Geophysical characteristics of Aswa shear, Nagasongola discontinuity and ring dyke complex in Uganda

During the years 2008–2012, the geology of most of Uganda was studied within the framework of the Sustainable Management of Mineral Resources Project (SMMRP). During the project, comprehensive airborne magnetic and radiometric surveys were flown over the entire country and geological, petrophysical, geochemical sampling, geological field studies and detailed geophysical field profiles were undertaken in selected sub-areas. This report concentrates on the geophysical properties of three major geophysical structures in the area considered during the project: the Pan-African (0.6–0.7 Ga) Aswa shear zone and Nagasongola discontinuity (suture), and the 1.36 Ga Uganda–Tanzania semi-circular ‘ring dyke’ complex. The geophysical profiles across the Aswa shear indicate that the fault zone dips steeply, at about 60° to NE. The structure represents a magnetic, gravimetric (density), radiometric and topographic discontinuity, all diminishing from SW to NE across the zone. The zone is also characterized by complex radiometric anomalies. A schematic reconstruction of the evolution of the Aswa shear zone on the magnetic map suggests a nearly 60 km sinistral horizontal component of displacement along the zone. The Nakasongola zone is another distinct magnetic, gravimetric and radiometric discontinuity, interpreted to represent a collision (suture) zone, where the northern, low-magnetic block has been thrust over the southern, denser and more magnetic block. Modeling of gravity and magnetic data are consistent with a geometry in which the southern, magnetic and high-density block dips gently to great depth beneath the northern block. Bedrock exposures in both the Aswa shear zone and Nagasongola zone areas indicate a very protracted and complex history of tectonic processes commencing in the Archaean–Paleoproterozoic era and culminating in Pan-African orogenies. Both, the Aswa shear zone and Nagasongola discontinuity are cut by continuous younger dykes that show no signs of disruption, indicating that these deformation zones are presently relatively inactive. Thus, the dating of these dykes can constrain estimates of minimum activation ages of the structures that they transect. The semicircular ring dyke complex in SW Uganda, which continues southwards across Lake Victoria to NW Tanzania, is most clearly visible in magnetic anomaly maps as curved anomalies with a diameter of about 450–650 km. Magnetic profile interpretations across the dykes in Tanzania and Uganda indicate that the dykes dip at about 30–40° towards the geometric center of the concentric rings, implying a cone shaped geometry for the dykes in three dimensions. Minimal offsets in anomalies cut by dykes indicate that the dykes were not intruded within fault zones associated with major dislocations, rather that they were emplaced within extensional fractures due to uplift above a mantle plume below the conical dyke complex. The present study, based on the results of the SMMRP, gives valuable, but still preliminary new information concerning large scale tectonic processes operating in the bedrock of Uganda. The structures described here can all be potentially related to hydrothermal events and enrichment of elements of economic interest.