Late Pleistocene-Holocene right-slip rate and paleoseismology of the Nayband fault, western margin of the Lut block, Iran

Mohammad Foroutan (1, 2) , Bertrand Meyer (1) , Michel Sébrier (1) , H. Nazari (3) , M. Murray (4) , Kristell Le Dortz (1) , M.A. Shokri (2) , Maurice Arnold (5) , G. Aumaître (5) , D. Bourles (5) , Karim Keddadouche (5) , S. Solaymani Azad (2) , M.J. Bolourchi (2)

1 iSTeP - Institut des Sciences de la Terre de Paris
2 Geological Survey of Iran
3 Research Institute for Earth Sciences
4 Nordic Laboratory fo Luminescence, Department of Geosciences
5 CEREGE - Centre européen de recherche et d'enseignement des géosciences de l'environnement

The 290-km-long, Nayband strike-slip fault bounds the western margin of the Lut block and cuts across a region thought to have been quiescent during the last few millennia. Cl-36 cosmic ray exposure (CRE) and optically stimulated luminescence (OSL) dating of cumulative geomorphic offsets are used to derive the long-term slip rate. The measured offsets at two sites along the fault range between 9 ± 1 m and 195 ± 15 m with ages from 6.8 ± 0.6 ka to ∼100 ka, yielding minimum and maximum bounds of late Pleistocene and Holocene slip rates of 1.08 and 2.45 mm yr-1, respectively. This moderate slip rate of 1.8 ± 0.7 mm yr-1, averaged over several earthquake cycles, is compared to the paleoseismic record retrieved from the first trench excavated across the fault. Combining the paleoseismic evidence with 18 OSL ages obtained from this trench site demonstrates the occurrence of at least four large (M ∼7) earthquakes during the last 17.4 ± 1.3 ka and of two older wearthquakes, one before ∼23 ka and another before 70 ± 5 ka. The exposed sediment succession also indicates a significant gap at the end of MIS-2 and the beginning of MIS-1. The age of the most recent regional incision is accurately bracketed between 6.1 ka and 7.4 ka. Sediments from the last ∼7 ka contain evidence of the three younger earthquakes. Interestingly, the penultimate and antepenultimate events occurred between 6.5 ± 0.4 ka and 6.7 ± 0.4 ka within a time interval lasting at most 1 ka whereas the most recent earthquake occurred within the last millennium. Such an irregular earthquake occurrence suggests the seismic behavior of the Nayband fault is not strictly time dependent but possibly related to clustering. From this and taking into account the occurrence of the most recent earthquake within the last 800 years, the imminence of an earthquake along the Nayband fault cannot be discarded. Although the most recent surface-rupturing event seems to have occurred after AD 1200, this event went unnoticed in the historical records. This provides a marked illustration of the incompleteness of the historical seismic catalogs in Central Iran, challenging any assessment of regional seismic hazard without appropriate geologic and geochronological information. Large and infrequent earthquakes are characteristic of the seismic behavior of the slow-slipping strike-slip faults slicing Central and Eastern Iran. Also, the slip rates summed across Central and Eastern Iran from the Iran Plateau up to the Afghan lowlands appear in agreement with the most recent GPS data.

Domaines: Sciences de la Terre

Mohammad Foroutan, Bertrand Meyer, Michel Sébrier, H. Nazari, M. Murray, et al.. Late Pleistocene-Holocene right-slip rate and paleoseismology of the Nayband fault, western margin of the Lut block, Iran. Journal of Geophysical Research : Solid Earth, 2014, 119 (4), pp.3517-3560. ⟨10.1002/2013JB010746⟩. ⟨hal-00992079⟩