Microstructural-deformation record of an orogen-parallel extension in the Vepor Unit, West Carpathians

Deformation microstructures of quartzo-feldspathic rocks have been used to analyse the orogen-parallel stretching of the Vepor Unit in the West Carpathians during Cretaceous convergence. This process occurs contemporaneously with the burial of the Vepor basement related to the overthrusting Palaeozoic volcano-sedimentary sequences of the Gemer Unit. The microstructures develop along a metamorphic gradient indicating an increase of pressure and temperature towards the structural footwall. The recrystallization of quartz occurs in the field of subgrain rotation recrystallization (SGR) while the plagioclase and K-feldspar disintegrate into a neocrystallized muscovite-albite matrix. Palaeopiezometry of the recrystallized quartz in conjunction with P–T estimates yield strain rates of 9 × 10−12 to 6 × 10−14 s−1, depending on temperature (380–480 °C) and choice of piezometer and flow law calibration. Strain analysis of the recrystallized quartz aggregates and quartz texture analysis indicate an overprint of the Variscan magmatic and high grade fabrics by a Cretaceous plane strain deformation resulting in vertical shortening and horizontal orogen parallel East–West stretching. Spatial distribution and conflicting sense-of-shear of the asymmetrical quartz lattice preferred orientation patterns indicate that the large-scale pure shear deformation of the Vepor Unit is locally accommodated by simple shear zones. Observed metamorphic gradients across the Vepor Unit are interpreted to result from later large-scale folding and heterogeneous exhumation of the orogen-parallel deformation fabric.