An Mw 6.5 earthquake devastated the town of Bam in southeast Iran on 26 December 2003. Surface displacements and decorrelation effects, mapped using Envisat radar data, reveal that over 2 m of slip occurred at depth on a fault that had not previously been identified. It is common for earthquakes to occur on blind faults which, despite their name, usually produce long‐term surface effects by which their existence may be recognised. However, in this case there is a complete absence of morphological features associated with the seismogenic fault that destroyed Bam.

The tectonic activity in the Alborz mountain range, northern Iran, is due both to the northward convergence of central Iran toward Eurasia, and to the northwestward motion of the South Caspian Basin with respect to Eurasia inducing a left-lateral wrenching along this range. These two mechanisms give rise to a NNE-SSW transpressional regime, which is believed to have affected the entire range for the last 5 ؎ 2 m.y. In this paper, we show that the internal domain of central Alborz is not affected by a transpressional regime but by an active transtension with a WNW-ESE extensional axis. We show that this transtension is young (middle Pleistocene). It postdates an earlier N-S compression and may have been initiated when the South Caspian Basin started moving. Consequently, our results suggest that the South Caspian Basin motion may have taken place more recently than previously proposed.

Extrusion tectonics and subduction in the eastern South Caspian region since 10 Email alerting services cite this article to receive free e-mail alerts when new articles www.gsapubs.org/cgi/alerts click Subscribe to subscribe to Geology www.gsapubs.org/subscriptions/ click Permission request to contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click viewpoint. Opinions presented in this publication do not reflect official positions of the Society. positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political article's full citation. GSA provides this and other forums for the presentation of diverse opinions and articles on their own or their organization's Web site providing the posting includes a reference to the science. This file may not be posted to any Web site, but authors may post the abstracts only of their unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make GSA, employment. Individual scientists are hereby granted permission, without fees or further requests to

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International audience¹⁰Be and ³⁶Cl cosmic ray exposure (CRE) and optically stimulated luminescence (OSL) dating of offset terraces have been performed to constrain the long-term slip-rate of the Dehshir fault. Analysis of cosmogenic ¹⁰Be and ³⁶Cl in 73 surface cobbles and 27 near-surface amalgams collected from inset terraces demonstrates the occurrence of a low denudation rate of 1 m.Ma⁻¹ and of a significant and variable inheritance from exposure prior to the aggradation of theses alluvial terraces. The significant concentrations of cosmogenic nuclides measured in the cobbles collected within the riverbeds correspond to 72 ± 20 ka of inheritance. The mean CRE age of the surface samples collected on the older terrace T3 is 469 ± 88 ka but the analysis of the distribution of ¹⁰Be concentration in the near-surface samples discard ages older than 412 ka. The mean CRE age of the surface samples collected on terrace T2 is 175 ± 62 ka but the ¹⁰Be depth profile discard ages older than 107 ka. For each terrace, there is a statistical outlier with a younger age of 49.9 ± 3.3 ka and 235.5 ± 35.4 ka on T2 and T3 respectively. The late sediments aggraded before the abandonment of T2 and inset levels, T1 b and T1a, yielded optically stimulated luminescence (OSL) ages of respectively 26.9 ± 1.3 ka, 21.9 ± 1.5 ka, and 10.0 ± 0.6 ka. For a given terrace, the OSL ages, where available, provide ages that are systematically younger than the CRE ages. These discrepancies between the CRE and OSL ages exemplify the variability of the inheritance and indicate the youngest cobble on a terrace, that minimizes the inheritance, is the most appropriate CRE age for approaching that of terrace abandonment. However, the upper bound on the age of abandonment of a terrace that is young with respect to the amount of inheritance is best estimated by the OSL dating of the terrace material. For such terraces, the CRE measurements are complementary of OSL dating and can be used to unravel the complex history of weathering and transport in the catchment of desert alluvial fans. This comprehensive set of dating is combined with morphological offsets ranging from 12 ± 2 m to 380 ± 20 m to demonstrate the Dehshir fault slips at a rate in the range 0.9 mm.yr⁻¹ - 1.5 mm.yr⁻¹. The variable inheritance exemplified here may have significant implications for CRE dating in arid endorheic plateaus such as Tibet and Altiplano

International audienceThe catastrophic 2003 Mw 6.6 Bam earthquake in southern Iran attracted much attention, and has been studied with an abundance of observations from synthetic aperture radar, teleseismic seismology, aftershock studies, strong ground motion, geomorphology, remote sensing and surface field work. Many reports have focused on the details of one or other data type, producing interpretations that either conflict with other data or leave questions unanswered. This paper is an attempt to look at all the available data types together, to produce a coherent picture of the coseismic faulting in 2003 and to examine its consequences for active tectonics and continuing seismic hazard in the region. We conclude that more than 80 per cent of the moment release in the main shock occurred on a near-vertical right-lateral strike-slip fault extending from the city of Bam southwards for about 15 km, with slip of up to 2 m but mostly restricted to the depth range 2–7 km. Analysis of the strong ground motion record at Bam is consistent with this view, and indicates that the extreme damage in the city can be attributed, at least in part, to the enhancement of ground motion in Bam because of its position at the end of the northward-propagating rupture. Little of the slip in the main shock reached the Earth's surface and, more importantly, aftershocks reveal that ~12 km vertical extent of a deeper part of the fault system remained unruptured beneath the coseismic rupture plane, at depths of 8–20 km. This may represent a substantial remaining seismic hazard to the reconstructed city of Bam. We believe that some oblique-reverse slip (up to 2 m, and less than 20 per cent of the released seismic moment) occurred at a restricted depth of 5–7 km on a blind west-dipping fault that projects to the surface at the Bam-Baravat escarpment, an asymmetric anticline ridge that is the most prominent geomorphological feature in the area. This fault did not rupture significantly at shallow levels in 2003, and it may also represent a continuing seismic hazard. Widespread distributed surface ruptures north of the city are apparently unrelated to substantial slip at depth, and may be the result of enhanced ground motion related to northward propagation of the rupture. The faulting at Bam may be in the early stages of a spatial separation (‘partitioning') between the reverse and strike-slip components of an oblique convergence across the zone. Such a separation is common on the continents, though in this case the slip vectors between the two faults differ only by ~20° as a substantial strike-slip component remains on the oblique-reverse fault. The Bam earthquake is one in a series of large earthquakes involving faulting along the western edge of the Lut desert. In addition to the unruptured parts of the faults near Bam itself, continuing and substantial hazard is represented by unruptured neighbouring faults, particularly blind thrusts along the Jebel Barez mountains to the south and strike-slip faulting at Sarvestan to the west

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