Incomplete mechanical decoupling and accelerated extension in the westernmost segment of central Tibet

The youngest tectonic structures in Tibet are a series of grabens trending approximately north-south. However, the grabens in central Tibet are less prominent than those in southern Tibet, which suggests that there is spatial variability in deformation mechanisms. The Chem Co half-graben is situated in the westernmost segment of central Tibet and is characterized by boundary normal faulting that occurred during the late Quaternary. The bedrock exposure ages, as recorded by cosmogenic nuclides 10Be and 26Al, vary linearly with elevation along the normal fault plane, yielding an average extension rate of 2.2−2.4 mm/yr. This millennial extension rate is ten times higher than the long-term rate (i.e., 0.3−0.4 mm/yr over millions of years), suggesting an episode of accelerated extension. We performed lithospheric flexure modeling to simulate extension of the westernmost segment of central Tibet. The results reveal heterogeneous σ2 stress concentrations at depth, suggesting that the accelerated extension is most plausibly the result of localized stress concentrations associated with lateral variations in crustal rheology. By comparing the material loss calculated from the extensional rates and crustal thinning, it seems that the eastward spread of crustal material is decoupled from the middle−lower crust but is affected by its upwelling. Therefore, the direct effects of convergence between India and Eurasia are less prominent in central Tibet, except for its western part, where rapid uplift continues owing to oblique collision.