Structure of the Grampian Caledonides
|Stephenson, D, and Gould, D. 1995. British regional geology: the Grampian Highlands. Fourth edition. Reprint 2007. Keyworth, Nottingham: British Geological Survey.|
Structure of the Grampian Caledonides
The general structure of the Grampian Caledonides has been described in Chapter 4. In that account the main fold structures and dislocations were outlined briefly in order to provide a background for the outcrop distribution and stratigraphy described in detail in Chapter 5. The more detailed structural accounts presented here involve careful consideration of the relative ages of the various structures, their relationships with each other, and hence the overall structural history of the area, which involves several phases of deformation. This history has been painstakingly pieced together and refined by a multitude of workers from the early days of C T Clough and E B Bailey to the present day. It is very much an ongoing process and ideas are constantly changing. No single hypothesis can account satisfactorily for all of the observed features but, although various controversies have raged over the years, there has also been a remarkable amount of agreement and consensus on many aspects.
In individual areas of study, separate identifiable episodes of deformation have been termed D1, D2, D3 ...... etc. in order of decreasing age. However, not all of these episodes are necessarily developed in all areas and, even within individual areas, the multiplicity of workers has resulted in differences in numbering of structural events. Consequently there is little consistency in the nomenclature of structural phases when comparing detailed studies of separate areas. Authors undertaking regional syntheses have attempted to solve this problem by disregarding phases of deformation whose effects can be shown to be of local extent only. The regional phases are then renumbered to produce a sequence of major events recognisable over wide areas and accepted by most authors. Thus, for example, the sequence D1 to D3 identified in the regional synthesis of the South-west Highlands by Roberts and Treagus (1977c), differs numerically from that used in detailed, more local studies within the same area by Roberts (1974a; 1976) and Treagus (1974). Even with this rationalisation, problems still exist on a regional scale with the result that different nomenclatures are still being perpetuated by various authors. One major problem arises from a variation in the number of recognisable major phases across the Tay Nappe. D1 and D2 are widespread events recognised by most authors. However, a D3 event associated with the development of the Tay Nappe in the Southern Highlands, becomes difficult to distinguish from D2 farther to the north-west. Consequently, workers in the Central Highlands such as Roberts and Treagus (1977c; 1979), Thomas (1979; 1980) and Treagus (1987) recognise only D1 and D2 and their main late-tectonic deformation is termed D3. Workers in the Southern Highlands (e.g. Harris et al., 1976; Bradbury et al., 1979; Harte et al., 1984; Mendum and Fettes, 1985) recognise three nappe-forming or modifying events and their main late-tectonic phase is D4. The latter nomenclature is more generally applicable and hence will be adopted in this account unless stated otherwise.
In most areas the structural development has thus been explained in terms of three or four major episodes of deformation which occurred during the Caledonian Orogeny. There is some evidence to suggest that still older events may be recognised in the Central Highland Migmatite Complex. During the first widespread (D1) deformation the major folds, together with accompanying slides, were initiated with a NE–SW trend. During D2 (or D2 and D3 of some authors) these folds were extensively modified in places to produce a complicated pattern of refolded nappes. In other places D2 folds form separate complexes of intermediate-scale folds, accompanied by slides which may be coincident with or extensions of those of D1 origin. Where a separate D3 phase is identified, it is seen to be broadly coincident with the peak of regional metamorphism and associated igneous intrusion, although it may be a little later in the north-east. Later phases are late-tectonic episodes which overprint the composite foliations of both the nappes and the separate D2 and D3 fold complexes. They are separated from the earlier movements by a significant time gap and seem to be the result of a change in tectonic regime from ductile folding to basement fracture and block uplift (Harte et al., 1984; Mendum and Fettes, 1985).
A diagrammatic structure of the Grampian Highlands is shown in (P915427). This is based largely on the diagram by Thomas (1979) for blocks A, B, C and D), extended to the north and north-east by incorporating results of published and unpublished work of various authors. For the purposes of the discussions in this chapter, the diagram has been divided into three complexes defined on a combination of structural and geographical criteria.
Southern Grampians Complex: the SE-facing folds to the south-east of the axis of the Loch Awe Syncline together with all the folds above and to the south-east of the Boundary Slide and its projected north-eastern continuation.
Western Grampians Complex: the apparently NW-facing folds to the north-west of the Loch Awe Syncline axis and the Ossian–Geal Charn Steep Belt, and the north-eastward continuation of these folds above the Fort William Slide through the Lochaber area.
Central Grampians Complex: the remainder of the Grampian Highlands, which comprises mainly the outcrop of the Grampian Group with those parts of the Lochaber Subgroup below the Boundary Slide. The northern part of this last area includes the Central Highland Migmatite Complex.
In the following sections, the structures within each complex are described from south-east to north-west, across the overall Caledonoid strike. Within each section, the structures are described firstly in the south-west, where the relatively simple fold geometry is essentially as described by E B Bailey, and then progressively through to the north-east with reference to the labelled blocks of P915427.