Pre-collisional ductile extension in the internal western Alps (Sesia zone, Italy)

Abstract

Early Alpine eclogites in the western Alps were formed in the Cretaceous prior to the main stage of continental collision in the Eocene. These early Alpine eclogites were exhumed to shallow crustal levels by the end of the Cretaceous, but Mesozoic (pre-collisional) extensional structures, required to unroof these rocks, are not common. Most of the extensional structures described in the western Alps are younger, they postdate Eocene continental collision or are coeval with it and may have helped the unroofing of the usually lower grade, collision-related, blueschist facies rocks. The Sesia zone, at the top of the western Alps structural pile, is a slice of a polymetamorphosed continental basement partly dominated by Eoalpine (130-110 Ma) eclogite facies mineral assemblages. The radiometric data show that these eclogites cooled below ca. 350°C at 60 Ma, so that the major part of the orogenic cycle, including burial, high-pressure metamorphism, cooling and decompression of this unit, was completed prior to continental collision. The present study analyses a NW-dipping, 2 km thick zone of medium- P ductile deformation that divides the Sesia zone into a lower, eclogite-bearing (EMS) unit and an upper blueschist facies unit (II DK). Ductile deformation was mainly confined to the footwall, EMS, whereas the hangingwall, II DK, preserves pre-Alpine structures and mineralogy and remained rigid. Rare eclogite facies nodules occur in this zone of ductile deformation, but medium- P dynamic recrystallization in the stability field of albite is predominant. Mineral chemistry and geothermobarometry of a metaquartz diorite, that occupies a significant part of this deformation zone, show that the ductile fabric in this rock formed at estimated P-T conditions of 8–10 kbar and 500°C. ‘S C’ fabric in the vicinity of the EMS-II DK contact indicates a top-to-the-WNW sense of motion, which is down-dip on the NW-dipping foliation in the region, and extensional with respect to the orientation of the NW-dipping tectonic contact. The interpretation of the medium- P fabric in the EMS as extensional is supported also by the fact that the hangingwall II DK has lower (Alpine) P-T. A WNW-directed Cretaceous extension of the type described in the present work may have dispersed the overthickened Adriatic margin towards the still open Piemont ocean. The later was then sufficiently large to provide the space which is required if the exhumation of deeply buried rocks is a result of horizontal spreading of orogenic wedges. However, the scale of the extensional deformation deduced from the metamorphic discontinuity across the II DK-EMS contact (ca. 2 kbar,) is certainly insufficent to account for the entire vertical motion implied by the emergence of the Sesia eclogites.