The Southern Uplands Terrane in Scotland – a notional controversy revisited
Development of a regional geological interpretation for the Southern Uplands Terrane has been driven historically by advances in graptolite biostratigraphy, sandstone petrography and turbidite sedimentology (particularly the recognition of reliable way-up criteria), then, more recently, by a growing appreciation of the nature of accretionary processes at active plate margins. The latter has led to the widely held view of the terrane as an archetypal fossil accretionary prism, built up by the accretion at the margin of Laurentia of the sedimentary cover of Iapetus oceanic crust, the cover being stripped from the oceanic crust during its subduction beneath Laurentia. The achievement of this broad consensus has not been without controversy, which still persists in respect of some aspects of the overall model. Suggestions that the northernmost (Ordovician) part of the terrane might have originated in a back-arc basin have been disproved by a combination of mineral dating and basin thermal history analysis utilizing clay mineralogy. Nevertheless, it remains possible for that part of the terrane to have a continental margin, forearc depositional history. The southernmost (mid-Silurian) part of the terrane most probably developed in a foreland basin setting, as a fold-and-thrust belt overriding Avalonia, following the closure of the Iapetus Ocean. Progress in understanding the origins of the Southern Uplands Terrane has been achieved only by the collaborative application of multidisciplinary techniques – biostratigraphic, sedimentary, structural, geochemical and geophysical. This review is a tribute to all those who have contributed.