Distribution of heat‐producing elements in the upper and middle crust of southern and west central Arizona: Evidence from the core complexes

Abstract

The amount and distribution of crustal heat production are a central component of all estimates of continental thermal structure, yet they remain in most cases an assumption rather than a constraint. This study utilizes the Catalina and Harquahala metamorphic core complexes as large exposures of the recent (<30 m.y. ago) upper and middle crust of the Arizona Basin and Range to gather primary heat production data. Samples were measured for their heat production, and their positions in the predetachment crust were reconstructed by restoring all major deformation associated with core complex formation, including subsequent block faulting. The restored part of the Catalina core complex spans a depth range from 3 to 13 ± 1 km, and the Harquahala core complex encompasses a depth range from the surface to 14 ± 3 km. Mylonitization probably has little systematic effect on the distribution of heat production in these core complexes. The depth distributions of heat production obtained in this study do not follow a smooth or systematic function but rather reflect a primary control exerted by local structural or magmatic history. Variogram analysis indicates that the scale of autocorrelation of heat production is variable but in most cases under 10 km. In each core complex, the amount of heat production observed plus that inferred to reside in the deeper crust is approximately 30 ± 5 μW/m3, roughly 50% higher than previous estimates. The difference can cause overestimates of 75–300°C in deep crustal temperatures if surface heat flow is assumed to be known. The use of a temperature‐ and pressure‐dependent function for thermal conductivity instead of a constant value causes an upward revision in estimated average geothermal gradients of approximately 2.5–4°C/km.