Abstract

The tectonic subsidence of many intracontinental basins and rifted margins is caused by the cooling of an initially hot lithosphere. The relationship between the surface heat flow and the rate of tectonic subsidence depends on the initial and boundary conditions, but useful mathematical relationships can be derived, which allow the heat flow to be computed directly from the subsidence history. If the heat flow at the base of the lithosphere is constant, and if horizontal heat transport can be neglected, the excess heat flow at the surface is proportional to the subsidence rate; it is approximately equal to 2.3 mW m⁻² when the rate of subsidence is 1 m My⁻¹. If the temperature is constant at the base of the lithosphere, the ratio of the surface heat flow to the rate of subsidence is always smaller; for short times, it is determined by the initial conditions; for times larger than 0.05 a²/k (a = lithospheric thickness, k = thermal diffusivity), the heat flow becomes proportional to half the subsidence rate (i.e., 1.15 mW m⁻² per m My⁻¹).

The analysis can be extended to include the effect of horizontal heat conduction. Relationships, similar to the one-dimensional ones, are derived between the Fourier components of the heat flow and of the subsidence rate, but one term must be added to account for horizontal heat transport. This term will be negligible for wavelengths longer than the lithospheric thickness; it also becomes smaller and negligible with increasing time.

Therefore, it is, in principle, always possible to estimate the paleoheat flow directly from the subsidence history when it is known that the tectonic subsidence results from thermal contraction only. The calculations will yield the heat flow exactly, provided that the flux is constant at the base of the lithosphere. If the base of the plate is isothermal, an upper limit on the surface heat flow will be obtained; an estimate that will be accurate, regardless of the initial conditions, can be obtained for periods longer than 15 My.