A structural and metamorphic study of Moine rocks between Loch Eil and Loch Eilt, Inverness-shire

Abstract

A structural and metamorphic study of Moine rocks between Loch Eil and Loch Eilt, Inverness-shire. "Research between Loch Eil and Loch Eilt, Inverness-shire on rocks from all three Divisions of the Moine Succession within the Caledonian Orogen, above and to the east of the Moine thrust zone, has revealed that, within the area, all three Divisions share a common sequence of polyphase deformation and metamorphism. All three Divisions have been metamorphosed during Precambrian and Caledonian tectono-thermal events.

The Ardgour granitic gneiss, a = 1000 Ma. pre-D₁ intrusion, sub-parallel but transgressing the junction between the central Glenfinnan and eastern Loch Eil Divisions, has undergone partial anatexis during D ₁, unlike the adjacent migmatitic meta-sediments. Major folds have not been recognised. Grenvillian ( = 1000 Ma.) D₂ deformation and high grade metamorphism has almost completely transposed the D₁ granitic gneiss foliation but there is no unequivocal evidence for D₂ partial anatexis. F₂ major folds occur in the vicinity of the granitic gneiss but not elsewhere.

Caledonian D₃ deformation has produced major F₃ sub-recumbent folds with curvilinear hinge lines, within the Glenfinnan and Loch Eil Divisions, above a flat lying major D₃ simple shear zone, the Sgurr Beag slide. Large scale WNW transport has carried these rocks over the underlying Morar Division rocks. Only at the highest levels of strain within the shear zone has the Grenville metamorphic assemblage re-equilibrated. Elsewhere the early isograds are folded during D₃.

Post-D₃ microdiorite sheet intrusions have suffered medium grade (amphi-bolite facies) metamorphism during D₄ deformation. D₄ heterogeneous, horizontally directed pure shear deformation, possibly above a flat lying decollement plane such as the Moine Thrust, has produced major upright folds of the D₃ Sgurr Beag slide and rotated Glenfinnan Division F₃ sub-recumbent isoclines towards vertical, to form the D₄ "steep belt".

D₅ deformation has produced sporadically distributed minor folds possibly related to regional warps of the earlier major structures.