Geography
students know that the glaciers carve a 'U' shaped valley. Thus it is
common perception that the glacier's limits end where the 'V' shaped
valley carved by a river starts. Is it really like that or the limit of
glaciation is controlled by some other natural agency too? This has been
baffling the geologists since quite some time. If there was another
agency, what it could be is yet another poser. Glaciers as such are a
storehouse of past climatic information. With advanced techniques it is
now possible to work out precisely the past climatic details by studying
the remnants of glacial sediments and moraines.
Such areas that contain the clues of the past are unfortunately remote
and quite inaccessible. It requires lot of grit and determination to
reach those places and search for the clues. A team of six determined
earth scientists and physicists including N. Juyal, M.G. Yadava and A.K.
Singhvi from Physical Research Laboratory (PRL), Ahmedabad; R.K. Pant
and N.K. Saini from Wadia Institute of Himalayan Geology, Dehradun and
N. Basavaiah of Indian Institute of Geomagnetism, Mumbai dared to
threadbare the area around Garbyang, Pithoragarh district in Uttarakhand
and the information they could gather has proved to be of immense value
Juyal states in a joint paper published by the team in the prestigious
journal PALAEO, that the higher Central Himalaya is a narrow strip of
mountainous terrain between the Great Himalayan Range and the
Indo-Tibetan water divide. This is a region that has preserved records
of ancient glaciation. It is significant to note here that glacial melt
water began to flow as rivers. The flow of these rivers got impounded,
because the downstream side was somehow lifted up. Such uplifts are
possible during massive earthquakes. Thus lakes were formed and such
lakes lasted for considerable time and were eventually drained. Garbyang
was one such lake. The drained waters of the lake exposed the
sedimentary record of the lake. Such unaltered pages of the climatic and
tectonic history of the Himalayas are highly sought for.
The Swiss launched a scientific expedition to the remote Himalayas in
1930s. Heim and Gansser two pioneering Swiss geologists found varve-like
deposits at Garbyang. Glacial varve (Swedish varv) are the fine clays
deposited in lakes by the glacial melt. These are deposited in fine
layers, each representing a year of glacial melt. In other words varves
or varvites (rocks) are useful tools to unfold the past glacial/climatic
history.
Varves are exposed between Chhiyalekh to Gunji in the Kali River valley,
record Juyal and his co-workers. Chhiyalekh pass at 3100 m is the divide
that defines the southern limit of U-shaped Garbyang basin. Varves or
the glacial lake sediments of Garbyang are estimated to be around 100 m
thick. However, the steep cliffs make most of the succession
inaccessible. Top 28 m part is accessible which was been studied by
Juyal et al. The type of varves at Garbayang are typical of deposition
in a lake that was already formed while the glacier existed, a fact
confirmed by the terminal moraine that proved to be much older than the
lake sediments.
Juyal and his team feel that the stream flow from the glacier was
blocked by the terminal moraine. This led to the formation of a lake,
which grew in size with the passage of time. Lake waters are always
placid and calm. Geologists term such environments as low energy
environments. In such conditions the finest of the sediments are
deposited in layers. Snow melt increases with the advent of summers and
thicker layers are formed, whereas during winters due to reduced snow
melt the layer of sediments are thinner. One can count in such varves
with ease, as to how many winters and summers the area has witnessed in
the past.
The natural dam formed due to accumulation of moraines can not last for
ever. The day it gives way, there is a sudden release of water from the
impounded lake. This caused sudden influx of water from the source area.
Naturally the energy regime of the lake had changed. In now the so
called high energy environment larger grains started to get deposited.
Thus Juyal reported gravel layers which marked the draining of the lake.
Trans Himadri Fault (THF) an ancient tectonic feature which was formed
some 23 MA is a feature that dominates the southern margin of Garbayang
lake. Geologists report that this fault though apparently dormant has
been reactivated several times in the past at 19 MA, 11MA and again at
the end of Pliocene, some 1.75 MA. Reactivation of a fault of such great
magnitude implies massive earth movements.
A minute study of the 28 m of the layers of fine lake sediments and the
thick moraines has yielded great clues.
Juyal and his co-workers conclude that during the past 1.75 million
years glaciation in the Central Himalayas was controlled by the
structure of the basin, which in turn depended on the tectonism. Thus
THF limited the advance of the glaciers. As the glaciers advanced
through the permissible space they generated plenty of glacial flour and
moraines. The terminal moraines at the THF were responsible for damming
of the melt water, leading to formation of the Garbayang Lake.
Earth's past climates deduced from the oxygen isotopes have revealed
alternating warm and cold cycles. Called as Marine Isotope Stages (MIS)
the climatic history has lots of fascinating things. For example in the
past 2.35 million years about fifty such cycles have been reported.
Though corroborative evidence for even half the numbers of such cycles
have yet to be obtained, yet the work of Juyal and his colleagues is a
step forward towards the past. According to them the sediments are
equivalent to MIS stage 4.
Detailed study of varves revealed high frequency, low magnitude climatic
oscillations between 18±3 and 13±2 thousand years (ka). Fine sediments
of varves indicate prominent cooling between 12 and 11ka which is
attributed to younger dryas. Younger Dryas represents the 'Big Freeze'
between 12700 to 11500 years before present. It was a global phenomenon
during which apparently everything remained frozen. Unlike today's
global warming, had the media been developed a global cooling would have
been the daily headlines!
Juyal and his colleagues observed seismicity with the help of preserved,
deformed layers of the varves at Garbayang. A dating of these sediments
indicated that THF was active between 20 and 17 ka and 14 and 13 ka. An
active THF is something quite serious. Because in case the fault is
reactivated to the scale on which it was operating in the days gone by,
it can cause massive destruction. In addition to Juyal et al past
seismicity in Grabayang area with the help of soft sediment deformation
has also been confirmed by B.S. Kotlia and K.S. Rawat of the Kumaon
University, Nainital. Kotilia and Rawat interpret liquefaction of sand
layers due to past earthquake/s.
The Himalayas, especially the Central Himalayas have lots of clues still
hidden under the sub-surface that need to be unearthed. These are the
clues about the past tremors that shook the Himalayas or even made them
rise. In the present context any information of such possibility is
significant, because we are populating the calm mountains vigorously. An
earthquake can lead to great loss of life in densely populated areas.
Relief measures in Himalayas become a problem in such an eventuality
because of disruption of communication links. It is better to take
precautions.
The designs of earthquake resistant houses at present are as per a
maximum anticipated shaking. The strength of the THF that has re-carved
the entire valleys shows that a dense population can be an invitation to
massacre if the fault decides to repeat its past.
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