Intrusion within a transtensional tectonic domain: the Čistá granodiorite (Bohemian Massif)—structure and rheological modelling

The Čistá granodiorite stock, which intrudes the Tis granite laccolith, and their Neoproterozoic country rocks of the Teplá–Barrandian zone were studied by means of structural analysis and anisotropy of magnetic susceptibility. The rocks were dated by the Pb–Pb evaporation on zircons, determining the age of the Čistá granodiorite and Tis granite at 373 Ma and 504 Ma, respectively. The structures in the country rocks are characterised by an axial cleavage dipping to the northwest and overprinting the folded bedding. S1 cleavage bears NE–SW-trending mineral lineations, which plunge to the northeast or southwest at shallow angles. The cleavage is deformed by D2 kink-bands, which indicate a vertical shortening and a NW–SE stretching. Both the Čistá granodiorite stock and the Tis granite are deformed by a sinistral transtensional shear zone. The geometry of this zone is compatible with the D2 brittle-ductile extensional kink-bands developed in the metasediments. According to microstructures in the granitoids affected by the shear zone, we show that the Tis granite was deformed in solid-state at high temperatures, whilst the Čistá granodiorite developed both the magmatic and high-temperature solid-state deformation fabric during its syntectonic cooling. Numerical thermal and rheological modelling of the Čistá granodiorite intrusion suggests that it caused an important weakening of the crust at the site of emplacement, which in combination with regional transtension explains that it became the site of a ductile shear zone.