VOLCANO-TECTONIC EVOLUTION OF THE CHYULU HILLS AND IMPLICATIONS FOR THE REGIONAL STRESS-FIELD IN KENYA

The 1.4 Ma to Holocene volcanic eruptive centers and flows in the Chyu lu Hills, southeastern Kenya, range in composition between foidites an d alkali basalts; they record the relation between linear volcanic cha ins, preexisting basement structures, geochemical evolution, and the e ffects of a changing regional tectonic stress field. Pleistocene volca nic centers follow northwest-trending Proterozoic joints and are compa tible with the early to middle Pleistocene east-northeast-west-southwe st-oriented regional position of the least compressive regional stress (S(hmin)) documented for the Kenya Rift. Because the angle between th e joints and S(hmin) was high, the ascent of magmas was facilitated by the preexisting basement structures. In contrast, Holocene north-nort heast-trending vents and associated normal faults cut older basement s tructures. The youngest faults and vents are approximately perpendicul ar to the present regional S(hmin) position of 135-degrees that has pr evailed in Kenya for approximately0.5 m.y. The position of the neotect onic stress field has resulted in a decreased angle between S(hmin) an d the joints and may have led to more difficult magma ascent, longer c rustal magma residence, a trend toward more rift-typical volcanic rock s, and finally new alignments of the volcanic fields.