Quantitative structural analyses and numerical modelling of ...

74 J. FRANĚK ET AL.weak lower and ⁄ or subducted HP crust is mechanicallydecoupled from middle-crustal units until thelater stages of its ascent. This is potentially indicatedby the discordance of early structures in the granuliteswith respect to the Monotonous and Varied Grouprocks. In these numerical models, rapid emplacementof these weak, hot rocks as structural domes in themiddle crust drives lateral flow, leading to extensionand ductile thinning above and adjacent to the dome.With these calculations in mind, the fan-like geometryof the S3 fabrics could have been due to the ductilethinning. Exhumation of the Moldanubian rocksfrom below the Tepla´-Barrandian Unit was suggestedby earlier authors (e.g. Scheuvens & Zulauf, 2000;Do¨ rr & Zulauf, 2008), but without the support ofstructural or seismic data covering the broaderMoldanubian domain.CONCLUSIONSThe structural evolution of the South Bohemiangranulites reveals a complexity not seen in the easternpart of the Moldanubian domain. The SouthBohemian granulites were exhumed along two distinctfabrics, the S2 and S3, instead of in a single verticalchannel known from the eastern Moldanubian(Schulmann et al., 2005, 2008). The S2 indicates adistinct older tectonic episode possibly related to anearly subduction period and emplacement of orogeniclower crust at the bottom of the orogenic root (Franeˇket al., 2011). The gravimetry and structural geologyindicate that the individual South Bohemian granulitemassifs reach several kilometres depth. A reflectionseismic profile additionally depicts a region of probablesteep fabrics through the crust below granulites.This vertical region of low reflectivity represents thetrace of the deformed granulite D3 crustal-scale ascentchannel that probably consists of additional deepergranulite bodies.Partially molten lower crust dominated by felsicgranulites was incorporated into middle crustal levelsin the form of a D3 syn-compressional crustal-scaledome at 342–337 Ma. After emplacement, the domeof felsic granulite deformed together with the surroundingmiddle crust. The subhorizontal S4 fabricimmediately reworked the S3 during the interval342–337 Ma, being induced by gravitational spreadingof the growing dome. During D4, the Tepla´-Barrandianupper crust slid to the NW away from the domeregion and allowed ductile thinning of the mid-crustallevel in the Moldanubian domain.This two-stage exhumation mechanism from HPconditions through the orogenic crust for felsic granulitesmay be applicable to exhumation of other HPfelsic rocks. The initial low viscosity probably enabledthe buoyancy-driven rise, while cooling and continuedshearing resulted in development of a large-scaleme´lange of granulite bodies dismembered withinmid-crustal rocks.The presence of low-density felsic granulites at thebottom of the crustal root is a key factor controllingexhumation of the orogenic lower crust and the tectonicstyle, both driven by buoyancy in addition totectonic far-field forces. The Bohemian Massif representsa field laboratory for many conceptual models toexplain the exhumation of orogenic lower crust ingeneral.ACKNOWLEDGEMENTSThe work was supported by a grant from the CzechScience Foundation (GACˇR 205 ⁄ 05 ⁄ 2187) and aninternal project of the Czech Geological Survey(326700). Visits by J. Franeˇk to ULP Strasbourg werefunded by the French Government Foundation(BGF). K. Schulmann and O. Lexa acknowledgefinancial support from the French National GrantAgency (no. 06-1148784) and from the Ministry ofEducation of the Czech Republic (grant MSM-0021620855). Prof. G. Manatschal is acknowledged forinspiring discussions. We are grateful for the constructiveand inspiring reviews of R. Jamieson andM. Pierce, and we thank M. Brown for careful editorialwork.REFERENCESAftalion, M., Bowes, D.R. & Vrána, S., 1989. Early carboniferousU-Pb Zircon age for Garnetiferous, Perpotassic Granulites,Blanský Les Massif, Czechoslovakia. Neues JahrbuchFur Mineralogie-Monatshefte, 4, 145–152.Beaumont, C., Jamieson, R.A., Nguyen, M.H. & Medvedev, S.,2004. Crustal channel flows: 1. Numerical models with applicationsto the tectonics of the Himalayan-Tibetan orogen.Journal of Geophysical Research B: Solid Earth, 109(B6),B06406.Beaumont, C., Nguyen, M.H., Jamieson, R.A. & Ellis, S., 2006.Crustal flow modes in large hot orogens. Geological Society ofLondon Special Publication, 268, 91–145.Beaumont, C., Jamieson, R.A., Butler, J.P. & Warren, C.J.,2009. Crustal structure: a key constraint on the mechanism ofultra-high-pressure rock exhumation. Earth and PlanetaryScience Letters, 287, 116–129.Behr, H.J., 1961. Beitrage zur petrographischen und tektonischenAnalyse des sachsischen Granulitgebirges. FrebiergerForschungshefte C, 119, 5–118.Behr, H.J., 1978. Subfluenz-Prozesse im Grundgebirgs-StockwerkMitteleuropas. Zeitschrift der Deutschen Gesellschaft fu¨rGeowissenschaften, 129, 283–318.Behr, H.J., 1980. Polyphase shear zones in the granulite beltsalong the margins of the Bohemian Massif. Journal of StructuralGeology, 2, 249–254.Behrmann, J.H. & Tanner, D.C., 1997. Carboniferous tectonicsof the Variscan basement collage in eastern Bavaria andwestern Bohemia. Geologische Rundschau, 86, S15–S27.Burg, J.P. & Podladchikov, Y., 1999. Lithospheric-scale folding:numerical modelling and application to the Himalayansyntaxes. International Journal of Earth Sciences, 88, 190–200.Carswell, D.A. & O’Brien, P.J., 1993. Thermobarometry andgeotectonic significance of high-pressure granulites – examplesfrom the Moldanubian Zone of the Bohemian Massif inLower Austria. Journal of Petrology, 34, 427–459.Ó 2010 Blackwell Publishing Ltd226