STRATIGRAPHIC AND STRUCTURAL CONSTRAINTS ON MECHANISMS OF ACTIVE RIFTING IN THE GREGORY RIFT, KENYA

A review of the stratigraphic and structural events associated with the early evolution of the Gregory Rift indicates that they are consistent with the active model of rifting. At present, there is no geochemical evidence to indicate unequivocally the presence of magma derived from asthenospheric material. However, the evidence for limited pre-rift uplift and the morphologic and geochemical features of the early basalts and phonolite flood lavas support the intrusion of an asthenospheric plume into the lithospheric upper mantle during mid-Miocene times. The scale of uplift (< 1 km) and the limited volumes of lava produced indicate that a small convective cell, with an initial diameter of perhaps only 100-150 km, rather than a major asthenospheric plume underlies the Gregory Rift. The subsequent rise of this small plume through the lithosphere to the base of the crust is documented by Late Miocene and Pliocene magmatic and rifting events. The presence of a relatively thick mechanical boundary layer beneath the Gregory Rift prior to rifting, indicated by the general absence of tholeiitic volcanism and low extension rates, is supported by geophysical and geochemical evidence for igneous underplating and significant magma fractionation within the lithosphere. There is a strong coincidence of location of magmatic activity and rifting with pre-existing zones of crustal weakness beneath the Gregory Rift. The rising plume was focused beneath a weak zone marking the contact between the reworked and buried margin of the Tanzanian craton and the adjacent Proterozoic mobile belt. Within this zone, major crustal-scale shear zones and thrusts accommodated limited lithospheric stretching and influenced the location and development of half-graben basins and transfer zones. Early basins were nucleated on NW-SE- and N-S-trending weaknesses. As the plume ascended and spread laterally, rifting propagated away from this zone and younger basins developed across both mobile belt and cratonic crust.