Caledonian ductile thrusting in a Precambrian metamorphic complex: The Moine of northwestern Scotland

Abstract

Structural models drawn from external thrust-belts have been applied to a part of the metamorphic Caledonides in northwestern Scotland. Internal deformation of the amphibolite-facies Moine rocks was accomplished primarily by northwest-directed ductile thrusting (“sliding”), which was succeeded within a significant portion of the Moine by upright folding above a basal detachment. The Moine can be regarded as a stack of three major nappes, each of which has distinctive structural, stratigraphic, and metamorphic characteristics. Caledonian shortening within the Moine is of the order of 100 km, yet geothermometry and geobarometry confirm that all tectonic units had equilibrated at similar mid-crustal depths during a Precambrian, “Ardgourian” metamorphic event (∼1000 Ma). Kyanite-grade Caledonian metamorphism implies that ductile thrusting and steep refolding took place beneath a considerable overburden, although an intervening stage of uplift is attested to by the Caledonian reworking of low-pressure thermal aureoles. Final uplift of the complex took place during or after emplacement onto the foreland by the Moine thrust and related structures. Prior to Caledonian tectonics, the Moine was only a few kilometres thick but hundreds of kilometres in lateral extent, perhaps as a result of post-Ardgourian, pre-Caledonian crustal stretching. The pre-existing geometry of the interface between Moine cover and Lewisian basement exerted a major control on Caledonian structural development.