In the southern Rocky Mountains and Rio Grande rift, rock cooling patterns from apatite fission-track (AFT) data spatially correlate with areas of voluminous middle Tertiary caldera-complex magmatism. We use thermochronology and gravity data to explore lithospheric modification by voluminous middle Tertiary magmatism. These data are not traditionally used to constrain magmatic processes, but in our study area they provide first-order constraints on the degree of mantle dedensification by basalt removal. We show that thermal isostatic responses to middle Tertiary magmatism drove spatially variable rock uplift and thermal perturbations that, coupled with variable exhumation, can explain AFT cooling and rock preservation patterns. We further argue that, if rock uplift exceeded exhumation, then surface uplift combined with magmatic weakening of the lithosphere could have influenced subsequent Neogene extension.