Over 50 new Nd isotope analyses are presented for high-grade orthogneisses from Algonquin Park and surrounding region to map major Grenvillian thrust boundaries. Nd model ages display a consistent geographical pattern that allows detailed mapping of the boundary between the Algonquin and Muskoka domains, here interpreted as the local trajectory of the Ottawan-age Allochthon Boundary Thrust (ABT). The ABT is underlain by a domain with Paleoproterozoic Nd model ages, interpreted as a tectonic duplex entrained onto the base of the main allochthon. The boundaries determined using Nd isotope mapping are consistent with field mapping and with remotely sensed aeromagnetic and digital elevation data. The precise location of the ABT can be observed in a road-cut on Highway 60, on the north shore of Lake of Two Rivers in the centre of Algonquin Park.