Basaltic Pulses and Lithospheric Thinning—Plio‐Pleistocene Magmatism and Rifting in the Turkana Depression (East African Rift System)

Abstract

AbstractThe East African Rift System (EARS) provides an opportunity to constrain the relationship between magmatism and plate thinning. During continental rifting, magmatism is often considered a derivative of strain accommodation—as the continental plate thins, decompression melting of the upper mantle occurs. The Turkana Depression preserves among the most extensive Cenozoic magmatic record in the rift. This magmatic record, which comprises distinct basaltic pulses followed by periods of relative magmatic quiescence, is perplexing given the lack of evidence for temporal heterogeneity in the thermo‐chemical state of the upper mantle, the nonexistence of lithospheric delamination related fast‐wave speed anomalies in the upper mantle, and the absence of evidence for sudden, accelerated divergence of Nubia and Somalia. We focus on the Pliocene Gombe Stratoid Series and show how lithospheric thinning may result in pulsed magma generation from a plume‐influenced mantle. By solving the 1D advection‐diffusion equation using rates of plate thinning broadly equivalent to those measured geodetically today we show that despite elevated mantle potential temperature, melt generation may not occur and thereby result in extended intervals of quiescence. By contrast, an increase in the rate of plate thinning can generate magma volumes that are on the order of that estimated for the parental magma of the Gombe Stratoid Series. The coincidence of large‐volume stratiform basalt events within the East African Rift shortly before the development of axial zones of tectonic‐magmatic activity suggests that the plate thinning needed to form these stratiform basalts may herald the onset of the localization of strain.