Velocity-density relationships and modeling the lithospheric density variations of the Kenya Rift

We have devised pressure-dependent and temperature-dependent linear velocity-density relationships from the crustal rocks in the Kenya Rift and applied the relationships to the crustal velocity model derived from the KRISP90 refraction experiment to compute densities and gravity anomalies. Our results indicate that pressure and temperature considerations reduce the misfit between the observed and the calculated gravity anomalies in the areas of large lateral pressure and temperature variations. Using a separate relationship involving velocity and density contrasts for the mantle of the Kenya Rift, the KRISP teleseismic velocity model, and the variation of the observed gravity anomalies, we derive a dρ/dV_p value of ̃0.05 (Mg s m^-3 km^-1) which results in the density contrast of ̃0.03 Mg m^-3 at ̃100 km depth beneath the rift and its flanks. Using the magnitude of the velocity and density contrasts, we conclude that the mantle at the depth of 100 km beneath the rift could be ̃200°C to 375°C hotter than the adjacent lithosphere (and much of it is considered subsolidus) and contains partial melt up to 2.5%. The amount of partial melt in the region of the Moho upwarp would be greater because both velocities and densities are lower in that region. Our results are consistent with the upwarping of the thermal boundary layer representing the lithosphere-asthenosphere boundary and the consequent partial melting of the lithosphere.