GEOLOGICAL EVOLUTION OF THE SOUTHEASTERN RED-SEA RIFT MARGIN, REPUBLIC OF YEMEN

The tectonic evolution of the southeastern margin of the Red Sea Rift in western Yemen has been investigated using a multidisciplinary field study of an east-west transect between Al Hudaydah and Sana'a. Slow s ubsidence of up to 1 km occurred over the area during a 100 m.y. perio d before rifting. There was a major episode of flood volcanism between ca. 30 and 20 Ma, and important extensional faulting began after the eruption of the volcanic rocks and ceased before middle to late Miocen e sediments and volcanic rocks were deposited unconformably on top of rotated fault blocks on the coastal Tihama Plain. Surface uplift has p roduced the Yemen highlands, whose highest peak reaches an elevation o f 3660 m. This is attributed to plume heating and eruption of >3000 m of volcanic rocks. Apatite fission-track ages indicate early to middle Miocene exhumational cooling ages, postdating the major volcanic phas e and contemporaneous with rifting. Volcanism was accompanied by empla cement of subvertical dike swarms, which generally strike north-northw est to northwest, broadly parallel to the Red Sea coastline. Major fau lts indicate northeast-southwest-directed extension. Large granitic sh eets and plutons (up to 25 km wide) intruded the volcanic rocks. Appro ximately 30 km of ex-tension has taken place across a 75-km-wide zone (beta = 1.7) in 6-8 m.y. The relative timing of volcanism followed by extension and uplift does not fit conventional models of passive or ac tive rifting. We suggest that the proto-Red Sea Rift was caused by reg ional plate stresses that exploited lithospheric weakening caused by t he Afar plume. Appreciable doming only occurred after the main episode of volcanism, which suggests that magmas extruded before maximum ther mal expansion of the lithosphere took place.