The Numba ductile deformation zone (northwest Cameroon): A geometric analysis of folds based on the Fold Profiler method

Abstract

The Numba ductile deformation zone (NDDZ) is characterised by folds recorded during the three deformation phases that affected the banded amphibole gneiss. Fold-shape analyses using the program Fold Profiler with the aim to show the importance of folding events in the structural analysis of the NDDZ and its contribution to the Pan-African orogeny in central Africa have been made. Classical field method, conic sections method and Ramsay’s fold classification method were applied to (i) have the general orientation of folds, (ii) analyze the fold shapes and (iii) classify the geometry of the folded bands. Fold axes in banded amphibole gneiss plunge moderately (\(<\)15\(^{\circ }\)) towards the NNE or SSW. The morphology of F\(_{1}\), F\(_{2}\) and F\(_{3}\) folds in the study area clearly points to (i) Z-shape folds with SE vergence and (ii) a dextral sense of shear motion. Conic section method reveals two dominant families: F\(_{1}\) and F\(_{3}\) folds belong to parabolic shape folds, while F\(_{2}\) folds belong to parabolic shape and hyperbolic shape folds. Ramsay’s scheme emphasizes class 1C (for F\(_{1}\), F\(_{2}\) and F\(_{3}\) folds) and class 3 (for F\(_{2}\) folds) as main fold classes. The co-existence of the various fold shapes can be explained by (i) the structuration of the banded gneiss, (ii) the folding mechanisms that associate shear with a non-least compressive or flattening component in a ductile shear zone and (iii) the change in rheological properties of the band during the period of fold formation. These data allow us to conclude that the Numba region underwent ductile dextral shear and can be integrated (i) in a correlation model with the Central Cameroon Shear Zone (CCSZ) and associated syn-kinematic intrusions and (ii) into the tectonic model of Pan-African belt of central Africa in Cameroon.