MESOZOIC COVER OVER NORTHERN ENGLAND - INTERPRETATION OF APATITE FISSION-TRACK DATA

Whether a significant thickness of Mesozoic cover rocks once rested up on the Palaeozoic rocks, which form the structural highs of the Englis h Lake District and the Northern Pennines, is a subject of long debate which has been revived recently by apatite fission track analysis (AF TA) studies. Evidence from adjacent basins suggests that 1200-1750 m o f Mesozoic cover was probably formerly present on the highs, prior to Cenozoic erosion. Higher estimates (c. 3000 m) of cover thickness, bas ed on such fission track-derived palaeotemperatures, result from under estimation or early Cenozoic surface temperatures and from not allowin g for the relatively low thermal conductivity of the eroded cover rock s. For similar reasons, apatite fission track estimates of eroded cove r rocks elsewhere in Britain may also prove to be overestimates.