Pan-African nappe system in southwest Nigeria: the Ife-Ilesha schist belt

Abstract

This paper presents new petrostructural data and a detailed geological map of the Ife-Ilesha area, a classical schist belt in southwest Nigeria. The overall syn-metamorphic flat-lying structures, recumbent folding and associated strong boudinage of the mafic and ultramafic rocks are the result of Pan-African syn-metamorphic stacking of thrust units above the Archæan basement. Supracrustal units are considered to be part of a former late Palæoproterozoic pericratonic cover, whereas mafic/ultramafic rocks may derive from a dismembered Proterozoic sill complex. It is proposed that this area is part of a Pan-African nappe system observed at mid-crustal levels. The northeastward tectonic vergence of units reconstructed from kinematic criteria is at variance with the southwestward motion of the nappes near the Pan-African suture 400 km to the west. Such stacking of units resulting from the same northeast-southwest compression at the scale of that part of Gondwana may correspond to the closure of a former basin floored by thinned Archæan crust with abundant magmatism of late Neoproterozoic age. Post-nappe upward doming of the slightly anatectic Archæan basement and upright folding of the already formed recumbent structures were linked to the initiation of the dextral strike-slip motion along the steep north-northeast-trending Ifewara mylonitic zone that compares well with similar shear zones of the Trans-Saharan Belt in the Tuareg Shield.