John Sutton

The pre-Torridonian metamorphic history of the Loch Torridon and Scourie areas in the North-West Highlands, and its bearing on the chronological classification of the Lewisian

Summary The paper describes the metamorphic history of two areas of Lewisian gneiss in the North-West Highlands of Scotland. Metamorphosed sedimentary rocks are described for the first time from the Loch Torridon area. It is shown that both in the Loch Torridon and the Scourie districts the gneisses have been produced in two separate periods of metamorphism, migmatization and deformation. These two metamorphic episodes, which are named the Scourian and Laxfordian episodes, are separated in time by a period of uplift and tension during which a series of uniform dolerite dykes was intruded. Since a very great interval of time appears to have elapsed between the two metamorphic episodes, it is suggested that the rocks produced during these episodes should be regarded as members, not of a single formation as heretofore, but of two distinct chronological units.

Crustal downfolding associated with igneous activity

Abstract The Ketilidian metamorphic rocks of South Greenland were intruded by post-orogenic, elongate, mushroom-shaped intrusions adjacent to which the country rocks have subsided at least 1 km. The form of the intrusions and their effects on the country rocks closely follow the theoretical predictions of Ramberg (1967). A model is developed to account for the structures observed from a consideration of the relative viscosities of the country rocks and the intrusive magmas and expected stress/strain rate relationships. Analogous subsidences, including grabens, associated with other intrusions of different physical properties and on different scales are reviewed. The general model derived is discussed in relation to phenomena such as the tilting of basalts to form linear features on the ocean floor.

V.—Further Observations on the Margin of the Laxfordian Complex of the Lewisian near Loch Laxford, Sutherland

The paper is concerned with one of the marginal parts of the Laxfordian complex of the Lewisian of north-western Scotland where the older Scourian complex ( c. 2500 million years) was reactivated during the Laxfordian orogeny ( c. 1600 million years). The effects of Laxfordian deformation are discussed with reference to the distortion of Scourian structures and post-Scourian dolerite dykes and to the orientation of newly-developed structures. The arrangement of structures N.E. of the Laxfordian metamorphic front is thought to result from rotation of pre-existing planar elements during compression on north-east-south-west lines, without the development of very large folds. The connection between the style and arrangement of Laxfordian structures and the metamorphic state of the rocks at the time of deformation is illustrated with reference to both local and regional variations, and some implications of the regional changes in the structural pattern are discussed.

The Structure and Stratigraphical Succession of the Moines of Fannich Forest and Strath Bran, Ross-Shire

Summary The order of deposition of metamorphosed Moinian sediments in an area a few miles east of the Moine thrust has been determined with the aid of current-bedding. The previously accepted succession is seen to be incomplete and inverted, and the structural interpretation based on it is therefore largely invalid. Within a succession some 13,000 feet thick, rocks formerly regarded as inliers of Lewisian appear at two horizons. The lower, forming the supposed inliers of Scardroy and Achnasheen, contains abundant banded hornblendic rocks; the upper, the so-called Fannich inlier, is dominantly siliceous. The major folds, with a wave-lenght of several miles, are overturned towards the west and are isoclinal only in the south. Their axes run, on the average, N.-S., but show much variation in the direction and angle of plunge. Relative variations in the plunge of an adjacent syncline cause the steep or inverted limb common to them both to change its thickness or even to disappear. Variations in plunge lead also to the formation of contemporaneous subsidiary flexures running NW.-SE. The relations of minor structures (small folds, cleavage and lineations) to the major fold system are discussed. I. Introduction The part of Wester Ross in the Northern Highlands of Scotland with which this paper deals (see Fig. 1) was mapped many years ago by the Geological Survey of Great Britain and described in two memoirs published in 1913 : one on the Fannich Mountains (explanation of sheet 92) and one on central Ross-shire (explanation of sheet 82). A small

The Deposition of the Upper Dalradian Rocks of the Banffshire Coast

Abstract Sedimentation structures in Upper Dalradian rocks belonging to the Banff Division of north-east Scotland suggest that the rocks were deposited in deep water by the action of turbidity currents. These structures, including grading of grits, greywackes and flags, and slumping, are described in detail and compared with the structures of sediments in other series of greywacke-facies. It is suggested that the change of sedimentary facies at the beginning of Upper Dalradian times may have been due to the emergence of a new land-mass on the north-western side of the basin of deposition, providing a rapid supply of coarse detritus. The uplifting of this land-mass might have been associated with an early phase of Caledonian folding and metamorphism.

STRUCTURES IN THE CALEDONIDES BETWEEN LOCH DUICH AND GLENELG, NORTH-WEST HIGHLANDS

At least three periods of folding affected the Moine and Lewisian rocks south of Loch Duich in the North-West Highlands before the formation of the Moine thrust. Two major folds are recognized and shown to belong to a set recognized elsewhere. The folding is frequently markedly disharmonic and is accompanied by sliding. The small-scale structures are described and figured. It is shown that the intensity of the regional metamorphism and related phenomena diminished during the folding. The changes in structural style resulting from this are described. A system of quartz-filled gashes in the Torridonian west of the Moine thrust is shown to be related to the Loch Alsh fold. Conjugate folds and other structures formed with the Moine thrust, are shown to be later than any of the large folds affecting the Moines.

