Palaeomagnetism and palaeogeography of the Southern Uplands block in Ordovician times

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Abstract
Synopsis: The major Ordovician igneous outcrops within the Southern Uplands were studied to evaluate the position of this region with respect to adjacent crustal plates during the later history of the Iapetus Ocean. Post-Arenig, pre-Caradocian gabbros and serpentinites of the Ballantrae Complex yield palaeomagnetic poles of 26°E, 10°S (dψ = 10.9, dχ = 18.8°) and 27°E, 12°S (dψ = 5.5, dχ = 9.7°) respectively after a.f. and thermal cleaning. Although 95 per cent confidence circles overlap with other British Ordovician data, magnetic inclinations are systematically shallower than contemporaneous results from England and Wales and correlate closely with other data from N of the Southern Uplands Fault line. A similar result is found from palaeomagnetic study of the (Lower Caradocian) Bail Hill Volcanics. The palaeomagnetic results support faunal and tectonic evidence that the major crustal separation in Ordovician times existed between the Southern Uplands and the northern Lake District. The visible ophiolite slices along the approximate line of the present Midland Valley represent pre-Llanvirnian obduction in a northerly direction of an earlier ocean basin.

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