trending magnetic features oblique to the trend of the axial trough break and offset the linear magnetic pattern (e.g. Allan, 1970; Hall et al., 1977). These features may be related to relatively small transform or accommodation zones (e.g. Martinez and Cochran,
1988). The area between the axial trough and the Red Sea coast is characterized by anomalies with smaller amplitude (300 nT) and longer wavelengths (semi-wavelength of
30 km) that trend parallel to the axial trough. Anomalies with variable amplitude (50 to
400 nT) and shorter wavelength (5 to 30 km) dominate throughout the Precambrian
Shield. Hall etACCEPTED al. (1977) report on a possible MANUSCRIPT fourth zone of distinct magnetic pattern with short wavelength (1 to 5 km) and variable amplitude (10 to 500 nT) anomalies along the coastal plains, that are seldom linked over long distances.
Various authors (e.g. Gridler and Styles 1974; Gridler and Underwood, 1985; Hall, 1989) suggest that the low amplitude magnetic anomalies overlying the majority of the Red Sea
Basin between the axial trough and the Precambrian Shield are similar to oceanic stripes.
However, other researchers have interpreted these low amplitude anomalies as large
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linear mafic plutons emplaced into extended continental crust (transitional crust), and that only the axial trough of the Red Sea is underlain by recent oceanic crust since 5 Ma (e.g.
Cochran, 1983; Bohannon, 1986) or 12 Ma (Izzeldin, 1987) .
Different magnetic models have been introduced by correlating observed magnetic data with synthetic sea-floor spreading profiles in order to define the type of the crust under the Red Sea shelves. Girdler and Styles (1974) were the first to propose two stages of sea floor spreading for the Red Sea (ca 41-34 Ma and ca 5 Ma; Fig. 13a). They supported this interpretation with drilling data near the Dahlak Islands, which showed the presence of a basaltic basement. Girdler and Styles (1974) also interpreted heat flow data from the eastern margin of the central Red Sea (Davies and Tramontini, 1970) and proposed that heat flow is consistent with older oceanic crust beneath the Dahlak Bank. The spreading rate, based on the ocean stripes width, was estimated at 14 mm/y (Gridler and Styles,
1974). Girdler and Underwood, (1985) later presented an alternate model in which ocean crust formed between ca 39-32 Ma.
By a compilation of existing magnetic data (table 3), Hall et al. (1977, 1989) suggest that the Red Sea was formed by a two-stage spreading process (ca 28-24 Ma and ca 5 Ma; Fig. 13b) and thatACCEPTED oceanic crust existed over MANUSCRIPT the full width of the Red Sea. The rate of spreading was established through comparisons between the magnetic reversal time scale (Heirtzler et al. 1968) and the Red Sea magnetic reversal stripes. The Oligocene spreading rate was estimated to be 20 mm/y, whereas the Pleistocene to recent spreading occurs at a rate of 8-9 mm/y (Hall et al., 1977). In contrast, Cochran (1983) and Martinez and Cochran (1988) argue that the magnetic anomalies over the southern Red Sea shelves are characterized by lower amplitude (less than 200 nT) and longer wavelength (20 to 50 km) than those observed at the axial trough and thus these anomalies may represent a
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wide zone of dike injection, rather than sea floor magnetic stripes. Furthermore, they interpret that the offshore southwestern Saudi Arabia magnetic anomalies (Hall et al.,
1977) are consistently offset slightly to the southwest of the gravity anomalies (Getting,
1977) and used this relationship to argue that magnetic anomalies are not related to sea- floor spreading.
In 2004, Cochran (2005) compiled a total of 152,885 total magnetic intensity shipboard measurements along the northern Red Sea (Fig. 11b). According to Cochran (2005), the data shows two patterns of magnetic anomalies. The southeast corner of the data shows a series of large amplitude that are distributed in a NW-SE direction while the remainder of the area (in the northern Red Sea) exhibits a relatively smooth magnetic anomaly. Unlike the southern Red Sea these anomalies are not linear. These magnetic anomalies are associated with high gravity anomalies suggesting that they are due to basement structures or intrusions of dense and magnetic plutons. Simple forward modeling of these anomalies suggests that their source could be three separate intrusive bodies into the evaporites sequence (Cochran, 1983; Martinez and Cochran, 1988).
Ground magnetic data collected in the Afar depression in 2009 (Bridges et al., 2012) (Fig. 10f) show a ACCEPTED pattern of anomalies that canMANUSCRIPT be interpreted as a magnetic stripe based on similarity of its pattern and amplitudes with magnetic anomalies from seafloor spreading areas. Bridges et al. (2012) interpreted this magnetic pattern as being due to incipient ocean crust formation within transitional continental crust before the onset of seafloor spreading.
Magnetic data covering the entire Red Sea was obtained from the marine-satellite magnetic anomaly data (World Digital Magnetic Anomaly Map project; Maus et al.,
2009) (Fig. 14b). This data shows that the magnetic field over the Red Sea varies between
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300 nT to -300 nT. Satellite-magnetic data was very effective in delineating oceanic/transitional crust and continental crust because it shows the presence of magnetic stripes. We present the tilt-derivative of the total magnetic intensity data (Fig. 14c). This type of filtering or data enhancement normalizes magnetic anomaly amplitudes and is effective for imaging subtle anomalies within shallow basement structure (Verduzco et al., 2004). The tilt-derivative is also effective because it highlights subdued amplitudes due to the burial of the magnetic basement, such as the basement buried beneath up to 4 km of sediments and evaporites of the Red Sea. Our interpreted structural map (Fig. 14d) reveals a series of linear magnetic anomalies parallel to the Red Sea axial trough and extending over the shelves. The magnetic stripes are offset by NE-SW or N-S faults, inferred from the steep gradients in the magnetic, gravity (Fig. 14 a; b) and bathymetric data (Fig. 1).
1.4 Tectonic Models
1.4.1 Pure shear model or symmetrical basin
Lowell and Genik (1972) initially proposed that extension along the Red Sea was driven by divergent convective flow in the asthenosphere (Fig. 15a). Oligocene extension resulted in aACCEPTED thinned continental lithosphere MANUSCRIPT with associated normal faulting in the southern Red Sea. Thereafter, continued rifting and thinning resulted in tectonic subsidence and the development of horsts and grabens with tilted blocks and leading to an extensive Miocene marine incursion. Lowell and Genik (1972) argued that the upper part of the continental crust was sufficiently thinned and breached to allow crustal intrusion of oceanic basalts in the last 4 Ma. Lowell and Genik (1972) recognized that there were seismic refraction anomalies along the west coast of the northern Red Sea, and the east coast of the central Red Sea. These anomalies were interpreted to represent isolated
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windows of oceanic crust. Furthermore, analysis of the Moho topography interpreted from seismic reflection sections at the northern end of the Danakil depression (Fig. 7c), suggests the region was experiencing recent rifting (Lowell and Genik, 1972). The
Danakil depression rift shows geometries consistent with an asymmetrical graben, which are deeper and most severely affected by faulting to the west, adjacent to the Ethiopian plateau. The graben becomes shallower to the east, probably representing the Danakil horst. Recent volcanic activity, coupled with evidence of recent faulting and seismic activity suggest that the Danakil depression is an active rift system that is parallel to the axial trough for at least 400 km (Lowell and Genik, 1972).
