Upper mantle structure beneath Cameroon from body wave tomography and the origin of the Cameroon Volcanic Line

Abstract

[1] The origin of the Cameroon Volcanic Line (CVL), a 1600 km long linear volcanic chain without age progression that crosses the ocean-continent boundary in west-central Africa, is investigated using body wave tomography. Relative arrival times from teleseismic P and S waves recorded on 32 temporary seismic stations over a 2-year period were obtained using a multichannel cross-correlation technique and then inverted for mantle velocity perturbations. The P and S wave models show a tabular low-velocity anomaly directly beneath the CVL extending to at least 300 km depth, with perturbations of −1.0 to −2.0% for P and −2.0 to −3.0% for S. The S wave velocity variation can be attributed to a 280 K or possibly higher thermal perturbation, if composition and other effects on seismic velocity are negligible. The near vertical sides of the anomaly and its depth extent are not easily explained by models for the origin of the CVL that invoke plumes or decompression melting under reactivated shear zones, but are possibly consistent with a model invoking edge-flow convection along the northern boundary of the Congo Craton lithosphere. If edge-flow convection in the sublithospheric upper mantle is combined with lateral flow channeled along a fracture zone beneath the oceanic sector of the CVL, then the oceanic sector can also be explained by flow in the upper mantle deriving from variations in lithospheric thickness.

Lithospheric instability and the source of the Cameroon Volcanic Line: Evidence from Rayleigh wave phase velocity tomography

“…This conclusion was further supported by the results of a more recent H-k stacking analysis of receiver functions using the same data set [Gallacher and Bastow, 2012]. Reusch et al [2010], using body wave tomography, imaged a uniform, vertically oriented, tabular low velocity zone directly below the CVL extending to at least 300 km depth. Using P-wave receiver functions to image the 410 and 660 km discontinuities, Reusch et al [2011] showed that the transition zone beneath the CVL has not been thinned, indicating that the tabular LVZ in the upper mantle does not extend as deep as the transition zone.…”

“…These slow velocities are separated from the fast velocities beneath the Congo Craton by a vertical boundary that extends to at least 200 km, although the velocity contrast across the boundary decreases with depth. These results, together with evidence from body wave [Reusch et al, 2010] and transition zone receiver function [Reusch et al, 2011] studies, create a clear model of a vertical low velocity zone beneath the CVL that extends from the base of the crust to depths between 200 and 300 km, but not into the upper mantle transition zone. These observations are used to evaluate candidate origin models for the formation of the CVL in section 5.2.…”

“…Although there is evidence that the deviation from regional and global average velocities decreases at greater depths, resolution at these depths is limited. Previous studies with resolution deeper in the upper mantle [Reusch et al, 2010] have found that low velocities beneath the CVL extend only to depths of 300-400 km and found no evidence for deflection in the depths of major mineral phase changes in the transition zone [Reusch et al, 2011], making such large-scale Rayleigh-Baynard convection cells an unlikely geodynamic source for the CVL.…”

“…Koch et al [2012] found no evidence for either pattern in their study, which provided constraints on the lateral variations in the fast propagation direction. Our study Reusch et al, 2010;after Ebinger and Sleep, 1998]. (c) Rayleigh-Baynard convection cell model [after Meyers et al, 1998].…”

“…We use data from a temporary passive seismic deployment installed across Cameroon from January 2005 through early 2007 [e.g., Tibi et al, 2005;Reusch et al, 2010Reusch et al, , 2011Tokam et al, 2010;Koch et al, 2012]. Eight broadband stations were installed in 2005 and operated for two years, and an additional 24 stations were installed in early 2006 and ran for a single year (Figure 1).…”