Red Sea: Structure & Tectonics
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Tectonics (from the Vulgar Latin tectonicus, meaning "building") is concerned with the processes which control the structure and properties of the Earth's crust, and its evolution through time. In particular, it describes the processes of mountain building, the growth and behavior of the strong, old cores of continents known as cratons, and the ways in which the relatively rigid plates that comprise the Earth's outer shell interact with each other. Tectonics also provides a framework to understand the earthquake and volcanic belts which directly affect much of the global population. Tectonic studies are important for understanding erosion patterns in geomorphology and as guides for the economic geologist searching for petroleum and metallic ores.

The divergent trends of the Red Sea depression and adjacent Precambrian basement clearly indicate that the Red Sea depression is not related to these early structures (Picard, 1970; Schürmann, 1966; Brown and Jackson, 1960). Recent structural maps (Brown, 1972; Choubert, 1968) further amplify the discordance and show that Paleozoic to Paleogene sediments and related structures cannot be part of the Red Sea evolution.

Uplift in Ethiopia and Arabia took place on an immense scale immediately after the late Eocene regression (Mohr, 1962). The Hail Arch, an early Paleozoic northtrending headland in the Precambrian basement of the Arabian shield was deformed during pre-Permian times and during late Cretaceous time was folded parallel to its N-S axis (Greenwood, 1972). Fractures that developed as part of the uplift provided feeder fissures for the Oligocene Trap Series in Ethiopia and Arabia. A monoclinal flexure developed along the eastern margin of the Red Sea as the Yemen-Hail arches formed. East of Jizan along the hinge lines of the monoclinal warp, zones of stretched, fractured rock were invaded by tholeiitic magma, forming local differentiated layered gabbros, granophyres, and diabase dike swarms ranging in age from 20 to 25 m.y. (Coleman and Brown, 1971). Uplift and arching in Ethiopia coincided with the intrusion of granite and rhyolite (22-25 m.y.) marginal to the Afar Depression (Barberi et al., 1972). In Ethiopia, continued uplift duirng the Oligocene and Miocene produced a complex pattern of major faults that gave rise to the downfaulted Afar.Depression, the Danakil horst, and the highlands west of the depression (Tazieff et al., 1972).

Deposition of the Red Series, confined to the Afar Depression, began in the late Oligocene and marked the warping of the depression (Bannert et al., 1970; Barberi et al., 1972). The normal faults bounding the depression are still active (Gouin, 1970). During Oligocene and Miocene time, uplift was sustained in the Yemen-Hail arches with continued monoclinal flexuring and gentle downwarping of the coastal plain. The thick Miocene clastic sections discovered in deep wells along the coastal plains were deposited unconformably on older downwarped units and demonstrate rapid erosion of the Yemen-Hail and Ethiopian arches (Gillmann, 1968). Steep dips of Jurassic and early Miocene sedimentary rocks towards the Red Sea axis along the monocline indicate downwarping of the Red Sea depression (Coleman and Brown, 1971; Gass and Gibson, 1969). Major normal faults have not been found along the eastern margin of the Red Sea, even though they are shown on numerous generalized maps (see, for instance, Mohr, 1962, p. 160; Picard, 1970; Holmes, 1965).

Whiteman's (1968, p. 235) comments on a presumably fault-bounded Red Sea are appropriate. "It is not possible here to give all the details relating to the nature of the bounding escarpments but analysis reveals that many of the faults shown on general maps do not exist on the ground and have, in fact, been put in on the assumption that most of the escarpments are of fault origin. It is indeed curious that almost every map of the Red Sea Depression showing faults shows a different pattern." Important to this argument is the recognition that the contacts between the Mesozoic and Tertiary sediments and the Precambrian basement are unconformities, and that the sediments dip steeply toward the axis of the depression. Perhaps some geologists hold to the view that the Red Sea is bounded by faults because the Suez and Aqaba grabens and the Afar Depression are all well-documented fault-bounded structures. The matching shorelines of the northern Red Sea along with the subparallel southern erosional scarps in the Arabian Peninsula and Ethiopia give quasitectonic evidence that permits this interpretation. Minor normal faults are associated with the monoclinal flexures along the Red Sea, and continued exploratory drilling may reveal major normal faults concealed by the thick Miocene evaporite-clastic deposits (Lowell and Genik, 1972). However, the Oligocene and Miocene development of the Red Sea depression deduced from the stratigraphic and structural record indicates that the northwest-southeast trending depression developed as a trough between the Arabian and African swells, rather than as a downfaulted block (Brown and Coleman, 1972).

Mobility of the Miocene evaporites is indicated by the numerous salt domes developed in the marginal parts of the Red Sea south of latitude 18°N (Brown, 1972). Salt domes may have begun to develop during the Pliocene and may be currently forming. Present-day subsurface salt flowage has been recorded at depths greater than 3000 meters (Frazier, 1970). At bottom-hole temperatures above 200°C, and under the pressure of thick overburdens, salt will act very nearly like soft butter. Salt could easily flow into the axial trough, and the irregular topography at latitude 16°40'N suggests that this may well have happened. Strong deformation of the top surface of the evaporites, as deduced from seismic profiles, clearly indicates salt flowage (Phillips and Ross, 1970).

Formation of the axial trough at the beginning of the Pliocene appears to have been related to a major rift, a marked departure from the Oligocene and Miocene development of the Red Sea depression. The physiography of the trough, a steep-walled depression with a floor of new basaltic crust, bears out the similarity to other active mid-ocean spreading centers. Concentration of seismic epicenters within the axial trough and alignment of active volcanoes along the trough axis also indicate present-day rifting. Solution of first motions on recent epicenters in the axial trough indicates movement along northeast-southwesttrending transform faults (Fairhead and Girdler, 1970; McKenzie et al., 1970). These first-motion solutions, combined with geologic studies, have shown that current left-lateral shear along the Dead Sea rift fits in with the general northeast movement of the Arabian plate away from Africa. In summary, synthesis of the present structural knowledge on the evolution of the Red Sea indicates at least two stages of development—pre-Miocene downwarping with crustal thinning and post-Miocene rifting.

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