S U M M A R YThe Doruneh fault, with a length of ∼600 km, is one of the longest, and most prominent, faults in Iran. It performs an important role in the regional tectonics, but has no record of large earthquakes. The geomorphology of the Doruneh fault contains numerous indications of cumulative left-lateral slip over various scales. We describe three sites where Late Quaternary landforms are displaced by the fault. (a) An incised alluvial fan near the village of Uch Palang is displaced by 800-850 m. (b) The Kuh-e Teagh-Ahmad fold is composed of folded Quaternary gravels and is displaced by ∼200-400 m. (c) A sequence of three terraces of the Shesh-Taraz river are displaced left-laterally by a maximum of 25 m. Infrared stimulated luminescence (IRSL) dating of the uppermost Shesh-Taraz river terrace gives a deposition age of ∼10 ka, which correlates with changes in global climate ∼10-12 ka ago, and provides a provisional slip-rate estimate of 2.4 ± 0.3 mm yr −1 . No major recent or historical earthquakes are recorded on the Doruneh fault. Relatively fresh scarps and partially infilled fractures appear to be the preserved surface ruptures from an earthquake event of unknown age. A series of small streams showing left-lateral displacements of 3 to 5.5 m (with an average slip of ∼4.7 m) record the possible magnitude of slip during this earthquake, which from scaling relationships would have had an M w of ∼7.5, and ruptured the fault over a length of >100 km. At the estimated slip-rate of ∼2.5 mm yr −1 , the average recurrence time between large-magnitude earthquakes on the Doruneh fault is ∼2000 yr.

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S U M M A R YRegional shortening is accommodated across NE Iran in response to the collision of Arabia with Eurasia. We examine how N-S shortening is achieved on major thrust systems bounding the eastern branch of the Alborz (east of 57 • E), Sabzevar and Kuh-e-Sorkh mountain ranges, which lie south of the Kopeh Dagh mountains in NE Iran. Although these ranges have experienced relatively few large earthquakes over the last 50 yr, they have been subject to a number of devastating historical events at Neyshabur, Esfarayen and Sabzevar. A significant change in the tectonics of the eastern Alborz occurs directly south of the Central Kopeh Dagh, near 57 • E. To the east, shortening occurs on major thrust faults which bound the southern margin of the range, resulting in significant crustal thickening, and forming peaks up to 3000 m high. Active shortening dies out eastward into Afghanistan, which is thought to belong to stable Eurasia. The rate of shortening across thrust faults bounding the south side of the eastern Alborz north of Neyshabur is determined using optically stimulated luminescence dating of displaced river deposits, and is likely to be 0.4-1.7 mm yr −1 . Shortening across the Sabzevar range 150 km west of Neyshabur has previously been determined at 0.4-0.6 mm yr −1 , although reassessment of the rate here suggests it may be as high as 1 mm yr −1 . Migration of thrust faulting into foreland basins is common across NE Iran, especially in the Esfarayen region near 57 • E, where the northward deflection of the East Alborz range reaches a maximum of 200 ± 20 km (from its presumed linear E-W strike at the beginning of the Oligocene). West of 57 • E, the tectonics of the Alborz are affected by the westward motion of the South Caspian region, which results in the partitioning of shortening onto separate thrust and left-lateral strike-slip faults north and south of the range. At the longitude of 59 • E, published GPS velocities indicate that 50 per cent of the overall shortening across NE Iran is accommodated in the Kopeh Dagh. The remaining 50 per cent regional shortening must therefore be accommodated south of the Kopeh Dagh, in the eastern Alborz and Kuh-e-Sorkh ranges. Assuming present day rates of slip and the fault kinematics are representative of the Late Cenozoic deformation in NE Iran, the total 200 ± 20 km N-S shortening across the eastern Alborz and Kopeh Dagh mountains since the beginning of uplift of the Kopeh Dagh basin would be accommodated in 30 ± 8 Ma. Although this extrapolation may be inappropriate over such a long timescale, the age is nevertheless consistent with geological estimates of post Early-to-Middle Oligocene (<30 Ma) for the onset of Kopeh Dagh uplift.

