An upper limit to palaeo heat flow: theory and examples from the Danish Central Graben

Abstract

The surface heat flow yields a condensed expression of the subsurface thermal regime. While the present heat flow can be measured as the product of thermal conductivity and temperature gradient, resolution of the past surface heat-flow variations calls for indirect methods such as the modelling of vitrinite reflectance and other organic maturity indicators, which represent integrals of the past temperature history. This paper argues that regarding the reflectance of vitrinite, which is the most common maturity indicator, the feature of the past thermal regime which can survive the effects of measuring and modelling uncertainties is the maximum temperature, present or past. How the maximum temperature was obtained is only rarely resolved. Given maximum temperature, burial history, thermal conductivity structure, and surface temperature history, the one-dimensional steady-state heat equation yields an approximate upper limit to the surface heat flow. Artificial examples show that the past heat flow may be similar to the maximum heat-flow constraint when the past was significantly warmer than the present. However, this requires a delicate balancing of burial history and heat-flow history to occur: the thermal relaxation rate of the hot past must compensate for heating due to burial. Generally, the past heat flow may be well below the maximum constraint. Examples from the Central Graben show that vitrinite reflectance samples of Upper Jurassic age constrain the maximum palaeo heat flow back to 150 Ma. This is due to the very rapid and significant sedimentation of Upper Jurassic sediments resulting in immediate deep burial of vitrinite. The data of the present study are consistent with a Early Cretaceous heat flow elevated by about 20–30% above the present, and with a heat flow throughout the Cenzoic only marginally higher than the present. However, the data are also fully consistent with a time independent background heat-flow history. This illustrates the limits of resolution in palaeo thermal reconstruction based on vitrinite reflectance and temperature data. The problem is one of finding the space of admissible solutions rather than one of finding a single solution.