STRAIN PARTITIONING IN THE MOINE NAPPE, NORTHERN SCOTLAND

The Moine nappe between the Ben Hope and Moine thrusts in northern Scotland displays strain partitioning with extreme flattening adjacent to the Ben Hope thrust and apparent constriction lower in the nappe package. Strathan Conglomerate quartzite and granite clasts immediately below the Ben Hope thrust in Strathan Bay form thin and laterally extensive sheets (e.g. 134:113:1), further from the thrust on Ben Hutig, clasts form roughly transport parallel rods (e.g. 21:4:1).

On Ben Hutig, D1 folds (m to 10s of m in scale) of the interbedded psammite and conglomerate layers with ESE to ENE trending axes have an axial planar foliation defined by moderately flattened clasts. D2 produced E to NE trending, overturned to recumbent, tight to isoclinal folds (cm to 10s of m in scale). F2 folds of clasts locally form rods where limbs are attenuated and cm scale sheath folds. An extension lineation is slightly oblique to F2 axes and wraps rods. At Strathan Bay, extremely flattened sheets vary in the degree of strain parallel to the thrust, and an extension lineation trends E. All samples show strong lattice preferred orientations, evidence for easy grain boundary migration recrystallization post-S1, and subsequent subgrain rotation recrystallization. These textures are consistent with amphibolite facies deformation during and post-dating D1 and somewhat lower temperatures during D2.

At Ben Hutig, F1 and F2 relationships are most compatible with shearing of preexisting moderately flattened clasts oriented oblique to the shear plane, formation of fold axes transitioning to rods parallel to the transport direction, and minor flattening. Shear parallel to the transport direction is evidenced by obliquely recrystallized grains, local sheath folds, and wrapping of L2 around rods. Minor flattening is evidenced in sections perpendicular to F2 axes by elongate recrystallized grains axial planar to F2 with local divergence. Thus, S tectonites near thrusts in the Moine nappe are caused by extreme flattening of clasts into sheets, whereas L tectonites result from distributed shear coupled with minor flattening, producing apparent constriction.