Horizontal and vertical thermal maturity patterns and time-temperature modeling of Cretaceous and Tertiary rocks in the San Juan Basin of southwestern Colorado and northwestern New Mexico indicate that the high levels of thermal maturity in the northern part of the basin are due to either (1) convective heat transfer associated with a deeply buried heat source located directly below the northern part of the basin or (2) the circulation of relatively hot fluids into the basin from a heat source north of the basin located near the San Juan Mountains. Vitrinite-reflectance (R_m) well profiles through Cretaceous and Tertiary rocks in the basin are commonly nonlinear, with two to four segments having different gradients. The different gradients most likely represent the combined effects of contrasting thermal conductivities associated with lithologic variations and differences in heat transfer processes (conductive versus convective). Time-temperature and kinetic modeling of nonlinear R_m profiles indicates that present-day heat flow is insufficient to account for the measured levels of thermal maturity. Furthermore, in order to match the nonlinear R_m profiles, it is necessary to assign artificially high thermal-conductivity values to some of the stratigraphic units. These unrealistically high thermal conductivities are interpreted as evidence of convective heat transfer.