Quantitative structural analyses and numerical modelling of ...

32 P. HASALOVÁ ET AL.The onset of the exhumation process is dated byzircon U–Pb ages of c. 340 Ma on felsic granulites,migmatites and mantle-derived syn-tectonicallyemplaced plutons (van Breemen et al., 1982; Kro¨ neret al., 1988; Holub et al., 1997; Schulmann et al.,2005). Tajčmanova´ et al. (2006) assigned metamorphicconditions of 840 °C at 18–19 kbar and 760–790 °C at10–13 kbar to relict steep granulitic fabrics whichoriginated by vertical extrusion of lower crust. Theseauthors also estimated the conditions of re-equilibrationof granulites associated with horizontal spreadingstage to 720–770 °C and 4–4.5 kbar. High pressurerocks of the Gfo¨ hl Unit are accompanied by largebodies of biotite–sillimanite Gfo¨ hl orthogneiss spatiallyassociated with K-feldspar–sillimanite paragneissesand leucocratic migmatites for which P–Tconditions of 7–10 kbar and 750 °C were estimated byRacek et al. (2006).The area of this study is located at the easternmosttermination of the Gfo¨ hl Unit (Fig. 1a). The main rocktype is represented by the Gfo¨ hl orthogneiss withprotolith ages 488 ± 6 Ma (U–Pb SHRIMP: Friedlet al., 2004) including small bodies of amphibolite,granulite, eclogite, ultrabasic rock and paragneiss. TheGfo¨ hl orthogneiss shows different stages of migmatizationcharacterized by the assemblage of Kfs +Pl + Qtz + Bt ± Grt ± Sill. This migmatizedorthogneiss complex is heterogeneously deformed bytop to the NE shearing along a large-scale, gentlydipping shear zone (Schulmann et al., 1994). Consequently,the northern margin of this complex is thrustover a footwall comprised of the Na´meˇsˇtÕ granulitebody and Neoproterozoic metagranites of the northeasterncontinental margin (Urban, 1992).STRUCTURAL EVOLUTIONMesoscopic structuresTwo major deformation events are recognized. Thefirst event (D 1 ) is represented by a steep, west-dippinghigh-grade foliation S 1 (Fig. 1b). This fabric is preservedin banded orthogneisses (type I), as an alternationof recrystallized monomineralic K-feldspar,plagioclase and quartz layers, separated by bands ofbiotite ± sillimanite (Fig. 2a). Lineation L 1 is locallymarked by alignment of biotite, sillimanite and byelongation of quartz and feldspar aggregates. Thisdeformation is attributed to an early stage of exhumationof the lower crust along a vertical channel(c)Type III(d)Type IV(b)Type II1 cm2 cm1cmd(a)Type Icba1cm1.0 mFig. 2. Schematic representation of the rock relationships at an outcrop scale and photographs of the individual rock types. (a) Bandedorthogneiss (type I); (b) stromatic migmatite (type II); (c) schlieren migmatite (type III); and (d) nebulitic migmatite (type IV). Theposition of this outcrop in the study area is shown in Fig. 1b.Ó 2007 Blackwell Publishing Ltd318