Regional magnetic and gravity map patterns, combined with modeling of magnetic and gravity data constrained by measured magnetic susceptibility and density information, are used to develop a better understanding of the three-dimensional character of major lithostratigraphic subdivisions (terranes) in southern New Brunswick. Models and long wavelength map patterns indicate that the boundaries between the Caledonia, Brookville, Kingston, and New River terranes are sub-vertical or dip slightly to the southeast, and that some map units in each terrane extend to depths ranging from 5000–8000 m. The Mascarene and St. Croix terranes are less well constrained than the more outboard terranes on the models, in part because of limited physical property data but also because of the presence of the Saint George Batholith, which obscures relationships among stratigraphic units. The models are consistent with the Mascarene terrane being a cover sequence over the boundary between the St. Croix and New River terranes, the specific location and nature of which is now revealed by the geophysical models. The terranes appear to be associated at depth with geophysically distinct bodies that are interpreted to correspond to the basements, from southeast to northwest, of Avalon terrane sensu stricto, Brookville–Bras d’Or terrane, and Exploits–Gander composite terrane. The latter “Gander basement” underlies the Kingston, New River, Mascarene, and St. Croix terranes.