Quantitative structural analyses and numerical modelling of ...

294 K. SCHULMANN ET AL.at 330–325 Ma and was associated with exhumation ofHP metamorphic rocks to the surface, as demonstratedby sedimentation of granulite pebbles and metamorphicmuscovite in the foreland basin. The earlier agesclustering around 340 Ma correspond to D 2 event, i.e.to the vertical material transfer associated with crustalscalefolding, evidence of which is partially preservedin flat-lying migmatites.The structure of the Moldanubian domain is consistentwith the Ôstage 3Õ of the channel-flow model – thehot fold nappe (Beaumont et al., 2006; Culshaw et al.,2006). In these channel-flow models, stage 3 coincideswith the arrival of a lower crustal block that forces weakmiddle and lower crust into large-scale gently inclinedfold nappes rooted in the thickened Moho.The hot fold nappe model can be applied to theMoldanubian root system, keeping in mind that themodel represents the result of a continuous 2D historywhereas the Moldanubian root system comprises anexhumation history in two stages that are kinematicallyindependent. Nevertheless, the hot fold nappemodel simulates well the relatively weakly deformedNE part of the Moldanubian system with well-preservedD 2 fabric, which reaches shallow crustal levelsand ultimately the surface (Figs 8 & 11). To the south,the extruded nappes were derived from more internaland hotter parts of the orogen. Increasingly largevolumes of weak lower crust were gradually transportedsouthwards over the Brunia continent leadingto an increase in the amount of lower crustal materialat the surface.The channel flow is heterogeneous, as predicted byBeaumont et al. (2001, 2006), because of the previoushistory, which generates important crustal strengthvariations. The implication of this model is that theheterogeneous crust makes the geometry and compositionof the channel flow similarly heterogeneous. Inthe Moldanubian case, the channel transportsdetached blocks and boudins of distinctly differentcompositions such as HP granulites and mid-crustalsegments that still preserve relicts of the D 2 fabric. Inthis southern part of the channel all isotopic systemsare re-equilibrated and the cooling ages are younger incomparison with the non-uniformly reset isotopicsystems in the north.CONCLUSIONSThis study has shown that exhumation of the orogeniclower crust in the eastern sector of the Variscan orogenicbelt is characterized by two independent stages.(1) The first stage is best preserved in the Lugiandomain, where it is characterized by an intra-crustalfolding that is responsible for vertical material transferassociated with exhumation of the deep orogenic lowercrust to shallower crustal levels corresponding topressures around 10 kbar at about 350 to 340 Ma. Thefolding and vertical extrusion events are followed by avertical shortening leading to development of subhorizontalfabrics at medium to low pressures. Theearly horizontal shortening was probably triggered bythe existence of a rigid Ordovician block, part of whichis preserved at the eastern boundary of the Lugiandomain. We suggest that this early exhumation eventwas related kinematically to Saxothuringian continentalsubduction to the east, creating a convergentcontinental accretionary wedge – the Lugian domain.Mechanical interactions with the large Brunia continentduring the exhumation process remain unconstrained.(2) The Moldanubian domain has steep fabrics insimilar orientations to those preserved in the Lugiandomain. Vertical material transfer during the LowerCarboniferous led to the development of alternationsof steeply inclined domains of lower and middle orogeniccrust, similar to Lugian domain. However, thistectonic event was followed by a NNE-directed subhorizontalshearing at about 330–325 Ma resultingfrom subsurface indentation by the Brunia continentalpromontory into the Moldanubian domain. The arrivalof the Brunia continent is responsible for the progressiveemplacement of hot fold nappes andheterogeneous channel flow in the rear (western) partof the system generating mixtures of middle and lowercrustal units with preserved early exhumation fabrics.ACKNOWLEDGEMENTSThis study was made possible thanks to the ANRproject ÔLFO in orogensÕ funding as well as to financialsupport of CNRS (UMRs 7516 and 7517) and theCzech Science Foundation (GACR 205 ⁄ 05 ⁄ 2187 andGACR 205 ⁄ 04 ⁄ 2065). Ondrej Lexa is indebted to theUniversite´ Louis Pasteur for covering his salary. Weare grateful to J. Platt, D. Grujic and an anonymousreviewer for constructive reviews. The editorial workof M. Brown is highly appreciated.REFERENCESAllemand, P. & Lardeaux, J. M., 1997. Strain partitioning andmetamorphism in a deformable orogenic wedge: Applicationto the Alpine belt. Tectonophysics, 280, 157–169.Andersen, T. B., Jamtveit, B., Dewey, J. F. & Swensson, E.,1991. Subduction and eduction of continental crust: majormechanisms during continent–continent collision andorogenic extensional collapse, a model based on the southNorwegian Caledonides. Terra Nova, 3, 303–310.Baratoux, L., Schulmann, K., Ulrich, S. & Lexa, O., 2005.Contrasting microstructures and deformation mechanisms inmetagabbro mylonites contemporaneously deformed underdifferent temperatures (c. 650 °C and c. 750 °C). In: DeformationMechanisms, Rheology and Tectonics: From Minerals tothe Lithosphere, Special Publication 243 (eds Gapais, D., Brun,J.-P. & Cobbold, P. R.), pp. 97–125. Geological Society,London.Beard, B. L., Medaris, L. G., Johnson, C. M., Brueckner, H. K.&Mı´ sarˇ, Z., 1992. Petrogenesis of Variscan high-temperaturegroup–A eclogites from the Moldanubian zone of the BohemianMassif, Czechoslovakia. Contributions to Mineralogy andPetrology, 111, 468–483.Ó 2007 Blackwell Publishing Ltd154