Discussion on the generation of the Tay Nappe, Scotland, by large-scale SE-directed shearing

Abstract

J. R Mendum & C. W. Thomas write: Krabbendam et al. 1997 present a revised model for the development of the Tay Nappe, in part based on their recent mapping around Glenshee. Although they consider this a new model, it follows those of numerous other authors, notably Harris et al. 1976 and Bradbury et al. 1979, but also Roberts & Treagus (1977), Treagus (1987) , Lambert & McKerrow (1976) and, ultimately, Sturt (1961) and Rast (1963) . Briefly, the Tay Nappe is considered to have developed by southeasterly directed D2 shearing of upright D1 structures; this shearing produced a grossly inverted stratigraphy in the ‘Flat Belt’. As limited space precludes a full discussion and presentation of alternative models, we comment on those aspects with which we have most difficulty. We find the evidence cited in support of their model is wanting in critical areas and we propose alternative interpretations. Our discussion is concentrated on the nature and geometry of D1 and D2 structures in the Dalradian of the Southern Highlands. D2 structures are generally well-developed in much of the Southern Highlands, but to understand the nature of, and relationships between, D1 and D2, it is necessary to examine localities where D1 is virtually unaffected by D2 and where D2 begins to modify D1. Such outcrops are confined mainly to the Highland Border ‘Steep Belt’, southeast of the Highland Border Downbend, and are not well-exposed in the Glenshee district. Critical sections that demonstrate these relationships occur north of Stonehaven, in Little Glenshee [NN9835],