The effect of tectonic boudinage and folding in a subducted mélange of the Alpine orogenic belt (Zermatt–Saas Zone, Italian Western Alps)

The Zermatt-Saas Zone is an eclogite-facies meta-ophiolite unit representing the fossil oceanic lithosphere of the Jurassic Tethys. In the Italian Northwestern Alps, the Zermatt-Saas Zone includes a chaotic rock unit, or melange, c. 40 m thick, interposed between serpentinites and calcschists. The melange consists of decimetre-scale ultramafic layers and boudins embedded in a serpentine + carbonate-rich matrix showing a block-in-matrix fabric. The melange has the same Alpine tectonometamorphic evolution as the surrounding rocks, starting with a prograde path developed under high-pressure (HP) conditions followed by a retrograde path during exhumation. The kinematic and metamorphic relations between inside- and outside-boudin foliations indicate that boudinage and shearing developed during the prograde HP path. Fluid-rock interaction enhanced shearing and focused ductile and brittle-ductile deformation along lithological contacts between rigid blocks and boudins and flowing carbonaceous matrix. Despite a pervasive orogenic evolution, the primary tectonosedimentary features of the melange are still recognizable in some outcrops and are attributed to an intra-oceanic (Jurassic) setting characterized by mass-transport processes. The present-day fabric of the studied melange unit thus results from the superimposition of the Alpine processes, responsible for fluid-assisted stratal disruption and mixing in the subduction channel, on the original stratigraphy formed during intra-oceanic gravitational processes.Supplementary material: Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6924080Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists