Not too old to rock: ESR and OSL dating methods reveal Quaternary activity of the Periadriatic Fault

doi:10.21203/rs.3.rs-3221175/v1

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Not too old to rock: ESR and OSL dating methods reveal Quaternary activity of the Periadriatic Fault

https://doi.org/10.21203/rs.3.rs-3221175/v1

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The Periadriatic Fault System (PAF) ranks among the largest post-collisional structures of the European Alps. Recent Global Satellite Navigation Systems data suggest that a fraction of the Adria-Europe convergence is still being accommodated in the Eastern Alps. However, the historical seismicity records along the easternmost segment of the PAF are ambiguous and instrumental records indicate that seismotectonic deformation is mostly concentrated in the adjacent Southern Alps and adjacent Dinarides. Both Electron Spin Resonance (ESR) and Optically Stimulated Luminescence (OSL) dating methods can be used as ultra-low temperature thermochronometers. Due to their dating range (a few decades to ~ 2 Ma) and low closure temperature (below 100°C), the methods have the potential for dating shear heating during earthquakes in slowly deforming fault zones, such as the PAF. Since the saturation dose of the quartz ESR signals is larger than quartz and feldspar OSL, ESR enables establishing a maximum age of the events (assuming the resetting during seismic events was at least partial), while OSL allows finding their minimum age when the signal is in saturation. We collected fault gouge samples from 3 localities along the easternmost segment of the PAF. For ESR, we measured the signals from the Al center in quartz comparing the results from the single aliquot additive dose (SAAD) and single aliquot regenerative (SAR) protocols. For OSL, we measured the Infrared Stimulated Luminescence (IRSL) signal at 50°C (IR₅₀) and the post-IR IRSL signal at 225°C (pIRIR₂₂₅) on potassium feldspar aliquots. Our dating results indicate that the studied segment of the PAF system accommodated seismotectonic deformation within a maximum age ranging from 1075 ± 48 to 541 ± 28 ka (ESR SAR) and minimum ages in the range from 196 ± 12 to 281 ± 16 ka (pIRIR₂₂₅). The obtained ages and the current configuration of the structure suggest that the studied segment of the PAF could be considered at least as a potentially active fault.

Periadriatic Fault System

Eastern Alps

Electron Spin Resonance

Optically Stimulated Luminescence

Active Fault

This article deals with the Quaternary activity of one of the longest faults in the European Alps that has not had strong instrumental earthquakes. A recurring problem in characterizing past and present regional deformation patterns in many regions is constraining the timing of fault activity. Notwithstanding purely academic aspects, the identification and characterization of active faults (e.g., in terms of slip rates and recurrence rates of damaging earthquakes) are of paramount importance from a societal perspective to assess the regional seismic hazard. Instrumental seismicity does not necessarily help to identify active faults, and geodetic monitoring may fail to recognize active fault segments in case of locked faults and low deformation rates. The problem may be exacerbated by long or unknown recurrence rates of large earthquakes in regions with low overall deformation rates such as the Alps and the Dinarides (Cheloni et al., 2014; Hintersberger et al., 2018; Grützner et al., 2021; Oswald et al., 2022), or intra-continental regions (e.g., 1811–1812 New Madrid earthquakes; Tuttle et al., 2002; 2001 Gujarat earthquake; Rajendran et al., 2008 2016 Peterman Ranges Earthquake; King et al., 2021). To date, there is no single definition for an “active fault”. According to the review of Slemmons & McKinney (1977), the most accepted definition is “a structure that shows evidence of Holocene displacement”, which conflicts in terms of temporal resolution with faults that have recurrence intervals of tens of thousands of years (Machette, 2000; Grützner et al., 2016, 2017). Four main criteria presented in Slemmons & McKinney (1977) and originally proposed by Willis & Wood (1924) could be used to assess the activity of a structure. These are either (1) offset during the present seismotectonic regime, (2) the potential for future offset, (3) evidence of recent activity, (4) and associated earthquake activity. Other definitions have been provided by the U.S. Nuclear Regulatory Commission (1997), which classifies a tectonic structure as of interest for seismic hazard when there is geological evidence for recurring deformation within approximately the last 0.5 Ma, at least once in the past 50 ka, or when it has been involved in deformation associated to the current tectonic regime. The International Atomic Energy Agency (2010) proposes guidelines to distinguish active structures depending on the tectonic setting, considering an Upper Pleistocene-Holocene time frame in interplate regions and a Pliocene-Quaternary time frame in intraplate regions.

Since instrumental data acquired via seismometers, satellite images, and Global Satellite Navigation Systems (GNSS) is only available for the last few decades, dating of recent brittle deformation can help determine whether a fault is (potentially) active. The most common techniques for finding the age of brittle deformation episodes in fault zones are either dating deformation associated with earthquakes in displaced or deformed soft sediments with luminescence or radiocarbon methods (e.g., McCalpin, 2009; Kondo et al., 2022), dating aseismically formed precipitations such as calcite veins and fibers using U/Th (e.g., Nuriel et al., 2012), or K/Ar dating of authigenic illite in fault gouge (e.g., Kralik et al., 1987; Pleuger et al., 2012; Torgersen et al., 2015; Ring et al., 2017; Vrolijk et al., 2018). These techniques either suffer from temporal limitations or rely on relative means to date the deformation episodes.

Both Electron Spin Resonance (ESR) and Optically Stimulated Luminescence (OSL) dating have been conventionally applied for dating sediments (e.g., Aitken, 1985; Schellmann et al., 2018; Roberts & Jacobs, 2018; Richter et al., 2022), and recently developed as ultra-low temperature thermochronometers (e.g., Guralnik et al., 2015; King et al., 2020). These methods utilize trapped charges within crystal lattice defects in minerals and measure a signal proportional to the amount of trapped electrons accumulated over time by exposure to natural irradiation (Rink, 1997; Aitken, 1998). The trapped charges can be released either during periods of increased temperature due to shear heating or due to the breaking of atomic bonds during frictional grain comminution (Fukuchi, 1992; Yang et al., 2019). Therefore, they potentially allow to directly date seismotectonic deformation in fault zones at near-surface conditions (e.g., Ikeya et al., 1982; Fukuchi 2001).

In this work, we present the first attempt to apply direct dating methods to constrain active deformation along the Periadriatic Fault system (PAF) in the European Alps by combining ESR and OSL dating on quartz and feldspar retrieved from fault gouges sampled in the core zone of the structure. The PAF represents an orogen-scale post-collisional fault system that was active mostly in the Oligocene to Miocene, but whose Quaternary activity is not well understood. Large instrumental earthquakes did not occur on this fault system, and historical earthquake records are ambiguous (Hammerl et al., 1994; Guidoboni et al., 2019; Caracciolo et al., 2021). Given the potential of the ESR and OSL dating methods to directly constrain the timing for seismogenic faulting episodes in the Quaternary, especially for slow-moving faults which are potentially still active, the present configuration of the PAF provides an opportunity to study the last seismogenic activity. For ESR, we compared the measurements obtained with the Single Aliquot Regenerative protocol and the Single Aliquot Additive Dose protocols. Our findings indicate that at least a partial reset of the system was achieved during the Quaternary due to seismic activity along the sampled portions of the structure.

