The ongoing collision of the Indian and Asian continents has created the Himalaya and Tibetan plateau through a range of deformation processes. These include crustal thickening, detachment of the lower lithosphere from the plate (delamination) and flow in a weakened lower crust 1‐6 . Debate continues as to which of these processes are most significant 7 . In eastern Tibet, large-scale motion of the surface occurs, but the nature of deformation at depth remains unresolved. A large-scale crustal flow channel has been proposed as an explanation for regional uplift in eastern Tibet 6,8,9 , but existing geophysical data 10,11 do not constrain the pattern of flow. Magnetotellurics uses naturally occurring electromagnetic waves to image the Earth’s subsurface. Here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20-40 km. The channels extend horizontally more than 800 km from the Tibetan plateau into southwest China. The electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12,13 that permits flow on a geological timescale. These findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux. Our results reveal the previously unknown complexities of these patterns of crustal flow. Many previous studies of the IndiaAsia

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Crustal deformation of the eastern Tibetan plateau revealed by magnetotelluric imaging

High topography in eastern Tibet is thought to have formed in response to weak lower crust flowing towards the plateau margin. Thermochronologic analyses of rocks exposed at the eastern plateau margin record periods of mountain growth early in the Indo-Asian collision, implying that crustal flow alone could not have created the high topography.

Two-phase growth of high topography in eastern Tibet during the Cenozoic

We process rigorously GPS data observed during the past 25 years from continental China to derive site secular velocities. Analysis of the velocity solution leads to the following results. (a) The deformation field inside the Tibetan plateau and Tien Shan is predominantly continuous, and large deformation gradients only exist perpendicular to the Indo‐Eurasian relative plate motion and are associated with a few large strike‐slip faults. (b) Lateral extrusions occur on both the east and west sides of the plateau. The westward extrusion peaks at ~6 mm/yr in the Pamir‐Hindu Kush region. A bell‐shaped eastward extrusion involves most of the plateau at a maximum rate of ~20 mm/yr between the Jiali and Ganzi‐Yushu faults, and the pattern is consistent with gravitational flow in southern and southeastern Tibet where the crust shows widespread dilatation at 10–20 nanostrain/yr. (c) The southeast borderland of Tibet rotates clockwise around the eastern Himalaya syntaxis, with sinistral and dextral shear motions along faults at the outer and inner flanks of the rotation terrane. The result suggests gravitational flow accomplished through rotation and translation of smaller subblocks in the upper crust. (d) Outside of the Tibetan plateau and Tien Shan, deformation field is block‐like. However, unnegligible internal deformation on the order of a couple of nanostrain/yr is found for all blocks. The North China block, under a unique tectonic loading environment, deforms and rotates at rates significantly higher than its northern and southern neighboring blocks, attesting its higher seismicity rate and earthquake hazard potential than its neighbors.

Present‐Day Crustal Deformation of Continental China Derived From GPS and Its Tectonic Implications

The Tibetan Plateau is expanding eastwards, but the modes of deformation are poorly understood. High-resolution seismic images from the region identify localized zones of weak crustal rocks as well as deep faults, implying that deformation occurs through a combination of crustal flow and movement of rigid blocks of crust.

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Eastward expansion of the Tibetan Plateau by crustal flow and strain partitioning across faults

[1] Understanding the geotectonic evolution of the southeastern Tibetan plateau requires knowledge about the structure of the lithosphere. Using data from 77 broadband stations in SW China, we invert Rayleigh wave phase velocity dispersion curves from ambient noise interferometry (T =1 0–40 s) and teleseismic surface waves (T =2 0–150 s) for 3‐D heterogeneity and azimuthal anisotropy in the lithosphere to ∼150 km depth. Our surface wave array tomography reveals (1) deep crustal zones of anomalously low shear wave speed and (2) substantial variations with depth of the pattern of azimuthal anisotropy. Upper crustal azimuthal anisotropy reveals a curvilinear pattern around the eastern Himalayan syntaxis, with fast directions generally parallel to the main strike slip faults. The mantle pattern of azimuthal anisotropy is different from that in the crust and varies from north to south. The tomographically inferred 3‐D variation in azimuthal anisotropy helps constrain the source region of shear wave splitting. South of ∼26°N (off the high plateau) most of the observed splitting can be accounted for by upper mantle anisotropy, but for stations on the plateau proper (with thick crust) crustal anisotropy cannot be ignored. On long wavelengths, the pattern of azimuthal anisotropy in the crust differs from that in the mantle. This is easiest explained if deformation varies with depth. The deep crustal zones of low shear wave speed (and, presumably, mechanical strength) may represent loci of ductile deformation. But their lateral variation suggests that in SE Tibet (localized) crustal channel flow and motion along the major strike slip faults are both important.

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Heterogeneity and anisotropy of the lithosphere of SE Tibet from surface wave array tomography

Sounding and study on electrical structure of the crust and upper mantle within the eastern border region of Qinghai-Tibet Plateau by using the magnetotelluric sounding (simply MT) method permitted us to understand the characteristics of specific electrical structure in the region. The sounding result clearly revealed that: (1) The Xianshuihe fault zone represents a large-scale lithospheric fault and is an important boundary fault of the rhombic Sichuan-Yunnan block. (2) The sounded region is a strong earthquake-prone zone. The different crustal media of blocks on both sides of the fault became an important deep background for the strong seismo-active zone. (3) A large-scale low-resistivity layer is found to exist at a depth more than ten kilometers beneath the northern part of the rhombic Sichuan-Yunnan block. Its electrical resistivity is only several to tens Ω · m. The layer northeastward extends down at an angle of 45°. It is related to an obstacle to the lateral squeeze of Qinghai-Tibet Plateau and eastward flow of mass by the rigid block. It is inferred from the characteristics of electrical property of deep media that the northern part of the recent rhombic Sichuan-Yunnan block is in a thermal state and is one of the recently fairly active blocks. (4) The lithosphere in the sounded region is gradually thickened from the western segment (northern Sichuan-Yunnan block) to east (Yangtze block).

Sounding of electrical structure of the crust and upper mantle along the eastern border of Qinghai-Tibet Plateau and its tectonic significance

Magnetotelluric signatures of Neoproterozoic subduction, and subsequent lithospheric reactivation and thinning beneath central South China

A regional structure close to the 3D/2D model and the distortion factors obtained from the Swift and Bahr's decomposition are used in analysis to the magnetotelluric data from West Sichuan-East Xizang sounding profile. The result shows that the structure of the region along this profile is simpler in the shallow part and relatively complex in the deep part. Taking the Ya'an line as a boundary, we find that the structure is simpler and more stable in the eastern part than that in the western part. The orientation of major structural axis is basically in nearly S-N direction and its deviation is less 30°. The decomposed parameters can well express several major faults. The sounding line 1 runs across three major tectonic units, which are cut by several major faults, so the structure is most complex. The structure along the sounding line 2 is close to 3D/2D model. And the sounding line 3 exhibits stronger static shift distortion.

Denghai Bai

Papers

Crustal deformation of the eastern Tibetan plateau revealed by magnetotelluric imaging

Sounding of electrical structure of the crust and upper mantle along the eastern border of Qinghai-Tibet Plateau and its tectonic significance

Magnetotelluric signatures of Neoproterozoic subduction, and subsequent lithospheric reactivation and thinning beneath central South China

Decomposition for the Impedance Tensor Distortion of Magnetotelluric Data from the West Sichuan‐East Xizang Profile