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Author's personal copyK. Schulmann et al. / C. R. Geoscience 341 (2009) 266–286 279trending belts of granulites, eclogites and peridotites(Fig. 3c) intimately associated with the (ultra-)potassicmagmatites [29,59]. The first granulite belt is representedby narrow strip of felsic granulites that occur at theSaxothuringian–Teplá-Barrandian boundary and it isinterpreted as an extrusion of the orogenic lower crustalong a large scale crustal shear zone at 340 Ma[71,163]. The coeval 340 Ma age of the Saxothuringianeclogites and of the felsic granulites [80,150] ledKonopásek and Schulmann [71] to propose a model ofsimultaneous exhumation of nappes derived from theSaxothuringian crust and viscous extrusion of orogeniclower crust from underneath the Teplá-Barrandian supracrustalunit (Fig. 4D) [36]. The second belt, recognizedeast of the Central Bohemian Plutonic Complex, i.e.,‘‘the magmatic arc’’, was exhumed along huge westdipping detachment zone [111,157,158], which was alsoresponsible for collapse of the upper part of the magmaticarc system and the downthrow of the whole Barrandiansection [115,157]. Such a huge vertical material transfer(Fig. 4D) could have been responsible for verticalexchange of the lower crustal and upper crustal materialin a range of 50 km with final throw of 15 km [110,162].The cooling ages from the lower crustal domain show thatthe granulites have crossed the 300 8C isotherm duringCarboniferous (330–310 Ma) [75,138] suggesting thetime at which the lower crustal bulge reached shallowposition in the upper plate.The third lower crustal belt rims the eastern marginof the Bohemian Massif, i.e. the boundary with theBrunia continent (Fig. 4D). Here the granulite fabricis also vertical and interpreted in terms of massivevertical exchanges with orogenic middle crust [123].The zone of the lower crustal bulge is interpreted as anenormous zone of vertical extrusion surrounded by amiddle crust coevally transported downwards in formof a crustal scale synform. The vertical materialtransfer along the Moldanubian lower crustal beltswas a matter of research for several Czech authorsduring the last five years [31,112,123,131,142]. Themodel of vertical extrusion is based on the concept ofbuckling of the lower and mid-crustal interfacefollowed by growth of crustal scale antiforms. Thisprocess is thought to be triggered by rheological andthermal instabilities in the arc region, while to the eastit is forced by rigid backstop, preserved only locally[130].However, the most important feature of the easternVariscan front is the development of horizontal fabricsin the Moldanubian root zone parallel to the Bruniacontinental margin. The intense deformation of theBrunia continent leading to the formation of theMoravo-Silesian imbricated nappe system was associatedwith the development of tectonically invertedBarrovian metamorphism (Fig. 4D) and formation ofcrustal mélange in its upper part (the MoravianMicaschist Zone). These phenomena, as well as mixingof HP rocks and migmatites in the overlyingMoldanubian nappe have been recently interpreted interms of indentation of the Brunia continent into the hotand thick continental root [123]. This lower crustalindentation and flow of hot lower crustal rocks insupracrustal levels are consistent with a model ofcontinental channel flow driven by the arrival of acrustal plunger, a model which is advocated for twodecades for the deformation of the Eastern Cordillera inthe Andes (e.g. [87]). Finally, the continuous load of theBrunia platform related to deep indentation process ledto the development and eastward propagation of theforeland basin. In our model, as the hot Moldanubianrocks advance over the Brunia platform, an imbricatedfootwall nappe system is generated and thrust over theprogressively buried foreland basin rocks.The time scale of exhumation of the orogenic lowercrust is well constrained due to a set of well dateddiachronous processes that start with the exhumation ofthe West-Bohemian granulites, growth of western andeastern orogenic lower crustal antiforms and a shallowchannel flow of partially molten lower crust associatedwith the inversion of the eastern foreland basin. All thatis linked with a major thermal event in the mantle asshown by the ages of a syn-extrusion high potassicmagmatism, the exhumation of large number of mantlefragments and the overall melting of the Moldanubiancrust. The time scale of all these processes wassurprisingly short, i.e., about of 20 Ma, and ranges from335 to 315 Ma (Fig. 5).4. Palaeographic constraints for Andean typeorogeny in the Bohemian MassifFinger and Steyrer [24] attempted to explain thetectonic processes at the Moldanubian–Moravianboundary with a plate tectonic model involving thesubduction of a Silurian–Devonian oceanic domainwestward beneath the Moldanubian zone. This conceptsupposes large rotation of the Brunia platform forming apart of an Old Red continent (Avalonia) around the coreof the Bohemian Massif during Visean times [140]. Themodel of westward subduction is also based on thepresence of Silurian MORB-type amphibolites [25]within the Gföhl Unit, which are interpreted as relics ofoceanic crust of the Rheic Ocean. In addition, theoccurrences of eclogites located along the eastern172