The PAF system is an orogen-scaled array of post-collisional, predominantly dextral, and mostly transpressive strike-slip faults in the European Alps. It formed in response to the NW-ward motion and counterclockwise rotation of the Adriatic plate with respect to Europe during the Alpine orogeny (Schmid et al., 1989; Fig. 1). The fault system can be traced for about 700 km along-strike the entire width of the orogen, starting from the N of Turin, Italy, to the Pannonian Basin in Hungary where it becomes buried below the Plio-Pleistocene sediments of the basin. The eastern PAF system (east of the Giudicarie Fault) also represents the division between the high-grade metamorphic Austroalpine units to the north and the low-grade South Alpine units to the south (Fig. 2). A suite of calc-alkaline intrusive bodies, usually referred to as Periadriatic plutons, delimits the PAF. These plutons are considered to have resulted from a magmatic episode triggered by slab breakoff of the southward subducting European plate during the Eocene-Oligocene (von Blanckenburg & Davies, 1995; Rosenberg, 2004).

According to different estimates, the PAF could have accommodated between 150 and 300 km of displacement between about 35 to 15 Ma (Laubscher, 1971; Schmid et al., 1989; Schmid & Kissling, 2000; Handy et al., 2005). The structure contributed to lateral extrusion in the Eastern Alps towards the Pannonian Basin during the late Oligocene-Miocene (Ratschbacher et al., 1991b). The northward motion of the southern Alpine indenter (the rigid northern tip of the Adriatic microplate) between 22 to 17 Ma led to an offset on the PAF by the NNE-SSW-trending sinistral Giudicarie Fault, modifying the originally straight E-W trend of the structure defined by the Insubric and Pustertal-Gailtal segments (e.g., Viola et al., 2001; Pomella et al., 2012; Sieberer, 2023; Fig. 1a). The minimum age for the offset has been established by K-Ar dating targeting clay minerals in fault gouges by Pleuger et al. (2012), which indicate displacement along the segment at around 17 Ma. This offset was simultaneous with south-directed thrusting in the eastern Southern Alps (Schönborn, 1999; Castellarin & Cantelli, 2000). The post-Oligocene displacement along the PAF segments east of the Giudicarie Fault likely did not exceed about 20 km (Ustaszewski et al., 2008), and was concurrent with at least 40 to 50 km N-S convergence accommodated along S-directed thrusts in the easternmost Southern Alps in Friuli (Nussbaum, 2000; Merlini et al., 2002; Moulin & Benedetti, 2018).

The Neogene deformation along the Pustertal-Gailtal segment of the PAF system has a prominently dextral transpressive character, especially in the region of the Karawanken Mountains (Polinski and Eisbacher, 1992; Nemes et al., 1997; Fodor et al., 1998; Vrabec and Fodor, 2006). In this segment, shortening is accommodated by slip partitioning between dextral strike-slip faults and N- and S-directed thrusts (Fig. 2). On the northern edge of the Karawanken Mountains, Mesozoic successions overthrust the Middle Miocene to Pliocene sedimentary infill of the Klagenfurt basin (Polinski & Eisbacher, 1992; Nemes et al., 1997). Apatite (U-Th-Sm)/He ages obtained along the fault fall between around 12 and 5 Ma and indicate Late Miocene to Pliocene cooling below 110–50°C, likely triggered by fault-related basement exhumation (Heberer et al., 2017). K-Ar dating of clay minerals in fault gouges hints at displacement along the structure in the westernmost part of the segment (Mauls, Italy) at around 16 Ma (Zwingmann & Mancktelow, 2004).

Available GNSS data (D’Agostino et al., 2008; Métois et al., 2015; Serpelloni et al., 2016; 2022; Sánchez et al., 2018 a, b) suggest that part of the present NW-SE convergence between stable Adria and Europe is still being accommodated along contractional faults in the Southern Alps and across the eastern PAF (Fig. 3). However, the seismic moment release along the eastern PAF system is considerably smaller than in the Southern Alps of Friuli, located south of the structure (Serpelloni et al., 2016; Reiter et al., 2018). It is also possible that this segment of the PAF system hosted a few strong historical earthquakes, such as the Mw∼7.0 event in 1348 and the Mw∼6.6 event in 1690 (AHEAD Group; Guidoboni et al., 2019), both with epicenters in Carinthia. However, recent studies place the epicenter to the south of the PAF towards Friuli (Hammerl, 1994; Caracciolo et al., 2021).

ESR and OSL dating methods are novel techniques that were developed and applied within the last few decades. They rely on the accumulation of unpaired electrical charges within traps (i.e., paramagnetic lattice defects) in mineral grains that can be released by different stimuli. These unpaired electrons are produced over time by the natural decay of radiogenic nuclides such as (²³⁸U and ²³²Th, their daughter nuclides, and ⁴⁰K). ESR was initially discovered by Zavoisky (1944; 1945), with the earliest attempts to use it as a geochronometer made by Zeller (1968), and the first successful dating application by Ikeya (1975). OSL was developed later by Huntley et al. (1985) and employed as a method for dating sediments (e.g., Aitken, 1998). In this section, we present a brief review of previous work dealing with the methods applied to date seismotectonic deformation, with an emphasis on ESR since it has been more widely tested and used as an absolute geochronometer to date deformation than OSL. Generally, works targeting active faults consistently indicate at least a partial system reset of the system.

Ikeya et al. (1982) and Miki & Ikeya (1982) for the first time studied the potential of ESR to constrain the timing of recent earthquakes in geologic timescales, using quartz grains from fault zone cores. Fault gouges are a suitable lithological target for the method because their mineral grains experience frictional heating and grain size comminution directly at the fault core during seismic slip (Sibson 1977, 1986; Heitzmann 1987; Fettes & Desmons, 2007). Both processes have been determined as necessary for a system reset. Originally, Miki & Ikeya (1982) argued that during deformation and frictional grain comminution, the E’ center in quartz would experience a reset or “annealing” due to dislocation creep or the breakup of molecular bonds (Buhay et al., 1988). Posteriorly, additional experimental evidence in favor of this mechanism was provided by other authors while discussing the same effect in the Al, OHC, and Ge centers (e.g., Schwarcz et al., 1987; Lee & Schwarcz, 1993; Lee & Schwarcz, 1994a). Shear heating also plays an important role in seismic slip as a mechanism to dissipate energy, which could lead to zeroing of the signals during an earthquake (e.g., Fukuchi, 1988; 1989; 1992). Experimental studies have determined that temperature could be the most influential factor for zeroing the signal from the Al center during a seismic event (Yang et al., 2019).

Establishing whether the system was totally or partially reset during a past seismic event is crucial to determine the significance of the obtained ages. The multiple-center and grain size plateau approaches were developed to aid this assessment. The multiple-center method was originally introduced by Fukuchi (1988) and relies on the assumption that discrepancies in the ages obtained for different centers could be the product of the uneven reset of the signals during posterior fault movements. Therefore, the agreement between ages obtained from different centers is considered a criterion for a complete reset of the system during the last movement along a fault.

The grain-size plateau method was proposed by Schwarcz et al. (1987), and further developed and applied in subsequent studies (e.g., Lee & Schwarcz, 1994b; Lee, 1994). Its basic principle states that after experiencing frictional deformation, grains below a critical radius would become fully reset. Thus, the distribution of ages from different grain size fractions would resemble a plateau ranging from younger ages in smaller particles to older ages in larger ones. The experimental basis for this distribution was initially presented by Buhay et al. (1988) and later expanded by other authors (Lee & Schwarcz, 1993; Lee, 1995; Lee & Schwarcz, 1996). An additional criterion for complete zeroing of the signal during an earthquake was presented by Lee & Schwarcz (1994b), assimilating the basic principle behind the multiple-center method. This consisted of measuring the distribution from different signals and observing if the youngest ages from at least two plateaus converge, meaning that said age could be interpreted as the time of the last activity of the fault.

