3D reconstruction of fabric and metamorphic domains in a slice of continental crust involved in the Alpine subduction system: the example of Mt. Mucrone (Sesia–Lanzo Zone, Western Alps)

Abstract

Geological mapping, multiscale structural analysis, and estimations of the degree of fabric evolution and of reaction progress allow the construction of a 3D quantitative model of structural and metamorphic gradients in a portion of continental crust deeply involved in the Alpine subduction system and mainly structured under eclogite-facies conditions before the continental collision. The investigated and modelled rocks outcrop in the surroundings of Mt. Mucrone, in the central Sesia-Lanzo Zone. The Geomodeller ® software allowed a quantitative 3D estimation of domains characterized by homogeneous fabric evolution and metamorphic reaction progress (DFE and DRP, respectively) for two of the seven structural and metamorphic imprints detected in this area: the D2-eclogitic and D5-greenschist stages, which are the most pervasive at km scale. Such a 3D modelling clarifies mutual relationships between fabric and metamorphic gradients and indicates that: (i) DFE and DRP are closely related regardless of the rock type; (ii) the syn-deformational thermal regime can influence the degree of metamorphic transformation, if DFE remains below the 60% threshold; (iii) the phase transitions can not be properly implemented in quantitative geodynamic modelling without considering the heterogeneity of reaction progress and fabric evolution.

Metamorphic Facies and Deformation Fabrics Diagnostic of Subduction: Insights From 2D Numerical Models

“…Therefore, metamorphic facies that attain a dominant mylonitic fabric are those characterized by temperatures lower than 600°C, where serpentine is stable, and high P/T ratios, such as epidote-eclogite, amphibole-eclogite and, mainly, blueschist facies (Figures 7-9). Pervasive mylonitic domains predicted under epidote-eclogite facies conditions agree with observations of the degree of fabric evolution in the continental crust deeply involved in the Alpine subduction system, where mylonitic fabrics are developed under epidote-eclogite facies in up to the 85% of the rock volume (Zucali et al, 2020). Moreover, the coexistence of mylonitic, tectonitic and coronitic, as predicted under blueschist metamorphic facies (Figures 7-9), has been observed in meta-intrusive rocks in the Sesia-Lanzo Zone (Corti et al, 2017) and in the Dent-Blanche Unit (Roda & Zucali, 2011).…”

Strain partitioning in host rock controls light rare earth element release from allanite-(Ce) in subduction zones

“…D3 and D4 stages ( P = 0.6 ± 0.10 GPa; T = 302 ± 24°C) consist of localised S3 foliation, shear zones, and open folds marked by greenschist-facies assemblages, D5 developed km-long brittle-to-ductile mylonitic shear zones at a shallow crustal level (Corti et al , 2017, 2018). In contrast with most of the Eclogitic Micaschists Complex, where the dominant fabric developed under eclogitic-facies conditions (Zucali and Spalla, 2011; Corti et al , 2019; Zucali et al , 2020), the dominant fabric of the Lago della Vecchia metagranitoids developed under the D2 blueschist-facies conditions. In response to deformation partitioning, D2 produced low-strain domains (i.e.…”

“…Wm). In addition, it should be considered that the positive correlation of the DFE and the DRP (degree of the metamorphic reaction progress, Corti et al , 2017, Zucali et al , 2020) implies that mylonitic rocks are enriched in weak mineral phases, such as Wm and Qz. In this context, the solid-state flow becomes easier as the deformation progresses.…”

“…1 b ), as defined by Corti et al (2017) and Zucali et al . (2020). While in mylonitic domains the rock fabric is related entirely to the new metamorphic conditions, in tectonitic to coronitic domains relic fabrics and mineral associations are preserved.…”

“…developed km-long brittle-to-ductile mylonitic shear zones at shallow crustal level (Corti et al, 2017(Corti et al, , 2018. In contrast with most of EMC, where the dominant fabric developed under eclogiticfacies conditions (Zucali and Spalla, 2011;Corti et al, 2019;Zucali et al, 2020), the dominant fabric of the Lago della Vecchia metagranitoids developed under the D2 blueschist-facies conditions. In response to deformation partitioning, D2 produced low-strain domains (i.e., coronitic domain) that are wrapped by intermediate (i.e., tectonitic domain) and high (i.e., mylonitic domain) finite strain domains ( Fig.…”