The closure of the Iapetus Ocean and Tornquist Sea: new palaeomagnetic constraints

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Abstract
Early Wenlock volcanic rocks from the East Mendips inlier, SW England, yield primary palaeomagnetic directions which indicate that southern Britain (Eastern Avalonia) occupied a palaeolatitude of 13 ± 5°S in Mid-Silurian (430 Ma) times. When combined with coeval data from Laurentia, the southern Laurentian margin (Scottish terranes), and Baltica, the collective data indicate that: (1) the British and Scandinavian sectors of the Iapetus Ocean were closed, to within the limits of the palaeomagnetic resolution, by the Early Wenlock; (2) Acadian deformation across Britain, which culminated in Early Devonian time, post-dated initial docking of Eastern Avalonia and Laurentia; (3) a previous palaeomagnetic requirement for the Tornquist Sea to remain open into Mid-Silurian times is removed, thereby reconciling palaeomagnetic and biogeographical constraints upon the convergence of southern Britain and Baltica. Recently-acquired palaeomagnetic data from southern Britain and Baltica have facilitated more reliable reconstructions of the early Palaeozoic palaeogeography of Northern Europe (Fig. 1). Palaeomagnetic data now record the progressive drift of southern Britain across the Iapetus Ocean during Ordovician times (Torsvik & Trench 1991; Channell et al. 1992 ) and a predominant northward drift and counterclockwise rotation of Baltica from Cambrian to Devonian times ( Torsvik et al. 1991 a Torsvik et al. 1992 a; Perroud et al. 1992 ). Nevertheless, details on the closure history of the Iapetus Ocean and Tornquist Sea (Fig. 1) have remained sketchy due to a sparseness of coeval Late Ordovician–Mid-Silurian data from the bordering continents. These palaeomagnetic uncertainties permit competing geological models. In this contribution, we address two outstanding problems relating to the closure