Tectonics or rebound: Pleistocene fault reactivation in the highest mountains of the Carpathians

The steep morphology, highest elevation within the Carpathians, and distinct fault borders of the Tatra Mts. support the assumption of recent tectonic activity. However, for decades, the Quaternary tectonic activity of the Tatra Mts. has been poorly evidenced, and a late Pleistocene fault rupture was discovered recently. Using the protected environment of the caves, we have utilized ²³⁰Th/U dating of damaged speleothems to extend the record of Quaternary deformation of the Tatra Mts. up to 0.5 Ma. The results from ten caves reveal five periods of increased activity at 465–332, 280–260, 204.8–188.8, 127–86.6, and 29.5–10.11 ka ago. Three mechanisms of fault reactivation were identified through paleostress analysis of cave passage offsets: (1) gravitational sliding; (2) NNE–SSW transpression, likely associated with the Alps–Carpathians–Pannonian (ALCAPA) NNE motion; and (3) tectonic-driven SE-trending compression that reactivated the strike–slip and the gently dipping oblique lower-order faults. The geochronological data indicate that these three stress regimes operated during the late Pleistocene; however, our fault-slip data do not allow us to identify the dominant process. Herein, we suggest that the fault slip was caused by the isostatic response of the Tatra block to the unloading of mountain glaciers, coupled with sediment evacuation overlapping with regional-scale tectonic processes. These processes are locally obliterated by debuttressing. We also show that the distances between the caves and possible seismic sources (i.e., the Sub-Tatric and Ružbachy faults) are sufficiently short to break speleothems in response to oscillation. Given that for most caves an earthquake of >M_w6.5 would be destructive, it seems co-seismic deformations in caves are highly likely considering the M_w > 7 potential of the Sub-Tatric Fault.