On the track of thrust faults within the Variscan basement of the Western Tatras: structural and geochronological approach

The crystalline basement of the Tatra Mountains belongs to the northernmost part of the Tatric unit of the Western Carpathians and is composed of pre-Mesozoic crystalline rocks, overlain by Mesozoic and Cenozoic sedimentary cover and nappes. Metamorphic rocks are the most abundant in the Western Tatra Mts. and display an inverted metamorphic sequence with high-grade rocks in the hanging wall (Upper Unit; peak conditions: 1.6 GPa, 750-800°C; Janák et al. 1996) and lower-grade rocks in the footwall (Lower Unit; peak conditions: 0.6-0.8 GPa, 640-660°C; Janák et al. 1996) separated by mid-crustal thrust fault. Those two basement units of contrasting pressure-temperature evolution are well documented in southern part of basement (i.e. in Slovakia). However, a presence of the Lower Unit in the northern part of the Western Tatras (i.e. in Poland) is highly debated.

To tackle this problem several field campaigns were carried out targeting an inferred thrust fault allegedly separating both basement units in the north. The field studies coupled with the structural analysis revealed presence of a wide high-strain zone, but no significant lithological difference across the zone on question. A set of three metasedimentary and one metaigneous rocks were collected along the profile cross-cutting the high-strain zone (from the bottom to the top: MB21-71, -82, -03, -32), and display a gradual increase in migmatization. The provenance study of detrital zircon shows no significant differences between the samples. The metasediments MB21-71, -82, -32 define prominent peaks at ca. 570-530 Ma, whereas MB21-32 shows additional younger peak at 520-500 Ma. Additionally, minor Palaeozoic (490-460 Ma;not in MB21-71), Proterozoic (ca. 680 Ma, 1500-1400 Ma, 2000 Ma), and Archean peaks (ca. 2800-2500 Ma) are present. The samples exhibit a metamorphic signature around 360-340 Ma too. The metaigneous rock MB21-03 yields U/Pb zircon age of ca. 490-480 Ma with only few Precambrian grains.

Our preliminary results coupled with a similar study from the southern side of the Western Tatras (Kohút et al. 2022) suggest that the observed similarities in the detrital zircon populations could indicate a similar protolith, thereof all samples could represent only one basement unit . This dataset helps to reconcile the nature of the Variscan basement assembly in the Western Tatra Mts. It also shows that the local tectonostratigraphy of the Tatra Mts. needs to be re-visited and re-evaluated.

Research funded by the National Science Centre, Poland, project no. 2021/43/B/ST10/02312 and supported by the Foundation for Polish Science stipend (M. Bukała). We also acknowledge the Tatra National Park for help and permission to conduct fieldwork.

References:

Janák, M., O'Brien, J.P., Hurai, V., & Reutel, C. (1996). Metamorphic evolution and fluid composition of the garnet-clinopyroxene amphibolites from the Tatras Mountains, Western Carpathians. Lithos, 39, 57-79.

Kohút M., Linnemann U., Hofmann M., Gärtner A., Zieger J. (2022) Provenance and detrital zircon study of the Tatric Unit basement (Western Carpathians, Slovakia). International Journal of Earth Sciences 111:2149-2168.