Detailed mapping of field relationships along the Red River fault reveals information about the distribution and magnitude of slip along the fault, about its interactions with other regional fault systems, and the relationship between river incision and growth of the eastern margin of the Tibetan Plateau. The Red River fault is complex, consisting of up to four strands, and is dominated by right-lateral strike-slip displacement. Evidence for an extensional component of displacement is strongest along the northern part of the fault, and decreases to the southeast, to zero southeast of a major bend in the fault. Results of this study indicate dextral displacement on the Red River fault is probably at least ∼40 km, 15–16 km of which predates incision of the Red River in Pliocene time or later, and probably also predates plateau growth and development of other regional fault systems. Long-term average slip rate on the Red River fault is a minimum of ∼5 mm/yr. However, regional active tectonics are characterized by rotation of the upper crust around the eastern Himalayan syntaxis, bounded to the east by the Xianshuihe-Xiaojiang fault system, which deflects but does not cut the Red River fault. The distributed nature of the Xianshuihe-Xiaojiang fault system as it approaches the Red River fault is important for accommodating shearing across the strong crustal anisotropy formed by the Red River fault and Ailao Shan shear zone. The Red River fault appears to terminate in the extending Dali fault system in northwest Yunnan. Pliocene surface uplift, river incision, and rotation around the eastern Himalayan syntaxis are inconsistent with the pre-Pliocene displacement on the Red River fault and lateral extrusion along the Ailao Shan shear zone advocated by the “extrusion model.” This suggests a significant change in crustal conditions on the southeast margin of the Tibetan Plateau in Pliocene time, possibly the result of lower-crustal flow.

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