HR: 08:00h
AN: V31F-01    [Abstracts]
TI: S-wave Velocity Structure, Mantle Xenoliths, and the Upper Mantle Beneath the Kaapvaal Craton
AU: Snoke, J A
EM: snoke@vt.edu
AF: Virginia Tech, Department of Geosciences, Blacksburg, VA 24061 United States
AU: Larson, A M
EM: alarson@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AU: * James, D E
EM: james@dtm.ciw.edu
AF: Carnegie Institution/DTM, 5241 Broad Branch Rd., N.W., Washington, DC 20015 United States
AB: Inversion of two-station Rayleigh wave fundamental-mode phase velocities across the undisturbed region of the southern Kaapvaal craton south of the Bushveld Province produces velocity-depth models quantitatively similar to those estimated from low-T mantle xenoliths brought to the surface in Cretaceous age kimberlite pipes erupted in the same region. The cratonic xenolith suite was previously analyzed thermobarometrically and chemically to obtain the equilibrium P-T conditions from which the seismic velocities and density of the cratonic mantle to about 180 km depth were calculated (James et al., G-cubed, January 3, 2004). As the xenoliths represent a snapshot of the mantle at the time of their eruption, comparison with recently recorded seismic data provides an opportunity to compare and contrast the independently gained results. We form a composite reference velocity model using xenolith values for the depth range 35-180 km with an interpolated join to PREM for the depth range 220 km to 500 km, and a regionally determined crustal model for the upper 35 km. This composite served as the base model for a Neighbourhood Algorithm surface-wave inversion using Rayleigh-wave phase velocities in the period range 16-170 seconds measured for five events along 16 two-station paths within the southern Kaapvaal Craton. Based on xenolith data, we constrain the Vp/Vs ratio in the inversion to vary from about 1.72 in the uppermost mantle to 1.78 at 180 km depth. The velocity structures determined by surface-wave inversion are consistent with those derived from the xenolith data, suggesting that the velocity structure (i.e., thermal structure) of the mantle to a depth of 180 km beneath the Kaapvaal craton today is similar to that at ~70-90 Ma, around the time of kimberlite eruption. Results from both surface wave inversion and xenolith calculations indicate that S-wave velocities decrease slightly with depth beneath the craton, from a value around 4.7 km/s in the uppermost mantle to about 4.6-4.65 km/s at a depth of 180-200 km. We performed an extensive series of tests based on a wide range of starting models, and in no instance was it possible to produce a low velocity zone with minimum Vs less than about 4.55-4.6 km/s. Additional analysis of synthetic models shows that for this surface-wave dataset of the southern Kaapvaal, there can be no low-velocity zone with Vs less than 4.5 km/s to depths of at least 200 km, such as has been proposed elsewhere to exist beneath southern Africa (e.g. Priestley, Lithos, 1999).
DE: 3909 Elasticity and anelasticity
DE: 7218 Lithosphere (1236)
DE: 7255 Surface waves and free oscillations
DE: 8103 Continental cratons
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005