Kinematic evolution of the Fodjomekwet-Fotouni Shear Zone (West-Cameroon): Implications for emplacement of the Fomopéa and Bandja plutons

The Fodjomekwet-Fotouni shear zone (FFSZ) belongs to the western part of the Central Cameroon Shear Zone (CCSZ). From the border to the core, the FFSZ is made of: (i) protomylonites, (ii) garnet-sillimanite mylonites and (iii) garnet-sillimanite ultramylonites. Structural data indicate (1) a N40°E mylonitic foliation trend (strongly dipping toward NE or SW) with strike varying between N34°E-78°NW at Fotouni and N52°E-61°SE at Fodjomekwet; (2) a stretching lineation (plunging toward NE or SW) with best line varying between 224/4 at Fotouni and 232/10 at Fodjomekwet; (3) sinistral (N-S) and dextral (NE-SW) kinematic markers, respectively correlated to the D2 and D3 of the CCSZ deformation phases. The presence of relic sinistral shear markers wrapped by dominant dextral shear markers rendered difficult the understanding of the FFSZ evolution. Anisotropy of magnetic susceptibility in the FFSZ shows that magnetic fabrics are characterized by dominant NE-SW and locally N-S to NNE-SSW trending foliation and lineation. These directions show that mylonitisation was initiated in the N-S direction during the sinistral syn-D2 shearing, evolved and reoriented toward NNE-SSW and NE-SW between 613 and 590 Ma. The sinistral shearing event was thereafter followed by a NE-SW trend dextral syn-D3 reactivation of the N50°E fault between 580 and 552 Ma. The sinistral phase seems to have favoured the evolution of fractures in pull-apart structures. The pull-apart opening of fracture enables the upwelling of magma and the emplacement of the Fomopéa and the Bandja plutons (FBP). This synkinematic emplacement is witnessed by the elongated shape and the NE-SW trending direction of these massifs. Kinematic emplacement of FBP controlled by active shear zone is comparable to syntectonic plutonism in eastern Nigeria and NE Brazil.