Author's personal copy280K. Schulmann et al. / C. R. Geoscience 341 (2009) 266–286Variscan front [73,132] can serve as an additionalargument for this subduction model.However, the palaeomagnetic investigations carriedout on granitoids of the Central Bohemian Pluton and itsextension to the east (the Nasavrky Plutonic Complex[58]) demonstrate that the Teplá-Barrandian, theMoldanubian as well as Brunia domains had a commongeodynamic behaviour since the Late Visean (335–330 Ma according to Edel et al. [22]). Therefore, theinterpretation of palaeomagnetic directions from Devoniansediments of the Brunovistulian platform[86,98,139] in terms of an oroclinal bending of theRhenohercynian domain along the Moldanubian corecannot be valid for several reasons: the consistency of these ‘‘Devonian’’ palaeomagneticdirections with Middle-Late Carboniferous directions(Cp directions at 330–325 Ma and B directions at320–315 Ma, Fig. 6) in the Moldanubian zonesuggests that the Devonian sediments overlying theBrunia continent were re-magnetized in Late Variscantime. Consequently, no relative rotation between theBrunia continent and the Moldanubian core can beconsidered prior to 330 Ma; magnetic overprinting of the Devonian sequences onthe Brunia continent is supported by importantdeformation and burial of Culm and Devoniansediments during Late Carboniferous deformation ofthe Brunia margin [30,120,126]. Hence, it is unlikelythat primary magnetizations could have survived sucha severe structural and thermal reworking of theDevonian sediments. We suggest that magnetic overprintingwas the result of the Carboniferous tectonothermalprocesses associated with the underthrusting ofthe Brunia platform together with Culm accretionarywedge [11] underneath the hot Moldanubian root zoneand with later uplift and erosion of the Moldanubian–Moravian nappe sequence.Fig. 6. a: paleomagnetic north directions obtained in Devonian (meta)sediments from the Teplá-Barrandian and Brunia zones (K: [86];N:[98];T:[139]; E:[21]) and from Early Carboniferous granitoids from Central Bohemian Pluton and Nasavrky Pluton [22]. Cn and Co directions representmagnetizations acquired at the end of the NW–SE shortening and the uplift of the Moldanubian root; Cp and B directions correspond to overprintsacquired during NNE–SSW compression and clockwise rotation of the Variscides. Note the similarity of ‘‘Devonian’’ directions with Carboniferousdirections; b: published mean virtual geomagnetic poles (VGPs) and associated apparent polar wander curve (grey dashed line) from Early Paleozoicrocks of the Bohemian Massif and mean poles from western Europe Variscides (stars with ages of magnetization) and associated apparent polarwandering curve (dark grey line) [22].Fig. 6. a : directions du nord paléomagmatique obtenues dans les (méta)sédiments dévoniens des zones Teplá-barrandienne et Brunia (K :[86] ;N :[98] ; T :[139] ; E :[21]) et dans les granitoïdes du Carbonifère inférieur et du Pluton Nasavrky [22]. Les directions Cn et Co représentent lesmagnétisations acquises à la fin du raccourcissement NW–SE et du soulèvement de la racine moldanubienne ; les directions Cp et B correspondent àdes réaimantations acquises durant la compression NNE–SSW et la rotation dans le sens des aiguilles d’une montre des Variscides. À noter, lasimilarité des directions « dévoniennes » et carbonifères ; b : pôles géomagnétiques virtuels (VGP) publiés, associés à la courbe de dérive apparentedes pôles (ligne en tiretés) pour les roches du Paléozoïque inférieur du Massif de Bohême et pôles moyens pour les Variscides d’Europe occidentale(étoiles avec âges de la magnétisation), associés à la courbe de dérive des pôles (ligne continue gris foncé) [22].173
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“It strikes me that all our knowl
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Contentsviii4.7 Schulmann, Konopás
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Dedicated to my wife Markéta, daug
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Foreword 2First topic “Quantitati
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Foreword 4source and freely availab
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1. Quantitative analysis of deforma
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Chapter 2Mechanisms of lower crusta
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2. Mechanisms of lower crustal flow
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Chapter 3Quantitative analyses ofme
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3. Quantitative analyses of metamor
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Bibliography 42Culshaw, N., Beaumon
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Bibliography 44sources and trigger
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Bibliography 46Skrzypek, E., Štíp
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ORIGIN OF FELSIC MIGMATITES 31Ó 20
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ORIGIN OF FELSIC MIGMATITES 33durin
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ORIGIN OF FELSIC MIGMATITES 35(a)Ty
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ORIGIN OF FELSIC MIGMATITES 37(a)Pl
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ORIGIN OF FELSIC MIGMATITES 39Grain
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ORIGIN OF FELSIC MIGMATITES 43(a)(b
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ORIGIN OF FELSIC MIGMATITES 45Fig.
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ORIGIN OF FELSIC MIGMATITES 47produ
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ORIGIN OF FELSIC MIGMATITES 49stron
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ORIGIN OF FELSIC MIGMATITES 51Cmı
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ORIGIN OF FELSIC MIGMATITES 53easte
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104 J. FRANĚK ET AL.in terms of th
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108 J. FRANĚK ET AL.(a)perthite po
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