The Ordovician history of the Iapetus Ocean in Britain: new palaeomagnetic constraints

Abstract

New late Tremadoc—early Arenig palaeomagnetic results from SW Wales imply that S Britain (part of Eastern Avalonia) occupied a southerly latitude of c . 60°S in early Ordovician times. When combined with Scottish Ordoviaan palaneomagnetic data, which indicate a 15°S latitude, the results indicate that the British sector of the Iapetus Ocean reached n latitudinal width of c . 5000 km in Tremadoc—Arenig times, which was reduced to c . 3000 km by Llanvirn—Llandeilo (mid-Ordovician) times. The new data resolve two previous controversies in Palaeozoic palaeogeography. First, the high southerly palaeolatitude links Avdonia to Gondwana, marginal to W Africa, thus reconciling codlicting reconstructions based upon either palaeomagnetic or faunal/facies evidence alone. Second, reliable Llanvim palaeomagnetic data imply that Avlonia had rifted northwards by Arenig time, whereas Armorica remained proximal to northern Africa throughout the Ordovician. The combined data therefore establish that Avdonia and Armorica formed separate micro-continents when rifting from Gondwana.