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 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 centers 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.
Highlights
A fundamental question in continental dynamics asks how strong lithospheric plates rupture, given the modest magnitude of available tectonic forces
Using a ray-theoretical approximation, we estimate that these maps satisfy approximately 80 per cent of the variance observed in the ambient-noise phase velocities, and 60 per cent of the variance in the individual ASWMS phase delays, compared to a regional 1-D starting model from the nearby Rukwa rift (Kim et al 2009)
We find that all period bands are sensitive to the presence of a sedimentary layer at the top of the crust
Summary
A fundamental question in continental dynamics asks how strong lithospheric plates rupture, given the modest magnitude of available tectonic forces (i.e. slab pull and ridge push; e.g. Forsyth & Uyeda1975; Bott 1991). A fundamental question in continental dynamics asks how strong lithospheric plates rupture, given the modest magnitude of available tectonic forces Lithospheric strain localization and strength reduction during early-stage rifting is achieved in models that include weakening mechanisms like magmatic intrusions to enable continental rupture (Buck 2004, 2006). In the past two decades, field, laboratory and modeling studies of mature continental rift systems.
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