Quantitative structural analyses and numerical modelling of ...

5 - 4 LEXA ET AL.: COLLISION IN WEST CARPATHIANSFigure 2. Idealized E-W cross section showing pre-Cretaceous structural and metamorphic pattern ofthe studied area. Insets show Variscan (Gelnica, Rakovec, and Klátov Formations) and Jurassic (Bôrkanappe) metamorphic PT conditions. The eastern ‘‘unknown’’ block represents hypothetical active marginduring closure of Meliata ocean. The legend is same as for Figure 1.metamorphic foliation and relics of sedimentary bedding aredipping conformably to the NNW. The metamorphic gradeand degree of structural transposition are gradually vanishingto the south.[10] Already consolidated Rakovec and Gelnica Groupsare discordantly covered by Early Carboniferous flysch andPermian clastics of ‘‘red bed’’ type. Sedimentation wasterminated by the development of Late Permian-EarlyTriassic evaporite formations. The metamorphism of Permianclastics yielded temperatures of 200–250°C [Šucha andEberl, 1992].2.1.3. Mesozoic Meliata Accretionary Wedge andSuperficial Triassic Nappe[11] Mesozoic sequences are located mainly along thesouthern margin of the Gemer Unit, locally rimming itsnorthern margin or forming klippens on the Gemer Unit.The bottom part of the Meliata accretionary wedge isformed by the sequence of thrust sheets (initially thesouthern slope of the European continent) consisting ofthinned continental margin, Permian clastics, Triassic limestonesand blueschist facies metabasalts (Figure 2) (380–460°C at 9–13 kbar [Faryad, 1995]).[12] These rocks are overlain by subduction-relatedmelange composed of deep-water oceanic sediments of theMeliata ocean and relics of oceanic mantle rocks. Theuppermost part of the accretionary wedge is composed ofTriassic (Turňa) carbonates, shales and pelagic sediments ofnorthern slope of the Apulian continent. The high pressurerocks of the Meliata accretionary wedge exhibit strongductile and polyphase deformation of all lithologies. Themylonitic foliation is generally dipping to the east, bears anintense southeast plunging stretching lineation and kinematicindicators, such as sigmoidal pebbles of metabasalts, whichsuggest top to the northwest transport. The Turňa shalesshow very low grade metamorphic overprint and locallydeveloped slaty cleavage subparallel to the bedding.[13] The accretionary wedge and the underlying GemerUnit are tectonically overlain by extensive horizontallylying Silica nappe derived from the Apulian shelf. The baseof the nappe is composed of Late Permian-Early Triassicevaporites and shales followed by Middle and Late Triassiccarbonates, which were affected by brittle faulting [Hók etal., 1995].2.2. Summary of Pre-Cretaceous TectonometamorphicEvolution of the Studied Area[14] The metamorphic and structural patterns describedabove are consistent with north-south polarity of the Variscanorogenic evolution marked by southward thrusting ofdeep-seated lower crustal and middle crustal complexesover low-grade foreland. The Vepor Unit is regarded as arelic of the Variscan internal domain in which the anatecticlower crust is thrust over the middle crustal Barroviancomplex. The original Paleozoic relationship between Veporand Gemer Units is largely obscured by later Mesozoicevolution. North dipping pre-Mesozoic fabrics and increasein metamorphic grade from the south to the north in both theGemer and Vepor Units suggest an existence of N-S polarityof Paleozoic orogeny [Faryad, 1990]. On the basis of thesedata, the Klátov and Rakovec groups may be interpreted asrelics of an Early Paleozoic basin underthrust beneath thehigh-grade rocks of the internal domain represented by theVepor Unit exposed in the north. In our model the wholenappe stack was further thrust over Early Paleozoic basinrepresented by the Gelnica Group, which is probablyunderlain by a Neo-Proterozoic basement (Figure 2). Thelack of thrust-related structures in Middle Carboniferousmetasediments indicates that the nappe stacking occurred inpre-Westfalian times.[15] The southward thrusting of high-grade gneisses overthe low-grade mostly metasedimentary foreland formed twopromontories separated by embayment of weakly metamorphosedsediments. This irregular geometric distribution ofgneissic complexes and soft sediments played a critical rolein the subsequent Mesozoic tectonic evolution.[16] The last major pre-Cretaceous tectonic event responsiblefor the final structural pattern was the Jurassic southeastwardsubduction (in recent coordinates) of the Meliataocean and the southern passive margin of the Europeanplatform. This process resulted in formation of an accretionarywedge and its northwestward obduction over both88