Stratigraphy, geochronology and volcano-tectonic evolution of the Kedong-Naivasha-Kinangop region, Gregory Rift Valley, Kenya

Twenty-two new K-Ar dates allow reorganization of the volcano-stratigraphy of the Kedong-Kinangop region on the east side of the central Kenya rift for the most recent 4 Ma, during which time most of the structural features evolved. Four principal phases of volcanic eruption were: widespread Kinangop tuff ash flows (3.7-3.4 Ma), Limuru flood trachytes (2.0-1.8 Ma), basalts and flood trachytes of the rift floor (1.65-0.9 Ma), Quaternary salic caldera volcanoes (0.4-0 Ma). K-Ar dating allows recognition of the Cobb Mountain normal polarity event at about 1.18 Ma, and dating of the Kedong flood at about 0.1 Ma. Major faulting occurred at 4-3 Ma, converting an early half-graben to a graben. Fault zones then migrated inwards progressively forming step-fault platforms at 3-2 Ma and 1.7 Ma, such that the graben narrowed with time. Extensive minor faulting of the rift floor occurred during the most recent 0.8 Ma. The average extension rate across the central rift was 3 mm a^-1 for the last 7 Ma, and is close to plate tectonic estimates. Individual fault strain rates are estimated at 0.1-0.4 mm a^-1. Faulting was episodic rather than continuous, and some episodes affected both sides of the rift. Volumes of magma erupted during 4-0 Ma require generation of at least 1.2×10⁵ km³ of primary magma in the 200 km length of the south-central rift if fractional crystallization is the dominant differentiation mechanism. This volume implies mantle upflow either as an asthenosphere 'plume', or as the result of lithosphere extension. The tectonic and magmatic sequence of events is proposed to result from lithosphere thinning, emplacement of large magma bodies in or near the lower crust, regional uplifts and superimposed limited lateral separation of the lithosphere as a result of plate motions.