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

[1] High topography in central Asia is perhaps the most fundamental expression of the Cenozoic Indo-Asian collision, yet anunderstandingof the timingandratesofdevelopmentof theTibetan Plateau remains elusive. Here we investigate the Cenozoic thermal historiesofrocksalongtheeasternmarginof theplateauadjacent to the Sichuan Basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed. Temperaturetime paths inferred from Ar/Ar thermochronology of biotite, multiple diffusion domain modeling of alkali feldspar Ar release spectra, and (U-Th)/He thermochronologyof zirconandapatite imply that rocks at the present-day topographic front of the plateau underwent slow cooling (<1 C/m.y.) from Jurassic times until the lateMioceneor earlyPliocene.The regional extent andconsistencyof thermal histories during this time period suggest the presence of a stable thermal structure and imply that regional denudation rateswere low (<0.1 mm/yr for nominal continental geotherms). Beginning in the late Miocene or early Pliocene, these samples experienced a pronounced cooling event (>30 –50 C/m.y.) coincident with exhumation from inferred depths of 8–10 km, at denudation rates of 1–2 mm/yr. Samples from the interior of the plateau continued to cool relatively slowly during the same time period ( 3 C/m.y.), suggesting limited exhumation (1–2 km). However, these samples recorda slight increase incooling rate (from<1 to 3 C/m.y.) at some time during the middle Tertiary; the tectonic significance of this change remains uncertain. Regardless, late Cenozoic denudation in this region appears to have been markedly heterogeneous, with the highest rates of exhumation focused at the topographic front of the plateau margin.We infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable SichuanBasin and thusmarks the onset of deformation related to the development of the Tibetan Plateau in this region. The present margin of the plateau adjacent to and north of the Sichuan Basin is apparently no older than the late Miocene or early Pliocene ( 5–12 Ma).