Structural analysis of a subduction-related contact in southern Sesia-Lanzo Zone (Austroalpine Domain, Italian Western Alps)

Abstract

A new foliation trajectory map at 1:10000 scale, represented here with an interpretative structural map, is derived from an original field analysis at 1:5000 scale in the southern Sesia-Lanzo Zone (SLZ). It shows the relative chronology of overprinting foliations, characterised by the mineral assemblages that mark superposed fabrics in each rock type. This map and the associated cross-sections, which synthesise the 3D structural outline of the tectonic contact between the Eclogitic Micaschists Complex (EMC), the Rocca Canavese Thrust Sheets and the Lanzo Ultramafic Complex, allow the correlation of the structural and metamorphic imprints that developed in these crustal and mantle complexes during Alpine subduction. Furthermore, the map and cross-sections allow the immediate perception of the metamorphic environments in which the structural imprints developed in each complex successively under eclogite, blueschist and greenschist facies conditions. The represented structural and metamorphic evolution of the southern end of the SLZ (internal Western Alps) has been inferred based on multiscale structural analysis. The dominant fabrics at the regional scale are two superposed mylonitic foliations that developed under blueschist and greenschist facies conditions, respectively. Metamorphic assemblages underlying the successive fabrics in the different metamorphic complexes allow us to identify contrasting metamorphic evolutions indicating that the tectonic contacts between the EMC, the Rocca Canavese Thrust Sheets and the Lanzo Ultramafic Complex developed under blueschist facies conditions and were successively reactivated during the greenschist facies retrogression.

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)

“…This pre-Alpine tectono-metamorphic evolution is preserved in the metamorphic complexes forming the SLZ: the Eclogitic Micaschist Complex (EMC), the Gneiss Minuti Complex (GMC), the II Dioritic-Kinzigitic Zone (IIDK), and the Rocca Canavese Thrust Sheets (RCT) (e.g. Compagnoni et al 1977;Pognante 1989aPognante , 1989bSpalla et al 1991;Cantù et al 2016;Roda et al 2018aRoda et al , 2019b. The IIDK, lying between EMC and GMC, consists of kilometric lenses in which Alpine eclogitic assemblages are not described, even if in the Vogna Valley the tectonic contact underlying the margin between IIDK and EMC is marked by eclogitefacies mylonites (Lardeaux 1981;Lardeaux et al 1982b).…”

“…On the other hand, eclogitic parageneses are widely described both in EMC and GMC, even though the GMC, which lies along the tectonic boundary with the Piemonte Zone, is widely re-equilibrated under greenschist-facies conditions, whereas the EMC, constituting the internal part of the SLZ, records the greenschist-facies re-equilibration mainly along discrete shear zones and shows a dominant metamorphic imprint under eclogite-facies conditions (e.g. Spalla et al 1983;Stünitz 1989;Spalla et al 1991;Giuntoli and Engi 2016;Cantù et al 2016).…”

“…Detailed multiscale correlation among superposed fabric elements and metamorphic assemblages is known to be of first-order importance to infer relationship between fabric and metamorphic gradients (e.g. Williams 1985;Stünitz 1989;Lardeaux and Spalla 1990;Passchier et al 1990;Johnson and Vernon 1995;Spalla et al 1999;Zucali et al 2002;Spalla and Zucali 2004;Gosso et al 2010;Salvi et al 2010;Hobbs et al 2011;Delleani et al 2012;Gosso et al 2015;Rebay et al 2015;Cantù et al 2016;Corti et al 2017Corti et al , 2018Delleani et al 2018).…”

Integrating X-Ray Computed Tomography With Chemical Imaging to Quantify Mineral Re-crystallization From Granulite to Eclogite Metamorphism in the Western Italian Alps (Sesia-Lanzo Zone)

“…The eclogitized ophiolitic relics of the Liguria-Piedmont Ocean, the Piedmont Zone, bound the external margin of the SLZ, while the SLZ internal margin is a thick mylonitic belt (the Canavese Line) ( Figure 1); the Canavese Line puts in contact the SLZ from the lower crustal rocks of the Southalpine Ivrea Zone (e.g., Bigi et al, 1990;Rebay et al, 2018;Balestro et al, 2019). The SLZ is subdivided into four main units ( Figure 1B): the Eclogitic Micaschist Complex (EMC), the Gneiss Minuti Complex (GMC), the II Dioritic-Kinzigitic Zone (IIDK) and the Rocca Canavese Thrust Sheets (RCT) (e.g., Compagnoni et al, 1977;Pognante, 1989a,b;Spalla et al, 1991;Cantù et al, 2016;Roda et al, 2018aRoda et al, , 2019. Eclogitic parageneses are widely described both in EMC and GMC with a strong difference in relative volume affected by the retrograde greenschist re-equilibration.…”

Analysis of fabric evolution and metamorphic reaction progress at Lago della Vecchia-Valle d’Irogna, Sesia-Lanzo Zone, Western Alps

“…The SLZ is classically divided into four main complexes ( Figure 1(B); Compagnoni et al, 1977;Gosso, 1977): (1) the II Dioritic-Kinzigitic Zone (IIDK) preserves a pervasive pre-Alpine HT metamorphic imprint; (2) the Gneiss Minuti Complex (GMC) and (3) the EMC display a dominant Alpine metamorphic imprint and locally preserve pre-Alpine HT relicts (Compagnoni et al, 1977;Gosso, Messiga, Rebay, & Spalla, 2010;Lardeaux, Gosso, Kiénast, & Lombardo, 1982;Manzotti et al, 2014;Zanoni, 2016); (4) the Rocca Canavese Thrust Sheets are characterized by a blueschist Alpine imprint with no eclogite-facies relicts (Cantù, Spaggiari, Zucali, Zanoni, & Spalla, 2016;Pognante, 1989;Spalla & Zulbati, 2003). The EMC and GMC consist of metapelite, paragneiss, metagranitoid, metabasite, and impure marble recording Alpine polyphasic deformation and eclogite-facies peak conditions.…”