The Geological Evolution of the Tibetan Plateau
The Tibetan Plateau is undoubtedly the most prominent and
distinguishable feature on the face of the globe, rightly known as the ‘Roof of
the World’. With an average elevation of
around 4500 meters (14,763 ft.) and covering an approximate 2.5 million sq. km
of area, it is the largest and highest plateau in earth’s entire geological
history. It is surrounded by the Himalaya-Karakoram complex in the south and west
that contains 14 major peaks of over 8000 meters including Mount Everest. To
the north, the plateau is bounded by the deserts of the Tarim Basin and Tsaidam
while a series of alternating deep forested valleys and high mountain ranges
marks its eastern periphery.
The geological evolution of the plateau and the ensuing tectonic
changes has been a matter of interest, debate and deliberation among the
scientific community. The general notion about the onset of its development,
even familiar to many schoolchildren, has been the collision of Indian plate
with the Eurasian plate at around 50 to 55 million years ago. This high-speed
collision due to the northward movement of Indian plate relative to the stable
Eurasia at a rate of 35-50 mm per year resulted in crustal shortening of the
plateau. The resultant high rate of uplift (upto 10 mm per year) led to the
eventual construction of the highly elevated Tibetan Plateau. Tibet has a thick
continental crust of about 65 km twice the average thickness of earth’s crust. During
the past 10 million years, the plateau experienced widespread extension (east
and west) that are expressed by a series of roughly north-south trending rifts,
which are a notable feature of the Tibetan Plateau.
Since the collision began, there have been a lot of tectonic
changes especially in the upper crust of the plateau leaving it as a collage of
continental fragments (called terranes) that were added successively to the
Eurasian plate during the Paleozoic and Mesozoic eras. The sutures (joints)
zones between these microplates consist of ophiolitic materials (volcanic
rocks) formed during the accretion of these crustal blocks. The main Tibetan
crustal blocks, from north to south, are the Kunlun Terrane, Songban-Kardze
Terrane, Jangthang Terrane and Lhasa Terrane.
Ideas behind the formation of the vast, elevated plateau?
There have been various theories proposed to explain the formation
of this immense thickness with micro scale variation upon them. An initial idea
was the concept of ‘distributed shortening’, of the Plateau by folding and
thrusting of its rocks. The faulting and subsequent movement of large masses of
rock stacked one on top of another leads to the thickening of the crust.
Later it was proposed that the under thrusting of the Indian
continental plate beneath the Tibetan Plateau leads to its subsequent uplift, a
theory known as ‘continental subduction’. This process is similar to pushing
one block of ice slab beneath another slab, causing it to rise upwards.
A more recent proposal, lower crustal flow, involves the
introduction of Indian crust beneath Tibet as melted rock, called magma.
Granitic melts derived from the subducting Indian crust rise into the overlying
Eurasian and transfer heat into the base of the Tibetan Plateau making it
buoyant to rise higher.
Active structures and
seismicity of the Tibetan Plateau
A series of tectonic deformation at varying scale in the plateau
results in various types of active structures including anticlines, synclines,
folds, left-slip faults, right-slip faults, strike-slip faults, thrust faults
and sutures that are all zones of seismic activities of varying degree. Thrust
faulting are formed at plate boundaries due to collision of plates; normal
faulting occur in the middle of the Tibetan plateau due to east-west extension;
and strike-slip faulting are a common phenomena at the eastward and south-eastward
movement of different fragmented blocks.
The data of focal mechanism, ground acceleration, GPS
vectors and earthquake catalogue suggests that due to the ongoing active
deformation across the plateau, there have been a swarm of seismic events in
the recorded history that are frequent and often severe. However, only few
events have been fatal such as the 1950Assam earthquake (Indo-Tibetan border) of
8.6 magnitude, which claimed atleast 1526 lives (although the casualties were
lesser on the Tibetan side) while a more recent event of 6.9 magnitude struck
Kyegudo (Yushul) area of western Tibet, claiming over 2,698 lives and injuring
12,135. Otherwise, the death toll during other events has been minimal due to very
low population density over the plateau.
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Fig. 2 The distribution of seismic events (over 5 magnitude) across the Tibetan Plateau between 1973 and 2012. Copyright: EDD, CTA |
The understanding of the geology and seismicity of the Tibetan
Plateau is important as it is closely related to the resources and development
in Tibet. The location of mineral deposits and prospects can be identified from
geological information while seismic data aids in recognizing hazards posed to
development projects such as construction of dams, railways, resettlement
housings etc. For example, the series of dams under construction on the
Yarlung-Tsangpo falls on the seismically active Gyatsa Canyon which experiences
strike-slip motion and is also located close to an active rift system in Woga, thus
posing a greater threat to the safety of the dam and people living in the
downstream regions. As millions of Tibetan nomads are pushed out of their
ancestral rangelands into urban ghettos, their vulnerability to earthquake hazard
is greatly increased if these concrete housing complexes are located close to
seismically active zones of the Tibetan Plateau.
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