Seismicity Patterns and Magmatic Processes in the Rwenzori Region, East-African Rift
Abstract
The 5000m high Rwenzori Mountains are situated within the western branch of the East African Rift System (EARS), at the border between Uganda and the Democratic Republic of Congo. They represent a basement block located within the rift valley whose origin and relation to the evolution of the EARS are highly puzzling. During a recent seismological campaign we located more than 800 earthquakes per month with magnitudes ranging from 0.5 to 5.1. Vertical sections across the northern parts of the Rwenzoris show, that west of the mountains (towards the rift valley) the focal depths range from 10 to 20 km, whereas the hypocentres go as deep as 30 km on the eastern side. This is in good agreement with Moho-depths derived from receiver functions and implies that all of these events are located within the crust. However, about 30 km east of the northern mountain ridge we located a cluster of 7 events that exhibit an anomalous depth of about 60 km. We can confidently locate these earthquakes within the mantle lithosphere beneath the rift. The existence of earthquakes at this depth is enigmatic, especially within a rifting regime were one expects hot and weak material relatively close to the surface. We think that these events are possibly related to the evolution of the Rwenzori Mountains. A recent hypothesis to explain the extreme uplift of the Rwenzori Mountains is rift induced delamination (RID) of mantle lithosphere. Here we show that the RID-process is indeed capable of explaining the seismic energy release in the mantle. However, in view of the specific hypocentral location of the event cluster, magmatic impregnation processes associated with dyke propagation into the mantle lithosphere may be a more realistic cause for seismic radiation at the observed depth. Crustal earthquakes northeast of the Rwenzori area are relocated with a double-difference algorithm to improve the spatial resolution of seismicity pattern. Several event clusters in the vicinity of the Fort Portal volcanic field form pipe-like structures with vertical extensions of 3 to 6 km and diameters of 1 to 2 km. In this region the rifting process is probably still in an early stage. The structures possibly indicate magmatic feeding channels through the crust that originate from the heated and impregnated lithospheric mantle.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.T31C2192L
- Keywords:
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- 7230 SEISMOLOGY / Seismicity and tectonics;
- 7230 SEISMOLOGY / Seismicity and tectonics;
- 8105 TECTONOPHYSICS / Continental margins: divergent