A 60.000-year tectonic record: speleoseismology insights from a fault zone

Northwestern Dinarides is a region of slow tectonic deformation with rates of 2-4 mm/year. Active deformations are largely accommodated by thrusting and dextral strike-slip faulting. The region exhibits moderate to strong seismicity and swarming events. However, the absence of Quaternary sediments across majority of fault scarps does not allow paleoseismic trenching on active fault segments in order to reconstruct a paleoearthquake record of the area. Even so, Northwestern Dinarides are an analogue for karstic phenomena, such as caves and abundance of speleothem forms in them. Generating the region as an ideal testing ground for speleoseismology. The investigated site of Postojna Cave is a 24 km long cave system, located in SW Slovenia, crosscut by the right-lateral strike slip Dinaric fault system (NW-SE striking). The karstic massif in which the cave evolved is enclosed by two major regional active faults, Idrija, Predjama faults and a smaller, active, Selce fault. Postojna Cave presents a diverse array of speleothem formations, characterized by their various morphologies. Some of these formations exhibit signs of deformation or breakage, with certain instances suggesting possible alteration induced by tectonic and seismic activities.

The investigated speleothems are located in an expansive cave chamber, on a subvertical fault zone with a Dinaric strike. The researched fault had a TM extensometer installed more than 20 years ago, to measure tectonic displacements within the fault. In the years 2009-2010 and in 2014 it exhibited displacements (tectonic transients) coinciding with major regional seismicity. The synchronous displacements and the abundance of deformed speleothems within a singular fault zone is why the location was chosen within the cave for sampling. Speleothems were sampled with a diamond corer in an overall distance of 50 m, along the strike of the fault. Specifically, fractures healed with speleothem in flowstone that is located directly within the fault core zone and on the hanging wall of the fault. Additionally, a few of youngest growth speleothems on fractured columns bridging the hanging wall and the footwall were included.

U-Th geochronology was done on nine sampled deformed speleothems using MC-ICP-MS. The results revealed ages from approximately 55,000 years BP to too recent for analysis (<0.5 ky). Two notable clusters of ages were recognized, 22 and 6.5 ky BP. The majority of the dated speleothems coincide with ages derived from local paleoseismic trenching data, younger than thresholds of 12 ky BP and 8.4 ky BP. Covering the age of youngest deformations on the near Selce fault (12 ky BP) and Predjama fault (8.4 ky BP). The most recent speleothem sample could potential represent deformations attributed to earthquakes that occurred approximately or are younger than 1500 CE. The most plausible interpretation of the dated deformed speleothems suggests that the ages at 22 ky BP and 6.5 ky BP may signify distinct tectonic deformation events, possibly indicative of paleoearthquakes. However, it is important that all speleothems dated as younger than the most recent deformations along the Selce and Predjama faults could potentially represent seismic or aseismic tectonic deformations.