A Thermal-Tectonic Model for High-Pressure Metamorphism and Deformation in the Sesia Zone, Western Alps

Abstract

The Sesia Zone, Western Alps, is a slice of Austroalpine continental crust that was once metamorphosed at T = 500°-560°C and P > 14-16 kbar at ~130-100 Ma B.P. Pervasive blueschist/eclogite facies assemblages record this event, and a later greenschist facies overprint is only well developed in the NW part of the zone. On the basis of thermal calculations, it is difficult to explain the preservation of high pressure assemblages in a simple model of underthrusting and subsequent uplift/erosion. An alternative model is proposed in which the Sesia Zone was emplaced by underthrusting of the Austroalpine continental margin prior to subduction of oceanic lithosphere of the Piemonte basin. Subduction of relatively cool oceanic lithosphere below the underthrust Sesia Zone then continued for a period of time (e.g., 20 Ma), thus suppressing thermal recovery of this latter unit. Exhumation of Sesia Zone rocks began during this subduction episode by uplift on the Insubric Line, erosion, and by underplating of oceanic material (Zermatt-Saas and Combin Zones) onto the base of the thickened continental crust. Pressure-temperature-time (PTt) paths can be calculated for this model which compare well with those estimated from petrological data. Differences in development of early metamorphism and deformation between upper and lower tectonic units are explained by this model on the basis of contrasting PTt paths and reaction kinetics. It is suggested that the regional pattern of greenschist overprinting was the result of differential uplift across the zone combined with the catalytic effect of deformation.