Quantitative structural analyses and numerical modelling of ...

J. KonopaÂsek et al. / Journal of Structural Geology 23 2001) 1373±1392 1391complex stress±temperature±time evolution Fig. 11). Asdiscussed above, the D2 structures are associated with westwardthrusting of the Lower Crystalline nappe over theSaxothuringian basement suggesting non-coaxial deformationwith E±W oriented s 1 direction.Strain analysis within D3 fold limbs indicates a N±Soriented horizontal compression s 1 and important verticalstress s 2 inhibiting elongation in the vertical direction.Then, the only elongation accommodating N±S horizontalcompression s 1 is possible in the horizontal E±W direction.This indicates an important role of rigid overburden forstresses acting in a low viscosity layer at depth. SubhorizontalN±S compression leads to lateral ¯ow of materialwithout the vertical component in the early stages ofD3 deformation. The same stress regime, however, isresponsible for the formation of kink band structures inthe hinge of the MeÏdeÏnec antiform. Kink band structuresrepresent a brittle-ductile regime Dewey, 1965, 1969)and, thus, indicate a decrease in temperature under thesame orientation of the D3 stress ®eld. It is, therefore,suggested that the horizontal N±S compression operatedduring an uplift of the whole region fromthe deep to thesupracrustal level.The presence of D4 kink bands must be associated withthe disappearance of horizontal stresses and increase insubvertical s 1 compression. If this change of stress regimewould occur at high temperature conditions, then the F 3folds would be refolded by large-scale ductile F 4 foldswith subhorizontal axial planes. Fig. 12 documents thatthe F 3 folding started at peak temperature conditions ca.6008C) enabling viscous buckling of the nappe sequencecaused by shortening in the N±S direction. This deformationphase continued during a temperature decreaseassociated with uplift, resulting in the development of D 3kink-bands and a crenulation cleavage in the hinge zones oflarge-scale anticlines. The latest F 4 phase occurred underrelatively low temperature conditions and indicates thedisappearance of horizontal stress, allowing vertical shorteningof steep D 3 fabric. These changes in stress can beinterpreted as an increase of the role of overburden aftertermination of the D3 compressive stress.8.6. Exhumation of eclogites and the importance ofextension in the Czech part of the KrusÏne hory Erzgebirge)MountainsIn contrast with the western Saxothuringian domain,where the boundary between eclogites-bearing nappes andsupracrustal autochthon can be easily identi®ed, a similarlimit is dif®cult to establish in the eastern KrusÏne horyMountains. Using structural and petrological criteria, wehave de®ned the boundary between the parautochthonousSaxothuringian metasediments and allochthonous eclogitesbearingnappe. Thrusting-associated structures developed inboth the parautocthonous and allochthonous units documentthat the nappe emplacement occurred in the middle crust at adepth corresponding to 13±15 kbar. Field observations,however, are not able to provide any information aboutthe mechanism of emplacement of eclogites from a depthcorresponding to 26 kbar to the base of non-eclogiticorthogneiss nappe. This work shows mechanical behaviourof the crust during and after the nappe emplacement witheclogites as a part of lithological assemblage. The exhumationof assembled parautochthonous and allochthonous unitsto supracrustal levels is associated with complex structuralreworking of originally simple fabric during the subsequentN±S shortening, which is responsible for the ®nal pattern ofthe central part of the KrusÏne hory Mountains in the CzechRepublic.There is a range of publications emphasising the role ofthe late Variscan extensional deformation for the ®naltectonic and metamorphic pattern of the German part ofthe eastern Saxothuringian domain Willner et al., 1994;Krohe, 1996, 1998; RoÈtzler et al., 1998). These interpretationsare based on regional distribution of metamorphicunits and orientation of shear structures in different areasof the Erzgebirge. In this study, we have examined a particularlysuitable area with steeply developed anisotropyaffected by vertical shortening and demonstrated that itachieves no more than ®rst percents of the bulk strain duringthis event. If the extensional tectonics would be a keyregime responsible for ®nal geometry of studied crystallinecomplexes then signi®cantly more important vertical shorteningshould be expected ®rst in areas with subverticallydeveloped anisotropy. Therefore, we suggest that the structuralpattern of the whole Saxothuringian domain should bere-evaluated in terms of detailed structural analysis todemonstrate the real signi®cance of late orogenic extensionin this part of the Bohemian Massif.AcknowledgementsWe are very grateful to John Cosgrove for helpfulcomments on an early version of the manuscript. Thepaper has also bene®ted from the comments of G. Oliverand W. Franke. We also thank Jaroslav Synek for drawingFig. 6. This work was funded by the Grant Agency of theCzech Republic, grant no. 205/96/0279.ReferencesCobbold, P.R., Cosgrove, J.W., Summers, J.M., 1971. Development ofinternal structures in deformed anisotropic rocks. Tectonophysics 121), 23±53.Dewey, J.F., 1965. Nature and origin of kink-bands. Tectonophysics 1,459±494.Dewey, J.F., 1969. The origin and development of kink bands in a foliatedbody. Geological Journal 6, 193±216.Flinn, D., 1962. On folding during three dimensional progressive deformation.Quarterly Journal of the Geological Society of London 118, 385±428.Franke, W., 1993. The Saxonian Granulites: a metamorphic core complex?Geologische Rundschau 82, 505±515.67