Mantle Structure and Flow Beneath an Early-Stage Continental Rift: Constraints From P Wave Anisotropic Tomography

To explore 3-D seismic velocity and radial anisotropy structures of the upper mantle and mantle transition zone beneath the Malawi and Luangwa rift zones of the East African Rift System, we conduct the first study of P wave anisotropic tomography using data recorded at 75 seismic stations including 34 stations that we installed along two profiles as part of the Seismic Arrays for African Rift Initiation experiment. Both rift zones are revealed to have normal or slightly low velocity anomalies in the lithosphere and upper asthenosphere. The surrounding cratonic lithosphere is characterized by high-velocity anomalies with amplitudes ranging from +1.0% to +2.0%. Negative radial anisotropy, which is indicative of upwelling or downwelling in the mantle, is mainly distributed beneath the rift zones, whereas the other areas mostly feature positive radial anisotropy that implies horizontal flow. A prominent circular low-velocity anomaly exists in the top 200 km of the upper mantle beneath the Rungwe Volcanic Province without obvious connections to the lower mantle. Combining the present findings with previous geodetic and tomography results, we interpret the Rungwe Volcanic Province magmatism as primarily due to decompression melting in response to lithospheric extension induced by the counterclockwise and clockwise rotations of the Victoria and Rovuma microplates, respectively, with respect to the Nubian plate. Isolated mantle upwelling, which is indicated by scattered low-velocity anomalies and negative radial anisotropy beneath the Malawi rift zone, may contribute to the incipient rifting.