DUPLEXING IN SOUTHERN TIBET AND BIRTH OF THE YARLUNG RIVER: A STRUCTURAL MODEL FOR DEVELOPMENT OF OLIGOCENE—MIOCENE RELIEF IN THE GANGDESE MOUNTAINS

Tectonic processes drive orography, affecting regional-to-global climate. Uplift of the Tibetan Plateau by collisional tectonic processes and development of the Plateau’s hydrologic system is a type example of the interplay between climate and tectonics. The Gangdese Mountains (Transhimalaya) are the southern boundary of internal drainage and the northern boundary of the Yarlung-Siang-Brahmaputra river watershed in southern Tibet. Geologic mapping from the Lazi and Dazhuka regions, approximately 350 km and 150 km west of Lhasa city, respectively, reveals a system of at least three, south-dipping reverse faults exposed along the southern flank of the Gangdese Mountains. This fault system comprises the Great Counter thrust (GCT), placing India-Asia suture zone rocks on Oligocene—Miocene conglomeratic strata, and two or more hanging-wall splays interpreted to sole into the GCT. Compilation of medium- to low-T thermochronology data from India-Asia suture zone strata and magmatic arc rocks in the Gangdese Mountains indicates widespread cooling during Oligocene—Miocene time, penecontemporaneous with deposition of the conglomeratic strata and GCT faulting. There is no discernible difference in the timing of cooling between hanging wall and footwall rocks of GCT fault system. Published seismic reflection data from the Gangdese Mountains reveals imbricated, north-dipping seismic reflections that shallow with depth, previously interpreted as a hinterland-dipping duplex. We propose that the GCT system is a foreland-dipping, passive-roof duplex that is kinematically linked with a hinterland-dipping duplex beneath the Gangdese mountains. In the Dazhuka Region, two north-dipping, brittle-ductile shear zones merge with the GCT updip, consistent with this interpretation.

The Yarlung-Siang-Brahmaputra river assumed its modern course during Neogene time, and likely existed beforehand in antecedent form. We propose that uplift of the Himalayas and corresponding topographic inversion in the Tethyan Himalaya colluded with Oligocene—Miocene duplexing along the Gangdese mountains to divert fluvial systems to the India-Asia suture zone, establishing the Yarlung River. Prior to this time, transverse rivers flowed from north to south across the Gangdese Mountains as evidenced by known and newly documented wind gaps.