Discussion:

       Data provided by various disciplines showed that the Tibetan puzzle may be solved by introducing not only a single but a combination of mechanisms. I propose a model that best fits the available data and literature. Up to now, numerous models including this kind of cross sectional sketches have been suggested for Tibet but none of them had included the complete available data set. For this exercise, I used all available data that show spatial changes across an ~ N-S cross section (geological mapping, seismic reflection, refraction, tomography,  MT survey, receiver function analysis, crustal and mantle anisotropy).  At first, I scaled all of these interpretations into same scale and then overlapped them to see how consistent they are with each other. At last, I end up with the model below (figure 31).

        Based on consistent data coming from various sources, I plotted a southward dipping intra-continental subduction under northern Tibetan plateau. Kunlun mountains and Qaidam basin where active thrusting observed, represent most probably, the surface expression of this subduction (figure 31). Although seismicity related to this process is absent due to high temperature conditions (~1300 C), it clearly shows strike-slip faulting that takes the horizontal motion related to the oblique convergence (slip partitioning). The vertical extent of these major strike-slip system is still under much debate.  In more recent works, there is an observable tendency to relate them with the ancient suture zones and intra- continental subduction. But there is still little direct evidence for this hypothesis. To test this possibility, we need more geophysical work concentrated on these faults.

        In addition to these conclusions,  correlation between inefficient Sn propagation, low Pn velocities and observed anisotropy supports a weak lower crust that may have lower crustal flow.  In crustal levels, MT work also suggests  localized partial melt and/or fluid (water ?) occurrence (figure 31).

        Briefly, the unique part of this model is the way how I interpreted the anisotropy results.  All the researchers that I know, assumed a more or less uniform plane for the upper most mantle anisotropy. Since we are not able to resolve the dip of the anisotropic plane but only the trend, these results can also be explained by anisotropic structures with spatially changing dips. If this is the case, anisotropy planes should became parallel or sub parallel to the subducting crust next to them.  By assuming two major subduction acting opposite to each other, I interpreted these results as it is seen in the below figure (figure 31). It is also worth noting that studies focusing on crustal anisotropy are still very limited. Although, some authors relate them to the shallow structures, they show large variations which makes generalizations difficult.

        Finally, the high velocity anomalies observed at great depths can be the suggestive of a broken slab derived from  the Indian plate that is sinking under the interior of the Lhasa terrain.  Theoretically, this process will cause asthenospheric upwelling which also explains the low velocity anomalies recorded at similar depths (200-300 km) beneath central and northern Tibet (figure 31).

 

Figure 31. Schematic lithospheric cross section representing recent structural pattern of Himalayan-Tibetan orogen. Data used for this construction, include most of the
published geophysical and geological work.
 
 
 
 


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