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Upper Mantle Structure of Southern Africa from Rayleigh Wave
Tomography with 2-D Sensitivity Kernels
Aibing Li (University of Houston)
A 3-D shear wave model in southern Africa has been developed from fundamental mode Rayleigh wave phase velocities,
which are computed at the period range of 20 to 167 s using a two-plane-wave tomography method. 2-D sensitivity kernels are
applied in the phase velocity inversion to account for finite-frequency effects, which are significant at periods greater than 100
s. The new model (Figure 1 and 2) confirms the first-order observations found by Li and Burke [2006], a fast mantle lid extending
to ~180 km depth and being underlain by a low velocity zone. One new feature in the model is the vertical alignment of a
shallow low velocity anomaly with a deep high velocity anomaly at the western Bushveld province. The alignment makes more
sense for interpreting the slow 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 southern end of the Kheiss belt to the northwest
of the Kaapvaal craton is also imaged for the first time. It suggests that the hot asthenosphere outside the craton could migrate
into the craton area through a weak channel and thermally erode the cratonic lithosphere from below. In addition, low velocity
anomalies from 100 to 180 km agree well with the localities of kimberlites erupted at 65-104 Ma in the Kaapvaal craton, providing
additional evidence for the depth extent of mantle xenoliths.
References
Li, A., and K. Burke (2006), Upper mantle structure of southern Africa from Rayleigh wave tomography, J. Geophys. Res., 111, B10303,
doi:10.1029/2006JB004321.
Jelsma, H. A., M. J. De Wit, C. Thiart, P. H. Dirks, G. Viola, I. J. Basson, and E. Anckar (2004), Preferential distribution along transcontinental
corridors of kimberlites and related rocks of Southern Africa, S. Afr. J. Geol., 107, 301-324.
Yang, Y., and D.W. Forsyth (2006), Regional tomographic inversion of amplitude and phase of Rayleigh waves with 2-D sensitivity kernels,
Geophys. J. Int., 166, 1148-1160.
Acknowledgements: Data used in this study are from the IRIS DMC. Yingjie Yang kindly provided the inversion codes with 2-D sensitivity kernels.
This research is supported by NSF grant EAR-0645503.
Figure 1. Shear-wave velocity variations at 4 depth ranges. Black squares delineate
the sites of kimberlites at 104 to 65 Ma based on Fig. 7c in Jelsma et al.
(2004), which show a better correlation with slow regions in shallow upper mantle
(a and b). Red lines show the locations of profiles in Figure. 2. Dashed lines
are tectonic boundaries.
Figure 2. Shear-wave velocity profiles at the depths of 50-300 km. A relative low
velocity zone appears on all profiles at roughly 160-260 km depths. (a-c) Velocity
profiles of AA', BB' and CC' from this study. (d) Shear-wave velocity profile along
CC' from the study of Li & Burke (2006). Note the good alignment of the shallow,
slow anomaly and the deep, fast anomaly at the distance of 8 degree in (c),
which is not well imaged in (d).
2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics | II-201