International audienceFieldwork complemented by SPOT image analysis throws light on current crustal shortening processes in the ranges of northeastern Tibet (Gansu and Qinghai provinces, China). The ongoing deformation of Late-Pleistocene bajada aprons in the forelands of the ranges involves folding, at various scales, and chiefly north-vergent, seismogenic thrusts. The most active thrusts usually break the ground many kilometres north of the range-fronts, along the northeast limbs of growing, asymmetric ramp-anticlines. Normal faulting at the apex of other growing anticlines, between the range fronts and the thrust breaks, implies slip on blind ramps connecting distinct active décollement levels that deepen southwards. The various patterns of uplift of the bajada surfaces can be used to constrain plausible links between contemporary thrusts downsection. Typically, the foreland thrusts and décollements appear to splay from master thrusts that plunge at least 15–20 km down beneath the high ranges. Plio-Quaternary anticlinal ridges rising to more than 3000 m a.s.l. expose Palaeozoic metamorphic basement in their core. In general, the geology and topography of the ranges and forelands imply that structural reliefs of the order of 5–10 km have accrued at rates of 1–2 mm yr−1 in approximately the last 5 Ma. From hill to range size, the elongated reliefs that result from such Late-Cenozoic, NE–SW shortening appear to follow a simple scaling law, with roughly constant length/width ratio, suggesting that they have grown self-similarly. The greatest mountain ranges, which are over 5.5 km high, tens of kilometres wide and hundreds of kilometres long may thus be interpreted to have formed as NW-trending ramp anticlines, at the scale of the middle–upper crust. The fairly regular, large-scale arrangement of those ranges, with parallel crests separated by piggy-back basins, the coevality of many parallel, south-dipping thrusts, and a change in the scaling ratio (from #5 to 8) for range widths greater than #30 km further suggests that they developed as a result of the northeastward migration of large thrust ramps above a broad décollement dipping SW at a shallow angle in the middle–lower crust. This, in turn, suggests that the 400–500 km-wide crustal wedge that forms the northeastern edge of the Tibet–Qinghai plateau shortens and thickens as a thickskinned accretionary prism decoupled from the stronger upper mantle underneath. Such a thickening process must have been coupled with propagation of the Altyn Tagh fault towards the ENE because most thrust traces merge northwestwards with active branches of this fault, after veering clockwise. This process appears to typify the manner in which the Tibet–Qinghai highlands have expanded their surface area in the Neogene. The present topography and structure imply that, during much of that period,

S U M M A R YWe have studied the Cenozoic and active tectonics of the north-eastern rim of Tibet west of the Yellow River (Gansu, China) where the western Haiyuan Fault enters the eastern Qilian Shan, a high mountainous region, which was the site of the 1927 May 23, M = 8-8.3, Gulang earthquake. Fieldwork, combined with analysis of aerial photographs and satellite images, reveals consistent cumulative left-lateral offsets of postglacial geomorphic features along the fault, but no recent rupture. West of the Tianzhu pull-apart basin, the levelling of offset-terrace risers implies Holocene horizontal and vertical slip rates on the steeply south-dipping, N11OE-striking fault of 11 f 4 and 1.3 f 0.3 mm yr-', respectively. The presence of subordinate, mostly normal, throws due to local changes in fault strike, and kinematic compatibility at the SW corner of the Tianzhu basin, constrains the azimuth of the fault-slip vector to be N110-115E. On the less prominent, N85-100E-striking Gulang Fault, which splays eastwards from the Haiyuan Fault near 102.2"E, less detailed observations suggest that the average Holocene left-slip rate is 4.3 f 2.1 mm yr-I with a minor component of =N-directed thrusting, with no recent seismic break either. East of =103"E, coeval slip on both faults thus appears to account for as much as 15 f 6 mm yr-' of left-lateral movement between NE Tibet and the southern edge of the Ala Shan Platform, in a N105 f 6E direction. West of -103"E, structural and geomorphic evidence implies that =NNE-directed shortening of that edge across the rising, north-eastern Qilian mountain ranges occurs at a rate of 4 f 2 mm yr-l, by movement on right-stepping thrusts that root on a 10-20" S-dipping dCcollement that probably branches off the Haiyuan Fault at a depth of 225 km. The existence of fresh surface breaks with metre-high free faces on a N-dipping, hanging-wall normal fault south of the easternmost, Dongqingding thrust segment, and of half-metre-high pressure ridges on that segment, indicates that the 1927 Gulang earthquake ruptured that complex thrust system. The =4 mm yr-' shortening rate is consistent with the inference that the thrusts formed and move as a result of orthogonal slip partitioning in a large restraining bend of the Haiyuan Fault.Based on a retrodeformable structural section, we estimate the cumulative shortening on the Qilian Shan thrusts, north of the Haiyuan Fault, to be at least 25 km. The finite displacements and current slip rates on either the thrusts or the left-lateral faults imply that Cenozoic deformation started in the Late Miocene, with slip partitioning during much of the Plio-Quaternary. Assuming coeval slip at the present rates on the Haiyuan and Gulang Faults in the last 8Ma would bring the cumulative left-lateral displacement between NE Tibet and the Ala Shan Platform 599 600 Y. Gaudemer et al. to about 120 km, consistent with the 95 f 15 km offset of the Yellow River ;ICI'OSS the Haiyuan Fault, but many times the offset (216 km) inferred on one rccent strand of that fault ...

Summary Late Pleistocene–Holocene sinistral slip‐rates on several segments of the Kunlun Fault in northeastern Tibet have been determined. These determinations are based on the measured displacement of alluvial surfaces whose surface ages were determined by cosmogenic 26Al and 10Be dating of quartz pebbles, and by 14C dating of charcoal. In the west, at three sites along the Xidatan–Dongdatan segment of the fault, near 94°E, terrace riser offsets ranging from 24 to 110 m, with cosmogenic ages ranging from ∼1800 to ∼8200 yr, yield a mean left‐lateral slip‐rate of 11.7 ± 1.5 mm yr−1. Field observations indicate minimum offsets of 9–12 m; this offset, when combined with the long‐term slip‐rate, indicates that great earthquakes (M ∼ 8) rupture this segment of the fault with a recurrence interval of 800–1000 yr. At two sites along the Dongxi–Anyemaqin segment of the fault, near 99°E, terrace riser offsets ranging from 57 to 400 m with 14C ages ranging from 5400 to 37 000 yr BP yield a minimum slip‐rate of ∼10 mm yr−1. At one site, the 1937 January 7, M=7.5 and the penultimate earthquakes produced 4 m of left‐slip and 0.4 m of reverse‐slip. The maximum recurrence interval of earthquakes with such characteristic slip is thus ∼400 yr. Farther east, near 100.5°E, along the Maqen segment of the fault, the 180 m offset of a lateral moraine, emplaced between the last glacial maximum (20 ka) and 11 100 yr BP, yields a mean slip‐rate of 12.5 ± 2.5 mm yr−1. The slip‐rates are constant, within uncertainty, throughout the 600 km of the Kunlun Fault that we studied. The average slip‐rate is 11.5 ± 2.0 mm yr−1. Extrapolating this rate to the reminder of the fault, we conclude that most (80 per cent) of the 300 morphological offsets measured in the field or on SPOT satellite images post‐date the Last Glacial Maximum. Most of the terraces we studied were deposited during the humid period of the Early Holocene Optimum (9–5 ka); the formation of younger terraces reflects Late Holocene climate change.

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