Abstract

The 1.4 Ma to Holocene volcanic eruptive centers and flows in the Chyulu Hills, southeastern Kenya, range in composition between foidites and alkali basalts; they record the relation between linear volcanic chains, preexisting basement structures, geochemical evolution, and the effects of a changing regional tectonic stress field. Pleistocene volcanic centers follow northwest-trending Proterozoic joints and are compatible with the early to middle Pleistocene east-northeast–west-southwest–oriented regional position of the least compressive regional stress (Shmin) documented for the Kenya Rift. Because the angle between the joints and Shmin was high, the ascent of magmas was facilitated by the preexisting basement structures. In contrast, Holocene north-northeast–trending vents and associated normal faults cut older basement structures. The youngest faults and vents are approximately perpendicular to the present regional Shmin position of 135° that has prevailed in Kenya for ∼0.5 m.y. The position of the neotectonic stress field has resulted in a decreased angle between Shmin and the joints and may have led to more difficult magma ascent, longer crustal magma residence, a trend toward more rift-typical volcanic rocks, and finally new alignments of the volcanic fields.

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