Contributions to the Elevation North America

Continental elevations result from a combination of buoyancy (i.e. compositional and thermal) and geodynamic forces. Thermal isostasy can produce nearly three kilometers of relief between cold shield platforms and hot rift zones. However, changes in bulk density and crustal thickness can potentially produce relief greater than nine kilometers; whereas, geodynamic contributions to elevation are frequently no greater than a few meters to a few kilometers. Therefore, elevation resulting from buoyant forces must first be removed before assessing the magnitude of geodyamic contributions to elevation. The extensive geologic and geophysical data coverage of North America as well as diverse tectonic settings are ideal for determining the buoyancy contributions to elevation and examining a range of possible geodynamic processes affecting elevation. Compositional buoyancy is removed for each of 15 tectonic provinces by determining the average bulk density and crustal thickness. An adjusted elevation is computed by equating the density-thickness product of an observed region to a standard crustal section (e.g. 40 km thick crust with average density of 2830 kg/m³). Mean province elevations are computed using the digital elevation model GTOPO30 with a spatial resolution of 1 km2. Crustal thickness is determined from seismic refraction models. Rock types are estimated from a combination of surface geology, drill cores, xenoliths, seismic refraction velocities, and tectonic history; densities are then estimated by correlating rock types to laboratory Vp-P-T-density investigations. Thermal buoyancy is removed by computing the difference between the integrated thermal structure of the province and a standard continental lithospheric geotherm (characteristic of surface heat flow 40 mW/m²) to 250 km depth. Heat flow is drawn from a global data set and supplemented with more recent heat flow data. Anomalous heat flow at individual sites are examined for possible disturbances resulting from thermal conductivity and heat production variations. Using the continental heat flow and elevation relation derived from this study, it is possible to identify province outliers where the thermal state is anomalous (transient, disturbed, etc.), the elevation is anomalous (dynamically supported, anomalous mantle, etc.) or both. Discriminating between these sources of elevation provide insights into the geodynamics of North America.