Cochran (1983) and Martinez and Cochran (1988) proposed extensional models in which different parts of the Red Sea (Northern, Central and Southern) represent successive phases in the continental rifting and the development of a continental margin (Fig. 15b;
Fig. 16). Common interpretation of the extensional evolution of the Red Sea suggest an initial period of late Oligocene or early Miocene diffuse extension characterized by rotational normal faulting and Miocene dike injection is followed by the initiation of sea- floor spreading at the southern Red Sea axial trough in the last 5 Ma. This spreading centre propagated northward through a transition zone preserved in the central Red Sea.
Martinez andACCEPTED Cochran (1988) suggested MANUSCRIPT propagation of sea floor spreading has affected the northern Red Sea where limited formation of new oceanic crust and the large separation of Arabia and Africa were accommodated by extension and thinning of the continental crust.
Berhe (1986) further conceptualized and further developed the rifting models of the southern Red Sea by taking into account diking period in southwestern Saudi Arabia
(Tihama Asir Complex) and K-Ar dating from southern Afar depression (at 14-11, 11-10,
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9-7, 5-4 and 1.6 Ma) (Fig. 15c). Berhe (1986) proposed that rifting in the Red Sea commenced at ca 25 Ma and was active at 14 Ma and 4 Ma. Continental breakup in the
Red Sea was initiated along a major NW-trending shear zone. Further extension was accommodated by normal faulting and repeated dyke injections with limited true oceanic crust accretion. Berhe (1986) interpreted basaltic volcanism identified in the Afar region as evidence for continuous volcanism, which contradicted the two stage sea-floor spreading models (e.g. Girdler and Styles, 1974). Berhe (1986) argued that any two stages model would require a portion of the African-Arabian plates along the central Red
Sea to move separately along major transform fault zones. Yet there is no evidence for such transform faults (Berhe, 1986). Berhe (1986) interpreted the ca 22.5 Ma (K-Ar;
Coleman et al. 1979) basalt dyke swarm of Tihama Asir Complex to represent an early phase of dyke injection associated with rapid thinning of the continental crust close to the coastline. In this model there was only limited oceanic crust formation in the Red Sea.
Bohannon (1986; 1989) and Bohannon and Eittreim (1991) proposed a model for the central and southern Red Sea in which a brief but intense time of extension (32 – 25 Ma) was preceded by a long period of magmatic expansion that resulted in extremely thin continental lithosphere at ca 24 Ma (Fig. 15f; Fig. 17). Mafic dikes along the southwestern ACCEPTED Saudi Arabia coast were interpretedMANUSCRIPT to mark the extent of the extensional phase. Most of the continental divergence was interpreted to be accommodated by disorganized plutonic expansion, which resulted in the development of magnetic anomalies and the development of thinner crust with time, leading to the present sea-floor spreading at 5 Ma. The cause of this voluminous melt production could be partially attributed to proximal hot spots, including the Afar depression (Bohannon 1989;
Bohannon and Eittreim; 1991). In this model, the rocks beneath the southern and central
Red Sea formed hybrid (transitional) crust, which varied between approximately 20 km
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thick near the continental edges and less than 10 km thick near the Red Sea axis
(Bohannon 1989; Bohannon and Eittreim, 1991). Major isostatic and thermal asymmetries are avoided in their model (Fig. 15f; Fig. 17), as they propose that early rifting was initiated when a major crustal detachment fault was activated and affected the whole crust from its root in the middle crust to the sedimentary cover.
Bosworth et al. (2005) presented a synthesis of the evolution of the Red Sea and interpreted a complex evolution of the Afar-Red Sea oceanic basins initiating with the arrival of the Afar plume at ~31 Ma. Plume magmatism was overprinted by Oligocene-
Miocene transition continental rifting (ca 27-24 Ma; Bosworth et al., 2005) at the junction of the present-day Gulf of Aden rift and the Afar rift. Renewed volcanism and basaltic dike emplacement along the entire length of the Red Sea occurred between 24 and 22 Ma
(Fig. 18). Widespread marine and evaporites deposition affected the Red Sea basin between ca 24 and 10 Ma, and seafloor spreading started at the southern Red Sea ca 5 Ma
(e.g., Cochran 1983). Extension in the northern Red Sea and the Gulf of Suez was attributed to continental rifting at ca 24 Ma, whereas extension at the Gulf of Aqaba occurred at ca 14 Ma based on observed stratigraphic, structural and volcanic relationships (Bosworth et al 2005), as well as fission track dates (e.g., Kohn and Eyal
1981). ACCEPTED MANUSCRIPT
1.4.2 Asymmetrical model or simple shear
Tectonic models became more sophisticated because of increased understanding of the role that low angle normal faults and asymmetrical extensional systems played in many extensional tectonic settings (e.g., Wernicke, 1985). Asymmetric extension models (e.g.,
Voggenreiter and Holtz, 1989; Division et al., 1989) have been proposed for the Red Sea based on simple lithospheric shear or a regional-scale low-angle normal fault of that
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breaks away at the surface on the African continent and projects to sub-lithospheric levels beneath the Arabian continent (Fig. 15d; e). Each of these models assume of multi-stage evolution: (1) an early stage of magmatic event lasted from the Oligocene to the Middle
Miocene at ca 14 Ma, based on the distribution of magmatism on either side of the rift, followed by 9-10 m.y of magmatic and tectonic quiescence and; (2) a late extension defined by the onset of sea-floor spreading in the southern Red Sea in the past 5 Ma.
In these models, Oligocene volcanism (Afar plume) marks the locus of initial mantle upwelling and magmatic underplating (e.g., Division et al., 1989). Early to Middle
Miocene volcanism along the edge of the Arabian plate is interpreted to represent the deactivation of a low-angle lithospheric shear zone at ca 14 Ma (Wernicke, 1985;
Voggenreiter et al., 1988). Such models were used to provide explanations for the asymmetrical distribution of volcanic rocks and the higher topography (up to 3 km) on the
Arabian side of the Red Sea, compared to lower elevations (2 km) on the African side.
Doglioni et al. (2003) suggested that the uplift of the eastern flank of the Red Sea could not be caused by thermal processes alone as predicted by asymmetrical extension models.
