Impacts of late Miocene normal faulting on Yarlung Tsangpo River evolution, southeastern Tibet

Abstract

Abstract The Jiacha Gorge in southeastern Tibet is the second-largest deeply incised gorge of the Yarlung-Tsangpo River, after the Tsangpo Gorge. A late Cenozoic N-S–trending normal fault, the Woka Rift fault, coincides with the western limit of the gorge. However, the relationship between the formation of the gorge, drainage evolution, and rift activity remains unclear. Analysis of the river long profile suggests that the Jiacha Gorge developed as a ~45-km-wide knickzone, rather than a local knickpoint. Projection of tributary stream profiles indicate significantly deeper incision in the gorge than in the downstream area, suggesting different controls on incision. Thermochronological data collected along two age-elevation profiles in the Woka Rift footwall and the Jiacha Gorge record rapid cooling at ca. 12–10 Ma followed by moderate cooling between ca. 10 and 7 Ma, with additional accelerated cooling after ca. 5 Ma in the gorge. We interpret late Miocene (ca. 12–10 Ma) rapid cooling to reflect the onset of east-west extension and normal faulting along the Woka Rift, whereas the Pliocene (post–ca. 5 Ma) accelerated cooling is inferred to be driven by river incision in the Jiacha Gorge. Geomorphic and sedimentologic observations suggest diversion of the Yarlung-Tsangpo River through the Jiacha Gorge, from an earlier more southerly course, after the onset of rifting. Therefore, we suggest that normal faulting of the Woka Rift anchored the Jiacha Gorge knickzone on its shoulder, while footwall uplift and drainage diversion led to enhanced incision, forming the steepest channel upstream of the Tangpo Gorge along the Yarlung-Tsangpo River.