Quantitative structural analyses and numerical modelling of ...

LEXA ET AL.: COLLISION IN WEST CARPATHIANS 5 - 5the Gemer and Vepor Units during the Late Jurassic (150–160 Ma [Dallmeyer et al., 1996; Faryad and Henjes-Kunst,1997; Maluski et al., 1993]) (Figure 2). In the southern partof the Gemer Unit, sedimentary bedding is well preservedand folded by large-scale open folds with N-S trendinghinges (Figure 3). This folding is connected with developmentof spaced cleavage steeply dipping to the east suggestingthat the thinned continental margin was intensivelyreworked during Jurassic subduction processes.3. Cretaceous Polyphase Structural Evolution:Collisional Stage[17] The Cretaceous collisional evolution of the Gemerand Vepor Units is marked by four major distinct tectonicevents: (1) formation of the Gemer Cleavage Fan (GCF)structure affecting the central part of the Gemer Unit,(2) extensional deformation of the western Vepor promontory,(3) transpressional shearing affecting the western Veporpromontory and development of the Trans-Gemer ShearZone (TGSZ), and (4) extrusion of the Gemer Unit over theeastern Vepor promontory along the Eastern Gemer Thrust(EGT).[18] In order to evaluate strain variations in conglomerateswe used published data [Németh et al., 1997], while forslates the degree of deformation associated with developmentof crenulation cleavage was evaluated using criteriaintroduced by Bell and Rubenach [1983]. In our work weused five stages of crenulation cleavage development[Passchier and Trouw, 1996, Figure 4.17ab] to semiquantitativelycharacterize the strain gradient in studied area.3.1. Early Cretaceous Deformation:Gemer Cleavage Fan[19] The Gemer Cleavage Fan represents the most spectacularstructure overprinting the pre-Mesozoic metamorphicfabric of the Gelnica and Rakovec Groups as well asJurassic fabric to the south. This cleavage forms an asymmetricpositive fan structure developed across the entirelength of the Gemer Unit (Figure 3). The intensity andmetamorphic grade of the cleavage are highest in a 5 kmwide, E-W oriented axial zone of the fan structure. Here, theonly relics of pre-Mesozoic fabric in the form of rootlessfolds are preserved in intensively developed steep slatycleavage corresponding to stages 4 and 5 of Bell andRubenach [1983] classification (Figure 3). Finite strainmeasurements of Németh et al. [1997] show oblate finitestrain symmetry and rather moderate strain intensities. Thiszone is characterized by the presence of numerous bodies ofgranites. These intrusions have been originally consideredCretaceous in age [Kantor, 1957; Máška, 1957; Vozár et al.,1996] and later, based on monazite (273 Ma [Finger andBroska, 1999]) and zircon U-Pb (275–245 Ma [Poller et al.,2002]) dating as Permian to Early Triassic. The maincleavage of the GCF is in the axial zone affected by kinkbands with kink planes perpendicular or oblique to stronglydeveloped vertical anisotropy (Figure 5a). These structuresare interpreted as a result of vertical shortening of steepcleavage associated with the evolution of GCF.[20] The intensity and degree of metamorphism of thenorth dipping, spaced cleavage of southern flank of GCFrapidly decrease to the south. The cleavage intensity correspondsto stages 1 and 2 of Bell and Rubenach’s [1983]classification. Here, the Cretaceous deformation is so weakthat the structures associated with Jurassic obduction ofMeliata accretionary wedge are well preserved. In thenorthern flank of GCF, the cleavage is dipping to the south.The dip angle gradually decreases to the north in conjunctionwith decreasing intensity of cleavage developmentcorresponding to the stages 1 and 2 of Bell and Rubenach[1983] (Figure 5b). In northern parts of the Gelnica Group,the north vergent kink bands developed mainly withinincompetent shales while competent lithologies like quartzitesand volcanics were gently folded by open folds withwavelengths of a few hundreds of meters (Figure 3a). Thedecrease in cleavage intensity is even more pronounced inamphibolite and greenschist facies metabasites of the RakovecGroup where a spaced to fracture cleavage is developedindicating very low finite strains [Passchier and Trouw,1996] (Figures 3a and 5c).[21] The Late Carboniferous and Permian cover of theGemer Unit, directly overlying the Rakovec Group, is alsoaffected by GCF. In this domain, the deformation is stronglyheterogeneous leading to the development of small-scale(up to several hundreds of meters) positive cleavage fanstructures. Carboniferous conglomerates contain pebbles offoliated metabasites of the Rakovec Group. However, thesecondary cleavage known from the Rakovec Group hasnever been discovered in the pebbles. This means that thesame post-Permian cleavage affects the Gemer Unit andLate Paleozoic sediments.[22] The features of GCF described above are developedprominently along the central part of the Gemer Unit(Figure 4a). Lateral extension of GCF toward western andeastern Vepor promontories (Figures 3a and 4b) is markedby change in cleavage trend, so that it becomes parallel totheir boundaries. In addition, the positive fan structuredisappears and the cleavage is dominantly vertical and veryintense. Triassic and Jurassic sediments of the Meliataaccretionary wedge in the front of the western Veporpromontory exhibit a strong cleavage development andreworking of Jurassic fabrics in continuation of GCF.Structural succession as well as 40 Ar/ 39 Ar cooling agesranging from 106 to 82 Ma from Gemer Unit [Dallmeyeret al., 1996] are our major arguments for Cretaceousshortening of the Gemer Unit associated with the developmentof GCF.3.2. The First Cretaceous Deformation:Vepor Extensional Mylonitization[23] In contrast to compressional deformation of theGemer Unit, the earliest Cretaceous structures developedin the Vepor basement are associated with extensionaltectonics (Figure 4a). This is manifested by mylonitizationof basement rocks and Permian-Triassic cover, characterizedby the development of anastomosed network ofgreenschist facies shear zones. The intensity of mylonitizationis highest in the cover sequences and gradually89