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S U M M A R YRecordings from a temporary seismic network in the Rwenzori region of the East African Rift show seven earthquakes at a depth range between 53 and 60 km. Evidence from receiver functions indicates a crustal thickness of about 32 km in this region suggesting that the earthquakes are located well within the mantle beneath the rift. The occurrence of faulting at this depth is of relevance for models of continental-rift initiation and related deep magmatic processes.

New structural and seismologic evidence from the Rwenzori Mountains, Uganda, indicate that continental rifts can capture and rotate fragments of the lithosphere while rift segments interact, in a manner analogous to the interaction of small-scale fractures. The Rwenzori Mountains are a basement block within the western branch of the East African Rift System that is located at the intersection of two rift segments and is apparently rotating clockwise. Structural data and new seismological data from earthquake epicentres indicate a large-scale, 20-km-long transsection fault is currently detaching the Rwenzori micro-plate on its northern margin from the larger Victoria plate (Tanzania craton), whereas it is already fully detached in the south. We propose that this fault develops due to the rotation of the Rwenzori block. In a numerical model we show how rift segment interaction, block rotation and the development of transsection faults (faults that cut through the Rwenzori Mountains) evolve through time. The model suggests that uplift of the Rwenzori block can only take place after the rift has opened significantly, and rotation leads to the development of transsection faults that connect two rift segments, so that the block is captured within the rifts. Our numerical model suggests that the majority of the uplift has taken place within the last 8 Ma.

Abstract.We have analysed the microseismic activity within the Rwenzori Mountains area in the western branch of the East African Rift. Seismogram recordings from a temporary array of up to 27 stations reveal approximately 800 events per month with local magnitudes ranging from -0.5 to 5.1. The earthquake distribution is highly heterogeneous. The majority of located events lie within faults zones to the east and west of the Rwenzoris with the highest seismic activity observed in the northeastern area, where the mountains are in contact with the rift shoulders. The hypocentral depth distribution exhibits a pronounced peak of seismic energy release at 15 km depth. The maximum extent of seismicity ranges from 20 to 32 km and correlates well with Moho depths that were derived from teleseismic receiver functions. We observe two general features: (i) beneath the rift shoulders, seismicity extends from the surface down to ca. 30 km depth; (ii) beneath the rift valley, seismicity is confined to depths greater than 10 km. From the observations there is no indication for a crustal root beneath the Rwenzori Mountains. The magnitude frequency distribution reveals a b-value of 1.1, which is consistent with the hypothesis that part of the seismicity is caused by magmatic processes within the crust. Fault plane solutions of 304 events were derived from P-polarities and SV/P amplitude ratios. More than 70 % of the source mechanisms exhibit pure or predominantly normal faulting. T-axis trends are highly uniform and oriented WNW-ESE, which is perpendicular to the rift axis and in good agreement with kinematic rift models. At the northernmost part of the region we observe a rotation of the T-axis trends to NEN-SWS, which may be indicative of a local perturbation of the regional stress field.

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