Continuous cratonic crust between the Congo and Tanzania blocks in western Uganda

Abstract

Western Uganda is a key region for understanding the development of the western branch of the East African rift system and its interaction with pre-existing cratonic lithosphere. It is also the site of the topographically anomalous Rwenzori Mountains, which attain altitudes of [5000 m within the rift. New structural and geochronological data indicate that western Uganda south and east of the Rwenzori Mountains consists of a WSW to ENE trending fold and thrust belt emplaced by thickskinned tectonics that thrust several slices of Proterozoic and Archaean units onto the craton from the south. The presence of Archaean units within the thrust stack is supported by new Laser-ICP-MS U-Pb age determinations (2637-2584 Ma) on zircons from the Rwenzori foothills. Repetition of the Paleoproterozoic units is confirmed by mapping the internal stratigraphy where a basal quartzite can be used as marker layer, and discrete thrust units show distinct metamorphic grades. The thrust belt is partially unconformably covered by a Neoproterozoic nappe correlated with the Kibaran orogenic belt. Even though conglomerates mark the bottom of the Kibaran unit, intensive brittle fault zones and pseudotachylites disprove an autochthonous position. The composition of volcanics in the Toro-Ankole field of western Uganda can be explained by the persistence of a cratonic lithosphere root beneath the northwardly thrusted Archaean and Palaeoproterozoic rocks of westernmost Uganda. Volcanic geochemistry indicates thinning of the lithosphere from [140 km beneath Toro-Ankole to ca. 80 km beneath the Virunga volcanic field about 150 km to the south. We conclude that the western branch of the East African rift system was initiated in an area of thinner lithosphere with Palaeoproterozoic cover in the Virunga area and has propagated northwards where it now abuts against thick cratonic lithosphere covered by a thrust belt consisting of gneisses, metasediments and metavolcanics of Neoarchaean to Proterozoic age.

Long-term cooling history of the Albertine Rift: new evidence from the western rift shoulder, D.R. Congo

“…Apart from the sandstone sample DRC09-46, all samples are of igneous or metamorphic origin, with AFT ages younger than the corresponding intrusion or metamorphic ages. The AFT age of the sandstone sample is 385 ± 60 Ma, which is older than the assigned stratigraphic Permian age (Lepersonne 1974).…”

“…Right simplified geological map of the Albertine Rift with main localities (modified after Ring 2008) of East Africa, where Archean and Proterozoic basement rocks are widely exposed. In the Albertine Rift, the cratonic crust between the Congo and Tanzania cratons consists of a late Archean (~2.6 Ga) WSW to ENE trending fold and thrust belt with a complex structural framework (Link et al 2010). Since the end of the Pan-African orogeny, the tectonic evolution of the African continent was dominated by rift processes (Delvaux 1991).…”

“…Locally, Phanerozoic sedimentary and volcanic rocks cover the Precambrian basement. Sandstones, shales and siltstones of Permo-Carboniferous age (Lepersonne 1974) are exposed in depressions along the easternmost margin of the Congo Basin (Figs. 2,3).…”

“…3 Selected rock types observed on the western rift shoulder and shore of Lake Albert. a Neogene sediments from the eastern shore of Lake Albert, similar to the sediments from the western shore (620 m asl); b shale with siltstones exposed in a depression along the easternmost margin of the Congo Basin (895 m asl, close to c); c thinly banked sandstones (935 m asl, DRC09-46), b, c Permo-Carboniferous in age (after Lepersonne 1974), d deeply weathered granites, Blue Mountains, close to DRC09-34 (1,505 m asl); e basic dike cutting through granites, Blue Mountains, close to DRC09-36 (1,453 m asl); f granitic gneiss with intense deformation, down-faulted block, Semliki rift valley, close to DRC09-22 (1,049 m asl) ◂ distinguished. The central Rwenzoris are further subdivided into a northern and a southern block (Bauer et al 2013).…”

Fault kinematics and stress fields in the Rwenzori Mountains, Uganda

“…Detailed structural mapping of mainly smaller brittle faults in the Rwenzori mountains reveals complex fault 51 systems in the central Rwenzoris (Koehn et al 2008; Link et al 2010) . Polyphase stress fields induced 52 northwards directed thrusting, at least two strike slip events with roughly N-S and E-W compression and 53 SW-NE striking normal faults (Sachau, Koehn, and Passchier 2011) .…”