New paleomagnetic constraints for the large-scale displacement of the Hronic nappe system of the Central Western Carpathians

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Abstract

The thin-skinned Hronic nappe system represents the structurally highest tectonic unit in the Late Cretaceous thrust-stack of the Central Western Carpathians. It mostly comprises a Permian volcano-sedimentary sequence and Triassic carbonate sediments which crop out in different parts of the Central Western Carpathians. We carried out a systematic paleomagnetic study on 24 Permian and 20 Triassic localities geographically distributed over 300 km in W-E direction. Several samples from each locality were drilled and oriented in-situ and specimens cut from them subjected to standard paleomagnetic and magnetic mineralogy experiments. The results were evaluated using principal component analysis, statistical evaluation of the characteristic remanences, and applying inclination-only and tilt tests. We documented the pre-tilting age of remanences for the majority of both the Permian and Triassic age groups. However, the latter was interpreted as remagnetized during the Cretaceous Normal Super-Chron in the course of nappe stacking between 90−80 Ma. The Permian group is exhibiting about 70°, the Triassic about 34° clockwise vertical axis rotations with respect to the present north. There is no indication in our data set for oroclinal bending of the Hronic Unit. We interpret the difference in clockwise rotations (about 36°) between Permian and 90−80 Ma as a clockwise block rotation taking place during major extensional and/or compressive events between stable Europe and Africa. Taking into consideration the well-documented counterclockwise rotation observed for the overstep sequences in the Central Western Carpathians and in the Pieniny Klippen Belt, the remagnetization of the Triassic sediments was closely followed by about 94° clockwise rotation. Research in progress will serve to decide if this large clockwise rotation involved the whole Central Carpathian nappe stack or part of this was due to the thin-skinned nappe emplacement of the Hronic Unit.

Introduction

The Western Carpathians form a northward convex, E-W trending mountain range, which is a part of the European Alpine orogenic system. Based on its structure and tectonic evolution it is divided into three main tectonic zones, namely the External, Central, and Internal Western Carpathians (Plašienka et al., 1997; Froitzheim et al., 2008; Plašienka, 2018). The Central Western Carpathians represent a nappe stack consisting of thick- and thin-skinned nappe units formed and thrust generally to the north-northwest (in recent coordinates) during the Late Cretaceous. The northern and northwestern part of the Central Western Carpathians, lying between the Čertovica thrust-fault and the Pieniny Klippen Belt (Fig. 1), is known as the Tatra-Fatra Belt comprising the Tatric-Fatric-Hronic nappe stack (Plašienka et al., 1997; Plašienka, 2018). The nappe stack is preserved in several fault bounded mega-anticlinal horst structures called the “core mountains” emerging from the sedimentary fill of the surrounding Paleogene and Neogene basins. The uplift of the core mountains, based on zircon and apatite fission-track data (e.g. Burchart, 1972; Kováč et al., 1994; Danišík et al., 2004; Králiková et al., 2016), started already in the Paleocene with the rapid acceleration since the Pliocene – Pleistocene.

Most of the published pre-Cenozoic paleomagnetic data from the Central Western Carpathians come from the Fatric Unit, mainly from the Polish part of the Tatry Mts. (Kądziałko-Hofmokl and Kruczyk, 1987; Kruczyk et al., 1992; Grabowski, 1995, 2000, 2005). They are complemented by sporadic data from the Tatric and Hronic units in the Tatry Mts. (Grabowski, 1997, 2000; Grabowski et al., 1999; Szaniawski et al., 2012) as well as from the Tatric and Fatric units of the remaining part of the “core mountains”: Nízke Tatry, Malá Fatra, Veľká Fatra (Kruczyk et al., 1992; Pruner et al., 1998; Szaniawski et al., 2020), Strážovské vrchy Mts. (Grabowski et al., 2009; Szaniawski et al., 2020) and Malé Karpaty Mts. (Grabowski et al., 2010). The majority of the reported paleomagnetic directions have been interpreted in terms of early pre- or syn-thrusting remagnetizations acquired during the Cretaceous Normal Super-Chron (Grabowski and Nemčok, 1999; Grabowski, 2000). Exceptions are the Lower Triassic siliciclastic deposits resting directly upon the Tatric crystalline basement (Szaniawski et al., 2012, 2020) and Berriasian pelagic limestones of the Fatric Unit (Grabowski, 2005; Grabowski and Pszczółkowski, 2006; Grabowski et al., 2009, 2010) where magnetization has been interpreted as primary. Unlike the Cenozoic paleomagnetic directions showing consistent 50−60 °CCW rotations in all principal tectonic units of the Western Carpathians (see comprehensive review by Márton et al., 2016), the pre-Cenozoic paleodirections display a more complex pattern. The distribution of paleodeclinations for the Tatric and Fatric units (both primary and secondary) and their apparent agreement with nappe transport trajectories was originally taken as a proof for oroclinal bending of the Central Western Carpathians or alternatively interpreted in terms of radial thrusting (e.g. Kruczyk et al., 1992). However, recently documented primary magnetizations from the Tatric cover Unit (Szaniawski et al., 2012; 2020) show concordant paleodeclinations relative to the present north, consistent through a considerable part of the Central Western Carpathians, and therefore challenge the oroclinal bending model.

