ON THE HEAT-FLOW VARIATION FROM ARCHEAN CRATONS TO PROTEROZOIC MOBILEBELTS

Heat how data from a number of continental shield regions show a trend of relatively low to high values from Archean cratons to bordering Pr oterozoic mobile belts. Of the two end-member explanations for this tr end, low heat production in Archean crust or a relatively thick craton ic lithosphere, the latter has come to be generally preferred. Such an explanation assumes a strong, one-to-one correspondence between the m antle component of surface heat flow and local lithospheric thickness. This assumption has been well validated below oceans and its applicab ility to continental lithosphere has gone largely unchallenged. It is, however, not fully valid. This is demonstrated through numerical mode ls that allow continents to form over a convecting. mantle. Model cont inents consist of a core region of thickened crust and mantle residuum and a peripheral region of thick crust, analogs to a craton and a mob ile belt, respectively. Despite a thicker thermal lithosphere in the c ore relative to the periphery, the equilibrium surface heat flux acros s 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 cond ition on the mantle below, Such a condition is associated with a weak correspondence between mantle heat flow and lithospheric thickness def ined thermally or mechanically. This general result, together with spe cific modeling results applied to heat flow data, suggests that variab le lithospheric thickness is most likely not the primary cause of heat flow variations near Archean cratons, leaving differing degrees of cr ustal heat production as the more likely candidate.