South of the Juan de Fuca-Pacific-North America triple junction located off northern Vancouver Island there is subduction, and to the north there is oblique convergence along the Queen Charlotte transform fault. In the mid-Tertiary, periods of extension, including crustal thinning and volcanism, formed Queen Charlotte Basin. Heat flow on northern Vancouver Island (67 mW/m2) is the same as in Queen Charlotte Basin to the north, but it decreases abruptly to the south over a distance of 70 km to an average value of 46 mW/m2. Compared to the rest of the island, the northern island has more subdued topography, a lower mean elevation, slightly higher Bouguer gravity values, and lower seismicity. Late Tertiary volcanics, which occur only on the northern part of the island, geochemically resemble the Tertiary Masset Formation which formed during periods of extension in Queen Charlotte Basin. These data are consistent with a Tertiary history including periods of extension for northern Vancouver Island, similar to Queen Charlotte Basin, and distinguish it from the rest of Vancouver Island. The southern boundary of this northern block extends northeastward from Brooks Peninsula, approximately above the inferred northern edge of the subducted Juan de Fuca plate and parallel to a series of linear magnetic anomalies associated with a northeasterly trending fault zone. The abrupt transition in observed heat flow is most likely associated with the edge of the subducted plate, which produces large temperature differences at depths of 30--40 km. The time constant for these thermal effects to reach the surface conductively indicates that this boundary has been stable over tens of millions of years, and therefore the location of the subducting plate's northern edge must have been stationary or moving very slowly over this period. Consequently it is likely that the triple junction has been located on average for the last 40 Myr off Brooks peninsula.