Imaging upper lithospheric structures of the Benue Trough and adjoining basement areas in Nigeria and Cameroon from satellite gravity data

Abstract

This study investigates the structures of upper lithosphere in the Benue Trough (BT) and its abutting basements in Nigeria and Cameroon through detailed analyses and interpretation of satellite Bouguer gravity anomaly (BA) data. Matched bandpass filtering, edge enhancements (tilt- and theta-derivatives), 3D inversion, and 2D modelling methods were applied to the BA data. The result from matched bandpass filtering reveals four gravity layer sources with their respective average depths, namely near-surface geological bodies (c. 0.6274 km), depth to basement (sediment thickness; c. 5.8921 km), intra-crustal boundary (c. 12.7538 km) and Moho (c. 27.980 km). Tilt- and theta-derivatives of the BA imaged numerous regional tectonic fabrics/structures exhibiting ENE – WSW, NNE –SSW, and NE – SW major trends coupled with N – S, NW – SE and E − W minor trends across the area, with the major regional tectonic trends mostly dominant in the BT and Adamawa Plateau/Cameroon Volcanic Line (AP/CVL) regions. Positive tilt cum positive theta derivatives of BA indicate that high-density (dense) geological bodies occur within the underlying sediments and crustal basements, particularly within the BT. 3D inversion result reveals a conspicuous and elongated (c. > 600 km long, 120 km wide) NE – SW orientated shallow (c. 23–30 ± 3 km) Moho structure beneath the BT and a prominent elongated, large (c. > 550 km long, 115 km wide) NE – SW striking deep (c. 34–44 ± 3 km) Moho structure below the AP/CVL region in the Western Cameroon Domain/Adamawa-Yade Domain (WCD/AYD) of Cameroon. Also, it shows that the Northern Nigerian Basement Complex (NNBC) and Eastern Nigerian Basement Complex (ENBC) regions are characterised by highly variable Moho morphology and depths (c. 26 to 36 ± 3 km). Generated 3D Moho model represents the first detailed Moho model for this studied area. The result from 2-D forward modelling of a NW – SE representative profile, constrained by known outcrop regional geology and available nearby seismic/teleseismic information, indicates the presence of two variable sedimentary sub-basins (c. 2–5 ± 0.4 km deep), a basement ridge and shallow Moho (c. 23–27 km ± 2 km) in BT region with occurrences of shallow intrusive bodies (e.g. granites), dense lower crustal body and deep Moho (c. 30–40 ± 2 km) underneath the basement regions, particularly in the AYD region. It also highlights that the underlying crust in BT is thinnest while thickest in basement regions of NNBC, ENBC, WCD (particularly AP/CVL area) and AYD along the NW – SE profile. The Moho depth values obtained in this study are within ±3.5 km with all those previously published from teleseismic works in the studied area. The imaged major NE – SW and ENE –WSW orientated tectonic fabrics/structures of the Precambrian crustal basement may have strongly influenced the location, emplacement and orientations of the BT, AP/CVL and Younger Granite complexes. Also, the BT developed in response to mechanical stretching (extension) of the lithosphere driven by regional far-field extensional stresses originating from remote plate tectonic/boundary forces coupled with some active-related effects (e.g. thermal upwelling of mantle materials) caused by changes in the dynamics of the sub-lithospheric mantle during the rifting in the Jurassic – Cretaceous.