A GEODYNAMIC MODEL OF THE WEST ANTARCTIC RIFT SYSTEM

Finite element models indicate that the tectonomagmatic evolution of the West Antarctic Rift System can be explained without the influence of extra-plate processes, such as changes in plate motion or impingement of a plume. The models simulate a broad zone of extension in West Antarctica during the Late Cretaceous Period followed by more focused extension within the Victoria Land Basin during the Cenozoic Era, consistent with the observed deformation history. Decompression melting of the asthenosphere is predicted to begin at approximately 50 Ma, ultimately producing ca. 7 x 10⁵ km³ of magmatic rocks beneath the West Antarctic Ice Sheet. The parameters found to exert primary control on the structural behavior of the models are the initial crustal thickness of West Antarctica, the mantle potential temperature (or mantle heat flux), and the distribution of heat producing elements in the West Antarctic crust. Models that reproduce the basic aspects of the tectonic evolution of the West Antarctic Rift system (initial diffuse extension followed by focused extension near the East Antarctica-West Antarctica boundary) require a pre-rift crustal thickness greater than 40 km in West Antarctica, a mantle potential temperature ≤ 1325º C (resulting in a mantle heat flux of less than 25 mW m^-2), and a greater abundance of heat producing elements in the West Antarctic lower crust in comparison to East Antarctica. Under these conditions, extension is initially distributed across the relatively warm (and hence weak) Ross Sea region of West Antarctica. The portion of West Antarctica immediately adjacent to East Antarctica is refrigerated by the cool East Antarctic craton, and remains relatively strong and undergoes no extension. As extension in West Antarctica progresses the heat producing crust thins. This leads to cooling and strengthening of the lithosphere in this region. As a consequence, the unthinned part of West Antarctica immediately adjacent to East Antarctica becomes the weakest region, resulting in cessation of extension in the central Ross Sea and focusing of extension in the Victoria Land Basin region.