We used data from the Tucson, Arizona, broad-band seismic station to analyze crustal and mantle structure under and adjacent to the Catalina metamorphic core complex. The data can be modeled with a simple two-layer crust having an average Moho depth of ∼30 km and seismic velocities consistent with a felsic composition. The lower crust thickens under the Catalina Mountains, depressing the Moho by ∼4.2 km. The crustal root can isostatically compensate the average topography of the Catalina Mountains, indicating that the depth of isostatic compensation is at or below the Moho and that the Catatina Mountains act as a coherent block bounded by high-angle normal faults. The presence of the crustal root is not consistent with models that predict upward bowing of the Moho under the Catalina metamorphic core complex. The presence of the root cannot rule out models of lower crustal flow during the formation of the core complex. However, the presence of a root favors a model of metamorphic core complex development described by crustal thickening in shortening episodes (crustal-welt model) followed by collapse of the crustal welt through extension.