Quantitative structural analyses and numerical modelling of ...

EXHUMATION IN LARGE HOT OROGEN 293represents a relict of the rigid backstop and the normal-senseshear zone in the west reflects the collapse ofvertically extruded material (Fig. 11b). Importantly,the U–Pb zircon data show Ordovician protolith agesfor both the felsic orogenic root (orogenic lower andmiddle crust) and the Ordovician mafic lower crustalblock to the east. These data, together with the coincidenceof the gravity and geological boundariesbetween the Lugian domain and the Brunia continent(Fig. 10), indicate that the transition from vertical tohorizontal fabric results from an intra-Lugianmechanical interaction and that the Brunia continentwas not involved in this process (Fig. 12).Heterogeneous channel flow and hot fold nappe fabricsin the Moldanubian domainThere are a number of differences associated with thedevelopment of the flat fabric in the Moldanubiandomain compared with the development of the flatfabric in the Lugian domain. In the southern part ofthe Moldanubian domain, the horizontal S 3 fabricdeveloped at decreasing pressure from south to north(Figs 4 & 5d) in conjunction with decreasing intensityof the D 3 reworking. This is associated with an increasein temperature and decrease in pressure in the orogenicmiddle crust concomitant with a decrease in temperatureand pressure of the orogenic lower crust. Inaddition, migmatized gneisses of the orogenic lowercrust commonly surround blocks and boudins of theorogenic middle crust that preserve evidence of theearly HP metamorphism. The D 3 deformation showsconstant NNE–SSW-oriented stretching and top-tothe-NNEthrust-related shear movement, which isconsistent with oblique transpressive deformation inthe adjacent Moravo–Silesian Zone. The commongeometry and oblique thrust kinematics of the D 3deformation are the chief features of the mechanicalinteraction between the Moldanubian domain, Moravo–SilesianZone and the Brunia continent. Thegeophysical observations confirm the hypothesis oflarge-scale displacement of the Brunia basementunderneath the Moldanubian domain, which interpretationlocates the sub-surface margin of the Bruniacontinental promontory far to the west from thepresent Moldanubian domain and Moravo–SilesianZone boundary.This highly non-coaxial deformation correlates withyoung cooling ages (330–325 Ma) for muscovite andbiotite 40 Ar ⁄ 39 Ar systems compared with older (350–340 Ma) hornblende 40 Ar ⁄ 39 Ar data, Sm–Nd coolingages from HP metamorphic rocks and metamorphicages from zircon, preserved mainly in the north.Muscovite and biotite from the adjacent Moravo–Silesian Zone nappes and the deformed Brunia continentshow similar young 40 Ar ⁄ 39 Ar cooling ages,which correspond to those of detrital muscovite fractionsfrom the Culm foreland basin. The detritalmuscovite shows convergence of the stratigraphic ageof sedimentation and the 40 Ar ⁄ 39 Ar cooling ages at330–325 Ma (F. Neubauer, pers. comm.). Pebbles ofgranulite and Mg-rich syenite (durbachite) in theforeland conglomerates have yielded similar informationfrom U–Pb dating of zircon, where two groups ofages occur at c. 340 and c. 325 Ma, the latter beingconsistent with the stratigraphic age of 330–320 Ma(Kotkova´ et al., 2007).We suggest that the D 3 deformation, which involvedsubhorizontal top-to-the-NE thrusting of the Moldanubiandomain over the Brunia continent, occurred(a)(c)Fig. 12. Sequence of block diagrams toshow the principal exhumation mechanismsand progressive tectonic evolution of theeastern margin of the Bohemia Massif. (a)Early stage of crustal folding and extrusionassociated with vertical material and heattransfer. (b) Second stage (c. 340 Ma) ofdevelopment of a flat fabric due to collapseof the vertical anisotropy – probably due toductile thinning mechanisms that accompanydetachments of the upper crust. (c)Weak orogenic root deformed by the Bruniacontinental promontory and the developmentof large hot nappes during D 3 at 330–325 Ma. The section shows a northwardattenuation of the crust together with anorthward-facing topographic slope withcoeval surface erosion in the North. Thelatter is consistent with the geometry of theBrunia continent.(b)Ó 2007 Blackwell Publishing Ltd153