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1969; SEARLE and ROSS, 1975). This reflector is interpreted as the top of the evaporite sectioncommonly correlated with the Miocene-Pliocene boundary. Within the axial trough where active plateaccretion has been taking place over the past 4-5 m.y., the evaporitic layer is absent except whereflowage has occurred from the sides. Here reflector S is not observed. The dominant structural stylesare normal faulting and salt diapirism, and they are often intimately associated with each other. Forexample, flowage in a piercement salt feature may be triggered by normal faulting from below (Fig.7b). In the southern part of the Red Sea normal faulting has become so highly developed that a riftstructure, a continuation of the Danakil Depression and the Gulf of Zula to the south, has beendeveloped, so that here double rifting occurs (LOWELL et al., 1975). Borehole results suggest thatthe Miocene salt layer is widespread and reaches 5 km in places.To summarize, the axial trough of the Red Sea is underlain by oceanic crust, the lateral extentof which however is still uncertain (GASS, 1977), although its extension almost to the two coastsappear likely. Over the axial trough, the marginal trough and the immediate continental regions,lithospheric thinning has taken place.RED SEA HOT BRINESAlong the axial rift of the Red Sea, hot brines have accumulated in a number of deeps rangingfrom about 19" to 24"N (fig. 1). These deeps located in isolated axial troughs and separated byinter-trough zones, are almost devoid of sediments; they have larger positive Bouguer anomaliescompared to the inter-trough zones, and they exhibit large lineated magnetic anomalies. Thus, theyrepresent sites of active plate accretion (BERTIN et al., 1979). Because of the density (and henceacoustic impedance) contrast between seawater and brine, false echoes are received on echo-soundingrecords across hot brine areas (Fig. 8; ROSS et al., 1973). The brine itself has temperatures rangingfrom slightly above ambient to over 65T, salinities of 100 to several hundred ppt, and an oxygencontent of about zero (BAUMANN et al., 1973; BACKER and SCHOELL, 1972; EMERY et al.,1969). In contrast, the deep waters above the brine usually have temperatures around 22"C, 40.6 pptsalinity, and 2 ml l-l oxygen. Chloride, sodium, potassium and calcium are enriched relative to oceanwater, while sulfate, carbonate and magnesium are depleted (BREWER and SPENCER, 1969).Particularly noteworthy is the enrichment of various heavy metals. Lead, manganese, iron and zincamong others are all enriched well over 1000 times in comparison to seawater (BROOKS et al.,1969). Temperature measurements in the Atlantis I1 and Chain Deeps, particularly from the upperconvection layer through the high temperature zone to the lower convection layer, suggest that watertemperature variations may be cyclic (HUNT et al., 1967; ROSS, 1972; SCHOELL andHARTMANN, 1978). This implies that the intensity of discharge or brine temperature fluctuationsmay also be cyclic and that the recharge action of the brine may be episodic.Sediments beneath the hot brine deeps can be divided into 7 facies types (BISCHOFF, 1969).Detrital material, which consists of coarse-grained sediments, remains of pelagic organisms plusdetrital quartz, feldspar and clay is the typical deposit outside the hot brine basins. Within the brinebasins, black fine-grained iron montmorillonite consisting of mostly clay with iron oxides, somesphalerite and a very high water content represents the uppermost sediment facies. Underlying this is alayer of amorphous goethite, fine to medium-grained and orange to yellow in color. The sulfide faciesis a black, fine-grained sediment not found outside the basin. It contains the highest concentration ofheavy metals that are present chiefly as sulfides, iron monosulfide, marmatite or sphalerite,chalcopyrite and pyrite. Within the iron montmorillonite and goethite amorphous facies are thin,semi-lithified, discontinuous beds of manganosiderite. In addition, massive, white, coarselycrystalline beds of anhydrite and black manganite also occur. Although the average metal content ofthese deposits is less spectacular than that of the brine, since very high concentrations are confined tothe sulfide-rich facies (BACKER and RICHTER, 1973; BACKER, 1976; BIGNELL and ALI, 1973;DEGENS and ROSS, 1976; HENRICKS et al., 1969), these metalliferous deposits are of economicsignificance, and efforts have been expended towards commercial exploitation (BACKER, 1979).The Red Sea hot brines certainly do not originate as a result of evaporation. This is evidencedby the differences in chemical composition between the brine waters and waters of the Dead Sea. Alsoin the Atlantis II Deep area, heavy oxygen and hydrogen isotopes are depleted in comparison to thewaters directly overlying the brine. Other hot brine deeps are enriched in these isotopes, however.312