An investigation of lower crustal deformation: Evidence for channel flow and its implications for tectonics and structural studies

Abstract

High-grade metamorphic terranes generally have similar deformation fabrics characterised by a shallowly dipping transposition foliation and recumbent folds. Deformation paths are typically non-coaxial and strain magnitudes extreme. We refer to this association of structures and metamorphism as the high-grade nappe association (HGNA), and argue that it is common, and represents crustal-scale (kilometres thick) shear zones. The structure of such rocks is generally interpreted as comprising stacked thrust sheets separated by thrusts. We concluded, however, that the ‘thrusts’, which are ubiquitously parallel to the transposition foliation, are not thrusts in the normal sense, but are various discontinuities, rotated by large pervasive shear strains. The thrust-like appearance may be further enhanced by late localised shear strain. The sense of shear may be constant across an HGNA shear zone, giving it the geometry and kinematics of a crustal-scale (kilometres thick) detachment zone. Alternatively it may reverse across the body, consistent with channel flow. Structural evidence therefore supports current ideas on the behaviour of the middle to lower crust during orogeny, and illustrates the deformation mechanisms involved. We describe the association with special reference to the Monashee complex, and discuss the implications of our interpretation for the kinematics of high-strain zones, palinspastic reconstruction, and interpretation of deformation fabrics at various scales.