The Rwenzori mountains lie in the western part of the East African Rift System (Fig. 1). The eastern and the western branch surround the Tanzania craton and the young rift faults shown in red follow mainly old Proterozoic mountain belts. The rift crosses the Toro belt in the northernmost part of the western branch and continues into the Archaean Congo Craton. Exactly at this location we find the Rwenzori basement block that was uplifted during extension up to 5000m. Extension in this area of the rift is about 2.1mm per year with the large Tanzania block rotating anticlockwise (Stamps et al., 2008). Figure 2 shows a close-up of the geology of the Rwenzori mountains illustrating how the block is mainly surrounded by rifts with young sediments shown in yellow colour. A basement bridge that connects the Rwenzori with the Ugandan platform only exists in the Fort Portal area northeast of the mountains. The green Paleoproterozoic units cross the Rwenzoris and make up the central high mountains (including mount Stanley with Margherita peak and mount Baker) where Amphibolite is very resistant to weathering. The topography of the mountains that was partly carved out by glaciers is illustrated in the digital height model in Figure 3. Field photographs in Figure 3 show how the landscape and vegetation changes. The west of the mountain is dominated by rain forest whereas the east shows Savanna. Hiking up the mountain leads through rain forest, bamboo forest, the cloud forest zone, the Afro-alpine zone with giant Lobelias up to the glacier zone with the Stanley plateau and the two highest peaks, Margherita and Alexander. We have developed numerical models to simulate how the Rwenzori basement block can be captured by propagating rift segments and how it is uplifted with the rift flanks. Figure 4 shows results of the numerical model where the Earth's crust is modelled as a thin elastic sheet that can fracture and produce brittle rifts and the lower crust is modelled as a visco-elastic sheet that is attached to the crust. Propagating rifts are shown in black and white vertical lines are used as markers to illustrate rotation of blocks. Rifts propagate towards each other, repel and capture basement blocks that are rotating clock- or anti-clockwise depending on the direction of rift overstep. From the simulations we argue that the Rwenzori mountains are captured by a southwards propagating lake Albert rift and a northwards propagating lake Edward rift and that the mountain is rotating clockwise in an opposite direction than the large Tanzania block (Fig. 1). The rotation of the two blocks leads to unusual fault networks around the mountains and produces forces that may add to the uplift of the block. In a second numerical model we simulate two-dimensional cross-sections through the Earth's lithosphere in order to understand how the rift flanks and the Rwenzori block are uplifted during extension. In our model the material behaves brittle and fractures on short time scales and flows in a viscous manner on long time scales. Figure 5 shows an example of a model outcome with two parallel rifts and an uplifted basement block (horst) in between. The crust is shown in yellow and blue colour and the mantle lithosphere in green, the model is 1:1 and has a width of 120km. The model is extended and produces two deep rift valleys, two rift flanks and an uplifted and tilted horst block. We propose that a combination of these two models, the rotation of the Rwenzoris and the rift flank uplift during stretching can produce the high mountains that we find.
Figure 1: Large scale map of the East African Rift System showing how the main branches run through Proterozoic mobile belts. Only the northern-most branch of the western rift crosses the Toro Belt and reaches into the Archaean Craton. Opening Vectors are in mm/year from Stamps et al., 2008.
Figure 2: Geological map of the Rwenzori area. Young rift-related faults are shown as black lines. Opening vector from Stamps et al., 2008.
Figure 3: Digital height model of the Rwenzori area shown in Fig. 2. Low areas in blue and high areas in red colour.
Figure 4: Numerical simulations of Koehn et al., 2008 show how propagating rift segments can capture basement blocks. The captured block rotate clock- or anti-clock wise depending on the direction of rift overstep. Rifts are shown in black, white lines are markers to illustrate the rotation.
Figure 5: Numerical simulations of an extending rift system with two rifts and a horst. The model represents cuts through the lithosphere with the mantle in green and the crust in yellow and blue colours. Extension is horizontal, the cross-section is on a 1:1 scale and the width of the model is 120 km. Faults are shown as black lines. In the model material can behave elastic and break on short time-scales and can flow in a viscous manner on long time-scales. Note how the lower crust is flowing but the upper mantle contains two small faults. Rift flanks and the central basement block are uplifted and partly tilted.