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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,

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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 ...

The processes which have governed the formation and evolution of large Tertiary strike‐slip faults during the penetration of India into eastern Asia are investigated by means of plane strain indentation experiments on layered plasticine models. The steady state deformation of plasticine obeys a power creep flow law false(trueε˙=Cfalse(Tfalse)σnfalse) . The stress exponent (n) is between 6 and 9 at 25°C. Uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening, a process observed for strain rates ranging from 3.5×10−5 to 3.6×10−3 s−1. Fault or shear zones form after only 7–10 % bulk strain. Subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine. A series of nine plane strain indentation experiments shows the influence of boundary conditions, as well as that of the internal structure of the plasticine model on the faulting sequence. The ubiquity of strain softening in experimental deformation of a variety of rocks, as well as the widespread occurrence of shear zones in nature suggest that long‐term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain. The deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision‐induced strikeslip faulting in Asia, particularly to total offsets and rates of movements on the faults. The experiments simulate the evolution of the western ends of the strike‐slip faults, which have probably been analogous to trench‐fault‐fault triple junctions. The experiments also illustrate mechanisms for the formation of extensional basins, such as the South China Sea, North China Basin, and Andaman Sea, near active continental margins. The basins, which appear to absorb terminal offsets along major strike‐slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts. The existence of distinct phases of strike‐slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous, non‐steady state deformation of the continental lithosphere.

RO1_PAC V2.3, a Repeat Orbit Interferometry package that allows topographic and surface change researchers to apply Interferometric Synthetic Aperture Radar (InSAR) methods, is now freely available to the community InSAR is the synthesis of conventional SAR and interferometry techniques that have been developed over several decades in radio astronomy and radar remote sensing. In recent years, it has opened entirely new application areas for radar in the Earth system sciences, including topographic mapping and geodesy. RO1_PAC, developed primarily to work with European Remote Sensing (ERS) satellite radar data, currently supports ERS‐1, ERS‐2, and Japanese Earth Resources Satellite (JERS) radar data, and is configurable to work with “strip‐mode” data from all existing satellite radar instruments. The first release of RO1_ PAC (V1.0) was made quietly in 2000, and roughly 30 groups in the academic and research community currently use it.

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