Evidences of crustal structures based on geophysical data integration in the Nkolafamba- Sangmelima area (South Cameroon)

Abstract

An integrated study combining the tensor audiomagnetotellurics (AMT), aeromagnetics, remote sensing and field geological data is used to better understand the structural geology in the Nkolafamba-Sangmelima region, a southern portion of the North Equatorial Fold (Panafrican) Belt -Congo Craton transition zone located between latitudes 02º45’N-04º00’N and longitudes 11º30’E-12º15’E. The lithology of the area comprises (i) the Yaounde Group (southern member of the Panafrican belt) represented by the Yaounde series (made up of migmatised gneisses, micaschists) and the Mbalmayo series; (ii) the Congo Craton, also referred to as the Ntem complex, which is made up of various granites and charnokites of the Tonalite-Trondheimite- Granodiorite suites. Three (or four) syn-, late to post-Panafrican deformation stages are identified in the NEFB whereas three stages supposedly related to pre-, syn to post Panafrican events affect the Congo Craton. The AMT data have been collected along 6 profiles striking N135º. From data analyses, the phase and apparent resistivity pseudosections revealed or confirmed that folding and many discontinuities affected the Yaounde series, showing that the area underwent intense tectonics and that the subsurface is made up of a mixture of both Panafrican belt and Congo Craton formations. The discontinuities have been interpreted as faults and/or fractures on geoelectrical sections; active faults being related to the conductive discontinuities. The highlighted faults correlate many fractures and faults observed on outcrops from Yaounde, Nkolafamba and Loum 2. They form a network of parallel faults symptomatic of NE-SW strike-slips in the area initially assumed to be coeval to the Central Cameroon Shear Zone. Field data confirm the results and aeromagnetics is used to check the south and east continuity of this strike-slips system. Aeromagnetics covers a contiguous area south and east of the AMT surveyed zone. The data deriving from a nationwide survey carried out in years 70s, have been provided as digital total magnetic intensity map at 1:50000 scale. The reduction to the magnetic equator highlighted a main NE-SW channel of strongly magnetised anomalies which presume the dominance of the NE-SW structural direction in the area. These anomalies are interpreted as lineaments, mafic to ultramafic or subcrustal intrusions affecting both the Yaounde Group and the Ntem complex. Analytic signal outlines magnetic domains sometimes matching known formations, potential mineral host including inferred greenstones belts relics. The difference in the anomalies (lineaments) density, due to difference in their nature of rocks, enabled to differentiate the Panafrican belt from the Ntem complex on the first vertical derivative map. Some lineaments are interpreted either as faults or contacts, including the faults outlining the Congo Craton/Panafrican belt thrust limit. The dominant directions of lineaments are NE-SW to ENEWSW, WNW-ESE to NW-SE and E-W. The multiscale edge analysis using the horizontal gradient magnitude isolates the major faults and contacts; these faults are vertical to sub-vertical and extend to depth located from 621 to 4000 m (or deeper) by the Euler 3D deconvolution method. The magnetic foliation map depicts folding subsequent to E-W, NNW-SSE and N-S compressions and various shear zones affecting the study area. The faults’ geometry is consistent with both vertical movements (related to basement’s collapses) and horizontal movements (wrenches) in both the Congo Craton and the Panafrican Belt. Vertical movements and the faults’ geometry has been confirmed by geophysical 2.75 modelling which neatly locates the Congo craton/Panafrican belt between Latitudes 03º20’N and 03º15’N eastward. Magnetic study corroborates the existence of NE-SW strike-slips south of the AMT investigated area and is validated by remote sensing. Remote sensing involves Landsat 8 OLI data over the entire study area; it is particularly efficient in lineaments detection, and serves as a validation tool. The remotely sensed lineaments interpreted as faults or contacts (total of 863) maintain the dominance of the NE-SW, NW-SE and N-S directions. However, an analysis based on length classified the NE-SW, NNW-SSE to WNW-ESE among the major lineaments, forming many shear zones and strike-slips. Remote sensing analysis validate the magnetic and the AMT faults, and establishes the continuity within the findings of the study which henceforth highlights a unique sinistral NE-SW shear zone affecting both the Panafrican belt and the Congo Craton as a result of pre-Neoproterozoic events followed by syn- and post-Neoproterozoic reactivations in a continent-continent convergence setting actually marked by WNW-ESE transpressions in the two units. The study correlate and upgrade the number of greenstones relics and the faults mapped in the Yaounde region, thus, it improves the geological map of the southern Cameroon and the information on the Panafrican belt/Congo Craton transition with positive moods for mineral exploration. Also, the integration of geosciences methods and data reveals to be consistent in investigation areas with a non-uniform geophysical surveying coverage.