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1 Department of Geological Sciences, University of Durham, Durham DHI 3LE, UK
2 Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK
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 N12ºE. 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. Thes e 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 120º triple junction during Iapetus rifting. but rather the exploitation of these pre-existing trends in the pre-Caledonian Atlantic borderlands during Iapetus opening.
Keywords: Ireland, Caledonian Orogeny, lineaments, granites.
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