The development of magmatism along the Cameroon Volcanic Line: Evidence from teleseismic receiver functions

The Cameroon Volcanic Line (CVL) in West Africa is a chain of Cenozoic volcanism with no clear age progression. The reasons for its existence are unclear, and the nature of its magmatic plumbing system is poorly understood. Specifically, whether or not the CVL crust presently contains melt and/or mafic intrusions, as is often observed at hot spots and rifts elsewhere, is presently unknown. To address this issue, we present a receiver function study of crustal structure using earthquakes recorded by the Cameroon Broadband Seismic Experiment. In regions of the CVL unaffected by Cretaceous extension associated with the breakup of Gondwana (e. g., the Garoua rift), Vp/Vs ratios are markedly low (network average similar to 1.74) compared to hot spots elsewhere, providing no evidence for either melt or cooled mafic crustal intrusions due to CVL magmatism. The character of P-to-S conversions from beneath the CVL also indicates that lower-crustal intrusions (often termed underplate) are not present beneath the region. Our observations thus corroborate earlier petrological studies that show CVL alkaline magmas fractionate in the mantle, not the crust, prior to eruption. Hypotheses for the formation of the CVL should not include markedly elevated upper-mantle potential temperatures, or large volumes of partial melt, both of which can explain observations at hot spots and rifts worldwide. The protracted, yet sporadic, development of small-volume alkali melts beneath the CVL may instead be explained better by lower melt volume mechanisms such as shear zone reactivation or lithospheric delamination.