Structural synthesis of S Staffordshire, UK: implications for the Variscan evolution of the Pennine Basin

Abstract

The presence of strata of Westphalian A to possibly Stephanian age in south Staffordshire, located at the southern margin of the Pennine Basin, provides a rare control on the timing of late Carboniferous deformation in Central England. During Westphalian A—C times, sedimentation in south Staffordshire was controlled by thermal subsidence, with a superimposed influence of synsedimentary normal and reverse faulting. Alkaline basic igneous activity occurred during Westphalian C times, synchronous with uplift of the southern margin of the Pennine Basin. Uplift culminated, during Westphalian C—D times, in a phase of Variscan inversion along basement lineaments and the development of a regional unconformity. Renewed subsidence and deposition of a relatively thick sedimentary sequence of Westphalian D to possibly Stephanian age was followed by a final phase of Variscan deformation, comprising possibly two pulses of uplift, tilting and erosion. Observations presented for south Staffordshire, in conjunction with studies from other parts of the Pennine Basin, indicate a complex interrelationship between rifting, thermal subsidence and Variscan basin inversion. Pulses of N—S extension and E—W to NW—SE compression are interpreted as the product of regional strike-slip deformation, superimposed upon a regime of thermal subsidence. Alkaline igneous activity is considered to be the product of mantle partial melting in response to limited decompression due to transtensional displacements on crustal thickness faults.

The identification of syn-depositionally-active structures in the coal-bearing Upper Carboniferous of Great Britain

“…Lavas, sills and small vents of Westphalian A to C age are found along the southern margin of the WalesLondon-Brabant High (Waters et al 1994); sills and lavas are found in the Westphalian C/D sequences of Oxfordshire, and in the earlier Westphalian of Berkshire (Poole 1977;Foster et al 1989). Igneous features are known to have produced a range of associated fault styles, for example radial and ring patterns around vents, including the crypto-volcanic vents interpreted by Barnett (1985).…”

“…In South Staffordshire, Barnsley (1964) identified the "Brinsford Axis", a small positive feature paralleling the local NNW structural trend, across which several Westphalian A and B sequences are attenuated. Waters et al (1994) discussed field relationships between Westphalian sequences and various structures towards the southern basin margin in south Staffordshire, and concluded that extensional and compressional regimes overlapped through much of the Westphalian, with Variscan compression eventually becoming dominant. Some specific active faults were inferred from depositional patterns.…”

“…Guion & Fielding (1988) summarized structures thought to have been active during the Westphalian, concluding that recognizable movement was more common in earlier Westphalian A times than later, although this may reflect the limited stratigraphical ranges that were studied. Later references include Giles (1989), Chisholm (1990), Waters et al (1994) and Rippon (1996), the last inferring a general lack of structural control, even by large structures such as sub-basin bounding faults, on most channel systems.…”

“…However, there will be areas where conformable successions span intervals that include local intra-Silesian unconformities elsewhere: these intervals might show more evidence of movement, as the unconformities will relate to regional tectonic pulses. Structural effects should be more noticeable near basin margins, where there is thinner mantling of basement structures: Waters et al (1994) concluded that in the Pennine Basin, growth folding was present in the thicker Westphalian sequences of the central basin areas, compared with the development of angular unconformities, associated with folding and faulting, towards the basin margin.…”

A second major fluvial sourceland for the Silesian Pennine Basin of northern England

“…By Namurian times the Pennine Basin was dominated by thermally induced subsidence (Leeder 1982;Collinson 1988 1990). By late Carboniferous times the influence of thrusting, to the south of the basin, resulted in flexurally induced subsidence (Leeder 1988: Waters et al 1994Corfield et al 1996).…”