ESR has been applied for dating seismotectonic deformation since the 1980s. The studied faults were usually active structures and the obtained ages corresponded to the maximum of the last complete system reset. The earliest applications of ESR to date seismogenic activity using minerals extracted from fault gouges were made in Japan. Ikeya et al. (1982) targeted the Atotsugawa Fault (Seismic slip in 1858; Takeuchi et al., 2003), where they obtained a Pleistocene age of seismic activity (~ 65 ka) measuring the E’ center in quartz. Later, Fukuchi et al. (1986) applied the method to the Itoigawa-Shizuoka Tectonic Line, measuring signals from the OHC, Ge, Al, and E’ centers in quartz. The obtained activity ages fall between 0.3–0.55 Ma and 1.5-2 Ma for different segments of the structure and generally agree with geological studies indicating activity during the Pleistocene. In a follow-up study, Fukuchi (2001) analyzed fault gouges from the Nojima Fault in Japan obtained from a core at a depth of 388.4 m, measuring the E’ and peroxy signals from quartz. Ages for seismogenic activity between 0.15–0.29 Ma and 0.39–0.73 Ma were obtained from different fault planes. However, since the fault is still active (1995 Kobe/Great Hanshin Earthquake), the ages do not reflect the time of the last seismogenic movement. (Matsumoto et al. (2001) studied core samples from the Nojima Fault (389.4 m depth), observing a decrease in the signal intensity from the Al and E’ centers, attributed to shear heating partially annealing the signals during seismic events.

The method was applied outside of Japan for the first time by Buhay et al. (1988) in the San Jacinto Fault Zone – California, where an activity age of 75 ka was obtained using quartz from fault gouge by measuring the signals from the OHC, Al, and Ti centers. Although the age was interpreted as a possible overestimation of the actual age of fault activity, it was also in agreement with geological observations indicating activity through the Pleistocene. Lee (1994) studied the fault zones in the Underground Rock Laboratory in Canada using the plateau method, obtaining ages for the E’ center of around 475 ka, 404 ka, and 219 ka. Lee (1994) and Lee & Schwarcz (1996) applied the grain-size plateau method to the San Gabriel Fault Zone in California using the signals from the E’ and OHC centers. Their results range between 1.17 Ma to 40 ka, with a clustering of the age distribution interpreted as periods of activity and quiescence. Their analysis implies that the youngest ages were obtained from reactivated segments of the fault.

Contrary to ESR, direct applications of OSL for dating deformation associated with tectonic slip using minerals from fault gouges are scarce. The method has been applied as a geothermometer by using the sensitization effect of quartz in two separate studies to study the annealing effect related to shear heating during seismic episodes. Rink et al. (1999) analyzed different grain-size fractions of quartz from fault gouge obtained from the Limerock Fault in California and observed a more effective zeroing in the finer grain-size fractions indicated by a sensitization effect in the OSL signal from the quartz. Spencer et al. (2012) studied drill core gouge samples (2,604 m depth) from the San Andreas Fault zone, using IRSL and thermoluminescence (TL) signals on poly-mineral and feldspar aliquots. Their dating results show a dispersion in ages ranging from decades to centuries, and temperature estimates for a system reset associated with rupture events in the range of 300–400°C. Tsakalos et al. (2020) dated with OSL gouge samples from the Nojima Fault Zone in Japan. Specifically, quartz grains in the grain size fraction of 4–11 µm from core (506.6 m depth) and surface samples were measured. The results indicate that at least a partial reset occurred during a past seismic event, with the most recent one taking place between 63 ka to 19 ka for the Nojima Fault, and 139 ka to 46 ka for the Asano fault. Since the fault zone is active, and the most recent seismic activity occurred in 1995 (M_w 6.9), their ages were interpreted as the maximum age of the last complete resetting event.

Suitable sampling localities were scouted using geological maps from the study area delimited in Fig. 1 (Rockenschaub & Nowotny, 2011 in Geologischen Österreich Bundesanstalt, 1989; Anderle, 1977a, b in Geologischen Österreich Bundesanstalt, 1989; Geološki zavod Slovenije, 2003; Bigi et al., 1990a, b) and several digital elevation models of spatial resolutions ranging from 30 m to 1 m (SRTM: NASA JPL, 2013; LIDAR: ARSO (2021); KAGIS, 2020). In total, three localities along the eastern PAF were studied, with their respective location details reported in Table 1 and the geology summarized in Fig. 4.

The first sampling locality is Mauls in Italy, to the east of the transition between the Giudicarie and the Pustertal-Gailtal segments of the PAF. In the area, the PAF marks the division between the high-grade metamorphic Austroalpine units to the north and a tonalite body belonging to the Periadriatic plutons in the south (Fig. 4b). The sampled outcrop contained gray to greenish cataclastic tonalites within a fault plane oriented 015/55 (nomenclature here and in the following: dip azimuth/dip angle) with slickensides indicating dextral slip (302/28) (Fig. 5a). The main fault plane contained ca. 1 m wide, pale gray fault gouge produced by the comminution of the tonalites (Fig. 5b). The second sampled locality is in Finkenstein, Austria. There, the PAF separates the Periadriatic tonalitic lamellae from limestones belonging to the South Alpine units, and it is overlain by potentially displaced Miocene sedimentary units (Fig. 4c). The sampled outcrop consisted of a body of limestone from the South Alpine units surrounded by blocks of tonalite. At the location, the fault core hosted a dark gray to green fault gouge between tonalites and limestones (Fig. 5c). The third locality is in Plešivec, Slovenia. Here, the PAF placed a tonalitic lamellar body in contact with Triassic limestones from the South Alpine units (Fig. 4d). In the outcrop, two fault planes were hosting a fault gouge within cataclastic tonalites in the vicinity of the contact with the limestones to the south (Fig. 5d-e). The fault planes were oriented 155/73 (Fig. 5e-f), consistent with the regional orientation of the PAF at map scale. The gouge exhibited a similarly oriented mm- to cm-spaced foliation oriented 140/65, forming a compositional layering defined by changes in coloration and possibly mineral components.

To sample the material in the field, we first removed the outermost, weathered portion of the gouge under direct sunlight exposure from each outcrop. Unweathered material was then collected using stainless steel cylinder tubes of 4.2 cm diameter and 15 cm length to avoid sunlight exposition. We collected additional material to determine the natural dose rate via gamma-ray spectrometry. The samples were collected at least 30 cm away from the adjacent wall rock to simplify the assumptions for the dose rate calculation. In cases where the fault core width was < 1 m, a dedicated wall-rock sample was collected for an extra dose rate calculation.

