The uppermost mantle shear wave velocity structure of eastern Africa from Rayleigh wave tomography: constraints on rift evolution

Abstract

An expanded model of the 3-D shear wave velocity structure of the uppermost mantle beneath eastern Africa has been developed using earthquakes recorded by the AfricaArray East African Seismic Experiment in conjunction with data from permanent stations and previously deployed temporary stations. The combined data set comprises 331 earthquakes recorded on a total of 95 seismic stations spanning Kenya, Uganda, Tanzania, Zambia and Malawi. In this study, data from 149 earthquakes were used to determine fundamental-mode Rayleigh wave phase velocities at periods ranging from 20 to 182 s using the two-plane wave method, and then combined with the similarly processed published measurements and inverted for a 3-D shear wave velocity model of the uppermost mantle. New features in the model include (1) a low-velocity region in western Zambia, (2) a high-velocity region in eastern Zambia, (3) a low-velocity region in eastern Tanzania and (4) low-velocity regions beneath the Lake Malawi rift. When considered in conjunction with mapped seismicity, these results support a secondary western rift branch striking southwestwards from Lake Tanganyika, likely exploiting the relatively weak lithosphere of the southern Kibaran Belt between the Bangweulu Block and the Congo Craton. We estimate a lithospheric thickness of ∼150-200 km for the substantial fast shear wave anomaly imaged in eastern Zambia, which may be a southward subsurface extension of the Bangweulu Block. The low-velocity region in eastern Tanzania suggests that the eastern rift branch trends southeastwards offshore eastern Tanzania coincident with the purported location of the northern margin of the proposed Ruvuma microplate. Pronounced velocity lows along the Lake Malawi rift are found beneath the northern and southern ends of the lake, but not beneath the central portion of the lake.

Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

“…13). Finally, the presence of a low-velocity mantle beneath the region is consistent with previous seismological results that demonstrated a marked decrease of surface waves velocity between ∼60 and 100 km depth (Fishwick & Bastow 2011;Weeraratne et al 2003;O'Donnell et al 2013).…”

Surface wave imaging of the weakly extended Malawi Rift from ambient-noise and teleseismic Rayleigh waves from onshore and lake-bottom seismometers

“…The localization of this large-magnitude, lowvelocity anomaly beneath the RVP matches lower resolution observations of low-velocity regions associated with the RVP (O'Donnell et al 2016) and other volcanic provinces along the Western Branch (Adams et al 2012;O'Donnell et al 2013). Although Western-rift eruptive centres commonly coincide with intrabasinal accommodation zones, our new results and earlier models suggest that the localization of magma is caused by processes at depth (Ruppel 1995;Corti et al 2002;Corti 2004).…”

“…A similar broadening and westward shift in the location of the RVP anomaly is observed in the Rayleigh-wave phase-velocity maps of both Adams et al (2012) and O'Donnell et al (2013). The observation of low velocities at depth outside the rift valley and volcanic province is qualitatively similar to observations from the Main Ethiopian Rift (MER), where low velocities extend west beneath the Ethiopian Plateau at depths >75 km (Bastow et al 2008;Gallacher et al 2016).…”

“…Weeraratne et al 2003;Pasyanos & Nyblade 2007;Fishwick 2010). Recent regional-scale studies using surface and body waves image a circular low-velocity region centred on the RVP within the uppermost mantle, which weakens and becomes more diffuse at greater depths (∼140 km; Adams et al 2012;O'Donnell et al 2013). The Pn/Sn study of O'Donnell et al (2016) attribute the RVP low-velocity anomaly to pervasive thermal alteration of the uppermost mantle and potentially the presence of melt.…”

Lithospheric instability and the source of the Cameroon Volcanic Line: Evidence from Rayleigh wave phase velocity tomography

“…Values and one standard deviation uncertainties for Table 3. Formal standard deviations from model parameters calculated using this methodology are frequently very small [e.g., Weeraratne et al, 2003;Adams et al, 2012;O'Donnell et al, 2013]. Therefore, for estimates on uncertainty that are likely more realistic, we refer the reader to the two sigma error estimates of our two-dimensional models ( Figure 5).…”