Strain partitioning in the Moine Nappe : northern Scotland




Collier, Sarah King

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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. aspect ratios of 134:113:1); further from the thrust on Ben Hutig, clasts form roughly transport parallel, WNW, rods (e.g. aspect ratios of 21:4:1). At Strathan Bay, extremely flattened sheets vary in the degree of strain parallel to the thrust, an extension lineation trends E-W, and folding is rare. On Ben Hutig, F1 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, recumbent to inclined, closed to tight folds (F2; cm to 10s of m in scale). F2 folds of clasts locally form rods where limbs are attenuated and cm scale sheath folds. Sheath folding (100s of m in scale) is also inferred from the regional swing in F2 fold axes. An extension lineation is slightly oblique to F2 axes and wraps rods. Samples from both areas 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 Strathan Bay, sheet formation is most compatible with intense flattening and coeval shear immediately below the Ben Hope Thrust. Flattening is evidenced by the equi-dimensional shape of the sheets in the X and Y directions and multiple orientations of elongate recrystallized quartz in sections perpendicular to the lineation. Shear parallel to the transport direction is evidenced by obliquely recrystallized grains in sections parallel to the lineation and strain variation in relation to the thrust. At Ben Hutig, rod formation is most compatible with shearing of preexisting moderately flattened clasts associated with F1 that are oriented oblique to the shear plane, formation of fold axes related to F2 sheath folds 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 L1 around rods. Minor flattening is evidenced in sections perpendicular to F2 axes by elongate recrystallized grains axial planar to recumbent F2 with local divergence due to axial rotation. Thus, S tectonites in the Moine nappe are the result of concentrated flattening of clasts into sheets with WNW directed shear whereas L tectonites result from distributed shear coupled with minor flattening, producing rods without constriction


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