5.1 Sample preparation

All samples were prepared at the facilities of the Leibniz Institute for Applied Geophysics (LIAG) – Hannover, in a dark laboratory under red light conditions to avoid optical bleaching of the signals. The coarse-grained (> 40 µm) portions of the samples were separated by decantation. Then, the coarse-grained fractions were dry-sieved to divide them into different grain-size ranges. Later, the grain-size fractions between 100–250 µm (63–300 µm in cases where the amount of material was considered insufficient) were selected for chemical treatment. Each sample was treated with sodium oxalate, 30% hydrochloric acid, and 30% hydrogen peroxide. The resulting material was then separated into groups of mineral phases using heavy liquid solutions of Na-polytungstate with densities between 2.62 g/cm³ − 2.7 g/cm³ for quartz and below 2.58 g/cm³ for K-feldspar. Afterward, the fraction corresponding to quartz was etched using 40% hydrofluoric acid for one hour, and any resulting carbonate precipitate was removed with 20% hydrochloric acid. Finally, the resulting quartz and the fraction corresponding to potassium feldspar were sieved separately into three grain-size ranges corresponding to 100–150 µm, 150–200 µm, and 200–250 µm.

5.2 Gamma-ray spectrometry sample preparation and dose rate measurements

The samples used to obtain the natural dose rates were prepared and measured at the facilities of LIAG. The initial material was dried at 110°C for 24 hours to obtain the natural water content by comparing the weight of the samples before and after drying. We assumed a 5% error in the natural water content. Then, the grain size was mechanically reduced to approximately 0.1 mm with a jaw crusher. Finally, 700 g of the resulting material was deposited in a sealed plastic container for four weeks to ensure equilibrium between ²²²Rn and its daughter nuclides.

An estimation of the radionuclide concentrations (²³⁸U, ²³²Th, and ⁴⁰K) was obtained by measuring the samples with an Ortec N-type HPGe gamma spectrometer. The dose rates for quartz and feldspar were calculated using the compositions and the conversion factors from Liritzis et al. (2013), the beta absorption factors from Guerin et al. (2012), the etching factor for beta absorption from Brennan (2003), the absorbed dose fraction of Rb from Readhead (2002), and the cosmic ray calculation from Prescott & Hutton (1994). We assumed no Rn loss and an etching depth for quartz of 10 µm. For feldspar, we assumed a 12.5 ± 0.5% K content to calculate the internal dose rate (Huntley & Baril, 1997). In cases where an extra wall rock sample was measured, we calculated the resulting dose rate following the procedure after Aitken (1985).

5.3 ESR measuring parameters and protocols

The ESR measurements were carried out using an X-band JEOL FA100 series spectrometer. The aliquots consisted of 60 mg quartz in the grain size range of 100–150 µm contained in quartz glass tubes of 2 inner mm diameter. During the measurements, the microwave power was set to 10 mW using a 100 kHz modulation, with magnetic sweeps of one minute and a sweep width of 30 mT. To obtain the spectra from the Al center, we measured three accumulations under temperatures of -160°C and − 156.5°C. The intensities from the Al center were calculated from the first positive peak (g ~ 2.02) to the last negative peak of the spectra (g ~ 1.99) and were then normalized using the 6th line of the Mn marker (g ~ 1.88), permanently inserted in the cavity. The Ti center could also be observed under the same conditions, but it was not recorded due to a very weak intensity. To consider the crystallographic anisotropy of quartz, we measured the intensities from three measurements after rotating the aliquot 60°. Finally, the mean intensity was calculated, and the uncertainty was taken as ± 1σ standard error.

The aliquots were irradiated using a custom-built X-ray radiation machine (Oppermann & Tsukamoto, 2015) with a maximum energy of 50 keV, providing a dose rate of 0.302 ± 0.014 Gy/s (Tsukamoto et al., 2021). Following the single aliquot regenerative (SAR) protocol established by Tsukamoto et al. (2015), the aliquots were preheated after laboratory irradiation to eliminate the thermally unstable portion of the signal. Measurements applying the single aliquot additive dose (SAAD) protocol were also carried out. Both protocols used to obtain the equivalent doses (D_e) are summarized in Fig. 6 and explained in detail with the performed quality tests in the following sections.

5.3.1 SAR protocol

For this protocol, we compare a laboratory-generated curve with the natural intensity measured from an aliquot. After the initial natural intensity measurement, the aliquot was annealed by subjecting it to a temperature of 420°C for 4 minutes. After that, several steps of irradiation, preheating, and measurements were performed until the saturation intensity was reached (i.e., the intensity at which the addition of further laboratory dose increments does not yield a stronger signal). Then, the residual intensity remaining after the initial annealing step was subtracted from each measurement, and the dose-response curve (DRC) was constructed by fitting the data points to a single (Eq. 1) or double-saturated exponential curve (Eq. 2), where the I parameters correspond to the saturation intensities and D₀ describes the characteristic saturation doses. The appropriate function was chosen based on the best fit to the dataset.

(1)\(f\left(x\right)=I(1-{e}^{\frac{-x}{{D}_{0}}})\)

(2)\(f\left(x\right)={I}_{1}\left(1-{e}^{\frac{-x}{{D}_{0a}}}\right)+{I}_{2}(1-{e}^{\frac{-x}{{D}_{0b}}})\)

Several D_e were obtained by running a Monte-Carlo simulation of 2000 repetitions to fit the dose-response curves to randomized data points within the standard error of the measurements. In each iteration, the natural intensity was projected onto the curve to obtain a D_e estimate. Then, the distribution of the values was analyzed to obtain the D_e, and the uncertainty was considered as the median absolute deviation of the distribution.

5.3.2 SAAD protocol

In this protocol, we calculated the accumulated dose since the last reset of the system, when the signal intensity is assumed as zero, using the current (natural) intensity. After measuring the natural intensity, several additional steps of irradiation, preheating, and measurements were subsequently performed. The intensity after annealing was subtracted from each measurement. Once the saturation intensity was reached, the dose-response curve was constructed by fitting the obtained data points to a single saturated exponential curve (Eq. 3), where I corresponds to the saturation intensity, c is a correlation coefficient, and D₀ corresponds to the dose at which saturation starts.

(3)\(f\left(x\right)=I-{ce}^{\frac{-x}{{D}_{0 }}}\)

The D_e was obtained by running a Monte-Carlo simulation of 2000 repetitions to fit the curve to randomized data points within the standard error of the measurements. In each iteration, the curve was extrapolated to the X-axis, where the intensity is zero, to obtain an estimate for the D_e. Afterward, the distribution of the values was analyzed to obtain the D_e, and the uncertainty was taken as the median absolute deviation of the distribution.

5.3.3 Preheating conditions test

A preheat conditions test was carried out to verify the temperature necessary to remove the thermally unstable part of the signal introduced after laboratory irradiation. For this, natural and laboratory-irradiated (~ 3,000 Gy) aliquots from the same sample were selected. Both were initially measured without preheating (room temperature preheat). Subsequent measurements were carried out after heating steps of 4 minutes at temperatures of 130°C, 160°C, 190°C, 220°C, and 250°C. After each heating step, the intensity from the Al center was calculated and plotted against the preheating temperature, comparing the curves from both aliquots.

