S U M M A R YThe Cameroon Volcanic Line (CVL) consists of a linear chain of Tertiary to Recent, generally alkaline, volcanoes that do not exhibit an age progression. Here we study crustal structure beneath the CVL and adjacent regions in Cameroon using 1-D shear wave velocity models obtained from the joint inversion of Rayleigh wave group velocities and P-receiver functions for 32 broad-band seismic stations deployed between 2005 January and 2007 February. We find that (1) crustal thickness (35-39 km) and velocity structure is similar beneath the CVL and the Pan African Oubanguides Belt to the south of the CVL, (2) crust is thicker (43-48 km) under the northern margin of the Congo Craton and is characterized by shear wave velocities ≥4.0 km s −1 in its lower part and (3) crust is thinner (26-31 km) under the Garoua rift and the coastal plain. In addition, a fast velocity layer (V s of 3.6-3.8 km s −1 ) in the upper crust is found beneath many of the seismic stations. Crustal structure beneath the CVL and the Oubanguides Belt is very similar to Pan African crustal structure in the Mozambique Belt, and therefore it appears not to have been modified significantly by the magmatic activity associated with the CVL. The crust beneath the coastal plain was probably thinned during the opening of the southern Atlantic Ocean, while the crust beneath the Garoua rift was likely thinned during the formation of the Benue Trough in the early Cretaceous. We suggest that the thickened crust and the thick mafic lower crustal layer beneath the northern margin of the Congo Craton may be relict features from a continent-continent collision along this margin during the formation of Gondwana.

[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.

S U M M A R YThe thickness of the mantle transition zone beneath Cameroon has been mapped using data from the 2005-2007 Cameroon Broadband Seismic Experiment to evaluate models for the origin of the Cameroon Volcanic Line (CVL). Some 2200 receiver functions have been stacked using a 3-D velocity model, revealing P s conversions from the mantle transition zone discontinuities at depths of ∼410 and 660 km. Results yield a nearly uniform transition zone thickness (251 ± 10 km) that is similar to the global average, implying that any thermal anomalies in the upper mantle beneath the CVL do not extend as deep as the transition zone. This finding, when combined with regional P and S velocity models of the mantle, supports an explanation attributing the origin of the CVL to upper mantle processes such as an edge flow convection cell in the upper mantle along the northwestern side of the Congo Craton lithosphere.

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SUMMARY The Cameroon Volcanic Line (CVL) is an 1800‐km‐long line of Cenozoic volcanoes that does not show a chronological progression consistent with hotspot‐related volcanism. We investigate seismic anisotropy to determine the upper‐mantle lattice preferred orientation and constrain the mantle flow pattern using a temporary array of 32 broad‐band seismographs deployed throughout Cameroon between 2005 and 2007 along with two additional permanent seismographs in adjacent countries. We determine the fast direction and lag time beneath each station by stacking SKS and SKKS splitting measurements from multiple events. The results indicate four regions with different splitting parameters. The Congo Craton in southern Cameroon and the Garoua rift region in northeast Cameroon have northeast–southwest‐oriented fast directions and split times of about 1 s. Between the Congo Craton and the CVL, in central Cameroon, the fast directions are variable and have small splitting times of 0.3 s or less. Along the CVL, where previous studies show a strong slow velocity anomaly in the mantle, the fast direction is oriented approximately north–south, with splitting times of about 0.7 s. We interpret measurements from southern Cameroon and northeast Cameroon as indications of lattice‐preferred orientation frozen into the Congo Craton and subcontinental lithosphere related to relict plate motion and deformation. The distinct pattern of splitting along the CVL suggests the existence of small‐scale convection in the asthenosphere related to the formation of the CVL, perhaps driven by the adjacent cold edge of the Congo Craton.