The Kamasia Range (or Tugen Hills) is an uplifted, faulted and arched block lying between the Elgeyo Escarpment, which forms the western boundary of the Kenya Rift, and the axial Baringo-Suguta trough between 0°15′N and 1°30′N in the northern part of the rift. Metamorphic basement rocks are exposed in the central part of the Kamasia area where they are overlain by 3000 m of Miocene lavas and sediments ranging in age from 16 Ma to 7 Ma. This is the thickest sequence of rocks of this age exposed in the Kenya Rift. Downwarping occurred from the beginning of this period, but the first major fault movements occurred at about 7 Ma (late Miocene) forming the Elgeyo and Kamasia fault-scarps. Later volcanicity was mainly confined to the area east of the Kamasia, although extensive flood trachyte lavas covered practically the whole area east of the Elgeyo at one time. A second period of major fault movement occurred in the late Pliocene-Pleistocene (2–0.5 Ma) uplifting the Kamasia Range, forming the western dip-slope and downfaulting the axial graben of the rift to the east.

The overall structure of the Kamasia is a broad arch cut by large faults, with maximum displacement of about 4000 m, just to the west of its axis, but in detail the structures are complex. The numerous faults are normal dip-slip type and show an en echelon and obliquely intersecting pattern with dominant directions 000–040° and 330–340°. Folding on axes perpendicular to the fault planes produced half-domes and basins indicating a secondary compressive stress directed along the rift. It is proposed that the majority of the faults developed as vertical fractures in horizontal rock sequences and were then rotated by continued extension and up-arching to produce complex tilted fault-block systems both synthetic and antithetic to the main rift structure.

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