Intrusion within a transtensional tectonic domain: the C Ï ista granodiorite (Bohemian Massif )Ðstructure and rheological modelling ZdeneÏk Venera a , Karel Schulmann a , Alfred KroÈner b a Institute of Petrology and Structural Geology, Faculty of Science, Universita Karlova, Albertov 6, Prague 128 43, Czech Republic b Institut fu Èr Geowissenschaften, Universita Èt Mainz, 55099 Mainz, Germany Received 2 December 1998; accepted 23 April 2000 Abstract The C Ï ista granodiorite stock, which intrudes the Tis granite laccolith, and their Neoproterozoic country rocks of the Tepla± 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 C Ï ista 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. S 1 cleavage bears NE±SW-trending mineral lineations, which plunge to the northeast or southwest at shallow angles. The cleavage is deformed by D 2 kink-bands, which indicate a vertical shortening and a NW±SE stretching. Both the C Ï ista granodiorite stock and the Tis granite are deformed by a sinistral transtensional shear zone. The geometry of this zone is compatible with the D 2 brittle-ductile extensional kink-bands developed in the metasediments. According to microstructures in the granitoids aected by the shear zone, we show that the Tis granite was deformed in solid- state at high temperatures, whilst the C Ï ista granodiorite developed both the magmatic and high-temperature solid-state deformation fabric during its syntectonic cooling. Numerical thermal and rheological modelling of the C Ï ista 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. 7 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction Magma ascent and emplacement in the crust occur in various deformation regimes which can be tenta- tively deduced from a structural study around the intrusion (Brun and Pons, 1981; Vigneresse, 1995a). An important factor controlling the emplacement of a pluton and its microstructural evolution, which is in turn related to the rate of magma cooling, is the depth of magma emplacement (Vigneresse, 1995b). This work deals with the structural relationships between a Variscan granodiorite pluton emplaced at a shallow crustal level, an early Palaeozoic (Caledonian) laccolith and the low-grade Neoproterozoic country rocks. We demonstrate that the granodiorite stock was emplaced along a strongly localised ductile transten- sional shear zone cross-cutting the granite laccolith. We correlate this shear zone aecting both granitoids with regionally distributed extensional deformation aecting the Neoproterozoic metasediment previously subjected to compression. An important objective of this study is understanding the strain localisation within the granodiorite stock during its intrusion into the previously undeformed granite laccolith. We present results of mesoscopic structural analysis of polyphase deformation carried out in both the country rocks and in the granodiorite intrusion. We accomplish this by a detailed anisotropy of magnetic susceptibility (AMS) study in igneous rocks, and by microstructural and microfabric analyses of the deformed granitoids. A combination of mesoscopic structural study, AMS and microstructural analysis enables us to recognise the transtensional mode of granodiorite emplacement and to distinguish whether Journal of Structural Geology 22 (2000) 1437±1454 0191-8141/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S0191-8141(00)00054-7 www.elsevier.nl/locate/jstrugeo