Sediment flux in the modern Indus River inferred from the trace element composition of detrital amphibole grains

Abstract

The drainage basin of the modern Indus River is composed of tectonic blocks that include oceanic arc units in the Indus Suture, reworked components of the Indian Plate and various parts of the former southern margin of Eurasia against which India collided. We investigate how these source units contribute to the modern bedload of the Indus River using the major and trace element composition of single detrital amphibole grains. Sediments from rivers eroding restricted drainage areas were analyzed to characterize the major potential source units. Samples taken at various localities downstream in the main Indus River allow the evolving provenance to be assessed. When coupled with existing bulk sediment Nd isotope data, a basic mass balance for the river can be constructed. The rapidly exhuming Southern Karakoram Metamorphic Complex is the dominant sediment source to the deep-sea Indus Fan. Although the Nanga Parbat Massif, located adjacent to the river's course is also rapidly exhuming, this is not an important source of sediment, probably because of its small area and the tectonic mechanism for its exhumation. The trace element composition of detrital amphibole grains can be a useful provenance indicator, especially in the identification of dominant sources in a complex mixed sediment environment. Trace element ratios Nb/Zr and Ba/Y were identified as being especially useful in discriminating detrital amphibole grains from various sources.