Correlation and Tectonic Implications of New Magnetic Lineations and Known Oceanic Fracture Zones of Gulf of Guinea Cul-de-Sac

Olufemi Oladapo Babalola

Interpretation of aeromagnetic data over the Nigerian continental margin reveals numerous lineations representing previously unknown marginal fracture zones and rifting-stage faults. Marine magnetic anomalies from published profiles enable the correlation of known oceanic fracture zones with the newly identified marginal fracture zones and faults.

Although the aeromagnetic lineations of southwestern Nigeria are not well developed, a west-southwest-east-northeast lineation just offshore Lagos is interpreted as a rift-stage fault. A discontinuous northeast-southwest lineation runs west of Benin City. East of Benin, a well-developed lineation, colinear with a steep magnetic gradient on marine profile that has not been identified as a fracture zone in previous studies, is also colinear with a fracture zone identified by previous investigators from proprietary data. The well-developed northeast-southwest lineation running through the Pennington River estuary is broken about 100 km inland from the coast before continuing toward Afikpo. This lineation is colinear with another steep magnetic gradient apparent on marine profiles, but wa not identified as a fracture zone by earlier workers.

The region of low-frequency magnetic intensity between the two preceding lineations is broken between the coast and the continental shelf by three shorter lineations and a fourth one whose significance is uncertain. It is yet to be ascertained which of the two short lineations southwest of the Forcados River estuary matches the sharp magnetic gradient mapped on marine profile. The magnetic gradient is colinear with the seismically identified Chain fracture zone farther seaward.

A short, well-developed northeast-southwest lineation runs from the continental shelf to the region of the Sangana River estuary. Farther offshore it is colinear with the Charcot fracture zone, as mapped by previous investigators from French institutions. Southeast of the Charcot trend, the lineations assume a more latitudinal trend with a well-developed east-northeast-west-southwest trend running through Port Harcourt. Farther south, three narrower lineations are present, but become less well developed toward the northeast.

The pattern of magnetic lineations and the geometry of positive and negative anomaly closures strongly suggest two distinct spreading directions: a northeast-southwest-oriented spreading over most of the Niger delta and Dahomey basin, succeeding an earlier, east-northeast-west-southwest spreading direction, localized in the southeastern Niger delta and probably in the Douala basin.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.