PRE-COLLISIONAL DUCTILE EXTENSION IN THE INTERNAL WESTERN ALPS (SESIAZONE, ITALY)

Early Alpine eclogites in the western Alps were formed in the Cretaceo us prior to the main stage of continental collision in the Eocene. The se early Alpine eclogites were exhumed to shallow crustal levels by th e end of the Cretaceous, but Mesozoic (pre-collisional) extensional st ructures, 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 h ave helped the unroofing of the usually lower grade, collision-related , blueschist facies rocks. The Sesia zone, at the top of the western A lps structural pile, is a slice of a polymetamorphosed continental bas ement partly dominated by Eoalpine (130-110 Ma) eclogite facies minera l assemblages. The radiometric data show that these eclogites cooled b elow ca. 350 degrees C at 60 Ma, so that the major part of the orogeni c cycle, including burial, high-pressure metamorphism, cooling and dec ompression 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). Ducti le deformation was mainly confined to the footwall, EMS, whereas the h angingwall, II DK, preserves pre-Alpine structures and mineralogy and remained rigid. Rare eclogite facies nodules occur in this zone of duc tile deformation, but medium-P dynamic recrystallization in the stabil ity field of albite is predominant. Mineral chemistry and geothermobar ometry of a metaquartz diorite, that occupies a significant part of th is deformation zone, show that the ductile fabric in this rock formed at estimated P-T conditions of 8-10 kbar and 500 degrees C. 'S-C' fabr ic 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-dip ping tectonic contact. The interpretation of the medium-P fabric in th e EMS as extensional is supported also by the fact that the hangingwal l II DK has lower (Alpine) P-T. A WNW-directed Cretaceous extension of the type described in the present work may have dispersed the overthi ckened 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 sprea ding of orogenic wedges. However, the scale of the extensional deforma tion deduced from the metamorphic discontinuity across the II DK-EMS c ontact (ca. 2 kbar,) is certainly insufficient to account for the enti re vertical motion implied by the emergence of the Sesia eclogites.