Crustal and Uppermost Mantle Structure Across the Tibet-Qinling Transition Zone in NE Tibet: Implications for Material Extrusion Beneath the Tibetan Plateau

East Asia is a tectonically active area on earth and has a complicated lithospheric deformation due to the western Indo-Asian continental collision and the eastern oceanic subduction mainly from Pacific plate. Till now, mantle dynamics beneath this area is not well understood due to its complex mantle structure, especially in the framework of global spherical mantle convection. Hence, a series of numerical models are conducted in this study to reveal the key controlling parameters in shaping the present-day observed mantle structure beneath East Asia under 3-D global mantle flow models. Global mantle flow models with coarse mesh are firstly applied to give a rough constraint on global mantle convection. The detailed description of upper mantle dynamics of East Asia is left with regional refined mesh. A power-law rheology and absolute plate field are applied subsequently to get a better constraint on the related regional mantle rheological structure and surficial boundary conditions. Thus, the refined and reasonable velocity and stress distributions of upper mantle beneath East Asia at different depths are retrieved based on our 3-D global mantle flow simulations. The derived large shallow mantle flow beneath the Tibetan Plateau causes significant lithospheric shear drag and dynamic topography that result in prominent tectonic evolution of this area. And the Indo&#8211;Asian collision may have induced mantle flow beneath the Indian plate and the different velocity structures between the asthenosphere and lithosphere indicate the shear drag of asthenospheric mantle. That may explain the reason that Indo&#8211;Asian collision has occurred for 50 Ma, and this collision can still continue to accelerate uplift in the Tibetan plateau. Finally, we also consider the possible implementations of 3-D numerical simulations combined with global lithosphere and deep mantle dynamics so as to discuss the relevant influences.

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Incision migration across Eastern Tibet controlled by monsoonal climate, not tectonics?

10.5194/egusphere-egu2020-444 ◽

2020 ◽

Author(s):

Katharine Groves ◽

Mark Allen ◽

Christopher Saville ◽

Martin Hurst ◽

Stuart Jones

Keyword(s):

Tibetan Plateau ◽

Transition Zone ◽

Channel Flow ◽

East Asian Monsoon ◽

Crustal Thickening ◽

Mean Annual Precipitation ◽

The Tibetan Plateau ◽

Geomorphic Indices ◽

Hypsometric Integral ◽

Early Cenozoic

The formation and uplift history of the Tibetan Plateau, driven by the India-Eurasia collision, is the subject of intense research. Geomorphic indices capture the landscape response to competition between climate and tectonics and reflect the spatial distribution of erosion. We analyse the link between climate and tectonics in the eastern part of the Tibetan Plateau using the mean annual precipitation, digital elevation data, and by calculating the geomorphic indices hypsometric integral (HI), surface roughness (SR) and elevation relief ratio (ZR). This is a region where competing tectonic models suggest either early Cenozoic plateau growth, or a late phase of crustal thickening, surface uplift and plateau growth driven by lower crustal flow (&#8220;channel flow&#8221;).Swath profiles of rainfall, elevation and the geomorphic indices were constructed, orthogonal to the internal drainage boundary. Each profile was analysed to find the location of maximum change in trend. A broad transition zone is present in the landscape, where changes in landscape and precipitation are grouped and in alignment. The zone cuts across structural boundaries. It represents, from East to West, a sharp decline in precipitation below ~650 mm/yr (interpreted as the western extent of the East Asian monsoon), a change from a high relief landscape to smoother elevations at 4500-5000 m, a transition to low HI (< 0.05), a decrease in SR and an increase in ZR. This zone is not a drainage divide: the main rivers have their headwaters further West, in the interior of the plateau.We argue that this geomorphic-climatic transition zone represents a change from incised to non-incised landscapes, the location of which is controlled by the western extent of the monsoon. Published low temperature thermochronology data suggest the plateau had reached its modern extent at the Eocene, but has been exhumed since ~15 Ma to the East of the transition zone, at least along major drainage networks. We therefore also suggest that the transition zone is the current position of a long-term wave of incision that has migrated from East to West, driven by late Cenozoic intensification of the monsoon climate. This work supports a model of early Cenozoic growth of the eastern Tibetan Plateau, superimposed by incision driven by climate change; it does not support the channel flow model.

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