Discussion of ‘low temperature thermal history of eastern Ireland: effects of fluid flow’ by A. A. McCulloch (1994)

Abstract

The discovery by McCulloch (1994a) of mid-Tertiary cooling along the Irish coast is potentially an exciting development. However, this discovery does not require revision of the interpretation of existing data from other areas to enhance its importance. McCulloch's paper gives a misleading picture of previous work on the application of AFTA, both in general and in specific application to north-west England. Three points should be made clear: 1. Rigorous interpretation of AFTA data from northwest England, based on a quantitative understanding of the system response, suggests that samples throughout the region began to cool from palaeotemperatures around 80–110°C or higher in the late Cretaceous to early Tertiary (70-60 Ma). Cooling to near-surface temperatures was not rapid and some samples remained sufficiently hot to produce appreciable annealing subsequent to the initial cooling. Some data have also suggested a possible earlier cooling episode, in the early Cretaceous, but the extent of this episode in north-west England is not yet clear. None of the data from north-west England shows any evidence of a discrete episode of cooling from maximum or peak palaeotemperatures beginning as late as 30 Ma. 2. Our preferred interpretation of all available AFTA and vitrinite reflectance data from north-west England and the East Irish Sea Basin, also supported by a range of independent evidence, involves heating due to a combination of burial by up to ∼2 km of additional section removed by Tertiary uplift and erosion and lateral transport of heat by fluids, probably induced by early Tertiary igneous activity in the Irish Sea Basin and elsewhere. 3. The available evidence suggests that chlorine is the only element that exerts a significant systematic chemical control on annealing rates of fission tracks in apatite. Quantitative modelling of fission track parameters, using kinetic models which have been rigorously developed and tested and which allow for the influence of C1, gives results which are compatible with geological annealing constraints and is an essential part of the quantitative interpretation of AFTA data. Any attempt to interpret data in the absence of quantitative modelling techniques is likely to lead to erroneous conclusions.