Oroclinal bending in the Caledonides of western Ireland: a mid-Palaeozoic feature controlled by a pre-existing structural grain

The Caledonides of the west of Ireland mark a segment of the Caledonian–Appalachian orogen where the structural grain swings from the NE–SW trend that typifies the Caledonides of northern Britain to an E–W orientation. The origin of this arcuate structure has previously been proposed to be either primary, reflecting the original geometry of promontories and embayments in the Laurentian margin in this sector of the orogen, or secondary, as evidenced by palaeomagnetically determined clockwise rotations in Silurian rocks in the region. We report the results of a palaeomagnetic investigation of the Early to middle Ordovician Mweelrea Ignimbrites (Llanvirn), from an accreted Taconic arc (the South Mayo Trough) in that part of the western Irish Caledonides that has an E–W trend. After removal of a low-temperature component of magnetisation, (L), interpreted to represent a viscous component of the earth’s present field, a high-temperature component of magnetisation, (H), was isolated at fifteen sites in four ignimbrite flows. H passes a palaeomagnetic fold test yielding a tilt-corrected mean remanence direction D D 132:6º, I D 29:1º, 95 D 17:9º which corresponds to a palaeopole at 36.4ºE, 9.9ºS. The pole is in reasonable agreement with established Early to middle Ordovician reference poles for Laurentia and indicates that no significant rotation occurred between the E–W South Mayo Trough and the Laurentian margin after accretion, and therefore that the curvature of this segment of the orogen is likely to be primary. This contrasts with other results from Silurian sequences to the south, which record significant post-Silurian clockwise rotations. Examination o