Basin thermal history favours an accretionary origin for the Southern Uplands terrane, Scottish Caledonides

Abstract

There is general agreement that the Siluro-Ordovician, Southern Uplands suspect terrane is an imbricate thrust belt formed adjacent to the Laurentian continental margin during closure of the Iapetus Ocean. However, there are conflicting models for its tectonosedimentary evolution, especially for the initial depositional environment. Was it a convergent forearc trench with subsequent incorporation of strata into a suprasubduction accretionary prism, or was it a back-arc or continental margin extensional basin subsequently caught up in a foreland fold and thrust belt? To address this issue the different thermal histories of the contrasting convergent and extensional sedimentary basin models have been investigated. Palaeogeothermal conditions in the Southern Uplands terrane, assessed from regional metamorphic patterns, clay mineral assemblages and K-white mica composition, rule out extension and support deposition in a low heat-flow, plate-convergent setting. There is no evidence for a change from extensional to convergent basin regimes from north to south, as required in some models. Instead, throughout the terrane, there is a close temporal and spatial relationship between deposition, burial metamorphism and tectonism that is elsewhere typical of accretion.