The Zermatt‐Saas ophiolite: the largest (60‐km wide) and deepest (c. 70–80 km) continuous slice of oceanic lithosphere detached from a subduction zone?

Abstract The Western Alps are a classic subduction‐related collisional orogen with well‐preserved, deeply subducted ophiolitic remnants of oceanic lithosphere. Some (e.g. Monviso, Voltri) were recognized as a palaeo‐subduction channel, with tectonic blocks showing a wide range of pressure–temperature conditions. We herein evaluate for the first time the metamorphic homogeneity of the extensive Zermatt‐Saas ophiolite. Zermatt‐Saas peak eclogitic assemblages are represented by omphacite–garnet ± phengite ± epidote ± lawsonite ± glaucophane in MORB‐derived metabasalts and garnet–chloritoid–talc ± lawsonite ± phengite in hydrothermalized metabasalts. Thermobarometric estimates with thermocalc and Raman Spectroscopy of carbonaceous material reveal homogeneous peak burial conditions at around 540 ± 20 °C and 23 ± 1 kbar. P – T paths indicate that the whole of the ophiolite, at least 60 km across, strikingly underwent similar burial and exhumation patterns and detached from the slab at depths around 80 km. The Zermatt‐Saas ophiolite thus appears to be the world’s largest oceanic lithosphere fragment exhumed from such depths, which provides important constraints on interplate coupling mechanisms.