Abstract

The timing of deformation in the Southern Uplands is constrained by tectonostratigraphical and intrusive relationships. Deformation migrated diachronously southwards, affecting the southern part of the Southern Uplands in the mid-Silurian. In contrast deformation in the Lake District cacan be shown by the same criteria to be Early Devonian. Thus, since the possibility of post-Emsian strike-slip along the Iapetus suture is remote, the regional tectonism cannot be related to a climactic episode due simply to closure of the Iapetus Ocean. Instead a southeast migrating deformation front is envisaged linking Silurian closure of Iapetus and the Southern Uplands backarc basin with Early Devonian collision farther south.

The timing and geometry of the main cleavage-producing deformation within the paratectonic Caledonides of northern Britain is generally debated in terms of closure of the Iapetus Ocean. Structures developed are related to accretion of a collage of terranes onto the southern margin of Laurentia, within a sinistral transpressive stress regime, during the collision of the three plates Laurentia, Baltica and Eastern Avalonia (Soper & Hutton 1984). However, within this overall model there is a considerable divergence of opinion as to the chronology and correlation of the main episode of deformation across the Iapetus suture collision zone. In north-west England and Wales, recent regional syntheses propose that the main structures were formed during an end-Silurian/Early Devonian collision and subsequent NW-directed underthrusting. This climaxed with the main cleavage-producing episode in the Emsian (Soper et al. 1987; McKerrow 1988) which has been correlated with the Acadian Orogeny of the

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