Rather, they proposed a simplified kinematic model where initial stretching of the lithosphere andACCEPTED associated asthenospheric MANUSCRIPT uplift and rift subsidence allowed high-density mantle rocks to rise (Doglioni et al. 2003). This model is supported by a surface wave tomographic model of the Earth’s upper 300 km that show a faster velocity and thicker lithosphere for the western limbs relative to the eastern limb whereas the upper asthenosphere is faster in the eastern limb (Panza et al. 2010). The initial phase of extension was followed by isostatic adjustments in response to lateral mantle flow with respect to a decoupled lithosphere (Doglioni et al. 2003). Flow of less dense depleted mantle generated in the Mid Ocean Ridge replaced dense undepleted asthenosphere
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residing beneath unstretched continental lithosphere of the northeastern shoulder of the
Red Sea. Doglioni et al. (2003) suggested that any asymmetry becomes significant only at mature stages of oceanization. Whilst this model is eloquent, it assumes that spreading is occurring on the length of the Red Sea. Recent studies (e.g., Cochran, 2005; Ligi et al.,
2012) have demonstrated that seafloor spreading is not occurring in the northern Red Sea.
1.4.3 Pull-apart model
A multi-stage pull-apart model (Fig. 19) was proposed by Makris and Rhim (1991) for the tectonic evolution of the Red Sea based on seismic reflection and refraction results from the northern Red Sea, magnetic data and heat flow observations from the axial trough, which exceed 300 Mw/m 2 (Girdler, 1970). Makris and Rhim (1991) suggest that strike-slip processes controlled the break-up of the Arabia and Africa plates based on pre- existing faults systems observed onshore.
These faults systems are: (1) the NW-trending Najd Shear System, which is preserved within the Arabian Shield and the Red Sea (Stern, 1985; Sultan et al., 1988); (2) the
Central African Fault Zone, which crosses the entire African continent in a E- to ENE- strike, meets the Red Sea coast in southern Egypt and Sudan, and extends across the Red Sea into ArabiaACCEPTED (Le Pichon and Francheteau, MANUSCRIPT 1978; Guiraud et al., 1985) and; (3) the Onib-Hamisana and Baraka suture zones, which trend parallel with the Red Sea coast around 23N and 20 N (Vail, 1983; Stern et al., 1986).
Makris and Rhim (1991) suggested that strike-slip movement along these fault systems created sharp plate boundaries that led to relatively local extension and thinning of the continental crust during the late Oligocene. This stage of lateral movement was followed by stretching of the continental crust and block faulting of the basement in the Gulf of
Suez between 19 and 15 Ma with an interpreted rate of extension up to 5 mm/y (Moretti
34
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and Coletta, 1987). Crustal extension in the Gulf of Suez region stagnated 15 Ma ago as the rate of extension reduced to <1 mm/y (Steckler et al., 1988). This stagnation marks the transition from the Suez stage to the Gulf of Aqaba stage ca 14 Ma (Makris and Rhim,
1991). Strike-slip movement commenced in the Gulf of Aqaba and had a strong influence on the northern and central Red Sea. This is supported by seismic data offshore of Egypt and Sudan which suggest the formation of oceanic crust over a distance of more than 50 km (Egloff et al. 1991; Rihm et al., 1991), which was used to support interpretations of an early pull apart tectonic model. Strike-slip motion influenced mainly the Egyptian and
Sudanese coastal areas while Arabia was separated from Africa by existing ocean crust formed during an earlier episode of pull-apart basins development (Makris and Rhim,
1991). The model requires crustal asymmetry on the eastern and western flanks of the
Red Sea (Makris and Rhim, 1991). The last stage of the tectonic model was sea floor spreading, which commenced in the south and propagated to the northern Red Sea
(Makris and Rhim, 1991).
1.5 Discussion
Despite the long held controversy about the rifting models, the opening of the Red Sea is complex ACCEPTED because multiple tectonic MANUSCRIPT drivers were operating including local forces associated with the arrival of the Afar plume, slab-pull forces related to the closure of the Neotethys and active subduction zone at Makran. In some models, plutonism was
emphasized as having a major role in thinning the continental lithosphere before
seafloor spreading had begun (e.g., Bohannon and Eittreim, 1991), whereas other
tectonic models have emphasized the role of mechanical stretching of the lithosphere
(e.g., Cochran and Martinez, 1988) or localized ocean crust formation (e.g., Makris and
Rhim, 1991). While some researchers (e.g., Hempton, 1987; McGuire and Bohannon,
35
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1989; Bosworth et al., 2005) suggested that rifting was principally passive and driven by
far-field stresses; other researchers interpret the rift as active and is driven by the arrival
of the Afar plume (e.g., White and McKenzie, 1989, 1995; Rogers 2006). However, it
seems most likely that the evolution of the Red Sea is the influenced by both active and
passive rifting (e.g., Davison et al., 1994; Drury et al, 1994; Zeyen et al., 1997;
Bellahsen et al., 2003; Ebinger and Belachew, 2010).
1.5.1 Model implications
Since the advent of plate tectonic theory the evolution of the Red Sea has been considered as an example of a juvenile ocean basin in the context of the Wilson cycle. A number of rifting models have been proposed including asymmetric extension models, symmetrical extension models, and pull-apart basins. Despite the advances in understanding the geological or the geophysical signatures of the Red Sea in the past 50 years there remains considerable conjecture related to: (1) the timing and extent of ocean basin initiation; (2) the timing and significance of mafic magma systems and; (3) the role of the Afar plume or far field tectonic forces in driving and focusing rifting and crustal extension.
The diversity of tectonic models for the evolution of the Red Sea can be largely attributed to the bias in ACCEPTEDthe geological and/or geophysical MANUSCRIPT data used in determining the evolution, or the geographic region of the Red Sea that was studied. For example, tectonic interpretations based on geophysical observation in the northern Red Sea are significantly different to those in the southern Red Sea (e.g., Hall, 1989; Makris and Rhimm, 1991;
Cochran and Karner, 2007). Similarly, interpretations based on geological data alone, such as early graben-horst models (e.g., Lowell and Genik 1972) did not consider the possibility of a shallow Moho topography beneath the Red Sea shelves and coastal plains
(Ghebrab, 1998) which is only evident in the seismic data (e.g., Mooney et al, 1985). As a
36
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way of explaining the significant crustal thinning required to explain the shallow Moho topography beneath the Red Sea shelves, Bohannon (1989) suggested a brief period of deformation and then magmatic rifting.
Most researchers agree that sea-floor spreading has been ongoing since ca 5 Ma (e.g.,
Lowell and Genik, 1972; Cochran, 1983; Bosworth et al., 2005), although there is no consensus about whether the Red Sea formed during a single-stage rift event or opened as a result of two discrete ocean spreading events. Further, termination of old sea floor spreading if it exists remains unexplained. The two stage spreading tectonic models were largely developed by interpretation of potential field and/or regional seismic data in the southern Red Sea (e.g., Girdler and Styles, 1974; Ahmed et al. 2013). In particular, the presence of basement rocks beneath the Farasan Bank that show characteristic of magnetic stripes and thinned crust were used to support the development of a small ocean in the southern Red Sea before the deposition of Miocene sedimentary shelves (Hall,
1989; Almalki et al., 2014) or simply the development of magnetic stripes in transitional crust (e.g., Ligi et al. 2012; Bridges et al. 2012).
