Journal de géologie et géophysique
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IFEWARA MYLONITE: Identifying the Neo-Tectonic Overprint Using Integrated Geophysical Methods

Akinde AS, Adepelumi AA and Dikedi PN

Integrated surface electrical resistivity and ground magnetic survey was carried out across the Ifewara Mylonite zone of Southwestern Nigeria. The previous remote sensing images and Proton Precision Magnetometer (PPM) data from previous observations along four traverses were used together to map the prominent fault/shear zone in the Precambrian Basement Complex. The aim of the research work was to delineate the shear zone as indicated by Mylonite. Dipole–Dipole DC resistivity profiling data were acquired at 10 m interval along four traverses established in approximately East-West direction across the location of suspected shear zone. Magnetic data were also acquired at 5 m interval along the traverses. Topographic corrections were made on the resistivity data, which were subsequently inverted using a Finite Difference (FD) and Finite Element (FE) approach. Numerical Modelling (NM) was also carried out to confirm the existence of the Mylonite. 2D subsurface resistivity structure of the Mylonite was obtained from the inversion of the numerically modelled data. The inverted resistivity sections obtained depicted three subsurface geologic strata comprising of top soil/overburden, weathered basement and the fresh bedrock. The magnetic highs observed along the four traverses correspond to a rock type of little or no magnetic properties (quartzite) and/or rock with a possible remnant magnetisation. The magnetic lows observed are presumably characterised by rocks in mafic mineral or quartzite with magnetic impregnation. High unit of basement rock located within distinct low resistivity region and interpreted as a shear zone—represents the location of Mylonite in Ifewara.

An overprint of Mylonite is indicative of a possible fault or shear zone presence. Anomalies noted in the generated magnetic profiles correlated with the shear zones indicated in dipole-dipole resistivity profiles. Furthermore, outcome of the NM results showed Shear zone or faults, covered by thin overburden thickness, producing good responses—this depicts high-resolution resistivity images. As the thickness of the overburden increases, the resolution of the resistivity images obtained dropped. The width of the shear zone or fault play some major roles in controlling image resolution. Validity of results obtained heightened by employing a Synthetic Modelling (SM) tool.