Aeromagnetic-based lithostructural interpretation of the ilesa basement complex, Southwestern Nigeria

Abstract

Quantitative lithostructural characterisation of the Ilesha Basement Complex in southwestern Nigeria remains poorly constrained despite its documented metallogeny. This study applies high-resolution aeromagnetic (HRAM) data, processed through Reduction-to-Equator (RTE), Total Horizontal Derivative (THDR), and Analytic Signal Amplitude (ASA) filters, to delineate magnetic domains, structural lineaments, and lithologic contacts. The survey was flown at 500 m line spacing and 80 m terrain clearance and processed using Oasis Montaj. Three magnetic domains are identified: high-amplitude anomalies (55.1-155 nT) in the northwest, attributed to amphibolite and mafic-intruded migmatitic gneiss; intermediate anomalies (6.0-55.1 nT) in the east, corresponding to quartzite and quartz-schist; and low-amplitude anomalies (-76.6 to 6.0 nT) in the south, reflecting granitic intrusions of low magnetic susceptibility. Manual THDR lineament extraction yielded 64 lineaments (2.5-16 km), dominated by NE-SW orientations (92%) linked to Pan-African thermotectonism (~600 Ma), with a subordinate E-W set (8%) interpreted as relict Liberian fabric (>2500 Ma). Sequential upward continuation at 500 m and 750 m indicates that brittle fault systems in the south-southwestern sector extend to depths of approximately 0.5-0.75 km; this estimate is preliminary and should be validated using Euler deconvolution or spectral analysis in future work. The conjugate fault geometry (sinistral NNW-SSE and dextral WNW-ESE) is consistent with NNE-directed Pan-African compression. The results establish a repeatable HRAM-based lithostructural framework for poorly exposed crystalline basement terrains of West Africa. It is recommended that follow-up studies apply Euler deconvolution and geochemical sampling of the identified ASA contact zones to validate the depth estimates and lithological attributions reported here. The findings have direct practical implications for groundwater exploration, mineral prospectivity assessment, and structural hazard mapping in the Ilesha schist belt and analogous basement terrains across West Africa.