Restoration of early-Variscan structures exposed along the Teplá shear zone in the Bohemian Massif: constraints from kinematic modelling

Abstract

Perpendicular or high-angle relationship of lineations is common in shear zones where synchronous fabric transposition and shearing occur. This paper documents a polyphase early-Variscan structural record exposing the transition from orogenic superstructure to infrastructure along a major detachment shear zone at the western margin of the Tepla–Barrandian domain, Bohemian Massif. On the scale of tens of kilometers this region shows major but continuous reorientation of S2/S3 intersection lineation from its NNE–SSW trends in the superstructure to the ESE–WNW trends in the infrastructure. A simple kinematic numerical model, which allows deformation parameters such as fabric attractor, deformation symmetry, and proportion of simple shear, to be varied, was formulated to explain the rotation of fabrics observed in the field. In line with the results of our kinematic modelling, these rotations are interpreted as a result of progressive transposition of the originally subvertical S2 foliation into the gently SE-dipping S3 marked by stretching at high angle to the incipient S2–S3 intersection. The mineral–fabric relations suggest that the S2 was associated with crustal thickening and is well preserved in the superstructure, while S3 was associated with exhumation of deeper parts of the infrastructure. The kinematic model confirmed the operation of major detachment shear zone and allowed a set of the most probable deformation parameters leading to successful simulation of the fabric geometries to be identified. In addition, the successful model parameters were used to restore the vertical structure of the studied region prior to exhumation on the basis of available pressure–temperature data. We believe that unlike the commonly used balanced cross-section restoration, our restoration technique can be successfully applied in orogen interiors if the mineral–fabric relations are well-constrained and high-quality pressure–temperature estimates exist.