Surface-wave imaging of the weakly-extended Malawi Rift from ambient-noise and teleseismic Rayleigh waves from onshore and lake-bottom seismometers.
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Date
2017
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Publisher
Geophysical Journal International
Abstract
Located at the southernmost sector of the Western Branch of the East African Rift System,
the Malawi Rift exemplifies an active, magma-poor, weakly extended continental rift. To
investigate the controls on rifting, we image crustal and uppermost mantle structure beneath
the region using ambient-noise and teleseismic Rayleigh-wave phase velocities between 9
and 100 s period. Our study includes six lake-bottom seismometers located in Lake Malawi
(Nyasa), the first time seismometers have been deployed in any of the African rift lakes. Noise
levels in the lake are lower than that of shallow oceanic environments and allow successful
application of compliance corrections and instrument orientation determination. Resulting
phase-velocity maps reveal slow velocities primarily confined to Lake Malawi at short periods
(T <= 12 s), indicating thick sediments in the border-fault bounded rift basin. The slowest
velocities occur within the Central Basin where Malawi Rift sedimentary strata may overlie
older (Permo-Triassic) Karoo group sediments. At longer periods (T > 25 s), a prominent
low-velocity anomaly exists beneath the Rungwe Volcanic Province at the northern terminus
of the rift basin. Estimates of phase-velocity sensitivity indicates these low velocities occur
within the lithospheric mantle and potentially uppermost asthenosphere, suggesting that mantle
processes may control the association of volcanic centres and the localization of magmatism.
Beneath the main portion of the Malawi Rift, a modest reduction in velocity is also observed
at periods sensitive to the crust and upper mantle, but these velocities are much higher than
those observed beneath Rungwe.
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Keywords
Seismic instruments; Seismic noise; Seismic tomography; Surface waves and free oscillations; Continental tectonics: extensional.