The structural development of the early Proterozoic Birimian and tarkwaian rocks of southwest Ghana, West Africa

Abstract

Rocks of early Proterozoic age belonging to the Birimian Supergroup and Tarkwaian Group make up a large part of the West African Craton. In southwest Ghana, the Birimian comprises a sequence of mostly fine-grained sedimentary/volcanoclastic rocks, which separate a series of four, northeast-trending volcanic belts. The Tarkwaian consists of coarse clastic sediments derived from Birimian and granitoid source rocks. Early formed structures in Birimian and Tarkwaian rocks include upright sub-horizontal to gently plunging folds with ongoing deformation resulting in the formation of higher strain zones located at the northwest margins of the volcanic belts. Deformation was principally one of flattering as revealed by the tight to isoclinal nature of the folds and absence of a pronounced mineral elongation lineation. Non-coaxial deformation is most clearly evident along the northwest margin of the Ashanti Belt where oblique thrusting of Birimian onto Tarkwaian rocks is observed. Deformation is attributed to a single progressive deformation event involving compression along a northeast-southwest directed axis that affected both Birimian and Tarkwaian rocks. Birimian sedimentary and volcanic rocks are intruded by the geochemically and texturally distinct Cape Coast and Dixcove granitoids. Field, petrographic and isotopic data have shown, contrary to previous interpretations, that the Dixcove granitoids within the volcanic belts were emplaced prior to regional deformation. A new model for the evolution of the Birimian and Tarkwaian is proposed and its implications are discussed.