Quantitative structural analyses and numerical modelling of ...

DTD 5ARTICLE IN PRESS18L. Baratoux et al. / Journal of Structural Geology xx (xxxx) 1–24Fig. 11. Plot of grain boundary preferred orientation (GBPO) of amphibole–amphibole, amphibole–plagioclase and plagioclase–plagioclase boundariesdisplaying weighted ratio of eigenvalues of inertia. The numbers represent the orientation of the eigenvector V 1 of GBPO with respect to the horizontal.Positive values indicate anticlockwise deviation. Samples from the pluton aureole are labelled Sil*.factor controlling the mode and style of folding of amineral fabric is the mechanical anisotropy of thematerial and this is defined by the ratio of two moduli,one a measure of the resistance to layer or fabricparallel compression (M) and the other to the resistanceto shear in the same direction (L).It can be shown that there is a direct link betweenthe anisotropy (M/L) and the competence contrast of abilaminate (m 1 /m 2 )(seePrice and Cosgrove, 1990) andthis allows one to use both theories to study foldingbehaviour. By analysing the geometries of the studiedfolds it is possible to determine which of the twotheoretical approaches is more appropriate. As shownearlier, the low F values of the folds from the garnet(low b 1 ) and staurolite zones (high b 1 ) are similar tothose in the range type 5a and 5b to type 4b (Fig. 6).The trend of the arrows in Fig. 7b (which indicates theratio of active buckling amplification and fold flatteningin the staurolite zone) is comparable with the trendsindicated in Fig. 6 for a multilayer (type 5b) with ahigh competence contrast between adjacent layers. Theconstruction of the dip isogons for the folds from thiszone shows alternations of layers with differentgeometries (classes 1B and 1C alternating with class3) and different curvature profiles of the hinge regions(Fig. 12), a pattern compatible with the folds of type5b. Using the classical theory of Ramberg (1963), thispattern can be interpreted in terms of folding of amultilayer sequence marked by alternation of layerswith a high competence contrast.However, as was shown above, the garnet zoneamphibolites are more or less compositionally homogeneousand do not show distinct compositional layering.Inspection of Fig. 7a shows that the folds in the garnet zonecould be considered to represent the early stages of activeand passive amplification observed in the staurolite zone.However, there is a marked difference in the density of thedata relating to the ‘competent’ and ‘incompetent’ membersof the multilayer. When the pattern of dip isogons for arepresentative fold profile for these folds is examined (Fig.12), it can be seen that there is an important increase of foldflattening component resulting in a convergence of shapes(and therefore dip isogon patterns) of adjacent layers.A comparison of the natural data from folds in thesillimanite zone (Fig. 7c) with the model folds in Fig. 6shows that the best fit is with type 2b folds. Thisgeometry is traditionally interpreted as indicating abilaminate with a low competence contrast. However,we note the strong asymmetry in the density of pointsindicating a lack of incompetent ‘layers’. This isconfirmed by inspection of the dip isogon pattern ofrepresentative folds from this zone (Fig. 12). Althoughthe pattern shows alternations of fold shapes of class 1Cwith those of class 3, it is obtained by analysing‘layers’ that are composed of several units. The absenceof alternations of clearly defined individual layerscombined with the high degree of flattening andrelatively low fold amplification indicate that thissystem approximates more closely to the model analysedby Biot than to a bilaminate.126