Elevated K/T palaeotemperatures throughout Northwest England: three kilometres of Tertiary erosion?

Apatite fission track analysis of samples from Northern and Central England demonstrates that most rocks presently at outcrop were subjected to palaeotemperatures greater than 90°C in the latest Cretaceous or early Tertiary. Available data indicate that Late Cretaceous/early Tertiary palaeogeothermal gradients were similar to present values, suggesting that heating was due to burial and that significant Tertiary erosion has removed approximately 3 km of overburden from the entire region. The Irish Sea and Cheshire Basins appear to have lost between 2.7 and 3.3 km of sediment but, more surprisingly, several kilometers of sedimentary cover may also have been removed from surrounding highs such as the Lake District and the Pennines. Further evidence for kilometre-scale erosion is found in fission track data from sediments in contact with the Tertiary Butterton dyke. While zircon fission track ages date the time of intrusion of the dyke at 61 ± 2 Ma, a reduced apatite age of 53 ± 1 Ma supports evidence that the present exposed levels experienced ambient temperatures of around 90°C at the time of intrusion. The cause of this widespread erosional event is considered to have been related to rifting in the North Atlantic and compression resulting from the Alpine orogeny. At 65 ± 5 Ma the timing and amount of cooling identified in this study fits well with a model of doming over a mantle hot-spot, immediately prior to rifting in the North Atlantic. However, associated with this rifting event was the Laramide pulse of the Alpine orogeny, which is known to have caused significant early Tertiary inversion of many basins in northwest Europe. The possibility that, by the end of the Cretaceous, up to 3 km of post-Triassic sediments lay across the Pennines, the Lake District and the Cheshire and Irish Sea basins demands considerable revision of ideas concerning the Late Cretaceous/early Tertiary geology of the region and leaves questions unanswered as to the tectonic and geomorphic processes which caused this regional event