Quantitative structural analyses and numerical modelling of ...

DTD 5ARTICLE IN PRESS20L. Baratoux et al. / Journal of Structural Geology xx (xxxx) 1–24stress but on the limbs it does not. The resulting stressgradients cause the migration of quartz from the limb to thehinge and mica from the hinge to the limb.Microstructural examination of the rocks in the presentstudy area shows an analogous situation in the amphiboleplagioclase rock. The resulting stress gradients establishedin both the amphibole and the plagioclase cause a migrationof plagioclase to the hinge and amphibole to the limbs,leading to metamorphic differentiation.We suggest that this process operated in the hinge zonesof some macrofolds leading to their thickening. Because ofstrain compatibility, this hinge thickening develops inalternating layers, thus producing patterns similar to thefolding of bilaminate (Fig. 12).In the hinge zones of folds in the staurolite zone aconspicuous reduction in the grain size of the amphiboleswith respect to the limb zones can be observed. This islocally connected with the reorientation of amphibolefragments, and we interpret this microstructure as beingthe result of the fracturing and rigid body rotation ofamphiboles as described by Nyman et al. (1992). Thedynamically recrystallized plagioclase in the hinge zonesalso shows a decrease in grain size with respect to the limb.In the hinges, both minerals have similar grain size, lowaspect ratio, very weak SPO and GBPO and exhibit animportant degree of mixing. We suggest that these featuresmay stimulate a change in deformation mechanism in thesedomains and that in the highly attenuated folds thefracturing of amphiboles and the dynamic recrystallizationof plagioclase switch to a type of granular flow facilitatingthe development of high strain intensities in these areas(Table 2). The changeover from dislocation creep dominatedflow to granular flows connected with a reduction ofgrain size and the mechanical mixing of minerals has beendescribed by several authors in greenschist facies metabasitemylonites (e.g. Stünitz, 1993; Berger and Stünitz,1996). As with the folds in the garnet zone we argue that thefabric on the limbs of the folds in the staurolite zonerepresents the original, pre-fold fabric.In contrast to the garnet and staurolite zones no bendingor fracturing of the amphibole lattice was noted in the hingezones of folds in the sillimanite zone. Instead the fold hingeareas in the sillimanite zone display crossover growths ofamphiboles and straight, well-equilibrated grain boundariesof all minerals. In addition, the amphibole grains show nosigns of internal deformation and the grain size is alwayshigher than that observed in the garnet and staurolite zones.All these criteria indicate that these grains developed by themechanism of nucleation and possibly syn-deformationalgrowth (Vernon, 1976; Rosenberg and Stünitz, 2003). Theplagioclase also shows well-equilibrated grain boundariesmeeting at triple point junctions and an almost uniformdistribution in the rock, features consistent with the highgradecrystallization of amphiboles and plagioclase (Brodieand Rutter, 1985). Comparison of the mineral microstructuresof the hinge zones and the limbs shows that the hingezones display higher aspect ratios, smaller grain sizes,similar SPO and like–like and unlike GBPOs (Table 2). Thisimplies that, during the development of the folds, recrystallizationoccurred simultaneously in both the hinge zonesand the limbs, producing an increase in elongation of theoriginal crystals in the hinge zone. This is because the grainsin both the limb regions and the hinge regions are parallel tothe axial plane, i.e. are perpendicular to the largest principalcompressive stress axis. This resulted in an increase inaspect ratio by the process of heterogeneous dissolutionaccompanied by recrystallization and grain growth, themechanism of schistosity transposition well known in highgrade schists and described by a number of authors (seePasschier and Trouw (1996, fig. 4.17) for a review). Unlikethe folds in the garnet and staurolite zones where weconsider that the fabric on the fold limbs represents the prefoldingmicrostructure, in the sillimanite zone it is modifiedby dissolution and growth process.In the contact aureole a similar type of microstructuralpattern to the sillimanite zone occurs but it is marked by acomplete loss of SPO and GBPO coupled with an importantgrain size increase (Table 2). These features are consistentwith there having been an important contribution from highgrade “static” recrystallization in both the hinge and limbdomains. However, we emphasise that one can still observesmaller grain sizes and aspect ratios for both amphibole andplagioclase in the hinge zones compared with the limbs.7. ConclusionsA summary of the fold analysis and the microstructuralstudies supporting the fold mechanics models is given inTable 2.The garnet grade region. The quantitative microstructuralanalysis of the limb zones in the folds in the garnetzone reveals the possible existence of a stress-supportingnetwork. This type of structure implies a relativelyhomogeneous stress distribution in the rock (Jordan,1988), which is controlled by the rheologically resistantamphiboles, with the plagioclase representing only weakpockets that deform to accommodate the deformationimposed by the strong amphibole. This is supported by thelarge grain size difference between the recrystallizedplagioclase and amphibole crystals, which can be interpretedin terms of a stress-supporting network with a highrheological contrast between the plagioclase and amphibole(Handy, 1990). In addition, the microstructural analysisshows that the amphibole stress-supporting frameworkcollapses in the hinge zones by brittle failure. This brittledeformation, which represents an extreme example of strainlocalization, tends to produce sharp-hinged chevron folds ona grain scale (Fig. 3a and b).The observed folding mechanism also gives rise tochevron folds on a macroscopic scale indicating that themechanical anisotropy of the rock was high. To interpret the128