Seismic anisotropy of upper mantle in eastern Tibetan Plateau and related crust-mantle coupling pattern

Abstract

regional digital networks, and portable broadband seismic networks deployed in Sichuan, Yunnan and Tibet, we obtained the SKS fast-wave direction and the delay time between fast and slow waves of each station by use of the stacking analysis method, and finally acquired the fine image of upper mantle anisotropy in the eastern Tibetan Plateau and its adjacent regions. We analyzed the crust-mantle coupling deformation on the basis of combining the GPS observation results and the upper mantle anisotropy distribution in the study area. The Yunnan region out of the plateau has different features of crust-mantle deformation from the inside plateau. There exists a lateral transitional zone of crust-mantle coupling in the eastern edge of the Tibetan Plateau, which is located in the region between 26° and 27°N in the west of Sichuan and Yunnan. To the south of transitional zone, the fast-wave direction is gradually turned from S60°-70°E in southwestern Yunnan to near EW in southeastern Yunnan. To the north of transitional zone in northwestern Yunnan and the south of western Sichuan, the fast-wave direction is nearly NS. From crust to upper mantle, the geophysical parameters (e.g. the crustal thickness, the Bouguer gravity anomaly, and tectonic stress direction) show the feature of lateral variation in the transitional zone, although the fault trend on the ground surface is inconsistent with the fast-wave direction. This transitional zone is close by the eastern Himalayan syntaxis, and it may play an important role in the plate boundary dynamics.upper mantle anisotropy, SKS wave, fast-wave direction, crust-mantle coupling, lithospheric deformation Since the 1990s, the broadband digital technology has made great progress in the research of the Earth sciences. The seismograms recorded by the broadband seismometers deployed in the world are widely applied to the study of seismology, deep structure and geodynamics. In this paper, we focus on the mantle anisotropy and related geodynamic issues. Generally speaking, the mantle anisotropy is determined by the lattice-preferred orientation of olivine crystals as a result of the mantle deformation. There are various causes resulting in the mantle deformation. Among them, the plate motion is a direct cause. The size and direction of the mantle anisotropy strongly depend on the velocity of the plate motion. In the various subjects of recent studies on geodynamics, the anisotropy in the mantle is one of effective ways to probe the complicated deep structure beneath the continent and its evolution, the crust-mantle coupling deformation, etc. [1,2] .

Crust-mantle coupling in North China: Preliminary analysis from seismic anisotropy

“…On the basis of SKS splitting information and surface GPS measurements, Wang et al [16] inferred that crust-mantle deformation within the Tibetan Plateau agrees with the strong coupling VCD crustmantle model. However, in the Yunnan region, outside the Tibetan Plateau, it is the decoupling SAF crust-mantle model that applies.…”

“…It is related to the establishment of a deep movement model for eastern Asia. It is an effective method that uses seismic anisotropy to study crustmantle coupling relationships [15,16]. Seismic anisotropy in the upper mantle can be related to deformation patterns of deep lithospheric media and reflect the movement and surrounding stress patterns of deep media that reach down to the upper mantle [14,17,18].…”

Azimuthal anisotropy of Rayleigh waves beneath the Tibetan Plateau and adjacent areas

“…Since the 1980s, many Chinese and foreign seismologists have studied the crust and upper mantle structure of the Tibetan Plateau by using seismic body wave and surface wave data [5][6][7][8][9][10][11][12][13][14][15][16][17][18] , and have studied the upper mantle anisotropy beneath the Tibetan Plateau using teleseismic SKS-wave splitting [19][20][21][22][23][24][25][26][27][28] . Hirn et al [20] considered seismic anisotropy as an indicator of mantle flow beneath Himalayas and Tibet, and they thought that the anisotropy of the Plateau is caused by the re-orientation of crystal lattice or the oriented arrangement of liquid-filled cracks due to differential shear deformation in the horizontal plane.…”

Seismic anisotropy inferred from direct <i>S</i>-wave-derived splitting measurements and its geodynamic implications beneath southeastern Tibetan Plateau

“…Previous studies on seismic anisotropy (McNamara et al, 1994;Sol et al, 2007;Huang et al, 2000Huang et al, , 2007Wang et al, 2007Wang et al, , 2008Chen et al, 2013;Zhao et al, 2014;Guilbert et al, 1996;Bai et al, 2009) that compared splitting parameters with APM, GPS, and structural and topographical features provide crucial information concerning the dynamic deformation pattern and possible linkage of the strength of coupling between the crust and lithospheric mantle of the southeastern or eastern Tibetan region. We observe a sharp transition in the spatial distribution of φ s from nearly W-E in the western part of the study region to nearly NW-SE or NNW-SSE near the southeastern Tibetan margin (Fig.…”