Relations between coalification and palaeogeothermics in Variscan and Alpidic foredeeps of western Europe

Abstract

The degree of coalification (“rank”) which may be measured as vitrinite reflectance (% Rm) under the microscope in almost all sedimentary rocks depends mainly on the maximum rock temperature — to a minor degree on the “cooking time”. It is for this reason that close relationships exist between coalification and palaeogeothermics. The degree of rank increases with depth (the “rank gradient”) commonly reflects the maximum geothermal gradient which was active in a certain profile and area. Palaeogeothermal gradients may be estimated from coalification gradients (measured in deep boreholes) if the burial history (m/Ma) is known.

In the subvariscan foredeep of the Ruhr Basin, coalification was completed before the Permian, due to the Asturian folding and uplift. The coalification gradients are one order of magnitude higher (0,5 – 1,0 % Rm/km) than in the foredeeps of the northern Alps and the northern Apennines (0,03 – 0,09 % Rm/km), although the depths of burial and the duration of heat exposure were similar. The reasons for the great difference are significantly higher geothermal gradients in the Ruhr Basin during the Upper Carboniferous. According to BUNTEBARTH et al. (1982) these gradients vary between 60 and 80° C/km, whereas in the foredeep molasses of the northern Alps and the northern Apennines they range between 19 and 23° C/km due to subduction and the resulting increase of crustal thickness. The high geothermal gradients of the Subvariscan foredeep (60 – 80° C/km against the present 30° C/km) suggest a thinner crust during the Carboniferous. Based on the correlation between heat flow density and crustal thickness reported by ČERMÁK (1979), values of 110 – 130 mW/m², estimated for the Ruhr Carboniferous, correspond to 20 – 22 km crustal thickness. Thus results of coalification studies agree with results that ZWART (1967, 1976) obtained from studies of the regional metamorphism of rocks within the Variscan orogen of Europe.