Quantitative structural analyses and numerical modelling of ...

292 K. SCHULMANN ET AL.example in the area of the Namche Barwa syntaxis(Burg et al., 1997).Based on shape analysis of macroscopic folds, Franeˇket al. (2006) proposed that the folding of lowercrustal layers occurred at granulite facies conditions bypassive amplification mechanisms. Consequently, theseauthors suggested that the folding started by thedevelopment of cuspate structures at the boundarybetween the lower and middle crust because of a lowviscositycontrast at this interface (cf. Kisters et al.,1996). Growth of the upward-pointing cusps wasreplaced by vertical extrusion as the whole domainbecame sufficiently shortened and the horizontalanisotropy was replaced by a vertical anisotropy. Atthat stage, the difference in the integrated vertical bulkviscosity between a weak lower crustal cusp and anadjacent stronger middle crustal lobe increased. Theseviscosity differences lead to strain partitioning andvertical extrusion of the weak orogenic lower crust butslower exhumation of adjacent middle crust (Jezˇeket al., 1998).To make the mechanism of vertical extrusion possiblea rigid floor is required (Schulmann et al., 2003),which is represented by a strong sub-root mantle in themodel proposed here. The strength of the sub-crustalmantle was modelled for a given thermal gradient byThompson et al. (2001) and Schulmann et al. (2002),who showed that at least 40 km of rigid mantle lithospherewas likely to have existed at the onset ofexhumation of the thermally weakened lower crust.The occurrence of slivers of mantle peridotite, whichhave sharp boundaries and discordant internal fabricswith respect to the flow fabric of the surroundinggranulites, within the orogenic lower crust providesfurther evidence for the existence of a strong mantleand implies an important strength contrast between themantle and the orogenic lower crust (Medaris et al.,2006). Although the granulite extrusion structuresdescribed in this study resemble extrusions of lowercrustdriven by density inversion (Martinez et al.,2001), they were controlled mostly by lateral shorteningforces in this case, as shown in Fig. 11a.Horizontal flow of orogenic lower crustA transition from vertical S 2 fabrics to horizontal S 3fabrics occurs in almost all examples of lower crustalvertical structures in the Moldanubian and Lugiandomains. Therefore, the main questions that arise are:is there a causal relationship between vertical extrusionand the development of horizontal flow? and what arethe driving forces for the deformation?Ductile thinning and collapse of vertical fabric in the LugiandomainA satisfactory tectonic model to explain the structural,petrological and geochronological data from theLugian domain is one involving folding of a layeredFig. 11. Bouguer anomaly map of the eastern margin of theBohemian Massif (provided by the Czech Geological Survey).Thick lines superimposed on the Bouguer anomaly map arelimits of geological unit boundaries and orogenic crustal levelsfrom Fig. 2. Br, Brunia continent; MDE, eastern branch of theMoldanubian domain; CMP, the Central Moldanubian pluton;MDW, western branch of the Moldanubian domain; LD, theLugian domain.orogenic crust followed by the asymmetrical northeastwardextrusion of the orogenic lower crust causedby the indentation of an Ordovician mafic lowercrustal block at c. 340 Ma. Extrusion of the orogeniclower crust was associated with the development of thehorizontal D 3 fabric accompanied with detachment ofthe western units and complete reworking of the orogenicmiddle crust in adjacent synforms. Consequently,the horizontal fabric in the Lugian domain cannothave been created in response to intracrustalflow because of lateral variations in lithostaticpressure ⁄ gravitational potential energy of thickenedcrust ⁄ lithosphere (Milnes & Koyi, 2000; Vanderhaeghe& Teyssier, 2001).A more likely explanation is a model in which thevertically moving material experiences a reversal in theprincipal strain-rate directions (Feehan & Brandon,1999), which was expressed as a switch from the verticalfabric at depth to the sub-horizontal fabric atshallow crustal levels (Ring & Brandon, 1999). However,such a model requires the presence of a thickcontinental accretionary wedge, as proposed by Platt(1986), which has a mixed flow field involving verticalthickening at depth and vertical thinning near thesurface.Previously Schulmann & Gayer (2000) interpretedthe Lugian domain as an obliquely convergent wedgedeveloped above the Saxothuringian subduction zone.In this model, the Ordovician mafic lower crustal blockÓ 2007 Blackwell Publishing Ltd152