From ductile to brittle shear zone - insights from the complex imbricated structure of the Internal Western Carpathians near Bretka village

The Western Carpathians represent the northernmost segment of the Alpine–Carpathian orogenic system and are characterized by a complex polyphase Variscan–Alpine evolution. The Internal Western Carpathians preserve remnants of the Triassic–Jurassic Meliata Ocean and record a long history of subduction, nappe emplacement and subsequent tectonic overprinting. One of the best exposures of this evolution occurs in the Bretka village area, where the Meliata Unit is bounded to the south by a prominent ductile-to-brittle shear zone. The Bretka shear zone is characterized by the close association of serpentinites, metabasalts, mylonitized carbonates and pseudotachylite-like rocks. In addition, evaporites, mainly anhydrite, are known from borehole data, suggesting that deformation was probably localized within an evaporitic mélange. The serpentinite body is interpreted as a tectonic block incorporated into this mélange. The most significant feature of the shear zone is the relationship between carbonate mylonites and pseudotachylite-like rocks, documenting the transition from ductile to brittle deformation. Mylonites developed within Anisian Steinalm limestones and display a gradual transition from undeformed limestones to completely mylonitized marbles towards the shear zone. The marbles exhibit alternating calcite- and dolomite-rich layers. Calcite forms a fine-grained recrystallized aggregate, whereas dolomite occurs as elongated and rotated porphyroclasts, indicating deformation temperatures of approximately 250–350 °C. Shear bands and asymmetric folds consistently indicate a top-to-the-north sense of shearing. The mylonites are cross-cut by numerous thin calcite veins oriented nearly perpendicular to the mylonitic foliation. Pseudotachylite-like rocks occur mainly within marble layers adjacent to serpentinite bodies and consist of a fine-grained matrix composed of chlorite, epidote and actinolite containing fragmented clasts of albite, calcite, titanite and relict magmatic pyroxene. The observed structures suggest a three-stage evolution of the shear zone. A progressive deformation in ductile conditions caused mylonitization of limestones. A subsequent seismic event caused brittle fragmentation of marble and local frictional melting at the contact with ultramafic rocks. Subsequent fluid infiltration produced an extensive network of thin calcite veins and altered the pseudotachylite-like material to chlorite-, epidote- and actinolite-bearing assemblages. These observations indicate that the mylonitization developed at temperatures below approximately 350 °C, corresponding to depths of roughly 12–14 km with subsequent brittle deformation in the upper parts of the crust. Although kinematic indicators consistently record mainly north-directed transport, both the shear zone and the surrounding rock complexes currently dip to the north. We therefore suggest that the shear zone originally formed during northward nappe emplacement and was subsequently rotated into its present orientation during later tectonic disintegration of the Meliata Unit.

Acknowledgments. The research was supported by the Slovak Research and Development Agency (APVV-21-0281 and SK-HU-24-0016) and the Slovak scientific grant agency VEGA (VEGA 1/0021/25).