NEW_Accomplishments.indd - IRIS

2006 IRIS 5-YEAR PROPOSAL SURFACE OF THE EARTH: GLOBAL STUDIES
S-Wave Velocity Structure Beneath the KAAPVAAL Craton
Angela Marie Larson, J. Arthur Snoke • Virginia Polytechnic Institute
David E. James • Carnegie Institution of Washington
Two-station Rayleigh-wave inversions for pure paths across the Kaapvaal craton of southern Africa yield velocity-depth
profiles consistent with seismic estimates obtained from approximately 100 mantle xenoliths brought to the surface in kimberlite
pipes. The cratonic xenoliths, all from the undisturbed region of the southern Kaapvaal craton and all less than 100 Ma in age,
had been previously analyzed thermobarometrically to obtain an equilibrium P-T profile of the cratonic mantle to about 180 km
depth. We use the seismic velocity-depth and density-depth profiles calculated on the basis of these P-T data and the mineral
modes and compositions of the xenoliths (James et al., 2004) to generate starting models for the inversion. The seismic velocity
and density-depth profiles from xenolith data are merged via smooth joins with those of the PREM global model for depths
of 220 km and greater. The inversion is based on a composite of 16 pure-path fundamental-mode Rayleigh-wave dispersion
curves from five events (Figure 1) across the southern Kaapvaal craton as shown in Figure 2A (Larson et al., 2005). The S-wave
velocity-depth models differ only slightly from the velocity-depth models estimated from mantle xenoliths (see Figure 2 and
caption for further explanation). S-wave velocities beneath the craton decrease slowly with depth to about 4.6 km/s at 200 km,
but never approach values that could be considered asthenospheric (i.e. 4.3–4.4
km/s). These results are consistent with other studies of cratonic areas showing
that low velocity zones tend to be very weak. Moreover, the fact that the surface
wave results are consistent with predictions from the xenolith data is a strong
indication that the velocity structure (i.e. thermal structure) of the upper 180 km
of the mantle beneath the Kaapvaal craton has not significantly changed over
the past 100 million years.
Figure 1. Epicenters of five events (stars) with great circle paths to Kaapvaal
array. Fundamental mode Rayleigh wave inversion was based on a composite
of sixteen pure-path two-station phase velocity dispersion curves with
distant station backazimuths within two degrees of the near station backazimuth
to the event (Figure 2).
James, D.E., F.R. Boyd, D. Schutt, D.R. Bell, and R.W. Carlson, Xenolith constraints on
seismic velocities in the upper mantle beneath southern Africa, G-cubed, 5, doi:10.1029/
2003GC000551 (Q01002), 1-32, 2004.
Larson, A.M., J.A. Snoke, and D.E. James, S-wave velocity structure beneath the Kaapvaal Craton
from surface-wave inversions compared with estimates from mantle xenoliths, Geophys.
J. Int. (submitted), 2005.
Snoke, J.A., and M. Sambridge, Constraints on the S-wave velocity structure in a continental
shield from surface wave data: comparing linearized least squares inversion and the directsearch
neighborhood algorithm, J. Geophys. Res., 107 (10.1029/2001JB000498 (8 pp.)),
2002.
Figure 2. (A) Map of southern Kaapvaal craton
indicating two-station Rayleigh-wave paths (blue
lines) and xenolith localities (colored diamonds).
(B) Neighborhood Algorithm (NA) fit (solid curve)
to the composite of 16 pure-path surface wave
dispersion curves (Snoke and Sambridge, 2002);
(Larson et al., 2005). (C) S-wave velocity-depth
profile from NA inversion of pure-path Rayleigh
wave data. Dashed line labeled XNLTH is the
velocity-depth profile obtained from Kaapvaal
craton xenolith data, merged with the global
PREM model for depths > 220 km as a starting
model. The maximum S-wave velocity around
4.75 km/s in the uppermost mantle decreases to
a minimum of about 4.6 km/s at 200 km depth.
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