EXHUMATION OF ULTRA-HIGH-PRESSURE METAMORPHIC TERRANES IN THE OVER-RIDING PLATE ABOVE RETREATING SUBDUCTION ZONES

Renewed roll-back of subducting oceanic lithosphere in front of an orogenic zone after an accretion event has the capacity to exhume very deep levels of the Earth's crust or lithosphere, because the crust and lithosphere can be severely extended in such environments. We discuss the role of lithospheric dislocations in this process, and how lithospheric dislocations that reverse their shear sense can be recognized, using the ultra-high-pressure slice at Lago di Cignana, Valtournenche, NW Italy as a case study. The UHP sliver occurs immediately beneath a major extensional shear zone (the Matterhorn Detachment), interleaved between the Zermatt Saas Unit and the Combin Shear Zone. The maximum metamorphic grade recorded by the Zermatt-Saas Unit is high-pressure (HP) eclogite facies, whereas rocks in the now dominantly greenschist facies Combin Shear Zone locally contains medium-pressure (MP) blueschist facies relicts. We suggest that the UHP unit is preserved as a thin tectonic slice because the locus of later extensional structures has not precisely followed the trajectory of older thrusts. The UHP slice is located in the lower boundary of a kilometre-scale shear zone that has sheared and retrogressed rocks from the Combin Zone. Fabrics and mineralogy reveal that the Combin shear zone has operated through blueschist facies conditions until greenschist facies metamorphic conditions were reached. Intense fabrics developed, commensurate with large shear strains associated with significant horizontal relative displacement. Structural geology suggests a history of large-scale overthrusting during which the UHP rocks were emplaced over the HP rocks of the Zermatt-Saas Unit. Tectonic inversion subsequent to the period of HP metamorphism led to the Alpine orogen being subject to large-scale (roughly NW-SE directed) horizontal stretching. This led to the formation of orogen-scale extensional shear zones, of which the Matterhorn Detachment may be one of the most important manifestations.