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Since no major shear, spreading, or subduction occurs between Nubia and Somalia, they argued thatthe evolutionary history of the Gulf of Aden must be similar to that of the Red Sea, Hence theyassumed a recent phase of spreading from 4.5 m.y. ago to the present at a rate of 1 c m yr1 and anearlier phase from the upper Oligocene (23.5 m.y. B.P.) to mid-Miocene (about 16 m.y. ago) (Fig.3c). Rates north and south of the Ridge were 1.1 and 1 c m yr-1 respectively. An even earlier phasefrom upper Eocene (43 m.y.) to lower Oligocene (35.5 m.y.) may also be interpreted. With thismodel, GIRDLER et al. (1980) constructed an isochron map for the western Gulf of Aden (Fig. 11).Here oceanic crust extends across almost the entire width, with an age of about 40 m.y. off theSomalian and the Arabian coasts. The Oligocene-Miocene spreading phase took place along a ridgeaxis lying in the south (thick line). A period of quiescence followed, lasting about 10 m.y., duringwhich about 1.5 km of sediment were deposited. The Plio-Pleistocene phase of the seafloor spreadingoccurred along an axis which is displaced 75 km to the north in the eastern part of the western Gulf.This axis has a strike of N 120"E. It is offset by a number of transform faults and bifurcates in thewest, one branch going towards the Strait of Bab-el-Mandeb and the other towards the Gulf ofTadjura. The bifurcation point may mark the triple junction between the Arabian, Somalian and NorthDana plates.To emphasize the difficulty encountered in unraveling the tectonic evolution of the Gulf ofAden, we should point out that COCHRAN, using the same data as GIRDLER et al. (COCHRAN,1982a) as well as other independent data (COCHRAN, 1981) concluded that the Gulf (and hence, byinference, the Red Sea) has undergone only one continuous episode of seafloor spreading, beginningabout 15 m.y. ago (Fig. 3d). The spreading, at least in the central Gulf of Aden, appears to beasymmetric. Noting the existence of a magnetic quiet zone landwards of the recognizable anomaliesand that the seaward boundary of this zone is marked by a noticeable increase in basement depth,COCHRAN argued that opening of the Gulf of Aden and the Red Sea started with diffuse extensioncharacterized by dike injection and normal faulting within a wide "rift zone". This extension amountsto about 100 km over at least 10 m.y. and is resp-onsible for the magnetic quiet zone (COCHRAN,1982b). The diffuse extension gave way eventually to spreading at a well-defined ridge axis, wherebymagnetic lineations could be generated.PLATE TECTONICS OF THE RED SEA AND GULF OF ADEN AREASAlthough the Red Sea has long been considered a classic example of an Ocean in its first stageof evolution, its tectonic history and the regional kinematic plate motions still remain an enigma.McKENZIE et al. (1970), FREUND (1970), GRIDLER and DARRACOTT(1972), LE PICHON eta!. (1973) md PJCHAROSON m d F!-L?RR!SON(1976) hme deve!=ped kinematic patterns based onthe assumption that the coastline or a particular isobath represents the continent-ocean structuralinterface. This implies that a large part of the Red Sea consists of oceanic crust. Others (ROSS andSCHLEE, 1973; LOWELL and GENIK, 1972; LOWELL et al., 1975; HUTCHINSON andENGELS, 1972) assumed that oceanic Crust is confined to the axial trough so that most of the Red Seaand the Afar Depression is underlain by thinned continental curst. Which of the above views is correctis still a subject of debate. In the following discussions we shall make use of the arguments of LEPICHON and FRANCHETEAU (1978).To establish the kinematic plate pattern in the Red Sea - Gulf of Aden - Levant area,seafloor spreading is assumed to be stable. This is reasonable because in the early stage of openingadjustments in the kinematic pattern are mechanically difficult and are unlikely as long as the twocontinen't-bearing lithospheres are in contact. The fact that transform faults especially in the Gulf ofAden do not indicate a change in trend implies that the poles of rotation have remained much the samethroughout time, i.e., the total poles of opening are approximately the same as the presentinstantaneous poles.To determine the instantaneous pole between Arabia and Nubia one can use the detailedbathymetric survey of the Valdivia and the resulting transform fault pattern between 19" and 23"N(PLAUMANN, 1975). Since this pattern is consistent with the pole at 36.5'N, 18"E determined byMcKENZIE et al. (1970) from a morphological fit of the two Red Sea coasts, one can accept this asthe correct instantaneous as well as total pole (Table 1). Note that this does not imply that the Red Seacoasts should be fitted together sometime in the geological past.315 I