Array observations of PKP phases across the Kenya Rift: Implications for structure and tectonics

PKP travel-time residuals from teleseisms recorded by a 100 x 110 km array centered on the south-central portion of the Kenya Rift show that variations in residuals are as large parallel to the rift as perpendicular to it. Asymmetry in residuals observed across the rift is consistent with half-grabens facing in opposite directions separated by an accommodation zone. The largest time residuals are offset at least 40-50 km west of the rift axis, supporting a rifting model in which a dominant half-graben fault is listric, penetrates the lithosphere, and triggers intrusion from asthenospheric sources. Linear regression of residuals and Bouguer gravity anomalies shows a slope of -0.015 +/- 0.002 s/mgal; the slopes of similar regressions for the tectonically inactive Midcontinent Rift and Sierra Nevada are -0.0025 +/- 0.001 s/mgal. One explanation for the observed factor of 5 difference is the presence of partial melt under the Kenya Rift system.