Dating syn-orogenic exhumation of subducted continental crust: The case of the Northern Apennines

Abstract

Dating deformation is key to unravel the evolution of orogens. Unfortunately, this is commonly challenged by the occurrence in the rock record of multiple overprinting deformation stages that lead to repeated blastesis and form multiple fabrics during both prograde and retrograde evolutions. Additionally, recrystallization is not always complete, which causes a mixture of inherited and neoblastic mineral phases. Therefore, an approach integrating age dating with microstructural and petrographic analyses is necessary to extract structurally constrained ages from the rock record. We used this approach to contribute to the long-lasting debate about exhumation of continental metamorphic units subducted to high-pressure conditions. We studied continental metasedimentary sequences of the Tuscan Metamorphic Units of the Northern Apennines (Italy) from two localities: the Island of Giglio to the west and the Monticiano-Roccastrada Unit to the east. We obtained two analytically distinct ⁴⁰Ar/³⁹Ar age ranges: an older 21–16 Ma age cluster is related to syn-orogenic top-to-the E contractional deformation occurring at ca. 1 GPa and 350 °C. A younger 15–11 Ma age group is related, instead, to exhumation to shallower structural levels and retrogression of these units to greenschist facies conditions. The interpretation of the results is based on white mica chemical compositions and rock fabrics. By integrating our results with published literature, we conclude that the investigated high-pressure continental metasedimentary sequences experienced early syn-orogenic exhumation from blueschist to greenschist facies conditions predominantly by a combination of extrusion and out- and in-sequence thrusting, with only minor contributions by extensional shearing during regional crustal thinning. Therefore, this study highlights the role of contractional fabrics in accommodating significant exhumation amounts of deeply subducted continental rocks.