Incision and differential bedrock uplift along the Indus River near Nanga Parbat, Pakistan Himalaya, from 10Be and 26Al exposure age dating of bedrock straths

Well preserved, river-cut bedrock surfaces (straths) abandoned by the progressive incision of the Indus River in northern Pakistan were dated using in situ cosmogenic 10Be and 26Al exposure age dating. Measurements on young and modern straths show no significant ingrowth of 10Be and 26Al, implying that strath exposure ages closely approximate their abandonment age. Strath exposure ages range up to 65 ka and increase with height above the river. The straths can be divided into two groups: low straths dated at about 7 ka, which yield very rapid bedrock incision rates (9–12 m/ka), and high straths >7 ka in age, which yield somewhat slower incision rates (1–6 m/ka). Incision rates for both groups increase by ∼1–3 m/ka downstream toward an active reverse fault, the Raikot fault, that forms the western border of the Nanga Parbat–Haramosh massif (NPHM). The downstream increases in incision rates for the low straths mimic downstream increases in the modern gradient, suggesting that bedrock incision rates have been proportional to stream power over the last 7 ka. Gradient increases are probably the result of the creation of knickpoints by bedrock uplift along the Raikot fault. The longitudinal profile and steep gorge of the Indus River in the study area indicate that bedrock incision appears to keep pace with differential vertical motions between the NPHM and surrounding regions. Thus, the 1–3 m/ka difference in incision rates likely reflects the differential bedrock uplift rate between the NPHM and the Skardu Basin. The differential incision rate is consistent with differential exhumation rates obtained from apatite fission track ages assuming geotherms of ≤80–90°C/km in the NPHM and 35°C/km in the Skardu Basin. This relation suggests that rapid bedrock incision of at least 1–3 m/ka has persisted within the NPHM for the last 0.5 Ma. The acceleration of incision rates throughout the study area since 15 ka is likely to be a climatic signal that records an increase in the discharge and/or sediment load of the Indus River in the study area related to deglaciation of the surrounding terrane.