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Here we report luminescence dating results on exhumed fault rocks from the North Tehran Fault (NTF, Iran), including the fault gouges. The NTF is an oblique-dip thrust fault expanding from west to east at the northern fringe of the megacity of Tehran. Although the fault is known to be active and a major threat to the city, only little is known about its earthquake history. We present first data on the timing of past fault motion using dating of fault rocks. Three sites have been selected for this study in which the fault damage zone is associated with Eocene volcaniclastic rocks (Karaj Formation). The fault gouge, transitional and damage zone were sampled for which we observed slip planes in the field. The fault rocks are composed of quartz and alkali feldspar favourable for luminescence dating. However, our study reveals that the quartz optically stimulated luminescence (OSL) signal of the Karaj Formation does not seem to have a measurable fast decaying signal component. Instead, the post-infrared stimulated luminescence at 225 °C (pIRIRSL 225) of polymineral fine-grain fraction appears to be the most feasible signal for these samples. The first luminescence-based results presented here comprise an age range from 45 ± 5 ka to 72 ± 13 ka for fault gouges and transitional zone. Since dating the transitional zone close to the fault gouge led to a younger age than the fault gouge at one site, we conclude that the sampling location plays a crucial role in dating exhumed fault rock using luminescence dating. The obtained ages are far older than those of the previous palaeoseismological studies that dated earthquake-related sedimentary features in the region. Therefore, more research is required to shed light on the resetting mechanism and condition of these fault rocks due to the recent activity of the NTF. [ABSTRACT FROM AUTHOR]

Luminescence dating is an effective means to investigate faults' past behaviours [1]. It has been successfully applied to offsets of geomorphic features or distinct stratigraphy in many tectonically active regions around the globe. This approach, among others, brought insights into the timing of past major earthquakes. The Central Alborz mountains in Iran are one of the tectonically most active regions globally, in which major earthquakes have been documented in historical records. This area encompasses the megacity of Tehran, which is crossed in the north by the North Tehran fault (NTF), a prominent oblique-slip fault zone extending over 68 km. Over the last decades, a few studies have been conducted to constrain past activity phases of the NTF during the late Quaternary [2,3]. However, due to its significant contribution to geohazard for Tehran, more studies are required to shed light on the NTF's behaviour, temporally and spatially. In this study, for the first time, we contribute to this demand by employing luminescence dating simultaneously on two different sample series: faulted colluvial deposits and fault gouges [4]. If successfully applied, this twofold approach can enable us to discuss the pros and cons of indirect and direct dating of past earthquakes at the NTF using luminescence techniques. We extracted the polymineral fraction (4-11µm) to probe the samples' general luminescence behaviour. Then, the equivalent dose will be determined through elevated post-infrared infrared stimulated luminescence (post-IR-IRSL) [5]. Combined with dose-rate measurements using gamma-ray spectrometry, the first ages will be established. We will share and discuss our preliminary findings about the applicability of luminescence dating in this geological setting, particularly for the fault gouge. [ABSTRACT FROM AUTHOR]