In any scenario, the northern sector of the Red Sea behaved differently as there is no clear geological or ACCEPTEDgeophysical evidence for theMANUSCRIPT existence of any magnetic stripes and possibly of any oceanic crust (Fig, 14b and c). Two stages spreading model (Fig. 14e) possibly bounded by transitional crust appear to offer a better explanation for the observed magnetic (e.g., Hall, 1989; Almalki et al., 2014), gravity (e.g., Gettings, 1977; Milkereit and Fluh, 1985; Mitchell and Park, 2014), and seismic data (e.g., Drake and Girdler,
1964; Davies and Tramontini, 1970; Ahmed et al. 2013) in the central and southern Red
Sea. In addition, two stage models are consistent with reconstructions of the Red Sea by
Sultan et al. (1992), which suggest that much of the Red Sea substrate must be oceanic
37
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crust. However, the two stage spreading model does not address the asymmetrical nature of the volcanic intrusion distribution and the topography on the flanks of the Red Sea.
Regardless of two stages or one stage spreading, some authors (e.g., Cochran 2005;
Cochran and Karner 2007) suggest that the northern Red Sea represent a transition from continental to recent oceanic extension. Likewise, in the Afar depression Lowell and
Genik (1972) for example, shows that there are small areas of ocean crustal accretion.
However, Bridges et al., (2012) argue that magnetic stripes can also form as a result of mafic underplating and mid crustal intrusion in transitional crust and thus not necessarily indicating sea-floor spreading. Almalki et al. (2014) demonstrated the presence of 7 stripes beneath the Farasan Bank in the southern Red Sea. If these stripes were to represent transitional crust alone this would be a remarkable observation. More mature ocean basins, such as the northern Atlantic Ocean, have been demonstrated to have one or two magnetic stripes associated with transitional crust (e.g., Bronner et al., 2011).
The asymmetrical extension models proposed for the Red Sea (e.g., Bohannon and
Eittreim 1991) suggest low-angle eastward-dipping detachment beneath the Red Sea to explain the basin geometry and the prevalence of off-axis magmatism on the Arabian flanks of theACCEPTED rift. However, this low angleMANUSCRIPT detachment has never been imaged in any seismic data and there is no evidence of this detachment on the surface (e.g., Rogers and Rosendahl, 1989). The pull-apart models (e.g., Makris and Rhim 1991) provide an explanation for asymmetrical magmatism and topography. However, seismic observations on the central and southern Red Sea eastern main troughs (6.69 – 6.8 km/s; offshore
Saudi Arabia) are similar to seismic velocities considered for this type of models to represent locally developed oceanic crust (6.5–6.7 km/s). Thus, the pull-apart model may apply to the asymmetry of northern Red Sea but appears to be untenable for the southern
38
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Red Sea. Although Makris and Rhim (1991) suggest that seismic interpretations show that the oceanic crust extends from the rift axis towards both flanks over 40 km beyond the latest recognizable magnetic stripe, this interpretation indicates that the formation of oceanic crust is symmetrical and at low spreading rates below 5 mm/y and spreading may have commenced between 12 and 9 Ma, earlier than the timing interpreted from the magnetic data.
Tectonic models proposing diffuse extension (e.g., Cochran 1983) interpreted crustal extension at depths greater than is permissible because the ductile rocks are eventually uplifted to shallow crustal levels where temperatures and pressures should be too low
(e.g., Ghebrab, 1998), given the geometric constraints on the shallow faults (e.g.,
Bohannon, 1989). In addition, large areas of the northern Red Sea are characterized by local subsidence (deeps) and abrupt boundaries at the continent-ocean interface from the
Gulf of Suez and Egypt into the Red Sea (e.g., Makris and Rhim 1991). Also, diffuse extension models do not address the probable magmatic consequences of uplifting large volumes of lithospheric mantle and asthenosphere under the axis of the rift (e.g.,
Bohannon, 1989).
It seems that ACCEPTEDmany of the models for the MANUSCRIPT evolution of the Red Sea have been developed within a specific location in the Red Sea and have been applied along the entire length without testing the models. In other words, many early models did not incorporate information from other parts of the Afro-Arabian rift system or adjacent plate boundaries, which play important roles in formulating some more recent models. Consequently it is possible that several different models may be required to explain the evolution and architecture of the Red Sea. For example, the two stage spreading model may be applicable to the southern and central Red Sea whereas the northern Red Sea - diffuse
39
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extensional models are applicable because there is little evidence for significant oceanic crust formation.
Recent advances in understanding the evolution of continental margins have demonstrated that the geometry and style of lithospheric extension can vary along the length of an oceanic basin (e.g., Huismans and Beaumont, 2011; 2014). Two end member modes of extensional margin are recognized. Type I margins are characterized by large exposure of large tracts of continental mantle lithosphere exposed at the seafloor (e.g.
North Atlantic), whereas type II margins are characterized by wide areas of thinned continental crust below which continental lower crust and mantle lithosphere is removed
(e.g. South Atlantic) (Huisman and Beaumont, 2011). The type of margin created can be attributed to variations in the rheological structure of the lithosphere. Numerical modelling of Huisman and Beaumont, (2011) suggest that type I margins are created when the upper lithosphere breaks-up before the lower lithosphere, whereas in type II margins the lower lithosphere separates before the upper lithosphere. The Red Sea represents one of the most juvenile examples of the incipient ocean initiation and many of the geophysical and geological features associated with type I and type II margins (e.g.,
Wright et al., 2006; 2012; Peron-Pinvidic and Manatschal, 2010) may not have been established withinACCEPTED the basin. There are neverthelesMANUSCRIPTs several characteristics of the southern
Red Sea that point to the possibility that it represents an incipient type I margin. For example, Almalki et al. (2014) suggest that several of the magnetic stripes at the southern
Red Sea, which correlate with a regional Bouguer gravity anomalies, represent transitional crust, which has been documented in the Northern Atlantic (Bronner et al.,
2011). Gravity modelling along the eastern margin of the Red Sea suggests a sharp step in crustal thickness and a rapid transition from continental crust to oceanic crust, which is a geometry identified in type I margins (Peron-Pinvidic and Manatschal, 2010).
40
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Furthermore, identification of transitional crust and the onset of diking in the Danakil
Depression of the Afar region suggest the onset of new ocean crust formation and the formation of micro-continent (Peron-Pinvidic and Manatschal, 2010) represented by the
Danakil Alps.
Lavier and Manatschal (2006) showed that the mode of extension may be time variable leading to different structures and extensional processes along the length of a rifted margin. Initial crustal extension (stage 1) is accommodated by listric faulting rooted in the middle crust and the development of basin forming grabens and half grabens.
