Geodynamic Constraints on the Tectonic Evolution of the West Antarctic Rift System

Finite element models show that the key aspects of the tectonic evolution of the West Antarctic Rift System were controlled primarily by the pre-rift thermal structure of the lithosphere. 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 best reproduce the basic aspects of the tectonic evolution of the West Antarctic Rift system (initial diffuse extension in the Late Cretaceous followed by focused Cenozoic extension near the East Antarctica-West Antarctica boundary) require a pre-rift crustal thickness greater than 40 km in West Antarctica, a mantle heat flux of less than 25 mW m-2 (corresponding to a mantle potential temperature ≤ 1325 °C), 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. The models predict decompression melting of the asthenosphere beginning at approximately 50 Ma, ultimately producing ca. 7 x 105 km3 of magmatic rocks beneath the West Antarctic Ice Sheet. The models also predict a modern surface heat flux of 112 mW m-2, in good agreement with the 115 mW m-2 measured in the AND-1B borehole beneath the McMurdo Ice Shelf by the ANDRILL Science Team.