The northern termination of the Ben More and Glencoul Thrusts, Assynt,
NW Scotland
Maarten Krabbendam & Graham Leslie
British Geological Survey, Murchison House, Edinburgh EH9 3LA, UK
The British Geological Survey has embarked on a project to revise all geological maps crossed by the Moine Thrust, starting with the Assynt Special Sheet. The Assynt Culmination in the Moine Thrust Zone exposes two substantial thrusts below the Moine Thrust: the Ben More Thrust (BMT) and the Glencoul Thrust. In central Assynt, the BMT is well defined and emplaces a sheet of Lewisian Gneiss overlain by parts of the Cambro-Ordovician sequence over the Glencoul Thrust Sheet and the ‘Sole Thrust Sheet’. However, to the north its course is less clear and several alternatives have been previously proposed.
North of Na Tuadhan, the BMT is cut by faults, but can be traced along the Leathad Riabhach gully, where it becomes a sub-vertical shear separating Lewisian on the NE side from quartzite on the SW side (see cross-section). This subvertical tract of the BMT follows Glen Beag, where it has a significant displacement (at least 300 m vertical; NE side up) but it does not displace the ‘Glencoul Thrust’ and the ‘Sole Thrust’ at the head of Loch Glencoul, only 3 km to the NW. This is interpreted to imply that along Leathad Riabhach and Glen Beag the BMT lies on a lateral ramp and that there is a branch line, where the BMT and the Glencoul Thrust meet, at the head of Loch Glencoul. This steep lateral ramp is a response to uplift as the Glencoul Thrust develops beneath. This interpretation necessitates that all rocks NE of Loch Glencoul and east of Loch Beag must belong to the BMT Sheet. Therefore, the Glencoul Thrust Sheet is much smaller than previously thought, whereas the BMT Sheet is much bigger, stretching from Loch Ailsh in the south to the Glendhu culmination.
The ‘unconformable’ contact of Lewisian Gneiss and Basal Quartzite within the BMT Sheet is marked by a shear zone, termed here the Coire a’ Mhadaidh Detachment. This shear zone is commonly marked by 1 or more metres of highly sheared, now schistose Lewisian gneiss. Shear sense indicators and linear fabrics typically show top-to-c. 280°N movement. The intensity of shearing shows that significant deformation occurred within the BMT Sheet. This ductile detachment is cut by at least two steep brittle extensional faults that locally cut out the Basal Quartzite and accommodate some differential uplift associated with the lateral ramp along Glen Beag.