Distorted drainage basins as markers of crustal strain east of the Himalaya

Abstract

Extensive lithospheric deformation on the eastern margin of the India/Eurasia collision has brought three major rivers into exceptionally close proximity. For over 300 km the Salween, Mekong, and Yangtze Rivers are only tens of kilometers apart, ∼10 times closer than rivers of comparable length elsewhere in the world. We view deeply incised river valleys as displacement markers that move with the underlying crust over time scales of at least tens of millions of years. Accordingly, the close spacing of these major rivers and their nearly parallel trends reflect the large crustal strains near the eastern Himalayan syntaxis. These river characteristics are consistent with two primary phases of deformation in this tectonic setting: NE‐SW shortening of ∼0.5 oriented radially from the eastern syntaxis, followed by dextral shear strain of 6±1 along a north trending zone. Pervasive shearing of this magnitude throughout the ∼150‐km‐wide region of anomalous drainage would account for ∼1000 km of northward motion of India relative to southern China. Directly east of this region, another ∼1000 km of northward motion is likely to have been accommodated by pervasive shearing through a zone hundreds of kilometers wide. Finally, we draw attention to the remarkable accord between the arcuate paths of each of the three great rivers in the broader southeastern Tibet region and small circles that curve around the eastern Himalayan syntaxis.