EROSION PATTERNS, TECTONIC UPLIFT AND SEDIMENT TRANSPORT IN THE INDUS RIVER DRAINAGE BASIN

The Indus River drains some of the world’s most impressive topography and feeds the erosional outwash into the Arabian Sea to form the Indus Fan. Understanding the growth of topography in the western Himalaya, Karakoram and Tibet is important to testing models of strain accommodation during India-Asia collision. This can only be done with the detrital record because erosion has largely removed the early uplift and exhumation record from the mountains themselves. Models explaining the relationship between tectonics, climate and erosion require the reconstruction of paleo-erosion patterns to compare with paleoceanographic models. A first step in this process is the reconciliation of modern tectonic uplift data with erosion rates and stream bedloads in the present Indus drainage basin. Ion microprobe analysis of Pb isotopes within single detrital K-feldspar grains taken from the different rivers that feed the Indus allow both the origin of the final sediment delivered to the modern Indus Fan to be determined, and also permits the isotopic heterogeneity of the orogen to be mapped. Although some of the major tectonic blocks have been characterized further east, little data exists from within the Indus drainage basin itself. Low, unradiogenic isotopic ratios typify the Kohistan and Dras arc units of the suture zone, and some parts of the Transhimalayan batholith. This reflects the modest crustal input to the petrogenesis of these units. Higher values are recorded from the Karakoram and Hindu Kush, reflecting a mixed mantle and crustal petrogenesis. The High Himalaya mostly yield still higher values. The most radiogenic grains are derived from Nanga Parbat, and presumably from the equivalent, if less metamorphosed, Lesser Himalaya. High values reflect the most reworked nature of High Himalayan granites and metamorphic rocks. The high isotopic ratios also make grains eroded from these sources easy to identify in mixed river and fan sediments. Although no K-feldspars with Nanga Parbat type Pb characteristics are known from the Pleistocene Indus Fan at ODP Site 720, they are seen in the lower reaches of the modern Indus River at Sukkur. Additional provenance data is available from the trace element composition of single amphibole grains, which also show systematic differences between different sources. These do not support Nanga Parbat being a major source of sediment, and suggest that the radiogenic Pb K-feldspar grains seen in the Indus River are derived from the Lesser Himalayan ranges. Amphibole populations in the main Indus River are dominated by erosion of the Karakoram and Hindu Kush, and are not significantly diluted by flow from the main foreland tributaries (Jellum, Chenab, Ravi and Sutlej). Himalayan source rocks contain less amphibole than those units within and north of the Indus Suture and consequently cannot dominate the net flux of this mineral group. The bulk Nd isotopic composition of the Indus Fan indicates that ~40% of the total sediment flux must be from Indian Plate sources. A combination of provenance methods suggests that sediment flux reflects both the rate of tectonic uplift and the area of a given source. The Indus Fan may preserve a good record of erosion of units north of the Indus Suture, contrasting with the Himalaya-dominated record of the Bengal Fan.