THE EFFECT OF TECTONIC BOUDINAGE AND FOLDING IN A SUBDUCTED MÉLANGE OF THE ALPINE OROGENIC BELT (ZERMATT-SAAS ZONE, ITALIAN NORTHWESTERN ALPS)

The Zermatt-Saas Zone in the Northwestern Alps is an eclogite-facies metaophiolite unit representing the remnant of the oceanic lithosphere of the Jurassic Tethys Ocean. In the southern Aosta Valley (Italian Northwestern Alps), the Zermatt-Saas Zone includes a composite chaotic rock unit, or mélange, ca. 40 m thick, interposed between serpentinite basement units and a metasedimentary cover unit made of calcschist. The mélange consists of dm-sized ultramafic layers and boudins embedded in a serpentine + carbonate-rich matrix showing a block-in-matrix fabric. The mélange unit shares the same Alpine tectono-metamorphic evolution with the surrounding serpentinites and calcschists. The polyphase orogenic history started with prograde deformation stages developed under high-pressure (eclogite- to blueschists-facies) conditions, followed by a retrograde path during the exhumation under greenschists-facies conditions. Intense shearing and boudinage played a primary role in determining the internal structure of the mélange. The kinematic and metamorphic coherence between inside- and outside-boudin foliation attests that the eclogitic foliation is contemporaneous with boudinage and shearing. Furthermore, fluid-rock interaction enhanced shearing and focused ductile and brittle/ductile deformation along lithological contacts between rigid blocks and boudins and flowing carbonaceous matrix. Later deformation phases and metamorphic recrystallization under greenschist facies conditions partially reworked the mélange structure and mineralogy during the exhumation stage. Despite a pervasive orogenic evolution, the primary tectono-sedimentary features of the mélange are still recognizable in some outcrops and attributed to an intraoceanic (Jurassic) setting characterized by mass transport processes. The present-day fabric of the studied mélange unit thus results from the superposition of the Alpine processes, responsible for fluid-assisted stratal disruption and mixing in a subduction channel environment, on the original stratigraphy built during intraoceanic gravitational processes.