The Franciscan Complex comprises several tectonostratigraphic terranes that were accreted to coastal northern California during late Mesozoic to Miocene time. Estimates of peak paleotemperature (based upon measurements of vitrinite reflectance) reveal spatial and temporal patterns which, when integrated with pressure-depth limits imposed by stratigraphic relations and inorganic metamorphic phases, enable reconstruction of past geothermal gradients. Geothermal gradients during metamorphism of matrix to the Mesozoic melange (the Central Belt) may have been unusually small; this condition can be explained by the depression of isotherms during subduction of old oceanic crust. However, the gradients offshore northern California increased during the Paleogene period (the Yager and Coastal terranes). During the middle Miocene epoch a pulse of hydrothermal discharge produced an overprint across the entire King Range terrane, and high regional heat flow continued through the remainder of the Cenozoic. This change in thermal regime must have been influnced by the proximity of the Pacific-Farallon ridge-and-transform system to the Franciscan-Cascadia subduction front. The thermal structure of the subduction complex was modified because of conductive and advective heat flux from the subducting oceanic lithospheric into the overriding accretionary wedge. ¿ American Geophysical Union 1989