The structure of the Moine and Dalradian Rocks in the Dalmally District of Argyllshire, Scotland

Abstract

Two phases of deformation are described from the area. D1 is related to the first phase rnovements widely rccognised in the Dalradian; D2 is related to more localised modifications of the D1 fold pattern.Four major D2 folds are recognised from the evidence of minor structures. Three of these, the Beinn Chuirn, the Ben Lui and the Ra Chreag Folds, had previously been considered as D1 structures, whilst the fourth, the Dalmally Fold, was unrecognised. These folds provide continuity between the southeast facing of the D1 folds of the Moine and Ballachulish Succession of Glen Orchy (Thomas and Treagus 1968) and the southeast facing of the D1 Tay Nappe (Shacketon 1958) in the Iltay SuFession above. The D2 deformatjon reaches a climax at the junction between the two Successions, the lltay Boundary Slide, which is now regarded as a D2 structure. The regional implications of these results are briefly considered.

The Dalradian rocks of the central Grampian Highlands of Scotland

“…It has been confirmed that the Ruskich Fold has a similar geometry and position in the structural pile to the Ben Lui Fold, which has also now been established as a D2 structure by Roberts and Treagus (1975).…”

“…They can be clearly correlated with the main regional D2 deformation phase. The entire outcrop of the Ben Challum Quartzite Formation lies on the right-way-up limb shared by the F2 Dalmally Antiform to the south (Figure 54a) and the F2 Ra Creag Synform to the north (Roberts and Treagus, 1975). This is supported by the dominant southerly vergence sense of minor fold-pairs observable at the site.…”

“…Previous authors have regarded this syncline and the complementary Glen Lochy Anticline as F1 structures (Cummins and Shackleton, 1955;Thomas and Treagus, 1968;Roberts and Treagus, 1975) but Tanner and Thomas (2010) have shown that it post-dates an earlier fabric (see below) and hence can be confidently assigned to the D2 regional deformation. However, only a small amount of deformation, and a gentle warping of the axial surface followed this main deformation event.…”

“…The geometrical relationships between the isoclinal folds and mineral lineations that pre-date the dominant D2 set ( Figure 56) are not clear, but both are interpreted to be of the regional D1 phase, and related to the regional major F1 fold, the Ardrishaig Anticline (Roberts and Treagus, 1975).…”

The Dalradian rocks of Scotland: an introduction

“…Black slaty pelites overlain by Ardrishaig Phyllite-type lithologies in the area around Dalmally can be equated with both the Easdale Slate to the west and the Ben Eagach Schist to the east, hence providing a crucial link (Roberts and Treagus, 1975). Throughout this area, a common succession may be recognized above the Boundary Slide-zone which, with only local absences, consists of the Killiecrankie Schist-Carn Mairg Quartzite-Ben Eagach SchistBen Lawers Schist formations.…”