Quantitative structural analyses and numerical modelling of ...

1390J. KonopaÂsek et al. / Journal of Structural Geology 23 2001) 1373±1392>>1σ 1 (N-S)σ 1 (vertical)temperature (oC)400 500 6002a1132b011depth (km)30 50Fig. 12. Diagramof the orientation of s 1 versus depth and temperature during D3 and D4 shows two possible scenarios of such an evolution. Path 1±2a±3shows N±S compression during exhumation of the studied area, which is demonstrated by progressive development of brittle-ductile D3 structures. After thedecreasing role of the N±S compression at shallow crustal levels and low temperature, the prevailing vertical compression will produce the D4 kink-bands.This scenario is consistent with ®eld observations. The path 1±2b shows the diminishing role of the N±S compressive stress and increase in the importance ofvertical compression caused by the overburden at high temperatures in depth. This evolution would result in refolding of the F 3 folds by ductile F 4 folds withsubhorizontal axial planes. The PT path adopted fromKonopaÂsek 2001).superimposed D3 deformation will produce strong oblatefabrics without macroscopically visible aggregate lineation.In the case of low D2 strain intensities Fig. 10b), the resultingfabric is marked by L±S symmetry with intermediateintensities and visible horizontal L 3 aggregate stretchinglineation former Y-axis).The ®eld observations suggest that the F 3 folds developedby mechanisms of buckling of a single layer of orthogneisssurrounded by micaschist. Strong ¯attening in the limbareas of the KlõÂnovec antiformalso indicates conditions ofmoderate viscosity contrast between strong orthogneissesand weak micaschists.8.4. The origin of the D3±D4 kink bands and other D4brittle-ductile structuresA question arises: were the above-described D4 kinkbandsdeveloped as post-D3 structures, or were they createdduring the early D3 deformation and then re-oriented duringthe D3 folding? As described above, the steep metamorphicfabric in metasediments between the northern limb of theKlõÂnovec antiformand the MeÏdeÏnec antiform, as well as thezone of metasediments in the southern limb of the KlõÂnovecantiform, are believed to represent the S 3 cleavage. Therefore,the kink-bands affecting the S 3 cleavage must be post-D3. This is the case of those kink-folds developed in thesteep southern limb of the MeÏdeÏnec antiformand in thesteep zones south of the KlõÂnovec antiform. On the otherhand, the kink-bands developed in the hinge zone of theMeÏdeÏnec antiformactually correspond to the axial cleavageof this structure and are, therefore, developed in the earlystages of D3 folding.If we accept a hypothesis that the kink-bands in metasedimentsclose to the KlõÂnovec and the MeÏdeÏnec antiformswere not generated in the same stress ®eld as the D3 largescale folds, another local source of stress must have existedto generate these structures. A possible source of stressduring the D4 is the weight of the overburden after therelaxation of the D3 stresses. This will produce subverticals 1 , which has been documented in the southern part of theKlõÂnovec antiform. Moreover, the expected deformationassociated with the relaxation of the D3 stresses is ratherlow. This is consistent with the development of kink bandsand brittle-ductile clevage in orthogneisses and metasediments.The amount of strain achieved during theirdevelopment is probably more than two orders of magnitudeless than the deformation associated with the D1±D3 deformationand, thus, rather insigni®cant at the scale of thestudied area.8.5. Variations of principal compression direction throughtimeThe analysis of the D2, D3 and D4 structures shows66