Shear Wave Speed Imaging of the Magma-poor Western Branch of the East African Rift

Abstract:

The East African Rift (EAR) is an active continental rift zone including the magma-poor western branch and the magma-rich eastern branch. Although it is known that melt products play an important role in initiating rifting in the magma-rich rifts, the mechanism initiating the magma-poor rifts is still under debate. To better image the melt distribution in the crust and the uppermost mantle beneath the region of the western branch of the EAR, this study inverts multiple data sets including Rayleigh wave phase dispersion and ZH ratios derived from ambient noise interferometry and teleseismic P waveforms. The measurements involve recordings from 232 broadband stations (17 networks) deployed between 1994 and 2019. The phase-velocity maps based on the Rayleigh wave dispersion in the period range of 5–45 seconds already indicate that the low-velocity zones are located in the vicinity along the rift axis of the western branch in the crust but are more concentrated at the northwestern flank of the Malawi Rift in the lower crust and the uppermost mantle. With additional measurements incorporated in the joint inversion from Rayleigh wave ZH ratios and teleseismic P waveforms, an improved shear wave speed model with more robust constraints on the sedimentary basement and Moho depths will provide important insight into the mechanism initiating the magma-poor rift, especially for the Malawi rift which has the best data coverage.