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Abstract

In areas where significant unconformities are present, palaeotemperatures derived from apatite fission-track analysis (AFTA) and vitrinite reflectance (VR) data through a vertical rock section can be used to estimate palaeogeothermal gradients and (by extrapolation to an assumed palaeo-surface temperature) amounts of exhumation (palaeo-burial). AFTA also provides a direct estimate of the timing of exhumation. These parameters can be used to reconstruct more complete histories than those based purely on the preserved rock record.

Precision and accuracy of these estimates are controlled by a range of theoretical and practical factors, perhaps the most important being the use of appropriate kinetic models. In extracting thermal history information from fission tracks in apatite, it is essential to use models that can describe variation in response between apatite grains within a sample. It is also important to recognize the limitations of the methods. AFTA and VR are dominated by maximum temperatures, preserving no information on events prior to a palaeo-thermal maximum. Recognition of this allows definition of key aspects of the history with greater precision.

Results from NW Europe define a series of regionally synchronous palaeo-thermal episodes, with cooling beginning in Early Cretaceous, Early Tertiary and Late Tertiary times. Latest results show that Early Tertiary palaeo-thermal effects in NW England can be understood as being due to a combination of higher basal heat flow and deeper burial, and emphasize the importance of obtaining data from a vertical sequence of samples. Comparison with similar results from other parts of the world suggests that events at plate margins exert a key influence on the processes responsible for regional exhumation, as recognized through Mesozoic and Cenozoic times across NW Europe.

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