139736eo.pdf (20MB) - Japan Oceanographic Data Center

THE RED SEAPHYSIOGRAPHYThe Red Sea is an elongated depression oriented northwest-southeast. It has a length of 1800km (2100 km including the Gulf of Suez), and a maximum width of almost 400 km. The maximumrecorded depth is 2920 m and the total water volume reaches 215,000 km3 (Fig. 1). At its southernend (16'-17'N) the coral-island Farasan and Dahlak Archipelagoes rise to slightly over 60 m above sealevel. Farther south, the Red Sea narrows to the Strait of Bab-el-Mandeb, to the north of which aseries of volcanic islands occupy the medial section. In the Red Sea proper a very distinct main troughextends northwards from the Zebayir Islands to the southern tip of the Sinai Peninsula. An axialgraben is cut into this main trough (Fig. 2; BACKER et al., 1975; LAUGHTON, 1970; ROSS et al.,1969), varying from 35 to 80 km in width, and up to 2100 m deeper than the adjoining seafloor of themain trough. In the northern part of the Red Sea, the axial trough is either very narrow (19 km at23'N) or may even be absent, being replaced by a U-shaped depression that is not fault-bounded.Displacement of the talwegs and abrupt termination of the longitudinal tensional faults suggest that theaxial trough is offset by a number of northeast-striking lineaments (BACKER et al., 1975). Theselineaments mark the location of transform faults and divide the medial part of the Red Sea between19.5' and 20.8'N into trough and inter-trough zones. The trough zones possess characteristics typicalof an active rift, while the inter-trough zones are devoid of large magnetic anomalies (ELTAYEB et al.,1979; SEARLE and ROSS, 1975) and have a surficial sedimentary layer of up to 1 km in thickness.Whether these inter-trough zones represent areas unaffected by the present seafloor spreading phase isstill a matter of debate.The Red Sea shelf is shallow and flat. North of Jiddah (about 22'N), Saudi Arabia, the shelfis narrow and interrupted by sharp topographic breaks. The adjoining coastal plain is likewise narrow,with terraces well above the present shoreline. South of Jiddah, both the shelf and the coastal plainbroaden until they merge imperceptibly with each other (Fig. 2). The general aspect is one ofemergence to the north, and of submergence to the south.On the Arabian Peninsula, a magnificent erosional escarpment sub-parallel to the coast extendsfrom the latitude of Jiddah to Yemen, forming the crest of the tilted Arabian block (Fig. 1). In Yemen,altitudes of over 3700 m are reached; a mean height of more than 2000 m is found in Saudi Arabia.No major normal faults parallel to this escarpment have been mapped between the coastal plain and thecrest. In Ethiopia a similar scarp bounds the coastal plain on the west, reaching altitudes of over 3000m. At the latitude of Massawa (15.7'N), the scarp turns south, leaving the Red Sea trend to join theEast African Rift. The Afar Depression borders the southwestern Red Sea. Here extensive volcanismand block faulting have given rise to a complicated morphology.Escarpments and volcanic rocks are absent on land bordering the Red Sea north of Jiddah(Fig. 1). Instead, the lower coastal slopes merge seaward as interfingering fans. In the extreme north,the Red Sea bifurcates into the Gulf of Suez and the Gulf of Aqaba, the former being controlledprimarily by normal faulting and the latter by left-lateral strike-slip faulting with a small extensionalcomponent. Narrow coastal plains ascend directly into fault-bounded basement blocks within a fewkilometers of the shoreline.GEOLOGYThe stratigraphic record shows that the Jurassic seas spread northward to the latitude ofMassawa (16'N) and southward to 29'N across the Sinai Peninsula. By the end of the Cretaceous theTethyan Sea had reached southwards to approximately 21'N (WHITEMAN, 1968; CARELLA andSCARPA, 1962). However, in the south, the Cretaceous marine facies is regressive, with theshoreline extending northwards only to 13'N. There is no evidence that the Red Sea depressionexisted during this time. Tectonic activity during the Mesozoic in and around the Red Sea was308