Cornwall and Princess Margaret arches are major regional uplifts in the Tertiary Eurekan orogen of the northeastern Canadian Arctic Islands and have influenced the development of adjacent synorogenic sedimentary basins. The arches are subparallel structures, about 200 km apart, and gravity and seismic-refraction data indicate that they are underlain at depth by crust-mantle upwarps. They may have developed as a result of crustal-scale folding during Eocene compression. Finite-element models for a layered quartz-diorite-olivine lithosphere rheology suggest that the horizontal stresses required are about 75 to 200 MPa. The strength of the continental lithosphere in the Eurekan orogen is affected by the existence of a thick succession of Paleozoic and younger sedimentary rocks in the crust and by the occurrence of a major igneous (thermal) event in the area 20-30 m.y. prior to the main phase of deformation.