Heat flow data from a number of continental shield regions show a trend of relatively low to high values from Archean cratons to bordering Proterozoic mobile belts. Of the two end-member explanations for this trend, low heat production in Archean crust or a relatively thick cratonic lithosphere, the latter has come to be generally preferred. Such an explanation assumes a strong, one-to-one correspondence between the mantle component of surface heat flow and local lithospheric thickness. This assumption has been well validated below oceans and its applicability to continental lithosphere has gone largely unchallenged. It is, however, not fully valid. This is demonstrated through numerical models that allow continents to form over a convecting mantle. Model continents consist of a core region of thickened crust and mantle residuum and a peripheral region of thick crust, analogs to a craton and a mobile belt, respectively. Despite a thicker thermal lithosphere in the core relative to the periphery, the equilibrium surface heat flux across a continent shows little variation. The finite thermal conductivity of buoyant continental material is at the heart of this behavior as it allows continents to enforce a spatially near-constant heat flux condition on the mantle below. Such a condition is associated with a weak correspondence between mantle heat flow and lithospheric thickness defined thermally or mechanically. This general result, together with specific modeling results applied to heat flow data, suggests that variable lithospheric thickness is most likely not the primary cause of heat flow variations near Archean cratons, leaving differing degrees of crustal heat production as the more likely candidate.¿ 1997 American Geophysical Union