TRIASSIC-JURASSIC RIFT-RELATED DEFORMATION AND TEMPERATURE-TIME EVOLUTION OF THE FOSSIL ADRIATIC MARGIN: A REVIEW FROM OSSOLA AND STRONA DI OMEGNA VALLEYS (IVREA-VERBANO ZONE)

Abstract

The direct observation and investigation of rift-related structures at the mesoscale is uncommon. Hence, detailed constraints on the evolution of the main faults and shear zones developed during crustal extension are not always available.The Ivrea-Verbano Zone, in the Italian Southern Alps, samples remnants of the former lower crust of the rifted margins surrounding the Alpine Tethys and therefore provides the opportunity to directly investigate rift-related tectonics. Here, several shear zones have been recognized and interpreted as related to Mesozoic rifting. However, even if there is a general agreement with this interpretation, the precise age of activity of many of those shear zones is not well constrained.In this paper we present a review of the Triassic-Jurassic geochronological and thermochronological data available for two sections of the Ivrea-Verbano Zone, the Strona di Omegna and Ossola valleys, where at least two extensional shear zones are exposed. Ductile deformation occurred under amphibolite-facies conditions and it was alternatively attributed to late Variscan deformation or to Triassic-Jurassic rifting-related tectonics.We discuss the available chronological data and the different interpretations provided for the shear zones considering also new geochronological studies on other lower crustal shear zones exposed in other sectors of the Ivrea-Verbano Zone. This review allows us to strengthen the more recent interpretations indicating that these shear zones are important tectonic structures related to Late Triassic-Jurassic deformation in the lower crust of the Adriatic margin.