Microstructural and quartz crystal fabric evidence for distributed WNW-NW directed ductile shear during the Caledonian orogeny, Ross of Mull, Scotland

Whether the contact between Moine (Tonian) metasedimentary rocks of the Morar and Glenfinnan groups on the Ross of Mull, Scottish Caledonides, is a stratigraphic transition, an unconformity or a ductile thrust is controversial. Quartz c-axis fabrics indicate that distributed WNW-NW directed ductile D2 shearing occurred within the Lagan Mor Formation (LMF, Morar-Glenfinnan transition zone) and the Scoor Pelite (SP, Glenfinnan Group) at deformation temperatures of 520–580 °C. Although the LMF-SP junction is marked by a macroscopic ‘platy’ zone, there is no obvious microtextural evidence for increasing strain magnitude and hence the nature of this contact remains uncertain. Metamorphic temperatures of 740-780 °C documented in structurally higher Glenfinnan Group rocks suggest that they were thrust over cooler and more foreland positioned Morar Group rocks. Whether this was accomplished by distributed shearing on a kilometres wide shear zone or a much narrow shear zone, such as the LMF-SP junction, is unknown. D2 shearing likely occurred at c. 450 Ma, and the D3 Assapol Synform on the Ross of Mull correlates with formation of the Northern Highlands Steep Belt at c. 448-432 Ma. We speculate that top-to-the WNW mylonite zones on the para-autochthonous Caledonian foreland of Iona formed during the same tectonic episode.