Continental thermal isostasy: 2. Application to North America

Abstract

Continental elevations can be partitioned into contributions from thermal and compositional buoyancy and geodynamic forces. In order to isolate the thermal component, elevations of 36 North American tectonic provinces are adjusted for the effects of compositional thickness and density variations by computing an isostatic adjustment relative to a standard crustal section (2850 kg m−3 density and 39 km thickness). Crustal densities are estimated using an empirical velocity‐density relationship. Compositional elevation adjustments applied to North American provinces range from ∼−1100 m in the southern Rocky Mountains to 2300 m in the Gulf of California, with uncertainties ranging from ∼200 m to >600 m. Theoretical thermal buoyancy is estimated by integrating the difference between a geotherm derived from observed values of heat flow and a reference geotherm. The best fitting continental heat flow–elevation model has a reference heat flow of 46.6 mW m−2 at 0 km and a 60:40 partitioning of surface heat flow between reduced and upper crustal radiogenic heat flow. Raw elevations of continental provinces show little correlation with heat flow, while compositionally adjusted elevations show a clear trend with ∼3 km difference between hot and cold provinces. A continental heat flow–elevation plot is used to identify outliers in adjusted province elevations. Anomalous elevations may reflect a nonsteady state thermal regime, dynamically supported elevation, anomalous mantle, or some combination of these states. Discriminating between these elevation sources provides insight into the geodynamics of North America, demonstrating the usefulness of this approach in continental geodynamic studies.