POST-RIFT VOLCANISM AND EXTENSION IN THE GUINEA PLATEAU, WEST AFRICA INTERPRETED FROM 3-D SEISMIC DATA: POTENTIAL LINK TO GRIMALDI SEAMOUNT-CHAIN MAGMATISM

Systems of rift-related fracture zones with two distinct orientations converge at the Guinea Plateau, West Africa and influence the geometry of the continental margin. One fracture zone orientation formed as a result of the Jurassic opening of the North Atlantic Ocean and the other from the Cretaceous opening of the South Atlantic Ocean. Detailed mapping on the Guinea Plateau using 3-D seismic data in two separate areas (Surveys A and B) highlights cylindrical features that are capped by a Late Paleocene unconformity. Most of these seismic anomalies range up to 2 km in diameter, but one 20-km-diameter feature covers the northwestern corner of one of the two 3-D seismic surveys. The larger structure, referred to informally as the Volcán Structure, has a large gravitational response that highlights its aerial extent, but lacks a clear magnetic signature. High seismic amplitudes, non-diapiric morphologies, and onlap onto the flanks suggest these features were created by extrusive volcanic events. Post-rift faulting strikes northwest in both 3-D surveys, suggesting a regional tectonic component. However, the differing characteristics, timing, and extension profiles of the faults suggest that the interpreted volcanism in Survey A and subregional mass-wasting in Survey B strongly influenced local faulting. The interpreted age of the volcanic field on the plateau is consistent with the Late Paleocene and younger Grimaldi seamounts toward the west; in addition, the Volcán Structure is on the same order of scale as the seamounts. Due to the spatial correlations and timing, the post-rift volcanism on the Guinea Plateau is likely linked to the same magmatism that created the Grimaldi seamount chain that exploited crustal weaknesses associated with the Guinea Fracture Zone.