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

Abstract

Structural mapping from the Flat Belt into the Tummel Steep Belt of the Dalradian Supergroup of the Central Highlands provides evidence for a new interpretation of the relationship between the two belts and their evolution. The open upright D₃ major folds of the Flat Belt intensify into the Steep Belt and are responsible for the steeper dips. These D₃ folds refold four newly recognized D₂ major folds. It is the recognition of the interference of these two sets of folds that leads to the radically revised structural interpretation presented here. A reconstruction of the pre-D₃ structural history shows that the stack of major D₂ folds, on the lower limb of the Tay Nappe, originally verged to the northwest, as did three D₁ fold-pairs.

A model is proposed for the D₂ deformation in a transpressional setting, simplified as partitioned into two sub-horizontal zones. The lower zone is represented by the pure-shear-dominated Tummel Belt with extension parallel to the regional orogenic trend; the upper zone is represented by the simple-shear-dominated Flat Belt where extension is perpendicular to that trend. The curvature of the minor D₂ 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.