One hundred twelve heat flow measurements were made on a 100-km-square grid of seismic reflection profiles centered over the intersection of the northern Juan de Fuca ridge and the Sovanco fracture zone. Profile spacing was roughly 10 km, and heat flow stations were located at each track line intersection. South of the fracture zone the crust is young (0--1.8 m.y.), sediment cover is incomplete, and on average, heat flow is very low in comparison with that predicted by any reasonable model for lithosphere creation. Station to station heat flow variability is high and uncorrelated with local topography or structure, and the nature and magnitude of variability cannot be accounted for by the thermal effects of either heat flow refraction or rapid sedimentation. These observations suggest that open groundwater circulation in effectively permeably basement rocks redistributes the heat and causes highly localized scatter in conductive measurements. A large portion of the total heat flow is ventilated directly to the sea. On the Explorer ridge flank, north of the fracture zone, the crust is older (3.3--6.6 m.y.) and fully sedimented. Large local variability in heat flow values still exists, but a relationship between heat flow and sediment thickness can be detected. On average, heat flow increases with decreasing sediment thickness, and although the variation is in the proper sense, the magnitude of the observed heat flow variation cannot be explained by simple conductive mechanisms, such as deposition of cold sediments. The observed relationship between heat flow and sediment thickness can be accounted for by widespread hydrothermal circulation in a highly heat transportive crust. Water temperatures at the base of the sediments are found to range between about 100¿ and 200¿C. A similar relationship between heat flow and sediment thickness is observed for the sediment-filled axial valley on the Juan de Fuca ridge, and basement temperatures estimated there range between about 100¿ and 300¿C. Basement temperatures estimated over the Juan de Fuca ridge flanks are on the average much lower and range between ≪25¿ and 200¿C. This is probably a result of groundwater ventilation through basement outcrops there. On the Explorer ridge flank, open circulation is prevented by the thick and continuous sediment cover, and the heat flow average over the area (5.7 HFU, where 1 HFU=1μcal cm 2s-1) should be a reliable estimate of the total conductive heat flux for this age crust (4.5 m.y. on average). This, with other reliable data points from the Pacific, supports a simple dependence of heat flow upon age of heat flow (HFU) =12.0/age1/2 (m.y.).