Determination of Past Heat Flow from Subsidence Data in Intracontinental Basins and Passive Margins

Abstract

The subsidence of rifted margins and of many intracontinental basins is caused by the cooling and thermal contraction of the initially hot lithosphere. Lithospheric cooling takes place in part through increased surface heat flow. Therefore, there is a relationship between the subsidence rate and the heat flow; this relationship depends on the initial and boundary conditions. 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, regardless of initial conditions; it is approximately equal to 2.3mWm−2 when the rate of subsidence is l m M.y.−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/κ (a = lithospheric thickness, κ = thermal diffusivity), the heat flow becomes proportional to half the subsidence rate (i.e. 1.15 mWm−2 per m M.y.−1).Horizontal transport of heat is conveniently included in the analysis in Fourier (or Hankel domain). The relationships between the Fourier components of the heat flow and the subsidence rate include an additional term accounting for horizontal heat conduction. This term is negligible for wavelengths longer than the lithospheric thickness; the effect of horizontal heat transport decreases and becomes negligible for longer time.Subsidence data from the North Sea and from the Pannonian basins have been used to compute paleo-heat flow. The examples demonstrate that, although heat flow can in principle be inferred, the method is sensitive to geologic noise (i.e. nonthermal subsidence) and it amplifies errors in the data; these difficulties can be removed provided that some smoothing of the data is performed. This can be achieved by averaging local subsidence histories. For the Pannonian Basin, subsidence rates and heat flow vary spatially, but the present average heat flow estimated from subsidence rate is compatible with the observed high value of the heat flow (100 mW m −2).