Structural relationships of the younger granites of Tirmini (Damagaram south-east, Niger)

Abstract

The Tirmini anorogenic ring complex is located approximately 25 km west of the town of Zinder, in the pan-African province of Damagaram-Mounio, the southeastern terminus of the Benin-Nigeria Shield. The aim of this study is to characterise the anorogenic deformation affecting the Tirmini younger granites. The petrography of Tirmini evolves from rhyolites to microgranites, granites and quartz alkaline syenite emplaced during anorogenic magmatism in the Permian (295 Ma). Very little structural data is available on the Tirmini complex. Only a cursory study of deformation has been carried out, without characterisation of the stages of anorogenic deformation or structural interpretation. The aim of this study is to characterise the structural evolution of the Tirmini younger granites. The methodology used is based on the exploitation of satellite imagery and mapping, supported by an analysis of deformation carried out in the field using Canvas software. The geological structures revealed include 360° schistosities, fracture schistosities, detachments (dexter and sinistral) anµ diaclases. The rheology of these structures has revealed two chronologically marked stages of anorogenic deformation (i) semi-ductile to brittle, Sd1, and (ii) frankly brittle, Sd2. The first stage of deformation is contemporaneous with magmatic activity, producing 360° schistosities linked to magma swelling and fracture schistosities (S1: N45°-N60° and S2: N75°-N95°) associated with the emplacement of granites and syenite. The second brittle stage, Sd2, is characterised by a system of conjugate dextral and sinistral detachments and two families of diaclases (F1: N45°-N60° and F2: N120°-N150°) associated with the emplacement of the pluton and the cooling of the magma.