『Abstract
The Indus River has been progressively transformed in the last
decades into a tightly regulated system of dams and channels,
to produce food and energy for the rapidly growing population
of Pakistan. Nevertheless, Indus River sands as far as the delta
largely retain their distinct feldspar- and amphibole-rich composition,
which is unique with respect to all other major rivers draining
the Alpine-Himalayan belt except for the Brahmaputra. Both the
Indus and Brahmaputra Rivers flow for half of their course along
the Indus-Asia suture zone, and receive major contributions from
both Asian active-margin batholiths and upper-amphibolite-facies
domes rapidly exhumed at the Western and Eastern Himalayan syntaxes.
Composition of Indus sands changes repeatedly and markedly in
Ladakh and Baltistan, indicating overwhelming sediment flux from
each successive tributary as the syntaxis is approached. Provenance
estimates based on our integrated petrographic-mineralogical data
set indicate that active-margin units (Karakorum and Transhimalayan
arcs) provide 〜81% of the 250±50×106 t of sediments
reaching the Tartbela reservoir each year. Partitioning of such
flux among tributaries and among source units allows us to tentatively
assess sediment yields from major subcatchments. Extreme yields
and erosion rates are calculated for both the Karakorum Belt (up
to 12,500±4700 t/km2 year and 4.5±1.7 mm/year for the
Braldu catchment) and Nanga Parbat Massif (8100±3500 t/km2
year and 3.0±1.3 mm/year). These values approach denudation rates
currently estimated for South Karakorum and Nanga Parbat crustal-scale
antiforms, and highlight the major influence that rapid tectonic
uplift and focused glacial and fluvial erosion of young metamorphic
massifs around the Western Himalayan Syntaxis have on sediment
budgets of the Indus system.
Detailed information on bulk petrography and heavy minerals of
modern Indus sands not only represents an effective independent
method to constrain denudation rates obtained from temperature-time
histories of exposed bedrock, but also provides an actualistic
reference for collision-orogen provenance, and gives us a key
to interpreting provenance and paleodrainage changes recorded
by clastic wedges deposited in the Himalayan foreland basin and
Arabian Sea during the Cenozoic.
Keywords: modern sands; bulk petrography; heavy minerals; sediment
budgets; collision orogens; Karakorum; Nanga Parbat; Himalaya
』
1. Introduction
1.1. Sampling and analytical procedures
2. The Indus Basin
2.1. Indus River upstream of Tarbela Dam
2.2. Indus River downstream of Tarbela Dam
2.3. Indus River across the plains
3. Geology of the Western Himalayan Syntaxis
3.1. Karakorum and Hindukush
3.2. Transhimalayan arcs and sutures
3.3. Northwestern Himalaya
3.4. Nanga Parbat Massif
3.5. Pakistan Himalaya
3.6. West Pakistan ranges
4. Composition of Indus sands
4.1. Karakorum and Hindukush tributaries
4.2. Ladakh and Kohistan tributaries
4.3. Himalayan tributaries
4.4. West Pakistan tributaries
4.5. Trunk river
4.6. Thal Desert
5. Sediment budgets
5.1. Heavy-mineral concentrations
5.2. Indus sands upstream of Tarbela Dam
5.3. Sediment yields and erosion rates
5.4. Indus sands downstream of Tarbela Dam
5.5. Himalayan tributaries of Punjab
5.6. The effect of chemical weathering
5.7. Total Indus budget
6. Symmetries of Himalaya sediment transport
7. Conclusions
Acknowledgments
Appendix A
References