Values of borehole temperature and %-smectite in illite/smectite mixed-layer clay provide important information about the basal heat flux history of the Nankai Trough subduction zone. Numerical simulations of sediment accumulation, heat conduction, and illite/smectite diagenesis permit comparisons of various thermal-history scenarios for ODP Site 1173. In our preferred models, basal heat flux during the first 10 Ma of sedimentation was near typical values for conductively cooling crust. During the last 5 Ma, basal heat flux was higher than expected. The anomalous heat flux was likely influenced by both off-axis volcanism and hydrothermal circulation.