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-App alachian orogen where the structural grain swings from the NE-SW trend that typifies the Caledonides of northern Britain to an E-W orientation. The or igin of this arcuate structure has previously been proposed to be either pr imary, reflecting the original geometry of promontories and embayments in t he Laurentian margin in this sector of the orogen, or secondary, as evidenc ed by palaeomagnetically determined clockwise rotations in Silurian rocks i n the region. We report the results of a palaeomagnetic investigation of th e Early to middle Ordovician Mweelrea Ignimbrites (Llanvirn), from an accre ted Taconic are (the South Mayo Trough) in that part of the western Irish C aledonides that has an E-W trend. After removal of a low-temperature compon ent 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 pala eomagnetic fold test yielding a tilt-corrected mean remanence direction D = 132.6 degrees, I = 29.1 degrees, alpha(95) = 17.9 degrees which correspond s to a palaeopole at 36.4 degrees E, 9.9 degrees S. The pole is in reasonab le agreement with established Early to middle Ordovician reference poles fo r Laurentia and indicates that no significant rotation occurred between the E-W South Mayo Trough and the Laurentian margin after accretion, and there fore that the curvature of this segment of the orogen is likely to be prima ry. This contrasts with other results from Silurian sequences to the south, which record significant post-Silurian clockwise rotations. Examination of all reliable palaeomagnetic data from the region indicates that the clockw ise rotations are restricted to the rocks south of the South Mayo Trough. W e propose that the rotations in the south are linked with the superposition of the pre-existing structural grain on the younger rock units during Late Silurian and Early Devonian sinistral transpression. (C) 1998 Elsevier Sci ence B.V. All rights reserved.