Pf. Green et al., Recognition of tectonic events in undeformed regions: contrasting results from the Midland Platform and East Midlands Shelf, Central England, J GEOL SOC, 158, 2001, pp. 59-73
Apatite fission-track and vitrinite reflectance data from Central England d
emonstrate 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 nort
h (East Midlands Shelf), to a tectonically stable platform in the south (Mi
dland Platform). AFTA data from the region reveal two discrete cooling epis
odes, 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 E
arly 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 145
0 m of additional section (1.1-2.2 km at +/- 95% confidence limits), subseq
uently removed by Tertiary erosion. In contrast, geological considerations
suggest a maximum overburden of 800-900 m above the base of the Lower Juras
sic in the vicinity of Rugby where palaeotemperatures at outcrop are simila
r to those near the Rufford-1 location. The discrepancy between stratigraph
ic and palaeo-thermal reconstruction of former burial depths, often noted i
n earlier studies, remains unresolved. The Late Tertiary episode is much le
ss 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 overbur
den may have been removed during the Late Tertiary. Results from the Apley
Barn Borehole (Oxfordshire) reveal a Late Tertiary palaeothermal episode ch
aracterized 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 n
o evidence of Early Tertiary effects. Major Early and Late Tertiary exhumat
ion was limited to regions underlain by older Palaeozoic basins while regio
ns overlying Palaeozoic basement were more stable, experiencing significant
ly 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 mecha
nisms of uplift and inversion, and highlight the importance of testing appa
rent stability using palaeo-thermal methods.