Kinematic evolution of the Pan-African shear zones of South Maradi, southern Niger

Abstract

The Pan-African Province (PAP) of south Maradi (Niger) corresponds to the northeastern part of the Benin-Nigerian Shield, which is part of the Pan-African Mobile Belt, located to the East of the West African Craton. This study focuses on the microstructural study supported by geochronological data of the South Maradi Pan-African shear zones. The structural analysis of the Pan-African shear bands, combined with the previous radiometric data allowed to highlight a polyphase history of this northeastern part of the Benin-Nigerian Shield. Two major tectonic periods are highlighted: A ductile to brittle Pan-African deformation stage (D1) relayed by a brittle post-Pan-African deformation stage (D2). The first phase D1 consists of two episodes noted D1a and D1b: (1) the D1a episode was a ductile pan-African deformation stage with a strong coaxial component, which favoured the development of symmetric fabrics of foliation. The tectonic regime was characterized, first by a NW-SE (~N140°) trending pure-shear-dominated shortening episode, which prevailed between 617.9 ± 2.8 to 589 ± 1.9 Ma. Then during episode D1a, the shortening direction evolved gradually from NW-SE to ~E-W (N90°-N70°). This change of direction is due to the constraint coming from the Pan-African orogeny. The deformation had progressively a rotational component and this is how D1b episode began. This latter was characterized by a brittle-ductile deformation, associated with the formation of mylonitic gneiss and micaschist. So, the counter clockwise sinistral rotation of the shortening axis Z (from N140° to N90°- N70°) reveals a Pan-African continuum of deformation between D1a and D1b episode. (2). Subsequently, a stage of brittle deformation D2 affected all the previous structures. This D2 stage is marked by a continental-scale system of conjugated strike-slip faults, NE-SW and NW-SE trending. This investigation contribute to knowledge of Pan-African kinematic evolution of Benin-Nigerian Shield, particularly its northeastern part (South Maradi).