Deformation history
Structural mapping has been performed in an area of the upper Valtournanche valley, where the local bedrocks are mostly serpentinites and metagabbros, with minor prasinites, calcschists, and rodingites (Fig. 2). Here outcrops are continuous, allowing walking correlation of superposed structures and continuous structural mapping to realize a detailed foliation trajectory map and cross sections up to ca. 2 km long, on which relationships between superposed fabric elements are reported.
Figure 2. Foliation trajectory map with interpreted lithostratigraphy of the meta-ophiolites of the upper Valtournanche (ZSZ).
Legend: 1 = serpentinites; 2 = metagabbros and minor amphibolites; 3 = calcschists; 4 = prasinites; 5 = rodingites. Topography redrawn from the technical map of Val d’Aosta Regional Administration. No hydrography is indicated. UTM coordinate system, map datum ED50, zone 32N. *1 and *2 are the locations of detailed structural sections of Fig. 6.
Here, four main ductile deformation stages (D1, D2, D3, and D4) have been recorded in serpentinites, metagabbros, and rodingites. The first three stages produce a foliation supported by distinctive metamorphic assemblages and mineral compositions in serpentinite (Rebay et al., 2012), whereas the last one is mainly responsible for open folds and a very localised disjunctive cleavage (Passchier & Trouw, 2005). The main structural feature of the area is the S1/S2 composite foliation, generally well developed in the serpentinites. S1 and S2 foliations can be distinguished only where they are superposed; in this case S2 occurs as a crenulation cleavage of S1.
- D1: S1 is a continuous sub-vertical foliation dipping mainly toward WSW and to a lesser extent toward ENE, because it has been dispersed by D2 folding (Fig. 3). The maximum concentration of S1/S2 composite foliation poles is consistent with that of S2 poles. Only locally S1 is dominant with respect to S2 (Fig. 4a).
Figure 3. Plots of the structural data
Density plots of linear (blue) and planar (red) structural data for foliations, fold axial planes, fold axes, and crenulation lineations of ductile structures in serpentinites, metagabbros, and rodingites.For D4 structures, pole to plane scattered plots are shown. Lower hemisphere, Schmidt equal area projection. “n” = number of data.
Figure 4. Structures in serpentinites
a) Pervasive S1 in serpentinites folded by D2 associated with an S2 crenulation cleavage. West of Gouffre de Busseraille; b) Late D2 normal shear zone in serpentinites west of Pesontsé. c) S3 disjunctive cleavage overprinting S2 in serpentinites, close to Varvoyes. d) S3 crenulation cleavage in serpentinites in the Bois Pancherot area. e) D4 folding of S2 in the serpentinites north of Pesontsé.
- D2: This group of structures mainly consists of up to metric folds, S2 foliation, AP2 fold axial plane, A2 fold axis, and L2 crenulation lineation. D2 folds are from open to isoclinal, generally associated with an axial plane foliation (S2) that is sub-horizontal and gently dipping to the W (Fig. 4a). The orientation of S2 is not very dispersed, because both D3 and D4 deformation stages produce a gentle folding (Fig. 3). AP2 fold axial planes gently dip to the WNW and L2 crenulation lineation and A2 fold axis shallowly plunge to the NNW or SSE (Fig. 3). Late D2 shear zones accommodate mainly a normal shearing, dip about 45° to the WNW (Fig. 4b) and locally are overprinted by D3 folds.
- D3: D3 metric folds are open (Fig. 4c) to tight. In average, the fold axial plane dips 50-70° to the W and both the fold axis and the crenulation lineation gently plunge to the N or S (Fig. 3). The S3 crenulation foliation (Fig. 4d) generally dips 60-70° to the W (Fig. 3) and locally is a disjunctive cleavage (Fig. 4c).
- D4: D4 mainly produces upright open folds (Fig. 4e), of metric size, and a rare S4 disjunctive cleavage develops in serpentinites. D4 produces open to tight cm-sized folds in well-foliated serpentinite. S4 and D4 axial planes are sub-vertical and generally trend NE – SW. D4 fold axes and crenulation lineations are mainly shallowly trending NNW – SSE (Fig. 3).