Biostratigraphical control of thrust models for the Southern Uplands of Scotland

Abstract

Graptolite biostratigraphy affords a robust and relatively accurate means of correlating Ordovician and Silurian hemipelagite and turbidite sequences and has been used to establish the structural development of the regional thrust belt in the Southern Uplands of Scotland. The overall structural pattern has long been recognised: greywackes within individual thrust slices, deposited within a relatively short time-interval, become sequentially younger southwards; each overlies the basal Moffat Shale Group which was deposited over a longer time. However, recent refinement of the graptolite biozonal scheme has allowed the better assessment of along-strike variations within the thrust belt which are here illustrated by two transects; one, based on work in the Rhins of Galloway and the Kirkcudbright areas (SW Southern Uplands), and the other in the Peebles-Hawick area (NE Southern Uplands). The SW transect most closely approximates to the regular pattern wherein a southward-propagating thrust-front incorporated sequentially younger greywacke units. The uniform geometry is interrupted only locally, towards the southern margin of the thrust belt, by a system of back-thrusts producing structural pop-ups. The NE transect departs from this regular model: a northern sector shows the orderly initiation of the thrust belt, but towards the SE a more irregular distribution of the thrust-slice agescan be best explained by outof- sequence movement. This transect also shows more repetitive imbrication of the same biostratigraphic interval than is apparent farther SW. In both transects the fundamental changes in thrustbelt geometry took place from mid-Llandovery times onwards, with a reversion to forward-breaking, in-sequence thrusting at the beginning of the Wenlock. The cause is a matter for speculation, but may be linked withthe accommodation of an obstacle to forward-thrust propagation. However it is recognised that such variationsin thrust geometry are a fundamental feature of most thrust belts and do not require a single regionally significant cause.

Regional conductivity data used to reassess Early Palaeozoic structure in the Northern Ireland sector of the Southern Uplands–Down–Longford terrane

“…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 (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.…”

“…1), the northern tracts of Caradoc and Ashgill (Upper Ordovician) age comprise the Leadhills Supergroup (Floyd 1996) Despite the broad, large-scale continuity of the structural tracts and their boundary faults there is considerable variation in the detail of the thrust architecture along strike. Rushton et al (1996) therein, but the effect on the Group's outcrop width is mitigated by a loss of older Gala Group tracts (Rushton et al 1996). So, for example, in the Rhins of Galloway the Gala Group outcrop has been divided into a series of structural tracts (Stone 1995) numbered in terms of age from 1 (the oldest, most northerly tract) to 8 (the youngest and most southerly tract; tracts 7 and 8 form the southern 25% of the Gala Group's outcrop width.…”

The Southern Uplands Terrane in Scotland – a notional controversy revisited

“…It also seems unlikely that oblique accretion would produce the distinctive cross-strike, time-stratigraphic pattern (Figure 2) that has become the terrane's leitmotif; more excision, duplication and interleaving of tracts would be expected under that circumstance. When disturbance of the sequential tract-incorporation pattern did occur in the late Llandovery (Ettrick and Hawick groups: Rushton et al 1996), followed by its subsequent re-establishment in the Wenlock (Riccarton Group: Kemp 1986), it most probably marked the effective closure of the Iapetus Ocean and the migration of the accretionary prism onto the Avalonian margin as a foreland fold and thrust belt. The Late Ordovician situation shown in Figure 12a has turbidite deposition on an extended fore-arc of thin continental crust.…”

“…Finally, Figure 12d illustrates the possible mid-Silurian situation following the elimination of the Iapetus Ocean by the 'soft' collision of Avalonia and Laurentia. The leading edge of the accretionary prism advanced onto the Avalonian margin, causing the Late Llandovery structural disruptions noted by Rushton et al (1996) before the forwardbreaking thrust pattern was re-established in a foreland basin setting. Shortening of the fore-arc basement continued and was accompanied by some north-directed thrusting at the rear of the accretionary prism.…”

“…The domainal nature of this transpressive deformation has been stressed, most recently by Holdsworth et al (2002b) who describe, from the Berwickshire coastal sections, its partitioning into strike-slip and contraction-dominated zones. The approximate temporal coincidence of this increase in transpression affecting the Hawick Group, with back-thrusting and out-of-sequence thrusting affecting the southernmost (youngest) Gala Group tracts, led to the suggestion by Rushton et al (1996) that during the Late Llandovery the accretionary thrust belt had to accommodate to an obstacle to its forward propagation. This may have been an isolated crustal feature -Novantia of Armstrong & Owen (2001) The Southern Uplands accretionary complex appears not to have suffered any substantial deformation as a result of the Laurentia-Avalonia collision, which accordingly has been widely described as 'soft'.…”