Episodic collisional orogenesis and lower crust exhumation during the Palaeoproterozoic Eburnean Orogeny: Evidence from the Sefwi Greenstone Belt, West African Craton

• Published in: Precambrian research 2019-06, Vol.325, p.88-110
• Main Authors: McFarlane, H.B., Ailleres, L., Betts, P., Ganne, J., Baratoux, L., Jessell, M.W., Block, S.
• Format: Article
• Language: English
• Subjects: Collisional orogenesis; Exhumation; Palaeoproterozoic; Sciences of the Universe; Sefwi Greenstone Belt; Transtension; West African Craton
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2019.02.012

•New tectono-metamorphic maps of Sefwi Greenstone Belt, West African Craton.•Evidence of episodic collision during Palaeoproterozoic Eburnean Orogeny.•Palaeoproterozoic expression of transtension-related lower crust exhumation. New constraints on the architecture, deformation sequence and tectonic evolution of the sparsely exposed Palaeoproterozoic Sefwi Greenstone Belt (ca. 2200–2070 Ma) in SW Ghana are derived from the integration of field observations, structural mapping and interpretation of regional airborne geophysical data. Our findings provide new insights into the tectonic evolution of the West African Craton during the Palaeoproterozoic Eburnean Orogeny (ca. 2150–2070 Ma). Domains of contrasting metamorphic grade comprising high-grade paragneisses and meta-volcanic rocks are tectonically juxtaposed against low-grade volcano-sedimentary packages. The earliest deformation event is characterised by a bedding-parallel high-grade metamorphic foliation (S1) that is axial planar to isoclinal folds formed during NNW-SSE shortening and late-D1 partial melting. This crustal thickening event generated biotite ± muscovite granites emplaced parallel to the NW margin of the Sefwi Belt between ca. 2092 and 2081 Ma, providing a minimum age for D1. D2 deformation is characterised by a moderate to steeply dipping, ENE-WSW to NE-SW striking, penetrative S2 mineral foliation. Within the Sefwi Belt, coaxial F2 fold axes and L2 stretching lineations pitch shallowly ENE-WSW to NE-SW, oblique to the major shear zones, indicative of ENE-WSW transtension and associated constrictional deformation. Differential exhumation of middle and lower crustal segments along NNE-striking D2 extensional detachments was coincident with sinistral oblique reactivation of NE-SW regional-scale shear zones. The timing of exhumation is constrained by metamorphic monazite U-Pb ages at ca. 2073 Ma. Subsequent E-W shortening is associated with dextral reactivation of regional NE-SW striking shear zones and refolding of earlier structures away from the major shear zones. We propose that the NW margin of the Sefwi Belt represents the collision (D1) of southern Ghana and central and NW Ghana/Ivory Coast which record different tectonic histories. Furthermore, we suggest the amalgamation of the West African Craton is the product of episodic collisional orogenesis, providing new insights into the evolution of orogenic processes during the Palaeoproterozoic.