Deformation of the Girvan succession, Southern Uplands
|Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
Deformation of the Girvan succession
Whilst the Southern Uplands accretionary complex was building up at the subducting margin of the Iapetus Ocean, the Ordovician to Silurian Girvan succession was accumulating on the subsiding margin of the Laurentian continent. The depositional setting was essentially a forearc basin between the Midland Valley volcanic arc, then rising to the north, and the developing accretionary complex to the south (P912315c). However, a note of caution must be added. Though this overall association of geological regimes fits the present-day outcrop pattern, it is highly unlikely that the original spatial relationships are preserved. The arc, forearc basin and accretionary complex all extended for considerable distances along strike, and substantial, sinistral strike-slip movement, post-Ordovician and particularly focused on the Southern Upland Fault, has brought about the juxtaposition currently seen.
For much of its history (and as described in Chapter 2) deposition of the Girvan succession was controlled by a series of major faults, throwing down to the south and sequentially stepping back into the Laurentian hinterland (P912324). There would undoubtedly have been some rotation of the early-deposited beds during subsequent fault growth, but there is only one stratigraphical break preserved in the Girvan succession. This coincides, approximately, with the Ordovician–Silurian boundary where the basal Silurian strata overlie the Ordovician at an unconformity with low-angle structural discordance.
The major structural disruption occurred in the mid to late Silurian, when the faults that had controlled sedimentation were reactivated as north-directed thrusts. The cause of this reversal is uncertain, but was most probably related to the collision, farther south, of Laurentia and Avalonia. The effect was to carry parts of the Girvan succession northward, over other parts of the succession and the underlying Ballantrae Complex. In the process, strata were rotated and folded so that steep dips are now commonly seen at outcrop. The thrust reactivation could have begun quite early in the Wenlock, with the youngest strata affected (and now preserved) being the Straiton Grits Formation, with an age equivalent to the murchisoni Biozone (P912323).
- Obduction of the Ballantrae Complex, Southern Uplands
- Deformation of the Girvan succession, Southern Uplands
- Southern Uplands accretionary complex
- Late Caledonian dyke swarms, Southern Uplands
- Late Caledonian plutonic rocks, Southern Uplands
Akhurst, M C, McMillan, A A, Kimbell, G S, Stone, P, and Merriman, R J. 2001. Silurian subduction-related assembly of fault-defined tracts at the Laurieston Fault, Southern Uplands accretionary terrane, Scotland, UK. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 91, 435–446.
Anderson, T B. 2001. Structural interpretations of the Southern Uplands Terrane. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 91, 363–373.
Anderson, T B, and Oliver, G J H. 1986. The Orlock Bridge Fault: a major late Caledonian sinistral fault in the Southern Uplands terrane, British Isles. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 77, 203–222.
Armstrong, H A, Owen, A W, Scrutton, C T, Clarkson, E N K, and Taylor, C M. 1996. Evolution of the Northern Belt, Southern Uplands: implications for the Southern Uplands controversy. Journal of the Geological Society of London, Vol. 153, 197–205.
Barnes, R P, and Stone, P. 1999. Trans-Iapetus contrasts in the geological development of southern Scotland (Laurentia) and the Lakesman Terrane (Avalonia). 307–323 in In sight of the Suture: the Palaeozoic geology of the Isle of Man in its Iapetus Ocean context. Woodcock, N H, Quirk, D G, Fitches, W R, and Barnes, R P (editors). Geological Society of London Special Publication, No. 160.
Barnes, R P, Lintern, B C, and Stone, P. 1989. Timing and regional implications of deformation in the Southern Uplands of Scotland. Journal of the Geological Society of London, Vol. 146, 905–908.
Kimbell, G S, and Stone, P. 1995. Crustal magnetisation variations across the Iapetus Suture Zone. Geological Magazine, Vol. 132, 599–609.
Leake, R C, and Cooper, C. 1983. The Black Stockarton Moor subvolcanic complex, Galloway. Journal of the Geological Society of London, Vol. 140, 665–676.
McCurry, J A, and Anderson, T B. 1989. Landward vergence in the Lower Palaeozoic, Southern Uplands–Down–Longford terrane. Geology, Vol. 17, 630–633.
Merriman, R J, and Roberts, B. 2001. Low-grade metamorphism in the Scottish Southern Uplands terrane: deciphering the patterns of accretionary burial, shearing and cryptic aureoles. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 91, 521–537.
Needham, D T. 2004. Deformation in Moffat Shale detachment zones in the western part of the Scottish Southern Uplands. Geological Magazine, Vol.141, 441–453.
Needham, D T, and Knipe, R J. 1986. Accretion- and collision-related deformation in the Southern Uplands accretionary wedge. Geology, Vol. 14, 303–306.
Phillips, E R, Barnes, R P, Boland, M P, Fortey, N J, and McMillan, A A. 1995. The Moniaive Shear Zone: a major zone of sinistral strike-slip deformation in the Southern Uplands of Scotland. Scottish Journal of Geology, Vol. 31, 139–149.
Rock, N M S, Gaskarth, J W, and Rundle, C C. 1986. Late Caledonian dyke-swarms in southern Scotland: a regional zone of primitive K-rich lamprophyres and associated veins. Journal of Geology, Vol. 94, 505–522.
Rushton, A W A, Stone, P, and Hughes, R A. 1996. Biostratigraphical control of thrust models for the Southern Uplands of Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 86, 137–152.
Thirlwall, M F. 1988. Geochronology of late Caledonian magmatism in Northern Britain. Journal of the Geological Society of London, Vol. 145, 951–967.