Depth-dependent anisotropy and rift-induced lithospheric delamination beneath the Rwenzori Mountains, East Africa (original) (raw)

Abstract

ABSTRACT Inferences of seismic anisotropy of the lithosphere and the deeper mantle are usually based on observations of shear-wave splitting from teleseismic phases. Therefore, depth variations of anisotropy are difficult to resolve. This is related to the fact that the effects of shear-wave splitting can be described by only two parameters, even in complex 3-dimensional media, provided that the dominant period of the incoming wave is sufficiently large. The Rwenzori region of the East African rift is characterized by strong seismic activity. Recently, we observed a cluster of unusually deep earthquakes with focal depths between 50 and 60 kilometers. Receiver-functions indicate a crustal thickness of about 32 km in this region, which is characterized by the extreme uplift (> 5000 m) of the Rwenzori Mountains. Numerical models show that lithospheric delamination induced by rifting may be the cause of the deep faulting. Measurements of shear-wave splitting from these earthquakes in combination with measurements from crustal and teleseismic events allow the discrimination between anisotropic contributions from the crust, mantle lithosphere, and asthenosphere. The results show that crustal anisotropy in the Rwenzori region is highly variable and relatively insignificant in comparison to anisotropy at greater depths that affects teleseismic phases. Delay-time measurements from the earthquakes in the upper mantle beneath the rift indicate that the most significant contribution to the observed shear-wave splitting for teleseismic phases is related to seismic anisotropy at depths greater than 60 kilometers.

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