Quantitative structural analyses and numerical modelling of ...

82 O. LEXA ET AL.(a)(a´)(b)(b´)(a)(b´)(b)(a´)Fig. 2. Geological maps of Blansky´ les and St. Leonhard granulite areas with schematic structural profiles. Two P–T space insetsshow the metamorphic evolution of lower crustal and middle crustal rocks (Petrakakis, 1997; Pitra et al., 1999; Scheuvens, 2002;Sˇtı´pska´ & Powell, 2005b; Racek et al., 2006, 2008). Protolith ages (Kro¨ ner et al., 2000; Friedl et al., 2004; Verner et al., 2008) ofmajor rock types are shown on the map.boundary between the root domain and the Bruniamicrocontinent is shifted 50–70 km westwards relativeto their contact on the surface suggesting that the highdensity basement rocks are covered by a thin sheet oflight granulites and migmatites in this area (Schulmannet al., 2008). North-west of the Moldanubian domainthere is an important gravity high corresponding to theNeoproterozoic basement of the Tepla´-BarrandianUnit. This is limited to the north by southeast dippingreflectors of the Tepla´ suture, which is characterized byhigh density eclogites and ultramafic rocks. The footwallof the suture corresponds to low density felsiccrust of the Saxothuringian basement.The seismic reflection and refraction sections andgravity modelling suggest a complex lithologicalstructure of the Moldanubian domain marked by a lowdensity, 5–10 km thick lower crustal layer locatedabove the Moho, a 5–10 km thick dense mafic layer, a10-km thick mid-crustal layer of intermediate densityand a locally developed 2–5 km thick low density layerat the top (Fig. 3). The low density lower crust correlateswell with low-P velocities in the range 6.0–6.4 km s )1 in the CEL09 section (Ru˚žek et al., 2007).Guy et al. (2010) proposed that the low density layerlocated above the Moho corresponds to felsic rocks,which are interpreted as deep crustal equivalents ofsurface outcrops of the Gfo¨ hl Unit. These authorsinterpreted the high density thick layer located abovelight granulites as an equivalent of the AGB (Fig. 3).The intermediate density layer forming recent uppercrust of the Moldanubian domain is interpreted asMonotonous and Varied group rocks, whereas the lowdensity rocks on the surface are directly correlated withexposures of the Gfo¨ hl Unit. It is suggested that thislayered structure of the Variscan crust reflects that ofthe original thick root, which was thinned by lateVariscan and Permian extensional processes (Burget al., 1994).Exhumation model connecting deep crustal geophysics andsurface geologyGuy et al. (2010) and Franěk et al. (2011a) proposed amodel connecting the deep structure of the Variscancrust with surface distribution of lower and mid-crustalrocks. The layered structure of the orogenic rootreported by geophysics thus represents a relict ofCarboniferous distribution of horizontally layeredcrust prior to exhumation. In addition, it is supposedthat the observed vertically layered distribution oforogenic lower crust surrounded by middle crustalunits reflects steepening of the deep crustal horizontallayering. The model connecting deep crustal layeringwith surface geology is based on several recent studiessuggesting that the granulites were exhumed alongsteep channels from lower crustal depth by a ductileextrusion mechanism (Sˇtı´pska´ et al., 2004; Schulmannet al., 2005; Franeˇk et al., 2006, 2011a; Tajcˇmanova´et al., 2006).In this view, the granulites represented, beforeexhumation, the structurally deepest orogenic lowercrust located at a depth of 60–70 km (18–20 kbar,Ó 2010 Blackwell Publishing Ltd184