Discussion on the Caledonian metamorphic core: an Alpine model

Abstract

D r . P. A. R. N ell writes: Bradbury (1985) presented a brave attempt at a crustal-scale model of the Dalradian. New work in the critical area of Glen Lyon ( Nell 1984 ) suggests that Dr Bradbury’s model of Dalradian structure needs revision. The concept of a Knapdale–Tummel downbend as an ‘antithetic rootless rotation zone’ is fundamental to Bradbury’s interpretation (e.g. Bradbury 1979a , Bradbury 1985 ; Bradbury et al. 1979 ); however, the evidence from Glen Lyon shows that this structure is not of regional importance. The Tay Nappe, Bradbury et al. 1979 proposed, was extruded by Dl–D3 pure shear in the Tummel steep belt accommodated by northwesterly-directed movement on the Boundary and Killiecrankie Slides, with rolling round the Tummel downbend. The major problems with this interpretation are: 1. The Boundary Slide cuts across folds associated with the early Tay Nappe. The D2 fold equivalent to the Ben Lui fold in Glen Lyon (the ‘net F2 antiform’, Fig. 1) folds a stack of F1 folds. These folds face southwards into the ‘flat belt’ of the Tay Nappe on the upper, gently dipping limb of the D2 antiform, but face upwards to the north-west on the lower limb. As the Boundary Slide is not folded along with these F1 folds but remains parallel to the S2 cleavage, and is folded by later folds, movement on the slide in the central Highlands is confined to D2 ‘times’. It is clear that on a larger scale, from Dalmally to Schichallion, and at Braemar, the Boundary Slide continues as an integral