PALEOMAGNETIC STUDY OF LIANDOVERY (LOWER SILURIAN) RED BEDS IN NORTH-WEST ENGLAND

High unblocking temperature magnetization components carried by haemat ite have been isolated from 12 sites of Llandovery (Lower Silurian) re d mudstones from NW England. The principal deformation phase in this a rea is Acadian (mid-Devonian), however, some faults (with related fold s) were also active during the later Variscan orogeny. The site-mean m agnetization directions pass a regional fold test at the 95 per cent c onfidence level, implying that the magnetization components pre-date A cadian folding. The overall mean direction (Declination: 43.4-degrees, Inclination: -24.1-degrees, alpha95: 12.4-degrees) gives a pole posit ion at 13.6-degrees-S, 313.9-degrees-E and a palaeolatitude of 12-degr ees-S. The low palaeolatitude implies closure of the Iapetus Ocean bet ween Eastern Avalonia and Laurentia by Late Llandovery time and is con sistent with a recently published palaeolatitude (13-degrees-S) for th e Wenlock (mid-Silurian) of southern Britain. Comparison of these valu es with the published palaeolatitude from NW England of 43-degrees-S f or Caradoc (Late Ordovician) time implies very rapid (at least 15 cm y r-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 Wal es, but is significantly different from a group of mid-Silurian poles comprising data from Scottish intrusions and from Swedish sediments. T his discrepancy may be due to Late Silurian/Early Devonian (pre-Acadia n) remagnetization of our sampling sites, or local clockwise rotation of the sampling sites during Acadian and/or Variscan deformation. Howe ver, the positive fold test, presence of reversals and local deformati on history tend to mitigate against these possibilities. The more like ly interpretation is that although Baltica/Scotland were one continent al block from the mid-Silurian (Scandian orogeny), Eastern Avalonia di d not become part of this block until latest Silurian.