Deformation temperatures, vorticity of flow, and strain in the Moine thrust zone and Moine nappe: Reassessing the tectonic evolution of the Scandian foreland–hinterland transition zone

Examination of deformation temperature, 3-D strain and flow vorticity (Wm) in mylonites from the Assynt-Loch More region of the Moine thrust zone (MTZ) allows quantitative kinematic and thermal characterization of shearing at the base of the Scandian (435–425 Ma) orogenic wedge. Quartz microstructures and c-axis fabric opening angles from mylonites in the immediate hangingwall and footwall to the Moine thrust suggest that deformation temperatures are highest in the eastern part of the Assynt region (including mylonites close to alkaline intrusive complexes) and decrease along strike both to the north (Stack of Glencoul – Loch More) and to the south (Knockan). Quartz c- and a-axis fabrics, together with limited 3-D strain data, indicate that deformation in both the footwall and hangingwall mylonites dominantly involved plane strain to general flattening, although domains of more constrictional flow are identified adjacent to thrust transport-parallel lineaments in the overlying Moine nappe. Rigid grain analyses indicate a remarkably constant flow vorticity for tens of kilometers along orogenic strike (40–60% pure shear) in both the hangingwall and footwall mylonites. Integration with previously reported strain and vorticity estimates from the Assynt region indicates a 50–75% sub-vertical shortening perpendicular to gently dipping foliation, that must have been accommodated by either volume loss or extrusion of material toward the synorogenic topographic surface. Extrusion implies a causal link between upper and lower crustal processes, with significant implications for the kinematic and geometric evolution of the Scandian wedge.