Shear wave model of southern Africa from regional Rayleigh wave tomography with 2-D sensitivity kernels

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SUMMARY

Rayleigh-wave phase velocities are computed at the period range of 20–167 s in southern Africa from a two-plane-wave (TPW) tomography method using 2-D, Born approximation sensitivity kernels. These phase velocities are compared with the results of the TPW method using Gaussian sensitivity functions and the correlation between the two methods is good up to 100 s. The correlation coefficient becomes less than 0.5 at periods greater than 111 s, indicating significant finite-frequency effects at long periods. The correlation of phase velocity between adjacent frequencies increases with period for inversions using the 2-D sensitivity kernels, while such a systematic trend is not observed for inversions using Gaussian sensitivity functions. In addition, low phase velocity anomalies at long periods (111–167 s) are better and more consistently imaged from inversions using 2-D sensitivity kernels. To account for finite-frequency effects and improve resolution, the 2-D sensitivity kernels are therefore recommended in regional TPW tomography.

A 3-D shear wave model is developed from the phase velocities obtained in southern Africa. The new model confirms the existence of a fast mantle lid underlain by a low-velocity zone revealed in a previous model constrained from the TPW method using Gaussian sensitivity functions. One new feature in this model is the vertical alignment of a shallow low-velocity anomaly with a deep high-velocity anomaly at the western Bushveld province. The alignment provides new evidence to support the interpretation of the slow anomaly as the result of high iron content from the Bushveld intrusion and the fast as a more depleted residual mantle. A low-velocity channel at the depths of 220–310 km from the Namaqua–Natal Belt to the northwest of the Kaapvaal Craton is also imaged. It suggests that the hot asthenosphere beneath the mobile belt can migrate into the craton area through a weak channel and produce thermal disturbance at the base of the craton. Further more, low-velocity anomalies from 100 to 140 km in the 3-D model agree well with the localities of kimberlites erupted at 65–104 Ma in the Kaapvaal Craton. This observation supports that kimberlites are generated in the lithosphere by thermal disturbance and provides additional evidence for the depth extent of mantle xenoliths.

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