Palaeomagnetic study of Llandovery (Lower Silurian) red beds in north-west England

Abstract

High unblocking temperature magnetization components carried by haematite have been isolated from 12 sites of Llandovery (Lower Silurian) red mudstones from NW England. The principal deformation phase in this area is Acadian (mid-Devonian), however, some faults (with related folds) were also active during the later Variscan orogeny. The site-mean magnetization directions pass a regional fold test at the 95 per cent confidence level, implying that the magnetization components pre-date Acadian folding. The overall mean direction (Declination: 43.4d, Inclination: -24.1d, α95: 12.4d) gives a pole position at 13.6dS, 313.9dE and a palaeolatitude of 12dS. The low palaeolatitude implies closure of the Iapetus Ocean between Eastern Avalonia and Laurentia by Late Llandovery time and is consistent with a recently published palaeolatitude (13dS) for the Wenlock (mid-Silurian) of southern Britain. Comparison of these values with the published palaeolatitude from NW England of 43dS for Caradoc (Late Ordovician) time implies very rapid (at least 15 cm yr−1) northward drift of Eastern Avalonia during Late Ordovician/Early Silurian time. The new pole position from the Llandovery red beds lies close to Late Silurian and Early Devonian poles from Scotland and Wales, but is significantly different from a group of mid-Silurian poles comprising data from Scottish intrusions and from Swedish sediments. This discrepancy may be due to Late Silurian/Early Devonian (pre-Acadian) remagnetization of our sampling sites, or local clockwise rotation of the sampling sites during Acadian and/or Variscan deformation. However, the positive fold test, presence of reversals and local deformation history tend to mitigate against these possibilities. The more likely interpretation is that although Baltica/Scotland were one continental block from the mid-Silurian (Scandian orogeny), Eastern Avalonia did not become part of this block until latest Silurian.