5.3.4 Isothermal annealing test

For the isothermal annealing test, four aliquots from the same sample were annealed at 420°C for 4 minutes and then irradiated to a dose of around 5,000 Gy. The aliquots were preheated to remove the unstable portion of the signal, and the initial intensity of the Al center was measured for each one. Then, each aliquot was heated for 4 minutes at a different temperature corresponding to 160°C, 190°C, 220°C, and 250°C, and the corresponding Al intensity was measured posteriorly. Successive steps of heating and measuring were carried out, maintaining the same heating temperature for each aliquot but doubling the heating time from the previous iteration until reaching a heating time of about two hours. After the measurements were finished, each intensity was normalized to the initial from the respective aliquot. We processed the data using the ESR MATLAB toolboxes after King et al. (2020). The curves were obtained by plotting the normalized intensities against the total annealing time for each temperature and fitting the data to the Gaussian model after Lambert (2018). The lifetime (\(\tau\)) was calculated using the parameters from the fit and the Eq. 4, where e⁻ corresponds to the charge of an electron in eV, k_{B is} the Boltzmann constant, E_t is the activation energy in eV, S is the frequency factor or electron escape probability in s^− 1, and T is the ambient temperature in K.

(4)\({\tau }_{T}={s}^{-1}{e}^{\frac{{E}_{t}*{e}^{-}}{{k}_{B}*T}}\)

5.3.5 Dose Recovery Test

To test whether the annealing step in the SAR protocol induces a sensitivity change, a dose recovery test was conducted. A recovery test in quartz ESR dating was originally introduced by Beerten et al. (2008). However, they heated the sample at 500°C for 1 hour before a dose was given, and its effectiveness to monitor any sensitivity change on natural aliquots in the SAR protocol was unclear. Tsukamoto et al. (2017), proposed a dose recovery test using modern sediment samples, and a similar approach was applied in subsequent studies (Richter & Tsukamoto, 2022; Richter et al., 2022). In this study, we followed a modified procedure proposed by Toyoda et al. (2009) it as a regenerative-additive method and demonstrated as a means for the dose recovery test by Fang & Grün (2020) and Richter et al. (2022). For this, the intensities measured using the SAAD protocol were compared to the calculated DRC from the SAR protocol. Apparent equivalent doses were calculated by projecting the SAAD intensities onto the SAR curve. The apparent equivalent doses were then compared to the added dose from each SAAD protocol measurement following the regenerative-additive dose plot. The slope of this plot is equivalent to the dose recovery ratio, and if this is close to unity, the SAR protocol is regarded to be robust. However, unlike the dose recovery test in luminescence dating, in which the dose recovery test is conducted using laboratory-bleached aliquots, it is difficult to reset the quartz ESR signals without heating the aliquots before a given dose is administered. Therefore, our dose recovery test may be affected by the signal saturation (i.e., the slope may not be linear).

5.4 OSL equipment, protocol, and data processing

For OSL, we used K-feldspar aliquots with a diameter of 2 mm and material in the grain size fraction of 100–150 µm, mounted on stainless steel disks with a diameter of 9.80 mm. The measurements were recorded with a Risø TL/OSL DA-20 reader available in the facilities of LIAG – Hannover. The reader is equipped with a beta ⁹⁰Sr/⁹⁰Y source that provides a dose rate of 0.119 ± 0.006 Gy/s. The light detection system of the reader includes a photomultiplier tube and blue detection filters (Schött BG-39 and Corning 7–59) of 380 and 460 nm. We used an extra neutral density filter (ND 1.0) in cases where the light level was too bright for the detector. For acquisition and initial processing, we used the Risø software suite. The R luminescence package after Kreutzer et al. (2012) was used to calculate the fading rate and corrected ages.

5.4.1 SAR Protocol

We measured the post-infrared infrared-stimulated luminescence signal at 225°C (pIRIR₂₂₅) after Buylaert et al. (2009) and the infrared-stimulated luminescence signal at 50°C (IR₅₀) after Thomsen et al. (2008) using a SAR protocol summarized in Table 2. The first step of the procedure consisted of preheating the aliquots at a temperature of 250°C for 60 seconds. Next, the measurements were carried out during a first IR stimulation for 100 seconds at 50°C, followed by a second IR stimulation for another 100 seconds at 225°C. Both included an initial dead time of 10 seconds. For all samples, a natural measurement was taken, and further measurements were recorded after giving doses of approximately 0 Gy, 110 Gy, 220 Gy, 560 Gy, 1130 Gy, and 2260 Gy. At the end of each run, we irradiated and measured a test dose of 90 Gy to correct the sensitivity. At the end of the whole set, a recycling dose of around 240 Gy was irradiated and measured to verify the validity of the sensitivity correction by the test dose signal.

The acceptance of the measurements was evaluated using two criteria after Murray & Wintle (2000). The first one corresponds to the recycling ratio, which is calculated by dividing the sensitivity-corrected intensity of the recycling dose measurement by the corresponding intensity obtained at the same dose value during the dose-response curve measurement run. An acceptable recycling ratio should be close to 1, with a margin of error of 10%. The second consists of the recuperation rate, which was obtained by dividing the sensitivity-corrected intensity of the zero-dose measurement by the sensitivity-corrected natural intensity. The recuperation rate should not exceed 5%.

The initial dose-response curves were constructed by fitting the data from at least three measured aliquots from each sample to a single saturated exponential function (Eq. 5), where I corresponds to the saturation intensity and D₀ is the dose at which saturation starts. A preliminary D_e was calculated for each sample using at least four natural measurements. The uncertainty corresponded to the standard error in the measurements. A fading test was performed using three aliquots from each sample. These were bleached and posteriorly irradiated with a known dose. Then, the signals were measured at different time intervals after the irradiation steps. The fading test data was processed using the R functions after Kreutzer et al. (2012) to calculate the g_{2 days} and recombination center density (Huntley, 2006). Finally, a correction after Kars et al. (2008) was applied using the parameter obtained from the fading test and the R functions after Kreutzer et al. (2012) to simulate the natural behavior of the dose-response curves considering the fading of the signals and then to calculate the corrected D_e.

(5)\(f\left(x\right)=I(1-{e}^{\frac{-x+c}{{D}_{0 }}})\)

When the effects of fading in the laboratory DRC are corrected, the saturation points of the curves usually occur at lower intensity and dose values, making it possible for DRC from samples with older ages to be considered in saturation. This can occur when the D_e cannot be determined because the natural intensity is larger than the saturation intensity of the DRC, or when the obtained D_e is larger than the D₀ of the DRC. In the second case, the saturation level can be estimated by dividing the natural intensity by the saturation intensity of the DRC.

The reliability of the applied protocols was confirmed by implementing a dose recovery test carried out using six aliquots from each sample. All the aliquots were previously bleached for four hours using a Hönle UVACUBE 400 solar simulator. To test the dose recovery ratio, half of the bleached aliquots were then given a dose similar to the natural D_e, and subsequently, the measurement protocol described above was re-applied. The residual dose after bleaching was measured using the other half of the aliquots. Finally, the dose recovery ratio was calculated by subtracting the residual dose from the irradiated D_e and then dividing the value by the irradiated dose (Eq. 6)

(6)\(DRR=\frac{{D}_{e Irradiated}-Residual}{{D}_{Irradiated}}\)

5.5 Age calculation

We calculated the ages following the standard procedure in trapped charge dating. Each D_e was divided by the respective dose rates for feldspar in OSL and for quartz in ESR (Eq. 7). The uncertainty of the ages was calculated by propagating the error from the dose rate and the D_e. For the samples that could be considered in saturation (i.e., D_e above two times D₀), we calculated a minimum age using the two times D₀ instead of the D_e (Eq. 8).

