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.

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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