A Global Perspective on the Composition of the Continental Crust from the Distribution of Heat Producing Elements: Comparison Between Heat Flow Data and Seismological Crustal Models.

Abstract:

The thermal evolution and the strength of continents critically depend on the amount and vertical distribution of heat producing elements in the crust. In turn, these two crustal characteristics provide constraints on the origin of the crust and its internal differentiation processes. Neither of them can be determined directly and they must be inferred from geochemical and geophysical data. One method relies on global crustal models such as CRUST1.0 which gives the structure of the continental crustal column on a $1^{o}x1^{o}$ grid. The crustal model is made up of sedimentary layers over basement consisting of upper, middle, and lower crust whose thicknesses and physical properties vary between provinces depending on age and tectonic type. One may then plug in the average values of heat production for these crustal layers from global geochemical studies. An alternative method consists of measuring the surface heat flow and subtracting the estimated Moho heat flux.

Using the global crustal models, we have calculated the crustal heat production for the continents. We have also averaged and placed on the same worldwide grid all the land heat flow measurements. After removing the mantle heat flux and excluding the tectonically active regions, we have compared these two maps. On the global scale, the model based heat production map shows much less variability ($\sigma = 7 mW~m^{-2}) than the map based on heat flow measurements ($\sigma = 14 mW~m^{-2}). Adjusting the heat production values to fit the heat flux measurements can cancel the difference between the means, but does not reduce the root mean square difference between the two sets. On a regional scale, abundant heat flow in southeastern Canada allow a very detailed comparison. Long wavelength trends of the heat flux data set are obliterated by the model. We attempted to change the crustal heat production values but found that it is impossible to obtain a good fit. Even when considering only the Archean Superior Province, using a single set of heat production values in the crustal model fails to predict the long and short wavelength variations of the surface heat flux.

It is impossible to predict crustal heat production from a generic crustal model without accounting for lateral differences in crustal heat production between the main geological terranes of a province.