Weakening of the middle crust delocalizes deformation and allows the middle crust to flow above a cooling and strengthening lower crust intruded with gabbro (stage 2 extension). The third stage of extension (stage 3) involves brittle faulting in the upper mantle and upper crust, exhumation of the middle crust and exhumation and juxtaposition of mantle next to middle crustal rocks leading to extreme crustal thinning and serpentinization, and the formation of transitional crust before the onset of the sea floor spreading. The Red Sea can be separated into three distinct segments. The southern Red
Sea is drifting, whereas the northern Red Sea is experiencing continental rifting (Cochran and Karner, 2007). The central Red Sea appears to demarcate a transition zone where isolated bathymetricACCEPTED deeps are interpreted MANUSCRIPT to mark the onset of crustal separation (Bonatti,
1985; Ligi et al., 2011). Correlation of the extensional architecture with the 3 stages of extension proposed by Lavier and Manatschal (2006) is speculative. Nevertheless, variations in the crustal architecture along the axis of the Red Sea appear to be broadly consistent with the extensional phases proposed by Lavier and Manatschal (2006).
Spreading in the southern Red Sea represents a segment that has undergone all three stages of extension. The northern Red Sea represent stage 1 extension, whereas the central Red Sea possibly represent a transition between stage 2 and stage 3, where by the
41
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deeps represent zones of extreme extension and the onset of mantle exhumation. Cochran and Karner (2007) presented a different viewpoint. They argued the northern Red Sea may never evolve into active spreading because of rheological differences between the southern and northern Red Sea. Although they consider the northern Red Sea may be undergoing less extension than the southern segment as a possible cause for the lack of sea floor spreading, their preferred interpretation is that the southern Red Sea is rheologically weaker compared to the northern Red Sea because of the influence of the
Afar Plume in the south (Cochran and Karner, 2007).
1.5.2 Geodynamic implication
The overall understanding of the boundaries of the Arabian Plate is well established (e.g.,
Bellahsen et al., 2003). This includes (1) the Red Sea that represents the western branch of the Afar rift system; (2) the southern arm of the Afar rift system is represented by the
Gulf of Aden; and (3) the northern and eastern boundaries of Bitlis, Zagros and Makran sutures that represent collision and remnant subduction zones related to the closure of the
Neoteyths ocean (e.g., Ghebreab, 1998; ArRajehi et al., 2010) (Fig. 1). The structural patterns in the Arabian Plate is complex due to the reorganization of lithospheric plates and plate movementsACCEPTED (e.g., Zeyen et al., MANUSCRIPT1997) as a consequence of continent collision at the northern plate boundary, the onset of escape tectonics within Eurasia to accommodate convergence between Arabia and Eurasia following initial collision between these plates
(Westaway, 1994), the arrival of the Afar plume (e.g., Jolivet and Faccenna, 2000), and the onset of rifting (e.g., Bosworth et al., 2005). The present day tectonism along the Red
Sea is also influenced by the East Anatolian Fault, the Dead Sea/Aqaba Transform and an active subduction zone at Makran (Bellahsen et al., 2003). Consequently, there are
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several interpretations of geodynamic or a combination of geodynamic processes that are influencing the evolution of the Red Sea.
The first stage of collision between Africa and Eurasia is estimated to have occurred at between 40 Ma (Hempton, 1987) and ca 35-30 Ma (Jolivet and Faccenna, 2000) in the western Mediterranean. This collision reduced the northward motion of Africa from 30 mm/y (before the collision) to <10 mm/y (after the collision). By ca 35 Ma a prominent salient of the African Plate had sutured with Asia (in Turkey). Further to the east subduction and consumption of the Neotethys beneath Zagros-Makran caused a counterclockwise rotation of the proto-Arabian plate, which culminated in the creation of the Arabian Plate (Jolivet and Faccenna, 2000; McQuarrie et al., 2003).
Contemporaneous with the onset of collision between Arabia and Eurasia was the arrival of the Afar Plume beneath the Africa-Arabia Lithosphere (e.g., Bosworth et al., 2005), focusing continental stretching between Africa and Arabia during the late Oligocene (e.g.,
Hempton et al., 1997). Slowing of the northward motion of the Arabian Plate as a result of collision to the north may have enhanced thermal weakening of the African/Arabian lithosphere because the Afar plume was allowed to reside beneath the lithosphere for a longer period of time. At the same time, the Carlsberg ridge (in the Indian Ocean) propagated westwardACCEPTED toward the weak MANUSCRIPT part of the African lithosphere above the Afar plume (Jolivet and Faccenna, 2000). Initial collision slowed the northward drift of
Africa/Arabia and allowed continental crust in the Red Sea/Gulf of Aden area to be stretched by northwest propagation of seafloor spreading in the Indian Ocean. Based on geological observations from drilling in the Red Sea, Hempton (1987) interpreted quiescent conditions after termination of rifting between Arabia and Africa allowing sedimentation to outpace subsidence (e.g., Prosser, 1993) throughout the Red Sea during the Miocene. The collision along the northern margin of the Arabian Plate drastically
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altered the boundary conditions at the northwest boundary of the Arabian plate (e.g.,
Bosworth et al. 2005) resulting in oblique collision between the Arabian Plate and the
Anatolian Plate (e.g. Beydoun 1999). Slab break off following collision may have initiated the Aqaba-Dead Sea strike-slip fault system and creation of the Sinai micro-plate
(e.g. Beydoun 1988).
Zeyen et al. (1997) argued that the Afar plume weakened the surrounding lithosphere based on geological and geophysical observations (Fig. 20). These observations include
(1) the thick Arabian crust (40 km) thinning rapidly to 10 km towards the Red Sea, with high seismic velocities in the lower crust (e.g., Mechie and Prodehl, 1988; Makris and
Ginzburg 1987); (2) the enormous volume of volcanic material erupted in the form of flood basalts.; and (3) the position of the triple junction and the high topographic features that cannot be explained by crustal thickening but can be caused by thermal buoyancy associated with plume activity (this can be traced on a large-scale topographic map)
(Zeyen et al., 1997). As a result there are several interpretation that propose that the rift system is driven by plume-related dynamic uplift (e.g., White and McKenzie, 1989, 1995;
Rogers, 2006).
Despite the longACCEPTED held viewpoint that mantleMANUSCRIPT plumes are major drivers of continental break-up (e.g., Story, 1995), there are numerous examples were plumes have not resulted in continental break (e.g., Siberian Traps: Hill, 1991) or that break-up is delayed with respect to plume arrival (e.g., Central Atlantic Traps: Courtillot et al., 1991). This suggests that far-field stresses are also required for plate fragmentation. Several contributions have linked the tectonic processes operating since the last 30 Ma in the Red
Sea with the closure of the Tethys Ocean (e.g. Hempton, 1987; McGuire and Bohannon,
1989; Bellahsen et al., 2003). However, the timing of the closure of the Tethys Ocean is
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subject to considerable controversy (e.g. Cherven, 1986; Tozer, 1989; McQuarrie et al.,
2003).
