Skip to Main Content
Book Chapter

The Moine thrust and the Scottish Caledonides

By
Mike Coward
Mike Coward
Search for other works by this author on:
Published:
January 01, 1988

In the Moine thrust zone, Proterozoic (Moine) metasedimentary rocks of the Caledonian belt are carried over the foreland sequence of Archaean to Proterozoic (Lewisian) gneiss, upper Proterozoic (Torridonian) sandstones, and the unconformable sequence of Cambro-Ordovician shelf quartzites and limestones. The thrust structures occur on all scales, from minor duplex zones a few centimeters across to large thrust sheets on a kilometer scale. The thrust sequence is generally piggy-back; thus, the easternmost (highest level) thrust formed first. The thrust direction was toward N65°W ±10°.

The lower thrust sheets have produced folds by the stacking of imbricate slices, although in Eriboll on the north coast, buckle folds are common in the hanging wall and footwalls of minor thrusts. The higher thrust sheets contain buckle folds, which locally are sometimes large-scale structures and generally oblique to thrust transport. This suggests that the buckle folds formed by differential movement, with the northern part of the thrust zone moving farthest to the west-northwest. Textural studies suggest that this movement in the north occurred at a slower rate, under more ductile conditions. Back thrusts and break-back faults developed at tip zones where the resistance to movement and/or fault propagation was high.

The amounts of displacement estimated from offsets of Lewisian structures are 35 to 45 km. Balanced cross sections in the region between Foinaven and Assynt show shortening values from 35 km+ to 55 km+. The Foinaven imbricates do not affect the lowest Cambrian strata or their basement; that is, the basement rocks must continue back some 55 km beneath the Moines, the distance equivalent to the restored middle to upper Cambrian. This implies that any crustal ramp to the Moine thrust zone must lie more than 55 km east of the present outcrop of the thrust. However, off the north coast of Scotland, deep seismic reflection profiles show moderately dipping reflections much farther to the northwest, and if these represent the crustal-scale ramp, this ramp must be offset by a major tear or transfer fault along the north Scottish coast.

In the southern part of Assynt, in the central part of the thrust zone, there are several extensional fault systems as well as a late (extensional?) fault at the local base of the Moines that cuts across earlier thrust and extensional faults. These extensional movements suggest a gravity-spreading mechanism for some Caledonide thrusting.

The timing of thrusting can be bracketed by the 430-Ma age for the Borralan igneous intrusion, which predates most major thrust movements, and by the Devonian age of molasse deposits. It is difficult to find the driving mechanism for the Moine thrust at this time.

You do not currently have access to this article.
Don't already have an account? Register
This Feature Is Available To Subscribers Only

Sign In or Create an Account

This PDF is available to Subscribers Only

View Article Abstract & Purchase Options

For full access to this pdf, sign in to an existing account, or purchase an annual subscription.

Subscribe Now