Distribution of heat production in two metamorphic core complexes, Basin and Range province, Arizona : quantitative constraints on models of regional thermal structure




Ketcham, Richard Alan, 1965-

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The amount and distribution of crustal heat production is a vital component of all estimates of continental thermal structure, yet it remains in most cases an assumption rather than a constraint. This study utilizes two Cordilleran metamorphic core complexes, the Catalina core complex and the Harquahala Mountains, as large and extensive exposures of the recent (<3O Ma) upper and middle crust of the southern Basin and Range of Arizona to gather primary heat production data. The depth distributions obtained do not follow a smooth or systematic function as predicted by models developed from the interpretation of linear relationships between surface heat flow and heat production; instead, they reflect a primary control exerted by the local structural or magmatic history. In each core complex, the amount of heat production observed plus that inferred to reside in the deeper crust is approximately 50% higher than predicted by standard estimates. This result is corroborated by variogram analysis of the data combined with published stochastic models. The difference results in overestimation of deep crustal temperatures by 150°C or more. Two-dimensional conductive thermal models of an evolving metamorphic core complex are utilized to provide insights into syn- and post-extensional thermal history. Model predictions of ancient cooling paths are remarkably consistent with estimates based on both structural and thermochronologic constraints, to the extent of reconciling seemingly conflicting data sets. The present-day heat flow patterns and low-relief Moho are most closely matched by a balanced geometry in which unroofing of the core complex is compensated by pure shear extension off-center, which may be analogous to models in which lower crustal flow compensates for tectonic denudation. The net amount of extension observed in Arizona core complexes does not appear to be sufficient to explain the high heat flow which characterizes the overall southern Basin and Range, indicating that additional sources of heat may be required. A new technique for determining concentrations of heat-producing elements in natural samples by gamma-ray scintillation spectrometry includes a test and partial correction for secular disequilibrium effects.