Pervasive horizontal fabric and rapid vertical extrusion: Lateral overturning and margin sub-parallel flow of deep crustal migmatites, northeastern Bohemian Massif

In the West Sudetes, northeastern Bohemia Massif, geochronometry provides evidence for repeated episodes of rapid cooling that contrasts sharply with an absence of structural evidence for significant tectonic exhumation by crustal extension. Instead, high-grade assemblages of the Orlica–Snieznik Complex have a regional sub-horizontal foliation and sub-horizontal lineations that trend parallel to narrow sub-vertical shear zones containing exhumed high-pressure assemblages. Mesoscopic petrofabrics combined with anisotropy of magnetic susceptibility (AMS) data from amphibolite facies to migmatitic meta-sedimentary and meta-igneous rocks reveal remarkably consistent average lineations that plunge shallowly to the SSW on both steep and sub-horizontal NNE-trending planar fabrics. The dominant SSW–NNE fabric orientation is parallel to the Bohemia–Brunia suture, which marks a major boundary along the eastern margin of the massif. The shape of the AMS ellipsoid is predominantly oblate, revealing flattened fabrics, with only local prolate ellipsoids. We envisage that the continental Brunian indentor operated as a rigid backstop and allowed the migmatized lower crustal orogenic root to be exhumed along the Bohemian margin shortly following terminal arc collision. Sub-vertical extrusion of the orogenic root was arrested in the mid-crust, where the lower ductile crust was laterally overturned at the base of rigid upper crustal blocks. Upon reaching the crustal high-strength lid the exhumed ductile mass of continental material laterally spread sub-parallel to the margin, underwent subsequent supra-Barrovian metamorphism, and quickly cooled. The application of AMS techniques to high-grade metamorphic rocks in concert with macroscopic structural observations is a powerful approach for resolving the deformation history of a terrane where visible rock fabrics can be tenuous.