Paleomagnetic results from volcanic rocks of the Shelve Inlier, Wales: evidence for a wide Late Ordovician Iapetus Ocean in Britain

Abstract

In order to shed light on the early Paleozoic paleogeography of Iapetus-bordering continents, samples for paleomagnetic study were collected from middle Ordovician (Llanvirn) volcanic rocks of the Shelve Inlier, Welsh Borderland. The folding within the inlier is constrained to late Ordovician (Ashgill) time, thus offering a fold test with tight age brackets. After stepwise thermal demagnetization, a pre-folding characteristic magnetization component was isolated from 95 specimens. The mean of 11 site mean directions for the characteristic magnetization gives D/I=116.4/+ 67.9(k = 89.4; α 95 = 4.9° ), and the paleomagnetic pole lies at 26.6°N, 36.1°E ( δp = 6.9°, δm = 8.2°). The fold test is positive at the 99% confidence level, indicating that the magnetization was acquired during the middle or late Ordovician. We infer from geological considerations that this magnetization is not a primary thermoremanence, but was acquired during late Ordovician time as a result of a thermochemical disturbance of that age. Our results indicate that southern Britain was near 51° south latitude during the later Ordovician. Since middle to late Ordovician paleolatitudes for Gondwana and Laurentia are well-determined, our result can be used to test various scenarios for the closure of Iapetus. We conclude that: (1) the Iapetus Ocean in Britain still had a latitudinal width of some 30° at this time, thus rendering a postulated Ordovician closure unlikely; (2) southern Britain, Avalon, and Hercynian Europe all occupied high latitudes during the later Ordovician, consistent with a paleoposition near the northern margin of Gondwana; and (3) the Acadian orogeny in Britain and North America could have been the result of the collision of Gondwana and Laurentia during early Devonian time.