(7)\(Age=\frac{ {D}_{e } \left(Equivalent dose\right)}{\dot{D }\left(Dose rate\right) }\)

(8)\(Min. Age=\frac{ {2*D}_{0 } \left(Saturation dose\right)}{\dot{D }\left(Dose rate\right) }\)

6.1 Dose Rates

The calculated dose rates for feldspar fall between 1.92 ± 0.12 to 4.72 ± 0.21 Gy/yr, and for quartz between 1.34 ± 0.06 to 3.99 ± 0.18 Gy/yr. Since the dose rates calculated for feldspar depend on the internal K content, they are generally larger than the ones calculated for quartz. A complete list of the results, including the respective contents of radiogenic nuclides and water, is reported in Table 3. The dose rates from the Mauls and Finkenstein localities present overall lower dose rates than the samples from Plešivec, which is possibly the result of differences in mineralogy between the gouges associated with the lithologies involved in comminution.

6.2 ESR Dating

A strong signal from the Al center was detected in the measured ESR spectra of the samples, usually accompanied by a very weak signal from the Ti center (Orange spectra in Fig. 7). Due to this effect, the measurements were recorded only from the Al center. The aliquots from the Mauls sample (4384) presented a subtle Fe³⁺ contamination, observed as a long-wavelength undulation superimposed on the spectra (Blue spectra in Fig. 7). However, the effect of the overprint was not strong enough to completely mask the signal of the Al center, meaning that the intensity could still be calculated. The optimal preheat conditions were tested before all the measurements were taken. The results from the preheat plateau temperature test are displayed in Fig. 8a. The plateau shows that the Al center intensity for the natural aliquot starts decaying at temperatures larger than 190°C. On the other hand, there are two decays for the laboratory-irradiated aliquot. The first one starts after preheating at 100°C where the thermally unstable portion of the signal starts attenuating, and the second one occurs above 160°C where partial annealing of the signal starts. Our comparison of the laboratory-irradiated and the natural aliquot presented in Fig. 8a shows that the intensities at the 160°C preheat temperature are comparable between both aliquots. Moreover, the anomalous peak introduced by laboratory irradiation is effectively removed after preheating at 160°C (Fig. 8b), which confirms it as the optimal preheat temperature.

The SAR dose-response curves were constructed by fitting the data points according to the exponential function that presented the best fit for the data. Therefore, the measurements from the samples belonging to the Mauls and Finkenstein localities were fitted to a single saturated exponential function, while the results from the Plešivec samples were fitted to a double saturated exponential function. The dose-response curves of the measured aliquots from each locality using the SAR and SAAD protocols are presented in Fig. 9, with their respective fit parameters in Supplement 1. It can be noted from Fig. 9 that the DRC from the SAAD protocol had a considerable amount of interpolation to obtain the D_e. In general, the interpolation parameters of the DRCs from the SAR protocol measurements have lower uncertainties compared to the ones estimated from the SAAD protocol measurements.

Overall, the D_e obtained using the SAAD protocol are larger than the ones obtained with the SAR protocol. For the latter, all the D_e belong to measurements with intensities below the saturation points of the dose-response curves. The D_e obtained using the SAR protocol range between 1,205 ± 23 Gy and 3,538 ± 62 Gy, while for the SAAD protocol, they range between 1,664 ± 23 Gy and 3,626 ± 120 Gy. The difference in D_e values between the measurements from both protocols is reflected in the calculated ages. The SAR protocol ages range from 541 ± 28 to 1,075 ± 48 ka, while the ages obtained with the SAAD protocol span from 1,521 ± 67 to 530 ± 32 ka. A complete overview of the calculated De values, their respective ages using both protocols and the saturation level of the natural intensity is presented in Table 4.

For our samples, the applied ESR dose recovery test shows that the relationship between the apparent D_e and the added dose is sample dependent. In the case of the Mauls (4384) and Finkenstein (4427) samples (Fig. 10a, b), the relationship can be defined by a linear function with dose recovery ratios corresponding to 0.94 ± 0.04 and 1.07 ± 0.23 respectively. However, for both of the Plešivec locality samples (Fig. 10c, d), the dose recovery ratios correspond to 0.88 ± 0.32 (sample 4429) and 0.66 ± 0.13 (sample 4430), and the distribution of the points in the plot could also be described by a saturated exponential curve.

The results from the isothermal annealing test performed using aliquots from the Plešivec locality sample are presented in Fig. 11. From the fit to the Gaussian model, we obtained an activation energy of 1.20 ± 0.08 eV, an Urbach band-tail width of 0.16 ± 0.01 eV, and a frequency factor of 1.25*10⁹ ± 7.57 s^− 1. Using these values and assuming an environmental temperature of 10°C, the calculated lifetime of the Al center corresponds to 4.23*10⁷ Ma.

6.3 OSL Dating

From the four available samples, three could be analyzed with OSL due to the lack of potassium feldspar in the sample from the Finkenstein locality (sample 4427). For the rest of the samples, sufficient material was available so that the IR₅₀ and pIRIR₂₂₅ signals could be successfully measured. After correcting the initial DRCs to consider the effect of anomalous fading, the simulated DRCs from both measured signals show a D_e either in saturation or very close to the saturation state (Fig. 12). The saturation rates for all measured samples exceed 78% (Table 5), confirming that most samples can be considered in saturation for the method (i.e., D_e>2D₀ or saturation rate > 85%). However, the DRCs allow to determine minimum ages using two times the characteristic saturation dose (2D₀). The results of the dose recovery test are presented in Table 5 and indicate good quality of the measurements, with recovery rates larger than 85% for both signals.

The obtained D_e from each OSL signal with the respective characteristic saturation doses of the DRCs and the calculated minimum ages are reported in Table 5, and the complete list of parameters of the DRCs with the respective Rho’ and g_{2 − days} used for the fading correction are available in Supplement 3. The measurements of the pIRIR₂₂₅ signal show DRCs in saturation, while the IR₅₀ signal D_e are close to the saturation state. For the Mauls sample, the pIRIR₂₂₅ signal presents a D₀ of 188 ± 1 Gy, while for the IR₅₀ signal, the D₀ is 322 ± 1 Gy. For the IR₅₀ signal, the D_e is 495 ± 28 Gy. In the samples from the Plešivec locality, the characteristic saturation doses are relatively close to each other. For sample 4429, the D₀ of the pIRIR₂₂₅ and IR₅₀ signals overlap within the margin of error with values of 620 ± 2 Gy and 623 ± 23 Gy, respectively. For the IR₅₀ signal, the D_e is 942 ± 34 Gy. For sample 4430, the characteristic saturation dose of the pIRIR₂₂₅ signal is 660 ± 2 Gy, while the IR₅₀ signal presents a larger value of 696 ± 2 Gy. For the IR₅₀ signal, the D_e is 1,064 ± 25 Gy.

The ages from both signals are calculated using the 2D₀ value and treated as minimum ages. In the case of the pIRIR₂₂₅ signal, this is because the DRCs are in saturation, while for the IR₅₀ signal the DRCs are close but still below saturation level but a larger fading correction factor (g_{2 days}) implies that the D_e is less reliable in older samples. For the Mauls sample, the variation in D₀ values results in a large minimum age variation, where the age is 196 ± 12 ka from the pIRIR₂₂₅ signal and approximately 335 ± 21 ka from the IR₅₀ signal. The ages from both signals for the two samples in the Plešivec locality fall within the margin of error between each other. For sample 4429, the ages are 281 ± 16 and 279 ± 13 for the pIRIR₂₂₅ signal and the IR₅₀ signal, respectively, while for sample 4430, the ages are 280 ± 13 for the pIRIR₂₂₅ signal and 295 ± 13 for the IR₅₀ signal.

