Obduction of the Ballantrae Complex, Southern Uplands

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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.

The following are related articles:

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

Obduction of the Ballantrae Complex

Geological sketch map of the Balcreuchan Port to Bennane Head area showing the structural repetition of the volcanosedimentary succession. P912322.
Possible model for the development of the various components of the Ballantrae Complex within an oceanic volcanic arc and backarc basin. P912321.

The Ballantrae Complex is an assemblage of mantle and crustal rocks formed in a variety of geotectonic settings: island arc, marginal basin and oceanic island. The various components were dismembered and juxtaposed during the obduction of the Ballantrae Complex onto the margin of Laurentia, which can be regarded as part of the Grampian Event. The initiation of obduction is dated by the radiometric (K-Ar) age of 478±8 Ma obtained from amphibolite within the dynamothermal metamorphic aureole at the base of the northern serpentinite belt. The graptolite fauna from mudstone associated with a serpentinite conglomerate at North Ballaird is likely to date ‘obduction-in-progress’ to the late Arenig. Obduction would then have been complete prior to the deposition of the Llanvirn basal beds of the Barr Group (the Kirkland Conglomerate Formation) on top of the assembled Ballantrae Complex. Detailed structure within the obducted ophiolite is difficult to unravel, but some indication of the structural complexity is shown by the tectonic repetition of a thin, fossiliferous mudstone–chert–lava succession between Bennane Head and Balcreuchan Port (P912322). The structural contacts between the different components of the complex are generally steep, and there is evidence for widespread strike-slip movement along the major faults.

Though there is no proof of the subduction polarity, it is conceptually easier to envisage southward subduction of marginal basin crust beneath an Iapetus island arc, since this would make arc–continent collision inevitable (P912321). To continue the closure of the Iapetus Ocean, collision and ophiolite obduction must then have been followed by a reversal in subduction direction (a well-established phenomenon elsewhere in the geological record), so that the ocean crust was lost northward beneath the Laurentian continental margin, now endowed with an attached ophiolite complex. Once northward subduction was established, the build-up of the Southern Uplands accretionary terrane could commence.

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