Tectonics and denudation adjacent to the Xianshuihe Fault, eastern Tibetan Plateau: Constraints from fission track thermochronology

Abstract

The Xianshuihe‐Xiaojiang fault system extending from eastern Tibet to central Yunnan, China, is a major left‐shear structural boundary, accommodating the clockwise rotation of crustal rocks between the Eastern Himalayan syntaxis and the South China Block. Zircon and apatite fission track (FT) data are reported for 111 samples of basement collected from both sides of the northern part of this fault and spanning 300 km across the eastern margin of the Tibetan Plateau, where its mean elevation drops from 3500 to 1500 m above sea level. The zircon FT ages define two fossil partial annealing zones at different elevations, one fossilised at circa 130 Ma, and the other at circa 21 Ma as a result of cooling probably via regional denudation. The apatite FT ages are mostly less than 25 Ma, but a few granitoids higher up on the plateau retain late Cretaceous apparent ages. Within the apatite FT data with Neogene ages, there may be several partial annealing zones, with mixture modeling identifying age components suggestive of discrete cooling phases at circa 22, 7 and 2 Ma. The first‐order pattern of the minimum amount of Neogene denudation for a section across the plateau margin immediately northeast of Xianshuihe Fault suggests that a relatively uniform 4–6 km has been eroded from the inner part of the plateau margin, increasing to 7–10 km at Kangding, where the oldest rocks (Precambrian) are exposed, and then decreasing markedly into Sichuan Basin. These new FT data combined with published FT data suggest that the present extent of the Tibetan Plateau was defined during the early Miocene.

Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from 40Ar/39Ar and (U‐Th)/He thermochronology

“…Cenozoic denudation has been heterogeneous across the eastern margin of the plateau adjacent to the Sichuan Basin, with high rates focused in a narrow zone along the present topographic front. These results are broadly consistent with the distribution of denudation inferred from apatite and zircon fission track ages in the Longmen Shan region [Arne et al, 1997] and along the margin of the plateau south and west of the Xianshuihe fault [Xu and Kamp, 2000].…”

“…Thus the variation in cooling ages between the topographic margin of the plateau and the interior appears to be a robust feature of both the apatite and zircon (U-Th)/He data. Similar variations in apatite fission track ages were observed in the Longmen Shan [Arne et al, 1997] and to the south, along the Xianshuihe fault [Xu and Kamp, 2000].…”

“…If the plateau has been growing east and northeast by the evacuation of lower crust from beneath the plateau, our results suggest that it did not begin to abut the Sichuan Basin until at least $11 -12 Ma (perhaps as young as 5 -6 Ma) and contradict the recent suggestion that the present configuration of the plateau margins was established by circa 20 Ma [Xu and Kamp, 2000].…”

“…As there are no major structural discontinuities between these samples and our plateau samples, this site probably represents a similar structural level. Second, a regional study of apatite fission track ages along the Xianshuihe fault [Xu and Kamp, 2000], south and west of the Longmen Shan region, concluded that much of the region was cooling slowly between circa 130 Ma and circa 22 Ma. Finally, a recent study of apatite (U-Th)/He ages in the same region [Clark et al, 2000] is consistent with slow cooling ($1°C/m.y.)…”

Late Miocene provenance change on the eastern margin of the Yinggehai-Song Hong Basin, South China Sea: Evidence from U–Pb dating and Hf isotope analyses of detrital zircons

“…For example, Clift and Sun (2006) suggested that the sedimentation rates in the Y-SH Basin increased sharply at the end of the Miocene. Clark et al (2005) used an initiation of the major river incisions to identify the timing of the initial uplift at 13e9 Ma and the high erosion rates between 0.23 and 0.47 mm/yr, and the result does coincide with the thermochronology data published by Xu and Kamp (2000) and Kirby et al (2002). Gong and Li (2004) found that the uplift of the Tibetan Plateau since 10 Ma has had an important influence on the climate and sedimentary environment of the South China Sea, and the thick infill of the Y-SH Basin has been supplied by paleo-Red River delta during the Late Miocene.…”