Gravity models of the Bushveld Complex – Have we come full circle?

• Published 2.5D Bushveld Complex gravity models were expanded into 3D.
• New density and crustal thickness information were used in new 3D models.
• 3D modelling with Moho included is required for modelling long wavelength anomalies.
• 3D gravity modelling with Moho included shows connected eastern and western lobes.

Gravity models reveal the 3-D extent of the mafic component of the Bushveld Complex, critical for understanding the origin of the world’s largest layered mafic intrusion and largest source of platinum-group elements (PGEs). New density information, broadband seismic data, borehole data and geological discoveries have improved the constraints on the gravity modelling. Furthermore, all of the models published up to now have been done in two or 2.5 dimensions which is not well suited to modelling the complex geometry of the Bushveld intrusion. Constrained three dimensional modelling takes into account effects of variations in geometry and geophysical properties of lithologies, providing better fits to the shape and amplitude of calculated fields. Gravity data reveal subsurface density contrasts to great depths and the significant density contrast between the mafic rocks of the Bushveld Complex and the surrounding granites and sediments, as well as contrasts across the crust–mantle boundary, make gravity modelling ideal for constraining the 3D geometry of the Bushveld Complex.The aim of this paper is to demonstrate the effect of the new constraints and use of full three dimensional modelling on gravity models of the Bushveld intrusion. We remodel previously published models using full three dimensional potential field modelling software to test the existing conceptual models in an equally conceptual way. Including the measured thicker crust underneath the Bushveld Complex necessitates the presence of dense material in the central area between the eastern and western lobes. The simplest way to achieve this is to model the Rustenburg Layered Suite as a single connected intrusion. This is similar to the first geometries suggested for the Bushveld Complex. In addition to these findings, variations in the lower crust and mantle densities also contribute to models of this scale and have to be considered.

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