THE CLOSURE OF THE IAPETUS OCEAN AND TORNQUIST SEA - NEW PALEOMAGNETIC CONSTRAINTS

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-degrees-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.