Palaeoseismic evidence for a medieval earthquake, and preliminary estimate of late Pleistocene slip-rate, on the Firouzkuh strike-slip fault in the Central Alborz region of Iran H. Nazari a,b,⇑ , J.-F. Ritz b , R.T. Walker c , R. Salamati d , M. Rizza b , R. Patnaik e , J. Hollingsworth f , H. Alimohammadian d , A. Jalali d , A. Kaveh Firouz d , A. Shahidi d a Research Institute for Earth Sciences, Geological Survey of Iran, P.O. Box 13185, 1494 Tehran, Iran b Laboratoire Géosciences Montpellier, UMR 5573, Université Montpellier 2, 34095 Montpellier Cedex 05, France c COMET+, Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK d Geological Survey of Iran, P.O. Box 13185, 1494 Tehran, Iran e Centre for Petroleum and Applied Geology, Panjab University, Chandigarh 160014, India f Géoazur, Les Lucioles, UMR 7329, Sophia-Antipolis, Nice, France article info Article history: Received 23 July 2013 Received in revised form 13 December 2013 Accepted 16 December 2013 Available online 27 December 2013 Keywords: Paleoseismology Slip-rate Medieval earthquake Firouzkuh Fault Iran abstract The 55 km-long Firouzkuh fault is located in the Central Alborz Mountains of Iran. It is a left-lateral fault, which dips to the south, and possesses a small dip-slip component of motion that we interpret to result from extension. The ratio of horizontal to vertical displacement across the fault, calculated from the cumulative displacement of landscape features, is 7.6. We provide constraints on the timing of the last earthquake on the Firouzkuh fault from two trenches (T1 and T2) across the fault zone, excavated in 2004, and located east of Firouzkuh city. The trenches expose faulted sedimentary deposits. Two opti- cally-stimulated luminescence (OSL) ages from sediments in the lower part of trench T1 date from the late Pleistocene (15.9 ± 0.9 ka and 27.1 ± 1.7 ka). The younger of the two dated units in T1 is displaced vertically across the fault by 2.2–4.4 m, from which we estimate a strike-slip displacement of 18.2– 33.4 m, and hence a average horizontal slip-rate of 1.1–2.2 mm/yr. The sediments exposed in T1 do not yield constraints on the most recent earthquake history. In trench T2, however, human skeletal remains of a middle aged male, which yield a radiocarbon age of 1159 ± 28 BP (corresponding to a mean calendar age of 791 AD), were found within a faulted alluvial layer at a depth of 60–70 cm from the sur- face. The existence of these medieval human places shows that a surface-rupturing earthquake occurred at some time after 1159 ± 28 BP. The amount of slip in each earthquake on the Firouzkuh fault is difficult to estimate, but assuming the entire 55 km fault length ruptures in each event, they will have had a maximum magnitude of 7.1. At our estimated late Quaternary slip-rate of 1.1–2.2 mm/yr magnitude 7.1 earthquakes, involving 1.2 m average displacement, would be expected to occur every 1100– 540 years. As the last earthquake on the Firouzkuh fault may be up to 700 years in age we suggest that the Firouzkuh fault is a major hazard for earthquakes in the near future. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The active faults within the Alborz Mountains of northern Iran pose a continuous hazard to local populations (Fig. 1a). Many ma- jor population centers – including the capital city of Tehran with a population of 12 million – are situated along the southern margin of the Alborz range and hence are at risk from future seismic events. Constraining the late Quaternary seismic history on individual faults within the Alborz is thus a high priority for under- standing seismic hazard within Iran. In addition, the major faults are also important elements in the deformation caused by the Ara- bia–Eurasia continental collision and details of their rate and style of slip are vital for understanding how they accommodate tectonic strain (e.g. Ritz et al., 2006; Nazari et al., 2009; Solaymani Azad, 2009; Landgraf et al., 2009;; Hollingsworth et al., 2010; Rizza et al., 2011). In this paper we investigate palaeoseismic evidence for the most recent surface-rupturing earthquake on the Firouzkuh left- lateral strike-slip fault (Fig. 1b and c). The Firouzkuh fault is one of a system of left-lateral strike-slip faults that run through the middle of the Alborz mountains and are thought to accommodate 1367-9120/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2013.12.018 ⇑ Corresponding author at: Research Institute for Earth Sciences, Geological Survey of Iran, P.O. Box 13185, 1494 Tehran, Iran. Tel.: +98 2166070518; fax: +98 2166070511. E-mail address: hamidnazari@hotmail.com (H. Nazari). Journal of Asian Earth Sciences 82 (2014) 124–135 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes