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Interpretations of data from the mantle support a rift setting for the Transantarctic Mountains (TAM) and the Ross Embayment. Much of this data is covered in the sections on the geologic setting and the crust, but the data have implications for the mantle in the TAM as well. Lower crustal xenoliths are covered in the geologic setting section. The chemistry of late Cenozoic volcanic rocks and the structure of the crust allow inferences to be made about the lower crust and mantle; these topics are covered in the section on the crust. Be sure to see these other sections for a more complete understanding of the lower crust and upper mantle in the TAM region.
Shear wave velocities beneath the Terror Rift in the Victoria Land Basin are up to 6% slow in the upper mantle compared to PREM (Preliminary Reference Earth Model). This slowness is illustrated for two earthquakes in Figure 22. The mantle is slowest in the depth range 40-100 km, and the velocity perturbation persists to greater than 160 km.
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| Figure 22. a) and b) Vertical component of seismic energy recieved at two stations in the TAM. c) and d) Velocity perturbations from the PREM model for the two arrivals shown in a and b. These ray paths pass beneath the active part of the rift system, the Terror Rift. Bannister et al., 2000. | ||||
S-wave velocity anomalies are considerably smaller beneath the TAM (Figure 23). The velocities are only 2% slower than PREM between 60 and 160 km. Note that the 160 km boundary is poorly constrained due to decreasing resolving power with depth of the fundamental mode energy (Bannister et al., 2000).
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| Figure 23. As for Figure 22, but for a path that passes beneath the TAM. Note that the slow velocity anomaly is significantly smaller beneath the TAM, but still present. Bannister et al., 2000. | ||||
Slow mantle can be caused by elevated temperatures, the presence of fluids, and/or the presence of partial melt. Elevated temperatures and partial melt are consistent with a rift setting. The slowest velocities occur beneath the Terror Rift, which is regarded as the active part of the rift system, whereas only moderately slow velocities occur beneath the TAM, which are in a rift shoulder position. Moderately slow velocities beneath the TAM suggest lateral heat conduction from the rift center to its flanks. This lateral heat conduction could be partly responsible for the uplift of the TAM (see Uplift Mechanisms page).