Geology Reference
In-Depth Information
8
A
Himalayan Monsoon
Rainfall and Topography
m
4
7
6
3
5
ridge
crests
4
3
2
1
0
monsoon precipitation
2
valley
bottoms
1
0
0
20
40
60
80
100
120
140
160
Distance (km)
Apatite Fission-Track Ages versus Rainfall
B
fission-
track
ages
4
3
Lesser
Himalaya
Greater
Himalaya
Tethyan
Himalaya
2
1
me
0
85
95
105
115
125
135
145
155
South
Distance (km)
North
Fig. 10.43
Modern climate, topography, and cooling ages in the central Himalaya.
Data from the Marsyandi catchment in central Nepal. A. Weather station data define a 10-fold gradient in monsoon
rainfall, with the maximum rainfall occurring
∼
15 km from the highest topography and on its upwind flank. Apatite
fission-track cooling ages (closure temperature
∼
°
C for very rapid cooling) display a large offset across the Main
Central Thrust (MCT), but no significant gradient to the north, where ages adjacent to the South Tibetan Detachment
(STD) are about as young as those adjacent to the MCT. CD: Chame detachment. Modified after Burbank
et al
. (2003)
and Blythe
et al
. (2007).
140
monsoonal rainfall is typical of spatial gradients
in individual storms. Analysis of storm data shows
that a difference does exist: large storms penetrate
farther into the range and deliver proportionately
greater amounts of rainfall (Craddock
et al
.,
2007). As a result, the north-south gradient in
rainfall delivered by large storms is only four-
fold across the range, rather than the 10-fold
annual gradient. But, even a four-fold gradient is
much greater than the gradient in cooling ages.
Most of the Marsyandi cooling ages, especially
in the north, come from valley bottoms. Are these
ages representative of mean bedrock erosion
rates? Young fission-track ages like these tend to
have large (
≥
50
%)
uncertainties that could mask
spatial gradients. Valley bottoms are sites where
hydrothermal fluids are concentrated, and a
higher flux should create steeper geothermal
gradients (Derry
et al
., 2009). The presence of
hot springs along the Marsyandi and the theory
behind fluxes of hot water suggest perhaps that
the ages are too young, although no clear reason
exists to suspect that the northern ages are more
perturbed by hydrothermal fluxes than are the
southern ages. Moreover, relief transects of ages
that rise far above the valley bottom in the south-
ern and mid parts of this study area (Blythe
et al
.,
2007; Whipp
et al
., 2007) suggest that long-term
erosion rates could be as fast or faster in regions
with less annual rainfall and smaller storm
rainfall. Additional relief transects in the arid
north, preferably using thermochronometers
with higher resolution, are needed to confirm the
proposed climate-erosion decoupling.
How might topography change in order to
maintain a persistent erosion rate, despite a
decrease in rainfall? Given that erosion rates in
this area far exceed documented rates of soil
production, most of the hillslope erosion is
driven by landsliding. Such slides occur when
