Kinematic Evolution of the Nyakong-Manyi Shear Zone (Adamawa, Cameroon): Constraints from Field Observations and Microstructures, and Implication for Metamorphic P-T-t Estimation

Abstract

The Nyakong-Manyi Shear Zone (NMSZ) is a NE-SW elongated corridor found to the northwest of the Foumban-Bankim Shear Zone (FBSZ) along the Central Cameroon Shear Zone. Controversial chronology models has been proposed for the kinematic evolution of the sinistral and dextral shear phases in the Tikar Plain, thus in the FBSZ; early dextral and late sinistral shear phases for some authors and early sinistral and late dextral shear for others. Moreover, the NMSZ kinematic evolution implication on the mylonitization P-T-t path in the area seem to be problematic and the present paper aim is to clear enough those problems; since this shear zone is the main mylonitic corridor that registered the left and right lateral movement in this area. The NMSZ comprises amphibolites, protomylonites, strict sensus mylonites (garnet-kyanite-sillimanite mylonite and garnet-pyroxene mylonite), ultramylonites kyanite-sillimanite and garnet-kyanite-sillimanite gneiss. Field structures testify that the investigated area recorded three deformation phases: (i) the D₁ deformation phase which is marked by NW-SE to N-S trending S₁ metamorphic foliation with low to moderate dips (15°–45°) that was transposed during the D₂ phase, is responsible for a regional metamorphism whose mineral paragenesis is garnet-kyanite-sillimanite; (ii) the early sinistral NNE-SSW to NE-SW shear phase D₂ marked by S₂ metamorphic and mylonitic foliations; responsible for, L₂ stretching mineral lineation, F₂ fold axes and B₂ boudins structures; (iii) the late dextral NE-SW shear phase D₃, characterized by F₃ folds, B₃ boudins and ductile dextral C₃ shear planes. Mineral paragenesis garnet + kyanite + sillimanite and microstructures within gneiss testify that this rock underwent high grade regional metamorphism whose peak conditions are estimated at 11.5–13.5 kbar/850–900 °C. After the peak of metamorphism gneiss was overprinted by high grade pressure mylonitization during the early sinistral and late dextral shear deformations. Microstructural data here indicate a high-grade mylonitization whose P-T conditions are estimated at least at around 10 kbar/750 °C attained during the D₂. Shear markers, indicates that the studied area underwent an intense mylonitization at deep crustal deformation level, probably at the ductile-brittle boundary structural level during a major dextral shear deformation.

Keywords

structural and microstructural features / pressure-temperature estimation / Nyakong-Manyi Shear Zone / Foumban-Bankim Shear Zone / Central Cameroon ShearZone / tectonics / petrology

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