Surface uplift of the southeast margin of the Tibetan Plateau is interpreted to have progressed from the northwest, near the Tibetan border, to the southeast, in the Red River region of the central Yunnan Province, China. This interpretation is based on existing thermochronologic data and new mapping and sedimentologic and paleobotanic data demonstrating incision in the headwaters of the Red River in Pliocene time or later. Together with previously published data demonstrating surface uplift and a gradient in crustal thickness in the absence of upper crustal shortening, this is strong evidence for growth of the southeast margin of the Tibetan Plateau through lower crustal flow. Displacement along the Ailao Shan–Red River shear zone slowed or ceased in early Pliocene time, and the Xianshuihe-Xiaojiang fault system initiated, accommodating diffuse deformation and rotation around the Eastern Himalayan syntaxis. We suggest a kinematic link between the change in mode of deformation and the introduction of a weak crustal layer through lower crustal flow.

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