Improved Mapping of the Structural Disposition of Some Younger Granite Ring Complexes of Nigeria Using High Resolution Aeromagnetic Data

Abstract

Surface and subsurface structural interpretation over a part of the Younger Granite province of Northern Nigeria using Aeromagnetic data was carried out to demonstrate the use of recently available high resolution aeromagnetic data in mapping precisely the Younger Granite. State of the art software packages were used in data reduction, processing/analysis and interpretation. Results of the aeromagnetic study show that the distinct magnetic anomalies clearly revealed in the area coincide spatially with outcropping ring complexes. Apart from outcropping ring complexes, some ring complexes without surface exposures (unexposed ring complexes) were also delineated. One of such ring complexes was studied in greater details and its lateral dimensions with that of the adjoining ring complexes were better resolved. The unexposed ring complex was found to have a surface area extent of approximately 169.5 km2 which is similar to that of Ririwai (180 km2), Banke (128 km2), Kudaru (174 km2) and Zuku (121 km2). The results of the 3D Euler solutions indicate that the depths of the ring complexes range from outcrop to 1800 m. For the inferred unexposed ring complex, the depth ranges from 5 to 1200 m. The Younger Granites show low Source Parameter Imaging (SPI) depth values of about 4 km below the sea level indicating deeper magnetic sources. Three (3) profiles drawn along the NESW, NNW-SSE and E-W directions on the Analytic Signal map of the study area gave an overburden thickness of less than 20 m that masked the unexposed ring complex with its mappable root situated at approximately 200 m below the sea level. The Ririwai, Tibchi, Banke, Amo, Kudaru and Zuku ring complexes have outcrops of approximately 1300 m, 950 m, 920 m, 1400 m, 1150 m and 1220 m respectively above mean sea level with their mappable roots situated at approximately 400 m, 180 m, 400 m, 800 m, 10 m and 300 m respectively below the sea level. It delineated numerous lineaments as structural features whose major trend is along the NE-SW direction. It was equally observed that the ring complexes of Northern Nigeria display diverse surface plans/shapes and exhibit both inward and outward dipping structural orientations with depth. The likely reason for the unexposed nature of the newly proposed ring complex could be as a result of partial subsidence of the enclosed block on which the extrusive edifice of the central volcano originally rested probably occasioned by excess space (much subsurface accommodation) for continuously rising magma within the country rock. Its geometry is supportive of this assertion as it revealed majorly outward dipping trend with depth along different axis. Another reason could be due to incomplete fluidization along the already formed ring fracture and poor assimilation of the country rock by the fluidizing agent amidst a slow/near quiescent piece meal stopping process during the central granite intrusion phase which usually marks the end of ring complex formation. Finally, this research has demonstrated the efficacy of aeromagnetic data in delineating both outcropping ring complexes as well as those without surface exposures (outcrops).