Till Sachau

Mountain-building under extension

A mechanism is presented which explains how intra-continental rifting can cause large topographic uplift. The effect is sufficient to account for the uplift of rift flanks and the very high and strongly localized uplift of the Rwenzori horst in the Western Branch of the East African Rift System. We propose that the uplift is generated by crustal bending, which is caused by a misfit of the lateral tensile stress between the upper and middle crust. The misfit is a function of different yield mechanisms when the upper crust breaks whereas the middle crust flows. Two independent numerical schemes confirm the suggested uplift mechanism. Both models—a 2 and 2.5 D elastoplastic lattice-particle model and a multilayer beam model—were used to calculate the surface topography as a result of lateral uniaxial extension. Using the fault geometry of the Rwenzori area, we find that the amount of topographic uplift is controlled by the viscosity and elasticity of the crust. The extreme uplift of the Rwenzori horst is—at least to some extent—a function of its considerably high elastic stiffness. The stiffness unites the two rifts that bound the Rwenzori horst and leads to an extremely high topography and a high Moho uplift in the center of the two rifts where the Rwenzori mountains sit.

Fault kinematics and stress fields in the Rwenzori Mountains, Uganda

The Rwenzori Mountains in western Uganda form an active rift-transfer zone in the western branch of the East African Rift System. Here we quantify local stress fields in high resolution from field observations of fault structures to shed light on the complex, polyphase tectonics expected in transfer zones. We apply the multiple inverse method, which is optimized for heterogeneous fault-slip data, to the northern and central Rwenzori Mountains. Observations from the northern Rwenzori Mountains show larger heterogeneity than data from the central Rwenzori, including unexpected compressional features; thus the local stress field indicates polyphase transpressional tectonics. We suggest that transpression here is linked to rotational and translational movements of the neighboring Victoria block relative to the Rwenzori block that includes strong overprinting relationships. Stress inversions of data from the central Rwenzori Mountains indicate two distinct local stress fields. These results suggest that the Rwenzori block consists of smaller blocks.

The Rwenzori Mountains, a Palaeoproterozoic crustal shear belt crossing the Albertine rift system

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.

Case studies and coupling of processes

This chapter with eight authored sections presents a selection of possible application of microdynamic simulation to address geological questions. The various processes that have been introduced in the previous chapter were used, sometimes with minor additions or modifications. Because processes in rocks never operate in isolation, the reader will see that the various authors in this chapter have combined two or more processes to simulate the microstructural development under investigation. As such the authors have fully taken advantage of the possibility of the Elle software to couple processes.