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

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