Mesostructure
Six groups of superposed ductile structures, named in the following D1 to D6, have been recognized during field mapping that was performed at 1:5.000 scale, with small areas mapped in greater detail (Delleani et al., submitted). They are all syn-metamorphic and Alpine in age (Delleani et al., 2010). Scattered pre-Alpine mineral and fabric relicts (pre-D1) are preserved in domains in which the early Alpine deformation is poor or absent, mainly in metaintrusive rocks.
The interpretative map of Fig. 2 synthesises the structural framework reconstructed in accordance to various extent with the following criteria: i) fold style of similar rock multilayers: ii) kinematic compatibilities of fold systems; iii) interference patterns; iv) symmetry of minor folds; v) orientation of structures; vi) different assemblages marking foliations (e.g. Connors & Lister, 1995; Hobbs et al., 1976; Johnson & Vernon, 1995; Passchier et al., 1990; Passchier & Trouw, 2005; Spalla, 1993; Turner & Weiss, 1963; Williams, 1985). The geological boundaries have been traced on the interpretative map according to two classes of confidence: objective, indicated with solid lines, and interpretative with dashed lines, on the base of exposure and amount of structural data reported on the objective map (Delleani et al., submitted). This map synthesizes the objective structural characters as foliation and fold axial surface trajectories, the sense of asymmetry of folds described by mineralogical or lithologic layerings, and interference patterns between superposed folds.
The degree of fabric evolution and peculiar structures developed during the successive metamorphic stages in metagranitoids and metasediments are shown in Figures 3 and 4, respectively.
Figure 3. Representative structures characterising low, medium and high degree (LD, MD and HD, respectively) of fabric evolution during the successive deformation stages (D1-D6) in the two main types of metagranitoids.
“Grey-type” metagranitoids: a) poorly-deformed, igneous texture in D1 LD; b) localized, progressive foliation developed close to a cm-thick syn-D1 mylonitic shear zone; c) poorly-deformed Qz igneous domains in D2 LD; d) D2 crenulation in hinge zone; e) weakly stretched igneous domains during D3; f) partially re-oriented igneous domains during D4 crenutalion; g) localized mm-thick shear zones in D5 HD domain; h) syn-D6 close fold. “Green-type” metagranitoids: i) 10-cm-spaced, discontinuous S1 foliation associated with D1 fold, emphasized by the contact with micaschists; l) mm-spaced S1 foliation and D1 fold, underlined by Qz-rich vein; m) syn-D2 parallel shape folding of S1; n) S1 + S2 composite fabric in D2 MD; o) mm-spaced S3 mylonitic foliation wrapping around 10-cm-thick eclogite boudin; p) gentle D4 folding with parallel shape; q) localized mm-thick shear zones in D5 HD; r) D6 gentle bending.
Figure 4. Representative structures characterising low, medium and high degree (LD, MD and HD, respectively) of fabric evolution during the successive deformation stages (D1-D6) in the two main types of country rocks.
Paragneisses: a) pre-Alpine compositional layering totally replaced by Amp-, Omp- and Grt-assemblages in poorly-deformed D1 domains; b) tight D1 folding of pre-Alpine alternating layers; c) mylonitic S1 foliation obliterating the pre-Alpine layering of paragneisses; d) parallel shape D2 folding; e) nearly isoclinal D2 folds with similar profile; f) cm-thick shear zone in D3 HD; g/h) open (g, LD) and tight (h, MD) D4 folds; i) cm-thick D5 shear zone in a Qz-rich layer of the paragneisses. Micaschists: l) rare preservation of continuous S1 foliation re-oriented by D2 crenulation; m) transposition of S1 foliation during the development of the new S2 axial plane foliation; n) S2 foliation crosscut by S3 in D3 HD; o) D4 gentle folding; p) D4 crenulation in a micaschist re-equilibrated under greenschist facies conditions; q) numerous D5 mm-thick shear zones marked by whitish Ab and reddish Fe-oxides; r) D6 kink fold associated with the 10-m-wavelenght D6 gentle bending.
