Fold mechanisms in the Canton Schist: constraints on the contribution of ¯exural ¯ow Aaron Stallard * , Ken Hickey 1 School of Earth Sciences, James Cook University, Townsville 4811, Australia Received 15 August 2000; revised 26 December 2000; accepted 31 January 2001 Abstract Fold mechanisms operating in the Canton Schist have been resolved using the geometric relationship between folds and spiral inclusion trailgeometries.Ofthefourend-memberfoldmechanisms,tangential±longitudinalstrainfoldingandslipfoldingareunabletoproducethe observed inclusion trail and fold geometries, but a combination of ¯exural ¯ow and pure shear folding is consistent with the geometric constraints. The maximum ¯exural ¯ow component during each fold event was determined from the geometric data. During F 3 and F 4 , ¯exural ¯ow produced #27%and #37%ofmeasuredlimbrotation,respectively,whichcorrespondstoamaximumof248 limb rotation by ¯exural¯owineachfoldevent.Tosatisfythegeometricconstraints,theremainderoflimbrotationmustbeaproductofpureshearfolding. Themaximumpossiblecomponentof¯exural¯owfoldingduringF 3 andF 4 increaseswithincreasedvariationinvorticitybetweenlayersin the rock mass. In the ¯exural ¯ow±pure shear model, a maximum of 28% of inclusion trail curvature is produced by rotation of porphyro- blasts relative to irrotational) fold limbs, with a minimum 72% curvature due to rotation of fold limbs relative to irrotational) porphyro- blasts. All model solutions produce less than < 88 syn-folding porphyroblast rotation relative to geographic coordinates. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Folding; Flexural ¯ow; Inclusion trails; Porphyroblasts; Pure shear 1. Introduction Foldingofnaturalrocksisgenerallydescribedintermsof four simple kinematic end-member models. They are tangential±longitudinal strain folding, pure shear folding, ¯exural ¯ow folding and slip folding. Determining the relative importance of these models during folding is a fundamental problem for structural geologists, and has been the subject of numerous theoretical and ®eld-based studies that seek to ®nd criteria that discriminate between them e.g. Lan and Hudleston, 1991; Davis, 1995; Hudleston et al., 1996; Bell and Hickey, 1997). Inclusion trail and fold geometries offer a means of distinguishing between the different models e.g. Williams and Jiang, 1999), as each of the four fold mechanisms produces a different geometric relationship between limb rotation and inclusion trail curvature in sub-spherical porphyroblasts Fig. 1). Tangential±longitudinal strain folds form by a bodily rotation, or spin, of the fold limbs, and all strain within a folded layer is coaxially accumulated. Porphyro- blastsdonotrotaterelativetothefoldedlayer,androtateby #908 relative to the axial plane of the developing fold Fig. 1b).Slipfoldingproducessimilar-typefoldsbysimpleshear parallel to the axial plane of the fold and potentially produces unlimited rotation of porphyroblasts relative to fold limbs and axial planes as limb rotation approaches 908 Fig. 1c). During ¯exural ¯ow, rotation of porphyro- blastsrelativetofoldlimbsisaresponsetovorticityinduced bysimpleshear,andthisrotationisoppositetothatresulting from body rotation of the fold limbs Fig. 1d). Pure shear folds result from passive ampli®cation of pre-existing de¯ections by homogeneous coaxial ¯ow and result in porphyroblast rotation relative to fold limbs only Fig. 1e). Each of these fold models produces different amounts of porphyroblast rotation, relative to fold limbs and axial planes,foragivenlimbrotation.Inrealrocks,theamountof syn-folding porphyroblast rotation relative to fold limbs can be determined from inclusion trail curvature for all porphyroblasts that grew synchronous with, or after in the case of a pre-existing porphyroblast) the fold event. Journal of Structural Geology 23 2001) 1865±1881 0191-8141/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII:S0191-814101)00032-3 www.elsevier.com/locate/jstrugeo * Corresponding author. Current address: Institute of Geosciences, Shizuoka University, Shizuoka 422-8529, Japan. Tel.: 181-54-237-1111; fax: 181-54-238-0491. E-mail address: aaron@se-geomail.sci.shizuoka.ac.jp A. Stallard). 1 Current address: Mineral Deposit Research Unit, Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC, Canada V6T 1Z4.