Seismic images of magmatic rifting beneath the western branch of the East African rift

Abstract

We have performed a tomographic study using a joint data set that includes local and teleseismic events, recorded by a temporary network in the western branch of the East African rift system. From the travel time residuals, we derive a three‐dimensional model of seismic P‐wave velocity anomalies for the crust and upper mantle down to a depth of 80 km. Particular attention is paid to the verification of the inversion results by various resolution tests. The results show that the eastern rift shoulder is characterized by relatively high seismic velocities. Lower velocities are obtained beneath the entire length of the rift valley and the Rwenzori Mountains. A prominent feature is observed north‐east of the mountain range: here we detected a vertically oriented, cylindrical low‐velocity anomaly with maximum amplitudes in the middle crust and the upper mantle lithosphere. We suggest that this anomaly indicates reservoirs of molten material related to the ongoing rifting process within this segment of the rift. Just above this anomaly, at depths between 5 and 16 km, earthquake swarms exist. The observed reduction in P‐wave velocity is used to provide constraints on the possible melt content and temperature anomaly in the uppermost mantle. The observed 3–5% P‐velocity decrease can be explained by melt fraction up to 2%–3.3% or alternatively by a temperature increase of at least 248 to 376 K and even higher‐temperature anomalies are possible if lower ambient temperatures in the reference mantle are assumed. Probably, the two effects act in combination.