Quantitative structural analyses and numerical modelling of ...

DTD 5ARTICLE IN PRESS2L. Baratoux et al. / Journal of Structural Geology xx (xxxx) 1–241964) and Fletcher (1995), the experimental studies byGhosh (1968), Dubey and Cobbold (1977), Dubey (1980),Mancktelow and Abbassi (1992) and Hobbs (1971) and thefield studies by Oertel and Ernst (1978) and Fowler andWinsor (1996)), this work is not directly applicable to thefolding of mineral fabrics where the concept of layerthickness and competence contrast are more difficult todefine. Fortunately a theory of the deformation ofmechanically anisotropic materials has been developed byBiot (1967), which enables the folding of mineral fabrics tobe considered.The application of modern computer graphics, inparticular geographic information systems (GIS) in quantitativepetrography, and computer techniques, which allowthe precise evaluation of grain shapes (Panozzo, 1983,1984), grain sizes and grain spatial distribution, haveprovided tools for the assessment of the relationshipbetween mineral fabrics and the folding that developswithin them. In this paper we present a method ofquantitative microstructural analysis that allows the semiquantitativeassessment of the mechanical anisotropy ofmineral fabrics. We demonstrate that quantitative computeraidedanalysis of natural fold shapes in conjunction with astudy of the internal microstructure allows a better understandingof the mechanical behaviour of naturally foldedmultilayers. Our observations show that it is possible todistinguish folds formed by active buckling from thosedominantly controlled by passive amplification. This can beclearly correlated with major variations in mineral microstructures,semi-quantitatively estimated mechanical anisotropyand the deformation mechanisms associated withfolding.The rock unit studied in this analysis consists ofmetabasites of relatively constant composition that showmetamorphic zonality ranging from the lower amphibolitefacies conditions in the east to the upper amphibolite faciesconditions in the west. The metamorphism wasaccompanied by the development of progressively evolvingmineral microfabrics forming a gently dipping to subhorizontalmetamorphic foliation. This consistently orientedbut systematically varying rock anisotropy has beensubsequently affected by horizontal compression responsiblefor the development of upright post-metamorphicfolds. In addition, a granite intruded into the western part ofthe amphibolite sheet and thermally influenced the host rockwithin a zone several kilometres wide. Field observationsshow a remarkable change in the style of the post-peakmetamorphic folding approaching the intrusion (Schulmannand Gayer, 2000) and the question arises as to whether thechange in fold style reflects the variation in theinherited mechanical anisotropy associated withthe east–west metamorphic–microfabric zonation,whether it is related to the lateral heat input from thewesterly granite, or both.The aims of this contribution are: (1) the quantitativecharacterization of the fold shapes and semi-quantitativedetermination of mechanical anisotropy based on quantitativemicrostructural analysis, (2) the determination ofpossible folding mechanisms using fold shape analysis aswell as the micro-deformational mechanisms associatedwith folding, and (3) the evaluation of the relative role of theinherited paleometamorphic microstructural zonation(mechanical anisotropy) and lateral heating, on folding.2. Geological settingThe northeastern margin of the Bohemian massif is builtup of the Neo-Proterozoic basement called Brunia and itssedimentary cover (Dudek, 1980) in the east, and a belt ofBrunia derived Variscan nappes (Silesian domain) with highgrade rocks of the orogenic root belonging to the Lugiandomain in the west (Fig. 1).The studied area is located in the Brunia derived parautochthoncalled the Desná dome, which consists ofmetamorphic Neo-Proterozoic basement core surroundedby a volcano-sedimentary Devonian envelope (Fig. 1b).Metabasites of the Jeseník amphibolite massif, the subjectof this study, form part of this Devonian sequence. The hugeaccumulation of basic rocks was formed during the EarlyDevonian rifting, as indicated by their tectono-stratigraphicposition and back-arc tholeiitic composition (Patočka andValenta, 1990). Metabasites are present on both the easternand western flanks of the Desná gneissic core. The basites atthe eastern margin show more tuffitic character while themain amphibolite body situated in the west developedmostly from basalts and possibly from gabbros as indicatedby preserved textures in the less deformed domains. Themetabasites are locally stratigraphically or tectonicallyintercalated with metapelites and quartzites of Pragian age(Chlupáč, 1994), which show the same tectono-metamorphichistory.2.1. Structural geologyThe complex polyphase tectonic evolution of the regioncomprises the early pre-Variscan deformations termed D 1 .The first Variscan structures are associated with theBarrovian metamorphism (termed D 2 ), and the late Variscanstructures (D 3 ), developed during the transpressionalregime. The most important D 2 structure is represented bya planar metamorphic fabric expressed as localized shearzones in the basement orthogneisses and as a penetrative,originally sub-horizontal metamorphic foliation (S 2 ) in allthe rocks of the Devonian cover. This fabric is now dippingto the NW or SE at moderate to steep angles as a result ofsubsequent large-scale folding (Fig. 2a). Within the cover,syn-schistose folds (F 2 ) as well as mineral lineation (L 2 ) arelocally well developed.The D 3 deformation had various effects on differentlithologies, depending mainly on the intensity of theprevious S 2 anisotropy (Schulmann and Gayer, 2000).110