Quantitative structural analyses and numerical modelling of ...

EXHUMATION IN LARGE HOT OROGEN 275mation event was related to almost vertical materialtransfer, which was driven by S-E directed oceanic andcontinental (Armorican–Saxothuringian) subductiondynamics during the Devonian and Tournasian toEarly Visean (Fig. 1). Following this early evolution,during the Middle to Late Visean indentation from theeast by a promontory of the Brunia continent at a highangle to the Saxothuringian subduction directionresulted in horizontal flow and transport of theextruded orogenic lower crust over the rigid continentas a hot fold nappe.GEOTECTONIC SETTINGSuess (1912, 1926) described the geology of the easternVariscan front and divided the crystalline complexes ofthe Bohemian Massif into two parts, an internalMoldanubian–Lugian domain and an external Moravo–SilesianZone (Figs 1 & 2). Dudek (1980) completedthis subdivision and defined a Brunia continentwith the Moravo–Silesian Zone as its westerndeformed margin. The eastern segment of the Bruniacontinent is built up of unmetamorphosed to weaklymetamorphosed Neoproterozoic granites, high-gradeschists and migmatites. This basement is unconformable,covered by Lower Carboniferous foreland basinsedimentary rocks, by Devonian shallow marine sedimentaryrocks, and locally by Cambro-Ordovicianclastic, pelitic metasediment rocks and bimodal metavolcanicrocks (Franke, 2000; Hartley & Otava, 2001).The Moravo–Silesian Zone represents a NE–SWtrendingbelt of sheared and metamorphosed rocksderived from the Brunia continent. This 300 km long,30)50 km wide belt consists of three NE–SW-elongatedtectonic windows emerging through structurallyoverlying high-grade rocks of the Moldanubian–Lugian domain: a southern Thaya window; a centralSvratka window; and a northern Silesian domain(Fig. 2). Schulmann et al. (2005) identified the Moldanubian–Lugiandomain in the Bohemian Massif asthe deep orogenic root system of the Variscan orogen(Fig. 2). The Elbe zone (Fig. 2) divides rocks of theMoldanubian–Lugian domain into two: a larger highgradeMoldanubian domain to the south and a smallerhigh-grade Lugian domain to the north (Suess, 1926).The Moldanubian domain (Figs 1 & 2) has beensubdivided into three major lithotectonic units (e.g.Fuchs, 1986), namely the amphibolite facies Monotonousand Varied Groups, which together with gneissesmake up the orogenic middle crust, and the predominantlygranulite facies Gfo¨ hl Unit, which is inferred tobe the orogenic lower crust (Fig. 2). The Varied Groupoutcrops structurally above the Monotonous Group,with a contact that is commonly marked by bodies ofgranite gneiss. The Monotonous Group consists ofmigmatitic paragneisses (metagraywacke) interlayeredwith granite orthogneisses, quartzites and rare eclogites(Dudek & Fediukova´ , 1974; Petrakakis, 1997). TheVaried Group consists of a thick sequence of paragneissesinterlayered with calcsilicate rocks, marbles,quartzites, graphite schists, amphibolites and felsicmetavolcanic rocks. The Gfo¨ hl Unit is composed oflarge areas of migmatitic granite gneiss, called theGfo¨ hl gneiss, and of areas of various highly anatecticmigmatites and paragneisses that are in placesaccompanied by migmatitic amphibolites at the base.The Gfo¨ hl Unit includes numerous bodies of Ky–Kfsfelsic granulite as well as tectonic lenses of eclogite andgarnet and ⁄ or spinel peridotite.The main part of the Lugian domain is composed ofmedium- to high-grade granite gneisses and metamorphosedvolcano-sedimentary rocks with Cambro–Ordovician protolith ages (Kro¨ ner et al., 2001, andreferences therein). The granite gneisses contain boudinsof eclogite and a belt of garnet–omphacite granulite,and are considered an equivalent of the Gfo¨ hlUnit, whereas the belts of medium-grade schists ofvolcano-sedimentary origin are regarded as an equivalentof either the Monotonous or the Varied Groups(Fig. 2). An Ordovician leptyno-amphibolite lowercrustal complex (the Stare´ Město belt) occurs in theeastern part of the Lugian domain (Sˇtı´ pska´ et al.,2001).DEFINITION OF BASEMENT AND OROGENICCRUSTAL LEVELSIn this section, multiple criteria, such as the peakpressure conditions attained by individual units, thecharacter and the age of the protolith and chronologyof metamorphic zircon, are used to decipher the relativevertical position of crustal units during the LowerPalaeozoic and the Devonian. This information is thenused to propose a model for the stratification of theorogenic crust along the eastern Variscan front.The Brunia continentBased on zircon protolith ages (Fig. 3a) and 40 Ar ⁄ 39 Arcooling ages ranging from 600 to 540 Ma, granites andmigmatites forming the basement of the Brunia continentare inferred to have originated during the Pan-African orogenic events (Van Breemen et al., 1982;Fritz et al., 1996; Finger et al., 2000; Friedl et al.,2004). Tectonic imbrication of Devonian metasedimentaryrocks and basement, and metamorphism tolower greenschist facies occurred at the western marginof the Brunia continent during the Carboniferous(Francu˚ et al., 2002). In the northern Silesian domain(Fig. 2), the metamorphic pattern of nappes derivedfrom the Brunia continent shows an inverted Barrovianzonation ranging from chlorite grade in the east tokyanite ⁄ sillimanite grade in the west (Sˇtı´ pska´ &Schulmann, 1995; Schulmann & Gayer, 2000). Thewestern termination of the Moravo–Silesian Zone inthe north reached eclogite facies conditions (Fig. 4;Sˇtı´ pska´ et al., 2006), and it is likely that the western tipof the Moravo–Silesian Zone in the south has beenÓ 2007 Blackwell Publishing Ltd135