P Wave Anisotropic Tomography of the SE Tibetan Plateau: Evidence for the Crustal and Upper‐Mantle Deformations

Abstract

The upper crust in the SE Tibetan Plateau is rotating around the eastern Himalayan syntax clockwise, and the western margin of the Yangtze Craton has been involved in the active tectonics. However, it is still unclear whether and how the deep crust and upper mantle respond to the plateau expansion. In this study we present a high‐resolution three‐dimensional model of P wave velocity tomography and azimuthal anisotropy in the crust and uppermost mantle beneath the SE Tibetan Plateau determined using traveltime data recorded by a dense seismic network. Widespread low‐velocity zones are revealed around a high‐velocity body in the deep crust and uppermost mantle beneath the southwest Yangtze Craton, where fast‐velocity directions of the azimuthal anisotropy are mostly parallel to the contour lines of the surface topography and the Moho depth along the plateau margin. These results indicate that gravitational potential plays an important role in the crustal and uppermost‐mantle deformations in the SE Tibetan Plateau. Meanwhile, the shallow crust may drop down to the deep crust and drive the ductile deep‐crustal material to intrude into the adjacent regions. The extruded crust is trapped in a deep‐crustal corner and obstructed by the surrounding strong blocks. The trapped crustal material may rise up, causing significant uplift and high heat flow at the surface.

P and S Wave Tomography Beneath the SE Tibetan Plateau: Evidence for Lithospheric Delamination

“…The picking accuracy is ~0.1 s for P wave arrivals and ~0.25 s for S wave arrivals. The data set has been used to determine a 3‐D P wave velocity model of the crust and upper mantle beneath the SE Tibetan Plateau by Huang et al (). All the local events have been well relocated with the obtained 3‐D velocity model (see Huang et al, , for details).…”

“…We inverted the local earthquake arrival times and teleseismic relative travel‐time residuals simultaneously for 3‐D P and S wave velocity (Vp, Vs) models of the crust and upper mantle beneath the SE Tibetan Plateau using the method of Zhao et al (, ). The starting 1‐D velocity model is modified from the AK135 velocity model (Kennett et al, ) according to the local travel‐time curves (see Huang et al, , for details; Figure S2). The Moho topography is derived from the surface wave and receiver function analyses (Pasyanos et al, ; Wang et al, ).…”

Sharp Lateral Moho Variations Across the SE Tibetan Margin and Their Implications for Plateau Growth

“…Specifically, the model of lower crustal flow explains the crustal thickening in the Tibetan margin well and has been supported by geophysical results in recent years, including high‐conductivity anomalies (e.g., Bai et al, 2010), low‐velocity anomalies (e.g., Bao et al, 2015; M. Chen et al, 2014; Fu et al, 2017; Z. Huang et al, 2018; Liu et al, 2014; W. Wang et al, 2014; Wei et al, 2013; Yao et al, 2008, 2010), and positive radial anisotropies ( V SH > V SV ) (H. Huang et al, 2010; Xie et al, 2017) and by GPS measurements (Shen et al, 2005). Previous receiver function studies have shown a gentle variation in the Moho topography from the Tibetan Plateau to the Yangtze Craton (e.g., Sun et al, 2012; W. Wang et al, 2017; X. Xu et al, 2013), consistent with the Moho variation predicted by the lower crustal flow model.…”

A Method for Estimating the Crustal Azimuthal Anisotropy and Moho Orientation Simultaneously Using Receiver Functions

“…We consider only a 4% anisotropy single-layer crust with a horizontal axis of symmetry in the S wave velocity. This anisotropy is considerably stronger than the average (Crampin, 1994;Z. Huang et al, 2018;Ji et al, 2015;Ji & Salisbury, 1993;Tatham et al, 2008;.…”

“…According to the recent revealed smoothed Moho topography (Z. Huang et al, 2018;Pasyanos et al, 2014;W. Wang et al, 2017), the Moho raises to the depth of~52 km approximately 70 km to the northwest (i.e., at 30.3°N to 99.5°E) and drops to a depth of~65 km approximately 70 km to the southeast (i.e., at 29.5°N to 101°E).…”