Quantitative structural analyses and numerical modelling of ...

54 J. FRANĚK ET AL.represented mostly by large bodies of HP felsic granuliteswhich form areas sometimes several 100 km 2 insize (OÕBrien & Carswell, 1993; Janousˇek & Holub,2007). These HP granulites are associated with otherHP rocks like mafic granulites, eclogites and garnetperidotites collectively forming the so-called Gfo¨ hlUnit (Fuchs, 1976). This unit does not form a continuousHP body, but occurs in three major belts, whichare surrounded by medium-grade rocks (Schulmannet al., 2009). The protolith of these rocks, their positionat lower crustal depths prior to exhumation andthe exhumation mechanisms represent major problemsof the Variscan belt (e.g. Behr, 1961, 1980). Schulmannet al. (2005) proposed a multistage model of rapidexhumation of orogenic lower crust associated withdevelopment of subvertical ascent channels triggeredby rapid amplification of initial instabilities in front ofan advancing continental buttress. Such a process isenabled by the extremely low viscosity of the Variscanorogenic lower crust at the bottom of the orogenic rootdue to its unusually hot thermal structure (e.g. OÕBrien,2008) combined with its felsic composition (Ru˚zˇeket al., 2007). This model is fundamentally similar tothat proposed by Weber & Behr (1983) who explainedthe exhumation of the Saxonian HP granulites byÔdiapiric foldingÕ. In their model, the deep granulitelayer tends to amplify and pierce through the weakermiddle crust during crustal shortening, to form largescalesteep folds bringing HP rocks to mid-crustallevels.The study region is located in the central part of theorogenic root, far from both the easterly continentalbuttress and westerly suture zone (Fig. 1). Here, weexamine a large portion of orogenic lower and middlecrust using both the published and unpublished part ofthe 9HR seismic line of Tomek et al. (1997). Theseismic profile is combined with a detailed structuralstudy along a SE–NW traverse evaluating mutualrelationships between middle orogenic crust and thethree major granulite massifs of South Bohemia, whichcompletes an earlier study of one of these massifs(Franeˇk et al., 2006). The profile line drawing is combinedwith detailed gravity forward modelling allowingestimation of a vertical ascent channel of orogeniclower crustal rocks. Finally, we discuss the developmentof flat-lying amphibolite facies fabrics throughoutthe traverse, which provides evidence of mid-crustalhorizontal flow. It is shown that this episode is connectedwith the collapse of the orogenic suprastructurerelated to final upwelling of a lower crustal dome.Despite disruption of the linear Variscan orogenictrend by late large-scale transcurrent shear zones (Edelet al., 2003), the wealth of quantitative geological datamakes the Bohemian Massif a suitable field laboratoryfor understanding processes related to exhumation oforogenic lower crust in large hot orogens. The fieldresults are suitable for testing the results from largescaleexhumation-related numerical models referencedabove.GEOLOGICAL SETTINGThe Bohemian Massif (Fig. 1) is traditionally dividedinto the Saxothuringian domain to the west, the Tepla´-Barrandian and Moldanubian domains in the centralpart of the Massif (Kossmat, 1927) and the Brunovistulian(Brunia) Neoproterozoic continent to the east(Dudek, 1980). Schulmann et al. (2005, 2009) interpretedthe Bohemian Massif as a Gondwana-derivedcollisional domain characterized by: (i) relicts of a twostageSE-directed subduction at the Saxothuringian–Tepla´-Barrandian boundary; (ii) a magmatic arcgenetically related to the subduction represented bythe Central Bohemian Plutonic Complex in the centre;and (iii) the rigid foreland represented by the Bruniamicroplate in the SE. In this concept, the Tepla´-Barrandian between the suture zone and the magmaticarc represents the fore-arc domain and the Moldanubiandomain between the magmatic arc and the Bruniamicroplate constitutes the shortened and thickenedintracontinental back-arc region. Large Variscanstrike-slip zones (e.g. the Elbe Fault Zone, seeFig. 1a,b) strike NW–SE and dismember the NNEtrending Variscan structure of the Bohemian Massif(Edel & Weber, 1995).Geology of the Saxothuringian, Teplá-Barrandian andMoldanubian domainsThe SE part of the Saxothuringian domain consists ofthe antiformal Erzgebirge Crystalline Complex, whichcan be divided into a lowermost para-autochtonousdomain that is overlain by crystalline nappes thatrecord peak HP metamorphism at 345–340 Ma(e.g. Franke, 2000; Konopa´sek & Schulmann, 2005).Towards the NW, the antiform passes into a Cambrianto Lower Carboniferous volcano-sedimentarysequence, which crops out in several large-scale latetectonic antiforms and synforms (Franke, 1993). Thesesupracrustal rocks are overthrust by oceanic crustalunits (Mu¨ nchberg, Frankenberg and Wildenfels klippen),metamorphosed at eclogite facies at c. 395–380 Ma (e.g. Dallmayer et al., 1995; Franke, 2000).The Saxonian granulite massif emerges as a large-scaleNE–SW elongated dome structure from below theSaxothuringian Basin (Du¨ rbaum et al., 1999) andconsists of HP felsic granulite, with subordinate lensesof mafic granulite and serpentinized peridotite (Ro¨ tzler& Romer, 2001). Kro¨ ner & Willner (1998) obtainedages of 485–470 Ma from zircon cores interpreted todate the protolith, while c. 340 Ma overgrowths wereinterpreted to date the time of peak metamorphism.The Tepla´-Barrandian domain is separated from theSaxothuringian domain by the Variscan SE-dippingsuture zone represented by the Maria´nske´ La´zněComplex (e.g. Zulauf et al., 1997). This unit consistsof serpentinites and metagabbros of Cambrian andOrdovician age (Timmermann et al., 2004), which werein part eclogitized before Devonian exhumation. TheÓ 2010 Blackwell Publishing Ltd206