Cenozoic tectonic and geomorphic evolution of the Red River Region, Yunnan Province, China

Abstract

(cont.) Xianshuihe-Xiaojiang fault system during growth of the southeast plateau margin. Cosmogenic ²⁶A1 and ¹⁰Be basin-wide erosion rate and burial ages indicate a background incision rate of [approximately] 0.05 to 0.10 mm/a, lower than the long-term incision rate minimum of [approximately] 0.26 mm/a. Cosmogenically-determined incision rate approximately doubles to [approximately] 0.20 mm/a in the region of maximum dip-slip displacement on the Red River fault. This thesis also develops a new cosmogenic tool for quantitative landscape analysis: using depth dependence data for multiple cosmogenic nuclides from a single site to constrain an erosion history. This method is applied in the Dry Valleys region of Antarctica.

This thesis outlines the Cenozoic development of the Red River region, exploring regional landscape evolution and tectonic accommodation of the India-Eurasia collision, focusing on the Oligo-Miocene, left-lateral Ailao Shan shear zone and the active, right-lateral Red River fault on the northeast margin of the shear zone, along which the Red River has incised a deep valley. Oligo-Miocene fluvial and alluvial conglomerates in the valley record shear zone unroofing: pervasive, syn-depositional shortening indicates transpressional exhumation. A low-relief landscape, developed in Late Miocene time, was probably uplifted in Pliocene time, triggering the incision of the Red River and isolating the low-relief landscape from modem base level. On the basis of stratigraphic data, river incision began in Pliocene time or later. Tributary longitudinal profiles indicate two-phase incision, the result of pulsed plateau growth or trunk channel adjustments to changing climate conditions. Paleo-Red River reconstruction indicates [approximately] 1400 m river incision, 1400-1500 m surface uplift and 750 m vertical displacement across the northern part of the Red River fault. Minimum right-lateral displacement on the fault is 40 km, 15-16 km of which predates river incision, plateau growth and development of other regional fault systems. Long term average slip-rate is a minimum of [approximately] 5 mm/yr. Rotation of a crustal fragment around the eastern Himalayan syntaxis, bounded on the east by the Xianshuihe-Xiaojiang fault system, causes deflection of the Red River fault, accommodated by distributed shear along strike of the Xianshuihe-Xiaojiang fault system. The Red River fault has decreased in regional importance since the initiation of the