In general, the equivalent doses and ESR ages (Table 4) obtained with the SAAD and SAR protocols are not in agreement. The SAAD DRC in Fig. 9 present a large extrapolation that introduces more uncertainty at the time of the D_e calculation, especially when dealing with older samples that present natural intensities closer to the saturation point of the curve. This results in SAAD ages that, when compared to their SAR counterparts, are older and present a larger uncertainty. One of the advantages of the SAR protocol over the SAAD protocol is that for the SAR protocol, it is not necessary to extrapolate a large portion of the DRC to obtain the equivalent dose. Despite the difficulty to account for sensitivity changes during the annealing step when applying the SAR protocol, our results from the dose recovery in Fig. 10 test indicate that the curves are comparable, and therefore, the results from the protocol are reliable. Since the obtained thermal lifetime of the Al center for the analyzed sample is above 10 times the dating limit for the method (Aitken, 1985), we can assume thermal stability for dating samples of the last couple of million years.

Our results from the OSL dating show that the pIRIR₂₂₅ signals in all samples are in saturation (Table 5, Fig. 12). In the case of the IR₅₀ signals, the analyzed samples are close but below saturation. However, for older samples (i.e., close to saturation) the fading correction factor for the IR₅₀ signal is larger than for the pIRIR₂₂₅ signal (in Supplement 3), meaning that the corrected D_e are less reliable than those obtained for the pIRIR₂₂₅ signal. In general, the saturated state of the signals could be the result of a large recurrence interval combined with only low-magnitude events that do not fully reset the system. Another possibility would be the complete lack of events during the effective datable time range of the method. Therefore, we consider the ages calculated from the pIRIR₂₂₅ signal (Table 5) as the minimum ages of the events.

Due to the limitations of the SAAD protocol when dealing with old samples, we consider the ages obtained with the SAR protocol ages would to be more reliable than the SAAD ages. However, the shear heating was likely not sufficient to fully reset the system by the last earthquake (e.g., Fukuchi 1988), which may also be the case during lower-magnitude earthquakes. In that scenario, the ages would correspond only to the maximum age of the last large earthquake (Tsukamoto et al., 2020). A combined approach with OSL allows us to narrow down the temporal range of the events due to the lower saturation limit of the system compared to ESR (Fig. 13). When the OSL signals are in saturation, the method can be used to constrain a minimum age for the last seismogenic activity.

Overestimation in the ESR ages could be accounted for by applying the SAR protocol to measure the Al center signal in the different grain sizes or by comparing the ages from different centers. This has been proposed in the past as the multiple center and grain size plateau approaches (Fukuchi 1988; Schwarcz et al., 1987; Lee & Schwarcz 1994, Lee 1994). However, the signal from the Ti center in our samples is weak, and the centers such as E’ and OHC do not strongly depend on shear heating to be effectively reset compared to the Al center (Yang et al., 2019). In this case, the grain size plateau approach using the Al center could still be applied. Since we assume at least a partial reset of the system, establishing a temperature estimate for shear heating along the fault plane would provide additional evidence for seismic slip. A potential way to obtain such estimates is to observe the sensitivity changes in the OSL signal with increasing heating temperature (e.g., Rink et al., 1999; Spencer et al., 2012). Another possibility to obtain temperature estimates would be applying a Raman spectroscopy geothermometer for organic matter (e.g., Henry et al., 2019) on cataclasites and gouges from the fault zone.

In the case of the studied segment of the PAF, the obtained ages indicate that seismic slip occurred along the structure during the Quaternary with a maximum age ranging from 1075 ± 48 to 541 ± 28 ka (ESR SAR) and minimum ages in the range from 196 ± 12 to 281 ± 16 ka (pIRIR₂₂₅). The individual ages are relatively congruent between localities along the fault (Fig. 14), and their differences could be attributed to different amounts of partial annealing of the system or differences in the timing of seismic slip. Nonetheless, our findings indicate that the easternmost segment of the PAF very likely has a longer seismic lifespan than previously indicated by the K-Ar ages from gouges obtained by Zwingmann & Mancktelow (2004) from Mauls, and Apatite (U-Th-Sm)/He cooling ages from In the Karawanken Mountains of c. 13 − 4 Ma which hint to fault activity of at least Late Miocene to Pliocene age (Heberer et al., 2017).

Considering most of the definitions of what makes a fault considered active, the narrowed-down period of seismic activity we obtained for this segment of the PAF implies that by some definitions the fault cannot be considered active. Our ages cannot account for Holocene displacement (Slemmons & McKinney, 1977), and although our minimum limit suggests activity younger than 0.5 Ma it is not possible to confirm how recurrent it was (U.S. National Regulatory Commission, 1997). However, there are four reasons why it could be considered at least potentially active. (1) The Eastern PAF is characterized by low seismicity and low deformation rates, similar to what usually occurs in intraplate settings. According to the definition of the International Atomic Energy Agency (2010), the seismically active period we dated for the structure fits within what could be considered active for such settings. (2) Earthquake ages obtained with trapped charged methods are generally overestimated even for active faults (e.g., Fukuchi et al., 1986; Buhay et al., 1988; Tsakalos et al., 2020), and do not necessarily reflect the last time of seismic slip. (3) The Eastern PAF is still geometrically compatible with the present-day stress regime (Fig. 3), implying the potential for future offset, which is one of the criteria to identify active faults defined by several authors (Willis & Wood, 1924; Slemmons & McKinney, 1977; U.S. Nuclear Regulatory Commission, 1997). (4) Other structures under similar tectonic settings (i.e., low deformation rates and scarce to inexistent historical and instrumental seismicity records) have recently shown seismic activity (e.g., Provence region, France: Ritz et al., 2020; Thomas et al., 2020; Eastern US: Figueiredo et al., 2022), meaning that a long recurrence interval could hinder the interpretation of active and potentially active faults. Spooner et al. (2019) have shown that present-day seismicity in the Eastern Alps correlates with the thickness and the density of the crust. However, in the interior of the Alps strong historical earthquakes are documented (Stucchi et al., 2012), and additional pre-historical events have been identified in lake paleoseismology (Daxer et al., 2020, 2022a, b; Oswald et al., 2022) and cave studies (Baroň et al., 2022). These observations are in line with our findings–large earthquakes do occur in the interior of the mountain chain, but they are rare and our instrumental and historical records are too short to capture the whole picture of seismicity. In the regional tectonic context, an active PAF will likely accommodate a small fraction of the Adria-Europe convergence, probably with a component of right-lateral slip as during the period of Neogene lateral extrusion (Ratschbacher et al., 1991b).

Optically Stimulated Luminescence and Electron Spin Resonance allow dating seismotectonic deformation directly at near-surface conditions in places where deformation rates are low or instrumental seismicity does not help to identify active faults. A lack of large instrumental earthquakes on the PAF system, combined with an ambiguous historical earthquake record, made the structure a suitable target to apply the methods to unravel the timing of seismotectonic deformation produced by the fault during the past couple of Ma. In this work, we introduced the first combined systematic approach of ESR and OSL dating on quartz and feldspar from fault gouges to date active deformation. Furthermore, we systematically compared two available methodological approaches of ESR to establish a meaningful age. Finally, we presented the first successful attempt at absolute dating for active deformation in the Eastern Alps along the PAF.

