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

Such an isotopic composition has lead to the speculation that the brines are derived from hydrothermalcirculation associated with plate accretion (BURKE et al., 1981; SCHOELL and FABER, 1976,1978; SCHOELL and RISCH, 1976). According to this view, along the immediate ridge flanks whichare devoid of sediment cover and where the pillow lavas, pillow breccias, hyaloclastites and minorflows are highly porous (though decreasingly so with depth), sea water permeates into the seafloor(BECKER et al., 1982). Chemical reactions take place between the basaltic material and the circulatingwater, resulting in the filling of cracks by precipitated minerals and geochemical alteration of the twointeracting components (ANDERSEN et al., 1982; EDMOND et al., 1982). These hydrothermalwaters emerge eventually along the ridge crest, sometimes in the form of black smokers. If the ridgecrest morphology permits them to accumulate, they form hot brine pools such as in the brine deeps ofthe Red Sea. Accordingly, for the Red Sea hot brines, these hydrothermal waters are paleo-waters ofthe Red Sea at various ages and reflect the isotopic composition at these ages. Figure 9 shows thechange of 018 concentrations in foraminifera in the Red Sea relative to present values (DEUSER andDEGENS, 1969; DEUSER et al., 1976). Since temperature variations in Red Sea waters are small,the A018 values [defined as 6018 (past)-- 60l8 (present)] represent approximately the variation in 0l8in seawater. From this figure, one may speculate that the Atlantis 11, Chain, Albatross and Discoverybrines are "light" Red Sea waters from previous pluvial periods. The Valdivia Deep may containpresent-day Red Sea deep water or Red Sea paleowater of present-day isotopic composition. TheErba, Port Sudan and Nereus deeps could represent Red Sea paleo-waters which have penetrated theupper crust during times when the isotopic composition of the Red Sea was significantly enriched inheavy isotopes compared to today. The brine waters of the Kebrit and Suakin deeps do not showoxygen and hydrogen isotopic compositions comparable to Red Sea paleo-waters. They may representaccumulations of interstitial waters whose isotopic ratios have been altered by low temperature mineralreactions (SCHOELL and FARBER, 1978).The mid-water density boundary separating normal Red Sea water from that of the hot brinepools is located approximately at the depth where Reflector S outcrops. It has also been noted that heatflow along the coast may yield temperatures in the order of 100°C at a depth of less than 2 km. Thesetwo observations suggest that hydrothermal discharge occurs principally along the flanks of the brinepool rather than in the central region of the axial zone. Hydrostatic pressure at times of sea level risemay induce a pulsating flow of formation waters entrapped in the sediments overlying Reflector S,whereby halite is leached from the underlying evaporite sequence in accordance with mineralequilibria. In view of the fact that freshwaters formed during pluvial times may also enter thesubmarine aquifers, mixing of waters of different isotopic compositions may occur and could accountfor the observed range in stable isotope values from saline waters found in the various brine deeps. Itis conceivable, however, that some of the heavy metals were leached from basaltic rock in theimmediate vicinity of the brine pools or were supplied during active spreading phases, thus implying asecondary source for part of the solutes from deeper regions.To summarize, the Red Sea brines could be paleowaters (or interstitial waters) which havecirculated in the upper lithosphere as hydrothermal waters. Their discharge or recharge mechanismmay be triggered by climatic changes and other exogenic factors (DEGENS and ROSS, 1970). Thisspeculative origin is further supported by the helium isotopic composition of brine waters, which atleast for the Atlantis I1 and Discovery deeps exhibit an enrichment in helium with the mantle heliumisotopic signature (LUPTON et al., 1977).THE GULF OF SUEZ AND THE LEVANT SHEAR SYSTEMThe Gulf of Suez is a graben with a long history of subsidence since the late Paleozoic (SAID,1962). It is underlain by continental crust, and the present tectonics may be one of right lateral shearwith normal faulting.The Gulf of Elat (Aqaba) is a transform boundary with a slight component of opening. It isabout 160 km long and 15-10 km wide. Along the shores of the Gulf, coastal plains are either verynarrow or are absent. On the western side, large alluvial fans extend into submarine cones incised bycanyons on sloping terraces. The eastern shores descend abruptly to deep basins. Three such deepbasins have been mapped in the center part of the Gulf (BEN-AVRAHAM et al., 1979a,b). They areelongated, en Cchelon, and reach a maximum depth of 1830 m. Structure within the Gulf is controlled313