Finding Quaternary Seismogenic Activity Along the Eastern Periadriatic Fault System: Dating of Fault Gouges via Trapped Charge Methods

The Periadriatic Fault System (PAF) is among the largest and most important post-collisional structures of the Alps; it accommodated between 150-300 km of right-lateral strike-slip motion between the European and Adriatic plates from about 35 until 15 Ma. Recent GPS data suggest that Adria-Europe convergence is still being accommodated in the Eastern Alps. However, according to instrumental and historical seismicity records, seismotectonic deformation is mostly concentrated in the adjacent Southern Alps. In this contribution, we present our first results for dating earthquakes along the PAF during the Quaternary by applying two trapped charge dating methods. Both Electron Spin Resonance (ESR) and Optically Stimulated Luminescence (OSL) are especially useful as ultra-low temperature thermochronometers due to their dating range (a few decades to ~1 Ma) and low closing temperature (below 100°C). We aim to show which segments of the PAF system accommodated seismotectonic deformation by directly dating quartz and feldspar from fault gouges. For ESR, we measure the signals from the Al center in quartz following the single aliquot additive (SAAD) and single aliquot regenerative (SAR) protocols, focusing on the 100-150 µm grain size fraction. For OSL, we measure the IRSL signal at 50°C (IR₅₀) and the post-IR IRSL signal at 225°C (pIRIR₂₂₅) on potassium feldspar aliquots of the 100-150 µm grain size fraction. Our ESR results indicate the PAF system accommodated seismotectonic deformation during the last 1 Ma, while the OSL signals for all samples were in saturation. The minimum ages obtained from OSL suggest that the events are likely not younger than 0.4 Ma. We also studied a segment of the nearby Lavanttal Fault, for which our ESR results suggest that the last earthquakes strong enough to produce sufficient shear heating to produce a partial reset on the geochronometer probably happened before 4 Ma.