Constraints on the formation and stability of continental crust from heat flow and heat production data.

Abstract:

The characteristics of stable continental crust are the end-result of a long series of magmatic and tectonic processes. Most studies of crustal formation have focussed on the composition and vertical stratification of the crust. Yet, when it comes to its thermal structure and mechanical strength, the thickness of the crust is an equally important variable. Heat flow and heat production data provide important constraints on all these characteristics as well as on crustal evolution. A global analysis of heat flow and thickness data shows that there is no correlation between these two variables. On a large scale, the bulk crustal heat production is not constant and its variations are partially compensated by changes of crustal thickness. On a smaller scale, within a geological province, the total crustal heat production varies by large amounts independently of crustal thickness changes. The range of Moho temperatures within a stable geological province may be as wide as $\approx$300K. The calculated variations in Moho temperatures explain seismic observations of P$_n$ and mantle shear wave velocities.

Archean provinces are characterized by low values of heat flow and heat production today. Accounting for the rundown of heat producing elements, however, one finds that, at the time of their stabilization, these provinces had almost the same heat flow and crustal heat production as younger ones. At that time, lower crustal temperatures were close to the solidus of typical continental assemblages. This indicates that the intrinsic thermal regime of the crust determines the conditions for crustal stability. Post-orogenic melting events may occur without external forcing and lead to the vertical differentiation of heat producing elements. The change in thermal structure leads to lower temperatures and higher mechanical strength in the new continental block. We show that crust that has the heat production characteristics of today's average continents cannot be thicker than about 50 km if it is to resist tectonic forces and/or gravitational collapse.