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

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

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12.07.2015 Views

intensity of magnetization which could be a result of destruction of coherently magnetized bodies byintense brecciation, by leaky intrusive activity along zones so narrow that the magnetizations areeffectively randomly distributed, by intrusion into sediments, or by virtue of the very differentmineralogy of fracture zone basalts. The axial anomalies are compatible with the seafloor spreadinghypothesis. From 16' to 19'N, ROESER (1975, 1976) identified the axial anomalies, therebyestablishing isochrons from 0-5 m.y., although the 5 m.y. isochrons are considered doubtful.Magnetic anomalies in the southernmost part of the Red Sea are not well established.Earlier studies suggested that the initial phase of spreading might have occurred between34-41 m.y. ago, from Late Eocene to Early Oligocene (GIRDLER and STYLES, 1974). Althoughthere is evidence that tectonic activity in the Red Sea and in the Gulf of Aden may have begun inJurassic or Cretaceous times, most investigators agree that separation started in the Miocene followingan Oligocene-Miocene phase of uplift and faulting (BEYDOUN, 1970). The pronounced phase ofOligocene and Miocene volcanism in the Red Sea area was probably the forerunner of the majorepisode of rifting in the Red Sea. The mid-Miocene age for the initiation of spreading also agrees withthe results of the Deep Sea Drilling Project in the Gulf of Aden, where 13 m.y. old sediments weredrilled @mediately above the acoustic basement close to the 2000 m isobath, just beyond the area ofcorrelatable magnetic anomalies (FISCHER, BUNCE et al., 1974). In addition, the volcanism thataccompanies initial fragmentation in the Afar Depression is earliest Miocene, 25 to 23 m.y. old(BARBER1 et al., 1975). Finally, the water depth over the marginal trough of the Red Sea (1-1.5km) is more compatible with a mid-Miocene age. The predicted depth to oceanic basement accordingto the cooling plate model for a 20 m.y. old lithosphere is about 4 km. Correcting for the sedimentcover, which includes up to 5 km of Miocene evaporites, the expected water depth would be 1.3-1.8km. On the other hand, should the crust be about 38 m.y. old (implying that the initiation of spreadingwas at the Eocene-Oligocene boundary), the theoretical corrected water depth would be 2-2.5 km.This gives a much larger discrepancy compared to observations. Of course, the alternative that theobserved depths are accounted for by a subsided, attenuated continental crust cannot, on the basis ofisostasy alone, be discounted.It should be noted that the development of the Red Sea inevitably has been influenced by thatof the Gulf of Aden. Recent thoughts on the Gulf suggest that the possibility of seafloor spreadingtaking place in a single phase starting about 16 m.y. cannot simply be dismissed. The implication isthat a similar single-phase development in the Red Sea also is possible.SEIS MICITYEarthquake activity in the Red Sea and Gulf of Aden is confined largely to the axial trough(Fig. 4). It is undoubtedly related to active seafloor spreading, although surprisingly few earthquakesappear to occur in the northern part of the Red Sea (FAIRHEAD and GIRDLER, 1971; McKENZIE etal., 1970; SYKES, 1968). The focal depths are all shallow, usually less than 100 km. The 3fault-plane solutions available so far indicate strike-slip motion in a NE-SW direction in the central andsouthern Red Sea, suggesting that the central rift may be offset by a number of transform faults,whereas the solution at the Gulf of Suez is characterized by normal faulting (McKENZIE et al., 1970;SYKES, 1968). While these solutions give an indication of the tectonic pattern of this area, they arenot accurate enough to be applied quantitatively. Epicenters in the Afar Depression are largely alongthe margin rather than being confined to the axial zone.GRAVITYBouguer gravity anomalies are positive over both the main and axial troughs. In cross-sectionthe axial trough is characterized by a slight minimum (ALLEN, 1970; FLEISCHER, 1969; PHILLIPSet al., 1969; PLAUMANN, 1975; QURESHI, 1971). Such a pattern is typical of mid-ocean ridgesand may be attributed to plate accretion. A maximum Bouguer value of 150 mgal has been measuredover the axial trough. However, over the inter-trough zone within the axial trough,310