In the Hronic Unit, Late Paleozoic volcanic and sedimentary rocks were the targets of the very early paleomagnetic studies in the Western Carpathians (see review by Krs et al., 1982). Results of these studies were among the first that have been interpreted in terms of large rotations within the Western Carpathians (Kotásek and Krs, 1965; Krs, 1966). The results were originally interpreted as CW vertical axis rotations. Later these paleomagnetic data were reinterpreted as CCW rotations (Márton et al., 1992; Krs et al., 1996), because some doubts had arisen about the sense of rotation in the light of the near-equatorial paleoposition of the studied rocks and the very similar angle of a Cenozoic CCW rotation documented for several areas in the Internal Western Carpathians (Márton et al., 2016 and references therein).

The main aim of the present study was to obtain positive proofs for the sense and amount of rotations in the Hronic Unit. Thus, we conducted a modern paleomagnetic study on the Late Paleozoic volcanic and sedimentary rocks of the basal part of the Hronic Unit (black dots in Fig. 1), on one hand and a new systematic research on the Mesozoic, mostly Triassic sediments of the same unit (Fig. 1). Our research focused on the Nízke Tatry Mts., where the Late Paleozoic and Triassic rocks are well exposed and accessible for sampling. Additionally, we collected samples from the western sector of the Central Western Carpathians (Malé Karpaty, Považský Inovec and Strážovské vrchy Mts.) in order to have a control on possible relative rotations between different partial nappes of the Hronic Unit, which could be attributed to oroclinal bending.

Section snippets

Geological background

The structurally lowermost tectonic unit of the Central Western Carpatians is the Tatric Unit. Its more frontal and distal elements, exposed in the Malé Karpaty, Považský Inovec and in the western part of the Malá Fatra Mts. (Fig. 1) are known as the Infra-Tatric Unit (Putiš, 1992; Putiš et al., 2008; Plašienka, 2018). The Tatric and Infra-Tatric units are composed of the Variscan crystalline basement and its Late Paleozoic and Mesozoic para-autochthonous, mostly sedimentary cover. It is

Paleomagnetic sampling

Altogether, we collected samples at 24 Late Paleozoic and 20 Mesozoic localities (Fig. 1, Fig. 2, Fig. 3, Fig. 4). Statistically acceptable directions were obtained for 20 Late Paleozoic (Table 1) and 16 Triassic localities (Table 2). The samples were drilled by using a portable water-cooled gasoline and an electric drill and oriented mainly by a magnetic compass or when the lithology or the situation required (e.g. closeness of a railway line) a sun compass was used. Special care was taken to

Laboratory methods

The samples were cut to standard-size specimens by a water-cooled wheel-saw. Usually two specimens from a sample were obtained. The natural remanent magnetization (NRM) was measured by using JR-4, JR-5, JR-5A, JR-6A spinner magnetometers in Budapest, Banská Bystrica and Warsaw, respectively. The magnetic susceptibility and anisotropy of the low-field susceptibility was measured by a KLY-2 kappabridge (Agico, Czech Republic). Then, specimens were stepwise demagnetized by either thermal

Magnetic mineralogy

Magnetic minerals in these rocks were identified by monitoring the magnetic susceptibility during heating-cooling runs from room temperature up to 700 °C. In some cases, the experiments started from liquid nitrogen temperature (e.g. Fig. 6, SMP160, 137).

In most of the basalts, and in intercalated tuffs (Fig. 6, SMP44), the magnetic mineral was identified as a slightly oxidized magnetite with Curie temperature a bit higher than 575 °C. Some exhibited the Verwey transition in the low-temperature

Discussion of the Permian paleomagnetic results

The results represent mostly volcanic rocks (lava flows, dykes, and in one case an intercalated tuff horizon) and red sediments. The positions of the lava flows were possible to measure directly from the attitude of intercalated tuffs or infer from the underlying and/or overlying sediments.

The paleomagnetic directions for the red sediments form two distinct groups (Fig. 10). Both are accompanied by those obtained for igneous rocks of Permian age. The larger group comprises all results from the

Conclusions

The presented paleomagnetic results from the Late Paleozoic and Triassic rocks of the Hronic nappe units demonstrate that:

  • 1)

    The Permian overall mean paleomagnetic direction is based on a robust set of data since it relies on locality mean directions, which are geographically distributed, represent different lithologies, and different carriers of the remanent magnetizations. Moreover, the paleomagnetic directions before tilt corrections are far from that of the present Earth’s magnetic field at

CRediT authorship contribution statement

Emő Márton: Conceptualization, Investigation, Formal analysis, Resources, Data curation, Writing - original draft, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Jozef Madzin: Conceptualization, Investigation, Formal analysis, Resources, Data curation, Writing - original draft, Writing - review & editing, Visualization, Project administration, Funding acquisition. Dušan Plašienka: Conceptualization, Investigation, Writing - review & editing,

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

This study was financially supported by the National Development and Innovation Office of Hungary (project K128625), the Slovak Research and Development Agency (projects APVV-0212-12, APVV-17-0170) and VEGA Agency (projects 2/0028/17, 1/0151/19). Constructive reviews by Miguel Garcés and an anonymous reviewer are gratefully acknowledged. We thank Tadeusz Sztyrak and Gábor Imre for technical assistance in the field and laboratory. Thanks to Tomáš Flajs (National Park Malá Fatra) for working

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