The demise of the Iapetus Ocean as recorded in the rocks of southern Scotland

The late Neoproterozoic to early Palaeozic Iapetus Ocean attained its maximum size towards the end of the Cambrian Period, separating Laurentia from Baltica to the east and Avalonia to the south. As the ocean began to close, subduction-related volcanic arcs formed at its margins, with vestiges now preserved in the late Cambrian to early Ordovician Ballantrae ophiolite complex. This was obducted in the mid-Ordovician as north-directed subduction of Iapetus oceanic crust was established beneath Laurentia. Thereafter, until the mid Silurian, a supra-subduction accretionary complex built up and now forms the Southern Uplands terrane. The accretionary complex grew through the southward-propagation of an imbricate thrust system that sequentially stripped the sedimentary cover from the subducting oceanic crust. This has produced a distinctive regional geology of linear, fault-bound tracts within which steeply inclined strata strike NE-SW; the oldest tract (Caradoc) forms the NW margin of the terrane whilst the youngest (Wenlock) forms the SE margin. Conversely, each individual tract youngs to the NW with a thin, basal development of graptolitic mudstone at its SE margin, conformably succeeded by a much thicker succession of turbiditic sandstone. As each tract was accreted its strata suffered deformation that was hence diachronous, earlier in the north than in the south. Eventually, in the mid Silurian, the Iapetus Ocean closed and the accretionary complex over-rode and depressed the margin of Avalonia, causing an abrupt increase there in Ludlow sedimentation rates. Deformation of the resulting strata 2 did not come until the early Devonian, ca. 400 Ma Acadian Orogeny, an event unrelated to the closing of Iapetus. The principal Acadian influence on the Southern Uplands terrane was sinistral transpression, with pre-existing structures re-activated and intensified, and granitic plutons and dyke swarms intruded.