Tectonic evolution of the Tombel graben basement, southwestern Cameroon

Planar structures (foliations and fractures) around the Tombel graben (southwestern end of the Central African Shear zone system) have been investigated and analyzed with the aim of unraveling the tectonic evolution of the basement. The foliations show two major trends, an older N-S-trending gneissose layering of uncertain age reworked by a later Pan-African (600 +/- 50 Ma) NE-SW ductile trend that is contemporaneous with sinistral shearing and mylonitization. The brittle phase characterized by NW-SE-trending open and partially filled fractures is younger than the mylonitization event and although it has not been dated, it is suggested that the origin of these fractures is linked to the onset of volcanism along the Cameroon volcanic line similar to 31 m.y. ago. The mylonitic foliation is recognized for the first time and supports a tectonic evolution model for theTombel graben in which ductile non-coaxial deformation was succeeded by brittle failure.