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Variation of Pn Velocity and Anisotropy Around the Mantle Wedge Beneath the Eastern Gangdese Belt, Southern Tibet

Session: Imaging Incipient and Fossil Subduction Zones [Poster]

Type: Poster

Date: 4/22/2021

Presentation Time: 03:45 PM Pacific

Description: 

Since Cenozoic, the Indian subcontinent and the Eurasian continent have been subducting continuously, which forms the present Tibetan Plateau. In the Yarlung Zangbo Suture Zone and its north and south sides, the crust and mantle materials were strongly compressed and metasomatized. Those not only created the Gangdese metallogenic belt, but also led the deep source seismicity. All of these suggest that the continent convergent mantle wedge, as the energy transfer and material exchange between crust and mantle, retains a large number of ‘relic’. Therefore, the exploration of structural features of uppermost mantle is a key to reveal the tectonic environment where crust-mantle structure and material evolved.

Pn wave is the main seismic phase at the uppermost mantle, and Pn tomography provides a possibility to study its structure. Base on the interstation Pn traveltime difference tomography, we used the data recorded by 113 seismograph, screened out 1640 rays, and obtained the eastern segment of Gangdese by inversion.

We carried out the Pn traveltime difference tomography calculation using an initial model with average Moho depth 75 km, which is obtained by receiver function method.

The variation of Pn velocity in the uppermost mantle of Gangdese ranges from 7.73 km/s to 8.43 km/s, showing strong lateral heterogeneity, which is consistent with the regional geological structure characteristics. Based on the results, it is inferred that ~ 31° N is an important deep fault / boundary. In the south of ~31°N, the low velocity anomaly under YGR divides the high velocity body on the south side into two high velocity bodies, Xianza-Nimu and South-Lhasa body. Combined with the results of regional geochemical exploration, we can speculate that it is the result of the enrichment of water-rich mantle material related to the subduction of Indian plate and Neo-Tethys Ocean. On the whole, the anisotropic variation of Pn wave velocities at the uppermost lithospheric mantle in the eastern Gangdese shows the evolution process of mantle wedge material in the subduction tectonic environment.

Presenting Author: Yulan Li

Student Presenter: Yes

Authors