Integrated geophysical mapping of the Ifewara transcurrent fault system, Nigeria

Abstract

Integrated geophysical methods involving magnetic and dipole–dipole resistivity (DDR) were conducted across a prominent zone of weakness clearly observable in Landsat MSS and SLAR images in the Precambrian basement complex of southwestern Nigeria. Up till now, the location and existence of this megascopic structure have not been confirmed using geophysical methods. With the objective of delineating this weak zone and its structural attributes, three traverses were established at 500 m intervals across it, and geophysical measurements were made at 10 m intervals along these traverses. Qualitative interpretation of the magnetic data obtained shows a diagnostic signature of a near-vertical fault, trending along a NNE–SSW direction. Also, the quantitative interpretation of the data using the non-linear least-squares regression technique indicates that the width of the magnetic anomaly ranges from 90 to 150 m, its dip angle varies between 75° and 85°; the anomaly is concealed by a regolith of approximately 15 m thickness. Furthermore, a 2D resistivity inversion of the field resistivity data reveals a three-layer model, representing thin resistive topsoil underlain by weathered bedrock, resistive bedrock with a distinct low resistivity zone located within the bedrock. The most plausible explanation for this low resistivity zone is that it was formed by shearing activities during Late Precambrian times. Conclusively, the integrated approach employed in this research confirms the existence of the supposed Ifewara shear zone (ISZ).