Origin of Kenya Rift Plateau‐type flood phonolites: Integrated petrologic and geophysical constraints on the evolution of the crust and upper mantle beneath the Kenya Rift

Abstract

A gravity profile across the Kenya Rift at the equator was modeled with recent results of petrologic and seismic investigations as constraints. This profile is dominated by a 600‐km‐wide gravity low of 100 × 10−5 m/s2 (100 mGal). The model incorporates the important concept of modification of the lower crust by magmatic processes. Alkali basaltic magmas are believed to have modified the lower crust in southern Kenya during early rift‐related basaltic volcanism (23–14 Ma) on the basis of (1) synchronous voluminous eruption of alkali basalt in northern Kenya; (2) Kenya Rift International Seismic Project 1990 (KRISP 90) seismic lines which delineate an ∼10‐km‐thick layer of basal lower crust in southern Kenya which is absent or very thin to the north; (3) results from high pressure/temperature experiments on a late Miocene Plateau phonolite and related geochemical modeling which indicate a lower crustal origin for these voluminous lavas (14–11 Ma) by fusion of alkali basaltic material; and (4) Plateau phonolite distributions that are conspicuously limited to the southern Kenya Rift above this anomalous lower crust. The gravity model features a lens of mafic intrusives (3000 kg/m3; constrained by petrologic arguments) at the base of the crust. It extends from 25 to 34 km depth beneath the rift valley and pinches out at a distance of about 250 km on each side of the rift. Surrounding lower crust (2850 kg/m3) is bounded by upper crust (2700 kg/m3) at about 15 km depth. The gravitational effect of the positive density contrast in the lower crust due to the lens of mafic intrusives is offset by an underlying wedge of anomalously low density mantle (3150 kg/m3). This wedge is about 300 km wide at the Moho and is relatively steep sided, in agreement with KRISP teleseismic results. East of the rift, this anomalous mantle is bounded by normal upper mantle (3260 kg/m3) extending to 100 km depth. West of the rift, normal upper mantle extends to 90 km depth. Within the rift valley, shallow horst and graben are indicated by the gravity data.