The Pennine Anticline, northern England – a continuing enigma?

Abstract

SUMMARY The westward-verging Pennine Anticline extends along the length of the central Pennines. It is an important Variscan fold of flexuous N–S trend developed on the Variscan Foreland. It defines the eastern margins of the Blackburn–Burnley and South Lancashire coalfields, and is commonly viewed as having been formed by the inversion of an underlying down-east Dinantian syn-depositional fault. Seismic reflection and potential field data provide some evidence on the deep structure of the anticline along much of its length. Modelling of gravity data does not provide unique solutions to basement and deep crustal structure beneath the Pennine Anticline. However, the models are consistent with both a basement/upper crustal density boundary and with variations in the Moho topography. In the southern part of the study area, these gravity data appear to define an important NNW–SSE trending, down-east fault that, when traced north, dies out and diverges from the Pennine Anticline. Where imaged by seismic reflection data, particularly over the Central Lancashire High, the fold is underlain by a high-angle reverse fault at basement level. Unusually in this part of northern England, this fault shows little evidence of a Dinantian extensional precursor, perhaps because its trend lay sub-parallel to the Dinantian extension direction. Although the structure remains enigmatic, it is suggested that the two en echelon anticlines mark folding in the Carboniferous cover rocks, developed above a through-going N–S zone of weakness in the basement comprising structures of varying trends. Under NW–SE directed Variscan compression, these structures