Anorogenic Deformations Associated with the Badaraka Alkaline Complex (Southeast-Damagaram, Niger)

The Badaraka complex is part of the Damagaram-Mounio anorogenic ring complexes linking the N-S alignment of the Air Massif complexes with those of Nigeria to the south. This Niger-Nigeria Province is a N-S alignment of complexes of Paleozoic to Mesozoic age. Badaraka is petrographically formed by an alkaline sequence evolving from trachyte to microsyenite and syenite typical of Anderson subsidence emplacement. The objective of this study is to determine the deformations associated with the Badaraka complex through microtectonic analysis. Structural analysis shows that the elliptical configuration of the Badaraka complex is the result of emplacement in an anorogenic crustal extension regime marked chronologically by two deformation phases: (i) semi-ductile to brittle D1 and (ii) frankly brittleD2. The first semi-ductile to brittle deformation phase D1 is related to piston collapse under an underlying magma chamber. It is not only responsible for the curved normal faults represented by normal fault mirrors but also for the fracture schistosities S1, N170° to N15° and S2 N75 to N95°. The latter are related to the setting of the granites that spread the trachytes. The second phase of frankly brittle deformation D2 is translated first by a system of conjugated sinister and dexter stripping which is at the origin of the elliptic form of the complex and end by a system of fracturing related to the cooling of the magma.