Our dating results suggest that the structure accommodated seismotectonic deformation associated with Adria-Europe convergence during the Quaternary, with a maximum age in the range of 1075 ± 48 to 541 ± 28 ka (ESR SAR) and minimum ages in the range from 196 ± 12 to 281 ± 16 ka (pIRIR₂₂₅). Therefore, in places where activity is not so high, ESR dating on quartz could be considered the preferred method for unraveling periods of seismogenic faulting, while OSL can be additionally applied to constrain the minimum ages even when the signals are in saturation. Therefore, a combined approach of the methods allows to narrow down the timing of seismotectonic deformation during the Quaternary. Considering the different definitions of what constitutes an active fault, the shear heating ages we obtained, and regional observations of the geometrical compatibility with the current stress regime, it would be possible to classify the Easternmost segment of the PAF as a potentially active fault.

The comparison between methodological approaches indicates that the SAR protocol presents systematic advantages over the classic SAAD protocol. The main advantage is a shorter uncertainty of the DRC parameters and the D_e, both associated with the lack of extrapolation for a large part of the curve when using the SAR protocol. This especially affects aliquots with measurements that have a DRC closer to a saturated state. However, changes in sensitivity after annealing are still unaccounted for when the SAR protocol is employed. Therefore, we applied a dose recovery test to verify the validity of the D_e. Still, it is recommended to apply the SAAD protocol in parallel with the SAR protocol to confirm the saturation level of the samples.

Since the ESR system was very likely incompletely reset during earthquakes along the PAF, the ESR ages we obtained are considered the maximum age of the last complete reset of the system. Finer particles are considered more effectively annealed during an earthquake by shear heating. In that sense, the grain size plateau method could be applied to account for age overestimations. The combination of other geothermochronometers such as the sensitization effect with temperature in OSL (e.g., Rink et al., 1999; Spencer et al., 2012), or Raman spectroscopy on carbonaceous matter (RSCM, e.g., Kouketsu et al., 2013; Lünsdorf et al., 2017) applied to the fault gouges could provide more insight into how effective shear heating was along the PAF during seismic slip, and the peak temperatures experienced during shear heating.

DRC: Dose Response Curve

D_e: Equivalent Dose

ESR: Electron Spin Resonance

GNSS: Global Satellite Navigation Systems

IRSL: Infrared Stimulated Luminescence

OSL: Optically Stimulated Luminescence

PAF: Periadriatic Fault

SAAD: Single Aliquot Additive Dose

SAR: Single Aliquot Regenerative

TL: Thermoluminescence

Supplements

Supplement 1 (.xlsx). Supplementary data of the ESR dating results. Curve parameters of the DRC from the measured ESR spectra with the SAR and SAAD protocols.

Supplement 2 (.png). Supplementary data of the Monte Carlo simulations for ESR. Comparison of the equivalent dose distributions resulting from the Monte Carlo simulation with the SAAD (blue distributions) and SAR (red distributions) protocols.

Supplement 3 (.xlsx). Supplementary data of the OSL dating results. Fit parameters of the laboratory DRC from the pIRIR₂₂₅ and IR₅₀ signals, Rho’, g value after two days, and fit parameters of the simulated natural DRC.

Availability of data and materials

The authors declare all data supporting the study are available in the document and the supplementary files. The datasets used and/or analyzed during the current study are also available from the corresponding author upon reasonable request.

Competing interests

The authors declare there are no competing interests.

Funding

Project funding by the Deutsche Forschungsgemeinschaft (DFG), project number 442590718 awarded to ST and KU.

Authors' contributions

Author #1 Erick Prince: Samples/field data acquisition; acquisition, analysis, and interpretation of data; draft of the manuscript.

Author #2 Sumiko Tsukamoto: Conception and design of the work; Samples/field data acquisition; assistance with interpretation of data; revisions to the manuscript.

Author #3: Christoph Grützner: Samples/field data acquisition; assistance with interpretation of data; revisions to the manuscript.

Author #4: Marko Vrabec: Samples/field data acquisition; revisions to the manuscript.

Author #5: Kamil Ustaszewski: conception and design of the work; Samples/field data acquisition; assistance with interpretation of data; revisions to the manuscript.

Acknowledgments

Project funding by the Deutsche Forschungsgemeinschaft (DFG), project number 442590718 awarded to ST and KU is gratefully acknowledged. The authors thank Gerlinde Ortner (GeoPark Carnic Alps) for her logistic support during fieldwork on Austrian territory, the laboratory technicians at LIAG for their help and advice in sample preparation and gamma spectrometry, Georgina King (University of Lausanne) for providing the ESR MATLAB Toolbox, and the student helpers involved in the field campaigns.

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Tables are available in Supplementary Files section.

GrpahicalAbstract.png
Supplement1.xlsx
Supplement 1 (.xlsx). Supplementary data of the ESR dating results. Curve parameters of the DRC from the measured ESR spectra with the SAR and SAAD protocols.
Supplement2.png
Supplement 2 (.png). Supplementary data of the Monte Carlo simulations for ESR. Comparison of the equivalent dose distributions resulting from the Monte Carlo simulation with the SAAD (blue distributions) and SAR (red distributions) protocols.
Supplement3.xlsx
Supplement 3 (.xlsx). Supplementary data of the OSL dating results. Fit parameters of the laboratory DRC from the pIRIR₂₂₅ and IR₅₀ signals, Rho’, g value after two days, and fit parameters of the simulated natural DRC.
Table1.xlsx
Table 1. Coordinates of the sampled localities.
Table2.xlsx
Table 2. Description of the protocol used to measure the pIRIR₂₂₅ and IR₅₀ OSL signals in potassium feldspar.
Table3.xlsx
Table 3. Radiogenic nuclide compositions, water content, and dose rates calculated for the fault gouge samples.
Table4.xlsx
Table 4. Results from ESR dating measuring the SAAD and SAR protocols on Qtz aliquots.
Table5.xlsx
Table 5. Results from OSL dating measuring the pIRIR₂₂₅ and IR₅₀ signals on Kfs aliquots.

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Editorial decision: Major Revision
25 Sep, 2023
Reviewers agreed at journal
16 Aug, 2023
Reviewers invited by journal
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Editor assigned by journal
03 Aug, 2023
First submitted to journal
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You are reading this latest preprint version

Not too old to rock: ESR and OSL dating methods reveal Quaternary activity of the Periadriatic Fault

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Abstract

Figures

1. Introduction

2. The Periadriatic Fault System

3. ESR and OSL for dating active deformation

4. Study Area and Sampling

5. Methodology

5.1 Sample preparation

5.2 Gamma-ray spectrometry sample preparation and dose rate measurements

5.3 ESR measuring parameters and protocols

5.3.1 SAR protocol

5.3.2 SAAD protocol

5.3.3 Preheating conditions test

5.3.4 Isothermal annealing test

5.3.5 Dose Recovery Test

5.4 OSL equipment, protocol, and data processing

5.4.1 SAR Protocol

5.5 Age calculation

6 Results

6.1 Dose Rates

6.2 ESR Dating

6.3 OSL Dating

7 Discussion

8 Conclusions

Abbreviations

Declarations

References

Tables

Supplementary Files

Status:

Version 1