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

generally weak or lacking, the only major exception being the beginning of the emplacement of thethick trap series in Yemen, where basalts are found interbedded with Cretaceous sedimentary rocks.The early Tertiary sedimentary record in the Red Sea area is rather incomplete. It is generallyaccepted that during the Eocene, transgressive marine deposits developed in the northern Sinai, inEgypt, and in northern Sudan (CARELLA and SCARPA, 1962; SAID, 1962; WHITEMAN, 1968).On the other hand, the Horn of Africa had completely emerged by the end of the Eocene. Depositionof the Red Series in the Afar Depression began in the late Oligocene (BARBER1 et al., 1972), andOligocene and Miocene marine sediments are found on the southern coast of the Arabian Peninsula.During the Oligocene, separate, extensive uplifts occurred on either side of the Red Sea. Thiswas accompanied by the outpouring of alkaline basalts that now form a thick trap series on the highplateaus behind the Arabian and Ethiopian scarps. The Miocene sedimentary sequence is characterizedby a great thickness of clastics (2-3 km) and evaporites (3-4 km) in the Red Sea marginal troughand the surrounding continents (BROWN, 1970; LOWELL and GENIK, 1972; ROSS et al., 1973).During this time, volcanic activity continued in the Afar Depression and in western Saudi Arabia. Asthe Red Sea depression subsided further, uplift continued in the Arabian-Yemen block on the east andthe Ethiopian segment on the west, It should be noted that during the deposition of the massive saltdeposits, the Red Sea probably remained connected to the open ocean. Tectonic subsidenceaccompanied by brine migration to the morphological deeps and final precipitation of salt minerals areresponsible for the observed evaporite sequence which locally reaches 5 km in thickness.At the beginning of the Pliocene a dramatic change in the sedimentary record occurred,whereby the Miocene evaporitic sequence gave way abruptly to marine oozes and marginal clastics.Within the Red Sea axial trough, a thin veneer of sediment directly overlies basalt that has extrudedinto the axial trough during active seafloor spreading (GASS, 1970; SCHILLING, 1969).MAGNETICSThe Red Sea has very large magnetic anomalies over the axial trough, with amplitudes of theorder of 2000 nT (nano-Teslas or gammas) diminishing northwards. In contrast, magnetic anomaliesover the main trough are much smaller, usually a few hundred nT (ALLEN, 1970; GIRDLER andSTYLES, 1974; PHILLIPS et al., 1969; PHILLIPS, 1970; ROESER, 1975, 1976; SEARLE andROSS, 1975). Apart from anomalies 1 and 2 in the axial trough, the magnetic pattern cannot becorrelated over the entire length of the Red Sea. In part this is due to the paucity of good magnetic dataextending from coast to coast, particularly in the northern Red Sea.Figure 3a shows a composite magnetic profile obtained from stacking 6 adjacent aeromagneticprofiles from the central Red Sea and then reducing it to the pole. Beneath this is a synthetic profilegenerated using the reversal time scale of TARLING and MITCHELL (1976). The magnetism isassumed to reside in layer 2 for the new crust in the axial trough and in layer 3 for the older crustbeneath the main trough. The composite and synthetic profiles are directly comparable only if twoseafloor spreading phases and a 70 km westward drift of the spreading axis - when the secondepisode of spreading began - are assumed (GIRDLER, 1978; NOY, 1978). In this model the morerecent spreading phase started about 4.5 m.y. ago and has continued up to the present at a rate of 0.7cm yr-l. The anomalies of the earlier phase suggest that it started about 30 m.y. B.P. (upperOligocene) and continued to about 15 m.y. B.P. (mid-Miocene). To illustrate the uncertainties in theinterpretation of these anomalies, a second model is shown in which the same dates are used for therecent phase of spreading but younger dates, namely all within the Miocene from 20 m.y. to about 6m.y. ago, for the earlier phase. Here a 27.5 km westward shift of the spreading center is required atthe re-initiation of spreading (NOY, 1978; Fig. 3b). From the magnetic anomalies alone it isimpossible to choose between these two possible models.In the northern Red Sea between 26" and 28'N a magnetic profile studied by GIRDLER andSTYLES (1976) suggests a low half-spreading rate of 0.5 cm yr-1 over the past few million years.Between 20.5' and 2'N in the axial trough, large magnetic anomalies have been interpreted bySEARLE and ROSS (1975) to represent an inter-trough zone (the Atlantis-Hatiba inter-trough zone)over which magnetic anomalies are absent. This absence has been explained by the magneticend-effect model whereby a number of short segments of spreading axis are assumed to be offset bysmall transform faults. These transform faults are associated with zones of very low (possibly zero)309