Thursday, 16 February 2012

The Geological Evolution and Seismicity of the Tibetan Plateau

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.
Fig.1 Active structures of the Tibetan Plateau (based on Taylor 2009) Copyright: EDD, CTA

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.
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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