The NE-SW Trending Fotouni and Fangam Basalt Cones: Evidence of Tectonic Control on Their Emplacement

Abstract

The Fotouni volcanic rocks outcrop within mylonitic corridor, in the southern domain of the continental part of Cameroon Line (CL). The NE-SW shape display by these volcanic rocks within the mylonitic corridor is an uncommon case along the CL. Field and petrographic data and spaces image analysis were performed in order to understand the NE-SW shape display by these rocks. Geochemical data display two distinct volcanic compositional groups: alkaline lavas and transitional lavas. Alkaline series ranges in composition from basanite, basalt to hawaiite and transitional series range from basalt, trachy-basalt to basaltic trachy-andesite.
 Field investigations show that the Fotouni, Fangam, Song and Mbeng basalts outcrops as NE-SW trending prismatic dykes which displaying well differentiated vertical tetragonal, pentagonal and hexagonal prisms within the Fotouni-Kékem shear zone (FFSZ). The overall NE-SW orientation of hills, where these prisms outcrop suggests the tectonic control on the emplacement of these rocks. Indeed, the activation of the FFSZ developed various fracture networks that facilitate magma uplift and the emplacement of the Fomopéa and Bandja plutons during the Pan-African orogeny. These Pan-African activations of the Fotouni fault weaken the crust during the Paleozoic. These weaken zones constitute path way for emplacement of volcanic dykes during the Tertiary within the NE-SW faults, probably facilitated by the general extension at the end of Cretaceous.