Structure of the crust beneath Cameroon, West Africa, from the joint inversion of Rayleigh wave group velocities and receiver functions

The 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 (Vs 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.