Pre-Alpine relicts (pre-D1)
- Metaintrusive and metasedimentary rocks preserve some mineralogical pre-Alpine relicts, such as Bt, Ksp, Aln, Ap and Zrn pre-Alpine relicts within metagranitoids of Mt. Mucrone and pinkish Grt cores and Aln occur within micaschists and paragneisses. In low-deformed domains, metagranitoids (Fig. 3a) and paragneisses (Fig. 4a) preserve pre-Alpine structural relicts such as inequigranular igneous texture. Primary igneous contacts between metre-thick aplitic and mesocratic dykes and metagranitoids are preserved.
Alpine deformations (D1-D6)
The first group of structures (D1) comprises isoclinal stretched similar folds and a pervasive axial plane foliation S1 (Figs. 3i, 3l, 4b and 4c), which is defined by SPO of Ph, Omp or Jd, Gln and Zo and by Grt-rich bands. S1 is a well-differentiated crenulation, showing a spaced to continuous character and is the most pervasive planar structure at the map scale in micaschists, paragneisses or metagranitoids. In metasediments, the degree of D1 fabric evolution ranges from stage 3 to stage 5 of the six stages decrenulation model of Bell and Rubenach (1983), whereas in metagranitoids it varies between stage 3 and 4 (Fig. 5) of the foliation development model of an originally isotropic rock, as described by Salvi et al. (2010).
D2 structures consist of isoclinal similar folds, metre to ten-metre in size, transposing the S1 foliation (Figs. 3c, 3d, 3m, 3n, 4d, 4e and 4l). An associated penetrative S2 foliation developed only in micaschists and is marked by SPO of Omp, Zo, Gln, Ph and Pg (Fig. 4m). In metasediments and metagranitoids the degree of D2 fabric evolution is similar to that of D1 stage, but the volume recording the more evolved stage of fabric development is sensibly smaller, as testified by the development of a penetrative foliation only in micaschists.
In poorly strained domains of metagranitoids, during deformation stages synchronous with the development of eclogite-facies assemblages, ten centimetre-thick shear zones developed (Fig. 3b), locally associated with Grt-bearing veins.
D3 structures principally consist of thick (up to 3 m) shear zones usually located at the main boundaries between different rocks (Fig. 3o). The associated S3 mylonitic foliation, occurring in these shear zones, is marked by Qz, Ph ± Pg, Amp ± Grt ± Chl, Czo and Ttn. Thicker shear zones locally occur along the boundaries between metagranitoids and country rocks, while in metagranitoids, paragneisses, eclogites and porphyric gneisses deformation and transformation are concentrated in cm-thick bands (Figs. 4f and 4n). During this stage, deformation is extremely localised and the degree of fabric evolution corresponds to stage 5-6, according to Bell & Rubenach (1983) and Salvi et al. (2010), in correspondence of D3 shear zone but never exceed stage 2 in the rest of the area (Fig. 3e).
D4 structures comprise tight to open up to ten metre scale folds (Figs. 3p, 4g, 4h and 4o), associated with an intense crenulation in the hinge zone (Figs. 3f and 4p). These parallel folds represent the most recurrent fold system. Where the fabric evolution exceeds stage 2, the HP-minerals underlying the earlier fabric elements are replaced, to various extent (from 10 to 70%), by Ab, green Amp, Chl, Ep and a new generation of white mica. D4 folds range from metre to hundred meters in size with a weakly SW dipping axial plane. No new foliation develops during D4 folding: this deformation stage is associated with the growth of greenschist-facies minerals.
In the D5 centimetre-thick shear zones (Figs. 3g, 3q, 4i and 4q), S5 foliation is defined by Qz, Ab, Chl, green Amp, new white mica and Fe-Ep.
Large-scale D6 structures consist of gentle folds that appear as undulations of earlier structures at the hundred metre-scale (Figs. 3h, 3r and 4r). They are chronologically joined with brittle-ductile small fractures and joint systems filled by Chl, Ab and Kfs. D6 axial planes are marked by growth of very fine-grained dark Amp, white mica, Ab, Chl and Kfs (Adl).
The orientations of the fabric elements, plotted on the Schmidt diagrams of Table 1, suggest that the superposed fold systems (D2-D1, D4-D2 and D6-D4) generally have axial surfaces intersecting at a high angle, and fold axes intersecting at a low angle. These geometric relationships between the four fold groups are indicated by consistent interference patterns of type 3 (Ramsay, 1967) both at the outcrop and map scale (Figs. 2, 3 and 4).
