Aspects of basin inversion in southern Britain

The Wessex Basin of southern England, a Permian to Cretaceous extensional basin, can be structurally divided into a set of constituent asymmetrical graben, bounded by major E–W-trending zones of en echelon syn-depositional normal faults. The graben were inverted in late Cretaceous and Tertiary times by compressive stresses oriented roughly north–south. Inversion structures fall into two related categories. Regional upwarps overlie earlier graben depocentres and were formed by bulk shortening of the graben-fill. Superimposed upon and geographically delimiting the regional upwarps, are roughly east-west trending linear zones of en echelon inversion structures. These coincide with the earlier graben-bounding faults and typically have the form of monoclinal or periclinal flexures each underlain by a partially reversed normal fault. The linear reverse fault/monocline inversion structures were a relatively inefficient method of basin shortening. Because upper crustal faults in the region steepen upward markedly, reversal of these faults under compression resulted in a shortening discrepancy at shallow depths. Locally, particularly in south Dorset, this was overcome by bucklefolding and low-angle reverse faulting which increased the amount of shortening attainable across the linear inversion structures. Elsewhere however, the shortening discrepancy was accommodated by bulk shortening and regional upwarp of the graben sedimentary-fill. This occurred preferentially in those graben containing young, poorly lithified and therefore weak sediments. Thus the early Cretaceous depocentres of the Weald and Channel basins were strongly upwarped, with axial uplifts of over 1000 m. Conversely, graben having older, more lithified sequences suffered little regional upwarp, shortening primarily by fault reversal along the linear inversion structures. The amount of crustal shortening which accompanied inversion was considerably less than the earlier crustal extension.