Structural characterization of the Pan-African Ndieki area in the Foumban-Bankim Shear Zone (West Cameroon): constraints from field observations and microstructures

摘要

The Ndieki area displays elongated NE-SW hill which crop out along the Foumban-Bankim Shear Zone. In petrographic point of view, it is made up of amphibolites, biotite-hornblende gneisses, and two-mica granites, all of which are wrapped by mylonites. Structural data indicate (1) NE-SW (N30 degrees E) to ENE-WSW (N70 degrees E) trending foliation, dipping moderately (50 degrees -60 degrees) towards NW; (2) stretching lineation striking NNE-SSW (N12E-N25E) and moderately (25 degrees -35 degrees) plunging towards SSW in mylonite and biotite-hornblende gneisses. It strikes NE-SW (N40E) and plunges strongly (66 degrees -81 degrees) towards SW in the two-mica granites; and (3) asymmetric folds (NE-SW) and sigma -type displaying dextral kinematics evolution in mylonites and biotite-hornblende gneisses, and N-S sinistral shear planes in mylonites and biotite-hornblende gneisses respectively correlated to the dextral transpressive and sinistral transcurrent deformation. All these rocks show that microstructures were acquired from the sub-magmatic to the solid-state deformation. The structural features show that the mylonitization was initiated in the early NE-SW dextral deformation event which has favored the evolution of fractures in pull-apart structures that induce the upwelling of magma and the emplacement of the Ndieki pluton, followed by the N-S trend sinistral shearing. Structural, kinematic features of the Ndieki pluton characterized deep crustal deformation, probably at the ductile-brittle boundary structural level during a major dextral shear deformation.