the Bouguer gravity reaches only 90 mgal. This reduced value can be attributed to the presence ofsediments in the inter-trough zone and the resulting lithospheric isostatic response, while elsewhere theaxial trough is devoid of sediments (SEARLE and ROSS, 1975).Modelling of Bouguer anomalies along a 900 km profile across the Red Sea at 20"N suggeststhat oceanic crust underlies almost the entire width of the Red Sea at this latitude, while the intrusivezone takes up half of the seafloor (BROWN and GIRDLER, 1982). Lithospheric thinning extends to120 and 180 km landward of the coasts.In contrast to the Red Sea, Bouguer anomalies in the Gulf of Aqaba reach -100 mgal.Likewise, negative values are typical of the Gulf of Suez. In both areas the negative anomalies areinterpreted as a result of transform faulting. Bouguer anomalies in the Afar Depression are largelynegative, apparently a result of relief. Positive anomalies are present only over volcanic centers andalong the coast to the northeast as well as parallel to the Danakil Horst. Their amplitudes, however,are less than those over the axial trough of the Red Sea.HEAT FLOWNearly 100 reliable heat flow measurements are now available from the Red Sea (ERICKSONand SIMMONS, 1969; GIRDLER et al., 1974; HAENEL, 1972; SCHLEUCH, 1973). They suggestthat the entire region is one of the high heat flow (Fig. 5). Over the axial trough an average of 467 f116 mW m-2 (around 11.1 HFU) has been observed, decreasing to 111 +_ 5 mW m-2 (2.64 HFU) 50to 270 krn from the spreading axis (GIRDLER and EVANS, 1977). The high variability over theaxial zone can be attributed to the effect of hydrothermal circulation in the intrusive zone and to theeffects of evaporitic flow as older crust fractures apart (GIRDLER and WHITMARSH, 1974). Heatflow along the coast, averaging 112 mW m-2, may provide a viable geothermal energy source. Atemperature of 100"C, for example, is reached at less than 2 km depth. The prevailing thermal gradientis about 40°C km-l or higher, suggesting that conditions are more inducive to the formation of naturalgas rather than oil for older source material, but may accelerate oil formation at shallow depths if thesource material is young (KLEMME, 1975).SEISMIC INVESTIGATIONSA number of seismic refraction profiles have been obtained in the Red Sea (DAVIES andTRAMONTINI, 1970; DRAKE and GIRDLER, 1964; FAIRHEAD, 1973; GINSBERG et al., 1981;GIRDLER, 1969; MAKRIS, 1982; PRODEHL, 1983; TRAMONTIM and DAVIES, 1969). However,their interpretation remains a controversy. Within the axial zone at a depth of about 5 km,velocities ranging from 6.6-7 km s-l are encountered (Fig. 6). They are interpreted to represent theoceanic layer (or layer 3) so that here oceanic crust is implied. Over the Red Sea marginal trough thereis a 2-5 km sedimentary cover, with velocities ranging from 3.54.5 km s-1. The materialcomprising this cover is considered to be evaporites and clastic and volcanic material. Still unresolvedis the nature of the material underlying this sediment cover. In a detailed survey area between 22" and23"N to the east of the axial trough, measured velocities of 6.6 km s-1 at an average depth of 4.6 kmhave been interpreted as oceanic crust (DAVIES and TRAMONTINI, 1970; TRAMONTINI andDAVIES, 1969). Others argue that this may represent one of the many greenstone belts within thePrecambrian (LOWELL et al., 1975). However, if we accept this latter interpretation, then the totalamount of opening in the Red Sea would be relatively small compared to that in the Gulf of Aden.This would imply that major tectonic movements have occurred in the East African Rift Systemcontradictory to geological evidence. In addition, as is mentioned earlier, the continental crustinterpretation contradicts results of gravity modelling at least at 20"N (BROWN and GIRDLER,1982). Around the margins of the Red Sea, velocities from 5.84-6.97 km s-1 have beenencountered, and they are interpreted as representing the Precambrian basement, hence implying acontinental crust.Continuous seismic reflection profiles in the Red Sea reveal the existence of a very prominentreflector, Reflector S, throughout the Red Sea except in the axial rift (Fig. 7a; LOWELL and GENIK,1972; LOWELL et al., 1975; PHILLIPS and ROSS, 1970; ROSS and SCHLEE, 1973; ROSS et al.,311

