Constraints on the thickness of mantle lithosphere involved in collisional orogenesis are fundamental for understanding the geodynamics of mountain building and the overall growth of continents by accretionary tectonics. Garnet peridotite and ultrahigh-pressure (UHP) crustal rocks provide such a constraint in many collisional orogens but have hitherto been unrecognized in western North America's Cordillera. Here we show the first evidence for exhumation of UHP (>2.8 GPa) garnet peridotite and eclogite and for deposition of these rocks as detritus in an Early Jurassic forearc basin (Laberge Group, Yukon Territory and British Columbia). Our results suggest that collision in this part of the North American Cordillera must have been thick skinned, involving a Proterozoic continental mass with a lithosphere >100 km (and possibly to 150 km) thick. Our discovery also provides insight into the vigor of uplift and erosion of deep-seated rocks in a nascent continental arc.