Quantitative structural analyses and numerical modelling of ...

DTD 5ARTICLE IN PRESSL. Baratoux et al. / Journal of Structural Geology xx (xxxx) 1–24 9Fig. 5. Field photographs of typical fold shapes: (a) garnet zone, (b) staurolite zone, (c) sillimanite zone and (d) sillimanite zone of the pluton aureole. Chevronfolds are typical for the garnet zone (a), flattened folds with high amplitude are characteristic of the staurolite zone (b), and similar folds with rather lowamplitudes are present in the sillimanite zone (c). Steep foliation with relic F 3 folds characterize the sillimanite zone in the pluton aureole (d).of the post buckle flattening). This graph shows therelationship between these two different expressions ofshortening. In order to illustrate the geometric implicationsof this graph we have plotted the fold patterns of the modelmultilayer sequences given by Ramsay and Huber (1987,pp. 414–415) (Fig. 6). This shows clearly the effect of theviscosity ratio (m 1 /m 2 ), layer thickness ratio (nZd 1 /d 2 ) andthe amount of superimposed strain on the folds positionon the graph. Folded incompetent layers plot in the left halfof the diagram and the slope of the data points shows thestrong negative correlation between the b 1 and F parameters.Folded competent layers plot in the right half of thediagram and the steeper slope of the data point trend shows aless pronounced positive correlation between the b 1 and Fparameters (Fig. 6).We have used these plots as standards with which wecompared the plots representing the natural folds from thestudy area. In addition to the qualitative estimate of theviscosity contrast and the proportion of incompetent tocompetent fraction, we can deduce semi-quantitatively theactive amplification of the buckle fold and the amount ofpost-buckle shortening. The histograms of the F values andb 3 /b 1 ratios for each of the folds studied are given in Fig. 7.4.2. Quantitative analysis of fold styles in the garnet zoneThe post peak metamorphic folds in the eastern part ofthe massif, i.e. in the garnet zone, display only a smallvariation in shape. The post buckle flattening is small, asdocumented by relatively low positive values of F (0.8–3)(Fig. 7a), which approximates to a parallel or weaklyflattened parallel fold. Very few folds in this domain belongto class 3A, i.e. folds that exhibit relatively small negativevalues of F (Fig. 7a). The dominance of class 1B (i.e.parallel) folds is important as it documents that activebuckling played a major role during the fold amplificationand that post-buckle flattening was subordinate. The shapesacquired from the harmonic Fourier analysis lie betweenchevron and sine wave, as can be seen from the histogram ofb 3 /b 1 (Fig. 5a), which has an average value of b 3 /b 1 ZK0.05. The absolute values of both F and b 1 are low(F max Z3; b 1max Z3), suggesting a low amount of deformationof the analysed rocks. A comparison of the measuredfold shapes plotted on the b 1 vs. F diagram with the modelmultilayer sequences (Fig. 6) indicates that the fieldexamples show a close resemblance to model folds (type5, i.e. folds characterized by a high viscosity ratio (m 1 /m 2 )117