The geology of lower Glen Lyon

• Main Author: Nell, P. A. R.
• Published: University of Manchester 1984
• Subjects: 550
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.582018

Glen Lyon forms one of the key areas of Dalradian geology, lying as it does between the inverted stratigraphy of the Tay Nappe 'flat belt', and the Boundary Slide. Historically, work by D. M. Ramsay (1959) interpreted the area as mostly lying on the upper inverted limb of a major recumbent syncline, analogous to the Ben Lui fold, considered then to be coeval with the Tay Nappe. Subsequent work (Treagus, 1964, Roberts and Treagus, 1954, etc.) has shown the Ben Lui fold to be later than the Ardrishaig Anticlin e in the South West Highlands, and resulted from the second deformation, and not the first, in the sequence of deformation episodes which has formed the present geometry of the Tay Nappe. To investigate the possible role of D2 folding in the central Grampians, lower Glen Lyon, an area of 130 km 2 between the Bridge of Balgie and Loch Tay Faults,was remapped at a scale of 1 : 10, 000 as far south as Ben Lawers,northwards to the Rannoch watershed. As a result of the more detailed coverage of the area, several modifications have been made to previous stratigraphic map s; and the structural history, in relation to the textural evidence, re-interpreted. Adjacent to the Boandary Slide, just east of the Bridge of Balgie fault, the Lochaber and Ballachulish subgroups have been recognised in a highly attenuated zone. These rocks have been found to be slid against the Carn Mairg Quartzite. The slide zone lies on the steep, highly-deformed. lower overturned limb of a major D2 compound antiform, cored by Ben Lui Schist and comprised of two antiforms and central synform. These folds fold earlier Fl folds which face SE on the upper inverted limb of the antiformal complex into the Tay Nappe. The course of all these folds, and the later folds which refold them, have been traced throughout lower Glen Lyon. The relationships of these folds are incompatible with the structural models proposed by Thomas. (1979, 1980) and Bradbury (1979)for the nearby areas of Drumochter and Pi tlochry. In the light of the structural evidence from Glen Lyon, the age of some folds has been revised and new fold-correlations9uggested. This has enabled a new model to be proposed for the orogenic development of the Grampian Highlands involving components of pure and simple shear for the extension of the Tay Nappe. The magnetics of the' serpentinites' in the region have been the subject of limited investigation, as has the geochemistry of all the meta-basites. Although the latter were introduced during Dalradian sedimentation in a 'back-arc' environment, the Glen Lyon 'serpentinite' in. particular appears to have been intruded later, possibly as late as during D 2