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 D 3 major folds of the Flat Belt intensify into the Steep Belt and are responsible for the steeper dips. These D 3 folds refold four newly 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 interpretation presented here. A reconstruction of the pre-D 3 structural history 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, 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 simpleshear-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.

Tectonic and kinematic evolution within mid-crustal orogenic root zones: a case study from the Caledonides of northwestern Ireland

“…Top-tothe-southeast (D3) deformation becomes increasingly intense towards the south with the present overall synformal geometry reflecting the interplay and interference between these deformation phases. There is thus no reversal in D2 and D3 shear across the supposed root zone, and no direct evidence of pure sheardominated deformation at greater structural depths as suggested by Treagus (1999) in the Central Highlands. Southeast-directed D3 shear may be interpreted as retrocharriage or 'back-folding' following the northdirected D2 crustal thickening associated with peak metamorphic conditions (e.g.…”

“…D2 deformation in deeper levels was associated with top-to-the-northwest shear, whilst at higher levels D2 shear was directed towards the southeast (Nell, 1986;Treagus, 1987;Krabbendam et al 1997). A recent model by Treagus (1999) suggests that D2 deformation is dominated by pure shear at deeper crustal levels, whilst higher levels display gravitydriven simple shear directed towards the southeast.…”

The Dalradian rocks of the north-east Grampian Highlands of Scotland

“…7). The tight, upright folds characteristic of the steep belt have been documented in the Central Grampian Highlands east of the Loch Tay Fault by Treagus (1999Treagus ( , 2000 and a similar structural pattern can be traced north-eastwards into the Kirkmichael-Glen Shee area (Crane et al, 2002) and on towards Braemar, where it is known as the Cairnwell Steep Belt (Upton, 1986).…”

“…Although the junction is recognized as a zone of high strain (Smith, 1980), the amount of excision could be relatively small because a through succession from the Grampian Group into the Appin Group can be demonstrated nearby in the area around Schiehallion (Treagus and King, 1978;Treagus, 1999). The fact that the Ballachulish Subgroup is present on the southern side of Glen Tilt ) also makes the tectonic break less important stratigraphically and the Glen Banvie ‗Series' was correlated tentatively with the Lochaber Subgroup by Smith (1980).…”