Superposed syn-metamorphic structures of the Alpine and pre-Alpine convergent cycles in the Southalpine basement of the Orobic Alps (Northern Italy)

摘要

New detailed maps of two key areas of the Southalpine basement unravel the superposition of pre-Alpine deformation stages associated with different metamorphic imprints, and distinguish Alpine thrust and fold structures involving basement and Permian-Mesozoic sedimentary cover. Basement rocks consist of metasediments (micaschists, quartzites and paragneisses) and metagranitoids and cover rocks comprise Permian volcanics and sediments. The boundaries between two tectono-metamorphic units have been distinguished at the map scale, thanks to the integrated use of structural and petrological analyses, on the basis of foliation trajectories supported by metamorphic assemblages. Two different pre-Alpine metamorphic evolutions characterise a basement portion that seemingly appears as a monotonous sequence dominantly affected by a greenschist-facies imprint. The volcano-sedimentary sequence of Permian age has been used as the time marker that separates Alpine from pre-Alpine superposed structures; Alpine deformation consists of two superposed groups of structures, the first of which is locally associated with a very low-grade metamorphic imprint and related at the regional scale to a south-verging thrust system.