Abstract

Structural sequences in Precambrian gneisses of Mingulay, Barra, South Uist, Benbecula, North Uist, Harris, and Lewis in the Outer Hebrides, Scotland, have been determined, compared, and used as a basis for correlation. At each locality examined, over a distance of 200 km across an orogenic belt, a sequence of at least six fold phases (F1–6) has been determined. The earliest recognized folds, denoted F1, are isoclinal and intrafolial and coeval with the dominant foliation. F2 folds are tight to isoclinal and both F1 and F2 folds show the highly variable orientation to be expected in rocks subjected to this high degree of polyphase deformation. F3 folds are closed, asymmetrical structures with syntectonic mineral growth and quartzo-feldspathic veins parallel to flat-lying axial surfaces. Both the axial surfaces and axes show variable orientation. F4 folds are SE.-trending, closed or open, steplike in style, on moderately steeply dipping axial planes paralleled by a fracture cleavage. F5 folds are upright, E-trending, closed or open structures whose vertical axial planes have controlled the uprise of granitic and pegmatitic veins. F6 folds are weakly developed open warps on NNE.-trending axial planes. A seventh set of structures, represented at several localities as a set of vertical, ENE.-trending fracture cleavages, is probably the expression of a very late waning phase of orogenesis.

The structures formed during each of the deformational phases are generally very nearly identical in terms of style, orientation, and intensity of development. For this reason it has been found possible to relate, with a high degree of certainty, the sequences from one area to the next, and so demonstrate the validity of the principle of correlation by structural sequence in gneisses.

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