Crustal thickening in Gansu-Qinghai, lithospheric mantle subduction, and oblique, strike-slip controlled growth of the Tibet plateau

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,

Active thrusting and folding in the Qilian Shan, and decoupling between upper crust and mantle in northeastern Tibet

Fieldwork south of the city of Gaotai (Gansu province, China) shows that active shortening of surface sediments in the foothills of the Yumu Shan, a large fore-mountain of the Qilian Shan, at the northeastern edge of Tibet, involves both overthrusting and flexural-slip folding. North of this mountain, we found and mapped a prominent north-facing thrust scarp that offsets a Holocene fan sloping gently (3.4°) to the north. Part of this scarp appears to be related to the M ≈ 7.5, 180 A.D. earthquake that may have led to the demise of the Han Dynasty city of Luo Tuo Chen, in the Hexi corridor. A set of 10, 100–150 m long profiles measured across this scarp, 3.2 m high on the average, can be made to fit the diffusion-degraded morphology of a surface break related to the 180 A.D. event using a value of about 3.3 m^2/10^3 yr for the mass diffusivity ϰ of fanglomerates in this part of Gansu province. Smaller mountain-facing scarps on a terrace-capped foothill result from bedding slip concurrent with active folding of underlying, steeply northdipping, Plioquaternary sandstone and conglomerate beds. Holocene uplift rates along the Yumu Shan, which is only one of the Qilian Shan ranges, are estimated to be between 0.4 and 1.9 mm/yr, which implies that much of the mountain formed in the Quaternary. The periclinal structure of the Plioquaternary envelope under which the Paleozoic core of the Yumu Shan plunges towards the west suggests that the whole 3200 m high mountain is a basement ramp anticline. Mountains striking parallel to the Yumu Shan, with similar structure and comparable or greater sizes north and south of the Hexi corridor probably also correspond to recent, crustal ramp anticlines. This implies that the wide, mountainous upper crustal wedge making the northeastern edge of the Tibet-Qinghai plateau is detached from the underlying lower crust and upper mantle

The SEXTANTS beamline at SOLEIL: a new facility for elastic, inelastic and coherent scattering of soft X-rays

11th International Conference on Synchrotron Radiation Instrumentation (SRI), Lyon, FRANCE, JUL 09-13, 2012International audienceSEXTANTS is a new SOLEIL beamline dedicated to soft X-ray scattering techniques. The beamline, covering the 50-1700 eV energy range, features two Apple-II undulators for polarization control and a fixed-deviation monochromator. Two branch-lines host three end-stations for elastic, inelastic and coherent scattering experiments

Postglacial left slip rate and past occurrence of M≥8 earthquakes on the Western Haiyuan Fault, Gansu, China

International audienceHigh-resolution (HR) air photographs and a 1-m horizontal and 2-m vertical resolution digital elevation model derived from them by stereophotogrammetry provide new constraints on the behavior of the western stretch of the active Haiyuan fault, in Gansu province, China. The photographs cover three swaths along the fault, each about 2-km-long and at least 500-m-wide, near the village of Songshan, at 103.5øE. This high-resolution data set is used to map and measure cumulative horizontal offsets of alluvial terraces and risers that range between 115 and la 135 m, and 70 and 90 m, at two sites. Dating these terraces with 14C yields minimum and maximum ages of 8400 and 7600, and 14,200 years B.P., respectively. This leads to a postglacial slip rate of 12±4 mm/yr, with a most likely minimum value of 11.6±1.1 mm/yr. The smallest stream offsets observed on the HR photographs range between 8 and 16 m and are interpreted as coseismic displacements of the last few earthquakes with M ≥ 8 that ruptured the 220-km-long Tianzhu gap of the fault, west of the Yellow River. Earthquakes of that size within this gap, which has been quiescent for at least 800 years, would recur at intervals of 1050±450 years