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,

Abstract.We address the problem of late Cenozoic uplift, erosion, and growth of northeastern Tibet by reconstructing, from isopach maps and drill holes, the vol-

Summary This work establishes estimates of mass accumulation rates in 18 mostly offshore sedimentary basins in Asia since the beginning of the Cenozoic, ≈ 66 Ma. The estimates were derived from isopach maps, cross‐sections and drill holes or stratigraphic columns assuming regional similarity of the strata. Average solid phase volumes and accumulation rates were calculated for nine epochs approximately corresponding to geological periods: Palaeocene ( ≈ 66–58 Ma), Eocene ( ≈ 58–37 Ma), Oligocene( ≈ 37–30 and 30–24 Ma), Miocene ( ≈ 24–17, 17–11 and 11–5 Ma), Pliocene ( ≈ 5–2 Ma) and Quaternary ( ≈ 2–0 Ma). These rates shed new light on the geological history of Asia since the onset of the collision of India with Asia ( ≈ 50 Ma). The overall average accumulation rates curve for Asian sedimentary basins since the beginning of the Tertiary shows an exponential form with slow accumulation rates (less than 0.5 × 106 km3 Myr− 1) until the beginning of the Oligocene, more than 15 Myr after the onset of the collision. From the Oligocene onwards rates increase quickly in an exponential manner, reaching their maximum values in the Quaternary (more than 1.5 × 106 km3 Myr− 1). From these observations we suggest that extrusion and crustal shortening are complementary processes that have been successively dominant throughout the India–Eurasia collision history. At smaller scales one may distinguish between independent histories at the subcontinental and basin scales. This permits a comparison of the relative importance of tectonic and climatic erosion processes affecting the different mountain belts of Asia during the Cenozoic.

No abstract

[1] The Ganga River is one of the main conveyors of sediments produced by Himalayan erosion. Determining the flux of elements transported through the system is essential to understand the dynamics of the basin. This is hampered by the chemical heterogeneity of sediments observed both in the water column and under variable hydrodynamic conditions. Using

Using depths and ages derived from isopachs, drill‐holes or cross‐sections, it is possible to reconstruct the space‐time depositional history of a sedimentary basin. Drill holes and cross‐sections give the local sedimentation history, while isopachs allow the definition of the spatial distribution of the sediments. Assuming several simple hypotheses, such as similarity of the strata and regional applicability of results derived from local analyses, one can reconstruct balanced maps of the solid (or grain) volumes, and hence the mass of sediments deposited during several time intervals since the Palaeogene. Applying this method to the Tarim and Dzungar basins (NW China), we estimate the total Cenozoic solid‐phase volume and mass of sediments stored to be 1358 ± 520 × 103 km3 (36.7 ± 14 × 1017 kg) and 172 ± 56 × 103 km3 (4.6 ± 1.5 × 1017 kg) respectively. The reconstruction also enables us to detect two main pulses in the sedimentation. The first, around 17 Ma, affected only the northern part of the Tarim Basin (also known as the Kucha or Kuche Depression) at the foot of the Tien Shan Mountains and supports the idea that the presently active shortening regime in that range started at that time. The second, 5 to 6 Ma, affected most of the depositional areas of the region and may have an even greater geographical extent. Assuming local isostasy, we estimate the volume of shortening induced by the rotation of the Tarim block relative to Siberia and stored in the range and adjacent basins to be between 1.15 × 106 and 4.23 × 106 km3. This corresponds to a clockwise rotation of between 2.5° and 8.7°. We use these results in two simple models of self‐similar growth of pyramidal topographies that approximately fit the eastern Tien Shan.

International audienceLarge rivers have been previously shown to be vertically heterogeneous in terms of suspended particulate matter (SPM) concentration, as a result of sorting of suspended solids. Therefore, the spatial distribution of suspended sediments within the river section has to be known to assess the riverine sedimentary flux. Numerous studies have focused on the vertical distribution of SPM in a river channel from a theoretical or experimental perspective, but only a few were conducted so far on very large rivers. Moreover, a technique for the prediction of depth-integrated suspended sediment fluxes in very large rivers based on sediment transport dynamics has not yet been proposed. We sampled river water along depth following several vertical profiles, at four locations on the Amazon River and its main tributaries and at two distinct water stages. Depending on the vertical profile, a one-to fivefold increase in SPM concentration is observed from river channel surface to bottom, which has a significant impact on the 'depth-averaged' SPM concentration. For each cross section, a so-called Rouse profile quantitatively accounts for the trend of SPM concentration increase with depth, and a representative Rouse number can be measured for each cross section. However, the prediction of this Rouse number would require the knowledge of the settling velocity of particles, which is dependent on the state of aggregation affecting particles within the river. We demonstrate that in the Amazon River, particle aggregation significantly influences the Rouse number and renders its determination impossible from grain-size distribution data obtained in the lab. However, in each cross section, the Rouse profile obtained from the fit of the data can serve as a basis to model, at first order, the SPM concentration at any position in the river cross section. This approach, combined with acoustic Doppler current profiler (ADCP) water velocity transects, allows us to accurately estimate the depth-integrated instantaneous sediment flux

A viscous fluid flowing over plastic grains spontaneously generates single-thread channels. With time, these laminar analogues of alluvial rivers reach a reproducible steady state, showing a well-defined width and cross section. In the absence of sediment transport, their shape conforms with the threshold hypothesis which states that, at equilibrium, the combined effects of gravity and flow-induced stress maintain the bed surface at the threshold of motion. This theory explains how the channel selects its size and slope for a given discharge. In this light, laboratory rivers illustrate the similarity between the avalanche angle of granular materials and Shields's criterion for sediment transport.