Moine Thrust zone mylonites at the Stack of Glencoul: II - results of vorticity analyses and their tectonic significance

Abstract

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.