Kinematically unrelated C—S fabrics: an Lexample of extensional shear band cleavage from the Veporic Unit (Western Carpathians)

Abstract

Discontinuous and kinematically unrelated C-S fabrics have been recognized along the contact between the Gemeric and Veporic Units in the Western Carpathians. The formation of S and C fabrics within orthogneiss, quartzite and chloritoid-kyanite schist of the Veporic Unit is associated with Cretaceous syn-burial orogen-parallel flow and subsequent exhumational unroofing. The formation of the two fabrics characterized by distinct quartz deformation microstructure and metamorphic assemblage is separated by an inter-tectonic growth of transversal chloritoid-, kyanite-, ± monazite-bearing assemblage. The monazite U-Th-Pb concordia age of 97 ± 4 Ma was obtained by the laser ablation ICP-MS dating method. The age of this inter-tectonic metamorphic stage together with existing 40 Ar/ 39 Ar ages on exhumation of the Veporic Unit indicate that despite the similar appearance to shear bands or C-S mylonites there is a time span of at least 10 Myr between the formation of homogeneous S fabrics and superposed discrete C fabrics in the studied rocks.

Inverse ductile thinning via lower crustal flow and fold‐induced doming in the West Carpathian Eo‐Alpine collisional wedge

“…This deformation culminates at the basement–cover boundary (Figure 2e) where the Sh gr 2A is associated with a zone of basement and cover mylonites and ultramylonites of up to hundred meters in width. These are rich in shear sense criteria such as apparent C/S fabrics formed by superposed Sh gr 2A /Sh am 1A (Figure 4d), sigmoidal clasts, rotated veins and mineral fabrics [ Hók et al , 1993; Plašienka , 1993; Putiš et al , 1997b; Bukovská et al , 2012]. This zone has been identified by the above mentioned authors as a major extensional detachment (Figure 3) and the highly deformed relics of the cover sequence as extensional allochthons.…”

“…The Sh am–gr 1A schistosity in the basement and cover was subsequently heterogeneously overprinted by the east dipping detachment fabric Sh gr 2A developed mainly in the eastern part of the contact domain (Figure 3) [ Hók et al , 1993; Plašienka , 1993; Bukovská et al , 2012]. The recumbent folds Fr gr 2A are rare in the basement and more frequent in the Veporic cover and Ochtiná nappe (Figure 4e).…”

Geological evolution of the southwestern part of the Veporic Unit (Western Carpathians): based on fission track and morphotectonic data

“…The Eo-Alpine metamorphism may be as old as ~ 115 Ma, as was revealed by 40 Ar/ 39 Ar and K-Ar ages (Maluski et al 1993;Kováčik et al 1996Kováčik et al , 1997 and Sm-Nd whole rock-garnet isochron (~ 109 Ma; Lupták et al 2004). The orogen-parallel extension of the Veporic Unit finished at latest by ~ 97 Ma, as is suggested by the post-kinematic growth of monazite in the southern Foederata cover sequences, revealed by the laser ablation ICP-MS dating (Bukovská et al 2013). At the same time, the northern part of the Veporic Unit still experienced thrusting and internal imbrications, related to the onset of underthrusting of the Fatric basement from the north, recorded by white mica 40 Ar/ 39 Ar ages of ~ 95-90 Ma from the lower-grade shear zones in the northern parts of the Veporic dome (Plašienka 2003;Putiš et al 2009).…”

“…The Alpine tectono-metamorphic phase is characterised by Cretaceous amphibolite facies conditions in the structural footwall, which gradually decrease to greenschist facies conditions towards the structural hanging wall Jeřábek et al 2008a). The Cretaceous metamorphism was associated with the development of a subhorizontal mylonitic fabric, which developed during E-W orogen-parallel stretching induced by the northward overthrusting of the Gemeric Unit (Lexa et al 2003;Jeřábek et al 2007Jeřábek et al , 2012Bukovská et al 2013). With the ongoing collision, this fabric was folded.…”

“…Model TL, Data TL -mean lengths of the model and data, and the standard deviations of length distributions. unroofing of the Veporic Unit from underneath the overlying Ochtiná Nappe and Gemeric Unit along a low-angle detachment shear zones (Hók et al 1993;Plašienka 1993;Jeřábek et al 2012;Bukovská et al 2013;Novotná et al 2015). Most probably, data obtained from newly formed phlogopite from the Muráň Nappe sole that yielded flat Ar-Ar spectra with plateau ages at 84 to 91 Ma (Milovský & Plašienka, 2007) could indicate unroofing of the Veporic Unit.…”