Quantitative structural analyses and numerical modelling of ...

DTD 5ARTICLE IN PRESS8L. Baratoux et al. / Journal of Structural Geology xx (xxxx) 1–24character of the metabasites compared with those in thegarnet and staurolite zones. Brownish green ferroanpargasitic grains of hornblende with aspect ratios of 2–4are arranged parallel to the S 2 fabric (Fig. 4c). They displaymutual equilibrated straight grain boundaries typical ofhigh-grade amphibolite textures (Brodie and Rutter, 1985).Unlike the staurolite zone where the plagioclase (An 30–60 )tends to form layers, we observe isolated grains andaggregates of plagioclase surrounded by elongate and welloriented crystals of amphibole (Fig. 3e). Chemical zonalityand a large span of plagioclase compositions evolvingtowards anorthite document prograde metamorphic growthassociated with dynamic recrystallization (Yund and Tullis,1991) (Fig. 4d).The hinge zones of micro-crenulations are very narrowand the amphibole grains tend to reorient parallel with thefold axial planes without any bending and fracturing (Fig.3f). The hornblende grains show straight boundaries, localdecussate structures, and a similar aspect ratio and size tothose within limb zones. Plagioclase reaches slightly higheraspect ratios and grain size compared with the limbs. Allthese features together with the absence of any compositionalzoning of the pargasitic hornblende (Fig. 4c)indicate grain growth related to prograde metamorphism(Vernon, 1976).3.4. Western area of the massif (the sillimanite zone of thepluton aureole)Unlike the zones described above, brown pargasitichornblende rich in Ti within the sillimanite zone of thepluton aureole, exhibits straight or slightly concaveequilibrated boundaries and fairly low aspect ratios (1.5–2). It is difficult if not impossible to distinguish in thinsection the S 2 and S 3 fabrics. In most of the studied samplesthe pargasitic hornblende is associated with plagioclase(An 30–60 ) of similar aspect ratios and size and arranged intoa regular ’static’ foam-like structure (Fig. 3g). The latterexhibits straight growth-related twins. Plagioclase-rich andamphibole-rich compositional bands are locally developed.Both minerals attain the same size ranging between 0.05 and0.5 mm. 1208 triple point junctions are formed byamphibole–amphibole, plagioclase–plagioclase, and evenamphibole–plagioclase grain boundaries. All of the featuresdescribed above are consistent with re-equilibration underhigh temperature conditions corresponding to the M 3 HT/LPmetamorphic overprint. The hinges of F 3 microfolds areextremely rare but if present they show very similarmicrostructural relations of hornblende and plagioclase tothat of the main fabric (Fig. 3h). A characteristic feature isthe growth of large elongate hornblende crystals parallel tothe axial plane of the F 3 microfolds. The texture of theserocks bears a remarkable resemblance to the upperamphibolite to granulite facies example of Brodie andRutter (1985, p. 155).4. Fold shape analysis4.1. Methods of quantitative analysisFor the purpose of the fold analysis, 3–6 photographs ofrepresentative fold types from each metamorphic zone(garnet, staurolite and sillimanite with a low degree of HToverprint) were selected (Fig. 5). The fold analysis could notbe carried out on the rocks from the pluton aureole becauseof the scarcity of macroscopic folds. The fold shapes,redrawn from photographs, were transformed by parallelprojection onto a plane perpendicular to the fold axis. Twoquantitative methods have been applied to quantify the foldshape: the method of Lisle (1997) based on Ramsay’s(1967) classification and the harmonic fold shape analysis ofHudleston (1973). The principles of these methods are givenin Appendix A.The method of Lisle is based on the polar projection ofthe normalized thickness of a folded layer. Each fold can becharacterized by a single number (the index F), whichexpresses the amount of flattening within each fold limb.Class 1 folds are characterized by positive F values (0 to N)and these give a measure of the amount of homogeneousflattening perpendicular to the axial plane required togenerate this fold shape from a parallel fold (class 1C forFO1, class 1B for FZ1 and class 1A for 0!F!1). Class 3folds are folds with strong thinning of the limbs (attenuatedfolds), typically developed in incompetent layers.These folds are characterized by negative F values (0 toKN). Lisle (1997) divided the field of class 3 folds intothree subfields with fold shape 3B marking the boundarybetween the newly defined 3A and 3C classes. Class 3Bfolds are defined as a ‘pure’ class 3 fold geometry fromwhich the other types (3A and 3C folds) develop by thesuperposition of flattening strains. Class 3A folds aregenerated from 3B by flattening in the direction normal tothe axial plane and have F values ranging from K1toKN.Class 3C folds are the result of flattening parallel to the axialplane and are relatively rare in the nature.Hudleston’s (1973) fold classification, based on theFourier harmonic analysis of folds, exploits the fact that thegeometry of a quarter wavelength is sufficient to characterizethat of the whole fold. The shape is approximated interms of its harmonics and it is found that the first two oddharmonics (coefficients b 1 and b 3 ) are sufficient toadequately describe the fold’s profile shape. b 1 expressesthe amplitude of the fold profile, b 3 is a measure of the‘angularity’ or ‘sharpness’ of the fold’s hinge. For sinewaves, b 3 Z0, for box-like folds b 3 O0 and for chevron-likefolds b 3 !0. The ratios of b 3 /b 1 describe a continuous seriesof shapes between the chevron and box-end members. Themethod for calculating b 1 and b 3 values is given inAppendix A.In the present analysis of fold shape, these two methodshave been combined by plotting b 1 values (a measure of theactive amplification of the fold) against F values (a measure116