Two-dimensional finite element models simulating extension of the West Antarctic Rift System (WARS) exhibit three classes of behavior, which are dependent upon the pre-rift thermal state of the upper mantle. All of the models begin with relatively cool East Antarctica lithosphere juxtaposed against warmer West Antarctica. The models all undergo an initial period of extension that is broadly distributed across the WARS. Class 1 models continue to extend in this way for more than 80 m.y. before abruptly developing a lithospheric neck at the edge of the model furthest from East Antarctica. The behavior of Class 1 models is dominated by a horizontal temperature gradient caused by juxtaposition of the warm WARS lithosphere against the cooler East Antarctica lithosphere. This produces a corresponding strength gradient, which causes a neck to eventually develop at the warm, weak edge of the model. Class 1 models have relatively high pre-rift temperatures at the base of the crust (>800 °C), which inhibits focusing of strain during the first 80 m.y. of extension. In Class 2 models the rift axis develops within the interior of the WARS. Class 2 models differ from Class 1 models in that the net heat production in the crust plays a larger role in determining the temperature at the top of the mantle prior to and during rifting. Necking at the edge of these models is inhibited because crustal thinning leads to cooling and strengthening of the lithosphere at the edge of the model. This causes the locus of extension to shift toward the weaker interior of the WARS. In Class 3 models, the rift axis forms where the pre-rift lithosphere transitions between relatively cool and thick East Antarctica and warmer and thinner West Antarctica. In these models, syn-extensional cooling and strengthening of the lithosphere causes the locus of strain to shift into the transitional region rather than the interior of the WARS. Class 3 models resemble the evolution of the WARS, which underwent a period of broad extension during the Late Cretaceous through late Paleogene Periods and more focused extension near the West Antarctica/East Antarctica boundary during the Neogene Period. All Class 3 models require the mantle potential temperature during the Late Cretaceous through Paleogene phase of broad extension to be no greater than 1270 °C, suggesting that an active mantle plume was not present beneath the WARS during the early stages of extension.

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