Using Continental Elevation to Estimate Heat Production Distributions

Elevation is useful constraint on the thermal structure of the lithosphere and is used extensively in studying oceanic lithosphere evolution. On continents, the use of elevation is limited because of the relative complexity of continental lithosphere thermal regimes, and in particular by uncertainties in lithospheric heat production distributions. We are exploring these uncertainties with sensitivity studies linking crustal and subcrustal heat production profiles with elevation. Construction of steady-state geotherms requires knowledge of four parameters: surface temperature; surface heat flow; thermal conductivity, k; and heat production, A. Surface temperature and heat flow are determined directly at the Earth's surface, but k and A must be estimated as a function of depth. We test the sensitivity of elevation to variations in k and A. Sensitivity tests are conducted by computing the elevation differences due to geotherms with heat flow values of 40-120 mW/m² and systematically varying k and A. The effect of varying k between 2.1 to 3.9 W/m/K result in elevation adjustments ranging from +1.1 to -0.6 km at 40 mW/m² and +0.2 to -0.2 km at 120 mW/m². Surface A₀ values used are computed by assuming a sub-crustal heat flow of 40 to 100% of the total heat flow. A(z) are then decreased through the crust via an exponential decreasing relationship. The remaing contribution to the surface heat flow are generated internally by HPE in the crust. Radical changes in A produce elevation differences of 3 km at 40 mW/m² and 0.25 km at 120 mW/m². Hence, heat production is the dominant uncertainty in interpreting the integrated thermal structure from elevation, particularly in regions of low heat flow. Since shields have had time to reach steady-state, variations in heat flow may come from variations in sub-crustal heat flow and variations in thermophysical parameters. Assuming sub-crustal heat flow is constant and all variations in heat flow and elevation between cratons arise from heat production, an estimate of A may be made by interpreting anomalous elevation. Results from an analysis of 12 cratonic regions reveal seven provinces with elevations more than 500 m above predicted, and one province with an elevation less than 500 m below expected. These elevation anomalies may be accounted for by reasonable local variations in heat production.