Quantitative structural analyses and numerical modelling of ...

5 - 2 LEXA ET AL.: COLLISION IN WEST CARPATHIANSmodel the finite strain pattern in time and space for complexboundary conditions.2. Geological Setting[5] The West Carpathians have been traditionally dividedinto the outer and inner structural zones. However, takinginto account the recent studies of Mesozoic evolution, thetriple division into Inner, Central and Outer West Carpathiansis commonly accepted (Figure 1) [see Plašienka etal., 1997]. The Outer West Carpathians (OWC) are representedby flysch dominated Paleogene formations andMesozoic rocks lacking pre-Mesozoic basement. The Innerand Central West Carpathians (IWC and CWC), separatedby a Late Jurassic oceanic suture, are composed of a pre-Late Cretaceous imbricated nappe system comprisingcrystalline basement units with characteristic Late Paleozoicand Mesozoic sequences [Matějka and Andrusov, 1931] andpostnappe Late Cretaceous to Neogene sedimentary andvolcanic formations.2.1. Pre-Cretaceous Geology of the Studied Area[6] The studied area is located in the southern part of theCWC and comprises three major lithological and tectonometamorphicunits (Figure 1). From the north to the southand from the bottom to the top they are (1) Variscancrystalline basement (Vepor Unit) with Late Paleozoic andMesozoic cover sequences, (2) Early to Late Paleozoic,basinal, mostly low grade turbiditic sequences (GemerUnit), and (3) Mesozoic accretionary wedge containingblueschist facies relics overlain by (4) flat, nonmetamorphosedSilica nappe.2.1.1. Variscan Crystalline Basement: The Vepor Unit[7] The crustal rocks of the Vepor basement are composedof two contrasting Variscan metamorphic domainsexhibiting pre-Alpine thrust tectonics. The structurallylower domain, generally dipping to the north, is composedof medium-grade schists exhibiting peak PT conditions inKy-St micaschists corresponding to maximum of 10 kbarand 550–600°C [Korikovskij et al., 1989; Méres andHovorka, 1991]. The classic Barrow type metamorphiczonation is difficult to establish due to the Alpine greenschistfacies overprint but in the eastern part of the Veporbasement (Čierna Hora Mountains), a metamorphic zonationranging from the biotite zone in the south to thestaurolite zone in the north is documented [Jacko et al.,1990; Korikovskij et al., 1990]. The structurally highercrystalline unit is represented by a domain of heterogeneouspara- and ortho-derived migmatites intruded byporphyritic to medium-grained peraluminous granites.The Variscan age of metamorphism is documented byTh-U-Pb dating of monazite (370–350 Ma [Janák et al.,2001a]) and by sporadically preserved 40 Ar/ 39 Ar coolingages ranging from 358 to 312 Ma [Dallmeyer et al.,1996]. High-grade fabrics represented by compositionallayering and stromatitic banding in migmatites are dippingeither to the NW or to the N under steep to mediumangles. Peak temperature conditions yield 680–730°C atpressures of 4–6 kbar (Figure 2) [Siman et al., 1996].These PT conditions most likely correspond to lateexhumation and decompression melting. This is deducedfrom other parts of West Carpathians where relics ofhigh-pressure assemblages preserved in similar types ofmigmatites were found [Hovorka and Méres, 1989;Janák et al., 1996].[8] The southern Vepor Variscan crystalline basement isunconformably covered by Late Carboniferous (Stephanian)sandstones and shales [Planderová and Vozárová, 1978].These metasediments were intruded by granitoids that wereresponsible for contact metamorphism ranging from biotiteto cordierite zones. Contact metamorphic conditions of500°C and 2 kbar were established by Vozárová [1990].Vozárová also suggests that contact metamorphism overprintedregional greenschist facies assemblages of Carboniferousmetasediments. The Vepor crystalline complexes aswell as the Late Carboniferous sequences are overlain byPermian and Triassic clastics and locally of Middle and LateTriassic carbonates.2.1.2. Early to Late Paleozoic Gemer Unit[9] The Gemer Unit consists of three principal groups thatdiffer in lithology and metamorphic grade. The amphibolitefacies metamorphic conditions are reported from the KlátovGroup [Faryad, 1990], which tectonically overlies thegreenschist facies metabasites and phyllites of the RakovecGroup. The Variscan metamorphic conditions correspond to440–480°C at 6–8 kbar for the northern part and 350–430°C at 4–5 kbar (Figure 2) for the southern part of theRakovec Group [Faryad and Bernhardt, 1996; Vozárová,1993]. The metamorphic foliation is represented by compositionallayering in metabasites dipping to the NNW undershallow to intermediate angles. The tectonically lowermostGelnica Group builds the major part of the Gemer Unit. Thelower part of this unit consists of a thick turbiditic sequenceof Ordovician age [Soták et al., 1999; Vozárová etal., 1999]gradually passing into volcanosedimentary sequencesaccompanied by products of massive rhyolite-dacite volcanismof probably Silurian-Devonian age [Cambel et al.,1990]. The top of the Gelnica Group is composed ofphyllites and black shales containing sporadic carbonatelenses. The metamorphic conditions indicate temperaturesof 350–400°C and pressures of 2.5–3.5 kbar [Faryad, 1992,1994] for regional metamorphism, which is overprintedby contact metamorphism (450–550°C and 1.5–2 kbar[Faryad, 1992]) associated with intrusion of granites incentral part of the Gelnica Group. Strong greenschist faciesFigure 1. (opposite) Geological and structural map of the studied area with trajectories of cleavage based on this work andthat of Snopko and Reichwalder [1970]. The inset in the upper left corner shows the Carpathian arc with location of thestudied area.86