Over 100 measurements of seafloor heat flow reveal that the accretionary complex adjacent to the Nicoya Peninsula is characterized by remarkably low heat flow; values over the accretionary prism average 28 mW/m2, and values in the trench and the ocean crust seaward of the trench average 14 mW/m2. We attribute the low heat flow to effective hydrothermal cooling of the upper crust on the subducting plate and suggest that extensional faults created by flexure of the lithosphere enhance hydrothermal circulation. Thermal models show that subduction of low temperature crust combined with significant frictional heating at the decollement can explain the low and uniform heat flow. Disparity between heat flow values observed on the lower trench slope with model results suggests upward advection of heat by porewater flux through broadly distributed conduits. ¿ American Geophysical Union 1996