Quantitative structural analyses and numerical modelling of ...

66 J. FRANĚK ET AL.At the top, the package terminates abruptly at 1 sTWT below the western edge of the Prachatice granulitemassif, probably on a local N–S fault. Thestrongest reflections in the profile appear below thePrachatice granulite massif; they diminish to the SE at5 s TWT, below the centre of the Blansky´ les granulitemassif. A parallel weaker package (G1) of12 km length appears attached to the top of thispackage of strong reflections. Except for these features,the crust below the granulites exhibits almost noreflectivity and there is only a weakly defined Moho(M4) at 12–13 s TWT. The reflections along the SEedge of the seismic profile are probably just artefacts ofthe migration procedure. This lack of reflections contrastswith a package of strong NW-dipping mantlereflections below 12s TWT (Cˇ. Tomek, personal communication;not shown in Fig. 10). The western part ofthe Moldanubian middle to lower crust exhibits uniformlydistributed horizontal reflections designated asLC, which appear in a lens-shaped domain60 · 15 km in size. There are several packages ofstrong reflections (Z1–Z4) in the underlying lowercrust that show apparent dips of 30–40° to the SE andcontinue through discontinuous MOHO reflections(M2 and M3) into the uppermost mantle.Interpretation of reflection seismic dataIn the high-grade Moldanubian domain, which recordspolyphase deformation, the reflection packages mayrepresent lithological layering, penetrative foliation,faults or boundaries of intrusive bodies. We try toovercome this ambiguity by careful comparison withthe surface extent of major lithological units and withour structural interpretations.The upper-crustal reflection packages (B1–B6) in theTepla´-Barrandian Unit correspond to Proterozoic lowgradesequence of siltstones interlayered with basaltsexposed on the surface that show SE-dipping schistosity.Termination of these packages at the margin ofthe Tepla´-Barrandian Unit may document sidewardintrusions of granitoid bodies related to the CentralBohemian Plutonic Complex, or reworking by laterVariscan deformation. The low reflectivity in thecrustal column directly below the Tepla´-Barrandian–Moldanubian boundary can be best explained by theoccurrence of Central Bohemian Plutonic Complexgranitoid intrusions at depth below the MonotonousGroup exposed at the surface.The anastomosing reflections in the upper crust ofthe western Moldanubian domain correlate well withthe geometry of the S4 fabrics as determined by ourfield research (Fig. 10a). This conformity suggests thatall the anastomosing reflections can be related to thesubsurface continuation of the S4 fabrics. The V1–V4reflection packages correlate well with surface exposuresof km-scale lenses of Varied Group rockssuggesting that the reflectors in this case representinterlayering of paragneisses with marbles, amphibolitesand other lithologies, which are oriented parallelto the S4 fabric. The K1 and K2 reflection packagesdocument probable subsurface occurrence of variedlithological intercalations at the western side of theMoldanubian domain. The nature of the straightreflection package (G2) dipping below the granulitemassifs remains ambiguous, because the observed highamplitude and length of reflections may be causedeither by lithological layering similar to that of theVaried Group or by a thick zone of intensive mylonitization.Based on the single seismic section, theorientation of the G2 reflectors can vary between asteeper dip to the NE through medium dips to the SEto steeper dips to the SW. Such orientations cannot bedirectly correlated to any structure mapped on thesurface. The straight geometry of the G2 packagesuggests that the corresponding reflectors were notsignificantly affected by the D4 deformation, or thatthey may represent an anomalously oriented D4structure. The low reflectivity in the crustal columnbelow the granulite massifs may be caused either bylack of reflection inducing inhomogeneities or extremesteepness of reflectors, which would not be detectableby seismic survey. The low energy of the shots cannotbe responsible for the lack of crustal reflections,because the much deeper mantle reflections have beenrecorded below this seismically transparent zone. Therest of the Moldanubian crystalline crust exhibits significantreflectivity, suggesting that the Moldanubiancrust in general contains abundant reflectors thatwould be recorded if they had a suitable attitude.Below the granulites the reflectors may have beenpartially destroyed by the rise of these HP rocks, but itis argued that this crustal section likely still containsnumerous reflectors similar to the rest of the Moldanubiandomain, but the attitude of these reflectors istoo steep to be detected by the seismic method. Incombination with the dominance of steep fabrics in theexposed granulites, the lack of reflections serves asindirect evidence for the dominance of steep foliationsin the seismically transparent zone throughout thecrust below the granulite massifs. The homogenouslydistributed reflections in the middle to lower crust,designated as LC in Fig. 10b, do not reach the surfacealong the 9HR line. Such middle to lower crustalreflections are classically, but without direct evidence,ascribed to subhorizontal intrusions. These horizontalreflections cannot represent the deeper continuation ofthe S4 fabric, because of the sharp transition zone withthe anastomosing upper-crustal S4-related reflectionsand the general dip of the S4 to the NW. The LCreflections are unlikely to represent a fabric older thanS4, because the D3 phase of horizontal shorteningleads to development of steep NE–SW trending mostlymigmatitic fabrics along the traverse. The abrupttransition in seismic fabric may also mark a rheologicaltransition between the Variscan upper and middle tolower crust, with the age of the LC reflectionsremaining unknown.Ó 2010 Blackwell Publishing Ltd218