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

Abstract

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, Þ 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 of all reliable palaeomagnetic data from the region indicates that the clockwise rotations are restricted to the rocks south of the South Mayo Trough. We 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.

A new Silurian palaeolatitude for eastern Avalonia and evidence for crustal rotations in the Avalonian margin of southwestern Ireland

“…It is suggested, therefore, that the offsets in the measured declinations across the Iapetus Suture are the result of local tectonic rotations, about vertical axes, as a result of Caledonian and/or Variscan deformation in southern Britain. Such local‐scale rotations have previously been documented, as a result of post‐Silurian deformation, on the Laurentian margin of the Iapetus (Mac Niocaill et al 1998).…”

Intersections of remanence small circles: new tools to improve data processing and interpretation in palaeomagnetism

“…Average standard deviation of remanence inclination is σ R = 8.4° (pre‐folding data sets only). Data from Gough & Opdike (1963), Klootwijk & Bingham (1980), Lin Jinlu & Watts (1988), Otofuji et al (1989), Sharps et al (1989), McFadden (1990), Channell et al (1992a,b), Chen et al (1992), Hirt et al (1992), Huang et al (1992), Klootwijk et al (1994), Thomas et al (1994), Cogné et al (1995), Haihong et al (1995), Patzelt et al (1996), Mac Niocaill et al (1998), Haubold et al (1999), Liu & Morinaga (1999), Thomas et al (1999), Uno (1999), Arriagada et al (2000), Huang et al (2000), Mac Niocaill (2000), Pueyo (2000), Henry et al (2001), Jordanova et al (2001), Waldhör et al (2001), Enkin et al (2000, 2002), Kravchinsky et al (2002), Schätz et al (2002), Zwing et al (2002), Enkin (2003), Halim et al (2003), Levashova et al (2003), Lewchuk et al (2003), Molina‐Garza et al (2003), Niitsuma et al (2003), Henry et al (2004), Pueyo et al (2004), Staiger (2004), Tamai et al (2004), Uno et al (2004), Huang et al (2005).…”

Mechanism of emplacement and crystallisation history of the northern margin and centre of the Galway Granite, western Ireland

“…Ryan et al (1995) have shown that this fault is a continuation of the Antrim-Galway and Southern Upland fault lines and was active at the initiation of intrusion of the main Galway Granite but that movement on it had ceased before the emplacement of the batholith ended, as is confirmed below. Neither they nor MacNiocaill et al (1998), who also appreciated its importance in controlling the emplacement of the granite, detailed how the Skird Rocks Fault generated the space for the granite. Unlike the Donegal Granite, in which the required space was demonstratably obtained in the gap between the exposed bifurcation and re-joining of a major sinistral fault, (Hutton 1982), no major faulting has been found along one side, the north side, of the Galway Granite.…”