Further constraints on the shear wave velocity structure of Cameroon from joint inversion of receiver function, Rayleigh wave dispersion and ellipticity measurements

To develop a new model of the Cameroon crust that would provide further constraints on its composition and help study regional tectonics, we simultaneously invert Rayleigh wave group and phase velocity dispersion, P-wave receiver function and Rayleigh wave ellipticity measurements using a joint inversion method based on the neighbourhood algorithm (NA). We obtain ensembles of 1-D isotropic Vs models along with layered Vp/Vs ratio and crustal thickness estimate beneath 32 broad-band seismic stations in the region. Due to the addition of ellipticity data, the resolutions of shallow structures are well improved in our new model, thereby showing a better spatial correlation with known tectonic features. By interpolating the ellipticity measurements and performing joint inversions with group and phase dispersion data at several grid nodes, we are able to image the geometry and constrain the locations of mafic bodies (Vs of 3.7-4 km s(-1)) that intruded into the upper crust (2-8 km) during the formation of Gondwana. Investigating the relationship between crustal parameters, we found a positive correlation between crustal thickness, Vp/Vs ratio, the Bouguer anomaly and topography. Specifically, we found that the inverted Moho depth and surface topography within the Adamawa Plateau and the Garoua Rift agrees with Airy isostasy. The inverted Vp/Vs ratio shows variation that ranged from 1.67 to 1.85 with uncertainties that are generally less than 0.05 at crustal depths. A regional averaged Vp/Vs ratio of 1.73 and a standard deviation of similar to 0.03 in the crust suggest a felsic to intermediate bulk crustal composition, indicating that the Cenozoic volcanism is most likely characterised by small volumes of mafic intrusion with limited alteration to the crust, thereby supporting plume-free hypothesis such as small-scale mantle convection for the origin of the Cameroon Volcanic Line.