Tectono-metamorphic evolution of UHP Zermatt-Saas serpentinites: a tool for vertical palaeogeographic restoration

Abstract

International Geology Review 42similar to those estimated for D2 in the surrounding serpentinites, which were dated at 65 ± 43 5.6 Ma. These results suggest that portions of ZSZ were subducted at high depth before 70 Ma 44 and widen the time span during which ZSZ recorded PT peak conditions. The comparison of 45 these data with results of a numerical model of an ocean-continent subduction system gives 46 insights on coupling stages of this UHP unit with the surrounding ZSZ rocks during the 47 Alpine convergence and vertical palaeogeography during different time steps.

Tectono-Metamorphic Evolution of Serpentinites from Lanzo Valleys Subduction Complex (Piemonte—Sesia-Lanzo Zone Boundary, Western Italian Alps)

“…The PTdt evolution of the Gias Vej serpentinites developed in a cold thermal regime (P/T<10 • C/km) during the subduction of the Tethys oceanic plate below the Adria plate, as generally interpreted in the literature synthesized in the geological setting. The exhumation of crust and mantle rocks during an oceanic subduction may occur within the mantle of the overriding plate, consequent to the rheological weakening induced by extensive serpentinization of the mantle wedge [8,29,115,[136][137][138][139][140]. To understand whether the metamorphic history of Gias Vej serpentinites can be achieved within a subduction system, we use a 2D finite element method to simulate an ocean-continent subduction, and we compare the PT peak data inferred from the serpentinites with the thermal evolution generated by the modeled subduction system.…”

“…The Piemonte Zone of the Western Alps comprises the ophiolitic remnants of the Ligurian-Piedmont Ocean, dismembered by the polyphase tectono-metamorphic evolution recorded during the Alpine convergence e.g., [1][2][3][4][5][6][7][8]. Even though some authors consider the ophiolites of the Piemonte zone as lithospheric fragments of a magma-poor ocean-continent transition zone of the Tethys e.g., [9][10][11], they are widely interpreted as derived from the Ligurian-Piedmont oceanic lithosphere, [7] and refs.…”

“…The Lago di Monastero-Punta Gias Vej area (Lanzo Valleys) offers the observation of the articulated boundary between the PZ and SLZ, a dense interlocking of ophiolitic and continental material, produced by the Alpine subduction-related structural evolution [13], in which slices of serpentinite are quite common (Figures 2 and 3a). As recently shown, in the axial portion of orogenic belts serpentinites are useful tools to unravel the oceanic and orogenic evolution of meta-ophiolite in subduction complexes e.g., [8,[18][19][20][21][22].…”

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

“…Following the pioneering works carried out in the seventies (e.g., England & Richardson, 1977; Oxburgh & Turcotte, 1970, 1971; Toksöz & Bird, 1977) several key advances have been made in numerical modeling methods that have improved the understanding of the thermo‐mechanical evolution of the oceanic and continental lithosphere during oceanic subduction and continental collision (e.g., Billen, 2008; Cloos, 1982, 1983, 1993; Cloos & Shreve, 1988a, 1988b; England & Thompson, 1984; Gerya & Stöckhert, 2006; Gerya et al., 2002; Luoni et al., 2020; Magni et al., 2014; Marotta & Spalla, 2007; Peacock, 1989, 1990b; Roda et al., 2011, 2012; Thompson, 1981; van Hunen & Allen, 2011). In particular, several models have shown the importance of dehydration‐hydration reactions (e.g., Arcay et al., 2005; Faccenda et al., 2009; Faccenda & Mancktelow, 2010; Guillot et al., 2001; Meda et al., 2010; Peacock, 1990a; Quinquis & Buiter, 2014; Rupke et al., 2004; Wang et al., 2019) on the mantle wedge dynamics, with the activation of short wavelength convective cells consequent to the serpentinization and the associated decrease in viscosity (Gerya et al., 2002; Hebert et al., 2009; Hirth & Kohlstedt, 2003; Honda & Saito, 2003; Meda et al., 2010; Regorda et al., 2017; Roda et al., 2010).…”

“…In the Alps, rocks deeply involved in the subduction complex and recording P‐T conditions compatible with blueschist and eclogite metamorphic facies can be observed in the axial zone of the chain, and generally show heterogeneous deformation with dominant tectonitic or mylonitic fabrics (e.g., Gasco et al., 2011; Lardeaux, 2014a; Malatesta et al., 2012; Oberhänsli & Goffé, 2004; Roda et al., 2021; Zucali et al., 2020). These rocks are often associated with serpentinites (e.g., Assanelli et al., 2020; Gasco et al., 2011; Lardeaux, 2014a; Malatesta et al., 2012; Manzotti et al., 2014; Roda et al., 2020), which have a crucial role in exhumation mechanisms (Gerya et al., 2002; Guillot et al., 2009; Luoni et al., 2020; Meda et al., 2010; Roda et al., 2012; Schwartz et al., 2001; Tamblyn et al., 2020; Warren et al., 2008; Yamato et al., 2007). Differently, Alpine HP metamorphism has not been recorded by rocks from the external domains of the Alps, which generally show a lower amount of finite deformation under P‐T conditions compatible with zeolite, prehnite‐pumpellyite, greenschist and epidote‐amphibolite metamorphic facies (e.g., Handy & Oberhänsli, 2004; Lardeaux, 2014a).…”