Quantitative structural analyses and numerical modelling of ...

290 K. SCHULMANN ET AL.top-to-the-NE-oriented thrusting in the stability fieldof cordierite at low pressures of 3–4 kbar.Structural and petrological studies of the southernand central Moldanubian domains show that similarP–T conditions around 7 kbar at 750 °C are characteristicfor all crustal levels during development of theflat S 3 fabric. Rocks from different depths were mixedand reworked together during the D 3 deformation, asevidenced by contrasting prograde P–T paths withdifferent pressure peaks. The increase of temperaturein the orogenic middle crust and the slight decrease oftemperature in the orogenic lower crust during retrogressionsuggest a mutual thermal equilibration duringdevelopment of the S 3 flat fabric.Structural observations show that all rocks werestrongly deformed during D 3 horizontal flow, althoughmore competent lithologies, mostly middle crust andsome granulites, retained their original steep S 2 fabric,whereas in the less competent, weak lower crust, thehorizontal top-to-the-NE D 3 ductile shearing dominates.During this process, the originally coherentorogenic middle and lower crust was disaggregated toform boudins and rootless folds in a pervasivelyflowing migmatitic matrix. We suggest that providingthe volume of orogenic middle crustal is high enough,the mineral assemblages from the D 2 vertical fabricsare preserved (Figs 6 & 9a). In contrast, the smallerboudins were completely re-equilibrated and evenheated in the flowing mass of hot orogenic lower crust(Figs 6 & 9b). The metamorphic conditions associatedwith the D 3 flow show clearly that pressure, temperatureand intensity of D 3 reworking decreases fromsouth to north across the whole continental margin.Based on dating of metamorphic zircon from granulites,the timing of HP metamorphism and S 2 fabricformation in the Moldanubian domain was estimatedby Schulmann et al. (2005) and Tajcˇmanova´ et al.(2006) to occur between 350 and 340 Ma. This agespan corresponds with cooling ages of minerals withhigh blocking temperatures in the northern Moldanubiandomain (Matte et al., 1990; Macintyre et al.,1992), and indicates that already during the D 2 stagean important cooling and exhumation of this area wastaking place, similar to the Lugian domain. Theyounger 330–325 Ma 40 Ar ⁄ 39 Ar cooling ages obtainedfor muscovite and biotite imply a second distinct periodof thermal reworking and isotopic resetting in theregion, correlated with the strong D 3 reworking in thesouth.Geophysical imagery of the subsurface shape of the Bruniacontinental promontoryIt is apparent that the thrust-related horizontal flow,which dominates the deformation of the Moldanubiandomain at around 325 Ma, does not exist in the Lugiandomain. This indicates a significant difference in thebulk exhumation processes between the two crustalsegments and the important involvement of the Bruniacontinent in the Moldanubian exhumation history. Inorder to discuss the relative contribution of the Bruniabasement in the tectonic evolution of the orogenic rootit is necessary to know the sub-surface extent of thebasement promontory underneath the orogenic rootrocks. Therefore, after the compilation of gravity data,a Bouguer anomaly map of the area (Fig. 10) wasproduced.At a large scale, the Bouguer anomaly map (Fig. 10)shows two main domains: (i) to the west, a domaincharacterized by low- and intermediate-gravity anomalies,inferred to be associated with rocks that have lowto intermediate densities; and (ii) to the east, a domainwith a succession of gravity highs inferred to be associatedwith significantly denser rocks. In the NE, thelow- to intermediate-gravity anomalies coincide withthe Lugian domain, whereas the gravity highs areassociated with the Brunia continent. The steep horizontalgradient in gravity reflects a steep boundarybetween the Lugian domain and the Brunia continent.In the SW, the Moldanubian domain west of theNNE–SSW-striking Central Moldanubian pluton ischaracterized by low- and intermediate-gravity anomaliessimilar to the Lugian domain. Similar to thenorth, we infer that the gravity highs over the Bruniacontinent in the SE represent mostly dense rocks.However, these gravity highs continue to the west, intothe eastern part of Moldanubian domain as far as theCentral Moldanubian pluton. Therefore, the observationmade in the north, where the gravity boundarycoincides with the geological boundary between theLugian domain and Brunia, is not valid in the south,where the geophysical boundary is located about 50–70 km west of the mapped geological boundary.Within the eastern Moldanubian domain, the rocksat outcrop are similar west and east of the SouthBohemian pluton, and rocks with high densities thatcould explain the gravity high in the eastern part of theMoldanubian domain are not represented at outcrop.Consequently, the dense rocks must be located at agreater depth. The gentle gradient of the gravity highsin the west indicates that the dense Brunia continentalpromontory dips towards the west, beneath the easternMoldanubian rocks. This interpretation is confirmedby preliminary 3D modelling, which shows that theboundary between the dense Brunia basement and thelow- to intermediate-density rocks of the Moldanubiandomain has a gentle dip down to a depth of some 2–3 km near the Central Moldanubian pluton, where itdips more steeply beneath the pluton (60–70°).Implication of Bouguer anomaly patternThis analysis of the Bouguer anomaly map in Fig. 10reveals striking differences between the Moldanubianand Lugian domains, which may be interpreted interms of the presence of Brunia basement under a thinlayer of Moldanubian rocks. This interpretation isconsistent with the suggestion that the S 3 fabric in theÓ 2007 Blackwell Publishing Ltd150