『Abstract
A fundamental objective in studies of climate-erosion-tectonics
coupling is to document convincing correlation between observable
indicators of these processes on the scale of a mountain range.
The eastern Himalaya are a unique range to quantify the contribution
of tectonics and climate to long-term erosion rates, because uniform
and steady tectonics have persisted for several million years,
while monsoonal precipitation patterns have varied in space and
time. Specifically, the rise of the Shillong plateau, the only
orographic barrier in the Himalayan foreland, has reduced the
mean annual precipitation downwind in the eastern Bhutan Himalaya
at the Miocene-Pliocene transition. Apatite fission-track (AFT)
analyses of 45 bedrock samples from an E-W transect along Bhutan
indicate faster long-term erosion rates outside of the rain shadow
in the west (1.0-1.8 mm/yr) than inside of it in the east (0.55-0.85
mm/yr). Furthermore, an AFT vertical profile in the latter segment
reveals a deceleration in erosion rates sometime after 5.9 Ma.
In this drier segment of Bhutan, there are remnants of a relict
landscape formed under a wetter climate that has not yet equilibrated
to the present climatic conditions. Uplift and preservation of
the paleolandscape are a result of a climate-induced decrease
in erosion rates, rather than of an increase in rock uplift rate.
This study documents not only a compelling spatial correlation
between long-term erosion and precipitation rates, but also a
climatically driven erosion-rate change on the scale of the eastern
Himalayas, a change that, in turn, likely influences that region's
recent tectonic evolution.
Keywords: tectonics; climate; erosion; paleolandscape; Himalaya;
apatite fission-track』
Introduction
Geological setting
An uplifted relict landscape in Bhutan
Orographic barrier in the foreland of the Bhutan Himalayas
Apatite fission-track analysis
Discussion
Conclusions
Acknowledgments
References cited