Abstract
Birimian units of the Beposo/Bomfa district of Ghana lie across the western boundary of the Ashanti volcanic belt. The established Upper/Lower Birimian stratigraphic contact (or volcanic belt/sedimentary basin boundary), has been redefined as a fault zone, part of a steep, NE trending, brittle-ductile shear zone. Siliciclastic lithofacies predominate, with meta-argillites spanning the boundary. Volcanics are absent. Along-strike discontinuity of the Upper Birimian volcanic belt as a stratigraphic unit is emphasised. Arkosic basin-fill sediments were derived from a granitic source to the west. An extensional, fault controlled intracratonic rift is inferred as the setting for the deposition of Birimian units. Rift closure was enacted by the Eburnian Orogeny, ∼2000 Ma.NW-SE directed shortening produced regional F 1, N40°E trending folds. Subsequently, cleavage parallel, lateral slip along pre-existing crustal weaknesses produced dextral shearing and NW trending, F 2, flexures and folds. Repeated reactivation and flushing of faults by mineralised fluids produced quartzitic tectonite units in which mylonite textures predominate. Late stage F 3 folding about WSW-ENE axes was post-dated by late orogenic, low angle thrusts (D 4). Tarkwaian sericite quartz schists and granodiorite plutons have been affected by all phases of deformation.
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