Les unités gneissiques et la zone de cisaillement crustal du Sud-Togo (Gneissic units and crustal shear zone of South Togo)

Abstract

This publication is the result of recent geological mapping in Togo. An important crustal shear-zone in the South Togo is reviewed and the overall structural evolution of the area is examined in the light of current ideas on the chain. The geological evolution of the area occurred in three tectono-metamorphic stages in which the foliation developed and was folded under upper amphibolite facies conditions, with anatexis and the emplacement of granites, followed by transcurrent shearing under retrograde metamorphic conditions. The shear-zones, recognised in southeastern Togo, are the southward continuation of the great submeridian shear zones of the Hoggar and the Adrar des Iforas. They take the form of kilometres wide, blasto to ultramylonitic zones in which a mylonitic foliation indicates dextral ductile deformation under greenschist facies conditions. The ductile deformation evolves towards late cataclasis in the center of the shear zones. The geological evolution of the South Togo accords with the geotectonic framework of the internal Pan-African zones of the central Hoggar and Nigeria, where a polycyclic tectono-metamorphic history, with important granitization and late shearing appears to be the general case.