The Boyndie syncline of the Dalradian of the Banffshire Coast

Summary The paper deals with the great fold affecting the Dalradian rocks exposed on the Banffshire coast, for which the name Boyndie syncline is now proposed. It is a large asymmetrical syncline whose axis runs north-north-east and emerges on the coast west of Banff. Its western limb is steeply inclined and, as Read showed, youngs eastward; over short stretches the rocks of this limb are intensely corrugated by subsidiary folds. The eastern limb is, as a whole, very gently inclined and is flexed to form the shallow Turriff syncline. Much of this limb is violently corrugated on a small scale. Variations in the attitude of the axial planes of small folds within the Boyndie syncline are described and discussed. It is suggested that the corrugation of the eastern limb is due to the squeezing out of incompetent beds from the core of the major fold. The central part of the steep western limb of the syncline has been intensely deformed and boudinage, mullioning and allied processes have led to the formation in this zone of a tectonic melange in which elonaated or lenticular masses of rock are enclosed in a less competent matrix. The axiil plunge of small-scale structures in this zone is very variable and is often steep. Preliminary petrofabric studies show that the preferred orientation of quartz is much stronger here than elsewhere in the fold. An oblique cleavage is imposed on rocks to the east and west of the intensely deformed sector. The possible significance of the structures seen on the Banffshire coast is discussed and their bearing on current views on the tectonics of North-East Scotland is debated.

The supposed Lewisian inlier Of Scardroy, Central Ross-Shire, And its relations with the surrounding Moine rocks

Summary The Scardroy area lies east of the Moine thrust in central Ross-shire and is composed predominantly of metamorphosed sediments. Over considerable areas the rocks are gneissose in appearance, and these gneisses, together with associated striped hornblende-granulites and basic and ultrabasic minor intrusions, were regarded by the Geological Survey as representatives of the basement on which the sediments of the Moine Series were laid down, and were correlated with the Lewisian gneisses west of the Moine thrust. The present paper brings forward evidence to show that the gneissose rocks were formed by recrystallization and felspathization of siliceous, semi-pelitic, calcareous and hornblendic rocks which are integral parts of the Moine Series. Detailed mapping shows that certain horizons can be traced continuously across the supposed line of unconformity. Movements accompanying the metamorphism of the Moine rocks led to the production of an anticline and syncline, overturned towards the east and pitching east of south. The minor structures and the axes of quartz girdles determined by fabric analysis pitch in the same direction. On every scale, the same structural pattern characterizes both the Moine and the supposed Lewisian rocks, and no traces of older structures have been detected in the latter. A tentative stratigraphical succession, established with the aid of sedimentary structures, suggests that the rocks hitherto regarded as Lewisian are younger than a belt of undoubtedly Moinian semi-pelitic granulites. In view of the facts given above, the writers conclude that the rocks of the Scardroy area belong to a single series and that no rocks of Lewisian age are present.

Development of the continental framework of the Atlantic

Abstract The hypothesis is advanced that the opening of the Atlantic through continental drift since the Mesozoic is a direct consequence of structural changes which began in the late Pre-Cambrian. The main stages in this process, which are recorded in the continental framework of the present-day Atlantic, can be summarised thus: 1. Development of a system of mobile belts in the late Pre-Cambrian. 2. Stabilisation of successive sections of this system accompanied by progressive restriction of plutonism, and uplift of stabilised mobile belts and. adjoining forelands. Between the Cambrian and the Triassic such uplifts progressively increased the extent of the continental crust above sea-level. Gondwanaland was uplifted in the Cambrian; a northern continent in the Devonian, and the intervening tract emerged above sealevel in the Upper Carboniferous and Permian. 3. The Triassic continent which resulted from these uplifts extended from the Arctic to the Antarctic. Subsidence within this extensive continent produced chains of elongated Mesozoic basins largely located above mobile belts stabilised earlier in the Phanerozoic. 4. Introduction of basaltic magma which had produced extensive dyke swarms and continental lava piles many kilometres thick within the uplifted continents from Carboniferous times onwards, led to the fragmentation of the continent and the opening of the Atlantic in the Cretaceous as new oceanic crust developed. The fractures up which magma was intruded were located for the most part within the Mesozoic basins, which in turn overlay stabilised Phanerozoic fold-belts whose disposition thus determined the shape of the Atlantic. The horizontal movements that produced the Atlantic carried crust and underlying mantle away from regions which had been previously uplifted after orogeny had ceased, towards regions where active orogeny continued. There is thus likely to be a common cause for the initiation of drift and the earlier diachronous ending of orogeny, which had stabilised the interior of Gondwanaland and Laurasia while leaving active mobile belts around the margins of the super-continents.

Metamorphism in Deep-Seated Zones of Transcurrent Movement at Kungwe Bay, Tanganyika Territory

Deformed rocks developed in zones where transcurrent movements took place during a late stage of the Ubendian orogeny are described from Kungwe Bay, western Tanganyika Territory. In areas where the regional metamorphism was still continuing, the mineral assemblages developed in the movement zones are the same as those developing regionally. Cataclastic textures characterize such movement zones. In other parts of the district, where the regional metamorphism had apparently come to an end, the zones of concentrated movement contain both mylonites and certain fine-grained deformed rocks which show no obvious cataclastic features. Some of the latter have textures recalling those of hornfelses and contain minerals such as garnet and hornblende which have remained stable during the deformation and have grown porphyroblastically. It is considered that these rocks result from deformation of the crust at depth, resulting in a local high-grade dislocation metamorphism to which heat released during the movements may have contributed. The significance of the deformed rocks and of the transcurrent movement system in which they occur is discussed, and a possible analogy with active seismic belts of the present day is mentioned.