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.