During the last decade various mechanisms that may play a role on the formation of Tibetan plateau, have been proposed to explain the complex deformation history. Although recent studies had improved our resolution, lack of complete data set and observed complexity still prevents us from discriminating among the different models. Theoretically, widely distributed high elevations in a collision zone like Tibet, can be generated by a number of competing and sometimes mutually exclusive mechanisms including underthrusting, distributed shortening, delamination, lower crustal flow, intracontinental subduction and continental extrusion (figure 24).
Figure 24. Schematic cartoons of tectonic models proposed to explain the thickening and uplift of the Tibetan crust.
In order to quantify the effects of some of these geodynamic models, various authors explored analytic solutions and physical experiments. Earlier analytic approaches considering plates as depth independent thin viscous sheets (England & McKenzie, 1982), later upgraded to 3 dimensional models with depth dependent Newtonian viscosity. Based on recent geophysical studies, Royden et al. (1997) assumed weak lower crust beneath the plateau. Deformation patterns along portions of the eastern plateau margin suggested that upper crustal deformation is decoupled from the motion of the underlying mantle by lower crustal flow (figure 25).
Figure 25. Surface deformation models for simulated India-Asia convergence. Each
figure represent a snapshot at different times (10, 20, 40 Ma) [Shen et al.2001].
More recently, a two dimensional thermo-mechanical laboratory model for subduction produced by push and pull forces was performed (Chemenda et al. 2000). Experiment was including a strong brittle upper crust, a weak ductile lower crust and a strong upper mantle underlined by a low viscosity asthenosphere. After testing many other parameter combinations, they conclude that subduction is very sensitive to rheology and lower crustal thickness (figure 26).
Figure 26. Time evolving shots of two analog experiment that simulate tectonic underplating of the continental
lithosphere with mantle break off [Chemenda et al. 2000].
Their results mainly show that tectonic underplating, delamination and slab-break off are fundamental events causing drastic effects on the evolutionary history of the convergent system. By combining these results with available data, they elaborate on an evolutionary model for Tibet (figure 27).
Figure 27. Evolutionary model for the Himalaya-Tibet system. (a) Subduction of the Indian continental crust ,
scraping of the sedimentary and upper crustal material from the subducting plate. Failure of the subducted crust ;
(b) Rapid uplift of the subducted crustal slice. Break-off the Indian mantle ; (c) Heating, weakening and uplift of
the remaining in the mantle crustal segment of the Indian margin; (d) Underplating of the Indian continental
lithosphere under Asia and initiation of delamination of the Indian lithospheric mantle; (e) Failure of the Indian
crust in front of the orogen and initiation of the MCT; (f) Replacement of the Asian mantle by the underplated
Indian crust ; (g) Formation of the South Tibet detachment (STD) and exhumation of the metamorphics in the
Crystalline Himalayas. (h) Present stage with main boundary thrust (MBT) [Chemenda et al. 2000].
Among the geodynamic models, continental extrusion may be the only one that includes large strike-slip faulting occuring far from the syntaxis. In early 1980's, they used a plasticine model to represent India penetrating further into Asia (Taponnier et al. 1982) (figure 28). Basically, the simulated southeastward motion of the southeast Asian landmass by means of strike-slip faulting and related block rotations. According to the theory as India penetrates further into Asia, the Himalaya mountains begins to rise and the land mass of Indochina starts to extrude southeast. During later stages continuous continental collision also results in successively younger strike-slip faults in the north (figure 28). Although this rigid block approach brings problems for explaining observed internal deformation, supporting data coming from surface faulting, earthquake mechanisms and some measured paleomagnetic rotations, caused extrusion theory to be widely accepted.
Figure 28. Plasticine model simulating the deformation due to Indo-Asian collision. Eastern side is left as a free boundary.
Offsets indicates strike-slip faulting [Tapponnier et al. 1982].
Nowadays, the same group of researchers make an effort to link internal shortening with extrusion related features. By compiling recent studies on Cenozoic deformation, magmatism and seismic structure, they formulate a time-dependent model for the growth of Tibetan plateau (Tapponnier et al. 2001)(figure 29). They introduced successive intracontinental subduction zones playing a dynamic role on maintaining stepwise growth and rise of the plateau in three main phases younging northward (figure 29, 30).
Figure 29. Simplified map of major tectonic boundaries and Tertiary faults in Tibet. Orange to pale yellow shades represent inferred
ages of principal plateau-building epochs. Dashed line shows the cross section location in figure 30 [Tapponnier et al. 2001].
Figure 30. Interpretative lithospheric cross section of Himalaya-Tibet orogen. Dashed contours and shades of blue in mantle show
velocity variation of shear-waves [Tapponnier et al. 2001].
In their model strike-slip faults develops at weakly welded sutures where oblique subduction is partitioned. In first order approximation, model seems working but there is still no solid data indicating the postulated intracontinental subduction zones.
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