Upper-mantle anisotropy beneath the Cameroon Volcanic Line and Congo Craton from shear wave splitting measurements

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 northeastsouthwest-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 northsouth, 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.