Fold mechanisms in the Canton Schist: constraints on the contribution of flexural flow

Abstract

Fold mechanisms operating in the Canton Schist have been resolved using the geometric relationship between folds and spiral inclusion trail geometries. Of the four end-member fold mechanisms, tangential–longitudinal strain folding and slip folding are unable to produce the observed inclusion trail and fold geometries, but a combination of flexural flow and pure shear folding is consistent with the geometric constraints. The maximum flexural flow component during each fold event was determined from the geometric data. During F3 and F4, flexural flow produced ≤27% and ≤37% of measured limb rotation, respectively, which corresponds to a maximum of 24° limb rotation by flexural flow in each fold event. To satisfy the geometric constraints, the remainder of limb rotation must be a product of pure shear folding. The maximum possible component of flexural flow folding during F3 and F4 increases with increased variation in vorticity between layers in the rock mass. In the flexural flow–pure shear model, a maximum of 28% of inclusion trail curvature is produced by rotation of porphyroblasts relative to (irrotational) fold limbs, with a minimum 72% curvature due to rotation of fold limbs relative to (irrotational) porphyroblasts. All model solutions produce less than ≈8° syn-folding porphyroblast rotation relative to geographic coordinates.