Abstract
This contribution discusses the development of the Palaeoproterozoic Buganda-Toro belt in the Rwenzori Mountains and its influence on the western part of the East African Rift System in Uganda. The Buganda-Toro belt is composed of several thick-skinned nappes consisting of Archaean Gneisses and Palaeoproterozoic cover units that are thrusted northwards. The high Rwenzori Mountains are located in the frontal unit of this belt with retrograde greenschist facies gneisses towards the north, which are unconformably overlain by metasediments and amphibolites. Towards the south, the metasediments are overthrust by the next migmatitic gneiss unit that belongs to a crustal-scale nappe. The southwards dipping metasedimentary and volcanic sequence in the high Rwenzori Mountains shows an inverse metamorphic grade with greenschist facies conditions in the north and amphibolite facies conditions in the south. Early D1 deformation structures are overgrown by cordierite, which in turn grows into D2 deformation, representing the major northwards directed thrusting event. We argue that the inverse metamorphic gradient develops because higher grade rocks are exhumed in the footwall of a crustal-scale nappe, whereas the exhumation decreases towards the north away from the nappe leading to a decrease in metamorphic grade. The D2 deformation event is followed by a D3 E-W compression, a D4 with the development of steep shear zones with a NNE-SSW and SSE-NNW trend including the large Nyamwamba shear followed by a local D5 retrograde event and D6 brittle reverse faulting. The Palaeoproterozoic Buganda-Toro belt is relatively stiff and crosses the NNE-SSW running rift system exactly at the node where the highest peaks of the Rwenzori Mountains are situated and where the Lake George rift terminates towards the north. Orientation of brittle and ductile fabrics show some similarities indicating that the cross-cutting Buganda-Toro belt influenced rift propagation and brittle fault development within the Rwenzori Mountains and that this stiff belt may form part of the reason why the Rwenzori Mountains are relatively high within the rift.
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Acknowledgments
We are very grateful for the suggestions of the reviewers Ruediger Killian and Enrique Gomez Rivas, which significantly enhanced the quality of the manuscript. We acknowledge funding of our research by DFG Grant KO 2463/4-2 of the Forschergruppe “Rift Dynamics, uplift and climate change in Equatorial Africa.” we thank the Uganda Wildlife Authority for their permission to work in the parks and the Ugandan National Council of Science and Technology. We acknowledge the use of the program Stereonet 6.3.2 from R.W. Allmendinger.
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Koehn, D., Link, K., Sachau, T. et al. The Rwenzori Mountains, a Palaeoproterozoic crustal shear belt crossing the Albertine rift system. Int J Earth Sci (Geol Rundsch) 105, 1693–1705 (2016). https://doi.org/10.1007/s00531-015-1167-1
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DOI: https://doi.org/10.1007/s00531-015-1167-1