The final closure of Neotethys is interpreted to have occurred during the Miocene (e.g.,
Hempton, 1987; Beydoun, 1999) but no later than 10 Ma (e.g., McQuarrie et al., 2003).
This is supported with recent apatite fission track data along the Bitlis-Zagros thrust zone that show the collision of Arabia with Eurasia occurred between ca 20-16 Ma (Okay et al.
2011). Cessation in the convergence between Arabia and Eurasia (Fig. 1) may have stalled rifting between the Arabian and African Plates (Almalki et al. 2014). This is supported by plate reconstructions coupled with geodetic observations that suggest that the Africa plate motion relative to Eurasia has been constant since 13 Ma but is 70% slower than the rate between 30 to 13 Ma (McQuarrie et al., 2003; McClusky et al., 2010;
ArRajehi et al., 2010). Almalki et al. (2014) speculated that stepping of the locus of extension from an Oligocene Mid Ocean Ridge to off-axis dikes along the Arabian margin at ca 24-22 Ma was caused by a decrease in convergence rate between Arabia and
Eurasia. Similar scenario has been suggested recently for the Andaman Basin (Morley and Alvey, 2015).
The tectonic ACCEPTEDreconstruction of Hempton MANUSCRIPT (1987) based on regional structures (Dead Sea fault; East and North Anatolian faults and Bitils/Zagros suture zone) and tectonic events supports geodynamic modeling results that suggest the Red Sea system has been controlled by the interaction and effects of the slab-pull forces between Arabia and
Eurasia and tensile stresses associated with the northwestward propagating Central Indian
Ridge spreading system since the last 30 Ma. Hempton (1987) proposed that continental stretching and dike injection was due to slab-pull and initial collision between Arabia and
Asia along the Bitlis/Zagros suture during the mid-Eocene. The geodynamics of the Red
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Sea were tested through the development of a series of 3D scaled analogue lithospheric experiments (Bellahsen et al., 2003). The results show that the orientation of the Red Sea and the Gulf of Aden could form by slab-pull far-field stresses superimposed on a zone of weakness, such as that generated by the Afar plume, without requiring involvement from the Central Indian Ridge as previously suggested (e.g., Hempton, 1987). Bellahsen et al.
(2003) experiments showed that simple oceanic subduction alone could not produce the forces required to extend the relatively cool and thick Arabian lithosphere and that a zone of weakness was required.
Bellahsen et al., (2003) proposed that slab-pull forces were guided through the slab-plate system and transmitted over large distance within the down-going plate, resulting in localized crustal extension in regions of pre-existing weakness such as older suture zones or mantle plume. Hence, the role of the Afar Plume was to localize crustal extension and ultimately plate fragmentation. This is supported by recent tectonic analysis of the broader Arabian Shield and adjacent Eurasian plate and GPS data (e.g., McQuarrie et al.,
2003; ArRajehi et al., 2010), which were used to interpret early Miocene (ca 24 ±2 Ma)
Red Sea opening with Arabia ‐Eurasia motion between 14.2 to 18.6 mm/yr (Fig. 21). The absence of observable acceleration of Arabia following separation from Africa suggest that the driverACCEPTED for opening of the Red MANUSCRIPT Sea and the Arabian motion is the negative buoyancy of the subduction zone beneath Makran and Zagros and consequently the Afar plume may have only played a minor role by weakening the continental lithosphere (e.g.,
Bosworth et al., 2005).
Renewed ridge spreading has resulted from a plate re-organization at ca 5 Ma, which has been attributed to the switch from subduction below the Bitlis/Zagros suture to oblique motion between Eurasia and Arabia at ca 6 Ma (Axen et al., 2001). This switch was
46
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driven by early Pliocene, "extrusion tectonics", which initiated in the thickened continental crust to north of the Bitlis/Zagros suture zone and allowed renewed independent and faster motion of Arabia relative to Africa, triggering renewed seafloor spreading in the Red Sea at ca 5 Ma (e.g., Cochran, 1983). The Arabian plate experienced a counterclockwise rotation of ~7 o around a rotational pole in the eastern Mediterranean
(e.g. Sultan et al. 1992; Westaway, 1994). The new motion coincides with a faster rate of slip along the Aqaba–Levant transform and a faster rate of separation between Arabia and
Africa (Voggenreiter and Holtz, 1989; Bosworth et al., 2005). Ongoing subduction, as suggested by the occurrence of deep earthquakes and arc volcanism (Quittmeyer et al.,
1984; Kazmi and Jan, 1997), still generates far-field stresses (Bellahsen et al., 2003), which drives present-day spreading in the Red Sea. The Red Sea is continuing to widen at a rate of around 7.0-8.5 mm/y (Rihm and Henke 1998) and 16 mm/y (Chu and Gordon,
1998). These rates are deduced from magnetic data at the current rift axis and are consistent with the Geodetic constraints between 7.1 and 15.4 mm/y at the northern and southern Red Sea respectively (ArRajehi et al., 2010) (Fig. 1).
The recent spreading of the Red Sea was accompanied by Quaternary to recent volcanic intrusions in the coastal plain of southwestern Saudi Arabia (e.g., Voggenreiter and Hotzl,
1989), in the ACCEPTEDAfar rift (e.g., Bronner et MANUSCRIPTal., 2011) and in the MER (e.g., Kendall et al.,
2005). It is important to note that the major phase of folding, uplift and volcanism in the
Zagros area took place from the Pliocene to Present (Desmons 1980). This period coincides with the most intense volcanic activity in the Afar Depression and continuous seafloor spreading in the Red Sea (Berhe, 1986). Magmatic intrusions in the last 5 Ma in the MER or the Afar depression have been used to suggest that magmatic processes may control the location of incipient sea floor spreading (e.g. Keranen et al., 2004). Recent dikes occurring in Saudi Arabia have been linked to tectonic adjustments within the
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crystalline rocks of the Arabian shield as they respond to stresses related to recent sea- floor spreading at the central Red Sea, to strike-slip motion along the Gulf of Aqaba/Dead
Sea transform fault, and to asthenosphere underflow and magmatic intrusions (Pallister et al., 2010). Further, recent interpretations by Chang et al. (2011) based on inversion of seismic travel times and waveforms beneath the Red Sea and Arabia provide an evidence for hot mantle material that extends eastward beneath western Arabia suggesting that the mantle flow from the Afar plume moves with Arabia in a north direction (Fig. 22). This may indicate that the tectonic driver for the recent dikes intrusion in Saudi Arabia may have been driven by this adjustment.
Acknowledgments
We would like to acknowledge King Abdulaziz City for Science and Technology for financial support for Khalid Almalki. The Geological Survey of Saudia Arabia is thanked for providing geological and geophysical information. The Regents of the University of
California and the World Data Centre for Geomagnetism in Edinburgh are thanked for providing theACCEPTED satellite geophysical and bathymetryMANUSCRIPT data. Peter Betts was supported by the Monash University Researcher Accelerator Program. Laurent Ailleres was supported by the Minerals Council of Australia. We especially thank William Bosworth for his constructive comments that greatly improved the paper, and the reviews of two anonymous reviewers, who improved the clarity of the paper. We also thank Eng. Hisham
Esmail for his support.
