Late Cenozoic alkali basalts and their interactions with the paleo-Hron River (Western Carpathians): New insights from geochronology and fluvial morphometric indices

Sporadic pulses of basaltic volcanism in the Western Carpathians during the Late Miocene to Middle/Late Pleistocene were contemporaneous with the formation of modern topography. Low-viscosity basaltic lavas entering river valleys influenced the evolution of drainage systems and the surrounding terrain. This study targets two important alkali basalt occurrences in the Western Carpathians – the Ostrá Lúka (OL) basalt complex and the Putikov vŕšok Volcano (PvV). These basalt complexes are unique in their spatial proximity to the Hron River valley, yet their age is unclear. We present new geochronological constraints for both basalt effusions and investigate their potential impact on the evolution of the Hron River valley. 40Ar/39Ar dating of plagioclase from two OL basalt samples yielded Late Tortonian plateau ages of 7.6 ± 0.1 Ma and 7.8 ± 0.1 Ma. The zircon (U-Th)/He age of 5.9 ± 0.6 Ma from one of the OL samples indicates post-eruption rejuvenation likely by younger lava flows. While fluvial morphometric indices extracted from the current Hron River channel in the OL area likely reflect younger uplift events, χ-elevation profiles of the Hron River left-side tributaries still indicate past drainage reorganisations. We hypothesise that the Hron River was diverted westward in the Late Tortonian when voluminous OL lavas blocked its earlier southward-trending paleo-valley. However, more data is required to confirm this hypothesis. Luminescence dating applied to sediments from a fluvial terrace underlying PvV basalt flow revealed minimum depositional ages of >165 ± 23 ka and > 163 ± 27 ka. These ages are older than the currently accepted OSL age (102 ± 11 ka) but consistent with the recently published K/Ar age of PvV basalts (450 ± 30 ka). The older terrace age suggests delayed incision and challenges the validity of the currently used classification of river terraces in the study area. The valley damming by lava flow followed by younger aggradation phase(s) can explain the seemingly low average incision rate (up to ∼0.03 mm yr−1) in this segment of the Hron River, which is supported by the linear inversion of its tributary profiles in the PvV area. •New robust 40Ar/39Ar and (U-Th)/He age constraints of the Ostrá Lúka alkali basalts•Revisited age limits of the Putikov vŕšok volcanic activity based on new OSL data•Interactions of basaltic lavas with the paleo-Hron revealed by morphometric indices•Valley damming and anomalous sediment supply were responsible for d