Thermal evolution of the lower crust beneath the Transantarctic Mountains

Abstract

Models of uplift, rifting, and heat transfer in the Transantarctic Mountains (TAM) rely on knowledge of the thermal evolution of the lower crust, but such information has remained elusive. Granulite xenoliths entrained in <2 Ma rift basalts at the TAM front chronicle the long-term thermal history of the deep crust and yield insight into the Cenozoic evolution of the TAM. Major-element thermobarometry of the xenoliths record elevated temperatures of 860–920 °C at ∼0.8 GPa. In situ coupled U-Pb and trace element zircon and titanite data reveal that the deep crust experienced intense heating at least twice: once at 850–900 °C ca. 500 Ma and again at 740–900 °C starting ca. 37 Ma. The Cenozoic temperature-time path indicates a high geothermal gradient beneath the TAM front—with 800–900 °C at 20–30 km depth and 900–1000 °C temperatures at the Moho (30–35 km depth)—and is interpreted to reflect heating by the adjacent West Antarctic Rift System. Despite elevated temperatures in the lower crust, minimal melting beneath the present-day TAM is inferred given that the crust is refractory, having been previously depleted during Ordovician magmatism. The identification of hot crust beneath the TAM forces revisions to estimates of the elastic thickness of the lithosphere, which underpin models invoking a flexural origin for the high elevations of the TAM. The observed Cenozoic heating trend supports models that suggest uplift was driven by a buoyant thermal anomaly beneath the TAM front. The finding of dry and strong deep crust is also in line with models wherein the TAM have maintained high elevations since the Mesozoic.