Wednesday, 4 January 2012

Rifting of the Tibetan Plateau

One of the prominent and distinguishable feature on the vast Tibetan Plateau is its spectacular rift valleys. Many scientists reckon that these rifts are formed due to the east-west extension of the crustal layer caused by active tectonic deformation. In this post, we will briefly discuss the formation of these rift valleys that adds to the grandeur of Tibet, the roof of the world.

Around 50 to 55 million years ago, Indian plate started colliding with the Eurasian plate, resulting in crustal shortening due to the northward movement of Indian plate relative to the stable Eurasia at a rate of 35-50 mm per year. The process led to the eventual construction of the Tibetan Plateau.

Fig. 1 Neo-tectonic map of the Tibetan Plateau showing active features. The north-south trending rifts are shown as green lines

In the past 10 million years, the plateau experienced widespread extension, which has been evident from Landsat imagery, fault-plane solutions of earthquakes, and field observations. This extension is expressed by a series of roughly north-south trending rifts, which are a notable feature of the Tibetan Plateau (Fig. 1). Many of the lakes have also been formed in the depression caused by the extension of crust.

Scientific knowledge about these structures has greatly improved since its initial recognition during late 1970s. The progress in knowledge about these structures can be credited to several detailed field investigations and improved quality of Landsat imagery, as well as availability of seismic reflection and refraction data.
Various studies have tried to constrain the timing of extension (east-west) in Tibet, e.g. the onset of east-west extension is constrained to be ~8 million years ago in Nyenchen Thangla region of southeast Tibet, while initiation of the Yadong-Gulu rift is constrained at about ~12 million years ago (Fig. 2). A north-south trending dike swarm which was dated to estimate the age of minor extension in southernmost Tibet near Shigatse is emplaced at ~18 million years ago. Unlike our knowledge about east-west extension in southern Tibet, the studies about the rifts in northern Tibet are based mainly on fault plane solutions of earthquakes and interpretations of Landsat images.

Fig. 2 The vast stretch of valley or Graben in the Yadong-Gulu Rift in front of the Nyenchen-Thangla Range. Photograph © A.B. Wittke

One of the foremost and common explanations of the Tibetan extension is gravitational spreading of the thick crust. More recently, the convective removal of the lower mantle lithosphere under the Tibetan plateau has been suggested as a mechanism to this extensional process. Other researchers argue that the formation of the Tibetan rifts is due to local boundary conditions such as oblique convergence or basal shear associated with subduction of the Indian plate under Tibet that led to the radial expansion and stretching along the Himalayan arc.

It is possible that any one or some combination of these processes together contributes to crustal extension in the Tibetan plateau. However, data about the timing of onset and vertical extent of the extension is limited and largely disputed.

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