the Bouguer gravity reaches only 90 mgal. This reduced value can be attributed to the presence ofsediments in the inter-trough zone and the resulting lithospheric isostatic response, while elsewhere theaxial trough is devoid of sediments (SEARLE and ROSS, 1975).Modelling of Bouguer anomalies along a 900 km profile across the Red Sea at 20"N suggeststhat oceanic crust underlies almost the entire width of the Red Sea at this latitude, while the intrusivezone takes up half of the seafloor (BROWN and GIRDLER, 1982). Lithospheric thinning extends to120 and 180 km landward of the coasts.In contrast to the Red Sea, Bouguer anomalies in the Gulf of Aqaba reach -100 mgal.Likewise, negative values are typical of the Gulf of Suez. In both areas the negative anomalies areinterpreted as a result of transform faulting. Bouguer anomalies in the Afar Depression are largelynegative, apparently a result of relief. Positive anomalies are present only over volcanic centers andalong the coast to the northeast as well as parallel to the Danakil Horst. Their amplitudes, however,are less than those over the axial trough of the Red Sea.HEAT FLOWNearly 100 reliable heat flow measurements are now available from the Red Sea (ERICKSONand SIMMONS, 1969; GIRDLER et al., 1974; HAENEL, 1972; SCHLEUCH, 1973). They suggestthat the entire region is one of the high heat flow (Fig. 5). Over the axial trough an average of 467 f116 mW m-2 (around 11.1 HFU) has been observed, decreasing to 111 +_ 5 mW m-2 (2.64 HFU) 50to 270 krn from the spreading axis (GIRDLER and EVANS, 1977). The high variability over theaxial zone can be attributed to the effect of hydrothermal circulation in the intrusive zone and to theeffects of evaporitic flow as older crust fractures apart (GIRDLER and WHITMARSH, 1974). Heatflow along the coast, averaging 112 mW m-2, may provide a viable geothermal energy source. Atemperature of 100"C, for example, is reached at less than 2 km depth. The prevailing thermal gradientis about 40°C km-l or higher, suggesting that conditions are more inducive to the formation of naturalgas rather than oil for older source material, but may accelerate oil formation at shallow depths if thesource material is young (KLEMME, 1975).SEISMIC INVESTIGATIONSA number of seismic refraction profiles have been obtained in the Red Sea (DAVIES andTRAMONTINI, 1970; DRAKE and GIRDLER, 1964; FAIRHEAD, 1973; GINSBERG et al., 1981;GIRDLER, 1969; MAKRIS, 1982; PRODEHL, 1983; TRAMONTIM and DAVIES, 1969). However,their interpretation remains a controversy. Within the axial zone at a depth of about 5 km,velocities ranging from 6.6-7 km s-l are encountered (Fig. 6). They are interpreted to represent theoceanic layer (or layer 3) so that here oceanic crust is implied. Over the Red Sea marginal trough thereis a 2-5 km sedimentary cover, with velocities ranging from 3.54.5 km s-1. The materialcomprising this cover is considered to be evaporites and clastic and volcanic material. Still unresolvedis the nature of the material underlying this sediment cover. In a detailed survey area between 22" and23"N to the east of the axial trough, measured velocities of 6.6 km s-1 at an average depth of 4.6 kmhave been interpreted as oceanic crust (DAVIES and TRAMONTINI, 1970; TRAMONTINI andDAVIES, 1969). Others argue that this may represent one of the many greenstone belts within thePrecambrian (LOWELL et al., 1975). However, if we accept this latter interpretation, then the totalamount of opening in the Red Sea would be relatively small compared to that in the Gulf of Aden.This would imply that major tectonic movements have occurred in the East African Rift Systemcontradictory to geological evidence. In addition, as is mentioned earlier, the continental crustinterpretation contradicts results of gravity modelling at least at 20"N (BROWN and GIRDLER,1982). Around the margins of the Red Sea, velocities from 5.84-6.97 km s-1 have beenencountered, and they are interpreted as representing the Precambrian basement, hence implying acontinental crust.Continuous seismic reflection profiles in the Red Sea reveal the existence of a very prominentreflector, Reflector S, throughout the Red Sea except in the axial rift (Fig. 7a; LOWELL and GENIK,1972; LOWELL et al., 1975; PHILLIPS and ROSS, 1970; ROSS and SCHLEE, 1973; ROSS et al.,311

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