The Caledonian strike-swing and associated lineaments in NW Ireland and adjacent areas: sedimentation, deformation and igneous intrusion patterns

Abstract

Abstraet: Part of the major swing in strike associated with the Greenland-Labrador Promontory of the North Atlantic Caledonides is exposed in considerable detail in the metasedimentary and meta-igneous rocks of the Neo-Proterozoic Dalradian Supergroup in northwest Ireland. We show that the strike swing controlled Dalradian stratigraphy and sedimentation patterns and is therefore at least c. 600 Ma in age. Our interpretation of the stratigraphy allows a reconstruction of this promontory in the Laurentian continental margin and shows that it separated a NE-trending. gently inclined and broad continental shelf in the north from an E-trending. more steeply inclined and narrower shelf to the south. Once established in the geometry of the Dalradian sequence, the strike swing and basement promontory had a major influence on Caledonian deformation patterns.It controlled: the nucleation of major folds: the geometry of the regionally developed primary cleavages; the facing direction of the early folds: and it also acted as a major buttress in perturbing and deflecting. on a regional scale, the orogenic transport vector.Accurate restoration of the sinistral displacement on a late Caledonian fault system shows that the apices of the strike swing at different stratigraphic levels, and their associated sedimentary facies changes, define a line trending approximately N12"E. This line is coincident with (again, after fault restoration): six out of the eight Devonian Donegal granites: the vast majority of the mantle-related, ultrabasic 'Appinite' suite (with most of these bodies concentrated at the intersection of this line and the NE-trending Main Donegal Granite shear zone): and the highest density of the Neo-Proterozoic basic tholeiite suite. These and other data are used to suggest that the Donegal Lineament is the . expression of a major steeply inclined fault in the basement beneath the Dalradian which is intimately related to the morphology of the promontory. The fault, which might be as old as 1800 Ma, probably reached down into the lithospheric mantle. controlling igneous activity, including the ascent, emplacement site and perhaps sources of the magmas. Another complementary strike swing and parallel lineament is described from the Dalradian rocks of the adjacent part of Scotland. It is suggested that these and other major lineaments (i.e old pre-Caledonian faults) which have either NNE. NE, or ESE trends reflect not so much the location of the northern British Isles at a 120" triple junction during Iapetus rifting. but rather the exploitation of these pre-existing trends in the pre-Caledonian Atlantic borderlands during Iapetus opening.

Gold potential of the Dalradian rocks of north-west Northern Ireland: prospectivity analysis using Tellus data

“…Similar features have been identified in the Dalradian of north-west Donegal and south-west Scotland (Hutton and Alsop 1996). This north-north-east-trending, pre-Dalradian structure, possibly as old as 1800 Ma, is likely to have controlled Dalradian sedimentation patterns and is associated with an increase in mantle-derived, tholeiitic magmatism and is responsible for a major change in bedding orientation and the dominant foliation in Dalradian rocks to the east of the Lack Inlier (Earls et al 1996b;Mitchell, 2004).…”

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

“…The agreement of the palaeomagnetic results from both Clare Island and Mweelrea with the palaeomagnetic directions from stable Laurentia points to the orogen having an original curved or arcuate structure. We postulate that this most likely relates to the presence of promontories and embayments in the Laurentian margin as proposed by Hutton and Alsop (1996). These promontories and embayments are likely to have controlled and=or modified, the style and geometry both of younger sedimentary basins and the structural grain of younger deformational structures.…”

“…On the basis of sedimentary facies changes in the Dalradian (Late Precambrian) of northwestern Ireland and restoration of Caledonian fault displacements in this sector of the orogen, Hutton and Alsop (1996) argued that the swing in strike is primary. They concluded that the arcuate form of the orogen most likely reflects 'pre-existing trends in the pre-Caledonian Atlantic borderlands'.…”

Refined model of incremental emplacement based on structural evidence from the granodioritic Newry igneous complex, Northern Ireland

“…Several deepseated crustal lineaments are thought to have influenced ascent and emplace ment of magma in the UK (Jacques and Reavy, 1994;Hutton and Alsop, 1996;Stevenson et al, 2006;Cooper et al, 2013). These lineaments are known primarily from geophysics (gravity and magnetic data) and are thought to trend approxi mately northsouth over much of the UK to the north of the Iapetus suture.…”

“…These lineaments are known primarily from geophysics (gravity and magnetic data) and are thought to trend approxi mately northsouth over much of the UK to the north of the Iapetus suture. The origin of the lin eaments is unclear, although they are proposed to represent preCaledonian faults (Jacques and Reavy, 1994;Hutton and Alsop, 1996;Cooper et al, 2013).…”