Deformation studies in the Irish Caledonides

Abstract

An interpretation of the major structure of parts of the Irish Caledonides is made, based on the results of mapping of the main early penetrative fabrics. Interpretation of major changes in fabric are attributed to strain variation, and models are developed to explain these changes. The regional nappe structures in the Dalradian rocks have been modified by (1) D 2 slide zones, and (2) a D 4 sinistral shear zone adjacent to the Main Donegal Granite in NW Donegal. In N Mayo the major structure is interpreted in terms of a steep dextral E-W shear zone modifying the early formed nappe pile. The strain history adjacent to the Iapetus suture in central Ireland is described and a wide zone of non-axial planar cleavage discussed. In SE Ireland the regional strain pattern is modified in the tectonic aureole of the Leinster Granite.

Late Caledonian granitoids and timing of deformation in the Iapetus suture zone of eastern Ireland

“…However, there is a unifying characteristic between these areas which suggests that this latter interpretation may not be the case. The evidence principally concerns the steeply inclined SI cleavage which, in a zone up to 50 km wide on either side of the supposed suture line, transects clockwise of the Fl fold axial surfaces (Soper & Hutton, 1984;Murphy, 1985;Cameron, 1981;Sanderson et al 1980). Consistent structural patterns are presumably caused by similar processes which, in this instance, are related to a regional sinistral transpression (Soper & Hutton, 1985) along discrete Caledonian faults.…”

Strike-slip terranes and a model for the evolution of the British and Irish Caledonides

“…cleavage transecting clockwise of Fj fold axes (Sanderson et al 1980;Cameron, 1981;Soper & Hutton, 1984;Murphy, 1985;Soper, 19866). Critical evidence for this association comes from the fact that cleavage transection occurs with the development of sub-horizontal stretching lineations (suggestive of transcurrent motions), whereas strictly axial planar cleavage is invariably related to the occurrence of steep, down-dip stretching lineations (suggestive of across-strike shortening and thrusting).…”

Tectonic relationships of the Leinster Granite, Ireland

“…, more oblate ellipsoids) than those for the surrounding country rocks, where some limited strain work has been camed out Sanderson et al 1980). Leake (1978), in a general review of Irish Caledonian granites, suggests that a spatial association exists between various lineaments and some of the Irish granites.…”