Abstract

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.

Department(s)

Geosciences and Geological and Petroleum Engineering

Comments

This study was supported by research grants from the National Program on Global Changing and Air-Sea Interaction (grant GASI-GEOGE-05), the National Natural Science Foundation of China (grant 41606043), the United States National Science Foundation (grants EAR-1009946 and EAR-1460516), the Japan Society for the Promotion of Science (JSPS grant 19H01996), and the Core Research Cluster of Disaster Science in Tohoku University (a Designated National University in Japan).

Keywords and Phrases

Anisotropic Tomography; Decompression Melting; Mantle Upwelling; The Rungwe Volcanic Province; Young Rift

International Standard Serial Number (ISSN)

0278-7407; 1944-9194

Document Type

Article - Journal

Document Version

Final Version

File Type

text

Language(s)

English

Rights

© 2020 American Geophysical Union, All rights reserved.

Publication Date

01 Feb 2020

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