Quantitative structural analyses and numerical modelling of ...

DTD 5ARTICLE IN PRESS12L. Baratoux et al. / Journal of Structural Geology xx (xxxx) 1–241 and 5 with an average value of around 3. Very few class3A folds are present. This indicates the important fact thatthere are relatively few incompetent layers and that a highdegree of post buckle flattening has occurred (as indicatedby the absence of parallel folds). The broad histogram of theb 1 /b 3 ratios, with no unequivocal peaks, reveals that theaverage fold shape approximates to that of a sine curve. Thegraph of b 1 vs. F (Fig. 7c) confirms the low number of class3A folds and the dominance of class 1C. It can be seen fromthis diagram that class 1C folds (FO1) have a low range ofb 1 values (between 1 and 4). In addition, we observe apositive correlation between the F and b 1 parameters. Theimplications of these observations are that although the foldamplification increases with the degree of post-buckleflattening, the maximum value of amplification is relativelylow. The measured fold assemblage may be compared withthe model folds of type 2b in Fig. 6, which indicate a lowviscosity ratio (m 1 /m 2 ) and a moderate value of n (d 1 /d 2 ).Compared with the folds in the staurolite zone where highamplification and low post-buckle flattening dominate, inthe sillimanite zone we observe limited amplification andpronounced post-buckle flattening. This indicates a changefrom folding dominated by active amplification (in thestaurolite zone) to dominantly passive fold amplification inthe sillimanite zone. We interpret this as reflecting animportant change in the rheological properties of the foldedmaterial rather than the result of differing amounts of finitestrain.4.5. Active buckling vs. post buckle flattening in the studiedfoldsThe fold geometries were examined in order to determinethe relative importance of active buckling and post-buckleflattening across the metamorphic zones of the studiedamphibolite unit. It has been shown that in the garnet zonefold amplification was dominated by active buckling withonly a small contribution from post-buckle flattening. In thestaurolite zone, although the folds have also experienced ahigh degree of amplification, this involved both activebuckling and post-buckle flattening. By comparing theseresults with those obtained from the model folds it can beargued that the folds in the staurolite zone are the equivalentof those in the garnet zone (model fold types 5a and 5b) buthave experienced a higher degree of finite strain. In thesillimanite zone the folds show a relatively lowamplification but important post-buckle flattening eventhough field observations suggest that the strainintensity of these folds and those of the staurolitezone are very similar. It is concluded that thedominance of flattening in the folds developed in thesillimanite zone, indicating that the folding wasdominated by passive amplification as opposed to thatwhich occurred in the garnet and staurolite zones.5. Quantitative analysis of rock anisotropyIn order to understand more fully the results of themesoscopic fold analyses discussed above, it is useful tostudy the petrofabrics of the folded units. The main goals ofthis study are to evaluate the mechanical anisotropy offolded systems and to determine the micro-deformationalmechanisms associated with folding.5.1. Methods of quantitative microstructural analysisApproximately 100 thin sections, collected from all threemetamorphic zones, have been studied in an attempt to showthe relationship between the microstructures and thefolding. Quantitative microstructural analysis has beenapplied to eight representative samples collected from thegarnet, staurolite and sillimanite zones and from the graniteaureole. Two thin sections, cut perpendicular to the F 3 foldaxes (YZ sections), were taken from the fold hinge and foldlimb in each zone, respectively. As noted above, macroscopicfolds are only present in the garnet to sillimanitezones. The degree of transposition of the original metamorphicfabric within the contact aureole was so high thatthe macroscopic folds are preserved only in domains withhigh lithological contrast. However, relics of rootlessmicroscopic folds can be seen in massive amphibolites inthin sections and, in this zone, it is these that are analysedfrom a microstructural point of view.A quantitative microstructural analysis of grains andgrain boundaries was carried out on the representativesamples by tracing and digitising the outlines of individualgrains using the ESRI ArcView 3.2 Desktop GIS environment.The map of grain boundaries was generated usingArcView extension Poly (Lexa, 2003). These data havebeen treated by MATLABe PolyLX Toolbox (Lexa, 2003)in which grain boundary and grain SPO were analysed usingthe moments of inertia ellipse fitting and eigen-analysis ofthe bulk orientation tensor techniques (Lexa, 2003).Digitised drawings of representative samples are shown inFig. 8 and the results from the quantitative microstructuralanalyses are presented in Table 1. The grain size of theminerals was calculated in terms of their Ferret diameter andthe resulting grain size distributions were statisticallyevaluated. The grain size statistics are summarized in Fig.9, which shows median values and the quartile difference ofthe Ferret diameters. In addition, a method of determiningthe orientation of grain boundaries and grain shapes,using the eigen-analysis technique, was applied in anattempt to quantify the bulk rock anisotropy. The results ofthe analysis of the bulk SPO of grains vs. their aspect ratio(R) are presented in Fig. 10. Grain boundary preferredorientation (GBPO) is presented using a diagram ofeigenvalue ratios (rZe 1 /e 2 ) and the orientation of thelargest eigenvector of the different types of grain boundaries(Fig. 11).The grain size analysis is a powerful technique for120