Tectonic Landforms and Active Faulting in the area surrounding Sumburu Hills， Northern Gregory Rift Valley， Kenya

Tectonic landforms and active faults in Sumburu Hills and Suguta Valley have been examined by means of aerial photographic interpretation and field work.The purpose of this paper is to discuss the characteristics of tectonic landforms, and the nature and cause of active faulting during the Quaternary.The main results are summarized as follows, (1) Active faults are almost all dip-slip normal, and are widely distributed trending NNE in parallel with Rift Valley.They are classified into I, II, III and IV, based on the degree of certainty of activity during the Quaternary.Landform of the eastern shoulder and median graben are largely controlled by active faulting and flexuring.(2) Active faults are classified into A, B, C, D and E types, based on their nature and cause.A type: distributes in the hills consisting of Miocene rocks as antithetic faults with downthrown to east side, and derived from reactivity of preexisted faults.B type: distributes in Tirr Tirr plateau, and forms horst-graben topography as a results of extension caused by updoming.C type: distributes along the boundary between west-end escarpment and median graben, and derived from boundary fault and flexure with large displacement downthrow to west side.D type: linearly distributes as Kangirinyang fault swarm cut many pyroclastic cones in Suguta Valley.Linear graben structures indicates the extensional axis along deep fracture associated with intrusion of basaltic magma.E type: distributes in the eastern part of median graben, and are short in length but tectonically young and active.(3) The intense faulting that largely determined a deep graben, has occurred since -81 -5C14-4around 3 Ma.Based on Yairi's method, the direction of horizontal extension in SugutaValley is estimated to be N 67W.This direction is at a right angle to the general trend of median graben.(4) Since the Miocene, the main area of faulting and flexuring have migrated with the time from east to west, and its horizontal migration amounts about 20km.The mean rate of migration reaches 2 mm per year and nearly equals to the spreading rate of East African Rift.