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THE CRUST BENEATH THE RED SEA - GULF OF ADEN RIFTSYSTEM:A REVIEW1HOW KIN WONG AND EGON T. DEGENSGeologisch-Palaontologisches Institut und Museum der Universitat HamburgBundestraae 55, D-2000 Hamburg 13, FRGABSTRACTThe Red Sea - Gulf of Aden - East African Rift System is one of the key areas to studyrift-drift processes. Within this single system, geodynamic processes ranging from uplift,block-faulting, volcanism, and seismic activity, to hydrothermal circulation and seafloor spreading arerepresented. In this paper we review the geological, geophysical, sedimentological and geochemicalcharacteristics of the Red Sea-Gulf of Aden, with special emphasis on the structure and evolution ofthe crust, as well as on their implications to continental rifting and plate accretion.INTRODUCTIONNorthern and northeastern Africa has always attracted the attention of geoscientists,particularly from the hydrogeological, tectonic and geomorphological points of view. With the recentpopularity of plate tectonics, this area together with its neighbours to the east and south has gainedeven greater significance. While the East African Rift is generally believed to be characterized bycontinental rifting, the Gulf of Aden and the Red Sea already have undergone active seafloorspreading. These three tectonic features meet at the Afar Depression, a triple junction with activevolcanism and earthquake activity attesting to ongoing tectonism.Rifting is a process generally believed to precede seafloor spreading. Studies of continentalrifts in Europe, North America and Africa (ILLIES and FUCHS, 1974; ILLIES and MUELLER,1970; MAGUIRE and KHAN, 1980; NOLET and MUELLER, 1982; PRODEHL, 1979; RAMBERGand NEUMANN, 1978) suggest that rifting may start with the diapiric intrusion of sub-lithosphericmaterial from a depth of several hundred kilometers into the lithosphere. A zone of tension isproduced along which partition of the continental crust eventually occurs. OXBURGH andTURCOTTE (1974), on the other hand, argued that the tensional failure at the earth's surface maysimply be a response to membrane stresses set up as the plate moves over a non-spherical earth. Otherunresolved questions include the spatial distribution of hypocenters in a rift system, the seismicvelocity structure of the crust and upper mantle within the graben and over its flanks, and the detailedstructure of the crust/mantle transition.The process of crustal formation at oceanic ridges is still not well-understood. It is generallyagreed that under the ridge crest, a mixture of mantle material and basaltic melt is being injected into thecrust. The basaltic melt, being lighter, migrates upwards and accumulates near the top of the injectiondiapir as magma chambers (BALLARD and VAN ANDEL, 1977; BRYAN and MOORE, 1977;LEWIS, 1983). These magma chambers are very limited in lateral extent (at most a few km), and it isunlikely that they represent a steady state phenomenon, certainly not in the Atlantic. The extrusion ofpillow basalts and lava flows from such magma chambers onto the seafloor is episodic, both in spaceand in time. Under the ridge axis, plate-driving forces produce an extensional strain which facilitateshydrothermal circulation, since crack propagation as a result of convective cooling is enhanced.Eventually, hydrothermal alteration of the crust and infilling of cracks by precipitation take place.Whether the Moho represents a petrologic boundary or is determined by the depth to whichhydrothermal penetration extends is still not resolved.Modified from a paper that appeared in Mitt.Geo1.-Palbnt. Inst., Univ. Hamburg, Georg Knetsch Heft 56, p. 53-94.307