THE EFFECTS OF EXTENSIONAL AND TRANSPRESSIONAL TECTONICS UPON THE DEVELOPMENT OF BIRIMIAN SEDIMENTARY FACIES IN GHANA, W-AFRICA - EVIDENCE FROM THE BOMFA-BEPOSO DISTRICT, NEAR KONONGO

Birimian units of the Beposo/Bomfa district of Ghana lie across the we stern boundary of the Ashanti volcanic belt. The established Upper/Low er Birimian stratigraphic contact (or volcanic belt/sedimentary basin boundary), has been redefined as a fault zone, part of a steep, NE tre nding brittle-ductile shear zone. Siliciclastic lithofacies predominat e, with meta-argillites spanning the boundary. Volcanics are absent. A long-strike discontinuity of the Upper Birimian volcanic belt as a str atigraphic unit is emphasised. Arkosic basin-fill sediments were deriv ed from a granitic source to the west. An extensional, fault controlle d intracratonic rift is inferred as the setting for the deposition of Birimian units. Rift closure was enacted by the Eburnian Orogeny, appr oximately 2000 Ma.NW-SE directed shortening produced regional F1, N40- degrees-E trending folds. Subsequently, cleavage parallel, lateral sli p along pre-existing crustal weaknesses produced dextral shearing and NW trending, F2, flexures and folds. Repeated reactivation and flushin g of faults by mineralised fluids produced quartzitic tectonite units in which mylonite textures predominate. Late stage F3 folding about WS W-ENE axes was post-dated by late orogenic, low angle thrusts (D4). Ta rkwaian sericite quartz schists and granodiorite plutons have been aff ected by all phases of deformation.