Orographic rainfall drives the Himalaya drainage divide to move north

Abstract

The Yarlung River flows between the east-trending Himalayan and Gangdese orogens, and across several north-trending rifts. It carries rich information on the Indian-Asian collisional tectonics and the Asian monsoon. The tectonic activity and the intensified monsoon would increase topographic uplift and rainfall, respectively, thereby driving drainage reorganization and divide migration. However, how and when the river network formed and adjusted remains controversial. Knowledge of its drainage-divide stability can provide a new and independent perspective for the river network reorganization and the tectonic and climatic changes behind it. Here, we reveal the drainage-divide stability of the Yarlung River using χ-plot and Gilbert metrics methods and simulate the river's formation process. We find that the Himalaya drainage divide is primarily moving north, which is driven by the spatiotemporal variations in precipitation across the Himalayas. The northward drainage-divide migration will further enhance the fluvial incision in the southern flank of the Himalayas. The drainage-divide migration and numerical simulation results support an early (>20 Ma) formation of the modern, orogen-parallel Yarlung River. The Gangdese drainage divide shows predominantly northward and southward migration of the Endorheic-Yarlung and Nu-Yarlung divides, respectively. Such mobility is controlled by base-level rise within the endorheic drainage and eastern downstream reaches of the Yarlung River, respectively. Moreover, the north-trending rifts separate the drainage divides into five zones, each with a distinct migration pattern. This demonstrates a surficial response to the along-strike tectonic variations, especially underthrusting Indian slab tears.