Crustal structure of the Gulf of Aden and the Red Sea

Tectonophysics, 20 (1973) 261-267 0 Elsevier Scientific Publishing Company,

CRUSTAL

STRUCTURE

Amsterdam

- Printed

in The Netherlands

OF THE GULF OF ADEN AND THE RED SEA

J.D. FAIRHEAD Newcastle upon Tyne (Great Britain)* (Received

December

5, 1971)

ABSTRACT Fairhead, J.D., 1973. Crustal structure of the Gulf of Aden and the Red Sea. In: S. Mueller (Editor), The Structure of the Earth’s Crust, based on Seismic Data. Tectonophysics, 20 (l-4): 26 l-267. Seismic refraction profiles now number 9 in the Gulf of Aden and 15 in the Red Sea with a further intensive study by the Cambridge University group between latitudes 22 and 23”N. The results of these surveys indicate that the main trough of the Gulf of Aden is underlain by oceanic crust while only the deep axial zone and a questionable amount of the main trough of the Red Sea are underlain by oceanic crust. Seismic reflexion profiles reveal the nature of Layer 1 and the upper surface of Layer 2. A strong subbottom reflector is found beneath the main trough of the Red Sea at 0.5 km but is found to be absent in the axial zone. This survey together with the refraction work and geological evidence suggests a complex history for the main trough of the Red Sea. Reflexion profiles and dredging in the Gulf of Aden indicate that the thickness of sediments increases away from the central rough zone and that the sediment is underlain by volcanic material.

THE GULF

OF ADEN

The structure

and evolution

of the Gulf of Aden has been extensively

studied and re-

viewed by Laughton et al. (1970). From the geophysical studies there is strong evidence that the major portion of the Gulf is floored by oceanic crust. Bathymetric charts compiled by Laughton (1966) and Laughton et al. (1970) show there are three physiographic provinces: (1) The narrow continental margins with prominent NE-SW scarps which continue across the Gulf along NE-SW strikes and suggest the continental margins could have once fitted together. (2) The main trough between the continental margins. (3) The central rough zone, along the centre of the main trough (indicated in Fig. 1 by the wavy lines) with in places a deep median valley. Seismic refraction studies (Laughton and Trarnontini, 1969) indicate oceanic crust to underlie the central rough zone of the main trough and to extend laterally as far as the

l

Present

address:

Department

of Earth

Sciences,

University

of Leeds, Leeds (Great

Britain).

J.U. FAIRHEAD

Fig. 1. The Gulf of Aden. The lpng dashed lines indicate marginal structures; short dashed tines connect structural features across the-Gulf and represent the direction of movement; wavy lines indicate the ex-. tent of the central rough zone; *re$resent earthqua& ep~~e~tres;+-...I represent the 1958 Vema-Atlantis seismic refraction profifes; ‘represent the 1967 Discovery seismic profiles. Velocities are in km/w. {Diagram after Girdler, 1965.1

continental margins. Eleven.profiles (nine are shown in Fig. 1 and the other two lie just to the east of the A&la-Fartak trench) giv&veIocity structures typical of oceanic crust. Layer 2 has a range of velocity from 3.94 to 5.3 km/set with a mean of4.42 ZIZ 0.47 (s.h.) km/set and layer 3 has a velocity range from 6.15 to 6.96 kmjsec with a mean of 6.62 f 0.24 (s.d.) km/set. Sub-crustal velocities range from 8.45 to 7.82 kmlsec for profiles well awvy from the ridge axis and velocities between 7.14 and 7.06 km/set are obtained for profiles close to the median valley . This observation is supported by the travel-time studies of Fairhead and Girdler (1971) for ear~~uake~ in both the Red Sea and Gulf of Aden and show maiked delays in the arrival times for ray paths along fhe ridge axis. Comparative geoiogy on either side of the Gulf (Beydoun, 1970) supports the hypothesis

263

CRUSTALSTRUCTUREOFTHEGULFOFADENANDTHEREDSEA that the continental SW direction.

margins were once joined and have subsequently

separated in a NE-

Magnetic anomalies have been found to parallel the median valley and have

been interpreted

using the sea-floor spreading model proposed by Vine and Mathews (1963)

The median valley is also associated with high heat flow (Von Herzen. 1963) and shallow seismicity (Roth&, 1954; Fairhead and Girdler, 1970) ( see Fig. 1). and represents the area in the Gulf where new crust is being generated. Le Pichon (1968) considered

the opening

of the Gulf of Aden to be a rotation about a pole 26”N 21”E and this coincides closely with the more recently determined pole by McKenzie et al. (1970) at 26.5”N 2 1.S”E. The earthquake mechanism determined by Sykes (1970) for the Alula-Fartak relative motion to the Arabian and Somalian plates that is consistent

trench event gives a with both the geologic;rl

and geophysical evidence.

