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y normal faulting. This is most intense in the basins and along the boundary faults. Diapirs havebeen observed in the southern part of the Gulf. A thick sedimentary fill consists mainly of alluvium onthe margins and turbidites and pelagic sediments in the basins. Seismic and gravity data suggest thatthe oldest sediments must lie at least 7 km below the present level of adjacent land (BEN-AVRAHAMet al., 1979a).The part of the Levant Shear System joining the Gulf of Aqaba to the Dead Sea is verynarrow. This area, known as the Araba Depression, is marked by well-documented left-lateralstrike-slip motion on either side (FREUND et al., 1970). To the north, the shear system widens toaccommodate the Dead Sea and continues northward along the Jordan Valley into the lakes of Tiberiasand Hula. North of Hula the depression reorients itself NNE, to pass between the Lebanon andAnti-Lebanon Mountains. At the latitude of Tripoli, the shear system changes to an almost N-S strike,cutting through the basalt of Homs into the Er Rharb Depression. Where the Levant Shear Systemends to the north is not clear. QUENNEL (1958) and FREUND et al. (1970) have presentedconvincing evidence that about 105 km of left-lateral movement has occurred along the Levant Shew,65 km of this in the Miocene, the remaining 40 km in the Plio-Pleistocene. Of this 105 km, only some60 km movement are taken up by faults in the Gulf of Elat (EYAL et al., 1981). This strike-slipmotion must be taken into consideration in any plate tectonic reconstruction of the area.THE GULF OF ADENThe Gulf of Aden links the East African Rift Valley System and the Red Sea to the CarlsbergRidge in the northwest Indian Ocean, which in turn is part of the worldwide mid-ocean ridge system.The axis of the Carlsberg Ridge is offset 310 km right-laterally along the Owen Fracture Zone at aboutlWN, 57"E, and continues into the Gulf of Aden as the Sheba Ridge (Fig. 10). Within the Gulf ofAden, this ridge is offset by a number of transform faults, the most significant of which occurs at 52'Ealong the Alula-Fartak Trench. Minor offsets have been mapped both magnetically and bathymetricallyto the west until the Gulf of Tadjura which marks the western end of the Gulf of Aden. The central riftof Sheba Ridge is characterized by high heat flow (VON HERZEN, 1963) and shallow seismicity(FAIRHEAD and GIRDLER, 1970).The crustal structure beneath the Gulf of Aden is clearly oceanic. This is demonstrated byseismic refraction results (LAUGHTON and TRAMONTINI, 1970; LAUGHTON et al., 1970:FAIRHEAD, 1973) which show that the measured velocities may be grouped into (Fig. 10):1) an unconsolidated sedimentary layer (1.83-3.08 km s-l),2) a layer of volcanic or consolidated sedimentary material (3.94-5.3 km s-I), and3) the oceanic layer or layer 3 (6.15-6.96 km s-l), below which the upper mantle hasa velocity ranging from 7.55-8.45 km s-l.Anomalously low velocity upper mantle has been found on the western end of the axis ofSheba Ridge (7.06-7.14 km s-I), In general, the crustal thickness is about 12 km, inclusive of thewater layer. This, together with the presence of layer 3, strongly suggests that the crust beneath theGulf of Aden is oceanic. Since profiles within 55 km of the south and north coasts suggest oceaniccrusts, the minimum crustal separation within the Gulf must be 260 km.Gravity data in the western Gulf of Aden also suggest that almost the entire seafloor isunderlain by oceanic crust (GIRDLER et al., 1980). The crustal thickness is only about 5 k m alongthe current spreading axis, where a discontinuous dramatic shallowing of the Curie isotherm possiblyoccurs (TAMSETT and GIRDLER, 1982). The lateral extent of the intrusion zone reaches about 75km. It overlies anomalous mantle at a depth of just over 10 km (GIRDLER et al., 1980).Just as in the Red Sea, lineated magnetic anomalies have been mapped in the Gulf of Aden(ALLEN, 1964; COCHRAN, 1981; GIRDLER and STYLES, 1978; GIRDLER et al., 1980; NOY,1978). However, their interpretation is not unambiguous, and their implications on the tectonicevolution of the Gulf are still debatable (GIRDLER and STYLES, 1982; COCHRAN, 1982a).According to GIRDLER et al. (1980) and GIRDLER and STYLES (1978), the very large amplitudeanomalies over the ridge axis is a result of the recent phase of seafloor spreading while the loweramplitudes over the marginal troughs are the result of an earlier phase of spreading. From magneticdata in the western part of the Gulf, GIRDLER and STYLES (1978), GIRDLER et al. (1980) andNOY (1978) deduced the seafloor spreading history in terms of a two-, possibly a three-stage model.314