Repeated slip along a major decoupling horizon between crustal-scale nappes of the Central Western Carpathians documented in the Ochtiná tectonic mélange

• Heterogeneous lithological record of blocks in matrix is interpreted as a mélange.
• Contrasting metamorphic conditions of blocks in the tectonic mélange were revealed.
• The mélange separates orogenic Gemer suprastructure and Vepor infrastructure.
• The mélange documents repeated slip along mechanically weak horizon.

The Ochtiná Unit is situated in the ENE–WSW-trending contact zone between two crustal-scale nappes, the upper Gemer Unit and the lower Vepor Unit, in the Central Western Carpathians, Slovakia. The Ochtiná Unit consists mainly of Carboniferous phyllitic schists and sandstones enclosing lenses of diverse lithological nature and contrasting metamorphic history. Peak PT conditions obtained by means of phase equilibrium modelling from lenses of amphibolite and chloritoid schist in this unit indicate 500–600 °C and 4–6.5 kbar and 500–520 °C and 9–11 kbar, respectively. These PT conditions contrast not only with the greenschist-facies metamorphism of dominant phyllite but also with each other documenting two distinct metamorphic field gradients related to Variscan and Alpine metamorphic events. Geochemical data reveal an affinity of the amphibolite lenses similar to Variscan rocks in the basement of the upper Gemer Unit and of the chloritoid schist similar to Alpine rocks in the cover of the lower Vepor Unit. Such heterogeneous lithological and metamorphic record is consistent with a block-in-matrix rock arrangement and the Ochtiná Unit is interpreted as deep seated tectonic mélange. The mélange evolved via repeated slip along the rheologically weak sediments of the Ochtiná Unit during the building and collapse of the Eo-Alpine orogenic wedge of the Central Western Carpathians. Deformation record indicates that the mélange separates two distinct structural domains marked by a decoupled behaviour, i.e. the orogenic suprastructure represented by the Gemer Unit and the infrastructure represented by the Vepor Unit. With this respect, the Ochtiná Unit represents an unusual example of a suprastructure–infrastructure transition zone with its position being controlled by the mechanical weakness of this sedimentary horizon and not by the temperature-dependent rheological transition.