Recognition of tectonic events in undeformed regions: contrasting results from the Midland Platform and East Midlands Shelf, Central England

Apatite fission-track and vitrinite reflectance data from Central England demonstrate how these techniques can reveal otherwise unrecognized tectonic and/or palaeothermal episodes in apparently tectonically stable areas. The results document the transition from an inverted basinal region in the north (East Midlands Shelf), to a tectonically stable platform in the south (Midland Platform). AFTA data from the region reveal two discrete cooling episodes, in the Early and Late Tertiary. Maximum palaeotemperatures from AFTA and VR data in outcrop samples define a consistent increase from less than or equal to 50 degreesC in Lower Cretaceous and Upper Jurassic units in the SE to around 80-90 degreesC in Triassic and older units in the NW. These Early Tertiary palaeotemperatures reflect a combination of deeper burial and elevated basal heat flow. Results from the Rufford-1 well define an Early Tertiary palaeogeothermal gradient of 40.5 degreesC km(-1) (32-50 degreesC km(-1) at +/- 95% confidence limits), corresponding to deeper burial by 1450 m of additional section (1.1-2.2 km at +/- 95% confidence limits), subsequently removed by Tertiary erosion. In contrast, geological considerations suggest a maximum overburden of 800-900 m above the base of the Lower Jurassic in the vicinity of Rugby where palaeotemperatures at outcrop are similar to those near the Rufford-1 location. The discrepancy between stratigraphic and palaeo-thermal reconstruction of former burial depths, often noted in earlier studies, remains unresolved. The Late Tertiary episode is much less well-constrained, but results from Rufford-1 may require between 910 and 1650 m of eroded section. Thus much of the total amount of removed overburden may have been removed during the Late Tertiary. Results from the Apley Barn Borehole (Oxfordshire) reveal a Late Tertiary palaeothermal episode characterized by a highly non-linear palaeotemperature profile which probably reflects local heating due to passage of hot fluids. Despite stratigraphic evidence for some Early Tertiary erosion results from this borehole show no evidence of Early Tertiary effects. Major Early and Late Tertiary exhumation was limited to regions underlain by older Palaeozoic basins while regions overlying Palaeozoic basement were more stable, experiencing significantly less exhumation. We suggest this reflects the preferential reactivation of the weaker basinal regions as a result of compressional events at plate margins. Our results emphasize the importance of incorporating results from both 'inverted' and 'non-inverted' areas in understanding the causal mechanisms of uplift and inversion, and highlight the importance of testing apparent stability using palaeo-thermal methods.