Origin of the Cameroon line of volcano-capped swells

Swells of the Cameroon Line, a 1000-km-long line of 10 volcano-capped swells, resemble other volcano-capped topographic and bathymetric swells on the African plate. However, individual swells of the Cameroon Line are about 10 times smaller in area and distinct in being arranged in a straight line, half on the continent and half on the ocean floor. These peculiarities are here interpreted as related to an underlying mantle plume (the 711 plume) at lat 7 degreesN, long 11.5 degreesE, and to the location of this plume area on the bisector of a right-angled bend in the continental margin. I suggest that circumstances that led to the development of the Cameroon Line included (1) the stress field in the neighborhood of the right-angled bend in the continental margin, which favored extension normal to the bisector, and (2) formation of a zone of extension, aligned with the bisector, that joined a point in the lithosphere over the 711 plume to the continental margin. The right-angled bend in the continental margin has existed since ca. 125 Ma, and the ages and alignment of a line of intrusions cutting the continental crust in Cameroon indicate that the 711 plume has been in its present position with respect to the continental margin for the past 65 m. yr. Nevertheless, the Cameroon Line of swells formed only at 30 Ma. Some change at 30 Ma appears to have triggered formation of the line. A new, platewide pattern of shallow-mantle convection developed under the African plate at 30 Ma. For that reason, I suggest that the trigger for formation of the Cameron Line was the establishment, as part of the new platewide convection system, of shallow-mantle convection localized under the zone of extension that joined the 711 plume to the right-angled bend in the continental margin. Changes at 30 Ma fostered rapid propagation of that zone of extension, which had been established in the continental lithosphere at 65 Ma, into the Gulf of Guinea for a horizontal distance of 650 km.