Large rivers and orogens: The evolution of the Yarlung Tsangpo–Irrawaddy system and the eastern Himalayan syntaxis

Ruth A.J Robinson,Cynthia A Brezina,Randall R Parrish,Matt S.A Horstwood,Nay Win Oo Nay Win Oo,Michael I Bird,Myint Thein Myint Thein,Abigail S Walters,Grahame J.H Oliver,Khin Zaw Khin Zaw

doi:10.1016/j.gr.2013.07.002

Abstract

The eastern Himalayan syntaxis has experienced some of the highest rates of deformation and erosion in the orogen during the Late Cenozoic, and the Yarlung Tsangpo, Brahmaputra, Irrawaddy, Salween, and Mekong rivers are the key erosional systems in that region. The Yarlung Tsangpo drains southern Tibet and the deep Siang River gorge through the eastern Himalayan syntaxis before joining the Brahmaputra in northeastern India. It has been proposed that the Yarlung Tsangpo drained into other large rivers of southern Asia, such as the Irrawaddy, Salween and Red River. We have used uranium/lead dating and hafnium measurements of detrital zircons from Cenozoic sedimentary deposits in Central Myanmar to demonstrate that the Yarlung Tsangpo formerly drained into the Irrawaddy River in Myanmar through the eastern syntaxis, and that this ancient river system was established by (at least) the Middle–Late Eocene. The Yarlung Tsangpo–Irrawaddy river disconnected in the Early Miocene driven by increased deformation in the eastern syntaxis and headward erosion by tributaries of the Brahmaputra. Our results highlight the significance of the sedimentary record of large orogen-parallel rivers and provide key chronological constraints on landscape evolution during the Early Miocene phase of the Himalayan orogeny.

Highlights

Recent research on large Asian river systems (Brookfield, 1998; Zeitler, 2001; Clark et al, 2004; Clift et al, 2006; Finnegan et al, 2008; Booth et al, 2009) has focused attention on their role in tectonics and tested how influential rivers are in controlling the location and magnitude of deformation
In order to test whether zircons derived from the Gangdese batholith, or other Lhasa terrane rocks, are contained in the sedimentary rocks of central Myanmar, and to establish whether a Yarlung Tsangpo– Irrawaddy river existed during Eocene to Miocene time, we compare our detrital zircon data with published U/Pb and Hf data for Transhimalayan batholiths in the Lhasa terrane, the eastern syntaxis, and within the current catchment of the Irrawaddy River in northeastern Myanmar (Fig. 1)
We propose that the breakdown of the Yarlung Tsangpo–Irrawaddy river occurred in the Early Miocene, and since the depositional age of the Miocene sedimentary rocks is not well constrained, we tentatively place the timing of the event to around 18 Ma based on the age of the youngest zircon in the dataset which has a Gangdese provenance

Summary

Introduction

Recent research on large Asian river systems (Brookfield, 1998; Zeitler, 2001; Clark et al, 2004; Clift et al, 2006; Finnegan et al, 2008; Booth et al, 2009) has focused attention on their role in tectonics and tested how influential rivers are in controlling the location and magnitude of deformation. The Mogok Metamorphic Belt occurs as narrow deformational zones (30–40 km wide) between the Central Myanmar Basin and the Shan Plateau of the Sibumasu block (Fig. 1), and is bounded by the Slate Belt in northeastern Myanmar (Mitchell et al, 2007) These regions contain intrusive rocks that are part of the Late Jurassic–Eocene magmatic arc which can be traced north through the eastern Himalayan syntaxis and into the Transhimalayan rocks of Tibet (Mitchell et al, 2007; Searle et al, 2007; Chiu et al, 2009; Mitchell et al, 2012). All of the magmatic arc rocks have similar chronologies, but differing geochemistry: the Gangdese is an I-type batholith, whereas the intrusive rocks of the eastern Transhimalayan, Mogok Metamorphic Belt and western Thailand (Fig. 1) are predominately S-type (Bertrand et al, 1999; Mitchell et al, 2007; Chiu et al, 2009; Ji et al, 2009; Searle et al, 2012)

Methods

Results

Discussion

Conclusion