Crustal Azimuthal Anisotropy Beneath the Southeastern Tibetan Plateau and its Geodynamic Implications

Abstract

The fast orientation and magnitude of crustal azimuthal anisotropy beneath the southeastern Tibetan Plateau and adjacent areas are measured by analyzing the sinusoidal moveout of the P to S converted phase from the Moho. Beneath the tectonically active plateau, the mean magnitude is 0.48 ± 0.13 s, which is about twice as large as that observed in the stable Sichuan Basin (0.23 ± 0.10 s). The two areas are separated by the Longmenshan fault zone, a zone of devastating earthquakes including the 12 May 2008 MW 7.9 Wenchuan earthquake. Fault orthogonal fast orientations observed in the southern Longmenshan fault zone, where previous studies have revealed high crustal Vp/Vs and suggested the presence of mid‐lower crustal flow, may reflect flow‐induced lattice preferred orientation of anisotropic minerals. Fault parallel anisotropy in the central segment of the fault zone is most likely related to fluid filled fractures, and fault perpendicular extensional cracks are probably responsible for the observed anisotropy in the northern segment. The crustal anisotropy measurements, when combined with results from previous studies, suggest the existence of mid‐lower crustal flow beneath the southeastern margin of the plateau. Comparison of crustal anisotropy obtained before and after the Wenchuan earthquake suggests that the earthquake has limited influence on whole crustal anisotropy, although temporal changes of anisotropy associated with the earthquake have been reported using splitting of shear waves from local earthquakes occurred in the upper crust.

Crustal Azimuthal Anisotropy Beneath the Central North China Craton Revealed by Receiver Functions

“…In contrast, the fast orientations for the upper and lower layers measured at five stations in the Zhangjiakou-Penglai Fault Zone are significantly different ( Figure 6). This nearly 90 • flip of the fast orientations is also found by direct S splitting from local events conducted in the Zhangjiakou-Penglai Fault Zone and active fault zones in Iceland (Crampin et al, 2002), as well as P m s moveout in the eastern Tibetan Plateau (Kong et al, 2016;Zheng et al, 2018). The 90 • difference between the fast orientation of the upper and lower layers could be caused by complex crustal structure beneath the fault zone with a broad shear zone at depth.…”

“…Such a spatial variation is consistent with results from a direct S splitting study (Gao et al, ), which shows that the average delay time in the central part of the fault zone is greater than 5 ms/km, whereas that in the area away from the fault is generally less than 3 ms/km. The inverse relationship between the δt measurements and the distance to the center of the fault zone, when combined with the observation that the fast orientations at the stations closest to the fault zone are parallel or subparallel to the strike of the fault zone (Figure ), may indicate a gradual intensification of fluid‐filled fractures toward the center of the active fault zone (Kong et al, ; Zheng et al, ). An alternative mechanism for the enhanced anisotropy along the fault zone is a broad zone of viscous deformation in the middle to lower crust, and a narrow zone of fluid‐filled fractures in the upper crust (e.g., Burgmann & Dresen, ).…”

“…This study takes advantage of a unique data set recorded by a dense portable array and the availability of a recently developed reverberation-removal technique to suppress the influence of the loose sedimentary layer on the RFs , for the purpose of providing constraints on the formation mechanisms of crustal anisotropy and crust-mantle deformation style in the CNCC, as well as characterizing possible effects of the tectonically active Zhangjiakou-Penglai Fault Zone on crustal anisotropy. Similar to most previous investigations utilizing RFs to quantify crustal anisotropy (e.g., Kong et al, 2016;Nagaya et al, 2008;Rumpker et al, 2014;Sun et al, 2012;Zheng et al, 2018), this study aims at quantifying the bulk crustal anisotropy under the assumption of a single horizontal axis of symmetry and a flat interface.…”

The 3D Seismic Azimuthal Anisotropies and Velocities in the Eastern Tibetan Plateau Extracted by an Azimuth‐Dependent Dispersion Curve Inversion Method

“…Pn waves travel mostly in the uppermost mantle, and therefore, the resulting azimuthal anisotropies are from the uppermost mantle. Pms phases from different directions can also be fitted with cosine functions to obtain the azimuthal anisotropies beneath stations (Sun et al, 2012; Sun et al, 2015; Zheng et al, 2018). Surface wave dispersions are the most widely used data to extract azimuthal anisotropies for different periods (Ceylan et al, 2012; Legendre et al, 2015; Su et al, 2008).…”

“…For example, joint analysis of GPS and SKS/SKKS splitting results shows that deformations in the crust and mantle are largely coupled in the eastern Tibetan Plateau, which is inconsistent with the predictions of the lower crust flow model (Chang et al, 2015; León Soto et al, 2012; Sol et al, 2007; Wang et al, 2008; Yang et al, 2018). On the other hand, based on anisotropies from Pms phases, some studies (Kong et al, 2016; Sun et al, 2012; Sun et al, 2015; Zheng et al, 2018) have suggested the existence of a lower crust flow in the region. Chen et al (2013) argued that a midlower crust flow may exist beneath the interior of the eastern Tibetan Plateau, but block extrusion resulting from pure shearing may occur in the Central Yunnan subblock.…”