Heat flow provinces reconsidered

Abstract

The concept of a heat flow province, an area in which there is apparently a linear relationship between near-surface heat flow and radiogenic heat generation, is considered. The relationship has been suggested for a wide variety of tectonic terrains, ranging in age from Archaean to Cenozoic. It is expressed as q = q 0 + A 0D in which q and A o are the measured heat flow and heat generation and q o and D are interpreted to be the uniform reduced heat flow from the mantle and lower crust, and D is a scale depth for the distribution of radiogenic elements. Reported values for D are in a remarkably narrow range, and are typically ∼ 10 km. However, there can be substantial uncertainties associated with the data. In particular, heat generation measurements from a limited number of sites may not adequately represent even a relatively uniform pluton. When the data are re-analysed with reasonable estimates of uncertainty associated with them, the calculated values of q o and D are, in most cases, significantly different from those calculated without regard to uncertainties in the data. Only the data from the Sierra Nevada batholith, a deep-seated plutonic complex, appear to satisfy the relationship even when their uncertainties are taken into account. With the re-analysis, D generally falls in the approximate range 10–20 km. This is also the range of depths to a commonly-observed discontinuity in the mid- to lower-crust, sometimes called the Conrad discontinuity, which suggests a possible correlation between the two. A model in which blocks of crust 10–20 km thick, depths being randomly arranged, are each assigned a uniform heat generation, randomly generated from a reasonable range, produces an apparent relationship between heat flow and heat generation that is at least as good as most of those deduced from real data. The model produces crustal temperatures that are generally considerably lower in the lower crust than those commonly calculated; this is consistent with the observations from seismic refraction that the depth to the Moho can vary widely even in stable shield environments. It is concluded that the concept of a heat flow province as originally expounded is invalid.