Short Paper: Timing and regional implications of deformation in the Southern Uplands of Scotland

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

In sight of the suture: the early Palaeozoic geological history of the Isle of Man

“…In summary then, it is most likely that both the D 1 and D2 phases on the Isle of Man occurred in latest Silurian or early Devonian time, between c. 420 and 400 Ma. This is essentially the same age as that deduced for the main deformation in the Lake District (Soper et al 1987;Hughes et al 1993) and slightly younger than the southernmost deformation in the Southern Uplands (Barnes et al 1989). The deformation in the Isle of Man is presumed to be driven by the culminating collision of Avalonia with Laurentia (Fig.…”

“…The contrast between these two hypotheses is, in fact, rather small if the Wenlock and Ludlow rocks of the Windermere Supergroup are themselves regarded as the diachronous southward advance of Southern Uplands deposits in a successor basin across the, now essentially closed, Iapetus Ocean (Barnes et al 1989;Kneller 1991; Fig. 2).…”

The Southern Uplands Terrane in Scotland – a notional controversy revisited

“…Barnes et al 1989) Conodonts were used by Armstrong et al (1996) to supplement the graptolite biostratigraphy of the Northern Belt and to strengthen correlation with the coeval Midland Valley successions, an important aspect of their fore-arc model. Their interpretation, and the broader regional synthesis by Armstrong & Owen (2001), was also aided by correlations of the shelly fauna, notably brachiopods.…”

“…Later phases of deformation are apparent locally, associated either with accommodation in the thrust hinterland commensurate with D 1 deformation at the thrust front (and so likely to be equally diachronous as D1 but less systematically so) or with intermittent sinistral shear imposed across the entire belt but focused into major strike-parallel fault zones. These post-D 1 deformation phases have been referred respectively to D 2 (co-axial with the gently plunging D 1 folds) and D 3 (sinistral, steeply plunging folds) following Barnes et al (1989), but their relationship is not everywhere strictly sequential.…”

Regional geochemical comparisons from the Lower Palaeozoic, Southern Uplands–Down-Longford terrane in Northern Ireland and Scotland

“…The sequential incorporation of tectonic tracts into an active thrust system means that the first deformation was diachronous, older in the north than in the south (Barnes et al 1989), and the structural profiles can vary along strike 4 (Rushton et al 1996). Each tract ideally comprises a very thin (generally <150 m) black shale sequence (the Moffat Shale Group) overlain by a very much thicker (up to about 2 000 m) sandstone succession.…”