Evidence of pre-oligocene emergence of the indian passive margin and the timing of collision initiation between India and Eurasia

Precise knowledge of the timing of Indo-Eurasian collision is prerequisite for understanding the subsequent evolution of the Himalayan- Tibetan orogenic system, yet the topic remains controversial despite decades of research. We present new data for the Upper Oligocene Basgo Formation of the Indus Basin of NW India that specifi cally address the proposal that collision initiated no earlier than the Eocene- Oligocene boundary. The Basgo Formation has been cited as the base of the Indus Group because of its previously assumed Maastrichtian age. This age has been revised to Upper Oligocene, but the stratigraphic location has not been re-evaluated. As such, it has been used as evidence of Oligocene-aged collision between India and Eurasia. Based on age constraints in the remainder of the Indus Group, we revise the stratigraphy and place it instead toward the top of the succession. We present evidence that the zircons in the Basgo sandstones originated from the Indian passive margin. Because conglomerate clasts are known to come from the Transhimalayan batholith to the north, our data support mixed provenance and require a minimum late Oligocene age for India-Eurasia collision in the NW Indian Himalaya. (U-Th)/He cooling dates for detrital zircons from the Basgo Formation range from 52.6 to 28.25 Ma, however, implying that their most probable source, the Indian passive margin, was emergent and eroding prior to Oligocene time due to collision. These data alone do not speak to whether the Basgo Formation records pre-Oligocene collision of India and Eurasia or India and the Transhimalayan Ladakh batholith, but as of the date of this publication, there is no evidence for Oligocene collision anywhere else in the Ladakh region. Thus, we interpret our data to demonstrate terminal collision between India and Eurasia prior to Oligocene time.