Quantitative structural analyses and numerical modelling of ...

Author's personal copyK. Schulmann et al. / C. R. Geoscience 341 (2009) 266–286 273is composed of kyanite-bearing micaschists that wereinterpreted by Suess [136] as a result of deep crustalretrogression (‘‘diaftorism’’) of the Moldanubiangneisses. Modern studies by Konopásek et al. [73] andŠtípská et al. [132] have shown that this zonecontains boudins of eclogites (Fig. 2; P = 16 kbar,T = 650 8C), HP granulites and peridotites embeddedin the metapelites. This zone, which contains elements ofboth Moravian and Moldanubian parentage, is thusregarded as a first order tectonic boundary.The underlying Moravo-Silesian Zone (Fig. 1) ischaracterized by two nappes composed of orthogneissesat the bottom and a metapelite sequence at the top.These nappes are thrust over the Neoproterozoicbasement which is often imbricated with its Pragianto Famennian cover [9,43]. A number of isotopic studies(e.g. [28]) show that the orthogneisses of the MoravianZone are derived from the underlying Brunia continent[20]. This 50 km wide and 300 km long zone of intensedeformation is marked by an inverted metamorphicsequence ranging from chlorite to kyanite-sillimanitezones and P–T conditions ranging from 5–10 kbar and550–650 8C (Fig. 2) [77,127]. The prograde metamorphismis interpreted as a result of continentalunderthrusting associated with intense top-to-the-NNEoriented shearing, development of sheath folds andgneissification of Brunia-derived granite protoliths[118,121]. The subsequent deformation is connectedwith recumbent folding and imbrication of Neoproterozoicgneisses with Devonian cover [119,120]. This laterphase is interpreted as late imbrications of metamorphosedBrunia crust in a kinematic continuum with earlyunderthrusting event. The cooling after the Barrovianmetamorphism is constrained at 340–325 Ma byhornblende Ar–Ar data [14,40] and monazite inclusionsin garnets yielding 340–330 Ma CHIME ages [78].2.7. The Brunia continentThe Brunia continental foreland (Fig. 1) originallycalled the Bruno-Vistulicum by Dudek [20] consists ofNeoproterozoic granitoids, migmatites and schists.They reveal the existence of a 680-Ma-old crust,intruded by 550-Ma-old granites [28,40]. This basementis unconformably overlain by Cambrian strata [7],Ordovician and shallow marine Lower Devonianquartzites and conglomerates followed by Givetiancarbonate platform sedimentation [3,68]. Since theEarly Carboniferous (350 Ma), foreland sedimentaryenvironment developed being accompanied by extensionalfaulting indicating the flexural subsidence of theBrunia margin [49,96]. From 345 until 300 Ma, a7.5 km thick Variscan flysch (Culm facies) wasdeposited onto the Brunia foreland. This sedimentationwas coeval with the onset of massive exhumation in theneighbouring Moldanubian domain and the continuousloading of the Brunia continent [49]. Low-grade sourcerocks associated with clastic muscovites dated at 345–330 Ma [117] gradually pass to a high-grade metamorphicsource material marked by pyrope-rich mineralfraction and granulite pebbles dated at 340–330 Ma[13,49,81,152]. Since 330 Ma, began also lateralshortening of the flysch basin marked by a deformationfront progressively migrating from eastward in conjunctionwith decreasing intensity of deformation[30,57]. Deformation of this flysch terminated atc. 300 Ma as indicated by Ar–Ar cooling ages of themetamorphosed Culm facies in the west [91] anddeformation of the Variscan molasse further east [11].3. Geodynamic evolution of the BohemianMassifThe spatial and temporal distribution of geologicalunits, magmatic fronts and metamorphic zones can beinterpreted in an evolutionary scheme very similar to thatcurrently reported from the Andean type orogeny. Thefollowing account provides a succession of tectonicevents that can be interpreted in terms of south-eastward(in the present-day coordinates) oceanic subductionunderneath an active continental margin, obduction ofthe Saxothuringian oceanic domain, formation of afore-arc region, growth of magmatic arc and developmentof a large-scale back-arc system on the continentallithosphere. The early oceanic subduction event couldhave been followed by a continental underthrusting of theArmorican plate leading to increased friction between theupper and lower plates, gradual flattening of thesubduction zone marked by eastward migration of arcand subsequent crustal thickening. The latter event couldhave been responsible for the development of a thickcontinental root due to thickening of the upper platerepresented by the Teplá-Barrandian and Moldanubiancrustal material. The final evolution is marked by thecontinental indentation of eastern Brunia continent into aweak orogenic root, exhumation of the Moldanubianorogenic lower crust, collapse of the Teplá-Barrandian lidand Moldanubian thrusting over the Brunia platform.3.1. Early Devonian oceanic subductionunderneath the active continental marginThe contact between the Saxothuringian-Armoricanplate and the overriding Teplá-Barrandian continent is166