The uppermost mantle shear wave velocity structure of eastern africa from rayleigh wave tomography: Constraints on rift evolution

An expanded model of the 3-D shear wave velocity structure of the uppermost mantle beneatheastern Africa has been developed using earthquakes recorded by the AfricaArray East AfricanSeismic Experiment in conjunction with data from permanent stations and previously deployedtemporary stations. The combined data set comprises 331 earthquakes recorded on a total of95 seismic stations spanning Kenya, Uganda, Tanzania, Zambia and Malawi. In this study, data from 149 earthquakes were used to determine fundamental-mode Rayleigh wave phasevelocities at periods ranging from 20 to 182 s using the two-plane wave method, and thencombined with the similarly processed published measurements and inverted for a 3-D shearwave velocity model of the uppermost mantle. New features in the model include (1) alow-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 LakeMalawi rift. When considered in conjunction with mapped seismicity, these results support asecondarywestern rift branch striking southwestwards from Lake Tanganyika, likely exploitingthe relatively weak lithosphere of the southern Kibaran Belt between the Bangweulu Block andthe Congo Craton. We estimate a lithospheric thickness of ~150-200 km for the substantialfast shear wave anomaly imaged in eastern Zambia, which may be a southward subsurfaceextension of the Bangweulu Block. The low-velocity region in eastern Tanzania suggests thatthe eastern rift branch trends southeastwards offshore eastern Tanzania coincident with thepurported location of the northern margin of the proposed Ruvuma microplate. Pronouncedvelocity lows along the Lake Malawi rift are found beneath the northern and southern ends ofthe lake, but not beneath the central portion of the lake.