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,