Seismic Anisotropy Beneath the Cameroon Volcanic Line from SKS and SKKS Splitting Analysis

The Cameroon Volcanic Line (CVL) is an 1800-km-long line of Cenozoic volcanism that runs from the island of Annobòn in the Atlantic to northeast Cameroon. It 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. For this study, we use a temporary array of 32 broadband seismographs deployed throughout Cameroon for 1 to 2 years between 2005 and 2007 along with two additional permanent seismographs in adjacent countries. Using the SKS splitting method described by Silver and Chan (1991) and the analysis program of George Helffrich, we investigate SKS and SKKS arrivals from 44 earthquakes with sources 95 to 145 degrees away and magnitudes greater than 6.0. First we determine the fast direction and lag time indicated beneath each station for each event, and then we stack the high-quality measurements for each station to find the final fast axis and lag time. We interpret the fast directions as indicating the direction of flow in the upper mantle, as suggested by laboratory experiments and previous studies of mantle xenoliths. Our results indicate four regions with different anisotropic parameters in the upper mantle. Two regions indicate anisotropy with a northeast-southwest-oriented fast direction and split time of about 1 s: the Congo craton in Southern Cameroon and the eastern end of the CVL. Between the Congo craton and the CVL, in central Cameroon, the fast direction is variable and has a small lag time of about 0.3 s. Along the CVL, the fast direction has an approximate north-south orientation, with a lag time of about 0.7 s. The splitting values in Southern Cameroon are consistent with large splitting times and NE directions found at GSN stations in Gabon and the Central African Republic and probably indicate a strong consistent anisotropic fabric frozen into the cratonic lithosphere. Splitting results along the CVL and between the CVL and the craton, although consistent between adjacent stations, show variable magnitudes and orientations. We interpret these results as indicating a complex pattern of present-day mantle flow in the asthenosphere related to the formation of the CVL. One possibility is small-scale convection induced by the cold edge of the Congo craton, although the splitting orientations do no show a simple pattern orthogonal to the cratonic edge.