Quantitative structural analyses and numerical modelling of ...

J. KonopaÂsek et al. / Journal of Structural Geology 23 2001) 1373±1392 1383the MeÏdeÏnec antiform. In this area, the S 2 fabric is completelyreworked into a steep to subvertical E±W trendingS 3 foliation. Here, a strong sub-horizontal stretching ofquartz aggregates can be observed in several places. In thehinge zone of the MeÏdeÏnec antiform, monoclinal or conjugatekink bands are developed affecting the early S 2 metamorphicfoliation of the garnetiferous micaschist andorthogneisses. These kink bands exhibit E±W trendingaxes and kink planes, bimodally distributed with respectto ¯at-lying S 2 foliation Fig. 5e). The geometry of thesekink bands suggests that they are genetically linked to theMeÏdeÏnec antiformal D3 structure but the mechanism and thekinematics of kinking will be discussed in a separate sectionbelow.The general structure of the studied domain is shown inblock-diagram Fig. 6), summarising lithological and structuralobservations discussed above as an effect of the D3folding superimposed on the D2 fabrics.6. Problem of the L 2 and L 3 lineation solved usingaggregate-shape analysisAn important feature is the parallelism of the L 2 minerallineation in the MeÏdeÏnec synforma D2 structure) with thelineation observed in micaschists and orthogneisses of boththe MeÏdeÏnec and the KlõÂnovec antiforms. If the WNW±ESEtrending L 2 lineation in the MeÏdeÏnec synformwere rotatedby late F 2 folding, the originally EW trending, gentlyplunging L 2 lineation would become subvertical Fig. 7).This can be observed in N±S trending, steep S 2 foliationin the southern part of the Oberwiesenthal structure. Subsequentrotation of the S 2 foliation by F 3 folds with a steephinge zone will keep the orientation of the L 2 in a subverticalposition Fig. 7) . The lineation in the steep limbsof the KlõÂnovec and the MeÏdeÏnec antiforms is always foundto plunge gently to the west, however, indicating that it wasformed during the D3 deformation L 3 lineation). Consequently,the change of the L 2 into an L 3 lineation must beassociated with complete modi®cation of the D2 fabricduring the D3 deformation. In order to investigate theproposed changes to the D2 fabric ellipsoid during the D3deformation, we have carried out a shape analysis of mineralaggregates in the orthogneisses fromthe MeÏdeÏnec synformand the MeÏdeÏnec and the KlõÂnovec antiforms.In order to determine the role of the F 3 folding inpossible fabric modi®cations, an aggregate-shape analysisof K-feldspar and quartz was carried out on the porphyriticorthogneisses. Selected porphyritic augen orthogneisseswere sampled in the hinge zones and limbs of both theMeÏdeÏnec and the KlõÂnovec antiforms, as well as in theMeÏdeÏnec synformand the Oberwiesenthal structure. Twosections in each sample were studied: a) perpendicular tometamorphic foliation and parallel to mineral lineation theXZ section of the ®nite strain ellipsoid), and b) perpendicularto both foliation and lineation the YZ sectionof the ®nite strain ellipsoid). The shapes of the K-feldsparand quartz aggregates were traced on transparent sheet,digitised and then analysed. Because of the large size ofmeasured aggregates, only a limited number of measurementscould be carried out on each sample Table 1). Finitestrain ratios were calculated fromthe two principal strainellipsoid planes using a harmonic mean method Lisle,1977; Ramsay and Huber, 1983, p. 80). These ratios wereFig. 7. Succession of the lower-hemisphere equal area projections shows schematically expected evolution of the orientation of the L 2 lineation during D2 andD3 folding. Early L 2 lineation is trending E±W being subhorizontal. During late F 2 folding, the L 2 lineation does not change the orientation, but becomessteeper. The successive F 3 folding changes the orientation of steep S 2 foliation; the L 2 lineation should rotate into N±S direction maintaining its steep plunge.In the D3 structures the MeÏdeÏnec and the KlõÂnovec antiforms), however, the lineation is always E±W oriented and mostly subhorizontal.59