Gravity modeling shows that the observed fluctuations in gravity over the Catalina-Rincon metamorphic core complex of southeastern Arizona are caused primarily by shallow crustal density contrasts. The Wilderness suite granites, the probable source of a 20-mgal low over the central part of the core complex, may extend to depths of 7–12 km below the surface. Seismic refraction information is consistent with the existence of a deep crustal root under the Catalina-Rincon core complex. We hypothesize that the voluminous Wilderness suite granites are related to a crustal thickening event that built the root in early Tertiary time. Modeling of flexural isostatic uplift, resulting from an excess crustal root, predicts two episodes of uplift for the Catalina-Rincon metamorphic core complex: (1) low-relief flexural uplift in the middle Tertiary and (2) complete uplift precipitated by high-angle Basin and Range faulting at 10–15 Ma. The uplift and exposure of core complexes, therefore, can be an isostatically driven process that occurs in regions of locally thickened crust.