Strain partitioning in the Moine Nappe, northernmost Scotland

Abstract

Extreme strain in the form of flattening or constriction during noncoaxial shear in ductile shear zones provides a record of ductile thrust system dynamics and the overall tectonic evolution. Within the Moine Nappe in northern Scotland, between the Ben Hope and Moine thrusts, the Strathan Conglomerate displays apparent strain partitioning with extreme flattening (e.g., laterally extensive sheets of deformed pebbles with aspect ratios of 134:113:1 and 88–92% estimated thinning) adjacent to the overlying Ben Hope Thrust and extreme constriction (e.g., rods with aspect ratios of 21:4:1 and estimated extension of 1000%) lower in the nappe package. We demonstrate that partitioning of strain is between its intensity and how deformation is manifested. Field, microstructural, and crystallographic orientation data from this study indicate that both areas were deformed by WNW-directed noncoaxial shear and coaxial flattening under amphibolite-facies conditions. Adjacent to the Ben Hope thrust, flattening was pervasive during noncoaxial shear, whereas beneath and within the Moine Nappe package, polyphase folding dominated. There, early, large-scale folds (F2) rotated into the transport direction. Subsequent transport-parallel (F3) folds and tubular sheath folds formed on the F2 limbs and were dismembered to form rods. No evidence of constriction is observed; instead, pervasive noncoaxial shear was accompanied by minor flattening under decreasing temperature conditions. Thus, these S-tectonites in the Moine Nappe are the result of concentrated flattening of pebbles into sheets during WNW-directed shear, whereas the L-tectonites result from heterogeneously distributed shear and folding, coupled with minor flattening, which produced rods without constriction.