A structural reinterpretation of the Tummel Belt and a transpressional model for evolution of the Tay Nappe in the Central Highlands of Scotland

Structural mapping from the Flat Belt into the Tummel Steep Belt of the Dal radian Supergroup of the Central Highlands provides evidence for a new inte rpretation of the relationship between the two belts and their evolution. T he open upright D-3 major folds of the Flat Belt intensify into the Steep B elt and are responsible for the steeper dips. These D-3 folds refold four n ewly recognized D-2 major folds. It is the recognition of the interference of these two sets of folds that leads to the radically revised structural i nterpretation presented here. A reconstruction of the pre-D-3 structural hi story shows that the stack of major D-2 folds, on the lower limb of the Tay Nappe, originally verged to the northwest, as did three D-1 fold-pairs. A model is proposed for the D-2 deformation in a transpressional setting, s implified as partitioned into two sub-horizontal zones. The lower zone is r epresented by the pure-shear-dominated Tummel Belt with extension parallel to the regional orogenic trend; the upper zone is represented by the simple -sheer-dominated Flat Belt where extension is perpendicular to that trend. The curvature of the minor D-2 fold hinges supports a more refined model of smoothly continuous partitioning of transpressive deformation between the basement and high-level, southeast-propagating nappes above the Flat Belt.