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

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

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South of 19"N, the McKENZIE pole no longer applies. The magnetic survey of ROESER(1975, 1976) suggests that the pole for the southern Red Sea must lie to the south. The best fit to hisisochrons gives a pole position of 7"N, 50.5"E, with a rate of opening for the past 2 m.y. of 5 x 10-7deg yrl. The corresponding half-spreading rate decreases from 0.75 cm yrl at 19"N to less than 0.6cm yrl south of 16"N.The pole just stated for the southern Red Sea has the important implication in that it predicts adifferent motion at 19"N from the Nubia-Arabia pole. We are therefore forced to conclude that a plateexists between Arabia and Nubia in the southern Red Sea, which as been named the North DanakilPlate, separated from Nubia by a zone of extension (Fig. 12). This extensional boundary is assumedto run along the western rift (discussed earlier from seismic data) through the Gulf of Zula to theDanakil Depression. As morphology suggests that the motion between the North Danakil and Nubianplates become negligible near 19"N since the zone of irregular topography terminates to the north near19"N, the rate of opening between Nubia and Arabia may be determined. This is 3.2 x deg yr-1,which corresponds to a spreading rate of 0.7 cm yr-l near 22"N and 0.5 cm yr-l at the northernextremity ofthe Red Sea.Since the orientation and motion along the Levant Shear System precludes it from being thetransform boundary between Nubia and Arabia, a separate plate, the Sinai Plate, must be assumed.The Sinai-Arabia pole may then be determined by assuming that the Levant Shear between the Lebanonand Anti-Lebanon Mountains and the Araba Depression lies on the same line of pure slip. This polecan thereby be estimated to be at 32.5"N, 4.4"W. The predicted motion along the Levant Shear is thenas follows: pure strike-slip in the Gulf of Aqaba and the Araba Depression, strike-slip with extensionin the Dead Sea and Jordan River area, compression with minor slip in Lebanon, pure strike-slip in theHoms region, and strike-slip with minor extension in Er-Rharb. The Arabia-Sinai rate of motion isthen determined by minimizing the motion between these two plates.The Sinai-Nubia pole obtained by vector composition can be determined to be at 35.9"N,42.6"E, with a rate of 1.65 x deg yr-l. A total rate of opening of 0.3 cm yrl in the Gulf of Suezin a WNW-ESE direction then results.The Arabia-Somalia pole (26.5"N, 21.5"E) can be determined from the magnetic anomalies inthe Gulf of Aden. The angular opening rate is 4 x deg yrl. The remaining Somalia-Nubia andNorth Danakil-Nubia poles can then be computed by vector addition.The total motion since early Miocene based on this kinematic pattern (Fig. 12) agreesreasonably well with the available data. Of particular interest is that the extension rates acrosscontinental grabens (e. . in the Ethiopian Rift), the western Red Sea Rift and in the Gulf of Suez is ofthe order of 0.3 cm yr- B . At this slow rate, features commonly associated with seafloor spreading arenot observed.REMARKS ON THE STRUCTURAL EVOLUTION OF THE RED SEA,GULF OF ADEN, AND EAST AFRICAN RIFT SYSTEMFigure 13 summarizes the structural evolution in the Red Sea, Gulf of Aden and the EastAfrican Rift. Both the Red Sea and the Gulf of Aden have been undergoing more-or-less synchronousseafloor spreading in one or more phases. In contrast, the East African Rift does not appear to becharacterized by active plate accretion, or else the accretion rate is so small that features common toactive spreading centers are not observable at present. Instead, intensive normal faulting, uplift andalkaline volcanism are the basic structural styles.Based on the idea of CLOOS (1939), we have proposed a sequence of events leading toseafloor spreading (DEGENS et al., 1971; WONG and VON HERZEN, 1974). These events are:1) uplift or arching caused by thermally activated volume increase resulting fromconvective upwelling in the asthenosphere (Fig. 14 I);2) block faulting and the propagation of normal faults from the top of the lithospheredownwards (Fig. 14 1);3) igneous and hydrothermal activity (regulated by the availability of meteoric water) andsubsequent erosion and non-marine deposition (Fig. 14 111);4) lithospheric thinning, possibly by divergent convective flow in the asthenosphere,perhaps accompanied by "diffuse extension" (COCHRAN, 198 1, 1982b);3.16

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