Discussion on the structure of the western part of the Southern Uplands of Scotland

Abstract

P. Stone & J.D. Floyd write: The Southern Uplands controversy (McKerrow 1987 and references therein) is addressed by Needham (1993) who details structural evidence in support of an origin for the terrane as a forearc accretionary prism. The structural histories demonstrated confirm previous accounts: a principal deformation phase was related to SE-directed thrusting with later accommodation deformation in the thrust hinterland contemporaneous with the southward-migrating principal phase at the thrust front. (cf. Needham & Knipe 1986; Barnes et al. 1989). Sinistral shear was imposed during the later stage of deformation as the stress regime became transpressive. Such a sequence is compatible with either of the models proposed for the evolution of the terrane: forearc accretionary prism (Leggett et al. 1979; Leggett 1987) or sequential back arc to foreland basin thrust belt (Stone et al. 1987; cf. Morris 1987). Since the original debate a general consensus has been reached that, within the Scottish sector, closure of the Iapetus Ocean culminated in the mid-Silurian. Thereafter the southern part of the Southern Uplands developed as a foreland basin as the leading edge of Laurentia overrode the Avalonian continental margin (e.g. Soper & Woodcock 1990 and references therein; cf. Stone et al. 1987, fig. 6). By that time any 'Southern Uplands' arc remnants could have been accreted at the Avalonian margin and thus would have similarly been overridden by the thrust belt developing ahead of Laurentia. The foreland basin, preceding that prograding thrust belt, migrated across the Iapetus Suture and is manifest in the