Along-strike variation in the stratigraphical and structural profile of the Southern Uplands Central Belt in Galloway and Down

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Abstract
The modern interpretation of the Southern Uplands depends on the recognition of laterally extensive, linear, fault-bounded tracts of northward-younging Lower Palaeozoic sediments with rare volcanics. The tracts become progressively younger southwards and are thought to have been sequentially accreted by northward underthrusting above a subduction zone. Detailed mapping and tentative correlation of three, well-exposed, coastal profiles through the Central Belt in SW Scotland and NE Ireland offers a new test of the Southern Uplands model. Comparison of the northern parts of each section indicates the presence of strike-parallel tracts with model structure. Moffat Shale outcrops associated with the tract-defining faults demonstrate a diachronous, incremental, southerly decrease in the age of the base of the overlying turbidites, essentially as in the Northern Belt. Southward this becomes markedly less pronounced, particularly in Down where the Central Belt is much wider than in Galloway. Distinctive Hawick Group lithologies permit the correlation of the southern parts of the sections. Large areas of predominantly southward-younging occur in each section, especially in Galloway where the southward-younging area is 12 km across. Here northerly-verging D 1 fold pairs are consistent with observed fault movement opposite in sense to the northerly underthrusting of the model. Back thrusting, in the style of the Pleistocene Cascadia Basin, is invoked to account for the landward-verging structure of these areas and to explain the narrowing of the Central Belt in Galloway. The structure of the Central Belt thus differs significantly from that of the Northern Belt and the accretionary prism model.