GEODYNAMIC MODELS OF THE WEST ANTARCTIC RIFT SYSTEM: IMPLICATIONS FOR THE THERMAL STATE OF THE WEST ANTARCTIC MANTLE

The West Antarctic Rift system (WARS), which underlays most of the Ross Sea and West Antarctic Ice Shelf, developed as a broad continental rift system beginning ca. 105 Ma. Since Oligocene time extension has focused in the relatively narrow Victoria Land Basin near the East Antarctica (EA)/West Antarctica (WA) suture. A transition from broad to narrow rifting is common in extensional systems, as evidenced by wide zones of transitional crust adjacent to the ocean-continent boundary on many rifted continental margins. However, the WARS is unusual in that 1) the transition to narrow rifting marks a shift in the extension loci rather than the more commonly observed progressive narrowing of the zone of extension, and 2) the rift ultimately focused near the EA/WA suture - a region presumed to have been strong in comparison to West Antarctica prior to rifting as it was transitional between the cool, strong East Antarctic craton and the more juvenile warmer, weaker West Antarctica lithosphere. We present a series of finite element models that show that thermal structure of the upper mantle plays a critical role in causing deformation to localize near the EA/WA suture. If the temperature at the top of the mantle, Tm, is greater than ~780°C, model behavior falls into two classes dependent primarily on the mantle potential temperature Tp. If Tp > 1310°C, the WARS remains hot and weak throughout rifting, producing a wide extensional province with no necking. If Tp < 1310°C, the rift behaves similarly to most rifted continental margins, beginning in a relatively wide zone (prescribed by the width of the initially weak WARS lithosphere), and progressively narrowing to a focused rift axis within the pre-weakened WARS, far from the EA/WA suture. A third type of behavior, similar to the WARS, is observed at intermediate Tp if Tm < 780°C. In these models, cooling during extension strengthens the WARS mantle sufficiently such that it becomes weaker than the transitional crust near the EA/WA suture. Deformation then shifts into this relatively narrow region, focusing stress and leading to rapid necking and formation of a narrow rift. Rift evolution similar to the WARS is only observed in models with Tp < 1340°C, suggesting a cool to moderately warm asthenosphere during most of the period of WARS tectonism.