Michael Lindenfeld

Crustal structure and high-resolution Moho topography across the Rwenzori region (Albertine rift) from P-receiver functions

Abstract The Rwenzori region, which is located between the Democratic Republic of Congo and Uganda, is part of the western branch of the East African Rift. With elevations of c. 5000 m a.s.l., the Rwenzori Mountains are situated between the Albert Rift and the Edward Rift segments and cover an area of approximately 120 km by 50 km. In this study we investigate the Moho topography beneath the Rwenzori region based on data from a network of 33 broadband seismic stations that were operated from September 2009 until August 2011. Variations of crustal thickness are obtained from the H-κ stacking method applied to P-receiver functions. We discuss the effect of low velocity layers within the crust on the determined Moho depths, which range from 20 km up to 39 km. The lack of a crustal root beneath the Rwenzori Mountains and its location in an extensional setting are contrary to the orogenesis generated by collisions of tectonic units. Our results indicate crustal thinning and provide evidence for the alternative mechanism of crustal bending, triggered by the tensile stress and the elasticity of the crust. Supplementary material: Examples and methods for identifying crustal structures and sediment layers are available at http://www.geolsoc.org.uk/SUP18801.

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.