Rotation of extension direction in the central Kenya Rift

Abstract

Displacement of stratigraphic units along major normal faults and the kinematics of minor faults show that in the central Kenya Rift the direction of extension rotated from east-west to northwest-southeast during the Pleistocene. Possibly controlled by lithospheric weaknesses in the Proterozoic basement, an early half graben bounded by east-northeast-dipping, northwest-and north-northwest-striking oblique normal faults developed between about 12 and 7 Ma. Between 5.5(?) and 4 Ma, west-southwest-dipping antithetic normal faults established a full graben, and after 3.3 Ma west-southwest-dipping and north-northwest-striking oblique normal faults separated a tectonically active inner rift from the intrarift Kinangop Plateau to the east. Analysis of minor fault data indicates a general north-south orientation for σ 2 until about 0.4 Ma; since then, a clockwise rotation of σ 2 into the north-northeast to northeast orientation appears to have taken place. This rotation caused dextral oblique reactivations along older normal faults and created new normal faults with dextrally oblique components. The new kinematic regime is compatible with regional seismicity, and possibly developed as a response to far-field tectonic stresses generated by spreading in the Gulf of Aden and the Red Sea.