SEISMOLOGICAL EVIDENCE FOR LITHOSPHERIC ALTERATION BY MELT INFILTRATION BENEATH THE EAST AFRICAN RIFT

A number of seismological studies in East Africa have focused on the northern and eastern branches of the East African Rift System (EARS). However, the seismic activity along the western branch is much more pronounced. Here, the Rwenzori Mountains, at the border between Uganda and the D.R. of Congo, are unusual as they are situated within the graben and reach elevations of up to 5100 m. We have analyzed data from local and teleseismic earthquakes to study the lithospheric structure beneath the rift and its surroundings. P-wave receiver functions reveal that the crustal thickness decreases from 32 km beneath the rift shoulders to about 24 km beneath the rift. Shear-wave splitting parameters from teleseismic phases are relatively uniform with fast polarizations oriented parallel to the rift axis and delay times of about 1 second. Splitting analyses from local events show that anisotropy within the crust and uppermost mantle does not contribute significantly to the observations. Also, the splitting observations cannot be explained by assumptions on asthenospheric mantle flow for the region. We therefore favor models of anisotropy within the lithosphere, most likely due to rift-parallel dykes or veins ascending from the asthenosphere and infiltrating the lower lithosphere to depths of about 50 km. The observation of earthquakes within the mantle at this depth may be indicative of magmatic fracturing during dyke emplacement. This interpretation is further supported by the analysis of S-wave receiver functions which provides evidence for two consecutive reductions in shear-wave speed at depths between 55-90 km and 140-200 km, respectively. Conversion depths decrease from the Tanzania craton towards the rift. We interpret the deeper discontinuity as the lithosphere-asthenosphere boundary, whereas the shallower discontinuity marks a melt-infiltration front, i.e. the upper boundary of the altered lower lithosphere.