From subduction channel to orogenic wedge: Exhumation recorded by orthogneiss microstructures in Erzgebirge, Bohemian Massif

Abstract

The Erzgebirge dome, exposing Proterozoic - Early Palaeozoic metagranitoids and metasedimentary rocks of the Saxothuringian domain in the Bohemian Massif, is formed by a tectonic stack of several UHP-HP and HT units exhumed during Variscan continental subduction-collision process. A systematic study of orthogneiss deformation microstructures carried out on a regional scale has been done to evaluate the extent and relative structural position of these units in the subduction complex. Five different microstructural types have been distinguished, revealing two overprinting sequences with 1) coarse grained microstructures in the parautochthonous Kateřina-Reitzenhain unit and 2) finer grained microstructures in the allochthonous HP-HT Gneiss Eclogite Unit. P-T conditions of the individual overprinting microstructures calculated by conventional thermobarometry suggest continuous deformation during exhumation along two contrasting P-T paths. The warmer one in the parautochthonous unit showed exhumation from ~13.5 kbar and ~820 °C to ~3 kbar and ~500 °C while the colder one in the allochthonous unit showed exhumation from ~16 kbar and ~740 °C to ~3 kbar and ~400 °C. The overprinting microstructures are associated with vertical shortening interpreted as ductile thinning. This deformation started at depths of ~10 kbar and led to a temporal elevation of the crustal heat flow affecting the exhumation P-T paths of the structurally deeper units. We propose a model of continuous underplating of the former accretionary wedge by deeply subducted and exhumed continental material building up the orogenic wedge. Progressive heating of the orogenic wedge is explained by heat advection related to fast exhumation of the UHP/HP-HT units from the subduction channel followed by ductile thinning.