Geology

Simplified geological map of Liberia

Liberia is underlain by the Man Shield, which comprises two major areas of Archaean and Palaeoproterozoic rocks. The Archaean basement, which is of Liberian age (2.5–3.0 Ga) and extends across central and western Liberia, is characterised by a granite-greenstone association that is dominated by granitoid gneisses and migmatites, which are infolded with supracrustal metavolcanic and metasedimentary rocks and intruded by a younger igneous complex. The supracrustal rocks form discontinuous narrow, elongate 'schist belts'. The metamorphic grade is generally amphibolite facies with greenschist facies dominating the schist belts.

The boundary between Archaean and Palaeoproterozoic-aged rocks (the Eburnean-age province, 1.8–2.5 Ga) is not well defined in eastern Liberia, but is generally considered to lie along the north-east-trending Cestos Shear Zone. The south-eastern part of this province in Liberia, extending west from Côte d'Ivoire to Greenville, consists of tightly folded paragneiss, migmatite and amphibolite. The north-western part of the province, to the north of Greenville, has similar lithologies and geophysical characteristics, but younger isotopic ages.

An extensive Palaeoproterozoic volcano-sedimentary sequence, the Birimian Supergroup, surrounds the Archaean basement of the Man Craton along its northern and eastern margins. The supracrustal rocks are surrounded by a regionally extensive granitoid complex. Although the genetic relationship between the Birimian sequence elsewhere in west Africa and the Eburnean-age province of Liberia is unclear, they are widely considered to be equivalent.

Rocks of Pan African age (approximately 550 Ma) underlie an elongate, fault-bounded zone along much of Liberia's coastline. They comprise metasedimentary and mafic meta-igneous rocks, containing granitic bodies and subordinate noritic intrusions. Phanerozoic rocks in Liberia include extensive north-west-trending Jurassic-age dolerite dykes, minor Palaeozoic and Cretaceous sandstones, and unconsolidated Quaternary deposits.

Multiple phases of deformation are present in the Precambrian rocks. The structural trend of the rocks in the Liberian and Eburnean-age provinces is principally north-east, whereas that of the Pan African-province is mainly north-west. A number of major north-east-trending faults in eastern Liberia are extensions of regional structures, which extend into Côte d'Ivoire and include the economically important Cestos, Dugbe, Dube and Juazohn shear zones. The Lofa River Shear Zone in north-west Liberia also trends north-east. The north-west-trending Todi Shear Zone marks the boundary of the Pan African province and comprises a series of south-west-dipping faults associated with intense zones of mylonite.

The extensive Archaean and Proterozoic terranes that are present in Liberia are highly prospective for many metals and industrial minerals, but an understanding of the detailed geology is poorly known.