Tectonic fabric of Sierra Leone, West Africa: implications for Mesozoic continental breakup

Abstract

A lineament map interpreted from Landsat images has been integrated with available geological and geophysical data from both offshore and onshore regions of Sierra Leone. The lineament patterns are related to intraplate and plate marginal reaction of pre-existing structures during Mesozoic rifting events that resulted in strong tectonic controls on magmatism. Of four main lineament trends, the NNW-SSE to N–S, NNE–SSW, and ENE–WSW trends are directly relatable to Archaean fabric in the Leo Uplift. A NW-SE trend reflects coast-parallel late Mesozoic dykes that follow the Rokelide Pan–African fabric (reactivated Archaean NNW and N–S trending structures). NW–SE trending faults defining offshore basins are segmented and offset by ENE–WSW trending continental extensions of Ocean fracture zones. Both the Guinea and Sierra Leone Fracture Zones have nucleated on ENE–WSW trending sinistral Archaean shear zones in the Leo Uplift. The four lineament trends focused Mesozoic magmatic events through protracted reactivation, Spatial and geometric relationships between the magmatic provinces and tectonic fabric indicate that intraplate deformation occurs far inland during rifting events. The angular relationships between the Permo–Triassic NE trending Guinea Belt, the Jurassic–Triassic NW–SE trending coast-parallel dykes, and the ENE–WSW trending Ocean fracture zones centred on the eariy Jurassic Freetown basic igneous complex, suggest that the Sierra Leone–Liberia continental margin evolved as an obliquely-sheared, rift–rift–transform passive margin during Mesozoic continental breakup.