Extreme uplift of the Rwenzori Mountains in the East African Rift, Uganda: Structural framework and possible role of glaciations

Abstract

The >5‐km‐high Rwenzori Mountains in the East African Rift form a promontory on the rift shoulder and are an extreme expression of rift‐mountain uplift. This study reports the pattern and the kinematics of major faults in the Rwenzori region. The fault pattern is characterized by a first set of N/NNE‐striking normal faults that are offset by mainly NE‐ to E‐striking faults. Fault‐slip data indicate a change in the extension direction from ENE/E to SE. The latter direction is in accord with earthquake focal data and borehole breakouts. Major faults with displacements >6 km occur at the boundaries of the Rwenzori Mountains; within the range minor faults with offsets ≤2 km have been mapped. Flexural isostatic rebound of the footwall of large‐displacement back‐to‐back normal faults bounding the Rwenzori horst and additional footwall uplift of a young crosscutting normal fault probably caused cumulative (surface) uplift of ∼3–4 km. I propose that the change in the kinematics of the rift faults and the largely contemporaneous onset of glaciation in the Rwenzori Mountains in the Middle Pleistocene were important for enhanced rock uplift. Glacial erosion rates were 1.5–4 mm/a and eroded 1–2 km of material off the top of the Rwenzoris. Glacial erosion and the retreat of the glaciers during interglacial periods caused removal of loads leading to isostatic rebound, which reduced horizontal stresses promoting normal faulting and enhanced rock uplift, thereby elevating the remaining terrain, transforming it into one with high peaks and deeply incised valleys.