『Abstract
The Brahmaputra River slices an exceptionally deep canyon through
the eastern Himalaya. Fission-track and laser-ablation U-Pb ages
of detrital zircon grains from the river document very rapid erosion
from this region and its impact on sediment fluxes downstream
in the Brahmaputra. Downstream from the canyon, 47％ of the detrital
zircons in the river's modern sediment load comprise a fission-track
age population averaging only 0.6 Ma. Equally young cooling ages
are reported from bedrock in the canyon through the Namche Barwa-Gyala
Peri massif but are absent from riverbank sands of major tributaries
upstream. Simple mixing models of U-Pb ages on detrital zircons
from samples taken above and below this massif independently suggest
that 45％ of the downstream detrital zircons are derived from the
basement gneisses extensively exposed in the massif. Constraints
on the extent of the source are provided by bedrock cooling ages
together with sediment-flux estimates at Pasighat, India, suggest
exhumation rates averaging 7-21 mm yr-1 in an area
of 〜3300 km2 centered on the massif. This rapid exhumation,
which is consistent with the very young cooling ages of the detrital
zircons from this area, produces so much sediment that 〜50％ of
the vast accumulation in the Brahmaputra system at the front of
the Himalaya comes from only 〜2％ of its drainage. This extreme
localization of rapid erosion, sediment evacuation, and bedrock
cooling bear on (1) common assumptions in geodynamic and geochemical
studies of the Himalaya about sources of sediment, and (2) plans
for hydroelectric development and flood management in southeastern
Tibet and the heavily populated areas of eastern India.
Keywords: Himalaya; Yarlung-Tsangpo River; Siang River; Brahmaputra
River; fission-track dating; U-Pb ICP-MS dating; detrital-mineral
thermochronology』
Introduction
Methods
Results from detrital zircon fission-track dating
Results from detrital zircon U-Pb dating
Exhumation rates in the Namche Barwa-Gyala Peri area
Conclusions
Acknowledgments
References cited