Quaternary tectonic reactivation from cave records and river anomalies in northern Spain

The identification of Quaternary faults remains particularly challenging especially in mature orogenic belts with low seismic activity. Consequently, it is crucial to develop approaches capable of both recognizing and dating faults with the potential to generate earthquakes and induce topographic modifications. Karst caves and relief analyses have been used to identify recent faults with considerable success; however, they are typically applied independently, and there are few methodologies that integrate them. To address this issue, we integrated cave geomorphology, sediment provenance analysis, and river anomaly assessment within a structural geology framework to investigate Quaternary faulting in the Cantabrian Mountains. The studied cave, situated in the northern border of an Alpine foreland basin, preserves allochthonous alluvial deposits related to a river located 280 m lower than the cavity. Fluvial aggradation would occur into the cave ca. 483-599 ka ago before significant river incision, enabling the inference of a maximum fluvial incision rate of 0.5-0.6 m·ka −1 . This value surpasses that of the fluvial network, likely due to the potential influence of two local faults linked to a Ksn river anomaly and other local factor, as the glacial deepening. These faults appear to have been intermittently active since the Chibanian and are associated with a regional fault connected to Quaternary rockfall processes and seismic activity in other areas. In conclusion, the effective combination of phreatic caves with speleothem ages and fluvial sediment provenance has facilitated the quantification of river incision, considering both regional and local factors, such as Quaternary fault activity in areas where the tectonic movements virtually ceased.