Quantitative structural analyses and numerical modelling of ...

EXTRUSIONOFLOWERCRUSTINVARISCANOROGEN 73developed during such an older episode, possibly inrelation to the early arc history. This model is stronglysupported by the numerical simulations of Gerya &Stockhert (2006), who proposed an origin for HPgranulites in a lithosphere-scale subduction wedge. Inthis model, the rocks of the footwall plate are draggedto depths corresponding to 2.0–2.5 GPa, exhumedbackwards by a return flow and accreted to the base ofthe hangingwall crust (Fig. 14a). Other possible modelsinclude those of Gerya et al. (2008), Warren et al.(2008) and Beaumont et al. (2009) for buoyancy-drivenexhumation of weakened continental rocks in a subductionchannel during the early stages of continentalcollision. Janousˇek & Holub (2007) suggested a similarearly evolution and pointed out the geochemicalaffinity of the felsic granulites with numerous pre-Variscan granitoids of the Saxothuringian domain, i.e.in the footwall of the Saxothuringian subduction. Weinfer that during the D2 deformation the granuliteswere partially exhumed in such a tectonic setting fromthe peak pressure of 1.8–2.0 GPa and accreted to thebase of the hangingwall lower crust (tentativelydepicted in Fig. 14a), because the following D3 deformationcommenced at lower granulite facies conditions(Fig. 3). Buoyancy of the weak felsic granulites presumablyplayed an important role during their transportand exhumation. Alternatively, the low viscosityand low density of the granulites could have allowedtheir rise vertically, through the mantle wedge above thesubduction zone, as suggested for example by Oncken(1998) for the Saxonian granulites. During the ascent,the granulites would also trap the characteristic bodiesof garnet and spinel peridotites. The very limited recordof the early S1 and S2 fabrics precludes distinctionbetween exhumation along the suture zone or thissecond case, as well as a more precise description of theearly granulite transport path.Carboniferous extrusion of granulites and collapse ofcrustal lidThe parallelism of the S3 and S4 strike with the twomain sutures in the Bohemian Massif – the Saxothuringiansubduction zone in the NW and the Moldanubian–Bruniaboundary in the SE (Fig. 1b), indicatesthat at least one of these collisional zones generatedhorizontal shortening inducing the D3 and D4 events.Both of them could have acted as a stiff indentorunderthrusting below the already juxtaposed Tepla´-Barrandian and Moldanubian domains. Geochronologicalarguments (e.g. Schma¨ dicke et al., 1995)indicate activity of the Saxothuringian subduction andsubsequent collision between 385 and 335 Ma (Figs 2& 12; e.g. Konopa´sek & Schulmann, 2005), a timerange involving the radiometric ages of Moldanubiandeformation. The easterly Brunia margin documentsprolonged underthrusting from 330 to 310 Ma (e.g.Hartley & Otava, 2001), too young to cause theMoldanubian shortening.The spatial abundance of S3 steep fabrics (Fig. 5)and their vertical extent (Fig. 10b) suggest homogeneousdevelopment of the steep foliation throughoutthe arc (the Central Bohemian Plutonic Complex) andback-arc domains (Moldanubian domain in the senseof Schulmann et al., 2005, 2009) in a NW–SE dextraltranspressional regime. In the Tepla´-Barrandian Unit,S3 is developed only at the eastern edge (e.g. Zˇa´k et al.,2005a) suggesting that the remote part of western forearcupper crust behaved as a stiff block during the D3.Strain localization below the magmatic arc wasresponsible for the unusually massive amplification ofF3 folds in the lower crust that resulted in localizedexhumation of the deep-seated granulites to midcrustallevels in the form of a 20 km wide ductileisoclinally folded subvertical fan-like structure (Figs 13& 14b–d). The adjacent mid-crustal units in this casewould be forced to develop marginal synclines aroundthe dome, to balance the voluminous vertical masstransfer through the ductile crust (Fig. 14c,d), draggingthe upper-crustal Palaeozoic Varied sequences tomid-crustal depths. This model for exhumation of thegranulites in many aspects is similar to that proposedby Behr (1978), Weber (1984) or Franke & Stein (2000)for exhumation of the Saxonian granulites, but it differsfrom the wide range of numerical models focusedon exhumation in collisional domains (e.g. Burg &Podladchikov, 1999; Gerya & Stockhert, 2006; Jamiesonet al., 2007).The specific tectonic history during exhumation ofgranulites in the Moldanubian domain results from acombination of buoyancy and the felsic composition,which together are responsible for their distinct andtransient mechanical behaviour (Franeˇk et al., 2011;Lexa et al., 2011). The abundance of felsic granulitesamong Variscan lower crustal rocks suggests thattheir presence at the base of continental crust isresponsible for the unique tectonic style presented inthis work. The D4 vertical shortening followedimmediately after the D3 granulite ascent, being mostpronounced near the margin of the upper-crustalTepla´-Barrandian block (Figs 13 & 14c,d). Here, theS4 was lubricated by late arc-related magmas andsyenite magma and attained characteristics of a normalshear zone with a dip-slip lineation (Zˇa´k et al.,2005a). This intensive vertical shortening in thecentral parts of the orogenic root can be explained bya ductile thinning mechanism in which the Tepla´-Barrandian suprastructure slid along the normalshear zone to the NW from the mid- and lowercrustal D3 dome described above.The D4 sliding was a mid-crustal expression ofgravitational spreading of the roof (suprastructure) ofthe crustal-scale dome cored by the granulites (infrastructure).The sliding was localized mainly at thethermally weakened volcanic arc, being driven by thefinal D3 dome amplification (Fig. 14c,d). Indeed,recent work by Gerya et al. (2008) and Beaumontet al. (2009) shows that rapidly exhumed, buoyant,Ó 2010 Blackwell Publishing Ltd225