John L. Roberts

Polyphase generation of nappe structures in the Dalradian rocks of the southwest Highlands of Scotland

Synopsis A regional synthesis of the Dalradian structures in the South-west Highlands of Scotland is attempted, based on the conclusion that the Iltay Boundary Slide passes into the Ballachulish Slide as it is traced around the Glen Orchy Antiform and the Stob Bhan Synform. On this framework the primary and secondary folds are traced throughout the South-west Highlands within the constraints imposed by the stratigraphy and structural knowledge. The primary folds associated with the development of the Tay Nappe at a high structural level are themselves affected by secondary folds at a deeper level. These folds define the Glen Orchy Nappe with an amplitude exceeding 25 km, underlying the Tay Nappe. The primary structures are traced through the secondary structures to root at depth within the Moine rocks lying between the Glen Orchy Antiform and the Kinlochleven Antiform. It is concluded that the polyphase development of nappes in the SW Dalradian is essentially a superficial phenomenon.

Oblique fold axes in the Dalradian rocks of the Southwest Highlands

Synopsis The development of folds with axes oblique to the northwest-southeast stretching-direction during the first two stages in the deformational history of Dalradian rocks in the Southwest Highlands is considered in terms of a quantitative model recently proposed by Sanderson (1973). This model allows the distribution of fold axes within the axial planes of the folds to be related to (1) the initial variability of the individual fold axes about a mean fold axis, (2) the initial attitude of the mean fold axis relative to the stretching-direction and (3) the amount of subsequent deformation within the axial planes of the folds. The various patterns of distribution shown by the fold axes in the Southwest Highlands are related to regional variations in (1) the initial attitude of the mean fold axis relative to the stretching-direction and (2) the amount of deformation within the axial planes of the folds. Measurement of strain indicators such as breccia fragments confirms that the strain ratios determined from the distribution of fold axes provide a reasonable estimate of the strain. Previous ideas concerning the development of these oblique folds are discussed in the light of the present work.

A revision of the stratigraphic succession of the East Mainland of Shetland.

Synopsis The succession of metamorphic rocks in the eastern part of the Mainland of Shetland is briefly described. Four main divisions are recognised. These rocks are compared in terms of lithology and sedimentary structures with the Moine and Dalradian rocks of Scotland. The westernmost division of the Shetland succession is correlated with the Moine while the other three divisions are considered to be Dalradian.

Sedimentary affiliations and stratigraphic correlation of the Dalradian rocks in the South-west Highlands of Scotland

Synopsis The concept of sedimentary affiliations (cf. the sedimentary associations of Knill 1959) is introduced, and three such affiliations (the black slate, the graded quartzite, and the greywacke affiliations) are described from the Dalradian metasediments younger than the Easdale Slates. The stratigraphic distribution of these affiliations is the basis for the proposed stratigraphic correlations. The difficulties of correlation across the Cowal Antiform and along the Highland Border are discussed. Current directions and facies changes allow the palaeogeography to be outlined. The distinction between the graded quartzite and greywacke affiliations provides evidence on the ‘greywacke problem’.

The intrusive form of some basalt dykes showing flow lineation

The dilational intrusion is described of some basalt dykes of Tertiary age into the Easdale Slates at Easdale, Argyllshire. The intrusive form of the dykes results from the walls of irregular fractures, guided by pre-existing planes of weakness, moving apart under magma pressure. The propagation of the fractures is discussed in relation to the external stress field controlling the intrusion of the dykes and the direction of opening of the fractures considered in relation to the magma pressure and the external stress field. The flow lineations developed at the margins of the dykes are discussed in terms of their mode of origin. Although the attitude of these lineations varies with the intrusive form of the dykes, the overall direction of flow of the magma was upwards to the north of northwest.

The Loch Nant area, Argyllshire, and its significance in the structural history of the Loch Awe Syncline

Synopsis The examination of a small area of Dalradian rocks near the axial zone of the Loch Awe Syncline, on its north-western limb, shows that an inverted succession faces upwards to the south-east on a primary slaty cleavage. No evidence has been found to support the pre-cleavage folding recognised from the same area by Harris and Berridge (1969), nor their re-interpretation of the stratigraphy. It is concluded that the succession established by Bailey (1913) still holds and that the Loch Awe Syncline is a primary structure, coeval with the nappe forming movements of the Scottish Highlands.