Geodynamic Constraints on the Source of Syn-Rift Magmas in the West Antarctic Rift System

Alkalic rocks appeared in Marie Byrd Land on the eastern flank of the West Antarctic Rift System (WARS) soon after extension began in the Late Cretaceous, and have been erupting on both flanks of the still-active rift since Eocene time. Petrogenetic models that have been proposed include melting by decompression of depleted asthenosphere, enriched domains within the asthenosphere, enriched domains within the lithosphere, active mantle plumes, fossil plume material entrained in the upper mantle, and enhanced melting due to small scale convection beneath the rift flanks. Geodynamic models that match the structural evolution of the rift require mantle temperatures similar to the global average, indicating that active plumes and melting of depleted asthenosphere are unlikely to contribute to Cretaceous and Paleogene magmatism. Enriched domains within the lithosphere are possible melt sources, but models show that these domains should melt soon after the onset of extension and become progressively less productive with time. Therefore, although enriched lithosphere may have contributed to Late Cretaceous syn-rift magmatism, it is an unlikely explanation for Cenozoic magmatism. The models show that small scale convection is possible with relatively cool geotherms, particularly if melt is present, as melt lowers the mantle viscosity and promotes convective flow. The models show that enriched domains in the asthenosphere are likely to begin to melt only after a finite amount of extension, thus providing a viable explanation for Paleogene and later magmatism within the rift. Our models thus suggest that enriched domains in the lithosphere could have contributed to early (Late Cretaceous) syn-rift magmatism in the WARS, with contributions from enriched domains in the asthenosphere dominating during the Cenozoic, possibly enhanced by small scale convection under the rift flanks. Melting of depleted asthenosphere is likely to contribute only to Neogene magmatism.