Synopsis

During its Lower Palaeozoic history, the Midland Valley of Scotland was subject to compression from the blocks on either side, confirming the views of Kennedy (1958). Cambrian–Llanvirn-aged rocks of the Hebridean craton formed part of a passive margin, which extended to the ESE and is now replaced by the metamorphic complexes of the Moine and Dalradian. Both blocks had undergone metamorphism, uplift and shedding of their covers elsewhere. The ophiolitic part of the Highland Border Complex, which may be a continuation of the structurally foreshortened and rotated Ballantrae Complex, was obducted at c.490 Ma and may have interacted with rocks of the Hebridean passive margin.

Ordovician rocks at Girvan are a small remnant of a one-time far larger forearc basin, which formed to the south of a major arc complex, originally situated in the Midland Valley and probably extending from somewhere in the North Sea to offshore Ireland and beyond. The Highland Border Complex includes rocks coeval with the Midland Valley arc at c.470 Ma that formed a back-arc basin.

The Southern Uplands have converged on the Midland Valley, with the forearc sequence apparently missing in Scotland. Part of that convergence history is recorded in the Silurian conglomerates of the southern Midland Valley. Silurian sequences in this area contain conglomerates and breccias, derived from the south that record the presence of an igneous and possibly also a metamorphic complex in the region now occupied by the trench sequence of the Southern Uplands. It is suggested that the trench was later thrust over this source block and that the Southern Upland Fault forms part of that thrust sequence.

The metamorphic complexes to the north, replacing the Hebridean passive margin, did not interact with the Midland Valley until Lower Devonian times. It follows that there was no mountain-building orogeny in Scotland, other than an arc, a sliver of trench, and numerous examples of crustal contraction by strike-slip and/or thrusting.

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