Utilizing Thermal Isostasy to Estimate Anomalous Crustal Radioactivity

Abstract:

Surface heat flow through Precambrian lithosphere ranges from ~20 mW m² in some parts of the Canadian Shield, southern India, and central Africa, to values in excess of 100 mW m² in parts of Australia. This range is nearly as large as seen within tectonically active regions. Yet both North America and Australia have large Precambrian regions unaffected by significant thermotectonic activity in the past 109 years. We examine the heat flow and elevation—corrected for crustal composition—for North America and Australia and interpret deviations from a predicted thermal isostatic relationship (calibrated to North America). Deviations from the thermal isostatic relationship can be in excess of ±1 km with many Australian regions exhibiting significant negative deviations from the thermal isostatic relationship, with the largest deviations occurring in regions with high heat flow (>60 mW m²). This observation is consistent with studies suggesting high heat production within the Australian upper crust. Our analysis provides a way to estimate the average upper crustal heat production and basal heat flow for these regions. In the most extreme case, we predict the average upper crustal heat production must be ~2.3 times higher than with sublithospheric heat flow similar to many other Precambrian regions. Analysis of additional Precambrian regions may help shed light on the global distribution of heat producing elements and conditions required for the stability and longevity of Precambrian lithosphere.