Detrital U–Pb rutile and zircon data show Indus River sediment dominantly eroded from East Karakoram, not Nanga Parbat

Abstract

Rapid uplift and exhumation is hypothesized to occur within focused zones of orogenic syntaxes which may dominate sediment flux from a mountain belt. The Namche Barwa-Gyala Peri Massif (NBGPM) in the Himalayan Eastern syntaxis is an example of such a high sediment production zone. Here we apply detrital zircon and rutile U–Pb dating to modern river sediments sampled up- and downstream of the Nanga Parbat-Haramosh Massif (NPHM) in the Western Himalayan syntaxis. Mass balancing of these data show that the total amount of sediment contributed to the trunk Indus from NPHM is relatively small, accounting for 9–10% of the total zircons reaching the Arabian Sea, and potentially much less when source fertility is taken into account. The sediment productivity of NPHM is high but likely less than half of that seen at NBGPM (30–56 Mt/yr vs. 72–211 Mt/yr). This discrepancy may be explicable by the drier climate, less prominent syntaxial knick-zone, and weaker spatial relationship between NPHM and the Indus River compared to its eastern twin. Our revised sediment flux estimate for NPHM implies regional modern exhumation rates of ∼3–5 mm/y, lower than estimated from the core massif since 1.7 Ma.