Late Variscan stress-field rotation initiating escape tectonics in the south-western Bohemian Massif: a far field response to late-orogenic extension

Abstract

The late Variscan tectonic evolution of the Muhlzone, located at the south-western margin of the Bohemian Massif, is characterized by subhorizontal magmatic shearing under low-differential stress conditions in intrusive and in situ granites of the South Bohemian Batholith. The magmatic shearing converted into ductile shearing and escape tectonics along conjugate strike-slip zones during the late stage of magma solidification (~325 Ma). Escape tectonics lasted for at least 30 Ma. The transition from subhorizontal to strike-slip shearing reflects the rotation of the palaeo-stress field by ~76° around an axis approximately parallel to σ 1 . Via a transient high-temperature stage of solid-state deformation the rotation caused σ 2 and σ 3 almost to swap positions during continuous contraction and cooling of the crust from >700 °C to >500 °C. The primary factor governing stress-field rotation in the Muhlzone were far-field effects of the regional tectonic evolution, specifically stress release by NE–SW crustal extension at the eastern margin of the Bohemian Massif. Extensional strain reduced the magnitude of the subhorizontal ENE–WSW oriented σ 2 to values below the magnitude of the steep σ 3 , which most likely remained constant. The σ 1 stress vector maintained a shallow SSE plunging orientation throughout the Late Variscan evolution. Stress-field rotation and the conversion from subhorizontal to steep shearing are favoured by deformation under low differential stress. High temperatures of metamorphism, or melt present in migmatites or plutons, provide the low shear strength that is required for deformation at low differential stress. Hence, in crust composed of high-grade metamorphic or partially molten rocks, small changes in the tectonic environment can be sufficient for stress-field rotation leading to significant changes in the tectonic style.