Structural evolution of the Nekézseny Fault – a displaced segment of the Dinaric-ALCAPA contact zone in NE Hungary (Bükk and Uppony Hills)

The Bükk and Uppony Hills (NE Hungary) are two adjacent structural units with correlations to the Northern Dinarides and Inner Western Carpathians (ALCAPA), respectively. These two units are separated by the Nekézseny Fault, which may therefore be considered as a presently displaced segment of the Dinaric-ALCAPA contact zone (Schmid et al. 2008). Along this contact zone, the Bükk-type Permo-Mesozoic formations are thrust over the Paleozoic and Senonian formations of the Uppony Unit (Schréter 1945). Despite of the Nekézseny Fault being a terrain boundary, its structural evolution has not been studied in details. Preliminary structural data suggested multiple faulting events between the latest Senonian and early Miocene (Fodor et al. 2005), however, the initial age of the contact zone has remained uncertain.

In this study a detailed structural analysis was carried out in order to understand the deformation geometry, kinematics and the timing of movements along the Nekézseny Fault. Our preliminary results show that the Nekézseny Fault developed in response to NW-SE shortening. Low-angle fractures within individual pebbles suggest an early (latest Cretaceous or early Paleogene) age for the NW-SE shortening, as pebble fracturing is limited to the early stage of diagenesis and requires soft or semi-consolidated fine-grained matrix.

The top-to-the-NW emplacement of the Bükk over the Uppony Unit was accompanied by the folding of the Senonian conglomerate in the footwall, where a large, almost isoclinal recumbent fold developed due to the estimated several km of displacement along the main contact zone. Despite of the similarity in the shortening directions, the top-to-the-NW shortening certainly post-dates the penetrative S-SE-vergent contractional structures present throughout the Bükk Hills, that are related to the latest Jurassic to Early Cretaceous nappe stacking and subsequent shortening (Csontos 1999). Microtectonic analysis of the Nekézseny Fault Zone proved that the main contact zone is a strongly distorted cataclastite zone, which suggests a late-stage low-temperature deformation. Similarly younger semi-ductile or low-temperature contractional structures (e.g. kink folds) were recognized in several parts of the Bükk Unit, all of which were dated tentatively to the late Cretaceous (Flórián-Szabó & Csontos 2002, Juhász 2020, McIntosh 2014, Koroknai et al. 2008, Scherman 2018). Our observations indicate that the top-to-the-NW displacement was much more extensive than previously thought and incorporated large part of the Bükk Unit. This shows that the top-to-the-NW displacement represents an important deformation phase, which should be integrated into the Mesozoic structural evolution of the Alpine-Dinaric area.

This study was supported by the research founds NKFIH OTKA 113013 and 134873, the ÚNKP-17-2 and ÚNKP-20-3 New National Excellence Program of the Ministry of Human Capacities.

References:
Csontos (1999): Bulletin of the Hungarian Geological Society 129, 4, 611-651.
Fodor et al. (2005): GeoLines 19: 141-161.
Juhász (2020): TDK thesis, ELTE, Budapest.
Koroknai et al. (2008): Journal of Structural Geology 30, 159-176.
McIntosh (2014): PhD thesis, University of Debrecen, Debrecen.
Scherman (2018): MSc thesis, ELTE, Budapest.
Schmid et al. (2008): Swiss Journal of Geosciences 101: 139-183.
Schréter (1945): Annual Report of the Geological Institute of Hungary, 1941-42: 197-237.