Role of pre-existing Precambrian structures and thermal anomaly in rift initiation and evolution - The Albertine and Rhino Grabens in Uganda

Abstract

We integrated Shuttle Radar Topography Mission Digital Elevation Models, airborne magnetic, radiometric, three-dimensional Full Tensor Gravity Gradiometry and Satellite gravity data to investigate the role of Precambrian structures in the evolution of the amagmatic Albertine-Rhino Grabens. The northern part of the Albertine-Rhino Graben extends within Mesoproterozoic Madi-Igisi fold belt wedged between two cratonic blocks. The southwestern part of the Albertine-Rhino Graben and the Edward-George Rift (EGR) extends within the Rwenzori and Kibara-Karagwe-Ankole orogenic belts. These extensional structures are separated by the ~5 km high Rwenzori Mountains and represent the northern segment of the Western Branch of the East African Rift System. No expression of surface volcanic activities is observed within the Albertine-Rhino Graben except the Toro-Ankole (TAVF) volcanic fields that are found within the EGR. Our results suggest that: (1) Strain localization, strain transfer, rift segmentation as well as rift termination against the Aswa Shear Zone (ASZ) as due to the presence of different Precambrian structures. (2) The ASZ extent is ~550 km in Uganda and South Sudan with a wider (~50 km) deformation belt and its evolution was due to E-W to NE-SW oblique collision between East and West Gondwana with strain localized at the boundary between the Saharan Metacraton and the Northern Uganda Terrane. (3) Rift initiation in the Albertine-Rhino Graben is associated with a thermal structure that shows shallow CPD and high heat flow beneath the EGR and TAVF. We observed a localized thin crust beneath the Rwenzori Mountains, the western Albertine border fault, EGR and TAVF due to the removal of sub-continental lithospheric mantle as a result of delamination. The northward migration of the mantle fluids within lithospheric scale Madi-Igisi Fold and Thrust Belt facilitated strain localization during Albertine-Rhino rift initiation.