Post-tectonic clastic dykes in the Dalradian of Scotland and Ireland: implications for delayed lithification and deformation of sediments

Abstract

Structures associated with soft-sediment deformation are preserved in the Neoproterozoic Dalradian Supergroup of SW Scotland and NW Ireland. Clastic dykes display a range of age relationships to regional Caledonian D1 folds and fabrics from pre-tectonic to hitherto unrecognized post-tectonic. Evidence for the post-D1 timing of some dykes includes the emplacement of centimetre-scale injections along regional S1 cleavage, the disorientation and transport of cleaved wall-rock clasts within larger dykes, and clastic dykes which markedly cross-cut and transect F1 fold hinges and axial planes. Collectively, these observations are compatible with the earliest regional (D1) structures deforming a sequence which contained locally overpressured and unlithified pockets of sediment. These critical relationships indicate that overpressured pockets of unlithified sediment were possibly retained within the Dalradian for significant periods of time spanning at least 120 Ma given existing isotopic constraints. Copyright © 2000 John Wiley & Sons, Ltd.