The Loch Quoich Line—a limit of early Palaeozoic crustal reworking in the Moine of the Northern Highlands of Scotland

Abstract

The Loch Quoich Line, a major structural line at least 60 km in length, within the Moine of the Northern Highlands of Scotland, has hitherto been variously interpreted as a major tectonic dislocation. the root zone of a nappe, and an unconformity. The work reported here was carried out at the type locality of the line, and was directed at resolving these ambiguities. The lithostratigraphic succession established by sedimentary structures shows that the rocks of the Glenfinnan Division are older than, but are stratigraphically continuous into, those of the Loch Eil Division. Rocks of the Loch Eil Division crop out extensively to the W of the Loch Quoich Line, and are not restricted in outcrop to the area E of the line, as was previously thought. The early (D 1 –D 2 ) structures were initially sub-horizontal. Steeply inclined, NNE-trending D 3 structures are weakly developed E of the axial trace of a major fold, the Beinn Beag Synform. In this area the metasediments remain essentially in their post-D 2 , gently inclined attitude. W of the axial trace of the synform, the earlier folds and fabrics were severely reworked into steep attitudes during D 3 , producing NNE-trending zones of 'platy' rocks, which mark regions of severe strain on D 3 fold limbs. Earlier (D 1 –D 2 ) structures are clearly preserved in D 3 hinge zones, which mark zones of low D 3 strain. It is concluded that the Loch Quoich Line coincides with the axial trace of the Beinn Beag Synform, and that it is the eastern limit of severe tectonic reworking (probably Caledonian) over-printing early (probably Precambrian) structures. Recognition of structural and stratigraphic continuity between the Glenfinnan and Loch Eil divisions, implies that the Loch Quoich Line cannot be correlated across the Great Glen Fault with the putative unconformity separating the Grampian (cover) and Central Highland (basement) divisions, in the Moine rocks of the Central Highlands.