CATASTROPHIC RELEASE OF HEAT AND FLUID-FLOW IN THE CONTINENTAL-CRUST

Heat may be released catastrophically from the continental crust by the onset of free convection in orogenic zones. Orogeny itself could create the conditions necessary for free convection by increasing the permeability of the crust. In the specific case of a continental collision zone, the conditions favoring free convection may move progressively into continental interiors, perhaps leading to repeated episodes of convection and related thermal events. Simple models and assumptions suggest that individual episodes of convective circulation and heat release persist for periods of time of the order of 10(6) yr or less. The rate and magnitude of energy release depend upon the initial thermal state and permeability of the crust and the depth of fluid circulation. Estimates made from typical parameters indicate that the total thermal energy released from the crust is approximately equivalent to an increase of background heat flow from about 60 to 200 mW/m2 for a period of 10(6) yr. A significant amount of this heat can be transported into the interior of a continent by a gravity-driven flow system if a continuous aquifer of sufficient permeability is present. The hypothesis could explain several phenomena attributed to tectonically driven fluids associated with the Alleghenian-Ouachita orogeny. These include the formation of Mississippi Valley-type lead-zinc deposits, widespread potassic alteration, and remagnetizations. Although speculative, the hypothesis is consistent with our current state of knowledge concerning the extent of fluid circulation in the continental crust.