Rapid exhumation of the upper Siang Valley, Arunachal Himalaya since the Pliocene

The work presents in situ low temperature thermochronological (Apatite Fission Track (AFT) and (U-Th)/He zircon (ZHe)) ages from the Siang Valley in the NE Himalayan syntaxial zone. Our results highlight the unique spatial erosion along the axial regions of the Siang antiform. All the ZHe cooling ages from the region range between ~ 14 and 9 Ma, except for a northernmost sample that is distinctly younger (~ 3.8 Ma). Results from AFT data (2.4 Ma to present) record Pleistocene cooling along the Siang River valley in the Siang antiform and reveal a general northward younging trend of ages with rapid cooling < 1.0 Ma in the northernmost part of Siang Valley. Thermal modelling of confined track data highlight the phase of relatively slow cooling (5-11 °C/Ma) above the PRZ of ZHe since ~ 14 Ma followed by a period of rapid cooling (40-100 °C/Ma) post ~ 1.5 Ma. Spatially coincident young cooling ages, steep slopes, higher fluvial incision potential and heavy precipitation indicates a balance between rapid rock uplift and erosion in the upper Siang Valley. Marked younging of the northernmost ZHe cooling age together with previously published young (< 10 Ma) detrital ZFT ages along the Siang Valley mark the southwestern limit of the rapid uplift related to the growth of the Namche Barwa antiform. The abrupt change in the ZHe cooling ages may indicate that the region is constrained to the south by an unmapped fault. The youngest AFT ages are reflected in a region receiving maximum modern-day average annual precipitation and highest specific stream power observed along the Siang River. We postulate that a strong precipitation gradient developed along the Siang River valley and advanced northward into the drier Tibetan plateau since the late Pliocene resulting in feedback between tectonics and erosion leading to the rapid cooling recorded by AFT ages.