TY - ABST
T1 - The extent of the Cratonic keel underneath the Southern African region: A 3D image using Finite-Frequency Tomograph
AU - Soliman, Mohammad Youssof Ahmad
AU - Bezada, Max
AU - Thybo, Hans
AU - Levander, Alan
PY - 2010/12
Y1 - 2010/12
N2 - We have re-examined the P body wave data from the South Africa Seismic Experiment (Carlson et al, EOS 77, 1996) across the Kaapvaal and Zimbabwe cratons and the Bushveld complex. Using finite-frequency kernels, we inverted the P-wave delay times to obtain 3-D images of compressional velocity perturbations in the mantle beneath the Southern African cratons to depths as great as the bottom of the transition zone. We utilized P-wave cross-correlation arrival times at three different frequency bands 1, 0.5 and 0.25 Hz. All P-wave arrivals were hand-processed. Crustal corrections were calculated using a crustal model we derived from PdS receiver functions using H-K analysis (Zhu and Kanamori, J. Geophys. Res. 105, 2000), and varied with azimuth. We chose damping for the inversion by examining the model norm-data misfit L curve. Checkerboard resolution tests showed good model recovery for 8°x8° anomalies with Vp variations of ±1.5%; 4°X4° checkerboards showed moderately good recovery. To try to isolate the depth extent of anomalies in the model we ran two suites of squeezing tests: 1) We reduced the maximum depth of the model from 1000 to 700 to 410 km. 2) For a 1000 km deep model we increased the damping parameter in the deeper layers progressively and examined the rms misfit and the imaged structure. Visual inspection of the tomographic images produced from these inversions showed that the lateral distribution of structures remained unchanged as the depth of the model was reduced and deep structures persist even with large damping, but with the rms error changing significantly. This indicates to us that the deeper structures are real. We have obtained a new P-wave tomography model showing the depth extent of the cratonic keel. We can distinguish the lateral and the vertical extent in Vp variations between the Archean and modified regions such as the Bushveld complex, and the mobile belts surrounding the cratons. The high velocity (+1.0%) cratonic roots extend to 220-250 km depth beneath the Kaapvaal and Zimbabwe cratons. Lower P-velocities are found under the Bushveld complex and the mobile belts. We also image significant positive and negative anomalies (±3.0%) unconnected to the cratons under the cratonic keels and in the transition zone. We are currently inverting the P wave data using different reference models, and examining the S-wave travel times.
AB - We have re-examined the P body wave data from the South Africa Seismic Experiment (Carlson et al, EOS 77, 1996) across the Kaapvaal and Zimbabwe cratons and the Bushveld complex. Using finite-frequency kernels, we inverted the P-wave delay times to obtain 3-D images of compressional velocity perturbations in the mantle beneath the Southern African cratons to depths as great as the bottom of the transition zone. We utilized P-wave cross-correlation arrival times at three different frequency bands 1, 0.5 and 0.25 Hz. All P-wave arrivals were hand-processed. Crustal corrections were calculated using a crustal model we derived from PdS receiver functions using H-K analysis (Zhu and Kanamori, J. Geophys. Res. 105, 2000), and varied with azimuth. We chose damping for the inversion by examining the model norm-data misfit L curve. Checkerboard resolution tests showed good model recovery for 8°x8° anomalies with Vp variations of ±1.5%; 4°X4° checkerboards showed moderately good recovery. To try to isolate the depth extent of anomalies in the model we ran two suites of squeezing tests: 1) We reduced the maximum depth of the model from 1000 to 700 to 410 km. 2) For a 1000 km deep model we increased the damping parameter in the deeper layers progressively and examined the rms misfit and the imaged structure. Visual inspection of the tomographic images produced from these inversions showed that the lateral distribution of structures remained unchanged as the depth of the model was reduced and deep structures persist even with large damping, but with the rms error changing significantly. This indicates to us that the deeper structures are real. We have obtained a new P-wave tomography model showing the depth extent of the cratonic keel. We can distinguish the lateral and the vertical extent in Vp variations between the Archean and modified regions such as the Bushveld complex, and the mobile belts surrounding the cratons. The high velocity (+1.0%) cratonic roots extend to 220-250 km depth beneath the Kaapvaal and Zimbabwe cratons. Lower P-velocities are found under the Bushveld complex and the mobile belts. We also image significant positive and negative anomalies (±3.0%) unconnected to the cratons under the cratonic keels and in the transition zone. We are currently inverting the P wave data using different reference models, and examining the S-wave travel times.
M3 - Conference abstract in journal
VL - 91
JO - Eos Trans. AGU, Fall Meet. Suppl.
JF - Eos Trans. AGU, Fall Meet. Suppl.
IS - 51
M1 - DI21A-1950
ER -