THEREDSEA The geophysical work in the Red Sea area has been summarised by Girdler ( 1969) and most aspects of the geology and geophysics of the Red Sea have been published following the discussion meeting in 1969 at the Royal Society of London (Philos. Trans. R. Sot. Lam’., Ser. A, 267: l-417). The Red Sea structures, unlike those of the Gulf of Aden, have been complicated by the complex sedimentary history, and the nature of the crustal structure beneath the main trough of the Red Sea is still in doubt, The Red Sea can be divided into three physiographic regions similar to the Gulf of Aden, these being the narrow continental margins, the main trough and the deep axial trough (Drake and Gridler, 1964). In the northern Red Sea (north of 25”N) the deep axial trough is absent and only the continental

margins and the main trough are observed. The shallow water along the Red Sea

coast has restricted

the oceanographic

surveys to the main trough. Seismic reflexion studies

by Knott et al. (1966) and Philips and Ross (1970) have shown the main trough to be covered by young sediments beneath which a strong reflector ‘S’ indicates a change in sedimentation to thicker layered sequences. The reflector S is observed across the main trough but is cut by many narrow crevasses (-100m deep). These crevasses have been tentatively correlated by Philips and Ross (1970) between five profiles to give linear features (represented by dashed lines in Fig. 2) striking east of north, paralleling the Gulf of Aqaba structures. In the central Red Sea the strong reflector S is only found beneath the main trough and is absent within the deep axial trough. The sediments above the reflector Sand the sedimentary feature beneath S either disappear close to the axial zone or outcrops along the walls of the deep axial trough. The interpretation of the reflexion profiles has been determined from land outcrops, boreholes (see Girdler, 1970; Frazier, 1970) and seismic refraction studies. Reflector S lies at a depth of 400 m (below the sea floor) and is either a discontinuity or unconformity between the Upper-Middle Miocene evaporites and limestones and the Pliocene and Pleistocene elastic sediments and reef limestones. The nature of the reflector S suggests

Fig. 2. The Red Sea. The northernmost seismic profile represents a 1967 Discovery profile; short darhtd lines represent crevasse lineations determined by reflexion studies; other symbols same as for Fig. 1. (Diagram after Girdler, 1969.)

265

CRUSTAL STRUCTURE 01: THE GULF OF ADEN AND THE RED SEA

the axial trough of the Red Sea formed since the Late Miocene when a permanent with the Gulf of Aden was established via the straits of Bab-el-Mandeb. tions probably Levantine

sea link

The crevasse linea-

represent strike-slip faulting similar in character to that observed along the

rift (Freund

Seismic refraction

et al., 1970). work by Drake and Girdler ( 1964) and Tramontini

both concur that the deep axial trough is underlain the main trough show both oceanic and continental

and Davies (1969)

by oceanic crust, but their results for crust. Drake and Girdler ( 1964) have

explained the velocity structures to indicate predominately sialic crust (mean velocity 5.86 km/set) overlain by evaporites, sediments and pyroclastics (mean velocity 4.07 km/set: while Tramontini

and Davies (1969) have explained

their results to indicate oceanic crust

(mean velocity 6.3 1 km/set ) overlaain by evaporites, etc. (veIocity 4.3 km/set).

Fig. 2 in-

dicates all the seismic results reported by Drake and Girdler (1964) plus the most northerly profile reported by Girdler (1969). The shaded area in the central Red Sea indicates the areas in which Tramontini and Davies (1969) carried out their detailed survey. Within this area twenty seismic refraction profiles were recorded and composite travel-time plots were constructed for both the deep axial trough and the main trough since individual profiles tended to give velocity structures that fluctuated greatly as each successive shot was fired. The composite plot for the axial trough gave a seismic velocity of 6.63 ? 0.16 km/set at a mean depth of 4.6 km and for the main trough gave a velocity of 6.31 ? 0.10 km/set. Additional geological and geophysical data relating to the structure summarised below.

