How did the Tibetan Plateau grow? A view from Eastern Tibet

Many (if not most) models of Tibetan Plateau growth infer a systematic expansion of the region of high elevations from the plateau interior to its margins. Whether driven by lower crustal flow or by the translation of crustal blocks along major translational faults, a general conclusion is that the high elevations along the eastern margin of the plateau reflect a Late Miocene (~ 10 Ma) event. Such an interpretation is consistent with timing of exhumation in major river canyons along the margin, and appeared to be consistent with the timing of uplift/exhumation in the ranges bounding the eastern plateau. Results from a series of vertical thermochronology transects requires us to rethink the history of exhumation in the Longman Shan and associated ranges. Specifically, results from an age-elevation transect detailing the cooling history of crystalline basement rocks in the Longman Shan (utilizing a range of thermochronometers) indicate significant cooling/exhumation beginning as early as 50 Ma. Our model results indicate that this exhumation rate increased in the Late Oligocene to Early Miocene (30 - 20 Ma), but a hiatus or slowing of exhumation in the mid Miocene is required to satisfy constraints from low-temperature thermochronometers. More rapid exhumation (continuing until the present), beginning about 12-10 Ma is consistent with these low-T data. One possible explanation for this pattern of cooling in the Neogene is that the present-day eastern margin to the Tibetan Plateau is a longer-lived topographic feature than normally assumed, and that as the plateau developed it grew in behind this bounding topographic feature. If true, this model has substantial implications for both patterns of growth of the plateau and also for directions and rates of sediment transport during the growth of the plateau.