Thrust to braiding transpression and transtension tectonics during the Paleoproterozoic evolution of the Birimian Greenstone Belt of Mako, Kédougou-Kéniéba Inlier, Eastern Senegal
Résumé
The structural cartography of the Birimian formations of the Mako area shows a polyphase deformation marked by variable structures such as imbricated shear zones, thrusts and reverse-shears, poly-foldings, “boudinages”, normal faults. The multiscale analysis of the various mapped structures combined with the satellite images allowed to distinguish three major phases of Eburnean deformations D1, D2 and D3. The D1 phase is compressive to thrust deformation due to SSE-NNW principal shortening direction which involves large overturned folds verging to the NW associated with minor thrust fault which are preserved in some lithologies (metabasalts, quartzites). The D2 phase is a sinistral transpressive deformation which comprises an early (D2a) stage followed by a late (D2b) stage. The early D2a stage is characterized by major NE-SW reverse-shears verging to the NW associated with NW-SE minor dextral shear zones. It is followed by D2b stage characterized by ENE-WSW shortening direction which creates major NNW-SSE sinistral reverse-shear zones. Interference between NE-SW and NNW-SSE major reverse-shear zone of D2a and D2b respectively creates an anatomizing pattern of the deformation. Shearing along D2 shear zone is associated with development of minor extensional zone which filled by varied magmatic products (mafic, intermediate and felsic). D3 phase is a transtension deformation associated with a dextral movement which creates conjugate normal faults with locally negative flower structures. D3 principal maximum stress is ENE-WSW oriented. The orientation of the main shortening direction of Eburnean phases D1, D3 and D3 is NNW-SSE, NW-SE and ENE-WSW, respectively. Such a pattern could be induced by an anticlockwise rotation of principal maximum stress in the Mako sector. The Eburnean granitoids emplaced before and during these three Eburnean orogenic phases.