Contraction of continental crust during orogeny results in elevated topography at the surface and a root at depth. Thermomechanical models suggest that root growth is enhanced by thickening of thermally softened thin lithosphere. A >400 km2 region of Archean gneiss in the Athabasca granulite terrane in the Canadian shield contains abundant mafic sills with mid-oceanic ridge basalt–like chemistry. Heat from the sills facilitated melting of supracrustal host rocks along a prograde pressure-temperature (P-T) path culminating at P > 1.4 GPa and T > 950 °C in the Neoarchean. A basalt sill, converted to eclogite near the base of the domain, exhibits positive Eu anomalies consistent with plagioclase accumulation at a shallow crustal level prior to burial. Eclogite facies metamorphism previously dated as 1.90 Ga is here revised to 2.54 Ga based on existing zircon dates from the sill and new monazite dates from the paragneiss that hosts the sill. The results suggest that upper crustal materials were thermally softened in a backarc setting prior to burial to lower crustal levels during orogenic root growth.