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Figure captions
Fig. 1: Bathymetry and elevation features of the Red Sea and surrounding area at 30-arc seconds resolution after Becker et al (2009). Tectonic framework of the Arabian plate and plate motion with residual velocities after Bellahsen et al. (2003) and ArRajehi et al.
(2010).
Fig. 2: Red Sea map showing wells location discussed in the text, geological features of the area and compilation of published 40 Ar/ 39 Ar dates for magmatism in Arabia, Africa and Afar modified after Bosworth et al. (2005).
Fig. 3: (A) Palaeogeographic history of the Red Sea basin after Meulenkamp and Sissingh
(2003). (B) Palaeoenvironmental interpretation maps of the Red Sea with relative time modified after Hughes and Beydoun (1992).
Fig. 4: Seismic reflection and refraction surveys location between 1964 and 2011.
Fig. 5: Interpreted seismic reflection sections of R.V. Chain cruise, Pliocene layers just above reflector (S) after Ross and Schlee (1977). Refer to figure 4 for location.
Fig. 6: Interpreted seismic lines (PV, PIV and PIII) after Rihm et al (1991). Interpreted seismic lines ACCEPTED(SI, SII and SIII) offshore SudanMANUSCRIPT after Egloff et al. (1991); Interpreted seismic lines (YV and YVI) offshore Yemen after Egloff et al. (1991). Refer to figure 4 for location.
Fig. 7: (A) Seismic reflection interpretation after Gaulier et al. (1988) based on some of the ESP seismic profiles. (B) Seismic reflection profile interpretation after Ligi et al.
(2012). (C) Seismic reflection section in western part of southern Red Sea showing western Danakil rift after Lowell and Genik (1972). Refer to figure 4 for location.
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Fig. 8: (A) Crustal section analysis for seismic lines V and VI and (B) Crustal section analysis for lines II and III after Mechie and Prodehl (1988). (C) Interpreted crustal model of the U.S. geological survey seismic profile after Mooney et al. (1985). (D) Interpreted crustal layers from seismic refraction survey (II, III, V and VI) after Bastow and Keir
(2011). Refer to figure 4 for location.
Fig. 9: Red Sea Bouguer anomaly completion map after Makris et al. (1991).
Fig. 10: (A) Two dimensional gravity model after Gettings (1977). (B) Gravity model of
Izzeldin (1987). (C), (D) & (E) Models of gravity profiles P1, P2 and P3 after Makris et al. (1991). (F) Forward model of magnetic profile across Afar depression after Bridges et al. (2012). Refer to figure 9 for location.
Fig. 11: (A) Bouguer gravity anomaly map contoured at 10 mGal for the northern Red
Sea after Cochran (2005); the data set was gridded with a 1km x 1km cell resolution. (B)
Total intensity magnetic shipboard measurements at the Northern Red Sea; magnetic anomaly map gridded with a 1 km 2 grid cell size after Cochran (2005).
Fig. 12: Magnetic anomaly map from a compilation of the Red Sea data, modified after
Hall et al. (1977). ACCEPTED MANUSCRIPT Fig. 13: Comparison of observed and simulated magnetic profiles across the southern Red Sea at 16 o N and 19.5 o N (A) after Girdler and Styles (1974) and (B) after Hall et al.
(1977) and Hall (1989).
Fig. 14: (A) Red Sea Marine-satellite free air gravity map from global 1-minute grids
Geosat and ERS-1 altimetry after Sandwell and Smith (2009). (B) Red Sea Marine- satellite magnetic anomaly map at 2-arc-minute resolution after Maus et al. (2009). (C)
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Tilt-derivative filter of the total magnetic intensity data. (D) Structural interpretation map of the Red Sea basement. (E) Proposed geologic map for the Red Sea.
Fig. 15: (A) Structure section from Ethiopian Plateaus across Danakil depression through southern Red Sea and southern Arabia after Lowell and Genik (1972). (B) Northern Red
Sea stage after Martinez and Cochran (1988). (C) Model of the southern Red Sea showing extension and dyke injection after Berhe (1986). (D) Simple shear model of the Red Sea after Voggenreiter et al (1988) and Voggenreiter and Hotzl (1989). (E) Schematic model for asymmetric volcanism and topography around the Red Sea after Dixon et al (1989).
(F) Model for the development of the Red Sea after Bohannon (1989).
Fig. 16: Development of the southern and central Red Sea continental margins and establishment of mid-ocean ridge spreading in 5 Ma after Cochran (1983).
Fig. 17: Summary of Red Sea history in stages after Bohannon and Eittreim (1991).
Fig. 18: Palinspastic restoration of the Red Sea system after Bosworth et al. (2005).
Fig. 19: Pull-apart model evolution of the Red Sea and major structures after Markis and
Rihm (1991).
Fig. 20: SketchACCEPTED of plume and rift evolution MANUSCRIPT after Zeyen et al. (1997).
Fig. 21: The evolution of Neotethys and the opening of the Red Sea after McQuarrie et al.
(2003).
Fig. 22: Shear wave velocity map of the Red Sea, Afar and the Arabian plate at 150 km depth and cross-sections of the S-wave across the Red Sea and Gulf of Aden after Chang et al. (2011).
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Table 1 : Available wells information from MPMR (1986).
