Yuanyuan V. Fu

Upper mantle discontinuity structure beneath eastern and southeastern Tibet: New constraints on the Tengchong intraplate volcano and signatures of detached lithosphere under the western Yangtze Craton

We present new constraints on the upper mantle transition zone structure beneath eastern and southeastern Tibet based on P wave receiver functions for a large broadband data set from two very dense seismic arrays. A clear depression of both the 410 km and 660 km discontinuities is detected west of the Red River fault relative to the east. The correlated topographic variations across the Red River fault are indicative of temperature changes in the upper mantle above the transition zone, which suggests that the fault is a deep-rooted structure that penetrates into the upper mantle and separates Indochina from South China. West of the Red River fault, the transition zone thickness under the Tengchong volcano is found to be normal compared to the global average. This strongly suggests that the intraplate volcano may originate from slab tearing of the eastward subducting Indian plate at shallow depths in the upper mantle rather than from dehydration of a flattened plate within the transition zone. Our results further show that the 660 km discontinuity is significantly depressed under the western Yangtze Craton and that the transition zone therefore thickens by up to 20 km. This thickening is suggestive of lowered temperatures associated with a remnant of detached lithosphere in response to overlying asthenospheric escape flow in and around the western Yangtze Craton. In addition, we find that the transition zone thickness beneath much of the Sichuan Basin is similar to the global average.

Lithospheric shear wave velocity and radial anisotropy beneath the northern part of North China from surface wave dispersion analysis

Rayleigh and Love wave phase velocities in the northern part of the North China are obtained from ambient noise tomography in the period range of 8–35 s and two plane wave earthquake tomography at periods of 20–91 s using data recorded at 222 broadband seismic stations from the temporary North China Seismic Array and permanent China Digital Seismic Array. The dispersion curves of Rayleigh and Love wave from 8 to 91 s are jointly inverted for the 3-D shear wave structure and radial anisotropy in the lithosphere to 140 km depth. Distinct seismic structures is observed from the Fenhe Graben and Taihang Mountain to the North China Basin. The North China Basin from the lower crust to the depth of 140 km is characterized by high-velocity anomaly, reflecting mafic intrusion and residual materials after the extraction of melt, and by strong radial anisotropy with Vsh > Vsv, implying horizontal layering of intrusion and alignment of minerals due to vigorous extensional deformation and subsequent thermal annealing. However, low-velocity anomaly and positive radial anisotropy are observed in the Fenhe Graben and Taihang Mountain, suggesting the presence of partial melt in the lithosphere due to the mantle upwelling and horizontal flow pull.

Radial anisotropy beneath northeast Tibet, implications for lithosphere deformation at a restraining bend in the Kunlun fault and its vicinity

Three-dimensional shear wave velocity and radial anisotropy models of the crust and upper mantle beneath the NE Tibetan plateau are constructed from new measurements of Love wave dispersions (20–77s) and previously obtained Rayleigh wave dispersions (20–87s) using a two-plane-wave method. The mid-lower crust is characterized with positive anisotropy (VSH > VSV) with large strength beneath the Qinling and Qilian Mountains and small values beneath the Anyemaqen Mountain. The large positive anisotropy can be explained by horizontal alignment of anisotropic minerals in the mid-lower crust due to crustal flow. The mantle lithosphere above 90 km is largely isotropic while weak positive anisotropy appears beneath 90 km, which probably marks the lithosphere-asthenosphere boundary (LAB). A low shear wave velocity anomaly and relatively negative radial anisotropy are imaged in the entire lithosphere beneath the restraining bend in the eastern Kunlun fault, consistent with a weak lithosphere experiencing vertical thickening under horizontal compression. The asthenosphere at the restraining bend is characterized by significant low velocity and positive radial anisotropy, reflecting that the asthenosphere here is probably hotter, has more melts, and deforms more easily than the surrounding region. We propose that the lithosphere at the restraining bend was vertically thickened and subsequently delaminated locally, and induced asthenosphere upwelling. This model explains the observations of velocity and anisotropy anomalies in the lithosphere and asthenosphere as well as geological observations of rapid rock uplift at the restraining bend of the Kunlun fault.

Lithospheric structure of the southeastern margin of the Tibetan Plateau from Rayleigh wave tomography

Lithospheric shear wave velocity beneath the southeastern margin of the Tibetan plateau is obtained from Rayleigh wave tomography using earthquake data recorded by the temporary ChinArray and permanent China Digital Seismic Array. Fundamental mode Rayleigh wave phase velocities at periods of 20-100 s are determined and used to construct the 3-D shear wave velocity model. Low-velocity anomalies appear along or close to the major faults in the middle crust and become a broad zone in the lower crust, suggesting block extrusion in the shallow crust and diffuse deformation in the lower crust, both of which play important roles in accommodating the collision between the Indian and Eurasian plates. A vertical low-velocity column beneath the Tengchong volcano is observed, which could be caused by upwelling of warm mantle due to the lithosphere extension in the Thailand rift basin to the south or by fluid-induced partial melting due to the subduction of the Burma slab. The western Yangtze craton is characterized by low velocity in the crust and uppermost mantle above the fast mantle lithosphere, indicating possible thermal erosion at the western craton edge resulted from the extrusion of the Tibetan Plateau. A low-velocity zone is imaged at the depths of 70-150 km beneath the eastern part of the Yangtze craton, which could be caused by small-scale mantle convection associated with the subduction of the Burma microplate and/or the opening of the South China Sea.

Origin of intraplate volcanism in northeast China from Love wave constraints

The tectonic source for widespread volcanism in northeast China has not been completely understood. We develop a 3-D SH velocity model in NE China that provides new constraints to the origin of the volcanism. The 3-D model is constructed from fundamental mode Love waves at the periods of 20-125 s recorded at 269 broadband seismic stations. The Changbai Mountain is characterized by a significant low velocity in the lower crust and uppermost mantle, which probably results from mantle upwelling due to the subduction of the Pacific plate. A fast and thin mantle lid of ~75 km is present beneath the Songliao Basin, indicating lithosphere extension from back-arc rifting. The slow velocity in the middle and fast velocities in the south and north at 75-115 km depths in the Songliao Basin suggest complex mantle flow with upwelling and downwelling. Unlike the other volcanic fields (Changbaishan volcano, Jingpohu volcano and Abaga volcano), the Halaha volcano has high velocity in the lower crust and upper mantle, implying a limited melt supply from mantle source recently. The subduction-induced upwelling leads to complicated small-scale mantle convection, which is responsible for the intraplate magmatism in northeast China.