The 2012 Mw 7.8 Haida Gwaii earthquake confirmed very oblique subduction and slip partitioning at the southern Queen Charlotte margin. In this study, we re‐examine the thermal regime near the earthquake using new model constraints and with the recognition that hydrothermal circulation in the subducting oceanic crust can significantly affect the margin thermal regime. The observed heat flow values are extremely high just seaward of the trench but decrease rapidly landward. We explain this pattern as the consequence of very vigorous hydrothermal circulation in the subducting oceanic crust. Using a finite‐element model, we simulate the thermal effect of the circulation using a high‐conductivity proxy that represents a very high Nusselt number in an aquifer along the top of the oceanic plate. Our thermal model indicates that the temperature at the intersection of the megathrust and the strike‐slip Queen Charlotte fault (QCF) just seaward of the coast is about 350° C, approximately the limit of seismogenic behavior, and cooler than previous models that did not include hydrothermal circulation. The change of plate motion kinematics across the QCF approximately coincides with a down‐dip transition of the thermally controlled seismogenic behavior of the megathrust. Seaward of the QCF, the shallow megathrust accommodates mainly the margin‐normal component of relative plate motion, with the strike‐slip component accommodated by the QCF. This portion of the megathrust exhibits stick slip and produced the 2012 Haida Gwaii earthquake. Landward of the QCF, the megathrust fully accommodates the very oblique motion of the oceanic plate beneath the continental crust and exhibits creep.