The structure and metamorphism of the moine succession in Ardnamurchan, West Highlands, Scotland with particular reference to the tectonics of the Morar anticline

H. O'Brien, B.
(1981) The structure and metamorphism of the moine succession in Ardnamurchan, West Highlands, Scotland with particular reference to the tectonics of the Morar anticline. PhD thesis, University of Liverpool.

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In the Ardnamurchan area the Moine Succession is divided into seven lithostratigraphical units. From W to E they are the Glenmore Psammite, the Carna Pelite, the Laga Psammite, the Dorlin Pelite, the Tarbert Psammite, the Mingarry Psammite and the Acharacle Complex. The rocks within these units are mostly metasedimentary schists and gneisses probably formed from sandstones, arkoses and siltstones. Where primary bedforms are preserved, they suggest that the sediments may have been originally deposited on either an alluvial fan or a marine delta. In places psammites and pelites occur in stratigraphically continuous, sedimentary sequences within the lithostratigraphical units. The contacts of the latter are thought, however, to be sharply bounded by brittle faults rather than depositional surfaces. T9gether the major units comprise an essentially tectonic succession 'of rqcks. Four phases of superimposed deformation are regionally recognizable in the Ardnamurchan area. The Dl-D4 deformations are identifiable in each of the lithostratigraphical units, although there is a significant spatial variation in the ubiquity and intensity of each deformation phase. The regional deformation of the Moine rocks in the thesis area is characteristically inhomogeneous with successive strains added incrementally. In the areas affected by large strains strong, composite LS-fabrics are developed. In the areas that suffered negligible strains, right-way-up rocks are commonly found displaying several ages of bedding-cleavage intersections and complex fold interference patterns are also present. Dl structures are usually modified or obliterated by the effects of later, superimposed strains. In the western and central parts of the area, however, Fl recumbent folds are found in extensive flat belts preserved as augen of low strain. Sl is observed as an axial planar crenulation cleavage or a strong shape fabric. Conjugate F2 folds are widespread and most take the form of either gently plunging, step-like asymmetrical folds or steeply plunging, isoclinal, similar folds. Variably penetrative, steeply dipping S2 cleavage is attendant to the F2 minor structures but, in general, it is not axial planar. 52 shape fabrics are produced on the steep limbs of some, but by no means all, F2 folds. Conjugate S3 cleavages are axial planar to sub-recumbent F3 folds. F3 asymmetrical folds are best developed in the areas least affected by D2 strain. There they refold steepened F1 folds and Sl ~leavage or transect the cores of F2 step-folds with long, flat limbs. Conjugate sets of open F4 folds refold all earlier folds and fabrics. They display variable amounts and directions of plunge. Axial planar 54 cleavage is normally widely spaced but in narrow zones in the E of the area it completely transposes earlier foliations. The Moine schists in the Ardnamurchan area have undergone a prolonged history of regional metamorphism which has locally been separated into three broad stages. Stage I occurred during the heating cycle of the metamorphism and culminated probably in the Precambrian with the formation of the climactic mineral assemblage. Stages 2 and 3 occurred at lower metamorphic grades during the cooling cycle and they are most likely both Caledonian events. The Stage 2 metamorphism was intimately associated with the development of the Dl to D4 structures and was the cause of an incomplete syntectonic hydration of the climactic assemblage. The Stage 3 metamorphism produced post-D4, randomly orientated porphyroblasts which, indicate that the progressive ,metamorphism during the cooling cycle outlasted the regional deformation. A chlorite retrogression affected all of the lithostratigraphical units in the thesis area after the stage 3 metamorphism. 'The shortening of the Moine Succession in the Ardnamurchan area ·is modelled in terms 'of the evolution of thrusts and nappes. Four fundamental episodes are re~ognized: (1) the formation and emplacement olthe nappes, (2) the uprighting and shortening of the nappes, (3) the backfolding of the nappes 'and (4) the tightening and shortening of the nappes.

Item Type: Thesis (PhD)
Depositing User: Symplectic Admin
Date Deposited: 20 Oct 2023 19:44
Last Modified: 20 Oct 2023 19:53
DOI: 10.17638/03175667
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