THE CALEDONIAN STRIKE-SWING AND ASSOCIATED LINEAMENTS IN NW IRELAND AND ADJACENT AREAS - SEDIMENTATION, DEFORMATION AND IGNEOUS INTRUSION PATTERNS

Part of the major swing in strike associated with the Greenland-Labrad or Promontory of the North Atlantic Caledonides is exposed in consider able detail in the metasedimentary and meta-igneous rocks of the Neo-P roterozoic Dalradian Supergroup in northwest Ireland. We show that the strike swing controlled Dalradian stratigraphy and sedimentation patt erns and is therefore al least c. 600 Ma in age. Our interpretation of the stratigraphy allows a reconstruction of this promontory in the La urentian continental margin and shows that it separated a NE-trending, gently inclined and broad continental shelf in the north from an E-tr ending. more steeply inclined and narrower shelf to the south. Once es tablished in the geometry of the Dalradian sequence. the strike swing and basement promontory had a major influence on Caledonian deformatio n patterns. It controlled: the nucleation of major folds; the geometry of the regionally developed primary cleavages: the facing direction o f the early folds; and it also acted as a major buttress in perturbing and deflecting. on a regional orogenic transport vector. Accurate res toration of the sinistral displacement on a late Caledonian fault syst em shows that the apices of the strike swing at different stratigraphi c levels, and their associated sedimentary facies changes, define a li ne trending approximately N12 degrees E. This line is coincident with (again. after fault restoration): six out of the eight Devonian Donega l granites: the vast majority of the mantle-related, ultrabasic 'Appin ite' suite (with most of these bodies concentrated at the intersection of this line and the NE-trending Main Donegal Granite shear zone): an d the highest density of the Neo-Proterozoic basic tholeiite suite. Th ese and other data are used to suggest that the Donegal Lineament is t he expression of a major steeply inclined fault in the basement beneat h the Dalradian which is intimately related to the morphology of the p romontory. The fault, which might be as old as 1800 Ma, probably reach ed down into the lithospheric mantle, controlling igneous activity, in cluding the ascent, emplacement site and perhaps sources of the magmas . Another complementary strike swing and parallel lineament is describ ed from the Dalradian rocks of the adjacent part of Scotland. It is su ggested that these and other major lineaments (i.e old pre-Caledonian faults) which have either NNE, NE, or ESE trends reflect not so much t he location of the northern British Isles at a 120 degrees triple junc tion during Iapetus rifting. but rather the exploitation of these pre- existing trends in the pre-Caledonian Atlantic borderlands during Iape tus opening.