Kinematic Coupling Between Hinterland and Foreland Deformation In the Northern Moine Thrust Zone, NW Scotland

The Moine thrust zone (MTZ) between the north coast southwards to Assynt displays the classic features of foreland deformation dominated mostly by brittle thrusts, whereas hinterland thrusting is associated with mylonite zones representing deformation at deeper crustal levels.

Quantitative vorticity (Wm) data from the mylonites (~100 samples) indicate the presence of a significant pure shear component (0.47-0.82; 68-38% pure shear), and integrated 3D strain and vorticity analyses on mylonitic Cambrian quartzites in the Loch Eriboll region indicate 24-56% vertical shortening sub-perpendicular to the flat lying mylonite belts and their associated thrusts, and sub-horizontal maximum principal stretches of 22-94% oriented parallel to the WNW trending thrust transport direction. These estimates account for ranges in estimated vorticity values and orogen-parallel stretching (non-plane strain), as indicated by 3D strain analyses, but constant volume is assumed for all deformation within the crystal plastic regime.

Transport parallel stretching within the crystal-plastic regime must be accommodated by either volume loss or extrusion of material towards the topographic surface, thereby directly or indirectly driving tectonic processes above the brittle-ductile transition zone. Interestingly, where hinterland mylonite zones are most prominently developed in Cambrian metasedimentary rocks (from the north coast to northern Assynt), the MTZ is characterized by a west-east succession of brittle to ductile thrusts with an estimated minimum total displacement of >30 km on the brittle structures and >100 km on the ductile structures. To the south of Assynt, no mylonites are recognized in the Cambrian succession, and the MTZ is typically marked by a single brittle fault. The high strain zone junction between these two regimes (Oykel constrictional zone) may represent a ductile lateral fault/shear zone to the north of which displacement was transferred from the extruding hinterland mylonites to the brittle thrusts in the foreland.