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Major and minor fracture analysis of the Tyndrum Fault Zone, Scotland, reveals a late Silurian history of transtensional deformation with opening across the zone as well as left-lateral strike-slip movements. The extensional phases are characterized by hydrothermal quartz veins and breccias associated with the early stages of precious-metal mineralization. The strike-slip movements are characterized by cataclastic textures and are associated with the later stages of the precious-metal mineralization. Further transtensional deformation in the Carboniferous is indicated by right-lateral strike-slip movements, associated with fractures throughout the zone containing both cataclastic and extensional hydrothermal quartz veins: the quartz veins are associated with base-metal mineralization. The pre-Devonian, WSW-ENE-directed, transtensional deformation of the zone is extrapolated to the whole Dalradian Terrane, the driving force being the gravitational collapse of the orogenic welt parallel to the tectonic trend. The necessary area increase is signified by the intrusion of the end-Caledonian granitic magmas and quartz-veins. The Carboniferous right-lateral movements resulting from N-S extension are related to similar movements, also transtensional, on the Great Glen Fault Zone and in the Midland Valley; the associated mineralization is related to a broader Dinantian base-metal mineralizing event.

The first section of the paper is concerned with a detailed account of the structure of the classic Schiehallion area. The account is extended to the flat belt via the Glen Lyon-Ben Lawers area. It is demonstrated that major D1 folds involve the whole of the Appin and Argyll groups and now face SE. However, the original D1 folds were probably open upward-facing folds which have been refolded and modified by the dominant D2 deformation. D2 is represented by an antiformal complex which culminates downwards in the Boundary Slide, a zone of strong deformation associated with NW translation of Dalradian over the ‘young Moine’ rocks. D3 has been responsible for the rotation of the originally flat D2 structures into their present ‘steep’ attitudes N of Glen Lyon. D4 is represented by the strike-swing of the lower Dalradian formations, possibly related to early movements on the two major faults that bound the area. The concept of a D1 Tay Nappe which roots in the ‘young Moine’ to the N is retained, although the scale and flat attitude of that fold is regarded as in part a product of D2.

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Two phases of deformation are described from the area. D1 is related to the first phase rnovements widely rccognised in the Dalradian; D2 is related to more localised modifications of the D1 fold pattern.Four major D2 folds are recognised from the evidence of minor structures. Three of these, the Beinn Chuirn, the Ben Lui and the Ra Chreag Folds, had previously been considered as D1 structures, whilst the fourth, the Dalmally Fold, was unrecognised. These folds provide continuity between the southeast facing of the D1 folds of the Moine and Ballachulish Succession of Glen Orchy (Thomas and Treagus 1968) and the southeast facing of the D1 Tay Nappe (Shacketon 1958) in the Iltay SuFession above. The D2 deformatjon reaches a climax at the junction between the two Successions, the lltay Boundary Slide, which is now regarded as a D2 structure. The regional implications of these results are briefly considered.

The stratigraphical continuity between the SW and Central Highlands of Scotland is clarified with particular reference to the lower Dalradian rocks. The structural disposition of the rocks reveals the presence of the same two structural elements as found in the SW Highlands, namely, the Ardrishaig anticline and the Ben Lui fold. The mushroom structure of the Central Highlands is rejected, while the Boundary Slide is shown to be a diffuse D 2 structure, which is probably of no great mechanical importance. (1977) have recently attempted to integrate the major folds and slides of the SW Highlands into a model that is reasonably coherent in terms of stratigraphy, fold geometry and deformation history. The area around Ben Lui formed the cornerstone of this correlation between the comparatively simple structures at a high level around Cowal, Loch Awe and Islay and the strongly deformed structures at a low level around Loch Creran and Loch Leven. We now extend the stratigraphy and, rather more tentatively, the fold traces from Ben Lui as far northeast as Schiehallion and Loch Tummel. Our conclusions are illustrated by a geological map and a series of structural cross-sections, as shown in Fig. 1, and should be compared with Thomas (this volume). Roberts & Treagus