GEODYNAMIC MODELS FAVOR PASSIVE RIFTING OF THE WEST ANTARCTIC RIFT SYSTEM

Finite element models simulating extension of the West Antarctic Rift System (WARS) exhibit three classes of behavior. All of the models begin with relatively cool East Antarctica lithosphere juxtaposed against warmer West Antarctica, and all undergo an initial period of extension that is distributed broadly across the WARS. The subsequent model evolution depends on the balance between mechanical weakening that results from thinning of the lithosphere and thermal strengthening that results from cooling of the upper mantle during extension. Cooling of the upper mantle in turn depends on the interplay between thinning of the radiogenic crust, which promotes cooling and strengthening of the lithosphere, and thinning of the entire lithosphere, which promotes warming and weakening above the rising asthenosphere. The behavior of Class 1 models is dominated by a horizontal temperature gradient caused by juxtaposition of the WARS lithosphere against cooler East Antarctica. The corresponding strength gradient, enhanced by mechanical weakening as the lithosphere thins, causes strain to become progressively more focused away from East Antarctica. Eventually, a narrow rift forms at the West Antarctica edge of the model. Class 2 models have a higher ratio of crustal heat production vs. mantle heat flow than Class 1 models. Thinning of the radiogenic crust results in cooling and strengthening of the lithosphere in the incipient neck at the edge of these models, causing rifting to eventually become focused in the weaker interior of the WARS. In Class 3 models, relatively low asthenosphere temperatures permit syn-extensional cooling and strengthening of the lithosphere throughout the WARS. In these models, the rift eventually localizes in the transitional region between East Antarctica and West Antarctica. Class 3 models resemble the evolution of the WARS, which underwent broad extension during the Late Cretaceous through late Paleogene Periods and focused rifting 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 a mantle plume was not present beneath the WARS during the early stages of extension.