The Altai range (western Mongolia) accommodates NNE-SSW shortening across the northern India-Eurasia collision zone by dextral slip on faults trending NNW-SSE, and anticlockwise, vertical-axis rotations of fault-bounded blocks. However, fault slip-rates and the way in which faulting evolves over time are poorly understood, and form the motivation for this study. We focussed on the HarUs-Nuur fault, a major transpressional fault bounding the eastern margin of the Altai. Three abandoned alluvial fan surfaces, each displaced right-laterally by the fault, were targeted for dating with cosmogenic 10 Be and quartz optically stimulated luminescence (OSL). The first surface (A2) shows an exponential decrease in 10 Be with increasing depth, with a significant inherited compo- * Corresponding author Material from the same sampling pit was dated at ∼19 ka with OSL, but we consider this younger age to be incorrect, possibly due to feldspar contamination or abnormal quartz OSL characteristics. The A2 surface is displaced by 175 m, implying a (maximum) dextral slip-rate of 2.4 ± 0.4 mm yr −1 . A second fan surface (F1) was dated at ∼6 ka with OSL and shows little variation in 10 Be with depth, consistent with this young age. The inherited component is higher than for A2, indicating contrasting levels of inheritance for different periods of fan aggradation. A final surface (F2) shows scattered 10 Be concentrations and lacks material suitable for OSL, so cannot be dated precisely. Using the total vertical displacement across the fault, we place the initiation of movement on the fault at ∼2 Ma, significantly later than the late Oligocene to Miocene (28-5 Ma) onset of shortening in the Altai region. This suggests that deformation in the Altai has widened over time to incorporate new faults at the range margins (such as Har-Us-Nuur), possibly because older faults in the range interior have rotated about vertical axes into orientations that require work to be done against gravity.

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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

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S U M M A R YWe investigate mountain building in the Altai range of western Mongolia, focusing on Baatar Hyarhan, a NW-trending massif bounded by active thrust faults. Our primary aims are to describe how thrusting has evolved over time, to calculate late Quaternary slip rates by dating offset alluvial markers with optically stimulated luminescence (OSL) and to compare these late Quaternary rates with measurements of deformation on decadal and geological timescales. Patterns of topography and drainage suggest that Baatar Hyarhan has grown in length and has propagated laterally from the SE towards the NW over time. On the NE side of the massif, the range-bounding Zereg fault appears active only along younger parts of Baatar Hyarhan; next to the oldest, SE part of the massif faulting has migrated into the adjacent Zereg Basin, where it has uplifted low, linear ridges of folded sediment, known locally as forebergs. On the SW side of the massif, only the range-bounding Tsetseg fault appears active. Using OSL, we establish ages of ∼15, ∼20 and ∼85 kyr for alluvial deposits cut by these faults. These ages are close to those of alluvial markers in the separate Gobi Altai range, suggesting that periods of fan and terrace formation may correlate over wide tracts of Mongolia, presumably under the primary control of climate. Combining our OSL ages with offsets measured with differential GPS, we calculate Late Quaternary slip rates across forebergs in the Zereg Basin and across the range-bounding Zereg and Tsetseg faults. Uncertainties in fault dip (due to lack of clear fault exposures) and burial ages (due to incomplete resetting of the luminescence clock) mean that the exact slip rates are poorly constrained. Nevertheless, the vertical displacement rates we calculate across the Zereg and Tsetseg range-front faults-0.2-0.6 and 0.1-0.4 mm yr −1 , respectivelyare at the lower end of long-term (∼5 Myr) estimates of 0.4-0.8 and 0.3-0.7 mm yr −1 , respectively. Vertical rates of deformation may, therefore, have remained constant over the past ∼5 Myr, but equally the late Quaternary rates might be lower than the geological ones. This possible discrepancy could be accounted for if some of the shortening has shifted away from the range-front faults onto other nearby structures. The forebergs in the eastern Zereg Basin are an obvious candidate, but they show at least 10 km cumulative shortening (which would take a few Ma to accumulate at late Quaternary rates) and cannot simply be regarded as the latest stage of outward mountain growth. The total Late Quaternary shortening rate across all three areas of faulting is 0.7-2.4 mm yr −1 , making up between one tenth and one third of the ∼7 mm yr −1 convergence across the whole Altai range.