A petrologic geotherm from a continental rift in Antarctica

Abstract

Most of our knowledge about geothermal gradients in continental-rift zones has been determined indirectly from heat-transfer calculations based on surface heat flow, inferred heat production, inferred heat-transfer mechanisms, etc. We document a petrologically determined crustal geotherm from a continental-rift environment in the Ross Sea and adjacent Transantarctic Mountains. This geotherm is based on thermobarometry of garnet, spinel, and olivine granulites included in Cenozoic alkaline volcanic rocks. The barometry yields pressures that are in good agreement with the geophysically-defined depth to the base of the crust in the Transantarctic Mountains (≈ 40–43 km) and the Ross Embayment (≈ 20–28 km). The thermometry yields very high temperatures (900–1000°C) in the middle and lower crust. These high temperatures and the shape of the geotherm clearly indicate that advection, rather than conduction, was the dominant heat-transfer mechanism in the lower and middle crust of this rift environment. Injection of alkaline basaltic magmas is the likely cause of the advective heat transfer. The data require the upper crust to have an average geothermal gradient of at least 50–100°C/km. Such a high geothermal gradient, without evidence of crustal melting, suggests that the lower crust was free of granitic compositions. It also presents strong support for thermal-expansion models for the uplift of the Transantarctic Mountains.