The Cameroon Line: Analysis of an intraplate magmatic province transecting both oceanic and continental lithospheres: Constraints, controversies and models

Volcanic risk perception in rural communities along the slopes of mount Cameroon, West-Central Africa.

The Cameroon Volcanic Line (CVL) straddles the continent-ocean boundary in West Africa but exhibits no clear age progression. This renders it difficult to explain by traditional plume/plate motion hypotheses; thus, there remains no consensus on the processes responsible for its development. To understand better the nature of asthenospheric flow beneath the CVL, and the effects of hotspot tectonism on the overlying lithosphere, we analyze mantle seismic anisotropy and seismicity. Cameroon is relatively aseismic compared to hotspots elsewhere, with little evidence for magmatism-related crustal deformation away from Mount Cameroon, which last erupted in 2000. Low crustal Vp/Vs ratios (∼1.74) and a lack of evidence for seismically anisotropic aligned melt within the lithosphere both point toward a poorly developed magmatic plumbing system beneath the CVL. Null SKS splitting observations dominate the western continental portion of the CVL; elsewhere, anisotropic fast polarization directions parallel the strike of the Precambrian Central African Shear Zone (CASZ). The nulls may imply that the convecting upper mantle beneath the CVL is isotropic, or characterized by a vertically oriented olivine lattice preferred orientation fabric, perhaps due to a mantle plume or the upward limb of a small-scale convection cell. Precambrian CASZ fossil lithospheric fabrics along the CVL may have been thermomechanically eroded during Gondwana breakup ∼130 Ma, with an isotropic lower lithosphere subsequently reforming due to cooling of the slow-moving African plate. Small-scale lithospheric delamination during the 30 Ma recent development of the line may also have contributed to the erosion of the CASZ lithospheric fossil anisotropy, at the same time as generating the low-volume alkaline basaltic volcanism along the CVL.

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The development of magmatism along the Cameroon Volcanic Line: Evidence from seismicity and seismic anisotropy

[1] 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 ∼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.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2011TC003028

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

Beach sediments collected along Atlantic Ocean of Cameroon were geochemically analysed using ICP–MS techniques to determine their contamination of heavy metal and related ecological risks. In this study, the selected metals displayed a declining trend of Fe > Mn > Ni > Cu > Co > Zn > Cr > Cd. Indices of pollution, such as factor of enrichment (EF) which values are generally more than 1.5, except for Mn; index of geo-accumulation (I-geo), where values of Co, Ni and Cu in sediments of the studied area were > 0, and other metals (Zn, Cr, Fe and Mn) were < 0; factor of contamination (CF) which values of all selected elements are between 1 and 3 except for Co and Ni which are up to 3; and load index of pollution (PLI), where values in all sampling sites are greater than 1, except one site. These results advocate a moderate contamination of the study sediments by metal with the exception of Mn. The potential ecological risk assessment (
 $$E_{\text{r}}^{\text{i}}$$
 and RI) indicates that this coast is a low-to-moderate-risk zone. PCA, cluster and correlation matrix indicate that heavy metals maintained fairly similar trends with both anthropogenic and natural sources. This study explained that this coastal area is not highly concentrated by heavy metals and revealed that the Atlantic coast of Limbe, Cameroon are moderately polluted by studied metals with is a low-to-moderate risk.

J.C. Delmond

Papers

Eruptive and earthquake activities related to the 2000 eruption of Mount Cameroon volcano (West Africa)