of the Red Sea are

(1) The 90-l 10 km horizontal displacement of Precambrian rocks along the Levantine rift includes a 40-50 km displacement of Miocene sediments (Freund et al., 1970). (2) The remarkable symmetrical coastlines, diverging at about 7” from north to south, have been shown to have a shoreline fit about a pole of rotation at 36.5”N 18”E (McKenzie et al., 1970). This pole has been modified to 32% 22”E by Freund (1970) to give a near shoreline fit so as to accomodate the various Precambrian structures found in the Red Sea depression, and the pole fits better the known geometry of the Levantine rift (Freund et al., 1970). (3) There is good agreement between the proposed direction mechanisms

(Fairhead,

of opening and the earthquake

1968; Fairhead and Girdler, 1970;McKenzie

et al., 1970). The seis-

micity of the Red Sea indicates the Red Sea is opening along the centre of the deep axial trough. (4) Linear magnetic anomalies are associated with the deep axial trough and indicate it has formed within the last 2-4 m.y. (Vine, 1966; Allan, 1970). The main trough, although not fully surveyed, is by comparison magnetically quiet, which may be the result of the large sedimentary overlay (Davies and Tramontini, 1970). (5) The gravity data indicate a broad positive Bouguer anomaly over the Red Sea and Afar depression compared to the negative Bouguer anomalies over the adjacent continental land masses. Although the models of Piilips et al. (1969), Tramontini

and Davies (1969)

266

J.D. FAIRHEAD

and Qureshi (197 1) are different. evaporites must be underlain tinental crust. (6) Hutchison

they show that beneath the main trough the low-density

by either a dense oceanic crust or an extremely

and Gass (197 1) have investigated

mafic and ultramafic

ciated with a small diatreme cone of Kod Ali lying close to the Ethiopian 41”49’E and consisting

almost entirely of pyroclastic

thinned con-

inclusions

asso-

coast at 13”57’%

debris. Silicic inclusions

are completely

absent. A diatreme vent similar to Kod Ali occurs in southern Arabia and has an abundance of both ultramafic nodules and granitic specimens. Although the apparent absence of the granitic material on Kod Ali does not necessarily mean an absence of continen tal crust from the whole of the southern Red Sea, it at least makes the presence of continental crust beneath Kod Ali doubtful. The interpretation of certain refraction profiles in Fig. 2 is difficult when the lower velocities lie about 6.3-6.4

km/set,

e.g., profile 177 and the mean velocity of 6.3 1 km/set

by Davies and Tramontini (1970) since the velocities may be caused by either continental or oceanic crust. However the distribution of seismic profiles in Fig. 2, indicating sialic crust. will allow a central zone along the length of the Red Sea to be underlain

by oceanic crust

and flanked to either side by thinned sialic crust. The area surveyed by Davies and Tramontini (1970) needs not to be incompatible with this. One of the arguments against oceanic crust in the northern

Red Sea is the lack of magnetic anomalies. Here the spreading rate is much

reduced compared to the central Red Sea and a mechanism of crustal thinning has been proposed (Girdler, 1970, fig. 1 I). In this area more refraction profiles and detailed magnetic surveys are required.

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Davies, D. and Tramontini, C., 1970. The deep structure of the Red Sea. Philos. Trans. R. SOC.Lond.. Ser. A, 267: 181-189. Drake, C.L. and Girdler, R.W., 1964. A geophysical study of the Red Sea. Geophys. J.R. Astron. SOC., 8: 473-495. Fairhead, J.D., 1968. The Seismicity of the East African Rift System 19.55 to 1968. Dissertation, University of Newcastle upon Tyne, Newcastle upon Tyne. Fairhead, J.D. and Giidler, R.W., 1970. The seimicity of the Red Sea, Gulf of Aden and Afar triangle. Philos. Trans. R. Sot. Lond., Ser. A, 267: 49-74.

Fairhead, J.D. and Girdler, R.W., 1971. The seimicity of Africa. Geophys. JR. Astron. Sot.. 24: 27 I 301. Frazier, S.B., 1970. Adjacent structures of Ethiopia; that portion of the Red Sea coast inchdir\gDahlak Kebir Island and the Gulf of Zula. Philos. Trans. R. Sot. Land., Ser. A, 267: 131-14 I. Freund, R., 1970. Plate tectonics of the Red Sea and East Africa. Nature, 228: 453. Freund, R., Garfunkeli, Z., and Zak, I., 1970. The shear along the Dead Sea rift. Phifos. Trans. R. SOC. Lond.. Ser. A, 267: 107-130.

Girdler, R.W., 1965. The role of translational and rotational movements in the formation of the Red Sea and Gulf of Aden. In: The WorM Rift System. Geol. Surv. Can., Pap. 66-14: 65-76.

CRUSTAL

STRUCTURE

OF THE GULF

OF ADEN AND THE RED SEA

261

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