Late-stage foreland growth of China's largest orogens (Qinling, Tibet): Evidence from the Hannan-Micang crystalline massifs and the northern Sichuan Basin, central China

This paper addresses the timing of final foreland growth of China's largest orogens: the Mesozoic Qin Mountains (Qinling) and the Cenozoic Tibetan Plateau. In particular, we ask when the front of the Qinling orogen fold-thrust belt was emplaced, and when the northern Sichuan Basin was affected by the eastward growth of the Tibetan Plateau. We employ zircon and apatite fission-track and (U-Th)/He dating in the Proterozoic crystalline rocks of the Hannan-Micang massifs and the sedimentary rocks of the northern Sichuan Basin. The Hannan-Micang rocks remained in the zircon fission-track partial annealing zone (240 +/- 30 degrees C) throughout the Paleozoic-Middle Triassic (481-246 Ma). From the late Middle Jurassic (ca. 165 Ma) to the early Late Cretaceous (ca. 95 Ma), enhanced cooling and exhumation, with rates of 1.2-2.5 degrees C/m.y. and 0.04-0.10 mm/yr, respectively, record propagation of the Qinling orogen into its leading foreland; the timing of foreland growth is supported by sedimentologic evidence, i.e., regional variation in sediment thickness and depocenter migration. Negligible cooling and exhumation since the Late Cretaceous (ca. 95 Ma) likely mark the end of the foreland fold-thrust belt formation and the subsequent persistence of a low-relief landscape that occupied extensive parts of central China; cooling and exhumation rates of 0.38-0.70 degrees C/m.y. and <0.02 mm/yr characterize this tectonic stagnation period. Accelerated cooling (4-5 degrees C/m.y.) since the Late Miocene (13-8 Ma), derived from apatite fission-track temperature-time path models, signifies involvement of the Hannan-Micang massifs and the northern Sichuan Basin into the eastward-growing Tibetan Plateau. Their inclusion into the plateau growth initiated faulting and stripped off 1.4-2.0 km of rock from the Hannan-Micang massifs and northern Sichuan Basin.