Well Name Total Lithology Thickness Basement depth (m) Rock (m) C1 (20) 3010 Halite 1200 — B1 (21) 2964 Halite 1080 Basalt Amber -1 (22) 3557 Halite 3340 Basalt Suri -7 (23) 2553 Limestone 57 Anhydrite, shale 2496 — Dhunishub -1 (54) 3867 Reefal limestone 169 Limestone, shale, some anhydrite 82 Shale, anhydrite, limestone, salt 99 Salt with some shale 3517 — Adel -2 (26) 2475 Reefal limestone 183 Anhydrite, shale 91 Shale with anhydrite and some salt 454 Salt with some anhydriteand shale 1747 — Secca Fawn -1 (24) 3363 Reefal limestone 411 Clay and marl 752 Anhydrite 1314 Salt, anhydrite and shale 877 Shale and anhydrite 9 — Zaydiya (34) 3020 Salt 728 — Salif -1 (32) 1524 Limestone, shale, salt — — Salif -2 (33) 2221 Limestone, shale, salt — — Hodeida -1 (36) 1730 Limestone, shale, salt — — Hodeida -2 (35) 2730 Limestone, shale, salt — — EL -Morgan 1 (1) 3146 Limestone, shale, salt — — Bargan -1 (4) 2892 Reefal limestone 314 Evaporite with sand and shale beds 2400 Granite Bargan -2 (5) 2786 Limestone, shale, salt — Granite ALkurmah-1 (7) 3010 Limestone, shale, salt Granite Hareed South-3ACCEPTED (3) 1684 MANUSCRIPT Granite Abu Shiban-2 (2) 2836 Granite Yuba 1 (6) 2271 Limestone, shale, salt — Granite Mansiyah 1(29) 3939 Reef limestone 691 — Shale and conglomeratic layers 2059 Salt 1222 Tokar -1 (19) 1538 — — — Durwara -2 (18) 4152 Limestone 1413 Evaporite 2159 Basalt Shale 580 Dungunab -1 (12) 1615 Salt 1065 Basalt 227 (10) 359 Salt 462 Basalt 228 (16) 325 Salt 108 — 229 (31) 212 Salt 29 — Bader-1 (8) 3347 Continental and reefal limestone 2528 Evaporite 733 Granite
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Zifaf -3 (27) 1463 Limestone, Shale, Salt — — Segid-1 (28) 465 Limestone, Shale, Salt — — Abu Shagara -1 (14) 2293 Igneous rocks at 2000 — Magherusun -1 (13) 2252 — — — Ghawwas -1 (15) 3466 — — — Durwara -1 (17) 2910 — — — ROS -95 -1 (37) 1951 — — — ROS X 94 -1 (38) 2900 — — — ROS Z 95 -1 (39) 3650 — — — ROS B 95 -1 (40) 4022 — — — ROS B 96 -1 (41) 4250 — — — QUSAIR A -1X (42) 5038 — — — QUSAIR B -1X (43) 4214 — — — RBOZ 10B -1 (44) 1746 — — — ABU MADD -1 (45) 1627 — — — MIKAWA -1 (46) 3362 — — — HALAIB -1 (47) 3596 — — — SUAKIN -1 (48) 2745 Anhydrite at 2745 — BASHAYER Z,1A (49) 2787 Anhydrite at 2787 — SAUTH SUAKIN -1 (50) 3712 Limestone at 3712 — DIGNA -1 (51) 2194 — — — J-1 (52) 3137 — — — MN -1 (53) 2870 — — — JAMBIYYA -1 (55) ? — — — ALMEETNAG -2 (56) 2042 — — — KATHIB -1 (57) 2459 — — — RAS SHAKE (58) ? — — — THIO -1 (59) 3119 — — — 225 (9) 230 Evaporite — — 226 (11) 14 Limestone — — 230 (30) 9 Limestone — — ACCEPTED MANUSCRIPT
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Table 2: Vema, Atlantis and Discovery survey results after Drake and Girdler (1964).
Profile No. Sedimentary Layer Crust Type 1st 2nd 3rd Continental Oceanic 185 2.00 ---- 4.30 5.93 ---- 184 ---- 3.55 4.40 5.83 ---- 183 ------3.92 ---- 7.34 182 ---- 3.68 4.85 6.09 ---- 180 -181 2.38 3.62 4.68 ---- 6.97 179 ---- 2.50 4.49 ---- 6.76 178 ---- 3.50 ------6.93 177 ---- 4.06 4.41 6.37 ---- 175 -176 1.87 2.50 3.30 5.91 ---- 174 ---- 2.43 3.68 ---- 7.16 173 ---- 2.75 4.48 ---- 7.31 172 ---- 2.33 3.49 5.53 ---- 171 ---- 2.22 3.53 5.80 ---- 170 1.78 ---- 3.71 5.92 ---- Range 1.78 -2.38 2.22 -4.06 3.3 -4.85 5.53 -6.38 6.93 -7.34
Table 3: Natural Environment Research Council survey result after Davies and Tramontini (1970) (velocity in km/s).
Station Water Thickness of Second Basement Dipping plane No. depth unconsolidated first layer apparent velocity for (km) layer sediment (~2.0 velocity velocity the basement km/s) (evaporite) (km) 1a 2.08 0.29 4.48 6.20 6.38 ab 2.26 0.10 4.52 6.57 2 1.95 ------6.57 ---- 3 2.06 ------5.72 ---- 4a 1.67 0.39 4.25 7.33 6.86 4b 1.82 0.06 4.01 6.45 5 ACCEPTED1.37 ---- MANUSCRIPT---- 7.13 ---- 6a 1.21 0.20 4.23 6.07 6.78 6b 1.12 0.12 4.09 7.65 7 1.19 0.16 4.12 7.51 ---- 8a 1.06 0.21 4.55 8.64 6.92 8b 1.00 0.23 4.40 5.77 9 0.97 0.22 4.27 ------10 0.88 0.28 4.31 5.96 ---- 11a 1.25 0.31 4.51 7.18 6.62 11b 0.97 0.19 4.41 6.14 12a 0.72 0.25 4.31 5.94 6.11 12b 0.75 0.27 4.22 6.3 13a 0.80 0.13 4.20 6.47 6.83 13b 0.84 0.33 3.99 7.22
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Table 4: Description of the magnetic surveys used in the compilation of Hall et al. (1977).
Date Code Area/Location Height Line Investigator (in meter) spacing (in meter) 1958 A5 4350 km 0 ---- Lamont Doherty 1959 A3 3000 km 0 ---- Cambridge Uni. 1961 A4 25 profiles 0 ---- NATO SACLANT 1962 C3 Offshore Saudi 300 ---- MPMR (Jizan) 1963 B3 Offshore Ethiopia 600 ---- Mobile Oil Co. o 17 N 1964 A7 Ten profiles 0 ---- Woods Hole Oceanog. Inst. 1964 -66 C4 Arabian Shield 300 ---- MPMR 1965 A6 Several profile 0 Lamont Doherty 1966 A2 Concentrated over 0 ---- Woods Hole deeps Oceanog. Inst. 1966 B2 Offshore Ethiopia 600 ---- Gulf Oil Co. o 16 N 1966 C5 Offshore Saudi 2000 2000 MPMR (Duba) 1966 C6 Offshore Arabia 2000 2000 MPMR (Ranak) 1967 A1 Two profiles 0 ---- I.O.S., UK 1968 B1 Ethiopia 1800 ---- Newcastle Uni, UK o 1968 B4/D4 Offshore Sudan 19 N 300 ---- Gontinental Oil Co. 1968 D2 Coast to Coast 19 oN 300 Project Magnet 1969 D3 Coast to Coast 21 oN 300 ---- MPMR 1970 C7 Offshore Saudi 450 2000 AUXERAP (Jizan) 1970 C8 Offshore Saudi 450 1500 AUXERAP (Alwajh) 1971 A8 Southern Red Sea 0 ----- Woods Hole Oceanog. Inst. o 1971 A10 Two profiles at 14 N 0 ------1971 C1/D1 Offshore Saudi 600 3000 Sun Oil Co. ACCEPTED(Allith) MANUSCRIPT 1971 C2 Offshore Saudi 600 3000 Sun Oil Co. (Yanbu) 1972 A11 Southern and central 0 ----- Glomar Challenger Red Sea
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