Abstract In this paper we review microstructural and petrofabric work carried out on the Moine Thrust zone and overlying thrust nappes. Our review is primarily historical, and starts with contributions made by both ‘amateur’ and ‘professional’ geologists from the 1880s through to the early 1920s during, and immediately following, the original field-mapping of the Moine Thrust zone by the Geological Survey. From the 1920s to the early 1950s contributions were first dominated by Geological Survey work on the microstructural and metamorphic transition between the thrust zone mylonites and the overlying Moine metasedimentary rocks. Subsequent university-based quartz petrofabric work, primarily focused on the Moines, would ultimately lead to the ‘Moine Petrofabric Controversy’ that ran from the late 1940s to the early 1960s. The later stages of this controversy overlapped, from the early 1950s–mid 1960s, with a phase of microstructural and quartz petrofabric work that concentrated on the thrust zone mylonites and immediately overlying Moine Schists. Our review concludes with an overview of microstructural, petrofabric and related strain analyses undertaken since the early 1970s, both within the Moine Thrust zone and its immediate foreland and in the overlying higher grade thrust sheets. Throughout our review we emphasize and track the changing tectonic interpretations that have been placed on available microstructural and petrofabric data.

Abstract The Moine Thrust Zone and overlying Moine Nappe represent classic ground in Scottish geology in which Neoproterozoic Moine metasedimentary rocks were translated towards the WNW over the Laurentian foreland comprising Lewisian basement gneisses and a Cambrian cover succession during Caledonian orogenesis at c. 430 Ma. Systematic mapping of both the Moine Nappe and underlying Moine Thrust Zone in the Loch Hope to Whiten Head region of Sutherland extends the tectonic stratigraphy developed in other parts of the thrust zone to the little-studied north coast section. An apparent continuum of deformation is recognized from the Moine Nappe down into the underlying thrust zone with a consistent gently ESE-dipping foliation associated with down-dip mineral and extension lineations. The polyphase fold history present within the Moine Nappe and the thrust zone results from progressive deformation during thrusting. Thrust-related fold axes and bedding-cleavage intersections are typically subparallel to the transport direction in high-strain zones, whereas gently north- or south-plunging attitudes are common in lower strain portions of the thrust zone. Thrusts are commonly deformed by folds developed within their footwalls, consistent with a foreland-propagating sequence of deformation. The newly defined Lochan Riabhach Thrust is interpreted as an ‘out-of-sequence’ structure that sliced at low angles across the thrust zone and toward the WNW after initial thrust stacking. Comparison with the downfaulted segment of the Moine Nappe and thrust zone exposed on the foreland at Faraid Head demonstrates cross-strike continuity of the thrust sheet template and structural history for at least 15 km to the NW.

Abstract Sharp lateral changes in structural geometry of ductile thrust stacks are not widely reported. A regional-scale lateral culmination wall forms the southern boundary of the Cassley Culmination in Moine rocks in the Caledonides of Sutherland, Northern Scotland. This culmination wall is part of the Oykel Transverse Zone (OTZ), a kilometre-scale shear zone characterized by constrictional finite strain fabrics aligned sub-parallel to the regional WNW-directed thrust transport direction. Main phase folds and fabrics in the transverse zone hanging wall are folded by main phase folds and fabrics in the footwall, thus recording foreland-propagating ductile deformation. South of the Cassley Culmination, shortening occurred uniformly, without development of discrete subsidiary thrusts; distributed deformation (fold development) alternated with localized thrusting within the culmination. The classic ESE-plunging mullions at Oykel Bridge are an integral part of the OTZ and were generated by constriction aligned sub-parallel to the transport direction. Constriction is attributed to differential, transtensional movement across the OTZ during culmination development. Subsequent formation of the underlying Assynt Culmination further accentuated upward-bulging of the Cassley Culmination, amplifying the lateral change across the transverse zone. The OTZ aligns with a pronounced gravity gradient on the south-western side of the Lairg gravity low. Interpretive modelling relates this gradient to a buried basement ramp that possibly controlled the location of the transverse zone.

Abstract The Moine Thrust zone (MTZ) marks the Caledonian foreland-to-hinterland transition zone at the base of the Scandian ( c . 430 Ma) orogenic wedge. In the Loch Eriboll region, the upper ductile part of the MTZ is composed in ascending order of two regionally extensive thrust sheets (Upper Arnaboll-Creag na Faoilin and Creagan) and is overlain by the Moine Nappe. Quartz crystal fabrics, kinematic vorticity (W m ), and strain estimates from the ductile thrust sheets in this region are used to determine how pure and simple shear components of deformation are partitioned, and indicate that these processes may be thermally, structurally, and lithologically dependent. At the lowest structural levels, quartzite and gneiss in the Upper Arnaboll-Creag na Faoilin (UA-CNF) thrust sheet yield rigid grain-based arithmetic mean minimum (W m min) and mean maximum (W m max) vorticity estimates of 0.57 and 0.67, respectively (60–53% pure shear). Creagan thrust sheet mylonites yield W m min and W m max estimates of 0.59 and 0.72 (59–48% pure shear). At the highest structural levels, Moine Nappe mylonites yield W m min and W m max estimates of 0.59 and 0.71 (59–49% pure shear). Quartz c - and a -axis fabrics qualitatively indicate an increase in non-coaxial deformation (top-to-the-west) traced towards structurally higher levels, which is accompanied by increases in deformation temperature ( c . 370 °C to c . 550 °C). Integrated strain and vorticity estimates indicate that significant sub-vertical foliation normal shortening has occurred as nappe stacking progressed.

Abstract This paper presents quantitative data from the Stack of Glencoul on flow vorticities associated with mylonite generation in the hanging wall and footwall of the Moine Thrust, using samples collected in a vertical traverse from 80 m above the thrust plane to 8.5 m beneath the thrust. Estimated vorticity numbers (Wm) in Moine pelites and psammites above the thrust range from 0.775–0.725 ( c . 43–47% pure shear component) increasing downwards to 0.83–0.75 (35–45% pure shear) at 10 cm above the thrust. Wm values in dynamically recrystallized Cambrian quartzites at 0.5–14.5 cm beneath the thrust range from 0.99–0.90 (10–30% pure shear). At 3.0–8.5 m beneath the thrust estimated Wm values are less than 0.75 in the quartzites, although there is some thin section-scale partitioning with Wm values of 0.75–0.65 (45–55% pure shear) in domains of dynamically recrystallized quartz and Wm values <0.65 (>55% pure shear) in domains of relict detrital quartz grains. Integration of strain and vorticity analyses indicates a vertical shortening of 50–75% in these gently dipping mylonites located at the base of the Moine Nappe. The tectonic implications of vertical shortening (thinning) and transport-parallel stretching at the base of the Moine Nappe are discussed.