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

SUMMARY An expanded model of the 3-D shear wave velocity structure of the uppermost mantle beneath easternAfricahasbeendevelopedusingearthquakesrecordedbytheAfricaArrayEastAfrican SeismicExperimentinconjunctionwithdatafrompermanentstationsandpreviouslydeployed 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 182s 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 secondarywesternriftbranchstrikingsouthwestwardsfromLakeTanganyika,likelyexploiting therelatively weak lithosphere ofthesouthernKibaran Beltbetween theBangweulu Blockand the Congo Craton. We estimate a lithospheric thickness of ∼150–200km 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.