Integrating Gravity and Magnetic Data with Remote Sensing in Structural Modelling of Benue Trough (Middle and Lower) of Nigeria

The Benue trough is a rift-related basin associated with interesting geological and structural features, but these features are barely observed on the surface due to sediment and vegetation cover. An integrated approach of combining remote sensing with gravity and magnetic data sets has been presented in this research. A new geology map was constructed for the area from the existing literature map and remote sensing data sets. Surface to near-surface/shallow lineaments and other non-linear geological bodies were mapped from remote sensing, residual gravity, and magnetic data sets while deep lineaments interpreted as basement lineaments were mapped from regional gravity and magnetic data sets. The study isolated tectonic-related lineaments (faults/fractures/shear zones) from non-tectonic (dykes and lithological contacts) and non-linear geological bodies (sills, volcanic rocks, plugs, etc.). The tectonic-related lineaments were interpreted as faults and grouped into four major types based on their orientations: the NE-SW, NW-SE, ENE-WSW, and N-S trending normal faults. This study interpreted two main tectonic regimes in the area; the extensional and the compression events. The extensional regime involved the formation of the NE-SW, ENE-WSW, NW-SE, and N-S trending normal faults during the Early Aptian (142 Ma), Late Aptian (120 Ma), the Albian Age (119-101 Ma), and the Maastrichtian (65 Ma) when the regional maximum extensional stress fields acted in NNW-SSE, N-S, NE-SW, and ENE-WSW extensional directions, respectively. The compressional regimes are characterized by the reversal in the stress field which occurred mainly during the Late Santonian (84 Ma). It is interpreted as responsible for basin inversion, fault reactivation, shortening, and the deformation of pre-existing sediments which produced several sub-basin, uplifted block, folds, etc.
The basement is estimated to have a maximum depth of 9 km and a shallowest depth estimate of 4 km within the basin based on the pseudo-gravity inversion method, and its morphology revealed several areas of basement subsidence and uplift. Four 2D gravity and magnetic model profiles were generated in the NW-SE trend direction and three major subsurface depth interfaces were identified as basement, Curie, and Moho depth interfaces with an average maximum depth of 9.65 km and minimum average depths of 12.50 km and 19.02 km, respectively, measured from topography surface. The model showed deepening of the trough to a maximum depth of about 10 km and shallowing of Moho to a depth of about 19 km, which generally indicated a thinned crust and Moho doming beneath the trough. The study also modelled a 3-D variation in density and susceptibility contrasts across the basin and obtained Moho interface from gravity data sets and areas of magmatic intrusions from magnetic data sets. The spatial distribution of the Moho interface was modelled and combined with the 3-D depth estimate from the pseudo-gravity inversion method to reveal the crustal structure for the whole trough. The crust is thinned to about 14 km in the northern part of the area while the crust thinned to about 9 km in the south especially at the centre of the trough. This has been interpreted to be because of the crustal extension during rifting. The amount of crustal extension varies between 93 km in the north and 142 km in the south giving the trough a ‘scissors style’ extensional opening while the rate of extension determined from stretching factor estimation increases from the Middle Benue Trough to the Lower Benue Trough. The lithosphere (the crust and upper mantle) analysed from the spectral technique has been interpreted to be stronger in the Middle Benue Trough with an average T_(e )value of 15 km than the Lower Benue Trough with an average T_(e )value of 10 km where T_(e ) acts as a proxy for the lithosphere strength. The thermal structure predicted from power spectrum and three-dimensional magnetic inversion techniques have been interpreted to influence the long-term strength of the lithosphere as well in this study. This research has further shown that the strength of the lithosphere and the Curie depth with values ranging between 5 km and 32 km, are directly related i.e., areas with high values of T_(e,) are correlated with areas of high Curie depth values even if the area is too small to map spatial variation of T_(e ). This study also showed that a mantle temperature of 1400 ℃ between Aptian and Albian times (125-100 Ma i.e., 25 Ma) produces about 9 km of magma melts from the lithosphere to be emplaced within the trough.