Trans-Iapetus contrasts in the geological development of southern Scotland (Laurentia) and the Lakesman Terrane (Avalonia)

Abstract

Abstract The Iapetus Ocean was a major feature separating Laurentia and Avalonia in the early Ordovician. The early Palaeozoic, Laurentian margin of the ocean is preserved in the UK in inliers in the Midland Valley of Scotland and in the Southern Uplands Terrane. The oldest rocks, of similar age to the deep-marine facies of the Skiddaw and Manx Groups opposite on the Avalonian margin, form the fragmentary ophiolitic sequences preserved in the Ballantrae and Highland Border complexes. These, together with equivalents displayed more extensively in Newfoundland, show that a succession of volcanic arcs and back arc basins formed and were accreted onto the Laurentian margin as a result of subduction during the late Cambrian and early Ordovician. In contrast, a suprasubduction zone extensional regime may have dominated the Avalonian margin at that time. Evidence for the progressive destruction of the ocean by northward directed subduction is preserved in the Caradoc-Wenlock rocks of the Southern Uplands. The tectonostratigraphic configuration, with fault-bound slices containing thick turbidite sandstone sequences, of individually restricted duration, resting southwards on progressively younger oceanic mudrocks, is suggestive of an accretionary complex, although the precise situation is debated. Interpretation of the structural complexity of the Southern Uplands is reliant upon extensive biostratigraphical data, prior to the availability of which the understanding was at a level comparable to that of the Manx Group at the present time. The late Llandovery-Wenlock Hawick and Riccarton Groups in the Southern Uplands continue the general tectonostratigraphical pattern. These younger turbidites are, however, of distinctive lithological character and show similarities with Wenlock to Ludlow sandstone-dominated sequences in the Windermere Supergroup in the southern Lake District and the Niarbyl Formation on the Isle of Man. This correlation between the Laurentian and Avalonian margins confirms that the Iapetus Ocean was no longer a significant feature by the mid-Silurian. The Southern Uplands accretionary thrust front migrated southwards on to the Avalonian foreland during the late Silurian as the Avalonian plate was subducted beneath Laurentia. Deformation in the Southern Uplands was largely complete prior to emplacement of c. 400 Ma granite plutons, whereas the Acadian deformation of the Skiddaw and Manx Groups was concentrated in the early Devonian at c. 390 Ma. However, similarities in structural style in the two areas seem to arise from the operation of similar mechanisms.