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 N, long 11.5 E, 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. Introduction: A Platewide Swell Population A platewide pattern of topographic and bathymetric swells set among irregularly shaped basins has long been a recognized peculiarity of the African plate (Holmes 1944; Nyblade and Robinson 1994). These swells have been interpreted as part of a shallowmantle convection system (e.g., Burke and Wilson 1972; McKenzie and Weiss 1975; England and Houseman 1984). My own interpretation suggests that the swells are presently rising, have all originated during the past 30 m.yr., and overlie massdeficient material in the upper mantle (Burke 1996). About 75 elliptical swells ranging from ca. 100 km to ca. 2000 km in maximum length are distinguishable (fig. 1). The exact number depends on whether Manuscript received December 7, 1999; accepted October 3, 2000. 1 Also: Visiting Scientist, Department of Terrestrial Magnetism and Geophysical Laboratory, Carnegie Institution of Washington, 5241 Broad Branch Road N.W., Washington, D.C. 20015, U.S.A.; e-mail: [email protected]. some of the smaller swells are grouped or treated as separate. Some of the swells lie on the continent and others on the ocean floor, where the size and shape of active swells are hard to define (cf. Nyblade and Robinson 1994). No swells straddle the continental margin. Volcanoes, or volcanic rocks younger than 30 Ma that have been mapped on about 60% of the swells, are generally concentrated close to swell crests. In contrast, young volcanoes and volcanic rocks, apart from carbonatites, are absent from swells on Africa’s cratons. Their absence is attributable to the thick, highly depleted mantle lithosphere under the cratons (Ashwal and Burke 1989). Either basalt has not been melted out of that lithosphere because it is too infertile or the top of any elevated asthenosphere under the swells is too deep to cross the basalt solidus. The Cameroon Line (figs. 1, 2) is a line of 10 of Africa’s volcano-capped swells, four of which lie on land and six in the Gulf of Guinea. Interest in the