Litho-structural characters of the Birimian belt of Banfora (Burkina Faso-West Africa)

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Résumé

The Banfora volcano-sedimentary belt is located in western Burkina Faso within the Léo-Man Shield of the West African Craton. It is characterized by lithological and structural diversity. Petrographic studies have identified, (i) volcanic rocks (andesites, basalts and volcanoclastites), (ii) metasedimentary rocks (argillites, siltstones, shales), (iii) felsic rocks (rhyolites, rhyodacites, dacites), (iv) basic and ultrabasic rocks (gabbro, pyroxenites) and (v) intrusive granitic rocks (granites, monzodiorites, granodiorites, quartz monzonites). The
structural analysis obtained from the interpretation of the geophysical data coupled with those of the field indicates a polyphase deformation regime associated with a regional shortening of E-W to ENE-WSW orientation. This structural context has made it possible to identify and confirm the existence of three major phases of deformation (D1, D2 and D3). It reflects an evolution from the ductile deformation regime to the brittle regime. The D1 phase, linked to an E-W to ENE-WSW shortening. This phase corresponds to the ductile deformation, with a mean orientation NS, NNE-SSW to NE-SW. It is responsible for the development of the S1 foliation. The D2 phase, with a mean NE-SW orientation, is characterized by a semi-ductile to brittle deformation manifested by a schistosity of S2 fractures. It is related to NW-SE stresses and develops mainly in shear corridors. The essentially brittle deformations, oriented NW-SE to E-W, are due to the shortening NE-SW to N-S and characterize the D3 phase. It is manifested by networks of fractures and takes up the sinister strikes of the D2 phase in local dextral shears. This last phase could be linked to the establishment of NE-SW oriented dolerite veins as well as injections of mineralized quartz veins and veinlets.All of these deformations testify to a polyphase regional stress field, consistent with the functioning of the regional shear zone “Greenville Ferkessédougou-Bobo-Dioulasso” (GFBSZ), oriented NS, NNE-SSW to NE-SW, which controls the tectonic structuring and metallogeny of the Banfora volcanosedimentary belt.

Mots-clés

Banfora, belt, shear, polyphase deformation, metallogeny