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McKenzie's (1978) widely accepted model for continental rifting predicts initial isostatic subsidence following instantaneous stretching of the lithosphere, assuming the initial crustal thickness is greater than 18 km. Clearly the Transantarctic Mountains (TAM), with elevations up to 4897 meters, flagrantly violate this prediction. However, the TAM also violate the assumption of instantaneous stretching. Rifting began in the Jurassic and continues to the present, a span of about 150 million years. One small range, the Ellsworth Mountains, has maintained at least 1.8 km of relief since the end of the Early Cretaceous (Fitzgerald and Stump, 1991). The long duration of rifting must be at least part of the explanation for the uplift of the TAM. This extended period of rifting would allow ample time for heat conduction laterally from the center of the rift to the flanks. Seismic evidence supports lateral heat conduction from the center of the rift to the flanks (Bannister et al., 2000). Hotter lithosphere beneath the rift flanks should cause uplift itself due to expansion, and it could also weaken the lithosphere and so allow other processes to become important. Thermal uplift by itself cannot explain all of the uplift of the TAM because this mechanism requires excessive stretching values in the mantle (van der Beek et al., 1994).
A large number of mechanisms have been proposed to explain the elevation of the TAM and the existence of basins on either side of the mountains. A location map that includes the Wilkes Basin is shown in Figure 28, a highly exaggerated profile is shown in Figure 29, and real profiles are shown in Figure 30.
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| Figure 28. Location map including the Wilkes Subglacial Basin. Contours are free-air gravity anomalies. ten Brink and Stern, 1992. | ||||
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| Figure 29. Cross-sections across the TAM and surrounding basins. Note extreme vertical exaggerations. Stern and ten Brink, 1989. | ||||
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| Figure 30. Stacked profiles across the TAM and Wilkes Basin at different lattitudes (shifted). Also shown is the modled flexural profile (open squares). Stern and ten Brink, 1989. | ||||
Stern and ten Brink (1989) explain the TAM and its surrounding basins using the model shown in Figure 31. In this model, the Wilkes Basin is interpreted to be a flexural basin produced by the load of the TAM.
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| Figure 31. Flexural uplift model for the TAM. In this model, the Wilkes Basin is interpreted to be a flexural feature ("outer low"), opposite in sense to a forebulge on a downgoing subducting plate (figure at top right). Forces (loads) included in the model are shown in b). Stern and ten Brink, 1989. | ||||
In this model, a fault cuts across the entire elastic lithosphere, effectively creating two broken plates. This fault cannot transmit shear stress, so both sides of the fault plane are free to move independently (Stern and ten Brink, 1989). The forces that account for surface uplift are shown in Figure 31. These forces are: 1) A thermal load beneath the TAM, 2) An end load beneath the TAM, and 3) Erosion of the TAM. The end load required by the numerical model is probably physically a Vening Meinesz-type load caused by unloading and isostatic rebound of the footwall of the normal fault (Heiskanen and Vening Meinesz, 1958). Unfortunately, Stern and ten Brink (1989) confuse rock uplift and surface uplift. Erosion is not capable of producing regional surface uplift, and it is surface uplift that they are modeling. The magnitudes of the forces used in their model are shown in Figure 32.
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| Figure 32. Magnitudes of loads (forces) used to match the profiles shown in Figure 30. The equivalent elastic thickness is also shown. Stern and ten Brink, 1989. | ||||
Figure 32 also shows the flexural rigidity used in the model. In order to fit the profiles shown in Figure 30, some extreme flexural rigidities must be used. The elastic thickness of the East Antarctic plate must be 115 km (flexural rigidity of 1025 N m) and it must decrease to 5 km at the plate edge. The elastic thickness in the Ross Embayment is required to be 19 km (flexural rigidity of 4 x 1022 N m). The 1025 N m flexural rigidity for the East Antarctic plate is one of the highest proposed for any location on Earth. Decreasing the flexural rigidity linearly beginning 130 km from the free edge seems somewhat drastic and even ad hoc (what is the significance of 130 km?). Despite these limitations, the recognition and treatment of a thermal load and a Vening Meinesz end load are useful. It is also interesting to treat the Wilkes Basin as a flexural basin caused by the load of the TAM.
Ferraccioli et al. (2001) propose a different mechanism for formation of the Wilkes Subglacial basin and the Adventure Trough. Based on gravity and magnetic data, they propose that these too are rift basins, bounded by large normal faults. Note that the age of these rifts is essentially unconstrained, however, so they may be much older than the TAM rifting. If the flexural basin interpretation put forth by Stern and ten Brink is viable, it may be possible to argue that older normal faults localized the position of the modern flexural basin. Figures 33 and 34 show the location of the basins to the west of the TAM, and a profile showing an interpretation for the Wilkes Basin.
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| Figure 33. Subglacial bedrock elevation map of Northern Victoria Land showing the depressions named the Wilkes Basin and the Adventure Trough. Ferraccioli et al., 2001. |
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| Figure 34. Rift model for the Adventure Trough and Wilkes Basin, based on gravity data. Note that this model does not attempt to explain the uplift of the TAM. Compare with Figure 31. Ferraccioli et al., 2001. |
Another important mechanism that likely contributes to the uplift of the TAM is thinning of the lithosphere beneath the TAM, probably by simple shear (van der Beek et al., 1994; Fitzgerald et al., 1986). A simple shear mechanism is suggested by the consistent asymmetry direction along the TAM. In this model, the crust thinned less than the mantle beneath the TAM because crustal thinning was offset into the Ross Embayment by the simple shear. The model and results are shown in Figure 35.
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| Figure 35. Lithospheric thinning by stretching ("necking") due to simple shear beneath the TAM. Models in which thinning occurs at about 20 km can match observed gravity data. van der Beek et al., 1994. | ||||
Thus thermal uplift, isostatic rebound after unloading, and thinning of the lithosphere due to asymmetric extension all likely cause part of the uplift of the TAM. The relative importance of each of these mechanisms remains largely unkown, although Stern and ten Brink (1989) suggest that the thermal load is 4 times greater than the rebound end load.
Numerous other models for uplift of the TAM have been proposed; these are summarized in Fitzgerald (1992). Magmatic additions to the crust beneath the TAM could play a part in elevating the mountains; this possibility is essentially unconstrained due to the lack of necessary data.