CONTROLS OF ASTHENOSPHERE TEMPERATURE AND VISCOSITY ON THE STRUCTURAL EVOLUTION OF THE WEST ANTARCTIC RIFT SYSTEM: INSIGHTS FROM 2D GEODYNAMIC MODELS

The West Antarctic Rift System (WARS) is a broad continental extensional province bounded by Marie Byrd Land to the east and the Transantarctic Mountains along the East Antarctic craton to the west. There are two main phases of extension. The first took place from Late Cretaceous to Paleogene, with extension distributed throughout West Antarctica. During the second phase in the Neogene, extension was focused along the margins of the WARS. Previous geodynamic models required a cool mantle (potential temperature ≤ 1543 K) to reproduce the WARS structural evolution, but those models did not include the asthenosphere. Here, finite element models including both the lithosphere and asthenosphere are used to simulate WARS extension. The models solve the Stokes and heat equations to capture the structural and thermal evolution of a 2-layer lithosphere extending at a rate of 7 mm/yr. We use dislocation and diffusion creep flow laws for the lithosphere and asthenosphere, respectively, with maximum stress defined by a plasticity limit. Prior to rifting, West Antarctica has a thicker model crust (45 km vs. 39 km) and thinner lithosphere (120 km vs. 180 km) than the East Antarctic craton. The models exhibit two classes of behavior dependent on asthenosphere viscosity. Class 1 (generated by low viscosity asthenosphere) is characterized by initial broad rifting throughout West Antarctica that becomes progressively focused in the central WARS as the model evolves. During early broad rifting a small convection cell (200 km by 200 km) forms in the asthenosphere beneath the craton-WARS boundary. This convection cell promotes delamination of the lower East Antarctic lithosphere along the boundary. Class 2 behavior (generated by relatively high viscosity asthenosphere) is characterized by initial broad extension of West Antarctica that transitions to narrow rifting focused along the craton-WARS boundary. The asthenosphere in Class 2 models forms two large convection cells (800 km x 200 km) beneath the lithosphere. No small convection cells or delamination of the lithosphere are observed. Class 2 models reproduce the structural evolution observed in the WARS. All Class 2 models have basal upper mantle temperatures < ~1700 K, implying the WARS upper mantle was not anomalously hot during Cretaceous rifting.