Early Tertiary paleo-thermal effects in Northern England: reconciling results from apatite fission track analysis with geological evidence

Abstract

The Paleozoic Lake District Block in northwest England has traditionally been thought of as tectonically stable since the Late Paleozoic, receiving only small thicknesses of Late Paleozoic to Mesozoic cover (although some workers have put forward different views). Apatite fission track analysis (AFTA) data from outcrop samples across the region reveal Early Tertiary paleotemperatures around 100 °C, requiring kilometre-scale Late Paleozoic and Mesozoic cover, removed during Tertiary uplift and erosion. With no evidence for elevated basal heat flow in NW England during the Early Tertiary, and no a priori justification for invoking it, earlier studies favoured an explanation involving burial by up to 3 km of overburden removed during Tertiary uplift and erosion. This conclusion was met with scepticism by many workers, and provoked a range of comments and criticisms, with a variety of alternative interpretations put forward, although these are also open to criticism. Results from the West Newton-1 hydrocarbon exploration well on the northern flank of the Lake District gave the first indication of a possibly more realistic interpretation, involving a combination of elevated heat flow and more restricted burial, but some aspects of the interpretation of these data were equivocal. More detailed sampling was therefore undertaken, in order to shed more light on the origin of the elevated Early Tertiary paleotemperatures observed across NW England. New AFTA data in outcrop samples from different elevations around Sca Fell (characterised by the highest elevations in the Lake District with the summit of Scafell Pike at 978 m asl) define an Early Tertiary paleogeothermal gradient of 61 °C/km, and require around 700 m of section removed from the summit during Tertiary uplift and erosion. These results, together with those from the West Newton-1 well, provide strong support for an interpretation involving Early Tertiary paleogeothermal gradients between 50% and 100% higher than present-day values, providing clear evidence of elevated basal heat flow during the Early Tertiary, contrary to earlier assumptions. Combined with amounts of section removed during Tertiary exhumation varying between ∼0.7 km (from mountain peaks) and ∼1.5–2 km (from coastal plains and glacial valleys near sea level) over the region, this interpretation finally provides a geologically plausible mechanism for the origin of the observed Early Tertiary paleo-thermal effects in NW England.