Geological constraints, structural evolution, and deep geology of the northwest Scottish Caledonides

Abstract

The front of the Caledonian orogenic belt in NW Scotland is marked by a zone of foreland thrusting which contains imbricates of Lewisian basement, Proterozoic cover and Cambrian shelf sediments. Faults within this Moine thrust belt, in most localities, propagated in a foreland directed sequence. Therefore the earliest of these thrusts was the Moine thrust, which carried a basement assemblage of Moine and Lewisian rocks over the Cambrian shelf. The development of large duplex and imbricate structures in the footwall postadted movement on this thrust. Hence a restoration of these imbricates provides a minimum estimate of the eastern extent of the Cambrian shelf. No synsedimentary or prethrust extensional features are recognised within the Cambrian succession which displays a remarkable stratigraphic and thickness consistency. This implies a corresponding consistency in crustal structure beneath these cover rocks immediately prior to Caledonian thrusting. A balanced cross section constructed across the northern part of the Moine thrust belt has given a restored width for the Cambrian shelf of 54 km. Geophysical data suggest that the present foreland crustal thickness is 28 km. If this is taken to be the crustal thickness beneath the Cambrian shelf during Caledonian thrusting, a minimum cross sectional area of 1512 km² of Lewisian rocks must remain beneath the present outcrop of the Moine thrust sheet. This foreland basement wedge may have been imbricated subsequent to the development of the Cambrian imbricates. Caledonian structures within the Moine thrust sheet are interpreted as forming an imbricate stack with the Moine thrust acting as a floor thrust. The ductile imbricate thrusts possibly roofed into the Naver slide. Late open folds, which warp foliation, slides, and metamorphic isograds within the Moine sheet, may represent culminations in the foreland Lewisian imbricates beneath the Moine thrust. These postulated culminations may have been sufficient to initiate gravity‐driven structures. This may explain late movements on high‐level faults within the Moine thrust belt.