Thermal and tectonic history of the East Midlands shelf (onshore UK) and surrounding regions assessed by apatite fission track analysis

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Abstract
Apatite fission track analysis has been used to study the thermal and tectonic history of the East Midlands Shelf and surrounding areas of onshore UK. Apatites from outcrops of Palaeozoic crystalline basement give fission track ages between 300 ± 29 and 420 ± 31 Ma, which are the oldest yet reported in the UK region and reflect the relative stability of this cratonic region. Mean track lengths in these samples are between 12 and 13 μm . Namurian and Triassic sandstones from outcrop give apatite fission track ages between c . 100 and c . 390 Ma, with mean track lengths generally between 10 and 12.5 μm . Distributions of single grain ages and track lengths suggest that all samples have been annealed (implying exposure to higher temperatures) since deposition, with maximum palaeotemperatures reached some time in the 50–80 Ma range prior to the onset of cooling. Samples from five exploration wells show consistent patterns, in which apatite fission track ages decrease sharply from around or just less than the stratigraphic age at near surface, to values in the 50–80 Ma range at depths of c . 1–2 km (present temperatures of c . 40–70 °C), showing that the samples studied have all been exposed to higher temperatures. Single grain ages and track length data suggest that maximum palaeotemperatures were attained at or before c . 60 Ma. Subsequent cooling was protracted and the data are compatible with two phases, in the early and mid-Tertiary. Palaeotemperature analysis suggests that palaeo-geothermal gradients were similar to present values, and that elevated palaeotemperatures were due to greater depth of burial, with cooling due to subsequent uplift and erosion. Estimates of uplift and erosion vary from c . 1–1.5 km on the east coast (around the Humber estuary) to c . 2.2 km in the East Midlands and southern Pennines. Compilation of palaeotemperature estimates from all samples analysed from the UK onshore region shows that Mesozoic burial and Tertiary uplift has affected a wide area, and that uplift was due to broad-scale, regional warping, with recognized ‘inversion axes’ being areas where uplift and erosion were particularly pronounced or where younger sediments and/or inverted basinal structures allow the identification of uplift and erosion. The recognition of Late Cretaceous/early Tertiary unconformities across wide areas of northwest Europe, both onshore and offshore, suggests that the regional uplift and erosion may have been far more widespread than previously recognized. This has important implications for hydrocarbon exploration in Northwest Europe.