Geology of the contact area between the Internal and External Nappe Zone of the Sardinian Variscan Belt (Italy): new insights on the complex polyphase deformation occurring in the hinterland-foreland transition zone of collisional belts

Figures & data

Figure 1. (a) Tectonic sketch map of the Sardinia Island (modified after Carosi et al., 2020; Montomoli et al., 2018). (b) Schematic geological cross-section (A-A’) along the Nappe Zone (modified after Carmignani et al., 1994; Conti et al., 1999, 2001).

Two different images and sketches describing the geographical, geological, and structural setting of the study area.

Figure 2. Outcrop and microscale view of the main structural features. The structural equal-areal projections (lower hemisphere) of the S₂ poles and L₂ (d), A₂ (e), A₃ (f) and A₄ (h) axes are shown. (a) Microscopic evidence of S₂ gradational crenulation cleavage. S₀ bedding and S₁ foliation, parallel to S₀, are also present. (b, c) S₂ spaced foliation at both meso- and microscale (XPL: crossed-polarized light). In the thin section, S₁ is marked by Wm + Chl. (d) Fine continuous S₂ foliation is defined by Wm + Chl (XPL). (e) F₂ fold, deforming the S₀//S₁ foliation in metasedimentary rocks belonging to the BU. (f,g) F₃ folds, deforming the S₂ foliation in metasedimentary rocks belonging to the MSU. No clear S₃ axial-plane foliation in the F₃ hinge zone is recognizable. (h) Late open folds (F₄) with sub-horizontal axes and axial planes deforming S₂ spaced foliation.

Six images showing the complex polyphase deformation history.

Figure 3. (a) Mesoscopic kinematic indicators of mylonites belonging to the Barbagia Thrust indicating a top-to-the S-SW sense of shear. (b,c) Structural variation going toward the Barbagia Thrust in both units. (b) variation from mylonites (on the left) to ultramylonites (on the right) in metavolcanic rocks belonging to the MSU. (c) Progressive variation of the foliation from spaced cleavage to a continuous cleavage moving toward the BT in metasedimentary rocks.

Three images showing the mesostructural evidence of shear deformation along with the BT and six images showing the variation of deformation and structural features going toward the BT from both units.

Figure 4. Synoptic reconstruction of the field and microstructural investigations of the deformation history of the BU and MSU.

Synoptic reconstruction of the deformation history.

Figure 5. Sketch of the structural evolution derived from the obtained data. (a) Structural features of the pre-, sin- and post-nappe stacking deformation under compressive conditions (D₁, D₂, D₃). (b) Simplified and not to scale structural features of the investigated sector. Far from the mylonitic zone, pre-D₂ structures are well-recognizable. The prominence of mylonite and, only in MSU, ultramylonite, characterizes the high-strain zone of the BT. The presence of S-SW dipping of the axial plane of F₂ folds due to the D₃ folding event has been highlighted.

Summary diagram of deformation history.

Carosi, R., Petroccia, A., Iaccarino, S., Simonetti, M., Langone, A., & Montomoli, C. (2020). Kinematics and timing constraints in a transpressive tectonic regime: The example of the Posada-Asinara shear zone (NE Sardinia, Italy). Geosciences, 10(8), 288. https://doi.org/10.3390/geosciences10080288

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Data Availability Statement

The authors confirm that the data supporting the findings of this study are freely available within the article and its supplementary materials.