DEEP LITHOSPHERIC STRUCTURE ACROSS THE CENTRAL AFRICAN SHEAR ZONE IN CAMEROON

Teleseismic P- and PKP-arrival times recorded by a network of 40 seism ic stations deployed along a 300 km profile across the Adamawa Plateau at the northern end of the Volcanic Line in central Cameroon provide constraints on lithospheric thickness and anisotropy within the subcru stal lithosphere. These data indicate a thinned lithosphere beneath th e Central African Shear Zone, where seismologically defined asthenosph ere upwells from a depth of about 190 km to about 120 km in a relative ly narrow belt. Thus it has only a low-amplitude effect on the observe d gravity anomalies; the Bouguer gravity high over the Garoua Rift is consistent with crustal thinning beneath it. An abrupt change of the l ithospheric thickness beneath the Northern Boundary Fault correlates w ith both the topographic relief and a distinct change of the orientati on of relatively high- and low-velocity directions, which we infer to be due to anisotropy within the subcrustal lithosphere. This fault may represent an important accretionary suture zone dividing lithospheric blocks that originated in different tectonic settings and acquired di fferent, frozen-in anisotropy.