A rock density model for geodynamic and tectonic studies of the Southern Benue Trough in Nigeria from a tailored gravity data

Abstract

The application of gravity data in mapping of the Earth’s subsurface has steadily been on the increase globally. Gravity information is more often used to understand mechanisms associated with rock formations, interpret underground faulting and fracturing zones as well as estimating depths of underlying geological structures. A rock density mapping could be used to estimate mineral deposits, differentiate lithofacies, understand the general dynamics of heat flow, interpret geomorphologies, and determine the size and characteristics of different types of rocks in the Earth’s crust. Recently, there is a growing trend of applying an apparent density mapping technique for the estimation of rock densities from gravity data. In this study, we compute a high-resolution tailored Bouguer gravity data over the Southern Benue Trough of Nigeria and use approximate rock density ranges of some common rock types and existing geological/geophysical information to understand geodynamic processes and tectonic events predominant within the study area. We further apply the inverse density deconvolution filter (i.e., the apparent density mapping technique) to a computed short-wavelength gravity component (realized from a gravity separation approach) in order to estimate rock densities. According to our estimates, the rock densities within the study area vary between 2.50 and 2.74 g/cm3, with minimum density values attributed to volcano-sedimentary deposits along the Cameroon Volcanic Line and maximum density values at the eastern and southern parts associated with mafic igneous rocks. A comparison of estimated rock densities with available in-situ rock density data showed slightly higher rock density estimates in most cases than the in-situ rock density values at 50 sample locations. However, estimated rock densities are within the range of density variations of sedimentary and basement rocks predominant in the study area. Our gravity and density maps reveal the geometry of main geological structures dominated by sedimentary basins, igneous intrusions, uplifts, volcanoes, and diapirs occurring within the study area. Folds, faults, and fractures forming ridges and troughs in different directions (mostly NE-SW) are also manifested in those maps. The compiled maps could identify these subsurface geological structures as well as reveal different erosion patterns and landforms characteristic of the study area. The revealed patterns of crustal deformations within the study area demonstrate compressional and extensional tectonic events which may have possibly led to the faulting and fracturing systems with thermal and chemical variations among ores and gangue minerals in the area. These findings confirm the different geomorphic processes and structural deformations well-known about the study area. In conclusion, we point out that a rock density model could be an essential tool for studying geodynamic processes and tectonic events in a region since it can demonstrate mechanisms of tectonic events, patterns of deformation regimes, and mineral prospectivity of such an area.   Keywords: gravity; rock densities; tectonics; geodynamics; density inversion; mineral deposits