Quantitative structural analyses and numerical modelling of ...

70 J. FRANĚK ET AL.and underlying dense Varied Group rocks. Theanomalies involved in our gravity model have smallareal extent and exhibit high gradients at their edges,implying that all of them are caused by upper-crustalheterogeneities. Removal of regional long-wavelengthgravity components caused by deeper heterogeneities isthen unnecessary and the original values of theBouguer anomalies have been used in the modellingthat follows. For the same reason, the model concernsonly upper-crustal bodies down to 10 km depth.The geometry of the Prachatice granulite massif hasbeen approximated in six vertical sections 40 km longand 10 km deep, trending NW–SE parallel to theseismic reflection line 9HR (Fig. 11a–c). Unlike thetwo-dimensional gravity model of Vra´na & Sˇra´mek(1999), the three-dimensional approach was chosenbecause the relationship of this granulite massif with itssurroundings suggests a complex non-cylindricalgeometry of structures on the scale of our model and acircular shape of the anomaly also means that a twodimensionalapproach is unsuitable. The model isconstrained by the surface extent of the geologicalunits, structural data extrapolated to 1–2 km depthand the reflection seismic line that cross-cuts thePrachatice granulite massif and the centre of the negativeanomaly. The database of rock densities(Appendix S2) from Hana´k et al. (unpublished data),Chlupa´cˇova´ et al. (unpublished data) and Blizˇkovsky´et al. (unpublished data) provided starting guesses,which were carefully adjusted when the constraintsdiscussed earlier in the article restricted changes ofgeometry for the modelled bodies. This is a commontechnique because rock densities vary in space, e.g. theVaried Group in surface section exhibits significantlithological change on a kilometre scale. The model wasdesigned to be as simple as possible to avoid ambiguoussolutions, but its simplicity naturally causes divergenceof the modelled gravity values from the measured valuesat the model edges. In our model, the Prachaticegranulite massif reaches a maximum depth of 5.7 km,having a thick lensoidal shape. It cannot extend belowthe strong reflection package, because such a packageof reflectors could not be expected in a lithologicallyhomogenous granulite massif. It also cannot form athinner lens, because then an unreasonably low densityfor the felsic granulite would be needed to produce themeasured gravity low. A spectacular detail is the cuspof high-density Varied Group rocks introduced into thegranulite massif from below in the hinge region of thelarge fold (e.g. at 15 km in section 3 in Fig. 11c).Inferring this structure is the only reasonable way toproduce the observed cranked gravity curves. It isprobably structurally analogous to a cusp at the SWedge of the Blansky´ les granulite massif, where a sheetof amphibolites with ultrabasites penetrates deeply insidethe granulite massif along the large fold axialplane. Our results differ from the two-dimensionalgravity model of the Prachatice granulite massif publishedby Vra´na & Sˇra´mek (1999) who expected arectangular rather than lensoidal shape of the massif,reaching to 9 km depth. The discrepancy is causedpartially by the different rock densities used andpartially by the two-dimensional approach. It is arguedthat our model approximates better the reality as aresult of calculation in three-dimensions as wellas using better structural constraints from the nearsurfacegeology.DISCUSSION – KINEMATIC AND MECHANICALSIGNIFICANCE OF THE DEFORMATION FABRICSTectonic significance of granulite facies fabricsThe granulite facies S1–S2 foliations represent aunique example of lower crustal fabrics in respect ofthe whole Bohemian Massif. The decompressional P–Tpath indicates that the S2 fabric originated duringexhumation. The highly discordant relationshipbetween the S2 granulite facies fabric and the S3amphibolite facies foliation is the most importantstructural observation from the South Bohemiangranulites (Franeˇk et al., 2006). The HP granulitesrecord the early deformation, which are not recordedin rocks of either the middle- or the upper-crustalunits. Therefore, there is a possibility that the granulitefacies D2 mylonitization is older and occurred in acrustal unit that was geographically remote comparedto the Lower Carboniferous D3–D4 history recordedin lower-, middle- and upper-crustal units (Fig. 12).Horizontal shortening and development of the regionalvertical fabricThe distribution of the steep S3 foliation in thegranulite massifs shows important variations, whichcan be interpreted as a result of large-scale folding ofgranulite massifs with steep N–S axial planes (Franěket al., 2006). The granulite bodies were morecompetent than the surrounding metasedimentaryrocks due to cooling after exhumation at later stagesof D3, which enabled fold amplification. The differencebetween the asymmetrically folded granulite S3fabrics and the straight NE–SW S3 fabric homogeneouslydeveloped in middle-crustal host rocks indicatesa non-coaxial dextral shear operating duringD3 in both the middle- and lower crustal units. Thesteep attitude of N–S axial planes rules out thepossibility that the S3 folding proceeded during thedevelopment of the flat S4 foliation. The axial planeslie close to the plane of maximum flattening of theinstantaneous D3 strain ellipsoid, while the generalNE–SW trend of the S3 fabric reflects the finitestrain orientation.When the Z-shaped Blansky´ les, the symmetricalKrˇisˇťanov and the V-shaped Prachatice granulitemassifs are unfolded to yield originally NE–SW strikingsheets, then the S3 generally dips to the west in theBlansky´ les, subvertically in the Krˇisˇtˇanov and to theÓ 2010 Blackwell Publishing Ltd222