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.
Law et al. (Tue,) studied this question.