Stress field changes in the Afro-Arabian rift system during the Miocene to Recent period

Abstract

We have investigated the Miocene to present-day stress fields of the Kenyan rift, the northern Red Sea, and the Gulf of Suez by integrating stratigraphically constrained fault movements, aligment of radiometrically dated dikes and volcanic vents, teleseismic earthquake focal mechanisms, and borehole breakouts in exploratory wells. Fault analysis and dike orientation suggest that in the central Kenyan rift the least horizontal stress direction ( S hmin) from 12 to 6 Ma was oriented NE-SW to ENE-WSW. In the northern Kenyan rift, 4.2–3.8 Ma feeder dikes of the Tirr Tirr trachytes also indicate NE-SW S hmin orientation. By about 2.6 Ma, faulting in the central Kenyan rift occurred in response to E-W extension. After ∼0.6 Ma, S hmin in the central Kenyan rift experienced further clockwise rotation into NW-SE orientation. Aligned vents of the late Quaternary volcanic fields of Mt. Marsabit, Nyambeni and Huri Hills, limited teleseismicity, and borehole breakouts in wells drilled to the east of the Kenyan rift, indicate that the present-day S hmin orientation is approximately NW-SE throughout most of Kenya. In the Gulf of Suez during most of the Middle Miocene-Early Pleistocene, S hmin was aligned NE-SW, normal to the rift axis. In the Late Pleistocene, but before 125 ka, the Gulf of Suez and northern Red Sea S hmin rotated counterclockwise into a N-S direction. Extensive breakout and limited teleseismicity data show that in the Gulf of Suez S hmin is presently N-S oriented. Our data suggest that local stress fields have rotated significantly during much of the Miocene to Recent tectonic history of the Afro-Arabian Rift System. The best documented stress field change occurred synchronously between the Tanzanian-Kenyan rifts in the south and the northern Red Sea-Gulf of Suez in the north during the Late Pleistocene. Two possible interpretations for this Late Pleistocene rotation of the stress field are: (1) a change in the dynamics of the Afro-Arabian Rift System affected this plate boundary over a region spanning 4000 km (forces generated within the rift system), or (2) motions along one of the distant African plate boundaries changed dramatically, resulting in stress field changes throughout the plate (forces generated outside the rift system). These two models can be tested by studies of Pleistocene to Recent stress fields outside the Afro-Arabian Rift System, and along the boundaries of the African plate.