The Tibetan
Plateau: Introduction
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.
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 of 8.6 magnitude claimed atleast 1526 lives (most
of them in the Indian side) while a more recent event of 6.9 magnitude struck
Kyegudo (Yushul)claiming over 2,698 lives with 12,135 injured. Usually, the
death toll during other events has been minimal due to very low population
density over the plateau.
The Tibetan Plateau is undoubtedly the most prominent and
distinguishable feature on the face of the earth, rightly known as the ‘Roof of
the World’. With an average elevation of
around 4500 meters (14,763 ft.) and covering around 2.5 million sq. km of area,
it is the largest and highest plateau in earth’s 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 Basin while a series
of alternating deep forested valleys and high mountain ranges marks its eastern
periphery.
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| Figure 1. A satellite's-eye view of the Tibetan Plateau. Image: NASA |
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 elevated Tibetan Plateau.
Fragmentation
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, Songban-Kardze, Jangthang
and Lhasa Terranes. Tibet has a thick continental crust of about 65 km twice
the average thickness of about 30 km. 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.
The formation of 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.
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| Figure 2. Illustration of different theories about the formation of the Tibetan Plateau |
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 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 plateau due to east-west extension; and
strike-slip faulting are a common phenomena at the eastward and south-eastward
movement of different blocks.
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| Figure 3. Active structures of the Tibetan Plateau. Black line show thrust faulting mainly along the Himalayas and northern parts; green lines depict normal faulting; and red lines indicate areas experiencing strike-slip faulting. |
The understanding of the geology and seismicity of Tibet is
important as it is related to the resources and development in Tibet. The
location of mineral deposits and prospects can be identified from geological
information and seismic data helps 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
also close to an active rift system in Woga, thus posing a big threat to the
people living in the downstream regions.
