Oligocene Orogen‐Parallel Extension in Southern Tibet During Indian Continental Subduction

The controversial history of Indian subduction beneath Asia is crucial to understand the Himalayan orogeny and more in general the geodynamic process of continental subduction. New key information is here presented from the Oligocene‐Pliocene Shiquanhe Basin located in the southwestern Tibetan Plateau. The alluvial‐fan, lacustrine, and braided‐river sediments of the Oligocene Rigongla Formation were non‐conformably deposited onto the Upper Cretaceous Gangdese granitoid rocks and fed from erosion of the batholith itself and of associated Paleogene Linzizong volcanic rocks. Stratigraphic evidence testifies to the development of an orogen‐parallel intracontinental rift along the retro‐side of the Gangdese arc in the Oligocene, at the same time as the Kailas basin formed along the pro‐side of the Gangdese arc. The subsidence of these twin basins may have been caused by steepening of the subducting Indian continental slab or by the passage of a wave of dynamic topography during continuing subduction. Plain Language Summary The convergence and collision of India and Asia leading to the formation of the Himalayan Mountains and Tibetan Plateau (“the roof of the world”) is one of the most significant geological events of the Cenozoic Era. Geophysical data show that rocks of the Indian continent lie beneath southern Tibet, but the early subduction history of India related to the initial topographic grow of the Himalaya remains unclear. We studied the Shiquanhe Basin in southwestern Tibet to investigate the continent's subduction history using stratigraphic, sedimentological, and provenance analyses. Our results indicate that an orogen‐parallel rift developed in southern Tibet during the Oligocene Epoch. Challenging earlier beliefs, we propose two alternative models to explain such event of tectonic extension: steepening of subduction angle or passage of a topographic wave across the subducting plate. The latter hypothesis would also explain the widespread uplift and lack of Oligocene sedimentary deposits recorded all across the front of the Himalayan range. Key Points Oligocene Rigongla Fm. testifies to orogen‐parallel extension along retro‐side of Gangdese arc Shiquanhe Basin passed from extension to compression in Oligocene to Miocene times Oligocene extension was caused by change of Indian subduction angle or wave of dynamic topography