An offshore geophysical survey of the Freetown igneous complex, Sierra Leone

Abstract

Gravity, magnetic and bathymetric surveys have been carried out to determine the offshore extent of the basic igneous intrusion exposed on the Freetown Peninsula, Sierra Leone. Although the westward continuation of the complex is not expressed bathymetrically, positive free-air anomalies show that it is an elongated feature, some 150 km in length, with a NW-SE trend. Gravity modelling indicates that the intrusion has a maximum width of about 50 km and reaches depths exceeding 17 km southwest of Freetown, where free-air values exceeding 150 mGal have been observed. The thickness of the body decreases towards the north as the top dips under the sediments of the outer continental shelf. To the west of the complex, a WNW-ESE gravity low on the outer shelf reflects a basement depression which is filled with almost 6 km of sediments. Large magnetic anomalies, with a total range of more than 1000 nT, are recorded over the Freetown intrusion. The dominant feature of the magnetic field is an area of negative anomalies within the complex, flanked by positive anomalies west of Freetown. Two-dimensional magnetic models show that the main part of the complex is normally magnetised but large positive magnetic anomalies also indicate the presence of reversely magnetised zones within the intrusion. The linearity and trend of the complex are consistent with an igneous origin which is closely related to Atlantic rifting and early movements along the Guinée Fracture Zone.